Secondary Facet-Selective Nucleation and Growth: Highly Oriented

Feb 12, 2009 - ABSTRACT: Recently, controlled growth of branched hierarchical crystals has been the focus of intensive research for their novel proper...
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Secondary Facet-Selective Nucleation and Growth: Highly Oriented Straight SnO2 Nanowire Arrays on Primary Microrods Jun Liu, Xiaolong Chen,* Wenjun Wang, Bo Song, and Qingsong Huang Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, P. R. China

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 4 1757–1761

ReceiVed July 22, 2008; ReVised Manuscript ReceiVed December 18, 2008

ABSTRACT: Recently, controlled growth of branched hierarchical crystals has been the focus of intensive research for their novel properties and potential applications. The key to synthesizing the branched nanostructures is to induce and control secondary nucleation on primary crystals. The metal catalysts and organic structure-directing agents are mainly utilized to induce the secondary nucleation and branch growth, however, nucleation sites are usually random because the metal catalysts and capping agents have been not controlled to deposit or etch on the expected facets of primary crystals. In this paper, we report that varying the ratio of oxygen to tin vapor in reaction can tune the growth rate of SnO2 along different directions in a vapor-solid (VS) process to alter the growth habit. By this new shape-controlled way, we can first control the synthesis of quasi-hexagonal primary microrods like SnO2 and then facet-selectively grow secondary nanobranches on them without use of any catalyst or surface modification. The controlled nucleation and the growth of nanowire arrays on primary crystals by this facile VS method just like SnO2 may provide a useful route for connecting nanobuliding blocks into some desired hierarchical nanostructures and nanodevices. Introduction Branched hierarchical nanostructures with tunable morphology have been a research focus recently because of their unique properties and potential applications in chemical sensors, optics, energy conversion, electronics, catalyst, medicine, and so on.1 The key to synthesizing the branched hierarchical nanostructures is to induce and control secondary nucleation on primary crystals. So far, the metal catalysts (such as noble metals and transition metals) have been used to deposit onto primary trunks and form the nucleation sites to induce the secondary nucleation and the subsequent branched growth by multistep vaporliquid-solid (VLS) process,2 or solution-liquid-solid (SLS) process very recently.3 Meanwhile, some organic structuredirecting agents (e.g., citrate, diamines, or diaminopropane) have been utilized to etch primary crystals to induce the nucleation sites for branched growth by a solution-phase method.4 Although these methods can induce the secondary nucleation, the nucleation sites are usually random because the metal catalysts and capping agents could not been controlled to deposit or etch on the desired facets of primary crystals. New methods enabling the secondary facet-selective nucleation and growth are still needed. An important semiconductor, SnO2, whose growth usually occurs along the [001] direction in fabrication of quasi onedimensional nanostructures, has been selected as a specific example for the secondary facet-selective nucleation and growth. At present, fewer papers have been reported for the creation of SnO2 oriented nanorod arrays only by template-assisted method or solution-based long time reaction method.5 In this communication, we report a facile vapor-solid (VS) process whereby the fiber direction switches from [001] to [010] and results in quasi-hexagonal microrods encompassed by a pair of {001} planes and other 4 equiv. {101} planes. The secondary nucleation and growth are then controlled to take place on the {001} planes only, leading to a new type branched nanostructure: the highly oriented straight SnO2 nanowire arrays on microrods. The key to the nanostructure is the realization of * Corresponding author. E-mail: [email protected].

facet-selective nucleation and growth by varying the ratio oxygen/tin vapor in the VS process. Experimental Section The primary microrods and secondary highly oriented nanowire arrays were skillfully accomplished by a simple two-step VS route. An alumina boat with pure Sn powders (4-5 g, 200 msh) is placed in the front of an alumina hollow tube with 25 mm in inner diameter and 22 cm in length. The alumina hollow tube together with the alumina boat was placed in the middle of a horizontal quartz tube furnace. In the first step, the quartz tube was pumped and then filled with argon and repumped. After repeating the operation three times, the quartz tube was full of argon atmosphere. The quartz tube was then heated to 900 °C in 1 h under an argon flow of 20 standard cubic centimeters per minute (sccm); therefore, the Sn vapor can form sufficiently due to the larger amount of Sn powder promptly vaporizing in a small confined space. After the temperature reached 900 °C, the sparse oxygen flow of 1 sccm under an argon flow of 80 sccm was suddenly introduced into the quartz tube to react with the excess Sn vapor for half an hour. In succession, in the second step, oxygen flow was adjusted to 20 sccm, while argon flow as carrier was kept at 80 sccm, and the reaction times were fixed from 5 min to 2 h at this step for nucleation and growth of nanobranch arrays on primary crystals. Finally, the oxygen flow was turned off and the furnace was naturally cooled. To observe the detailed growth process, we checked and characterized products obtained at the end of the first step and obtained at different reaction durations in the second step.

Results and Discussion SnO2 is an important semiconductor with a band gap 3.6 eV at room temperature. It has a rutile-type structure (P42/mnm) with lattice constants a ) 4.738 Å and c ) 3.188Å.6 Novel SnO2 nanostructure-based devices have been developed and gained considerable attention.7 Figure 1a shows the primary microrods at the first growth stage and Figure 1b shows the morphology of these rods in an enlarged magnification. It can be seen that these rods exhibit quasi-hexagonal prisms (see the hexagonal cross section shown in inset to Figure 1b also) with several micrometers in diameter and several hundred micrometers in length. The X-ray diffraction (XRD) pattern of the primary microrods can be well indexed to a tetragonal cell with

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Figure 1. The SEM images of the straight primary quasi-hexagonal SnO2 rods.

Figure 2. (a) XRD pattern of the primary quasi-hexagonal SnO2 rods; (b) XRD pattern of the branched nanostructures.

a ) 4.739 Å and c ) 3.189Å, in good agreement with the standard data (ICDD-PDF 41-1445), confirming these rods are SnO2 in rutile structure (Figure 2a). Compared with the XRD pattern of the branched structures (Figure 2b) and standard rutile SnO2, the XRD pattern of the primary microrods (Figure 2a) shows the {101} diffraction peak with a substantial increase in intensity compare to the {110} diffraction peak. And according to the extinction rule, the {001} have not X-ray diffraction in rutile SnO2. A striking feature in morphology is that the six side surfaces can be divided into two categories: two rough surfaces with many parallel stripes and four smooth surfaces. To distinguish these side planes, we performed the Raman scattering measurements, because these rods are too thick (with

several microns in diameter) to be directly observed by TEM. The examinations are arranged by an incident light perpendicular to a primary SnO2 rod with the polarization of light perpendicular (Figure 3a) or parallel (Figure 3b) to the lengthy direction of SnO2 rod. Three Raman-active modes are observed at 475, 634, and 776 cm-1 in Figure 3a, which are in agreement with the reported Eg, A1g, and B2g for rutile SnO2, respectively.8 In comparison, we cannot observe the 475 cm-1 peak in Figure 3b. Considering the Eg mode corresponding to the oxygen atom vibrating along the c axis, it can be deduced that the c axis is perpendicular to the fiber direction of the rod. We will show the c axis, which is perpendicular to the two rough side surfaces {001} below. In the second step, when we increased the ratio oxygen/tin vapor by enhancing of oxygen flow while keeping the temperature constant, white particles and small rods are observed on the two rough surfaces (images a and b in Figure 4), whereas nothing is on the four smooth surfaces, suggesting secondary facet-selective nucleation and growth occur. With the growth continuing, these nascent rods are well aligned to be perpendicular to the primary quasi-hexagonal rods but have a tetragonal column in morphology (Figure 4c). Upon further growth, they develop into about 10 µm long and submicrons wide secondary branches (images d and e in Figure 4). Enlarged top-view SEM image (Figure 4f) presents that two branches may degenerate together to make a new larger one, leading to the number of branches decreasing, and reveals that each of the four columnar facets of the branches is at an angle of 45° with the direction of the primary rods, suggesting the secondary rods have a certain crystallographic relationship to the primary ones. Figure 5a shows a TEM image of a typical nanobranch vertically grown on the rough facet. No catalyst nanoparticle such as Sn is on its tip, and thus the growth of branches should follow the VS mechanism. High-resolution transmission electron microscopy (HRTEM) image (Figure 5b) reveals that the secondary rods are single crystals and the spacing of lattice fringes along the fiber direction is 0.33nm, corresponding to that for {110} plane. The in situ electron diffraction pattern can be well indexed and the fiber direction is easily determined to be along the c axis. Accordingly, the side facets of these tetragonal columns are {110} or {100}. The elongated quasi one-dimensional fibers along the c axis and side surfaces encompassed by {110} are common in morphology for rutile SnO2 and have already been synthesized via many routes.9 Considering the fact that the secondary rods with fiber direction along [001] only grow vertically on the two rough surfaces of the primary rods, homogeneous epitaxy should occur. This means that these two rough surfaces of the primary rods are {001}. Moreover, the side faces {110} are orientated to be at an angle of 45° to the fiber direction of primary rods, we can deduce that the primary

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Figure 3. Raman spectra of a single primary quasi-hexagonal SnO2 rod. (a) Polarization of incident light perpendicular to the length direction of SnO2 rod; (b) polarization of incident light parallel to the length direction of SnO2 rod.

Figure 4. Growth process of SnO2 branched nanostructures (SEM images). (a) secondary nucleation is formed, which only located on the two rough symmetry surfaces ({001} facets); (b) magnified SEM image shows white particles are located on the two rough surfaces (c) the first nascent oriented branches emerged on the two {001} facets; (d) a large scale synthesis and the final morphology of branched nanostructures; (e) enlarged side-view SEM image shows the highly oriented nanobranch arrays. (f) enlarged top-view SEM image shows the aligned branches are perpendicular to the rough surfaces and each of the four columnar facets of the branches is orientated to be at an angle of 45° to the fiber direction of the primary rods (the [010] direction) and two branches may grow together to make a new larger one and the number of branches decreased.

rods are elongated along the a or the b axis (Figure 6). The other four smooth surfaces are probably {101} according the angle shown in the inset to Figure 1b. This is also manifested in the XRD pattern (Figure 2a) where the {101} diffraction peak show a substantial increase in intensity, suggesting a strong preferred orientation along {101}, and the rough faces {001} have not no diffraction peak according to the extinction rule.

As is well-known, the (001) facet has a bigger oxygen atom density than the (100) or (010) facets for rutile SnO2. It is expected that more oxygen will be consumed in growth along [001] than along [100]. This is the reason why the fast crystal growth is along [001] direction in high ratio oxygen/tin vapor atmosphere as evidenced by many observations. In our experiments, however, sparse oxygen was first introduced to react with

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Figure 5. (a) TEM image of the branched nanostructure; (b) HRTEM image and the Fourier-transform spectrum of the lattice image (inset) shows the nanobranch is a single crystal and grows along the [001] direction.

Figure 6. Scheme of the branches vertically on the {001} surfaces of a primary rod. The branches have four side surfaces of {110}, each of them have an angle of 45° from the direction of the primary rod. It can deduce that the primary rods are elongated along the a axis or the b axis.

excess Sn vapor at the early stage, the growth along [001] will be significantly restrained to an extent so that the fast growth direction switches to [100] or [010]. As a result, SnO2 elongates along [100] or [010] and the {001} and {101} surfaces survive, leading to the unusual primary quasi-hexagonal SnO2 rods. Recently, a similar phenomenon was reported that NO2 can control the shape of Pd nanocrystal by varying the growth rate of different facets due to preferential interactions with these facets.10 Some surfactants (e.g., citrate, poly and bromide) were also used to control the size and shape of nanocryatals via a similar mechanism.11 Here, ratio of oxygen concentration to tin vapor is found to play a vital role in controlling the shape of SnO2 primary rod by regulating the growth rate for different facets. Ying and co-workers reported that SnO2 nanowhiskers with growth direction along [001] were synthesized under a constant flow of 99% nitrogen and 1% oxygen at 800 °C.12 This result does not contradict our arguments as the ratio of oxygen to tin vapor, in our experiments, is expected to be lower than that because our SnO2 rods were synthesized at 900 °C. During the second growth stage, the oxygen becomes richer with respect to the tin vapor. Under such a high ratio oxygen/ tin vapor atmosphere, the fast growth direction of SnO2 then switches to [001]. Among all facets of the primary rods, the {001} are the most favored facets as substrates for the nucleation and growth of the secondary rods. The nucleation and growth on other facets are suppressed because of the larger lattice mismatch and relatively lower surface energy. Higher surface energy of {001} surfaces may add to initiate the secondary nucleation only on the two rough surfaces ({001} facets). This

Figure 7. Schematic illustration of facet-selective nucleation and branched growth. The two symmetry rough surfaces with parallel stripes are {001}, and the other smooth side surfaces are {101}. The fiber direction of the primary rod is the a or b axis, the branches grow along the c axis and side surfaces encompassed by {110}.

is the so-called selective-nucleation. With the growth continuing, the first nascent oriented branches emerge and grow along the normal direction of {001} facets, and finally develop into a highly orientated secondary nanorod array. The secondary facetselective nucleation and growth process for the array are schematically shown in Figure 7. Herein, the c-elongated tetragonal rods with stable side facets of {110} can confer to the branch arrays a promising material for photocatalytic and sensing devices, since the {110} facets have the best efficiency for chemisorption and dissociation of oxygenated compounds at the SnO2 interface due to the lowest interatomic distances between tin atoms with respect to {101} and {001} facets.7c,13 Some papers suggested that the stronger the excitonic PL spectrum, the larger the content of oxygen vacancy or defect, and the higher the photocatalytic activity.14 Figure 8 shows comparative room-temperature photoluminescence (PL) spectra of the SnO2 hierarchical nanostructures, SnO2 nanobelts, and a standard SnO2 powder under the same experimental conditions and all excited at 325 nm (the spectra are thus comparable). Emissions at 550 nm are observed in the SnO2 nanobelt sample and hierachical nanostructure sample, whereas no emissions are observed in the powder sample. Moreover, we found that intensity in emission peak at this wavelength is about 6 times larger in hierachical nanostructure samples than in their nanobelt counterpart. The emission at 550 nm in SnO2 generally originates from defect electronic states formed by so-called “bridging” oxygen vacancies,15 in which the most of electrons are excited to the bulk shallow energy level and then return to the intragap level of SnO2 corresponding to oxygen surface vacancies via radiative transitions. Therefore,

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Figure 8. Comparative room-temperature photoluminescence (PL) spectra of the SnO2 hierarchical nanostructures, SnO2 nanobelts and a standard SnO2 powder under the same experimental conditions and all excited at 325 nm.

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we have good reason to think our hierachical nanostructures so fabricated are full of oxygen defects and may have attractive luminescence properties and photocatalytic property.

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Conclusions In conclusion, our results indicate that varying the ratio of oxygen to tin vapor in a reaction system can tune the growth rate of SnO2 along different directions to control the final crystal morphologies. In the new shape-controlled way, we synthesize primary quasi-hexagonal microrods and then facet-selectively grow secondary nanobranches on them without use of any catalyst or surface modification. And the {001} facets of primary crystal perform as a perfect substrate for SnO2 nanorod arrays. The controlled nucleation and growth of nanowire arrays on primary crystals by this facile VS method just like SnO2 may provide a useful route for connecting nanobuliding blocks into some desired hierarchical nanostructures and nanodevices. Acknowledgment. This work is supported by The National Basic Research Program of China (973 Program) Grant 2007CB936300, the National High Technology Research and Development Program of China (863 Program) Grant 2006AA03A107 and the National Natural Science Foundation of China (Grant 50702073).

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