The Study of Water Photodecomposition to H2 : Au-loaded Titanate Preparation, Fe2O3/Si Multibandgap Films Design and Fe2O3 Film Preparation
Date Issued
2005
Date
2005
Author(s)
Tai, Yu-Wei
DOI
zh-TW
Abstract
This thesis was focused on the development of novel photo-converters by the photo-catalytic and photo- electrochemical systems for water splitting to hydrogen. In the photo-catalytic system, the effects of gold loading on the perovskite titanate substrate (K2La2Ti3O10) for water splitting were studied. In the photo-electrochemical system, the device of Fe2O3/Si multiple bandgap films was designed and the preparations of the p-type and n-type Fe2O3 films were studied.
In the photo-catalytic system, the factors of the loading processes and pretreatment procedures on Au/K2La2Ti3O10 were investigated. Also, a preliminary comparison of the activities of nano-gold and the reduced (then partially re-oxidized) nickel for water splitting under UV or visible light were made. It was found that Au/K2La2Ti3O10 prepared by an incipient wetness impregnation process possessed a better activity for water splitting than that prepared by a deposition process. This was because a better crystallinity of K2La2Ti3O10 was preserved from the impregnation process than that from the deposition. Moreover, the activity of Au/K2La2Ti3O10 can be increased significantly, after the reduction of gold ions to nano-gold metal through some pretreatment processes. When compared with the best metal–titanate catalyst reported in the literature (i.e., Ni/K2La2Ti3O10), Au/K2La2Ti3O10 possessed a lower hydrogen production rate in UV region and a higher one in visible region. This may be because Ni/K2La2Ti3O10 preserved a better crystallinity of K2La2Ti3O10 to produce more electron-hole pairs in UV region and Au/K2La2Ti3O10 had an absorption in the visible region from plasma resonance on the nano-gold surface, rather than on the Ni surface.
There were two bottlenecks in the photo-catalytic system, which were the separation of the mixing product (H2/O2) and the low theoretical quantum efficiency of the photo-catalyst under solar light. To improve the quantum efficiency for water splitting, the device of multiple bandgap films in the photo-electrochemical system was designed. Fe2O3 and Si were chosen as the top and bottom materials, and ITO was chosen as the transparent middle layer for ohmic contact. The theoretical quantum efficiency of this device for solar-to-electricity was raised to 32% from 24% of the single crystal Si solar cell. Due to the n-type intrinsic behavior of Fe2O3 and the maturity of the Si and ITO technology, the p-Fe2O3 film preparation was the key research. The p-Fe2O3 film was fabricated by RF magnetron sputtering process and the p-Fe2O3 target. The preparation of p-Fe2O3 pellets was studied for the p-Fe2O3 target preparation. From our results of the p-Fe2O3 pellets preparation, it was demonstrated that the p-Fe2O3 pellet could be made by Mg doping, under the conditions of three atmospheres oxygen pressure at 900℃ for 20 h.
In order to fabricate p-type and n-type Fe2O3 films with good photo-efficiencies, some sputtering factors were investigated. The p-type dopant of Fe2O3 films was the Mg atom and the n-type one was the Si or Ti atom. The results were shown that the plasma composition was an important factor for sputtering the p-Fe2O3 film. The Fe2O3 films with p-type photocurrent were obtained only by sputtering in the O2 plasma gas. The pure Fe2O3 films transformed from p-type to n-type after the post-annealing treatment, due to the release of the interstitial O2 atoms by structure compression. The better n-type photocurrent can be obtained by post-annealing in three atmospheres oxygen pressure. Moreover, the more Mg doping levels the Fe2O3 film contained, the higher p-type photocurrent was obtained. On the contrary, the rising effect of the n-type photocurrent was obtained by doping with the Si and Ti atoms. It has been concluded that the higher p-type photocurrent density of 0.02 mA/cm2 at -0.4V was obtained by the 2at% Mg-Fe2O3 film which was sputtered under O2 plasma gas. The higher n-type photocurrent density of 0.36 mA/cm2 at +0.6V was obtained by the 1at% Ti-Fe2O3 film which was sputtered under O2 plasma, followed by the post-annealing treatments in air and in three atmospheres oxygen pressure. The higher real quantum yield of solar-to-electricity and quantum efficiency of solar-to-hydrogen for the above-mentioned 1at% Ti-Fe2O3 film were 1.8% and 0.44%, respectively. The devices of pn and pin junction Fe2O3 films were fabricated on ITO glass substrate by RF magnetron sputtering process. None of them had the phenomenon of solid state solar cell. Compared with the amorphous Si film (a-Si), the quantum yield of the Fe2O3 film was much lower than that of the a-Si film, probably due to the indirect bandgap property of the Fe2O3 film.
The main contributions of this thesis were listed as below: (1) the quantum yield of K2La2Ti3O10 was increased in the visible region by gold loading, (2) the cheaper device of the Fe2O3/Si multiple bandgap films was designed to obtain the higher theoretical quantum efficiency, (3) the Fe2O3 films with the higher p-type and n-type photocurrents were sputtered by doping with Mg and Ti atoms, respectively.
Subjects
水分解
氫能
光催化系統
奈米金
層狀鈦觸媒
光電化學系統
多重能隙
薄膜
太陽能電池
塊材
射頻磁控濺鍍法
氧化鐵
p型
n型
鎂
矽
鈦
Water splitting
Hydrogen
Photo-catalytic
Au
Perovskite titanate
Photo-electrochemical
multi-bandgap
thin film
solar cell
pellet
RF sputtering
Fe2O3
p-type
n-type
Magnesium
Silicon
Titanium
SDGs
Type
thesis
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