Cobalt Oxide Thin Films Research Articles

Pen plotter printing of Co3O4 thin films: features of the microstructure, optical, electrophysical and gas-sensing properties

Abstract Сombining the sol–gel method and pen plotter printing, using the hydrolytically active heteroligand complex [Co(C5H7O2)2-x(C4H9O)x] as a component of a new functional ink, Co3O4 thin films were produced on the surface of substrates of various types. The influence of synthesis conditions and print modes on the microstructure, optical, electrophysical and sensor properties of planar nanomaterials was comprehensively studied. The optical band gap energies associated with charge transfer for transitions O2-→Co3+ and O2-→Co2+ of the obtained thin films were evaluated. Using Kelvin probe force microscopy and impedance spectroscopy, the electrophysical characteristics (the electronic work function of the film surface, the temperature dependence of electrical conductivity, and the activation energy of electrical conductivity) of the obtained Co3O4 films were determined. The sensitivity of printed cobalt(II, III) oxide thin films with respect to various gases (Н2, СН4, СО and NO2) was studied. It was established that the obtained samples demonstrated the highest sensor response when detecting CO and NO2 in the operating temperature range of 150–200°С. The prospects of the proposed synthesis method and printing technology when forming Co3O4 thin-film nanostructures to create electrodes of supercapacitors and receptor components of resistive CO and NO2 gas sensors were shown.

Surfactant free stable cobalt oxide nanocolloid in water by pulsed laser fragmentation and its thin films for visible light photocatalysis

Stable, pure nanocolloid of cobalt oxide (Co2O3) in water was prepared by pulsed laser fragmentation in liquid (PLFL) technique. Cobalt oxide powder with black color turned to golden yellow for the nanocolloid after laser fragmentation in water. The structure and morphology of the nanoparticles were analyzed by transmission electron microscopy. Spherical nanoparticles of cobalt oxide (∼10−20 nm) were obtained by PLFL. Nanostructured cobalt oxide thin films were fabricated by spray deposition of the cobalt oxide nanocolloid onto heated glass substrates. Scanning electron microscopy analysis showed that the thin films surfaces were uniform with some porosity, layered nature that had a transformation on thermal treatments. Raman spectroscopic results matched with the peaks for Co2O3 in literature. XPS results of the surface of the films exhibited a difference in oxygen content which was confirmed by XPS mapping. Photocatalytic activities for the as prepared and annealed thin films were studied using methylene blue as a contaminant under irradiation of visible light from a solar simulator. The results showed good visible light photocatalytic activity for the cobalt oxide nanostructured thin films and an enhancement with increase in the annealing temperature as compared to the commercial cobalt oxide powder.

Investigation of Supercapacitive Behaviour of Electrodeposited Cobalt Oxide Thin Film by Potentiostatic Mode

Investigation of Supercapacitive Behaviour of Electrodeposited Cobalt Oxide Thin Film by Potentiostatic Mode

Structured cobalt oxide catalysts for VOC abatement: the effect of preparation method.

Magnetron sputtering was employed for the deposition of cobalt oxide thin films on stainless steel meshes. Catalysts prepared by sputtering in inert and oxidation atmosphere were compared with those obtained by electrochemical deposition and hydrothermal synthesis. Systematic characterization using X-ray diffraction, scanning electron microscopy, N2 physisorption, infrared spectroscopy, Raman spectroscopy, and temperature-programmed reduction by hydrogen allowed detailed monitoring of their physicochemical properties. Ethanol gas-phase oxidation was employed as a model reaction to reveal the catalytic performance of the catalysts. It was shown that the catalyst prepared by magnetron sputtering in oxidation atmosphere exhibited the best mechanical stability among all studied catalysts. Moreover, its catalytic activity was 18 times higher than that of pelletized commercial cobalt oxide.

Effect of annealing temperature on the thermal transformation to cobalt oxide of thin films obtained via chemical solution deposition

An ammonia-free chemical solution deposition formulation is employed to synthesize cobalt hydroxide thin films. The films are subsequently annealed at different temperatures to obtain cobalt oxide (Co3O4). The reaction solution is composed solely of cobalt sulfate and triethanolamine. The annealing temperature has a strong effect on the properties of thin films. X-ray diffraction shows that the films are amorphous despite the thermal treatment; the morphology for both as-deposited and annealed films are found to be homogeneous, compact, with no evident changes. X-ray photoelectron spectroscopy reveals that the as-deposited thin films are mainly composed of cobalt hydroxide. After annealing, the films transform into cobalt oxide (Co3O4) thin films. The optical spectra confirm the X-ray photoelectron spectra, due to the appearance of characteristic features of Co3O4 on both characterization methods. The energy band gap is estimated to lie between 2.1 and 3.0 eV. The resistivity of the annealed films ranges from 4.29 × 103 to 1.46 × 104 Ω⋅ cm. Finally, with the work function and ionization energy, we propose an experimental band diagram as a function of annealing temperature, showing a p-type nature for the cobalt oxide films.

Statistical, morphological, and corrosion behavior of PECVD derived cobalt oxide thin films

Experimental parameters have direct influences on materials properties and therefore their applications. The effect of plasma power on the properties of cobalt oxide thin films, prepared using plasma-enhanced chemical vapor deposition technique, on stainless steel substrates have been addressed in this paper. The structural, morphological, and compositional properties of these films were investigated by means of X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) technique. The XRD patterns demonstrated the growth of polycrystalline Co3O4 thin film with a cubic spinel structure such that the intensity of (511) and (311) peaks increase as the plasma power increases to 100 W. It is observed that crystallite size increases by increasing the plasma power and the maximum crystallite size is found to be 64.8 nm for 100 W. The AFM results illustrate that the surface roughness and grain size increase by increasing the plasma power, and the film deposited at lower plasma power has more uniform and smoother surface, mainly owing to the increase in surface diffusion that in turn causes the coalescence of the grains. The results of XPS spectra indicated the formation of Co3O4 thin films on stainless steel substrates and there were no other elements other than Co, O in the XPS spectra. Additionally, stereometric analysis and fractal dimension of the 3-D surface microtexture of the AFM micrographs were analyzed and the Kolmogorov–Smirnov test was used to assess the normal distribution of quantitative variables. The results of statistical analysis corroborated the experimental results and proved that the surface roughness increased upon an increase in plasma power. Moreover, the corrosion behavior and the surface morphology of the cobalt oxide thin films were investigated using the potentiodynamic method and scanning electron microscopy. The results of these analysis proved that as the plasma power increases the corrosion resistance improves against the H2SO4. The sample which deposited at 100 W plasma power has the minimum corrosion current and the corrosion resistance of steel substrate was improved by controlling the anodic reactions resulted from a protective Co3O4 thin film. These results are useful for building and designing stainless steel devices in corrosive environments.

Molar optimization of MnO2 to form composite with Co3O4 by potentiodynamic electrodeposition for better electrochemical characterizations

Manganese incorporated cobalt oxide thin films were deposited on stainless steel by using potentiodynamic electrodeposition via aqueous route. The structural elucidation reveals face-centered cubic Co3O4 and orthorhombic MnO2 having polycrystalline nature. FESEM and TEM show porous granular surface morphology along with nano-spikes. AFM image exhibits granular morphology. Optimized samples were studied for further electrochemical characterizations. All CV curves show mixed capacitive behavior. As compared to others, 1% manganese incorporation electrode shows maximum specific capacitance 605.39 F/g at 2 mV/s in 1M KOH. Chronopotentiometric charge–discharge studies report power density 18.12 kW/kg, energy density 33.7 Wh/kg and columbic efficiency 73.89%. To know the internal resistive properties of the electrode, the electrochemical impedance analysis was carried out in the frequency range 1 mHz to 1 MHz. Using Nyquist plot, the observed internal resistance is ~ 0.78 Ω. ZsimpWin software was used to develop Randle’s equivalent circuit to search the circuitry parameters associated with the cell.

Two Kinds of Metastable Structures in an Epitaxial Lanthanum Cobalt Oxide Thin Film.

Thin films have attracted much interest because they often have novel properties different from those of their bulk counterparts. In this work, we tune two metastable states in three kinds of lanthanum cobalt oxide thin films by electron beam irradiation and record their dynamic transition process in situ in a transmission electron microscope. The lanthanum cobalt oxide thin films exhibit a homogeneous microstructure in the initial state and then transfer to a stripelike superstructure with 3a0 periodicity (a0 is the perovskite lattice parameter), further developing into a superstructure with 2a0 periodicity in dark stripes (brownmillerite structure). To explore the inherent energy discrepancy within the two metastable states, we perform first-principles calculations on a LaCoO3-δ (0 ≤ δ ≤ 0.5) thin film system by geometry optimization. The calculation results suggest that the forming energy of the 3a0 periodicity stripelike structure is a little lower than that of the 2a0 periodicity in the LaCoO3-δ thin film. Our work explains why the two stripelike structures coexist in lanthanum cobalt oxide thin films and extends prospective applications related to oxygen vacancies in thin films.

(Invited) Tracking Structures in Solar Fuels Catalysis: In-Situ X-Ray Structure Characterization of Interfacial Water-Splitting Molecular and Thin-Film Catalysts

A key challenge for the design of efficient photoelectrochemical devices that employ nanoscaled interfacial designs for solar energy-to-fuels conversion lies in resolving atomic structures and dynamics at the active interfaces, and relating these to the complex cascade of events which includes excited-state charge separation, charge-accumulation, and multi-step energy conversion and catalysis. Our research programs have been developing in-situ, time-resolved X-ray techniques aimed at the resolution of atomic scale structures that underlie interfacial charge transfer and photo-driven water-splitting catalysis under conditions relevant to photoelectrochemical function for both interfacial thin-films and molecular photosensitizers and catalysts bound to semiconductor surfaces. This presentation will discuss our development of X-ray techniques aimed at the resolution of atomic scale structures that underlie interfacial charge transfer and photo-driven water-splitting catalysis under conditions relevant to photoelectrochemical function. We have developed high surface area TiO2 semiconductor and ITO and IZO conducting oxide porous assemblies that duplicate oxide-supported dye and catalyst architectures widely used in dye sensitized solar cells (DSSC) and photoelectrochemial (PEC) electrodes, but have been tailored to allow for interfacial X-ray structure characterization. Thin-Film Metal-Oxide Water Oxidation Catalysts. Non-noble-metal, thin film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of L-edge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, while CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4p-3d hybridization through Co-O-Co bonding was detected by RXES and the inter-site dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure while CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance OEC performance.The combined electronic and structural analyses demonstrate that hole transfer to catalytic sites in amorphous cobalt oxide thin films is the rate-limiting step for electrochemical water-splitting rather than the multi-step catalytic events themselves. The macroscopic catalytic properties of the thin films were found to be correlated to the electronic structures measured at the atomic scale for the metal-oxo cluster domains. These results show the interplay between intrinsic catalytic activity and charge transport properties of semiconductor thin-film catalysts. In-situ Structure-Function Analysis of Molecular Water Oxidation Catalysts. Building from the microporous electrodes for in-situ X-ray analyses of amorphous oxide thin-film OECs, we have extended this approach by the development of nano-porous electrode architectures which enable the use of combined PDF and X-ray spectroscopy analyses of coordination structures for molecular photosensitizers and catalysts in solution and when bound to semiconductor oxide surfaces. PDF measurements require PEC oxide supports with sufficiently low background scattering to permit detection of the surface bound complexes. We have found that atomic layer deposition of semiconductor oxides on anodic aluminium oxide templates, AAO, provides a suitable layered, high-surface area architecture. PDF patterns measured for the N3 dye, cis-bis(isothiocyanato) bis(2,2’-bipyridyl-4,4’-dicarboxylato) ruthenium(II), bound to an amorphous TiO2 surface reveal details on outer sphere ligand structures. Each of the PDF peaks can be assigned to the atom pair distances calculated from crystal and DFT structures. Measurements on ruthenium photosensitizer complexes bound to the TiO2 surface resolve distortions in the surface-coordinating ligand structures. On-going work is investigating the correlation between ligand structural distortion and the metal-to-ligand charge-transfer (MLCT) states with interfacial charge injection function.

Influence of pH and annealing on the optical and electrochemical properties of cobalt (III) oxide (Co3O4) thin films

Cobalt (III) oxide (Co3O4) thin films were deposited on glass and FTO substrates using chemical bath deposition method at a temperature of 80 °C in the alkaline bath maintaining the pH values of 10 and 12. X-ray diffraction result reveals that the crystallinity increased with increasing pH and annealing. Scanning electron microscopy (SEM) revealed features of flower-like particles, which transformed into a needle and rod-like structures after annealing. The optical studies showed a higher absorption in the visible region, which decreases as the wavelength increases, energy band gap range of 1.3 eV–1.85 eV and a remarkable refractive index of 1.40–1.75. The electrochemical testing of the electrodes obtained the highest specific capacitance of 1567F/g, the energy density of 245.98 (Wh/Kg) and power density of 2.08 (W/Kg) for pH 12 annealed. Specific capacitances, power densities and energy densities values obtained from electrochemical studies suggest that the films are suitable for use as supercapacitors electrodes materials.

Metal–Organic Frameworks as Porous Templates for Enhanced Cobalt Oxide Electrocatalyst Performance

Water oxidation to dioxygen is an essential energy capture process in biological systems mediated by the oxygen-evolving complex. It continues to attract interest as a half-reaction in electrochemical water-splitting to hydrogen and dioxygen in abiotic systems. There is a corresponding interest in improving the activity of nonprecious metal electrocatalysts for this reaction. One of the most well-known water oxidation catalysts is amorphous cobalt oxide. This catalyst can operate in water at neutral pH with low overpotentials and is known to continually reform under catalytic conditions, a type of self-healing. The active sites for the electrocatalytic reaction are thought to be adjacent octahedral cobalt sites in the bulk material, meaning that only the surface of the material is active. Thus, catalytic activity could be improved by increasing the surface area to volume ratio of the active material. To this effect, codeposited microporous materials such as metal organic frameworks (MOFs) can be used to control the morphology of cobalt oxide thin films. This study employs a highly stable MOF, UiO-66, to act as a templating agent for an electrodeposited cobalt oxide film. By depositing this porous crystalline MOF on the electrode surface prior to the electrochemical growth of the catalyst, we show the catalytic current density of the cobalt oxide material can be increased relative to the unmodified material. Through cyclic voltammetry (CV), controlled potential electrolysis (CPE), and electron microscopy studies, we show that the increase in current density is likely due to an increase in the effective surface area of the catalyst resulting from catalyst deposition around the MOF particles.

Study of structural phase transition in nanocrystalline Cobalt Oxide thin films by pulsed laser deposition

In this work, nanocrystalline cobalt oxide thin films have been deposited on fused quartz substrates using pulsed laser deposition followed by thermal treatment at different annealing temperatures. The structural properties were characterized by XRD and revealed a phase transition from cubic CoO to spinel Co3O4 structure in the films annealed at 873–1073 K, whereas mixed phases of CoO and Co3O4 were obtained in films annealed at 773 K, morphological properties obtained by SEM revealed nanometric spherical-like particulates, the optical properties were analyzed by UV–vis spectroscopy and revealed a redshift in band gap. A mathematical model was proposed to find empirical relations to evaluate band gap, crystallite size, and microstrain as a function of annealing temperature, these relations show great coincidence with the experimental data and the data reported in the literature.

RETRACTED ARTICLE: Surface morphology, optical properties and Urbach tail of spray deposited Co3O4 thin films

Cobalt oxide (Co3O4) thin films were deposited onto glass substrates by a simple spray pyrolysis technique using Co (CH3COO)2 .4H2O as a precursor material. The as-deposited films were characterized using field emission scanning electron microscope (FESEM) and UV–visible spectrophotometer. FESEM images showed uniform and well-aligned nanofiber growth of Co3O4 thin film. The average crystallite size was found to increase from 15 to 25 nm with the increase of substrate temperature. The increase in average crystallite size may be due to the agglomeration and recrystallization of cobalt oxide nanoparticles. The absorption peaks obtained at 355 nm and 540 nm were attributed to the transitions taking place between oxygen and cobalt charge transfer (O2− → Co3+ and O2− → Co2+). The energy band gap was found to decrease from 2.35 to 2.03 eV and Urbach energy (Eu) increased from 0.73 eV to 0.84 eV with the increased substrate temperatures from 250 °C to 350 °C. The optical absorption coefficient exhibited an exponential dependence of photon energy following Urbach’s rule, and Urbach tail was dependent on lattice defects. The direct band gap of the Co3O4 thin film clearly showed a red shift with increasing temperature and provided an absorption favorable for bio-sensing applications.

Electrochromic and photoluminescence properties of cobalt oxide thin films prepared by spray pyrolysis

Cobalt oxide thin films were prepared by a facile spray pyrolysis technique, using a perfume atomizer with an aqueous solution of hydrated cobalt chloride salt with a molar concentration of 0.025 M as a source of cobalt. The films were deposited onto glass substrates at temperature of 350 °C. The structural, morphological, and electrochromic properties of the obtained films were studied. It was found from X-ray diffraction analysis that the films were polycrystalline in nature with spinel-type cubic structure and preferred orientation along [111] direction. The Scanning Electron Microscopy images revealed a porous structure with the average grain size around 200 nm. The cyclic voltammetry measurements revealed that Cobalt oxide thin film is an anodically coloring electrochromic material with a transmittance variation in the visible range of 31%, and a fast response time (about 2 seconds) and a good cycling stability. These electrochromic performances make cobalt oxide thin film an attractive material for using as an anodic electrochromic material in smart windows devices. The photoluminescence spectra exhibited a strong emission in the visible region confirming the good crystallinity properties of Co3O4 thin films.

Atomic layer deposition of cobalt(II) oxide thin films from Co(BTSA)2(THF) and H2O

In this work, we have studied the applicability of Co(BTSA)2(THF) [BTSA = bis(trimethylsilyl)amido] (THF = tetrahydrofuran) in atomic layer deposition (ALD) of cobalt oxide thin films. When adducted with THF, the resulting Co(BTSA)2(THF) showed good volatility and could be evaporated at 55 °C, which enabled film deposition in the temperature range of 75–250 °C. Water was used as the coreactant, which led to the formation of Co(II) oxide films. The saturative growth mode characteristic to ALD was confirmed with respect to both precursors at deposition temperatures of 100 and 200 °C. According to grazing incidence x-ray diffraction measurements, the films contain both cubic rock salt and hexagonal wurtzite phases of CoO. X-ray photoelectron spectroscopy measurements confirmed that the primary oxidation state of cobalt in the films is +2. The film composition was analyzed using time-of-flight elastic recoil detection analysis, which revealed the main impurities in the films to be H and Si. The Si impurities originate from the BTSA ligand and increased with increasing deposition temperature, which indicates that Co(BTSA)2(THF) is best suited for low-temperature deposition. To gain insight into the surface chemistry of the deposition process, an in situ reaction mechanism study was conducted using quadrupole mass spectroscopy and quartz crystal microbalance techniques. Based on the in situ experiments, it can be concluded that film growth occurs via a ligand exchange mechanism.

Efficient perovskite solar cells with negligible hysteresis achieved by sol–gel-driven spinel nickel cobalt oxide thin films as the hole transport layer

Current–voltage hysteresis is a critical issue that impacts the photovoltaic performance of perovskite solar cells, and thus, it is imperative to develop high-efficiency perovskite solar cells without hysteresis behavior.

CADMIUM-DOPED Co3O4 THIN FILMS: SYNTHESIS AND CHARACTERIZATION

This paper contains the results of the structural and spectroscopic characterizations of undoped and Cadmium-doped cobalt oxide thin films with different Co/Cd molar ratios (3%, 5%, 7% and 9%). The nanosized undoped and cadmium-doped Co3O4 thin layers were prepared using sol–gel process and deposited on glass substrates by dip coating. The changes caused by the incorporation of cadmium at different levels of doping have been highlighted by the techniques UV–Visible (UV–Vis) spectroscopy, Infrared (IR) spectroscopy, X-ray diffraction (XRD) measurements, SEM coupled EDX and impedance spectroscopy. From the UV–Vis spectroscopy analysis, it was found that all the films are two direct bandgap energies. The optical transmittance and the bandgap energy decrease with increase in Cd concentration. The XRD spectra confirm the films were polycrystalline with a cubic spinel structure. The results of the impedance spectroscopy show that the equivalent circuit of the synthesized samples is an RC parallel circuit.

Electrodeposition of nano crystalline cobalt oxide on porous copper electrode for supercapacitor

The cobalt oxide thin films were prepared by electrodeposition methods on to porous copper substrate. As-deposited films were obtained as Co3O4 films with the help of heat treatment at 300 °C for 2 h. Their structural and surface morphological properties were investigated by using X-ray diffraction, Fourier transformation analysis and field emission scanning electron micrograph (FESEM), atomic force microscope, energy dispersive X-ray analysis techniques (EDAX). The X-ray diffractogram reveals the formation of cobalt hydroxide CoOOH with orthorhombic crystal structure and cobalt oxide Co3O4 with cubic crystal structure. The FESEM micrographs confirms the morphology of prepared films, also EDAX spectra confirms the presence of Co and O elements of the solid films. The electrochemical performance of the films was studied in aqueous 1 M KOH electrolyte using cyclic voltammetry. The cyclic voltammograms exhibits symmetric nature and increase in capacitance with respect to the film thickness. The maximum specific capacitance of cobalt hydroxide CoOOH is 602 F g−1 and cobalt oxide Co3O4 is found to be as 630 F g−1. The above investigation shows that low-cost cobalt oxide electrode can be a potential application in supercapacitor.

Nickel Doping on Cobalt Oxide Thin Film Using by Sputtering Process-a Route for Surface Modification for p-type Metal Oxide Gas Sensors

This study proposes a route for surface modification for p-type cobalt oxide-based gas sensors. We deposit a thin layer of Ni on the Co oxide film by sputtering process and annealed at 350 °C for 15 min in air, which changes a typical sputtered film surface into one interlaced with a high density of hemispherical nanoparticles. Our in-depth materials characterization using transmission electron microscopy discloses that the microstructure evolution is the result of an extensive inter-diffusion of Co and Ni, and that the nanoparticles are nickel oxide dissolving some Co. Sensor performance measurement unfolds that the surface modification results in a significant sensitivity enhancement, nearly 200% increase for toluene (at 250 °C) and CO (at 200 °C) gases in comparison with the undoped samples.

Cobalt oxide thin films for high capacity and stable Li-ion battery anode

Here, we report reactive DC-sputter deposited Co3O4 thin films as a promising and stable Li-ion battery anode. Thin films were deposited on stainless steel by reactive sputtering of cobalt target in O2 atmosphere. X-ray diffraction and X-ray photo electron spectroscopy confirm the formation of Co3O4 crystal structure and absence of other impurities. The electron microscopy analysis shows a columnar growth morphology of the thin films while high resolution images reveal that the film is composed of ultra-small nanoparticles of average size of 5 nm. Fabricated half cells upon cycling between 3.0 and 0.01 V exhibit a stable capacity of 1125 mAh/g at a current density of 1 A/g for 100 cycles. Moreover, the electrode exhibited excellent rate capability and stability at higher rates; at current density of 10 A/g, a capacity close to 1000 mAh/g was observed. The excellent cycling stability of the cell was further confirmed by cycling at a high rate of 25 A/g (28 C) wherein the same was able to retain a capacity of 330 mAh/g even at the end of 1800 cycles. This enhanced performance could be related to the formation of 5-nm primary particles and columnar growth morphology, capable of reducing the lithium ion diffusion lengths and thus offered better kinetics even at high rates.