Valence Band Edge Positions Research Articles

Nanoporous Anodic Bismuth Oxide Photo-Anodes

Metal-oxide semiconductors have received a wide attention as photo anodes since the demonstration of water splitting by TiO2 electrode under UV illumination [1]. Significant research efforts were focused on developing various semiconductors with band-gap engineering for efficient photo water splitting. Bismuth oxide (Bi2O3) with a band gap of 2.8eV is a promising photo anode which exhibits good electrical conductivity, oxygen ion conductivity, and high dielectric permittivity. Due to its distinctive properties, Bi2O3 finds applications in a wide range of areas like gas sensors, optical coatings, microelectronics, photocatalysts, solid state electrolytes, superconductors etc. In spite being a non-toxic material with appropriate band gap and valence band edge position (+3.13 V vs. NHE), Bi2O3 demonstrates a poor hydrogen evolution due to its lower conduction band edge position (+0.33 V vs. NHE). The general strategies employed to overcome these limitations were simultaneous doping and nano-sizing of the material. In the present investigation, Bi2O3 nanoporous films were synthesized by electrochemical anodization in the electrolyte solutions containing citric acid, ethylene glycol and glycerol. Nanoporous bismuth oxide films were formed by anodizing bismuth circular discs of 3 mm thick and 12.7 mm diameter. Anodization was carried out at various potentials ranging from 3 V to 60 V for different time durations ranging from 0.5 to 2 h. After anodization, the samples were thermally annealed at 200 °C for 2h. The influences of anodization time, electrolyte concentration and applied voltages on morphology have been investigated in this study. Potentiodynamic, potentiostatic, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis studies were carried out with and without illuminated conditions. All the depositions were examined under a FEI Quanta 200F scanning electron microscope. Figure 1 shows the nanoporous morphology of the oxide layer formed at 3V for 30minutes in citric acid electrolyte. The diameters of the pores were in the range of 20 -50 nm and total thickness of the film was about 500nm. It was observed that pore diameter and film thickness changed with the change in applied potential, time and electrolyte concentration. Photo electrochemical studies were carried out using a potentiostat (Gamry, Reference 600) with platinum as the counter electrode. Potential Vs current plots were constructed by scanning the potential of the sample from the open circuit potential to 0.5 V at a scan rate of 5mV/s in 1 M KOH solution. Potentiostatic measurements were carried out in 1 M KOH solution at 0.2 V and 0.5 VAg/AgCl. A solar simulator with an AM 1.5 filter was used for illuminating the samples. The thickness of the oxide layer increased with the increase in the anodization potential. The photo current density of the nanoporous bismuth oxide increased with increase in the thickness of the oxide. The dark current density decreased with the increase in thickness of the oxide layer. The maximum photo current density (Iilluminated-Idark) recorded at an applied potential of 0.5 VAg/AgCl was 1 mA/cm2 for the sample anodized at 20 V. The sample anodized at 60 V showed about 20 µA/cm2 dark current density and 0.8 mA/cm2 photo current density at 0.5 VAg/AgCl. The photo activity of the nanoporous bismuth oxide is comparable to that of nanotubular TiO2 oxide photo anodes. The nanoporous Bi2O3 contained a defect concentration in the range of 7x1016 – 4x1018 cm-3 under dark condition for various anodized conditions. Upon illumination, the defect density increased to values in the range of 4x1017 to 2x1019 cm-3. A detailed discussion will be provided in the final presentation on the photo electrochemical behavior of the Bi2O3 nanoporous structure as a function of morphology, and defect and electronic structures. References A. Fujishima and K. Honda, Nature 238 (1972) 37

Synergistic effect between carbon dopant in titania lattice and surface carbonaceous species for enhancing the visible light photocatalysis

Visible light-sensitive carbon doped titanium dioxide (C-TiO2) was prepared by grinding anatase TiO2 with anhydrous D-glucose solution as carbon source followed by calcination. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET surface area measurements, scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), FTIR analysis and photoluminescence (PL) techniques. The results confirmed the interstitial incorporation of carbon atoms in the TiO2 lattice via OTiC and TiOC surface states. The calculation of valence band (VB) edge position of C-TiO2 by using electronegativity values shows cathodic shift with increase in the carbon concentration and this renders high oxidative power for photogenerated holes. The observed new electronic state above the VB edge was responsible for the electronic origin of band gap narrowing and visible light photoactivity of C-TiO2. The carbon atom was also present as carbonaceous species on the surface which acts as sensitizer. The photocatalytic activity of C-TiO2 was evaluated for the degradation of 4-chlorophenol under both UV and solar irradiation. The undoped TiO2 showed better activity under UV light whereas C-TiO2 showed higher photocatalytic activity under visible light. The pretreatment of C-TiO2 with UV light reduced the visible light activity due to the removal of surface carbonaceous species. The synergistic effect of surface carbonaceous species along with interstitial carbon is discussed in detail and accounted for visible light activity.

Computational Search for Single-Layer Transition-Metal Dichalcogenide Photocatalysts

Some of the members of the family of single-layer transition-metal dichalcogenides have recently received a lot of attention for their promising electronic properties, with potential applications in electronic devices. In this work, we focus on the stability of the dichalcogenides and determine their potential for photocatalytic water splitting. Using a first-principles design approach, we perform a systematic theoretical study of the dichalcogenides MX2 (M = Nb, Mo, Ta, W, Ti, V, Zr, Hf, and Pt; X = S, Se, and Te). First, we use a van der Waals functional to accurately calculate their formation energies. The results reveal that most MX2 have similar formation energies to those of single-layer MoS2 and WS2, implying the ease of mechanically exfoliating a single-layer MX2 from their layered bulk counterparts. Next, we use the many-body G0W0 approximation to obtain the band structures, finding that about half of the MX2 are semiconductors. We then calculate conduction and valence band edge positions by comb...

Single-Layer Group-III Monochalcogenide Photocatalysts for Water Splitting

The recent synthesis of single-layer GaS and GaSe opens the question of stability for other single-layer group-III monochalcogenides (MX, M = Ga and In, X = S, Se, and Te) and how the dimension reduction affects the properties of these materials. Using a first-principles design approach, we determine that the single-layer group-III monochalcogenides exhibit low formation energies and are suitable for photocatalytic water splitting. First, density-functional calculations using a van der Waals functional reveal that the monochalcogenides have formation energies similar to that of single-layer MoS2, implying the ease of mechanically extracting single-layer monochalcogenides from their layered bulk counterparts. Next, calculations using a hybrid density functional and the quasiparticle many-body G0W0 approximation determine the conduction and valence band edge positions. Comparing the band edge positions with the redox potentials of water shows that single-layer monochalcogenides are potential photocatalysts ...

Experimental study of contamination and cleaning of in-vessel mirrors for ITER optical diagnostics on T-10 and QSPA-T facilities

Abstract ITER-relevant hydrocarbon films deposited under controlled conditions in the SOL of T-10 tokamak during working and cleaning discharges and by using QSPA-T plasma gun at plasma shots onto W and graphite targets were investigated by XPS, XAES and ellipsometry. XPS survey, valence band and C1s, O1s core level spectra showed differences in chemical composition and electronic structure as between T-10 and QSPA-T films and between T-10 films obtained in working and cleaning discharges. Hydrogen content evaluation in the films was made independently by analysis of XPS valence band edge position and ellipsometry data. The Csp 2 carbon content was determined from XAES CKLL spectra. Classification of T-10 and QSPA-T films was undertaken using ternary phase diagram (H, Csp 2 and Csp 3 ). It was shown that all studied hydrocarbon films are positioned between diamond- and polymer-like a-C:H films. The observed difference between XPS and ellipsometry evaluations of hydrogen content may be connected with strong structural non-uniformity of the films along thickness.

Mineralization of Phenol and 4-Chlorophenol Induced by Visible Light and Assisted by Semiconducting β-Bi2O3

Intensely yellow β -bismuth trioxide in the form of about 41 nm large crystallites has been prepared by thermal decomposition of (BiO)2CO3 at 400 ◦C. From the band gap energy of 2.3 eV obtained under the assumption of an indirect band-to-band transition and the quasi-Fermi level of −0.28±0.02 V (vs. NHE) a valence band edge position of 2.02 V is calculated. This semiconducting oxide assists a Vis light (λ ≥455 nm) aerial mineralization of the aqueous model pollutants phenol and 4-chlorophenol exhibiting maximum rates at a bismuth oxide concentration of 2.0 g L−1. In the absence of oxygen no degradation occurs indicating that β -Bi2O3 does not act as oxidizing agent. The mineralization suffers from photocorrosion of β -Bi2O3 generating a mixture of α-Bi2O3 and various bismuth carbonates as observed upon multiple use of the oxide in the mineralization of phenol.

Grain Boundary Effect on the Dielectric Properties of Nanocrystalline β-CuSCN

The physics of grain boundaries in semiconductors is of interest both from a fundamental as well as technical point of view because they play crucial role in controlling properties. The grain boundary effect in the nanocrystalline materials are more because the volume fraction of atoms lying at the grain boundaries of the nanocrystalline materials are more as compared with conventional coarse-grained polycrystalline materials. The most attractive hole transporting electrolyte material of dye sensitized solar cell is CuSCN because of its band gap, valence band edge position and p-type semiconducting nature. We have synthesised nanocrystalline beta phase of CuSCN using co-precipitation technique. The grain boundary effect on the dielectric behaviour has been investigated using impedance spectroscopy in a wide frequency range 1 Hz to 1 MHz under dc bias voltages 0 V to -4.2 V. The results are analyzed with double (or back-to-back) Schottky grain boundary barrier model and discussed in detail.

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.

Gate current and oxide reliability in p/sup +/ poly MOS capacitors with poly-Si and poly-Ge/sub 0.3/Si/sub 0.7/ gate material

Fowler-Nordheim (FN) tunnel current and oxide reliability of PRiLOS capacitors with a p/sup +/ polycrystalline silicon (poly-Si) and polycrystalline germanium-silicon (poly-Ge/sub 0.3/Si/sub 0.7/) gate on 5.6-nm thick gate oxides have been compared. It is shown that the FN current depends on the gate material and the bias polarity. The tunneling barrier heights, /spl phi//sub B/, have been determined from FN-plots. The larger barrier height for negative bias, compared to positive bias, suggests that electron injection takes place from the valence band of the gate. This barrier height for the GeSi gate is 0.4 eV lower than for the Si gate due to the higher valence band edge position. Charge-to-breakdown (Q/sub bd/) measurements show improved oxide reliability of the GeSi gate on of PMOS capacitors with 5.6 nm thick gate oxide. We confirm that workfunction engineering in deep submicron MOS technologies using poly-GeSi gates is possible without limiting effects of the gate currents and oxide reliability.

Photoelectron spectroscopy study of amorphous silicon-carbon alloys deposited by plasma-enhanced chemical vapor deposition

X-ray photoelectron spectroscopy (XPS) coupled with Fourier transform infrared (FTIR) and optical transmission spectroscopy (OTS) has been used for the characterization of silicon-carbon alloys (a-Si1−xCx: H, F) deposited via plasma, by varying the CH4 amount in SiF4–CH4–H2 feeding mixture. XPS measurements have shown that carbon-rich a-Si1−xCx: H, F alloys include large amounts of fluorine (>11 at. %), which make the films susceptible to the air oxidation. In addition, the effect of the alloying partner carbon on the valence band (VB) and on the VB edge position of amorphous silicon is also described.

Hole injection into single- and polycrystalline silicon electrodes

The purpose of this paper is to examine hole injection into either the valence band of single-crystal silicon or into the grain-boundary levels of polycrystalline silicon from various oxidizing ions in solution. The results for single crystals did not follow the simple picture one gets from comparing the redox potentials to the position of the silicon valence band edge. It is concluded that the band edges are shifted up by a hydride layer on the silicon surface. A solution of ferric EDTA (ethylene diamine tetraacetic acid) and 1% HF did not exhibit injection into the valence band and was used to test for hole injection into grain-boundary levels. This grain-boundary injection was definitely observed but exhibited a marked time dependence (decay). This is best interpreted as a form of hydrogen passivation (by the cathodic injection of protons).