Morphology control of cathodically deposited TiO2 films

This work demonstrates that the microstructure of TiO₂ film can be designed and controlled by adjusting the temperature and cycle number of cathodic deposition in a solution containing TiCl₃ and NaNO₃. The redox interactions between TiCl₃ and NO₃⁻ are investigated by in situ ultraviolet-visible (UV-vis) absorption spectroscopy. Linear sweep voltammetry (LSV) is employed to study the NO₃⁻ reduction and to clarify the deposition behavior of TiO₂ in the designed plating solution. The decrease in TiO₂ deposition rate with the TiO₂ thickness may be due to the poor electron conductivity of TiO₂ depressing the generation rate of OH⁻ from the NO₃⁻ reduction. The morphology and size of TiO₂ aggregates are strongly influenced by varying the deposition temperature from 5 to 50 °C and a maximal rate of TiO₂ deposition is obtained at 25-35 °C. TiO₂ deposited at 25 °C is the roughest with a roughness factor (Ra) of ca. 67 nm. This study provides a useful method to control the morphology and deposition rate of TiO₂ film for practical photoelectrochemical applications.     

Enhanced inactivation of E. coli bacteria using immobilized porous TiO2 photoelectrocatalysis

Immobilized TiO₂ nanotube electrodes with high surface areas were grown via electrochemical anodization in aqueous solution containing fluoride ions for photocatalysis applications. The photoelectrochemical properties of the grown immobilized TiO₂ film were studied by potentiodynamic measurements (linear sweep voltammetry), in addition to the calculation of the photocurrent response. The nanotube electrode properties were compared to mesoporous TiO₂ electrodes grown by anodization in sulfuric acid at high potentials (above the microsparking potential) and to 1 g/l P-25 TiO₂ powder. Photocatalyst films were evaluated by high resolution SEM and XRD for surface and crystallographic characterization. Finally, photoelectrocatalytic application of TiO₂ was studied via inactivation of E. coli. The use of the high surface area TiO₂ nanotubes resulted in a high photocurrent and an extremely rapid E. coli inactivation rate of ∼10⁶ CFU/ml bacteria within 10 min. The immobilized nanotube system is proven to be the most potent electrode for water purification.     

Chitosan/TiO2 nanocomposite pervaporation membranes for ethanol dehydration

Novel chitosan/titanium dioxide (CS/TiO₂) nanocomposite membranes were prepared using tetrabutyl titanate (TBT) as precursor and acetyl acetone as chelating agent by in situ sol-gel process, and characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetry (TG). The characterization results demonstrated that nano-sized TiO₂ particles dispersed homogeneously within the CS matrix, which could be assigned to the hydrogen and titanoxane bonds formed between CS and TiO₂. Moreover, the pervaporation performance of these membranes was investigated using the separation of ethanol-water mixture as model system. Compared with CS/TiO₂ hybrid membranes prepared by blending method, most of CS/TiO₂ nanocomposite membranes prepared by in situ sol-gel process exhibited higher permeation flux and separation factor under the identical conditions. Among all the prepared membranes, CS/TiO₂ nanocomposite membrane containing 6 wt% TiO₂ exhibited the best pervaporation performance, whose averaged permeation flux and separation factor were 0.340 kg m⁻² h⁻¹ and 196 for 90 wt% aqueous solution of ethanol at 80 °C, respectively.     

Improved performance of dense TiO2/CdSe coupled thin films by low temperature process

Dense TiO₂ and TiO₂/CdSe coupled nanocrystalline thin films were synthesized onto ITO coated glass substrate by chemical route at relatively low temperature (≤100 °C). TiO₂ films were nanocrystalline and crystallinity disappears after CdSe deposition as evidenced by X-ray powder diffraction. Surface morphology and physical appearance of films were studied from SEM and actual photo-images, reveals dense nature of TiO₂ (10-12 nm spherical grains, faint violet) and CdSe (80-90 nm spherical grains, deep brown), respectively. Presence of two absorption edges in UV spectra implies existence of separate phases rather than composite formation. TiO₂ film was found to have higher water contact angle (71°) than TiO₂/CdSe (61°) and CdSe (56°). I-V and stability tests of photo-electrochemical cells were performed with TiO₂ and TiO₂/CdSe film electrodes (under light of illumination intensity 80 mW/cm²) in lithium iodide as an electrolyte using two-electrode system.     

LiNi0.8Co0.15Al0.05O2 cathode materials prepared by TiO2 nanoparticle coatings on Ni0.8Co0.15Al0.05(OH)2 precursors

Synthesis, electrochemical, and structural properties of LiNi_0.8Co_0.15Al_0.05O₂ cathodes prepared by TiO₂ nanoparticles coating on a Ni_0.8Co_0.15Al_0.05(OH)₂ precursor have been investigated by the variation of coating concentration and annealing temperature. TiO₂-coated cathodes showed that Ti elements were distributed throughout the particles. Among the coated cathodes, the 0.6 wt% TiO₂-coated cathode prepared by annealing at 750 °C for 20 h exhibited the highest reversible capacity of 176 mAh g⁻¹ and capacity retention of 92% after 40 cycles at a rate of 1C (=190 mA g⁻¹). On the other hand, an uncoated cathode showed a reversible first discharge capacity of 186 mAh g⁻¹ and the same capacity retention value to the TiO₂-coated sample at a 1C rate. However, under a 1C rate cycling at 60 °C for 30 cycles, the uncoated sample showed a reversible capacity of 40 mAh g⁻¹, while a TiO₂-coated one showed 71 mAh g⁻¹. This significant improvement of the coated sample was due to the formation of a possible solid solution between TiO₂ and LiNi_0.8Co_0.15Al_0.05O₂. This effect was more evident upon annealing the charged sample while increasing the annealing temperature, and at 400 °C, the coated one showed a more suppressed formation of the NiO phase from the spinel LiNi₂O₄ phase than the uncoated sample.     

Electrochemically multi-anodized TiO2 nanotube arrays for enhancing hydrogen generation by photoelectrocatalytic water splitting

An enhanced hydrogen production by photoelectrocatalytic water splitting was obtained using extremely highly ordered nanotubular TiO₂ arrays in this work. Highly ordered TiO₂ nanotube arrays with a regular top porous morphology were grown by a facile and green three-step electrochemical anodization. The well ordered hexagonal concaves were uniformly distributed on titanium substrate by the first anodization, served as a template for further growth of TiO₂ nanotubes. As a result, the TiO₂ nanotube arrays constructed through the third anodization showed appreciably more regular architecture than that of the sample by conventional single anodization under the same conditions. The enhanced photoelectrochemical activity was demonstrated through the hydrogen generation by photoelectrocatalytic water splitting, with an exact H₂ evolution rate up to 420 μmol h⁻¹ cm⁻² (10 mL h⁻¹ cm⁻²) in 2 M Na₂CO₃ + 0.5 M ethylene glycol. The photocurrent density of the third-step anodic TiO₂ nanotubes is about 24 mA cm⁻² in 0.5 M KOH, which is 2.2 times higher than that of the normal TiO₂ nanotubes (∼11 mA cm⁻²) by a single electrochemical anodization.     

Monolithic-like TiO2 nanotube supported Ru catalyst for activation of CH4 and CO2 to syngas

Highly-ordered TiO₂ nanotube arrays (TiNTA) were prepared by an electrochemical anodization method and used as the carrier material to load 1 wt.% Ru. The Ru/TiNTA catalyst was then applied to the combination reactions of the partial oxidation of methane reaction (POM) with the carbon dioxide reforming with methane reaction (CRM) for syngas production. In comparison with the commercial TiO₂ powder (P25) supported 1 wt.% Ru catalyst, Ru/TiNTA shows higher activity and much better stability. The superior performance of Ru/TiNTA is attributed to the specific monolithic-like structure and confinement effect of TiNTA.     

Lithium-ion battery anode properties of TiO2 nanotubes prepared by the hydrothermal synthesis of mixed (anatase and rutile) particles

From mixed (anatase and rutile) bulk particles, anatase TiO₂ nanotubes are synthesized in this study by an alkaline hydrothermal reaction and a consequent annealing at 300-400 °C. The physical and electrochemical properties of the TiO₂ nanotube are investigated for use as an anode active material for lithium-ion batteries. Upon the first discharge-charge sweep and simultaneous impedance measurements at local potentials, this study shows that interfacial resistance decreases significantly when passing lithium ions through a solid electrolyte interface layer at the lithium insertion/deinsertion plateaus of 1.75/2.0 V, corresponding to the redox potentials of anatase TiO₂ nanotubes. For an anatase TiO₂ nanotube containing minor TiO₂(B) phase obtained after annealing at 300 °C, the high-rate capability can be strongly enhanced by an isotropic dispersion of TiO₂ nanotubes to yield a discharge capacity higher than 150 mAh g⁻¹, even upon 100 cycles of 10 C-rate discharge-charge operations. This is suitable for use as a high-power anode material for lithium-ion batteries.     

TiO2 nanotubes manufactured by anodization of Ti thin films for on-chip Li-ion 2D microbatteries

The use of self-organized TiO₂ nanotube arrays electrochemically grown onto Si is investigated for the fabrication of an alternative electrode dedicated to on-chip Li-ion 2D microbatteries. Discharge/charge curves and cycling performance are studied in lithium-anode electrochemical test cells for both amorphous and crystalline titania nanotubes. At 5 μA cm⁻² amorphous TiO₂ nanotube layers onto Si deliver a maximum areal capacity of 89 μAh cm⁻² in the first reversible discharge and 56 μAh cm⁻² over 50 cycles. We demonstrate that these nanostructured thin film electrodes showing such electrochemical performances are compatible with IC technology.     

Bioactive and electrochemical characterization of TiO2 nanotubes on titanium via anodic oxidation

The selection of bioactive and electrochemically stable materials for implants having effective corrosion resistance during long-term use in the body is essential. In this study, the bioactive and electrochemical properties of titanium implant materials with a nanotube surface treatment and various types of post-treatments were examined. Two types of amorphous TiO₂ nanotubes were grown homogeneously on the surface: one with a larger diameter (approximately 85 nm) and one with a smaller diameter (approximately 50 nm). Amorphous TiO₂ nanotubes were partially crystallized to anatase and rutile by heat treatment at 500 °C for 2 h. The corrosion potential (E_corr) of the heat-treated sample (HT) had a novel value of 0.102 V due to the stable TiO₂ crystal phase compared to the −0.106 V observed in the anodic oxidation sample (AN). The corrosion current density (I_corr) ranged from 0.20 to 0.64 μA/cm² according to the post-treatment conditions. However, at 0.6 V, where a passive layer had formed, the corrosion resistance of the HT was approximately ten times that of the AN and untreated (UT) samples. After evaluating the hydroxyapatite (HA)-forming ability by immersion in a simulated body fluid (SBF) solution, the CP process induced the adsorption of Ca and P onto HT. A comparison of the time-dependent amount of Ca and P adsorption showed that Ca adsorption plays a role in determining the rate at which hydroxyapatite (HA) is formed. For the induction of HA formation, a level of Ca adsorption above a critical level is required.     

Enhancing the performance of dye-sensitized solar cells based on an organic dye by incorporating TiO2 nanotube in a TiO2 nanoparticle film

The photoelectrochemical properties of a high molar extinction coefficient charge transfer organic dye containing thienylfluorene segment called FL, and the effect of incorporating TiO₂ nanotube (TiNT) in TiO₂ nanoparticle film along with the above dye on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were investigated. The influence of soaking time of the TiO₂ electrode in dye solution and the effect of varying its concentration, on the solar cell efficiency was also studied. Cyclic voltammetric (CV) analysis revealed the linear relationship between the anodic peak current and the scan rate, indicating a surface-confined diffusion process.The surface morphology of TiNT was characterized using SEM, TEM and XRD. The open-circuit voltage (V_OC) of the DSSC increased with the increase in the wt% of TiNT and shows optimal value at about 5 wt%, which is correlated with the suppression of the electron recombination as found out from the electron lifetime studies.The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance (R_ct) and electron lifetime under different ratios of the TiNT/nanoparticle. The electron lifetimes of the DSSCs based on FL and N3 dye were very close to one another and the DSSC based on the FL showed respectable photovoltaic performance of ca. 7.8% under the light intensity of 100 mW cm⁻² (AM 1.5G).     

Photoelectro-Fenton combined with photocatalytic process for degradation of an azo dye using supported TiO2 nanoparticles and carbon nanotube cathode: Neural network modeling

In this work, treatment of an azo dye solution containing C.I. Basic Red 46 (BR46) by photoelectro-Fenton (PEF) combined with photocatalytic process was studied. Carbon nanotube-polytetrafluoroethylene (CNT-PTFE) electrode was used as cathode. The investigated photocatalyst was TiO₂ nanoparticles (Degussa P25) having 80% anatase and 20% rutile, specific surface area (BET) 50 m²/g, and particle size 21 nm immobilized on glass plates. A comparison of electro-Fenton (EF), UV/TiO₂, PEF and PEF/TiO₂ processes for decolorization of BR46 solution was performed. Results showed that color removal follows the decreasing order: PEF/TiO₂ > PEF > EF > UV/TiO₂. The influence of the basic operational parameters such as initial pH of the solution, initial dye concentration, the size of anode, applied current, kind of ultraviolet (UV) light and initial Fe³⁺ concentration on the degradation efficiency of BR46 was studied. The mineralization of the dye was investigated by total organic carbon (TOC) measurements that showed 98.8% mineralization of 20 mg/l dye at 6 h using PEF/TiO₂ process. An artificial neural network (ANN) model was developed to predict the decolorization of BR46 solution. The findings indicated that artificial neural network provided reasonable predictive performance (R² = 0.986).     

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

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

Amorphous Ru1−yCryO2 loaded on TiO2 nanotubes for electrochemical capacitors

Amorphous Ru_1−yCr_yO₂/TiO₂ nanotube composites were synthesized by loading different amount of Ru_1−yCr_yO₂ on TiO₂ nanotubes via a reduction reaction of K₂Cr₂O₇ with RuCl₃·nH₂O at pH 8, followed by drying in air at 150 °C. Cyclic voltammetry and galvanostatic charge/discharge tests were applied to investigate the performance of the Ru_1−yCr_yO₂/TiO₂ nanotube composite electrodes. For comparison, the performance of amorphous Ru_1−yCr_yO₂ was also studied. The results demonstrated that the three dimensional nanotube network of TiO₂ offered a solid support structure for active materials Ru_1−yCr_yO₂, allowed the active material to be readily available for electrochemical reactions, and increased the utilization of active materials. A maximum specific capacitance 1272.5 F/g was obtained with the proper amount of Ru_1−yCr_yO₂ loaded on the TiO₂ nanotubes.     

Selective catalytic reduction of NOX with ammonia over Mn-Ce-OX/TiO2-carbon nanotube composites

Mn-Ce-O_X catalysts loaded on TiO₂-carbonaceous materials were prepared by sol-gel method. Selective catalytic reduction of NO_X was conducted in a fixed-bed flow-reactor over catalysts coated on aluminum plates. A de-NO_X efficiency of more than 90% was obtained over the Mn-Ce-O_X/TiO₂-carbon nanotubes (CNTs) catalyst between 75 °C and 225 °C under a gas hourly space velocity (GHSV) of ~ 36,000 h⁻¹. This activity improvement is attributed to the increase of the BET surface area, and the occurrence of reaction between adsorbed NO_X and NH₃. Moreover, the de-NO_X efficiency was increased to 99.6% by adding 250 ppm SO₂ between 100 °C and 250 °C.     

Low temperature and microwave dielectric properties of TiO2/ZBS glass composites

TiO₂ based ceramic/glass composites were prepared by a non-reactive liquid phase sintering (NLPS) using zinc borosilicate (ZBS) glass having the deformation temperature of 588 °C. The compounds of Zn₂SiO₄ and Zn₄B₆O₁₃ were formed after the sintering process, indicating that the ZBS glass was a non-reactive one in this system. For TiO₂/50 vol% ZBS glass composite, the two-stage sintering behavior was conducted as the sintering temperature increased. The former might be correlated to the NLPS process and the latter appeared to be related to the crystallization. The dielectric constant (?_r) was mainly affected by the porosity and obeyed the logarithmic mixing rule. The quality factor (Q × f₀) showed an increase and then a steep decrease after the maximum at 850 °C. TiO₂/50 vol% ZBS glass composite sintered at 900 °C demonstrated 36 in the dielectric constant (?_r) and 7500 GHz in the quality factor (Q × f₀) for an application to LTCC filters.     

Enhanced efficiency in dye-sensitized solar cells based on TiO2 nanocrystal/nanotube double-layered films

A new bilayer-structured film with TiO₂ nanocrystals as underlayer and TiO₂ nanotubes as overlayer was fabricated. The resultant double-layer TiO₂ (DL-TiO₂) film could significantly improve the efficiency of dye-sensitized solar cells (DSSCs) owing to its synergic effects, i.e. effective dye adsorption mainly originated from TiO₂ nanocrystal layer and rapid electron transport in one-dimensional TiO₂ nanotube layer. The overall energy-conversion efficiency (η) of 6.15% was achieved by the formation of DL-TiO₂ film, which is 44.7% higher than that formed by pure nanocrystalline TiO₂ (NC-TiO₂) film and far larger than that formed by nanotube TiO₂ (NT-TiO₂) film (η = 0.37%). The charge recombination behavior of cells was investigated by electrochemical impedance spectra, and the results showed that DL-TiO₂ film-based cell possessed the lowest transfer resistance and the longest electron lifetime. The incident-photon-to-current efficiency spectra indicated that the broad bands covered almost the entire visible spectrum from 400 to 700 nm with the maxima of 57.3%, 40.3%, and 2.2% at a wavelength of ∼530 nm for DL-TiO₂-, NC-TiO₂-, and NT-TiO₂-based solar cells, respectively. It is expected that the double-layer film electrode can be extended to other composite films with different layer structures and morphologies for enhancing the efficiencies of DSSCs.     

Hierarchical structured TiO2 nano-tubes for formaldehyde sensing

Hierarchical structured TiO₂ nano-tubes were prepared following a two-step method: the highly ordered uniform TiO₂ nanotube arrays were first grown by the conventional electrochemical anodization of the Ti metal sheet followed by mechanical milling of the as-fabricated TiO₂ nanotube arrays. The obtained nanotubes with a length around 400 nm and opening diameter ∼100 nm were formed mixed with the spherical TiO₂ single crystals with a diameter around 10 nm indicating hierarchical nanostructure. The as-synthesized TiO₂ hierarchical nanotubes based resistive-type chemical sensor exhibits good sensitivity to formaldehyde at room temperatures with or without UV-irradiation. The response of the sensor increased almost linearly as a function of the concentration of formaldehyde from 10–50 ppm under UV irradiation. The response of the sensor to different relative humidity and other possible interferents such as ammonia, methanol and alcohol was investigated. The larger response of the sensor to formaldehyde relative to these interferents is suggested to be due to the deeper diffusion of formaldehyde into the TiO₂ nanotubes.     

Iron oxide supported sulfated TiO2 nanotube catalysts for NO reduction with propane

Sulfated TiO₂ nanotubes and a series of iron oxide loaded sulfated TiO₂ nanotubes catalysts with different iron oxide loadings (1 wt%, 3 wt%, 5 wt% and 7 wt%) were prepared and calcined at 400 °C. The physico-chemical properties of the catalysts were studied by using XRD, N₂-physisorption, Raman spectroscopy, SEM-EDX, TEM, XPS, and pyridine adsorption using FTIR and H₂-TPR techniques. It was observed that iron oxide was highly dispersed on the sulfated TiO₂ nanotube support due to its strong interaction. The activity of these catalysts in the catalytic removal of NO with propane was also studied in the temperature range of 300–500 °C. Highest activity (90% NO conversion) was observed with 5 wt% iron oxide supported on sulfated TiO₂ catalyst at 450 °C. Selective catalytic reduction of NO activity of the catalysts was correlated with iron oxide loading, reducibility, and the Brönsted and Lewis acid sites of the catalysts. The catalyst also showed good stability under studied reaction conditions that no deactivation was observed during the 50 h of reaction.     

Study on photocatalytic deposition of bismuth onto nanocrystalline TiO2 by quartz crystal microbalance and electrochemical method

In situ techniques of quartz crystal microbalance (QCM), differential pulse voltammetry (DPV) and amperometric measurement were employed to investigate the adsorption Bi(III) ions and the photocatalytic deposition Bi process at the surface of nanocrystalline TiO₂. It was obtained that the adsorption of Bi(III) ions onto nanocrystalline TiO₂ accords with the pseudo-second-order reaction and the reaction rate constant k was about 13.3 g mol⁻¹ min⁻¹. In addition, the photocatalytic deposition of Bi onto the surface of TiO₂ was further investigated. It was found that photocatalytic deposition rate at the surface of TiO₂ was enhanced by increasing pH value or initial concentration of Bi(III) ions. The influence of organic hole-scavegeners on the photocatalytic deposition of Bi was also investigated, and it was obtained that formic acid may be the best for the photocatalytic reduction of Bi. The mass ratio between the Bi(III) and Bi metal deposition was calculated as 7.48:1. Therefore, it can be concluded that QCM, DPV and amperometric measurement may be effective and reliable for the investigation of the photocatalytic deposition of Bi onto the surface of nanocrystalline TiO₂.