以簡易綠色的反溶劑程序製備具可見光活性的ZnO奈米晶體

Abstract

因擁有特殊的光電特性和優異的物理及化學性質，ZnO奈米晶體被廣泛應用在許多領域，然而，ZnO的能隙過大，約3.35電子伏特，無法有效捕捉太陽光能量的缺點，進一步限制了它在光電轉換上的應用，因此，如何使ZnO奈米晶體吸收可見光是為其能否更全面發展的重要關鍵。此論文研究提出一個對環境友善的反溶劑程序，來製備過渡金屬離子摻雜的ZnO奈米晶體，合成方法為藉由一種稱為深共熔溶劑(deep eutectic solvent，DES)的室溫離子液體來溶解ZnO粉末，將此溶有ZnO的DES注入對ZnO不具溶解度的非良溶劑中（例如水）時，ZnO會因溶解度的急劇下降而析出成長，若在非良溶劑中事先置入金屬離子，例如Cu2+、Ni2+ 與Co2+，ZnO因反溶劑程序析出而成長的同時，會與事先置入的金屬離子摻合，進而成功獲得具金屬離子摻雜的ZnO奈米晶體。由於所摻雜的金屬離子能在ZnO能隙中產生新的缺陷能階來作為電子躍遷的跳板，此金屬離子摻雜的ZnO樣品能夠有效吸收可見光而展現光催化活性，然而，此缺陷能階亦會在載子傳遞過程中捕捉電子，進而降低樣品的光電轉換效率。光電化學水分解實驗結果顯示，Cu2+摻雜ZnO樣品的光活性在Cu2+濃度為2.0 at%時達到最高，過量的Cu2+摻雜會因大量電子被捕捉於缺陷能階中而降低整體光催化表現。此外，由於Cu2+與ZnO之間的顯著sp-d的相互作用，Cu2+摻雜的ZnO樣品在室溫下表現出明顯的磁滯現象，其飽和磁化量隨著Cu2+濃度的增加而增大。

With the particular optoelectronic characteristics and excellent physicochemical properties, ZnO nanocrystals have been applied to extensive fields. However, the large band gap of ZnO (~3.35eV) prohibits it from effective light absorption under sunlight irradiation, which further limits its applicability in relevant photoelectric processes. To render visible light absorption thus becomes an essential task for the further advancement of ZnO. In this work, an environmentally benign antisolvent method has been developed to prepare transition metal ion-doped ZnO nanocrystals. A room-temperature ionic liquid, known as deep eutectic solvent (DES), was used as the solvent to dissolve ZnO powders. Upon the introduction of ZnO-containing DES into a bad solvent which shows no solvation to ZnO (e.g. water), ZnO was precipitated and grown due to the dramatic decrease of solubility. By adding transition metal ions such as Cu2+, Ni2+ and Co2+ in the bad solvent, the growth of ZnO in antisolvent process was accompanied by metal ion doping, resulting in the formation of metal ion-doped ZnO nanocrystals. The thus-obtained metal ion-doped ZnO showed additional absorption band in visible range (400-800 nm), attributable to the doped ions which invoke the interband transitions within the energy gap of ZnO to enable photoresponse to visible light. The doped ions may generate dopant states to trap charge carriers in the charge transfer process, leading to a depressed photoconversion efficiency for ZnO. Photoelectrochemical water splitting experiments showed that the photoactivity of the Cu2+-doped ZnO achieved the highest at the Cu2+ concentration of 2.0 at%, above which the photocatalytic performance was declined as a result of the significant charge carrier trapping at the dopant states. Furthermore, owing to the significant sp-d interaction between Cu2+ and ZnO, the Cu2+-doped ZnO samples exhibited obvious hysteresis loop at 300K with the saturated magnetization increasing with the increase of Cu2+ concentration.