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A simple microwave-based route for size-controlled preparation of colloidal Pt nanoparticles
Materials Letters 61 (2007) 1873 – 1875 www.elsevier.com/locate/matlet
A simple microwave-based route for size-controlled preparation of colloidal Pt nanoparticles Yonglan Luo School of Chemistry and Chemical Industry, China West Normal University, Nanchong 637002, Sichuan, PR China Received 20 May 2006; accepted 23 July 2006 Available online 17 August 2006
Abstract A simple route for the preparation of colloidal Pt nanoparticles is demonstrated. The formation of such colloids occurs in a single-step process, involving the direct microwave-based heat-treatment of an aqueous solution containing H2PtCl6 and 3-thiophenemalonic acid. In the synthesis, the particle size can be controlled by the molar ratio of the reactant agents. © 2006 Elsevier B.V. All rights reserved. Keywords: Microwave; Pt nanoparticles; Size control
1. Introduction Colloidal metal nanoparticles are of great interest because they can be used not only as catalysts [1], photocatalysts [2], sensors [3] and ferrofluids [4], but they also can be used in the fields of optical and electronic [5] as well as in magnetic devices [3]. Such nanoparticles also hold promise for use as advanced materials with novel properties and have potential applications in the fields of physics, chemistry, biology, medicine, material science and their different interdisciplinary fields [6]. As a result, considerable attention has been paid to the synthesis and characterization of colloidal metal nanoparticles [7]. Up to now, a great deal of methods for the preparation of metal nanoparticles have been developed [8]. However, nanoparticles tend to be fairly unstable in solution and therefore, special precautions have to be taken to avoid their aggregation or precipitation during the preparation of such colloidal particles in solution. To obtain stable colloids, the most effective and common strategy is the introduction of a protective agent in the reaction system [9]. Microwave chemistry has experienced an exponential growth rate, both in the industry and academia. Microwave irradiation as a fast, simple, and efficient heating method has
E-mail address: [email protected]. 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.166
been widely used in chemistry since 1986 [10,11]. More recently, Liao has successful prepared polyelectrolyte-protected gold nanoparticles by microwave-based thermal method, carried out by heating a polyelectrolyte–HAuCl4 aqueous solution [12]. However, to the best of my knowledge, there is no paper reporting on the one-step preparation of size-controlled colloidal Pt nanoparticles via a microwave-based thermal route. In this letter, I develop a simple route for the preparation of colloidal Pt nanoparticles in a single-step process, involving the direct heat-treatment of an aqueous solution containing H2PtCl6 and 3-thiophenemalonic acid in a microwave oven for several minutes. It is also found that the particle size can be controlled by the molar ratio of 3-thiophenemalonic acid to H2PtCl6. 2. Experimental procedures H2PtCl6 and 3-thiophenemalonic acid were purchased from Aldrich. All reagents were used as received without further purification. The water used was purified through a Millipore system. Three colloidal Pt samples were prepared as follows: Firstly, 200 μL of 0.06 M H2PtCl6 aqueous solution was diluted to 10 mL by the addition of water, and then 0.12 M 3thiophenemalonic acid aqueous solution was introduced into such solution with molar ratios of 2.5:1, 5:1, and 10:1 of 3thiophenemalonic acid to H2PtCl6 (corresponding to samples 1, 2 and 3, respectively). The resulting solution was irradiated in a
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Y. Luo / Materials Letters 61 (2007) 1873–1875
Fig. 1. Typical TEM images of Pt nanoparticles obtained with molar ratios of (A) 2.5:1, (B) 5:1 and (C) 10:1 of 3-thiophenemalonic acid to H2PtCl6.
domestic microwave operated at 100% power of 300 W for 8 min. The as-prepared colloidal solutions were stored at room temperature and used for further characterization. UV–vis experiments were carried on a CARY 500 Scan UV–vis–near infrared (UV–vis–NIR) spectrophotometer. Samples for TEM characterization were prepared by placing a drop of colloidal Pt solution on a carbon coated copper grid and dried at room temperature. TEM measurements were made on a JEOL 2010 transmission electron microscopy operated at an accelerating voltage of 200 kV. X-ray photoelectron spectrum (XPS) was acquired on an ESCLAB MKII using Mg as the exciting source. 3. Results and discussion The formation of Pt nanoparticles was further confirmed by TEM. Fig. 1 shows typical TEM images of three colloidal Pt samples thus formed. When the molar ratio of 3-thiophenemalonic acid to H2PtCl6 is 2.5:1, it is found that the Pt nanoparticles mainly consist of particles with a diameter of ∼3 nm and some large particles with a diameter of ∼ 5 nm are also observed, as shown in Fig. 1A. When the molar ratio is increased to 5:1, it is clearly seen that most of the Pt particles are ∼ 2 nm in diameter, as shown in Fig. 1B. When the molar ratio is further increased to 10:1, the Pt nanoparticles further become smaller (∼ 1–1.5 nm in diameter), as shown in Fig. 1C. Obviously, the size of such colloidal Pt nanoparticles can be controlled by the molar ratio of
Fig. 2. UV–vis absorption spectra indicative of Pt nanoparticles formation as a function of time in a 3-thiophenemalonic acid–H2PtCl6 aqueous solution with an initial molar ratio of 2.5:1 of 3-thiophenemalonic acid to H2PtCl6 under microwave irradiation with a time interval of 1 min.
the reactants and increasing the molar ratio of 3-thiophenemalonic acid to H2PtCl6 results in decreasing particle size. The formation of Pt nanoparticles can be attributed to the direct redox between 3-thiophenemalonic acid and H2PtCl6, because there are no other reducing reagents in the system. In this study, Pt nanoparticles cannot form after a 3-thiophenemalonic acid–H2PtCl6 aqueous solution has been stored for several months at room temperature in the presence of microwave irradiation, indicating that microwave irradiation can considerably accelerate chemical processes. It can be attributed to the fact that microwave irradiation is a fast, simple and efficient heating method and therefore, rapidly increases the temperature of the solution. As a result, the electron transfer (ET) rate from Pt (IV) to 3-thiophenemalonic acid is accelerated markedly (roughly doubles the reaction rate by every 10 K). The formation process of Pt nanoparticles can be explained as follows: During the microwave irradiation, H2PtCl6 is reduced by 3-thiophenemalonic acid to form Pt atoms first. With the elapsed time, new Pt atoms are generated in this system and nucleation occurs as the concentration of Pt atoms reaches a critical supersaturation, resulting in the formation of nuclei. The nuclei grow to nanoscale particles by further addition of free Pt atoms in the solution. It is also worthwhile mentioning that such colloidal Pt nanoparticles are quite stable, which indicates that 3-thiophenemalonic acid serves as an effective protective agent for Pt nanoparticles. This observation may be attributed to that the sulfur atom in 3thiophenemalonic acid has a very strong nucleophilicity with lone-pair electrons and such lone-pair electron can form a type of donor–
Fig. 3. XPS spectra of the precipitate obtained from centrifuging colloidal Pt sample 1.
Y. Luo / Materials Letters 61 (2007) 1873–1875
acceptor complex with the Pt atom on the particle surface, giving 3thiophenemalonic acid-protected Pt nanoparticles. The increase of the molar ratio of 3-thiophenemalonic acid to H2PtCl6 results in decreasing particle size which can be attributed to the fact that a higher molar ratio increases the number of 3-thiophenemalonic acid molecules available to interact with Pt nanoparticles and thus a more effective protective ability for such Pt particles can be expected [13]. The formation process of Pt nanoparticles was further traced by the UV–vis absorption method. Fig. 2 shows the time-dependent absorption spectra collected during the whole microwave irradiation process with a time interval of 1 min. With the elapsed time, the peak at 257 nm gradually disappears, which can be attributed to the gradual reduction of PtCl2− 6 by 3-thiophenemalonic acid. When the solution is irradiated for 8 min, this peak completely disappears and a spectrum that has absorption in all ranges of the UV–vis spectrum and at the same time, the absorption increases gradually with the decrease of wavelength, is observed. In addition, further microwave irradiation gives no change of such spectra. These observations demonstrate that the PtCl2− 6 ions in the solution have been totally reduced to form Pt nanoparticles [14]. To further confirm the formation of metal Pt from H2PtCl6, XPS was used to identify the change in oxidation states for Pt after the microwave irradiation reaction had occurred. Fig. 3 shows the XPS spectra of the precipitate obtained from centrifuging colloidal Pt sample 1. It is reported that the 4f7/2 and 4f5/2 binding energies for polycrystalline Pt are at 71.3 and 74.7 eV, respectively [15]. However, the 4f7/2 and 4f5/2 binding energies for the Pt nanoparticles prepared in this present study are at 72.0 and 75.2 eV, respectively, which are higher than those reported values [15]. This shift may be attributed to the formation of a donor–acceptor complex of the sulfur atom in 3thiophenemalonic acid with the Pt atom on the particle surface. It is worthwhile mentioning that further adding a strong reducing reagent such as NaBH4 into the colloidal solution thus formed cannot lead to further change of both the solution color and corresponding UV–vis spectra, indicating that the Pt salts have been completely reduced by 3thiophenemalonic acid.
4. Conclusion Radiating an aqueous solution containing 3-thiophenemalonic acid and H2PtCl6 in a microwave oven for several minutes
1875
results in the rapid formation of stable, 3-thiophenemalonic acidprotected colloidal Pt nanoparticles. It is found that the particle size can be controlled by the molar ratio of 3-thiophenemalonic acid to H2PtCl6. It not only provides a simple and rapid methodology for the preparation of Pt nanoparticles, but also can be used as a general strategy for the one-step preparation of 3thiophenemalonic acid-protected noble metal nanoparticles for applications. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15]
H. Hirai, H. Wakabayashi, M. Komiyama, Chem. Lett. 1983 (1983) 1047. A.P. Brugger, P. Cuendet, M. Gratzel, J. Am. Chem. Soc. 103 (1981) 2923. J.M. Thomas, Pure Appl. Chem. 60 (1988) 1517. E.P. Wohlharth (Ed.), Ferromagnetic Materials, vol. 2, North-Holland, Amsterdam, 1980. G. Schon, U. Simon, Colloid Polym. Sci. 273 (1995) 202. J.H. Fendler (Ed.), Nanoparticles and Nanostructured Films, VCH, Weinheim, 1998. (a) J. Kiwi, M. Gratzel, J. Am. Chem. Soc. 101 (1979) 7214; (b) G. Schmid, Chem. Rev. 92 (1992) 1709; M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc., Chem. Commun. 801 (1994). A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 102 (2002) 3757. N.R. Rao, G.U. Kulkarni, P.J. Thomas, P.P. Edwards, Chem. Soc. Rev. 29 (2000) 27. R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge, J. Rousell, Tetrahedron Lett. 27 (1986) 279. X. Xu, W. Yang, J. Liu, L. Lin, Adv. Mater. 12 (2000) 195. F. Liao, Chem. Lett. 33 (2004) 1020. R.M. Crooks, M. Zhao, L. Sun, V. Chechik, L.K. Yeung, Acc. Chem. Res. 34 (2001) 181. T. Teranishi, M. Hosoe, T. Tanaka, M. Miyake, J. Phys. Chem., B 103 (1999) 3818. E. Drawdy, G.B. Hoflund, S.D. Gardner, E. Yngvadottir, D.R. Schryer, Surf. Interface Anal. 16 (1990) 369.
A simple microwave-based route for size-controlled preparation of colloidal Pt nanoparticles Yonglan Luo School of Chemistry and Chemical Industry, China West Normal University, Nanchong 637002, Sichuan, PR China Received 20 May 2006; accepted 23 July 2006 Available online 17 August 2006
Abstract A simple route for the preparation of colloidal Pt nanoparticles is demonstrated. The formation of such colloids occurs in a single-step process, involving the direct microwave-based heat-treatment of an aqueous solution containing H2PtCl6 and 3-thiophenemalonic acid. In the synthesis, the particle size can be controlled by the molar ratio of the reactant agents. © 2006 Elsevier B.V. All rights reserved. Keywords: Microwave; Pt nanoparticles; Size control
1. Introduction Colloidal metal nanoparticles are of great interest because they can be used not only as catalysts [1], photocatalysts [2], sensors [3] and ferrofluids [4], but they also can be used in the fields of optical and electronic [5] as well as in magnetic devices [3]. Such nanoparticles also hold promise for use as advanced materials with novel properties and have potential applications in the fields of physics, chemistry, biology, medicine, material science and their different interdisciplinary fields [6]. As a result, considerable attention has been paid to the synthesis and characterization of colloidal metal nanoparticles [7]. Up to now, a great deal of methods for the preparation of metal nanoparticles have been developed [8]. However, nanoparticles tend to be fairly unstable in solution and therefore, special precautions have to be taken to avoid their aggregation or precipitation during the preparation of such colloidal particles in solution. To obtain stable colloids, the most effective and common strategy is the introduction of a protective agent in the reaction system [9]. Microwave chemistry has experienced an exponential growth rate, both in the industry and academia. Microwave irradiation as a fast, simple, and efficient heating method has
E-mail address: [email protected]. 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.166
been widely used in chemistry since 1986 [10,11]. More recently, Liao has successful prepared polyelectrolyte-protected gold nanoparticles by microwave-based thermal method, carried out by heating a polyelectrolyte–HAuCl4 aqueous solution [12]. However, to the best of my knowledge, there is no paper reporting on the one-step preparation of size-controlled colloidal Pt nanoparticles via a microwave-based thermal route. In this letter, I develop a simple route for the preparation of colloidal Pt nanoparticles in a single-step process, involving the direct heat-treatment of an aqueous solution containing H2PtCl6 and 3-thiophenemalonic acid in a microwave oven for several minutes. It is also found that the particle size can be controlled by the molar ratio of 3-thiophenemalonic acid to H2PtCl6. 2. Experimental procedures H2PtCl6 and 3-thiophenemalonic acid were purchased from Aldrich. All reagents were used as received without further purification. The water used was purified through a Millipore system. Three colloidal Pt samples were prepared as follows: Firstly, 200 μL of 0.06 M H2PtCl6 aqueous solution was diluted to 10 mL by the addition of water, and then 0.12 M 3thiophenemalonic acid aqueous solution was introduced into such solution with molar ratios of 2.5:1, 5:1, and 10:1 of 3thiophenemalonic acid to H2PtCl6 (corresponding to samples 1, 2 and 3, respectively). The resulting solution was irradiated in a
1874
Y. Luo / Materials Letters 61 (2007) 1873–1875
Fig. 1. Typical TEM images of Pt nanoparticles obtained with molar ratios of (A) 2.5:1, (B) 5:1 and (C) 10:1 of 3-thiophenemalonic acid to H2PtCl6.
domestic microwave operated at 100% power of 300 W for 8 min. The as-prepared colloidal solutions were stored at room temperature and used for further characterization. UV–vis experiments were carried on a CARY 500 Scan UV–vis–near infrared (UV–vis–NIR) spectrophotometer. Samples for TEM characterization were prepared by placing a drop of colloidal Pt solution on a carbon coated copper grid and dried at room temperature. TEM measurements were made on a JEOL 2010 transmission electron microscopy operated at an accelerating voltage of 200 kV. X-ray photoelectron spectrum (XPS) was acquired on an ESCLAB MKII using Mg as the exciting source. 3. Results and discussion The formation of Pt nanoparticles was further confirmed by TEM. Fig. 1 shows typical TEM images of three colloidal Pt samples thus formed. When the molar ratio of 3-thiophenemalonic acid to H2PtCl6 is 2.5:1, it is found that the Pt nanoparticles mainly consist of particles with a diameter of ∼3 nm and some large particles with a diameter of ∼ 5 nm are also observed, as shown in Fig. 1A. When the molar ratio is increased to 5:1, it is clearly seen that most of the Pt particles are ∼ 2 nm in diameter, as shown in Fig. 1B. When the molar ratio is further increased to 10:1, the Pt nanoparticles further become smaller (∼ 1–1.5 nm in diameter), as shown in Fig. 1C. Obviously, the size of such colloidal Pt nanoparticles can be controlled by the molar ratio of
Fig. 2. UV–vis absorption spectra indicative of Pt nanoparticles formation as a function of time in a 3-thiophenemalonic acid–H2PtCl6 aqueous solution with an initial molar ratio of 2.5:1 of 3-thiophenemalonic acid to H2PtCl6 under microwave irradiation with a time interval of 1 min.
the reactants and increasing the molar ratio of 3-thiophenemalonic acid to H2PtCl6 results in decreasing particle size. The formation of Pt nanoparticles can be attributed to the direct redox between 3-thiophenemalonic acid and H2PtCl6, because there are no other reducing reagents in the system. In this study, Pt nanoparticles cannot form after a 3-thiophenemalonic acid–H2PtCl6 aqueous solution has been stored for several months at room temperature in the presence of microwave irradiation, indicating that microwave irradiation can considerably accelerate chemical processes. It can be attributed to the fact that microwave irradiation is a fast, simple and efficient heating method and therefore, rapidly increases the temperature of the solution. As a result, the electron transfer (ET) rate from Pt (IV) to 3-thiophenemalonic acid is accelerated markedly (roughly doubles the reaction rate by every 10 K). The formation process of Pt nanoparticles can be explained as follows: During the microwave irradiation, H2PtCl6 is reduced by 3-thiophenemalonic acid to form Pt atoms first. With the elapsed time, new Pt atoms are generated in this system and nucleation occurs as the concentration of Pt atoms reaches a critical supersaturation, resulting in the formation of nuclei. The nuclei grow to nanoscale particles by further addition of free Pt atoms in the solution. It is also worthwhile mentioning that such colloidal Pt nanoparticles are quite stable, which indicates that 3-thiophenemalonic acid serves as an effective protective agent for Pt nanoparticles. This observation may be attributed to that the sulfur atom in 3thiophenemalonic acid has a very strong nucleophilicity with lone-pair electrons and such lone-pair electron can form a type of donor–
Fig. 3. XPS spectra of the precipitate obtained from centrifuging colloidal Pt sample 1.
Y. Luo / Materials Letters 61 (2007) 1873–1875
acceptor complex with the Pt atom on the particle surface, giving 3thiophenemalonic acid-protected Pt nanoparticles. The increase of the molar ratio of 3-thiophenemalonic acid to H2PtCl6 results in decreasing particle size which can be attributed to the fact that a higher molar ratio increases the number of 3-thiophenemalonic acid molecules available to interact with Pt nanoparticles and thus a more effective protective ability for such Pt particles can be expected [13]. The formation process of Pt nanoparticles was further traced by the UV–vis absorption method. Fig. 2 shows the time-dependent absorption spectra collected during the whole microwave irradiation process with a time interval of 1 min. With the elapsed time, the peak at 257 nm gradually disappears, which can be attributed to the gradual reduction of PtCl2− 6 by 3-thiophenemalonic acid. When the solution is irradiated for 8 min, this peak completely disappears and a spectrum that has absorption in all ranges of the UV–vis spectrum and at the same time, the absorption increases gradually with the decrease of wavelength, is observed. In addition, further microwave irradiation gives no change of such spectra. These observations demonstrate that the PtCl2− 6 ions in the solution have been totally reduced to form Pt nanoparticles [14]. To further confirm the formation of metal Pt from H2PtCl6, XPS was used to identify the change in oxidation states for Pt after the microwave irradiation reaction had occurred. Fig. 3 shows the XPS spectra of the precipitate obtained from centrifuging colloidal Pt sample 1. It is reported that the 4f7/2 and 4f5/2 binding energies for polycrystalline Pt are at 71.3 and 74.7 eV, respectively [15]. However, the 4f7/2 and 4f5/2 binding energies for the Pt nanoparticles prepared in this present study are at 72.0 and 75.2 eV, respectively, which are higher than those reported values [15]. This shift may be attributed to the formation of a donor–acceptor complex of the sulfur atom in 3thiophenemalonic acid with the Pt atom on the particle surface. It is worthwhile mentioning that further adding a strong reducing reagent such as NaBH4 into the colloidal solution thus formed cannot lead to further change of both the solution color and corresponding UV–vis spectra, indicating that the Pt salts have been completely reduced by 3thiophenemalonic acid.
4. Conclusion Radiating an aqueous solution containing 3-thiophenemalonic acid and H2PtCl6 in a microwave oven for several minutes
1875
results in the rapid formation of stable, 3-thiophenemalonic acidprotected colloidal Pt nanoparticles. It is found that the particle size can be controlled by the molar ratio of 3-thiophenemalonic acid to H2PtCl6. It not only provides a simple and rapid methodology for the preparation of Pt nanoparticles, but also can be used as a general strategy for the one-step preparation of 3thiophenemalonic acid-protected noble metal nanoparticles for applications. References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15]
H. Hirai, H. Wakabayashi, M. Komiyama, Chem. Lett. 1983 (1983) 1047. A.P. Brugger, P. Cuendet, M. Gratzel, J. Am. Chem. Soc. 103 (1981) 2923. J.M. Thomas, Pure Appl. Chem. 60 (1988) 1517. E.P. Wohlharth (Ed.), Ferromagnetic Materials, vol. 2, North-Holland, Amsterdam, 1980. G. Schon, U. Simon, Colloid Polym. Sci. 273 (1995) 202. J.H. Fendler (Ed.), Nanoparticles and Nanostructured Films, VCH, Weinheim, 1998. (a) J. Kiwi, M. Gratzel, J. Am. Chem. Soc. 101 (1979) 7214; (b) G. Schmid, Chem. Rev. 92 (1992) 1709; M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc., Chem. Commun. 801 (1994). A. Roucoux, J. Schulz, H. Patin, Chem. Rev. 102 (2002) 3757. N.R. Rao, G.U. Kulkarni, P.J. Thomas, P.P. Edwards, Chem. Soc. Rev. 29 (2000) 27. R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge, J. Rousell, Tetrahedron Lett. 27 (1986) 279. X. Xu, W. Yang, J. Liu, L. Lin, Adv. Mater. 12 (2000) 195. F. Liao, Chem. Lett. 33 (2004) 1020. R.M. Crooks, M. Zhao, L. Sun, V. Chechik, L.K. Yeung, Acc. Chem. Res. 34 (2001) 181. T. Teranishi, M. Hosoe, T. Tanaka, M. Miyake, J. Phys. Chem., B 103 (1999) 3818. E. Drawdy, G.B. Hoflund, S.D. Gardner, E. Yngvadottir, D.R. Schryer, Surf. Interface Anal. 16 (1990) 369.