The investigation of the physical properties and hydrogenated mechanism of TM5Si4 (TM=Ti,Zr, Hf)

The outstanding physical properties make TM₅Si₄ silicides become the potential silicon-based transition-metal ultrahigh-temperature materials. In present work, we adopt the first-principles scheme to explore the structural stability, mechanical properties and explain the hydrogenated mechanism of Ti₅Si₄, Zr₅Si₄ and Hf₅Si₄ using the electronic structures. And the investigation increases the theoretical support for the developments and applications of TM₅Si₄ silicides. Three hydrogenated models have shown that the hydrogen displays the stability for hydrogenated TM₅Si₄ compounds. Furthermore, the introduction of hydrogen occupation has weakened the elastic properties of TM₅Si₄. The metallic property of TM₅Si₄ and three hydrogenated models was confirmed by the electronic structures. The localized hybridization between hydrogen and TM₅Si₄ confirm the hydrogenated structural stability.     

The effects of dynamic structural transformations on hydrogenation properties of Mg and MgNi thin films

Mg capped with Al and Ti thin layers and Mg_xNi films have been sputter-deposited on quartz substrates and hydrogenated at 600 kPa for 250 °C. A complete fast transformation of metallic into hydride phase is registered for the films, demonstrating the dynamic state of the internal microstructure under hydrogenation. It leads to local and long-range restructuring and the fast hydrogenation rate is attributed to the fast hydrogen uptake and transport along columns and grain boundaries of nanocrystallites. A slow H-loading is observed when the dynamic structural relaxation processes are suppressed by internal and external inhomogeneities such as barrier layers on the surface, new phases in the bulk and impurities. A partial transformation of metallic into hydride phase is registered when the structural formations newly nucleated at the initial stages of hydrogenation suppress dynamic processes and prevent the H-uptake.     

Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2

PtSe₂ monolayer is previously predicted to be a two-dimensional water-splitting photocatalyst. However, the weak van der Waals (vdW) interaction between H₂O and the basal surface of PtSe₂ significantly undermines its photocatalytic water-splitting activities. In this work, we explore the possibility of various intrinsic defects of PtSe₂ in remedying this deficiency on the basis of first-principles calculations. It is interesting to find that the introduction of Pt@Se, Se@Pt, and Se interstitial defect not only fully retain the water redox abilities of pure PtSe₂ and realize spatial separation of photogenerated electrons and holes, but also can extend optical absorption range and absorption coefficients. Moreover, introduction of the three kinds of defects increase the initial weak vdW interactions between H₂O and the PtSe₂ surface to different extent. In particular, Pt@Se anti-site defect transform the initial weak vdW to strong chemical interaction between H₂O and PtSe₂ surface, and function as active reaction site. These insights demonstrate that introduction of intrinsic defects, especially the Pt@Se anti-site defect, are effective means for improving the photocatalytic water-splitting activities of PtSe₂ monolayer.     

Imaging the hydrogenation of Mg thin films

Among the metal hydride materials, magnesium (Mg) and its alloys show excellent performance for hydrogen storage. The main drawback is the slow hydrogen absorption and desorption kinetics, the sole barrier to commercial adoption. In this work we use Mg thin films as model materials in order to study these kinetics, and observe the growth process of the hydride. Palladium (Pd) is used as a catalyst coating for improving the conditions of hydrogenation. The hydride formation is followed by in-situ X-ray diffraction. Microscopic imaging of the co-existence of Mg and MgH₂ is presented. The microstructure change is clearly visible in the micrographs, despite the fact that sample preparation damages the hydride phase. The transformation from columnar grains of the as-deposited Mg thin film, to a grainy equi-axed structure film indicate that the hydride is observed. The hydride is immediately formed at the interface between the Pd and the Mg thin film and grows in a layer-like reaction towards the substrate (SiO₂). These combined techniques provide an efficient methodology to follow the kinetics of hydride formation within the layer, and study further the diffusion coefficients and mechanism of hydrogenation.     

Theoretical study of the rutile based semiconductor with visible-light responsive photocatalytic activity for water splitting

The cluster expansion formalism combined with the accurate first-principles calculations were performed to investigate the stabilities, the electronic structures and the optical absorption of the rutile based semiconductor TiPt₇O₁₆. Our comprehensive calculations have predicted that TiPt₇O₁₆ is the most stable structure among the hundreds of the configurations of Pt substituted rutile TiO₂ (Ti_1-xPt_xO₂ (R), x = 0–1). More importantly, the accurate HSE06 calculations indicate that TiPt₇O₁₆ is a potential visible-light responsible photocatalyst with the band gap of 1.70 eV. In addition, the Pt-terminated (010) and Ti & Pt-terminated (011) surfaces are suitable for catalyzing the overall water splitting, due to their band edges span over the required redox potential.     

Effect of annealing on the electrical, optical and structural properties of hydrogenated amorphous silicon films deposited in an asymmetric R.F. plasma CVD system at room temperature

This paper reports the effect of annealing on hydrogenated amorphous silicon films (a-Si : H) deposited by r.f. self-bias technique on cathode in an asymmetric r.f. plasma CVD system at room temperature. Detailed study of the variation of the dark and photoconductivity (σ_D and σ_ph) as a function of temperature and light intensity, surface morphology, hydrogen evolution, optical absorption, subgap absorption and related parameters, thermal and structural disorder on the optical-absorption edge, IR vibrational modes and bonded hydrogen content have been carried out on unannealed and annealed samples at different temperatures (T_a) from 100°C to 550°C. It is found that the values of σ_ph increase and that of Urbach energy (E_o), subgap defect density (N_d) and the polyhydride to monohydride ratio decrease upto T_a=250°C and beyond 250°C the values of σ_ph decrease and that of E_o, N_d and the polyhydride to monohydride ratio increase. The best opto-electronic properties with much improved σ_ph and σ_ph/σ_D and dominant monohydride bonding are obtained after annealing the room temperature deposited film at 250°C for 1 h. The σ_D data obeys a Meyer Neldel rule in annealed a-Si : H films. The value of optical band gap is found to be related to the E_o and the hydrogen content. The Urbach energy (E_o) which is a measure of the disorder is the sum of structural and thermal disorder. The structural disorder part decreases with the annealing temperature upto 300°C and thereafter it increases. The curves of optical absorption coefficient versus photon energy at different T_a converge to a common point.     

Hydrogenation effect on structural,electrical and optical properties of CdS thin films for solar cell

Adsorption effects would be expected to be of considerable importance with thin films because of the changes in electron location accompanying adsorption. The effects of hydrogenation on structural, optical and electrical properties of the CdS thin films have been reported. GIXRD patterns shows that films have polycrystalline nature with a hexagonal structure. The optical band gap increased after hydrogenation of the film. The variation of conductivity of CdS films have been investigated depending upon the applied voltage at room temperature. The resistivity increased after hydrogenation of the films. Hydrogenated thin films can be used in solar cells because hydrogen plays an important role to modify the physical properties.     

First-principles study on the dehydrogenation properties and mechanism of Al-doped Mg2NiH4

Mg₂NiH₄, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg₂NiH₄, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg₂Ni and a destabilization of the hydride Mg₂NiH₄, which significantly alters the hydrogen desorption enthalpy ΔH_des for the reaction Mg₂NiH₄ → Mg₂Ni + 2H₂. A desirable enthalpy value of ∼0.4 eV/H₂ for application can be obtained for a doping level of x ≥ 0.35 in Mg_2−xAl_xNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg₂Ni and Mg₂NiH₄ exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg₂NiH₄ is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg₂Ni and Mg₃AlNi₂ as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg₂NiH₄ can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.     

Influence of hydrogen dilution on structural, electrical and optical properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PE-CVD)

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited from pure silane (SiH₄) and hydrogen (H₂) gas mixture by conventional plasma enhanced chemical vapour deposition (PE-CVD) method at low temperature (200 °C) using high rf power. The structural, optical and electrical properties of these films are carefully and systematically investigated as a function of hydrogen dilution of silane (R). Characterization of these films with low angle X-ray diffraction and Raman spectroscopy revealed that the crystallite size in the films tends to decrease and at same time the volume fraction of crystallites increases with increase in R. The Fourier transform infrared (FTIR) spectroscopic analysis showed at low values of R, the hydrogen is predominantly incorporated in the nc-Si:H films in the mono-hydrogen (SiH) bonding configuration. However, with increasing R the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (SiH) to di-hydrogen (SiH₂) and (SiH₂)_n complexes. The hydrogen content in the nc-Si:H films decreases with increase in R and was found less than 10 at% over the entire studied range of R. On the other hand, the Tauc's optical band gap remains as high as 2 eV or much higher. The quantum size effect may responsible for higher band gap in nc-Si:H films. A correlation between electrical and structural properties has been found. For optimized deposition conditions, nc-Si:H films with crystallite size 7.67 nm having good degree of crystallinity (84% ) and high band gap (2.25 eV) were obtained with a low hydrogen content (6.5 at%). However, for these optimized conditions, the deposition rate was quite small (1.6 Å/s).     

The effect of hydrogen on the mechanical properties of FeTi for hydrogen storage applications

In this paper, the density functional theory (DFT) within the generalized gradient approximation (GGA) was used. The single crystal elastic constants for the intermetallic FeTi and its hydrides FeTiH and FeTiH₂ are successfully obtained from the stress–strain relationship calculations and the strain energy-strain curves calculations, respectively. The shear modulus, Young's modulus, Poisson's ratio and shear anisotropic factors are also calculated. The bulk moduli derived from the elastic constants calculations of the cubic FeTi, orthorhombic P222₁ FeTiH and Cmmm FeTiH₂ are calculated. For cubic FeTi compound, the bulk modulus is in a good agreement with both theoretical results and experimental data available in the literature. More importantly, it is found that, the insertion of hydrogen into the FeTi crystal structure causes an increase in the bulk modulus. From the analysis of shear-to-bulk modulus ratio, it is found that FeTi compound and its hydrides are ductile and that this ductibility, changes with changing the concentration of hydrogen.     

Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principles calculations

The structure, stability, dehydrogenation thermodynamic and kinetic properties of MgH₂ hydride under different biaxial strain conditions were investigated by using first-principles calculations based on the density functional theory (DFT). The results show that either biaxial tensile or compressive strain is likely to cause the structural deformation of MgH₂ crystal, and its lattice distortion becomes severe with increasing magnitude of strain. Due to the contribution of strain energy, the biaxial strain not only weakens the structural stability of MgH₂, but also lowers its hydrogen desorption enthalpy and dehydrogenation temperature. Furthermore, the diffusion activation energy of hydrogen atom in MgH₂ host is also decreased, which results in a remarkable improvement of dehydrogenation properties. Noticeably, the effect of tensile strain in improving dehydrogenation thermodynamics is relatively superior to that of compressive one, while the role of the latter in enhancing dehydrogenation kinetics is relatively stronger than that of the former. Further analysis of electronic structures suggests the strain-induced changes in structural and dehydrogenation properties of MgH₂ are closely associated with the value of total densities of states at the Fermi level as well as the bonding electrons number below Fermi level. These results provide an insight for developing better MgH₂-based nanocomposite hydrogen storage materials by introducing suitable interface misfit strain.     

Influence of noble metals on the electronic and optical properties of LiH hydride: First-principles calculations

Lithium hydride (LiH) has attracted attention because of its high density hydrogen storage energy. However, the poor dehydrogenation properties cannot be used as an effective hydrogen storage material. In this paper, we apply the first-principles calculations to study the influence of noble metals on the electronic and optical properties of LiH hydride. Here, five noble metals TM(TM = Ag, Au, Pd, Pt and Ru) are considered. The calculated result shows that the noble metals are thermodynamic stability in LiH hydride. In particular, it is found that only the Pt-doped LiH is a dynamical stability compared to the other noble metals doping. Here, the calculated band gap of the pure LiH is 3.002 eV. Interestingly, these noble metals are beneficial to improve the electronic transfer (near Fermi level) of LiH because the introduction of noble metal induces the H-1s state to the Fermi level, making the band gap of the noble metal doped LiH disappear. In addition, we study the influence of noble metal Pt on the optical properties of LiH. It is found that the Pt doping can enhance the optical activity of LiH for visible light and infrared light, presumably caused by the addition of d state.     

Influence of the S/Cd ratio on the luminescent properties of chemical bath deposited CdS films

Usually as-grown chemical bath deposited Cadmium sulfide (CdS) samples do not show luminescence at room temperature because of the high density of recombination centers due to many defects generated during the growth process, particularly for chemical bath deposited CdS films. The change of the S/Cd ratio allows to control the density of defects giving rise to a better quality CdS films which can show luminescence at room temperature. Depending on the S/Cd ratio an evolution and improvement of the photoluminescence signal at room temperature is observed.     

The effect of Ti atom on hydrogenation of Al(111) surface: First-principles studies

First-principles calculations were performed to investigate hydrogen dissociation and subsequent diffusion over both clean and Ti-doped Al(111) surfaces. The calculations show that it is energetically favorable to dope the surface or subsurface layer of Al(111) with Ti atom. Through calculations on the detailed process associated with hydrogen dissociation and diffusion, we found that Ti doping will decrease the hydrogen dissociation barrier by about 0.6 eV. Additionally, the mobility of hydrogen atoms on surface will be easier if Ti atom is placed in subsurface layer instead of top surface layer. The present results further contribute towards understanding the improved kinetics observed in recycling of hydrogen in Ti-doped NaAlH₄.     

The effect of heteropolyacids and isopolyacids on the properties of chemically bath deposited CdS thin films

The deposition of CdS films on ITO/glass substrates from a chemical bath containing cadmium acetate, ammonia, ammonium acetate and thiourea has been carried out with and without small amounts of heteropolyacids (HPA) (phosphotungstic acid (PTA): H₃[PW₁₂O₄₀], silicotungstic acid (STA): H₄[SiW₁₂O₄₀], phosphomolybdic acid (PMA): H₃[PMo₁₂O₄₀]) and isopolyacids (IPA) (tungstic acid (TA): H₂WO₄ and molybdic acid (MA): H₂MoO₄) for different deposition times. The chemical, morphological, structural and optical properties of the films have been determined. The composition in sulphur and in cadmium of the films’ surface and volume was determined for various HPA and IPA used in the deposition bath. The HPA and IPA which give the thickest film with the biggest grain size were deduced. The optical transmission at 400 nm of CdS films deposited with STA at short time (20 min) (50%) is higher than those of CdS deposited at longer time (6 h) (7%). The optical transmission of CdS deposited with STA at short time is higher (50%) than that of CdS deposited without STA (20%). The performances of heterojunctions CdS/CdTe solar cells fabricated from CdS films deposited with and without STA and CdTe films deposited without STA have been determined. It was shown that the CdS/CdTe heterojunction solar cells fabricated from CdS films deposited with STA exhibited better photon collection efficiency and solar cell efficiency (η=6%) than CdS/CdTe heterojunction solar cells fabricated from CdS films deposited without STA (η=3.3%).     

Processing effects on the structure of CdTe, CdS and SnO2 thin films

Thin films of CdTe and CdTe/CdS and SnO₂ used for heterojunction solar cells were deposited on glass substrates. The effects resulting from the processing with thermal heat and CdCl₂ treatments are investigated. The optical properties are determined by photoluminescence (PL) and transmission spectra. The compositional changes within the CdTe film structures are studied by 2 MeV ⁴He⁺ beam using the Rutherford backscattering (RBS) technique. The optical and the RBS data are then correlated to the evolution of high-efficiency solar cells.     

The effect of microstructure on the hydrogenation of Mg/Fe thin film multilayers

Nanoconfined magnesium hydride can be simultaneously protected and thermodynamically destabilized when interfaced with materials such as Ti and Fe. We study the hydrogenation of thin layers of Mg (<14 nm) nanoconfined in one dimension within thin film Fe/Mg/Fe/Pd multilayers by the optical technique Hydrogenography. The hydrogenation of nanosized magnesium layers in Fe/Mg/Fe multilayers surprisingly shows the presence of multiple plateau pressures, whose nature is thickness dependent. In contrast, hydrogen desorption occurs via a single plateau which does not depend on the Mg layer thickness. From structural and morphological analyses with X-ray diffraction/reflectometry and cross-section TEM, we find that the Mg layer roughness is large when deposited on Fe and furthermore contains high-angle grain boundaries (GB's). When grown on Ti, the Mg layer roughness is low and no high-angle GB's are detected. From a Ti/Mg/Fe multilayer, in which the Mg layer is flat and has little or no GB's, we conclude that MgH₂ is indeed destabilized by the interface with Fe. In this case, both the ab- and desorption plateau pressures are increased by a factor two compared to the hydrogenation of Mg within Ti/Mg/Ti multilayers. We hypothesize that the GB's in the Fe/Mg/Fe multilayer act as diffusion pathways for Pd, which is known to greatly alter the hydrogenation behavior of Mg when the two materials share an interface.     

Chemically sprayed ZnO:F thin films deposited from diluted solutions: Effect of the time of aging on physical characteristics

Transparent and conductive fluorine-doped zinc oxide (ZnO:F) thin films were deposited on glass substrates by the chemical-spray technique starting from a diluted solution of zinc acetate and hydrofluoric acid. The effect of the aging time of the starting solution on the electrical, structural, morphological and optical characteristics of ZnO:F thin films was observed and analyzed. The resistivity of the ZnO:F thin films decreases as a more aged solution is used, reaching a saturation value of 6×10⁻² Ω cm. X-ray diffraction reveals that the films are polycrystalline in nature with a (1 0 0) preferential growth in almost all the cases. High-resolution scanning electron microscopy clearly reveals that the films are composed of nanoparticles of spherical shape, whose average diameter is in the order of 15 nm that matches well with the crystallite size calculated from X-ray diffraction. This result shows that fluorine incorporation effectively inhibits grain growth. This, in turn, produces a porous structure. Also, the increase in the time of aging enhances slightly the transmittance of the films.     

The effect of oxygen vacancies on the structure and electrochemistry of LiTi2(PO4)3 for lithium-ion batteries: A combined experimental and theoretical study

We report that a partially oxygen deficient LiTi₂(PO₄)₃ shows a much better rate capability as a cathode material for lithium-ion batteries compared to stoichiometric LiTi₂(PO₄)₃. A combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemistry, and first-principles calculations was used to determine and rationalize the structural and electrical changes that occur with different heat treatment atmospheres. XRD and XPS experiments confirmed that some Ti⁴⁺ transformed to Ti³⁺ in oxygen deficient LiTi₂(PO₄)₃ heat treated under N₂; Ti³⁺ was detected and the lattice parameter increased compared to that of LiTi₂(PO₄)₃. Electrical conductivity measurements indicated an increase in the electronic conductivity of nearly two orders of magnitude for the oxygen deficient LiTi₂(PO₄)₃ sample compared to LiTi₂(PO₄)₃. First-principles calculations suggest that the oxygen vacancies could be formed in LiTi₂(PO₄)₃ under oxygen-poor conditions, and this may significantly decrease the donor levels of other possible donor defects and thus improve the electronic mobility.     

Optical and structural properties of the sol-gel-prepared ZnO thin films and their effect on the photocatalytic activity

ZnO thin films were obtained by the sol-gel method, using the dip-coating procedure. Glass slides were used as substrates. The sintering temperature (T_s) was varied in the range of 200-600 °C in intervals of 50 °C, in an open atmosphere. Films with 1 and 5 coatings were prepared for each T_s. An increase of the grain size from 10 to 34 nm as the T_s increased was observed from X-ray diffraction measurements. The thickness of the films prepared starting from five coatings, decreased by 36% when T_s increased, and denser films were obtained. This result was corroborated with the refractive index values, calculated from the UV-Vis transmission spectra. The films were tested as a photocatalyst by the photobleaching of methylene blue in an aqueous solution under UV light exposure during 5 h. The photocatalytic activity (PA) increased with T_s, around 72% for the films with one coating and 66% for those with five coatings. The samples with one coating and a T_s=500 °C showed the best PA. However, the glass substrate had a negative effect on the PA for T_s>500 °C, even when the surface morphology of the samples showed an increase in roughness when T_s increased. The observed negative effect can be due to the presence of an amorphous compound formed by Si, Zn and O at the glass-ZnO interface.