Mixed Oxide Thin Films Research Articles

Optical and mechanical properties of Sol-gel prepared Titania (TiO2) –Silica (SiO2) mixed thin films ‘as prepared at 300 K’ without any post heat treatment

The mixed metal oxide (MMO) thin films like titania-silica (TiO2–SiO2) prepared by sol-gel technology have shown a variety of interesting optical, mechanical and thermo-mechanical properties; these thin films are being used in a variety of applications. In the present investigation, the TiO2–SiO2 (TS) thin films (on glass substrates) are prepared at 300 K from non-aqueous individual sols of TiO2 and SiO2 of 0.1 molar (TS-1) and 0.3 molar (TS-3) concentrations; no post heat treatment has been conducted on these films. The thicknesses of the two layer (2 l) and four layer (4 L) thin films prepared by dip coating are in the range 27 nm to 170 nm. The optical and mechanical properties of these ‘as-deposited’ thin films have been evaluated. The ‘as-deposited’ TS thin films show an optical transparency (in the visible region) of ∼90% and optical band gaps are in the range 4.44 eV to 4.50 eV. The optical density has been calculated from the refractive index measurements. The refractive index of TS-1 thin films of thickness 27 nm is practically constant at 1.61 over the wavelength range 400–750 nm. Micro-cracks have been observed (Scanning electron micrographs) in TS-1-4L and TS-3-4L thin films. These micro cracks are more significant in TS-3-4L samples than those of TS-1-4L samples. These cracks are not seen in all other samples (TS-1-2L and TS-3-2L). These mixed titania-silica thin films show higher adherence and higher hardness compared to those of TiO2 thin films. The Raman spectroscopy measurements show the existence of multiple phases of Ti–O–Si. It is known that in the multi-phase structures, the phase boundaries hinder crack propagation and enhances the resistance to plastic deformation enhancing the mechanical hardness of these TS thin films.

Synthesis and characterization of zinc-tin-mixed oxides thin films

The mixed oxides thin films were prepared by spray pyrolysis method on glass substrates using different molar ratios of zinc and tin in the starting solution Zn/(Zn + Sn) = 25 at.%, 50 at.%, and 60 at.%. The structural, morphological and optical properties were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), Scanning Electron Microscopy (SEM) and spectrophotometry. XRD analysis indicate the coexistence of both the hexagonal phase of ZnO and the tetragonal phase of SnO2 for all mixed thin films. Microstructural parameters such as micro-strain and crystallite size suggest the good crystallinity of mixed thin film at 60 at.%. Optical analysis by means of transmittance T (λ) and reflectance R (λ) measurements allowed us to determine the optical constants for single and mixed oxides. The refractive index dispersion was adequately described by the Wemple-Di-Domenico model. The values of oscillator energy E0, dispersion energy Ed, high-frequency dielectric constant ε∞ as well as the ratio of the carrier concentration to the effective mass N/m∗ were estimated according to both Wemple-Di-Domenico and Spitzer–Fan models.

Reversible and repeatable phase transition at a negative temperature regime for doped and co-doped spin coated mixed valence vanadium oxide thin films.

Smooth, uniform mixed valance vanadium oxide (VO) thin films are grown on flexible, transparent Kapton and opaque Al6061 substrates by the spin coating technique at a constant rpm of 3000. Various elements e.g., F, Ti, Mo and W are utilized for doping and co-doping of VO. All the spin coated films are heat treated in a vacuum. Other than the doping elements the existence of only V4+ and V5+ species is noticed in the present films. Transmittance as a function of wavelength and the optical band gap are also investigated for doped and co-doped VO thin films grown on a Kapton substrate. The highest transparency (∼75%) is observed for the Ti, Mo and F (i.e., Ti–Mo–FVO) co-doped VO system while the lowest transparency (∼35%) is observed for the F (i.e., FVO) doped VO system. Thus, the highest optical band gap is estimated as 2.73 eV for Ti–Mo–FVO and the lowest optical band gap (i.e., 2.59 eV) is found for the FVO system. The temperature dependent phase transition characteristics of doped and co-doped VO films on both Kapton and Al6061 are studied by the differential scanning calorimetry (DSC) technique. Reversible and repeatable phase transition is noticed in the range of −24 to −26.3 °C.

Reduction of Bias and Light Instability of Mixed Oxide Thin-Film Transistors

Despite their potential use as pixel-switching elements in displays, the bias and light instability of mixed oxide semiconductor thin-film transistors (TFTs) still limit their application to commercial products. Lack of reproducible results due to the sensitivity of the mixed oxides to air exposure and chemical contamination during or after fabrication hinders any progress towards the achievement of stable performance. Consequently, one finds in literature several theories and mechanisms, all justified, but most of them conflict despite being on the same subject matter. In this study, we show that under an optimized fabrication process, which involves the in situ passivation of a mixed oxide semiconductor, we can reduce the bias and light instability of the mixed-oxide semiconductor TFTs by decreasing the semiconductor thickness. We achieve a negligible threshold voltage shift under negative bias combined with light illumination stress when the mixed oxide semiconductor thickness is around three nanometers. The improvement of stability in the thin mixed-oxide semiconductor TFTs is due to a reduced number of oxygen-vacancy defects in the bulk of the semiconductor, as their total number decreases with decreasing thickness. Under the optimized fabrication process, bulk, rather than interfacial defects, thus seem to be the main source of the bias and light instability in mixed oxide TFTs.

CeO2–TiO2 mixed oxide thin films with enhanced photocatalytic degradation of organic pollutants

A series of CeO2–TiO2 mixed oxides were prepared by the peroxo sol–gel method. X-ray diffraction, transmission electron microscopy, ultraviolet–visible spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy were used to investigate the characteristics of CeO2–TiO2 sols and thin films in order to determine the influence of adding CeO2 to TiO2 on the photocatalytic degradation of methylene blue aqueous solution under both ultraviolet and visible light irradiation. The pH values of the as-prepared CeO2–TiO2 sols were neutral; the as-prepared sols contained nanocrystals in colloidal suspensions, and there was no subsequent calcination process. It was observed that the highest photocatalytic degradation activities with respect to methylene blue under both UV and visible light irradiation were exhibited for an optimum CeO2–TiO2 weight ratio of 0.05. The high photocatalytic activity was because of changes in the spectral absorption of material, which was attributed to the heterojunction formed by TiO2 and CeO2 networks via Ti–O–Ce bonds after addition of CeO2. Thus, the presence of CeO2 in TiO2 significantly enhanced the photocatalytic activity.

Electrochromic W1–x–yTixMoyO3 Thin Films Made by Sputter Deposition: Large Optical Modulation, Good Cycling Durability, and Approximate Color Neutrality

Tungsten oxide thin films are used in electrochromic devices such as variable-transmittance “smart windows” for energy efficient buildings with good indoor comfort. Two long-standing issues for WO3 thin films are their limited durability under electrochemical cycling and their blue color in transmission. Here, we show that both of these problems can be significantly alleviated by additions of titanium and molybdenum. We found that ∼300 nm-thick films of sputter deposited W1–x–yTixMoyO3 are able to combine a midluminous transmittance modulation of ∼70% with good color neutrality and durability under extended electrochemical cycling. The Ti content should be ∼10 at. % in order to achieve durability without impairing transmittance modulation significantly, and the Mo content preferably should be no larger than 6 at. % in order to maintain durability. Hence, our results give clear guidelines for making three-component mixed-oxide thin films that are suitable for electrochromic “smart windows”.

Spray Pyrolysis Deposition of Single and Mixed Oxide Thin Films

The influence of processing parameters is investigated on the structural characteristics of single and mixed oxides produced by spray pyrolysis technique. The films were synthesized by spraying precursor solutions through a noz-zle onto a heated alumina substrate. The precursor consisted separately of aqueous solutions of tin chloride for SnO2 and zinc chloride for ZnO for single oxide cases, and aqueous solutions of tin chloride and indium nitrate for SnO2 + In2O3 and zinc chloride and indium nitrate solutions for ZnO + In2O3 for mixed oxide cases. The substrate temperature was varied accordingly for each single and mixed case. The films produced were characterized by X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy. The results indicate that a non-homogenous film is formed at low temperature for both single oxides considered. The temperature has significant effect on the composition of the synthesized films of both single oxides below 450℃. The results for mixed oxides show that the best homogeneous films are obtained for 80 wt% ZnO + 20 wt% In2O3, and 80 wt% SnO2 + 20 wt% In2O3.

Electrochemical behavior of spray deposited mixed nickel manganese oxide thin films for supercapacitor applications

The relatively low energy density is a crucial issue obstructing the growth of supercapacitors as energy storage devices. Various tactics have been developed to enhance the energy density. Nickel manganese oxide thin films have been synthesized by spray pyrolysis on preheated amorphous and FTO coated glass substrates. These films are characterized for structural, morphological and optical properties using X-ray diffraction, scanning electron microscopy and UV–Vis spectrophotometer. XRD study illustrates formation of spinel cubic NiMn2O4 as prominent phase with crystalline size 11 nm. SEM shows fractured surface morphology; wherein small pores on the surface are covered by the particles. Optical band gap of 1.98 eV is observed. The electrochemical performance of NiMn2O4 electrode is studied by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements. The maximum specific capacitance is found to be 460 Fg−1 at scan rate of 5 mVs−1. The excellent rate capability and long term stability suggests NiMn2O4 thin film to be good candidate for electrochemical supercapacitors.

Transparent conducting ZnO-CdO mixed oxide thin films grown by the sol-gel method

Mixed oxides of zinc and cadmium with different proportions were deposited on ordinary glass substrates using the sol-gel spin coating method under optimized deposition conditions using zinc acetate dihydrate and cadmium acetate dihydrate as precursors. X-ray diffraction patterns confirmed the polycrystalline nature of the films. A combination of cubic CdO and hexagonal wurtzite ZnO phases was observed. The oxidation states of Zn, Cd and O in the deposited films were determined by X-ray photoelectron spectroscopic studies. Surface morphology was studied by scanning electron microscopy and atomic force microscopy. The compositional analysis of the thin films was studied by secondary ion mass spectroscopy. The transmittance of the thin films was measured in the range 300–800nm and the optical bandgap was calculated using Tauc’s plot method. The bandgap decreased from 3.15eV to 2.15eV with increasing CdO content. The light emission properties of the ZnO:CdO thin films were studied by photoluminescence spectra recorded at room temperature. The current-voltage characteristics were also assessed and showed ohmic behaviour. The resistance decreased with increasing CdO content.

Optimization of RF Sputtered Ag-Doped BaTiO3-CuO Mixed Oxide Thin Film as Carbon Dioxide Sensor for Environmental Pollution Monitoring Application

Thin films of BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -CuO doped with Ag have been evaluated for their carbon dioxide sensing characteristics. The effect of various parameters, such as Ag doping, annealing conditions, film thickness, electrode dimensions, and operating temperature on sensitivity has been explored. The thin films in the range of 250 nm, with 1% Ag doping and no post-deposition annealing, display the best sensitivity at an optimum operating temperature of 250 °C. We correlate sensitivity with material characteristics as determined by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope, and electron probe microscopic analysis. Sensitivity to CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> increases with the reduction in electrode dimensions, with 500 nm electrodes giving the best sensitivity of 19% for 350 ppm and 80% for 1000 ppm of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . The sensor shows very low cross sensitivity to CO, NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> .

Optical and IR studies on (MoO3)1−x–(WO3)x mixed oxide thin films

Optical and IR studies on (MoO3)1−x–(WO3)x mixed oxide thin films

Reversible Magnetization Modulation in Ordered Mesoporous Mixed Metal Oxide Thin Films By Double Layer Charging and Topotactic Lithium Insertion

Mesostructured metal oxide thin films have received much attention in recent years. The primary reason for this is the fact that many such materials have been shown to outperform their bulk counterparts. Energy storage and conversion is one of the fields where this is very evident. Advances in polymer templating over the past 10 years have enabled the preparation of various thin film materials with cubic and 2D-hexagonal pore structures and different ordering lenghts.[1] Their formation relies in principle on the coassembly of either sol-gel precursors or preformed building blocks with an amphiphilic polymer structure-directing agent. Despite the progress made so far, complex metal oxides (mixed metal oxides) often exhibit an ill-defined mesoporous morphology after removal of the polymer template and thermally-induced crystallization because of their limited stability. Here, we describe the evaporation-induced self-assembly (EISA)[2] synthesis of cubic mesoporous mixed metal oxide thin films with >15 nm diameter pores and nanocrystalline walls by using a large polyisobutylene-block-poly(ethylene oxide) diblock copolymer. We focus specifically on perovskite-type La1–x Sr x MnO3 (LSMO) and La1–y Ca y MnO3 (LCMO) as well as spinel-type LiFe5O8. These materials were characterized by various state-of-the-art techniques, including electron microscopy, GISAXS, RBS, XRD and XPS, and were found to be of high quality after calcination at elevated temperatures. For LSMO and LCMO, we show that the charge carrier density, and therefore the magnetization, can be modulated through double layer charging (i.e., electrostatic hole- or electron-doping).[3] Magnetization modulation values of up to 10% have been achieved, the highest thus far reported for electrolyte-gated mixed-valence manganese oxides. Apart from that, we show that the room temperature magnetization of LiFe5O8 can be tuned by topotactic insertion/extraction of Li-ions into/from the spinel lattice and provide insights into the mechanism. Both processes allow for control of magnetism, are highly reversible, and strongly benefit from the high surface-to-volume ratio of the films. [1] C. Sanchez, C. Boissiere, D. Grosso, C. Laberty, and L. Nicole, Chem. Mater.20, 682 (2008). [2] C. J. Brinker, Y. F. Lu, A. Sellinger, and H. Y. Fan, Adv. Mater. 11, 579 (1999). [3] C. Reitz, P. M. Leufke, R. Schneider, H. Hahn, and T. Brezesinski, Chem. Mater. 26, 5745 (2014).

Structural and optoelectronic characteristics of nanocrystalline silicon oxide film as absorber layer for thin film solar cells

The mixed phase nanocrystalline silicon oxide (nc-SiOx:H) thin films have been prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) system from SiH4, H2 and CO2 mixture. The effect of oxygen incorporation on the structural and opto-electrical properties of nc-Si:H films was investigated. As the CO2 flow rate increased, the crystalline volume fraction in the films decreased, while the optical band gap increased slowly. During the phase transition, both oxygen and hydrogen content increased drastically with the increasing CO2 flow rate, while the Urbatch energy and microstructure factor showed obvious reduction, which indicated an enhancement of the medium range order of the films and good passivation of the inner voids. Device quality nc-SiOx:H film was fabricated with low microstructure factor of 0.32 and high crystalline volume fraction around 50%, along with wide optical band gap of ∼1.75 eV, low dark-conductivity σd ∼7.6 × 10−6 S/cm and high photosensitivity of about 2 × 102. On flat substrate a solar cell was prepared with high Voc of 0.62 V and Jsc of 14.57 mA/cm2. These results revealed the potential of nc-SiOx:H film as an absorber layer for thin film solar cells.

Tailoring the pseudocapacitive behavior of electrochemically deposited manganese-nickel oxide films

Manganese-nickel mixed oxide thin films were deposited by anodic electrodeposition on stainless steel substrate from an acetate solution using a potentiodynamic technique, at scan rate of 100mVs−1 and room temperature. The effect of electrolyte pH, varied in the range from 4 to 7, on composition, morphology and capacitance behavior of oxide thin films was investigated. The nickel content in the oxide increased with increasing deposition pH, allowing to investigate the effect of the oxide composition on the capacitive behavior of as-grown manganese-nickel mixed oxides. Oxide films deposited from the electrolyte at pH 6, having a composition close to Ni0.10Mn0.90Ox showed the highest specific capacitance and the lowest charge transfer resistance. After annealing, the oxide had a complex structure of composite nature, consisting of intermixed amorphous and nanocrystalline phases. A birnessite type oxide with turbostratic disorder was identified as the major phase, in the presence of nickel hydroxide as a finely dispersed second phase. Annealing caused a drastic reduction of the charge transfer resistance and a limited increase of the specific capacitance, probably as the result of diverging effects on oxide properties, i.e. enhanced conductivity and porosity sintering. Cycle life testing of this material revealed a 25% increase of the specific capacitance over 5,000 cycles to a final value of 225Fg−1 (1M Na2SO4, 50mVs−1, 0.11mgcm−2 mass loading).

The influence of the pressure on the microstructure of yttria-stabilized zirconia thin films deposited by dual magnetron sputtering

Mixed oxide thin films, such as yttria-stabilized zirconia, deposited by dual reactive magnetron sputtering on a non-rotating substrate show a typical microstructure of bended, or tilted columns. Two effects define the tilt. The first effect is the compositional gradient over each column which results in a different lattice spacing. To accommodate this difference, the column bends. As such, the chemical composition has a major influence on the final columnar tilt. The second effect is ballistic shadowing which is controlled by the pressure-distance product. At higher pressure-distances, this second effect plays a more prominent role, and a different behaviour of the columnar tilt as a function of the film composition is noticed. The experimental trends can be understood by the use of a particle trajectory code which provides the angular and energy distribution of the atoms to a ballistic aggregation Monte Carlo code simulating the resulting microstructure.

Influences of post-annealing temperature on the structural and electrical properties of mixed oxides (CuFeO2 and CuFe2O4) thin films prepared by spray pyrolysis technique

P-type mixed oxides (CuFeO2 and CuFe2O4) transparent conducting thin films have been successfully deposited on p-type Si (111) substrates at 450°C by spray pyrolysis deposition (SPD) and annealed at 800°C for 2h. The crystal structure, surface morphology and electrical property have been investigated. It is observed that the CuFeO2 and CuFe2O4 thin films as deposited and annealed, have polycrystalline hexagonal structure and the crystallite size increases by annealing processes. The electrical property of the Ni/CuFeO2/Si Metal–Semiconductor–Metal (MSM) photo detectors was investigated using the current–voltage (I–V) measurements. The barrier heights ϕΒ of Ni/CuFeO2/Ni MSM thin films of as deposited and annealed on Si substrates were calculated and its values are 0.478, 0.345eV, respectively with an applied bias voltage of 3V.

Structural characterization and electrochemical properties of cerium–vanadium (Ce–V) mixed oxide films synthesized by chemical route

Mixed oxide thin films of cerium and vanadium were obtained by chemical bath method from Ce(NO3)3 and 6H2O, NH4VO3 with distilled water as solvent in the presence of polyethylene glycol (PEG) as surfactant and dip coated on non-conducting quartz substrate. The optical, structural and morphological properties depend on post-deposition temperature. No remarkable effect was observed upon the addition of surfactant. The amorphous to polycrystalline transition of the films revealed that the inclusion of CeO2 was responsible for the widening of the energy bandgap 3.8eV. The supercapacitive behavior of the electrode was evaluated using cyclic voltammetry (CV) and galvanostatic charge discharge techniques in 1M LiClO4. The specific capacitance of 179F/g was achieved for the mixed oxide electrode at a scan rate of 20mV/s.

Effect of Annealing on the Development of Fully Transparent Ternary V-O-Na-Si Mixed Metal Oxide Thin Films from Polymer-Assisted Dip-Coated V2O5

Both transparent oxides and transparent conductive oxides are of particular research interest for future applications in flexible, optically transparent thin film transistors and luminescent devices. We report the formation of a transparent oxide material based on the interdiffusion of Na-O and Si-O species with dip-coated V2O5 thin films on a borosilicate glass substrate. The deposition process used a facile solution processed dip-coating technique in the high-rate draining regime. Liquid precursors of vanadium alkoxide and alkoxide-polyethylene glycol mixtures were used for thin film deposition. We examine the effect of annealing condition on the phase conversion process, morphology and optical transmittance due to the conversion of the V2O5 films to completely transparent ternary mixed metal oxide thin films. The work also examines the role of polymer-assisted deposition on the development of the V-O-Na-Si transparent thin films during different annealing conditions. Polymer-assisted V2O5 thin films on glass are shown to convert to optically clear thin films during annealing, with a transparency >95% across the visible spectrum, and a blue-shift of the absorption edge to maintain >90% transparency at 380 nm.

Molecular Insight in Structure and Activity of Highly Efficient, Low-Ir Ir–Ni Oxide Catalysts for Electrochemical Water Splitting (OER)

Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally prepared Ir-Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photoelectrochemical cells (PEC). Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over pure Ir oxide. In situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anticorrelated. To uncover activity and stability controlling parameters, the Ir-Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an intuitive model for the formation of the catalytically active state of the bimetallic Ir-Ni oxide surface, we identify the coverage of reactive surface hydroxyl groups as a suitable descriptor for the OER activity and relate it to controllable synthetic parameters. Overall, our study highlights a novel, highly active oxygen evolution catalyst; moreover, it provides novel important insights into the structure and performance of bimetallic oxide OER electrocatalysts in corrosive acidic environments.

Effect of W–Ti target composition on the surface chemistry and electronic structure of WO3–TiO2 films made by reactive sputtering

Tungsten–titanium (W–Ti) mixed oxide thin films were fabricated using reactive sputtering of W–Ti alloy targets with Ti content ranging from 0 to 30wt%. The effect of target composition on film structure, surface/interface chemistry and chemical valence state of the W and Ti cations was investigated in detail. All films were amorphous in nature as confirmed by X-ray diffraction analyses. For growth times of 1h, the thicknesses of the W–Ti–O films decreased significantly from 150 to 35nm with increasing Ti content in the target, confirming that the oxide film growth behavior is dependent on the sputter-target composition. The chemistry and composition of the films probed using X-ray photoelectron spectroscopy (XPS) confirms the existence of W and Ti in their highest oxidation states of 6+ and 4+, respectively. Quantification of binding energy shifts for W and Ti core-level transitions confirms the formation of WO3–TiO2 composite oxide films. Depth profiles confirm and validate film uniformity, as well as film thickness differences and variable oxidation behavior of W and Ti in the films.