Oxygen-rich Ti1−xO2 pillar growth at a gold nanoparticle–TiO2 contact by O2 exposure

In-situ gas-injection transmission electron microscopy revealed that a pillar grew at the edge of the interface of a gold nanoparticle and a TiO₂ substrate during exposure to O₂ gas at 100 Pa. The pillar was found to have a titanium-deficient chemical composition of Ti_1 ? xO₂ (x > 0) by electron energy loss spectroscopy (EELS). The spectra showed a chemical shift of oxygen and titanium ions to have ionic states of Ti³⁺ and O^y? (y < 3/2). The formation of the Ti_1 ? xO₂ at the contact edge of gold–Ti_1 ? xO₂ interface is discussed from the perspective of an O₂ affinity, which plays an important role in CO oxidation process of supported gold particle.     

Surface oxides of Ir(111) prepared by gas-phase oxygen atoms

The Ir(111) surface is oxidized with gas-phase oxygen atoms under vacuum condition to achieve an oxidation level beyond its saturation coverage for chemisorption. Two surface oxides, rutile IrO₂ of (100) domain and corundum Ir₂O₃ of (001) domain, have been grown at 550 K with different oxygen exposure of 3.6 × 10⁵ L and 7.2 × 10⁵ L respectively. The temperature programmed desorption (TPD) experiment of rutile IrO₂(100) shows its desorption curve (at 4 K s^? 1) peaks at 750 K, followed by a long tail of less pronounced desorption features. On the other hand, TPD of corundum Ir₂O₃(001) displays a symmetric trace, peaking at 880 K. Carbon monoxide titration experiments show that adsorbed CO reduces corundum Ir₂O₃(001) at 400 K, but CO does not adsorb on rutile IrO₂(100) and no reduction reaction occurs. Evidently, among the two surface oxides, corundum Ir₂O₃(001) involves in catalysis of carbon monoxide oxidation, while rutile IrO₂(100) does not. The formation of two surface oxides is also compared, we conclude that the atom arrangement favors Ir₂O₃(001) at the oxide/metal interface.     

Structural and electrochemical properties of LiNi0.5Mn0.5 − xAlxO2 (x = 0, 0.02, 0.05, 0.08, and 0.1) cathode materials for lithium-ion batteries

Layered LiNi_0.5Mn_0.5 ? xAl_xO₂ (x = 0, 0.02, 0.05, 0.08, and 0.1) series cathode materials for lithium-ion batteries were synthesized by a combination technique of co-precipitation and solid-state reaction, and the structural, morphological, and electrochemical properties were examined by XRD, FT-IR, XPS, SEM, CV, EIS, and charge–discharge tests. It is proven that the aliovalent substitution of Al for Mn promoted the formation of LiNi_0.5Mn_0.5 ? xAl_xO₂ structures and induced an increase in the average oxidation number of Ni, thereby leading to the shrinkage of the lattice volume. Among the LiNi_0.5Mn_0.5 ? xAl_xO₂ materials, the material with x = 0.05 shows the best cyclability and rate ability, with discharge capacities of 219, 169, 155, and 129 mAh g^? 1 at 10, 100, 200, and 400 mA g^? 1 current density respectively. Cycled under 40 mA g^? 1 in 2.8–4.6 V, LiNi_{0. 5}Mn_0.45Al_0.05O₂ shows the highest discharge capacity of about 199 mAh g^? 1 for the first cycle, and 179 mAh g^? 1 after 40 cycles, with a capacity retention of 90%. EIS analyses of the electrode materials at pristine state and state after first charge to 4.6 V indicate that the observed higher current rate capability of LiNi_{0. 5}Mn_0.45Al_0.05O₂ can be understood due to the better charge transfer kinetics.     

Field-assisted sustainable O− ion emission from fluorine-substituted 12CaO·7Al2O3 with improved thermal stability

We examined the electric field-assisted thermionic emission of atomic oxygen radical anion (O^?) in a vacuum from fluorine-substituted derivatives of 12CaO·7Al₂O₃ (C12A7) with a composition of (12 ? x)CaO·7Al₂O₃·xCaF₂ (0 ≤ x ≤ 0.8). Unsubstituted C12A7 easily decomposed into 5CaO?3Al₂O₃ (C5A3) and 3CaO?Al₂O₃ (C3A) above 830 °C during the emission experiment in a vacuum. The decomposition temperature range became narrower as the amount of F^? ion substitution increased, e.g. the sample with x = 0.4 kept a single phase after the emission experiment at 900 °C. The emitted anionic species from the x = 0.4 sample were dominated by O^? ions (～ 92%) together with a small amount of O₂^? ions (～ 4%) and F^? ions (～ 4%). The absence of an O₂ gas supply to the opposite side of the emission surface led to a nearly steady co-emission of O^? ions and electrons with a ratio of < 1/1. The O₂ gas supply markedly enhanced the O^? ion emission, and suppressed the electron emission. A sustainable and high-purity O^? ion emission with a current density of 11 nA cm^? 2 was achieved at 830 °C with the supply of 40 Pa O₂ gas. The similarity in these emission features to the unsubstituted C12A7, together with the improved thermal stability demonstrates that the F^? ion-substituted C12A7 is a promising material for higher intensity O^? ion emission at higher temperatures.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.     

Diffusion-promoted-desorption mechanism for D2 desorption from Si(100) surfaces

Temperature-programmed-desorption (TPD) spectra and isothermal desorption rates of D₂ molecules from a Si(100) surface have been calculated to reproduce experimental β_1, A-TPD spectra and isothermal desorption rate curves. In the diffusion-promoted-desorption (DPD) mechanism, hydrogen desorption from the Si(100) (2 × 1) surfaces takes place via D atom diffusion from doubly-occupied Si dimers (DODs) to their adjacent unoccupied Si dimers (UODs). Taking a clustering interaction among DODs into consideration, coverages θ_DU of desorption sites consisting of a pair of a DOD and UOD are evaluated by a Monte Carlo (MC) method. The TPD spectra for the β_1, A peak are obtained by numerically integrating the desorption rate equation R = ν_A exp(? E_d, A / k_BT)θ_DU, where ν_A is the pre-exponential factor and E_d, A is the desorption barrier. The TPD spectra calculated for E_d, A = 1. 6 eV and ν_A = 2.7 × 10⁹ /s are found to be in good agreement with the experimental TPD data for a wide coverage range from 0.01 to 0.74 ML. Namely, the deviation from first-order kinetics observed in the coverage dependent TPD spectra as well as in the isothermal desorption rate curves can be reproduced by the model simulations. This success in reproducing both the experimental TPD data and the very low desorption barrier validates the proposed DPD mechanism.     

An oxygen permeable membrane La0.8Sr0.2(Ga0.8Mg0.2)1 − xCrxO3 − δ for a reduced atmosphere

Mixed electron hole and oxide ion conducting perovskite-type oxides, La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ (0 ≤ x ≤ 1.0)_, were prepared by solid state reaction. The phase stability and the oxygen permeation properties of the oxides were examined as a function of the content of Cr. La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ has a perovskite related tetragonal phase with x = 0.1 to 0.8. The total electrical conductivity of La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ increases with increasing x. The oxygen permeation flux across the La_0.8Sr_0.2(Ga_0.8Mg_0.2)_1 ? xCr_xO_3 ? δ membranes at higher temperatures increases with x up to x = 04. The maximum oxygen permeation flux of 1.6 × 10^? 7 mol^? 1 cm^? 2 at 1100 °C in a oxygen activity gradient of air/10^? 2 Pa is observed in La_0.8Sr_0.2(Ga_0.8Mg_0.2)_0.6Cr_0.4O_3 ? δ. This perovskite-type oxide is stable under an oxygen partial pressure of 7 × 10^? 10 Pa at 1000 °C.     

Conductivity of SnP2O7 and In-doped SnP2O7 prepared by an aqueous solution method

SnP₂O₇ and In-doped SnP₂O₇ have been prepared by an aqueous solution method using (NH₄)₂HPO₄ as phosphorous source. It was found that the solid solution limit in Sn_1 ? xIn_x(P₂O₇)_1 ? δ was at least x = 0.12. All pyrophosphates in the Sn_1 ? xIn_x(P₂O₇)_1 ? δ (x ≤ 0.12) series exhibit 3 × 3 × 3 superlattice structures. The conductivities of Sn_0.92In_0.08(P₂O₇)_1 ? δ in air are 6.5 × 10^? 6 and 8.0 × 10^? 9 S/cm at 900 and 400 °C, respectively, when prepared by an aqueous solution method and annealed at 1000 °C. The conductivity of undoped SnP₂O₇ is slightly lower. However, it was also found that the low-temperature conductivities of pyrophosphates annealed only at 650 °C are several orders of magnitude higher than those annealed at 1000 °C, which could be related to a trace amount of an amorphous secondary phase. The peak conductivity was in this case observed at around 250 °C, which is the same temperature as previously observed in In-doped SnP₂O₇ although the conductivity is still three orders of magnitude lower in the present study. These differences can be related to large differences in particle size and morphology, and all in all, the conductivities of SnP₂O₇-based materials are very sensitive to the synthetic history.     

Oxygen nonstoichiometry and defect equilibrium in La2 − xSrxNiO4 + δ

Nonstoichiometric variation of oxygen content in La_2 ? xSr_xNiO_4 + δ (x = 0, 0.1, 0.2, 0.3, 0.4) and decomposition P(O₂) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range between 873 and 1173 K and the P(O₂) range between 10^? 20 and 1 bar. La_2 ? xSr_xNiO_4 + δ showed the oxygen excess and the oxygen deficient compositions depending on P(O₂), temperature, and the Sr content. The value of partial molar enthalpy of oxygen approaches zero as δ increases in the oxygen excess region, which indicate that the interstitial oxygen formation reaction is suppressed as δ increase. The relationship between δ and logP(O₂) were analyzed by two types of defect equilibrium models. One is a localized electron model, and the other is a delocalized electron model. Both models can well explain the oxygen nonstoichiometry of La_2 ? xSr_xNiO_4 + δ with a regular solution approximation.     

The effect of chemical pressure in rutheno-cuprates

We have studied the effect of negative chemical pressure in the RuGd_1.5(Ce_0.5?xPr_x)Sr₂Cu₂O_10?δ with Pr content of 0.0 ? x ? 0.2. This is also investigated using the bond length results obtained from the Rietveld refinement analysis. The c parameter and cell volume increase with x for 0.0 ? x ? 0.15. The width of the resistivity transition also increases with Pr concentration, indicating higher inhomogeneity and oxygen deficiency. The difference in the ionic valences of Pr^3+,4+ and Ce⁴⁺ causing different hole doping, the difference in the ionic radii, and oxygen stoichiometry affect the superconducting transition. The magnetoresistance shows a cusp around 135 K which lies between the antiferromagnetic and ferromagnetic transition temperatures, which is probably due to the presence of a spin glass region. There exist two magnetic transition temperatures for 0.0 ? x ? 0.2 which respectively change from T_M = 155 K to 144 K and from T_irr = 115 K to 70 K. The magnetization versus applied magnetic field isotherms at 77 K and 300 K show that the remanent magnetization and coercivity are lower for samples with higher Pr content.     

Effect of Cr substitution on the superconducting and magnetic properties of RuSr2Eu1.5Ce0.5Cu2O10−δ

In this paper we investigate the properties of polycrystalline series of Ru_1?xCr_xSr₂Eu_1.5Ce_0.5Cu₂O_10?δ (0.0 ? x ? 0.40) by resistivity, XRD and dc magnetization measurements. EuRu-1222 is a reported magneto superconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity occurs in Cu–O₂ planes below T_c ? 40 K. The exact nature of Ru spins magnetic ordering is still being debated and no conclusion has been reached yet. In this work, we found the superconducting transition temperature T_c = 20 K from resistivity and dc magnetization measurements for pristine sample. DC magnetization measurements exhibited ferromagnetic like transition for all samples.     

Interfacial reactions of Ba2YCu3O6+z with coated conductor buffer layer,LaMnO3

Chemical interactions between the Ba₂YCu₃O_6+x superconductor and the LaMnO₃ buffer layers employed in coated conductors have been investigated experimentally by determining the phases formed in the Ba₂YCu₃O_6+x–LaMnO₃ system. The Ba₂YCu₃O_6+x–LaMnO₃ join within the BaO–(Y₂O₃–La₂O₃)–MnO₂–CuO_x multi-component system is non-binary. At 810 °C (p_O2 = 100 Pa) and at 950 °C in purified air, four phases are consistently present along the join, namely, Ba_2?x(La_1+x?yY_y)Cu₃O_6+z, Ba(Y_2?xLa_x)CuO₅, (La_1?xY_x)MnO₃, (La,Y)Mn₂O₅. The crystal chemistry and crystallography of Ba(Y_2?xLa_x)CuO₅ and (La_1?xY_x)Mn₂O₅ were studied using the X-ray Rietveld refinement technique. The Y-rich and La-rich solid solution limits for Ba(Y_2?xLa_x)CuO₅ are Ba(Y_1.8La_0.2)CuO₅ and Ba(Y_0.1La_1.9)CuO₅, respectively. The structure of Ba(Y_1.8La_0.2)CuO₅ is Pnma (No. 62), a = 12.2161(5) Å, b = 5.6690(2) Å, c = 7.1468(3) Å, V = 494.94(4) Å³, and D_x = 6.29 g cm^?3. YMn₂O₅ and LaMn₂O₅ do not form solid solution at 810 °C (p_O2 = 100 Pa) or at 950 °C (in air). The structure of YMn₂O₅ was confirmed to be Pbam (No. 55), a = 7.27832(14) Å, b = 8.46707(14) Å, c = 5.66495(10) Å, and V = 349.108(14) Å³. A reference X-ray pattern was prepared for YMn₂O₅.     

Electrochemical supercapacitor studies of hierarchical structured Co2+-substituted SnO2 nanoparticles by a hydrothermal method

Hierarchical structured Co-doped SnO₂ nanoparticles are prepared by a low temperature hydrothermal process. The structural and surface morphologies of the SnO₂ and Sn_1?xCo_xO₂ nanoparticles are studied by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The Sn_1?xCo_xO₂ nanoparticles form with a tetragonal rutile structure during the hydrothermal process without further calcination. The pseudocapacitance behavior of the Sn_1?xCo_xO₂ nanoparticles is characterized by cyclic voltammetry (CV) in 1.0 M H₂SO₄ electrolyte. The specific capacitance (SC) is found to increase with an increase in cobalt content. A maximum SC of 840 F g^?1 is obtained for a Sn_0.96Co_0.04O₂ composite at a 10 mV s^?1 scan rate.     

Oxygen non-stoichiometry and electrical conductivity of Pr2−xSrxNiO4±δ with x = 0–0.5

Oxygen non-stoichiometry and electrical conductivity of the Pr_2−xSr_xNiO_4±δ series with x = 0.0–0.5 were investigated in Ar/O₂ (pO₂ = 2.5 to 21 000 Pa) within a temperature range of 20–1000 °C. The equilibrium values of oxygen non-stoichiometry and electrical conductivity of these nickelates were determined as functions of temperature and oxygen partial pressure (pO₂). The nickelates with x = 0–0.5 appear to be p-type semiconductors in the investigated temperature and pO₂ ranges. The nickelates with x = 0.3–0.5 show very feebly marked pO₂ dependencies of the conductivity. Pr_1.7Sr_0.3NiO_4±δ shows the anomalies of the conductivity versus oxygen partial pressure which can be related to the orthorhombic–tetragonal crystal structure transformations. The conductivity of the Pr_2−xSr_xNiO_4±δ samples correlates with the average oxidation state of the nickel cations. The samples with x = 0.5 have the highest nickel oxidation state (≈ 2.5+), the highest [Ni³⁺]/[Ni²⁺] ratio close to 1 and show the highest conductivity (≈ 120 S/cm) in the whole pO₂ and temperature ranges investigated.     

Photochemistry of methyl bromide on the α-Cr2O3(0001) surface

The photochemical properties of the Cr-terminated α-Cr₂O₃(0001) surface were explored using methyl bromide (CH₃Br) as a probe molecule. CH₃Br adsorbed and desorbed molecularly from the Cr-terminated α-Cr₂O₃(0001) surface without detectable thermal decomposition. Temperature programmed desorption (TPD) revealed a CH₃Br desorption state at 240 K for coverages up to 0.5 ML, followed by more weakly bound molecules desorbing at 175 K for coverages up to 1 ML. Multilayer exposures led to desorption at ~ 130 K. The CH₃Br sticking coefficient was unity at 105 K for coverages up to monolayer saturation, but decreased as the multilayer formed. In contrast, pre-oxidation of the surface (using an oxygen plasma source) led to capping of surface Cr³⁺ sites and near complete removal of CH₃Br TPD states above 150 K. The photochemistry of chemisorbed CH₃Br was explored on the Cr-terminated surface using post-irradiation TPD and photon stimulated desorption (PSD). Irradiation of adsorbed CH₃Br with broad band light from a Hg arc lamp resulted in both photodesorption and photodecomposition of the parent molecule at a combined cross section of ~ 10^? 22 cm². Photodissociation of the CH₃–Br bond was evidenced by both CH₃ detected in PSD and Br atoms left on the surface. Use of a 385 nm cut-off filter effectively shut down the photodissociation pathway but not the parent molecule photodesorption process. From these observations it is inferred that d-to-d transitions in α-Cr₂O₃, occurring at photon energies < 3 eV, do not significantly promote photodecomposition of adsorbed CH₃Br. It is unclear to what extent band-to-band versus direct CH₃Br photolysis play in CH₃–Br bond dissociation initiated by more energetic photons.     

Synthesis and electrical transport properties of Lu2+xTi2−xO7−x/2 oxide-ion conductors

The Lu_2+xTi_2−xO_7−x/2 (x = 0; 0.052; 0.096; 0.286; 0.44; 0.63; 33.3–49 mol% Lu₂O₃) nanoceramics with partly disordered pyrochlore-type structure are prepared by sintering freeze-dried powders obtained by a co-precipitation technique with 1600 °C annealing. Similar to pyrochlore-like compositions in the zirconate system, some of the new titanates are good oxide-ion conductors in air. The new solid-state electrolytes have oxide-ion conductivity in the interval of 1.0 × 10^− 3 – 2.5 × 10^− 3 S/cm at 740 °C in air. This value of conductivity is comparable with that of ZrO₂/Y₂O₃ ceramics. The conductivity of Lu_2+xTi_2−xO_7−x/2 depends on the chemical composition. The highest ionic conductivity is exhibited by nearly stoichiometric Lu_2+xTi_2−xO_7−x/2 (x = 0.096; 35.5 mol% Lu₂O₃) material containing ∼ 4.8 at.% Lu_Ti anti-site defects.     

Interactions between glycine and amorphous solid water nanoscale films

The interactions of glycine (Gly) with amorphous solid water (ASW) nanolayers (≤ 100 ML), vapor-deposited on single crystalline AlO_x surfaces at 100 K, have been investigated by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K-edge, temperature-programmed thermal desorption (TPD), X-ray photoelectron spectroscopy (XPS), and temperature-dependent work function measurements. Gly-on-ASW, ASW-on-Gly, and Gly on top of ASW-on-Gly ultrathin films have been fabricated. In contrast to the uniform ASW films grown directly on the hydrophilic AlO_x, water molecules adsorb on the hydrophobic Gly films in the form of 3D ASW clusters. This leads to significant differences in the NEXAFS and work function data obtained from ASW-on-AlO_x and ASW-on-Gly films, respectively. Furthermore, these structural differences influence the chemical state of Gly molecules (neutral vs. zwitterionic) adsorbed on top of ASW films. N1s XPS measurements revealed an increased amount of neutral Gly molecules in the film top-deposited on the ASW-on-Gly structure in comparison to the neutral Gly in the films directly condensed on AlO_x or grown on the ASW substrate. H₂O TPD spectra demonstrate that the crystallization and desorption processes of ASW are affected in a different way by the Gly layers, top-deposited on to ASW-on-AlO_x and ASW-on-Gly films. At the same time, Gly adlayers sink into the ASW film during crystallization/desorption of the latter and land softly on the alumina surface in the form of zwitterionic clusters.     

Methylene bromide chemistry and photochemistry on rutile TiO2(110)

The chemistry and photochemistry of methylene bromide (CD₂Br₂) on the rutile TiO₂(110) surface was probed using temperature programmed desorption (TPD). CD₂Br₂ desorbed in three desorption states at 145, 160 and 250 K tentatively assigned to desorption from the multilayer, from an η¹-CD₂Br₂ species and a bridging η²-CD₂Br₂ species, respectively. The latter two TPD states presumably involve binding of CD₂Br₂ molecules to the surface through Br coordination at five-coordinate Ti⁴⁺ surface sites. The 160 and 250 K TPD states saturated at coverages of 1.0 and 0.33 ML, respectively, where 1 ML is equivalent to the surface Ti⁴⁺ site density (5.2 × 10¹⁴ cm^? 2). No thermal decomposition of CD₂Br₂ was observed on either the clean surface or with preadsorbed O₂. UV irradiation of CD₂Br₂ on TiO₂(110) resulted in predominately photodesorption, with trace amounts of photodecomposition evidenced in TPD. The rate of CD₂Br₂ photodesorption from TiO₂(110) occurred with a low cross section (~ 2 × 10^? 21 cm²) similar to that expected from direct optical excitation of CD₂Br₂. This observation suggests that charge carriers generated in TiO₂(110) were no more effective in activating adsorbed CD₂Br₂ molecules than would be expected through direct molecular excitation. These findings suggest that photocatalytic destruction of halocarbons such as CD₂Br₂ on TiO₂ may preferentially occur though indirect processes (such as OH radical attack) as opposed to direct electron transfer processes involving charge carriers generated in TiO₂ by bandgap excitation.     

Effect of Fe doping on the room temperature ferromagnetism in chemically synthesized (In1−xFex)2O3 (0 ⩽ x ⩽ 0.07) magnetic semiconductors

Observation of room temperature ferromagnetism in Fe doped In₂O₃ samples (In_1−xFe_x)₂O₃ (0 ⩽ x ⩽ 0.07) prepared by co-precipitation technique is reported. Lattice parameter obtained from powder X software shows distinct shrinkage of the lattice constant indicating an actual incorporation of Fe ions into the In₂O₃ lattice. X-ray diffraction data measurements show that the entire sample exhibits single phase polycrystalline behavior. SEM micrographs showed the prepared powder was in the range 25–36 nm. SEM EDS mapping showed the presence of Fe and In ions in the Fe doped In₂O₃ sample. The highest remanence magnetization moment (6.624 × 10⁻⁴ emu/g) is reached in the sample with x = 0.03.     

Synthesis and visible light photocatalysis water splitting property of chromium-doped Bi4Ti3O12

Photocatalysts Bi₄Ti_3 ? xCr_xO₁₂(x = 0.00, 0.06, 0.15, 0.30, 0.40, and 0.50) with perovskite structure were synthesized by sol–gel method and their electronic structures and photocatalytic activities were investigated. The Bi₄Ti_2.6Cr_0.4O₁₂ photocatalyst exhibited the highest performance of H₂ evolution in methanol aqueous solution (58.1 μmol h^? 1 g^? 1) under visible light irradiation (λ > 400 nm) without a co-catalyst, whereas no H₂ evolution is observed for Bi₄Ti₃O₁₂ under the same conditions. The UV–vis spectra indicated that the Bi₄Ti_2.6Cr_0.4O₁₂ had strong photoabsorption in the visible light region. The results of density functional theory (DFT) calculation illuminate that the conduction bands of Bi₄Ti₃O₁₂ are mainly attributable to the Ti 3d + Bi 6p orbitals, and the valence bands are composed of O 2p + Bi 6s hybrid orbitals, while the conduction bands of chromium-doped Bi₄Ti₃O₁₂ are mainly attributable to the Ti 3d + Bi 2p + Cr 3d orbitals, and the O 2p + Cr 3d hybrid obitals are the main contribution to the valence band.