PdO/TiO2半導體氧化物異質接合結構之有機分子光催化分解研究

Abstract

我們利用反應性濺鍍的方式沉積氧化鈀片狀結構(PdO nanoflake)於分散在二氧化矽基板上的二氧化鈦(TiO2, Degussa P25)奈米顆粒上，並研究PdO/TiO2 type -II異質接合的複合光觸媒，在水溶液中光催化分解亞甲基藍(Methylene blue, MB)。在全波段光照下，PdO/TiO2複合光觸媒表現出比純P25光觸媒好兩倍的光催化效率，這樣的結果來自於兩半導體間異質接合形成的能障(Energy barrier)，造成光激發載子能夠快速的被分離，而MB光催化分解的過程也被提出。在只有可見光照射下雖然可激發PdO，但PdO/TiO2複合光觸媒未能表現出明顯的光催化反應，我們認為原因是不平衡的載子濃度，造成次費米能階(Quasi-Fermi level)的位置無法滿足光降解MB時起始反應的氧化還原電位。這樣的PdO/TiO2複合光觸媒也能在多次的光催化反應後，表現出極佳的化學穩定性。

We prepared PdO nanoflakes by reactive sputter deposition to form a type-II heterostructure with TiO2 nanoparticles (Degussa, P25) on the SiO2 substrate for the study of photocatalytic decomposition of methylene blue (MB) molecules. The PdO/TiO2 heterostructure has a photocatalytic activity toward MB decomposition under white light illumination as larege as twice that of bare TiO2 nanoparticles. The enhancement of the photocatlaytic activity is ascribed to the fast photogenerated carrier separation resulting from the potential barrier formed at the heterojunction between the two semiconductor oxides. A photocatalytic reaction mechanism for the MB decomposition is proposed. The PdO/TiO2 heterostructure exhibits little photocatalytic activity toward the MB decomposition under visible light illumination even thoung PdO can be photoexcited by visible light. We ascribe the photocatalytic inactivity to the mismatch between quasi-Fermi levels of nonequlibrium photogenerated carriers in the heterostructure and the redox potential of primary surface reactions leading to the MB decompostion. This composite material also shows chemical stability after 10 cycle photocatalysis reactions.