Oxygen Vacancy Sites Research Articles

Interfacial MoO2 nanograins assembled over graphitic carbon nanofibers boosting efficient electrocatalytic reduction of nitrate to ammonia

Electrocatalytic nitrate reduction reaction (NO3-RR) for sustainable carbon-free hydrogen carrier (NH3) production from polluted NO3- sources of industrial wastewater is very promising. Mo-based catalysts are defined as popular electrocatalytic materials, but the rational performance achievement of Mo-based catalysts in the NO3-RR remains limited. In this study, to overcome the NO3-RR activity limitation, the interfacial MoO2 nanograins assembled on the N-functional carbon nanofibers (defined as MoO2/C) were constructed by electrospun and controlled graphitizing process. The comparative catalysts dominated by molybdenum oxides namely MoO2/MoO3 were synthesized by controlled calcination in air atmosphere. The MoO2 nanograins anchored in situ on carbon nanofibres could afford more interfacial active regions and accelerate the electron transfer efficiency of the electrocatalytic process by graphitic carbon fiber network. More importantly, MoO2/C was endowed with more oxygen vacancy sites which can trap the free NO3- ions over the reactive sites to enhance the surface interaction of NO3- with catalytic sites during the reaction process. The experimental results showed that the electrocatalytic nitrate reduction performance of the catalyst MoO2/C obviously outperformed that of MoO2/MoO3 in the light of the atomic activity efficiency with a near 7.5 folds enhancement. The MoO2/C affords the highest NH3 yield of 4838.5 µg h−1 mgcat−1 and Faraday efficiency of 30 % at –0.9 V vs.RHE and was endowed with 50 h of continuous reaction stability, proving the potential in practical electrochemical water treatment involving nitrate contamination.

Activity and adsorption behavior of oxygen on rutile TiO2(110)

The activity and adsorption behavior of oxygen on rutile TiO2(110) (RTiO2(110)) were investigated using the temperature programmed desorption (TPD) method with methanol (CH3OH) as the probe molecule. By controlling the coverage of molecular O2 on the surface via increasing or decreasing O2 exposure, two chemisorbed O2 species on the surface are confirmed, one at the bridging oxygen vacancy (OV) site (O22-/Ov) and the other at the five-fold coordinated titanium (Ti5c) site (O22--/Ti5c). At low O2 exposure, O22--/OV is the main species on the surface, which only leads to the O–H bond cleavage of CH3OH, producing methoxy groups (CH3O). However, after the OV sites are nearly filled by O2 at about 0.1 L O2 exposure, O2/Ti5c species begins to appear on R-TiO2(110) surface, resulting in the formation of formaldehyde (CH2O) via the reaction of O2/Ti5c species with CH3OH or CH3O to break the C-H bond at low surface temperature. Moreover, the yield of CH2O increases linearly with that of H2O. In addition, when the 1 L O2 covered surface is irradiated with 355 nm UV irradiation to desorb and dissociate O2/Ti5c species, the yield of CH2O decreases linearly with that of H2O. Further analysis suggests that the charge state of O2/Ti5c may not change as the exposure of O2 changes on the R-TiO2(110) surface, and O2 is most likely to adsorb on the Ti5c sites in the form of O22-, not O2−. The result not only advances our understanding on the adsorption state of O2 on TiO2, but also provides clues for low temperature C–H bond activation with O2 on TiO2.

Reactivity of surface oxygen vacancy sites and frustrated Lewis acid-base pairs of In2O3 catalysts in CO2 hydrogenation.

The effects of oxygen vacancy (VO) formation energy and surface frustrated Lewis acid-base pairs (SFLPs) on the CO2 hydrogenation activity of In2O3 catalysts were studied using density functional theory calculations. The VO formation energy of 2.8-3.3 eV was found to favor HCOO formation, whereas the presence of SFLPs is conducive to CO formation.

Silver nanoparticle-decorated NiFe2O4/CuWO4 heterostructure electrocatalyst for oxygen evolution reactions.

In this work, Ag nanoparticles decorated with NiFe2O4/CuWO4 heterostructure were synthesized using the step-wise precipitation method. The influence of varying Ag loading on the NiFe2O4/CuWO4 heterostructure and its electrochemical OER performance was extensively studied in 1 M KOH electrolyte. The obtained LSV profile was analyzed to determine the overpotential, Tafel slope, and onset potential. The heterostructure with an optimal Ag loading of 5 wt% required the least overpotential (1.60 V vs. RHE) for generating a current density of 10 mA cm-2 with a lower Tafel slope of 44.5 mV dec-1, indicating its faster OER kinetics. Furthermore, the composite remained stable over a period of 24 hours with a minimum rise in the overpotential after the stability test. The enhanced OER performance of the as-prepared catalyst can be attributed to the presence of multiple metallic elements in the Ag-loaded NiFe2O4/CuWO4 composite, which created a diverse array of oxygen-vacant sites with varying reactivity, enhancing the charge-transfer kinetics; and thus contributing to the overall efficiency of OER. Therefore, optimizing the Ag concentration and engineering a microstructure represents an encouraging strategy for developing cost-effective catalysts for next-generation energy-conversion applications.

The dynamic anti-corrosion of self-derived space charge layer enabling long-term stable seawater oxidation

Developing corrosion-resistant oxygen evolution electrocatalysts that can sustain seawater electrolysis is crucial but challenging for hydrogen production. Herein, we develop a bimetallic oxyhydroxide electrocatalyst with self-derived selenate space charge layer (SeO42− SCL) by in-situ electrochemically reconstructing cobalt-doped nickel diselenide (Co-NiSe2) pre-catalyst, enabling long-term stability for seawater electrolysis. In-situ experiments and theoretical results reveal the promoting effect of cobalt-doping on the reconstruction of NiSe2 and generation of dynamically stable oxygen vacancy sites. Importantly, the SeO42− SCL derived from the reconstruction process shows a dynamic anti-corrosion behavior, thus protecting metal species from dissolution and meanwhile without blocking the diffusion and adsorption of reactive species. Consequently, a two-electrode cell assembled by this Co-NiSe2 pre-catalyst as an anode, reaches an industrial current density (500 mA cm−2) at a cell voltage of 1.70 V, and that works stably for over 1500 h in alkaline seawater, which is of significance for promoting the practicality of low-cost catalysts.

Enhancing the ketonization ability of CeO2 for deoxygenation of biomass-derived sugars by Fe doping

Ketonization of oxygenated compounds in pyrolytic products of biomass is an effective means for deoxygenation and upgrading of bio-oils. However, the ketonization potential of biomass-derived sugars and the mechanism underneath remain unclear. On this basis, a series of Fe doped CeO2 catalysts were synthesized and employed for catalytic ketonization of xylan. Characterization of the catalysts proved the successful formation of the Fe-CeOx solid solution. Doping an appropriate amount of Fe (20–33%) ions on CeO2 could obviously increase the number of oxygen vacancy and basic sites by up to 56.31% and 29.6%, respectively. During the catalytic reaction of xylan, pure CeO2 exhibited a good ketonization ability during xylan pyrolysis, and the introduction of a certain amount of Fe further enhanced the ability. The highest yield of ketones was obtained by 33%Fe-CeO2, which was 38% higher than pure CeO2. The selectivity to linear ketones (acetone and 2-butanone) reached a maximum of 85% for 57%Fe-CeOx. Thus, oxygen-rich compounds had been converted to low oxygen containing and high-quality ketone components. Introducing Fe into CeO2 favored the production of ethylene glycol rather than 2,3-dihydroxy propanal, and thus promoted the yield of acetone. This investigation provides fundamentals for the deoxygenation of biomass through catalytic ketonization, aiming at the production of value-added ketone products.

Exploring Key Factors for Oxygen Evolution Reaction on La2-X Sr x NiO4+Δ by Oxygen Defect Engineering

IntroductionDevelopment of Oxygen Evolution Reaction (OER) catalyst is an important research issue for green hydrogen production by water splitting. Perovskite oxides have attracted attention as OER catalysts which can achieve both low cost and high catalytic activity. So far, some hypotheses have been proposed to understand key factors of catalytic activity of perovskite catalyst. Suntivich et al. reported volcano-like relationship between the OER activity and the number of the e g-electron of the B-site transition metal and the highest catalytic activity is achieved when the number of the e g-electron is close to 1.21. Besides, oxygen vacancies act as electrochemically active sites by incorporating reaction intermediates directly2,3. However, it is difficult to distinguish the contribution of electric state and oxygen defects on OER activity because both electronic state and oxygen defect concentration change simultaneously. For instance, the oxygen vacancy formation creates two electrons to maintain charge neutrality (2TMTM ×+OO ×↔2TMTM '+V O ∙∙+1/2 O2(g)).The aim of this study is to clarify the key factors improving OER catalytic activity. For the aim, we controlled the Ni valence state and oxygen defect concentration in layered perovskite oxide La2-x Sr x NiO4+δ by tuning Sr content (x) and the amount of oxygen defect (δ) independently (Fig. 1-a), to reveal the influence of transition metal electronic state and oxygen defects on OER activity. La2-x Sr x NiO4+δ has two types of oxygen defects: oxygen vacancy (V O ∙∙) and unoccupied interstitial site (V i ×). Comparing OER activity of oxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) allowed us to evaluate the influence of Ni valence state on OER activity while no oxygen vacancy and fully unoccupied interstitial sites ([V O ∙∙]=0,[V i ×]=2.0). Additionally, comparing OER activity of La2NiO4.1 (Ni2.2+,[V O ∙∙]=0,[V i ×]=1.9), La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) and La1.6Sr0.4NiO3.9 (Ni2.2+,[V O ∙∙]=0.1,[V i ×]=2.0) allowed us to evaluate the influence of oxygen defects on OER activity with negligible effect of Ni valence state.ExperimentalLa2-x Sr x NiO4+δ (x=0, 0.2, 0.4) were synthesized by Pechini method. The oxygen content of La2-x Sr x NiO4+δ was controlled by annealing them under suitable oxygen partial pressure and temperature and then quenched based on oxygen nonstoichiometric data4. Obtained samples were characterized by XRD, XAS and SEM.For the OER test, the catalyst ink was prepared by mixing the catalyst powder, acetylene black, K+-exchanged Nafion and tetrahydrofuran. 6.4 μl of the catalyst ink was sonicated and dropped onto the glassy-carbon rotating disk electrode. The electrochemical measurements were performed in O2-saturated 0.1 M KOH. The disk electrode potential was controlled between 0.3 and 0.9 V versus Hg/HgO reference electrode filled with 0.1 M KOH at a scan rate of 10 mVs-1.Result and DiscussionOxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) showed similar catalytic activity regardless of Ni valence (Fig. 1-b). This implies that the electronic state of Ni has insignificant influence on catalytic activity when the sample has almost no oxygen vacancy. The observed tendency is inconsistent with the Suntivich’s result that the relationship between OER activity and e g filling1.La2NiO4.1 (Ni2.2+,[V O ∙∙]=0,[V i ×]=1.9) and La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) which have different amount of unoccupied interstitial sites and almost no oxygen vacancy showed similar OER activity (Fig. 1-c). In contrast, La1.6Sr0.4NiO3.9 (Ni2.2+,[V O ∙∙]=0.1,[V i ×]=2.0) which has oxygen vacancy and fully unoccupied interstitial sites showed much better OER activity than La1.8Sr0.2NiO4 (Ni2.2+,[V O ∙∙]=0,[V i ×]=2.0) which has almost no oxygen vacancy and fully unoccupied interstitial sites (Fig. 1-c). These results indicate that oxygen vacancy has high OER activity as suggested in earlier works2.3, while unoccupied interstitial site which has available space for oxygen intercalation has negligibly low catalytic activity.The difference between oxygen vacancy and interstitial site is the presence of direct bonding with B-site cation. Reaction intermediates in oxygen vacancies can form direct bonding with Ni, while that in the interstitial site cannot. As suggested in the previous studies, a higher hybridization promotes charge transfer between the transition metal and reaction intermediate and effective charge transfer improves OER activity1,5.SummeryIn this work, to reveal the influence of transition metal electronic state and oxygen defects on OER activity, we have synthesized La2-x Sr x NiO4+δ , and controlled Ni valence state and oxygen defect concentration independently by tuning x and δ. It was revealed that oxygen vacancy boosts OER activity while influence of Ni valence state and unoccupied interstitial sites is insignificant.

Improving the Cyclic Reversibility of Layered Li-Rich Cathodes by Combining Oxygen Vacancies Introduction and Surface Fluorination

Layered-type Li-rich cathode materials have attracted significant attention for the next generation Li-ion batteries, but their poor reversibility in terms of both voltage and capacity upon cycling are as prominent as their high capacity. Irreversible oxygen redox activity and surface deterioration have been deemed as the root cause and direct cause for their poor performance, respectively. To mitigate those issues, we introduce substantial amounts of oxygen vacancies into layered-type Li1.2Ni0.2Co0.2Mn0.4O2 by using CaH2 as a reduction agent. Reduced samples show higher reversible capacity due to the increased cation redox participation, but still sustain poor cyclic performance. With further fluorination to reduced Li1.2Ni0.2Co0.2Mn0.4O2-x by using NH4HF2, the reversible capacity reached 270 mAh/g and maintained its 99% after 100 cycles. STEM-EELS detects deeper F signals rather than just on surface as direct fluorination methods show, and HAXPES patterns indicate there are both ionic and covalent fluorine coordination. These results demonstrate that by the combination of oxygen deficiency introduction and surface fluorination, some F- ions could occupy the sites of oxygen vacancies near the surface rather than sole LiF generation on the surface, which illuminates a new strategy to modify the cathode materials for Li-ion batteries. The Ni0.2Co0.2Mn0.4(OH)1.6 precursor was prepared by the coprecipitation of NiSO4, CoSO4 and MnSO4, and then mixed with LiOH. The mixture was calcined with oxygen flow at 900 ℃ to obtain pristine Li1.2Ni0.2Co0.2Mn0.4O2. The pristine Li1.2Ni0.2Co0.2Mn0.4O2 was reduced by the appropriate amount of CaH2 in a vacuumed glass tube at 260 ℃, then the reduced Li1.2Ni0.2Co0.2Mn0.4O1.85 was mixed with various amounts of NH4HF2 and sintered at 450 ℃ with argon flow. Hard X-ray photoemission spectroscopy (HAXPES, at BL46XU in SPring-8) was used to investigate the coordination state of fluorine ions. The electrodes of different samples were prepared by slurrying the active material, acetylene black, and PVDF with a weight ratio of 8:1:1, then coating on the Al foil. The 2032-type coin cells were assembled for the electrochemical test. The counter electrode was lithium metal and the electrolyte was 1 M LiPF6 in EC and EMC solvent (3:7 by volume). The charge-discharge test was operated on an automatic cycling and data recording system (HJ1001SD8, Hokuto Denko). Fig. 1a shows the F 1s HAXPES spectra of fluorinated samples which were synthesized by subjecting reduced Li1.2Ni0.2Co0.2Mn0.4O1.85 to chemical fluorination using NH4HF2. When F is 0.1 stoichiometry, F ions are more covalent which implies they could mainly occupy the oxygen vacancy sites near surface. Further fluorination increased the intensity of LiF on surface. The cyclic performance of four samples is shown in Fig. 1b. The pristine sample suffered from the severe capacity decay whose drastic oxygen redox activity is deemed to cause various deterioration [1]. Direct fluorination of pristine sample had some positive effect to the cyclic performance because of the reported interface stabilization [2]. The initial capacity of the reduced sample is higher than pristine sample owing to the increased cation redox contribution, but its cyclability is still poor especially after long cycles. The important point to note is the combination of reduction and fluorination significantly improved both the capacity and cyclic performance upon 100 cycles, and the coordination and role of fluorine ions could be different from the conventional fluorination strategy. Figure 1

High Reliability Tungsten-Doped Indium Zinc Oxide Thin Film Transistors for Display Applications

Recently, flat panel display technologies are based on amorphous silicon (a-Si), polycrystalline silicon (poly-Si) or amorphous oxide semiconductor (AOS) thin film transistors (TFTs) technology. In order to reach higher performance for ultra-high resolution display applications, the material selection for the channel layer is vital. AOS TFTs drawn lots of attention because of its advantages such as low leakage current, high driving current and high uniformity [1,3]. In this work, we presented a novel amorphous oxide semiconductor (AOS) material of gallium-free a-InWZnO thin film transistors (IWZO TFTs) which is very promising to be applied for ultra-high resolution display technology. Experiment In this work, we demonstrated the structure of a-IWZO TFTs processes. Figure 1 shows the schematic of the TFT cross-section. After growing a 550 nm thermal oxide layer as buffer layer, a 25-nm-thick of molybdenum was deposited by DC-sputter as the bottom gate electrode. A 10-nm-HfO2 was deposited by plasma-enhanced atomic layer deposition (PEALD) system as high-κ gate dielectric (GI). Then, post deposition annealing (PDA) was carried out to enhance the quality of the insulator layer. Next, an ultra-thin body a-IWZO was deposited by RF-sputter as channel layer. Finally, source and drain were defined by lift-off process and the gate contact via was etched by dry etching to finish the device fabrication. Result and Discussion Figure 1(b) shows the negative gate bias stress (NGBS) transfer curve (ID-VG) of a-IWZO TFTs with passivation layer. The experimental results have shown that passivation layer could keep the back channel of IWZO channel from exposing to oxygen and hydrogen from the atmosphere. Without the passivation layer, the back channel would contact moisture leading the diffused hydrogen atoms to fill oxygen vacancy sites and de-trap free carriers, causing the VTH shifts negatively. The ultra-thin a-IWZO TFTs with passivation layer exhibited a low subthreshold swing (S.S.) of 89.7 mV/dec, high on/off current ratio (ION/IOFF) of 2.1×107 and small threshold voltage shift (ΔVth) of -0.097 V during NGBS of -2MV/cm with 2000s. To sum up, the passivation layer could enhance the a-IWZO TFTs reliability. Reference [1] Kamiya, H. Hosono, NPG Asia Mater., vol. 2, pp. 15, 2010.[2] Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, nature, vol. 432, no. 7016, pp. 488-492, 2004.[3] Y. Kuo, C. M. Chang, I. H. Liu, and P. T. Liu, Scientific reports, vol. 9, no. 1, pp. 1-7, 2019. Figure 1

Bi-doped CuCeO2−x nanorod catalysts for photothermal CO-PROX in H2-rich streams: Enhanced oxygen vacancy and light absorption capacity

The wide band gap of ceria limited the light absorption and photothermal conversion of copper-ceria catalyst for photothermal preferential oxidation of CO (CO-PROX). In the present work, doping Bi element was adopted to regulate the energy band and catalytic performance of CuCeO2−x nanorod catalysts for photothermal CO-PROX. Bi/CuCeO2−x nanorods were synthesized by co-precipitation hydrothermal and deposition precipitation method. The Bi/CuCeO2−x catalyst with the molar ratio of Bi/Ce 2.5% exhibited the best catalytic performance and the CO conversion reached 90% under 2.5 suns with a surface temperature of 120 °C. The doped Bi species were incorporated into the CeO2 lattice and promoted the formation of oxygen vacancy sites and the strong Cu-Ce interaction, as revealed by XRD, XPS, Raman and H2-TPR results. In addition, doping Bi reduced the band gap of ceria nanorod from 2.77 to 2.47 eV and remarkably enhanced the UV–vis light response and photogenerated carrier separation of Bi/CuCeO2−x-2.5.

Synergistic mechanism of surface oxygen vacancies and metal sites on Al-substituted NiFe2O4 during peroxymonosulfate activation in the solid-water interface for 2,4-D degradation

In this study, Al-doped NiFe2O4 (NiAlxFe2-xO4) was synthesized by the isomorphic substitution strategy, which was further employed for peroxymonosulfate (PMS) activation to remove 2,4-dichlorophenoxyacetic acid (2,4-D) in water. Doping of Al could construct oxygen vacancies (OVs) in the structure of NiAlxFe2-xO4, which further facilitates the exposure of metal sites for PMS activation. The removal rate of 2,4-D increased greatly compared to the NiFe2O4-activated process. By the density functional theory (DFT) process, it was verified that Al was more likely to replace Fe rather than Ni. OVs sites have the lowest adsorption energy of H2O (Eads = -0.95 eV), and easily formed surface hydroxyl groups (–OH), which were further replaced by PMS (HSO5-). The formed surface complex Me-O-O-SO3- was readily activated via electron transfer between O-O bond, thus promoting the cleavage of it and inducing SO4·- generation. This process induced the high performance of the NiAlxFe2-xO4/PMS system for effective 2,4-D degradation. Furthermore, the purified water showed relatively low toxicity. The present study proposed new insights into the interface mechanism of PMS triggering.

MgAl layered double hydroxide (LDH) for promoting ammonia synthesis in non-thermal plasma: Role of surface oxygen vacancy

Here, the MgAl layered double hydroxide (LDH) supports and relevant metal on LDH catalysts (i.e., Ni, Co, and Ru) were prepared and investigated under non thermal plasma (NTP) conditions to probe the role of surface oxygen vacancy (OV) in NTP-assisted ammonia (NH3) synthesis. The findings show that OV on the LDH carrier is highly beneficial to NH3 formation under NTP conditions, and concentration of OV on the LDH can be regulated by the post-synthesis calcination and hydrogen plasma etching. Additionally, loading of active metal species on the LDH could promote the NH3 synthesis further due to presence of multiple reaction pathways and the synergy between the surface OV and metal sites in such NTP-catalytic systems. As the result, the catalysts developed by this work showed high ammonia synthesis rates of 4.42−4.52 mmol g−1 h−1 and energy efficiencies of 1.67−1.71 gNH3 kWh−1, respectively. The findings of the work pave the way for the rational design and optimization of highly efficient catalysts with dual active sites for intensifying the NTP-catalytic ammonia synthesis.

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.

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.

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.

Self-assembly tourmaline@BiFeO3 composites with enhanced polarization for dual-selective C3H6O and H2S detection

Ferroelectric oxides with great polarization have attracted much attention due to the polarization modulated carrier behaviors causing high gas sensitivity in detecting hazardous toxic gases. In this work, tourmaline@BiFeO3 (T@BFO) composites were fabricated by the sol-gel method based on the electrostatic self-assembly strategy for the C3H6O and H2S detection. It is found that BFO nanoparticles are uniformly distributed on tourmaline due to the strong electrostatic field of tourmaline. In comparison with pure BFO, the T@BFO composites showed an improved gas sensitivity. More importantly, the sensor based on 3.0-T@BFO can effectively detect C3H6O at 150 °C and H2S at 210 °C in limits of detection as low as 50 and 7 ppb, respectively. The results exhibit that 3.0-T@BFO composites have lower activation energies (Eact) of C3H6O at 150 °C and H2S at 210 °C, which endow 3.0-T@BFO with dual-selectivity for C3H6O and H2S. The Ex-situ XPS and density functional theory calculations further reveal that tourmaline activates the sensing site of BFO to achieve selective adsorption of C3H6O at Bi, Fe, and oxygen vacancies sites and H2S at surface lattice oxygens, Fe and oxygen vacancies sites. These results provide an efficient strategy to design practice multifunctional and highly sensitive gas sensors for detecting hazardous toxic gases.

Rationally designed oxygen vacant TiO2 decorated with covalent organic framework for enhanced electrocatalytic nitrogen reduction to ammonia

This study focuses on the rational design of uniformly distributed oxygen vacant TiO2−x nanoparticles supported on an anthraquinone-based covalent organic framework (TiO2−x@COF-Aq), aiming to address the challenges of Haber-Bosch process. The N-philic nature of Ti and the presence of abundant oxygen vacant sites on the TiO2−x@COF-Aq promote the adsorption of nitrogen, facilitating the electrocatalytic NRR process. The theoretical study reveals that TiO2−x@COF-Aq suppresses the H+ adsorption, facilitates oxygen vacancy formation and favors the N2 adsorption at oxygen vacant sites. The synthesized nanocatalyst exhibits many folds higher electrochemical NRR activity than bulk oxygen vacant TiO2−x with an ammonia yield of ∼30 μg mg-1 h–1 and ∼16% Faradaic efficiency. Long term stability study manifests the robustness and industrial significance of the catalyst. The overall synergism between the stabilized nanoparticle and functionalized COF has established the fundamental understanding of the proton coupled electron transfer for N2 fixation through experiments and theoretical findings.

Engineering linker-defects of MIL-101 series metal organic frameworks for boosted Yb(III) adsorption

Developing the highly efficient adsorbents for improved recovery of rare earth elements from industrial wastewater is significant. Therefore, two defective MIL-101 series, named as MIL-101(Cr)-D and MIL-101(Fe)-D, were fabricated by introducing benzoic acid as the second ligand to coordinate with metal clusters. The controlled samples were characterized by XRD, ATR-IR, TGA, EPR, and BET tests, verifying the presence of oxygen-vacancy sites and the integrity of crystalline structure. In addition, the adsorption kinetics, the adsorption isotherms, the influences of pH and different REE ions on adsorption performance, and the cycling stability of defective and normal MIL-101s were carefully discussed and compared. The results showed that the adsorption capacities of MIL-101(Fe)-D10 and MIL-101(Cr)-D20 were 27.88 and 37.45 mg·g−1 respectively for Yb(III) capture at 30 °C and pH = 6, were much higher than those of the corresponding normal MIL-101. Furthermore, the adsorption mechanisms of two defective MIL-101s have been analyzed by EDS, mapping and XPS, confirming the surface complexation interactions between Yb(III) and O-vacancy sites originated from Cr-O and Fe-O clusters. Meanwhile, the excellent reusability of MIL-101(Cr)-D10 was validated by the successive twelve adsorption recycles and XRD analysis. Hence, this work supplies the feasible and facile strategy for manufacturing the missing-linker defective MOFs, endowing these adsorbents with the enhanced adsorption performance towards Yb(III) from aqueous solution.

Meta-generalized gradient approximation time-dependent density functional theory study of electron trapping in Hf- and Zr-doped lutetium oxide: influencing the oxygen vacancy.

This work analyzes the effects of a nearby Hf or Zr dopant on the electron density trapped at an oxygen vacancy site. The two metals are among the dopants used to achieve thermoluminescence and energy storage in phosphors based on cubic lutetium oxide (c-Lu2O3). The presence of oxygen vacancies is anticipated in those phosphors. If the dopant is located outside the immediate surroundings of the vacancy site, the resulting optical trap depth is similar to that of the isolated oxygen vacancies (1.6-1.7 eV versus 1.7 eV). If the dopant is one of the four metal cations surrounding the vacancy site, the corresponding trap depth is 2.0-2.1 eV. Using time-dependent density-functional theory calculations, it was found that the excitation of the vacancy-trapped electrons can take two forms: a local excited state at the vacancy site can be formed, or an electron transfer to Hf might occur. With charge compensation in mind, several structures with three defects were analyzed: the dopant cation, the vacancy and an interstitial oxygen (Hf/Zr plus a Frenkel pair). These last two systems with the dopant in a +4 oxidation state and a single electron trapped at the vacancy site correspond to zero total charge, while another electron can be trapped. The vacancy site is expected to trap the electron, not the dopant. The composite defects of the dopant and Frenkel pair are thus considered the most likely electron traps in cubic Lu2O3:Hf and cubic Lu2O3:Zr.

Solvent-free cross aldol condensation of aldehydes and ketones over SrMo1-xNixO3-δ perovskite nanocrystals as heterogeneous catalysts

Aldol condensation is arguably one of the most fascinating reactions that leads to the formation of C–C bonds. Its use in the pharmaceutical industry to synthesis complex drugs from simple aldehydes and ketones has become of paramount importance. Although this is one reaction that has lured a lot of attention, not enough has been explored in heterogeneous catalysis. In this work we have successfully synthesized multicationic perovskites via the soft-template method and characterized them thoroughly. The synthesized perovskite nanocrystals were found to have small SBET however their catalytic application in the conversion of benzaldehyde (BAL) in the aldol condensation with diethyl ketone (DEK) was found to be astonishing. The synthesis was confirmed using many techniques, from determining the oxidation states of the materials using XPS. This gave access to determine the coordination of the metals in the perovskite lattice and also qualitatively assess the oxygen environments that exist. The oxygen vacancies and SBET were used to assess the activity of the perovskite catalysts in the cross-aldol condensation reaction. The optimal conditions for this aldol condensation were found to be 120 °C after 25 h with no solvent using SrMo0.5Ni0.5O3-δ inorganic perovskite which had the highest amount of oxygen vacant sites which gave a conversion of 88 % and an 82 % selectivity towards the desired cross-aldol condensation product. The use of dimethylformamide (DMF) for this reaction is discouraged as it reacts with BAL to produce a higher amide.