Vacancy Population Research Articles

There are plenty of atomic vacancies at the bottom

Atomic vacancies are primordial for controlling the photocatalytic activity of perovskites. On the one hand, they are useful adsorption sites and help narrow the band gap, but they may also trap the electron-hole pair created after light absorption. Its precise control, although essential, is hardly achieved with traditional methods. In this work, an alternative experimental approach is employed to finely control the atomic vacancy population of SrTiO3 nanoparticles for application in the photodegradation of methylene blue. The atomic vacancies are created through Au7+ irradiation using different fluencies. The irradiated samples exhibit a photodegradation efficiency up to three times higher than the case without irradiation. X-ray absorption spectroscopy (XAS) measurements show that the local atomic order decreases with the ion irradiation, but it increases again after the photodegradation reaction. After the reaction, new surface chemical components appear in the X-ray photoelectron spectroscopy (XPS) measurements. These come due to the atomic vacancy diffusion from the inner to the surface region, as explained by Density Function Theory (DFT) calculations. The improvement in the photodegradation results is closely related to these atomic vacancies at the surface, as demonstrated by in situ Small Angle X-Ray Scattering (SAXS) measurements, and these enable the H2O dissociation, thus forming radicals that desorb for degrading the methylene blue molecule.

Na Vacancy-Driven Phase Transformation and Fast Ion Conduction in W-Doped Na3SbS4 from Machine Learning Force Fields.

Solid-state sodium batteries require effective electrolytes that conduct at room temperature. The Na3PnCh4 (Pn = P, Sb; Ch = S, Se) family has been studied for their high Na ion conductivity. The population of Na vacancies, which mediate ion diffusion in these materials, can be enhanced through aliovalent doping on the pnictogen site. To probe the microscopic role of extrinsic doping and its impact on diffusion and phase stability, we trained a machine learning force field for Na3-x W x Sb1-x S4 based on an equivariant graph neural network. Analysis of large-scale molecular dynamics trajectories shows that an increased Na vacancy population stabilizes the global cubic phase at lower temperatures with enhanced Na ion diffusion and that the explicit role of the substitutional W dopants is limited. In the global cubic phase, we observe large and long-lived deviations of atoms from the averaged symmetry, echoing recent experimental suggestions. Evidence of correlated Na ion diffusion is also presented that underpins the suggested superionic nature of these materials.

Defect characterizations of N-rich GaNAs ternary alloys

This paper presents a comprehensive analysis of defects in As-diluted GaN alloys. GaNAs crystal layers with an arsenic content of 1.8, 2.9, 4.1, and 5.5 % are grown on GaN buffer layers using the metalorganic vapour-phase epitaxy (MOVPE) method. Since these alloys have potential in electronics due to the modification of the electronic structure caused by As, it is extremely important to determine their quality. Complementary techniques for the characterization of the alloys are used. Densities of screw and edge dislocations are obtained using X-ray diffraction measurements performed, respectively, from the surface and edge of samples. In turn, a population of vacancies is determined by Doppler broadening variable energy positron annihilation spectroscopy (DB-VEPAS) and variable energy positron lifetime positron annihilation spectroscopy (VEPALS), which provide the distribution of vacancies as a function of depth. An increase in dislocation density is observed accompanied by a rise in the concentration of vacancy agglomerations and reduction in Ga-vacancy-hydrogen associates in the function of As-content.

Discarded vehicle tires and their association with mosquito vector abundance across socioenvironmental gradients in New Orleans, LA.

Discarded vehicle tires serve as habitat for mosquito vectors. In New Orleans, Louisiana, discarded tires are an increasingly important public concern, especially considering that the city is home to many medically important mosquito species. Discarded tires are known to be associated with mosquito abundance, but how their presence interacts with other socioenvironmental gradients to influence mosquito ecology is poorly understood. Here, we ask whether discarded tire distribution could be explained by social factors, particularly median income, home vacancy and human population density, and whether these factors interact with urban heat islands (UHI) to drive mosquito vector assemblages. We surveyed tire piles across the city and adult mosquitoes in 12 sites, between May and October of 2020. We compared this data with the social indicators selected and UHI estimates. Our results show that median income and human population density were inversely related to tire abundance. Tire abundance was positively associated with Aedes albopictus abundance in places of low heat (LS) severity. Heat was the only predictor for the other monitored species, where high heat corresponded to higher abundance of Aedes aegypti, and LS to higher abundance of Culex quinquefasciatus. Our results suggest that low-income, sparsely populated neighborhoods of New Orleans may be hotspots for discarded vehicle tires, and are associated with higher abundances of at least one medically important mosquito (Ae. albopictus). These findings suggest potential locations for prioritizing source reduction efforts to control mosquito vectors and highlight discarded tires as a potential exposure pathway to unequal disease risk for low-income residents.

Pivotal role of water vapor–mediated defect engineering on SrTiO3 nanofiber toward efficient photocatalytic water splitting

The oxygen vacancies (OVs) have played key roles as either charge carrier traps for inhibiting recombination or directly as active sites for photocatalytic redox reactions. In this study, the population and types of oxygen vacancies within the electrospun SrTiO3 nanofibers were modulated through water vapor and air atmosphere annealing respectively. Combining the distinctive 1D nanofibrous structure with optimized OVs, the water vapor–mediated OVs-rich SrTiO3 nanofibers achieved a significant enhancement in specific surface area and porosity, effectively suppressing the recombination of photogenerated carriers. The OVs can effectively promote the conversion of photon excitons into charge carriers, accelerate the carrier separation, and facilitate surface redox reaction. The optimized water vapor–mediated STO-W-700 exhibited abundant active sites and rapid surface charge separation/migration across the nanofibers, leading to an exceptional hydrogen production performance of 296.82 μmolg/h. During the photocatalytic process, the OVs act synergistically with nearby metal sites to optimize the adsorption energy of reactants. This work not only provides an effective pathway for manipulating oxygen vacancies through water vapor–mediated defect engineering strategy, but also reveals a new water splitting mechanism, laying the foundation for further research and development of efficient solar energy conversion technologies.

Curved plane increases the d vacancy population of Pd for green production of cyclohexanone

Curved plane increases the d vacancy population of Pd for green production of cyclohexanone

Nanostructured Pr-Rich CexPr1-xO2-δ Mixed Oxides for Diesel Soot Combustion: Importance of Oxygen Lability.

Soot combustion experiments with 5%O2/He were conducted using model soot, and four distinct compositions of CexPr1-xO2-δ oxides of varying nominal cerium compositions (x = 0, 0.2, 0.3, and 1) were prepared. The catalyst samples were comprehensively characterized using techniques such as XRD, Raman spectroscopy, HR-TEM, N2 adsorption at -196 °C, XPS, O2-TPD, H2-TPR, and work function measurements. The Pr-rich compositions, ranging from Ce0.3Pr0.7O2-δ to PrO2-δ, resulted in a significant increase in the total evolved O2 amounts and enhanced catalyst reducibility. However, a decrease in the textural properties of the catalysts was noted, which was particularly important for the pure praseodymia under the synthesis route conducted. The catalytic activity was investigated under the two following contact modes of mixing between soot and catalyst: loose and tight. The results revealed that the catalytic performance is associated with the surface contact in tight contact mode and with the combination of surface/subsurface/bulk oxygen mobility and the BET surface area in loose contact mode. Notably, the temperatures estimated at 10% and 50% of the conversion (T10 and T50) parameters were achieved at much lower temperatures than the uncatalyzed soot combustion, even under loose contact conditions. Specifically, the 50% conversion was achieved at 511 °C and 538 °C for Ce0.3Pr0.7O2 and Ce0.2Pr0.8O2, respectively. While no direct correlation between catalytic activity and work function was observed, a significant relationship emerges between work function values and the formation of oxygen vacancies, whatever the conditions used for these measurements. On the other hand, the ability to generate a high population of oxygen vacancies at low temperatures, rather than the direct activation of gas-phase O2, influences the catalytic performance of Pr-doped ceria catalysts, highlighting the importance of surface/subsurface oxygen vacancy generation, which was the parameter that showed a better correlation with the catalytic activity, whatever the soot conversion value or the mode of contact considered.

Precise Tailoring of Lithium-Ion Transport for Ultralong-Cycling Dendrite-Free All-Solid-State Lithium Metal Batteries.

All-solid-state lithium metal batteries can address crucial challenges regarding insufficient battery cycling life and energy density. The demonstration of long-cycling dendrite-free all-solid-state lithium metal batteries requires precise tailoring of lithium-ion transport of solid-state electrolytes (SSEs). In this work, a proof of concept is reported for precise tailoring of lithium-ion transport of a halide SSE, Li3InCl6, including intragranular (within grains) but also intergranular (between grains) lithium-ion transport. Lithium-ion migration tailoring mechanism in crystals is developed by unexpected enhanced Li, In, and Cl vacancy populations and lower energy barrier for hopping. The lithium-ion transport tailoring mechanism between the grains is determined by the elimination of voids between grains and the formation of unexpected supersonic conducting grain boundaries, boosting the lithium dendrite suppression ability of SSE. Due to boosted lithium-ion conduction and dendrite-suppression ability, the all-solid-state lithium metal batteries coupled with Ni-rich LiNi0.83Co0.12Mn0.05O2 cathodes and lithium metal anodes demonstrate breakthroughs in electrochemical performance by achieving extremely long cycling life at a high current density of 0.5 C (2000 cycles, 93.7% capacity retention). This concept of precise tailoring of lithium-ion transport provides a cost, time, and energy efficient solution to conquer the remaining challenges in all-solid-state lithium-metal batteries for fast developing electric vehicle markets.

Enhanced thermoelectric performance of SnSe by controlled vacancy population

The thermoelectric performance of SnSe strongly depends on its low-energy electron band structure that provides high density of states in a narrow energy window due to the multi-valley valence band maximum (VBM). Angle-resolved photoemission spectroscopy measurements, in conjunction with first-principles calculations, reveal that the binding energy of the VBM of SnSe is tuned by the population of Sn vacancy, which is determined by the cooling rate during the sample growth. The VBM shift follows precisely the behavior of the thermoelectric power factor, while the effective mass is barely modified upon changing the population of Sn vacancies. These findings indicate that the low-energy electron band structure is closely correlated with the high thermoelectric performance of hole-doped SnSe, providing a viable route toward engineering the intrinsic defect-induced thermoelectric performance via the sample growth condition without an additional ex-situ process.Graphical

Kinetics of substitutional Xe and self-interstitial Mo in γ U–10Mo: a molecular dynamic study

Diffusion is one among multiple reasons responsible for the micro-structural changes instigating substantial alterations in the macroscopic properties of materials. Simulating irradiation effects offers necessary insights into the production of radiation damages and their impact on different characteristics of nuclear fuels. Atomistic simulations were performed to obtain the migration energy and diffusion pre-factor of a single Xenon (Xe) substitutional as a function of percent atomic vacancies concentration in γ Uranium 10 wt% Molybdenum (γ U–10Mo). We observed a decrease and an increase in the migration energy and diffusion pre-factor respectively with an increase in the population of vacancies which proved that vacancy diffusion is a true mechanism of Xe in metals. Nudged elastic band (NEB) calculations were involved to obtain the migration energy of a self-interstitial Mo for intra- and inter-planar transitions. We observed the migration energy of Mo substantially higher than that of Xe which indicated that Xe is more mobile than Mo in bcc γ U–10Mo.

CuCeOx/CuO Catalyst Derived from the Layered Double Hydroxide Precursor: Catalytic Performance in NO Reduction with CO in the Presence of Water and Oxygen.

Valencies of metal species and lattice defects, such as oxygen vacancies, play a pivotal role in metal oxide-catalyzed reactions. Herein, we report a promising synthetic strategy for preparing CuO-supported CuCeOx catalysts (CuCeOx/CuO) by calcination of a hydrotalcite precursor [Cu6Ce2(OH)16]CO3·nH2O. The structural and chemical properties of catalysts were characterized by XRD, ICP-AES, TEM, TPR, NH3-TPD, XPS, Raman spectroscopy, and N2 adsorption, which revealed that the thermal pretreatment in an oxidative atmosphere caused segregation and reconstitution processes of the precursor, resulting in a mesoporous catalyst consisting of well-dispersed CuO-supported CuCeOx clusters of 1.8-3.2 nm in size with a high population of oxygen vacancies. The as-prepared catalyst shows excellent catalytic performance in the reduction of NO by CO in the absence as well as in the presence of water and oxygen. This behavior is attributed to its high oxygen defect concentration facilitating the interplay of the redox equilibria between Cu2+ and reduced copper species (Cu+/Cu0) and (Ce4+/Ce3+). The high surface population of oxygen vacancies and in situ-generated metallic copper species have been evidenced by Raman spectroscopy and X-ray photoelectron spectroscopy. The layered double hydroxide-derived CuCeOx/CuO also showed good water tolerance and long-term stability. In situ infrared spectroscopy investigations indicated that adsorbed hyponitrite species are the main reaction intermediates of the NO conversion as also corroborated by theoretical simulations.

The effect of quenching on the hinderance of the NbC growth process in a model microalloyed steel

ABSTRACT The effect of vacancies on the evolution of strain-induced NbC precipitates (SIP) was studied in a model austenitic Fe–30Ni–0.1Nb-0.1C steel deformed at 925 °C. Precipitation growth and coarsening characteristics have been investigated in detail by employing high resolution transmission electron microscopy. In general, the SIP preferentially nucleated on the nodes of the periodic dislocation networks constituting microband walls and grew over the varying holding time following the conventional precipitation growth and coarsening theory. Nevertheless, the current results demonstrated unexpected sluggishness in the process of growth and coarsening of the precipitates on the introduction of intermediary population of vacancies by a quenching step added in the thermomechanical processing schedules. It has been suggested that the formation of stable vacancy-solute complexes may reduce the effective availability of solutes for the precipitate growth.

Charge quenching at defect states in transition metal dichalcogenide–graphene van der Waals heterobilayers

The electronic and spin properties of van der Waals materials are greatly influenced by the atomic features of the interface. Combining $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ methods, model Hamiltonian, and quantum master equation, the authors calculate the electronic transition times and the dynamics of electron population and depopulation of chalcogen vacancies at the interface between graphene and transition metal dichalcogenides. Their findings give insight into the semiclassical dynamics, reveal the interplay between lattice symmetry and spin through the spin-orbit interaction, and examine its impact on the charge quenching.

Spinel ferrite catalysts for CO2 reduction via reverse water gas shift reaction

The production of CO via Reverse Water Gas Shift (RWGS) reaction is a suitable route for CO2 valorization. In this study a series of modified spinels AB2O4 (A site = Ni, Zn and Cu and B site=Fe) are investigated as RWGS catalysts and their structure-to-function relationships derived from the changes on the A-site cation are rationalized. For all ferrite systems, the RWGS reaction the process main activity and selectivity is governed by the B-site cation, but the variations on the A-site metals determines catalysts’ structural features and stability in the reaction. Among the catalyst series, superior RWGS performance displayed the ferrites modified with Cu and Ni associated to the greater oxygen vacancy population for those spinels enabled by the partial allocation on Fe3+ cations into the tetrahedral sites.

Ternary CuCrCeOx Solid Solution Enhances N2‐Selectivity in the NO Reduction with CO in the Presence of Water and Oxygen

AbstractCo‐doping of ceria with Cu and Cr forming a ternary CuCrCeOx solid solution enhances N2‐selectivity as well as NO and CO conversions compared to the corresponding singly doped ceria, CuCeOx and CrCeOx, and pristine ceria, in the absence as well as presence of water and oxygen. The comparative study of these solid‐solution catalysts indicates that the doping of ceria results in the coupling of different transition metal redox cycles, which has a strong effect on the surface population of oxygen vacancies. The temperature required to reach 95 % N2‐selectivity is shown to be nearly linearly correlated to the concentration of oxygen vacancy defects. The oxygen defect population is highest for the ternary CuCrCeOx solid solution, which exhibits the best catalytic performance, reaching 100 % N2‐selectivity at around 100 °C. Three redox cycles (CrVI/CrIII, CuII/CuI, and CeIV/CeIII) have been identified in this catalyst leading to a synergistic effect. The ternary CuCrCeOx catalyst exhibits good water tolerance and long‐term stability while the SO2 tolerance is limited due to selective blocking of the active surface oxygen defects forming SO32− species, resulting in catalyst deactivation.

Bismuth oxyhalide quantum dots modified sodium titanate necklaces with exceptional population of oxygen vacancies and photocatalytic activity

We here demonstrate the controlled synthesis of BiOBr quantum dots (QDs) decorated Na2Ti3O7 necklaces via a hydrothermal transformation of sodium titanate nanotubes. The BiOBr QDs are deposited on the surface of Na2Ti3O7 necklaces, forming a heterogeneous interface of BiOBr(200) and Na2Ti3O7(110), which promotes the separation efficiency of photogenerated charges, thus resulting in its superior catalytic performance in the photo-oxidation of benzyl alcohol. The BiOBr/Na2Ti3O7-1.0 exhibiting highest oxygen defect population gives best photocatalytic activity with a promising conversion rate of 3.32 mmolreacted BA gcatal.−1h−1, which is substantially higher than the corresponding reported photocatalysts. DFT results corroborate the superior performance of BiOBr/Na2Ti3O7 is mainly due to the formation of a built-in electric field and given efficiently the charge transfer between BiOBr(200) and Na2Ti3O7(110). In all, this study reports a simple in-situ hydrothermal growth protocol to efficiently construct BiOBr/Na2Ti3O7 heterojunction composites and offers guidelines for design of a new synthetic strategy to prepare efficient photocatalysts.

Ab initio calculations for void swelling bias in α - and δ -plutonium

Void swelling can develop in materials under persistent irradiation when nonequilibrium vacancy and self-interstitial populations migrate under sufficiently asymmetric interaction biases. In conventional metals, the propensity is determined to the first approximation by comparing point-defect relaxation strains. We thus present DFT-based calculations of structures and formation energies and volumes of point defects in the $\ensuremath{\alpha}$ and the $\ensuremath{\delta}$ phases of plutonium. We discuss the pros and cons of various levels of electronic structure theory: spin polarization, spin-orbit coupling, and orbital polarization. Our results show that lattice defects in $\ensuremath{\delta}$-Pu, in contrast to most fcc metals, have surprisingly small formation volumes. Equally unexpected are the large defect formation volumes found in the low-symmetry $\ensuremath{\alpha}$-Pu phase. Both these unusual properties can be satisfactorily explained from defect-induced spin/orbital moment formation and destruction in the Pu phases. Surprisingly, the point defects in $\ensuremath{\alpha}$-Pu are found to induce far larger transformation of the local electronic structure than in $\ensuremath{\delta}$-Pu. When we use the calculated defect properties to estimate the classic void swelling bias in each of the phases, we find it to be unusually small in $\ensuremath{\delta}$-Pu but likely much larger in $\ensuremath{\alpha}$-Pu. Hence, swelling rates and mechanisms can diverge dramatically between the different phases of Pu. Especially in the transient regime before the formation of large defect clusters, the swelling rate of $\ensuremath{\alpha}$-Pu can reliably be expected to be much larger than $\ensuremath{\delta}$-Pu. However, accurate forecasts over longer times will require the conventional void-swelling theory to be modified to handle the complexities presented by the different Pu phases. As a case in point, we show the possible anomalous temperature dependence of vacancy properties in $\ensuremath{\delta}$-Pu, caused by entropic contributions from defect-induced spin-lattice fluctuations. Such complications may affect defect-defect interactions and thus alter the void swelling bias.

Optimizing the oxide support composition in Pr-doped CeO2 towards highly active and selective Ni-based CO2 methanation catalysts

In this study, Ni catalysts supported on Pr-doped CeO2 are studied for the CO2 methanation reaction and the effect of Pr doping on the physicochemical properties and the catalytic performance is thoroughly evaluated. It is shown, that Pr3+ ions can substitute Ce4+ ones in the support lattice, thereby introducing a high population of oxygen vacancies, which act as active sites for CO2 chemisorption. Pr doping can also act to reduce the crystallite size of metallic Ni, thus promoting the active metal dispersion. Catalytic performance evaluation evidences the promoting effect of low Pr loadings (5 at% and 10 at%) towards a higher catalytic activity and lower CO2 activation energy. On the other hand, higher Pr contents negate the positive effects on the catalytic activity by decreasing the oxygen vacancy population, thereby creating a volcano-type trend towards an optimum amount of aliovalent substitution.

A parallel discrete dislocation dynamics/kinetic Monte Carlo method to study non-conservative plastic processes

Non-conservative processes play a fundamental role in plasticity and are behind important macroscopic phenomena such as creep, dynamic strain aging, loop raft formation, etc. In the most general case, vacancy-induced dislocation climb is the operating unit mechanism. While dislocation/vacancy interactions have been modeled in the literature using a variety of methods, the approaches developed rely on continuum descriptions of both the vacancy population and its fluxes. However, there are numerous situations in physics where point defect populations display heterogeneous concentrations and/or non-smooth kinetics. Here, a kinetic Monte Carlo (kMC) approach for modeling vacancy transport in response to arbitrary stress fields is used. Vacancies are treated as point particles and are coupled to the dislocation substructure representing a deformed material via an advection term defined by the local stress gradients. The stress fields and the dislocation substructure are evolved using a discrete dislocation dynamics (DDD) module. To extend the coupled model to the treatment of large systems, we have implemented it in the massively-parallel DDD code ParaDiS. To avoid numerical incompatibilities associated with merging deterministic (DDD) and stochastic (kMC) integration algorithms, we cast the entire elasto-plastic-diffusive problem within a single stochastic framework, taking advantage of a parallel kMC algorithm to evolve the system as a single event-driven process. The large-scale implementation enables the study of the evolution of a variety of dislocation-defect scenarios governed by non-conservative transport kinetics. After carrying out an exhaustive numerical and computational analysis of our parallel algorithm, we show results that emphasize situations where inhomogeneous vacancy dynamics are of relevance, and compare discrete kinetics to continuum solutions for several cases.

Continuous dimethyl carbonate synthesis from CO2 and methanol over BixCe1\u2212xO\u03b4 monoliths: Effect of bismuth doping on population of oxygen vacancies, activity, and reaction pathway

We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide in the absence of a dehydrating agent. BixCe1−xOδ nanocomposites of various compositions (x = 0.06–0.24) were coated on a ceramic honeycomb and their structural and catalytic properties were examined. The incorporation of Bi species into the CeO2 lattice facilitated controlling of the surface population of oxygen vacancies, which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts. The DMC production rate of the BixCe1−xOδ catalysts was found to be strongly enhanced with increasing Ov concentration. The concentration of oxygen vacancies exhibited a maximum for Bi0.12Ce0.88Oδ, which afforded the highest DMC production rate. Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140 °C and a gas-hourly space velocity of 2,880 mL·gcat−1·h−1. In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy (−OCH3) species. The activation of CO2 to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.