As-deposited TiO2 Films Research Articles

The formation mechanism of (001) preferred orientation for anatase TiO2 film prepared by DC pulsed magnetron sputtering

TiO2 films were deposited by DC pulsed magnetron sputtering with different sputtering power density (SPD). Then, as-deposited TiO2 films were annealed at 600 °C and the anatase TiO2 films were obtained. XPS results showed Ti/O atom ratio was kept at 1/(1.94 ± 0.04) for all as-deposited and annealed TiO2 films. XRD results showed that the annealed TiO2 films displayed obvious anatase (001) preferred orientation, which was deposited with SPD in 0.83 W/cm2. The cross-section TEM results showed that annealed TiO2 film with anatase (001) preferred orientation consisted of a small quantity of columnar crystals and a majority of equiaxed crystals, which [001] crystallographic axis was parallel to the normal direction of film. AFM results revealed that with SPD in 0.83 W/cm2, the dominated diffusion effect controlled TiO2 film growth process with Volmer-Weber mode, which contained the nano-size anatase crystal nuclei. The [001] crystal orientation of anatase crystal nuclei was uniform parallel to the normal direction of TiO2 film growth surface. During the annealing process, the nano-size anatase crystal nuclei grew and coalesced into the large anatase grains without the change of [001] crystal orientation, which resulted in TiO2 film with obvious (001) preferred orientation.

Studies on high refractive index amorphous TiO2 thin film for possible improvement of light extraction efficiency in organic light emitting diodes

Studies on low-cost sol–gel spin-coated TiO2 thin film with high refractive index (HRI), which may be used as an intermediate layer to enhance the light extraction efficiency (LEE) of a typical bottom-emitting organic light emitting diode (OLED), are reported. The TiO2 solution is prepared using titanium tetra iso-propoxide, acetic acid, and ethanol. Spin coating method is used to deposit TiO2 thin films on glass substrate. Different optical characterizations of as-deposited and annealed (150°C, 300°C, and 450°C) TiO2 thin films on glass substrate are done, from which different properties of the film are derived. Ellipsometric measurement shows shrinkage in the thickness of the as-deposited TiO2 films after annealing at different temperatures. X-ray diffraction reveals amorphous TiO2 formation for all the samples. The RI of the coated film increases with the increase in annealing temperatures. Its value at 633 nm wavelength for the as-deposited film is found to be 2.1, which is quite high and it is seen that this RI can be further increased to 2.78 by annealing the samples at 450°C. This value is comparatively high compared to several other reported values of other researchers. The as-deposited sample reveals highest porosity, which further decreases with rise in annealing temperature. Our calculation of LEE for a typical OLED with the intermediate layer of this HRI as-deposited TiO2 film shows improvement of the LEE. However for annealed films, the experimentally obtained thicknesses are not adequate for this improvement, but it is shown that by increasing the film thickness, further improvement of LEE is possible.

Effect of Annealing Temperature on Spatial Atomic Layer Deposited Titanium Oxide and Its Application in Perovskite Solar Cells.

In this study, spatial atomic layer deposition (sALD) is employed to prepare titanium dioxide (TiO2) thin films by using titanium tetraisopropoxide and water as metal and water precursors, respectively. The post-annealing temperature is varied to investigate its effect on the properties of the TiO2 films. The experimental results show that the sALD TiO2 has a similar deposition rate per cycle to other ALD processes using oxygen plasma or ozone oxidant, implying that the growth is limited by titanium tetraisopropoxide steric hindrance. The structure of the as-deposited sALD TiO2 films is amorphous and changes to polycrystalline anatase at the annealing temperature of 450 °C. All the sALD TiO2 films have a low absorption coefficient at the level of 10−3 cm−1 at wavelengths greater than 500 nm. The annealing temperatures of 550 °C are expected to have a high compactness, evaluated by the refractive index and x-ray photoelectron spectrometer measurements. Finally, the 550 °C-annealed sALD TiO2 film with a thickness of ~8 nm is applied to perovskite solar cells as a compact electron transport layer. The significantly enhanced open-circuit voltage and conversion efficiency demonstrate the great potential of the sALD TiO2 compact layer in perovskite solar cell applications.

Investigation of the Influences of Post-Thermal Annealing on Physical Properties of TiO2 Thin Films Deposited by RF Sputtering

For this study, titanium dioxide (TiO2) thin films were deposited on glass substrates at room temperature by the RF magnetron sputtering technique. The preparation parameters that offer better control and reproducibility of film fabrication were first optimized. Then, the effects of post-deposition annealing temperature at 350, 450, and 550°C on the microstructure, surface morphology, and optical properties of the prepared films were investigated using X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV–Visible (UV–Vis) spectrophotometry. Interestingly, XRD analysis shows that as-deposited and annealed TiO2 film possess anatase crystal structure only with a preferential orientation along the (101) plane. The intensity of the (101) diffraction peak and crystallite size are found to increase with increasing annealing temperature, which indicates an improvement in the crystallinity of the films. Raman spectra confirm that all samples possess anatase phase and the crystallinity is enhanced with increasing thermal annealing. From the analysis of SEM and AFM images, it is revealed that the heat treatment significantly affects the morphology, grain size, and surface roughness of the TiO2 films. The UV–Vis spectroscopy analysis shows that as-deposited TiO2 thin film is highly transparent in the visible region with an average transmittance about 84%, whereas the transmission decreases slightly with an increase in annealing temperature. Moreover, the optical band gap energy shows a red shift with increasing the annealing temperature.

A simple route to prepare anatase TiO2 film onto polyimide substrate by DC pulsed magnetron sputtering

TiO2 film was deposited onto polyimide substrate by DC pulsed magnetron sputtering with different sputtering power densities. XRD results showed that with increasing the sputtering power density from 0.83 W/cm2 to 5.00 W/cm2, the crystallinity of as-deposited TiO2 film improved monotonously. And 5.00 W/cm2 was determined as the critical sputtering power density in which the highest temperature the substrate could withstand. Then, XRD and TEM results showed that the as-deposited TiO2 film could have a better anatase structure by providing an auxiliary heating to the substrate at the highest temperature during TiO2 film deposition. Combining the sputtering parameters, as well as DC pulsed voltage waveform, the reason for as-deposited TiO2 film crystallization was investigated systematically.

High rate deposition of photoactive TiO2 films by hot hollow cathode

In this paper we present a plasma deposition technique that allows the reactive deposition of oxide layers with extremely high deposition rate. The new approach combines reactive sputtering by DC hollow cathode discharge with thermal evaporation from the hot surface of the hollow cathode. As an example of successful fast deposition, photoactive films of titanium dioxide (TiO2) with various thicknesses were deposited using this technique. The uncooled titanium nozzle served as a hot hollow cathode and simultaneously as an inert gas (Ar) inlet. The reactive gas (O2) was introduced into the vacuum chamber through a separate inlet. During deposition, the temperature of the titanium hollow cathode reached up to 1600 °C, depending on the discharge parameters. This made it possible to combine the ion sputtering of hot titanium cathode with its thermal surface evaporation, which significantly increased the TiO2 deposition rate. The highest achieved deposition rate was 567 nm/min (34 μm/h), which (with respect to the geometry of this process) corresponds to total volume of the deposited TiO2 material 1.2 mm3/min per 1 kW of absorbed power. Despite extremely high thermal flux to the substrate, TiO2 films were successfully deposited even on temperature-sensitive PET foil. The as-deposited and post-annealed TiO2 films prepared on fluorine doped tin oxide (FTO) substrates and glass were subject to further analyses including X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and photoelectrochemical (PEC) measurements. Whereas the as-deposited TiO2 films had an amorphous (or nearly amorphous) structure, which exhibited only weak photoactivity, after annealing their PEC activity increased by an order of magnitude.

Influence of the Anatase and Rutile phases on the luminescent properties of rare-earth-doped TiO2 films

For a long time, the development of new and more efficient light-emitting materials has been the driving force to investigate the presence of trivalent rare-earth (RE3+) ions into solid hosts. It happens because RE3+ ions provide efficient and well-defined optical transitions whose intensities are determined by phenomena regarding excitation−recombination mechanisms or, ultimately, by the RE3+ atomic surroundings. Motivated by these aspects, this paper presents a comprehensive study of the influence exerted by TiO2 (under the amorphous and crystalline forms) onto the light emission due to Sm3+ and Tm3+ ions. To this end, Sm-, Tm-, and SmTm-doped TiO2 films were prepared and their structural−electronic properties were investigated by optical spectroscopic techniques − following appropriate thermal annealing treatments. As a result of the deposition method and conditions, the as-deposited (stoichiometric) TiO2 films present an amorphous structure and single RE concentrations around 0.5 at.%. Also, whereas thermal annealing the films at increasing temperatures resulted in almost no changes in their elemental composition, extensive variations were observed in their atomic structure and RE3+-related light emission characteristics. Based on the experimental results, it became clear the role played by the amorphous or crystalline (Anatase or Rutile) structures of the TiO2 films onto the RE3+-related luminescence features, and the likely mechanisms behind the phenomenon are presented and discussed in detail.

Hydrogen-Accelerated Phase Transition and Diffusion in TiO2 Thin Films

TiO2 thin films have received attention as intermetallic diffusion barriers in hydrogen-permeable membranes used for the extraction of purified H2 from chemical reactors. We report the fabrication of thin (∼100 nm) TiO2 films on Si(111) substrates by pulsed laser deposition. Anatase and rutile phases are found to coexist in the as-deposited TiO2 films by X-ray diffraction. Upon H2 annealing at 773–973 K, the phase transformation from anatase to rutile occurs and completes at a strongly (100–380 K) reduced temperature as compared to bulk TiO2. Atomic force microscopy shows that the surface roughness of the thin films increases only slightly (from 3 to 6 nm) in this H-accelerated phase transition. The hydrogen depth distributions in the TiO2 thin films are revealed with resonant 1H(15N,αγ)12C nuclear reaction analysis and evaluated in terms of a one-dimensional diffusion model, which allows the determination of the temperature-dependent diffusion coefficient of hydrogen as D(T) = (5.27 ± 0.89) × 10–8 exp(−0...

Semi-automatic spray pyrolysis deposition of thin, transparent, titania films as blocking layers for dye-sensitized and perovskite solar cells.

For proper function of the negative electrode of dye-sensitized and perovskite solar cells, the deposition of a nonporous blocking film is required on the surface of F-doped SnO2 (FTO) glass substrates. Such a blocking film can minimise undesirable parasitic processes, for example, the back reaction of photoinjected electrons with the oxidized form of the redox mediator or with the hole-transporting medium can be avoided. In the present work, thin, transparent, blocking TiO2 films are prepared by semi-automatic spray pyrolysis of precursors consisting of titanium diisopropoxide bis(acetylacetonate) as the main component. The variation in the layer thickness of the sprayed films is achieved by varying the number of spray cycles. The parameters investigated in this work were deposition temperature (150, 300 and 450 °C), number of spray cycles (20–200), precursor composition (with/without deliberately added acetylacetone), concentration (0.05 and 0.2 M) and subsequent post-calcination at 500 °C. The photo-electrochemical properties were evaluated in aqueous electrolyte solution under UV irradiation. The blocking properties were tested by cyclic voltammetry with a model redox probe with a simple one-electron-transfer reaction. Semi-automatic spraying resulted in the formation of transparent, homogeneous, TiO2 films, and the technique allows for easy upscaling to large electrode areas. The deposition temperature of 450 °C was necessary for the fabrication of highly photoactive TiO2 films. The blocking properties of the as-deposited TiO2 films (at 450 °C) were impaired by post-calcination at 500 °C, but this problem could be addressed by increasing the number of spray cycles. The modification of the precursor by adding acetylacetone resulted in the fabrication of TiO2 films exhibiting perfect blocking properties that were not influenced by post-calcination. These results will surely find use in the fabrication of large-scale dye-sensitized and perovskite solar cells.

Effect of substrate type on the electrical and structural properties of TiO2 thin films deposited by reactive DC sputtering

Electrical and structural properties of TiO2 thin films deposited at room temperature by reactive DC sputtering have been investigated on three different substrates: high resistivity (>1000 Ω cm) float zone Si(1 1 1), float zone Si(1 0 0) and alkali free glass. As-deposited TiO2 films on glass substrate showed extremely high resistivity of (∼5.5 × 103 Ω cm). In contrast, lower resistivities of ∼2 Ω cm and ∼5 Ω cm were obtained for films on Si(1 1 1) and Si(1 0 0), respectively. The as-deposited films were found to be oxygen-rich amorphous TiO2 for all the substrates as evidenced by X-ray photoemission spectroscopy and X-ray diffraction. Subsequent annealing led to appearance of anatase TiO2 on Si but not on glass. The surface of as-deposited TiO2 on Si was found to be rougher than that on glass. These results suggest that the big difference of electrical resistivity of TiO2 would be related with existence of more anatase nuclei forming on crystalline substrates, which is consistent with the theory of charged clusters that smaller clusters tend to adopt the substrate structure.

Laser induced photocatalytic activity enhancement of TiO2 thin films.

In this paper, the photocatalytic activity enhancement of TiO2 thin films was realized by laser irradiation. The H2 yield of the as-irradiated film is 79 μmol/(h*m2), which is 33% more than that of the as-deposited TiO2 film. Spectrophotometer, X-ray diffraction and Raman system were employed to characterize the samples. The results showed that both the scanning rate and line spacing of the laser modification have effects on photocatalytic activity. It suggests that a phase junction is formed between the amorphous and rutile phases. The increment of H2 generation could be attributed to the alignment of Fermi levels in the phase junction.

Very thin thermally stable TiO2 blocking layers with enhanced electron transfer for solar cells

Very thin TiO2 blocking layers (BLs) are important components for achieving high solar power conversion efficiencies (PCEs) in the dye-sensitized solar cells, and particularly perovskite solar cells (PSCs). When reasonably thin, TiO2 BLs prevent recombination of photogenerated charges at the conductive fluorine-doped tin oxide (FTO) glass substrate used in these devices. However, all previous attempts to generate efficient TiO2 BLs have been hampered by an insufficient charge transfer rate at quasi-amorphous TiO2 and very low thermal stability, leading to the loss of blocking properties after thermal calcination. In this work, we report the deposition of homogenous very thin (∼30nm) TiO2 BLs by combining advanced high impulse power magnetron sputtering (HiPIMS) and additional bipolar medium-frequency (MF) magnetron co-sputtering. The as-deposited TiO2 films were shown to provide excellent blocking properties which were preserved even after thermal treatment at 450°C. Moreover, TiO2 BLs thermally treated at 450°C show a well-developed rutile structure and 70 times higher photocurrents compared to the as-deposited layers. This work opens possibilities for the utilization of very thin TiO2 layers in solar cell technologies providing a double mode of action: blocking functionality and efficient electron transport.

Surface Properties Contrast between Al Films and TiO2 Films Coated on Magnesium Alloys by Magnetron Sputtering

Al films and TiO2 films were separately coated on AZ31 magnesium alloy substrates by means of magnetron sputtering method. The surface properties of the samples were explored and compared. Scanning electron microscope (SEM) observed the compact structure characteristics of as-deposited Al films and TiO2 films. After heat treatment under 200oC for half an hour the films kept the compact structure characteristics and there didn’t exist structural defects on the surface of the films. Nano-indentation measurement results display that the micro-hardness of as-deposited Al film and TiO2 film reaches about 1.90 Gpa and 1.51 Gpa, separately. Al film is a bit harder than TiO2 film. Finally, the corrosion experiments in simulated body fluid initially indicate the different corrosion properties for Al film and TiO2 film. Al film presents more effective anti-corrosion properties than TiO2 film.

Deposition of photocatalytic anatase titanium dioxide films by atmospheric dielectric barrier discharge

In this study, TiO2 films are easily deposited in a single-step process from titanium tetraisopropoxide (TTIP) using a dielectric barrier discharge (DBD) device under atmospheric pressure at room temperature. The surface morphology of the as-deposited TiO2 films is observed by scanning electron microscopy (SEM). The as-deposited TiO2 films exhibit nanometer-sized particles with a lot of agglomerates. The cross section images indicate that the deposition rate of TiO2 films can reach about 70nm·min−1, which is much faster than that of conventional low pressure chemical vapor deposition (CVD) (typically <4nm·min−1). In order to confirm the structure composition and optical properties of the films, glancing incidence X-ray diffraction (GIXRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) are employed. The as-deposited TiO2 films are partly crystalline which show a good photocatalytic activity in the degradation of methylene blue under UV irradiation. After annealing at 673K, the TiO2 films are in the anatase phase and composed by homogeneous nanoparticles. The photocatalytic degradation rate of methylene blue of the TiO2 films after annealing (~0.6h−1) is almost 4 times higher than that of the as-deposited TiO2 films (~0.16h−1). The photocatalytic degradation rate of the TiO2 film after annealing is the same as the one of the commercial anatase TiO2 spin coated film when the error bar is taken into account. The TiO2 films show excellent photocatalytic stability during 3 recycling photocatalytic measurements.

The annealing temperature dependence of anatase TiO2 thin films prepared by the electron-beam evaporation method

In this paper, we report on titanium dioxide (TiO2) thin films deposited by an electron beam evaporation method on quartz glass substrates (15 × 15 × 2 mm3 in size), followed by post-annealing at 300 °C to 600 °C for an annealing time of up to 2 h. The substrate temperature during the film deposition was kept at 150 °C. The effect of post-growth thermal annealing on the structural and optical properties of TiO2 thin films were systematically studied as a function of annealing temperature. We found that the as-deposited TiO2 films are amorphous in structure, while the films started to crystallize into the anatase phase when annealed at temperatures ≥450 °C. An increase in annealing temperature results in a decrease of transmittance percentage and also in optical band gap energy. The refractive indices of the films were evaluated from the measured transmittance spectra using the envelope method. An increase in the refractive index with an increase of annealing temperature was observed.

Integrated Nano- and Macroscale Investigation of Photoinduced Hydrophilicity in TiO2 Thin Films.

The hydrophilicity of titanium dioxide has been investigated for films, deposited on glass by e-beam evaporation, being exposed to UV radiation and subjected to thermal annealing. The wettability alteration has been showed to depend upon both treatments, and insights into how to introduce more stable hydrophilicity into these films have been presented for the sake of boosting their commercial value. Observations from multiple length scales to assess the wetting behavior of as-deposited and high-temperature annealed samples were assessed through macroscopic measurements, i.e., water contact angle measurements, showing that the annealed crystalline samples, treated at 500 °C, are much more hydrophilic (SCA ≈ 20°) than as-deposited TiO2 films (SCA ≈ 90°), and the nanoscopic experiments performed by amplitude modulation (AM) atomic force microscopy (AFM) indicated that this increased hydrophilicity is related to an enhanced adhesion force and surface energy, resulting in the partial crystallization of TiO2 and the consequent formation of crystals on its surface rather than being related to morphologic differences. XRD and Raman measurements have highlighted that the crystallinity of the TiO2 film is crucial in determining its hydrophilicity, in good agreement with the AFM study. The results also indicated that, after irradiation, the samples treated at 500 °C preserve their hydrophilicity for a significant time compared to previous studies.

Amorphous TiO2 Compact Layers via ALD for Planar Halide Perovskite Photovoltaics.

A low-temperature (<120 °C) route to pinhole-free amorphous TiO2 compact layers may pave the way to more efficient, flexible, and stable inverted perovskite halide device designs. Toward this end, we utilize low-temperature thermal atomic layer deposition (ALD) to synthesize ultrathin (12 nm) compact TiO2 underlayers for planar halide perovskite PV. Although device performance with as-deposited TiO2 films is poor, we identify room-temperature UV-O3 treatment as a route to device efficiency comparable to crystalline TiO2 thin films synthesized by higher temperature methods. We further explore the chemical, physical, and interfacial properties that might explain the improved performance through X-ray diffraction, spectroscopic ellipsometry, Raman spectroscopy, and X-ray photoelectron spectroscopy. These findings challenge our intuition about effective electron selective layers as well as point the way to a greater selection of flexible substrates and more stable inverted device designs.

Preparation and Characterization of SiO&lt;sub&gt;2&lt;/sub&gt;-Coated TiO&lt;sub&gt;2&lt;/sub&gt; Thin Films via Sol-Gel Dip Coating Technique

In the present research, nanocrystalline TiO2 thin films have been prepared via a sol-gel method using Ti {OCH(CH3)2}4 act as a precursor. The morphological effect of sets of as-deposited and annealed samples (100, 300, 500 and 700°C for 1 hour) and thickness variation on the structural properties were investigated by Atomic Force Microscopy (AFM) measurements. The observation showed that the size of grain increases with film thickness, indicating that the size of the particles in the films can be effectively controlled by changing the temperature of thermal treatment. AFM topology images of the as-deposited and annealed TiO2 films at 100 and 300°C indicates the films are rather smooth, compact, bulky, not uniform and column-like structure. TiO2 thin films annealed at 500 and 700°C showed homogenous morphology surface with regular size of spherical particles.

Preparation and properties of ZrO2 and TiO2 films and their nanolaminates by atomic layer deposition

Nanolayers of ZrO2 and TiO2 and their nanolaminates were grown on silicon wafer by an atomic layer deposition process. Precursors of tetrakis (dimethylamido) zirconium and tetrakis (dimethylamido) titanium were used to deposit ZrO2 and TiO2 films at different temperatures, respectively. The XRD results show that ZrO2 films began to crystallize at the deposition temperature of 300°C, and the cubic phase was first formed when the thickness of the film reached 16.5nm, and then monoclinic phase appeared in the film when the thickness further increased. The as-deposited TiO2 films exhibited anatase phase at the deposition temperature range of 50–300°C. AFM morphologies indicate that both ZrO2 and TiO2 films were uniform and smooth. Nanolaminates of ZrO2 and TiO2 were obtained by deposition of ZrO2 and TiO2 layers at 300°C alternatively with precise layer thickness control. The results show that the nanolaminates exhibited a giant dielectric constant of 850 at low frequency which was much higher than the individual dielectric constants of ZrO2 and TiO2. The nanolaminates of ZrO2 and TiO2 with the giant dielectric constant have potential applications for high-k dielectric materials.

TiO2 thin films with rutile phase prepared by DC magnetron co-sputtering at room temperature: Effect of Cu incorporation

The thin films for pure TiO2 and that incorporated with Cu ion were deposited by DC magnetron co-sputtering with Ar gas. The crystal texture, surface morphology, energy gap and optical properties of the prepared films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectrometer (XPS), UV–vis spectrophotometer, and Raman spectroscopy. The results show that as-deposited TiO2 film mainly possesses anatase structure at room temperature with pure Ar gas, but the introduction of Cu can alter the phase structure of crystallite TiO2. XRD patterns and Raman spectra indicate that the Cu incorporation with high concentration (ACu/ATi+ACu≈20%) favors the formation of rutile phase. Moreover, the Cu incorporation into TiO2 lattice induces band gap narrowing. Band structures and density of states have been analyzed based on density functional theory (DFT) and periodic models in order to investigate the influence of the Cu incorporation on the electronic structure of TiO2. Both experimental data and electronic structure calculations evidence the fact that the change in film structure from the anatase to the rutile phase can be ascribed to the possible incorporation of Cu1+ in the sites previously occupied by Ti4+, and the presence of Cu results in important effect on the electronic states, which is mainly related to the 3d Cu orbitals in the gap and in the vicinity of the valence band edges for TiO2.