MXenes: From Discovery to Applications of Two-Dimensional Metal Carbides and Nitrides 9789814877954

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MXenes MXenes

MXenes

From Discovery to Applications of

Two-Dimensional Metal Carbides and Nitrides

edited by

Yury Gogotsi

Published by Jenny Stanford Publishing Pte. Ltd. 101 Thomson Road #06-01, United Square Singapore 307591 Email: [email protected] Web: www.jennystanford.com British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. MXenes: From Discovery to Applications of Two-Dimensional Metal Carbides and Nitrides Copyright © 2023 Jenny Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher. Cover image: Courtesy of Younghee Lee, CEO/Design Director at Scientific Graphic & Journal Cover Artwork, Korea. ISBN 978-981-4877-95-4 (Hardcover) ISBN 978-1-003-30651-1 (eBook)

Contents Preface

Part I: Introduction 1. The Rise of MXenes

xxvii

3

Yury Gogotsi and Babak Anasori

Part II: Discovery 2. Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

15

Michael Naguib, Murat Kurtoglu, Volker Presser, Jun Lu,

Junjie Niu, Min Heon, Lars Hultman, Yury Gogotsi, and

Michel W. Barsoum

3. Two-Dimensional Transition Metal Carbides

31

Michael Naguib, Olha Mashtalir, Joshua Carle, Volker Presser, Jun Lu, Lars Hultman, Yury Gogotsi, and Michel W. Barsoum

3.1 3.2

3.3 3.4

Introduction Results and Discussion 3.2.1 Ti2AlC 3.2.2 Ta4AlC3 3.2.3 TiNbAlC and (V0.5,Cr0.5)3AlC2 3.2.4 Ti3AlCN Conclusions Materials and Methods 3.4.1 Synthesis of MAX Phases and Exfoliation

into MXenes 3.4.2 Characterization

32

34

38

39

42

44

46

47

47

48

vi Contents

4. Two-Dimensional, Ordered, Double Transition Metals

Carbides

55

Babak Anasori, Yu Xie, Majid Beidaghi, Jun Lu, Brian C. Hosler,

Lars Hultman, Paul R. C. Kent, Yury Gogotsi, and

Michel W. Barsoum

4.1 4.2

4.3 4.4

Introduction Results and Discussion 4.2.1 Theoretical Prediction of Double

Transition Metal MXenes 4.2.2 Synthesis of Double Transition Metal

MXenes 4.2.3 Electrochemistry of Mo2TiC2Tx Conclusions Materials and Methods 4.4.1 Synthesis of MAX Phases 4.4.2 Synthesis of MXenes 4.4.3 Preparation and Testing of LIB Electrodes 4.4.4 Delamination of Mo2TiC2Tx and

Preparation of MXene ‘Paper’ 4.4.5 Electrochemical Capacitor Fabrication

and Testing 4.4.6 Microstructural Characterization 4.4.7 Density Functional Theory Simulations

5. Synthesis of Two-Dimensional Titanium Nitride Ti4N3

56

58

58

61

65

69

70

70

70

71

71

71

72

72

79

Patrick Urbankowski, Babak Anasori, Taron Makaryan,

Dequan Er, Sankalp Kota, Patrick L. Walsh, Mengqiang Zhao,

Vivek B. Shenoy, Michel W. Barsoum, and Yury Gogotsi

6. Synthesis of Mo4VAlC4 MAX Phase and Two-Dimensional

Mo4VC4 MXene with Five Atomic Layers of Transition Metals 95

Grayson Deysher, Christopher Eugene Shuck,

Kanit Hantanasirisakul, Nathan C. Frey, Alexandre C. Foucher,

Kathleen Maleski, Asia Sarycheva, Vivek B. Shenoy, Eric A. Stach,

Babak Anasori, and Yury Gogotsi

6.1

Introduction

96

Contents

6.2

Results and Discussion

6.2.1

Synthesis and Structural Characterization

6.2.4

Thermal Analysis

6.2.2 6.2.3

6.3 6.4

6.2.5 6.2.6

101

Electrical and Optical Properties

109

Compositional Characterization Density Functional Theory

Experimental Methods 6.4.2

6.4.3

Synthesis of Mo4VAlC4 MAX

Synthesis of Mo4VC4 MXene

116

116

116

117

119

Optical Properties

6.4.9

112

Compositional Characterization

6.4.7 6.4.8

107

118

Structural Characterization

6.4.6

103

Mo4VC4 Film Preparation

6.4.4 6.4.5

99

Microscopy

Conclusions

6.4.1

99

Microscopy

Electrical Properties Thermal Analysis

6.4.10 Density Functional Theory Calculations

Part III: Properties

7. Electronic and Optical Properties of 2D Transition Metal Carbides and Nitrides

119

119

120

121

121

121

135

Kanit Hantanasirisakul and Yury Gogotsi

7.1 Introduction

136

7.4 Computational Studies of Electronic Properties

149

7.6 Effects of Surface Terminations on Electronic Properties

161

7.2 Synthesis and Processing

7.3 Structure and Surface Chemistry

7.5 Experimental Measurements of Electronic and

Transport Properties

139

143

155

vii

viii

Contents

7.7 Electromagnetic Interference Shielding

Properties 7.8 Sensors 7.8.1 Pressure and Strain Sensors 7.8.2 Molecular Sensing 7.9 MXene Heterostructures 7.10 Optoelectronic Properties: Transparent

Conductive Thin Films 7.11 Nonlinear Optical Properties 7.12 Plasmonic Properties 7.13 Light-to-Heat Conversion and Photothermal

Therapy Applications 7.14 Conclusions and Perspectives

8. Elastic Properties of 2D Ti3C2Tx MXene Monolayers

and Bilayers

163

167

167

169

173

178

182

184

186

189

207

Alexey Lipatov, Haidong Lu, Mohamed Alhabeb, Babak Anasori, Alexei Gruverman, Yury Gogotsi, and Alexander Sinitskii

8.1 8.2 8.3 8.4

Introduction Results Discussion Materials and Methods 8.4.1 Synthesis of Ti3C2Tx 8.4.2 Materials Characterization 8.4.2.1 Scanning electron microscopy 8.4.2.2 Atomic force microscopy 8.4.3 Analysis of Force-Indentation Curves

9. Control of MXenes’ Electronic Properties through

Termination and Intercalation

208

210

218

219

219

220

220

220

220

227

James L. Hart, Kanit Hantanasirisakul, Andrew C. Lang,

Babak Anasori, David Pinto, Yevheniy Pivak, J. Tijn van Omme,

Steven J. May, Yury Gogotsi, and Mitra L. Taheri

9.1 Introduction 9.2 Results

228

230

Contents

9.2.1 Sample Synthesis and Experimental

Approach

9.2.2 Adsorbed Species

9.3 9.4

230

233

9.2.3 Intercalation

234

Methods

245

9.2.4 Termination

Discussion

9.4.1 Syntheses of MXenes

9.4.2 Electron Microscopy and Spectroscopy

239

244

245

247

9.4.3 In situ Heating and Biasing

247

9.4.5 Low Temperature Electronic Transport

249

10. High-Temperature Behavior and Surface Chemistry

of Carbide MXenes Studied by Thermal Analysis

257

9.4.4 Thermogravimetric-Mass Spectrometry

Analysis

9.4.6 X-ray Photoelectron Spectroscopy 9.4.7 X-ray Diffraction

248

249

250

Mykola Seredych, Christopher Eugene Shuck, David Pinto,

Mohamed Alhabeb, Eliot Precetti, Grayson Deysher,

Babak Anasori, Narendra Kurra, and Yury Gogotsi

10.1 Introduction 10.2 Results and Discussion 10.2.1 Ti3C2Tx Etched in HF of Different

Concentrations 10.2.2 Ti3C2Tx Etched in Mixed Acids 10.2.3 Ti3C2Tx Films 10.2.4 M2CTx MXenes: Mo2CTx and Nb2CTx 10.2.5 Mo2CTx and Nb2CTx Films 10.2.6 Chemical and Thermal Stability

Implications 10.3 Summary 10.4 Experimental Section 10.4.1 Synthesis of Ti3C2Tx and Fabrication of

Ti3C2Tx Film

258

260

262

264

265

266

267

268

270

271

271

ix

x

Contents

10.4.2 Synthesis of Ti3C2Tx Using HF/H2SO4 or

HF/HCl 10.4.3 Synthesis of Mo2CTx Multilayer Powder

and 2D Mo2CTx Film 10.4.4 Synthesis of Nb2CTx Multilayer Powder

and 2D Nb2CTx Film 10.4.5 Thermal Analysis−Mass Spectrometry

11. Electrochromic Effect in Titanium Carbide MXene

Thin Films Produced by Dip-Coating

272

272

273

273

281

Pol Salles, David Pinto, Kanit Hantanasirisakul,

Kathleen Maleski, Christopher E. Shuck, and Yury Gogotsi

11.1 Introduction 11.2 Results and Discussion 11.2.1 Thin Film Processing by Dip-Coating

and Characterization 11.2.2 In situ Electrochemical and Optical Characterization of Ti3C2Tx Thin Films 11.2.3 Understanding the Mechanism Involved

in the Electrochromic Changes 11.3 Conclusions 11.4 Experimental Section

12. Effects of Synthesis and Processing on Optoelectronic

Properties of Titanium Carbonitride MXene

282

284

284

286

290

295

296

303

Kanit Hantanasirisakul, Mohamed Alhabeb, Alexey Lipatov,

Kathleen Maleski, Babak Anasori, Pol Salles, Chanoknan Ieosakulrat,

Pasit Pakawatpanurut, Alexander Sinitskii, Steven J. May,

and Yury Gogotsi

12.1 Introduction 12.2 Experimental Section 12.3 Results and Discussion 12.3.1 Synthesis and Delamination 12.3.2 Optoelectronic Properties 12.3.3 Electronic and Transport Properties 12.4 Conclusions

304

307

311

311

318

321

325

Contents

13. Raman Spectroscopy Analysis of the Structure and Surface Chemistry of Ti3C2Tx MXene

333

Asia Sarycheva and Yury Gogotsi 13.1 Introduction

334

13.4 Materials

349

13.2 Results and Discussion 13.3 Conclusions

13.4.1 Raman Spectrometer

Part IV: Synthesis and Processing

14. Intercalation and Delamination of Layered Carbides

and Carbonitrides

336

348

349

359

Olha Mashtalir, Michael Naguib, Vadym N. Mochalin,

Yohan Dall’Agnese, Min Heon, Michel W. Barsoum, and

Yury Gogotsi

14.1 Introduction 14.2 Results

14.2.1 Intercalation of MXenes

14.2.2 Molecular Dynamics (MD) Simulations 14.2.3 Delamination of MXene

14.2.4 Energy Storage Applications of

Delaminated MXene

14.3 Discussion 14.4 Methods

14.4.1 Intercalation of f-MXene

14.4.2 De-intercalation of MXene 14.4.3 Delamination of MXene

360

361

361

365

366

368

369

370

370

370

371

14.4.4 Preparation of Pressed MXene Discs

371

14.4.7 Preparation of Lithium Coin Cells

372

14.4.5 Physical Characterization

14.4.6 Electrochemical Characterization 14.4.8 MD Simulations

371

372

373

xi

xii

Contents

15. Conductive Two-Dimensional Titanium Carbide

‘Clay’ with High Volumetric Capacitance

379

Michael Ghidiu, Maria R. Lukatskaya, Meng-Qiang Zhao,

Yury Gogotsi, and Michel W. Barsoum

16. Amine-Assisted Delamination of Nb2C MXene for

Li-Ion Energy Storage Devices

401

Olha Mashtalir, Maria R. Lukatskaya, Meng-Qiang Zhao,

Michel W. Barsoum, and Yury Gogotsi

17. Guidelines for Synthesis and Processing of

Two-Dimensional Titanium Carbide

415

Mohamed Alhabeb, Kathleen Maleski, Babak Anasori,

Pavel Lelyukh, Leah Clark, Saleesha Sin, and Yury Gogotsi

17.1 Introduction 17.2 Discussion of Methods 17.2.1 General Synthesis and Processing of

Ti3C2Tx MXene 17.2.2 Choice of Materials 17.2.2.1 Choice of Ti3AlC2 precursor 17.2.2.2 Choice of etchants and

intercalants 17.2.3 Choice of the Synthesis Method 17.2.4 HF Etching Protocol 17.2.5 In situ HF Formation 17.2.6 Bifluoride-Based Etchants 17.2.7 Fluoride-Based Salt Etchants 17.2.8 Choice of Intercalation Method 17.2.9 Dimethyl Sulfoxide 17.2.10 Tetraalkylammonium Hydroxides 17.2.11 Lithium Ions 17.2.12 Processing, Deposition, and Storage 17.2.13 Sonication and Size Selection 17.2.14 Deposition of Flakes 17.2.15 Substrate Functionality

416

420

420

421

421

423

423

423

427

427

428

431

431

432

434

437

437

438

440

Contents

17.2.16 Storage of Material 17.2.17 Characterization Methods 17.3 Summary

18. Selective Etching of Silicon from Ti3SiC2 (MAX) to Obtain 2D Titanium Carbide (MXene)

440 441 442

451

Mohamed Alhabeb, Kathleen Maleski, Tyler S. Mathis, Asia Sarycheva, Christine B. Hatter, Simge Uzun, Ariana Levitt, and Yury Gogotsi

19. Additive-Free MXene Inks and Direct Printing of Micro-Supercapacitors

463

Chuanfang (John) Zhang, Lorcan McKeon, Matthias P. Kremer, Sang-Hoon Park, Oskar Ronan, Andrés Seral-Ascaso, Sebastian Barwich, Cormac Ó Coileáin, Niall McEvoy, Hannah C. Nerl, Babak Anasori, Jonathan N. Coleman, Yury Gogotsi, and Valeria Nicolosi

19.1 Introduction 19.2 Results 19.2.1 Solvent Selection Criteria 19.2.2 Formulation of Inkjet-Printable MXene Organic Inks 19.2.3 All-MXene Inkjet-Printed Patterns 19.2.4 Extrusion Printing of All-MXene Patterns 19.2.5 Charge-Storage Performance of Printed MSCs 19.3 Discussion 19.4 Methods 19.4.1 Preparation of Ti3C2Tx Aqueous Inks 19.4.2 Preparation of Ti3C2Tx Organic Inks 19.4.3 Inkjet Printing of Micro-Supercapacitors and Resistors 19.4.4 Extrusion Printing of MicroSupercapacitors 19.4.5 Materials Characterization 19.4.6 Electrochemical Characterization

464 466 466 468 470 473

475 477 478 478 478 479 479 480 480

xiii

xiv

Contents

20. Additive-Free MXene Liquid Crystals and Fibers

487

Jizhen Zhang, Simge Uzun, Shayan Seyedin, Peter A. Lynch,

Bilen Akuzum, Zhiyu Wang, Si Qin, Mohamed Alhabeb,

Christopher E. Shuck, Weiwei Lei, E. Caglan Kumbur,

Wenrong Yang, Xungai Wang, Genevieve Dion, Joselito M. Razal,

and Yury Gogotsi

20.1 Introduction 20.2 Results

20.2.1 MXene Liquid Crystals 20.2.2 Pure LC MXene Fibers

20.2.3 Pure LC MXene Fiber Properties

20.3 Conclusion 20.4 Methods

488

491

491

496

501

505

506

20.4.1 Synthesis of Ti3C2 MXene

506

20.4.3 Synthesis and Delamination of Mo2Ti2C3

and Ti2C MXenes Inks

507

20.4.2 Preparation of L-Ti3C2 and S-Ti3C2

MXene Inks

20.4.4 Wet-Spinning of Pure MXene Fibers 20.4.5 Characterization

21. Scalable Manufacturing of Free-Standing, Strong

Ti3C2Tx MXene Films with Outstanding Conductivity

507

508

509

519

Jizhen Zhang, Na Kong, Simge Uzun, Ariana Levitt, Shayan Seyedin, Peter A. Lynch, Si Qin, Meikang Han, Wenrong Yang, Jingquan Liu, Xungai Wang, Yury Gogotsi, and Joselito M. Razal

22. Scalable Synthesis of Ti3C2Tx MXene

539

Christopher E. Shuck, Asia Sarycheva, Mark Anayee, Ariana Levitt, Yuanzhe Zhu, Simge Uzun, Vitaliy Balitskiy, Veronika Zahorodna, Oleksiy Gogotsi, and Yury Gogotsi

22.1 22.2 22.3 22.4

Introduction Results and Discussion Conclusion Experimental Section

540

543

552

553

Contents

Part V: Composites 23. Flexible and Conductive MXene Films and Nanocomposites with High Capacitance

563

Zheng Ling, Chang E. Ren, Meng-Qiang Zhao, Jian Yang, James M. Giammarco, Jieshan Qiu, Michel W. Barsoum, and Yury Gogotsi

23.1 Introduction 23.2 Results and Discussion 23.2.1 Conductive, Flexible, Free-Standing Ti3C2Tx Films 23.2.2 Conductive, Flexible, Free-Standing Ti3C2Tx/PDDA and Ti3C2Tx/PVA Composites 23.2.3 Mechanical Properties of the Ti3C2Tx and Ti3C2Tx/PVA Films 23.2.4 Capacitive Performance of Ti3C2Tx-Based Films 23.3 Conclusion 23.4 Materials and Methods 23.4.1 Preparation of MXene-Based Nanocomposites 23.4.2 Fabrication of Free-Standing Ti3C2Tx and Its Composite Films 23.4.3 Mechanical Testing 23.4.4 Electrochemical Testing

24. Flexible MXene/Graphene Films for Ultrafast Supercapacitors with Outstanding Volumetric Capacitance

565 567

567

569

571

573 575 576 576

576 577 577

583

Jun Yan, Chang E. Ren, Kathleen Maleski, Christine B. Hatter, Babak Anasori, Patrick Urbankowski, Asya Sarycheva, and Yury Gogotsi

24.1 Introduction 24.2 Results and Discussion 24.3 Conclusions

584 587 601

xv

xvi

Contents

24.4 Experimental Section 24.4.1 Preparation of Delaminated Ti3C2Tx MXene Solution 24.4.2 Fabrication of Flexible MXene/rGO Hybrid Films 24.4.3 Material Characterizations 24.4.4 Electrochemical Measurements

25. Cold Sintered Ceramic Nanocomposites of 2D MXene and Zinc Oxide

602

602 602 603 604

609

Jing Guo, Benjamin Legum, Babak Anasori, Ke Wang, Pavel Lelyukh, Yury Gogotsi, and Clive A Randall

26. Colloidal Gelation in Liquid Metals Enables Functional Nanocomposites of 2D Metal Carbides and Lightweight Metals

625

Vladislav Kamysbayev, Nicole M. James, Alexander S. Filatov, Vishwas Srivastava, Babak Anasori, Heinrich M Jaeger, Yury Gogotsi, and Dmitri V. Talapin

26.1 Introduction

26.2 Results and Discussion

26.2.1 Preparation and Exfoliation of MXene Sheets

26.2.2 Liquid Metals as Particle Dispersion Media

26.2.3 Colloidal Gelation in Liquid Metals

626 629

629 630 633

26.2.4 MXene/Mg−Li Composite

635

26.4.1 MXene Exfoliation via Minimally Intensive Layer Delamination

643

26.3 Conclusions 26.4 Methods

26.4.2 Intercalation with Tetramethylammonium Hydroxide

26.4.3 Mg−Li Alloy with Ti3C2Tx MXenes

26.4.4 Al-Doped Mg−Li Alloy with Ti3C2Tx MXenes

642 643

644

644

645

Contents

26.4.5 Ga Liquid Metal with Ti3C2Tx/TMA+ MXenes 26.4.6 Rheology Measurements 26.4.7 Mechanical Testing 26.4.8 Synchrotron X-ray Diffraction 26.4.9 Transmission Electron Microscopy 26.4.10 Focused Ion Beam-Scanning Electron Microscopy

Part VI: Energy Storage

27. MXene: A Promising Transition Metal Carbide Anode for Lithium-Ion Batteries

645 646 646 646 646 647

655

Michael Naguib, Jérémy Come, Boris Dyatkin, Volker Presser, Pierre-Louis Taberna, Patrice Simon, Michel W. Barsoum, and Yury Gogotsi

27.1 Introduction 27.2 Experiment 27.2.1 Synthesis of Exfoliated Ti2C 27.2.2 Characterization 27.2.3 Electrochemical Testing 27.3 Results and Discussions 27.4 Conclusions

28. Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide

656 657 657 657 658 658 662

665

Maria R. Lukatskaya, Olha Mashtalir, Chang E. Ren, Yohan Dall’Agnese, Patrick Rozier, Pierre Louis Taberna, Michael Naguib, Patrice Simon, Michel W. Barsoum, and Yury Gogotsi

29. 2D Metal Carbides and Nitrides for Energy Storage

677

Babak Anasori, Maria R. Lukatskaya, and Yury Gogotsi

29.1 Introduction

29.2 Synthesis of MXenes

29.2.1 Etching with Hydrofluoric Acid

678

679 683

xvii

xviii

Contents

29.2.2 Etching in the Presence of a Fluoride

Salt

683

29.3.1 Structure of the MXene Layer

685

29.2.3 Delamination

29.3 Structure and Properties

29.3.2 Surface Terminations

29.3.3 Effect of Synthesis Conditions on MXene Quality and Terminations

29.3.4 Stability

684

685

685

687

690

29.3.5 Physical and Mechanical Properties

691

29.4.2 MXene-Based Electrochemical Capacitors

698

29.4 Energy Storage Applications of 2D Carbides 29.4.1 MXenes in Batteries

29.5 Applications other than Energy Storage 29.6 Conclusions

29.7 Gaps in the Current Knowledge

30. Ultra-High-Rate Pseudocapacitive Energy Storage in

Two-Dimensional Transition Metal Carbides

695

695

703

705

707

723

Maria R. Lukatskaya, Sankalp Kota, Zifeng Lin, Meng-Qiang Zhao, Netanel Shpigel, Mikhael D. Levi, Joseph Halim, Pierre-Louis Taberna, Michel W. Barsoum, Patrice Simon, and Yury Gogotsi

30.1 Introduction 30.2 Theoretical Capacitance and Voltage Window 30.3 Electrode Design for High Volumetric

Performance 30.4 Electrode Design for High-Rate Performance 30.5 Conclusions 30.6 Methods 30.6.1 Synthesis of Ti3C2Tx 30.6.2 Synthesis of Mo2CTx 30.6.3 Preparation of Ti3C2 ‘Paper’ Electrodes 30.6.4 Preparation of Macroporous MXene

Electrodes 30.6.5 Preparation of Ti3C2Tx Hydrogels

724

726

729

730

733

734

734

735

735

735

736

Contents

30.6.6 Electrochemical Measurements 30.6.7 Capacitance Calculations 30.6.8 Characterization of Structure and Properties 30.6.9 Characterization of Macroporous Electrodes 30.6.10 Processing Control for Hydrogel Films

31. Thickness-Independent Capacitance of Vertically Aligned Liquid-Crystalline MXenes

737 737

738 738 739

745

Yu Xia, Tyler S. Mathis, Meng-Qiang Zhao, Babak Anasori, Alei Dang, Zehang Zhou, Hyesung Cho, Yury Gogotsi, and Shu Yang

32. High Capacity Silicon Anodes Enabled by MXene Viscous Aqueous Ink

759

Chuanfang (John) Zhang, Sang-Hoon Park, Andrés Seral-Ascaso, Sebastian Barwich, Niall McEvoy, Conor S. Boland, Jonathan N. Coleman, Yury Gogotsi, and Valeria Nicolosi

32.1 Introduction 32.2 Results and Discussion 32.2.1 MXene Ink Characterization 32.2.2 Electrode Fabrication and Characterization 32.2.3 Electrical and Mechanical Characterization 32.2.4 Electrochemical Characterization of nSi/MXene Anodes 32.2.5 Performance of Gr-Si/MX-C Anode 32.2.6 Comparison with Published Data 32.3 Conclusion 32.4 Methods 32.4.1 MXene Ink Preparation 32.4.2 Electrode Fabrication 32.4.3 Material Characterization 32.4.4 Electrochemical Characterization

760 762 762 764 766 768 771 773 774 775 775 775 776 776

xix

xx

Contents

33. Influences from Solvents on Charge Storage in

Titanium Carbide MXenes

783

Xuehang Wang, Tyler S. Mathis, Ke Li, Zifeng Lin, Lukas Vlcek,

Takeshi Torita, Naresh C. Osti, Christine Hatter,

Patrick Urbankowski, Asia Sarycheva, Madhusudan Tyagi,

Eugene Mamontov, Patrice Simon, and Yury Gogotsi

33.1 Introduction

784

33.2 Distinct Charging Processes for Different Solvent Systems

786

33.4 Full Desolvation towards Enhanced Energy

Storage in MXene

795

33.3 Molecular Arrangements of Electrolytes in

MXenes 33.5 Conclusions 33.6 Methods

790

798

798

33.6.1 Preparation of Ti3C2 Colloidal Solution

798

33.6.3 Preparation of Macroporous Ti3C2

799

33.6.5 Material Characterization

800

33.6.2 Preparation of Vacuum-Filtered Ti3C2

Thin Film 33.6.4 Preparation of Graphene–CNT

Composites for Positive Electrode 33.6.6 Electrode Preparation 33.6.7 Electrochemical Tests

33.6.8 Calculations for the Electrochemical

Tests 33.6.9 MD Simulations

Part VII: Biomedical, Environmental, and Catalytic Applications

34. Charge- and Size-Selective Ion Sieving through Ti3C2Tx

MXene Membranes Chang E. Ren, Kelsey B. Hatzell, Mohamed Alhabeb, Zheng Ling,

Khaled A. Mahmoud, and Yury Gogotsi

799

800

802

802

803

804

813

Contents

35. Single Platinum Atoms Immobilized on an MXene as an Efficient Catalyst for the Hydrogen Evolution Reaction 829

Jinqiang Zhang, Yufei Zhao, Xin Guo, Chen Chen, Chung-Li Dong,

Ru-Shi Liu, Chih-Pin Han, Yadong Li, Yury Gogotsi,

and Guoxiu Wang

35.1 Introduction 35.2 Results

35.2.1 Synthesis and Structural Characterization

of Mo2TiC2Tx–PtSA 35.2.2 Electronic States of Atoms in

Mo2TiC2Tx–PtSA

35.2.3 Mechanistic Study on Electrochemical

Exfoliation and Pt Single-Atom

Immobilization 35.2.4 Electrochemical HER Evaluation of

Mo2TiC2Tx–PtSA

35.2.5 DFT Calculation of Mo2TiC2O2–PtSA

towards HER

35.3 Conclusions 35.4 Methods

35.4.1 Synthesis of Mo2TiC2Tx MXene

35.4.2 Delamination of Mo2TiC2Tx MXene

Using Organic Solvent 35.4.3 Synthesis of Mo2TiC2Tx–VMo

830

832

832

835

836

838

841

843

844

844

844

844

35.4.4 Synthesis of Mo2TiC2Tx–PtSA

845

35.4.7 DFT Calculations

847

35.4.5 Characterization

35.4.6 Electrochemical Measurements

36. MXene Molecular Sieving Membranes for Highly Efficient Gas Separation

845

846

853

Li Ding, Yanying Wei, Libo Li, Tao Zhang, Haihui Wang, Jian Xue, Liang-Xin Ding, Suqing Wang, Jürgen Caro, and Yury Gogotsi

36.1 Introduction 36.2 Results

854

856

xxi

xxii

Contents

36.2.1 Preparation of MXene Nanosheets

36.2.2 Preparation of 2D MXene Membranes 36.2.3 Gas Separation Performance of 2D

MXene Membranes 36.2.4 Gas Separation Mechanism

36.3 Discussion 36.4 Methods

856

856

858

860

861

863

36.4.1 Preparation of the MXene Membranes

863

36.4.3 Gas Permeation Measurements

864

36.4.2 Characterization of the MXene

Nanosheets and Membranes 36.4.4 MD Simulations

37. MXene Sorbents for Removal of Urea from Dialysate:

A Step toward the Wearable Artificial Kidney

864

866

875

Fayan Meng, Mykola Seredych, Chi Chen, Victor Gura, Sergey

Mikhalovsky, Susan Sandeman, Ganesh Ingavle,

Tochukwu Ozulumba, Ling Miao, Babak Anasori, and

Yury Gogotsi

37.1 Introduction

37.2 Results and Discussion

876

880

37.2.1 Interaction between Urea and MXenes

880

37.2.4 Assessment of MXene Ti3C2Tx

Biocompatibility

890

37.4.1 Materials

894

37.2.2 Urea Adsorption from Aqueous Solution 37.2.3 Urea Adsorption from Dialysate

37.3 Conclusions

37.4 Experimental Section

37.4.2 Adsorption of Urea from Aqueous

Solution

37.4.3 Adsorption of Urea from Dialysate

881

888

893

894

895

897

Contents

38. A Gel-Free Ti3C2Tx-Based Electrode Array for High-Density, High-Resolution Surface Electromyography

903

Brendan B. Murphy, Patrick J. Mulcahey, Nicolette Driscoll,

Andrew G. Richardson, Gregory T. Robbins, Nicholas V. Apollo,

Kathleen Maleski, Timothy H. Lucas, Yury Gogotsi,

Timothy Dillingham, and Flavia Vitale

38.1 Introduction

38.1.1 High-Density Surface Electromyography

38.1.2 Materials and Design Strategies for sEMG 38.1.3 Fabrication of Ti3C2Tx MXene HDsEMG

Arrays

38.2 Results and Discussion

38.2.1 Impedance Measurements in Saline and

on Human Skin

38.2.2 Baseline sEMG Recording

904

904

906

909

911

911

915

38.2.3 High-Resolution Mapping of Muscle

Activation

917

38.4.1 Ti3C2Tx HDsEMG Array Fabrication

921

38.3 Conclusion

38.4 Experimental Section

38.4.2 Impedance Spectroscopy

38.4.3 Testing the Effects of Skin Treatment on Skin Impedance 38.4.4 sEMG Recordings 38.4.5 sEMG Analysis

Part VIII: Applications in Optics, Electronics and Sensing 39. Electromagnetic Interference Shielding with 2D Transition Metal Carbides Faisal Shahzad, Mohamed Alhabeb, Christine B. Hatter,

Babak Anasori, Soon Man Hong, Chong Min Koo, and Yury Gogotsi

921

921

923

923

924

924

933

xxiii

xxiv

Contents

40. 2D Titanium Carbide for Wireless Communication

949

Asia Sarycheva, Alessia Polemi, Yuqiao Liu, Kapil Dandekar,

Babak Anasori, and Yury Gogotsi

40.1 Introduction

40.2 Results and Discussion 40.3 Materials and Methods

40.3.1 MXene Synthesis

40.3.2 Ti3C2 Spraying on PET

40.3.3 Measuring Sheet Resistance 40.3.4 Determining Thickness

40.3.5 Scanning Electron Microscopy 40.3.6 Atomic Force Microscopy 40.3.7 UV-vis Spectroscopy 40.3.8 X-ray Diffraction

40.3.9 Electrodynamic Simulations

41. Metallic Ti3C2Tx MXene Gas Sensors with Ultrahigh Signal-to-Noise Ratio

950

952

963

963

963

964

964

964

965

965

965

965

971

Seon Joon Kim, Hyeong-Jun Koh, Chang E. Ren, Ohmin Kwon, Kathleen Maleski, Soo-Yeon Cho, Babak Anasori, Choong-Ki Kim, Yang-Kyu Choi, Jihan Kim, Yury Gogotsi, and Hee-Tae Jung

41.1 Introduction

972

41.4 Methods

983

41.2 Results and Discussion 41.3 Conclusions

41.4.1 Synthesis of Ti3C2Tx

974

983

983

41.4.2 Preparation of Black Phosphorus, MoS2,

and Reduced Graphene Oxide Solutions

984

41.4.4 Gas Delivery System and Resistance

Measurements

985

41.4.3 Ti3C2Tx, BP, MoS2, and RGO Film Fabrication

and Transfer onto a Sensor Electrode 984

41.4.5 Noise Power Spectral Density

Measurements

985

Contents

41.4.6 Binding Energy Calculations via Density

Functional Theories 41.4.7 Characterization

986

986

42. Surface-Modified Metallic Ti3C2Tx MXene as Electron

Transport Layer for Planar Heterojunction Perovskite

Solar Cells

993

Lin Yang, Chunxiang Dall’Agnese, Yohan Dall’Agnese, Gang Chen, Yu Gao, Yoshitaka Sanehira, Ajay Kumar Jena, Xiao-Feng Wang, Yury Gogotsi, and Tsutomu Miyasaka

42.1 Introduction 42.2 Results and Discussion 42.2.1 Characterization of Ti3C2Tx Nanosheets

and UV-Ozone Treated Ti3C2Tx Films 42.2.2 Photovoltaic Characterization 42.3 Conclusion 42.4 Experimental Section 42.4.1 Materials 42.4.2 Preparation of Ti3C2Tx MXene

Hydrocolloid 42.4.3 Preparation of Ti3C2Tx MXene Films 42.4.4 Device Fabrication 42.4.5 Thin Film Characterization 42.4.6 Device Characterization

43. Anomalous Absorption of Electromagnetic Waves by

2D Transition Metal Carbonitride Ti3CNTx

994

996

996

1000

1005

1005

1005

1006

1006

1006

1007

1007

1013

Aamir Iqbal, Faisal Shahzad, Kanit Hantanasirisakul,

Myung-Ki Kim, Jisung Kwon, Junpyo Hong, Hyerim Kim,

Daesin Kim, Yury Gogotsi, Chong Min Koo

44. Beyond Ti3C2Tx: MXenes for Electromagnetic Interference Shielding

1029

Meikang Han, Christopher Eugene Shuck, Roman Rakhmanov, David Parchment, Babak Anasori, Chong Min Koo, Gary Friedman, and Yury Gogotsi

44.1 Introduction

1031

xxv

xxvi

Contents

44.2 Results and Discussion 44.3 Conclusions 44.4 Methods 44.4.1 Materials 44.4.2 Synthesis of MAX Powders 44.4.3 Synthesis of MXenes 44.4.3.1 Synthesis of Ti3C2Tx, Ti2CTx,

and Ti3CNTx 44.4.3.2 Synthesis of Mo2TiC2Tx,

Mo2Ti2C3Tx, and Nb4C3Tx 44.4.3.3 Synthesis of TiyNb2−yCTx 44.4.3.4 Synthesis of NbyV2−yCTx 44.4.4 Fabrication of MXene Films 44.4.4.1 Spin-casting 44.4.4.2 Spray-coating 44.4.4.3 Vacuum-assistant filtration 44.4.5 Characterization

Index

1033

1043

1044

1044

1044

1045

1045

1045

1046

1046

1046

1046

1047

1047

1047

1053

Preface Nanoscale materials or nanomaterials have been in the center of materials research since the discovery of fullerenes in 1985. After the discovery of carbon nanotubes in the early 90s, the research largely focused on one-dimensional materials. The observation of interesting physics in separated graphene sheets in 2004 brought two-dimensional (2D) materials to limelight. While the 20th century was the century of plastics and semiconductors, the 21st century looks like the century of nanomaterials so far. We are observing a shift from the use of conventional bulk materials and device manufacturing by subtractive processes to additive manufacturing and self-assembly of materials, structures and devices from nanoscale building blocks. The key role in shaping the future of materials belongs to 2D materials, which can be assembled into dense and strong heterostructures by using flat layers as building blocks—think of building a house using bricks, eventually with organic or inorganic “mortar.” Assembly from nanoscale “bricks” allows us to create materials with the required combinations of properties. The variety of 2D materials has been quickly increasing, including numerous 2D building blocks with rich chemistry, that can be used to assemble materials and devices for advanced technologies. Among the fastest growing 2D families are carbides and nitrides of transition metals known as MXenes. In 2D MXene with a general formula of Mn+1XnTx, n + 1 (n = 1 to 4), layers of early transition metals are interleaved with n layers of carbon or nitrogen. The Tx in the formula represents surface terminations, such as O, OH, halogens or chalcogens, which are bonded to the outer M layers. The transformative synthesis of the first MXene (2D titanium carbide, Ti3C2Tx) was performed at Drexel University in 2011 by selective etching of Ti3AlC2 MAX phase, as described in Chapter 2. Since that time, more than 50 MXene compositions have been published. At least a hundred of stoichiometric carbide and nitride MXene compositions are

xxviii

Preface

theoretically possible. More than a thousand compositions can be made, if surface terminations are added, and an infinite number of solid solutions and high-entropy MXenes can be created. They offer not only unique combinations of properties but also a way to finely tune the properties by varying the structure and composition, opening a new era of chemically tunable nanomaterials. The large family of MXenes and their unique combination of properties open the doors to a variety of different applications, many of which are described in this book. At the time of publication of this book, close to 14,000 publications from 87 countries with the word “MXene” in the abstract could be identified in Web of Science. The numbers of publications and citations are growing with acceleration. Multiple journals have published focus and virtual issues on MXenes. International conferences on MXenes are organized annually, in addition to dedicated symposia at professional society meetings, such as Materials Research Society (MRS) and American Chemical Society (ACS). Rapid expansion of the field and enormous interest in MXenes worldwide justify the publication of this book. My research on MXenes over the years was supported by the U. S. Department of Energy, starting with the initial discovery (Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract No. DE-AC02-05CH11231, Subcontract 6951370 under the Batteries for Advanced Transportation Technologies (BATT) Program) to study of fundamental properties of MXenes (DE-SC0018618) and exploration of their energy-related applications within the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Office of Science, Office of Basic Energy Sciences. Support from the U. S. National Science Foundation, Division of Materials Research (DMR-2041050 and other grants) for fundamental and applied research on MXenes is equally appreciated. I am grateful to other funding agencies, private foundations, and companies for supporting our MXene research projects in the past decade; they are acknowledged separately at the end of each chapter of this book.

Preface

This book compiles the most important research from my multidisciplinary team and numerous collaborators worldwide. It reports on the discovery and the rise of MXenes. Synthesis and processing of MXenes, their properties and incorporation into polymer, ceramic and metal matrices to produce composites are described. Applications of MXenes in energy storage, electrocatalysis, functional textiles, optics, electronics, communication, sensing, healthcare, as well as environmental technology are discussed in 44 chapters on more than 1000 pages. This book will appeal to anyone interested in the synthesis, properties, and applications of MXenes and nanomaterials in general.

Yury Gogotsi Charles T. and Ruth M. Bach Distinguished University Professor Director, A.J. Drexel Nanomaterials Institute Drexel University Philadelphia, Pennsylvania, USA March 29, 2023

xxix

Part I Introduction

Chapter 1

The Rise of MXenes Yury Gogotsia and Babak Anasoria,b aDepartment

of Materials Science & Engineering, Drexel University,

Philadelphia, Pennsylvania 19104, USA

bA.J. Drexel Nanomaterials Institute, Drexel University,

Philadelphia, Pennsylvania 19104, USA

[email protected], [email protected]

For the past 15 years, starting with the discovery of the unique physical properties of single-layer graphene, two-dimensional (2D) materials have been widely researched. This interest led to both a new wave of research on known 2D materials, such as metal dichalcogenides and boron nitride, and the discovery of many new 2D materials [1, 2]. Although many of these materials remain subjects of purely academic interest, others have jumped into the limelight due to their attractive properties, which have led to practical applications. Among the latter are carbides and nitrides of transition metals known as MXenes (pronounced “maxenes”), a fast-growing family of 2D materials. In a 2D flake of MXene, n + 1 (n = 1−3) layers of early transition metals Reprinted from ACS Nano, 13(8), 8491−8494, 2019. MXenes: From Discovery to Applications of Two-Dimensional Metal Carbides and Nitrides Edited by Yury Gogotsi Text Copyright © 2019 American Chemical Society Layout Copyright © 2023 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4877-95-4 (Hardcover), 978-1-003-30651-1 (eBook) www.jennystanford.com

4

The Rise of MXenes

(M, elements in blue in Fig. 1.1) are interleaved with n layers of carbon or nitrogen (X, elements in gray in Fig. 1.1), with a general formula of Mn+1XnTx. The Tx in the formula represents the surface terminations, such as O, OH, F, and/or Cl (elements in orange in Fig. 1.1), which are bonded to the outer M layers [3]. Atomic schematics of three types of MXenes are shown at the bottom of Fig. 1.1a. The variety of compositions and structures of MXenes has led to the formation of a large and rapidly expanding family of 2D materials. MXenes (Fig. 1.1a), their precursor MAX phases, and intercalated metal ions in MXenes (Fig. 1.1b) serve as embodiments of the fundamental principles of chemistry, showing how the elements can be used as building blocks to form a variety of nanomaterials. To mark the 150th anniversary of Dmitri Mendeleev’s now-iconic periodic table of the elements, the United Nations General Assembly and UN Educational, Scientific, and Cultural Organization have proclaimed 2019 the International Year of the Periodic Table of Chemical Elements [4]. The MAX and MXene compositions nicely illustrate the power of the periodic table. In the past 2 years, the MXenes field has seen significant increases in the numbers of research areas and publications [5]. The 2nd International Conference on MXenes, held in May 2019 at the Beijing University of Chemical Technology in Beijing, China, attracted 450 participants, more than double the number of attendees of the first conference, organized at Jilin University in Changchun, China, a year earlier, an indication of the quickly growing interest in this family of materials. The 2nd International Conference on MXenes covered numerous aspects of the basic science and applications of MXenes, including synthesis, structure, and properties, as well as applications in energy storage and conversion, environment and catalysis, separation membranes, medicine, optics, and electronics. This variety of topics represents a major expansion in MXene applications compared to the first conference, which focused on MXenes for energy. MXene symposia in Europe, the United States, and China are scheduled for the end of this year and spring and summer 2020, respectively.

The Rise of MXenes

(a)

(b)

Figure 1.1 Periodic tables showing compositions of MXenes and MAX phases. (a) Elements used to build MXenes. The bright blue elements represent MXenes that have not been yet experimentally confirmed. The schematics of three typical structures of MXenes are presented at the bottom. (b) Elements used to build MAX phases, MXenes, and their intercalated ions. The elements with blue striped background are only reported in MXene precursors (MAX phases), and their MXenes have not yet been synthesized. The elements on the red background are the A elements in MAX phases that can potentially be selectively etched to make MXenes. The green background shows the cations that have been intercalated into MXenes to date. As per the legend at the bottom, 1M and 1A indicate the formation possibility of a single (pure) transition metal and A element MAX phase (and MXene). SS indicates the existence of solid solutions in transition metal atomic planes (blue) or A element planes (red); 2M shows the formation possibility of an ordered double-transition metal MAX phase or MXene (either in-plane or out-of-plane) [5].

5

6

The Rise of MXenes

Several factors are contributing to the rapid expansion of the field. The first evidence for significant growth of MXenes is the number of research institutions that are studying MXenes and have already published their work in peer-reviewed journals (more than 750 institutions from 50 countries) [5]. This intense research has led to fast growth in the number of synthesized compositions. The first MXene (Ti3C2Tx) was discovered at Drexel University in 2011 [6], with no prior prediction for the stability of such 2D compounds. Since then, more than 30 MXene compositions have been published (marked in blue in Fig. 1.2), and dozens more have been explored by computational methods (marked in gray in Fig. 1.2). A unique feature of MXenes comes into play when two transition metals are mixed in a MXene structure. In addition to the formation of expected solid solutions, such as (Ti,Nb)CTx (marked in green in Fig. 1.2), transition metals can form ordered structures in a single 2D MXene flake, either by forming atomic sandwiches of transition metals planes (for n ≥ 2) such as Mo2TiC2Tx, or in-plane (n = 1) ordered structures such as (Mo2/3Y1/3)2CTx. Although ordered MXenes were synthesized in 2014 and reported in 2015 [7], many new compositions have been synthesized in this subfamily of MXenes (marked in red in Fig. 1.2). The rush in synthesizing new ordered 2D carbide phases brought excitement to the MAX phase research community. Researchers who have been studying MAX phases for the past two decades started to develop new MXene precursors, mainly MAX phases but also other layered carbides and nitrides. Since 2017, researchers have synthesized about 30 new ordered double-transition metal MAX phases and explored their properties, including their magnetic characteristics [8−10]. Transition metals that are only reported in MAX phases are marked with blue stripes in Fig. 1.1b. In the past 2 years, computational studies on MXenes and their precursors have predicted hundreds of possible compositions [10−14]. The formation of solid solutions on M and/or X sites offers possibilities for the synthesis of an infinite number of nonstoichiometric MXenes and an attractive opportunity to finely tune properties by mixing different transition metals or creating carbonitrides. There are ongoing attempts to produce 2D borides and thereby to add another X element to the system.

The Rise of MXenes

Figure 1.2 MXene compositions reported to date. The top row shows structures of mono-M MXenes; the second row shows double-M solid solutions (SS). The double-M SS compositions are marked in green. The third row shows ordered double-M MXenes, and their compositions are marked in red. The fourth row shows an ordered divacancy structure, which has only been reported for the M2C MXenes, making a M1.33C composition due to ~33 atom % of vacancies in the M layers, and their compositions are marked in pink. This table includes both experimentally (marked in blue) and theoretically (marked in gray) explored compositions of MXenes. Surface terminations are not shown. Adapted with permission from Ref. [3]. Copyright 2017 Springer Nature.

Another important advance witnessed in the past year is the development of processes for fluoride-free synthesis of MXenes. Most of the initially published MXene synthesis routes involved fluoride-containing compounds, either aqueous or molten salts [3]. An electrochemical fluoride-free synthesis route, for example, in dilute hydrochloric acid, was recently reported for Ti2CTx MXene synthesis [15]. However, scaling up selective electrochemical etching methods to large production volumes may be a challenging task. In 2019, Huang et al. used molten

7

8

The Rise of MXenes

ZnCl2 salt to synthesize new MAX phases as well as fluorine-free MXenes [16]. This method can significantly widen experimental research on MXenes, as those who are interested in MXenes but do not want to work with any hydrofluoric acid (HF)-containing or HF-forming chemicals in their laboratories can now synthesize MXenes.

Figure 1.3 Explored applications and properties of MXenes to date. The center pie chart shows the ratio of publications in each explored application/ property of MXenes with respect to the total number of publications on MXenes. The middle pie chart ring, with the same colors, shows the starting year for exploration of each application/property of MXenes. NB: although there may be one or two papers published prior to the mentioned year, we considered a year with several significant publications as the starting (breakthrough) year for each slice. The outer ring shows the ratio of publications on Ti3C2Tx MXene versus all other MXene compositions (M2XTx, M3X2Tx, and M4X3Tx) [5].

The Rise of MXenes

MXenes have a unique combination of properties, including the high electrical conductivity and mechanical properties of transition metal carbides/nitrides; functionalized surfaces that make MXenes hydrophilic and ready to bond to various species; high negative zeta-potential, enabling stable colloidal solutions in water; and efficient absorption of electromagnetic waves, which have led to a large number of applications. MXene applications are presented as the center pie chart in Fig. 1.3 [5]. The second ring in Fig. 1.3 shows the year in which the first papers reported on each application. The first explored application of MXenes was in energy storage, which remains a large proportion of MXene activities. The use of MXenes in the biomedical field, although only 2 years old, has become one of the hottest research topics with studies on photothermal therapy of cancer, theranostics, biosensors, dialysis, and neural electrodes [5, 17]. Another area in which MXene research is taking over from other nanomaterials is in electromagnetic applications, including electromagnetic interference shielding and printable antennas [18]. In other fields, including electronic and structural applications, most of the published studies are theoretical with relatively few experimental papers, and many predicted properties, such as ferromagnetism or topological insulators, have yet to be validated experimentally. To date, more than 70% of all MXene research has focused on the first discovered MXene, Ti3C2Tx. The exploration of this MXene is so extensive that, for many researchers, the name MXene has become synonymous with Ti3C2Tx, and they use it without specifying the composition, which can be confusing as there are numerous structures and compositions of MXenes. At least 100 stoichiometric MXene compositions and a limitless number of solid solutions offer not only unique combinations of properties but also a way to tune them by varying ratios of M or X elements. The large, underexplored family of MXenes and their unique combination of properties opens the door to a variety of different applications, and the possibilities of new compositions make us believe that we are still in the early days of MXene research and many exciting discoveries are to come.

9

10

The Rise of MXenes

Acknowledgments Y.G. would like to thank the U.S. Department of Energy for continuous funding of his research on MXenes from the initial discovery (Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies under Contract No. DE-AC02-05CH11231, subcontract 6951370 under the Batteries for Advanced Transportation Technologies (BATT) Program) to exploration of energy-related applications via the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the Office of Science, Office of Basic Energy Sciences. Research of Y.G. and B.A. on fundamental properties of MXenes is currently supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Grant No. DE-SC0018618.

References

1. Zhang, H. Ultrathin Two-Dimensional Nanomaterials. ACS Nano 2015, 9, 9451−9469. 2. Wee, A. T. S., Hersam, M. C., Chhowalla, M., Gogotsi, Y. An Update from Flatland. ACS Nano 2016, 10, 8121−8123.

3. Anasori, B., Lukatskaya, M. R., Gogotsi, Y. 2D Metal Carbides and Nitrides (MXenes) for Energy Storage. Nat. Rev. Mater. 2017, 2, 16098.

4. Seijo, B. C. 2019: The Year the Periodic Table Gets Its Due. C&EN 2019, 97, 24−25.

5. Anasori, B., Gogotsi, Y. 2D Metal Carbides and Nitrides (MXenes), Structure, Properties and Applications; Springer: Berlin, 2019.

6. Naguib, M., Kurtoglu, M., Presser, V., Lu, J., Niu, J., Heon, M., Hultman, L., Gogotsi, Y., Barsoum, M. W. Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248−4253.

7. Anasori, B., Xie, Y., Beidaghi, M., Lu, J., Hosler, B. C., Hultman, L., Kent, P. R. C., Gogotsi, Y., Barsoum, M. W. Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes). ACS Nano 2015, 9, 9507−9516.

8. Tao, Q., Lu, J., Dahlqvist, M., Mockute, A., Calder, S., Petruhins, A., Meshkian, R., Rivin, O., Potashnikov, D., Caspi, E. a. N., Shaked, H., Hoser, A., Opagiste, C., Galera, R.-M., Salikhov, R., Wiedwald, U., Ritter, C., Wildes, A. R., Johansson, B., Hultman, L., Farle, M., Barsoum,

References

M. W., Rosen, J. Atomically Layered and Ordered Rare-Earth i-MAX Phases: A New Class of Magnetic Quaternary Compounds. Chem. Mater. 2019, 31, 2476−2485.

9. Petruhins, A., Lu, J., Hultman, L., Rosen, J. Synthesis of Atomically

Layered and Chemically Ordered Rare-Earth (RE) i-MAX Phases;

(Mo2/3RE1/3)2GaC with RE=Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Mater.

Res. Lett. 2019, 7, 446−452.

10. Dahlqvist, M., Lu, J., Meshkian, R., Tao, Q., Hultman, L., Rosen, J. Prediction and Synthesis of a Family of Atomic Laminate Phases with Kagomé-Like and In-Plane Chemical Ordering. Sci. Adv. 2017, 3, No. e1700642. 11. Ashton, M., Hennig, R. G., Broderick, S. R., Rajan, K., Sinnott, S. B. Computational Discovery of Stable M2AX Phases. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 94, 054116.

12. Dahlqvist, M., Petruhins, A., Lu, J., Hultman, L., Rosen, J. The Origin of Chemically Ordered Atomic Laminates (i-MAX); Expanding the Elemental Space by a Theoretical/Experimental Approach. ACS Nano 2018, 12, 7761−7770. 13. Rajan, A. C., Mishra, A., Satsangi, S., Vaish, R., Mizuseki, H., Lee, K.-R., Singh, A. K. Machine-Learning Assisted Accurate Band Gap Predictions of Functionalized MXene. Chem. Mater. 2018, 30, 4031−4038.

14. Frey, N. C., Wang, J., Vega Bellido, G. I. n., Anasori, B., Gogotsi, Y., Shenoy, V. B. Prediction of Synthesis of 2D Metal Carbides and Nitrides (MXenes) and their Precursors with Positive and Unlabeled Machine Learning. ACS Nano 2019, 13, 3031−3041.

15. Sun, W., Shah, S., Chen, Y., Tan, Z., Gao, H., Habib, T., Radovic, M., Green, M. Electrochemical Etching of Ti2AlC to Ti2CTx (MXene) in LowConcentration Hydrochloric Acid Solution. J. Mater. Chem. A 2017, 5, 21663−21668. 16. Li, M., Lu, J., Luo, K., Li, Y., Chang, K., Chen, K., Zhou, J., Rosen, J., Hultman, L., Eklund, P., Persson, P. O. Å., Du, S., Chai, Z., Huang, Z., Huang, Q. An Element Replacement Approach by Reaction with Lewis Acidic Molten Salts To Synthesize Nanolaminated MAX Phases and MXenes. J. Am. Chem. Soc. 2019, 141, 4730−4737.

17. Cheng, L., Wang, X., Gong, F., Liu, T., Liu, Z. 2D Nanomaterials for Cancer Theranostic Applications. Adv. Mater. 2019, 1902333.

18. Sarycheva, A., Polemi, A., Liu, Y., Dandekar, K., Anasori, B., Gogotsi, Y. 2D Titanium Carbide (MXene) for Wireless Communication. Sci. Adv. 2018, 4, No. eaau0920.

11

Part II Discovery

Chapter 2

Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2 Michael Naguib,a,b Murat Kurtoglu,a,b Volker Presser,a,b Jun Lu,c Junjie Niu,a,b Min Heon,a,b Lars Hultman,c Yury Gogotsi,a,b and Michel W. Barsouma aDepartment

of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA bA.J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, Pennsylvania 19104, USA cDepartment of Physics IFM, Linkoping University, Linkoping 58183, Sweden

[email protected], [email protected]

Typically two-dimensional (2D) free-standing crystals exhibit properties that differ from those of their 3D counterparts [1]. Currently, however, there are relatively few such atomically layered solids [2–5]. Here, we report on 2D nanosheets, composed of a few Ti3C2 layers and conical scrolls, produced by the room temperature exfoliation of Ti3AlC2 in hydrofluoric acid. The large elastic moduli predicted by ab initio simulation, and the possibility of varying their surface chemistries (herein they are terminated by hydroxyl and/or fluorine groups) render these Reprinted from Adv. Mater., 23(37), 4248−4253, 2011.

MXenes: From Discovery to Applications of Two-Dimensional Metal Carbides and Nitrides Edited by Yury Gogotsi Text Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Layout Copyright © 2023 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4877-95-4 (Hardcover), 978-1-003-30651-1 (eBook) www.jennystanford.com

16

Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

nanosheets attractive as polymer composite fillers. Theory also predicts that their bandgap can be tuned by varying their surface terminations. The good conductivity and ductility of the treated powders suggest uses in Li-ion batteries, pseudocapacitors, and other electronic applications. Since Ti3AlC2 is a member of a 60+ group of layered ternary carbides and nitrides known as the MAX phases, this discovery opens a door to the synthesis of a large number of other 2D crystals. Arguably the most studied freestanding 2D material is graphene, which was produced by mechanical exfoliation into single-layers in 2004 [1]. Some other layered materials, such as hexagonal BN [2], transition metal oxides, and hydroxides [4], as well as clays [3], have also been exfoliated into 2D sheets. Interestingly, exfoliated MoS2 single layers were reported as early as in 1986 [5]. Graphene is finding its way to applications ranging from supercapacitor electrodes [6] to reinforcement in composites [7]. Although graphene has attracted more attention than all other 2D materials combined, its simple chemistry and the weak van der Waals bonding between layers in multilayer structures limit its use. Complex, layered structures that contain more than one element may offer new properties because they provide a larger number of compositional variables that can be tuned for achieving specific properties. Currently, the number of non-oxide materials that have been exfoliated is limited to two fairly small groups, hexagonal van der Waals bonded structures (e.g., graphene and BN) and layered metal chalcogenides (e.g., MoS2, WS2, etc.) [8]. It is well established that the ternary carbides and nitrides with a Mn+1AXn formula, where n = 1, 2, or 3, M is an early transition metal, A is an A-group (mostly groups 13 and 14) element, and X is C and/or N, form laminated structures with anisotropic properties [9, 10]. These, so-called MAX, phases are layered hexagonal (space group P63/mmc), with two formula units per unit cell (Fig. 2.1a). Near-close-packed M-layers are interleaved with pure A-group element layers, with the X-atoms filling the octahedral sites between the former. One of the most widely studied and a promising member of this family is Ti3AlC2 [11, 12] (Fig. 2.1a). Over 60 MAX phases are currently known to exist [9].

Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

Figure 2.1 Schematic of the exfoliation process for Ti3AlC2. (a) Ti3AlC2 structure. (b) Al atoms replaced by OH after reaction with HF. (c) Breakage of the hydrogen bonds and separation of nanosheets after sonication in methanol.

The Mn+1Xn layers are chemically stable. By comparison, because the A-group atoms are relatively weakly bound, they are the most reactive species. For example, heating Ti3SiC2 in a C-rich atmosphere results in the loss of Si and the formation of TiCx [13]. When the same compound is placed in molten cryolite [14] or molten Al [15], essentially the same reaction occurs: the Si escapes and a TiCx forms. In the case of cryolite, the vacancies that form lead to the formation of a partially ordered cubic TiC0.67 phase. In both cases, the high temperatures led to a structural transformation from a hexagonal to a cubic lattice and a partial loss of layering. In some cases, such as Ti2InC, simply heating in vacuum at ≈800 °C, results in loss of the A-group element and TiCx formation [16]. Removing both the M and A elements from the MAX structure by high-temperature chlorination results in a porous carbon known as carbide-derived carbon with useful and unique properties [17, 18]. Mechanical deformation of the MAX phases, which is mediated by basal dislocations and is quite anisotropic, can lead to partial delamination and formation of lamellas with thicknesses that range from tens to hundreds of nanometers [19]. However, none of the MAX phases have ever been exfoliated into few-nanometerthick crystalline layers reminiscent of graphene. Furthermore,

17

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Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

as far as we are aware, there are no reports on the selective room temperature or moderate-temperature liquid or gas-phase extraction of the A-group layers from the MAX phases and/or their exfoliation. Here, we report the extraction of the Al from Ti3AlC2 and formation of a new of 2D material (Fig. 2.1b,c) that we propose to call “MXene” to emphasize its graphene-like morphology. Based on the results presented below it is reasonable to conclude that the following simplified reactions occur when Ti3AlC2 is immersed in HF: Ti3AlC2 + 3HF = AlF3 + 3/2H2 + Ti3C2 Ti3C2 + 2H2O = Ti3C2(OH)2 + H2

Ti3C2 + 2HF = Ti3C2F2 + H2

(2.1)

(2.2)

(2.3)

Reaction (2.1) is essential and is followed by Reaction (2.2) and/or (2.3). In the remainder of this paper we present evidence for the aforementioned reactions and that they result in 2D Ti3C2 exfoliated layers with OH and/or F surface groups (Fig. 2.1b,c). Reaction (2.2) and (2.3) are simplified in that they assume the terminations are –OH or –F, respectively, when in fact they most probably are a combination of both. In order to understand the dominant reaction, density functional theory (DFT)-based geometry optimizations were carried out on both hydroxylated (Reaction 2.2) and fluorinated (Reaction 2.3) MXene layers and theoretical X-ray diffraction (XRD) patterns of the optimized structures were compared to the experimental XRD results. A summary of the results is shown in Table 2.1. The Ti3AlC2 structure is composed of individual Ti3C2 layers separated by Al atoms. When Reaction 2.1 takes place, Al atoms are removed from between the layers, resulting in the exfoliation of individual Ti3C2 layers from each other due to the loss of metallic bonding holding them together when the Al atoms are present. The exfoliated 2D Ti3C2 layers possess two exposed Ti atoms per unit formula that should be satisfied by suitable ligands. Since the experiments were conducted in an aqueous environment rich in fluorine ions, hydroxyl and fluorine are the most probable

Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

ligands. Modeling of each case was conducted by attaching respective ligands to the exposed Ti atoms followed by full geometry optimizations. Table 2.1 Summary of the DFT calculation results

Unit cell parameters (Å)

Volume change

Formula

a=b

c

Ti3AlC2 (Exp.)

3.080

18.415

–

Ti3C2

3.048

15.006

–19%

21.541

+16%

Ti3AlC2

Ti3C2(OH)2 Ti3C2F2

3.058 3.059 3.019

18.554 19.494

–

+5%

XRD pattern of the initial Ti2AlC-TiC mixture after heating to 1350 °C for 2 h resulted in peaks that corresponded mainly to Ti3AlC2 (bottom curve in Fig. 2.2a). When the Ti3AlC2 powders were placed into the HF solution, bubbles, presumed to be H2, were observed, suggesting a chemical reaction. Ultrasonication of the reaction products in methanol for 300 s resulted in significant weakening of the XRD peaks and the appearance of an amorphous broad peak around 24° 2θ (top diffractogram in Fig. 2.2a). Exfoliation leads to a loss of diffraction signal in the out-of-plane direction, and the nonplanar shape of the nanosheets results in broadening of the peaks corresponding to in-plane diffraction. When the same powders were cold pressed to 1 GPa into free-standing, 300 μm thick and 25 mm diameter discs (Fig. 2.2e), their XRD patterns showed that most of the nonbasal plane peaks of Ti3AlC2, most notably the most intense peak at ≈39° 2θ, disappear (middle curve in Fig. 2.2a). On the other hand, the (001) peaks, such as the (002), (004), and (0010), broadened, lost intensity, and shifted to lower angles compared to their location before treatment. Using the Scherrer formula [20], the average particle dimension in the [0001] direction after treatment is estimated to be 11 ± 3 nm, which corresponds to roughly ten Ti3C2(OH)2 layers. To identify the peaks, we simulated the XRD patterns of hydroxylated Ti3C2(OH)2, (red curve in center of

19

20

Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

Fig. 2.2a) and fluorinated Ti3C2F2, structures (gold curve in center of Fig. 2.2a). Clearly, both were in good agreement with the XRD patterns of the pressed sample (purple curve in Fig. 2.2a); the agreement was better with the former. The disappearance of the most intense diffraction peak of Ti3AlC2 at 39° 2θ and the good agreement between the simulated XRD spectra for Ti3C2(OH)2 and the experimental results provides strong evidence of the formation of the latter. The presence of OH groups after treatment was confirmed by Fourier transform infrared (FTIR) spectroscopy.

Figure 2.2 Analysis of Ti3AlC2 before and after exfoliation. (a) XRD pattern for Ti3AlC2 before HF treatment, simulated XRD patterns of Ti3C2F2 and Ti3C2(OH)2, measured XRD patterns of Ti3AlC2 after HF treatment, and exfoliated nanosheets produced by sonication. (b) Raman spectra of Ti3AlC2 before and after HF treatment. (c) XPS spectra of Ti3AlC2 before and after HF treatment. (d) SEM image of a sample after HF treatment. (e) Cold-pressed 25 mm disk of etched and exfoliated material after HF treatment.

Geometry optimization of the hydroxylated (Fig. 2.3f) and fluorinated structure resulted in 5% and 16% expansion of the original Ti3AlC2 lattice, respectively (Table 2.1). If Al atoms were simply removed and not replaced by functional groups, the DFT optimization caused the structure to contract by 19%, which is not observed. This is quite reasonable since the exposed Ti atoms

Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

on the MXene surfaces are unstable in air and should be satisfied by suitable ligands. The increase of the c-lattice parameters upon reaction (Fig. 2.2a) is thus strong evidence for the validity of Reaction 2.2 and 2.3. In particular, the calculated XRD diffractograms of the geometry-optimized structure of the hydroxylated MXene shows a close match with the experimental XRD diffractogram of the treated powders. Although it is reasonable to assume that Reaction 2.2 is more probable than Reaction 2.3, a mixture of hydroxyl and fluorine cannot be ruled out. Raman spectra of Ti3AlC2 before and after HF treatment are shown in Fig. 2.2b. Peaks I, II, and III vanished after treatment, while peaks IV, V, and VI merged, broadened, and down-shifted. Such downshifting has been observed in Raman spectra of very thin layers of inorganic layered compounds [21]. The line broadening and the spectral shifts in the Raman spectra are consistent with exfoliation and are in agreement with the broadened XRD profiles. In analogy with Ti3SiC2 [22], peaks I to III in Fig. 2.2b can be assigned to Al–Ti vibrations, while peaks V and VI involve only Ti–C vibrations. The fact that only the latter two exist after etching confirms both the mode assignments and, more importantly, the loss of Al from the structure. The Ti 2p X-ray photoelectron spectroscopy (XPS) results before and after treatment are shown in Fig. 2.2c. The C 1s and Ti 2p peaks before treatment match previous work on Ti3AlC2 [23]. The presence of Ti–C and Ti–O bonds was evident from both spectra, indicating the formation of Ti3C2(OH)2 after treatment. The Al and F peaks (not shown) were also observed and their concentrations were calculated to be around 3 at% and 12 at%, respectively. Aluminum fluoride (AlF3), a reaction product, can probably account for most of the F signal seen in the spectra. The O 1s main signal (not shown at ≈530.3 cm–1) suggest the presence of an OH group [24]. A SEM image of an ≈1500 μm3 Ti3AlC2 particle (Fig. 2.2d) shows how the basal planes fan out and spread apart as a result of the HF treatment. X-ray energy-dispersive spectroscopy (EDAX) of the particles showed them to be composed of Ti, C, O, and F with little or no Al. This implies that the Al layers were replaced by oxygen (i.e., OH) and/or F. Note that the exfoliated particles

21

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Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

maintained the pseudoductility of Ti3AlC2 and could be easily cold pressed into freestanding disks (Fig. 2.2e). This property may prove to have importance in some potential applications, such as anodes in Li-ion batteries.

Figure 2.3 Exfoliated MXene nanosheets. (a) TEM images of exfoliated 2D nanosheets of Ti–C–O–F. (b) Exfoliated 2D nanosheets. Inset shows SAD pattern confirming hexagonal symmetry of the planes. (c) Single- and double-layer MXene sheets. (d) HRTEM image showing the separation of individual sheets after sonication. (e) HRTEM image of bilayer Ti3C2(OH)xFy. (f) Atomistic model of the layer structure shown in (e). (g) Calculated band structure of single-layer MXene with –OH and –F surface termination and no termination (Ti3C2), showing a change from metal to semiconductor as a result of change in the surface chemistry.

TEM analysis of exfoliated sheets (Fig. 2.3a,b) shows them to be quite thin and transparent to electrons because the carbon grid is clearly seen below them. This strongly suggests a very thin foil, especially considering the high atomic number of Ti. The corresponding selected area diffraction (SAD; inset in Fig. 2.3b) shows the hexagonal symmetry of the planes. EDAX of the same flake showed the presence of Ti, C, O, and F. Figure 2.3c,d show cross-sections of exfoliated single- and double-layer MXene sheets. Figure 3e,f show high-resolution TEM images and a

Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

simulated structure of two adjacent OH-terminated Ti3C2 sheets, respectively. The experimentally observed interplanar distances and angles are found to be in good agreement with the calculated structure. Figure 2.4a,b show stacked multilayer MXene sheets. The exfoliated layers can apparently also roll into conical shapes (Fig. 2.4d); some are bent to radii of less than 20 nm (Fig. 2.4e). Note that if the Al atoms had been replaced by the C atoms, the concomitant formation of strong Ti–C bonds, for example, when Ti3SiC2 reacts with cryolite at 900 °C [14], exfoliation would not have been possible. It follows that the reaction must have resulted in a solid in which the Ti–Al bonds are replaced by much weaker hydrogen or van der Waals bonds. This comment notwithstanding, the EDAX results consistently show the presence of F in the reaction products implying that, as noted above, the terminations are most likely a mixture of F and OH. The presence of up to 12 at% F has also been confirmed using XPS. In the latter case, however, some of it could originate from AlF3 residue in the sample. Lastly, it is instructive to point out the similarities between MXene and graphene, which include (i) the exfoliation of 2D Ti3C2 layers (Fig. 2.4a,b) into multilayer sheets that resemble exfoliated graphite [25] and (ii) the formation of scrolls (Fig. 2.4d,e). Additionally, as the cross-sectional TEM image (Fig. 2.4e) shows, some nanosheets were bent to radii less than 20 nm without fracture, which is evidence for strong and flexible Ti3C2 layers. Similar scrolls were produced by sonication of graphene [26, 27]. We assume that the sonication used for exfoliation caused some nanosheets to roll into scrolls, as schematically shown in Fig. 2.4f. Multilayer structures may be used, for example, as hosts for Li storage. DFT calculations at 0 K and in Li-rich environments show that the formation of Ti3C2Li2 as a result of the intercalation of Li into the space vacated by the Al atoms (Fig. 2.4c) assuming Reaction (2.4), Ti3C2 + 2Li = Ti3C2Li2

(2.4)

has an enthalpy change of 0.28 eV. One possible reason for the positive value maybe the fact that Li has an atomic radius of

23

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Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

145 pm, whereas that of Al is 125 pm. The structure shown in Fig. 2.4c would provide a capacity of 320 mAh g–1, which is comparable to the 372 mAh g–1 of graphite for LiC6. The elastic modulus of a single, exfoliated Ti3C2(OH)2 layer, along the basal plane, is calculated to be around 300 GPa, which is within the typical range of transition metal carbides and significantly higher than most oxides and clays [3]. And while the 300 GPa value is lower than that of graphene [7], the existence of surface functional groups for the treated powders should ensure better bonding to, and better dispersion in, polymer matrices if these exfoliated layers are used as reinforcements in polymer composites. It is also fair to assume the bending rigidity of the Ti3C2 layers to be significantly higher than graphene. It is important to note here that the Ti3C2 sheets were much more stable than graphene sheets under the 200 kV electron beam in the TEM experiment.

Figure 2.4 TEM images and simulated structures of multilayer MXene. (a) TEM images for stacked layers of Ti–C–O–F. Those are similar to multilayer graphene or exfoliated graphite that finds use in electrochemical storage. (b) The same as (a) but at a higher magnification. (c) Model of the Li-intercalated structure of Ti3C2 (Ti3C2Li2). (d) Conical scroll of about 20 nm in outer diameter. (e) Cross-sectional TEM image of a scroll with an inner radius of less than 20 nm. (f) Schematic for MXene scroll (OH-terminated).

Experimental Section

DFT calculations also predict that the electronic properties of the exfoliated layers are a function of surface termination (Fig. 2.3g). The calculated band structure of a single Ti3C2 layer resembles a typical semimetal with a finite density of states at the Fermi level. Indeed, the resistivity of the thin disk shown in Fig. 2.2e is estimated to about an order of magnitude higher than the same disc made with unreacted Ti2AlC powders, which translates to a resistivity of ≈0.03 μΩ m. This low resistivity should prove beneficial in applications such as Li-ion batteries (Fig. 2.4c) or pseudocapacitor electrodes, replacing layered transition metal oxides [28], which show useful redox properties and Li-intercalation [29] but have low electrical conductivities. When terminated with OH and F groups, the band structure has a semiconducting character, with a clear separation between valence and conduction bands of 0.05 eV and 0.1 eV, respectively (Fig. 2.3g). Thus, it is reasonable to assume that it would be possible to tune the electronic structure of exfoliated MAX layers by varying the functional groups. This behavior may be useful in certain electronic applications, such as transistors, where the use of graphene [30] and MoS [31] has been successfully demonstrated. In conclusion, the treatment of Ti3AlC2 powders for 2 h in HF results in the formation of exfoliated 2D Ti3C2 layers. The exposed Ti surfaces appear to be terminated by OH and/or F. The implications and importance of this work extend far beyond the results shown herein. As noted above, there are over 60 currently known MAX phases and thus this work, in principle, opens the door for formation of a large number of 2D Mn+1Xn structures, including the carbides and nitrides of Ti, V, Cr, Nb, Ta, Hf, and Zr. The latter could include 2D structures of combination of M-atoms, e.g., Ti0.5Zr0.5InC [32] and/or different combinations of C and N, such as Ti2AlC0.5N0.5 [33], if the selective chemical etching is extended to other MAX phases. We currently have solid evidence for the exfoliation of Ta4AlC3 into Ta4C3 flakes.

Experimental Section

Powder of Ti3AlC2 was prepared by ball-milling Ti2AlC (>92 wt%, 3-ONE-2, Voorhees, NJ) and TiC (99%, Johnson Matthey Electronic,

25

26

Two-Dimensional Nanocrystals Produced by Exfoliation of Ti3AlC2

NY) powders in a 1:1 molar ratio for 24 h using zirconia balls. The mixture was heated to 1350 °C for 2 h under argon, Ar. The resulting loosely held compact was crushed using a mortar and pestle. Roughly 10 g of powders are then immersed in ≈100 mL of a 50% concentrated HF solution (Fisher Scientific, Fair Lawn, NJ) at room temperature for 2 h. The resulting suspension was then washed several times using deionized water and centrifuged to separate the powders. In some cases, to align the flakes and produce free-standing discs, the treated powders were cold pressed at a load corresponding to a stress of 1 GPa in a steel die. X-ray diffraction (XRD) patterns were obtained with a powder diffractometer (Siemens D500, Germany) using Cu Kα radiation and a step scan of 0.02° with 1 s per step. Si powder was added to some samples as an internal standard. A scanning electron microscope, (SEM, Zeiss Supra 50VP, Germany) was used to obtain high-magnification images of the treated powders. Transmission electron microscopes (JEOL JEM-2100F and JEM 2100, Japan; FEI, Tecnai G2 TF20UT FEG, Netherlands) operating at 200 kV were used to characterize the exfoliated powders. Chemical analysis in the TEM was carried out using an ultrathin window X-ray energy dispersive spectrometer (Mahwah, NJ). The TEM samples were prepared by deposition of the flakes from an isopropanol suspension on a lacey-200 mesh carbon-coated copper grid. Raman spectroscopy of the cold-pressed samples was carried out on a microspectrometer (inVia, Renishaw plc, Gloucestershire, UK) using an Ar ion laser (514.5 nm) and a grating with 1800 lines mm–1. This corresponds to a spectral resolution of 1.9 cm–1 and a spot size of 0.7 μm in the focal plane. XPS (using a PHI 5000, ULVAC-PHI, Inc., Japan) was used to analyze the surfaces of samples before and after exfoliation. Theoretical calculations were performed using DFT using the plane-wave pseudopotential approach, with ultrasoft pseudopotentials and Perdew Burke Ernzerhof (PBE) exchange Wu–Cohen (WC) correlation functional, as implemented in the CASTEP code in Material Studio software (Version 4.5). A 8 × 8 × 1 Monkhorst–Pack grid and plane-wave basis set cutoff of 500 eV were used for the calculations. Exfoliation was modeled by first removing Al atoms from the Ti3AlC2 lattice. Exposed Ti atoms

References

located on the bottom and top of the remaining Ti3C2 layers were saturated by OH (Fig. 2.1b) or F groups, followed by full geometry optimization until all components of the residual forces became less than 0.01 eV Å–1. Equilibrium structures for exfoliated layers were determined by separating single Ti3C2 layers by a 1.2 nm thick vacuum space in a periodic supercell followed by the aforementioned full geometry optimization. Band structures of the optimized materials were calculated using a k-point separation of 0.015 Å–1. The elastic properties of the 2D structures were calculated by subjecting the optimized structure to various strains and calculating the resulting second derivatives of the energy density.

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC0205CH11231, Subcontract 6951370 under the Batteries for Advanced Transportation Technologies (BATT) Program. M.K. was supported by Gurallar Co., Turkey. V.P. was supported by the Alexander von Humboldt Foundation. The authors are thankful to Dr. V. Mochalin for help with FTIR analysis. L.H. acknowledges support from the Swedish Foundation for Strategic Research, the Knut and Alice Wallenberg Foundation, a Swedish Government Strategic Grant, and an European Research Council Advanced Grant.

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12. N. V. Tzenov, M. W. Barsoum, J. Am. Ceram. Soc. 2000, 83, 825. 13. T. El-Raghy, M. W. Barsoum, J. Appl. Phys. 1998, 83, 112.

14. M. W. Barsoum, T. El-Raghy, L. Farber, M. Amer, R. Christini, A. Adams, J. Electrochem. Soc. 1999, 146, 3919.

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16. M. W. Barsoum, J. Golczewski, H. J. Siefert, F. Aldinger, J. Alloys Compd. 2002, 340, 173. 17. Y. Gogotsi, A. Nikitin, H. Ye, W. Zhou, J. E. Fischer, B. Yi, H. C. Foley, M. W. Barsoum, Nat. Mater. 2003, 2, 591.

18. G. Yushin, E. N. Hoffman, M. W. Barsoum, Y. Gogotsi, C. A. Howell, S. R. Sandeman, G. J. Phillips, A. W. Lloyd, S. V. Mikhalovsky, Biomaterials 2006, 27, 5755. 19. M. W. Barsoum, M. Radovic, in Encyclopedia of Materials: Science and Technology, (Eds: K. H. J. Buschow, W. C. Robert, C. F. Merton, I. Bernard, J. K. Edward, M. Subhash, V. Patrick), Elsevier, Oxford 2004, 1.

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Chapter 3

Two-Dimensional Transition Metal Carbides Michael Naguib,a,b Olha Mashtalir,a,b Joshua Carle,a Volker Presser,a,b Jun Lu,c Lars Hultman,c Yury Gogotsi,a,b and Michel W. Barsouma aDepartment of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA bA.J. Drexel Nanotechnology Institute, Drexel University, Philadelphia, Pennsylvania 19104, USA cDepartment of Physics, IFM Linkoping University, Linkoping 58183, Sweden

[email protected], [email protected]

Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (>60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti2AlC, Ta4AlC3, (Ti0.5,Nb0.5)2AlC, (V0.5,Cr0.5)3AlC2, and Ti3AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying Reprinted from ACS Nano, 6(2), 1322−1331, 2012.

MXenes: From Discovery to Applications of Two-Dimensional Metal Carbides and Nitrides Edited by Yury Gogotsi Text Copyright © 2012 American Chemical Society Layout Copyright © 2023 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4877-95-4 (Hardcover), 978-1-003-30651-1 (eBook) www.jennystanford.com

32

Two-Dimensional Transition Metal Carbides

between 10 and 72 h followed by sonication. The removal of the “A” group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.

3.1 Introduction

Two-dimensional (2-D) materials, such as graphene, are known to have unique properties [1–4] that, in turn, can potentially lead to some promising applications [5–12]. Over the years, other 2-D materials with different chemistries have been synthesized by exfoliation of layered 3-D precursors such as boron nitride [13], metal chalcogenides (e.g., MoS2 [14, 15], WS2 [16, 17]), oxides, and hydroxides [18–20]. In most, if not all, of these cases, the initial bonding between the layers was relatively weak, making the structure amenable to exfoliation. As far as we are aware, and until our recent work [21], the exfoliation of layered solids with strong primary bonds had not been reported. Very recently, we reported on the exfoliation of the layered transition metal carbide, Ti3AlC2 [21]. A schematic of the exfoliation process is shown in Fig. 3.1. We note that Ti3AlC2 is a member of a large family of layered hexagonal (space group P63/mmc) ternary metal carbides and nitrides referred to as the MAX phases. The term MAX phases reflects the chemical composition: Mn+1AXn, where n = 1, 2, or 3 (M2AX, M3AX2, or M4AX3, etc.), “M” is an early transition metal, “A” is an A group (mostly groups 13 and 14) element, and “X” is C and/or N [22]. These solids combine unusual and sometimes unique properties, as they are easily machinable and, in addition to being highly damage tolerant, extremely thermal shock resistant [23]. Some MAX phases, most notably, Ti3AlC2, are also quite oxidation resistant, especially when compared to their chemically related binary carbides [24]. In general, the MAX phases are chemically quite stable, but the A layers are chemically more reactive because they are

Introduction

relatively weakly bonded when compared to the M–X bonds. At high temperatures, the MAX phases partially decompose according to the following reaction [25]. Mn+1AXn = Mn+1Xn + A

(3.1)

Such high decomposition temperatures, however, induce recrystallization and the Mn+1Xn layers turn into nonlayered, bulk 3-D cubic carbides and/or nitrides with rock-salt structures with some ordering of the vacancies on the X sites [25–27]. It is important to note that the bonding in the MAX phases is a combination of metallic, covalent, and ionic bonding, and the bonding strength is, in most cases, quite strong [25]. Many of these compounds, especially the Al-containing ones, were fabricated at temperatures as high as 1600 °C. For example, the Ti3AlC2 powders tested in our previous work were synthesized at 1350 °C [21], and bulk Ti2AlC samples are hot pressed at 1600 °C [28].

Figure 3.1 Schematic for the exfoliation process of MAX phases and formation of MXenes.

33

34

Two-Dimensional Transition Metal Carbides

Our first attempt to exfoliate MAX phases was carried out by immersing Ti3AlC2 powders in 50% hydrofluoric acid, HF, at room temperature for 2 h. This procedure resulted in the selective etching of the aluminum, Al, layers and their replacement by hydroxyl, OH, and fluorine, F, surface groups. Besides nanosheets, we also observed scrolls, nanotubes, and multilayers of Ti3C2 after sonication. In that work, and given that over 60 MAX phases are known to exist, we speculated that Ti3C2 could very well represent a member of a much larger family of 2-D transition metal carbides and/or nitrides. To emphasize their similarity to graphene, we proposed to label these 2-D solids “MXenes”. The purpose of this paper is to show that indeed it is possible to exfoliate a number of chemically quite diverse, Al-containing MAX phases. In all cases, the operative reactions are presumed to be Mn+1AlXn + 3HF = AlF3 + Mn+1Xn + 1.5H2

(3.2)

Mn+1Xn + 2HF = Mn+1XnF2 + H2

(3.4)

Mn+1Xn + 2H2O = Mn+1Xn(OH)2 + H2

(3.3)

The following MAX phases were chosen for study: Ti2AlC, Ta4AlC3, Ti3AlCN, (V0.5, Cr0.5)3AlC2, and (Ti0.5, Nb0.5)2AlC, henceforth referred to as TiNbAlC. The first two were chosen to show that it is possible to exfoliate both M2AX (211) and M4AX3 (413) phases, in addition to the already exfoliated M3AX2 (312) phase. The (V0.5, Cr0.5)3AlC2 and TiNbAlC compositions were chosen to show that the M element needs neither to be confined to Ti nor be a single element; Ti3AlCN was chosen to show that the X element need not be confined to C but can also be a mixture of C and N.

3.2 Results and Discussion

The HF treatment process yield, Y, was obtained by measuring the initial powder weight before HF treatment (Wi ) and the weight, Wf, after HF treatment (i.e., after several iterations of washing and subsequent drying). Y was then calculated to be the ratio Wf/Wi × 100%. We note that the values reported in

Results and Discussion

Table 3.1 are, thus, most likely lower than the actual synthesis yields because some powder is inevitably lost in the washing steps. Table 3.1 List of MAX phases exfoliated in this work and exfoliation process parametersa c Lattice constant (nm) HF conc. Time Before After Domain Yield Associated HF HF size (nm) (wt%) figures Compound (%) (h) Ti2AlC

10

10

1.36

1.504

6

60

2A, 3A, and 4C

Ta4AlC3

50

72

2.408

3.034 2.843

38

90

2B, 3B, 4D, 5D, 6A–D, and 8A

(V0.5Cr0.5)3 AlC2

50

69

1.773

2.426

28

NA

inset 3C and 7C,D

Ti3AlC2

50

2

1.842

2.051

11

TiNbAlC Ti3AlCN aThe

50 30

28 18

1.379 1.841

1.488 2.228

5

7

80

80

100

2C, 3C, 4E, 5C and 7A,B

2D, 3D, 4F, 5B, and 8B

4A,B

particle size for all MAX phases was 200 GPa), as determined in nanoindentation [12, 13] and wrinkling experiments [14], and are about one-fifth of the Young’s modulus of pristine graphene. GO sheets can also be processed into mechanically stable macroscopic structures, such as “graphene oxide paper” [15], or used as the reinforcement in various polymer matrix composites [3, 4]. In searches for other 2D crystals with promising elastic characteristics, it is natural to consider transition metal carbides (TMCs), which are known for their exceptional bulk mechanical properties [16]. Since 2011, TMCs have been available in a 2D form, known as MXenes [17]. More than 20 different MXenes have been synthesized by selective metal extraction and exfoliation of ternary TMCs and nitrides, known as MAX phases, in fluorine-containing etchants [18], and many other MXenes have been studied theoretically [18, 19]. MXenes have a general formula of Mn+1XnTx, where M represents a transition metal (Ti, Zr, V, Nb, Ta, Cr, Mo, Sc, etc.), X is carbon or nitrogen, and n = 1, 2, or 3 [20]. This chemical synthesis of MXenes adds surface functionalities such as fluorine, oxygen, and hydroxyl groups, denoted as Tx in the MXenes’ general formula. Examples of widely studied MXenes include Ti3C2Tx, Ti2CTx, Nb2CTx, V2CTx, Mo2TiC2Tx, and Nb4C3Tx, all of which have surface functional groups. Similar to GO, the synthesis of MXenes is scalable [21], and materials are processable in water and a variety of polar organic solvents [22]. Ti3C2Tx is the first discovered and the most widely studied MXene material to date. It shows higher electrical conductivity than solution-processed graphene [23], outstanding electrochemical properties [24], and great promise for various applications ranging from energy storage [18] to electromagnetic interference shielding [25]. Large uniform monolayer Ti3C2Tx flakes of several square micrometers in lateral size can now be

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Elastic Properties of 2D Ti3C2Tx MXene Monolayers and Bilayers

prepared in high yields [21, 23], but the mechanical properties of Ti3C2Tx monolayers, or any other MXenes for that matter, have not been measured yet. To date, only a few theoretical studies are available on mechanical properties of MXenes [26–30], predicting them to be stiffer than their MAX phase precursors [31] and bulk TMCs [16]. MXene paper and composites have been tested [32], but their properties are determined by weak interfaces. Here, we report mechanical measurements of the elastic modulus and breaking strength of monolayer and bilayer Ti3C2Tx MXene flakes by the atomic force microscopy (AFM) indentation. We also compare the MXene flakes to GO flakes, as both materials can be solution-processed because of their surface functionalization and are often discussed with regard to similar applications, such as conductive coatings, filtration membranes, composites, porous scaffolds, and energy storage [3, 18]. As shown in Fig. 8.1A, a monolayer of Ti3C2Tx consists of three layers of close-packed Ti atoms stacked in the ABC ordering, with carbon atoms occupying the octahedral sites; the flakes are terminated with –F, –O, or –OH groups. Our results show that a single layer of Ti3C2Tx has an effective elastic modulus of 330 ± 30 GPa, which exceeds considerably the mean values previously found in the nanoindentation experiments on GO and rGO monolayers [12, 13] and other solution-processed 2D materials.

8.2 Results

The synthesis of Ti3C2Tx was performed by in situ hydrofluoric acid (HF) etching of aluminum from Ti3AlC2, as described by Alhabeb et al. [21] and Lipatov et al. [23]. This method produces high-quality MXene flakes with lateral sizes up to 10 mm [23]. The final product is a dark green solution of Ti3C2Tx flakes in water, which could be directly drop-casted on a Si/SiO2 substrate with prefabricated microwells. However, this deposition method yields fractured and surface-contaminated flakes after drying (Fig. S1). Flake fracture is caused by the hydrophilicity of Ti3C2Tx and the high surface tension of water, which drags MXene flakes into wells upon drying. Flakes that only partially cover microwells

Results

survive drying, but become crumpled and therefore unusable for indentation experiments.

Figure 8.1 Preparation of MXene membranes. (A) Structure of a Ti3C2Tx monolayer. Yellow spheres, Ti; black spheres, C; red spheres, O; gray spheres, H. (B) Scheme of the polydimethylsiloxane (PDMS)–assisted transfer of MXene flake on a Si/SiO2 substrate with prefabricated microwells. See text for details. (C) SEM image of a Ti3C2Tx flake covering an array of circular wells in a Si/SiO2 substrate with diameters of 0.82 mm. (D) Noncontact AFM image of Ti3C2Tx membranes. (E and F) Height profiles along the dashed blue (E) and red (F) lines shown in (D).

We developed a deposition technique, which produces very clean, tautly stretched Ti3C2Tx membranes (Fig. 8.1B). First, MXene solution is drop-casted on a PDMS support and air-dried, leaving multiple MXene flakes on a surface. The surface of a PDMS support with MXene flakes is washed with running deionized (DI) water to remove possible salt contaminants from the original

211

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Elastic Properties of 2D Ti3C2Tx MXene Monolayers and Bilayers

solution. After drying, the PDMS support is placed on a Si/SiO2 substrate with prefabricated microwells with flakes facing down. No pressure was applied to the support to avoid damaging the flakes. Then, the PDMS film is gently peeled from the substrate, leaving some of the MXene flakes on the SiO2 surface. The rationale behind this technique is that hydrophilic MXene flakes should have stronger attractive interaction with the hydrophilic silica surface than with the hydrophobic PDMS. This method should also be applicable to other solution-processed 2D materials, such as GO, and MXenes other than Ti3C2Tx. We also tested several other approaches for membrane fabrication, but all of them had certain drawbacks, while the direct PDMS transfer method consistently produced MXene membranes of excellent quality. A representative scanning electron microscopy (SEM) image in Fig. 8.1C shows a transferred MXene flake that fully covers five microwells. According to noncontact AFM images (Fig. 8.1D), the MXene membranes are stretched across the openings and adhere to the well walls because of the attractive interaction between MXene flakes and SiO2 (see the AFM height profile in Fig. 8.1E). The step height at the edge of the flake shown in Fig. 8.1D is about 3.0 nm (Fig. 8.1F), which includes trapped water molecules between the flake and the substrate. This flake folds at its edge, which adds a layer to itself, and measuring its height reveals a thickness of 1.6 nm for noncontact AFM mode (Fig. 8.1F), which matches the thickness that we observed in AFM measurements of Ti3C2Tx monolayers in our previous study [23]. While the procedure that we used for the synthesis of Ti3C2Tx provides monolayer flakes at high yields, we also found several suspended bilayer structures, which consisted of two monolayer flakes overlapping over a well. Since the thickness of Ti3C2Tx monolayers is an important parameter for analysis of the results of nanoindentation experiments, it is necessary to comment on the limitations of AFM for the determination of thicknesses of monolayers of 2D materials. For example, while the nominal thickness of graphene is 0.335 nm, in various experiments, the AFM measurements of monolayer graphene flakes produced thickness values in the range of 0.4 to 1.7 nm, as summarized by Shearer et al. [33]. This inaccuracy could be affected by a number of factors that include the AFM imaging mode (tapping, contact, etc.), tip-

Results

surface interactions, presence of various surface adsorbates, and trapped interfacial molecules, among others. Therefore, nominal thicknesses, rather than AFM-measured thicknesses, were used in other works on mechanical indentation of 2D materials for calculations of mechanical characteristics [1, 5]. Here, likewise, AFM produced a largely overestimated thickness value of 1.6 nm, and for the Young’s modulus calculation, we instead used the thickness of a Ti3C2Tx monolayer of 0.98 nm, which was determined by atomically resolved transmission electron microscopy (TEM) and supported by theoretical calculations [34, 35]. It should be pointed out that high-resolution TEM is a preferred method for the determination of the thickness of a Ti3C2Tx monolayer compared to x-ray diffraction (XRD) analysis. When Ti3C2Tx is produced in a bulk form, XRD could be used to determine the interlayer spacing between the MXene sheets; this spacing can vary considerably, depending on the amount and chemical nature of species intercalated between the sheets [36]. However, in the monolayer Ti3C2Tx membrane, there are no interlayer spacings with intercalated species, and similarly to AFM, the XRD measurements may overestimate the nominal thickness of a MXene monolayer. The scheme of the nanoindentation experiment is shown in Fig. 8.2A. The surface of a substrate was scanned for MXene flakes suspended over wells using AFM in tapping mode. At least two AFM scans of the same well were performed to confirm that no drift of a sample occurred. Then, the AFM tip was positioned directly in the center of a selected well and slowly moved downward, providing controlled stretching of a MXene flake. Two to four cycles of loading and unloading were performed on the same MXene flake, with an incremental loading increase of 50 nN (see the corresponding curves in Fig. 8.2B). The bottom inset in Fig. 8.2B illustrates the behavior of the membrane in the beginning of the indentation experiment. The tip first snaps down to the membrane attracted by van der Waals forces and then begins to deflect the membrane as the tip presses downward. We extrapolate the linear force versus deflection (F-δ) dependence before snapping until it crosses the curve and consider this point as a center of origin where the force and displacement are both zero, which is necessary to obtain the correct F-δ relationship. The extension and retraction curves

213

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Elastic Properties of 2D Ti3C2Tx MXene Monolayers and Bilayers

within each loading cycle, as well as the curves for different loads, retrace each other, indicating high elasticity of MXene flakes and that no flake detachment occurs during the measurements. The fracture of this bilayer Ti3C2Tx membrane occurred at a load force of about 200 nN and a deflection of 38 nm. Even at the maximum deflection, the center of the membrane is far from the bottom of the well that is ≈300 nm deep. The AFM tip punctures the membrane, leaving a small hole as seen in the top inset in Fig. 8.2B. Unlike in graphene membranes [1], in which the breaks extend over the entire well (Fig. S3, B and C), the punctures in suspended Ti3C2Tx flakes were very local, in agreement with noncatastrophic fracture of MXene sheets predicted by molecular dynamics simulations [37]. We consider the system under investigation as isotropic because of circular wells, the spherical tip, and close-packed structure of Ti3C2Tx. Therefore, we can parametrize the membrane using Young’s modulus EYoung, Poisson’s ratio v (0.227 for Ti3C2Tx [38]), and thickness h and fit the experimental F-δ data using the formula 2D F =  2D 0 + E

q3  3 r2

q3 3



(8.1)

2D is the F =  0  +E where represents 2 prestress in the membrane, E r 2D elastic modulus, and r is the radius of the well [1, 5]. The dimensionless constant q is related to v as q = 1/(1.049 − 0.15v − 0.16v2) = 0.9933. The first term in Eq. 8.1 corresponds to the prestretched membrane regime and is valid for small loads. The second term for the nonlinear membrane behavior is characterized by a cubic F ~ δ3 relationship with a coefficient of E2D, which dominates at large loads. The applicability of this formula is demonstrated in the inset in Fig. 8.2C, where the F–δ dependence for a bilayer Ti3C2Tx flake is shown in logarithmic scale. At small loads (less than 10 nN), the dependence is linear and shown by the blue solid line, while above 10 nN, the coefficient is 3 (red solid line), meaning that the dependence is cubic. The latter fits a considerable amount of the experimental data, confirming that most of the mechanical response is expected to be in the region characterized by a cubic F ~ δ3 relationship. It is possible to use this relationship to determine the corresponding coefficient 2D

2D

Results

E2D with high precision. Other nonlinear effects in F-δ dependence can be ignored if the AFM tip radius is much smaller than the radius of a well, that is, rtip ≪ r [1, 5]. In our case, the diameter of the well measured by SEM is a = 2r = 820 nm, and according to the manufacturer’s specifications, the AFM tip radius is 7 nm, which results in rtip/r = 0.017 ≪ 1. Figure 8.2C shows the experimental and fitting curves for single- and double-layer MXene flakes. Good fitting (R2 > 0.995 for all measurements) is an indicator that the model is appropriate.

Figure 8.2 Elastic response and indentation test results. (A) Scheme of nanoindentation of a suspended Ti3C2Tx membrane with an AFM tip. (B) Force-deflection curves of a bilayer Ti3C2Tx flake at different loads. The bottom inset is a detailed view of the same curves showing the center of origin. The top inset shows AFM image of the fractured membrane. (C) Comparison of loading curves for monolayer (1L) and bilayer (2L) Ti3C2Tx membranes and the least squares fit to the experimental indentation curves by Eq. 8.1. Hole diameter is 820 nm. The inset shows the same experimental curve for bilayer Ti3C2Tx in logarithmic coordinates. The curve shows a linear behavior in the first 10 nm of indentation (blue line) and approaches the cubic behavior at high loads (red line). (D) Histogram of elastic stiffness for monolayer and bilayer membranes. Solid lines represent Gaussian fits to the data. (E) Histogram of pretensions of monolayer membranes. (F) Histogram and Gaussian distribution of breaking forces for monolayer membranes. Tip radius is 7 nm.

215

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Elastic Properties of 2D Ti3C2Tx MXene Monolayers and Bilayers

In our experiments, we measured 18 membranes from 16 different monolayer Ti3C2Tx MXene flakes. For each membrane, two curves at different loads were collected before the rupture, totaling 36 experimental points. For monolayer MXene membranes, the E2D elasticity ranged from 278 to 393 N/m, with an average of 326 ± 29 N/m (Fig. 8.2D). A narrow distribution of the experimental E2D values was achieved even though the measurements were performed on 16 different flakes. For each MXene flake that covered two wells, the nanoindentation experiments were even more reproducible. For one pair of wells covered by the same flake, we found E2D elasticities of 344 and 341 N/m, and for another such pair of wells, we measured E2D values of 318 and 323 N/m (Fig. S2). These results show great reproducibility of data measured within the same flake. The corresponding distribution of membrane3 3pretensions is q 2D E 2Dthe 2range from 0.14 shown in Fig. 8.2E. The valuesFof=  0  lay+ in r to 0.34 N/m for monolayer MXene flakes, showing strong interaction between the membrane and the well walls. These values are comparable with those obtained for graphene and MoS2 membranes [1, 5]. The procedure used for the synthesis of Ti3C2Tx provides monolayer flakes [23]. Two monolayer flakes may overlap or one flake may fold on top of a well, in both cases resulting in bilayer membranes. The F-δ curve for one of the bilayer Ti3C2Tx membranes is presented in Fig. 8.2C, in comparison with the curve for a monolayer MXene flake. We measured four different bilayer membranes, collecting a total of 10 experimental points, which are presented in the histogram plot in Fig. 8.2D. The E2D values determined for bilayer Ti3C2Tx flakes ranged from 632 to 683 N/m, with an average of 655 ± 19 N/m. This number is exactly twice that determined for monolayer MXene membranes, suggesting a strong interaction between layers that is likely associated with the hydrogen bonding between the Ti3C2Tx surface groups. Similar effects were observed for overlapping GO membranes [12] and multilayer h-BN flakes [8], where a strong interaction between layers was caused by either hydrogen bonding or interlayer B-N interaction, respectively. In contrast, multilayer graphene exhibits lower E2D than expected from multiplying monolayer E2D by the number of layers due

Results

to weaker interlayer interaction and therefore a greater tendency of layers to slide relative to each other upon indentation [1, 8]. Suspended MXene membranes can be deformed elastically up to a certain stress when mechanical failure occurs. Figure 8.2F presents the distribution of fracture forces for monolayer MXene membranes ranging from 50 to 102 nN and averaging at Ff = 77 ± 15 nN. As shown in the inset in Fig. 8.2B, the fracture occurred in the center of a membrane where the stress was applied by the AFM tip. We can extract the maximum stress at the central part of the sheet using the expression for the indentation of a linearly elastic circular membrane under a spherical indenter [39]  2D max =

F f E 2D 4rtip

(8.2)

In our work, we used a2Ddiamond AFM tip with a radius of FE 7 nm, rendering  2D to bef between 14 and 20 N/m. On average, max = 4 r these values correspond totip5.2% of the Young’s modulus E2D for monolayer MXene membranes, which is lower than the theoretical upper limit of a material’s breaking strength [37] due to the presence of defects in the material [40].

Figure 8.3 Comparison of indentation tests on Ti3C2Tx with other 2D materials. (A) Comparison of experimental F-δ curves for monolayer graphene and Ti3C2Tx membranes. (B) Comparison of effective Young’s moduli for several 2D materials: GO [12], rGO [13], MoS2 [5], h-BN [8], and graphene [1]. In this chart, we compare values produced on membranes of monolayer 2D materials in similar nanoindentation experiments.

217

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Elastic Properties of 2D Ti3C2Tx MXene Monolayers and Bilayers

Considering that graphene is a benchmark 2D material, we decided to directly compare F-δ curves for suspended Ti3C2Tx and graphene monolayers (Fig. 8.3A). The elasticity of graphene in our experiment was found to be 341 ± 28 N/m, obtained from three monolayer membranes (see details in Fig. S3), which is very close to the previously reported values of 340 ± 50 N/m [1] and thus reconfirms the validity of our experimental approach.

8.3 Discussion

The effective Young’s modulus EYoung and breaking strength σmax F f E 2D can be calculated from E2D and  2D , respectively, by dividing max = 4 rtip them by the membrane’s thickness. As we explained previously, for the Young’s modulus calculation, we used the nominal thickness of a Ti3C2Tx monolayer of 0.98 nm, which was obtained from high-resolution TEM analysis and theoretical calculations [34, 35]; the same approach was used in other works on indentation of 2D materials [1, 5]. The effective Young’s modulus for MXene membranes is 333 ± 30 GPa, and the breaking strength is 17.3 ± 1.6 GPa (taking the average tip radius of 7 nm). It is interesting to note that according to the molecular dynamics simulations, Ti3C2 has a Young’s modulus of 502 GPa [27]. As expected, the experimentally determined value for Ti3C2Tx of 333 ± 30 GPa is lower because of surface functionalization and the presence of defects. However, the difference in the Young’s moduli of the “ideal” Ti3C2 and the experimentally realized Ti3C2Tx is not as dramatic as in the case of graphene and GO (1050 GPa versus 210 GPa). This could be rationalized by the fact that surface functionalization has a stronger effect on the mechanical properties of one-atom-thick monolayer graphene compared to thicker Ti3C2Tx flakes. In the future studies, it would be interesting to compare mechanical properties of Ti3C2Tx with different functional groups and, ultimately, without functionalization. It should be pointed out that in Ti3C2Tx monolayers used in this study, a considerable fraction of the nominal thickness of 0.98 nm is occupied by the surface functionalities [34], such as –F and –OH, and thinner flakes of pristine Ti3C2 are expected to

Materials and Methods

have a Young’s modulus approaching the theoretically predicted value of 502 GPa [27]. Comparison of EYoung values with other benchmark 2D materials is presented in Fig. 8.3B. At 330 ± 30 GPa, the effective Young’s modulus of Ti3C2Tx MXene exceeds the previously reported mean values for GO, rGO, and MoS2 that were produced in similar nanoindentation experiments [5, 12, 13] but is lower than those of h-BN and graphene [1, 8]. While the values for graphene, h-BN, and MoS2 were reported for defect-free flakes, the Ti3C2Tx MXene flakes tested here are solution-processed. There is potential to develop methods to synthesize Ti3C2Tx flakes of higher quality to reach a larger Young’s modulus closer to the theoretical value. In addition, Ti3C2Tx is just one of more than 20 synthesized MXenes, and MXenes with a different number of atomic layers or a different transition metal may have higher elasticity. This study suggests great potential of MXenes for structural composites, protective coatings, nanoresonators, membranes, textiles, and other applications that require bulk quantities of solution-processable materials with exceptional mechanical properties.

8.4 Materials and Methods 8.4.1 Synthesis of Ti3C2Tx

MAX phase precursor, Ti3AlC2, was produced as described elsewhere [17, 21]. Ti3C2Tx MXene was synthesized via selective etching of Al from Ti3AlC2 using in situ HF etchant solution as described elsewhere [21]. The etchant solution was prepared by adding 0.8 g of LiF to 10 ml of 9 M HCl and allowing the solution to mix thoroughly at room temperature for a few minutes. After that, 0.5 g of Ti3AlC2 was slowly added over the course of 5 min to avoid initial overheating due to the exothermic nature of the reaction. Then, the reaction was allowed to proceed at ambient conditions (~23°C) under continuous stirring (550 rpm) for 24 hours. The resulting MXene was repeatedly washed with DI water until an almost neutral pH (≥6) was achieved. The product was then collected using vacuum-assisted filtration through a polyvinylidene difluoride membrane (0.45-mm pore size; Millipore)

219

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Elastic Properties of 2D Ti3C2Tx MXene Monolayers and Bilayers

and dried in a vacuum desiccator at room temperature for 24 hours. To delaminate 0.2 g of Ti3C2Tx, a freshly produced powder was redispersed in 50 ml of DI water and stirred continuously for 1 hour. Then, the Ti3C2Tx solution was centrifuged at 3500 rpm, and the supernatant, a dark green colloidal solution of MXene, was collected. Previous studies have shown that this solution contains primarily monolayer flakes [23, 40].

8.4.2 Materials Characterization

8.4.2.1 Scanning electron microscopy SEM analysis was performed using a Zeiss Supra 40 Field-Emission SEM at an accelerating voltage of 5 kV.

8.4.2.2 Atomic force microscopy

Surface topography imaging and force-indentation curve measurements were performed on an Asylum Research MFP3D system. Single-crystal diamond tips (D80, SCD Probes) with tip radii of 5 to 10 nm and a spring constant of ~3.5 N/m, according to the manufacturer’s specifications, were used for force-indentation experiments. The spring constant of each AFM cantilever was calibrated via thermal noise method [41] before indentation experiments. During the force-indentation experiments, the z-piezo displacement speed was controlled at a rate of 100 nm/s. Different rates ranging from 50 to 1000 nm/s were also tested and showed no clear difference for the forceindentation curves.

8.4.3 Analysis of Force-Indentation Curves

During the indentation experiments, the cantilever bending and z-piezo displacement were recorded as the tip moved downward. The cantilever bending was calibrated by measuring a forcedisplacement curve on a hard Si/SiO2 surface in advance. The loading force was obtained by multiplying the cantilever bending by the cantilever spring constant, and the deflection of the membrane was obtained by subtracting the cantilever bending from the z-piezo displacement.

Supplementary Materials

In the real force-deflection data, there is a negative force section due to the tip jump-to-surface effect, where the tip snaps down to the membrane attracted by van der Waals forces when it is very close to the surface. We extrapolated the zero force line in the force-deflection dependence before snapping until it crossed the curve and considered this point as a center of origin where the force and displacement are both zero, which is necessary to obtain the correct force-deflection relationship.

Acknowledgments

Funding: This work was supported by the NSF through ECCS1509874 with a partial support from the Nebraska Materials Research Science and Engineering Center (DMR-1420645). The materials characterization was performed in part in the Nebraska Nanoscale Facility: National Nanotechnology Coordinated Infrastructure and the Nebraska Center for Materials and Nanoscience, which are supported by NSF (ECCS-1542182) and the Nebraska Research Initiative. M.A. was supported by the Libyan-North America Scholarship Program funded by the Libyan Ministry of Higher Education and Scientific Research. B.A and Y.G. were supported by U.S. Army Research Office grants W911NF-17-S-0003 and W911NF-17-2-0228. Author contributions: A.S., B.A., and Y.G. initiated the project. M.A., B.A., and Y.G. synthesized and characterized MAX and MXene samples. A.L. fabricated MXene and graphene membranes. H.L. and A.G. performed AFM imaging and nanoindentation experiments. A.L. performed the data analysis. A.L. and A.S. wrote the manuscript with contributions from all other authors. Y.G. and A.S. supervised the project. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Supplementary Materials

Supplementary material for this article is available at http:// advances.sciencemag.org/cgi/content/full/4/6/eaat0491/DC1

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Elastic Properties of 2D Ti3C2Tx MXene Monolayers and Bilayers

Fig. S1. Ti3C2Tx MXene membranes prepared by drop-casting

from an aqueous solution. Fig. S2. Mechanical properties of Ti3C2Tx

MXene monolayer on a single flake.

Fig. S3. Mechanical properties of graphene monolayers.

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Chapter 9

Control of MXenes’ Electronic Properties through Termination and Intercalation James L. Hart,a Kanit Hantanasirisakul,a,b Andrew C. Lang,a Babak Anasori,a,b David Pinto,a,b Yevheniy Pivak,c J. Tijn van Omme,c Steven J. May,a Yury Gogotsi,a,b and Mitra L. Taheria aDepartment of Materials Science & Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA bA.J. Drexel Nanomaterials Institute, Drexel University, Philadelphia, Pennsylvania 19104, USA cDENSsolutions, Informaticalaan 12, Delft 2626ZD, The Netherlands

[email protected], [email protected]

MXenes are an emerging family of highly-conductive 2D materials which have demonstrated state-of-the-art performance in electromagnetic interference shielding, chemical sensing, and energy storage. To further improve performance, there is a need to increase MXenes’ electronic conductivity. Tailoring the MXene surface chemistry could achieve this goal, as density functional theory predicts that surface terminations strongly influence MXenes’ Fermi level density of states and thereby MXenes’ electronic conductivity. Here, we directly correlate MXene surface Reprinted from Nat. Commun., 10, 522, 2019.

MXenes: From Discovery to Applications of Two-Dimensional Metal Carbides and Nitrides Edited by Yury Gogotsi Text Copyright © 2019 American Chemical Society Layout Copyright © 2023 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4877-95-4 (Hardcover), 978-1-003-30651-1 (eBook) www.jennystanford.com

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de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. Furthermore, we show that intercalation can induce transitions between metallic and semiconductor-like transport (transitions from a positive to negative temperature-dependence of resistance) through inter-flake effects. These findings lay the groundwork for intercalation- and termination-engineered MXenes, which promise improved electronic conductivity and could lead to the realization of semiconducting, magnetic, and topologically insulating MXenes.

9.1 Introduction

Discovered in 2011, MXenes are a rapidly growing family of 2D transition metal carbides, nitrides, and carbonitrides with the general formula Mn+1XnTx (n = 1, 2, or 3; M = transition metal, e.g., Ti, V, Nb, Mo; X = C and/or N; T = surface termination, e.g., –OH, –F, =O) [1–5]. MXenes are formed by selective etching parent ternary carbide MAX compounds to remove the A-group element, e.g., Ti3AlC2 (layered MAX) → Ti3C2Tx (2D MXene) [6, 7]. For both device applications and fundamental studies, MXene samples are generally thin films comprised of many MXene flakes, though some studies have focused on single-layer MXene [8, 9]. In contrast to most other 2D materials, MXenes offer an attractive combination of high electronic conductivity, hydrophilicity, and chemical stability [1–5, 10–13]. With these properties, MXenes show exceptional promise in areas including electromagnetic interference shielding [14, 15], wireless communication [16], chemical sensing [17–20], energy storage [21–23], optoelectronics [24–27], triboelectrics [28–30], catalysis [31–33], and conformal/ wearable electronics [34]. Performance in these applications is directly related to electronic conductivity, and as such, there is motivation to further increase the MXene metallic conductivity. In parallel, there is excitement surrounding the potential realization of semiconducting MXenes, which are predicted to be excellent materials for spintronics and thermoelectrics [35–37].

Introduction

To meet these demands on MXenes’ electronic properties, researchers have mostly focused on the development of new Mn+1Xn chemistries. So far, over 30 MXenes have been synthesized, but the first discovered MXene—Ti3C2Tx—remains the most conductive [13]. Recently, certain Mo- and V-based MXenes have garnered interest due to their so-called semiconductor-like behavior [38–40], i.e., a negative temperature-dependence of resistance (dR/dT). However, the cause of negative dR/dT in these MXenes remains unclear, and their underlying electronic structure is debated [38, 41]. A potentially more useful approach to control MXenes’ conductivity is to manipulate their surface chemistry. Surface terminations, which are introduced during MXene synthesis [6], have been predicted to control metalto-insulator transitions [37, 38] and to affect functional properties such as magnetism [1, 42], Li-ion capacity [43], catalytic performance [31, 44], band alignment [45], mechanical properties [46], and predictions of superconductivity [47]. While promising, these predicted effects lack direct experimental confirmation [48, 49]. An additional mechanism which can affect MXene conductivity is intercalation. Intercalants are not thought to alter the intra-flake (intrinsic) MXene properties, but for multi-layer samples, intercalation can increase device resistance by over an order of magnitude [27, 40, 41, 50–52]. This effect is generally attributed to intercalants increasing the inter-flake spacing and thereby the inter-flake resistance. For any multi-layer MXene sample, intercalation, termination, and Mn+1Xn chemistry all contribute to the measured electronic conductivity, and this convolution of effects greatly complicates experimental interpretation. As a result, our understanding and ability to control MXenes’ electronic properties are lacking. To address this challenge, we perform in situ vacuum annealing (up to 775 °C) and electrical biasing of MXenes within the transmission electron microscope (TEM). We observe deintercalation and surface de-functionalization with in situ electron energy loss spectroscopy (EELS) and ex situ thermogravimetric analysis with mass spectroscopy (TGA-MS). Importantly, we utilize low-dose direct detection (DD) EELS [53] to avoid electron beam-induced specimen damage [54] (Supplementary

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Fig. 1). With this approach, we correlate the desorption of –OH, –F, and =O termination species with increased MXene conductivity. Additionally, we report transitions from ensemble semiconductor-like (negative dR/dT) to metallic behavior in Ti3CNTx and Mo2TiC2Tx after the de-intercalation of water and organic molecules. This work furthers our fundamental understanding of conduction through MXene films and opens the door to intercalation- and termination-engineered MXenes.

9.2 Results

9.2.1 Sample Synthesis and Experimental Approach We investigated three MXenes with diverse macroscopic electronic transport behavior: Ti3C2Tx, Ti3CNTx, and Mo2TiC2Tx (Table 9.1). Ti3C2Tx (its structure is shown schematically in the Fig. 9.1a top inset) is the most studied MXene and is known to be metallic and highly conductive [7]. Density functional theory (DFT) studies have consistently predicted that surface functionalization reduces the Ti3C2Tx density of states (DOS) at the Fermi level (EF), suggesting a decrease in the charge carrier density and thereby a decrease in the conductivity [43, 55, 56]. Understanding of the next two MXenes—Ti3CNTx and Mo2TiC2Tx—is limited. The structure of Ti3CNTx is similar to Ti3C2Tx but with a mixture of C and N on the X-sites (Fig. 9.1b top inset). DFT predicts Ti3CNTx to be metallic for all terminations [55, 57, 58], but to date, this MXene has only shown semiconductor-like transport (unpublished results). Mo2TiC2Tx is an ordered, double transition metal MXene analogous to Ti3C2Tx but with the outer Ti layers replaced by Mo layers [59] (Fig. 9.1c top inset). Terminations have been predicted to induce metallic, semiconducting [38], and topologically insulating [60] states in this MXene. Experimentally, Mo2TiC2Tx shows semiconductor-like behavior in its as-prepared state [38], but it is unclear if this behavior is due to intrinsic (Mn+1XnTx stoichiometry) or extrinsic (intercalation, inter-flake hopping) effects. We produced the Ti3C2Tx and Ti3CNTx samples through etching of their 3D parent MAX phases, i.e., Ti3AlC2 and Ti3AlCN,

Results

in a mixture of LiF and HCl. This process results in –OH, –F, and =O terminations and H2O and Li+ intercalation [6, 23]. Mo2TiC2Tx was instead produced through etching of Mo2TiAlC2 in HF and delaminating via tetrabutylammonium hydroxide (TBAOH) intercalation [59]. This method reduces the concentration of –F termination and results in tetrabutylammonium (TBA+) and H2O intercalation [40, 61]. TBA+ is a large organic ion which can significantly increase the inter-flake spacing and electrical resistance [6, 40, 52]. To directly compare H2O and TBA+ intercalation, we additionally studied Ti3CNTx prepared with HF etching and TBAOH delamination. For clarity, this sample is referred to as Ti3CNTx (TBA+).

Figure 9.1 Evolution of MXene electronic properties with in situ vacuum annealing. Resistance versus temperature measurements are shown for Ti3C2Tx (a), Ti3CNTx (b), and Mo2TiC2Tx (c). The measurements were conducted during various vacuum annealing steps performed in the TEM. Each vacuum annealing step is represented with a different color and symbol. For each annealing step, both the heating and cooling curves are shown. In all cases, the resistance decreased during annealing, hence, the cooling curve is always beneath the heating curve. The atomic structures of the various MXenes are shown as insets. The initial state schematics (top schematics) show Ti3C2Tx and Ti3CNTx with intercalated water molecules on their surfaces, and Mo2TiC2Tx is shown with water and TBA+ molecules. With annealing, the MXene sample resistance is affected by the loss of adsorbed species, intercalants, and terminating species. Some of these processes are shown schematically. The resistance data in this figure are also shown in Supplementary Figs. 13–15, where the resistance is plotted as a function of annealing time.

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Table 9.1 Summary of in situ heating and biasing results MXene chemistry

Termination

Ti3C2Tx

(OH)0.4F0.4O0.5 → F0.2O0.5 H2O, Li+ → Li+

Ti3CNTx Ti3CNTx

(TBA+)

Mo2TiC2Tx

Intercalation

(OH)0.9F0.5O0.7 → F0.2O0.7 H2O, Li+ → Li+ (OH)0.6F0.1O1.2 → O1.2

(OH)0.5F0.01O1.5 → O0.8

H2O, H2O,

TBA+ TBA+

Resistance dR/dT (Ω) M → M 41 → 10 S→M

→ none S → S

→ none S → M

159 → 27

3330 → 290

2500 → 387

In each cell, the arrow symbol represents changes induced during in situ vacuum annealing up to temperatures of 775, 700, 750, and 775 °C for Ti3C2Tx, Ti3CNTx, Ti3CNTx(TBA+), and Mo2TiC2Tx, respectively. In the dR/dT column, M indicates metallic conduction (positive dR/dT) while S indicates semiconductor-like conduction (negative dR/dT). Initial termination concentrations were determined through ex situ X-ray photoelectron spectroscopy (XPS) measurements (Supplementary Tables 3–6 and Supplementary Fig. 12). The loss of intercalants and –OH terminations was determined from ex situ TGA-MS, and the reduction in –F and =O terminating species was measured with in situ EELS.

After synthesis, MXene films were spray-casted [24] onto MEMS (microelectromechanical systems)-based nanochips [62] designed for heating and biasing within the TEM column (Supplementary Fig. 2). The MXene films were at least several flakes thick (Supplementary Figs. 1 and 2), and the electrode spacing (~20 μm) was considerably larger than the MXene flake diameters (~100 nm up to 2–3 μm). As such, sample resistance measurements were dependent upon both intra- and inter-flake contributions. The cross-sectional sample areas were not welldefined, so we report the relative change in sample resistance and not the absolute resistivity. TEM imaging and electron diffraction confirmed the MXene structure both before and after in situ annealing at ≥700 °C (Supplementary Fig. 2). The temperature-dependent resistance of each sample was initially measured in ambient atmosphere from room temperature (RT) up to 75 °C in order to understand the as-prepared sample properties. In agreement with previous results, Ti3C2Tx displayed metallic behavior while Ti3CNTx and Mo2TiC2Tx displayed semiconductor-like (negative dR/dT) behavior [27, 38] (Table 9.1 and Fig. 9.1). After measurement of the as-prepared MXene

Results

electronic properties, the samples were inserted into the TEM and vacuum annealed at temperatures up to 775 °C. Before providing a detailed analysis of our in situ heating and biasing experiments, we first summarize our two main findings. First, we observed transitions from semiconductor-like to metallic behavior in Ti3CNTx and Mo2TiC2Tx after the annealinginduced loss of intercalated species. These transitions reveal the intra-flake metallicity of Ti3CNTx and Mo2TiC2Tx and demonstrate that intercalants can cause negative dR/dT in multi-layer MXenes. Second, high temperature annealing and the partial loss of surface terminations (Fig. 9.1, bottom insets) increased the conductivity of all three MXenes. This finding is in agreement with past predictions that non-terminated MXenes exhibit metallic behavior with high carrier concentrations [43, 55, 56, 59]. In the following, we describe the in situ heating and biasing data in greater detail; results are organized based on the different mechanisms which influence MXene electronic properties.

9.2.2 Adsorbed Species

Prior to thermal annealing, insertion of Ti3C2Tx and Ti3CNTx into the TEM vacuum (~10−5 Pa) caused an immediate reduction in resistance (Fig. 9.1a,b and Supplementary Fig. 3). For both of these samples, the resistance decreased roughly 20% after 150 s of insertion into the TEM. This change in resistance is attributed to the loss of adsorbed atmospheric species, e.g., H2O and O2. These species are known to cause doping in 2D materials such as graphene [63], and similar effects have previously been reported in Ti3C2Tx [8, 25]. We note that both of these samples were only intercalated with H2O and Li+. For Mo2TiC2Tx intercalated with TBA+, the sample resistance did not significantly change upon exposure to the TEM vacuum (Fig. 9.1c and Supplementary Fig. 3). This observation suggests an increased effect of the large TBA+ molecule relative to H2O, specifically, that TBA+ intercalation limits the film resistance and masks the effect of atmospheric species desorption on the electronic resistance. The effect of adsorbed species on the resistance of Ti3CNTx(TBA+) could not be determined, since the sample resistance prior to in situ vacuum annealing was too high to be accurately measured.

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9.2.3 Intercalation For all studied MXenes, de-intercalation significantly increased electronic conductivity. We first consider the role of H2O, which began to de-intercalate at lower temperatures than TBA+. For all samples, mass spectroscopy (MS) showed a large peak in the H2O (mass to charge ratio (m/e) = 18) ion current centered at ~150 °C, indicating H2O de-intercalation. This behavior is shown for Ti3CNTx in Fig. 9.2a, and full TGA-MS results for all samples are presented in Supplementary Fig. 4. In situ EELS data is consistent with the de-intercalation of H2O during low temperature annealing. Figure 9.2b shows the O K-edge of Ti3CNTx after various in situ annealing steps; three distinct peaks are observed. For =O or –OH bonded with surface Ti, these three peaks are expected, with peak 1 arising due to hybridization between the termination moiety and the Ti 3d orbitals [64]. Conversely, the O K-edge of water does not contain peak 1 [65]. With annealing, peaks 2 and 3 decrease with respect to peak 1, signifying the loss of intercalated H2O. The loss of =O or –OH terminations cannot explain the observed changes in fine structure, as surface de-functionalization would produce a more uniform decrease in the O K-edge intensity. By annealing samples at 200 °C, we isolate the influence of H2O de-intercalation on the Ti3C2Tx and Ti3CNTx electronic properties from any effects of surface de-functionalization. To prove this point, we consider the de-functionalization of –OH, the least stable termination species and thus the first to desorb [12, 66]. The MS –OH (m/e = 17) ion current in Fig. 9.2a shows two peaks for Ti3CNTx. The first peak perfectly mirrors the H2O ion current and is thus attributed to de-protonated H2O (Supplementary Fig. 4); the second peak at ~375 °C is attributed to –OH termination loss. Our conclusion that –OH terminations are stable at 500 °C. This is likely the case for Ti3CNTx(TBA+), since this MXene does not show metallic behavior even after annealing at 750 °C. To the best of our knowledge, these results constitute the first direct experimental correlation of MXene surface chemistry and electronic conductivity. Previous DFT studies have predicted that termination of Ti3C2Tx and Ti3CNTx with –OH, –F and/or =O significantly alters the electronic states near EF [7, 43, 55, 56, 66]. For non-terminated Ti3C2 and Ti3CN, DFT predicts a local maximum in the DOS at EF, but with complete surface functionalization, the DOS(EF) is greatly reduced. Consequently, surface functionalization may alter the electronic resistance through a decrease the carrier concentration, n. To test the validity of this proposed mechanism, we analyzed the concurrent changes in the Ti3C2Tx and Ti3CNTx resistance and dR/dT with in situ annealing. We assume that the metallic intra-flake conductivity of these MXenes can be described by the Drude equation, and that over the temperature range of our in situ TEM experiments, electron-phonon scattering is approximately linear in temperature (Fig. 9.1) [76, 77]. With these assumptions, the Drude equation predicts that an increase in n with annealing will produce a proportional decrease in the resistance and dR/dT, i.e., DR∝DdR/

Results

dT∝(n1/n2−1), where n1 and n2 are the carrier concentrations before and after annealing, respectively (Supplementary Note 1). To visualize this behavior, we define η as the ratio of the proportional change in the RT dR/dT to the proportional change in the RT resistance for a given annealing step. A value of η = 1 is predicted for a change in resistance driven solely by a change in the intra-flake carrier concentration. For annealing Ti3C2Tx and Ti3CNTx at high temperatures, η ~ 1, supporting the claim that surface de-functionalization increases the MXene conductivity through an increase in n (Fig. 9.4).

Figure 9.4 Analysis of concurrent resistance and dR/dT changes with annealing. For a given annealing step, η is the ratio of the proportional change in the RT dR/dT to the proportional change in the RT resistance (Supplementary Note 1). A positive value of η (indicating a decrease in both the dR/dT and the resistance) is consistent with a change in the intra­ flake resistance, and a negative value of η (indicating an increase in dR/dT and a decrease in resistance) is consistent with a change to the inter-flake resistance. For a change in resistance solely due to a change in the intra-flake carrier concentration, the Drude formula predicts that η = 1. The colored lines are a guide to the eye. Error bars represent the measurement standard error accounting for the linear fit to the dR/dT data and assuming a base uncertainty of 1.2% in the resistance measurements. For the inset equation, n1 and n2 refer to the intra-flake carrier concentrations before and after an annealing step, respectively. See Supplementary Fig. 5 for dR/dT and η analysis for all studied MXenes.

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In contrast to the η ~ 1 behavior for high temperature annealing, η is negative for annealing Ti3C2Tx and Ti3CNTx at low temperatures (η 500 °C (Supplementary Fig. 5).

9.3 Discussion

In this study, we vacuum annealed multi-layer MXene samples within the TEM and measured 4, 6, >10, and 6 times increases in the conductivity of Ti3C2Tx, Ti3CNTx, Ti3CNTx (TBA+), and Mo2TiC2Tx, respectively. With annealing, we studied de-intercalation and surface de-functionalization with both in situ and ex situ spectroscopic techniques. By correlating this chemical analysis with in situ resistance and dR/dT measurements, we were able to delineate the effects of intercalation and surface termination on MXene electronic properties. Considering the role of intercalants first, we found that both H2O and TBA+ intercalants increase sample resistance and can induce negative dR/dT. Vacuum annealing caused de-intercalation, significant decreases in sample resistance, and the onset of metallic conduction (except for Ti3CNTx(TBA+)). The effects of TBA+ intercalation on electronic properties were far greater than that of H2O, and even after annealing at >750 °C, lingering effects of TBA+ intercalation persisted. These findings are relevant for the

Methods

optimization of MXene devices where large metallic conductivity is required and the increased inter-flake spacing associated with intercalation does not affect performance, e.g., wireless communication and wearable electronics. The ability to control dR/dT through intercalation could find use in multi-functional sensors or in the development of MXene films with arbitrary dR/ dT values. Additionally, we note the striking similarities between the intercalated Ti3CNTx and Mo2TiC2Tx samples measured here with previous reports of semiconductor-like transport in Mo- and V-based MXenes [38–40]. Confirmation of intrinsic semiconducting MXene behavior will require temperature-dependent resistance measurements of single-flake MXene devices, which is beyond the scope of this study. Regarding the effects of termination, vacuum annealing was shown to cause partial surface termination removal and increases in the MXene electronic conductivity. Oxygen terminations were more stable than –F terminations, and Mo2TiC2Tx experienced a greater degree of surface de-functionalization than the Ti-based MXenes. These findings provide an avenue to further improve performance in MXene applications such as electromagnetic interference shielding and optoelectronics. For other applications, e.g., triboelectrics and catalysis, improved conductivity is desired, but the functionalized MXene surfaces offer chemical benefits. Hence, further analysis is needed to understand how partial de-functionalization affects performance in these areas. From a broader perspective, our findings provide a first step towards realizing non-functionalized MXenes and termination-engineered MXenes, which are predicted to exhibit magnetism [56], fully spin-polarized transport [42], semiconducting behavior [36, 38], and nontrivial topological order [60, 78].

9.4 Methods

9.4.1 Syntheses of MXenes Ti3C2Tx MXene was synthesized by selective etching of Ti3AlC2 MAX phase (Materials Research Center, Ukraine) in a mixture of LiF and HCl [6]. Specifically, 0.5 g of Ti3AlC2 powder was allowed

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to react with a premixed etchant (0.8 g of LiF and 10 mL of 9 M HCl) for 24 h at room temperature. Then the acid mixture was washed with 150 mL of deionized water for 3–5 cycles until the pH of the supernatant reached the value of 5. After that, the mixture was hand-shaken for 10 min followed by centrifugation at 3500 rpm for 10 min to remove unreacted MAX particles and reaction by-products. The dark supernatant was centrifuged at 3500 rpm for another 1 h to yield delaminated Ti3C2Tx solution. Ti3CNTx MXene was synthesized from Ti3AlCN MAX [11] phase by two different routes. For the first route, the synthesis protocol is similar to the Ti3C2Tx protocol mentioned earlier except that the reaction mixture was stirred at 500 rpm at 40 °C for 18 h. For the second route, etching is performed in HF acid and delamination is achieved with molecular intercalation. Initially, 2 g Ti3AlCN was etched in 20 mL of ~30% HF acid for 24 h at room temperature. The reaction mixture was washed with 150 mL of deionized water for 3–5 cycles until the pH of the supernatant reached the value of 5. The resulting mixture was filtered through a filter paper (3501 Coated PP, Celgard, USA) to collect multi-layer Ti3CNTx powder. To delaminate the Ti3CNTx powder, 1 g of dry powder was stirred in a mixture of 9 mL of water and 1 mL of tetrabutylammonium hydroxide (TBAOH) solution (48 wt%, Sigma Aldrich) for 24 h. The mixture was hand-shaken for 10 min and washed with 3–5 cycles of deionized water. After neutral pH was reached, the mixture was centrifuged at 3500 rpm for 10 min to remove unreacted MAX particles and reaction by-products. The brownish supernatant was centrifuged at 3500 rpm for another hour to yield delaminated Ti3CNTx solution. Mo2TiC2Tx was synthesized from Mo2TiAlC2 MAX phase [59]. To produce the MAX phase, Mo, Ti, Al, and graphite powders (Alfa Aesar, Ward Hill, MA) were mixed in a ratio 2:1:1.1:2 and ball milled for 18 h in a plastic jar with zirconia balls. The powder mixture was transferred in an alumina crucible and held at 1600 °C for 4 h (5 °C min−1, Ar). The resulting MAX block was drilled and sieved (400 mesh, particle size 6. Then, powders were collected by vacuum-assisted filtration. Delamination of 1 g of Ti3C2Tx powder was achieved via intercalation of tetramethylammonium hydroxide (TMAOH, 25 wt % aqueous solution, Sigma-Aldrich) followed by the collection of the stable

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aqueous colloidal solution of Ti3C2Tx used to fabricate films via vacuum-assisted filtration. The synthesis and delamination of Ti3C2Tx are described step-by-step elsewhere [5].

10.4.2 Synthesis of Ti3C2Tx Using HF/H2SO4 or HF/HCl

Similar to the synthesis of Ti3C2Tx using 5 wt % HF, 1 g of Ti3AlC2 was etched using 20 mL of etchant solution at ambient temperature for 24 h. The acid mixture etchant was made by adding 2 mL of HF (50 wt %, Acros Organics) to either 4 mL of H2SO4 (96 wt %, Fisher) or 12 mL of HCl (37 wt %, Fisher) [13]. Washing and delamination steps are similar to those employed for Ti3C2Tx etched in pure HF. Ti3C2Tx multilayer powders and films were stored in vacuum and then vacuum-dried at 55 °C for 48 h before TA−MS analysis.

10.4.3 Synthesis of Mo2CTx Multilayer Powder and 2D Mo2CTx Film

Mo2Ga2C MXene precursor was synthesized as per the protocol described elsewhere [52]. Two grams of Mo2Ga2C precursor was etched in 20 mL of 50% HF for 120 h at 50 °C under stirring. After etching, the solution was washed by centrifugation at 3500 rpm for 5 min, decantation of the acidic supernatant, and addition of 300 mL of deionized water. This step was repeated five times until the supernatant reached pH >6. Finally, the multilayer Mo2CTx MXene was filtered through a Millipore 0.45 μm cellulose acetate membrane (Millipore). The collected powder was dried at 55 °C under vacuum for 48 h. To prepare a film of Mo2CTx from MXene single-layer flakes, 1 g of the multilayer powder was delaminated in a mixture of 4 mL of TMAOH and 6 mL of D.I. H2O for 18 h under stirring. After delamination, the solution was washed two times by centrifugation at 3500 rpm for 20 min, decantation of the basic supernatant, and addition of 50 mL of water until the supernatant reached pH OCV) and blue cross marks indicate cathodic potentials (EWE < OCV). Probing the transmittance (%T) spectral response from 280 to 1000 nm to (b) cathodic potentials and (c) anodic potentials, with black arrows showing the direction of change from OCV to the extreme potential and insets showing the %T reversibility to OCV.

An important parameter of an electrochromic device is the switching rate, which is the time needed to switch from one color to the other, or from minimal to maximal transmittance at a speciϐic wavelength of interest [5]. In Fig. 11.3, the smooth and immediate switching rate of the Ti3C2Tx electrochromic device (device conϐiguration in Fig. 11.1a,b) at different potentials from 0.0 to −1.0 V/Ag was displayed using 1 m H3PO4 aqueous electrolyte (instead of H3PO4/PVA gel electrolyte, to avoid any

Results and Discussion

possible diffusion limitation of the gel). The switching rate was investigated at 450 nm, the region in the spectrum where Ti3C2Tx had the broadest shift in transmittance (up to 20%T) (see Fig. 11.2b). It is worth noting that the switching could be performed at any wavelength, and often may be application dependent. When a smooth change of potential is applied (through CV from 0.0 to −1.0 V/Ag at 50 mV s–1), control over the transmittance shift based on the potential is demonstrated (Fig. 11.3a). However, when the potential was abruptly changed from 0.0 to −1.0 V/Ag (by chronoamperometry), ≈20% change in transmittance was observed in 0.6 s (Fig. 11.3b). Metal oxides, such as tungsten oxide, have a switching rate of a few seconds to one minute and an optical modulation of up to 85% [14]. Some polymer-based electrochromic devices have been shown to switch in ≈10 ms with a switching contrast from 70% to 90%; however, they need to be combined with metal grids and complex nanostructures to obtain such a fast rate [45]. In our study, fast switching rates can be obtained without the need of an external current collector because of the metallic conductivity of Ti3C2Tx [37]. However, when high currents occur (intense current spikes, 10–15 mA cm–2; Fig. S6, Supporting Information), resulting from the immediate switch of potential, the Ti3C2Tx thin ϐilm degrades after a few cycles and the ratelifetime performance will need further device engineering and optimization in future studies.

Figure 11.3 Switching rate of Ti3C2Tx electrochromic device in 1 m H3PO4 aqueous electrolyte in a three-electrode configuration. The rate was probed by monitoring the change in transmittance at 450 nm (T450 nm) when the potential was swept from 0.0 to −1.0 V/Ag, applied by (a) cyclic voltammetry at 50 mV s–1 and (b) chronoamperometry. The potenƟal applied to the device is represented by the blue trace and the measured T450 nm by the black trace. Inset in (b) shows shiŌ of transmiƩance for switch rate calculation.

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11.2.3  Understanding the Mechanism Involved in the Electrochromic Changes To understand the mechanism of these changes, in situ electrochemical Raman spectroscopy and in situ XRD were used, allowing for observation of the chemical and structural changes of the device during cycling in H3PO4/PVA gel electrolyte (Fig. 11.4; Fig. S7, Supporting Information). XRD was analyzed in the 2 region between 4° and 8°, corresponding to the (002) peak of Ti3C2Tx, to probe the effect of the lattice expansion or contraction due to intercalation/deintercalation of the electrolyte ions and water molecules at different applied potentials. Comparing the XRD patterns of the device without and with electrolyte, a shift of the (002) peak was observed, corresponding to an increase of the c-lattice parameter from 28.8 to 30.4 Å (2 from 6.07° to 5.85°), indicating intercalation of the electrolyte (Fig. 11.4a) [46]. The higher initial c value in the Ti3C2Tx ϐilm is due to water remaining intercalated from the dip-coating process [47, 48]. When potentials were applied, a shift of the (002) peak was only observed for anodic potentials (−0.1 to 0.2 V/Ag), where the expansion is diminished (c = −0.6 Å) (Fig. 11.4b). However, the expansion upon intercalation for cathodic potentials (−0.1 to −0.8 V/Ag) is not signiϐicant compared to the cycling of Ti3C2Tx with other electrolytes [46], and suggests the origin of the optical peak shift is not because of the intercalation/deintercalation of H+ ions alone. Therefore, we turned our attention to the relationship between the pseudocapacitive nature of Ti3C2Tx and the electrochromic properties observed. The pseudocapacitive mechanism relies on the reduction and oxidation of Ti–O/Ti–OH terminations, and the variation of the oxidation state of Ti in Ti3C2Tx [34, 35, 37]. Demonstrated by Hu et al., the change of surface terminations of Ti3C2Tx from –O to –OH when a cathodic potential is applied can be followed using in situ Raman spectroscopy [33]. The scattering peak at 723 cm–1 is assigned to the out-of-plane vibration of a C–Ti bond surrounded by an O-termination, such as in Ti3C2O2, whereas the peak at 708 cm–1 corresponds to that of C–Ti in a Ti3C2O(OH) environment [49]. While applying a cathodic potential,

Results and Discussion

the environment of the Ti transition metal atoms progressively changes from –O to –OH, inducing a down shift of the peak. This effect on the Raman shift of 723 cm–1 vibrational mode was observed for acidic electrolyte (H2SO4) but not for neutral electrolyte (MgSO4) [33].

Figure 11.4 Investigation of the electrochromic mechanism of the Ti3C2Tx electrode in H3PO4/PVA gel in three-electrode configuration by in situ X-ray diffraction (XRD) (a, b) to study the structural changes and in situ Raman spectroscopy (c, d) to study the chemical changes. Panels (a) and (c) show XRD patterns and Raman spectra, respectively, of the electrode before (orange trace) and after (black trace) addition of electrolyte. The XRD patterns and Raman spectra recorded at different potentials (0.2 to −0.8 V/Ag) are shown in (b) and (d), respectively.

Similarly, in situ electrochemical Raman spectroscopy was performed in a three-electrode conϐiguration (Fig. S7, Supporting Information). Figure 11.4c shows a Raman spectrum for Ti3C2Tx (deconvoluted in Fig. S3c and Table S1 in the Supporting Information). The addition of the H3PO4/PVA gel electrolyte had no effect on the Raman spectra, suggesting that

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the preintercalation observed in XRD does not modify the surface chemistry of Ti3C2Tx. On the other hand, Fig. 11.4d shows a proportional shift of the peak from 723 to 708 cm–1 while applying a cathodic potential from 0 to −0.8 V/Ag, respectively. Combined with the absence of signiϐicant variation of the c-lattice parameter under similar conditions, these observations indicate that the shift of the UV–vis–NIR peak in in situ electrochemistry is due to the pseudocapacitive properties of the Ti3C2Tx. Recently, El-Demellawi et al. [43], demonstrated a shift of ≈0.3 eV for the surface plasmon at 1.7 eV of Ti3C2Tx ϐlakes upon annealing up to 900 °C. This shift was attributed to the modiϐication of the surface terminations of Ti3C2Tx (in that case desorption of ϐluorine (F) groups) which involved the increase of the metal-like free electron density. Following the Planck–Einstein equation, the surface plasmon could be converted to the vis–NIR absorption peak observed for Ti3C2Tx. In addition, an energy shift of +0.3 eV corresponds to a wavelength shift of −110 nm, similar to the results shown in this study with H3PO4/PVA gel electrolyte (Fig. 11.2b). Therefore, controlling the surface terminations reveals a possibility of tuning the surface plasmon resonance and the resulting electrochromic behavior. To corroborate our hypothesis different aqueous electrolytes were tested to probe the effect of the anion (H3PO4 vs H2SO4) and the effect of the cation (H2SO4 vs MgSO4). In the case of H2SO4 electrolyte, the CV shows a large increase of the Faradaic current for cathodic potentials (Fig. 11.5a), similar to the behavior of H3PO4 (Fig. 11.2a), relating to the pseudocapacitive mechanism [34]. In accordance with the optical changes occurring during electrochemical cycling in H3PO4 electrolyte, H2SO4 electrolyte devices showed absorption peak shifts of 100 nm and T770 nm ≈12% for cathodic potentials (from 34% at −0.16 V to 46% at −1.0 V/Ag; Fig. 11.5b) and small changes for anodic potentials (Fig. 11.5c). On the other side, when changing the electrolyte to MgSO4, the CV was rectangular (Fig. 11.5d), indicative of an electrical double layer capacitance [34]. Probing the optical changes, devices fabricated with MgSO4 electrolyte show a blueshift with a lower magnitude ( = 35 nm, T770 nm ≈3%) (Fig. 11.5e,f).

Results and Discussion

Figure 11.5 In situ electrochromic study of Ti3C2Tx in H2SO4 and MgSO4 aqueous electrolytes in a three-electrode configuration. (a) Cyclic voltammogram of the device in H2SO4, where blue cross marks indicate cathodic potentials (EWE < OCV) and red cross marks indicate anodic potentials (EWE > OCV). Probing the UV–vis–NIR transmittance spectral response from 280 to 1000 nm to (b) cathodic potentials (reversibility to OCV is shown in the inset) and (c) anodic potentials; with black arrows showing the direction of change from OCV to the extreme potential applied. (d) Cyclic voltammogram of the device in MgSO4, and (e) UV–vis–NIR spectra recorded at cathodic potentials (reversibility to OCV is shown in the inset) and (f) anodic potentials.

To emphasize the different optoelectrochemical behavior between acidic (1 M H2SO4 and H3PO4) and neutral (1 M MgSO4) electrolytes, the energy (in eV) of the absorption peak as a function of the applied potential was plotted in Fig. 11.6a. Two clear trends are observed when EWE < OCV (cathodic potentials) and EWE > OCV (anodic potentials) for all the three tested electrolytes, where the energy associated with the absorption peak follows a linear trend with the applied potential (total energy change for acidic electrolytes schematized in Fig. 11.6b). For EWE > OCV, the slope of energy change is similar for all the three electrolytes, emphasizing the negligible effect of the anion intercalation on MXene optical properties in this potential range. Focusing on EWE < OCV, where the most important optical

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changes occur, here again both acidic electrolytes (H3PO4 and H2SO4) showed similar effects (Table S2, Supporting Information). Considering the difference in energy between OCV and the most negative cathodic potential applied (EWE − OCV = −0.8 V), the Ti3C2Tx ϐilms in acidic electrolytes had a total shift of ≈0.25 eV, however with the MgSO4 electrolyte the shift was only about ≈0.08 eV. These results indicate that the nature of the cation plays an important role in the electrochromic properties of Ti3C2Tx. In case of acidic electrolytes, the observed shifts are 3 times higher than the neutral electrolyte, corroborating that protons and the redox mechanism play a signiϐicant role in electrochromic performance of MXene devices. The coupling between redox and optical properties shown in this study suggest the process could be further optimized if the surface functionalities and transition metal of MXene were controlled.

Figure 11.6 (a) Comparison of the change in absorption peak position of UV–vis–NIR spectra (corresponding wavelength plotted in energy, eV) for Ti3C2Tx MXene with different electrolytes under potenƟal. (b) Schematic of the energy change of the absorption peak as a funcƟon of the applied potential for acidic electrolytes.

Conclusions

It has been demonstrated that Ti3C2Tx MXene can be used as an active material in an electrochromic device. Because the MXene structure and composition has a direct effect on their optical properties (compare, e.g., Ti3C2Tx and Ti2CTx [21]) devices with a variety of electrochromic properties should be possible. As a proof of concept, Ti3CNTx MXene was also studied and has demonstrated an even larger shift of the absorption peak than Ti3C2Tx (Fig. S8, Supporting Information). This work opens a new avenue for the use of MXene family of materials, with more than 30 members already available, to be further developed as optic, photonic, and electrochromic materials.

11.3 Conclusions

Ti3C2Tx thin ϐilms have been fabricated by an optimized dip-coating method, obtaining a maximum FoMe of 17. The electrochromic behavior of the thin ϐilms has been studied in a three-electrode conϐiguration by in situ UV–vis–NIR spectroscopy, observing a shift of the absorption peak and change of transmittance, which is proportional to the cathodic potentials applied. These optical changes are dependent on the electrolyte, where the largest change was observed with acidic electrolytes (T770 nm ≈12%,  ≈100 nm) compared to neutral electrolyte (T770 nm ≈3%,  35 nm). Using in situ XRD and in situ Raman spectroscopy, the mechanism of the electrochromic behavior has been attributed to the pseudocapacitive change of the MXene surface functionalities (Ti–O to Ti–OH) upon reduction. It is believed that the surface plasmon related to the absorption peak in the visible region is affected by tuning the metal-like free electron density of the MXene, which increases when a cathodic potential is applied, and this phenomenon is further ampliϐied by the pseudocapacitive mechanism. These results suggest that the electrochromic change of the ϐilms could be improved by controlling the surface functionalities of Ti3C2Tx. Due to changes in optical properties with MXene composition, MXene electrochromic devices with different colors could be produced. Owing to about 30 MXenes available with different optical and electrochemical properties (and millions of compositions

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possible), a variety of electrochromic behaviors might be achieved by varying the MXene composition and electrode conϐiguration.

11.4 Experimental Section

Preparation of Ti3C2Tx: All chemical reagents were used as received without further puriϐication. Ti3AlC2 MAX phase powder was obtained from Y-carbon Ltd., Ukraine and sieved (particle size < 40 μm). Ti3C2Tx MXene was synthesized by selective etching of the aluminum from the MAX, following minimally intensive layer delamination (MILD) protocol previously described by Alhabeb et al. [21]. Brieϐly, Ti3AlC2 powder (1 g) was slowly added to an etchant solution containing of lithium ϐluoride salt (1 g) (LiF, Alfa Aesar, 98+%) dissolved in hydrochloric acid (20 mL, 9 m) (HCl, Fisher, technical grade, 35–38%) under stirring. The reaction was stirred for 24 h at 35 °C. The resulting acidic solution was washed with deionized water, by consecutive centrifugation (5 min at 3500 rpm) and decantation of the clear supernatant, until a pH of 6 or more was reached. When pH ≥ 6, delamination occurred, a stable dark supernatant of Ti3C2Tx was obtained and was collected by centrifuging for 30 min at 500 rpm. Smaller MXene ϐlakes (≈0.5 μm) were prepared by sonication of the obtained colloidal solution in an ice bath for 30 min under inert gas bubbling to avoid oxidation. The resulting colloidal dispersion was then centrifuged at 3500 rpm for 20 min, and the supernatant was collected. The concentration of Ti3C2Tx solution was measured by ϐiltering a known volume of colloidal dispersion through a polypropylene ϐilter (3501 Coated PP, Celgard LLC, Charlotte, NC), followed by overnight drying under vacuum and weighing. Thin Films Preparation by Dip-Coating: Glass substrates of 2.5 × 7.5 cm2 size (Fischer Scientiϐic) were cleaned in bath sonication with a soap solution (Hellmanex III, Fisher Scientiϐic) followed by consecutive sonication in deionized water and ethanol for 5 min each and then dried with compressed air. Then, a plasma treatment (Tergeo Plus, Pie Scientiϐic) at 50 W with a mixture of O2 and Ar (3 and 5 sccm) for 5 min was applied

Experimental Section

to the substrates for further cleaning and to improve their hydrophilicity. Finally, as-prepared substrates were coated with MXene thin ϐilm by dip-coating technique. An automated dipcoater (PTL-MM01 Dip Coater, MTI Corporation) was used to control the dipping/withdrawing speed and distance. The substrates were immersed in the colloidal solution for 3 min, pulled out at a constant speed of 2 mm s–1, and dried in air at room temperature. In case of multiple dipping (up to ϐive), the substrate was left to dry between each dip for 5 min. The ϐilm on the back side of the substrate was erased using ethanol. The parameters studied during optimization of the technique were: MXene concentration (1–10 mg mL–1), number of dips (1–5) and MXene ϐlake size. The obtained thin ϐilms were kept in a vacuum desiccator overnight before characterization. Material Characterization: The particle size of MXene in a colloidal solution was measured by dynamic light scattering (DLS, Zetasizer Nano ZS, Malvern Panalytical). The optical spectra of the MXene thin ϐilms was measured in the range of 280–1000 nm by UV–vis–NIR spectroscopy (Evolution 201 UV–vis–NIR spectrophotometer, Thermo-Fischer scientiϐic). The sheet resistance was measured with a four-point probe (ResTest v1, Jandel Engineering Ltd., Bedfordshire, UK) with a probe distance of 1 mm, measuring at ϐive different spots for each sample and taking the averaged result. The top view of the MXene coatings were imaged using a scanning electron microscope (Zeiss Supra 50VP, Germany). Roughness and thickness of the ϐilms were analyzed by optical proϐilometer (Zygo Corporation, Middleϐield, USA). Raman spectroscopy was done using an inverted reϐlection mode with a Renishaw microscope (2008, Glouceshire, UK), equipped with 50× objective and a LEICA CTR6000 setup with 633 nm laser, 1800 lines mm–1, grating at 10% of maximum intensity. Spectra were collected with an accumulation time of 120 s and three accumulations. XRD was conducted on a Rigaku Smartlab operating at 40 kV and 40 mA. Each scan was collected from 4° to 8° (2θ) with a step size of 0.02° at 5 s per step, on MXene ϐilms or loose MAX powder. Fabrication of MXene Electrochromic Device: To study the electrochromic properties of the MXene thin ϐilms, symmetric three-electrode cells were used. The working electrode and

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counterelectrode were MXene thin ϐilms on glass substrate with copper tape on one side to make the electrical contact. A silver wire was used as pseudo-reference electrode and a Teϐlon mask was used as mask to create an electrolyte reservoir between the electrodes with an area 3.7 cm2. For single-electrode in situ optoelectrochemical study (UV–vis–NIR spectroscopy), a 0.5 cm diameter hole was made on the Ti3C2Tx CE (see Fig. 11.1), to ensure the UV–vis–NIR characterization of the WE only. For in situ XRD measurements, a PET foil was used as WE substrate instead of glass to improve the collected signal. For in situ Raman spectroscopy measurements, MXene was deposited on a glass cover slide and used as a WE (for schematic of the three setups see Fig. S7 in the Supporting Information). The electrolytes used were phosphoric acid in polyvinyl alcohol gel (H3PO4/PVA gel), sulfuric acid (H2SO4, Fisher Scientiϐic 98%) and magnesium sulphate (MgSO4, Fisher Scientiϐic all with a concentration of 1 M. To obtain the H3PO4/PVA gel, PVA (1 g) (Alfa Aesar, 98%) was dissolved in deionized H2O (10 mL) by stirring at 80 °C for 3 h. Then concentrated H3PO4 (1 g, 0.6 mL) (Alfa Aesar) was added to the obtained PVA gel and stirred for 30 min at room temperature to obtain H3PO4/PVA gel [50]. In situ UV–vis–NIR, XRD, and Raman Spectroscopy Experiments: For in situ electrochemical measurements with UV–vis–NIR spectroscopy, XRD, and Raman spectroscopy, the systems were precycled 5 times by cyclic voltammetry at 20 mV s–1 to determine the potential window of the device. Then, chronoamperometry (CA) were acquired for different potentials applied for a period of 15 min each, during the time needed to measure the spectra of the corresponding techniques (UV–vis–NIR spectroscopy, XRD, and Raman spectroscopy). In the case of UV–vis–NIR spectroscopy, the uncoated glass slide was used for the blank. The change of transmittance was measured at 770 nm (T770 nm), comparing the spectra at OCV and at the applied potential. Three different electrolytes were compared: H3PO4/PVA gel, H2SO4, and MgSO4. To calculate the switching rate, the time needed to switch transmittance at 450 nm (T450 nm) was measured when chronoamperometry from 0.0 to −1.0 V/Ag was applied, with an aqueous H3PO4 electrolyte. The time measured corresponds to 90% of the total change of transmittance. To evaluate the dynamic

References

response of the device in case of a continuous potential perturbation, T450 nm was also followed while cycling the working electrode through a CV between 0.0 and −1.0 V/Ag at 50 mV s–1. In the case of Raman spectroscopy and XRD analysis, the only electrolyte used was H3PO4/PVA gel. The conditions followed for in situ Raman spectroscopy and XRD were the same conditions used for thin ϐilm characterization.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author (https://onlinelibrary.wiley.com/action/downl oadSupplement?doi=10.1002%2Fadfm.201809223&ϐile=adfm201 809223-sup-0001-S1.pdf; https://onlinelibrary.wiley.com/action/ downloadSupplement?doi=10.1002%2Fadfm.201809223&ϐile=adf m201809223-sup-0001-S1.mp4).

Acknowledgements

The authors acknowledge Asia Sarycheva and Tyler Mathis for the help operating the Raman spectrometer. The authors also acknowledge the Core Research Facilities (CRF) at Drexel University for providing access to SEM and XRD. P.S. was supported by the French Network RS2E through a scholarship for the master program Materials for Energy Storage and Conversion (MESC).

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16. F. Shahzad, M. Alhabeb, C. B. Hatter, B. Anasori, S. Man Hong, C. M. Koo, Y. Gogotsi, Science 2016, 353, 1137.

17. M. Naguib, M. Kurtoglu, V. Presser, J. Lu, J. Niu, M. Heon, L. Hultman, Y. Gogotsi, M. W. Barsoum, Adv. Mater. 2011, 23, 4248.

18. K. Hantanasirisakul, Y. Gogotsi, Adv. Mater.30, 2018, 1804779.

19. J. Halim, K. M. Cook, M. Naguib, P. Eklund, Y. Gogotsi, J. Rosen, M. W. Barsoum, Appl. Surf. Sci. 2016, 362, 406.

20. I. Persson, L.-Å. Näslund, J. Halim, M. W. Barsoum, V. Darakchieva, J. Palisaitis, J. Rosen, P. Persson, 2D Mater. 2018, 5, 015002.

21. M. Alhabeb, K. Maleski, B. Anasori, P. Lelyukh, L. Clark, S. Sin, Y. Gogotsi, Chem. Mater. 2017, 29, 7633.

22. K. Maleski, C. E. Ren, M. Zhao, B. Anasori, Y. Gogotsi, ACS Appl. Mater. Interfaces 2018, 10, 24491.

23. C. Zhang, B. Anasori, A. Seral-Ascaso, S. H. Park, N. McEvoy, A. Shmeliov, G. S. Duesberg, J. N. Coleman, Y. Gogotsi, V. Nicolosi, Adv. Mater. 2017, 29, 1702678.

24. A. Lipatov, M. Alhabeb, M. R. Lukatskaya, A. Boson, Y. Gogotsi, A. Sinitskii, Adv. Electron. Mater. 2016, 2, 1600255. 25. C. J. Zhang, S. Pinilla, N. McEvoy, C. P. Cullen, B. Anasori, E. Long, S.-H. Park, A. Seral-Ascaso, A. Shmeliov, D. Krishnan, C. Morant, X. Liu, G. S. Duesberg, Y. Gogotsi, V. Nicolosi, Chem. Mater. 2017, 29, 4848.

26. K. Hantanasirisakul, M. Q. Zhao, P. Urbankowski, J. Halim, B. Anasori, S. Kota, C. E. Ren, M. W. Barsoum, Y. Gogotsi, Adv. Electron. Mater. 2016, 2, 1600050.

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Chapter 12

Effects of Synthesis and Processing on Optoelectronic Properties of Titanium Carbonitride MXene Kanit Hantanasirisakul,a,b Mohamed Alhabeb,a,b Alexey Lipatov,c Kathleen Maleski,a,b Babak Anasori,a,b Pol Salles,a,b Chanoknan Ieosakulrat,a,d Pasit Pakawatpanurut,d Alexander Sinitskii,c Steven J. May,b and Yury Gogotsia,b aA.J.

Drexel Nanomaterials Institute, Drexel University,

Philadelphia, Pennsylvania 19104, USA

bDepartment of Materials Science & Engineering, Drexel University,

Philadelphia, Pennsylvania 19104, USA

cDepartment of Chemistry and Nebraska Center for Materials and Nanoscience,

University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0304, USA

dCenter of Sustainable Energy and Green Materials,

Center of Excellence for Innovation in Chemistry,

and Department of Chemistry, Faculty of Science, Mahidol University,

Bangkok 10400, Thailand

[email protected]

MXenes, a relatively new class of two-dimensional (2D) transitionmetal carbides, carbonitrides, and nitrides, exhibit unique properties such as high electronic conductivity, a wide range of optical characteristics, hydrophilicity, and mechanical stability. Because Reprinted from Chem. Mater., 31(8), 2941–2951, 2019.

MXenes: From Discovery to Applications of Two-Dimensional Metal Carbides and Nitrides Edited by Yury Gogotsi Text Copyright © 2019 American Chemical Society Layout Copyright © 2023 Jenny Stanford Publishing Pte. Ltd. ISBN 978-981-4877-95-4 (Hardcover), 978-1-003-30651-1 (eBook) www.jennystanford.com

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of the high electronic conductivity, MXenes have shown promise in many applications, such as energy storage, electromagnetic interference shielding, antennas, and transparent coatings. 2D titanium carbide (Ti3C2Tx, where Tx represents surface terminations), the first discovered and most studied MXene, has the highest electronic conductivity exceeding 10 000 S cm–1. There have been several efforts to alter the conductivity of MXenes, such as manipulation of the transition-metal layer and control of surface terminations. However, the impact of the C and N site composition on electronic transport has not been explored. In this study, the effects of synthesis methods on optoelectronic properties of 2D titanium carbonitride, Ti3CNTx, were systematically investigated. We show that Ti3CNTx, which hosts a mix of carbon and nitrogen atoms in the X layer, has lower electronic conductivity and a blue shift of the main absorption feature within the UV−visible spectrum, compared to Ti3C2Tx. Moreover, intercalants such as water and tetraalkylammonium hydroxides decrease the electronic conductivity of MXene due to increased interflake resistance, leading to an increase in resistivity with decreasing temperature as observed in ensemble transport measurements. When the intercalants are removed, Ti3CNTx exhibits its intrinsic metallic behavior in good agreement with Hall measurements and transport properties measured on single-flake field-effect transistor devices. The dependence of conductivity of Ti3CNTx on the presence of intercalants opens wide opportunities for creating MXene-based materials with tunable electronic properties.

12.1 Introduction

Two-dimensional (2D) transition-metal carbides, carbonitrides, and nitrides, collectively known as MXenes, have shown great promise in various applications, including energy storage, electromagnetic interference shielding, transparent conductive coatings, photothermal therapy, catalysis, water desalination, and as a metamaterial [1–7]. MXenes have a general formula of Mn+1XnTx, where M is an early transition metal, X is C and/or N, and Tx represents surface terminations, such as O and F, and

Introduction

n = 1−3. To date, nearly 30 MXene compositions with different properties have been experimentally synthesized and dozens more have been predicted to be stable and studied theoretically. Among them, 2D titanium carbide, Ti3C2Tx, is the first and most studied MXene. It has high metallic conductivity exceeding 10 000 S cm–1 in thin films [8]. The performance of MXenes in the above-described applications is related to their electronic conductivity, which can be improved by different approaches, such as increasing the lateral flake size, minimizing the defects and impurities, and modification of surface terminations [9–11]. Density functional theory (DFT) studies have predicted that the electronic and optical properties of MXenes strongly depend on transition metals, X elements, and surface terminations [12, 13]. For example, it has been experimentally shown that the composition of outer atomic layers (M′) in the ordered doubletransition-metal MXenes, M2′M″C2 and M2′M2″C3, has a strong influence on their electronic properties [14]. Moreover, the effects of surface terminations and intercalation on the electronic properties of MXenes have been recently reported [11, 15, 16]. However, there is no experimental evidence that tuning of the electronic and optical properties of MXenes can be achieved by manipulating the X elements. In most bulk transition-metal carbides and carbonitrides, the electronic conductivity increases with nitrogen content because of increased density of states (DOS) at the Fermi level (EF) resulting from the extra electrons of the nitrogen atoms [17–20]. In bulk-layered ternary carbides and/or nitrides (known as MAX phases, where A is a group IIIA−VIA element), which are precursors for MXene synthesis, the electronic conductivity is of metallic character with high DOS at the EF [21, 22]. It has been shown that the electronic properties of the MAX phases can be controlled by manipulating the three components of the structure, that is, transition metals, A elements, and X elements [23–25]. Although alteration of the transition metal and/or X layer affects the DOS at EF of the MAX phases, the change in the A elements (e.g., formation of a binary solid solution) shows little effect on electronic and thermal properties aside from increased residual resistivity, resulting from solid solution scattering [24]. The

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effects of N-substitution on the electronic properties of the MAX phases are apparent when comparing Ti2AlC, Ti2AlC0.5N0.5, and Ti2AlN. Ti2AlC0.5N0.5 showed the highest resistivity followed by Ti2AlC and Ti2AlN, and the DOS at EF slightly increases with N-substitution. Similar trends were observed for Ti3AlC2 and Ti3AlCN systems [25]. However, for carbonitride MXenes, it was predicted that the DOS at EF of Ti3CNTx and Ti3N2Tx MXenes is lower than that of their Ti3C2Tx counterparts, yet these materials are metallic with high DOS at EF. The Fermi level is positioned at a band that contains mainly Ti 3d states, whereas the C 2p and N 2p states are found between −4 to −2 eV and −6 to −3 eV below the Fermi level, respectively [13]. Unlike the MAX phases, the contribution of the N 2p band in carbonitride and nitride MXenes to the DOS at EF is almost negligible. However, this difference in electronic properties between bulk carbonitrides and 2D carbonitride MXenes has not been investigated experimentally. Therefore, it might be possible to alter the electronic conductivity of Ti3C2Tx by partial substitution of carbon atoms with nitrogen atoms in the X layer—forming Ti3CNTx. Titanium carbonitride MXene has been reported to have very attractive energy storage and electrocatalytic properties [26–28], so understanding and controlling its conductivity is of great practical interest. Moreover, we have recently shown that the electronic properties of Mo2CTx and V2CTx MXenes can be tuned by formation of Mo2N and V2N via high-temperature ammoniation [29]. Similar to its Ti3C2Tx carbide counterpart, Ti3CNTx can be synthesized by etching Ti3AlCN in (i) hydrofluoric acid (HF) followed by intercalation of organic intercalants, such as hydrazine or tetrabutylammonium hydroxide (TBAOH), to delaminate the 2D MXene layers [30–32] or (ii) in a mixture of lithium fluoride (LiF) and hydrochloric acid (HCl), forming HF in situ, without subsequent use of any organic intercalant [26]. For both synthesis methods, Ti3CNTx showed much lower conductivity compared to Ti3C2Tx obtained by similar methods, although the LiF + HCl method produced MXenes with higher electronic conductivity compared to HF etching [9, 26, 30]. However, there is no published report on the effects of synthesis and processing methods on the optoelectronic properties of Ti3CNTx MXene.

Experimental Section

In this work, we studied the optoelectronic properties of a titanium carbonitride MXene, Ti3CNTx, and the effects of material synthesis and processing on those properties. We showed that the presence of nitrogen atoms in the X layer results in lower electronic conductivity and a blue shift of the main absorption peak in UV−visible spectra compared to Ti3C2Tx. Moreover, we found that the macroscopic electronic transport behavior of Ti3CNTx is largely governed by interflake electron hopping processes rather than intrinsic electron transport within a flake. Single-flake measurements reveal intrinsic metallic behavior of the material. This work demonstrates the effects of X elements, synthesis, and processing on optoelectronic properties of MXenes, providing more opportunities to tune those properties.

12.2 Experimental Section

Ti3AlCN MAX powder was prepared following a procedure reported elsewhere [32]. Briefly, elemental Ti (Alfa Aesar, 99.5 wt % purity), AlN (Sigma-Aldrich, 99 wt % purity), and graphite (Alfa Aesar, 99 wt % purity; particle size < 48 μm) with a molar ratio of 3:1:1 were ball-milled for 18 h in a plastic jar with 10 mmdiameter zirconia milling balls at 50 rpm. The powder mixture was heated under Ar flow (100 mL min–1) in a tube furnace at 10 °C min–1 to 1500 °C and held for 2 h. After cooling down to room temperature, the resulting block of Ti3AlCN MAX was crushed via drill-milling and sieved through a 400-mesh sieve (particle size < 38 μm). Ti3CNTx MXene was produced by two etching routes, via a mixture of LiF and HCl and via HF. For the LiF + HCl route, 0.5 g of Ti3AlCN powder was slowly added to a mixture of 0.8 g of LiF (Alfa Aesar) and 10 mL of 9 M HCl (Fisher Chemical), and the solution was stirred at 500 rpm at 35 °C for 24 h. Then, the mixture was washed by adding 150 mL of deionized (DI) water and centrifuged at 3500 rpm (2300 rcf) for 5 min followed by decantation of the clear supernatant. The washing process was repeated four to five times until the pH of the supernatant became close to neutral and the supernatant became dark, as a result of partial delamination of Ti3CNTx flakes (d-Ti3CNTx). The dark

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supernatant was decanted and ∼10 mL of DI water was added to the sediment. The mixture was bath-sonicated (2510, Branson) at 40 kHz for 30 min or manually shaken for 15 min and centrifuged at 3500 rpm for 1 h. The supernatant, consisting mainly of single-layer or few-layer flakes of Ti3CNTx, was collected for further processing. For the HF route, 2 g of MAX powder was slowly added to 20 mL of 30 wt % HF aqueous solution (Acros Organics). The mixture was stirred at 500 rpm at room temperature (20 °C) for 18 h and washed in the same way as the LiF + HCl method. When the pH of the supernatant became close to neutral, the multilayered Ti3CNTx (ML-Ti3CNTx) was collected by a vacuum-assisted filtration. An additional 200 mL of DI water was used to rinse the powder. To delaminate the HF-etched Ti3CNTx, 1g of the powder was added to a mixture of 5 mL of tetramethylammonium hydroxide (TMAOH) (25 wt % in water, Sigma-Aldrich) or TBAOH (48 wt % in water, Sigma-Aldrich) and 5 mL of DI water. The mixture was stirred for 12 h at room temperature and then washed with repeated centrifugation (50 mL of DI water for two to three centrifugation times or until the pH of the solution was close to neutral). Finally, ∼10 mL of DI water was added to the sediment, the mixture was bath-sonicated for 30 min, and centrifuged at 3500 rpm for 1 h to separate single- and few-layer flakes from nonexfoliated multilayered MXenes. The Ti3CNTx thin films were prepared by a spin-casting technique using a spin coater (Laurell Technologies, Model WS-650 Hz, USA). The delaminated MXene solution with a concentration of 5 or 20 mg mL was applied to an oxygen plasmatreated microscope slide and spun at various spin speeds from 1000 to 10 000 rpm for 30 s. The films were subsequently dried at 5000 rpm for 15 s. The free-standing MXene films were prepared by vacuum-assisted filtration of the delaminated solution on a porous membrane (3501 Coated PP, Celgard, USA). Sheet resistance was measured using a four-point probe (ResTest v1, Jandel Engineering Ltd., Bedfordshire, UK) with 1 mm distance between the probes. The measurement was repeated five times at the four corners and the middle of each film. The average values were reported.

Experimental Section

UV−visible spectroscopy measurements were conducted using a spectrometer (Evolution 201, Thermo Scientific), scanning from 200 to 1000 nm and normalized at 264 nm. A microscope glass slide and DI water were used as blanks for thin-film and solution measurements, respectively. The extinction coefficients (e) were calculated by the Beer−Lambert law [A = eCl, where A is the measured absorbance, C is the concentration (g L–1) of MXene in solution determined through gravimetric analysis, and l is the optical path length (1 cm)]. The as-produced solutions were serial-diluted, and extinction coefficients were calculated from the extinction value at the peak maxima in the visible region (l max). Dynamic light scattering (DLS) of the d-Ti3CNTx solutions was performed using a Zetasizer Nano ZS (Malvern Panalytical) in a polystyrene cuvette. The average particle size was taken over a total of five measurements from each sample. Scanning electron microscopy (SEM) images were taken with an electron microscope (Zeiss Supra 50VP). The Ti3CNTx MXene flakes were also visualized using a FEI Tecnai Osiris scanning transmission electron microscope equipped with a HAADF detector and a X-FEG high brightness Schottky field emission gun. The accelerating voltage was 200 kV. X-ray diffraction (XRD) patterns were recorded by a powder diffractometer (Rigaku Smart Lab, USA) with Ni-filtered Cu Kα radiation operated at 40 kV and 15 mA. A step size of 0.03° and 0.5 s dwelling times were used to collect the patterns. Atomic force microscopy (AFM) images were recorded by a Bruker AFM Multimode 8 using a driving frequency of 249 kHz, driving amplitude of 73.85 mV, and scanning frequency between 0.6 and 0.9 Hz. Transport property measurements were conducted in a physical property measurement system (PPMS) (Quantum Design EverCool II). Free-standing MXene films with a size of 5 × 5 mm were wired to the PPMS sample puck using a silver wire and a conductive silver paint (Ted Pella, CA). Temperature-dependent resistivity was recorded from 10 to 300 K in a low-pressure helium environment (∼10 Torr) using the four-point configuration. The film thicknesses were measured by an electronic micrometer (MDH-25M, Mitutoyo, IL). The geometry of the samples and probe spacing were taken into account in resistivity calculation

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following the standard method [33]. Hall measurements were performed at various temperatures using van der Pauw configuration. The magnetic field was swept between −30 and 30 kOe, and the Hall voltage was measured two to five times at the same magnetic field value. The effect of magnetoresistance was removed by subtracting the Hall resistance in the positive magnetic field regime with their respective negative field values and dividing the difference by 2. X-ray photoelectron spectroscopy (XPS) spectra were collected in a spectrometer (Physical Electronics, VersaProbe 5000, Chanhassen, MN) using a 100 μm monochromatic Al Kα X-ray beam. Photoelectrons were collected at a takeoff angle of 45° between the sample surface and the hemispherical electron energy analyzer. Charge neutralization was applied using a dualbeam charge neutralizer, irradiating low-energy electrons and ion beams. Survey scans were obtained from 1000 to −4 eV binding energy using a pass energy of 117.4 eV, whereas the highresolution scans were obtained within the interested regions using a pass energy of 23.5 eV. Quantification and deconvolution of the core-level spectra were performed using a software package (CasaXPS Version 2.3.16 RP 1.6). Background contributions to the measured intensities were subtracted using a Shirley function. Thermogravimetric and mass spectroscopy analyses were performed using a Discovery SDT 650 thermal balance connected to a Discovery mass spectrometer (TA Instruments, DE). The free­ standing MXene films with masses around 5−10 mg were dried in a vacuum desiccator at room temperature for at least 24 h prior to the measurement. Then, they were packed in a 90 μL alumina pan and heated to 1500 °C at a constant heating rate of 10 °C min–1 in He atmosphere (100 mL min–1). The furnace was purged with a 100 mL min–1 flow of He gas for 1 h before the analysis to remove air residue. For single-flake device fabrication, the Ti3CNTx MXene solution was drop-cast onto a Si/SiO2 substrate and dried in air, leaving multiple MXene flakes on the substrate. Individual monolayer flakes were identified by optical microscopy and then used for device fabrication. Similar to other 2D materials deposited on the Si/SiO2 substrate, MXene flakes show color contrast in the optical microscope. Multilayered and folded flakes exhibit higher

Results and Discussion

contrast compared with monolayer flakes, which we used to differentiate between the number of layers. For device fabrication, we selected uniformly colored flakes with the lowest contrast, which were confirmed to be Ti3CNTx monolayers by AFM imaging. A Zeiss Supra 40 field-emission scanning electron microscope and a Raith pattern generator were used for electron beam lithography to pattern electrodes on Ti3CNTx MXene flakes. Deposition of 3 nm of Cr at a rate of 0.2 Å s–1 and 20 nm of Au at a rate of 0.3 Å s–1 was performed using an AJA electron beam evaporation system at a base pressure of ∼7 × 10–9 Torr. The electrical characterization of MXene devices was performed in a Lake Shore TTPX cryogenic probe station at a base pressure of 2 × 10–6 Torr using an Agilent 4155C semiconductor parameter analyzer.

12.3 Results and Discussion

12.3.1 Synthesis and Delamination The XRD pattern of the Ti3AlCN MAX powder used in this study is shown in Fig. S1a. The peak positions and intensity are in good agreement with the previous report [32]. We note that there is a small amount of M4AX3 and M2AX phases in the powder as indicated by their (002) peaks at 7.5° and 13.0°, respectively. Moreover, we cold-pressed this Ti3AlCN powder as well as Ti3AlC2 powder with pressure ranging from 250 to 900 MPa and compared the electronic conductivity and density of the resulting MAX powder pellets as shown in Fig. S1b. Although the conductivity of Ti3AlC2 and Ti3AlCN pellets increased with increasing the applied pressure because of more efficient interparticle contacts at the grain boundaries, the difference between Ti3AlC2 and Ti3AlCN lies within the error of the measurement. This result and that reported earlier show minimal effect of nitrogen substitution on the electronic conductivity of the Ti3AlC2 and Ti3AlCN MAX phases [25, 34]. A schematic representation of the two synthesis routes and the structures of the resulting products is shown in Fig. 12.1. In the LiF + HCl route, the resulting MXene sheets are intercalated by water molecules and solvated Li+ ions, whereas in the HF +

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Optoelectronic Properties of Titanium Carbonitride MXene

TMAOH route, the films are intercalated by water molecules and tetramethylammonium cations (TMA+). Similar to Ti3C2Tx, Ti3CNTx was readily delaminated without the need of sonication when a mixture of LiF and HCl with a molar ratio of LiF to MAX powder exceeding 7.5:1 was used in the etching step [35]. SEM and AFM were used to confirm successful delamination of Ti3CNTx via the LiF + HCl route (Figs. 12.2a−c and S2), the HF + TMAOH route (Fig. 12.2d), and the HF + TBAOH route (Fig. 12.2e). Thin and transparent flakes (marked with orange arrows) with lateral size around 300 nm for sonicated solution and up to 10 μm for the LiF + HCl route without sonication were observed over a large area on a porous anodized alumina membrane (Fig. 12.2b−c). A cross-sectional SEM image of a ∼2.5-μm-thick free-standing film prepared by vacuum filtration of d-Ti3CNTx (LiF + HCl) solution is shown in Fig. 12.2f. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) of Ti3CNTx (LiF + HCl) without sonication also confirm delamination, yielding flakes with lateral size larger than 10 μm with a typical hexagonal symmetry of MXenes (Fig. 12.3). The size distribution curves obtained from DLS of d-Ti3CNTx solutions (Fig. 12.4a) are shown in Fig. 12.4b. The DLS results are in good agreement with the flake size observed in SEM and TEM, where the peaks in DLS results are at ∼300 nm for the sonicated solution and at ∼1 and 10 μm for nonsonicated solution. For both synthesis methods, the obtained flakes are free of impurity particles, such as oxide or remaining salt, as can be seen from the clean surface and edges of the flakes in the SEM, TEM, and AFM results in Figs. 12.2, 12.3, and S2. Nonetheless, small amounts of impurity nanoparticles were observed in the delaminated solution, especially when organic bases were used in the delamination step (not shown). The thickness of a single MXene flake was measured by AFM and was found to be close to 2.5 nm (Fig. S2). The thickness of Ti3CNTx measured in this work was similar to that of Ti3CNTx and Ti3C2Tx reported earlier [9, 26, 36]. We also observed a step of 1.5 nm when there was a stack of flakes on top of each other. However, the measured thickness values are still larger than that proposed by the DFT calculation and high-resolution TEM images (∼1 nm) [37]. The increased thickness is likely due to water and/or other adsorbed molecules trapped between the

Results and Discussion

flakes and the substrate, which was also reported for graphene and other 2D materials [38, 39].

Figure 12.1 Schematic of the two synthesis routes used for producing Ti3CNTx. Using a mixture of LiF and HCl results in Li+ and H2O intercalation (upper route), which leads to more compactly stacked MXene sheets compared to when TMAOH or TBAOH are used as intercalants (lower route). Ti, C, N, Al, O, H, F, and Li atoms are represented by yellow, black, blue, gray, red, white, green, and purple spheres, respectively. Electron paths are represented by red and green arrows.

Figure 12.2 (a−c) SEM images of d-Ti3CNTx flakes from the LiF + HCl route with and without sonication. (d,e) SEM images of delaminated Ti3CNTx flakes from the HF + TMAOH and HF + TBAOH routes with sonication, respectively. The flakes are marked with orange arrows to be distinguishable from the porous alumina membrane used for imaging. (f) Cross-sectional SEM image of a 2.5-μm-thick free-standing film prepared by vacuum filtration of d-Ti3CNTx (LiF + HCl) solution.

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Figure 12.3 (a) TEM image of a Ti3CNTx (LiF + HCl) single flake. (b) SAED pattern showing typical hexagonal symmetry of the flake.

Figure 12.4 (a) Optical image of d-Ti3CNTx and d-Ti3C2Tx solution produced by LiF + HCl method. (b) DLS data of d-Ti3CNTx solution obtained from different routes. The nonsonicated LiF + HCl route has a larger particle size of 1 μm and a peak of more than 10 μm, which is the upper limit of the DLS machine.

XRD patterns of free-standing films obtained by vacuumassisted filtration of the delaminated solution produced by each synthesis method before and after annealing are presented in Fig. 12.5a. For the as-produced films, the (002) peaks of the LiF + HCl, HF + TMAOH, and HF + TBAOH films are located at 6.8°, 6.8°, and 4.7°, respectively, corresponding to the interlayer distances (d) of 13.0, 13.0, and 18.7 Å, respectively. We used TBAOH to further intensify the effects of intercalated ions on

Results and Discussion

transport properties as will be discussed in the following sections. For the TMAOH-delaminated film, there is a shoulder centered at 5.5°, corresponding to the interlayer distance of 16.1 Å. From the (002) peaks of the as-produced LiF + HCl and HF + TMAOH samples, their interlayer distance is ∼13 Å because of Li+ and water intercalation, in good agreement with the previous reports [30, 40]. On the other hand, TMA+ and TBA+ intercalation resulted in larger interlayer distances of 16.1 and 18.7 Å, respectively, which are similar to that reported previously [31]. The increases of ∼3 and ∼6 Å for the samples intercalated by TMA+ and TBA+, respectively, agree well with the size of the cations [41].

Figure 12.5 (a) XRD patterns showing the (002) reflection of the Ti3CNTx films produced by different methods and post-synthesis treatments. (b and d) TGA−MS thermogram of the Ti3CNTx (LiF + HCl) and Ti3CNTx (HF + TMAOH), respectively. (c and e) N 1s high-resolution XPS spectra of the Ti3CNTx (LiF + HCl) and Ti3CNTx (HF + TMAOH), respectively.

After drying at 150 °C under vacuum for 18 h, a negligible shift was observed in the (002) peak of the Ti3CNTx (LiF + HCl) sample. However, when the sample was heated at 400 °C in Ar atmosphere for 3 h, the peak shifted to 8.64° (Fig. 12.5a, top

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Optoelectronic Properties of Titanium Carbonitride MXene

pattern). This result suggests that Ti3CNTx synthesized by this method has a stronger affinity to the intercalated water molecules compared to Ti3C2Tx, where the (002) peak was reported to be around 8.6° after drying only at 120 °C under vacuum [35]. This conclusion is supported by the thermogravimetric analysis coupled with mass spectroscopy (TGA−MS) data shown in Fig. 12.5b, where water desorption (indicated by the m/z = 18) was not complete until close to 250 °C. The reason for this phenomenon is not clear at the moment and was not observed in the HF-etched samples. After annealing at 400 °C, the (002) peaks of the LiF + HCl, HF + TMAOH, and HF + TBAOH samples were at 8.6° (10.3 Å), 8.0° (11.0 Å), and 6.8° (13.0 Å), respectively. The large interlayer spacing of the HF + TMAOH and HF + TBAOH samples after annealing suggests incomplete removal of the organic cations. However, for the HF + TMAOH sample after drying at 400 °C, a small peak at 8.6° was also observed, suggesting that some of the TMA+ cations were removed by thermal annealing. TGA−MS results of the HF + TMAOH and the HF + TBAOH samples shown in Figs. 12.5d and S3a, respectively, show that the organic cations decomposed slightly after 400 °C, which agree with the smaller (002) peak shifts for these two samples compared to the LiF + HCl one. We also tested thermal stability of the TBAOH aqueous solution and found that the sample decomposed at ∼250 °C as revealed by the thermogram shown in Fig. S3b. The increase of the decomposition temperature of approximately 150 °C of the intercalated organic species indicates a strong interaction between TMA+ and TBA+ ions and the MXene surface, which might be responsible for incomplete thermal decomposition. Independent of the synthesis method, Ti3CNTx MXene starts to decompose close to 900 °C forming CO gas (m/z = 28). Interestingly, no NOx gas (m/z = 30, 46, etc.) was detected after MXene decomposition. The XRD patterns of the sample annealed at 1500 °C in He environment (Fig. S4a) shows only peaks corresponding to TiN and some trace of anatase and rutile, in contrast to TiC formed when Ti3C2Tx is annealed at 1500 °C. This result suggests that formation of TiN is thermodynamically more favorable compared to TiC at high temperatures.

Results and Discussion

Figure 12.6 (a) UV−visible spectra of d-Ti3CNTx solutions produced using LiF + HCl (black) and HF + TMAOH (red) routes compared to Ti3C2Tx (LiF + HCl) solution (blue). The solution concentration is around 1.5 × 10–2 mg mL–1. (b) Calibration curves of the solutions obtained using their respective peak absorption in the visible region. (c) UV−visible spectra of spin-casted Ti3CNTx (LiF + HCl) films. The optical image of the films is shown in the inset. Drexel University logo is used with permission from Drexel University. (d) Film transmittance as a function of sheet resistance for the Ti3CNTx samples prepared using two etching routes. Ti3CNTx (HF + TMAOH) thin films are about 10 times more resistive than Ti3CNTx (LiF + HCl) films.

XPS was used to analyze chemical composition and C/N ratio of the Ti3CNTx prepared by different methods. Shown in Fig. 12.5c,e are high-resolution XPS scans of N 1s region of Ti3CNTx prepared from the LiF + HCl and the HF + TMAOH routes, respectively. For the LiF + HCl sample, four peaks were used to fit the spectra: TiCN (396.6 eV), N−Ti−Tx (397.2 eV), −NR2 (R = C, H) or N−O (399.3 eV), and N−TiOx (401.0 eV). Similar peak assignments were used for the HF + TMAOH and HF + TBAOH samples (Fig. S7) except the high binding energy peak of 401.8 eV

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Optoelectronic Properties of Titanium Carbonitride MXene

was assigned to NCR4 (R = C, H). Detailed XPS peak fits of all regions are presented in Figs. S5−S7 and Tables S1−S3. TiCN was observed in all samples at approximately 20 at. %. Interestingly, the N−O bond was observed in both samples, suggesting the possible formation of oxynitrides. When comparing atomic concentrations obtained from the XPS spectra, we observed that the C/N ratio is not unity and varies in a range close to 1.2−1.5:0.5 (see Tables S1−S3). We believe that the different C/N ratios can affect the optoelectronic properties of Ti3CNTx and other carbonitride MXenes in general, as will be discussed in the next section. The absence of Al confirms successful etching, washing, and delamination steps for both synthesis methods, as they were adequate to remove any Al-containing impurities. From the Ti 2p region, it was observed that Ti in the HF + TMAOH and HF + TBAOH samples has higher contribution from Ti4+ (458.4 eV) of 17 and 26 at. %, respectively, compared to that of the LiF + HCl sample (13 at. %). The Ti4+ contribution could come from the O-terminated MXene surface and/or a small amount of TiO2 nanoparticles present in the free-standing film samples used in XPS analysis. This might be due to treatment of the sample with bases, similar to that observed for the KOH- and NaOH-treated Ti3C2Tx [42, 43].

12.3.2 Optoelectronic Properties

UV−visible spectroscopy was used to study the optical properties of Ti3CNTx prepared by different methods. Figure 12.6a shows absorption spectra of d-Ti3CNTx solutions prepared by LiF + HCl and HF + TMAOH methods compared to a solution of d-Ti3C2Tx prepared by the LiF + HCl method [35]. A clear distinction can be observed when comparing Ti3CNTx and Ti3C2Tx. The local absorption maxima are located at 670 and 770 nm for Ti3CNTx (LiF + HCl) and Ti3C2Tx (LiF + HCl), respectively. Ti3CNTx (HF + TMAOH) did not show a clear absorption peak, but a broader shoulder at around 670 nm. The difference in absorption peaks translates into different colors of the two MXenes, that is, brownish for Ti3CNTx and dark green for Ti3C2Tx as shown in Fig. 12.4a. The full UV−visible spectra recorded over wavelengths between 200 and 1000 nm are shown in the inset of Fig. 12.6a.

Results and Discussion

It is worth mentioning that Ti3CNTx shows higher optical absorption in the near-infrared region compared to Ti3C2Tx, which could be useful for photothermal therapy applications. We also found that the absorption peak of Ti3CNTx made by LiF + HCl from different batches of Ti3AlCN MAX powder showed a shift in peak position ranging from 670 to 730 nm (not shown). A conclusive reason for this shift could not be drawn at this point and is beyond the scope of this study. However, we believe that it is related to the C/N ratio variation in the Ti3AlCN MAX precursor. Further studies will be conducted to verify this hypothesis. In general, the results presented here show the tunability of optical properties of MXenes by manipulating their X elements. Figure 12.6b presents a plot of the absorbance at the local absorption maxima (670 nm for Ti3CNTx and 770 nm for Ti3C2Tx) as a function of the solution concentration and the extinction coefficients were calculated to be 27.6, 27.0, and 32.4 L g–1 cm–1 for Ti3CNTx (LiF + HCl), Ti3CNTx (HF + TMAOH), and Ti3C2Tx (LiF + HCl), respectively. Although the etching methods only slightly affected the absorption coefficient of Ti3CNTx, the resulting absorption coefficients were lower than that of Ti3C2Tx, which is beneficial for transparent conductive coating applications. The transparent films of Ti3CNTx were prepared by a spincasting technique using d-Ti3CNTx solution with a concentration between 5 and 20 mg mL–1. As the TBAOH delamination route did not yield high-concentration solutions, we only compared the optoelectronic properties of thin films made by LiF + HCl and HF + TMAOH routes. When using 5 mg mL–1 solution of Ti3CNTx (LiF + HCl), the transmittance of the thin films at 550 nm ranged from ∼75 to 94%, when the spin speed increased from 1000 to 10 000 rpm. To obtain less transparent and more conductive films, 20 mg mL–1 solution was used (Fig. 12.6c). The thickness of the film with 80% transmittance was 9 nm with a surface roughness of 2.7 nm, while that of the film with 94% transmittance was 2.5 nm. The latter film mainly had a monolayer coverage of the MXene flakes as confirmed by AFM images (Fig. S8). Although the thickest film we obtained with the 20 mg mL–1 solution has a transparency of 40%, thicker films can be prepared by repeating the spin-casting process or using a higher solution concentration. Note that a high spin speed of

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Optoelectronic Properties of Titanium Carbonitride MXene

more than 5000 rpm is required to obtain homogeneous coating when using high concentration of the MXene solution because of its viscoelastic property [44]. Moreover, we note that MXene in solution tends to agglomerate when the concentration of the solution exceeds 20 mg mL–1, which may cause inhomogeneity of the resulting thin films. Next, we studied the optoelectronic properties of the Ti3CNTx thin films by comparing their optical transparency measured by UV−visible spectroscopy and sheet resistance measured in a four-point probe configuration. The plots of transmittance as a function of sheet resistance of the thin films prepared from Ti3CNTx (LiF + HCl) and Ti3CNTx (HF + TMAOH) are shown in Fig. 12.6d. At the same transparency, the films made from the Ti3CNTx (HF + TMAOH) solution were approximately 1 order of magnitude less conductive than those made from the LiF + HCl solution. The sheet resistance of the thin films with transparency between ∼40 and ∼94% prepared from the Ti3CNTx (LiF + HCl) solution ranges from 0.1 to 10 kΩ sq–1, whereas that of films prepared from the Ti3CNTx (HF + TMAOH) solution range from 3 to 10 000 kΩ sq–1. The electrical figure of merit (FoMe; sDC/sop, where sDC is the electronic conductivity and sop is the optical conductivity at 550 nm) of each film was calculated using the



s

.5 op equation T550nm  1 188 Rs s DC



2

, where T550 nm is the transmittance

of the film at 550 nm and Rs is the sheet resistance (Ω sq–1) [45]. The films prepared from the Ti3CNTx (LiF + HCl) solution and the Ti3CNTx (HF + TMAOH) solution have FoMe values of 2 ± 0.81 and 0.1 ± 0.027, respectively. The large difference in the electrical conductivities of the films prepared from the Ti3CNTx (LiF + HCl) and the Ti3CNTx (HF + TMAOH) solutions is ascribed to differences in their surface chemistries and largely to intercalation of TMA+ cations, which expands the interlayer distance, suppressing inter-flake electron transport. However, we do not eliminate the possibility that the Ti3CNTx (HF + TMAOH) films could be more oxidized because of prolonged exposure to basic solution as suggested by the XPS results. Moreover, it was reported for Ti3C2Tx that HF etching results in more defective MXene flakes, which could potentially affect the electronic properties of MXenes [46]. The Ti3CNTx (LiF + HCl) thin films were less

Results and Discussion

conductive than the Ti3C2Tx films of similar thickness that were prepared by a similar method [8, 47]. This is possibly due to reduced electron density of Ti d-electrons induced by the presence of highly electronegative nitrogen atoms in the X layer, as predicted by the DFT calculations [13]. Although nitrogen has one extra electron compared to carbon, its electrons are localized in a lower energy band far away from the Fermi level, which are not active in electronic transport [13]. Moreover, random distribution of the nitrogen atoms in the X layer and atomic defects could act as scattering centers, reducing electron mobility [13, 25, 48]. To study the stability of the Ti3CNTx thin films prepared by different methods, the resistance of the films with different thicknesses was recorded while the films were exposed to ambient conditions (20 °C, laboratory air). For Ti3CNTx (LiF + HCl) and Ti3CNTx (HF + TMAOH) films with 8 nm nominal thickness, the sheet resistance increased 1.25 and 1.5 times their original values after 30 h, respectively, as shown in Fig. S9. For thinner films, the sheet resistance increased drastically over time, that is, 2.5 times for the Ti3CNTx (HF + TMAOH) film with ∼4 nm thickness and 3.5 times for the Ti3CNTx (LiF + HCl) film with ∼2 nm thickness. The Ti3CNTx thin films are less stable compared to Ti3C2Tx, where only 10% change was observed in a singleflake device over 30 h and a negligible change in resistance was observed in a 25 nm-thick film over 150 min [9, 47]. We also observed that vacuum annealing of the freshly made thin films of Ti3CNTx at 150 °C caused the sheet resistance to increase, opposite to that reported for Ti3C2Tx [8]. The increase of sheet resistance was only observed for the very thin films (