Highly photocatalytically active cobalt-doped ZnO (ZnO:Co) nanorods have been prepared by a facile hydrothermal process. X-ray diffraction, X-ray photoelectron spectroscopy, Raman scattering and UV-vis diffuse reflectance spectroscopy confirmed that the dopant ions substitute for some of the lattice zinc ions, and furthermore, that Co²⁺ and Co³⁺ ions coexist. The as-prepared ZnO:Co samples have an extended light absorption range compared with pure ZnO and showed highly efficient photocatalytic activity, only requiring 60 min to decompose ∼93% of alizarin red dye under visible light irradiation (λ > 420 nm). The photophysical mechanism of the visible photocatalytic activity was investigated with the help of surface photovoltage spectroscopy. The results indicated that a strong electronic interaction between the Co and ZnO was present, and that the incorporation of Co promoted the charge separation and enhanced the charge transfer ability and, at the same time, effectively inhibited the recombination of photogenerated charge carriers in ZnO, resulting in high visible light photocatalytic activity. Open image in new window

The composite Bi₂O₃/ZnO photocatalyst, a novel p–n type heterojunction with different molar ratios of Bi to Zn, has been fabricated using a hydrothermal method. The XRD, XPS, UV-vis DRS, SEM, HRTEM studies were used to characterize the as-obtained products. The photocatalytic activities of the p-Bi₂O₃/n-ZnO heterojunctions were investigated for their efficiency on the degradation of alizarin red (AR) dye under visible light irradiation (λ > 420 nm), and the results showed that the composites possessed remarkable photocatalytic activities, which were conducive to their separation, recycling, and reuse. The photogenerated charge-transfer properties were tested by surface photocurrent (SPC) and surface photovoltage (SPV) experiments and results show that the interfacial electric field located between Bi₂O₃ and ZnO played an important role in promoting the separation of photogenerated electron and hole pairs. These results are helpful to design and construct high efficiency heterogeneous semiconductor photocatalysts.

A visible-light-active ZnO photocatalyst system in the presence of manganese ions (Mn/ZnO) was prepared via a simple and rapid approach. XRD, XPS, Raman scattering and UV-Vis DRS confirmed the manganese exists in multivalent forms (Mn³⁺/Mn²⁺) in the ZnO lattice, furthermore, ZnO light absorption is extended to the visible region. The photocatalytic activities of the catalysts were evaluated by measuring the photodegrading efficiency of 2,4-dichlorophenol (DCP) under visible light irradiation. With an optimal molar ratio of 5% in Mn/ZnO the highest rate photodegradation was achieved under the experimental conditions. We have characterized the separation and transfer behavior of the photogenerated charges in the visible region by means of surface photovoltage (SPV), surface photocurrent (SPC) and transient photovoltage (TPV) techniques. Based on the comprehensive investigation of the photovoltaic properties of Mn/ZnO photocatalyst, we illustrate the behavior of photogenerated charges have distinct effects on the photocatalytic activity. It is demonstrated that the incorporation of multivalent Mn in ZnO promoted the separation of photogenerated charges, inhibited the recombination of photogenerated carriers, and thus prolonged the charges lifetime to participate in the photocatalytic reaction, resulting in highly efficient photocatalytic activity, which is attributed to the formation of a strong electronic interaction between the multivalent Mn and ZnO.

In₂O₃-sensitized flowerlike ZnO with visible light photoelectric response properties were synthesized by a facile two-step process, and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX, HRTEM and UV-vis diffuse reflectance spectroscopy. The results revealed that In₂O₃ nanoparticles have grown and eventually coalesced on the surface of the flowerlike ZnO successfully, and the samples exhibited significant response to visible light. The photoelectric gas-sensing of the In₂O₃-sensitized ZnO was also studied to formaldehyde (HCHO) under 460 nm light irradiation at room temperature with the help of surface photocurrent technique. It was found that ZnO sensitized with In₂O₃ could enhance the gas response to HCHO under the visible light illumination. This may be due to the fact that the composite structure of In₂O₃–ZnO extends the photo absorbing range to visible light area, inhibits the recombination of photo-generated electrons and holes, and thus increases the utilization of photo-generated carriers in photoelectric gas detection, resulting in the higher sensing response in some extent. The gas response to 5 ppm and 100 ppm formaldehyde can reach to 19% and 419% under visible light irradiation at room temperature, respectively. These results should be valuable for designing a new type of visible-light assisted gas sensor at room temperature.