Aqueous TiO2 Suspensions Research Articles

Experimental Photodegradation of OPs and CBs by Advanced Oxidation Techniques in the Aqueous TiO2 Suspensions

The reaction of pesticides with the OH radical, is the most dominant annihilation reaction ofthe atmosphere. Phosmet, Azamethiphos, Coumaphos and Temephos were used as organophosphorouspesticides, and Methiocarb, Carbofuran and Pirimicarb compounds were used as carbamate pesticideswith the aim of investigating the kinetics of the reactions of Organophosphorous (OPs) and Carbamate(CBs) Pesticides with the OH radicals. In aqueous TiO2 suspensions, the degradation reactions viaphotocatalytic-effect of the seven pesticides selected as pollutants were analysed, and the impact of theinitial concentration was calculated. All experiments were conducted in the same photoreactor and underthe same conditions as three repetitions. The optimum amount of photocatalyst for the studied pesticidewas determined and TiO2 concentration for all experiments were taken as 0.2 g/100 mL. Considering thatorganic pollutants are present in water in trace amount, the initial concentrations for all organiccompounds were added so as to be1.0x10-4 molxL-1, into the suspensions and the solution containing theorganic pollutant. In this study, degradation speed of pesticides were determined experimentally, and thefastest degrading pesticide was tried to be found out.

Prediction of the optical properties of TiO2 suspension by coupling the classical particle agglomeration model with the discrete dipole approximation algorithm and experimental verification

Nanofluids are widely used in the field of energy and environment. However, the optical properties of such multiphase suspension are quite complex. Even for the well-studied TiO2 suspension, theoretical investigation of its optical parameters is quite rare, which remarkably restricts the extension of their application especially in the field of solar energy and optofluidics. Therefore, in this paper, the optical properties of TiO2 suspension were studied by coupling the classical particle agglomeration model and the discrete dipole approximation algorithm. The absorption and scattering coefficients of TiO2 aqueous suspension have been calculated and verified by experimental results. The effects of cluster compactness, size and other factors on the phase function were analyzed. Our work can also provide a reference for the study of optical properties of other nanofluids.

Elucidating the particle concentration-dependent flow behavior of aqueous TiO2 suspensions based on polymer dispersant structures and interparticle surface distances

Flowing behavior of concentrated aqueous TiO2 suspensions was evaluated in relation to the structure of polymeric dispersants, their adlayer thickness on particle surface, and particle concentrations. Comb-type polymeric dispersants comprising polyethylene glycol-based side chains with different combinations of unit number and side chain length were investigated. Based on particle concentration–apparent viscosity curves of the suspensions with TiO2 nanoparticles saturated with dispersants, maximum packing fraction and polymer adlayer thickness were estimated through the Krieger–Dougherty model. When the steric repulsive force of polymer dispersant dominated van der Waals attractive forces, the flow curves of the suspension exhibited hysteresis characteristics as the average particle surface distance approached the adlayer thickness of polymer dispersants. Additionally, by introducing the concept of relative surface distance, which is the average particle surface distance divided by the adlayer thickness, the particle concentration dependency of the suspension viscosities with different dispersants followed a unified trend.

Direct photoconversion of nitrite to dinitrogen on Pd/TiO2 coupled with photooxidation of aquatic pollutants

Nitrate (NO3−) and nitrite (NO2−) are ubiquitous aquatic pollutants and should be controlled for water quality. Although diverse methods of nitrate/nitrite reduction have been developed, most require chemical reductants and external energy inputs. Nitrite is oxidized by photogenerated hydroxyl radicals (•OH) in an aqueous suspension of Pd-loaded TiO2 (Pd/TiO2) with producing nitrate as a major product. The main photoconversion product of nitrite is changed from nitrate to N2 on Pd/TiO2 in the presence of aquatic pollutants (arsenite (As(III)) or 4-chlorophenol (4-CP)) which can suppress the photooxidation of NO2− by scavenging holes/•OH. The photogenerated H2 on Pd/TiO2 serves as a main reductant and was immediately consumed for reducing nitrite to N2. The presence of As(III) or 4-CP also enhanced the durability of Pd/TiO2 by efficiently scavenging hole/•OH that may oxidize Pd nanoparticles. The proposed process can be a viable treatment option for nitrite-contaminated water containing various aquatic pollutants.

Simple preparation of broadband UV filters based on TiO2 coated with aqueous extracts of native trees from the Chaco region of Argentina.

The nativeforest of northwestern Argentina,as part of the Chaco region, is a rich andunexploited source of phytochemical compounds for medicinal/cosmetic applications. In the present study, fruit, leaf, branch, and bark organs of the native trees Sarcomphalus mistol (Mistol, M) and Schinopsis lorentzii (Quebracho Colorado santiagueño, QC) were harvested, and aqueous plant extracts (PE) were prepared. The spectroscopic (UV-Vis absorbance, diffuse reflectance, ATR-FTIR) and antioxidant (TEAC, Folin-Ciocalteu) properties of PE were characterized and used as TiO2 coating material to obtain a series of TiO2@PE nanocomposites. These materials showed almost null photocatalytic activity compared to aqueous suspensions of bare TiO2, displaying yellowish to brownish coloration and high long-term stability in both freshwater and seawater model solutions. The loss of photocatalytic activity in TiO2@PE was associated with the combination of the internal filter effect and the antioxidant/radical capacity exerted by the phytochemicals of the PE coating,with higher broadband photoprotection for the nanocomposites prepared with QC extracts. Thus, this study shows the potential capacity of the forest resources of the Chaco region of Argentina for the development of new cosmetic and/or sun protection formulations.

Glycerol as an excellent hydrogen and electron source for photocatalytic hydrogenation of nitrobenzene in water

The development of a new method for effective utilization of glycerol dissolved in water is needed. In this study, glycerol was used not as an energy source and a starting compound but as an electron and hydrogen source for photocatalytic hydrogenation of nitrobenzene in an aqueous suspension of TiO2 under various conditions. The function of glycerol as an electron and hydrogen source was compared with that of other C3-alcohols and glycols. The results showed that glycerol works most efficiently as an electron and hydrogen source and that electrons extracted from glycerol are directly used for the hydrogenation without formation of other products such as H2. The photocatalytic hydrogenation was also examined under solar light with and without stirring. Important factors for the electron and hydrogen source in the photocatalytic reaction in an aqueous medium are discussed on the basis of the results.

Synthesis, characterization, and photocatalytic performance of methyl orange in aqueous TiO2 suspension under UV and solar light irradiation

Nano-crystalline titanium dioxide (TiO2) photocatalyst has been extensively utilized in the last few decades because of its strong oxidizing power, redox selectivity, and higher photocatalytic performance. In this work, TiO2 nanoparticle (NPs) was synthesized, characterized, and applied for the degradation of methyl orange (MO) dye in an aqueous solution under UV and solar light irradiations. The XRD results showed dominated anatase phase (82%) with a smaller crystalline size (18.23 nm). A rough surface with a uniform spherical shape was observed by SEM analysis. FTIR and UV–Vis/DRS spectroscopy confirmed the stretching vibration of the hydroxyl groups and higher absorption edge towards longer wavelength with smaller bandgap energy (3.12 eV) of TiO2 lattice structure. The activity of TiO2 NPs was carried out from the degradation of MO dye as a model contaminant for textile effluent in presence of UV/solar irradiation. The photodegradation efficiency of MO dye (15 mg L−1) was observed at 85.90% and 96.38% for 0.150 g and 0.030 g of TiO2 NPs at natural pH (7.0), after 5 h of UV and solar light irradiation. The respective rate constants were determined to be 0.0063 min−1 and 0.01 min−1 for UV and simulated solar light irradiation respectively, and exhibited the best-fitted kinetic model of pseudo-first-order. The photodegradation efficiency was increased with the decrease of pH, while efficiency decreased with an increase in initial dye concentration. The obtained result from TiO2 NPs was compared with commercially available Degussa P25 catalyst. The photocatalyst reusability test and reaction mechanism were also evaluated. The study suggests that TiO2 NPs are a promising photocatalyst that can be used for the degradation of organic dye compounds from textile wastewater towards the sustainable management of environment.

Colloidal Stability and Aggregation Mechanism in Aqueous Suspensions of TiO2 Nanoparticles Prepared by Sol-Gel Synthesis.

Understanding the colloidal stability and aggregation behavior of TiO2 nanoparticles in aqueous suspension is a prerequisite to tune supracolloidal structure formation. While the aggregation mechanism for dried TiO2 nanopowders is well documented, there is still work to be done to understand TiO2 nanoparticle aggregation in suspension. Therefore, this work focuses on the colloidal stability and aggregation mechanism of TiO2 nanoparticle aqueous suspensions prepared using a straightforward one-step sol-gel-based approach over a concentration range of 0.5-5 wt %. Fully crystalline nanoparticles consisting primarily of anatase were obtained. After assessing the colloidal stability of the as-prepared suspensions, small-angle X-ray scattering coupled with fractal analysis was carried out. This analysis showed, for the first time, how the TiO2 nanoparticle aggregation mechanism─predicted by the diffusion limited cluster-cluster aggregation (DLCA) and diffusion limited particle-cluster aggregation (DLA) theories─depends directly on the starting concentration in the aqueous suspensions. We found that concentrated suspensions favored DLA, while dilute suspensions tend to follow the DLCA mechanism. The effect of the aggregation mechanism on the aggregate shape is also discussed.

Morphological Characterization of Selected Natural and Calcined TiO2 Photocatalysts Using SEM, XRD and BET

The present work characterized commercial TiO2 powder photo-catalysts (P25, AV01, S7001, PC100 & PC500) using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), and Brauner, Emmer and Teller (BET) techniques to determine crystalline size(nm), particle size(nm) and specific surface area(m2/g) of natural and calcined TiO2 catalyst particles. TiO2 particulate layers were prepared by deposition from aqueous TiO2suspensions c=10g.L-1 on degreased microscopic glass of dimensions 75mm × 25mm calcined at 300°C for 2hrs. The calcined layers were scanned to obtain cross-sectional images and reveal particle size and catalyst morphology. The results indicate a crystalline/surface area ratio as follows; [(P25=28nm/50m2/g); (AV01=169nm/10m2/g); (S7001=9nm/250m2/g) ;(PC100=198nm/79.5m2/g) ;(PC500=173.6nm/91.3m2/g)]. Particle size of AV01 powder catalyst (100-150 nm) was larger compared to the other catalysts., catalyst P25, PC100, PC500 and S7001 appear to have almost similar particle sizes of between (50-100 nm). Comparing the observed particle sizes using the SEM technique with those calculated using XRD, it can be concluded that these values are very similar. X-Ray Diffraction patterns indicated that the crystalline structure AV01, S7001, PC100 and PC500 comprises of pure anatase phase. Catalyst P25 catalyst powder consisted of a crystal structure 74% anatase and 26% rutile content. Crystalline sizes were calculated by Scherer formula using a special programme Analytical X`pert.PRO + high score plus. It can be concluded that smaller crystalline size favours increased surface areas. An optimum balance between surface area and crystallinity is desirable to obtain maximum semi-conductor properties since the predominant electron/hole recombination depends on relative particle size/surface area ratios.

Modeling the photocatalytic oxidation of carboxylic acids on aqueous TiO2 suspensions and on immobilized TiO2-chitosan thin films in different reactor geometries irradiated by UVA or UVC light sources

Photocatalytic reactor models incorporating radiation transport parameters and intrinsic kinetic parameters of photocatalytic decomposition of single-compound and mixtures of oxalic (OA) and formic acid (FA), in both slurry suspensions and immobilized photocatalytic thin films, were validated in two well-mixed annular photoreactors geometries irradiated with either UVA or UVC radiation. The six-flux absorption scattering model (SFM) predicted the radiation field in a titanium dioxide (TiO2 P25) catalyst suspension, the local volumetric rate of photon absorption (LVRPA) at each point of the reactor and, after volumetric integration, the lumped VRPA. The model combining the lumped VRPA, the kinetics models describing the adsorption and photocatalytic decomposition of OA and FA and the material balance of the reacting species was fitted to the experimental results of photocatalytic decomposition of OA and FO in a slurry TiO2 suspension, to determine the OA and FA intrinsic reaction kinetics parameters. Such intrinsic parameters were then used with the average surface rate of photon absorption (SRPA) to model and predict the photocatalytic oxidation of OA and FA on non-porous TiO2–chitosan films immobilized on glass plates immersed in the photoreactor. The models for both slurry suspensions and immobilized photocatalysts predicted the degradation and mineralization of OA and FA and of a mixture of them under diverse experimental conditions. This study demonstrates a systematic methodology for determining intrinsic reaction kinetics rate parameters of water contaminants that can also be used to represent the photocatalytic oxidation of such contaminants in irradiated slurries and in immobilized photocatalytic thin films and in any photoreactor geometry.

Using an acoustic levitator to investigate the drying kinetics and solids forming process of individual droplets during spray drying

AbstractSpray drying is a widely used process to turn slurries into dry powders and is especially important for thermally-sensitive materials, that are often found in the food or pharmaceutical industry. However, detailed insight into the drying kinetics during spray drying is difficult to investigate due to the boundary conditions in a spray drying tower. As a result, there is a lack of important information on the drying process and subsequent solidification of individual droplets. In this context, an experimental setup for a droplet positioned in a stationary ultrasonic field of an acoustic levitator is designed to enable a non-contacting measurement of the drying kinetics and the subsequent solidification process. To generate a comparable situation like in a real spray drying process, the droplet is positioned in an airflow, where air temperature, humidity, and velocity can be adjusted over wide range. Using an infrared camera to measure the surface temperature and a Complementary Metal Oxide Semiconductor (CMOS) camera for object recognition, the droplet can be observed continuously and drying kinetics of the droplet can be determined from the measured surface temperature and decreasing droplet size. Result of a 10 wt.% aqueous micro particle TiO2 suspension are reported and show that the investigated method is a very valuable and fast tool to safely scale-up spray drying systems very close to real process conditions. Especially when only small sample amounts are available in an early development stage.

Photocatalytic reduction of chlorate in aqueous TiO2 suspension with hole scavenger under simulated solar light

This work investigates photocatalytic reduction of aqueous chlorate by using commercial P25 TiO2 in the presence of hole scavengers under simulated solar light. It compares the photocatalytic ability of P25 TiO2 to reduce chlorate in the presence or absence of a hole scavenger (methanol) with that of bismuth oxyhalides (BiOBr, TiO2-BiOBr, BiOCl, TiO2-BiOCl, and BiOI). Bismuth oxyhalides have more interest as promising photocatalysts because they have a narrow band gap and are more responsive to visible light. However, in this study, P25 TiO2 was a more effective photocatalyst for chlorate reduction in the presence of methanol than bismuth oxyhalides. Also, this study examines the effectiveness of methanol, ethanol, and formate as hole scavengers during chlorate reduction. The presence of methanol enhanced chlorate reduction more than that of formate and ethanol. Finally, this work estimates the impacts of operating parameters such as photocatalyst dose, initial chlorate concentration, solution pH, methanol concentration, and light intensity on chlorate removal and studies the reaction mechanism for chlorate reduction in the system of TiO2/chlorate/methanol (photocatalyst/contaminant/reagent) under simulated solar light. The highest chlorate removal of 97.5% was achieved using 1 g/L of TiO2 with initial chlorate concentration of 0.012 mM and methanol concentration of 2 mM at the pH of 5.4 after 180 min of radiation. The major product of chlorate photoreduction was chloride ion.

Optically Transparent Colloidal Dispersion of Titania Nanoparticles Storable for Longer than One Year Prepared by Sol/Gel Progressive Hydrolysis/Condensation.

The molecular catalyst sensitized system (MCSS), where an excited molecular catalyst adsorbed on a semiconductor such as TiO2 injects electrons to the conduction band of the semiconductor leading to hydrogen evolution/CO2 reduction coupled with an oxidation of water on the molecular catalyst, has been one of the most probable candidates in the approach to artificial photosynthesis. For a full utilization of visible light, however, a serious light scattering of the aqueous suspension of TiO2 in the visible region, which is generally experienced, should be avoided. Here, we report a preparation of optically transparent colloidal dispersion of TiO2 by the sol/gel reaction of TiCl4 through progressive hydrolysis/condensation under the basic condition without any calcination processes. The TiO2 nanoparticles (TiO2(NPs)) obtained were characterized as an amorphous particle (∼10-15 nm) having a microcrystal domain of anatase within several nm by XRD, Raman spectroscopies, XRF, XAFS, TG/DTA, and HRTEM, respectively. The energy-resolved distribution of carrier electron traps in TiO2(NPs) as a fingerprint of TiO2 was characterized through reversed double-beam photo-acoustic spectroscopy to have a close similarity to that of TiO2(ST-01) as well as the observation of carrier traps by transient absorption spectroscopy. Though the powder TiO2(NP) itself was not dispersed well in aqueous solution, the wet TiO2(NPs) as prepared before being dried up provided a completely transparent aqueous dispersion under the acidic condition (1 M HCl). Addition of methanol enabled the colloidal dispersion (TiO2(NPs, MeOH/H2O, 0.1 M HCl)) to keep the optical transparency for longer than 1 year (550 days), which is the first example of TiO2 dispersion storable for such a long period. TiO2(NPs, MeOH/H2O) exhibited a moderate photocatalytic reactivity of H2 evolution with a quantum yield of ∼2.6% upon 365 nm light irradiation. An optically transparent thin film of TiO2(NPs, MeOH/H2O) was also successfully prepared on a glass plate to exhibit an enhanced hydrophilicity upon UV light irradiation.

Photocatalytic Partial Oxidation of Tyrosol: Improving the Selectivity Towards Hydroxytyrosol by Surface Fluorination of TiO2

The photocatalytic partial oxidation of tyrosol to hydroxytyrosol, the most powerful and expensive natural antioxidant currently known, has been studied in irradiated aqueous suspensions of TiO2 as the photocatalyst. Three TiO2 samples, two commercial (Merck and P25® Evonik) and a home prepared one, have been used bare or surface modified. The influence of surface hydroxylation, surface fluorination, and silver nanoparticles decoration on the selectivity towards hydroxytyrosol has been investigated. The higher the hydroxyl group density, the lower the selectivity. On the other hand, fluorination improved the selectivity towards hydroxytyrosol for all of the samples. In particular, selectivity values up to 10%, i.e. ca. 10 times higher than those obtained with bare sample, were obtained in the presence of fluorinated P25 Evonik TiO2. It is proposed that the higher production of hydroxyl radicals and the lower hydrophilicity of the surface of the fluorinated sample favour hydroxylation of tyrosol and desorption of hydroxytyrosol, respectively, thus hindering the overoxidation of tyrosol. The presence of silver on fluorinated TiO2 was detrimental and it did not significantly improve the selectivity even on bare TiO2. Even if the selectivity hereby reported is relatively low, results show that the surface modification of the photocatalyst and the fluidodynamic optimization of the reacting system could make viable this interesting photocatalytic route of synthesis, which can be used also for the enrichment of some commercial polyphenolic solutions in the very valuable hydroxytyrosol.

Photocatalytic Degradation of Aqueous Rhodamine 6G Using Supported TiO2 Catalysts. A Model for the Removal of Organic Contaminants From Aqueous Samples.

As a model for the removal of complex organic contaminants from industrial water effluents, the heterogeneous photocatalytic degradation of Rhodamin 6G was studied using TiO2-derived catalysts, incorporated in water as suspension as well as supported in raschig rings. UV and Visible light were tested for the photo-degradation process. TiO2 catalysts were synthesized following acid synthesis methodology and compared against commercial TiO2 catalyst samples (Degussa P25 and Anatase). The bandgap (Eg) of the TiO2 catalysts was determined, were values of 2.97 and 2.98 eV were obtained for the material obtained using acid and basic conditions, respectively, and 3.02 eV for Degussa P25 and 3.18 eV for anatase commercial TiO2 samples. Raschig rings-supported TiO2 catalysts display a good photocatalytic performance when compared to equivalent amounts of TiO2 in aqueous suspension, even though a large surface area of TiO2 material is lost upon support. This is particularly evident by taking into account that the characteristics (XRD, RD, Eg) and observed photodegradative performance of the synthesized catalysts are in good agreement with the commercial TiO2 samples, and that the RH6G photodegradation differences observed with the light sources considered are minimal in the presence of TiO2 catalysts. The presence of additives induce changes in the kinetics and efficiency of the TiO2-catalyzed photodegradation of Rh6G, particularly when white light is used in the process, pointing toward a complex phenomenon, however the stability of the supported photocatalytic systems is acceptable in the presence of the studied additives. In line with this, the magnitude of the chemical oxygen demand, indicates that, besides the different complex photophysical processes taking place, the endproducts of the considered photocatalytic systems appears to be similar.

Contribution of adsorption and photo catalysis for the elimination of Black Eriochrome (NET) in an aqueous medium-Optimization of the parameters and kinetics modeling

The adsorption and photocatalytic degradation of Black Eriochrome (NET), a dye selected as a model organic pollutant, has been investigated in aqueous suspension of TiO2 as photo catalyst under UV irradiation. The elimination of NET, an organic compound widely used in the textile industry, was realized by photo degradation. The advanced oxidation process (AOP) concerns the active species generated in the landfill, mainly the HO• radicals. These species induce highly oxidizing properties for the destruction of the organic waste. The adsorption and photo catalytic degradation of NET has been investigated in aqueous TiO2suspension under UV irradiation. To determine the equilibrium time, fine TiO2 powder was dispersed in NET solution under continuous stirring; the NET concentration gradually decreases and stabilizes after 20 min of contact. The modeling of this kinetic followsthe pseudo-first order kinetic model with a high determination coefficient (R2 = 0.999) and a capacity qe,cal in good agreement with the experimental data. The photo degradation experiments were undertaken to evaluate the effect of the analytical parameters influencing the NET removal efficiency. It has been observed that under optimal conditions (pH 12, adsorbent dosage 1 g/L, stirring speed 200 rpm and contact time 120 min), 92.9 % removal efficiency of NET was obtained for a concentration of 46 mg/L.The photocatalytic degradation rate was favored for high NET concentrations of solution in agreement with the Langmuir-Hinshelwood (L-H) model.

A study of photocatalytic degradation of methomyl and its commercial product Lannate-90

Photocatalytic degradation of the carbamate insecticide methomyl and its commercial product Lannate-90 in aqueous suspension of TiO2 and ZnO using an Osram ultra-vitalux' lamp (315-400 nm) was investigated. The objective of the investigation was to study and compare the rates of photochemical degradation of the pure active compound and its formulated product in order to find out the impact of inert ingredients on degradation processes. The photochemical degradation of pure methomyl was found to be slower than the degradation of its commercial product Lannate-90. Photodegradation was investigated by UV-Vis spectrometry and HPLC, while mineralization was monitored using IC and TOC analyses. Light absorption capabilities of catalysts were characterized by ultraviolet-visible diffuse reflectance spectroscopy method, while the stability of catalysts was studied by infrared analyses. Degradation products of methomyl and its formulated product were identified by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) results and tentative photodegradation pathways were proposed.

Photocatalytic degradation of swine wastewater on aqueous TiO2 suspensions: optimization and modeling via Box-Behnken design

Heterogeneous photocatalysis is a promising technology to treat many industrial wastewaters. To date, this potential has not been proven with wastewaters from agricultural origins, such as swine wastewater. In this work, the photocatalytic degradation of swine wastewater was studied by applying a response surface methodology based on the Box-Behnken design. The interactive effects of the variation of factors such as photocatalyst dosage (X1), wastewater concentration (X2), and irradiation time (X3) were analyzed to identify the optimal operating conditions for COD reduction. A second-order polynomial accurately represented organics degradation with a high adjusted R-squared (0.9666). The main effects of factor X2 and the quadratic effects of factors X2 and X3 were the most significant for COD reduction. The optimal conditions for COD degradation were 1.16 g L−1 for photocatalyst dosage, 1.68% for wastewater concentration, and irradiation time of 9.2 h. These results have been validated in a confirmation experiment and COD removal reached 91.7% (98.1 % predicted). Based on the Langmuir—Hinshelwood model, the reaction rate constant was 3.9×10−3 min−1. Besides, FTIR analysis indicated that Aeroxide® TiO2 reusability may be possible, especially for low wastewater concentrations. Heterogeneous photocatalysis can be applied as a technology for the integrated treatment of industrial wastewaters resulting from swine production.

Tuning friction and slip at solid-nanoparticle suspension interfaces by electric fields

We report an experimental Quartz Crystal Microbalance (QCM) study of tuning interfacial friction and slip lengths for aqueous suspensions of TiO2 and Al2O3 nanoparticles on planar platinum surfaces by external electric fields. Data were analyzed within theoretical frameworks that incorporate slippage at the QCM surface electrode or alternatively at the surface of adsorbed particles, yielding values for the slip lengths between 0 and 30 nm. Measurements were performed for negatively charged TiO2 and positively charged Al2O3 nanoparticles in both the absence and presence of external electric fields. Without the field the slip lengths inferred for the TiO2 suspensions were higher than those for the Al2O3 suspensions, a result that was consistent with contact angle measurements also performed on the samples. Attraction and retraction of particles perpendicular to the surface by means of an externally applied field resulted in increased and decreased interfacial friction levels and slip lengths. The variation was observed to be non-monotonic, with a profile attributed to the physical properties of interstitial water layers present between the nanoparticles and the platinum substrate.

Trace Amount CoFe2O4 Anchored on a TiO2 Photocatalyst Efficiently Catalyzing O2 Reduction and Phenol Oxidation.

Semiconducting TiO2 is the most studied photocatalyst for organic oxidation by O2. To accelerate the reaction, a cocatalyst for O2 reduction or for organic oxidation is often used, but the bifunctional one is rare. Herein we report a spinel CoFe2O4 (CF) efficiently catalyzing O2 reduction and phenol oxidation on TiO2 in aqueous suspensions at pH 3-11. The composite materials (CF/TiO2) were made by depositing 0-5 wt % CF onto TiO2 through a hydrothermal method. Solid characterization showed that CF nanoparticles (5 nm) homogeneously distributed in CF/TiO2, whereas the TiO2 phase remained unchanged in crystal structure and crystallite size. For phenol oxidation under UV light, CF was nearly not active, but 0.01 wt % CF/TiO2 was more active than TiO2, by approximately a factor of 3.6. Such trend in activity among the catalysts was also observed from the photocatalytic reduction of O2 to H2O2, from the electrochemical reduction of O2, and from the photoelectrochemical oxidation of phenol and H2O. An open-circuit potential and photoluminescence measurement suggest that there is an interfacial electron transfer from TiO2 to CF, followed by O2 reduction. Accordingly, a possible mechanism is proposed, involving CF catalysis for O2 reduction and phenol oxidation. Then the mutual promotion between electron and hole transfer results in great enhancement in the efficiency of charge separation and hence in the rate of chemical reaction. Because spinel compounds have rich composition and unique structures, they are worthy of being further investigated as cocatalysts of a semiconductor photocatalysis.