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Structural and optical characterization of colloidal Se nanoparticles prepared via the acidic decomposition of sodium selenosulfate
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Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 169–174
Structural and optical characterization of colloidal Se nanoparticles prepared via the acidic decomposition of sodium selenosulfate Alexander L. Stroyuk a,∗ , Alexandra E. Raevskaya a , Stepan Ya. Kuchmiy a , Vladimir M. Dzhagan b , Dietrich R.T. Zahn c , Steffen Schulze c a
Pysarzhevski Institute of Physical Chemistry, National Academy Science of Ukraine, Nauky Avenue 31, Kyiv 03028, Ukraine b Institute of Semiconductors Physics of National Academy Science of Ukraine, Nauky Avenue 41, Kyiv 03028, Ukraine c Institut f¨ ur Physik, Technische Universit¨at Chemnitz, Reichenhainer Straße 70, D-09107 Chemnitz, Germany Received 19 October 2007; received in revised form 29 January 2008; accepted 31 January 2008 Available online 10 March 2008
Abstract Colloidal selenium nanoparticles (NPs) were synthesized via acidic decomposition of sodium selenosulfate. The effects of synthesis and postsynthesis treatment conditions on the size, structure and size distribution of the Se nanoparticles are discussed. It is shown that the decomposition of sodium selenosulfate with non-oxidative acids (e.g., HCl) in aqueous solutions of polymers (sodium polyphosphate, gelatin, polyvinyl alcohol, polyethyleneglycole) and surfactants (sodium dodecylsulfonate, cetylpyridinium chloride) results in the formation of amorphous 25–200 nm Se nanoparticles converting upon ageing at 90 ◦ C into trigonal 150–250 nm Se nanocrystals. Optical properties (absorption and Raman spectra) of freshly prepared and aged Se nanoparticles both in colloidal solutions and in polymeric (polyvinyl alcohol) films are analyzed. © 2008 Elsevier B.V. All rights reserved. Keywords: Selenium nanoparticles; Polymer films; Selenosulfate; Raman spectroscopy
1. Introduction Semiconductor nanoparticles (NPs) are of increasing interest for both fundamental and applied research because of their unique electronic and thermodynamic properties as a result of their small dimensions, metal selenides as well as selenium NPs having apparently the greatest application potential [1–4]. Selenium exhibits a combination of many interesting and useful properties [5–9], for example, a relatively low melting point, a high photoconductivity, catalytic activity with respect to hydration and oxidation reactions, high piezoelectric, thermoelectric, and nonlinear optical responses, diverse biological impacts. It is reasonable to expect that the availability of low-dimensional selenium nanostructures will introduce new types of applications, or enhance the performance of currently existing devices as a result of various size effects [3,8,10]. Selenium NPs, especially those synthesized in aqueous solutions via “green” techniques and encapsulated with non-toxic ∗
Corresponding author. Tel.: +380 44 525 0270. E-mail addresses: [email protected], [email protected] (A.L. Stroyuk). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.01.055
organic polymers (e.g., gelatin, polyvinyl alcohol, polyacrylamide or dextrines) are expected to have high potential for biological and medical applications as a strong antioxidant and nutritional additive [11–13]. Selenium exists in several allotropic forms mainly with chain-based trigonal structure (t-Se) and ring-based monoclinic structure (m-Se) [14–16]. Each selenium atom is covalently bonded in the chain to two nearest neighbors with a bond angle ˚ [14–16]. The polymorphs of 100◦ and a bond length of 52.3 A are distinguished by the correlation between neighboring dihedral angles. Depending on this correlation, Se can form either trans- (chain-like) or cis (ring-like) configurations, the energy difference between the two configurations being only 0.03 eV [15,16]. Crystalline t-Se is the most stable form at ambient conditions. Stable Se NPs of monoclinic structure in polymeric matrix have been recently synthesized and their optical and vibrational properties were reported [17,18]. A sixfold enhancement of the emission in Se NPs, as compared to the bulk selenium, has been reported in [19]. Here we report on a new convenient synthesis method of colloidal Se NPs via acidic decomposition of sodium selenosulfate. The effects of a stabilizer nature, synthesis and post-synthesis
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treatment conditions on the structural and spectral characteristics of Se NPs are discussed. Vibrational properties of selenium NPs encapsulated in polymer matrices are studied.
multichannel detection. All Raman spectra were recorded on film samples in a backscattering geometry, using various wavelengths of Ar+ and Kr+ lasers for excitation. Resolution for the Raman spectra was 2 cm−1 .
2. Experimental 3. Results and discussion Se powder, anhydrous sodium sulfite, gelatin, polyvinyl alcohol (PVA), sodium polyphosphate (SPPh), cetylpyridinium chloride (CPyCl), sodium dodecyl sulfate (SDS), and polyethyleneglycol with the average molecular weight of 600 g/mole (PEG600) were purchased from Aldrich and used without additional purification. Sodium selenosulfate solutions were prepared dissolving Se powder in a hot (96–98 ◦ C) aqueous Na2 SO3 solution at the ratio [Se]:[Na2 SO3 ] = 1:4. A stock 0.2 M Na2 SeSO3 solution containing 0.6 M Na2 SO3 was used in the experiments. Colloidal selenium solutions were prepared via acidic decomposition of the sodium selenosulfate by HCl in aqueous solution of a surfactant or a polymer. In a typical procedure, to prepare 50 mL 5 × 10−3 M Se sol in 10 wt.% gelatin, to 48.7 mL of aqueous 10% gelatin solution 1.25 mL 0.2 M Na2 SeSO3 solution and 0.1 mL concentrated HCl were successively added at intense refluxing. The resulting solution was then cooled to 5–7 ◦ C to form a gel which was cut into small pieces and dialyzed at 5–7 ◦ C against doubly-distilled water for 4–5 days. The residues of inorganic substances (NaCl, Na2 SO3 and HCl) were removed from the gel in the course of the dialysis. No selenium was found in the distilled water used for the dialysis. The purified gel was then used as is or dissolved in hot water (or a solution of other polymeric stabilizer or a surfactant) to prepare the solutions with necessary Se concentration. The gel was also used for the preparation of films on glass substrates. A 4.0 cm × 2.5 cm piece of microscopy glass plate was cleaned successively with concentrated H2 O2 and H2 SO4 . Then 2.5 mL of the dialyzed hot viscous 5 × 10−3 M Se solution in 10% gelatin was deposited onto the glass plate. The plate with the solution was kept drying in a ventilated box at room temperature for 5–7 days. It was found that concentration-normalized absorption spectra of the films and their parental Se solutions were identical. The PVA films were prepared in a similar way using non-purified selenium sols. The XRD spectra were registered using a DRON-3M diffractometer with copper K␣ irradiation (λ = 0.1541 nm). The samples for the XRD experiments were prepared precipiting Se NPs from the solutions with acetone. Absorption spectra of the solutions and the films were recorded using a Specord 210 spectrophotometer. TEM measurements were performed using a Philips CM 20 FEG 200 keV transmission electron microscope equipped with a Gatan GIF imaging filter. To prepare samples for TEM experiments, a dialyzed 5 × 10−3 M colloidal Se solution was diluted to 5 × 10−4 M and subjected to the ultrasonic treatment. Then a drop of this solution was deposited onto a carbonized copper grid. Size distributions profiles of Se NPs were plotted as a result of counting of 200–220 individual particles in the TEM images. Raman spectra were recorded at room temperature using a triple monochromator Raman system (Dilor XY) equipped with a charge-coupled device (CCD) camera for the
The majority of the methods of colloidal selenium preparation are based on selenite or selenous acid reduction with various agents (hydrazine, glucose, NaBH4 , SnCl2 , sodium thiosulfate, etc.) [7–9,16,19,20]. Here we propose a simple method of colloidal Se NPs synthesis using an “indirect” reduction of bulk selenium. The term “indirect” means that we do not reduce selenium species to form selenium NPs but instead reduce bulk Se crystals with sodium sulfite to obtain the precursor, Na2 SeSO3 , which can be easily, with non-oxidative acids (HCl, H3 PO4 , etc.), converted into the Se NPs: Se(bulk) + Na3 SO3 → Na3 SeSO3
(1)
Na2 SeSO3 + 2HCl → 2NaCl + H2 SeSO3
(2)
H2 SeSO3 → Se(nano) + SO2 + H2 O
(3)
The color of the prepared colloidal Se solutions varies with the nature and content of a stabilizer used from orange to reddishbrown. It was found that the parameters, affecting noticeably the optical properties of the colloidal Se particles, are the synthesis temperature and the stabilizer nature, while the variations in the concentrations of the stabilizer, Na2 SeSO3 and HCl do not significantly influence the color of the resulting Se colloids. 3.1. Structural characterization of Se NPs To obtain information about the structure and the size of Se NPs, two sets of samples were prepared for XRD and TEM experiments, the first one prepared from the freshly synthesized Se colloid and the second one—from a solution aged for 2 h at 90 ◦ C. It was found that while the freshly prepared Se NPs are amorphous (a-Se), the Se NPs aged at elevated temperatures are crystalline, the peaks in their XRD spectrum (Fig. 1) at 23.5◦ (plane index 100), 29.7◦ (1 0 1), 41.4◦ (1 1 0), 43.6◦ (1 0 2), 45.4◦ (1 1 1), 48.0◦ (2 0 0), 51.7◦ (2 0 1), and 56.0◦ (0 0 3) being characteristic for the trigonal Se (t-Se) crystals [8,9,19,21]. Fig. 2 presents the TEM images and the size distribution charts for the freshly prepared Se NPs (a) and the ones aged at elevated (85–90 ◦ C) temperature (b). It can be seen from Fig. 2a that the freshly prepared Se colloid consists of spherical NPs with a wide and non-symmetrical size distribution in the range of 25–200 nm with approximately half of the Se mass concentrated in the form of 25 to 40 nm particles. Ageing of the Se colloid at elevated temperatures results in disappearance of small (d < 50 nm) selenium NPs with the simultaneous enlargement of bigger NPs indicating the Ostwald ripening [3] governing the Se NPs growth in the system under investigation. An almost Gaussian shape of the size distribution profile of the aged Se NPs also speaks in favour of this mechanism of NPs growth.
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Fig. 1. XRD spectrum of Se NPs aged at 90 ◦ C for 2 h.
3.2. Absorption spectra of colloidal Se solutions Fig. 3 shows the absorption spectra of colloidal Se solutions prepared under various conditions. The optical absorption spectra of the Se NPs synthesized using various stabilizing additives (SDS, CPyCl, SPPh, PEG600) are shown in Fig. 3a. The additives were chosen to represent various types of colloidal stabilizers—anionic (SDS) and cationic (CPyCl) surfactants, inorganic polymers (SPPh) and organic polymers of increasing (from PEG600 to PVA to gelatin) molecular weight. Fig. 3b shows the variation of the absorption spectrum of gelatin-stabilized Se NPs with increasing synthesis temperature. Earlier optical reflectivity and absorption measurements revealed that trigonal (sometimes called hexagonal) selenium
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is an indirect gap semiconductor [22,23] with the absorption edge at ∼1.6 eV [8] and the lowest direct optical transition at 1.95–2.10 eV (300 K), depending on measurement wavelength and polarization [24,25]. The indirect transitions in t-Se, which also determine the band gap, start at 1.6 eV and the lowest direct optical transition is at 2.39 eV [18]. As the lowest direct transition in t-Se has been ascribed to interchain interaction, this explains the similarity of the absorption spectrum of a-Se to that of single Se chains observed in [25], since in a-Se the ordering and therefore the interchain interaction are lost. Bulk a-Se was reported to have Eg close to 1.95 eV [25,26], but in recent studies of ∼200 nm-large a-Se particles pronounced peaks were reported at much lower energies with the tail extending even further into the infrared domain of the spectrum [7,27]. It can be seen from Fig. 3 that the absorbance of Se NPs decreases gradually towards the long-wave side of the spectrum without any particular spectral features near the band edge. This agrees well with the well-known fact that the absorbance of Se NPs near the threshold originates from the indirect interband electronic transitions [8], in which case the following dependence between the absorption coefficient α and the excitation energy hν is expected: α=A
(hν − Eg )2 hν
(4)
The coefficient α can be calculated as α = 2303 D ρ C−1 l−1 [3] where D(hν) is the optical density of the solution at given λ, ρ is the density of the bulk selenium (g cm−3 ), C is the Se concentration in g cm−3 , l is the optical cuvette thickness. The coefficient A in the expression (4) gives the transition probability. So, for the indirect transitions the long-wave tail of the absorption spectrum is expected to be linear when plotted in the coordinates
Fig. 2. TEM images and the size distributions of the freshly prepared (a) and aged at 85–90 ◦ C (b) Se NPs.
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1
2
3
1.0
Stabilizer S [S] (M or wt.%) Synthesis temperature (◦ C)
Gelatin
1.0
Gelatin
1.0
SDS CPyCl SPPh PVA PEG600
0 0.1 1.0 10.0
0.1
0.1 0.1 5 × 10−3 0.1 0.1
25
4 27 45 80 90
25
Eg (eV) 1.67 1.75 1.70 1.65 1.85 1.75 1.67 1.66 1.65 1.68 1.47 1.55 1.55 1.44
Note: Eg determination error is ±0.01.
Fig. 3. (a) Absorption spectra of the colloidal Se solutions prepared using various stabilizing agents: SDS (curve 1), CPyCl (2), SPPh (3) and PEG600 (4). Concentrations of Se NPs and the stabilizers are indicated in Table 1 (series 3). (b) Absorption spectra of 2% gelatine-stabilized Se colloids prepared at different temperature: 4 ◦ C (1), 27 ◦ C (2), 45 ◦ C (3), 80 ◦ C (4) and 90 ◦ C (5). Inset: curve 2 in the coordinates (α E)1\2 − hν. Experimental conditions are indicated in Table 1 (series 2). Cuvette length l = 1.0 cm.
(α hν)1\2 − hν. This was found to be the case (inset in Fig. 3b) for the most part of the curve representing the absorption spectrum of Se NPs prepared at 27 ◦ C. The same correlations take place for the most of the absorption spectra of Se NPs under investigation. Zeroing the dependence (α hν)1\2 − hν to the ordinate axis one can calculate the value Eg + Eph , where Eph is the phonon energy in Se NPs. From the Raman spectroscopy results discussed below Eph was evaluated to be ≈0.03 eV. The energies of the indirect interband transition Eg of Se NPs, calculated from the absorption spectra using the above approach, are summarized in Table 1. Analysis of the data presented in Table 1 shows that the Eg values for the prepared Se NPs are in most cases close or slightly higher than Eg of the bulk selenium. The cases, when Eg determined from the absorption spectra of Se colloids is smaller than 1.6 eV (in case of the stabilization with PVA, SPPh and
PEG600), originate most probably from the hindering of the edge of the fundamental absorption band of Se NPs by a plasmon resonance band [28,29], as is evidently seen, for example, from the spectral curve 4 in Fig. 3a. Fig. 3b and Table 1 (batch No. 3) show that the increase in the synthesis temperature from 4 to 90 ◦ C results in a bathochromic shift of the absorption edge of the gelatin-stabilized Se NPs and a decrease in Eg from 1.85 to 1.65 eV. The exciton Borh radius in the trigonal selenium being as small as 1.55 nm [23], the observed variations in the optical spectra of selenium NPs presented here can hardly originate from changes in the electronic properties of Se NPs due to variation in the size of Se nanoparticles along with the synthesis temperature. It has been shown [7,28,29] that these spectral effects are accounted for by the Rayleigh scattering amplification with the growth of the size of Se NPs and can be satisfactorily described by Mie theory. Other parameters of the synthesis, such as the nature of a stabilizer and its content, have weak influence upon the optical properties of Se NPs. 3.3. Vibrational spectra of the PVA film hosting Se NPs The absorption spectrum of a PVA film containing Se NPs reveals two broad shoulders (Fig. 4), identically to that of the PVA-stabilized colloidal Se solution. The trigonal structure of the Se NPs formed was established from the Raman spectra taken at different excitation wavelengths. The spectrum of the bare Se NPs excited with λ = 647.1 nm of the Kr+ laser reveals a strong narrow feature at 235 cm−1 with a weak broad shoulder at the high-frequency side (Fig. 5). The Raman scattering is assumed to be non-resonant under this excitation wavelength that falls below the absorption edge of the NPs (Fig. 4). The fitting of the spectra with two Lorentzians gave peaks with ν equal to 233.7 and 236.7 cm−1 correspondingly (Fig. 6a), and a full width at half maximum Γ of each peak being ≈5 cm−1 .
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Fig. 4. Optical absorption spectra of a PVA film with Se NPs. The arrows show the laser lines wavelengths used for recording Raman spectra.
Fig. 5. Raman spectra of Se NPs for λexc = 647.1 nm (1) and 476.5 nm (2). The inset shows the main peak in more detail.
The peak positions obtained are very close to the frequency values reported for asymmetric bond-stretching E mode and to the symmetric bond stretching A1 mode of trigonal Se, respectively [30–33]. The spectra, obtained under the excitation with a number of green-to-violet lines of the Ar+ laser (528.7–457.9 nm) in resonance with the lower maximum in the absorption spectrum, contain the sharp peak at ∼237 cm−1 , with its Γ decreasing with λexc , and a weak and broad structure between 400 and 500 cm−1 . In Fig. 5 the spectrum for λexc = 476.5 nm is shown as an example. We attribute the peak at 237 cm−1 to the A1 -mode, with a possible small contribution of E-mode. The decreasing of the contribution of E-mode with decreasing λexc can be responsible for the observed narrowing of the whole peak. A broad (Γ ≈ 25 cm−1 ) high-frequency shoulder also appears at these excitations, being most pronounced at λexc = 647.1 and 501.7 nm (Fig. 6b). The maximum of this broader feature is ∼260 cm−1 and thus coincides with the Raman active phonon frequency in single Se chains [34,35]. The width of the E and A1 phonon peaks – equal 8–10 cm−1 – is smaller than previously reported for nm-sized NPs [17,30] but larger than the natural (bulk) phonon linewidth (5 cm−1 at 300 K) [30].
The broad structure between the 400 and 500 cm−1 is obviously a superposition of the second order phonon peaks: 2E, 2A1 and that corresponding to the single chain mode at ∼260 cm−1 . The frequencies of the 2E- and 2A1 -related modes were found for bulk t-Se at 438 and 455 cm−1 , respectively [31]. The replica of the single-chain mode is expected close to 500 cm−1 [36]. The enhancement of the second to first order peak intensity ratio for resonant as compared to non-resonant (476.5 and 647.1 nm, Fig. 5) excitation is observed. The fact that different vibrational modes dominate at nonresonant and resonant excitation, with the non-resonant peak intensity being comparable to the resonant one, needs to be studied yet. Similar observations were made for 4 nm Se NPs [17]. The authors observed peaks a 256 cm−1 for resonant excitation with λexc = 476.5 nm and at 260 cm−1 for λexc = 514.5 nm much below the Eg and assigned them respectively to E1 mode, redshifted due to the phonon confinement, and some non-optical mode. Due to larger NP size in the present study, the frequencies of the phonon modes are not apparently affected by the phonon confinement, but the resonance effects are strong, as observed for single Se chains and rings as well [37].
Fig. 6. Raman spectra of Se NPs λexc = 647.1 nm (a) and 501.7 nm (b) along with fitting Lorentz shapes.
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4. Conclusions A new convenient method for the preparation of selenium sols and the polymer films hosting Se nanoparticles through the acidic decomposition of sodium selenosulfate in aqueous solutions has been proposed. The effects of the stabilizer nature and content as well as the synthesis temperature upon the optical band gap of Se nanoparticles have been studied. It has been found that the post-synthesis thermal treatment of the gelatinstabilized Se nanoparticles results in the crystallization of the initially amorphous Se nanoparticles into the trigonal form, growth of the average nanoparticles size and narrowing of the size distribution. The structure of the phonon spectrum of Se nanocrystals embedded in polyvinyl alcohol films, as a function of the excitation wavelength, has been analyzed in details. Acknowledgement V. Dzhagan is grateful to the Alexander von Humboldt Foundation for financial support during his research stay at Chemnitz University of Technology. References [1] N.L. Rosi, C.A. Mirkin, Chem. Rev. 105 (2005) 1547. [2] S.P. Gubin, N.A. Kataeva, G.B. Khomutov, Russ. Chem. Bull. Int. Ed. 54 (2005) 827. [3] S.V. Gaponenko, Optical Properties of Semiconductor Nanocrystals, University Press, Cambridge, 1996. [4] C.M. Welch, R.G. Compton, Anal. Bioanal. Chem. 384 (2006) 601. [5] Selenium, in: R.A. Zingaro, W.C. Cooper (Eds.), Van Nostrand Reinhold, New York, 1974. [6] L.I. Berger, Semiconductor Materials, CRC Press, Boca Raton, FL, 1997. [7] Z.-H. Lin, C.R.C. Wang, Mater. Chem. Phys. 92 (2005) 591. [8] B. Gates, B. Mayers, B. Cattle, Y. Xia, Adv. Funct. Mater. 12 (2002) 219. [9] B.T. Mayers, K. Liu, D. Sunderland, Y. Xia, Chem. Mater. 15 (2003) 3852. [10] A. Umehara, S. Nitta, H. Furukawa, S. Nonomura, Appl. Surf. Sci. 119 (1997) 176.
[11] M.P. Rayman, Lancet 356 (2000) 233. [12] B. Huang, J. Zhang, J. Hou, C. Chen, Free Radical Biol. Med. 35 (2003) 805. [13] J. Zhang, H. Wang, X. Yan, L. Zhang, Life Sci. 76 (2005) 1099. [14] R.S. Oremland, M.J. Herbel, J.S. Blum, et al., Appl. Environ. Microbiol. 70 (2004) 52. [15] H.D. Sun, Z.K. Tang, W.M. Zhao, G.K.L. Wong, Appl. Phys. Lett. 71 (1997) 2457. [16] X. Hu, J.C. Yu, Q. Li, J. Mater. Chem. 16 (2006) 748. [17] M. Rajalakshmi, A.K. Arora, Nanostruct. Mater. 11 (1999) 399. [18] M. Rajalakshmi, A.K. Arora, Solid State Commun. 110 (1999) 75. [19] H. Zhang, D. Yang, Y. Ji, X. Ma, J. Xu, D. Que, J. Phys. Chem. B 108 (2004) 1179. [20] U. Jeong, T. Herricks, E. Shahar, Y. Xia, J. Am. Chem. Soc. 127 (2005) 1098. [21] Y. Chen, W. Zhang, Y. Fan, X. Xu, Z. Zhang, Mater. Chem. Phys. 98 (2006) 191. [22] J. Stuke, in: R.A. Zingaro, W.C. Cooper (Eds.), Selenium, Van Nostrand Reinhold, New York, 1974, p. 148. [23] S. Tutihasi, I. Chen, Solid State Commun. 5 (1967) 255. [24] W. Henrion, Phys. Stat. Sol. 12 (1965) K113. [25] V.N. Bogomolov, S.V. Kholodkevich, S.G. Romanov, L.S. Agroskin, Solid State Commun. 47 (1983) 181. [26] P. Nagels, E. Sleeck, R. Callaerts, E. Marquez, et al., Solid State Commun. 102 (1997) 539. [27] J.S. Zhang, X.V. Gao, L.D. Zhang, Y.P. Bao, Biofactors 15 (2001) 27. [28] J. Dauchot, A. Watillon, J. Colloid Interface Sci. 23 (1967) 62. [29] A. Watillon, J. Dauchot, J. Colloid Interface Sci. 27 (1968) 507. [30] A. Mooradian, G.P. Wright, in: W.C. Cooper (Ed.), The Physics of Selenium and Tellurium, Pergamon, Oxford, 1969. [31] G. Lucotsky, A. Mooradian, W. Taylor, G.B. Wright, R.C. Keezer, Solid State Commun. 5 (1967) 113. [32] R.M. Martin, G. Lucotsky, K. Helliwell, Phys. Rev. B 13 (1976) 1383. [33] D. Nesheva, I.P. Kotsalas, C. Raptis, E. Vateva, J. Non-Cryst. Sol. 224 (1998) 283–290. [34] I.L. Li, S.C. Ruan, Z.M. Li, J.P. Zhai, Z.K. Tang, Appl. Phys. Lett. 87 (2005) 071902. [35] V.V. Poborchii, A.V. Kolobov, K. Tanaka, Appl. Phys. Lett. 72 (1998) 1167. [36] L. He, Z.X. Shen, G. Gu, L. Qin, S.H. Tang, Chem. Phys. Lett. 300 (1999) 504–508. [37] I.L. Li, Z.K. Tang, J. Appl. Phys. 95 (2004) 6364.
Colloids and Surfaces A: Physicochem. Eng. Aspects 320 (2008) 169–174
Structural and optical characterization of colloidal Se nanoparticles prepared via the acidic decomposition of sodium selenosulfate Alexander L. Stroyuk a,∗ , Alexandra E. Raevskaya a , Stepan Ya. Kuchmiy a , Vladimir M. Dzhagan b , Dietrich R.T. Zahn c , Steffen Schulze c a
Pysarzhevski Institute of Physical Chemistry, National Academy Science of Ukraine, Nauky Avenue 31, Kyiv 03028, Ukraine b Institute of Semiconductors Physics of National Academy Science of Ukraine, Nauky Avenue 41, Kyiv 03028, Ukraine c Institut f¨ ur Physik, Technische Universit¨at Chemnitz, Reichenhainer Straße 70, D-09107 Chemnitz, Germany Received 19 October 2007; received in revised form 29 January 2008; accepted 31 January 2008 Available online 10 March 2008
Abstract Colloidal selenium nanoparticles (NPs) were synthesized via acidic decomposition of sodium selenosulfate. The effects of synthesis and postsynthesis treatment conditions on the size, structure and size distribution of the Se nanoparticles are discussed. It is shown that the decomposition of sodium selenosulfate with non-oxidative acids (e.g., HCl) in aqueous solutions of polymers (sodium polyphosphate, gelatin, polyvinyl alcohol, polyethyleneglycole) and surfactants (sodium dodecylsulfonate, cetylpyridinium chloride) results in the formation of amorphous 25–200 nm Se nanoparticles converting upon ageing at 90 ◦ C into trigonal 150–250 nm Se nanocrystals. Optical properties (absorption and Raman spectra) of freshly prepared and aged Se nanoparticles both in colloidal solutions and in polymeric (polyvinyl alcohol) films are analyzed. © 2008 Elsevier B.V. All rights reserved. Keywords: Selenium nanoparticles; Polymer films; Selenosulfate; Raman spectroscopy
1. Introduction Semiconductor nanoparticles (NPs) are of increasing interest for both fundamental and applied research because of their unique electronic and thermodynamic properties as a result of their small dimensions, metal selenides as well as selenium NPs having apparently the greatest application potential [1–4]. Selenium exhibits a combination of many interesting and useful properties [5–9], for example, a relatively low melting point, a high photoconductivity, catalytic activity with respect to hydration and oxidation reactions, high piezoelectric, thermoelectric, and nonlinear optical responses, diverse biological impacts. It is reasonable to expect that the availability of low-dimensional selenium nanostructures will introduce new types of applications, or enhance the performance of currently existing devices as a result of various size effects [3,8,10]. Selenium NPs, especially those synthesized in aqueous solutions via “green” techniques and encapsulated with non-toxic ∗
Corresponding author. Tel.: +380 44 525 0270. E-mail addresses: [email protected], [email protected] (A.L. Stroyuk). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.01.055
organic polymers (e.g., gelatin, polyvinyl alcohol, polyacrylamide or dextrines) are expected to have high potential for biological and medical applications as a strong antioxidant and nutritional additive [11–13]. Selenium exists in several allotropic forms mainly with chain-based trigonal structure (t-Se) and ring-based monoclinic structure (m-Se) [14–16]. Each selenium atom is covalently bonded in the chain to two nearest neighbors with a bond angle ˚ [14–16]. The polymorphs of 100◦ and a bond length of 52.3 A are distinguished by the correlation between neighboring dihedral angles. Depending on this correlation, Se can form either trans- (chain-like) or cis (ring-like) configurations, the energy difference between the two configurations being only 0.03 eV [15,16]. Crystalline t-Se is the most stable form at ambient conditions. Stable Se NPs of monoclinic structure in polymeric matrix have been recently synthesized and their optical and vibrational properties were reported [17,18]. A sixfold enhancement of the emission in Se NPs, as compared to the bulk selenium, has been reported in [19]. Here we report on a new convenient synthesis method of colloidal Se NPs via acidic decomposition of sodium selenosulfate. The effects of a stabilizer nature, synthesis and post-synthesis
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treatment conditions on the structural and spectral characteristics of Se NPs are discussed. Vibrational properties of selenium NPs encapsulated in polymer matrices are studied.
multichannel detection. All Raman spectra were recorded on film samples in a backscattering geometry, using various wavelengths of Ar+ and Kr+ lasers for excitation. Resolution for the Raman spectra was 2 cm−1 .
2. Experimental 3. Results and discussion Se powder, anhydrous sodium sulfite, gelatin, polyvinyl alcohol (PVA), sodium polyphosphate (SPPh), cetylpyridinium chloride (CPyCl), sodium dodecyl sulfate (SDS), and polyethyleneglycol with the average molecular weight of 600 g/mole (PEG600) were purchased from Aldrich and used without additional purification. Sodium selenosulfate solutions were prepared dissolving Se powder in a hot (96–98 ◦ C) aqueous Na2 SO3 solution at the ratio [Se]:[Na2 SO3 ] = 1:4. A stock 0.2 M Na2 SeSO3 solution containing 0.6 M Na2 SO3 was used in the experiments. Colloidal selenium solutions were prepared via acidic decomposition of the sodium selenosulfate by HCl in aqueous solution of a surfactant or a polymer. In a typical procedure, to prepare 50 mL 5 × 10−3 M Se sol in 10 wt.% gelatin, to 48.7 mL of aqueous 10% gelatin solution 1.25 mL 0.2 M Na2 SeSO3 solution and 0.1 mL concentrated HCl were successively added at intense refluxing. The resulting solution was then cooled to 5–7 ◦ C to form a gel which was cut into small pieces and dialyzed at 5–7 ◦ C against doubly-distilled water for 4–5 days. The residues of inorganic substances (NaCl, Na2 SO3 and HCl) were removed from the gel in the course of the dialysis. No selenium was found in the distilled water used for the dialysis. The purified gel was then used as is or dissolved in hot water (or a solution of other polymeric stabilizer or a surfactant) to prepare the solutions with necessary Se concentration. The gel was also used for the preparation of films on glass substrates. A 4.0 cm × 2.5 cm piece of microscopy glass plate was cleaned successively with concentrated H2 O2 and H2 SO4 . Then 2.5 mL of the dialyzed hot viscous 5 × 10−3 M Se solution in 10% gelatin was deposited onto the glass plate. The plate with the solution was kept drying in a ventilated box at room temperature for 5–7 days. It was found that concentration-normalized absorption spectra of the films and their parental Se solutions were identical. The PVA films were prepared in a similar way using non-purified selenium sols. The XRD spectra were registered using a DRON-3M diffractometer with copper K␣ irradiation (λ = 0.1541 nm). The samples for the XRD experiments were prepared precipiting Se NPs from the solutions with acetone. Absorption spectra of the solutions and the films were recorded using a Specord 210 spectrophotometer. TEM measurements were performed using a Philips CM 20 FEG 200 keV transmission electron microscope equipped with a Gatan GIF imaging filter. To prepare samples for TEM experiments, a dialyzed 5 × 10−3 M colloidal Se solution was diluted to 5 × 10−4 M and subjected to the ultrasonic treatment. Then a drop of this solution was deposited onto a carbonized copper grid. Size distributions profiles of Se NPs were plotted as a result of counting of 200–220 individual particles in the TEM images. Raman spectra were recorded at room temperature using a triple monochromator Raman system (Dilor XY) equipped with a charge-coupled device (CCD) camera for the
The majority of the methods of colloidal selenium preparation are based on selenite or selenous acid reduction with various agents (hydrazine, glucose, NaBH4 , SnCl2 , sodium thiosulfate, etc.) [7–9,16,19,20]. Here we propose a simple method of colloidal Se NPs synthesis using an “indirect” reduction of bulk selenium. The term “indirect” means that we do not reduce selenium species to form selenium NPs but instead reduce bulk Se crystals with sodium sulfite to obtain the precursor, Na2 SeSO3 , which can be easily, with non-oxidative acids (HCl, H3 PO4 , etc.), converted into the Se NPs: Se(bulk) + Na3 SO3 → Na3 SeSO3
(1)
Na2 SeSO3 + 2HCl → 2NaCl + H2 SeSO3
(2)
H2 SeSO3 → Se(nano) + SO2 + H2 O
(3)
The color of the prepared colloidal Se solutions varies with the nature and content of a stabilizer used from orange to reddishbrown. It was found that the parameters, affecting noticeably the optical properties of the colloidal Se particles, are the synthesis temperature and the stabilizer nature, while the variations in the concentrations of the stabilizer, Na2 SeSO3 and HCl do not significantly influence the color of the resulting Se colloids. 3.1. Structural characterization of Se NPs To obtain information about the structure and the size of Se NPs, two sets of samples were prepared for XRD and TEM experiments, the first one prepared from the freshly synthesized Se colloid and the second one—from a solution aged for 2 h at 90 ◦ C. It was found that while the freshly prepared Se NPs are amorphous (a-Se), the Se NPs aged at elevated temperatures are crystalline, the peaks in their XRD spectrum (Fig. 1) at 23.5◦ (plane index 100), 29.7◦ (1 0 1), 41.4◦ (1 1 0), 43.6◦ (1 0 2), 45.4◦ (1 1 1), 48.0◦ (2 0 0), 51.7◦ (2 0 1), and 56.0◦ (0 0 3) being characteristic for the trigonal Se (t-Se) crystals [8,9,19,21]. Fig. 2 presents the TEM images and the size distribution charts for the freshly prepared Se NPs (a) and the ones aged at elevated (85–90 ◦ C) temperature (b). It can be seen from Fig. 2a that the freshly prepared Se colloid consists of spherical NPs with a wide and non-symmetrical size distribution in the range of 25–200 nm with approximately half of the Se mass concentrated in the form of 25 to 40 nm particles. Ageing of the Se colloid at elevated temperatures results in disappearance of small (d < 50 nm) selenium NPs with the simultaneous enlargement of bigger NPs indicating the Ostwald ripening [3] governing the Se NPs growth in the system under investigation. An almost Gaussian shape of the size distribution profile of the aged Se NPs also speaks in favour of this mechanism of NPs growth.
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Fig. 1. XRD spectrum of Se NPs aged at 90 ◦ C for 2 h.
3.2. Absorption spectra of colloidal Se solutions Fig. 3 shows the absorption spectra of colloidal Se solutions prepared under various conditions. The optical absorption spectra of the Se NPs synthesized using various stabilizing additives (SDS, CPyCl, SPPh, PEG600) are shown in Fig. 3a. The additives were chosen to represent various types of colloidal stabilizers—anionic (SDS) and cationic (CPyCl) surfactants, inorganic polymers (SPPh) and organic polymers of increasing (from PEG600 to PVA to gelatin) molecular weight. Fig. 3b shows the variation of the absorption spectrum of gelatin-stabilized Se NPs with increasing synthesis temperature. Earlier optical reflectivity and absorption measurements revealed that trigonal (sometimes called hexagonal) selenium
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is an indirect gap semiconductor [22,23] with the absorption edge at ∼1.6 eV [8] and the lowest direct optical transition at 1.95–2.10 eV (300 K), depending on measurement wavelength and polarization [24,25]. The indirect transitions in t-Se, which also determine the band gap, start at 1.6 eV and the lowest direct optical transition is at 2.39 eV [18]. As the lowest direct transition in t-Se has been ascribed to interchain interaction, this explains the similarity of the absorption spectrum of a-Se to that of single Se chains observed in [25], since in a-Se the ordering and therefore the interchain interaction are lost. Bulk a-Se was reported to have Eg close to 1.95 eV [25,26], but in recent studies of ∼200 nm-large a-Se particles pronounced peaks were reported at much lower energies with the tail extending even further into the infrared domain of the spectrum [7,27]. It can be seen from Fig. 3 that the absorbance of Se NPs decreases gradually towards the long-wave side of the spectrum without any particular spectral features near the band edge. This agrees well with the well-known fact that the absorbance of Se NPs near the threshold originates from the indirect interband electronic transitions [8], in which case the following dependence between the absorption coefficient α and the excitation energy hν is expected: α=A
(hν − Eg )2 hν
(4)
The coefficient α can be calculated as α = 2303 D ρ C−1 l−1 [3] where D(hν) is the optical density of the solution at given λ, ρ is the density of the bulk selenium (g cm−3 ), C is the Se concentration in g cm−3 , l is the optical cuvette thickness. The coefficient A in the expression (4) gives the transition probability. So, for the indirect transitions the long-wave tail of the absorption spectrum is expected to be linear when plotted in the coordinates
Fig. 2. TEM images and the size distributions of the freshly prepared (a) and aged at 85–90 ◦ C (b) Se NPs.
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1
2
3
1.0
Stabilizer S [S] (M or wt.%) Synthesis temperature (◦ C)
Gelatin
1.0
Gelatin
1.0
SDS CPyCl SPPh PVA PEG600
0 0.1 1.0 10.0
0.1
0.1 0.1 5 × 10−3 0.1 0.1
25
4 27 45 80 90
25
Eg (eV) 1.67 1.75 1.70 1.65 1.85 1.75 1.67 1.66 1.65 1.68 1.47 1.55 1.55 1.44
Note: Eg determination error is ±0.01.
Fig. 3. (a) Absorption spectra of the colloidal Se solutions prepared using various stabilizing agents: SDS (curve 1), CPyCl (2), SPPh (3) and PEG600 (4). Concentrations of Se NPs and the stabilizers are indicated in Table 1 (series 3). (b) Absorption spectra of 2% gelatine-stabilized Se colloids prepared at different temperature: 4 ◦ C (1), 27 ◦ C (2), 45 ◦ C (3), 80 ◦ C (4) and 90 ◦ C (5). Inset: curve 2 in the coordinates (α E)1\2 − hν. Experimental conditions are indicated in Table 1 (series 2). Cuvette length l = 1.0 cm.
(α hν)1\2 − hν. This was found to be the case (inset in Fig. 3b) for the most part of the curve representing the absorption spectrum of Se NPs prepared at 27 ◦ C. The same correlations take place for the most of the absorption spectra of Se NPs under investigation. Zeroing the dependence (α hν)1\2 − hν to the ordinate axis one can calculate the value Eg + Eph , where Eph is the phonon energy in Se NPs. From the Raman spectroscopy results discussed below Eph was evaluated to be ≈0.03 eV. The energies of the indirect interband transition Eg of Se NPs, calculated from the absorption spectra using the above approach, are summarized in Table 1. Analysis of the data presented in Table 1 shows that the Eg values for the prepared Se NPs are in most cases close or slightly higher than Eg of the bulk selenium. The cases, when Eg determined from the absorption spectra of Se colloids is smaller than 1.6 eV (in case of the stabilization with PVA, SPPh and
PEG600), originate most probably from the hindering of the edge of the fundamental absorption band of Se NPs by a plasmon resonance band [28,29], as is evidently seen, for example, from the spectral curve 4 in Fig. 3a. Fig. 3b and Table 1 (batch No. 3) show that the increase in the synthesis temperature from 4 to 90 ◦ C results in a bathochromic shift of the absorption edge of the gelatin-stabilized Se NPs and a decrease in Eg from 1.85 to 1.65 eV. The exciton Borh radius in the trigonal selenium being as small as 1.55 nm [23], the observed variations in the optical spectra of selenium NPs presented here can hardly originate from changes in the electronic properties of Se NPs due to variation in the size of Se nanoparticles along with the synthesis temperature. It has been shown [7,28,29] that these spectral effects are accounted for by the Rayleigh scattering amplification with the growth of the size of Se NPs and can be satisfactorily described by Mie theory. Other parameters of the synthesis, such as the nature of a stabilizer and its content, have weak influence upon the optical properties of Se NPs. 3.3. Vibrational spectra of the PVA film hosting Se NPs The absorption spectrum of a PVA film containing Se NPs reveals two broad shoulders (Fig. 4), identically to that of the PVA-stabilized colloidal Se solution. The trigonal structure of the Se NPs formed was established from the Raman spectra taken at different excitation wavelengths. The spectrum of the bare Se NPs excited with λ = 647.1 nm of the Kr+ laser reveals a strong narrow feature at 235 cm−1 with a weak broad shoulder at the high-frequency side (Fig. 5). The Raman scattering is assumed to be non-resonant under this excitation wavelength that falls below the absorption edge of the NPs (Fig. 4). The fitting of the spectra with two Lorentzians gave peaks with ν equal to 233.7 and 236.7 cm−1 correspondingly (Fig. 6a), and a full width at half maximum Γ of each peak being ≈5 cm−1 .
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Fig. 4. Optical absorption spectra of a PVA film with Se NPs. The arrows show the laser lines wavelengths used for recording Raman spectra.
Fig. 5. Raman spectra of Se NPs for λexc = 647.1 nm (1) and 476.5 nm (2). The inset shows the main peak in more detail.
The peak positions obtained are very close to the frequency values reported for asymmetric bond-stretching E mode and to the symmetric bond stretching A1 mode of trigonal Se, respectively [30–33]. The spectra, obtained under the excitation with a number of green-to-violet lines of the Ar+ laser (528.7–457.9 nm) in resonance with the lower maximum in the absorption spectrum, contain the sharp peak at ∼237 cm−1 , with its Γ decreasing with λexc , and a weak and broad structure between 400 and 500 cm−1 . In Fig. 5 the spectrum for λexc = 476.5 nm is shown as an example. We attribute the peak at 237 cm−1 to the A1 -mode, with a possible small contribution of E-mode. The decreasing of the contribution of E-mode with decreasing λexc can be responsible for the observed narrowing of the whole peak. A broad (Γ ≈ 25 cm−1 ) high-frequency shoulder also appears at these excitations, being most pronounced at λexc = 647.1 and 501.7 nm (Fig. 6b). The maximum of this broader feature is ∼260 cm−1 and thus coincides with the Raman active phonon frequency in single Se chains [34,35]. The width of the E and A1 phonon peaks – equal 8–10 cm−1 – is smaller than previously reported for nm-sized NPs [17,30] but larger than the natural (bulk) phonon linewidth (5 cm−1 at 300 K) [30].
The broad structure between the 400 and 500 cm−1 is obviously a superposition of the second order phonon peaks: 2E, 2A1 and that corresponding to the single chain mode at ∼260 cm−1 . The frequencies of the 2E- and 2A1 -related modes were found for bulk t-Se at 438 and 455 cm−1 , respectively [31]. The replica of the single-chain mode is expected close to 500 cm−1 [36]. The enhancement of the second to first order peak intensity ratio for resonant as compared to non-resonant (476.5 and 647.1 nm, Fig. 5) excitation is observed. The fact that different vibrational modes dominate at nonresonant and resonant excitation, with the non-resonant peak intensity being comparable to the resonant one, needs to be studied yet. Similar observations were made for 4 nm Se NPs [17]. The authors observed peaks a 256 cm−1 for resonant excitation with λexc = 476.5 nm and at 260 cm−1 for λexc = 514.5 nm much below the Eg and assigned them respectively to E1 mode, redshifted due to the phonon confinement, and some non-optical mode. Due to larger NP size in the present study, the frequencies of the phonon modes are not apparently affected by the phonon confinement, but the resonance effects are strong, as observed for single Se chains and rings as well [37].
Fig. 6. Raman spectra of Se NPs λexc = 647.1 nm (a) and 501.7 nm (b) along with fitting Lorentz shapes.
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4. Conclusions A new convenient method for the preparation of selenium sols and the polymer films hosting Se nanoparticles through the acidic decomposition of sodium selenosulfate in aqueous solutions has been proposed. The effects of the stabilizer nature and content as well as the synthesis temperature upon the optical band gap of Se nanoparticles have been studied. It has been found that the post-synthesis thermal treatment of the gelatinstabilized Se nanoparticles results in the crystallization of the initially amorphous Se nanoparticles into the trigonal form, growth of the average nanoparticles size and narrowing of the size distribution. The structure of the phonon spectrum of Se nanocrystals embedded in polyvinyl alcohol films, as a function of the excitation wavelength, has been analyzed in details. Acknowledgement V. Dzhagan is grateful to the Alexander von Humboldt Foundation for financial support during his research stay at Chemnitz University of Technology. References [1] N.L. Rosi, C.A. Mirkin, Chem. Rev. 105 (2005) 1547. [2] S.P. Gubin, N.A. Kataeva, G.B. Khomutov, Russ. Chem. Bull. Int. Ed. 54 (2005) 827. [3] S.V. Gaponenko, Optical Properties of Semiconductor Nanocrystals, University Press, Cambridge, 1996. [4] C.M. Welch, R.G. Compton, Anal. Bioanal. Chem. 384 (2006) 601. [5] Selenium, in: R.A. Zingaro, W.C. Cooper (Eds.), Van Nostrand Reinhold, New York, 1974. [6] L.I. Berger, Semiconductor Materials, CRC Press, Boca Raton, FL, 1997. [7] Z.-H. Lin, C.R.C. Wang, Mater. Chem. Phys. 92 (2005) 591. [8] B. Gates, B. Mayers, B. Cattle, Y. Xia, Adv. Funct. Mater. 12 (2002) 219. [9] B.T. Mayers, K. Liu, D. Sunderland, Y. Xia, Chem. Mater. 15 (2003) 3852. [10] A. Umehara, S. Nitta, H. Furukawa, S. Nonomura, Appl. Surf. Sci. 119 (1997) 176.
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