Introduction Of Oxygen Vacancies Research Articles

Boosting photocatalytic CO2 reduction by tuning photogenerated carrier kinetics in two-dimensional WOx/BiOCl S-scheme heterojunction with oxygen vacancies

Photocatalytic CO2 reduction driven by sustainable solar energy is a highly desirable route to achieve carbon recycling, to which charge separation in photocatalysts holds the key. In this work, two-dimensional (2D) WO3·H2O nanosheets with rich oxygen vacancies (Vo-WOx) were combined with 2D BiOCl nanosheets to form a 2D/2D Vo-WOx/BiOCl S-scheme heterojunction by a self-assembly process. The 2D/2D stacked structure combined with the introduction of oxygen vacancies in Vo-WOx could improve the light harvesting of the designed photocatalytic system. The dual transfer pathways in the S-scheme junction were induced by the fact that the photogenerated electrons on the conduction band and defect energy level of the Vo-WOx transfer across the interface to combine with the BiOCl, leading to an improved kinetic process of photogenerated carriers. Moreover, the S-scheme configuration can retain a strong redox potential for satisfying the thermodynamic requirement of CO2 reduction. As a result, the optimized 30Vo-WOx/BiOCl composite can achieve the highest CO yield rate of 8.82 μmol h−1 from CO2 reduction without any sacrifice reagents, which was 4.69 and 3.08 times higher than that of pure BiOCl and WO3/BiOCl without oxygen vacancy. This work provides a new view of defect engineering in S-scheme heterojunction for pursuing a highly efficient photocatalytic CO2 reduction system.

100% N2O inhibition in photocatalytic NOx reduction by carbon particles over Bi2WO6/TiO2 Z-scheme heterojunctions

The chemically inert property of nitrous oxide (N2O) has made it difficult to be reduced to benign dinitrogen during (photo) selective catalytic reduction (SCR) of NOx. The introduction of oxygen vacancies and BrØnsted acid sites to thermal catalysts is found to effectively inhibit N2O formation. However, the efficiency is still far from satisfactory and the inhibition of N2O in photocatalytic selective reduction of NOx has not been investigated yet. Herein, 100% inhibition of N2O formation during photo-SCR of NOx with carbon particles is successfully achieved via the fabrication of Bi2WO6/TiO2 Z-scheme heterojunction photocatalyst. The completely inhibited N2O formation is attributed to the rapid charge separation and stronger N2O adsorption on the (101) plane of TiO2 according to the density functional theory calculations and experimental results. The presence of water vapor prolongs the reaction time for NOx photo-reduction of carbon particles but has no significant effect on carbon oxidation rate. Introduction of heterostructure interface effectively accelerates photo-induced charge carrier separation and transfer, thereby enhancing photocatalytic efficiency for NOx reduction with carbon particles. The optimized 0.2 Bi2WO6/TiO2 (molar ratio) composite achieves the highest formal electron/photon quantum efficiency of 0.036, which is 3.3 times that of TiO2. The results provide an alternative perspective on the catalyst fabrication via the simple heterojunction materials for toxic byproduct control in the field of pollutants removal.

Characteristics of (Sr0.92Y0.08)0.85Ti1-XNixO3-

Perovskite with A-site deficiency(A1-xBO3) is promising candidate due to introduction of oxygen vacancy for ionic conductivity and nano-sized exsolution of B component with metal phase for catalytic activity. In this study, Ni-doped Sr0.92Y0.08TiO3 perovskite has been investigated to improve the catalytic activity for CO2 dry internal reforming of methane in solid oxide fuel cells. (Sr0.92Y0.08)0.85Ti1-xNixO3- \U0001d6ff (x=0.05, 0.10, 0.15, 0.20) perovskite with A-site deficiency is prepared by pechini method and compared to stoichiometric Sr0.92Y0.08Ti1-xNixO3- \U0001d6ff (SYTN15) perovskite. The physical and physicochemical characteristics of the synthesized powders are analyzed by GC, XRD, BET, TEM, XPS, and TPO. (Sr0.92Y0.08)0.85Ti0.85Ni0.15O3- \U0001d6ff (SYTN(+)15) shows the highest methane conversion with 93% and carbon dioxide conversion with 86%. In addition, the SYTN(+)15 results to the lowest decreasing rate of the methane conversion with 1.86% drop for 50h. The SYTN(+)15 shows better catalytic performance by 10% for dry reforming of methane (DRM) comparing to the SYTN15. The cell performances of the SYTN(+)15 and the SYTN15 are 60.05mW/cm2 and 28.50mW/cm2, respectively. Consequently, A-site deficiency catalyst, SYTN(+)15 exhibits excellent catalytic activity for DRM and sufficient electrochemical property for SOFC anode materials.

Construction of the novel polyimide/Bi2MoO6-OVs p-n type heterojunction aerogel photocatalysts to enhance the photodegradation on organic pollutants driven by the internal electric field

To photodegrade the organic pollution, the Polyimide/Bi 2 MoO 6 -OVs p-n type aerogel heterojunction photocatalysts with rich pore structure and large surface area were synthesized by chemical amide reaction and supercritical ethanol drying technique successfully. The introduction of oxygen vacancies can be beneficial to the activation of molecular oxygen, which can inhibit photo-generated charge recombination to improve photocatalytic efficiency. Furthermore, the formation of Mo-N chemical bond between the coordination of N atoms of heptazine units and Mo atoms of Bi 2 MoO 6 is confirmed by structure analysis. And the ability of the heterojunction photocatalyst to degrade RhB is improved significantly, which is due to photogenerated electron-hole pairs can be quickly separated under the internal electric field within the space charge region under visible light. Simultaneously, the SEM images show the flocculent PI polymer is coated on the surface of flower-like Bi 2 MoO 6 -OVs microsphere, which can expand the light absorption range and enlarge the surface area. This work provides an innovative perspective to construct polymer aerogel with inorganic materials heterojunction photocatalyst with superior photocatalytic activity. • The introduction of oxygen vacancies to Bi 2 MoO 6 is beneficial to the activation of molecular oxygen. • The flocculent PI polymer coated on the surface of flower-like Bi 2 MoO 6 -OVs microsphere is prepared. • The Mo-N chemical bond formation, rich pore structure and large surface area improve photodegradation efficiency.

Prestrain Programmable 4D Printing of Nanoceramic Composites with Bioinspired Microstructure.

Four-dimensional (4D)printing enables programmable, predictable, and precise shape change of responsive materials to achieve desirable behaviors beyond conventional three-dimensional (3D) printing. However, applying 4D printing to ceramics remains challenging due to their intrinsic brittleness and inadequate stimuli-responsive ability. Here, this work proposes a conceptional combination of bioinspired microstructure design and a programmable prestrain approach for 4D printing of nanoceramics. To overcome the flexibility limitation, the bioinspired concentric cylinder structure in the struts of 3D printed lattices are replicated to develop origami nanoceramic composites with high inorganic content (95wt%). Furthermore, 4D printing is achieved by applying a programmed prestrain to the printed lattices, enabling the desired deformation when the prestrain is released. Due to the bioinspired concentric cylinder microstructures, the printed flexible nanoceramic composites exhibit superior mechanical performance and anisotropic thermal management capability. Further, by introducing oxygen vacancies to the ceramic nanosheets, conductive nanoceramic composites are prepared with a unique sensing capability for various sensing applications. Hence, this research breaks through the limitation of ceramics in 4D printing and achieves high-performance shape morphing materials for applications under extreme conditions, such as space exploration and high-temperature systems.

The insight into effects of oxygen vacancy on the activation of peroxymonosulfate via MOF-derived magnetic CoFe2O4 for degrading organic contaminants

Herein, MOF-derived magnetic CoFe2O4 with oxygen vacancy was successfully synthesized to activate peroxymonosulfate (PMS) for the effective degradation of emerging organic contaminants. The introduction of oxygen vacancy not only increased the specific surface area of CoFe2O4, but also reduce the charge transfer resistance. The CoFe2O4 with oxygen vacancy showed low charge transfer resistance and oxidation potential. Benefitting from the existence of oxygen vacancy, the PMS could be activated quickly by CoFe2O4 to produce SO4•−. As expected, developed CoFe2O4/PMS oxidation system displayed the excellent degradation ability toward various typical organic pollutants, and the organic pollutant could be mineralized into CO2, H2O and other inorganic molecules. Interestingly, both redox couples of Fe3+/Fe2+ and Co3+/Co2+ played a critical role in the PMS activation process. The SO4•−, OH•, O2•− and 1O2 were major active species in the CoFe2O4/PMS system, which were responsible for the degradation of organic pollutants. Furthermore, proposed CoFe2O4/PMS oxidation system also presented a wide pH range of applicability. The existence of magnetic property was convenient for the recovery of catalyst. All in all, this study provided a promising catalyst for the activation of PMS, and obtained results injected some new insights into the research on advanced oxidation processes (AOPs).

Origin of defects induced large flexoelectricity in ferroelectric ceramics

Defects have been regarded as playing a critical role in the functionalities of many solid dielectrics. However, the contribution of defects to the specific coupling between strain gradient and electric polarization (i.e., flexoelectricity) has not yet been thoroughly understood. Herein, we selected the typical ferroelectric ${\mathrm{BaTiO}}_{3}$ (BTO) ceramics and introduced oxygen vacancies and trapped charge defects by using stoichiometric and nonstoichiometric Fe dopants, respectively. Compared with the pure BTO ceramics, the flexoelectric coefficients of stoichiometric Fe-doped BTO ceramics were increased by fivefold (from 9.5 to 65 \ensuremath{\mu}C/m) while that of the nonstoichiometric counterparts almost keep stable. The results show that the oxygen vacancies rather than trapped defects make a remarkable contribution to the enhancement of flexoelectricity, and this is explained by the reorientation of the defect dipoles formed by the oxygen vacancies. The result presented in this work not only benefits the understanding of the mechanism of flexoelectricity but also provides a feasible strategy to design flexoelectric materials and related devices with high flexoelectric coefficients.

Thermoelectric properties of oxygen-deficient spinel-type ZnxFe3−xO4−δ prepared by carbothermal reduction and spark plasma sintering

This paper describes the effect of oxygen vacancies, which are induced by a carbothermal reduction process and spark plasma sintering (SPS) treatment, on the thermoelectric properties of spinel-structured zinc ferrites ZnxFe3−xO4 (0.2 ≤ x ≤ 1.0). Characterization of the oxygen-deficient ZnxFe3−xO4−δ materials revealed that Zn2+ preferentially occupy the octahedral B-sites and that valence modification of Fe3+ to Fe2+ occurred, resulting in an excess of n-type carriers at B-sites and an increased power factor. In addition, Co substitution enhanced the power factor for the materials over the entire investigated temperature range. Compared with the carbothermal reduction process, SPS treatment effectively introduced oxygen vacancies within a short time and resulted in a material with a substantially improved electrical conductivity.

Introduction of oxygen vacancy to manganese ferrite by Co substitution for enhanced peracetic acid activation and 1O2 dominated tetracycline hydrochloride degradation under microwave irradiation

High microwave-response cobalt-substituted manganese ferrite (CMFO-0.5) was successfully synthesized as a heterogeneous catalyst for efficient peracetic acid (PAA) activation and tetracycline hydrochloride (TCH) degradation with singlet oxygen (1O2) as the dominated reactive oxidized species (ROS). The removal efficiency of TCH could reach 98.16% within 6 min under microwave irradiation when the CMFO-0.5 was added at 20 mg/L. It's found that the Co substitution could produce the oxygen vacancies (OVs), improve the microwave (MW) absorbing performance and enhance the internal electron transfer efficiency of materials. The phenomenon why 1O2 as the dominated ROS rather than hydroxyl radical (•OH) and organic radicals (R-O•) would be explained by the following aspects: the oxygen adsorbed on the OVs can accept the electron transformed from PAA to form superoxide radical (•O2−), which will disproportionate to form 1O2; the energy generated by the non-thermal effect of MW can dissociate PAA to generate peroxy-group for 1O2 generation. Furthermore, the possible TCH degradation pathways were proposed based on DFT theory calculations and product identification, and the toxicity predictions of the degradation products were also performed by the Ecological Structure-Activity Relationship Model (ECOSAR) software. Additionally, the decrease of acute toxicity of treated TCH, excellent stability and strong resistance towards water matrix fully demonstrate the superiority of the proposed system for practical application in wastewater treatment.

Dual-site oxygen activation for enhanced photocatalytic aerobic oxidation by S-scheme Ni2P/Bi3O4Br-OVs heterojunction

Solar-driven activation of molecular oxygen (O2) offers an appealing strategy to realize aerobic oxidation catalysis at room temperature, while the efficiency of photocatalytic O2 activation is severely limited by lacking O2 adsorption sites and poor carrier utilization. Herein, an efficient Ni2P/Bi3O4Br-OVs heterojunction is firstly proposed to overcome the limitation by the dual-site design of oxygen vacancies (OVs) and on Ni2P cocatalyst. By the joint observations from spectroscopic measurements and theoretical simulations, the introduction of OVs and Ni2P on Bi3O4Br nanosheets can not only optimize the light absorption, facilitate the separation and transfer of photogenerated charge carrier through the S-scheme mechanism, but also provide active sites for effective adsorption and activation of oxygen molecules, thereby contributing to highly efficient generation of •O2− radicals. Benefiting from the more oxidative active species of •O2− and h+, the Ni2P/Bi3O4Br-OVs photocatalyst achieves near-unity conversion rate and selectivity in aerobic oxidation of sulfide to sulfoxide, and it also exhibits outstanding photoactivity in the aerobic degradation of phenolic pollutants under visible light. This study showcases a feasible O2 activation approach by defect and heterojunction engineering, and provides a new perspective for designing high-performance photocatalysts in the field of aerobic oxidation catalysis.

Enhancing the PEC water splitting performance of In2O3 nanorods by a wet chemical reduction

Generating oxygen vacancies is an effective way to improve photoelectrochemical (PEC) water splitting performance of semiconductor materials owing to the formation of shallow donor level and the supply of additional electron donor. Herein, oxygen vacancies were introduced into In2O3 nanorods by a hydrothermal reduction method using NaBH4 solution as the reductant, and the effects of hydrothermal reduction time on the oxygen vacancy concentration, optoelectronic property and PEC water splitting activity over In2O3 nanorods were systematically investigated. The results of LSV, EIS and MS showed that the reduced samples displayed superior PEC performance and In2O3-x-1 exhibited a highest photocurrent density of 0.97 mA cm−2 at 1.23 V vs. RHE under the irradiation of visible light, which was roughly 4 times of bare In2O3. The remarkable performance of In2O3-x-1 is mainly ascribed to the introduction of oxygen vacancies, which leads to better light absorption capacity, higher carrier concentration, and increased electron transport efficiency at the electrode/electrolyte interface.

Photoelectrocatalytic detoxification and cytotoxicity analysis of deoxynivalenol over oxygen vacancy-engineered WO3-x film with low bias

A surface oxygen vacancy-engineered WO3-x film with enhanced photoelectrocatalytic (PEC) activity for the detoxification of deoxynivalenol (DON) was successfully synthesized via an improved sol-gel method. The presence of oxygen vacancy was verified by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectra and electron spin resonance spectroscopy. The introduction of oxygen vacancy into WO3-x film substantially enhanced its absorption in visible region, and facilitated its transfer and separation of photogenerated charges. This made the WO3-x film to have excellent PEC activity for the detoxification of DON under visible light illumination at a low bias of +0.3 V. The percentage of DON removed was up to 86.4 % after being treated with PEC for 2 h. When the bias potential applied was further increased, there were no obvious changes in the degradation efficiency of the film. The cell viabilities were dramatically enhanced with prolonged time, which were over 95 % after 2 h PEC treatment compared to the control. Through the analysis of eight intermediates that were separated and identified by high performance liquid chromatography-mass spectrometry, it can be concluded that the reduction of DON and the cleavage of double bond at C9C10, epoxy group at C1213 and hydroxyl group at C3 in DON could greatly reduce the toxicity, and the detoxification mechanism of this PEC system was proposed. This work can provide further insights into DON detoxification using PEC system with low bias.

2D black NiO with oxygen vacancies/1D CdS nanocomposites for efficient visible-light-driven photocatalytic H2 production

Constructing highly active and stable earth-abundant heterojunction to replace expensive metals is regarded as an effective strategy in the photocatalytic H2 production system. Herein, a NiO with oxygen vacancies and broad absorption in the visible light band is prepared and the introduction of oxygen vacancies leads to the enhancement of catalytic activity and electrical conductivity of NiO. Meanwhile, a 2D NiO/1D CdS heterojunction is constructed in different atmospheres for photocatalytic H2 production. Different calcination atmospheres will result in distinct surface components, which further brings about a vast difference in activity. The best NiO/CdS catalyst calcined in Ar flow shows a dramatic Hydrogen evolution rate of 15.6 mmol·g−1·h−1 under visible light irradiation, which is comparable to CdS with photo-deposited palladium. The apparent quantum efficiency exceeded 15 % under visible light at 420 nm and 475 nm. Further investigation with contact potential differences (CPD) and in-situ Kelvin probe force microscopy (KPFM) technology indicates the dynamic process of photogenerated carrier transfer from CdS to NiO under illumination, which explains the reason for the high-efficiency utilization of photogenerated charge and high hydrogen production activity.

Oxygen vacancy-mediated amorphous GeOx assisted polysulfide redox kinetics for room-temperature sodium-sulfur batteries

The practical applications of room-temperature sodium-sulfur (RT Na-S) batteries have been greatly hindered by the natural sluggish reaction kinetics of sulfur and the shuttle effect of sodium polysulfide (NaPSs). Herein, oxygen vacancy (OV)-mediated amorphous GeOx/nitrogen doped carbon (donated as GeOx/NC) composites were well designed as sulfur hosts for RT Na–S batteries. Experimental and density functional theory studies show that the introduction of oxygen vacancies on GeOx/NC can effectively immobilize polysulfides and accelerate the redox kinetics of polysulfides. Meanwhile, the micro-and mesoporous framework, acting as a reactor for storing active S, is conducive to alleviating the expansion of S during the charging/discharging process. Consequently, the S@GeOx/NC cathode affords a reversible capacity of 1017 mA h g−1 at 0.1 A g−1 after 100 cycles, outstanding rate capability of 333 mA h g−1 at 10.0 A g−1 and long lifespan cyclability of 385 mAh g−1 at 1 A g−1 after 1200 cycles. This work furnishes a new way for the rational design of metal oxides with oxygen vacancies and boosts the application for RT Na-S batteries.

Theoretical investigations of oxygen vacancy effects in nickel-doped zirconia from ab initio XANES spectroscopy at the oxygen K-edge.

In this study, we present theoretical X-ray absorption near-edge structure (XANES) spectra at the K-edge of oxygen in zirconia containing Ni dopant atoms and O vacancies at varying concentrations. Specifically, our model system consist of a supercell composed of a zirconia (ZrO2) matrix containing two nickel dopants (2Ni), which substitute two Zr atoms at a finite separation. We found the 2Ni atoms to be most stable in a ferromagnetic configuration in the absence of oxygen vacancies. In this system, each Ni atom is surrounded by two shells of O with tetrahedral geometry, in a similar way as in bulk cubic zirconia. The oxygen K-edge XANES spectrum of this configuration shows a pre-edge peak, which is attributable to dipole transitions from O 1s to O 2p states that are hybridized with unoccupied Ni 3d states. The intensity of this pre-edge peak, however, reduces upon the introduction of a single vacancy in the 2Ni-doped zirconia matrix. The corresponding ground state remains ferromagnetic, while one of the nickel atoms adopts a trigonal bipyramidal geometry, and the other one remains in a tetrahedral geometry. Furthermore, the introduction of two vacancies in the 2Ni-doped zirconia results in the two Ni atoms having distorted octahedral and trigonal bipyramidal geometries and being coupled antiferromagnetically in the ground state. Additionally, the oxygen K-edge XANES spectrum shows a further decrease in the intensity of the pre-edge peak, compared to the case of a single vacancy. Thus, the changes in the intensity of the pre-edge peak evidence a major structural change in the local environment around nickel atoms and, by extension, in the zirconia matrix. This change is due to the structural disorder induced by the 2Ni dopants and the O vacancies. Furthermore, the analysis of the XANES signatures shows that the oxidation state of nickel atoms changes with the introduction of oxygen vacancies. Our study therefore shows a possibility to control the oxidation state and magnetic order in a typical diluted magnetic oxide. Such a finding may be crucial for spintronics-related applications.

Bifunctional OER/NRR Catalysts Based on a Thin-Layered Co3O4-x/GO Sandwich Structure.

Due to ample low-coordinated surface atoms, a distorted lattice endows thin-layered transition metal oxides with a flexible electronic structure, making them the ideal candidates for overall ammonia synthesis. This work proposes a novel and facile method for the controllable synthesis of thin-layered Co3O4 catalysts with graphene as a conductive matrix to further enhance the overall N2 fixation performance. X-ray photoelectron spectroscopy (XPS) and synchrotron radiation X-ray absorption spectroscopy (XAS) demonstrate that the sandwiched Co3O4-x/GO catalysts enable exposure of more coordination unsaturated active sites, resulting in numerous oxygen vacancies. With a higher conductivity and distorted crystalline structure, excellent electrochemical NRR activity is realized with a NH3 production rate of 5.19 mmol g-1 h-1 and a Faradaic efficiency of 10.68% at -0.4 V vs reversible hydrogen electrode (RHE). The density functional theory (DFT) calculation demonstrates that introducing oxygen vacancies in thin-layered cobalt oxides could result in an increased density of states (DOS) near the Fermi level, which would accelerate the NRR rate-determining step. Charge transfer could be accelerated through a weak Co 3d-N 2p σ hybrid bond with a lower energy level. No obvious performance decay could be found after six cycles. Furthermore, the sandwiched Co3O4-x/GO catalyst exhibits a low overpotential of 280 mV@10 mA cm-2 and an outstanding durability for the anode OER, even better than those of the benchmark RuO2. Such an inexpensive sandwiched transition metal oxide catalyst shows great potential in the field of overall N2 fixation.

A‐Site Substitution Motivating the Phase Transformation of LaNiO3 Perovskite for High‐Performance Artificial N2 Fixation

AbstractElectrocatalytic N2 reduction reaction (NRR) to NH3 under ambient conditions is an attractive strategy compared to the traditional Haber‐Bosch process. However, there still remains a great challenge due to the high activation barrier of N2 and consequent sluggish kinetics. Herein, A‐site substitution of LaNiO3 perovskites by Sr‐doping is proposed which can lead to phase transformation and to the formation of oxygen vacancies, significantly improving the NRR performance of La1‐xSrxNiO3 (LSNO). The appropriate oxygen vacancy concentration can be modulated by changing the stoichiometric ratio of La and Sr elements. LSNO‐III (x=0.5) delivers the highest ammonia production rate of 55.9 μg h−1 mg−1cat. at −0.7 V and Faraday efficiency of 23.17 % at −0.5 V versus the reversible hydrogen electrode in 0.1 M Na2SO4. Density functional theory (DFT) calculations further demonstrate that introduction of oxygen vacancies decreases the distance of the d‐band center to the Fermi energy level, which not only facilitates adsorption and activation of N2 molecules and subsequent hydrogenation reactions, but also further optimizes reaction pathways in NRR.

Effect of oxygen vacancy defects on electronic and optical properties of MgO monolayers: First principles study

The optoelectronic properties induced by oxygen vacancy defects in MgO (111) monolayers have been studied using hybrid level of DFT method. HSE calculations show significant reduction in electronic band gap of MgO monolayer as a result of introduction of oxygen vacancies. The pristine monolayer has a wide band gap (4.84 eV, indirect) semiconducting behaviour, which changes to 2.97 eV (indirect) and 2.28 eV (direct) with increment in oxygen vacancy defect concentrations of 6.25 % and 12.5 %, respectively. Consequently, presence of oxygen vacancies leads to energy red shift of the observed optical phenomena with reference to the pristine monolayer. Most importantly, the di-vacancy system with two consecutive vacancy sites displays the strongest optical absorption and also becomes optically responsive over the spectral range from visible to ultraviolet region in the electromagnetic spectrum. Thus creation of oxygen vacancies in MgO monolayers may be a fruitful technique for achieving a suitable solar energy material.

Single transition metal atom stabilized on double metal carbide MXenes for hydrogen evolution reaction: a density functional theory study

Double metal carbide MXenes have received considerable attention for use in renewable energy storage technologies in recent years. Here, density functional theory calculations were performed to investigate the hydrogen evolution reaction (HER) performances of double metal carbide MXenes (Cr2TiC2O2 and the Mo2TiC2O2 monolayer (ML) modified by a single transition metal (TM) atom (Co, Ni, Cu, Zn, Rh, Pd, Ag, Cd, Ir, Pt, Au, or Hg)). The thermodynamic stabilities of TM adatoms on pristine Cr2TiC2O2 and Mo2TiC2O2 MLs and defective Cr2TiC2O2 and Mo2TiC2O2 MLs with oxygen vacancies (denoted as TM-M′2TiC2O2−δ , where M′ = Cr or Mo) were calculated, and the results indicated that the introduction of oxygen vacancies can increase the stabilities of TM adatoms. The TM modification on the MXenes surface tuned the Gibbs free energy of hydrogen adsorption G H, and the G H values of Au-, Co-Cr2TiC2O2−δ , and Ni-Mo2TiC2O2−δ closed to zero values (0.07, −0.07, and 0.02 eV, respectively). The presence of TM adatoms also changed the reaction mechanism of H2 desorption and reduced the activation barrier for H2 production to improve the catalytic efficiency. Nine TM-M′2TiC2O2−δ systems were found to have both HER activity and a lower H2 desorption reaction barrier than the pristine MXenes. Systematic electronic structure analyses showed that charge rearrangement through TM modification could tune the number of electrons gained by the surface O atom and have an impact on the hydrogen adsorption strength. These results indicated that TM surface modification is an effective method for improving the HER catalytic activity of double TM carbides.

Atomic‐Layer‐Deposited Oxygen‐Deficient TiO2 on Carbon Cloth: An Efficient Electrocatalyst for Nitrogen Fixation

AbstractElectrochemical nitrogen reduction reaction (NRR) is a promising alternative to the traditional Haber‐Bosch process for ammonia production, but plagued by the high bonding energy of N≡N and the hydrogen evolution side reaction. Recent studies show that enhancing the adsorption and activation of N2 on the catalyst is the key to improving the NRR performance, and the introduction of oxygen vacancies on the surface of metal oxides is an effective way to design high‐performance catalysts. Here, TiO2 with different thicknesses was deposited on conductive carbon cloth by atomic layer deposition, and then oxygen vacancies were introduced on the catalyst surface by an annealing process. The abundant oxygen vacancies on TiO2 surface are found to be beneficial for enhancing the NRR activity of the catalysts. Moreover, the NRR activity is significantly influenced by the thickness of the deposited TiO2 layer, and an over‐thick TiO2 layer leads to a decrease in the conductivity of the catalysts and hinders the charge transfer process. With the optimum thickness, the atomic‐layer‐deposited oxygen‐deficient TiO2 catalysts can achieve a Faradaic efficiency of 31.6 % and a yield of 3.4 μg h−1 cm−2 for ammonia production in 0.5 M LiOH aqueous electrolyte at 0 vs. RHE. It is anticipated that ultrathin semiconducting oxide catalysts synthesized by atomic layer deposition will have broad and promising applications for efficient electrochemical NRR. Introduction