The present work deals with the photoelectrochemical hydrogen production over the spinel ZnCr₂O₄. The photoactivity is dependent on the synthesis conditions and the oxide has been prepared by nitrate way in order to produce homogeneous powder with large active surface. The transport properties indicate p-type conductivity with activation energy of 0.21 eV. A corrosion potential of 0.404 V_SCE and an exchange current density of 50 µA cm^-2 have been determined from the semi logarithm plot. The photocurrent onset potential, assimilated to the flat band potential, was found to be -0.39 V_SCE. ZnCr₂O₄/S₂O₃^2- is a self driven system where absorption of light promotes electrons into the conduction band with a potential (-1 V) sufficient to reduce water into hydrogen. The activity shows a tendency toward saturation whose deceleration is the result of the competitive reductions of end products namely S₂O₆^2- and S₂O₄^2- with water. A comparative study with CuCr₂O₄is reported. © 2008 International Association for Hydrogen Energy.

The Cd²⁺ photo-electrodeposition was successfully carried out in air-equilibrated aqueous CuFeO₂ suspension. The delafossite CuFeO₂ is p-type semiconductor characterized by a low optical gap, properly matched to the sun spectrum, and a long term chemical stability in neutral solution. It has been elaborated by the sol-gel technique where the specific surface area is increased via the surface/bulk ratio. The TG/DSC plots and IR spectra show that the solid phases are formed only at temperatures exceeding 400 and at 700 °C, the system is mixed phases. When fired at 950 °C under nitrogen flow, the delafossite has been identified (CuO + CuFe₂O₄ ? CuFeO₂ + 1/2O₂). All the XRD lines index in a hexagonal unit cell with the lattice constants a = 284.2 and c = 169.4 pm. The photocurrent onset potential (+0.35 V_SCE) is close to the flat band potential (+0.23 V_SCE) determined from the capacitance measurement. CuFeO₂/Cd²⁺ solution is a self photo-driven system, the absorption of light promotes electrons into CuFeO₂-CB with a potential (-0.93 V_SCE) sufficient to reduce Cd²⁺. This occurs because of the dark Cd²⁺ adsorption on the surface powder. The system was optimized with respect to the following physical parameters : pH 6.8, Cd²⁺ (100 ppm) and a mass concentration C_m (1 mg catalyst/ml solution). The hetero-system CuFeO₂/TiO₂ has been also reported for a comparative purpose. Prolonged irradiation (>50 min) was accompanied by a pronounced decrease in the rate of Cd-deposition owing to the competitive water reduction. Indeed, the generated bi-functional CuFeO₂/Cd particles account for the low over-potential of hydrogen and favour its evolution in aqueous solution. © 2008 Elsevier B.V. All rights reserved.

The photocatalytic H₂ evolution over CuCr₂O₄ has been studied in connection with some physical properties. The black oxide is classified as a narrow band gap semiconductor crystallizing in the normal spinel structure. The photoactivity is dependent on the preparative conditions through the surface area of the powder and CuCr₂O₄ has been synthesized by nitrate route. The conductivity varies exponentially with temperature indicating an extrinsic conductivity between mixed valences states Cr^3+/4+located in octahedra sharing common corners. The oxide exhibits p-type conductivity with an activation energy of 0.20 eV and a hole mobility of 2.14 × 10^-8 cm²/V s thermally activated. CuCr₂O₄ shows an excellent chemical stability, a corrosion potential of -0.775 V_SCE and an exchange current density of 16 µA cm^-2 have been determined from the semilogarithm plot. The capacitance measurement (C^-2-V), performed in basic electrolyte, exhibits a linear behavior from which a flat band potential of 0.48 V_SCE was obtained. CuCr₂O₄ is photoelectrochemically stable by hole consumption reactions involving X^2-(=SO₃^{2 -} and S₂ O₃^{2 -}) as hole scavengers. The best catalytic performance for H₂-photoproduction was obtained in (NaOH 0.5 M, Na₂S₂O₃ 0.025 M) with an average rate of 0.013 cm³ h^-1 (mg catalyst)^-1 and a quantum efficiency of 0.2% under polychromatic light. S₂ O₃^{2 -} is oxidized to SO₃^{2 -} and subsequently to SO₄^{2 -} as proven by the dual slopes in the Tafel plot. The system shows a tendency towards saturation whose deceleration is the result of the competitive reductions of the end products namely S₂ O₆^{2 -} and S₂ O₄^{2 -} with the water discharge. © 2008 Elsevier Ltd. All rights reserved.

The removal of NO₃^- by a CuCrO₂-mediated catalytic photoprocess was investigated under mild conditions. The delafossite CuCrO₂
exhibits an excellent chemical stability over the whole pH range and
relatively a large specific surface area when elaborated by chemical
means. The optical gap was found to be 1.28 eV, close to the optimal
value required for terrestrial applications. The conduction band is
located at -1.06 V_SCE and thus lies below the NO₃^-
level leading to a thermodynamically favorable nitrate reduction under
visible light. The possibility of identifying the reaction products via
the intensity-potential characteristics was explored. The reaction of NO₃^-
proceeds in competition with the water reduction and seems to be the
major reason in the regression of the photoactivity during prolonged
testing. The complete conversion occurs in less than 4 h with a
quantum efficiency of 2%.

Photocatalytic H₂-production has been realized over active CuInSe₂, synthesized by the fusion technique. The material crystallizes in the chalcopyrite structure and exhibits p-type conductivity ascribed to copper deficiency. An optical gap of 0.95 eV was determined from the reflectance diffuse spectrum. The electrical conductivity follows an Arrhenius-type law with activation energy of 23 meV in conformity with polaron hopping. The slope and the intercept of the Mott-Schottky plot gave a holes density N_A of 5.9 × 10¹⁸ cm^-3 and a flat band potential of -0.36V_SCE, in perfect agreement with the photo-onset potential V_on (-0.35V_SCE). Hence, the conduction band, located at -1.29V_SCE, allows a spontaneous H₂ liberation upon visible light. In aqueous solutions, the material is stabilized by hole consumption involving X^2- species (double bond, longS^2- and SO₃^2-). H₂formation would become thermodynamically easy in alkaline media and the best photoactivity was obtained in thiosulfate electrolyte (10^-2 M S₂O₃^2-, 0.5 M KOH) with an evolution rate of 0.009 ml mg^-1 h^-1. The light induced electron transfer through the interface involves two steps mechanism where S₂O₃^2- is oxidized to SO₃^2- and SO₄^2- by successive reactions. Such results are corroborated by the semi logarithmic plots and photocurrent-photopotential characteristics. The influence of pH was studied with S₂O₃^2- The tendency towards saturation for prolonged irradiation is attributed to competitive reductions of the end products S_n^2- and S₂O₆^2- with water and to the yellow color of polysulfide S_n^2-. © 2008 Elsevier B.V. All rights reserved.

CuFeO₂ single crystal, synthesized by the flux method, is a narrow band gap semiconductor crystallizing in the delafossite structure with a direct optical transition of 1.63 eV. The oxide exhibits a good chemical stability ; the semi-logarithmic plot gave an exchange current density of 0.60 µA cm^-2 in KCl (0.5 M) electrolyte. CuFeO₂ shows p-type conductivity ; the origin of acceptors Cu²⁺ results from oxygen insertion in the layered lattice where most of excess holes are trapped in surface-polaron states. The electrochemical study is confined in the (a,b) plane and reversible oxygen intercalation is evidenced from the intensity potential characteristics. The detailed photoelectrochemical studies have been reported for the first time on the single crystal. The photocurrent is ascribed to the transfer Cu⁺:3d ?3d. The capacitance measurement (C^-2-V) shows a linear behavior from which a flat band potential of +0.54 V_SCE and a density N_A of 1.60 × 10¹⁸ cm^-3 were determined. The valence band, located at 5.33 eV below vacuum, is made up of Cu-3d orbital typical of delafossite oxides. The Nyquist plot shows a semicircle attributed to a capacitive behavior with a low density of surface states within the gap. The centre is localized below the real axis with an angle of 16.2° ascribed to a constant phase element (CPE), a single barrier of the junction CuFeO₂/electrolyte and one relaxation time of the electrical equivalent circuit. © 2008 Springer-Verlag.

The delafossite CuCrO₂ is a promising candidate for the visible light driven catalysis. The NO₂^- removal by photoelectrochemical process is studied under mild conditions, close to that encountered in the natural environment. CuCrO₂exhibits a long term chemical stability with a corrosion rate of 0.34 µmol m^-2 year^-1 in KCl (0.5 M). A forbidden band of 1.3 eV has been evaluated from the diffuse reflectance spectrum. The flat band potential (-0.07 V_SCE) determined from the Mott-Schottky plot is close to the photocurrent onset potential (0 V_SCE). Hence, the conduction band is positioned at -1.08 V_SCEand thus lies below the NO₂^- level leading to a feasible reduction upon visible illumination. The conversion occurs in less than 5 h with a quantum efficiency of  0.5%. The possibility of identifying the reaction products via the intensity-potential characteristics was explored by using standard solutions. The decrease of the conversion rate over time is attributed to the competitive water reduction. In absence of catalyst, NO₂^- is oxidized to NO₃^- in air equilibrated solution and the reaction follows a first order kinetic with a half life of 21 h, NO₃^- has been identified by iodometry through copper titration. © 2008 Springer Science+Business Media B.V.

Effect of support on the crystalline structure, the acid-base properties and activity of iron based systems in the CO + H₂O reaction. The water gas shift reaction was studied in the temperature range of 350-450 °C over iron based catalysts exempt of Cr₂O₃. The basic properties and CO conversion of these catalysts are given in the following sequence : Fe₃O₄/MgO > Fe₃O₄/TiO₂ > Fe₃O₄ > Fe₃O₄/SiO₂. The basic catalysts are consequently more active than acid ones. On the other hand, the DRX results showed a strong crystalline fraction of MgO in the Fe₃O₄/MgO catalyst calcined at 400 °C. Hence, Fe₃O₄/MgO generates an MgO reservoir able to feed the catalyst surface with formate species during the water gas shift reaction. © 2008 Académie des sciences.

Photocatalytic H₂-production has been observed over FeCr₂O₄, synthesized from Cr(NO₃)₃ and Fe(NO₃)₃. The oxide crystallizes in the normal spinel structure and the transport properties exhibit p-type conductivity ascribed to metal deficiency. An optical gap of 1.33 eV is determined from the reflectance diffuse spectrum. From photoelectrochemical measurements, a flat band potential of +0.23 V_SCE is found. The valence band, made up of cationic 3d orbital, is located at 5.2 eV (0.45 V_SCE) below vacuum. Hence, the conduction band (-0.98 V_SCE) allows a thermodynamically feasible H₂ liberation under visible light. In aqueous electrolytes, the oxide is stabilized by hole consumption involving X^2- species and the best photoactivity for H₂-production is obtained in S^2- solution with an evolution rate of 8.26 cm³ g^-1 h^-1. The tendency towards saturation is due to the competitive reduction of the end products namely S and SO with water and to the yellow color of S. On the other hand and for a comparative purpose, the catalytic activity of unreduced FeCr2O4 for the water-gas shift reaction (CO + H2O ? CO2 + H2) is studied in the temperature range 523-723 K. At 723 K, the rate of H2-formation and the degree of CO conversion are respectively 280 µmol g-1 s-1 and 44%. © 2008 Elsevier Ltd. All rights reserved.

The metal ions M²⁺ (Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Ag⁺) are potentially toxic. Their electro deposition has been carried out in aqueous air-equilibrated CuCrO₂ suspension upon visible illumination. The delafossite CuCrO₂ is p-type semiconductor characterized by a low band gap (1.28 eV) and a long-term chemical stability. The corrosion rate is found to be 10^-2 µmol m^-2 month^-1 in aqua regia. The oxide has been elaborated through nitrate route where the specific surface area is increased via the surface/bulk ratio. A correlation exists between the dark M²⁺ adsorption, the redox potential of M^2+/0 couple and the conduction band of CuCrO₂ positioned at -1.06 V_SCE. Ag⁺ cannot be photoreduced because of its positive potential located far above the valence band. By contrast, Zn²⁺ is efficiently deposited due to the large driving force at the interface. The improved photoactivity of copper with a deposition percentage (90%) is attributed to the strong dark adsorption onto the surface catalyst. The results indicate a competitive effect with the water reduction ; it has been observed that the M²⁺ deposition goes parallel with the hydrogen evolution. Such behavior is attributed to the low H₂ over voltage when ultra fine aggregate of M islands are photodeposited onto CuCrO₂ substrate. © 2008 Elsevier B.V. All rights reserved.

CuAlO₂ single crystal elaborated by the flux method is a narrow band gap semiconductor crystallizing in the delafossite structure (SG R3¯m). Oxygen insertion in the layered lattice generates p-type conductivity where most holes are trapped in surface-polaron states. The detailed photoelectrochemical characterization and electrochemical impedance spectroscopy (EIS) have been reported for the first time on the single crystal. The study is confined in the basal plan and reversible oxygen insertion is evidenced from the intensity potential characteristics. The oxide is characterized by an excellent chemical stability ; the semi-logarithmic plot gave a corrosion potential of-0.82 V_SCE and an exchange current density of 0.022 µA cm^-2 in KCl (0.5 M) electrolyte. The capacitance measurement (C^-2-V) shows a linear behavior from which a flat band potential of +0.42 V_SCE and a doping density N_A of 10¹⁶ cm^-3 have been determined. The valence band, located at 5.24 eV (0.51 V_SCE) below vacuum, is made up of Cu-3d orbital. The Nyquist plot exhibits a pseudo-semicircle whose center is localized below the real axis with an angle of 20°. This can be attributed to a single relaxation time of the electrical equivalent circuit and a constant phase element (CPE). The absence of straight line indicates that the process is under kinetic control. © 2008 Elsevier B.V. All rights reserved.

The electrical conductivity and thermoelectric power of polycrystalline Ba_0.98La_0.02SnO_3-d, prepared under O₂-free atmosphere, have been measured between 4.2 and 300 K. The existence of onsite bipolaron is due to the tendency to Sn³⁺ ions to disproportionate into mixed valences Sn²⁺ and Sn⁴⁺ in equivalent sites. The data indicates electron degeneracy like conduction with polaron conductivity and a finite density of states at the Fermi level. The thermal evolution of the conductivity follows an Arrhenius type law with activation energy of 1 meV and electron mobility thermally activated. The conduction mechanism changes to a variable range hopping at low temperatures. From photo electrochemical measurements, the band gap E_g was found to be 3.26 eV and the transition is directly allowed ; further indirect interband transition occurs at 2.82 eV. The valence band edge, located at 7.4 eV below vacuum, is made up from O:2p orbital typical of perovskite oxides. The respective values of the slopes (231 and 1104 mV dec^-1) and the exchange current density J (0.339 mA cm^-2) in the semi-logarithmic plot predict long-lived material. As expected, the title oxide exhibits a pH-dependent flat band potential V_fb and at pH 13.60 (KOH 1 M), the current-potential characteristic gave a V_fb-value of -0.03 V_SCE, close to that obtained from the capacitance measurement (+0.05 V_SCE). The C^-2-potential plot displays a straight linear part with a positive slope confirming the n-type conductivity and a uniform doping density (N_d = 7.24 × 10²⁰ cm^-3). The utilization of BaSnO₃ as coating for PEC electrode is suggested. © 2007 Elsevier B.V. All rights reserved.

Photo-assisted H₂ evolution has been realized over the new heterosystem CuFeO₂/SnO₂ without any noble metal and was studied in connection with some physical parameters. The delafossite CuFeO₂ has been prepared by thermal decomposition from various salts. The polarity of generated voltage is positive indicating that the materials exhibit p-type conductivity whereas the electroneutrality is achieved by oxygen insertion. The plot of the logarithm (conductivity) vs. T^-1 gives average activation energy of 0.12 eV. CuFeO₂ is a narrow band gap semiconductor with an optical gap of 1.32 eV. The oxide was characterized photoelectrochemically ; its conduction band (-1.09 V_RHE) is located below that of SnO₂ (-0.86 V_RHE) at pH 13.5 itself more negative than the H₂O/H₂ level leading to a thermodynamically favorable H₂ evolution under visible irradiation. The sensitizer CuFeO₂, working as an electron pump, is stable towards photocorrosion by hole consumption reactions involving the reducing agents X^2- (=S₂O₃^2-and SO₃^2-). The photoactivity was dependent on the precursor and the best performance (0.026 ml h^-1 mg^-1) was obtained in S₂O₃^2- (pH 13.5) over CuFeO₂ synthesized from nitrate with a mass ratio (CuFeO₂/SnO₂) equal to unity. A quantum yield of 0.5% was obtained under polychromatic light. H₂ liberation occurs concomitantly with the oxidation of S₂O₃^2- to dithionate and sulfate. The tendency towards saturation, in a closed system, is mainly ascribed to the competitive reduction of the end product S₂O₆^2-. © 2008 International Association for Hydrogen Energy.

CuInSe₂ ingots were elaborated by direct reaction in evacuated quartz ampoule. The material is polycrystalline and crystallizes in the chalcopyrite structure (SG : I over(4, ¯) 2 d). The chemical composition was determined by the energy dispersive spectroscopy and RBS technique. The analysis indicates a stoichiometric compound with small copper deficiency and a uniform bulk composition. A band gap at 0.94 eV was evaluated from the diffuse reflectance spectrum. The electrical properties were investigated and an activation energy of 23 meV was obtained in the heating direction from the Arrhenius type law. The thermal variation of the thermo power indicates p-type conduction with polaron hopping. The compound is stable in aqueous electrolyte with a dissolution rate of 8 µmol cm^-2 month^-1. A corrosion potential of +0.20 V_SCE and an exchange current density of 0.318 mA cm^-2 were determined from semi logarithmic plot in KCl (1 M). The current-potential characteristic gave a potential V_on of onset photocurrent of -0.60 V_SCE, close to the flat band potential (V_fb = -0.58 V_SCE), determined by extrapolating the linear part to C^-2 = 0 in the Mott Shottky plot. The doping density (N_A = 3.8 10¹⁷ cm^-3) was calculated from the slope. The Nyquist plot shows a semicircular arc whose center is localized below the real axis with an angle of 0.086 rad. This can be attributed to single barrier of the junction SC/electrolyte and single relaxation time of the electrical equivalent circuit. © 2007 Elsevier B.V. All rights reserved.

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 S^2- media was found to be 0.54 ; such result was corroborated by semi-logarithmic plots. © 2007 Elsevier B.V. All rights reserved.

The morphology, optical characterization and transport properties of dense material CuIn₃Se₅ have been reported. CuIn ₃Se₅ was synthesized by the fusion technique in a sealed quartz ampoule. The ingots were polycrystalline and the phase crystallizes in a P-chalcopyrite structure. The XRD pattern confirms the formation of the ordered vacancy compound. The composition (Cu = 13.3 at%, In = 31.5 at%, Se = 55.2 at%), established by energy dispersive spectroscopy, is close to the stoichiometric formulation. A band gap of 1.19 eV has been obtained from the diffuse reflectance spectrum. CuIn₃Se₅ exhibits semi-conducting behavior with an activation energy of 0.52 eV. The thermal variation of the thermopower indicates that the charge carriers are electrons and the conduction occurs predominantly by small polaron hopping. The dissolution rate, determined through dissolved copper, was found to be 1.2 ?mol m^-2/year in the KCl electrolyte. The results give deeper insights into the photoelectrochemical properties investigated for the first time on the dense material. The capacitance measurement indicates linear behavior from which a flat band potential V_fb of -0.22 V_SCE and a doping density N _D of 3.75 × 10¹⁶ cm^-3 were determined. The energy band diagram shows the potentiality for solar energy conversion. The complex impedance pattern is circular in the low frequency region and tends to a straight line in the high frequency one, attributed to the ionic diffusion. © 2008 IOP Publishing Ltd.

The transport phenomena through ion exchange membrane may be improved by the application of different forces inside or outside the systems. The electrical current, produced by synthetic photo electrode, is applied to enhance the separation of metallic ions. The cation exchange membrane placed between the two compartments permitted the separation and concentration of the ions M²⁺ (=Cu²⁺, Zn²⁺ and Ni²⁺). The movements of M²⁺from the aqueous solution to the membrane and inside the membrane are followed by the flux determinations and the potential measurements. In this study, the three cations are studied alone or as binary and ternary systems. It was observed that the illuminated CdS electrode leads to an increasing of the transport flux at least 10 times more than that calculated in the obscurity. Moreover, in all the cases the potential of the first specie electrode M/M²⁺ in the feed compartment increases until a maximal value reached at 100 min above which it undergoes a diminution. The membrane is more selective to Zn²⁺ ; this selectivity decreases in the binary and ternary systems. On the other hand, the photo electrochemical properties of the hexagonal CdS showed strong light absorption of wave lengths shorter than 520 nm. The shape of the intensity potential curve under illumination and the negative flat band potential (-1.02 V_SCE) provide unambiguous evidence of n-type conductivity. The conduction band, located at -1.25 V_SCE leads to a thermodynamically M²⁺ reduction under visible light and accentuates the diffusion process. © 2007 Elsevier B.V. All rights reserved.

The catalytic electrodeposition of a variety of metal ions M²⁺ (Zn²⁺, Cd²⁺, Hg²⁺, Ni²⁺, Pb²⁺, Cu²⁺, Ag⁺), all of environmental issue, was performed under visible light in aqueous air-equilibrated CuFeO₂ suspensions. The delafossite CuFeO₂ was synthesized by nitrate way and the surface/bulk ratio is greatly increased. In the dark, it exhibits a long term chemical stability with a corrosion rate of 1.3 × 10^-8 mol m^-2 year^-1 in KCl (1 M). The black oxide, with a gap of 1.32 eV, determined from the reflectance diffuse spectrum, was characterized by photoelectrochemistry. The flat band potential was found to be 0.16 V_SCE and a correlation exists between the redox potential of M^2+/0 couple and the conduction band of CuFeO₂located at -1 V_SCE. Ag⁺ is slightly photoelectrodeposited because of its low adsorption and its positive potential. By contrast, Cu²⁺ and Cd²⁺ are efficiently reduced owing to the strong dark adsorption and the large driving force at the interface. The quantum yield of M²⁺ deposition works out to be 0.1% ; the metal deposition proceeds competitively with the water reduction and decelerates over time. The specie showing the high H₂-evolution exhibits the lower M²⁺ reduction and the medium pH should make the M²⁺ deposition easier. The best photocatalytic H₂-evolution (0.18 ml g^-1 min^-1) was obtained in solution containing Cd²⁺ (30 ppm). This is a surprising result and merits a more detailed investigation. © 2007 Elsevier B.V. All rights reserved.

PbS was prepared by a simple chemical method for use as a photosensitizer with TiO₂. Its physico-chemical characterisation reveals a semiconducting behaviour with a band gap energy of 0.41 eV showing efficient visible light absorption. The valence and conduction band energy levels are 
 0.78 and -1.19 V, respectively. Interparticle electron injection (IPEI) from photoactivated PbS to inactivated TiO₂ was demonstrated by the increase of eosin degradation under visible light. Different parameters affecting the photoactivity such as the PbS amount, eosin concentration and initial pH were investigated. Optimal degradation of eosin occurs at a PbS/TiO₂ ratio of 10:90, an eosin concentration of 30 mg l^-1, a pH 7, and when using a 2× 200 W tungsten lamp. These conditions result in a constant decomposition rate of 0.116 min^-1 corresponding to an eosin half-life of 6.4 min. In this case, the rate of degradation of eosin is respectively 20 and 40 times greater than when in contact with pure TiO₂ or through direct photolysis. The photoeffeciency of PbS(10%)/TiO₂ was compared to that of CdS(10%)/TiO₂, Bi₂S₃(10%)/TiO₂, Cu₂O(10%)/TiO₂ and Bi₂O₃(10%)/TiO₂ using visible and UV-vis light. PbS(10%)/TiO₂ has the best efficiency regardless of the wavelength of light used. The contribution of the electromotive force and the band gap of the narrow band gap semiconductor to the photoactivity improvement were discussed. © 2007 Elsevier B.V. All rights reserved.

Poly (acrylic acid-co-crotonic acid) copolymers with various compositions of acrylic acid (AA) and crotonic acid (CA), (3.0 = AA = 4.5 g and 0.5 = CA = 2.0 g), were prepared by free radical polymerization using toluene as solvent and azobis-iso-butyro-nitrile (AIBN) as initiator. The average molecular weights were determined from the intrinsic viscosity measurements in NaNO₃ (1 M) as supporting electrolyte. The thermal analysis (DSC and TGA) of copolymers indicate a hydrophilic behaviour and a thermal stability in the temperature range (100-220 °C) above which they undergo an irreversible degradation. The synthesized copolymers are good fixation agents that can be used in the recovery of heavy metals, known for their high toxicity in aqueous media. In a second part, we investigated the complexes formation of metal ions M(II) (M = Pb, Cd and Hg) with the copolymer (D) of composition 3.0 g (AA)/2.0 g (CA). The pH titration of the copolymer alone and in presence of M(II) were realized through the batch method to determine the acid-base properties and complex-forming of ligand copolymers. The results show that for Pb(II), the predominant complex specie is PbA₂ with a stability constant pK₂ = -8.82. On the contrary, with Cd(II) and Hg(II) neither MA nor MA₂ becomes predominant and the stability increases as follows : Pb(II)>Cd(II) Hg(II). © 2007 Elsevier Ltd. All rights reserved.

The transport properties of Sr_0.98La_0.02SnO_3-d in the system Sr_1-xLa_xSnO_3-d, after which the pyrochlore La₂Sn₂O₇ appears, were investigated over the temperature range 4.2-300K. The oxide was found to be n-type semiconductor with concomitant reduction of Sn⁴⁺ into Sn²⁺. The magnetic susceptibility was measured down to 4.2K and is less than 3x10^-5 emu cgs mol^-1 consistent with itinerant electron behavior. The electron is believed to travel in a narrow band of Sn:5s character with an effective mass 4mo. The highest band gap is 4.32eV and the optical transition is directly allowed. A further indirect transition occurs at 4.04eV. The electrical conductivity follows an Arrhenius-type law with a thermal activation of 40meV and occurs by small polaron hopping between nominal states Sn^4+/2+. The linea...

SrPbO_3-d is n-type, narrow-bandgap semiconductor crystallizing in a distorted perovskite structure. The electrical conductivity varies with temperature but in a nonmonotonic manner. At high temperatures, the thermal evolution of the conductivity follows an Arrhenius-type law with an activation energy of 0.11 eV attributed to mixed valency states Pb^4+/2+ and leading to a dielectric constant of 13.6. The sign of thermopower S is negative, indicating that the mobile carriers are electrons, which is ascribed to oxygen extraction. The electron hopping occurs between similar electronic localized states in the vicinity of the mobility edge with an electron mobility of 10-2 m2 V-1 s-1 that is thermally activated. The variation S (T) suggests a finite density of states at the Fermi level. At low te...

CuMnO₂ is prepared via Cu⁺->Li⁺ exchange in molten copper (I) chloride. It crystallizes in a monoclinic structure (SG C2/m) where the MnO₆octahedra elongation is ascribed to the Yahn-Teller (Y-T) effect of Mn3+ ions. From chemical analysis, the oxide is more accurately formulated as CuMnO_2.01. Above 250 °C, it undergoes a reversible transition to spinel Cu_x Mn_3-xO₄ and beyond 940 °C it converts back to Cu_1.1Mn_0.9O₂. Extrapolation of high-temperature magnetic data indicates an effective moment of 5.22 uB, and a T-intercept O_p of -450 K, consistent with high spin (HS) configuration Mn³⁺ and strong antiferromagnetism in the basal plans...

A novel oxide AgMnO₂ was prepared from LiMnO₂ via Ag⁺->Li⁺ exchange in the eutectic melt AgNO^3-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 (X/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 O_p = -436 K and a magnetic mome...

Here we report the synthesis and photo electrochemical properties of super oxides CuYO_2.50 and CuYO_2.25 prepared from the delafossite CuYO₂, respectively, by thermal oxidation at 380°C under O2-flow and soft chemistry in NaBrO solution (5N). Their applications as catalysts for H₂ evolution upon visible light were investigated. The oxygen insertion was accompanied by partial oxidation of Cu⁺. For CuYO_2.25, the chemical analyses revealed the presence of mixed valent states containing at least formally an equal number of C^u+ and Cu²⁺. The thermal analysis (TGA) under reducing atmosphere indicates that oxygen is inserted in different crystallographic sites, for CuYO_2.25 it exhibits a two-step reduction mechanism with restoration of the parent oxide. In air, CuYO_2+x is thermally stable up ...

The formation of a CuAl_1-xMn_xO₂ solid solution for a limited range of x values (0 < x < 0.2). Magnetic studies show that Mn³⁺ ions are in the HS state and that the predominant interactions are antiferromagnetic. The oxides are semiconductors with an activation energy decreasing with x.

A new photochemical heterosystem Bi₂S₃/TiO₂ immersed in solutions of X^2- (=S^2- or S₂O₃^2-) as reducing agents for generated minorities is proposed. Bi₂S₃, synthesized via solvent thermal process, has a good crystallinity and exhibits n-type conduction as evidenced from photocurrent-potential characteristics. The flat band potential of Bi₂S₃ (-0.83V_SCE) is pH-insensitive and shifts negatively with increasing S^2- concentration by 53mV/pS. The heterosystem shows a much higher activity than Bi₂S₃ alone where efficient H2-photoevolution can be realized with concomitant oxidation of X^2-. The optimal value of the mass ratio (Bi₂S₃/TiO₂) was equal to unity. In S₂O^32- electrolyte, the photoactivity peaks at pH 13.7 with an evolution rate of 2.9mmolmg-1h-1 and decreases gradually to zero near pH 7. T...

Excellent quality CuLaO² has been synthesized through solid reaction in evacuated silica tubes and the delafossite structure was refined in the space group Formula Not Shown . The oxide is thermally stable up to 200degreeC, above 610degreeC it converts irreversibly to CuLa₂O₄. The conduction band (-1.99V_SCE) lies below H₂O/H₂ level leading to a thermodynamically favorable H2-photoevolution with concomitant oxidation of X^2-(=S^2- or SO^32-). The surface platinization improved the activity by increasing the rate constants of X^2- oxidation. The redox kinetic is relatively fast with SO^32- whereas the electrochemical system S^2-/Pt behaves irreversibly and where the polarization is mainly of activation. The charge carriers move in a narrow conduction band made up primarily of Cu 3d orbitals. The ...

CuAlO₂/TiO₂ heterojunction was prepared and studied for its potential application as dispersed photoelectrode capable to generate H₂ under visible light. Transport properties of delafossite CuAlO₂, i.e. thermoelectric power and electrical conductivity were studied and correlated to an optical and photoelectrochemical characterization to establish energetic diagram of the heterosystem CuAlO₂/TiO₂. The valence and the conduction bands were estimated to be respectively of -0.05 and of -1.34V/SCE, which permit electron injection from activated CuAlO₂ to an activated TiO₂. This fact permitted a physical separation of charge carriers. Photocatalytic experiment performed using S^2- and S₂O₃^2- showed efficiency respectively of 5265 and 60mmol and then after 40min of irradiation by visible light. Th...

BaSnO₃, crystallizing in a cubic perovskite structure, is a polaron hopping semiconductor. It can be made conducting by reducing a small fraction of Sn⁴⁺ to Sn₂₊ under O^2- free atmosphere. This can be achieved through the solid solution Ba^1-xLaxSnO₃ which is mixed phase for x> 0.02, a behavior readily understood in term of the lone pair cation Sn2+. The magnetic susceptibility was measured down to 4.2 K and is less than 1.7x10-5 emu cgs mol-1 consistent with collective electron behavior. The Mossbauer spectroscopy at room temperature exhibits a wide unsplit peak with a quadrupole doublet of 3.18 mm sec^-1 and an isomer shift of 0.12 mm sec^-1 characteristic of Sn⁴⁺, corroborating the delocalization of the stereo chemical pair 5s². The band gap Eg was found to be 3.12 ; further interband transition, indirectly allowed, occurs at 2.85 eV. The transport properties of Ba_0.98La_0.02SnO_3-d, indicate a degenerate n-type conductivity s^300K = 4.03 (Ohm cm)-1, little temperature dependent, with an activation energy ?E of 1 meV and an electron mobility µ_300K 0.1 cm² (V s)^-1, thermally activated. The conduction occurs by small polaron hopping between mixed valences tin ions. The observed conductivity is greater than that coming from La-substitution with one electron doping implying the existence of oxygen vacancies. The electrons are believed to travel in Sn-5s conduction band with an effective mass of 3.9 mo. A predominant variable range hopping is predicted from the non linearity of Ln s vs T^-1 plot. The electron localization may be attributed to the random distribution of lanthanum as well as oxygen vacancies.

TiO₂, CdS and coupled CdS/TiO₂ powders with CdS percentage varied from 5 to 50% (w/w) have been prepared by a sol-gel method. These pure and composite semiconductors were tested for the photocatalytic degradation of an anionic azo-dye, i.e. Orange II in aerated aqueous suspension under UV-vis and visible (l>400nm) illumination. The photocatalytic efficiency of the CdS/TiO2 couples is found to strongly depend on the proportion of CdS. Under UV-vis light, CdS/TiO₂ couples containing a low proportion of CdS are found to be as efficient as pure TiO₂ which shows the best photocatalytic activity. Under visible illumination, these formers exhibit faster degradation rate than both isolated components of the composite photocatalyst. The results obtained under both illumination conditions are compar...

The new oxide CuLaO_2.62, synthesized through solid reaction under O₂ atmosphere from CuLaO₂ at 380°C , crystallizes in a pseudodelafossite structure. The formulation comes from thermal analysis and iodometric titrations. It exhibits a p-type semi-conductivity ; the resistivity p_300K (2485 Ohm cm) and the thermopower S_300K (93µVK^-1) suggest a mechanism conduction by small polarons hopping between Cu^2+/3+. The material is thermally stable up to 500°C beyond which it converts irreversibly to CuLa₂O₄ . Its photocatalytic ability, in contact with S^2- and SO₃^2- aqueous solutions for H₂ production, has been evaluated in connection with some physical parameters. The title oxide i...

Mixed valencies system Sr_1-xLaxPbO_3-d (0x0.15) has been prepared by solid state reaction at 900°C. All solutions crystallize in a distorted perovskite structure and were carefully examined by chemical analysis and X-ray diffraction. The La³⁺-substitution was checked via the linear dependence of lattice constants on x-value, i.e. Vegard’s law. The expansion of the unit cell volume involves a simultaneous increase of all the parameters. The magnetic susceptibilities are slightly temperature dependent, dominated by core diamagnetism giving an effective mass m* of 9. The transport properties were investigated from 300K down to liquid helium. The resistivity was found to change markedly with the rate substitution x and the oxides behave as degenerate semi-conductors with very weak activati...

We have been studying the technical feasibility of a photochemical H₂ evolution based on a dispersion of CuCrO₂ powder in aqueous electrolytes containing various reducing agents ( S^2-, SO₃^2- and S2O₃^2- ). The title oxide combines a fair resistance to corrosion with an optimal band gap Eg of 1.32eV. The intercalation of a small amount of oxygen should be accompanied by a partial oxidation of Cu⁺ into Cu²⁺ implying a p-type semiconductivity. The S^2- oxidation inhibits the photocorrosion and the H₂ evolution increases parallel to polysulfides S_n^2- formation. Most of H₂ is produced when p-CuCrO₂ is connected to n-Cu₂O formed in situ. H₂ liberation proceeds mostly on CuCrO₂ while the oxidation of S_2- takes place over Cu₂O surface and the hetero system Cu...

A new photochemical system based on a CuYO₂ dispersion in aqueous S^2- or SO₃^2- solution as hole scavengers is proposed. The delafossite CuYO₂, doped with calcium, is a low p-type semiconductor with a hole mobility of 7.3x10^-7m²V^-1s^-1. The band gap is 3.50eV and the transition is directly allowed. The valence band edge, located at 5.23eV below vacuum, is made up from Cu-3d type typical of delafossite oxides. The flat band potential V_fb ( +O.17V_SCE ) lies below the H₂O/H₂ potential permitting a spontaneous H₂-formation under band gap illumination in aqueous S^2- or SO₃^2- electrolytes. p-CuYO₂ loaded with n-Cu₂O exhibited a higher performance with a H₂ evolution rate of 0.4cm³h^-1 in 0.1MS^2- (pH 13.4). The decrease of photoactivity over time is due to the formation of yellow polysul...

The transport properties of oxygen-deficient perovskite SrPbO_3-d with mixed lead valency were investigated down to 4.2 K. The small d-value (0.059), determined from iodometry, is due to the inert lone pair Pb²⁺ that does not enjoy regular octahedral coordination in spite of collective electron behaviour. The oxide exhibits a temperature-independent magnetic susceptibility consistent with itinerant electrons. The sign of carriers like polarons is that of n-type conductivity coming from the balance charge via oxygen extraction. The thermal variation of conductivity and thermopower reveal the existence of an energy gap. The conduction mechanism occurs by polaron hopping in conformity with small activation energy. The metal–insulating transition seems to be of Anderson type, resulting from the disorder of oxygen vacancies. At low temperature, the conductivity was fitted to a variable range hopping . A comparison with SrSnO₃ will be reported. The covalency of Sn–O raises the antibonding conduction state of 5s parentage and increases the forbidden gap from 1.78 to 3.30 eV..

The photocatalytic ability of CuM₂O₄ (M=Al, Cr, Mn, Fe and Co) crystallizing with spinel-type structure has been evaluated according to the H₂-evolution. The oxides are black and displayed a semiconducting behavior where the electronic balance comes from a small over stochiometry (CuCo₂O_4.016). Electron hopping occurs between similar sites in normal spinel with a low activation energy whereas larger energies were found in inverse spinel. The electrodes were characterized photoelectrochemically and acquired the characteristic of p-type semiconductors. The conduction band edge, determined from intensity-potential curves, is located below the H₂O/H₂ potential leading to a thermodynamically favorable H₂-liberation under visible light. The photoactivity was dependent on preparative conditions a...

Solid solutions of formula Cu_1+xMn_1-xO₂ (0 < x < 0.2) were synthesized by solid state reaction in silica sealed tubes. They crystallize with the monoclinic crednerite structure (space group C2/m). The stability domain in air is quite narrow and a phase diagram is proposed and compared with previous results. Magnetic study confirmed the HS state of Mn³⁺ ions and revealed that the predominant interactions are antiferromagnetic. Their strength decreases with x, which can be ascribed to a dilution effect, and long-range 3D magnetic ordering observed for CuMnO₂, disappears for x>0.05. The crednerite solid solutions are p-type semiconductors. Modeling the thermoelectric power behavior suggests that charge carriers are Cu²⁺ holes diffusing in Cu layers for small x values and Mn⁴⁺ holes d...

The small band gap CuAlO₂, as prepared, is a p-type semiconductor with a low mobility polarons. Its catalytic ability for H₂ production upon visible light was studied in connection with its synthesis method. The potential of the conduction band (-1.63 V_SCE) lies below H₂O/H₂ level leading to a spontaneous H₂ -evolution. An enhancement has been made in the preparation of active oxide by coprecipitation. The highest H₂ production, with a generation rate of 0.19ml h^-1 mg^-1 , has been obtained in a 0.1 MS^2- solution at pH 13.72 and increases parallel to polysulfide Sn^2- formation (nS^2-+2(n-1)H₂O -> nS^2- + (n-1) H₂ + 2(n-1) OH^-) ; both reactions occur simultaneously and no photoactivity was observed below pH 7 indicating that For...

The properties of CuFeO₂ have been studied according to the catalytic hydrogen production upon visible light. CuFeO₂ with a low band gap Eg, a good chemical stability and a suitable flat band potential appears as a suitable candidate. The potential of photoelectrons allows favorably a thermodynamically H₂-evolution from alkaline thiosulfate S₂O₃^2- solution. There is a major difference between pure and loaded oxide with some metal catalysts. Our best results have been obtained with unloaded CuFeO₂ at 50 °C and pH 13.60. Thiosulfate S₂O₃^2- ions can be oxidized to sulfite SO₃^2- and subsequently to sulfate SO₄^2- and the electronic exchange occurs via mediation of surface states. The quite high H2-formation at the beginning shows a tendency towards saturation, it competes with SO₃^2- produced by parallel oxidation of S₂O₃^2-.

The solar cells collecting grids present a serious problem and more particularly under solar concentration. Our contribution in this article is to seek the best compromise between shadow effect and series resistance effect. The cell considered here is of Si (silicon) type, n⁺p with circular geometry (radius a = 4.9 cm ), a silver metallization (p_m = 1.6 x 10^-6 Ohm cm), and a contact resistivity of p_C = 10^-5 Ohm cm. Our calculations are made under the condition of AM1.5 with 1 sun concentration.The various power losses caused by this grid are:losses due to the grid shadow, losses in grain boundaries due to the metal/semiconductor contact,power dissipated in the resistance of layer between bars, andlosses in the grid metallization.We summarize the results of our model by:For a sheet res...

Cu₂O/TiO₂, Bi₂O₃/TiO₂ and ZnMn₂O₄/TiO₂ heterojunctions were studied for potential applications in water decontamination technology and their capacity to induce an oxidation process under VIS light. UV-vis spectroscopy analysis showed that the junctions-based Cu₂O, Bi₂O₃ and ZnMn₂O₄are able to absorb a large part of visible light (respectively, up to 650, 460 and 1000nm). This fact was confirmed in the case of Cu₂O/TiO₂ and Bi₂O₃/TiO₂ by photocatalytic experiments performed under visible light. A part of the charge recombination that can take place when both semiconductors are excited was observed when a photocatalytic experiment was performed under UV-vis illumination. Orange II, 4-hydroxybenzoic and benzamide were used as pollutants in the experiment. Photoactivity of the junctions was f...

Alkaline (Li, Na, K) modified titanium dioxide nanoparticles were prepared by sol-gel route and by impregnation technology. XRD analysis showed that the crystallinity level of catalysts is largely dependent on both the nature and the concentration of alkaline. Indeed, the best crystallinity is obtained for Li-doped TiO₂ and is lowest for K-doped TiO₂. In general, it was found that the anatase phase is stabilised by the alkaline. pH of zero charge has been found to be in the range 6.6-7.9. Depending on the alkaline concentration, the charge compensation may occur at the surface of the catalyst in acidic and basic environment. For photocatalytic test, Malachite green oxalate, 4-hydroxybenzoic acid and benzamide were used as representative organic pollutants. The catalysts prepared by the imp...

Bi₂S₃/TiO₂ and CdS/TiO₂ heterojunctions were prepared by direct mixture of both constituents and by precipitation of the sensitizer with commercial TiO₂ at different concentrations. UV-Vis spectroscopy analysis showed that the junctions-based Bi₂S₃ and CdS are able to absorb a large part of visible light (respectively up to 800 and 600nm). This fact was confirmed by photocatalytic experiment performed under visible light. A part of charge recombination that can take place when both semiconductors are excited was observed when photocatalytic experiment was performed under UV-Vis illumination. Orange II, 4-hydroxybenzoique acid (4-HBZ) and benzamide (BZ) were used as pollutants. Photoactivity of the junctions was found to be strongly dependent on the substrate. The different phenomena taking...

Photocatalytic hydrogen production induced under illumination using a new oxide CuMnO₂ has been studied. The applied catalyst was without noble metal. The hydrogen was generated from aqueous solution containing either S^2- or SO^₃2-. The conduction band of CuMnO₂, with a value of -0.88V/SCE, allows a spontaneous hydrogen formation. The highest hydrogen evolution rate was 0.95x10^-2mlmg^-1h^-1 on the crednerite CuMnO₂-E obtained by exchange reaction with SO₃^2- as hole scavenger. When S^2- was used, the hydrogen production reaches a saturation attributed to the competitive reduction of yellow polysulfides S_n^2- as well as their strong light absorption in the visible region. There was no photoactivity observed below pH 12...

Nanosized titanium oxide (TiO₂) powders were prepared by sol-gel route. The preparation parameters are optimized by means of Malachite green oxalate degradations. All catalysts were analyzed by X-ray diffraction (XRD) and BET technique. The best catalyst was compared to TiO₂-P25 using two other types of pollutant, 4-hydroxy benzoic and benzamide (BZ) and their performance was found to be strongly dependant on the pollutant’s type...

Nanosized titanium oxide (TiO₂) powders were prepared by emulsification-gelation technology, hydrolysis and reflux process. Starting precursor ( Tetraisopropylorthotitanate Ti(OC₃H₇)₄ ) was modified by 1,4 cyclohexane diol (C₆H₁₄O₂), hydrogen peroxide (H₂O₂) and Triethanol amine (C₆H₁₅NO₃). Titanium tetrachloride was also used as precursor. The effect of methanol on ethanol solvent was also investigated. The powders were analyzed by X-ray diffraction (XRD) and BET technique. Scanning electron microscopy observation was used for agglomerate observations. The photoactivity of the products obtained was taken as efficiency of Benzamide photodegradations.

Two narrow band gap-semiconductors AMn₂O₄ (A=Cu and Zn) with p-type semiconductivity and crystallising in the spinel structure have been studied for photoassisted hydrogen production by visible light in aqueous solution containing sulfide S^2- or sulfite SO₃^2-. The highest H₂ evolution rate, obtained under illumination with platinized Cu-oxide, was found to be 0.26×10^-2 mlmg^-1 h^-1 in a 0.1 M SO₃^2- alkaline solution and does not depend on specific area. H₂ evolves over oxide whereas oxidation of S^2- and/or SO₃^2- occurs on platinum that behaves as a counter electrode. When sulfide is used as hole scavenger, the hydrogen production decreases over time and reaches a zero value after 3 h because of the formation of coloured polysulfides S_n^2-, which acts as an optical filter and compete with water reduction...

Films of polycrystalline Bi₂S₃ have been prepared onto bismuth and platinum substrates by electrodeposition from an aqueous sulfide bath. The films were thin, uniform and well adhered. Bi₂S₃ is a direct band gap semiconductor with a value of 1.28 eV optimally matched with the solar spectrum. The photoelectrochemical study was undertaken for the generation of hydrogen by using illuminated n-Bi₂S₃ particles ; it was found that hydrogen evolution depends highly on the synthesis method of powder. Impregnation of platinum onto Bi₂S₃ shows a production enhancement of about 25%. The most active photocatalyst, prepared by a solvent thermal process and loaded with Pt in 0.1 M S^2- alkaline electrolyte, yields 2.13×10^-2 ml mg^-1 of H₂ after 4 h of irradiation with the visible output of a 500 W halogen lamp...