Micro-EIS of anodic thin oxide films on titanium for capacitor applications

The present work focuses on the investigation of the effect of the different crystallographic orientation of titanium grains on the formation of anodic oxide films and consequently their dielectric and semiconductive properties. By using a microcapillary cell the formation process and the electrochemical impedance spectroscopy (EIS) can be performed at high lateral resolution on variously orientated single grains of polycrystalline titanium. The oxide films were potentiodynamically formed by cyclovoltammetry. EIS measurements immediately followed by the oxide formation were used for a detailed investigation of the film properties, in particular, the relative permittivity ?_r and the donor concentration N_D. In contrast to the most publications it was found that under the chosen conditions the crystallographic orientation of titanium substrate has no significant influence on the oxide thickness d, the relative permittivity ?_r or on the donor concentration N_D of the oxide films. The relative permittivity ?_r is approximately 50. The donor concentration depends on the film thickness and amounts to approximately 3 × 10¹⁸ cm⁻³ in minimum.     

Thick titania films for semiconductor photocatalysis

A brief overview of work carried out by this group on thick (>1 μm), optically clear, robust titania films prepared by a sol–gel method, as well as new results regarding these films, are described. Such films are very active as photocatalysts and able to destroy stearic acid with a quantum yield of 0.32%. The activity of such films is largely unaffected by annealing temperatures below 760 °C, but is drastically reduced above this temperature. The drop in photocatalyst activity of such films as a function of annealing temperature appears to correlate well with the change in porosity of the films and suggests that the latter parameter is very important in deciding the overall activity of such films. The importance of porosity in semiconductor photocatalysed cold combustion may be due to the effect it has on access of oxygen to the active sites, rather like the effect the position of a fire grate (open or closed) has on the rate of burning, i.e., hot combustion, that takes place in a fireplace.     

Hydrogen evolution reaction on anodic titanium oxide films

Oxide titanium films were prepared using linear potential sweeps up to different limiting potential values between 0 and 60 V in 0.5 M H₂SO₄. The electronic characteristics of these films were investigated through impedance measurements during their formation. The hydrogen evolution reaction (HER) in either HClO₄ or H₂SO₄ solutions in the pH range between 1 and 4 was studied on the different oxide films. The obtained experimental results could be explained assuming that the recombination of adsorbed hydrogen atoms is the rate determining step (rds), and that the Temkin type isotherm applies to the adsorbed intermediates. The log i vs E plot after correction for diffusion control in the bulk gives a slope of 2.3 RT/F.The influence of the physico-chemical properties of the oxide films on the HER has also been studied from the changes in the current—potential profiles. The oxide film thickness changes the HER overpotential, but the slope of the log i vs E relationship remains independent of it.     

外场辅助光催化机理及降解有机污染物研究进展

光催化技术能够利用太阳能生成自由基对有机污染物进行矿化降解，具有绿色、节能、无污染的优势，被认为是最有前景的高级氧化技术，目前研究集中于提升光催化技术的降解效率。而采用电场、超声场等外场辅助的方法能够通过提高溶液传质效率、抑制载流子复合、提高自由基产率等途径，进一步提高光催化效率。综述了电场、热场、微波场、超声场、磁场5种外场各自辅助光催化降解有机污染物的研究进展和应用现状，重点介绍了外场提升光催化降解效率的主要机理，归纳了每种外场辅助技术的优缺点，对其在应用中面临的问题进行总结，并对未来研究和发展方向进行了展望。     

Reversible reactions within anodic oxide films on titanium electrodes

Evidence is presented for a reversible reaction occurring within the bulk of the surface oxide film on Ti electrodes at negative potentials.

High frequency impedance measurements give results characteristic of a bulk phenomenon and clearly distinguish the behaviour of filmed Ti electrodes from that of some other “valve” metals at low potentials.

The potential/pH dependences of the impedance minima are discussed in terms of various models for the reversible formation of a hydrated oxide in the bulk of the oxide film.     

溶胶凝胶法制备平整TiO_2薄膜及其表征

以钛酸四丁酯为前驱体,通过溶胶凝胶法制备表面平整、致密的TiO2薄膜,并且通过热重分析仪(TGA)、X射线衍射仪(XRD)、衰减全反射红外线光谱分析仪(ATR IR)、扫描电子显微镜(SEM)、原子力显微镜(AFM)和接触角测量仪等研究TiO2薄膜的晶相、组成和表面的微观结构、润湿性。利用TiO2薄膜表面部分覆盖的十八烷基三氯硅烷自组装单分子层在紫外光照下的降解来研究其光催化性能,并且用水在该薄膜上的接触角的变化来表征十八烷基三氯硅烷的降解量。研究结果表明:TiO2薄膜具有很低的粗糙度,透明的锐钛矿相薄膜在较弱的紫外光照强度下具有较好的光催化性能,在自清洁和光学薄膜领域有潜在的应用前景。     

Photocatalytic properties of visible-light enabling layered titanium oxide/tin indium oxide films

Visible-light enabling titanium oxide/tin indium oxide (TiO₂/ITO) thin films deposited on unheated glass slides with prolonged deposition duration were investigated in this study. Structural properties characterized by X-ray diffraction (XRD), Raman spectra and scanning electron microscopy (SEM) showed typical polycrystalline structure with primary anatase phase along with elongated pyramid-like grains lying on the film surface and densely packed columnar structure from cross-sectional profile. The XRD preferential peak of (2 1 1) and the Raman peak intensity at 640 cm⁻¹ dramatically increased without noticeable broadening and shift as the deposition time was prolonged beyond 2 h. This implies that more perfectly crystalline structure, less internal stress, and comparatively larger grains were obtained by this technique. The Ti2p_3/2 and O1s XPS peaks shifted toward higher binding energy suggest that the local chemical state was influenced by the prolonged deposition duration in the film, which resulted in red shift of absorption threshold into visible-light region. Under ultra-violet (UV) and visible-light illumination, the visible-light enabling film exhibited the best photocatalytic activity on MB degradation with the rate-constant of about 0.231 h⁻¹. Hydrophilic conversion rate was estimated to be 8.14 × 10⁻³ deg⁻¹ min⁻¹ and long-term UV-induced hydrophilicity of 10° in the dark storage up to 72 h was observed. In addition to its inherent characteristics of the layered TiO₂/ITO structure on hole/electron separation, all these could be attributed to more perfectly formed crystalline structure, densely packed columnar crystals and the surface roughness along with its enlarged surface area.     

纳米二氧化钛光催化剂的可控水解制备与研究

以TiCl4为原料,采用可控水解法制备了TiO2光催化剂,探讨了体系的pH值、温度、浓度等对TiO2催化活性的影响,并通过XRD和TEM对优化条件下制备的产品的晶型和形貌做了表征。结果表明:当溶液pH值为6、体系温度在15℃、四氯化钛和乙醇的体积比为1︰3时,得到的TiO2为锐钛矿型,平均粒径在13nm左右,且分散性能好、催化活性高。     

Spray deposited titanium oxide thin films as passive counter electrodes

Titanium dioxide (TiO₂) thin films were deposited from methanolic solution onto fluorine doped tin oxide coated conducting glass substrates by spray pyrolysis technique. The electrochemical properties of TiO₂ thin films were investigated using cyclic voltammetry, chronoamperometry, chronocoulometry and iono-optical studies, in 0.1N H₂SO₄ electrolyte. Performance of the films deposited at three different substrate temperatures, viz. 350, 400 and 450 °C is discussed in view of their utilization in electrochromic devices, as counter electrode. The magnitude of charge storage capacity, Q/t (4.75-6.13 × 10⁻³ mC/(cm² nm)) and colouration efficiency (3.2-4.3 cm²/mC) of TiO₂ rank these films among the promising counter electrodes in electrochromic devices.     

Tungsten oxide nanowire-reduced graphene oxide aerogel for high-efficiency visible light photocatalysis

A light, 3-D, porous aerogel was fabricated by way of a simple approach from 1-D tungsten oxide nanowires and 2-D reduced graphene oxide sheets. The as-prepared graphene oxide, tungsten oxide nanowires, and tungsten oxide-reduced graphene oxide (W₁₈O₄₉-RGO) aerogel were characterised. The photocatalytic activities of as-prepared aerogel under visible light irradiation were investigated through the degradation of six different organic dyes including Rhodamine B, reactive black 39, reactive yellow 145, weak acid black BR, methyl orange, and weak acid yellow G. In comparison with the pure W₁₈O₄₉ nanowires, the prepared W₁₈O₄₉-RGO aerogel had significantly improved photocatalytic efficiency. Also, the photocatalysis of W₁₈O₄₉-RGO aerogel maintained its efficiency after 30 cycles for each of the six dyes. The photocatalytic mechanism was studied by adding hole and radical scavengers: the results confirmed that the holes generated in W₁₈O₄₉-RGO aerogel played a key role in the visible light photocatalytic process.     

PVDF/ZnO composite films for photocatalysis: A comparative study of solution mixing and melt blending methods

Heterogeneous photocatalysis represents a solution for several environmental problems. However, achieving photocatalyst separation from reaction media on a large scale remains a challenge, one that might be overcome by the immobilization of photocatalysts into supports. To this end, composites of polyvinylidene fluoride and zinc oxide (ZnO) were prepared by three different techniques: solution mixing (SM) followed by Nonsolvent Induced Phase Separation; and melt blending at both low- and high-shear rates (LS and HS) in a rheometer and mixing chamber, respectively, to compare the resultant morphology in photocatalysis. Photocatalytic efficiency was assessed by methylene blue (MB) discoloration in a batch reactor liquid phase and by resazurin (RZ) reduction. The changes promoted by nanoparticle inclusion, processing conditions and UV effect were demonstrated by FTIR-ATR, XRD, DSC, and SEM. Results showed that ZnO incorporation was successful under all processing conditions, providing effective photocatalytic composites. However, samples prepared by SM had a twofold increase in discoloration efficiency and fourfold increase in surface photoactivity, when compared with LS or HS-produced samples, explained by its higher porosity of 88% ± 1.3%. The covering of the photocatalyst surface was also evident on SEM analysis for melt blended samples, further contributing to reduction in their photocatalytic activity.     

Soft X-ray absorption study of tantalum incorporation in titanium oxide films: Impact of flash-lamp annealing

The impact of Ta incorporation (up to ~21 at.%) in titanium dioxide (TiO₂) films subjected to post-deposition millisecond-range flash-lamp annealing (FLA) is addressed. Phase formation with short-range information was established by means of soft X-ray absorption near-edge structure (XANES) in combination with standard X-ray diffraction. As-grown films are X-ray amorphous, but display a significant structural improvement upon FLA. Up to relatively large Ta concentrations (~ 12 at.%), FLA can be used to effectively incorporate Ta into a nanocrystalline anatase TiO₂ phase, although its structural quality deteriorates progressively with the Ta content. For the intermediate Ta range between 12 and 17 at.%, the structure of the FLA films is highly disordered, being unable to overcome the initial distorted arrangement. In any case, rutile- or Ta₂O₅-like environments emerge for low and high contents, respectively. Finally, for the highest Ta content (~21 at.%), the formation of good-quality nanocrystalline Ta₂O₅ phase occurs after FLA. As assessed by XANES, the structural evolution upon FLA seems to be determined by the initial (amorphous) structure. Lastly, all the samples are highly transparent from the visible to the near-infrared region, and the band-gap can be tailored from ~3.2 to ~3.8 eV with increasing the amount of incorporated Ta.     

Formation of porous anodic titanium oxide films in hot phosphate/glycerol electrolyte

The present study reveals the formation of porous anodic films on titanium at an increased growth rate in hot phosphate/glycerol electrolyte by reducing the water content. A porous titanium oxide film of 12 μm thickness, with a relatively low content of phosphorus species, is developed after anodizing at 5 V for 3.6 ks in 0.6 mol dm⁻³ K₂HPO₄ + 0.2 mol dm⁻³ K₃PO₄/glycerol electrolyte containing only 0.04% water at 433 K. The growth efficiency is reduced by increasing the formation voltage to 20 V, due to formation of crystalline oxide, which induces gas generation during anodizing. The film formed at 20 V consists of two layers, with an increased concentration of phosphorus species in the inner layer. The outer layer, comprising approximately 25% of the film thickness, is developed at low formation voltages, of less than 10 V, during the initial anodizing at a constant current density of 250 A m⁻². The pore diameter is not significantly dependent upon the formation voltage, being ∼10 nm.     

Evolution of TiOx-SiOx nano-composite during annealing of ultrathin titanium oxide films on Si substrate

This paper discusses the formation of the TiO_x-SiO_x nano-composite phase during annealing of ultrathin titanium oxide films (~27 nm). The amorphous titanium oxide films are deposited on silicon substrates by sputtering. These films are important for high-k dielectrics and sensing applications. Annealing of these films at 750 °C in the O₂ environment (for 15–60 min) resulted in the polycrystalline rutile phase. The films exhibit Raman peaks at 150 cm⁻¹ (B_1g), 435 cm⁻¹ (E_g), and 615 cm⁻¹ (A_1g) confirming the rutile phase. The signature TO (1078 cm⁻¹) and LO (1259 cm⁻¹) infrared active vibrational modes of Si–O–Si bond confirms the presence of silicon-oxide. The X-ray photoelectron spectra of the TiO_x films show multiple peaks corresponding to Ti metal (453.8 eV); Ti⁴⁺ state (458.3 eV (Ti 2p_3/2) and 464 eV (Ti 2p_1/2)); and Ti³⁺ state (456.4 eV (Ti 2p_3/2) and 460.8 eV (Ti 2p_1/2)). The O1s XPS spectra peaks at 530–533 eV can be attributed to Ti–O and Si–O bonds of the TiO_x-SiO_x nano-composite phase in the annealed films. The depth profiling XPS study shows that the top surface of the annealed film is mainly TiO_x and the amount of SiO_x increases with the depth.     

Vertically aligned zinc oxide nanosheet for high-performance photocatalysis of water pollutants

The present work is focused on the synthesis of morphology-controlled vertically aligned Zinc oxide (ZnO) nanosheets (NSs) using a high power low-temperature sonochemical process. The synthesis process is a facile and low-temperature process, which can be able to produce large-area, homogeneously-grown, vertically-aligned ZnO NSs with high surface to volume ratio. Also, using the same process, we have developed a low-temperature in-situ method of the magnesium (Mg), doped ZnO NSs with identical structure and morphology. The process is versatile, scalable and can be adopted for synthesizing various other doped morphology controlled nanosheets synthesis. The morphology and structure were confirmed using X-ray diffraction technique, FESEM, TEM/HRTEM while surface composition, bonding and doping were confirmed using XPS and UV–VISIBLE spectroscopy. We obtained 2D few layered thick ZnO nanosheets of flake size 2–5 μm with bandgap ∼3.38 eV, which was increased to ∼3.7 eV due to the Mg doping. Furthermore, we demonstrated the comparative photocatalytic activity of the as-synthesized pristine and doped ZnO nanosheets and found that Mg-doping promotes the faster catalytically active redox reaction, hence; the dye degradation time was 15 min faster compared to the pristine ZnO NSs under identical conditions. We believe that the rate-enhancement in photocatalytic dye degradation of Mg-doped ZnO NSs is obtained due to the faster hole transport properties and low recombination rate.     

Variation of the dielectric constant of anodic oxide films on titanium with oxygen evolution

Titanium was anodized potentiodynamically in acidic solution of pH 3 (Na₂SO₄ + H₂SO₄) in the potential region before and at oxygen evolution. The oxide film growth efficiency in the range of potential above oxygen evolution was investigated using galvanostatic charging curves. A value of 62% for the efficiency was estimated. The conductivity characteristics of the oxide film formed in the two regions were investigated on the basis of the variation of the dielectric constant. Values of 18.2 and 39.7 for the dielectric constant were obtained for oxides formed in the region before and at the oxygen evolution potential, respectively.     

Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell

0-18 nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nafion 117 membranes, and used as thin spacer electrolyte layers between the Nafion and a 3 nm Pt catalyst film. Electrochemical characterisation of the films in terms of oxygen reduction activity, high frequency impedance and cyclic voltammetry in nitrogen is performed in a fuel cell at 80 °C and full humidification. Titanium oxide films with thicknesses up to 18 nm are shown to conduct protons, whereas zirconium oxide and tantalum oxide block proton transport already at a thickness of 1.5 nm. The performance for oxygen reduction is higher for a bi-layered film of 3 nm platinum on 1.5 or 18 nm titanium oxide, than for a pure 3 nm platinum film with no spacer layer. The improvement in oxygen reduction performance is ascribed to a higher active surface area of platinum, i.e. no beneficial effect of combining platinum with zirconium, tantalum or titanium oxides on the intrinsic oxygen reduction activity is seen. The results suggest that TiO₂ may be used as electrolyte in fuel cell electrodes, and that low-temperature proton exchange fuel cells could be possible using TiO₂ as electrolyte.     

Fabrication of a new type of organic-inorganic hybrid superlattice films combined with titanium oxide and polydiacetylene

We fabricated a new organic-inorganic hybrid superlattice film using molecular layer deposition [MLD] combined with atomic layer deposition [ALD]. In the molecular layer deposition process, polydiacetylene [PDA] layers were grown by repeated sequential adsorption of titanium tetrachloride and 2,4-hexadiyne-1,6-diol with ultraviolet polymerization under a substrate temperature of 100°C. Titanium oxide [TiO₂] inorganic layers were deposited at the same temperatures with alternating surface-saturating reactions of titanium tetrachloride and water. Ellipsometry analysis showed a self-limiting surface reaction process and linear growth of the nanohybrid films. The transmission electron microscopy analysis of the titanium oxide cross-linked polydiacetylene [TiOPDA]-TiO₂ thin films confirmed the MLD growth rate and showed that the films are amorphous superlattices. Composition and polymerization of the films were confirmed by infrared spectroscopy. The TiOPDA-TiO₂ nanohybrid superlattice films exhibited good thermal and mechanical stabilities.