Eugene Oh

Precursor Effects of Citric Acid and Citrates on ZnO Crystal Formation

We have studied the precursor effects of citric acid and various citrates-including triethyl citrate, tripotassium citrate, trisodium citrate and triammonium citrate-on the formation of ZnO crystals in alkaline solution. These citrate-related chemicals could be divided into three groups (group A, triethyl citrate; group B, tripotassium citrate and trisodium citrate; and group C, citric acid and triammonium citrate) based on their activity for modifying the ZnO growth direction and solution pH dependency on their concentration. We could obtain ZnO structures with various distinct morphologies by simply changing the concentration of citric acid or citrate additive dissolved in the alkaline reaction solution. On the basis of the results, we propose the growth mechanisms underlying the formation of the various ZnO structures in the absence and presence of citric acid or citrate additives.

Synthesis of hierarchical hexagonal zinc oxide/zinc aluminium hydroxide heterostructures through epitaxial growth using microwave irradiation

We present a simple, fast and reproducible method for synthesising zinc oxide (ZnO)/zinc aluminium hydroxide (ZnAl : LDH) hierarchical heterostructures with various secondary ZnO morphologies. ZnAl : LDHs, support materials, were synthesised by microwave irradiation at 95 °C for 20 min. Microwave-assisted secondary growth formed ZnO nanorod/ZnAl : LDH heterostructures through introducing the as-produced ZnAl : LDHs into the reaction solution. ZnO nanotube/ZnAl : LDH heterostructures were synthesised by aging the ZnO nanorod/ZnAl : LDH structures. 2D ZnO film/ZnAl : LDH sandwich-like heterostructures were formed by secondary ZnO growth in a citrate anion-containing solution. The secondary structures grew epitaxially and perpendicularly along the [0001] direction (c-axis orientation) on the ZnAl : LDH surfaces. On the basis of the results, we propose the mechanisms for the epitaxial growth of the various ZnO structures on the ZnAl : LDH surfaces.

Turning refuse plastic into multi-walled carbon nanotube forest

Abstract A novel and effective method was devised for synthesizing a vertically aligned carbon nanotube (CNT) forest on a substrate using waste plastic obtained from commercially available water bottles. The advantages of the proposed method are the speed of processing and the use of waste as a raw material. A mechanism for the CNT growth was also proposed. The growth rate of the CNT forest was ∼2.5 μm min−1. Transmission electron microscopy images indicated that the outer diameters of the CNTs were 20–30 nm on average. The intensity ratio of the G and D Raman bands was 1.27 for the vertically aligned CNT forest. The Raman spectrum showed that the wall graphitization of the CNTs, synthesized via the proposed method was slightly higher than that of commercially available multi-walled carbon nanotubes (MWCNTs). We expect that the proposed method can be easily adapted to the disposal of other refuse materials and applied to MWCNT production industries.

Selective Synthesis of SiC and SiOx Nanowires by Direct Microwave Irradiation

We report a facile and novel method for the selective growth of SiC and SiOx nanowires by direct microwave irradiation on a Si substrate under atmospheric pressure. Selective synthesis of the amorphous SiOx or crystalline SiC nanowires was achieved by varying the gas composition in the reactor. The proposed method requires a short reaction time (2 min) with no need for vacuum equipment because the reaction is carried out under atmospheric pressure. High-resolution transmission electron microscope images revealed that the SiOx nanowires were amorphous and that the SiC nanowires were crystalline. Raman spectra showed features typical of nano-sized SiC. On the basis of our results, we propose a growth mechanism of SiC and SiOx nanowires synthesized by direct microwave irradiation.

NO sensing characteristics of ZnO nanorod prepared by ultrasound radiation method

ZnO sensors were fabricated from ZnO nanorods prepared by ultrasound radiation method and their gas sensing properties were investigated for NO, (CH3)3N, H2S, CO and CH3SH. In a procedure, zinc nitrate hydrate [Zn(No3)2ldr6H2O] and hexamethyleneteramine [C6H12N4,] were dissolved in deionized water and then the solution was irradiated with high intensity ultrasound radiation for lh by employing a direct immersion titanium horn. The lengths of ZnO nanorod were from 250 nm to 500 nm and the diameters were from 40 nm to 80 nm. The size of ZnO nanorod can be controlled by concentration of solution. The sensing characteristics of these nanostructures were investigated for three kinds of sensor fabricated by different concentrations (0.01, 0.005 and 0.001 M) of solution. We observed that the property of sensors was influenced by the morphology. Crystal structure and ceramic microstructure of ZnO nanorod were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM).