Tunable photocatalytic selectivity of fluoropolymer PVDF modified TiO2

Fluoropolymer poly-vinylidene-fluoride modified TiO₂ (PVDF/TiO₂) were prepared via a simple chemisorption approach and characterized by thermo gravimetric analyse, transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy and photoluminescence spectra. The modified mechanism and the photocatalytic selectivity of the PVDF/TiO₂ were studied. The existence of Ti-F coordination bond on the interface between TiO₂ and PVDF was confirmed. For the PVDF modification, the photocatalytic degradation (PCD) of cationic dye was greatly enhanced, and the PCD of anionic dye was obviously inhibited. PVDF/TiO₂ shows high photocatalytic selectivity than that of TiO₂ by degrading mixed solution of cationic dyes MB and anionic dyes MO. The selectivity can be tuned by changing the PVDF modification amount.     

The enhanced photocatalytic activity of CdS/TiO2 nanocomposites by controlling CdS dispersion on TiO2 nanotubes

CdS/TiO₂ nanocomposites were prepared via a simple wet chemical method, and characterized through X-ray diffraction (XRD) and transmission electron microscopy (TEM). Their ability to degrade Acid Rhodamine B was investigated under visible light irradiation. The results indicate that CdS/TiO₂ nanocomposite with a mass ratio of 4:1(TiO₂:CdS) showed high photocatalytic activity and the CdS loaded on TiO₂ nanotube surface exhibited a hexagonal phase. The dispersion of CdS on TiO₂ nanotube surface had an important effect on the degradation efficiency of pollutant, which provides a strategy for practical industry application.     

Enhancement of the photocatalytic activity of TiO2 nanoparticles by surface-capping DBS groups

TiO₂ nanoparticles capped with sodium dodecylbenzenesulfonate (DBS) are synthesized by a sol-hydrothermal process using tetrabutyl titanate and DBS as raw materials. The effects of surface-capping DBS on the surface photovoltage spectroscopy (SPS), photoluminescence (PL) and photocatalytic performance of TiO₂ nanoparticles are principally investigated together with their relationships. The results show that the surface of TiO₂ nanoparticles can be well capped by DBS groups while the pH value and added DBS amount are controlled at 5.0 and 2% of TiO₂ mass weight, respectively, and the linkage between DBS groups and TiO₂ surfaces is mainly by means of quasi-sulphonate bond. The intensities of SPS and PL spectra of TiO₂ obviously decrease after DBS-capping, while the activity can greatly increase during the photocatalytic degradation of Rhodamine B (RhB) solution, which are mainly attributed to the electron-withdrawing character of the DBS groups. Moreover, the enhancement of photocatalytic activity of DBS-capped TiO₂ is also related to the increase in the capability for adsorbing RhB.     

Synthesis of fluorinated TiO2 hollow microspheres and their photocatalytic activity under visible light

Fluorinated TiO₂ hollow microspheres with three-dimensional hierarchical architecture were prepared by solvothermally treatment using solid microspheres as precursor. The obtained solid and hollow TiO₂ microspheres were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS), UV-Vis diffuse reflectance spectrum (DRS) and photoluminescence (PL) spectra. The photocatalytic activity of as-prepared solid and hollow TiO₂ microspheres was determined by degradation of methyl orange (MO) under visible light irradiation. The results showed that the surface fluorination, the existence of accessible mesopores channels, and the increased light harvesting abilities could remarkably improve the photocatalytic activity of TiO₂ hollow microspheres.     

Effect of structure on the photocatalytic activity of Pt-doped TiO2 nanotubes

Highly ordered TiO₂ nanotubes with different tube length were fabricated by anodization using C₂H₂O₄·2H₂O containing 0.5 wt.% NH₄F (electrolyte A) and anhydrous dimethyl sulfoxide containing 1% HF (electrolyte B), respectively. Then cathodic reduction method was used to dope Pt in TiO₂ nanotubes in chloroplatinic acid. The results indicated that cathodic reduction could efficiently platinize TiO₂ nanotubes. Pt-doped TiO₂ nanotubes with the longer length had the higher photocatalytic activity for degrading methyl orange under UV and visible irradiation. The longer tube length has a positive effect on the photocatalytic activity of Pt-doped TiO₂ nanotubes. Besides, as the content of anatase further decreases, the photocatalytic activity drops gradually due to the reduction reaction in the surface area.     

表面桥氧空位对甲醛在二氧化钛表面吸附的影响

Oxygen vacancy (O_v) has significant influence on physical and chemical properties of TiO₂ systems,especially on surface catalytic processes.In this work,we investigate the effects of O v on the adsorption of formaldehyde (HCHO) on TiO₂(110) surfaces through firstprinciples calculations.With the existence of O_v,we find the spatial distribution of surface excess charge can change the relative stability of various adsorption configurations.In this case,the bidentate adsorption at five-coordinated Ti (Ti_5c) can be less stable than the monodentate adsorption.And HCHO adsorbed in O_v becomes the most stable structure.These results are in good agreement with experimental observations,which reconcile the long-standing deviation between the theoretical prediction and experimental results.This work brings insights into how the excess charge affects the molecule adsorption on metal oxide surface.     

Effect of surface species on Cu-TiO2 photocatalytic activity

The Cu-TiO₂ nanoparticles with different Cu dopant content were prepared by sol-gel method. The structure of the as-prepared catalysts and the surface species of Cu-TiO₂ were determined using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and diffuse reflection spectroscopy (DRS). The relationship between the photocatalytic activity and the surface species of Cu-TiO₂ was revealed via the measurement of surface photovoltage spectroscopy (SPS) as well as the degradation of the rhodamine B (RhB). The experimental results suggest that the Cu-TiO₂ photocatalysts with appropriate content of Cu (about 0.06 mol%) possess abundant electronic trap, which effectively inhibits the recombination of photoinduced charge carriers, improving the photocatalytic activity of TiO₂. While at high Cu dopant region (>0.06 mol%), the excessive oxygen vacancies and Cu species can become the recombination centers of photoinduced electrons and holes. Meanwhile, at heavy Cu doping concentration, excessive P-type Cu₂O can cover the surface of TiO₂, which leads to decrease in the photocatalytic activity of photocatalyst. The photocatalytic experimental results are in good agreement with the conclusions of SPS measurements, indicating that there is a close relationship between the photocatalytic activity and the intensity of SPS spectra.     

Improvement in the photocatalytic activity of TiO2 by the partial oxidation of the C impurities

TiO₂ was treated by water in an ultrasonic bath, resulting in the enhancement of the photocatalytic activity for the decomposition of methylene blue under UV and visible light irradiation. No change in the crystallinity and optical properties of TiO₂ by the H₂O-treatment was observed. The X-ray photoelectron spectroscopy (XPS) and FT-IR data revealed that the C impurities were oxidized by this treatment, indicating that the change in the structure of the C impurities plays a pivotal role in the photocatalytic activity of TiO₂.     

CH3OH/TiO2(110)界面的光电子能谱研究

Methanol/TiO₂(110) is a model system in the surface science study of photocatalysis where methanol is taken as a hole capture. However, the highest occupied molecular orbital of adsorbed methanol lies below the valence band maximum of TiO₂, preventing the hole transfer. To study the level alignment of this system, electronic structure of methanol covered TiO₂(110) surface has been measured by ultraviolet photoelectron spectroscopy and the molecular orbitals of adsorbed methanol have been clearly identified. The results indicate the weak interaction between methanol and TiO₂ substrate. The static electronic structure also suggests the mismatch of the energy levels. These static experiments have been performed without band gap excitation which is the prerequisite of a photocatalytic process. Future study of the transient electronic structure using time-resolved UPS has also been discussed.     

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.     

Preparation, characterization and visible-light photocatalytic activity of AgI/AgCl/TiO2

In this paper, a novel composite photocatalyst AgI/AgCl/TiO₂ was prepared by ion exchange method and characterized by XRD, SEM and UV-Vis spectrometry. The as-prepared AgI/AgCl/TiO₂ composites show much higher photocatalytic activity than AgCl/TiO₂ and AgI/TiO₂ under visible-light irradiation (λ > 400 nm) in the process of methyl orange (MO) degradation. When the molar percentage of AgI to initial AgCl is 20% (sample SE-20%), the maximal degradation efficiency of MO has reached 85.8% after irradiation for 120 min. The enhancement of photocatalytic activity of the composite photocatalyst AgI/AgCl/TiO₂ will be attributed to its good absorption in the visible-light region, especially low recombination rate of the electron-hole pairs based on the photoluminescence (PL) spectra investigation of AgI/AgCl/TiO₂ and the matching band structures of AgI, AgCl and TiO₂. The detection of reactive species by radical scavengers displays that O₂⁻ and H₂O₂ are the main reactive species for the degradation of MO under visible-light irradiation. Moreover, PL analysis by using terephthalic acid (TA) as a probe molecule further reveals that OH can be negligible for the degradation of MO.     

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.     

Synthesis and characteristics of natural zeolite supported Fe-TiO2 photocatalysts

Natural zeolite supported Fe³⁺-TiO₂ photocatalysts were synthesized for the sake of improving the recovery and photocatalytic efficiency of TiO₂. The as-prepared materials were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible diffuse reflection spectroscopy (UV-vis DRS), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). Methyl orange was used to estimate the photocatalytic activity of the samples. The results showed that zeolite inhibited the growth of TiO₂ crystallite sizes. The Fe³⁺ concentration played an important role on the microstructure and photocatalytic activity of the samples. The iron ions could diffuse into TiO₂ lattice to the form Fe-O-Ti bond and gave TiO₂ the capacity to absorb light at lower energy levels. The photocatalytic activity of the samples could be enhanced as appropriate dosages of Fe³⁺ were doped.     

Effects of surface-modification with Bi2O3 on the thermal stability and photoinduced charge property of nanocrystalline anatase TiO2 and its enhanced photocatalytic activity

Bi₂O₃ surface-modified TiO₂ nanoparticle has been synthesized by sol-hydrothermal processes, followed by post-treatment with an appropriate amount of bismuth nitrate solution, and also characterized by XRD, Raman, BET, TEM, FT-IR, XPS, UV-vis DRS and SPS techniques. The effects of the surface-modification with Bi₂O₃ on the thermal stability, photoinduced charge separation and photocatalytic activity for degrading rhodamine B (or phenol) under ultraviolet (or visible) irradiation are investigated in detail, along with their relationships and the activity enhancement mechanisms are also suggested. The results show that the modification with Bi₂O₃ can improve the thermal stability of the as-prepared anatase crystallites, consequently enhancing the anatase crystallinity so as to promote the photoinduced charge separation. And the modification with Bi₂O₃ also extends the optical response range. It can be concluded that the activity enhancement of surface-modified TiO₂ is mainly attributed to the increase in the photoinduced charge separation rate and to the extent of the optical response range, compared with un-modified ones. Moreover, the inhibition phase transformation mechanism related to Bi₂O₃ is suggested.     

From anodic TiO2 nanotubes to hexagonally ordered TiO2 nanocolumns

We report on the formation of hexagonally ordered TiO₂ nanocolumnar layers by electrochemical oxidation in a fluoride containing electrolyte, using self-organizing nanotube formation conditions at elevated potentials and low temperatures. The influence of the substrate temperature on the nanocolumn morphology and composition is investigated and characterized by FE-SEM and EDX. The origin of these nanocolumns can be attributed to a thickening of the inner tube wall of the double wall structure of self-organized TiO₂ nanotubes. Furthermore, a transition from nanocolumnar to nanotubular structure can be established by changing the applied voltage or applying a post-immersion treatment.     

Characterization and photocatalytic activity of Zn–TiO2/AC composite photocatalyst

Activated carbon (AC) supported Zn²⁺–TiO₂ photocatalyst was prepared by sol–gel method. The prepared samples were characterized by X-ray diffraction, scanning electron micrograph, nitrogen absorption, diffuse reflectance UV/VIS and X-ray photoelectron spectroscopy. Using toluene as a pollution target, the photocatalytic activity of photocatalyst was evaluated. The results showed that prepared photocatalyst was obviously helpful for the removal of toluene in air. The photocatalytic degradation of toluene by Zn²⁺–TiO₂/AC reached 100% for 40 min and remained 75% after 160 min, while degradation by TiO₂ was only 30%. It indicated that the photocatalytic activity of prepared photocatalyst was enhanced. It is due to Zn²⁺-doping increased the oxidation and reduction of hole–electron pairs, which was the important factor in heterogeneous photocatalysis.     

Surface state of TiO2 treated with low ion energy plasma

The effect of low pressure radio frequency (rf) plasma treatment on TiO₂ surface states has been studied using X-ray photoelectron spectroscopy. Three different oxidation states of oxygen in untreated TiO₂ powder were observed, which suggests the existence of adsorbed water and carbon on the surface. The ratio of oxygen to titanium (O/Ti) was decreased for the low ion dose plasma treated samples due to desorption of water from the surface. In the case of Ti 2p about 20% of surface states were converted to Ti³⁺ 2p_3/2 state after plasma treatment with a very good stability, whereas untreated TiO₂ remained mostly as Ti⁴⁺ state. A rapid decrease in the ratio of carbon to titanium (C/Ti) at TiO₂ surface was also observed after plasma treatment and more than 90% of carbon atoms were removed from the surface. Therefore, the plasma treatment of TiO₂ has advantages to surface carbon cleaning, increasing O⁻ and Ti³⁺ surface states, hence improving the activity of TiO₂ for different environmental, energy and biological applications.     

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.     

Influence of deposition atmosphere on photocatalytic activity of TiO2/SiOx double-layers prepared by RF magnetron sputtering

TiO₂/SiO_x double-layers have been prepared at room temperature by RF magnetron sputtering. The TiO₂ top-layer was deposited in an Ar atmosphere, while the SiO_x bottom-layer was deposited in an Ar/O₂ atmosphere. Samples were characterized using X-ray photoelectron spectroscopy, scanning electron microscopy, atomic force microscopy, and photoluminescence techniques. The photocatalytic activity of the samples was evaluated by the photodegradation of methylene blue; the results showed that the photocatalytic activity of the TiO₂/SiO_x double-layers was superior to that of the TiO₂ single-layers. The presence of the SiO_x bottom-layer improved the photocatalytic activity of the TiO₂ layer because it may act as a trap for electrons generated in the TiO₂ layer thus preventing electron-hole recombinations.     

Photoelectrochemical property and photocatalytic activity of N-doped TiO2 nanotube arrays

N-doped TiO₂ nanotube arrays (NTN) were prepared by anodization and dip-calcination method. Hydrazine hydrate was used as nitrogen source. The surface morphology of samples was characterized by SEM. It showed that the mean size of inner diameter was 65 nm and wall thickness was 15 nm for NTN. The ordered TiO₂ nanotube arrays on Ti substrate can sustain the impact of doping process and post-heat treatment. The atomic ratio of N/Ti was 8/25, which was calculated by EDX. Photoelectrochemical property of NTN was examined by anodic photocurrent response. Results indicated the photocurrent of NTN was nearly twice as that of non-doped TiO₂ nanotube arrays (TN). Photocatalytic activity of NTN was investigated by degrading dye X-3B under visible light. As a result, 99% of X-3B was decomposed by NTN in 105 min, while that of TN was 59%.