Electric-field-induced self-assembled one-dimensional nanostructures in CdX and HgX (X=Se, Te) thin films

Self-assembling due to the presence of electric field during deposition in the spray deposited nanocrystalline CdX and HgX (X=Se and Te) thin films is reported. The films are deposited using solvothermally synthesized nanoparticles dispersed in 1-butanol and sprayed on the glass substrates at 200 °C without any voltage as well as by applying a voltage to the nozzle. The electron microscopy studies reveal the formation of nanorods for the films deposited with the voltage. The X-ray diffractograms of CdSe films deposited without voltage and with voltage show hexagonal crystal structure. On the other hand, CdTe films deposited without voltage show cubic crystal structure and the films deposited with voltage show hexagonal crystal structure. HgX films deposited without and with voltage show cubic crystal structure and there is no nanorod formation below 700 V. HRTEM studies show the growth direction of the CdX nanostructures. In view of the self-assembly observed under voltage, the induced dipole moment and the resultant electrostatic interaction are expected to be the driving force for the growth of nanorods. Blueshift in the band gap is observed for all the films deposited without and with voltages, and it is attributed to the quantum confinement effect due to the formation self-assembled one-dimensional nanostructures.     

Performance evaluation of molybdenum dichalcogenide (MoX2; X= S,Se, Te) nanostructures for hydrogen evolution reaction

Hydrogen evolution reaction (HER) using transition metal dichalcogenides (TMDs) have gained interest owing to their low-cost, abundancy and predominant conductivity. However, forthright comparisons of transition metal chalcogenides for HER are scarcely conducted. In this work, we report the synthesis of series of molybdenum chalcogenide nanostructures MoX₂ (X = S, Se, Te) via a facile hydrothermal method. Used as an electrocatalyst for HER, MoS₂ nanograins, MoSe₂ nanoflowers and MoTe₂ nanotubes could afford the benchmark current densities of 10 mA cm⁻² at the overpotentials of −173 mV, −208 mV and −283 mV with the measured Tafel slope values of 109.81 mV dec⁻¹, 65.92 mV dec⁻¹ and 102.06 mV dec⁻¹, respectively. Besides other factors influencing HER, the role of electronic conductivity in HER of these molybdenum dichalcogenides are elucidated. In addition, the presented molybdenum dichalcogenides in this work are also complimented with robustness as determined from high-current density stability measurements.     

Electrodeposition of HgCdTe thin films

Hg_xCd_1−xTe thin films in the compositional range x=0.05–0.6 were grown on SnO₂-coated glass substrates using the electrodeposition technique. The electrochemical bath consisted of aqueous solutions of CdCl₂, HgCl₂ and Te reacted with HNO₃, along with a complexing agent CH₃CN (acetonitrile). The conditions for the growth of HgCdTe from such a bath were ascertained with the help of cyclic voltammetry (CV) curves. Studies indicate that single-phase HgCdTe can be electrodeposited from the bath only if it is thoroughly mixed over a period of time, prior to the deposition. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy and electron microprobe studies. XRD data show that single-phase material was obtained over the whole range of compositions studied. The films were polycrystalline having the cubic fcc structure and the (1 1 1) orientation. The best stoichiometry HgCdTe films grown from the baths of composition X=0.15, 0.3 and 0.6 were obtained at deposition potentials 0.65, 0.7 and 0.75 V (versus saturated calomel electrode (SCE)) respectively, the corresponding film compositions being x=0.16, 0.4 and 0.6, respectively.     

Novel Janus 2D structures of XMoY (X,Y = O,S, Se,Te) composition for solar hydrogen production

The successful fabrication of H-phase Janus transition metal dichalcogenides (TMDs) has received considerable interest due to its great potential in photocatalytic applications. Here, new A′-XMoY (X/Y = O, S, Se, Te) Janus-type structures belonging to the family of TMDs were theoretically investigated for the first time in terms of photocatalytic water splitting via DFT calculations. For all compounds, the Raman spectra were calculated. The SMoO, SeMoO, SMoSe, SMoTe and SeMoTe compounds are dynamically stable and are semiconductors. Among all considered structures SMoTe is the most promising candidate for solar hydrogen production because valence and conduction bands perfectly engulf the redox potentials of water at both neutral and acidic media, opposite to SMoSe, SMoO, SeMoO suitable only in the acidic media, and SeMoTe – in the neutral media. Moreover, A′-SMoTe demonstrates the outstanding values of the solar-to-hydrogen (STH) conversion efficiencies of 54.0 and 67.1 for neutral and acidic media.     

Electrodeposition of ZnO thin films on n-Si(1 0 0)

In this study, ZnO thin films have been deposited onto monocrystalline n-type Si(1 0 0) by electrodeposition at different applied potentials. XRD shows a preferential orientation (0 0 0 2) that increases when the applied cathodic potential increases. The XPS analysis presents a Zn/O composition close to stoichiometric. SEM micrographs show a compact structure with localized platelets with a grain size of about 10 μm. However, crystallite size determined by the Scherrer method shows a size close to 2.50×10⁻² μm, then the grains can be considered as clusters of crystallites. Optical measurements were made on samples deposited on ITO/glass through the same procedures giving a band gap of 3.3 eV in agreement with the reported values for ZnO at room temperature.     

Cd(Zn,S)Se quaternary thin films for electrochemical photovoltaic cell application

In this study, Cd_1−xZn_xS_ySe_1−y (0 ≤ x = y ≤ 0.35) photoelectrodes are deposited via inexpensive facile chemical bath deposition. The effects of Zn and S doping on the compositional, microstructural, electrical, and optical properties of thin films were analysed. The electrochemical photovoltaic (EPV) cell of configuration Cd_1−xZn_xS_ySe_1−y/0.25M sulfide/polysulfide/C was assembled to examine the different performance parameters in light and in dark conditions. An EPV cell fabricated with the Cd_1−xZn_xS_ySe_1−y (0 ≤ x = y ≤ 0.075) photoelectrode exhibited a maximum photoconversion efficiency of 3.18%. This performance can be attributed primarily to the enhanced light-absorption ability of the material because of the enhanced rough microstructure and low recombination of photo-injected electrons with the electrolyte. The photovoltaic (PV) performance is significantly enhanced after doping CdSe with Zn and S.     

Catalytic activity for hydrogen evolution reaction of Janus monolayer MoXTe (X=S,Se)

At present, the precious metal Pt is a common catalyst for large-scale hydrogen evolution reaction (HER) production of hydrogen, but due to its high price and scarcity, finding an innovative catalyst has become the key to electrocatalytic hydrogen evolution. Here, the HER electrocatalytic activity of Janus MoXTe (X = S, Se) monolayers was investigated through first-principles calculations. Mo vacancy, X vacancy and Te vacancy were introduced into 2H, 1T, and 1T’ phase respectively and their stability was studied. The results show that the introduction of vacancy can improve the electrocatalytic hydrogen evolution performance. Particularly, the Gibbs free energies (ΔG_H) of Te vacancy of 2H phase MoSTe and MoSeTe are close to zero (ΔG_H = 0.03, −0.05 eV, respectively), and has the highest exchange current density. We further find that the conductivity of 2H phase MoSTe and MoSeTe is enhanced after introducing Te vacancy. In details, H get 1.86 and 1.43 e on V_Te in 2H phase MoSTe and MoSeTe. The bond between S and H is more stable, H is better adsorbed on the catalyst, and the performance is improved. Our research provides a strategy for designing MoXTe monolayer electrocatalysts, which are predicted to be employed in HER catalysts with low cost and high performance.     

Electrodeposition and characterization of ZnSe semiconductor thin films

In this work, results on the preparation and characterization of ZnSe thin films obtained by electrodeposition are presented. Voltammetric curves were recorded in order to characterize the electrochemical behavior of the Zn⁺²/SeO₂ system on different substrates. Thin films were deposited potentiostatically from an unstirred, deareated aqueous solution onto titanium, glass substrates coated with fluorine doped tin oxide and ITO glass substrates. The effect of main parameters such as the deposition potential, SeO₂ concentration and annealing on film composition and structure were analyzed. The as-grown and treated layers were characterized by X-ray energy dispersive analysis, X-ray diffraction, scanning electron microscopy and photoelectrochemical studies. Optical measurements were done on these samples which gave a clear band edge near 2.6 eV quite close to the accepted room temperature value of 2.7 eV for ZnSe.     

Depth profiling of Cu(In,Ga)Se2 thin films grown at low temperatures

In order to understand the effect of the process temperature on the growth of Cu(In,Ga)Se₂ (CIGSe) thin films via the 3-stage co-evaporation process, absorber layers have been fabricated on glass using a set of different maximum process temperatures in the nominal temperature range between 330 and 525 °C. Using energy dispersive X-ray analysis, depth profiles could be recorded on cross-sections of finished devices and were correlated to the device performance. The effect of the process temperature on the gallium gradient in the CIGSe layer is evident in the gallium distribution across the depth of the device.     

Hydrothermal preparation of MoX2 (X = S,Se, Te)/TaS2 hybrid materials on carbon cloth as efficient electrocatalyst for hydrogen evolution reaction

The prominent features-based two-dimensional (2D) materials have proven themselves as efficient as well as robust electrocatalysts for the hydrogen evolution reaction (HER) owing to their low-cost, abundance and predominant conductivity. In this work, we report the synthesis of series of molybdenum chalcogenide nanostructures MoX₂ (X = S, Se, Te), hybridized with TaS₂ nanosheets, via a facile hydrothermal method, on self-supported carbon cloth electrode. Used as an electrocatalyst for HER, hybrid phase MoSe₂/TaS₂/CC electrode with a Mo/Se ratio of 1:1.5 exhibits the best HER performance, which could afford the benchmark current densities of 10 mA/cm² at the overpotentials of 75 mV with the measured Tafel slope values of 54.7 mV/dec. In addition, the presented molybdenum dichalcogenides in this work are also complimented with robustness as determined from durability and air stability measurements. The unique aspects of these unique hybrids, such as 1T and 2H phases hybridized MoS₂ and MoSe₂, semimetallic nature 1T′-MoTe₂ petal clusters and strong interface interaction between MoX₂ (X = S, Se, Te) and conductive TaS₂ nanosheets, cause superior HER catalytic performance.     

Two-dimensional ABC3 (A = Sc,Y; B = Al,Ga, In; C = S,Se, Te) with intrinsic electric field for photocatalytic water splitting

Hydrogen production from water photolysis is considered to be one of the main approaches to obtain clean energy in the future. Two-dimensional (2D) materials with intrinsic vertical electric field can break the band gap limitation (>1.23 eV) of photocatalysts in traditional theory, and broaden the spectrum that can be used for photocatalytic water splitting, such as infrared light. Inspired by the ferroelectric phase α-B₂C₃ (B = Al, Ga, In; C = S, Se, Te), we replaced one of the B atoms with IIIB elements (namely Sc and Y) of the same valency, and successfully predicted 18 kinds of ABC₃ (A = Sc, Y; B = Al, Ga, In; C = S, Se, Te) monolayers with high stability. Encouragingly, 15 predicted ABC₃ monolayers possess direct band gap features in contrast to that of α-B₂C₃, which all exhibit indirect band gap feature. Due to the large intrinsic vertical electric field, the ABC₃ monolayers hold great potential for efficient photocatalytic water splitting in acidic and alkaline as well as neutral environments. The rectified solar-to-hydrogen (STH) efficiencies of 14 ABC₃ MLs were larger than 20%. Excitingly, the STH efficiency limits of YGaTe₃ and YInTe₃ even reach 35.97% and 36.94% using the full solar spectrum, which breaks the conventional theoretical efficiency limit. In addition, considerable light absorption and large carrier mobility are confirmed in ABC₃ MLs. These interesting findings render ABC₃ monolayers a strong candidate for future photovoltaic and nanoelectronic applications.     

Study of one-step electrodeposition condition for preparation of Culn(Se, S)2 thin films

Some condition parameters for the one-step electrodeposition to prepare the quaternary CuIn(Se, S)₂ thin films were studied. It was found that filtering out the precipitation in the source solution after several hours from the preparation of the solution was good to obtain a given composition quaternary films. The copper to indium ratio of the films changed drastically depending on the applied potential. It became easy to control the film composition in a relatively wide range of the applied potential by decreasing selenium concentration in the source solution.     

MS2 (M = W, Mo) photosensitive thin films for solar cells

Photosensitive textured WS₂ and MoS₂ films can be obtained by the techniques of reactive sputtering and solid state reaction, as long as the substrates used are each coated with a 10–20 nm Ni layer. When MS₂ (M = W, Mo) layers are deposited onto these substrates and then annealed for half an hour at 1073k in an argon atmosphere, textured films crystallized in the 2H-MS₂ structure are obtained, with their c crystallite axes perpendicular to the plane of the substrate. The films are nearly stoichiometric. The crystallinity enhancement of the films can be attributed to an improvement in the crystallization process related to liquid NiS phases present at the grain boundaries during annealing. Residual phases (Ni_xS_y; Ni;…) are distributed at the grain boundaries and do not strongly disturb the properties of the WS₂ and MoS₂ films. The optical absorption spectra are similar to those of single crystals, and the high photosensitivity of the films is attributed to a grain size enhancement by the NiS phase.     

Crystallisation of indium-tin-oxide (ITO) thin films

The goal of this study has been to investigate the microstructure changes of ITO films and their effects on the physical properties of the material in the amorphous to crystalline transition. ITO films deposited by electron-beam deposition directly onto substrates held at temperature up to 150 °C have an amorphous structure. They were highly absorbing and have poor electrical properties. During the thermal crystallisation process, the resistance decreases, and a simultaneous variation in the optical transmission occurs. In particular, as the film resistivity decreases, the visual appearence of the films change optically from very dark brown to transparent. Correspondingly, microstructure changes from amorphous to microcrystalline, and eventually to polycrystalline. It was found that the electron-beam induced crystallization process observed in the microscope can be defined as an initial gradual ordering, followed by a classical nucleation and growth. Before crystals can develop, ‘germ nuclei’ must be present in the parent phase. These nuclei act as centres for the crystallisation. Because of experimental spatial-resolution limitations, it is difficult to observed the formation of the initial, stable-crystal nuclei. After crystals have ordered, with dimensions of several unit cells, the crystallisation process progresses from these nuclei. The electron-beam crystallised particles are highly ordered and include both cubic and hexagonal phases of In₂O₃.     

Fabrication of Cu(In,Ga)Se2 thin films solar cell by selenization process with Se vapor

CIGS films were prepared on Mo-coated glass by sputtering and selenization processes. The metallic precursors were selenized under higher pressure in selenium vapor instead of H₂Se. In order to improve the performance of CIGS thin film solar cells, the morphologies of CIGS thin films were studied carefully by various temperature profiles. The relationship between temperature decrease rate and fill factor (FF) of solar cells was investigated thoroughly. On the other hand the value of open circuit voltage (V_oc) was improved by increasing the gallium content near the surface of CIGS thin film. A glass/Mo/CIGS/CdS/ZnO cell was fabricated and the conversion efficiency of 9.4% was obtained without antireflective film.     

Computational evaluation of 2D metal-organic frameworks with TMX4-centers (X = O,S and Se) for CO2 electroreduction

Electrochemical CO₂ reduction into value-added fuels and chemicals is regarded as a highly efficient way to achieve a carbon neutral cycle. Recently, two-dimensional metal-organic frameworks (2D MOFs) have attracted much attention in CO₂ reduction reaction (CO₂RR). Herein, we employed density functional theory (DFT) to study the catalytic performance of 48 kinds of the π-d conjugated 2D layered MOFs, i.e., TM-BHX, composed of transition metal ions and multidentate organic ligands, such as benzenehexaol (BHO), benzenehexathiol (BHT) and benzenehexaselenolate (BHS), for CO₂RR. By investigating the thermodynamic stability and electrochemical stability, conductivity and the free energy change of the first hydrogenation step (CO₂ + H⁺ + e^? → 1COOH or CO₂ + H⁺ + e^? → 1HCOO), nine TM-BHX were selected from 48 MOFs, including TM-BHT (TM = Cr, Fe, Co, Ru, Rh, Ir) and TM-BHS (TM = Ru, Rh, Ir). Possible reaction pathways of CO₂ reduction into C1 products were explored to determine the CO₂RR mechanism. Our results showed that among 9 candidates, Cr-, Fe-, Co-BHT, and Ir–BHS not only exhibit high activity with low limiting potential (?0.30, ?0.29, 0, and ?0.49 V, respectively), but also have high CO₂RR selectivity with the positive value of U_L(CO₂) – U_L(H₂), so they are promising CO₂RR electrocatalysts. This work provides a new kind of 2D MOFs as efficient CO₂RR electrocatalysts for experimental research.     

Preparation and characterization of Cu(In,Ga)(Se,S)2 thin films from electrodeposited precursors for hydrogen production

Semiconducting Cu(In,Ga)(Se,S)₂ thin films were made from electrodeposited Cu(In,Ga)Se₂ precursors, followed by physical vapor deposition of In₂S₃, Ga, and Se. The bandgaps of these materials were found to be between 1.6 and 2.0 eV, which spans the optimal bandgap necessary for application for the top junction in photovoltaic multijunction devices and for unassisted water photolysis. These films were characterized by electron-probe microanalysis, scanning Auger spectroscopy, X-ray diffraction, and photocurrent spectroscopy.     

Preparation of Cu(In,Ga)(S,Se)2 thin films by sequential evaporation and annealing in sulfur atmosphere

Cu(In,Ga)(S,Se)₂ thin films with high Ga/III ratio (around 0.8) were prepared by sequential evaporation from CuGaSe₂, CuInSe₂, In₂Se₃ and Ga₂Se₃ compounds and then annealing in H₂S gas atmosphere. The annealing temperature was varied from 400 to 500 °C. These samples were characterized by means of XRF, EPMA, XRD and SEM. The S/(S+Se) mole ratio in the thin films increased with increase in the annealing temperature, keeping the Cu, In and Ga contents nearly constant. The open circuit voltage increased and the short circuit current density decreased with increase in the annealing temperature. The best solar cell using Cu(In,Ga)(S,Se)₂ thin film with Ga/(In+Ga)=0.79 and S/(S+Se)=0.11 annealed at 400 °C demonstrated V_oc=535 mV, I_sc=13.3 mA/cm², FF=0.61 and efficiency=4.34% without AR-coating.     

A superstrate solar cell based on In2(Se,S)3 and CuIn(Se,S)2 thin films fabricated by electrodeposition combined with annealing

Stacked thin films composed of In₂(Se,S)₃ and CuIn(Se,S)₂ layers were grown on a fluorine-doped tin oxide (FTO)-coated glass substrate using electrodeposition of the corresponding selenide (In₂Se₃ and CuInSe₂) precursors followed by annealing in H₂S flow (5% in Ar). Structural characterizations of both layers revealed that the resulting film quality strongly depended on annealing conditions of both CuIn(Se,S)₂ and In₂(Se,S)₃ layers: a compact and uniform film was obtained by annealing both layers at 400 °C. Performance of Au/CuIn(Se,S)₂/In₂(Se,S)₃/FTO superstrate-type solar cells also followed these structural characteristics, i.e., a preliminary conversion efficiency of 2.9% was obtained on the device based on 400 °C-annealed In₂(Se,S)₃ and CuIn(Se,S)₂ layers.