- Email: [email protected]
Direct dissolution of Au nanoparticles induced by potassium ferricyanide
Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 207–210
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Brief note
Direct dissolution of Au nanoparticles induced by potassium ferricyanide Junfeng Zhai, Yueming Zhai, Shaojun Dong ∗ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, Jilin, PR China
a r t i c l e
i n f o
Article history: Received 4 June 2008 Received in revised form 6 October 2008 Accepted 15 October 2008 Available online 6 November 2008 Keywords: Potassium ferricyanide Dissolution Prussian blue analogue Au nanoparticles
a b s t r a c t In this work, we studied the reaction between Au nanoparticles (Au NPs) and [Fe(CN)6 ]3− by the UV–vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy. The absorption peak of Au NPs disappeared after adding [Fe(CN)6 ]3− and the XPS data conformed the formation of [Au(CN)2 ]− . The results demonstrated that [Fe(CN)6 ]3− could induce the dissolution of Au NPs, where the CN− from the dissociation of [Fe(CN)6 ]3− played an important role. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Due to the large surface-to-volume ratio, metal nanoparticles have been widely used in catalysis [1,2]. Since reducing the metal particle size to nanoscale will make the surface atoms more active, whether the catalysis takes place at nanoparticle surfaces or in solutions using a complex made by the nanoparticles is still not clear. For revealling the exact mechanism of catalysis, it is necessary to study the reaction between metal nanoparticles and each reactant separately [3,4]. Au nanoparticles (Au NPs) have been used as catalysts in the electron transfer reaction between thiosulfate and hexacyanoferrate III ([Fe(CN)6 ]3− ) and fabrication of Prussian blue (PB) shell on Au NPs [5,6], however, to the best of our knowledge, the reaction between Au NPs and [Fe(CN)6 ]3− has not yet been exploited in detail, which could help us to look insight into the catalytic mechanism and the instability of Au NPs in catalytic reactions. In this work, UV–vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were employed to study the reaction between Au NPs and [Fe(CN)6 ]3− . The results presented here demonstrated that Au NPs can be dissolved in the presence of [Fe(CN)6 ]3− and no Prussian blue analogues were observed as that in the case of Pt nanoparticles [4]. CN− ions resulted from the dissociation of [Fe(CN)6 ]3− are proposed to be responsible for the dissolution of Au NPs and the possible dissolution mechanism was proposed.
2.1. Reagents
∗ Corresponding author. Tel.: +86 431 85262101; fax: +86 431 85689711. E-mail address: [email protected] (S. Dong). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.10.040
Potassium ferricyanide, trisodium citrate dehydrate and hydrogen tetrachloroaurate (HAuCl4 ·3H2 O) were analytic grade and used as received. All chemicals above were obtained from Chinese chemical reagents companies. Sodium borohydride (NaBH4 ), polyvinylpyrrolidone (PVP, Mw = 40,000), poly (sodium 4-styrenesulfonate) (PSS, Mw = 70,000) and poly ethylenimine (PEI, Mw = 25,000) were purchased from Aldrich Chemical Company, Inc. Deionized water was used for all experiments. 2.2. Synthesis of Au NPs All glassware used for colloid synthesis was cleaned in a bath of freshly prepared aqua regia (3:1 HCl:HNO3 ) and rinsed throughout with water before use. Citrate-protected Au NPs were synthesized according to previous work [7], in brief, by heating a 50 mL aqueous solution of 0.01% HAuCl4 to boiling with vigorous stirring and then adding 2.6 mL aqueous solution of 1% sodium citrate, maintaining the mixture for 10 min and cooling at room temperature. PVP-protected Au NPs was synthesized by adding 1 mL 1% sodium citrate to a 50 mL boiling mixture containing 0.0500 g PVP and 0.01% HAuCl4 under vigorous stirring, maintaining the mixture for 10 min and cooling at room temperature. The synthesis procedure for PSS-protected Au NPs is the same as that for PVP-protected Au NPs, except the protecting reagent changed to 0.0520 g PSS.
208
J. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 207–210
PEI-protected Au NPs was synthesized by adding 1 mL 0.075% NaBH4 to a 50 mL mixture of 0.0532 g PEI and 0.01% HAuCl4 under vigorous stirring and maintaining the mixture for 20 min. All the colloid solutions were stored at 4 ◦ C. 2.3. Instruments Samples for TEM were prepared by placing a drop of solution on carbon-coated copper grid and were examined by using a JEOL 2010 transmission electron microscopy operated at 200 kV. X-ray photoelectron spectroscopy measurements (XPS) were carried out on a thermo ESCALAB 250 photoelectron spectrometer with an Al K␣ X-ray radiation as the X-ray source for excitation. UV–vis absorption spectra were recorded on a Cary 500 UV–vis-NIR spectrometer. 3. Results and discussions Noble metal nanoparticles such as Au, Ag and Cu display a strong absorption band in visible range that is not present in spectrum of the bulk metal. Such resonance is attributed to the collective oscillations of surface electrons induced by visible light and often coined as surface plasmon resonance (SPR). Since the optical properties of these metal nanoparticles are strongly dependent on their shapes, sizes and especially sensitive to the surrounding environment [8,9], UV–vis spectrometry is an efficient method to study the reactions of noble metal nanoparticles [10–13]. Here, after the addition of K3 [Fe(CN)6 ], the SPR band of citrate-protected Au NPs centered at 520 nm, showed clearly damping in intensity and completely disappeared after 6 h, indicating the dissolution of Au NPs induced by [Fe(CN)6 ]3− , as shown in Fig. 1. Moreover, it is well known that the insoluble product of surface reactions can be confined on the surfaces where the reactions occur [14,15], for instance, Ag NPs were used to react with CSe2 for fabricating Ag/Ag2 Se core shell nanoparticles [16], where an obvious red shift of Ag SPR band was observed because of the formation of Ag2 Se shell on Ag NPs. Since the dissolution reaction is a surface reaction and no obvious SPR band shift was observed in Fig. 1, the product of the dissolution reaction should be soluble [8,9,17]. It is worth to point that the SPR bands of Au NPs protected respectively by PVP, PEI and PSS also disappeared after adding K3 [Fe(CN)6 ] (see Figs. S1–S3, supporting information), indicating polymer-coated Au NPs can also be effectively dissolved by K3 [Fe(CN)6 ], and the reaction rates are dependent on the polymer used.
Fig. 1. UV–vis absorption spectra of Au NPs (solid) before and after reaction with K3 [Fe(CN)6 ] for about 30 s (dash), 4 h (dot) and 6 h (dash dot), [Au] = 2.4 × 10−4 M, [K3 [Fe(CN)6 ]] = 1.1 × 10−3 M. The inset is the photographs of Au NPs before (left) and after (right) reaction with K3 [Fe(CN)6 ] for 6 h. Au NPs was protected by citrate.
Fig. 2. (a) XPS spectra of Au NPs before (A) and after reaction (B) with K3 [Fe(CN)6 ] for 7 h. (b) XPS spectra of N and Fe on Au NPs surfaces after the reaction with K3 [Fe(CN)6 ] for 1 h.
The chemical states of Au element before and after reacting with K3 [Fe(CN)6 ] were characterized by XPS spectrometry. Au NPs showed Au 4f7/2 and Au 4f5/2 at 82.9 and 86.5 eV, respectively. After the reaction with [Fe(CN)6 ]3− for 7 h, Au 4f7/2 and Au 4f5/2 shifted to 85.2 and 88.8 eV, respectively, as shown in Fig. 2a. Binding energy is known for its sensitivity to the chemical state of element, the positive shift in binding energy of Au thus indicates the oxidation of Au NPs [18,19], which confirms the results of UV–vis spectrometry. The reaction time of 7 h was chosen to ensure the complete reaction of Au NPs with [Fe(CN)6 ]3− . Fig. 2b shows the XPS data of Au NPs reacting with K3 [Fe(CN)6 ] for 3 h and then collected by centrifugation and washed with water for several times. No peaks for Fe2p and N1s were detected on Au NPs surfaces, indicating the product of the reaction is soluble, which is identical with the results of UV–vis spectrometry. Fig. 3 shows the TEM images of Au NPs before and after reaction with K3 Fe(CN)6 for 3 h. The average size of Au NPs decreased from 14 to 12 nm and the size distribution increased after the reaction. The observed decrease in size directly demonstrates the dissolution of Au NPs induced by K3 Fe(CN)6 , The increase in size distribution may result from the different reaction rates of Au NPs with different sizes, as described by Henglein [20]. The excellent stability of Au can be interpreted by its quite positive redox potential and makes it the suitable coinage metal. Because of their ability to form complexes with Au and thus decrease the redox potential of Au [21–23], some nucleophilic ligands, including SCN− , CN− and S2 O3 2− , have been used to dissolve Au. [Fe(CN)6 ]3− , due to its dissociation in solution [24], can actually
J. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 207–210
209
Fig. 3. TEM images of the Au NPs before (A) and after (B) the reaction with K3 [Fe(CN)6 ] for 3 h.
present two different coordinate sites, N and C, and the corresponding ligands are [Fe(CN)6 ]3− and CN− , respectively. It is well known that [Fe(CN)6 ]3− can form PB analogous complexes with transition metals and thus show new absorption bands in the range of 400–800 nm [4,25,26]. Here, if [Fe(CN)6 ]3− is the ligand of Au, a PB analogous complex can thus be expected, however, no new absorption peaks appeared during the reaction, as shown in Fig. 1. Moreover, PB analogous complexes are insoluble, which is in opposition to the results from UV–vis spectrometry and XPS. In contrast, cyanide complex of Au is well known as a soluble complex, and Lyon and co-worker [27] recently reported the dissolution of Au NPs by KCN, where the SPR band showed significant damping in intensity and no new absorption band appeared during the reaction. It can be found that the evolutions in the UV–vis absorption spectra of Au NPs dissolved by K3 [Fe(CN)6 ] and KCN are identical. Therefore, we propose that CN− is the ligand of Au in our system. In order to further confirm our proposal, the evolution in UV–vis spectra of hydrogen tetrachloroaurate (HAuCl4 ) after adding K3 Fe(CN)6 was monitored (Fig. 4). It can be observed that no new absorption band appeared after the addition of [Fe(CN)6 ]3− , indicating that no bond was formed between [Fe(CN)6 ]3− and Au. Namely, [Fe(CN)6 ]3− cannot act as the ligand of Au. Additionally, a slight blue shift in the absorption band of [AuCl4 ]− occurred with increase in time, as shown in Fig. 4. Since the exact maximum absorption positions for Au complexes are related to the ligands [28], the blue shift of the absorption positions may indicate the change in ligand for Au, that is, CN− takes place of Cl− to act as ligand of Au.
Fig. 4. UV–vis spectra of HAuCl4 before (black) and after addition of K3 [Fe(CN)6 ] for 2 min (red), 3 h (blue) and 15 h (green). Spectrum of K3 [Fe(CN)6 ] (pink). [HAuCl4 ] = 1.4 × 10−4 M, [K3 [Fe(CN)6 ]] = 4.0 × 10−4 M. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Two oxidants are actually present in our system. One is the [Fe(CN)6 ]3− ([Fe(CN)6 ]3− + e− = [Fe(CN)6 ]4− , E0 = 0.385 V (vs. SHE)) and the other is the dissolved O2 (O2 + 2H2 O + 4e− = 4OH− , E0 = 0.401 V (vs. SHE)). The reaction between Au NPs and K3 Fe(CN)6 for 3 h were carried out in the presence and absence of oxygen and the corresponding UV–vis spectra are shown in Fig. 5. The damping of SPR band was obvious and the approximate damping factors of 2.21 and 4.44 can be derived in the absence and presence of oxygen, respectively. Considering that the concentration of oxygen and K3 Fe(CN)6 in Au colloid solution is 2.4 × 10−4 and 1 × 10−3 M [29], respectively, it can thus be concluded that there is no evident difference in the kinetics of charge transfer from Au to oxygen and [Fe(CN)6 ]3− in the presence of nucleophilic ligand. Early work reported the redox potential of [Au(CN)4 ]− / [Au(CN)2 ]− lied between 0.53 and 0.59 V [30], which is positive than that of [Fe(CN)6 ]3− /[Fe(CN)6 ]4− and O2 /OH− . Therefore, formation of [Au(CN)2 ]− is efficient in thermodynamics. Moreover, after the reaction with [Fe(CN)6 ]3− for 7 h, the peaks of Au 4f7/2 and Au 4f5/2 appeared at 85.2 and 88.8 eV, respectively, which are identical with that of [Au(CN)2 ]− [31,32]. According to the discussion above, the possible dissolution mechanism of Au NPs induced by K3 Fe(CN)6 can be described by Eqs. (1)–(3) [Fe(CN)6 ]3− = [Fe(CN)6−n ](3−n)− + nCN− −
Au + 2CN + [Fe(CN)6 ]
3−
−
= [Au(CN)2 ] + [Fe(CN)6 ]
(1) 4−
(2)
4Au + 8CN− + O2 + 2H2 O = 4[Au(CN)2 ]− + 4OH−
(3) ]3−
and Eqs. where Eq. (1) represents the dissociation of [Fe(CN)6 (2) and (3) are the dissolution process of Au NPs. It should be mentioned that the Eq. (3) also represents the well known dissolution
Fig. 5. UV–vis absorption spectra of Au NPs before (solid) and after reaction for 3 h with K3 [Fe(CN)6 ] in the absence (dash) and presence (dot) of oxygen, [Au] = 2.4 × 10−4 M, [K3 [Fe(CN)6 ]] = 1.1 × 10−3 M.
210
J. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 207–210
process of Au induced by KCN. It should be pointed that although the [Fe(CN)6 ]3− possesses very low dissociation constant, the reaction (2) and (3) could promote the dissociation of [Fe(CN)6 ]3− . Our results allow us to suppose another possible route to generate PB/Au core shell structures in the mixture of Fe3+ and [Fe(CN)6 ]3− [6], that is, [Fe(CN)6 ]3− is firstly reduced to [Fe(CN)6 ]4− at Au NPs surfaces, which in turn reacts with remained Fe3+ to yield PB. Moreover, in charge transfer reaction between thiosulfate and [Fe(CN)6 ]3− , the instability of Au NPs may be contributed to the reaction between Au NPs and [Fe(CN)6 ]3− , and the [Au(CN)2 ]− may be the intermediate, however, due to the complexity of catalytic reactions, more experiments are needed to demonstrate these supposes. 4. Conclusion The reaction between Au NPs and [Fe(CN)6 ]3− was studied by UV–vis absorption spectroscopy, XPS and TEM. The decreased particle size and absorption peak intensity of Au NPs after adding [Fe(CN)6 ]3− demonstrated that [Fe(CN)6 ]3− could induce the dissolution of Au NPs. XPS data conformed the formation of [Au(CN)2 ]− after the reaction. According to our results, a possible dissolution mechanism was proposed, in brief, CN− from the dissociation of [Fe(CN)6 ]3− acts as the ligand of Au, and the remained [Fe(CN)6 ]3− and oxygen as the oxidants. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 20675076, 20575064). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2008.10.040. References [1] R. Narayanan, M.A. El-Sayed, Effect of catalysis on the stability of metallic nanoparticles: Suzuki reaction catalyzed by PVP-palladium nanoparticles, J. Am. Chem. Soc. 125 (2003) 8340–8347. [2] J.M. Thomas, B.F.G. Johnson, R. Raja, G. Sankar, P.A. Midgley, High-performance nanocatalysts for single-step hydrogenations, Acc. Chem. Res. 36 (2003) 20–30. [3] R. Narayanan, M.A. El-Sayed, Effect of nanocatalysis in colloidal solution on the tetrahedral and cubic nanoparticle shape: electron-transfer reaction catalyzed by platinum nanoparticles, J. Phys. Chem. B 108 (2004) 5726–5733. [4] M.A. Mahmoud, M.A. El-Sayed, Reaction of platinum nanocatalyst with the ferricyanide reactant to produce Prussian blue analogue complexes, J. Phys. Chem. C 111 (2007) 17180–17183. [5] P.L. Freund, M. Spiro, Colloidal catalysis: the effect of sol size and concentration, J. Phys. Chem. 89 (1985) 1074–1077. [6] J.D. Qiu, H.Z. Peng, R.P. Liang, J. Li, X.H. Xia, Synthesis, characterization, and immobilization of Prussian blue-modified au nanoparticles: application to electrocatalytic reduction of H2 O2 , Langmuir 23 (2007) 2133–2137. [7] Y. Jin, Y. Shen, S. Dong, Electrochemical design of ultrathin platinum-coated gold nanoparticle monolayer films as a novel nanostructured electrocatalyst for oxygen reduction, J. Phys. Chem. B 108 (2004) 8142–8147.
[8] L.A. Lyon, D.J. Pena, M.J. Natan, Surface plasmon resonance of au colloidmodified au films: particle size dependence, J. Phys. Chem. B 103 (1999) 5826–5831. [9] A.C. Templeton, J.J. Pietron, R.W. Murray, P. Mulvaney, Solvent refractive index and core charge influences on the surface plasmon absorbance of alkanethiolate monolayer-protected gold clusters, J. Phys. Chem. B 104 (2000) 564–570. [10] D. Eck, C.A. Helm, N.J. Wagner, K.A. Vaynberg, Plasmon resonance measurements of the adsorption and adsorption kinetics of a biopolymer onto gold nanocolloids, Langmuir 17 (2001) 957–960. [11] T. Linnert, P. Mulvaney, A. Henglein, Surface chemistry of colloidal silver: surface plasmon damping by chemisorbed iodide, hydrosulfide (SH-), and phenylthiolate, J. Phys. Chem. 97 (1993) 679–682. [12] S. Underwood, P. Mulvaney, Effect of the solution refractive index on the color of gold colloids, Langmuir 10 (1994) 3427–3430. [13] G.C. Papavassiliou, Optical properties of small inorganic and organic metal particles, Prog. Solid State Chem. 12 (1979) 185–271. [14] J. Gao, B. Zhang, X. Zhang, B. Xu, Magnetic-dipolar-interaction-induced selfassembly affords wires of hollow nanocrystals of cobalt selenide, Angew. Chem. Int. Ed. 45 (2006) 1220–1223. [15] Y.G. Sun, B.T. Mayers, Y.N. Xia, Template-engaged replacement reaction: a onestep approach to the large-scale synthesis of metal nanostructures with hollow interiors, Nano Lett. 2 (2002) 481–485. [16] H. Tan, S. Li, W.Y. Fan, Core-Shell, Hollow nanocrystal formation via small molecule surface photodissociation; Ag/Ag2 Se as an example, J. Phys. Chem. B 110 (2006) 15812–15816. [17] S. Link, M.A. El-Sayed, Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles, J. Phys. Chem. B 103 (1999) 4212–4217. [18] J.C. Fuggle, E. Kallne, L.M. Watson, D.J. Fabian, Electronic structure of aluminum and aluminum-noble-metal alloys studied by soft-X-ray and X-ray photoelectron spectroscopies, Phys. Rev. B 16 (1977) 750. [19] H. Kitagawa, N. Kojima, T. Nakajima, Studies of mixed-valence states in threedimensional halogen-bridged gold compounds, Cs2 AuIAuIIIX6 , (X = Cl, Br or I). Part 2. X-ray photoelectron spectroscopic study, J. Chem. Soc., Dalton Trans. (1991) 3121–3125. [20] A. Henglein, P. Mulvaney, T. Linnert, Chemistry of Agn aggregates in aqueoussolution—nonmetallic oligomeric clusters and metallic particles, Faraday Discuss. (1991) 31–44. [21] T. Pal, N.R. Jana, T.K. Sau, Nucleophile induced dissolution of gold, Corros. Sci. 39 (1997) 981–986. [22] S. Zheng, Y.Y. Wang, L.Y. Chai, Research status and prospect of gold leaching in alkaline thiourea solution, Miner. Eng. 19 (2006) 1301–1306. [23] G. Senanayake, Review of rate constants for thiosulphate leaching of gold from ores, concentrates and flat surfaces: effect of host minerals and pH, Miner. Eng. 20 (2007) 1–15. [24] X. Wu, M. Cao, C. Hu, X. He, Sonochemical synthesis of Prussian blue nanocubes from a single-source precusor, Cryst. Growth Des. 6 (2006) 26–28. [25] M. Yamada, T. Sato, M. Miyake, Y. Kobayashi, Temporal evolution of composition and crystal structure of cobalt hexacyanoferrate nano-polymers synthesized in reversed micelles, J. Colloid Interf. Sci. 315 (2007) 369–375. [26] A.P. Baioni, M. Vidotti, P.A. Fiorito, E.A. Ponzio, S.I.C. de Torresi, Synthesis and characterization of copper hexacyanoferrate nanoparticles for building up long-term stability electrochromic electrodes, Langmuir 23 (2007) 6796–6800. [27] N. Singh, L.A. Lyon, Au nanoparticle templated synthesis of pNIPAm nanogels, Chem. Mater. 19 (2007) 719–726. [28] A.K. Gangopadhayay, A. Chakravorty, Charge transfer spectra of some gold(III) complexes, J. Chem. Phys. 35 (1961) 2206–2209. [29] M. Huang, Y. Shao, X. Sun, H. Chen, B. Liu, S. Dong, Alternate assemblies of platinum nanoparticles and metalloporphyrins as tunable electrocatalysts for dioxygen reduction, Langmuir 21 (2005) 323–329. [30] A.E. Remick, Studies on oxidation–reduction mechanism. I. Potential humps and the equivalence change principle, J. Am. Chem. Soc. 69 (1947) 94–95. [31] A. Warshawsky, N. Kahana, V. Kampel, I. Rogachev, R.M. Kautzmann, J.L. Cortina, C.H. Sampaio, Ion exchange resins for gold cyanide extraction containing a piperazine functionality, 2 study of the gold extraction reaction, Macromol. Mater. Eng. 286 (2001) 285–295. [32] D. Jermakowicz-Bartkowiak, B.N. Kolarz, W. Tylus, Sorption of aurocyanide and tetrachloroaurate onto resin with guanidine ligand—an XPS approach, Polymer 44 (2003) 5797–5802.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Brief note
Direct dissolution of Au nanoparticles induced by potassium ferricyanide Junfeng Zhai, Yueming Zhai, Shaojun Dong ∗ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, Jilin, PR China
a r t i c l e
i n f o
Article history: Received 4 June 2008 Received in revised form 6 October 2008 Accepted 15 October 2008 Available online 6 November 2008 Keywords: Potassium ferricyanide Dissolution Prussian blue analogue Au nanoparticles
a b s t r a c t In this work, we studied the reaction between Au nanoparticles (Au NPs) and [Fe(CN)6 ]3− by the UV–vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy. The absorption peak of Au NPs disappeared after adding [Fe(CN)6 ]3− and the XPS data conformed the formation of [Au(CN)2 ]− . The results demonstrated that [Fe(CN)6 ]3− could induce the dissolution of Au NPs, where the CN− from the dissociation of [Fe(CN)6 ]3− played an important role. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Due to the large surface-to-volume ratio, metal nanoparticles have been widely used in catalysis [1,2]. Since reducing the metal particle size to nanoscale will make the surface atoms more active, whether the catalysis takes place at nanoparticle surfaces or in solutions using a complex made by the nanoparticles is still not clear. For revealling the exact mechanism of catalysis, it is necessary to study the reaction between metal nanoparticles and each reactant separately [3,4]. Au nanoparticles (Au NPs) have been used as catalysts in the electron transfer reaction between thiosulfate and hexacyanoferrate III ([Fe(CN)6 ]3− ) and fabrication of Prussian blue (PB) shell on Au NPs [5,6], however, to the best of our knowledge, the reaction between Au NPs and [Fe(CN)6 ]3− has not yet been exploited in detail, which could help us to look insight into the catalytic mechanism and the instability of Au NPs in catalytic reactions. In this work, UV–vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were employed to study the reaction between Au NPs and [Fe(CN)6 ]3− . The results presented here demonstrated that Au NPs can be dissolved in the presence of [Fe(CN)6 ]3− and no Prussian blue analogues were observed as that in the case of Pt nanoparticles [4]. CN− ions resulted from the dissociation of [Fe(CN)6 ]3− are proposed to be responsible for the dissolution of Au NPs and the possible dissolution mechanism was proposed.
2.1. Reagents
∗ Corresponding author. Tel.: +86 431 85262101; fax: +86 431 85689711. E-mail address: [email protected] (S. Dong). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.10.040
Potassium ferricyanide, trisodium citrate dehydrate and hydrogen tetrachloroaurate (HAuCl4 ·3H2 O) were analytic grade and used as received. All chemicals above were obtained from Chinese chemical reagents companies. Sodium borohydride (NaBH4 ), polyvinylpyrrolidone (PVP, Mw = 40,000), poly (sodium 4-styrenesulfonate) (PSS, Mw = 70,000) and poly ethylenimine (PEI, Mw = 25,000) were purchased from Aldrich Chemical Company, Inc. Deionized water was used for all experiments. 2.2. Synthesis of Au NPs All glassware used for colloid synthesis was cleaned in a bath of freshly prepared aqua regia (3:1 HCl:HNO3 ) and rinsed throughout with water before use. Citrate-protected Au NPs were synthesized according to previous work [7], in brief, by heating a 50 mL aqueous solution of 0.01% HAuCl4 to boiling with vigorous stirring and then adding 2.6 mL aqueous solution of 1% sodium citrate, maintaining the mixture for 10 min and cooling at room temperature. PVP-protected Au NPs was synthesized by adding 1 mL 1% sodium citrate to a 50 mL boiling mixture containing 0.0500 g PVP and 0.01% HAuCl4 under vigorous stirring, maintaining the mixture for 10 min and cooling at room temperature. The synthesis procedure for PSS-protected Au NPs is the same as that for PVP-protected Au NPs, except the protecting reagent changed to 0.0520 g PSS.
208
J. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 207–210
PEI-protected Au NPs was synthesized by adding 1 mL 0.075% NaBH4 to a 50 mL mixture of 0.0532 g PEI and 0.01% HAuCl4 under vigorous stirring and maintaining the mixture for 20 min. All the colloid solutions were stored at 4 ◦ C. 2.3. Instruments Samples for TEM were prepared by placing a drop of solution on carbon-coated copper grid and were examined by using a JEOL 2010 transmission electron microscopy operated at 200 kV. X-ray photoelectron spectroscopy measurements (XPS) were carried out on a thermo ESCALAB 250 photoelectron spectrometer with an Al K␣ X-ray radiation as the X-ray source for excitation. UV–vis absorption spectra were recorded on a Cary 500 UV–vis-NIR spectrometer. 3. Results and discussions Noble metal nanoparticles such as Au, Ag and Cu display a strong absorption band in visible range that is not present in spectrum of the bulk metal. Such resonance is attributed to the collective oscillations of surface electrons induced by visible light and often coined as surface plasmon resonance (SPR). Since the optical properties of these metal nanoparticles are strongly dependent on their shapes, sizes and especially sensitive to the surrounding environment [8,9], UV–vis spectrometry is an efficient method to study the reactions of noble metal nanoparticles [10–13]. Here, after the addition of K3 [Fe(CN)6 ], the SPR band of citrate-protected Au NPs centered at 520 nm, showed clearly damping in intensity and completely disappeared after 6 h, indicating the dissolution of Au NPs induced by [Fe(CN)6 ]3− , as shown in Fig. 1. Moreover, it is well known that the insoluble product of surface reactions can be confined on the surfaces where the reactions occur [14,15], for instance, Ag NPs were used to react with CSe2 for fabricating Ag/Ag2 Se core shell nanoparticles [16], where an obvious red shift of Ag SPR band was observed because of the formation of Ag2 Se shell on Ag NPs. Since the dissolution reaction is a surface reaction and no obvious SPR band shift was observed in Fig. 1, the product of the dissolution reaction should be soluble [8,9,17]. It is worth to point that the SPR bands of Au NPs protected respectively by PVP, PEI and PSS also disappeared after adding K3 [Fe(CN)6 ] (see Figs. S1–S3, supporting information), indicating polymer-coated Au NPs can also be effectively dissolved by K3 [Fe(CN)6 ], and the reaction rates are dependent on the polymer used.
Fig. 1. UV–vis absorption spectra of Au NPs (solid) before and after reaction with K3 [Fe(CN)6 ] for about 30 s (dash), 4 h (dot) and 6 h (dash dot), [Au] = 2.4 × 10−4 M, [K3 [Fe(CN)6 ]] = 1.1 × 10−3 M. The inset is the photographs of Au NPs before (left) and after (right) reaction with K3 [Fe(CN)6 ] for 6 h. Au NPs was protected by citrate.
Fig. 2. (a) XPS spectra of Au NPs before (A) and after reaction (B) with K3 [Fe(CN)6 ] for 7 h. (b) XPS spectra of N and Fe on Au NPs surfaces after the reaction with K3 [Fe(CN)6 ] for 1 h.
The chemical states of Au element before and after reacting with K3 [Fe(CN)6 ] were characterized by XPS spectrometry. Au NPs showed Au 4f7/2 and Au 4f5/2 at 82.9 and 86.5 eV, respectively. After the reaction with [Fe(CN)6 ]3− for 7 h, Au 4f7/2 and Au 4f5/2 shifted to 85.2 and 88.8 eV, respectively, as shown in Fig. 2a. Binding energy is known for its sensitivity to the chemical state of element, the positive shift in binding energy of Au thus indicates the oxidation of Au NPs [18,19], which confirms the results of UV–vis spectrometry. The reaction time of 7 h was chosen to ensure the complete reaction of Au NPs with [Fe(CN)6 ]3− . Fig. 2b shows the XPS data of Au NPs reacting with K3 [Fe(CN)6 ] for 3 h and then collected by centrifugation and washed with water for several times. No peaks for Fe2p and N1s were detected on Au NPs surfaces, indicating the product of the reaction is soluble, which is identical with the results of UV–vis spectrometry. Fig. 3 shows the TEM images of Au NPs before and after reaction with K3 Fe(CN)6 for 3 h. The average size of Au NPs decreased from 14 to 12 nm and the size distribution increased after the reaction. The observed decrease in size directly demonstrates the dissolution of Au NPs induced by K3 Fe(CN)6 , The increase in size distribution may result from the different reaction rates of Au NPs with different sizes, as described by Henglein [20]. The excellent stability of Au can be interpreted by its quite positive redox potential and makes it the suitable coinage metal. Because of their ability to form complexes with Au and thus decrease the redox potential of Au [21–23], some nucleophilic ligands, including SCN− , CN− and S2 O3 2− , have been used to dissolve Au. [Fe(CN)6 ]3− , due to its dissociation in solution [24], can actually
J. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 207–210
209
Fig. 3. TEM images of the Au NPs before (A) and after (B) the reaction with K3 [Fe(CN)6 ] for 3 h.
present two different coordinate sites, N and C, and the corresponding ligands are [Fe(CN)6 ]3− and CN− , respectively. It is well known that [Fe(CN)6 ]3− can form PB analogous complexes with transition metals and thus show new absorption bands in the range of 400–800 nm [4,25,26]. Here, if [Fe(CN)6 ]3− is the ligand of Au, a PB analogous complex can thus be expected, however, no new absorption peaks appeared during the reaction, as shown in Fig. 1. Moreover, PB analogous complexes are insoluble, which is in opposition to the results from UV–vis spectrometry and XPS. In contrast, cyanide complex of Au is well known as a soluble complex, and Lyon and co-worker [27] recently reported the dissolution of Au NPs by KCN, where the SPR band showed significant damping in intensity and no new absorption band appeared during the reaction. It can be found that the evolutions in the UV–vis absorption spectra of Au NPs dissolved by K3 [Fe(CN)6 ] and KCN are identical. Therefore, we propose that CN− is the ligand of Au in our system. In order to further confirm our proposal, the evolution in UV–vis spectra of hydrogen tetrachloroaurate (HAuCl4 ) after adding K3 Fe(CN)6 was monitored (Fig. 4). It can be observed that no new absorption band appeared after the addition of [Fe(CN)6 ]3− , indicating that no bond was formed between [Fe(CN)6 ]3− and Au. Namely, [Fe(CN)6 ]3− cannot act as the ligand of Au. Additionally, a slight blue shift in the absorption band of [AuCl4 ]− occurred with increase in time, as shown in Fig. 4. Since the exact maximum absorption positions for Au complexes are related to the ligands [28], the blue shift of the absorption positions may indicate the change in ligand for Au, that is, CN− takes place of Cl− to act as ligand of Au.
Fig. 4. UV–vis spectra of HAuCl4 before (black) and after addition of K3 [Fe(CN)6 ] for 2 min (red), 3 h (blue) and 15 h (green). Spectrum of K3 [Fe(CN)6 ] (pink). [HAuCl4 ] = 1.4 × 10−4 M, [K3 [Fe(CN)6 ]] = 4.0 × 10−4 M. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Two oxidants are actually present in our system. One is the [Fe(CN)6 ]3− ([Fe(CN)6 ]3− + e− = [Fe(CN)6 ]4− , E0 = 0.385 V (vs. SHE)) and the other is the dissolved O2 (O2 + 2H2 O + 4e− = 4OH− , E0 = 0.401 V (vs. SHE)). The reaction between Au NPs and K3 Fe(CN)6 for 3 h were carried out in the presence and absence of oxygen and the corresponding UV–vis spectra are shown in Fig. 5. The damping of SPR band was obvious and the approximate damping factors of 2.21 and 4.44 can be derived in the absence and presence of oxygen, respectively. Considering that the concentration of oxygen and K3 Fe(CN)6 in Au colloid solution is 2.4 × 10−4 and 1 × 10−3 M [29], respectively, it can thus be concluded that there is no evident difference in the kinetics of charge transfer from Au to oxygen and [Fe(CN)6 ]3− in the presence of nucleophilic ligand. Early work reported the redox potential of [Au(CN)4 ]− / [Au(CN)2 ]− lied between 0.53 and 0.59 V [30], which is positive than that of [Fe(CN)6 ]3− /[Fe(CN)6 ]4− and O2 /OH− . Therefore, formation of [Au(CN)2 ]− is efficient in thermodynamics. Moreover, after the reaction with [Fe(CN)6 ]3− for 7 h, the peaks of Au 4f7/2 and Au 4f5/2 appeared at 85.2 and 88.8 eV, respectively, which are identical with that of [Au(CN)2 ]− [31,32]. According to the discussion above, the possible dissolution mechanism of Au NPs induced by K3 Fe(CN)6 can be described by Eqs. (1)–(3) [Fe(CN)6 ]3− = [Fe(CN)6−n ](3−n)− + nCN− −
Au + 2CN + [Fe(CN)6 ]
3−
−
= [Au(CN)2 ] + [Fe(CN)6 ]
(1) 4−
(2)
4Au + 8CN− + O2 + 2H2 O = 4[Au(CN)2 ]− + 4OH−
(3) ]3−
and Eqs. where Eq. (1) represents the dissociation of [Fe(CN)6 (2) and (3) are the dissolution process of Au NPs. It should be mentioned that the Eq. (3) also represents the well known dissolution
Fig. 5. UV–vis absorption spectra of Au NPs before (solid) and after reaction for 3 h with K3 [Fe(CN)6 ] in the absence (dash) and presence (dot) of oxygen, [Au] = 2.4 × 10−4 M, [K3 [Fe(CN)6 ]] = 1.1 × 10−3 M.
210
J. Zhai et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 335 (2009) 207–210
process of Au induced by KCN. It should be pointed that although the [Fe(CN)6 ]3− possesses very low dissociation constant, the reaction (2) and (3) could promote the dissociation of [Fe(CN)6 ]3− . Our results allow us to suppose another possible route to generate PB/Au core shell structures in the mixture of Fe3+ and [Fe(CN)6 ]3− [6], that is, [Fe(CN)6 ]3− is firstly reduced to [Fe(CN)6 ]4− at Au NPs surfaces, which in turn reacts with remained Fe3+ to yield PB. Moreover, in charge transfer reaction between thiosulfate and [Fe(CN)6 ]3− , the instability of Au NPs may be contributed to the reaction between Au NPs and [Fe(CN)6 ]3− , and the [Au(CN)2 ]− may be the intermediate, however, due to the complexity of catalytic reactions, more experiments are needed to demonstrate these supposes. 4. Conclusion The reaction between Au NPs and [Fe(CN)6 ]3− was studied by UV–vis absorption spectroscopy, XPS and TEM. The decreased particle size and absorption peak intensity of Au NPs after adding [Fe(CN)6 ]3− demonstrated that [Fe(CN)6 ]3− could induce the dissolution of Au NPs. XPS data conformed the formation of [Au(CN)2 ]− after the reaction. According to our results, a possible dissolution mechanism was proposed, in brief, CN− from the dissociation of [Fe(CN)6 ]3− acts as the ligand of Au, and the remained [Fe(CN)6 ]3− and oxygen as the oxidants. Acknowledgments This work was supported by National Natural Science Foundation of China (Nos. 20675076, 20575064). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2008.10.040. References [1] R. Narayanan, M.A. El-Sayed, Effect of catalysis on the stability of metallic nanoparticles: Suzuki reaction catalyzed by PVP-palladium nanoparticles, J. Am. Chem. Soc. 125 (2003) 8340–8347. [2] J.M. Thomas, B.F.G. Johnson, R. Raja, G. Sankar, P.A. Midgley, High-performance nanocatalysts for single-step hydrogenations, Acc. Chem. Res. 36 (2003) 20–30. [3] R. Narayanan, M.A. El-Sayed, Effect of nanocatalysis in colloidal solution on the tetrahedral and cubic nanoparticle shape: electron-transfer reaction catalyzed by platinum nanoparticles, J. Phys. Chem. B 108 (2004) 5726–5733. [4] M.A. Mahmoud, M.A. El-Sayed, Reaction of platinum nanocatalyst with the ferricyanide reactant to produce Prussian blue analogue complexes, J. Phys. Chem. C 111 (2007) 17180–17183. [5] P.L. Freund, M. Spiro, Colloidal catalysis: the effect of sol size and concentration, J. Phys. Chem. 89 (1985) 1074–1077. [6] J.D. Qiu, H.Z. Peng, R.P. Liang, J. Li, X.H. Xia, Synthesis, characterization, and immobilization of Prussian blue-modified au nanoparticles: application to electrocatalytic reduction of H2 O2 , Langmuir 23 (2007) 2133–2137. [7] Y. Jin, Y. Shen, S. Dong, Electrochemical design of ultrathin platinum-coated gold nanoparticle monolayer films as a novel nanostructured electrocatalyst for oxygen reduction, J. Phys. Chem. B 108 (2004) 8142–8147.
[8] L.A. Lyon, D.J. Pena, M.J. Natan, Surface plasmon resonance of au colloidmodified au films: particle size dependence, J. Phys. Chem. B 103 (1999) 5826–5831. [9] A.C. Templeton, J.J. Pietron, R.W. Murray, P. Mulvaney, Solvent refractive index and core charge influences on the surface plasmon absorbance of alkanethiolate monolayer-protected gold clusters, J. Phys. Chem. B 104 (2000) 564–570. [10] D. Eck, C.A. Helm, N.J. Wagner, K.A. Vaynberg, Plasmon resonance measurements of the adsorption and adsorption kinetics of a biopolymer onto gold nanocolloids, Langmuir 17 (2001) 957–960. [11] T. Linnert, P. Mulvaney, A. Henglein, Surface chemistry of colloidal silver: surface plasmon damping by chemisorbed iodide, hydrosulfide (SH-), and phenylthiolate, J. Phys. Chem. 97 (1993) 679–682. [12] S. Underwood, P. Mulvaney, Effect of the solution refractive index on the color of gold colloids, Langmuir 10 (1994) 3427–3430. [13] G.C. Papavassiliou, Optical properties of small inorganic and organic metal particles, Prog. Solid State Chem. 12 (1979) 185–271. [14] J. Gao, B. Zhang, X. Zhang, B. Xu, Magnetic-dipolar-interaction-induced selfassembly affords wires of hollow nanocrystals of cobalt selenide, Angew. Chem. Int. Ed. 45 (2006) 1220–1223. [15] Y.G. Sun, B.T. Mayers, Y.N. Xia, Template-engaged replacement reaction: a onestep approach to the large-scale synthesis of metal nanostructures with hollow interiors, Nano Lett. 2 (2002) 481–485. [16] H. Tan, S. Li, W.Y. Fan, Core-Shell, Hollow nanocrystal formation via small molecule surface photodissociation; Ag/Ag2 Se as an example, J. Phys. Chem. B 110 (2006) 15812–15816. [17] S. Link, M.A. El-Sayed, Size and temperature dependence of the plasmon absorption of colloidal gold nanoparticles, J. Phys. Chem. B 103 (1999) 4212–4217. [18] J.C. Fuggle, E. Kallne, L.M. Watson, D.J. Fabian, Electronic structure of aluminum and aluminum-noble-metal alloys studied by soft-X-ray and X-ray photoelectron spectroscopies, Phys. Rev. B 16 (1977) 750. [19] H. Kitagawa, N. Kojima, T. Nakajima, Studies of mixed-valence states in threedimensional halogen-bridged gold compounds, Cs2 AuIAuIIIX6 , (X = Cl, Br or I). Part 2. X-ray photoelectron spectroscopic study, J. Chem. Soc., Dalton Trans. (1991) 3121–3125. [20] A. Henglein, P. Mulvaney, T. Linnert, Chemistry of Agn aggregates in aqueoussolution—nonmetallic oligomeric clusters and metallic particles, Faraday Discuss. (1991) 31–44. [21] T. Pal, N.R. Jana, T.K. Sau, Nucleophile induced dissolution of gold, Corros. Sci. 39 (1997) 981–986. [22] S. Zheng, Y.Y. Wang, L.Y. Chai, Research status and prospect of gold leaching in alkaline thiourea solution, Miner. Eng. 19 (2006) 1301–1306. [23] G. Senanayake, Review of rate constants for thiosulphate leaching of gold from ores, concentrates and flat surfaces: effect of host minerals and pH, Miner. Eng. 20 (2007) 1–15. [24] X. Wu, M. Cao, C. Hu, X. He, Sonochemical synthesis of Prussian blue nanocubes from a single-source precusor, Cryst. Growth Des. 6 (2006) 26–28. [25] M. Yamada, T. Sato, M. Miyake, Y. Kobayashi, Temporal evolution of composition and crystal structure of cobalt hexacyanoferrate nano-polymers synthesized in reversed micelles, J. Colloid Interf. Sci. 315 (2007) 369–375. [26] A.P. Baioni, M. Vidotti, P.A. Fiorito, E.A. Ponzio, S.I.C. de Torresi, Synthesis and characterization of copper hexacyanoferrate nanoparticles for building up long-term stability electrochromic electrodes, Langmuir 23 (2007) 6796–6800. [27] N. Singh, L.A. Lyon, Au nanoparticle templated synthesis of pNIPAm nanogels, Chem. Mater. 19 (2007) 719–726. [28] A.K. Gangopadhayay, A. Chakravorty, Charge transfer spectra of some gold(III) complexes, J. Chem. Phys. 35 (1961) 2206–2209. [29] M. Huang, Y. Shao, X. Sun, H. Chen, B. Liu, S. Dong, Alternate assemblies of platinum nanoparticles and metalloporphyrins as tunable electrocatalysts for dioxygen reduction, Langmuir 21 (2005) 323–329. [30] A.E. Remick, Studies on oxidation–reduction mechanism. I. Potential humps and the equivalence change principle, J. Am. Chem. Soc. 69 (1947) 94–95. [31] A. Warshawsky, N. Kahana, V. Kampel, I. Rogachev, R.M. Kautzmann, J.L. Cortina, C.H. Sampaio, Ion exchange resins for gold cyanide extraction containing a piperazine functionality, 2 study of the gold extraction reaction, Macromol. Mater. Eng. 286 (2001) 285–295. [32] D. Jermakowicz-Bartkowiak, B.N. Kolarz, W. Tylus, Sorption of aurocyanide and tetrachloroaurate onto resin with guanidine ligand—an XPS approach, Polymer 44 (2003) 5797–5802.