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Efficient and Stable Silicon Photocathodes Coated with a Vertically Standing Nano-MoS2 Films for Solar Hydrogen Production Ronglei Fan, Jie Mao, Zhihao Yin, Jiansheng Jie, Wen Dong, Liang Fang, Fengang Zheng, and Mingrong Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15854 • Publication Date (Web): 27 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017
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ACS Applied Materials & Interfaces
Efficient and Stable Silicon Photocathodes Coated with a Vertically Standing Nano-MoS2 Films for Solar Hydrogen Production Ronglei Fan,† Jie Mao,‡ Zhihao Yin,† Jiansheng Jie,‡ Wen Dong,† Liang Fang,† Fengang Zheng† and Mingrong Shen*
,†
†
Department of Physics, Optoelectronics and Energy, Jiangsu Key Laboratory of Thin Films, and
‡
Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for
Carbon-based Functional Materials and Devices, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China.
ABSTRACT: Water splitting in a photoelectrochemical cell has recently received intense research, which converts sunlight into hydrogen energy. Silicon is suitable as a viable light-harvesting material for constructing such cell, however needs to improve its stability and explore cheap and efficient co-catalyst. Here we fabricate highly efficient and stable photocathodes by integrating crystalline MoS2 catalyst with ∼2nm Al2O3 protected n+p-Si. Al2O3 acts as both protective and passivative layer of the Si surface, while the sputtering method using a MoS2 target along with a post-annealing leads to a vertically standing, conformal and crystalline nano-MoS2 layer on Al2O3/n+p-Si photocathode. Efficient (0.4 V vs. RHE onset potential and 35.6 mA/cm2 saturated photocurrent measured under 100 mA/cm2 Xe lamp illumination) and stable (above 120 h continuous water splitting) photocathode was obtained, which opens the door for the MoS2 catalyst to be applied in photoelectrochemical hydrogen evolution in a facile and scalable way.
KEYWORDS: photocathode, silicon, MoS2 catalyst, photoelectrochemical properties, Al2O3 layer
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INTRODUCTION The fabrication of efficient, stable and cheap photocathodes is important in photoelectrochemical (PEC) H2 generation.1,2 Silicon (band-gap ~1.12 eV) is an suitable semiconductor for photocathode since it is rich on earth and absorbs a significant fraction of the solar energy and the position of its conduction band edge is higher than the potential of hydrogen evolution reaction (HER) during water splitting.3 However, there are two challenges of utilizing Si for HER. One is that Si is easily oxidized during the PEC reaction due to the oxygen in the solution, and the electrically insulating SiO2 will result in device failure.4,5 Therefore, thin protective layers such as TiO2, Al2O3 and Ni/NiOx are used to suppress the oxidation while the interfacial charge transfer of photogenerated carriers is allowed.5-8 Another challenge is that the HER kinetics on the Si electrode surface is rather slow. Pt is reported as the efficient and stable catalyst for the photocathodes. However, it is a rare and expensive resource. For solar hydrogen production to be cost competitive, the HER catalysts need to be efficient, inexpensive and scalable.9 Recently, many new classes of inorganic HER catalysts have been reported, composed of only Earth-abundant elements.10 However, coupling these catalysts onto photocathodes is less studied. In addition to the synthetic difficulties, such coupling is challenging because in many cases the photoelectrodes and catalysts are chemically incompatible, which induces interfacial defects and stability issues.11 Several
groups
have recently tried to load the photocathodes with non-noble metal catalysts for efficient H2 generation,7,12,13 however, the stability and PEC reactivity are far from satisfactory. Fabrication of low-cost, stable and efficient photocathodes for practical applications needs further research. Molybdenum sulfide (MoS2) has been studied as the most attractive candidate for cheap HER electrocatalyst.14-18 The bulk MoS2 is not catalytic, but its edge site shows highly catalytic activity.17,19 Therefore, producing MoS2 with morphologies that can maximize catalytically active edge sites has been of great interest. Recently, molybdenum sulfide catalysts have been incorporated on Si photocathode 2
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successfully
by
different
methodologies
such
as
simple
dropcasting,20
electrodeposition,21 or sulfidization of Mo.22 However, their chemical stability and PEC performance are far from perfect. Typically, Seger et al. electrodeposited amorphous MoSx on Ti-protected n+p-Si photocathode. 0.33 V vs. RHE (RHE: Reversible Hydrogen Electrode; V vs. RHE: VRHE) onset potential and 16 mA/cm2 saturated photocurrent were obtained under red light with AM1.5 cut-off (
