Growth and Photoelectrical Properties of Reactive Sputtered PdO Nanoflakes

• Main Authors: Huang, Chien-Jung, 黃建融
• Other Authors: Pan, Fu-Ming
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/17544160656816341672

博士 === 國立交通大學 === 材料科學與工程學系 === 99 === In the thesis, we deposited palladium oxide nanoflake thin films by reactive sputter deposition (RSD) and various properties were studied including electron field emission, photoconduction and photocatalysis. The PdO thin film RSD-deposited on the Pt substrate had a flake-like morphology. The nanosized flake had a core-shell structure with a single Pd grain encapsulated by a crystalline PdO surface layer. The formation of the PdO capped nanoflakes was a result of a large interfacial stress built up between PdO and Pt due to lattice mismatch. Field emission characteristics of the nanoflakes were studied and a field enhancement factor of 791 was obtained. PdO nanoflakes were also deposited on a SiO2 substrate by reactive sputter deposition and photoconduction characteristics of the thin films were examined. PdO thin films deposited at 25 oC is composed of bowed nanoflakes standing on the SiO2 substrate, which have a single-crystalline structure after thermal anneal at 400 oC. The 400 oC-annealed nanoflake thin film has a bandgap energy in the red-light range (~2.06 eV), and exhibits a very sensitive photoresponse upon UV (365 nm) illumination. The high photoresponse sensitivity of the 400 oC-annealed nanoflake thin film is ascribed to a lower density of recombination centers and traps due to an excellent crystallinity and a high carrier extraction efficiency due to a low electrical resistivity. A slight decrease in the photocurrent density during the initial stage of the UV illumination is attributed to adsorption of O2- anions on the 400oC-annealed nanoflakes. We deposited PdO nanoflakes by RSD on TiO2 nanoparticles dispered 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 UV-vis 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.