Hydrothermal preparation of high saturation magnetization and coercivity cobalt ferrite nanocrystals without subsequent calcination

In this work, CoFe2O4 nanocrystals with high saturation magnetization (Ms) and high coercivity (Hc) have been fabricated via a simple hydrothermal method and without subsequent calcination. The resulting CoFe2O4 nanocrystals are characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, differential scanning calorimetry and vibrating sample magnetometry. The results indicate that CoFe2O4 nanocrystals are single crystal and the average crystallite size is increasing with the hydrothermal temperature. The electron micrographs show that the nanocrystals are well-dispersed and possess uniform size. The shape of CoFe2O4 nanocrystals is transformed from spherical into rod by increasing the hydrothermal temperature. The nanocrystals show relatively high Ms of 74.8 emu g−1 and Hc of 2216 Oe, as compared to previous reported results. The obtained results reveal the applicability of this method for efficiently producing well crystallized and relatively high magnetic properties CoFe2O4 nanocrystals as compared to other methods. More importantly, it does not require further calcination processes

Quantum entrapment and valence charge polarization in Ag, Cu, Pt, and Rh nanoclusters

Atomic under-coordination and non-bonding electrons are extensively used in nanomaterials and nanostructures. The bonds between the under-coordinated sites follow the rule of relaxation dynamics, although quantum confinement (QC) theory, Coulomb blockade, and size-dependent dynamic effects cannot describe the change in Hamiltonian and other magnitudes. The effect of under-coordinated atom on the electronic structures of nanomaterials was calculated in this study using the bond-order-length-strength (BOLS) correlation and non-bonding electron polarization (NEP) notations. The Hamiltonian perturbation of the atomic under-coordination entrapped the core electrons and polarized the valence charge. Consistency between the BOLS-NEP notation and density functional theory (DFT) calculations on Ag, Cu, Pt, and Rh nanoclusters with cuboctahedral (COh) and Marks decahedral (M-Dh) structures confirmed that the shorter and stronger bonds between atomic under-coordination induced local densification, quantum entrapment, and valence charge polarization. The strong localization determined the intriguing catalytic, magnetic, and plasmonic attributes of these metallic nanoclusters. The effect of excess charge states from (+2) to (-2) was determined using DFT calculations and BOLS correlation theories on the metallic nanoclusters with COh and M-Dh structures. Consistency between DFT calculations and experimental observations confirmed our BOLS predictions including the local bond length relaxation, charge densification, quantum entrapment, valence band polarization, and magnetization of the metallic nanoclusters with negative, neutral, and positive excess charge states. Magnetization behavior was observed in the even (positive/negative) excess charge states for all metallic nanoclusters and the odd (positive/negative) excess charge states in the Pt and Rh nanoclusters. By contrast, non-magnetization behavior was observed in the odd (positive/negative) excess charge states in the Ag and Cu nanoclusters. Furthermore, Rh and Pt were regarded as donors and acceptors in the catalytic reactions, respectively. This work elucidated the development of cationic, neutral, and anionic metallic nanoclusters for catalyst and magnetic applications.DOCTOR OF PHILOSOPHY (EEE

Effect of atomic under-coordination on the properties of Ag and Cu nanoclusters

Density functional theory calculations have been carried out to investigate the effect of the atomic under-coordination on the bond contraction, lattice strain, and electron configuration of Cuboctahedral and Marks decahedral structures of silver and copper nanoclusters. Our calculated results are consistent in trend with experimental measurements including extended X-ray-absorption fine structure (EXAFS), scanning tunneling microscope/spectroscopy (STM/S), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectra (UPS). This agreement approved the prognostications made on the bond-order-length-strength (BOLS) correlation and nonbonding electron polarization (NEP), suggesting that atomic under-coordination at the surface of nanoclusters cause bond contraction, which then leads to lattice strain, charge densification, core electron entrapment, as well as polarization of valence charge. The results of this work will contribute to the understanding of the intriguing properties of Ag and Cu nanoclusters.Published versio

A review on the role of materials science in solar cells

The demand for energy of modern society is constantly increasing. The desire for environmental-friendly alternative energy resources with the least dependency on fossil fuels is growing. Solar energy is an important technology for many reasons and is worthy of urgent attention. Indeed, it has experienced rapid growth over the last few years. It is expected to become truly main stream when the breakeven of high performance is achieved and its cost becomes comparable with other energy sources. Various approaches have been proposed to enhance the efficiency of solar cells. This paper reviews some current initiatives and critical issues on the efficiency improvement of solar cells from the material sciences and chemistry perspectives

Catalytic and Magnetic Behaviors of Excessively Charged Silver, Copper, Platinum, and Rhodium Atomic Clusters

Spin-resolved density functional theory examination revealed that 0, ±1e, and ±2e additional surface charge has impact to the bond-electron-energy relaxation, and the associated catalystic and magnetic properties of M_<i>N</i> atomic clusters (M = Ag, Cu, Pt, and Rh; <i>N</i> = 13–147 atoms). Trends consistency between calculations and experimental observations confirmed our predictions [Chem. Rev. 2015, 115, 6746] that the shorter-and-stronger bonds between undercoordinated atoms induce the local densification and quantum entrapment of the core electrons, which in turn polarize the valence electrons dominating the catalytic and magnetic performance of the clusters. The excessive charge states has an important impact to the magnetization of M_<i>N</i>. Specifically, even excessive charge states addition induces magnetization of all the M_<i>N</i> clusters while odd excessive charge addition affects only Pt and Rh clusters. Observations provide insight on the relationship between excessive charge and the intrinsic effect of the atomic undercoordination on the catalytic and magnetic properties of M_<i>N</i> nanoclusters

The Role of Physical Techniques on the Preparation of Photoanodes for Dye Sensitized Solar Cells

Dye sensitized solar cells (DSSCs) have attracted numerous research, especially in the context of enhancing their efficiency and durability, due to the low-cost and environmentally friendly nature of photovoltaic (PV) technology. The materials in DSSCs are vital towards the realization of these goals, since many of the important components are influenced by their respective preparation and deposition methods. This review aims to detail the research and development aspects of the different physical methods with the purpose of evaluating their prospects and corresponding limitations. The diversity of consideration and criteria includes thin film applications, material characteristics, and process technology that need to be taken into account when selecting a specific deposition method. Choosing a deposition method is not as simple as it seems and is rendered quite complicated due to various factors. Usually, a researcher will evaluate techniques based on factors such as the different preparations and deposition technology with materials’ and substrates’ type, specified applications, costs, and efficiencies

Size and crystallinity-dependent magnetic properties of CoFe2O4 nanocrystals

CoFe2O4 nanocrystals were synthesized by a wet chemical coprecipitation approach. In order to investigate the effect of degree of crystallinity and mean crystallite size of CoFe2O4 nanocrystals on the magnetic properties, a series of CoFe2O4 samples with different degree of crystallinity and mean crystallite size were produced by varying the synthesis and subsequent calcination temperatures. The higher synthesis and subsequent calcination temperatures have resulted in greater degree of crystallinity and bigger mean crystallite size of CoFe2O4 nanocrystals. The VSM studies showed that the saturation magnetization (Ms) and coercivity (Hc) of the CoFe2O4 nanocrystals possessed a linear relationship with the mean crystallite size

In Situ Formation of Nano Ni-Co Oxyhydroxide Enables Water Oxidation Electrocatalysts Durable at High Current Densities

The oxygen evolution reaction (OER) limits the energy efficiency of electrocatalytic systems due to the high overpotential symptomatic of poor reaction kinetics; this problem worsens over time if the performance of the OER electrocatalyst diminishes during operation. Here, a novel synthesis of nanocrystalline Ni-Co-Se using ball milling at cryogenic temperature is reported. It is discovered that, by anodizing the Ni-Co-Se structure during OER, Se ions leach out of the original structure, allowing water molecules to hydrate Ni and Co defective sites, and the nanoparticles to evolve into an active Ni-Co oxyhydroxide. This transformation is observed using operando X-ray absorption spectroscopy, with the findings confirmed using density functional theory calculations. The resulting electrocatalyst exhibits an overpotential of 279 mV at 0.5 A cm-2 and 329 mV at 1 A cm-2 and sustained performance for 500 h. This is achieved using low mass loadings (0.36 mg cm-2 ) of cobalt. Incorporating the electrocatalyst in an anion exchange membrane water electrolyzer yields a current density of 1 A cm-2 at 1.75 V for 95 h without decay in performance. When the electrocatalyst is integrated into a CO2 -to-ethylene electrolyzer, a record-setting full cell voltage of 3 V at current density 1 A cm-2 is achieved.Natural Sciences and Engineering Research Council (NSERC) of Canada Vanier Canada Graduate Scholarship Canadian Centre for Electron Microscopy Advanced Photon Source, an Office of Science User Facility U.S. Department of Energy (DOE) Office of Science Argonne National Laboratory. Grant Number: DE-AC02-06CH11357 Engineered Nickel Catalysts for Electrochemical Clean Energy. Grant Number: RGPNM 477963-2015 Natural Sciences and Engineering Research Council of Canad