Photocatalytic Activity For O2 Evolution Research Articles

Enhanced Visible Light Response of TiO2 Codoped with Cr and Ta Photocatalysts by Electron Doping

The importance of electron concentration in rutile TiO2 photocatalysts motivated us to apply H2 reduction treatment to visible-light-responsive TiO2 photocatalysts sensitized by chromium ion doping. We found that H2 reduction treatment of rutile TiO2 particles codoped with Ta and Cr (TiO2:Ta/Cr) enhanced photocatalytic activity for O2 evolution by water oxidation under visible irradiation (>2.2 eV). The enhanced visible light activity of H2-treated TiO2:Ta/Cr was attributed to the increase of electron concentration, which was confirmed by UV–vis diffuse reflectance and electron spin resonance (ESR) spectroscopy. The H2-treated TiO2:Ta/Cr photocatalyst was repeatedly used in aqueous media in spite of the presence of doped electrons. Photoluminescence and transient absorption spectroscopies revealed that electron doping with H2 treatment decreased the midgap states working as deep traps of photoexcited electrons and increased the accumulation of the photoexcited electrons in the conduction band. The optimiz...

Effects of Coapplication of Rh-Doping and Ag-Substitution on the Band Structure of Li2TiO3 and the Photocatalytic Property

Formation of impurity levels by doping and control of valence bands by substitution with suitable elements into photocatalyst materials are useful strategies for sensitization of metal oxide photocatalysts with a wide band gap to visible light. In the present study, we demonstrated energy gap narrowing of a metal oxide photocatalyst by coapplication of the doping and the substitution. Rh-doped and Ag(I)-substituted Li2TiO3 (AgLi1/3Ti2/3O2:Rh, 2.2–2.3 eV) was successfully synthesized from Rh-doped Li2TiO3 (Li2TiO3:Rh, 3.0–3.2 eV) with a molten AgNO3 treatment. The AgLi1/3Ti2/3O2:Rh possessed a narrower energy gap than either Li2TiO3:Rh or Ag(I)-substituted Li2TiO3 (AgLi1/3Ti2/3O2, 2.8 eV). The AgLi1/3Ti2/3O2:Rh showed the activity for photocatalytic O2 evolution from an aqueous AgNO3 solution under irradiation of light with wavelength longer than 480 nm under which AgLi1/3Ti2/3O2 did not show the activity. The photocatalytic activity of AgLi1/3Ti2/3O2:Rh for the sacrificial O2 evolution became ∼4 times gre...

Photochemical Water Oxidation in a Buffered Tris(2,2'-bipyridyl)ruthenium-Persulfate System Using Iron(III)-Modified Potassium Manganese Oxides as Catalysts.

Study of manganese oxides for electrocatalytic and photocatalytic oxidation of water is an active area of research. The starting material in this study is a high-surface-area disordered birnessite-like material with K+ in the interlayers (KMnOx). Upon ion-exchange with Fe3+, the disordered layer structure collapses (Fe(IE)MnOx), and the surface area is slightly increased. Structural analysis of the Fe(IE)MnOx included examination of its morphology, crystal structure, vibrational spectra, and manganese oxidation states. Using the Ru(bpy)32+–persulfate system, the dissolved and headspace oxygen upon visible light photolysis with highly dispersed Fe(IE)MnOx was measured. The photocatalytic activity for O2 evolution of the Fe(IE)MnOx was three times better than KMnOx, with the highest rate being 9.3 mmolO2 molMn–1 s–1. The improvement of the photocatalytic activity was proposed to arise from the increased disorder and interaction of Fe3+ with the MnO6 octahedra. As a benchmark, colloidal IrO2 was a better photocatalyst by a factor of ∼75 over Fe(IE)MnOx.

Effects of Zirconium Doping Into a Monoclinic Scheelite BiVO4 Crystal on Its Structural, Photocatalytic, and Photoelectrochemical Properties.

Effects of zirconium (Zr) doping into BiVO4 powder on its structural properties and photocatalytic activity for O2 evolution were examined. The formation of BiVO4 powder crystallized in a monoclinic scheelite structure (ms-BiVO4) was achieved when the sample was doped with a relatively small amount of Zr. The photocatalytic activity of Zr-doped ms-BiVO4 powder was much higher than that of non-doped ms-BiVO4. However, further doping caused a reduction of photocatalytic activity for O2 evolution due to the occurrence of structural alterations into tetragonal scheelite and tetragonal zircon structures. Similar effects of Zr doping were also observed for the photoelectrochemical (PEC) system based on BiVO4 thin films doped with various amounts of Zr. Thus, Zr doping was confirmed to be effective for improvements of photocatalytic and PEC functions of BiVO4 for water oxidation.

Preparation of Mo- and W-doped BiVO4 fine particles prepared by an aqueous route for photocatalytic and photoelectrochemical O2 evolution

Abstract Mo- and W-doped BiVO4 fine particles with a diameter of 50–200 nm were prepared from Bi2O3 and, Mo- and W-doped V2O5 by an aqueous route using an aqueous acetic acid solution. The Mo- and W-doped BiVO4 calcined at 673 K was elongate polyhedral particles grown along with the b-axis, while the non-doped BiVO4 prepared by the same method was featureless particles. X-ray absorption near edge structure (XANES) and electron spin resonance (ESR) measurements revealed that the Mo6+ and W6+ ions were doped at V sites, resulting in the formation of V4+ in BiVO4. The Mo- and W-doped BiVO4 powders showed lower activity for photocatalytic O2 evolution than the non-doped BiVO4. In contrast, the Mo- and W-doped BiVO4 photoelectrodes fabricated by an electrophoresis method using these fine particles gave higher photoelectrochemical performance due to a positive effect of the increase in the n-type character by Mo- and W-doping.

Effects of donor doping and acceptor doping on rutile TiO2 particles for photocatalytic O2 evolution by water oxidation

Crystalline defects of photocatalyst particles may be considered to be the recombination center of photoexcited electrons and holes. In this study, we investigated the photocatalytic activity of cation-doped rutile TiO2 photocatalysts for O2 evolution from an aqueous silver nitrate solution under ultraviolet light irradiation. The photocatalytic activity of rutile TiO2 was enhanced by donor doping of Ta5+ and Nb5+ with a valence higher than that of Ti4+, regardless of increased density of electrons and Ti3+ species (an electron trapped in Ti4+ sites). Conversely, acceptor doping of lower valence cations such as In3+ and Ga3+ decreased photocatalytic activity for O2 evolution by water oxidation. The doping of equal valence cations such as Sn4+ and Ge4+ hardly changed the activity of non-doped TiO2. This study demonstrates that Ti3+ species, which is a crystalline defect, enhanced the photocatalytic activity of semiconductor oxides, for example rutile TiO2 with large crystalline size.

KCl flux-induced growth of isometric crystals of cadmium-containing early transition-metal (Ti4+, Nb5+, and Ta5+) oxides and nitridability to form their (oxy)nitride derivatives under an NH3 atmosphere for water splitting application

In this study, we have attempted to experimentally validate the results of previous theoretical calculations predicting the possible formation of the CdTiO3−xNy, CdNbO2N, and CdTaO2N phases by applying conventional one- and two-step fabrication methods under an NH3 flow. For the two-step method, CdTiO3, Cd2Nb2O7, and Cd2Ta2O7 crystals were first grown by a KCl flux method, and the effects of solute concentration and cooling rate on the crystal growth were studied. The formability of their (oxy)nitride derivatives was investigated by changing the nitridation temperature (750–950°C) and time (1–10h) of oxide precursors. It was found that the CdTiO3−xNy, CdNbO2N, and CdTaO2N phases cannot be formed by the applied conventional methods due to the low volatilization temperature of cadmium and the susceptibility of titanium and niobium to reduction under an NH3 atmosphere. Under high-temperature NH3 atmosphere, only Cd2Ta2O7 was fully converted to single-phase Ta3N5. The results from the photocatalytic O2 evolution test of bare and CoOx-loaded Ta3N5 crystalline structures, converted from Cd2Ta2O7 (Cd–Ta3N5) and Na2CO3-treated Ta2O5 (Na–Ta3N5) and Cd2Ta2O7 (Na–Cd–Ta3N5) crystals by nitridation at 850°C for 20h under an NH3 flow, revealed that the CoOx-loaded Ta3N5 showed more than two times higher O2 evolution rate (655μmol), whereas the CoOx-loaded Cd–Ta3N5 and Na–Cd–Ta3N5 exhibited nearly four (501μmol) and three (422μmol) times higher O2 evolution rates at 5h compared with their bare counterparts. An improved photocatalytic activity for O2 evolution is related to the higher density of nucleation centers of CoOx nanoparticles in the form of dangling bonds in porous Ta3N5 structures and long-lived photogenerated holes, as attested by time-resolved absorption spectroscopy.

Photocatalytic Properties of Layered Metal Oxides Substituted with Silver by a Molten AgNO3 Treatment.

K4Nb6O17 (BG: 3.67 eV) and Na2W4O13 (BG: 3.12 eV) layered oxide photocatalysts with wide band gaps were treated with a molten AgNO3 to substitute K+ and Na+ with Ag+, resulting in red-shifts of absorption edges in diffuse reflectance spectra. A part of Na+ ions in the interlayer of Na2W4O13 was substituted with Ag+ ions by the molten AgNO3 treatment with keeping the layered structure. Both Ag(I)-substituted K4Nb6O17 and Na2W4O13 showed photocatalytic activities for O2 evolution from aqueous solutions containing a sacrificial reagent utilizing the absorption bands newly formed by the Ag(I)-substitution. Notably, the Ag(I)-substituted Na2W4O13 produced O2 under visible light irradiation. When ball-milled Na2W4O13 was treated with a molten AgNO3, the Ag(I)-substitution rate increased. The Ag(I)-substituted Na2W4O13 with ball-milling showed higher photocatalytic activity for O2 evolution than that without ball-milling. Z-schematic water splitting proceeded under visible light irradiation by combining the Ag(I)-substituted Na2W4O13 of an O2-evolving photocatalyst with Ru-loaded SrTiO3 doped with Rh of a H2-evolving photocatalyst.

Multi-shelled CeO2 hollow microspheres as superior photocatalysts for water oxidation

A general self-templating method is introduced to construct triple-shelled CeO₂ hollow microspheres, which are composed of tiny CeO₂ nanoparticles. When the triple-shelled CeO₂ hollow microspheres are used as photocatalysts for direct water oxidation with AgNO₃ as the electron scavenger, excellent activity and enhanced stability for O₂ evolution are achieved, in contrast with commercial CeO₂ nanoparticles, single-shelled CeO₂ hollow microspheres and double-shelled CeO₂ hollow microspheres. Such an outstanding performance is attributed to the unique properties of the triple-shelled CeO₂ hollow microspheres including more efficient multiple reflections of the incident light by the inner shells, the larger surface area and more active sites for improving separation of electron-hole pairs, and the more curved surfaces unfavorable for deposition of in situ generated Ag nanoparticles.

Enhancement of Photocatalytic Water Oxidation by the Morphological Control of LaTiO2N and Cobalt Oxide Catalysts

Controlled morphological and structural variations of LaTiO2N and CoOx nanoparticles loaded as a cocatalyst increased the photocatalytic O2 evolution. Four different LaTiO2N samples were prepared from two varied La2Ti2O7 precursors by thermal ammonolysis with and without flux. The most effective transformation from La2Ti2O7 into LaTiO2N in terms of crystallinity and nitrogen contents was obtained in the following two cases: flux-assisted ammonolysis of La2Ti2O7 characterized by a comparatively small particle size and a large surface area and ammonolysis without flux of highly crystalline La2Ti2O7. In both cases, high activity for O2 evolution under visible-light illumination (λ ≥ 420 nm) (∼50 μmol h–1) was obtained. Loading LaTiO2N with CoOx enhanced the photocatalytic activity for O2 evolution, although the effect of CoOx depended on the morphologies of LaTiO2N as a support. The promotional effect was most pronounced for LaTiO2N with a skeletal morphology, because such LaTiO2N contain most unsaturated bo...

Template-free synthesis of BiVO4 nanostructures: I. Nanotubes with hexagonal cross sections by oriented attachment andtheir photocatalytic property for water splitting under visible light

Monoclinic structured BiVO4 nanotubes have been selectively prepared by a template-free and surfactant-free solvothermal process in anethanol–H2O mixed solvent. Interestingly, the nanotubes obtained atVethanol:Vwater = 3:1 have hexagonal cross sections with long lengths ofapprox. 1.2 µm, side lengths of approx. 200 nm and wall thicknesses of approx. 30 nm. A possible orientedattachment growth mechanism has been proposed based on the results of time-dependentexperiments. Nanoplates were firstly formed by aggregation of primary nanocrystallites andthen self-assembly converted to hexagonal-prismatic nanotubes via the correspondingoriented attachment mechanism. During this process, the presence of ethanol and thevolume ratio of ethanol and water play important roles in the formation of the nanotubeswith hexagonal cross sections. UV–vis spectroscopy was further employed to estimate thebandgap energy of the novel structure, which is larger (2.53 eV) than that for the bulkBiVO4. The valence band edge position for the as-synthesized material was estimated to be 2.8 eV,which is positive enough for water oxidation. The photocatalytic activity forO2 evolution from water splitting over the samples under visible light(λ>420 nm) irradiation wasinvestigated by using AgNO3 as a sacrificial reagent. Experiment results indicate that theas-synthesized nanotubes exhibit higher photocatalytic activities forO2 evolution thanthat of bulk BiVO4, which is attributed to the special tubular structure morphology.

Investigations of Electronic Structures and Photocatalytic Activities under Visible Light Irradiation of Lead Molybdate Replaced with Chromium(VI)

Abstract Chromium-replaced PbMoO4, PbMo1−xCrxO4, prepared by an aqueous reflux method, showed a photocatalytic activity for O2 evolution from an aqueous solution containing an electron accepter, such as Ag+ or Fe3+ under visible light irradiation (vis) (λ ≥ 420 nm). PbMo0.98Cr0.02O4 (Energy gap: 2.26 eV) showed the highest activity for the O2 evolution under vis and UV + vis light irradiation. The activity of PbMo0.98Cr0.02O4 was one order of magnitude higher than that of PbMoO4 under UV + vis light irradiation. The crystal structures of PbMo1−xCrxO4 were studied by the Rietveld analysis for powder X-ray diffraction. PbMo1−xCrxO4 had a scheelite-type structure when x was equal to or smaller than 0.2. The energy band structures of PbMo1−xCrxO4 were calculated based on density functional theory using the structural parameters refined by the Rietveld analysis. It was revealed that the top of the valence band of PbMoO4 consisted of O2p and Pb6s orbitals and the accepter level was composed of a Cr3d orbital in the forbidden band below the conduction band consisting of a Mo4d orbital. It was concluded that the visible light response was due to the excitation from the Pb6s + O2p valence band to the Cr3d accepter level.

Photocatalytic behavior of a new series of photocatalysts, Ba2Bi(V)Bi(III)O6 and Ba2Sb(V)Bi(III)O6

Oxide photocatalysts, Ba2Bi(V)Bi(III)O6 and Ba2Sb(V)Bi(III)O6, with the same monoclinic structure, were synthesized by a solid state reaction method. Their band gaps were determined by UV-Vis diffuse reflectance spectroscopy to be about 2.2 and 1.7 eV, respectively. These photocatalysts showed photocatalytic H2 evolution from an aqueous CH3 OH solution under visible light irradiation. The rates of H2 evolution are 0.49 and 0.67 μmol/h for Ba2Bi(V)Bi(III)O6 and Ba2Sb(V)Bi(III)O6, respectively. They also showed activity for photocatalytic O2 evolution from an aqueous AgNO3 solution under visible light irradiation and the rates of O2 evolution were found to be 1.6 and 2.7 μ mol/h, respectively.

Nickel and either tantalum or niobium-codoped TiO2 and SrTiO3 photocatalysts with visible-light response for H2 or O2 evolution from aqueous solutions

TiO2 (band gap = 3.0 eV) and SrTiO3 (band gap = 3.2 eV) codoped with nickel and either tantalum or niobium ions showed photocatalytic activities for O2 evolution from an aqueous silver nitrate solution and H2 evolution from an aqueous methanol solution under visible light irradiation (lambda > 420 nm). The visible-light responses were due to the charge-transfer transition from the electron donor levels formed by the 3d orbitals of doped Ni2+ to the conduction bands of the host materials. Codoping of a tantalum or niobium ion made the absorption intensity in the visible light region strong for TiO2 doped with nickel ions, resulting in the enhancement of the photocatalytic activity for O2 evolution under visible light irradiation. SrTiO3 codoped with nickel and tantalum showed higher activity for H2 evolution than SrTiO3 doped with only nickel. This is due to the fact that codoped Ta5+ ions played a role in charge compensation, resulting in the suppression of the formation of Ni3+ ions, which are expected to trap photogenerated electrons.

Photocatalytic Decomposition of Organic Contaminants by Bi2WO6Under Visible Light Irradiation

An oxide photocatalyst Bi2WO6 with corner-shared WO6 octahedral layered structure was synthesized. Its band gap was determined to be 2.69 eV from UV–vis diffuse reflectance spectra. The photocatalyst showed not only the activity for photocatalytic O2 evolution with the initial evolution rate of 2.0 μmol/h but also the activity of mineralizing both CHCl3 and CH3CHO contaminants under visible light irradiation. Meanwhile, wavelength dependence of CH3CHO decomposition was observed, which indicated that the photocatalytic activity of the photocatalyst was in good agreement with its light-absorption ability.

Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates

α-AgVO3, β-AgVO3, Ag4V2O7 and Ag3VO4 prepared by precipitation and solid-state reactions showed intense absorption bands in the visible light region due to band gap transitions. Comparison of a diffuse reflectance spectrum of α-NaVO3 with that of α-AgVO3 with a diopside-type structure revealed that a band gap (2.5 eV) of α-AgVO3 was 0.6 eV smaller than that (3.1 eV) of α-NaVO3. The electronic band structure study using a plane-wave based density functional method indicated that the decrease in the band gap of α-AgVO3 was due to Ag 4d orbitals which formed a valence band at a more negative level than O 2p orbitals. Among α-AgVO3, β-AgVO3, Ag4V2O7 and Ag3VO4, only Ag3VO4 showed a photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under visible light irradiation. Holes photogenerated in Ag3VO4 can migrate to the reaction sites on the surface more easily than those of other silver vanadates, because the content of silver forming a valence band is large. It resulted in that holes photogenerated in Ag3VO4 are able to oxidize H2O to form O2.

LaTiO2N as a Visible-Light (≤600 nm)-Driven Photocatalyst (2)

LaTiO2N, a Ti-based oxynitride, was studied as a visible-light (420−600 nm)-driven photocatalyst. DFT calculation for LaTiO2N indicated that the top of the valence band consists predominantly of N 2p orbitals with a small contribution by O 2p orbitals, while the bottom of the conduction band is made up entirely of empty Ti 3d orbitals. LaTiO2N was synthesized from a La2Ti2O7 precursor at 1123 K under NH3 flow. The relationship between the preparation condition and the photocatalytic activities of H+ reduction to H2 and H2O oxidation to O2 was examined. It was found that the photocatalytic activity for O2 evolution increased with nitridation time, reaching a maximum at 72 h. Further treatment beyond 72 h lowered the rate of O2 evolution. The rate of H2 evolution was almost independent of nitridation time. The relationship between nitridation time and the rate of O2 evolution is discussed on the basis of X-ray photoelectron spectroscopy (XPS) analyses.

Development of Photocatalyst Materials for Water Splitting with the Aim at Photon Energy Conversion.

A photocatalyst which has extensively been studied so far is TiO2 with a 3.0-3.2eV band gap. Well-known photocatalysts with visible-light response are only Pt/CdS and WO3. Thus, photocatalyst materials mainly employed have been so limited. In such a background, new photocatalysts with high activity have recently been developed. Alkali and alkaline earth tantalates have arisen as a new group of photocatalyst materials for water splitting into H2 and O2 under ultra-violet light irradiation. They showed the activities even without co-catalysts such as Pt, being different from titanate photocatalysts. When NiO co-catalysts were loaded on tantalate photocatalysts, except for LiTaO3, the photocatalytic activities were drastically increased. Among the tantalates, NiO/NaTaO3 showed the highest activity. Moreover, the activity of NiO/NaTaO3 was improved by La-doping. On the other hand, highly crystalline BiVO4 powders with scheelite (monoclinic) and zircon type (tetragonal) structure were selectively synthesized by an aqueous process. The BiVO4 powder with the scheelite structure showed a high photocatalytic activity for O2 evolution in the presence of sacrificial reagent (Ag+) under visible light irradiation (λ>420nm). The photocatalytic activity of the BiVO4 powder prepared by the aqueous process was much higher than that of BiVO4 prepared by a conventional solid state reaction. Zn0.957Cu0.043S (band gap: 2.5eV) and Zn0.999Ni0.001S (band gap: 2.3eV) photocatalysts showed high activities for H2 evolution from an aqueous K2SO3 and Na2S solution under visible light irradiation without co-catalysts such as Pt. ZnNb2O6, Bi2W2O9, Bi2WO6, and Bi3TiNbO9 consisting of ions with d10 and s2 configuration were also active for H2 or O2 evolution from aqueous solutions containing sacrificial reagents.

ChemInform Abstract: Effect of Surface Hydroxyl Density on Photocatalytic Oxygen Generation in Aqueous TiO2 Suspensions.

AbstractTiO2 powders with various surface hydroxyl densities are prepared by thermal treatment (7 h at 600‐1000 °C) and their activity for photocatalytic O2 evolution in aqueous suspensions is studied in the presence of various electron acceptors (AgNO3, K2PtCl4 and KBrO3).