Semiconductor photocatalysis provides potential solutions for many energy and environmental-related issues. Recently, various semiconductors with hierarchical nanostructures have been fabricated to achieve efficient photocatalysts owing to their multiple advantages, such as high surface area, porous structures, as well as enhanced light harvesting. ZnO has been widely investigated and considered as the most promising alternative photocatalyst to TiO₂. Herein, we present a review on the fabrication methods, growth mechanisms and photocatalytic applications of hierarchical ZnO nanostructures. Various synthetic strategies and growth mechanisms, including multistep sequential growth routes, template-based synthesis, template-free self-organization and precursor or self-templating strategies, are highlighted. In addition, the fabrication of multicomponent ZnO-based nanocomposites with hierarchical structures is also included. Finally, the application of hierarchical ZnO nanostructures and nanocomposites in typical photocatalytic reactions, such as pollutant degradation and H₂ evolution, is reviewed.

A facile method was developed to synthesize γ‐MnO₂ with different structures and surface properties in this paper. γ‐MnO₂ was prepared by oxidation of MnSO₄ with Na₂S₂O₈ at 90^°C for 2.0 h. γ‐MnO₂ formed at initial pH 1.0 (M1) and 8.5 (M2) was composed of MnO_1.93 · 0.23H₂O and MnO_1.96 · 0.18H₂O, respectively. The higher ratio of pyrolusite in M2 (P_r = 43.90%) than that in M1 (P_r = 24.86%) indicated that compared with M1, M2 would absorb more protons since the planar oxygen atoms in pyrolusite were incompletely coordinated and liable to absorb the protons. Meanwhile, the higher oxidation valence of Mn in M2 than that in M1 revealed that the Mn atoms in M2 were more liable to draw the electron density from the surface oxygen atoms in hydroxyl groups. The structural and compositional differences between M1 and M2 were the major reasons why M2 possessed a higher surface potential and a weaker ability to absorb Zn²⁺ ions.

In this paper, the influence of alkyl trimethyl ammonium bromides (C_nH_2n+1(CH₃)₃NBr, n = 10, 12, 14, 16, 18, abbreviated as ATAB) on the formation of alpha calcium sulfate hemihydrate (α-CaSO₄·0.5H₂O) whiskers under a hydrothermal condition (135 °C, 3.0 h) was analyzed. Specifically, it focuses on cetyl trimethyl ammonium bromide (C₁₆H₃₃(CH₃)₃NBr, abbreviated as CTAB). The rising CTAB concentration from 0 to 9.2 × 10⁻⁴ mol·L⁻¹ led to the increase of the average aspect ratio of α-CaSO₄·0.5H₂O whiskers from 80 to 430, since the selective adsorption of CTAB on the negatively-charged side facets of the whiskers inhibited the growth of the whiskers along the direction normal to the lateral facets. The further increase of CTAB concentration above the critical micelle concentration (abbreviated as CMC) showed little effect on the morphology of α-CaSO₄·0.5H₂O whiskers, considering that CTAB tended to form micelles instead of being adsorbed on the whisker surfaces. Similar phenomena were observed in other ATABs (n = 10, 12, 14, 18).

CaSO₄ whiskers (CSW) can be used to reinforce PVC matrix to produce light and strong composites. However, the weak interfacial interaction between the smooth CSW and PVC matrix limited the fabrication of PVC composite with perfect mechanical properties. In this work, CaCO₃ nanoparticles were coated on CSW surface by wet modification of CSW in Na₂CO₃ solution at 80°C, which increased the surface roughness of CSW from 56.8 nm to 115.6 nm. The use of the hierarchical CSW rather than the raw CSW in the fabrication of CSW/PVC composite led to the increase of the flexural strength from 86.3 MPa to 113.2 MPa and the impact strength from 56.7 kJ·m⁻² to 82.5 kJ·m⁻² owing to the enhanced mechanical interlocking between CSW and PVC matrix.

The influence of NH₄Cl on hydrothermal formation of CaSO₄·0.5H₂O whiskers from CaSO₄·2H₂O precursor at 135°C was investigated in this paper. Compared with the blank experiment, the presence of 3 × 10⁻² mol·L⁻¹ NH₄Cl led to the increase of the lengths of the whiskers from 50 to 160 μm to 150 to 300 μm and the decrease of the diameters from 1.0 to 1.5 μm to 0.2 to 0.5 μm. The dissolution of CaSO₄·2H₂O was accelerated by the complex interactions with NH₄Cl and the soluble cations, which led to the decrease of the induction time for the occurrence of α‐CaSO₄·0.5H₂O from 46 minutes to 34 minutes and the formation of CaSO₄·0.5H₂O whiskers with high aspect ratios. Furthermore the critical supersaturation for the formation of α‐CaSO₄·0.5H₂O was investigated.