Controlled Synthesis and Flexible Self-Assembly of Monodisperse Au

Feb 13, 2017 - Moreover, unprecedented binary nanocrystal SLs can be achieved ... Liu Huang , Xiaodong Wan , Hongpan Rong , Yuan Yao , Meng Xu , Jia ...
0 downloads 0 Views 3MB Size
Download PDF
Article pubs.acs.org/cm

Controlled Synthesis and Flexible Self-Assembly of Monodisperse Au@Semiconductor Core/Shell Hetero-Nanocrystals into Diverse Superstructures Liu Huang,† Jiaojiao Zheng,† Lingling Huang,‡ Jia Liu,† Muwei Ji,† Yuan Yao,§ Meng Xu,† Jiajia Liu,† Jiatao Zhang,*,† and Yadong Li∥ †

Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China ‡ School of Optoelectronics, Beijing Institute of Technology, Beijing 100081, China § Institute of Physics, Chinese Academy of Science, Beijing 100190, China ∥ Department of Chemistry and Collaborative Innovation Center for Nanomaterial Science and Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Monodisperse nonepitaxially grown Au@MX (MX = Ag2S, ZnS, or CdS) core/shell hetero-nanocrystals (HNCs) with controlled crystallinity, composition, anisotropically shaped Au cores but isotropic overall morphologies were successfully prepared. By using C18 acid/alkali co-capping surface ligands, the as-prepared Au@MX HNCs can selfassemble into large-scale two-dimensional monolayer and three-dimensional multilayer superlattices (SLs) on both flexible (graphene) and rigid (Si wafer or glass) substrates. Moreover, unprecedented binary nanocrystal SLs can be achieved through synergistic self-assembly of Au@MX HNCs together with Au NCs (or CdS NCs) into different intricate patterns. Experimental evidence and finite-element method theoretical simulations indicate that the Au@CdS HNC SL film taking up a closest-packing mode has a rate of photocurrent generation much higher than that of its disordered−assembled counterpart, probably because of the enhanced plasmon−exciton coupling permitted by the large-scale ordered nanopatterning.

■

optical Stark effect and spin manipulation.8 With improved plasmon−exciton coupling and an enhanced lifetime of excitons, the nonepitaxially grown Au@semiconductor HNCs have also shown great promise for applications in solar light harvesting and conversion. 9,10 To attain more flexible plasmon−exciton coupling in Au@semiconductor HNC systems, maneuvering the nanogaps between the metal and semiconductor is greatly important considering that the localized surface plasmon resonance (LSPR) and quantum confinement effect can be strongly enhanced when the distance between the two components is comparable to the characteristic lengths of related physical interactions.11 To fully exploit the unique characteristics and accelerate their application in bulk-scale optoelectronic devices, the large-scale assembly of nonepitaxially grown core/shell HNCs into superstructures through the bottom-up strategy is strongly

INTRODUCTION

Integrating multiple components within individual nanocrystals (NCs) is an efficient way to achieve close conjunction and synergistic interaction of materials with different chemical and physical properties at the nanoscale.1−6 Conventional methods for constructing such hetero-nanocrystals (HNCs) are mainly based on epitaxial growth, which often results in unintentional interface and crystalline imperfections and accordingly performance degradation of devices. To overcome this limitation, recently, a nonepitaxial growth strategy that allows synthesis and precise control of high-quality monocrystalline HNCs has been developed.7,8 In particular, on the basis of this strategy, plasmonic metal@semiconductor core/shell structured HNCs with an elaborately engineered interface and a perfect monocrystalline shell can be achieved to maximize the positive interactions between the plasmon and exciton for nanobiophotonics and nanoenergy applications. For example, these Au@semiconductor core/shell HNCs prepared through nonepitaxial growth have been demonstrated to display a superior plasmon−exciton resonant nature toward the tailoring of the © 2017 American Chemical Society

Received: January 5, 2017 Revised: February 12, 2017 Published: February 13, 2017 2355

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363

Article

Chemistry of Materials

of a Au@Ag2S colloidal solution in toluene was mixed with 0.1 mL of OA and 0.05 mL of OAm (2:1 OA:OAm mole ratio) while being magnetically stirred for 5 min to recap the colloid surface with ligands. Then, 2 mL of the methanol solution containing 0.04 g of Cd(NO3)2· 4H2O or Zn(NO)3·6H2O was added. After the mixture had been stirred for 10 min, 0.06 mL of TBP was added to the solution described above, and the resulting mixture was heated at 80 °C for 2 h while being magnetically stirred. The as-prepared Au@MX core/shell HNCs with OA/OAm co-capping ligands were washed three times with ethanol, and the final precipitates were dispersed in toluene for further assembly. With regard to controlling the shell thickness of Au@CdS HNCs, the experimental procedure was the same except that the reaction parameters in Au@Ag HNC synthesis were carefully adjusted, wherein the molar ratio of the AgNO3 precursor to Au seed was found to be vitally important to the final shell thickness. In addition, the reaction time could also determine the thickness of the CdS shell. Synthesis of Monodisperse Ag-Doped CdS QDs. The Agdoped CdS QDs for BNSL assembly were synthesized using the method developed in our previously published work.23 Similar to the shell of Au@CdS HNCs, these QDs were synthesized via the sequential in situ sulfuration and cation exchange reaction of the Ag NCs. In this study, monodisperse Ag NCs (∼5 nm) were prepared first using the method described by Li et al.,24 which can be further used in the preparation of amorphous Ag2S NCs by the sulfuration with an S precursor in a molar ratio of 1:5 at 50 °C. The obtained Ag2S NCs (0.035 mmol) were dispersed in 6 mL of toluene with 0.2 mL of OA and 0.1 mL of OAm. After that, 1 mL of the methanol solution containing 0.1 g of Cd(NO3)2·4H2O was added. After the mixture had been magnetically stirred for 10 min, 0.1 mL of TBP was added, and the mixture was heated at 80 °C for 2 h while being magnetically stirred. The as-prepared CdS QDs with OA/OAm co-capping ligands were washed three times with ethanol and redispersed in toluene for BNSL assembly. The resulting CdS QDs contained 1% Ag dopant. Self-Assembly of As-Prepared Monodisperse HNCs by the Solvent Evaporation-Assisted Dip-Coating Method. Dip-coating is a widely used and simple technique for manufacturing colloidal NCs films.25,26 Several kinds of rigid or flexible substrates, such as multigraphene (deposited on a transmission electron microscopy grid), ITO and FTO glass, silicon wafers, and carbon thin films, were vertically dipped into a Au@MX HNC colloidal suspension in toluene with concentrations of 0.3−1.5 mg/mL. Prior to the deposition of NCs, the substrates were immersed in the colloids without being disturbed for at least 6 h. By slow evaporation of toluene in air, controllable 2D or 3D ordered patterns were spontaneously deposited on the substrates as a result of the self-assembly of Au@MX HNCs. During this process, the withdrawal speeds of the substrates were 10 nm/s, and the temperature of the whole system was 35 °C. Generally, monolayer and multilayer assemblies can be obtained from diluted and concentrated HNC colloids, respectively. To tune the thickness of the films, a multistep approach was employed in this experiment in which the high-concentration HNC colloidal suspensions (>0.6 mg/mL) were first utilized to manufacture the multilayer films via the dipcoating process. Next, these colloids were diluted by adding small amounts of toluene approximately equivalent to 20% of the initial colloid volume followed by dip-coating for the formation of SLs. In the remaining steps, the diluting/dip-coating process outlined above was repeated until the monolayers were obtained. Note that herein the concentrations of these colloids were decreased to ∼0.3 mg/mL. In this way, a series of assemblies with different thicknesses was successively prepared. The BNSLs were obtained by the co-assembly of Au@CdS HNCs with Au NCs at a concentration ratio of 1:7, and the co-assembly of Au@ZnS HNCs with Ag-doped CdS QDs at a concentration ratio of 1:3, where Au NCs were the core precursors of Au@CdS HNCs. For the BNSLs, all the samples (including Au@CdS HNCs, Au@ZnS HNCs, Au NCs, and Ag-doped CdS QDs) were washed three times with ethanol and redispersed in toluene. Under similar capping conditions, the use of a toluene solvent guaranteed the good dispersity of the colloidal components mentioned above, and this was considered

desired. Generally, there are two kinds of strategies for accomplishing assembly, namely, self-assembly driven by noncovalent interactions (e.g., van der Waals force, dipolar interaction, entropic force, etc.)12−17 and template-assisted assembly.18,19 The self-assembly of nonepitaxially grown Au@ semiconductor HNCs into well-defined two-dimensional (2D) and three-dimensional (3D) complex structures on diverse substrates may stimulate unprecedented LSPR-induced collective plasmonic applications. Furthermore, the synergistic assembly of Au@semiconductor core/shell HNCs together with Au NCs to form binary nanocrystal superlattices (BNSLs) can provide a new way to create the binary cooperative systems where the nanogaps between intercomponents are dominant for their overall performance. Recently, we have reported that on the basis of the nonepitaxial growth method, Au@semiconductor core/shell HNCs with oleic acid (OA) and oleylamine (OAm) co-capping ligands could be obtained through sequential sulfuration and cation exchange reactions starting from Au@Ag HNCs. In this article, through modification of such a nonepitaxial growth method, monodisperse Au@semiconductor core/shell HNCs with controlled crystallinity and shell thickness, varied compositions (such as Ag2S, CdS, and ZnS), and anisotropically shaped Au cores but isotropic overall morphologies were prepared first. By using acid/alkali co-capping ligands, these nonepitaxially grown core/shell HNCs were further arranged into diverse single-layer 2D and multiple-layer 3D superlattices (SLs) with controllable close-packing modes on either rigid or flexible substrates. According to theoretical simulation of electromagnetic field enhancement of such large-scale SLs, the photoelectric property of the ordered assembly films with a controlled packing mode was preeminently improved compared to that of the random assembly films. Moreover, we found that with the similar capping ligands, Au@semiconductor HNCs could be assembled together with monodisperse Au NCs or CdS quantum dots (QDs) to form diverse BNSL coassemblies.

■

EXPERIMENTAL SECTION

Chemicals and Materials. Tetrachloroauric(III) acid hydrate (HAuCl4·4H2O), silver nitrate (AgNO3, ≥99%, A.R.), cadmium nitride tetrahydrate [Cd(NO3)2·4H2O, 99.999%], zinc nitrate hexahydrate [Zn(NO)3·6H2O, 99.99%], sodium sulfide nonahydrate (Na2S·9H2O, 99.99%), sodium sulfite anhydrous (Na2SO3, ≥98.0%), oleylamine (OAm, approximate C18 content of 80−90%), oleic acid (OA, technical grade, 90%), sublimed sulfur (99.5%), and tributylphosphine (TBP, 95%) were obtained from Aladdin Reagent. The solutions, including those of methanol (anhydrous, 99.8%), ethanol (anhydrous, ≥99.5%), and toluene (anhydrous, 99.8%), were purchased from Sinopharm Chemical Reagent. High-purity water was obtained by PURELAB Ultra (resistivity of >18 MΩ cm). Synthesis of Monodisperse Au@Ag2S Core/Shell HNCs through Nonepitaxial Growth. The preparations of monodisperse Au core nanoparticles and Au@Ag core/shell HNCs as well as the S precursor solution with an OA:OAm concentration ratio of 2:1 were conducted following the previous work.7,20,21 First, 8 mL of a Au@Ag colloidal suspension in toluene at the desired concentration was added to a glass bottle. While the suspension was being magnetically stirred, 1 mL of the S precursor solution was introduced at room temperature to obtain a dark green Au@Ag2S colloidal solution. The as-obtained Au@Ag2S solution was washed with ethanol by size selection22 and collected by centrifugation at 5000 rpm for 10 min. Finally, the precipitated HNCs with OA/OAm co-capping ligands were redispersed in 10 mL of toluene. Synthesis of Monodisperse Au@MX Core/Shell HNCs through Nonepitaxial Growth. Typically, the as-prepared 10 mL 2356

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363

Article

Chemistry of Materials

Figure 1. (A) HAADF-STEM image of as-prepared Au@CdS HNCs with a qusi-spherical core and a hexagonal wurtzite lattice. (B−D) TEM images of as-prepared Au@MX core/shell HNCs with controlled shell compositions (Ag2S shell, ZnS shell, and CdS shell, respectively), different core sizes, and different shell thicknesses. (E) TEM images of as-prepared Au@ZnS HNCs with anisotropically shaped Au cores and uniform ZnS shells. The scale bar is 50 nm. a great advantage for the formation of different patterned BNSLs by using the same dip-coating method. To obtain a homogeneous colloid for BNSLs, the two NC species were mixed, ultrasonically dispersed, and held quietly for 12−20 h. Photocurrent Response Measurements. The FTO photoanodes were fabricated by the dip-coating method (ordered assembly) and drop aggregation, followed by annealing at 400 °C in N2 for 1 h. The areas of obtained films are near 1 cm × 1 cm. The electrolyte here consists of a 0.5 M Na2S and 0.5 M Na2SO3 water solution (pH 13.6). For the test, the photoanodes were irradiated from the front side. The system was irradiated with a 300 W Xe lamp with an optical filter (λ > 420 nm). All measurements were conducted with a CH Instruments 760D potentiostat in a three-electrode setup with a Pt counter electrode (2 cm × 3 cm) and saturation mercury reference electrode (SCE). Characterization of Structure and Morphology. Lowresolution transmission electron microscopy (LRTEM) performed on a JEOL JEM-1200EX instrument working at 80 kV and highresolution transmission electron microscopy (HRTEM) performed on an FEI Tecai G2 F20 S-Twin instrument working at 200 kV were utilized to characterize the details of morphology and interfacial lattice of the prepared HNCs as well as their self-assemblies. Scanning electron microscopy (SEM) images were obtained using a Hitachi FESEM 4800 instrument. Finite-Element Method Simulation. The full-wave numerical simulations are performed using the commercial software COMSOL Multiphysics based on the finite-element method (FEM) to simulate the field enhancement of light matter interaction through the core/ shell Au@CdS HNC assemblies. All the structures are excited by linearly polarized light with wavelengths of 400 and 500 nm. The permittivity of Au (and its refractive index) is deduced from the Drude−Lorentz model fitted to the experimental data.27 The refractive index of the CdS is fixed to 2.7. The interval distances between the nanoparticle assemblies are estimated to be 2.2 nm. All the NCs are

surrounded with an air environment. To suppress the noise reflected or scattered from the simulated boundaries, the outside boundaries of the calculated area were set to the perfectly matched layer.

■

RESULTS AND DISCUSSION Structure and Morphology of As-Prepared Au@MX Core/Shell HNCs. Using a modified nonepitaxial strategy, through in situ sulfuration of Au@Ag HNCs and further cation exchange reaction, monodisperse Au@CdS or Au@ZnS HNCs were prepared. Au NCs 8−12 nm in size were chosen as the core materials because of their unique size-, shape-, and dielectric environment-dependent plasmonic properties. Such initial Au NCs with a high monodispersity were prepared, followed by being uniformly coated with a Ag overlayer. The resulting Au@Ag HNCs were transformed into Au@Ag2S HNCs with amorphous shells (by reaction with organic sulfur precursors) and then subsequently converted to Au@CdS or Au@ZnS HNCs with monocrystalline shells through cation exchange at 80 °C. The flexible control and precise tailoring of the shell composition, crystallization, thickness, and interface between Au and chalcogenide components in Au@MX core/ shell HNCs were achieved by engineering the reaction kinetics during synthesis. For example, the enhanced reaction temperature and modified TBP concentration are critical for obtaining monodisperse Au@MX core/shell HNCs with good crystallization. The final Au@MX HNCs were sterically stabilized with hydrophobic OA and OAm molecules, which formed the acid/alkali co-capping surface. The high-angle annular darkfield scanning TEM (HAADF-STEM) image (Figure 1A) strongly substantiates the single-crystalline feature of the CdS 2357

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363

Article

Chemistry of Materials

Figure 2. (A) Scheme of the dip-coating process. The Au@MX HNCs were capped with OA/OAm as the ligands. The OA:OAm mole ratio is 2:1. (B and C) TEM and STEM images of the monolayer (column a), double-layer (column b), and multilayer (column c) SLs by self-assembly of asprepared Au@ZnS (a, 9 nm core, 2 nm shell; b and c, 8 nm core, 7 nm shell) and Au@CdS (a, 12 nm core, 2 nm shell; b, 10 nm core, 5 nm shell; c, 12 nm core, 4 nm shell) HNCs. (D) Stacking models of Au@MX HNC assembly.

Au seed as well as the reaction time, we could easily achieve the starting Au@Ag HNCs with different core sizes and shell thicknesses. Then the following in situ conversion could easily maintain the morphologies. Thus, the final Au@MX HNCs with different core sizes and shell thicknesses could be obtained reasonably. As shown in panels B and C of Figure 1, the shell

shell that forms a hexagonal wurtzite lattice [the lattice spacing of 0.316 nm was indexed as the (101) plane of the wurtzite lattice], and the well-defined and sharp heterointerface between the Au core and CdS shell where the lattice mismatch between the two constituents was up to 43%. By independently adjusting the molar ratio between the AgNO3 precursor and 2358

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363

Article

Chemistry of Materials

Figure 3. TEM [A(a,b), B(a), and C(a)] and STEM [A(c). B(b,c), C(b,c)] images of large-scale 3D SLs. The sizes of cores in panels A−C are 8, 11, and 8 nm, respectively. The thicknesses of shells in panels A−C are 7, 3, and 6 nm, respectively. The insets are their corresponding FFT patterns.

way to manipulate the dielectric environment surrounding the surface of plasmonic Au NCs and can strongly facilitate the assembly of Au@semiconductor HNCs into large-scale SLs because of their high monodispersity. Self-Assembly of As-Prepared Au@MX Core/Shell HNCs. As illustrated by LRTEM in Figure 1B−E and Figure S1, the as-prepared HNCs with different shell thicknesses all exhibited a strong tendency to self-assemble. Through the modified dip-coating method, monodisperse Au@MX HNCs could be self-assembled into macroscale 2D monolayer and 3D multilayer SLs by adjusting the colloid concentrations. The dip-coating process is simple and versatile for film formation based on careful control of the solvent evaporation speed. The temperature, humidity, vapor pressure, and withdrawal speed are the facile operation factors to control. Murray’s group had studied the long-range order supperlattice on the wafer substrate via such a method.25 The speed of solvent volatilization that influences the progress of drying plays a crucial role in formation of the final film. For the as-obtained nonepitaxially grown Au@HNCs with OA/OAm co-capping ligands, which could be dispersed well in toluene here, we chose a vertical dip-coating technique in a sealed box with a controlled pulling speed to obtain large-scale SLs, as shown in Figure 2A. With precise control of operating temperature (here

thicknesses with good uniformity could be modulated between 2 and 7 nm. The overall sizes of Au@MX HNCs were 13−22 nm (notably, the sizes of the core and shell shown in these figures are statistical averages). Interestingly, it was noticed that though the majority of Au cores existed in random polyhedral shapes, the generated HNCs exclusively displayed a uniform spherical morphology with a well-controlled shell composition and crystallinity. Such observations paralleled those of Au@MX HNCs, which also afforded a high-quality crystalline structure (encompassing both the shell and interface domains), as well as an isotropic spherical morphology in spite of the anisotropic shapes of Au cores (Figure 1E and Figure S1). As far as we know, this is one advantage over all other heteroepitaxial processes. It should be mentioned that the possible existence of a Au−Ag alloy thin layer at the interface in Au@Ag HNCs (also converted into the MX shell during subsequent cation exchange) could intensify the sharpness and randomness of Au cores in the final products. 28,29 This underpinned the distinctive benefit associated with the nonepitaxial growth method; namely, the overall morphology of core/shell HNCs is independent of the core shapes, which thus can initiate a more flexible way to manipulate the dielectric environment surrounding the surface of plasmonic Au NCs. Such a benefit can initiate a more flexible 2359

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363

Article

Chemistry of Materials

Figure 4. (A) STEM (a), TEM (b), calculated E-field norm (c, top, volts per meter), and current density norm (c, bottom, amperes per square meter) of BNSLs from Au@CdS HNCs (12 nm core, 4 nm shell) and Au NCs (12 nm) with a CaCu5 pattern. The illuminating linearly polarized light used for the calculation is 400 nm. (B) STEM (a) and TEM (b) images of BNSLs from Au@CdS HNCs and Au NCs with a ncp pattern. (C) TEM image of Au@ZnS HNCs (8 nm core, 6 nm shell) and Ag-doped CdS QDs (5 nm) with another ncp pattern. The inset shows that the Agdoped CdS QD trace addition changed the Au@ZnS HNCs packing pattern from hcp to bbc. The inserted schematics in panels A(a), B(a), and C highlight the packing pattern of the binary building blocks.

35 °C) and better control of the solvent evaporation rate, the HNCs were deposited onto the substrate slowly into large area ordered assemblies. In this work, the monolayer, double-layer, and multilayer assemblies were successfully obtained through a multistep colloid diluting/dip-coating approach, by which the thickness of the films mainly depended on the concentration of the HNC colloids. With a well-defined low concentration, the HNC monolayer SLs with micrograde could be obtained (seen in Figure 1 and column a of Figure 2B,C). At the monolayer, each individual HNC was surrounded by six other HNCs on the same plane, exhibiting the closest stacking structure. The average interparticle distance of these SLs with acid/alkali binary co-capping ligands is ∼2.2 nm, consistent with the existence of the C18 organic surface capping ligands.30 The double-layer and multilayer SLs of HNCs were formed with an increase in colloid concentration. From the double-layer SLs of Au@ZnS and Au@CdS HNCs displayed in column b of Figure 2, the feature of individual core/shell HNCs could be clearly identified. For multilayer SLs, face center cubic (fcc) and hexagonal compact packing (hcp) are the only two modes of closest packing. Here, such Au@MX HNCs with acid/alkali cocapping in toluene present dominant hcp stacking in their multilayer SLs, as shown in column c of panels B and C of Figure 2. Such assemblies of Au@HNCs from monolayers to multilayers were consistent with the stacking models in Figure 2D, where the features could be more realized. The characteristic ABAB-type hcp can be clearly observed, and the (001) planes were revealed. The multilayer SLs of Au@ZnS, Au@CdS, and Au@Ag2S HNCs could be achieved on large scales, as seen in Figure 3. The macroscale 3D SL of Au@ZnS HNCs (Figure 3A) exhibited a terracelike structure, which implied a growth process via layer-by-layer assembly.31 One can see that some cracks could be discerned in this SL. Along the crack edges, the

HNCs were still arranged in an orderly fashion [Figure 3A(b)]. Such finely structured cracks could thus be considered to define the grain boundaries of the Au@ZnS HNC SLs.32,33 The terracelike structure and boundary-defined cracks were also found in Au@CdS HNC multilayer SLs (Figure 3B). In column a of Figure 3C, one can see that monolayer and double-layer structure coexist in the same SL (generated from Au@Ag HNCs). Columns b and c of Figure 3C show that the Au@ Ag2S HNC multilayer SL was formed with well-arranged boundaries, and the minor other stacking patterns existed in this SL. In Figure 3, the inserted FFT patterns all corresponding to the (001) plane of the hexagonal structure revealed the packing geometry of the large-scale Au@MX HNC SLs. As described above, the good uniformity of shape and size is the key factor for their close packing that potentially maximizes the utilization of space and minimizes energy consumption. The noncovalent interactions, such as van der Waals force and dipole−dipole interaction arising from the uniform capping of HNCs with OA/OAm binary ligands, together with the electrostatic force caused by the cations, probably accounted for the driving force for hcp self-assembly of as-prepared HNCs.34,35 Binary Assembly of Au@MX HNCs with Au NCs or CdS QDs into BNSLs. BNSLs, combining two types of NCs into a well-defined superlattice structure, are considered to have collective properties of individual components and are anticipated to provide a programmable way to design metamaterials. Several close-packing and non-close-packing (ncp) periodic and even aperiodic quasicrystalline phases of BNSLs have been demonstrated by the Murray group and the Talapin group.36,37 However, most of the investigated BNSLs consisted of two single-component NCs without the involvement of multiple-component HNCs.12,32,36−38 Here, we performed the first exploratory study of fabrication of 2360

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363

Article

Chemistry of Materials ordered BNSLs combining Au@CdS HNCs and Au NCs or doped CdS QDs, with the aim of offering a new platform for achieving unprecedented plasmon−exciton coupling and light− matter interactions by exploiting the prominent advantages of Au@MX core/shell HNCs. As illustrated in Figure 4, Au@CdS HNCs (12 nm core, 4 nm shell) and Au NCs (12 nm) with a concentration ratio of 1:7 can co-assemble into BNSLs with a dominant CaCu5-type structure and a minor fraction of non-close-packed patterns (Figure 4A,B and Figures S2 and S3). The coexistence of two stacking modes could be a result of the locally decreased concentration of Au@CdS HNCs during the SL growth process.36 It is noteworthy that the CaCu5 BNSLs demonstrated in this study can be achieved on a micrometer scale [Figure 4A(b)]. The maximal packing density and size ratio of these CaCu5 BNSLs were determined to be 0.7 and 0.63, respectively, coinciding with those of the CaCu5-type BNSLs previously reported.38 The TEM and STEM images of the CaCu5 sturcture characteristic of (001) plane projection are shown in columns a and b of Figure 4A, while the non-closepacked BNSL phases are shown in Figure 4B. Similar nonclose-packed structures of single-nanocrystal superlattices have been demonstrated in other materials system and were explained by rotational patterns of hexagonal close-packed monolayers.38,39 By virtue of the FEM simulation shown in column c of Figure 4A and Figures S4 and S5, the CaCu5 BNSL structure possesses a uniform and significant E-field enhancement between the neighboring nanoparticles upon being illuminated with linearly polarized light (λ = 400 and 500 nm). The enhanced E-field under 400 nm light irradiation could reach 105 orders of magnitude, while the current density can be as high as 1010. With an understanding that plasmons of gold NCs can efficiently promote exciton dissociation in neighboring semiconductors and then enhance the generation of free charge carriers,10,40,41 the obtained simulation results can reflect an enhanced uniform plasmon−exciton coupling in this structure. Besides the combination of Au@MX core/shell HNCs and plasmonic Au NCs, the Au@ZnS HNCs (8 nm core, 6 nm shell) and Ag-doped CdS QDs (5 nm) with a concentration ratio of 1:3 were co-assembled into another periodic ncp BNSL structure (Figure 4C). It should be noted that the pattern mode of the obtained non-close-packed BNSLs was sensitive to the size of the QDs. In addition to nonclose packing, we found that the existence of QDs or other secondary building blocks can have an impact on the packing mode of close-packed HNC SLs. As shown in the inset of Figure 4C, the addition of a small amount of CdS QDs into a Au@ZnS HNC colloid could cause the packing mode of HNC SL to change from hcp to bcc. Such an intentional minor disturbance via the introduction of guest impurities may be another promising way to regulate the packing mode of host NCs. Self-Assembly of As-Prepared Au@MX Core/Shell HNCs on Different Rigid/Flexible Substrates. A prerequisite for fabrication of HNC-based optoelectronic devices is the feasible integration of HNC SLs on given rigid/flexible substrates on a large scale. Here, we demonstrate that the Au@ MX HNC superlattice structures can easily grow on both rigid (such as silicon wafer, and glass) and flexible (such as conductive polyethene membrane and carbon film) substrates without the requirement of additional substrate pretreatment. Figure 5A shows the TEM image of Au@CdS HNC SLs that were deposited on a flexible carbon film substrate. It can be

Figure 5. (A) TEM images of Au@ZnS HNC (8 nm core, 6 nm shell) self-assembly with a monolayer on flexible multi-graphene. (B and C) SEM images of Au@ZnS HNC self-assembly with 2D monolayer and 3D multilayer SLs, respectively, on rigid silicon wafers.

seen that the wrinkled carbon thin film was coated with an ordered self-assembled monolayer. At the folded area, the monolayer and multilayer superlattices both existed and firmly adhered on the substrate forming a darker region. The SEM image in Figure 5B shows the micrometer-scale monolayer of self-assembled Au@CdS HNCs on a rigid silicon substrate. With a change in the concentrations of Au@CdS HNCs, ordered multilayer assemblies could be easily formed on the silicon wafer, as confirmed by the large-scale SEM images in Figure 5C. The successful integration of the macroscopic superlattice from Au@MX plasmonic hetero-nanocrystals with either 2D or 3D structures, on any desired substrates, may lead to a variety of optoelectronic and plasmonic applications. Photocurrent Response of Au@MX Core/Shell HNC Assemblies. A previous study has verified the direct coupling between the plasmon and exciton in epitaxial Au@PbS HNCs.42 To probe the effect of interparticle arrangement of HNCs on plasmon−exciton coupling, we compared the photocurrent response of Au@CdS HNC SL bulk films taking a closest-packing mode with that of those formed by random droplet aggregation. Both samples held an area near 1 cm × 1 cm and were annealed at 400 °C for 1 h in N2 before the photocurrent tests. Figure 6A shows that the photocurrent generation efficiency of Au@CdS HNC-assembled films in the closest-packing mode could reach 0.05 mA cm−2 and was much higher than that of those in a disordered arrangement. These experimental results probably suggested that the closestpacking mode of Au@CdS HNCs can enhance plasmon− exciton coupling and therefore improve their photoelectric performance. From FEM simulation, we investigated the E-field enhancement within such ordered Au@CdS core−shell HNC assemblies. From Figure 6B and Figure S6, we can see that field enhancement can reach as high as 104, with the hot spots located between the CdS shell layers. Such assemblies can permit photoinduced current generation due to ordered boundedness, while the disordered assemblies cannot. In 2361

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363

Article

Chemistry of Materials

Figure 6. (A) Photocurrent responses of Au@CdS HNC dip-coating ordered assembly films and disordered aggregation at 1 V vs RHE (300 W Xe lamp; λ > 420 nm). (B) Calculated E-field norm (top, volts per meter) and current density norm (bottom, amperes per square meter) of Au@CdS HNC monolayer and trilaminar ordered assemblies.

■

addition to this, the interparticle connectivity might be improved by such an ordered arrangement, and the transport of charge carriers could be facilitated between HNCs as well as within the photoanode.43 It is worth mentioning that both the E-field enhancement and the current density of CaCu5 BNSLs are higher than those of a Au@CdS HNC ordered assembly by 105 and 1010 orders of magnitude, respectively (Figure 4, Figure 6, and Figures S4−S6). Because of the uniform plasmon− exciton coupling of binary NLs, the CaCu5 BNSLs could be predicted to have a higher photoelectric generation efficiency. Besides the photocurrent enhancement by plasmon−exciton coupling, the wavelength of incident light here is >420 nm, which illustrated that the photoinduced electron/hole separation of Au@CdS HNCs was caused and enhanced by Au core SPR-induced E-field enhancement, which is different from the case for those of CdS quantum dots. On the other hand, because visible light makes up a large part of sunlight, such Au@semiconductor HNC ordered assembly would be more capable of utilizing solar energy.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00046. Figures S1−S6 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lingling Huang: 0000-0002-3647-2128 Jiatao Zhang: 0000-0001-7414-4902 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21322105, 91323301, 51372025, 61505007, and 51501010).

■

CONCLUSIONS In summary, we show the preparation and diverse assembly of monodisperse Au@MX (MX = Ag2S, CdS, or ZnS) core/shell HNCs with controlled crystallinity, composition, shell thickness, and randomly anisotropic Au cores but isotropic overall morphologies. With acid/alkali binary co-capping ligands on the surface, as-prepared Au@MX HNCs could self-assemble into controlled 2D and 3D SLs on either flexible or rigid substrates and even co-assemble with Au NCs or CdS QDs into various BNSLs. Moreover, the multilayer film formed by ordered assembly of Au@CdS HNCs in closest-packing mode afforded a photocurrent generation efficiency much higher than that of its randomly assembled counterpart. These experimental results combined with theoretical simulations suggested that large-scale closest packing of metal−semiconductor HNCs is propitious for enhancing plasmon−exciton coupling, which in turn can improve their photoelectrically related properties. We envision that the versatile nanoscale organization of nonepitaxially grown Au@MX HNCs into macroscopic superstructures can promote their exploitation in both fundamental studies and technological applications.

■

REFERENCES

(1) Milliron, D. J.; Gur, I.; Alivisatos, A. P. Hybrid organic Nanocrystal solar cells. MRS Bull. 2005, 30, 41−44. (2) Srivastava, S.; Kotov, N. A. Composite Layer-by-Layer (LBL) Assembly with Inorganic Nanoparticles and Nanowires. Acc. Chem. Res. 2008, 41, 1831−1841. (3) Jiang, R.; Li, B.; Fang, C.; Wang, J. Metal/Semiconductor Hybrid Nanostructures for Plasmon-Enhanced Applications. Adv. Mater. 2014, 26, 5274−5309. (4) Beveridge, J. S.; Buck, M. R.; Bondi, J. F.; Misra, R.; Schiffer, P.; Schaak, R. E.; Williams, M. E. Purification and Magnetic Interrogation of Hybrid Au-Fe3O4 and FePt-Fe3O4 Nanoparticles. Angew. Chem., Int. Ed. 2011, 50, 9875−9879. (5) Xu, R.; Wu, R.; Shi, Y. M.; Zhang, J. F.; Zhang, B. Ni3Se2 nanoforest/Ni foam as a hydrophilic, metallic, and self-supported bifunctional electrocatalyst for both H2 and O2 generations. Nano Energy 2016, 24, 103−110. (6) Wu, R.; Zhang, J. F.; Shi, Y. M.; Liu, D.; Zhang, B. Metallic WO2Carbon Mesoporous Nanowires as Highly Efficient Electrocatalysts for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 6983− 6986. 2362

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363

Article

Chemistry of Materials (7) Zhang, J. T.; Tang, Y.; Lee, K.; Ouyang, M. Nonepitaxial Growth of Hybrid Core-Shell Nanostructures with Large Lattice Mismatches. Science 2010, 327, 1634−1638. (8) Zhang, J. T.; Tang, Y.; Lee, K.; Ouyang, M. Tailoring LightMatter-Spin Interactions in Colloidal Hetero-Nanostructures. Nature 2010, 466, 91−95. (9) Lambright, S.; Butaeva, E.; Razgoniaeva, N.; Hopkins, T.; Smith, B.; Perera, D.; Corbin, J.; Khon, E.; Thomas, R.; Moroz, P.; Mereshchenko, A.; Tarnovsky, A.; Zamkov, M. Enhanced Lifetime of Excitons in Nonepitaxial Au/CdS Core/Shell Nanocrystals. ACS Nano 2014, 8, 352−361. (10) Razgoniaeva, N.; Lambright, S.; Sharma, N.; Acharya, A.; Khon, E.; Moroz, P.; Razgoniaev, A.; Ostrowski, A.; Zamkov, M. Exciton Generation in Semiconductor Nanocrystals via the Near-Field Plasmon Energy Transfer. J. Phys. Chem. C 2015, 119, 15562−15571. (11) Su, B.; Guo, W.; Jiang, L. Learning from Nature: Binary Cooperative Complementary Nanomaterials. Small 2015, 11, 1072− 1096. (12) Walker, D. A.; Kowalczyk, B.; de la Cruz, M. O.; Grzybowski, B. A. Electrostatics at the nanoscale. Nanoscale 2011, 3, 1316−1344. (13) Henzie, J.; Gruenwald, M.; Widmer-Cooper, A.; Geissler, P. L.; Yang, P. Self-Assembly of Uniform Polyhedral Silver Nanocrystals into Densest Packings and Exotic Superlattices. Nat. Mater. 2012, 11, 131− 137. (14) He, L.; Wang, M.; Ge, J.; Yin, Y. Magnetic Assembly Route to Colloidal Responsive Photonic Nanostructures. Acc. Chem. Res. 2012, 45, 1431−1440. (15) Wang, P.-p.; Sun, H.; Ji, Y.; Li, W.; Wang, X. Three-Dimensional Assembly of Single-Layered MoS2y. Adv. Mater. 2014, 26, 964−969. (16) Radha, B.; Senesi, A. J.; O’Brien, M. N.; Wang, M. X.; Auyeung, E.; Lee, B.; Mirkin, C. A. Reconstitutable Nanoparticle Superlattices. Nano Lett. 2014, 14, 2162−2167. (17) Luo, D.; Yan, C.; Wang, T. Interparticle Forces Underlying Nanoparticle Self-Assemblies. Small 2015, 11, 5984−6008. (18) Rycenga, M.; Camargo, P. H. C.; Xia, Y. Template-Assisted SelfAssembly: A Versatile Approach to Complex Micro- and Nanostructures. Soft Matter 2009, 5, 1129−1136. (19) Debije, M. G.; Verbunt, P. P. C. Thirty Years of Luminescent Solar Concentrator Research: Solar Energy for the Built Environment. Adv. Energy. Mater. 2012, 2, 12−35. (20) Zhang, J. T.; Tang, Y.; Weng, L.; Ouyang, M. Versatile Strategy for Precisely Tailored Core@Shell Nanostructures with Single Shell Layer Accuracy: The Case of Metallic Shell. Nano Lett. 2009, 9, 4061− 4065. (21) Shen, C.; Hui, C.; Yang, T.; Xiao, C.; Tian, J.; Bao, L.; Chen, S.; Ding, H.; Gao, H. Monodisperse Noble-Metal Nanoparticles and Their Surface Enhanced Raman Scattering Properties. Chem. Mater. 2008, 20, 6939−6944. (22) Fanizza, E.; Depalo, N.; Clary, L.; Agostiano, A.; Striccoli, M.; Curri, M. L. A Combined Size Sorting Strategy for Monodisperse Plasmonic Nanostructures. Nanoscale 2013, 5, 3272−3282. (23) Liu, J.; Zhao, Q.; Liu, J.-L.; Wu, Y.-S.; Cheng, Y.; Ji, M.-W.; Qian, H.-M.; Hao, W.-C.; Zhang, L.-J.; Wei, X.-J.; Wang, S.-G.; Zhang, J.-T.; Du, Y.; Dou, S.-X.; Zhu, H.-S. Heterovalent-Doping-Enabled Efficient Dopant Luminescence and Controllable Electronic Impurity Via a New Strategy of Preparing II-VI Nanocrystals. Adv. Mater. 2015, 27, 2753−2761. (24) Li, L.; Hu, F.; Xu, D.; Shen, S.; Wang, Q. Metal ion redox potential plays an important role in high-yield synthesis of monodisperse silver nanoparticles. Chem. Commun. 2012, 48, 4728− 4730. (25) Gaulding, E. A.; Diroll, B. T.; Goodwin, E. D.; Vrtis, Z. J.; Kagan, C. R.; Murray, C. B. Deposition of Wafer-Scale SingleComponent and Binary Nanocrystal Superlattice Thin Films Via DipCoating. Adv. Mater. 2015, 27, 2846−2851. (26) Yang, J.; Choi, M. K.; Kim, D. H.; Hyeon, T. Designed Assembly and Integration of Colloidal Nanocrystals for Device Applications. Adv. Mater. 2016, 28, 1176−1207.

(27) Rakic, A. D.; Djurisic, A. B.; Elazar, J. M.; Majewski, M. L. Optical Properties of Metallic Films for Vertical-Cavity Optoelectronic Devices. Appl. Opt. 1998, 37, 5271−5283. (28) Motl, N. E.; Bondi, J. F.; Schaak, R. E. Synthesis of Colloidal AuCu2S Heterodimers via Chemically Triggered Phase Segregation of AuCu Nanoparticles. Chem. Mater. 2012, 24, 1552−1554. (29) Gao, C.; Hu, Y.; Wang, M.; Chi, M.; Yin, Y. Fully Alloyed Ag/ Au Nanospheres: Combining the Plasmonic Property of Ag with the Stability of Au. J. Am. Chem. Soc. 2014, 136, 7474−7479. (30) Zhang, X.; Lv, L.; Ji, L.; Guo, G.; Liu, L.; Han, D.; Wang, B.; Tu, Y.; Hu, J.; Yang, D.; Dong, A. Self-Assembly of One-Dimensional Nanocrystal Superlattice Chains Mediated by Molecular Clusters. J. Am. Chem. Soc. 2016, 138, 3290−3293. (31) Goubet, N.; Richardi, J.; Albouy, P.-A.; Pileni, M.-P. Which Forces Control Supracrystal Nucleation in Organic Media? Adv. Funct. Mater. 2011, 21, 2693−2704. (32) Dong, A. G.; Chen, J.; Vora, P. M.; Kikkawa, J. M.; Murray, C. B. Binary Nanocrystal Superlattice Membranes Self-Assembled at the Liquid-Air Interface. Nature 2010, 466, 474−477. (33) Bodnarchuk, M. I.; Kovalenko, M. V.; Pichler, S.; Fritz-Popovski, G.; Hesser, G.; Heiss, W. Large-Area Ordered Superlattices from Magnetic Wustite/Cobalt Ferrite Core/Shell Nanocrystals by Doctor Blade Casting. ACS Nano 2010, 4, 423−431. (34) Quan, Z.; Xu, H.; Wang, C.; Wen, X.; Wang, Y.; Zhu, J.; Li, R.; Sheehan, C. J.; Wang, Z.; Smilgies, D.-M.; Luo, Z.; Fang, J. SolventMediated Self-Assembly of Nanocube Superlattices. J. Am. Chem. Soc. 2014, 136, 1352−1359. (35) Zhuang, Z.; Peng, Q.; Zhang, B.; Li, Y. Controllable synthesis of Cu(2)S nanocrystals and their assembly into a superlattice. J. Am. Chem. Soc. 2008, 130, 10482−10483. (36) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. Structural Characterization of Self-Assembled Multifunctional Binary Nanoparticle Superlattices. J. Am. Chem. Soc. 2006, 128, 3620−3637. (37) Talapin, D. V.; Shevchenko, E. V.; Bodnarchuk, M. I.; Ye, X.; Chen, J.; Murray, C. B. Quasicrystalline Order in Self-Assembled Binary Nanoparticle Superlattices. Nature 2009, 461, 964−967. (38) Bodnarchuk, M. I.; Kovalenko, M. V.; Heiss, W.; Talapin, D. V. Energetic and Entropic Contributions to Self-Assembly of Binary Nanocrystal Superlattices: Temperature as the Structure-Directing Factor. J. Am. Chem. Soc. 2010, 132, 11967−11977. (39) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Self-Organization of Nanosized Gold Particles. Chem. Mater. 1998, 10, 922−926. (40) Zhang, W.; Saliba, M.; Stranks, S. D.; Sun, Y.; Shi, X.; Wiesner, U.; Snaith, H. J. Enhancement of Perovskite-Based Solar Cells Employing Core-Shell Metal Nanoparticles. Nano Lett. 2013, 13, 4505−4510. (41) Chen, M.; Shao, L.; Kershaw, S. V.; Yu, H.; Wang, J.; Rogach, A. L.; Zhao, N. Photocurrent Enhancement of HgTe Quantum Dot Photodiodes by Plasmonic Gold Nano rod Structures. ACS Nano 2014, 8, 8208−8216. (42) Lee, J.-S.; Shevchenko, E. V.; Talapin, D. V. Au-PbS Core-Shell Nanocrystals: Plasmonic Absorption Enhancement and Electrical Doping via Intra-Particle Charge Transfer. J. Am. Chem. Soc. 2008, 130, 9673−9675. (43) Gujral, S. S.; Simonov, A. N.; Higashi, M.; Abe, R.; Spiccia, L. Optimization of Titania Post-Necking Treatment of TaON Photoanodes to Enhance Water-Oxidation Activity under Visible-Light Irradiation. ChemElectroChem 2015, 2, 1270−1278.

2363

DOI: 10.1021/acs.chemmater.7b00046 Chem. Mater. 2017, 29, 2355−2363