The global health-threatening crisis from the COVID-19 pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), highlights the scientific and engineering potentials of applying ultraviolet (UV) disinfection technologies for biocontaminated air and surfaces as the major media for disease transmission. Nowadays, various environmental public settings worldwide, from hospitals and health care facilities to shopping malls and airports, are considering implementation of UV disinfection devices for disinfection of frequently touched surfaces and circulating air streams. Moreover, the general public utilizes UV sterilization devices for various surfaces, from doorknobs and keypads to personal protective equipment, or air purification devices with an integrated UV disinfection technology. However, limited understanding of critical UV disinfection aspects can lead to improper use of this promising technology. In this work, fundamentals of UV disinfection phenomena are addressed; furthermore, the essential parameters and protocols to guarantee the efficacy of the UV sterilization process in a human-safe manner are systematically elaborated. In addition, the latest updates from the open literature on UV dose requirements for incremental log removal of SARS-CoV-2 are reviewed remarking the advancements and existing knowledge gaps. This study, along with the provided illustrations, will play an essential role in the design and fabrication of effective, reliable, and safe UV disinfection systems applicable to preventing viral contagion in the current COVID-19 pandemic, as well as potential future epidemics.

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Converting a practically limitless energy source, such as sunlight, into chemical energy with very little or no carbon footprint will pose a major challenge in the coming decades. The technology exists to convert solar energy into chemical fuels, such as hydrogen, through overall water splitting to produce hydrogen and oxygen. However, the photocatalytic efficiency is still below the feasibility limit. Many photocatalyst materials have been developed for solar hydrogen generation through water splitting in the last four decades. Gallium-zinc oxynitride (GaN:ZnO) solid solution is reported to be a suitable photocatalyst for overall water splitting and to have the highest photocatalytic activity. The traditional synthesis method contains difficulties and inefficiencies, such as long nitridation of starting materials at high temperatures and low Zn content of the synthesized photocatalyst. A number of experiments have been conducted in recent years to develop new synthesis approaches. In this article, a comprehensive review of various synthesis techniques of GaN:ZnO solid solution, along with their advantages and disadvantages, are presented. This information is essential for improving the efficiency of the synthesis techniques of GaN:ZnO solid solution and its photocatalytic activity for overall water splitting.

ZnO nanoparticles and nanowires decorated with platinum nanoparticles in different loading concentrations were prepared to detect low concentrations of NO2 under 365-nm ultraviolet-light emitting diode (UV-LED) irradiation at room temperature. Solution precipitation, self-assembly crystallization, and photo-deposition techniques were used to synthesize the sensing material. The synthesized sensors at different stages of development were characterized by XRD, HRTEM and FE-SEM analyses. Although the sensing response of the pristine ZnO nanoparticles was 0.41 for NO2 detection, the response improved significantly to 1.5 using ZnO nanowires in the identical photo-activation settings of 365 nm UV wavelength and 25 mW/cm2 irradiance. The decoration of the surface of the ZnO nanowires with Pt nanoparticles further enhanced the sensing performance, whereas the 0.1wt% Pt-decorated ZnO nanowire sensor exhibited a response of 4.33 with a 140-s response time, which is more than one order of magnitude higher and 50s faster than the response generated by the ZnO nanoparticles. This improved performance is attributed to the role of Pt active sites in promoting NO2 adsorption on the ZnO nanowires’ surface as well as enhancing the layer electron utilization.

Gallium zinc oxynitride (GaN:ZnO) solid solution is one of the most promising visible-light activated photocatalysts due to high stability in water splitting reaction conditions and controllable band edge potentials. To further enhance the activity of this photocatalyst by facilitating the photo-induced charges separation, a nanocomposite of GaN:ZnO solid solution photocatalyst and graphene oxide (GO) nanosheets was fabricated through facile ultrasonication. Structural, optical, and electrochemical characterization of the prepared composite revealed the effective interaction between composite components and significant improvement in charge separation. The synthesized composite with a defined amount of GO demonstrated superior activity for overall water splitting under visible light irradiation with significantly higher apparent quantum efficiency.

One of the most promising methods for conversion and storage of solar energy is in the form of the chemical bonds of an energy carrier, such as hydrogen or light hydrocarbons. However, the traditional methods to harness and store solar energy are simply too expensive to be implemented on a large scale. It has been documented that the recombination of photo-induced charge carriers is the greatest source of inefficiency in photocatalytic systems. In the last decade, graphene derivatives and their functionalized nanostructures were extensively utilized for various roles to improve the efficiency of photocatalytic solar fuel generation. These include photocatalyst/redox active sites via band gap and defect density engineering, charge acceptor due to their excellent carrier mobility, a solid-state charge mediator by electronic band alignment, and light absorber by taking advantage of their photoluminescence characteristics at the nanoscale. This chapter aims to provide an authoritative and in-depth review on the properties and application of graphene derivatives, as well as the recent advances in the design of graphene-based photocatalytic systems. The knowledge extracted from the presented materials can be applied to other applications dealing with surface chemistry, interfacial science, and optoelectronic device fabrication.

Surface modified gallium-zinc oxynitride solid solution exhibited outstanding stability and visible-light activity for water splitting. However, the considerable rate of photo-induced charge recombination and the low surface area of the bulk photocatalyst limited its performance. Here, an efficient technique is proposed for the synthesis of a nanoporous oxynitride photocatalyst and its graphene-hybridized material. The nanoporous oxynitride photocatalyst was prepared via a nanoscale solid-state route, using microwave irradiation as an intermolecular-state activation method, Ga 3+ /Zn 2+ layered double hydroxide as an atomic-level uniform mixed-metal precursor, and urea as a non-toxic ammonolysis soft-template.The graphene-hybridized photocatalyst was fabricated using a facile electrostatic self-assembly technique. The photocatalytic activity of the synthesized graphene hybridized nanoporous oxynitride photocatalyst was systematically improved through shortening the majority-carrier diffusion length and enhancing the density of active hydrogen evolution sites within the quasi-three-dimensional nanostructure, reaching 7.5-fold sacrificial photocatalytic hydrogen evolution, compared to the conventional 1 wt% Rh-loaded oxynitride photocatalyst. † Electronic supplementary information (ESI) available: (1) High-angle annular dark eld (HAADF) tomography video, showing the nanoporous structure of the prepared GaZnON photocatalyst. (2) Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the prepared graphene oxide (GO) nanosheets. See

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