Controllable Assembly of Ordered Semiconductor Ag2S

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NANO LETTERS

Controllable Assembly of Ordered Semiconductor Ag2S Nanostructures

2003 Vol. 3, No. 1 85-88

Feng Gao,† Qingyi Lu,† and Dongyuan Zhao* Molecular Catalysis and InnoVatiVe Materials Lab, Department of Chemistry, Fudan UniVersity, Shanghai 200433, P. R. China Received September 25, 2002; Revised Manuscript Received November 1, 2002

ABSTRACT A direct and simple one-step route has been demonstrated to control the organization of semiconductor Ag2S nanocrystals. Several superstructures including hexagonal, quasi-orthogonal, and chain-like arrays assembled from Ag2S nanoparticles, and quasi-orthogonal and hexagonal structures assembled from Ag2S nanorods have been synthesized at low temperature by directly using inorganic compounds AgNO3 and Na2S as the reactants and dodecylthiol as the structure-directing agent. This method is very simple and general and has no requirement of the presynthesis process of Ag2S nanoparticles or nanorods.

Research in nanomaterials is rapidly expanding into the assembly of well-ordered two- and/or three-dimensional (2D and/or 3D) superstructures because they are critically important to chemical, optical, magnetic, and electronic nanodevices and would provide possibilities to probe brand-new properties and applications resulted from the spatial orientation and arrangement of the nanocrystals.1 Many efforts have been focused on the preparation of self-organized lattices of metal and oxide nanoparticles including silver,2 gold,3 cobalt,4 indium,5 γ-Fe2O3,6 cobalt oxide,7 and BaTiO3.8 The assembly of uniform semiconductor chalcogenide nanocrystals, such as CdS, CdSe, and Ag2S nanoparticle arrays, usually requires the presynthesis of uniform nanoparticles, followed by the organization process by surfactants or ligands.9-11 Chemical fabrication of 1D nanostructures has been a very active field during the past decade due to the outstanding optical and electronic properties of 1D nanostructures.12 These 1D systems offer fundamental scientific opportunities for studying the influence of size and shape with respect to physical properties.13 Moreover, if such nanorods can be ordered and assembled into an appropriate superstructure, then a host of nanoelectronic applications could be envisioned.14 However, to now, relatively few reports have appeared concerning the assembly of nanorods to ordered superstructures,15-19 which may be due to the difficulty of the organization of 1D anisotropic nanostructures. Li et al.16 synthesized linear chains and rectangular superlattices of BaCrO4 by using reverse micelles and microemulsion droplets containing fixed concentrations of barium and chromate ions. Alivisatos et al. described a mixed surfactant* Corresponding author. E-mail: [email protected] † These authors contributed equally to this work. 10.1021/nl025811a CCC: $25.00 Published on Web 11/16/2002

© 2003 American Chemical Society

assisted organometallic precursor method for the assemblies of CdSe17 and Co18 nanorods. In this communication, we report a general, controllable synthesis and assembly of monodispersive semiconductor Ag2S nanoparticles and nanorods in one step at low temperature by a simple and direct approach. In the process, dodecylthiol was used as a structure-directing agent and simple inorganic compounds AgNO3 and Na2S as reactants directly, and the presynthesis of the initial Ag2S nanoparticles or nanorods was not required. Several ordered superstructures including hexagonal, quasi-rectangular and chain-like structures from Ag2S nanoparticles and quasi-orthogonal and hexagonal structures from Ag2S nanorods have been successfully obtained by varying the temperature or the reactant concentration. In a typical synthetic process, 30 mL 0.027 M AgNO3 aqueous solution (0.80 mmol) and 30 mL 0.067 M dodecylthiol toluene-solution (2.0 mmol) were mixed to form a yellowy micellar solution under intensively stirring at room temperature (the molar ratio of the surfactant/silver ion is 2.5). 30 min later, 10 mL 0.33 M Na2S (3.3 mmol) aqueous solution was dropped into the microemulsion. Immediately after Na2S addition, the micellar solution color changes from yellowish to black, suggesting the formation of colloidal Ag2S particles. The mixed solution was stirred at room temperature for 24 h and then moved into a Telfon bottle. After the Telfon bottle was maintained at 80 °C for 40 h, it was by then cooled to room temperature. The collected black solid-state product was washed with benzene, distilled water and absolute alcohol for several times and dried at room temperature. The obtained superstructure of the solid product has been characterized by small-angle X-ray diffraction, recorded on a German Bruker D4 X-ray diffractometer with

Figure 1. TEM images of the sample prepared with low molar ratio of dodecylthiol/silver ion (2.0 mmol:0.8 mmol) at aging temperature of 80 °C.

Ni-filtered Cu KR radiation. The morphology, crystallinity, size, and organization behavior of Ag2S nanocrystals have been determined by transmission electron microscopy (TEM), accompanied by selected area electron diffraction (SAED), carried out on a Japan JEOL JEM2011 transmission electron microscopy. The obtained black powders are dispersed in alcohol by ultrasound and then a drop of this solution is deposited on an amorphous carbon film on Cu grid for TEM observation. In our experiments, we found that the synthetic conditions including the synthetic temperature, the molar ratio of dodecylthiol to silver ion, and the silver ion concentration have important effects on the morphology and the assembly behavior of the nanocrystals. Transmission electron microscopy (TEM) image (Figure 1a) of the sample prepared with low dodecylthiol/silver ion molar ratio (2.0 mmol:0.8 mmol) at aging temperature of 80 °C reveals a 2D hexagonal-like ordered superstructure of monodispersive Ag2S nanoparticles. Its selected area electron diffraction (SAED) pattern (Figure 1b) exhibits polycrystalline diffraction rings, which can be indexed to monoclinic Ag2S. The clear lattice planes shown in a highresolution TEM image (Figure 1c) directly confirm the high crystallinity of Ag2S nanoparticles. The high magnification TEM image (Figure 1d) shows that these nanoparticles are monodispersive with an average diameter of 6.5 nm and an interparticle spacing of 1.0 nm. Its Fourier transform power spectrum (Figure 1e) displays ordered hexagonal-like spot arrays, further indicating the formation of the hexagonal Ag2S superstructure. The small-angle X-ray diffraction (SAXRD) pattern (Supporting Information 1) of the sample shows several diffraction peaks, which can be approximately indexed as face-center-cubic (fcc) superstructure with the d111 spacing of 6.6 nm. The obtained superlattice constant correlates with the average spacing between the particle centers observed from TEM images (Figure 1a,d). These results are consistent with the viewpoint reported in many previous works.4,10 The TEM image (Figure 2a) of the sample prepared with low dodecylthiol/silver molar ion ratio at room temperature shows a parallel-arrayed nanochain-like Ag2S structure. High magnification TEM image (Figure 2b) further reveals that these Ag2S nanochains are formed by arranged quasinanoparticles with an average diameter about 3.0 nm. The distance spacing between the centers of the adjacent 86

Figure 2. TEM images of the sample prepared with low molar ratio of dodecylthiol/silver ion at different aging temperatures. (ac) at room temperature; (d) at 160 °C.

Figure 3. TEM images of the sample prepared at aging temperature of 80 °C with high molar ratio of dodecylthiol/silver ion (5.0) but different silver ion concentrations: (a-d) 4.0 mmol:0.8 mmol; (eg) 2.0 mmol:0.4 mmol. Inset is the magnified image of the nanorod.

nanochains is about 4.3 nm. The two different periods indicate that this sample has a 2D orthogonal-like superstructure. Its Fourier transform power spectrum (Figure 2c) displays orthogonal-arrayed spots with the axis ratio of 1.40, which is consistent with the result obtained from TEM images. The SAXRD pattern for this sample (Supporting Information 2) shows two diffraction peaks centered at d ) 5.1 and 3.5 nm, respectively, which might come from the periods between nanochains and nanoparticles. The periods obtained from TEM images are a little smaller than those from SAXRD, which might be due to the constriction during the TEM sample preparation.4,10 On the contrary, the sample prepared at high temperature (160 °C) consists of discrete 1D chains with an average diameter of 180 nm, assembled by disordered nanoparticles with nonuniform sizes in the range of 6 to 15 nm (Figure 2d). These results indicate that high temperature does not benefit the formation of the ordered structures, which may be attributed to the rapid growth of Ag2S particles at high temperature. TEM image (Figure 3a) shows that the sample prepared with high dodecylthiol/silver ion molar ratio (such as 5.0) consists of a quasi-ordered superstructure formed by parallelarrayed nanorods. The high resolution TEM image (Figure 3b) further confirms the single crystalline nature of the Ag2S nanorod. Based on the TEM observation (Figure 3c), these nanorods are about 3.5 nm in diameter and 6.0 nm in length. The spacings between two parallel nanorods are not very uniform and are about 2.4 nm in diameter and 3.6 nm in length. Its Fourier transform power spectrum (Figure 3d) confirms that these nanorod arrays have a quasi-orthogonal Nano Lett., Vol. 3, No. 1, 2003

superstructure. The sample’s SAXRD pattern (Supporting Information 3) reveals three broad diffraction peaks at d spacing of 10.1, 6.4 and 3.2 nm, respectively. The two former peaks may come from the two different periods of nanorod arrays, and the last one peak at d ) 3.2 nm might be the higher index diffraction of the period of 6.4 nm. The formation of the nanorod arrays might be explained from the liquid crystal template effect.20 High surfactant/inorganic species molar ratio might profit the formation of the liquid crystal arrays and result in the 1D growth of Ag2S nanocrystals.20 To maintain the rod morphology but adjust the organization behavior, we maintained the dodecylthiol/silver ion molar ratio but decreased the silver ion concentration to 0.4 mmol and also obtained the semiconductor Ag2S superstructure. The sample’s SAXRD pattern (Supporting Information 4) shows a diffraction peak at d ) 8.0 nm and a broad peak with a maximum around d ) 3.9 nm, suggesting that the sample also has an ordered nanostructure. TEM image (Figure 3e) displays a 2D hexagonal ordered superstructure similar to that shown in Figure 1a, but its SAED pattern displays a quite different symmetry pattern with two dispersed and elongated spots, suggesting that the Ag2S nanocrystals might have an anisotropic shape and perpendicular arrangement on the substrate.21 The high magnification TEM image (Figure 3g) of the superstructure edge exhibits a column-like particle with diameter of 6.6 nm and length of 11.4 nm, which is consistent with the result obtained from the SAED pattern and further indicates that the hexagonal superstructure might be assembled by short nanorods.16,17 In our experiments, we have changed the synthetic temperature, the molar ratio of dodecylthiol to silver ion, and the silver ion concentration. At room temperature, high silver ion concentration (0.8 mmol) and low molar ratio of dodecylthiol to silver ion (2.5), the product shows a parallelarrayed nanochain-like Ag2S structure. When the synthetic temperature increases to 80 °C but with other conditions unchanged, the obtained product displays a 2D hexagonallike ordered superstructure assembled by monodispersive sphere-like nanoparticles. Further increase of the synthetic temperature results in the formation of a 1D chain-like structure assembled by nonuniform and relatively big nanoparticles. Keeping the synthetic temperature at 80 °C and silver ion concentration at 0.8 mmol but increasing the ratio of dodecylthiol to silver ion from 2.5 to 5.0, the product changes to be a 2D quasi-orthogonal superstructure assembled by monodispersive nanorods. Decreasing the silver ion concentration from 0.8 mmol to 0.4 mmol, the produced sample also displays 2D hexagonal-like ordered superstructure, but some different SAED patterns indicate that the superstructures might be assembled by nanorods.16,17 All the experimental conditions including the synthetic temperature, the molar ratio of dodecylthiol to silver ion, and the silver ion concentration affect the formation of these structures due to the fact that our method is a one-step synthetic process without the presynthesis of uniform nanoparticles. The addition of dodecylthiol is no doubt a key factor to the assembly of nanocrystals. As previously noted Nano Lett., Vol. 3, No. 1, 2003

in the literature,2 dodecylthiol might be a linking agent to organize uniform nanoparticles. Without dodecylthiol, these superstructures could not be obtained. The interaction between the capping groups, -SH, and the silver ions leads the produced nanoparticles to get together and organize gradually. The molar ratio of dodecylthiol to silver ion would affect the assembly behavior of surfactant dodecylthiol and then the morphology of the particles. According to previous literature,22 at low surfactant concentration, surfactant can assemble to sphere-like micelles, while at relatively high concentration, surfactant tends to form rod-like micelles, which might act as the template for the formation of nanorod Ag2S. So we think that the product obtained at high molar ratio of dodecylthiol to inorganic ion has rod-like morphology might be due to the template-limited effect which comes from the surfactant assembly at high concentration. The silver ion concentration is another factor to the assembly of nanocrystals. At high silver ion concentration, the growth rate of Ag2S nuclei might be faster than their assembly rate, so these nuclei tend to form nanorods which then gradually organize to form 2D quasi-orthogonal superstructures. However, at low silver ion concentration, the growth rate of Ag2S nuclei might be slow, and these nuclei gradually assemble to 2D superstructures and at the same time they might grow and form rod-like shapes perpendicular to the ordered plane. The synthetic temperature mainly affects the particle size and homogeneity; however, these qualities also have a great effect on their assembly behaviors. For example, in our experiments the products produced under different synthetic temperatures have different superstructures, even at the same concentrations of dodecylthiol and silver ion. When the synthetic temperature is low (room temperature), the formed nanoparticles are very small and gradually organize to form a 2D quasi-rectangular superstructure. When the synthetic temperature is moderate, the formed particles are very uniform in both size and morphology and easily selfassemble to 2D hexagonal structure. Further increase of temperature leads to the particle growth, which is disadvantageous to the 2D assembly and results in the formation of chain-like structures. In conclusion, the synthesis and simultaneous assembly of monodispersed Ag2S nanoparticles and nanorods to quasihexagonal and orthogonal superstructures have been achieved through a simple route at low temperature without a presynthesis process for uniform nanocrystals. This direct and general route allows us to control the crystal morphology and their organization behavior and may be useful to synthesize other ordered semiconductor nanocrystal arrays. Acknowledgment. The work was supported by NNSC (Grants 20173012, 29925309, 20101002), Shanghai NanoTech. Center (0152 nm029), State Key Basic Res. Prog. (2000CB6105). Supporting Information Available: Four small angle XRD patterns for the as-synthesized products. 87

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Nano Lett., Vol. 3, No. 1, 2003