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Characterization of silver sulfide nanoparticles synthesized by a simple precipitation method
Materials Letters 59 (2005) 529 – 534 www.elsevier.com/locate/matlet
Characterization of silver sulfide nanoparticles synthesized by a simple precipitation method G.A. Martı´nez-Castan˜o´na,b, M.G. Sa´nchez-Loredoc, H.J. Dorantesd, J.R. Martı´nez-Mendozab, G. Ortega-Zarzosab, Facundo Ruizb,* a Centro de Investigacio´n en Materiales Avanzados, Chihuahua, Chih., Me´xico Facultad de Ciencias, Universidad Auto´noma de San Luis Potosı´, Zona Universitaria, Me´xico c Instituto de Metalurgia, Universidad Auto´noma de San Luis Potosı´, Me´xico d ESIQIE, Instituto Polite´cnico Nacional, Me´xico
b
Received 16 June 2004; received in revised form 6 October 2004; accepted 20 October 2004 Available online 30 October 2004
Abstract Silver sulfide nanoparticles with different sizes were synthesized using a simple aqueous precipitation. Particles were obtained in the presence of three stabilizing agents controlling thus particle size and agglomeration. The particles obtained were characterized using XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal and spectroscopy techniques. We observed a bsize quantizationQ effect reflected in a shift on the band gap value of smaller sample obtained. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Semiconductors
1. Introduction Semiconductor nanoparticles have been investigated over the past years due to their potential applications in microelectronics and due to their specific optic, electronic and catalytic properties. Methods to prepare semiconductor nanoestructures can be classified into physical methods and chemical methods. Physical methods use molecular beam and lithography techniques obtaining well defined nanostructures which are used in high precision electronic and magnetic measurements [1]. Among many chemical routes to synthesize semiconductor nanoparticles [2], the colloidal one represents an option due to their versatility and their relative facility; using this technique, one can obtain almost any material choosing adequate reactants and controlling the reaction parameters. Silver sulfide can be * Corresponding author. Tel.: +52 444 8262319; fax: +52 444 8262321. E-mail address: [email protected] (F. Ruiz). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.10.043
used as giant magnetoresistor (GMR) [3]; silver sulfide thin films with a silver excess can be used as detectors in the infrared region [4] and silver sulfide clusters are used in photographic sensibility [5]. There are many and different methods to synthesize silver sulfide nanoparticles, some can give silver sulfide particles with cube [6] or dendrite forms [7]; silver sulfide particles that are bioactive can also be obtained [8,9], silver sulfide nanoparticles under 15 nm in size were synthesized [10–12] and recently, simpler methods are appearing [13]. Each method has advantages and produces particles with particular properties. In this work, silver sulfide nanoparticles were synthesized using a simple aqueous precipitation. Three stabilizing agents were used in order to prevent particle growth and agglomeration: Triton X-100, a very wellknown surfactant with an hydroxyl functional group; mercaptoacetic acid and 3-mercapto-1,2-propanediol, both containing a thiol functional group. We used these agents to probe that the functional group plays an important role in the prevention of particle growth and agglomeration.
530
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
out on a Perkin Elmer TGA 7, using nitrogen as purging gas, at a scanning rate of 10 8C/min. Differential thermal analysis (DTA) was obtained on a Perkin Elmer instrument model DTA 7 using nitrogen as purging gas, also at a scanning rate of 10 8C/min. The morphology was observed by scanning (SEM) and transmission electron microscopy (TEM). SEM images were obtained using a XL-30 scanning electron microscope (Philips, Netherlands). EDS measurements (energy-dispersive X-ray spectrometry) were carried out using an X-ray microanalyzer (DX-4I, EDAX) built on the scanning electron microscope. TEM images were obtained using a JEOL JEM 2000 FXII using an accelerating voltage of 200 kV. Samples for TEM were prepared by placing a drop of the sample suspension on a copper grid. Optical analysis was made in an Oceanoptics S2000 and NIR spectra were obtained using an NIR Ocean Optics system. Fig. 1. X-ray diffraction pattern of the sample synthesized using 3mercapto-1,2-propanediol as stabilizing agent.
3. Results and discussion Also, we determined the optical band gap using the simple diffuse reflectance spectroscopy.
2. Experimental section 2.1. Synthesis Silver nitrate was dissolved in 1 L of deionized water to a concentration of 8 mM. This solution was placed in a 2-L reaction vessel. Under stirring (200 rpm) 10 mmol of Triton X-100, 3-mercapto-1,2-propanediol or mercaptoacetic acid was added and the mixture was stirred for an additional 5 min. Ammonium sulfide (4 mmol) dissolved in 100 mL of deionized water was added dropwise under ambient conditions. When using 3-mercapto-1,2-propanediol or mercaptoacetic acid as protective reagents, this method yields dispersions that are stable even during months. For comparison purposes, an additional sample was obtained following the same route but using no stabilizing agent. Hereafter, the particles prepared without stabilizing agent will be referred as Ag2S, silver sulfide powders prepared in the presence of Triton X-100 as Ag2S-T; in the presence of mercaptoacetic acid as Ag2S-M and those obtained in presence of 3-mercapto-1,2-propanediol as Ag2S-P. 2.2. Characterization When applicable, SEM characterization was carried out using the stable dispersions. XRD, thermal and spectroscopic analyses were made using the dried powders obtained by precipitation by means of ionic strength adjustment. X-ray diffraction patterns were recorded with a Rigaku 2200 powder X-ray diffractometer. The nickel filtered Cu Ka (k=1.5418 2) radiation was used at 36 kV and 30 mA. Thermogravimetric analysis (TGA) was carried
Fig. 1 shows the diffraction pattern of the sample Ag2S-P. All patterns obtained agree with the published data (JCPDS 14-072). The difference between the diffraction patterns of the different samples is only the peak broadening due to the particle size. The nanoparticles synthesized are in the monoclinic form which is the stable form of silver sulfide at room temperature. Average particle size was calculated using Scherrer’s equation and the results are reported in Table 1. Differences in particle agglomeration were observed by SEM. The samples named Ag2S and Ag2S-T show strong agglomeration (Fig. 2a and b). In Fig. 3, a closer view of these samples is presented, and we can see that they are formed by nanometrical particles; the observed particle size in these samples is not consistent with those presented in Table 1 probably due to a different size distribution in both samples, the average particle size obtained by analysis made by X-ray diffraction is considered over a large amount of particles, which is very difficult to do in SEM analysis, particle size observed in Fig. 3 represents a very small fraction of the sample. The particles named Ag2S-M and Ag2S-P are loose agglomerated (Fig. 2c and d). Particle morphology was observed using TEM. Images of sample Ag2S-M are presented in Fig. 4, the particles present ellipsoidal and spherical morphology and their sizes are between 20 and 50 nm. Table 1 Average particle size calculated using Scherrer’s equation Synthesis conditions Sample
pH
Stabilizing agent
Particle size (nm)
Ag2S Ag2S-T Ag2S-M Ag2S-P
7 7.5 1.9 2
Without stabilizing agent Triton X-100 Mercaptoacetic acid 3-mercapto-1,2-propanediol
64.6 74 52 30.8
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
531
Fig. 2. Scanning electron microscopy images of the Ag2S samples synthesized using as stabilizing agent: (a) Without stabilizing agent; (b) Triton X-100; (c) Mercaptoacetic acid and (d) 3-mercapto-1,2-propanediol.
TGA measurements were made to reveal the presence of any chemically adsorbed species (Fig. 5). The samples Ag2S and Ag2S-T present a mass loss of 0.55% and 1.5%, respectively, the small mass loss observed is probably due to desorption of water molecules from the particles surface. The sample Ag2S-M shows a mass loss of 3.5% between 75 and 250 8C probably due to desorption of mercaptoacetic acid molecules. The sample Ag2S-P gives a mass loss of 2% in two steps; the first step occurred between 85 and 210 8C and the second step between 215 and 350 8C. These results agree with those reported by Vossmeyer et al. [14], they obtain CdS clusters using the same stabilizing agent and observed a mass loss between 250 and 300 8C, this mass loss is achieved in several steps and can be ascribed to the desorption of 3-mercapto-1,2-propanediol molecules. To study the phase transitions involved, DTA measurements were done. The results for the samples Ag2S-T and Ag2S-P are presented in Fig. 6. For sample Ag2S-T, the
DTA curve has three endothermic peaks at 180, 600 and 805 8C, corresponding to the phase transitions of the three polymorphs of Ag2S [15]: monoclinic acanthite, stable up to 176 8C; body-centered cubic argentite, stable from 176 8C to a temperature between 586 and 622 8C; above which the stable form is a face-centered cubic polymorph. The peak at 805 8C corresponds to the decomposition of silver sulphide. Ag2S and Ag2S-M samples show similar results. DTA curve for sample Ag2S-P presents four endothermic peaks at 178, 280, 587 and 806 8C, three of them corresponding to the phase transitions already mentioned, and the peak at 280 8C corresponding to desorption of stabilizer molecules, this result is in agreement with the one obtained using TGA analysis. The small differences in transition temperatures could be due to size effect or to deviations in stoichiometry of the samples. Stabilizing agents surround the particles creating a layer that prevents against growth and agglomeration. The main
Fig. 3. Scanning electron microscopy images showing a closer view of the samples synthesized using as stabilizing agent: (a) Without stabilizing agent and (b) Triton X-100. We can see that the agglomerates are formed by nanometric units.
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G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
Fig. 4. Transmission electron microscopy images of sample Ag2S-M.
factor discriminating the effectiveness of the various ligands is the difference in Lewis base character, requiring a firmly bound moiety to stabilize growth [18]. Triton X100 is not an effective stabilizing agent in the synthesis of silver sulfide nanoparticles because its hydroxyl group has
little affinity for the silver species; this is the reason why sample Ag2S-T presents no adsorption and the particles are strongly agglomerated. Because Ag(I) is a soft acid, the stability of the moiety would increase with increasing ligand softness, so it is expected that sulphur-containing
Fig. 5. TGA curves for samples (a) Ag2S, (b) Ag2S-T, (c) Ag2S-M, and (d) Ag2S-P.
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
533
Fig. 6. DTA curves for samples (a) Ag2S-T and (b) Ag2S-P.
ligands would be good capping agents, therefore, mercapto acetic acid and 3-mercapto-1,2-propanediol, with a thiol group, have high affinity for silver species and are thus more efficient as stabilizing agents, being capable to adsorb on the particles surface. This adsorption can be detected by TGA, and effects of this adsorption can be observed in size reduction and effective dispersion (SEM and XRD results). A fundamental property of semiconductor compounds is the band gap, the energy separation between the filled valence band and the empty conduction band. Silver sulfide is a direct band gap semiconductor with a band gap value of 1 eV (near infrared region) in the bulk form. We calculated the band gap of sample Ag2S-P in order to determine if there is a bsize quantizationQ effect in the synthesized materials. Optical excitation of electrons across the band gap is strongly allowed, producing an abrupt increase in absorptivity at the wavelength corresponding to the gap energy.
This feature in the optical spectrum is known as the optical absorption edge. Diffuse reflectance spectroscopy can be used to determine the optical absorption edge [16,17]. The ideal diffuse reflectance spectrum consists of a nearly flat, low absorbance region at long wavelengths that abruptly transforms to a steeply rising absorption edge at shorter wavelengths. The most direct way of extracting the optical band gap is to simply determine the wavelength at which the extrapolations of the base line and the absorption edge cross. Fig. 7 shows the NIR analysis using this method, the band gap value obtained was 1.08 eV, this value is shifted compared with the bulk value and this could be a consequence of a bsize quantizationQ effect in the sample. On other hand, when a graph is plotted between (ahm)2/3 or [hm ln{(R max R min)/(R R min)}]2/3, and hm (as abscissa), a straight line is obtained, where R max and R min are maximum and minimum values of reflectance, R is the reflectance at a given photon energy, hm. The extrapolation of the straight line to (ahm)2/3=0 axis gives the value of the band gap of the sample. This method does not give good results with our samples. As expected, reduction in particle size gives a shift in the optical band gap of the Ag2S-P sample. It is evident that particle size, agglomeration degree and optical properties are strongly dependent of the kind of sulphur-containing reagent applied.
4. Conclusion
Fig. 7. Energy band gap of the sample Ag2S-P obtained using NIR spectroscopy.
Silver sulfide nanoparticles with different sizes and agglomeration degree were synthesized using a simple aqueous solution method. This method is fast, no expensive and it can be performed under ambient conditions. We used three different stabilizing agents (Triton X-100, 3-mercapto1,2-propanediol or mercaptoacetic acid), and we found that under the same reaction conditions, the most effective is 3mercapto-1,2-propanediol, which gives us particles with an average size of 30.8 nm. Reduction in particle size gives a
534
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
shift in the optical band gap of the particles. Finally, an important practical result concerning the synthesis method is that the obtained nanoparticles are produced in gram scale.
References [1] A. Eychmqller, J. Phys. Chem., B 104 (2000) 6514. [2] G.C. Hadjipanayis, R.W. Siegel, Nanophase Materials, Kluwer Academic Publisher, Netherlands, 1994, pp. 1 – 19. [3] N. Harpeness, O. Palchik, A. Gedanken, V. Palchik, S. Amiel, M.A. Slifkin, A.M. Weiss, Chem. Mater. 14 (2002) 2094. [4] M. Brelle, J. Zhang, L. Nguyen, R. Mehra, J. Phys. Chem., A 103 (1999) 10194. [5] D. Brqhweiler, R. Seifert, G. Calzaferri, J. Phys. Chem., B 103 (1999) 6397. [6] M. Wu, X. Pan, X. Qian, J. Yin, Z. Zhu, Inorg. Chem. Commun. 7 (2004) 359.
[7] J. Xiao, Y. Xie, R. Tang, W. Luo, J. Mater. Chem. 12 (2002) 1148. [8] M. Meziani, Y. Sun, J. Am. Chem. Soc. 125 (2003) 8015. [9] M. Brelle, J. Zhang, L. Nguyen, R.J. Mehra, Phys. Chem., A 103 (1999) 10194. [10] D. Brqhwiler, C. Leiggener, S. Glaus, G. Calzaferri, J. Phys. Chem., B 106 (2002) 3770. [11] G. Calzaferri, C. Leiggener, S. Glaus, D. Schqrch, K. Kuge, Chem. Soc. Rev. 32 (2003) 29. [12] G. Calzaferri, D. Brqhweiler, S. Glaus, D. Schqrch, A. Currao, C. Leiggener, J. Imaging Sci. Technol. 45 (2001) 331. [13] X. Changqi, Z. Zhicheng, Y. Qiang, Mater. Lett. 58 (2004) 1671. [14] T. Vossmeyer, L. Katsikas, M. Giersig, I. Popovik, K. Diesner, A. Chemseddine, A. Eychmqller, H. Weller, J. Phys. Chem. 98 (1994) 7665. [15] M.B. McNeil, B.J. Little, J. Am. Inst. Conserv. 31 (1992) 355. [16] S. Boldish, W. White, Am. Mineral. 83 (1998) 865. [17] V. Kumar, S. Sharma, T. Sharma, V. Singh, Opt. Mater. 12 (1999) 115. [18] M. Green, N. Allsop, G. Wakefield, P. Dobson, J.L. Hutchison, J. Mater. Chem. 12 (2002) 2671.
Characterization of silver sulfide nanoparticles synthesized by a simple precipitation method G.A. Martı´nez-Castan˜o´na,b, M.G. Sa´nchez-Loredoc, H.J. Dorantesd, J.R. Martı´nez-Mendozab, G. Ortega-Zarzosab, Facundo Ruizb,* a Centro de Investigacio´n en Materiales Avanzados, Chihuahua, Chih., Me´xico Facultad de Ciencias, Universidad Auto´noma de San Luis Potosı´, Zona Universitaria, Me´xico c Instituto de Metalurgia, Universidad Auto´noma de San Luis Potosı´, Me´xico d ESIQIE, Instituto Polite´cnico Nacional, Me´xico
b
Received 16 June 2004; received in revised form 6 October 2004; accepted 20 October 2004 Available online 30 October 2004
Abstract Silver sulfide nanoparticles with different sizes were synthesized using a simple aqueous precipitation. Particles were obtained in the presence of three stabilizing agents controlling thus particle size and agglomeration. The particles obtained were characterized using XRD, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal and spectroscopy techniques. We observed a bsize quantizationQ effect reflected in a shift on the band gap value of smaller sample obtained. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Semiconductors
1. Introduction Semiconductor nanoparticles have been investigated over the past years due to their potential applications in microelectronics and due to their specific optic, electronic and catalytic properties. Methods to prepare semiconductor nanoestructures can be classified into physical methods and chemical methods. Physical methods use molecular beam and lithography techniques obtaining well defined nanostructures which are used in high precision electronic and magnetic measurements [1]. Among many chemical routes to synthesize semiconductor nanoparticles [2], the colloidal one represents an option due to their versatility and their relative facility; using this technique, one can obtain almost any material choosing adequate reactants and controlling the reaction parameters. Silver sulfide can be * Corresponding author. Tel.: +52 444 8262319; fax: +52 444 8262321. E-mail address: [email protected] (F. Ruiz). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.10.043
used as giant magnetoresistor (GMR) [3]; silver sulfide thin films with a silver excess can be used as detectors in the infrared region [4] and silver sulfide clusters are used in photographic sensibility [5]. There are many and different methods to synthesize silver sulfide nanoparticles, some can give silver sulfide particles with cube [6] or dendrite forms [7]; silver sulfide particles that are bioactive can also be obtained [8,9], silver sulfide nanoparticles under 15 nm in size were synthesized [10–12] and recently, simpler methods are appearing [13]. Each method has advantages and produces particles with particular properties. In this work, silver sulfide nanoparticles were synthesized using a simple aqueous precipitation. Three stabilizing agents were used in order to prevent particle growth and agglomeration: Triton X-100, a very wellknown surfactant with an hydroxyl functional group; mercaptoacetic acid and 3-mercapto-1,2-propanediol, both containing a thiol functional group. We used these agents to probe that the functional group plays an important role in the prevention of particle growth and agglomeration.
530
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
out on a Perkin Elmer TGA 7, using nitrogen as purging gas, at a scanning rate of 10 8C/min. Differential thermal analysis (DTA) was obtained on a Perkin Elmer instrument model DTA 7 using nitrogen as purging gas, also at a scanning rate of 10 8C/min. The morphology was observed by scanning (SEM) and transmission electron microscopy (TEM). SEM images were obtained using a XL-30 scanning electron microscope (Philips, Netherlands). EDS measurements (energy-dispersive X-ray spectrometry) were carried out using an X-ray microanalyzer (DX-4I, EDAX) built on the scanning electron microscope. TEM images were obtained using a JEOL JEM 2000 FXII using an accelerating voltage of 200 kV. Samples for TEM were prepared by placing a drop of the sample suspension on a copper grid. Optical analysis was made in an Oceanoptics S2000 and NIR spectra were obtained using an NIR Ocean Optics system. Fig. 1. X-ray diffraction pattern of the sample synthesized using 3mercapto-1,2-propanediol as stabilizing agent.
3. Results and discussion Also, we determined the optical band gap using the simple diffuse reflectance spectroscopy.
2. Experimental section 2.1. Synthesis Silver nitrate was dissolved in 1 L of deionized water to a concentration of 8 mM. This solution was placed in a 2-L reaction vessel. Under stirring (200 rpm) 10 mmol of Triton X-100, 3-mercapto-1,2-propanediol or mercaptoacetic acid was added and the mixture was stirred for an additional 5 min. Ammonium sulfide (4 mmol) dissolved in 100 mL of deionized water was added dropwise under ambient conditions. When using 3-mercapto-1,2-propanediol or mercaptoacetic acid as protective reagents, this method yields dispersions that are stable even during months. For comparison purposes, an additional sample was obtained following the same route but using no stabilizing agent. Hereafter, the particles prepared without stabilizing agent will be referred as Ag2S, silver sulfide powders prepared in the presence of Triton X-100 as Ag2S-T; in the presence of mercaptoacetic acid as Ag2S-M and those obtained in presence of 3-mercapto-1,2-propanediol as Ag2S-P. 2.2. Characterization When applicable, SEM characterization was carried out using the stable dispersions. XRD, thermal and spectroscopic analyses were made using the dried powders obtained by precipitation by means of ionic strength adjustment. X-ray diffraction patterns were recorded with a Rigaku 2200 powder X-ray diffractometer. The nickel filtered Cu Ka (k=1.5418 2) radiation was used at 36 kV and 30 mA. Thermogravimetric analysis (TGA) was carried
Fig. 1 shows the diffraction pattern of the sample Ag2S-P. All patterns obtained agree with the published data (JCPDS 14-072). The difference between the diffraction patterns of the different samples is only the peak broadening due to the particle size. The nanoparticles synthesized are in the monoclinic form which is the stable form of silver sulfide at room temperature. Average particle size was calculated using Scherrer’s equation and the results are reported in Table 1. Differences in particle agglomeration were observed by SEM. The samples named Ag2S and Ag2S-T show strong agglomeration (Fig. 2a and b). In Fig. 3, a closer view of these samples is presented, and we can see that they are formed by nanometrical particles; the observed particle size in these samples is not consistent with those presented in Table 1 probably due to a different size distribution in both samples, the average particle size obtained by analysis made by X-ray diffraction is considered over a large amount of particles, which is very difficult to do in SEM analysis, particle size observed in Fig. 3 represents a very small fraction of the sample. The particles named Ag2S-M and Ag2S-P are loose agglomerated (Fig. 2c and d). Particle morphology was observed using TEM. Images of sample Ag2S-M are presented in Fig. 4, the particles present ellipsoidal and spherical morphology and their sizes are between 20 and 50 nm. Table 1 Average particle size calculated using Scherrer’s equation Synthesis conditions Sample
pH
Stabilizing agent
Particle size (nm)
Ag2S Ag2S-T Ag2S-M Ag2S-P
7 7.5 1.9 2
Without stabilizing agent Triton X-100 Mercaptoacetic acid 3-mercapto-1,2-propanediol
64.6 74 52 30.8
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
531
Fig. 2. Scanning electron microscopy images of the Ag2S samples synthesized using as stabilizing agent: (a) Without stabilizing agent; (b) Triton X-100; (c) Mercaptoacetic acid and (d) 3-mercapto-1,2-propanediol.
TGA measurements were made to reveal the presence of any chemically adsorbed species (Fig. 5). The samples Ag2S and Ag2S-T present a mass loss of 0.55% and 1.5%, respectively, the small mass loss observed is probably due to desorption of water molecules from the particles surface. The sample Ag2S-M shows a mass loss of 3.5% between 75 and 250 8C probably due to desorption of mercaptoacetic acid molecules. The sample Ag2S-P gives a mass loss of 2% in two steps; the first step occurred between 85 and 210 8C and the second step between 215 and 350 8C. These results agree with those reported by Vossmeyer et al. [14], they obtain CdS clusters using the same stabilizing agent and observed a mass loss between 250 and 300 8C, this mass loss is achieved in several steps and can be ascribed to the desorption of 3-mercapto-1,2-propanediol molecules. To study the phase transitions involved, DTA measurements were done. The results for the samples Ag2S-T and Ag2S-P are presented in Fig. 6. For sample Ag2S-T, the
DTA curve has three endothermic peaks at 180, 600 and 805 8C, corresponding to the phase transitions of the three polymorphs of Ag2S [15]: monoclinic acanthite, stable up to 176 8C; body-centered cubic argentite, stable from 176 8C to a temperature between 586 and 622 8C; above which the stable form is a face-centered cubic polymorph. The peak at 805 8C corresponds to the decomposition of silver sulphide. Ag2S and Ag2S-M samples show similar results. DTA curve for sample Ag2S-P presents four endothermic peaks at 178, 280, 587 and 806 8C, three of them corresponding to the phase transitions already mentioned, and the peak at 280 8C corresponding to desorption of stabilizer molecules, this result is in agreement with the one obtained using TGA analysis. The small differences in transition temperatures could be due to size effect or to deviations in stoichiometry of the samples. Stabilizing agents surround the particles creating a layer that prevents against growth and agglomeration. The main
Fig. 3. Scanning electron microscopy images showing a closer view of the samples synthesized using as stabilizing agent: (a) Without stabilizing agent and (b) Triton X-100. We can see that the agglomerates are formed by nanometric units.
532
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
Fig. 4. Transmission electron microscopy images of sample Ag2S-M.
factor discriminating the effectiveness of the various ligands is the difference in Lewis base character, requiring a firmly bound moiety to stabilize growth [18]. Triton X100 is not an effective stabilizing agent in the synthesis of silver sulfide nanoparticles because its hydroxyl group has
little affinity for the silver species; this is the reason why sample Ag2S-T presents no adsorption and the particles are strongly agglomerated. Because Ag(I) is a soft acid, the stability of the moiety would increase with increasing ligand softness, so it is expected that sulphur-containing
Fig. 5. TGA curves for samples (a) Ag2S, (b) Ag2S-T, (c) Ag2S-M, and (d) Ag2S-P.
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
533
Fig. 6. DTA curves for samples (a) Ag2S-T and (b) Ag2S-P.
ligands would be good capping agents, therefore, mercapto acetic acid and 3-mercapto-1,2-propanediol, with a thiol group, have high affinity for silver species and are thus more efficient as stabilizing agents, being capable to adsorb on the particles surface. This adsorption can be detected by TGA, and effects of this adsorption can be observed in size reduction and effective dispersion (SEM and XRD results). A fundamental property of semiconductor compounds is the band gap, the energy separation between the filled valence band and the empty conduction band. Silver sulfide is a direct band gap semiconductor with a band gap value of 1 eV (near infrared region) in the bulk form. We calculated the band gap of sample Ag2S-P in order to determine if there is a bsize quantizationQ effect in the synthesized materials. Optical excitation of electrons across the band gap is strongly allowed, producing an abrupt increase in absorptivity at the wavelength corresponding to the gap energy.
This feature in the optical spectrum is known as the optical absorption edge. Diffuse reflectance spectroscopy can be used to determine the optical absorption edge [16,17]. The ideal diffuse reflectance spectrum consists of a nearly flat, low absorbance region at long wavelengths that abruptly transforms to a steeply rising absorption edge at shorter wavelengths. The most direct way of extracting the optical band gap is to simply determine the wavelength at which the extrapolations of the base line and the absorption edge cross. Fig. 7 shows the NIR analysis using this method, the band gap value obtained was 1.08 eV, this value is shifted compared with the bulk value and this could be a consequence of a bsize quantizationQ effect in the sample. On other hand, when a graph is plotted between (ahm)2/3 or [hm ln{(R max R min)/(R R min)}]2/3, and hm (as abscissa), a straight line is obtained, where R max and R min are maximum and minimum values of reflectance, R is the reflectance at a given photon energy, hm. The extrapolation of the straight line to (ahm)2/3=0 axis gives the value of the band gap of the sample. This method does not give good results with our samples. As expected, reduction in particle size gives a shift in the optical band gap of the Ag2S-P sample. It is evident that particle size, agglomeration degree and optical properties are strongly dependent of the kind of sulphur-containing reagent applied.
4. Conclusion
Fig. 7. Energy band gap of the sample Ag2S-P obtained using NIR spectroscopy.
Silver sulfide nanoparticles with different sizes and agglomeration degree were synthesized using a simple aqueous solution method. This method is fast, no expensive and it can be performed under ambient conditions. We used three different stabilizing agents (Triton X-100, 3-mercapto1,2-propanediol or mercaptoacetic acid), and we found that under the same reaction conditions, the most effective is 3mercapto-1,2-propanediol, which gives us particles with an average size of 30.8 nm. Reduction in particle size gives a
534
G.A. Martı´nez-Castan˜o´n et al. / Materials Letters 59 (2005) 529–534
shift in the optical band gap of the particles. Finally, an important practical result concerning the synthesis method is that the obtained nanoparticles are produced in gram scale.
References [1] A. Eychmqller, J. Phys. Chem., B 104 (2000) 6514. [2] G.C. Hadjipanayis, R.W. Siegel, Nanophase Materials, Kluwer Academic Publisher, Netherlands, 1994, pp. 1 – 19. [3] N. Harpeness, O. Palchik, A. Gedanken, V. Palchik, S. Amiel, M.A. Slifkin, A.M. Weiss, Chem. Mater. 14 (2002) 2094. [4] M. Brelle, J. Zhang, L. Nguyen, R. Mehra, J. Phys. Chem., A 103 (1999) 10194. [5] D. Brqhweiler, R. Seifert, G. Calzaferri, J. Phys. Chem., B 103 (1999) 6397. [6] M. Wu, X. Pan, X. Qian, J. Yin, Z. Zhu, Inorg. Chem. Commun. 7 (2004) 359.
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