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.     

Fabrication of polymer blend composites based on [PVA-PVP](1−x):(Ag2S)x (0.01 ≤ x ≤ 0.03) with small optical band gaps: Structural and optical properties

In this paper composite materials, based on polymer blends of polyvinyl alcohol (PVA): polyvinyl pyrrolidone (PVP) with small optical band gap, has been studied. Silver sulfide (Ag₂S) semiconductor particles have been synthesized in PVA:PVP blend host polymer, using in situ method. X-ray diffraction (XRD) analyses and Fourier transform infrared (FTIR) spectroscopy for the composite samples were carried out. From the XRD pattern, distinguishable crystalline peaks caused by the Ag₂S semiconductor particles were observed. From the result of FTIR spectroscopy, the intensity of the FTIR bands were shifted and increased, revealing the occurrence of interactions between the PVA:PVP blend system and Ag₂S particles. The composite samples were found to exhibit absorption spectra that cover UV–visible to near infrared regions. The absorption edge was found to be 5 eV for pure PVA:PVP system and shifted to 1.15 eV for incorporated PVA:PVP with 3 M of Ag₂S. The refractive index was also evaluated for the samples and observed to be increased from 1.15 to 1.52 as doping increased to the highest. A linear relationship between the refractive index and the filler fraction has been reported. Theoretical discussion of optical dielectric loss, which is a crucial parameter for the band gap estimation, was given. The achieved results reveal that spectra of the optical dielectric loss (ɛ_i) can be used to study the band gap structure and Tauc's model can be important in determining the types of electronic transition. The optical band gap was found to decrease from 5.2 eV for the pure PVA:PVP to 1.1 eV for doped PVA:PVP with 3 M of Ag₂S. Such reduction can be associated with the increase of optical dielectric constant. Finally, the correlation between optical dielectric constant and density of states was discussed.     

Structural,electronic and optical features of molybdenum-doped bismuth vanadium oxide

Molybdenum (Mo)-doped bismuth vanadium oxide (BiVO₄) powders were prepared by a sol–gel method. X-ray diffraction (XRD) patterns and micro-Raman vibrational bands confirm the monoclinic scheelite phase. Molybdenum doping of BiVO₄ matrix was confirmed from XRD by higher angle 2θ shift of the characteristic peak (−121) and from Raman showing lower frequency shift of dominant band from 831 to 822 cm⁻¹ which corresponds to V–O symmetric stretching mode. EPR investigations confirmed the substitution of Mo in the crystalline sites of monoclinic BiVO₄. SEM analysis showed spherical shaped particles around 100–200 nm with weak agglomerated particles. Homogeneous presence of molybdenum in BiVO₄ matrix was confirmed from STEM analysis. Diffuse reflectance spectra showed higher absorption in the range of 550–850 nm and optical band gap energies were calculated by using the Kubelka–Munk formula, i.e, 2.46 eV for 2 wt% Mo–BiVO₄ and 2.48 eV for undoped BiVO_4. This confirms that, Mo–BiVO₄ particles have almost the same energy band gap but induce higher absorption in the visible light region as compared to undoped material.     

Morphologies and optical properties of mixed tin oxysulfide produced by evaporation condensation of SnS

The goal of the present study was to use the evaporation-condensation method of SnS to synthesize mixed novel tin oxysulfide nanostructures. Structural, morphological and optical properties of these nanomaterials were investigated. Synthesis was carried out in horizontal tube furnace using Au coated quartz and Si(100) substrates. The substrates were set along 1–25 cm from the centered alumina boat. The deposition was observed mainly in two regions. Region A was close to the boat and extended approximately 6 cm from it and region B started approximately 15 cm from the center of the boat and extended over 10 cm towards the end of the tube. EDAX and XRD analysis revealed formation of mixed tetragonal SnO₂ and tetragonal tin oxide sulfate, SnO₂SO₄, in region A whereas mixed orthorhombic SnS and tetragonal tin oxide sulfate in region B. SEM investigations showed randomly oriented nanowires and cotton candy morphologies for samples prepared in regions A and B, respectively. Moderate transmittance, high refractive index (2.218 at 550 nm) and two optical band gaps of 1.87 eV and 3.35 eV were obtained for samples prepared in region A whereas high transmittance, moderate refractive index and optical band gap of 1.82 eV were obtained for the samples prepared in region B. The obtained results, especially the new oxysulfide samples, may have find applications in various fields.     

Effects of Sb,Zn doping on structural,electrical and optical properties of SnO2 thin films

Highly transparent, low resistive pure and Sb, Zn doped nanostructured SnO₂ thin films have been successfully prepared on glass substrates at 400° C by spray pyrolysis method. Structural, electrical and optical properties of pure and Sb, Zn doped SnO₂ thin films are studied in detail. Powder X-ray diffraction confirms the phase purity, increase in crystallinity, size of the grains (90–45 nm), polycrystalline nature and tetragonal rutile structure of thin films. The scanning electron microscopy reveals the continuous change in surface morphology of thin films and size of the grains decrease due to Sb, Zn doping in to SnO₂. The optical transmission spectra of SnO₂ films as a function of wavelength confirm that the optical transmission increases with Sb, Zn doping remarkably. The optical band gap of undoped film is found to be 4.27 eV and decreases with Sb, Zn doping to 4.19 eV, 4.07 eV respectively. The results of electrical measurements indicate that the sheet resistance of the deposited films improves with Sb, Zn doping. The Hall measurements confirm that the films are degenerate n-type semiconductors.     

Electronic band structure and optoelectronic properties of double perovskite Sr2MgMoO6 through modified Becke-Johnson potential

The crystal structure, electronic and optical properties of double perovskite Sr₂MgMoO₆ have been calculated by using the full-potential linear augmented plane wave (FP-LAPW) method. The band structure and density of states (DOS) were carried out by the modified Becke–Johnson (mBJ) exchange potential approximation based on the density functional theory (DFT). The calculated band structure shows a direct band gap (Γ–Γ) of 2.663 eV for Sr₂MgMoO₆. The compound Sr₂MgMoO₆ has a triclinic structure with the space group I-1, the lattice parameters a=5.5666 Å, b=5.5661 Å and c=7.9191 Å, which are used in our calculations. The optical parameters, like dielectric constant, refractive index, reflectivity and energy loss function were also calculated and analyzed. This work provides the first quantitative theoretical prediction of the optical properties and electronic structure for the triclinic phase of Sr₂MgMoO₆.     

Low temperature synthesis of pure anatase carbon doped titanium dioxide: An efficient visible light active photocatalyst

Low temperature pure anatase Carbon Doped Titanium Dioxide (C-TiO₂) is successfully synthesized by using starch as an effective, economical, and nonhazardous carbon source. The synthesized C-TiO₂ has been further characterized by X-Ray Diffraction, SEM, TEM, BET, XPS and UV- DRS techniques, which reveal that the particles are crystalline with spherical morphology, high surface area and an optical band gap of 2.79 eV for C-TiO₂ calcined at 400 °C. Furthermore photocatalytic degradation of Rhodamine B dye was carried out using as-prepared C-TiO₂ under visible light irradiation. Prepared C-TiO₂ calcined at 200 °C and 400 °C show higher degradation efficiency (85% and 100% in 120 min respectively) as compared to that of undoped TiO₂ and commercial Degussa P-25. Result shows that the C-TiO₂ containing lower carbon percentage has higher photocatalytic activity. Thus enhanced photocatalytic activity of C-TiO_2, may be due to synergic effect of carbon doping and [101] facet enhanced synthesis of anatase C-TiO₂.     

Two novel Bi-based oxychloride photocatalysts: Synthesis,optical property and visible-light-responsive photocatalytic activity

Two novel visible-light-responsive bismuth oxychloride photocatalysts Bi₂EuO₄Cl and Bi₂NdO₄Cl have been successfully developed via a solid-state reaction route. Their crystal structures and optical properties were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), diffuse reflectance spectra (DRS), and photoluminescence (PL) spectra. Fascinatingly, both the compounds possess considerable optical absorption in a broad region ranging from UV light to visible light. The indirect-transition optical band gaps of Bi₂EuO₄Cl and Bi₂NdO₄Cl are estimated to be 2.21 and 1.89 eV, respectively. For the first time, their photocatalytic activities were determined by photodecomposition of methylene blue (MB) in aqueous solution under visible light (λ>420 nm). The results revealed that both Bi₂EuO₄Cl and Bi₂NdO₄Cl can be used as effective visible-light-driven photocatalysts. In addition, theoretical calculations on the electronic structure, orbital constitutions and optical absorption of Bi₂NdO₄Cl were also performed. These findings shed light on the exploration of new photocatalytic materials activated by visible light.     

Effect of polyvinyl alcohol on the shape,photoluminescence and photocatalytic properties of PbMoO4 microcrystals

For this study, lead molybdate (PbMoO₄) microcrystals were prepared by the co-precipitation method and processed using a conventional hydrothermal method at 100 °C for 10 min with polyvinyl alcohol (PVA) as the capping agent. These microcrystals were structurally characterized by X-ray diffraction (XRD) and micro-Raman spectroscopy, and their morphology was investigated by field-emission gun scanning electron microscopy (FEG-SEM). The optical properties were analyzed by ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns and MR spectrum indicate that the PbMoO₄ microcrystals have a scheelite-type tetragonal structure. FE-SEM images reveal that the PVA promotes the aggregation of several octahedrons and the formation of large porous stake-like PbMoO₄ microcrystals which are related to the oriented attachment growth process. Moreover, the effect of the capping agent hinders the growth of a large amount of micro-octahedrons which can be verified with by several nanocrystals on large crystals. Intense green PL emission was observed at room temperature for PbMoO₄ microcrystals which are related to structural defects at medium range and intermediary energy levels between the valence band (VB) and the conduction band (CB). Photocatalytic activity was observed for PbMoO₄ as a catalyst in the degradation of the rhodamine B (RhB) dye, achieving total degradation after 90 min under UV-light.     

Thermally induced structural changes and optical properties of tin dioxide nanoparticles synthesized by a conventional precipitation method

Tin dioxide (SnO₂) nanoparticles were synthesized by a conventional precipitation method using the reaction between tin chloride pentahydrate and ammonia solutions. The obtained powders were calcined at varied temperatures from 300 to 1050 °C, and then characterized by using thermogravimetric analysis, differential thermal analysis and Fourier transformation infrared spectroscopy. The average crystallite size, determined by x-ray diffraction, was found to be in the range of 3.45–23.5 nm. The analysis exhibited a tetragonal phase. The activation energy of crystal growth was calculated and found to be 12.12 kJ/mol. The microstructure of nanoparticles was examined by high resolution transmission electron microscopy. Optical properties were investigated by a UV–vis absorption spectrophotometer. The calculated optical band gap lies between 4.75–4.25 eV as a result of increasing the calcination temperatures and crystallite size.     

Spectroscopic studies of sol–gel grown CdS nanocrystalline thin films for optoelectronic devices

CdS is one of the highly photosensitive candidate of II–VI group semiconductor material. Therefore CdS has variety of applications in optoelectronic devices. In this paper, we have fabricated CdS nanocrystalline thin film on ultrasonically cleaned glass substrates using the sol–gel spin coating method. The structural and surface morphologies of the CdS thin film were investigated by X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) respectively. The surface morphology of thin films showed that the well covered substrate is without cracks, voids and hole. The round shape particle has been observed in SEM micrographs. The particles sizes of CdS nanocrystals from SEM were estimated to be～10–12 nm. Spectroscopic properties of thin films were investigated using the UV–vis spectroscopy, Photoluminescence and Raman spectroscopy. The optical band gap of the CdS thin film was estimated by UV–vis spectroscopy. The average transmittance of CdS thin film in the visible region of solar spectrum found to be～85%. Optical band gap of CdS thin film was calculated from transmittance spectrum ～2.71 eV which is higher than bulk CdS (2.40 eV) material. This confirms the blue shifting in band edge of CdS nanocrystalline thin films. PL spectrum of thin films showed that the fundamental band edge emission peak centred at 459 nm also recall as green band emission.     

Optical characterization of mechanically alloyed PbSnS3 nanocrystals

The present study explains the preparation of PbSnS₃ nanocrystals using mechanical alloying as the processing technique and elemental powders as the starting material. The elemental powders of Pb, Sn and sublimed sulphur (S) were mechanically alloyed for 40 h. Phases evolved during mechanical alloying are explored by X-ray diffraction (XRD). The morphology and microstructural features have been investigated using High-resolution Transmission Electron Microscopy (HRTEM). UV–Vis–NIR spectroscopy has been used to measure the optical absorption characteristics. The mechanically alloyed powders show particle sizes in the range of 3–12 nm. The absorption edge extends from 503 nm to the visible region to 1360 nm near infrared region with a sudden change in slope at 860 nm, indicating the feature of indirect band gap semiconductors. The intersection point with the x-axis of extrapolating (αE)² line as a function of E gives a direct band gap of 1.33 eV and an indirect gap of 0.59 eV.     

First principles calculations of structural,electronic and optical properties of zinc aluminum oxide

A first principles study of structural, electronic and optical properties of zinc aluminum oxide (ZnAl₂O₄) by means of the full potential linear augmented plane wave method is presented. The local density approximation is used for the exchange-correlation potential. A direct band gap of 4.19 eV, in agreement with experiment (E_g=3.9 eV), was determined. ZnAl₂O₄ is transparent in the visible spectral region; the excitonic transition associated with the fundamental band gap is 4.17 eV. The refractive index value is 1.74 in the ultraviolet spectral region.     

Improvement of 1.53 μm emission and energy transfer of Yb3+/Er3+ co-doped tellurite glass and fiber

To improve the 1.53 μm band emission of Er³⁺, the trivalent Yb³⁺ ions were introduced into the Er³⁺ single-doped tellurite glass with composition of TeO₂–ZnO–La₂O₃, a potential gain medium for Er³⁺-doped fiber amplifier (EDFA). The improved effects were investigated from the measured 1.53 μm band and visible band spontaneous emission spectra together with the calculated 1.53 μm band stimulated emission (signal gain) spectra under the excitation of 975 nm laser diode (LD). It was found that Yb³⁺/Er³⁺ co-doping scheme can remarkably improve the visible band up-conversion and the 1.53 μm band fluorescence emission intensity, and meanwhile improves the 1.53 μm band signal gain to some extent, which were attributed to the result of the effective energy transfer of Yb³⁺:²F_5/2 + Er³⁺:⁴I_15/2 → Yb³⁺:²F_7/2 + Er³⁺:⁴I_11/2. The quantitative study of energy transfer mechanism was performed and microscopic energy transfer parameters between the doped rare-earth ions were determined. In addition, the spectroscopic properties of Er³⁺ were also investigated from the measured absorption spectrum according to the Judd–Ofelt theory, and the structure behavior and thermal stability of the prepared tellurite glass were analyzed based on the X-ray diffraction (XRD) and differential scanning calorimeter (DSC) measurements, respectively.     

First principles study of electronic and optical properties of the ternary SnSb4S7 using modified Becke-Johnson potential

The electronic and optical properties of SnSb₄S₇ compound are calculated by the full-potential linearized augmented plane-wave (FP-LAPW) method. The density of states (DOS) is carried out by the modified Becke-Johnson (mBJ) exchange potential approximation based on density functional theory (DFT). The compound SnSb₄S₇ has a monoclinic structure with the space group P2₁/m with lattice parameters of a=11.331 Å, b=3.865 Å and c=13.940 Å. The band gap is calculated to be 0.8 eV. The optical parameters, like dielectric constant, refractive index, reflectivity and energy loss function were also calculated and analyzed. The present work provides information about variation of the electronic and optical properties which reveals that SnSb₄S₇ is suitable for optoelectronic devices.     

Structural,electrical and optical properties of sintered SnO2 pellets

Tin dioxide (SnO₂) powder was prepared by the co-precipitation method using SnCl₂ solution as a precursor. The powder was then pelletized and sintered. Structural characterization of the samples with XRD confirmed that all the pellets were of SnO₂ having polycrystalline nature with the crystallite size of the order of 90 nm. SEM-EDAX was used to confirm the morphology and composition of the samples. The measurements of electrical properties were carried out in the frequency range of 100 Hz to 100 kHz at various fixed temperatures from 40 °C to 160 °C. The a.c. conductivity and the dielectric constant were found to be dependent on both frequency and temperature. The frequency and temperature dependent conduction properties of SnO₂ are found to be in accordance with correlated barrier hoping model. Infrared and visible spectroscopic studies show that a strong vibration band characteristic of the SnO₂ stretching mode was present at around 620 cm^?1 and the samples exhibited optical transmittance in the visible range.     

Effect of Bi doping on structural,morphological, optical and ethanol vapor response properties of SnO2 nanoparticles

The gas sensing behavior of thick films of Bi doped SnO₂ has been investigated towards ethanol vapor. The screen printing technique was used to prepare the thick films. The films were sintered at 650 °C for 2 h. The structural, surface morphological, optical and gas sensing properties of undoped and Bi doped SnO₂ thick films have been studied. X-ray diffraction and Raman spectroscopy confirmed that the films consisted exclusively of tetragonal tin oxide, without any impurity phases. FE-SEM studies revealed the formation of highly porous microstructure with grain size in few tens of nanometers. From the optical studies, the band gap was found to be decreased with bismuth doping (3.96 eV for undoped, 3.83 eV, 3.71 eV and 3.6 eV for 1 mol%, 2 mol% and 3 mol% Bi, respectively). The 3 mol % Bi doped SnO₂ thick films exhibited the highest sensitivity to 100 ppm of ethanol vapor at 300 °C. The effect of microstructure on sensitivity, response time and recovery time of the sensor was studied and discussed.     

Investigation of the properties of ferromagnetic ZnO:Cr2O3 nanocomposites

Present work focuses on the structural, optical and magnetic properties of ZnO:Cr₂O₃ nanocomposites. ZnO nanoparticles were synthesized and the structure was confirmed using powder x-ray diffraction. ZnO nanoparticles was grown in the hexagonal wurtzite structure with the preferential orientation along (101) plane. ZnO:Cr₂O₃ composites have been synthesized by doping different concentration of Cr₂O₃ (1, 3 and 5 wt%) into ZnO. The incorporation of Cr₂O₃ was confirmed using Fourier transformed infrared spectroscopy. UV–visible absorption spectra have been observed and interpreted for the determination of optical constants of ZnO:Cr₂O₃ composites. The optical constants like optical band gap, refractive index were determined and the effect of Cr₂O₃ on these constants was investigated. Relation between optical band gap and the refractive index were obtained. Magnetic studies using vibrating sample magnetometer reveal the ferromagnetism at 150 K in the composites with 3 and 5 wt% of Cr₂O₃.     

Effects of Cu/In ratio and annealing temperature on physical properties of dip-coated CuInS2 thin films

CuInS₂ thin films were prepared by sol–gel dip-coating method on glass substrates using 0.75, 1 and 1.25 ratios of Cu/In in the solution. The prepared films were annealed at 380 °C, 420 °C and 460 °C for 30 min under argon environment. The structural, optical, morphological and composition properties of those were investigated by X-ray diffraction (XRD), UV–vis transmittance spectroscopy and scanning electron microscopy with an energy dispersive X-ray spectrometer. The XRD results showed that the films exhibit polycrystalline tetragonal CuInS₂ phase with (112) orientation. According to the EDX results the Cu/In ratios of the films were respectively 0.65, 0.92 and 1.35 for the Cu/In ratios of 0.75, 1 and 1.25 in the solutions. The optical band gap was found to be between 1.30 eV and 1.43 eV, depending on Cu/In ratio.     

Characterization of cadmium sulfide/titania heterostructure films by utilizing microstructure and optical characteristics

Engineering and controlling the bandgap of semiconducting metal oxide (TiO₂) to enhance photoactivity under visible light is challenging. Impact of the changing CdS thickness (50–150 nm) on the structure and optical properties of the CdS/TiO₂ heterostructure films (HSFs) which fabricated by pulsed laser deposition (PLD) was observed. XRD, FE-SEM, AFM, UV–vis and PL spectroscopy measurements were utilized to characterize structural and optical behaviors of the films. XRD measurement shows gradual increments of the lattice constants of the films with the increase of CdS thickness. The mean values of the calculated lattice constants and cell volume (V) were a=b=0.3785 nm, c=0.9475 nm and V=13.58 nm³ respectively. The average of crystallite sizes estimated for TiO₂ and CdS/TiO₂ at various CdS thickness is 12.20, 13.49, 24.24 and 43.10 nm. FESEM images prove the high quality nanocrystalline nature of the films without cracks and dislocation. The root means square roughness of the films was increased with the increase of CdS thickness as showed by AFM images. UV–vis measurement reveals an improvement in the optical absorbance of HSFs in the range of 380–550 nm due to presence of CdS. Interestingly, the PL intensity was enhanced by a factor of nineteen compare to pure TiO₂ attributed to the charge carrier recombination in the band gap. The current results suggest that possibility to improve the optical and structural properties of the TiO₂ films and also it possible to fabricate high quality CdS/TiO₂ HSFs by variation of the CdS thickness.