Nanocrystalline Titania Films Research Articles

Photodegradation of Basic Blue by highly efficient nanocrystalline titania films

Transparent nanocrystalline titania films have been deposited on glass slides by using sol–gel procedures carried out in the presence of surfactant Triton X-100. Films were calcined at 550°C to ensure destruction of all organic residues but they still retained structures that consist of small nanoparticles and very high active surface areas, larger than expected for such high heating temperatures. Characterization of the films has been made by various techniques. These films are very efficient for photodegradation of the dye Basic Blue, either in air or in aqueous solutions.

Electron Transfer Dynamics in Quantum Dot/Titanium Dioxide Composites Formed by in Situ Chemical Bath Deposition

We present experimental transient absorption (TA) results on the dynamics of electron relaxation and electron transfer in cadmium sulfide quantum dots (QDs) grown by chemical bath deposition techniques on nanocrystalline oxide substrates. The quantum dots are prepared in situ in nanocrystalline titanium dioxide (TiO2) and zirconium oxide (ZrO2) films. The conduction band offset between the CdS QDs and TiO2 allows for efficient electron injection from the photoexcited QDs into the conduction band of TiO2. An unprecedented peak is seen in the transient absorption spectrum, which may be used to track the spectral response of the QD/TiO2 composites. Dynamic measurements in the visible and mid-IR are used to evaluate the time scale of the electron injection process. The TA dynamics for these systems are found to be multiexponential. A comparison of the TA dynamics for CdS/TiO2 and CdS/ZrO2 composites in the visible and mid-IR region indicates that electron transfer occurs on the time scale of ∼10−50 ps.

Investigations of Dye-Sensitised Titania Solar Cell Electrode Using Confocal Laser Scanning Microscopy

We have characterised a dye-sensitised nanoporous nanocrystalline titania film used in prototype photoelectrochemical solar cell production. The film was sensitised with two types of ruthenium (II) based dyes for different times, between 0.5 and 24 hours. The penetration and coverage for each type of dye was studied using Confocal Laser Scanning Microscopy (CLSM). Varying laser powers were applied and fluorescence images from the film have been analysed. Both dyes were found to percolate through the whole of the nanoporous film irrespective of the dyeing time but the amount of dye increased with dyeing time. The CLSM is shown to be a valuable tool for the investigation of dye penetration and coverage in porous films.

Engineering the nanocrystalline structure of TiO2 films by aerodynamically filtered cluster deposition

We have produced nanocrystalline titanium dioxide films with different structures (anatase or rutile) by depositing mass selected clusters from the gas phase. Nanoparticles are produced by a pulsed microplasma cluster source and are selected by aerodynamic separation effects. We have characterized nanocrystalline films by Raman spectromicroscopy and transmission electron microscopy, showing that the films assembled with very small clusters have a predominant rutile phase, whereas larger clusters form films with anatase structure. Our observations suggest that phonon confinement effects are responsible for a significant shift and broadening observed for the Raman peaks.

Characterization of a commercial dye-sensitised titania solar cell electrode

We have characterised a dye-sensitised nanoporous nanocrystalline titania film used in prototype photoelectrochemical solar cell production. From transmission electron microscopy the particles were seen as mixtures of tetrahedral and rhombohedral geometries with size distribution in the range between 10 and 25 nm. These particles were identified by X-ray diffraction as nanocrystals of anatase and brookite phases. The film was sensitised with a ruthenium (II) based chromophore for different times (between 0.5 and 24 h) and the penetration and coverage of the dye was studied using secondary ion mass spectroscopy. The dye was found to percolate through the whole of the titania film and was distributed uniformly. Using Rutherford backscattering, the composition of the film was determined and found to be 1 wt% Ru on maximum sensitisation. The optical properties of the dye-sensitised films were also measured which resulted in an increase of absorbance and a decrease of transmittance for dyeing times up to 8 h. Beyond this time the values remained unchanged and thus a semi-transparent film with luminous transmittance between 0.12 and 0.60 were obtained.

Preparation of nanocrystalline titania films by pulsed laser deposition at room temperature

Titanium oxide (titania) nanoparticles/thin films were deposited on a non-heated substrate by the ablation of a rutile TiO 2 target in Ar atmosphere using an ArF excimer laser under off-axis configuration. The ambient pressure was changed to evaluate its influence on the morphology and crystal structure of the deposited titanium oxide. The ablated species tend to form thin film or nanoparticles embedded in the thin film at low growth pressure while moderate pressure favors the formation of monodispersed nanoparticles. Transmission electron microscopic (TEM) results showed that an increase of Ar pressure from 13.3 to 133 Pa leads to an increase in mean diameter of the primary nanoparticle from 5.9 to 32 nm and reached a maximum. The further increase of the growth pressure leads to a decrease in mean diameter of the primary nanoparticles. The selected area electron diffraction (SAED) rings and X-ray diffraction (XRD) peaks were observed for the titania prepared in the Ar pressure range of 133 Pa and 1.06 kPa at room temperature. Under the pressure range higher than 1.06 kPa or lower than 133 Pa the products were less-oxidized amorphous phase, TiO 2− x .

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.

Near-infrared sensitization solar cell with the electrode of aluminium phthalocyanine adsorbed on nanocrystalline titanium dioxide film

The dye-sensitized solar cell using visible and near-infrared sensitization of nanocrystalline TiO 2 films by aluminium phthalocyanine, aluminium 2,9,16,23-tetraphenoxy-29H, 31H-phthalocyanine hydroxide (AlTPPc), was developed and its photoelectrochemical properties were investigated. The short-circuit photocurrent (ISC) was 0.026 mA cm-2, the open-circuit photovoltage ( V OC) was 186 mV, and the fill factor (FF) of solar cell using AlTPPc adsorbed on nanocrystalline TiO 2 film electrode was estimated to be 40.4%, respectively. By using AlTPPc adsorbed on nanocrystalline TiO 2 film electrode, photovoltage and photocurrent were higher compared with that of only nanocrystalline TiO 2 film electrode and were maintained under near-infrared irradiation. Thus, the solar cell using the near-infrared sensitization by AlTPPc adsorbed on nanocrystalline TiO 2 film electrode was developed.

Porous nanocrystalline titania films by plasma electrolytic oxidation

Nanocrystalline titania films were prepared by plasma electrolytic oxidation of a titanium alloy at 200–350 V in a Na 2CO 3 electrolytic solution using a pulsed power supply. XRD, EDS and Field Emission (FE) SEM were employed to characterize the phase, composition, and microstructure of the films. Vickers indentation, nanoindentation and adhesion–tension test were used to evaluate the mechanical properties of the films. The phase, pore size and thickness of the films strongly depend on the applied voltage consistent with the previous reports. The films prepared at 350 V were porous with ∼1 μm pores and the pore walls were composed of 10–20 nm rutile crystallites. The hardness, Young's modulus and bond strength of the film were 0.9±0.2 GPa, 32±4 GPa and 37±3 MPa, respectively. The film exhibited a significant plasticity and ductility compared to the conventional coarse-grained titania ceramics.

Charge-transfer studies of iron cyano compounds bound to nanocrystalline TiO(2) surfaces.

Nanocrystalline (anatase) titanium dioxide films have been sensitized to visible light with K(4)[Fe(CN)(6)] and Na(2)[Fe(LL)(CN)(4)], where LL = bpy (2,2'-bipyridine), dmb (4,4'-dimethyl-2,2'-bipyridine), or dpb (4,4'-diphenyl-2,2'-bipyridine). Coordination of Fe(CN)(6)(4-) to the TiO(2) surface results in the appearance of a broad absorption band (fwhm approximately 8200 cm(-1)) centered at 23800 +/- 400 cm(-1) assigned to an Fe(II)-->TiO(2) metal-to-particle charge-transfer (MPCT) band. The absorption spectra of Fe(LL)(CN)(4)(2-) compounds anchored to TiO(2) are well modeled by a sum of metal-to-ligand charge-transfer (MLCT) bands and a MPCT band. Pulsed light excitation (417 or 532 nm, approximately 8 ns fwhm, approximately 2-15 mJ/pulse) results in the immediate appearance of absorption difference spectra assigned to an interfacial charge separated state [TiO(2)(e(-)), Fe(III)], k(inj) > 10(8) s(-1). Charge recombination is well described by a second-order equal concentration kinetic model and requires milliseconds for completion. A model is proposed wherein sensitization of Fe(LL)(CN)(4)(2-)/TiO(2) occurs by MPCT and MLCT pathways, the quantum yield for the latter being dependent on environment. The solvatochromism of the materials allows the reorganization energies associated with charge transfer to be quantified. The photocurrent efficiencies of the sensitized materials are also reported.

Study of anatase to rutile phase transition in nanocrystalline titania films

Nanocrystalline titania films were prepared by a complexing agent-assisted sol-gel dip-coating process. The effect of acetylacetone, diethanolamine and polyethylene glycol on the structure of the heat-treated titania films was examined by Raman and FTIR spectroscopy and X-ray diffraction. The effect the complexing agents have on the anatase to rutile phase transition during the heat treatment process is studied. The understanding of this effect is expected to enhance our capacity to tailor the composition and morphology of films and thus their properties. The Raman and the infrared spectra of nanocrystalline titania films and the changes induced by the heat treatment were also investigated. We have also studied the size of the crystallites in TiO2 films and its dependence on the type of complexing agent used.

Study of the phase transition and the thermal nitridation of nanocrystalline sol–gel titania films

Nanocrystalline titania films were prepared by a complexing agent-assisted sol–gel method and converted to titanium nitride by a thermal nitridation process. The effect of acetylacetone (AcAc), diethanolamine (DEA) and acid catalysts (HCl and HNO 3) on the structure and morphology of the heat-treated titania films and on their nitridation products was examined by FTIR spectroscopy, X-ray diffraction (XRD) and atomic force microscopy (AFM). The carbothermal reduction of titania during the nitridation process with the formation of carboxynitrides has been considered. The results showed that the oxide to nitride transition strongly depends on the complexing agent used to prepare the titania films. The XRD results indicated the dependence of the lattice parameter of the nitridation product on the complexing agent or acid catalyst: AcAc and DEA lead to TiN x with a lattice parameter α close to the theoretical value, while with HCl the lattice parameter was found sensibly lower showing the presence of an oxynitride. These results are accounted for by the effect of complexing agents and acid catalysts on the size of both TiO 2 and TiN grains and the different reactivity of the anatase and rutile phases. The possibility of tailoring the composition and morphology of TiN films by using complexing agents is envisaged.

IR Spectroscopy of Surface Water and Hydroxyl Species on Nanocrystalline TiO2 Films

The speciation of water and hydroxyl groups bound to the surface of a nanocrystalline titania film has been investigated by in-situ infrared spectroscopy as a function of temperature. Calibration of the absorbance of the δ(H2O) mode at 1625 cm-1 by thermogravimetry has enabled an estimation of the concentration of surface H2O present during thermal dehydration of the films, which varied from 5 to 0.65 molecules per nm2 over the temperature range 27−150 °C. Two types of coordinated H2O and both terminally bound and bridging hydroxyls have been identified by the temperature-dependent behavior of their corresponding O−H stretching modes. Hydrogen bonding was observed between coordinated H2O and terminally bound hydroxyls (ν(OH) = 3730 cm-1), whereas bridging hydroxyls (ν(OH) = 3670 cm-1) do not appear to be affected by similar H-bonding.

Functionalising nanocrystalline TiO2 films: dye sensitised solar cells and optical biosensors

The fabrication of porous, nanocrystalline titanium dioxide (TiO2) films is considered first, and their application as dye sensitised photovoltaic devices is summarised. The electron transfer and transport dynamics within such devices is then discussed in more detail, focusing on electron transfer between the TiO2 and the molecular dyes, and the role of intra band trap states in the TiO2. The authors conclude by introducing a new potential application of such nanocrystalline TiO2 films, as a substrate for enzyme immobilisation in bioanalytical devices.

Nanocrystalline Titania Films and Particles by Chemical Vapor Synthesis

Nanocrystalline Titania Films and Particles by Chemical Vapor Synthesis

Nanocrystalline Titania Films and Particles by Chemical Vapor Synthesis

Thick crystalline titania films with grain sizes below 30 nm, as well as nanocrystalline titania particles with sizes below 10 nm, are prepared by a modified CVD method called chemical vapor synthesis (CVS). Pyrolysis of titanium tetraisopropoxide (TTIP) is performed in a hot-wall reactor, in a mixed helium/oxygen atmosphere, using liquid precursor delivery. Film thicknesses of up to 150 μm on silicon (100) substrate materials are achieved. The influence of process parameters (particularly reactor position) on the microstructure and growth rate is examined. Process temperatures, starting at film-forming (CVD) conditions and going up to exclusive particle formation (CVS) are investigated. Grain size, particle size, and crystallographic phases are determined by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Electron Injection and Recombination in Dye Sensitized Nanocrystalline Titanium Dioxide Films: A Comparison of Ruthenium Bipyridyl and Porphyrin Sensitizer Dyes

This paper is concerned with the parameters influencing the interfacial electron transfer kinetics, and therefore the sensitizing efficiency, for different sensitizer dyes adsorbed to nanocrystalline titanium dioxide films. We consider three sensitizer dyes: Ru(2,2‘-bipyridyl-4,4‘-dicarboxylate)2-cis-(NCS)2 (Ru(dcbpy)2(NCS)2) and zinc and free base tetracarboxyphenyl porphyrins (ZnTCPP & H2TCPP). These dyes were selected as they exhibit large differences in their oxidation potentials and photophysics, while retaining similar carboxylate groups for binding to the TiO2 surface. For example, whereas the photophysics of Ru(dcbpy)2(NCS)2 in solution is dominated by ultrafast ( 1 ns) π* singlet excited states and only weak singlet/triplet mixing. The ground and excited-state oxidation potentials also differ by up to ...

Dynamics of Electron Injection in Nanocrystalline Titanium Dioxide Films Sensitized with [Ru(4,4‘-dicarboxy-2,2‘-bipyridine)2(NCS)2] by Infrared Transient Absorption

We have used femtosecond pump−probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4‘-dicarboxy-2,2‘-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge-transfer absorption peaks and probing the absorption by injected electrons in the TiO2 at 1.52 μm and in the range of 4.1−7.0 μm, we have directly observed the arrival of electrons injected into the TiO2 film. Our measurements indicate an instrument-limited ∼50 fs upper limit on the electron injection time. We have compared the infrared transient absorption for noninjecting systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2 and found no indication of electron injection at probe wavelengths in the mid-IR (4.1−7.0 μm).

Vibrational Spectroscopic Study of the Coordination of (2,2‘-Bipyridyl-4,4‘-dicarboxylic acid)ruthenium(II) Complexes to the Surface of Nanocrystalline Titania

The coordination of photosensitizing Ru(II) dyes to a nanocrystalline titania film, as employed in the Gratzel solar cell, has been examined by vibrational spectroscopy. The major infrared bands of the adsorbed dyes have been assigned by comparison with spectra (IR and Raman) of the parent dye molecules, and suggest a bidentate chelate or bridging coordination to the TiO2 surface via two carboxylate groups per dye molecule.

Charge Recombination Kinetics in Dye-Sensitized Nanocrystalline Titanium Dioxide Films under Externally Applied Bias

The kinetics of charge recombination were determined following low intensity optical excitation of RuII(2,2‘-bipyridyl-4,4‘dicarboxylate)2(NCS)2 sensitized nanocrystalline TiO2 films as a function of externally applied bias voltage V. These kinetics were found to be multiexponential. The median time (t50%) for charge recombination was found to depend strongly upon bias voltage, reducing from t50% ∼ 200 ms for V = 0 V versus Ag/AgCl to < 30 ns for V ≤ −500 mV. This reduction in t50% was found to follow the equation t50% ∝ exp(qV/1.7kBT). These results are consistent with the charge recombination kinetics being strongly dependent upon the accumulation of electrons in intraband/conduction band states of the TiO2 film. For biases V ≤ −400 mV, the rate of charge recombination was found to exceed the rate of rereduction of the dye cation by an I-/I3- redox couple in solution.