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专家视点 | 王智杰研究员、雷勇教授：迈向太阳能分解水工业化应用的重要一步
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迈向太阳能分解水工业化应用的重要一步 利用太阳光照射光催化剂分解纯水制取氢气（H2：O2摩尔比 = 2 : 1），将太阳能转化为可储存和运输的氢能，是实现“液态阳光”能源计划最为理想的方法之一。关于光催化分解水能否实用化，现阶段存在两大挑战：①光催化剂的量子效率；②光催化剂的响应波长的拓展。 近日，中国科学院半导体研究所王智杰研究员团队和德国伊尔梅瑙工业大学雷勇教授团队在《Journal of Semiconductors》上撰写news and views文章《A close step towards industrialized application of solar water splitting》，介绍了改性SrTiO3量子效率提高到接近100%（350-360 nm波长响应），再次告诉业界，接近100%的量子效率（全分解水效率）是可能的。 尽管一些研究者已经实现了可见光响应的全解水，其效率和稳定性仍需提升。为了满足光解水工业化应用的需求，体系量子效率需要接近100%，太阳能转化效率需要超过10%。由于化学反应能量势垒和相应逆反应的限制，光解水的效率和稳定性达不到太阳能电池的水平。有鉴于此，东京大学K.Domen课题组使用传统SrTiO3作为模型，通过Al掺杂，熔盐法处理，不同晶面助催化剂修饰实现了接近100%的量子效率（Nature, 2020, 581, 411）。上述工作使光解水工业化应用的实现迈出了重要一步。 Full Text The efficient utilization of solar energy and the decomposition of water into oxygen and hydrogen are the current hot spots in the community of research, with great application promise. Hydrogen is an important clean fuel and widely used in the fuel cell. Oxygen is a necessary gas for human survival. It is of significance for manned space exploration to achieve efficient oxygen acquisition and gas recycling in confined space. As early as the 1970s, Fujishima et al. proposed a scheme of using TiO2 to decompose water by photoelectrochemistry[1]. However, TiO2 has such a large bandgap that it can only absorb UV light, which merely accounts for a small part of solar radiation. It’s difficult to meet the comprehensive requirements of high efficiency, low cost and high stability for TiO2. For this reason, researchers have adopted various approaches to improve the separation efficiency of charge carriers, focusing on morphology, electronic structure and interface characteristics of materials[2, 3]. Although solar water splitting uses abundant resources such as solar energy and water, it is difficult to reach the level of industrial application for the current energy conversion efficiency. The bottleneck of its development is the insufficient light absorption in visible light and near infrared region and the low efficiency of photocatalytic reaction. Modification of the surface and interface of photocatalyst can overcome these limitations effectively[4]. In recent years, in order to improve the efficiency of surface/interface carrier generation and separation, researchers have focused on the design of semiconductor multilayer heterojunctions and nanostructures to improve the efficiency of hydrogen production from solar water splitting. Nanostructures are designed to reduce the diffusion length of minority carriers. The size and morphology of nanostructures are important factors affecting the photocatalytic activity of catalysts, where nanostructures with high surface area can provide more photocatalytic sites[1, 5]. There are still many problems in improving the photocatalytic performance of nanomaterials[6]. If H2 and O2 with a stoichiometric ratio of 2 : 1 can be efficiently produced by overall water splitting without the dependence on sacrificial agent, hydrogen could actually replace conventional fossil fuels, and this is the ultimate goal researchers are trying to achieve. Onishi's group used SrTiO3–NiO to decompose water vapor into hydrogen and oxygen under the ultraviolet light produced by high-pressure mercury lamps, which achieved overall water splitting for the first time and started the research upsurge[7]. Zou et al. used Ni doped InTaO4 to achieve hydrogen production from overall water splitting in visible light, where the quantum efficiency is of 0.66%[8]. Domen’s group prepared GaN:ZnO solid solution with visible light response to achieve stable overall water splitting under visible light, which opened the avenue of non-oxide photocatalyst for overall water splitting[9]. Kang’s group prepared the metal-free nanocomposite of carbon dot and carbon nitride for photocatalytic overall water splitting under visible light, where the quantum efficiency at 420 nm was as high as 16% and the solar energy conversion efficiency in the full-spectrum was up to 2%[10]. Although some researchers have achieved overall water splitting with visible light response, the efficiency and stability still need to be improved. The EQE needs to be close to 100% and solar conversion efficiency needs to be over 10% for the industrialized application of solar water splitting. As the drawbacks of energy potential barrier of chemical reaction and the according reverse reaction, the efficiency and stability of water splitting can not be as high as solar cells. To this end, Domen et al. used traditional SrTiO3 as the model to achieve photocatalytic water splitting with a quantum efficiency of almost unity[11], which can make a close step towards industrialized application of it. As the overall water splitting is an uphill reaction (Gibbs energy of +237 kJ/mol) consisting of multiple charge carriers transfer processes, to achieve a 100% IQE (internal quantum efficiency), the HER (hydrogen evolution reaction) and OER (oxygen evolution reaction) must proceed without any reverse reaction. Herein, Domen et al. constructed highly active HER and OER cocatalysts on Al doped SrTiO3 site-selectively to increase the EQE to the upper limit by a stepwise photodeposition method. As shown in Fig. 1, the results showed that the EQE values of the optimized sample at 350, 360, and 365 nm were determined to be 95.7%, 95.9% and 91.6%, respectively, which are the highest values reported so far for photocatalytic water splitting. The IQE should be close to 100% in the wavelength region of 350–360 nm on account of that the EQE exceeded 95%. In this case, a Rh core/Cr2O3 shell structure was formed on the surface of Al doped SrTiO3 through a two-step photodeposition, where the Rh promotes both the HER and the oxygen reduction reaction (ORR, a major backward electron transfer process), whereas the Cr2O3 shell inhibits only the ORR by blocking the access of evolved O2 to the surface of the Rh core[12]. Another cocatalyst of CoOOH promotes the OER, thus, the efficiency of the optimized photocatalyst of Rh/Cr2O3/CoOOH/SiTiO3:Al is boosted. Figure 1. Photocatalytic water-splitting activities. (a) Time course of H2 andO2 evolution on SrTiO3:Al loaded with various cocatalysts during photoirradiation. Left, loaded with Rh (0.1 wt%)/Cr2O3 (0.05 wt%) by two-step photodeposition. Middle, loaded with Rh (0.1 wt%)/Cr2O3 (0.05 wt%)/CoOOH (0.05 wt%) by three-step photodeposition. Right, loaded with Rh (0.1 wt%)-Cr (0.1 wt%) oxide by co-impregnation. (b) Ultraviolet-visible diffuse reflectance spectrum of bare SrTiO3:Al (black solid line) and wavelength dependence of external quantum efficiency (EQE) during water splitting on Rh (0.1 wt%)/Cr2O3 (0.05 wt%)/CoOOH (0.05 wt%)-loaded SrTiO3:Al (red symbols). The Rh/Cr2O3 cocatalyst was found to be preferentially deposited on the (100) crystal facets, while the CoOOH cocatalyst was primarily located in the (110) direction, as illustrated in Fig. 2. The observed anisotropic deposition of cocatalysts can be attributed to a charge rectification effect inside each photocatalyst particle induced by an internal electric field, which originates from the work function difference between the respective facets. Increasing the work function difference obviously increases the extent of anisotropic charge separation, as well as the concentration of electrons at the {100} facets and of holes at the {110} facets, as seen from Fig. 3. Surface dipoles resulting from unbalanced cation/anion ratios on distinct facets may induce this work function difference even without forming junctions or composites. Figure 2. Transmission electron microscopy. (a) Selected-area electron diffraction pattern obtained from SrTiO3:Al loaded with Rh (0.1 wt%)/Cr2O3 (0.05 wt%)/CoOOH (0.05 wt%). (b) Corresponding transmission electron microscopy image of a particle. (c) Particle morphology and crystal orientation. Figure 3. Simulations of photocarrier distributions in SrTiO3:Al particles. (a) Mapping of conduction-band energy, Ec. (b) Density of electrons (e-), n. (c) Density of holes (h+), p. (d) Energy band diagram. (e) Electron and hole densities as functions of position (x′, y′) with work function difference ΔWel = 0.2 eV. (f) Effect of ΔWel on electron-to-hole-density ratio at the {100} and {110} facets. This study demonstrates overall water splitting with an IQE close to unity by applying aluminium doping for defect suppression, flux treatment to improve the crystallinity, and a Cr2O3 shell to inhibit ORR, in addition to facet engineering, and thus gives a definitive answer to this problem. The selection of high-performance cocatalysts for the HER and OER is another important aspect. Recently, Ta3N5 and Y2Ti2O5S2 have been reported to split water into hydrogen and oxygen under visible light[13, 14]. These materials absorb visible light with wavelengths of up to 600 and 640 nm, respectively, and the STH (solar-to-hydrogen) efficiency can reach 10% once the EQE is improved to a level similar to that of SrTiO3:Al. References: [1]Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature, 1972, 238, 37 [2]An X, Li T, Wen B, et al. New insights into defect-mediated heterostructures for photoelectrochemical water splitting. Adv Energy Mater, 2016, 6, 1502268 [3]Wang X D, Xu Y F, Rao H S, et al. Novel porous molybdenum tungsten phosphide hybrid nanosheets on carbon cloth for efficient hydrogen evolution. Energ Environ Sci, 2016, 9, 1468 [4]Wu B, Liu D, Mubeen S, et al. Anisotropic growth of TiO2 onto gold nanorods for plasmon-enhanced hydrogen production from water reduction. J Am Chem Soc, 2016, 138, 1114 [5]Zhang L, Ye X, Boloor M, et al. Significantly enhanced photocurrent for water oxidation in monolithic Mo:BiVO4/SnO2/Si by thermally increasing the minority carrier diffusion length. Energ Environ Sci, 2016, 9, 2044 [6]Luo J, Steier L, Son M K, et al. Cu2O nanowire photocathodes for efficient and durable solar water splitting. Nano Lett, 2016, 16, 1848 [7]Domen K, Naito S, Soma M, et al. Photocatalytic decomposition of water vapour on an NiO–SrTiO3 catalyst. J Chem Soc Chem Commun, 1980, 12, 543 [8]Zou Z, Ye J, Sayama K, et al. Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst. Nature, 2001, 414, 625 [9]Maeda K, Takata T, Hara M, et al. GaN:ZnO solid solution as a photocatalyst for visible-light-driven overall water splitting. J Am Chem Soc, 2005, 127, 8286 [10]Liu J, Liu Y, Liu N, et al. Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science, 2015, 347, 970 [11]Takata T, Jiang J, Sakata Y, et al. Photocatalytic water splitting with a quantum efficiency of almost unity. Nature, 2020, 581, 411 [12]Maeda K, Domen K. Photocatalytic water eplitting: Recent progress and future challenges. J Phys Chem Lett, 2010, 1, 2655 [13]Wang Z, Inoue Y, Hisatomi T, et al. Overall water splitting by Ta3N5 nanorod single crystals grown on the edges of KTaO3 particles. Nat Catal, 2018, 1, 756 [14]Wang Q, Nakabayashi M, Hisatomi T, et al. Oxysulfide photocatalyst for visible-light-driven overall water splitting. Nat Mater, 2019, 18, 827 1980年6月生。中国科学院半导体研究所研究员，博士生导师，中科院特聘研究员。 2004年在浙江大学获得学士学位； 2009年在中国科学院半导体研究所获得博士学位； 2009年8月-2015年2月分别在美国怀俄明大学、迈阿密大学、密歇根大学以及德国伊尔梅诺理工大学从事科研工作； 2015年加入中国科学院半导体研究所材料科学重点实验室工作，并于2017年任材料科学重点实验室主任。 长期从事新能源领域和光电技术领域中载流子输运动力学及其高效器件的研究。 迄今在Energy Environ. Sci.、Nat. Commun.、J. Am. Chem. Soc.、Angew. Chem.等国际权威期刊发表SCI收录论文80余篇。现任德国物理学会会员，担任Journal of Physics D: Applied Physics, 及Journal of Semiconductors 编委。多次参与主办国内、国际会议，出任组委秘书，并在国际学术会议上担任分会主席。承担科研项目多项，涵盖国家自然科学基金项目、美国自然科学基金项目、国家重点研发计划纳米专项、前沿科学重点研究计划项目、北京市自然科学基金项目等。 Prof. Yong Lei（雷勇教授）是德国伊尔梅瑙工业大学的终身教授。2003到2006年作为洪堡学者在德国卡尔斯鲁厄理工大学进行科学研究。之后在德国明斯特大学担任研究团队负责人(2006-2009)和青年教授(2009-2011)。2011至今在德国伊尔梅瑙工业大学担任终身教授、应用纳米物理研究团队负责人。 雷勇教授长期从事基于模板的功能微纳结构、表面结构和图案化及在能源器件和光电器件中的研究，在多元和表面纳米结构化、钠离子和钾离子电池等领域具有国际性研究声誉。迄今为止，雷勇教授共发表了214篇SCI文章，很多发表在顶级期刊上，包括影响因子大于20的论文18篇（17篇通讯作者），影响因子10-20的论文有70篇（55篇通讯作者），如Nat. Nanotech., Nat. Commun., JACS, Angew. Chem., Adv. Mater., Energy Environ. Sci., Chem. Soc. Rev., Prog. Mater. Sci., Adv. Energy Mater., Mater. Today, Adv. Funct. Mater., ACS Nano, Nano Lett., Nano Today.等。雷教授目前是期刊Advanced Energy Materials（影响因子25.245）的Editorial Advisory Board Member，Energy & Environmental Materials的Associate Editor，期刊Carbon Energy和InfoMat的Advisory Board Member。作为项目负责人，雷勇教授主持了多项欧洲和德国的大型研究项目，包括欧洲研究委员会（ERC）的享有很高声誉的ERC研究项目、德国联邦教育及研究部（BMBF）重大研究项目、德国研究基金会（DFG）和德国大众基金会。 点击阅读王智杰研究员和雷勇教授文章： A close step towards industrialized application of solar water splitting Jun Liu, Zhijie Wang, Yong Lei J. Semicond. 2020, 41(9): 090401 doi: 10.1088/1674-4926/41/9/090401
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