Francis Opoku

Understanding the mechanism of enhanced charge separation and visible light photocatalytic activity of modified wurtzite ZnO with nanoclusters of ZnS and graphene oxide: from a hybrid density functional study

Recently, semiconductor photocatalysts have received significant interest in addressing the global energy and environmental crisis. Despite the great potential application of zinc oxide (ZnO), the utilisation efficiency of solar energy is limited only to the ultraviolet (UV) region. Therefore, our research interest is centred on the development of a model hybrid ZnO-based photocatalyst material with efficient photocatalytic performance and stability. A hybrid ternary ZnS/graphene oxide (GO)/ZnO system, where the band edges of the individual components in the heterostructure will have a step-wise structure for harvesting a broader portion of the solar spectrum, is an excellent solution. Herein, we explore the charge transfer, the improved photocatalytic mechanism, and the electronic and interfacial properties of the hybrid van der Waals (vdWs) ZnS/GO/ZnO heterostructure for the first time by carrying out comprehensive hybrid density functional theory calculations. Our results reveal the existence of vdWs interaction, enhance charge transfer, narrow the band gap and show remarkable improvement in the visible light photocatalytic activity of the heterostructures compared to pure ZnO. This enhancement is ascribed to the electron acceptor–transporter role played by the GO sheet in the interfacial layer of ZnS and ZnO. According to adhesion energy results, the monolayers are in contact and form stable heterostructures. The calculated charge density difference shows interlayer charge transfer from the ZnO(001) surface to the ZnS(110) surface through the GO sheet. Most significantly, the ZnS/GO/ZnO heterostructures exhibit a type-II band alignment, which significantly restrains the recombination of charge carriers. The band edge positions of ZnS/GO/ZnO with enough driving force for electron and hole transfer are well aligned for the feasibility of splitting water into H2 and O2, as well as for photodegrading pollutants in the water system. Therefore, the vdWs ZnS/GO/ZnO heterostructures appear as a new type of photocatalyst material for solar energy application. This study illustrates the usefulness of using low cost GO as an interfacial electron acceptor–donor to boost the photocatalytic performance of ZnO-based photocatalysts. The findings in this study provide a theoretical basis for developing highly efficient ZnO-based photocatalysts, as well as attract broad interest in VdWs heterostructure research in photocatalysis.

Role of MoS2 and WS2 monolayers on photocatalytic hydrogen production and the pollutant degradation of monoclinic BiVO4: a first-principles study

The global dependence on exhaustible fossil fuel resources has made the search for an alternative renewable and sustainable fuel more urgent. Photocatalysis has gained increasing consideration as a promising technology to solve problems associated with solar energy conversion. Fabricated m-BiVO4-based heterostructures have shown improved photocatalytic activity for hydrogen evolution and pollutant degradation; however, a deeper understanding of the photocatalytic mechanism and the role of the monolayers is still lacking. Moreover, no theoretical studies have been carried out on MS2/m-BiVO4(010) heterostructures. In the present study, the roles of MoS2 and WS2 monolayers loaded onto a m-BiVO4 surface for active photocatalytic hydrogen production and pollutant degradation are explored using first-principle studies. Herein, hybrid density functional calculations and a long-range dispersion correction method were used to investigate the charge transfer, electronic properties, photocatalytic activity and mechanism of the MS2/m-BiVO4(010) heterostructures. The results showed a narrow band gap, built-in potential and a type-II band alignment for the MS2/m-BiVO4(010) heterostructures compared to pure m-BiVO4, which favour the separation and transfer of charge carriers and visible-light-driven activity. The MoS2/m-BiVO4 heterostructure showed a suitable band edge for hydrogen production and pollutant degradation compared to the WS2/m-BiVO4 heterostructure. This improvement was attributed to the role of the MoS2 monolayer as an electron donor, the many reactive sites on the MoS2 surface and the enhanced electron/hole pair separation of charge carriers at the MoS2/m-BiVO4(010) interface. Considering that the MS2 monolayer coupled with m-BiVO4 can restrain the electron–hole recombination rate without lattice distortion indicates that the heterostructure approach is better than the doping approach. Based on the analysis of the electronic properties, the MS2/m-BiVO4(010) heterostructures were shown to fit within the acceptable band gap and built-in potential range. The proposed theoretical design paves a way for the effective and large-scale fabrication of m-BiVO4-based photocatalyst for solar energy conversion and environmental remediation applications.

Metal Oxide Polymer Nanocomposites in Water Treatments

Recently, several pollutants such as dyes, pharmaceuticals and phenolic compounds, which can cause toxic effects to human health, have identified in water resources. Water pollution has extensively studied and several conventional techniques, such as chemical treatment, adsorption, biological treatment, and membrane-based separation, have adopted for pollutants removal from wastewater/ water resources. However, these techniques had led to the production of soluble refractory organic compounds and healththreatening bacteria that are hard to be removed. Recently, photocatalysis has considered as one of the most viable technology for water treatment using sunlight to eliminate harmful bacteria and pollutants owing to its cost-effectiveness and high efficiency. Metal oxide and polymers have become promising materials for water treatment owing to their properties, such as surface mobility, large surface area and superb magnetic and optical properties. This book chapter discusses recent design and synthesis of visible light response polymer/metal oxide nanocomposite through several synthetic strategies for water treatment. The results show that the polymer-metal oxide nanocomposite possesses a superior photodegradation activity toward pollutants under simulated visible light. Major challenges in polymer-metal oxide nanocomposite synthesis and future research perspectives for developing alternate synthesis methodologies are also discussed.