ZnO nanoparticles (NPs) are synthesized by deoxidizing ZnO powder in a vacuum drying process. This process reduces the size of the NPs and increases the concentration of oxygen vacancies on their surfaces. ZnO NPs with sufficient oxygen vacancies are highly effective for the photodecomposition of methylene blue (MB) dye in water under ultraviolet irradiation. The MB degradation efficiency exceeds 99 percent after 50 min of light irradiation, and the catalytic property of the NPs remains stable over several complete degradation cycles. It is revealed that the concentration of oxygen vacancies on the surface, and the photocatalytic activity, are both higher for smaller NPs. Oxygen vacancies reduce the recombination rate of photo-generated charge carriers by capturing the electrons and hence, improve the efficiency of redox reactions. In addition, a smaller particle size leads to a larger specific surface area and a higher photonic efficiency for the ZnO NPs.

An reduced graphene oxide (RGO) resistive gas sensor was prepared to detect ammonia at room temperature, the result indicated that the desorption of gas (NH ${}_3$ ) molecules from a graphene-based sensor was difficult, which lead to a baseline drift. The responses of different concentrations were compared and studied. It was found that both the response rate and its acceleration were affected by the gas concentration. An Intermolecular Forces Based Model was established to explain the adsorption and desorption dynamic response curves. A new method was proposed based on this model. The first and second derivative extrema (FSDE) of the response curve can be attained quickly to calibrate the gas concentrations. The experiment results demonstrated that this new method could eliminate the baseline drift and was capable of increasing the efficiency of gas calibration significantly.

The construction of heterostructured photocatalyst with an appropriate energy band structure will help realize highly efficient photo-excited charge separation. In this study, ternary CuO/CeO₂/ZnO nano-particle (NP) composites were synthesized by a facile two-step sol-gel method, which exhibit significantly enhanced photocatalytic degradation performance for various organic pollutants under UV and visible light excitation. The photo-responses to both UV and visible light, as well as the visible light absorption and utilization rates of ZnO are found to be synergistically intensified by CeO₂ and CuO co-coupling. The first-order kinetic constants (K) of 3%CuO/CeO₂/ZnO for methylene blue (MB) degradation are ~3.9, ~4.1 and ~4.8 times higher than ZnO under UV light, visible light and simulated sunlight illumination, respectively. The roles of CuO and CeO₂ in optical properties and photo-degradation under UV and visible light were explored. Besides, the photogenic holes (h⁺) of ZnO, CeO₂, and the produced hydroxyl radicals (·OH) are proved to be the main active species under UV light. Dissimilarly, under visible light, the superoxide radicals (·O₂⁻) formed by the reactions between oxygen molecules and the photo-generated electrons (e⁻) of CuO moving towards the catalysts surface are also found to be important for promoting dye decomposition. The improved photo-responses, the well-matched band structure that facilitates charge transfer processes, and the highly efficient utilization of the photo-excited carriers of the ternary nano-heterostructure are suggested to be the key factors for the remarkable enhancement of photocatalytic performance of ZnO nano-photocatalyst. This work offers a low-cost strategy to acquire highly active UV and visible light-driven photocatalyst.

Heterostructured photocatalysts have shown an enormous potential in photocatalytic degradation of organic pollutants in wastewater. However, the efficacy of such heterojunction on the photocatalytic degradation behaviors has not yet been fully revealed. This work aims to demonstrate a specific photocatalytic degradation behavior of ZnO-based heterostructured nanocomposites toward methyl orange (MO) and methylene blue (MB) dyes based on a systematically comparative investigation for their physical and chemical properties. A series of low-cost and efficient ZnO-based heterostructured nanocomposite photocatalysts including ZnO/CuO, ZnO/TiO₂ and ZnO/SnO₂ with 3 and 10 mol% of CuO/TiO₂/SnO₂ were synthesized by a simple strategy to combine the modified polymer-network gel and traditional sol-gel methods. The physical and chemical properties were analyzed using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), X-ray photoelectron spectra (XPS), ultraviolet-visible (UV-Vis) absorption spectra, photoluminescence (PL), surface photovoltage (SPV), electrochemical impedance spectroscopy (EIS) and zeta potential. Owing to the fast interfacial charge transfer at the heterojunction, all the three ZnO-based nanocomposite catalysts exhibited higher efficient separation of photogenerated electrons and holes, delivering an enhanced photocatalytic activity for the degradation of organic dyes compared with pure ZnO. Three photocatalysts of ZnO/3 %-CuO, ZnO/3 %-TiO₂ and ZnO/10 %-SnO₂ (marking as ZC3, ZT3 and ZS10, respectively) were capable of achieving the complete degradation of 4 mg/L concentration of MB dye within 50 min, and the first two could degrade MO within 80 min. However, the degradation rate of MO by ZS10 became significantly slower. For MO and MB degradation, the active species of photogenerated holes (h_hν⁺) and superoxide radicals (·O₂^-) play the predominant roles, respectively, followed by hydroxyl radicals (·OH). The differences in heterojunction configuration and dominant active species result in a specific photocatalytic degradation behavior of ZnO-based composite nanostructures. The generation of the active species are influenced by the heterojunction configurations, of which the essence is that the different band alignments can results in the differences of interfacial charge transfer behaviors, and thus selective generation of the active species such as h_hν⁺, ·O₂^- and ·OH. Importantly, this work offers a fundamental understanding for specific photocatalytic degradation of the different heterojunction nanostructures towards the different organic dyes.

RGO/SnO ${}_2$ nanocomposites were prepared by a simple blending method and then airbrushed on interdigitated electrodes to obtain the corresponding gas sensors. The characterizations of SEM, TEM, Raman, XRD and FTIR were used to characterize the microstructures, morphologies and surface chemical compositions of the nanocomposites, indicating that the two materials coexist in the composite films and the concentration of surface defects is affected by the amount of SnO ${}_2$ nanoparticles. It is also found that the room temperature sensing performance of RGO to NO ${}_2$ can be improved by introducing appropriate amount of SnO ${}_2$ nanoparticles. The enhanced NO ${}_2$ sensing properties are attributed to the rough surface structure and increased surface area and surface defects of the nanocomposite films. Since further reduction of RGO, heat treating the sensing films resulted in a decrease in the response and recovery times of the sensors. Furthermore, the sensor annealed at 200 ${}^{\circ }$ C exhibited a small baseline drift, wide detection range, good linearity, high stability and better selectivity.