Electronic Structure Energy Research Articles

In Ukrainian

The results of an experimental study of nickel nanostructures formation in Ni х InSe intercalates are presented. The surface topography and crystallography study data and also electron energy structure of interlayer (0001) cleavage surface of InSe layered crystals, equilibrium thermodynamically intercalated by nickel (Ni 3 d InSe intercalates) were obtained by methods of scanning tunneling microscopy and scanning tunneling spectroscopy (STM / STS) and also low energy electrons diffraction (LEED). It is determined that nickel is placed in the interlayer gaps of Ni х InSe intercalates and, therefore, is observed at the interlayer (0001) cleavage surfaces thus forming Ni 3 d /InSe(Ni)(0001) nanosystem and occurs in the fine - dispersed phase of nickel metal clusters on the (0001) cleavage surface. The concentrations of nickel metal clusters on the cleavage (0001) surfaces in Ni 0 . 75 InSe intercalates are estimated at the level of 0.8–1.25 % by STS and computer analysis method of the current-voltage characteristics in Ni 3 d /InSe(Ni)(0001) nanosystems.

Density functional theory calculations of the water interactions with ZrO2 nanoparticles Y2O3 doped

Development of a new electricity generation techniques is one of the most relevant tasks, especially nowadays under conditions of extreme growth in energy consumption. The exothermic heterogeneous electrochemical energy conversion to the electric energy through interaction of the ZrO2 based nanopowder system with atmospheric moisture is one of the ways of electric energy obtaining. The questions of conversion into the electric form of the energy of water molecules adsorption in 3 mol% Y2O3 doped ZrO2 nanopowder systems were investigated using the density functional theory calculations. The density functional theory calculations has been realized as in the Kohn-Sham formulation, where the exchange-correlation potential is approximated by a functional of the electronic density. The electronic density, total energy and band structure calculations are carried out using the all-electron, full potential, linear augmented plane wave method of the electronic density and related approximations, i.e. the local density, the generalized gradient and their hybrid approximations.

Enhanced Photocatalytic Activity of Zn–Ag–In–S Semiconductor Nanocrystals with a Dumbbell-Shaped Heterostructure

Nanocrystals with a heterojunction of type-II band alignment have attracted much interest for the construction of solar energy conversion systems because photogenerated electrons and holes can be effectively separated at the heterojunction. Here, we report the preparation of heterostructured multinary nanocrystals composed of ZnS–AgInS2 solid solution (ZAIS), which have tunability of the electronic energy structure depending on their chemical composition. Ellipsoidal ZAIS nanocrystal domains with various compositions were epitaxially grown on both termini of preformed ZAIS nanorods as seeds. The resulting nanocrystals had a dumbbell shape composed of two ellipsoidal nanocrystals (ca. 4–6 nm in width × 7–11 nm in length) connected by a nanorod (ca. 4 nm in width × 16–23 nm in length). Because the Zn fraction in the chemical composition was larger in the rod part than in the two tip parts in the nanodumbbell, heterojunctions of type-II or quasi-type-II were formed at the interface between the rod and tip pa...

Experimental and Computational Analysis of the Formation of Surface Oxygen Functional Groups during Iron Catalyzed Char Gasification with CO2

ABSTRACTAn experimental and computational study was carried out to evaluate the role of iron in coal gasification with CO2. Scanning electron microscopy, energy dispersive X-rays, Mössbauer, X-ray fluorescence, thermogravimetric analysis, and electronic structure methods were used. It was found that even though reaction rates for the catalyzed system are higher, apparent activation energies are not statistically different from the ones obtained for the non-catalyzed reaction. This behavior may be due to an increase in the concentration of surface oxygen complexes when iron is present. Several reaction pathways were proposed by molecular modeling using density functional theory. The presence of iron favors the formation of surface oxygen complexes from a thermodynamic point of view, which agrees with experimental results. It is suggested that iron would increase the concentration of active sites and, as a consequence, there is a rise in the reaction rate with CO2 in coal gasification with no significant changes in activation energies.

The chirality of metal-encapsulated silicene-like nanotubes

ABSTRACTThe energy stability and electronic structures of (3,3) armchair metal-encapsulated SiNTs (M@SiNTs) are systematically studied by using DFT-GGA calculations. Results show that most the metal-encapsulated SiNTs with the same space group can be stabilized through doping by metal atoms with silicene-like structures. Compared with the different chirality, (4,0) zigzag M@SiNTs are more stable than (3,3) armchair M@SiNTs. The band gaps of (3,3) and (4,0) SiNTs are 1.06 eV and 0.06 eV, they are both semiconductors. But most (3,3) armchair and (4,0) zigzag M@SiNTs are conductors, except (3,3) Sn@SiNT, (4,0) K@SiNT and (4,0) Ba@SiNT are semiconductors with small value of band gaps. The interaction between encapsulated metal and silicon atoms makes out orbital hybridizations, and decreases the gap between valence band and conduction band. Finally, electronic structural analysis shows that metal atoms gain electrons and Si atoms lose electrons as a whole, some electrons transfer from Si to metal atoms.

The use of AES and EELS for complex analysis of two-dimensional coatings and their growth process

Additional possibilities for complex analysis of two-dimensional coatings (thickness <1nm or <10ML) grown by physical vapor deposition (PVD) on a single-crystal silicon substrate under two deposition regimes have been revealed: 1) low-temperature (at a low beam temperature) and 2) high-temperature (at an elevated temperature of the beam), respectively. Coatings, including those in the form of pure metal and a silicide mixture, and their interface with the substrate have been analyzed by Auger electron spectroscopy (AES) and characteristic electron energy loss spectroscopy (EELS). A technology of the deposition from a ribboned source has been developed to ensure both deposition regimes. The conventional uses of AES are limited to the characterization of elemental composition, electron energy structure and coating thickness. For EELS, the conventional uses are the determination of phase types (valence electron density) and phase formation stages. The simultaneous use of the two methods and the choice of equal (and minimal) probing depths, ~ 2.5nm (primary electron energy 300eV), provided new possibilities for studying subnanometric two-dimensional coatings, in particular, for comparison of coating composition and density. The chosen probing depth also made it possible to characterize the interface between the coating and the substrate. At the same time, the use of similar probing depths made allowed using the thickness of the coating obtained from AES data for analyzing EELS data. In addition, other possibilities have been considered, i.e., the use of the following dependences: a) the energy of the plasmon satellite of the Auger peak vs the thickness of the coating for analyzing changes in the electron density in the near-interface layer of silicon; b) the attenuation of the Auger signal generated by marker atoms at the interface between the coating and the substrate for localizing the adsorption sites of the deposited atoms; c) the intensity and energy of the loss peaks in the EELS as a function of the primary electron energy for depth profiling of coating compositions. The use of two depth attenuation functions for two probing depths has enabled a quantitative Auger analysis of binary coatings. These advantages have provided the possibility of a more complete characterization of two-dimensional coatings and their interfaces with substrates, as well as the coating formation processes. In particular, it has for the first time become possible to identify the wetting nanophase layer of metals on silicon substrates, to investigate the process of its formation and to show how its composition depends on vapor-phase physical deposition modes.

Faraday rotation by the undisturbed bulk and by photoinduced giant polarons in EuTe

A quantum mechanical model is developed for the Faraday effect in europium telluride, for photons of energy within the transparency gap. The model is based on the well known band edge electronic energy states in EuTe. A concise expression for the Verdet constant is obtained, determined by few parameters already available in the literature. The Verdet constant adopted here, defined by the ratio between the Faraday rotation angle and the magnetization, is in effect temperature independent. Its dependence on the photon energy and applied magnetic field is in excellent agreement with published results. Below 3 T the Verdet constant is also nearly independent on field, but above 3 T at low temperatures it increases due to the band gap redshift. The model is used to calculate the photoinduced Faraday rotation associated with photoinduced giant magnetic polarons in EuTe. The theoretical photoinduced Faraday rotation excitation describes quite well the main features seen experimentally. Due to the common band-edge electronic energy structure, the model reported here could be extended to all other europium chalcogenides.

The use of AES and EELS for complex analysis of two-dimensional coatings and their growth process

Additional possibilities for complex analysis of two-dimensional coatings (thickness &lt;1 nm or &lt;10 ML) grown by physical vapor deposition (PVD) on a single-crystal silicon substrate under two deposition regimes are revealed: 1) low-temperature (at low beam temperature) and 2) high-temperature (at an elevated temperature of the beam), respectively. Coatings, including in the form of pure metal and a silicide mixture, and their interface with the substrate were analyzed by Auger electron spectroscopy (AES) and characteristic electron energy loss spectroscopy (EELS). To ensure both deposition regimes, a technology of the deposition from the ribboned source was developed. The traditional use of AES is limited to determining the composition of the elements, the energy electronic structure, and the thickness of the coating. And EELS — the types of phases (the density of valence electrons) and the stages of their formation. The simultaneous use of both methods and the choice of equal (and minimal) probing depths, ~ 2.5 nm (primary electron energy 300 eV), provided new possibilities for studying subnanometric two-dimensional coatings, in particular, — for comparison of the composition of coatings and their density. The chosen probing depth made it possible to characterize also interface between coating and substrate. At the same time, the same probing depth made it possible to use the thickness of the coating obtained from the AES data to analyze the data of the EELS. In addition, other possibilities are considered. This is the use of dependencies: a) the energy of the plasmon satellite of Auger peak, depending on the thickness of the coating, for analyzing changes in the electron density in the near-interface layer of silicon; B) attenuation of the Auger signal generated by marker atoms at the interface between the coating and the substrate to localize the places of adsorption of deposited atoms; and c) the intensity and energy of the loss peaks in the EELS in dependence on the primary-electron energy for profiling the composition of coatings over the depth. The use of two attenuation functions for two depths of probing provided a quantitative Auger analysis of binary coatings. All this made it possible to characterize more fully both the two-dimensional coatings themselves and the interface layer of the substrate, as well as the processes of their formation. And, in particular, this made it possible to identify for the first time the wetting nanophase layer of metal on a silicon substrate, to investigate the process of its formation and to show how its composition depends on the modes of vapor-phase physical deposition.

Experimental and theoretical study of the electronic energy structure of phosphorus containing sulfides InPS4, Tl3PS4 and Sn2P2S6

The electronic energy structures of three phosphorus-containing sulfides are studied from the experimental K and L2,3 X-ray emission spectra, K absorption spectra of sulfur and phosphorus, X-ray photoelectron spectra, and also quantum chemical calculations based on density functional theory (DFT). The fullpotential and all-electron quantum chemical calculations are carried out using the LAPW+lo basis set implemented in the WIEN2k software package [1]. The following exchange-correlation potentials are used for the calculations: PBE, PBE+U, and mBJ [2]. The spin-orbit coupling of Tl 5d3/2 and 5d5/2 electronic states are taken into account in Tl3PS4. All the specific features of the electronic energy structures of the compounds under study are determined in the valence and conduction bands near the Fermi level. The obtained band gaps E g are in good agreement with the literature experimental data.

Strain-magneto-optics of a magnetostrictive ferrimagnetic CoFe2O4

Significant magnetoreflection of unpolarized light in magnetostrictive ferrimagnetic single crystal of CoFe2O4 has been revealed in the infrared spectral range. The magnetoreflection effect in CoFe2O4 is up to 4% in a relatively small magnetic field. We experimentally demonstrate clear correlation between magnetostriction and magnetoreflection of light in CoFe2O4 ferrite. The influence of a magnetic field on specular reflection is likely to be indirect: application of a magnetic field results in strong strains and deformations of the crystal lattice, which lead to the change in electron energy structure and hence reflection spectrum of CoFe2O4 crystal - strain-magneto-optics.

Near-IR Photoelectrochemical Response of Colloidally Synthesized (Cu,Ag)2ZnSnS4 solid Solution nanocrystals

Introduction Colloidally synthesized nanocrystals (NCs) of multinary semiconductors have attracted much attention due to their light-harvesting properties and tunable electronic energy structure depending on their size and composition. One of important application of colloidal semiconductor NCs is a precursor to prepare semiconductor films with homogeneous composition across whole films via spin-coating. So far, a lot of study have been reported for the fabrication of photovoltaic devices using these colloidal nanocrystals as a precursor ink. Cu2ZnSnS4 (CZTS), a multinary chalcogenide with p-type semiconductor property, has been intensely studied as a new type of photovoltaic materials owing to their optimal properties for solar energy conversion, such as high absorption coefficient, being composed of earth-abundant and non-toxic elements, and band gap energy of 1.4-1.5 eV. In our previous papers, we successfully synthesized CZTS nanocrystals via thermal decomposition of precursors in hot organic solution and investigated their photoelectrochemical properties depending on the particle size and composition.[1-3] Recently, Kudo and co-workers reported that the photocatalytic activities for hydrogen evolution can be greatly enhanced by formation of solid solution between CZTS and Ag2ZnSnS4 (AZTS) in bulk system.[4] The hydrogen evolution rate under visible-light irradiation of CZTS-AZTS photocatalyst particles was 150 times higher than that of pure CZTS particles due to increased ability for photocatalytic reduction and oxidation reaction with increased energy gap by making solid solution. Therefore the doping of Ag+ into CZTS nanocrystal to form solid solution is a promising strategy to enhance their photo-properties without changing nanocrystal size, but such attempt has not been carried out. In this study, we prepare the solid solution NCs of CZTS-AZTS uniformly dispersed in organic solution and report their photoelectrochemical properties. Experimental CZTS-AZTS solid solution ((Cu1-x Ag x )2ZnSnS4; CAZTS) NCs were prepared via thermal decomposition of metal diethyl dithiocarbamate precursors in hot oleylamine under N2 atmosphere. After removal of largely aggregated particles, the resulting NCs were dissolved in chloroform. Photoelectrochemical properties of CAZTS NCs immobilized on ITO electrodes were measured in an aqueous solution containing 0.2 mol dm-3 Eu(NO3)3 as an electron scavenger. The Ag/AgCl electrode and Pt wire were used as reference and counter electrodes, respectively. Results and discussions The X-ray diffraction patterns of thus-obtained NCs were assignable to kesterite crystal structure similar to that of CZTS and individual peaks were shifted to lower angle with increase in the Ag content in NCs, except for NCs with x = 1.0 in which sub-peaks assignable to orthorhombic AZTS were additionally observed. These indicated that the obtained NCs were not a mixture of pure CZTS and AZTS NCs but a solid solution between CZTS and AZTS. TEM measurement revealed that thus-obtained particles had polygonal shape with average diameter of ca. 10 nm, regardless of x value. The optical bandgap of CAZTS NCs in chloroform, estimated from absorption onset energy, was blue-shifted with an increase in Ag content from 1.1 eV (x = 0) to 2.0 eV (x = 1.0). We determined the composition-dependent electronic energy structure of CAZTS NCs with photoelectron yield spectroscopy. The energy level of valence band edge of CAZTS NCs became slightly higher from +0.8 to +0.2 V vs. Ag/AgCl with increasing Cu content in the products, while that of conduction band edge was nearly constant at ca. -1.0 ~ -1.1 V, irrespective of x value. CAZTS thin films were prepared onto ITO electrodes by spin-coating chloroform solutions containing corresponding NCs at 1000 rpm for 10 second, followed by heating the electrodes at 300°C. The irradiation (λ > 350 nm) to thus-obtained films under appropriate potential application produced cathodic photocurrents except for NCs of x = 1.0 (AZTS), the magnitude being increased with negative shift of the electrode potential. The observed behavior was similar to that of a p-type semiconductor photoelectrode. On the other hand, AZTS NCs-immobilized ITO electrodes exhibited anodic photocurrent like as an n-type semiconductor as reported in our previous paper.[5] Action spectra of the photocurrent were in good agreement with absorption spectra of CAZTS NPs used, indicating that CAZTS NPs effectively worked as photovoltaic materials driven by the irradiation of visible and near-IR lights. Reference [1] T. Kameyama, et al., J.Mater. Chem., 2010, 20, 5319. [2] H. Nishi, et al., Phys. Chem. Chem. Phys., 2013, 16, 672. [3] H. Nishi, et al., J. Phys. Chem. C, 2013, 117, 21055. [4] A. Kudo, et al., Chem. Mater., 2010, 22, 1402. [5] T. Sasamura, et al., Chem. Lett., 2012, 41, 1009.

(Invited) Tunable Photoelectrochemical Property of ZnTe-AgInTe2 Solid Solution Nanocrystals in the Near-IR Region

I-III-VI2 semiconductor nanocrystals (NCs), including CuInS2, Cu(In,Ga)Se2, and AgInS2, have received intense attention as light-absorbing materials in solar energy conversion systems owning to their unique physicochemical properties, such as multiple exciton generation and hot-carrier extraction, originated from quantum size effect. In addition, I-III-VI2 semiconductors can easily form solid solutions with II-VI or other I-III-VI2 semiconductors, resulting in wide tunability in their electronic energy structure with chemical composition. So far we have successfully prepared ZnS-AgInS2 [1] and ZnSe-AgInSe2 [2] solid solution NCs and controlled their optical properties in visible-light region by changing their chemical compositions. Recently, much attention has been paid to near-infrared light (NIR)-absorbing semiconductor NCs for the application to solar energy conversion systems. We also reported the synthesis of highly NIR-luminescent AgInTe2 NCs of band gap energy at about 1.1 eV.[ 3] In this study, we report solution phase synthesis and photoelectrochemical property of ZnTe-AgInTe2 solid solution ((AgIn)xZn2(1 - x)Te2, ZAITe) NCs having a tunable optical properties in near-infrared light region. ZAITe NCs were synthesized by a thermal reaction of corresponding metal acetates and a Te precursor in 1-dodecanethiol at 180°C for 180 min. Photoelectrochemical properties of ZAITe NCs were measured in an aqueous solution containing 0.2 mol dm-3 Eu(NO3)3as an electron scavenger. The Ag/AgCl electrode and Pt wire were used as reference and counter electrodes, respectively. Prepared ZAITe NCs exhibited broad peaks in their XRD patterns, each peak position being gradually shifted to higher diffraction angle from the corresponding peak of hexagonal AgInTe2 to hexagonal ZnTe with an increase in Zn content in NCs, that is, with a decrease in the x value. This indicated the formation of solid solution NCs between ZnTe and AgInTe2. TEM measurement revealed that rod-shaped NCs were formed with width of ca. 4 nm and length of ca. 11 nm. The optical property of ZAITe NCs was controllable in the near-IR region by changing the composition of solid solution. The absorption onset of ZAITe NCs was blue-shifted from 1040 to 770 with a decrease in x from 1.0 to 0.25, indicating that their energy gap was enlarged from 1.2 to 1.6 eV with an increase in the Zn content in ZAITe NCs. These NCs exhibited near-band-edge PL peak located near the corresponding absorption onset, except for x = 0.25. The main PL peak observed for NCs with x= 0.25 was largely red-shifted from their absorption onset and then could be assigned to the emission with donor-acceptor pair recombination or the emission from defect sites. Photoelectron yield spectroscopy measurement enable to determine the composition-dependent electronic energy structure of ZAITe NCs. The energy level of conduction band edge (ECB) of ZAITe NCs became slightly higher from -3.4 to -3.1 eV vs. vacuum level with increasing Zn content in the products, while that of valence band edge (EVB) was nearly constant at ca. -4.7 eV, irrespective of xvalue. The photoelectrochemical properties of ZAITe NCs immobilized on ITO electrodes were measured in an Eu(NO3)3 aqueous solution by the irradiation with Xe lamp light. Cathodic photocurrents were observed regardless of xvalue, indicating that ZAITe NCs exhibited a photoresponse similar to a p-type semiconductor. The photoresponse was observed with near-IR light irradiation and then the photocurrent action spectra were in good agreement with absorption spectra of ZAITe NCs used. These demonstrate the potential use of ZAITe NCs as photovoltaic materials in near-infrared region. Reference [1] T. Torimoto, et al., J. Phys. Chem. C 2015, 119, 24740. [2] T. Kameyama, et al., J. Phys. Chem. C, 2014, 118, 29517. [3] T. Kameyama, et al., Nanoscale, 2016, 8, 5435.

Relaxor ferroelectricity, ferromagnetic and optical second harmonic properties in lanthanum lithium niobate (La0.05Li0.85NbO3) nanoparticles

Relaxor ferroelectricity, ferromagnetism and Second Harmonic Generation properties were founded and studied as a function of a reduction heat treatment at 650°C in a Ar-5%H2 atmosphere in stoichiometric La0.05Li0.85NbO3 nanoparticles of 40nm. A diffuse dielectric anomaly related with relaxor behavior from 25°C to 800°C in a frequency range from 100Hz to 1MHz was founded. It also shows ferromagnetic anhysterestic type and ferroelectric hysteresis loops at room temperature with a magnetic spin remnant of 2.5×10−3emu/g and polarization saturation of 0.235μC/cm2, remnant polarization of 0.141μC/cm2, coercive field of 1.35kV/cm, respectively. It shows very good second harmonic generation signal at 450nm and 500nm. High Resolution Transmission Electron Microscopy, X-ray Photoelectron Spectroscopy and Raman spectroscopy, indicate an ABO3 perovskite structure, new electronic binding energy structure for La (5s, 4d), Li (1s), Nb (4s, 3d, 4p) and oxygen (1s, 2s) and new vibrations modes on octahedron NbO6 related to multiferroic single phase nanoparticles, respectively.

Predicting the equilibrium solubility of solid polycyclic aromatic hydrocarbons and dibenzothiophene using a combination of MOSCED plus molecular simulation or electronic structure calculations

ABSTRACTMethods to predict the equilibrium solubility of non-electrolyte solids are important for the design of novel separation processes. Here we demonstrate how conventional molecular simulation free energy calculations or electronic structure calculations in a continuum solvent, here SMD or SM8, can be used to predict parameters for the MOdified Separation of Cohesive Energy Density (MOSCED) method. The method is applied to the solutes naphthalene, anthracene, phenanthrene, pyrene and dibenzothiophene, compounds of interested to the petroleum industry and for environmental remediation. Adopting the melting point temperature and enthalpy of fusion of these compounds from experiment, we are able to predict equilibrium solubilities. Comparing to a total of 422 non-aqueous and 193 aqueous experimental solubilities, we find the proposed method is able to well correlate the data. The use of MOSCED is additionally advantageous as it is a solubility parameter-based method useful for intuitive solvent selection and formulation.

Ab initio calculation and synchrotron X-ray spectroscopy investigations of tin oxides near the Sn L 3 absorption edges

The results of theoretical and experimental investigations of the electron-energy structure of the conduction band of tin oxides have been presented. The Sn L3 X-ray absorption near-edge fine structure (XANES) has been calculated for the first time for single crystals of metallic tin and tin monoxide, as well as for the orthorhombic and tetragonal phases of tin dioxide, using the linearized augmented plane wave (LAPW) method. The fine structure of the XANES spectra has been compared with the specific features of the energy distribution of the local partial densities of states of the tin compounds under investigation. A joint analysis of the results of the simulation and the experimental X-ray synchrotron Sn L3 XANES spectra of commercial bulk samples of metallic tin and tin oxides SnO and SnO2 has been carried out.

(Invited) Shape-Dependent Photocatalytic H2 Evolution Activity of ZnS-AgInS2 Solid Solution Nanoparticles

Nanoscale materials exhibit size- and shape-dependent physicochemical properties due to both the quantum size effect and the difference in the crystal facets exposed to the solution. So far the preparation for Cd-based II-VI nanoparticles with anisotropic shapes has been intensively investigated via solution phase synthetic methods. Especially rod-shaped nanoparticles have attracted much attention because of unique optical and electronic properties originating from their one-dimensional particle shape, such as polarized emission, improved solar cell performance, and higher photocatalytic activity. On the other hand, recent research efforts have been focused on developing high-quality semiconductor nanoparticles containing no toxic elements such as Cd or Pb. Nanoparticles composed of I, III, and VI group elements, such as AgInS2, CuInS2, and Cu(In,Ga)Se2, have been a promising alternative to Cd-based particles, because their light absorption properties and electronic energy structure could be controlled by modifying the size and shape of nanoparticles as well as by making solid solutions with II-VI group semiconductors. We have reported(1) that nanoparticles of low-toxic ZnS-AgInS2 solid solution ((AgIn)xZn2(1-x)S2, ZAIS), prepared by thermal decomposition of corresponding metal acetates and thiourea in a hot organic solution, exhibited tunable optical properties depending on their chemical composition and particles size. Furthermore these spherical ZAIS nanoparticles worked as an efficient photocatalyst for H2evolution. However little is known about the dependence of photocatalytic activity on the particle shape. In this study, we prepare ZAIS particles with different anisotropic shapes to enhance their photocatalytic activity. ZAIS nanoparticles with anisotropic shapes were prepared by thermal decomposition of corresponding metal acetates and sulfur compounds of elemental sulfur and 1,3-dibutylthiourea in oleylamine at 250 oC. The obtained crude particle mixture was subjected to size-selective precipitation to isolate ZAIS particles with individual anisotropic shapes. The chemical composition of thus-obtained particles was controlled by changing metal precursor ratio. For comparison, spherical ZAIS particles were prepared as reported previously.(1) The photocatalytic activity of ZAIS nanoparticles was investigated for H2 evolution from aqueous solution as a model reaction. ZAIS particles suspended in a 2-propanol/water mixture solution containing Na2S as a hole scavenger was irradiated with Xe lamp light (λ> 350 nm). The heat treatment at 250 oC for 3 min produced rod-like particles with length of ca. 40 nm and width of ca. 5 nm. With further heating for 10 min, rice-shaped particles were formed as a by-product, the dimensions of which were length of ca.18 nm and width of ca. 9 nm. Rice-shaped particles could be isolated from rod particles by size-selective precipitation. Figure 1a shows TEM images of purified ZAIS particles with rod and rice shapes, as well as spherical particles for comparison. XRD patterns of individual kinds of particles well agreed with wurtzite crystal structure of ZAIS solid solution. The chemical composition of obtained ZAIS particles were almost stoichiometric and the fraction of Zn2+in total metal ions was tunable depending on the ratio of metal precursors in preparation. The irradiation of ZAIS particles suspended in 2-propanol/water solution containing S2- as a sacrificial electron donor induced H2 evolution. Figure 1b shows the time courses of the amount of H2 evolved by irradiation to ZAIS particles with various shapes, in which the x value, representing the solid solution composition of particles, was similar to 0.35-0.45. Regardless of particle shapes, the amount of H2 evolution was linearly increased with elapse of irradiation, indicating that each kind of ZAIS particles efficiently worked as a photocatalyst. However the photocatalytic activity was greatly dependent on the shape of particles, though the Eg of particles was similar to ca.2.3-2.5 eV. ZAIS particles with rod-like and spherical shapes exhibited much higher H2evolution rate than rice-like particles. The highest activity was obtained for rod-like particles, which was ca. 1.5 times larger than that of spherical ones. Since rod-like ZAIS exhibited longer PL lifetime than spherical particles, we concluded that the enhanced photocatalytic activity of rod-like ZAIS particles originated from the change in lifetime of photogenerated charge carriers with the particle size and/or shape. Reference (1) T. Torimoto, et al., J. Phys. Chem. C 119, 24740-24749 (2015). Figure 1

Photoluminescence and Photoelectrochemical Properties of ZnTe-AgInTe2 Solid Solution Nanocrystals

Introduction Much attention has been paid to near-infrared light (NIR)-absorbing and -emitting semiconductor nanocrystals (NCs) due to their unique physicochemical properties, such as tunable energy structure depending on their size and multiple exciton generation. The most widely studied NIR responsive NCs are based on II-VI and IV-VI semiconductors such as CdTe, PbS, and PbSe. However, their high toxicity is an obstacle to practical application of these NCs, because of potential risks for the human health and the environment. Recently, increasing attention has been paid to I-III-VI2 semiconductor NCs such as CuInS2, AgInSe2, and AgInTe2 [1 -2 ] for the application to in vivo imaging or photoenergy conversion system, because they contain no highly toxic heavy metals and have intense photoluminescence (PL) and absorption in near-infrared region. Furthermore their absorption and emission wavelength can be varied by making solid solutions between I-III-VI2 and II-VI semiconductors. We have successfully prepared nanocrystals of ZnS-AgInS2 [2] and ZnSe-AgInSe2 [3] solid solutions and controlled the optical properties by changing their chemical composition. In this study, we report a solution phase synthesis of ZnTe-AgInTe2 solid solution ((AgIn)xZn2(1 - x)Te2, ZAITe) NCs having a tunable optical properties in near-infrared light wavelength region and their photoelectrochemical properties. Experimental ZAITe NCs were synthesized by a thermal reaction of corresponding metal acetates and a Te precursor in 1-dodecanethiol at 180°C for 180 min. Photoelectrochemical properties of ZAITe NCs were measured in an aqueous solution containing 0.2 mol dm-3 Eu(NO3)3as an electron scavenger. The Ag/AgCl electrode and Pt wire were used as reference and counter electrodes respectively. Results and discussions TEM observations revealed that rod-shaped NCs with width of ca. 4 nm and length of ca. 11 nm were formed regardless of chemical composition of particles, that is, the x value. Thus-obtained NCs exhibited broad peaks in their XRD patterns, each peak position being gradually shifted to higher diffraction angle from the corresponding peak of hexagonal AgInTe2 with a decrease in the x value. Figure 1 shows the absorption and PL spectra of ZAITe NCs. The absorption onset was blue-shifted from 1100 to 810 nm with a decrease in x from 1.0 to 0.25. These results indicated that the obtained NCs were composed of ZAITe solid solution, the composition of which could be controlled by varying the fraction of Zn precursor in preparation, and that their energy gap could be varied depending on their chemical composition. The NCs exhibited near-band-edge PL peak located near the corresponding absorption onset except for x = 0.25, the peak wavelength being blue-shifted with a decrease in x value. The main PL peak observed for NCs with x = 0.25 was largely red-shifted from their absorption onset, suggesting that electronic states in band gap, such as donor and acceptor sites or surface defect sites, were formed at the xvalue smaller than 0.5. Layer-by-layer deposition enabled the accumulation of ZAITe NCs on an ITO electrode. The thus-obtained particle films exhibited a photoresponse similar to p-type semiconductor electrode regardless of x value. Action spectra of the photocurrent were in good agreement with absorption spectra of ZAITe NCs immobilized. The potentials of the valence band and conduction band edges were successfully determined from the onset potentials of cathodic photocurrents and the energy gaps estimated from absorption spectra. ZAITe NCs had a tunable electronic energy structure depending on xvalue. These demonstrate the potential use of ZAITe NCs as photovoltaic materials in near-infrared region. Reference [1] M. Langevin et al., Nanoscale Res. Lett., 2015, 10, 255. [2] T. Kameyama, et al., Nanoscale, 2016, 8, 5435. [3] T. Torimoto, et al., J. Am. Chem. Soc., 2007, 129, 12388. [4] T. Kameyama, T. Torimoto, et al., J. Phys. Chem. C, 2015, 119, 24740. Figure 1

Photoelectrochemical Properties of Colloidally Synthesized (Cu,Ag)2ZnSnS4 solid Solution nanocrystals

Introduction Multinary semiconductors of Cu2ZnSnS4 (CZTS) composed of earth-abundant and non-toxic elements have been intensively studied as a new type of potential photovoltaic materials for the application to thin film solar cells[1] and photocatalyst for hydrogen evolution[2] due to its promising optical properties for absorbing solar light, such as band gap energy of 1.4-1.5 eV and high absorption coefficient. Recently, Kudo and co-workers reported that the photocatalytic activities for hydrogen evolution can be greatly enhanced by formation of solid solution between CZTS and Ag2ZnSnS4 (AZTS).[2]The hydrogen evolution rate under visible-light irradiation of CZTS-AZTS photocatalyst particles was 150 times higher than that of pure CZTS particles due to an increase in conduction band edge energy by making solid solution. Nanometer-sized crystals of such photoactive semiconductors have also attracted much attention due to their light-harvesting properties and tunable electronic energy structure depending on their size. In addition, semiconductor nanocrystals (NCs) have opened up new ways to generate multiple charge carriers with a single photon. In our previous papers, we successfully synthesized CZTS NCs via thermal decomposition of precursors in hot organic solution and investigated their photoelectrochemical properties depending on the particle size and composition.[3-5] The incorporation of Ag+into CZTS nanocrystal solid solution is expected to enhance photo-properties of nanoparticles, but such attempt has not been carried out. In this study, we prepare CZTS-AZTS NCs uniformly dispersed in organic solution and report their photoelectrochemical properties. Experimental CZTS-AZTS solid solution ((Cu1-x Ag x )2ZnSnS4; CAZTS) NCs were prepared via thermal decomposition of metal diethyl dithiocarbamate precursors in hot oleylamine under N2 atmosphere. After removal of largely aggregated particles, the resulting NCs were dissolved in chloroform. Photoelectrochemical properties of CAZTS NCs immobilized on ITO electrodes were measured in an aqueous solution containing 0.2 mol dm-3 Eu(NO3)3 as an electron scavenger. The Ag/AgCl electrode and Pt wire were used as reference and counter electrodes respectively. Results and discussions TEM measurement revealed that thus-obtained particles were polygonal shape with average diameter of ca. 10 nm regardless of x value. Their chemical composition could be controlled between 0 < x < 1 by varying the Cu/Ag ratio in precursors. The X-ray diffraction patterns of NCs were assignable to kesterite-type crystal structure and each peak was shifted to lower angle with increase in the Ag content in NCs, indicating that the obtained NCs were not a mixture of CZTS and AZTS NCs but a solid solution between CZTS and AZTS. Figure 1 shows absorption spectra of CAZTS NCs in chloroform. The absorption onset of CAZTS NCs was blue-shifted with an increase in Ag content. The bandgap of the NCs was tunable from 1.1 eV (x = 0) to 2.0 eV (x = 1.0) only by changing value of x. To investigate the photoelectrochemical properties of CAZTS NCs, the NCs were immobilized onto ITO electrodes with spin-coating CAZTS NCs chloroform solution at 1000 rpm for 10 second, followed by heating the electrode at 300°C. Irradiation (λ > 350 nm) to thus-obtained films was carried out in an aqueous solution containing Eu(NO3)3 as an electron scavenger. Cathodic photocurrents were observed for CAZTS NC electrodes except for NCs of x = 1.0 (AZTS) under applied electrochemical bias, the magnitude being increased with negative shift of the electrode potential. The observed behavior was similar to that of a p-type semiconductor photoelectrode. On the other hand, AZTS NCs-immobilized ITO electrodes exhibited anodic photocurrent like as an n-type semiconductor as reported in our previous paper.[6]Action spectra of the photocurrent were in good agreement with absorption spectra of CAZTS NPs used, indicating that CAZTS NPs effectively worked as photovoltaic materials driven by irradiation of visible and near-IR light. Reference [1] D. Aldakov, et al., J. Mater. Chem. C, 2013, 1, 3756. [2] A. Kudo, et al., Chem. Mater., 2010, 22, 1402. [3] T. Kameyama, et al., J.Mater. Chem., 2010, 20, 5319. [4] H. Nishi, et al., Phys. Chem. Chem. Phys., 2013, 16, 672. [5] H. Nishi, et al., J. Phys. Chem. C, 2013, 117, 21055. [6] T. Sasamura, et al., Chem. Lett., 2012, 41, 1009. Figure 1

Modelling of free positron states in TiHx

Electron energy structure, positron spectrum and positron characteristics of α-Ti and α-TiH0.125 were calculated. Self-consistent calculations of the band structure were performed by the linear muffin-tin orbital method in the atomic sphere approximation. Modelling has been made on low content of hydrogen into α-Ti with expanded close-packed hexagonal cell inclusive 8 titanium atoms. Variation of sphere radiuses permitted to consider anisotropy and spherical symmetry of potential. Positron potential and positron wave function were calculated on a base of self-consistent electron density. Then positron probability of existence into TiHx lattice and lifetime were founded. Theoretical calculation indicated a satisfactory agreement of positron characteristics absolute values with the experimental data is achieved but the tendency of values with hydrogen defects increasing is not. The reason of divergence is discussed. On the basis of experimental data and theoretical calculations it was shown that different hydrogen atom states demonstrate the different influence in the lifetime spectra.

Diagnosing the Fine Structure of Electron Energy Within the ECRIT Ion Source**supported by National Natural Science Foundation of China (No. 11475137)

The ion source of the electron cyclotron resonance ion thruster (ECRIT) extracts ions from its ECR plasma to generate thrust, and has the property of low gas consumption (2 sccm, standard-state cubic centimeter per minute) and high durability. Due to the indispensable effects of the primary electron in gas discharge, it is important to experimentally clarify the electron energy structure within the ion source of the ECRIT through analyzing the electron energy distribution function (EEDF) of the plasma inside the thruster. In this article the Langmuir probe diagnosing method was used to diagnose the EEDF, from which the effective electron temperature, plasma density and the electron energy probability function (EEPF) were deduced. The experimental results show that the magnetic field influences the curves of EEDF and EEPF and make the effective plasma parameter nonuniform. The diagnosed electron temperature and density from sample points increased from 4 eV/2×1016 m−3 to 10 eV/4×1016 m−3 with increasing distances from both the axis and the screen grid of the ion source. Electron temperature and density peaking near the wall coincided with the discharge process. However, a double Maxwellian electron distribution was unexpectedly observed at the position near the axis of the ion source and about 30 mm from the screen grid. Besides, the double Maxwellian electron distribution was more likely to emerge at high power and a low gas flow rate. These phenomena were believed to relate to the arrangements of the gas inlets and the magnetic field where the double Maxwellian electron distribution exits. The results of this research may enhance the understanding of the plasma generation process in the ion source of this type and help to improve its performance.