Todays, photocatalytic processes are commonly used as promising technology for the degradation and removal of organic pollutants from the environments. There is a wide variety of photocatalytic processes that are mostly used for aqueous media in a batch reactor. In this regard, the application of such processes for purifying air streams is still a challenge due to the very short exposure time. In the application of photocatalytic processes for the removal of organic compounds, the air stream containing the pollutants is passed through a photocatalyst filter, which results in a very short contact time between the pollutant molecules and the photocatalyst surface. This is considered a major limitation for the application of these technologies in air. Previous studies have showed that the photocatalytic processes are able to remove the VOCs from air streams with a limited efficiency in the range 50–70% depending on the air flow rate and used source light (Cao et al. 2000; Liang et al. 2010). However, many efforts have been made to improve the efficiency of the photocatalytic processes for air purifying (Long et al. 2012; Samarghandi et al. 2017). Due to the advantages of the photocatalytic processes, many researchers have recently focused on the use of ultraviolet (UV) along with catalyst for the inactivation of pathogenic microorganisms in water as an on-site disinfection method (De Vietro et al. 2019; Gerrity et al. 2008). Few studies have investigated the application of these technologies for inactivation of viruses in air. As we know, with growing concerns about viral pandemics in the last year, and the importance of using ventilation systems for reducing the transmission of these diseases, there is an increasing interest in the use of photocatalytic processes for the removal and reducing the load of airborne microbial agents. For this reason, different types of photocatalytic processes can be used for inactivation of viruses in air due to their organic nature as follows:
As previously mentioned, various materials have been used as photocatalytic, which are typically made of metal oxides, metal sulfides, oxysulfides, oxynitrides, and composites (Sakka 2013). Among them, TiO2 has been more commonly used for the sterilization of microbial agents and organic compounds (Li et al. 2018). For instance, in Kim et al.’s study, TiO2/UV photocatalyst showed the highest activity for inactivation of airborne MS2 viruses (Kim and Jang 2018). In order to use TiO2 as a photocatalyst in air purifiers, it should be used in the form of a filter similar to other filter type, which provides enough surfaces for passing a determined volume of polluted air and trapping pollutants in the air stream. For this reason, in the Kim et al.’s study, four types of catalyst frame shapes, including two mm-pleated, five mm-pleated, spiral, and flat sheet types, were applied with a thickness of 0.2 mm of titanium sheets, and it was found that the 2 mm pleated catalyst offered the highest efficiency for the inactivation of airborne MS2 viruses (Kim and Jang 2018). They also used a vacuum UV (VUV, wavelength ≤200 nm) for the photocatalysis of the target virus. The VUV refers to the wavelength range below ≈ 200 nm. In the vacuum UV range, light is strongly absorbed in air and thereby can improve the performance of the photocatalytic process for air applications. In this regard, a previous study reported a higher disinfection efficacy for vacuum UV in comparison with UV light on Bacillus subtilis spores (Wang et al. 2010). Zan et al. (2007) also investigated the efficiency of nano-TiO2 particles and TiO2-coated ceramic plates as photocatalytic processes on the inactivation of hepatitis B virus (HBV). However, in the Zan et al.’s study, HBV surface antigen in aqueous solution was removed using the photocatalytic process, and it cannot be generalized to the origin virus. They reported that both TiO2 suspension and TiO2-coated ceramic plates were capable of destroying most HBV antigens under weak ultraviolet light, weak sunlight, or indoor sunlight (Zan et al. 2007).
In another study, a thin film of TiO2-coated glass (T3 sample, 50 × 50 mm) along with a low intensity of UV-A (0.01 mW cm−2) was successfully applied for inactivation of influenza virus (Nakano et al. 2012a). TiO2 in combination with other compounds such as copper(II) has been applied for the inactivation of airborne viruses. For instance, in a recent study, Cu(II)–TiO2 nanocomposite was synthesized and used for viral inactivation in air, which showed suitable antiviral activity (Liu et al. 2015). Recently, many efforts have been made to improve the photocatalytic characteristics of commercial TiO2 as a photocatalyst for inactivation of microbial agents. In a related study, Ahmed et al. introduced a simple method for the synthesis of anatase TiO2 quantum dots using a single step microwave–hydrothermal method, which offered a higher energy gap compared to the commercially available anatase TiO2 nanoparticles. They also reported an increase in the absorbance of ultraviolet light for superior photocatalytic inactivation ability of E. coli (Ahmed et al. 2019). In Daikoku et al.’s study on the inactivation of aerosol-associated influenza by photocatalysis using an air cleaner and the porous ceramic substrate, due to the use of porous ceramic coated with nanoTiO2, the viral inactivation efficiency of photocatalysis was greatly attributed to the surface area of the used coated ceramic, and the photocatalytic air cleaner showed a high efficiency for the decomposition of organic chemicals including acetaldehyde and dioxins and inactivation of aerosol-associated influenza virus in air stream (Daikoku et al. 2015). Moreover, Nakano et al. also reported that TiO2 thin film could disinfect influenza virus through degradation of viral proteins, in which this inactivation effect depended on the UV irradiation time and its intensity (Nakano et al. 2012b). Therefore, it can be concluded that TiO2 thin film can be successfully used for disinfection of the influenza virus in the air, which can be also used for other airborne viruses and hinder viral transition through air. Moreover, doping TiO2 with other metals like Pt, Ag, Pd, and Au has been recently introduced as effective strategy to enhance its photocatalytic activity. The application of metal nanoparticle provides a high specific surface area to generate special barrier, which results in an increase in the photocatalytic activity of the basic TiO2 through the improvement of charge separation rate. It has been also reported that in the decoration of TiO2 with Au and Ag, the localized surface plasmon resonance as an additional effect leads to a strong absorption of the visible light, which improves the photocatalytic activity of the decorated TiO2 (Yu et al. 2017).
Silver (Ag) has been widely used as an antibacterial materials due to its high surface area and chemo-physical properties (Marambio-Jones and Hoek 2010). Ag+ and silver nanoparticles can inactivate the viruses and denature their enzymes through the reaction with sulfhydra, amino, carboxyl, phosphate, and imidazole groups. In this case, Joe et al. (2016) synthesized silver nanoparticles using spark discharge generation (SDG) system and then coated the particles onto the air filter surface to provide antiviral ability (Joe et al. 2016). SDG is one of the mechanical methods that uses a high voltage to a pair of close-set silver electrodes, in which a spark is produced between the two electrodes. In this system, under the effect of the electric field, the accelerated ions and electrons affect the surface of the silver node and results in the vaporization of the electrode surface. The metallic vapors cool downstream from the spark. Eventually, nucleation condensation and coagulation of the silver nanoparticles occurred (Byeon et al. 2008; Joe et al. 2016). In addition, silver-doped titanium dioxide (Ag/TiO2) nanocomposites with various silver contents have been also developed for oxidation of microbial agents. Moongraksathum et al. reported that the presence of silver on TiO2 significantly improved the virucidal effects (greater than 99.99%) under UV-A irradiation (Moongraksathum et al. 2019). Therefore, coating Ag with TiO2 can be used as absolution for improving the photocatalytic properties of Ag in the inactivation of viral particles in air. For instance, Mehdizadeh and Tavangar showed that the presence of 1.5% Ag coated on N/TiO2 (with 2:1 mole ratio) resulted in the highest activity under visible and ultraviolet irradiation in the photocatalytic processes (Mehdizadeh and Tavangar 2017). Moreover, Liga et al. demonstrated that the inactivation rate of MS2 was enhanced by more than 5-fold through silver-doped titanium dioxide nanoparticles (nAg/TiO2), depending on the base TiO2 material, and the inactivation efficiency increased with increasing silver content. They also reported that the inactivation efficiency of MS2 virus in drinking water using the nAg/TiO2 nanoparticles was attributed to increased production of hydroxyl-free radicals under the effect of UV-A irradiation provided by four 8 W UV-A lamps (in a wavelength range 315–400 nm) (Liga et al. 2011). Therefore, it can be concluded by doping TiO2 with Ag nanoparticles; a synergic effect occurs between them for inactivating viruses under UV radiation. It has been also reported that TiO2 doped with silver causes a slight increase in virus adsorption on the catalyst surface and also increases the photocatalytic inactivation of viruses through the increase of the production of hydroxyl-free radicals (Liga et al. 2011). It should be noted that most of the mentioned studies have focused on the photocatalytic inactivation of viruses in water and aqueous solutions, and very few research have investigated the effects of such processes for inactivation of viral agents in air.
Some recent studies have evaluated the role of copper (Cu) in the photocatalysis of various organic compounds in air (Arana et al. 2005; Papadaki et al. 2019; Tri et al. 2019; Wu et al. 2018). It has been reported that the presence of Cu in the catalyst surface could modify the interactions of the organic pollutants with the catalysts surface and thereby improves the process efficiency (Araña et al. 2005). Cu is most commonly used as doping agent with TiO2 and ZnO photocatalyst in order to improve their photocatalytic activity. In a related research, it was reported that the Cu-doped ZnO exhibits 3.5-fold higher photocatalytic activity compared to pure corn seed-shaped ZnO (Kadam et al. 2017). Cu has also been used in Cu-TiO2 nano-fibers under visible light for inactivation of viral agents, in which the Cu-TiO2 nano-fibers offered a suitable efficiency for the inactivation of bacteriophage f2 under visible light (Zheng et al. 2018). Cuprous oxide (Cu2O) as a semiconductor with optical-band-gap absorption has been also applied as a promising photocatalyst (Singh et al. 2018). To the best of our knowledge, until now, there has been no study evaluating the use of Cu2O as photocatalytic for the inactivation of virus in air. Based on the previous studies, the efficient antiviral properties of Cu2O can convert it as a suitable alternative for rapid inactivation of viral agents in air stream with continuous flows through photocatalytic processes. CuxO/TiO2 photocatalyst has been also applied as an antivirus agent through denaturalizing the protein of the virus in the photocatalytic oxidation process (Abidi et al. 2019). Cu(II) species in the CuxO can act an electron acceptor through photo-induced interfacial charge transfer, resulting in the production of Cu(I) species with antivirus activity and holes with high oxidation potential under visible light irradiation in the valence band similar to TiO2. The CuxO/TiO2 photocatalyst can maintain its antiviral effect under dark conditions, due to the remaining active Cu (I) species. Hence, it has been suggested that the proposed CuxO/TiO2 photocatalyst can decrease the risk of transmission of viral infection in indoor air by applying such coating photocatalyst (Miyauchi et al. 2020).