Energy Band Positions Research Articles

Investigations on spectroscopic properties of Dy3+ doped zinc telluro-fluoroborate glasses for laser and white LED applications

A new series of Zinc telluro-fluoroborate glasses (ZTFB) doped with Dy3+ ions with the chemical composition (30−x) B2O3 + 30TeO2 + 16ZnO + 10ZnF2 + 7CaF2 + 7BaF2 + xDy2O3 (where x = 0.05, 0.1, 0.25, 0.5, 1 and 2 in wt%) have been synthesized by the conventional melt quenching technique and the prepared glass samples were characterized through XRD, FTIR, UV-Vis-NIR optical absorption, photoluminescence and decay spectral measurements. The XRD patterns have been recorded to confirm the amorphous nature. The FTIR spectral measurements were carried out to identify the functional groups present in the title glasses. From the energy band positions of the absorption spectra, bonding parameters (β¯ and δ) have been calculated to identify the covalent/ionic nature of the metal-ligand bond present in the prepared glasses. The Judd-Ofelt (JO) intensity parameters Ωλ (λ = 2, 4 and 6) have been calculated from the oscillator strengths of the different absorption bands to understand the nature of the ligand environment around the Dy3+ ions. The radiative parameters such as transition probability (A), stimulated emission cross-section (σPE), radiative lifetime (τR) and branching ratios (βR) corresponding to the 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, and 4F9/2 → 6H11/2 emission transitions have been determined using JO parameters and the refractive index values. The CIE chromaticity coordinates were calculated from the luminescence spectra to explore the dominant emission colour of the title glasses and the obtained results were discussed in detail and reported.

Preparation of hierarchical micro/nanostructured Bi2S3-WO3 composites for enhanced photocatalytic performance

Novel hierarchical micro/nanostructured photocatalysts composed of WO3 microspheres and needle-like Bi2S3 nanocrystals were successfully synthesized by a simple combination of hydrothermal and post-calcination method. The as-obtained Bi2S3-WO3 composites was applied for photocatalytic removal of Rhedamine B (RhB) organic pollution in water under visible light irradiation. The morphology and structure of the as-obtained samples were characterized by XRD, SEM, TEM, N2 adsorption–desorption techniques, UV–vis DRS, XPS and PL spectra. It was found that 70%-Bi2S3-WO3 has the highest photocatalytic activity and more than 90% (over 3 times than pure WO3) of the RhB solution were degraded in 100 min. The degradation kinetics data were found to be consistent with the Langmuir-Hinshelwood (L-H) kinetics model. Furthermore, the kinetic rate of 70%-Bi2S3-WO3 composites is 9 times more than the pure WO3. The modification of WO3 by deposited Bi2S3 could improve charge separation and transfer with the increase of special surface area and higher utilization of visible light. These favorable factors lead to significantly enhanced visible photocatalytic performance; and Bi2S3-WO3 composites had an excellent stability in cycling experiments. At last, a tentative mechanism of enhanced photocatalytic activity for Bi2S3-WO3 composite was discussed according to the calculation of the energy band position.

Synthesis, surface and optical properties of Ag2CaV4O12 nanoparticles for dye removal under visible irradiation

Ag-containing compounds are regarded as potential candidates for the efficient photocatalyst driven by visible-light. In this work, nano-sized Ag2CaV4O12 was prepared via a modified Pechini synthesis. The detailed structural refinement of X-ray diffraction (XRD) pattern indicates that the Ag2CaV4O12 crystallizes in tetragonal system with the space group of P4/nbm (125), Z = 2. The surface property of this vanadate was measured by the scanning electron microscope (SEM), energy dispersive spectra (EDS), transmission electron microscopy (TEM), and nitrogen adsorption-desorption isotherm. The sample has an efficient optical absorption in the wavelength from UV to visible region characterized by its narrow band-gap energy of 2.231 eV with an indirect allowed property. The valence band maximum is composed of multiple states of Ag-d and O-p resulting in a narrow band. The excellent photocatalytic activity of Ag2CaV4O12 nanoparticles was tested by the photodegradation of rhodamine B (RhB), methylene blue (MB) and phenol solutions excited by visible-light. The photocatalytic mechanism was suggested according to experimental results and the band energy positions.

Highly photoactive Brookite and Anatase with enhanced photocatalytic activity for the degradation of indigo carmine application

TiO2 Brookite, and Anatase powders were prepared by a facile and low temperature solution based method. XRD diffractometry was performed to confirm their phase composition and crystallinity. Morphology was further confirmed by TEM. N2 adsorption was used to determine surface area and pore size distribution. The difference in capacitive properties of both materials and the determination of the flat band position was obtained by Mott-Schottky analysis. Diffuse reflectance and Tauc plots allowed to obtain the band gap value. The degradation of Indigo Carmine dye with both catalysts in a photo-electrochemical microreactor was assessed by UV–vis, and Raman spectroscopy. Both materials were more active than the benchmark commercial Degussa P25 sample in the order P25<Anatase<Brookite and explained in terms of energy band positions.

Hydrogen evolution from solar water splitting on nanostructured copper oxide photocathodes

Copper oxide has received much attention as a photocatalyst for solar water splitting, since it has a band gap of ∼2.0 eV with favourable energy band positions for water cleavage; it is abundant and environmentally friendly. In this paper, we report the preparation of copper oxide thin films on copper substrate with different morphology by a solution route in large scales. The method was simple, low cost, and can be completed in the absence of any surfactant. Copper oxide films were characterised by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), Fourier transform-infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The XRD analysis and FT-IR analysis confirm the formation of copper oxide films. SEM images show gradual development of hierarchical structures of copper oxide with different morphology. The films were successfully tested as photocathodes in a photoelectrochemical cell, and their photocatalytic activity was evaluated by testing the solar water splitting. The photocatalytic activity of these films was found to be size-dependent and films with smaller dimensions demonstrated a significant increase in photocatalytic activities during solar water splitting process. Based on the above discussion, we believe that these samples are attractive candidates as a visible-light-driven photocatalyst.

Surface Modification or Doping of WO&lt;sub&gt;3&lt;/sub&gt; for Enhancing the Photocatalytic Degradation of Organic Pollutant Containing Wastewaters: A Review

Tungsten trioxide (WO3) is an oxygen deficient metal oxide and well known semiconductor with a small band gap of between 2.4 and 2.8 eV. It is also used as a photo-catalyst for degradation of organic pollutants present in aqueous environment. It has stable physico-chemical properties and shows strong absorption of solar spectrum and thus can be used in visible-light driven photocatalysis. WO3 has a conduction band (ECB) of +0.4 V versus NHE (normal hydrogen electrode) at pH = 0. Therefore, pure WO3 has lower light energy conversion efficiency as compared to other widely used photocatalysts such as zinc oxide (ZnO) and titanium oxide (TiO2). This is because the reduction potential of the electrons in WO3 is low due to its low conduction band level. O2 cannot be efficiently trapped in the conduction band electrons to yield superoxide radicals and fast recombination of charge carriers takes place resulting in lesser photocatalytic activity of WO3. However, holes in the valence band (EVB = +3.1 V) are energetically favorably situated to oxidize water to hydrogen. To modify the energy band position and reduce the charge carrier recombination, doping or surface modification of WO3 is necessary. This review article demonstrates the effect of dopants (low band semiconductor catalyst) on the surface modification of WO3 to enhance the photo catalytic activity which helps in degradation of the organic pollutants present in the wastewater.

Electrochemical Deposition of Cu2O Thin Film As Efficient Photocatalyst for Water Splitting Reaction

Solar water splitting can form the basis of a sustainable hydrogen based energy economy, however the system requires optimisation in terms of cost and performance on a scale commensurate with the global energy demand. Photocatalysts consist of solar spectrum absorber semiconductor materials and co-catalysts as the redox mediator for hydrogen and oxygen evolution reactions. The key factors for efficient solar water splitting process to occur with such photocatalysts are a semiconductor with a sufficiently narrow band gap to absorb visible lights, a suitable thermodynamic potential for the redox reactions, and stability against photocorrosion. However, current photocatalysts also suffer from undesirable photoelectron-hole (charge carrier) recombination within the semiconductor due to their short diffusion path lengths and lifetimes, which results in fewer than 10% of the incident photons being used for water splitting. Cuprous Oxide (Cu2O), one of the few naturally occurring p-type metal oxides, is the most promising candidate for an efficient photocathode because of its earth abundance, a band gap of 2.0 eV, which enables efficient visible light absorption, a scalable CMOS compatible deposition technique (e.g., electrodeposition), and favourable energy band position i.e. conduction band lies 0.7 eV negative of the hydrogen evolution potential that drives off half of the water reduction reaction without of any external bias. However, the application of Cu2O as photocathode is limited by two drawbacks; fast photoelectron–hole recombination owing to short carrier diffusion length and self-photo corrosion in aqueous solution under illumination since the potentials of Cu2O reduction and oxidation to metallic Cu and CuO, respectively, lie within the bandgap. Hence, the solar-to-fuel conversion efficiency on Cu2O depends on how fast the photoelectron can be extracted to the reduction reaction sites and inhibition of electrolyte access to Cu2O surface. A thin protective coating and a heterostructure in the seed layer of the electrochemically deposited Cu2O film enhance the overall photocatalytic performance versus the Cu2O film alone. The thickness of the Cu2O film was optimised to avoid the charge carrier recombination towards the efficient and stable photocatalytic performance. The photocatalytic performance of Cu2O thin film is shown in Fig 1, in which the film performance is unexpectedly high and only dropped to 22% after the first cycle at 0.0 VRHE, after which it stabilises. This presentation will discuss our current research to develop cheap and efficient photocatalysts and their enhancement mechanism towards photocatalytic water splitting reactions. Figure 1

Light emission from CdTe based quantum well structures with embedded ultrathin MnTe layers

The exciton luminescence of CdTe based quantum well structures with the embedded ultrathin MnTe layers is studied. It appears that the energy position and width of the exciton emission bands as well as their lifetime and temperature behavior depend strongly on the real profiles of manganese distribution in the direction of the structure growth. The relative contributions of free and localized states to the low temperature exciton luminescence show that the manganese incorporation is favorable to smooth the CdTe/Cd1−xMgxTe interfaces.

Synthesis, surface properties and optical characteristics of CuV2O6 nanofibers

In3+-doped CuV2O6 nanofibers were prepared via the hydrothermal synthesis method, which produced fibers with a typical diameter of 100 nm, and a length of 1–5 μm. The nanofibers grew in a preferred [020] direction. The crystal phase together with the structure was studied via X-ray polycrystalline diffraction (XRD) and the Rietveld refinement. The surface characteristics of this nanostructure were measured with a scanning electron microscope (SEM), energy dispersive spectra (EDS), transmission electron microscopy (TEM), and N2–adsorption–desorption isotherms. Photo-activities were evaluated by optical absorption, luminescence, and decay behaviors. The band-gap structures and positions were investigated. The vanadate has an efficient optical absorption from the UV to the visible wavelength region with an indirect allowed transition characterized by the narrow gap energy of 1.96 eV. The photocatalysis was investigated by the photo-degradation of RhB solutions irradiated by visible light. Correspondingly, CuV2O6:In3+ nanofibers possess quenched luminescence and have a more efficient photocatalytic activity on the RhB degradation. Photocatalytic mechanisms were proposed based on the experimental results, the band-energy positions, and the trapping experiments. The coexistence of V4+/V5+ ions and induced-color centers was discussed on the proposed photocatalytic mechanism. The results demonstrated the promising potency of such In3+-doped CuV2O6 nanofibers for technological applications due to their high photo-activity and good cycling performance with the fiber morphology.

Modeling of Cr3+ doped d-gluconic acid monohydrate

Zero field splitting parameters (ZFSPs) D and E of Cr3+ ion doped d-gluconic acid monohydrate (DGAM), C6H12O7H2 O are determined using the superposition model. The theoretical ZFSPs for Cr3+ in DGAM single crystal are compared with the experimental values obtained by the electron paramagnetic resonance. There is a good agreement between the two. The energy band positions of the optical absorption spectra of Cr3+ in DGAM single crystal calculated with the crystal field analysis package are in good match with the experimental values.

Synthesis, structural and optical properties of nanoparticles (Al, V) co-doped zinc oxide

The synthesis by the sol–gel method, structural and optical properties of ZnO, Zn0.99Al0.01O (AlZ), Zn0.9V0.1O (VZ) and Zn0.89Al0.01V0.1O (AlVZ) nanoparticles was reported. The approach was slow release of water for hydrolysis by esterification reaction followed by a supercritical drying in ethyl alcohol. After thermal treatment at 500∘C in air, the obtained nanopowders were characterized by various techniques such as transmission electron microscopy, X-ray diffraction and photoluminescence (PL) spectroscopy. The structural properties showed that the ZnO nanoparticles with an average particle size of 25 nm exhibit hexagonal wurtzite structure. From the optical studies, it was found that the optical band gap was located between 2.97 and 3.17 eV. The obtained electrical properties showed the potential application of the samples in optoelectronic devices. The powder of AlVZ presented a strong luminescence band in the visible range. The PL band energy position presented a small blue shift with the increase of measurement temperature. Different possible attributions of this emission band will be discussed.

AgV7O18: A new silver vanadate semiconductor with photodegradation ability on dyes under visible-light irradiation

A new silver vanadate semiconductor AgV7O18 was prepared via the hydrothermal synthesis method. The sample crystallized in typical nanorods with a big length/diameter ratio and large specific surface area. The detailed band structures were reported. AgV7O18 nanorods have small band energy of 1.837eV with indirect allowed electronic transitions. The narrowed band-gap is due to the hybridization between the Ag-d and O-p states in the valence band. The photocatalysis was investigated by photo-degradation of Rhodamine-B (RhB) irradiated by visible light. AgV7O18 has efficient photodegradation on RhB. The photocatalysis was discussed according to the band energy positions and the trapping experiments.

Enhanced visible light photocatalytic performances of self-assembled hierarchically structured BiVO4/Bi2WO6 heterojunction composites with different morphologies

A possible mechanism for enhanced photocatalytic activity over self-assembled hierarchical BiVO4/Bi2WO6 composites was proposed according to calculated energy band positions.

Use of delafossite oxides CuCr1-xGaxO2 nanocrystals in p-type dye-sensitized solar cell

Pure phase of CuCr1-xGaxO2 (x = 0.10, 0.20, 0.30, 0.40, 0.50.) nanocrystals with the crystal size ranging from 15 nm to 50 nm were synthesized through hydrothermal method. The crystal structure, morphology, element composition information, optical properties and valence band energy position of these delafossites were analyzed by XRD, SEM/EDS, TEM/EDS, XPS/UPS, and Uv–vis measurements. It was found that the Ga3+ ions were perfectly substituted at Cr3+ in CuCrO2 lattice and finally formed the CuCr1-xGaxO2 solid solution. The crystal size of CuCr1-xGaxO2 enlarged from 15 nm to 50 nm with the increasing of Ga composition x from 0.10 to 0.50. The optical transmittance of these CuCr1-xGaxO2 films approached about 60–75% in the visible region and the direct band gap values around 3.25–3.30 eV. The valence band energy position of these CuCr1-xGaxO2 nanocrystals ranged from 5.29 to 5.44 eV, which is much lower than traditional NiO nanoparticles used in p-type dye sensitized solar cells (DSSCs). Moreover, these CuCr1-xGaxO2 nanocrystals were applied as photocathodes in p-type DSSC devices, and the CuCr0.90Ga0.10O2 based solar cell show the best performance (134 mV, 1.56 mA cm−2, 0.10%) after roughly optimizing the composition and the thickness of photocathode films.

Zero field splitting parameters of Cr3+ doped potassium sodium dl-tartrate tetrahydrate

The zero field splitting parameters (ZFSPs) D and E of Cr3+ ion doped potassium sodium dl-tartrate tetrahydrate (PSTT), KNaC4H4O6·4H2O are calculated using the superposition model. The theoretically calculated ZFSPs for Cr3+ in PSTT crystal are compared with the experimental values obtained by electron paramagnetic resonance. Theoretical ZFSPs are in good agreement with the experimental values. The energy band positions of optical absorption spectra of Cr3+ in PSTT single crystal calculated with the CFA package agree well with experimental values.

Photoreduction Synthesis of Hierarchical Hematite/Silver Nanostructures for Photoelectrochemical Water Splitting

AbstractHematite is a good candidate for solar water splitting because of its earth‐abundance, moderate band gap, and electrochemical stability. However, the improper energy band position to absorb the visible region of sunlight has limited any increase of the efficiency. In this study, a facile solution‐based photoreduction process is employed to fabricate a hematite/silver hierarchical nanostructure. By irradiating hydrothermally grown hematite nanowire with UV light, Ag nanoparticles are synthesized rapidly on the surface of the hematite nanowire, the size of which is easily controllable by changing the UV irradiation time. The enhanced optical performance of the resultant hierarchical nanostructure is confirmed by various analytical tools that include UV/Vis absorption, photoluminescence, and Raman spectroscopy together with the theoretical estimation through finite‐difference time‐domain simulation. Furthermore, the hierarchical hematite/silver nanostructure is applied as a photoanode in a photo‐electrochemical cell for water splitting with enhanced efficiency.

A controlled anion exchange strategy to synthesize Bi2S3 nanoparticles/plate-like Bi2WO6 heterostructures with enhanced visible light photocatalytic activities for Rhodamine B

Bi2S3/Bi2WO6 semiconductor heterostructures with enhanced visible light photocatalytic activity were firstly prepared through a facile in-situ anion exchange approach at relatively low temperature, using thioacetamide as sulphur source and Bi2WO6 nanosheets as precursors. Bi2S3 nanoparticles decorated on the surface of 2D plate-like Bi2WO6 nanosheets were analyzed by using X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy etc. Photocatalytic experiments indicate that such Bi2S3/Bi2WO6 composites possess higher photocatalytic activity for the degradation of Rhodamine B under visible light irradiation in comparison with pure Bi2S3 or Bi2WO6. Among Bi2S3/Bi2WO6 composites with different contents of Bi2S3, the Bi2S3/Bi2WO6 composite with 30% Bi2S3 exhibits the highest photocatalytic efficiency. Based on the calculated energy band positions and photoluminescence spectrum, it was postulated that enhanced photocatalytic property could be ascribed to the reduction of the recombination rate of photogenerated electron–hole pairs and the heterojunction interface. Moreover, these results suggest that the heterostructured photocatalysts can be potentially applied in environmental protection and waste water treatment.

Synergetic effects of Sr-doped CuBi2O4 catalyst with enhanced photoactivity under UVA- light irradiation.

Sr-doped CuBi2O4 micro-particles were successively synthesized via a solid-state technique and were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and UV-vis diffuse reflectance spectroscopy (UV-vis-DRS) techniques. Results show that Sr-doped CuBi2O4 was crystallized with a spinel-type structure and tetragonal crystal system, and the band gap energy was about 1.35eV. The as-prepared Sr-doped CuBi2O4 treated at 573°C for 12h exhibited the highest efficiency, as a result of 97.22% of CR degradation within 220min, which is approximately 31 times greater than CR photodegradation when catalyzed by CuBi2O4 (3.13%) and about 2.3 times superior than that catalyzed by the untreated Sr-doped CuBi2O4 sample (42.08%). Pseudo-first-order kinetic model gave the best fit, with highest correlation coefficients (R (2) = 0.94-0.97). The Sr-doping and extending reaction time up to 12h could be effective in producing Sr-doped CuBi2O4 materials that delay electron-hole recombination, thereby increasing the lifetime of the electron electron-hole separation and support the charge carrier transfer to the catalyst surface. On the basis of the calculated energy band positions, superoxide radical anions (O2 (•-)) were the main oxidative species responsible for the photocatalytic degradation of CR dye solution.

Visible-light driven generation of reactive radicals over BiFeO3/TiO2 nanotube array: experimental evidence and energetic mechanism

BiFeO3-based materials have attracted broad interests due to their multiferric and photoactive properties, but there remains some controversial uncertainty about the electronic structures of various nanoparticles of BiFeO3. In this work, BiFeO3 nanoparticles smaller than 100 nm have grown into TiO2 nanotube array by an ultrasonic-immersion method. Some evidences on the energy band position of BiFeO3 nanoparticles in BiFeO3/TiO2 nanotube array were from the visible-light driven photo responses. Positive surface photovoltage polarity of the composite is the same as that of the pure TiO2 nanotube array, which reveals the photo-induced electron transfer from BiFeO3 nanoparticles to TiO2 nanotubes. Furthermore, both ·OH and ·O2 − radicals could be detected by the electron paramagnetic resonance (EPR) spin-trapping method. According to the EPR result, the valence band edge of BiFeO3 nanoparticles could be deduced as more positive than the potential at which ·OH radical could be generated. A scheme on the energy band alignment and photo-induced charges transfer processes is then proposed based on the observed visible-light photo activity of BiFeO3/TiO2 nanotube array.

Controlled preparation of hollow SnO2@M (M = Au, Ag) heterostructures through template-assist method for enhanced photocatalysis

Abstract According to relative energy band positions between noble metals of Au, Ag, and wide bandgap semiconductor SnO 2 , Au, and Ag nanoparticles (NPs) modified hollow SnO 2 NPs were designed. Firstly, the hollow SnO 2 NPs have been synthesized by employing the silica template-assisted hydrothermal methods, then Au and Ag were coated on the surface of SnO 2 to form the core–shell structures, respectively. The morphology, composition, and optical properties of as-prepared samples have been investigated by various analytic methods. Subsequently, the photocatalytic reaction to Rhodamine B (RhB) dye under simulated-sunlight irradiation was employed to investigate their photocatalytic performance, the results demonstrate the metal–semiconductor with direct Schottky contact exhibited remarkable photocatalytic activity. The finite difference time domain (FDTD) calculation results illustrate the introduced and decorated Au and Ag NPs could enhance the harvesting abilities for visible light. We envision the aforementioned synthesized strategy and catalytic results are used to the designing and application of other metal–semiconductor system.