The oxidation of titanium thin films in phosphoric medium

The properties of TiO₂ thin film grown under potentiostatic mode are investigated by the linear voltammetry, photo-electrochemical technique and capacitance measurements. The anodic film thickens up to ～10 nm by diffusion-controlled process and the growth obeys to inverse and direct logarithmic laws respectively in acidic and alkaline solutions. The effect of the anion on the stability of the films is discussed. The film grown at pH 2 has the best corrosion resistance, as evidenced from the electro-kinetic parameters. The electrochemical impedance spectroscopy (EIS) reveals the contribution of the bulk effect. The experimental data are fitted by shifting the centre of the semi-circle down the real axis and interpreted in terms of constant phase element (CPE). The Mott-Schottky plots exhibit a linear behavior, characteristic of n type conductivity and the flat band potential follows nearly a Nerstian slope (?0.056 V pH^?1), the H₃O⁺ specie is identified as potential determining specie.     

Adsorption of Zn2+ ions onto NaA and NaX zeolites: Kinetic,equilibrium and thermodynamic studies

The adsorption of Zn²⁺ onto NaA and NaX zeolites was investigated. The samples were synthesized according to a hydrothermal crystallization using aluminium isopropoxide (Al[OCH(CH₃)₂]₃) as a new alumina source. The effects of pH, initial concentration, solid/liquid ratio and temperature were studied in batch experiments. The Freundlich and the Langmuir models were applied and the adsorption equilibrium followed Langmuir adsorption isotherm. The uptake distribution coefficient (K_d) indicated that the Zn²⁺ removal was the highest at minimum concentration. Thermodynamic parameters were calculated. The negative values of standard enthalpy of adsorption revealed the exothermic nature of the adsorption process whereas the negative activation entropies reflected that no significant change occurs in the internal structure of the zeolites solid matrix during the sorption of Zn²⁺. The negative values of Gibbs free energy were indicative of the spontaneity of the adsorption process. Analysis of the kinetic and rate data revealed that the pseudo second-order sorption mechanism is predominant and the intra particle diffusion was the determining step for the sorption of zinc ions. The obtained optimal parameters have been applied to wastewater from the industrial zone (Algeria) in order to remove the contained zinc effluents.     

Synthesis and physical properties of new oxide AgMnO<Subscript>2</Subscript>

A novel oxide AgMnO₂ was prepared from LiMnO₂ via Ag⁺ → Li⁺ exchange in the eutectic melt AgNO₃-KNO₃. It crystallizes in a monoclinically distorted unit cell (SG C2/m) caused by the Jahn-Teller (J-T) ion Mn³⁺ (3d ⁴). The structure was refined by isotypy with the crednerite CuMnO₂. There are two long axial Mn–O of 264.2(0) pm and four equatorial bonds of 192.7(3) pm and Mn–O–Mn adjoining (83.07°) are bent below the ideal angle. The thermal variation of the magnetic susceptibility (χ/T ⁻¹) obeys a Curie-Weiss law with manganese in a trivalent, high spin (HS) state accommodated in elongated MnO₆ octahedra (14.8%). Direct coupling between Mn³⁺ involves negative exchange interactions through long-range antiparallel moments with a temperature θ _p = −436 K and a magnetic moment of 5.26 μ_B/Mn³⁺ slightly larger than the spin only moment. The title oxide is stable in air up to ∼680 °C before it decomposes into metal silver. It displays a semi-conducting behavior with an activation energy of ∼0.45 eV, characteristic of a conduction by low mobility polarons between Ag^+/2+ where nearly all polarons are bonded. The photoelectrochemical properties of AgMnO₂ have been investigated by photocurrent technique in 1 M KOH. The cathodic photocurrent J _ph provides unambiguous evidence of p-type character attributed to oxygen insertion (0.025 oxygen by formula unit) as required by the charge compensating mechanism. The valence band is made up of Ag−4d wave functions positioned at ∼5.14 eV below vacuum. A comparison with CuMnO₂ was also reported.     

Visible light induced H2PO(4)(-) removal over CuAlO2 catalyst

The delafossite CuAlO₂ is successfully used for the visible light driven H₂PO₄⁻ reduction. It is prepared from the nitrates decomposition in order to increase the ratio of reaction surface per given mass. CuAlO₂ is a narrow band gap semiconductor which exhibits a good chemical stability with a corrosion rate of 1.70 μmol year⁻¹ at neutral pH. The flat band potential (+0.25 V_SCE) is determined from the Mott-Schottky characteristic. Hence, the conduction band, positioned at (−1.19 V_SCE), lies below the H₂PO₄⁻ level yielding a spontaneous reduction under visible illumination. The photocatalytic process is investigated under mild conditions and 30% conversion occurs in less than ∼6 h with a quantum efficiency of ∼0.04% under full light. The concentration decreases by a factor of 39% after a second cycle. The photoactivity follows a first order kinetic with a rate constant of 6.6 × 10⁻² h⁻¹. The possibility of identifying the reaction products via the intensity-potential characteristics is explored. The decrease of the conversion rate over illumination time is due to the competitive water reduction.     

Photoelectrochemical properties of anodic silver sulphide thin films

We have investigated the semi-conducting properties and photoelectrochemical characterization of Ag₂S thin films electrodeposited galvanostatically onto silver substrate from alkaline S²⁻ bath. Films grown with a low current density not exceeding 20 μA cm⁻² are uniform and well adhered. An optimal thickness of 1.34 μm has been determined. At high current density, oxygen evolution occurs simultaneously and provokes crashing of films. From photoelectrochemical measurements, the band gap was found to be 1.85 eV and the transition is indirectly allowed. The Mott-Schottky plot exhibits a linear behavior, characteristic of n-type conductivity, from which a flat band potential of − 1.20 V_SCE and a donor density of 5.63 × 10¹⁶ cm⁻³ were determined. The conduction band, located at 3.28 eV, is made up of mainly Ag-5s wave function. Ag₂S is long lived and under illumination, it is stabilized by holes consumption reactions involving S_n²⁻/S²⁻ redox couple. A conversion efficiency of 1.1% and a fill factor of 0.27 have been obtained. The electrochemical impedance spectroscopy, measured over a wide frequency range (10⁻²-10⁵ Hz), reveals contribution of the bulk effect. The experimental data are modeled by shifting the centre of the semi−circle down the real axis and interpreted in terms of constant phase element due mainly to the porosity and inhomogeneity of the film.     

Study of Optical Properties of Nanocrystalline Zinc Phosphide Thin films

Silicon - Zinc phosphide (Zn3P2), a II–V group semiconductor compound has long been considered as a very interesting and suitable candidate for several applications especially in...     

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.     

Bulk p-CuInSe2 photo-electrochemical solar cells

Dense CuInSe₂ of high quality, prepared by the fusion technique in evacuated quartz ampoule from stoichiometric melt, crystallizes in the chalcopyrite structure. Compositional analysis carried out by secondary ion mass spectrometry (SIMS) and energy dispersive spectroscopy (EDS) indicates a uniform distribution of elements through the depth and a composition close to the stoichiometry. The diffuse reflectance spectrum gives a band gap at 0.94 eV. The electrical conductivity follows an Arrhenius-type law with activation energy of 23 meV in conformity with polarons hopping. Above 320 °C, CuInSe₂ undergoes an irreversible oxidation. The thermal variation of the thermopower indicates p-type behavior attributed to copper deficiency and a hole mobility μ_300 K of 0.133 cm² V⁻¹ s⁻¹, thermally activated. In KCl media, the compound exhibits an excellent chemical stability with a corrosion rate of 8 μmol cm⁻² month⁻¹. The photo-electrochemical properties, investigated for the first time on the ingots, confirm the p-type conductivity. From the capacitance measurements, the flat band potential (V_fb=−0.62V_SCE) and the holes density (N_A=4×10¹⁷ cm⁻³) were determined. The valence band, located at 4.43 eV below vacuum, is made up of mainly Se orbital with little admixture of Cu character. The change of the electrolyte causes a variation in the potential V_fb (dV_fb/dpH=−0.058 V pH⁻¹) indicating strong OH⁻ adsorption. The fill factor in S²⁻ media was found to be 0.54; such result was corroborated by semi-logarithmic plots.     

Visible light degradation of Orange II using xCuyOz/TiO2 heterojunctions

Cu₂O/TiO₂, Cu/Cu₂O/TiO₂ and Cu/Cu₂O/CuO/TiO₂ heterojunctions were prepared and studied for their potential application as photocatalysts able to induce high performance under visible light. Orange II was used as a representative dye molecule. The effect of the amount and composition of the photosensitizers toward the activation of TiO₂ was studied. In each case, the global mechanism of Inter Particle Electrons Injection (IPEI) was discussed. The highest photocatalytic activity was observed for the system Cu/Cu₂O/CuO (MB2 catalyst) under visible light (t_1/2 = 24 min, k = 159.7 × 10⁻³ min⁻¹) and for the heterojunction cascade Cu/Cu₂O/CuO/TiO₂ (MB2 (50%)/TiO₂) under UV–vis light (t_1/2 = 4 min, k = 1342 × 10⁻³ min⁻¹). In the last case, the high performance was attributed firstly to the electromotive forces developed under this configuration in which CuO energy bands mediate the electrons transfer from Cu₂O to TiO₂. The formation of monobloc sensitizers also accounts for the decrease of the probability of the charges lost. It was demonstrated that “Cu₂O/CuO” governs the capability of the heterojunction cascade and Cu does not play a significant role regardless of the heterojunction cascade efficiency. The electrical energy consumption per order of magnitude for photocatalytic degradation of Orange II was investigated for some representative catalytic systems. Visible/MB2 and UV/vis MB2 (50%)/TiO₂ exhibited respectively 0.340 and 0.05 kWh m⁻³ demonstrating the high efficiency of the systems.     

Photoelectrochemical properties of doped polyaniline: Application to hydrogen photoproduction

The semi conducting properties of doped polyaniline (emeraldine-salt, PANI) elaborated by chemical route are investigated by the photo-electrochemical technique. The band gap is found to be 1.48 eV and the transition is directly allowed. The electrical conduction obeys to an exponential law with activation energy of 0.13 eV. p-type conductivity is evidenced from the cathodic photocurrent. The energy band diagram clearly shows the spontaneous hydrogen photo evolution. The potential of the conduction band of PANI (−0.93 V_SCE) determined from the capacitance measurements is suitably positioned with respect to H₂O/H₂ level (−0.66 V_SCE). Therefore, the photocatalytic properties of this material has been evaluated according to the hydrogen generation. The best performance is achieved at pH ∼7 with a liberation rate of 0.113 mL h⁻¹ (mg catalyst)⁻¹ and a quantum efficiency of 0.18% under visible light (29 mW cm⁻²). An increase of 56% is obtained on the hetero-system PANI/TiO₂.