Nanocrystalline Anatase Films Research Articles

Enhanced surface hydroxylation of nanocrystalline anatase films improves photocurrent output and electron lifetime in dye sensitized solar cell photoanodes

Anatase TiO2 nanocrystalline films were prepared from commercial (Alfa Aesar 5nm and 10nm) and aqueous-solution synthesized precursors (aqueous TiO2) by screen printing with the purpose of elucidating the influence of surface hydroxylation on dye sensitized solar cell (DSSC) performance. These two commercial TiO2 sources were selected as they possessed similar nanocrystallite size and mesoporous film interfacial surface area with our in-house variety. In this way it was possible to distinguish the effect of surface hydroxylation on DSSC photocurrent output and electron lifetime from other film surface properties. Their morphological features were characterized in terms of particle size, shape, Brunauer–Emmett–Teller (BET) surface area, topography and their surface hydroxyl groups quantified using thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). The electrochemical properties of the DSSCs composed of the aqueous TiO2, Alfa Aesar 5nm and 10nm photoelectrodes were probed via electrochemical impedance spectroscopy (EIS) and open circuit voltage decay (OCVD). The combined EIS and OCVD results showed that photoelectrodes composed of surface hydroxyl-rich TiO2 nanocrystalline material exhibit longer electron lifetime attributed to enhanced surface-dye binding that suppresses interfacial recombination. As a result of the stabilized surface hydroxyl-dye (N719) binding, the photocurrent output from the OH-rich anatase photoanodes (aqueous and Alfa Aesar 5nm TiO2,) was a remarkable 85–108% higher than that of the OH-poor film (Alfa Aesar 10nm) despite their equivalent surface area and porosity.

Multifunctional nature of UV-irradiated nanocrystalline anatase thin films for biomedical applications

Anatase is known to decompose organic material by photocatalysis and to enhance surface wettability once irradiated by ultraviolet (UV) light. In this study, pulse magnetron-sputtered anatase thin films were investigated for their suitability with respect to specific biomedical applications, namely superhydrophilic and biofilm degrading implant surfaces. UV-induced hydrophilicity was quantified by static and dynamic contact angle analysis. Photocatalytic protein decomposition was analyzed by quartz crystal microbalance with dissipation. The surfaces were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The radical formation on anatase, responsible for photocatalytic effects, was analyzed by electron spin resonance spectroscopy. Results have shown that the nanocrystalline anatase films, in contrast to reference titanium surfaces, were sensitive to UV irradiation and showed rapid switching towards superhydrophilicity. The observed decrease in carbon adsorbents and the increase in the fraction of surface hydroxyl groups upon UV irradiation might contribute to this hydrophilic behavior. UV irradiation of anatase pre-conditioned with albumin protein layers induces the photocatalytic decomposition of these model biofilms. The observed degradation is mainly caused by hydroxyl radicals. It is concluded that nanocrystalline anatase films offer different functions at implant interfaces, e.g. bedside hydrophilization of anatase-coated implants for improved osseointegration or the in situ decomposition of conditioning films forming the basal layer of biofilms in the oral cavity.

Chemical bath deposition of thin TiO 2-anatase films for dielectric applications

Titania thin films were prepared on bare Si and Pt/Ti/SiO 2/Si substrates by chemical bath deposition (CBD) from solutions of a titanium peroxo complex and subsequent calcinations at 350 and 700 °C, respectively. The CBD process allowed deposition on both uncoated and metal-coated Si substrates with the same deposition rate. Optimization of the annealing process yielded uniform and crack-free nanocrystalline anatase films. The influence of the film thickness, irradiation of visible light, measuring frequency, temperature and substrate on the dielectric properties will be discussed in the paper. Films with a final thickness of about 600 nm showed comparably high relative permittivity of 31.8 on silicon and of 52.7 on Pt/Ti/SiO 2/Si substrates, respectively. The present route provides anatase thin films with higher dielectric constants than classical sol–gel routes and is therefore a promising candidate for potential applications in large scale integration.

Synthesis of titanium oxide layers on glass substrates with aqueous refluxed sols (RS) and photocatalytic activities

Nanocrystalline anatase films were pre- pared on glass substrates by the dip-coating method from aqueous refluxed sols and calcined at 100, 300 and 500 � C. FT-IR, TEM, SEM, XRD, UV-VIS and XPS analysis were used to characterize the microstructure, phase evolution and chemical composition of the refluxed sols (RS) sols and TiO2 films. The photocat- alytic activities of the TiO2 films were evaluated by photocatalytic degradation of aqueous methyl orange solutions. It was found that the RS sols were flavescent, alkalescent and contained anatase crystals. The TiO2 thin films contained not only Ti and O elements, but also a small amount of N and Na elements. The TiO2 films were transparent and its maximal light transmit- tance exceeded 80% under visible light region. Cal- cined at 300 � C, the TiO2 thin film showed the highest photocatalytic activity, when the calcination tempera- ture is higher than 300 � C, the decrease in photocata- lytic activity is due to sintering and growth of TiO2 crystallites resulting in the decrease of surface area. The results suggested that RS sols are promising novel precursors for synthesizing TiO2 films, and refluxing technique is quite efficient in preparing titania nano- crystalline films.

Low-temperature preparation of nanocrystalline anatase films through a sol-gel route

Nanocrystalline anatase (TiO2) films were prepared at very low temperature through a sol-gel route using titanium isopropoxide and hydrogen peroxide in ethanol. Crystallization occurred after film deposition at 35°C in an atmosphere saturated with water vapor. Both thin and thick films of nanocrystalline anatase were prepared. Observed particle size in crystallized films is approximately 20–40 nm as measured with AFM. No residual organic material was apparent through FTIR after crystallization occurred. Dynamic light scattering studies performed on this system indicate that particle size measured in solution is strongly dependent on the amount of agitation samples received prior to measurement.

Formation of nanoporous and nanocrystalline anatase films by pyrolysis of PEO–TiO 2 hybrid films

Poly(ethylene oxide)–titania (PEO–TiO 2) organic–inorganic hybrid films were synthesized through a sol–gel process and spin coating. The mixed precursor sols were aged for 6 and 12 h, respectively, prior to the spin coating. Hybrid films were crystallized to the anatase phase first and then further heat treated at elevated temperatures for different time periods to analyze the anatase-to-rutile phase transformation. Quantitative X-ray diffraction (Q-XRD) results were employed for Johnson–Mehl–Avrami (JMA) kinetic analyses and the Avrami exponent ( n) was determined for each film. The Arrhenius plots were created based on the JMA plots and the activation energy ( Q) values for the phase transformation were determined for each film. The difference in the phase transformation kinetics in the films was discussed based upon the difference in the activation energy values and thus the molecular structures of each hybrid film. Nanoporous and nanocrystalline anatase films were successfully achieved through the retarded phase transformation during the pyrolysis of the organic–inorganic hybrid films. Owing to the maximized surface area these nano-structured anatase films are highly expected to show the enhanced photocatalytic efficiency compared to common TiO 2 films as well as sintered bulk forms.

Electrolytic deposition of titania films as interference coatings on biomedical implants: Microstructure, chemistry and nano-mechanical properties

TiO 2 films with uniform thickness were electrolytically deposited on AISI 316L stainless steel and Ti6Al4V substrates for potential use as color coded biocompatible coatings on biomedical implants. Deposition occurred via a peroxoprecursor method from solutions containing TiCl 4 and H 2O 2. By optimizing electrolyte formulation and deposition parameters, thin stoichiometric titania films with almost uniform thickness-dependent interference colors, similar as known from the color anodization processes of Ti-alloys, were obtained. Crack-free films were found up to 140 nm on AISI 316L and up to 190 nm on Ti6Al4V substrates. After thermal annealing at 450 °C of as-deposited amorphous peroxotitanium hydrate films, Raman and transmission electron microscopy showed highly stoichiometric, nanocrystalline anatase films. Chemical depth profiling was performed by glow-discharge optical emission spectrometry (GD-OES), showing clearly a densification and loss of water during annealing. On AISI 316L, GD-OES revealed stoichiometric TiO 2 films containing a small Fe (3–4 at.%) and Cr (1 at.%) contamination due to thermal diffusion from the substrate. On Ti6Al4V, the comparison between electrolytic TiO 2 films and color-anodization in different sulfuric and phosphoric acid containing electrolytes showed significant higher purity of electrolytic films, absent of V, Al, S, P contaminations as they were found in anodic oxides (4–6 at.% Al, 1–2 at.% V), especially V and S being problematic in biomedical applications. Annealing greatly increased the mechanical properties of the green films. A nano-hardness of 5.5–6.6 GPa, elastic modules close to substrate modules, excellent adhesion and very ductile behavior were found from nanoindentation and scratch tests. Based on thickness uniformity, high purity and good mechanical properties, electrolytic TiO 2 films are not only attractive as biocompatible colored coatings on non-anodizable biomedical alloys such as AISI 316L and CoCrMo, but also for Ti-alloys that are anodized for protective as well as coding reasons prior to implantation.

Photocatalytic activity of nanocrystalline TiO 2-based films produced by ESAVD method

Nanocrystalline TiO 2-based films have been deposited using a novel and cost-effective Electrostatic Spray Assisted Vapour Deposition (ESAVD) method for photocatalysis applications. The deposited films mainly have anatase phase with very small amounts of rutile or brookite. This paper reports the photocatalytic activity of the nanostructured TiO 2-based films under solar light irradiation. The influence of PEG additive and Fe doping on the film photocatalytic efficiency will be presented, as well as the structural changes with the additive and dopant. The results demonstrate that ESAVD process can produce nanocrystalline anatase films with good photocatalytic activity, and further enhancement of photocatalytic efficiency can be achieved by introducing appropriate Fe doping.

Hydrothermal processing of nanocrystalline anatase films from tetraethylammonium hydroxide peptized titania sols

Nanocrystalline anatase films were obtained by hydrothermally treating dip-coated TiO 2 gel films onto silicon substrates from tetraethylammonium hydroxide (TENOH) peptized titania aqueous sols diluted with different solvents (isoproponol, acetone and propanediol) at 240°C for 2 h. SEM, AFM and XRD analysis were used to characterize the microstructure and phase evolution of the films. The highest degree of homogeneity and good wettability towards the substrate could only be obtained with mixed solvents of acetone and isopropanol, yielding denser structure with homogeneously distributed fine grains among all the three solvents. A uniform distribution of fine grains of around 50 nm was obtained after hydrothermal treatment, while the film calcined at 500°C/2 h displayed larger grains of around 80 nm and were not so uniformly developed. All these results suggest that (a) TENOH peptized titania sol is a promising novel stock precursor for synthesizing TiO 2 films; (b) hydrothermal technique is quite efficient in preparing titania nanocrystalline films compared with calcination method.

Dye Sensitization of Natural Anatase Crystals with a Ruthenium-Based Dye

The sensitization of natural semiconducting anatase crystals from two different locations was investigated using a ruthenium-based dye (cis-di(thiocyanato)-bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II), also known as N3). A maximum incident-photon to sensitized photocurrent conversion efficiency of was measured. The crystal face dependence of the sensitization efficiency was also investigated as well as the adsorption kinetics for N3. The efficiency of sensitization of the (101) face was approximately one order of magnitude higher than for the (001) face. Various surface binding geometries for N3 on the two anatase crystal surfaces are discussed. Sensitization yields that varied by a factor of four were also observed on (101) faces depending on the identity of the crystal. Examination of the morphology of the crystal surfaces with atomic force microscopy and scanning electron microscopy indicated that differences in sensitization efficiency on different faces and crystals were not solely the result of differences in microscopic surface areas. The rise time for sensitized photocurrents decreased with higher light intensities and higher N3 coverages suggesting that trap states, associated with trapping and detrapping of injected electrons, can be saturated. The comparison of the sensitization behavior between anatase single crystals (natural and synthetic) and nanocrystalline anatase films revealed that the absorbed photon-to-current efficiency of the nanocrystalline films is approximately three to seven times greater than on single crystals. © 2002 The Electrochemical Society. All rights reserved.

Synthesis of nanocrystalline titania films by micro-arc oxidation

Nanocrystalline titania films were synthesized by micro-arc oxidation of titanium substrates in an electrolytic solution using a pulsed power supply. X-ray diffraction (XRD) indicated that the deposited films consisted of a high crystalline anatase phase. Field emission scanning electron microscopy (FESEM) showed that the films were macro-porous with 1–2 μm pores and the matrix was composed of 10–20 nm grains. The adhesion–tension test showed that the films had an adhesive strength of 20±2MPa. Such firmly adhesive, porous and nanocrystalline anatase films are expected to have significant applications as orthopaedic/dental implants and catalysts.

Redox targeting of oligonucleotides anchored to nanocrystalline TiO2 films for DNA detection.

DNA immobilization on mesoporous nanocrystalline anatase films produces a striking enhancement of the oxidation current, which is a result of targeting guanine by a ruthenium-based redox mediator. The amperometric response can be used to monitor hybridization, rendering this system attractive for DNA chips.

Lithium insertion into self-organized mesoscopic TiO 2 (anatase) electrodes

Self-organized nanocrystalline anatase films were prepared by a sol–gel technique with subsequent hydrothermal growth of crystals in basic solutions. Hydrothermal treatment at 190–230°C leads to the formation of compact films with narrow pore size distribution. Materials prepared at higher temperatures show more open structure with lower specific surface area. The electrochemical activity of the self-organized electrodes decreases with decreasing specific surface area. Ordered electrodes showed higher insertion capacity by ca. 15% compared to those of non-ordered electrodes with the same specific surface area. Insertion coefficients, x, exceeding 0.5 were observed for very thin (0.2 mg/cm 2) self-organized films with specific surface area higher than 60 m 2/g. However, these values were not reproduced for thicker films. The insertion rate depends on the inserted charge with a maximum at x∼0.1. It indicates that only a part of the total electrode–electrolyte interface is available for charge transfer reaction. The total insertion capacity is therefore affected by propagation of the electrochemically active surface within the film.

Characterization by resonance Raman spectroscopy of sol–gel TiO 2 films sensitized by the Ru(PPh 3) 2(dcbipy)Cl 2 complex for solar cells application

The development of TiO 2 photoelectrochemical cells is largely dependent on the elaboration of efficient dyes molecules to sensitize the nanocrystalline titania surfaces. In the present work, a new system functioning on this basis is proposed: an Ru(II) complex bearing two triphenylphosphine (PPh 3) and one 2,2′-bipyridyl-4,4′-dicarboxylate (dcbipy) chromophoric ligands, deposited on TiO 2 prepared through a simple sol–gel process is tested in a wet regenerative photoelectrochemical cell. The grafting of the complex on TiO 2 surface is characterized using Raman spectroscopy. The measurement of the full-width at half-maximum (FWHM) of the TiO 2 lower frequency E g Raman line allowed to compare the crystallite sizes of the sol–gel films (13–14 nm) with those of nanocrystalline anatase films (7–8 nm). Very intense Raman bands were observed (ex situ and in situ) for the Ru(PPh 3) 2(dcbipy)Cl 2 complex chemisorbed on TiO 2. The most important vibrations were unambiguously assigned to the PPh 3 or to dcbipy ligands. Altering the potential applied to the electrode for a given laser excitation energy can selectively enhance the vibrational modes of PPh 3. A reversible shift of the dcbipy Raman lines was also observed. The electronic absorption spectrum and the electrochemical data are taken into account in order to explain the obtained results for the chemisorbed dye on both nanocrystalline and sol–gel films.

Fast Electrochromic Switching with Nanocrystalline Oxide Semiconductor Films

Forward biasing of transparent nanocrystalline (anatase) films in lithium ion‐containing organic electrolytes leads to rapid and reversible coloration due to electron accumulation and Li+ intercalation in the anatase lattice. Absorption of >90% light throughout the visible and near IR can be switched on and off within a few seconds. The nanocrystalline morphology of the film plays a primary role in enhancing the electrochromic process.