Hydrogen is an efficient carbon-free energy carrier, offering several advantages over fossil fuels such as a high energy density of about 120 MJ/Kg, no greenhouse gases emission and clean by product after delivering energy by thermal or electrochemical combustion with oxygen. Many research efforts have been made to develop an efficient and sustainable way for hydrogen generation. Compared to the conventional hydrogen generation by fossil fuel consumed processes of electrolysis and steam reformation of natural gases, hydrogen generation by photoelectrochemical (PEC) water splitting cells using solar radiation is essentially clean. However, two major issues, namely, stability and efficiency, of the PEC water splitting cells still remain challenging. The stability of PEC water splitting cells is determined by the photo-anticorrosive ability of the electrodes, and thus electrodes made of photo-anticorrosive materials are necessary. On the other hand, the efficiency of PEC water splitting cells has been limited by the electrode performance in absorbing light, separating charges and transporting carriers.

To address these limitations, nanostructured photoelectrodes made of one-dimensional semiconductor nanowire array on a conductive film are expected to enable superior performances over their film counterparts. First, the nanowire array realizes an anti-reflection surface to increase the light absorption. Second, the nanowire array gives large surface areas that enhance chemical reaction for water decomposition. Finally, the diameters of the nanowires are in nanoscale, and thus reduce the carrier diffusion length in the diameter directions of the nanowires leading to improved charge separation at the nanowire surface areas to support an enhanced chemical reaction of water decomposition. Therefore, the fabrication of functional semiconductor nanowire array structure is important. However, single semiconductor material does not simultaneously fulfill all the requirements, say good chemical stability, strong light absorption property, suitable energy band position and easy fabrication of functional nanostructures. To this end, the composite core-shell nanowire heterostructures consisting of two semiconductor materials are proposed in my research to satisfy those requirements for stable and efficient PEC water splitting.

In the first part of my research, a nanowires-on-a-film structure consisting of dense and vertically aligned ZnO nanowires on a ZnO film was synthesized using a-plane sapphire as a substrate and Au nanoparticles as growth catalysts through a one-step chemical vapor deposition (CVD) process. This simple CVD process enables the growth of a dense ZnO nanowire arrays on a ZnO film in the wurtzite phase with a single domain texture and highly oriented along the ZnO c-axis direction over large areas. This result is evidenced from the systematic studies of the crystal texture quality of the ZnO nanowires-on-a-film sample by scanning transmission electron microscopy, selected area electron diffraction technique, X-ray θ-2θ scan, diffraction pole figure and rocking curve analyses.

The epitaxial mechanism of the single-domain texture and hightly c-oriented ZnO nanowire array on a ZnO film structure was further elucidated. A single-crystalline ZnAl2O4 buffer layer was formed between the epi-ZnO film and the a-plane sapphire substrate. The single crystal and well oriented ZnAl2O4 buffer layer (with aligned crystallographic orientations of the Al2O3 [11-20] parallel to the ZnAl2O4 [0-21] and parallel to the ZnO [0001]) provides an improved in-plane symmetry, reduces lattice mismatches and therefore favors the epitaxial growth of the high-quality ZnO nanowire on a ZnO film structure. The elucidated role of the ZnAl2O4 buffer layer in supporting the epitaxial growth of the ZnO nanowires and the ZnO film on the a-plane sapphire may have a profound impact on the growth techniques of single-domain and highly c-oriented ZnO films and nanostructures.

Moreover, the growth mechanism of the ZnO nanowires and the ZnO film is clarified by the CVD study of growing the ZnO nanowires and ZnO film using different amount of Au catalysts. It is evidenced that the length of the fabricated ZnO nanowires largely increases with the increased amount of Au catalysts and thus the one-dimensional ZnO nanowires is very likely grown by a Au-sacrificial VLS growth process. In comparison, the growth of ZnO film is very likely governed by a vapour-solid (VS) process, since the thickness of the film has a "saturated" value of ~ 2 μm when the amount of Au used is more than 1 nm for the CVD growth. With the understanding of the epitaxial and growth mechanism of the ZnO nanowires and ZnO film, the large-scale and homogeneous fabrication of the ZnO nanowires-on-a-flim structure composed of vertically aligned ZnO nanowires having different lengths on a thick ZnO film has been achieved.

The optical and electronic properties of the ZnO nanowire array on a ZnO film structure under the growth condition of 2 nm Au catalyst were characterized by the low-temperature photoluminescence (PL) spectroscopy and electrochemical impedance spectroscopy. Free exciton and bound exciton emissions along with the phonon replica emissions were clearly observed in the low-temperature PL spectrum, indicating a good optical property and a high crystalline quality of the fabricated ZnO nanowire arrays on a ZnO film structure. An n-type semiconductor behaviour and the carrier density of ~ 10(17) cm(-3) were obtained from the electrochemical impedance analysis of the ZnO nanowire array on a film sample.

In the second part of my research, a dense ZnO-ZnGa2O4 core-shell nanowire array on a film structure was synthesized using the fabricated ZnO nanowires-on-a-film samples as the substrates in the second-step CVD process. The process realizes a homogeneous growth of the core-shell nanowires over a large area on the substrate and enables the electrical contact of the core-shell nanowires through their underneath film of the same material.

The ZnO cores and ZnGa2O4 shells of the core-shell nanowires are of single crystal quality and have aligned crystallographic orientations of the ZnO cores [0001] parallel with the ZnGa2O4 shells [111] as evidenced from XRD θ-2θand transmission electron microscopy (TEM) analyses. The ultraviolet-visible diffuse reflectance analyses showed two sharp near-band-edge absorptions of the ZnO cores and the ZnGa2O4 shells. This reconfirms the core-shell nanowire structure. In addition, the electrochemical impedance analyses and photocurrent-voltage scan were used to identify the flatband potential, carrier density and interface band bending of the ZnO-ZnGa2O4 core-shell nanowire array samples. An n-type semiconductor property, a flat-band potential of ~ -0.4 V (versus NHE) and a carrier density of ~ 10(19) cm(-3) were obtained for the ZnO-ZnGa2O4 core-shell nanowires. Compared to the ZnO nanowires, the enhanced cathodic flatband potential of the ZnO-ZnGa2O4 core-shell nanowire array provides a more suitable energy band position for the water splitting chemical reaction. Also, the increased carrier density of the core-shell nanowire array samples improves the conductivity, leading to a possible reduction of the ohmic energy loss in the PEC cell circuit. The amperometric study of the ZnO-ZnGa2O4 core-shell nanowire array sample showed clear on-off current cycles with switching on and off the light illumination. A very low current density less than 10(-4) mA/cm2 was obtained in the dark condition, while a stable and large photocurrent density of 1.2 mA/cm2 was obtained with a bias of 1.23 VRHE under a 300 W Xenon lamp illumination. This stable photocurrent observed under the continuous light illumination reveals that the ZnGa2O4 shells worked as an anti-corrosive outer-layer of the ZnO nanowires and enhanced the stability of the photoanode in contrast to the ZnO or n-doped ZnO nanowires whose PEC performances can be deteriorated under the light illumination.

In the third part of my research, a nitridation process was carried out on the ZnO-ZnGa2O4 core-shell nanowire array on a film structure for the fabrication of ZnO-ZnGaON core-shell nanowire arrays. After the nitridation, the morphology of the dense nanowire array structure was maintained in the SEM observation. The energy dispersive spectrometer analysis examined on the surfaces of the core-shell nanowires revealed the existence of element nitrogen in the sample. The ultraviolet-visible diffuse reflectance analyses showed two absorption shoulders of the core-shell sample. The absorption in the UV region is likely attributed to the Ga and/or N doped ZnO cores and the absorption in the visible region is attributed to the ZnGaON material. A visible light sensitive photocurrent was obtained for the ZnO-ZnGaON core-shell nanowire sample when used as a photoanode in PEC water splitting cell with the on-off cycles of the visible light illumination (λ > 420 nm). The photocurrent of the fabricated ZnO-ZnGaON nanowire photoanode was ~ 20 times larger than the photocurrent obtained with the ZnO-ZnGa2O4 nanowire sample under visible light illumination (λ > 420 nm). Recently, an optimized nitridation was conducted on the ZnO-ZnGa2O4 core-shell nanowire sample, which largely increased the conductivity of the fabricated ZnO-ZnGaON sample as estimated from the two point measurement. A best delivered photocurrent density of ~ 1.7 mA/cm2 was obtained with a nitridated ZnO-ZnGa2O4 nanowire photoanode under one sun light illumination (AM 1.5G solar simulator). The obtained large photocurrent of one nitridated ZnO-ZnGa2O4 nanowire photoanode is likely due to the appropriate nitridation treatment by which the ZnO-ZnGaON nanowire array was formed with a good conductivity. I expect the ZnO-ZnGaON nanowire photoanode fabricated with the optimized nitridation process will contribute efficiently to solar PEC water splitting.

金属酸化物ナノワイヤアレイは,高密度,大表面積の半導体光デバイスの実現に向けて研究がなされている.例えば,発光素子や光検出器,レーザ,太陽電池,センサ類である.ナノワイヤアレイをデバイスに組み込む際,各々のナノワイヤを電気的に接続する技術が必要となる.これまでに,金属膜や半導体膜を用いたナノワイヤアレイ膜によって,ナノワイヤアレイの電気的接続を実現する方法が研究されているが,異種材料の接合による障壁効果のため,ナノワイヤアレイ膜の光電特性に課題がある.本論文において,鐘苗君は単一の作製ステップで,高品質な単結晶ZnOナノワイヤアレイ膜を作製する方法について述べている.さらに,単結晶ZnOナノワイヤアレイ膜を,光触媒的水分解による水素発生に用いる光アノードに応用している.

第1章は,「General introduction(序論)」であり,垂直配向ナノワイヤアレイ膜の光触媒的水分解応用について述べている.金属酸化物ナノワイヤアレイ膜の材料として用いられるTiO2,αiFe2O3,ZnOについてまとめ,本研究でナノワイヤアレイ膜の材料にZnOを選んだ理由を述べている.また,ナノワイヤアレイ膜の作成における主要な課題のひとつを,単一材料から成る単結晶構造を持つナノワイヤアレイ膜の実現であるとしている.

第2章は,「Epitaxial growth of ZnO nanowire array on a film structure in a single crystal domain quality(垂直配向性ZnOナノワイヤアレイ膜のエピタキシャル成長)」であり,配向性ナノワイヤアレイ膜の作成プロセスを述べている.サファイア基板上におけるZnOのエピタキシャル成長により,垂直配向性ZnOナノワイヤアレイの試作に成功した.ZnO膜とサファイア基板との境界における固相拡散反応によって中間層が形成される.この中間層が,ZnOナノワイヤの垂直配向性エピタキシャル成長を可能にする.さらに,金触媒の量を調整することで,高密度垂直配向性ZnOナノワイヤアレイ膜を実現した.試作したナノワイヤアレイ膜を複数の方法により分析し,ナノワイヤアレイ膜が単一材料からなる単結晶であることを示した.

第3章は,「Optical and electrical properties of the single-domain ZnO nanowire array on a film structure(単結晶ZnOナノワイヤアレイ膜の光学的および電気的特性)」であり,試作したナノワイヤアレイ膜の光学的および電気的特性について述べている.低温フォトルミネッセンス分光法により得られた自由励起子発光から,試作したナノワイヤアレイ膜の単結晶構造が高品質であることを示した.また,電気化学インピーダンス分光法により,試作したナノワイヤアレイ膜の表面特性が,光触媒的水分解による水素発生に適していることを示した.

第4章は,「Dense ZnO-ZnGa2O4 core-shell nanowire array on a film structure from synthesis to optical properties(高密度ZnO-ZnGa2O4コア-シェル型ナノワイヤアレイ膜の光学的特性)」であり,高密度ZnO-ZnGa2O4コア-シェル型ナノワイヤアレイ膜を試作し,光学的特性を分析した.試作したZnO-ZnGa2O4ナノワイヤアレイ膜が,垂直配向性かつ高品質な単結晶構造を有することを示している.

第5章は,「Dense ZnO-ZnGa2O4 core-shell nanowire array on a film for stable and efficient photoelectrochemical water splitting(高密度ZnO-ZnGa2O4ナノワイヤアレイ膜の安定的高効率光電気化学水分解応用)」であり,ZnO-ZnGa2O4ナノワイヤアレイ膜の水分解用光電気化学セルにおける光アノード応用について述べられている.ZnOナノワイヤがZnGa2O4で覆われたコア-シェル構造によりZnOナノワイヤを光腐食から保護した.試作したコア-シェル型ナノワイヤアレイ膜により,紫外線領域において,安定した光触媒的水分解を実現した.

第6章は,「Dense ZnO-ZnGaON nanowire array of a film structure for visible-light photoelectrochemical water splitting(高密度ZnO-ZnGaONナノワイヤアレイ膜の光電気化学水分解応用)」であり,コア-シェル型ナノワイヤアレイ膜を用いた可視光領域における光触媒的水分解について述べている.シェル構造の窒化により可視光の吸収を可能にした.試作した窒化コア-シェル型ナノワイヤアレイ膜により,吸収波長帯を可視光領域に拡張し,高効率な光触媒的水分解反応を実現した.

第7章は,「General Conclusion(結論)」であり,本論文全体の結論を述べている.

以上,本論文では,高品質な単結晶ZnOナノワイヤアレイ膜を光アノードに用いた高効率な太陽光水分解を提案・実証している.サファイア基板との固相拡散反応で形成される中間層によるZnOのエピタキシャル成長と,金触媒成長によるZnOナノワイヤおよびZnO膜の成長制御により,効率的な光吸収,キャリア輸送を可能とする光アノードの作成に成功している.ZnGaOシェルの形成による対光腐食性と,光アノード構造の窒化による可視光吸収性により得られた,高密度ZnO-ZnGaONコア-シェル型ナノワイヤアレイ膜を光アノードとした光触媒的水分解性能は非常に高く,太陽光模擬(AM1.5G)の条件下において,最高で1.7 mA/cm2の電流密度を記録している.この電流密度は,これまでに報告されている最大の電流密度2.2 mA/cm2に匹敵する値である.

よって,本論文は博士 (工学) の学位請求論文として合格と認められる.