Confined conversion of CuS nanowires to CuO nanotubes by annealing-induced diffusion in nanochannels
© Mu and He; licensee Springer. 2011
Received: 2 September 2010
Accepted: 16 February 2011
Published: 16 February 2011
Copper oxide (CuO) nanotubes were successfully converted from CuS nanowires embedded in anodic aluminum oxide (AAO) template by annealing-induced diffusion in a confined tube-type space. The spreading of CuO and formation of CuO layer on the nanochannel surface of AAO, and the confinement offered by AAO nanochannels play a key role in the formation of CuO nanotubes.
Well-aligned semiconductor one-dimensional (1D) nanostructures have attracted extensive attention in the last decade owing to their great potential in novel optoelectronic nanodevices, such as laser diodes, field effect transistors, light-emitting diodes, and sensors . Copper oxide (CuO) is a p-type semiconductor with a narrow band gap, and is a candidate material for photothermal and photoconductive applications [2, 3]. Moreover, it is potentially a useful component in the fabrication of sensors, field emitters, lithium-CuO electrochemical cells, cathode materials, and high Tc-superconductors [4, 5]. Its crystallinity, size, and shape and stoichiometry play a key role in these applications. Considerable efforts have been devoted to overcoming numerous challenges associated with efficient, controlled fabrication of these nanostructures via chemical or physical approaches. Thus far, well-aligned 1 D CuO nanostructures have been obtained using techniques such as thermal evaporation [2, 6], electrospinning , MOCVD , and sol-gel process . CuO nanowires were also prepared by conversion from their nanoscale analogs of copper hydroxide at elevated temperatures [10–14]. In this study, a novel approach for the preparation of CuO nanotubes via confined conversion from CuS nanowires by annealing-induced diffusion in nanochannels is reported.
Recently, prior studies including these of the authors have reported the preparation of metal sulfide nanowires by chemical precipitation in anodic aluminum oxide (AAO) channels under ambient conditions [15, 16]. In this article, the authors report on the synthesis of CuO nanotubes using CuS nanowires embedded in AAO as precursor. Not only the structure but also the morphology of product could be selectively controlled via this method. The conversion too was easily performed. This approach may be extended to the synthesis of various metal oxide nanotubes by annealing their precursor nanowires embedded in AAO template, and the precursor can be sulfides, carbonates, and oxalates, which can be readily transformed into oxides at elevated temperatures.
AAO templates used were prepared by aluminum anodic oxidation as described previously . In brief, electropolished aluminum foil was anodized in aqueous oxalic acid (4%) at a constant voltage of 40 V for several hours to prepare AAO templates of 50-nm pores using a H-type cell. After the anodization, the remaining aluminum was etched by a 20% HCl + 0.2 M CuCl2 mixed solution, and the barrier layer was dissolved by 5% phosphoric acid.
In a typical synthesis of CuS nanowires, one half-cell of the H-type cell was filled with aqueous (NH4)2S of 0.01 M, and the other was filled with aqueous CuSO4 of stoichiometric concentration. After reaction for 12 h, the AAO template embedded with CuS nanowires were detached and thoroughly washed with deionized water and subsequently annealed in muffle furnace in air at 650°C for 1-20 h.
Crystallographic and purity information on as-prepared metal sulfide nanowires were obtained using powder X-ray diffraction (XRD). The XRD analyses were performed using a Philip X'Pert PRO SUPER çA rotation anode with Ni-filtered Cu Kα radiation (λ = 1.5418 Å). Identical slit width and accelerating voltage were used for all the samples.
CuS nanowires and CuO nanotubes were observed on a field emission scanning electron microscopy (SEM) instrument (FE-SEM Leo 1550) operated at an acceleration voltage of 10 kV. The CuS nanowires and CuO nanotubes were recovered by dissolving the AAO membrane in 2 M aqueous NaOH for 2 h at room temperature. The products were obtained by centrifugation followed by washing three times with deionized water and dried in air. Samples were dropped onto silicon wafer which was ultimately attached onto the surface of SEM specimen stage. For the analysis of nanowire arrays, membranes were initially attached to a piece of silicon wafer by conductive double-sided carbon tape. They were immersed in 0.2 M aqueous NaOH for 1 h in order to partially remove the template, creating aligned nanowires/nanotubes. After washing with deionized water followed by air-drying, the specimens were subsequently mounted onto a SEM specimen stage for imaging.
Specimens for transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations were prepared by dropping the as-prepared nanowires/nanotubes onto carbon-coated copper grids followed by drying. TEM images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2100 TEM, and HRTEM images were obtained on a JEOL JEM-2100F TEM.
Results and discussion
In summary, CuO nanotubes were successfully converted from CuS nanowires embedded in AAO template by annealing-induced diffusion in a confined tube-type space. The spreading of CuO and formation of CuO layer on the nanochannel surface of AAO and the confinement offered by AAO nanochannels play a key role in the formation of CuO nanotubes. Preliminary results showed that the present conversion by annealing-induced confined diffusion of sulfide nanowires to oxide nanotubes might be readily extended to other precursors that can thermally decompose to form corresponding oxides, including carbonates and oxalates, and thus opening up a new viable route to prepare nanotubes of various oxides. Since the CuO nanotubes grew with the assistance of AAO template, their diameter and pore size could be feasibly tuned by changing the electrochemical parameters used during the fabrication of the AAO template. It is expected that such CuO nanotubes may offer exciting opportunities for applications in catalysis, electrochemistry, superconductivity, and super-hydrophobic coating. Furthermore, CuO nanotubes with large specific surface areas may also be applied in sensor applications.
anodic aluminum oxide
selected area electron diffraction
scanning electron microscopy
transmission electron microscopy
This study was supported by NSFC (Grant No. 21003142) and the Knowledge Innovation Program of the Chinese Academy of Sciences (CAS) (Grant No. KSCX2-YW-G-059).
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