Zinc oxide (ZnO) nanostructures have been studied extensively in the past 20 years due to their novel electronic, photonic, mechanical and electrochemical properties. Recently, more attention has been paid to assemble nanoscale building blocks into three-dimensional (3D) complex hierarchical structures, which not only inherit the excellent properties of the single building blocks but also provide potential applications in the bottom-up fabrication of functional devices. This review article focuses on 3D ZnO hierarchical nanostructures, and summarizes major advances in the solution phase synthesis, applications in environment, and electrical/electrochemical devices. We present the principles and growth mechanisms of ZnO nanostructures via different solution methods, with an emphasis on rational control of the morphology and assembly. We then discuss the applications of 3D ZnO hierarchical nanostructures in photocatalysis, field emission, electrochemical sensor, and lithium ion batteries. Throughout the discussion, the relationship between the device performance and the microstructures of 3D ZnO hierarchical nanostructures will be highlighted. This review concludes with a personal perspective on the current challenges and future research.

ZnO 3D hierarchical structures assembled by different low-dimensional building blocks (nanosheet, nanoneedle) have been successfully synthesized at room temperature without the addition of organic agents. The morphology of the hierarchical structures can be well controlled by simply changing the concentration of the reactant. On the basis of the experimental results, a possible mechanism for the formation of the ZnO 3D hierarchical structures is proposed. The room temperature photoluminescence spectra of the ZnO 3D hierarchical structures show that the relative intensity of UV emission increases from nanosheets to the thin needle flowers and can be tuned by controlling the morphology. Importantly, the thin needle flowers sample exhibits an enhanced photocatalytic performance as compared with the other ZnO nanostructures due to structural features and good optical quality.

Au/TiO₂ composites were synthesized by a two-step hydrothermal approach and their photocatalytic and electrochemical properties were investigated. It is found that Au/TiO₂ composites with a large specific surface area, stability and catalytic activity of Au nanoparticles (NPs) demonstrate enhanced photocatalytic activity compared to pure TiO₂ microspheres. Furthermore, the electrochemical properties of the Au/TiO₂ composite electrode demonstrate its ability to electrocatalyze the oxidation of hydrogen peroxide and substantially increase the response current. In addition, this modified electrode also exhibits a very rapid and sensitive response towards glucose detection at the low operating potential. The modified electrode shows a linear response from 1.0 to 15 mM with a reproducible sensitivity of 1350 μA mM⁻¹ cm⁻² within a response time of less than 6 s. The photocatalytic and electrochemical activity of Au/TiO₂ composites provides a new platform for environmental remediation and biomedical applications. All these results reveal that Au/TiO₂ composites exhibit great prospects for developing efficient non-enzymatic biosensors.

3D anatase TiO₂ hollow microspheres assembled with high-energy {001} facets have been synthesized by a one-pot solution method and their lithium storage capacity are investigated. The structural and compositional analysis of the mesoporous TiO₂ product has been performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) along with selected area electron diffraction (SAED). The Bruauer–Emmett–Teller (BET) specific surface area has been calculated by the nitrogen isotherm curve and pore size distribution of TiO₂ microspheres has been determined by the Barret–Joyner–Halenda (BJH) method. It has been found that the as-prepared TiO₂ electrodes delivered high capacity, good cycling stability and rate capability. The improved electrochemical performance is attributed to the hollow nature and exposed high-energy {001} facets of the 3D assembled structure. Therefore, such a structure can be considered to be an attractive candidate as an electrode material for LIBs.