A. N. Red’kin

Vapor phase synthesis of aligned ZnO nanorod arrays from elements

A vapor phase process is described for the synthesis of one-dimensional zinc oxide nanocrystal arrays. Using this process, well-aligned ZnO nanorod arrays are grown on (100) Si substrates with no catalyst. Cathodoluminescence, x-ray diffraction, and electron microscopy results demonstrate that the nanorods are characterized by high purity, stoichiometric composition, and perfect crystal structure.

Adsorption properties of carbon nanotubes depending on the temperature of their synthesis and subsequent treatment

A correlation between the temperature of the synthesis of carbon nanotubes (CNT) by method of catalytic pyrolysis from ethanol on a nickel catalyst and their ability for the modification of oxidation under various conditions and also their adsorption capacity to a number of metal ions is demonstrated. The study of infrared spectra in correlation with the total acidity of the CNT synthesized and modified under various conditions has shown that the samples synthesized at 400°C possess the maximum ability for modification. This can be explained by the higher deficiency of their structures at low synthesis temperatures. Adsorption isotherms of silver, copper, lead, cadmium, zinc, iron, and magnesium ions under batch and dynamic modes were studied, depending on pH and element concentration in the solution. It was found that CNT synthesized at 400°C and treated with conc. HNO3 in an autoclave at 110–120°C possess the maximum adsorption capacity of all the studied elements, which exceeds the capacity of active coal, traditionally used for these purposes, by several times. The attained adsorption capacity is 5–10 times higher than that reported in the literature for carbon nanotubes in relation to the same elements. Prospects of using CNT as collectors for the preconcentration of trace impurities in spectroscopic methods of analysis are considered.

Synthesis of fibrous carbon nanomaterials from ethanol vapor on a nickel catalyst

The processes underlying the synthesis of fibrous carbon nanomaterials via nickel-catalyzed pyrolysis of ethanol vapor are investigated. The IR spectra of the gas released from the reactor during synthesis indicate that, in the initial stages of the process, ethanol molecules decompose into simpler species: CH4, CO, and H2. In the temperature range 400–600°C, carbon monoxide disproportionation plays a key role in carbon deposition. Varying the process conditions, one can obtain carbon nanofibers of various thicknesses or carbon nanotubes.

Elemental vapor-phase synthesis of nanostructured zinc oxide

We systematize experimental data on the elemental vapor-phase synthesis of zinc oxide nanocrystal arrays on substrates. This process may yield nanostructures differing in shape and dimensions, in particular, well-aligned ZnO nanorod arrays. A model is proposed in which aligned zinc oxide nanorod arrays may grow by the vapor-liquid-solid (VLS) mechanism, and liquid zinc nanodroplets forming on the substrate surface at the beginning of the process catalyze one-dimensional growth. The VLS process is accompanied by zinc oxide deposition onto the lateral surface of the nanorods from the vapor phase. The relative rates of these processes influence the shape of the nanorods and the thickness of the polycrystalline underlayer. Optimizing the deposition conditions, one can grow uniform arrays of aligned high-quality ZnO nanorods with no catalysts and with no special substrate preparation steps.

Luminescence of CVD ZnO nanocrystals of different shapes

The effect of the shape and dimensions of zinc oxide nanocrystals on the threshold and directionality of UV lasing in the nanocrystals has been investigated. The results indicate that hexagonal columnar nanocrystals have the lowest optical threshold pump power at nanoresonator diameters of 100–200 nm, comparable to the absorption path length of the excitation light. The laser radiation is shown to have a mode structure, being directed predominantly along the prism axis. The lasing mechanism in prismatic nanocrystals differs from that in hexagonal-pyramidal nanocrystals. Electron microscopy results show that the ZnO nanocrystals are highly uniform in shape and size and that these parameters depend on the chemical vapor deposition conditions.

Luminescence of ZnO nanorods grown by chemical vapor deposition on (111) Si substrates

ZnO nanorods have been grown on (111) Si substrates by chemical vapor deposition in a horizontal reactor, with no catalyst. The nanorods grown far from the outlet end of the reactor are larger in size, have a higher structural perfection, and exhibit more efficient room-temperature edge luminescence in comparison with the nanorods grown at the outlet end. The low-temperature cathodoluminescence spectrum of the nanorods also depends on their position in the reactor during growth, which is interpreted in terms of the density of native defects. The nanorods exhibit room-temperature stimulated emission in the excitonic spectral region.

Quartz crystal microbalance determination of vapors of volatile organic compounds on carbon nanotubes under batch conditions

Specific features of the sorption of vapors of monoatomic aliphatic alcohols and aromatic compounds on coatings formed from carbon nanotubes of various genesis studied by the method of piezoelectric micro weighing are considered. The morphology of the coatings is considered in dependence on the conditions of synthesis and aftertreatment of carbon nanotubes. A new method of the processing of signals from an array piezoelectric sensors is proposed; it allows an increase in the selectivity of the determination of volatile organic compounds in an equilibrium gas phase by 2–2.5 times. The advantages of the application of carbon nanotubes as sorption coatings in comparison to the standard polymeric sorbents for piezoelectric micro weighing are demonstrated: the detection limit decreases by 4–15 times and the duration of measurement by 5–55 times.

Effect of chemical etching on the luminescent properties of zinc oxide nanorods

Vertically aligned ZnO nanorods grown on (100) Si substrates have been found to have a polycrystalline zinc oxide underlayer. After etching in a hydrochloric acid solution, the nanorods had smaller dimensions and pointed ends. As shown by exciton spectroscopy, the nanorods had a higher structural perfection and better luminescent properties in comparison with the underlayer. The 4.2-K luminescence spectra of the nanorods exhibit violet emission due to bound and interacting excitons. The low-temperature edge emission of the ZnO underlayer is shown to consist of lines due to electron-hole plasma recombination.

Electrical conductivity and optical properties of thin carbon films grown from ethanol vapor

Transparent, ultrathin carbon films have been grown through the pyrolysis of ethanol vapor at a reduced pressure on copper substrates at temperatures from 600 to 950°C. The electrical conductivity of the films increases with deposition temperature. Depending on deposition temperature, ethanol vapor pyrolysis may follow different mechanisms and the carbon deposition process has different key features, which influence the properties of the films. In the range 600–750°C, ethanol vapor pyrolysis is a catalytic process, which results in selective growth of a thin carbon film with an optical transmittance of ∼95% only on the copper surface. At higher temperatures, carbon deposition is nonselective, and the resultant films are darker. The carbon deposition mechanism is discussed in relation to the ethanol vapor pyrolysis temperature. The present results suggest that carbon deposition from ethanol vapor is a promising approach to producing transparent, conductive carbon films.

Vapor phase synthesis of nanofibrous carbon materials from water-ethanol mixtures

We present a detailed study of the pyrolytic synthesis of nanofibrous carbon materials (NCMs) from ethanol and various water-ethanol mixtures on a nickel catalyst at temperatures from 400 to 700°C. In the synthesis from 96% ethanol, the initial deposition rate increases with temperature, but this is accompanied by a substantial decrease in catalyst life. The addition of water to the reaction system considerably increases the catalyst life. One possible reason for this is that water vapor prevents carbonization of the catalyst particles. At the same time, above 600°C the rate of NCM deposition from a 50% water-ethanol mixture is considerably slower. At 550°C and lower temperatures, the rate of NCM deposition from water-ethanol mixtures (based on ethanol consumption) changes insignificantly. Our results demonstrate that NCMs can be synthesized even from very dilute (down to 10 vol %) aqueous ethanol solutions. The participation of water vapor in chemical processes that take place in the reaction zone is discussed.