The structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition by GISAXS and Raman spectroscopy

Thin silicon films were deposited by the plasma-enhanced chemical vapour deposition method using microwave (MW) and standard radio frequency (RF) gas discharge in silane gas diluted by hydrogen in the range that produces a mixture of amorphous and crystalline phases. The samples were analysed by Raman spectroscopy and grazing incidence small-angle X-ray scattering (GISAXS), while the threshold for the transition between the amorphous and crystalline phase was checked by the change in electrical conductivity. The crystalline fraction, estimated by Raman spectroscopy, varied between 0% and 70% while the individual crystal sizes were between 3 and 9 nm. However, the size distribution was broad suggesting also the existence of smaller and larger crystals.The “particles” observed by GISAXS, most probably voids, were in the range between 2 and 12 nm. The voids in samples deposited by MW plasma were larger when closer to the surface. Their shape indicated the formation of a columnar structure perpendicular to the surface, more pronounced at higher temperature. The samples deposited by RF plasma and low power had spherically symmetric “particles” with uniform size across the depth of the samples. An increase of the RF power resulted in the formation of a columnar structure parallel to the surface. The observed differences are discussed in relation to the difference in growing kinetics of the used deposition methods.     

Self-assembled ionic liquid synthesis of nitrogen-doped mesoporous TiO2 for visible-light-responsive hydrogen production

Nitrogen-doped mesoporous TiO₂ has been synthesized by a simple solvent evaporation-induced self-assembly method using a nitrogen-containing ionic liquid concurrently as a nitrogen source and mesoporous template. After being evaporated and subsequently calcined at various temperatures (300–900 °C), the synthesized samples were thoroughly characterized by X-ray diffraction (XRD), Raman, small-angle X-ray scattering patterns (SAXS), N₂ adsorption-desorption isotherms, X-ray photoelectron (XPS) and UV–Vis diffuse reflectance (UV–Vis DR) spectroscopies. The obtained results suggest that the calcination temperature greatly influences the crystallization of TiO₂, formation of mesoporous structure, specific surface area and N-doping amounts. Among the fabricated photocatalysts, the samples calcined at 600 °C exhibit superior photocatalytic performance for hydrogen production in water/methanol solution under visible light illumination if compared to other synthesized samples and commercial TiO₂ (Degussa P25). The finding is possibly due to the synergy of more N-doping amounts on the well-defined mesoporous TiO₂ with highly anatase crystal phase and moderate surface area in the catalysts.     

Synchrotron-based spectroscopy for the characterization of surfaces and interfaces in chalcopyrite thin-film solar cells

The experimental CISSY setup was constructed for the surface and interface analysis of chalcopyrite Cu(In_1−xGa_x)(S_ySe_1−y)₂ “CIGSSe” thin-film solar cells operable as laboratory surface analysis system using commercial X-ray and UV sources or as end station at the BESSY synchrotron facility. It houses an X-ray spectrometer for X-ray emission (XES) and an electron analyzer for photoemission (PES) spectroscopy measurements. The techniques deliver information about the chemical and electronic sample structure on a complementary depth scale. With probing depths up to half a micrometer, XES provides information of the near-surface sample bulk. PES in contrast only probes the first monolayers of a sample and hence, is very surface sensitive. The special feature of the CISSY setup is the unique combination of these spectroscopies with in-system sputter and wet-chemical preparation capabilities. Furthermore, a programmable sample manipulator enables laterally resolved measurements or measurements while the sample is constantly moved, reducing damage of radiation sensitive material, as e.g. organics. This arrangement allows for the characterization of real-world sample surfaces and interfaces prepared under controlled conditions such as vacuum or inert gas. The main focus of the CISSY experiments is a better understanding of the solar cell functioning mechanisms by combining the knowledge of the chemical and electronic structures gained by spectroscopic investigation of involved components and the physical device properties.The paper will give an overview of important results from eight years of CISSY research. In that period of time CISSY has tackled three main research areas: (i) the CIGSSe absorber layer; (ii) the buffer/absorber interface; and (iii) laterally resolved and time-resolved measurements, particularly of chalcopyrite thin-film solar cell components and structures. This review will focus on some examples out of these topics including results which could only be obtained by the combination of in-system preparation and analysis capabilities as realized by the CISSY setup.     

Experimental study of graded bandgap Cu(InGa)(SeS)2 thin films grown on glass/molybdenum substrates by selenization and sulphidation

High-performance Cu(InGa)(SeS)₂ (CIGSS) thin film absorbers with an intentionally graded bandgap structure grown by a two-stage method have been studied. Materials obtained from Showa Shell Sekiyu K.K., Japan have been grown using selenization and sulphidation of the Mo/Cu–Ga/In stacked precursors. Full characterizations have been carried out using X-ray diffraction, Raman, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence, inductively coupled plasma mass spectroscopy, glow discharge optical emission spectroscopy (GDOES) and photoelectrochemical (PEC) techniques to study various properties. The material layers were found to be polycrystalline with the (1 1 2) preferred orientation, and the largest grains were about 2 μm. Raman measurements show the presence of at least five different phases within the material. XPS confirmed the copper depletion and the richness of sulphur at the top surface region. Although the PEC studies indicate the overall electrical conductivity of the layer as p-type, GDOES profiling reveals the segregation of different phases at different depths suggesting the possibility of having buried junctions within the material itself. The results are presented together with suggestions for further improvements of CIGSS solar cell material.     

FT-IR and micro-Raman spectroscopic characterization of minerals in high-calcium coal ashes

Two high-calcium coal ashes were prepared in a muffle furnace at 815 °C. The mineral matter in both coal ashes was characterized by Fourier transform infrared (FT-IR) spectroscopy, micro-Raman spectroscopy, and X-ray diffraction (XRD). The overlapping bands of original FT-IR spectra were resolved into individual ones by using second derivative method. The presence of two most intense absorption bands (1154 and 1120 cm^?1) in the original spectra indicate that both coal ashes contain high levels of anhydrite, consistent with the XRD result. The presence of amorphous silica and metakaolinite was found from the second derivative spectra. Calcite and anhydrite in both ashes show marked Raman bands at 1086 and 1017 cm^?1, respectively. In addition, the Raman intensity of anatase in both ashes is very strong, due to the high polarizability of TiOTi. FT-IR and micro-Raman spectroscopy are complementary for the identification of anhydrite in coal ashes. Moreover, a combination of both spectroscopic techniques can provide more information on mineral composition and structure as compared to XRD, since XRD fails to identify the presence of amorphous silica, metakaolinite, and anatase.     

Corrosion resistance of pyrolytic graphite in LiCl-KCl-UCl3 molten salt for pyrochemical reprocessing application

Pyrolytic graphite (PyG) is a highly oriented, dense and crystalline form of graphite, which exhibits superior air oxidation resistance, corrosion resistance, and favourable mechanical, thermal, and electrical properties compared to conventional graphite materials. It is proposed as the material of construction for high-temperature molten LiCl-KCl for pyrochemical reprocessing of metallic fuel. In the present study, long-term corrosion behaviour of PyG in LiCl-KCl molten salt with 5?wt-% UCl₃ was evaluated by immersion studies at 873?K for 2000?h, under inert argon atmosphere. Characterisation of PyG before and after molten salt exposure was carried out using X-ray diffraction technique, scanning electron microscope with energy dispersive spectroscopy, laser Raman spectroscopy and X-ray photo-electron spectroscopy. The results revealed the superior corrosion resistance and excellent phase stability of PyG with negligible weight change and no appreciable change in the surface chemistry and morphology up to the exposure time of 2000 h.     

High visible-light photocatalytic hydrogen evolution of C,N-codoped mesoporous TiO2 nanoparticles prepared via an ionic-liquid-template approach

A simple and novel method was developed to fabricate carbon and nitrogen codoping mesoporous TiO₂ (CNMT-x) through evaporation induced self-assembly by using an ionic liquid simultaneously as carbon, nitrogen sources and a mesopore creator. These CNMT-x samples were fully characterized by a series of spectroscopic and analytical techniques, such as small- and large-angle X-ray diffraction (XRD), N₂ adsorption–desorption isotherms, transmission electron microscopy (TEM), Raman, ultraviolet–visible (UV–vis) diffuse reflectance and X-ray photoelectron (XPS) spectroscopies. The obtained results showed that CNMT-0.75 catalysts exhibited the optimal photocatalytic hydrogen generation activity in water/methanol sacrificial reagent system under visible light irradiation, which was highly superior to those of conventionally prepared C-doped and commercial TiO₂. The superior photocatalytic performances observed for CNMT-x could be attributed to the synergistic effects of C,N-codoping with high surface area of mesostructured TiO₂.     

Structural analysis of PdRh/C and PdSn/C and its use as electrocatalysts for ethanol oxidation in alkaline medium

The Pd/C, PdRh(50:50)/C and PdSn(50:50)/C nanomaterials were used as electrocatalysts for ethanol (EtOH) oxidation in Direct Ethanol Fuel Cell (DEFC) in an alkaline medium. This work aims to provide a complete physical characterization of the nanomaterials, elucidate the bifunctional mechanism concerning ethanol oxidation reaction and understand the influence of carbon – metal bonding in the electrochemical activity. These nanomaterials were investigated by X-ray photoelectron spectroscopy (XPS) and revealed that the atomic percentage of the surface is different of those obtained by Energy Dispersive X-ray spectroscopy (EDS). Raman spectroscopy showed a bonding between palladium and carbon atoms which can play a decisive role in the performance of the materials. Attenuated Total Reflectance technique coupled to the Fourier Transform Infrared spectroscopy (ATR-FTIR) made possible to investigate the oxidation products originated by the ethanol oxidation, and all the electrocatalysts showed the presence of acetaldehyde, carbonate ions, acetate and carbon dioxide, suggesting that the mechanism of oxidation is incomplete. Among all the nanomaterials studied, PdSn(50:50)/C showed the best electrochemical and Fuel Cell's results. It is about 33% better than Pd/C. The micrographs obtained by Transmission Electron Microscopy (TEM) revealed some agglomerate regions, but they are consistent with the literature data.     

Growth and characterization of stepwise flash evaporated CuInTe2 thin films

CuInTe₂ films grown by stepwise flash evaporation onto glass and silicon substrates held at 573 K were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), Rutherford backscattering spectrometry (RBS) and Raman spectroscopy. XRD and TEM studies showed the formation of single-phase polycrystalline CuInTe₂. Results of the RBS measurements showed the films to be near-stoichiometric and negligible diffusion of elements across the CuInTe₂/Si interface. Various lattice vibrational modes identified by Raman measurements were found to match well with those reported for single-crystal CuInTe₂, confirming the crystalline quality of the CuInTe₂ thin films.     

A novel co-electrodeposited Co/MoSe2/reduced graphene oxide nanocomposite as electrocatalyst for hydrogen evolution

In this study, a simple and fast electrochemical method was employed to synthesis molybdenum diselenide thin film. The morphology, structure and chemical composition of the nanocomposites were investigated by field emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The progressive effects of transition metal ions including Ni, Cu, and Co were surveyed on the hydrogen evolution activity of MoSe₂ thin films. Co/MoSe₂ nanocomposite thin films has significant electrocatalytic activity as compared to other samples, In order to achieve higher performance, preparing Co/MoSe₂/RGO nanocomposite thin film, two strategies including layer by layer electrodeposition and co-electrodeposition has been employed. The presence of reduced graphene oxide leading to the onset potential shifts to more positive values and increase the current density. Also, results showed that the Co/MoSe₂/RGO nanocomposite prepared by co-electrodeposition exhibits the best electrochemical hydrogen evolution at onset potential of −0.18 with an overpotential of −0.45 V.     

A glassy carbon electrode modified with tailored nanostructures of cobalt oxide for oxygen reduction reaction

Herein we report on various surface morphological characteristics of the synthesized cobalt oxide (Co₃O₄) nanostructures obtained by means of facile one-step hydrothermal method for oxygen reduction reaction (ORR). The synthesized nanostructures of Co₃O₄ were adequately characterized by field emission scanning electron microscopy (FESEM) fitted with Energy-dispersive X-ray spectroscopy (EDX) elemental mapping, X-ray diffraction (XRD) and Raman techniques. The electrochemical studies were carried out to analyse the performance of as-synthesized catalysts for ORR by cyclic voltammetry (CV), and chronoamperometric (CA) techniques. A higher electrocatalytic response was observed for Co₃O₄ nanocubes compared with all the other controlled electrodes by CV with a current density of 0.69 mA/cm² at a potential value of −0.46 V. The as-synthesized material showed adequate tolerance against methanol observed by CV in the presence of 0.5 M methanol, and good stability when compared with commercial Pt/C catalyst using the CA technique.     

Oxygen-doped activated carbon fiber cloth as electrode material for electrochemical capacitor

Novel oxygen-doped activated carbon fiber cloths (OACFC), with different compositions of surface oxygen functionalities, have been prepared by direct electrooxidative/reductive methods in an undivided electrolytic cell filled with high purity water without a supporting electrolyte under high voltage conditions. The morphology and surface chemical composition of the materials have been investigated by SEM, Raman and XPS spectroscopies. They revealed an electrochemical erosion of the CF surface upon activation, concomitant with a strong change of the D/G ratio of characteristic Raman bands and the surface O/C atomic ratio, respectively. Thus pretreated material was tested as electrodes for an electrochemical capacitor by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy measurements in 3.75 M H₂SO₄. The performance of the electrochemical capacitor based on modified carbon electrodes was compared to that of an analogous device with unmodified carbon. The measurements revealed altered electrochemical behavior of the OACFC in terms of the determined capacitances. The proposed activation method is also superior to other electrochemical activation procedures, since it uses much less energy per CF surface or mass.     

ORR performance evaluation of Al-substituted MnFe2O4/ reduced graphene oxide nanocomposite

Active and durable oxygen reduction reaction (ORR) catalysts are of utmost importance to realize the commercialization of hydrogen fuel cells and metal-air batteries. Al-substituted MnFe₂O₄-based ternary oxide and reduced graphene oxide (MAF-RGO) nanocomposite is synthesized using an in-situ co-precipitation followed by a hydrothermal process and verified for ORR electrocatalysis in the alkaline electrolyte (0.1 M KOH). MAF-RGO is first analyzed using physicochemical characterization tools including X-ray diffraction, Raman spectroscopy, sorption studies, electron microscopy, X-ray photoelectron spectroscopy, etc. Further, the characteristic ORR peak centered at 0.56 V vs. reversible hydrogen electrode (RHE) in cyclic voltammetry (CV) studies confirms the electrocatalytic performance of MAF-RGO. The ORR onset potential of 0.92 V vs. RHE is obtained in linear sweep voltammetry (LSV) measurement at 1600 rpm in O₂-saturated electrolyte exhibiting an improved ORR performance as compared to the commercial electrocatalyst. The reduction kinetics is observed to follow the desirable near 4-e^- mechanism. In addition, the electrocatalyst exhibits improved relative current stability of 86% and methanol poisoning resistance of 82%, which is better in comparison to the standard Pt/C. The observed electrochemical performance results from the synergism between the oxygen vacancy-rich Al-substituted metallic oxide active species and the functional group enriched predominantly mesoporous RGO sheets with excellent electrical conductivity. The introduction of metallic species enhanced the inter-planar spacing between graphitic sheets easing the maneuver of reactant species through the electrocatalyst and accessing more ORR-active sites. This study establishes the potency of mixed transition metal oxide/nanocarbon composites as durable high-performance ORR-active systems.     

Optical spectroscopy study of nc-Si-based p–i–n solar cells

In the present study we analyzed nanocrystalline silicon (nc-Si)-based p–i–n thin film structures (SiC/nc-Si/n-doped amorphous Si) on glass produced by radio-frequency plasma-enhanced chemical vapor deposition. The crystallinity of the nc-Si layer was modified by varying the deposition conditions ([SiH₄]/[H₂] ratio in the plasma and radio-frequency power). Structural properties of the samples (crystalline fraction and crystal size distribution) were inferred by Raman spectroscopy. Different optical spectroscopy methods were combined for the determination of the optical constants in different spectral ranges: spectrophotometry, ellipsometry and photothermal deflection spectroscopy. Characterization results evidence that the optical properties of the nc-Si layers are strongly connected with the layer structural properties. Thus, the correlation between density of defects, Urbach energy, band-gap and line-shape of dielectric function critical points with the crystalline properties of the films is established.     

A hydrothermal route for constructing reduced graphene oxide/TiO2 nanocomposites: Enhanced photocatalytic activity for hydrogen evolution

A series of reduced graphene oxide/TiO₂ (RGO/TiO₂) nanowire microsphere composites were synthesized with a facile one-step hydrothermal method using TiCl₃ and graphene oxide (GO) as the starting materials, during which the formation of TiO₂ and the reduction of GO occur simultaneously. The obtained nanocomposites were characterized with X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, respectively. UV–vis absorption spectra showed that the absorption edges of TiO₂ were extended into visible light region with the addition of RGO. The photocatalytic activities of the samples with and without Pt as cocatalysts were evaluated by hydrogen evolution from water photo-splitting under UV–vis light illumination. Enhanced photocatalytic properties were observed for the as-prepared RGO/TiO₂ nanocomposites. The amount of hydrogen evolution from the optimized photocatalyst reached to 43.8 μmol h⁻¹, which was about 1.6 times as high as that of bare TiO₂. The results shown here indicate a convenient and applicable approach to further exploitation of high activity materials for photocatalytic water splitting applications.     

Characteristics and structural change of layered polysilane (Si6H6) anode for lithium ion batteries

Layered polysilane (Si₆H₆) has a graphite-like structure with higher capacity than crystalline silicon. The rate of increase of the thickness of a layered polysilane electrode after 10 charge-discharge cycles was smaller than that for a Si powder electrode, although the layered polysilane electrode has higher capacity. The structural changes of electrochemically lithiated and delithiated layered polysilane at room temperature were studied using scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Layered polysilane became amorphous by insertion of lithium to 0 V, whereas insertion of lithium into crystalline silicon produces Li₁₅Si_4. Layered polysilane maintained an amorphous state during lithium insertion and deinsertion, whereas silicon changed between Li₁₅Si₄ and amorphous Li_xSi, which explains the smaller volume change of a layered polysilane electrode compared with a Si powder electrode.     

Carbothermal synthesis of Sn-based composites as negative electrode for lithium-ion batteries

The composite [Sn-BPO₄/xC] to be used as negative electrode material for the storage of electrochemical energy was obtained by dispersing electroactive tin species onto a BPO₄ buffer matrix by carbothermal reduction of a mixture of SnO₂ and nanosized BPO₄. This composite material was thoroughly characterized by X-ray diffraction, Scanning Electron Microscopy, ¹¹⁹Sn Mössbauer spectroscopy and Raman spectroscopy. The electrochemical tests of this new material highlight its very interesting electrochemical properties, i.e., a discharge capacity of 850 mAh g⁻¹ for the first cycle and reversible capacity around 585 mAh g⁻¹ at C/5 rate. These electrochemical performances are attributed to the very high dispersion and stabilisation of tin metal particles onto the BPO₄ matrix. The irreversible capacity observed for the first charge/discharge cycle is due the reduction of interfacial Sn^II species and to the passivation of the anode surface by liquid electrolyte decomposition (formation of the SEI layer).     

Pt–Co cathode electrocatalyst behaviour viewed by in situ XAFS fuel cell measurements

The paper presents a preliminary structural investigation of the 20% Pt–Co (1:1) alloy on Vulcan XC-72 catalyst using X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). XAS results have been obtained ex situ and in situ using a specially optimized for XAS measurement fuel cell (down to 6 keV). The results are compared with those obtained for pure Pt catalyst on the same carbon support under the same working conditions.     

Synthesis,characterization and electrochemical properties of La2-xEuxNiO4+δ Ruddlesden-Popper-type layered nickelates as cathode materials for SOFC applications

Eu doped La₂NiO₄ powders, with the general formula La_2-xEu_xNiO_4+δ denoted as LENO_x (for x = 0, 0.2, 0.4, 0.6 and 0.8), were synthesized via the mechanical milling reaction method. The Eu³⁺ doping content has a remarkable influence on structural and electrochemical properties. The phase identification and morphology were studied by X-ray diffraction (XRD), Raman spectroscopy, Infrared spectroscopy (IR), A laser size analyzer and scanning electron microscopy (SEM). Lattice parameters were calculated using the Rietveld method. It was observed that the lattice parameter values in LENO_x systems varied with the amount of Eu³⁺. The latter was symmetrically deposited by spin coating on both surfaces of an Ce_0.8Sm_0.2O_1.9 (SDC) electrolyte and studied using AC impedance spectroscopy. The electrochemical properties were studied using two-probe impedance spectroscopy and results showed that the ASR of LENO_x was enhanced by the Eu³⁺ dopant content x. Results also showed that LNEO_0.2 had the lowest Area specific resistance (ASR) at 700 °C and it was therefore concluded that doping with the appropriate amount of Eu³⁺ can further improve the properties of a nickelate cathode.     

Simple synthesis and characterization of SrSnO3 nanoparticles with enhanced photocatalytic activity

SrSnO₃ nanoparticles with peanut-like morphologies were synthesized by a simple wet chemical reaction. These peanut-shaped SrSnO₃ were formed by the fusion of two or more nanoparticles with an average size of 45 nm. The resulting powders were characterized in detail using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Moreover, the photocatalytic activity for hydrogen evolution from pure water was investigated under UV light irradiation. The peanut-shaped nanoparticles exhibited a much higher photocatalytic activity compared to SrSnO₃ powder synthesized by a solid-state reaction. This was attributed to their higher structural order, caused by the formation of a carbonate-free pure phase, as well as their higher surface area resulting from the decrease in the particle size.