Band Edge Potential Research Articles

Flat-Band Potentials of Molecularly Thin Metal Oxide Nanosheets.

Exfoliated nanosheets derived from Dion-Jacobson phase layer perovskites (TBAxH1-xA2B3O10, A = Sr, Ca, B = Nb, Ta) were grown layer-by-layer on fluorine-doped tin oxide and gold electrode surfaces. Electrochemical impedance spectra (EIS) of the five-layer nanosheet films in contact with aqueous electrolyte solutions were analyzed by the Mott-Schottky method to obtain flat-band potentials (VFB) of the oxide semiconductors as a function of pH. Despite capacitive contributions from the electrode-solution interface, reliable values could be obtained from capacitance measurements over a limited potential range near VFB. The measured values of VFB shifted -59 mV/pH over the pH range of 4-8 and were in close agreement with the empirical correlation between conduction band-edge potentials and optical band gaps proposed by Matsumoto ( J. Solid State Chem. 1996, 126 (2), 227-234 ). Density functional theory calculations showed that A-site substitution influenced band energies by modulating the strength of A-O bonding, and that subsitution of Ta for Nb on B-sites resulted in a negative shift of the conduction band-edge potential.

(Invited) Photoelectrochemical Systems for Sunlight Driven Fuel Production

Photoelectrochemistry and photocatalysis have been attracted huge attentions as a promising mean to convert solar energy to storable and conveyable energy carriers such as hydrogen, formic acid, and methylcyclohexane (MCH). Hydrogen is the most promising candidate among the energy carriers because of its simple production through decomposition of abundant water and usability in combustion engines and fuel cells with no emissions except for water. In the present study, we investigated hydrogen evolution through overall water splitting, and MCH evolution through reduction of toluen in existence of water as a reductant without external bias voltage. Photoelectrochemical (PEC) water splitting using the combination of photocathode and photoanode is one of the promising means to produce hydrogen via water splitting under sunlight. The configuration can relax the criteria for the photoelectrode used in the PEC cell and allows us to use photocatalytic materials with narrower bandgap resulting in utilization of larger wavelength region in sunlight spectrum. Considering the PEC cell composed of combined photocathode and photoanode, both photocathode and photoanode should show enough large photocurrent at around 0.6 V vs. reversible hydrogen electrode (RHE) under sunlight. We found that the solid solution of ZnSe and Cu(In, Ga)Se2 (ZnSe:CIGS) with optimal composition shows relatively large cathodic photocurrent with high onset potential under simulated sunlight of AM1.5G. As shown in figure 1, ZnSe poses proper band edge potentials. However, its bandgap is not enough narrow to capture sunlight efficiently. On the other hand, CIGS have enough narrow bandgap of 1.1 eV while its potential of valence band maximum is too shallow resulting in not enough high onset potential of cathodic photocurrent to construct PEC cell with photoanode as discussed above. To achieve photocathode material with preferable band edge potentials for water splitting and sunlight absorption, ZnSe:CIGS was investigated. The solid solution with optimal composition of ZnSe0.15:CIGS0.85 act as photocathode and its cathodic photocurrent is largely enhanced by surface modifications with CdS and Pt as the case of other chalcogenide photocathodes. The PEC cell composed of ZnSe:CIGS based photocathode and BiVO4 based photoanode in side-by-side configuration showed stoichiometric water splitting under simulated sunlight with solar-to-hydrogen conversion efficiency of ~1% without external bias voltage.As discussed above, hydrogen is the promising energy carrier while the energy concentration of hydrogen gas is very thin in atmospheric pressure. Thus, pressure rising and/or liquefaction are indispensable to convey and to store. MCH is the one of the promising hydrogen carrier because of its outstanding high hydrogen concentration comparable to liquefied hydrogen. We investigated MCH production through an addition reaction of toluene with existence of reductant, water, using two-chamber PEC cell combined with membrane-electrode assembly. The PEC cell with SrTiO3 photoanode prepared by particle transfer method and MEA composed of Pt loaded carbon and anion exchange membrane showed MCH production spontaneously. We will discuss about PEC MCH production in detail in the presentation. Figure 1

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...

Surface emission enhancement for deep ultraviolet AlGaN-based LEDs using triangular shaped quantum wells

The optical polarization properties of AlGaN/AlN conventional rectangular shaped and triangular shaped quantum wells (QWs) were investigated using the modified theoretical model based on the effective mass equation. The calculated results show that there is more emission component with TE polarization from triangular shaped QWs than that from conventional QWs for same peak emission wavelength, which is beneficial to the surface emission of Al-rich AlGaN-based light-emitting diodes (LEDs). Furthermore, the AlGaN/AlN triangular shaped QWs have shorter critical wavelength for polarization switching between dominant TE and TM emissions than conventional QWs. This is because that valence subband structure changes for the special band edge potential in AlGaN/AlN triangular shaped QWs. So, the polarization control to enhance the surface emission of DUV AlGaN-based LEDs can be realized by using triangular shaped QWs structure.

A graphene-coupled Bi2WO6 nanocomposite with enhanced photocatalytic performance: a first-principles study.

An experimentally synthesized graphene/Bi2WO6 composite showed an enhancement of the visible-light photocatalytic activity, while the underlying mechanism is not known. Here, first-principles calculations based on density functional theory were performed to explore the various properties of the graphene/Bi2WO6(010) composite aiming at gaining insights into the mechanism of its photocatalytic activity. The stability, electronic properties, charge transfer, and visible-light response were investigated in detail on the Bi2WO6(010) surface coupled with graphene. An analysis of charge distribution and Bader charge shows that there is a strong covalent bonding between graphene and the Bi2WO6(010) surface. The covalent interaction induces a small bandgap in graphene. The interband transition of graphene and the surface states of the Bi2WO6(010) surface would cause the absorption spectrum of graphene/Bi2WO6(010) to cover the entire visible-light region and even the infrared-light region. The photogenerated electrons flow to graphene from the conduction band of Bi2WO6 under the built-in electric field and band edge potential well. Thus, graphene serves as a photogenerated electron collector and transporter which significantly reduces the probability of electron-hole recombination and increases catalytic reaction sites not only on the surface of graphene but on also the surface of Bi2WO6. The decrease of charge recombination is possibly responsible for the enhancement of the visible-light photocatalytic activity of the graphene/Bi2WO6(010) nanocomposite.

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.

Tuning of photocatalytic reduction by conduction band engineering of semiconductor quantum dots with experimental evaluation of the band edge potential.

The conduction band edge (CBE) of WO3 quantum dots (QDs) was determined experimentally and engineered by size control to around one nanometer. Photocatalytic reactions of proton and molecular oxygen were tuned by changing the CBE of WO3-QDs.

Design of a Monolithic Photoelectrochemical Tandem Cell for Solar Water Splitting with a Dye-sensitized Solar Cell and WO3/BiVO4Photoanode

Photoelectrochemical cell (PEC) is one of the attractive ways to produce clean and renewable energy. However, solar to hydrogen production via PEC system generally requires high external bias, because of material`s innate electronic band potential relative to hydrogen reduction potential and/or charge separation issue. For spontaneous photo-water splitting, here, we design dye-sensitized solar cell (DSSC) and their monolithic tandem cell incorporated with a photoanode. has high conduction band edge potential and suitable band gap (2.4eV) to absorb visible light. To achieve efficient photoanode system, electron and hole mobility should be improved, and we demonstrate a tandem cell in which film is connected to cobalt complex based DSSC.

Effect of Polarity on Photoelectrochemical Properties of Polar and Semipolar GaN Photoanode

In this study, the photoelectrochemical (PEC) properties of polar GaN were compared to those of semipolar GaN. GaN samples were grown on sapphire by metal–organic chemical vapor deposition (MOCVD). Various electrolytes, namely, aqueous solutions of H2SO4 (pH 0.2), Na2SO4 (pH 6.0), and NaOH (pH 14.0), were used for investigating the effect of pH. The band-edge potentials of polar and semipolar GaN calculated from the Mott–Schottky plots indicates that these potentials can split water and followed the Nernst equation for each electrolyte. Further, the PEC properties of GaN with different polarities were investigated under 100 mW illumination. The photocurrent density of polar GaN was found to be higher than that of semipolar GaN. This result was attributed to the polarization effect of polar GaN. Interestingly, all GaN samples showed a divergent trend in the two-electrode method, similar to a real device.

Reduced Graphene Oxide Composite of Gallium Zinc Oxynitride Photocatalyst with Improved Activity for Overall Water Splitting

AbstractGallium zinc oxynitride (GaN:ZnO) solid solution is one of the most promising visible‐light activated photocatalysts due to high stability in water splitting reaction conditions and controllable band edge potentials. To further enhance the activity of this photocatalyst by facilitating the photo‐induced charges separation, a nanocomposite of GaN:ZnO solid solution photocatalyst and graphene oxide (GO) nanosheets was fabricated through facile ultrasonication. Structural, optical, and electrochemical characterization of the prepared composite revealed the effective interaction between composite components and significant improvement in charge separation. The synthesized composite with a defined amount of GO demonstrated superior activity for overall water splitting under visible light irradiation with significantly higher apparent quantum efficiency.

Assessing photocatalytic power of g-C3N4 for solar fuel production: A first-principles study involving quasi-particle theory and dispersive forces.

First-principles quasi-particle theory has been employed to assess catalytic power of graphitic carbon nitride, g-C3N4, for solar fuel production. A comparative study between g-h-triazine and g-h-heptazine has been carried out taking also into account van der Waals dispersive forces. The band edge potentials have been calculated using a recently developed approach where quasi-particle effects are taken into account through the GW approximation. First, it was found that the description of ground state properties such as cohesive and surface formation energies requires the proper treatment of dispersive interaction. Furthermore, through the analysis of calculated band-edge potentials, it is shown that g-h-triazine has high reductive power reaching the potential to reduce CO2 to formic acid, coplanar g-h-heptazine displays the highest thermodynamics force toward H2O/O2 oxidation reaction, and corrugated g-h-heptazine exhibits a good capacity for both reactions. This rigorous theoretical study shows a route to further improve the catalytic performance of g-C3N4.

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.

Electronic Structure and Optical Properties of Nb2O5 Photo-Catalyst Calculated By Density Functional Method

Recently, there has been attempts to increase the photo-catalytic activities of Nb2O5 with the addition of carbon modifications [1-4]. It has been proposed that carbonate species on the modified Nb2O5 act as surface sensitizer, which extends the photo-catalytically active region of Nb2O5 into the visible range [1-3]. Experiments have shown the photo-catalytic activity of these carbon-modified Nb2O5 structures are higher than of non-carbon-modified Nb2O5 and carbon-modified meso-porous TiO2 [1]. These carbon-modified Nb2O5 structures are promising candidates for photo-catalytic water splitting and more research is needed to understand these materials at the atomic scale. Here, we propose a DFT study of carbon-modified Nb2O5 (001) plane. The aim of this study is to determine the source of activity gain through electronic structure calculations. First, ground state, density of states, energy band structures, and optical properties of carbon-modified Nb2O5 are calculated. The electronic interaction between carbon and Nb2O5 will also be examined to determine its contribution to modify adsorption of intermediate species such as O2, H2, and OH. Band edge potentials and band gaps as calculated indicate the potential for driving the water cleavage reaction using solar energy. These results as presented point to a set of design guidelines for optimizations of photo-catalytic activity of Nb2O5.

Defect-meditated efficient catalytic activity toward p-nitrophenol reduction: A case study of nitrogen doped calcium niobate system

This work reported on the synthesis of a series of nitrogen doped Ca2Nb2O7 with tunable nitrogen content that were found to be efficient and green noble-metal-free catalysts toward catalytic reduction of p-nitrophenol. XPS and ESR results indicated that the introduction of nitrogen in Ca2Nb2O7 gave rise to a large number of defective nitrogen and oxygen species. Defective nitrogen and oxygen species were found to play synergetic roles in the reduction of p-nitrophenol. The underlying mechanism is completely different from those reported for metallic nanoparticles. Moreover, the more negative conduction band edge potential enabled nitrogen doped Ca2Nb2O7 to show photo-synergistic effects that could accelerate the reduction rate toward p-nitrophenol under UV light irradiation. This work may provide a strategy for tuning the catalytic performance by modulating the chemical composition, electronic structure as well as surface defect chemistry.

Effect of the Si/TiO2/BiVO4 heterojunction on the onset potential of photocurrents for solar water oxidation.

BiVO4 has been formed into heterojunctions with other metal oxide semiconductors to increase the efficiency for solar water oxidation. Here, we suggest that heterojunction photoanodes of Si and BiVO4 can also increase the efficiency of charge separation and reduce the onset potential of the photocurrent by utilizing the high conduction band edge potential of Si in a dual-absorber system. We found that a thin TiO2 interlayer is required in this structure to realize the suggested photocurrent density enhancement and shifts in onset potential. Si/TiO2/BiVO4 photoanodes showed 1.0 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode (RHE) with 0.11 V (vs RHE) of onset potential, which were a 3.3-fold photocurrent density enhancement and a negative shift in onset potential of 300 mV compared to the performance of FTO/BiVO4 photoanodes.

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.

Role of CeO2 as oxygen promoter in the accelerated photocatalytic degradation of phenol over rutile TiO2

In this work, we report that the nonstoichiometric CeO2 prepared at low temperature has ability to store and release oxygen to rutile TiO2, improving phenol degradation in aerated aqueous suspension under UV light. The biphase oxide was prepared by mixing individual oxides together, without obvious changes of TiO2 phase, in terms of the crystallite size, surface area and band gap energy. As the amount of CeO2 in CeO2/TiO2 increased, the photocatalytic activity of the mixed oxide increased, and then decreased. The optimal CeO2 loading was about 1.5wt%, which increased the activity of TiO2 by 60%. Strikingly, as the synthesis temperature for CeO2 and TiO2 increased, the activity enhancement of TiO2 decreased and increased, respectively. Moreover, CeO2 was also positive to the production of H2O2 over the irradiated rutile TiO2. However, a similar loading of CeO2 onto anatase and P25 TiO2 led to slight decrease in the photocatalytic activity for phenol degradation. By using silver ions as electron scavengers, none of the above TiO2 samples showed an activity enhancement on the addition of CeO2. Although these oxides have different conduction band edge potentials, the possible charge transfer between TiO2 and CeO2 is excluded. A plausible mechanism responsible for the observed activity difference among the samples is proposed.

Porous Graphitic Carbon Nitride: A Possible Metal-free Photocatalyst for Water Splitting

Hydrogen generation through photocatalytic water splitting with the aid of renewable solar energy is an important step toward the development of sustainable and alternative energy. In the present study, using the first-principles calculations, we have explored the s-triazine based two-dimensional porous graphitic carbon nitride (g-CN) materials as a potential photocatalyst for water splitting. For calculating the band structures more accurately, we have employed hybrid density functionals. The calculated band gap of the single layer g-CN is found to be 2.89 eV, which decreases to ∼2.75 eV in multilayered structure. To improve the visible light activity, the effect of doping with different nonmetals on the electronic structure has been investigated. Among the different dopants studied, phosphorus is found to be more effective to reduce the band gap to 2.31 eV. The band edge potentials obtained from density functional calculations are corrected for vacuum potentials. The band alignments with respect to the ...

Perovskite oxide nanosheets with tunable band-edge potentials and high photocatalytic hydrogen-evolution activity.

Perovskite nanosheets of HCa(2-x)Sr(x)Nb3O10 and HCa2Nb(3-y)Ta(y)O10 with controlled band-edge potentials were prepared. They worked as highly efficient heterogeneous photocatalysts for H2 evolution from a water/methanol mixture under band-gap irradiation. The activity was found to depend on the composition. The highest activity was obtained with HCa2Nb2TaO10 nanosheets, recording an apparent quantum yield of approximately 80% at 300 nm, which is the highest value for a nanosheet-based photocatalyst reported to date.

Mono- and co-doped NaTaO3 for visible light photocatalysis.

Electronic structures of doped NaTaO3 compounds are of significant interest to visible light photocatalysis. This work involves the study of the band gap, band edge potentials, and thermodynamic stability of certain mono-doped and co-doped NaTaO3 systems, using DFT-PBE as well as hybrid (PBE0) functional calculations. Doping of certain non-magnetic cations (Ti, V, Cu, Zn, W, In, Sn, Sb, Ce, and La), certain anions (N, C, and I), and certain co-dopant pairs (W-Ti, W-Ce, N-I, N-W, La-C, Pb-I, and Cu-Sn) is investigated. Our calculations suggest that substitutional doping of Cu at the Ta site, Cu at the Na site, and C at the O site narrows the band gap of NaTaO3 to 2.3, 2.8, and 2.1 eV, respectively, inducing visible light absorption. Additionally, passivated co-doping of Pb-I and N-W narrows the band gap of NaTaO3 to the visible region, while maintaining the band potentials at favorable positions. Hybrid density of states (DOS) accurately describe the effective band potentials and the location of mid-gap states, which shed light on the possible mechanism of photoexcitation in relation to the photocatalysis reactions. Furthermore, the thermodynamic stability of the doped systems and defect pair binding energies of co-doped systems are discussed in detail. The present results provide useful insights into designing new photocatalysts based on NaTaO3.