Insight into the Growth Mechanism and Photocatalytic Behavior of Tubular Hierarchical ZnO Structures: An Integrated Experimental and Theoretical Approach

The hydrothermal synthesis of zinc oxide (ZnO) particles from zinc acetylacetonate monohydrate in “pure” aqueous solution and in aqueous NaOH solution at 90 °C is reported. The structural and morphological properties of ZnO particles were investigated by powder X-ray diffraction, X-ray absorption spectroscopy (XAS), field emission scanning electron microscopy, and transmission electron microscopy. The effect of NaOH on the growth mechanism and photocatalytic performance of hierarchical ZnO structures was investigated. The experimental findings, supported by results of quantum chemical calculations at the level of density functional theory, were used to propose the mechanism of nucleation and preferential growth of finely tuned hollow and nonhollow ZnO structures and their effects on the photocatalytic activity. The calculations indicate that the process of ZnO nucleation in “pure” aqueous solution mainly proceeds by the reaction of small monomers, while tetramers play a crucial role in aqueous NaOH solution. Both the preferred ZnO nanostructure and microstructure growth processes are driven by O–H···O hydrogen bonds as controlling elements. The calculated values of the EO···H interaction indicate a stronger interaction via O–H···O hydrogen bonds in “pure” aqueous media (EO···H = −11.73 kcal mol–1) compared to those obtained in aqueous NaOH solution (EO···H = −8.41 kcal mol–1). The specific structural motif of the (ZnO–H2O)12 dodecamers with calculated negative ΔG*INT free release energy indicates that the formation of anisotropic nanocrystalline ZnO with the c-axis as the primary growth direction is spontaneous and accelerated exclusively in “pure” aqueous solution, whereas it is an unfavorable endergonic process in aqueous NaOH solution (ΔG*INT > 0). Efforts have been made to determine the photocatalytic efficiency of the ZnO samples based on the XAS measurements. ZnO particles obtained in “pure” aqueous solution show the highest photocatalytic activity due to the presence of a larger amount of oxygen vacancies.

本文报道了以一水合乙酰丙酮锌为前驱体，分别在"纯"水溶液与氢氧化钠水溶液中，于90℃下水热合成氧化锌（ZnO）颗粒的研究。采用粉末X射线衍射、X射线吸收光谱（X-ray absorption spectroscopy, XAS）、场发射扫描电子显微镜以及透射电子显微镜对所得氧化锌颗粒的结构与形貌特性进行了表征。研究了氢氧化钠对分级结构氧化锌生长机制与光催化性能的影响。结合密度泛函理论水平下的量子化学计算结果对实验数据进行佐证，本文提出了精准调控的空心与实心氧化锌结构的成核及择优生长机制，并阐明了该类结构对光催化活性的影响。计算结果表明，"纯"水溶液中氧化锌的成核过程主要通过小分子单体的反应进行，而氢氧化钠水溶液中四聚体则发挥关键作用。氧化锌纳米结构与微观结构的择优生长过程均以O–H···O氢键作为调控因素。EO···H相互作用的计算值显示，"纯"水介质中通过O–H···O氢键形成的相互作用更强（EO···H = -11.73 千卡·摩尔⁻¹），显著高于氢氧化钠水溶液中的对应值（EO···H = -8.41 千卡·摩尔⁻¹）。(ZnO–H₂O)₁₂十二聚体的特定结构基元具有计算得到的负ΔG*INT释放自由能，这表明以c轴为主要生长方向的各向异性纳米晶氧化锌仅能在"纯"水溶液中自发形成并加速生长；而在氢氧化钠水溶液中，该过程为不利的吸能过程（ΔG*INT > 0）。本文还基于XAS测试结果测定了各氧化锌样品的光催化效率。在"纯"水溶液中合成的氧化锌颗粒因含有更多氧空位，展现出最优的光催化活性。