Latest developments in nano-photoactivation opened up new avenues for organic compounds

The modern world is still not very good at keeping environmental and health problems under control, despite the rapid advancements in technology and industrial development that are occurring. The current COVID-19 pandemic is the best example; it has made us realise that the modern world should also take care of the development of novel technologies, materials, and medical innovations to control such health- and environment-related issues [1]. This realisation has made us realise that the modern world should also take care of the development of novel technologies, materials, and medical innovations. Several undesired components that are present in the environment can either directly or indirectly have an impact on human health. In this particular setting, many microbial pathogens that are present in the environment, such as viruses, bacteria, protozoa, and so on, can sometimes pose a risk to human health and cause deadly infectious diseases [1,2]. Recent developments suggest that methods and materials based on nanotechnology could be alternative options with the huge potential for controlling such bacterial and viral outbreaks [3,4,5,6]. These outbreaks have been a serious problem and have increased at a concerning rate over the past few decades [2]. One of the singular processes that takes place in the presence of sun radiation [7,8] is known as photocatalysis. Photocatalysis makes use of nano- photocatalysts. Due to the fact that there is unused solar energy on Earth, this process holds great potential for the management of environmental problems and the enhancement of the health of the general population [9,10]. In the field of degradation of toxic or harmful organic compounds and gases [11,12,13]. Another application is the photocatalytic viral and bacterial disinfection of water, air, or surfaces, which, in the end, protects the environment and improves human health .Under the influence of sunlight, a process known as photocatalytic removal or disinfection of such species is a promising method that is also kind to the environment. The technique uses photocatalysts that are appropriate for the job. In addition to this, it has proven to be very cost-effective and has great promise in open environments [1,9]. In recent years, various different types of metal oxide semiconductor photocatalysts, such as TiO2, ZnO, CuO, WO3, and others, have been created to function as visible active photocatalysts. Their properties have been enhanced through a number of changes, which have made it possible for them to function more effectively in the presence of solar light in the photocatalytic degradation and disinfection of chemical and biological species, respectively . These photocatalysts have oxidative capabilities, which are demonstrated by the photocatalytic formation of harmful reactive oxygen species (ROS), as shown in Figure 1. This allows for the photo-degradation and inactivation of such species in both an outdoor and an indoor setting. It has been discovered to be highly helpful for the treatment of a variety of bacterial and viral infections, including measles, influenza, herpes, Ebola, current COVID-19, and many others [1,2], as is schematically represented in Figure 2.

In depth discussions of each of these photocatalytic substances are provided below.

2. NANOPHOTOCATALYST BASED ON A SEMICONDUCTOR

In order to facilitate organic processes via photocatalysis, semiconductor-based materials have been the subject of substantial research. When a semiconductor is exposed to light, it takes it in. The photons elevate electrons from the valence band to the conduction band. The resulting electron-hole pair (e—-h+) may recombine, resulting in the waste of light energy as heat, or it may interact with electron

A variety of semiconductors (including titanium dioxide (TiO2), zinc oxide (ZnO), amorphous iron oxide (a-Fe2O3), and wolframite (WO3)) have been studied for use as photocatalysts. Among them, TiO2 has been the subject of the greatest research into heterogeneous photocatalysis because to its high stability, low toxicity, chemical inertness, and resistance to photocorrosion (Fujishima et al., 2000). But TiO2 absorbs UV light due to its high band gap (3.0–3.2 eV). Because ultraviolet light makes up such a small percentage of the solar spectrum, researchers have focused on creating visible-light-absorbing photocatalysts. 2.1 Nano-Composite of Semiconductors and Graphene Graphene (GR) is "a single carbon layer in the graphite structure," whose properties can be described "by analogy to a polycyclic aromatic hydrocarbon of quasi unlimited dimension," according to the official definition (McNaught and Wilkinson, 1997). In this allotrope, sp2 hybridised carbon is arranged in a regular hexagonal lattice, making up the structure of the crystalline phase. Novoselov et al. were the first to synthesise it in 2004. Exfoliation of graphite was mechanically separated from the bulk material (Novoselov et al., 2004).

Nanocomposites of graphene and semiconductor materials have semi-conductor nanoparticles uniformly anchored to 2-D graphene. Graphene oxide (GO) or pure graphene (GR) can serve as a precursor (GO). Composites made of graphene have been shown to have superior photocatalytic activity compared to those made of semiconductors and carbon nanotubes. The advantages of GR over CNT were demonstrated in a study that compared the two nanostructures. In terms of controlling the nanocomposite's morphology and increasing TiO2's photocatalytic effectiveness, GR proved superior to CNT (Y. Zhang et al., 2011). However, antecedents of the components and the route taken to anchor the com- ponents on each other are crucial factors in determining the composite's catalytic activity.

photocatalysis The visible-light photo-catalytic activities of the nanocomposite are improved by the inclusion of graphene for the following reasons: (i) Metal-to-carbon (M-C) and metal-oxide-carbon (M-O-C) connections are formed when semiconductors and graphene contact. New energy levels may result from such interactions. Deoxygenation of GO occurs during its reduction to RGO in RGO composites generated from GO precursors. This is counteracted by the diffusion of semiconductor surface lattice oxygen atoms into chemical interactions with RGO as nanoparticle development continues. (ii) Graphene's high conductivity and electron mobility lengthen the lifetime of photogenerated holes and electrons that would otherwise recombine in a matter of nanoseconds (An and Yu, 2011; Long et al., 2013; Ng et al., 2010). As a result, the mean free path of electrons is lengthened, allowing them to be conducted across a wider region of the carbon framework, which improves the possibility of the carbon's interplay with substrates. (iii) Graphene provides structural integrity and increases the photocatalyst's surface area. Graphene's 2-D conjugated p- electron system enhances substrate adsorption on the nanocomposite through its interactions with the substrate.

posites

To selectively oxidise alcohols to aldehydes and catalyse the epoxidation of alkenes, graphene works as a photosensitizer in GR-ZnS nanocom- posite (Y. Zhang et al., 2012a).

ii) Selective reduction reactions The reduction of nitro compounds is a common method for preparing amines. In turn, amines play a crucial role as building blocks in the production of pharmaceuticals and colourants. These are also employed in the removal of CO2 and H2S from gas streams. After 7 minutes of irradiation with microwave energy at 108 °C, alumina-supported hydrazine in the presence of Fe(III) compounds has delivered an outstanding yield (89%) for this reaction (Vass et al., 2001). These rapid and substantial yields have not enhancing their functionality. In particular, photocatalysts have garnered the interest of the scientific community due to their ability to facilitate the selective reduction of molecules like 4-nitroaniline, which have seen relatively little use from other types of catalytic systems.

4. CONCLUSIONS AND PERSPECTIVES

This review has covered the fabrication, mechanism, and use of nanophotocatalysts for performing selective organic transformations. The vast number of nanophotocatalysts developed thus far for organic conversions were also categorised according to their composition and mechanism to facilitate their study. Even though the literature review showed that advances are being made to fabricate materials with better photocat- alytic properties, much more research is desired in this field. Most importantly, the stability and the performance of these catalysts need to be monitored in natural sunlight. The catalysts have been promi- nently applied for selective oxidation of alcohols and reduction in nitroaromatics. It cannot be denied that strict vetting criteria were used in conjunction with these conversions. However, more could be done to expand the range of reactions for which these photocatalysts are used. The composition, nature of used support, size of nanoparticles, shape of nanoparticles, polarity of reactant, irradiation intensity, and solvent were found to have the greatest impact on the catalytic activity. Better photocatalysts can be created by keeping an eye on and manipulating these parameters. Studies on graphene and semiconductor-based nanocomposites dominate the literature, but metal-organic frameworks (MOFs) and plasmon-based photocatalysts are quickly emerging as promising nanophotocatalysts. Newer kinds of arrangements of components in the composites are being developed. The composites are being extended from binary to distinct merits of each component can be tapped to the fullest. It is beyond doubt that the conservation of earth‘s non-renewable resources is the need of the hour. The development of low-cost, reliable nanophotocatalysts will open up new opportunities to convert solar energy to chemical energy and can prove to be an enormously crucial step toward solving the energy dilemma of the globe.

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Research Scholar, Sunrise University, Alwar, Rajasthan