Symmetry-Breaking Segmented Selenium Nanorods and Nanowires

Apr 8, 2015 - The latter included well-known vapor–liquid–solid (VLS),(3) ... such as forming several metal chalcogenides, Ag2Se, CdSe, ZnSe, and ...
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Symmetry-Breaking Segmented Selenium Nanorods and Nanowires: Synthesis via Coupling of Nanorods Ming-Han Liu,*,† Yu-Tzu Liu, and C. R. Chris Wang* Department of Chemistry and Biochemistry, National Chung Cheng University, Min-Hsiung, Chia-Yi, 62102, Taiwan S Supporting Information *

ABSTRACT: We present a facile route to prepare selenium nanorods (SeNRs) with alternating symmetry-breaking segments dispersed in aqueous solution. The key roles in this preparation include an assistance provided by the carboxymethyl−cellulose (CMC) and a careful control in particle’s nucleation and growth. Thus, prepared symmetry-breaking segmented SeNRs, namely, SBS-SeNRs, show discrete trigonal and quasi-trigonal phases in the form of repeating quasitrigonal−trigonal−quasi-trigonal, (tq−t−tq)n, where tq stands for the quasi-trigonal phase in the space group of P321 and t is the trigonal phase in the space group of P3121. Evidence from the particle dimensions analysis during the growth and the result of post-synthesis surface modification showed that the SBS-SeNRs grow via two different particle couplings. The first one was found in the early stage that very short monoclinic Se nanorods couple together with a simultaneous phase transition to trigonal SeNR. The second coupling occurs at the later stage by the coupling of the (tq−t−tq)-SeNRs, which are formed via a growth of symmetry-breaking Se at both ends of the t-SeNR. Meanwhile, we designed an experiment by conducting the silica coating via sol−gel process to further prove the symmetry-breaking segmented nanostructure. The coating proceeded preferentially on the trigonal segments of SBS-SeNRs rather than the quasi-trigonal segments to form a float-like core−shell nanostructure, as well as a string of beads-like one in the case of selenium nanowires (SeNWs). The synthesis of SBS-SeNRs demonstrates the segmented nanomaterials are no longer limited in the combination of different compositions, but opens up a possibility for the versatile intriguing segmented nanomaterials in the future.

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Their catalytic properties10c,11a,d were often reported for such unique nanostructures with two or more distinct materials. For example, segmented (Pt/Au) rod-shaped particles, which were prepared by the electrochemical method, promoted autonomous movement through oxygen formation as propulsion in decomposition of hydrogen peroxide.11d Selenium (Se) is known as not only an essential trace element with wide physiological functions but also a chemopreventive agent to prevent cancer.12 For example, the functionalized Se nanoparticles exhibited anticancer synergism which can induce apoptosis in the cancer cells.12a Other than that, in the field of semiconductors, selenium was recognized as an interesting semiconductor which has been applied in various aspects ranging from photographic exposure meter,13 semiconductor rectifier,14 and solar cell15 to catalysis.16 It is known that elemental selenium owns extensive chemical reactivities toward either oxidation or reduction, such as forming several metal chalcogenides, Ag2Se, CdSe, ZnSe, and Bi2Se3.17 The approaches to prepare 0D and 1D Se nanomaterials without any stabilizing agents have been reported by the use of either physical18 or chemical19 methods. In order to improve their uniformity in dimensions, stabilizing or capping agents

INTRODUCTION Nanomaterials have attracted much attention in broad disciplines due to their fruitful physical and chemical properties that differ extremely from those of their bulk analogs in wide potential applications.1 Literature has reported numerous synthetic strategies for various nanostructures. Among them, one-dimensional (1D) nanomaterials which display interesting properties and potential applications in nanoscale electronic and optoelectronic devices2 were usually prepared through either template or template-free methods. The latter included well-known vapor−liquid−solid (VLS),3 vapor−solid (VS),4 and solution−liquid−solid (SLS)5 mechanisms, and the former were conducted with the assistance of either hard or soft templates. In the cases of hard-template based syntheses, porous materials such as AAO6 and SBA-157 were often utilized to prepare various structured nanorods or nanowires. For soft templates as the directing agents, surfactants or block copolymers can confine the particle growth via self-assembly. A well-known example is the gold nanorods prepared via hexadecyltrimethylammonium bromide (C16TABr) stabilization.8 Moreover, for high quality semiconductor nanowire CdE (E = S and Te), some nanoparticles, i.e., Bi and core/shell-type Au/Bi, are utilized as critical nanocatalysts for improving the growth of nanowire.9 Interestingly, recent synthetic schemes were reported to prepare multicomponent nanomaterials, such as Janus nanoparticles10 and segmented nanorods/wires.11 © 2015 American Chemical Society

Received: December 9, 2014 Revised: March 12, 2015 Published: April 8, 2015 2832

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Chemistry of Materials were normally used.20,21 However, a lack of a good control in the particle dimensions as well as aspect ratios is still a challenging task so far. Moreover, among the previously mentioned methods, the crystal phase of the final product selenium nanomaterials was all identified to be the thermally most stable trigonal phase,22 namely, t-Se. It is believed to be attributed to the relatively energetic conditions, e.g., sonication19a and hydrothermal treatment.6a,19b,d Such experimental conditions favored the formation of t-Se. Up to date, to the best of our knowledge, there is no literature that has declared using either hard- or soft-templates as capping materials to synthesize (i) length-controlled Se nanoparticles, ranging from nanorods with several tens nanometers in length to nanowires with several micrometers, and (ii) the segmented 1D nanomatrials with the same material and the coexistence of two different crystal symmetries. To achieve the goals, we chose a two-step synthetic scheme with a mild crystal growth condition to ensure both a fine control in the nucleation-andgrowth processes and the stabilization of symmetry-breaking trigonal (i.e., quasi-trigonal) segments, denoted tq-Se along with the t-Se. The quasi-trigonal phase was characterized by the breaking of a threefold screw axis along c-axis in the primitive phase (t-Se). The space group of quasi-trigonal Se thus displays P321 by breaking the higher P3121 symmetry of t-Se. The segmented Se nanostructure with two coexisted crystal symmetries is called (tq−t−tq)-SeNRs. The growth mechanism for the symmetry-breaking segmented SeNRs and SeNWs, denoted SBS-SeNRs and SBS-SeNWs, were investigated and proposed via (high-resolution) TEM, and powder X-ray diffraction analyses. The structures of the SBS-SeNRs and SBS-SeNWs were further probed by silica coating to form interesting nanoparticles of “float-like” SeNR@SiO2 and “string of beads-like” SeNW@SiO2.

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purified by centrifugation with deionized water for further TEM/ HRTEM characterization. Characterization. The crystal structure and phase composition of each sample were investigated by using Shimadzu XRD-6000 with Cu Kα radiation (λ = 1.54184 Å). Morphology, size and local structure of the selenium nanoparticles were examined by transmission electron microscope (TEM) operated at 200 kV (JEOL, JEM-2010, and FEI, Tecnai-F20).

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RESULTS AND DISCUSSION Fine-controlled selenium nanorods (SeNRs) were synthesized via a simple two-step solution method (Scheme 1). It consists Scheme 1. Synthetic Strategy of (tq−t−tq)-SeNRs with CMC StabilizationA

A

Stage I stands for the formation process of spherical-like amorphous seeds operated at room temperature. Stage II represents the couplingand-growth process runs at an elevated temperature.

of seed formation of amorphous selenium nanoparticles, i.e., aSeNPs at the first step, followed by a subsequent growth to form shorter monoclinic SeNRs (m-SeNRs) and then symmetry-breaking segmented SeNRs (SBS-SeNRs), i.e., (tq− t−tq)-SeNRs. In such a synthetic system, CMC was employed as the morphology-and-crystal phase stabilized agent to govern the growing fortune of m-SeNRs. CMC is a cellulose derivative, a type of polysaccharides, in which part of the hydroxyl groups were replaced by carboxymethyl groups (−CH2−COOH) in the backbone consisting of several hundred linked glucose units. Cellulose acted as a morphology-directing agent to prepare selenium nanobelts.23 A hyperbranched polysaccharide was demonstrated to be a stabilizer to fabricate highly stable selenium nanoparticles.24 It has been clearly shown25 that the selenium atoms of selenite ions can readily bound to the hydroxy groups of CMC through intermolecular hydrogen bonds (O−H···Se), which provides a flexible scaffold to stabilize freshly reduced selenium atoms. Therefore, CMC is a suitable agent to stabilize selenium structure while selenium atoms are freshly formed from the reduction of selenites. It was anticipated that CMC can extend its stabilizing capability to the thermodynamically less stable monoclinic crystal phase. Accordingly, the phase transition process of selenium can effectively either be slowed down or terminated at intermediate steps against mild reaction conditions (practical condition: 70 °C, 1 h). Selenium nanoparticles were formed in the presence of CMC through the following chemical reactions:

EXPERIMETAL SECTION

Chemicals. Carboxymethly-cellulose (CMC, sodium salt, low viscosity, Sigma-Aldrich), selenium dioxide (SeO2, 98%, SigmaAldrich), (3-mercaptopropyl)trimethoxy-silane (MPTMS, 95%, Sigma-Aldrich), sodium silicate solution (ca. 27% SiO2 in 14% NaOH(aq), Sigma-Aldrich), and sodium borohydride (NaBH4, powder, Riedel-deHaën) were used without further purification. Preparation of Selenium Nanorods (SeNRs). Selenium nanorods were prepared through a two-step synthetic process. It began with dissolving 0.082 g of SeO2 into 10 mL of 3.9 wt % CMC solution (i.e., dissolved 0.4058 g of CMC into 10 mL of deionized water) to form the selenite aqueous solution. Then, we added freshly prepared 1 mL of 0.74 mM NaBH4 solution dropwise into the above selenite solution at room temperature under constant stirring, marking the first step. The color of the solution changed from pale yellow to brick red during the addition of the reducing agent. Subsequently, the mixed solution was placed into the oven at 70 °C for 1 h of reaction. For controllable SeNRs, the reaction time at room temperature, also called seeding time, was controlled in the range from 2 to 8 min, resulting in the mean length of SeNRs from 200 nm to 1.2 μm at the end of reaction. In addition, selenium nanowires were also obtained via the same method, but the seeding time was restricted within 1 min. Prior to further characterizations, the as-synthesized SeNRs were washed and purified with deionized water by centrifugation. Preparation of SeNR@SiO2 Nanoparticles. SeNR@SiO2 nanoparticles were prepared by a simple sol−gel process. Briefly, a 100 μL of SeNRs solution was dissolved in a 50 mL vial containing 25 mL of deionized water, followed by MPTMS modification via 1 mL of ethanol solution of 5 × 10−2 mM MPTMS. After 30 min of MPTMS modification as a mediated layer, 200 μL of 0.05 wt % silicate solution was added into the above solution for another 30 min modification. At the end of reaction, the SeNR@SiO2 particles were washed and

SeO2 + H 2O → H 2SeO3

(1)

NaBH4 + 2H 2SeO3 → 2Se + NaOH + B(OH)3 + 2H 2O (2)

Prior to the second step of the growth at elevated temperature, the time of the stirred solution at room temperature, namely, seeding time, was carefully controlled for the fine-tuned SeNRs with different aspect ratios. After that, to produce SBS-SeNRs, the whole reaction solution was placed into a 70 °C oven for coupling processes and the formation of 2833

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Chemistry of Materials tq-Se. Supporting Information Figure S1 indicates that the selenium nanoparticles have an almost spherical form at the final of seeding step. The corresponding electron diffraction (inset of Supporting Information Figure S1) reveals these Se nanoparticles are amorphous, i.e., glassy selenium. Subsequently, these glassy selenium nanoparticles easily transformed to either monoclinic phase or trigonal phase at the end of reaction, which is dictated by practical reaction conditions, especially reaction temperature. The transition between two phases will occur when temperature is higher than 70 °C.26 Thus, the crucial temperature for the control of SBS-SeNRs/ SeNWs is certainly selected to be 70 °C. It is well-known that selenium typically exists in three allotropes except glassy selenium, including α-monoclinic, β-monoclinic, and trigonal allotropes. These Se allotropes are also packed in the structures of rings and chains for monoclinic and trigonal phases, respectively. Through the thermodynamic calculation based on experimental data, it has been concluded that the phase transition of selenium from monoclinic to trigonal can be achieved due to the negative heat of reaction under ambient temperature.26a Hence, it is definite that monoclinic phases are a metastable phase compared with the trigonal phase. To examine the synthetic feasibility of SBS-SeNRs, a drop of this reaction solution was quenched in cold water for further TEM characterization. Supporting Information Figure S2a reveals that plenty of rod-like selenium nanoparticles existed at 10 min of growth time. The crystal phase of rod-like selenium nanoparticle (red square in Supporting Information Figure S2a) was characterized to β-monoclinic phase via highresolution TEM analysis (Supporting Information Figure S2b). Meanwhile, some of selenium nanoparticles showing spheroid-like morphologies were examined and certainly assigned to the α-monoclinic phase (Supporting Information Figure S3). In terms of long-term crystal stability, the thermodynamically unstable monoclinic selenium will spontaneously transform to the trigonal phase. According to previous studies in α-27 and β-28 monoclinic phases of Se, it was thus concluded that the typical crystal phase transformation of Se is as follows: α-monoclinic → β-monoclinic → trigonal.22 As a result, these immature nanorods grew from amorphous spherical nanoparticles and subsequently elongated along the c-axis via interparticle coupling (discussed below). These wellcoupled m-SeNRs formed at the early stage further converted to trigonal phase before the growth of quasi-trigonal segments at both tips. Owing to CMC-stabilization and relatively lower reduction potential, these quasi-trigonal segments of SBSSeNRs were supposedly retained at the later stages of the growth process. These SBS-SeNRs were collected and purified by centrifugation with deionized water for characterization. Figure 1 shows large-scale TEM images of selenium nanorods at different seeding times from 2 to 8 min with the same time interval of 2 min. The corresponding mean length of nanorods in Figure 1a−d are 1170 ± 246, 633 ± 101, 446 ± 99, and 252 ± 29 nm, respectively. Their corresponding aspect ratios (length/diameter) and mean diameters are shown in Supporting Information Figure S4a,b, respectively. It is clearly reasonable that much less precursor remaining in the reaction solution led to a rather low ability for SeNRs elongation while the seeding time is increased. Based on the statistical results over the above samples, CMC as stabilizer effectively confined the SeNR diameter around 20 nm (Supporting Information Figure S4b). Even under the condition with less than 1 min seeding time, the synthesized SeNWs showed a quite uniform

Figure 1. TEM images of selenium nanorods prepared at room temperature with the same growth time of 1 h via different seeding time of (a) 2 min, (b) 4 min, (c) 6 min, and (d) 8 min. The corresponding mean length of (a) to (d) are 1170 ± 246 nm, 633 ± 101 nm, 446 ± 99 nm, and 252 ± 29 nm, respectively.

diameter at 20 ± 3.6 nm (not shown here). Interestingly, the length distribution drastically narrowed down with increasing seeding time and finally toward a steady deviation around 11.5% (obtained from statistical result of Figure 1d). In comparison with other solution-based syntheses either for 1D trigonal19b,c,29 or monoclinic30 Se nanoparticles, the facile approach for fine-controlled SeNRs in different aspect ratios is the first time they have been developed under a relatively lower temperature reaction process. More interestingly, these selenium nanorods have a discontinuity of crystal symmetry, i.e., (tq−t−tq)-SeNRs. The preparation method is unique not only in preparation of chalcogenide 1D nanomaterials but also in synthesis of segmented 1D nanomaterials.11 Evolution of Symmetry-Breaking Segmented SeNRs. The spherical SeNPs at the seeding stage had an amorphous state (Supporting Information Figure S1). After that, the selenium nanoparticles kept growing to form rod-like nanoparticles while the solution was placed in a 70 °C oven for further growth. To investigate the evolution of selenium crystal phases during growth process, the XRD patterns at different growth times were carried out via quenching process (Figure 2). The XRD patterns at early stages of 5 (Figure 2i) and 10 (Figure 2ii) min indicate three phases. The phases are αmonoclinic (JCPDS card No. 24-1202), β-monoclinic (ICSD PDF No. 01-073-6182 24-714), and trigonal (JCPDS card No. 06-0362) phases. When the growth time extended to 20 min (Figure 2iii), both the intensities of α- and β-monoclinic phases manifestly decreased but the trigonal phase remained, indicating that the thermodynamically unstable crystal phases, i.e., α-m and β-m, underwent a phase transformation into trigonal phase. When extending the growth time to 40 min (Figure 2iv), only the intensity of the trigonal diffraction significantly increased, and none of α- and β-monoclinic diffraction could be found at this stage. At the end of 60 min 2834

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Figure 2. Time-evolution of XRD patterns from the quenched selenium samples with 4 min seeding time and different growth times of (i) 5 min, (ii) 10 min, (iii) 20 min, (iv) 40 min, and (v) 60 min.

growth process (Figure 2v), the intensity of the t(100) representative diffraction peak increased drastically. The phenomenon means that the selenium nanorods with a preferential orientation laid on the glass slide upon XRD characterization. An interesting question arose from the XRD pattern of 60 min: Was phase transition of later formed selenium indeed slowed down? An experiment had been done to answer the question: It is that the 60 min sample was particularly characterized again by using double integration time for collecting the diffraction data, which showed two clearly characteristic diffraction peaks at 21.0° and 29.9° corresponding to the (021) plane of α-monoclinic phase and the (1̅22) plane of β-monoclinic phase, respectively (Supporting Information Figure S5). It could be reasonably expected that the collected SeNRs exist some crystallinity-imperfect segments (discussed below), i.e., quasi-trigonal segments, which are ascribed to an inadequate crystal-healing time for phase transition. At the same time, a series of samples with different growth times were examined by TEM and statistically analyzed. Interestingly, the sample at 15 min of growth time showed a remarkable phenomenon in their distribution plots of length versus diameter (Figure 3, first row). The distribution diagram of the 15 min sample reveals two distributions separated by a gap (Figure 3, gap A), indicating an unusual crystal growth. The center populations of two different lengths of selenium nanorods are at 61.4 ± 13.1 nm and 126.5 ± 15.0 nm. The mean length of the latter is almost two times longer than the former, and no other population is in between. The unusual phenomenon was rationally speculated that the longer SeNRs originated from the coupling of two shorter selenium nanorods. At the 20 min sample, the population at 126.5 ± 15.0 nm disappeared, and hence the size of the gap became larger around ca. 120 nm (Figure 3, gap B), which means that a new unit of the shorter SeNR (ca. 61.4 ± 13.1 nm) conjugated the coupling of two nanorods, forming the elongated SeNR with triple length. According to the corresponding XRD result (Figure 2iii), it is clear that these m-SeNRs via twice coupling had simultaneously converted to the trigonal phase and underwent further growth. In comparison with the 15 min sample, the population of the shorter SeNRs significantly decreased, meanwhile, resulting in the increasing population of

Figure 3. Particle dimension statistics on the diameters vs lengths of Se nanoparticles with the seeding time of 2 min and different growth times: 15, 20, and 25 min. The inserts are the corresponding TEM images for 15 and 20 min of growth time, respectively. (Blue circles and brown squares represent the spherical and rod-like Se nanoparticles, respectively. The gaps marked A and B indicate the evidence of interparticle coupling for two and three Se nanorods, respectively.).

spherical SeNPs. The depreciating of the above populations is ascribed to the occurrence of the Ostwald ripening process. When the solution went through the growth process for 25 min, the Ostwald ripening was still proceeding. Nevertheless, the twice coupled SeNRs kept growing in the solution. A number of reactions, especially in surface catalytic reactions, usually depend strongly on their exposed planes of crystals.31 Accordingly, the SBS-SeNRs should supposedly exist because of the difference of reaction activity between trigonal and quasitrigonal phases. Therefore, an oxidation reaction of thiols catalyzed by selenium16b/selenite32 was chosen as a demonstration to distinguish the trigonal segments from selenium nanorods. It has been proposed that selenite can catalyze the oxidation of thiol molecules (denoted RSH), resulting in the transformation of RSH to the RSSR molecular group.32 Besides, selenium can also catalyze the thiol molecules not only to form disulfide bonds, i.e., RSSR, but to produce HSe− anions, and subsequently the HSe− anions would be oxidized by oxygen.16b According to the above studies, it can be realized that the selenium surface is very active to thiol molecules whether in the form of selenium or selenite. 3-Mercaptopropyltrimethoxysilane molecules, denoted MPTMS, are a suitable candidate for the surface modification of selenium nanorods. In the demonstration of MPTMS modification (Figure 4a,b), the trigonal segments could be obviously distinguished from SBSSeNRs via the fast Fourier transform (i.e., FFT) analyses 2835

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Scheme 2. Proposed Formation Mechanism of Selenium Nanorods via Inter-Nanorod Coupling to the Formation of (Repeating) tq−t−tq Crystalline-Segmented Selenium Nanorodsa

Figure 4. TEM images of SeNRs prepared from the sample with 6 min seeding, followed by a MPTMS modification. (a) SeNRs with a lump of MPTMS modification on the trigonal segments of SeNRs. (b) Enlarged image of single MPTMS-modified SeNR (dashed circle: lump location of MPTMS). (c) FFT image converted from the solid box of part b, showing that the reciprocal pattern follows up the reflection condition of typical trigonal selenium, i.e. l = 3n. (d) FFT image converted from dashed box of part b, revealing that four additional spots (dashed circles) equally separated at both sides of origin. (SeNR@MPTMS particles have been carefully examined, and the e-beam condition is mild enough without noticeably disturbing the crystal structure.)

a The schematic mechanism is described based on the case of (tq−t− tq)n-SeNRs (n = 3−4) ca. 1−2 μm in length.

as building units for the first-order coupling. Afterward, those m-SeNRs were elongated not only via the covalent bonding of freshly formed Se atoms at the both ends of β-monoclinic segments but via the first-order coupling. These longer mSeNRs around 200 nm in length originated from internanorods coupling of three m-SeNRs that simultaneously underwent a phase transition from m-Se to t-Se (Scheme 2c). Apparently, the coupling process dominated the elongation of selenium nanorods at the early stage of reaction. Due to a limited growing time for crystal self-healing, the freshly reduced selenium and smaller Se nanoparticles obtained via Ostwald process would rush into growth on and/or conjugation with tSeNR (ca. 200 nm) without precisely adjusting atom positions and, therefore, lost the threefold screw axis of trigonal phase, leading to quasi-trigonal segments conjugated with trigonal SeNRs, i.e., (tq−t−tq)-SeNRs (Scheme 2d). At the final stage of the growth process, these (tq−t−tq)-SeNRs will couple to each other, leading to the repeated (tq−t−tq)-SeNRs/SeNWs, denoted (tq−t−tq)n-SeNRs/SeNWs (n ≥ 2) (Scheme 2e). Silica Preferential Modification. As we mentioned above, MPTMS molecules were utilized to modify the surface of selenium nanorods, showing a lump of MPTMS molecules accumulated preferentially on the trigonal segments of SeNRs (Figure 4a,b). To expand nanostructural versatility, the SBSSeNRs were utilized as a unique hard-template to produce core−shell nanostructures via silica modification. MPTMS molecules not only modified the selenium surface as a mediated layer for silica condensation but decorated the SBS-SeNRs to form a unique float-like core−shell nanostructure. The appropriate (tq−t−tq)-SeNRs only containing a trigonal segment were modified by MPTMS, followed by silica condensation, as shown in Figure 5a. The float-like core− shell nanostructure of SeNR@SiO2 was named by nanofloats. Interestingly, using the elongated SBS-SeNR around ca. 2.8 μm, i.e., (tq−t−tq)5-SeNR, as template for silica modification did show a 1D string of beads-like core−shell nanostructure with six accumulations of silica beads on its surface (Figure 5b).

(Figure 4c,d). In Figure 4d, there are four additional reciprocal spots equally separated at both sides of origin, which is a clear evidence that the symmetry from naked segments (i.e., quasitrigonal Se) had a lower symmetry than trigonal, hence, the reflection-forbidden spots corresponding to (001) and (002) can be obtained. Some nearly transparent patches on the nanorod, shown in Figure 4b, were ascribed to e-beam damage during high-resolution e-beam alignment. A number of highresolution images taken from MPTMS-modified and naked areas of SBS-SeNRs were analyzed to exclude the alternation of crystal symmetries from electron illumination. These analyzed results simultaneously confirm that the preferential modification was localized on trigonal segments rather than quasitrigonal segments. However, so far we cannot definitely determine what reason governs the preferential accumulation of MPTMS molecules on trigonal segments. Nevertheless, the uneven MPTMS modification opens up an opportunity for the fabrications of various core−shell nanostructures with alternating structures on the surface (discussed in the later paragraph). Formation Mechanism. To elucidate the formation of SBS-SeNRs, CMC molecules play a crucial role during the reaction process because of the appropriate interaction between Se atoms and hydroxyl groups of its side chains. At the first step, namely, seeding step, selenious acid was reduced by sodium borohydride to form selenium nanomaterial. When the reaction system reached a supersaturated condition, spherical glassy selenium nuclei formed (Scheme 2a). After this stage, the solution was placed into a 70 °C oven for further growth, which was called the growth step. At the early stage of the growth process, the glassy selenium nuclei converted to spheroid-like m(α)-SeNPs, and then to form rod-like m(β)-SeNRs with a length of ca. 61 nm (Scheme 2b). The m-SeNRs were regarded 2836

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully acknowledge the funding of this work by MOST (Ministry of Science and Technology). Dr. M.-H. Liu thanks Dr. Ming-Wen Chu (Center for Condensed Matter Sciences, National Taiwan University), Prof. Ju-Hsiou Liao (Department of Chemistry and Biochemistry, National Chung Cheng University) and Dr. Heather F. Greer (EaStCHEM, School of Chemistry, University of St Andrews) for HRTEM assistance and valuable discussions in crystallography.

Figure 5. TEM images of silica preferential modification on the trigonal segments of (repeating) tq−t−tq symmetry-breaking segmented selenium nanorods after (a) 1° and (b) 2° inter-SeNR coupling.

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CONCLUSIONS A new crystal growth of 1D nanomaterial for symmetrybreaking segmented selenium nanorods (SBS-SeNRs) through internanorods coupling has been developed and shown in tq− t−tq alternating crystal structure. These dimension-controlled SBS-SeNRs were prepared under relatively lower reaction temperature ca. 70 °C via a facile solution-based method. The trigonal segments of SBS-SeNRs were formed via the first-order internanorods coupling of monoclinic segments and simultaneously accompanied by a m → t phase transition. The trigonal segments further served as intermediates for subsequent growing of quasi-trigonal selenium to produce (tq−t−tq)SeNRs. The quasi-trigonal segments of SBS-SeNRs were effectively retained against realistic growth condition by means of the assistance of CMC. The SBS-SeNRs subsequently underwent the second-order internanorods coupling to form (tq−t−tq)n-SeNR with repeating tq−t−tq structures. Through the assistance of suitable stabilizer under a mild condition, the unusual growth mechanism could probably be extended to the design of segmented 1D nanomaterials containing several discontinuous segments. MPTMS modification on (tq−t−tq)SeNRs demonstrates the reactivity difference between trigonal and quasi-trigonal crystal segments, resulting in an uneven coating. When the same notion was applied on the (tq−t−tq)nSeNR, the elongated (tq−t−tq)n-SeNR will form in a string of beads-like core−shell nanostructure. It appears that the symmetry-breaking segmented SeNRs could provide a special segmented platform to preferentially transform to other segmented materials via different reactivities on crystal structures of selenium.

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ASSOCIATED CONTENT

* Supporting Information S

Additional TEM images, statistical analyses, and XRD characterization. This material is available free of charge via Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(C.R.C.W.) E-mail: [email protected]. *(M.-H.L.) E-mail: [email protected]. Present Address †

(M.-H.L.) Center for Condensed Matter Sciences, National Taiwan University, Taipei, 10617, Taiwan. Notes

The authors declare no competing financial interest. 2837

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on April 8, 2015 with minor text errors in the Abstract. The corrected version was published ASAP on April 9, 2015.

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