Influence of transition metal-doped titanium(IV) dioxide on the photodegradation of polyethylene

The effects of adding dispersed powders of various forms of titanium(IV) dioxide on the photodegradation of polyethylene have been examined from the following points of view: effect of crystal form, concentration of pigment, transition metal ion, dopant concentration, calcination temperature of pigment, and pigment coating.The rate of photodegradation of polyethylene is reduced by adding certain grades of TiO₂ such as coated TiO₂ particles or TiO₂ doped with small percentages of Cr or Mn ions. The rate is increased on adding TiO₂ doped with V and especially Mo or W ions. The anatase form of TiO₂ is more photoactive than the rutile form, and the effect of increasing the calcination temperature of the pigment is to reduce photoactivity by boosting the rutile fraction. The concentration dependences of the degradation rates are complex, but can be directly related to the percentage of anatase achieved after calcination. Even the most aggressive of the metal-doped pigments are less photoactive than the Degussa P25 material, containing both rutile and anatase.     

Effect of polycrystalline structure of TiO2 particles on the light scattering efficiency

The opacifying power of synthesized polycrystalline TiO2 particles in a cellulose matrix was found experimentally and theoretically to be superior to that of a commercial rutile pigment, depending on crystal structure of the synthesized particles. The crystal structure of the particles was varied by calcination of amorphous titania nanoparticles at different temperatures and was characterized using SEM, TEM, and XRD. Polycrystalline anatase pigments had less opacifying power than commercial rutile, while polycrystalline pigments containing a one-to-one mixture of anatase and rutile had similar opacifying power as the commercial pigment if they have a similar overall particle size. The polycrystalline rutile pigments composed of a linear linkage of several individual rutile crystals gave 6% more opacity than the commercial rutile pigment. Theoretical light scattering calculations using the T-matrix method showed the light scattering efficiency of linearly arranged polycrystalline rutile particles to depend on number and size of crystals composing the particle. It is suggested that the efficiency of rutile pigments can be increased dramatically by controlling both the primary crystal size and the overall particle size. It is believed that the greater than expected light scattering efficiency of the biphasic pigment results from reflection and refraction of light at the grain boundaries between crystals of different phase, which have different refractive indices.     

Enhanced Photoactivity in Bilayer Films with Buried Rutile–Anatase Heterojunctions

Herein, we study the photoactivity of anatase–rutile bilayer thin films consisting of an anatase overlayer of variable thickness from some tenths to some hundred nanometers deposited onto a rutile thin film. As references single anatase layers of equivalent thickness were deposited onto silicon. All the films were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. The photoactivity of the samples was assessed by following the evolution with the UV illumination time of both the wetting angle on the thin film surface and the decoloration of a dye in a water solution. While a similar efficiency is found for the first type of experiments irrespective of the anatase thickness, in the second type a maximum in the photoactivity is found for a thickness of the anatase layer of about 130 nm. This enhanced photoactivity in bilayer systems with a buried anatase–rutile heterojunction is related to the formation of different Schottky potential barriers in the anatase layer, depending on its thickness and the substrate (i.e. rutile or SiO₂) where it is deposited.     

Heats of immersion of titanium dioxide pigments in alcohol—water mixtures

The heats of immersion of partially dried anatase and rutile pigments in mixtures of water with methanol, ethanol, and n-propanol were measured by a differential calorimetric method. The anatase heats of immersion could best be explained by assuming preferential adsorption of the alcohols, the effect being greatest for n-propanol The rutile pigment, however, appeared to adsorb water preferentially in methanol—water and ethanol—water mixtures over the whole concentration range. In propanol—water mixtures the rutile pigment preferentially adsorbed water below an alcohol mol fraction of 0.25, and preferentially adsorbed propanol at mol fractions of alcohol greater than 0.25. The differences in behaviour between the two pigments may be explained qualitatively from the point of view of their surface morphology.     

Three-phase junction for modulating electron–hole migration in anatase–rutile photocatalysts

The heterophase solid–solid junction as an important type of structure unit has wide applications for its special mechanics and electronic properties. Here we present a first three-phase atomic model for the anatase–rutile TiO₂ heterophase junction and determine its optical and electronic properties, which leads to resolution of the long-standing puzzles on the enhanced photocatalytic activity of anatase–rutile photocatalysts. By using a set of novel theoretical methods, including crystal phase transition pathway sampling, interfacial strain analysis and first principles thermodynamics evaluation of holes and electrons, we identify an unusual structurally ordered three-phase junction, a layer-by-layer “T-shaped” anatase/TiO₂-II/rutile junction, for linking anatase with rutile. The intermediate TiO₂-II phase, although predicted to be only a few atomic layers thick in contact with anatase, is critical to alleviate the interfacial strain and to modulate photoactivity. We demonstrate that the three-phase junction acts as a single-way valve allowing the photogenerated hole transfer from anatase to rutile but frustrating the photoelectron flow in the opposite direction, which otherwise cannot be achieved by an anatase–rutile direct junction. This new model clarifies the roles of anatase, rutile and the phase junction in achieving high photoactivity synergistically and provides the theoretical basis for the design of better photocatalysts by exploiting multi-phase junctions.     

Influence of transition metal-doped titanium(IV) dioxide on the photodegradation of polystyrene

The effects of adding dispersed powders of various forms of titanium(IV) dioxide on the photodegradation of polystyrene have been examined using FT-IR spectroscopy from the following points of view: effect of crystal form, concentration of pigment, transition metal ion, dopant concentration, calcination temperature of pigment, and pigment coating.The rate of photodegradation of polystyrene is reduced by adding certain grades of TiO₂ such as coated TiO₂ particles or TiO₂ doped with small percentages of Cr or Mn ions. The rate is increased on adding TiO₂ doped with V and especially Mo or W ions. The anatase form of TiO₂ is more photoactive than the rutile form, as is the effect of increasing the calcination temperature of the pigment. The concentration dependences of the degradation rates are complex, but can be directly related to the percentage of anatase achieved after calcination. Even the most aggressive of the metal-doped pigments are less photoactive than a Degussa P25 material, containing rutile and anatase.     

Photocatalytic coatings for environmental applications

A series of nano- and micronparticle-grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photocatalytic activites have been determined in a series of paint films by FTIR, chalking, color, gloss change and weight loss after artifical weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge-carrier dynamics in the pigments. Pigment particle size, surface area and properties clearly play an important role in dispersion and any polymer-pigment interactions. Photooxidation studies on several types of paint films show a clear demarcation between nanoparticle- and pigmentary-grade titanium dioxide, with the former being more active because of their greater degree of catalytic surface activity. The photosensitivity of titanium dioxide is considered to arise from localized sites on the crystal surface (i.e. acidic OH), and occupation of these sites by surface treatments inhibits photoreduction of the pigment by ultraviolet radiation; hence, the destructive oxidation of the binder is inhibited. Coatings containing 2-5% by weight alumina or alumina and silica are satisfactory for general-purpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7-10% weight. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminum phosphates of other metals. For example, the presence of hydrous alumina particles lowers van der Waals forces between pigments particles by several orders of magnitude, decreasing particle-particle attractions. Hydrous aluminum oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilization in various water- and oil-based paint media in comparison with conventional organic stabilizers. Hindered piperidine stabilizers are shown to provide no additional benefits in this regard, often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated in this study. In the former case, surface erosion is shown to be controlled by varying the ratio of admixture of durable pigmentary-grade rutile (heavily coated) and a catalytic-grade anatase nanoparticle. For environmental applications in the development of coatings for destroying atmospheric pollutants such as nitrogen oxide gases (NO(X)), stable substrates are developed with photocatalytic nanoparticle-grade anatase. In this study, porosity of the coatings through calcium carbonate doping is shown to be crucial in the control of the effective destruction of atmospheric NO(X) gases. For the development of microbiological substrates for the destruction of harmful bacteria, effective nanoparticle anatase titania is shown to be important, with hydrated high surface area particles giving the greatest activity.     

Photo-stabilising action of metal chelate stabilisers in polypropylene: Part VIII—Pigment interactions

The interaction of four metal chelates—Cyasorb UV 1084, Irganox 1425, Irgastab 2002 and Sanduvor NPU—with five commercial pigments, anatase and rutile titanium dioxide, cadmium yellow, Vynamon Blue B and Monolite Yellow G, is examined using infra-red spectroscopy. Individually, four pigments offer some stabilisation to processed polypropylene. Their order of stabilisation is Monolite Yellow G > Vynamon Blue B > cadmium yellow ～ rutile titanium dioxide. Anatase titanium dioxide showed mild sensitising action. When mixed with the metal chelates their order of stabilisation is variable. The effect of a commercial antioxidant, Irganox 1010, is also examined. It is found that the antioxidant tends to suppress any synergistic effects that may be present between the chelate and the pigment.     

Photochemical processes involving TiO2 pigments and TiO2–light stabilizer combinations in commercial polyolefins

The photoactivity of titania pigments at different loadings in both polyethylene and polypropylene is examined by phosphorescence and carbonyl formation and a correlation between the two observed. A nickel (II) chelate stabilizer proved more effective than a hydroxybenzophenone stabilizer in the presence of anatase. This difference in stabilizing efficiency appears to be due to more efficient quenching of the excited state of the anatase by the nickel chelate than by the hydroxybenzophenone.     

Photocatalytic activity of TiO2 prepared by microwave/sol–gel method

This study addresses the photodegradation efficiency of bisphenol A (4,4′-isopropylidenephenol, BPA) and the generation of hydroxyl radicals using the UV/TiO₂ system. Photocatalysts were formed by the microwave/sol–gel method and TiO₂ was prepared with anatase and rutile phases with a high surface area. The hydroxyl radicals generated by the preparation of TiO₂ by a microwave/sol–gel process, and its photoactivity, exceeded those of TiO₂ prepared using a sol–gel process.     

Enhanced photoactivity of neodymium doped mesoporous titania synthesized through aqueous sol–gel method

Nanosize neodymium doped titania has been prepared by hydrolysis of titanium oxychloride followed by peptisation under acidic condition. The anatase to rutile phase transformation temperature was found to increase by 150 °C as a result of neodymium doping. The doped sample shows 10 times higher surface area than the undoped one after calcining at 700 °C. All the samples calcined at 500, 600 and 700 °C show type IV isotherm, which is characteristic of mesoporous material. The pore size distribution curves also show that the pores are in mesoporous region. Further, the neodymium doped titania shows increased photoactivity than the undoped titania with respect to decomposition of methylene blue when subjected to UV light. The transmission electron micrograph indicates that a nanocrystalline doped titania is obtained through the present method. The effect of neodymium doping on the anatase phase stability, specific surface area and photoactivity are reported.     

Photocatalytic Activity in the 2,4-Dinitroaniline Decomposition Over TiO2 Sol-Gel Derived Catalysts

Titania was prepared by the sol-gel method from titanium alkoxide. Depending on the pH of the gelling solution, specific surface areas between 88 and 10 m²/g were obtained. The band gap (E _g) of the samples was found between 3.05 and 3.32 eV. In samples gelled at pH5 and 9 and calcined at 400°C only anatase phase is observed, while for pH3 and pH7 brookite, anatase and rutile or anatase-rutile phases coexist. It was found that the photoactivity in the 2,4-dinitroanailine decomposition depends on the E _g and on the crystalline phases. The highest activity corresponds to the catalysts having the lowest E _g and more than one crystalline phases co-existing.     

The influence of concentration of doping elements on anatase–rutile transformation at the synthesis of rutile pigments (Ti,Cr,Nb)O2 and their pigmentary properties

An analysis of the effects of dopants concentration and different starting titanium compounds on the anatase to rutile phase transformation at the synthesis of rutile pigments Ti_1?3xCr_xNb_2xO_2±δ is presented in this study. The main goal was to analyze reaction mixtures for x = 0.05 (previous study) and 0.30 by simultaneous TG–DTA analysis and to determine the temperature of anatase–rutile transition. For x = 0.05, initial temperatures 760–830 °C are needful for a formation of rutile structure. The temperature is the lowest for the hydrated Na₂Ti₄O₉ paste (760 °C) and similar for other starting compounds of titanium. But for x = 0.30, the anatase–rutile transition begins at higher temperatures 910–1,030 °C because of high-Nb content, which is the inhibitor of this modification change. In addition, we found the influence of calcination temperatures (850, 900, 950, 1000, 1050, 1100, and 1150 °C) on color properties and particle size distribution of these materials prepared from anatase TiO₂ and with x = 0.30. Selected pigments were also analyzed by X-ray powder diffraction.     

Bi2O3 /TiO2复合纳米颗粒的可见光光催化性能

采用溶胶与水热相结合的方法合成了具有可见光光催化活性的复合纳米颗粒Bi2O3/TiO2,并对其进行了X射线衍射、透射电镜、X射线光电子能谱、紫外-可见漫反射谱、红外光谱、低温N2吸附脱附及电子顺磁共振分析。结果表明,复合少量的氧化铋可显著抑制TiO2由锐钛矿到金红石的相转移过程,并将光吸收范围扩展到可见光区。可见光照射下(λ>420 nm),利用电子顺磁共振技术检测到明显的羟基自由基(.OH)信号。铋的最佳掺杂量为Bi/Ti质量比2.0%,适量铋的掺入能显著改善锐钛矿TiO2的结晶度,抑制光生电子-空穴对的复合,提高光催化量子效率。通过可见光照射下,4-氯酚的降解实验测试Bi2O3/TiO2复合纳米颗粒的可见光光催化活性。同时,利用气-质联用仪对4-氯酚降解过程的中间产物进行了测定,并提出可见光照射下的Bi2O3光敏化机理。     

Transition metal-doped titanium(IV) dioxide: Characterisation and influence on photodegradation of poly(vinyl chloride)

The effects of transition metal dopants (V(IV), V(V), Mn(II), Cr(III), Mo(V), and W(V)), introduced into TiO₂, upon the rate of photodegradation of poly(vinyl chloride) (PVC) films containing TiO₂ have been measured. The rates were determined mainly by monitoring carbonyl group formation. In another set of experiments, the rates of chloride ion release from irradiated PVC particles suspended in water undergoing agitation with air or O₂ in the presence of particles of doped TiO₂ were measured electrochemically. The doping of TiO₂ (rutile) with Cr(III), V(V) or Mn(II) reduces the photoactivity of the pigment, while doping by Mo(V) or W(V) enhances its photoactivity; the results obtained from carbonyl index measurements are paralleled closely by those from chloride ion release. Even the most aggressive doped pigments were less reactive than Degussa P25 pigment, while the greatest protection to PVC film was offered by TiO₂ particles coated with Al₂O₃ or SiO₂. Overall, the photoactivity of doped TiO₂ is a complex function of dopant concentration, the energy levels of the dopants in the TiO₂ lattice, their d electronic configuration and their local distribution. Photoactivity is also linked to other factors such as crystal type, particle size distribution and surface area. There is a clear relationship between the tendency of the dopant to induce the rutile-to-anatase transition and its effect in enhancing the photoactivity of the pigment. The characterisation of the doped pigments was achieved using X-ray powder diffraction, EPR spectroscopy, diffuse reflectance UV-vis spectrophotometry, scanning optical and electron microscopy and particle size analysis using LALLS.     

CARRIER PHOTOGENERATION IN POLY（N-VINYL-CARBAZOLE）-BASED PHOTOCONDUCTIVE THIN-FILM DEVICES

In this paper, photoexcitation processes in the bilayer devices based on inorganic materials and poly（N-vinylcarbazole） （PVK） were investigated. In order to clarify the roles of inorganic materials in photoconductive properties of bilayer devices, TiO2 and ZnS were chosen to combine with PVK. A model for generation of photocurrent （Iph） in single layer device of PVK was obtained. It is deduced that the recombination rate constant （Pcomb） and the ionization rate constant （y） ofexcitons should be considered as the most important factors for Iph. For inorganic materials （TiO2 or ZnS）/PVK bilayer devices, in reverse bias of-4 V, the photocurrent of 115 mA/cm^2 in the TiO2/PVK device was observed, but the photocurrent in the ZnS/PVK device was only 10 mA/cma under the illumination light of 340 nm and the light intensity of 14.2 mW/cm^2. The weaker photocurrent is attributed to the absorption of ZnS within UV region and the energy offset at the interface between PVK and ZnS, which impedes the transport of charge carriers.     

含蒽醌基单偶氮染料光电导性质构效研究

由1-氨基蒽醌和不同偶合基合成了九个含蒽醌基的单偶氮染料,并对其进行了 元素分析、红外光谱和紫外光谱表征。紫外光谱研究结果表明,偶合基酰胺芳环上 取代基对偶氮染料的吸收光谱影响不大。当偶合基由2-羟基-3-氨替酰胺萘酚（ Azol～8)变为2-羟基-11氢-苯并咔唑-3-氨替酰胺萘酚（Azo9)时,偶氮染料的吸收 红移了130 nm,最大吸收出现在662 nm处,成为近红外染料。偶氮染料光电导性质 的研究表,偶合基酰胺芳环上取代基吸电子效应越强,光敏性越好,且与Hammett 常数σ有良好的相关性。值得注意的是萘环上扩大一个咔唑环（Azo9),使偶氮染 料的光敏性提高了几十倍,表明在萘环上扩大共轭环是提高偶氮染料光敏性的一个 良好途径。     

用表面处理法降低TiO2粉末光活性问题的研究

本工作采用了两种简便的方法即用不同金属盐和金属氧化物对两种晶型(锐钛矿型和金虹石型)TiO₂粉末进行表面处理来降低TiO₂的光活性并对两种表面处理方法的优缺点作了比较,在评价TiO₂光活性时采用异丙醇光氧化反应的方法。实验证明,用金属盐表面处理TiO₂,其光活性有明显降低。用金属氧化物对TiO₂进行表面处理,在氧化物/TiO₂比例适当时也可以得到与金属盐表面处理TiO₂的同样效果。此外,我们还对TiO₂表面处理后能降低其光活性的作用机制问题进行了讨论。     

Curious behaviors of photogenerated electrons and holes at the defects on anatase,rutile, and brookite TiO2 powders: A review

Photocatalytic reactions are governed by photogenerated charge carriers upon band gap excitation. Therefore, for better understanding of the mechanism, the dynamics of photocarriers should be studied. One of the attractive materials is TiO₂, which has been extensively investigated in the field of photocatalysis. This review article summarizes our recent works of time-resolved visible to mid-IR absorption measurements to elucidate the difference of anatase, rutile, and brookite TiO₂ powders. The distinctive photocatalytic activities of these polymorphs are determined by the electron-trapping processes at the defects on powders. Powders are rich in defects and these defects capture photogenerated electrons. The depth of the trap is crystal phase dependent, and they are estimated to be < 0.1 eV, ∼0.4 eV and ∼0.9 eV for anatase, brookite, and rutile, respectively. Electron trapping reduces probability to meet with holes and then elongate the lifetime of holes. Therefore, it works negatively for the reaction of electrons but positively works for the reaction of holes. In the steady-state reactions, both electrons and holes should be consumed. Hence, the balance between the positive and negative effects of defects determines the distinctive photocatalytic activities of anatase, rutile, and brookite TiO₂ powders.     

Investigation of visible light active Ag sensitized mixed phase TiO2 photocatalyst for solar energy application

Visible photo-active anatase and mixed phase Ag/TiO₂ photocatalysts were prepared using sol–gel method with 1.5 wt% Ag concentration. Due to the large band gap, pristine titania (anatase) is mainly active in the UV light and the photoactivity is limited. But in the presence of both anatase (~3.2 eV) and rutile (~3.0 eV) phases in TiO₂, the catalyst is expected to show enhancement in the photoactivity due to mixed phase junction. The Ag particles are used to act as electron sink and to swiftly transport the photo-generated electrons and, consequently, lessen the recombination rate. The synergistic effect of surface plasmon resonance (SPR) enhanced local field due to Ag particles and the existing mixed phase of TiO₂ is expected to provide a boost in its photoactivity in the visible region. In the present work the mixed phase Ag/TiO₂ photocatalyst showed large enhancement in the degradation of methylene blue compared to all the reference systems.