Review of Recent Developments in the Fabrication of ZnO/CdS Heterostructure Photocatalysts for Degradation of Organic Pollutants and Hydrogen Production

Researchers have recently paid a lot of attention to semiconductor photocatalysts, especially ZnO-based heterostructures. Due to its availability, robustness, and biocompatibility, ZnO is a widely researched material in the fields of photocatalysis and energy storage. It is also environmentally beneficial. However, the wide bandgap energy and quick recombination of the photoinduced electron-hole pairs of ZnO limit its practical utility. To address these issues, many techniques have been used, such as the doping of metal ions and the creation of binary or ternary composites. Recent studies showed that ZnO/CdS heterostructures outperformed bare ZnO and CdS nanostructures in terms of photocatalytic performance when exposed to visible light. This review largely concentrated on the ZnO/CdS heterostructure production process and its possible applications including the degradation of organic pollutants and hydrogen evaluation. The importance of synthesis techniques such as bandgap engineering and controlled morphology was highlighted. In addition, the prospective uses of ZnO/CdS heterostructures in the realm of photocatalysis and the conceivable photodegradation mechanism were examined. Lastly, ZnO/CdS heterostructures' challenges and prospects for the future have been discussed.

The doping of B in ZnO@CdS for enhanced visible-light H2 production

The visible-light water splitting enhancement of B in ZnO@CdS was systematically studied and the B doped ZnO@CdS rods (B-ZnO@CdS) showed an excellent H2 generation rate of 13.1 mmol h-1g-1, mostly...

Fast photodegradation of antibiotics and dyes by anionic surfactant-aided CdS/ZnO nanodispersion

Photocatalytic technology has broad applications in energy and environmental science. In this study, we synthesized a type II heterojunction CdS/ZnO nanodispersion by means of one-pot precipitation. Different from previous studies,...

Transformation of Wurtzite ZnO to a New Triclinic Nanoporous ZnO Phase via Hydrothermal Treatment with Metformin for Designing Proton Conducting Material

Zinc oxide is one of the most widely studied semiconductor metal oxides, which predominantly crystallizes as hexagonal wurtzite and often cubic zinc-blende phases. Here we report the transformation of the highly stable wurtzite ZnO to a new triclinic phase NZO-2 by using metformin as a template during post-synthesis hydrothermal treatment. This crystalline phase of the material NZO-2 has been identified through the refinement of the powder XRD data. NZO-2 possesses porous rod like particle morphology consisting of the self-assembly of 3-7 nm size spherical nanoparticles and interparticle nanoscopic voids spaces. NZO-2 has been surface phosphorylated and the resulting material displayed good proton conductivity. Further, NZO-2 displayed ultra-low band gap of 1.74 eV, thereby responsible for red emission under high energy laser excitation and this may open new opportunities in optoelectronic application of ZnO.

Z-Scheme Photocarrier Transfer Realized in Tungsten Oxide-Based Photocatalysts by Combining with Bismuth Vanadate Quantum Dots

Multicomponent photocatalysts with a Z-scheme charge transfer are promising in converting solar to hydrogen fuel because of their significantly improved light absorption and restrained photocarrier recombination while keeping their redox capacity. In this work, a composite photocatalyst of BiVO₄ quantum dot-decorated WO₃ nanosheet arrays was synthesized and investigated. The existence of the Z-scheme charge transfer behavior was confirmed by the redox probe technique. Such a Z-scheme charge transfer makes it possible to generate hydrogen without bias. An optimized photocatalyst produces a hydrogen generation rate of 0.75 μmol/h without bias and a photocurrent of 1.91 mA/cm² at 1.23 V versus RHE, which is about 70% higher than that of pure WO₃. We attributed these improvements to the enhanced light absorption, extended conduction band level of BiVO₄, as well as the unique charge transfer behavior in the Z-scheme structure. This work presents a generalizable method to improve the redox capacity of a variety of semiconductors through rationally selecting the building material blocks in view of energy levels.

Developing the Ternary ZnO Doped MoS2 Nanostructures Grafted on CNT and Reduced Graphene Oxide (RGO) for Photocatalytic Degradation of Aniline

Transition metal sulfide semiconductors have achieved significant attention in the field of photocatalysis and degradation of pollutants. MoS2 with a two dimensional (2D) layered structure, a narrow bandgap and the ability of getting excited while being exposed to visible light, has demonstrated great potential in visible-light-driven photocatalysts. However, it possesses fast-paced recombination of charges. In this study, the coupled MoS2 nanosheets were synthesized with ZnO nanorods to develop the heterojunctions photocatalyst in order to obtain superior photoactivity. The charge transfer in this composite is not adequate to achieve desirable activity. Therefore, heterojunction was modified by reduced graphene oxide (RGO) nanosheets and carbon nanotubes (CNTs) to develop the RGO/ZnO/MoS2 and CNTs/ZnO/MoS2 ternary nanocomposites. The structure, morphology, composition, optical and photocatalytic properties of the as-fabricated samples were characterized through X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), Energy-Dispersive X-ray (EDX), elemental mapping, Photoluminescence (PL), Ultraviolet-Visible spectroscopy (UV-VIS), and Brunauer-Emmett-Teller (BET) techniques. The photo-catalytic performance of all samples was evaluated through photodegradation of aniline in aqueous solution. The combination of RGO or CNTs into the ZnO/MoS2 greatly promoted the catalytic activity. However, the resulting RGO/ZnO/MoS2 ternary nanocomposites showed appreciably increased catalytic performance, faster than that of CNTs/ZnO/MoS2. Charge carrier transfer studies, the BET surface area analysis, and the optical studies confirmed this superiority. The role of operational variables namely, solution pH, catalyst dosage amount, and initial concentration of aniline was then investigated for obtaining maximum degradation. Complete degradation was observed, in the case of pH = 4, catalyst dosage of 0.7 g/L and aniline concentration of 80 ppm, and light intensity of 100 W. According to the results of trapping experiments, hydroxyl radical was found to be the main active species in the photocatalytic reaction. Meanwhile, a plausible mechanism was proposed for describing the degradation of aniline upon ternary composite. Moreover, the catalyst showed excellent reusability and stability after five consecutive cycles due to the synergistic effect between its components. Total-Organic-Carbon concentration (TOC) results suggested that complete mineralization of aniline occurred after 210 min of irradiation. Finally, a real petrochemical wastewater sample was evaluated for testing the catalytic ability of the as-fabricated composites in real case studies and it was observed that the process successfully quenched 100% and 93% of Chemical Oxygen Demand (COD) and TOC in the wastewater, respectively.