Enhanced photocatalytic activity of TiO2 nano-structured thin film with a silver hierarchical configuration

TiO₂ sol-gels with various Ag/TiO₂ molar ratios from 0 to 0.9% were used to fabricate silver-modified nano-structured TiO₂ thin films using a layer-by-layer dip-coating (LLDC) technique. This technique allows obtaining TiO₂ nano-structured thin films with a silver hierarchical configuration. The coating of pure TiO₂ sol-gel and Ag-modified sol-gel was marked as T and A, respectively. According to the coating order and the nature of the TiO₂ sol-gel, four types of the TiO₂ thin films were constructed, and marked as AT (bottom layer was Ag modified, surface layer was pure TiO₂), TA (bottom layer was pure TiO₂, surface layer was Ag modified), TT (pure TiO₂ thin film) and AA (TiO₂ thin film was uniformly Ag modified). These thin films were characterized by means of linear sweep voltammetry (LSV), X-ray diffraction (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy and transient photocurrent (I_ph). LSV confirmed the existence of Ag⁰ state in the TiO₂ thin film. SEM and XRD experiments indicated that the sizes of the TiO₂ nanoparticles of the resulting films were in the order of TT > AT > TA > AA, suggesting the gradient Ag distribution in the films. The SEM and XRD results also confirmed that Ag had an inhibition effect on the size growth of anatase nanoparticles. Photocatalytic activities of the resulting thin films were also evaluated in the photocatalytic degradation process of methyl orange. The preliminary results demonstrated the sequence of the photocatalytic activity of the resulting films was AT > TA > AA > TT. This suggested that the silver hierarchical configuration can be used to improve the photocatalytic activity of TiO₂ thin film.     

Surface modification of nanocrystalline anatase with CTAB in the acidic condition and its effects on photocatalytic activity and preferential growth of TiO2

The nanocrystalline anatase TiO₂, which was synthesized by a sol-hydrothermal process in advance, has successfully modified with cetyltrimethylammonium bromide (CTAB) in the acidic condition as well as in the basic condition. On the basis of the measurements of infrared spectrum and X-ray photoelectron spectroscopy of the resulting TiO₂, together with the phase-transfer experiments, it is suggested that the modification mechanism in the acidic condition is closely related to Br⁻. Interestingly, compared with un-modified TiO₂, the modified TiO₂ exhibits high photocatalytic activity for degrading Rhodamine B (RhB) solution, especially for that modified in the acid. The enhanced photocatalytic activity of modified TiO₂ in the acid is attributed to the role that the Br⁻ can easily capture photo-induced holes and then form active Br, consequently effectively inducing photocatalytic oxidation reactions, based on the surface photovoltage responses of the resulting TiO₂. After that, a one-pot sol-hydrothermal route at the temperature as low as 80 °C is developed to directly synthesize CTAB-modified nanocrystalline TiO₂ with a little preferred growth along 〈0 0 1〉 direction, which can be easily dispersed in the organic system and possess good photocatalytic performance. This work provides a feasible strategy to further improve the photocatalytic performance of nanocrystalline anatase and to synthesize TiO₂ nanocrystals with preferential growth.     

Efficient one-pot synthesis of Ag nanoparticles loaded on N-doped multiphase TiO2 hollow nanorod arrays with enhanced photocatalytic activity

The simultaneous Ag loaded and N-doped TiO₂ hollow nanorod arrays with various contents of silver (Ag/N-THNAs) were successfully synthesized on glass substrates by one-pot liquid phase deposition (LPD) method using ZnO nanorod arrays as template. The catalysts were characterized by Raman spectrum, field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscope (HRTEM), ultraviolet-vis (UV-vis) absorption spectrum and X-ray photoelectron spectroscopy (XPS). The results suggest that AgNO₃ additive in the precursor solutions not only can promote the anatase-to-rutile phase transition, but also influence the amount of N doping in the samples. The photocatalytic activity of all the samples was evaluated by photodegradation of methylene blue (MB) in aqueous solution. The sample exhibited the highest photocatalytic activity under UV light illumination when the AgNO₃ concentration in the precursor solution was 0.03 M, due to Ag nanoparticles acting as electron sinks; When the AgNO₃ concentration was 0.07 M, the sample performed best under visible light illumination, attributed to the synergetic effects of Ag loading, N doping, and the multiphase structure (anatase/rutile).     

TiO2 thick films supported on reticulated macroporous Al2O3 foams and their photoactivity in phenol mineralization

TiO₂ thick films deposited on macroporous reticulated Al₂O₃ foams with pore size of 10 ppi and 15 ppi were prepared using dip coating from slurries of Aeroxide^® P25 nanopowder and precipitated titania. All prepared films have sufficiently good adhesion to the surface of the substrate also in case of strongly cracked films. No measurable release of deposited TiO₂ after repeated photocatalytic cycles was observed. The photocatalytic activity was characterized as the rate of mineralization of aqueous phenol solution under irradiation of UVA light by TOC technique. The best activity was obtained with Aeroxide^® P25 coated Al₂O₃ foam with the pore size of 10 ppi, annealed at 600 °C. The optimal annealing temperature for preparation of films from precipitated titania could be determined at 700 °C. Films prepared by sol-gel deposition technique were considerably thinner compared to coatings made of suspensions and their photocatalytic activity was significantly smaller.     

Preparation and dispersive mechanism of highly dispersive ultrafine silver powder

Using ascorbic acid as the reducing agent, AgNO₃ as the source of Ag, the ultrafine silver powder was prepared by liquid-phase reduction method. The optimal conditions to prepare the ultrafine silver powder were obtained by studying the effects of following factors, such as the selection of dispersant, the doses of dispersant and pH, on the dispersibility of silver powder under other constant conditions. The pure ultrafine silver powder with quasi-spherical shape and mean size of 1.15 μm was synthesized under the optimal conditions of polyvinyl alcohol (PVA) as disperser, PVA/AgNO₃ mass ratio of 4:100 and pH 7 while maintaining other conditions exactly in the same circumstances, such as AgNO₃ concentration of 0.20 mol L⁻¹, ascorbic acid concentration of 0.15 mol L⁻¹ and reaction temperature of 40 °C. The ultrafine silver powder was characterized by SEM and XRD. And a PVA dispersive mechanism for preparing highly dispersive ultrafine silver powder, proved by the ultraviolet spectra, is that PVA absorbed on the surface of silver particles by coordination bond preventing the silver particles from diffusion and aggregation. In addition, the steric effect may help to reduce aggregation.     

IR and XPS investigation of visible-light photocatalysis—Nitrogen-carbon-doped TiO2 film

To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Nitrogen and carbon doping TiO_2−x−yN_xC_y films were obtained by heating the TiO₂ gel in an ionized N₂ gas and then were calcined at 500 °C. The TiO_2−x−yN_xC_y films have revealed an improvement over the TiO₂ films under visible light (wavelength, 500 nm) in optical absorption and photocatalytic activity such as photodegradation of methyl orange. X-ray photoemission spectroscopy, infrared spectrum and UV-visible (UV-vis) spectroscopy were used to find the difference of two kinds of films. Nitrogen and carbon doped into substitutional sites of TiO₂ has been proven to be indispensable for band-gap narrowing and photocatalytic activity.     

Photocatalytic behavior of heavy La-doped TiO2 films deposited by pulsed laser deposition using non-sintered target

We have investigated the control of photocatalytic behavior under deposited conditions of non-sintered target of different molar ratios with TiO₂ and La₂O₃ from 1:0 to 1:2 for heavily La doping, and post-annealing temperature from 600 °C to 1000 °C for crystallizing by pulsed laser deposition. We have successfully crystallized heavily La-doped TiO₂ films with post-annealing temperature over 800 °C and with molar ratio of TiO₂:La₂O₃ over 1:1 on a quartz substrate. Heavily La-doped TiO₂ films are observed the decomposition of methylene blue and a water-splitting reaction in photocatalytic behavior under Xe light irradiation. When stoichiometric La-doped TiO₂ (TiO₂:La₂O₃ = 1: 1) is synthesized with heat-treatment at 900 °C, the best results are obtained under photocatalytic behavior and pure La₂Ti₂O₇ crystalline were obtained.     

Visible-light-activated Ce-Si co-doped TiO2 photocatalyst

To enhance the visible photocatalytic activity and thermal stability of TiO₂, Ce-Si co-doped TiO₂ materials were synthesized through a nonaqueous method of which the purpose was to reduce the aggregation between TiO₂ particles. The obtained materials maintained anatase phase and large surface area of 103.3 m² g⁻¹ even after calcined at 800 °C. The XPS results also indicated that Si was weaved into the lattice of TiO₂, and Ce mainly existed as oxides on the surface of TiO₂ particles. The doped Si might enhance surface area and suppress transformation from anatase to rutile, while the doped Ce might cause visible absorption and inhibit crystallite growth during heat treatment. Evaluated by decomposing dye Rhodamine B, visible photocatalytic activity of Ce-Si co-doped TiO₂ was obviously higher than that of pure TiO₂ and reached the maximum at Ce and Si contents of 0.5 mol% and 10 mol%.     

Sunlight photocatalytic activity of CdS modified TiO2 loaded on activated carbon fibers

To improve the photocatalytic application performances of TiO₂, in this work, firstly CdS modified Degussa P25 TiO₂ (CdS/TiO₂) composites were prepared by two methods, sol-gel method and precipitation method. Next they, sol-gel-CdS/TiO₂ (sg-CdS/TiO₂) and precipitation-CdS/TiO₂ (pp-CdS/TiO₂), were loaded on activated carbon fibers (ACFs) by dip-coating method using the sodium carboxymethyl cellulose as adhesives. The composites were characterized by XRD, UV-vis absorbance spectra, SEM, EDS and BET. The photocatalytic activities under sunlight were investigated by the degradation of methylene blue. The results showed that CdS/TiO₂ composites were mainly composed of anatase-TiO₂ and little CdS cubic phases. The absorption wavelengths of sg-CdS/TiO₂ and pp-CdS/TiO₂ composites were extended to 590 nm and 740 nm, respectively. The absorption edge had a pronounced ‘red shift’. From EDS analysis, the elemental contents of CdS/TiO₂ were mainly Ti and O and a small quantity of S and Cd. CdS/TiO₂ loaded on ACFs were in the form of small clusters, but not very uniform; compared with the original ACFs, the surface area and pore volume of CdS/TiO₂/ACFs decreased slightly, respectively, while the average pore diameter was not changed. The photodegradation rate of methylene blue under sunlight with CdS/TiO₂/ACFs composites was markedly higher than that of P25-TiO₂/ACFs, and the effect of pp-CdS/TiO₂/ACFs composites was better than that of sg-CdS/TiO₂/ACFs, when irradiated for 180 min, and the photodegradation rate of methylene blue reached to 90.1%. The photodegradation kinetics of the methylene blue fitted with the Langmuir-Hinshelwood equation. The apparent reaction rate constants of sg-CdS/TiO₂/ACFs and pp-CdS/TiO₂ were 0.0105 min⁻¹ and 0.0146 min⁻¹, respectively, which were about 1.3-1.7 times as large as that of P25-TiO₂/ACFs.     

The enhanced conductivity and stability of AZO thin films with a TiO2 buffer layer

Aluminum doped zinc oxide (AZO) films were substitutes of the SnO₂:F films on soda lime glass substrate in the amorphous thin-film solar cells due to good properties and low cost. In order to improve properties of AZO films, the TiO₂ buffer layer had been introduced. AZO films with and without TiO₂ buffer layer were deposited on soda lime glass substrates by r.f. magnetron sputtering. Subsequently, one group samples were annealed in vacuum (0.1 Pa) at 500 °C for 120 s using the RTA system, and the influence of TiO₂ thickness on the properties of AZO films had been discussed. The XRD measurement results showed that all the films had a preferentially oriented (0 0 2) peak, and the intensity of (0 0 2) peak had been enhanced for the AZO films with TiO₂ buffer layer. The resistivity of TiO₂ (3.0 nm)/AZO double-layer film is 4.76×10⁻⁴ Ω cm with the maximum figure merit of 1.92×10⁻² Ω⁻¹, and the resistivity has a remarkable 28.7% decrease comparing with that of the single AZO film. The carrier scattering mechanism of TiO₂ (3.0 nm)/AZO double-layer film had been described by Hall measurement in different temperatures. The average transmittance of all the films exceeded 92% in the visible spectrum. Another group samples were heat treated in the quartz tube in air atmosphere, and the effect of TiO₂ thickness on thermal stability of AZO films had been discussed.     

Reasons for the deactivation of Pt/TiO2 photocatalyst treated by inert gas N2

The effect of N₂ treatment on the photocatalytic activity of Pt⁰/TiO₂ was investigated. The results showed that after treatment at 500 °C in N₂, 70% of the photocatalytic activity of 1.0 wt.% Pt⁰/TiO₂ was lost by the evaluation of photocatalytic oxidation reaction of C₃H₆. Transmission electron microscopy (TEM) and Ar⁺ ion sputtering tests revealed that in the course of high-temperature N₂ treatment, the size of Pt⁰ particles on TiO₂ increases and a strong interaction between metal and support, i.e. Pt⁰ particles encapsulated by Ti_xO_y, happens, which are the reasons for the deactivation of Pt⁰/TiO₂ photocatalyst treated by high-temperature N₂.     

Low temperature synthesis of iodine-doped TiO2 nanocrystallites with enhanced visible-induced photocatalytic activity

Iodine-doped TiO₂ nanocrystallites (denoted as I-TNCs) were prepared via a newly developed triblock copolymer-mediated sol-gel method at a temperature of 393 K. I-doping, crystallization and the formation of porous structure have been simultaneously achieved. The obtained particles were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis spectrophotometer. The results indicated that the as-prepared I-TNCs possessed a diameter of ca. 5 nm with anatase crystalline structure and a specific surface area of over 200 m² g⁻¹. The presence of iodine expanded the photoresponse in visible light range, and led to enrich in surface hydroxyl group on the TiO₂ surface. Compared with the commercial photocatalyst P25, the I-TNCs significantly enhanced the photocatalytic efficiency in the degradation of rhodamine B and 2,4-dichlorophenol, and the I-TNCs with 2.5 mol% doping ratio exhibited the best photocatalytic activity.     

Influence of growth temperature of TiO2 buffer on structure and PL properties of ZnO films

A series of ZnO films with TiO₂ buffer on Si (1 0 0) substrates were prepared by DC reactive sputtering. Growth temperature of TiO₂ buffer changed from 100 °C to 400 °C, and the influence on the crystal structures and optical properties of ZnO films have been investigated. The XRD results show that the ZnO films with TiO₂ buffer have a hexagonal wurtzite structure with random orientation, and with the increase of growth temperature of TiO₂ buffer, the residual stresses were released gradually. Specially, the UV emission enhanced distinctly and FWHMs (full width half maximum) decreased linearly with the increasing TiO₂ growth temperature. The results all come from the improvement of crystal quality of ZnO films.     

Sputter deposition of high transparent TiO2−xNx/TiO2/ZnO layers on glass for development of photocatalytic self-cleaning application

In this study, TiO_2−xN_x/TiO₂ double layers thin film was deposited on ZnO (80 nm thickness)/soda-lime glass substrate by a dc reactive magnetron sputtering. The TiO₂ film was deposited under different total gas pressures of 1 Pa, 2 Pa, and 4 Pa with constant oxygen flow rate of 0.8 sccm. Then, the deposition was continued with various nitrogen flow rates of 0.4, 0.8, and 1.2 sccm in constant total gas pressure of 4 Pa. Post annealing was performed on as-deposited films at various annealing temperatures of 400, 500, and 600 °C in air atmosphere to achieve films crystallinity. The structure and morphology of deposited films were evaluated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The chemical composition of top layer doped by nitrogen was evaluated by X-ray photoelectron spectroscopy (XPS). Photocatalytic activity of samples was measured by degradation of Methylene Blue (MB) dye. The optical transmittance of the multilayer film was also measured using ultraviolet-visible light (UV-vis) spectrophotometer. The results showed that by nitrogen doping of a fraction (∼1/5) of TiO₂ film thickness, the optical transmittance of TiO_2−xN_x/TiO₂ film was compared with TiO₂ thin film. Deposited films showed also good photocatalytic and hydrophilicity activity at visible light.     

Role of Cl ions in photooxidation of propylene on TiO2 surface

The effect of Cl⁻ ions on photooxidation of propylene on TiO₂ semiconductor was investigated. Cl⁻/TiO₂ catalysts were prepared by annealing Degussa P25 TiO₂ in the gas flow of N₂ and Cl₂ under 100-400 °C. The photocatalytic oxidation of propylene was carried out in a continuous flow system, with the chromatograph to analyze the products on line. The experimental results showed that the activity of Cl⁻/TiO₂ catalysts increased as heat-treated temperature decreased. The activity of the sample heat-treated at 100 °C was about two times higher than that of pure TiO₂. Moreover, as to TiO₂, the main product of the propylene photocatalytic oxidation was CO₂, but with Cl⁻/TiO₂ catalysts, not only CO₂ but also trace CO was determined. The adsorbed species on TiO₂ surface before and after reaction were analyzed by X-ray photoelectron spectroscopy (XPS) and thermogravimetric/differential thermal analyses (TG-DTA) coupled to a mass spectrometer (MS). XPS analysis showed that there was Cl⁻ absorbed on the Cl⁻/TiO₂ surface, and the absorption amount of Cl⁻ decreased after the photooxidation reaction of propylene. TG-DTA-MS analysis confirmed chlorine absorbed on the surface of TiO₂ in the form of Cl⁻ ion. These results illuminated that absorbed Cl⁻ on the surface of TiO₂ formed a weak physical absorption on TiO₂ at low temperature, and subsequently participated in the photooxidation of propylene, finally removed from TiO₂ surface.     

Photocatalytic decomposition of methylene blue and 4-chlorophenol on nanocrystalline TiO2 films under UV illumination: A ToF-SIMS study

The photocatalytic degradation of methylene blue and 4-chlorophenol on nanocrystalline TiO₂ (nc-TiO₂) under UV irradiation was investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). Nanocrystalline TiO₂ films were prepared from suspensions containing TiO₂-crystallites of different average sizes, the smallest one being 12 nm. The organic substances (either methylene blue or 4-chlorophenol) were applied to these films. The specimens were studied in the pristine state and upon UV exposure. The UV illuminations were carried out both under atmospheric conditions and in situ under ultrahigh vacuum in the ToF-SIMS instrument. Distinct mass signals from the parent molecules and from fragment ions are observed for the as-prepared samples. Upon irradiation with UV light under atmospheric conditions, the surface composition is significantly changed, an observation ascribed to photocatalytic reactions induced by UV photons: the parent molecule signals are strongly diminished whereas fragmentation products are identified to be present at the TiO₂ surfaces. UV irradiations carried out under different vacuum conditions in the ToF instrument (ultrahigh vacuum, air or oxygen adsorption) indicate that varying ambient conditions may influence the photocatalytic reaction on the nanocrystalline TiO₂ films.     

TiO2 activation using acid-treated vermiculite as a support: Characteristics and photoreactivity

Vermiculite was treated by sulfuric or nitric acid aqueous solutions with different concentration. These modified materials as the promising supports, were used to immobilize TiO₂. TiO₂ was prepared by the precursor, which was obtained by substituting partly isopropyl alcohol with Cl⁻ in titanium chloride {[Ti(IV)(OR)_nCl_m] (n = 2-3, m = 4 − n)}. The TiO₂/vermiculite composites were characterized by X-ray diffraction, scanning electron microscopy, and the nitrogen absorption. Their photocatalytic activity was evaluated by removal of methylene blue (MB). The pure anatase type crystalline phase was well deposited on the supports. The concentrations of acid for treatment had a significant influence on pore sizes and surface area of vermiculite. The treatment process changed microstructure of vermiculite, modified its characteristics, and farther improved the catalytic activity and absorption capacity of TiO₂/vermiculite composites. The treatment effect of nitric acid was superior to that of sulfuric acid.     

Nanostructure of photocatalytic TiO2 films sputtered at temperatures below 200 °C

The article reports on correlations between the process parameters of reactive pulsed dc magnetron sputtering, physical properties and the photocatalytic activity (PCA) of TiO₂ films sputtered at substrate surface temperature T_surf ≤ 180 °C. Films were deposited using a dual magnetron system equipped with Ti (Ø50 mm) targets in Ar + O₂ atmosphere in oxide mode of sputtering. The TiO₂ films with highly photoactive anatase phase were prepared without a post-deposition thermal annealing. The decomposition rate of the acid orange 7 (AO7) solution during the photoactivation of the TiO₂ film with UV light was used for characterization of the film PCA. It was found that (i) the partial pressure of oxygen pO₂ and the total sputtering gas pressure p_T are the key deposition parameters influencing the TiO₂ film phase composition that directly affects its PCA, (ii) the structure of sputtered TiO₂ films varies along the growth direction from the film/substrate interface to the film surface, (iii) ∼500 nm thick anatase TiO₂ films with high PCA were prepared and (iv) the structure of sputtered TiO₂ films is not affected by the substrate surface temperature T_surf when T_surf < 180 °C. The interruption of the sputtering process and deposition in long (tens of minutes) pulses alternating with cooling pauses has no effect on the structure and the PCA of TiO₂ films and results in a decrease of maximum value of T_surf necessary for the creation of nanocrystalline nc-TiO₂ film. It was demonstrated that crystalline TiO₂ films with high PCA can be sputtered at T_surf ≤ 130 °C. Based on obtained results a phase zone model of TiO₂ films was developed.     

AgSbSe2 and AgSb(S,Se)2 thin films for photovoltaic applications

Silver antimony selenide (AgSbSe₂) thin films were prepared by heating sequentially deposited multilayers of antimony sulphide (Sb₂S₃), silver selenide (Ag₂Se), selenium (Se) and silver (Ag). Sb₂S₃ thin film was prepared from a chemical bath containing SbCl₃ and Na₂S₂O₃, Ag₂Se from a solution containing AgNO₃ and Na₂SeSO₃ and Se thin films from an acidified solution of Na₂SeSO₃, at room temperature on glass substrates. Ag thin film was deposited by thermal evaporation. The annealing temperature was 350 °C in vacuum (10⁻³ Torr) for 1 h. X-ray diffraction analysis showed that the thin films formed were polycrystalline AgSbSe₂ or AgSb(S,Se)₂ depending on selenium content in the precursor films. Morphology and elemental analysis of these films were done using scanning electron microscopy and energy dispersive X-ray spectroscopy. Optical band gap was evaluated from the UV-visible absorption spectra of these films. Electrical characterizations were done using Hall effect and photocurrent measurements. A photovoltaic structure: glass/ITO/CdS/AgSbSe₂/Al was formed, in which CdS was deposited by chemical bath deposition. J-V characteristics of this structure showed V_oc = 435 mV and J_sc = 0.08 mA/cm² under illumination using a tungsten halogen lamp. Preparation of a photovoltaic structure using AgSbSe₂ as an absorber material by a non-toxic selenization process is achieved.     

Study on the properties and charge generation in dye-sensitized n-TiO2|dye|p-CuI solid state photovoltaic solar cells

Transparent semiconducting copper iodide (CuI) films were prepared by XeCl Excimer laser and their characteristics are investigated. These films exhibited optical transmittance over 80% in the wavelength range from 400 to 900 nm and minimum resistivity of about 2 kΩ cm⁻¹. The optical absorption of the these films shows a remarkable blue shift compared to that of polycrystalline of CuI, which can be explained from the viewpoint formation of ultra fine of CuI grains. The titanium dioxide (TiO₂) films have been prepared by sol-gel method. The properties of pulsed laser deposited CuI and TiO₂ films in power output of n-TiO₂|dye|p-CuI cells is studied. An efficient charge generation is observed through the illumination of TiO₂ layer of the fabricated n-TiO₂|dye|p-CuI solid state photovoltaic solar cells. From the current-voltage characteristics, the fill factor and power conversion efficiency were about of 45 and 3%, respectively. The maximum photo-current of about 12.5 mA/cm² and photo-voltage of 475 mV under AM 1.5 conditions were obtained for the n-TiO₂|dye|p-CuI solid states photovoltaic solar cells with good reproducibility. Adsorbed dye molecules to the TiO₂ surface act as a relay, especially under illumination through TiO₂ layer in the wave range region of 300-400 nm.