Space charge analysis in doped zinc phthalocyanine thin films

We present an improved method for the determination of the space charge density in organic semiconductors used as active layers in Schottky barriers. These measurements provide a powerful tool for the interpretation of basic properties such as the rectifying effect, doping process and carrier trapping mechanisms of films together with a way to assess the potential for sensor applications. Metal/molecular semiconductor Schottky junctions were prepared on zinc phthalocyanine layers doped by a controlled exposure to the ambient air. The organic material is deposited on aluminium or heavily doped silicon substrates, in order to make a Schottky barrier (film thickness around 1 μm). An ohmic contact is obtained by a gold deposition on the strongly doped side of the molecular material. We have investigated the current-voltage and capacitance-voltage characteristics. The results are interpreted in terms of a space charge region at the interface with the substrate, followed by an extended semi-insulating layer.The contribution of these two regions to the total impedance is analyzed in well improved conditions of measurements.     

Modified salp swarm algorithm for global optimisation

Neural Computing and Applications - Salp swarm algorithm (SSA) is a newly swarm-based metaheuristic algorithm that simulate the swimming and foraging behaviour of salps in oceans so to search for...     

Physical and photoelectrochemical characterizations of hematite α-Fe2O3: Application to photocatalytic oxygen evolution

The physical properties and photoelectrochemical characterization of α-Fe₂O₃, synthesized by co-precipitation, have been investigated in regard to solar energy conversion. The optical gap is found to be 1.94 eV and the transition is indirectly allowed. The chemical analysis reveals an oxygen deficiency and the oxide exhibits n-type conductivity, confirmed by a negative thermopower. The plot log σ vs 1/T shows linearity in the range (400-670 K) with the donor levels at 0.14 eV below the conduction band and a break at ∼590 K, attributed to the ionization of the donors. The conduction occurs by small polaron hopping through mixed valences Fe^2+/3+ with an electron mobility μ_400 K of 10⁻³ V cm² s⁻¹. α-Fe₂O₃ exhibits long term chemical stability in neutral solution and has been characterized photoelectrochemically to assess its activity as bias-free O₂-photoanode. The flat band potential V_fb (−0.45V_SCE) and the electron density N_D (1.63 × 10¹⁸ cm⁻³) were determined, respectively, by extrapolating the linear part to C⁻² = 0 and the slope of the Mott Schottky plot. At pH 6.5, the valence band (+1.35V_SCE) is suitably positioned with respect to the O₂/H₂O level (+0.62 V) and α-Fe₂O₃ has been evaluated for the chemical energy storage through the photocatalytic reaction: (, ΔG = 213.36 kJ mol⁻¹). The best photoactivity occurs in solution (0.025 M, pH 8) with an oxygen rate evolution of 7.8 cm³ (g catalyst)⁻¹ h⁻¹.     

Physical and photo-electrochemical characterizations of α-Fe2O3. Application for hydrogen production

The optical, electrical and photo-electrochemical properties of dense hematite α-Fe₂O₃ have been studied for the photo-catalytic hydrogen production. The band gap was evaluated at 1.96 eV from the diffuse reflectance spectrum and the transition is directly allowed; further indirect transition occurs at 2.04 eV. The oxygen deficiency permits the altering of the transport properties and the oxide exhibits n type behavior with activation energy of 0.11 eV. α-Fe₂O₃ is found to be photo-electrochemically active. The flat band potential V_fb (−0.51 V_SCE) and the density N_D (19.12 × 10¹⁹ cm⁻³) were obtained respectively by extrapolating the linear part to C⁻² = 0 and the slope of the Mott–Schottky plot. The complex impedance pattern is circular in the high frequency region followed by a straight line in the low frequency one, a behavior attributed to the Warburg ionic diffusion. The conduction band edge (−0.62 V_SCE) lies below the H₂O/H₂ level (−0.50 V_SCE) and Fe₂O₃ offers the possibility to be used as hydrogen photocathode. The best activity was obtained in SO₃²⁻ (0.5 M, pH 13.8) solution with a rate evolution of 6 ml (g catalyst)⁻¹ min⁻¹.     

Enhanced photocatalytic hydrogen production under visible light over a material based on magnesium ferrite derived from layered double hydroxides (LDHs)

The magnesium ferrite derived from layered double hydroxides (molar ratio Mg/Fe = 2), synthesized by coprecipitation method is found to be an active photocatalyst for hydrogen production from water under visible light. The structural, morphological, and optical properties of the material are characterized by powder X‐ray diffraction, scanning electron microscopy, and Fourier transform infrared and UV‐Vis diffuse reflectance spectroscopy. The results indicated that the material has small particles with a diameter of ~1.8 nm and a specific surface area above 60 m² g^?1. The optical properties revealed semiconducting properties with band gap energy of 1.74 eV, showing an efficient visible light absorption. The cyclic voltammetry indicated that the photoelectrochemical response of the material is characterized by type p conductivity. Furthermore, the solid exhibited a high photoactivity toward the reduction of water, which is attributed to the efficient separation and transportation of the photogenerated charge carriers. Under visible light, the best performance is achieved at pH 10 with a hydrogen liberation amount and quantum efficiency of 223 mol and 0.5%, respectively, after 1 h of irradiation. Copyright © 2014 John Wiley & Sons, Ltd.     

Hydrogen evolution under visible light over the heterojunction p‐CuO/n‐ZnO prepared by impregnation method

The semiconducting properties of the heterojunction CuO/ZnO, synthesized by impregnation method from nitrates, are studied for the first time to assess its feasibility for the hydrogen production under visible light, an issue of energy concern. CuO exhibits a direct optical transition at 1.33 eV, due to Cu²⁺: 3d orbital splitting in octahedral site, and possesses a chemical stability in the pH range (4–14). The Mott–Schottky plot in (Na₂SO₄, 0.1 M) medium indicates p‐type conduction with a flat band potential of 0.70 V_SCE and a holes density of 1.35 × 10¹⁷ cm^?3. As application, hydrogen evolution upon visible light is demonstrated on the heterojunction ×%CuO/ZnO (x = 5, 10 and 20 wt.%). The best performance occurs at pH ~12 with an evolution rate of 4.8 cm³ min^?1 (g catalyst)^?1 and a quantum yield of 0.12%. The improved activity is attributed to the potential of the conduction band of CuO (?1.34 V_SCE), more negative than that of ZnO, the latter acts as electrons bridge to water molecules. The presence of SO₃^2? reduces the recombination process, thus resulting in more H₂ evolution. Copyright © 2015 John Wiley & Sons, Ltd.     

Photo‐induced hydrogen on iron hexagonal mesoporous silica (Fe‐HMS) photo‐catalyst

The iron hexagonal mesoporous silica (Fe‐HMS)‐n photocatalyst, where n is the molar ratio Si/Fe in the precursor gel (=50), has been successfully synthesized at an ambient temperature using hexadecylamine as template agent. The material was characterized by X‐ray diffraction, N₂ adsorption measurement Brunauer, Emmet, Taller (BET) and Barrett–Joyner–Halenda theory, UV–Vis spectroscopy, high‐resolution transmission electron microscopy and electron spin resonance. The electrical conductivity follows an Arrhenius‐type law with activation energy of 0.04 eV. Fe₂O₃ is uniformly dispersed on the HMS surface; it works synergistically to make Fe‐HMS photoelectrochemically active. The flat band potential (?0.75 V_SCE) is higher than the potential of hydrogen generation (?0.50 V_SCE at pH~7), and the material shows high efficiency for hydrogen evolution upon visible light. The photoactivity in Na₂SO₄ and Na₂SO₃ (0.1 M) solution was found to be 136 and 175 µmol g^‐1 min^‐1, respectively under full light (29 mW cm^‐2). The tendency to saturation, observed only in SO₃^2‐ electrolyte, is ascribed to the competitive reduction of the end product, namely S₂O₆^2‐ with the water reduction. Copyright © 2011 John Wiley & Sons, Ltd.