Dissolution kinetics of titanium dioxide nanoparticles: the observation of an unusual kinetic size effect

Different types of industrially produced titanium dioxide nanoparticles and a precipitated titanium dioxide have been dissolved in aqueous NaCl solutions at temperatures of 25 and 37 degrees C. The titanium concentration in solution with regard to dependence on time has been determined up to 3000 h after starting the dissolution experiment. The effect of particle size, pH value, temperature, background electrolyte concentration, and mass concentration of titanium dioxide exposed to the liquid phase has been studied. The nanoparticles have been characterized by N2 physisorption measurements and XRD. The total dissolved titanium in solution has been determined by adsorptive stripping voltammetry (AdSV) and inductively coupled plasma mass spectrometry (ICP-MS). A new kinetic size effect has been observed. It turns out that this effect can be explained by applying an already existing phenomenological thermodynamic and kinetic model. The model describes all possible phenomena in a colloidal dispersion, nucleation, growth of particles, Ostwald ripening, and dissolution of particles using a uniform concept.     

Influence of the Metal Work Function on the Photocatalytic Properties of TiO2 Layers on Metals

The photocatalytic properties of titanium dioxide (TiO₂) layers on different metal plates are investigated. The metal–semiconductor interface can be described as a Schottky contact, and is part of a depletion layer for the majority carriers in the semiconductor. Many researchers have demonstrated an increase in the photocatalytic activity, due to the formation of a metal–semiconductor contact that are obtained by deposition of small metal islands on the semiconductor. Nevertheless, the influence of a Schottky contact remains uncertain, sparking much interest in this field. The immobilization of nanoparticulate TiO₂ layers by dip‐coating on different metal substrates results in the formation of a Schottky contact. The recombination rate of photoinduced electron–hole pairs decreases at this interface provided that the thickness of the thin TiO₂ layer has a similar magnitude to the depletion layer. The degradation of dichloroacetic acid in aqueous solution and of acetaldehyde in a gas mixture is investigated to obtain information concerning the influence of the metal work function of the back contact on the efficiency of the photocatalytic process.     

半导体电极的平带电位

平带电位(E_(fb))是半导体/电解质溶液体系的重要概念,是半导体电极在平带状态时的电极电位,它是半导体电极特有的可以实验测定的物理量。利用Mott-Schottky曲线以及光电化学等方法可以测定平带电位,判断半导体的类型以及估算半导体的载流子浓度,其数值可用于推测半导体的能级结构,确定半导体材料的价带或导带能级位置。这对于与太阳能开发利用相关的半导体光催化和光电化学研究都是非常重要的。本文分析了半导体电极的能带弯曲及影响因素,首次提出半导体界面层内费米能级弯曲,阐明半导体电极平带电位的物理意义及其测定方法,以帮助初学者理解和应用平带电位。     

Adsorption behavior of carboxymethyl starch on titanium dioxide surfaces

The adsorption of carboxymethyl starch (CMS) on titanium dioxide surface from aqueous solution of electrolyte was investigated by adsorption and electrokinetics mobility measurements. Zeta potential measurements showed that the addition of CMS resulted in a shift of isoelectric point to the more acidic region, indicating the adsorption of CMS from the aqueous solution onto titanium dioxide surface. The positively charged and hydrophilic surface sites of titanium dioxide favor the adsorption of CMS molecules. The adsorption capacity of CMS on titanium dioxide surface was found to be controlled by the number of functional group on CMS that promotes surface charge CMS adsorption in agreement with Langmuir isotherm. For the adsorption of CMS, the pseudo-second-order kinetics of chemical reaction provides the best correlation of the experimental data.     

Advances in Photoelectrocatalysis with Nanotopographical Photoelectrodes

The design of photoelectrodes for high efficiency solar fuel energy conversion devices is based on the search for adequate surface conditioning to achieve efficient light harvesting, stability, minimized surface recombination losses and high electron‐transfer rates at the electrolyte interface. An overview on established and novel approaches is given. A recent viable solution is provided by electroplating of nanoscale catalytic metals on passivated semiconductor surfaces, thereby forming reactive centers and avoiding contact between the semiconductor surface and the electrolyte. At these nano‐dimensioned Schottky‐type junctions, light‐induced excess minority carriers are scavenged and transferred to the electrolyte. Various possible device configurations are outlined and envisaged systems for hydrogen or oxygen evolution and carbon dioxide reduction are presented. The role of ultrathin passivating films is emphasized and methods to fabricate open as well as compact conformal films are described.     

Fe~(3+)离子对二氧化钛光电化学催化性能的影响

应用电化学阳极氧化法制备Ti上多孔状纳米晶TiO2薄膜,以及不同Fe3+离子掺杂量的二氧化钛薄膜.研究了Fe3+离子掺杂对二氧化钛薄膜吸收光谱和光催化活性的影响,发现Fe3+的掺杂使薄膜的吸收带边发生红移,在可见光照射下其光催化活性也有一定的提高.     

Bubble coalescence during acoustic cavitation in aqueous electrolyte solutions

Bubble coalescence behavior in aqueous electrolyte (MgSO(4), NaCl, KCl, HCl, H(2)SO(4)) solutions exposed to an ultrasound field (213 kHz) has been examined. The extent of coalescence was found to be dependent on electrolyte type and concentration, and could be directly linked to the amount of solubilized gas (He, Ar, air) in solution for the conditions used. No evidence of specific ion effects in acoustic bubble coalescence was found. The results have been compared with several previous coalescence studies on bubbles in aqueous electrolyte and aliphatic alcohol solutions in the absence of an ultrasound field. It is concluded that the impedance of bubble coalescence by electrolytes observed in a number of studies is the result of dynamic processes involving several key steps. First, ions (or more likely, ion-pairs) are required to adsorb at the gas/solution interface, a process that takes longer than 0.5 ms and probably fractions of a second. At a sufficient interfacial loading (estimated to be less than 1-2% monolayer coverage) of the adsorbed species, the hydrodynamic boundary condition at the bubble/solution interface switches from tangentially mobile (with zero shear stress) to tangentially immobile, commensurate with that of a solid-liquid interface. This condition is the result of spatially nonuniform coverage of the surface by solute molecules and the ensuing generation of surface tension gradients. This change reduces the film drainage rate between interacting bubbles, thereby reducing the relative rate of bubble coalescence. We have identified this point of immobilization of tangential interfacial fluid flow with the "critical transition concentration" that has been widely observed for electrolytes and nonelectrolytes. We also present arguments to support the speculation that in aqueous electrolyte solutions the adsorbed surface species responsible for the immobilization of the interface is an ion-pair complex.     

4,4′-Diaminodiphenylsulfone; an efficient co-additive in dye-sensitized nanocrystalline TiO2 solar cells

The effect of 4,4′-diaminodiphenylsulfone (DADPS) as a co-additives in iodide/triiodide redox electrolyte on the performance of dye-sensitized solar cell based on the 2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid (TPA) and 2-cyano-3-(2΄-(5΄,10΄,15΄,20΄- tetraphenylporphyrinato Zinc (II)yl) acrylic acid (Zn-1) dyes have been investigated. Compared withstandard electrolyte (0.1 M LiI, 0.05 M I₂, 0.6 M 1-butyl-2,3-dimethylimidazolium iodide (BDMII) and TBP (0.5, 1 M)), adding 0.5 M of DADPS, as a co-additive, into the electrolyte system has caused increasing short current density (J_sc) and open-circuit potential (V_oc) consequently the energy conversion (η) improved. Electrochemical impedance results indicate adsorbing of DADPS on the titanium dioxide surface leads to an increase in the lifetime (τ). Adsorbing of DADPS on the semiconductor surface retards the interfacial charge recombination that has a beneficial effect on the V_oc and J_sc. The results showed that amine groups of DADPS attach to the conduction band (CB) of TiO₂ film and suppress the electron recombination process and as a result, it can be applied as a promising co-additive in DSSCs.     

Thin‐Layer Cyclic Voltammetric and Scanning Electrochemical Microscopic Study of Antioxidant Activity of Ascorbic Acid at Liquid/Liquid Interface

An electron transfer reaction between ascorbic acid (H₂A) in an aqueous solution and oxidizing agent in an organic solution immiscible with water has been studied by thin‐layer cyclic voltammetry (TLCV) for charge transfer at the interface between two immiscible electrolyte solutions (ITIES). As an antioxidant, H₂A provide electrons through the aqueous/organic interface to reduce Fc⁺ and the procedure has been proved to be a one electron process again. In this work, the first combination of TLCV and scanning electrochemical microscopy (SECM) was achieved and showed a reasonable agreement between the results from the two different approaches. Otherwise, lower concentration ratios Kr of aqueous to organic reactants was adopted, which is given as evidence to the proposed procedure of Barker.     

Direct measurement of the temperature coefficient of the electron quasi-fermi level in dye-sensitized nanocrystalline solar cells using a titanium sensor electrode

A novel type of dye-sensitized cell (DSC) with a passivated titanium sensor electrode located on top of the nanocrystalline titanium dioxide layer has been used to study the temperature dependence of the electron quasi-Fermi level relative to the I3-/I- redox-Fermi level under short circuit conditions. The results show that the Fermi level decreases with increasing temperature (-1.76 meV K(-1)) as predicted for diffusive electron transport at short circuit. A smaller temperature dependence (-0.25 meV K(-1)) of the position of the TiO2 conduction band relative to the I3-/I- redox-Fermi level was deduced from the shifts in the trap distribution. An expression for the temperature dependence of the open circuit voltage, U(photo), has been derived. The experimentally observed temperature dependence of U(photo) gave values of the activation energy (0.25 eV) and preexponential factor (10(8) s(-1)) for the transfer of electrons from the conduction band of the nanocrystalline TiO2 to triiodide ions.     

Features of the occurrence of electrochemical processes in contact of sodium chloride solutions with the surface of superhydrophobic coatings on titanium

A joint analysis of the results of electrochemical studies and the evolution of the parameters of a sodium chloride solution droplet in contact with the coating under test reveals the pattern of changes in the surface state which result from the electrochemical reactions and adsorption-desorption processes at the coating/electrolyte interface. Features of the corrosion process are studied on titanium samples with different protective layers on the surface: (1) a natural oxide, (2) a coating prepared via plasma electrolytic oxidation (PEO coating), (3) a PEO coating with a hydrophobic layer, and (4) a PEO coating with a superhydrophobic nanocomposite layer. The best protective properties in a chloride-containing electrolyte are exhibited by the superhydrophobic nanocomposite coating. The mechanism of corrosion protection of this coating is formulated.     

Electokinetic and adsorption properties of different titanium dioxides at the solid/solution interface

Six samples of titanium dioxide of different phase compositions and specific surface areas have been characterized by XRD, Raman-and FTIR spectroscopy, adsorption of nitrogen, electrophoresis. Adsorption of Zn(II) ions at the TiO₂/NaCl aqueous solution interface as well as the effect of adsorption on the structure of electrical double layer have been studied. The influence of ionic strength, pH and presence of ions on the adsorption of Zn(II) ions at the TiO₂/NaCl solution interface have also been investigated. The zeta potential, surface charge density, parameters of adsorption edge pH_50% and ΔpH_10–90% for different concentrations of basic electrolyte have been determined. Studied unpurified samples showed lower values of isoelectric point pH_iep compared with literature data due to the presence of anion impurities. The antibate dependence between pH_iep values and particle size has been established. Adsorption of Zn(II) ions using monophase samples is completed at a lower pH than for the biphase TiO₂. Appearance of the point CR3 is associated with the charge turnover from positive to negative at high values of pH and formation of Zn(OH)₂.      

Photocatalyzed reaction of indole in an aqueous suspension of titanium dioxide

The photocatalyzed oxidation of indole (1) in an aqueous suspension of titanium dioxide has been investigated and an attempt has been made to identify the products formed during the photooxidation process by gas chromatographic–mass spectrometric (GC–MS) analysis. Photolysis of an aqueous solution of indole (1) in the presence of titanium dioxide and oxygen led to the formation of 2,3-dihydroindole-2-one (6) and 1H-indole-2,3-dione (7). A probable pathway for the formation of these products has been proposed.     

Determination of lead and cadmium in titanium dioxide by differential pulse anodic stripping voltammetry

A procedure for the simultaneous determination of lead and cadmium in TiO(2) by differential pulse anodic stripping voltammetry (ASV) has been developed. The key feature of the method is the use of triethanolamine (TEA) to remove titanium interference: TiO(2) undergoes acidic digestion with HF/H(2)SO(4) at atmospheric pressure, TEA is added to the HCl solution of the residue and the solution is analysed using a standard ASV instrumentation, equipped with a hanging mercury drop electrode. The calibration curves for both lead and cadmium are linear up to 50 mugl(-1) of solution, and the detection limits are 1 mugl(-1), corresponding to 1 mugg(-1) of TiO(2). Method reliability was tested by comparing the results with those given by electrothermal atomic absorption spectroscopy. The method has been successfully applied for determination of both contaminants in powdered titanium dioxide (raw materials) and in titanium dioxide-containing cosmetics (sunscreen products).     

Charge-tranfer reactions of oxygen ions and titanium ions at titanium oxide films

The partial current densities for the transfer of titanium(IV) and oxygen ions, and of electrons at the interface between the electrolyte and the titanium(IV) oxide layer on titanium were measured as functions of total current density and pH value. It is shown theoretically, how conclusions regarding the mechanisms of the transfer reactions of both ions can be drawn from various relations between the ionic partial current densities and their dependence on solution composition, even if the electric potential difference at the oxide interface cannot be measured. Mechanisms for the transfer reactions of titanium(IV) and oxygen ions are derived from the experiments.     

Photocatalytic degradation and adsorption of 2-naphthol on suspended TiO2 surface in a dynamic reactor

The photocatalytic oxidation of 2-naphthol has been investigated at room temperature in a dynamic photoreactor with system UV/O2 (air) and aqueous suspension of titanium dioxide TiO2 irradiated under a variety of conditions. The kinetics of disappearance of pollutant were affected by several operating parameters such as TiO2 mass, concentration of the substrate and reaction pH. The experiments were measured by high performance liquid chromatography. A Langmuir-Hinshelwood model was found to be accurate for photocatalytic degradation and indicates that adsorption of the solute on the surface of semiconductor particles plays a role in photocatalytic reaction.     

Titanium dioxide coated surfaces for capillary electrophoresis and capillary electrochromatography

The potential of titanium dioxide coatings to control the electroosmotic flow and to affect the migration behavior of analytes in capillary electrophoresis and open-tubular capillary electrochromatography was evaluated. The inner wall of a fused-silica capillary was applied with a solution of a titanium peroxo complex, followed by heating at an elevated temperature. The resultant product was ascertained to be titanium dioxide in a crystalline form of anatase by the results of Fourier transform-infrared spectrometry (FT-IR), X-ray photoelectron spectroscopy, and Raman spectroscopy. The capillary thus made had anodic or cathodic electroosmotic flow, depending on the pH and composition of the background electrolyte used. The titanium dioxide surface of the capillary was readily modified by a silanizing reagent. The performance of the titanium dioxide surfaces with or without chemical modifications was examined with inorganic anions, neutral compounds and peptides.     

Photoinduced dissolution of titanium dioxide by oxalic acid

The photoinduced dissolution of titanium dioxide is observed upon the UV irradiation of aqueous solutions of degassed suspensions of this semiconductor in the presence of oxalic acid. A mechanism is proposed involving the photoinduced and dark dissolution of titanium dioxide with the formation of titanium(III) and titanium(IV) oxalates, respectively.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 6, pp. 545–547, November–December, 1994.     

A novel method for measuring zeta potentials of solid–liquid interfaces

A novel method for measuring the zeta potential of PDMS–electrolyte solution interface is presented in this paper. When an electrolyte solution passes the electrode coated with PDMS film, an electrical current will be induced due to the electrostatic potential difference between the PDMS–air interface and the PDMS–electrolyte solution interface. Based on this principle, the relationship between the measured electrical signal and the zeta potential of PDMS–electrolyte solution interface is experimentally investigated. The results show that the magnitude of the measured signal is linearly proportional to this potential difference. An empirical correlation between the zeta potential and the measured voltage signal was obtained. Good agreement was found when comparing the zeta potential calculated by this empirical equation with that reported in the published journal papers. The zeta potential measurement method presented in this paper is simple and accurate and can be used for measuring zeta potentials of other dielectric–electrolyte interfaces.     

锑化镓的光助微刻蚀及其表面氧化物的研究

用电化学和光电化学方法研究锑化镓表面的腐蚀以及锑化镓表面氧化膜的生成和溶解，锑化镓电极在一定电势下生成的氧化膜，用俄歇能谱证明，其主要成分为难溶的氧化锑，此氧化膜的存在抑制了锑化镓的进一步腐蚀，同时亦使锑化镓的半导体光电化学性能大为减弱，通过激光微刻蚀及电子显微镜的观察，在刻蚀剂中添加酒石酸，柠檬酸和氢氟酸等试剂，可使刻蚀形得改善，实验研究了锑化镓的平带电势的测定。