TiO2 Thin Films Research Articles

Laser Direct Writing of Crystallized TiO2 by Photothermal Effect Induced by Gold Nanoparticles

Near‐infrared (NIR) laser annealing is successfully used to crystallize TiO2 thin films from a sol–gel solution deposited on gold nanoparticle arrays (AuNPs). The AuNPs are used as nano‐heaters allowing a local temperature increase up to 500 °C in the film. The temperature reached under the laser is deduced from the presence of the anatase phase in the samples obtained by laser exposure, showing that crystallized TiO2 can be obtained by the photothermal effect. Different analytical techniques supported this study, such as grazing X‐ray diffraction (GIXRD), UV–vis, and Raman spectroscopy. The temperature increase is confirmed by a numerical model that emphasizes the role of NPs coupling in the photothermal effect. Direct laser patterning by NIR laser and in combination with Deep‐UV photolithography (DUV) are demonstrated. This fabrication method opens new perspectives in applications such as photonics, photocatalysis, or biosensing.

Influence of Ho Substitution on Structural, Morphological, and Optical Properties of Anatase Ti1-xHoxO2 (x= 0.0, 0.01, 0.02, 0.03) Thin Films

Ti1-xHoxO2 (x= 0.0, 0.01, 0.02, 0.03) thin films are synthesized by a sol-gel method and deposited by a dip-coating technique on the glass substrates. The films’ crystal structures are examined by an X-ray diffraction technique, while the morphological properties are investigated by scanning electron microscopy and atomic force microscopy. UV-Vis and photoluminescence spectrophotometry are used to analyze the optical properties. Based on the X-ray diffraction patterns, all the films belong to the anatase phase. It is observed that the surface characteristics, such as the morphology, film thickness, and roughness change significantly with the holmium substitution. The optical investigations reveal that the transmittance, band gap energies, and luminescence properties can be adjusted by the holmium substitution. According to the current study, holmium substituted TiO2 thin films with improved optical properties may be a suitable candidate for applications that require a wide band gap and high optical transparency as well as luminescence properties.

Enhanced, selective, and room temperature detection of ethanol vapor by RF-sputtered TiO2 thin films

Enhanced, selective, and room temperature detection of ethanol vapor by RF-sputtered TiO2 thin films

Swift heavy ion irradiation induced modifications on the structure, optical and electrical properties of pulsed laser deposited anatase TiO2 thin films

The swift heavy ions of Ag7+ (100 MeV) were used to irradiate the anatase TiO2 thin films prepared by pulsed laser deposition (PLD) technique on quartz substrates. The effect of ion fluences (1×1012, 1×1013 and 1×1014 ions/cm2) on the structure, surface morphology, optical and electrical properties are investigated through several analytical techniques such as XRD, Micro-Raman, FTIR, XPS, AFM, FESEM, UV–visible and Hall measurements. The analysis of XRD data shows a gradual shift in the (101) plane toward higher diffraction angle on increasing the ion fluence which increases the lattice strain in the films. The EDS spectra reveal a modest Ti-rich composition with a significant shift following irradiation for both as-grown and irradiated films. The development of hillocks at increased ion fluence and considerable grain splitting following irradiation are observed in SEM micrographs. AFM images exhibit increase in surface roughness with the increase in ion fluence. Moreover, there is a significant drop in the band gap of the irradiated thin films from 3.92 to 3.44 eV with the increase in ion fluence. The various electrical parameters such as carrier concentrations (n), Hall mobility (μ), electrical resistivity (ρ), Hall coefficient (RH), etc., are determined by the Hall measurements at room temperature. This study indicates that the SHI irradiation plays a significant role in tunning the physical properties of anatase TiO2 thin films.

Development of direct current magnetron sputtered TiO[formula omitted] thin films as buffer layers for copper indium gallium diselenide based solar cells

We describe the development and characterization of direct current magnetron sputtered titanium dioxide thin films from ceramic targets mixed with metallic titanium particles. The aim of this article is to assess their suitability for the application as buffer layer in copper indium gallium diselenide (CIGSe) based solar cells. The absorber material is produced in a semi-industrial, roll-to-roll hybrid sputter co-evaporation process. A potential strategy to modify electro-optical properties of TiO2 thin films by controlling the concentration of defects, such as oxygen vacancies, is investigated by X-ray photoelectron spectroscopy (XPS), transmission/reflection spectroscopy, spectroscopic ellipsometry, and cathodoluminescence. The presence of oxygen in the plasma during the sputter deposition process has a crucial impact on the electron transport mechanism in the studied devices. The source of the changed device characteristic can be found in modified band discontinuities at the absorber–buffer interface and in an oxidized CIGSe surface, indicated by the presence of cationic selenium, as detected by XPS. We sketch a tentative band alignment diagram and outline the range of possible modifications. The harmful effect of oxygen in the plasma forces the omission of oxygen in the process and restricts the possibilities to modify the material properties of the TiO2 layers, and consequently the prospects for an application on CIGSe. Nevertheless, the results of this work depict the principal eligibility, albeit optimization on both sides of the absorber–buffer interface are inevitable for an improved cell performance.

Deposition of thin TiO2 films on fluorophlogopite particle surface by CVD in fluidized bed reactor to prepare silver pearlescent pigments

Two critical parameters: Glossiness and chromaticity space of pearlescent pigments were evaluated based on crystallinity and grain size. Both the roughness and the vertical distance evolution along the surface of deposited TiO2 film are affected by the crystallinity. The optimal crystallinity of TiO2 was deposited at the temperature of 350 °C. The TiCl4 concentration effectively changed the grain size. However, the high flow rate of fluidizing gas and optimal gas–solid ratio (<0.11 m3/(kg h)) restricted the change of TiCl4 concentrations in fluidized bed reactor. In this case, the grain size of the deposited TiO2 film was about 55 nm. Depending on the grain size, the color of pearlescent pigments was adjusted by controlling the film thickness based on TiO2 concentration. Finally, the silver pearlescent pigment with high glossiness was prepared by chemical vapor deposition method in a fluidized bed reactor.

Enhancing Oxygen Sensing Precision Through Gate-Controlled TiO2 Field Effect Transistors Under Ultraviolet Excitation

We report on the fabrication and characterization of a novel oxygen sensor based on a TiO2 thin film field effect transistor (FET) deposited on a silicon substrate with an oxide layer by magnetron sputtering. TiO2 is a n-type semiconductor with a wide band gap, which allows the formation of oxygen vacancies or adsorbed oxygen species on its surface under ambient conditions. These oxygen-related defects act as electron traps that modulate the electrical conductivity of the TiO2 film. Under ultraviolet (UV) irradiation, at 310 nm wavelength, the photogenerated carriers in the TiO2 film are captured by the oxygen defects, resulting in a decrease of the film resistance that depends on the oxygen concentration. We demonstrate that the sensitivity and resolution of the oxygen sensor can be enhanced by applying a positive gate voltage to the FET device. The photocurrent variation per unit of oxygen concentration (ΔIphoto/ΔCPO) increases from 1.08 at VG =0 V to 2.5 at VG= 20 V in the range of 5%–20% oxygen concentration. The gate voltage also extends the controllable range of oxygen defects and photocurrent. Our study provides a new insight into the design and optimization of gas sensors based on TiO2 thin film FETs.

Enhancement of the efficiency of solar collector by SiO2, TiO2, and ZnO thin films layers

Most scientific researchers are motivated to develop alternative energy applications and raise their efficiency as a result of rising global warming and pollution from traditional energy sources. In this study, Nanocoating thin films of ` have been prepared on different substrates (stainless steel 316 L, commercially pure Ti, Si-Wafer(100) and glass). Magnetron sputtering deposition technique at 2.8 × 10–7 Torr vacuum has been used to synthesize the nanocoated thin films. Scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), Atomic Force Microscopy (AFM), and UV–visible radiation tests have been used to characterize the thin films deposited. The investigation results showed that the thin films deposited do not have microcracks or defects with a uniform and homogenous structure at 400 nm magnification. It has a nanoparticle grain size of 60–90 nm with a crystal structure for TiO2, SiO2, and ZnO films. ZnO films have coarser grain and more aggregation than the other films. The highest efficiency of the collector was recorded for a collector integrated with ZnO thin-film. The efficiency of the ZnO-coated collector was about 66 % at midday compared to the conventional system (no thin film), which was 39 %.

Phase Change-Induced Magnetic Switching through Metal–Insulator Transition in VO2/TbFeCo Films

The ability to manipulate spins in magnetic materials is essential in designing spintronics devices. One method for magnetic switching is through strain. In VO2 on TiO2 thin films, while VO2 remains rutile across the metal–insulator transition, the in-plane lattice area expands going from a low-temperature insulating phase to a high-temperature conducting phase. In a VO2/TbFeCo bilayer, the expansion of the VO2 lattice area exerts tension on the amorphous TbFeCo layer. Through the strain effect, magnetic properties, including the magnetic anisotropy and magnetization, of TbFeCo can be changed. In this work, the changes in magnetic properties of TbFeCo on VO2/TiO2(011) are demonstrated using anomalous Hall effect measurements. Across the metal–insulator transition, TbFeCo loses perpendicular magnetic anisotropy, and the magnetization in TbFeCo turns from out-of-plane to in-plane. Using atomistic simulations, we confirm these tunable magnetic properties originating from the metal–insulator transition of VO2. This study provides the groundwork for controlling magnetic properties through a phase transition.

Compositional Modification of Epitaxial Pb(Zr,Ti)O3 Thin Films for High‐Performance Piezoelectric Energy Harvesters

AbstractIn this study, Piezoelectric energy harvesters (PEHs) are fabricated based on epitaxial Pb(Zr,Ti)O3 (PZT) thin films deposited by an RF magnetron sputtering with varied Zr/[Zr+Ti] ratio. For a compatibility with micro‐electromechanical systems, the epitaxial PZT thin films are deposited on Si substrates (PZT/Si). The morphotropic phase boundary (MPB) are formed in the compositional range of 0.44 ≤ Zr/[Zr+Ti] ≤ 0.51 of the epitaxial PZT/Si, which is far broader than that of the bulk PZT. Meanwhile, effective transverse piezoelectric coefficient (|e31,f|) values are evaluated by the direct and converse piezoelectric effects using the unimorph cantilever method. Among the compositions, the rhombohedral‐dominant MPB (MPB‐R) of Zr/[Zr+Ti] = 0.51 exhibits a direct |e31,f| of 10.1 C m−2 and a relative permittivity (εr) of 285, leading to the maximum figure of merit of 40 GPa in this study. On the other hand, the maximum converse |e31,f| of 14.0 C m−2 is measured from the tetragonal‐dominant MPB (MPB‐T) of Zr/[Zr+Ti] = 0.44. At a resonance frequency, the MPB‐T presents a high output power density of 301.5 µW−1/(cm2 g2) under an acceleration of 3 m−1 s−2, which is very promising for the high‐performance PEH applications.

Rapid post-annealing effect on the TiO2-based electrochromic films

The electrochromic (EC) window is a promising energy-saving smart window that can vary between three states: transparent, translucent, and opaque by applying low voltages. One of the potential EC materials is known as titanium dioxide (TiO2) due to its unique chemical and physical properties. Conventional post-annealing has been commonly employed for phase transition in EC thin films, but it suffers from drawbacks such as long treatment time and high energy consumption. Hence, in this study, TiO2 thin films were deposited on indium-doped tin oxide (ITO) and fluoride-doped tin oxide (FTO) glass substrates using a sol-gel spin coating technique, followed by a rapid post-annealing interval of 5 min at temperatures ranging from 300 to 450 °C. The optical, morphological, structural, and EC properties of TiO2 thin films were investigated. The measurement depicted that rapid post-annealing was a cost-effective and time-saving method for realising high-quality TiO2 EC thin films. Hence, this approach reduces the overall fabrication time and minimises energy consumption, rendering it an environmentally friendly process. The successful deployment of TiO2 in EC applications paves the way for integrating EC and photovoltaic technologies in energy-saving window glass applications, considering TiO2 is a crucial element in photovoltaic devices.

Determination of Non-linear Refractive Index of Pure SnO2 and TiO2 Doped SnO2 Thin Films Using Z-scan Technique

In this research work the non-linear refractive indices n2 was determined for the pure SnO2 and TiO2 doped SnO2 thin films by using the Z-scan technique. We have used a continuous wave Ar-ion laser with incident wavelength of 514 nm as the source. n2 was measured by varying three quantities: the incident laser intensity, thickness of the pure SnO2 films and doping concentration of TiO2 in SnO2 films. From the experimental results, the values of n2 were found to be increasing with the increase of thickness of the pure SnO2 films and also with the increase of doping concentration of TiO2 in SnO2 thin films. Dhaka Univ. J. Sci. 71(2): 104-110, 2023 (July)

Influence of the Cu doping on the physical and H2 gas sensing properties of TiO2

Due to poor selectivity, existing H2 sensors based on pure TiO2 nanostructures do not exhibit good efficiency at low gas concentrations. Therefore, many studies are carried out to improve the gas sensor properties of TiO2s. In this study, the structural and H2 gas sensing parameters of Cu doped TiO2 thin films obtained by using sputtering method were investigated as a function of Cu doping. In the co-sputtering system, direct current (DC) and radio frequency (RF) sources were used to simultaneously deposit of the Ti (fixed at 100 W in DC) and Cu (25, 35 and 45 W in RF). The structural, optical and morphological properties of the films were discussed in the light of XRD, XPS, UV-VIS, SEM and AFM results. XRD results revealed that the produced thin films have anatase TiO2 structure and the crystallite sizes in the most dominant A(110) plane are in the range of 14–18 nm. According to XPS results, copper ratios in Cu25, Cu35 and Cu45 samples were calculated as 0.433%, 1.598% and 2.233%, respectively. It was also found that CuO and Cu2O formations were formed in addition to the anatase phase only in Cu45 sample from these films. It was determined from the 2D and 3D AFM images that the surface roughness values of the films decreased from 63.92 nm to 12.99 nm with increasing Cu doping. In addition to the structural properties of the produced films, hydrogen gas detection performances at 1000 ppm gas concentration and 200 °C temperature were investigated with the current sensitive gas sensing system. The results reveal that Cu doping significantly improves the H2 gas responses and response times of TiO2s. Among the films produced, the highest gas response (2284) and shortest times (τres:128s, τrec:129s) was obtained in Cu45 thin film. This increase in gas responses was explained by the oxygen vacancy model and the results are presented by comparing pristine TiO2 and other 3d transition metal doped TiO2s in the literature.

Ecofriendly alkali metal cations diffusion improves fabrication of mixed-phase titania polymorphs on fixed substrate by chemical vapor deposition (CVD) for photocatalytic degradation of azo dye

Controlling the nanoscale synthesis of semiconductor TiO2 on a fixed substrate has fascinated the curiosity of academics for decades. Synthesis development is required to give an easy-to-control technique and parameters for TiO2 manufacture, leading to advancements in prospective applications such as photocatalysts. This study, mixed-phase TiO2(B)/other titania thin films were synthesized on a fused quartz substrate utilizing a modified Chemical vapor depodition involving alkali-metal ions (Li+, Na+, and K+) solution pre-treatment. It was discovered that different cations promote dramatically varied phases and compositions of thin films. The films had a columnar structure with agglomerated irregular-shaped particles with a mean thickness of 800–2000 nm. Na+ ions can promote TiO2(B) more effectively than K+ ions, however Li+ ions cannot synthesize TiO2(B). The amounts of TiO2(B) in thin films increase with increasing alkali metal (K+ and Na+) concentration. According to experimental and DFT calculations, the hypothesized TiO2(B) production mechanism happened via the meta-stable intermediate alkaline titanate transformation caused by alkali-metal ion diffusion. The mixed phase of TiO2(B) and anatase TiO2 on the fixed substrate (1 × 1 cm2) obtained from Na+ pre-treated procedures showed significant photocatalytic activity for the degradation of methylene blue. K2Ti6O12, Li2TiO3, Rutile TiO2, and Brookite TiO2 phase formations produced by K+ and Li + pretreatment are low activity photocatalysts. Photocatalytic activities were more prevalent in NaOH pre-treated samples (59.1% dye degradation) than in LiOH and KOH pre-treated samples (49.6% and 34.2%, respectively). This revealed that our developed CVD might generate good photocatalytic thin films of mixed-phase TiO2(B)/anatase TiO2 on any substrate, accelerating progress in future applications.

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.