Diffusion-Induced Void Evolution in Core–Shell Nanowires: Elaborated View on the Nanoscale Kirkendall Effect

In 2006, our group reported the fabrication of ultra-long single-crystal ZnAl₂O₄ spinel nanotubes, starting from ZnO/Al₂O₃ core–shell nanowires, involving the nanoscale Kirkendall effect. In this feature paper, we introduce our up-to-date results on the design of porous and hollow 1D nanostructures following this solid-state interfacial reaction route. In particular, we present our recent understandings on void evolution induced by the unbalanced diffusion. We first give a short introduction on the distinctions of core–shell nanowires in interfacial nanoreactions, in contrast to other systems. Then we discuss the roles of desorption, stress, and/or defects on void evolution and nanotube formation. Our results demonstrate that the stress- and defects-engineered diffusion mechanism can be used to specifically design hollow nanostructures, which will open up a new window in exploitation of the Kirkendall effect at the nanoscale.     

Fabrication of platinum nanoparticles and nanowires by electron beam lithography (EBL) and nanoimprint lithography (NIL): comparison of ethylene hydrogenation kinetics

Electron beam lithography (EBL), size reduction lithography (SRL), and nanoimprint lithography (NIL) have been utilized to produce platinum nanoparticles and nanowires in the 20–60-nm size range on oxide films (SiO₂ and Al₂O₃) deposited onto silicon wafers. A combination of characterization techniques (SEM, AFM, XPS, AES) has been used to determine size, spatial arrangement and cleanliness of these fabricated catalysts. Ethylene hydrogenation reaction studies have been carried out over these fabricated catalysts and have revealed major differences in turnover rates and activation energies of the different nanostructures when clean and when poisoned with carbon monoxide. The oxide-metal interfaces are implicated as important reaction sites that remain active when the metal sites are poisoned by adsorbed carbon monoxide.     

Morphology of aluminium oxide nanostructures after calcination of arc discharge Al–C soot

The morphology of nanostructures that were formed during the calcination of aluminium–carbon composite and synthesized by the method of electric arc spraying was studied. It was shown that based on the aluminium content in the sprayed electrode and the buffer gas pressure, nanostructures of different morphologies are formed: chains of γAl₂O₃ nanoparticles, hollow γAl₂O₃ nanoparticles, γAl₂O₃ nanotubes, and hollow nanoparticles with inner partitions.     

Synergetic effect of combined use of Cu–ZnO–Al2O3 and Pt–Al2O3 for the steam reforming of methanol

Commercial Cu–ZnO–Al₂O₃ catalysts are used widely for steam reforming of methanol. However, the reforming reactions should be modified to avoid fuel cell catalyst poisoning originated from carbon monoxide. The modification was implemented by mixing the Cu–ZnO–Al₂O₃ catalyst with Pt–Al₂O₃ catalyst. The Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture created a synergetic effect because the methanol decomposition and the water–gas shift reactions occurred simultaneously over nearby Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalysts in the mixture. A methanol conversion of 96.4% was obtained and carbon monoxide was not detected from the reforming reaction when the Pt–Al₂O₃ and Cu–ZnO–Al₂O₃ catalyst mixture was used.     

Fabrication of aluminum carbide nanowires by a nano-template reaction

Large quantities of aluminum carbide nanowires have been prepared by the in situ synthesis of carbon nanotubes within Al powder and heat treatment of the obtained nanotube/Al powder. Scanning and transmission electron microscopy, selected area diffraction, and X-ray powder diffraction have been used to characterize the heat-treated composite powders and the initial one. The results showed that the Al₄C₃ nanowires with diameters ranging from 7 to 25 nm and lengths ranging from 1 to 5 μm are single-crystal. Transmission electron microscopy images indicated that growth of the Al₄C₃ nanowires occurs initially by the formation of a thin, uniform carbide coating and that further growth proceeds by inward growth of this coating with a concomitant consumption of the carbon nanotube until a solid Al₄C₃ nanowire is formed. Reactive wetting kinetics between nanotubes and Al were believed to be responsible for the growth mechanism of Al₄C₃ nanowires.     

A new method of low-temperature methanol synthesis on Cu/ZnO/Al2O3 catalysts from CO/CO2/H2

A new synthesis method of low-temperature methanol proceeded on Cu/ZnO/Al₂O₃ catalysts from CO/CO₂/H₂ using 2-butanol as promoters. The Cu/ZnO/Al₂O₃ catalysts were prepared by co-impregnation of r-Al₂O₃ with an aqueous solution of copper nitrate and zinc nitrate. The total carbon turnover frequency (TOF), the yield and selectivity of methanol were the highest by using the Cu/ZnO/Al₂O₃ catalyst with copper loading of 5% and the Zn/Cu molar ratio of 1/1, which precursor were not calcined, and reduced at 493 K. The activity of the catalysts increased due to the presence of the CuO/ZnO phase in the oxidized form of impregnation Cu/ZnO/Al₂O₃ catalysts. The active sites of the Cu/ZnO/Al₂O₃ catalyst for methanol synthesis are not only metallic Cu but also special sites such as the Cu–Zn site, i.e. metallic Cu and the Cu–Zn site work cooperatively to catalyze the methanol synthesis reaction.     

CO2 Hydrogenation to Methanol Over Cu/ZnO/Al2O3 Catalysts Prepared by a Novel Gel-Network-Coprecipitation Method

A novel gel-network-coprecipitation process has been developed to prepare ultrafine Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from CO₂ hydrogenation. It is demonstrated that the gel-network-coprecipitation method can allow the preparation of the ultrafine Cu/ZnO/Al₂O₃ catalysts by homogeneous coprecipitation of the metal nitrate salts in the gel network formed by gelatin solution, which makes the metallic copper in the reduced catalyst exist in much smaller crystallite size and exhibit a much higher metallic copper-specific surface area. The effect of the gel concentration of gelatin on the structure, morphology and catalytic properties of the Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from hydrogenation of carbon dioxide was investigated. The Cu/ZnO/Al₂O₃ catalysts prepared by the gel-network-coprecipitation method exhibit a high catalytic activity and selectivity in CO₂ hydrogenation to methanol.     

CO/FTIR Spectroscopic Characterization of Pd/ZnO/Al2O3 Catalysts for Methanol Steam Reforming

An as-synthesized 8.8wt% Pd/ZnO/Al₂O₃ catalyst was either pretreated under O₂ at 773 K followed by H₂ at 293 K or under H₂ at 773 K to obtain, respectively, a supported metallic Pd° catalyst (Pd°/ZnO/Al₂O₃) or a supported PdZn alloy catalyst (PdZn/ZnO/Al₂O₃). Both catalysts were studied by CO adsorption using FTIR spectroscopy. For the supported PdZn alloy catalyst (PdZn/ZnO/Al₂O₃), exposure to a mixture of methanol and steam, simulating methanol steam reforming reaction conditions, does not change the catalyst surface composition. This implies that the active sites are PdZn alloy like structures. The exposure of the catalyst to an oxidizing environment (O₂ at 623 K) results in the break up of PdZn alloy, forming a readily reducible PdO with its metallic form being known as much less active and selective for methanol steam reforming. However, for the metallic Pd°/ZnO/Al₂O₃ catalyst, FTIR results indicate that metallic Pd° can transform to PdZn alloy under methanol steam reforming conditions. These results suggest that PdZn alloy, even after an accidental exposure to oxygen, can self repair to form the active PdZn alloy phase under methanol steam reforming conditions. Catalytic behavior of the PdZn/ZnO/Al₂O₃ catalyst also correlates well with the surface composition characterizations by FTIR/CO spectroscopy.     

Methanol Synthesis

Methanol, like ammonia, is one of the key industrial chemicals produced by heterogeneous catalysis. As with the original ammonia catalyst (Fe/K/Al₂O₃), so with methanol, the original methanol synthesis catalyst, ZnO, was discovered by Alwin Mittasch. This was translated into an industrial process in which methanol was produced from CO/H₂ at 400?°C and 200 atm. Again, as with the ammonia catalyst where the final catalyst which is currently used was achieved only after exhaustive screening of putative “promoters”, so with methanol, exhaustive screening of additives was undertaken to promote the activity of the ZnO. Early successful promoters were Al₂O₃ and Cr₂O₃ which enhanced the stability of the ZnO but not its activity. The addition of CuO was found to increase the activity of the ZnO but the catalyst so produced was short lived. Current methanol synthesis catalysts are fundamentally Cu/ZnO/Al₂O₃, having high CuO contents of?~60?% with ZnO?~?30?% and Al₂O₃?~?10?%. Far from promoting the activity of the ZnO by incorporation of CuO, the active component of these Cu/ZnO/Al₂O₃ catalysts is Cu metal with the ZnO simply being involved as the preferred support. Other supports for the Cu metal, e.g. Al₂O₃, MgO, MnO, Cr₂O₃, ZrO₂ and even SiO₂ can also be used. In all of these catalysts the activity scales with the Cu metal area. The original feed has now changed from CO/H₂ to CO/CO₂/H₂ (10:10:80), radiolabelling studies having provided the unlikely discovery that it is the CO₂ molecule which is hydrogenated to methanol; the CO molecule acts as a reducing agent. The CO₂ is transformed to methanol on the Cu through the intermediacy of an adsorbed formate species. These Cu/ZnO/Al₂O₃ catalysts now operate at?~230° and between 50 and 100 atm. This important step change in the activity of methanol synthesis has resulted in a significant reduction in the energy required to produce methanol. The “step change” however has been incremental. It has been obtained on the basis of fundamental knowledge provided by a combination of surface science techniques, e.g. LEED, scanning tunnelling microscope, TPD, temperature programmed reaction spectroscopy, combined with catalytic mechanistic studies, including radiolabelling studies and chemisorption studies including reactive chemisorption studies, e.g. N₂O reactive frontal chromatography.     

Tailoring UV emission from a regular array of ZnO nanotubes by the geometrical parameters of the array and Al2O3 coating

Self-aligned and equal-spaced zinc oxide (ZnO) nanotube arrays were fabricated with anodic aluminum oxide (AAO)-assisted growth and the ALD technique. The near band-edge (NBE) emission was strongly affected by the nanotube's geometrical parameters, such as a packing density and thickness of the nanotube walls. The NBE emission was further enhanced with Al₂O₃ coating. The effect was analyzed by X-ray photoelectron spectroscopy (XPS) and ascribed to the surface defect passivation and a ZnAl₂O₄ spinel formation. The NBE emission enhancement was greater in ZnO nanotubes with thicker walls. A smaller UV enhancement factor was explained by less uniform and integral Al₂O₃ coverage of the ZnO nanotubes with thinner walls; this, possibly induced a variation of the Al₂O₃ refractive index along the nanotubes. As a result, the optical conditions at the ZnO/Al₂O₃/air interfaces was changed and the light extraction efficiency was reduced in the latter samples.     

一维氧化铝基纳米材料的制备及其形成机理

采用二次阳极氧化法制备得到孔径为60 nm的氧化铝模板。将氧化铝模板放入NaOH溶液中腐蚀约4 min左右,制备得到了氧化铝纳米管、纳米线的混合物。将沉积有InSb纳米线的氧化铝模板放入NaOH溶液中进行腐蚀5～8 min后,制备得到了InSb/Al₂O₃纳米同轴电缆。在制备基础上,讨论了氧化铝纳米管、纳米线及InSb/Al₂O₃纳米同轴电缆的形成机理。     

Role of ZnO in Cu/ZnO/Al2O3 catalyst for hydrogenolysis of butyl butyrate

For hydrogenolysis of butyl butyrate (BB), a series of Cu/ZnO/Al₂O₃ catalysts with different metal compositions were prepared, and characterized by N₂O chemisorption for measuring Cu surface area and by chromatographic experiment for determining the heat of BB adsorption. As a result, the presence of ZnO in Cu-based catalysts was found to enhance the catalytic activity of Cu due to dual function of ZnO. The Cu surface area was linearly correlated with the butanol productivity, demonstrating that ZnO exerts the structural function in Cu/ZnO/Al₂O₃ catalysts. Additionally, the role of ZnO as a chemical contributor was revealed such that its presence leads to lower activation energy of the surface reaction, thus resulting in higher Cu catalytic activity obtained at a low temperature such as 200 °C. Consequently, optimizing the Cu/Zn ratio in Cu/ZnO/Al₂O₃ catalyst is required to tune its structural and chemical characteristics of Cu metals, and thus to obtain a higher activity on the hydrogenolysis reaction.     

A Simple Approach for the Formation of Oxides,Sulfides, and Oxide–Sulfide Hybrid Nanostructures

The synthesis of various types of metal oxide and metal sulfide nanostructures through the thermal decomposition of single-source molecular precursors is presented. By varying the reaction conditions, such as solvent, precursor concentration, temperature, and time, we were able to control the shape of Fe₃O₄ (rectangles and belt-like structures), CoO nanofibers, and SnO₂ nanowires. Two sulfides are also presented: CdS filling a vertically aligned ZnO nanowire array (to create a hybrid structure) and SnS wires. The formation of various phases of iron oxide and cobalt oxide is also shown.     

Synthesis of Al2O3 nanowires by heat-treating Al4O4C in a carbon-containing environment

A novel route was developed for the synthesis of Al₂O₃ nanowires by heat-treating Al₄O₄C via Si atom doping in a furnace with a carbon heater. The nanowires had dimensions of 20–60 nm in diameter and a few hundred microns in length, and were tunable by adjusting the heating temperature. Al₂O₃ nanowires exhibited a curved, or twisted, structure and the interface of the bending part had a defective microstructure. The study of their growth mechanism indicates that the Al₂O₃ nanowires grew by a vapour–liquid–solid (VLS) growth process.     

Catalytic growth of semiconductor micro- and nano-crystals using transition metal catalysts

The catalytic reaction concept was introduced in the growth of semiconductor micro- and nano-crystals. It was found that gallium nitride (GaN) micro- and nano-crystal structures, carbon nanaotubes, and silicon carbide (SiC) nanostructures could be efficiently grown using transition metal catalysts. The use of Ni catalyst enhanced the growth rate and crystallinity of GaN micro-crystals. At 1,100 ‡C, the growth rate of GaN micro-crystals grown in the presence of Ni catalyst was over nine times higher than that in the absence of the catalyst. The crystal quality of the GaN microcrystals was almost comparable to that of bulk GaN. Good quality GaN nanowires was also grown over Ni catalyst loaded on Si wafer. The nanowires had 6H hexagonal structure and their diameter was in the range of 30–50 nm. Multiwall nanotubes (MWNTs) were grown over 20Fe : 20Ni : 60Al₂O₃ catalyst. However, single wall nanotubes (SWNTs) were grown over 15Co : 15Mo : 70MgO catalyst. This result showed that the structure of CNTs could be controlled by the selection of catalysts. The average diameters of MWNTs and SWNTs were 20 and 10 nm, respectively. SiC nanorod crystals were prepared by the reaction of catalytically grown CNTs with tetrametysilane. Structural and optical properties of the catalytically grown semiconductor micro- and nano-crystals were characterized using various analytic techniques. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Synthesis of multi-walled carbon nanotube bundles on the Fe-Co-Mo/Al2O3 catalyst

The Fe-Co-Mo/Al₂O₃ catalyst was synthesized by ??wet combustion?? with the use of metal nitrates and citric acid. Multi-walled carbon nanotubes obtained by the catalytic pyrolysis of propylene were shown to grow on this catalyst in the form of bundles. The influence of process parameters on the properties of the nanotubes was studied.     

Diameter- and length-dependent self-organizations of multi-walled carbon nanotubes on spherical alumina microparticles

Multi-scale hybrid structures of multi-walled carbon nanotubes (MWCNTs) and micrometric alumina particles (μAl₂O₃) have been produced. The hybrid structures are obtained by in situ grafting carbon nanotubes (CNTs) on spherical μAl₂O₃ particles using an easy chemical vapor deposition method, without any pre-patterned catalyst treatment. The study of the influence of temperature and hydrogen ratio shows three regular hybrid structures defined according to CNT arrangement on μAl₂O₃. Furthermore, the organization modes demonstrate that the hybrid structures are strongly dependent on the diameter, length and area number density of CNTs. This dependency has been explained using a proposed nano-cantilever beam model. It evaluates the maximum deflection of one CNT due to weak van der Waals interactions. The model analysis shows that the MWCNT organizations are a result of varying competitive interactions between MWCNT rigidity and their attractive forces on the large curvature surface of μAl₂O₃. In addition, the influence of specific characteristics of μAl₂O₃ on hybrid structures is also discussed.     

Ni- and Fe-based catalysts for hydrogen and carbon nanofilament production by catalytic decomposition of methane in a rotary bed reactor

Four catalysts, consisting of Ni, Ni:Cu, Fe or Fe:Mo as the active phase and Al₂O₃ or MgO as a textural promoter, were tested for the catalytic decomposition of methane in a rotary bed reactor, obtaining both CO₂-free hydrogen and carbon nanostructures in a single step. Hydrogen yields of up to 14.4 Ndm³ H₂·(h·g_cat)^− 1 were obtained using the Ni-based catalysts, and methane conversions above 80% were observed with the Fe-based catalysts. In addition to hydrogen production, the Ni-based catalysts allowed the large-scale production of fishbone-like carbon nanofibres, whereas the use of the Fe-based catalysts promoted the production of carbonaceous filaments having a high degree of structural order, consisting of both chain-like carbon nanofibres and carbon nanotubes.     

Formation of carbon nanotubes from the carbon monoxide disproportionation reaction over Co/Al2O3 and Co/SiO2 catalysts

The ammonia method has been successfully used for preparing thermostable and well dispersed alumina‐supported catalysts with a surface average size of cobalt particle D _{s= 5.7} nm. The disproportionation reaction of CO over this Co/Al₂O₃ catalyst and a similar Co/SiO₂ catalyst leads to the formation of carbon nanotubes demonstrating the same morphology. The amount of nanotubes over Co/Al₂O₃, however, is much larger than that obtained over Co/SiO₂, because of a faster ageing in the latter solid. Similar support effects have already been reported for other catalytic reactions involving carbon oxides. This revised version was published online in July 2006 with corrections to the Cover Date.     

Modulation of wall thickness of Al-doped ZnO nanotubes by nanolamination of atomic layer deposition for oxygen sensing

The high surface-to-volume ratio and feature dimensions of the gas sensors are the key factors for improving the gas response. In this study, a novel method to prepare an Al-doped ZnO (AZO) nanotube oxygen sensor with tunable wall thickness is reported via the ZnO–Al₂O₃ nanolamination of atomic layer deposition (ALD) using tris(8-hydroxyquinoline) gallium nanowire (GaQ₃NW) as a template. The ALD of Al₂O₃ significantly enhances wall uniformity and decreases the wall thickness of the AZO nanotubes. In addition, the incorporation of Al₂O₃ allows full coverage of AZO on GaQ₃NWs. With an increase in the Al₂O₃ fraction, the carrier concentration increases, but the depth of the depletion layer and gas response of the nanotube sensor are reduced. The gas response of the nanotubes is inversely proportional to wall thickness, suggesting that it is a function of the surface-to-volume ratio. When the wall thickness is decreased to 12 nm, the gas response of AZO nanotubes with 2% Al increases significantly to 7. This can be explained by the grain control model, because thin wall leads to the formation of fully charge-depleted nanotubes.