NixCo3-xO4表面修饰对CdSe/TiO2纳米管阵列光电化学氧化水活性的影响

利用氨挥发诱导法在CdSe/TiO2纳米管阵列表面负载一层NixCo3-xO4。采用SEM、XRD、XPS、UV-Vis对样品进行表征,通过线性扫描伏安法测定光阳极的释氧电势来评价其光电水氧化活性。结果表明:表面NixCo3-xO4是尖晶石结构;相对于CdSe/TiO2纳米管阵列光阳极,NixCo3-xO4/CdSe/TiO2光阳极能将光电氧化水的过电势降低430 mV。Ni离子的引入使得NixCo3-xO4表面富含三价阳离子(Ni3+,Co3+),从而促进CdSe/TiO2光阳极光电水氧化的进行。     

Effect of Calcination Temperature on Surface Oxygen Vacancies and Catalytic Performance Towards CO Oxidation of Co3O4 Nanoparticles Supported on SiO2

Co₃O₄/SiO₂ catalysts for CO oxidation were prepared by conventional incipient wetness impregnation followed by calcination at various temperatures. Their structures were char-acterized with X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and X-ray absorption fine structure (XAFS) spectroscopy. Both XRD and Raman spectroscopy only detect the ex-istence of Co₃O₄ crystallites in all catalysts. However, XPS results indicate that excess Co²⁺ ions are present on the surface of Co₃O₄ in Co₃O₄(200)/SiO₂ as compared with bulk Co₃O₄. Meanwhile, TPR results suggest the presence of surface oxygen vacancies on Co₃O₄ in Co₃O₄(200)/SiO₂, and XAFS results demonstrate that Co₃O₄ in Co₃O₄(200)/SiO₂ con-tains excess Co²⁺. Increasing calcination temperature results in oxidation of excess Co²⁺ and the decrease of the concentration of surface oxygen vacancies, consequently the for-mation of stoichiometric Co₃O₄ on supported catalysts. Among all Co₃O₄/SiO₂ catalysts,Co₃O₄(200)/SiO₂ exhibits the best catalytic performance towards CO oxidation, demon-strating that excess Co²⁺ and surface oxygen vacancies can enhance the catalytic activity of Co₃O₄ towards CO oxidation. These results nicely demonstrate the effect of calcination temperature on the structure and catalytic performance towards CO oxidation of silica-supported Co₃O₄ catalysts and highlight the important role of surface oxygen vacancies on Co₃O₄.     

Tailoring Oxygen Vacancy on Co3O4 Nanosheets with High Surface Area for Oxygen Evolution Reaction?

Electrochemical water splitting requires efficient water oxidation catalysts to accelerate the sluggish kinetics of water oxidation reaction. Here, we designed an efficient Co₃O₄ electrocatalyst using a pyrolysis strategy for oxygen evolution reaction (OER). Morphological characterization confirmed the ultra-thin structure of nanosheet. Further, the existence of oxygen vacancies was obviously evidenced by the X-ray photoelectron spectroscopy and electron spin resonance spectroscopy. The increased surface area of Co₃O₄ ensures more exposed sites, whereas generated oxygen vacancies on Co₃O₄ surface create more active defects. The two scenarios were beneficial for accelerating the OER across the interface between the anode and electrolyte. As expected, the optimized Co₃O₄ nanosheets can catalyze the OER efficiently with a low overpotential of 310 mV at current density of 10 mA/cm² and remarkable long-term stability in 1.0 mol/L KOH.     

一锅法制备Co3O4/g-C3N4复合光催化剂及其光催化性能

以三聚氰胺和六水合氯化钴为原料,一锅法制备Co_3O_4负载的多孔石墨相氮化碳(Co_3O_4/g-C_3N_4)复合光催化材料。采用X射线衍射(XRD)、傅里叶变换红外(FT-IR)光谱、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、紫外-可见漫反射光谱(UV-Vis DRS)、光致发光光谱(PL)等手段对其结构和光学特性进行表征。以盐酸四环素(TC)为目标污染物,评价了不同负载量Co_3O_4/g-C_3N_4复合光催化剂的可见光催化性能。结果表明,所制备的Co_3O_4/g-C_3N_4复合光催化剂为多孔结构,其比表面积较大,并在可见光区域具有显著的吸收。利用原位生成的Co_3O_4纳米粒子在氮化碳表面形成异质结构,可有效转移光生载流子,降低光生电子-空穴的再结合率,从而提高光催化活性。并且存在最佳Co_3O_4复合量,当六水合氯化钴加入量为三聚氰胺的8%(w/w)时,所制备的复合光催化剂CoCN-8具有最佳的光催化性能。在可见光的照射下,60 min内可降解85%的TC,而同样条件下,纯g-C_3N_4仅降解23%的TC。     

Active Electron Density Modulation of Co3O4-Based Catalysts Enhances their Oxygen Evolution Performance

Despite the fact that many strategies have been developed to improve the efficiency of the oxygen evolution reaction (OER), the precise modulation of the surface electronic properties of catalysts to improve their catalytic activity is still challenging. Herein, we demonstrate that the surface active electron density of Co₃O₄ can be effectively regulated by an argon-ion irradiation method. X-ray photoelectron and synchrotron x-ray absorption spectroscopy, UV photoelectron spectrometry, and DFT calculations show that the surface active electron density band center of Co₃O₄ has been upshifted, leading to a significantly enhanced absorption capability of the oxo group. The optimized Co₃O₄-based catalysts exhibit an excellent overpotential of 260 mV at 10 mA cm⁻² and Tafel slope of 54 mV dec⁻¹, superior to the capability of the benchmark RuO₂, representing one of the best Co-based OER catalysts. This approach could guide the future rational design and discovery of ideal electrocatalysts.     

Preparation of nano-Co3O4-coated Albizia procera-derived carbon by direct thermal decomposition method for electrochemical water oxidation

It is obtained that nano-Co₃O₄-coated carbon prepared by thermal decomposition of Co(NO₃)₂·6H₂O at 300 °C on home-made Albizia procera (Roxb.) leaves derived carbon is an efficient electrocatalyst for electrochemical water oxidation in 0.1 M NaOH (aq.) solution. The loading of nano-Co₃O₄ on the carbon was changed by varying the amount of precursor of cobalt (100–1000 mg) and using a constant amount of the carbon (200 mg) during thermal decomposition. The prepared samples were characterized by physical techniques, including X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), thermo-gravimetric analysis (TGA), fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), diffuse reflectance spectroscopy (DRS), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). XRD, TEM, FESEM, and EDS confirmed the formation of uniformly distributed nanoparticles of single-phase Co₃O₄ on the surface of carbon. The XRD data reveals formation of nano-Co₃O₄ with average particle sizes in the range of 9–17 nm. The FESEM micrographs demonstrate that Co₃O₄ nanoparticles, having irregular morphology, are uniformly and densely covered on the surface of supporting carbon.. The prepared samples were immobilized on the filter paper derived carbon electrode (FPCE) to study their electrocatalytic properties toward water oxidation. The cyclic voltammetric studies showed that the nano-Co₃O₄-C prepared using 400 mg of Co(NO₃)₂·6H₂O (nano-Co₃O₄-C-400), which possesses meso- and macropores with BET surface area of 192.4 m²/g, reaches a current density of 28 mAcm⁻² at 1.5 V and electrochemical water oxidation starting potential of 0.7 V. In this work, it is also shown that the current densities, at 1.5 V, increase by increasing the amount of cobalt oxide in the prepared samples though. The nano-Co₃O₄-C-400 catalyst shows optimum performance for electrochemical water oxidation in terms of starting water oxidation potential, reasonable amount of Co₃O₄ and moderate level of current density at 1.5 V.     

Efficient Selective Oxidation of Aromatic Alkanes by Double Cobalt Active Sites over Oxygen Vacancy-rich Mesoporous Co3O4

The development of efficient catalyst for selective oxidation of hydrocarbon to functional compounds remains a challenge. Herein, mesoporous Co₃O₄ (mCo₃O₄-350) showed excellent catalytic activity for selective oxidation of aromatic-alkanes, especially for oxidation of ethylbenzene with a conversion of 42 % and selectivity of 90 % for acetophenone at 120 °C. Notably, mCo₃O₄ presented a unique catalytic path of direct oxidation of aromatic-alkanes to aromatic ketones rather than the conventional stepwise oxidation to alcohols and then to ketones. Density functional theory calculations revealed that oxygen vacancies in mCo₃O₄ activate around Co atoms, causing electronic state change from Co³⁺_(Oh)→Co²⁺_(Oh). Co²⁺_(Oh) has great attraction to ethylbenzene, and weak interaction with O₂, which provide insufficient O₂ for gradual oxidation of phenylethanol to acetophenone. Combined with high energy barrier for forming phenylethanol, the direct oxidation path from ethylbenzene to acetophenone is kinetically favorable on mCo₃O₄, sharply contrasted to non-selective oxidation of ethylbenzene on commercial Co₃O₄.     

Applying Co3O4@nanoporous Carbon to Nonenzymatic Glucose Biofuel Cell and Biosensor

A novel hierarchically nanoporous carbon (NPC) derived from Al‐based porous coordination polymer is prepared by two‐step carbonization method for immobilization of the Co₃O₄ in the application of the nonenzymatic biofuel cells and biosensors. The structure and morphology are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high‐resolution transmission electron microscopy (HRTEM), and X‐ray diffraction (XRD). Brunauer‐Emmett‐Teller (BET) is to characterize the porous nature of the NPC, and X‐ray photoelectron spectroscopy (XPS) is to characterize the composition of Co₃O₄@nanoporous carbon (Co₃O₄@NPC). Without collapse in the high carbonization temperature (above 1600 °C), the NPC maintains the nanoporous structure and high specific surface area of 1551.2 m² g⁻¹. In addition, the NPC is composited with Co₃O₄ by hydrothermal method to form the Co₃O₄@NPC. When tested as the nonenzymatic electrocatalyst for glucose oxidation reaction (GOR), the Co₃O₄@NPC exhibits higher response to glucose, in which the current shifts up by 64 %, than pure Co₃O₄ in 0.1 M KOH. The limit of detection is 0.005 mM (S/N=3) and response time is within 3 s. The detection range can be divided into two sections of 0.02–1.4 mM and 1.4–10.7 mM with the sensitivity of 249.1 μA mM⁻¹ cm⁻² and 66.6 μA mM⁻¹ cm⁻², respectively. A glucose fuel cell is constructed with the Co₃O₄@NPC as the anode and Pt/C catalyst as the cathode. The open‐circuit potential of the nonenzymatic glucose/O₂ fuel cell was 0.68 V, with a maximum power density of 0.52 mW cm⁻² at 0.27 V. This work may contribute to exploring other nanoporous carbons for application in glucose fuel cells and biosensors.     

Structure Sensitivity of CO Oxidation on Co3O4: A DFT Study

The reaction mechanism of CO oxidation on the Co₃O₄ (110) and Co₃O₄ (111) surfaces is investigated by means of spin‐polarized density functional theory (DFT) within the GGA+U framework. Adsorption situation and complete reaction cycles for CO oxidation are clarified. The results indicate that 1) the U value can affect the calculated energetic result significantly, not only the absolute adsorption energy but also the trend in adsorption energy; 2) CO can directly react with surface lattice oxygen atoms (O_2f/O_3f) to form CO₂ via the Mars–van Krevelen reaction mechanism on both (110)‐B and (111)‐B; 3) pre‐adsorbed molecular O₂ can enhance CO oxidation through the channel in which it directly reacts with molecular CO to form CO₂ [O₂(a)+CO(g)→CO₂(g)+O(a)] on (110)‐A/(111)‐A; 4) CO oxidation is a structure‐sensitive reaction, and the activation energy of CO oxidation follows the order of Co₃O₄ (111)‐A(0.78 eV)>Co₃O₄ (111)‐B (0.68 eV)>Co₃O₄ (110)‐A (0.51 eV)>Co₃O₄ (110)‐B (0.41 eV), that is, the (110) surface shows higher reactivity for CO oxidation than the (111) surface; 5) in addition to the O_2f, it was also found that Co³⁺ is more active than Co²⁺, so both O_2f and Co³⁺ control the catalytic activity of CO oxidation on Co₃O₄, as opposed to a previous DFT study which concluded that either Co³⁺ or O_2f is the active site.     

A Facile Preparation of the Co3O4 Villiform Nanostructure

A Co₃O₄ villiform nanostructure was prepared by the aid of the cobalt oxalate precursor and characterized with x-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscopy (SEM) and UV-vis spectrum. It is found that the villiform structure obtained consists of Co₃O₄ nanorods with diameters of 50–80 nm. Moreover, the UV-vis absorptions of the villiform Co₃O₄ show the apparent blue shifts by comparison with the bulk Co₃O₄, indicating a quantum size effect similar to the free Co₃O₄ nanoparticles.     

Comparative study of Co3O4-ZSM-5 catalysts synthesized by different hydrothermal methods for the catalytic oxidation of dichloromethane

In this work, various Co₃O₄-ZSM-5 catalysts were prepared by the microwave hydrothermal method (MH-Co₃O₄@ZSM-5), dynamic hydrothermal method (DH-Co₃O₄@ZSM-5), and conventional hydrothermal method (CH-Co₃O₄/ZSM-5). Their catalytic oxidation of dichloromethane (DCM) was analyzed. Detailed characterizations such as X-ray diffractometer (XRD), scanning microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), H₂ temperature-programmed reduction (H₂-TPR), temperature-programmed desorption of O₂ (O₂-TPD), temperature-programmed desorption of NH₃ (NH₃-TPD), diffuse reflectance infrared Fourier-transform spectra with NH₃ molecules (NH₃-DRIFT), and temperature-programmed surface reaction (TPSR) were performed. Results showed that with the assistance of microwave, MH-Co₃O₄@ZSM-5 formed a uniform core-shell structure, while the other two samples did not. MH-Co₃O₄@ZSM-5 possessed rich surface adsorbed oxygen species, higher ratio of Co³⁺/Co²⁺, strong acidity, high reducibility, and oxygen mobility among the three Co₃O₄-ZSM-5 catalysts, which was beneficial for the improvement of DCM oxidation. In the oxidation of dichloromethane, MH-Co₃O₄@ZSM-5 presented the best activity and mineralization, which was consistent with the characterizations results. Meanwhile, according to the TPSR test, HCl or Cl₂ removal from the catalyst surface was also promoted in MH-Co₃O₄@ZSM-5 by their abundant Brønsted acid sites and the promotion of Deacon reaction by Co₃O₄ or the synergistic effect of Co₃O₄ and ZSM-5. According to the results of in situ DRIFT studies, a possible reaction pathway of DCM oxidation was proposed over the MH-Co₃O₄@ZSM-5 catalysts.     

Investigation of CO and formaldehyde oxidation over mesoporous Ag/Co3O4 catalysts

CO and formaldehyde (HCHO) oxidation reactions were investigated over mesoporous Ag/Co₃O₄ catalysts prepared by one-pot (OP) and impregnation (IM) methods. It was found that the one-pot method was superior to the impregnation method for synthesizing Ag/Co₃O₄ catalysts with high activity for both reactions. It was also found that the catalytic behavior of mesoporous Co₃O₄ and Ag/Co₃O₄ catalysts for the both reactions was different. And the addition of silver on mesoporous Co₃O₄ did not always enhance the catalytic activity of final catalyst for CO oxidation at room temperature (20 °C), but could significantly improve the catalytic activity of final catalyst for HCHO oxidation at low temperature (90 °C). The high surface area, uniform pore structure and the pretty good dispersion degree of the silver particle should be responsible for the excellent low-temperature CO oxidation activity. However, for HCHO oxidation, the addition of silver played an important role in the activity enhancement. And the silver particle size and the reducibility of Co₃O₄ should be indispensable for the high activity of HCHO oxidation at low temperature.     

Facile Synthesis of Graphene/Cobalt Oxide Nanohexagons for the Selective Detection of Dopamine

This work presents a simple green approach for the chemical synthesis of cobalt oxide nano hexagons (Co₃O₄ NHs) with an average size of 160±40 nm incorporated graphene nanosheets (GR). The techniques used to confirm the formation of GR−Co₃O₄ NHs are transmission electron microscopy (TEM), energy‐dispersive X‐ray spectroscopy (EDX), and X‐ray diffraction spectroscopy (XRD). The dopamine (DA) sensor was fabricated by drop casting GR−Co₃O₄ NHs on the pre‐cleaned glassy carbon electrode (GCE). GR−Co₃O₄ modified GCE displayed a sensitive and selective electrochemical determination of DA compared to only GR and Co₃O₄ NHs modified GCE. Our fabricated sensor showed a wide linear range from 0.2 to 3443 μM with low limit of detection (84 nM) towards the determination of DA. The sensitivity of our fabricated sensor was calculated to be 108 μA mM⁻¹ cm⁻². As well, a significant storage stability, repeatability and reproducibility were attained by GR−Co₃O₄ NHs modified GCE. Human urine samples were targeted for the demonstration of practicality of our sensor.     

Fabrication of Co3O4 particles-modified multi-walled carbon nanotube and poly(phenosafranine) composite electrode for selective and sensitive rutin detection

The preparation and characterisation of a new composite electrode with Co₃O₄ particles-modified multi-walled carbon nanotube (MWCNT) and poly(phenosafranine), as well as its novel application for the voltammetric detection of rutin was described. The resulting composite electrode was characterised using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). In the optimised experimental conditions, the oxidation peak current (I_pa) of rutin showed a linear increase in concentration, between 0.008–0.6 and 0.80–6.0 μmol L⁻¹, with a detection limit of 0.00379 μmol L⁻¹. Due to its good selectivity and stability, the composite electrode was successfully applied in detecting rutin in pharmaceutical formulations.     

Facile synthesis of ultrafine Co3O4 nanocrystals embedded carbon matrices with specific skeletal structures as efficient non-enzymatic glucose sensors

A facile, effective, and environmentally friendly method has been adopted for the first time to prepare tiny Co₃O₄ nanocrystals embedded carbon matrices without using surfactants, harmful organic reagents or extreme conditions. Structural characterizations reveal that the size-controlled Co₃O₄ nanocrystals are uniformly dispersed on carbon matrices. Electrochemical measurements reveal that Co₃O₄-ordered mesoporous carbon (OMC) can more efficiently catalyze glucose oxidation and acquire better detection parameters compared with those for the Co₃O₄-macroporous carbon, Co₃O₄-reduced graphene oxide, and free Co₃O₄ nanoparticles (NPs) (such as: the large sensitivity (2597.5 μA cm⁻² mM⁻¹ between 0 and 0.8 mM and 955.9 μA cm⁻² mM⁻¹ between 0.9 and 7.0 mM), fast response time, wide linear range, good stability, and surpassingly selective capability to electroactive molecules or Cl⁻). Such excellent performances are attributed to the synergistic effect of the following three factors: (1) the high catalytic sites provided by the uniformly dispersed and size-controlled Co₃O₄ nanocrystals embedded on OMC; (2) the excellent reactant transport efficiency caused by the abundant mesoporous structures of OMC matrix: (3) the improved electron transport in high electron transfer rate (confinement of the Co₃O₄ NPs in nanoscale spaces ensured intimate contact between Co₃O₄ nanocrystals and the conducting OMC matrix). The superior catalytic activity and selectivity make Co₃O₄-OMC very promising for application in direct detection of glucose.     

Mn3O4 nanoplates and nanoparticles: Synthesis, characterization, electrochemical and catalytic properties

Mn₃O₄ hexagonal nanoplates and nanoparticles were synthesized via a solvent-assisted hydrothermal oxidation process at low temperature and a solvothermal oxidation method, respectively. The synthesized product was characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), electron diffraction (ED), Fourier transform infrared (FT-IR) spectroscopy. Their capability of catalytic oxidation of formaldehyde to formic acid at room temperature and atmospheric pressure and electrochemical properties by cyclic voltammogram (CV) were compared. The results showed that Mn₃O₄ hexagonal nanoplate is a better catalyst, and the hexagonal nanoplates and nanoparticles modified electrodes blended with carbon black have a higher specific capacitance.     

In situ Fourier Transform Infrared Spectroscopy Diagnostic for Characterization and Performance Test of Catalysts

The present work establishes a systematic approach based on the application of in-situ Fourier transform infrared spectroscopy (FTIR) for the investigation of the crystal structure, thermal stability, redox behavior (temperature-programmed reduction/temperatureprogrammed re-oxidation) as well as the catalytic properties of Co₃O₄ thin films. The syntheses of Co₃O₄ were achieved by chemical vapor deposition in the temperature range of 400-500℃. The structure analysis of the as-prepared material revealed the presence of two prominent IR bands peaking at 544 cm^-1 (υ₁) and 650 cm^-1 (υ₂) respectively, which originate from the stretching vibrations of the Co-O bond, characteristic of the Co₃O₄ spinel. The lattice stability limit of Co₃O₄ was estimated to be above 650℃. The redox properties of the spinel structure were determined by integrating the area under the emission bands υ₁ and υ₂ as a function of the temperature. Moreover, Co₃O₄ has been successfully tested as a catalyst towards complete oxidation of dimethyl ether below 340 ℃. The exhaust gas analysis during the catalytic process by in situ absorption FTIR revealed that only CO₂ and H₂O were detected as the final products in the catalytic reaction. The redox behavior suggests that the oxidation of dimethyl ether over Co₃O₄ follows a Mars-van Krevelen type mechanism. The comprehensive application of in situ FTIR provides a novel diagnostic tool in characterization and performance test of catalysts.     

High-valance molybdenum doped Co3O4 nanowires: Origin of the superior activity for 5-hydroxymethyl-furfural oxidation

Co₃O₄ has been widely explored in electrocatalytic 5-hydroxymethyl-furfural (HMF) oxidation. However, the poor intrinsic ability has seriously limited its electrochemical ability. Heteroatom-doping is an efficient method to enhance the electrocatalytic ability of catalyst by regulating electronic structure. Herein, we have modulated the electronic structure of Co₃O₄ by high valance Mo⁶⁺-doping. With the introduction of Mo⁶⁺, the content of Co²⁺ was increased and metal-oxygen bond was strength. Electrochemical results suggested that the electrocatalytic ability of Co₃O₄ towards HMF oxidation has been dramatically improved and reaction kinetics has been fasten. Theoretical calculations demonstrated that the surrounding cobalt sites after Mo⁶⁺-doping with assembled electron has a strong adsorption ability towards HMF molecule leading to more favourable oxidation of HMF. Post characterizations demonstrated pristine Co₃O₄ structure was kept after electrolysis cycles and CoOOH active species were formed. This work provides a valuable reference for developing efficient heteroatom-doped electrocatalysts for HMF oxidation.     

Extending Metal‐to‐Polyoxometalate Charge Transfer Lifetimes: The Effect of Heterometal Location

In an effort to develop robust molecular sensitizers for solar fuel production, the electronic structure and photodynamics of transition‐metal‐substituted polyoxometalates (POMs), a novel class of compound in this context, was examined. Experimental and computational techniques including femtosecond (fs) transient absorption spectroscopy have been used to study the cobalt‐containing Keggin POMs, [Co^IIW₁₂O₄₀]^6? ( 1 a ), [Co^IIIW₁₂O₄₀]^5? ( 2 a ), [SiCo^II(H₂O)W₁₁O₃₉]^6? ( 3 a ), and [SiCo^III(H₂O)W₁₁O₃₉]^5? ( 4 a ), finding the longest lived charge transfer excited state so far observed in a POM and elucidating the electronic structures and excited‐state dynamics of these compounds at an unprecedented level. All species exhibit a bi‐exponential decay in which early dynamic processes with time constants in the fs domain yield longer lived excited states which decay with time constants in the ps to ns domain. The initially formed states of 1 a and 3 a are considered to result from metal‐to‐polyoxometalate charge transfer (MPCT) from Co^II to W, while the longer‐lived excited state of 1 a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion ( 1 a ) is far longer‐lived (τ=420 ps in H₂O; τ=1700 ps in MeCN) than that of 3 a (τ=1.3 ps), in which the single Co^II atom is located in a pseudo‐octahedral addendum site. Short‐lived states are observed for the two Co^III‐containing heteropolyanions 2 a (τ=4.4 ps) and 4 a (τ=6.3 ps) and assigned solely to O→Co^III charge transfer. The dramatically extended lifetime for 1 a versus 3 a is ascribed to a structural change permitted by the coordinatively flexible central site, weak orbital overlap of the central Co with the polytungstate framework, and putative transient valence trapping of the excited electron on a single W atom, a phenomenon not noted previously in POMs.     

Nickel‐substituted cobalt ferrite nanoparticles supported on arginine‐modified graphene oxide nanosheets: Synthesis and catalytic activity

The amino acid arginine was used to modify the surface of graphene oxide nanosheets and then nickel‐substituted cobalt ferrite nanoparticles were supported on those arginine‐grafted graphene oxide nanosheets (Ni_0.5Co_0.5Fe₂O₄@Arg–GO). The prepared Ni_0.5Co_0.5Fe₂O₄@Arg–GO was characterized using flame atomic absorption spectroscopy, inductively coupled plasma optical emission spectrometry, energy‐dispersive spectroscopy, Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, Raman spectroscopy, X‐ray diffraction, thermogravimetric analysis, scanning electron microscopy and transmission electron microscopy. The application of Ni_0.5Co_0.5Fe₂O₄@Arg–GO as a catalyst was examined in a one‐pot tandem oxidative cyclization of primary alcohols with o ‐phenylenediamine to benzimidazoles under aerobic oxidation conditions. The results showed that 2‐phenylbenzimidazole derivatives were successfully achieved using Ni_0.5Co_0.5Fe₂O₄@Arg–GO nanocomposite catalyst via the one‐pot tandem oxidative cyclization strategy.