Global environmental pollution and energy supply demand have been regarded as important concerns in recent years. Metal oxide semiconductor photocatalysts is a promising approach to apply environmental remediation as well as fuel generation from water splitting and carbon dioxide reduction. ZnO nanostructures have been shown promising photocatalytic activities due to their non-toxic, inexpensive, and highly efficient nature. However, its wide band gap hinders photo-excitation for practical photocatalytic applications under solar light as an abundant, clean and safe energy source. To overcome this barrier, many strategies have been developed in the last decade to apply ZnO nanostructured photocatalysts under visible light. In this review, we have classified different approaches to activate ZnO as a photocatalyst in visible-light spectrum. Utilization of various nonmetals, transition metals and rare-earth metals for doping in ZnO crystal lattice to create visible-light-responsive doped ZnO photocatalysts is discussed. Generation of localized energy levels within the gap in doped ZnO nanostructures have played an important role in effective photocatalytic reaction under visible-light irradiation. The effect of dopant type, ionic size and its concentration on the crystal structure, electronic property and morphology of doped ZnO with a narrower band gap is reviewed systematically. Finally, a comparative study is performed to evaluate two classes of metals and nonmetals as useful dopants for ZnO nanostructured photocatalysts under visible light.

Analyzing the commercialization potential of g-C3N4photocatalysts for solar H2generation from an economic viewpoint and for large-scale production.

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ZnO/CuO nanofibers, with different CuO concentrations, were fabricated by one-step electrospinning of the polymer precursor and annealing in air. Scanning electron microscopy (SEM) showed smooth and beadless morphology for the synthesized nanofibers, while X-ray diffraction (XRD) analysis revealed formation of hexagonal and monoclinic crystalline structure phases for ZnO and CuO nanofibers, respectively. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of CuO on the surface of ZnO nanofibers. For further confirming the formation of chemical bonds, Fourier transform infrared (FT-IR) spectroscopy was employed. The effect of Cu contents in the overall electronic band structure of ZnO was explained by density functional theory (DFT) calculations. Diffuse reflectance spectroscopy (DRS) showed that the ZnO band gap energy reduced with increasing amount of CuO contents due to the presence of the Cu­(3d) energy states above the valence band. Comparing the photocatalytic activity of ZnO/CuO nanofiber samples with different CuO concentrations under similar sunlight irradiation conditions revealed that the ZnO/(0.5 wt %) CuO sample exhibited the highest performance among all samples. This was explained by an effective suppression of electron–hole recombination as verified by both photoluminescence (PL) and photocurrent density measurements. By means of charge carrier scavengers, it was found that holes and hydroxyl radicals are the main surface species for photocatalytic degradation of methylene blue (MB) over the ZnO/(0.5 wt %) CuO nanofiber. Furthermore, the optimized sample demonstrated great activity for the degradation of bisphenol A (BPA) with a rate constant of 3.4 × 10–2 min–1. Finally, a photocatalytic degradation mechanism based on the main reactive oxygen species (ROS) and calculated band positions was proposed.