Assembly of Ultrathin Gold Nanowires: From Polymer Analogue to

Mar 6, 2017 - Ultrathin nanowires (NWs) are considered to be ideal building blocks for the assembly of complex nanostructures toward future nanodevice...
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Assembly of Ultrathin Gold Nanowires: From Polymer Analogue to Colloidal Block Yuan Chen,† Yawen Wang,§ Jian Peng,† Qingchi Xu,‡ Jian Weng,*,† and Jun Xu*,‡ †

Department of Biomaterials, College of Materials, Xiamen University, Xiamen 361005, China Department of Physics, Research Institute for Biomimetics and Soft Matter, Fujian Provincial Key Laboratory for Soft Functional Materials, Xiamen University, Xiamen, 361005, China § Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China ‡

S Supporting Information *

ABSTRACT: Ultrathin nanowires (NWs) are considered to be ideal building blocks for the assembly of complex nanostructures toward future nanodevices. The polymer/ particle duality of ultrathin NWs plays an important role in the study of solution phase self-assembly behavior of ultrathin NWs; yet it has not been fully exploited. Herein, we demonstrate the effects of the polymer/particle duality of ultrathin NWs on the morphologies of assembled complex nanostructures. The length of ultrathin AuNWs directly correlates with the flexibility of NWs and affects the polymer-like assembly of NWs, while the concentration of surfactants determines interfacial tension and ligand−solvent interactions and affects both polymer-like and colloidal assembly of NWs. By fine-tuning these two factors, ultrathin AuNWs can swing between “soft” and “hard” building blocks, and highly uniform nanorings, nanograins, nanobundles, and superlattice-like nanospheres are obtained. The different assembly behavior of long and short NWs can be considered as two components to construct anisotropic complex nanostructures, in analogy with the fabrication of polymer−inorganic nanoparticle hybrid nanostructures. We synthesized anisotropic structures of Au nanodiamond rings and nanonecklaces by the coassembly of polymer-like long NWs with particle-like short NWs or Au nanoparticles. This strategy could potentially be extended to the organization of anisotropic complex nanostructures with other ultrathin NW systems in the future. KEYWORDS: ultrathin gold nanowires, polymer-like assembly, emulsion, anisotropic nanostructures, self-assembly

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More importantly, the 1D property of ultrathin NWs also allows them to be treated as “soft” building blocks, which can undergo conformational change during the assembly process. Several groups illustrated that inorganic ultrathin NWs less than 5 nm or even in the subnanometer scale, such as Bi2S3,26,27 In2S3,28 and GdOOH,29 were analogous to macromolecules or linear polymers and displayed excellent flexibility. These highly flexible ultrathin NWs could be assembled in a colloidal system and demonstrated conformational diversity of the assembled structures.23,24 Obviously, ultrathin NWs are both polymer-like (“soft”) and colloidal nanoparticle-like (“hard”) in the solution phase and can be considered as both linear polymer analogues and colloidal nanoparticles. Generally, the polymer/particle duality of ultrathin NWs is directly related to their length (assuming

ltrathin nanowires (NWs) are of particular interest among various inorganic nanostructures owing to their one-dimensional (1D) structure and outstanding physical and chemical properties.1−10 In practice, they are usually assembled into highly ordered architectures, where the size- and shape-dependent properties can couple, and widely applied in sensors, electronics, photonics, and bioelectronics.11−19 One of the key issues in fabricating the nanowire-based devices is to assemble the NWs in a controlled manner. In a colloidal system, a tremendous number of methods have been developed to assemble ultrathin NWs over the past decades. One common approach is to treat the ultrathin NWs as normal nanoparticles and apply similar assembly strategies. In this way, the NWs are considered as “hard” building blocks, which would not change shape during the assembly process. Several simple nanostructures, including 1D bundles, 2D films, and nanosheets, can be obtained by interface assembly, ligandinduced self-assembly, colloidal self-assembly, etc.20−25 © 2017 American Chemical Society

Received: November 18, 2016 Accepted: March 6, 2017 Published: March 6, 2017 2756

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Figure 1. Schematic illustration of the assembly of ultrathin AuNWs in a solvent-shifting emulsion system.

(Figure 2). In brief, 150 μL of oleylamine (OAm) and 3 mg of HAuCl4·3H2O were mixed in 2.5 mL of tetrahydrofuran (THF)

the NW has a constant diameter). When the NW is long enough, the aspect ratio is high, leading to large conformational entropy. In this situation, the NWs display polymer-like high flexibility and can be easily manipulated (bent, rotated, coiled, and so on) in the exogenous process. As the length decreases, the polymer-like properties of the NWs gradually fade, while inherent colloidal properties gradually arise. When the NWs are short to a certain degree, they appear as “hard” building blocks, and the colloidal behavior becomes dominant. That means assembly of short NWs is merely governed by the interactions of NW−NW, NW−medium, and NW−surfactant (such as electrostatic interactions, van der Waals forces, and entropic forces),30−32 and the external factors resulting in intra-NW deformation are not considered.16 The polymer/particle duality plays an important role in determining the morphologies of ultrathin NW assembled complex nanostructures, but has been seldom considered in solution systems, let alone the effects of the duality on the morphologies of assembled nanostructures.28,29 To precisely control the assembly of ultrathin NWs, it is of great importance to understand the mechanism of how the polymer/particle duality affects the morphologies of the complex nanostructures based on ultrathin NWs. Herein, we systematically study the effects of polymer/ particle duality on the morphologies of ultrathin NW assembled complex nanostructures (Figure 1) based on previously reported solvent-shifting emulsion systems.33−35 In the current system, the length of ultrathin AuNWs directly correlates with the flexibility of the NWs and affects the polymer-like assembly of NWs, whereas the concentration of surfactants determines the interfacial tension and ligand−solvent interactions and, thus, affects both polymer-like and particle-like assembly of NWs. Thus, by varying the length of the NW and the concentration of surfactants, the flexibility of NWs, interfacial compression, as well as ligand−solvent interactions is finetuned to obtain uniform nanorings, nanograins, nanobundles, and superlattice-like nanospheres. In addition, long and short ultrathin NWs that have different assembly behaviors can be considered as two different components to construct anisotropic complex nanostructures, such as nanodiamond rings and nanonecklaces.

Figure 2. TEM images of (a) AuNW-I, (b) AuNW-II, and (c) AuNW-III and respective length distributions. (d) Viscosity versus shear rate of AuNW-I, AuNW-II, and AuNW-III and OAm (6 wt %).

to form a mixture solution. Into three of the above-mentioned solutions, 0, 100, and 200 μL of H2O was introduced to generate solutions 1, 2, and 3, respectively. After that, 100 μL of triisopropylsilane (TIPS) was added to each solution, and the color of the solutions changed from orange to yellow due to the reduction of Au(III) to Au(I) (Figure S1a,b). Dark brown solutions (Figure S1c) were obtained after incubation at room temperature for another 5 h, and Au NWs can then be precipitated with ethanol. The AuNWs obtained from solutions 1, 2, and 3 are hereafter referred to as AuNW-I, AuNW-II, and AuNW-III, respectively. Figure 2a−c presented TEM images of the as-synthesized AuNW-I, AuNW-II, and AuNW-III. It was clear that AuNWs in all these samples are ultrathin with a diameter of 2−3 nm (Figure S2). Figure S3a,b show the HRTEM of a typical ultrathin Au NW (AuNW-II). The NW was single-crystalline with the Au(111) planes perpendicular to the wire axis, which

RESULTS AND DISCUSSION Ultrathin AuNWs with a diameter of 2−3 nm were first synthesized following the reported method with slight modification:36 the length (LAuNW) was precisely controlled ranging from a few hundred nanometers to a few micrometers by introducing different amounts of H2O during synthesis 2757

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Figure 3. SEM and TEM images of different AuNW-based assembly structures: (a, b) Au nanorings; (c, d, e) Au nanograins; (f, g) Au superlattice nanospheres. Images of the typical nanoring, nanograins, and superlattice-like nanospheres were inset in (b), (c), and (g), respectively (see Figure S3 for the corresponding HRTEM images).

was consistent with previous reports.36,37 As shown in Figure 2a, without introduction of H2O, all ultrathin Au NWs are longer than 1 μm (Figure S4, the statistics of the ultralong NWs was based on the most intuitive approach). Meanwhile, the measured mean LAuNW from TEM images are around 598 nm for AuNW-II (Figure 2b) and 128 nm for AuNW-III (Figure 2c), respectively. The fluidic behavior of the as-synthesized ultrathin AuNWs was next investigated to disclose the difference in polymer-like properties among the three types of NWs. As shown in Figure 2d, the concentrated (3 mg/mL) AuNW solution (including AuNW-I, AuNW-II, and AuNW-III) demonstrated typical nonNewton fluid behavior with clear shear thinning at high shear rates. The results are in good agreement with the rheological properties of polymers,38 indicating that the ultrathin structure endowed the AuNWs with a viscous property when dispersed in a nonpolar solvent. Thus, the ultrathin AuNWs could be considered to be polymer-like, and the longitudinal extension of NWs was similar to that of a linear polymer chain. The ultrathin NWs also exhibited length-dependent fluidic behavior, which is normally observed in a linear polymer system. In polymer physics, the polymer chain length is an important factor affecting viscosity. There exists a relationship between melt viscosity η and the polymer chain length Z roughly as η ∼ Z3.2; that is, the longer the molecules, the higher the viscosity would be at a given concentration.39 In our experiments, at low shear rates, longer NWs had higher viscosity, while shorter NWs had lower viscosity. This observation was consistent with classical polymer theory. Dynamic light scattering (DLS) was further used for confirming the length difference among AuNW-I, AuNW-II, and AuNW-III. As shown in Figure S5, the measured hydrodynamic radius is 184.4 nm for AuNW-I, 64.6 nm for AuNW-II, and 48.4 nm for AuNW-III, respectively. Such results showed the same trends as that obtained from TEM images and rheograms of AuNWs, indicating the length difference among AuNW-I, AuNW-II, and AuNW-III. The length difference in DLS and TEM images suggested Au NWs adopted different conformations in THF.29

The surface plasma resonance (SPR) absorption peak of AuNWs (Figure S6) also supported the length difference among AuNW-I, AuNW-II, and AuNW-III. In the IR region, the AuNWs with different lengths appear in the longitudinal SPR absorption peak.40 For AuNW-III (mean length around 128 nm), the peak position is observed in the near-IR region (1000−2000 nm) (Figure S6) as reported previously.40 When the length increased (AuNW-II, mean length around 598 nm), the SPR peak was shown significantly red-shifted to the mid-IR and far-IR regions, where the signal was detected in the mid-IR (∼6 μm), but the far-IR signal was now shown due to equipment limitation. However, we did not observe a significant band in the UV−vis, near-IR, and mid-IR regions for AuNW-I (mean length >1 μm), and the SPR peak of this superlong AuNW was supposed to appear in the far-IR region. Subsequently, the assembly of ultrathin AuNWs was conducted using a previously reported solvent-shifting method (Figure 1). Under vigorous stirring, Triton X-100 was quickly added into the purified AuNWs to generate emulsions. During the emulsification process, the THF was gradually excluded from the OAm oil droplet to water, resulting in the compression of the confined AuNWs, well-known as the “solvent-shifting” method.33 The mixture was then heated at 60 °C for 2 h to remove the residual THF, causing further contraction of AuNW bundles and giving the final assembled nanostructures. We first studied the effect of the LAuNW on morphologies of the AuNW assemblies in the absence of Triton X-100 (CTriton X‑100 = 0). As shown in Figure S7a,h,o, the final structures of the AuNW assemblies are independent of LAuNW, and all three types of AuNWs yield disordered structures composed of randomly aggregated NWs. In the absence of surfactant, the assembly system consists only of OAm-stabilized AuNWs, THF, and H2O, resulting in formation of Pickering emulsion.41 No doubt, interfacial tension between AuNWs and oil droplets was extremely high, and the immiscible system (OAm and H2O) confined Au NWs at the droplet interface, no matter how long the NWs were. Meanwhile, the van der Waals interaction among NWs became the dominated force to 2758

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phase transition and the decrease of contact angle (θow, where an oil−water interface meets the AuNW surface, as illustrated in Figure 4).

prevent the sliding of NWs to rearrangements and induced aggregation of AuNWs to minimize their surface energy. Once the THF evaporated, the solvent-shifting process further compressed the NWs, and the van der Waals interaction among NWs became even stronger. As a result, the trapped NW assembled into a disordered aggregation (Figure S7a,h,o). When 4 mM (Figure S7c,j,q) to 8 mM (Figure S7d,k,r) of CTriton X‑100 was introduced into the system, the morphologies of the NW assemblies start to exhibit a distinct length-dependent effect. Consistent with a previous report,42 the longest NWs (AuNW-I > 1 μm) gave nanorings with a diameter of about 786 nm in high yield (Figure 3a,b). The inset TEM image suggests that the Au nanoring is composed of parallel-packed AuNWs bundles. In addition, from HRTEM images (Figure S3c,d), the single NW could be identified with the diameter of ∼2 nm and has the same lattice orientation as the synthesized Au NWs (Figure S3b), confirming that nanorings are composed of AuNWs. As LAuNW decreases to about 598 nm, a large area of uniform grain-like ellipsoidal nanostructures with a mean length of 439 nm and width of 231 nm (Figure S8) was observed (Figure 3c,d). Close inspection of the nanograins reveals that they are also composed of parallel-packed ultrathin AuNWs (Figure 3e, Figure S3e,f, and Figure S9). Figure S9 presents the two ends of a single nanograin, and the obviously curved AuNWs could also be identified in the middle part of the nanograin. Further reducing LAuNW to about 128 nm (AuNWIII) leads to formation of spherical assemblies with a mean diameter of about 200 nm (Figure 3f). TEM (Figure 3g) and HRTEM (Figure S3g,h) characterization confirmed that the solid nanosphere has a superlattice-like structure that is composed of highly ordered short AuNWs, although there were tiny fusions during the process of the assembly of AuNWIII. Au nanorings, Au nanograins, and Au superlattice-like nanospheres were further characterized by DLS and smallangle X-ray scattering (SAXS). As shown in Figure S10, the hydrodynamic radius is 326.8 nm for Au nanorings, 515.2 nm for Au nanograins, and 1380.1 nm for superlattice-like nanospheres, respectively. The size of each type of assembly obtained from DLS was consistent with that observed from TEM images, suggesting a morphological difference among the AuNW-I, AuNW-II, and AuNW-III assemblies (Figure S10). For SAXS characterization, a peak was observed at q = 88.1 Å−1 for Au nanorings, 90.3 Å−1 for Au nanograins, and 87.4 Å−1 for superlattice-like nanospheres, respectively. The positions of the peaks in the SAXS spectra roughly corresponded to 0.71, 0.69, and 0.71 nm of center-to-center distance between the neighboring AuNWs (d = 2π/q),43 which was in accordance with those measured from the TEM. The SAXS patterns (Figure S11 insets) display bright diffraction rings, revealing highly ordered assembles within the nanostructure.44 The assembly condition of the system with Triton X-100 was similar to that in previously reported emulsion systems.35 Small OAm/THF oil droplets formed first at the initial stage of mixing; meanwhile the ultrathin NWs tended to form “‘stacked’” parallel bundles because of van der Waals interactions. The formed bundles with partial wettability in both immiscible fluid phases (THF/OAm and H2O) could stay at the interface with the stabilization of Triton X-100. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) supported the presence of Triton X-100 on the surface of the AuNW assemblies (Figures S12 and S13). Upon heating, the complete evaporation of THF led to the successful

Figure 4. Schematic illustration for the process of phase transfer of ultrathin AuNWs from THF/OAm oil droplet to H2O and the change of contact angle θow depending on the concentration of Triton X-100 (CTriton X‑100).

Initially the oil droplets are large enough to allow the NWs to be straight. If the NWs are long enough, as the oil droplets shrink, they would have to coil, leading to nanorings with multiloops in water. From the energy viewpoint, the coiling process is driven by the phase transfer energy, Etransfer,45 while van der Waals interaction (between opposing monolayers of OAm on the surface of ultrathin AuNWs) provides the stacking energy, Estacking, to stabilize the ring structures (eq 1). The transfer energy can be express as eq 2, where Ainter(dbundle) is the interfacial area as a function of diameter of the Au NW bundle, γow is the surface tension, and θow is the contact angle. For Estacking (eq 3), σ stands for the van der Waals interaction between the unit area of monolayers of OAm and Acontact(dbundle) represents the contact area of AuNW bundles as a function of diameter of a Au NW bundle.29 Estrain is expressed as eq 4 (see Supporting Information for details), where E is the Young’s modulus of the AuNW bundle, k is the curvature, and I0 = πR4AuNW/4 is the geometrical moment of inertia of a bundle.46−48 When Etransfer and Estacking triumph over strain energy Estrain generated from the induced curvature, AuNW bundles prefer to be bent and remained inside of the oil droplet to minimize their exposure to water. ΔE = Etransfer + Estacking − Estrain > 0

(1)

Etransfer = A inter (dbundle)γow(1 − |cos θow|)2

(2)

Estacking = σAcontact (dbundle)

(3)

Estrain

⎤ ⎡ ⎛ ⎞2 ⎛ ⎞ dbundle d ⎢3⎜ 3 bundle ⎟ +⎜ ⎟− ⎥ =⎢ ⎜ DAuNW ⎟ DAuNW ⎟ ⎜ 4 4 ⎥⎥ ⎝ dAuNW + 2 ⎠ ⎢⎣ ⎝ dAuNW + 2 ⎠ ⎦ EI0k 2/2

(4)

From eq 4, it can be seen that the strain energy of the NWs is proportional to k2. Therefore, for long NWs, the formation of a large ring is favorable. With a large radius R, AuNW bundles can have low bending curvature (k) and thus low Estrain stored in the bent NWs (Figure 3a,b). As a result, with proper CTriton X‑100, the longest NWs (AuNW-I) would yield nanoring structures, demonstrating the typical polymer-like assembly behavior. When LAuNW decreases, the obtained nanoring structures would have to either adopt fewer loops and/or reduce its radius 2759

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ACS Nano to remain stable. Decreasing the number of loops would decrease the stacking energy, Estacking, while a larger k would result in a higher strain energy, Estrain, both of which would decrease the final ΔE. Therefore, a small ring with a higher curvature and a large ring composed of fewer loops are both unfavorable. Instead, AuNWs preferred to bend in a low curvature state to reduce its strain energy, leading to the formation of nanograins (Figure 3c−e). It is also noticed that there exist many twisted bundles and gaps on the surface of a grain, which are produced by the volume expansion during the bending process (Figure S14). This strongly supports that the nanograins are formed by bending of NWs instead of stacking them. The intrinsic colloidal properties of AuNWs also gradually increase with the decrease of LAuNW. To minimize the exposure to water, the AuNWs tend to assemble into a thicker bundle. A combination of polymer-like assembly and colloidal assembly led to the formation of a nanograin structure. The van der Waals interactions among NWs provided additional energy to stabilize the obtained nanograins. When the OAm was further removed by washing with ethanol three times, the nanograin structure could be partially destroyed (Figure S15). When LAuNW continues to decrease (AuNW-III), the curvature resulting from bending the short NWs would be even higher when a quantity of bending energy is needed to overcome the strain energy (assuming a short NW bundle could form a nanoring). Consequently, the nanograin morphology became unfavorable. The NWs behave similarly to the colloidal nanorods, and a colloidal assembly arose. Instead of nanorings or nanograins, the superlattice-like nanospheres (Figure 3f,g) were obtained, demonstrating the intrinsic colloidal self-assembly behavior of the shortest NWs. The surfactant concentration was further adjusted to investigate the morphological transformation behavior of the NWs during the phase transition process. At a CTriton X‑100 larger than 20 mM, assemblies of the shortest AuNWs remained nanospheres, while the assemblies of longer AuNWs (AuNW-I and AuNW-II) become NW bundles, both with a diameter of about 200 nm but a length of about 3.8 and 2.3 μm, respectively. In emulsion systems, the higher the concentration of surfactant, the lower the interfacial interaction and the smaller the contact angle θow would be (Figure 4). Thermodynamically, when the interfacial interaction or the contact angle decreases, the energy of the phase transition Etransfer decreases accordingly, until it could not overcome the induced strain energy (eq 2). Hence, at the high surfactant condition, the long AuNWs remained in a straight state to form NW bundles rather than bending into rings or grains (Figure S7e,f,g). However, for the short NWs (AuNW-III), since they already possess a very high induced strain energy, a further increase of the surfactant concentration would not “convert” their colloidal properties much, and superlattice-like nanospheres were the dominant assemblies (Figure S7s,t,u). As discussed in the previous section, both LAuNW and CTriton X‑100 are critical to the morphology of the AuNW assembly in an emulsion system. A phase diagram showing the morphology transition from seven different CTriton X‑100 values as a function of LAuNW values is presented here (Figure 5, the corresponding SEM image for each point is shown in Figure S7). A number of trends could be illustrated. First, at a low CTriton X‑100 value (1 mM) or without surfactant, AuNWs formed random aggregation with disordered structures

Figure 5. Phase diagram of the morphological transition for AuNW assembled nanostructures plotted by LAuNWs vs CTriton X‑100. Different structures are denoted by different shapes: ⊗ (random aggregation); ⧫ (nanograin); ◎ (nanoring); ● (superlattice-like nanosphere); and thick bold line (nanobundle). The scale bars are 500 nm.

regardless of LAuNW, because the low CTriton X‑100 could not provide adequate stabilization for oil/THF droplets and the AuNWs had no choice to serve as stabilizers themselves. The van der Waals interaction among NWs and the surface tension between oil/H2O, independent of LAuNW, “locked” the NW from rearrangement, resulting in the disordered structure. A moderate CTriton X‑100 (4−8 mM) could provide sufficient phase transition energy to drive the polymer-like assembly of the highly flexible ultralong NWs (AuNW-I, LAuNW > 1 μm), which coil into nanorings. Other products, such as racket shapes, eight shapes, and twisted rings, also provided evidence of polymerlike assembly with conformational diversity of ultralong NWs (Figure S16). NWs with shorter length (AuNW-II, LAuNW ≈ 598 nm) exhibit both polymer-like and particle-like behavior, and nanograins formed as a result of competition between polymer-like assembly and colloidal assembly. When LAuNW decreases to ∼128 nm (AuNW-III), colloidal assembly became predominant over polymer-like assembly and gave superlatticelike nanospheres. At high CTriton X‑100 (>8 mM), surface tension at the oil−water interface was further reduced by the surfactant; therefore the energy of the phase transition became too small. As a result, medium to long NWs (AuNW-I, AuNW-II) tended to form straight bundles in solution rather than bend into rings or grains. We further studied the effect of AuNW concentration on the morphologies of the assembled nanostructures. As shown in Figures S17, S19, and S20, at low CTriton X‑100 (1 mM) and high CTriton X‑100 (40 mM), the morphologies for each type of AuNW assemblies were not significantly affected by the concentration of AuNWs. At a moderate C Triton X‑100 (8 mM), the morphologies of both AuNW-I and AuNW-II assemblies were slightly changed with AuNW concentration. For AuNWI, decreasing the concertation of NW (Figure S17g−j) resulted in nanorings with a smaller diameter and thinner rim, whereas increasing the concertation of NW resulted in nanorings with a larger diameter and thicker rim (Figure S18). For AuNW-II, when we reduced the concentration of AuNW-II to 10 μL (1/ 40 of normal concentration), the morphology of assemblies 2760

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ACS Nano became cage-like nanostructures (Figure S19g). Increasing the concentration of AuNW-II resulted in the same nanograins (Figure S19l) as that obtained from normal NW concentration (Figure S19k). Generally, decreasing the concentration of NW decreased the diameter of Au NW bundles. To respond to the decreasing diameter of the AuNW bundle, the assembled structures would have to increase their curvature to store more strain energy (according to eq 4). Thus, small rings (AuNW-I, Figure S17g) or nanostructures with higher curvature (Au NWII, Figure S19g) were favorable. On the contrary, increasing the diameter of AuNW bundles resulted in nanorings with a large radius (Figure S18) or nanostructures with a lower curvature to lower the strain energy. Such results suggest that the assembly behaviors of all three types of AuNW are not significantly affected by the AuNW concentration. Theoretically, the persistence length of a single AuNW is around 758 nm (see the Supporting Information for details) and is significantly different from the length of AuNW-I, AuNW-II, and AuNW-III. Therefore, the assembly behavior of these three types of AuNWs are thought to be less affected by the variation of diameter of the AuNW bundles (depending on the AuNW concentration). As illustrated by the phase diagram in Figure 5, at a moderate CTriton X‑100, AuNW assembly could be fine-tuned between a polymer-like assembly and a colloidal assembly by controlling the length of NWs. Co-assembly of long polymer-like NWs and short colloidal NWs could also be considered as an analogue to the coassembly of polymer and inorganic nanoparticles.49−53 Anisotropic nanostructures are expected from the coassembly process. Indeed, we demonstrate the formation of two diamond ring structures and a necklace structure with the solvent-shifting method. The wire-decorated nanodiamond ring (W-nanodiamond ring) in Figure 6a was assembled by combining AuNW-I and AuNW-II. In particular, the ring could be divided into two parts: a bundle gathered by short wires and a ring coiled by long wires (Figure 6b). For comparison, we also assembled AuNW-I and Au nanoparticles (which have typical colloidal dominated assembly behavior) in the same condition, and a particle-decorated nanodiamond ring (P-nanodiamond ring) can be obtained (Figure 6c). As shown in Figure 6d, the “diamond” part of the nanodiamond ring was formed with Au nanoparticle aggregates. The formation of a nanodiamond ring in both ways indicated that, like nanoparticles, short NWs also gave superlattice-like structures through colloidal assembly. In order to minimize surface energy, the “diamond” part assembled on the “ring” part rather than isolated in the solution. In another case, when a high CTriton X‑100 was applied, the compression energy in the solution system was not enough to impel long wire coiling, leading to formation of a necklacelike structure (nanonecklace, Figure 6e,f). This facile preparation process envisaged a bright blueprint for developing various anisotropic hybrid structures through tuning colloidal properties and polymer-like properties.

Figure 6. (a) SEM and (b) TEM images of a W-nanodiamond ring; (c) TEM image of a P-nanodiamond rings; (d) TEM image of a single P-nanodiamond ring; (e) SEM image of a nanonecklace; (f) TEM image of a nanonecklace. Inset: Enlarged TEM image of the designated spot.

like long NWs and colloidal-dominated short NWs) to form a hybrid, making the polymer-like and colloidal assembly coexist in a solution system. This scalable and facile coassembly route creates two types of nanodiamond rings and nanonecklaces. Thus, a deep understanding of the inorganic ultrathin NW assembly in a colloidal system will be highly beneficial in developing other types of functional nanoassemblies, particularly for future anisotropic hybrid materials with two different components.

METHODS Materials. Gold(III) chloride trihydrate was purchased from Sigma-Aldrich. Oleylamine was purchased from J&K Scientific Co., Ltd. (Beijing, China). Triisopropylsilane was purchased from Alfa Aesar. Other chemicals were purchased from Xilong Chemical Co., Ltd. (Shantou, China). All chemicals were used without further purification. Preparation of Ultrathin AuNWs. A 100 μL amount of OAm and 3 mg of HAuCl4·3H2O were added into 2.5 mL of THF to form a yellow solution. Then 150 μL of TIPS was added to the mixture, and this solution was incubated at room temperature for 5 h to obtain AuNW-I (the longest). For the synthesis of different lengths of NWs and Au nanoparticles, H2O must be properly increased (details will be discussed in the following parts). For the synthesis of AuNW-II, AuNW-III, and Au nanoparticles, 100, 200, and 500 μL of H2O were respectively used, while other materials and conditions remained unchanged. A 400 μL amount of as-synthesized AuNW solution was

CONCLUSION In summary, we systematically studied the effects of polymer/ particle duality of ultrathin AuNWs on the morphologies of assembled complex nanostructures. A series of different structures including nanorings, nanograins, superlattice-like nanospheres, and nanobundles are generated by tuning the length of AuNWs and the concentration of Triton X-100. Furthermore, we demonstrate the use of ultrathin inorganic NWs with two distinctive natures of characteristics (polymer2761

DOI: 10.1021/acsnano.6b07777 ACS Nano 2017, 11, 2756−2763

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ACS Nano purified with ethanol (EtOH) and redispersed in 400 μL of THF for further use. Preparation of Au Nanorings, Nanograins, Straight Nanobundles, and Superlattice-like Nanospheres. A 1 mL amount of Triton X-100 (8 mM) aqueous solution was quickly injected into purified AuNW-I under vigorous stirring. The resulting mixture was heated at 60 °C for 2 h without sealing to evaporate THF to obtain Au nanorings. Au nanograins, straight nanobundles, and superlattice-like nanospheres were prepared by the same method except using different length NWs or different concentrations of Triton X-100. Preparation of Wire-Diamond Rings (W-Diamond Rings) and Nanoparticle-Diamond Rings (N-Diamond Rings). A 100 μL amount of as-synthesized AuNW-I was mixed with 100 μL of AuNWII; then 1 mL of Triton X-100 (8 mM) aqueous solution was quickly injected into purified AuNW-I under vigorous stirring. The resulting mixture was heated at 60 °C for 2 h without sealing to evaporate THF to obtain W-diamond rings. For the synthesis of N-diamond rings, 100 μL of AuNW-I was mixed with 100 μL of Au nanoparticles, while other materials and conditions remained unchanged. Preparation of Nanonecklaces. A 100 μL amount of assynthesized AuNW-I was mixed with 100 μL of AuNW-II. Then 1 mL of Triton X-100 (20 mM) aqueous solution was quickly injected into purified AuNW-I under vigorous stirring, and other steps were the same as those for the preparation of Au nanorings. Characterization. Scanning electron microscope (SEM) images were obtained under SU-70 with an accelerating voltage of 20 kV. The samples were prepared by dropping 20 μL solutions of the products on a silicon wafer. Transmission electron microscope (TEM) and highresolution TEM (HRTEM) images were taken from a EM-2100 operated at 200 kV. The viscosity curve was measured from an Anton Paar MCR302 rotational rheometer using a CP25-2-SN rotor with a 0.1 mm gap. The concentration of AuNWs was 3 mg/mL, and that of OAm was 6 mg/mL. For DLS spectra, they were all obtained from a laser light scattering spectrometer (ALV/DLS/SLS-5022F) using a solid-state He−Ne laser (output power of 22 mW at λ = 632.8 nm) and a digital time correlator (ALV-5000/EPP). The optical absorption spectra were carried out with a UV−vis spectrometer (UV-2550), and FTIR was collected from a Fourier transform infrared spectrometer (Nicolet IR200). SAXS patterns were generated using a Xeuss 2.0 system (Xenocs SA, Grenoble, France). Thermal gravimetric analysis was carried on an SDT-Q600 instrument at a heating rate of 10 °C min−1 under a nitrogen atmosphere.

Universities (20720150016, 20720150017), National Key Scientific Research Projects (2014CB932004), and the 111 Project (B16029).

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07777. Photographic images, TEM images, SEM images of AuNW assemblies with different AuNW concentrations and different surfactant concentrations, SEM images of other control experiments, DLS, FTIR, TGA, SAXS, and derivation of strain energy (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jun Xu: 0000-0002-3096-3217 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was financially supported by the National Nature Science Foundation of China (21401153, 21503175, 31371005), the Fundamental Research Funds for the Central 2762

DOI: 10.1021/acsnano.6b07777 ACS Nano 2017, 11, 2756−2763

Article

ACS Nano

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DOI: 10.1021/acsnano.6b07777 ACS Nano 2017, 11, 2756−2763