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To evaluate its photoactivity under solar light, ZnO with a nano morphology with crystallites sizes ranging from 2.5 to 5 µm was prepared by the acetate decomposition (ZnO-Nit) by one-step calcination. The powders were characterized by X-ray diffraction and transmission electron microscopy. The optical band gap of ZnO-Ac (3.23 eV) was determined from the diffuse reflectance and the transition is directly allowed, assigned to the charge transfer O2−: 2p → Zn2+: 4s. The capacitance-potential (C−2–E) characteristic plotted at pH ~ 7 shows n type behaviour with a conduction band (− 0.85 VSCE), made up of Zn2+: 4s orbital and positive of the O2/O2• level and a valence band (2.56 VSCE) deriving mostly from O2−: 2p orbital whose potential is less anodic than the •OH/H2O level. So, ZnO was successfully used for the degradation of two hazardous dyes namely the Reactive Blue 19 (RB 19) by the radicals •OH and O2•− under sunlight. Indeed, RB 19 is a vinyl sulphone dye, difficult to oxidize because its anthrax quinone stabilized by mesomery. The best performance is due to enhanced active surface area (59 m2 g−1). The RB 19 oxidation follows a zeroth order kinetic with a rate constant of 7.3 × 10–3 mol L−1 min−1 for a concentration 10 mg L−1. The RB 19 elimination was followed by chemical oxygen demand and a photodegradation mechanism was proposed to explain the high performance of the catalyst upon solar light. For a comparative purpose, the oxidation of Rhodamine B is also reported.

The solid solution NiFe2-xMnxO4 (0 ≤ x ≤ 2) is synthesized by sol gel method and the physical properties are investigated for the first time. The thermal analysis shows that the phases are formed above 400 °C. The oxides crystallize in an inverse cubic spinel whose lattice constant (a: 0.83381–0.83985 nm) increases only slightly up to x = 1.6, according to the Vegard’s law; such result is supported by the FTIR spectra. The UV-Visible spectroscopy shows both direct (1.00–1.56 eV) and indirect optical transitions (0.39–1.65 eV) due to the crystal field splitting of 3d metal. Field-dependent magnetization of the solid solution was measured at 300 K in the region (±20 kOe) and the end member NiFe2O4 exhibits a high magnetism with a saturation magnetization (20 emu/g), comparable to that reported previously. The saturation magnetization varies between 0.5 and 20 emu/g while the coactivity fluctuates between 110 and 239 Oe. The thermal variation of the electrical conductivity indicates a conduction mechanism by low polaron hopping which follows an exponential law with variable activation energies (Ea: 0.12–033 eV). The thermo-power is positive and nearly constant (S300K: 144–130 µVK−1), indicating p type conduction with a mobility more a less constant (1.5–8 × 10−6 V2 cm−1 s−1). The hole densities (NA × 1016: 0.26–2.17) are determined electrochemically from the capacitances measurements in neutral medium (Na2SO4 0.5 M). The flat band potential (Efb: −0.04 to −0.29 V) does not change significantly indicating that the valence band derives mostly from 3d orbital.

NiFe2−xMnxO4 (0 ≤ x ≤ 2) is prepared by sol–gel route and the physical properties are studied for the first time. The thermal analysis shows that the phases are formed at 600 °C. The oxides crystallize in an inverse spinel with a cubic symmetry whose lattice constant (a: 0.83381–0.83985 nm) augments slightly up to x = 1.6, according to the Vegard’s law; such result is corroborated by the FTIR spectroscopy. The diffuse reflectance shows direct (1.00–1.56 eV) and indirect (0.39–1.65 eV) optical transitions due to the lift of degeneracy of 3d orbital. Field-dependent magnetization of NiFe2−xMnxO4 was measured at 300 K in the range (± 20 kOe) and NiFe2O4 shows a strong magnetism with a saturation magnetization of 20 emu/g. The saturation magnetization varies from 0.5 to 20 emu/g, whereas the coercivity fluctuates between 110 and 239 Oe. The variation of the electrical conductivity with temperature shows a conduction mechanism by small polaron hopping which obeys an exponential law with activation energies (Ea: 0.12–033 eV). The thermo-power is positive and almost constant (S300K: 144–130 µV K−1), indicating p type comportment with a mobility more a less constant (1.5–8 × 10–6 V2 cm−1 s−1). The hole concentrations (NA × 1016: 0.26–2.17) are computed from the capacitances measurements in Na2SO4 solution (0.5 M). The flat band potentials (Efb: − 0.04 to − 0.29 V), determined electrochemically, does not vary significantly indicating that the valence band derives mainly from 3d character.

The Pr2NiO4/SnO2 heterojunction with a mass ratio equal to unity was tested with success for the hydrogen production under visible light irradiation. Pr2NiO4, prepared by nitrate route, crystallizes in a tetragonal symmetry with K2NiF4 type structure. The physical, electrical and photo-electrochemical characterizations are correlated to show the feasibility of Pr2NiO4 for the hydrogen formation under visible light. The enhanced hydrogen production activity is due to electron injection of activated conduction band Pr2NiO4-CB (−1.53 VSCE) into SnO2-CB (−0.87 VSCE) which acts as an electron pump, resulting in better water reduction. The band gap of the semiconductor Pr2NiO4 is 1.81 eV with a direct optical transition. Pr2NiO4 acquired p type conductivity, due to oxygen insertion in the layered lattice with an activation energy of 0.09 eV. The flat band potential (Efb, 0.18 VSCE), very close to the photocurrent onset potential (0.13 VSCE) and the density of the holes (NA, 1.85 1020 cm−3) were obtained from the Mott-Schottky characteristic. H2 production rate of 24.3 μmol g−1 min−1 is obtained with a quantum yield of 1.45% within 30 min under optimal conditions (1 mg of catalyst/mL, pH ~12 and 50 °C) in presence of S2O2−3 as hole scavenger under visible light flux of 29 mW cm−2.

The photoelectrochemical ability of Ag(5%)/NiFe2O4 for H2 evolution under visible light irradiation was studied. The hetero-junction prepared by sol gel method shows a much higher activity than the physical mixture (hetero-system: Ag + NiFe2O4). The electrons of NiFe2O4 activated by visible light are focalized toward Ag clusters, intimately contacted to the spinel. The bifunctional crystallite is generated by incorporation of ultrafine Ag crystallites, thereby avoiding high over-voltages. The spinel was characterized by thermal analysis, X-ray diffraction and scanning electron microscopy. The optical gap of NiFe2O4 (1.59 eV) is due to the crystal field splitting of d-d transition of Fe3+: 3d orbital in octahedral site. p-type conductivity of NiFe2O4 was demonstrated from the capacitance measurement in alkaline KOH solution (0.1 M). The conduction band (−0.87 V) is more cathodic than the H2 evolution and the hetero-junction was successfully used for the photochemical hydrogen production under visible light irradiation. The photo-activity is dependent on the nature of the hole scavenger (NO2−, Fe(CN)64− and I−). NO2− is a hazardous ion and gave the best activity, due to its optimal band bending of 0.5 V and is converted to nitrate. An evolution rate of {0.17 cm3 H2 (g catalyst)−1 mn−1} was obtained at pH ∼ 7 with an apparent quantum yield of 1.3%.

The layered niobate Sr0.5Nb3O8·1.7H2O is synthesized by soft chemistry in aqueous electrolyte via Sr2+ → H+ exchange between strontium nitrate and niobic acid HNb3O8·H2O. The material is identified by X-ray diffraction using Rietveld refinement, thermal analysis (TG/DSC) and optical measurements. The semiconducting and photoelectrochemical properties are investigated for the first time. The band gap of Sr0.5Nb3O8·1.7H2O is evaluated at 3.67 eV, and the transition is directly allowed due to the charge transfer O2−: 2p → Nb5+: 4d. The thermal variation of the electrical conductivity shows that 4d electrons are localized and the data are fitted by a small-polaron hopping model: σ = σo exp {− 0.13 eV/kT}. The capacitance measurement done in the ionic electrolyte (Na2SO4, 10−2 M) indicates n-type semiconductivity with a flat band potential of − 0.09 VSCE. The conduction band, made up of Nb5+: 4d orbital, is located at − 0.22 VSCE. As application, Rhodamine B (RhB) is oxidized by photocatalysis on Sr0.5Nb3O8·1.7H2O through O2· radicals; 56% of the initial concentration (10 mg L−1) is eliminated after 3 h under solar light (90 mW cm−2), and the Rh B oxidation follows a first-order kinetic with a rate constant of 0.246 h−1.

The oxidation of Rhodamine B (Rh B), a hazardous dye, was investigated under sunlight over the Keggin type heteropolysalt Ag3PMo12O40. The compound prepared by precipitation from the heteropolyacid H3PMo12O40 and silver salicylate, was characterized by single-X-ray diffraction, SEM analysis, Infrared and 31P NMR spectroscopy and diffuse reflectance. The optical properties indicated two allowed transitions, direct (1.79 eV) and indirect (1.12 eV). The Mott–Schottky characteristic plotted in Na2SO4 electrolyte (0.5 M) showed n-type behavior with a flat band potential of 0.25 VSCE. The electrochemical impedance spectroscopy showed the predominance of the bulk material with an intrinsic impedance (Rb = 247 Ω cm2). Rh B was successfully oxidized by O2· radical with an enhanced activity assigned to the conduction band (0.05 VSCE/4.80 eV) deriving from Mo: 5s orbital, more cathodic than the O2·/O2 couple. The degradation kinetic followed a first order model with a half-life of 70 min under an average solar flux of 930 mW cm−2.

The hybrid semiconductor CYCHA-CoAl2P3O12·mH2O was elaborated by hydrothermal route at 453 K in presence of cyclohexylamine (CYCHA) as organic template. The X-ray diffraction pattern of the blue product indicates a zeolite type CAP-CHA 5 topology which crystallizes in a hexagonal system (SG:R $$ \overline{3} $$ ) with the lattice constants: a = 13.5494 (3) A, c = 15.2795 (5) A. The optical, physical and photo-electrochemical characterizations of the material were investigated. The diffuse reflectance indicates an optical transition at 1.84 eV, directly allowed due to the crystal field splitting of Co 3+ in tetrahedral coordination. The electrical conduction occurs by low polaron hopping with activation energy of 37 meV. The conduction band is formed by Co3+: e.g. orbital (5.15 eV) below vacuum. The valence band, deriving mostly from O2−: 2p orbital with a potential (6.85 eV), is adequately located with respect to O2/H2O level (~ 1.40 VSCE), thus leading to an oxidation of water under illumination. The best photoactivity was found at neutral pH (~ 7) with an oxygen evolution rate of 38 µmol min−1 g−1 and a quantum yield of 0.40%. The as-synthesized material also showed adsorption capacity of 90% of the methyl violet dye in aqueous solution with a half life of 43 min.

The complete solid solution CdxZn1-xS (0 ≤ x ≤ 1) prepared by ultrasound-assisted route is used for the H2 formation upon visible light illumination. The correlation of chemical and physical characterizations permits to assess the feasibility of the system for the photocatalytic hydrogen evolution. The compounds crystallize in a cubic structure (x

The physical and photoelectrochemical properties of the spinel ZnCr2O4 prepared by Sol Gel method are investigated. The thermal analysis indicates that the cubic spinel is formed beyond 500 °C with a lattice constant of 0.8328(3) nm. The Rietveld texture analysis indicates the absence of inter-granular residual stresses. The attenuated transmission reflectance confirmed the phase formation and forbidden band is found to be 1.85 eV. The valence and conduction bands have respectively Cr3+-t2g Cr3+-eg character. The magnetic curves M(H) of ZnCr2O4 exhibit a low hysteresis and the magnetization rises as the applied field up to 19 kOe with no saturation. The electrical properties of ZnCr2O4 indicate semiconducting behavior with activation energy of 43 meV while the thermo-power characterizes hole doped spinel. The capacitance measurement (C−2 - E) in Na2SO4 solution (0.1 M) shows p type conduction with a flat band potential (Efb) of −0.061 VSCE and a hole density (NA) of 2.7 × 1018 cm-3. The semicircle in the Nyquist plot is assigned to the bulk contribution (7700 kΩ cm2) with a depletion angle of 4° due to a constant phase element. As application, orange II (OII) is successfully oxidized on the spinel under solar and artificial lights. The conduction band is cathodic enough to generate radicals O2 which mineralizes OII. The best activity occurs at pH ∼7 under solar light (44%) and the process follows a first order kinetic with a half-life of 170 min. As expected, the COD removal efficiency increases, with irradiation time and reaches 20% after 1 h under solar light.

Ponceau 4R is successfully oxidized on the hetero-system ZnCo2O4/ZnO under the solar light. The spinel with a nano-morphology is elaborated by the sol gel method at ~ 850 °C. The X-ray diffraction pattern exhibits narrow peak characteristics that reveals a good crystallization. The capacitance−2–potential (C−2–E) plot of the semiconductor ZnCo2O4 indicates p type behavior from which a flat band potential of + 0.30 VSCE is determined. The energy band diagram, built from the physic-chemical properties, clearly predicts the electron transfer from the conduction band of ZnCo2O4 toward dissolved oxygen via ZnO. Indeed, the ZnCo2O4/ZnO composite improves the photocatalytic performance, where the colloidal photochemical hetero-system is successfully used for the light induced Ponceau 4R oxidation. The spinel dose and Ponceau 4R concentration are optimized. The conversion rate is controlled by UV–Visible spectrophotometry and, under the ideal conditions, the oxidation of 70% of Ponceau 4R (15 ppm) is obtained in aerated solution for less than 4 h when exposed to the solar light. The oxidation obeys a first order kinetic with a half-photocatalytic life of 130 min (k ~ 52 × 10−4 min−1).

Nanocrystalline lithium manganese oxide (LiMn2O4), synthesized by the sol-gel method, crystallizes in the spinel structure. The physical, electrical, and photoelectrochemical properties were studied for the photocatalytic degradation of methyl orange, a toxic compound, under solar irradiation. The diffuse reflectance spectrum allowed the direct band gap (1.99 eV) to be calculated. The Fourier transform IR spectrum contained all the characteristic peaks of the spinel LiMn2O4, which exhibited n-type behavior. The thermal evolution of the electrical conductivity exhibited an Arrhenius-type behavior with an activation energy of 0.27 eV. The Mott-Schottky curve allowed the determination of the flat band potential (−0.24 V vs. the saturated calomel electrode) as well as the carrier density (1.78 × 1021 cm−3). The detoxification of water containing methyl orange with LiMn2O4 as a photocatalyst was successfully completed. Total degradation for an initial concentration of 10−4 M methyl orange at pH ∼3 and 25 °C was obtained in less than 5 h under solar irradiation. No adsorption was obtained in the dark in the presence of LiMn2O4.

The spinel NiMn2O4 prepared by sol-gel is applied for the hydrogen liberation upon visible light. The oxide crystallizes in a cubic symmetry with a crystallite dimension of 63 nm, smaller than that obtained by transmission electron microscopy (TEM; 470 nm), indicating crystallites agglomeration. p-type conductivity is demonstrated by the positive thermo power (+130 µV at 300 K), with a hole mobility of 2.09 × 10−4 cm2 V−1 s−1 and an activation energies of 70 meV. The optical gap (1.13 eV), attributed to 3d orbital splitting, is determined from the diffuse reflectance and is appropriately matched to the sun spectrum. The physical and chemical characterizations are correlated to show the feasibility of NiMn2O4 for the hydrogen formation. The capacitance measurement gives a flat band potential of −0.20 VSCE and a hole density of 0.29 × 1016 cm−3. The potential of the conduction band (−1.26 VSCE) is below that of H2O/H2 level (∼−0.40 VSCE), allowing H2-liberation upon visible irradiation. The NiMn2O4 mass, pH and concentration of the hole scavenger S2O32− are optimized. Under optimized conditions, the H2volume liberation reaches 77 µmol with rate of 0.024 µmol mg−1 min−1.

The composition Ba(Ti0.950Sc0.025Nb0.025)O3 (BTSN) belongs to the (1 − x)BaTiO3–xBaSc1/2Nb1/2O3 system (x = 0.05). The room temperature X-ray diffraction (XRD) analysis refined by the Rietveld method shows a tetragonal perovskite structure. The dielectric study, carried out on the ceramics in the ranges (77–500 K) and (102–2 × 105 Hz), exhibits a normal ferroelectric behavior. The ferroelectric-paraelectric phase transition appears at the Curie temperature (TC) of 338 K with a wide space charge region. The maximal dielectric permittivity (er) is characterized by a broader peak than that observed in BaTiO3 at the temperature Tc. The slight deviation from the Curie-Weiss law is typical of ferroelectric materials with a diffuse phase transition. The polarization-electric field (P-E) loop of BTSN is in good agreement with the XRD analysis. The BTSN ferroelectric ceramic exhibits interesting photoelectrochemical properties with an optical gap of 2.60 eV and a flat band potential of −0.58 VSCE. We successfully tested the photocatalytic oxidation of amoxicillin, a pharmaceutical antibiotic currently released in water as pollutant on BTSN. The oxidation follows a first order kinetic with a half-life of 123 min under solar light and a conversion yield of 72%.

Benzamide is successfully degraded on the novel heterosystem NiMn2O4/TiO2 under visible light. The nanosized spinel is synthesized by the sol–gel method at ~850 °C. The X-ray diffraction pattern shows narrow peaks and the oxides are well crystallized. The Mott–Schottky plot (C−2−E) of NiMn2O4 is characteristic of p-type conductivity from which a flat-band potential of −0.20 VSCE is obtained. The energy-band diagram, built from the physicochemical characterizations, predicts the electron transfer from the conduction to dissolved oxygen via TiO2. The loading of TiO2 with NiMn2O4 enhances the photoactivity and NiMn2O4 islands achieve a colloidal photochemical heterosystem, tested successfully for the light-induced benzamide degradation. The spinel dose and benzamide concentration are optimized. Under the ideal conditions, the rate of the benzamide disappearance is controlled by high-performance liquid chromatography. A conversion of 85% is reported in aerated benzamide solution (15 ppm) in less than 2 h under artificial light. This conversion rate increases up to 94% under solar light and the oxidation obeys to a first-order kinetics with a half-life of 53 min.

Polycrystalline sample Ba(Ti0.9Sc0.05Nb0.05)O3 (BTSN) has been synthesized by solid-state reaction. The effects on the symmetry and dielectric properties of simultaneous cationic substitution of scandium and niobium in the titanium site of BaTiO3 were investigated. The room-temperature X-ray diffraction revealed a cubic perovskite phase after sintering at 1350 °C. The dielectric properties in the ranges (80–445 K) and (102–106 Hz) were studied. A broad dielectric band anomaly coupled with the shift of dielectric maxima toward higher temperatures with increasing frequency indicates a diffuse phase transition with a relaxor behavior. The parameters of the diffuse phase transition were evaluated from the linear plot of the modified Curie–Weiss law and a good fit to the Vogel–Fulcher relation corroborates the relaxor nature. The relaxor ferroelectric is characterized by a spontaneous polarization which should promote the separation of electron/hole (e−/h+) pairs and favors the photocatalytic properties in the nanodomains. The electrochemical impedance spectroscopy, measured over the region (1 mH–105 Hz) at pH ~ 7, shows the predominance of the bulk contribution. This BTSN ferroelectric ceramic possesses attractive photoelectrochemical properties with an optical gap of 2.54 eV and a flat band potential of 0.45 V SCE . As application, the oxide was successfully tested for the photooxydation under solar light of amoxicillin (AMX), a currently used antibiotic. Indeed, the energy band diagram indicates an electron transfer from the conduction band to dissolved oxygen, forming O2 · radicals, responsible of the AMX degradation. The AMX concentration was followed by high performance liquid chromatography. A conversion of 92% is recorded in AMX solution (50 mg L−1) within 4 h under solar light and the kinetic obeys to a first order model with a rate constant of 7.73 × 10−3 min−1.

To assess its photocatalytic activity under sunlight, ZnO nano powder with crystallites sizes in the range (18–29 nm) was synthesized by decomposition from nitrate (ZnO-Nit) and acetate (ZnO-Ac) precursors by one-step calcinations. The powders were characterized by X-ray diffraction, transmission electron microscopy and Fourier-transform infrared spectroscopy. The optical band gap of ZnO-Ac, determined from the diffuse reflectance, was found to be 3.23 eV and the transition is directly allowed, due to the charge transfer O2−: 2p → Zn2+: 4s. The capacitance-potential (C−2 – E) characteristic plotted at pH ∼7 shows n type behaviour with a conductionband (−0.42 VSCE), made up of Zn2+: 4s orbital and positive of the O2/O2 level and a valence band (2.81 VSCE) deriving mostly from O2−: 2p orbital whose potential is less anodic than the OH/H2O level. So, ZnO was successfully used for the degradation of the Reactive Blue 19 (RB 19) a hazardous dye, by both radicals O2 −and OH under solar light. Indeed, RB 19 is a vinylsulphone dye, difficult to eliminate by chemical oxidation because its anthraquinone structure is highly stabilized by resonance. The best photoactivity occurs over ZnO-Ac owing to its large active surface area (59 m2 g−1). The oxidation obeys a zero order kinetic with a rate constant of 7.3 × 10−3 mol L−1 min−1. The elaborated material showed an outstanding recyclability.

La2NiO4 elaborated by nitrate route gives a positive result when used as a catalyst for the hydrogen photo-evolution. The prepared oxide crystallizes in the tetragonal K2NiF4 structure I4/mmm. The BET method was used to determine the specific surface area (4.91 m2/g); such value could suggest an interesting catalytic activity. Optical, electrical and photo-electrochemical properties have been studied to elaborate the energy diagram, a preamble for the photocatalysis. The band gap (Eg) has been evaluated at 1.51 eV and the transition is directly allowed. La2NiO4 is p-type semiconductor assigned to O2− intercalation in the two-dimensional crystal lattice; activation energy of 0.13 eV in the heating direction from the Arrhenius type law. The flat band potential Efb (−0.45 VSCE) and the density of the holes NA (1.13 × 1021 cm-3) were computed from the capacitance measurement. The conduction band illustrated in the energetic diagram for La2NiO4 (−1.77 VSCE) offers the possibility of H2 generation upon visible light. The best performance occurs at pH 13 in the presence of S2O3-3 as a hole scavenger. The H2 generation of the hetero-system (La2NiO4/SnO2) is significantly improved compared to La2NiO4. The higher photoactivity is ascribed to the electrons injection from the conduction band (La2NiO4-CB = −1.77 V/SCE), acting as generator and electrons pump, to (SnO2-CB = 0.35 V/SCE) inducing a better water reduction. The H2 evolution rate reaches 15.1 μmol g−1 min−1.

This work is devoted to the preparation and characterization of the spinel NiAl 2 O 4 prepared by nitrate route and its application for the hydrogen evolution under visible illumination. The X-ray diffraction revealed a cubic symmetry and a crystallite size of 40 nm, determined from the full width at half maximum. The oxide possesses a forbidden band of 1.84 eV and the transport properties indicate p -type conduction. The photo-electrochemical characterization gives a conduction band of −1.79 V SCE and the spinel is tested with success for the hydrogen formation under visible irradiation (29 mW cm −2 ). An evolution rate of 65 μL g −1 min −1 and a quantum yield of 0.15% are obtained within 30 min under optimal conditions (0.5 mg of catalyst/mL, pH ∼ 12 and 50 °C) in presence of SO 3 2− as holes scavenger.

CdS nanocrystalline thin films were synthesized by chemical bath deposition for different deposition times (10–75 min) and the semiconducting and photo-electrochemical properties were investigated. The results indicate that the deposition rate decreases with the deposition time, due to the depletion of Cd2+ and S2− ions near the glass subtract. The structure is characterized by X-ray diffraction; the films crystallize in a cubic blende structure with (111) as a preferential orientation and the crystallite size varies between 13 and 35 nm. The surface morphology, studied by atomic force microscopy, indicates that the growth process occurs through ion by ion mechanism because of the stability of the complex Cd(NH3)4 (~7), the small deposition rate corroborates this result. The UV visible spectra of the films indicate a high transparency (80%). The transmittance data analysis give a variable forbidden band (2.2–2.4 eV), closely related to the deposition time and agree with the disorder range (230–500 meV). The capacitance–voltage measurement exhibits a linear behavior, characteristic of n type conduction, from which a flat band potential of −0.92 VSCE is determined. The Nyquist plot shows a semicircle assigned to the bulk contribution (=2.5 Ω cm2); the center is located below the real axis (−10°), ascribed to a constant phase element (CPE). A fill factor (FF) of 0.37 and energy conversion of 0.38% are obtained under visible illumination. As application, eosin solution (15 ppm) i oxidized under visible light; the best activity is obtained at pH ~6.3. 80% of eosin disappears after 1 h of exposition and the kinetic obeys to a first order model with a rate constant of 4.5 × 10−2 min−1.

In this study, we are interested by the caesium-tin mixed salt of 1-vanado-11-molybdophosphoric acid (CsSnPMo11VO40) with Cs/Sn ~ 1, micro and mesoporous material with a large specific surface area and a good thermal stability. Caesium atoms in Cs3HPMo11VO40 are partially substituted by tin in order to facilitate the electron transfer in the redox processes. The synthesis and characterization by BET measurement, inductively coupled plasma (ICP), X-ray diffraction (XRD), 31P NMR, thermal analysis (TGA and DTA), UV–Vis and Raman spectroscopies, infrared technique of the Keggin-type compound CsSnPMo11VO40 are reported. The forbidden band (1.72 eV) is due to the crystal field splitting of Mo6+: 4d orbital in octahedral site and the compound exhibits a chemical stability at neutral pH and semiconducting properties due to molybdenum mixed valences. The intensity-potential (J-E) characteristic shows a small hysteresis while the capacitance measurement (C−2 - E) of the junction CsSnPMo11VO40/electrolyte in Na2SO4 electrolyte exhibits a rectifying behavior with a Schottky type barrier; the positive slope is characteristic of n type conduction with a flat band potential of −0.30 VSCE. The electrochemical impedance spectroscopy (EIS) shows two overlapped semicircles attributed to the bulk contribution (Rb=168 Ω cm2) and grains boundaries (Rgb=806 Ω cm2). The depletion angle (−16°) is ascribed to a constant phase element (CPE, 67 µF cm−2). The photosensitivity in the visible light involves d–d transition and the energy diagram shows the feasibility of the material for the HCrO4- reduction upon visible light (109 mW cm−2). The disappearance rate is found to be 4 mmol mn−1 (mg catalyst)−1 and the chromate reduction obeys to a first order kinetic with a half-life of 83 mn.

The substituted pyrochlore La 1.9 Ba 0.1 Sn 2 O 7 was prepared by nitrate route and the physical and electrochemical characterizations were investigated. The optical gap is found to be 3.01 eV and the conductivity is characteristic of a semiconducting behavior with activation energy of 0.42 eV. The conduction band of La 1.9 Ba 0.1 Sn 2 O 7 (−2.64 V SCE ), predicted from electro negativities of the constituent atoms, is close to that determined experimentally (−2.99 V SCE ). The capacitance measurement (C −2 –V) exhibits a linear behavior, characteristic of p type conductivity, from which a flat band potential of −0.05 V SCE and hole density N D of 8.42 × 10 14 cm −3 were determined, in agreement with the small electrical conductivity (σ 300K ∼3 × 10 −6 Ω −1 cm −1 ). The Nyquist plot showed a semicircle which is a contribution of capacitance and resistance in parallel. A bulk resistance contribution (R b = 71 kΩ cm 2 ) and a low density of surface states are observed. The center of the arc is localized below the real axis with a depletion angle of 14° ascribed to a constant phase element (CPE). As application, chromate was successfully converted to Cr 3+ on the pyrochlore under sunlight. ∼1 h was necessary to reach the adsorption equilibrium for an initial chromate concentration of 10 −4 M. The conduction band is more cathodic than the potential of HCrO 4 − which is spontaneously reduced under solar light. The best performance is obtained at pH ∼ 3 with a catalyst dose of 1 mg/mL. 40% of chromate disappears with a quantum yield of 0.53% in less than 2 h. The process obeys to a first order kinetic with an apparent rate constant of 2.3 × 10 −3 min −1 .

Cs1·48Sb0·78PMo11VO40 (CsSbPMoV) was prepared and characterized by many physico-chemical techniques. The spectroscopies 31P MAS-NMR, FT-IR) and X-ray diffraction showed that the salt is pure, of Keggin-type crystallizing in a cubic structure.The UV–Vis spectrum revealed the presence of molybdenum and antimony with mixed valence states, Mo(VI)/Mo(V) and Sb(V)/Sb(III). The optical properties showed two transitions: directly (1.20eV) and indirectly (1.38eV) allowed. The intensity-potential (J- E), plotted in Na2SO4 (0.1 ​M), exhibited a sluggish electrochemical system. The capacitance−2– potential (C−2 - E) curve showed n-type behavior with a flat band potential (Efb) of 0.10 ​V and a conduction band positioned at 4.65 ​eV, below vacuum (−0.1 VSCE), composed mostly of (Mo, Sb) 5s wave functions, negative of the •OH/H2O level. Therefore, CsSbPMoV was successfully tested for theoxidation of Orange II (OII), a hazardous dye, by •OH and O2•radicals under solar light. The discoloration obeys a first order kinetic model with an abatement yield of 68% and a photo catalytic half-life (t1/2) of 60 ​min.

Na2MnPO4F is synthesized by hydrothermal route at 453 K and the physical properties and photo-electrochemical characterizations are reported. The compound crystallizes in a monoclinic system (SG: P 21/n) with the lattice constants: a=13.7132 A, b=5.3461 A, c=13.7079 A, β=119.97°. The UV–visible spectroscopy shows an indirect optical transition at 2.68 eV; a further direct transition occurs at 3.70 eV, due to the charge transfer O2−: 2p → Mn2+: eg. The thermal variation of the electrical conductivity is characteristic of a semiconducting behavior with activation energy of 39 meV and an electron mobility (µ318 K=5.56×10−4 cm2 V−1 s−1), thermally activated. The flat band potential (+0.47 VSCE) indicates that the valence band derives mainly from O2−: 2p orbital with a small admixture of F− character while the conduction band is made up of Mn2+: t2g orbital. The electrochemical impedance spectroscopy shows the contribution of both the bulk and grains boundaries. The photocatalytic performance of Na2MnPO4F for the degradation of Rhodamine B (RhB) is demonstrated on the basis of the energy diagram. 88% of the initial concentration is degraded under UV light and the oxidation follows a first order kinetic with a rate constant of 0.516 h−1. Neither adsorption nor photolysis is observed. The photoactivity results from the electron transition from the hybridized band (O2−, F−) to the Mn2+: eg orbital, occurring in the UV region. The catalyst was subjected to three successive photocatalytic cycles, thus proving its long term stability.

The spinel ZnFe 2 O 4 prepared from nitrate precursors is used for the reduction of chromate under visible light using TiO 2 as mediator. A gap of 1.97 eV is obtained for ZnFe 2 O 4 from the diffuse reflectance and the crystallite size (70–130 nm) is determined from TEM measurement. The current–potential characteristic indicates a good electrochemical stability. The photo electrochemical properties permit the construction of the energy diagram of the hetero-system ZnFe 2 O 4 /TiO 2 /HCrO 4 − solution. The conduction band of the spinel (−1.47 V SCE ) favors the physical separation of the electron/hole pairs and the electrons injection into TiO 2 . The best composition ZnFe 2 O 4 (75%)/TiO 2 (25%) is used to catalyze the reaction (2 HCrO 4 − + 3 C 2 H 4 O 4 + 1.5 O 2 + 8 H + → 2 Cr 3+ + 6 CO 2 + 11 H 2 O). 60% of Cr(VI) are reduced after 3 h of visible light irradiation and the activity obeys a first order kinetic with a half-life of 70 min. The photocurrent–photovoltage characteristic of the spinel gives an optical-to-electrical energy efficiency of 0.28% and a fill factor of 0.49.

The photo-electrochemical properties of the libethenite Cu 2 (PO 4 )(OH) were studied to assess the oxidation of orange G (OG) under both solar and artificial lights. The compound was synthesized by hydrothermal route and characterized by X-ray diffraction, scanning electron microscopy, thermal and chemical analyses, UV–vis diffuse reflectance and electrochemical impedance spectroscopy. The libethenite possesses an optical gap of 1.52 eV and exhibits n type semiconducting behavior with an activation energy of 20 meV. On the basis of photo-electrochemical properties, it was concluded that the conduction band of the material (=4.81 eV/+0.30 V RHE ) permits the formation of O 2 ·− radicals for the OG oxidation. A maximum elimination of 94% was obtained upon sunlight within 120 min exposure. The injection process of orange G into the conduction band of the libethenite is confirmed by the photocurrent measurements at different concentrations. The decolorization process proceeds like a photo-Fenton reaction, based on copper ions. Referring to the energetic diagram, a degradation mechanism is proposed. Cu 2 PO 4 (OH) catalyst permits to activate H 2 O 2 and initiate the decomposition of OG molecules. The OG removal follows a pseudo-first order kinetic with an apparent rate constant of 6.2×10 −3 min −1 under sunlight.

New niobate Ca0.5Nb3O8.1.5H2O was elaborated by soft chemistry via Ca2+ → H+ exchange between calcium nitrate and niobic acid H3ONb3O8. The compound was identified by X-ray diffraction using the Rietveld refinement; it was characterized by thermal analysis (TG), Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. The layered compound resembles closely to the orthorhombic niobate KNb3O8. Two optical transitions indirectly (3.10 eV) and directly (3.34 eV) allowed were obtained from the diffuse reflectance. The photo-electrochemical properties are studied for the first time. The oxide exhibits n type conductivity with a flat band potential (Efb) of -0.12 VSCE and an electrons density (ND) of 6.51 × 1015 cm−3. The valence band formed from Nb5+: 4d orbital, is positioned at 7.59 eV below vacuum (2.84 VSCE). As application, Rhodamine B (Rh B, 10 ppm) a hazardous dye was successfully oxidized on Ca0.5Nb3O8.1.5H2O by •OH radicals under solar light; a conversion rate of 48% was obtained after 140 min. and the Rh B discoloration follows a first order kinetic with a half-life of 150 min.

The photoelectrochemistry of the hetero-junction Ag/NiFe2O4, elaborated by sol gel route, was successfully used for the nitrite oxidation under visible light illumination. The electrons of NiFe2O4 activated by visible light are transferred toward Ag particles, strongly contacted to NiFe2O4. The hetero-junction is generated by incorporating ultrafine Ag clusters, thereby precluding high over-voltages. The spinel was studied by thermal analysis, X-ray diffraction and scanning electron microscopy. The optical band gap of NiFe2O4 (1.63 eV) is assigned to the lifting of degeneracy of the Fe3+: 3d orbital by oxygen crystal field splitting in octahedral site. p-type conduction of NiFe2O4 was shown from the (capacitance−2 – potential)characteristic in KOH electrolyte (0.1 M). The valence band (+0.70 V) is more anodic than the potential of NO2− oxidation and the hetero-junction was successfully tested for the conversion of NO2− to NO3− under visible light irradiation. NO2− is a hazardous ion and the photocatalytic performance is due to the optimal band bending. The rate conversion {0.085 μmol NO2−(g catalyst)−1 mn−1} was quantified indirectly through the evolved H2 volume.

The spinel ZnFe2O4 prepared by nitrate route is used as dispersed photons collector capable to sensitize TiO2 under visible light and to reduce Cr(VI) into trivalent state. The transport properties, optical and photo-electrochemical characterizations are correlated, to build the energetic diagram of the hetero-system ZnFe2O4/TiO2/CrO4 − solution. A gap of 1.97 eV is obtained for the spinel from the diffuse reflectance. The conduction band of ZnFe2O4 (−1.47 V SCE) favors the electrons injection into TiO2, permitting a physical separation of the charge carriers. The oxidation of oxalic acid by photoholes prevents the corrosion of the spinel. The best configuration ZnFe2O4 (75 %)/TiO2 (25 %) is used to catalyze the downhill reaction (2HCrO4 − + 3C2H4O4 + 1.5O2 + 8H+ → 2Cr3+ + 6CO2 + 11 H2O, ∆G° = −557 kcal mol−1). 60 % of Cr(VI) are reduced after 3 h of visible light illumination and the photoactivity follows a first-order kinetic with a half-life of 70 min. The water reduction competes with the HCrO4 − reduction which is the reason in the regression of the photoactivity; a hydrogen evolution rate of 24 µmol mg−1 h−1 is obtained.

The continuous solid solution NiFe2−xMnxO4 (0 ≤ x ≤ 2) prepared by sol–gel is successfully tested for the hydrogen production under visible light. All spinels crystallize in a cubic structure with a particle size ranging from 30 to 63 nm. They exhibit p-type conductivity with an average thermopower of ∼135 μV K−1, a hole mobility of ∼9 × 10−5 cm2 V−1 s−1 and an activation energies in the range (0.12–0.33 eV). The optical transition of the most active photocatalyst NiFe1.6Mn0.4O4 (1.53 eV) is well matched to the solar spectrum. The chemical and physical properties are correlated to assess the feasibility of the spinels for the hydrogen liberation. The conduction band (−1.54 VSCE) is located below the H2O/H2 potential (∼−0.8 VSCE), allowing H2-evolution under visible light. The spinel dose, pH and S2O32− concentration are optimized. Under the ideal conditions, the volume of H2 liberation reaches 124 μmol at saturation after 20 min of irradiation.

Ba2SnO4 is synthesized by nitrate route, and the physical and photoelectrochemical properties are investigated. The oxide crystallizes in the perovskite structure with a quadratic symmetry. UV-visible spectroscopy shows an optical transparency over the visible region with a direct transition of 3.18 eV. The magnetic susceptibility (~2.2 × 10−5 emu cgs mol−1) is in conformity with collective electrons. The thermal variation of the conductivity indicates a conduction mechanism by adiabatic polaron hopping σ = σ o exp (1.5 meV kT−1) with an electron mobility (μ 300K) of 5.43 × 10−4 cm2 V−1 s−1. The sign of hole-like small polarons is that of n-type carriers originating from oxygen off-stochiometry. The thermal variation of the thermo power indicates a finite density of state at the Fermi level. A pronounced dip, associated with a phonon drag contribution, is observed near 30 K. The nonlinear dependence of log σ vs. T −1 is due to the transition to variable range hopping at low temperature. The Mott-Schottky characteristic in KOH medium confirms the n-type conductivity and gives a flat band potential of −0.91 VSCE and electron density of 1.50 × 1018 cm−3. The Nyquist plot shows the contribution of both bulk and grain boundaries whereas the straight line at 45° at low frequencies corresponds to the Warburg diffusion. The data are modeled to an equivalent circuit with a constant phase element (CPE).

We have investigated the semiconducting and photoelectrochemical properties of SnO films grown potentiostatically on tin substrate. The oxide is characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The anodic process corresponds to the formation of SnO· n H 2 O pre-passive layer that is removed upon increasing potential due to surface etching at the metal/oxide interface. SnO films deposited for long durations (>50 mn) are uniform and well adhered; they thicken up to ~50 nm by diffusion-controlled process and the growth follows a direct logarithmic law. The thickness is determined by coulometry and the X-ray diffraction indicates the tetragonal SnO phase (SG: P4/mmm) with a crystallite size of 32 nm. The Mott–Schottky plot is characteristic of n type conductivity with an electrons density of 5.72×10 18 cm −3 , a flat band potential of −0.09 V SCE and a depletion width of ~10 nm. The valence band, located at 5.91 eV below, vacuum is made up of hybridized O 2− :2 p Sn 2+ :5 s while the conduction band (4.45 eV) derives from Sn 2+ :5 p orbital. The electrochemical impedance spectroscopy (EIS) measured in the range (10 −2 –10 5 Hz) shows the contribution of the bulk and grain boundaries. The energy band diagram predicts the photodegradation of methylene blue on SnO films. 67% of the initial concentration (10 mg L −1 ) disappears after 3 h of exposure to visible light (9 mW cm −2 ) with a quantum yield of 0.072.

The perovskite BaSn0.95Co0.05O3−δ, prepared by nitrate route, crystallizes in a cubic symmetry with a slight isotropic contraction of the host lattice. The oxygen crystal field splits the Co3+: 3d orbital by a value of 2.02 eV with a low spin configuration of Co3+ ( t 2g 6 e g 0 ). The average magnetic susceptibility (~2×10−5 emu cgs mol−1) is temperature independent, in conformity with itinerant electrons behavior. The sample is nominally non-stochiometric and the conductivity follows a thermally activated hopping of lattice polaron with an activation energy of 19 meV. The thermal variation of the thermo-power suggests a finite density of states at the Fermi level. The zero temperature conductivity is finite and the charge carriers in localized states are thermally excited into extended states above the mobility edge. A variable range hopping is predicted at low temperature from the non-linearity of ln(σ) versus T−1 plot while the hump observed near 25 K is ascribed to the phonon drag. The photo-electrochemical characterization in KOH medium indicates p type behavior with a flat band of −0.21VSCE and a donor density of 6.95×1020 cm−3. The electrochemical impedance spectroscopy is undertaken and the response is modeled by an equivalent circuit with the predominance of the bulk contribution. The depressed semicircle is attributed to the faradic charge transfer with a constant phase element (CPE) while the straight line at low frequencies is due to the Warburg diffusion.

The chromate reduction is successfully achieved over the new heterosystem La2CuO4/SnO2 under solar light. La2CuO4 is a narrow band gap semiconductor with a direct optical transition of 1.27 eV. Its photoelectrochemical characterization is undertaken to predict the electrons injection in SnO2, the latter acts as electrons bridge to adsorbed HCrO4−. The Mott-Schottky plot of La2CuO4 is characteristic of p-type conductivity with a flat band potential of −0.46 VSCE. Hence, the conduction band (−1.62 VSCE) is cathodic enough to reduce HCrO4− into Cr3+. The chrono-potentiometry shows that the adsorption is reached after ∼ 40 min. and averages 15% for an initial concentration of 30 mg/L at pH = 4. The photoactivity is strongly enhanced in presence of oxalic acid as holes scavenger, which hinders the recombination of electron/hole pairs. A complete reduction (30 mg/L) occurs in less than 2 h under solar irradiation. The kinetic of the chromate reduction is well described by the Langmuir-Hinshelwood model with an apparent rate constant of 0.23 mn−1. The saturation beyond 75 min is due to the competitive water reduction which inhibits considerably the chromate reduction. The reutilization of the heterosystem shows a negligible deactivation effect and the heterosystem could be promising for the water depollution. © 2014 American Institute of Chemical Engineers Environ Prog, 2014 34: 744–750, 2015

The transport properties of the delafossite CuAlO2 single crystal, grown by the flux method, are confined in ∞[AlO2] layers extending in the (001) plans. The dielectric properties are measured up to 490 K in the frequency range (102–105 Hz). The small variation of the dielectric loss tan(δ) is attributed to the wide space charge region. The linear plot log (conductivity) vs. 1000/T follows an Arrhenius type law and the results are discussed in terms of electron hopping among localized states. The activation energy (0.18 eV) gives an effective mass of 16m0 indicating that the levels in the vicinity of the Fermi level are strongly localized. Hence, the increase of the conductivity (σ) results from a thermal activation of the mobility (μ300 K = 1.2 × 10−5 cm−2 V−1 s−1). The sign of hole like small polarons is that of p type carriers originating from oxygen intercalation. The thermopower is little temperature dependent and characteristic of non degenerate conductivity with a low holes concentration and a large concentration of surface states within the gap region.

The metal organic framework (MOF) Cu3PO4(C2N3H2)2OH, synthesized by hydrothermal route, is characterized physically and photoelectrochemically. The compound crystallizes in a monoclinic symmetry with a layered structure. UV–visible spectroscopy shows an optical transition of 2.58 eV. The thermal variation of the electrical conductivity indicates a semiconducting behavior with a conduction mechanism dominated by small polaron hopping: σ=σ0 exp {0.44 eV/kT} and electron mobility (µ300 K=0.3 cm2 V−1 s−1) thermally activated. The interfacial capacitance at pH ~7 is characteristic of n type behavior with a flat band potential of +0.40 VSCE, an electrons density of 2×1015 cm−3 and a space charge region of 10 µm. The semicircle at high frequencies in the Nyquist plot is not centered on the real axis and the depletion angle (17°) is due to a constant phase element. The impedance data at low frequencies indicate a diffusion-controlled process. As application, the photochemical oxygen evolution is successfully achieved under visible light with a liberation rate of 0.70 cm3 min−1 g−1 and a quantum yield of 0.13%. The reaction ( 2 SO 3 2 − +H2O→ S 2 O 3 2 − +O2+2OH, ΔG0=90.68 kcal mol−1) can be evaluated for the chemical storage.

The semiconducting properties of the perovskite LaCoO3, prepared by nitrate route, are investigated for the first time by the photo-electrochemical technique. The oxide shows a direct optical transition at 1.33 eV, due to Co3+: 3d orbital splitting in octahedral site and possesses a chemical stability over a fair pH range (4–14). The conductivity follows an exponential type law with a hole mobility (8.3 × 10−2 cm2 V−1 s−1), thermally activated. The Mott–Schottky plot in KOH medium is characteristic of p type conduction with a flat band potential of 0 VSCE and a holes density of 1.35 × 1017 cm−3. The electrochemical impedance spectroscopy reveals the predominance of the bulk and grains boundaries contributions with a constant phase element and a multi-relaxation type nature. As application, the hydrogen evolution upon visible light is demonstrated on the hetero-junction LaCoO3/SnO2. The best performance occurs at pH ∼ 12.8 with an evolution rate of 0.25 cm3 min−1 (mg LaCoO3)−1 and a quantum yield of 0.11%. The improved activity is attributed to the wide depletion width of ∼10 nm and the potential of the conduction band of LaCoO3 (−1.34 VSCE), more negative than that of SnO2, the latter acts as electrons bridge for the interfacial water reduction. The relevance of 3d orbital of the performance of semi conducting photoelectrode is discussed.

The influence of the thickness on the optical, structural and morphological properties of CdS thin films elaborated by chemical bath deposition is studied. The deposition time varies between 15 and 75 min and the optical measurements show that the films have high optical transmission (85%). The band gap presents a maximum of 2.44 eV for a deposition time of 30 min which corresponds to a lower disorder. The deposition rate ( n type conductivity with a flat band potential of −0.95 V SCE and an electron density of 1.43×10 25 m −3 . The Nyquist plot shows the predominant contribution of the grain boundaries; the data are modeled to an equivalent circuit with a constant phase element. As application, orange II is oxidized via radicals and the best photoactivity is obtained at pH~7. 68% of orange II disappears in ~1 h under solar light (AM 1). The oxidation follows a zero order kinetic with a rate constant of 0.11 mol L −1 min −1 .