Vacant Sites Research Articles

BaTiO3-xNy: Highly Basic Oxide Catalyst Exhibiting Coupling of Electrons at Oxygen Vacancies with Substituted Nitride Ions.

The base strength of oxide catalysts is controlled by the electron charge distribution between cations and anions, with unsaturated oxygen ions that have lone pair electrons typically acting as basic sites. Substitution of oxide ions with anions that have different valences, such as nitride and hydride ions, can often generate basic sites. It is plausible that electrons trapped at oxygen vacancy sites could provide increased electron density and shift the highest occupied molecular orbital energy levels of anions upward in the case that the oxygen vacancies couple with surface-substituted anions. The present work demonstrates that high catalytic basicity can be obtained via site-selective doping of anions at face-sharing Ti2O9 dimer sites with oxygen vacancies in BaTiO3-x. This improved basicity stems from the coupling of substituted nitride ions to electrons at oxygen vacancies. The oxynitride BaTiO3-xNy was found to contain nitride ions that have increased electronic charge density on the basis of such interactions. Enhanced surface basicity following doping with nitride ion was also confirmed by CO2 temperature-programmed desorption and infrared spectroscopy in conjunction with the adsorption of CHCl3. The strong Lewis base sites resulting from the formation of the oxynitride evidently facilitated the catalytic activation of C-H bonds to promote Knoevenagel condensation reactions between aldehydes and active methylene compounds with pKa values of up to 28.9.

Synergic effect of CuS and MgO for boosting adsorption-photocatalytic activity of S-doped biochar

Adsorption coupled with photocatalysis is a promising route to remove dyes from wastewater. Biochar was modified with S, CuS, and MgO (CuS–Mg/S-BC) through one-pot sulfurization to boost the adsorption and photocatalytic removal of rhodamine B (RhB). The abundant functional groups as well as vacant sites formed at the junction interface among S-doped biochar, CuS, and MgO favored RhB adsorption and photon capture, and this tight interface increased the separation and transfer of photoinduced e−-h+ pairs. Hence, CuS–Mg/S-BC exhibited greater removal efficiency than S-doped biochar, 2-CuS/S-BC (The biochar is loaded with 5 % CuS), and b-Mg/S-BC (The biochar is loaded with 5 % MgO). Langmuir and second-order models well described the adsorption isotherms and kinetics, respectively of CuS–Mg/S-BC composites. 2 b-CuS-Mg/S-BC (Biochar is loaded with both 5 % CuS and MgO) exhibited a removal efficiency of 95.70 % within 120 min, and qe estimated from the second-order model was 981.67 mg g−1. For a 220 mg L−1 RhB solution, the removal efficiency and the adsorption efficiency decreased from 95.70 % to 87.01 % and from 83.54 % to 76.17 %, respectively, after seven cycles.

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio‐Inspired Bimetallic MXene Electrocatalyst

AbstractMo‐ and Fe‐containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3RR) catalyzed by an Fe‐substituted two‐dimensional molybdenum carbide of the MXene family, viz., Mo2CTx : Fe (Tx are oxo, hydroxy and fluoro surface termination groups). Mo2CTx : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2CTx) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 μmol h−1 mg−1, respectively, for the reduction of NO3− to NH4+ in acidic media and 70 % and 12.9 μmol h−1 mg−1 in neutral media. Regardless of the media, Mo2CTx : Fe outperforms monometallic Mo2CTx owing to a more facile reductive defunctionalization of Tx groups, as evidenced by in situ X‐ray absorption spectroscopy (Mo K‐edge). After surface reduction, a Tx vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3RR in close analogy to the prevailing mechanism of the natural Mo‐based nitrate reductase enzymes.

Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst.

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3 RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo2 CTx : Fe (Tx are oxo, hydroxy and fluoro surface termination groups). Mo2 CTx : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2 CTx ) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 μmol h-1 mg-1 , respectively, for the reduction of NO3 - to NH4 + in acidic media and 70 % and 12.9 μmol h-1 mg-1 in neutral media. Regardless of the media, Mo2 CTx : Fe outperforms monometallic Mo2 CTx owing to a more facile reductive defunctionalization of Tx groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a Tx vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3 RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.

Nitrogen and oxygen binary vacancies modulated g-C3N4/BiO2−x for visible-NIR-driven nitrogen photofixation

In order to achieve efficient development of green and sustainable industries and improve energy efficiency, the growth of highly efficient catalysts with photoresponse from ultraviolet (UV) to near-infrared (NIR) region is of great significance for the reduction of nitrogen to ammonia. Herein, we reported binary nanohybrids composed of nitrogen vacancy-rich two dimensional (2 D) g-C3N4 and oxygen vacancy-rich 2 D BiO2−x (CN/BiO). Both experimental results and DFT calculations indicated that the synergistic effect of the heterostructure in CN/BiO effectively suppresses the recombination of photogenerated electron-hole pairs. Advancing from the efficient charge separation, N2 adsorbed on the nitrogen vacancy and oxygen vacancy sites can effectively receive high-energy electrons, which in turn promote NH3 production. Impressively, the CN/BiO exhibited remarkable ammonia yield rate of 602.1 μmol g−1 h−1 with > 420 nm light irradiation. More crucially, superior activity under NIR light was indeed accomplished for CN/BiO with the NH3 production rates up to 89.4 μmol g−1 h−1 using > 780 nm light irradiation. This study provided a novel synthetic route to prepare highly active and full-spectrum light-driven heterojunction photocatalysts toward sustainable and scalable solar-to-NH3 production.

Nonthermal plasma induced NiS/UiO-66-NH2 for boosting visible-light photo-Fenton degradation of tetracycline hydrochloride

Heterojunctions constructed from metal organic frameworks (MOF) and metal sulfides are the potential candidates for solar-light driven photo-Fenton elimination of antibiotics from wastewater. To boost the solar-light induced photo-Fenton degradation of tetracycline hydrochloride (TCH) over UiO-66, NH2-functionlized UiO-66 (UiO-66-NH2) was in-situ loaded with NiS (NiS/NU-66) via the nonthermal plasma sulfurization. For comparison to UiO-66-NH2, NiS/NU-66 had the lower adsorption capacity while the higher photo-Fenton activity for TCH removal. The removal efficiency of NiS/NU-66 climbed and then declined with an increase in NiS content, and the best removal efficiency (84.92 ± 2.55 %) for 110 mg L−1 TCH solution was obtained by c-NiS/NU-66 in 150 min. The sulfur and oxygen defects as well as the plentiful functional groups over NiS/NU-66 were favorable for providing the abundant vacant sites of TCH adsorption and photons capture. In addition, the rapid separation and migration of photo-generated e−/h+ pairs were achieved at the tight junction interface between NiS and UiO-66-NH2, enlarging the visible-light absorption capacity. In NiS/NU-66-assisted photocatalytic system, H2O2 was efficiently convert to •OH by e− and Ni2+/Ni3+ couples in solar light region. Hence, •OH, •O2− and h+ played the vital roles in this photo-Fenton system.

High-Entropy Co-Free O3-Type Layered Oxyfluoride: A Promising Air-Stable Cathode for Sodium-Ion Batteries.

Sodium-ion batteries have recently emerged as a promising alternative to lithium-based batteries, driven by an ever-growing demand for electricity storage systems. The present workproposes a cobalt-free high-capacity cathode for sodium-ion batteries, synthesized using a high-entropy approach. The high-entropy approach entails mixing more than five elements in a single phase; hence, obtaining the desired properties is a challenge since this involves the interplay between different elements. Here, instead of oxide, oxyfluoride is chosen to suppress oxygen loss during long-term cycling. Supplement to this, lithium isintroduced in the composition to obtain high configurational entropy and sodium vacant sites, thus stabilizing the crystal structure, accelerating the kinetics of intercalation/deintercalation, and improving the air stability of the material. With the optimization of the cathode composition, a reversible capacity of 109mAhg-1 (2-4V) and 144mAhg-1 (2-4.3V) is observed in the first few cycles, along with a significant improvement in stability during prolonged cycling. Furthermore, in situ and ex situ diffraction studies during charging/discharging reveal that the high-entropy strategy successfully suppresses the complex phase transition. The impressive outcomes of the present work strongly motivate the pursuit of the high-entropy approach to develop efficient cathodes for sodium-ion batteries.

Paper-based biomimetic test-strip for CA15-3 with coloured readout

The combination of molecularly-imprinted polymers (MIPs) on paper substrates for capturing a cancer antigen 15–3 (CA15-3) with the traditional coloured transduction scheme of enzyme-linked immunosorbent assay (ELISA) using 3,3′,5,5′-tetramethylbenzidine (TMB), horseradish peroxidase (HRP) and H2O2 is presented here for the first time.Here, the paper surface was modified with a silane derivative containing an amine function that allows subsequent binding of 3-aminophenylboronic acid (3-APBA). The target protein, CA15-3, was then bound via the boronic groups of APBA. The empty space around CA15-3 was filled by polymerization of dopamine. Finally, the CA15-3 template was removed by breaking the imine function to create vacant sites for which CA15-3 has a high affinity. Binding of CA15-3 was detected by oxidation of TMB substrate by HRP in the presence of H2O2.The results showed that the MIP was able to selectively recognize CA15-3 within 3–500 U mL−1, even in complex samples. Quantitative data were extracted by analysing the colour coordinates of images captured with a smartphone using ImageJ. This selective behaviour was confirmed by comparison with a control material (non– polymer, NIP).Overall, the described approach has advantages over conventional ELISA, namely low cost and high stability, which is due to the use of MIP as a trapping element acting as a selective pre-concentration point of CA15-3. To our knowledge, this is the first biomimetic ELISA (B-ELISA) on paper substrate used for macromolecules such as proteins. We believe that this approach has the potential to be applied to other protein disease biomarkers, making it a suitable tool for screening glycoproteins at the point-of-care.

Symmetric bipolar resistive switching in copper oxide nanostructure/ITO lateral device under exposure to atmospheric oxygen and application in artificial synaptic devices

Capacitively coupled resistive switching behaviour was observed in a planar-geometry two-terminal device based on nanostructures of copper oxide as active material and indium tin oxide (ITO) as electrodes. When tested in ambient atmosphere, current-voltage I−V characteristic was found to be symmetric with pronounced hysteresis loop signifying two different states of resistance, namely the low resistive state (LRS) and high resistive state (HRS). Capacitive effect was apparent in the low voltage regime due to the presence of offset voltage at zero current and offset current at zero bias. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of native oxygen vacancies in copper oxide. Additionally, exposure to atmospheric oxygen played the dominant role in creation of excess vacancy sites by electrochemical reaction near positive electrode. Creation of excess density of oxygen vacancy near the electrodes in conjunction with trapping and detrapping of electrons from these defect sites is perceived as the prime mechanism for the observed switching behaviour. The device ON/OFF ratio, ION/IOFF∼1.5 was found to be stable over at least 100 continuous cycles. When tested under vacuum, the device I−V characteristics was linear in shape without any hysteresis signifying ohmic transport. Analysis of ultraviolet photoelectron spectroscopy (UPS) showed the Fermi energy level of copper oxide is at ∼ 4.69 eV, which is in a close match with the work function of ITO-electrode. The essential synaptic characteristics such as learning and forgetting curves and paired pulse facilitation (PPF) are established. The nonlinearity factors (NLF) are very low indicating the copper oxide-based devices are promising candidates for neuromorphic applications.

Unraveling the Role of Metal Vacancy Sites and Doped Nitrogen in Enhancing Pseudocapacitance Performance of Defective MXene.

Nitrogen-doped titanium carbides (MXene) films exhibit extraordinary volumetric capacitance when high-concentration sulfuric acid electrolyte is utilized owing to the enhancement of pseudocapacitance. However, the energy storage mechanism of nitrogen-doped MXene is unclear due to the complex electrode structure and electrolyte ions' behavior. Here, based on pristine MXene (Ti3C2O2), three different MXene structures are constructed by introducing metal vacancy sites and doped nitrogen atoms, namely, defective MXene (Ti2.9C2O2), nitrogen-doped MXene (Ti3C2O1.9N0.1), and nitrogen-doped MXene with metal vacancy sites (Ti2.9C2O1.9N0.1). Then, the density functional theory (DFT)-based calculations coupled with the effective screening medium reference interaction site method (ESM-RISM) are applied to reveal the electrochemical behavior at the electrode/electrolyte interfacial area. Through analyzing the electronic structure, electrical double-layer capacitance (EDLC), and equilibrium potential of the pseudocapacitance reaction, the specific effect of structural changes on their performance can be clarified: metal vacancy sites can reduce the potential difference of gap layer (Outer Helmholtz plane) at charged state and increase the electronic capacity of Ti, which can be used to explain the high pseudocapacitance, low charge transfer resistance and high-rate capacity properties of nitrogen-doped MXene observed in experiments.

Tailoring the gas sensing characteristics of ZnO and ZnS monolayers toward CO2, CO and N2 upon introducing defects and substitutional functionalization

This work aims to design two-dimensional (2D) catalysts based ZnO and ZnS for CO2 reduction reaction (CRR), nitrogen reduction reaction (NRR), CO2 and N2 captures and sensors. Cheap and abundant elements which are Mg, Cl, and N are doped into Zn, S, and O sites to tune the electronic properties of the ZnO and ZnS monolayers. Also, influences of O, S and Zn vacancies on the electronic properties and gas adsorption are examined as well. Adsorption energy, interaction energy, distortion energy, density of states, Bader charge, shift of Fermi level, and work function of the modified surfaces are studied via density functional theory (DFT) performed on VASP. From the results, Mg, Cl, and N doping as well as vacant sites can extremely change electronic properties, i.e., semiconductive to metallic behavior, the shift of Fermi level and the variation of work function. Some modified ZnS and ZnO monolayers can be good candidates CRR and NRR catalysts since neither too strong nor too light binding of CO2, CO and N2. The best catalysts for this study is (Mg, VZn)-ZnO and Cl-ZnS because both can be sensors and electrochemical catalysts due to the proper binding strength for CO2, CO and N2 adsorption. The change in the electronic properties leading to the change in gas adsorption shows that the ZnO and ZnS modification by cheap-element doping and vacancy-site creation are potential ways to improve their properties, extend their applications and lead to the invention promising catalysts for electrochemical reactions, gas sensors and gas storages.

Molecular dynamic simulation of the influence of vibration effects on scratching processes in Varied crystal orientations

The research delves into the uncharted terrain of crystal orientation’s effect on high-frequency vibration-assisted processing of single-crystal copper, employing molecular dynamics to devise non-vibration, one-dimensional (1D), and two-dimensional (2D) vibration-assisted scratching models. The innovative discovery is the ‘peak-shaving’ effect, invoked by high-frequency vibration, which significantly mitigates surface irregularities on single-crystal copper, enhancing surface quality and material plasticity, thereby facilitating machinability. A key revelation is the superior efficacy of 2D vibration in material fortification relative to 1D vibration. Another novel finding is the amplified plasticity of single-crystal copper with a (111) crystal orientation under vibration-assisted excitation, linked to the varying directions of dislocation slip contingent upon crystal orientations. The pioneering observation that the induction of vibration during scratching dynamically propels dislocation defect structures, leading to the generation of a significant volume of vacant and interstitial atomic sites, underscores the pronounced influence of 2D vibration. This research contributes invaluable microscopic perspectives into the operative mechanism of crystal orientation’s impact on high-frequency vibration-assisted processing.

Electronic, mechanical, and vibrational properties of calcium lanthanum sulfide solid solution: A DFT study

The combination of optical and mechanical properties of calcium lanthanum sulfide (CLS) makes it an attractive material for optical applications. CLS is a γ-phase La2S3/CaS solid solution above 50:50 La2S3/CaS. Experiments have characterized the effect of composition on the structural, thermal, and optical properties. However, the effect of impurities and other defects, such as porosity, on resulting properties is difficult to deconvolute. We used the density functional theory (DFT) simulations to characterize the effect of La2S3/CaS relative fractions on structural, electronic, mechanical, and vibrational properties of single crystalline CLS solid solutions. We find a decrease in the stiffness with CaS fraction and confirm the experimentally observed increase in the lattice parameter with La2S3 fraction. We also characterized the point defects, including sulfur vacancies, oxygen substituting sulfur, oxygen, and sulfur impurities on lanthanum sublattice vacant sites, and sulfur substituting calcium (vS, OS, OLa, and SLa, respectively). Our defect studies in 90:10 CLS find the neutral charge state configuration to be energetically favorable for all tested configurations except for the sulfur vacancies (vS).

Photosynthesis of Benzonitriles on BiOBr Nanosheets Promoted by Vacancy Associates

AbstractPhotocatalytic organic functionalization reactions represent a green, cost‐effective, and sustainable synthesis route for value‐added chemicals. However, heterogeneous photocatalysis is inefficient in directly activating ammonia molecules for the production of high‐value‐added nitrogenous organic products when compared with oxygen activation in the formation of related oxygenated compounds. In this study, we report the heterogeneous photosynthesis of benzonitriles by the ammoxidation of benzyl alcohols (99 % conversion, 93 % selectivity) promoted using BiOBr nanosheets with surface vacancy associates. In contrast, the main reaction of catalysts with other types of vacancy sites is the oxidation of benzyl alcohol to benzaldehyde or benzoic acid. Experimental measurements and theoretical calculations have demonstrated a specificity of vacancy type with respect to product selectivity, which arises from the adsorption and activation of NH3 and O2 that is required to promote subsequent C−N coupling and oxidation to nitrile. This study provides a better understanding of the role of vacancies as catalytic sites in heterogeneous photocatalysis.

Photosynthesis of Benzonitriles on BiOBr Nanosheets Promoted by Vacancy Associates.

Photocatalytic organic functionalization reactions represent a green, cost-effective, and sustainable synthesis route for value-added chemicals. However, heterogeneous photocatalysis is inefficient in directly activating ammonia molecules for the production of high-value-added nitrogenous organic products when compared with oxygen activation in the formation of related oxygenated compounds. In this study, we report the heterogeneous photosynthesis of benzonitriles by the ammoxidation of benzyl alcohols (99 % conversion, 93 % selectivity) promoted using BiOBr nanosheets with surface vacancy associates. In contrast, the main reaction of catalysts with other types of vacancy sites is the oxidation of benzyl alcohol to benzaldehyde or benzoic acid. Experimental measurements and theoretical calculations have demonstrated a specificity of vacancy type with respect to product selectivity, which arises from the adsorption and activation of NH3 and O2 that is required to promote subsequent C-N coupling and oxidation to nitrile. This study provides a better understanding of the role of vacancies as catalytic sites in heterogeneous photocatalysis.

Site-occupancy factors in the Debye scattering equation. A theoretical discussion on significance and correctness.

The Debye scattering equation (DSE) [Debye (1915). Ann. Phys. 351, 809-823] is widely used for analyzing total scattering data of nanocrystalline materials in reciprocal space. In its modified form (MDSE) [Cervellino et al. (2010). J. Appl. Cryst. 43, 1543-1547], it includes contributions from uncorrelated thermal agitation terms and, for defective crystalline nanoparticles (NPs), average site-occupancy factors (s.o.f.'s). The s.o.f.'s were introduced heuristically and no theoretical demonstration was provided. This paper presents in detail such a demonstration, corrects a glitch present in the original MDSE, and discusses the s.o.f.'s physical significance. Three new MDSE expressions are given that refer to distinct defective NP ensembles characterized by: (i) vacant sites with uncorrelated constant site-occupancy probability; (ii) vacant sites with a fixed number of randomly distributed atoms; (iii) self-excluding (disordered) positional sites. For all these cases, beneficial aspects and shortcomings of introducing s.o.f.'s as free refinable parameters are demonstrated. The theoretical analysis is supported by numerical simulations performed by comparing the corrected MDSE profiles and the ones based on atomistic modeling of a large number of NPs, satisfying the structural conditions described in (i)-(iii).

Revealing the atomic mechanism of diamond–iron interfacial reaction

AbstractDiamond, with ultrahigh hardness, high wear resistance, high thermal conductivity, and so forth, has attracted worldwide attention. However, researchers found emergent reactions at the interfaces between diamond and ferrous materials, which significantly affects the performance of diamond‐based devices. Herein, combing experiments and theoretical calculations, taking diamond–iron (Fe) interface as a prototype, the counter‐diffusion mechanism of Fe/carbon atoms has been established. Surprisingly, it is identified that Fe and diamond first form a coherent interface, and then Fe atoms diffuse into diamond and prefer the carbon vacancies sites. Meanwhile, the relaxed carbon atoms diffuse into the Fe lattice, forming Fe3C. Moreover, graphite is observed at the Fe3C surface when Fe3C is over‐saturated by carbon atoms. The present findings are expected to offer new insights into the atomic mechanism for diamond‐ferrous material's interfacial reactions, benefiting diamond‐based device applications.

Breaking the Selectivity Barrier: Reactive Oxygen Species Control in Photocatalytic Nitric Oxide Conversion

AbstractSemiconductor photocatalytic technology holds promise in efficiently reducing low concentrations of gaseous nitric oxide (NO). However, the suboptimal selectivity in NO removal, leading to the undesired production of NO2 byproducts, poses a challenge. In this study, a defective CdS/Na2Ti3O7 heterostructure is rationally designed with strong electronic interaction and intimate interface contact for promoting charge transfer kinetics. This design refines reactive oxygen species (ROS) generation, resulting in an impressive 81% NO elimination and 99.7% selectivity toward nitrates. Detailed mechanistic studies reveal an intriguing catalysis scenario in which reactant molecules are selectively adsorbed and activated at different sites. Anionic vacancies on the CdS/Na2Ti3O7 surface render the activation of molecular O2 to reactive superoxide radicals (O2 −) species. Furthermore, intrinsic surface basicity and O vacancy sites of the Na2Ti3O7 cocatalyst facilitate the capture and activation of acidic nitrogen oxides (NOx) molecules as nitrate species, contributing to enhanced catalytic activity and selectivity. As a result, anionic vacancies and basic sites over CdS/Na2Ti3O7 heterostructure synergistically regulate the NOx oxidation pathway, refining the products toward nitrate with remarkable selectivity. These insights guide the development of advanced photocatalytic systems for environmental remediation, highlighting the importance of managing ROS production for efficient pollutant removal.

The effect of vacant sites on the magnetic properties of a germanene nanostructure: Monte Carlo simulations

The effect of vacant sites on the magnetic properties of a germanene nanostructure: Monte Carlo simulations

Mn-Modified ZnO Nanoflakes for Optimal Photoelectrochemical Performance Under Visible Light: Experimental Design and Theoretical Rationalization.

Doping of zinc oxide (ZnO) with manganese (Mn) tunes midbandgap states of ZnO to enhance its optical properties and makes it into an efficient photoactive material for photoelectrochemical water splitting, waste removal from water, and other applications. We demonstrate that ZnO modified with 1 at. % Mn exhibits the best performance, as rationalized by experimental, structural, and optical characterization and theoretical analysis. ZnO doped with the optimal Mn content possesses improved light absorption in the visible region and minimizes charge carrier recombination. The doping is substitutional and creates midgap states near the valence band. Mn atoms break localized charge traps at oxygen vacancy sites and eliminate photoluminescence peaks associated with oxygen vacancies. The optimal performance of Mn-modified ZnO is demonstrated with the photodegradation of Congo red and photoelectrochemical water splitting.