Nanocrystalline TiO2 thin films for NH3 monitoring: microstructural and physical characterization

Nanocrystalline titanium oxide thin films have been deposited by spin coating technique and then have been analyzed to test their application in NH₃ gas-sensing technology. In particular, spectrophotometric and conductivity measurements have been performed in order to determine the optical and electrical properties of titanium oxide thin films. The structure and the morphology of such material have been investigated by X ray diffraction, Scanning microscopy, high resolution electron microscopy and selected area electron diffraction. The X-ray diffraction measurements confirmed that the films grown by this technique have good crystalline tetragonal mixed anatase and rutile phase structure. The HRTEM image of TiO₂ thin film showed grains of about 50–60 nm in size with aggregation of 10–15 nm crystallites. Selected area electron diffraction pattern shows that the TiO₂ films exhibited tetragonal structure. The surface morphology (SEM) of the TiO₂ film showed that the nanoparticles are fine with an average grain size of about 50–60 nm. The optical band gap of TiO₂ film is 3.26 eV. Gas sensing properties showed that TiO₂ films were sensitive as well as fast in responding to NH₃. A high sensitivity for ammonia indicates that the TiO₂ films are selective for this gas.     

Effect of substrate temperature on structural, morphological and optical properties of crystalline titanium dioxide films prepared by DC reactive magnetron sputtering

Titanium dioxide (TiO₂) thin films have been deposited with various substrate temperatures by dc reactive magnetron sputtering method onto glass substrate. The effects of substrate temperature on the crystallization behavior and optical properties of the films have been studied. Chemical composition of the films was investigated by X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) analysis of the films revealed that they have polycrystalline tetragonal structure with strong (101) texture. The surface morphological study revealed the crystalline nature of the films at higher substrate temperatures. The TiO₂ films show the main bands in the range 400–700 cm^?1, which are attributed to Ti–O stretching and Ti–O–Ti bridging. The transmittance spectra of the TiO₂ thin film measured with various substrate temperatures ranged from 75 to 90 % in the visible light region. The optical band gap values of the films are increasing from 3.44 to 4.0 eV at growth temperature from 100 to 400 °C. The structural and optical properties of the films improved with the increase in the deposition temperature.     

Influence of calcination ambient and film thickness on the optical and structural properties of sol-gel TiO2 thin films

Influence of both calcination ambient and film thickness on the optical and structural properties of sol-gel derived TiO₂ thin films have been studied. X-ray diffraction results show that prepared films are in an anatase form of TiO₂. Films calcined in argon or in low vacuum (∼2 × 10⁻¹ mbar) are found to be smaller in crystallite size, more transparent at low wavelength region of ∼300-450 nm, denser, have higher refractive index and band gap energy compared to air-calcined films. Scanning electron microscopic study reveals that surfaces of TiO₂ films calcined in argon or in low vacuum are formed by densely packed nano-sized particulates. Presence of voids and signs of agglomeration can be seen clearly in the surface microstructure of air-calcined films. In the thickness range ∼200-300 nm, band gap energy and crystallite size of TiO₂ films remain practically unaffected with film thickness but refractive index of thinner film is found to be marginally higher than that of thicker film. In this work, it has been shown that apart from temperature and soaking time, partial pressure of oxygen of the ambient is also an important parameter by which crystallite size, microstructure and optical properties of the TiO₂ films may be tailored during calcination period.     

Optimization of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin film absorber layer growth without sulphurization using triethanolamine as complexing agent for thin film solar cells applications

Quaternary kesterite Cu₂ZnSnS₄ (CZTS) thin films have been prepared via a simple spin-coating technique based on a sol–gel precursor of 2-methoxyethanol solution with metal salts and thiourea. Solution processed CZTS thin film growth parameters using complexing agent triethanolamine (TEA) have been investigated. Effects of complexing agent TEA on structural, morphological, optical, electrical and photovoltaic properties of CZTS thin films were systematically investigated. X-ray diffraction and Raman spectroscopy studies reveal that amorphous nature of CZTS thin film changes into polycrystalline with kesterite crystal structure with optimized TEA concentartion. Surface morphology of CZTS films were analyzed by field emission scanning electron microscope and atomic force microscope, which revealed the smooth, uniform, homogeneous and densely packed grains and systematic grain growth formation with varying TEA concentrations. UV–Vis spectra revealed a direct energy band gap ranging from 1.78 to 1.50 eV, which was found to depend upon the TEA concentration. X-ray photoelectron spectroscopy demonstrated stoichiometric atomic ratios of multicationic quaternary CZTS thin film grown without sulphurization. p-type conductivity was confirmed using Hall measurements and the effect of varying concentartion of TEA on electrical and photovoltaic properties are studied. The SLG/FTO/ZnO/CZTS/Al thin film solar cell is fabricated with the CZTS absorber layer grown at optimized TAE concentration of 0.06 M. It shows a power conversion efficiency of 0.87% for a 0.16 cm² area with V_oc = 0.257 mV, J_sc = 8.95 mA/cm² and FF?=?38%.     

Low temperature stabilized rutile phase TiO2 films grown by sputtering

The deposition of rutile phase TiO₂ films on unheated substrates by radio frequency magnetron sputtering is elaborated. The effect of total pressure and O₂/Ar flow ratio on the growth of rutile film on different substrates has been studied thoroughly. The development of crystalline phase along with film deposition rate, surface morphology, optical transmission and band gap were also investigated for various growth conditions. It was found that the rutile phase crystallinity increased with decrease in total pressure and increase in O₂ flow. In addition, the grown rutile films have interesting optical characteristics such as high transmittance (~ 85%) and high refractive index (~ 2.7) with a band gap about 3.2 eV.     

Improving conversion efficiency of dye-sensitized solar cells by metal plasma ion implantation of ruthenium ions

Dye-sensitized solar cells (DSSC) are based on the concept of photosensitization of wide-band-gap mesoporous oxide semiconductors. At present, DSSC have ventured into advanced development and pilot production. Our current research emphasizes on improvements on titanium dioxide (TiO₂) photosensitivity under visible light irradiation by using metal plasma ion implantation (MPII). The anatase TiO₂ electrode was prepared via a sol-gel process and deposited onto indium-tin oxide glass substrates. Subsequently, the as-deposited TiO₂ films were subjected to MPII at 20 keV in order to incorporate ruthenium (Ru) atoms onto the TiO₂ surface layer. The Ru-implanted TiO₂ thin film possessed nanocrystalline Ru clusters of 20 nm in diameter and distributed in near surface layer of TiO₂ films. The Ru clusters showed effective in both prohibiting electron-hole recombination and generating additional Ru-O impurity levels for the TiO₂ band gap structure. A significant reduction of TiO₂ band gap energy from 3.22 to 3.11 eV was achieved, which resulted in the extension of photocatalysis of TiO₂ from UV to Vis regime. A small drop of photoelectric performance of 8% was obtained due to the incorporation of Ru atoms in the surface layer of TiO₂, a similar side effect as observed in the Fe-implanted TiO₂. However, the overall retention of the photocatalysis capability is as high as 92% when switch from UV to Vis irradiation. The improvement of the photosensitivity of TiO₂ DSSC by means of metal plasma ion implantation is promising.     

Synthesis of V-doped TiO2 films by chemical bath deposition and the effect of post-annealing on their properties

Amorphous composite films, composed of a Ti_1 − xV_xO₂ solid-solution phase and a V₂O₅ phase, were produced by chemical bath deposition and subsequently air-annealed at various temperatures up to 550 °C. The microstructure and chemical composition of the as-prepared and annealed films were investigated by a combinatorial experimental approach using Scanning electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy. Ultraviolet-Visible Spectrometry was applied to determine the optical band gap of the as-prepared and annealed films. It followed that the incorporation of vanadium in the as-deposited films reduces the optical band gap of TiO₂ from about 3.8 eV to 3.2 eV. Annealing of the films up to 350 °C leads to slight increase of band gap, as attributed to a reduction of the defect density in the initially amorphous oxide films due to the gradual development of long-range order and a concurrent reduction of the V⁴⁺-dopant concentration in the Ti_1 − xV_xO₂ solid-solution phase. The films crystallized upon annealing in air at 550 °C, which resulted in drastic changes of the phase constitution, optical absorbance and surface morphology. Due to the lower solubility of V⁴⁺ in crystalline TiO₂, V⁴⁺ segregates out of the crystallizing Ti_1 − xV_xO₂ solid-solution phase, forming crystalline V₂O₅ at the film surface.     

Growth of stoichiometric TiO2 thin films on Au(100) substrates by molecular beam epitaxy

The present study reports on the growth of thin TiO₂ films onto Au(100) single crystals by Ti evaporation in a reactive O₂ atmosphere at two different substrate temperatures: room temperature (RT) and 300 °C. The growth of the oxide films was monitored by means of X-ray photoemission spectroscopy, while the valence and conduction band electronic structure was investigated by UV and inverse photoemission spectroscopy, respectively.The TiO₂ film grows epitaxially on the Au(100) substrate at 300 °C exhibiting the rutile (100) surface. The evolution of the Ti 2p lineshape with the oxide coverage shows the presence of reduced oxide species (characterized by Ti^3 + ions) at the Au(100) interface. A crystalline and stoichiometric TiO₂ oxide is produced at high substrate temperature, while growth at RT gives a measurable concentration of defects. Post growth annealing in ultra-high vacuum of the RT grown film increases this concentration, while subsequent annealing in O₂ atmosphere restores the sample to the as-grown conditions.     

Influence of film thickness and annealing atmosphere on the structural, optical and luminescence properties of nanocrystalline TiO2 thin films prepared by RF magnetron sputtering

TiO₂ thin films were deposited onto quartz substrates by RF magnetron sputtering. Inorder to investigate the effect of film thickness on the structural and optical properties, films were deposited for different time durations, and post-annealed at 873 K. The influence of annealing atmosphere (air/oxygen) on the film properties was also investigated. The films were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV–vis spectroscopy and photoluminescence (PL) spectroscopy. Films deposited at different time durations are amorphous-like in nature. From XRD patterns it can be inferred that deposition for longer duration is essential for achieving crystallisation in TiO₂ thin films prepared by RF magnetron sputtering. The films exhibited good adherence to the substrate and are crack free as revealed by SEM images. Film thickness was found to increase with increase in sputtering time. The optical band gap of the films was found to decrease with increase in film thickness, which is consistant with XRD observations. Film thickness did not show any significant variation when annealed in both air and oxygen. Defect related PL emission in the visible region (blue) was observed in all the investigated films, which suggests the application of these films in optoelectronic display devices.     

Post-deposition treatment behaviour of zinc oxide thin films in hydrogen peroxide solution

Abstract

Zinc oxide thin films with c axis orientation were grown on Si (100) by radio frequency magnetron sputtering in mixture of argon and oxygen environment using Zn target. These films are treated in hydrogen peroxide (H₂O₂) solution at room temperature for one hour and two hours. During this study it has been found that post-deposition treatment of ZnO films improves the film quality. Structural, electrical and optical properties have been compared before and after H₂O₂ treatment by X-ray diffraction analysis, ac conductivity, band gap and refractive index. The full width at half maximum decreases after post-deposition process, which improves the crystal quality. The relief in stress has been observed but films are not fully stress free. The film after treatment becomes highly insulating having resistivity of the order of 10¹⁴ Ω cm^?1 and can be used for piezoelectric applications. The increase in the band gap and refractive index, near to bulk value, has been observed after post-deposition treatment indicating the increase in grain size and crystal quality.     

Dimensional effects observed for the electrical, dielectrical and optical properties of TiO2 DC magnetron thin films

In this paper, we present the dimensional effects observed for TiO₂ films deposited by DC circular magnetron sputtering. TiO₂ thin films are deposited in an Al–TiO₂–Al structure to investigate their non-linear characteristics, from which the carrier effective mass and the barrier potential at the Al–TiO₂ interface is calculated, and an important relationship between the effective carrier mass and film thickness is observed. The dielectric constant of TiO₂ thin films is also investigated, and is observed to vary with TiO₂ film thickness. Further, TiO₂ band gap is observed to vary with film thickness.     

Influence of Nb dopant on the structural and optical properties of nanocrystalline TiO2 thin films

In this study, preparation of Nb-doped (0-20 mol% Nb) TiO₂ dip-coated thin films on glazed porcelain substrates via sol-gel process has been investigated. The effects of Nb on the structural, optical, and photo-catalytic properties of applied thin films have been studied by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Surface topography and surface chemical state of thin films was examined by atomic force microscope and X-ray photoelectron spectroscopy. XRD and Raman study showed that the Nb doping inhibited the grain growth. The photo-catalytic activity of the film was tested on degradation of methylene blue. Best photo-catalytic activity of Nb-doped TiO₂ thin films were measured in the TiO₂-1 mol% Nb sample. The average optical transmittance of about 47% in the visible range and the band gap of films became wider with increasing Nb doping concentration. The Nb⁵⁺ dopant presented substitutional Ti⁴⁺ into TiO₂ lattice.     

Enhanced photovoltaic performance of quantum dot sensitized solar cells with Ag-doped TiO2 nanocrystalline thin films

Ag-doped titanium dioxide (TiO₂) nanocrystalline thin films have been prepared by the sol–gel dip coating method and used as photoanode to fabricate quantum dot sensitized solar cells. The X-ray diffraction studies reveal the formation of anatase phase without any impurity phase. The surface morphology studied using scanning electron microscope shows uniform distribution of particles. The optical band gap was found to be 3.5 and 3.4 eV for CdS quantum dot sensitized TiO₂ and CdS quantum dot sensitized Ag-doped TiO₂ thin film respectively. The Ag-doped TiO₂ based solar cell exhibited a power conversion efficiency of 1.48 % which is higher than that of TiO₂ (0.9 %).     

Ag surfactant effects of TiO2 films prepared by sputter deposition

The surfactant effect of Ag on the thin film structure of TiO₂ by radio frequency magnetron sputtering has been investigated. Comparisons between the atomic force microscopy images revealed that the surface roughness of TiO₂ film mediated by Ag was smaller than that of the TiO₂ film without Ag. The surface segregation effect of Ag was confirmed using X-ray photoelectron spectroscopy. The results of X-ray diffraction revealed that the initial deposition of a 0.4 nm thick Ag surfactant layer onto a Fe buffer layer prior to the deposition of the TiO₂ film reduced the rutile (110) growth and enhanced the anatase (100) growth. It was concluded that Ag was an effective surfactant for changing the thin film structure of TiO₂ on the Fe buffer layer. The photocatalytic effect of the fabricated TiO₂ film was also investigated using the remote oxidation process. TiO₂ films with the Ag surfactant exhibited higher photocatalytic activity than conventionally deposited TiO₂ films.     

Ag surfactant effects of TiO2 films prepared by sputter deposition

The surfactant effect of Ag on the thin film structure of TiO₂ by radio frequency magnetron sputtering has been investigated. Comparisons between the atomic force microscopy images revealed that the surface roughness of TiO₂ film mediated by Ag was smaller than that of the TiO₂ film without Ag. The surface segregation effect of Ag was confirmed using X-ray photoelectron spectroscopy. The results of X-ray diffraction revealed that the initial deposition of a 0.4 nm thick Ag surfactant layer onto a Fe buffer layer prior to the deposition of the TiO₂ film reduced the rutile (110) growth and enhanced the anatase (100) growth. It was concluded that Ag was an effective surfactant for changing the thin film structure of TiO₂ on the Fe buffer layer. The photocatalytic effect of the fabricated TiO₂ film was also investigated using the remote oxidation process. TiO₂ films with the Ag surfactant exhibited higher photocatalytic activity than conventionally deposited TiO₂ films.     

Electronic structure of the surface of ruthenium — titanium oxide anodes. X-ray electron and modulation spectroscopy

Ruthenium — titanium oxide films of various composition have been studied by a high resolution electron microscopy method. X-ray electron spectroscopy and electroreflection. It is shown that the composition of the films is non-uniform because of the presence of the ruthenium oxide phase in the form of small particles (5–30 nm) dispersed in the TiO₂. As a result of the interaction of these particles with the defective sections of the TiO₂ phase, an impurity (contact) band is formed inside the latter. Such contact microareas have an energy gap less than in TiO₂. It is this microheterogeneous structure (and consequently the spottiness of the film surface) that determines the electronic properties of the films. Potential barriers of different amplitudes are formed in films with a composition below the critical (x_c < 30 mol.% RuO₂) at the interface between the contact microareas or TiO₂ and the ruthenium oxide particles. It is these barriers that determine the surface band structure of these films and are responsible for the two observed flat — band potentials being considerably different from the TiO₂ one. When the critical concentration is achieved, the barriers disappear and the Fermi level of the films drops into the contact band, which brings together the ruthenium oxide particles (they form an infinite conductive cluster).     

Effect of annealing on microstructural and optoelectronic properties of nanocrystalline TiO2 thin films

In this study, semi-transparent nanostructured titanium oxide (TiO₂) thin films have been prepared by sol–gel technique. The titanium isopropoxide was used as a source of TiO₂ and methanol as a solvent and heat treated at 60°C. The as prepared powder was sintered at various temperatures in the range of 400–700°C and has been deposited onto a glass substrate using spin coating technique. The effect of annealing temperature on structural, morphological, electrical and optical properties was studied by using X-ray diffraction (XRD), high resolution transmittance electron microscopy (HRTEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), dc resistivity measurement and optical absorption studies. The XRD measurements confirmed that the films grown by this technique have good crystalline nature with tetragonal-mixed anatase and rutile phases and a homogeneous surface. The HRTEM image of TiO₂ thin film (annealed at 700°C) showed grains of about 50–60?nm in size with aggregation of 10–15?nm crystallites. Electron diffraction pattern shows that the TiO₂ films exhibited a tetragonal structure. SEM images showed that the nanoparticles are fine and varies with annealing temperature. The optical band gap energy decreases with increasing annealing temperature. This means that the optical quality of TiO₂ films is improved by annealing. The dc electrical conductivity lies in the range of 10^?6 to 10^?5?Ω?cm^?1 and it decreases by the order of 10 with increase in annealing temperature from 400°C to 700°C. It is observed that the sample Ti700°C has a smooth and flat texture suitable for different optoelectronic applications.     

TiO2 thin films for dyes photodegradation

The aim of the present study is to investigate the influence of the TiO₂ specific surface (powder, film) on the photocatalytic degradation of methyl orange. Porous TiO₂ films were deposited on transparent conducting oxide substrates by spray pyrolysis deposition. The films were characterized by X-ray diffraction (XRD), Scanning Electronic Microscopy, and the UV-Vis spectroscopy. The XRD spectra of nanoporous TiO₂ films revealed an anatase, crystalline structure that is known as the most suitable structure in photocatalysis. The average thickness of the films was 260 nm and the measured band gap is 3.44 eV. The influence of the operational parameters (dye concentration, contact time) on the degradation rate of the dye on TiO₂ was examined. There were calculated the kinetic parameters and the process efficiency. Using thin films of TiO₂ is technologically recommended but raises problems due to lowering the amount of catalyst available for the dye degradation.     

Enhancement of photosensitivity by annealing in Bi2S3 thin films grown using SILAR method

Bi₂S₃ thin films were grown by successive ionic layer adsorption and reaction method (SILAR) onto the glass substrates at room temperature. The as prepared thin film were annealed at 250 °C in air for 30 min. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and electrical measurement systems. The X-ray diffraction patterns reveal that Bi₂S₃ thin film have orthorhombic crystal structure. SEM images showed uniform deposition of the material over the entire glass substrate. The optical energy band gap observed to be decreased from 1.69 to 1.62 eV for as deposited and annealed films respectively. The I–V measurement under dark and illumination condition (100 W) show annealed Bi₂S₃ thin film gives good photoresponse as compared to as deposited thin film and Bi₂S₃ thin film exhibits photoconductivity phenomena suggesting its useful in sensors device. The thermo-emf measurements of Bi₂S₃ thin films revealed n-type electrical conductivity.     

Cr-doped TiO2 thin films deposited by RF-sputtering

Cr-doped TiO₂ thin films with different band gaps were prepared. Higher Cr doping was beneficial to the formation of the rutile-TiO₂ phase over the anatase-TiO₂ phase. A 4.8% Cr-doped thin film indicated a band gap of 2.95 eV, which was lower than the band gap of the rutile-TiO₂. Cr doping was accompanied by the formation of not only the rutile-TiO₂ phase but also the Cr₂O₃ phase, lead to the degradation of the hydrophilicity. The TiO₂ thin films with the mixed phase were not desirable to improve the hydrophilicity.