Magnetic and photocatalytic properties of nanocrystalline ZnMn<SUB>2</SUB>O<SUB>4</SUB>

The present study describes the synthesis of ZnMn2O4 nanoparticles with the spinel structure. These oxide nanoparticles are obtained from the decomposition of metal oxalate precursors synthesized by (a) the reverse micellar and (b) the coprecipitation methods. Our studies reveal that the shape, size and morphology of precursors and oxides vary significantly with the method of synthesis. The oxalate precursors prepared from the reverse micellar synthesis method were in the form of rods (micron size), whereas the coprecipitation method led to spherical nanoparticles of size, 40-50 nm. Decomposition of oxalate precursors at low temperature (~ 450&#176;C) yielded phase pure ZnMn2O4 nanoparticles. The size of the nanoparticles of ZnMn2O4 obtained from reverse micellar method is relatively much smaller (20-30 nm) as compared to those made by the co-precipitation (40-50 nm) method. Magnetic studies of nanocrystalline ZnMn2O4 confirm antiferro-magnetic ordering in the broad range of ~ 150 K. The photocatalytic activity of ZnMn2O4 nanoparticles was evaluated using photo-oxidation of methyl orange dye under UV illumination and compared with nanocrystalline TiO2

Crystal Growth and Unusual Electronic Transport Properties of Some Reduced Molybdenum Oxides with Bi-Octahedral Mo10 Clusters

Single crystals of AM05O3 (A = Ca, Sr, La-Gd), suitable for electrical conductivity measurements have been grown by high temperature and fused salt electrolytic techniques. The structures of all of these compounds are dominated by the presence of bi-octahedral clusters of Mo atoms joined together parallel to the monoclinic a axis, forming infinite chains. Temperature dependent electrical resistivity measurements on AMo5Og (A = La, Ce, Pr, Nd, Sm) show anomalous metal-semiconductor transitions near 180 and 30 K. The resistivities of the Eu and Gd analogues are different, in that the former is semiconducting while the latter shows a weak anomaly ~ 110 K. The Ca and Sr analogues are also semiconducting in the range 20-300 K. The electrical conductivity of these phases appears to be closely related to the inter-cluster separation and the number of metal-cluster electrons. The magnetic susceptibility of these compounds show no anomalies at the temperatures corresponding to the transitions seen in their electrical resistivities. The magnetic susceptibility of LaMosOg shows a small decrease in the !y (dy/dT) vs T plot in the vicinity of ~ 150 K

A new low temperature methodology to obtain pure nanocrystalline nickel borate

The present study focuses on the development of a new route for the synthesis of pure nickel borate nanoparticles using reverse micellar route. Nickel borate nanoparticles (25 nm) were synthesized from a precursor (obtained by reverse micellar route) containing both nickel and boron (nickel nitrate and sodium borohydride as starting materials). Decomposition of the precursor at a temperature of ~800 °C yielded pure nickel borate nanoparticles. This was confirmed by powder X-ray diffraction and transmission electron microscopy. These nanoparticles show an antiferromagnetic ordering with Nèel temperature of 47 K

Magnetic and electrochemical properties of nickel oxide nanoparticles obtained by the reverse-micellar route

Homogeneous nanoparticles of nickel oxide (NiO) of 25 nm size with narrow size distribution have been synthesized by the reverse-micellar route using CTAB (cetyltrimethyl ammonium bromide) as the surfactant. FTIR studies show a broad absorption around 405-415 cm−1 and a weak absorption around 82 cm−1 corresponding to a surface-active mode, which is indicative of the nanocrystalline nature of the oxide. Magnetization studies show nearly temperature independent paramagnetism with high magnetic moment compared to bulk NiO. Cyclic voltammetric (CV) studies show well-resolved anodic peak (Ni+2→Ni+3+e−;) at 0.45 V at a scan rate of 1 mV/s, which shifts to 0.47 V at the scan rate of 100 mV/s. The cathodic peak (Ni+3+e−→Ni+2) is also observed at 0.34 V, which remains at the same position irrespective of the sweep rate. The quasireversible nature of the voltammograms suggests the usefulness of these materials for ultracapacitor applications

Cu-Co-Ni alloys: an efficient and durable electrocatalyst in acidic media

We have developed efficient nanostructures of Cu–Co–Ni alloy with varied stoichiometry as an alternative to the costly Pt-based alloys for hydrogen evolution reaction (HER). These nanoparticles were synthesized using the reverse micellar method. The size of the alloy nanoparticles varied from 40 to 70 nm. An enhanced catalytic activity as evident from high current density was observed for these Cu–Co–Ni (111) alloys which follows the Volmer–Heyrovsky mechanism. They have excellent stability (up to 500 cycles) and significant activity in acid media which might be due to the low hydrogen binding energy

Variable-temperature Infrared Spectroscopic Study of some Molybdenum Bronzes : Evidence for Electron-Phonon Coupling

An infrared vibrational spectroscopic study of some molybdenum oxide bronzes, commonly termed the &apos;blue&apos; bronzes, the &apos; red &apos; bronzes and the &apos; purple&apos; bronzes has been made. The charge density wave driven metal-tosemiconducting phase transition, mediated through electron-phonon coupling, is evidenced in the infrared spectra of the blue bronze studied as anomalous intensity behaviour on some of the internal modes, specifically in the lower-frequency Mo-0-Mo stretching region. Significant intensity differences are observed between these different classes of bronzes: a qualitative explanation for these differences is given. Non-topotactic chemical insertion of, amongst others, alkali metals into a host lattice, such as molybdenum trioxide (MOO,), yields a number of &apos;bronzes&apos;. The most common classes of molybdenum oxide bronzes are the &apos;blue&apos; bronzes Mov,Mo03 (M = K, Rb and Tl), the &apos;red&apos; bronzes Mo.,,MoO3 (M = Li, K, Rb, Cs and 7&apos; 1) and the &apos;purple&apos; bronzes M,~,Mo,017 (M = Li, Na, K and Tl). Specifically, this paper is concerned with the bronzes containing those cations which are shown in bold. X-Ray studies have shown that blue bronzes (M = K,&apos; Rb2 and T13) and the red bronzes (M = K : Rb,5 C S ,~ and T16&quot;) both form layered structures comprising infinite sheets of MOO, octahedra. In contrast, the purple bronzes (M = K7 and Na8) possess slab-like layered structures, each slab being formed from both MOO, octahedra and MOO, tetrahedra. Within a class of bronze, the structures are usually either identical or very similar. This is certainly true for those cations mentioned above. The exception, however, is the lithium ion; both Li,,,,MoO~ and Li,.,Mo,O~(: possess atypical structures and atypical physical properties of their particular classes. The blue bronze system, especially Ko.,MoO3, has been intensively investigated. Diffuse X-ray studies by Pouget et al. &apos; &apos; demonstrated a metal-to-semiconductor transition of the Peierls type towards an incommensurate semiconducting state at 180 K. Dumas et found that the blue bronze exhibited non-linear transport, suggesting that it was due to the sliding of the charge density wave (CDW) whilst Brusetti et al. &apos;, detected a large electrical anisotropy in the plane of the layered structure. Optical reflectivity measurements by Travaglini et all4 confirmed that the blue bronze is a quasione-dimensional metal above 180 K. In contrast, the red bronze (Ko.,~MOO,) is exclusively semic~nducting.&apos;~&quot;** The purple bronzes of Na, K and TI are somewhat different in that they are quasi-two-dimensional in their electrical properties and demonstrate a charge density wave driven metal-to-metal phase transition. On the other hand, Li,.,Mo,O, has been described as a quasi-one-dimensional metal, although recently reported X-ray absorption edge measurements indicate considerable dispersion in two dimensions.&apos; However, whereas the phase transition from the metallic to semiconducting state is independent of M in the blue bronzes, and has no effect on the semiconducting behaviour in the red bronzes, in the purple bronzes, the metal-to-metal phase transition is found to be dependent on the M ion (24, 80, and 120 K for Li,~,Mo,017,&apos;5&apos; N~,~,Mo,O,-,&apos;~~ and K ,~, M o ,~~ ,15e respectively). Vibrational spectroscopic studies of the bronzes are mostly limited to the blue bronzes, and in particular to Ko.3MoO3. Sat0 et aZ.17 first reported the existence of a soft phonon mode above the Peierls transition temperature whilst more accurate neutron data obtained by Pouget et al.&quot; yielded values for the soft mode frequency and damping coefficient as a function of temperature. Raman data&apos;, located the A + phase mode of the CDW below the phase transition temperature at ca. 6-7 meV (ca. 50 cm-&apos;). Reflectivity2&apos; measurements in the far infrared (ca. 2 meV, CQ. 16 cm-&apos;) showed the development of the CDW A -mode near the phasetransition temperature, gaining considerable intensity with decreasing temperature. More recently, a few studies of other bronzes have been reported. Massa2 &apos; described the temperature dependence of the Raman spectra of the blue bronze Rb,.,MoO,. Weak peaks centred around 950 cm-&apos; became Raman-active below 180 K, whilst a number of heavily screened phonons in the 525-675 cm-&apos; region were noted in the low-temperature spectrum. Jandl et ~1 .~~ reported an infrared reflectance study of Rbo.,MoO,. None of the phonons predicted by group theory were observed whilst in the metallic state (&gt; CQ. 180 K) although many bands became apparent at 9 K (the semiconducting state). We note, however, that their grouptheoretical prediction for the number of infrared vibrations is incorrect; their analysis appears to assume discrete MOO, units and uses a centred unit cell (vibrational analyses are always carried out using a primitive unit cell). have reported optical reflectivity measurements for MMo,O,, (M = Li, Na, K) at 300 and 6 K. The existence of charge-density waves in these materials has been investigated and the low dimensionalities of these compounds have been confirmed. A more complete review of the electrical and optical properties of the blue bronzes may be found in the review by Schlenker and Dumas24 whilst Greenblatt25 has given a review of structural, electrical, magnetic and thermal properties of many classes of bronzes and related Magneli structures. Degiorgio et Experimental Bronzes were grown by a gradient flux technique.26 Before use, the materials were washed thoroughly with 5% hydrofluoric acid and then annealed at 373 K for 12 h before being cooled to ambient temperature at a rate of 0.1 &quot;C min-&apos;. Infrared measurements were made with a Nicolet 740 infrared spectrometer using KBr as supporting matrix. Spectral resolution was 2 cm-&apos; for all measurements. Lowtemperature measurements were made using an Oxford Instruments cryostat using liquid nitrogen as refrigerant

Nanorods of manganese oxalate: a single source precursor to different manganese oxide nanoparticles (MnO, Mn<SUB>2</SUB>O<SUB>3</SUB>, Mn<SUB>3</SUB>O<SUB>4</SUB>)

Nanorods of anhydrous manganese oxalate were prepared by the reverse-micellar method using CTAB as the surfactant. Manganese oxalate precursor was used to synthesize single phase nanoparticles of various manganese oxides such as MnO, Mn2O3 and Mn3O4 under specific reaction conditions. Both MnO (28 nm) and &#945;-Mn2O3 (50 nm) are stabilized as cubic phase. &#945;-Mn2O3 shows a weak antiferromagnetic transition (TN = 80 K), while the spinel Mn3O4 (100 nm) particles show a ferrimagnetic transition at 43 K

Design of anisotropic Co₃O₄ nanostructures: control of particle size, assembly, and aspect ratio

Alignment of Co₃O₄ nanoparticles to yield a range of anisotropic nanostructures with a controlled aspect ratio (1:5 to 1:13) was possible from a single nanostructured precursor (cobalt oxalate) that was obtained by a microemulsion method under controlled kinetic parameters and the temperature of decomposition. The role of the cationic surfactant is critical for obtaining the anisotropic nanostructures of Co₃O₄ comprising an assembly of Co₃O₄ nanoparticles (diameter 5–50 nm). The shape (from spherical to highly elongated) and size of individual nanoparticles could be varied by the temperature of decomposition. Higher temperature led to elongated and larger particles. All of the nanorods showed antiferromagnetic behavior with a decrease in Neel temperature (TN) with decrease in the average size of the individual oxide nanoparticles. The variation of TN with the particle diameter follows the finite-size scaling relation. These nanostructures were found to be excellent electrocatalysts for oxygen evolution reaction (OER), with a high current density of 104 mA/cm²

Binary Fe−Co Alloy Nanoparticles Showing Significant Enhancement in Electrocatalytic Activity Compared with Bulk Alloys

Microemulsion-based synthesis of Fe−Co alloy nanoparticles has been reported for the first time. Spherical, uniform, and highly monodisperse nanoparticles of Fe75Co25, Fe67Co33, Fe50Co50, and Fe33Co67 with an average size of 20, 25, 10, and 40 nm, respectively, were synthesized. These nanoparticles crystallize in a body-centered cubic cell. A higher cobalt content led to the formation of biphasic mixtures. Energy-dispersive X-ray spectroscopy studies confirmed the Fe/Co ratios. Nanoparticles of the Fe33Co67 alloy show higher hydrogen and oxygen evolution efficiencies (over 100 times) compared with other Fe−Co alloys of nanocrystalline or bulk form. The Fe−Co alloy nanoparticles also show ferromagnetism

Development of a microemulsion-based process for synthesis of cobalt (Co) and cobalt oxide (Co<SUB>3</SUB>O<SUB>4</SUB>) nanoparticles from submicrometer rods of cobalt oxalate

Rod-shaped nanostructures of cobalt oxalate dihydrate were synthesized at room temperature by the microemulsion (reverse micellar) route. These rods are highly uniform in length and can be modified with temperature (from ~6.5 μm at 50 °C to ~2.5 μm at 150 °C) while keeping the diameter nearly constant (200-250 nm). Thermal decomposition of these rods in a controlled atmosphere (air and H2) leads to nanoparticles of Co3O4 and Co, respectively, while in a helium atmosphere a mixture of Co and CoO nanoparticles is obtained. Co3O4 nanoparticles (~35 nm) were slightly agglomerated, while Co nanoparticles were monodispersed and highly uniform (~25 nm). The oxalate rods and Co3O4 nanoparticles show an antiferromagnetic ordering at 54 and 35 K, respectively