Abstract
Many visible light-driven photocatalysts have been developed recently because they possess the advantage to utilize the visible light of solar spectrum more efficiently. In addition, combination of two photocatalysts is further studied based on the mechanism of Z-scheme system. In this system, two semiconductor-based photocatalysts with relatively suitable band positions are chosen to address the drawback of self-recombination of electrons and holes in a single photocatalyst. In this research, visible light-driven Ta3N5 is the main material to be investigated, and WO3 is further combined with Ta3N5 in order to achieve higher efficiency in photocatalysis. The synthesized powders of two semiconductors were examined to study the performance of H2 evolution. Furthermore, considering that the existence of a shuttle redox mediator might compete with the photoexcited electrons in the H2-evolution photocatalyst, different forms of catalyst, i.e., powder, film, and microporous structure, were prepared and the performances in photocatalytic reaction were examined and discussed. Ta3N5 - WO3 heterojunctions on silicon (Si) wafer and polysulfone (PSF) hollow fiber were fabricated by sol-gel and atomic layer deposition (ALD). The excellent conformal film of Ta3N5 was obtained by ALD using the precursors of PDMAT and NH3, with the lengths of pulse - purge - pump time of each half-reaction being 2 - 10 - 2 s and 2.5 - 10 - 2 s, respectively. The as-deposited heterojunctions were subjected to various annealing conditions. The crystalline phases were examined by X-ray diffraction (XRD). For Ta3N5 - WO3 on Si, the major phases were Ta3N5 and substoichiometric WO2.72 after rapid thermal annealing (RTA). While for Ta3N5 - WO3 on PSF, the major phase was Ta3N5 together with a minor WO3 phase after annealing in a tube furnace. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET) analyses were conducted to determine the morphology, elemental composition, and specific surface area, respectively. The results of the performances of various photocatalysts demonstrate that applying the Ta3N5 - WO3 heterojunction exhibited enhanced efficiency compared with utilizing only one single photocatalyst. Furthermore, loading with a proper amount of Pt as co-catalyst also enhanced the performance. In the test of photocatalytic dye degradation, the absorbance of methylene blue (MB) solution was almost decomposed to 0 after 1 h of irradiation by AM 1.5 using the Ta3N5 - WO3 powders as catalyst. The test in photocatalytic water splitting also indicated that the direct Z-scheme system of Ta3N5@WO3@Si with 50 cycles of Pt deposition by ALD performed extremely good efficiency. The H2 production reached the value of 3072 μmol g-1 without applying the filter. Therefore, combination of Ta3N5 with WO3 based on the Z-scheme system was proved to be a promising photocatalyst.