利用一步驟陽離子交換法製備奈米棒異質結構

Abstract

半導體奈米材料在受光照射後，其所激發出之電子電洞對，若能導出於材料便可被廣泛應用於多種光催化反應中，例如分解環境中的污染物與水分解產生氫氣等。由兩種不同半導體材料所組成之異質結構，若其相對能帶結構呈現合適配對時，經照光後所產生的電子電洞能有效被轉移與分隔在兩種材料端，如此可降低電子電洞直接複合的機率，因此能大幅增進整體結構運用於光催化系統中之效能。本論文發現在陽離子交換法中以Ag2Se奈米棒作為模板製備ZnSe奈米棒時，如果額外加入適量的H2O，會伴隨著ZnO顆粒的沉積，進而形成具ZnO顆粒修飾的ZnSe奈米棒。由於相對能帶結構之關係，經照光後所產生之電子會轉移至ZnO端，而電洞則留在ZnSe端，使ZnSe–ZnO奈米棒展現優異之載子分離效果。相較於純ZnSe奈米棒與相關商用品光觸媒，此ZnSe–ZnO奈米棒異質結構展現較佳之光催化分解染料效能，此乃因ZnSe/ZnO異質界面能有效促進材料內部之電子電洞分離效率所致。本論文使用時間解析螢光光譜技術，來對此異質結構中電子由ZnSe轉移至ZnO端之速率常數作定量量測，結果顯示當加入的H2O含量為0.8 vol%時，所沉積的ZnO顆粒具有合適數量，故樣品展現最高的電子轉移速率常數分離效率；當H2O的添加量超越此最佳值時，會有過多的ZnO顆粒沉積於ZnSe奈米棒表面，此時轉而顯著的載子再複合會消耗載子數量，進而導致電子轉移速率常數降低，此結果與其應用於光催化分解染料時的速率常數變化趨勢一致，亦即由0.8 vol% H2O所製備之ZnSe–ZnO樣品具有最高的光催化活性。另一方面，本論文亦開發一個以L-Cysteine當作還原劑的陽離子置入程序，以Se奈米棒作為模板，成功獲得各種在一般製程下難以合成的硒化物奈米棒，包含CuSe與Bi2Se3。

The charge carriers generated from irradiated semiconductors are widely utilized in many photocatalytic reactions such as the elimination of environmental pollutants and hydrogen production from water splitting. To effectively carry out photocatalytic reactions, the band structure of semiconductors must be modulated to afford favorable charge transfer and achieve spatial charge separation. In this work, a novel one-step cation exchange method has been developed to prepare ZnO-decorated ZnSe nanorods (ZnSe–ZnO NRs). In a typical procedure, ZnSe NRs were prepared by conducting cation exchange reaction on Ag2Se NRs with Zn2+. With extra addition of H2O in the cation exchange process, formation of ZnSe nanorods would be accompanied by the deposition of ZnO nanocrystals, resulting in the formation of ZnSe–ZnO NRs. Through modulating the concentration of H2O added, the amount of ZnO nanocrystals decorated on ZnSe NRs could be readily controlled. Due to the staggered band offset, the photoexcited electrons of ZnSe would preferentially transfer to ZnO, simultaneously leaving photogenerated holes at ZnSe to achieve pronounced charge carrier separation. Time-resolved photoluminescence spectroscopy was used to quantitatively analyze the electron transfer from ZnSe to ZnO for the as-prepared ZnSe–ZnO NRs. A highest electron-transfer rate constant was observed for ZnSe–ZnO NRs with 0.8 vol% H2O, above which a reduced electron-transfer rate constant was recorded as a result of the consumption of charge carriers from the later-emerging electron-hole recombination process. The carrier dynamics results were fundamental consistent with those of performance evaluation in photocatalytic dye degradation, in which ZnSe–ZnO NRs with 0.8 vol% H2O exhibited the highest photocatalytic activity. On the other hand, by using L-cysteine as the reducing agent and Se nanorods as the growth template, we have developed an ion-insertion process to prepare a variety of functional selenide NRs that are difficult to be obtained from the typical methods, including CuSe and Bi2Se3.