Electrochemical Crystal Growth and Physical Properties of Titanium Oxides and Oxyfluorides

神奈川大学博士（工学）Thesis or Dissertation本博士論文では，さまざまなチタン酸化物・酸フッ化物の単結晶試料を定電圧下における高温容融塩電解により育成し，得られた結晶の結晶構造，電子特性，磁気特性，光学特性および水素生成光触媒活性を調査した．本手法によって得られた結晶は他の手法で育成することは国難であり，結晶にドープされたd電子が化合物の物性に大きく影響することを明らかにしている．本論文は6章構成をとり，第1章は総説，第2-5章は各論，第6章は全体総括となっている．以下，第2-5章の内容について簡潔に記す．なお，各項目に付されている数字は基になった発表論文の番号と対応している．In this dissertation, single crystals of various titanium oxides and oxyfluorides were grown employing high-temperature constant-voltage electrolysis of TiO2 with A2MoO4(A = Li, Na, K, Cs)-based melts. Structural, electronic, magnetic, and optical properties, and photocatalytic H2 evolution activities of the resultant crystals were extensively investigated. The described method is highly effective in obtaining electron-doped titanium oxides and oxyfluorides with precisely controlled chemical compositions and crystal structures. Throughout the dissertation, the author emphasizes that crystals obtained herein are never obtained by any other techniques, and the doped d electrons significantly impact the properties of the materials. Chapter 1 describes a brief overview of titanium oxides, mixed-anion compounds, and crystal growth techniques. The objectives of the dissertation are also mentioned in this chapter. In Chapter 2, needle-like crystals of hollandite-type CsxTi8O16 were successfully obtained by constant-voltage electrolysis of TiO2 with the Cs2MoO4 melt. Depending on the applied voltages, the resultant crystals exhibit distinct properties, either electrical insulators with optical transparency or semiconductors with metallic luster. Chapter 3 deals with the crystal growth of titanium oxides containing lithium, sodium, and potassium by using the A2MoO4(A = Li, Na, K) melt. Crystals of KxTi8O16(hollanditetype),Na2+xTi6O13, and Li2+xTi3O7 (ramsdellite-type) are successfully grown. These compounds unexceptionally crystallize in one-dimensional tunnel structures with various tunnel shapes depending on the alkali metals incorporated. In Chapter 4, the crystal growth of titanium oxyfluorides is attempted by using the eutectic Li2MoO4-LiF melt. Rocksalt-type Li2Ti(O,F)3 (F/Ti ~ 0.15) crystals are obtained by precisely tuning the applied voltage. The crystals were found to produce H2 more efficiently from aqueous methanol under UV light than a nondoped oxyfluoride Li5Ti2O6F reference. Chapter 5 reports the successful growth of Na/Mo/F-codope CaTiO3 crystals by using the Na2MoO4-NaF-CaMoO4 melt. The dopant concentration is controlled by the applied voltage values. Finally, the concluding remarks and suggestions for future works are given in Chapter 6.doctoral thesi