(This is a sample cover image for this issue. The actual cover is not yet available at this time.)

Transcription

1 (This is a sample cover image for this issue. The actual cover is not yet available at this time.) This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Materials Letters 88 (2012) Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: Optical properties and self-assembly of Ag 2 S nanoparticles synthesized by a one-pot method Miao Wang a,1, Yu Wang a,1, Aiwei Tang a,b,n,xuli b, Yanbing Hou b, Feng Teng b a Department of Chemistry, School of Science, Beijing JiaoTong University, Beijing , PR China b Key Laboratory of Luminescence and Optical Information, Ministry of Education, School of Science, Beijing JiaoTong University, Beijing , PR China article info Article history: Received 21 June 2012 Accepted 7 August 2012 Available online 18 August 2012 Keywords: Nanoparticles Optical properties Self-assembly Superlattice structures abstract Spherical silver sulfide (Ag 2 S) nanoparticles have been synthesized using a simple and one-pot route, which is a direct heating process using silver acetate and n-dodecanethiol as raw materials.transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and selected area electron diffraction (SAED) were used to confirm the size, morphology and crystal structure of as-obtained Ag 2 S nanoparticles. The size of the products can be tuned by controlling the reaction time, and the growth process of the samples has been studied by employing UV vis absorption spectra. Furthermore, these Ag 2 S nanoparticles have a high tendency to self-assemble into two- or three-dimensional superlattice structures. This one-pot approach may be extended to synthesis of other metal sulfides nanoparticles. & 2012 Elsevier B.V. All rights reserved. 1. Introduction As an important type of colloidal inorganic nanoparticles, metal sulfide nanoparticles have exhibited obvious sizedependent optical and electrical properties, which have been widely used in light-emitting diodes (LED) [1,2], solar cells [3,4], biological cells [5] and memory devices [6,7]. Recently, silver sulfide (Ag 2 S) nanoparticles have attracted much attention due to their potential applications in photoconductors, solar cells, nearinfrared photo-detectors [8], and so on. More importantly, Ag 2 Sis less toxic as compared to other nanoparticles such as cadmium sulfide (CdS), lead sulfide (PbS), etc. Since the Ag 2 S nanoparticles possess so many applications and non-negligible advantages, different approaches, such as the hydrothermal route [9], the solvothermal method [10] and single-source precursor routes [11], and so on, have been developed for synthesis of Ag 2 S nanoparticles. For instance, Wang s group developed a strategy for obtaining single-crystalline, monodispersed Ag 2 S nanoparticles with a size of 10 nm using a single source precursor route, and the NIR emission at 1058 nm was first observed [11]. Furthermore, Wang and other co-workers prepared sub-micrometer Ag 2 S particles with normal and flattened rhombic dodecahedral morphologies through a facile hydrothermal method [9]. However, the single-source precursor synthetic method mentioned above is not suitable for the large-scale synthesis of high-quality nanoparticles, and by the hydrothermal route it is difficult to control the shape and size of the nanoparticles. Therefore, it remains a challenge to develop a simple and low-cost approach for synthesis of Ag 2 S nanoparticles on a large scale. In this paper, a one-pot approach was adopted to prepare high-quality Ag 2 S nanoparticles without any extra steps for presynthesis of metal precursors. By prolonging the reaction time, the size of the nanoparticles is increased, and the growth process was monitored by UV vis spectra. More interestingly, the asobtained Ag 2 S nanoparticles could easily self-assemble into twoor three-dimensional superlattice structures. 2. Experimental section Synthesis of Ag 2 S nanoparticles: In a typical synthesis, 3 mmol of Ag(OAc) and 20 ml of DDT were added into a three-necked n Corresponding author at: Department of Chemistry, School of Science, Beijing JiaoTong University, Beijing , PR China. Tel.: þ address: awtang@bjtu.edu.cn (A. Tang). 1 These authors contributed equally to this work. Scheme 1. Illustration of synthesis of Ag 2 S nanoparticles by a one-pot approach X/$ - see front matter & 2012 Elsevier B.V. All rights reserved.

3 M. Wang et al. / Materials Letters 88 (2012) flask at room temperature, and then the mixture was degassed by nitrogen for 15 min to remove the oxygen. Afterwards, the mixture was slowly heated to 205 1C under N 2 and kept at that temperature for 100 min. Then the solution was naturally cooled to room temperature; the products were washed and precipitated using chloroform and ethanol three times. Finally, the samples were re-dispersed into chloroform or dried in vacuum for subsequent characterization. The synthetic approach of Ag 2 S nanocrystals by direct heating Ag(OAc) in DDT is illustrated in Scheme 1. Sample characterization: The TEM of Ag 2 S nanoparticles was studied using a Hitachi-7650 transmission electron microscope operating at an accelerating voltage of 100 kv. The HRTEM images, SAED and energy dispersive X-ray spectroscopy (EDS) patterns were recorded using a JEM-2010 transmission electron microscope working at an accelerating voltage of 200 kv. XRD patterns of the nanoparticles were measured using a Bruker D8 Discover X-ray Diffractometer with a CuKa radiation source (l¼ Å). The UV vis absorption spectra of the Ag nanospheres in chloroform were obtained on a Shimadzu-UV 3101 spectrophotometer. All the measurements were carried out under ambient conditions at room temperature. Fig. 1. XRD patterns of Ag 2 S nanoparticles (100 min), and the inset shows the corresponding SAED patterns. Fig. 3. UV vis spectra of Ag 2 S nanoparticles obtained at different reaction times. Fig. 2. TEM images of Ag 2 S nanoparticles for different reaction times: (a) 60 min; (b) 100 min; (c) HRTEM image of Ag 2 S nanoparticles shown in (b); and (d) EDS patterns of Ag 2 S nanoparticles.

4 110 M. Wang et al. / Materials Letters 88 (2012) Fig. 4. TEM images of typical superlattice structures of Ag 2 S nanoparticles obtained at 60 min. 3. Results and discussion The crystal phase of the as-obtained Ag 2 S nanoparticles was confirmed by the XRD patterns, which are shown in Fig. 1. It can be observed that all the diffraction peaks in the XRD patterns match well with those of monoclinic Ag 2 S (JCPDS Card no ), and no other peaks from Ag appear in the XRD patterns, which suggest the pure phase of our samples. In addition, several peaks are broadened due to the nanocrystalline nature of the samples [11]. The SAED pattern displayed in the inset of this figure shows several distinct rings corresponding to monoclinic Ag 2 S, which is in agreement with the XRD results. Fig. 2(a) and (b) depicts the TEM images of Ag 2 S nanoparticles obtained at 60 min and 100 min, respectively. It is clearly seen that the Ag 2 S nanoparticles are spherical with a relatively narrow size distribution, and the mean size is increased from nm at 60 min to nm as the reaction time is prolonged. The corresponding size distribution histograms are given in Fig. S1 in the supporting information. The HRTEM image (Fig. 2c) of Ag 2 S nanoparticles obtained from 100 min shows the well-resolved lattice fringes, suggesting the highly crystalline nature of the asobtained samples. The inter-plane distance of the lattice fringes is about nm, which is ascribed to that of the (112) or (121) plane of monoclinic Ag 2 S (JCPDS Card no ). To further confirm the formation of Ag 2 S nanoparticles, energy dispersive X-ray spectroscopy (EDS) was also performed to determine the chemical composition of the as-prepared Ag 2 S nanoparticles (shown in Fig. 2d), and the atomic ratio of Ag to S is about 1.96:1, very close to the stoichiometric composition of Ag 2 S, which verifies the formation of Ag 2 S nanoparticles. Fig. 3 presents the UV vis spectra of the Ag 2 S nanoparticles dispersed in chloroform obtained from different reaction times. At the beginning of the reaction, the Ag DDT complex is formed, which acts as the precursor to decompose into Ag 2 S nanoparticles. For the sample obtained from 10 min, an obvious absorption peak at 344 nm appeared, which is attributed to the chargetransfer excitation of Ag DDT complex [12]. The XRD patterns of the samples obtained at 10 min and 30 min confirmed the formation of Ag DDT complex, which are shown in Fig. S2 in the supporting information. As the reaction time is prolonged, the intensity of the peak at 344 nm gradually becomes weaker, and a broad band at 490 nm appears. When the reaction time is prolonged to 100 min, the peak at 344 nm disappears and the band at 490 nm is dominant in the absorption spectrum, which may originate from the band gap of the Ag 2 S nanoparticles. Compared to the band gap (E g ¼1.1 ev) of bulk Ag 2 S [12], an obvious blue-shift in the UV vis absorption can be observed due to quantum-confinement effects. More interestingly, the spherical Ag 2 S nanoparticles have a high tendency to self-assemble into three-dimensional (3D) superlattice structures by slow evaporation of carrier solvent from a dense solution. Fig. 4(a) and (b) shows the typical TEM images of superlattice structures of Ag 2 S nanoparticles; irregularly shaped 3D superlattice structures with clearly defined edges are observed, and the domain size of the superlattice structures can reach up to several micrometers (Fig. S3 in the supporting information). It is noted that highly-ordered arrangement of Ag 2 S nanoparticles is formed on the edge of the samples. As stated in previous reports, the presence of excess alkylthiol played an important role in the formation of highly-ordered superlattice structures [13]. With respect to Ag 2 S nanoparticles capped by DDT, the capping ligands on the surface of Ag 2 S nanoparticles promote the superlattice formation due to the cohesive interaction through interdigitation or interpenetration of alkyl chains of thiol-molecules [14]. 4. Conclusion In summary, we report a reproducible and simple strategy to prepare monodispersed Ag 2 S nanoparticles, in which DDT acted not only as the sulfur source, but also the capping-agent and reaction medium. By adjusting the reaction time, the size of the Ag 2 S nanoparticles can be easily controlled, and the origin of the absorption band changed from that of the Ag DDT complex to band gap of Ag 2 S nanoparticles. The as-obtained Ag 2 S nanoparticles can easily self-assemble into 3D superlattice structures. Acknowledgments This work was partly supported by the research projects of National Science Foundation of China (no ),the National Science Foundation for Distinguished Young Scholars of China (no ), and Basic Scientific Research Fund of Beijing JiaoTong University (2010JBZ006, 2011JBM301). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at

5 M. Wang et al. / Materials Letters 88 (2012) References [1] Sun QJ, Wang YA, Li SL, Wang DY, Zhu T, Xu J, et al. Nat Photon 2007;1: [2] Tang AW, Teng F, Xiong S, Wang Y, Feng B, Hou YB. J Electrochem Soc 2008;155:K [3] Huynh WU, Dittmer JJ, Alivisatos AP. Science 2002;295: [4] Tang AW, Teng F, Jin H, Gao YH, Hou YB, et al. Mater Lett 2007;61: [5] Li YL, Jing LH, Qiao RR, Gao MY. Chem Commun 2011;47: [6] Tang AW, Teng F, Hou YB, Wang YS, Wang ZG, et al. Appl Phys Lett 2010;96: [7] Tang AW, Teng F, Liu J, Wang YC, Peng HS, Hou YB, et al. J Nanopart Res 2011;13: [8] Zhou XD, Shi HQ, Huang DM, Jia SM, Fu X, Jiao K. Mater Lett 2008;62: [9] Yu C, Leng M, Liu MZ, Yu Y, Liu D, Wang C. Cryst Eng Comm 2012;14: [10] Ma DK, Hu XK, Zhou HY, Zhang JH, Qian YT. J Cryst Growth 2007;304: [11] Du YP, Xu B, Fu T, Cai M, Le F, Wang QB, et al. J Am Chem Soc 2010;132: [12] Schaaff TG, Rodinone AJ. J Phys Chem B 2003;107: [13] Zhang YG, Li XM, Xu WW, Li S, Wang HZ, Li LS. Mater Lett 2012;67: [14] Tang AW, Qu SC, Hou YB, Teng F, Wang YS, Wang ZG. J Solid State Chem 2011;184: