Formation Of Recombination Centers Research Articles

Synthesis and characterization of CuO/Co0304 and CuO/Fe:O; composites and their potential application in the photocatalytic CO2 reduction process

Cupric oxide is a prominent material used as a photocatalyst due to its narrow bandgap; coupling it with other metal oxide semiconductors improves its efficiency due to the favored charge transference. This work reports the synthesis of the composites CuO/CosO4 and CuO/Fex0s, prepared in a sol-gel and hydrothermal two-step methodology to disperse the cocatalyst particles over CuO. The effect of the cocatalyst's concentration over CuO in its structural, optical, and photocatalytic properties was analyzed. A better distribution of the Fe203 particles over CuO was observed, which resulted in the largest cfficiency in the photocatalytic CO2 reduction to formic acid. Despite this, increasing the cocatalyst concentration reduces the photocatalytic activity due to the surface saturation, probably causing the formation of recombination centers. The presented methodology represents a low-cost way to obtain highly efficient composites in photocatalytic reductive processes.

Theoretical insights of codoping to modulate electronic structure of hbox {TiO}_2 and hbox {SrTiO}_3 for enhanced photocatalytic efficiency

hbox {TiO}_2 and hbox {SrTiO}_3 are well known materials in the field of photocatalysis due to their exceptional electronic structure, high chemical stability, non-toxicity and low cost. However, owing to the wide band gap, these can be utilized only in the UV region. Thus, it’s necessary to expand their optical response in visible region by reducing their band gap through doping with metals, nonmetals or the combination of different elements, while retaining intact the photocatalytic efficiency. We report here, the codoping of a metal and a nonmetal in anatase hbox {TiO}_2 and hbox {SrTiO}_3 for efficient photocatalytic water splitting using hybrid density functional theory and ab initio atomistic thermodynamics. The latter ensures to capture the environmental effect to understand thermodynamic stability of the charged defects at a realistic condition. We have observed that the charged defects are stable in addition to neutral defects in anatase hbox {TiO}_2 and the codopants act as donor as well as acceptor depending on the nature of doping (p-type or n-type). However, the most stable codopants in hbox {SrTiO}_3 mostly act as donor. Our results reveal that despite the response in visible light region, the codoping in hbox {TiO}_2 and hbox {SrTiO}_3 cannot always enhance the photocatalytic activity due to either the formation of recombination centers or the large shift in the conduction band minimum or valence band maximum. Amongst various metal-nonmetal combinations, hbox {Mn}_text {Ti}hbox {S}_text {O} (i.e. Mn is substituted at Ti site and S is substituted at O site), hbox {S}_text {O} in anatase hbox {TiO}_2 and hbox {Mn}_text {Ti}hbox {S}_text {O}, hbox {Mn}_text {Sr}hbox {N}_text {O} in hbox {SrTiO}_3 are the most potent candidates to enhance the photocatalytic efficiency of anatase hbox {TiO}_2 and hbox {SrTiO}_3 under visible light irradiation.

Bismuth sulphide decorated ZnO nanorods heterostructure assembly via controlled SILAR cationic concentration for enhanced photoelectrochemical cells

The current study investigates Bi2S3 thin films coated on ZnO NRAs with varying cationic concentrations through ionic layer adsorption and reaction (SILAR) technique. XRD patterns reveal that Bi2S3 is successfully synthesised and exhibits orthorhombic structure on the wurtzite ZnO NRAs. The band gap energy (Eg) of Bi2S3/ZnO NRAs shows a notable red shift with increasing cationic concentration. The photocurrent density increases significantly with increasing concentration from 1 mM to 3 mM before decreases at higher concentration due to agglomeration of Bi2S3 NPs and formation of recombination centres. The hybrid photoanode Bi2S3/ZnO NRAs at 3 mM exhibits the highest photocurrent value (1.92 mA cm−2), which is about six times greater than that of plain ZnO NRAs (0.337 mA cm−2). The high photoconversion efficiency value of 1.65% versus 0.5 V Ag A−1g−1C−1l−1 is obtained by Bi2S3/ZnO NRAs (3 mM) in comparison with pristine ZnO NRs, mainly due to the stepwise band alignment edge and significant enhancement of morphological and optical properties. The study reveals that controlling the cationic concentration can potentially improve the photoconversion efficiency.

Mixed Valence Tin Oxide Sn3O4: A Visible-Light Driven Semiconductor for Photocatalytic Water Splitting Under Visible Light Irradiation

Water splitting using a particle photocatalyst with visible light has attracted interest, because this will realize efficient conversion of the solar energy to hydrogen fuels for sustainable energy managements. Many water-splitting photocatalysts, such as metal-doped oxides, metal-(oxy)nitrides, and metal-oxysulfides, exhibit water reduction and oxidation ability when absorbing visible light. However, in such water-splitting photocatalysts, solely doping and/or replacement of foreign element at host material has been attempted to prepare visible-light-driven photocatalysts. The doping and/or replacement induces defect sites and charge imbalance into photocatalyst, resulting in the formation of recombination centers between excited electron and holes, in return to create new valence state. Therefore, the development of new native visible-light-sensitive photocatalyst for water splitting without doping of foreign elements is a major challenge for photocatalyst research. Herein, we report that an undoped metal oxide, Sn3O4, can efficiently catalyze Hydrogen (H2)-evolution in the presence of a sacrifice agent and, furthermore, can split water to H2 and O2 under irradiation of visible light by two-step photoexcitation (Z-scheme) systems using Sn3O4 as H2-evolving photocatalyst, combined with an appropriate O2-evolving photocatalyst in the presence of iodate/iodide (IO3 −/I−) as an electron mediator. We prepared nanocrystals of Sn3O4 by hydrothermal synthesis using sodium citrate as a ligand [1]. The Sn3O4 significantly catalyzed hydrogen evolution in aqueous solution in the presence of sacrifice agent under irradiation of visible light (λ>420nm), even without Pt co-catalyst. Pt co-catalyst was critical for Sn3O4 material to exhibit higher photocatalytic activity, so far in this study. The apparent quantum yield was 0.2% for high purity Sn3O4 and 1.5% for Pt loaded-Sn3O4, competitive to that of visible-light-sensitive doped metal oxide and (oxy)nitrides photocatalyst. Simultaneous evolution of H2 and O2 was attempted by using Sn3O4 as H2-evolving photocatalyst, combined with an appropriate O2-evolving photocatalyst (BiVO4, WO3 etc.,). By employing the appropriate combination of Sn3O4 and the redox couple (IO3 −/I−), simultaneous evolution of H2 and O2 stably proceeded with higher rates. [1] M. Maidhily, T. Tanabe, et al., ACS Appl. Mater. Interfaces 2014, 6, 3790.

Proton-Induced Trap States, Injection and Recombination Dynamics in Water-Splitting Dye-Sensitized Photoelectrochemical Cells.

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize a sensitized metal oxide and a water oxidation catalyst in order to generate hydrogen and oxygen from water. Although the Faradaic efficiency of water splitting is close to unity, the recombination of photogenerated electrons with oxidized dye molecules causes the quantum efficiency of these devices to be low. It is therefore important to understand recombination mechanisms in order to develop strategies to minimize them. In this paper, we discuss the role of proton intercalation in the formation of recombination centers. Proton intercalation forms nonmobile surface trap states that persist on time scales that are orders of magnitude longer than the electron lifetime in TiO2. As a result of electron trapping, recombination with surface-bound oxidized dye molecules occurs. We report a method for effectively removing the surface trap states by mildly heating the electrodes under vacuum, which appears to primarily improve the injection kinetics without affecting bulk trapping dynamics, further stressing the importance of proton control in WS-DSPECs.

Fe-doped TiO<sub>2</sub>/SiO<sub>2</sub> Aerogel Microspheres for Visible Light Photocatalysis Degradation of Methylene Blue

In this paper, we present a strategy for the preparation of Fe-doped TiO2/SiO2aerogel microspheres (Fe-TSAMs) for visible light photocatalysis degradation of methylene blue. A water- in-oil (W/O) emulsion of Span 80–Tween 85/n-heptane–n-butanol/FeCl3-doped TiO2/SiO2alcoholic sols–formamide was obtained. Wet TiO2/SiO2gel microspheres were prepared by the sol-gel process in the W/O emulsion, then they were dried at ambient pressure condition to prepare Fe-TSAMs. The Fe-TSAMs were characterized by N2adsorption and desorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that the resultant Fe-TSAMs possess nanoporous network structure comprising TiO2and SiO2nanoparticles with diameters of 10-20 nm. The Fe-TSAMs have a mean diameter of about 100 μm, an apparent density of 0.26 g/cm3, a specific surface area of 360 m2/g, a pore volume of 1.77 cm3/g and the most probable pore size of 18.7 nm with the porosity of 87.7%. By using methylene blue as a simulated aqueous pollutant, the visible light photocatalysis properties of the Fe-TSAMs were investigated. The results indicate that low concentration Fe ion doping (5 wt%) could avoid the formation of recombination centers for photo-generated electron/hole pairs. Compared with commercial P25 TiO2powders, Fe-TSAMs exhibit excellent photocatalytic ability for degradation of methylene blue under visible light region. The Fe-TSAMs could be reclaimed conveniently and the reused Fe-TSAMs also have a good photocatalytic activity.

Optoelectronic properties of thin film CdS formed by ultraviolet and infrared pulsed-laser deposition

Thin CdS films on glass are formed by ultraviolet (UV) and infrared (IR) pulsed-laser deposition (PLD) at 355 and 1064 nm with a repetition rate of 10 Hz and a pulse width of 10 ns and 150–180 μs, respectively. The applied laser fluence is kept in the range of 2–4 J cm−2. By UV-PLD, the orientation of the c-axis with respect to the glass surface is adjustable via laser fluence, resulting in perpendicular and parallel oriented films at 2 and 4 J cm−2, respectively. The orientation of IR-PLD samples is maintained perpendicularly, independent of the fluence. The optoelectronic properties of the films are studied by photocurrent (PC) and photoluminescence (PL) spectroscopy at 300 K. The PC of the UV-PLD samples reflects the turn of the c-axis, i.e. the absorption anisotropy, of CdS. The IR-PLD films, however, do not show PC because of high dark conductivity. The UV-PLD samples show PL in the range (2.27–2.45 eV). The emission below 2.45 eV is caused by formation of recombination centers. Notably, the film formed at 3 J cm−2 emits the spectral sum of the films formed at 2 and 4 J cm−2. The IR-PLD samples show green emission (2.493 eV) clearly above the CdS bandgap due to band filling. The results pave the way for the creation of smart photonic gratings, which exhibit locally tunable optoelectronic properties.

Electric Field Induced Recombination Centers in GaAs

We investigated the lateral electric field effect in a GaAs/AlGaAs single quantum well and in AlGaAs and GaAs single layers. We observed the quenching of photoluminescence spectra at low temperatures even when the electric field was much lower than that required for the dissociation of the excitons. To explain this phenomenon we propose a model of new recombination center formation by considering the displacement of charged atoms. We also performed theoretical calculations. By ab initio calculations, we found that a meta-stable state exists when negatively charged Ga atoms are displaced to the interstitial sites. The calculated potential barrier height from the stable state to the meta-stable state was as high as 0.5 eV. However, the meta-stable to stable transition barrier height was only 0.05 eV. These results are consistent with the experimental results.

Evaluation of hydrogen-sensitive switching device by capacitance-voltage method

Abstract To investigate the hydrogen-sensitive switching properties of a device with a Pd-insulator-(n)Si-(p + )Si structure, four devices with different thicknesses of either the Pd electrode or the insulator layer have been fabricated and subjected to measurements of the current-voltage ( I – V ) and capacitance-voltage ( C - V ) characteristics in an atmosphere containing various concentrations of H 2 in air. The I – V characteristics reveal negative-resistance switching properties with the switching voltage being dependent on the hydrogen gas concentration and the device used. The work function of Pd, estimated from the C - V characteristics, decreases with increasing hydrogen concentration, almost independent of the device used. It is shown that the change of Pd work function upon exposure to H 2 can induce an effective decrease in switching voltage when the Pd electrode and the insulator layer are sufficiently thin and thick, respectively. These phenomena are explained by assuming the formation of recombination centres in the insulator layer.

Improved delineation technique for two dimensional dopant profiling

A chemical delineation technique suitable for two dimensional dopant profiling has been investigated and improved. The formation of equi-concentration lines in p-doped areas during chemical etching due to drastic changes of the etching rate as function of dopant concentration has been studied. Formation of p-n junction, degeneration of the semiconductor, and formation of recombination centers are discussed as possible cause of these changes. In addition, a new etching procedure for delineation of equi-concentration lines in n-doped regions was successfully tested.

Formation of Recombination Centers in Epitaxial GaAs due to Rapid Changes of the Growth Velocity

not Available.

Reliability of low-cost CdS/Cu2S solar cells

Accelerated life tests show that CdS/Cu2S solar-cell lifetimes increase with lower temperature, cyclic light and a nitrogen-gas ambient. A normal Arrhenius type of temperature dependence was found. Extrapolated lifetime for cells kept below 50°C in nitrogen gas with earth deployment exceeded twenty years. The probable decay mechanisms are thermal diffusion, formation of recombination centres and voltage-induced decomposition.