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Spectroscopy of anion on colloidal silver particles: Chemiadsorption effect
Chemical Physics 332 (2007) 284–288 www.elsevier.com/locate/chemphys
Spectroscopy of anion on colloidal silver particles: Chemiadsorption effect Aiping Zhang, Yan Fang
*
Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Capital Normal University, Beijing 100037, PR China Received 1 September 2006; accepted 8 December 2006 Available online 29 December 2006
Abstract Laser-ablated silver particles were prepared to characterize chemiadsorption of nucleophilic anion and its evocable spectroscopy. Besides the gradual decrease of particles’ plasmon resonance absorption with the increase of anion, some new absorption bands, assigned to different mechanisms, appeared and shifted due to chemiadsorption effect and aggregation of particles. At the same time, gradual changes and new appearance of luminescence bands indicate that, the hybridization of surface energy states on modified metal surfaces did happen. A chemiadsorption model and a hybridization mechanism were assumed to explain these phenomena. 2006 Elsevier B.V. All rights reserved. Keywords: Silver particle; Chemiadsorption; Hybridization; Spectroscopy; Anion
1. Introduction Although colloidal metals in aqueous solution are recognized with a huge amount of interface, their fascinating chemical and physical properties have not been well documented in recent literature. In the case of silver, two kinds of classical spectral techniques have always been used to characterize its surface properties: (1) surface-enhanced Raman scattering by chemisorbed molecules, which can be found in a large amount of reports [1–5] and (2) observations of shape and position changes of the surface plasmon absorption band caused by an oscillation of electron gas in the particles [6–8]. Recently, a new method named resonance scattering spectroscopy [9–11] was used for colloidal metals studies. These studies indicate that nanoparticles and superamolecules in big size are fundamentals cause to exhibit resonance scattering spectrum, the influence factors on resonance scattering spectra include particle shape, diameter, rigidity, refraction, index, absorption properties, *
Corresponding author. Tel.: +86 10 68902965; fax: +86 10 68982332. E-mail addresses: [email protected], [email protected] (Y. Fang). 0301-0104/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.12.013
etc., [12–15]. At present, studies about resonance scattering are not much. And there is, however a need to more fully characterizes its mechanism and factors to which it is influencing. It was shown that small concentration of nucleophilic anion in silver sol could drastically change shape and position of the plasmon absorption band. Many mechanisms [12–19] have been proposed to explain these phenomena, and one may suggest that [16] unoccupied orbits, into which a nucleophilic reagent can denote electron pairs, do exist on the metal surface, and these orbits can be easily affected by the nucleophilic adsorbates. The consequence is not only a change in the plasmon resonance absorption of colloidal particles but also a change in their other surface spectral activity [18]. It is postulated that chemiadsorption of solute molecules changes the electronic properties of small metal particles, a dipolar layer on the surface being produced and the potential of Fermi level being changed. Presently, noble metal colloids are often prepared by chemical deoxidization methods, and the presence of residual oxidation products and extraneous ions in colloids can cause uncontrolled aggregation and diffusive double layer on metal surface, which makes the discussions of added
A. Zhang, Y. Fang / Chemical Physics 332 (2007) 284–288
adsorbates more difficult and less conviction. Therefore, it is essential to prepare ‘chemically pure’ metal colloids to avoid interference of extraneous ions in the application of spectroscopy investigations of targeted molecules/anions. In this work, Ag plate was ablated in deionized water using Nd:YAG pulse laser, and finally ‘chemically pure’ Ag colloids were obtained. Then, different concentrations of nucleophilic anions ðIO 3 Þ were added into these colloids to detect the chemiadsorption and aggregation effects of colloidal silver by spectral methods. UV–vis absorption and photoluminescence spectra were investigated by taking nanoparticles and clusters as a whole. Discussed with experimental evidence, a chemiadsorption model and a hybridization mechanism were proposed to explain these obvious phenomena. 2. Experimental 2.1. Materials KIO3 (Analytical grade reagents) purchased from Beijing Chemical Company were used as received, without further purification. All the other chemicals used in the experiments were analytical grade reagents with deionized water used for solution preparation. 2.2. Preparation of Ag colloids Silver plate (2 mm thickness, 99.99%, Aldrich) was purified using diluted nitric acid. Then, Ag plate was rinsed in ultrasonic washer dipped in deionized water for 20 min. After that, the plate was fixed at the bottom of a quartz cell filled with deionized water in a depth of 10 mm above it.
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The laser beam was adjusted by a 200 mm focusing lens to make the laser facula on Ag plate to be about 2 mm (diameter). The irradiation by 1064 nm laser from Nd:YAG pulse laser with 10 Hz repetitions was continued for 60 min. Finally, buff silver colloids were obtained with their mean diameter is about 15 nm (Fig. 1a). And their characteristic absorption spectrum showed a band centered at 393 nm (see Fig. 4a). 2.3. Apparatus and measurements The transmission electron micrograph (TEM) images of Ag colloids were taken with an H-600 TEM made by the Hitachi Corporation after placing several drops of Ag sol on Ni–Cu grid. The Raman spectra were obtained by the RFS 100/s Bruker NIR-FT spectrophotometer. The operated wavelength is 1064 nm. The resolution was 2 cm1 and a 180 geometry was employed. The output laser power, which could not induce a change of the adsorbate-substrate system, was 150 mw in the case of the colloidal solution, and 50 mw in the case of the solid powder. Emission and absorption spectra were recorded on a fluorolog-3 spectrometer (Jobin Yvon) and a Shimadzu Model UV-2401PC UV–vis spectrometer, respectively. All emission spectra were received with the excited wavelength fixed at 320 nm, and recorded from 320 nm to 900 nm. All measurements were carried out at room temperature on a spectrophotometric 1 cm · 1 cm quartz cuvette. 3. Results and discussion The sizes of laser-ablated silver particles were measured by TEM. It can be seen from Fig. 1a that these particles
1 Fig. 1. TEM images of laser-ablated Ag colloids containing different concentrations of IO 3 , c(anion)/mol L : (a) 0.000; (b) 0.001; (c) 0.005; (d) 0.010; (e) 0.020.
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according to which interactions of surface atoms with nucleophilics, leads to excess negative charges in the metal interior, and these charges can be picked up by electron acceptor (for example O2). This process is schematically depicted in Fig. 3. As can be seen from Fig. 3, a surface atom carrying a nucleophilic anion (X) acquires a small positive charge d+, and the interior of colloidal particles receives a corresponding negative charge d. Ag Particles carrying nucleophilic adsorbates on their surface are very reactive toward O2, as a result, O2 picks up the excess negative charge on particles when in the presence of air (‘oxidation’ of metal surface) and thus, allowing further chemiadsorptions of surface atoms until the whole particle is oxidized. Fig. 3 also shows the shift of Fermi potential at equilibrium before and after anions adsorbed. When IO 3 added into Ag sol, the buff solution became blue immediately, and a much stronger and permanent color appeared with the increase of ½IO 3 . The presence of IO 3 leads to silver colloidal particles displaying some notable changes of well-known plasmon resonance absorption. From Fig. 4a, only one prominent absorption at 393 nm was present in the range of 300–900 nm before adding IO 3 . After IO3 added, the 393 nm absorption band decreased dramatically, meanwhile a new significant band
755
were relatively same in size and were almost near spherical. After the addition of IO 3 , chemiadsorption and aggregation effects occurred and thus influenced the diameters and shapes of particles. The higher concentration of added IO 3 was, the more aggregation of silver atoms/particles did occur, resulting in an increase of particles’ diameters. Particle sizes were measured according to the statistical analysis of large number (200–250) of particles. When the concentration of IO 3 were 0.000, 0.001, 0.005, 0.010, 0.020 mol L1, the average diameters of silver particles were measured as 15, 21, 29, 34, 43 nm, respectively (Fig. 1). The FT-Raman spectrum of solid KIO3, the FT-SERS 1 spectrum of IO 3 (0.001 mol L ) on laser-ablated Ag particles is respectively presented in Fig. 2a and b. It is obvious that this Ag sol is a good SERS-active substrate; and shifts of most SERS bands, compared with the corresponding bands in the Raman spectrum of solid KIO3 (Fig. 2a), indicates that it is a chemical interaction between anions and metals, which is in accordance with that of Ref. [19]. When increasing the concentration of IO 3 , SERS bands appeared the same as Fig. 2b. The mechanism of chemiadsorption and subsequent oxidation on metal surface was proposed previously [20],
Absorption Intensity (a.u.)
747 735
b
a e 760 835
450 352
787 810
198
328 305 372
175
Intensity (a.u.)
393
a 300 500
1000
1500
2000
Wave Length/nm Fig. 2. Raman spectrum of solid KIO3 (a), and FT-SERS spectrum of 1 IO 3 (0.001 mol L ) on laser-ablated Ag particles (b).
400
500
600
700
800
900
Wave Length/nm Fig. 4. UV–vis absorption spectra of laser-ablated Ag colloids containing 1 different concentrations of IO 3 c(anion)/mol L : (a) 0.000; (b) 0.001; (c) 0.005; (d) 0.010; (e) 0.020.
Fig. 3. Schematic presentation of surface chemiadsorption by a nucleophilic regent X and final oxidation of the silver particle by oxygen.
A. Zhang, Y. Fang / Chemical Physics 332 (2007) 284–288
where the aspect ratio is L/d and x, y is always called to be length and radial axis, respectively. The new band in long wavelength can be ascribed to the longitudinal resonance of electric field vector along the length axis [26–28]. Because the electric field vector can be decomposed into E? and Ek, in which E? is perpendicular to the length and Ek is parallel with it, the Ek excites the longitudinal resonance along the length at longer wavelength. According to Kerker’ theory [27], the longitudinal plasmon resonance position of the silver linear aggregation particles would shift to longer wavelength with the increase of aspect ratio (L/d). In our work, when ½IO 3 increased, longer and longer nanorods formed due to linear aggregation effect and higher aspect ratio of L/d thus occurred. As a result, the longitudinal resonant position shifted to red from 760 to 835 nm gradually. Two new absorptions at 450 and 352 nm were recorded after adding IO 3 , and these bands can be seen to in-plane and out-of-plane quadrupole resonance, respectively, according to other authors [28]. The evolution of absorption features of silver particles was studied as a function of particle size and shape due to aggregation effect. As the particle size increased from 15 to 43 nm, 450, 393 and 352 nm absorption bands remained almost stable at the same position with their intensity decreased slowly, accompany with 760 nm band observed to shift to longer wavelength. Luminescence experiments were carried out on the same samples as absorption. The emission spectra, shown in Fig. 5, clearly indicated two emission bands centered at 352 and 467 nm, respectively before anion added. These two luminescence bands were considered to be intra-band transition associated with surface plasmon resonance of Ag sol [15]. It may indicate the presence of two distinct emission centers or one center with complex excited state structure on silver surfaces. When IO 3 added and with the increase of its concentration, 467 nm emission band decreased gradually while 352 nm band shifted to a lower
467
364
Intensity (a.u.)
at 760 nm appears and red-shifted to 835 nm with the increase of ½IO 3 . It is known that for a metal particle different plasmon resonant modes can be excited at different dimensions [21–24]. For metal nanorod, the relationship between the aspect ratio and the optical absorption peak can be described by Gan’s theory, which introduced a geometrical factor pi for different dimensions [25]. The x and y axes of the nanorod are identical and correspond to the nanorode diameter (d), whereas the z-axis represents the nanorod length (L). The geometrical factors pi are given by 1 e2 1 1þe ln pz ¼ 1 ð1Þ 2e 1e e2 1 pz ð2Þ px ¼ py ¼ 2 2 1=2 L d2 e¼ ð3Þ L2
287
a
e
e
419
a
352
350
400
450
500
550
600
650
Wave Length/nm Fig. 5. Photoluminescence emission spectra of laser-ablated Ag colloids 1 containing different concentrations of IO 3 , c(anion)/mol L : (a) 0.000; (b) 0.001; (c) 0.005; (d) 0.010; (e) 0.020.
energy state (364 nm) with its intensity increased quickly. In the meantime, a new increasing band at 419 nm appeared and showed stronger and stronger. It is known that two properties of metal particles determine its spectra properties [6,20]: (1) the density of free electrons on silver surfaces, which also determines the position of Fermi level of particles; (2) the effective size of colloidal particles. Both properties are influenced by chemisorbed species. Therefore, red shift and broadening of the surface plasmon absorption bands (Fig. 4) of silver particles were interpreted as a downward shift or more negative potential of Fermi level due to the adsorption of IO 3. Consider a small spherical metal particle embedded in an environment of anions with different dielectric constant, energy states on metal surface can be easily influenced by the ‘chemical’ contributions by these adsorbates and even by the inter-band transitions between particles via anions [29,30]. The hybridization models for the plasmon response of core/shell structure have been used to explain changed spectral properties by many authors [10,11]. The adsorption mechanism of anion on metal surface proposed previously [20] is similar as the hybridization model considering the core/shell complex structure. In our experiments, luminescence signals of particles adsorbed by anions have similar effect as metal core/shell structures of others [10]. In addition, plasmon hybridizations of metal colloids have been reported by other authors [12,13,15]. In our case, UV–vis signals showed the existence of chemiadsorption shell of anions. So, the shift of metal particles’ surface energy states induced by adsorbed anions could be considered as energy hybridization effect on metal surface as others [10–15,29]. Therefore, it is postulated that chemiadsorption of IO 3 changed the electronic properties of metal particles, a dipolar layer on the surface being produced and the potential of Fermi level being changed. It is also postulated that electron-transfer (CT) reactions did
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A. Zhang, Y. Fang / Chemical Physics 332 (2007) 284–288
b
a
S2 S'2
S2 S'2
S'1 S1 S'1
S1
352nm
364nm 419nm
S0 467nm
Fig. 6. Energy states hybridization (a) and emission transitions (b) proposed for sliver colloid before and after the addition of IO 3.
occur between surface-modified metal particles and adsorbates, which leads to the hybridization of surface energy states. And because of the hybridization effect, new emission centers assigned to these new energy states occurred. Thus, both absorption and luminescence spectra changed a lot between absence and in the present of IO 3 . The hybridization of energy states and emission transitions process could be sketched by the four-state model of luminescence and be shown in Fig. 6a and b. 4. Conclusion ‘Chemically pure’ Ag colloids were prepared by ablating Ag plate in deionized water using Nd:YAG pulse laser. Then, different concentrations of nucleophilic ions ðIO 3Þ were added into the mixture to detect the interactions and spectral changes of colloidal silver. Gradual changes and new appearances of both absorption and luminescence indicate the chemisorption and hybridization occurred on metal surfaces. It is deduced that chemiadsorption of IO 3 changes the electronic properties of silver surfaces, a dipolar layer on the surface being produced and the potential of Fermi level being changed, which can be seen as hybridization effect between surface energy states. Discussed with classical theory and evidence, hybridization model and an energy transition depiction were proposed to explain the obvious spectra phenomena. Acknowledgements The authors are grateful for the support of this research by the National Natural Science Foundation of China and Natural Science Foundation of Beijing. References [1] R.A. Alvarez-Puebla, E. Arceo, P.J.G. Goulet, J.J. Garrido, R.F. Aroca, J. Phys. Chem. B 109 (2005) 3787.
[2] D.S. dos Santos, P.J.G. Goulet, N.P.W. Pieczonka, O.N. Oliveira, R.F. Aroca, Langmuir 20 (2004) 10273. [3] X.C. Yang, Y. Fang, J. Phys. Chem. B 107 (2003) 10100. [4] R.A. Alvarez-Puebla, D.S. Dos Santos, R.F. Aroca, Analyst 129 (2004) 1251. [5] E.C. Le Ru, M. Meyer, P.G. Etchegion, J. Chem. Phys. B 110 (2006) 1944. [6] H.B. fu, J.N. Yao, J. Am. Chem. Soc. 123 (2001) 1434. [7] R.L. Zong, J. Zhou, Q. Li, B. Du, B. Li, M. Fu, X.W. Qi, L.T. Li, S. Buddhudu, J. Phys. Chem. B 108 (2004) 16713. [8] Y. Fang, J. Chem. Phys. 108 (1998) 4315. [9] L.A. Peyser, A.E. Vinson, A.P. Bartko, R.M. Dickson, Science 291 (2001) 103. [10] E. Prodan, C. radloff, N.J. Halas, P. Nordlander, Science 302 (2003) 419. [11] C.L. Nehl, N.K. Grady, G.P. Goodrich, F. Tam, N.J. Halas, J.H. Hafner, Nano Lett. 4 (2004) 2355. [12] Z.L. Jiang, Z.W. Feng, T.S. Li, F. Li, F.X. Zhong, J.Y. Xie, X.H. Yi, Sci. Chin. B 44 (2001) 175. [13] Y.Q. He, S.P. Liu, L. Kong, Z.F. Liu, Spectrochim. Acta A 61 (2005) 2861. [14] R.F. Pastrnack, P.J. Collings, Science 269 (1995) 935. [15] Z.L. Jiang, W.F. Yuan, H.C. Pan, Spectrochim. Acta A 61 (2005) 2488. _ [16] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607. [17] A.P. Zhang, Y. Fang, Acta Phys. Chim. Sin. 22 (2006) 756. [18] A. Henglein, T. Linert, P. Mulvaney, Ber. Bunsenges. Phys. Chem. 94 (1990) 1449. [19] X.L. Yie, W.J. Deng, E.J. Liang, W. Kerker, Acta Phys. Sin. 47 (1997) 2130. [20] P. Mulvaney, T. Linnert, A. Henglein, J. Phys. Chem. 95 (1991) 7843. [21] C.S. Ah, S.D. Hong, D.J. Jand, J. Phys. Chem. B 105 (2001) 7871. [22] M.S. Sander, R. Gronsky, Y.M. Lin, M.S. Dresselhaus, J. Appl. Phys. 89 (2001) 2733. [23] F.J. Gracia de Abajo, A. Howie, Phye. Rev. B 65 (2002) 115418. [24] M. Maillard, S. Giorgio, M.P. Pileni, J. Phys. Chem. B 107 (2003) 2466. [25] B.M.I. van der Zande, M.R. Bohmer, L.G.J. Fokkink, C. Schonenberger, Langmuir 16 (2000) 451. [26] A. Henglein, J. Phys. Chem. 97 (1993) 5457. [27] M. Kerker, J. Colloid Interf. Sci. 105 (1985) 5. [28] D.S. Tsai, C.H. Chen, C.C. Chou, Mater. Chem. Phys. 90 (2005) 361. [29] E. Zaremba, B.N.J. Persson, Phys. Rev. B 35 (1987) 596. [30] B.N.J. Persson, Surf. Sci. 281 (1993) 153.
Spectroscopy of anion on colloidal silver particles: Chemiadsorption effect Aiping Zhang, Yan Fang
*
Beijing Key Laboratory for Nano-Photonics and Nano-Structure, Capital Normal University, Beijing 100037, PR China Received 1 September 2006; accepted 8 December 2006 Available online 29 December 2006
Abstract Laser-ablated silver particles were prepared to characterize chemiadsorption of nucleophilic anion and its evocable spectroscopy. Besides the gradual decrease of particles’ plasmon resonance absorption with the increase of anion, some new absorption bands, assigned to different mechanisms, appeared and shifted due to chemiadsorption effect and aggregation of particles. At the same time, gradual changes and new appearance of luminescence bands indicate that, the hybridization of surface energy states on modified metal surfaces did happen. A chemiadsorption model and a hybridization mechanism were assumed to explain these phenomena. 2006 Elsevier B.V. All rights reserved. Keywords: Silver particle; Chemiadsorption; Hybridization; Spectroscopy; Anion
1. Introduction Although colloidal metals in aqueous solution are recognized with a huge amount of interface, their fascinating chemical and physical properties have not been well documented in recent literature. In the case of silver, two kinds of classical spectral techniques have always been used to characterize its surface properties: (1) surface-enhanced Raman scattering by chemisorbed molecules, which can be found in a large amount of reports [1–5] and (2) observations of shape and position changes of the surface plasmon absorption band caused by an oscillation of electron gas in the particles [6–8]. Recently, a new method named resonance scattering spectroscopy [9–11] was used for colloidal metals studies. These studies indicate that nanoparticles and superamolecules in big size are fundamentals cause to exhibit resonance scattering spectrum, the influence factors on resonance scattering spectra include particle shape, diameter, rigidity, refraction, index, absorption properties, *
Corresponding author. Tel.: +86 10 68902965; fax: +86 10 68982332. E-mail addresses: [email protected], [email protected] (Y. Fang). 0301-0104/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2006.12.013
etc., [12–15]. At present, studies about resonance scattering are not much. And there is, however a need to more fully characterizes its mechanism and factors to which it is influencing. It was shown that small concentration of nucleophilic anion in silver sol could drastically change shape and position of the plasmon absorption band. Many mechanisms [12–19] have been proposed to explain these phenomena, and one may suggest that [16] unoccupied orbits, into which a nucleophilic reagent can denote electron pairs, do exist on the metal surface, and these orbits can be easily affected by the nucleophilic adsorbates. The consequence is not only a change in the plasmon resonance absorption of colloidal particles but also a change in their other surface spectral activity [18]. It is postulated that chemiadsorption of solute molecules changes the electronic properties of small metal particles, a dipolar layer on the surface being produced and the potential of Fermi level being changed. Presently, noble metal colloids are often prepared by chemical deoxidization methods, and the presence of residual oxidation products and extraneous ions in colloids can cause uncontrolled aggregation and diffusive double layer on metal surface, which makes the discussions of added
A. Zhang, Y. Fang / Chemical Physics 332 (2007) 284–288
adsorbates more difficult and less conviction. Therefore, it is essential to prepare ‘chemically pure’ metal colloids to avoid interference of extraneous ions in the application of spectroscopy investigations of targeted molecules/anions. In this work, Ag plate was ablated in deionized water using Nd:YAG pulse laser, and finally ‘chemically pure’ Ag colloids were obtained. Then, different concentrations of nucleophilic anions ðIO 3 Þ were added into these colloids to detect the chemiadsorption and aggregation effects of colloidal silver by spectral methods. UV–vis absorption and photoluminescence spectra were investigated by taking nanoparticles and clusters as a whole. Discussed with experimental evidence, a chemiadsorption model and a hybridization mechanism were proposed to explain these obvious phenomena. 2. Experimental 2.1. Materials KIO3 (Analytical grade reagents) purchased from Beijing Chemical Company were used as received, without further purification. All the other chemicals used in the experiments were analytical grade reagents with deionized water used for solution preparation. 2.2. Preparation of Ag colloids Silver plate (2 mm thickness, 99.99%, Aldrich) was purified using diluted nitric acid. Then, Ag plate was rinsed in ultrasonic washer dipped in deionized water for 20 min. After that, the plate was fixed at the bottom of a quartz cell filled with deionized water in a depth of 10 mm above it.
285
The laser beam was adjusted by a 200 mm focusing lens to make the laser facula on Ag plate to be about 2 mm (diameter). The irradiation by 1064 nm laser from Nd:YAG pulse laser with 10 Hz repetitions was continued for 60 min. Finally, buff silver colloids were obtained with their mean diameter is about 15 nm (Fig. 1a). And their characteristic absorption spectrum showed a band centered at 393 nm (see Fig. 4a). 2.3. Apparatus and measurements The transmission electron micrograph (TEM) images of Ag colloids were taken with an H-600 TEM made by the Hitachi Corporation after placing several drops of Ag sol on Ni–Cu grid. The Raman spectra were obtained by the RFS 100/s Bruker NIR-FT spectrophotometer. The operated wavelength is 1064 nm. The resolution was 2 cm1 and a 180 geometry was employed. The output laser power, which could not induce a change of the adsorbate-substrate system, was 150 mw in the case of the colloidal solution, and 50 mw in the case of the solid powder. Emission and absorption spectra were recorded on a fluorolog-3 spectrometer (Jobin Yvon) and a Shimadzu Model UV-2401PC UV–vis spectrometer, respectively. All emission spectra were received with the excited wavelength fixed at 320 nm, and recorded from 320 nm to 900 nm. All measurements were carried out at room temperature on a spectrophotometric 1 cm · 1 cm quartz cuvette. 3. Results and discussion The sizes of laser-ablated silver particles were measured by TEM. It can be seen from Fig. 1a that these particles
1 Fig. 1. TEM images of laser-ablated Ag colloids containing different concentrations of IO 3 , c(anion)/mol L : (a) 0.000; (b) 0.001; (c) 0.005; (d) 0.010; (e) 0.020.
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according to which interactions of surface atoms with nucleophilics, leads to excess negative charges in the metal interior, and these charges can be picked up by electron acceptor (for example O2). This process is schematically depicted in Fig. 3. As can be seen from Fig. 3, a surface atom carrying a nucleophilic anion (X) acquires a small positive charge d+, and the interior of colloidal particles receives a corresponding negative charge d. Ag Particles carrying nucleophilic adsorbates on their surface are very reactive toward O2, as a result, O2 picks up the excess negative charge on particles when in the presence of air (‘oxidation’ of metal surface) and thus, allowing further chemiadsorptions of surface atoms until the whole particle is oxidized. Fig. 3 also shows the shift of Fermi potential at equilibrium before and after anions adsorbed. When IO 3 added into Ag sol, the buff solution became blue immediately, and a much stronger and permanent color appeared with the increase of ½IO 3 . The presence of IO 3 leads to silver colloidal particles displaying some notable changes of well-known plasmon resonance absorption. From Fig. 4a, only one prominent absorption at 393 nm was present in the range of 300–900 nm before adding IO 3 . After IO3 added, the 393 nm absorption band decreased dramatically, meanwhile a new significant band
755
were relatively same in size and were almost near spherical. After the addition of IO 3 , chemiadsorption and aggregation effects occurred and thus influenced the diameters and shapes of particles. The higher concentration of added IO 3 was, the more aggregation of silver atoms/particles did occur, resulting in an increase of particles’ diameters. Particle sizes were measured according to the statistical analysis of large number (200–250) of particles. When the concentration of IO 3 were 0.000, 0.001, 0.005, 0.010, 0.020 mol L1, the average diameters of silver particles were measured as 15, 21, 29, 34, 43 nm, respectively (Fig. 1). The FT-Raman spectrum of solid KIO3, the FT-SERS 1 spectrum of IO 3 (0.001 mol L ) on laser-ablated Ag particles is respectively presented in Fig. 2a and b. It is obvious that this Ag sol is a good SERS-active substrate; and shifts of most SERS bands, compared with the corresponding bands in the Raman spectrum of solid KIO3 (Fig. 2a), indicates that it is a chemical interaction between anions and metals, which is in accordance with that of Ref. [19]. When increasing the concentration of IO 3 , SERS bands appeared the same as Fig. 2b. The mechanism of chemiadsorption and subsequent oxidation on metal surface was proposed previously [20],
Absorption Intensity (a.u.)
747 735
b
a e 760 835
450 352
787 810
198
328 305 372
175
Intensity (a.u.)
393
a 300 500
1000
1500
2000
Wave Length/nm Fig. 2. Raman spectrum of solid KIO3 (a), and FT-SERS spectrum of 1 IO 3 (0.001 mol L ) on laser-ablated Ag particles (b).
400
500
600
700
800
900
Wave Length/nm Fig. 4. UV–vis absorption spectra of laser-ablated Ag colloids containing 1 different concentrations of IO 3 c(anion)/mol L : (a) 0.000; (b) 0.001; (c) 0.005; (d) 0.010; (e) 0.020.
Fig. 3. Schematic presentation of surface chemiadsorption by a nucleophilic regent X and final oxidation of the silver particle by oxygen.
A. Zhang, Y. Fang / Chemical Physics 332 (2007) 284–288
where the aspect ratio is L/d and x, y is always called to be length and radial axis, respectively. The new band in long wavelength can be ascribed to the longitudinal resonance of electric field vector along the length axis [26–28]. Because the electric field vector can be decomposed into E? and Ek, in which E? is perpendicular to the length and Ek is parallel with it, the Ek excites the longitudinal resonance along the length at longer wavelength. According to Kerker’ theory [27], the longitudinal plasmon resonance position of the silver linear aggregation particles would shift to longer wavelength with the increase of aspect ratio (L/d). In our work, when ½IO 3 increased, longer and longer nanorods formed due to linear aggregation effect and higher aspect ratio of L/d thus occurred. As a result, the longitudinal resonant position shifted to red from 760 to 835 nm gradually. Two new absorptions at 450 and 352 nm were recorded after adding IO 3 , and these bands can be seen to in-plane and out-of-plane quadrupole resonance, respectively, according to other authors [28]. The evolution of absorption features of silver particles was studied as a function of particle size and shape due to aggregation effect. As the particle size increased from 15 to 43 nm, 450, 393 and 352 nm absorption bands remained almost stable at the same position with their intensity decreased slowly, accompany with 760 nm band observed to shift to longer wavelength. Luminescence experiments were carried out on the same samples as absorption. The emission spectra, shown in Fig. 5, clearly indicated two emission bands centered at 352 and 467 nm, respectively before anion added. These two luminescence bands were considered to be intra-band transition associated with surface plasmon resonance of Ag sol [15]. It may indicate the presence of two distinct emission centers or one center with complex excited state structure on silver surfaces. When IO 3 added and with the increase of its concentration, 467 nm emission band decreased gradually while 352 nm band shifted to a lower
467
364
Intensity (a.u.)
at 760 nm appears and red-shifted to 835 nm with the increase of ½IO 3 . It is known that for a metal particle different plasmon resonant modes can be excited at different dimensions [21–24]. For metal nanorod, the relationship between the aspect ratio and the optical absorption peak can be described by Gan’s theory, which introduced a geometrical factor pi for different dimensions [25]. The x and y axes of the nanorod are identical and correspond to the nanorode diameter (d), whereas the z-axis represents the nanorod length (L). The geometrical factors pi are given by 1 e2 1 1þe ln pz ¼ 1 ð1Þ 2e 1e e2 1 pz ð2Þ px ¼ py ¼ 2 2 1=2 L d2 e¼ ð3Þ L2
287
a
e
e
419
a
352
350
400
450
500
550
600
650
Wave Length/nm Fig. 5. Photoluminescence emission spectra of laser-ablated Ag colloids 1 containing different concentrations of IO 3 , c(anion)/mol L : (a) 0.000; (b) 0.001; (c) 0.005; (d) 0.010; (e) 0.020.
energy state (364 nm) with its intensity increased quickly. In the meantime, a new increasing band at 419 nm appeared and showed stronger and stronger. It is known that two properties of metal particles determine its spectra properties [6,20]: (1) the density of free electrons on silver surfaces, which also determines the position of Fermi level of particles; (2) the effective size of colloidal particles. Both properties are influenced by chemisorbed species. Therefore, red shift and broadening of the surface plasmon absorption bands (Fig. 4) of silver particles were interpreted as a downward shift or more negative potential of Fermi level due to the adsorption of IO 3. Consider a small spherical metal particle embedded in an environment of anions with different dielectric constant, energy states on metal surface can be easily influenced by the ‘chemical’ contributions by these adsorbates and even by the inter-band transitions between particles via anions [29,30]. The hybridization models for the plasmon response of core/shell structure have been used to explain changed spectral properties by many authors [10,11]. The adsorption mechanism of anion on metal surface proposed previously [20] is similar as the hybridization model considering the core/shell complex structure. In our experiments, luminescence signals of particles adsorbed by anions have similar effect as metal core/shell structures of others [10]. In addition, plasmon hybridizations of metal colloids have been reported by other authors [12,13,15]. In our case, UV–vis signals showed the existence of chemiadsorption shell of anions. So, the shift of metal particles’ surface energy states induced by adsorbed anions could be considered as energy hybridization effect on metal surface as others [10–15,29]. Therefore, it is postulated that chemiadsorption of IO 3 changed the electronic properties of metal particles, a dipolar layer on the surface being produced and the potential of Fermi level being changed. It is also postulated that electron-transfer (CT) reactions did
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b
a
S2 S'2
S2 S'2
S'1 S1 S'1
S1
352nm
364nm 419nm
S0 467nm
Fig. 6. Energy states hybridization (a) and emission transitions (b) proposed for sliver colloid before and after the addition of IO 3.
occur between surface-modified metal particles and adsorbates, which leads to the hybridization of surface energy states. And because of the hybridization effect, new emission centers assigned to these new energy states occurred. Thus, both absorption and luminescence spectra changed a lot between absence and in the present of IO 3 . The hybridization of energy states and emission transitions process could be sketched by the four-state model of luminescence and be shown in Fig. 6a and b. 4. Conclusion ‘Chemically pure’ Ag colloids were prepared by ablating Ag plate in deionized water using Nd:YAG pulse laser. Then, different concentrations of nucleophilic ions ðIO 3Þ were added into the mixture to detect the interactions and spectral changes of colloidal silver. Gradual changes and new appearances of both absorption and luminescence indicate the chemisorption and hybridization occurred on metal surfaces. It is deduced that chemiadsorption of IO 3 changes the electronic properties of silver surfaces, a dipolar layer on the surface being produced and the potential of Fermi level being changed, which can be seen as hybridization effect between surface energy states. Discussed with classical theory and evidence, hybridization model and an energy transition depiction were proposed to explain the obvious spectra phenomena. Acknowledgements The authors are grateful for the support of this research by the National Natural Science Foundation of China and Natural Science Foundation of Beijing. References [1] R.A. Alvarez-Puebla, E. Arceo, P.J.G. Goulet, J.J. Garrido, R.F. Aroca, J. Phys. Chem. B 109 (2005) 3787.
[2] D.S. dos Santos, P.J.G. Goulet, N.P.W. Pieczonka, O.N. Oliveira, R.F. Aroca, Langmuir 20 (2004) 10273. [3] X.C. Yang, Y. Fang, J. Phys. Chem. B 107 (2003) 10100. [4] R.A. Alvarez-Puebla, D.S. Dos Santos, R.F. Aroca, Analyst 129 (2004) 1251. [5] E.C. Le Ru, M. Meyer, P.G. Etchegion, J. Chem. Phys. B 110 (2006) 1944. [6] H.B. fu, J.N. Yao, J. Am. Chem. Soc. 123 (2001) 1434. [7] R.L. Zong, J. Zhou, Q. Li, B. Du, B. Li, M. Fu, X.W. Qi, L.T. Li, S. Buddhudu, J. Phys. Chem. B 108 (2004) 16713. [8] Y. Fang, J. Chem. Phys. 108 (1998) 4315. [9] L.A. Peyser, A.E. Vinson, A.P. Bartko, R.M. Dickson, Science 291 (2001) 103. [10] E. Prodan, C. radloff, N.J. Halas, P. Nordlander, Science 302 (2003) 419. [11] C.L. Nehl, N.K. Grady, G.P. Goodrich, F. Tam, N.J. Halas, J.H. Hafner, Nano Lett. 4 (2004) 2355. [12] Z.L. Jiang, Z.W. Feng, T.S. Li, F. Li, F.X. Zhong, J.Y. Xie, X.H. Yi, Sci. Chin. B 44 (2001) 175. [13] Y.Q. He, S.P. Liu, L. Kong, Z.F. Liu, Spectrochim. Acta A 61 (2005) 2861. [14] R.F. Pastrnack, P.J. Collings, Science 269 (1995) 935. [15] Z.L. Jiang, W.F. Yuan, H.C. Pan, Spectrochim. Acta A 61 (2005) 2488. _ [16] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607. [17] A.P. Zhang, Y. Fang, Acta Phys. Chim. Sin. 22 (2006) 756. [18] A. Henglein, T. Linert, P. Mulvaney, Ber. Bunsenges. Phys. Chem. 94 (1990) 1449. [19] X.L. Yie, W.J. Deng, E.J. Liang, W. Kerker, Acta Phys. Sin. 47 (1997) 2130. [20] P. Mulvaney, T. Linnert, A. Henglein, J. Phys. Chem. 95 (1991) 7843. [21] C.S. Ah, S.D. Hong, D.J. Jand, J. Phys. Chem. B 105 (2001) 7871. [22] M.S. Sander, R. Gronsky, Y.M. Lin, M.S. Dresselhaus, J. Appl. Phys. 89 (2001) 2733. [23] F.J. Gracia de Abajo, A. Howie, Phye. Rev. B 65 (2002) 115418. [24] M. Maillard, S. Giorgio, M.P. Pileni, J. Phys. Chem. B 107 (2003) 2466. [25] B.M.I. van der Zande, M.R. Bohmer, L.G.J. Fokkink, C. Schonenberger, Langmuir 16 (2000) 451. [26] A. Henglein, J. Phys. Chem. 97 (1993) 5457. [27] M. Kerker, J. Colloid Interf. Sci. 105 (1985) 5. [28] D.S. Tsai, C.H. Chen, C.C. Chou, Mater. Chem. Phys. 90 (2005) 361. [29] E. Zaremba, B.N.J. Persson, Phys. Rev. B 35 (1987) 596. [30] B.N.J. Persson, Surf. Sci. 281 (1993) 153.