CRYSTAL GROWTH & DESIGN
Micropatterns of Ag2Se Nanocrystals
2004 VOL. 4, NO. 3 509-511
Sudip K. Batabyal,† C. Basu,*,† A. R. Das,‡ and G. S. Sanyal§,# Department of Solid State Physics, Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata, 700 032, India, and Department of Chemistry, University of Kalyani, Nadia, 741235, India Received November 25, 2003;
Revised Manuscript Received March 4, 2004
ABSTRACT: Silver selenide (Ag2Se) nanocrystals were synthesized solvothermally using a single source precursor silver selenite (Ag2SeO3) in the presence of dimethyl formamide (DMF) in an autoclave. The effect of time and temperature on the crystalline and morphological behavior of the products was investigated. The interaction of Ag2SeO3 with DMF resulted in the formation of Ag2Se nanoparticles, whereas mixed solvent of DMF and a few aliphatic alcohols (1:1 (v/v), at 165 °C for 24 h) yielded novel microstructures. The mixed combination containing methanol-DMF yielded fractal-like morphology, while hollow needle-shaped structures resulted from a ethanolDMF mixture. Rod shaped structures having riblike morphology were obtained from propanol-DMF mixed solvent. Introduction Investigations on semiconductor nanostructures have been the focus of intense activity in recent years because of the potential applications of these materials in various fields,1 such as solar cells, pigments, IR detectors, luminescence devices, and optical fiber communication. However, toward realization of technologically useful nanostructures spatial orientation and arrangement of the materials in addition to size and shape play crucial roles. Considerable research attention has been devoted to the development of well-ordered patterns of nano structures.2-4 In this context, silver selenide (Ag2Se) which exhibits a number of interesting and useful characteristics is conspicuous for attracting vigorous research attention.5-8 The high-temperature phase of the material (β-Ag2Se) is a superionic conductor5 that is useful in solid electrolyte in the case of photo chargeable secondary batteries. On the other hand, the low-temperature variety (R-Ag2Se) is a narrow band gap semiconductor which finds wide application as photosensitizer in photographic films or thermochromic materials.6 The R-Ag2Se is also a promising material for thermoelectric applications because of its relatively high Seebeck coefficient, low lattice thermal conductivity, and high electrical conductivity.7 Besides, large magnetoresistance has also been reported for a nonstoichiometric derivative of this solid.8 These characteristic properties and novel application potentials have prompted considerable efforts for the synthesis of Ag2Se with different morphologies.9-12 Biljana Pejova reported9 a thin flim deposition technique of (111) textured Ag2Se thin film from aqueous ammoniacal solution of silver nitrate and sodium selenosulfate. Glanville synthesized10 Ag2Se nanowires * To whom correspondence should be addressed: Prof. C. Basu, Department of Solid State Physics, Indian Association for the CultivationofScience,Jadavpur,Kolkata-700032,India.E-mail:
[email protected]; fax: 91 33 24732805; phone: 91 33 24734971. † Department of Solid State Physics, Indian Association for the Cultivation of Science. ‡ Polymer Science Unit, Indian Association for the Cultivation of Science. § University of Kalyani. # Present address: 39 South End Park, Kolkata-700029, India.
Table 1. Results of As-Synthesized Ag2Se in Neat DMF sample name
temp (°C)
duration of heating (h)
particle size measured from XRD (nm)
particle size measured from TEM (nm)
X1 X2 X3 X4
165 165 200 250
24 48 24 24
25 37 39 40
15 30 62 30
reacting silver nitrate with Se nanowire. Recently, Qian et al. obtained β-Ag2Se tubular crystals11 through hydrothermal growth and also microrods of Ag2Se arranged in dendritic structures12 via glycothermal synthesis. Various methods which require the synthesis of initial nanoparticles have been reported13 for the formation of patterned nanostructures. However, to date we are not aware of any detailed study on the micropattern formation of silver selenide. This communication deals with an investigation on the nanocrystallinity of solvothermally produced silver selenide from a single source precursor silver selenite (Ag2SeO3) in dimethyl formamide (DMF) and on its micropattern formation in mixed solvents containing DMF and a few normal aliphatic alcohols. The synthetic method is simple without requiring any template for the growth of nanostructure and simultaneous self-assembly into various patterns. Experimental Procedures Neat DMF solvent completely reduces Ag2SeO3 (prepared from silver nitrate and sodium selenite) to nano crystalline Ag2Se at a moderately high temperature (200/250 °C) and pressure in an autoclave. The chemical reaction probably takes place as
Ag2SeO3 + 3HCON(CH3)2 f Ag2Se + 3(CH3)2NH + 3CO2 The reduction in DMF was investigated at different temperatures (165/200/250°C) and times (24/48 h). The results are incorporated in Table 1. At 165 °C, the reduction was not complete in 24 or 48 h, and Ag2SeO3 was apparently present in the final product along with Ag2Se (Figure 1). But at 200/ 250 °C and 24 h the reduction was complete; the final product contained only Ag2Se (Figure 1). Sodium selenite (Na2SeO3) was prepared by dissolving selenium dioxide (SeO2) in sodium hydroxide solution. Silver
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Figure 1. XRD patterns of Ag2Se nanoparticles synthesized in DMF at different times and temperatures. nitrate (AgNO3) solution was added slowly to the Na2SeO3 solution when white Ag2SeO3 precipitated out. The precipitate was filtered, washed with water, and then vacuum-dried at 60°C. In a typical experiment 0.5 g of dried Ag2SeO3 was mixed with 25 mL of DMF in a Teflon-lined stainless steel autoclave (capacity 50 mL) and the whole mass stirred for about 10 min. The autoclave was next placed inside an air-oven at a prefixed temperature (165/200/250 °C) and heated for a particular period of time (24/48 h). The oven was then allowed to cool slowly to room temperature. The precipitate obtained was filtered out from the solvent, washed with water, and dried in a vacuum oven (60°C). The effects of a few normal aliphatic alcohols in the presence of DMF (1:1 v/v mixture) on the size and morphology of the product were studied under the solvothermal condition of 165 °C and 24 h, total volume of the mixed solvent being maintained the same (25 mL). The morphologies in different alcohols were observed to be different; the precipitates in neat DMF and DMF-butanol, DMF-hexanol and DMF-octanol mixtures were powdery, wooly in a DMF-methanol mixture, and tiny rodlike in a DMF-ethanol and DMF-propanol mixtures, respectively. Attempts to synthesize Ag2Se nanocrystals using DMFwater mixtures apparently resulted in the separation of elemental selenium mixed with Ag2Se. The prepared samples were characterized by X-ray diffraction method operating on a Rich, Seifert XRD 3000P X-ray diffractometer with graphite monochromated Cu-KR radiation (λ ) 1.5418 Å). The morphology of the final products was determined from transmission electron microscopic images taken by a Hitachi H-800 transmission electron microscope (TEM) at 50 KV accelerating potential. Scanning electron microscopic (SEM) images and energy-dispersive X-ray analysis (EDXA) were recorded on a JEOL GSM-5800.
Results and Discussion Figures 1 and 2 show the XRD patterns of as-prepared Ag2Se samples in DMF, and DMF-alcohol mixtures under the different conditions mentioned. The broadened nature of these peaks indicates that the grain sizes of the samples are in the nanometer scale. Average sizes of the particles estimated from Scherrer equation (as well as from TEM measurement) are given in Table 1. While majority of the peaks could be indexed to the orthorhombic Ag2Se phase in agreement with the reported data in the literature, there are peaks marked with an asterisk (*) attributable to residual Ag2SeO3 impurity which, however, disappears in ×3 and ×4 (Figure 1) as the temperature is raised to 200 and 250
Batabyal et al.
Figure 2. XRD patterns of as-synthesized Ag2Se in different mixed solvents.
Figure 3. TEM micrograph of Ag2Se nanoparticle obtained using neat DMF at (a) 200 °C, 24 h; (b) 165 °C, 24 h.
°C, respectively. Spectrum (a) (Figure 2) represents the standard spectrum of JCPDF data sheet (PDF No. 24/ 1041) of orthorhombic Ag2Se. The XRD pattern of Ag2Se prepared using neat DMF solvent in an autoclave at 165 °C for 24 h is contained in the spectrum b, whereas spectra (c-h) represent the XRD profiles of Ag2Se obtained using mixed solvents of DMF and methanol, ethanol, propanol, butanol, hexanol, and octanol, respectively; spectra of all but DMF-methanol /DMFbutanol mixtures indicate impurity due to Ag2SeO3 marked with asterisk. No characteristic peak of elemental Ag or Se was observed. EDXA for the sample in DMF-methanol mixture confirms that the obtained product consists of silver and selenium and that the stoichiometry closely agrees with Ag2Se. The TEM micrograph of the final product using neat DMF exhibits no micropattern formation. Figure 3 shows the TEM micrograph of Ag2Se nanoparticle obtained by 24 h heating of the reaction mixture in neat DMF at (a) 200 °C and (b) 165 °C. Figure 4 shows optical micrograph (a) and scanning electron micrograph (b) of Ag2Se microcrystals synthesized using DMF-methanol mixed solvent. The micrograph clearly exhibits fractallike growth of the crystals. The morphology of Ag2Se synthesized using DMF-ethanol mixture is quite different from that prepared through DMF-methanol combination; hollow needlelike structures of micrometer diameter were observed when the former solvent mixture was used (Figure 5a). Figure 5b represents the surface profile of such hollow needles; close observation reveals that these needles are bundles of thinner ones.
Micropatterns of Ag2Se Nanocrystals
Crystal Growth & Design, Vol. 4, No. 3, 2004 511
Figure 6. Scanning electron micrograph of as synthesized Ag2Se from a DMF-propanol mixed solvent (a) a few rods, (b) surface profile, and (c) lateral growth. Figure 4. Optical micrograph (a) and scanning electron micrograph (b) of fractal-like Ag2Se synthesized using DMFmethanol mixed solvent.
Figure 5. Scanning electron micrograph of hollow needlelike Ag2Se synthesized using a DMF-ethanol mixed solvent. (a) a few Ag2Se needles; (b) a closer view of Ag2Se needle of circular cross-section (the upper inset shows rectangular cross-section, and the lower one represents surface profile of Ag2Se needles).
The cross-section of those needles is of two types: a few are circular, while the others are rectangular. The length of the large needle is of the order of 300 µm and diameter 90 µm, whereas the diameter of the smaller needle is of the order of 1 µm. In the case of a DMFpropanol mixture the product contains tiny rods mixed with a little powder. The SEM micrograph (Figure 6) shows the riblike structure of the rods. There is a tendency of lateral growth formation on the central rod. The SEM micrographs of the Ag2Se obtained using DMF-hexanol or DMF-octanol mixtures do not exhibit any type of microstructure formation; instead, powderlike morphology is observed. In the case of a octanol mixture the powder contains some tiny rods of dimension 0.5 to 1 µm. Conclusion Ag2Se nanocrystals were prepared by the interaction of DMF and silver selenite, while the mixed solvents
(DMF-methanol/ethanol/propanol) yielded patterned microstructures of Ag2Se. The results lead us to believe that nanoparticles were formed by the reductive action of DMF on Ag2SeO3 and that the particles self assembled in microstructural patterns in the presence of a few straight chain aliphatic alcohols (methanol /ethanol /propanol). Although the exact mechanism is not clear to us, it can be said that in methanol, ethanol, as well as propanol self-assembly of Ag2Se nanoparticles is favored yielding different patterns; however, no pattern formation is observed in the mixed combinations of DMF with butanol, hexanol, or octanol.
References (1) Rao, C. N.; Cheetham, A. K. J. Mater. Chem. 2001, 11, 2887. (2) Zhu, H. W.; Xu, C. L.; Wu, D. H.; Wei, B. Q.; Vajtai, R.; Ajayan, P. M. Science 2002, 296, 884. (3) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano Lett. 2003, 3, 235. (4) Batabyal, Sudip K.; Basu, C.; Sanyal, G. S.; and Das, A. R. Mater. Lett. 2003, 58, 169. (5) Shimojo, F.; Okazaki, H. J. Phys. Soc. Jpn. 1993, 62, 179. (6) Lewis, K. L.; Pitt, A. M.; Wyatt-Davies, T.; Milward, J. R. Mater. Res. Soc. Symp. Proc. 1994, 374, 105. (7) Ferhat M.; Nagao, J. J. Appl. Phys. 2000, 88, 813. (8) Xu, R.; Husmann, A.; Rosenbaum, T. F.; Saboungi, M. L.; Enderby, J. E.; Littlewood, P. B. Nature 1997, 390, 57. (9) Pejova, B.; Najdoski, M.; Grozdanov, I.; Dey, S. K. Mater. Lett., 2000, 43, 269. (10) Glanville, Y. J.; Narehood, D. G.; Sokol, P. E.; Amma A.; Mallouk, T. J. Mater. Chem. 2002, 12, 2433. (11) Hu, J.; Deng, B.; Lu, Q.; Tang, K.; Jiang, R.; Qian, Y.; Zhou, G.; Cheng, Hao. Chem. Commun. 2000, 715. (12) Shen, G.; Chen, D.; Tang, K., Jiang, X.; Qian, Y. Chem. Lett., 2003, 32, 210. (13) Lu, Q.; Gao, F.; Zhao, D. Angew. Chem., Int. Ed. 2002, 41, 1932.
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