氧化鈀奈米薄片之成長機制與光電特性之研究

Abstract

於本論文中，我們利用反應性濺鍍法成長氧化鈀薄膜並且研究期成長機制與光電特性，包含場發射、光感應與光催化反應。當氧化鈀成長於白金基板上時，此氧化鈀具有片狀之形貌。此奈米薄片具有核殼結構，其結構為一層氧化鈀包覆一顆單晶鈀。我們將此被氧化鈀層包覆之奈米薄片的成因歸咎於氧化鈀與白金基材之間晶格不匹配產生之介面壓應力所形成。測試此奈米薄片之場發射特性並發現其場強因子約為791。氧化鈀薄片亦成長於二氧化矽基板上並研究其光感應特性。於室溫下所成長之氧化鈀薄膜亦具有奈米薄片結構並垂直站立於二氧化矽基板上。於400 oC下退火後，此奈米薄片具有單晶結構。於400 oC下退火後之氧化鈀薄膜之能隙坐落於紅光區(約2.06 eV)，並且於紫外光(365 nm)照射下展現極佳之光敏感度。造成此高之光敏感度乃由於此薄膜具有極佳之結晶性，因此減少電子電洞對覆合中心。此外，此薄膜亦具有較低電阻，因此具有較高之載子傳輸效率。於紫外光起始照射時，電流密度有些許下降之趨勢，此為O2-離子吸附於此400 oC退火後之氧化鈀薄片上所造成。 我們亦製備了PdO/TiO2異質結構以探討其光催化分解亞甲基藍特性。於紫外光-可見光照射下，相較於純TiO2奈米球，PdO/TiO2異質結構對於亞甲基藍具有兩倍之光觸媒活性。此提高之光觸媒活性乃歸功於此兩半導體氧化物異質接合所形成之位能障。此對於亞甲基藍分解之光觸媒反應機制亦於本文中探討。於可見光照射下，雖然PdO仍可被激發，但此PdO/TiO2異質結構對於亞甲基藍僅有微弱之觸媒活性。我們將其歸因於此異質結構中非平衡之光激發載子的次費米能階不匹配所造成。而表面分解亞甲基藍反應的氧化還原電位不匹配亦是其因素之一。

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.