Anatase TiO2 Films Research Articles

TiO2 films prepared using plasma ion assisted deposition for photocatalytic application

Anatase TiO2 films were synthesized on glass substrates using a plasma ion assisted deposition (PIAD) without external heating or subsequent annealing. The low temperature PIAD process produced crystalline anatase films with increased hardness and Young’s modulus, reduced film surface roughness and improved optical properties compared with those deposited without ion assistance, as well as a possibility to be applied to a wider range of substrates. Photocatalytic characterization showed that significant increases in photocatalytic stability and efficiency were achieved after using the PIAD process, although surface roughness and porosity of the films decreased slightly.

Electronic and optical properties of TiO2 and its polymorphs by Z-scan method

TiO2 is a material which has attracted considerable attention from the scientific community for its innumerable properties. TiO2 is known to exist in nature in three different crystalline structures: rutile, anatase, and brookite. Anatase and rutile TiO2 films have been widely characterized for their potential applications in solar cells, self-cleaning coatings, and photocatalysis. In the present report, the third-order nonlinear susceptibilities of TiO2 and its polymorphs, anatase, and rutile, prepared by the sol—gel technique followed by heat treatment are investigated using the Z-scan technique at a wavelength of 532 nm with a duration of 7 ns. Imaginary and real values of χ(3) for amorphous, anatase, and rutile are also calculated and found to be 5 × 10−19 m2/V2, 27 × 10−19 m2/V2, 19 × 10−19 m2/V2, respectively. It is found that the values of the optical constants of amorphous TiO2 after heat treatment vary considerably. It is assumed that this could be due to the variation in the electronic structure of TiO2 synchronous with the formation of its polymorphs, anatase, and rutile. Amorphous TiO2 is marked by the localization of the tail states near the band gap, whereas its crystalline counterparts are characterized by completely delocalized tail states.

Photocatalytic activity of anatase and rutile TiO2 epitaxial thin film grown by pulsed laser deposition

Epitaxial rutile-TiO2(011) and anatase-TiO2(001) films have been grown by pulsed laser deposition on Al2O3(1¯102) and LaAlO3(100), respectively. For low growth rates of ~0.0012nm/s and growth temperatures of 600°C high quality films are obtained. In particular the anatase films exhibit 100nm wide flat terraces for 10–15nm film thickness. Atomic steps of both ~0.25 and ~0.5nm are measured, suggesting the existence of two different terminations, although only a 4×1 reconstruction is observed in reflection high-energy electron diffraction. The rutile films exhibit coherent twin domain boundaries, which is likely the reason for smaller terrace sizes compared to anatase films. Photocatalytic activity of the films is measured by decomposition of methyl orange solution under UV-irradiation. The anatase films are about twice as active as the rutile films. For both films the activity does not change significantly for films of 10nm and thicker. For rutile films the activity is also compared to that of single crystal rutile TiO2(011) samples. Both rutile-films and single crystals exhibit very similar activity, indicating the good quality of the films. Post-growth annealing of the films in air at 600°C decreases the photocatalytic activity of the films. On the other hand, reduction of thin films by vacuum annealing to 800°C does not change the photocatalytic activity of the films significantly compared to the as grown films. This is in contrast to single crystal rutile samples whose photocatalytic activity for the decomposition of methyl orange almost doubles after reduction by high-temperature annealing in vacuum compared to the stoichiometric bulk-samples. The differences between films and bulk samples may hint differences in defect formation due to the substrate interface.

Sol–gel derived nanocrystalline TiO2 thin films: A promising candidate for self-cleaning smart window applications

In the present work, anatase TiO2 films are prepared by sol–gel spin coating method. The structural and optical properties of the films have been studied at different post-annealing temperatures. The photocatalytic activity and electrochromic performance of the films are investigated. The films annealed at 400°C exhibit the highest photocatalytic activity with a rate constant of 4.56×10−3min−1. The electrochromic performance for the films annealed at 400°C expressed in terms of difference in optical density (ΔOD) at 550nm between coloured and bleached state is 0.5493. This combination of photocatalysis and electrochromism makes the sol–gel derived titania thin films as promising candidates for self-cleaning smart window applications.

Electrodeposition of TiO2 Using Ionic Liquids

We report on the electrodeposition of TiO2 upon transparent conducting oxide (ITO) by using a milder electrochemical bath based on room-temperature ionic liquids (1-ethyl-3-methyl imidazolium-bis(trifluoromethylsulfonyl)imide)) and titanium butoxide as a soluble precursor. By taking advantage of oxygen reduction which proceeds about 500 mV before titanium +IV reduction, wellcrystallized thin films of anatase TiO2 are tailored under potentiostatic control at −1.5 V (vs. AgCl/Ag) / 100◦C followed by a rapid post-annealing treatment at 500◦C in air.

Structural and optical properties of ITO/TiO2 anti-reflective films for solar cell applications.

Indium tin oxide (ITO) and titanium dioxide (TiO2) anti-reflective coatings (ARCs) were deposited on a (100) P-type monocrystalline Si substrate by a radio-frequency (RF) magnetron sputtering. Polycrystalline ITO and anatase TiO2 films were obtained at room temperature (RT). The thickness of ITO (60 to 64 nm) and TiO2 (55 to 60 nm) films was optimized, considering the optical response in the 400- to 1,000-nm wavelength range. The deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), and atomic force microscopy (AFM). The XRD analysis showed preferential orientation along (211) and (222) for ITO and (200) and (211) for TiO2 films. The XRD analysis showed that crystalline ITO/TiO2 films could be formed at RT. The crystallite strain measurements showed compressive strain for ITO and TiO2 films. The measured average optical reflectance was about 12% and 10% for the ITO and TiO2 ARCs, respectively.

Anatase TiO2 Films with Dominant {001} Facets Fabricated by Direct-Current Reactive Magnetron Sputtering at Room Temperature: Oxygen Defects and Enhanced Visible-Light Photocatalytic Behaviors

A TiO2 film with dominant anatase {001} facets is directly prepared by direct-current reactive magnetron sputtering at room temperature without using morphology-controlling agents. The formation mechanism of anatase TiO2 films with dominant {001} facets is explained by the competition between thermodynamics and ion impinging in the deposition process. The crystalline TiO2 film shows a superior photocatalytic efficiency for the degradation of Rhodamine B under UV-visible (λ > 250 nm) lights. Furthermore, a comparable photodegradation of Rhodamine B is also found on the TiO2 film surface by using visible (λ > 420 nm) lights. During film growth, the surface bombarded by high energy of ions yields plenty of oxygen defects, which can enhance the photocatalytic activity of the films irradiated under visible light.

Nanocrystalline Rutile Electron Extraction Layer Enables Low-Temperature Solution Processed Perovskite Photovoltaics with 13.7% Efficiency

We demonstrate low-temperature (70 °C) solution processing of TiO2/CH3NH3PbI3 based solar cells, resulting in impressive power conversion efficiency (PCE) of 13.7%. Along with the high efficiency, a strikingly high open circuit potential (VOC) of 1110 mV was realized using this low-temperature chemical bath deposition approach. To the best of our knowledge, this is so far the highest VOC value for solution-processed TiO2/CH3NH3PbI3 solar cells. We deposited a nanocrystalline TiO2 (rutile) hole-blocking layer on a fluorine-doped tin oxide (FTO) conducting glass substrate via hydrolysis of TiCl4 at 70 °C, forming the electron selective contact with the photoactive CH3NH3PbI3 film. We find that the nanocrystalline rutile TiO2 achieves a much better performance than a planar TiO2 (anatase) film prepared by high-temperature spin coating of TiCl4, which produces a much lower PCE of 3.7%. We attribute this to the formation of an intimate junction of large interfacial area between the nanocrystalline rutile TiO2 and the CH3NH3PbI3 layer, which is much more effective in extracting photogenerated electrons than the planar anatase film. Since the complete fabrication of the solar cell is carried out below 100 °C, this method can be easily extended to plastic substrates.

Electrical properties of anatase TiO2 films by atomic layer deposition and low annealing temperature

In this paper, the authors studied anatase TiO2 films, fabricated by using atomic layer deposition and postdeposition annealing (PDA). The as-grown TiO2 films were of high purity; the carbon and nitrogen contents were within the x-ray photoelectron spectroscopy detection limit of 3–5 at. %. The anatase TiO2 film fabricated by PDA at 500 °C in O2 had a very high dielectric constant of >30 and was of high quality because it exhibited no hysteresis at its flatband voltage (Vfb) and contained negligible defect charge. The positive Vfb shift of anatase TiO2 (0.08 V), caused by the bottom interface dipole at a TiO2/SiO2 interface, was much smaller than those of Al2O3 (0.72 V) and HfO2 (0.29 V). However, the maximum Vfb change of the anatase TiO2 was greater than those of HfO2 and HfSiOx because the TiO2 contained more oxygen than the other materials.

Rice-like brookite titania as an efficient scattering layer for nanosized anatase titania film-based dye-sensitized solar cells

Rice-like brookite TiO2 particles with high phase purity and ∼600 nm particle diameter were synthesized through a hydrothermal method, and then bilayer TiO2 film-based dye-sensitized solar cells (DSSCs) were fabricated by using photoanode consisting of nanosized anatase TiO2 film as underlayer and the above brookite TiO2 submicrometer particle film as overlayer. The effects of the brookite overlayer on the light scattering and photoelectrochemical performances of the anatase TiO2 film-based DSSCs were investigated by using UV–vis diffused reflectance absorption spectra (DRS), open-circuit voltage decay (OCVD) curves and electrochemical impedance spectra (EIS) measurements. It is found that the bilayer film-based DSSCs have much better light harvesting capability, higher open-circuit voltage and longer electron lifetimes than the single anatase TiO2 film-based one. After optimizing the thicknesses of underlayer and overlayer, the corresponding bilayer TiO2 film-based DSSC gives conversion efficiency up to 8.83%, with a 29% improvement in the efficiency as compared to that (6.82%) of the single film-based one under AM 1.5G one sun irradiation. The present results represent a clear advance towards efficient light scattering materials for the nanosized TiO2 film-based solar cells with low-cost and high efficiency.

Mechanism of insulator-to-metal transition in heavily Nb doped anatase TiO2

Heavily Nb-doped anatase TiO2 (TNO) thin films were prepared by pulsed dc magnetron sputtering using an Nb-doped TiO2 target. The as-grown films exhibit high resistivity whose resistance decreases by ∼2 × 104-fold upon vacuum annealing. The ∼40% Nb-doped anatase TiO2 film shows a low resistivity of 5.7 × 10−4 Ω cm and a high electron concentration of 3.07 × 1021 cm−3. Combined in situ x-ray photoelectron spectroscopy (XPS) measurement and density-functional theory (DFT) calculations show that oxygen interstitial (Oint) and Nb interstitial (Nbint) defect clusters introduce localized shallow p-type accepter states that trap the electrons and reduce the conductivity. These defect clusters can be eliminated by vacuum annealing which is companied by outward diffusion of Nb. As a result, the trapped electrons backfill the Ti sites which are delocalized to promote conductivity.

Effect of laser wavelength on phase and microstructure of TiO2 films prepared by laser chemical vapor deposition

Rutile and anatase TiO2 films were prepared by laser chemical vapor deposition using CO2 and Nd:YAG lasers. The effects of laser wavelength on the phase, orientation, and microstructure of these TiO2 films were investigated. Using a CO2 laser, single-phase rutile TiO2 films were obtained at 826–1225K. These films showed a (100) orientation and a dense structure. The highest deposition rate was 83μmh−1 at 1070K. Using a Nd:YAG laser, the phase of the TiO2 films changed from rutile to anatase with increasing deposition temperature from 852 to 1230K. The rutile TiO2 films showed a (100) orientation with a columnar structure, while the anatase TiO2 films exhibited a (001) orientation with a cauliflower-like structure. Using a Nd:YAG laser, the highest deposition rates for rutile and anatase TiO2 films were 142 and 40μmh−1, respectively.

Ab initio study on magnetism at TiO2/SrTiO3 interface

Using density functional theory calculations, we have studied the electronic structure and magnetic properties of the TiO2/SrTiO3 interface with oxygen vacancy. We find that the oxygen vacancy at the interface may be responsible for the unexpected ferromagnetism in pristine anatase TiO2 film grown on SrTiO3 substrate. Structural relaxation has a profound impact on the local magnetic moment. After relaxation, the system can transform from a low spin state into a high spin state with the increasing Ti–Ti bond length. Isolated oxygen vacancy produces a magnetic moment of 0.36μB. The magnetic moment mainly comes from the d-orbitals of Ti atoms adjacent to the oxygen vacancy. We also find that the two oxygen vacancies at the interface are always coupled ferromagnetically.

Thickness dependent optical and eosin-Y sensitized solar cells characteristics of nc-a TiO2 films

Herein, we report the thickness dependent optical and eosin-Y dye sensitized solar cell (DSSC) characteristics of nanocrystalline anatase (nc-a) TiO2 films deposited on indium tin oxide (ITO) substrates by using sol–gel dip-coating method with C12H28O4Ti (TTIP) (0.05 and 0.10M) as Ti-precursor. The as-deposited films were heated at 400°C/2h in air. The XRD and Raman spectroscopy characterization studies revealed the formation of pure single anatase TiO2 phase in resultant films. The HRTEM studies reflected uniform size distribution of spherical TiO2 nanoparticles (average size ∼20nm) with crystalline grains of diameters=15–20nm in a structured matrix. This is consistent with XRD results of crystallite size=10–25nm. A profound effect of variation of film thickness on optical and eosin-Y sensitized photovoltaic characteristics of nc-a TiO2 films is observed. With increasing the film thickness, the % optical transmission (T), absorption coefficient (α), refractive index (n) and optical conductivity (σopt) are found to be decreasing whereas, porosity ratio (% P) and photoconversion efficiency (η) are found to be increasing. The optical indirect band gap is found to be between 3.20 and 3.21eV. The better photovoltaic performance: open-circuit voltage of 0.554V, short circuit current of 1.291mA, fill factor of 41.57% and conversion efficiency (η) of 1.10% is obtained for eosin-Y DSSC using redox polyiodide (1M KOH+1M KI+0.5M iodine) electrolyte in actonitrile.

Rutile TiO2-based perovskite solar cells

A perovskite solar cell based on rutile TiO2 film was prepared and its photovoltaic performance was compared to an anatase TiO2-based perovskite solar cell. Rutile TiO2 nanoparticles with aspect ratio of 0.2 (20 nm wide and 100 nm long) were prepared by hydrolysis of TiCl4 at ambient temperature. Anatase TiO2 nanoparticles with diameter of about 50 nm were hydrothermally synthesized. The annealed rutile film showed porosity of 60.6%, while lower porosity of 49.1% was detected in the anatase TiO2 film. CH3NH3PbI3 perovskite was deposited on TiO2 film using either a one-step spin coating or two-step dipping method. 2,2′,7,7′-Tetrakis(N,N-p-dimethoxy-phenylamino)-9,9′-spirobifluorene (spiro-MeOTAD) was used as a hole transporting material. One-step deposition led to average power conversion efficiency (PCE) of 8.19% from the rutile-perovskite solar cells and 7.23% from the anatase-perovskite solar cells, while two-step deposition resulted in higher average PCE of 13.75% for the former device and 13.99% for the latter one. Regardless of the deposition methodologies, the rutile–perovskite solar cell showed generally higher Jsc and lower Voc. Slower electron transport and longer electron lifetime were observed for the rutile-based perovskite solar cell than for the anatase-based one. Although the same perovskite material was used for both rutile and anatase TiO2, the difference in electronic behavior indicates that photo-excited electrons are in part injected to TiO2 and the extent of electron injection can be influenced by the crystal phase of TiO2. Despite longer electron lifetime, the slightly lower voltage of the rutile-based device might be due to the fact that the amount of injected electrons was relatively larger for rutile than anatase, leading to a lower Fermi energy level at equilibrium between TiO2 and perovskite. Using a 260 nm-thick rutile TiO2 film, the highest PCE of 14.46% was achieved by depositing CH3NH3PbI3 using a two-step method, in which photocurrent density of 20.02 mA cm−2, open-circuit voltage of 1.022 V and fill factor of 0.71 were demonstrated.

Synthesis and characterization of mixed phase anatase TiO2 and sodium-doped TiO2(B) thin films by low pressure chemical vapour deposition (LPCVD)

Mixed phase anatase TiO2 and TiO2(B) thin films were synthesized using Low Pressure Chemical Vapour Deposition (LPCVD) onto glass substrates.

Atmospheric growth and strong visible luminescence of anatase titanium oxide films with various orientations

Oriented anatase TiO2 films with visible luminescence and various nano-morphologies have been grown by TiCl4–O2–Ar reactive plasma vapor deposition.

The Gas Nucleation Process Study of Anatase TiO2 in Atmospheric Non-Thermal Plasma Enhanced Chemical Vapor Deposition

The gas phase nucleation process of anatase TiO2 in atmospheric non-thermal plasma enhanced chemical vapor deposition is studied. The particles synthesized in the plasma gas phase at different power density were collected outside of the reactor. The structure of the collected particles has been investigated by field scanning electron microscope (FESEM), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The analysis shows that uniform crystalline nuclei with average size of several nanometers have been formed in the scale of micro second through this reactive atmospheric plasma gas process. The crystallinity of the nanoparticles increases with power density. The high density of crystalline nanonuclei in the plasma gas phase and the low gas temperature are beneficial to the fast deposition of the 3D porous anatase TiO2 film.

Electrical Characterization of Sol-Gel Derived TiO2Film on c-Si Substrate by Admittance Measurement

Transport and storage properties of sol-gel synthesized, namely, dip coating technique, titanium dioxide (TiO2) thin film over crystalline silicon (c-Si), has been investigated by means of current-voltage (I-V) and admittance analysis within different ambient. Considering the work function of anatase TiO2film, determined by both FTIR and TG/DTA analysis, silver (Ag) as front metal electrode was chosen to hinder a barrier for charge carriers. Electrical analysis implied that Ag/TiO2/c-Si structure was actually constituted by Ag/TiO2/native silicon dioxide (SiO2)/c-Si [SIS] structure, in which SiO2layer was identified by FTIR analysis. Consequently, the electrical features of the film were interpreted in terms of SIS diode that is capable of explaining C-V features.

TiO2 films prepared by DC reactive magnetron sputtering at room temperature: Phase control and photocatalytic properties

Precise control of the crystal phase of TiO2 by sputtering at room temperature is particularly important for practical applications. In this paper, nanocrystalline TiO2 films were deposited on glass and silicon substrates using DC reactive magnetron sputtering without external heating. The phase composition and preferred orientation of the TiO2 films could be controlled by adjusting the negative bias voltages. It was found that TiO2 films deposited at free negative bias voltage or ground exhibited an anatase single phase with a strong (101) preferred orientation, while the preferred orientation transformed into (211) when the negative bias voltage reached −50V. A further increase in the negative bias voltage induced the formation of an additional rutile phase, thus generating a mixed phase of anatase and rutile. A pure rutile phase was obtained at high negative bias voltage (≥−100V). The rutile phase with (101) preferred orientation could also be obtained by increasing the bias voltages above −125V. In addition, the TiO2 films of different phases performed different photocatalytic activities. The anatase TiO2 films showed higher photocatalytic activity than rutile phase films. Particularly, anatase TiO2 films with (211) preferred orientation showed a significant photocatalytic degradation of organic dye (Rhodamine B), the degradation of which reached over 90% after an irradiation time of 2h.