鈦酸鍶-鈮酸銀固溶體層狀薄膜結構之光吸收與光電流表現

The Optical Absorption and Photocurrent Response of Layered Structures Based on SrTiO3-AgNbO3 Solid Solution Thin Films

林易生(Yi-Sheng Lin)

指導教授 ： 謝宗霖

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.10433

摘要

本文探討鈣鈦礦氧化物薄膜AgNbO3、SrTiO¬¬3及其固溶體(SrTiO3)x(AgNbO3)1-x (x =0.25、0.5、0.75) 作為異相反應光觸媒(heterogeneous photocatalyst)，應用於水分解(water splitting)產氫之表現，所使用的製程為溶膠-凝膠法(sol-gel process)以及旋轉塗布法(spin-coating method)，並透過三極式電化學裝置所量測之光電流值來觀察水分解效率。
本研究著眼於(SrTiO3)x(AgNbO3)1-x薄膜在不同固溶比例下，其能帶結構的變化，並利用此變化為基礎進行不同的光電極設計，有效提升水分解效率。實驗結果顯示，固溶體薄膜之能隙值介於AgNbO3 (Eg = 2.8 eV)及SrTiO3 (Eg = 3.28 eV)兩純薄膜之間，且其吸收光譜(UV-Vis spectroscopy)隨著x值的增加呈現藍移的趨勢；另一方面，透過紫外光電子能譜儀(ultraviolet photoelectron spectroscopy)的觀測，此一系列固溶體薄膜之費米能階(Fermi level)與價帶頂端(the top of valence band)之電位差值，亦會隨x值的增加而有減少的變化。
鑑於固溶比例可調控能帶結構此一特點，本研究進一步利用此結果，將不同成分的(SrTiO3)x(AgNbO3)1-x薄膜進行層狀堆疊，觀察異質接面(heterojunction)對光電流表現的影響。重大實驗結果有二：其一，SrTiO3/AgNbO3層狀結構(將SrTiO3堆疊於AgNbO3之上)的光電流密度優於單一成份的SrTiO3與AgNbO3薄膜；其二，SrTiO3/(SrTiO3)0.75(AgNbO3)0.25/AgNbO3與SrTiO3/(SrTiO3)0.5(AgNbO3)0.5/AgNbO3之光電流密度較SrTiO3/AgNbO3更為出色。此二成果指引設計優秀光觸媒材料所需之可行取徑：首先，異質接面所形成的內建電場，可幫助光致激發電子電洞對快速分離，降低再複合的機率；再者，將不同能隙值材料由寬至窄能隙做循序漸進之堆疊，將能有效利用光能，提升光致載子進行水分解反應的可能；最後，利用固溶體晶體結構的過渡性以及能帶結構的可調控性，將能作為純物質結構(SrTiO3/AgNbO3)之匹配層，優化層狀結構的品質，降低光致載子受阻之機率。

關鍵字

SrTiO3 ； AgNbO3 ； (SrTiO3)x(AgNbO3)1-x ； 水分解 ； 光電流 ； 異質接面

並列摘要

In this study, we chose perovskite oxide thin films — (SrTiO3)x(AgNbO3)1-x (x = 0; 0.25; 0.5; 0.75; 1) to discuss the application of heterogeneous photocatalyst in the field of water splitting for hydrogen harvesting. The sol-gel method and spin-coating process were used to synthesize the thin films, and “photocurrent density” was measured by a three-electrode electrochemical cell to observe the efficiency of water splitting.
The first part of the thesis was focused on the band structure evolution of (SrTiO3)x(AgNbO3)1-x. It was found that the band-gap energy of the solid solution was between AgNbO3 and SrTiO3, and exhibited average blue shift with the increase of the value x. Furthermore, by means of UPS analysis, the difference in potential between Fermi level and the top of valence band would decrease with the x value enhancement.
Secondly, we investigated the effect of “heterojunction” on photocurrent responses through multiple layered-structure arrangements. There were two essential results: first of all, the photocurrent density of SrTiO3/AgNbO3 was superior to the pure components; second, SrTiO3/(SrTiO3)x(AgNbO3)1-x/AgNbO3 (x = 0.75, 0.5) performed higher photocurrent density than SrTiO3/AgNbO3. The results demonstrated several ideas of photoelectrode design for the course of outstanding photocurrent performance: initially, the “build-in electrical field” formed at the interface of different components is able to restrain recombination of photo-induced carriers; further, stacking materials of different band-gap energy can enhance light absorbance; last but not least, solid solution can be an intermediate matching layer to improve interface quality, which has consequential influence on conductivity of photo-induced carriers.

並列關鍵字

SrTiO3 ； AgNbO3 ； (SrTiO3)x(AgNbO3)1-x ； water splitting ； photocurrent ； heterojunction

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