Na2S Aqueous Solution Research Articles

Modification of back interfacial contact with MoO3 layer in situ introduced by Na2S aqueous solution for efficient kesterite CZTSSe solar cells

A high-efficiency CZTSSe solar cell was achieved through the modification of the back interfacial contact with a MoO3 layer synthesized in situ using Na2S aqueous solution.

Synthesis of bone-shaped Au–Ag2S metal-semiconductor nanorods and their photocatalytic application

In this work, we describe an aqueous synthetic strategy for producing bone-shaped Au − Ag2S metal-semiconductor hybrid nanorods. Specifically, Au nanorods were first synthesized and used as seeds to synthesize bone-shaped Au − Ag nanorods by selectively overgrowing Ag shells at both ends of Au nanorods. Through the site-selective sulfidation strategy using Na2S aqueous solution, the Au − Ag bimetallic nanorods were transformed into Au–Ag2S metal–semiconductor hybrid nanorods. When used as photocatalysts for the photooxidative degradation of methylene blue (MB) under sunlight irradiation, the Au–Ag2S hybrid nanorods showed excellent photocatalytic performance due to the synergistic action of metal and semiconductor components.

Minimalist preparation and photocatalysis properties of a spinyball-like indium-based composite suitable for different light conditions

The CdIn2S4/In2S3/CaIn2S4 composites were successfully prepared by an extremely simple programmed temperature hydrothermal method, which present a spinyball-like structure with a micro-sphere diameter of about 2.5–3 µm. X-ray diffraction analysis (XRD) results show that CaIn2S4 and CdIn2S4 exist in the form of cubic phase, and In2S3 exists in the form of cubic β-In2S3 in the composites. By discussing the relationship between the change of strain gravity and the change of cell parameters of CaIn2S4 after compounding In2S3 and CaIn2S4, the effects of crystal lattice change, crystal face growth and inhibition on the photocatalytic activity were studied. Meanwhile, through the photoluminescence spectra (PL), electrochemical impedance spectra (EIS), and transient photocurrent response analysis, the recombination of CaIn2S4, In2S3, and CdIn2S4 can accelerate the charge transfer and inhibit the recombination of electron–hole pairs in the CdIn2S4/In2S3/CaIn2S4 composites, so that higher photocatalytic performance can be obtained. Using methyl orange (MO) as the degradation object, the photo-degradation properties of the as-composites were studied and compared, and the results show that spinyball-like In2S3/CaIn2S4 and CdIn2S4/In2S3/CaIn2S4 exhibit high photocatalytic activity for degradation of MO under visible light and simulated sunlight, respectively. Furthermore, with Pt loaded as a co-catalyst, in Na2S aqueous solution, the hydrogen production rate of CdIn2S4/In2S3/CaIn2S4 photocatalysts is 74.9 μmol·g−1 (8 h), which is 8 times that of the commercially available TiO2. Furthermore, based on the results of the trapping experiment of CdIn2S4/In2S3/CaIn2S4, a possible photocatalytic mechanism was proposed, and the effect of multi-path photo-generated electron migration in the photocatalytic process was explained.

Preparation of CoSOH/Co(OH)2 composite nanosheets and its catalytic performance for oxygen evolution

Zn-Co(OH)2 precursor was hydrothermally deposited on carbon paper at 120°C, using cobalt nitrate and zinc nitrate as raw materials. Then Zn-Co(OH)2 was etched and partially sulfided into CoSOH/Co(OH)2 with 5 mol/L NaOH and 1 mol/L Na2S aqueous solution at room temperature. The catalytic performance in oxygen evolution reaction (OER) was investigated. XRD, SEM, TEM, XPS were used to characterize the microstructure, physical and chemical properties of the catalyst. The results show that the used method can etch Zn atoms, create oxygen vacancies and dope sulfur. The oxygen vacancies and doped sulfur play a positive role in enhancing the OER performance. In addition, amorphous CoSOH also exhibited better OER activity. The synergy between CoSOH and Co(OH)2 finally induces the best catalytic properties (overpotential η=310 mV, Tafel slope b=90 mV/dec) and long-term electrochemical stability, that is CoSOH/Co(OH)2 possess superior electrocatalytic oxygen evolution performance.

Photoluminescence of n-InP (100) surface passivated with aqueous Na2S solution

Passivation of the n-InP(100) surface with an aqueous solution of sodium sulphide (Na2S) has been investigated by spectroscopic photoluminescence (PL). After sulphide treatment the PL signal intensity of InP increased significantly and this increase is dependent on duration of sulphide passivation. The correlation of the increase in the PL intensity after sulphide treatment with the variation of the solution pH in contact with InP(100) surface has been established, that expect the possibility for optimization of the passivation process by monitoring the change of the solution pH value in the course of sulphide treatment of the semiconductor.

Effect of Na2S soaking on the properties of Cu2ZnSn(S,Se)4 films and solar cells

In this paper, we propose a Na2S soaking process (used as post-treatment process for the precursor films) for the non-vacuum prepared Cu2ZnSn(S,Se)4 (CZTSSe) films. The CZTSSe film prepared without this process show triple-layer structure in which fine-grain layer and obvious layer-boundaries exist. By soaking the precursor film in Na2S aqueous solution with suitable concentration, the fine-grain layer in the triple-layer structure is substituted by a small-grain layer and the layer-boundaries become continuous. We find the quantum efficiencies of the solar cells in long wavelength region are improved by the Na2S soaking. Therefore, the short-circuit currents of the solar cells are significantly increased. By using the Na2S soaking process, CZTSSe solar cells with the best efficiency of 11.5% have been prepared.

Enhanced Power Conversion Efficiency of Dye-Sensitized Solar Cells by Band Edge Shift of TiO2 Photoanode.

By simple soaking titanium dioxide (TiO2) films in an aqueous Na2S solution, we could prepare surface-modified photoanodes for application to dye-sensitized solar cells (DSSCs). An improvement in both the open-circuit voltage (Voc) and the fill factor (FF) was observed in the DSSC with the 5 min-soaked photoanode, compared with those of the control cell without any modification. The UV–visible absorbance spectra, UPS valence band spectra, and dark current measurements revealed that the Na2S modification led to the formation of anions on the TiO2 surface, and thereby shifted the conduction band edge of TiO2 in the negative (upward) direction, inducing an increase of 29 mV in the Voc. It was also found that the increased FF value in the surface-treated device was attributed to an elevation in the shunt resistance.

Synthesis of ZnS nanoparticles of required size by precipitation in aerosol microdroplets

ABSTRACTZinc sulphide (ZnS) nanoparticles were prepared by an aerosol method from zinc acetate and sodium sulphide (Na2S) aqueous solutions. Aqueous solution of zinc acetate was dispersed into the form of microdroplets, which were introduced by airflow to vigorously stirred aqueous solution of Na2S, which was in excess. Microdroplets served as microreactors, so the reaction took place only in limited volume. Particle size distribution was studied by transmission electron microscopy and by dynamic light scattering measurements. In this work, the equation that allows us to predict the final size distribution of ZnS nanoparticles using exact concentration of zinc acetate was derived and ZnS nanoparticles with predicted mean particle diameter around 50 and 70 nm were successfully synthetised.

Contactless Measurement of Electron Concentration in Undoped Homoepitaxial InSb Layers

The photoreflectance spectra of undoped InSb grown by the molecular beam epitaxy method on the n+-InSb substrate have been measured with a Fourier-transform infrared (FTIR) spectrometer. The intensity of the surface electric field has been determined from the period of the Franz–Keldysh oscillations observed at low temperatures. Since the value of the Fermi level pinning has been stabilized by treating the samples in an aqueous solution of Na2S, the field intensity depends mainly on the concentration of free carriers. The influence of the temperature of preliminary annealing of the substrate on the electron concentration in the epitaxial layer has been observed.

The structural and the photoelectrochemical properties of ZnO-ZnS/ITO 1D hetero-junctions prepared by tandem electrodeposition and surface sulfidation: on the material processing limits.

ZnO–ZnS 1D hetero-nanostructures were prepared by an easy and scalable processing route. It consists of ZnO nanorod electrodeposition on ITO substrate and surface sulfidation by ion exchange in an aqueous Na2S solution. Increasing the treatment contact time (tc) from 8 to 48 h involves different ZnS growth mechanisms leading to different structural and microstructural rod characteristics, even if the overall size does not change significantly. Grazing X-ray diffraction, high-resolution microscopy, energy-dispersive spectrometry and X-ray photoelectron spectroscopy describe the outer surface layer as a poly- and nanocrystalline ZnS blende shell whose thickness and roughness increase with tc. The ZnO wurtzite–ZnS blende interface goes from continuous and dense, at short tc, to discontinuous and porous at long tc, indicating that ZnS formation proceeds in a more complex way than a simple S2−/O2− ion exchange over the treatment time. This feature has significant consequences for the photoelectrochemical performance of these materials when they are used as photoanodes in a typical light-assisted water splitting experiment. A photocurrent (Jp) fluctuation of 45% for less than 5 min of operation is observed for the sample prepared with a long sulfidation time while it does not exceed 15% for that obtained with a short one, underlining the importance of the material processing conditions on the preparation of valuable photoanodes.

Coordination of the chemical and electronic processes in GaSb(100) surface modification with aqueous sodium sulfide solution

GaSb(100) surface electronic passivation with aqueous Na2S solution is driven by the redox reaction of gallium sulfide and elemental antimony formation.

Influence of the Reduction of Graphene Oxide with Hydroiodic Acid on the Structure and Photoactivity of CdS–rGO Hybrids

The temperature used in the chemical reduction of graphene oxide (GO) with hydroiodic acid has a significant influence on the removal of surface oxygenated functional groups, on the residual iodine species and on the rupture, stacking and graphitization of the graphene sheets in the reduced graphene oxides. The modification in the characteristics of the reduced graphene oxides induces changes in the surface area, the exposition of reduced graphene oxide entities and in the concentration of small CdS nanocrystals with strong confinement effect on the CdS-reduced graphene oxide hybrids. The hybridization of the reduced graphene oxide with CdS modifies in different way their photocatalytic behavior for hydrogen production from aqueous solutions of Na2S and Na2SO3 under simulated sunlight irradiation. Only the hybrid formed between the CdS and the reduced graphene oxide treated at higher temperature showed improved hydrogen production rate respect to the bare CdS reference associated with the better conductivity of the reduced graphene oxide and with the increase in the concentration of small CdS nanocrystals sith strong confinement effect observed in the hybrid.

Multiple exciton generation for photoelectrochemical hydrogen evolution reactions with quantum yields exceeding 100%

Multiple exciton generation (MEG) in quantum dots (QDs) has the potential to greatly increase the power conversion efficiency in solar cells and in solar-fuel production. During the MEG process, two electron–hole pairs (excitons) are created from the absorption of one high-energy photon, bypassing hot-carrier cooling via phonon emission. Here we demonstrate that extra carriers produced via MEG can be used to drive a chemical reaction with quantum efficiency above 100%. We developed a lead sulfide (PbS) QD photoelectrochemical cell that is able to drive hydrogen evolution from aqueous Na2S solution with a peak external quantum efficiency exceeding 100%. QD photoelectrodes that were measured all demonstrated MEG when the incident photon energy was larger than 2.7 times the bandgap energy. Our results demonstrate a new direction in exploring high-efficiency approaches to solar fuels. Multiple exciton generation has been shown to improve the performance of quantum-dot-based solar cells. Yan et al. now apply it to photoinduced hydrogen production and present a system using PbS quantum-dot photoelectrodes that yields an external quantum efficiency of over 100%.

Effect of the Support Nanostructure (Nanofibers and Nanotubes) on the Photoelectrochemical Performance of TiO2-CdO@CdS Semiconducting Architectures

Nanofibers and nanotubes of TiO2 were decorated with CdO@CdS type core@shell particles, via a facile route consisting of cadmium electrodeposition on previously formed TiO2 nanostructures, its thermal oxidation to obtain CdO and later surface sulfidation by ion-exchange in Na2S aqueous solution. The diffraction patterns (XRD) of the resulting hetero-nanostructures confirm the formation of the cubic CdO and CdS phases, whereas their Photoelectron spectra (XPS) indicate the sole presence of CdS on the surface. Interestingly, tested as photoanodes in a home-made photoelectrochemical (PEC) cell, they exhibit better performances, toward water oxidation and hydrogen generation than pristine TiO2 nanostructures. These improvements are attributed to a higher visible light absorbance and a convenient band alignment between TiO2-CdO and CdO-CdS. Moreover, although TiO2 nanofibers present a higher surface area to be coupled with CdO@CdS, TiO2 nanotubes coupled with CdO@CdS achieved the maximum photoresponse at −0.25 V vs SCE, under 1 sun illumination. It indicates that this nanostructure, because of its ability for directional electron transport, is the most suitable matrix to build efficient TiO2-CdO@CdS photoanodes for PEC use.

Fast and Simple Construction of Efficient Solar‐Water‐Splitting Electrodes with Micrometer‐Sized Light‐Absorbing Precursor Particles

Micrometer‐sized light‐absorbing semiconductor particles (usually prepared by high temperature synthetic techniques) hold the desirable merits of high crystallinity, low concentrations of bulk defects, and a decreased grain boundary density to reduce bulk recombination of photocarriers. However, solar‐water‐splitting electrodes assembled using them as precursors always produce very low photocurrents. This could be due to the lack of an effective fabrication and/or modification protocol applicable to assemble these micrometer‐sized semiconductor particles into suitable electrode configurations. A fast and simple fabrication scheme of drop‐casting followed by the necking treatment is developed to enable the micrometer‐sized precursor particles derived photoelectrodes to deliver appreciable photocurrent densities (>1 mA cm−2). By applying this fabrication scheme, photoelectrodes of solid‐state reaction derived Mo doped BiVO4 (≈4 μm, modified with oxygen evolution catalysts) and commercial WO3 (size ranging from 100 nm to >10 μm) have yielded photocurrent densities higher than 1 mA cm−2, while the photoelectrode composed of commercial CdSe (≈10 μm) is able to produce a photocurrent density higher than 5 mA cm−2 (in a Na2S aqueous solution). This strategy provides a new possible way, in addition to the predominant route of nanostructuring, to construct efficient solar‐water‐splitting electrodes.

Synthesis of CuInS2 by Sulfide Ion‐Exchange of CuIn(OH)5 Precursors

CuInS2 semiconductor was synthesized from CuIn(OH)5 precursors via facile anion exchange in aqueous Na2S solution at room temperature. CuIn(OH)5 was prepared from copper and indium salts and NaOH by co‐precipitation in aqueous solution as a polycrystalline. The sulfide exchange reaction did not alter the copper to indium ratio. The reaction resulted in stoichiometric amounts of sulfur in the CuInS2 sample at room temperature. Fourier transform infrared spectra and ion chromatography were used to analyze the impurities, including chloride and nitrate in the prepared samples. CuInS2 showed a chalcopyrite crystal structure after annealing at 500°C under 4% hydrogen‐mixed argon atmosphere. The bandgap of the CuInS2 thin film device was estimated to be 1.46 eV.

Photocatalytic hydrogen evolution from water splitting using Cu doped ZnS microspheres under visible light irradiation

Cu/ZnS microspheres have been successfully synthesized using microwave irradiation method without using any template. Cu/ZnS microspheres were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), inductively coupled plasma optima optical emission spectrometer (ICP-OES), X-ray photoelectron spectroscopy (XPS), diffused reflectance spectroscopy (DRS), and electrochemical impedance spectroscopy (EIS) methods. Tuning of band gap from 3.43 to 2.36 eV was successfully achieved upon doping copper (0–10%) into ZnS. The photocatalytic activity was investigated by photosplitting of water containing an aqueous Na2S solution under visible light irradiation. Among the prepared photocatalysts, the hydrogen evolution rate reaches the maximum of about 973.1 μmol h−1 g−1 for 2.0 mol% Cu2+ ion doped ZnS. Moreover, Cu/ZnS microspheres were found photocatalytically stable during the 48 h test runs.

Photocatalytic performance of Pd decorated TiO2–CdO composite: Role of in situ formed CdS in the photocatalytic activity

We report an enhanced photocatalytic activity for TiO2–CdO composite for hydrogen generation from aqueous solution of Na2S + Na2SO3 compared to pure CdO and TiO2. An in situ conversion of CdO to CdS is observed during the photocatalytic reaction resulting in the formation of a CdO–CdS–TiO2 ternary composite, which play a role in the enhanced activity. Presence of Pd as a co-catalyst in the composite increases the hydrogen production nearly ten times. As prepared composite exists as anatase phase of TiO2 and cubic phase of CdO. Characterization of the catalyst by powder XRD, SEM and TEM suggest that a nanosized TiO2 exists as a dispersed phase on aggregated CdO. UV-visible absorption spectrum of the composite shows features of both CdO and TiO2. Composite sample exhibits increased photocurrent response and has increased fluorescence lifetime for the charge carriers compared to pure CdO and TiO2. A detailed characterization of the used catalysts clearly shows the formation of CdS phase besides TiO2 and CdO. This is the first report on the photocatalytic activity of TiO2–CdO system for hydrogen generation from water due to in situ bulk formation of CdS during the photocatalytic reaction. The in situ formed CdS in the composite enhances the photocatalytic activity in two ways: (i) CdS, whose conduction band is at higher negative potential than CdO and TiO2, can reduce H+ more efficiently and (ii) efficient separation of photogenerated charges can occur in this composite system, resulting in increased lifetime for the charge carriers.

Construction of carbon-nickel cobalt sulphide hetero-structured arrays on nickel foam for high performance asymmetric supercapacitors

NiCo2S4 and carbon-NiCo2S4 hetero-structured nanosheet arrays have been firstly synthesized on nickel foams in a facile hydrothermal and CVD method. The products NiCo2S4 and carbon-NiCo2S4 are obtained in Na2S aqueous solution in a hydrothermal reaction from precursor of NiCox(OH)y and carbon-NiCo2O4 through anion exchange respectively, which are characterized by XRD, TEM, Raman analysis. The obtained binary metal sulphide NiCo2S4 nanosheet delivers better electrochemical activity and higher mass loading compared with NiCo2O4. In addition, carbon introduced enhances the capacitive behaviours and cycle performance for enhanced electric conductivity and stable architecture. The integrated carbon-NiCo2S4 electrode exhibits high areal capacitance of 8.33Fcm−2 and ultrahigh specific capacitance of 1893.2Fg−1 with a potential window of 0.8V and high mass loading of 4.4mgcm−2 and good cycle performance in a three-electrode system. The asymmetric supercapacitor is further assembled using carbon-NiCo2S4 as the positive electrode and activated carbon as the negative electrode. The integrated asymmetric supercapacitor demonstrates a high energy density of 68.82Whkg−1 at a power density of 47.83Wkg−1, which confirms its practical applicability.

CdS-loaded flexible carbon nanofiber mats as a platform for solar hydrogen production

Photocatalysis-based solar hydrogen (H2) has garnered attention as a renewable energy carrier, yet the technology is still immature. For practical operation of photocatalytic H2 production, we fabricated a novel heterojunction of CdS-loaded flexible carbon nanofiber (CNF) mats. CNF mats were prepared by electrospinning and carbonization. CdS nanoparticles were deposited in alcohol and water solutions by the successive ionic layer adsorption and reaction (SILAR) method. The former solution was found to be more effective in uniformly dispersing CdS particles in the CNF mats. When pristine CNF mats were treated with acid (hereafter CdS/a-CNF), the degree of CdS particle distribution was further enhanced. Moreover, H2 production was improved by a factor of 3 in an aqueous solution of Na2S (0.1 M) and Na2SO3 (0.1 M) under visible light (λ > 420 nm). Heat-treatment of CdS/a-CNF mats at 400 °C dramatically increased H2 production by 3.6 times due to the improved crystallinity of the CdS particles. In this heterojunction, the CNF mats have three roles: They work as a multi-scalable support for CdS particles, enhance photogenerated charge transfers, and catalyze H2 evolution.