Modification strategies of TiO2 for potential applications in photocatalysis: a critical review

TiO₂ has received tremendous attention owing to its potential applications in the field of photocatalysis for solar fuel production and environmental remediation. This review mainly describes various modification strategies and potential applications of TiO₂ in efficient photocatalysis. In past few years, various strategies have been developed to improve the photocatalytic performance of TiO₂, including noble metal deposition, elemental doping, inorganic acids modification, heterojunctions with other semiconductors, dye sensitization and metal ion implantation. The enhanced photocatalytic activities of TiO₂-based material for CO₂ conversion, water splitting and pollutants degradation are highlighted in this review.     

Sol–gel doped TiO<Subscript>2</Subscript> nanomaterials: a comparative study

Among the great number of sol–gel prepared nanomaterials, TiO₂ has attracted significant interest due to its high photocatalytic activity, excellent functionality, thermal stability and non-toxicity. The photocatalytic degradation of pollutants using un-doped and doped TiO₂ nanopowders or thin films is very attractive for applications in environmental protection, as a possible solution for water purification. The present work describes a comparative structural and chemical study of un-doped TiO₂ and the corresponding S- and Ag-doped materials. The photocatalytic activity was established by testing the degradation of organic chloride compounds from aqueous solutions. Sol–gel Ag-doped TiO₂ coatings, prepared by co-gelation and sol–gel Ag-doped TiO₂ coatings obtained from nanopowders were also compared. Their structural evolution and crystallization behaviour (lattice parameters, crystallite sizes, internal strains) with thermal treatment were followed by thermal analysis, X-ray diffraction, transmission electron microscopy, atomic force microscopy and specific surface areas measurements. X-ray photoelectron spectroscopy analyses were performed to characterize the surface composition and S or Ag speciation, which was used to interpret the catalytic data.     

High photocatalytic activity and stability for decomposition of gaseous acetaldehyde on TiO2/Al2O3 composite films coated on foam nickel substrates by sol-gel processes

A set of anatase titanium dioxide (TiO₂) films coated on foam nickel that modified by Al₂O₃ films as transition layer (indicated as TiO₂/Al₂O₃ films) were synthesized via sol-gel route. The bulk and surface properties of the TiO₂/Al₂O₃ films were characterized by thermal gravimetric and differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), and BET. The photocatalytic activities of TiO₂/Al₂O₃ films were investigated based on the degradation of gaseous acetaldehyde under ultraviolet (UV) irradiation. The foam nickel is a promising substrate material in practical applications because of its excellent hydrodynamic properties for gas passing. The TiO₂/Al₂O₃ composite films showed much higher photocatalytic activity and stability for degradation of gaseous acetaldehyde than the onefold TiO₂ films. The significant enhancement in photocatalytic activity and stability can be ascribed to the coating of Al₂O₃ transition layer, which concentrates the target substances around TiO₂ particles and increases the specific surface area (SSA) of the substrate (the SSAs of bare foam nickel and Al₂O₃ modified foam nickel are 0.12 and 113.7 m²/g, respectively) to provide more sites for TiO₂ loading.     

Hydrothermal synthesis of TiO2 nanorods: formation chemistry,growth mechanism,and tailoring of surface properties for photocatalytic activities

Titanium dioxide (TiO₂) is one of the best semiconductor photocatalysts with optical band gap of 3.2 eV. The optical band gap and photocatalytic properties could be further tuned by tailoring shape, size, composition, and morphology of the nanostructures. Hydrothermal synthesis methods have been applied to produce well-controlled nanostructured TiO₂ materials with different morphologies and improved optoelectronic properties. Among various morphologies, one-dimensional (1D) TiO₂ nanostructures are of great importance in the field of energy, environmental, and biomedical because of the directional transmission properties resulting from their 1D geometry. Particularly, TiO₂ nanorods (NRs) have gained special attention because of their densely packed structure, quantum confinement effect, high aspect ratio, and large specific surface area that could specially improve the directional charge transmission efficiency. This results in the effective photogenerated charge separation and light absorption, which are really important for potential applications of TiO₂-based materials for photocatalytic and other important applications. In this review, hydrothermal syntheses of TiO₂ NRs including the formation chemistry and the growth mechanism of NRs under different chemical environments and effects of various synthesis parameters (pH, reaction temperature, reaction time, precursors, solvents etc.) on morphology and optoelectronic properties have been discussed. Recent developments in the hydrothermal synthesis of TiO₂ NRs and tailoring of their surface properties through various modification strategies such as defect creation, doping, sensitization, surface coating, and heterojunction formation with various functional nanomaterials (plasmonic, oxide, quantum dots, graphene-based nanomaterials, etc.) have been reported to improve the photocatalytic activities. Furthermore, applications of TiO₂ NRs/tailored TiO₂ NRs as superior photocatalysts in degradation of organic pollutants and bacterial disinfection have been discussed with emphasis on mechanisms of action and recent advances in the fields.     

钛铁矿制备二氧化钛-四氧化三铁复合材料及其光催化应用

TiO_2因具有多种优异的特性被广泛应用在半导体光催化领域,但是纳米结构的TiO_2颗粒细微,在进行光催化反应之后,难以回收再利用。本文以廉价钛铁矿为原料制备光催化剂TiO_2,同时利用副产物铁合成Fe_3O_4,并采用简单温和的浸渍法制备Fe_3O_4/TiO_2磁性复合材料。通过XRD、FT-IR、SEM、EDS等手段对材料形态结构进行表征分析,并以光降解有机污染物若丹明B为探针反应,考察其光催化性能。结果表明,质量比为1∶10的Fe_3O_4/TiO_2复合材料结构稳定、分散均匀,具有最优的光催化活性(波长356nm下反应3h,若丹明B降解率达到64.0%),并表现出良好的重复性。同时,动力学结果显示降解符合一级反应动力学。     

Enhanced photocatalytic reduction of Cr(VI) by manganese-doped anatase titanium dioxide

Nano-TiO₂ is frequently used as an optimal photocatalyst, since it is nontoxic, low cost, and environmentally friendly, especially for its photocatalytic oxidation action. However, its photocatalytic reducing action has not been widely researched. In this study, TiO₂ doped with different concentrations of manganese was prepared by the sol–gel method and characterized using different techniques to analyze the surface structure, phase composition, and surface elements of the different materials. To investigate the photocatalytic activity, Mn–TiO₂ was used for photocatalytic reduction of Cr(VI). Moreover, various organic pollutants were added to determine whether they enhanced the photocatalytic reduction of Cr(VI). The experiments indicated that the presence of Mn in TiO₂ could enhance its photocatalytic reduction action, especially at 0.02 % molar ratio. Manganese ions doped in TiO₂ behaved as electron accumulation sites. In addition, pH value, and photocatalyst dosage were investigated to analyze their effects on the photocatalytic reduction action. The results show that lower pH value improved the efficiency of photocatalytic reduction; there were no significant changes in the photocatalytic reduction rate with dosage above 1.0 g/L. In the presence of different electron donors (organic pollutants as hole scavengers), the photocatalytic reduction of Cr(VI) was generally improved. In short, manganese-doped TiO₂ exhibited improved photocatalytic reduction activity, especially in cooperation with various organics.     

钛基TiO2纳米管阵列电极的光电催化性能

应用电化学阳极氧化法在纯Ti基底上制备高度有序的TiO2纳米管阵列,考察了Ti/TiO2光阳极的光电化学响应.以苯酚溶液为目标污染物,研究Ti/TiO2电极的光电催化性能,并与光催化性能进行比较.结果表明,该电极光电催化性能优于光催化性能.施加0.6 V电压时,光电催化性能最好.电化学阻抗谱分析显示,光电催化和光催化降解过程的速控步骤均为表面反应步骤,外加偏压减小了界面电荷转移阻抗,提高了光生载流子的分离效率.     

PVA在提高TiO2/CdSe异质结光催化活性与抗光腐蚀性能方面的应用及机理

通过阳极氧化法和电化学沉积制备了TiO_2/CdSe异质结膜,并通过旋涂结合后续热处理的方法,在TiO_2/CdSe异质结膜上制备适量脱水态的聚乙烯醇(PVA)来提高TiO_2/CdSe异质结抗光腐蚀性能。采用XRD,SEM,FTIR,UV-Vis,PL,电化学测试,光催化降解罗丹明B等方法对样品的晶体结构、微观形貌、光电化学性能、光催化性能等进行了表征,并通过测定光降解体系中Cd2+的浓度,研究了纳米复合材料的抗光腐蚀性能。结果表明,与TiO_2/CdSe相比,TiO_2/CdSe/PVA纳米复合材料不仅具有更好的可见光光催化活性,还具有良好的可见光光催化稳定性和抗光腐蚀性能。同时,PVA的存在对光催化反应中的二次污染物Cd2+也有抑制作用。     

Characterization of Titanium Dioxide Doped with Nitrogen and Sulfur and its Photocatalytic Appraisal for Degradation of Phenol and Methylene Blue

The degradation of organic dyes in the presence of modified TiO₂ is still under intensive investigation. We report here an evaluation of the photocatalytic activity of nitrogen‐ (N‐) and sulfur‐ (S‐) doped TiO₂ for the degradation of phenol and methylene blue (MB). N‐doped TiO₂ (N–TiO₂), S‐doped TiO₂ (S–TiO₂), and N–S‐doped TiO₂ (N–S–TiO₂) were prepared using the sol–gel method. The photocatalytic activity was evaluated in a batch reactor using phenol and MB as models of pollutants. In addition, this investigation was performed using a household lamp as the visible light source. Properties of the synthesized materials in terms of Brunauer–Emmett–Teller (BET) surface analysis, field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), and photocatalytic ability were examined. Our study shows that N–S–TiO₂ exhibits better photocatalytic degradation ability for all the considered dyes compared to the other doped TiO₂ materials. In conclusion, we have successfully prepared and evaluated the photocatalytic activity of N‐ and S‐doped TiO₂ for the degradation of phenol and MB using an ordinary household lamp.     

RE/TiO2光催化剂的谱学特性与气相光催化性能的研究

采用改进的sol-gel法和浸渍法制备了TiO₂掺杂稀土离子La³⁺、Y³⁺、Gd³⁺、Er³⁺、Nd³⁺、Pr³⁺的RE/TiO₂光催化剂,运用FTIR、XRD、TEM、低温氮吸附/脱附、TG/DTA、UV-Vis DRS、表面光电压谱(SPS)等进行表征,以气相光催化降解乙烯、溴代甲烷作为探针反应,阐明了RE/TiO₂光催化剂的谱学特性与气相光催化性能的关系。结果显示,稀土离子掺杂后,TiO₂的锐钛矿含量增加,比表面积增大,粒径变小,吸收边发生蓝移,表面光电压的响应阈值增大,此外,Pr³⁺除外的其它稀土离子掺杂的TiO₂的表面光电压信号增强;光催化降解实验表明,与纯TiO₂相比,La³⁺、Y³⁺、Gd³⁺、Er³⁺、Nd³⁺掺杂TiO₂样品上乙烯、溴代甲烷的光催化活性均有不同程度的增强,而且表现出较强的矿化能力。但是,掺杂Pr³⁺的TiO₂的光催化性能降低恰好对应较弱的表面光电压信号。所以,本文认为提高光生电子-空穴对的分离效率是改善光催化性能的关键因素。     

Photocatalytic degradation of azo dyes catalyzed by Ag doped TiO2 photocatalyst

In this paper, photocatalytic degradation of commercial textile azo dyes catalyzed by titanium dioxide and modified titanium dioxide with Ag metal (1% w/w) in aqueous solution under irradiation with a 400 W high-pressure mercury lamp is reported. The effect of various parameters such as irradiation time of UV light, amount of photocatalyst, flow rate of oxygen, pH and temperature for the Ag-TiO₂ photocatalyst were investigated. Kinetic investigations of photodegradation indicated that reactions obey improved Langmuir-Hinshelwood model and pseudo-first-order law. The rate constant studies of photocatalytic degradation reactions for Ag-TiO₂ and TiO₂ photocatalysts indicated that in all cases the rate constant of the reaction for Ag-TiO₂ was higher than that of TiO₂.     

Fabrication and photocatalytic activity of high-efficiency visible-light-responsive photocatalyst ZnTe/TiO2 nanotube arrays

A new ZnTe modified TiO₂ nanotube (NT) array catalyst was prepared by pulse potential electrodeposition of ZnTe nanoparticles (NPs) onto TiO₂ NT arrays, and its application for photocatalytic degradation of anthracene-9-carboxylic acid (9-AnCOOH) was investigated. The even distribution of ZnTe NPs was well-proportionately grown on the top surface of the TiO₂ NT while without clogging the tube entrances. Compared with the unmodified TiO₂ NT, the ZnTe modified TiO₂ NT (ZnTe/TiO₂ NT) showed significantly enhanced photocatalytic activity towards 9-AnCOOH under simulated solar light. After 70 min of irradiation, 9-AnCOOH was degraded with the removal ratio of 45% on the bare TiO₂ NT, much lower than 80%, 90%, and 100% on the ZnTe/TiO₂ NT with the ZnTe NPs prepared under the pulsed “on” potentials of −0.8, −1.0, and −2.0 V, respectively. The increased photodegradation efficiency mainly results from the improved photocurrent density as results of enhanced visible-light absorption and decreased hole-electron recombination due to the presence of narrow-band-gap p-type semiconductor ZnTe.     

非共价修饰碳纳米管/二氧化钛复合材料的合成及性能

采用溶胶-凝胶法在聚乙烯吡咯烷酮(PVP)非共价修饰的碳纳米管表面均匀沉积二氧化钛粒子制得纳米复合材料。用TEM、XRD、FTIR、N2吸脱附等对复合材料进行了表征。结果表明:纳米二氧化钛纳米粒子均匀沉积在被修饰碳纳米管表面,且二氧化钛为纯锐钛矿晶体结构,没有金红石和板钛矿相。非共价修饰碳纳米管/二氧化钛复合材料具有良好的介孔结构,其孔径分布主要集中在6～10 nm,且比表面积与纯的二氧化钛相比明显增大,在紫外光照射下降解亚甲基蓝,相比纯的二氧化钛和碳纳米管/二氧化钛,具有较高的催化活性。     

Photocatalytic activity of Au/TiO<Subscript>2</Subscript> nanocomposite for azo-dyes degradation

The structural properties of Au/TiO₂ catalyst were studied by X-ray diffraction, UV-visible diffuse reflectance, photoluminescene, scanning transmission and electron microscope, and temperature programmed reduction. The photocatalytic activity of the catalysts was evaluated for the degradation of various azo-dyes such as methylene blue, methyl orange, reactive blue-4, and eosin-B under solar irradiation. It was found that TiO₂ catalyst modified with gold exhibits higher percentage of degradation compared to starting TiO₂. For example, TiO₂ showed 35% of methyl orange degradation whereas gold modified TiO₂ possessed 82%. Effect of different parameters such as pH and dye concentration has been evaluated and the photocatalytic activity was correlated with physico-chemical properties. The dye degradation rate followed first order kinetics.     

Fluorinated Mesoporous Anatase TiO<Subscript>2</Subscript> Microspheres with High Surface and Enhanced Photocatalytic Activity for the Degradation of Methyl Orange

Highly crystalline mesoporous TiO₂ microspheres with areas up to 122 m²/g and tunable pore size have been prepared through a combined sol–gel and solvothermal processes. The concentration of NaF exhibits a great effect on the morphology, crystallinity, crystal size and photocatalytic activity of the TiO₂ microspheres. A higher NaF concentration results in an increase in the average crystal size and pore size, whereas whereas it caused a decrease in the specific surface areas. All fluorinated TiO₂ microspheres showed a higher photocatalytic activity than P25 and pure TiO₂ microspheres obtained in the absence of NaF due to the effect of surface fluorination on the photoactivity of TiO₂. The approach described in this study provides a simple method to synthesize the micrometer-sized hierarchical structure of mesoporous TiO₂ microspheres that are ready for practical applications such as environmental pollutants removal and solar cell because these high active materials can be easily separated.     

Degradation kinetics of pollutants present in a simulated wastewater matrix using UV/TiO<Subscript>2</Subscript> photocatalysis and its microbiological toxicity assessment

Water pollution is one of the major concerns over long-term sustainability of the environment. Effective and efficient treatment of polluted wastewater is still a serious challenge for global researchers. In the last 2–3 decades, due to the incessant emergence of micropollutants in surface and ground water bodies, several endeavors have been made to resolve the water pollution issues either through chemical, physical and biological degradation processes or through removal/separation processes using different adsorbents and membranes. It has been found that most of the studies are mainly limited to single or binary pollutant analysis in a pure water matrix. Therefore, in this novel investigation, a mixture of five different pollutants has been studied for UV/TiO₂-based photocatalytic degradation. In the present study, a commercially available TiO₂, an antibiotic, i.e. Ciprofloxacin and four different synthetic dyes, i.e. Rhodamine B, Methylene Blue, Methyl Orange and Amaranth have been used as a photocatalyst, a pharmaceutical and various industrial dyes, respectively, in a batch photocatalytic reactor system with a stirrer. It is important to note that the commercial TiO₂ photocatalyst has also been characterized with the help of several characterization techniques. The present study is mainly focused on the degradation of different micropollutants present in the simulated wastewater matrix and their individual degradation kinetics. It is interesting to observe that MB and RhB have shown the maximum degradation followed by CIP (96.21, 96.15 and 89.62%, respectively). In addition, a microbiological assay has also been performed to check the toxicity variation in the degraded products. It is quite interesting to observe that the simulated wastewater matrix has completely lost its microbial toxicity within 120 min of UV/TiO₂-based photocatalytic treatment. Finally, total organic carbon evaluations of various treated samples have also been performed and the obtained results substantiate the theory of assimilable organic carbon.     

Application of an Ion Beam Technique for the Design of Visible Light-Sensitive, Highly Efficient and Highly Selective Photocatalysts: Ion-Implantation and Ionized Cluster Beam Methods

As a new concept for the design of effective photocatalysts, an ion beam technology using accelerated metal ions, a metal ion implantation and an ionized cluster beam (ICB) method, have been applied to design unique photocatalysts. The metal ion implantation of TiO₂ and Ti-containing zeolites with highly accelerated metal ions (V⁺, Cr⁺, etc.) is useful in designing photocatalysts that can operate efficiently under visible light irradiation. Also, an ICB method with a low acceleration energy is useful in preparing transparent TiO₂ thin films on various types of substrates for the efficient photocatalytic degradation of pollutants diluted in water and air. The combination of the ICB method and metal ion implantation can develop the TiO₂ thin films that are able to operate not only under UV light but also under visible light irradiation.     

TiO2-石墨烯光催化剂：制备及引入石墨烯的方法对光催化性能的影响

利用溶胶-凝胶法原位制备了二氧化钛/石墨烯(TiO2-GE)复合光催化剂,研究了纯TiO2以及不同方法制备的TiO2-GE复合光催化剂对亚甲基蓝及罗丹明B光催化降解性能。结果表明:石墨烯的引入提高了TiO2的光催活性,这主要是得益于石墨烯优异的电子传输性能及较好吸附特性。不同方法制备TiO2-GE复合催化剂的光催化活性也存在较大差别。原位制备的TiO2-GE复合光催化剂表现出最佳的光催化活性。     

Development of TiO<Subscript>2</Subscript> photocatalysts suitable for practical use and their applications in environmental cleanup

Recently, environmental disruption is proceeding on a global scale through the consumption of huge amounts of fossil fuels and the emission of various chemical substances. However, these substances resist bio-treatment. TiO₂ generates electrons and holes by irradiation with light. Most organic micro-pollutants, including dioxins, are decomposed into carbon dioxide and water by the effect of the holes with high oxidative potential. By using such a photocatalytic reaction, various applications are feasible for environmental cleanup. In general, TiO₂ powder has been utilized as photocatalyst, although TiO₂ powder photocatalyst has several disadvantages: (1) it is difficult to handle, (2) photocatalytic reaction is slow and it takes a lot of time for treatment and (3) it is difficult to apply to plastics and textiles, because the photocatalyst decomposes them. We have developed a photocatalyst suitable for practical use and have developed high-activity photocatalysts such as TiO₂ photocatalytic transparent film, photocatalytic silica-gel, apatite-coated TiO₂ photocatalyst usable for plastics and textiles, photocatalytic paper, photocatalytic blue charcoal and photocatalytic oxygen scavenger. The application of these high-activity photocatalysts has been studied in deodorization, anti-bacterial, self-cleaning, anti-stain, water treatment, air purification such as photocatalytic decomposition of dioxins and VOC, and NO _x removal. Now various photocatalytic articles using these new photocatalyst materials are on the market in Japan. Photocatalytic technology can create many valuable products for environmental use all over the world.     

Efficient and Robust Cu/TiO2 Nanorod Photocatalysts for Simultaneous Removal of Cr(VI) and Methylene Blue under Solar Light

Because of its large bandgap, TiO₂ can function only under UV light. TiO₂ surface modification with noble metal nanoparticles can extend the light absorption from UV to visible light region and enhance the photocatalytic quantum yield. In this work, TiO₂ nanorods (Cu/TiO₂) modified by copper nanoparticles were prepared by a one‐step solvothermal method at low cost. The resultant Cu/TiO₂ nanorods show excellent synergistic effect in the oxidation of methylene blue (MB) and the reduction of aqueous Cr(VI) under solar light irradiation. Mechanistic investigation suggests that the Cr(VI) species could effectively scavenge the electrons from MB in the presence of the as‐prepared photocatalyst, leading to the simultaneous removal of both pollutants. Being economically viable, environmentally sustainable, and highly efficient, the proposed photocatalyst holds promise for technologies involving simultaneous organic degradation and heavy metal removal in wastewater treatment.