Preparation and characterization of TiO2/Ti film electrodes by anodization at low voltage for photoelectrocatalytic application

Multiporous TiO₂/Ti film electrodes were prepared by different anodic oxidation processes at low voltage, in which the micro-structured TiO₂ thick films were prepared in H₂SO₄–H₂O₂–H₃PO₄–HF solution for 2 h and the nano-structured TiO₂ thin films were prepared in H₃PO₄–HF solution for 30 min with post-calcination. Both types of TiO₂/Ti films were characterized by scanning electron microscopy and X-ray diffraction analysis. The photocatalytic (PC) and photoelectrocatalytic (PEC) reactivity of the TiO₂/Ti electrodes were evaluated in terms of bisphenol A (BPA) degradation in aqueous solution. The experimental results demonstrated that the nano-structured TiO₂/Ti thin-film electrodes had higher reactivity in the BPA degradation reaction. The PEC degradation of BPA was further studied using different cathodes, either a reticulated vitreous carbon (RVC) electrode or a platinum (Pt) electrode. The experimental results confirmed that the efficiency of BPA degradation could be significantly enhanced in the TiO₂/Ti–RVC reaction system due to the generation of H₂O₂ on the RVC cathode. It is believed that such a H₂O₂-assisted TiO₂ PEC oxidation process may have good potential for water and wastewater treatment.     

Large-scale preparation of nanoporous TiO2 film on titanium substrate with improved photoelectrochemical performance

Fabrication of three-dimensional TiO₂ films on Ti substrates is one important strategy to obtain efficient electrodes for energy conversion and environmental applications. In this work, we found that hierarchical porous TiO₂ film can be prepared by treating H₂O₂ pre-oxidized Ti substrate in TiCl₃ solution followed by calcinations. The formation process is a combination of the corrosion of Ti substrate and the oxidation hydrolysis of TiCl₃. According to the characterizations by scanning electron microscopy (SEM), X-ray diffraction (XRD), and diffuse reflectance spectroscopy (DRS), the anatase phase TiO₂ films show porous morphology with the smallest diameter of 20 nm and possess enhanced optical absorption properties. Using the porous film as a working electrode, we found that it displays efficient activity for photoelectrocatalytic decolorization of rhodamine B (RhB) and photocurrent generation, with a photocurrent density as high as 1.2 mA/cm². It represents a potential method to fabricate large-area nanoporous TiO₂ film on Ti substrate due to the scalability of such chemical oxidation process.     

Immobilization of horseradish peroxidase in three-dimensional macroporous TiO2 matrices for biosensor applications

Three-dimensional macroporous TiO₂ was synthesized by a sol-gel procedure using polystyrene colloidal crystals as templates. SEM showed that a face-centered cubic (FCC) 3D macroporous structure was obtained. Horseradish peroxidase (HRP) was successfully immobilized on the surface of an optically transparent electrode (OTE). Photoelectrochemical properties were characterized using a three electrodes system and an ultraviolet lamp. The HRP/TiO₂/OTE displays a rapid photocurrent response, approximately 178.7 nA, under UV illumination (380 nm). The sensitivity of H₂O₂ detection was 70.04 μA/mM without UV illumination, and it increased to 102.97 μA/mM when illuminated by UV. The amperometric response was also enhanced. The high response was due to the good biocompatibility of TiO₂ and excellent photoelectrical property and the large effective surface of the three-dimensionally ordered macroporous structure.     

Effect of TiO2 supporting layer on Fe2O3 photoanode for efficient water splitting

For an electrochemical water splitting system, titanate nanotubular particles with a thickness of ∼700 nm produced by a hydrothermal process were repetitively coated on fluorine-doped tin oxide (FTO) glass via layer-by-layer self-assembly method. The obtained titanate/FTO films were dipped in aqueous Fe solution, followed by heat treatment for crystallization at 500 °C for 10 min in air. The UV–vis absorbance of the Fe-oxide/titanate/FTO film showed a red-shifted spectrum compared with the TiO₂/FTO coated film; this red shift was achieved by the formation of thin hematite-Fe₂O₃ and anatase-TiO₂ phases verified using X-ray diffraction and Raman results. The cyclic voltammetry results of the Fe₂O₃/TiO₂/FTO films showed distinct reversible cycle characteristics with large oxidation–reduction peaks with low onset voltage of I–V characteristics under UV–vis light illumination. The prepared Fe₂O₃/TiO₂/FTO film showed much higher photocurrent densities for more efficient water splitting under UV–vis light illumination than did the Fe₂O₃/FTO film. Its maximum photocurrent was almost 3.5 times higher than that obtained with Fe₂O₃/FTO film because of the easy electron collection in the current collector. The large current collection was due to the existence of a TiO₂ base layer beneath the Fe₂O₃ layer.     

Electrochemical stability of citrate-capped gold nanoparticles electrostatically assembled on amine-modified glassy carbon

The stability of citrate-capped gold nanoparticles assembled on covalently attached ethylenediamine films on glassy carbon was probed using cyclic voltammetry and scanning electron microscopy. The accessible potential windows in 0.01 M HClO₄ and phosphate buffer (pH 7.4) were found to be very similar to those for re-constructed polycrystalline bulk gold. At the nanoparticle electrodes, gold is dissolved at high positive potentials and re-deposited at lower potentials resulting in the formation of fewer, larger particles. The electrografted amine layer does not limit the stability range and citrate-capping groups do not stabilise the nanoparticles to oxidation. In addition to the structural stability of the nanoparticle electrodes, preliminary investigations of the stability of their activity for voltammetric analyses were also undertaken. Repeated voltammetric reduction of H₂O₂ at the nanoparticle electrodes showed excellent reproducibility indicating that the nanoparticle surface maintained a constant activity for the redox process. In contrast, a re-constructed polycrystalline bulk gold electrode did not exhibit a stable response under the same conditions.     

Preparation and photocatalytic activity of Keggin-ion tungstate and TiO2 hybrid layer-by-layer film composites

Keggin ions (PW₁₂O₄₀³⁻ (PW₁₂), SiW₁₂O₄₀⁴⁻ (SiW₁₂), H₂W₁₂O₄₀⁶⁻ (H₂W₁₂)) and TiO₂ hybrid thin films were prepared using the layer-by-layer method. Their photocatalytic activities were investigated using gaseous 2-propanol decomposition. All films were transparent in the visible wavelength range. For 2-propanol decomposition, H₂W₁₂ was the most effective for the combination with TiO₂ despite having the smallest TiO₂ deposition amount. The photocatalytic activity of the PW₁₂–TiO₂ hybrid film was increased 2.3 times by visible light with UV illumination. This increase was less remarkable for hybrid films of other Keggin ions, suggesting that the visible light excitation of reduced PW₁₂ plays an important role in the enhancement of 2-propanol decomposition.     

Fluorescence spectra of poly(di-n-hexylsilane)/TiO2 nanoparticle hybrid film

The interaction between poly(di-n-hexylsilane) (PDHS) and TiO₂ nanoparticle was studied based on the temperature dependence of the fluorescence of a PDHS/TiO₂ nanoparticle hybrid film. The polysilane is a suitable probe to investigate a guest polymer-host matrix interaction because the photophysical properties of polysilanes remarkably depend on the conformation of the σ-conjugated Si-Si chain. The PDHS/TiO₂ nanoparticle hybrid film showed a fluorescence band assigned to a disordered structure even at 80 K whereas only the fluorescence band of an ordered structure was observed for the PDHS film at 80 K. The disordering of the Si-Si main chain was explained by the perturbation of the n-hexyl side chain in the neighborhood of the TiO₂ nanosurface. The non-radiative deactivation of the excited state via the disorder-induced local potential minima was suggested by the temperature dependence of the fluorescence intensities of the disordered and ordered structures in the temperature region from 80 to 160 K.     

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.     

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.     

Photoelectrochemical application of nanotubular titania photoanode

Titania/titanium (TiO₂/Ti) electrodes with tailored surface structure have been fabricated by galvanostatic-potentiostatic anodization process. Highly ordered titania nanotubes array can be prepared by electrolyzing titanium foil at 20 V for 40 min in HF-H₃PO₄ electrolyte. Comparatively, micro-structured and crystallized TiO₂ multiporous film can be prepared at 20-40 V for 6 h in H₂SO₄-H₃PO₄-H₂O₂-HF electrolyte. The morphological characteristics and crystal behaviors of both nanotubular and micro-structured TiO₂ films are investigated by field emission scanning electron microscopy and X-ray diffraction measurement. Photoelectrochemical properties of TiO₂/Ti film electrodes are examined by anodic photocurrent response and cyclic voltammetry measurement. Photocatalytic and photoelectrocatalytic application are investigated by using either nanotubular TiO₂/Ti thin-film or micro-structured TiO₂/Ti thick-film electrodes as photoanodes for recalcitrant organic pollutant degradation.     

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).     

Electrochemical polymerization and characterization of a poly(azulene)-TiO<Subscript>2</Subscript> nanoparticle composite film

A poly(azulene)-TiO₂ composite film (PAz-TiO₂) was synthesized electrochemically by oxidation of azulene in an electrolyte medium containing TiO₂ nanoparticles. Polymerization was performed under magnetic stirring in an acetonitrile solution containing tetrabutylammonium hexafluorophosphate as the electrolyte salt. Influence of the concentration of TiO₂ in the reaction suspension on the electrochemical and optical properties and on the structure of the composite films was studied by cyclic voltammetry, ex situ Raman and FTIR reflection spectroscopy and in situ UV–vis and FTIR spectroelectrochemical techniques. Morphology of the composite films was studied by Scanning Electron Microscopy and the amount and distribution of the TiO₂ nanoparticles within the polymeric matrix by Inductively Coupled Plasma Mass Spectrometry with laser ablation. Addition of TiO₂ in the reaction suspension had a small catalytic activity for the polymerization of Az. Inclusion of TiO₂ nanoparticles in PAz did not affect the voltammetric behavior or the chemical structure of the formed polymer films. However, a different chain conformation and morphology of the film was formed when synthesized in presence of TiO₂ compared to the plain PAz film. It was also found that the film morphology was more homogeneous when the concentration of TiO₂ was ≥10 mM in the polymerization solution than films polymerized without any TiO₂.     

Preparation of dye sensitized solar cell by using supercritical carbon dioxide drying

Dye-sensitized solar cells (DSSC) derived from TiO₂ aerogel film electrodes were fabricated. TiO₂ aerogels were obtained by using sol–gel method and supercritical carbon dioxide (sc-CO₂) drying. First, TiO₂ wet gels were obtained by sol-gel method. Then, the solvents in the TiO₂ wet gels were replaced by acetone. The TiO₂ aerogels were obtained by using sc-CO₂ drying from the TiO₂ wet gels. The conditions of sc-CO₂ drying were at 313, 323 K and 7.8–15.5 MPa. The electrodes with TiO₂ aerogel films were obtained by deposition of the aerogels on glass substrates. The electrodes with TiO₂ aerogel films and a commercial particle film of various thickness were obtained by repetitive coatings and calcinations. The amount of dye adsorbed on the TiO₂ films with sc-CO₂ drying was higher than that of commercial particle film. The amount of dye adsorbed on the TiO₂ films increased with increasing surface area of the TiO₂ film. DSSCs were assembled by using the TiO₂ aerogel film electrodes and their current–voltage performance was measured. The power performance of DSSC made by supercritical drying was higher than that of commercial particles. The DSSC with the film electrode made at 313 K and 15.5 MPa showed the best power performance (J_sc = 7.30 mA/cm², V_oc = 772 mV, η = 3.28%).     

Photocatalytic Decomposition of Formic Acid Under Visible Light Irradiation Over V-ion-implanted TiO2 Thin Film Photocatalysts Prepared on Quartz Substrate by Ionized Cluster Beam (ICB) Deposition Method

Unique visible-light-responsive TiO₂ photocatalysts (λ>450 nm) were successfully developed by implantation of V ions into the TiO₂ thin films prepared on a quartz substrate by an ionized cluster beam (ICB) deposition method. After V ions implantation into TiO₂ thin film, the photocatalytic activity of the thin films for the decomposition of formic acid into CO₂ and H₂O was found to proceed efficiently under visible light irradiation longer than 450 nm. The TiO₂ thin film photocatalysts were characterized by XRD, UV-vis, XPS, FE-SEM and AFM.     

Novel dynamic effects in electrocatalysis of methanol oxidation on supported nanoporous TiO2 bimetallic nanocatalysts

New dynamic aspects of the catalysis of methanol oxidation reaction (MOR) have been studied using quantum mechanical calculations applied to the support-catalyst cluster interactions and surface diffusivity of adsorbed intermediates. For very small catalyst-support clusters, we have found a strong enhancement of the ligand effect for bimetallic catalysts of the type Pt_nM_m attributed to the decreased local density of states near the Fermi level of Pt atoms neighboring the additive metal atom M. This enhancement results in a decreased barrier for surface diffusion of adsorbed CO_ad through the cooperative diffusion mechanism, based on structural relaxation of the catalyst-support cluster, proposed in this work. The strong ligand effect dominates over the Schwoebel potential and trapping well effects, being responsible for accumulation of poisoning intermediates at step sites on the catalyst surface and gradual decrease of catalytic activity with decreasing size of catalyst nanoparticles. The lattice relaxation and strong ligand effects in small catalyst-support clusters lead to lower adsorption energy for CO_ad and thus, to higher reactivity and mobility of reactants and intermediates. The experimental investigations included submonolayer films of bi-functional catalysts (PtRu, PtFe) deposited on novel nanostructured supporting materials, designed with the goal of achieving high variability of their electronic and chemical properties to influence the catalytic activity of sub-monolayer catalyst. The mesoscopic TiO₂ supporting film formation was investigated using EQCN, pulse voltammetric and AFM techniques. The conditions for the formation of monodispersed TiO₂ nanoparticles with regular nanopores (nanotubes), 20-80 nm in diameter, were described. It follows from EQCN and voltammetric measurements and AFM image analysis that the nanopores are formed by a dissolution-precipitation mechanism. The catalysts, Pt and PtRu, deposited on supporting nanoporous TiO_2−x films, were used to study MOR. A lower poisoning effect for cluster PtRu on a TiO_2−x support film than that for unsupported PtRu or bare Pt catalysts has been observed. These effects have been attributed to differences in CO_ad binding energy and lowering of activation energy for surface mobility leading to a more facile 2D diffusion of CO_ad from Pt sites to Ru and the supporting TiO_2−x. The substrate-catalyst interactions were further investigated using quantum mechanical calculations performed for a model TiO₂ nano-ring (representing an orifice of a TiO_2−x nanotube studied experimentally) with adlayers of Pt, Ru and Fe catalysts. We have found unusually strong electron delocalization effects for Pt₂Fe₂ clusters on (TiO₂)₄ as compared to (TiO_2−x)₄Pt₂Ru₂. We have also analyzed various states in surface diffusion of CO_ad on bimetal clusters supported on (TiO₂)_n and observed considerable dynamic widening of metal-to-metal atom distances induced by CO adsorption (up to 9% for Pt-Pt distance and up to 15% for Fe-Fe distance). We propose that this new dynamic effect leading to cooperative surface diffusion may be further explored in designing novel nanoparticle catalysts.     

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.     

Photocatalytic decomposition of NO on titanium oxide thin film photocatalysts prepared by an ionized cluster beam technique

Transparent TiO₂ thin film photocatalysts were prepared on transparent porous Vycor glass (PVG) by an ionized cluster beam (ICB) method. The UV‐VIS absorption spectra of these films show specific interference fringes, indicating that uniform and transparent TiO₂ thin films are formed. The results of XRD measurements indicate that these TiO₂ thin films consist of both anatase and rutile structures. UV light (λ > 270 nm) irradiation of these TiO₂ thin films in the presence of NO led to the photocatalytic decomposition of NO into N₂, O₂ and N₂O. The reactivity of these TiO₂ thin films for the photocatalytic decomposition of NO is strongly dependent on the film thickness, i.e., the thinner the TiO₂ thin films, the higher the reactivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ferrocenium hexafluorophosphate-induced nanofibrillarity of polyaniline-polyvinyl sulfonate electropolymer and application in an amperometric enzyme biosensor

The formation of nanofibrillar polyaniline-polyvinyl sulfonate (Pani-PVS) composite by electropolymerization of aniline in the presence of ferrocenium hexafluorophophate (FcPF₆) and its application in mediated-enzyme biosensor using the horseradish peroxidase/hydrogen peroxide (HRP/H₂O₂) enzyme-substrate system is reported. The electropolymerization was carried out at glassy carbon electrodes (GCE) and screen printed carbon electrodes (SPCE) in a strongly acidic medium (HCl). Scanning electron microscopy (SEM) images showed that 100 nm diameter nanofibrils were formed on the SPCE in contrast to the 800-1000 nm cauliflower-shaped clusters which were formed in the absence of FcPF₆. A model biosensor (GCE//Pani-PVS/BSA/HRP/Glu), consisting of horseradish peroxidase (HRP) immobilized by drop coating atop the GCE//Pani-PVS in the presence of bovine serum albumin (BSA) and glutaraldehyde (glu) in the enzyme layer casting solution, exhibited voltammetric responses characteristic of a mediated-enzyme system. The biosensor response to H₂O₂ was very fast (5 s) and it exhibited a detection limit of 30 μM (3σ) and a linearity of up to 2 mM (R² = 0.998). The relatively high apparent Michaelis-Menten constant value () of the sensor indicated that the immobilized enzyme was in a biocompatible microenvironment. The freshly prepared biosensor was successfully applied in the determination of the H₂O₂ content of a commercial tooth whitening gel with a very good recovery rate (97%).     

Influence of transition metal doping on the structural,optical, and magnetic properties of TiO2 films deposited on Si substrates by a sol–gel process

Transition metal (TM)-doped TiO₂ films (TM = Co, Ni, and Fe) were deposited on Si(100) substrates by a sol–gel method. With the same dopant content, Co dopants catalyze the anatase-to-rutile transformation (ART) more obviously than Ni and Fe doping. This is attributed to the different strain energy induced by the different dopants. The optical properties of TM-doped TiO₂ films were studied with spectroscopic ellipsometry data. With increasing dopant content, the optical band gap (E_OBG) shifts to lower energy. With the same dopant content, the E_OBG of Co-doped TiO₂ film is the smallest and that of Fe-doped TiO₂ film is the largest. The results are related to electric disorder due to the ART. Ferromagnetic behaviors were clearly observed for TM-doped TiO₂ films except the undoped TiO₂ film which is weakly magnetic. Additionally, it is found that the magnetizations of the TM-doped TiO₂ films decrease with increasing dopant content.     

pH-controllable bioelectrocatalysis of glucose by glucose oxidase loaded in weak polyelectrolyte layer-by-layer films with ferrocene derivative as mediator

Oppositely charged poly(allylamine hydrochloride) (PAH) and hyaluronic acid (HA) were assembled into {PAH/HA}_n layer-by-layer (LBL) films on pyrolytic graphite (PG) electrodes. Glucose oxidase (GOD) in solution was then loaded into the films, designated as {PAH/HA}_n-GOD. When the {PAH/HA}_n-GOD film electrodes were placed in pH 5.0 buffers containing ferrocenedicarboxylic acid (Fc(COOH)₂) and glucose, a well-defined and large cyclic voltammetric (CV) oxidation wave of glucose catalyzed by GOD immobilized in the films and mediated by Fc(COOH)₂ in solution was observed. However, when the films were placed in pH 9.0 buffers containing the same amount of Fc(COOH)₂ and glucose, the electrocatalytic response was quite small. The bioelectrocatalysis for the film system was at the “on” state at pH 5.0 and at the “off” state at pH 9.0. This pH-sensitive “on-off” behavior was reversible and could be repeated for several times. The possible mechanism of the pH-switchable bioelectrocatalysis was explored and discussed, and should be mainly attributed to the different electrostatic interaction between Fc(COOH)₂ and the films at different pH. This work provides a novel model to realize pH-controllable bioelectrocatalysis based on the enzyme-loaded LBL assembly films, and may guide us to develop the tunable electrochemical biosensors based on electrocatalysis with immobilized enzymes.