Pure Mesoporous TiO2 Research Articles

Construction of oxygen-rich vacancies and heterojunctions coupled with different morphologies of CeO2/mesoporous TiO2 framework structures for efficient photocatalytic CO2 reduction performance.

The coupling of efficient adsorption and effective charge separation with photocatalysts enables the use of sunlight for photocatalytic reduction of carbon dioxide (CO2) into high-value-added products. In this work, we used a straightforward solid-phase hydrothermal technique to build an oxygen-vacancy-rich, heterogeneous interface-coupled CeO2/mesoporous TiO2 framework structural system. The heterogeneous structure was constructed by introducing oxygen-vacancy-rich CeO2 into mesoporous TiO2, which may encourage the transfer of charges and increase the number of active sites and CO2 adsorption by utilizing the coupled synergistic effect of oxygen vacancies and heterogeneous interfaces, and it can also regulate the pathway of the photocatalytic reaction and the selectivity of the products. The composite of CeO2 with different morphologies and oxygen-rich vacancies regulated the system's active sites and degree of exposure and enhanced photocatalytic CO2 reduction. The highest CO yield of 6.25 mmol gcat-1 was obtained by use of the rod CeO2/mesoporous TiO2 composite photocatalyst (R-CeO2/TiO2), and this yield was 1.6 times higher than that of pure mesoporous TiO2 and 1.84 times higher than that of pure R-CeO2. Also, the product selectivity increased by 4.3% compared to a single sample. Combining the Mott-Schottky plot results and the energy-barrier perspective to further explore the photocatalytic reduction of the CO2 reaction mechanism as well as the product selectivity, it appears that the construction of the composite system of oxygen-rich vacancies and heterogeneous boundary-coupled photocatalysis provides a practical pathway for the photocatalytic reaction, which may contribute to the photocatalytic reaction's high efficiency and yield selectivity.

Preparation, Characterization and Evaluations of Carbon-Doped Ag/Fe/TiO₂ Mesoporous Nanocomposite Photocatalyst for Degradation of Methylene Blue and Congo Red.

Carbon doped silver/iron/TiO₂ nanocomposite is synthesized via the solvothermal technique. Titanium tetraisopropoxide is used as a TiO₂ source. The composite samples are characterized by different physicochemical methods, including nitrogen adsorption-desorption analysis, transmission electron microscope, scanning electron microscope, X-ray diffraction, photoluminescence, UV-vis, Fourier-transform infrared, and Energy dispersive X-ray spectroscopy. The nanocrystalline structure of the samples with anatase phase having a tetragonal shape is shown by the XRD and TEM analysis. The photo-absorption boundary of pure TiO₂ expands into the visible light region due to composite formation, shown by analysis of UV-vis data. An increase in the degree of electron-hole couple segregation is shown via photoluminescence analysis. N₂ adsorption-desorption analysis manifests the higher surface area of samples along with mesoporous nature. The high photodegradation action is shown by the composite samples as compared to pure mesoporous TiO₂.

Modification of electron-transport layers with mixed RGO/C60 additive to boost the performance and stability of perovskite solar cells: A comparative study

In perovskite solar cells (PSCs), the transportation of photogenerated electrons can be controlled by the modification of electron-transport layers. In the present study, TiO 2 doped with reduced graphene oxide (RGO) and C 60 was used as an electron-transport layer in hole-transport-free porous perovskite solar cells. The PSCs were fabricated in the laboratory by a single-step method. As the measurements showed, the use of a mixed electron-transport layer in perovskite cells helps to decrease the absorption in perovskite layers in the visible range, while the electron transport is increased. The scanning electron microscopy images and the X-ray diffraction patterns also showed that large perovskite crystals have fewer holes and better morphology than the reference cells fabricated with pure mesoporous TiO 2 . Appropriate amounts of RGO and C 60 were used to compare the parameters of the modified fabricated cells with those of the reference cells. Moreover, the stability of the fabricated cells was examined by the comparison of them to the reference cells. This comparative study showed an increase of efficiency from 5.15% to 9.10% for the untreated and the treated samples respectively. Less cell destruction was also observed over a 32-day period for the treated samples, indicating the improvement of the shelf stability of PSCs through the suggested ETL modification. • TiO 2 doped with reduced graphen oxide (RGO) and C 60 as an electron transport layer in perovskite solar cells. • Decreased Perovskite layer absorption in the visible range by using mixed electron transport layer. • Elimination of Perovskite interface defects by using a mixed electron transport layer.

The optical and electrical properties regulation of TiO2 mesoporous thin film in perovskite solar cells

In this work, we investigate how TiO2 nanotube and nanosheet composite mesoporous TiO2 layers can improve the power conversion efficiency (PCE) of perovskite solar cells (PSCs) with respect to pure mesoporous TiO2. Pure and nanotube/nanosheet composite TiO2, nanostructure of three different concentrations (0.5, 1, and 3 mol%) were synthesized by a simple, hydrothermal method and characterized by XRD, SEM, transmission electron microscopy (TEM), and UV–Vis diffuse reflection spectroscopy. TiO2 nanotubes and TiO2 nanosheets were incorporated into the TiO2 porous layer of perovskite solar cell, which outperform cells using pure TiO2 in several ways: higher open-circuit voltage (Voc) (1.06 V), power conversion efficiency (PCE) (15.58%), short-circuit current (Jsc), and fill factor (FF). These properties improvements are attributed to the better properties of modulated TiO2 as compared to TiO2 such as better optical transmission properties, faster electron transfer rate and higher electron lifetime. This work provides a simple and effective strategy to improve TiO2 porous layer, which is significant for the study of the cell porous layer in perovskite solar.

Aminosilicate modified mesoporous anatase TiO2@graphene oxide nanocomposite for dye-sensitized solar cells

Highly ordered aminosilicate modified mesoporous anatase TiO2 graphene oxide (mTiO2@GO/NH2) nanocomposite microspheres are prepared by hydrothermal method and are investigated using various spectral and microscopic techniques. X-ray photoelectron spectroscopy and Fourier-transform infrared spectral analyses confirmed the presence of NH2 groups in the mTiO2@GO/NH2 nanocomposites. The mTiO2@GO/NH2 exhibits lower photoluminescence intensity than bare mesoporous TiO2 microspheres, disclosing an efficient photoelectron transfer from the mesoporous TiO2 to GO through aminosilicate chain. Atomic force microscopic analysis reveals that mTiO2@GO/NH2 thin film has highly ordered porous structure. The highly porous mTiO2@GO/NH2 exhibits good dye loading for light harvesting and allow good electrolyte contact, which minimizes the back-electron transfer between photoanode and oxidized dye molecules. The dye-sensitized solar cell (DSSC) based on mTiO2@GO/NH2 nanocomposite photoanode, with optimum amount of (3-aminopropyl)triethoxysilane (APTES), delivers an overall photo-conversion efficiency of 5.11%, which is 2.5-fold higher than that of the DSSC based on pure mesoporous TiO2 microspheres (mTiO2 MS) photoanode. The significant photovoltaic performance of the present DSSC based on mTiO2@GO/NH2 photoanode is due to synergistic effects between the TiO2 and GO heterojunction.

Dehydration of Alginic Acid Cryogel by TiCl4 vapor: Direct Access to Mesoporous TiO2 @C Nanocomposites and Their Performance in Lithium-Ion Batteries.

A new strategy for the synthesis of mesoporous TiO2 @C nanocomposites through the direct mineralization of seaweed-derived alginic acid cryogel by TiCl4 through a solid/vapor reaction pathway is presented. In this synthesis, alginic acid cryogel can have multiple roles; i) mesoporous template, ii) carbon source, and iii) oxygen source for the TiO2 precursor, TiCl4 . The resulting TiO2 @alginic acid composite was transformed either into pure mesoporous TiO2 by calcination or into mesoporous TiO2 @C nanocomposites by pyrolysis. By comparing with a nonporous TiO2 @C composite, the importance of the mesopores on the performance of electrodes for lithium-ion batteries based on mesoporous TiO2 @C composite was clearly evidenced. In addition, the carbon matrix in the mesoporous TiO2 @C nanocomposite also showed electrochemical activity versus lithium ions, providing twice the capacity of pure mesoporous TiO2 or alginic acid-derived mesoporous carbon (A600). Given the simplicity and environmental friendliness of the process, the mesoporous TiO2 @C nanocomposite could satisfy the main prerequisites of green and sustainable chemistry while showing improved electrochemical performance as a negative electrode for lithium-ion batteries.

Investigation of Gd-doped mesoporous TiO2 spheres for environmental remediation and energy applications

Abstract The TiO2 is considered by many as the most versatile material with a very large number of applications in many fields that includes photocatalysis and dye-sensitized solar cell (DSSC). But it is active mainly in the UV region of the spectrum due to its large band gap, around 3.2 eV, which causes low photon absorption. This limitation can be overcome by doping TiO2 with a lanthanide which enhances the absorption of visible photons. In this report, we investigated pure mesoporous TiO2 and Gd-doped mesoporous TiO2 spheres to analysis its photocatalytic and energy conversion efficiency. Gadolinium was chosen due to its stable 4f orbitals state of Gd3+ ions. The XRD spectra and Raman spectra analysis confirms the presence of TiO2. It was noted that only anatase was in all the synthesised samples. The morphological analysis confirms FESEM, HRTEM and Element mapping analysis confirms the formation of mesoporous structures with gadolinium incorporated on the mesosphere's surface. The synthesis samples' photocatalytic activity was studied and sample with 1 mmol of Gd was found to perform best among all the samples. A DSSC study was also conduct and Gd-doped TiO2 was found to out-perform the TiO2.

Applying BaTiO3-coated TiO2 core–shell nanoparticles films as scaffold layers to optimize interfaces for better-performing perovskite solar cells

In this paper, we replaced mesoporous TiO2 nanoparticles scaffold layers by BaTiO3-coated TiO2 core–shell nanoparticles films which obtained by treating pure mesoporous TiO2 layers with 1.0 wt% barium nitrate solution, successfully realized the aim of optimizing interfaces bonding at TiO2/CH3NH3PbI3. Ultrathin BaTiO3 shell layer can combine better with CH3NH3PbI3 layer so as to reduce the existence of carrier recombination centers. Moreover, better optical absorption and larger fill factor were obtained in this manner by the reason of larger CH3NH3PbI3 grain size and fewer crystal boundaries. Furthermore, photoluminescence spectra and electrochemical impedance spectroscopy verified that our core–shell scaffold material contributes to accelerate carrier separation and retard carrier recombination. As a result, average power conversion efficiency enhanced from 11.20 to 13.76% under ambient conditions, which realized almost a quarter improvement than the devices based on pure mesoporous TiO2 layers. Such results have a certain guiding effect on solving interface defects and carrier recombination.

Novel mesoporous NiO/TiO2 nanocomposites with enhanced photocatalytic activity under visible light illumination

The development of active semiconductor photocatalysts with desirable material properties and efficient carrier transformation is indispensable for better photocatalyst utilization and performance optimization. Here, we demonstrate the synthesis of novel mesoporous NiO-TiO2 nanocomposites at different NiO contents through a modified sol–gel method in the presence of triblock copolymer as a structure directing agent, followed by a simple ultrasonication process for enhancement of the photocatalytic properties under visible light illumination. The results indicated that XRD patterns of the NiO/m-TiO2 nanocomposites at different NiO contents show a biphasic anatase-rutile structure. TEM images of the NiO/m-TiO2 nanocomposites showed the formation of very fine TiO2 nanoparticles and NiO sheet-like structure with particle sizes ranging between 10 and 15 nm and 30–50 nm, respectively. The mesoporous NiO/TiO2 nanocomposites have large surface areas of 111.3 m2/g and wide pore volumes of 0.376 cc/g. NiO/m-TiO2 nanocomposites exhibit highly enhanced photocatalytic performance for Methylene Blue (MB) decomposition under visible light illumination, compared with either pure mesoporous TiO2 or pure NiO nanoparticles. The optimum NiO loading content was amounted to be 0.5 wt%, giving a photodegradation rate of 3.3 × 10−7 mol L−1 min−1 with a photocatalytic efficiency of 90%, exceeding that of pure NiO and pure mesoporous TiO2 by more than 10 and 1.5 times, respectively. Additionally, the optimal photocatalyst exhibited extremely high degradation ability towards the gemifloxacin mesylate (GFM: antibiotic drug), complete degradation of molecule was obtained after 180 min. The enhancement of the photocatalytic performance of prepared photocatalysts is explained by the synergistic effect, high surface area, low bandgap values, meso-structure and the formation of p-n junction at the interface of NiO and TiO2. The facile synthesis route, along with superior photocatalytic activity and proper reusability make the current mesoporous NiO/TiO2 nanocomposite a potential candidate in photocatalysis related fields.

Positive Ni(HCO3)2 as a Novel Cocatalyst for Boosting the Photocatalytic Hydrogen Evolution Capability of Mesoporous TiO2 Nanocrystals

A highly efficient hybrid photocatalytic system consisting of Ni(HCO3)2 and mesoporous TiO2 microrods was successfully fabricated by a hydrothermal approach. The involvement of Ni(HCO3)2 cocatalyst resulted in significant improvement of the hydrogen evolution on the host TiO2 photocatalyst under artificial sunlight irradiation. The optimized hybrid photocatalysis system with 2.5% (mole ratio) Ni(HCO3)2 loading displayed the highest hydrogen evolution rate of 1798.0 μmol g–1 h–1, which was more than 18 times higher than that of pure mesoporous TiO2. On the basis of the photoluminescence spectroscopy and photoelectrochemical measurement results, it is proposed that the enhancement of photocatalytic hydrogen evolution capability on the TiO2–Ni(HCO3)2 host photocatalyst–cocatalyst is not only attributed to the efficient separation of photoinduced electron–hole pairs, but also derived from the Ni(HCO3)2 cocatalyst acting as active reaction sites toward hydrogen production.

3D Macro-Mesoporous TiO2-Graphene Oxide (GO) Composite with Enhanced Catalytic Performance in the Epoxidation of Styrene and its Derivatives

It has been widely accepted that graphene oxide shows promising potential on facilitating electron transfer which can enhance catalytic properties. In this work, a 3D ordered macro-mesoporous titania composites with graphene oxide (TiO2-GO) were synthesized through a confinement self-assembly method and applied to the epoxidation reactions of styrene and its derivatives. XRD, BET, SEM, TEM and XPS were used to characterize the hierarchically ordered composites. The incorporation of GO in the TiO2-GO composite made the catalyst structure rather stable and dramatically improved the catalytic activity compared with pure mesoporous TiO2, the conversion of styrene in epoxidation raised from 53.68 % to 93.34 % as the TiO2 combined with GO. Based on the experimental results, the mechanism was proposed to study the π-π interaction between GO and styrene, which was further confirmed by Photoluminescence (PL) analysis, polarization and cyclic voltammetry curves. In short, combining GO with metallic oxide materials will enhance the catalytic performance in epoxidation reactions and such designed strategy can extend to other catalytic reactions.

Preparation and photocatalytic activity of B–N co-doped mesoporous TiO2

B-doped and B–N co-doped mesoporous TiO2 photocatalysts were synthesized by a fast sol–gel method. The prepared samples were characterized by X-ray diffraction (XRD), UV–vis diffuse reflectance spectrum (DRS), transmission electron microscopy (TEM), N2 adsorption–desorption, and X-ray photoelectron spectroscopy (XPS). It is found that the average crystallite size, pore size, and specific surface area of the co-doped TiO2 are 9.1nm, 16.8nm, and 125.4m2/g, respectively. Compared with that of the pure mesoporous TiO2, the absorption band edge of B–N co-doped sample exhibits an evident red-shift and the absorption intensity of visible region increases obviously. The experimental results show that a structure of Ti–N–B–O is formed on the surface of the photocatalyst, indicating a synergistic effect of two dopants enhancing the photocatalytic activity, which was evaluated by the degradation of methyl blue aqueous solution under UV and visible light irradiation.

Visible-Light Active Mesoporous Ce Incorporated TiO<SUB>2</SUB> for the Degradation of 4-Chlorophenol in Aqueous Solution

Efficient visible-light active mesoporous Ce incorporated TiO2 nanoparticles have been prepared by sol-gel method using Pluranic P123 as the structure directing agent. Low angle XRD and BET analysis revealed the mesoporous nature of the nanoparticles. The incorporation of Ce4+ into TiO2 and Ti4+ into CeO2 is evident from the slight shift in their respective XRD patterns. XPS results exhibited +4 oxidation state for Ce and Ti ions. UV-DRS analysis of Ce incorporated TiO2 demonstrated red shift in the absorbance spectrum of TiO2, which is mainly due to the formation of Ce impurity states below the conduction band edge of TiO2. The photocatalytic activity of mesoporous Ce incorporated TiO2 nanoparticles has been evaluated in the degradation of 4-chlorophenol in aqueous solution under solar light illumination. Pure mesoporous TiO2 showed poor visible-light activity due to its small absorption of sun light. Mesoporous Ce incorporated TiO2 photocatalysts exhibited the highest photocatalytic activity compared to pristine mesoporous TiO2 and CeO2. The incorporation of Ce4+ in TiO2 played a major role in the enhancement of photocatalytic activity under sun light.

Preparation, Characterization and Photocatalytic Activity of Lanthanum Doped Mesoporous Titanium Dioxide

Lanthanum doped mesoporous titanium dioxide photocatalysts with different La content were synthesized by template method using tetrabutyltitanate (Ti(OC4H9)4) as precursor and Pluronic P123 as template. The catalysts were characterized by thermogravimetric differential thermal analysis, N2 adsorption-desorption measurements, X-ray diffraction, and UV-Vis adsorption spectroscopy. The effect of La3+ doping concentration from 0.1% to 1% on the photocatalytic activity of mesoporous TiO2 was investigated. The characterizations indicated that the photocatalysts possessed a homogeneous pore diameter of about 10 nm with high surface area of 165 m2/g. X-ray photoelectron spectroscopy measurements indicated the presence of C in the doped samples in addition to La. Compared with pure mesoporous TiO2, the La-doped samples extended the photoabsorption edge into the visible light region. The results of phenol photodecomposition showed that La-doped mesoporous TiO2 exhibited higher photocatalytic activities than pure mesoporous TiO2 under UV and visible light irradiation.

Probing Porosity and Pore Interconnectivity in Self-Assembled TiO2–Graphene Hybrid Nanostructures Using Hyperpolarized 129Xe NMR

Hyperpolarized (HP) 129Xe NMR was used to probe the porosity and interconnectivity of pores in self-assembled hybrid TiO2–graphene nanostructures. We have demonstrated that HP 129Xe NMR is a powerful technique in probing any changes in porosity and interconnectivity of the pores caused by the addition of a small amount of functionalized graphene sheets (FGSs) (1% weight percent) into the network of mesoporous TiO2. To obtain the information on the changes in porosity and interconnectivity of the pores caused by the addition of a small amount of FGSs, a comparative study has been carried out by acquiring HP 129Xe NMR spectra under identical experimental conditions for both pure mesoporous TiO2 and hybrid TiO2–FGSs. The HP 129Xe NMR results from our comparative study suggest that TiO2 and graphene are mixed uniformly on the nanoscale and the resulting hybrid nanostructure has better channel connectivity between different domains, enhancing the transport property for Li-insertion/extraction.

Mesoporous TiO 2− xA y (A = N, S) as a visible-light-response photocatalyst

Mesoporous TiO 2− x A y (A = N, S) thin films were fabricated using thiourea as a doping resource by a combination of sol-gel and evaporation-induced self-assembly (EISA) processes. The results showed that thiourea could serve two functions of co-doping nitrogen and sulfur and changing the mesoporous structure of TiO 2 thin films. The resultant mesoporous TiO 2− x A y (A = N, S) exhibited anatase framework with a high porosity and a narrow pore distribution. The formation of the O–Ti–N and O–Ti–S bonds in the mesoporous TiO 2− x A y (A = N, S) were substantiated by the XPS spectra. A new bandgap in visible light region (520 nm) corresponding to 2.38 eV could be formed by the co-doping. After being illuminated for 3 h, methyl orange could be degraded nearly completely by the co-doped sample under both ultraviolet irradiation and visible light illumination. While pure mesoporous TiO 2 could only degrade 60% methyl orange under UV illumination and showed negligible photodegradation capability in the visible light range. Furthermore, the photo-induced hydrophilic activity of TiO 2 film was improved by the co-doping. The mesoporous microstructure and high visible light absorption could be attributed to their good photocatalytic acitivity and hydrophilicity.

Preparation of Highly Ordered Mesoporous Al2O3/TiO2 and Its Application in Dye-Sensitized Solar Cells

Highly ordered mesoporous Al(2)O(3)/TiO(2) was prepared by sol-gel reaction and evaporation-induced self-assembly (EISA) for use in dye-sensitized solar cells. The prepared materials had two-dimensional, hexagonal pore structures with anatase crystalline phases. The average pore size of mesoporous Al(2)O(3)/TiO(2) remained uniform and in the range of 6.33-6.58 nm while the Brunauer-Emmett-Teller (BET) surface area varied from 181 to 212 m(2)/g with increasing the content of Al(2)O(3). The incorporation of Al content retarded crystallite growth, thereby decreasing crystallite size while simultaneously improving the uniformity of pore size and volume. The thin Al(2)O(3) layer was located mostly on the mesopore surface, as confirmed by X-ray photoelectron spectroscopy (XPS). The Al(2)O(3) coating on the mesoporous TiO(2) film contributes to the essential energy barrier which blocks the charge recombination process in dye-sensitized solar cells. Mesoporous Al(2)O(3)/TiO(2) (1 mol % Al(2)O(3)) exhibited enhanced power conversion efficiency (V(oc) = 0.74 V, J(sc) = 15.31 mA/cm(2), fill factor = 57%, efficiency = 6.50%) compared to pure mesoporous TiO(2) (V(oc) = 0.72 V, J(sc) = 16.03 mA/cm(2), fill factor = 51%, efficiency = 5.88%). Therefore, the power conversion efficiency was improved by approximately 10.5%. In particular, the increase in V(oc) and fill factor resulted from the inhibition of charge recombination and the improvement of pore structure.

Role of phosphorus in synthesis of phosphated mesoporous TiO 2 photocatalytic materials by EISA method

The phosphated mesoporous TiO 2 (PMT) were synthesized by using evaporation-induced self-assembly approach (EISA) with phosphorus content from 1 to 15 mol%. The X-ray diffraction and N 2 adsorption–desorption isothermal results reveal that the incorporating of phosphorus is of benefit to improving the thermal stability and enhancing the surface area of mesoporous TiO 2 by constraining the growth of anatase crystallite. XPS confirms the phosphorus in the calcined PMT exists as amorphous titanium phosphate in a pentavalent-oxidation state (P 5+) and embedded into the nanocrystalline anatase TiO 2. In photodegradation gas phase acetaldehyde, the photocatalytic activity of PMT samples is higher than that of pure mesoporous TiO 2 and P25. It is believed that the enhancing photocatalytic activity of phosphated mesoporous TiO 2 is mainly caused by two factors relative with the incorporating of phosphorus in framework.

Phosphatization: A promising approach to enhance the performance of mesoporous TiO 2 anode for lithium ion batteries

We report phosphatization is a promising method to enhance the performance of mesoporous TiO 2 anode for lithium ion batteries. The resulting phosphated mesoporous TiO 2 possessed higher reversible capacity and better cycling stability than the pure mesoporous TiO 2. When cycled at 30 mA/g between 3.0 and 1.0 V, the initial capacity of phosphate mesoporous TiO 2 was 249 mA h/g, significantly higher than that of pure mesoporous TiO 2 (204 mA h/g). After 40 cycles, the capacity retention ratio of phosphate mesoporous TiO 2 reached 83.7%, while pure mesoporous TiO 2 had merely a capacity retention ratio of 62.3%. We believe that this phosphatization process could be used to enhance the electrochemical performance of other metal oxides for lithium ion batteries.

The structural, physical and photocatalytic properties of the mesoporous Cr-doped TiO 2

A visible-light-active mesoporous Cr-doped TiO 2 photocatalyst with worm-like channels was synthesized using an evaporation-induced self-assembly approach and characterized by X-ray powder diffraction, nitrogen adsorption–desorption, X-ray photoelectron spectroscopy, transmission electron microscope, and UV–vis diffuse reflectance, respectively. The effect of Cr 3+ doping concentration on the photocatalytic activity of mesoporous TiO 2 was investigated from 0.1 to 1 mol%. The characterizations indicated that the photocatalysts possessed a homogeneous pore diameter of about 8 nm with high surface area of 117 m 2/g and a crystalline anatase pore wall doped by Cr 3+. Compared with pure mesoporous TiO 2, the Cr-doped TiO 2 extended the photoabsorption edge into the visible light region. The results of gaseous acetaldehyde photodecomposition showed that mesoporous Cr-doped TiO 2 exhibited higher photocatalytic activities than pure mesoporous TiO 2 and nonporous Cr-doped TiO 2 under visible light irradiation.