Abstract
The artificial photosynthesis technology known as the Honda-Fujishima effect, which produces oxygen and hydrogen or organic energy from sunlight, water, and carbon dioxide, is an effective energy and environmental technology. The key component for the higher efficiency of this reaction system is the anode electrode, generally composed of a photocatalyst formed on a glass substrate from electrically conductive fluorine-doped tin oxide (FTO). To obtain a highly efficient electrode, a dense film composed of a nanoparticulate visible light responsive photocatalyst that usually has a complicated multi-element composition needs to be deposited and adhered onto the FTO. In this study, we discovered a method for controlling the electronic structure of a film by controlling the aerosol-type nanoparticle deposition (NPD) condition and thereby forming films of materials with a band gap smaller than that of the prepared raw material powder, and we succeeded in extracting a higher current from the anode electrode. As a result, we confirmed that a current approximately 100 times larger than those produced by conventional processes could be obtained using the same material. This effect can be expected not only from the materials discussed (GaN-ZnO) in this paper but also from any photocatalyst, particularly materials of solid solution compositions.
Highlights
The nanoparticle deposition (NPD) we previously developed[7,8,9] is capable of forming a film of a nano-ceramic particulate structure with high crystallinity at low temperature, and it can be considered to be optimal for the production of artificial photosynthesis anode electrodes
A GaN-ZnO-based material is a solid solution of hexagonal wurtzite structure crystals that can continuously change its composition as Ga-Zn and N-O are substituted in the same crystal structure
It is known that a GaN-ZnO solid solution has a narrower band gap than pure GaN or ZnO, and its absorption wavelength edge can be expanded to the long wavelength side as the amount of ZnO dissolved into the GaN increases[10]
Summary
Enhancement of photocatalytic properties using controlled NPD. A GaN-ZnO-based material is a solid solution of hexagonal wurtzite structure crystals that can continuously change its composition as Ga-Zn and N-O are substituted in the same crystal structure. The described simulation results agree with the experimental results shown in Fig. 1(b) in that the decrement of the band gap between the raw material powder and the NPD film is larger for the GaN-ZnO solid solution than for pure GaN. The VBM is not significantly different between the raw material powder and the NPD film, as shown, but a composition dependence can be detected, and the VBM shows a maximum value (higher side of the graph) at 45% ZnO This results from the effect of the combined bonding of the Zn, O, Ga, and N electron orbitals in the VBM, as described earlier. We studied the process with GaN-ZnO, which is a representative visible light responsive photocatalyst, but the results presented in this paper are applicable to various other photocatalysts and make a great contribution to the development of high-efficiency artificial photosynthesis anode electrodes
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