Synthesis and characterization of the semiconductor Li0.93Cu0.07Nb3O8: Application to oxygen evolution under visible light

The semiconductor Li_0.93Cu_0.07Nb₃O₈ is prepared by soft chemistry in aqueous electrolyte via Cu²⁺ → Li⁺ exchange between copper nitrate and LiNb₃O₈. The substituted niobate crystallizes in an orthorhombic symmetry and the semiconducting and photo-electrochemical properties are investigated for the first time. The oxide exhibits a dark brown color and the UV–Visible spectroscopy gives an optical gap of 1.42 eV, due to the crystal field splitting of Cu²⁺ in octahedral site. The thermal variation of the conductivity shows that Nb: 4d-electrons are localized and the data are fitted by a small-polaron hopping model σ = σ_o exp {−0.053 eV/kT} with a carrier density thermally activated. The capacitance measurement done in ionic electrolyte (Na₂SO₄, 10⁻² M) indicates n type semiconductor with mixed valences Nb^5+/4+, due to the hetero-valent substitution Li⁺/Cu²⁺, with a flat band potential of 0.28 V_SCE and electrons density of 2.17×10¹⁷ cm⁻³. The Nyquist diagram shows mainly the bulk contribution with a diffusion process. The valence band (6.39 eV below vacuum) derives from O^2-: 2p orbital with a small admixture of Cu²⁺: 3d character while the conduction band is made up of Nb⁵⁺: 4d orbital. The material is successfully tested for the oxygen generation with an evolution rate of 87 µmol mn⁻¹ g⁻¹ under visible light (29 mW cm⁻²) and a quantum yield of 0.35%.     

Synthesis and semiconducting properties of Na2MnPO4F. Application to degradation of Rhodamine B under UV-light

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 Å, b=5.3461 Å, c=13.7079 Å, 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⁺: e_g. The thermal variation of the electrical conductivity is characteristic of a semiconducting behavior with activation energy of 39 meV and an electron mobility (µ_318 K=5.56×10⁻⁴ cm² V⁻¹ s⁻¹), thermally activated. The flat band potential (+0.47 V_SCE) 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⁺: t_2g 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⁺: e_g orbital, occurring in the UV region. The catalyst was subjected to three successive photocatalytic cycles, thus proving its long term stability.     

Electrochemical growth of tin(II) oxide films: Application in photocatalytic degradation of methylene blue

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·nH₂O 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 5.72×10¹⁸ 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 O²⁻:2p Sn²⁺:5s while the conduction band (4.45 eV) derives from Sn²⁺:5p orbital. The electrochemical impedance spectroscopy (EIS) measured in the range (10⁻²–10⁵ 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.     

Preparation and characterization of the semiconductor CuMnO2 by sol-gel route

Nano crystallites of the crednerite CuMnO₂ are prepared by sol–gel method with two-step annealing process. The powder heated at 450 °C under air flow shows a mixture of CuO, Mn₂O₃ and Cu_xMn_3−xO₄. However, when calcined at 900 °C under N₂ atmosphere, the crednerite CuMnO₂ with a monoclinic structure (space group: C2/m) is obtained. The Raman spectrum shows a single peak at 679 cm⁻¹ assigned to A_1g mode whereas the infrared analysis confirms the linearity of CuO₂³⁻ units. The optical transition at 1.70 eV, determined from the diffuse reflectance is attributed to the inter-band d-d transition of Cu⁺ ion. The oxide exhibits semiconducting properties with an activation energy of 0.21 eV. The photo-electrochemical measurement shows p-type conduction due to O²⁻ insertion in the two dimensional lattice. The flat band potential (+0.12 V_SCE), indicates a cationic character of both valence and conduction bands deriving from Cu⁺: 3d orbital.     

Semiconducting properties of hydrothermally synthesized libethenite application to orange G photodegradation

The photo-electrochemical properties of the libethenite Cu₂(PO₄)(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 O₂^·− 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. Cu₂PO₄(OH) catalyst permits to activate H₂O₂ 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.     

Sulfur stoichiometry driven chalcopyrite and pyrite structure of spray pyrolyzed Cu-alloyed FeS2 thin films

We report on fabrication of Cu_xFe_1−xS₂ (CFS) thin films using chemical spray pyrolysis followed by post-sulfurization. Post-sulfurized CFS films were grown with compact and good crystalline texture. The sulfur stoichiometry in CFS films was found to be crucial for determination of its crystal structure. The sulfur deficient CFS films were driven to chalcopyrite CFS (CH-CFS) structure whereas the sulfur cured CFS films were grown with Cu-incorporated pyrite CFS (P-CFS) structure which was confirmed by X-ray diffraction and Raman spectroscopy analysis along with UV–vis spectroscopy measurement. Electrical characterizations of both types of CFS films revealed p-type conductivity with carrier concentration in the range of 10¹⁸–10²⁰ cm⁻³ and mobility of 0.5–9 cm² V⁻¹ s⁻¹. The band gaps of CFS films of CH-CFS structure (0.885–0.949 eV) were found to be less than that of P-CFS structure (0.966–1.156 eV), which indicates its potential application for thermoelectric and photovoltaic devices.     

Preparation and characterization of the brownmillerite Sr2Co2O5 as novel photocatalyst in the hydrogen generation

Sr₂Co₂O₅ is a semiconductor belonging to the brownmillerite family; it is prepared by nitrate route and the photo-electrochemical properties are assessed for the first time for the photocatalytic hydrogen production. Thermal analysis indicates the formation of the semiconductor phase at 750 °C. An optical transition at 1.10 eV, directly allowed is obtained from the diffuse reflectance spectrum, due to the internal Co³⁺: d-d transition in octahedral coordination. A flat band potential of 0.037 V_SCE is determined in KOH solution (0.1 M) from the Mott-Schottky characteristic and the results are relevant for the water reduction. The conduction band of Sr₂Co₂O₅ (−0.85 V_SCE), deriving from Co³⁺: 3d orbital is more cathodic than the potential of H₂O/H₂ couple and hydrogen is successfully evolved under visible light. A rate evolution of 68 µmol (g catalyst)⁻¹ min⁻¹ at pH ∼ 12 and a light-to-chemical energy efficiency of 0.82% are determined.     

A study on phase transformation of SnOx thin films prepared by reactive magnetron sputtering

In the paper, SnO_x thin films were deposited by reactive magnetron sputtering from a tin target in O₂ containing working gas. The evolution from Sn-containing SnO to tetravalent SnO₂ films was investigated. The films could be classified into three groups according to their optical band gaps, which are E_g<2.5 eV, E_g=3.0–3.3 eV and E_g>3.7 eV. The electric measurements show that high conductivity can be obtained much easier in SnO₂ than in SnO films. A high electron mobility of 15.7 cm² V⁻¹ s⁻¹, a carrier concentration of 1.43×10²⁰ cm⁻³ and a resistivity of 2.8×10⁻³ Ω cm have been achieved in amorphous SnO₂ films. Films with the optical band gap of 3.0–3.3 eV remain amorphous though the substrate temperature is as high as 300 °C, which implies that °btaining high mobility in p-type SnO is more challenging in contrast to n-type SnO₂ films.     

Theoretical insight into the electronic and photocatalytic properties of Cu2O from a hybrid density functional theory

A comprehensive first-principle investigation, based on hybrid density functional theory, produces strong evidence that the Cu₂O band-edges do satisfy the requirements of the H⁺/H₂ and O₂/H₂O redox levels, demonstrating that it has enough driving force for photocatalytic overall water splitting. The calculated band gap of Cu₂O is 2.184 eV, which is consistent with the experimental value of 2.17 eV. The highly dispersive s–s hybrid states at the conduction band bottom result in a small effective mass of the electron, which is favorable to carrier separation and the carrier transfer to surface, and thus facilitate the reduction of H⁺ to H₂. The strong optical absorption of Cu₂O is beneficial to overall water splitting under visible light irradiation. Possible reasons for no observation of H₂ in some experiments are also discussed. The results address the ongoing controversy associated with photocatalytic overall water splitting of Cu₂O.     

Up-scalable synthesis of size-controlled copper ferrite nanocrystals by thermal treatment method

Close-packed cubic copper ferrites (CuFe₂O₄) nanoparticles were synthesized using an effective thermal-treatment method directly from an aqueous solution containing copper and iron nitrates as metal precursors and poly(vinyl pyrrolidone) as a capping agent. The FTIR spectra of the calcined samples revealed the vibration bands of Fe–O and Cu–O at 315 and 535 nm respectively. The structural, morphological, optical and magnetic properties of the nanocrystal powder samples were analyzed using various characterization techniques. The powder X-ray diffraction unveiled the formation of spinel phase of CuFe₂O₄ with the average particle size determined from TEM images increased from 24 to 34 nm at the calcination temperatures between 773 and 1173 K. The band gap calculated using Kubelka–Munk function from the UV–visible diffuse reflectance spectra decreased from 2.64 to 2.45 eV with increasing calcination temperature. The electron spin resonance (ESR) spectroscopy confirmed the presence of unpaired electrons in the calcined samples. The g-factor increased from 2.10497 to 2.57056 and the resonance magnetic field decreased from 3.11599×10⁻⁷ to 2.55161×10⁻⁷ A/m with increasing calcination temperature.     

Red luminescence of spherical CuxZn1−xS semiconductor nanoparticles using 2-mercaptoethanol as capping agent

Cu²⁺-doped (0–2 at%) ZnS nanoparticles stabilized by 2-mercaptoethanol (2-ME) were successfully prepared using wet precipitation route in aqueous solution. The structural and optical characteristics were studied by various techniques. XRD pattern showed zinc blende cubic structure of Cu²⁺-doped ZnS with grain size of 4±0.5 nm. Spherical shape and well distribution of particles is confirmed by TEM, SEM and STM microscopy. Copper doping were identified by elemental dispersive (EDS) spectrometry. UV–vis spectroscopy revealed strong confinement effect due to blue shift in absorption shoulder peak as compared to bulk ZnS. Red luminescence band at～657 nm on Cu²⁺ doping may be arising from recombination of electrons at sulfur vacancies (V_s) and Cu(t₂) states formed at ZnS band gap. Optimum concentration of 0.25 at% (red band) of Cu²⁺ doping was selected by the observed enhanced PL emission.     

Magnetic,optical, microscopic and electrical behavior of Ca2−xYxCo2O5 prepared by a molten flux method

In this present study, we have reported the preparation of yttrium doped polycrystalline Ca_2−xY_xCo₂O₅ (x=0.0–1.0) material by a molten flux method and its various properties like electrical, optical, dielectric and magnetic behaviors. Characterization techniques have been adopted to confirm its physical nature and properties. X-ray diffraction results confirmed the crystal structure of prepared Ca_2−xY_xCo₂O₅ as orthorhombic and the scanning electron microscope pictured the presence of platelet-shaped particles with the dimensions of 150–300 nm. It also reveals the state of higher concentration of yttrium (Y³⁺) controls the grain size of Ca_2−xY_xCo₂O₅ ceramics. Further, we find out that the higher concentration of yttrium (Y³⁺) increases the optical band gap due to the occurrence of metal–insulator transition and also the same in electrical resistivity from 0.2 mΩ cm to 0.5 mΩ cm, which is due to the replacement of holes by Y³⁺ ions. The result of dielectric studies proves that the conduction mechanism of yttrium doped calcium cobalt oxide is due to space charge polarization. The magnetic saturation behavior shows the decreasing area in the hysteresis curve while the Y³⁺ concentration is increased, which is due to the phase transition of ferromagnetic to paramagnetic.     

Phase transition and surface morphology effects on optical,electrical and lithiation/delithiation behavior of nanostructured Ce-doped V2O5 thin films

Cerium doped V₂O₅ thin films were prepared by the sol−gel process. X-ray diffraction analysis revealed the phase transition from α-V₂O₅ orthorhombic to β-V₂O₅ tetragonal structure by annealing at 400 °C. The SEM and AFM images revealed that annealing temperature changed the surface morphology of the V₂O₅ films from fiber like wrinkle network to elongated sheets. Also, the particle shape was significantly influenced by Ce doping and a nanorod-like morphology was formed at 1.5 mol% Ce−doped V₂O₅. Power spectral density analysis indicated that surface roughness and fractal dimension of β−V₂O₅ increase by Ce doping. Optical measurement showed that the band gap narrowing (from 2.68 to 2.28 eV) occurred when the annealing temperature and dopant concentration increased. The variation of activation energy of the films was explained based on the small polaron hopping mechanism. The α−V₂O₅ film showed enhanced lithium−ion storage capacity compared to pristine β−V₂O₅ film and 1 mol% Ce−doped α−V₂O₅ thin film revealed the best ion storage capacity (Q_a=207.19 mC/cm², I_max=4.13 mA/cm² at scan rate of ν=20 mV/s).     

Mixing Y(RE)BCO (RE=Nd,Sm) superconductors by crystal pulling method

Y_1−xRE_xBa₂Cu₃O_7−δ [Y(RE)BCO or REsBCO] superconductors were prepared by the crystal pulling method. The RE mixing content x in the crystal can be effectively controlled by the processing temperature. With an increase in processing temperature, the RE concentration in the liquid increased, which resulted in the higher RE substitution content in single crystals. The higher critical temperatures T_c above 92 K could be obtained by an optimized oxygenation treatment. The T_c value of Y(RE)BCO tends to be insensitive to the growth atmosphere of the oxygen partial pressure under a certain limit of the RE content. The Y_.727Nd_.273Ba₂Cu₃O_7−δ and the Y_.941Sm_.059Ba₂Cu₃O_7−δ samples show values of the critical current density J_c are about 2×10⁴ Acm⁻² at 1.2 T and 2.8×10⁴ Acm⁻² at 1.1 T for H//c, respectively, indicating that mixing REs 123 have obvious effects on J_c–H curves since RE ions are likely to substitute at both Y²⁺ and Ba²⁺ sites.     

Model based precise analysis of the injection currents in Al/ZrO2/Al2O3/ZrO2/SiO2/Si structures for use in charge trapping non-volatile memory devices

Metal/insulator/Silicon (MIS) capacitors containing multilayered ZrO₂/Al₂O₃/ZrO₂/SiO₂ dielectric were investigated in order to evaluate the possibility of their application in charge trapping non-volatile memory devices. The ZrO₂/Al₂O₃/ZrO₂ stacks were deposited by reactive rf magnetron sputtering on 2.4 nm thick SiO₂ thermally grown on p-type Si substrate. C–V characteristics at room temperature and I–V characteristics recorded at temperatures ranging from 297 K to 393 K were analyzed by a comprehensive model previously developed. It has been found that Poole-Frenkel conduction in ZrO₂ layers occurs via traps energetically located at 0.86 eV and 1.39 eV below the bottom of the conduction band. These levels are identified as the first two oxygen vacancies related levels in ZrO₂, closest to its conduction band edge, whose theoretical values reported in literature are: 0.80 eV, for fourfold, and 1.23 eV, for threefold coordinated oxygen vacancies.     

Structure,optical property,and photo-catalytic activity of solid-solution β-AgAl1−xGaxO2 under visible light

Visible-light responsive β-AgAl_1−xGa_xO₂ photocatalysts were prepared through a sol-gel method combined with a cation exchange reaction. The samples were characterized by X-ray diffraction, scanning electron microscopy, UV–vis absorption spectrum, and photoluminescence spectrum. The results demonstrated that the solid-solution β-AgAl_1-xGa_xO₂ with delafossite structure showed high crystallinity. The microparticles with smooth surface were polycrystal and had polyhedron crystal morphologies. The optical band gap of the β-AgAl_1−xGa_xO₂ was modulated in the range of 2.31–2.70 eV; meanwhile the photoluminescence spectra showed a red shift by varying the molar ratio of Ga/Al from 0.25 to 4 attributed to the decrease of band gap. The photo-catalytic degradation of methyl orange was studied under visible light irradiation (wavelength>420 nm). The degradation process followed the first-order kinetics. The β-AgAl_0.2Ga_0.8O₂ with the reaction rate of 0.0402 min⁻¹ showed the highest photo-catalytic activity. This might be resulted from the narrow band gap of 2.31 eV for the β-AgAl_0.2Ga_0.8O₂ which could utilize visible light efficiently to photo-generate electrons and holes. The β-AgAl_1−xGa_xO₂ had potential applications in degrading organic dyes by using visible light efficiently.     

2,2-Dicyanovinyl-end-capped oligothiophenes as electron acceptor in solution processed bulk-heterojunction organic solar cells

Three 2,2-dicyanovinyl (DCV) end-capped A-π-D-π-A type oligothiophenes (DCV-OTs) containing dithieno[3,2-b:2′,3′-d]silole (DTSi), cyclopenta[1,2-b:3,4-b′]dithiophene (DTCP) or dithieno[3,2-b:2′,3′-d]pyrrole (DTPy) unit as the central donor part, mono-thiophene as the π-conjugation bridge were synthesized. The absorption spectroscopies, cyclic voltammetry of these compounds were characterized. Results showed that all these compounds have intensive absorption band over 500–680 nm with a LUMO energy level around −3.80 eV, which is slightly higher than that of [6,6]phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM, E_LUMO = −4.01 eV), but lower than that of poly(3-hexylthiophene) (P3HT, E_LUMO = −2.91 eV). Solution processed bulk heterojunction “all-thiophene” solar cells using P3HT as electron donor and the above mentioned oligothiophenes as electron acceptor were fabricated and tested. The highest power conversion efficiency (PCE) of 1.31% was achieved for DTSi-cored compound DTSi(THDCV)₂, whereas PTB7:DTSi(THDCV)₂ based device showed slightly higher PCE of 1.56%. Electron mobilities of these three compounds were measured to be around 10⁻⁵ cm² V⁻¹ s⁻¹ by space charge limited current method, which is much lower than that of PC₆₁BM, and was considered as one of the reason for the low photovoltaic performance.     

Defect characterization of Tl4GaIn3Se2S6 layered single crystals by photoluminescence

Photoluminescence (PL) spectra of Tl₄GaIn₃Se₂S₆ layered crystals grown by the Bridgman method have been studied in the energy region of 2.02–2.35 eV and in the temperature range of 16–45 K. A broad PL band centered at 2.20 eV was observed at T=16 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.1 to 149.9 mW cm⁻² range. Radiative transitions from shallow donor level located at 10 meV below the bottom of conduction band to moderately deep acceptor level located at 180 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of present work and previously reported paper on thermally stimulated current measurements carried out below room temperature. Analysis of the transmission and reflection measurements performed in the wavelength range of 400–1030 nm at room temperature revealed the presence of indirect transitions with 2.22 eV band gap energy.     

Structural,optical and transport properties of SxZnO

In this paper, S-doped ZnO (S_xZnO) was prepared using sol-gel method at different S amounts. The structural, optical and transport properties were investigated. The introduction of S atoms into the ZnO network was found to lower the crystallization level which results in reducing the crystallite size up to x=0.3. The doping process is confirmed by the observed peak at ~610 cm⁻¹ in the ATR spectrum related to the Zn-S linking. EDX mapping shows a homogeneous distribution of S atoms on the particles surface. The best compromise between the band gap (Eg=2.96 eV), the charge carriers (N_A=2.139×10²² cm⁻³), the conductivity (σ=5.56×10⁻⁴ Ω⁻¹ m⁻¹) and the mobility (µ=16.26×10⁻¹⁴ m² V⁻¹ s⁻¹) is obtained for x=0.1. The conduction mechanism is assumed by small hopping polaron. The S-doping has impacted positively the photocatalytic activity of ZnO, with particularly high performance for S_0.2ZnO.     

The effect of external stress on the properties of AlGaAs/GaAs single quantum well laser diodes

The change of spectrum of the AlGaAs/GaAs single quantum well laser diode is measured under the application of uniform uniaxial in-plane tensile and compressive stress. In the range of the tensile stress we apply (up to 597 MPa), the wavelength increases linearly at a rate of 5.3 nm GPa⁻¹. The energy band gap decreases with the tensile stress with the slope of −10 meV GPa⁻¹, which is close to the theoretical change of the heavy hole band edge with respect to the conduction band edge. There is a shorter wavelength peak existing on the spectrum as the tensile stress increases, suggesting a transition from the conduction band to a higher energy valence band. For the compressive stress (up to −516 MPa), the wavelength decreases with the stress, but it shows an abrupt reduction from −162 to −200 MPa. The threshold current also varies as a result of the change of the energy band structure.