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Controllable synthesis of floatable nanocrystalline Ag2S and Ag by a silane coupling agent-modified solvothermal method
Materials Research Bulletin 47 (2012) 3732–3737
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Controllable synthesis of floatable nanocrystalline Ag2S and Ag by a silane coupling agent-modified solvothermal method Xuxin Fan, Xu Qin, Liqiang Jing *, Yunbo Luan, Mingzheng Xie Key Laboratory of Functional Inorganic Materials Chemistry, Heilongjiang University, Ministry of Education, School of Chemistry and Materials Science, Harbin 150080, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 December 2011 Received in revised form 28 April 2012 Accepted 11 June 2012 Available online 18 June 2012
Nanocrystalline Ag2S and Ag have been synthesized by a silane coupling agent (SCA) modified solvothermal method, followed by thermal treatment in N2 atmosphere. The key of this method is to select bifunctional SCAs with HS– or NH2– ends, which easily couple Ag+ prior to the solvothermal process, while the (H3CO)3–Si– ends of SCA easily change into (HO)3–Si– ends by hydrolysis reactions. These factors are favorable for the production of nanocrystalline Ag2S and Ag with small nanoparticle size during the solvothermal processes, meanwhile distributed homogeneously in the networks resulting from the polymerization of (HO)3–Si– groups. The formed networks effectively inhibit the aggregation and growth of Ag2S and Ag crystallites and keep their monodispersion. The resulting spherical Ag2S and Ag nanoparticles with average diameter of about 10 nm are floatable in water, attributed to the small nanoparticle size and to the formed silane-coupling link. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Microstructure
1. Introduction Nano-sized materials have been studied extensively in recent years owing to their special physical and chemical properties [1]. Many reports have focused on the synthesis of metal chalcogenide nanoparticles and their assembled structures [2–5]. Among many metal chalcogenide nanoparticles, silver sulfide nanoparticle has attracted considerable attention because of its being potential prospective optoelectronic and thermoelectric materials, such as photoanodes, photoconductors, IR detectors and superionic conductors [6–8]. Ag2S nanoparticles with variable sizes have been prepared via various methods, such as one-step surfactant-assisted solvothermal, gamma-ray irradiation and template-free solution routes [9–11]. However, sizes of Ag2S particle obtained by the methods mentioned above are rather large, within the range of micrometer size in general. Although there have been several reports on the synthesis of small-sized Ag2S nanocrystals up to day [12,13], the synthetic processes are complicated and costly, which restricts greatly wide practical application. In addition to metal chalcogenide nanoparticles, metal nanoparticles (NPs) [14,15] are widely applied in different fields, such as catalysis [16] and optical sensing [17,18]. Compared with other metal nanoparticles, silver nanoparticles have been extensively studied because of its cheapness, antibacterial properties [19] and surface-enhanced Raman scattering (SERS) [20]. Ag nanoparticles
* Corresponding author. Tel.: +86 451 86608610. E-mail address: [email protected] (L. Jing). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.032
have always been prepared via various methods, such as photoreduction [21], chemical reduction [22,23] and so on. However, it is difficult for those methods to control the Ag particle size with uniform dispersion. Since sizes of the nanomaterials play important roles in the practical applications, it is necessary to develop a feasible synthetic route for controlling over size of nanoparticles easily. On the basis of the above analysis, to develop simple synthetic pathways to prepare Ag2S and Ag nanoparticles are very important and quite meaningful. Generally speaking, large size of particle prepared is attributed to the tiny solubility product of itself in the aqueous solution [24] and to choose a suitable chelating agent to react with the reactant is a feasible method to reduce its concentration. Sun et al. [25] reported the synthesis of BiVO4 by introducing EDTA as strong chelators to greatly decrease the free Bi3+ content in the solution, leading to the formation of small-sized BiVO4. Naturally, it is expected that Ag+ concentration is possibly one of the important factors influencing nucleation and subsequent growth of nano-sized Ag2S and Ag, especially for Ag2S because of its very low-solubility products (Ksp(Ag2S) = 6.3 10 50) [9]. Thus, the key of the synthetic method is to control Ag+ concentration. Silane coupling agent as a kind of functional molecule has been extensively utilized in the modification of material surfaces, which is valuable technique to design and prepare new composite and functional materials [26]. However, using silane-coupling agents to control particle size in synthetic process has seldom been reported. (H3CO)3–Si– ends of silane coupling agents easily form (HO)3–Si– groups by hydrolysis reactions, further producing –Si–O–Si–connections by polymerization reactions. The produced
X. Fan et al. / Materials Research Bulletin 47 (2012) 3732–3737
–Si–O–Si– connections may effectively impede rapid growth of nanocrystallites. Functional groups, such as HS– and NH2–, may control Ag+ concentration in the synthetic processes by effectively coupling Ag+ ions. KH590 with HS– ends and KH550 with NH2– ends, as typical SCAs used widely, may achieve our goal to control Ag+ concentration. To the best of our knowledge, they have seldom been reported to use them to control inorganic nanoparticle size so far. In this paper, we report a novel approach to prepare small-sized Ag2S and Ag nanocrystals by a SCA modified solvothermal method for the first time. The KH590 and KH550 play important roles in the synthetic processes so that sizes of Ag2S and Ag nanoparticles are obviously decreased. Interestingly, the resulting Ag2S and Ag nanocrystals can be stably floatable in water, which may be useful for practical applications. This simple synthetic method would be extended to fabricate other Ag-containing nanostructured materials.
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(a)
(b)
2. Experimental 2.1. Synthesis of materials All chemicals are of analytical-grade reagents and are used without further purification. All the annealed procedures are carried out under nitrogen atmosphere. In a typical synthetic procedure, 2.94 10 3 mol silver nitrate (AgNO3) is dissolved in 12.5 ml 95% ethanol by sonication for several minutes (solution A). Another mixture consists of 12.5 ml of toluene and a desired amount of KH590 (or KH550) (solution B). Solution B is then added dropwise to solution A under vigorously stirring at room temperature, producing a kelly gel (or suspension). After continuously stirring for 60 min, 25 ml of the resulting mixture is sealed in a 50 ml Teflon-lined stainless steel autoclave and then maintained at 160 8C for 6 h. The transparent solution is poured out and the products are dried at 80 8C in air, the sample is referred to as SA80(A-80). To improve Ag2S or Ag crystal quality, the as-prepared Ag2S or Ag is annealed at certain temperature for 120 min. The sample is referred to as SA-X (A-X), in which SA(A) means Ag2S(Ag), and X represents the thermal treatment temperature in N2. 2.2. Characterization of materials The samples are characterized by X-ray Powder Diffraction (XRD) with a Rigaku D/MAX-rA powder diffractometer (Japan), using Cu Ka radiation (l = 0.15418 nm), and an accelerating voltage of 30 kV and emission current of 20 mA are employed. Scanning electron microscopy (SEM) observations are carried out on a Philips XL-30-ESEM-FEG operated at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) observations are carried out on a JEOL 1200EX operated at an accelerating voltage of 100 kV. The BET surface area is evaluated by a ST-2000 constant volume adsorption apparatus. The Fourier transform infrared (FTIR) spectra of the samples are collected with a Bruker Equinox 55 Spectrometer, using KBr as diluents. The surface composition and elemental chemical state of the samples are examined by X-ray photoelectron spectroscopy (XPS) using a Kratos-AXIS ULTRA DLD apparatus with Al (Mono) X-ray source, and the binding energies are calibrated with respect to the signal for adventitious carbon (binding energy = 284.6 eV). 3. Results and discussion 3.1. Measurements of XRD, TEM and SEM The crystal phase composition of the products is examined by powder X-ray diffraction (XRD). The main diffraction peaks
Fig. 1. XRD patterns of the Ag2S (a) and Ag (b) nanoparticles (*, Ag2S; ^, SiC; ! Ag).
showed in Fig. 1a can be well indexed as monoclinic a-Ag2S (JCPDS Card No.14-0072) [27–29]. In addition, the characteristic peaks of SiC can be detected in the sample annealed at the 800 8C. In Fig. 1b, the peaks (2u) at 38.18, 44.48 and 64.28 are attributed to the (1 1 1), (2 0 0), and (2 2 0) reflections of face-centered cubic (fcc) structured Ag, respectively [30]. As the thermal treatment temperature increases, the XRD peaks of Ag2S or Ag become slightly high, indicating that the Ag2S or Ag crystallite sizes do not change obviously. It is suggested that the used KH590 or KH550 SCA play important roles in inhibiting the growth of nanocrystalline Ag2S or Ag. According to the Scherrer equation, it is evaluated that the crystallite sizes of resulting Ag2S and Ag are within 8– 11 nm and 13–16 nm, respectively. It should be pointed that silver sulfate can be produced if the corresponding precursors are calcined at certain temperature for 2 h in air instead of in N2 (see Supporting information SI-Fig. S1). Silver sulfide has been known to be a mixed ionic–electronic conductor at high temperature (above 200 8C) [31]. It exists in three different allotropic forms. a-Ag2S is the monoclinic phase and stable up to 178 8C. a-Ag2S easily transforms to body-centered cubic (bbc) b-Ag2S over 178 8C, and the bbc phase transforms to a face-centered cubic (ffc) g-Ag2S phase over 600 8C [32]. Surprisingly, Ag2S nanoparticles we prepared still keep the monoclinic phase at 600 8C and 800 8C. Zhang et al. [33] reported the phase transformation started to form at the interfaces between the nanoparticles in the agglomerated TiO2 nanoparticles with increasing thermal treatment temperature. Similarly, it is suggested that the KH590 SCA with (H3CO)3–Si– effectively inhibit the interface contacts between the Ag2S crystallites, leading to the phase change inhibition of Ag2S from a to b and g phase. The TEM photographs of the Ag2S (a) and Ag (b) samples are shown in Fig. 2, indicating that the morphologies of the samples are spherical. Surprisingly, it can be seen that the particle sizes of the SA-600 and the SA-800 samples do not change a lot compared
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Fig. 2. TEM photographs of the Ag2S (a) and Ag (b) samples.
to that of the SA-400 samples. This would be beneficial to still keep large surface areas of the samples by thermal treatment at high temperature. The SAED patterns in the inset show the polycrystal structure. Based on TEM images of Ag nanocrystals, similar results could be obtained. It is seen that the nanoparticle sizes of resulting Ag2S and Ag are within 8–11 nm and 13–16 nm, respectively, with increasing thermal temperature. These results are consistent with the XRD analyses. Fig. 3 shows scanning electron microscopy (SEM) images of SA400 and SA-600 samples. From the pictures, Ag2S nanoparticles seem to be spherical, and are covered by a large amount of the sponge-like substances. As the thermal treatment temperature increases, the sponge-like substances gradually decrease. In addition, from the analysis of SEM images of Ag, similar results could be obtained. Interestingly, the samples could be stably floatable in water (SI-Fig. S2). It is assumed that the floatable feature is attributed to the sponge-like substances. From the BET surface areas of the Ag2S samples prepared at different annealing temperatures (SI-Table 1), even after annealing temperature at 800 8C, the samples still remain high surface areas, which may be associated with the sponge-like substances. From the table, the
BET surface areas slowly reduce with increasing annealing temperature, indicating Ag2S crystallite sizes slightly change. It is in accord with the XRD and SEM results. 3.2. Measurements of IR and XPS Fig. 4 shows the FT-IR spectra of the Ag2S (a) and Ag (b) samples. The peaks at around 500 cm 1 may result from resulting nanocrystalline Ag2S or Ag. The IR peaks at 1000–1145 and 780–800 cm 1 are attributed to the asymmetric stretching vibration, symmetric and stretching vibration of the group Si– O–Si, respectively, along with the IR peaks at 1090–1120 cm 1 dues to the vibrations of the of C–Si–O group [34–36]. A series of bands at around 2870–2935 cm 1 due to the vibrations of methylene –(CH2)3– [37,38] and the peaks at about 1230– 1275 cm 1 are due to the vibrations of Si–CH3 [34]. And, the peaks located at 886 cm 1 result from SiC in Fig. 4a [39]. As the thermal treatment temperature increases, the 2870–2935 cm 1 peak gradually decreases, indicating –(CH2)3– in –Si(CH2)3–O– Si(CH2)3– network structure is gradually decomposed, leading to the SiC formation at the high temperature, which is supported by
Fig. 3. SEM images of SA-400 and SA-600 samples.
X. Fan et al. / Materials Research Bulletin 47 (2012) 3732–3737
(a) SA-80
T%
SA-400
SA-600
SA-800
500
1000
1500
2000
2500
3000
3500
4000
Wavelength / cm-1
(b)
T%
A-80
A-400
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SEM and XRD results. The IR peaks at about 1630 and 3430 cm 1 are ascribed to surface hydroxyl and adsorbed water molecules, respectively. The peaks at about 1464 cm 1 are due to the vibrations of NHx in Fig. 4b [37]. X-ray photoelectron spectra (XPS) of SA-600 sample are shown in Fig. 5. The binding energies of Ag3d5/2 and Ag3d3/2 are 367.6 and 373.6 eV, respectively, indicating that the chemical valences of Ag are +1 valence, while that of S2p is 163.2 eV, demonstrating that S is 2 valence in the sample. These results are consistent with the reported data of Ag2S [9]. The strong peak (in Fig. 5c) at 103.0 eV corresponds to Si2p in –Si–O–Si– [40]. The C1s XPS peak shown in Fig. 5d is located at about 284.6 eV, assigned to the C1s in C–C in – Si(CH2)3–O–Si(CH2)3– networks [40]. The O1s XPS spectra (in Fig. 5e) are at least involved with two kinds of O chemical states according to the binding energy range, including hydroxyl groups and Si–O–Si linkages. The OH signal is closely related to the hydroxyl groups resulting mainly from the chemisorbed water and its peak position is at about 531.7 eV [41]. The Osi signal is attributed to Si–O–Si linkages in –Si(CH2)3–O–Si(CH2)3– networks and its peak position is at about 532.5 eV [42]. For XPS spectra of Si2p, C1s and O1s, similar results could also be obtained in the resulting Ag sample, as shown in SI-Fig. S3. In addition, the N1s XPS peak is located at about 398.4 eV and 400.7 eV, which are the characteristic peaks of N1s in NHx species in SI-Fig. S3 [37]. It is deduced that the N species derives from the NH2 function group in the KH550 molecule. Therefore, the XPS results further support those of FT-IR spectra.
A-600 3.3. Analysis of synthetic mechanism
A-800 500
1000
1500
2000
2500
Wavelength /
3000
3500
4000
cm-1
Fig. 4. FT-IR spectra of the Ag2S (a) and Ag (b) samples.
Accepted widely, sizes of nanocrystals can be affected through a delicate balance of the nucleation and growth rate [23]. On the basis of the above discussion, a possible mechanism for the formation of size-controlled nanocrystalline Ag2S and Ag is suggested, as shown in Scheme 1, involved with three steps. Firstly, prior to the solvothermal process, Ag(I) reacts easily with thiolate anion or NH2– groups of SCAs [43], which effectively controls the Ag+ concentration by forming the Ag(I)–thiolate (or – NH2–) complex. Wu et al. [44] reported the synthesis of smallsized Hg and HgS by formation of Hg(II)–thiolate complex, by
Coupling process Hydrolysis reactions HS or NH2 (CH2 )3
Si
(OCXHY)3
Solvothermal
Annealing treatment
process
:S- or NH2
:Ag+
:-(CH2)3-Si-OH
:Ag2S or Ag nanoparticles
:Si
: single Ag2S or Ag molecule
(X=1 or 2; Y=3 or 5)
Scheme 1. Schematic of the formation of Ag2S or Ag.
X. Fan et al. / Materials Research Bulletin 47 (2012) 3732–3737
3736
(a)
(b)
S2p
Intensity / a.u.
Intensity / a.u.
Ag3d
158 160 162 164 166 168 170 172 174 176
364 366 368 370 372 374 376 378 380 382
Bingding energy / eV
Bingding energy / eV Si2p
(d)
C1s
Intensity / a.u.
Intensity / a.u.
(c)
96
98 100 102 104 106 108 110 112 114 116
278 280 282 284 286 288 290 292 294 296
Bingding energy / eV
Bingding energy / eV
(e) Intensity / a.u.
O1s
OSi
OH
524 526 528 530 532 534 536 538 540 542 544
Binging energy / eV Fig. 5. X-ray photoelectron spectra (XPS) of SA-600 sample.
controlling Hg2+ concentration in the solution. Thus, we suppose that the small-sized Ag or Ag2S have a similar formation process. It is suggested that it is effective to impede the nucleation of the Ag2S and Ag by forming the Ag(I)–thiolate (or –NH2–) complex. Meanwhile, a large number of Si–OH groups are produced by hydrolysis reactions of SCAs. Then, in the solvothermal processes, – Si(CH2)3–O–Si(CH2)3– networks are formed by –Si–OH polymerization reactions, meanwhile nanocrystalline Ag2S and Ag are produced and distributed homogeneously in –Si(CH2)3–O– Si(CH2)3– networks. Finally, in the thermal treatment process in N2, the –Si(CH2)3–O–Si(CH2)3– networks formed in the solvothermal process effectively inhibit the growth of the nanocrystalline Ag2S and Ag. As the thermal treatment temperature rises, organic C–C chains content in –Si(CH2)3–O–Si(CH2)3– networks gradually decreases, leading to sponge-like substances capping the Ag2S and Ag nanoparticles. It is expected that the formed networks not only effectively inhibit the aggregation and growth of Ag2S and Ag crystallites, but also keep their high monodispersion after high temperature thermal treatment. Xia et al. [45] have reported that a nanoparticle reaches a size where the electrostatic repulsion energy between nanoparticles and the newly arrived nanoparticles
outside equals the attractive energy associated with van der Waals interactions. Differently, we suppose the synthesis of small-sized Ag2S and Ag is mainly due to formation of –Si(CH2)3–O–Si(CH2)3– networks as a thin shell layer in our works. The following experimental results further support the suggested synthetic mechanism. The larger the mole ratio of Ag (silver nitrate) to N (KH550), the smaller the crystallite size of the resulting Ag nanoparticles, as shown in SI-Fig. S4. If KH590 or KH550 is replaced of trimethoxypropylsilane, single Ag is obtained with large size in SI-Fig. S5. If the mole ratio of Ag to S is stoichiometric, the Ag–Ag2S mixed crystallites can be detected with the rising annealing temperature (SI-Fig. S6). Therefore, it is necessary for the production of only Ag2S to select an excess of S2 . However, crystal SiC can be detected in the SA-600 sample (SI-Fig. S7) when the mole ratio of Ag to S is 2:5. Interestingly, the resulting spherical Ag2S and Ag nanoparticles are floatable in water, attributed to the small nanoparticle size and to the formed silane-coupling link. This based on the above characterization results. Moreover, they exhibit strong adsorptive ability for colored dyes. Thus, it suggests that the as-prepared Ag2S and Ag nanoparticles have great potential for applications in the
X. Fan et al. / Materials Research Bulletin 47 (2012) 3732–3737
environmental purification and monitoring, which is under carrying out. 4. Conclusions In summary, floatable silver sulfide and silver nanoparticles have been successfully fabricated via a SCA modified solvothermal method, followed by thermal treatment at different temperatures in N2 atmosphere. It is worth note that the KH590 or KH550 plays important roles in controlling crystallite growth, favorable for the production of nanocrystalline Ag2S and Ag with small nanoparticle size during the solvothermal processes. The formed networks effectively inhibit the aggregation and growth of Ag2S and Ag crystallites and keep their monodispersion by forming a thin shell layer after high temperature thermal treatment in N2 atmosphere. The floatable features of resulting spherical Ag2S and Ag nanoparticles are attributed to the small nanoparticle size and to the formed silane-coupling link. It is suggested that this simple synthetic method would be extended to fabricate other Agcontaining nanostructures with floatable features. Acknowledgements This work is financially supported from the National Nature Science Foundation of China (No. 21071048), the Chang Jiang Scholar Candidates Programme for Provincial Universities in Heilongjiang, the Science Foundation of Harbin City of China (No. 2011RFXXG001), and the Program for Innovative Research Team in Heilongjiang University (Hdtd2010-02), for which we are very grateful. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2012. 06.032. References [1] M.H. Chen, L. Gao, Mater. Lett. 60 (2006) 1059–1062. [2] C.C. Wu, H.F. Cho, W.S. Chang, T.C. Lee, Chem. Eng. Sci. 65 (2010) 141–147. [3] S.H. Shen, L.J. Guo, X.B. Chen, F. Ren, S.S. Mao, Int. J. Hydrogen Energy 35 (2010) 7110–7115. [4] X.H. Zhang, D.W. Jing, L.J. Guo, Int. J. Hydrogen Energy 35 (2010) 7051–7057. [5] K.M. Parida, N. Biswal, D.P. Das, S. Martha, Int. J. Hydrogen Energy 35 (2010) 5262–5269.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Controllable synthesis of floatable nanocrystalline Ag2S and Ag by a silane coupling agent-modified solvothermal method Xuxin Fan, Xu Qin, Liqiang Jing *, Yunbo Luan, Mingzheng Xie Key Laboratory of Functional Inorganic Materials Chemistry, Heilongjiang University, Ministry of Education, School of Chemistry and Materials Science, Harbin 150080, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 December 2011 Received in revised form 28 April 2012 Accepted 11 June 2012 Available online 18 June 2012
Nanocrystalline Ag2S and Ag have been synthesized by a silane coupling agent (SCA) modified solvothermal method, followed by thermal treatment in N2 atmosphere. The key of this method is to select bifunctional SCAs with HS– or NH2– ends, which easily couple Ag+ prior to the solvothermal process, while the (H3CO)3–Si– ends of SCA easily change into (HO)3–Si– ends by hydrolysis reactions. These factors are favorable for the production of nanocrystalline Ag2S and Ag with small nanoparticle size during the solvothermal processes, meanwhile distributed homogeneously in the networks resulting from the polymerization of (HO)3–Si– groups. The formed networks effectively inhibit the aggregation and growth of Ag2S and Ag crystallites and keep their monodispersion. The resulting spherical Ag2S and Ag nanoparticles with average diameter of about 10 nm are floatable in water, attributed to the small nanoparticle size and to the formed silane-coupling link. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures B. Chemical synthesis C. Microstructure
1. Introduction Nano-sized materials have been studied extensively in recent years owing to their special physical and chemical properties [1]. Many reports have focused on the synthesis of metal chalcogenide nanoparticles and their assembled structures [2–5]. Among many metal chalcogenide nanoparticles, silver sulfide nanoparticle has attracted considerable attention because of its being potential prospective optoelectronic and thermoelectric materials, such as photoanodes, photoconductors, IR detectors and superionic conductors [6–8]. Ag2S nanoparticles with variable sizes have been prepared via various methods, such as one-step surfactant-assisted solvothermal, gamma-ray irradiation and template-free solution routes [9–11]. However, sizes of Ag2S particle obtained by the methods mentioned above are rather large, within the range of micrometer size in general. Although there have been several reports on the synthesis of small-sized Ag2S nanocrystals up to day [12,13], the synthetic processes are complicated and costly, which restricts greatly wide practical application. In addition to metal chalcogenide nanoparticles, metal nanoparticles (NPs) [14,15] are widely applied in different fields, such as catalysis [16] and optical sensing [17,18]. Compared with other metal nanoparticles, silver nanoparticles have been extensively studied because of its cheapness, antibacterial properties [19] and surface-enhanced Raman scattering (SERS) [20]. Ag nanoparticles
* Corresponding author. Tel.: +86 451 86608610. E-mail address: [email protected] (L. Jing). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.06.032
have always been prepared via various methods, such as photoreduction [21], chemical reduction [22,23] and so on. However, it is difficult for those methods to control the Ag particle size with uniform dispersion. Since sizes of the nanomaterials play important roles in the practical applications, it is necessary to develop a feasible synthetic route for controlling over size of nanoparticles easily. On the basis of the above analysis, to develop simple synthetic pathways to prepare Ag2S and Ag nanoparticles are very important and quite meaningful. Generally speaking, large size of particle prepared is attributed to the tiny solubility product of itself in the aqueous solution [24] and to choose a suitable chelating agent to react with the reactant is a feasible method to reduce its concentration. Sun et al. [25] reported the synthesis of BiVO4 by introducing EDTA as strong chelators to greatly decrease the free Bi3+ content in the solution, leading to the formation of small-sized BiVO4. Naturally, it is expected that Ag+ concentration is possibly one of the important factors influencing nucleation and subsequent growth of nano-sized Ag2S and Ag, especially for Ag2S because of its very low-solubility products (Ksp(Ag2S) = 6.3 10 50) [9]. Thus, the key of the synthetic method is to control Ag+ concentration. Silane coupling agent as a kind of functional molecule has been extensively utilized in the modification of material surfaces, which is valuable technique to design and prepare new composite and functional materials [26]. However, using silane-coupling agents to control particle size in synthetic process has seldom been reported. (H3CO)3–Si– ends of silane coupling agents easily form (HO)3–Si– groups by hydrolysis reactions, further producing –Si–O–Si–connections by polymerization reactions. The produced
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–Si–O–Si– connections may effectively impede rapid growth of nanocrystallites. Functional groups, such as HS– and NH2–, may control Ag+ concentration in the synthetic processes by effectively coupling Ag+ ions. KH590 with HS– ends and KH550 with NH2– ends, as typical SCAs used widely, may achieve our goal to control Ag+ concentration. To the best of our knowledge, they have seldom been reported to use them to control inorganic nanoparticle size so far. In this paper, we report a novel approach to prepare small-sized Ag2S and Ag nanocrystals by a SCA modified solvothermal method for the first time. The KH590 and KH550 play important roles in the synthetic processes so that sizes of Ag2S and Ag nanoparticles are obviously decreased. Interestingly, the resulting Ag2S and Ag nanocrystals can be stably floatable in water, which may be useful for practical applications. This simple synthetic method would be extended to fabricate other Ag-containing nanostructured materials.
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(a)
(b)
2. Experimental 2.1. Synthesis of materials All chemicals are of analytical-grade reagents and are used without further purification. All the annealed procedures are carried out under nitrogen atmosphere. In a typical synthetic procedure, 2.94 10 3 mol silver nitrate (AgNO3) is dissolved in 12.5 ml 95% ethanol by sonication for several minutes (solution A). Another mixture consists of 12.5 ml of toluene and a desired amount of KH590 (or KH550) (solution B). Solution B is then added dropwise to solution A under vigorously stirring at room temperature, producing a kelly gel (or suspension). After continuously stirring for 60 min, 25 ml of the resulting mixture is sealed in a 50 ml Teflon-lined stainless steel autoclave and then maintained at 160 8C for 6 h. The transparent solution is poured out and the products are dried at 80 8C in air, the sample is referred to as SA80(A-80). To improve Ag2S or Ag crystal quality, the as-prepared Ag2S or Ag is annealed at certain temperature for 120 min. The sample is referred to as SA-X (A-X), in which SA(A) means Ag2S(Ag), and X represents the thermal treatment temperature in N2. 2.2. Characterization of materials The samples are characterized by X-ray Powder Diffraction (XRD) with a Rigaku D/MAX-rA powder diffractometer (Japan), using Cu Ka radiation (l = 0.15418 nm), and an accelerating voltage of 30 kV and emission current of 20 mA are employed. Scanning electron microscopy (SEM) observations are carried out on a Philips XL-30-ESEM-FEG operated at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) observations are carried out on a JEOL 1200EX operated at an accelerating voltage of 100 kV. The BET surface area is evaluated by a ST-2000 constant volume adsorption apparatus. The Fourier transform infrared (FTIR) spectra of the samples are collected with a Bruker Equinox 55 Spectrometer, using KBr as diluents. The surface composition and elemental chemical state of the samples are examined by X-ray photoelectron spectroscopy (XPS) using a Kratos-AXIS ULTRA DLD apparatus with Al (Mono) X-ray source, and the binding energies are calibrated with respect to the signal for adventitious carbon (binding energy = 284.6 eV). 3. Results and discussion 3.1. Measurements of XRD, TEM and SEM The crystal phase composition of the products is examined by powder X-ray diffraction (XRD). The main diffraction peaks
Fig. 1. XRD patterns of the Ag2S (a) and Ag (b) nanoparticles (*, Ag2S; ^, SiC; ! Ag).
showed in Fig. 1a can be well indexed as monoclinic a-Ag2S (JCPDS Card No.14-0072) [27–29]. In addition, the characteristic peaks of SiC can be detected in the sample annealed at the 800 8C. In Fig. 1b, the peaks (2u) at 38.18, 44.48 and 64.28 are attributed to the (1 1 1), (2 0 0), and (2 2 0) reflections of face-centered cubic (fcc) structured Ag, respectively [30]. As the thermal treatment temperature increases, the XRD peaks of Ag2S or Ag become slightly high, indicating that the Ag2S or Ag crystallite sizes do not change obviously. It is suggested that the used KH590 or KH550 SCA play important roles in inhibiting the growth of nanocrystalline Ag2S or Ag. According to the Scherrer equation, it is evaluated that the crystallite sizes of resulting Ag2S and Ag are within 8– 11 nm and 13–16 nm, respectively. It should be pointed that silver sulfate can be produced if the corresponding precursors are calcined at certain temperature for 2 h in air instead of in N2 (see Supporting information SI-Fig. S1). Silver sulfide has been known to be a mixed ionic–electronic conductor at high temperature (above 200 8C) [31]. It exists in three different allotropic forms. a-Ag2S is the monoclinic phase and stable up to 178 8C. a-Ag2S easily transforms to body-centered cubic (bbc) b-Ag2S over 178 8C, and the bbc phase transforms to a face-centered cubic (ffc) g-Ag2S phase over 600 8C [32]. Surprisingly, Ag2S nanoparticles we prepared still keep the monoclinic phase at 600 8C and 800 8C. Zhang et al. [33] reported the phase transformation started to form at the interfaces between the nanoparticles in the agglomerated TiO2 nanoparticles with increasing thermal treatment temperature. Similarly, it is suggested that the KH590 SCA with (H3CO)3–Si– effectively inhibit the interface contacts between the Ag2S crystallites, leading to the phase change inhibition of Ag2S from a to b and g phase. The TEM photographs of the Ag2S (a) and Ag (b) samples are shown in Fig. 2, indicating that the morphologies of the samples are spherical. Surprisingly, it can be seen that the particle sizes of the SA-600 and the SA-800 samples do not change a lot compared
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Fig. 2. TEM photographs of the Ag2S (a) and Ag (b) samples.
to that of the SA-400 samples. This would be beneficial to still keep large surface areas of the samples by thermal treatment at high temperature. The SAED patterns in the inset show the polycrystal structure. Based on TEM images of Ag nanocrystals, similar results could be obtained. It is seen that the nanoparticle sizes of resulting Ag2S and Ag are within 8–11 nm and 13–16 nm, respectively, with increasing thermal temperature. These results are consistent with the XRD analyses. Fig. 3 shows scanning electron microscopy (SEM) images of SA400 and SA-600 samples. From the pictures, Ag2S nanoparticles seem to be spherical, and are covered by a large amount of the sponge-like substances. As the thermal treatment temperature increases, the sponge-like substances gradually decrease. In addition, from the analysis of SEM images of Ag, similar results could be obtained. Interestingly, the samples could be stably floatable in water (SI-Fig. S2). It is assumed that the floatable feature is attributed to the sponge-like substances. From the BET surface areas of the Ag2S samples prepared at different annealing temperatures (SI-Table 1), even after annealing temperature at 800 8C, the samples still remain high surface areas, which may be associated with the sponge-like substances. From the table, the
BET surface areas slowly reduce with increasing annealing temperature, indicating Ag2S crystallite sizes slightly change. It is in accord with the XRD and SEM results. 3.2. Measurements of IR and XPS Fig. 4 shows the FT-IR spectra of the Ag2S (a) and Ag (b) samples. The peaks at around 500 cm 1 may result from resulting nanocrystalline Ag2S or Ag. The IR peaks at 1000–1145 and 780–800 cm 1 are attributed to the asymmetric stretching vibration, symmetric and stretching vibration of the group Si– O–Si, respectively, along with the IR peaks at 1090–1120 cm 1 dues to the vibrations of the of C–Si–O group [34–36]. A series of bands at around 2870–2935 cm 1 due to the vibrations of methylene –(CH2)3– [37,38] and the peaks at about 1230– 1275 cm 1 are due to the vibrations of Si–CH3 [34]. And, the peaks located at 886 cm 1 result from SiC in Fig. 4a [39]. As the thermal treatment temperature increases, the 2870–2935 cm 1 peak gradually decreases, indicating –(CH2)3– in –Si(CH2)3–O– Si(CH2)3– network structure is gradually decomposed, leading to the SiC formation at the high temperature, which is supported by
Fig. 3. SEM images of SA-400 and SA-600 samples.
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(a) SA-80
T%
SA-400
SA-600
SA-800
500
1000
1500
2000
2500
3000
3500
4000
Wavelength / cm-1
(b)
T%
A-80
A-400
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SEM and XRD results. The IR peaks at about 1630 and 3430 cm 1 are ascribed to surface hydroxyl and adsorbed water molecules, respectively. The peaks at about 1464 cm 1 are due to the vibrations of NHx in Fig. 4b [37]. X-ray photoelectron spectra (XPS) of SA-600 sample are shown in Fig. 5. The binding energies of Ag3d5/2 and Ag3d3/2 are 367.6 and 373.6 eV, respectively, indicating that the chemical valences of Ag are +1 valence, while that of S2p is 163.2 eV, demonstrating that S is 2 valence in the sample. These results are consistent with the reported data of Ag2S [9]. The strong peak (in Fig. 5c) at 103.0 eV corresponds to Si2p in –Si–O–Si– [40]. The C1s XPS peak shown in Fig. 5d is located at about 284.6 eV, assigned to the C1s in C–C in – Si(CH2)3–O–Si(CH2)3– networks [40]. The O1s XPS spectra (in Fig. 5e) are at least involved with two kinds of O chemical states according to the binding energy range, including hydroxyl groups and Si–O–Si linkages. The OH signal is closely related to the hydroxyl groups resulting mainly from the chemisorbed water and its peak position is at about 531.7 eV [41]. The Osi signal is attributed to Si–O–Si linkages in –Si(CH2)3–O–Si(CH2)3– networks and its peak position is at about 532.5 eV [42]. For XPS spectra of Si2p, C1s and O1s, similar results could also be obtained in the resulting Ag sample, as shown in SI-Fig. S3. In addition, the N1s XPS peak is located at about 398.4 eV and 400.7 eV, which are the characteristic peaks of N1s in NHx species in SI-Fig. S3 [37]. It is deduced that the N species derives from the NH2 function group in the KH550 molecule. Therefore, the XPS results further support those of FT-IR spectra.
A-600 3.3. Analysis of synthetic mechanism
A-800 500
1000
1500
2000
2500
Wavelength /
3000
3500
4000
cm-1
Fig. 4. FT-IR spectra of the Ag2S (a) and Ag (b) samples.
Accepted widely, sizes of nanocrystals can be affected through a delicate balance of the nucleation and growth rate [23]. On the basis of the above discussion, a possible mechanism for the formation of size-controlled nanocrystalline Ag2S and Ag is suggested, as shown in Scheme 1, involved with three steps. Firstly, prior to the solvothermal process, Ag(I) reacts easily with thiolate anion or NH2– groups of SCAs [43], which effectively controls the Ag+ concentration by forming the Ag(I)–thiolate (or – NH2–) complex. Wu et al. [44] reported the synthesis of smallsized Hg and HgS by formation of Hg(II)–thiolate complex, by
Coupling process Hydrolysis reactions HS or NH2 (CH2 )3
Si
(OCXHY)3
Solvothermal
Annealing treatment
process
:S- or NH2
:Ag+
:-(CH2)3-Si-OH
:Ag2S or Ag nanoparticles
:Si
: single Ag2S or Ag molecule
(X=1 or 2; Y=3 or 5)
Scheme 1. Schematic of the formation of Ag2S or Ag.
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(a)
(b)
S2p
Intensity / a.u.
Intensity / a.u.
Ag3d
158 160 162 164 166 168 170 172 174 176
364 366 368 370 372 374 376 378 380 382
Bingding energy / eV
Bingding energy / eV Si2p
(d)
C1s
Intensity / a.u.
Intensity / a.u.
(c)
96
98 100 102 104 106 108 110 112 114 116
278 280 282 284 286 288 290 292 294 296
Bingding energy / eV
Bingding energy / eV
(e) Intensity / a.u.
O1s
OSi
OH
524 526 528 530 532 534 536 538 540 542 544
Binging energy / eV Fig. 5. X-ray photoelectron spectra (XPS) of SA-600 sample.
controlling Hg2+ concentration in the solution. Thus, we suppose that the small-sized Ag or Ag2S have a similar formation process. It is suggested that it is effective to impede the nucleation of the Ag2S and Ag by forming the Ag(I)–thiolate (or –NH2–) complex. Meanwhile, a large number of Si–OH groups are produced by hydrolysis reactions of SCAs. Then, in the solvothermal processes, – Si(CH2)3–O–Si(CH2)3– networks are formed by –Si–OH polymerization reactions, meanwhile nanocrystalline Ag2S and Ag are produced and distributed homogeneously in –Si(CH2)3–O– Si(CH2)3– networks. Finally, in the thermal treatment process in N2, the –Si(CH2)3–O–Si(CH2)3– networks formed in the solvothermal process effectively inhibit the growth of the nanocrystalline Ag2S and Ag. As the thermal treatment temperature rises, organic C–C chains content in –Si(CH2)3–O–Si(CH2)3– networks gradually decreases, leading to sponge-like substances capping the Ag2S and Ag nanoparticles. It is expected that the formed networks not only effectively inhibit the aggregation and growth of Ag2S and Ag crystallites, but also keep their high monodispersion after high temperature thermal treatment. Xia et al. [45] have reported that a nanoparticle reaches a size where the electrostatic repulsion energy between nanoparticles and the newly arrived nanoparticles
outside equals the attractive energy associated with van der Waals interactions. Differently, we suppose the synthesis of small-sized Ag2S and Ag is mainly due to formation of –Si(CH2)3–O–Si(CH2)3– networks as a thin shell layer in our works. The following experimental results further support the suggested synthetic mechanism. The larger the mole ratio of Ag (silver nitrate) to N (KH550), the smaller the crystallite size of the resulting Ag nanoparticles, as shown in SI-Fig. S4. If KH590 or KH550 is replaced of trimethoxypropylsilane, single Ag is obtained with large size in SI-Fig. S5. If the mole ratio of Ag to S is stoichiometric, the Ag–Ag2S mixed crystallites can be detected with the rising annealing temperature (SI-Fig. S6). Therefore, it is necessary for the production of only Ag2S to select an excess of S2 . However, crystal SiC can be detected in the SA-600 sample (SI-Fig. S7) when the mole ratio of Ag to S is 2:5. Interestingly, the resulting spherical Ag2S and Ag nanoparticles are floatable in water, attributed to the small nanoparticle size and to the formed silane-coupling link. This based on the above characterization results. Moreover, they exhibit strong adsorptive ability for colored dyes. Thus, it suggests that the as-prepared Ag2S and Ag nanoparticles have great potential for applications in the
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environmental purification and monitoring, which is under carrying out. 4. Conclusions In summary, floatable silver sulfide and silver nanoparticles have been successfully fabricated via a SCA modified solvothermal method, followed by thermal treatment at different temperatures in N2 atmosphere. It is worth note that the KH590 or KH550 plays important roles in controlling crystallite growth, favorable for the production of nanocrystalline Ag2S and Ag with small nanoparticle size during the solvothermal processes. The formed networks effectively inhibit the aggregation and growth of Ag2S and Ag crystallites and keep their monodispersion by forming a thin shell layer after high temperature thermal treatment in N2 atmosphere. The floatable features of resulting spherical Ag2S and Ag nanoparticles are attributed to the small nanoparticle size and to the formed silane-coupling link. It is suggested that this simple synthetic method would be extended to fabricate other Agcontaining nanostructures with floatable features. Acknowledgements This work is financially supported from the National Nature Science Foundation of China (No. 21071048), the Chang Jiang Scholar Candidates Programme for Provincial Universities in Heilongjiang, the Science Foundation of Harbin City of China (No. 2011RFXXG001), and the Program for Innovative Research Team in Heilongjiang University (Hdtd2010-02), for which we are very grateful. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2012. 06.032. References [1] M.H. Chen, L. Gao, Mater. Lett. 60 (2006) 1059–1062. [2] C.C. Wu, H.F. Cho, W.S. Chang, T.C. Lee, Chem. Eng. Sci. 65 (2010) 141–147. [3] S.H. Shen, L.J. Guo, X.B. Chen, F. Ren, S.S. Mao, Int. J. Hydrogen Energy 35 (2010) 7110–7115. [4] X.H. Zhang, D.W. Jing, L.J. Guo, Int. J. Hydrogen Energy 35 (2010) 7051–7057. [5] K.M. Parida, N. Biswal, D.P. Das, S. Martha, Int. J. Hydrogen Energy 35 (2010) 5262–5269.
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