case, the reduction of metal ions occurs exclusively on ... tion of colloidal metal films with the preset thickness ... Catalytic Activity of Colloidal Platinum Films.
Colloid Journal, Vol. 67, No. 3, 2005, pp. 357–362. Translated from Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 398–403. Original Russian Text Copyright © 2005 by Rudoy, Sukhov, Dement’eva, Abkhalimov, Vereshchagina, Kartseva, Ershov.
Metal Nanoparticles on Polymer Surfaces: 5. Catalytic Activity of Colloidal Platinum Films Incorporated in Polystyrene Surface Layer V. M. Rudoy, N. L. Sukhov, O. V. Dement’eva, E. V. Abkhalimov, O. F. Vereshchagina, M. E. Kartseva, and B. G. Ershov Institute of Physical Chemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119991 Russia Received December 1, 2004
Abstract—The fundamental possibility to enlarge Pt nanoparticles in monolayer ensembles formed on polystyrene surfaces by the adsorption from hydrosol in solution of isopropanol and K2PtCl4 is demonstrated for the first time. The enlargement of “seeding” nanoparticles is performed after their preliminary incorporation (partial embedding) into the polymer surface layer by the annealing of a system within the range between “surface” and “bulk” glass transition temperatures of polystyrene. It is shown that a colloidal film of metallic platinum with a thickness up to 200 nm is formed in the course of enlargement and it is mechanically fixed in the polymer surface layer. Such a system exhibits, over a long time, high catalytic activity in the model reaction of methyl viologen reduction with hydrogen.
INTRODUCTION Considerable recent attention has been focused on the problem of the creation of nanostructured “twodimensional” systems by the “enlargement” of metal (gold and silver) nanoparticles preliminarily adsorbed on various substrates in solution containing the ions of corresponding metal and weak reducer (e.g., see [1–4]). In this case, the reduction of metal ions occurs exclusively on the surfaces of seeding nanoparticles acting as catalysts. This approach is rather promising for the preparation of colloidal metal films with the preset thickness and chemical composition (in particular, films of bimetallic particles). Such systems possess a number of specific properties (optical, electronic, etc.) [1–4] and can be applied in optoelectronics, for creating chemical and biological sensors, catalysts, etc. One of the main problems encountered upon the preparation of such structures is the preliminary attachment of a monolayer of seeding nanoparticles to the substrate. In the majority of cases, this is realized due to electrostatic interactions between particles and substrate [1–4]. Meanwhile, it is more reasonable sometimes to form nanoparticle ensembles on the surface of inert glassy polymers, because in this case there is a possibility for their “mechanical” attachment to the substrate surface. Such a possibility resulted from the difference in glass transition temperatures (Tg) of a polymer in the bulk and near the free surface was studied by us earlier [5, 6]. According to experiments, the nanoparticles of gold citrate hydrosol with a diameter of 18 ± 2 nm embed in polystyrene (PS) surface layer by 3–4 nm already after 1 h-annealing of a system at 60°C (i.e., at a temperature
that is much lower than Tg of block polystyrene) [6]. Simultaneously, it was demonstrated that, in principle, gold nanoparticles thus incorporated into the PS surface layer could be enlarged in the mixed solution of chloroauric acid and hydroxylamine [6]. In our previous work [7], we reported the results of tentative experiments into the study of catalytic activity of monolayer ensembles of platinum nanoparticles deposited onto the PS surface. It was established that such a system is a highly effective catalyst of the model reaction of methyl viologen reduction with hydrogen. Unfortunately, the part of Pt nanoparticles were “washed-off” from the PS surface due to their weak interaction with the polymer, thus decreasing the reaction rate constant by approximately 1.5 times upon the repeated use of such a catalytic system. This work has two goals. The first one is the development of the procedure for the enlargement of Pt nanoparticles incorporated into the PS surface layer to form the colloidal metal film. The second goal is the study of the catalytic activity of such a system. EXPERIMENTAL The object of the study was platinum in a nanosized state; the catalytic reduction of methyl viologen (dimethyl-4,4'-bipyridine) with hydrogen in aqueous alkaline solutions was the model reaction for studying the colloidal platinum properties on a polymer substrate [8]. Platinum hydrosol was prepared by the reduction of aqueous 1 × 10–4 M K2PtCl4 solution containing 2 × 10–3 M sodium salt of ethylenediaminetetraacetic acid with hydrogen. Sizes of platinum nanoparticles adsorbed on the PS surface and the structure of colloi-
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dal film formed at the initial stage of particle enlargement (see below) were determined using an SMM-2000 atomic force microscope (AFM) (MIET, Moscow, Russia). To process images obtained in contact mode, we used an SM2000 program. According to atomic force microscopy, synthesized platinum hydrosol was characterized by a fairly wide particle size distribution. The shape of nanoparticles was spherical; their mean diameter was equal to 30 ± 10 nm. Polystyrene with a molecular mass of 226 000 (Mw/Mn = 2.19, Tg ≅ 105°ë) was used as a substrate. Polystyrene films were prepared by the spin coating on the surfaces of polished silicon wafers with a size of 1 × 1 cm2. For this purpose, we used PS toluene solution with a concentration of 2 wt %. Samples were exposed to the air over 1–2 days to evaporate a solvent and then were annealed in the air for 6 h at 120 ± 1°C. Platinum particles were deposited onto a polymer substrate placing a hydrosol droplet on its surface. Two hours after, the remaining hydrosol was removed with the filtering paper. To incorporate adsorbed metal nanoparticles into the surface layer of a polymer matrix, the samples were annealed at 60°C for 5 min or 1 h. However, experiments demonstrated that, in the latter case, nanoparticles incorporated into PS are characterized by a rather low catalytic activity. Therefore, all data reported below refer to the samples prepared at 5 min time of nanoparticle embedding. The reaction between hydrogen and methyl viologen (MV2+) catalyzed by the colloidal platinum was carried out in a special small all-glass vessel equipped with an additional “finger” with quartz optical cell. The essence of experiment was as follows. Four milliliters of water was poured into the vessel and a silicon wafer covered with PS was dipped into the vessel. The monolayer of seeding platinum nanoparticles was preliminarily incorporated into the PS surface layer. After system degassing, 0.5 ml of NaOH solution and 0.5 ml of 1 × 10–2 M methyl viologen solution were added and then hydrogen was let under a pressure of 1 atm. Optical absorption spectra were recorded with a Specord UV-VIS spectrophotometer under continuous stirring of a solution. The kinetics of methyl viologen reduction was measured recording an increasing (with time) inten+. sity of the absorption band of radical-cation MV with a maximum at 600 nm (ε600 = 1.1 × 104 l M–1 cm–1) [9]. The catalytic activity of colloidal platinum film prepared by the enlargement of nanoparticles in the initial monolayer was studied in the same manner. The reduction of platinum ions (see below) was also controlled spectrophotometrically by a decrease in optical density D of a solution in the absorption region of Pt2+ ions (λ = 200 nm). To study the sample surfaces after performing the catalytic experiments, a Nanoscope IIIa atomic force microscope (Digital Instruments, USA) was used. Measurements were performed in a contact mode. In this
case, for the processing of AFM images and their graphic representation, we used FemtScan-001 software [10]. RESULTS AND DISCUSSION As was mentioned above, by present, the procedures were developed for the enlargement of nanoparticles that were preliminarily adsorbed on a substrate in a solution containing the ions of corresponding metal. However, these procedures were implemented only for Au and Ag particles. We elaborated similar procedure as applied to platinum nanoparticles. This procedure is based on the use of mixed aqueous solution of K2PtCl4 and isopropanol. Experiments were performed as follows: a silicon wafer covered with the PS film containing seeding platinum nanoparticles preliminarily fixed in its surface layer was placed for a given time into aqueous solution (5 ml) containing 1 × 10–4 M K2PtCl4 and 1 × 10−3 M isopropanol. The results of AFM analysis demonstrated that the enlargement of platinum nanoparticles incorporated into the PS surface layer really proceeds in the solution of K2PtCl4 and isopropanol. It should be emphasized that, in the absence of nanoparticles, the solution is stable and no reduction of platinum ions takes place (analogous picture was observed in [11] when studying the enlargement of gold nanoparticles in HAuCl4 and NH2OH mixed solution). Thus, the Pt2+ ions are reduced directly on the surface of metal particles. Evidently, such a selective reduction of platinum ions is explained by the fact that nanoparticles incorporated into a polymer matrix catalyze the oxidation of isopropanol and the reduction of Pt2+ ions. In practice, the process does not proceed in the absence of air oxygen. Seemingly, the coupled reactions of the catalytic oxidation of isopropanol and the reduction of air oxygen take place with the participation of colloidal platinum as a pool, i.e., a peculiar accumulator and a carrier of electrons from alcohol to oxygen. In this case, each nanoparticle acts as a microelectrode on which the electrochemical reaction is realized. Coupled alcohol oxidation and oxygen reduction can be expressed by the following reactions: n(CH3)2CHOH + (Ptcoll) ⇒ n(CH3)2CO (or other products) + ( Pt coll ) ( Pt coll )
n–
n–
(1)
+ 2nH+,
+ 0.25nO2 + 0.5nH2O ⇒ (Ptcoll) + nOH–. (2)
The reduction of Pt2+ ions on a “microelectrode” resulted in the particle enlargement is related to the proceeding of cathode reaction: ( Pt coll )
n–
0
+ mPt2+ ⇒ [(Pt + m Pt ) coll ]
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Fig. 1. The AFM images of the polystyrene surface with adsorbed platinum nanoparticles after (a) the annealing of a system at 60°C for 5 min and (b) subsequent exposure of a sample to a solution containing K2PtCl4 and isopropanol for 24 h. Corresponding cross sections of surface relief are shown in the bottom part of a figure. Scan area is 10 × 10 µm2.
Thus, the reduction of Pt2+ ions on metal nanoparticles turned out to be thermodynamically advantageous. The nanoparticles are capable of catalyzing such surface reduction reactions, which according to thermodynamic considerations do not proceed in a solution bulk or proceed too slowly. This is caused by the difference in the potentials of Pt2+/Pts and Pt2+/Pt0 redox pairs on nanoparticle surfaces and in the solution bulk, respectively. Indeed, standard potential E0(Pt2+/Pts) equals 1.2 V and potential E0(Pt2+/Pt0) is equal to –1.2 V. (When calculating the latter potential, we took into account the correction for the sublimation energy at a room temperature.) Let us now pass to the results of the AFM study of platinum colloidal films formed during the nanoparticle enlargement. Unfortunately, it seems impossible to determine with an adequate accuracy the depth of the particle embedding in the PS surface layer during the annealing at 60°C due to a rather wide particle size distribution for the initial platinum hydrosol. Nevertheless, we believe that, indeed, such an annealing leads to the nanoparticle incorporation into a polymer matrix. In particular, this is confirmed by the invariable positions of particles upon the sample scanning in an AFM in contact mode. (At the same time, as was shown in [5, 12], COLLOID JOURNAL
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nanoparticles simply deposited onto PS easily move on its surface under the action of the AFM cantilever tip.) Partial embedding of particles in a polymer during the system annealing is also supported by the results of catalytic experiments (see below). The comparison of AFM images presented in Fig. 1 makes it possible to estimate an increase in the size of platinum particles at the intermediate stage of reduction. As follows from Fig. 1a, the maximal size of seeding Pt nanoparticles equals about 45 nm. After 24 h exposure of a sample to the solution containing K2PtCl4 and isopropanol, the isolated structures with a height of 90–100 nm, i.e., of much larger sizes, are formed on the PS surface. It was shown earlier that the process of nanoparticle enlargement on the surface of glassy polymer is determined by the packing density of nanoparticles in their initial monolayer and by the intensity of metal–polymer interaction [6]. In this case, this interaction is fairly weak and the reduction of Pt2+ ions takes place only on the surfaces of seeding particles, whereas new nanoparticles are not formed directly on polystyrene substrate (see also [6]). Evidently, in this situation, it is rather difficult to form a really continuous metal film even at a
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methyl viologen with hydrogen as a model reaction. The reduction of MV2+ ion with molecular hydrogen in an alkaline aqueous solution catalyzed by the colloidal platinum can be described by the following overall equation:
D200 1.6 1.4 1.2
2MV2+ + H2 ⇒ 2 MV
1.0 0.8 0
200
400
600
800
1000
1200 t, h
Fig. 2. Kinetics of a decrease in the optical absorption of Pt2+ ions caused by their reduction on the platinum nanoparticles in the presence of isopropanol.
D600 0.3 2 0.2
1
0.1
200
400
600
800
1000 1200 1400 t, s
Fig. 3. Kinetics of an increase in the absorption of radical– +
.
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sufficiently large coverage of the surface with seeding nanoparticles. According to the results of spectrophotometric measurements (see Fig. 2), the whole of Pt2+ ions that are present in a solution contacting with initial nanoparticle ensemble are reduced over approximately 30 days. During this time, visually continuous nonuniform colloidal platinum film of such a large thickness1 is formed that to study this film by the AFM technique turned out to be impossible. The next stage of a work consisted in the study of the catalytic properties of platinum colloidal film incorporated into the PS surface layer using the reduction of 1 In
.
+ 2H+.
(4) +
0.6
0
+
the approximation of “continuous layer”, the calculated thickness of metal film is equal to 168 nm.
.
The standard potential of the MV2+/ MV * redox pair is equal to –0.4 V [13], i.e., it is more negative than the potential of 2H+/H2 pair that is equal to zero. Therefore, the reaction reversed to reaction (4) proceeds in neutral and acidic media. However, the potential of hydrogen system in alkaline medium reaches the value of approximately –0.8 V due to a decrease in the concentration of H+ ions. Under these conditions, reaction (4) becomes preferable. It is established that, in the presence of the monolayer of platinum particles incorporated into the surface layer of polymer matrix, the product of the reduction of +. MV2+, cation–radical MV , is formed in aqueous alkaline methyl viologen solution after letting the molecular hydrogen to the reaction system. The cation– radicals are registered by the appearance of characteristic absorption with a maximum at 600 nm [7]. The reaction proceeds at a fairly low rate (Fig. 3, curve 1). After the 24-h enlargement of nanoparticles by the procedure described above, the rate of a process increases approximately threefold (Fig. 3, curve 2). Figure 4 demonstrates the kinetics of the reduction of MV2+ with hydrogen catalyzed with the colloidal film formed on the PS surface after the enlargement of seeding Pt nanoparticles during 8 days. It is seen that the catalytic reaction is practically ended instantaneously (Fig. 4, curve 1). After the removal of the plate with a catalyst from the optical cell, the charge of a fresh portion of a solution, the dipping of a plate, and the repeated reduction, the reaction rate decreases significantly (Fig. 4, curve 2). Such a multiple “cyclic” use of a sample leads to a decrease in the rate of model reaction to a practically constant value, which was reached already in the third cycle (see curve 3, Fig. 4) and then it was changed only insignificantly with an increase in the number of cycles up to ten. Probably, the observed effect is explained by the loose, “two-level” structure of colloidal film formed during the prolonged enlargement. Seemingly, two competing processes proceed in a system during the enlargement of nanoparticles incorporated into the PS surface layer. The first process is the enlargement of particles and their gradual coalescence. The second process is the formation and growth of “secondary” platinum nanoparticles on the surface of seeding particles, which can be attributed to the surface partial blocking with bulky stabilizing ions. (Such a possibility follows from the fact that, already at a relatively short time of enlargement, formed Pt structures are rather loose (see Fig. 1b).) This second process can become COLLOID JOURNAL
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361
2000 4000
200
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1000 1200 t, s
Fig. 4. Kinetics of an increase in the absorption of radical– +
.
cations MV in a solution (after letting the hydrogen to a system) in the presence of colloidal platinum film incorporated into the PS surface layer: (1) colloidal film formed after 8 days of nanoparticle enlargement in solution containing K2PtCl4 and isopropanol, (2) the same sample after the replacement of spent solution with a fresh portion, and (3) the same sample after the second replacement of spent solution with a fresh portion.
dominant with time because of a decrease in the curvature radius of growing seeding nanoparticles. The gradual transfer of such secondary platinum nanoparticles, which are weakly attached to the surface of colloidal film, to a liquid phase should result in the observed decrease of the rate of catalytic process upon the multiple replacement of “spent” methyl viologen solution. These assumptions are confirmed by the following experimental fact: the spectrometric analysis of a solution probe from the reaction vessel after the completion of reduction of methyl viologen demonstrated the presence of platinum nanoparticles in the probe. Note that the catalyst conserves its fairly high activity even after the transfer of a part of metallic platinum from the surface of colloidal film to the solution (cf. curve 2 in Fig. 3 and curve 3 in Fig. 4). Thus, a considerable part of particles comprising the colloidal film formed upon the enlargement of seeding nanoparticles appeared to be incorporated in the PS surface layer. This assumption is also indirectly confirmed by the examination of sample surface after its use in catalytic experiments. The presence of metallic platinum in the form of two “fractions” was disclosed on the PS substrate upon taking the AFM image shown in Fig. 5. The first fraction represented a loose substance that can be easily scrubbed by the cantilever tip during scanning the sample surface. The second fraction represents a rather large “monolith” Pt nanoparticles embedded in the surface layer of polymer matrix. Unfortunately, limitations of the AFM technique that were also mentioned in our previous work [6] did not allow us to obtain sufCOLLOID JOURNAL
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2000 0
nm
Fig. 5. 3D AFM image of colloidal platinum film incorporated into the polystyrene surface layer after tenfold use of a sample in the catalytic reaction.
ficiently complete information on the structure of such a colloidal film. Its analysis is to be reported in the next communication using independent research techniques. CONCLUSIONS (1) The procedure for the enlargement of platinum hydrosol particles in the presence of K2PtCl4 and isopropanol was developed. The mechanism of the catalytic reduction of Pt2+ ions on the surface of colloidal metal particles, based on the coupled reactions of the catalytic oxidation of alcohol and the reduction of air oxygen with the participation of platinum colloids as a pool, i.e., a peculiar accumulator of electrons and their carrier from alcohol to oxygen, was proposed. (2) The proposed procedure was used to prepare platinum colloidal film mechanically attached to a polymer carrier by the enlargement of metal nanoparticles preliminarily incorporated into the polystyrene surface layer. (3) It was shown that the platinum colloidal film thus formed is characterized by the catalytic activity in the reduction of methyl viologen with hydrogen over long time. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project nos. 03-03-32562 and 04-03-32274 and by the Russian Academy of Sciences (Program “Fundamental Problems in Physics and Chemistry of Nanosized Systems and Nanomaterials”). The support of the Foundation of Russian Science for O.V. Dement’eva is gratefully acknowledged.
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