Valence Band Edge Potential Research Articles

Photocatalytic activity of MAl2O4 (M = Mg, Sr and Ba) for hydrogen production

This work reports the hydrogen production of aluminate spinel type materials, MAl2O4 (M=Mg, Sr, and Ba), prepared by solid state method. Characterization was carried out by means of X-ray diffraction (XRD), scanning electronic microscopic (SEM), UV–vis spectroscopy, BET method, chronoamperometry, and EIS. Through electrochemical impedance and UV–vis spectroscopy it was possible the determination of the valence and conduction band edge potential, to define the transfer charge mechanism during the hydrogen production reaction. The photocatalytic measurements coupled with a gas chromatographic system indicated that MgAl2O4 showed the best rate of 7.3μmol·g−1·h−1 for magnesium aluminate, without any co-catalyst or sacrificial reagent. The tendency of the photocatalytic activity for hydrogen production in this work is as follows: MgAl2O4>BaAl2O4>SrAl2O4. When Al2O3 was used as co-catalyst mixed with Magnesium aluminate (MgAl2O4), the photoactivity was enhanced, increasing up to 9 times the hydrogen production (64 vs. 7.3μmol·g−1·h−1). Additionally, photo-corrosion test indicated that all samples were chemically stable under irradiation conditions.

New Dion–Jacobson Phase Three-Layer Perovskite CsBa2Ta3O10 and Its Conversion to Nitrided Ba2Ta3O10 Nanosheets via a Nitridation–Protonation–Intercalation–Exfoliation Route for Water Splitting

We report on the synthesis of new Dion–Jacobson phase three-layer perovskite CsBa2Ta3O10 crystals by conventional solid-state reaction and fabrication of their nitrided Ba2Ta3O10 nanosheets via a nitridation–protonation–intercalation–exfoliation route for visible-light-driven photocatalytic water splitting application. The two-dimensional crystal structure of CsBa2Ta3O10 refined by the Rietveld refinement method belongs to the simple tetragonal system and consists of corner-shared distorted TaO6 octahedra and Ba cations in cuboctahedral sites forming triple-layer perovskite slabs. The valence and conduction band edge potentials estimated by density functional theory with respect to vacuum scale and normal hydrogen electrode indicate that CsBa2Ta3O10 is a UV-light-active semiconductor that can be used for photocatalytic water splitting and decomposition of organic pollutants. The projected electronic wave functions contour plot of CsBa2Ta3O10 in the (1 0 0) plane exhibited two different Ta–O bonds. The nit...

Density Functional Theory Calculation of the Band Alignment of (101̅0) In(x)Ga(1-x)N/Water Interfaces.

Conduction band edge (CBE) and valence band edge (VBE) positions of InxGa1-xN photoelectrodes were computed using density functional theory methods. The band edges of fully solvated GaN and InN model systems were aligned with respect to the standard hydrogen electrode using a molecular dynamics hydrogen electrode scheme applied earlier to TiO2/water interfaces. Similar to the findings for TiO2, we found that the Purdew-Burke-Ernzerhof (PBE) functional gives a VBE potential which is too negative by 1 V. This cathodic bias is largely corrected by application of the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional containing a fraction of Hartree-Fock exchange. The effect of a change of composition was investigated using simplified model systems consisting of vacuum slabs covered on both sides by one monolayer of H2O. The CBE was found to vary linearly with In content. The VBE, in comparison, is much less sensitive to composition. The data show that the band edges straddle the hydrogen and oxygen evolution potentials for In fractions less than 47%. The band gap was found to exceed 2 eV for an In fraction less than 54%.

Room temperature synthesis of Bi4O5I2 and Bi5O7I ultrathin nanosheets with a high visible light photocatalytic performance.

Bi4O5I2 and Bi5O7I ultrathin nanosheets are prepared via a facile route at room temperature. The composition, morphology, energy bands and photocatalytic properties of the materials are investigated systematically using different characterization techniques. The results reveal that Bi4O5I2 and Bi5O7I can be easily obtained by adjusting the OH(-) concentration at different pH values. Both Bi4O5I2 and Bi5O7I have the morphology of ultrathin nanosheets and a high surface area. With the increase of the Bi content, the adsorption edge of the samples shifts towards a shorter wavelength and the valence band edge potential becomes more positive. Thus, they exhibit a superior photocatalytic efficiency for the degradation of phenol under visible light irradiation compared to BiOI.

Preparation of Silver Carbonate and its Application as Visible Light-driven Photocatalyst Without Sacrificial Reagent.

Visible light-driven photocatalyst is the current research focus and silver oxyacid salts with p-block elements are the promising candidates. In this research, Ag2 CO3 was prepared by a facile precipitation method and used to degrade the pollutants from waters. The results revealed that the silver carbonate with monoclinic structure quickly decomposed methyl orange and rhodamine B in less than 15 min under visible light irradiation. When it was recycled six times, the degradation of methyl orange still can reach 87% after 30 min. The calculated band gap of Ag2 CO3 was 2.312 eV with Valence band edge potential of 2.685 eV and Conduction band 0.373 eV vs NHE, which endowed the excellent photo-oxidation ability of silver carbonate. Photogenerated holes and ozone anion radicals were the primary active species in the photo-oxidization degradation of dye. The generation of metallic silver resulted from photocorrosion and the consequent reduction in the ozone anion radical amount led to the performance degradation of Ag2 CO3 . The simple preparation method and high photocatalytic performance of Ag2 CO3 increases its prospect of application in future.

Photoelectrochemistry of n-type antimony sulfoiodide nanowires

In the presented work the photoelectrochemical properties of SbSI along with the electronic structure (i.e. conduction and valence band edge potentials as well as conductivity type) of sonochemically obtained nanowires are discussed for the first time. The spectroscopic investigations indicate interesting optical properties, including surface isotope effect and excitonic emission. The photoelectrochemical investigation of SbSI revealed the occurrence of the photoelectrochemical photocurrent switching effect. It may be defined as a change in photocurrent direction (generated at the illuminated semiconducting electrode immersed in electrolyte) due to an appropriate polarization of the electrode versus the reference electrode. It is often observed for semiconductors as a result of the reduction of molecular oxygen dissolved in the electrolyte. However, in the case of SbSI, the photocurrent switching was recorded regardless of the presence of molecular oxygen in the electrolyte, probably due to the reduction of triiodide species formed at anodic polarization of the SbSI electrode, in an iodide-containing electrolyte. The switching potential (i.e. the potential where anodic-to-cathodic photocurrent transition occurs) equals to ca. 0.4 V versus standard hydrogen electrode, which is close to the formal potential of the I−/I3− redox couple. Therefore, this semiconducting material is of potential interest for the construction of new photovoltaic systems, novel optoelectronic switches and logic devices.

Fabrication of a three-dimensional BiOBr/BiOI photocatalyst with enhanced visible light photocatalytic performance

Three-dimensional (3D) BiOBr/BiOI hierarchical microsphere photocatalysts were successfully synthesized via a facile one-pot solvothermal method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS) and N2 adsorption–desorption isotherms. The photocatalytic activity was evaluated by the photocatalytic degradation of Rhodamine B (RhB) and tetracycline (TC) under visible light irradiation (λ>420nm). The 50% BiOBr/BiOI composite showed a higher photocatalytic removal efficiency of RhB and TC than 10% and 90% BiOBr/BiOI composites. RhB (C0=15mg/L) was degraded completely within 40min and over 83% of TC (C0= 40mg/L) was degraded within 50min using a catalyst quantity of 0.4g/L of 50% BiOBr/BiOI. The enhanced photocatalytic activity of the 50:50 composite was attributed to effective separation of photogenerated carriers and the balance between enhanced visible light absorption and decreased valence band (VB) edge potential.

Electron Paramagnetic Resonance Investigation of Charge Transfer in TiO2(B)/Anatase and N–TiO2(B)/Anatase Mixed-Phase Nanowires: The Relative Valence and Conduction Band Edges in the Two Phases

Regarding how photogenerated charge carriers are transferred in TiO2(B)/anatase mixed-phase nanowires, no unified conclusion has been reached. Electron paramagnetic resonance (EPR) spectroscopy is employed to investigate the vectorial charge transfer in this material. When the material is subjected to UV irradiation, we show that holes stimulated in anatase are transferred to TiO2(B) by comparing EPR-detected amounts of trapped hole O– accumulated on TiO2(B) with X-ray diffraction (XRD)-determined TiO2(B) bulk phase compositions. Under visible-light irradiation which only activates the TiO2(B) phase, we unambiguously show that electron transfer occurs from TiO2(B) to anatase. Without intervention of other charge carriers generated by bandgap excitation, we monitor exclusively the fate of conducting electrons generated by specific excitation of N– midgap states of TiO2(B) with holes localized on the N atom in N-doped TiO2(B)/anatase. The result again clearly demonstrates that electrons migrate from TiO2(B) to anatase. Time-dependent decay of the N•-hole EPR signal shows that it is difficult for the transferred electron to return to TiO2(B). Both higher conduction band and valence band edge potentials in TiO2(B) than the corresponding ones of anatase are implicated. This study helps to point the way toward future development of TiO2(B) nanowire based material for photovoltaic and photocatalytic applications.

ZnIr2O4: An efficient photocatalyst with Rashba splitting

Semiconductor-based photocatalysts nowadays are of central interest for the splitting of water into hydrogen and oxygen. However, the efficiency of the known materials is small for direct utilization of the solar energy. Using first-principles calculations, we show that ZnIr2O4 can overcome this shortage. Modified Becke-Johnson calculations give an indirect band of 2.25 eV, which can be reduced to the visible energy range by S doping. For 25% S doping we find a direct band gap of 1.25 eV and a Rashba spin splitting of 220 meV Å. The valence band edge potential is 2.89 V against the standard hydrogen electrode, which is sufficient for photocatalytic water oxidation and pollutant degradation. The optical absorption of S-doped ZnIr2O4 is strongly enhanced, making the material an efficient photocatalyst for visible light.

β-Ga2O3 Single Crystal as a Photoelectrode for Water Splitting

We report the photoelectrode properties of an n-type β-Ga2O3 single crystal in aqueous solutions. The conduction and valence band-edge potentials were found to be 1.1 V higher and 2.5 V lower than the H+/H2 and O2/H2O redox potentials, respectively. Photocurrent drastically increased as photogenerated carriers were excited at a photon energy higher than the fundamental absorption edge of 4.7 eV. The incident photon-to-current conversion efficiency was 36% at 5.2 eV. Gaseous oxygen and hydrogen evolved from the photoelectrode and Pt counter electrode, respectively. The stoichiometric water splitting was demonstrated by applying an external bias of 1 V.

Cross‐Linked g‐C3N4/rGO Nanocomposites with Tunable Band Structure and Enhanced Visible Light Photocatalytic Activity

Cross-linked rather than non-covalently bonded graphitic carbon nitride (g-C3 N4 )/reduced graphene oxide (rGO) nanocomposites with tunable band structures have been successfully fabricated by thermal treatment of a mixture of cyanamide and graphene oxide with different weight ratios. The experimental results indicate that compared to pure g-C3 N4 , the fabricated CN/rGO nanocomposites show narrowed bandgaps with an increased in the rGO ratio. Furthermore, the band structure of the CN/rGO nanocomposites can be readily tuned by simply controlling the weight ratio of the rGO. It is found that an appropriate rGO ratio in nanocomposite leads to a noticeable positively shifted valence band edge potential, meaning an increased oxidation power. The tunable band structure of the CN/rGO nanocomposites can be ascribed to the formation of C-O-C covalent bonding between the rGO and g-C3 N4 layers, which is experimentally confirmed by Fourier transform infrared (FT-IR) and X-ray photoelectron (XPS) data. The resulting nanocomposites are evaluated as photocatalysts by photocatalytic degradation of rhodamine B (RhB) and 4-nitrophenol under visible light irradiation (λ > 400 nm). The results demonstrate that the photocatalytic activities of the CN/rGO nanocomposites are strongly influenced by rGO ratio. With a rGO ratio of 2.5%, the CN/rGO-2.5% nanocomposite exhibits the highest photocatalytic efficiency, which is almost 3.0 and 2.7 times that of pure g-C3 N4 toward photocatalytic degradation of RhB and 4-nitrophenol, respectively. This improved photocatalytic activity could be attributed to the improved visible light utilization, oxidation power, and electron transport property, due to the significantly narrowed bandgap, positively shifted valence band-edge potential, and enhanced electronic conductivity.

Electronic structure and optical properties of Ag3PO4 photocatalyst calculated by hybrid density functional method

The electronic structure and optical properties of Ag3PO4 were studied by hybrid density functional theory. The results indicated that the band gap is 2.43 eV, which agrees well with the experimental value of 2.45 eV. The conduction bands of Ag3PO4 are mainly attributable to Ag 5s and 5p states, while the valence bands are dominated by O 2p and Ag 4d states. The highest valence band edge potential was 2.67 V (vs. normal hydrogen electrode), which has enough driving force for photocatalytic water oxidation and pollutants degradation. The optical absorption spectrum showed that Ag3PO4 is a visible light response photocatalyst.

Preparation and visible light photocatalytic activity of Zn 1− xFe xO nanocrystalline

Zn 1− x Fe x O nanocrystallines are prepared by precipitation method, and characterized by X-ray powder diffraction (XRD), Transmission electron microscopy (TEM), and UV–vis diffuses reflectance spectroscopy. Zn 1− x Fe x O nanocrystallines show high photocatalytic activities for xanthate degradation at pH 7 under visible light irradiation. Zn 0.96Fe 0.04O nanocrystallines exhibit the highest photocatalytic efficiency, which is attributed to the balance between visible light absorption and valence band edge potential.

Photoelectrochemical characterization and photocatalytic properties of mesoporous TiO2/ZrO2 films

Optically transparent, crack‐free mesoporous titania and zirconia‐doped titania thin film photocatalysts were fabricated by sol‐gel technique, using nonionic amphiphilic block copolymer Pluronic P123 as template. The structural and optical properties of these films were characterized using SEM, low‐angle XRD, and UV/Vis spectroscopy, hexane adsorption investigation. Band gap energy and the position of flatband potentials were estimated by photoelectrochemical measurements. Enhancing of photocatalytic activity of zirconia‐doped films relative to pure TiO2 originates from an anodic shift of the valence band edge potential. Catalytic activity of mesoporous TiO2 and TiO2/ZrO2 (5–50% of ZrO2) films in the processes of CrVI to CrIII photoreduction and 2,4‐dinitroaniline photooxidation correlates with crystalline size and growth with increasing of specific surface area of the samples.

Temperature induced restoration of fluorescence from oxidised single-walled carbon nanotubes in aqueous sodium dodecylsulfate solution

Fluorescence intensity from single walled carbon nanotubes in aqueous sodium dodecylsulfate solution is shown to be strongly temperature dependent in the range 5-60 degrees C. Emission peaks corresponding to particular nanotube species which have been quenched due to oxidation by aerated water show sharp transitions at distinct temperatures as the sample is heated. The temperature at which the transition occurs is found to be species dependent and has been interpreted as being proportional to the valence band edge potential of the nanotube. The results are explained quantitatively using the Nernst equation to measure the increase in reduction potential of the solution as the temperature is raised. The removal of dissolved O(2) and CO(2) is thought to be a significant driving force in the reversal of the redox reaction which causes oxidation of the nanotubes.

Photoinduced halophenol formation in the presence of iron(III) species or cadmium sulfide

Phenol photo-induced decomposition has been investigated in the presence of iron(III) oxide or iron(III) ions at different pH, paying particular attention to the evolution of intermediates. The addition of halides causes a modest decrease in phenol degradation rate and starts additional transformation pathways. Chloride induces the formation of chlorinated compounds (2- and 4-chlorophenol), whose concentrations are strongly dependent on pH. Analogously, bromide ions lead to the formation of bromophenols. Similar investigations have been performed by adopting cadmium sulfide as a photocatalyst, whose valence band edge potential ( E VB°) allows the oxidation of bromide but not chloride. The addition of chloride does not lead to chlorophenols, while bromide causes the formation of bromophenols, thus confirming the possibility of obtaining selective abiotic halogenation of phenol.

Photoelectrochemistry and interfacial energetics of titanium dioxide photoelectrodes in fluoride-containing solutions

Current-potential and ac impedance measurements were used to characterize the semiconductor/solution interface for TiO{sub 2-x} and TiO{sub 2-x}F{sub x} single-crystal electrodes in anhydrous HF/NaF and acetonitrile/tetraethylammonium fluoride (TEAF) solutions. In HF/NaF, photoelectrochemical evolution of elemental fluorine occurs at potentials positive of the flatband potential, E{sub fb}, which is +0.6 V vs the H{sup +}/H{sub 2} formal potential. Photocorrosion occurs readily in this medium with TiO{sub 2-x} but not with TiO{sub 2-x}F{sub x}. No photocurrent is observed even at very positive potentials in acetonitrile/TEAF, and Mott-Schottky plots show that E{sub fb} is {minus}2.10 V vs NHE in this medium and that the valence band-edge potential is therefore negative of the F{sup {minus}}/F{sub 2} formal potential. The ideal photoelectrochemical behavior and low photocorrosion rate observed with TiO{sub 2-x}F{sub x} are attributed to a low density of interband surface states ((3-7) {times} 10{sup 11} cm{sup {minus}2}), tentatively assigned to Ti{sup 3+} deep levels. No oxygen vacancy states are detected at the TiO{sub 2-x}F{sub x} surface, in either HG/NaF or acetonitrile/TEAF, by the ac impedance technique.