In situ study of nucleation and growth dynamics of Au nanoparticles on MoS2 nanoflakes

Song, Boao; He, Kun; Yuan, Yifei; Sharifi-Asl, Soroosh; Cheng, Meng; Lu, Jun; Saidi, Wissam A.; Shahbazian-Yassar, Reza

doi:10.1039/C8NR03519A

Title: In situ study of nucleation and growth dynamics of Au nanoparticles on MoS₂ nanoflakes

Journal Article · Thu Jul 19 00:00:00 EDT 2018 · Nanoscale

DOI:https://doi.org/10.1039/C8NR03519A· OSTI ID:1472115

^[1]; He, Kun ^[1]; Yuan, Yifei ^[2]; Sharifi-Asl, Soroosh ^[1];

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Univ. of Illinois at Chicago, Chicago, IL (United States)
Univ. of Illinois at Chicago, Chicago, IL (United States); Argonne National Lab. (ANL), Argonne, IL (United States)
Argonne National Lab. (ANL), Argonne, IL (United States)
Univ. of Pittsburgh, Pittsburgh, PA (United States)

Two-dimensional (2D) substrates decorated with metal nanoparticles offer new opportunities to achieve high-performance catalytic behavior. However, little is known on how the substrates control the nucleation and growth processes of the nanoparticles. This paper presents the visualization of dynamic nucleation and growth processes of gold nanoparticles on ultrathin MoS₂ nanoflakes by in situ liquid-cell transmission electron microscopy (TEM). The galvanic displacement resulting in Au nuclei formation on MoS₂ was observed in real time inside the liquid cell. We found that the growth mechanism of Au particles on pristine MoS₂ is in between diffusion-limited and reaction-limited, possibly due to the presence of electrochemical Ostwald ripening. A larger size distribution and more orientation variation is observed for the Au particles along the MoS₂ edge than on the interior. Differing from pristine MoS₂, sulfur vacancies on MoS₂ induce Au particle diffusion and coalescence during the growth process. Density functional theory (DFT) calculations show that the size difference is because the exposed molybdenum atoms at the edge with dangling bonds can strongly interact with Au atoms, whereas sulfur atoms on the MoS₂ interior have no dangling bonds and weakly interact with gold atoms. In addition, S vacancies on MoS₂ generate strong nucleation centers that can promote diffusion and coalescence of Au nanoparticles. Furthermore, the present work provides key insights into the role of 2D materials in controlling the size and orientation of noble metal nanoparticles vital to the design of next generation catalysts.

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Research Organization:: Argonne National Lab. (ANL), Argonne, IL (United States)

Sponsoring Organization:: National Science Foundation (NSF); University of Pittsburgh; USDOE

Grant/Contract Number:: AC02-06CH11357; 4J-30361

OSTI ID:: 1472115

Alternate ID(s):: OSTI ID: 1464297

Journal Information:: Nanoscale, Vol. 10, Issue 33; ISSN 2040-3364

Publisher:: Royal Society of ChemistryCopyright Statement

Country of Publication:: United States

Language:: English

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Cited by: 28 works

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References (67)

General synthesis of noble metal (Au, Ag, Pd, Pt) nanocrystal modified MoS ₂ nanosheets and the enhanced catalytic activity of Pd–MoS ₂ for methanol oxidation Yuwen, Lihui; Xu, Fei; Xue, Bing Nanoscale, Vol. 6, Issue 11 https://doi.org/10.1039/C3NR06084E	journal	January 2014
Generalized Gradient Approximation Made Simple Perdew, John P.; Burke, Kieron; Ernzerhof, Matthias Physical Review Letters, Vol. 77, Issue 18, p. 3865-3868 https://doi.org/10.1103/PhysRevLett.77.3865	journal	October 1996
Electron microscopy of specimens in liquid de Jonge, Niels; Ross, Frances M. Nature Nanotechnology, Vol. 6, Issue 11, p. 695-704 https://doi.org/10.1038/nnano.2011.161	journal	October 2011
Defect-induced conductivity anisotropy in MoS $_{2}$ monolayers Ghorbani-Asl, Mahdi; Enyashin, Andrey N.; Kuc, Agnieszka Physical Review B, Vol. 88, Issue 24 https://doi.org/10.1103/PhysRevB.88.245440	journal	December 2013
Projector augmented-wave method Blöchl, P. E. Physical Review B, Vol. 50, Issue 24, p. 17953-17979 https://doi.org/10.1103/PhysRevB.50.17953	journal	December 1994
Frontiers of in situ electron microscopy Zheng, Haimei; Meng, Ying Shirley; Zhu, Yimei MRS Bulletin, Vol. 40, Issue 1 https://doi.org/10.1557/mrs.2014.305	journal	January 2015
Small Adsorbate-Assisted Shape Control of Pd and Pt Nanocrystals Chen, Mei; Wu, Binghui; Yang, Jing Advanced Materials, Vol. 24, Issue 7 https://doi.org/10.1002/adma.201104145	journal	January 2012
Nanoscale Imaging of Lithium Ion Distribution During In Situ Operation of Battery Electrode and Electrolyte Holtz, Megan E.; Yu, Yingchao; Gunceler, Deniz Nano Letters, Vol. 14, Issue 3 https://doi.org/10.1021/nl404577c	journal	February 2014
Adsorption of Polyvinylpyrrolidone on Ag Surfaces: Insight into a Structure-Directing Agent Al-Saidi, W. A.; Feng, Haijun; Fichthorn, Kristen A. Nano Letters, Vol. 12, Issue 2 https://doi.org/10.1021/nl2041113	journal	January 2012
Microlandscaping of Au Nanoparticles on Few-Layer MoS ₂ Films for Chemical Sensing Lu, Junpeng; Lu, Jia Hui; Liu, Hongwei Small, Vol. 11, Issue 15 https://doi.org/10.1002/smll.201402591	journal	January 2015
Observation of Single Colloidal Platinum Nanocrystal Growth Trajectories Zheng, H.; Smith, R. K.; Jun, Y. -w. Science, Vol. 324, Issue 5932 https://doi.org/10.1126/science.1172104	journal	June 2009
Direct in Situ Observation of Nanoparticle Synthesis in a Liquid Crystal Surfactant Template Parent, Lucas R.; Robinson, David B.; Woehl, Taylor J. ACS Nano, Vol. 6, Issue 4 https://doi.org/10.1021/nn300671g	journal	March 2012
From ultrasoft pseudopotentials to the projector augmented-wave method Kresse, G.; Joubert, D. Physical Review B, Vol. 59, Issue 3, p. 1758-1775 https://doi.org/10.1103/PhysRevB.59.1758	journal	January 1999
Electrochemical Ostwald Ripening of Colloidal Ag Particles on Conductive Substrates Redmond, Peter L.; Hallock, A. J.; Brus, Louis E. Nano Letters, Vol. 5, Issue 1 https://doi.org/10.1021/nl048204r	journal	January 2005
Direct Observation of Aggregative Nanoparticle Growth: Kinetic Modeling of the Size Distribution and Growth Rate Woehl, Taylor J.; Park, Chiwoo; Evans, James E. Nano Letters, Vol. 14, Issue 1 https://doi.org/10.1021/nl4043328	journal	December 2013
A selective and efficient electrocatalyst for carbon dioxide reduction Lu, Qi; Rosen, Jonathan; Zhou, Yang Nature Communications, Vol. 5, Issue 1 https://doi.org/10.1038/ncomms4242	journal	January 2014
Electrochemical Ostwald ripening and surface diffusion in the galvanic displacement reaction: control over particle growth Ghosh, Tapas; Karmakar, Prasanta; Satpati, Biswarup RSC Advances, Vol. 5, Issue 114 https://doi.org/10.1039/C5RA20297C	journal	January 2015
Enhanced Electrocatalytic Properties of Transition-Metal Dichalcogenides Sheets by Spontaneous Gold Nanoparticle Decoration Kim, Jaemyung; Byun, Segi; Smith, Alexander J. The Journal of Physical Chemistry Letters, Vol. 4, Issue 8 https://doi.org/10.1021/jz400507t	journal	March 2013
Interfacial Properties of Monolayer and Bilayer MoS2 Contacts with Metals: Beyond the Energy Band Calculations Zhong, Hongxia; Quhe, Ruge; Wang, Yangyang Scientific Reports, Vol. 6, Issue 1 https://doi.org/10.1038/srep21786	journal	March 2016
Electrochemical properties of small clusters of metal atoms and their role in the surface enhanced Raman scattering Plieth, W. J. The Journal of Physical Chemistry, Vol. 86, Issue 16 https://doi.org/10.1021/j100213a020	journal	August 1982
Binding of Polyvinylpyrrolidone to Ag Surfaces: Insight into a Structure-Directing Agent from Dispersion-Corrected Density Functional Theory Saidi, Wissam A.; Feng, Haijun; Fichthorn, Kristen A. The Journal of Physical Chemistry C, Vol. 117, Issue 2 https://doi.org/10.1021/jp309867n	journal	January 2013
Direct in Situ Determination of the Mechanisms Controlling Nanoparticle Nucleation and Growth Woehl, Taylor J.; Evans, James E.; Arslan, Ilke ACS Nano, Vol. 6, Issue 10 https://doi.org/10.1021/nn303371y	journal	September 2012
Epitaxial Growth of Hetero-Nanostructures Based on Ultrathin Two-Dimensional Nanosheets Tan, Chaoliang; Zhang, Hua Journal of the American Chemical Society, Vol. 137, Issue 38 https://doi.org/10.1021/jacs.5b03590	journal	September 2015
Surface lattice-engineered bimetallic nanoparticles and their catalytic properties Wu, Jianbo; Li, Panpan; Pan, Yung-Tin (Frank) Chemical Society Reviews, Vol. 41, Issue 24 https://doi.org/10.1039/c2cs35189g	journal	January 2012
Au Nanoparticle-Modified MoS ₂ Nanosheet-Based Photoelectrochemical Cells for Water Splitting Yin, Zongyou; Chen, Bo; Bosman, Michel Small, Vol. 10, Issue 17 https://doi.org/10.1002/smll.201400124	journal	March 2014
Synthesis of Large-Area MoS₂ Atomic Layers with Chemical Vapor Deposition Lee, Yi-Hsien; Zhang, Xin-Quan; Zhang, Wenjing Advanced Materials, Vol. 24, Issue 17, p. 2320-2325 https://doi.org/10.1002/adma.201104798	journal	March 2012
Direct-write liquid phase transformations with a scanning transmission electron microscope Unocic, Raymond R.; Lupini, Andrew R.; Borisevich, Albina Y. Nanoscale, Vol. 8, Issue 34 https://doi.org/10.1039/C6NR04994J	journal	January 2016
Nanoparticle Silver Catalysts That Show Enhanced Activity for Carbon Dioxide Electrolysis Salehi-Khojin, Amin; Jhong, Huei-Ru Molly; Rosen, Brian A. The Journal of Physical Chemistry C, Vol. 117, Issue 4 https://doi.org/10.1021/jp310509z	journal	January 2013
Heteroepitaxial Growth of Gold Nanostructures on Silicon by Galvanic Displacement Sayed, Sayed Y.; Wang, Feng; Malac, Marek ACS Nano, Vol. 3, Issue 9 https://doi.org/10.1021/nn900685a	journal	August 2009
Selective Decoration of Au Nanoparticles on Monolayer MoS2 Single Crystals Shi, Yumeng; Huang, Jing-Kai; Jin, Limin Scientific Reports, Vol. 3, Issue 1 https://doi.org/10.1038/srep01839	journal	May 2013
Uniform Au@Pt core–shell nanodendrites supported on molybdenum disulfide nanosheets for the methanol oxidation reaction Su, Shao; Zhang, Chi; Yuwen, Lihui Nanoscale, Vol. 8, Issue 1 https://doi.org/10.1039/C5NR06077J	journal	January 2016
A climbing image nudged elastic band method for finding saddle points and minimum energy paths Henkelman, Graeme; Uberuaga, Blas P.; Jónsson, Hannes The Journal of Chemical Physics, Vol. 113, Issue 22, p. 9901-9904 https://doi.org/10.1063/1.1329672	journal	December 2000
Opportunities and challenges in liquid cell electron microscopy Ross, F. M. Science, Vol. 350, Issue 6267 https://doi.org/10.1126/science.aaa9886	journal	December 2015
Graphene–inorganic nanocomposites Bai, Song; Shen, Xiaoping RSC Adv., Vol. 2, Issue 1 https://doi.org/10.1039/C1RA00260K	journal	January 2012
Nanoporous Ionic Organic Networks: Stabilizing and Supporting Gold Nanoparticles for Catalysis Zhang, Pengfei; Qiao, Zhen-An; Jiang, Xueguang Nano Letters, Vol. 15, Issue 2 https://doi.org/10.1021/nl504780j	journal	January 2015
The kinetics of precipitation from supersaturated solid solutions Lifshitz, I. M.; Slyozov, V. V. Journal of Physics and Chemistry of Solids, Vol. 19, Issue 1-2 https://doi.org/10.1016/0022-3697(61)90054-3	journal	April 1961
NIH Image to ImageJ: 25 years of image analysis Schneider, Caroline A.; Rasband, Wayne S.; Eliceiri, Kevin W. Nature Methods, Vol. 9, Issue 7 https://doi.org/10.1038/nmeth.2089	journal	June 2012
Electrochemical growth of composite hydroxyapatite coatings for controlled release Fu, Cong; Song, Boao; Wan, Chaoyi Surface and Coatings Technology, Vol. 276 https://doi.org/10.1016/j.surfcoat.2015.06.007	journal	August 2015
Advances in sealed liquid cells for in-situ TEM electrochemial investigation of lithium-ion battery Wu, Fan; Yao, Nan Nano Energy, Vol. 11 https://doi.org/10.1016/j.nanoen.2014.11.004	journal	January 2015
Multigrain Platinum Nanowires Consisting of Oriented Nanoparticles Anchored on Sulfur-Doped Graphene as a Highly Active and Durable Oxygen Reduction Electrocatalyst Hoque, Md Ariful; Hassan, Fathy M.; Higgins, Drew Advanced Materials, Vol. 27, Issue 7 https://doi.org/10.1002/adma.201404426	journal	November 2014
Anomalous Growth and Coalescence Dynamics of Hybrid Perovskite Nanoparticles Observed by Liquid-Cell Transmission Electron Microscopy Qin, Fuyu; Wang, Zhiwei; Wang, Zhong Lin ACS Nano, Vol. 10, Issue 11 https://doi.org/10.1021/acsnano.6b04234	journal	September 2016
Density Functional Theory Study of Nucleation and Growth of Pt Nanoparticles on MoS ₂ (001) Surface Saidi, Wissam A. Crystal Growth & Design, Vol. 15, Issue 2 https://doi.org/10.1021/cg5013395	journal	January 2015
Galvanic displacement as a route to highly active and durable extended surface electrocatalysts Alia, S. M.; Yan, Y. S.; Pivovar, B. S. Catal. Sci. Technol., Vol. 4, Issue 10 https://doi.org/10.1039/C4CY00736K	journal	January 2014
Pushing the Envelope of In Situ Transmission Electron Microscopy Ramachandramoorthy, Rajaprakash; Bernal, Rodrigo; Espinosa, Horacio D. ACS Nano, Vol. 9, Issue 5 https://doi.org/10.1021/acsnano.5b01391	journal	May 2015
Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set Kresse, G.; Furthmüller, J. Physical Review B, Vol. 54, Issue 16, p. 11169-11186 https://doi.org/10.1103/PhysRevB.54.11169	journal	October 1996
Control of Electron Beam-Induced Au Nanocrystal Growth Kinetics through Solution Chemistry Park, Jeung Hun; Schneider, Nicholas M.; Grogan, Joseph M. Nano Letters, Vol. 15, Issue 8 https://doi.org/10.1021/acs.nanolett.5b01677	journal	July 2015
A general one-step approach for in situ decoration of MoS ₂ nanosheets with inorganic nanoparticles Cheng, Zehong; He, Benzhao; Zhou, Li Journal of Materials Chemistry A, Vol. 3, Issue 3 https://doi.org/10.1039/C4TA04946B	journal	January 2015
Electron microscopy of whole cells in liquid with nanometer resolution Jonge, N. d.; Peckys, D. B.; Kremers, G. J. Proceedings of the National Academy of Sciences, Vol. 106, Issue 7 https://doi.org/10.1073/pnas.0809567106	journal	January 2009
On the interpretation of the forbidden spots observed in the electron diffraction patterns of flat Au triangular nanoparticles Reyes-Gasga, J.; Gómez-Rodríguez, A.; Gao, Xiaoxia Ultramicroscopy, Vol. 108, Issue 9 https://doi.org/10.1016/j.ultramic.2008.03.005	journal	August 2008
Synthesis of {111}-Faceted Au Nanocrystals Mediated by Polyvinylpyrrolidone: Insights from Density-Functional Theory and Molecular Dynamics Liu, Shih-Hsien; Saidi, Wissam A.; Zhou, Ya The Journal of Physical Chemistry C, Vol. 119, Issue 21 https://doi.org/10.1021/acs.jpcc.5b01867	journal	May 2015
Facet development during platinum nanocube growth Liao, H. -G.; Zherebetskyy, D.; Xin, H. Science, Vol. 345, Issue 6199 https://doi.org/10.1126/science.1253149	journal	August 2014
Scattering and Interference in Epitaxial Graphene Rutter, G. M.; Crain, J. N.; Guisinger, N. P. Science, Vol. 317, Issue 5835, p. 219-222 https://doi.org/10.1126/science.1142882	journal	July 2007
Controlled Electroless Deposition of Noble Metal Nanoparticle Films on Germanium Surfaces Porter, Lon A.; Choi, Hee Cheul; Ribbe, Alexander E. Nano Letters, Vol. 2, Issue 10 https://doi.org/10.1021/nl025677u	journal	October 2002
Experimentally Validated Structures of Supported Metal Nanoclusters on MoS ₂ Shi, Yongliang; Song, Boao; Shahbazian-Yassar, Reza The Journal of Physical Chemistry Letters, Vol. 9, Issue 11 https://doi.org/10.1021/acs.jpclett.8b01233	journal	May 2018
Trends in the Adsorption and Growth Morphology of Metals on the MoS ₂ (001) Surface Saidi, Wissam A. Crystal Growth & Design, Vol. 15, Issue 7 https://doi.org/10.1021/acs.cgd.5b00269	journal	June 2015
Oxygen reduction reaction activities for Pt-enriched Co/Pt(111), Co/Pt(100), and Co/Pt(110) model catalyst surfaces prepared by molecular beam epitaxy Yamada, Y.; Miyamoto, K.; Hayashi, T. Surface Science, Vol. 607 https://doi.org/10.1016/j.susc.2012.08.016	journal	January 2013
Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points Henkelman, Graeme; Jónsson, Hannes The Journal of Chemical Physics, Vol. 113, Issue 22 https://doi.org/10.1063/1.1323224	journal	December 2000
Solution-phase epitaxial growth of noble metal nanostructures on dispersible single-layer molybdenum disulfide nanosheets Huang, Xiao; Zeng, Zhiyuan; Bao, Shuyu Nature Communications, Vol. 4, Issue 1 https://doi.org/10.1038/ncomms2472	journal	February 2013
Nanocomposites of Two-Dimensional Molybdenum and Tungsten Dichalcogenides with Metal Particles: Preparation and Prospects for Application Lampeka, Ya. D.; Tsymbal, L. V. Theoretical and Experimental Chemistry, Vol. 51, Issue 3 https://doi.org/10.1007/s11237-015-9410-1	journal	July 2015
Quantitative Description of Crystal Nucleation and Growth from in Situ Liquid Scanning Transmission Electron Microscopy Ievlev, Anton V.; Jesse, Stephen; Cochell, Thomas J. ACS Nano, Vol. 9, Issue 12 https://doi.org/10.1021/acsnano.5b03720	journal	October 2015
Visualization of Electrode–Electrolyte Interfaces in LiPF ₆ /EC/DEC Electrolyte for Lithium Ion Batteries via in Situ TEM Zeng, Zhiyuan; Liang, Wen-I; Liao, Hong-Gang Nano Letters, Vol. 14, Issue 4 https://doi.org/10.1021/nl403922u	journal	March 2014
Operando liquid cell electron microscopy of discharge and charge kinetics in lithium-oxygen batteries He, Kun; Bi, Xuanxuan; Yuan, Yifei Nano Energy, Vol. 49 https://doi.org/10.1016/j.nanoen.2018.04.046	journal	July 2018
Multistep nucleation of nanocrystals in aqueous solution Loh, N. Duane; Sen, Soumyo; Bosman, Michel Nature Chemistry, Vol. 9, Issue 1 https://doi.org/10.1038/nchem.2618	journal	October 2016
Reducing Dzyaloshinskii-Moriya interaction and field-free spin-orbit torque switching in synthetic antiferromagnets Chen, Ruyi; Cui, Qirui; Liao, Liyang Nature Communications, Vol. 12, Issue 1 https://doi.org/10.1038/s41467-021-23414-3	journal	May 2021
Electronic structure of AlFeN films exhibiting crystallographic orientation change from c- to a-axis with Fe concentrations and annealing effect Tatemizo, Nobuyuki; Imada, Saki; Okahara, Kizuna Scientific Reports, Vol. 10, Issue 1 https://doi.org/10.1038/s41598-020-58835-5	journal	February 2020
Direct Observation of Aggregative Nanoparticle Growth: Kinetic Modeling of the Size Distribution and Growth Rate Woehl, Taylor J.; Park, Chiwoo; Evans, James E. Microscopy and Microanalysis, Vol. 20, Issue S3 https://doi.org/10.1017/s1431927614009799	journal	August 2014
Nanoscale Imaging of Lithium Ion Distribution During In Situ Operation of Battery Electrode and Electrolyte Holtz, Megan E.; Yu, Yingchao; Gunceler, Deniz arXiv https://doi.org/10.48550/arxiv.1311.6490	text	January 2013

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Title: In situ study of nucleation and growth dynamics of Au nanoparticles on MoS2 nanoflakes

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Title: In situ study of nucleation and growth dynamics of Au nanoparticles on MoS₂ nanoflakes