Đang tải... (xem toàn văn)
... crystals of LTMDs, the PEC properties of atomically thin crystals of LTMDs remain elusive In this thesis, the preparation and characterization of large area atomically thin films of molybdenum disulfide. .. performance of MoS thin films These include application of a proper bias and chemical passivation of the samples VI List of Tables Chapter Introduction Table 1 Performance and characteristic of. .. 3.6.2 Effect of pre-annealing on the quality of MoS films 41 3.6.3 Effect of sulfurization temperature on the quality of MoS films 44 Chapter Photocurrent measurement of MoS thin films and
PREPARATION OF ORIENTED MOLYBDENUM DISULFIDE THIN FILMS FOR PHOTOELECTROCHEMICAL ENERGY HARVESTING APPLICATIONS CHEN ZIMEI (B.Sc LANZHOU UNIV) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2014 DECLARATION I hereby declare that this thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously Acknowledgement First of all, I would like to express my sincere gratitude to my supervisor Prof Goki Eda, for providing me with the opportunity to learn and work in the lab, for his invaluable guidance, constructive advice and for his understanding and kindness throughout the last two years I would also like to express my special thanks to Laurie for her helpful guidance and initial training in photoelectrochemical measurement, for her kindness and encouragement to me I could not have done my work without the help of my colleagues First, I would like to express my great thanks to Dr Weijie Zhao, who was always patient to answer my questions about Raman and photoluminescence spectroscopy and always gave me helpful suggestions when I encountered different problems Also, great thanks to Dr Minglin Toh, for his guidance and encouragement to me when I faced difficulties I would also express my appreciation to Dr Ivan Verzhbitskiy, who was always glad to answer my various questions about Raman system and give me instructions; and Dr Henrik Schmidt, for his help to solve the communication problem between monochromator and computer and helpful guidance on Labview program as well as lock-in amplifier I would also thank Dr Shisheng Li and Dr Francesco Giustiniano, for their kindness and helpful suggestion At the same time, I would like to give my great thanks to Leiqiang, for his helpful guidance on the data processing and plotting; to Kiran, who always gave me support and guidance on the synthesis and always encouraged me; to Shunfeng, who was glad to help me with the transfer technique and always brings laughter to the lab; to Rajeev, I for his help with the AFM; to Xiuyuan, for his instant assistance when I needed help It was a great pleasure for me to work with such warm and cooperative team, and learn from them Finally, I would like to express my thanks to my family and my friends, thank you for your unswerving support throughout the years II Table of Contents Acknowledgement I Summary V List of Tables VII List of Figures .VIII List of Symbols .XIII Chapter Introduction 1.1 Layered transition metal dichalcogenides (LTMDs) 1.3 Photoelectrochemistry of layered transition metal dichalcogenides 1.4 Motivation Chapter Photoelectrochemistry instrumentation 2.1 Working principle of a photoelectrochemical cell for solar energy conversion 2.2 Photoelectrochemical techniques 11 2.2.1 Photocurrent spectroscopy 11 2.2.2 Photocurrent spectroscopy instrumentation 12 2.2.3 Lock-in technique in photocurrent measurement 14 2.2.4 Interpretation of the results: IPCE and APCE 15 Chapter MoS2 thin film preparation and characterization 17 3.1 Introduction to the chemical exfoliation of MoS 17 3.2 Experimental procedure of chemical exfoliation of MoS 18 3.3 Results and discussion 20 3.3.1 Morphologies-Optical, SEM and AFM imaging 20 3.3.2 UV-Vis absorbance spectra of chemically exfoliated MoS 23 3.3.3 Raman spectra of chemically exfoliated MoS 26 3.3.4 Photoluminescence spectra of chemically exfoliated MoS 28 3.4 Sulfurization of thermally evaporated MoO3 30 3.5 Experimental procedure for the sulfurization and thermal evaporation of MoO 31 3.6 Results and discussion 34 3.6.1 MoS2 films obtained from sulfurization of MoOx films with various thicknesses 34 3.6.2 Effect of pre-annealing on the quality of MoS films 41 3.6.3 Effect of sulfurization temperature on the quality of MoS films 44 Chapter Photocurrent measurement of MoS thin films and relevant issues 48 4.1 Experimental methods 48 4.1.1 MoS2 photoelectrode preparation 48 4.1.2 Photocurrrent measurement 50 4.2 Results and discussions 51 4.2.1 Measurement issue - selection of reference frequency in lock-in technique for photocurrent measurement 52 4.2.2 Mechanism of recombination, charge separation and charge carrier diffusion in MoS2 thin film photoelectrochemical cell 53 Chapter Conclusions and Outlook 57 5.1 Conclusions 57 5.2 Outlook 59 III References 61 Appendix 71 IV Summary Group VI layered transition metal dichacogenides (LTMDs) as a group of semiconductors with intriguing properties have been studied for decades Due to the suitable band gap for solar energy absorption and extremely good stability in various aqueous and non-aqueous electrolyte solutions, these materials have been considered as good candidates of semiconductor electrodes for photoelectrochemical (PEC) solar cells However, the scalable synthesis of bulk single crystals LTMDs remains a challenge, limiting the development of their application in solar cells Two-dimensional, atomically thin sheets of group VI LTMDs exhibit attractive physical properties that are absent in their bulk form due to quantum confinement effects and change in crystal symmetry Large area atomically thin films of LTMDs can be fabricated with potentially scalable techniques, providing an opportunity for their application in solar energy conversion However, the photoelectrochemical properties of atomically thin LTMDs remain to be investigated In this project, large area atomically thin films of MoS2 were produced by lithium-assisted chemical exfoliation and sulfurization of thermally evaporated MoO x thin films The morphology of MoS films was characterized by optical microscopy, scanning electron microscopy and atomic force microscopy The quality of films was characterized by UV-Vis absorbance spectroscopy, Raman spectroscopy and photoluminescence spectroscopy Photocurrent measurements were conducted in a three-electrode configuration with lock-in technique to investigate the photoelectrochemical behavior of MoS2 thin film photoelectrodes The measurements were performed by varying the electrolyte and the V thickness of samples However, no effective photocurrent signal was successfully detected Finally, several possible approaches were proposed to further improve the photocurrent performance of MoS thin films These include application of a proper bias and chemical passivation of the samples VI Chapter Conclusions and Outlook 5.1 Conclusions This thesis presented the work on the synthesis, characterization and study of the PEC properties of atomically thin films of MoS MoS2 films were produced through lithium- intercalation assisted chemical exfoliation of commercial MoS powder and sulfurization of thermally evaporated MoO x film For the chemically exfoliated MoS , thin films with various thicknesses were obtained by varying the concentration of the suspension filtered The morphology was characterized by optical microscope and scanning electron microscope, confirming that the films were composed of randomly stacked flakes with sizes ranging from 100 nm to 700 nm AFM analysis confirmed that most of the MoS2 sheets obtained by exfoliation are monolayers UV-Vis absorbance spectroscopy, Raman spectroscopy and photoluminescence spectroscopy were utilized to characterize the lattice and electronic band gap structure as well as the quality of MoS2 thin films The Raman shift of the E1 2g and A1g modes with increasing film thickness was compared with that of mechanically exfoliated samples The thickness dependence of these Raman peaks was significantly weaker than the trends observed in mechanically exfoliated samples This finding indicates that the stacking order in the restacked monolayer sheets is not well defined For MoS2 films synthesized via sulfurization of evaporated MoO x , control of the film thickness was also achieved by controlling the initial thickness of the MoO x precursor film Pre-annealing conditions of the MoO x films and sulfurization temperature were 57 varied in order to improve the film quality It was found that the pre-annealing at 200 °C in either nitrogen or air does not help improving the quality of the samples On the other hand, sulfurization at 850 °C led not only to a smoother film but also to a less disordered material, as evidenced by sharper vibrational modes and a significantly enhanced photoluminescence intensity Hence, 850 °C is found to be the optimal growth temperature for moderate quality MoS films To perform photocurrent measurements, chemically exfoliated MoS2 films were deposited on FTO conductive substrates On the other hand, chromium / gold on quartz were used as the conductive film instead of FTO substrates for the MoS films obtained by sulfurization of MoO x However, due to the shrinking of the gold film at the growth temperature, MoS2 films on chromium / gold electrodes were not able to maintain a steady value of dark current This was probably due to the rough surface and exposed gold particles which would induce electrolysis reaction during the measurement, resulting in an invalid photocurrent measurement No significant photocurrent signals could be isolated from the current base line (dark current) and the experimental noise This may be attributed to the limitations from both the measurement conditions and the quality of the samples The conflict between the long response time of the photocurrent and the requirement on the range of reference frequency for an accurate lock- in measurement limits the collection of the real photocurrent signals On the other hand, the imperfect van der Waals plane of chemically exfoliated MoS2 films with various surface states acting as recombination centers may lead to weak photocurrent response Furthermore, the insufficiently thick 58 space charge layer may result in inefficient charge separation 5.2 Outlook Although uniform, oriented thin films of MoS were successfully produced, the quality of the films still need to be improved This may be achieved further optimizing the synthesis protocols For example, the effect of the flow rate of the carrier gas during sulfurization, substrate, heating and cooling rate on the crystallinity may be investigated With respect to the PEC application of MoS film synthesized by sulfurization of MoOx to the production of photoelectrodes, an alternative metal layer may be considered Alternatively, the MoS film may be transferred from the growth substrate to the conductive substrate With this approach, the MoS films may retain the original uniformity and quality similarly to those on Si/SiO or quartz substrates In order to improve the photocurrent measurement, optimization of the measurement conditions with lock-in technique is required However, selection of a proper reference frequency remains a challenge due to uncertainty in the response time of the photocurrent in MoS2 thin films The response time may be investigated by applying a bias to enhance the photocurrent signal As for the surface states affecting the samples, specific chemical passivation could be used to minimize the recombination process, as shown in previous work on bulk crystals, such as treatment in a saturated solution of bis 1,2-iphenylphosphine or with ethane 4-ter-butyl pyridine,66 or induce organic molecules such as the disodium salt of the ethylene diamine tetracetic acid as blocking 59 agents of the defective sites.67,68 The photoelectrochemistry of atomically thin transition metal dichalcogenides remain largely unexplored Further investigations on atomically thin films of WS2 , MoSe2 and WSe2 should reveal their potential in applications Additionally, the study of photoelectrodes based on a composite of wide band gap oxides such as TiO and LTMD thin films could be another promising direction 60 References Wilson, J A.; Yoffe, A D., The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties Advances in Physics 1969, 18 (73), 193-335 Novoselov, K S.; Jiang, D.; Schedin, F.; Booth, T J.; Khotkevich, V V.; Morozov, S V.; Geim, A K., Two-dimensional atomic crystals Proceedings of the National Academy of Sciences of the United States of America 2005, 102 (30), 10451-10453 Coleman, J N.; Lotya, M.; O’Neill, A.; Bergin, S D.; King, P J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R J.; Shvets, I V.; Arora, S K.; Stanton, G.; Kim, H.-Y.; Lee, K.; Kim, G T.; Duesberg, G S.; Hallam, T.; Boland, J J.; Wang, J J.; Donegan, J F.; Grunlan, J C.; Moriarty, G.; Shmeliov, A.; Nicholls, R J.; Perkins, J M.; Grieveson, E M.; Theuwissen, K.; McComb, D W.; Nellist, P D.; Nicolosi, V., Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials Science 2011, 331 (6017), 568-571 Liu, K.-K.; Zhang, W.; Lee, Y.-H.; Lin, Y.-C.; Chang, M.-T.; Su, C.-Y.; Chang, C.-S.; Li, H.; Shi, Y.; Zhang, H.; Lai, C.-S.; Li, L.-J., Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates Nano Letters 2012, 12 (3), 1538-1544 Zhan, Y.; Liu, Z.; Najmaei, S.; Ajayan, P M.; Lou, J., Large-Area Vapor-Phase Growth and Characterization of MoS Atomic Layers on a SiO2 Substrate Small 2012, (7), 966-971 van der Zande, A M.; Huang, P Y.; Chenet, D A.; Berkelbach, T C.; You, Y.; Lee, 61 G.-H.; Heinz, T F.; Reichman, D R.; Muller, D A.; Hone, J C., Grains and grain boundaries in highly crystalline monolayer molybdenum disulphide Nat Mater 2013, 12 (6), 554-561 Lee, Y.-H.; Zhang, X.-Q.; Zhang, W.; Chang, M.-T.; Lin, C.-T.; Chang, K.-D.; Yu, Y.-C.; Wang, J T.-W.; Chang, C.-S.; Li, L.-J.; Lin, T.-W., Synthesis of Large-Area MoS2 Atomic Layers with Chemical Vapor Deposition Advanced Materials 2012, 24 (17), 2320-2325 Roxlo, C B.; Chianelli, R R.; Deckman, H W.; Ruppert, A F.; Wong, P P., Bulk and surface optical absorption in molybdenum disulfide Journal of Vacuum Science & Technology A 1987, (4), 555-557 Mak, K F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T F., Atomically Thin MoS2 : A New Direct-Gap Semiconductor Physical Review Letters 2010, 105 (13), 136805 10 Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F., Emerging Photoluminescence in Monolayer MoS Nano Letters 2010, 10 (4), 1271-1275 11 Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single- layer MoS2 transistors Nat Nano 2011, (3), 147-150 12 Lee, H S.; Min, S.-W.; Chang, Y.-G.; Park, M K.; Nam, T.; Kim, H.; Kim, J H.; Ryu, S.; Im, S., MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap Nano Letters 2012, 12 (7), 3695-3700 13 Pu, J.; Yomogida, Y.; Liu, K.-K.; Li, L.-J.; Iwasa, Y.; Takenobu, T., Highly Flexible MoS2 Thin-Film Transistors with Ion Gel Dielectrics Nano Letters 2012, 12 (8), 62 4013-4017 14 Bertolazzi, S.; Brivio, J.; Kis, A., Stretching and Breaking of Ultrathin MoS ACS Nano 2011, (12), 9703-9709 15 Zhao, W.; Ghorannevis, Z.; Chu, L.; Toh, M.; Kloc, C.; Tan, P.-H.; Eda, G., Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2 ACS Nano 2012, (1), 791-797 16 Tonndorf, P.; Schmidt, R.; Böttger, P.; Zhang, X.; Börner, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D R T.; Michaelis de Vasconcellos, S.; Bratschitsch, R., Photoluminescence emission and Raman response of monolayer MoS , MoSe2 , and WSe2 Opt Express 2013, 21 (4), 4908-4916 17 Yin, Z.; Li, H.; Li, H.; Jiang, L.; Shi, Y.; Sun, Y.; Lu, G.; Zhang, Q.; Chen, X.; Zhang, H., Single-Layer MoS2 Phototransistors ACS Nano 2011, (1), 74-80 18 Gratzel, M., Photoelectrochemical cells Nature 2001, 414 (6861), 338-344 19 Priambodo, P S.; Sukoco, D.; Purnomo, W.; Sudibyo, H.; Hartanto, D., Electric Energy Management and Engineering in Solar Cell System 2013 20 Tributsch, H.; Bennett, J C., Electrochemistry and photochemistry of MoS layer crystals I Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1977, 81 (1), 97-111 21 Tributsch, H., The MoSe2 Electrochemical Solar Cell: Anodic Coupling of Electron Transfer to d → d Photo- Transitions in Layer Crystals Berichte der Bunsengesellschaft für physikalische Chemie 1978, 82 (2), 169-174 22 Gobrecht, J.; Tributsch, H.; Gerischer, H., Performance of Synthetical n-MoSe2 in 63 Electrochemical Solar Cells Journal of The Electrochemical Society 1978, 125 (12), 2085-2086 23 Tributsch, H., Electrochemical solar cells based on layer-type transition metal compounds: Performance of electrode material Solar Energy Materials 1979, (3–4), 257-269 24 Kline, G.; Kam, K.; Canfield, D.; Parkinson, B A., Efficient and stable photoelectrochemical cells constructed with WSe and MoSe2 photoanodes Solar Energy Materials 1981, (3), 301-308 25 Fan, F.-R F.; White, H S.; Wheeler, B L.; Bard, A J., Semiconductor ele ctrodes 31 Photoelectrochemistry and photovoltaic systems with n- and p-type tungsten selenide (WSe2 ) in aqueous solution Journal of the American Chemical Society 1980, 102 (16), 5142-5148 26 Kline, G.; Kam, K K.; Ziegler, R.; Parkinson, B A., Further studies of the photoelectrochemical properties of the group VI transition metal dichalcogenides Solar Energy Materials 1982, (3), 337-350 27 Djemal, G.; Müller, N.; Lachish, U.; Cahen, D., Photoelectrochemical cells using polycrystalline and thin film MoS2 electrodes Solar Energy Materials 1981, (4), 403-416 28 Razzini, G.; Lazzari, M.; Bicelli, L P.; Levy, F.; De Angelis, L.; Galluzzi, F.; Scafè, E.; Fornarini, L.; Scrosati, B., Electrochemical solar cells with layer-type semiconductor anodes Performance of n-MoSe2 cells Journal of Power Sources 1981, (4), 371-382 64 29 Lewerenz, H J.; Heller, A.; DiSalvo, F J., Relationship between surface morphology and solar conversion efficiency of tungsten diselenide photoanodes Journal of the American Chemical Society 1980, 102 (6), 1877-1880 30 Gobrecht, J.; Gerischer, H.; Tributsch, H., Electrochemical Solar Cell Based on the d-Band Semiconductor Tungsten-Diselenide Berichte der Bunsengesellschaft für physikalische Chemie 1978, 82 (12), 1331-1335 31 Kautek, W.; Gobrecht, J.; Gerischer, H., The Applicability of Semiconducting Layered Materials for Electrochemical Solar Energy Conversion Berichte der Bunsengesellschaft für physikalische Chemie 1980, 84 (10), 1034-1040 32 Audas, R.; Irwin, J C., Investigation of the performance of an MoS2‖I −/I2 ‖C electrochemical solar cell Journal of Applied Physics 1981, 52 (11), 6954-6960 33 Decker, F.; Scrosati, B.; Razzini, G., Photoelectrochemical Solar Cells Based on Molybdenum and Tungsten Dichalcogenides In Photoelectrochemistry and Photovoltaics of Layered Semiconductors, Aruchamy, A., Ed Springer Netherlands: 1992; Vol 14, pp 121-154 34 Parrot, J E., Photo and thermoelectric effects in semiconductors: J Tauc: Pergamon Press, Oxford, 1962 pp 260, 60s Solid-State Electronics 1962, (6), 422-423 35 Ryvkin, S M., Photoelektrische Erscheinungen in Halbleitern Akademie-Verlag: Berlin, 1965 36 Moss, T S B G J E B., Semiconductor opto-electronics Butterworths: London, 1973 65 37 Gerischer, H., Electrochemical photo and solar cells principles and some experiments Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1975, 58 (1), 263-274 38 Nozik, A J., Photoelectrochemistry: Applications to Solar Energy Conversion Annual Review of Physical Chemistry 1978, 29 (1), 189-222 39 Lohmann, F., Fermi-Niveau und Flachbandpotential von Molekülkristallen aromatischer Kohlenwasserstoffe Zeitschrift Naturforschung Teil A 1967, 22, 843 40 Note, A., About Lock-in Amplifiers Stanford Research Systems: Sunnyvale, CA, USA Available online: http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/AboutLIAs.pdf (accessed on August 2013): 1999 41 Ramakrishna Matte, H S S.; Gomathi, A.; Manna, A K.; Late, D J.; Datta, R.; Pati, S K.; Rao, C N R., MoS2 and WS2 Analogues of Graphene Angewandte Chemie International Edition 2010, 49 (24), 4059-4062 42 Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M., Photoluminescence from Chemically Exfoliated MoS2 Nano Letters 2011, 11 (12), 5111-5116 43 Joensen, P.; Frindt, R F.; Morrison, S R., Single- layer MoS2 Materials Research Bulletin 1986, 21 (4), 457-461 44 Divigalpitiya, W M R.; Frindt R F.; Morrison, S R., Inclusion Systems of Organic Molecules in Restacked Single-Layer Molybdenum Disulfide Science 1989, 246 (4928), 369-371 66 45 Py, M A.; Haering, R R., Structural destabilization induced by lithium intercalation in MoS2 and related compounds Canadian Journal of Physics 1983, 61 (1), 76-84 46 Wypych, F.; Schollhorn, R., 1T-MoS2 , a new metallic modification of molybdenum disulfide Journal of the Chemical Society, Chemical Communications 1992, (19), 1386-1388 47 Tsai, H.-L.; Heising, J.; Schindler, J L.; Kannewurf, C R.; Kanatzidis, M G., Exfoliated−Restacked Phase of WS2 Chemistry of Materials 1997, (4), 879-882 48 Mattheiss, L F., Energy Bands for 2H-NbSe2 and 2H-MoS2 Physical Review Letters 1973, 30 (17), 784-787 49 Acrivos, J V.; Liang, Y.; Wilson, J.; Yoffe, A., Optical studies of metal-semiconductor transmutations produced by intercalation Journal of Physics C: Solid State Physics 1971, 4, L18-L20 50 Yoffe, A D., Layer Compounds Annual Review of Materials Science 1973, (1), 147-170 51 Coehoorn, R.; Haas, C.; Dijkstra, J.; Flipse, C J F.; de Groot, R A.; Wo ld, A., Electronic structure of MoSe2 , MoS2 , and WSe2 I Band-structure calculations and photoelectron spectroscopy Physical Review B 1987, 35 (12), 6195-6202 52 Lee, C.; Yan, H.; Brus, L E.; Heinz, T F.; Hone, J.; Ryu, S., Anomalous Lattice Vibrations of Single- and Few-Layer MoS2 ACS Nano 2010, (5), 2695-2700 53 Yang, D.; Sandoval, S J.; Divigalpitiya, W M R.; Irwin, J C.; Frindt, R F., Structure of single-molecular-layer MoS2 Physical Review B 1991, 43 (14), 67 12053-12056 54 Li, T.; Galli, G., Electronic Properties of MoS Nanoparticles The Journal of Physical Chemistry C 2007, 111 (44), 16192-16196 55 Lin, Y.-C.; Zhang, W.; Huang, J.-K.; Liu, K.-K.; Lee, Y.-H.; Liang, C.-T.; Chu, C.-W.; Li, L.-J., Wafer-scale MoS2 thin layers prepared by MoO sulfurization Nanoscale 2012, (20), 6637-6641 56 Gutiérrez, H R.; Perea-López, N.; Elí as, A L.; Berkdemir, A.; Wang, B.; Lv, R.; López-Urí as, F.; Crespi, V H.; Terrones, H.; Terrones, M., Extraordinary Room-Temperature Photoluminescence in Triangular WS Monolayers Nano Letters 2012, 13 (8), 3447-3454 57 Kam, K K.; Parkinson, B A., Detailed photocurrent spectroscopy of the semiconducting group VIB transition metal dichalcogenides The Journal of Physical Chemistry 1982, 86 (4), 463-467 58 Dinh, H.; Chen, Z.; Miller, E., Photoelectrochemical Water-splitting: Standards, Experimental Methods, and Protocols SPRINGER VERLAG GMBH: 2013 59 Guo, X.- Z.; Luo, Y.-H.; Zhang, Y.-D.; Huang, X.-C.; Li, D.-M.; Meng, Q.-B., Study on the effect of measuring methods on incident photon-to-electron conversion efficiency of dye-sensitized solar cells by home- made setup Review of Scientific Instruments 2010, 81 (10), - 60 Xue, G.; Yu, X.; Yu, T.; Bao, C.; Zhang, J.; Guan, J.; Huang, H.; Tang, Z.; Zou, Z., Understanding of the chopping frequency effect on IPCE measurements for dye-sensitized solar cells: from the viewpoint of electron transport and extinction 68 spectrum Journal of Physics D: Applied Physics 2012, 45 (42), 425104 61 Bard, A J.; Fan, F.-R F.; Gioda, A S.; Nagasubramanian, G.; White, H S., On the role of surface states in semiconductor electrode photoelectrochemical cells Faraday Discussions of the Chemical Society 1980, 70 (0), 19-31 62 Kautek, W.; Gerischer, H., The photoelectrochemistry of the aqueous iodide/iodine redox system at n-type MoSe2 -electrodes Electrochimica Acta 1981, 26 (12), 1771-1778 63 King, L A.; Zhao, W.; Chhowalla, M.; Riley, D J.; Eda, G., Photoelectrochemical properties of chemically exfoliated MoS Journal of Materials Chemistry A 2013, (31), 8935-8941 64 Kautek, W.; Gerischer, H.; Tributsch, H., The Role of Carrier Diffusion and Indirect Optical Transitions in the Photoelectrochemical Behavior of Layer Type d-Band Semiconductors Journal of The Electrochemical Society 1980, 127 (11), 2471-2478 65 Wang, R.; Ruzicka, B A.; Kumar, N.; Bellus, M Z.; Chiu, H.-Y.; Zhao, H., Ultrafast and spatially resolved studies of charge carriers in atomically thin molybdenum disulfide Physical Review B 2012, 86 (4), 045406 66 Parkinson, B A.; Furtak, T E.; Canfield, D.; Kam, K.-K.; Kline, G., Evaluation and reduction of efficiency losses at tungsten diselenide photoanodes Faraday Discussions of the Chemical Society 1980, 70 (0), 233-245 67 Razzini, G.; Peraldo Bicelli, L.; Pini, G.; Scrosati, B., Electrochemical Solar Cells with Layer-Type Semiconductor Anodes: Chemical Treatments of the Crystal Surface 69 Journal of The Electrochemical Society 1981, 128 (10), 2134-2137 68 Sakata, T.; Janata, E.; Jaegermann, W.; Tributsch, H., Time-Resolved Photocurrent of WSe2 Photoanode Studied with a Nanosecond Pulse Laser Journal of The Electrochemical Society 1986, 133 (2), 339-345 70 Appendix Figure A (a) AFM image of the evaporated MoOx film with a thickness of nm and the roughness of the film is ~ 0.3 nm (b) A selected cross-sectional height profile showing the thickness of the MoOx film (c) AFM image of the MoS2 film from sulfurization of MoOx film with the thickness of nm The thickness of MoS2 film is about nm, consistent with the original MoOx film The roughness of the MoS2 film is ~ 0.6 nm (d) A selected cross-sectional height profile showing the thickness of the MoS2 film Figure A (a) Raman spectrum of a MoOx film collected with a 532 nm laser excitation wavelength, only Si peaks are seen in the spectrum (b) Raman spectrum of bulk MoO powder 71