Changes In Chemical State Research Articles

Oxidation of aluminum thin films protected by ultrathin MgF2 layers measured using spectroscopic ellipsometry and X-ray photoelectron spectroscopy

To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF2, are used to protect aluminum mirrors against oxidation. In this study, we present, for the first time, combined X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometric (SE) studies of aluminum oxidation as a function of MgF2 overlayer thickness (thickness 0-5 nm). Dynamic SE tracks the extent of oxide growth every ca. 2s over a period of several hours after the evaporated Al + MgF2 bilayer is removed from the deposition chamber. Aluminum oxidation changes under the fluoride layer were quantitatively verified with XPS. Changes in chemical state from Al metal to Al oxide were directly observed. Oxide growth is computed from relative XPS peak areas as corrected for electron attenuation through the MgF2 overlayer. An empirical formula fits time-dependent data for aluminum surfaces protected by MgF2 as a function of MgF2 layer thickness: aluminum-oxide thickness = kSE*log(t)+bSE. The slope depends only on MgF2 thickness, decreasing monotonically with increasing MgF2 thickness. This method of employing SE coupled with XPS can be extendable to the study of other metal/overlayer combinations.

Post-illumination activity of Bi2WO6 in the dark from the photocatalytic \u201cmemory\u201d effect

Photocatalysts with the photocatalytic “memory” effect could resolve the intrinsic activity loss of traditional photocatalysts when the light illumination is turned off. Due to the dual requirements of light absorption and energy storage/release functions, most previously reported photocatalysts with the photocatalytic “memory” effect were composite photocatalysts of two phase components, which may lose their performance due to gradually deteriorated interface conditions during their applications. In this work, a simple solvothermal process was developed to synthesize Bi2WO6 microspheres constructed by aggregated nanoflakes. The pure phase Bi2WO6 was found to possess the photocatalytic “memory” effect through the trapping and release of photogenerated electrons by the reversible chemical state change of W component in the (WO4)2− layers. When the illumination was switched off, Bi2WO6 microspheres continuously produced H2O2 in the dark as those trapped photogenerated electrons were gradually released to react with O2 through the two-electron O2 reduction process, resulting in the continuous disinfection of Escherichia coli bacteria in the dark through the photocatalytic “memory” effect. No deterioration of their cycling H2O2 production performance in the dark was observed, which verified their stable photocatalytic “memory” effect.

Chemical-State-Dependent Free Energy Profile from Single-Molecule Trajectories of Biomolecular Motors: Application to Processive Chitinase

The mechanism of biomolecular motors has been elucidated using single-molecule experiments for visualizing motor motion. However, it remains elusive how changes in the chemical state during the catalytic cycle of motors lead to unidirectional motions. In this study, we use single-molecule trajectories to estimate an underlying diffusion model with chemical-state-dependent free energy profile. To consider nonequilibrium trajectories driven by the chemical energy consumed by biomolecular motors, we develop a novel framework based on a hidden Markov model, wherein switching among multiple energy profiles occurs reflecting the chemical state changes in motors. The method is tested using simulation trajectories and applied to single-molecule trajectories of processive chitinase, a linear motor that is driven by the hydrolysis energy of a single chitin chain. The chemical-state-dependent free energy profile underlying the burnt-bridge Brownian ratchet mechanism of processive chitinase is determined. The novel framework allows us to connect the chemical state changes to the unidirectional motion of biomolecular motors.

Interfacial Structure Change and Selective Dissolution of Columbite–(Fe) Mineral during HF Acid Leaching

The goal of the paper is to study the charge transfer and reactions at the columbite-(Fe) (FeNb2O6) mineral surface during the HF leaching process. In this paper, X-ray photoelectron spectroscopy (XPS), leaching experiments, and density functional theory (DFT) calculations were used to study the surface element adsorption, charge distribution, chemical state, and energy changes of the mineral surface during the process of leaching columbite–(Fe) with different concentrations of hydrofluoric acid. The results showed that as the concentration of F atoms was increased during the acid leaching process, the Nb–O bond was more likely to be broken than the Fe–O bond; the amount of charge transferred from Nb atom to F atom (0.78 e–0.94 e/atom) was greater than that from Fe atom to the F atom (0.25 e–0.28 e/atom), so it was determined that compared to Fe atoms, it was easier for the Nb atoms to bind to F. The results of XPS analysis showed that the electron binding energies of Nb5+–O, Fe3+–O, and Fe2+–O bonds on the mineral surface increased sequentially, and the M–O bond broke during the acid leaching process, forming more stable M–F bonds. Therefore, the Nb5+–F bonds were easier to form a stable structure. Combined with the ICP results, it was found that in the filtrate after 5M HF and 10M HF acid leaching minerals, c(Nb)/c(Fe) were 2.69 and 2.95, respectively, and the concentration ratio of Nb to Fe element in the mineral was 2 which was lower than 2.69 and 2.95, confirming the result of DFT calculation and illustrating that Nb atoms in columbite-(Fe) mineral were more soluble than Fe atoms.

Analysis of Photo-Intercalation Towards Photo-Charging Lithium-Ion Battery

The energy conversion of light energy into electricity is one of the most promising studies to meet the increasing world energy demands. Solar cell is one of the spread energy devices where light energy converts to electrical energy to be used directly or to be stored as chemical energy for later use by using a rechargeable battery. However, the latter case including two steps of energy conversion in the basic solar cell drives to the low efficiency during device-to-device transfer. The efficient establishment of solar cell technology on a global scale requires an efficient improvement concerning materials and devices to increase the power conversion efficiency. The integration of light-harvesting solar energy with the battery system was expected to enhance the performance of the light energy conversion.The photo-electrochemical reaction of lithium-ion insertion and deinsertion has been reported by employing dye-sensitized battery material under light irradiation [1]. The light energy generated electron-hole pairs with the holes aiding the chemical conversion. Nevertheless, energy efficiency and reaction kinetics should be improved by investigating the appropriate anode material for the system. By utilizing the photo-intercalation and photo-deintercalation concept that has been proposed by H. Tributsch in 1983 [2], the light energy can induce the transfer of ions between the electrode and electrolyte when there is a difference of fermi level energy between electrolyte and electrodes in semiconductor materials. Hence, to investigate the basic fundamental studies of the photo-intercalation reaction, active battery materials and semiconductors were utilized in the photochemical cell. A photo-sensitized LiFePO4 has been prepared to be applied as the working electrode, while a silicon p-type semiconductor has a role as the counter electrode in three electrodes battery system. The electrochemical transparent cell was utilized in order to observe the electrical current response from the energy produced by the reaction that takes place under the light irradiation.The experiment was done by observing the current response of the LiFePO4/Si and the potential changes of LiFePO4 vs. Li+/Li full cell system under light irradiation. There was no electrochemical activation involved in the system. When there was no light irradiation, there was no current response observed. Meanwhile, there was a distinct current and voltage changes marked once the light was irradiating the photochemical cell. For further analysis, the chemical state changes of the electrode materials in the photo-electrochemical system were investigated by conducting an experiment using X-ray absorption spectroscopy. The experiment was done after the irradiation of light energy towards the material. It showed that there was peak changes shape of the X-ray absorption spectrum which corresponds to the lithiation of silicon. By this result, we acquired the possibility of advancing the design principles for photo-assisted rechargeable batteries by using prospective materials.Reference:[1] A. Paolella, et al, Nature Communication, 8 , (2017).[2] H. Tributsch, Solid State Ionics, 9-10 , 41-58 (1983).

The Activation of Ti-Zr-V-Hf Non-Evaporable Getter Films with Open-Cell Copper Metal Foam Substrates.

Secondary electron emission (SEE) inhibition and vacuum instability are two important issues in accelerators that may induce multiple effects in accelerators, such as power loss and beam lifetime reduction. In order to mitigate SEE and maintain high vacuum simultaneously, open-cell copper metal foam (OCMF) substrates with Ti-Zr-V-Hf non-evaporable getter (NEG) coatings are first proposed, and the properties of surface morphology, surface chemistry and secondary electron yield (SEY) were analyzed for the first time. According to the experimental results tested at 25 °C, the maximum SEY () of OCMF before and after Ti-Zr-V-Hf NEG film deposition were 1.25 and 1.22, respectively. The XPS spectra indicated chemical state changes of the metal elements (Ti, Zr, V and Hf) of the Ti-Zr-V-Hf NEG films after heating, suggesting that the NEG films can be activated after heating and used as getter pumps.

Thermoplastic polyurethane-cellulose nanocomposite for transparent armour: Characterisation of adhesion and thermal aging

Environmental degradation of transparent thermoplastic polyurethane (TPU) is an issue of growing concern. Measured increased in the mechanical properties of nano-reinforced TPUs have been reported, however, limited research is available regarding their adhesive properties and resistance to environmental aging. By incorporating cellulose nanocrystals (CNCs) into a TPU via planetary ball milling, an increase to both the mechanical and adhesive properties was obtained. The TPU/CNC nanocomposite showed bond integrity and lap shear strengths were increased by 34% and 25% respectively relative to the unmodified nanocomposite, and further improvements in mechanical performance were seen. The TPU/CNC nanocomposite showed higher melting temperature and increased melting enthalpy indicating improved thermal aging resistance. The restriction of TPU segments and changes in chemical bond state and crystallinity were identified as key mechanisms driving performance improvements.

The effect of natural salt of various concentrations on the chernozem chemical properties

In the laboratory experiment, the effect of natural salt with herbicidal activity on the chemical properties of the soil is evaluated. To study the mechanism of action of natural salt with a concentration of 1, 5, 10, 15, and 20% on the plant-soil system and the change in chemical state, untreated natural brines of the Troitsk deposit in the Krasnoyarsk Territory were used. Studies have found that the use of natural salt as a herbicide determined alkalinization of the soil by 0.2-0.7 units pH while maintaining a neutral reaction of the soil solution, increasing the amount of exchange bases by 1.0 mmol / 100g, changing the composition of the aqueous extract. The maximum amount of toxic salts with a predominance of NaCl in the aqueous extract was found in the case of a 5% concentration of natural salt. A salt concentration of 5 and 10% determined strong salinization of the soil, 15 and 20% - strong. The concentration of natural salt of 5% and above determined the possibility of further soil alkalinization.

Chemical-State-Dependent Free Energy Profile from Single-Molecule Trajectories of Biomolecular Motors: Application to Processive Chitinase.

The mechanism of biomolecular motors has been elucidated using single-molecule experiments for visualizing motor motion. However, it remains elusive that how changes in the chemical state during the catalytic cycle of motors lead to unidirectional motions. In this study, we use single-molecule trajectories to estimate an underlying diffusion model with chemical-state-dependent free energy profile. To consider nonequilibrium trajectories driven by the chemical energy consumed by biomolecular motors, we develop a novel framework based on a hidden Markov model, wherein switching among multiple energy profiles occurs reflecting the chemical state changes in motors. The method is tested using simulation trajectories and applied to single-molecule trajectories of processive chitinase, a linear motor that is driven by the hydrolysis energy of a single chitin chain. The chemical-state-dependent free energy profile underlying the burnt-bridge Brownian ratchet mechanism of processive chitinase is determined. The novel framework allows us to connect the chemical state changes to the unidirectional motion of biomolecular motors.

Temperature-programmed reduction of model CuO, NiO and mixed CuO–NiO catalysts with hydrogen

Samples of model CuO, NiO and CuO–NiO catalysts were synthesized by calcination of individual copper and nickel hydrated nitrates, and according to a technique of their co-calcination. They were characterized by XRD, Raman spectroscopy, TEM and XPS. Solid solutions of Cu2+ in NiO and Ni2+ in CuO in the CuO–NiO sample were shown to be formed. Reduction of CuO, NiO and CuO–NiO oxides as model catalytic systems was studied by in situ XRD and TPR-H2. Reduction of CuO–NiO system was found to take place at notably lower temperatures than in the case of individual CuO and NiO. In situ XRD studies showed that the NiO–CuO reduction begins from copper oxide reduction evidenced by a sharp hydrogen uptake peak in the TPR-H2 curve. Kinetics of reduction of CuO, NiO and CuO–NiO oxides by H2 was investigated in detail. It was shown that apparent activation energy and pre-exponential factor obtained by Kissinger method for copper oxide reduction in CuO–NiO, compared to pure copper oxide, increased from 38 to 54 kJ mol−1 and from 6.4 to 13.0 (ln A) correspondingly. This is connected with formation of solid solution of Ni2+ in CuO that results in a significant growth of nucleation sites. As compared to individual nickel oxide, reduction of NiO in CuO–NiO is characterized by considerably lower activation energy. This is most probably caused by changes of chemical state of Ni2+ as a result of copper introduction into nickel oxide structure and formation of solid solution of Cu2+ in NiO.

Synthesis mechanism of amorphous Si2BC3N powders: Structural evolution of 2Si‐BN‐3C mixtures during mechanical alloying

AbstractA detailed knowledge of structural evolution during mechanical alloying (MA) is of interest and critical for understanding of synthesis mechanism, optimization of milling process, and control of milling products. The structural evolution of the 2Si‐BN‐3C mixture with a molar ratio of Si:BN:C = 2:1:3 during MA was studied by investigating the changes of phases, morphologies, elemental distributions, microstructures, and chemical bond states using XRD, SEM‐EDS, TEM, XPS, and Raman. With the increases in milling time, the particle sizes of the milled 2Si‐BN‐3C powders first decrease to submicrometers (~1 hour), slightly increase afterward (~3 hours), and eventually decrease to nanoscales gradually reaching equilibrium (≥10 hours). Depending on the intrinsic crystal structures of themselves, h‐BN and graphite are almost amorphized after milling of 10 hours, while amorphization of c‐Si takes at least 20 hours. The same milling parameters can provide amorphous Si2BC3N powders, but incompletely amorphous (partially crystalline) SiC powders, which is mainly due to the dilution effect of B and N atoms on the atomic concentration of Si and C hindering the microdiffusion and subsequent mechanochemical reaction of Si and C. Crystallite refinement–induced amorphization is the major synthesis mechanism of amorphous Si2BC3N powders. This work would offer more insight into the MA synthesis of multiple component materials, especially Si‐based brittle systems.

Physicochemical properties of Cr-doped TiO2 nanotubes and their application in dye-sensitized solar cells

Doping is always known as an effective way to modify the properties of a semiconductor. In this work, Cr-doped TiO2 nanotubes (Cr-TNTs) with different Cr contents were synthesized via a microwave-assisted hydrothermal method. The changes in crystal structures, chemical and electronic states, and the band gap structure of the obtained Cr-doped TiO2 nanomaterials were analyzed, thereby confirming the effect of the doping on physicochemical properties of the products. Moreover, the existence of Ti3+ sites and oxygen vacancies have significantly influenced the separation of electron-hole pairs as predicted by the photoluminescence spectra. The EIS results indicated that Cr-doping on TNT made a significant contribution to reducing the recombination rate of photogenerated charge carriers, leading to a prolonged lifetime of photogenerated electrons. The Cr-TNT based dye-sensitized solar cell (DSSC) containing 7.5 atomic percentage of Cr/Ti has demonstrated the best efficiency on the solar light conversion among the prepared samples.

Real time measurements of UV-vis diffuse reflectance of silver nanoparticles on gallium oxide photocatalyst

Ag loaded Ga2O3 photocatalysts are well known to be highly active for the CO2 reduction with water to CO. However, Ag changes its chemical state during the reaction, resulting in the decrease of its photocatalytic activity. To examine the chemical state change during the photocatalytic CO2 reduction, we have attempted a real time observation of the change in the chemical and physical states of Ag co-catalyst loaded on Ga2O3 by monitoring UV–vis reflectance spectra under the atmosphere simulating the photocatalytic CO2 reduction. In UV–vis spectra, two characteristic absorptions appeared at around 450 nm and 600 nm corresponding to resonance absorptions of localized surface plasmon (LSPR) of Ag nanoparticles (Ag-NPs) and Ag in metallic state. The LSPR absorption appeared as light irradiation started and grew with the time. It was revealed that Ag on Ga2O3 was initially in the oxidized state and reduced by the light irradiation to be Ag-NPs. Further light irradiation enlarged the size of Ag-NPs to be the metallic state especially under atmosphere with water. Under CO2 atmosphere, Ag-NPs remained small after further light irradiation. These results suggest that Ag initially loaded on Ga2O3 in the oxidized state was reduced to be dissolved as Ag+ in water and precipitated as Ag-NPs. Then, some of Ag-NPs aggregated to be larger Ag particles or in the metallic state. Such reduction process depends on gas species of the system, i.e. water promoted the formation of Ag-NPs while CO2 suppressed the formation Ag-NPs.

Electrochemically Induced Structural and Morphological Evolutions in Nickel Vanadium Oxide Hydrate Nanobelts Enabling Fast Transport Kinetics for High-Performance Zinc Storage.

Suitable intercalation cathodes and fundamental insights into the Zn-ion storage mechanism are the crucial factors for the booming development of aqueous zinc-ion batteries. Herein, a novel nickel vanadium oxide hydrate (Ni0.25V2O5·0.88H2O) is synthesized and investigated as a high-performance electrode material, which delivers a reversible capacity of 418 mA h g-1 with 155 mA h g-1 retained at 20 A g-1 and a high capacity of 293 mA h g-1 in long-term cycling at 10 A g-1 with 77% retention after 10,000 cycles. More importantly, multistep phase transition and chemical-state change during intercalation/deintercalation of hydrated Zn2+ are illustrated in detail via in situ/ex situ analytical techniques to unveil the Zn2+ storage mechanism of the hydrated and layered vanadium oxide bronze. Furthermore, morphological development from nanobelts to hierarchical structures during rapid ion insertion and extraction is demonstrated and a self-hierarchical process is correspondingly proposed. The unique evolutions of structure and morphology, together with consequent fast Zn2+ transport kinetics, are of significance to the outstanding zinc storage capacity, which would enlighten the mechanism exploration of the aqueous rechargeable batteries and push development of vanadium-based cathode materials.

Simple Cosolvent-Treated PEDOT:PSS Films on Hybrid Solar Cells With Improved Efficiency

Conductivity improvement of poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films up to 938.22 S·cm−1 was achieved by simply introducing the ethylene glycol (EG) as cosolvent for preparation. By conducting detailed characterizations on the film thickness, carrier concentrations, crystallinity, chemical state, and structural change of PEDOT chains, we revealed that the structural conformation was promoted from benzoid to the quinoid arrangement of PEDOT chains through the optimal EG additions, leading to the formation of the connected and aligned grains that was mediated by the dipolar–dipolar interaction, and therefore facilitated the charge hopping for allowing the improved film conductivity. The employment of such EG-treated PEDOT:PSS films with both enhanced conductivity and beneficial optical transmittance was demonstrated, indicating the sound design of hybrid solar cells with efficiency of 12.89%, which was compatible to the higher performance, low cost, and reliable cell manufacturing.

In situ work-function measurement during chemical transformation of MoS2 to MoO3 by ambient-pressure x-ray photoelectron spectroscopy

In this study, the oxidation of a two-dimensional (2D) MoS2 was performed as an alternative route for the synthesis of a 2D-layered MoO3 structure with high work function (WF) and hole mobility. The proposed method can also be used to tune the electronic properties (WF and bandgap) of MoO3/MoS2 composite-based semiconductors. By ambient pressure x-ray photoelectron spectroscopy (AP-XPS), in situ monitoring of the WF and chemical state of the surface was carried out during the oxidation of MoS2 to MoO3 layers. By heating the MoS2 sample in an O2 + Ar gas environment, the chemical transformation of the MoS2 to a MoO3/MoS2 composite layer and eventually to MoO3 was observed. The chemically transformed MoO3 film had a properly layered structure, according to cross-sectional transmission electron microscopy and high-resolution grazing-incidence x-ray diffraction analyses. During the oxidation, the WF change according to the change in surface chemical state was simultaneously measured using Ar gas as a surface potential probe. This study demonstrates the capability of AP-XPS for the monitoring and optimization of the conditions for chemical transformation (oxidation) to achieve desired physical properties (e.g. WF).

Enhanced dielectric properties and grain boundary potentials in sulfur-doped CaCu3Ti4O12 thin films

CaCu3Ti4O12 (CCTO) has been reported to possess a colossal dielectric constant owing to the intrinsic interfacial polarization via charge accumulations across the grain boundary. Herein, we explore the effects of unusual anion-doping on the dielectric properties of sputter-deposited CCTO thin films using an example of sulfur-doping. A post-annealing process of the films was utilized in a flowing H2S atmosphere for the sulfur-doping. The incorporation of sulfur into the perovskite structure was evidenced with the changes in chemical states, such as the reduced cations of Cu+ and Ti3+, the increased concentration of oxygen vacancies, and the formation of S-O bonds. The sulfurized CCTO thin films demonstrated an enhanced relative permittivity of ∼620 at 100 Hz, which is substantially better than that of the unsulfurized film. Direct measurement of the grain-boundary potential using Kelvin probe force microscopy suggests that the enhanced relative permittivity is associated with an increased Schottky barrier height.

Study of the calcium and phosphorus exchange dynamics in germinating oilseeds

The article is aimed at unifying technological approaches to production of functional drinks from hemp and soybean seeds. We studied the change in chemical states of calcium and phosphorus during swelling and germination, as one of the factors that enable the osmoregulation of the protein complex and increase stability of plant dispersions from bioactivated seeds. Seeds at rest, swelling seeds and germinating seeds and aqueous dispersions based on them were studied. The sources of insoluble and soluble forms of macronutrients in the cellular structures of bioactivated seeds were studied. It has been established that swelling forms of calcium included in the general metabolism become mobile. A further increase in calcium content in the aqueous dispersion up to 98.5% is accompanied by a rapid increase in conditionally soluble calcium (14.6 times) and an increase in stability of the system. Calcium metabolism in seeds is accompanied by changes in the phosphorus-salt composition. Within 48 hours, with an insignificant (4.7%) decrease in total phosphorus, an increase in soluble inorganic phosphorus (67.5%) and a decrease in organic phosphorus (51.9%) were observed. The appearance of inorganic phosphates and calcium ions in cell juice can be associated with the hydrolysis of phytin. In such forms, digestibility of phosphorus and calcium increases. Similar results were obtained for dispersions from germinating soybean seeds.

In situ AP-XPS analysis of a Pt thin-film sensor for highly sensitive H2 detection

In situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) combined with resistivity measurement was performed for a Pt thin-film H2 gas sensor. We experimentally demonstrate that the chemical state of the Pt surface changes under working conditions, and it directly links to the sensing performance. Moreover, the operating principle is discussed at the atomic scale.

Observation of Thermal Stability and Structure-selective Chemical State Change of Lithium Ion Secondary Batteries

Observation of Thermal Stability and Structure-selective Chemical State Change of Lithium Ion Secondary Batteries