Co3O4 Surface Research Articles

Identifying Highly Active and Selective Cobalt X-Ides for Electrocatalytic Hydrogenation of Quinoline.

Earth-abundant Co X-ides are emerging as promising catalysts for the electrocatalytic hydrogenation of quinoline (ECHQ), yet challenging due to the limited fundamental understanding of ECHQ mechanism on Co X-ides. This work identifies the catalytic performance differences of Co X-ides in ECHQ and provides significant insights into the catalytic mechanism of ECHQ. Among selected Co X-ides, the Co3O4 presents the best ECHQ performance with a high conversion of 98.2% and 100% selectivity at ambient conditions. The Co3O4 sites present a higher proportion of 2-coordinated hydrogen-bonded water at the interface than other Co X-ides at a low negative potential, which enhances the kinetics of subsequent water dissociation to produce H*. An ideal 1,4/2,3-H* addition pathway on Co3O4 surface with a spontaneous desorption of 1,2,3,4-tetrahydroquinoline is demonstrated through operando tracing and theoretical calculations. In comparison, the Co9S8 sites display the lowest ECHQ performance due to the high thermodynamic barrier in the H* formation step, which suppresses subsequent hydrogenation; while the ECHQ on Co(OH)F and CoP sites undergo the 1,2,3,4- and 4,3/1,2-H* addition pathway respectively with the high desorption barriers and thus low conversion of quinoline. Moreover, the Co3O4 presents a wide substrate scope and allows excellent conversion of other quinoline derivatives and N-heterocyclic substrates.

Hydroxylation of an ultrathin Co3O4(111) film on Ir(100) studied by in situ ambient pressure XPS and DFT

In the present work, we have studied the interaction of water with spinel cobalt oxide (Co3O4), an effect which has been considered a major cause of its catalytic deactivation. Employing a Co3O4(111) model thin film grown on Ir(100) in (ultra)high vacuum, and ambient pressure X-ray photoelectron spectroscopy (APXPS), hydroxylation in 0.5 mbar H2O vapor at room temperature was monitored in real time. The surface hydroxyl (OH) coverage was determined via two different models based (i) on the termination of a pristine and OH-covered Co3O4(111) surface as derived from density functional theory (DFT) calculations, and (ii) on a homogeneous cobalt oxyhydroxide (CoO(OH)) overlayer. Langmuir pseudo-second-order kinetics were applied to characterize the OH evolution with time, suggesting two regimes of chemisorption at the mosaic-like Co3O4(111) film: (i) plateaus, which were quickly saturated by OH, followed by (ii) slow hydroxylation in the “cracks” of the thin film. H2O dissociation and OH formation, blocking exposed Co2+ ions and additionally consuming surface lattice oxygen, respectively, may thus account for catalyst deactivation by H2O traces in reactive feeds.

Oxygen vacancies are generated in the inner layer of the core-shell structure by in-situ electrochemical activation to promote electrochemical energy storage

Alkaline zinc ion batteries have attracted attention as potential energy storage devices but remain challenging due to the low utilization rate of active substances, poor conductivity, and few active sites. In order to solve these shortcomings, we grow ultrathin nickel-cobalt layered double hydroxides (NiCo LDH) by electrodeposition on Co3O4 surfaces, and generated abundant oxygen vacancies in the inner layer of the core-shell structure by electrochemical method. The special core-shell structure improves the contact between the active material and the electrolyte, bringing a positive effect on ion diffusion and charge transfer during the energy storage process. At the same time, the abundant oxygen vacancies not only effectively improve the conductivity and the number of active sites of the material, but also improve the valence state conversion ability of the cobalt element. As expected Co3O4-OV/NiCo LDH exhibits a specific capacity of 338.47 mAh g−1 at a current density of 1 A g−1 and a high average discharge potential of 1.65 V relative to Zn2+/Zn. Furthermore, the characterizations combined with density functional theory (DFT) calculations indicate that the generation of oxygen vacancies improves the conductivity and the number of active sites which results in excellent reversible electrochemical activity. This work not only provides new insight into designing advanced electrodes by introducing oxygen vacancies but also opens up avenues for the development of high-performance zinc-cobalt batteries.

Target-triggered oxygen vacancy increase of Co3O4 nanoparticles with promoted peroxidase-like activity for specific turn-on colorimetric sensing of uranyl ions

As the most stable form of uranium in water and soil, uranyl ions (UO22+) possess strong biological toxicity and radioactivity, causing a great threat to human health. Therefore, it is of significance to develop reliable methods for monitoring uranyl ions in the environment. Here we propose a “light-up” colorimetric strategy based on target-accelerated peroxidase-mimicking activity of Co3O4 for the specific detection of uranyl ions. Original Co3O4 nanoparticles exhibit a certain peroxidase-like activity in catalyzing colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to blue oxTMB with the participation of H2O2. When UO22+ is introduced, it interacts with the nanozyme rapidly and specifically and promotes the latter’s catalytic ability via increasing active oxygen vacancies on Co3O4 surface. According to such a phenomenon and mechanism, selective determination of UO22+ with favorable performance is achieved, with a linear measurement range of 0.2–10 μM and a detection limit of 0.08 μM. Reliability and practicability of the proposed method are demonstrated by analyzing the pollutant in several environmental water matrices. Our work not only offers a novel, efficient yet convenient approach for measuring UO22+, but may also inspire the exploration of new nanozyme-based sensing principles and strategies for other analytical applications.

Surface dynamics and electrochemical examination of Co3O4 films by iron doping

This study focused on Co3O4 films, which were prepared by cost-effective chemical bath deposition on In:SnO2 (ITO) substrates with iron doping concentrations ranging from 2 to 6 mol %. Structural properties were investigated by XRD as well as nanotexture of Fe: Co3O4 films was captured via SEM and detailed fractal analysis was analyzed in each prepared film. Effective using of prepared Fe: Co3O4 electrodes for electrochemical charge storage applications has been examined by using CV and EIS. From x-ray patterns, spinel cubic structure of Co3O4 was observed in all samples, while peaks with Co2O3 and substrate indexed peaks were also shown. Pure and iron doped Co3O4 surfaces have spherical agglomerative forms while porous structures were observed in 4% Co3O4 samples. Redox peaks induced by Faradaic reactions in the CV plots present pseudo- capacitive nature for all electrodes and improves charge transfer process in 4% Co3O4 and 6% Co3O4 from EIS measurements. Additionally, using scaling theory, the coverage ratio, fractal dimensions, cluster sizes and interface critical exponent values of the superficial hetero morphology of the samples are calculated. While the coating rate decreases according to the iron concentration, fractal dimensions increase. However, as the number of clusters increases, the average cluster size decreases. The interface critical exponent value shows an irregular change.

Efficient photoreduction of CO2 to tunable syngas by Zn-doped Co3O4: Tuning binding strength to versatile reaction intermediates simultaneously

Photochemical reduction of CO2 (CO2RR) to syngas is an attractive approach for carbon–neutral recycling. Manipulating binding strength to reactive species on catalyst surfaces is challenging but essential for achieving efficient syngas production with a desired CO/H2 ratio. Here, we demonstrate that the binding strength to key reaction intermediates (*CO2, *H and *CO) on the Co3O4 surface can be fine-tuned by incorporating varying amount of Zn atoms. Dedicated experiments and theoretical simulations unveil that Zn doping allows for simultaneous control of both CO2 adsorption/activation and the adsorption affinity of *CO on Zn-Co3O4 catalysts, resulting in a significant enhancement in CO2 to CO conversion. Moreover, Zn doping can alter the hydrophilicity of the catalyst surface, affecting the water adsorption behavior and adjusting the hydrogen evolution reaction (HER) activity. As a result, the CO/H2 ratio can be readily tuned via varying the Zn doping level. This study presents an effective and economical method approach to manipulate the electronic structure of catalyst surfaces for controlling the kinetics of CO2RR and HER reactions, providing valuable insight into the design of highly active transition metal oxides (TMOs) based photocatalysts for syngas production with a tailored CO/H2 ratio.

A DFT investigation on surface and defect modulation of the Co3O4 catalyst for efficient oxygen evolution reaction

The electrolysis of a water for hydrogen production is a promising way to produce clean energy, but the sluggish oxygen evolution reaction (OER) limits the overall efficiency of water electrolysis. In this work, we investigated the water oxidation pathways on the perfect and defect Co3O4(111) surfaces by using density functional theory (DFT) calculations. We found that for the perfect surface the free energy barrier of the potential determining step (PDS) in the adsorbate evolution mechanism (AEM) of water is lower than that in the lattice oxygen mechanism (LOM). For the defect surfaces, cobalt vacancies are more easily formed than oxygen vacancies. The Co vacancy promotes the formation of *OH, changes the PDS of the LOM and AEM, and reduces the free energy barrier of both PDS. The PDS of the LOM pathway on the VCo2Co3O4(111) surface is the coupling step of the O adatom and lattice oxygen, which promotes the LOM process. Different from the OER mechanism on the perfect surface and the defect surface with Co vacancy, the LOM is perferred to occur on the defect surface with O vacancy. This work may provide new insight into the relationship between the surface structure and OER activity surface of the Co3O4 catalyst and help to design the efficient OER catalysts by surface and vacancy engineering.

Enhancing low-temperature CO removal in complex flue gases: A study on La and Cu doped Co3O4 catalysts under real-world combustion environment

Designing CO oxidation catalysts for complex flue gases conditions is particularly challenging in fire scenarios. Traditional flue gas simulations use a few representative gases but often fail to adequately evaluate catalyst performance in real-world combustion conditions. In this study, we developed doping strategies using La and Cu to enhance the water resistance of Co3O4 catalysts. Catalyst 0.1La-Co3O4-CuO/CeO2 exhibits exceptional low-temperature catalytic activity, achieving 100% conversion at 130 °C. This enhancement is largely due to the introduction of La, which increases the active Co3+/Co2+ ratio and suppresses hydroxyl group formation on the Co3O4 surface. Cu doping also changes the Co3O4 lattice structure, forming Cu+ as active sites and enhancing the activity at low temperatures. For the first time, steady-state tube furnace and fixed bed were employed to evaluate the catalytic performance of CO in actual combustion atmosphere. Catalyst 0.1La-Co3O4-CuO/CeO2 maintains excellent catalytic efficiency (T100 = 120 °C) under well-ventilated conditions. However, its activity significantly decreases in poorly ventilated environments, due to the competitive adsorption of small molecules at active sites, such as acetone, commonly found in smoke. This study provides valuable insights for designing water-resistant, low-temperature, non-noble metal catalysts and offers a methodology for evaluating CO catalytic activity in real-world environments.

Computational screening and experimental fabrication of MoSe2/3d–TMOS heterojunction for room-temperature gas sensor

Heterogeneous engineering is an efficient method to overcome the poor adsorption performance of MoSe2 materials. In this work, the screening process for a suitable transition metal oxide semiconductor with 3d orbital (3d–TMOS) was performed based on the work of adhesion and band edges positions with reference to the MoSe2 material. Then, the gas sensor performance of the suitable composite system was further tested and discussed. The results show that the Cr2O3, Fe2O3 and Co3O4 surface present good interfacial binding strength with MoSe2 compared with other 3d–TMOSs. And the MoSe2/Co3O4 system was selected considering the suitable band edges positions. The as-fabricated MoSe2/0.5–Co3O4 nanocomposite exhibits excellent sensitivity to NO2 at room temperature. Moreover, the MoSe2/0.5–Co3O4 nanocomposite exhibits excellent linear response (R2 = 0.985), selectivity, repeatability, and long–term stability to NO2. The improvement mechanism of gas sensor performance can be mainly attributed to the fast charge extraction rate of type-II heterojunction and greatly sensitive of built-in potential barrier to the carrier density in the MoSe2/0.5–Co3O4 nanocomposite. This research provides a promising method to select the candidates for gas sensor using the combined DFT calculations and experiments.

Co3O4 (111) surfaces in contact with water: molecular dynamics study of the surface chemistry and structure at room temperature.

In this work, we have used ab initio molecular dynamics at room temperature to study the adsorption and dissociation of a thin water film on Co3O4 (111) surfaces, considering the O-rich and Co-rich terminations named as the A-type and B-type surface terminations, respectively. We investigate the occupation of active sites, the hydrogen bond network at the interface and the structural response of the surfaces to water adsorption. On both terminations, water adsorbs via a partial dissociative mode. The contact layer is populated by molecular water as well as OH groups and surface OH resulting from proton transfer to the surface. The B-termination is more reactive, with a higher degree of dissociation in the contact layer with water (46%). On the B-terminated surface, water barely adsorbs on the Co2+ sites and almost exclusively binds and dissociates on the Co3+ sites. The interaction with the surface consists mostly of Co3+-Ow bonds and proton transfer exclusively to the 3-fold unsaturated surface Os1. Hydrogen bonds between water molecules in the aqueous film dominate the hydrogen bond network and no hydrogen bonds between water and the surface are observed. The A-terminated surface is less reactive. Water binds covalently on Co2+ sites, with a dissociation degree of 13%. Proton transfer occurs mostly on the 3-fold unsaturated surface oxygens Os1. Besides, short-lived surface OH arising from proton transfer to 3-fold unsaturated surface oxygens Os2 is observed. H-bonding to surface Os1 and Os2 constitutes 12.7% and 19.8% of the H-bond network, respectively, and the largest contribution is found among the water molecules (67.4%). On both surfaces, the coordination number of the active sites drives the relaxations of the outermost atom positions to the their bulk counterparts. The occupation of active sites on B-termination could reach up to 3 adsorbates per Co3+ leading to a binding motif in which the Co is octahedrally coordinated and which was observed experimentally.

A comparative DFT study of HCHO decomposition on different terminations of the Co3O4(110) surface.

Density functional theory calculations have been performed to compare the HCHO decomposition on Co3O4(110)-A and (110)-B terminations. The results showed that the energy barriers of the two C-H bond cleavages of HCHO on the (110)-A termination were lower than those on the (110)-B termination, suggesting that the (110)-A termination had stronger HCHO decomposition ability than the (110)-B termination. Electronic structures revealed that the stronger HCHO decomposition ability of the (110)-A termination might be ascribed to the strong covalent bond between HCHO and the (110)-A termination, as well as the higher d-band center of Co3+ ions on the (110)-A termination. Furthermore, we proposed that the preparation of Co3O4 under oxygen-rich growth conditions was beneficial to HCHO decomposition because the (110)-A termination was more stable under oxygen-rich conditions.

Oxygen vacancies tuning over Co3O4 nanosheet preferentially growing (2 2 0) facet for efficient Hg0 removal from flue gas

Oxygen vacancy is very important for the Hg0 oxidation process due to it can capture and activate gaseous oxygen forming surface chemisorbed oxygen to oxidize Hg0 to HgO. Precise control of oxygen vacancies amount is a big challenge in this field. For Co3O4, oxygen vacancy can be generated by Co3+ reduction to Co2+ and Co3+ mainly exist in (220) facet. Thus, in the present work, Co3O4 nanosheets preferentially growing (220) facet with more Co3+ were synthesized and CO was used as reduction agent to reduce Co3O4 at different reduction condition. After that, Co3O4 nanosheets with different amount of oxygen vacancies were obtained. XPS, Raman and PL results show that, the amount of oxygen vacancies increase firstly and then decrease with the increase of reduction temperature and time. CO can reduce Co3+ to Co2+ forming oxygen vacancies. However, when the reduction temperature and time are at a high level, Co3O4 will be over reduced to CoO leading to a decrease of oxygen vacancies. DFT confirm that, when Co3O4 is over reduced, O2 will hard to adsorb on Co3O4 surface and lattice oxygen will be formed even after O2 adsorption. Co3O4-Te300Ti2 has an optimal reduction degree leading to higher oxygen vacancy concentration and better O2 adsorption and activation ability. Therefore, Co3O4-Te300Ti2 has a high Hg0 oxidation efficiency which is over 96 % in a wide temperature range of 150–300 °C at a high GHSV of 180,000 h−1.

Linker regulation of CoO@Cu/C derived from self-assembly of MOF to enhance catalytic activity of organic contaminants

The reduction of hazardous organic contaminants in agricultural wastewater to their corresponding amines is a key procedure in the fine chemical industry for pharmaceuticals, polymers, agrochemicals, and dyes. However, their effective and selective reduction reactions require compressed hydrogen at high temperatures, which are expensive and limited in supply. In this study, we present a novel approach using a layer-by-layer (LBL) assembly of copper metal–organic frameworks (MOFs) to prepare an earth-abundant, highly stable plasmonic nano-photocatalyst (i.e., Cu nanoparticles (NPs)) over Co3O4 nanocubes (indicated as CoO@Cu/C). The catalyst was produced by thermally treating the prepared core–shell material. Herein, highly monodispersed Cu NPs with an average size of 5 nm were embedded in the carbon shell on the surface of CoO. This unique composition resulted in a significant enhancement in the catalytic performance, yielding a remarkable efficiency (≈100 % after 60 s) and exceptional selectivity (≈98 %). Consequently, the reusable and sustainable CoO@Cu/C catalyst exhibited brings unattainable a remarkable catalytic performance and consistent activity even after six cycles in water owing to this unique composition of the homogeneously dispersed Cu-NPs inside the carbon shell. This, in turn, resulted in highly effective adsorption characteristics of the carbon matrix and high catalytic performance of ultra-small Cu-NPs on the CoO surface. Moreover, the activity of this catalyst is highly effective. This study presents an effective strategy for obtaining remarkable catalytic performance and selectivity via the coordination activation of Cu-NPs on the CoO surface.

Epitaxial Growth of Cobalt Oxide Thin Films on Sapphire Substrates Using Atmospheric Pressure Mist Chemical Vapor Deposition

This study demonstrated the epitaxial growth of single-phase (111) CoO and (111) Co3O4 thin films on a-plane sapphire substrates using an atmospheric pressure mist chemical vapor deposition (mist-CVD) process. The phase structure of the grown cobalt oxide films was manipulated by controlling the growth temperature and process ambient, confirmed through X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Furthermore, the electrical properties of Co3O4 films were significantly improved after thermal annealing in oxygen ambient, exhibiting a stable p-type conductivity with an electrical resistivity of 8.35 Ohm cm and a carrier concentration of 4.19 × 1016 cm−3. While annealing CoO in oxygen atmosphere, the Co3O4 films were found to be most readily formed on the CoO surface due to the oxidation reaction. The orientation of the atomic arrangement of formed Co3O4 was epitaxially constrained by the underlying CoO epitaxial layer. The oxidation of CoO to Co3O4 was largely driven by outward diffusion of cobalt cations, resulting in the formation of pores in the interior of formed Co3O4 films.

First-Principles Study of Oxygen Evolution Reaction on Ir with Different Coordination Numbers Anchoring at Specific Sites of Co3O4 (111) Surface

First-Principles Study of Oxygen Evolution Reaction on Ir with Different Coordination Numbers Anchoring at Specific Sites of Co3O4 (111) Surface

Switching the Oxygen Evolution Mechanism on Atomically Dispersed Ru for Enhanced Acidic Reaction Kinetics.

Designing stable single-atom electrocatalysts with lower energy barriers is urgent for the acidic oxygen evolution reaction. In particular, the atomic catalysts are highly dependent on the kinetically sluggish acid-base mechanism, limiting the reaction paths of intermediates. Herein, we successfully manipulate the steric localization of Ru single atoms at the Co3O4 surface to improve acidic oxygen evolution by precise control of the anchor sites. The delicate structure design can switch the reaction mechanism from the lattice oxygen mechanism (LOM) to the optimized adsorbate evolution mechanism (AEM). In particular, Ru atoms embedded into cation vacancies reveal an optimized mechanism that activates the proton donor-acceptor function (PDAM), demonstrating a new single-atom catalytic pathway to circumvent the classic scaling relationship. Steric interactions with intermediates at the anchored Ru-O-Co interface played a primary role in optimizing the intermediates' conformation and reducing the energy barrier. As a comparison, Ru atoms confined to the surface sites exhibit a lattice oxygen mechanism for the oxygen evolution process. As a result, the delicate atom control of the spatial position presents a 100-fold increase in mass activity from 36.96 A gRu(ads)-1 to 4012.11 A gRu(anc)-1 at 1.50 V. These findings offer new insights into the precise control of single-atom catalytic behavior.

Single W, Cu, Co atom oxide embedded in Co3O4-rGO for boosting electrocatalytic oxygen evolution reaction and for photodegradation of tetracycline

One of the most important phenomena in developing renewable and sustainable energy methods is electrocatalytic water splitting. Higher loadings of single metal atom oxide (SMAO) (M: W, Cu, and Co) in Co3O4-rGO catalysts are credited with boosting their efficiency in electrocatalytic oxygen evolution reaction (OER) and photodegradation of tetracycline (TC). In this work, we investigated the catalytic activity of electrocatalysts for OER by preparing ultrathin Co3O4 nanosheets and reduced graphene oxide (rGO) embedded with SMAO (rGO-ED-Co3O4-WCuCo-30) using a simple one-pot sonication process. The well-designed rGO-ED-Co3O4-WCuCo-30 electrocatalyst allows for the delivery of current densities of 10 and 100 mA cm−2 at overpotentials of 291 and 365 mV, respectively, with a relatively modest Tafel slope of 57.9 mV dec−1 during OER. The densely embedded SMAO particles on the Co3O4 surface are responsible for this behavior because they increase the electrochemical surface area with a large number of active sites in a synergistic manner. The rGO-ED-Co3O4-WCuCo-30 catalyst was used effectively in OER with outstanding long-term stability for more than 24 h. The photocatalytic results further show that the activity of Co3O4 was significantly enhanced by the addition of SMAO, with the TC degradation efficiency of 97.57%.

Improved polymer chain movement in Poly(Vinyl Alcohol)–Potassium hydroxide gel polymer electrolytes with nanostructured Cobalt(II,III) oxide fillers for supercapacitor

Designing high-performance polymer-based electrolytes is crucial for enhancing the conductivity of electrochemical capacitors. In this study, a highly conductive polymer/inorganic hybrid electrolyte was developed to improve the ionic conductivity by increasing the free volume between the polymer chains. The filler promotes the movement of OH− ions in the polymer gel electrolyte and enhances the device performance by facilitating the rapid movement of the polymer chains. In particular, rod-like or other similarly shaped fillers promote the formation of ionic conduction networks to improve the ionic conductivity. The use of the urchin-shaped Co3O4 (U–Co3O4) filler (4 wt%) facilitates a high ionic conductivity of 2.61 × 10−2 S cm−1 at room temperature because the U–Co3O4 surface acts as a conductive network in the polymer matrix contributing to the rapid ionic transport. Therefore, an optimal filler morphology is for increasing the performance of gel polymer electrolytes.

First Principle Calculations of Magnetic Proximity Effect Induced Valley Splitting on Heterointerface WS<sub>2</sub>/CoO(111)

The first principle density functional theory calculation has been done for calculating electronic and valleytronic properties on WS2/CoO(111) heterointerface. We have performed the structural optimization of the WS2/CoO(111) heterointerface and obtained the most stable structures by evaluating the binding energy between WS2 and CoO(111) surface. Electronic and valleytronic properties can be understood by band structures, density of states, and spin texture of the crystal models. The results show that non-magnetic WS2 becomes ferromagnetic because of the interfacial effect with the CoO(111) surface. The presence of CoO(111) near the WS2 gives magnetic induction which breaks the time-reversal symmetry preserved on non-magnetic WS2. Accordingly, the largest valley degeneracy is observed at Q and Q’ point in the unoccupied state, with the valley splitting up to 186 meV. Furthermore, the out-of-plane spin texture has also been calculated and the results show that spin configuration at Q and Q’ have opposite signs (up and down, respectively) indicating that the valley couple occurs on the heterointerface. Our results suggest that WS2/CoO(111) is a promising candidate for valleytronic applications.

Surface enrichment of lanthanum on Co3O4 for stable chemical looping combustion

This study introduced a durable oxygen carrier with surface enrichment of lanthanum on Co3O4 for chemical looping combustion. It employed the surface enrichment in order to optimize the usage of rare earth lanthanum with secured stability of oxygen carriers. The formation of LaCoO3 perovskite on the Co3O4 surface imparts strong durability and overcomes deactivation of Co3O4 while maintaining CO2 selectivity due to its enhanced sintering resistance. Lanthanum dopants were optimized at 10 wt%. Co3O4 with 10 wt% of lanthanum had 93.1% and 95.4% average CO2 selectivity and CH4 conversion at 800 °C for 50 cycles, respectively. The surface enrichment provides not only resistance to sintering but also high oxygen storage capacity for long-term cyclic chemical looping combustion.