Developments in photocatalytic antibacterial activity of nano TiO<Subscript>2</Subscript>: A review

TiO₂, which is one of the most explored materials, has emerged as an excellent photocatalyst material for environmental and energy fields, including air and water purification, self-cleaning surfaces, antibacterial and water splitting. This review summarizes recent research developments of TiO₂-based photocatalyst used for photocatalytic antibacterial applications. Several strategies to enhance the efficiency of TiO₂ photocatalyst are discussed, including doping with metal ions, noble metals, non-metals, and coupling with other materials. The mechanism of photocatalytic antibacterial activity in the presence of nano-sized TiO₂ is also discussed. The modified TiO₂ photocatalyst significantly inhibits the growth of bacterial cells in response to visible light illumination. TiO₂ photocatalysis appears to be promising as a route of advanced oxidation process for environmental remediation.     

Preparation of TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> composite oxide and its photocatalytic degradation of rhodamine B

The composite semiconductor photocatalyst TiO₂/SiO₂ was prepared by template-hydrothermal method using carbon spheres as the template. The structural and optical properties of TiO₂/SiO₂ were characterized by XRD, SEM, BET, UV–Vis DRS, TG-DTA, PL techniques. The formation of hydroxyl radical on the surface of TiO₂/SiO₂ was studied with terephthalic acid as the probe molecule, combined with fluorescence technique. The results showed that the specific surface area of TiO₂/SiO₂ composite was 327.9 m²/g, and the specific surface area of TiO₂/SiO₂ was larger than that of pure TiO₂. Photocatalytic degradation of rhodamine B showed that TiO₂/SiO₂ composite oxide under visible light illumination 40 min, the degradation rate was 98.6 % and the degradation rate of pure TiO₂ was only 11.9 %. The apparent first-order rate constant of TiO₂/SiO₂ was 33 times that of pure TiO₂ and more than 6 times that of P25 when the molar ratio of Ti to Si was 1:1 under visible light irradiation. Moreover, it’s also as much as 5 times that of pure TiO₂ and is more than 1 times that of P25 under UV light irradiation 25 min. Based on the experimental results, ^·O₂ ^? and h⁺ were suggested to be the major active species which was responsible for the degradation reaction. The increased photocatalytic activity of TiO₂/SiO₂ may be mainly attributed to effectively suppressing the recombination of hole/electron pairs. After the photocatalyst TiO₂/SiO₂ was reused 5 times, the degradation rate of rhodamine B could reach 89.2 % under visible light irradiation. Moreover, The composite semiconductor photocatalyst TiO₂/SiO₂ was selective towards the degradation of rhodamine B.     

Template-free preparation of TiO<Subscript>2</Subscript> microspheres for the photocatalytic degradation of organic dyes

TiO₂ microspheres were successfully synthesised by simple solution phase method by using various amount of titanium butoxide as precursor. The prepared TiO₂ were characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance absorption spectra (UV-DRS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). XRD analysis revealed that the as-synthesized TiO₂ microsphere poses an anatase phase. The photocatalytic degradation experiments were carried out with three different dyes, such as methylene blue, brilliant black, reactive red-120 for four hours under UV light irradiation. The results show that TiO₂ morphology had great influence on photocatalytic degradation of organic dyes. The experimental results of dye mineralization indicated the concentration was reduced by a high portion of up to 99% within 4 hours. On the basis of various characterization of the photocatalysts, the reactions involved to explain the photocatalytic activity enhancement due to the concentration of titanium butoxide and morphology include a better separation of photogenerated charge carriers and improved oxygen reduction inducing a higher extent of degradation of aromatics.     

Highly efficient photocatalytic degradation of methylene blue using carbonaceous WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> composites

Recent improvements in the performance of photocatalysts made it possible to tackle pollution through environment friendly methods. This study investigates the modification of the photocatalytic activity of TiO₂ by employing WO₃ and conductive polymers, namely, polyaniline (Pani) and polypyrrole (Ppy). Basing on our previous improvement of TiO₂ using a conductive polymer and activated carbon (AC), this study determines the activated carbon forms of TiO₂. The prepared composites are characterized using X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared, thermogravimetric analysis, Brunauer–Emmet–Teller, and UV–Vis spectroscopy. The specific surface area of the mesoporous composites is as follows: WO₃/TiO₂·AC (Pani) > WO₃/TiO₂·AC (Ppy) > WO₃/TiO₂·Pani > WO₃/TiO₂·Ppy (127 > 98 > 68 > 44 m² g^?1), which exhibited a similar trend to the photocatalytic performances (100 > 95 > 91 > 72 % conversion rate). This result could be attributed to higher porosity, surge of charge separation, and photo-responding range extension induced by the synergistic effect of WO₃, conducting polymers, and TiO₂ in the samples.     

Photoelectrocatalytic degradation of phenol using a TiO<Subscript>2</Subscript>/Ni thin-film electrode

As a new type of photoelectrode, TiO₂/Ni thin-film electrode was prepared by dip-coating technique. The structural and surface morphology of electrode was examined by X-ray diffraction (XRD) and scanning electron microscope (SEM). Effects of initial phenol concentration, pH value, number of film layers, voltage of electrical bias applied, variation of inorganic salt type and types of dissolved gas on the photoelectrocatalytic (PEC) degradation of phenol using ultraviolet (UV) illuminated TiO₂/Ni thin-film electrode were investigated. The mechanism of PEC degradation of phenol was also studied by analyzing reaction intermediates.     

Characteristics and photocatalytic degradation of methyl orange on Ti-RH-MCM-41 and TiO<Subscript>2</Subscript>/RH-MCM-41

Our purpose was to synthesize, characterize and test photodegradation of methyl orange on two catalysts containing 10 wt% titanium supported on mesoporous MCM-41 synthesized with rice husk silica. The first catalyst was Ti-RH-MCM-41 prepared by adding tetrabutyl orthotitanate (TBOT) in a synthetic gel of RH-MCM-41, and the second catalyst was TiO₂/RH-MCM-41 prepared by grafting TBOT on the preformed RH-MCM-41. The mesoporous structures were observed on both catalysts and they had surface area of 1,073 and 1,006 m²/g. The Ti in Ti-RH-MCM-41 was in the form of Ti(IV) with tetrahedral geometry residing in the mesoporous structure. This form was less active for photodegradation of methyl orange than the other one. The Ti in TiO₂/RH-MCM-41 was anatase titania with octahedral geometry located outside the mesoporous framework. This form was more an active phase for the photodegradation and the reaction parameters on this catalyst were further investigated. The optimum catalyst weight to methyl orange volume ratio was 5 g/L and the optimum initial concentration of the dye was 2.0 ppm. The degradation rate obeyed pseudo-first order and the adsorption of methyl orange on TiO₂/RH-MCM-41 obeyed Langmuir isotherm.     

Enhancement of photocatalytic disinfection of surface modified rutile TiO<Subscript>2</Subscript> nanocatalyst

We demonstrate a facile modification of rutile TiO₂ and its anti-bacterial activity under solar irradiation. The modification of rutile TiO₂ was done by microwave-assisted hydrothermal process with hydrogen peroxide (H₂O₂) as a solvent. The structural properties were analyzed by using XRD and Raman studies. The modified rutile TiO₂ shows no change either crystalline phase or crystalline size. The formation of Ti-OH was observed in Raman study. The TEM analysis shows modification on the surface of rutile TiO₂. The photocatalytic disinfection of Escherichia coli (E. coli) under solar irradiation shows double-times better performance by modified rutile TiO₂ compared to rutile TiO₂. The enhancement of anti-bacterial activity was attributed surface modification and Ti-OH.     

Solar photocatalytic detoxification of cyanide by different forms of TiO<Subscript>2</Subscript>

The photocatalytic efficiencies of TiO₂ nanocrystals of different modifications (anatase, rutile, P25 Degussa, Hombikat), to oxidize cyanide ion and subsequently the cyanate also, under natural sunlight at 950±25W m⁻² in alkaline solution have been compared. The oxides have been characterized by powder XRD, UV-visible diffuse reflectance and impedance spectroscopies. Under identical solar irradiance, the reaction follows Langmuir-Hinshelwood kinetics on cyanide, and depends on the apparent area of the catalyst bed and dissolved oxygen. However, the adsorption of cyanide on TiO₂ in dark is too small to be measured analytically. The photocatalytic activity of TiO₂ is not solely governed by the band gap or charge-transfer resistance or capacitance or phase composition but is in accordance with the specific surface area or the average crystallite size; rutile is an exception.     

Facile synthesis of TiO<Subscript>2</Subscript>/ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocomposite by sol-gel auto combustion method for superior visible light photocatalytic efficiency

Photocatalytic composite materials having photon absorption capability in the range of visible light were synthesized by loading TiO₂ (5, 10, 15, and 20 wt%) on ferrite nanocomposites by sol-gel auto-combustion method. The synthesized nanocomposites were analyzed using X-ray diffraction, Transmission electron microscopy, diffuse reflectance spectroscopy and N₂ adsorption techniques. The generation of photo active hydroxyl radicals for all the synthesized composites was found higher under the irradiation of red LED (RLED irradiation) which was confirmed by degradation of rhodamine B dye under irradiation of RLED. Photocatalytic activity of the synthesized nanocomposites was also carried out under irradiation of ultraviolet (UVLED) and blue (BLED) light emitting diodes, which is comparatively less than for the reaction under red LED irradiation. The operational parameters like catalyst amount, pH and concentration of dye solution were studied and ESI-MS degradation pathway is proposed by analyzing the degraded samples.     

P123 assisted morphology-engineered and hierarchical TiO<Subscript>2</Subscript> microspheres for enhanced photocatalytic activity

In this study, hierarchical titanium dioxide (TiO₂) microspheres with controlled morphology derived from calcination treatment of hierarchical titanate microspheres were fabricated. The obtained hierarchical TiO₂ microspheres with diameters of 1 to 2 µm were composed of polycrystalline anatase nanosheets with thickness of 10 nm. The morphology was manipulated by simply adjusting the molar ratio of tetrabutyl titanate/P123. At a low molar ratio of 17.04, TiO₂ microspheres composed of a large number of nanosheets closely packed together were obtained. At a high molar ratio of 34.08, TiO₂ hybrid architectures with polycrystalline anatase hierarchical microspheres and single-crystal anatase mesoporous (approximately 5 nm) nanospheres were obtained. Investigations on evolution formation revealed that P123 played a key role in the formation of a well-defined hierarchical structure. The photocatalytic performances of the obtained samples were investigated by the degradation of methylene blue and papermaking wastewater. When compared with commercial P25, the obtained hierarchical TiO₂ microspheres exhibit superior photocatalytic activity, high degradation efficiency, and good reproducibility. The product with hybrid architectures exhibited the highest photocatalytic activity. The chemical oxygen demand and the chroma removal rate of papermaking wastewater achieved 85.5 and 100%, respectively, after 12 h of photodegradation.     

Degradation kinetics of recalcitrant organic compounds in a decontamination process with UV/H<Subscript>2</Subscript>O<Subscript>2</Subscript> and UV/H<Subscript>2</Subscript>O<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> processes

In this study, degradation aspects and kinetics of organics in a decontamination process were considered in the degradation experiments of advanced oxidation processes (AOP),i.e., UV, UV/H₂O, and UV/H₂O,/TiO₂ systems. In the oxalic acid degradation with different H₂O₂ concentrations, it was found that oxalic acid was degraded with the first order reaction and the highest degradation rate was observed at 0.1 M of hydrogen peroxide. Degradation rate of oxalic acid was much higher than that of citric acid, irrespective of degradation methods, assuming that degradation aspects are related to chemical structures. Of methods, the TiO₂ mediated photocatalysis showed the highest rate constant for oxalic acid and citric acid degradation. It was clearly showed that advanced oxidation processes were effective means to degrade recalcitrant organic compounds existing in a decontamination process.     

Influence of heat treatment on thermally-reduced graphene oxide/TiO<Subscript>2</Subscript> composites for photocatalytic applications

Thermally-reduced graphene oxide/TiO₂ composites (TRGO/Ti) were prepared by the thermal reduction of graphene oxide/TiO₂ composite that was obtained from a simple, environmentally friendly, one-step colloidal blending method. The changes in structural and textural properties as well as their corresponding photocatalytic activities were investigated as a function of calcination temperature. The presence of stacked TRGO sheets significantly retarded both the aggregation and the crystalline phase transformation of TiO₂ as increasing the temperature from 200 to 600 °C. TRGO/Ti composites exhibited higher photocatalytic activity for the degradation of methylene blue in comparison with pure TiO₂ due to the increase in specific surface area and the formation of π-π conjugations between dye compounds and aromatic regions of TRGO. However, increasing the calcination temperature resulted in the lower photoactivity and slower kinetics, which can be ascribed to the decrease in surface area, the reduction of oxygen vacancies, and the loss of functional groups at the edges or on the basal planes of the TRGO sheets.     

Effects of surfactants on the preparation of TiO<Subscript>2</Subscript> nanoparticles in microwave-assisted sol-gel process and their photocatalytic activity

Nanosized TiO₂ particles were prepared through facile sol-gel reaction by using microwave-assisted method. To investigate the effects of surfactants on the formation of TiO₂, various additives (PVP, Triton X-100 and P123) were employed. The diameter of synthesized titania spheres could be controlled from 105 to 380 nm. The TiO₂ particles prepared with P123 triblock copolymer showed large surface area and high pore volume. It was attributed to the fact that the pore site, where the surfactant template initially existed, was generated upon calcination process. The characteristics of prepared TiO₂ nanoparticles were analyzed by using FE-SEM, TEM, XRD, FT-IR and N₂ adsorption-desorption. As an application of prepared composites for water treatment, their photocatalytic performances for the degradation of methylene blue dye were examined by using UV-vis spectrophotometer under room light irradiation. The prepared TiO₂ particles with Triton X-100 and P123 exhibited higher performance for methylene blue photo-degradation than that of P25. It was attributed to the effects of large specific surface area and high porosity.     

A photocatalytic performance of TiO<Subscript>2</Subscript> photocatalyst prepared by the hydrothermal method

Anatase phase nanocrystalline TiO₂ powders were prepared by hydrothermal method with the TTIP (titanium tetra isopropoxide) at 200 oC in a stirred autoclave system. The effects of synthesis conditions on the physical properties of catalyst were investigated by using XRD, SEM, DLS, DSC and BET. The TiO₂ powders obtained from the optimum condition showed uniform spherical shape, crystalline structure, submicron size with a sharp size distribution and few agglomerates. The optimum synthesis conditions were obtained within the covered experimental ranges. The photocatalytic activity of TiO₂ powders prepared by the hydrothermal method was tested for photooxidation of methyl orange.     

Effect of TiO<Subscript>2</Subscript> loading on the activity of V/TiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> in the catalytic oxidehydrogenation of ethylbenzene with carbon dioxide

TiO₂-Al₂O₃ mixed oxides with different compositions ranging from 40wt-% to 95wt-% of TiO₂ were prepared by sol-gel method and impregnated with different amounts of VO _x . Supports and catalysts were characterized by X-ray diffraction (XRD), physisorption, temperature preprogrammed reduction (H₂-TPR), and ammonia temperature programmed desorption (NH₃-TPD). TiO₂ content in the support had obvious effect on the crystal structure, texture characteristic, acid property, and catalytic activity in dehydrogenation of ethylbenzene (EB) with carbon dioxide. The highest catalytic activity was acquired when the TiO₂ content was 50 wt-%.     

Adsorption of thallium cations on RuO<Subscript>2</Subscript>–TiO<Subscript>2</Subscript> electrodes

The electrochemical ion-exchange properties of RuO₂–TiO₂ film electrodes with different composition have been studied in acidic and alkaline media. Thallium-cation uptake has been observed only from the latter and its extent was found to be a function of electrode potential and composition. At potentials near 0.0 V (RHE), the amount of adsorbed Tl⁺ exhibited a maximum, and decreased with increasing potential, reaching a broad minimum in the range 0.4–0.8 V. A further increase in the electrode potential, above about 1.0 V, led to an increase of adsorbed thallium species, essentially due to deposition of a few layers of Tl(III) hydroxide. In fact, the release of the latter species was found to be much slower than that of thallium ions adsorbed at 0.0 V. For the latter, in turn, the double injection/ejection mechanism, currently accepted to explain the charge-storage in oxide electrodes, seems to be confirmed. The high Γ values attained at 0.0 V indicate that the large ionic radius of Tl⁺ does not prevent its diffusion through the thinner pore texture of the oxide coatings, possibly because of its poor hydration, related with lower charge density at the ion surfaces.     

Li<Subscript>2</Subscript>Cu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> + SiO<Subscript>2</Subscript>/Ti compositions for the catalytic afterburning of diesel soot

Two methods were used to obtain a catalytically active oxide coating on the surface of titanium for the catalytic afterburning of diesel soot: plasma electrochemical formation of an oxide film on the surface of titanium and extraction pyrolytic deposition of the Li₂Cu₂(MoO₄)₃ compound. The Li₂Cu₂(MoO₄)₃/TiO₂ + SiO₂/Ti compositions synthesized by the single-step extraction pyrolytic treatment of the oxidized surface of titanium ensured a high burning rate of soot of ∼300°C. The subsequent deposition of Li₂Cu₂(MoO₄)₃ lowers the activity of the catalyst, due probably to the growth of molybdate phase crystallites and the filling of open oxide film pores. Double lithium-copper molybdate is able to reduce appreciably the concentration of CO in the oxidation products of soot. The advantages of these methods are the possibility of forming high-cohesion durable coatings on surfaces of any complexity, the simplicity of their implementation, and high productivity and low cost. The obtained results can be recommended for use in developing methods for creating composite coatings on catalytic soot filters.