| Energy crisis and environmental pollution are one of the critical issues to restrict Chinese economic and social development. Therefore novel and clean energy has aroused widespread attention. Among these clean energies, solar energy is rich, economic and inexhaustible. Hydrogen widely exists in nature, and a large amount of hydrogen is widely reserved in water on earth. Since 1972, K. Honda and A. Fujishima found that hydrogen can be produced on the platinum electrode in the case of applied bias under light irradiation, scientists began to extensively investigate semiconductor-based photoelectrochemical water splitting. WO3 as an important semiconductor has been used in photo-degradation and water splitting due to its advantages of good visible light absorption, cheap, non-toxic and stability. However, WO3 also has some defects. In particular, the light-generated electron and hole has high recombination probability, and the charge mobility is also realtively slow, thus normally limiting the practical application of WO3. Therefore, in this thesis, BiVO4/Bi2S3 and In2S3 is proposed to modify WO3 film for forming heterojunctions, which could efficiently improve the photoelectrochemical water splitting performance. The photoelectrochemical behaviors were investigated and the inherent charge transfer mechanism in these heterojunctions are discussed. The main research works include: 1. The preparation of WO3 photoelectrodes modified by BiVO4/Bi2S3 and the study of photoelectrochemical water splitting performanceA vertical array of WO3 nanoplates thin film was prepared by hydrothermal, in which BiVO4 was grown on the surface of WO3 film by spin-coat followed by Bi2S3 deposition with successive ionic layer adsorption and reaction（SILAR） after annealing the WO3/BiVO4 film, and then the sample was annealed at vacuum to prepare high performance electrode. Vertically orientated WO3 nanoplates film of the composite structure modified by BiVO4/Bi2S3 was confirmed by morphology and structural characterization. The characterization results show that the photocurrent density of WO3/BiVO4/Bi2S3 is 1.2 mA cm-2 at the potential of 1.23 V（vs. RHE）, which is as large as 2.14 times of the plain WO3. ABPE is 0.29 % at the potential of 0.49 V（vs. RHE）, which is more negative than the plain WO3 nanoplates, offering 10 times better performance and indicating that the composite structure of WO3/BiVO4/Bi2S3 enhances aquatic negative oxygen kinetics rate. The enhancement mechanism of the photoanode is analyzed by IPCE and EIS, revealing that the composite structure has the high quantum efficiency in the visible light region and promotes photoelectron-hole separation effectively than pore WO3. 2. WO3/In2S3 heterojunction preparation and related photoelectrochemical behaviorsA vertical array of WO3 nanoplates thin film was prepared by hydrothermal approach, followed by deposition of In2S3 with different cycles of successive ionic layer adsorption and reaction（SILAR） after annealing the WO3 film, and then a high performance WO3/In2S3 photoelectrode was obtained by a vacuum heat treatment at 250 ℃.Vertically, the composed WO3 nanoplates film structure is formed by modification of In2S3, which was confirmed by morphology and structural characterization. Results show that the photocurrent density of WO3/In2S3 made by continuous 90 cycles of sedimentary In2S3 is 2.2 mA cm-2 at a potential of 1.23 V（vs. RHE）, which is larger than that of the plain WO3 by 2.14 times. ABPE is 0.29 % at the potential of 0.75 V（vs. RHE）, which is more negative than the plain WO3 nanoplates with increased performance by 7.25 times than the plain WO3 nanoplates. Photocurrent density can reach the maximum value of 1.2 cm-2 mA when depositing In2S3 by continuous 90 cycles, which is 2 times higher than WO3. This indicates that that the composite structure of WO3/In2S3 enhances oxygen evolution kinetics rate. The enhancement mechanism is further investigated by IPCE and EIS, revealing that WO3/In2S3 has a high quantum efficiency in the visible light region while promoting photoelectron-hole separation effectively than plain WO3. The heterojunction structure expands the range of visible light absorption and speeds up the photoelectron and hole separation.In summary, this work accomplished a WO3 compose with low band gap width semiconductor,which can be used to improve photoelectrochemical water splitting efficiency, mainly due to enhanced dynamics of charge separation and transport and expanded visible light absorption range. |
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