Co-doped TiO2 Thin Films Research Articles

Phase growth control in low temperature PLD Co: TiO2 films by pressure

This paper reports on the structural and optical properties of Co-doped TiO2 thin films grown onto (0001)Al2O3 substrates by non-reactive pulsed laser deposition (PLD) using argon as buffer gas. It is shown that by keeping constant the substrate temperature at as low as 310°C and varying only the background gas pressure between 7Pa and 70Pa, it is possible to grow either epitaxial rutile or pure anatase thin films, as well as films with a mixture of both polymorphs. The optical band gaps of the films are red shifted in comparison with the values usually reported for undoped TiO2, which is consistent with n-type doping of the TiO2 matrix. Such band gap red shift brings the absorption edge of the Co-doped TiO2 films into the visible region, which might favour their photocatalytic activity. Furthermore, the band gap red shift depends on the films’ phase composition, increasing with the increase of the Urbach energy for increasing rutile content.

Oxygen vacancy induced phase formation and room temperature ferromagnetism in undoped and Co-doped TiO2 thin films

TiO2 and Co-doped TiO2 (CTO) thin films deposited at various oxygen partial pressures by pulsed laser deposition exhibit room temperature ferromagnetism (RTFM) independent of their phase. Films deposited at 0.1 mTorr oxygen partial pressure show a complete rutile phase confirmed from glancing angle x-ray diffraction and Raman spectroscopy. At the highest oxygen partial pressure, i.e. 300 mTorr, although the TiO2 film shows a complete anatase phase, a small peak corresponding to the rutile phase along with the anatase phase is identified in the case of CTO film. An increase in O to Ti/(Ti+Co) ratio with increase in oxygen partial pressure is observed from Rutherford backscattering spectroscopy. It is revealed from x-ray photoelectron spectroscopy (XPS) that oxygen vacancies are found to be higher in the CTO film than TiO2, while the valency of cobalt remains in the +2 state. Therefore, the CTO film deposited at 300 mTorr does not show a complete anatase phase unlike the TiO2 film deposited at the same partial pressure. We conclude that RTFM in both films is not due to impurities/contaminants, as confirmed from XPS depth profiling and cross-sectional transmission electron microscopy (TEM), but due to oxygen vacancies. The magnitude of moment, however, depends not only on the phase of TiO2 but also on the crystallinity of the films.

Preparation and characterization of Co-doped TiO2 materials for solar light induced current and photocatalytic applications

Different amounts of Co-doped TiO2 powders and thin films were prepared by following a conventional co-precipitation and sol–gel dip coating technique, respectively. The synthesized powders and thin films were subjected to thermal treatments from 400 to 800 °C and were thoroughly investigated by means of X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive analysis with X-rays, FT-infrared, FT-Raman, diffuse reflectance spectroscopy, ultraviolet–visible spectroscopy, BET surface area, zeta potential, flat band potential measurements, band-gap energy, etc. The photocatalytic ability of the powders was evaluated by methylene blue (MB) degradation studies. The thin films were characterized by photocurrent and ultraviolet–visible (UV–Vis) spectroscopy techniques. The characterization results suggest that the Co-doped TiO2 powders synthesized in this study consist mainly anatase phase, and possess reasonably high specific surface area, low band gap energy and flat band potentials amenable to water oxidation in photoelectrochemical (PEC) cells. The photocatalytic degradation of MB over Co-doped TiO2 powders followed the Langmuir–Hinshelwood first order reaction rate relationship. The 0.1 wt.% Co-doped TiO2 composition provided the higher photocurrent, n-type semi-conducting behavior and higher photocatalytic activity among various Co-doped TiO2 compositions and pure TiO2 investigated.

Effects of Co doping on structure and optical properties of TiO2 thin films prepared by sol–gel method

Co-doped TiO2 thin films were synthesized on quartz substrates by sol–gel method. Atomic force microscopy results indicate that the surfaces of the Ti1−xCoxO2 (0≤x≤0.10) films become smooth and compact with increasing Co content. X-ray diffraction results show that all the films are rutile phase structure and Co doping leads to lattice contraction. X-ray photoelectron spectroscopy results reveal that the predominant oxidation state of Co is divalent. Peak positions of Raman-active modes (B2g, A1g and Eg) shift to lower frequency with increasing Co content. The refractive index n at 670nm from transmittance spectra increases with increasing Co content. The OBG varies between 3.10 and 3.26eV. Note that optical band gap (OBG) first increases and then decreases with increasing Co content, reaching its maximum value when x is 0.03. These results suggest that the increasing mechanism of OBG is related to the decrease of grain size, compressive stress, and reduction of rutile TiO2, and the decreasing mechanism of OBG is involved with defect and impurity. The competition of the two mechanisms leads to the strange change of OBG.

Enhanced Dopant Solubility and Visible-Light Absorption in Cr–N Codoped TiO2 Nanoclusters

A major obstacle toward employing TiO2 as an efficient photoactive material is related to its large optical band gap, strongly limiting visible light absorption. Substitutional doping with both donors and acceptors (codoping) potentially leads to a significant band gap reduction, but the effectiveness of the codoping approach remains limited by the low solubility of dopants inside TiO2. Here we show that nanostructured Cr and N codoped TiO2 thin films can be obtained by supersonic cluster beam deposition (SCBD) with a high concentration of dopants and a strongly reduced band gap. Complementary spectroscopic investigations show that doping effectively occurs in substitutional lattice sites, inducing dopant levels in the gap that are remarkably delocalized. The high surface-to-volume ratio, typical of SCBD nanostructured films, likely facilitates the dopant incorporation. The present results indicate that SCBD films are highly promising photoactive nanophase materials.

Evaluation of the structural, optical and photocatalytic properties of nitrogen-fluorine co-doped TiO2 thin films

Nanostructured TiO2 films were synthesized by a modified sol–gel method using Pluronics P-123 (EO20PO70EO20) as templating agent, titanium n-butoxide [Ti(OC4H9)4] as inorganic precursor, and ammonium fluoride (NH4F) as the source of N and F dopant atoms in order to prepare sols of x(NF) TiO2 (x=2, 5, 10, 20 wt%). The thin film preparation was made by spin coating, followed by calcination at 400 °C. The as-prepared TiO2 and xNF-TiO2 films were characterized by XRD, Raman spectroscopy, FTIR, TGA-DTA, SEM, UV–Vis diffuse reflectance spectroscopy and EPR. XRD and Raman spectroscopy show that the crystalline structure of these samples consists exclusively of the anatase phase. The band gap (Eg) values for the doped 10 and 20 NF-TiO2 film systems were found to be significantly smaller than those corresponding to the rest of the other TiO2 films. The photocatalytic properties of these films are investigated by following the degradation of methyl orange in aqueous solution under UV irradiation. The photodecomposition is mainly a direct function of the amount of NF present in the TiO2 matrix. The 20NF-TiO2 sample shows the highest activity of all the samples studied.

Photocatalytic activity of (sulfur, nitrogen)-codoped mesoporous TiO2 thin films

Nitrogen and sulfur co-doped mesoporous TiO2 thin films were fabricated using thiourea as a doping resource by the combination of the sol–gel and evaporation-induced self-assembly (EISA) processes. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption, and UV–vis spectra were performed to characterize the as-synthesized mesoporous TiO2 materials. The XPS result shows that O–Ti–N and O–Ti–S bonds in the (S, N)-codoped mesoporous TiO2 were formed. The resultant mesoporous (S, N)-codoped TiO2 exhibited anatase framework with a high porosity and a narrow pore distribution. After being illuminated for 3 h, methyl orange (MO) could be degraded completely by the co-doped sample under the ultraviolet irradiation, whereas mesoporous TiO2 film without doping could only degrade 60% MO. After being illuminated by visible light, the water contact angles of the mesoporous co-doped TiO2 samples decreased slightly, but the pure TiO2 mesoporous film exhibited no change in the hydrophilicity.

Determination of Co spin state in rutile Co:TiO2

X-ray absorption near-edge structure (XANES) and first-principles calculations are employed to determine the Co spin state in the Co-doped TiO2 thin films. Based on the experiments and the theoretical calculations, we find that the transition of Co 1s electron to the unoccupied Co 3d eg states is clarified from the pre-edge single-peak in the Co K-edge XANES spectrum of the Co-doped TiO2 thin film, and that the Co 3d t2g states are occupied while the eg states are partially unoccupied. This suggests a low-spin configuration of the substitutional Co2+ in the Co-doped TiO2 thin film. The studies demonstrate the ability of Co K-edge XANES to provide information on spin state of Co ions in the Co-doped TiO2.

Co-doped rutile TiO2-δ thin films studied by XANES and first principles calculations

The x-ray absorption near-edge structure (XANES) and first-principles calculations were employed to study the local structure and magnetic origin of Co-doped rutile TiO2 thin film. It is found that the experimental Co K-edge XANES can be reproduced by the calculated spectrum of substitutional Co (CoTi) with O vacancy (VO), suggesting the coexistence of CoTi and VO in this system. First-principles total energy calculations further reveal that the O vacancy prefers to reside near the Co ion and substantially facilitates the impurity doping. The electronic structure analysis indicates that the O vacancy can mediate the ferromagnetic interaction between the CoTi ions and thus is responsible for the observed room-temperature ferromagnetism in the Co-doped TiO2-δ thin films.

V and Ga Co-doping effect on optical absorption properties of TiO2 thin films

In this work, V and Ga co-doped TiO2 thin films were obtained by pulsed laser deposition (PLD) using sintered ceramic targets, followed by short term annealing at 1000°C. The structures of the thin films were characterized by X-ray diffraction (XRD) and the optical absorption properties of the films were investigated using UV-VIS spectrophotometer. A narrow energy band gap of about 1.5 eV was obtained in a typical co-doped film with 1 at.% V and 0.5 at.% Ga as dopants. The analysis on the chemical states of Ti and Ga elements by X-ray photoelectron spectroscopy (XPS) revealed that the Ga doping plays an important role in the band gap narrowing.

STEM Z-contrast and Electron Energy Loss Spectroscopy Characterization of Co-doped TiO2 Thin Films

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006

Preparation and characterization of silver and indium vanadate co-doped TiO2 thin films as visible-light-activated photocatalyst

Novel visible-light-activated photocatalytic Ag/InVO4-TiO2 thin films were developed in this paper through a sol-gel method from the TiO2 sol containing Ag and InVO4. The photocatalytic activities of Ag/InVO4-TiO2 thin films were investigated based on the oxidation decomposition of methyl orange in aqueous solution. The Ag/InVO4-TiO2 thin films were characterized by X-Ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectroscopy (UV-vis). The results revealed that the Ag/InVO4-TiO2 thin films extended the light absorption spectrum toward the visible region; the Ag and InVO4 co-doped thin films significantly enhanced the methyl orange photodegradation under visible light irradiation. It has been confirmed that the Ag/InVO4-TiO2 thin films could be excited by visible light (E < 3.2 eV). The significant enhancement in the Ag/InVO4-TiO2 photo activity under visible light irradiation can be ascribed to the simultaneous effects of doped noble metal Ag by acting as electron traps and InVO4 as narrow band gap sensitizer.

Magnetic properties of epitaxial Co-doped anatase TiO2 thin films with excellent structural quality

The heteroepitaxy of Co-doped anatase TiO2 on LaAlO3(001) has been refined with the goal of determining the relationship between structural quality and magnetic ordering. By significantly reducing the deposition rate and substrate temperature, well-ordered Co:TiO2 films with high crystalline quality were obtained by oxygen-plasma-assisted molecular beam epitaxy, as characterized by x-ray diffraction. These films exhibit uniform Co doping, with no evidence of Co segregation or secondary phases throughout the film depth or on the surface. Despite the improvement in crystalline quality and Co distribution, the films exhibit negligible ferromagnetism, with saturation moments of only ∼0.1μB∕Co. This loss of ferromagnetism is in stark contrast to faster-grown Co:TiO2 films, where higher growth rate and substrate temperature typically result in lower crystalline quality, a highly nonuniform Co distribution, and average saturation moments of ∼1.2μB∕Co. The presence of ferromagnetism in faster-grown Co:TiO2 does not appear to arise from intrinsic point defects present in the bulk material, such as charge-compensating oxygen vacancies, but is instead attributed to the presence of extended structural defects.

Optical and magnetic properties of Co-doped TiO2 thin films grown by pulsed laser deposition

Co-doped TiO2 thin films were grown by pulsed laser deposition onto (0001)Al2O3 substrates, heated at 310°C, using a KrF excimer laser. The ablation of polycrystalline targets of TiO2 rutile containing a few mol% of Co3O4 was performed in a background gas mixture of argon and hydrogen at total pressure of 10Pa, for H2 flow rates of 0.0, 0.1 and 0.2sccm. X-ray diffraction analysis showed that the as-deposited films consist of TiO2 rutile-based nanocrystallites with preferred orientation along the [101] direction. The chemical composition of the samples was evaluated by Rutherford backscattering measurements, revealing that Ti distribution is homogeneous on the surface and along film depth, and that the Co:Ti ratio is 0.03:0.97 for all the films investigated. In this paper, we focus on the influence of hydrogen addition to argon background gas on the optical properties of the Co-doped TiO2 films. Results of magnetic characterisation will also be presented.

Structural and magnetic properties of cobalt-doped rutile thin films deposited on TiN–Si

Highly oriented Co-doped rutile TiO2 thin film on silicon has been achieved by using a TiN buffer layer. The direct deposition of TiO2:Co film on TiN–Si yields (110)-oriented film structure having two equivalent structural domains which are perpendicular to each other. Interestingly, no ferromagnetism is observed in this film structure down to 10K, the reason for which is attributed to the formation of cobalt silicide in the silicon. On the other hand, when the TiO2:Co film is deposited on TiO2, formed by in situ oxidation of TiN–Si, cobalt appears highly mobile leading to segregation at the TiO2–Si interface along with the formation of cobalt clusters in the TiO2:Co film region. This is responsible for the ferromagnetism in this case.

Binding state and microstructure analyses of Co-doped TiO2 thin film

Multilayered Co-doped TiO2 thin films with postannealing treatment were investigated. Based on x-ray photoelectron spectroscopy, Co(II) binding state was found after annealing. Its intensity increases with the annealing temperature. Postannealing in an ultrahigh vacuum system is an efficient way to drive Co atoms to diffuse into TiO2. It is interesting to find that samples with partial cosputtering structure have a much stronger Co(II) peak than those of pure multilayer structure. Transmission electron microscope patterns show that the annealed films have a polycrystal rutile-TiO2 phase. The saturation magnetization of annealed sample at 150 K is 1.325 uB per Co atom, which is close to the value expected for low-spin Co(II).

Growth and properties of Co-doped TiO2 thin films grown on buffered Si substrate

Epitaxial growth of a Co-doped anatase TiO2 thin film on silicon has been achieved by using SrTiO3/TiN bilayers as a buffer. All layers were sequentially formed by pulsed laser deposition. X-ray φ scan and selective area electron diffraction patterns revealed that SrTiO3/TiN buffer layers were grown with a cube-on-cube orientation relationship with silicon. Ferromagnetism was observed at room temperature with a coercivity of around 607 Oe. However, the ferromagnetism is attributed to the highly cobalt enriched TiO2 clusters which nucleate after a certain growth of film, as observed by atomic/magnetic force microscopy as well as cross-sectional transmission electron microscopy.

Substrate effects on the room-temperature ferromagnetism in Co-doped TiO2 thin films grown by pulsed laser deposition

Co:TiO 2 films were fabricated by laser ablation on Si, LaAlO3 (LAO), and SrTiO3 (STO) substrates from a ceramic target. Films on all types of substrates have Curie temperature (TC) above 400 K. All films are highly crystallized with different structures. While films on Si substrates are rutile, films on LAO and STO substrates are single phased anatase. Due to the different lattice mismatch, films grown under the same growth conditions on Si, LAO, and STO substrates have different saturation magnetization and coercivity. While Co atoms are mostly localized near the surface of the films, magnetic measurements suggested that the ferromagnetism unlikely originates from Co clusters.

Co distribution in ferromagnetic rutile Co-doped TiO2 thin films grown by laser ablation on silicon substrates

Pure rutile Co-doped TiO2 films were fabricated by the pulsed-laser-deposition technique on silicon substrates from a ceramic target. Under the right fabrication conditions, Co concentration in the films could be almost the same as in the target, and films under various conditions all are ferromagnetic well above room temperature. Even though Rutherford backscattering spectroscopy measurements show that Co atoms mostly localize near the surface of the films and exist less in deeper levels, other experimental evidence shows that the ferromagnetism does not come from Co segregations, but from the Co-doped TiO2 matrix. Rutile Ti1−xCoxO2 thin films grown by a very simple technique on low-cost silicon substrates showing a Curie temperature (TC) above 400 K appear to be very attractive to applications.

Investigations on the nature of observed ferromagnetism and possible spin polarization in Co-doped anatase TiO2 thin films

High-quality epitaxial thin films of Co-doped anatase TiO2 (Co:TiO2) were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior altered from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like pattern. Electrical conductivity and saturation magnetization increased, seemingly consistent with the picture of carrier-induced ferromagnetism. However, we also found that the spatial distribution of Co ions became highly nonuniform and the chemical state of Co ions changed from ionic to metallic. All of these PO2 dependences, even including the transport and the magnetic properties, can be explained in terms of the formation of cobalt clusters, whose existence was clearly demonstrated by transmission–electron–microscope studies. Our work clearly indicates that the cobalt clustering will result in the room-temperature ferromagnetism observed in our Co:TiO2 films. To check the possible spin polarization of carriers in Co:TiO2 films, we also fabricated a heterojunction composed of a ferromagnetic Co:TiO2, an insulating SrTiO3, and a ferromagnetic half-metallic (La,Ba)MnO3 layer. When the magnetic field was varied, we could not observe any changes in its I–V characteristic curves, which suggests that there might be little spin-polarization effect in the anatase Co:TiO2 layer.