Zinc Oxide—From Synthesis to Application: A Review - MDPI
Two organic structure-directing agents (sodiumtartarate and trisodium citrate) were used to synthesizeCoO nanoparticles with differentmorphologies. The calcinations of the cobalt-tartarate complexproduced block morphologies of CoO, whereasthose of cobalt-citrate complexes produced nanospheres. The changein morphologies may be due to the varying coordination of organicstructure-directing ligands. Cobalt with citrate ligands formwater-soluble coordination complexes and display variedcoordination modes depending on the conditions (). Conversely, the addition of tartarateimmediately produces water insoluble precipitates (). The coordination differences oforganic ligands have previously been used to modulate themorphologies of nanoparticles ().The crystal structures of the products were verified by PXRD. shows the PXRD pattern ofCoO-NP-B andCoO-NP-S. The diffraction peaks typical offace-centered cubic CoO are evident, andare concordant with those of standard CoOcubic structures (JCPDS card no. 42–1467) (). The absence of impurity peaksindicated the formation of pure CoO. Themarked intense single PXRD peak at 220 planes forCoO-NP-B indicated adequate crystallinityof the sample with flat morphology. The PXRD ofCoO-NP-S also determined the crystallinity,although this was decreased compared withCoO-NP-B, and the increased number of peakswas due to the various crystallographic faces present in spheremorphologies. shows theFE-SEM images of the CoO productsCoO-NP-s andCoO-NP-B synthesized by the calcination ofcitrate () and tartarate() cobalt complexes,respectively. CoO-NP-S showed the presenceof aggregated spherical nanostructures. The high magnificationimage shows the aggregation of many smaller spheres (40–60 nm)().CoO-NP-B exhibits clear blocks with 100–200nm thickness and a length of ≥1 µm. The high magnification imageshows small pores on the sides of the blocks ().
Nano Materials Chemistry Lab. - 성균관대학교
Cobalt oxide (CoO) is ananti-ferromagnetic p-type semiconductor (with direct optical bandgaps at 1.48 and 2.19 eV), which is considered to be among the mostpromising functional materials as it has gas-sensing, catalytic andelectrochemical properties. CoO hastherefore been widely investigated for its potential application insolid state sensors, electrochromic devices and heterogeneouscatalysts, as well as in lithium batteries (–).Nanostructured CoO materials weredemonstrated to have magnetic, optical, field emission andelectrochemical properties that are used in various devices(,–).Notably, the shape and size-dependent properties of inorganicnanomaterials have been the subject of numerous investigations,with the aim of synthesizing CoOnanomaterials with controlled size and shape (–).Several techniques have been reported for the synthesis ofCoO nanostructures with differentmorphologies, including the nano-casting method for producingCoO nanowires (), the surfactant-based template approachfor constructing CoO nanoboxes (), the mechano-chemical reaction methodfor the synthesis of CoO nanoparticles(), and the thermal decompositionand oxidation method for the growth of CoOnanorods (). Cobalt nanowires areconverted to CoO nanotubes by calcinationat 600°C in air for an extended period of time (), followed by heating of a cobalt foilfor 12 h in air at 350°C which leads to the growth ofCoO nanowalls (). This nanostructuredCoO has been investigated for a range ofshape and size-dependent properties.
(ZnO) nanorod, also known as , has a direct of 3.37 , which is similar to that of , and it has an excitation of 60 meV. The optical bandgap of ZnO nanorod can be tuned by changing the morphology, composition, size etc. Recent years, ZnO nanorods have been intensely used to fabricate nano-scale electronic devices, including , ultraviolet , , and ultra-bright (LED). Various methods have been developed to fabricate the single crystalline, ZnO nanorods. Among those methods, growing from vapor phase is the most developed approach. In a typical growth process, ZnO vapor is condensed onto a solid substrate. ZnO vapor can be generated by three methods: thermal evaporation, chemical reduction, and (VLS) method. In the thermal evaporation method, commercial ZnO powder is mixed with SnO2 and evaporated by heating the mixture at elevated temperature. In the chemical reduction method, zinc vapor, generated by the reduction of ZnO, is transferred to the growth zone, followed by reoxidation to ZnO. The VLS process, originally proposed in 1964, is the most commonly used process to synthesize single crystalline ZnO nanorods. In a typical process, catalytic droplets are deposited on the substrate and the gas mixtures, including Zn vapor and a mixture of CO/CO2, react at the catalyst-substrate interface, followed by nucleation and growth. Typical metal catalysts involve , , , and . ZnO nanowires are grown epitaxially on the substrate and assemble into monolayer arrays. Metal-organic chemical vapor deposition () has also been recently developed. No catalyst is involved in this process and the growth temperature is at 400 ~500 °C, i.e. considerably milder conditions compared to the traditional vapor growth method. Moreover, metal oxide nanorods (ZnO, CuO, Fe2O3, V2O5, others) can be simply made by heating initial metal in air in a process. For example, to make a dense "carpet" of CuO nanorods it was found to be enough to heat Cu foil in air at 420 °C.