Photocatalytic degradation of organic compounds diluted in water using visible light-responsive metal ion-implanted TiO2 catalysts: Fe ion-implanted TiO2

The application of metal ion-implantation method has been made to improve the electronic properties of the TiO₂ photocatalyst to realize the utilization of visible light. The photocatalytic properties of these unique TiO₂ photocatalysts for the purification of water have been investigated. By the metal ion-implantation method, metal ions (Fe⁺, Mn⁺, V⁺, etc.) are accelerated enough to have the high kinetic energy (150 keV) and can be implanted into the bulk of TiO₂. TiO₂ photocatalysts which can absorb visible light and work as a photocatalyst efficiently under visible light irradiation were successfully prepared using this advanced technique. The UV-Vis absorption spectra of these metal ion-implanted TiO₂ photocatalysts were found to shift toward visible light regions depending on the amount and the kind of metal ions implanted. They were found to exhibit an effective photocatalytic reactivity for the liquid-phase degradation of 2-propanol diluted in water at 295 K under visible light (λ>450 nm) irradiation. The investigation using XAFS analysis suggested that the substitution of Ti ions in TiO₂ lattice with implanted metal ions is important to modify TiO₂ to be able to adsorb visible light.     

TaON and Ta3N5 as new visible light driven photocatalysts

TaON and Ta₃N₅, Ta⁵⁺-based (oxy)nitrides, were studied as visible light driven photocatalysts. Under visible light irradiation (λ≥420 nm), the (oxy)nitrides oxidize water to O₂ and reduce H⁺ to H₂ in the presence of sacrificial reagents (Ag⁺ and methanol). TaON oxidizes water into O₂ efficiently, with a maximum quantum yield of 10%. The photocatalytic reactions proceed via the bandgap transitions (E_g, TaON: 2.5 eV, Ta₃N₅: 2.1 eV) without any noticeable degradation of the catalysts. The small energy gaps of TaON and Ta₃N₅ are ascribed to the valence band structures consisting of N 2p orbitals.     

Ternary Ag@SrTiO3@CNT plasmonic nanocomposites for the efficient photodegradation of organic dyes under the visible light irradiation

In this research, carbon nanotube (CNT)-modified plasmonic silver-strontium titanate (Ag@ SrTiO₃) nanocomposites for the degradation of the organic dye were prepared by the sol-gel method. The characterization of all products was carried out using the X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), N₂ adsorption-desorption test (BET), field emission-scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV–visible diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, electrochemical impedance spectroscopy (EIS), and transient photocurrent (TPC) studies. It was found that the incorporation of Ag in and introducing CNT into the SrTiO₃ nanoparticles reduced the crystallite size to 21 nm and the band gap energy to 2.7 eV. The Reduced PL peak intensity, increased photocurrent value, and reduced charge transfer resistance approved that the Ag@SrTiO₃@CNT nanocomposite had a greater charge transfer efficiency than other samples. The optimal dosage of the photocatalyst, for the complete degradation of 5 ppm of the methylene blue (MB) solution after 30 min of the visible light irradiation, was decided as 0.5 g/L. Besides, in the experimental environment, the Ag@SrTiO₃@CNT sample illustrated the most significant photocatalytic performance of the degradation of methyl orange (MO) and Rhodamine B (RhB) dyes. The detailed mechanism and kinetics of the degradation procedure were clarified. Finally, the prepared system displayed increased stability and reusability in the entire cyclic degradation experiment.     

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.     

Photocatalytic degradation of organic pollutants catalyzed by layered iron(II) bipyridine complex–clay hybrid under visible irradiation

An organic–inorganic layered hybrid was prepared by intercalation of Fe(bpy)₃²⁺ into laponite clay. UV–vis diffuse reflectance, X-Ray diffraction, and SEM confirmed the intercalation and the strong host–guest interaction of Fe(bpy)₃²⁺ molecules with the clay matrix. Compared with laponite, the hybrid formed a solid layered structure due to the linking of laponite platelets by Fe(bpy)₃²⁺ molecules. Upon visible light irradiation (λ > 420 nm), the hybrid was found to be highly effective for the degradation of nonbiodegradable cationic organic pollutants such as Rhodamine B (RhB) and N,N-dimethylaniline by activating H₂O₂ at neutral pH values, but inactive toward anionic organic compounds such as Orange II and Sulforhodamine-B. The adsorption and degradation of organics on the hybrid could be controlled by changing the pH value of the suspension. The total organic carbon (TOC) removal yield of RhB was 41%. pH effect trials and the final degraded products further indicate that unless the target is adsorbed onto the clay layers the reaction could not occur. Neither OH nor OOH/O₂⁻ EPR signals were detected during the reaction. The solid support of laponite not only alters the photochemical properties of Fe(bpy)₃²⁺ but also provides a rigid microenvironment for the enrichment of local substrate molecules and thus enhances the interaction of the active center with the substrate.     

C_3N_4-BiVO_4复合光催化剂可见光催化降解甲基橙

采用水热合成法制备C_3N_4-BiVO_4复合光催化剂,以甲基橙为目标污染物,研究催化剂用量、甲基橙溶液初始浓度和pH值、NaCl用量对甲基橙脱色率的影响,并通过C_3N_4-BiVO_4复合光催化剂的循环使用实验,考察其重复使用性能。结果表明,在甲基橙初始浓度20 mg·L~(-1)、复合光催化剂用量3.0 g·L~(-1)及弱酸性条件下,光照反应6 h,目标污染物甲基橙脱色率达98.81%,溶液中的NaCl对催化剂降解甲基橙有抑制作用。催化剂重复使用5次后,溶液脱色率约80%,表明催化剂性能较稳定,可重复使用。     

Enhanced performance of g-C3N4/TiO2 photocatalysts for degradation of organic pollutants under visible light☆

Photocatalytic degradation is one of the most promising remediation technologies in terms of advanced oxida-tion processes (AOPs) for water treatment. In this study, novel graphitic carbon nitride/titanium dioxide (g-C3N4/TiO2) composites were synthesized by a facile sonication method. The physicochemical properties of the photocatalyst with different mass ratios of g-C3N4 to TiO2 were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), N2 sorption, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and UV–vis DRS. The photocatalytic performances were evaluated by degradation of methylene blue. It was found that g-C3N4/TiO2 with a mass ratio of 1.5:1 exhib-ited the best degradation performance. Under UV, the degradation rate of g-C3N4/TiO2 was 6.92 and 2.65 times higher than g-C3N4 and TiO2, respectively. While under visible light, the enhancement factors became 9.27 (to g-C3N4) and 7.03 (to TiO2). The improved photocatalytic activity was ascribed to the interfacial charge transfer between g-C3N4 and TiO2. This work suggests that hybridization can produce promising solar materials for envi-ronmental remediation.     

Nanocomposites of SnO2 and g-C3N4: Preparation,characterization and photocatalysis under visible LED irradiation

Tin dioxide nanoparticles were prepared in the presence of graphitized carbon nitride (g-C₃N₄) forming nanocomposites with different contents of SnO₂ up to 40 %. G-C₃N₄ was synthetized by heating of melamine at 550 °C in the open air and Sn²⁺ ions were precipitated by sodium hydroxide in g-C₃N₄ aqueous dispersions. Resulting mixtures were dried by freezing at ?20 °C and calcined at 450 °C to obtain SnO₂/g-C₃N₄ nanocomposites.The nanocomposites were characterized by common characterization methods in solid state and in their aqueous dispersions using dynamic light scattering (DLS) analysis and photocatalysis. SnO₂ nanoparticles in the nanocomposites were found to have an average size of 4 nm, however, those precipitated without g-C₃N₄ had an average size of 14 nm. Separation of photoinduced electron and holes via heterojunction between SnO₂ and g-C₃N₄ was demonstrated by photocatalytic decomposition of Rhodamine B (RhB) under LED visible irradiation (416 nm) and photocurrent measurements. The most photocatalytically active nanocomposite contained 10 % of SnO₂. Graphitized carbon nitride was assumed to serve as a template structure for the preparation of SnO₂ nanoparticles with a narrow size distribution without using any stabilizing additives.     

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.     

The kinetics of phenol decomposition under UV irradiation with and without H2O2 on TiO2, Fe–TiO2 and Fe–C–TiO2 photocatalysts

H₂O₂ used in the photo-Fenton reaction with iron catalyst can accelerate the oxidation of Fe²⁺ to Fe³⁺ under UV irradiation and in the dark (in the so called dark Fenton process). It was proved that conversion of phenol under UV irradiation in the presence of H₂O₂ predominantly produces highly hydrophilic products and catechol, which can accelerate the rate of phenol decomposition. However, while H₂O₂ under UV irradiation could decompose phenol to highly hydrophilic products and dihydroxybenzenes in a very short time, complete mineralization proceeded rather slowly. When H₂O₂ is used for phenol decomposition in the presence of TiO₂ and Fe–TiO₂, decrease of OH radicals formed on the surface of TiO₂ and Fe–TiO₂ has been observed and photodecomposition of phenol is slowed down. In case of phenol decomposition under UV irradiation on Fe–C–TiO₂ photocatalyst in the presence of H₂O₂, marked acceleration of the decomposition rate is observed due to the photo-Fenton reactions: Fe²⁺ is likely oxidized to Fe³⁺, which is then efficiently recycled to Fe²⁺ by the intermediate products formed during phenol decomposition, such as hydroquinone (HQ) and catechol.     

Photocatalytic activities of AgSbO3 under visible light irradiation

A novel visible light sensitive photocatalyst, AgSbO₃ was prepared by a conventional solid-state reaction method. This oxide belonging to a cubic-pyrochlore structure can absorb visible light with wavelength up to about 480 nm. From the band structure calculation, we found that the top of the valence band consists of the hybridized Ag 4d and O 2p orbitals and the bottom of the conduction band mainly consists of the Ag 5s and the Sb 5s orbitals. Photocatalytic activities were evaluated using O₂ evolution from an aqueous silver nitrate solution and decomposition of gaseous 2-propanol under visible light irradiation. We found that AgSbO₃ shows a higher O₂ evolution activity than WO₃ and 2-propanol can be mineralized by the AgSbO₃ photocatalysis under visible light irradiation.     

An efficient electron transfer at the Fe/iron oxide interface for the photoassisted degradation of pollutants with H2O2

Fe-200 was synthesized through the calcination of iron powder at 200 °C for 30 min in air. On the basis of characterization by X-ray diffraction and X-ray photoelectron spectroscopy, Fe-200 had a core–shell structure, in which the surface layer was mainly composed of Fe₂O₃ with some FeOOH and FeO, and the core retained metallic iron. The kinetics and mechanism of the interfacial electron transfer on Fe-200 were investigated in detail for the photoassisted degradation of organic pollutants with H₂O₂. Under deoxygenated conditions in the dark, the generation of hydroxyl radicals in aqueous Fe-200 dispersion verified that galvanic cells existed at the interface of Fe⁰/iron oxide, indicating the electron transfer from Fe⁰ to Fe³⁺. Furthermore, the effects of hydrogen peroxide and different organic pollutants on the interfacial electron transfer were examined by the change rate of the Fe³⁺ concentration in the solution. The results indicated that hydrogen peroxide provided a driving force in the electron transfer from Fe²⁺ to Fe³⁺, while the degradation of organic pollutants increased the electron transfer at the interface of Fe⁰/iron oxide due to their reaction with OH.     

Photocatalytic degradation of bisphenol A and dye by graphene-oxide/Ag3PO4 composite under visible light irradiation

A facile, one-step synthesis of graphene-oxide (GO)/Ag₃PO₄ was prepared. The as-prepared samples were characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and fourier transformed infrared (FT-IR) spectroscopy. The SEM image indicated that Ag₃PO₄ particles were mainly distributed on the surface of GO sheets uniformly. DRS analysis revealed that the samples had good visible light response. The photocatalytic activity of the composites was evaluated by the degradation of Rhodamine B (RhB) and Bisphenol A (BPA). The results indicated that the photocatalytic performance of GO/Ag₃PO₄ was greatly enhanced after introduction of GO. The photocatalytic degradation efficiency of colorless chemical pollutants (BPA) over GO/Ag₃PO₄ was higher than that of Ag₃PO₄, and the possible degradation path was proposed by liquid chromatography mass spectrometry (LC-MS) analysis. Moreover, the photocatalytic stability was discussed by XRD and FT-IR spectroscopy analysis. Based on the experimental results, a possible visible-light photocatalytic degradation mechanism was also discussed.     

Efficient degradation of 4-nitrophenol by using functionalized porphyrin-TiO2 photocatalysts under visible irradiation

The effect of a series of carboxylic acids (C₂–C₈), as solvents for the preparation by flame spray pyrolysis of LaCoO₃ catalyst for the flameless combustion of methane, has been investigated. Acetic acid showed to be unsatisfactory from several points of view: low phase purity of the catalyst, higher amount of unburnt carbonaceous residua, lower catalytic activity and low thermal stability. By increasing the carbon chain length of the solvent, the consequent increase of flame temperature led to an increase of crystal phase purity and of particle size and to a decrease of specific surface area of the catalyst. Catalytic activity showed only marginally affected by the last parameter, phase purity seeming more important. Thermal resistance showed directly related to flame temperature, i.e. to the combustion enthalpy of the solvent, but a relatively high amount of residual organic matter can negatively affect this property.     

Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation

Cerium dioxide was prepared by the precipitation method and found to be an efficient photocatalyst to degrade azodyes under visible light irradiation. Nonbiodegradable azodyes acid orange 7 (AO7) was selected as modal target to examine the photocatalytic activity of CeO₂. AO7 could be efficiently degraded in aqueous suspension of CeO₂ under visible light illumination. The catalyst was characterized by X-ray diffraction (XRD), N₂ sorption, transmission electron microscopic image (TEM) and UV/vis absorption spectrum techniques. AO7 solution was quickly decolorized and partly mineralized under visible light irradiation with existing CeO₂. The photodegradation rate of this azodye catalyzed by CeO₂ is much faster than those occurring on commercial titania (Degussa P25) under otherwise identical conditions of visible light irradiation. Experiments were conducted to examine the adsorption mode of acid orange 7 on CeO₂ and adsorption capacity at different pH values. The possible degradation pathway has been proposed for the photocatalytic degradations by using certain radical scavengers and gas chromatography–mass spectrometry (GC–MS) to determine intermediates. The enhanced photoactivity of the lanthanide oxide CeO₂ was attributed to the superior adsorption capacity and special 4f electron configuration.     

TiO2 photocatalyst loaded on hydrophobic Si3N4 support for efficient degradation of organics diluted in water

TiO₂ photocatalyst loaded on Si₃N₄ (TiO₂/Si₃N₄) was prepared by a conventional impregnation method and its photocatalytic performance for the degradation of organics (2-propanol) diluted in water was compared with that of TiO₂ photocatalysts (TiO₂/SiO₂, TiO₂/Al₂O₃, and TiO₂/SiC) loaded on various types of supports (SiO₂, Al₂O₃, and SiC). The formation of the well-crystallized anatase phase of TiO₂ was observed on the calcined TiO₂/Si₃N₄ photocatalyst, while a small anatase phase of TiO₂ was observed on the TiO₂/SiC photocatalyst and amorphous TiO₂ species was the main component on the TiO₂/SiO₂ and TiO₂/Al₂O₃ photocatalysts. The measurements of the water adsorption ability of photocatalysts indicated that the TiO₂/Si₃N₄ photocatalyst exhibited more hydrophobic surface properties in comparison to other support photocatalysts. Under UV-light irradiation, the TiO₂/Si₃N₄ photocatalyst decomposed 2-propanol diluted in water into acetone, CO₂, and H₂O, and finally, acetone was also decomposed into CO₂ and H₂O. The TiO₂/Si₃N₄ photocatalyst showed higher photocatalytic activity than TiO₂ photocatalyst loaded on other supports. The well-crystallized TiO₂ phase deposited on Si₃N₄ and the hydrophobic surface of Si₃N₄ support are important factors for the enhancement of photocatalytic activity for the degradation of organic compounds in liquid-phase reactions.     

Long-term degradation of Ta2O5-doped Bi2O3 systems

Bismuth oxide in δ-phase is a well-known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). 5-10 mol% Ta₂O₅ are doped into Bi₂O₃ to stabilize δ-phase by solid state reaction process. One Bi₂O₃ sample (7.5TSB) was stabilized by 7.5 mol% Ta₂O₅ and exhibited single phase δ-Bi₂O₃-like (type I) phase. Thermo-mechanical analyzer (TMA), X-ray diffractometry (XRD), AC impedance and high-resolution transmission electron microscopy (HRTEM) were used to characterize the properties. The results showed that holding at 800-850 °C for 1 h was the appropriate sintering conditions to get dense samples. Obvious conductivity degradation phenomenon was obtained by 1000 h long-term treatment at 650 °C due to the formation of α-Bi₂O₃ phase and Bi₃TaO₇, and 〈1 1 1〉 vacancy ordering in Bi₃TaO₇ structure.     

Supported Fe/C and Fe/Nafion/C catalysts for the photo-Fenton degradation of Orange II under solar irradiation

The physicochemical properties and photocatalytic behavior of Fe/C and Fe/Nafion/C fabrics have been investigated under laboratory conditions (small vial, artificial light) and in large-scale photo-reactors under sunlight to achieve low-cost decontamination of textile and agro-chemical industry effluent wastewater. Fe-ions deposited onto chemically treated C fabrics or encapsulated in Nafion thin films cast directly onto the carbon fabrics are efficient in decomposing H₂O₂ used as an oxidant in the photo-assisted abatement of non-biodegradable azo-dyes used in textiles. The Orange II taken as a model pollutant quickly fades under solar irradiation even at an initial pH 6. This range of pH is not possible with the homogeneous photo-Fenton process. In contrast, the use of supported catalysts on C fabrics allows the costly pH adjustment to be avoided. This makes catalyst recovery and economical decontamination of wastewater containing non-biodegradable pollutants possible.     

Effect of Ta2O5 in (Ca,Si, Ta)-doped TiO2 ceramic varistors

TiO₂ varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO₂ ceramics were investigated. The ceramics have nonlinear coefficients of α = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁴. Experimental evidence shows that small quantities of Ta₂O₅ improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta₂O₅ leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10⁴ and tg δ = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO₂–0.8 mol% Ta₂O₅ ceramic is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta⁵⁺ for Ti⁴⁺.