Journal of Colloid and Interface Science 369 (2012) 117–122 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis A novel method for the templated synthesis of Ag2S hollow nanospheres in aqueous surfactant media Rajib Ghosh Chaudhuri, Santanu Paria ⇑ Department of Chemical Engineering, National Institute of Technology, Rourkela 769 008, Orissa, India a r t i c l e i n f o Article history: Received 15 September 2011 Accepted 28 November 2011 Available online 16 December 2011 Keywords: Core/shell Hollow nanoparticles Ag2S Quantum yield a b s t r a c t Ag2S is an important direct semiconductor material that receives considerable research interest because of its low toxicity and high chemical stability. This work reports an easy and novel route for the synthesis of hollow Ag2S particles by a sacrificial core method in surfactant containing aqueous media. Sulfur is used as a sacrificial core in this method and removed by dissolving in carbon disulfide. Core sulfur particles were synthesized in situ by acid catalyzed reaction of sodium thiosulphate in aqueous surfactant media. The particles were characterized by using different instrumental techniques, showing 67% improved light emission capacity in terms of quantum yield compared to solid Ag2S particles. The same route is also suggested to prepare other nanoparticles. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Hollow nanoparticles are currently of great interest in nanotechnology. These particles are always superior to normal nanoparticles of the same material in a wide range of applications such as drug or gene delivery [1,2], bioencapsulation [3], controlled release [4,5], medical diagnostics [6,7], lithium-ion batteries [8,9], and catalysis [10,11] because of high surface to volume ratio, low density, low refractive index, and thermal expansion coefficient. Recently, there is an immense interest among the researchers to find more friendly routes for the synthesis of hollow nanoparticles, to produce hollow nanoparticles easily for different applications. The existing routes for hollow nanoparticles synthesis are mostly focused on either single step method [12,13] or sacrificial core method [14,15]. Single step method is too specific to get a wide range of materials [12,13,16]. In contrast, core/shell method using a sacrificial core can be used for a wide variety of hollow nanoparticles [15,17–19]. In general, an easy technique of core removal is always preferable; therefore, the core selection is very important. The organic cores such as surfactant micelles [20,21], vesicles [22,23], or widely used polymers [15,17–19] are removed by solvent washing or calcinations at high temperature, whereas the inorganic cores especially metals, silica, are generally removed by acid or alkali leaching [14,24–26]. The core particles are generally selected depending on the sensitivity of the shell materials toward high temperature, acid, or alkali, as well as on the selectivity of coating after proper surface modification (if required). In the core/shell method, the core ⇑ Corresponding author. Fax: +91 661 246 2999. E-mail addresses: sparia@nitrkl.ac.in, santanuparia@yahoo.com (S. Paria). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.11.064 particles are either synthesized in situ [14,18,19,21,23–28] or sometimes supplied externally [15,17,29–32]. Ag2S is an important direct semiconductor material with band gap 1 eV [33–35], having excellent photoelectric and thermoelectric properties [36], good chemical stability [37,38], and negligible toxicity to leaving organisms [39–41]; because of these properties, Ag2S is used in various commercially available optical and electronic devices [35,42,43]. Although there are many literature available on Ag2S nanoparticles, only limited studies are carried out on hollow particles [37,44,45]. Recently, hollow/solid Ag2S/Ag heterodimers show enhanced antimicrobial property against Escherichia coli [38]. Surfactant mediated one-pot synthesis of core and core/shell nanoparticles in aqueous media may improve the basic understanding in colloid and interface science. The stability of colloidal core particles in the presence of surfactants molecules in aqueous media is important to get stable and small size core particles. The formation of core/shell nanoparticles is mainly driven by the opposite surface charge of the core particles as well as by the heterogeneous nucleation at the core particle surface. The present study reports a novel and easy route, using sulfur as a sacrificial core for the preparation of hollow Ag2S nanoparticles in aqueous media, and the same route can also be used for the preparation of other hollow nanoparticles. Previously reported sacrificial cores were removed by either dissolution or calcination for the synthesis of hollow nanoparticles; however, using the present route, the same core can be removed by both dissolution and calcination, depending on the properties of shell material. Although this study reports the core removal by dissolution method, we also prepared hollow TiO2 nanoparticles in our laboratory by calcination and will be communicated soon. As a result, the method is more generalized to prepare a wide variety of hollow nanoparticles. To 118 R. Ghosh Chaudhuri, S. Paria / Journal of Colloid and Interface Science 369 (2012) 117–122 the best of our knowledge, similar synthesis route has not been reported till date for the preparation of hollow nanoparticles. 2. Experimental section Na2 S2 O3 þ 2HNO3 ! 2NaNO3 þ H2 SO3 þ S # ðcoreÞ ð1Þ AgNO3 þ CTABþ Br ! CTAþ þ AgBr # ðshellÞ ð2Þ The required chemicals were purchased from the following sources: sodium thiosulphate (Na2S2O3, 5H2O) from Rankem, cetyltrimethyl ammonium bromide (CTAB) with 99% purity from Sigma Aldrich, nitric acid (HNO3) from Merck, AgNO3 from Ranbaxy with 99.9% purity, and carbon disulfide (CS2) from Merck with 99% purity. All the chemicals were used as it is received without any further purification. Ultrapure water of 18.2 MX cm resistivity and pH 6.4–6.5 (Sartorius, Germany) was double distilled again and used for all the experiments. The constant temperature 28 ± 0.5 °C was maintained throughout the experiments. Additionally, in the next step of aqueous alcohol washing of the core/shell nanoparticles, the reactivity of the sulfur nanoparticles increases because of the presence of alcohol; as a result, elemental sulfur atoms of the core surface react with AgBr and convert to Ag2S. There is also an another possibility of the formation of Ag2S from the dissociation of silver thiosulphate complex which formed in the presence of un-reacted thiosulphate ions present in the media [37,44]. The negatively charged complex [AgðS2 O3 Þ ; AgðS2 O3 Þ3 2 ] may preferably adsorb on the surfactant modified core surface, and subsequently generate S2 ion after hydrolysis of the complex, that can further react with Ag+ to form Ag2S. Finally, when the core/ shell particle is treated with CS2, AgBr is completely converted to Ag2S as well as the core is also dissolved to form hollow Ag2S particles. The probable reactions can be presented as: 2.2. Synthesis techniques AgðS2 O3 Þ ; AgðS2 O3 Þ3 ƒƒƒƒƒƒ! Ag2 S þ H2 SO3 þ H2 O ð3Þ Sulfur nanoparticles as core were synthesized from HNO3 catalyzed reaction of sodium thiosulphate in the presence of CTAB solution according to our previous study [46]. After the completion of core formation, the solution was sonicated by a probe type sonicator using 260 W for 20 min, and finally, AgNO3 solution was added slowly stepwise in situ under continuous stirring condition for uniform shell coating on the core surface. After a waiting period of 60 min, the particles were separated by centrifugation at 25,000 rpm for 20 min, washed thrice by water ethanol mixture (1:1, v/v) and, finally, treated with CS2 for the complete conversion of AgBr to Ag2S, as well as to remove the core to form Ag2S hollow particles. 2AgBr þ CS2 ! Ag2 S þ CS þ Br2 ð4Þ nAg2 S ! ðAg2 SÞn ð5Þ nCS ðCSÞn ð6Þ 2.1. Materials 2.3. Particle characterization Particle size measurement was carried out by dynamic light scattering (DLS) using Malvern Zeta Size analyzer, UK (Nano ZS), with the help of cumulant fitting model and intensity distribution within the media. The size and shape of the particles were observed under a scanning electron microscope (JEOL JSM-6480LV) and transmission electron microscope (Tecnai S-twin). Core/shell and hollow particles were also characterized by UV–vis–NIR Spectroscopy (Shimadzu-3600), fluorescence spectroscopy (Hitachi7000), TGA (Shimadzu), and X-ray diffraction (XRD) (Philips, PW 1830 HT). Hþ ;C2 H5 OH 3.2. Effect of AgNO3 concentration on the core/shell particles size and shell thickness The control on the size as well as the shell thickness of hollow nanoparticles is very important for different applications. In this study, fixed size core particles (60 ± 6.1 nm) were used as a template obtained by HNO3 catalyzed disproportionation reaction of 5 mM thiosulphate solution in CTAB micellar media, and after the core/shell formation, the final size was again measured to get the shell thickness. While increasing AgNO3 concentration, the final size of the core/shell particles and the shell thickness increase continuously (Fig. 1). To support the formation of core/shell particles over the possibility of formation of separate solid particles of shell material, we also studied the effect of AgNO3 concentration on size of the pure AgBr particles formation and observed that the particle size decreases with the increasing AgNO3 concentration. Whereas for the core/shell nanoparticles, as the particle size increases with the increasing reactant concentration, indirectly indicates that the core is coated by AgBr. At 0.01 mM AgNO3 concentration, the core/shell 3. Results and discussions 200 60 Particle Size (nm) Sulfur is a neutral element, having almost zero surface charge (2.17 mV) in aqueous media [46]. In the presence of CTAB (2.8 mM) solution, CTAB molecules are adsorbed on the sulfur particle surface through the tailgroup, and a uniform positive surface charge of +26.0 to +30.0 mV is developed on the core particle surface. In aqueous media, AgBr particles show negative surface charge of 15 to 25 mV. This difference in surface potential is the driving force to form S/AgBr core/shell nanoparticles, whenever AgNO3 is added to the CTAB stabilized suspension of core particles. The cationic surfactant (CTAB) plays here important roles, such as act as a capping agent to generate lower size core particles [46], prevent agglomeration, surface modifier for the uniform and favorable complete coating of core particles, and a reactant by supplying the counter Br ions to form AgBr. The reactions for core and shell formation can be written as: 150 50 100 Particle Size 40 Shell Thickness 30 20 50 Shell Thickness (nm) 70 3.1. Mechanism of particles formation 10 0 0 0.005 0.01 0 0.015 AgNO Concentration (mM) 3 Fig. 1. Effect of silver nitrate concentration on the overall particle size and shell thickness for a fixed sized core particles (60 nm). 119 R. Ghosh Chaudhuri, S. Paria / Journal of Colloid and Interface Science 369 (2012) 117–122 particle size obtained was 170 ± 10.4 nm measured by DLS with a narrow distribution, as well as a very small peak near to 90 nm (as shown in Fig. S1, Supplementary data). The size of the pure AgBr particles synthesized from same AgNO3 concentration was also about 90 ± 5.2 nm. These results indicate that during the second step of the synthesis, mostly core/shell particles were formed with a very few separate pure AgBr particles. 3.3. Characterization of particles by UV–vis spectroscopy To support indirectly the formation of core/shell and conversion of AgBr shell to Ag2S, UV–vis spectroscopy was used for different particles such as pure core (S), pure shell materials (AgBr), coated particles (S/AgBr), mixture of core and shell particles (S + AgBr) prepared individually, core/shell particles after aqueous ethanol (1:1, v/v) wash (S/AgBr–Ag2S), and core/shell particles after CS2 wash (expected to be hollow Ag2S). UV spectra (as in Fig. 2a) of single particles clearly show that both are having peak at 271 nm wavelength with higher absorbance value of sulfur (S = 0.935, AgBr = 0.358) as indicated by the spectra (i and ii). Pure S synthesized in aqueous media is not having any specific peak but shows high absorbance below 300 nm wavelength, and the presence of peak at 271 nm for both the single particles in CTAB media may be because of the adsorption of CTAB molecules on the surface of the particles. The absorbance values obtained for different materials are compared as follows: S > S/AgBr > (S + AgBr) > AgBr at 271 nm wavelength. From the spectra, it was observed that after the addition of AgNO3, there was a change in the absorbance. However, to confirm whether AgBr really coats on the S particles surface or they formed separate AgBr particles; two different spectra S/AgBr (denoted as (iii) in Fig. 2a) and S + AgBr (denoted as (iv) in Fig. 2a) are compared. It is observed that the absorbance (0.66) of the mixed particles (S + AgBr) is very close to that of average absorbance of two single particles ((0.935 + 0.358)  0.5 = 0.6465); the average is taken because the particles were mixed in equal volumes. However, the core/shell particles show higher absorbance (0.817) indirectly indicates that these particles are different from the mixture of S and AgBr particles. Interestingly, a significant change in the spectra from the original was observed when the S/AgBr particles were treated 1.0 (a) Absorbance 0.8 with aqueous alcohol (1:1) and CS2 as shown in spectra (v and vi), respectively. These two spectra do not show any absorbance peak at 271 nm. The absorbance of these two particles decreases below 300 nm wavelength, but it increases above that wavelength especially in the visible wavelength region compared with the remaining other four spectra. The absorbance of hollow nanoparticles (after CS2 treatment) is even higher compared with that of the aqueous ethanol treated S/AgBr particles above 480 nm wavelength. The photograph of six different solutions, one can also easily see the difference in physical appearance of all the samples, indicates that they are different (Fig. 2b). 3.4. Characterization of the particles by fluorescence spectroscopy Photoluminescence properties of all the particles were also studied by the help of fluorescence spectroscopy to see the differences. The samples were excited at 270 nm wavelength and measured the emission spectra for each samples (as shown in Fig. 3). All samples except sulfur show emission spectra at 543.0, 547.4, 543.8, and 543.4 nm wavelength with an impressive full width at half-maximum (FWHM) as small as 5.1, 10.1, 9.6, and 9.5 nm for pure AgBr, S/AgBr, aqueous ethanol treated S/AgBr, CS2 treated S/ AgBr respectively, which could be attributed to the narrow size distribution of the particles. While comparing the emission spectra of as prepared S/AgBr and alcohol washed same particles, there is an increase in intensity after the alcohol wash. However, pure AgBr and S/AgBr are having almost same intensity but at different wavelength that may be because of the presence of sulfur that change the effective band gap of AgBr particles. This intensity change can be attributed to partial conversion of AgBr shell to Ag2S during alcohol wash. The AgBr is an indirect semiconductor material of band gap 2.5 eV, whereas Ag2S is a direct semiconductor material of band gap 1 eV. Since Ag2S is a direct semiconductor, it is expected to have high intensity of emission light compared with that of AgBr, which is an indirect semiconductor. On the other hand, although Ag2S is a low band gap material, still the emission at the 543 nm (2.28 eV) is observed, as the shell is formed by the deposition of small sized particles (5–10 nm, discussed later in Section 3.6). The presence of these small size particles leads to quantum confinement, and as a result, the particles emit high energy light, although overall particle (shell) size is large. The further increase in emitted light intensity after CS2 treatment is attributed to complete conversion of Ag2S from AgBr as well as to the complete removal of core (S) material. In addition, aqueous ethanol (i) 0.6 0.4 1600 (v) (iv) 50 1200 (vi ) 40 (iv ) 360 400 440 480 520 560 600 Wavelength (nm) Intensity 320 (iii) 30 (ii) 280 (iv) 60 1400 (iii ) 0.2 0 70 1000 20 10 800 (iii) 0 600 784 798 812 826 840 400 200 0 520 (i) 530 540 (ii) 550 560 570 580 590 600 Wavelength (nm) Fig. 2. (a) UV–vis spectra of pure S (i), AgBr (ii), S/AgBr (iii), S + AgBr (iv), S/AgBr after alcohol wash (v), and hollow Ag2S (vi); (b) physical appearances of the samples. Fig. 3. Emission spectra of AgBr (i), S/AgBr before (ii) and after washing (iii) with aqueous ethanol solution, and hollow Ag2S (iv), when excited at 270 nm wavelength. 120 R. Ghosh Chaudhuri, S. Paria / Journal of Colloid and Interface Science 369 (2012) 117–122 treated S/AgBr and CS2 treated S/AgBr particles show an extra peak at higher wavelength at 817.8 (1.516 eV) and 818.2 nm (1.515 eV), respectively, may be because of bulk Ag2S particles. Quantum yield (QY) was calculated for hollow Ag2S and pure Ag2S particles synthesized by the same route using phenol as a standard material (QY = 0.14) [47]. The QYs of the hollow and solid Ag2S particles are 0.89 and 0.22, respectively, clearly shows 67% enhancement of QY for hollow particles (as shown in Supplementary data). 3.5. Characterization of the particles by TGA and XRD TGA also indicates the indirect proof for AgBr coating on the core sulfur particles (as in Fig. S3, Supplementary data). The XRD analysis of S, S/AgBr, and S/AgBr after aqueous alcohol washing and S/AgBr particles after CS2 treatment is a direct evidence of AgBr to Ag2S conversion (Fig. 4). The positions and intensities of the diffraction peaks of core sulfur particles are in good agreement with the literature values for orthorhombic or a-phase sulfur with S8 structure (08-0247 from JCPDS PDF Number). The XRD pattern of S/AgBr shows the identical peak for sulfur particles is minimized, and the pattern is mainly dominated by the peak of AgBr (79-0149 from JCPDS PDF Number) at 30.95° (2h), good agreement with the literature value for cubic structure. The pattern also contains another identical peaks for cubic AgBr at 27.44° and 31.82° (14-0255 from JCPDS PDF Number), and an identical peak at 38.09° of Ag2S in cubic phase (01-1151 from JCPDS PDF Number) may be formed via Ag-thiosulphate complex [Ag(S2O3) or AgðS2 O3 Þ3 2 ]. However, after washing of S/AgBr particles with aqueous ethanol, the XRD pattern changes totally, as the characteristic peak of AgBr is vanished and more new prominent peaks of Ag2S appear at 31.47°, 34.37°, 37.71° and 40.52° (2h). The appearance of new peaks was because of the presence of monoclinic Ag2S confirmed with literature value (14-0072 from JCPDS PDF Number). The formation of Ag2S after aqueous ethanol treatment confirms once again by XRD as speculated before qualitatively. The XRD pattern of alcohol washed S/ AgBr particles after treating with CS2 shows 100% matching with the literature of monoclinic Ag2S (09-0422 from JCPDS PDF Number), and this confirms the final product is completely Ag2S nanoparticles. 3.6. Characterization of the particles by SEM and TEM 120 100 2 ** * 80 The SEM images confirm that the AgBr coated core/shell particles are almost spherical shape as shown in Fig. 5a. The magnified Hollow Ag S * * 60 40 20 0 350 S/AgBr after wash 300 250 * * * * 200 150 Intensity (a.u) 100 * 50 0 6000 S/AgBr before wash 5000 4000 3000 # 2000 ^ 1000 ## 0 12000 S 10000 8000 6000 4000 2000 0 20 25 30 35 40 2θ / o 45 50 55 60 Fig. 4. XRD pattern of core (S), core/shell S/AgBr before and after alcohol wash, and hollow Ag2S particles [ and ^ represent monoclinic and cubic Ag2S, respectively and # for cubic AgBr]. Fig. 5. (a) SEM image of core/shell (S/AgBr) particles, (b) FESEM image hollow Ag2S particles. Inset shows EDX analysis of hollow particles. Particles were synthesized from 5 mM thiosulphate and 0.1 mM AgNO3 concentration. R. Ghosh Chaudhuri, S. Paria / Journal of Colloid and Interface Science 369 (2012) 117–122 121 Fig. 6. TEM image of (a) S/AgBr, (b) close-up image of naked S, (c) close-up image of S/AgBr, and (d) hollow Ag2S particles synthesized from 5 mM thiosulphate and 0.1 mM AgNO3 concentrations. FESEM images show that the surfaces of core/shell and hollow particles are not very smooth; probably, the shell was formed by deposition of very small sized (5–10 nm) nanoparticles on the core surface (Fig. 5b). The EDX of hollow Ag2S particles shows the presence of Ag and sulfur of atomic ratio (Ag:S) 1.7:1, close to that of Ag2S. As shown in the TEM image (Fig. 6a) of the S/AgBr particles after washing, there is the contrast difference in the figure indicates the formation of core/shell particles. The lattice fringe and the lattice spacing for pure S is 0.39 nm, which are shown in figure (Fig. 6b). In the previous section, the XRD data indicate that the core is highly crystalline in nature but after coating crystalline nature of the core/shell and even after CS2 treatment, hollow Ag2S particles decreases with low intensity XRD peak. This can be attributed to the formation of shell by the deposition of very small sized nanoparticles. The high resolution TEM images also show the same results (Fig. 6c); without coating, the lattice fringe of the sulfur particles is clear, but because of the coating of less crystalline Ag2S material, the sharpness lattice fringes decreases. The high resolution magnified distinct image of a hollow particle (as shown in Fig. 6d and Fig. S4, Supplementary data). There is also a good agreement between the TEM results and the DLS data for the size and shell thickness of the hollow particles. [37,44] or template free additive (thiourea) [45] in aqueous media. They were able to produce Ag2S hollow structure of rhombic [37], hexagon with a hole [44], network-like or a nanotube [45]; in contrast, closed hollow nanospheres can be prepared using the propose route. Different characterization techniques such as UV–vis spectrometry, fluorescence spectroscopy, XRD and TEM were also confirmed the formation of S/AgBr core/shell as well as hollow Ag2S. Shell thickness can be controlled by maintaining the AgNO3 concentration. The previously reported studies mostly focused on the formation of hollow particles without showing any improved properties on hollow structure. In this paper, we studied the light emission property of hollow Ag2S particles and compared it with that of the same solid particles. The hollow Ag2S nanoparticles showed a 67% higher quantum yield compared with the solid Ag2S nanoparticles. More importantly, this route can also be useful to prepare a wide range of hollow nanoparticles. The whole process (formation of core, core/shell, and hollow particles) may contribute to improve the understanding of stabilization colloidal particles through surfactant adsorption, formation of core/shell structure, nucleation, and growth of nanoparticles. 4. Conclusions The financial support from the Department of Science and Technology (DST) under Nanomission, New Delhi, India, Grant No. SR/ S5/NM-04/2007, for this project is gratefully acknowledged. We also acknowledge Saha Institute of Nuclear Physics (SINP) and Indian Association for the Cultivation of Science (IACS), Kolkata, India, for giving the opportunity to access their TEM and FESEM facility respectively. In this paper, a sacrificial core of sulfur nanoparticles is used to synthesize Ag2S hollow nanospheres via a wet chemical method at room temperature. There are only a few literature available on hollow Ag2S nanomaterials [37,44,45], where the hollow structures were obtained using a template of surfactant micelle (CTAB) Acknowledgments 122 R. Ghosh Chaudhuri, S. Paria / Journal of Colloid and Interface Science 369 (2012) 117–122 Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.11.064. References [1] L. Wang, J. Bao, F. Zhang, Y. Li, Chem. – Eur. J. 12 (2006) 6341. [2] W. Lu, G. Zhang, R. Zhang, L.G. Flores II, Q. Huang, J.G. Gelovani, C. Li, Cancer Res. 70 (2010) 3177. 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