A comparison of the electrochemical and physicochemical behavior of cobalt-based oxides with spinel structure MCo 2 O 4 (M = Mn, Fe, Co, Ni, and Zn) was conducted to investigate the effect of the oxidation state and cation distribution in the spinel on the electrocatalytic activity of the oxygen evolution reaction (OER) in an alkaline solution. Various spinel MCo 2 O 4 electrocatalysts were synthesized by a facile microwave-assisted synthesis and low-temperature annealing. The overpotential of these MCo 2 O 4 electrocatalysts for the OER is comparable to the reported overpotentials of catalysts based on cobalt oxides. From the findings, the catalytic activity of OER decreases in the order of ZnCo 2 O 4 > NiCo 2 O 4 > FeCo 2 O 4 > Co 3 O 4 > MnCo 2 O 4 . It was revealed that the active sites are controlled by the balance of M 3+ /M 2+ cation distribution in octahedral and tetrahedral sites and bythe bond strength between M and oxygen atoms at the catalyst surface from the direct combination of in situ X-ray absorption fine structure (XAFS) spectroscopy with the electrochemical experiments to track the oxidation state and the structural changes of electrocatalysts before and after the exposure to the OER conditions. This study provides insights into the effects of cation distributions on the OER activity and demonstrates a promising method for determining the fundamental mechanism of cationsubstituted cobalt oxides for OER.

Spinel ferrite MFe2O4 (M = Mn, Fe, Co, Ni, and Zn) nanoparticles, coordinated with oleylamine, oleic acid, and trioctylphosphine oxide, were effectively synthesized by the microwave-assisted rapid synthesis in 1-dodecanol. Spinel ferrite nanoparticles were structurally analyzed using HRTEM, SAED, HAADF-STEM, XRD, XRF, XPS, and EXAFS measurements, which suggested that MFe2O4 nanoparticles have 3.2–4.3 nm in diameter and a spinel cubic structure with space group Fd3̅m. The metal composition retains an initially charged molar ratio (Fe: M = 2: 1), and both corresponding elements exist within a particle. EXAFS analysis performed quantitative determination of cation distribution in the tetrahedral and octahedral sites of the spinel structure to gain a deep insight into the influence of the cation distribution (i.e., the degree of inversion) to their magnetic properties. Compared with the magnetic properties of the bulk spinel structure, small spinel ferrite particles at nanoscale showed some promising applications for the magnetic data storage, catalysts, sensors, and ferrofluids, due to the significant effect of their particle sizes, surface coordinating environments, and the cation distributions.

Cobalt oxide (CoO) and nickel oxide (NiO) nanoparticles, coordinated by oleylamine, oleic acid, and trioctylphosphine oxide, were prepared as antiferromagnetic materials in 1-octanol or 1dodecanol by rapid synthesis under microwave irradiation. Structural analysis of the finally obtained oxide nanoparticles was performed by means of X-ray diffraction, high-resolution transmission electron microscopy, and extended X-ray absorption fine structure measurements, which suggested that the higher temperature in 1-dodecanol (at 523 K) than in 1-octanol (at 463 K) enhanced the reaction efficiency in the preparation of NiO nanoparticles. The dependence of reaction conditions (reaction temperature, starting organometallic precursor, solvent, additives, etc.) on the structure (particle size and polydispersity) was demonstrated. The magnetic properties of CoO and NiO were examined by means of a vibrating sample magnetometer, which indicated the antiferromagnetic structure of the pure CoO and NiO nanoparticles.

Unprotected" metal and alloy nanoclusters prepared using the alkaline-ethylene glycol method (AEGM), stabilized by adsorbed solvent molecules and simple ions, have been widely applied in the development of high-performance heterogeneous catalysts and the exploration of the effects of metal particle size and composition, surface ligands of support, and modifiers on the catalytic properties of heterogeneous catalysts. The formation process and mechanism of such unprotected metal nanoclusters need to be further investigated. In this study, the formation process and mechanism of unprotected Pt and Ru nanoclusters prepared with AEGM were investigated by in situ quick Xray absorption fine spectroscopy (QXAFS), in situ ultraviolet-visible (UV-Vis) absorption spectroscopy, transmission electron microscopy, and dynamic light scattering. It was discovered that during the formation of unprotected Pt nanoclusters, a portion of Pt(IV) species was reduced to Pt(II) species at room temperature. With increasing temperature, Cl − coordinated to Pt ions was gradually replaced with OH − to form intermediate platinum complexes, which further condensated to form colloidal nanoparticles. Obvious scattering signals of the colloidal nanoparticles could be observed in the UV-Vis absorption spectra of the reaction system before the formation of Pt-Pt bonds, as revealed by QXAFS measurements. In situ QXAFS analysis revealed that Pt nanoclusters were derived from the reduction of Pt oxide nanoparticles. The average particle size of the nanoparticles obtained by heating the reaction mixture for 15 min at 80 C was 3.7 nm. High resolution transmission electron microscopy (HRTEM) images showed that the spacing between the crystal planes of the nanoparticles was 0.249 nm, indicating that the intermediate nanoparticles were platinum oxide. As the reaction proceeded, the average size of the nanoparticles decreased to 2.4 nm, and two types of nanoparticles were observed having different contrasts, corresponding to Pt metal nanoclusters standing on the intermediate metal oxide nanoparticles as confirmed by HRTEM images. When the reaction time was further extended, the average size of nanoparticles decreased to 1.4 nm, and the observed lattice spacing of the nanoparticles was the same as that of Pt( 111) crystal plane at 0.227 nm, indicating that the final products were Pt metal nanoclusters. In general, when metal oxides are reduced to metal nanoclusters, the density of the nanoparticles will increase, whereas the volume will decrease. Moreover, as shown in this study, the formation of multiple small metal nanoclusters standing on one metal oxide nanoparticle was also observed in TEM photographs. Thus, compared with the size of the initial nanoparticles, the average size of the final metal nanoclusters was significantly reduced. On the other hand, during the formation of unprotected Ru metal nanoclusters, Cl − in RuCl3 was first replaced with OH − to form Ru(OH) 6 3− , which further condensated to form Ru oxide nanoparticles, and unprotected Ru met...