Stability Of Vacancies Research Articles

Interfacial bonding of hydroxyl-modified g-C3N4 and Bi2O2CO3 toward boosted CO2 photoreduction: Insights into the key role of OH groups

The development of heterogeneous photocatalysts with superior photogenerated charge separation and CO2 activation is a key challenge for artificial photosynthesis. Herein, novel hydroxyl-modified g-C3N4/flower-like Bi2O2CO3 composites (OH-CN/BOC) with covalently bonded heterointerfaces were fabricated through a direct mechanical mixing approach. The bonded samples exhibited remarkable CO2 photoreduction activity under visible light. The CO production rate of the optimized sample was 91.8, 18.2, 8.6, and 6.1 times greater than those of BOC, CN, OH-CN, and CN/BOC, respectively, reaching 26.69 μmol g-1h−1; and it remained stable after four cycles. The experimental and DFT studies reveal that the introduction of OH groups on CN leads to the chemical bonding of CN and BOC, induces stable surface oxygen vacancies (OVs) on BOC, and enhances the interaction between the catalyst with CO2 and H2O molecules, hence greatly improving the CO2 photoreduction activity of OH-CN/BOC. This work provides new insights and potential strategies for constructing high-quality interfacial heterojunctions with strong chemical bonds to facilitate the photocatalytic performance.

Effects of vacancies on the electronic structures and photocatalytic properties of g-C3N4

The geometric structures, electronic structures and photocatalytic properties of g-C3N4 with different vacancies are studied by the first-principles calculations. These calculations suggest that introducing different vacancies can cause the relaxation of these adjacent atoms, the formation of dangling bonds, and the deformation of these building blocks. And these band gaps of the heptazine-based graphitic C3N4 sheets are narrowed by different vacancies. The energetic preferability of the heptazine-based g-C3N4 sheets with different vacancies is assessed as functions of the environmental factors, including nitrogen partial pressure and temperature. It is indicated that VC2 and VN2 are the respectively stable vacancies over the investigated nitrogen partial pressure. A transition from VN2 to VC2 should occur at ultrahigh vacuum pN2≥∼10−23atm at 1000 K, and move to higher nitrogen partial pressure with the increasing temperature. According to the calculated absorption coefficient, these g-C3N4 sheets with VC2 and VN2 vacancies have larger absorption than the perfect structure over the visible wavelength range, suggesting that vacancies can increase the catalytic efficiency of graphitic C3N4. These results show that introducing defects, especially vacancies, is an effective approach to enhance the sunlight absorption efficiency of graphitic carbon nitride.

Stabilize the oxygen vacancies in Bi2SiO5 for durable photocatalysis via altering local electronic structure with phosphate dopant

Oxygen vacancies in catalysts can bring new properties, especially the enhanced catalytic efficiencies that are absent in their pristine counterparts. However, the deactivation of oxygen vacancies inevitably limits the durability of catalytic performance. Herein, a model of photocatalytic NO oxidation over Bi2SiO5 catalyst is selected to unravel the pivotal role of dopants in enhancing the long-term stability of oxygen vacancies. The introduction of phosphate dopants could stabilize oxygen vacancies via turning the local electronic structure. The phosphate dopants evoke the enhanced electron redistribution and cause the charge compensation to unsaturated coordination atoms around the oxygen vacancies. The unique electronic structure modulated by dopants could facilitate the adsorption and activation of H2O and O2, impeding those atoms from filling into oxygen vacancies. This work provides an innovative route for rationally altering the local electronic structure of catalysts toward efficient and sustainable photocatalytic applications.

In-situ synthesis of metal Bi to improve the stability of oxygen vacancies and enhance the photocatalytic activity of Bi4O5Br2 in H2 evolution

Bi4O5Br2 has received considerable interests for energy conversion and environment remediation in recent years. Fabrication of oxygen vacancies (OVs) is effective to improve the photocatalytic activity of Bi4O5Br2. However, OVs are instable and easy to deactivate during continuous photocatalytic process. In this paper, Bi4O5Br2 modified by metal Bi and OVs (Bi-OVs-Bi4O5Br2) have been prepared to improve the stability of OVs and the photocatalytic activity in H2 evolution. Bi-OVs-Bi4O5Br2 exhibited excellent photocatalytic activity with a H2 evolution rate of 67.9 μmol·g−1·h−1, which was 2.1 times higher than that of pure Bi4O5Br2. The enhancement may be attributed to the narrowed band gap, facilitated separation of photogenerated charge carriers, increased adsorption sites for H2O and reduced H2 adsorption energy on the surface of Bi4O5Br2. Notably, the metal Bi could act as reactive sites to activate H2O, so as to avoid OVs to be filled with H2O. As a result, Bi-OVs-Bi4O5Br2 exhibited high photocatalytic stability in continuous test. Results from this study clarified the intrinsic functionality of metal Bi and OVs in Bi4O5Br2 and provided new insights into rational design of highly-efficient Bi-based photocatalyst.

Fluorine-induced oxygen vacancies on TiO2 nanosheets for photocatalytic indoor VOCs degradation

Photocatalytic degradation of indoor VOCs has great prospects for addressing human health issues in daily life. Here, TiO2 containing fluorine ions (F−) on the surface was reduced by hydrogen to obtain the oxygen vacancy-rich sample F-TiO2−x, which exhibited excellent performance in the photocatalytic VOCs degradation. The characterization results showed that the strong electron repulsion between F− and Ti3+ encouraged metallization to promote photogenerated charges separation. The DFT calculation results proved that the F− on the surface of TiO2 was beneficial to the formation of surface oxygen vacancies, and the synergistic effect of oxygen vacancies and F− on F-TiO2−x could improve the adsorption and activation of H2O molecules, which was also effective for oxygen molecules. All of these were beneficial to the formation of active free radicals. Moreover, the presence of surface F- could effectively promote the stabilization of oxygen vacancies.

Controlling electrical and optical properties of wurtzite CdxZn1−xO with high Cd contents via native defects manipulation by low-temperature annealing

Bandgap energies in wurtzite (WZ) structured CdxZn1−xO alloys are known to decrease with increasing Cd content (x). Our previous work demonstrated that WZ-CdxZn1−xO alloys with a high Cd content of x ∼ 0.6 and a low gap of 2 eV can be stabilized by oxygen interstitials when grown in an O-rich environment. However, such O-rich WZ-CdxZn1−xO alloys have poor electrical properties due to compensating native defects. In this work, we synthesized pure WZ phase CdxZn1−xO thin films with different Cd contents by magnetron sputtering in an oxygen-rich environment. Changes in structural, electrical, and optical properties of these O-rich wurtzite CdxZn1−xO after rapid thermal annealing were investigated. While alloys with a low Cd composition of 0.2 can maintain a pure wurtzite structure up to 500 °C, phase separation occurs at a lower annealing temperature of ∼400 °C for Cd-rich (x = 0.6) films. Isochronal and isothermal annealing studies reveal the kinetics of native defects in these alloys. Highly mobile hydrogen interstitial donor defects, oxygen interstitials, and more stable cation vacancies outdiffuse sequentially as the annealing temperature increases from <300 to >400 °C. By exploiting the difference in the energy barrier between acceptor defects removal and phase separation, a pure wurtzite phase alloy with a low bandgap of 2 eV and decent electrical properties was realized by annealing O-rich WZ-Cd0.6Zn0.4O at 300 °C with an extended annealing duration of >100 s. These results demonstrate a practical way to obtain low-gap oxide semiconductors with strong optical absorption and controllable electrical conductivities.

High Configuration Entropy Activated Lattice Oxygen for O2 Formation on Perovskite Electrocatalyst

AbstractThe single‐phase oxides with elemental complexity and compositional diversity, usually named high entropy oxides, feature homogeneously dispersed multi‐metallic elements in equiatomic concentration. The unusual properties of high entropy oxides endow their potential application in clean‐energy‐related electrocatalysis. However, the possible fundamental relationship between configuration entropy and the underlying catalytic mechanism is still not well understood and established. Herein, a high entropy perovskite cobaltate consisting of five equimolar metals in the B‐site (Mg, Mn, Fe, Co, and Ni) is employed as an electrocatalyst for oxygen evolution reaction (OER). The configuration entropy serves as an effective tool to promote the intrinsic activity of the Co reactive site and manipulate the OER mechanism. The high entropy cobaltate demonstrates a lower overpotential of 320 mV at a current density of 10 mA cm−2, outperforming other counterparts. The X‐ray spectroscopies disclose the synergistic charge‐exchange effect among different cations and the formation of a new oxygen hole state. Combinatorially computational and experimental results unveil the enigma that the high configuration entropy leads to the random occupation of cations, facilitates the surface reconstruction, and benefits the formation of stable surface oxygen vacancies. Owing to these merits, the O2 formation is found to be kinetically favorable via the lattice oxygen mechanism.

Acid‐ and Base‐Stable Cs2Pt(Cl,Br)6 Vacancy‐Ordered Double Perovskites and Their Core–Shell Heterostructures for Solar Water Oxidation

The stability of the absorber materials in an aqueous medium is the key to developing successful photoelectrochemical (PEC) solar fuel devices. The halide perovskite materials provide an opportunity to tune desired optoelectronic properties and show very high photovoltaic power conversion efficiency. However, their stability is poor as they decompose instantly in an aqueous electrolyte medium. Here the most stable vacancy ordered double perovskites Cs2PtCl6 and Cs2PtBr6, which remain intact in a wide range of pH values between 1 and 13 is reported. These materials also possess excellent absorption properties covering a significant portion of the visible spectrum. Like conventional ABX3 materials, these ultrastable materials offer tunability in optical properties via mixed halide sites. Through anion exchange, the conversion of Cs2PtCl6 to Cs2PtBr6 through core–shell conversion mechanism is shown. The latter led to the formation of type‐II heterostructures. The electrochemical properties of these materials are investigated in detail and their ability to carry out solar water oxidation on an unprotected photoanode, with photocurrent density of >0.2 mA cm−2 at 1.23 V (vs. RHE) is demonstrated.

Stability Mechanism of Low Temperature C2H4-SCR with Activated-Carbon-Supported MnO x -Based Catalyst.

Manganese-based catalysts have shown great potential for use as a hydrocarbon reductant for NOx reduction (HC-SCR) at low temperatures if their catalytic stability could be further maintained. The effect of CeO2 as a promoter and catalyst stability agent for activated carbon supported MnOx was investigated during low temperature deNOx based on a C2H4 reductant. The modern characterization technology could provide a clear understanding of the activity observed during the deNOx tests. When reaction temperatures were greater than 180 °C and with ceria concentrations more than 5%, the overall NO conversion became stable near 70% during long duration testing. In situ DRIFTS shows that C2H4 is adsorbed on the Mn3Ce3/NAC catalysts to generate hydrocarbon activated intermediates, R-COOH, and the reaction mechanism followed the E-R mechanism. The stability and the analytical data pointed to the formation of stable oxygen vacancies within Ce3+/Ce4+ redox couplets that prevented the reduction of MnO2 to crystalline Mn2O3 and promoted the chemisorption of oxygen on the surface of MnOx-CeOx structures. Based on the data, a synergetic mechanism model of the deNOx activity is proposed for the MnOx-CeOx catalysts.

A Wearable Respiration Sensor for Real-Time Monitoring of Chronic Kidney Disease.

Human respiration is accompanied with abundant physiological and pathological information, such as the change in ammonia (NH3) content, which is related to chronic kidney disease (CKD); hence, monitoring the breathing behavior helps in health assessment and illness prediction. In this work, a wearable respiration sensor based on CeO2@polyaniline (CeO2@PANI) nanocomposites that underwent a hydrogen plasma treatment is developed. The results unambiguously show that the response of the corresponding nanocomposites is significantly enhanced from 165 to 670% to 100 ppm NH3 compared to the counterpart that did not undergo hydrogen plasma treatment and even reaches 24% to 50 ppb NH3, suggesting its fascinating capability of detecting the trace level of NH3 in human breathing. The superior response for NH3 is ascribed to the stable oxygen vacancies produced by the hydrogen plasma treatment. Furthermore, the clinical tests for patients with uremia suggest that the as-designed sensor has potential applications in clinical monitoring for CKD. Herein, this work offers a new strategy to obtain respiration sensors with high performance and provides a feasible approach for health evaluation and disease monitoring of patients with CKD.

Role of Ni on the helium diffusion, stability and energetics of vacancy–type clusters in Fe–6.25Cr–3.13Ni (at.%) ternary alloys: From first-principles

Ab initio methods based on DFT are utilized to study the role of Ni addition on the interstitial helium diffusion, energetics and stability of HenVm (n = 0–6, m = 0–4) clusters in austenitic Fe–6.25Cr–3.13Ni (at.%) ternary alloys. The limit vacancy concentration considered in our simulation will be as close to the real irradiation conditions as possible. The presence of Ni obviously hinders the long-range migration of an individual helium between tetrahedral and octahedral interstitials, or between tetrahedral gaps, but greatly promotes the movement between octahedral gaps, and the travel between octahedral (tetrahedral) gaps cannot cross the tetrahedral (octahedral) interstitials. The addition of Ni enhances the attractions between interstitial helium, drives the short-range migration and tends to form small helium bubbles; on the other hand, it seriously hinders the trapping of helium by multiple-vacancies, and reduces the risk of helium bubble nucleation. In addition, the effects of Ni on the stability and energetics of helium vacancy complexes are evaluated, and it is confirmed that Ni improves the stability of the cluster models and enhances the irradiation expansion resistance of the alloy configurations, although it is not conducive to the formation of HenVm clusters. Our findings provide a theoretical reference for understanding the thermal helium desorption spectra and helium-induced embrittlement or hardening under irradiation conditions.

Direct Observation of PFIB-Induced Clustering in Precipitation-Strengthened Al Alloys by Atom Probe Tomography.

The effect of sample preparation on a pre-aged Al–Mg–Si–Cu alloy has been evaluated using atom probe tomography. Three methods of preparation were investigated: electropolishing (control), Ga+ focused ion beam (FIB) milling, and Xe+ plasma FIB (PFIB) milling. Ga+-based FIB preparation was shown to introduce significant amount of Ga contamination throughout the reconstructed sample (≈1.3 at%), while no Xe contamination was detected in the PFIB-prepared sample. Nevertheless, a significantly higher cluster density was observed in the Xe+ PFIB-prepared sample (≈25.0 × 1023 m−3) as compared to the traditionally produced electropolished sample (≈3.2 × 1023 m−3) and the Ga+ FIB sample (≈5.6 × 1023 m−3). Hence, the absence of the ion milling species does not necessarily mean an absence of specimen preparation defects. Specifically, the FIB and PFIB-prepared samples had more Si-rich clusters as compared to electropolished samples, which is indicative of vacancy stabilization via solute clustering.

Vacancy formation energies and migration barriers in multi-principal element alloys

Multi-principal element alloys (MPEAs) continue to garner interest as structural and plasma-facing materials due to their structural (phase) stability and increased resistance to radiation damage. Despite sensitivity of mechanical behavior to irradiation and point-defect formation, there has been scant attention on understanding vacancy stability and diffusion in refractory-based MPEAs. Using density-functional theory, we examine vacancy stability and diffusion barriers in body-centered cubic (Mo0.95W0.05)0.85Ta0.10(TiZr)0.05. The results in this MPEA show strong dependence on environment, originating from local lattice distortion associated with charge-transfer between neighboring atoms that vary with different chemical environments. We find a correlation between degree of lattice distortion and migration barrier: (Ti, Zr) with less distortion have lower barriers, while (Mo, W) with larger distortion have higher barriers, depending up local environments. Under irradiation, our findings suggest that (Ti, Zr) are significantly more likely to diffuse than (Mo, W) while Ta shows intermediate effect. As such, material degradation caused by vacancy diffusion can be controlled by tuning composition of alloying elements to enhance creep strength at extreme operating temperatures and harsh conditions.

CrOx decoration on Fe/TiO2 with tunable and stable oxygen vacancies for selective oxidation of glycerol to lactic acid

Non-noble metal-based catalysts catalyze the conversion of glycerol to lactic acid.

Theoretical exploration of mechanical, electronic structure and optical properties of aluminium based double halide perovskite.

The mechanical, electronic structure and optical properties of aluminium based double halide perovskite were calculated by density functional theory. The formation energy and elastic constant confirm the stability of the cubic perovskite materials. The materials are all ductile and suitable for flexible photovoltaic and optoelectronic devices. The band gap values vary from 0.773 eV to 3.430 eV, exactly corresponding to the range of ideal band gap values for good photoresponse. The band structure analysis shows that all the materials possess small effective mass, which indicates a good transport of carriers. And these materials have a broad energy range of optical absorption for utilization and a detector of photons. Moreover, less expensive K2AgAlBr6 were investigated for comparison with materials containing a cesium element, and according to the results, is also a candidate for photoelectronic devices due to the similar properties.

Effect of Defects on Optical, Electronic, and Interface Properties of NiO/SnO2 Heterostructures: Dual-Functional Solar Photocatalytic H2 Production and RhB Degradation.

Photocatalytic H2 evolution and organic pollutant oxidation have witnessed a radical surge in recent times. However, this integration demands spatial charge separation and unique interface properties for a trade-off between oxidation and reduction reactions. In the current work, defect engineering of NiO/SnO2 nanoparticles aided in altering the optoelectronics and interface properties and enhanced photocatalytic activity. After annealing the catalysts in a N2 atmosphere, the hydroxyl groups were replaced by water molecules through surface modification. The photoexcited holes accumulated on SnO2 break the water molecules and facilitate the reduction of protons on NiO; this is known as spatial separation. Meanwhile, direct hole oxidation, an oxygen reduction reaction, ensures the degradation activity in this 2-fold system. By defect engineering, the limitations of SnO2 such as higher H2O adsorption, wide bandgap (reduced from 3.02 to 1.88 eV), and electronic properties were addressed. The H2 production in the current work has attained a value of 3732 μmol/(g h), which is 2.9 times that of the previous best reported under sunlight. Recyclability tests confirmed the stability of vacancies by promoting the reoxidation of defect states during photocatalytic activity. Additionally, efforts were made to study the effect of defect density on the photocurrent, the electrical resistance, and the mechanism of photocatalytic reactions. Electrochemical characterizations, UPS, XPS, UV-DRS, and PL were employed to understand the influence of defects on the bandgap, charge recombination, charge transport, charge carrier lifetime, and the interface properties that are responsible for photocatalytic activity. In this regard, it was understood that maintaining the optimal defect concentration is important for higher photocatalytic efficiencies, as the defect optimality preserves key photocatalytic properties. Apart from characterizations, the photocatalytic results suggest that excess defect density triggers the undesired thermodynamically favored back reactions, which greatly hampered the H2 yield of the process.

Using Emerging Hot Spot Analysis to Explore Spatiotemporal Patterns of Housing Vacancy in Ohio Metropolitan Statistical Areas

We highlight the use of a newer method—emerging hot spot analysis of space-time cubes from defined locations—for examining the spread of housing vacancy in large Ohio MSAs. Using this method, we discovered that many Ohio MSAs concurrently experienced spread, contraction, and vacancy stabilization in census tracts located adjacent to, or within close proximity of, one another. These results indicate that vacancy proliferation is not solely a matter of geographic determinism, whereby high vacancy in one tract predicts high vacancy in neighboring tracts in future years. We also found that vacancy spread at the tract level is associated with population dynamics at the neighborhood, city, and MSA levels. Our findings suggest that vacancy reduction initiatives should account for population trends at various geographic scales, not just physical conditions within a particular neighborhood or tract.

Correlating electronegativity in bimetallic oxygen carriers for chemical looping combustion

Metallic tailoring of oxygen carrier plays a crucial role within chemical looping combustion (CLC) applications, which can be correlated to the redox reaction enhancement and sustainability. Such attributes arise from the electric valence of the material constituents, which can bring about superior oxygen vacancy and structural stability. In the concurrent study, the role of electronegativity of the bimetallic carriers upon CLC performance was scrutinized based on various bimetallic metal oxides composition. Two metallic systems were examined in the current study: Iron based oxides and copper-based oxides, where the secondary metallic element within each of the examined oxide carrier is composed of one of the 5 metallic elements, predefined for the purpose of electronegativity variation. The secondary metallic element include: Calcium, Lanthanum, Magnesium, Manganese, and Chromium. Several gas byproducts were monitored during the course of the CLC reaction, via mass spectrometer, in order to quantify the variations incurred as a result of different metallic composition inclusion. Additionally, electron microscopic techniques, such as the SEM, were utilized in the study, so as to visualize the structural alteration of sintered materials. The experimental results indicate a higher reactivity with the fuel at a higher electronegativity difference within the bimetallic structure, in the case of iron-based oxide system. Also, copper-based oxide system was associated with higher carbon dioxide production with higher selectivity, relative to iron-based oxide systems.

Activation of Molecular O2 on CoFe2 O4 (001) Surfaces: An Embedded Cluster Study.

Dioxygen activation pathways on the (001) surfaces of cobalt ferrite, CoFe2O4, were investigated computationally using density functional theory and the hybrid Perdew‐Burke‐Ernzerhof exchange‐correlation functional (PBE0) within the periodic electrostatic embedded cluster model. We considered two terminations: the A‐layer exposing Fe2+ and Co2+ metal sites in tetrahedral and octahedral positions, respectively, and the B‐layer exposing octahedrally coordinated Co3+. On the A‐layer, molecular oxygen is chemisorbed as a superoxide on the Fe monocenter or bridging a Fe−Co cation pair, whereas on the B‐layer it is adsorbed at the most stable anionic vacancy. Activation is promoted by transfer of electrons provided by the d metal centers onto the adsorbed oxygen. The subsequent dissociation of dioxygen into monoatomic species and surface reoxidation have been identified as the most critical steps that may limit the rate of the oxidation processes. Of the reactive metal‐O species, [FeIII−O]2+ is thermodynamically most stable, while the oxygen of the Co−O species may easily migrate across the A‐layer with barriers smaller than the associative desorption.

Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2 surfaces

To study the dependence of the relative stability of surface (V A ) and subsurface (VB ) oxygen vacancies with the crystal facet of CeO2, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce3+ polarons, are facet- and concentration-dependent. At the higher vacancy concentration, the V A is more stable than the V B at the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both the V A and the VB have similar stability. The stability of the V A vacancies, compared to that of the V B , is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce3+−Ce3+ interactions.