酸碱度调控氧化锌纳米材料形貌及其光催化还原CO2研究

利用水热法合成了形貌可控的氧化锌(ZnO)微纳材料。利用X射线衍射(XRD),扫描电子显微镜(SEM),透射电子显微镜(TEM),紫外-可见(UV-Vis)分光光谱和比表面积分析(BET)等技术对所制备的ZnO材料进行了表征。不同酸碱度(pH值)条件下,经过500 ℃退火2 h制备的ZnO均为纤锌矿结构。随着前驱液pH值的增加,所得ZnO从片状晶体变为棒状晶体。片状ZnO主要暴露极性晶面,棒状ZnO主要暴露非极性晶面。从生长角度考虑,在溶液为弱酸性条件下(pH 6.5),溶液中游离的氯离子(Cl^-)抑制了ZnO在锌极性面({Zn²⁺}_{crystal surface})的生长,水热反应产物为片状Zn₅(OH)₈Cl₂·H₂O,退火后得到微孔片状ZnO;当溶液中添加氢氧根(OH^-)后,锌离子(Zn²⁺)被络合为四羟基锌络合离子(Zn(OH)₄^2-),该络离子促进了ZnO在{Zn²⁺}_{crystal surface}的生长,从而得到棒状晶体。利用上述催化剂,在氙灯照射下进行光催化还原二氧化碳实验,发现极性面较多的片状ZnO具有更高的光催化性能。

Controllable Modulation of Morphology and Photocatalytic Performance of ZnO Nanomaterials via pH Adjustment

ZnO microstructures and nanostructures with controlled-morphology were synthesized by the hydrothermal method. All samples were prepared using precursors at different pH values and then annealed at 500 ℃ for 2 h. The samples were characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-Vis), and BET specific surface area measurement. All samples were confirmed by XRD to be wurtzite ZnO. As the pH value of the precursor increased, sheet-like ZnO disappeared and rod-like ZnO was produced. The major surfaces of sheet-like and rod-like ZnO were polar and nonpolar crystal faces, respectively. At pH 6.5, Cl^- was adsorbed onto the (002) polar face and inhibited the growth along the polar crystal face ({Zn²⁺}_{crystal surface}). A microporous sheet ZnO was formed by annealing the obtained sheet-like Zn₅(OH)₈Cl₂·H₂O. When OH^- was added into the precursor, Zn(OH)₄^2- was generated via coordination with Zn²⁺, which was adsorbed onto the (002) polar face and promoted growth along the polar crystal face. Rod-like ZnO was thus produced. The obtained ZnO could photocatalytically reduce CO₂ under illumination. Sheet-like ZnO exhibited better photocatalytic performance than rod-like ZnO. This may be because the polar crystal face shows better photocatalytic activity than the unpolar crystal face.