Photocatalytic activity of titanium dioxide coatings: Influence of the firing temperature of the chemical gel

Heterogeneous photocatalysis can be exploited for the decomposition of micro-organisms which have developed on the surfaces of building materials. In this work, the efficiency of titanium dioxide coatings on fired clay products is examined. The sol–gel method is used to synthesize a fine TiO₂ powder with a specific surface area of 180 m² g^?1. Thermal treatment of the chemical gel at 340 °C leads to crystallisation in the anatase phase and with further temperature increase, crystallite growth. For thermal treatments in the range 580–800 °C, there is a progressive transition from anatase to rutile. However, despite a decrease in specific surface area of the powder attributed to aggregation/agglomeration, the coherent domain size deduced from X-ray diffraction measurements remains almost constant at 23 nm. Once the transition is completed, increase of thermal treatment temperature above 800 °C leads to further crystallite growth in the rutile phase. The thermally treated titania powders were then sprayed onto fired clay substrates and the photocatalytic activity was assessed by the aptitude of the coating to degrade methylene blue when exposed to ultraviolet light. These tests revealed that the crystallite size is the important controlling factor for photocatalytic activity rather than the powder specific surface area or the anatase/rutile polymorph ratio.     

Enhanced photocatalytic activity of electrospun TiO2/ZnO nanofibers with optimal anatase/rutile ratio

The photocatalytic characteristics of the TiO₂/ZnO nanofibers synthesized by electrospinning followed by calcinating at different temperatures to alter the anatase-to-rutile ratio are investigated. The results demonstrate that the photocatalytic activity of TiO₂/ZnO nanofibers is enhanced by optimizing the anatase/rutile ratio among the trade-off effects of the band-gap energy, the electron/hole recombination rate, and the surface area. When calcined at 650 °C, the TiO₂/ZnO nanofibers with optimal anatase/rutile ratio (48:52) balancing these trade-off effects have the highest photocatalytic efficiency both in the degradation of RhB in liquid and conversion of NO gas.     

Feasibility of using a rotating packed bed in preparing coupled ZnO/SnO2 photocatalysts

In this work, coupled ZnO/SnO₂ photocatalysts were prepared in a rotating packed bed (RPB) via co-precipitation. The precursors of coupled ZnO/SnO₂ photocatalysts were formed from solutions of zinc sulfate, tin tetrachloride and sodium hydroxide. The calcinations of these precursors yielded coupled ZnO/SnO₂ photocatalysts. The effect of calcination temperature on the characteristics and photocatalytic activity of coupled ZnO/SnO₂ photocatalysts was studied. The photocatalytic activity of coupled ZnO/SnO₂ photocatalysts was evaluated using the photocatalytic decolorization of methylene blue. The experimental results reveal that coupled ZnO/SnO₂ photocatalysts that were obtained by calcination at 600 °C for 10 h were the most efficient in decolorizing methylene blue.     

Synthesis and photocatalytic performance of hollow sphere particles of SiO2-TiO2 composite of mesocellular foam walls

Hollow Microspheres of SiO₂-TiO₂ photocatalysts whose walls are made up of mesoporous cellular foams were synthesized with the aid of hexane as a swelling agent and P123 as a pore template by an emulsion templating method. Pore structure of materials and crystal phase of titanium oxide was tailored by hydrothermal and calcination temperature during synthesis of samples. The samples were characterized with field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N₂ adsorption–desorption experiments, X-ray photoelectron spectroscopy (XPS) and X ray diffraction (XRD) techniques. The effect of pore structure and titania phase on photoactivity were evaluated by methylene blue (MB) degradation test under UV light as well. Results showed that hydrothermal temperature during synthesis process has a significant effect on pore and window sizes of mesostructured cellular foam. Interestingly, for the sample hydrothermally treated at higher temperature (130 °C), anatase to rutile transformation was avoided after calcination treatment as high as 800 °C. The highest photocatalytic activity was detected from the sample hydrothermally treated at 130 °C and calcined at 800 °C for which the highest degree of crystallinity and anatase phase as well as enhanced pore connectivity was obtained.     

Enhanced photocatalytic degradation of rutile/anatase TiO2 heterojunction nanoflowers

Rutile/anatase TiO₂ heterojunction nanoflowers were prepared via a facile one-step hydrothermal approach using titanium tetrachloride and urea as the raw materials, cetyl trimethyl ammonium bromide (CTAB) as the template. The prepared TiO₂ nanoflowers were characterized by XRD, SEM, TEM and BET analyses. The photocatalytic performance of the as-prepared TiO₂ samples for methyl blue degradation under simulated solar light was investigated. TiO₂ heterojunction nanoflowers with mixed rutile/anatase phase (prepared with 3 mmol CTAB) give the highest photocatalytic activity. In addition, TiO₂ nanoflowers show excellent stability after 9 cycles under the same conditions. These results suggested that the mixed phase anatase/rutile TiO₂ heterojunction nanoflowers have great potential for the future photodegradation of real dye waste water.     

Photocatalytic Nb2O5-doped TiO2 nanoparticles for glazed ceramic tiles

TiO₂ nanoparticles are currently used as coating for self-cleaning building products. In order to achieve high self-cleaning efficiency for outdoor applications, it is important that titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in nano-range, so that a large enough surface area is available for enhanced catalytic performance. In this work, TiO₂ nanoparticles doped with 0–5 mol% Nb₂O₅ were synthesized by co-precipitation. Nb₂O₅ postponed the anatase to rutile transformation of TiO₂ by about 200 °C, such that after calcination at 700 °C, no rutile was detected for 5 mol% Nb₂O₅-doped TiO₂, while undoped TiO₂ presented 90 wt% of the rutile phase. A systematic decreasing on crystallite size and increasing on specific surface area of TiO₂ were observed with higher concentration of Nb₂O₅ dopant. Photocatalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under ultraviolet and daylight illumination and compared to commercial standard catalyst (P25). The results showed enhanced catalysis under UV and visible light for Nb₂O₅-doped TiO₂ as compared to pure TiO₂. In addition, 5 mol% Nb₂O₅-doped TiO₂ presented higher photocatalytic activity than P25 under visible light. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap.     

Synergic effect of cation doping and phase composition on the photocatalytic performance of TiO2 under visible light

The synergic effect of cation doping and phase composition for the further improvement of the photocatalytic activity of TiO₂ under visible light is reported for the first time. Fe^3 + and Sn^4 + co-doped TiO₂ with optimized phase composition were synthesized through a simple soft-chemical solution method. The visible-light-driven photocatalytic activity of Fe^3 + and Sn^4 + co-doped TiO₂ was 5 times of that of Evonik P25 TiO₂ using degradation of methylene blue as model reaction. The synthesized photocatalysts were characterized by powder X-ray diffraction, UV–Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, ¹¹⁹Sn Mössbauer spectroscopy, and X-ray absorption fine structure spectroscopy. It is indicated that Sn^4 + doping can facilitate the phase transition from anatase to rutile. The different ratios of anatase and rutile can be achieved by tuning the amount of Sn^4 + doped into the lattice. Furthermore, the doping of Sn^4 + into TiO₂ lattice can stabilize the phase composition when Fe^3 + is co-doped. In the Fe^3 + and Sn^4 + co-doped TiO₂, Sn^4 + is mainly used to tune and stabilize the phase composition of TiO₂ and Fe^3 + acts as a doping cation to narrow the band gap of TiO₂. Both band gap and phase composition of TiO₂ can be tuned effectively by the simultaneous introduction of Fe^3 + and Sn^4 +. The synergic effect of optimized phase composition (anatase/rutile = 25/75) and narrowed band gap should be the two main reasons for the promoted photocatalytic activity of TiO₂ under visible light.     

Suspension high velocity oxy-fuel spraying of a rutile TiO2 feedstock: Microstructure,phase evolution and photocatalytic behaviour

Nano-structured TiO₂ coatings were produced by suspension high velocity oxy fuel (SHVOF) thermal spraying using water-based suspensions containing 30 wt% of submicron rutile powders (~180 nm). By changing the flame heat powers from 40 kW to 101 kW, TiO₂ coatings were obtained with distinctive microstructures, phases and photocatalytic behaviour. Spraying with low power (40 kW) resulted in a more porous microstructure with the presence of un-melted nano-particles and a lower content of the anatase phase; meanwhile, high powers (72/101 kW) resulted in denser coatings and rougher surfaces with distinctive humps but not necessarily with a higher content of anatase. Linear sweep voltammetry (LSV) was used to evaluate the photocatalytic performance. Surprisingly, coatings with the lowest anatase content (~20%) using 40 kW showed the best photocatalytic behaviour with the highest photo-conversion efficiency. It was suggested that this was partially owing to the increased specific surface area of the un-melted nano-particles. More importantly, the structural arrangement of the similarly sized TiO₂ nano-crystallites between rutile and antase phases also created catalytic “hot spots” at the rutile−anatase interface and greatly improved the photo-activity.     

Hydrothermal processing and in situ surface modification of metal oxide nanomaterials

In situ surface modification of TiO₂ and ZnO metal oxide particles has been carried out under hydrothermal conditions within a wide range of temperature and pressure (T = 150–400 °C; P = up to 20 MPa). The influence of the surfactant and selective doping with active metal ions on the crystal size, morphology, and photocatalytic activity of TiO₂ and ZnO particles has been carried out. A systematic characterization of the product has been carried out using powder XRD, FTIR, TGA, SEM/TEM, and UV–vis spectroscopy. Similarly the photocatalytic activity in these metal oxides varies with the size, shape and dopant metals.     

TiO2 nanofibers doped with rare earth elements and their photocatalytic activity

In the present study rare earth doped (Ln³⁺–TiO₂, Ln = La, Ce and Nd) TiO₂ nanofibers were prepared by the sol–gel electrospinning method and characterized by XRD, SEM, EDX, TEM, and UV-DRS. The photocatalytic activity of the samples was evaluated by Rhodamine 6G (R6G) dye degradation under UV light irradiation. XRD analysis showed that all the synthesized pure and doped titania nanofibers contain pure anatase phase at 500 °C but at 700 °C it shows both anatase and rutile phase. XRD result also shows that Ln³⁺-doped titania probably inhibits the phase transformation. The diameter of nanofibers for all samples ranges from 200 to 700 nm. It was also observed that the presence of rare-earth oxides in the host TiO₂ could decrease the band gap and accelerate the separation of photogenerated electron–hole pairs, which eventually led to higher photocatalytic activity. To sum up, our study demonstrates that Ln³⁺-doped TiO₂ samples exhibit higher photocatalytic activity than pure TiO₂ whereas Nd³⁺-doped TiO₂ catalyst showed the highest photocatalytic activity among the rare earth doped samples.     

Effect of heterojunction on photocatalytic properties of multilayered ZnO-based thin films

In the present study, the effects of the heterojunctions on the optical and structural characteristics and the resulting photocatalytic properties of multilayered ZnO-based thin films were investigated. The junctions were composed of semiconducting ZnO nano-porous films coated on the In₂O₃ and SnO₂ counterpart layers. The multilayered ZnO films based on the triple-layered Ag-doped indium oxide (AIO)/tin oxide (TO)/zinc oxide (ZnO), indium oxide (IO)/Ag-doped tin oxide (ATO)/zinc oxide (ZnO), indium oxide (IO)/tin oxide (TO)/zinc oxide (ZnO) and tin oxide (TO)/indium oxide (IO)/zinc oxide (ZnO) have been fabricated by subsequent sol–gel dip coating. Their structural and optical properties combined with photocatalytic characteristics were examined toward degradation of Solantine Brown BRL (C.I. Direct Brown), an azo dye using in Iran textile industries as organic model under UV light irradiation. Effects of operational parameters such as initial concentration of azo dye, irradiation time, solution pH, absence and presence of Ag doping and consequent of sublayers on the photodegradation efficiencies of ZnO nultilayered thin films were also investigated and optimum conditions were established. It was found that the photocatalytic degradation of azo dye on the composite films followed pseudo-first order kinetics. Photocatalytic activity of AIO/TO/ZnO interface composite film was higher compared with other films and the following order was observed for films activities: AIO/TO/ZnO > IO/TO/ZnO > ATO/IO/ZnO > TO/IO/ZnO. Differences in the film efficiencies can be attributed to differences in crystallinity, interfacial lattice mismatch, and surface morphology. Besides, the presence of Ag doping between layers that may act as trap for electrons generated in the ZnO over layer thus preventing electron–hole recombination.     

Synthesis and photocatalysis of TiO2 hollow spheres by a facile template-implantation route

Process variables such as reaction temperature (55 to 90 °C), calcination temperature (450 to 750 °C), and concentration of TiCl₄ precursor (26 to 105 mM) have been examined in order to tailor the surface area, crystallite size, and the anatase/rutile ratio of the polycrystalline TiO₂ microcapsules prepared by a template-implantation route in heptane solvent. The hollow capsules are all non-aggregating with nanoporous shell structure. Among the process variables examined, the Brunauer–Emmett–Teller (BET) surface area and the anatase/rutile ratio are found critically dependent on the reaction temperature, in which a reduced reaction temperature (from 90 to 55 °C) leads to a higher BET value (from 8.4 to 36.4 m^?2 g^?1), a predominant anatase phase (weight fraction of the anatase phase increases from 0.20 to 0.84), and an improved photodegradation of aqueous methylene blue (MB) dye under UV exposure (the degradation rate increases from 0.5×10^?2 to 5.5×10^?2 min^?1).     

One-step preparation of size-defined aggregates of TiO2 nanocrystals with tuning of their phase and composition

One-step route based on the thermal decomposition of the double salt (NH₄)₂TiO(SO₄)₂ (ammonium titanyl sulfate, ATS) is presented to prepare size-defined aggregates of Ti-based nanoparticles with structural hierarchy. The component of Ti-based networks is tunable from anatase/rutile TiO₂, nitrogen-doped TiO₂, TiN_xO_1−x, to TiN depending on the atmospheres and reaction temperatures. The as-prepared Ti-based powders were characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (DRS), and BET surface area techniques. It is found that TiO₂ in the predominant rutile phase could be achieved by the thermal decomposition of ATS in flowing Ar gas. Furthermore, the nitrogen-doped TiO₂, TiN_xO_1−x solid solution and TiN were prepared by the thermal decomposition of ATS in flowing NH₃ gas by varying the temperatures. The network of anatase TiO₂ with a specific surface area up to 64 m² g⁻¹ contains large mesopores with a mean diameter of ca. 15 nm, and the large pore size allows more accessible surface and interface available for the photocatalytic degradation of large-molecule dyes. The photocatalytic activity of the prepared TiO₂ and nitrogen-doped TiO₂ under UV–vis light irradiation is compared to Degussa P-25 using the photocatalytic degradation of methylene blue (MB) as a model reaction. The anatase TiO₂ nanoparticles derived from one-step route show the highly efficient photocatalytic activity for the degradation of MB in comparison with Degussa P-25. The presence of large-sized rutile in the TiO₂ powder decreases the specific surface area and thus the powder exhibits a lower photocatalytic activity.     

Enhanced photocatalytic activity of TiO2-ZnO thin films deposited by dc reactive magnetron sputtering

The effect of deposition conditions on the photocatalytic activity of TiO₂-ZnO thin films was studied. By using a (Ti)₉₀-(Zn)₁₀ alloy target, the samples were deposited at room temperature on glass substrates by dc reactive magnetron sputtering and post-annealed in air at 500 °C. The dependence of the physical properties of the films on the O₂/Ar gas ratio and the deposition working pressure was investigated. XRD patterns showed mainly the formation of the anatase phase of TiO₂. Optical absorption measurements exhibited a blue shift of the band-gap energy with increasing working pressure. XPS spectra indicated the presence of the Ti⁴⁺ and Zn²⁺ oxidation states, which correspond to TiO₂ and ZnO, respectively. The chemical state of Ti was further analyzed by means of the modified Auger parameter, α’, which gave a value of ca. 873 eV. The photocatalytic property of the films was assessed by the degradation of a methylene blue aqueous solution. The maximum photocatalytic performance was observed for the samples deposited at 3.0 mTorr and O₂/Ar gas ratio of 10/90. These results are explained in terms of the structural, optical, and morphological properties of the films.     

HIPed TiO2 dense pellets with improved photocatalytic performance

The effects of heating method and temperature on physical, structural and photocatalytic behaviors of TiO₂ pellets prepared by conventional heating and hot isostatic pressing have been evaluated. The pellets of submicron TiO₂ powders were heated to 600, 650, 700, 750 and 1000 °C using both processing methods in order to compare anatase to rutile phase transformation and densification behaviors. Bulk densities and porosities were calculated using the Archimedes method. XRD analysis were performed to calculate anatase/rutile ratios. Microstructures were characterized using SEM. Photocatalytic experiments have been performed under full spectrum irradiation. Degraded methylene blue samples were periodically monitored through UV–vis spectrophotometer to determine degradation kinetics. Anatase to rutile transformation is slightly faster and densification is better for lower temperatures for conventional heating, however HIPing gives better densification above 750 °C as it also retards rutile transformation. Mixed phase structures and HIPed samples showed the best photocatalytic performance which makes this method advantageous.     

Photocatalytic activity of rubber sheet impregnated with TiO2 particles and its recyclability

The preparation of rubber sheet impregnated with titanium dioxide particles is presented. This method is simple and low cost based on the use of commercial TiO₂ powder directly mixing with rubber latex (60% HA) and distilled water. The morphology and roughness of the sheet surface increased with increasing amount of distilled water. Sheet impregnated with anatase (Imp-An) showed uniform, small grains with dense structure and well surface coverage more than one with P25 (Imp-P25). Their photocatalytic activities were evaluated using methylene blue (MB) as a model organic dye compound. These impregnated sheets could degrade MB solution under UV-light irradiation. Comparing with the commercial TiO₂ samples in powder form (anatase from Carlo Erba and Degussa P25) the efficiencies of photocatalytic degradation of MB fall in the decreasing order as: P25 (powder) > anatase (Carlo Erba) (powder) > Imp-An sheet > Imp-P25 sheet. However, the impregnated sheet has an advantage over the loose powder that the catalyst sheet can be recovered after used and can be reused.     

Structural,optical and photocatalytic properties of hafnium doped zinc oxide nanophotocatalyst

A series of novel hafnium (Hf) doped ZnO nanophotocatalyst were synthesized using a simple sol–gel method with a doping content of up to 6 mol%. The structure, morphology and optical characteristics of the photocatalysts were characterized by XRD, SEM, TEM, FTIR, XPS, DRS and PL spectroscopy. The successful synthesis and chemical composition of pure and doped ZnO photocatalysts were confirmed by XRD and XPS. DRS confirmed that the spectral responses of the photocatalysts were shifted towards the visible light region and showed a reduction in band gap energy from 3.26 to 3.17 eV. Fluorescence emission spectra indicated that doped ZnO samples possess better charge separation capability than pure ZnO. The photocatalytic activity of Hf-doped ZnO was evaluated by the methylene blue (MB) degradation in aqueous solution under sunlight irradiation. Parameters such as irradiation time and doping content were found effective on the photoactivity of pure ZnO and Hf-doped ZnO. The photocatalysis experiments demonstrated that 2 mol% Hf-ZnO exhibited higher photocatalytic activity as compared to ZnO, ZnO commercial and other hafnium doped ZnO photocatalysts and also revealed that MB was effectively degraded by more than 85% within 120 min. The enhanced photoactivity might be attributed to effective charge separation and enhanced visible light absorption. It was concluded that the presence of hafnium within ZnO lattice could enhance the photocatalytic oxidation over pure ZnO.     

Synthesis and photocatalytic activity of ring-like anatase TiO2 with {001} facts exposed

Developing photocatalysts with specific morphology and good photocatalytic activities promises good opportunities to discover the geometry-dependent properties. In the present work, ring-like anatase TiO₂ with dominant {001} facets exposed were successfully synthesized via a one-pot solvothermal process of tetrabutyl titanate and hydrofluoric acid solution at 180 °C for 8 h. We found that hydrofluoric acid plays an important role in the formation of ring-like TiO₂. The morphology and microstructure were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauer–Emmett–Teller N₂ gas adsorption–desorption isotherms. The photocatalytic activity was evaluated by photocatalytic oxidation degradation of methylene blue aqueous solution under UV light. Results showed that ring-like TiO₂ with {001} facets exposed exhibited an excellent photocatalytic activities due to its unique structure: Nanosheets with hole.     

Synergy effect in the photocatalytic degradation of methylene blue on a suspended mixture of TiO2 and N-containing carbons

N-containing carbon materials were obtained from waste plum stones submitted to pyrolysis under Ar flow at 700 °C or to activation under steam at 800 °C and enriched with nitrogen by heating in a NH₃/air mixture at 270 °C or in NO at 300 °C. In situ mixtures of TiO₂ and carbons were prepared by the slurry method and methylene blue photodegradation was chosen as a model reaction to verify the influence of N-containing carbons on the photocatalytic activity of TiO₂ under artificial visible light irradiation. From the kinetics of methylene blue degradation an important synergy effect between both solids was detected with a remarkable increase up to a factor of 5.3 higher in the photocatalytic activity on TiO₂–C than that on TiO₂ alone. A mechanism for the photoassisting role of N-containing carbons upon the photoactivity of TiO₂ under visible light is discussed.     

Photooxidation of iodide ion on some semiconductor and non-semiconductor surfaces

In 80% aqueous ethanol, TiO₂ (anatase), ZrO₂, ZnO, V₂O₅, Fe₂O₃ and Al₂O₃ photocatalyze the oxidation of iodide ion but CdO and CdS do not; the wavelength of illumination is 365 nm. However, Fe₂O₃ fails to bring in a sustainable photocatalysis in 60% aqueous ethanol. The photooxidation of iodide ion on TiO₂, ZrO₂, ZnO, V₂O₅ and Al₂O₃ in 60% aqueous ethanol was studied as a function of [I⁻], amount of catalyst suspended, airflow rate, light intensity and solvent composition. The metal oxides examined show sustainable photocatalytic activity. Iodine formation is larger with illumination at 254 nm than at 365 nm. The mechanisms of photocatalysis on semiconductor and non-semiconductor surfaces have been discussed. Photocatalytic generation of iodine has been analyzed using a kinetic model. The photocatalytic efficiencies are of the order V₂O₅ > TiO₂ > ZrO₂ > ZnO > Al₂O₃ and V₂O₅ > TiO₂ > ZrO₂ > ZnO=Fe₂O₃ > Al₂O₃ in 60% and 80% aqueous ethanol.