Surface Oxidation State Research Articles

A general way to manipulate electrical conductivity of graphene

Electrical conductivity of graphene is one of the most important factors to dominate its applications, which is much lower than conventional good conductors including cooper in spite of its excellent electron mobility, limited by its low carrier density. In this work, a general way to improve electrical conductivity of graphene is proposed via introducing Cu NPs, which are rich in free electrons, into the well-crystalized laser-induced graphene (LIG). The LIG/Cu composite films, with an average diameter of 10 nm of Cu NPs evenly dispersed, were prepared in a laser induction process. It is worth mentioning that the electrical conductivity of porous graphene composited with Cu NPs is increased up to 0.37 × 107 S m−1, which is 3000 times that of pure LIG. To make clear mechanism of this notable phenomenon, the fine structure of Cu-graphene interface is characterized by X-ray Absorption Fine Structure (XAFS) spectroscopy, based on which, Density Function Theory (DFT) calculations are further adopted to reveal the influence of interface structure on electrical conductivity. It is revealed that Cu NPs with surface oxidation state (Cu2+) are most conducive to forming stable bonds with graphene, which will facilitate the electrons transfer from Cu to graphene. As a result, high carrier density and mobility are simultaneously realized in the graphene film, which finally leads to significant electrical conductivity enhancement. The results are of great significance in manipulating electrical conductivity of graphene with respect to various applications.

Synergic effects of Sn4+ doping on the nonlinear optical limiting properties of SnxCd1-xMoO4 nanostructures for optoelectronic applications

There is a rising demand in the development of novel nonlinear optical materials comprised out of transition metal molybdates owing to their fascinating optical and luminescent properties, resulting in wide application in future optoelectronic and optical limiting devices. Herein, we report non linear optical properties of novel tin (Sn4+) doped Cadmium molybdate (CdMoO4) nanostructures synthesized via chemical precipitation technique. The X-ray diffraction (XRD) pattern confirms that pristine as well as tin (Sn4+) doped Cadmium molybdate (CdMoO4) are highly crystalline with a tetragonal structure. Raman spectroscopy further confirms the structure and phase formation of SnxCd1-xMoO4 nanostructures. From the UV–Visible absorption spectra, it is observed that the ground state absorption increases in the UV region after the addition of the Sn4+, and the bandgap is found to be decreased. The morphology and particle size was analysed using Transmission electron microscopy (TEM). The surface binding energy and oxidation state was analysed by X-ray photo electron spectroscopy. The non-linear optical properties was studied by z scan technique using Q switched pulsed Nd YAG laser. Experimental results suggests, that two photon absorption mechanism is responsible for the observed nonlinearity. It is found that two photon absorption (β) is increased from 2.51 to 955.5×10−11 and third order susceptibility (χ(3)) enhances from 10.36 to 1480×10−13esu on Sn doping. The optical limiting threshold of pure and SnxCd1-xMoO4 nanostructures was also studied and found to be in the range of 41.45 to 11.92 J/cm2. These results clearly show that SnxCd1-xMoO4 nanostructures as a potential candidate for photonic, optical limi,ting and optoelectronic devices.

Thickness scaling down to 5 nm of ferroelectric ScAlN on CMOS compatible molybdenum grown by molecular beam epitaxy

We report on the thickness scaling behavior of ferroelectric Sc0.3Al0.7N (ScAlN) films grown on Mo substrates by molecular beam epitaxy. Switchable ferroelectricity is confirmed in ScAlN films with thicknesses ranging from 100 to 5 nm. An increase in coercive field and a significant diminution of remnant polarization are found when the ferroelectric layer is scaled down to below 20 nm. Notably, a switching voltage of 2–3.8 V and saturated remnant polarization of ∼23 μC/cm2 are measured in 5 nm thick ScAlN. X-ray diffractions and transmission electron microscopy studies indicate that the increase in coercive field and diminishment in switchable polarization can be closely linked to the surface oxidation and strain state in ultrathin ScAlN films. This work sheds light on the fundamental thickness scaling fingerprints of ScAlN thin films and represents an important step for next-generation compact and power-efficient devices and applications based on nitride ferroelectrics.

Tuning Surface Oxidation States of Nickel Oxide for Efficient Inverted Perovskite Solar Cells

Nickel oxide (NiOx), as an inorganic p-type semiconductor, has been widely adopted as hole-transporting layers in perovskite solar cells. Despite its superior material stability, the poor charge extraction and multi-vacancies greatly restrict the photovoltages and efficiencies. Here, we propose a facile method to tune the surface oxidation states of NiOx films by a lithium salt treatment for photovoltage enhancement. X-ray photoelectron spectroscopy measurement indicates that the lithium treatment only reduces the Ni3+ sites at the top region rather than in the bulk of NiOx films. This graded distribution allows more efficient charge extraction at the NiOx/perovskite interface as revealed by photoluminescence studies. Through a combination of capacitance–voltage and drive-level capacitance profiling measurements, we confirm enhanced built-in potentials and decreased interface defect densities in lithium-modified devices. Further modifying the interface with a self-assembly monolayer, the energy offsets at the interface can be largely reduced. Based on these enhanced properties, the modified devices achieve a high-power conversion efficiency of 22.4% (0.07 cm2) with a 120 mV enhancement in photovoltage in comparison with the untreated devices.

Tailoring the magnetic landscape in Al-doped LaMnO3: an experimental and computational perspective

We have reported the synthesis, structural and magnetic properties of LaAl x Mn1−x O3 (x = 0.05, 0.15, 0.25) in this article. We have synthesized these compounds through the Sol-gel citrate method and performed the Rietveld refinement of X-ray diffraction data to determine the lattice parameters. The surface elemental composition and oxidation states of LaAl0.25Mn0.75O3 are investigated using X-ray photoelectron spectroscopy (XPS) in a wide energy window of 0–1200 eV. The magnetic study shows the ferromagnetic transition of these materials. To understand the nature of magnetization from the experiments, we have studied the first principle density functional theory (DFT) and Monte-Carlo simulation. From the DFT calculations, we have confirmed the ferromagnetic structure in the ground state and studied the electronic structure of these materials. The Monte Carlo simulation has been done through the anisotropic Ising model to analyze the origin of magnetic phase transition. We have determined the anisotropy and the interaction constants from the DFT calculations. The double exchange interaction is mainly responsible for the ferromagnetic ground state.

Fine-tuning surface oxidation states of ruthenium nanoparticles to enhance hydrogen electrode reactions

The oxidation state of surface atoms on small-sized Ru-NPs can be precisely tuned by NP sizes. The intrinsic activities of the HER/HOR strongly depend on the surface Ru–O structure.

EMISSIVITY OF METAL SURFACE COATINGS

As it is known, emissivity is paramount for accurate temperature measurement using IR thermography. The emissivity of metals is particularly interesting due to its variability in relation to surface conditions (surface roughness and oxidation state), viewing angle, temperature, and wavelength. In the frame of this paper, knowledge regarding metal surface coatings and its effect on emissivity has been gained in an effort to improve risk assessment of workplace burns arising from hot metal surfaces. The present study examined the emissivity of two-component polyurethane metal covering coat on steel and grey cast iron depending on temperature and coating thickness. It was concluded that metal coating leads to moderated metal heating and increases constancy of emissivity compared to uncoated metal surfaces.

Correlating Oxidation State and Surface Ligand Motifs with the Selectivity of CO2 Photoreduction to C2 Products

AbstractThe design for non‐Cu‐based catalysts with the function of producing C2+ products requires systematic knowledge of the intrinsic connection between the surface state as well as the catalytic activity and selectivity. In this work, photochemical in situ spectral surface characterization techniques combined with the first principle calculations (DFT) were applied to investigate the relationships between the composition of surface states, coordinated motifs, and catalytic selectivity of a titanium oxynitride catalyst. When the catalyst mediates CO2 photoreduction, C2 product selectivity is positively correlated with the surface Ti2+/Ti3+ ratio and the surface oxidation state is regulated and controlled by coordinated motifs of N−Ti‐O/V[O], which can reduce the potential dimerization energy barriers of *CO−CO* and promote spontaneous formation of the subsequent *CO−CH2* intermediate. This phenomenon provides a new perspective for the design of heterogeneous catalysts for photoreduction of CO2 into useful products.

Correlating Oxidation State and Surface Ligand Motifs with the Selectivity of CO2 Photoreduction to C2 Products.

The design for non-Cu-based catalysts with the function of producing C2+ products requires systematic knowledge of the intrinsic connection between the surface state as well as the catalytic activity and selectivity. In this work, photochemical in-situ spectral surface characteristic technique combined with the first principle calculations (DFT) were applied to investigate the relationships among the composition of surface states, coordinated motifs, and catalytic selectivity of the titanium oxynitride catalyst. By adopting the process of CO2 photoreduction, we found that C2 product selectivity is positively correlated with the surface Ti2+/Ti3+ ratio while the surface oxidation state is regulated and controlled by coordinated motifs of N-Ti-O/V[O], which can reduce the potential dimerization energy barriers of *CO-CO* and promote the spontaneous formation of the subsequent *CO-CH2* intermediate. On the contrary, relatively high or low N/O coordination ratios will cause imbalance of coordination motifs in reducing surface states and abate the hydrogenation barriers of the methoxy group, thus facilitating methane formation. Such phenomenon provides a new design perspective and by such, researchers make a step close to the accurate production of photo-transformed CO2 products.

Synthesis of Silicotungstic Acid/Ni-Zr-O Composite Nanoparticle by Using Bimetallic Ni-Zr MOF for Fatty Acid Esterification

In this study, the bimetallic Ni-Zr MOF-derived nickel-zirconium oxide (Ni-Zr-O)-impregnated silicotungstic acid (HSiW) nanocomposite catalyst (HSiW@Ni-Zr-O) was prepared via a hydrothermal procedure followed by a pyrolysis treatment, and its structural, morphological, and surface components and oxidation states were characterized by using XRD, FTIR, TPD-NH3, SEM, TEM, N2 physisorption, and XPS analyses. Then, the nanocomposite catalysts were successfully applied to the esterification of oleic acid (OA) with methanol. According to its characteristics, the obtained HSiW@Ni-Zr-O-1 catalyst would generate larger pores, a higher acidity, and active interfaces at the calcining temperature of 300 °C. Therefore, HSiW@Ni-Zr-O-1 exhibits an excellent catalytic activity of 95.2% under optimal reaction conditions. Additionally, the catalyst can be reused with a good catalytic activity after nine cycles. This study highlights the opportunity of using bimetallic MOFs as precursors to the synthesis of highly nanoporous metal oxide, which supports the larger-industrial scale production of biofuels.

Powdered MnySb1–yOx Catalysts for Cerium-Mediated Oxygen Evolution in Acidic Environments

MnySb1–yOx powders with a series of compositions were evaluated as catalysts for chemical water oxidation in aqueous perchloric, sulfuric, or methanesulfonic acid. O2(g) evolved spontaneously over MnySb1–yOx catalyst powders that had been suspended in solutions that were pre-loaded with Ce4+ ions. The rate of O2 evolution depended on the amount, as well as the oxidation state, of the Mn in the powder. The highest O2 evolution rate was observed from the most Mn-rich catalyst, which had an effective surface oxidation state of Mn2.9+ in its rest state. The facile synthetic accessibility of such catalysts in powder form constitutes a step toward replacing Ir or Ru in Ce-mediated oxygen evolution in decoupled water splitting systems, as well as toward developing inks of earth-abundant catalysts for preparation of catalyst-coated membranes used in conventional proton-exchange membrane electrolyzers.

Pd supported on defective TiO2 polymorphic mixtures: Effect of metal-support interactions upon glycerol selective oxidation

Palladium catalysts supported on defective mixes of anatase, TiO2 (II) and rutile crystalline phases, previously obtained by high-energy ball milling, were synthesized and tested for glycerol selective oxidation. A deep characterization of these unusual materials was carried out to elucidate catalytic and physicochemical features. Electron density transfer from support to metal or vice versa, depending on the polymorphs present, could not only alter palladium particle sizes and its surface oxidation state but also reducibility and oxygen mobility of catalysts. Furthermore, acid-base properties achieved also influenced catalytic activity under mild conditions of liquid-phase glycerol oxidation. A conversion of 94% and a selectivity to glyceric and lactic acids of 48% and 22% respectively were obtained for the Pd catalyst supported on mechanochemically activated anatase. The presence of several polymorphs in a metal oxide support could therefore benefit or handicap catalytic cycle for a particular reaction. Metal-support interactions play a key role in heterogenous catalysts and thus the rational design of supports comes on the scene.

Low-frequency acoustic irradiation coupled photocatalytic degradation of dye pollutant using LaNi0.5Co0.5O3/g-C3N4 nanocatalyst

BackgroundSonophotocatalysis is a contemporary energy transfer approach employed for wastewater remediation. MethodsIn this study, we describe the fabrication of a sequence of LaNi0.5Co0.5O3/g-C3N4 Z-scheme catalysts with varying amounts of g-C3N4 via a facile impregnation procedure. To comprehend the crystallite structure, microscopic surface, element composition, and chemical oxidation state of the LaNi0.5Co0.5O3/g-C3N4, several characterization methodologies were performed. Significant findingsThe findings demonstrate that the heterojunctions formed between the rhombohedral perovskite-type LaNi0.5Co0.5O3 crystal and g-C3N4 nanosheets boost visible light absorption, aid in the separation of photogenerated electron-hole pairs, and play a critical role in suppressing their recombination thereby substantially improving catalytic activity. Furthermore, the sonophotocatalytic efficacy of these nanohybrids was examined against the removal of Congo Red (CR) dye employing ultrasonic-assisted UV photon stimulation. Consequently, LaNi0.5Co0.5O3/g-C3N4-25 wt.% displayed the optimum degradation, with approximately 83% of CR degraded after 120 min with a first-order kinetic rate of 0.01545 min−1, which was about 3.1 and 5.9 times greater than pristine LaNi0.5Co0.5O3 and g-C3N4, respectively advocating a Z-scheme charge carrier mechanism for the degradation process. The synergistic effect of sonolysis and photolysis has been discussed, and a suitable mechanism was proposed which validates LaNi0.5Co0.5O3/g-C3N4 composite as an affordable and efficient catalyst for environmental remediation.

Eu(III)–BaTiO3 nanoparticles and BaTiO3/TiO2/Ti sheets; photocatalytic and electrocatalytic CO2 reduction

Perovskite barium titanate (BaTiO3, BTO) has shown a high potential as catalysts. Eu(III) ion has been employed as a unique red-luminescence activator in a host metal oxide. Herein, Eu(III)-doped BTO nanoparticles (NPs) were synthesized, and their photocatalytic CO2 reduction activities and products were examined. Major CO2 reduction products were observed to be CH3OH, CO, and CH4. Doped Eu(III) ions were used as a tracer to discuss the roles of dopants by measuring photoluminescence (PL) properties and PL lifetimes. Electrocatalytic CO2 reduction performances were evaluated using BTO electrodes directly grown on a TiO2/Ti sheet, and Cu-deposited BTO electrodes. H2 was a major product and formate was significantly increased upon Cu deposition on the surface. Surface oxidation states of BTO with dopant concentration and overlayer Cu metal were fully discussed by X-ray photoelectron spectroscopy. The surface electronic structure was altered by doping and metal deposition, and thereby the selectivity and productivity for CO2 reduction were varied. The unique physicochemical properties and CO2 reduction demonstration tests provide significant information on understanding CO2 reduction mechanism and the development of BTO-based catalysts for energy and environment.

Oxidation States Regulation of Cobalt Active Sites through Crystal Surface Engineering for Enhanced Polysulfide Conversion in Lithium-Sulfur Batteries.

In this work, unique Co3 O4 /N-doped reduced graphene oxide (Co3 O4 /N-rGO)composites as favorable sulfur immobilizers and promoters for lithium-sulfur (Li-S) batteries are developed. The prepared Co3 O4 nanopolyhedrons (Co3 O4 -NP) and Co3 O4 nanocubes mainly expose (112) and (001) surfaces, respectively, with different atomic configurations of Co2+ /Co3+ sites. Experiments and theoretical calculations confirm that the octahedral coordination Co3+ (Co3+ Oh ) siteswith different oxidation states from tetrahedralcoordination Co2+ sites optimize the adsorption and catalytic conversion of lithium polysulfides. Specially, the Co3 O4 -NP crystals loaded on N-rGO expose (112) planes with ample Co3+ Oh active sites, exhibiting stronger adsorbability and superior catalytic activity for polysulfides, thus inhibiting the shuttle effect. Therefore, the S@Co3 O4 -NP/N-rGO cathodes deliver excellent electrochemical properties, for example, stable cyclability at 1C with a low capacity decay rate of 0.058% over 500cycles, superb rate capability up to 3C, and high areal capacity of 4.1mAhcm-2 . This catalyst's design incorporating crystal surface engineering and oxidation state regulation strategies also provides new approaches for addressing the complicated issues of Li-S batteries.

Preparation, Characterization and Stability Studies of Gold Nanoparticles Capped by 1,2,3‐Triazole‐Based Mesoionic Carbenes

AbstractThe preparation of stable and size‐uniform AuNPs (AuNPs=gold nanoparticles) has gained extensive attention. Strongly coordinating 1,2,3‐triazole‐based mesoionic carbenes are suitable capping agents to stabilize AuNPs, but such chemistry has been rarely explored. In this work, a series of newly synthesized triazolin‐5‐ylidenes with varying wingtips have been exploited as the capping ligands. Two different synthetic approaches have been explored and compared, which employed either gold(I) complexes or gold(III) species as the precursors. The gold nanomaterials synthesized via these two routes show different characteristics in terms of particle size, surface oxidation state as well as colloidal stability. It was found that the carbene with two long aliphatic wingtips is the best capping ligand giving relatively stable and monodisperse AuNPs. In addition, one of the gold nanomaterials, NP‐IIc, has exhibited good stability in the presence of biologically relevant agents including PBS, cysteine and glutathione, which implies its potential biological applications.

Characterization of IASCC crack tips extracted from neutron-irradiated flux thimble tube specimens in view of a probabilistic fracture model

This study reports the properties of irradiation assisted stress corrosion crack tips extracted by means of focused-ion beam from 60 to 80 dpa neutron-irradiated O-ring specimens tested under straining conditions under a pressurized-water reactor environment. Various crack tip morphologies and surrounding deformation features were analyzed as a function of applied stress, surface oxidation state and loading form - constant versus cyclic. All investigated cracks exhibit grain boundary oxidation in front of the crack tip, with the extent of oxidation being proportional to applied stress. These findings clearly demonstrate that, under the subcritical crack propagation regime, the grain boundary oxide grows faster than the crack. On the other hand, crack tips appertaining to specimens with removed oxide layer at the outer surface show comparatively less oxidation at the crack tip, which could indicate towards crack initiation from regions that exemplify lower stress, such as the O-ring inner surface. Cyclic loading is found to have a more pronounced effect on the crack tip microstructure, demonstrating increased deformation twinning and α′-martensitic transformation, which signifies towards an increased susceptibility to intergranular failure. Still, the extent of crack tip grain boundary oxidation in this case agrees well with expected values for maximum stress applied during cyclic loading. All results are interpreted based on the probabilistic subcritical crack propagation mechanism and provide strong support to a stress-driven internal oxidation model.

In Situ Periodic Regeneration of Catalyst during CO2 Electroreduction to C2+ Products.

Developing electrocatalytic reactions with high-efficiency can make important contributions to carbon neutrality. However, poor long-term stability of catalysts is a bottleneck for its practical application. Herein, an "in situ periodic regeneration of catalyst (PR-C)" strategy is proposed to give long-term high efficiency of CO2 electroreduction to generate C2+ products over Cu catalyst by applying a positive potential pulse for a short time periodically in the halide-containing electrolyte. The high Faradaic efficiency (81.2 %) and current density (22.6 mA cm-2 ) could be maintained completely at least 36 h, while the activity and selectivity decreased continuously without using the PR-C method. Control experiments and operando characterization demonstrated that the surface structure and oxidation state of Cu could be recovered periodically by the PR-C method, which was beneficial for CO2 activation and C-C coupling.

In Situ Periodic Regeneration of Catalyst during CO2 Electroreduction to C2+ Products

AbstractDeveloping electrocatalytic reactions with high‐efficiency can make important contributions to carbon neutrality. However, poor long‐term stability of catalysts is a bottleneck for its practical application. Herein, an “in situ periodic regeneration of catalyst (PR‐C)” strategy is proposed to give long‐term high efficiency of CO2 electroreduction to generate C2+ products over Cu catalyst by applying a positive potential pulse for a short time periodically in the halide‐containing electrolyte. The high Faradaic efficiency (81.2 %) and current density (22.6 mA cm−2) could be maintained completely at least 36 h, while the activity and selectivity decreased continuously without using the PR‐C method. Control experiments and operando characterization demonstrated that the surface structure and oxidation state of Cu could be recovered periodically by the PR‐C method, which was beneficial for CO2 activation and C−C coupling.

Morphology and facet tailoring of CaSnO3 assembled in molten salt with defect-mediated photocatalytic activity

Modulation of perovskite oxide toward controlled active facets and defects has attracted much attention. However, the influence of facets and defects on the photocatalytic activity are still ambiguous, especially for the alkaline-earth stannates. In this work, a high-efficiency CaSnO3 semiconductor photocatalyst was prepared by a facile one-step molten salt method without adding a capping agent. By simply varying the solutes, three kinds of CaSnO3 with different morphology were successfully synthesized, including sphere, cube, and cuboctahedron. Meanwhile, both the facet exposure and formation of defects affected the photocatalytic performance of CaSnO3 samples. The formation mechanism for various CaSnO3 morphology and their facet-dependent photocatalytic performances were explored in detail, which indicated that the formation of {100} facets was beneficial for the photocatalytic performance rather than {111}. Although these samples showed similar absorption edges, the varying amount of oxygen vacancy was one of the reasons for the diverse photocatalytic activity. The distortion of crystal structure of CaSnO3 was due to the formation of reduced Sn, which could influence the bond and angle of Sn–O–Sn and subsequently the photocatalytic activity. Different scavengers were also used to identify the role of active species in the photo-degradation process. The relationships between surface electronic structure, oxygen vacancy, and oxidation state of B site with the photocatalytic activity of tin stannate were clearly revealed.