Ahmad Umar

Zinc oxide nanonail based chemical sensor for hydrazine detection

Using ZnO nanonails, a hydrazine electrochemical sensor has been fabricated, for the first time, which showed a high and reproducible sensitivity of 8.56 microA cm(-2) microM(-1) with a response time less than 5 s, a linear range from 0.1 to 1.2 microM and a correlation coefficient of R = 0.999. The limit of detection (LOD), based on the S/N ratio, was estimated to be 0.2 microM.

Growth of aligned ZnO nanorods and nanopencils on ZnO/Si in aqueous solution: growth mechanism and structural and optical properties

Well-aligned ZnO nanorod and nanopencil arrays were synthesized in a high density on ZnO/Si substrate by a low-temperature aqueous solution technique. Detailed structural characterizations revealed that the as-synthesized nanorods and nanopencils were single crystalline, with a hexagonal phase, and with growth along the [0001] direction. The room-temperature photoluminescence spectra showed a strong ultraviolet emission at 381 nm, a weak blue band at 460 nm, and a broad green emission at 580 nm. A detailed growth mechanism has been proposed for the formation of nanorods and nanopencils based on the different crystallographic habits of wurtzite hexagonal ZnO.

ZnO nanosheet networks and hexagonal nanodiscs grown on silicon substrate: growth mechanism and structural and optical properties

Two-dimensional ZnO nanosheet networks composed of many thin and uniform hexagonal-shaped ZnO nanosheets and ZnO nanodiscs were prepared in a large scale on silicon substrate through thermal evaporation using ZnCl2 and O2 as source materials for Zn and oxygen, respectively, without the use of metal catalysts or additives. Detailed structural studies indicated that the synthesized products are single crystalline with wurtzite hexagonal structure. Raman scattering of the synthesized products confirmed that the as-grown structures have good crystal quality with a hexagonal wurtzite phase. Room temperature photoluminescence spectra showed a strong green band with a suppressed UV emission from the ZnO nanosheet networks, but on the other hand a dominant and strong near band edge emission with a much suppressed deep level emission was observed in the nanodiscs. The growth mechanism of these structures is also discussed in detail.

Ultra-sensitive hydrazine chemical sensor based on high-aspect-ratio ZnO nanowires

High-aspect-ratio ZnO nanowires based ultra-sensitive hydrazine amperometric sensor has been fabricated which showed a high and reproducible sensitivity of 12.76 microAcm(-2)nM(-1), detection limit, based on S/N ratio, 84.7 nM, response time less than 5s, linear range from 500 to 1200 nM and correlation coefficient of R=0.9989. This is the first report in which such a very high-sensitivity and low detection limit has been achieved for the hydrazine sensors by using ZnO nanostructures modified electrodes. Therefore, this work opens a way to utilize simply grown ZnO nanostructures as an efficient electron mediator to fabricate efficient hydrazine sensors.

Flower-shaped ZnO nanostructures obtained by cyclic feeding chemical vapour deposition: structural and optical properties

Flower-shaped ZnO nanostructures were grown on Si(100) and Si(111) substrates by cyclic feeding chemical vapour deposition without the use of a metal catalyst. The structures obtained on the Si(100) substrate exhibited triangle-shaped leaves with lengths and diameters in the ranges 300-400 and 90-130 nm, respectively. Uniformly shaped leaves with hexagonal facets originating from one centre were observed on the flower-shaped structures grown on the Si(111) substrate. Transmission electron microscopy and selected area electron diffraction patterns showed that these structures are highly crystalline, with the wurtzite hexagonal phase, and preferentially oriented in the c-axis direction. Unlike star-shaped ZnO nanostructures grown on Au-coated silicon substrates, the flower-shaped nanostructures showed sharp and strong UV emission at 378 nm and broad and weak green emission at 520 nm, indicating a good crystal quality and few structural defects.

Optical and field emission properties of single-crystalline aligned ZnO nanorods grown on aluminium substrate

Optical and field emission (FE) properties of aligned single-crystalline ZnO nanorods, grown on aluminium substrate at 530 °C by the non-catalytic thermal evaporation process, have been examined. Raman-scattering and room-temperature PL spectra exhibit a strong and sharp optical phonon E2 mode at 437 cm−1 and a strong ultraviolet emission at 381 nm, respectively. The FE characterization shows that a turn-on field for the vertically aligned nanorods was 5.8 V µm−1 and the emission current density reached to 0.061 mA cm−2 at an applied electrical field of 9.0 V µm−1 and shows no saturation. The field enhancement factor β was estimated, from the F–N plot, to be about ~2.081 × 103.

Growth and optical properties of large-quantity single-crystalline ZnO rods by thermal evaporation

Large-quantity synthesis of single-crystalline perfectly hexagonal-shaped ZnO rods has been achieved through the thermal evaporation of metallic zinc powder in the presence of oxygen without the use of metal catalyst or additives. Detailed structural analyses confirm that the structures formed exhibit a wurtzite hexagonal phase and are preferentially oriented along the c-axis direction. Low-temperature photoluminescence shows that the exciton emission bound-to-neutral donor (D0X) and free-exciton dominate the edge emission region while the donor–acceptor pair (DAP) and its replica dominate the lower energy regions. With increased temperature, D0X decreases abruptly and is then suppressed, but with further increase in temperature (>100 K) the photoluminescence spectra are dominated by the DAP.

High-sensitive glutamate biosensor based on NADH at Lauth's violet/multiwalled carbon nanotubes composite film on gold substrates.

A highly sensitive amperometric L-glutamate biosensor based on the electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide has been developed on Lauths Violet (known as thionine)/multiwalled carbon nanotubes (Th-MWCNTs) composite film, which is used as a mediator and an enzyme immobilization matrix. The glutamate biosensor, which is fabricated by immobilizing glutamate dehydrogenase (GLDH) on the surface of Th-MWCNTs, displayed a precipitous response (ca. 3 s), a low detection limit (15.9 nM), a wide linear dynamic range (0.1 to 500 microM), and high sensitivity of 281.6 microAmM(-1) cm(-2), higher biological affinity, as well as good stability and repeatability. Interferences from other biological compounds were also studied for the fabricated sensor. The Th-MWCNTs system exemplifies a simple and efficient approach to the assimilation of GLDH and electrodes, which can provide analytical access to a large group of enzymes for wide range of bioelectrochemical applications in health care fields.

Synthesis of ZnO nanowires on steel alloy substrate by thermal evaporation: Growth mechanism and structural and optical properties

ZnO nanowires having a diameter in the range of 15–40 nm and several tens of micrometers in length were grown on steel alloy substrates by the thermal evaporation technique without the use of any catalyst or additives. A detailed structural analysis revealed that the as-grown ZnO nanowires are single crystalline with wurtzite hexagonal structures and preferentially oriented in the c-axis direction. Origination of a strong and sharp Raman-active E2 mode at 436.6 cm-1 indicated that the grown ZnO nanowires have good crystal quality with the hexagonal wurtzite phase. Photoluminescence spectra also exhibited a sharp and strong peak in UV and a suppressed and weak band in the visible region, confirming the good optical properties and less structural defects for the deposited products. Additionally, a systematic growth mechanism is also proposed in detail to acquire a better understanding for the growth of nanowires on steel alloy substrate.

Complex nanostructures of ZnO: growth and properties

Variety of complex ZnO nanostructures such as flower-shaped structures, hierarchical and star-shaped nanostructures have been grown by the novel cyclic feeding chemical vapour deposition (CFCVD) process on various substrates at low-temperatures of 475°C to 550°C. Metal organic source, diethyl zinc (DEZn) and oxygen gas was used as source materials for zinc and oxygen, respectively for the growth of ZnO nanostructures synthesised by CFCVD process. The selected area electron diffraction (SAED) pattern of the flower-shaped structures confirmed that the grown products are single-crystalline ZnO. In addition to the ZnO nanostructures grown CFCVD process, comb-like ZnO structures were also synthesised, in a high density, via simple thermal evaporation process by using metallic zinc powder and oxygen as source materials for zinc and oxygen, respectively. The X-ray diffraction pattern of the synthesised comb-like structures exhibited that these structures are possessing single-crystallinity and wurtzite hexagonal phase of ZnO.