TiO2 Thin Films Research Articles

Enabling real-time detection of photocatalytic reactions by a re-useable micro-reactor

We present a highly sensitive and versatile Si-based μ-reactor for photocatalytic experiments. The μ-reactor can be operated in front side (through a borosilicate window) as well as backside (through the catalyst support) illumination. The μ-reactor is sealed by compressing the Si-based microchip, a parafilm gasket and the supported catalyst by four screws. This design allows for fast and reliable assembly of the μ-reactor and the microchip is re-usable for several independent experiments, making the presented μ-reactor ideal for catalyst screening studies. We analyze volatile reactants on-line by connecting the μ-reactor to a mass-spectrometer. The limiting capillary of the μ-reactor allows for nearly 100% collection efficiency (i.e. every produced molecule will be collected) by limiting the gas flow to the mass-spectrometer—enabling highly sensitive measurements and an absolute calibration. Finally, the small height of the reactor volume results in very fast response times, which make the reactor suitable for studies of transient activity phenomena—including catalyst activation and deactivation. We demonstrate these capabilities of the μ-reactor by performing CO oxidation experiments on a TiO2 thin film with subsequent calibration of the chip. Based on a mathematical model of the μ-reactor we additionally present a method for determining the residence time, the reaction volume and the working distance.

High Oxygen Sensitivity of TiO2 Thin Films Deposited by ALD.

The gas sensitivity and structural properties of TiO2 thin films deposited by plasma-enhanced atomic layer deposition (ALD) were examined in detail. The TiO2 thin films are deposited using Tetrakis(dimethylamido)titanium(IV) and oxygen plasma at 300 °C on SiO2 substrates followed by annealing at temperatures of 800 °C. Gas sensitivity under exposure to O2 within the temperature range from 30 °C to 700 °C was studied. The ALD-deposited TiO2 thin films demonstrated high responses to O2 in the dynamic range from 0.1 to 100 vol. % and low concentrations of H2, NO2. The ALD deposition allowed the enhancement of sensitivity of TiO2 thin films to gases. The greatest response of TiO2 thin films to O2 was observed at a temperature of 500 °C and was 41.5 arb. un. under exposure to 10 vol. % of O2. The responses of TiO2 thin films to 0.1 vol. % of H2 and 7 × 10-4 vol. % of NO2 at a temperature of 500 °C were 10.49 arb. un. and 10.79 arb. un., correspondingly. The resistance of the films increased due to the chemisorption of oxygen molecules on their surface that decreased the thickness of the conduction channel between the metal contacts. It was suggested that there are two types of adsorption centers on the TiO2 thin films surface: oxygen is chemisorbed in the form of O2- on the first one and O- on the second one.

Supported MOCVD TiO2 Thin Films Grown on Modified Stainless Steel Mesh for Sensing Applications.

Among semiconductor metal oxides, that are an important class of sensing materials, titanium dioxide (TiO2) thin films are widely employed as sensors because of their high chemical and mechanical stability in harsh environments, non-toxicity, eco-compatibility, and photocatalytic properties. TiO2-based chemical oxygen demand (COD) sensors exploit the photocatalytic properties of TiO2 in inducing the oxidation of organic compounds to CO2. In this work, we discuss nanostructured TiO2 thin films grown via low-pressure metal organic chemical vapor deposition (MOCVD) on metallic AISI 316 mesh. To increase the surface sensing area, different inorganic acid-based chemical etching protocols have been developed, determining the optimal experimental conditions for adequate substrate roughness. Both chemically etched pristine meshes and the MOCVD-coated ones have been studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) microanalysis, and X-ray photoelectron spectroscopy (XPS). We demonstrate that etching by HCl/H2SO4 at 55 °C provides the most suitable surface morphology. To investigate the behavior of the developed high surface area TiO2 thin films as COD sensors, photocatalytic degradation of functional model pollutants based on ISO 10678:2010 has been tested, showing for the best performing acid-etched mesh coated with polycrystalline TiO2 an increase of 60% in activity, and degrading 66 µmol of MB per square meter per hour.

Preparation of Transparent ZIF-8/TiO2 Nanocomposite Thin Films for Photocatalytic Applications

Transparent ZIF-8/TiO2 nanocomposite thin films were prepared by a two-stage dip-coating method. TiO2 was first deposited on glass substrates by sol-gel dip-coating. Heat treatment temperature, number of layers and doping metal type/level were optimized in the first step. In the next step, ZIF-8 was grown by solvent based crystallization method on TiO2 layers. Cu, Ce, Fe or Zn doped TiO2 thin films were prepared in order to increase the photocatalytic performance of ZIF-8/TiO2 nanocomposite. The highest photocatalytic methylene blue degradation activities were obtained with the ZIF-8/TiO2 nanocomposite thin films prepared by using 1% Cu or 1% Ce doped TiO2 thin films as the substrates. Both films exhibited 19% dye removal in 1 hour under 254 nm LED light irradiation whereas the dye removal efficiencies were 36% and 29%, respectively, in 1 hour under 365 nm LED light irradiation.

Atomic Layer Deposition of Defective Amorphous TiOx Thin Films with Improved Photoelectrochemical Performance.

A proper control of defects in TiO2 thin films is challenging work for enhancing the photoelectrochemical (PEC) efficiency in water splitting processes. Additionally, a deep understanding of how defects affect the PEC performance of TiO2 thin films is of great interest for achieving better performance. With these aims, we prepared defective amorphous TiOx thin films at various growth temperatures by atomic layer deposition using tetrakis(dimethylamido)titanium as the Ti precursor. Careful X-ray photoelectron spectroscopy and electron spin resonance spectroscopy analyses revealed that the defect concentration in the TiOx thin films can be controlled by adjusting the growth temperature during the ALD process. We also evaluated the light absorption properties of the deposited TiOx thin films using ultraviolet-visible absorption spectroscopy. And it was found that the TiOx thin film deposited at a growth temperature of 200 °C exhibited the highest defect concentration and the highest photocurrent density of 0.051 mA/cm2 at 1.23 V vs reversible hydrogen electrode (RHE) compared to those of the other films. The light absorption efficiency, photogenerated charge separation efficiency, and charge transfer efficiency of defective amorphous TiOx thin films were carefully studied to understand the correlation between the defect concentration in the prepared TiOx thin film and its PEC activity. This study provides insight into the PEC properties of defective amorphous ALD-TiOx thin films.

Relationship between oxidation, stresses, morphology, local resistivity, and optical properties of TiO2, Gd2O3, Er2O3, SiO2 thin films on SiC

The relationship between internal mechanical stresses, surface morphology, nanoscale electrical properties, and optical characteristics in TiO2, Gd2O3, Er2O3, and SiO2 thin films on SiC substrates was investigated. The oxide films were synthesized using the rapid thermal annealing and analyzed through scanning spreading resistance microscopy, photoluminescence, and absorption spectroscopy. Tensile stresses were found in the films, they are attributed to thermal and lattice mismatch, oxidation, and grain boundaries. These stresses influence on surface morphology, resistivity variations, and photoluminescence intensity. Surface roughness and grain structure were found to correlate with variations in resistivity, which were attributed to conductive pathways along grain boundaries and possible metallic phases. Photoluminescence intensity was also observed to correlate with estimated lattice mismatch strain. Gd2O3/SiC exhibited the fewest defects, while Er2O3 and TiO2 showed more, with Er2O3 being the most mismatched and roughest. The results indicate that internal strains in oxide thin films on SiC substrates can influence on surface morphology, leading to formation of defects and spatial inhomogeneity. These fluctuations in local conductivity and luminescence center density have significant implications for dielectric and optical applications. The study provides insights for future processing refinements to mitigate internal strains and enhance the performance of oxide thin films in semiconductor and optical technologies.

Study of Atomic Layer Deposition Nano-Oxide Films on Corrosion Protection of Al-SiC Composites.

In recent years, aluminum matrix composites (AMCs) have attracted attention due to their promising properties. However, the presence of ceramic particles in the aluminum matrix renders AMCs a high corrosion rate and makes it challenging to use traditional corrosion protection methods. In this study, atomic layer deposition (ALD) techniques were used to deposit HfO2, ZrO2, TiO2, and Al2O3 thin films on AMC reinforced with 20 vol.% SiC particles. Our results indicate that the presence of micro-cracks between the Al matrix and SiC particles leads to severe micro-crack-induced corrosion in Al-SiC composites. The ALD-deposited films effectively enhance the corrosion resistance of these composites by mitigating this micro-crack-induced corrosion. Among these four atomic-layer deposited films, the HfO2 film exhibits the most effective reduction in the corrosion current density of Al-SiC composites in a 1.5 wt% NaCl solution from 1.27 × 10-6 A/cm2 to 5.89 × 10-11 A/cm2. The electrochemical impedance spectroscopy (EIS) investigation shows that HfO2 deposited on Al-SiC composites has the largest Rp value of 2.0 × 1016. The HfO2 film on Al-SiC composites also exhibits effective inhibition of pitting corrosion, remaining at grade 10 even after 96 h of a salt spray test.

Degradation of binary mixtures of water pollutants in a photocatalytic microreactor: Competitive kinetics with explicit radiation absorption effects

The photocatalytic degradation of 17-α-ethinylestradiol (EE2) and phenol in aqueous solutions was studied using a continuous flow, planar microreactor under simulated sunlight. A TiO2 thin film was employed as photocatalyst. The degradation rates of single compounds and binary mixtures were well described by the Langmuir-Hinshelwood equation, including competitive terms for the companion pollutant and the intermediate products, and the effect of photon absorption by the photocatalyst. The absorbed radiation in the microreactor was calculated from the optical properties of the TiO2 film and the application of the Net Radiation method. The estimation of the kinetic parameters was carried out by numerical integration of coupled mass balance equations for EE2 and phenol employing the Runge-Kutta method, and adjusting the kinetics parameters with a nonlinear least-squares solver. The objective of this routine was to minimize the sum of the squares of the differences between the simulated and experimental outlet concentrations of EE2 and phenol under different operating conditions. Using this approach, intrinsic kinetic parameters were obtained. The model was able to predict the degradation of both organic pollutants under different inlet concentrations, irradiation levels, and fluid flow rates. This methodology can be extended to assess the simultaneous degradation of multi-component systems.

Improvement of the photovoltaic performance of perovskite solar cells by modification of electron transport layer using Mg-doped Tio2, Cs2Co3, and bilayer Mg-doped Tio2/Cs2Co3

In this literature, the enhancement of electron transport layers (ETLs) in perovskite solar cells (PSCs) by modifying Tio2 thin layers with Magnesium (Mg) and Cesium Carbonate (CS2CO3) was studied. We aimed to modify lattice and surface properties for improved ETL performance. The impact of various doping ratios and coating speeds using UV–visible spectra, scanning electron microscope (SEM), and electron chemical impedance spectrum (EIS) were analyzed. An optimized thin film, 1.5% Mg-doped Tio2, was identified with increased transmission, wider band gap, enhanced conductivity, and reduced resistivity. The bilayer Mg-doped Tio2/CS2CO3 structure shows significant surface modifications via SEM. The optimized thin films including pure Tio2 (60 mm/s), Mg-doped Tio2 (1.5%), Tio2-CS2CO3 (60 mm/s), and Mg-doped Tio2/CS2CO3 were integrated into perovskite solar cells (PSCs), achieving superior efficiency. Introducing Magnesium (Mg) dopants to TiO2 thin films enhanced the band gap energy, influencing energy levels within. This optimized the alignment of energy states at the TiO2-perovskite interface. As a result, the open-circuit voltage (Voc) of the perovskite solar cells improved, reflecting the potential difference between illuminated electrons and the external circuit. This enhancement, due to Mg doping, facilitated efficient charge carrier separation and extraction, boosting overall cell performance. Bilayer Mg-doped Tio2/CS2CO3 showed the best performance with open-circuit voltage (Voc), short-circuit density (Jsc), fill factor, and power conversion efficiency (PCE) 0.98 v, 16.02 mA/cm2, 59%, and 9.37% respectively.

Optically Transparent TiO2 and ZnO Photocatalytic Thin Films via Salicylate-Based Sol Formulations

Sol compositions for transparent TiO2 and ZnO photocatalytic thin film deposition are of interest for the wet-chemical fabrication of self-cleaning coatings. The choice of stabilizing agent is crucial for the sol film-forming properties, with acetylacetone and monoethanolamine conventionally employed for TiO2 and ZnO deposition sols, respectively. Salicylic acid (SA), capable of chelating both Ti(IV) and Zn(II) precursors, remains underexplored. This study presents novel SA-based sol formulations for the deposition of both TiO2 and ZnO films, based on titanium tetraisopropoxide (TTIP) and zinc acetate dihydrate (ZAD) precursors, in a fixed 1:3 (TTIP:SA) and 1:2 (ZAD:SA) ratio, and isopropanol solvent, varied across the 1:10 to 1:20 precursor-to-solvent ratio range. Fourier-Transform Infrared Spectroscopy analysis and Density Functional Theory computations confirmed the formation of H2Ti[SA]3 and Zn[SA]2·2H2O complexes. Scanning Electron Microscopy, X-ray diffraction, and Ultraviolet-Visible spectroscopy were employed to study the structural and optical properties of the dip-coated films, revealing dense TiO2 (86–205 nm) and ZnO (35–90 nm) layers of thickness proportional to the salicylate concentration and transmittance in the 70–90% range. Liquid-phase Methylene blue (MB) photooxidation experiments revealed that all films exhibit photocatalytic activity, with ZnO films being superior to TiO2, with 2.288 vs. 0.366 nm h−1 cm−2 MB removal rates.

Synergistic effects of Mg doping on TiO2 for improved toxic gas sensing performance at room temperature

The gas sensing characteristics of magnesium (Mg)-doped titanium dioxide (TiO2) films were investigated using a spray pyrolysis method. TiO2 Thin films with varying Mg doping concentrations (0, 2.5, and 5 weight percentages) were deposited and tested for their gas detection ability to organic compounds such as ethanol, butanol, toluene, xylene, and formaldehyde at room temperature. Results disclosed that introducing Mg into TiO2 enhanced the gas sensing characteristics, particularly for formaldehyde. Mg-doped TiO2 film improved the change in electrical resistance during gas adsorption, leading to an increased response in formaldehyde detection. Additionally, XRD revealed the crystal structure, while Raman spectroscopy provided insights into molecular vibrational modes of the fabricated films. FESEM allowed for high-resolution imaging of surface morphology, and atomic force microscope assessed surface roughness and other properties of the as deposited samples. UV-Vis spectroscopy was utilized to examine the optical characteristics. The collective results strongly indicated that the introduction of Mg significantly improved the gas-sensing capabilities of TiO2 films, making them highly promising for various gas-sensing applications.

Effect of alkaline dopant Ba2+on TiO2 thin films: Insights of synthesis and properties

Ba doped TiO2 thin films are fabricated by the sol-gel dip-coated method. X-ray diffraction confirms the formation of the anatase phase. With the increase of Ba doping, the size of the crystallite decreases. Crystallite size is in the range of 11–10 nm. The bandgap of pure TiO2 and Ba doped TiO2 thin films is in the range of 3.15–2.94 eV and is beneficial to enhance solar cell efficiency and water treatment. The hopping process explains dielectric characteristics that obey the Maxwell-Wagner model and Koop's theory. Their low dielectric constant is applicable to high-frequency devices. The ferromagnetism was recorded for all Ba-doped TiO2 thin films. The antibacterial activity of Ba-doped TiO2 is higher than that of pure TiO2, owing to the smaller particle size and larger specific surface area, which results in increased surface reactivity to light and water adsorption. Ba-doped TiO2 can be applied in nanomedicine.

Temperature Dependence of Structure, Dielectric and Energy Storage Properties in Bi3.15Nd0.85Ti3O12 Thin Films

In this work, 80-nm Bi3.15Nd0.85Ti3O12 thin films with different annealing temperature have been successfully fabricated on p-type silicon substrates by sol-gel method. The effect of annealing temperature on the microstructure, dielectric properties, and energy-storage performances of the thin films were investigated in detail. The prepared films at 750 °C had a layered perovskite structure with an excellent crystalline morphology, featuring a high dielectric constant of 43.8 and a low dielectric loss < 2% at 1 kHz, which lead the significantly improvement of breakdown strength to increase the energy storage properties. The maximum recoverable energy density of 4.3 J/cm3 with an efficiency of 70.7% under the electric field of 1.90 MV/cm has been obtained for the Bi3.15Nd0.85Ti3O12 film capacitors at 750 °C. These properties indicate that Bi3.15Nd0.85Ti3O12 system is a promising environmentally friendly lead-free ferroelectric.

Influence of Lattice Mismatch on Structural and Functional Properties of Epitaxial Ba0.7Sr0.3TiO3 Thin Films.

Substrate-induced strains can significantly influence the structural properties of epitaxial thin films. In ferroelectrics, this might lead to significant changes in the functional properties due to the strong electromechanical coupling in those materials. To study this in more detail, epitaxial Ba0.7Sr0.3TiO3 films, which have a perovskite structure and a structural phase transition close to room temperature, were grown with different thicknesses on REScO3 (RE-rare earth element) substrates having a smaller lattice mismatch compared to SrTiO3. A fully strained SrRuO3 bottom electrode and Pt top contacts were used to achieve a capacitor-like architecture. Different X-ray diffraction techniques were applied to study the microstructure of the films. Epitaxial films with a higher crystalline quality were obtained on scandates in comparison to SrTiO3, whereas the strain state of the functional layer was strongly dependent on the chosen substrate and the thickness. Differences in permittivity and a non-linear polarization behavior were observed at higher temperatures, suggesting that ferroelectricity is supressed under tensile strain conditions in contrast to compressive strain for our measurement configuration, while a similar reentrant relaxor-like behavior was found in all studied layers below 0°C.

Ethanol sensing performance of sol gel driven spin coated TiO2 thin film

Pure TiO2 thin film was fabricated on glass substrate using the sol–gel spin coating technique. The TiO2 based gas sensing element was used to study the sensing behavior of ethanol at room temperature. The film was characterized by using XRD, FESEM, and Two-point probe method. The XRD diffraction pattern confirmed the formation of the anatase phase and has porous morphology. It was found that TiO2 thin film had the good sensor response to ethanol gas. The ethanol response for 100 ppm was 2.05, which increased significantly to 5.84 for 500 ppm of ethanol. The response and recovery time of thin film element for 500 ppm of ethanol was 120 and 156 s, respectively. The stability of this thin film sensor element was tested over a period of one year. It was found that there was no deterioration in sensor characteristics of thin film element.

Effect of fluoric acid concentration on the structural, optical, and photocatalytic properties of TiO2 thin films

Effect of fluoric acid concentration on the structural, optical, and photocatalytic properties of TiO2 thin films

Farklı Kalınlıklarda Üretilen TiO2 İnce Filmlerinin Boya Duyarlı Güneş Pili Performansına Etkisi

Dye-sensitized solar cells (DSSC) are known as 3rd generation solar cells. One of the most important parameters affecting the performance of DSSCs is the thin film thickness that forms the photoanode layer. In this study, we examined how 38, 60 and 76 µm thick TiO2 thin films change dye-sensitized solar cell performance. The highest efficiency (4.73%) was seen in the solar cell with 38 µm thin film thickness. In addition, the mineralogical and morphological analyses of the produced TiO2 nanopowders were performed with X-ray diffraction (XRD) and Scanning electron microscopy (SEM). XRD analyses showed that TiO2 was in the anatase crystal phase. SEM photographs confirmed the formation of microspheres in close contact with each other.

The growth mechanisms of TiO2 film onto PET surfaces by atomic layer deposition

Atomic layer deposition (ALD) was used to coat a polyethylene terephthalate (PET) polymer substrate with TiO2 film. The TiO2 was grown onto the surface with better film coverage by using thicker ALD deposition. Further evaluation on the coated substrates indicated that the reactive sites of –C=O that existed on PET surface played a significant contribution to facilitating the initial ALD growth of the TiO2 thin film. The chemical composition of the coated substrates was characterised using energy dispersive X-ray spectroscopy, which showed that increasing the TiO2 film thickness increased the Ti element content. Two growth mechanisms, namely, diffusion growth and direct coordination through the precursor coordination with surface reactive sites of –C=O with product release occurred simultaneously in the initial growth of TiO2 coating onto PET polymer by ALD. The surface alteration of the coated ALD was characterised by Fourier transform infrared spectroscopy, which showed that the hydroxyl –OH groups emerged in the TiO2 ALD film.

Structural, electronic and optical characteristics of TiO2 and Cu-TiO2 thin films produced by sol-gel spin coating

Sol-gel spin coating was employed to deposit titanium dioxide (TiO2) and Cu-TiO2 composites on glass substrates. Cu was utilized as a dopant in varying amounts. The structural, morphological, optical, and dielectric characteristics of the produced samples were evaluated using a variety of techniques. XRD analysis was used to confirm the anatase (TiO2) phase's existence. After adding Cu(NO3)2.3(H2O) to TiO2, it was observed that Cu atoms were mostly distributed on the TiO surface, resulting in a decrease in the particle size, as revealed by the TEM images. Raman Spectroscopy and FTIR analysis indicates the expansion of the lattice TiO2 with Cu amount and the formation of vibrating Ti-O-Ti bonds respectively. With increasing dopants, spherical nanoparticles begin to form and orient themselves to aggregate, as shown in the AFM's noticeable shift in roughness. The UV–vis spectroscopy revealed a shift of the absorption peak toward visible range that signified an increase in the bandgap after doping. The theoretical study using quantum espresso ab initio simulation and DFT + U correction were used to derive the electronic band gap energy values which are consistent with our experimental results.

Optical absorption enhancement of TiO2 via plasmonic effect of gold nanoparticles in volatile organic compounds medium

Controlling volatile organic compounds (VOC) is of utmost importance due to their adverse impacts on human health and environment. Photocatalytic degradation using TiO2 nanoparticles has emerged as a promising approach for VOC control. However, the limited photocatalytic activity of TiO2 under visible light owing to its wide bandgap presents a significant challenge. To address this limitation, the integration of plasmonic materials with TiO2 enables efficient hot electron injection into TiO2 under visible light. In this study, we investigate the influence of gold nanoparticles (AuNP) on TiO2-based structures, namely AuNP-decorated TiO2 thin films, Au@TiO2 core@shell structures, and AuNP-loaded TiO2 nanoparticles, using finite difference time domain (FDTD) simulations. Simulation results demonstrate that the presence of AuNP in TiO2 structures reduces the bandgap and enhances light absorption. Moreover, the localized electric field enhancement observed at the AuNP-TiO2 interface indicates the potential for effective hot electron injection into TiO2. Additionally, simulations conducted in a VOC medium demonstrate higher absorption compared to vacuum conditions. This work provides insights into the mechanisms underlying previously published experiments and proposes an appealing design for the development of highly efficient photocatalytic materials under visible light.