Preparation of TiO2 nanofilm via sol–gel process and its photocatalytic activity for degradation of methyl orange

The preparation of TiO₂ nanofilm was conducted on common glass via the sol–gel process. Glacial acetic acid and diethanolamine were used as inhibitors to prepare acidic and alkaline TiO₂ sol, respectively. XRD, SEM, and EDS characterization showed that the film prepared from acidic TiO₂ sol had a narrow particle size distribution of 15–30 nm and relatively poor particle crystallization while in the case of the film from alkaline TiO₂ sol the nanoparticles were in a wide range of 10–80 nm and well crystallized. The photolysis evaluation through MO degradation revealed that the film from acidic sol possessed apparently better photocatalytic activity than that from alkaline sol. Heat treatment with longer time led to a 50% increase of the photocatalytic activity for the film.     

Impact of supercritical drying and heat treatment on physical properties of titania/silica aerogel monolithic and its applications

TiO₂–SiO₂ monolithic aerogels were homogeneously prepared using sol–gel method. Critical point of drying of TiO₂–SiO₂ gels with ethanol was studied for 30, 60, 90 and 120 min. Subsequently, the gels were dried with supercritical ethanol, resulting in amorphous aerogels that crystallized following heat treatment at 550 °C from 1 to 5 h. The TiO₂–SiO₂ aerogels were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements. The molar ratio of SiO₂:TiO₂ was 6 and the synthetic strategy revealed that TiO₂–SiO₂ aerogel, had a surface area 868 m²/g, particle size 40 nm, density 0.17 g/cm³ and 80% porosity. The finding indicated that from economic point of view, TiO₂–SiO₂ gel should be supercritical dried for 30 min and heat-treated for 5 h. The TiO₂–SiO₂ aerogel monoliths photocatalyst synthesized using sol–gel method provided insight into the characteristics that make a photocatalyst material well-suited for photodegradation of phenol and cyanide in an industrial waste stream containing Cl⁻, S²⁻ and NH₄⁺. Interestingly, after multiple reuse cycles (i.e. ≥7), photodegradation systems with regenerated photocatalyst showed a slightly decreasing of photoactivity 2–4%. The overall kinetics of photodegradation of either phenol or cyanide using TiO₂–SiO₂ aerogel photocatalyst was found to be of first order.     

Synthesis and characterization of mesoporous Ta2O5–TiO2 photocatalysts for water splitting

Two series of Ta₂O₅–TiO₂ photocatalysts (Ta:Ti = 4:1, 1:1 and 1:4) were prepared by sol–gel technique applying triblock copolymer of Pluronic P123 and were tested in platinized form (0.3 wt.%) in photodecomposition of water under ultraviolet and visible light (λ > 300 nm). It was found the mesoporous character of tantalum containing catalysts with relatively high surface area (100–130 m² g⁻¹) of these samples. However, higher concentration of TiO₂ in mixed oxides leads to the destruction of mesoporous character of synthesized photocatalysts. All samples were characterized with thermogravimetry, XRD, N₂ physisorption, DR-UV–vis and FTIR spectroscopy. The mixed oxides of Ta₂O₅–TiO₂ system showed much lower band-gap than pure Ta₂O₅ and relatively high activity in platinized state in photocatalytic hydrogen generation under visible. Doping of pure oxides and mixed systems with sulfur resulted in lowering of the band-gap values below 3 eV and much better activity in H₂ evolution reaction. Non-platinized photocatalysts showed activity in liquid phase cyclohexene photooxidation at 305 K.     

Tungsten and nitrogen co-doped TiO2 nano-powders with strong visible light response

A two-step method, combining with sol–gel and mechanical alloying (MA) method, was used to fabricate the tungsten and nitrogen co-doped TiO₂ nano-powders ((W, N) co-doped TiO₂ NPs). The (W, N) co-doped TiO₂ NPs showed strong absorbance in visible range, as long as 650 nm. Enhanced photocatalytic activities under visible light irradiation were also observed from the results of photodegradation experiments and chemical oxygen demand (COD) analysis. Physical, chemical, and optical properties of the samples were investigated. Possible reasons for the enhanced photocatalytic activities were analyzed based on the experimental results. Oxygen vacancies detected by electron spin response (ESR) spectra, acting as trapping agencies for electrons (e⁻) to produce active oxygen species (O²⁻), were proved to be the main cause for the improved photocatalytic performances.     

Superhydrophilic Cu-doped TiO2 thin film for solar-driven photocatalysis

Nanosized Cu-doped TiO₂ film was prepared by the sol–gel spin coating technique. XPS analysis showed that Cu atoms had been successfully doped into TiO₂ lattice, which hence modified the surface chemical composition. As a result, the Cu-doped TiO₂ thin film possessed a superhydrophilic surface with a water contact angle (WCA) only 5.1° and exhibited excellent anti-fogging behavior. The Cu-doped TiO₂ thin film also exhibited a much better photocatalytic activity than the reference TiO₂ thin film, as evaluated by the degradation of 10 mg/L methylene blue (MB) solution under simulated solar-driven irradiation.     

Synthesis of hectorite–TiO2 and kaolinite–TiO2 nanocomposites with photocatalytic activity for the degradation of model air pollutants

We studied the synthesis and photocatalytic activity of small-sized TiO₂ supported on hectorite and kaolinite. Deposition of TiO₂ on the clay mineral surface was conducted by using a sol–gel method with titanium isopropoxide as precursor. Anatase TiO₂ particles formation was achieved by hydrothermal treatment at 180 °C. Material characterization was conducted using XRD, SEM, XPS, ICP-OES, BET and porosimetry analysis. fficiency in synthesizing clay–TiO₂ composites depended strongly on the clay mineral structure. Incorporation of anatase in hectorite, an expandable clay mineral, was found to be very significant (> 36 wt.% Ti) and to be followed by important structural changes at the clay mineral surface. Instead, no major structural modifications of the clay were observed for kaolinite–TiO₂, as compared with the untreated material. Photocatalytic performance of clay–TiO₂ composites was evaluated with ATR-FTIR following the oxidation of adsorbed toluene and d-limonene, two model air pollutants. In either case, the photocatalytic removal efficiency of these hydrophobic substrates by the synthesized clay–TiO₂ composites was comparable to that observed using pure commercial TiO₂ (Degussa P25).     

Efficient decomposition of organic compounds with FeTiO3/TiO2 heterojunction under visible light irradiation

FeTiO₃/TiO₂, a new heterojunction-type photocatalyst working at visible light, was prepared by a simple sol–gel method. Not only did FeTiO₃/TiO₂ exhibit greatly enhanced photocatalytic activity in decomposing 2-propanol in gas phase and 4-chlorophenol in aqueous solution, but also it induced efficient mineralization of 2-propanol under visible light irradiation (λ ≥ 420 nm). Furthermore, it showed a good photochemical stability in repeated photocatalytic applications. FeTiO₃ showed a profound absorption over the entire visible range, and its valence band (VB) position is close to that of TiO₂. The unusually high photocatalytic efficiency of the FeTiO₃/TiO₂ composite was therefore deduced to be caused by hole transfer between the VB of FeTiO₃ and TiO₂.     

Electrochemical promotion of the CO2 hydrogenation reaction using thin Rh, Pt and Cu films in a monolithic reactor at atmospheric pressure

A monolithic electropromoted reactor (MEPR) with up to 22 thin Rh/YSZ/Pt or Cu/TiO₂/YSZ/Au plate cells was used to investigate the hydrogenation of CO₂ at atmospheric pressure and temperatures 220–380 °C. The Rh/YSZ/Pt cells lead to CO and CH₄ formation and the open-circuit selectivity to CH₄ is less than 5%. Both positive and negative applied potentials enhance significantly the total hydrogenation rate but the selectivity to CH₄ remains below 12%. The Cu/TiO₂/YSZ/Au cells produce CO, CH₄ and C₂H₄ with selectivities to CH₄ and C₂H₄ up to 80% and 2%. Both positive and negative applied potential significantly enhance the hydrogenation rate and the selectivity to C₂H₄. It was found that the addition of small (0.5 kPa) amounts of CH₃OH in the feed has a pronounced promotional effect on the reaction rate and selectivity of the Cu/TiO₂/YSZ/Au cells. The selective reduction of CO₂ to CH₄ starts at 220 °C (vs 320 °C in absence of CH₃OH) with near 100% CH₄ selectivity at open-circuit and under polarization conditions at temperatures 220–380 °C. The results show the possibility of direct CO₂ conversion to useful products in a MEPR via electrochemical promotion at atmospheric pressure.     

Oxidation of NO in gas phase by Au–TiO2 photocatalysts prepared by the sol–gel method

This work describes an innovative nanosemiconductor system, based on Au–TiO₂ for UV photo-assisted oxidation of nitrogen monoxide (NO). The synthesis of these materials was carried out by the sol–gel method. Titanium(IV) isopropoxide and HAuCl₄ were the precursors of the photocatalyst, which was prepared in acid conditions. The catalysts were characterized by the following techniques: BET, XRD, UV–vis and dark-field TEM. The evaluation of the photocatalytic activity was performed in situ using an FTIR spectrometer with high sensitivity and a UV spectrometer (365 nm) after 60 min at atmospheric pressure and room temperature. The NO + O₂ mixture concentration was 150 ppm. The photocatalytic conversion of nitrogen monoxide (NO) was studied by FTIR, which reached 85% in 60 min. The semiconductor type materials exhibited an enhanced photoactivity when compared with our reference TiO₂.     

Escherichia coli inactivation by N, S co-doped commercial TiO2 powders under UV and visible light

Commercial anatase TiO₂ powders (Tayca TKP101, TKP102) were ground with thiourea and annealed at 400 and 500 °C. Diffuse reflectance spectra (DRS) showed that the doping with thiourea shifted the TiO₂ absorption towards the visible region. The absorption was observed to increase with increasing annealing temperature. Using the Kubelka–Munk relations, it was possible to determine the band-gap of the doped TiO₂. Doped Tayca TiO₂ TKP101 showed a band-gap of 2.12 and 2.24 eV calcined at 400 and 500 °C, respectively. Doped Tayca TiO₂ TKP102 calcined at 400 and 500 °C showed in both cases a band-gap of 2.85 eV. X-ray photoelectron spectroscopy (XPS) revealed that these doped TiO₂, TKP101 annealed at 400 °C and TKP102 annealed at 400 and 500 °C present interstitial N-doping while doped TKP101 annealed at 500 °C showed a peak characteristic of substitutional N-doping. S-doped materials calcined at 500 °C presented only anionic S-doping. Nitrogen adsorption studies (BET) showed a loss of specific surface area (SSA) in annealed TiO₂ samples. N- and S co-doped materials showed suitable photocatalytic activity under UV illumination towards Escherichia coli inactivation and also under visible light irradiation (400–500 nm). Applying different annealing temperatures led to a variety of structures for N and S incorporated in the crystalline network. TiO₂ upon annealing showed a varying degree of hydroxylation and particles sizes. This seems to affect the trapping and transfer of the charge carriers generated under light and the semiconductor performance.     

TiO2 nanotubes and CNT–TiO2 hybrid materials for the photocatalytic oxidation of propene at low concentration

In this work, we investigated titanium dioxide (TiO₂) nanotubes and CNT–TiO₂ hybrid materials for the photocatalytic oxidation (PCO) of propene at low concentration (100 ppmv) in gaseous phase. The materials were prepared via sol–gel method using sacrificial multi-walled carbon nanotubes (CNT) as templates and subsequent heat treatments to obtain the desired crystalline phase (anatase, rutile or a mixture of both) and eventually to remove the carbon template. We also studied rutile nanotubes for the first time and demonstrate that the activity strongly depends on the crystalline composition, following rutile < anatase < anatase/rutile mixture. The enhanced activity of the anatase–rutile mixture is attributed to the decrease in the electron–hole pair recombination due to the multiphasic nature of the particles. The key result of this work is the exceptional performance of the CNT–TiO₂ hybrid, which yielded the highest observed photocatalytic activity. The improved performance is attributed to synergistic effects due to the hybrid nature of the material, resulting in small anatase crystalline sizes (CNT act as heat sinks) and a reduced electron–hole pair recombination rate (CNTs act as electron traps). These results demonstrate the great potential of hybrid materials and stimulate further research on CNT-inorganic hybrid materials in photocatalysis and related areas.     

Preparation of short, robust and highly ordered TiO2 nanotube arrays and their applications as electrode

The short, robust and highly ordered TiO₂ nanotube arrays (TNAs) electrode was prepared by sonoelectrochemical anodization of titanium in HF–H₂O electrolyte solution (referred as short TNAs, STNAs). The self-organized arrays of titania nanotubes of approximately 12–65 nm in diameter and 75–280 nm in length can be synthesized at anodic voltage of 5–20 V. The electron transport process within the STNAs electrode was much favorable in comparison with that for the long TNAs electrode synthesized by conventional magnetic agitation technique (referred as long TNAs, LTNAs), as confirmed by the obviously enhanced photocurrent response of STNAs electrode either in 0.02 M Na₂SO₄ electrolyte solution or in different concentrations of glucose solution or under different intensities of UV illumination. To investigate their photoelectrochemical applications, degradation of tetracycline, a typical pharmaceutical and personal care products (PPCPs), was carried out using photoelectrocatalytic (PEC) means, comparing with electrochemical (EC) and photocatalytic (PC) processes. The kinetic constant of the PEC process of STNAs electrode was 3.17 times as high as its PC process. The color removal rate of tetracycline by STNAs electrode achieved 81% within 3 h, which was 21% higher than that for LTNAs electrode. In degrading tetracycline, 41% of TOC was mineralized using the STNAs electrode against 23% using LTNAs electrode under similar conditions. Such kind of titania nanotubes will have many potential applications in various areas as an outstanding photoelectrochemical material.     

Photo-catalytic degradation of Rhodamine B on C-, S-, N-, and Fe-doped TiO2 under visible-light irradiation

C-, S-, N-, and Fe-doped TiO₂ photocatalysts were synthesized by a facile sol–gel method. The structure and properties of catalysts were characterized by N₂ desorption–adsorption, X-ray diffraction (XRD), UV–vis spectroscopy, and X-ray photoelectron spectroscopy (XPS). Results revealed that the surface area of the multi-doped TiO₂ was significantly increased and the crystallite size was smaller than the pure TiO₂ obtained by a similar route. Compared with TiO₂, the peak position in doped-TiO₂ XRD patterns was slightly shifted, which could be attributed to the distortion by the substitution of carbon, nitrogen, and sulfur dopants for some oxygen atoms and Fe³⁺ for Ti⁴⁺ in the lattice of TiO₂. These substitutions were confirmed by XPS. In addition, these dopants were responsible for narrowing the band gap of TiO₂ and shifting its optical response from ultraviolet (UV) to the visible-light region. The photocatalytic reactivities of these multi-doped TiO₂ catalysts were investigated by degrading Rhodamine B (RB) in aqueous solution under visible-light irradiation (λ > 420 nm). It was found out that the reactivity was significantly enhanced and the catalyst doped with nitrogen, carbon, sulfur, and 0.3 wt% iron had the highest photocatalytic activity.     

Carbon nanotubes/titanium dioxide (CNTs/TiO2) nanocomposites prepared by conventional and novel surfactant wrapping sol–gel methods exhibiting enhanced photocatalytic activity

In this work, a conventional sol–gel method was used to prepare CNTs/TiO₂ nanocomposites with different carbon loading in the range up to 20% CNTs/TiO₂ by weight. The bare CNTs (multi-walled), and the composites were characterized by a range of analytical techniques including TEM, XRD, BET and TGA–DSC. The results show the successful covering of the CNTs with discrete clusters of TiO₂ and bare CNTs surfaces which after annealing at 500 °C led to mesoporous crystalline TiO₂ (anatase) clusters. The photocatalytic activities of the nanocomposites were monitored from the results of the photodegradation of methylene blue (MB). The optimum CNTs/TiO₂ ratio in the composites prepared by conventional sol–gel method was found to be in the range from 1.5% to 5% by weight under the experimental conditions investigated. The maximum increase in activity was found to be 12.8% compared to the pure TiO₂ sample.In contrast, the synthesis of CNTs/TiO₂ nanocomposites by a novel surfactant wrapping sol–gel method [B. Gao, C. Peng, G.Z. Chen, G. Li Puma, Appl. Catal. B: Environ. 85 (2008) 17.] led to a uniform and well-defined nanometer-scale titania layer on individual CNTs. The nanocomposites were found to enhance the initial oxidation rate of methylene blue by onefold compared to the pure TiO₂ sample. This larger degree of rate enhancement is attributed to the supporting role of the CNTs and surface properties prepared by this novel modified sol–gel method.     

Sn-containing layered double hydroxides as precursors for nano-sized ZnO/SnO2 photocatalysts

Sn⁴⁺-containing LDH was prepared using the co-precipitation method at constant pH, and characterized using X-ray diffraction, UV–vis diffuse reflectance spectroscopy and TG/DTG methods. The obtained product was further exposed to different thermal treatments in order to obtain nano-sized coupled ZnO/SnO₂ systems with enhanced photocatalytic performances than the ones obtained by mixing the two semiconductor oxides. The formation of a well-defined ZnO/SnO₂ system and the crystallite size, fully investigated using XRD, micro-Raman scattering and UV–vis DR techniques, were found to be influenced by the nature of the precursors and the calcination temperature. The photocatalytic activity of the ZnO/SnO₂ systems, evaluated for the photodegradation of methyl orange (MO) dye, was studied as a function of the initial pH, catalyst loading and the calcination temperature. The metal dispersion supplied by layered structures proved to be an advantage when preparing coupled ZnO/SnO₂ systems, the photocatalytic activity being 2.3 times higher comparing with the physical mixtures performances. The maximum photocatalytic activity of the coupled ZnO/SnO₂ system having a layered precursor was observed when using neutral pH, at a catalyst loading of 1 g/L calcined at 600 °C for 4 h.     

CO hydrogenation to iso-C4 hydrocarbons over CeO2–TiO2 catalysts

CeO₂–TiO₂ (Ce:Ti = 0.25–9, molar ratio) catalysts were synthesized by a sol–gel method and the catalytic performances were evaluated in the selective synthesis of isobutene and isobutane from CO hydrogenation under the reaction conditions of 673–748 K, 1–5 MPa and 720–3000 h⁻¹. The physical properties, such as specific surface area, cumulative pore volume, average pore diameter, crystal phase and size, of the catalysts were characterized by N₂ adsorption/desorption and XRD. All the CeO₂–TiO₂ composite oxides showed higher surface areas than pure TiO₂ and CeO₂. No TiO₂ phase was detected on the samples of CeO₂–TiO₂ in which TiO₂ contents were in the range of 10–50 mol%. Crystalline Ce₂O₃ was detected in CeO₂–TiO₂ (8:2). The reaction conditions, temperature, pressure and space velocity, had obvious influences on the CO conversion and distribution of the products over CeO₂–TiO₂ (8:2) catalyst.     

Co–SiO2 aerogel-coated catalytic walls for the generation of hydrogen

Cobalt–silica (Co–SiO₂) aerogel coatings were successfully grown on the walls of ceramic monolith channels, thus resulting in structured catalytic wall materials useful for gas–solid reactions. The preparation involved the synthesis in situ of SiO₂ gels by the sol–gel hydrolysis and condensation of tetraethylorthosilicate (TEOS), quenching at the gelation point, incorporation of Co by impregnation, and extraction of the solvent by supercritical drying. Characterization of the aerogel-coated monoliths revealed an excellent dispersion and homogeneity of the aerogel and good adherence properties. The catalytic performance of these materials in the ethanol steam reforming reaction addressed to obtain hydrogen was studied at atmospheric pressure by carrying out consecutive cycles at 473–773–473 K with a C₂H₅OH:H₂O  1:3 (molar) mixture and stability tests, and the results were compared with those obtained over monoliths prepared by conventional washcoating methods from Co–SiO₂ xerogel. Co–SiO₂ aerogel catalytic walls were about four times more active for hydrogen generation at 623 K than conventional monoliths. An unusual rapid activation of aerogel-coated monoliths was attained at 580–590 K, even after several cycles and oxidation treatments at 563–613 K, which was attributed to highly dispersed cobalt particles and higher effective diffusivity of reaction species due to high porosity and larger average pore size. The reproducible low-temperature activation of Co–SiO₂ aerogels supported on ceramic monoliths may be useful for the practical application of fuel reformers to the on-site generation of hydrogen from ethanol.     

Effect of TiO2 particle size on the photocatalytic reduction of CO2

Pure TiO₂ anatase particles with a crystallite diameters ranging from 4.5 to 29 nm were prepared by precipitation and sol–gel method, characterized by X-ray diffraction (XRD), BET surface area measurement, UV–vis and scanning electron microscopy (SEM) and tested in CO₂ photocatalytic reduction. Methane and methanol were the main reduction products. The optimum particle size corresponding to the highest yields of both products was 14 nm. The observed optimum particle size is a result of competing effects of specific surface area, charge–carrier dynamics and light absorption efficiency.     

Structure and performance of a novel TiO2-phosphonate composite photocatalyst

An innovative SiO₂-PO₄³⁻-TiO₂ photocatalyst is presented which is able to bond TiO₂ to Raschig rings (RR). Evidence for the formation on the catalyst surface of PO stretching bands near 1200–1250 cm⁻¹ is presented by FTIR spectroscopy. The TiO₂ Degussa P25 on the catalyst surface (RR) was further characterized by high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction showing that the composite catalyst prepared at 500 °C does not alter the particle size or crystallographic composition of the TiO₂ Degussa P25 particles. The Ti- and P-distribution of the catalyst surface overlayers was obtained by Ar-sputtering eroding up to 100 topmost catalyst layers. By atomic force microscopy (AFM) the root mean square roughness (Rq) or rugosity of 771 nm and an average height of the catalyst layer of 1.52 μm were found on the glass surface. The root mean square roughness Rq varies very little in value before and after the photocatalysis indicating that the sample porosity is conserved during 4-CP photodegradation. The disappearance kinetics of 4-chlorophenol (4-CP) on the SiO₂-PO₄³⁻-TiO₂ composite occurred within 15 min and was faster than the 45 min needed with suspensions of TiO₂ Degussa P25 (1 g L⁻¹). The SiO₂-PO₄³⁻-TiO₂ photocatalyst was able to degrade repetitively 4-CP solutions without loss of activity. The effect of the light intensity, oxidant concentration and 4-CP concentration on the photodegradation kinetics was investigated and is reported in this study.     

Photocatalytic mineralization of phenol catalyzed by pure and mixed phase hydrothermal titanium dioxide

The photocatalytic mineralization of phenol catalyzed by pure (anatase, rutile) and mixed phase hydrothermal TiO₂ was studied in aqueous solution employing different oxidative agents, H₂O₂ and O₂. In the case of H₂O₂, rutile particles, having large dimensions and high aspect ratio (size: 30–70 nm × 150–350 nm), display the highest catalytic activity due to their low tendency to recombine electrons and holes generated by UV irradiation. By using water dissolved gaseous O₂, the catalytic TiO₂ activity generally decreases and rutile displays the lowest efficacy. In fact, oxygen preferentially chemisorbs at the surface of the nanosized particles of anatase (5–15 nm) and acts as effective electron scavenger, inhibiting the electron-hole recombination. The number of electron and hole traps (Ti³⁺, O₂⁻ and O⁻) and the rate of formation of the short-lived hydroxyl radicals OH under UV irradiation, were evaluated by electron paramagnetic resonance (EPR). A correlation was suggested among the amount of the charge carrier centers, the rate of formation of OH radicals and the catalyst photoactivity. This confirms that the photocatalytic properties depend on the possibility that electrons and holes separately interact with the oxidative agents at the TiO₂ surface, inducing the formation of OH radicals.