Technological watch

AbstractThe COVID-19 pandemic refocused scientists the world over to produce technologies that will be able to prevent the spread of such diseases in the future. One area that deservedly receives much attention is the disinfection of health facilities like hospitals, public areas like bathrooms and train stations, and cleaning areas in the food industry. Microorganisms and viruses can attach to and survive on surfaces for a long time in most cases, increasing the risk for infection. One of the most attractive disinfection methods is paints and coatings containing nanoparticles that act as photocatalysts. Of these, titanium dioxide is appealing due to its low cost and photoreactivity. However, on its own, it can only be activated under high-energy UV light due to the high band gap and fast recombination of photogenerated species. The ideal material or coating should be activated under artificial light conditions to impact indoor areas, especially considering wall paints or frequent-touch areas like door handles and elevator buttons. By introducing dopants to TiO2 NPs, the bandgap can be lowered to a state of visible-light photocatalysis occurring. Naturally, many researchers are exploring this property now. This review article highlights the most recent advancements and research on visible-light activation of TiO2-doped NPs in coatings and paints. The progress in fighting air pollution and personal protective equipment is also briefly discussed.

Graphical AbstractIndoor visible-light photocatalytic activation of reactive oxygen species (ROS) over TiO2 nanoparticles in paint to kill bacteria and coat frequently touched surfaces in the medical and food industries.

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Learn more IntroductionNosocomial infections account for 7% of hospitalizations in developed countries and 10% in undeveloped countries. Bacteria like Pseudomonas aeruginosa can survive on inanimate surfaces for days to months and Candida albicans from 1 to 120 days.1,2,3 Viruses mostly do not last that long, with survival times ranging from a few hours to a few weeks. These pathogenic microorganisms and viruses (MoVs)4,5,6 may cause life-threatening infections when entering the body through wounds, surfaces, or devices like catheters and intravenous lines. Furthermore, they can be self-transferred by patients from their own hands to the mucosa of the mouth or nose. MoVs can be transported in three main ways: aerosolized, for example, through flushing toilets;7, 8 kinematic, for example, an infected hand touching a surface,5 and hydrodynamic, for example, the practice of dowsing certain vegetables in water and then freezing it to keep them crisp has been known to introduce waterborne bacteria to food.9

Healthcare-associated infections (HAIs) are connected to environmental surfaces, potential reservoirs for pathogens allowing transmission from surfaces to patients and vice versa. Polymicrobial biofilms are a densely packed community of organisms (fungi, bacteria, and viruses) that exist at a phase or density interface. They are embedded within an extracellular matrix, often consisting of polysaccharides.10 In the human body, they shield the pathogenic microorganisms from antibodies and immune cells, resulting in chronic infections that are difficult to eliminate.11 They can also be found on medical implants and inanimate surfaces, including food processing surfaces, walls, ceramics, steel, etc. To compound the apparent threat of these biofilms, they are known to enhance the production of biofilm masses of other species. They can have an adverse effect on disinfection and cleaning practices.12,13,14,15 Biofilms do not only grow on wet surfaces; they can also be found in intensive care units and other surfaces in hospitals, as was shown by several research groups in the past.16,17,18,19

From the above, it is evident that there is a need for some form of surface protection or improvement to prevent microorganisms from attaching to them. However, the development of antimicrobial, antiviral surface coatings must consider that fungi, bacteria, and viruses interact differently with a surface. This is discussed in detail in the review of Tolker-Nielsen.20

The adherence of bacteria, for example, will be assisted by hydrophobic flagella. Very soon after the initial contact, the attachment can become gradually stronger within minutes by developing strategies to remove the obstructive water on the inanimate surface (see Fig. 1 for the steps followed during biofilm formation).21, 22

Fig. 1Schematic representation of the steps followed during biofilm formation. Reprinted from: Achinas, S. et al. A brief recap of microbial adhesion and biofilms. Appl. Sci. 2019, 9, 2801,21 with permission from MDPI

Full size imageFor the attachment of fungi to surfaces, hydrophobicity also plays a role. For example, Candida albicans cells may alter the features of their cell wall to attach efficiently to surfaces with different physical or chemical properties and even change their surface feature from hydrophobic to hydrophilic depending on the temperature they finds themselves in.23

The persistence of viruses on surfaces is affected by the material’s porosity, humidity, temperature, light conditions, the presence of biofilms, and pH.24 The potential of viral spreading via contaminated surfaces depends mainly on the total amount of virus deposited and the ability of the virus to maintain infectivity while it is in the environment. Attachments are primarily driven by electrostatic forces between ionizable amino acids or negative charges in the capsid.25

Limiting the continuous growth of pathogens on surfaces reduces the risk of transmission. Disinfection of surfaces is a big business. It is estimated that the global antimicrobial market size is more than USD 8 billion and will reach more than USD 20 billion by 2028.26

Although antimicrobial resistance is a natural phenomenon,27 modern malpractice accelerated it. The cost to healthcare systems due to resistance is a serious problem worldwide. In 2016, a United Nations Bulletin estimated that antimicrobial resistance is directly responsible for more than 700 000 deaths annually.28 The threat is not only related to microorganisms. The recent SARS-COVID-19 pandemic is an indication of that. Recent research has shown that the virus can survive for 28 days at 20°C on glass, stainless steel, and banknotes.29

Kumaravel et al.30 mentioned in a recent review that antimicrobial coatings with innovative, environmentally responsive, multifunctional features, such as continuous antimicrobial protection with scratch, abrasion, chemical and stain resistance with easy cleaning properties, offer opportunities to fight microbial resistance and provide better protection against viral outbreaks. Accordingly, the manufacturing sector urgently needs visible-light-activated antimicrobial coatings that do not contain expensive additives like Ag. This is driven by the highest regulatory standards in health care, the food industry, construction, electronics, med-tech, pharma, public infrastructure, and home environments.

Wang et al.31 recently reviewed the literature on commercial coatings for uses in spacecraft and space stations. The growth of biofilms in spacecraft is extenuated by the higher CO2 concentrations, microgravity conditions, and high humidity. To complicate matters, the resistance of biofilms increases as the number of microorganisms in the film increases. This forces the use of combined antimicrobial strategies using different types of agents. Naturally, photocatalytic paints and coatings that generate ROS in visible-light conditions could be excellent foils to bacterium resistance.

Sun and co-workers discussed the lessons learned from the COVID-19 pandemic in terms of antiviral surfaces in a recent review article32 and concluded that surfaces that provide antiviral ability through an excitonic effect to generate localized heat, light, free radicals, and free charges and carriers to kill or interfere with the adhesion and replication of the viruses and germs are probably one of the best and cheapest methods to combat future virus pandemics (see Fig. 2 for an illustration of promising antiviral coatings). Talebian et al.24 proposed that future technologies should focus on disinfection using metal oxide NPs because of their inherent broad range of antiviral activities, persistence, and efficacy at low dosages. They went a step further to suggest that the detection of the virus via nanotechnology could facilitate faster and more accurate identification even at the early stages of the infection due to the versatility of surface modification of nanoparticles.

Fig. 2Promising antiviral coatings based on the selection of materials and engineering of surface nanostructures and the antiviral action mechanisms. Reprinted from: Sun, Z. et al. Future antiviral surfaces: Lessons from COVID-19 pandemic. Sustain. Mater. Technol. 2020, 25, e00203,32, (copyright 2020) with permission from Elsevier

Full size imageVery shortly after the onset of the COVID-19 pandemic, NanoTechSurface, Italy, fabricated a robust and self-cleaning formula comprised of titanium dioxide and silver ions for disinfecting surfaces.33 Similarly, FN Nano Inc., USA, developed a photocatalytic paint based on titanium dioxide nanoparticles,24 which can destroy organic compounds like viruses on the surface upon exposure to light, damaging the viral membrane.

Recently, TiO2 nanoparticles were shown effective against HCoV-NL63 under various humidity conditions. Khaiboullina et al.34 showed that TiO2 nanoparticles retain virucidal efficacy even at very high humid conditions (85% relative humidity), predicating for broader use of TiO2 NPs coatings on outdoor surfaces. They illustrated that TiO2-coated surfaces have a viral inactivation property even on dried virus droplets.

Much research is being conducted to develop antimicrobial and antiviral coatings or paints for near-patient surfaces in hospitals and for applications in the food industry. Most of this research is focused on using various nanomaterials or combinations of them with metals or polymers. Of these materials, TiO2 is perhaps the most well known. It is a photocatalyst that exists in three main crystallographic forms, namely anatase, brookite, and rutile. The bandgap of these forms varies between 3 and 3.2 eV rendering it ultraviolet-active but inactive in the visible-light region.35 The disinfection nature of titania is well established to be about three times more effective than chlorine and 1.5 times more than ozone.36 It can kill a wide range of bacteria, fungi, and viruses.37, 38

When exposed to ultraviolet light, it breaks down water vapor in the air to produce free oxygen radicals that will attack whatever is on the surface, including organisms like those mentioned before (see Fig. 3 for the proposed mechanism).39 A recent review article discusses the mechanisms of disinfection using TiO2 in detail.40 In short, TiO2 activates valence band electrons (e?) which move to the conduction band, generating an exciton pair and leaving a positively charged hole (h+) in the valence band. The e? and h+ can recombine and either undergo recombination to radiatively or non-radiatively dissipate the excess energy as heat or light. This process can happen in the bulk of the NP or on the surface. The excitons can move to the surface and produce reactive oxygen species (ROS) by reacting with oxygen. The oxygen radicals formed can further react with H2O to produce hydroxide radicals. The biocidal action of TiO2 materials is thus ascribed to ROS, which breaks down the cell membrane of biomatter, leading to lipid peroxidation41,42,43,44 and eventually interfering with cellular respiration, inactivating a wide range of organisms.

Fig. 3Schematic representation of the mechanism followed by TiO2 for bacterial disinfection using either UV or visible light. Reprinted from Fisher, M.B. et al., Nitrogen and copper doped solar light active TiO2 photocatalysts for water decontamination, Appl. Catal. B, 2013, 130–131, 8–13.39 Copyright (2013), with permission from Elsevier

Full size imageThere are several reasons why TiO2 NPs are so sought after. They have high photoreactivity, are inexpensive, are very stable chemically, and have a self-cleaning ability which prevents the formation of biofilm masses on the surface.45, 46 They do have drawbacks too. Their fast electron–hole recombination, which corresponds to the degradation of the photoelectric energy into heat, meaningfully restricts the photooxidation rate of biomaterials or organic compounds on the surface.47, 48 To overcome this, TiO2 has been used with metal ions, noble metals, non-metals, other metal oxides, and polymeric structures like g–C3N4,49,50,51,52 to name a few, with improved results.

Due to the abovementioned, it is evident that there are opportunities to use TiO2 NPs in the healthcare environment, and unsurprisingly, they are considered by many researchers. There are many examples in the literature, ranging from applications where TiO2 was coated onto suture material for wound healing, on titanium implants, combined with nanofibers loaded with antibiotics as a controlled drug delivery system, used for self-sterilizing catheter coatings, on stainless steel as coatings and as paint additives or surface coatings.53,54,55,56,57,58,59,60,61

The physicochemical characteristics of NPs play a significant role in their efficacy against pathogenic microorganisms. Small-sized NPs boost antimicrobial and antibacterial properties due to the increase in the surface area-to-volume ratio; on the contrary, smaller particles are more toxic to mammalian cells, so increased leaching needs to be considered.62

The surface vacancy of NPs also plays a decisive role in biological activity. For example, the oxygen vacancy in ZnO NPs could be fine-tuned by doping on the Zn and O site by aliovalent substitution using, for example, N.63 This resulted in an increased generation of ROS. This was also seen for MoS2 nanosheets, where an increase in nanoholes led to increased antibacterial activity. Unlike known antibacterial mechanisms, these nanoholes serve as electron donors to biofilms, leading to increased electron transport capacity and effectively destroying proteins, intercellular adhered polysaccharides, and extracellular DNA.63, 64

The surface morphology and crystallographic planes of NPs have importance in their all-over function. For example, spherical Ag nanoparticles were more effective against Klebsiella pneumoniae than rod-shaped silver NPs. Similarly, Ag nanoplates which formed a (111) lattice plane, exhibited the most potent bactericidal effect on E. coli.65 Additionally, it was shown that the antibacterial activity of Cu2O nanocrystals is also facet dependent, with the octahedron morphology (exposing the (111) facet) displaying a higher antibacterial activity than the cubic morphology (exposing only the (100) facet).66

Several strategies prevent microorganisms from remaining on surfaces or destroying them. These include the self-explanatory kill-on-contact approaches, the release of antimicrobial substances by the coating over time, surfaces with high hydrophobicity or hydrophilicity, nanoprotrusions, and/or combinations of two or more of these methods,26 as discussed in detail in a recent review by Birkett et al. which covered many different types of nanomaterials and their antimicrobial applications.

Antifungal and antibacterial surfaces use similar destruction mechanisms, namely the generation of ROS and toxic ions. Antiviral actions of surfaces are categorized into six basic types.67 Ionic surfaces such as metals and polyethylenimines degrade the RNA/DNA, photosensitizing materials producing ROS (such as TiO2),68 adsorbing surfaces which cause membrane disruption through dehydration, sharp nanostructured surfaces like graphene cause membrane rupture through puncturing, controlled release of virucides by hydrogels and inactivating surfaces that contain biopolymers which can bind to the membrane or capsid protein.67

The recent review by Chen et al. discussed the incorporation strategies of TiO2 and other nanoparticles in paint and how inorganic binders can prevent the photodegradation of organic binders or other molecules in the paint.69 They also addressed the old argument about which of hydrophobic or hydrophilic surfaces are most efficient. On the one hand, hydrophobic surfaces could prevent biofouling or fungi and bacterial attachment. Conversely, hydrophilic surfaces promote better contact with biomatter and have improved moisture trapping essential for generating ROS.

Coatings that can improve the air quality in buildings in visible light have also been employed. In one such example, tungsten-doped TiO2 nanoparticles incorporated into a coating for medium-density fiberboards showed higher activity in NOx degradation than commercial products.70

Over the years, the development of paints moved toward using waterborne formulations instead of traditional organic solvent-borne options. These are safer for the environment and have less odor. Waterborne paints typically contain additives, binders, dyes, fillers, pigments, plasticizers, and solvents. The addition of TiO2 NPs to paints is not without challenges. The NPs can photodegrade the organic binders in the paint, and in some cases, the binders can trap NPs so that ROS cannot be generated.71 There is a lot to consider in coatings design; hydrophilicity versus hydrophobicity, washability, efficacy, leaching, and durability are just a few. Of these factors, durability is possibly the most important, as it has a direct financial impact. For example, when a coating of anatase/rutile TiO2 was tested in a real-life scenario (42% anatase and 38% rutile)72 and coated on outdoor limestone surfaces, it was found that the photocatalytic ability was almost totally reduced after one year. Also, the surface color of the limestone was changed by the end of the term of exposure. The massive influence of environmental factors such as humidity, temperature, and concentration of pollutants in the microstructure of the coating could influence photocatalytic activity. While these factors will be more limited in an indoor scenario, they cannot be ignored.73,74,75

The low-cost and anticorrosive properties of TiO2, its photocatalytic efficacy, and its ability to deactivate viruses, bacteria, and fungi make it one of the most attractive materials for surface coatings which, in many cases, must be spread over vast surface areas. The increased interest in TiO2 NPs as coatings is demonstrated by the increase in articles per year from just under 20,000 to almost 50,000 from 2010 to date (Fig. 4).

Fig. 4Diagrams reporting the number of publications per year for the reported time ranges. (adapted from Web of Science, Clarivate Analytics; date of search: May 10, 2022)76 using the following combinations of topic keywords: (a) TiO2 or titanium dioxide and coatings or paints and antibacterial or antibacterial or antimicrobial or antimicrobial or antifungal or antifungal or antivirus or antiviral and visible light; the number of publications corresponding to the orange portion of the bar have been obtained narrowing the search adding UV as a topic keyword

Full size imageWith this review, we intend to highlight the latest research on NPs with applications in indoor facilities like hospitals or food processing areas focusing on TiO2-containing paints or coatings and the strategies followed to make them more effective in visible-light conditions while maintaining their durability.

Toward visible-light activity–dopingMetal dopants: reducing band gap energyIt was mentioned before that the large bandgap of pristine TiO2 NPs (3–3.2 eV) makes it impossible to employ its photocatalytic properties in visible-light conditions; this can only be achieved by the incorporation of dopants that affect the electronic band structure enough to promote visible-light absorption and a red light shift in the bandgap. The valence band of titania consists of hybridized states of oxygen 2p orbitals and titanium 3d orbitals, while the conduction band consists of titanium 3d orbitals. TiO2 can be doped with either metals or non-metals or combinations of them.

The doping of TiO2 with transition elements with partially filled d-orbitals alters the charge transfer properties in such a way that photogenerated carriers are successfully separated, producing a shift in absorption properties. This happens when Ti4+ is replaced in the TiO2 lattice by a transition metal, creating a new energy state in the band gap of TiO2. The localized d-electron state of the transition metals captures electrons from the titania valence band, suppressing the recombination of electrons and photogenerated holes. Transition metal ions like Mo6+ have a very similar radius to Ti4+, affording easy substitution and a narrow bandgap.77

The incorporation of rare earth metals into the TiO2 crystal lattice can also activate visible light since the large mismatch between the charge and ionic radii between the dopants and the titania affords lattice defects. Rare earth dopants with 4f, 5d, and 6s electrons introduce impurity energy levels by introducing orbitals between the conduction and valence bands which act as trapping centers for photogenerated species.78, 79

Yu and co-workers synthesized V-doped TiO2 NPs by the solgel hydrolysis method using titanium butoxide (Ti(OBu)4) and vanadyl acetylacetonate (VO(acac)2) as the precursor and dopant, respectively. The bandgap energy of 0.45% V-doped TiO2 NPs was reduced to 2.34 eV with a distinct redshift from 3.25 eV. The photocatalytic activity of these NPs exhibited two times higher enhanced visible-light-induced photocatalytic performance compared to the pristine TiO2 NPs when used to reduce methylene blue.80

Some noble-metal NPs (Ag, Au, Cu, Pt and Pd) can absorb light from the visible to the near-infrared range due to localized surface plasmon resonance (LSPR). The term LSPR describes the oscillation of metal particle-free electrons. Free electrons are set into oscillatory vibrations when NPs are irradiated with a resonant frequency similar to the oscillating frequency.81

When dopants like Au and Ag NPs are used, LSPR generates hot electrons and holes in the TiO2 conduction band under visible light. The holes can capture conduction electrons of TiO2, reducing the charge recombination processes.

Non-metal dopants: reducing band gap energyThe incorporation of non-metals also resulted in the narrowing of the band gap energy and, accordingly, having a positive effect on the photocatalytic activity of the doped TiO2 composite in visible light. The ionic radii of nitrogen and oxygen are comparable; thus, doping TiO2 via the replacement of oxygen with nitrogen does not have a high formation energy barrier.

When nitrogen is incorporated into the crystal lattice of TiO2, its spectral response is extended to the visible region82, 83 because of the position of the N 2p state above the valence band.84 When TiO2 fiber is doped with N during the synthesis, the band gap energy was reduced from 3.39 to 3.01 eV. This resulted in the improved photocatalytic degradation of methylene blue under visible light.85 In a review, Du et al. emphasized that doping TiO2 with N reduces the recombination efficiency of photoinduced charge carriers.86 However, the photocatalytic reaction rates of these N-doped TiO2 NPs are still low due to poor visible-light absorption (thus, ideal band-to-band absorption is not yet achieved).

Doping TiO2 with sulfur has been reported to be either cationic or anionic. Thus, sulfur can replace either Ti ions (see Fig. 5 for comparing the pristine and substituted TiO2 crystal cell unit) or lattice oxygen, respectively.87, 88 Both cases resulted in improved photocatalytic activity. The visible-light activity of S-doped TiO2 is caused by the band gap narrowing from mixing the S 3p and O 2p states. It has been shown that S-doped TiO2 can be effectively used as visible-light photocatalysts to kill bacteria (M. lylae).89

Fig. 5The cell unit of (left) pristine rutile TiO2 and (right) cell unit where one of the oxygen molecules was replaced with a sulfur molecule. Dark spheres: Ti; white spheres: O; gray spheres: S. Reprinted from Umebayashi, T. et al. Sulfur-doping of rutile-titanium dioxide by ion implantation: Photocurrent spectroscopy and first-principles band calculation studies. J. Appl. Phys. 2003, 93, 5156–5160.87 Copyright (2003), with permission from the American Institute of Physics

Full size imageThe band gap energy can be drastically reduced when TiO2 is doped with red phosphorus (P4) from 3.2 eV to 2.5 eV.90 It is proposed that the redshift obtained for the band gap energy of black phosphorus-doped TiO2 is due to the replacement of Ti4+ with pentavalent P.91 A review article is available on the band gap narrowing and photocatalytic activity of C-, N-, and S-doped TiO2.92

Doping of photocatalysts with carbon dots (CDs) has been reported to improve their ability to absorb long-wavelength photons in the visible-light region93 by inhibiting the recombination of the photoelectron–hole pairs.

CDs were grown on TiO2 sheets during a hydrothermal process using ammonium citrate (see Fig. 6 for an illustration of the preparation).94 The CDs act as solid-state electron mediators, producing a highly effective visible-light photocatalyst to degrade aquatic pollutants.

Fig. 6An illustration of the procedure used to prepare carbon dots/TiO2 sheets. Reprinted from: Shen, S. et al. Construction of carbon dots-deposited TiO2 photocatalysts with visible-light-induced photocatalytic activity to eliminate pollutants. Diam. Relat. Mater. 2022, 124, 108,896,94 (copyright 2022) with permission from Elsevier

Full size imageWhen macro-mesoporous TiO2 nanospheres were doped with carbon CDs (30% doping), they formed nanocomposites which were more successful in degrading methylene blue under visible light than the undoped TiO2 sample.95 In a different study, carbon quantum dots (CQDs) were randomly embedded in mesoporous TiO2 using a solgel-based approach without destroying the mesopores.96 This enhanced the visible-light photocatalytic activity of the TiO2 composite, as shown by the 98% removal of methylene blue compared to the 5% removal using the pristine TiO2 under the same conditions. The deposition of CQDs on TiO2 also resulted in an effective photocatalyst for wastewater treatment.97

The preparation of a CQD-modified TiO2 composite via a hydrothermal reaction resulted in a photocatalyst which could evolve hydrogen under visible light four times more efficiently than pure TiO2.98 It was proposed that the CQDs photosensitized the TiO2 to be able to respond under visible light, resulting in a dyad structure which could evolve hydrogen. When this CQD-modified TiO2 composite was exposed to UV–visible irradiation, the CQD took on a different role, acting as an electron reservoir. This results in the more efficient separation of photoelectron–hole pairs of TiO2.

Some researchers doped the CQDs with other non-metals, such as S, N and P, and then incorporated them on TiO2, resulting in even more superior photocatalysts.99, 100

Photocatalytic activityNanomaterials exhibit exceptional photocatalytic activity making them a choice material for applications such as environmental remediation, decomposition/degradation of harmful substances (such as dyes, industrial effluent, bacteria, organic toxins, and metal ions), and renewable energy sources. Factors influencing the photocatalytic ability of nanoparticles like TiO2 include surface area, crystallinity, crystal phase, crystal shape (facet and morphology), crystallite size, and dopants.101

The review of Padmanabhan et al. discussed how the different facets of TiO2 NP surfaces could be engineered to obtain better photocatalysis.102 To improve the visible-light photocatalytic activities of the TiO2 (101) surface, Han et al. used DFT calculations to study TiO2(101) facets doped with 4d transition metal atoms by systematically investigating the geometric structures, doping methods, and the optical properties of the doped surfaces. They found that the visible-light absorption can be enormously increased by doping with Y, Zr, Nb, Mo, and Ag and only weakly increased by doping with other 4d transition metals. Y and Ag-doped NPS showed the most improvement of the TiO2(101) surfaces among all the elements studied.103

By replacing the microstructural building units of hard biotemplated TiO2 typically obtained from solgel methods, Jiang et al.104 used solvothermal techniques to prepare both convex and concave nanotextured surfaces impregnated on biotemplates (tobacco stems) using tetrabutyltitaniumdioxide, glycerine or HF, respectively, with isopropanol as solvent. The convex material had the highest visible-light photoactivity for the reduction of tetracycline, more than 20 times faster than the new TiO2 material. XPS measurements showed that the catalysts had advantageous carbon impurities obtained from the biomass during synthesis. Also, the bandgap values of this material were calculated as 2.89 eV, confirming the sensitization of biomass carbon dopants.

The photocatalytic performance of TiO2 can be enhanced even further by incorporating a dopant. Ye et al. produced C–TiO2 nanocomposites by a facile calcination approach and acid etching using starch as the carbon source and Fe2O3 as a graphitization catalyst precursor. After calcination at 800°C, the iron species were removed by washing with hot HCl. The photocatalytic degradation rate of tetracycline was six times higher than pristine TiO2 under visible light. Also, after seven cycles, the material showed no decrease in photocatalytic activity.105

Lee et al. synthesized a hybrid nanocomposite composed of one-dimensional N-doped TiO2 nanotubes (N-TNTs) and two-dimensional graphitic carbon nitride nanosheets (g–CNNs) to obtain visible-light photocatalysis. They got 98% degradation of rhodamine B after 150 min of exposure to solar lighting. Moreover, the composites were still as effective after four cycles, indicating durability and stability.106

When designing a photocatalyst containing at least two entities, attention to optimization of the molar ratio and the effect of calcination temperature on the photocatalytic properties is an important aspect often under-investigated. N-doped TiO2 NPs synthesized via the glycerol-assisted solgel technique and with variation nitrogen-to-titanium (N:Ti) molar ratio, calcination temperature, calcination duration and TiO2 loading were investigated for the photodegradation of formaldehyde vapor under visible light. All the N-doped catalysts exhibited a narrow bandgap (2.64–2.50 eV) and small particle sizes ranging from 23.12 to 25.17 nm. The photodegradation results were the highest (70.59%) for an N:Ti molar ratio of 20:1. N-doped TiO2 calcined at 300°C for 1 h provided the highest catalytic efficiency.

It is widely accepted that anatase TiO2 has better photocatalytic properties, but rutile TiO2 is cheaper and chemically more stable. By incorporating Mn and graphene (G) into TiO2 nanowires (T(G–Mn) NW) in a two-step process (facile electrospinning followed by a hydrothermal process),107 it is possible to manipulate the material’s crystal structure by changing the annealing temperature (see Figs. 7a–7f for the SEM images). At 550°C, a mixture of rutile and anatase was confirmed by powder diffraction methods. By elevating the temperature to 800°C, only pure rutile T(G–Mn) NW was obtained. Interestingly, this was also the photocatalyst with the superior photocatalytic performance under visible-light reduction of NOx (see Figs. 7g and 7h for the comparative graph of photocatalytic efficiency). The authors contributed this to the variations in oxygen vacancy concentrations and the Ti–O defects that arise because Mn and G are included in the crystal lattice.

Fig. 7SEM images of T(G-Mn)1 annealed at (a) 550°C and (d) 800°C, T(G-Mn)2 annealed at (b) 550°C and (e) 800°C, and T(G-Mn)3 annealed at (c) 550°C and (f) 800°C, showing the structure manipulation. The graphs indicate the photocatalytic efficiency for the removal of NOx by different T(G-Mn) annealed at (g) 550°C and (h) 800°C. Reprinted from: Lee, J.-C., et al. Manganese and graphene included titanium dioxide composite nanowires: fabrication, characterization and enhanced photocatalytic activities. Nanomaterials 2020, 10, 456,107 (copyright 2020) with permission from MDPI

Full size imageCombining Cu2O, graphene, and TiO2 also gives excellent photocatalytic properties and good stability. TiO2/G/Cu2O nanosheets were fabricated on carbon fiber in a three-step process and evaluated the degradation of rhodamine B (RhB) in visible-light conditions.108 After 180 min, the RhB was degraded by 80%, compared to TiO2/Cu2O nanosheets which degraded RhB by 40%. The graphene acts as an electron sink and accepts photoelectrons, preventing the formation of photoelectron-hole pairs.

Fe/TiO2 and Co/TiO2 NPs were synthesized by solgel methods, and their photocatalytic activity under visible-light degradation of carbamazepine has been evaluated and compared to pristine TiO2 NPs.109 Under UV-A light, the 1 wt% Fe/TiO2 NPs outperformed the other materials substantially, reaching 96.9% degradation after 240 min. However, when the same reactions were performed in visible-light conditions, all three catalysts performed poorly, with only 12.54% of carbamazepine degradation obtained after 240 min using the 1 wt% Fe/TiO2 NPs (again, the best performer).

The physicochemical properties of noble-metal nanoparticles determine the photocatalytic activity of TiO2-modified NPs. TiO2 modified with mono- and bimetallic nanoparticles of Pt, Cu, and Ag were prepared using chemical and thermal reduction methods. Their photocatalytic activity was examined for 2-propanol oxidation and hydrogen generation processes. The effect of size, metal type, and content of the different NPs on biocidal activity was also evaluated. The synthesis method significantly influenced the size of the nanoparticles but was also determined by the type of metal used. For example, the thermal reduction method produced smaller NPs than the chemical methods when Pt was used, but the opposite was found for Ag. Using light-emitting diodes, the biocidal test results indicated that Ag NPs obtained by chemical methods had the highest activity.

To develop less laborious processes to modify the surface of anatase TiO2 with gold or silver, Salomatina et al. added the calculated amount of NP precursors to acetic acid solutions of chitosan and then dispersed TiO2 particles in the solution, followed by enzymatic destruction of chitosan by chitosanase. As a result, Au and Ag NPs of small-size parameters were deposited on the TiO2 surface. As expected, the photocatalytic activity of the prepared NPs was not as effective under visible light as with UV light. This was attributed to the fact that the excitation of electrons in the valence band under UV light was imparted with excess energy, which increased their lifetimes at impurity-defect levels and in the conduction band.110

Zhang et al.111 fabricated a composite photocatalyst composed of polyethylene terephthalate (PET) filaments loaded with Ag–N co-doped TiO2 nanoparticles and sensitized with the water-insoluble disperse blue 183 dye for applications in water purification. For this review, the superior photocatalytic activity of the dye-sensitized Ag–N co-doped TiO2-coated PET filaments need mentioning as this technology can be transferred to indoor applications. The photodegradation results of methyl orange (MO) dye solution showed that the positive holes, ·OH–and ·O2? radicals, were the main reactive radical species under visible-light catalysis. Further, these filaments did not lose photocatalytic activity under repetitive experiments.

The microbial synthesis of nanoparticles is gaining increasing attention due to its more straightforward, greener, and economical approach. The synthesis of titanium dioxide embedded silver oxide nanocomposite structures (AgO/Ag2O@TiO2) using a cell-free growth culture supernatant of the bacteria Alcaligenes aquatilis was recently reported.112 Briefly, the cell-free supernatant was obtained by centrifugation after growing the culture in a nutrient broth at room temperature. After this, AgNO3 was added and stirred to reduce the silver. The formed nanoparticles were washed and added to more of the supernatant and hexafluorotitanate (K2TiF6) and kept at room temperature for 4 h while stirring. The fabricated photocatalyst had a band gap of 1.75 eV and was able to degrade the Reactive Blue-220 dye almost completely under visible light in 90 min.

Antibacterial activity: crystal properties and dopants for visible-light photocatalytic disinfectionTitanium dioxide nanoparticles are considered attractive antibacterial materials since they are chemically stable, not toxic, economical to produce, and, most importantly, they are photocatalytically active, thus producing ROS. Damp environments are excellent sources for microbe growth, assisting the need for water during ROS production.

The efficiency of H2O adsorption on TiO2 surfaces determines the generation of ROS and, in turn, antimicrobial effects. Theoretical and experimental studies showed that the molecular adsorption of H2O prefers to take place on anatase (101) surfaces, but the chemical reactivity for water decomposition occurs in the order (001)?>?(100)?>?(110)?>?(103)?>?(101).113 The surface potentials of the facets determine whether a facet will become an oxidation site or a reduction site during photocatalysis. For example, (110) is the reduction and (001)/(111) are the oxidation sites in faceted rutile crystals, respectively. Furthermore, the charge separation of photogenerated species is strongly facet dependent. In other words, the morphology of TiO2 NPs determines their photocatalytic behavior. Phadmanabahn et al.102 discussed how this morphology can be controlled by introducing high-energy facet stabilizing agents like amino or carbonyl groups, promoting the growth of certain facets above others.102 Recently, Chen et al. showed how vital the facet-dependent contact of TiO2 with graphene is for the photolytic behavior (see Fig. 8 for the proposed mechanism).114 The morphology of TiO2/graphene hybrids synthesized via a hydrothermally modified solgel method can be varied between nanoellipsoids with high-energy facets and nanoellipsoids exposing low-energy facets by just changing the quantity of water that determines the hydrolytic steps during crystal growth, as was shown by Phadmanabahn et al. recently.115

Fig. 8The proposed mechanism of facet-dependent contact 3D/2D heterojunctions for photocatalytic reactions. Reprinted from: Chen, L. et al. One-step solid-state synthesis of facet-dependent contact TiO2 hollow nano cubes and reduced graphene oxide hybrids with 3D/2D heterojunctions for enhanced visible photocatalytic activity. Appl. Surf. Sci. 2020, 504, 144,353,114 (copyright 2020) with permission from Elsevier

Full size imageThe effective generation of ROS can also be obtained by combining the charge separation properties of graphene with the surface plasmon resonance effects facilitated by Ag NPs. In this regard, an Ag/TiO2/rGO nanohybrid (rGO?=?reduced graphite oxide) was recently reported for its excellent antibacterial properties and activity toward E. coli and S. aureus and its self-cleaning ability. A 100% disinfection of the bacterial environments was obtained within 180 min of visible-light irradiation.116

Coating photocatalytic TiO2 NPs via an aerosol method to glass resulted in 99% efficiency antibacterial and antibiofilm activities against S. aureus.117

The antimicrobial efficacy of copper (Cu)-doped TiO2 (Cu-TiO2) was evaluated against E. coli and Staph. aureus under visible-light irradiation. The doping of TiO2 was obtained with a solgel method, using 0.5 mol% Cu. UV–Vis results indicated that the band gap was reduced to 2.8 eV. Through density functional theory (DFT) studies, the existence of oxygen vacancies created by the substitution of Ti4+ by Cu+ and Cu2+ ions was confirmed. A significantly high bacterial inactivation (99.9999%) was attained in 30 min of visible-light irradiation by Cu-TiO2.118

In another study, silver and gold were used to modify commercial titania. They were tested for their antibacterial (Escherichia coli (E. coli)) and antifungal (Aspergillus niger (A. niger), Aspergillus melleus (A. melleus), Penicillium chrysogenum (P. chrysogenum), Candida albicans (C. albicans)) activity under visible-light irradiation and in the dark. The Ag-modified NPs showed remarkably high antibacterial activity and decomposed bacterial cells under visible-light irradiation. The gold-modified samples were almost inactive against bacteria in the dark but showed a significant bactericidal effect under visible-light irradiation. This was attributed to the plasmonic excitation of titania by the localized surface plasmon resonance of gold. The antifungal activity tests showed efficient suppression of mycelium growth by bare titania and suppression of mycotoxin generation and sporulation by gold-modified titania.119

On their own, CQDs have been reported to show bacteriostatic and bactericidal properties under photodynamic conditions.120 The combination of CQDs and TiO2 has resulted in better photocatalytic antibacterial activity than pristine TiO2.97 The antibacterial properties of CQDs-TiO2 reached 90.9% and 92.8% efficiency against the Gram-negative, Gram-positive E. coli and S. aureus strains under visible-light irradiation (see Fig. 9 for the TEM images of the cells).121 The CQDs-TiO2 nanocomposite could be recycled seven times.

Fig. 9The TEM images of E. Coli cells after introducing TiO2 and CQDs-TiO2 and irradiation with visible light. Reprinted from: Yan, Y. et al. Carbon quantum dot-decorated TiO2 for fast and sustainable antibacterial properties under visible light. J. Alloys Compd. 2019, 777, 234–243,121 (copyright 2019) with permission from Elsevier

Full size imageDopants in coatings and paintsSeveral paints on the market worldwide are already enriched with TiO2 or doped TiO2 to degrade priority pollutants like NO. Unfortunately, while many of these paints can degrade NO by more than 80%, the formation of NO2 as a by-product is either ignored or neglected. To demonstrate this, Kotzias et al. used TiO2 doped with manganese (0.1 wt%) and were able to degrade NO up to 95% in indoor conditions using only visible light with a conversion efficiency of NO to NO2 of 2%. This was compared to commercial products, some of which showed up to 8% conversion (see Fig. 10 for the comparative graph).122 This shows the importance of focusing on photocatalytic efficiency and minimizing the emission of harmful substances.

Fig. 10Graphs showing the percentage conversion from NO to NO2 of several commercial products under visible-light illumination. Adapted from: Kotzias, D. et al. Smart surfaces: photocatalytic degradation of priority pollutants on TiO2-based coatings in indoor and outdoor environments-principles and mechanisms. Materials (Basel). 2022, 15, 402,122 (copyright 2022) MDPI

Full size imageThe synthesis of a superhydrophobic WO3–TiO2 nanorod (MWT) dispersed in polydimethylsiloxane (PDMS) for use as a building coating by spraying methods produced superior durability and antifouling properties even after 450 days of practical application on an outdoor surface. The excellent self-cleaning property of the coating was confirmed by the removal and resistance to the adhesion of powder particles. The degradation efficiency under visible light decreased by 4.74% after the first five cycles due to the adhesion of oxidation products. This was restored after flushing with water.123

Salvadores et al. formulated photocatalytic paints using pristine anatase or carbon-doped anatase. The paints were tested in indoor and outdoor environments against acetaldehyde and NO, respectively. A photoreactor was irradiated with fluorescent lamps with wavelengths varying from 310 to 710 nm for the indoor experiment. The formulation of a carbon-doped sample showed the highest acetaldehyde conversion, reaching almost 60% in 60 min. The outdoor experiments under UV light showed that the conversion ability of the different paints all decreased substantially with time, possibly due to oxidation products forming on the surfaces.124, 125

Currently, most commercial self-cleaning window surface coatings can only be activated by UV light. Recently, Peeters et al. manufactured a transparent photocatalytic self-cleaning Au/TiO2 coating that is activated in normal light conditions.126 Pre-fabricated gold nanoparticles were made compatible with the organic medium of a TiO2 solgel coating suspension, resulting in a one-pot coating suspension. Homogeneous, smooth, highly transparent, and photoactive gold-embedded anatase thin films were obtained through spin coating methods. A clear redshift (see Fig. 11 for the UV–Vis spectra) of the surface plasma resonance band was observed for embedded Au nanoparticles (626 nm) compared to the colloidal suspension of PVP-stabilized Au nanoparticles (521.5 nm). The fact that the Au NPs were partially or fully embedded in the TiO2 matrix enhanced its photocatalytic performance. It should protect it against the detachment often observed when NPs are not embedded in the matrix.

Fig. 11Graphs showing the normalized UV–vis absorption spectra of (a) the colloidal Au NPs suspensions before (blue curve) and after (red curve) ligand exchange and (b) of the Au NPs modified films on glass with different loadings. A comparison of the two graphs clearly shows the redshift. Reprinted from: Peeters, H. et al. Plasmonic gold-embedded TiO2 thin films as photocatalytic self-cleaning coatings. Appl. Catal. B Environ. 2020, 267, 118,654,126 (copyright 2020) with permission from Elsevier (Color figure online)

Full size imageMoongraksathum et al. demonstrated the antiviral capability of a silver-doped TiO2 coating prepared using a solgel method and achieved photocatalytic activity under UVA and visible-light irradiation. A 1% wt concentration of Ag in the TiO2 solgel formed the most photoactive coatings. They tested the antiviral capability of their coating (on a glass substrate) against influenza A and enterovirus. They achieved a?>?99.99% (>?4.17-log) reduction in viral activity after irradiation with a 15 W UVA lamp for 20 min.127

Salvadores et al.124 used undoped and carbon-doped TiO2 in different amounts in the formulation of water-based and pseudo-paints and tested them for degradation of acetaldehyde under indoor conditions using visible light. They also evaluated the degradation of NOx under outdoor conditions using UV light. All the carbon-doped samples could degrade the two contaminants in different light conditions. The paint with the maximum amount of carbon-doped TiO2 produced the best conversion efficiency but not the highest quantum efficiency. This was obtained in paint with less doped carbon, making it the optimal formulation for energy use.

In another study, nanosized CuxO clusters were grafted onto TiO2 to provide antibacterial properties under dark conditions through the Cu(I) species in the clusters. Visible-light photocatalysis was afforded by the Cu(II) species, but the particles’ color turned brown as the copper concentration increased. This is one of the challenges of doped NPs for paint applications.52 However, Bucuresteanu et al. seem to have overcome this and produced a washable paint containing 2% Cu-doped TiO2 manufactured by a solgel process in a paint factory. The paint was tested and compared to standard painted areas in a community hospital over one year and showed a complete decrease to having zero microorganisms in the catalytically painted ward. In contrast, the contamination in the regular wards remained the same.128

Co-doping TiO2 with Cu and N via a solgel process for photocatalytic coatings on glass surfaces revealed that the doping narrowed the band gap energy and the antibacterial properties of TiO2 against E. coli and S. aureus increased with increased dopant concentration (see Fig. 12 for cell culture images).129

Fig. 12Images of the S. aureus cells after irradiation with light from a methane halide lamp radiation (a) the uncoated sample, (b) the undoped coating, and (c) the 0.75% Cu–N-doped coated sample. Reprinted from: Tahmasebizad, N. et al. Photocatalytic activity and antibacterial behavior of TiO2 coatings co-doped with copper and nitrogen via sol–gel method. J. Sol–Gel Sci. Technol. 2020, 93, 570–578,129 (copyright 2020) with permission from Springer Link

Full size imageA thin coating of TiO2 NP was prepared by aerosol flame synthesis and direct thermophoretic deposition, which resulted in the formation of a superhydrophylic coating.130 This coating could be activated by standard room illumination to inhibit Staphylococcus aureus.

When surfaces such as medical grade stainless steel 316L were coated with TiO2 or SiO2-TiO2 using soft lithographic and Dip-Pen Nanolithographic methods, a reduction of 60% bacterial (Streptococcus mutans) adhesion to the surface was observed.131 Additionally, bacterial adhesion was reduced even further when exposed to UV light.

Dopants and incorporation into textilesIncorporating nanomaterials into textile surfaces has initiated the development of new advanced nanocomposite textile products. Preparing TiO2 NPs containing textiles is relatively uncomplicated; however, insufficient anchoring of the TiO2 NPs to certain fibers i