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Fabrication of Barbed-Shaped SnO@SnO2 Core/Shell Nanowires Arunas Jagminas,*,† Francisco M. Morales,‡ Kestutis Mazeika,† Giulio P. Veronese,§ Jonas Reklaitis,† Juan G. Lozano,‡ Jose M. Manuel,‡ Rafael García,‡ Marija Kurtinaitien_e,† Remigijus Jusk_enas,† and Dalis Baltrunas† †
State Research Institute Center of Physical Sciences and Technology, Savanoriu 231, LT-02300 Vilnius, Lithuania Dpto Ciencia de los Materiales e Ingeniería Metalurgica y Química Inorganica, University of Cadiz, 11510-Puerto Real-Cadiz-Spain § Istituto per la Microelettronica ed i Microsistemi del CNR, via Gobetti 101, 40129 Bologna, Italy ‡
ABSTRACT: The interest in core/shell nanowires (Nws) is stimulated by the importance of fabricating materials possessing unique magnetic, catalytic, or optical properties relative to ones of their bulk parents. Here, the fabrication of a novel design of core-shell SnO@SnO2 Nws via heat treatment of Sn Nws, fully encapsulated inside the pores of alumina matrix, is demonstrated. Under this scenario, densely packed arrays of barbed-shaped Nws, possessing extremely high surface areas, have been fabricated. 119 Sn M€ossbauer effect spectroscopy, high-resolution transmission electron microscopy (HRTEM), field-emission scanning electron microscopy (FESEM), and X-ray diffraction spectroscopy (XRD) were used to study the morphology and composition of the end-products. In addition, electron energy-loss spectroscopy (EELS) was successfully applied here to detect the ionization edges of tin and oxygen in the core and shell part of single Nw.
1. INTRODUCTION During the recent two decades, nanometer-scaled crystals of almost any binary semiconductor compound have been fabricated demonstrating that their physical and chemical properties are drastically different from their bulk counterparts.1-7 Besides the disparity in size, change in shape also results in different electronic states and energy band gaps in semiconducting nanospecies.8 Nanowires constitute a separate group of 1D nanostructures finding prospective applications in novel optical, electronic, and magnetic devices. The protocols for fabrication of most nanowired metals, semiconductors, and conductive polymers with well-controlled dimensions have already been proposed using a variety of techniques such as electrodeposition, chemical vapor deposition, electron beam evaporation, molecular beam epitaxy, metal-organic chemical vapor deposition (MOCVD), sol-gel, and so forth.9-12 Recently, a great consideration was devoted to Nws composed of more than one solid phase13-16 because the modification of the Nws surface opened new perspectives to their exciting applications in sensing and optoelectronic devices, nanomedicine, solar and fuel cells. In particular, of special interest are core/shelled Nws in which the core and shell thicknesses can be independently varied.17-20 Such Nws offer a number of advantages over Nws based on single compounds or alloys. For example crystalline-Si core/amorphous-Si shell Nws have demonstrated enhanced photoconversion efficiency,19 Ge Nws coated with carbon sheath possess a significant melting point reduction allowing us to form higher quality defect free Nws,21 and Ge@Si Nws can be used to develop gigahertz transistors due r 2011 American Chemical Society
to the high-mobility carrier gas accumulation at their radial heterojunction.22 In this way, the physicochemical properties of semiconducting Nws (band gap, NIR absorption, etc.) can be tuned by varying the diameter of their core and the thickness of shells. Tin oxides have in particular attracted much attention due to their broad applications in the fabrication of photocatalysts and selective gas sensors,23,24 lithium secondary batteries,25,26 optical waveguides,27 heat mirrors, and electrochromic windows. 28 Moreover, because of their chemical stability and wide band gap at room temperature (3.6-4.0 eV29), SnO2 thin films are promising for high-efficiency solar cells,30 short wavelength optoelectronics,31 and blue photoluminescence devices.32 Additionally, tin-based oxides are promising candidates to replace carbon-based anode materials because of their large capacity for lithium insertion by the formation of a Li4.4Sn alloy.33 During the past decade, SnO2 nanotubes,34 nanoparticles and nanorods,35 and nanobelts36 with new and prospective nanometer-scaled properties have been fabricated by some of the previously commented methods such as sol-gel process,37 hydrothermal synthesis,35,36 CVD,38,39 MOCVD,40 etching of tin foils in alkaline solution,41 or shape-preserving thermal oxidation of Sn0 Nws.42,43 It is worth noticing that a large surface area of the Nws is a key factor, allowing us to generate a strong
Received: November 9, 2010 Revised: January 12, 2011 Published: February 28, 2011 4495
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response of intercalating ions in SnO2 gas sensors and doublelayer charging. Core-shelled SnO2 Nws filled with various materials have also been created and investigated since the electrical conductivity of nanocrystalline SnO2 depends strongly on its surface states resulting in the space-charge and band modulation.43-47 The common way to produce core/shell Nws containing crystalline SnO2 is the oxidation of Sn Nws by heat treatment in oxygen environment. As it has been demonstrated, this procedure results only in shape preserving SnO242 or SnO@SnO243 Nws formation. However, there is a great need to develop reliable synthesis methods of obtaining densely packed SnO2 Nw arrays with a high surface area and hence new properties for eventual applications. Here, we report a route for the fabrication of novel design barbed-shaped Nw arrays of metal oxides, in particular SnOcore/SnO2-shell (SnO@SnO2). It is based on the filling of alumina template pores with metallic tin by electrodeposition, a subsequent sealing in boiling water of the alumina pores, which remained empty after tin deposition thus encapsulating Sn Nws, and a later heat treatment of such alumina matrix at a proper protocol. Hence, besides a host material for the deposition of densely packed metal Nws, the alumina surroundings act as a source of oxygen for the generation of a core and a shell with different states of metal oxidation, particularly SnO@SnO2. Contrary to other reports, that presented shape-preserving thermal oxidation of free-standing42 and not fully encapsulated43 tin Nws, this fabrication scenario allows us to form arrays of mechanically stable semiconducting Nws possessing a novel design and extremely high surface area with predetermined size and spacing. The composition and morphology of the obtained initial and end-products are studied in detail by 119Sn M€ossbauer spectroscopy (MS), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM), field-emission scanning electron microscopy (FESEM), and X-ray diffraction spectroscopy (XRD). In addition, electron energy-loss spectroscopy (EELS) was successfully applied here to detect the Sn and O ionization edges and hence the distinct regions of compositions for single Nws.
overgrowth of powdered tin onto the specimen surface began. For the preparation of the specimens, devoted to M€ossbauer spectroscopy investigations, the deposition solution additionally contained ∼10 wt % of 119Sn isotope salt. The alumina templates, encased with Sn Nws, were then sealed in boiling water for 45 min, fully encapsulating the deposited material and annealed. Thermal treatment was conducted in an open programmable oven (Zhermack) at temperatures ranging between 300 and 600 ((2) °C for the time of 0.5 to 3 h using a 10°/min ramping. The mechanical thinning and Arþ ion milling were performed in preparations of bulk solid specimens before alumina etching for high-resolution TEM observations, whereas free-standing Nws after template dissolution and washings were dispersed in ethanol and deposited on a grid covered with holey carbon for TEM inspections. Characterizations. The shape, composition and microstructure of Sn Nws encapsulated within the alumina pores were investigated before and after heat treatment in cross sections and planar views of the template, and following its dissolution, using a FESEM microscope (model LEO 1530) and a JEOL 2010FEG TEM microscope operating at 72 and 200 keV, respectively. The latter is coupled to an EELS spectrometer, model GIF 2002, and the spectra were collected in STEM mode with a collection semiangle of 24.36 mrad. The energy resolution, estimated from the full width at half-maximum of the zero loss peak, is 1.2 eV. X-ray diffraction studies were performed using a diffractometer D8 (Bruker AXS, Germany) equipped with a G€obel mirror (primary beam monochromator) for CuKR radiation. A step-scan mode was used in the 2Θ range from 15 to 60° with a 0.04° step and a counting time of 5 to 30 s per step. The alumina templates intended for XRD investigations were separated from the electrode surface by one-side sequential etching of the electrode window in a solution of 1.5 M NaOH and then in 0.1 M CuCl2 þ 10% HCl. M€ossbauer spectra (MS) were recorded in a constant acceleration mode using a spectrometer Wissenschaftliche Elektronik GMBH and a Ca119SnO3 source. The velocity scale was calibrated relative to CaSnO3. All experimental spectra were fitted to Lorentzian lines using a least-squares fitting algorithm.
2. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION
Preparation. All chemicals were chemical grade reagents from Aldrich and used as received. Deionized water obtained from a Milli-Q Millipore water system was used for the preparation of all solutions and washings. High-ordered structure alumina templates were produced in laboratory using the twostep anodization technique proposed by Masuda and Fukuda.48 The anodizing was carried out either in 0.5 M H2SO4 at 25 V and 3 °C for 2 h or in 1.2 M H2SO4 at 15 V and 17 °C for 1 h, forming alumina templates with average pore diameter (Øpore) 23 and 15 nm, respectively. High-purity (99.99 at%) Al foil specimens with a size of 45 45 mm2 and 100 μm thick, annealed at 500 °C for 3 h were used. The as-prepared templates in 0.5 M H2SO4 were sonicated in 2.0 M H2SO4 at 30 °C for 20 min to widen the pores up to Øpore 28 nm and to thin the barrier layer. To fill completely the alumina pores with tin, alternating current (ac, 50 Hz) deposition at a constant average current density of 2.75 mA 3 cm-2 was conducted in an aqueous solution consisting of 0.05 M SnSO4, 0.03 M tartaric acid, C4O6H6, and 0.015 M hydrazine sulfate, acidified to pH 1.15 using H2SO4, until the
Fabrication Scenario. We found here that three preconditions are required and satisfied seeking to fabricate the barbedshaped Nw arrays inside the alumina pores successfully: (i) a complete filling of the alumina pores by metal, (ii) the sealing of the alumina pores that remain empty after the depositions in boiling water or water steam, and (iii) the heat treatment of the encapsulated metal Nws at temperatures much higher than the melting point of the deposited metal. Two kinds of porous alumina having a 2D porous network with an average pore diameter of 15 and 28 nm and length of ∼15 μm were chosen here to prove this claim. The good uniformity and complete filling of the alumina pores with Sn0 Nws in these experiments were verified by means of FESEM images of alumina templates in cross-section geometry (part a of Figure 1), and following the template etching (part b of Figure 1). The average diameter of the resulting cylindrical Nws for 15 and 25 V alumina templates equaled to 14.5 and 28 nm, respectively. Part c of Figure 1 shows the different aspect of the Nws encased in the alumina matrix during their progress upon annealing. Moreover, parts d, e, and f 4496
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Figure 2. XRD spectra of the alumina matrix with encapsulated tin Nws before and after annealing at indicated temperatures for 2 (a) and 3 h (b). The average diameter of alumina pores (Øpore) is 15 nm.
Figure 1. Microstructure of tin and tin oxide Nws. (a) Cross-sectional FESEM view of an alumina template encased with tin Nws by ac electrodeposition. The average alumina pore diameter is 28 nm. Before annealing, the alumina was sealed in boiling water for 45 min. (b) A panoramic view of free-standing tin Nws arrays after alumina etching before annealing. (c) Encased Nws after matrix annealing at 500 °C for 2 h. (d, e, f) Typical TEM views at different magnifications of tin Nws disengaged from the alumina template pores after matrix annealing at 500 °C for 2 h: Single barbed-shaped Nws are visible.
of Figure 1 show TEM micrographs at different magnifications of free-standing long Nws after heat treatment and liberation from the matrix, displaying typical barbed straight-long shapes. This collection of results illustrates that the surface of these Nws after template annealing exhibits a particular topography. As seen, numerous dendrite-like thorns, up to 11-25 nm of length (parts c-f of Figure 1), depending on the initial Nws diameter (parts a and b of Figure 1) are formed. This implies a novel design of barbed-shaped Nws, the thicker the Nws, the larger and bigger the thorns are. Furthermore, it is reasonable to note that the
diameter, length, and spacing of barbed-shaped Nws, as well as their height can simply be controlled through the morphology and thickness of the alumina matrix. Heat-Treatment Effects. To follow the compositional and structural changes of the encapsulated tin Nws upon heattreatment, X-ray diffraction analyses were used. Figure 2 shows the XRD spectra of the alumina template, encased with tin Nws by prolonged ac deposition, before and after the annealing treatment in air at various temperatures (Tann). The position and relative intensity of all diffraction peaks for the as-deposited product (labeled as “Initial” in part a of Figure 2) are in good agreement with standard tin (PDF file no. 4 - 673) confirming the metallic nature of Sn0 Nws. The strongest peaks of this diffractogram can be indexed as 200, 101, 220, 211, and 301 reflections of body-centered tetragonal tin. The annealing of this specimen results in a change of the alumina color from deep black toward puce and gold tints, as well as a decrease in the Sn0 diffraction peaks and the appearance and enhancement of new ones. The first diffraction peak associated with SnO2 appeared for the samples annealed at Tann = 400 °C. Note that all peaks attributable to Sn0 disappeared only for Tann g 500 °C, for example at a temperature much higher than the melting point for the bulk metallic tin (231.9 °C). In this case, the diffraction lines at 2Θ = 26.58, 33.87, and 51.76° can be assigned to a tetragonal rutile-type crystalline phase of Sn(IV) oxide, SnO2 (PDF file no. 41-1445). In the case of the long annealing for 2 h at 500 °C (templates with pore diameters of 15 nm), the crystallite size of SnO2 particles was calculated from the XRD line broadening using the TOPAS software. On the basis of the Rietveld structure refinement Scherrer equation, the average crystallite size is about 9.7 ( 1.5 nm. It is worth noticing that the XRD pattern of specimen treated at 450 °C, except of the diffraction lines attributable to SnO2, contained additional peaks with positions depending on the annealing time (τann). These lines could be assigned to crystalline phase of Sn(II) oxide, SnO (PDF file no. 06-0395) if a more prolonged heating, that is, τann = ∼3 h, is used (part b of Figure 2). Nevertheless, the peak positions of these lines were found to be slightly shifted toward lower values of diffraction angles, indicating a small distortion of the 4497
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The Journal of Physical Chemistry C crystalline lattice from its nominal unstrained bulk values, since a = 3.802 and c = 4.900 Å were found instead of a0 = 3.802 and c0 = 4.836 Å corresponding to the relaxed crystal. To qualitatively and quantitatively further assess the effect of annealing on the transformation of the Sn0 nanowires encased within the alumina pores by electrodeposition, the M€ossbauer spectroscopy (MS) was performed, and the typical MS spectra of the products heated at different temperatures are presented in Figure 3. From these results, a singlet MS line (part a of Figure 3) with the isomer shift, δ, equal to 2.49 ( 0.02 mm 3 s-1, characteristic of the as-deposited nanowired material, is attributed to β-Sn. In the case of annealing at Tann g 450 °C, the resulting M€ossbauer spectra are mainly composed of two broadened doublets characteristic of SnO and SnO2 (parts b and c of Figure 3). The quadrupole splitting (Δ) and the isomer shift (δ) values for tin oxides in annealed specimens well coincides with the known ones only for the crystalline SnO2.49 For SnO, in all annealed specimens the quadrupole splitting (Δ = 2.05 ( 0.08 mm s-1) and isomer shift (δ = 2.87 ( 0.05 mm s-1) differ from those of a stable tetragonal (black) SnO, with Δ = 1.34 ( 0.01 mm s-1 and δ = 2.60 ( 0.01 mm s-1 (parts f and g of Figure 3). The larger Δ and δ values of SnO for annealed specimens imply probably the formation of either amorphous SnO,49 Δ = 2.87 mm s-1, or orthorhombic (nonstable red) SnO,50 Δ = 1.99 mm s-1, or 5SnO 3 H2O,51 Δ = 2.04 mm s-1. Because well-defined MS parameters exist only for stable tetragonal (black) SnO and X-ray data indicate the formation of tetragonal (black) SnO, we suppose that differences in SnO MS parameters are due to formation of a nanocrystalline nonstoichiometric form of tetragonal SnO with possible contribution of amorphous SnO as intermediate oxide phase. Novel Design. From the TEM images of the Sn/SnO2 blends (parts c-f of Figure 1) it is possible to infer that the Sn rodshaped Nws transformed into a barbed wire-type product upon the heat treatment, becoming 2 to 3 times thicker at the thorn sites. Furthermore, after the alumina template dissolution, numerous washings, and centrifugations, these Nws remain long and straight, which implies an excellent mechanical hardness. Our calculations indicate a significant surface area increase of the encapsulated Sn Nws upon annealing. For these calculations, experimentally determined average size of thorns (∼15 nm in height and ∼10 nm in diameter at the base and ∼11 nm in height and 5 nm in diameter at the base) were used for 25 and 15 nm diameter of Sn Nws, respectively. The average quantity of thorns for 100 nm Nw length was 13 and 20 for 25 and 15 Nws calcinated at 500 °C for 1.5 h. Hence, assuming even a simple pyramidal shape of thorns, where the surface area is equal to πr(r2 þ h2)1/2, the increase in size of encapsulated Sn Nws upon annealing was approximated to at least 1.5 times. However, there are further aspects we should consider in connection with this estimation of the surface area change. At first, the shape of thorns is more complex. At second, as seen from TEM images, the roughness of barbed-shaped Nws surface is much higher than that Sn Nws allowing assume a 3-4 times increase in surface area. As far as we know, such previously unthinkable nanotechnological architecture of Nws has never been observed before. TEM Studies Indicating SnO@SnO2 Core/Shell Morphology. Part a of Figure 4 shows a planar view of TEM micrograph displaying alumina grains where the darker one is oriented along the [0001] direction (see its electron-diffraction pattern with
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Figure 3. 119Sn M€ossbauer transmission spectra (MS) of the Sn Nws encapsulated within the alumina pores by electrodeposition before and after oxidation. (a) Room temperature MS for as-deposited arrays. (b, c) MS collected at indicated temperatures for specimens calcinated at 500 °C for 2 h. (d ,e) MS collected at indicated temperatures for specimens calcinated at 500 °C for 0.5 h. Øpore = 28 nm. (f) MS for a Sn Nw array encapsulated inside the pores of an alumina matrix, after subsequent sealing and 2 h annealing at 450 °C. (g) The same MS recording conditions as in (f) but for commercial SnO powder.
hexagonal symmetry in the insert). For the same TEM preparation, part b of Figure 4 shows a Z-contrast HAADF-STEM image demonstrating that the positions of the pores (brighter contrasts) are filled with a material with heavier atoms (Sn) than those of the surroundings with a lower atomic mass. Unfortunately, this technique is not sensitive to very light atoms such as O. Furthermore, to corroborate the existence and location of different oxide phases (SnO and/or SnO2), we have performed EELS (Figure 5) and HRTEM (part c of Figure 4) measurements in different positions across alumina-encapsulated single Nw after heat treatment at Tann 500 °C for τann 2 h. It has been previously reported52 that EELS can be used to search differences in the near edge structure of SnO and SnO2 with a view to distinguish them unambiguously. Whereas the oxygen K edge appears at the same position for each phase, a chemical shift of Sn M4,5 of about 3.5 eV is found between phases with Sn in (II) and (IV) oxidation states. For this purpose, EELS line scans were carried out along heated Nws, and for the profile shown in part b of Figure 4, two EELS spectra are presented in Figure 5. Here, the red dotted line corresponds to a spectrum taken from the edge of the Nw, whereas the black solid line is a spectrum recorded from its core. Although the O K-edges (the main peaks in the expanded spectra) mostly overlap with the M4 and M5 edges of tin (insert), they can be utilized to adjust possible energy miscallibrations. In this way, it was reported that the O K-edge should remain at the same energy position for both 4498
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Figure 5. Characteristic EELS spectra from the core and the shell of annealed tin Nws, encapsulated in the alumina matrix after calcination at 500 °C in air for 2 h. (a) EELS taken from the inner (red dotted line) and outer (black line) parts of an encased Nw within the 480 to 570 eV range, calibrated with the O K-edges. The insert is a magnified region of the same spectra demonstrating a chemical shift of the M5 energy peaks of tin in the core (SnO) and shell (SnO2) of the Nw.
Figure 4. TEM, HAADF-STEM, and HRTEM images of encapsulated Sn Nws (Øpore = 15 nm) after calcination in matrix. (a) Plan view TEM overview and electron diffraction pattern of alumina grains of the matrix showing a large variety of diameters for the pores. (b) Z-contrast image showing that the pores are filled with Sn oxide and the matrix is composed of Al oxide. A line profile for which a set of EELS spectra were recorded is shown. (c) High-resolution TEM image of crystalline planes of SnO2 taken from a single Nw barb.
SnO and SnO2 phases. Thus, variations observed due to high tension fluctuations in the electron microscope were corrected to recalibrate both spectra by aligning the oxygen peak at 532 eV. In this case, the position of the M5 peak of Sn is different for each spectrum, which indicates the existence of two different phases: SnO located at the core and SnO2, at the edges of the Nw (at energies about 486 eV for (II)SnO and 490 eV for (IV)SnO2), which is in good agreement with previous results.53 Therefore, this is an empirical proof of the existence of a layer mainly formed of crystalline SnO2 species that envelope the core of SnO Nw. Thus, the HRTEM image recorded near the edge of an isolated Nw (part c of Figure 4) indicates the crystalline nature of the oxide shell thorns. After measuring the visible periodic contrast, it appears that most of them are spaced about 3.34 Å, which is exclusive of the (110) family of planes of the SnO2 phase and far away from other interplanar parameters associated with β-Sn or SnO. In conclusion, the shell and thorns are composed of nanocrystalline regions probably only of the tin dioxide phase, and as an example, the portion with resolved atomic columns is shown in the figure. Kinetics of Barbed Nws Formation. It is worth pointing out here that, in contrast to calcination of free-standing Nws in air,42 the heat-treatment of encapsulated β-Sn Nws within the Tann and τann limits of this study results always in the formation of SnO@SnO2 core/shell Nws. For the same Tann, the contents of formed SnO and SnO2 phases, depend on the τann and the
Figure 6. Kinetics of the oxidation of encapsulated β-Sn Nws. Variations of the M€ossbauer subspectra areas, corrected by the M€ossbauer effect probability (fx), if fSnO = 0.12 and fSnO2 = 0.3,35 for SnO and SnO2 Nw components in the specimens versus annealing time and initial diameter of Sn Nws: (1, 2) 14.5 nm; (3, 4) 28 nm. Tann = 500 °C.
diameter of tin Nws (ØSn) in a manner presented in Figure 6. The empirical dependences of the composition of heat-treated tin Nws on the heating time enable some interesting conclusions. It seems that the maximum contamination of SnO for ØSn = 14.5 nm and ØSn = 28 nm Nw arrays and Tann = 500 °C is observed after 1.5 and 2 h heating, respectively. Upon further heating, the content of SnO decreases and the core becomes thinner, which can probably be linked to the disappearance of the β-Sn component in the central part of the barbed-shaped Nws. In addition, it can be perceived that the formation of SnO2 species during annealing proceeds throughout the formation of an intermediate Sn2þ ionized state as in the case of free-standing β-Sn Nws calcinations in air.42 It can be also expected that, due to difficulties arisen from the oxygen species diffusion, the oxidation degree of the resulting products ought to vary crosswise forming a SnO (core) and SnO2 (shell) composite. Mechanism of Barbed Nws Formation. The fact that the shape of Sn Nws made a significant change with respect to the annealed product indicates that the calcination of the alumina encapsulating matrix provides a unique reaction environment. This effect differs from the annealing of iron Nws, encapsulated fully into the same alumina matrix at approximately a same 4499
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The Journal of Physical Chemistry C temperature resulting in the formation of FeAl2O4 material.54 The formation of spinel structures by calcination of alumina matrices filled with Fe Nws implied that metals, encased in the alumina pores can easily diffuse inside the alumina cell walls upon the heat treatment. The differences in the behavior between Fe Nws and the Sn Nws observed here may simple be explained by incapacity of tin atoms to form spinels. The experimental findings of this study also indicate that the formation of barbed-shaped Nws proceeds at Tann g 400 °C through spontaneous diffusion of molten tin streams inside the alumina cell walls in random directions, because of a strong radial pressure that induced zones to diffuse into the alumina cell walls forming the barbs. Also, the oxidation of tin Nws encapsulated fully in the alumina milieu suggest the usage of its oxygen atoms through intimate contact between the molten tin and alumina material consisting of Al2O2.81 3 (SO4)0.19 3 0.38 H2O.55 This effect can be explained by the differences in the thermal expansion coefficients of alumina (∼8.8 10-6 K-1) and Sn0 (∼2.1 10-5 K-1) because the expansion of completely encapsulated Sn0 Nws must produce a radial pressure to the matrix walls with annealing temperature increase. It is reasonable to suspect that, in line with experimental findings, much higher Tann than the melting point of tin are required to destroy the alumina cell walls due to polycrystalline nature of Sn0 Nws in which always exist liquidlike spaces between nanocrystallites to release the stress from the lattice expansion.56 As reported by Cai et al.,57 the radial pressure can be relaxed by the axial expansion if alumina pores remained opened, as in the case of the report.43 We note also that utilization of alumina oxygen atoms forming tin oxides may further increase the volume of Nws and pressure to walls.
4. CONCLUSIONS The synthesis of a novel design of core-shell SnO@SnO2 Nws is demonstrated via heat treatment of Sn Nws encapsulated inside the pores of an alumina matrix. We believe that the large surface area barbed-shaped SnO@SnO2 Nw arrays presented herein offer further advantages for novel gas-sensing applications. Essentially, the same strategy could be applied to grow the assembly of barbed Nws of other oxides whose metals do not form spinel-type compositions, but further study is needed for this purpose. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Fax: þ370 52649774. Tel.: þ370 52648891.
’ ACKNOWLEDGMENT A.J., K.M., J.R., R.J., and D.B. acknowledge support for this work from the Lithuanian State Science and Studies Foundation, grant no. C-07035. J.G.L. and J.M.M. thank Junta de Andalucía for the financial support. TEM work was carried out at the DME SCCYT of UCA. ’ REFERENCES (1) Alivisatos, A. P. Science 1996, 271, 933. (2) Ozin, G. A. Adv. Mater. 1992, 4, 612. (3) Li, J.; Wang, L.-W. Phys. Rev. B 2005, 72, 125325. (4) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353.
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