Cr-doped TiO2 Research Articles

Photocatalytic Activities of Transition Metal-Doped TiO<sub>2</sub> Nano-Powders by Sonochemical Method and their Physicochemical Properties

With respect to metal doping into TiO2, the doping limits for V, Cr, Zr, Nb, Mo, and W are predicted to be higher than other transition metals according to the binding energy calculations in a unit cell model of anatase TiO2, which suggests that Cr and W can be doped into anatase structure more easily than Ag or Pt, for example. Our investigation has twofold research objectives. One is to prepare metal-doped TiO2 nano-powders from these transition metals, and the other is to test photocatalytic activity of each resulting powder. For the former, sonochemical process has been used to produce Cr-doped, W-doped, Ag-doped, and Pt-doped TiO2 nanoparticles. For the latter, we have performed photocatalytic oxidation of methylene blue. The combined results of the morphology and photocatalytic activites have enabled characterization of the physicochemical properties of these transition metal-doped nanoparticles.

Visible-light absorption and photocatalytic activity of Cr-doped TiO 2 nanocrystal films

Chromium doped titanium dioxide (TiO 2) nanocrystal films with various doping concentration have been successfully prepared by a sol–gel dip-coating process. These films have been characterized by XRD, XPS, AFM, and UV–vis absorption spectroscopy. It is found that Cr doping can effectively reduce the transition temperature of anatase to rutile phase as well as the grain size. The absorption edges of TiO 2 thin films shift towards longer wavelengths (i.e. red shifted) from 375 nm to about 800 nm with increasing Cr concentration, which greatly enhances TiO 2 nano-materials on the absorption of solar spectrum. The appearance of UV–vis absorption features in the visible region can be ascribed to the newly formed energy levels such as Cr 2p level and oxygen vacancy state between the valence and the conduction bands in the TiO 2 band structure. The enhancement of the photocatalytic properties is observed for Cr-doped TiO 2 thin film.

Cr-doped TiO2 (rutile): Ferromagnetism in bulk form?

Cr-doped rutile samples in powder and single crystal form have been prepared and studied with particular emphasis on their compositional, structural, and magnetic properties. Both powders and single crystals were characterized using x-ray diffractometry and vibrating sample magnetometry, while crystals were also studied using the x-ray Laue technique, scanning electron microscopy, and energy dispersive x-ray analysis. The results indicate weak ferromagnetic behavior at room temperature (RT) in Cr-doped TiO2 powders having 2 at. % Cr, while powders with higher amounts of Cr (e.g., 8 or 12 at. %) showed no evidence of ferromagnetism. Following successful floating zone crystal growth of Cr-doped rutile crystals in argon, magnetic measurements taken from the as-grown crystals revealed no indication of RT ferromagnetism over the composition range 2 to 12 at. % Cr. A solubility limit for Cr into the rutile matrix of 3 at. % Cr at RT is suggested.

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 2.95eV, 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.

USE OF NANOSIZED CHROMIUM DOPED TiO<sub>2</sub> SUPPORTED ON ZEOLITE FOR METHYLENE BLUE DEGRADATION

The photocatalytic degradation of methylene blue dye under visible light has been investigated using chromium modified titanium dioxide supported on zeolite (Cr-TiO2/zeolite). The photocatalyst was prepared by sol-gel method and characterized by X-ray diffraction and SEM. The rate of photodegradation of dye was monitored spectrophotometrically. The effect of pH, dye concentration, amount of photocatalyst and intensity of light on the rate of photocatalytic reaction was observed. The results showed that the use of Cr-doped TiO2 increased the rate of photocatalytic degradation of methylene blue as compared to untreated TiO2. The photocatalytic mechanism of Cr-TiO2 catalyst has been tentatively discussed. Keywords: Methylene blue, zeolite, chromium, photocatalytic degradation

Formation of Cr-modified silicide coatings on a Ti–Nb–Si based ultrahigh-temperature alloy by pack cementation process

Cr-modified silicide coatings were prepared on a Ti–Nb–Si based ultrahigh temperature alloy by Si–Cr co-deposition at 1250 °C, 1350 °C and 1400 °C for 5–20 h respectively. It was found that both coating structure and phase constituents changed significantly with increase in the co-deposition temperature and holding time. The outer layers in all coatings prepared at 1250 °C for 5–20 h consisted of (Ti,X) 5Si 3 (X represents Nb, Cr and Hf elements). (Ti,X) 5Si 4 was found as the only phase constituent in the intermediate layers in both coatings prepared at 1250 °C for 5 and 10 h, but the intermediate layers in the coatings prepared at 1250 °C for 15 and 20 h were mainly composed of (Ti,X) 5Si 3 phase that was derived from the decomposition of (Ti,X) 5Si 4 phase. In the coating prepared at 1350 °C for 5 h, single (Ti,X) 5Si 3 phase was found in its outmost layer, the same as that in the outer layers in the coatings prepared at 1250 °C; but in the coatings prepared at 1350 °C for 10–20 h, (Nb 1.95Cr 1.05)Cr 2Si 3 ternary phase was found in the outmost layers besides (Ti,X) 5Si 3 phase. In the coatings prepared at 1400 °C for 5–20 h, (Nb 1.95Cr 1.05)Cr 2Si 3 ternary phase was the single phase constituent in their outmost layers. The phase transformation (Ti,X) 5Si 4 → (Ti,X) 5Si 3 + Si occurred in the intermediate layers of the coatings prepared at 1350 and 1400 °C with prolonging co-deposition time, similar to the situation in the coatings prepared at 1250 °C for 15 and 20 h, but this transformation has been speeded up by increase in the co-deposition temperature. The transitional layers were mainly composed of (Ti,X) 5Si 3 phase in all coatings. The influence of co-deposition temperature on the diffusion ability of Cr atoms was greater than that of Si atoms in the Si–Cr co-deposition processes investigated. The growth of coatings obeyed inverse logarithmic laws at all three co-deposition temperatures. The Si–Cr co-deposition coating prepared at 1350 °C for 10 h showed a good oxidation resistance due to the formation of SiO 2 and Nb, Cr-doped TiO 2 scale after oxidation at 1250 °C for 10 h.

Preparation and Photoelectrochemical Activity of Cr-Doped TiO2 Nanorods with Nanocavities

Cr-doped TiO2 nanorods with nanocavities were synthesized by a facile hydrothermal treatment and heating in air. The samples were characterized respectively by means of X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). XRD patterns indicated that all the samples were anatase crystalline. HRTEM results and the electron diffraction patterns illustrated that the TiO2 nanorods possessed the single-crystalline structure. TEM images confirmed that there were different types of nanocavities inside the nanorods, such as a circle, hexagon, and rectangle. XPS results suggested that Cr elements were successfully doped into the TiO2 nanorods after hydrothermal and most Cr congregated on the surface in the form of Cr2O3 after heating. The optical properties of the samples were studied with a UV−vis spectrometer. The photoelectrochemical activity of the Cr-doped TiO2 nanorods thin film was better than that of the commercial anatase TiO2 partic...

Effects of Single Metal-Ion Doping on the Visible-Light Photoreactivity of TiO2

Titanium dioxide (M-TiO2), which was doped with 13 different metal ions (i.e., silver (Ag+), rubidium (Rb+), nickel (Ni2+), cobalt (Co2+), copper (Cu2+), vanadium (V3+), ruthenium (Ru3+), iron (Fe3+), osmium (Os3+), yttrium (Y3+), lanthanum (La3+), platinum (Pt4+, Pt2+), and chromium (Cr3+, Cr6+)) at doping levels ranging from 0.1 to 1.0 at. %, was synthesized by standard sol−gel methods and characterized by X-ray diffraction, BET surface area measurement, SEM, and UV−vis diffuse reflectance spectroscopy. Doping with Pt(IV/II), Cr(III), V(III), and Fe(III) resulted in a lower anatase to rutile phase transformation (A−R phase transformation) temperature for the resultant TiO2 particles, while doping with Ru(III) inhibited the A−R phase transformation. Metal-ion doping also resulted in a red shift of the photophysical response of TiO2 that was reflected in an extended absorption in the visible region between 400 and 700 nm. In contrast, doping with Ag(I), Rb(I), Y(III), and La(III) did not result in a red shift of the absorption spectrum of TiO2. As confirmed by elemental composition analysis by energy dispersive X-ray spectroscopy, the latter group of ions was unable to be substituted for Ti(IV) in the crystalline matrix due to their incompatible ionic radii. The photocatalytic activities of doped TiO2 samples were quantified in terms of the photobleaching of methylene blue, the oxidation of iodide (I−), and the oxidative degradation of phenol in aqueous solution both under visible-light irradiation (λ > 400 nm) and under broader-band UV−vis irradiation (λ > 320 nm). Pt- and Cr-doped TiO2, which had relatively high percentages of rutile in the particle phase, showed significantly enhanced visible-light photocatalytic activity for all three reaction classes.

Undoped and Cr-doped TiO 2 thin films obtained by spray pyrolysis

Undoped and chromium doped titanium oxide thin films were fabricated by spray pyrolysis by using a solution of titanium tetrachloride and ethyl alcohol. The films have been deposited on heated glass substrates at 373 K. After annealing for 90 min at 723 K, the initially amorphous films became polycrystalline with a predominant anatase structure and average crystallite sizes depending on dopant (Cr) concentration. The repartition of chromium impurities in the matrix of titanium oxide films, analyzed by electron paramagnetic resonance and X-ray photoelectron spectroscopy showed that the entrance of chromium into the anatase structure is mainly achieved by substitution. A decrease in unit cell parameters ratio ( c/ a) with the increase of chromium content sustains this assertion. The wetting properties of the titanium oxide films were evaluated from contact angle measurements between de-ionized water and films surface during- and post-irradiation with UV light. The correlation between the concentration of the dopant, film structure, surface morphology and wettability characteristics is discussed.

Synthesis, characterization of B-doped TiO2 nanotubes with high photocatalytic activity

Cr-doped TiO2 nanotubes (Cr/TiO2 NTs) with high photocatalytic activity were prepared by the combination of sol–gel process with hydrothermal treatment. XRD, TEM and UV–vis DRS techniques were employed for microstructural characterization. TEM images show that Cr/TiO2 NTs are in good tubular structure and have diameter of about 10 nm. The Cr doping induces the shift of the absorption edge to the visible light range and the narrowing of the band gap. The photocatalytic experiment reveals that the photocatalytic performance of TiO2 NTs can be improved by the doping of chromium ions.

Density Functional Characterization of the Electronic Structure and Visible‐Light Absorption of Cr‐Doped Anatase TiO2

To evaluate the electronic and optical properties of Cr-doped anatase TiO(2), three possible Cr-doped TiO(2) models, including Cr at a Ti site (model I), Cr at a Ti site with an oxygen vacancy compensation (model II), and an interstitial Cr site (model III), are studied by means of first principles density functional theory calculations. In model I, the splitting behavior of the Cr 3d states and the insulating properties are successfully depicted by the GGA+U method, from which it is proposed that Cr at a Ti site should exist as Cr(4+) instead of the generally believed Cr(3+). As a result, the electron transitions between these impurity states, the conduction band (CB), and the valence band (VB), as well as the d-d transitions between occupied and unoccupied Cr 3d states, provide a reasonable explanation for the experimentally observed major and minor absorption bands. In models II and III, the impurity states and associated optical transition processes-as well as the corresponding electron configurations-are examined.

Visible-light Photocataltic Activity of Cr-doped TiO<SUB>2</SUB> Nanoparticles Synthesized by Flame Spray Pyrolysis

Visible-light Photocataltic Activity of Cr-doped TiO<SUB>2</SUB> Nanoparticles Synthesized by Flame Spray Pyrolysis

Hydrogen production by photocatalytic water-splitting using Cr- or Fe-doped TiO2 composite thin films photocatalyst

Abstract Cr- or Fe-ion-doped TiO2 thin films have been synthesized by radio-frequency magnetron sputtering and a sol–gel method to study hydrogen generation by photocatalytic water-splitting under visible light irradiation. The doping method, dopant concentration, charge transfer from metal dopants to TiO2, and type of dopants used for modification of TiO2 were investigated for their ability to enhance photocatalytic activity. UV–Visible spectra show that the metal-doped-TiO2 obtained by sputtering is much more efficient than that obtained by the sol–gel technique at inducing a red shift of the absorption edge in the visible light range. Low concentration metal ion doping must be done near the conducting indium tin oxide (ITO) – TiO2 interface to avoid the formation of recombination centers for photo-generated electron–hole pairs. H2 production rate (μmol/h) is higher for Fe-doped TiO2 (15.5 μmol/h) than for Cr-doped TiO2 (5.3 μmol/h) due to the ability of Fe ions to trap both electrons and holes, thus avoiding recombination, while Cr can only trap one type of charge carrier. A constant H2 generation rate is obtained for long periods of time by all the investigated TiO2 films because of the separate evolution of H2 and O2 gases, thus eliminating the back-reaction effect.

Effect of Cationic or Anionic Dopants on Optical and Photocatalytic Properties of TiO2 Nanopowders made by Flame Spray Synthesis (FSS)

AbstractTiO2, TiO2-1at.% W and TiO2-1at.% Cr were produced from metal-organic precursors by flame spray synthesis (FSS). TiO2-0.5at.% N was obtained by ammonolysis of FSS made TiO2 nanopowder in a rotating tube furnace under NH3 atmosphere. According to the X-ray diffraction (XRD) analysis, anatase is the predominant phase in all samples. Diffusive reflectance and the resulting band gap energy (Eg) were determined by diffusive reflection spectroscopy (DRS). Additional impurity bands at 2.43 and 2.57 eV for N- and Cr-doped TiO2, respectively have been observed. The impurity band formed in the band gap resulted in increase of the light absorption in the visible range. The photocatalytic performance of the nanopowders under ultraviolet (UV, 290-410 nm) and visible light irradiation (Vis, 400-500 nm) was studied by the degradation of methylene blue (MB) in aqueous suspensions. It was found that all types of dopants influence the structure, interaction with the visible light as well as photocatalytic activity. Among all nanopowders, TiO2-W exhibited the best photoactivity, much higher than the commercial TiO2-P25 nanopowder. The optimum of the photodecolourization was obtained for 0.7 and 1 at.% W.

Photocatalysts of Cr Doped TiO2 Film Prepared by Micro Arc Oxidation

A series of Cr doped TiO2 films were prepared by micro arc oxidation (MAO) using an electrolyte of Na3PO4+K2Cr2O7. X-ray diffraction and scanning electron microscopy revealed that the films mainly consisted of anatase phase with a porous surface morphology. The films have an excellent photocatalytic effect for degradation of methylene blue and decomposition of water under visible light illumination. This arises from the formation of Cr3+/Cr4+ and oxygen vacancy energy levels owing to Cr doping. The former reduces the electron-hole recombination chance, while the latter generates a new gap between the conduction band (CB) and valence band (VB) of TiO2, which lowers the photo energy of the excited electron in the VB to the oxygen vacancy states. The mechanisms for film synthesis during the MAO process are also presented.

Electrical and optical properties of Cr2−xTixO3 thin films

Cr2−xTixO3 thin films with 0 ⩽ x ⩽ 1.82 were prepared on sapphire substrates by aerosol assisted chemical vapour deposition. The films in the 0 ⩽ x ⩽ 1.6 composition range grow preserving the α-chromite structure with a lattice constant that changes as a function of the titanium concentration [Ti]. However, films with x > 1.6 start to develop a crystalline phase that can be identified as Cr-doped TiO2. For the Cr2−xTixO3 films a Ti3+ → Cr3+ substitution is deduced from the crystalline structure and confirmed by XPS experiments. The increment in the lattice parameter agrees with the reduction in the optical band gap which goes from 3.50 eV to 2.70 eV for films with x = 0 and x ∼ 1.6, respectively. At room temperature the electrical conductivity decreases as a function of the [Ti]. The thermal stimulated conductivity indicates a trap level at ∼0.55 eV inside the gap in all the samples and the behaviour of the σ–T curves at higher temperatures indicates that the transport properties undergo a transition from a variable range hopping process to a more complicated process as the [Ti] increases.

Characteristics of chromium-doped titanium oxide coatings synthesized by cathodic arc deposition

Cr-doped titanium oxide (Cr/TiO 2) thin films were deposited on stainless steel and glass substrates by using the cathodic arc deposition technique. The characteristics of undoped and Cr-doped TiO 2 thin films were compared by using several analytical approaches. The undoped TiO 2 film as an as-deposited coating had an anatase structure, while the Cr/TiO 2 film had an amorphous structure, as revealed by X-ray diffraction. After 3 h of heat treatment at 450 °C, the undoped and Cr-doped titanium oxide films became mixed anatase and rutile phases. The microstructure observed under a scanning electron microscope of the Cr-doped TiO 2 film was more compact than that of the undoped film. UV–VIS spectroscopy indicated that chromium doping shifted the absorption edge of the titanium oxide film from the UV region to the visible region. Chromium doping extends the optical absorption range to the visible region and reduces the optical band gap of as-deposited films from 3.2 to 2.2 eV. The drop in the water contact angle from 28° to zero following visible light irradiation implies that the Cr/TiO 2 thin film becomes a better photocatalytic material because of better hydrophilic property.

Photocatalytic properties of Cr-doped TiO 2 films prepared by oxygen cluster ion beam assisted deposition

We prepared Cr-doped titanium dioxide (TiO 2) films by oxygen (O 2) cluster ion beam assisted deposition method, and investigated photocatalytic properties of the films as well as crystallographic property, optical property and surface morphology. The films prepared at a substrate temperature below 200 °C were found to be amorphous from the X-ray diffraction measurement. For the substrate temperatures such as 300 °C and 400 °C, the films exhibited rutile and/or anatase structures. The film surface measured by the atomic force microscope (AFM) was smooth at an atomic level. Furthermore, the optical band gap decreased with increase of Cr-composition, and it was approximately 3.3 eV for the non-doped films, 3.2 eV for the 1% Cr-doped films and 3.1 eV for the 10% Cr-doped films, respectively. With regard to the photocatalytic properties of the Cr-doped TiO 2 films, we measured the change of contact angle as well as the photocatalytic degradation of methylene blue by the UV light irradiation. Compared with the non-doped films, the 1% Cr-doped films prepared at a substrate temperature of 400 °C showed high degradation efficiency. In addition, the contact angle of the 1% Cr-doped films with an initial value of 60° decreased to 10° by the UV light irradiation for 20 min, and the films exhibited the predominant properties of photocatalytic hydrophilicity even for the UV light irradiation with longer wavelengths.

Charge Transport in Cr-Doped Titanium Dioxide

The present work reports the effect of chromium on the mobility terms for electrons and electron holes for TiO2 at 1273 K. These data were determined by using the concentration data from the defect disorder diagram derived by the authors and the electrical conductivity data for Cr-doped TiO2 reported by Carpentier et al. [J. Phys. Chem. Solids 1989, 50, 145]. It is shown that chromium incorporates into the TiO2 lattice according to two different mechanism depending on chromium concentration. In the concentration range 1−3 atom %, chromium incorporation leads to the formation of acceptors, which are compensated by tri-valent titanium interstitials. However, in the range 4−5 atom %, the acceptor-type defects formed by chromium incorporated in the titanium sites are compensated by oxygen vacancies. The incorporation of chromium results in a relatively insignificant increase of the mobility of electrons from μn = 0.5 × 10-5 m2 V-1 s-1 for undoped TiO2 to μn = 1.3 × 10-5 m2 V-1 s-1. The incorporation of Cr res...

Chromium-doped titanium dioxide thin-film photoanodes in visible-light-induced water cleavage

A new method for the synthesis of nanocrystalline anatase Cr-doped TiO 2 colloids from nanotubes utilizing the hydrothermal ion-intercalation (HII) method has been devised using hydrothermal treatment in an acidic environment. To investigate the photoelectrochemical reaction that occurs when water is split into H 2 and O 2 under visible light, a Cr-doped TiO 2 thin film served as a model photoelectrode. The photoelectrochemical activity of Cr-doped TiO 2 was higher than that of the undoped sample. The photoelectrochemical activity of photoelectrodes increased drastically with increased chromium doping. A red shift in the band gap was induced by Cr doping of TiO 2. It was revealed that the Cr-doped TiO 2 photoelectrode was able to utilize a wide range of light in the visible region of the spectrum. At high Cr concentrations, the lower photoelectrochemical activity is attributed to the effect of Cr 3+ ion recombination and excess Cr 3+ ions forming secondary phase Cr 2O 3.