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One-Step Electrosynthesis of Bimetallic Au-Pt Nanoparticles on Indium Tin Oxide Electrodes: Effect of the Deposition Parameters Barbara Ballarin,*,† Massimo Gazzano,‡ Erika Scavetta,† and Domenica Tonelli† Dipartimento di Chimica Fisica ed Inorganica and INSTM-UdR Bologna, UniVersita` di Bologna, V.le Risorgimento 4, 40136 Bologna, Italy, and ISOF-CNR, V. Selmi, 40126 Bologna, Italy ReceiVed: April 8, 2009; ReVised Manuscript ReceiVed: July 15, 2009
Bimetallic Au-Pt nanoparticles (Au-PtNPs) were prepared by one-step electrochemical reduction of KAuBr4 and K2PtCl4 precursors in acidic media. Characterization of Au-PtNPs was made by scanning electron microscopy, X-ray diffraction, UV-vis spectroscopy, and cyclic voltammetry. The effects of gold and platinum precursors molar ratio and the deposition time were studied. Pt and Au nanoparticles (PtNPs and AuNPs) were also electrosynthesized for comparison. The results show that the bimetallic system can be obtained only with 1:1 or 2:1 Au/Pt molar ratio; all the other ratios afford the electrosynthesis of PtNPs and AuNPs in separate domains. The deposition time seems to affect preliminarily the percentage of surface coverage and not the composition of the nanoparticles. The electrocatalytic behavior of bimetallic Au-PtNPs catalyst for methanol oxidation was investigated in alkaline conditions; the results show a higher catalytic efficiency for Au70Pt30 composition. 1. Introduction Metal nanoparticles have drawn considerable interest in various fields of science and engineering because of their unique physical and chemical properties, leading to potential applications in electronics, in optical and magnetic devices,1,2 and for catalytic and electroanalytical purposes.3 Several approaches have been employed to synthesize and immobilize the metal nanoparticles, with predetermined size, crystallographic orientation, and geometry, onto an electrode surface as, for example, self-assembly liquid phase methods based on peculiar binder molecules or seed mediated growth methods.4-6 However, these routes are often somewhat complex and time-consuming, thereby limiting their applicability to the mass production of electrochemical sensors. In contrast, electrochemical methods have been proven to have some advantages over chemical ones in the synthesis of size-selective or shape-controlled highly pure metal nanomaterials7 because of their easy and fast procedures. Moreover, by varying the deposition potential, concentration, pH of buffer solution, and deposition time, it is possible to control the supersaturation conditions and, therefore, the size and distribution of the nanoparticles directly at the electrode interface. Among metal nanoparticles, the bimetallic ones have recently drawn attention not only for their promising catalytic performance but also for the lower price, for the difference in electronic and optical properties compared to those of corresponding monometallic nanoparticles, and for the high stability in many electro-oxidation reactions. In particular, Au-PtNPs have been reported in the literature as promising electrocatalysts for fuel cells or for biosensors applications.8-10 There are several methods to generate bimetallic nanoparticles, which in general can be divided into two groups: dry and wet methods.11 Between * To whom correspondence should be addressed. E-mail: ballarin@ ms.fci.unibo.it. Fax: +39-051-2093690. † Universita` di Bologna. ‡ ISOF-CNR.
the latter, different attempts were made to follow the preparation prevalently via the chemical reductive route.12-15 The electrochemical deposition process represents an alternative simple and direct procedure for the fabrication of nanostructured bimetallic hybrid film directly on the electrode surface, but until now, very few articles are reported in the literature.16 In this paper, we report the preparation of bimetallic Au-PtNPs prepared by electrochemical reduction of the two metal ions. The nanoparticles were electrodeposited on indium tin oxide (ITO) electrodes from aqueous solutions and were characterized by UV-vis spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and cyclic voltammetry (CV). The influence of some parameters such as Au/Pt molar ratio and deposition time on the particles size and distribution is discussed. For comparison, Pt and Au nanoparticles have also been electrosynthesized. The electrocatalytic activity of the Au-PtNPs-ITO electrode toward methanol oxidation in alkaline media was also tested. 2. Experimental Section A. Chemicals. KAuBr4 (99%), K2PtCl4 (98%), H2SO4, and KI (99.7%) were analytical grade (Aldrich) and used without further purification. The solutions were prepared with Milli-Q water (18 MΩ, Millipore). The buffer solution (PBS, pH 4.0) was prepared from a potassium hydrogen phosphate. B. Instruments and Measurements. Electrochemical measurements were performed using a potentiostat (Autolab PGSTAT20) in a conventional three-electrode cell configuration with ITO (surface area ) 1.28 cm2) as the working electrode, a saturated calomel electrode as the reference electrode, and a platinum wire as the counter electrode. All the electrochemical measurements were conducted at room temperature in nitrogensaturated solutions. The elemental composition and the morphological characterization of Au-PtNPs were investigated using a scanning electron microscope (EVO 50-RENISHOW) operating at 15 kV equipped with an energy-dispersive spectrometer.
10.1021/jp903247u CCC: $40.75 2009 American Chemical Society Published on Web 08/05/2009
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Figure 1. SEM images of (A) PtNPs, (B) AuNPs, (C) Au1Pt1, (D) Au1Pt2, (E) Au1Pt4, and (F) Au2Pt1 nanoparticles electrodeposited on ITO electrodes. The inset shows the corresponding images at 70000×.
ITO electrode surface coverage was calculated from SEM images by using the surface analysis program ImageJ. X-ray diffraction patterns were carried out on a X’PertPro PANalytical diffractometer using Cu KR radiation (λ ) 1.5418 Å), performing steps of 0.066° (2θ) and counting 400 s/step in the 2θ angular region between 30° and 85°. The electrodes were placed on a flat sample holder and directly analyzed. The cell parameters were calculated as the best values that fit the distances of the reflections (111), (200), (220), and (311) for the pure metals phases and from the interplanar distance of the (111) peak for the alloys. The crystal sizes perpendicular to 111 planes were calculated by using the Scherrer method with the empirical constant K ) 0.94.17 UV-vis spectrophotometry was performed using a HewlettPackard 8453 diode array spectrophotometer.
C. Au-Pt Nanoparticles Preparation. Before modification, the ITO electrode was polished by ultrasound alternate treatments for 15 min in soap water, acetone, and ethanol solution and then dried at room temperature. Au-PtNPs were electrodeposited onto the ITO surface from 0.5 M H2SO4 and 100 µM KI solution containing KAuBr4 and K2PtCl4, at a fixed potential of -0.155 V for different deposition times (200, 500, and 800 s, respectively). For comparison, the deposition of PtNPs and AuNPs was conducted on ITO electrodes from a 1 × 10-3 M KAuBr4 or K2PtCl4 solution in 0.5 M H2SO4 and 100 µM KI at a fixed potential of -0.045 V (Au) or -0.155 V (Pt) for 500 s. 3. Results and Discussion A. Au/Pt Molar Ratio Effect. To investigate the effect of the Au/Pt molar ratio on the electrodeposited bimetallic nano-
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Figure 2. SEM images (20000×) and EDS mapping analysis (Au and Pt elements) of samples Au1Pt1 and Au1Pt2.
Figure 3. XRD patterns of AuNPs (red) and PtNPs (blue) grown on the ITO substrate (black). The marked area is enlarged in the inset. The angular values (deg) and the indices of the planes that generate the reflections are also reported, while ITO peaks are marked by asterisks.
particles, Au-PtNPs were electrodeposited on ITO electrodes at a fixed potential of -0.155 V for 500 s from a 0.5 M H2SO4 and 100 µM KI solution containing KAuBr4 and K2PtCl4 with different molar ratios (i.e., 1:1, 1:2, 1:4, and 2:1) (starting solutions: 1 × 10-3 and 2 × 10-3 M KAuBr4; 1 × 10-3, 2 × 10-3, or 4 × 10-3 M K2PtCl4) (samples Au1Pt1, Au1Pt2, Au1Pt4, and Au2Pt1, respectively). In all cases the formation of homogeneous films was observed on the ITO electrode surface. SEM images of PtNPs and AuNPs are shown in parts (A) and (B) of Figure 1 (see inset): spherical shape particles are observed in the former case whereas a roselike morphology is present in the latter one. Parts (C)-(F) of Figure 1 display the SEM images of Au1Pt1, Au1Pt2, Au1Pt4, and Au2Pt1 samples: in all cases the electrodeposition affords an almost homogeneous coverage of
Figure 4. XRD patterns of Au1Pt1, Au1Pt2, Au1Pt4, and Au2Pt1 nanoparticles on ITO electrode surface. ) ) Au-Pt alloy peaks; * ) ITO peaks.
nanoparticles with the presence of bigger, roselike systems. The EDS analysis (effected on SEM at higher magnification) and the mapping Au and Pt elements over the sample (see Figure 2, reported as example) revealed that the presence of bimetallic Au-PtNPs started to be observed when Au and Pt were present in the deposition solution with a 1:1 molar ratio (sample Au1Pt1) and became more evident by increasing the Au amount to 2:1 (sample Au2Pt1). In contrast, when the Pt concentration in the deposition solution was increased (i.e., 2 or 4 × 10-3 M), separate PtNPs and AuNPs domains formed on the ITO surface and a solid deposit (revealed to be metallic gold) was obtained at the bottom of the cell. Our results suggest that composite Au-PtNPs can be obtained employing solution with an Au/Pt molar ratio equal to or higher than 1 and that the morphology and composition of bimetallic nanoparticles are strongly dependent on the molar ratio Au/Pt.18 XRD was also carried out to study the crystalline nature of the samples. Figure 3 shows the XRD patterns of AuNPs or PtNPs.
One-Step Electrosynthesis of Au-Pt Nanoparticles
Figure 5. Typical cyclic voltammograms for (A) AuNPs, (B) PtNPs, (C) Au1Pt1, Au2Pt1, and (D) Au1Pt2 and Au1Pt4 samples obtained in 0.1 M PBS, pH 4.0. Potential sweep rate: 50 mV s-1. Electrode area ) 1.28 cm2.
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Figure 6. (A) Cyclic voltammograms obtained in 0.1 M PBS, pH 4.0. Potential sweep rate: 50 mV s-1. Electrode area ) 1.28 cm2; XRD patterns of (B) Au2Pt1-200, Au2Pt1-500, and Au2Pt1-800 and (C) Au1Pt1-200, Au1Pt1-500, and Au1Pt1-800 samples. * ) ITO peaks; ) ) Au-PtNPs alloy peaks.
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Figure 7. Cyclic voltammograms in 0.5 M KOH at (A) PtNPs, (B) Au2Pt1-500, (C) Au1Pt1-500, and (D) AuNPs before and after 1.35 M MeOH addition. Scan rate ) 50 mV s-1, whereas no differently indicated steady state cycles are reported.
TABLE 1: Parameters Obtained from XRD Data; SD in Parentheses peaks position (2θ/deg) sample crystal sizea name (111) (200) (220) (311) [nm] (SD) Au 38.2 Pt 39.9 Au1Pt1 38.2 39.5 Au2Pt1 38.7 38.2c Au1Pt2 39.8 Au1Pt4 39.8
44.4 46.3 44.4
64.6 67.6 64.6
77.6 81.3 77.6
45.0 44.4c 46.4 46.3
65.6 64.6 67.6 67.6
78.7 77.6 81.3 81.2
39(5) 13(2) 37(4) 11(2) 22(3) 11(2) 12(2)
a (nm)
compositionb
0.4078(5) 0.3920(2) 0.4078(1) 0.3951 0.4032(2) 0.4071(2) 0.3919(2) 0.3927(2)
Au100 Pt100 Au Au20Pt80 Au70Pt30 Au Pt(Au)d Pt (Au)d
TABLE 2: Parameters Obtained from XRD Data; SD in Parentheses sample name
deposition surface a [nm] crystal size [nm] time (s) coverage (%)a (SD) (SD) compositionb
Au2Pt1-200
200
14.1
Au2Pt1-500
500
28.7
Au2Pt1-800
800
33.1
Au1Pt1-200
200
11.4
Au2Pt1 0.4032(2) 0.4032(2) 0.4071(2) 0.4032(6) Au1Pt1 0.3952 0.4076(1) 0.3951 0.4078(1) 0.3952 0.4077(4)
16(2)
23(4)
Au70Pt30 Auc Au70Pt30 Auc Au70Pt30
31(4) 11(2) 37(4) 12(2) 15(2)
Au20Pt80 Au Au20Pt80 Au Au20Pt80 Au
22(3)
a Calculated by Scherrer equation. b Au % estimated by cell parameters. c Shoulder peak. d Very small diffraction peaks of Au are still evident; Pt is the main system present.
Au1Pt1-500
500
20.5
Au1Pt1-800
800
28.3
In both cases the peaks can be indexed as (111), (200), (220), and (311) for the face-centered-cubic phase, as evidenced in the inset of the figure. The difference in the unit cell volume of the two metals is big enough to appreciate a good shift in the angular position of the reflections. The peaks due to the ITO layer were easily identified and are marked by asterisks in all the figures. XRD profiles and the relative parameters of Au1Pt1, Au1Pt2, Au1Pt4, and Au2Pt1 samples are reported in Figure 4 and Table 1. Au1Pt1 XRD profile exhibits a new broad but fairly symmetric peak at an angular value of 39.5°, which is intermediate between (111) peaks of Au and Pt, and a shoulder at about 46° near the base of the peak due to the ITO substrate. The presence of these new reflections suggests an alloy-type phase19 in which Pt and Au atoms are tightly mixed: moreover, the determination of the lattice parameter a ) 0.3951 nm allowed assignment of a composition of Au20Pt80.
a Estimated by SEM images. b Au % estimated by cell parameters; determined only from the position of the (111) reflection. c Very small diffraction peaks of Au are still evident.
For samples Au1Pt2 and Au1Pt4 the simultaneous presence of gold and platinum as pure phases was detected, in perfect agreement with what was observed by SEM/EDS microscopy. In contrast, in the case of the Au2Pt1 sample, single peaks at intermediate positions with respect to those of Au and Pt appear again, indicating that an alloy phase is present. The value of the lattice parameter a ) 0.4032 nm corresponds to a composition of Au70Pt30 that lies in the bimetallic miscibility gap. Moreover, the higher intensity of the alloy peaks in the Au2Pt1 samples gives evidence that a greater amount of alloy phase is formed with respect to that found in the Au1Pt1 case. The size distribution of the systems studied20 gives further evidence of the bimetallic phase formation: the particle size is
One-Step Electrosynthesis of Au-Pt Nanoparticles considerably smaller when a second metal is present, as occurred for the samples Au1Pt1 and Au2Pt1, if compared with AuNPs, whereas there is no change in the case of Au1Pt2 and Au1Pt4 with respect to PtNPs. Moreover, the decrease of the particle size seems to be correlated with the increase of Pt % in the composite, as appeared for AuNPs, Au70Pt30, and Au20Pt80 samples in which a change from 39 to 11 nm occurs. No dampening on the Au plasmon resonance band (PSR), due to the introduction of Pt,21 expected for physical mixtures of individual AuNPs and PtNPs22 is observable from the UV-vis spectra of Au2Pt1 and Au1Pt1 samples (not reported), but a significant blue shift in the peak position occurred with the change in the alloy composition (i.e., from 529 to 487 nm). The shift of PSR band by increasing the Pt % confirms the formation of bimetallic particles and can be attributed to the chemical interaction of the two metals.22-24 Cyclic voltammetry was performed in nitroge- saturated 0.1 M phosphate buffer solution (pH 4.0) over a potential region of -0.20 to +1.40 V (chosen on the basis of the system stability) at a scan rate of 50 mV s-1 to electrochemically characterize the nanoparticles immobilized on the ITO surface (Figure 5). Parts (A) and (B) of Figure 5 show the typical voltammetric paths obtained within this potential range, for AuNPs and PtNPs.25 Au1Pt1 and Au2Pt1 CV paths (Figure 5 C) present the predominance of the response specific to Au whereas the absence of this response on Au1Pt2 and Au1Pt4 samples may be indicative of a predominance of Pt on the electrode surface (Figure 5 D).26 B. Electrodeposition Time Effect. The effect of the electrodeposition time (200, 500, or 800 s) was investigated only for the Au/Pt ratios, giving composite bimetallic nanoparticles (samples Au1Pt1and Au2Pt1, respectively). The voltammetric paths relative to the Au2Pt1 samples obtained at different deposition times were similar (Figure 6 A) and showed an increase of both the capacitive and faradic current attributable to the increase of the nanoparticles surface coverage, as estimated from SEM images. The XRD patterns show the presence of the Au-PtNPs alloy peaks at the same angular position independently from the deposition time (see Figure 6 B and data reported in Table 2); this is indicative of a constant composition of Au70Pt30 for all the bimetallic samples. A small amount of Au phase is still present in the samples electrosynthesized at shorter times (200 and 500 s), whereas a pure alloy is obtained only at 800 s, as evidenced from the arrow in Figure 6 B. Au1Pt1-200, Au1Pt1-500, and Au1Pt1-800 samples still present CV paths similar to each other with an increase in the current amount, in accordance with the increase of the surface coverage (see Table 2). Again, the XRD patterns (Figure 6C) show the alloy peaks ()) at the same angular positions independently from the deposition time with a composition of Au20Pt80 attributable to all the samples. However, in this case, a pure alloy phase has never been obtained, even at 800 s, and the Au diffraction peaks, predominant at lower deposition times, are still present. C. Electrocatalytic Properties for Methanol Oxidation. The electrochemical measurements were carried out in a nitrogen-saturated 0.5 M KOH aqueous solution in the absence and presence of 1.35 M methanol for Au2Pt1 and Au1Pt1 samples (parts (B) and (C) of Figure 7); the responses of Pt and Au nanoparticles are reported as a comparison (parts (A) and (D), respectively, of Figure 7). In the potential range investigated, on a PtNPs electrode, no significant voltammetric signal has been registered whereas the MeOH addition caused the presence of the catalytic response at lower potential27,28 (onset at +0.39 V), well visible in the first cycle, which became lower and lower
J. Phys. Chem. C, Vol. 113, No. 34, 2009 15153 upon successive scans (see Figure 7A, n ) 4 cycles). In contrast, on Au2Pt1 and Au1Pt1 samples, before methanol addition, a stable and constant signal was obtained after ca. 10 cycles, showing a broad oxidation wave in the potential range +0.20 to +0.60 V and a reduction peak at ca. +0.00 V attributable to the formationofgoldsurfaceoxidesandtheirsubsequentreduction.25,29,30 The methanol addition causes the presence of a new stable anodic wave around a potential of +0.20 V, with an onset potential at +0.00 V, clearly shifted toward negative values with respect to that observed for the Au-oxide formation, which is attributable to the electrocatalytic oxidation of the substrate.25,31 Moreover, the decrease of the cathodic waves in the presence of methanol confirms that the electrogenerated oxides species are involved in the electrocatalytic oxidation and that the surface oxides act as an electron-transfer mediator in the oxidation process.25,31 It is worth noting that, at Au-PtNPs/ITO electrodes, the onset of the solvent oxidation, in the presence of methanol, is shifted toward more positive potential of about 100 mV with respect to that observed with AuNPs, allowing a larger separation from the catalytic peak. With regard to the catalytic efficiency, defined as (icat - iblank)/ iblank where iblank is the current before methanol addition, the following trend has been observed: Au2Pt1 > Au > Au1Pt1 > Pt (calculated data: 2.91 > 1.99 > 0.77 > 0.28, respectively). Taking into account the different surface coverage of the electrodes (Au2Pt1 ) 28.7%, Au ) 6.4%, Au1Pt1 ) 20.5%, Pt ) 14%), we can deem that higher catalytic activity is obtained for AuPt catalyst with 70% Au (Au2Pt1 sample) whereas a lower amount (i.e., 20%, Au1Pt1 sample) appears to be quite comparable to that corresponding to the PtNPs catalyst. 4. Conclusion Bimetallic Au-PtNPs have been electrosynthesized on ITO surfaces. The analytical techniques utilized to investigate the morphology and composition of the nanoparticles have confirmed that composite systems, with an alloy-type structure, have been obtained only if the Au/Pt molar ratio in the starting solution was 1:1 or 2:1. The resulting composition was Au20Pt80 and Au70Pt30. The deposition time does not effect the composition of Au-PtNPs, even if a pure alloy is generally obtained at longer times (800 s). When an excess of Pt ions was employed in the electrolytic solution, the formation of AuNPs and/or PtNPs in separate domains occurred. The solvent oxidation is shifted toward more positive values on Au-Pt with respect to Au nanoparticles. Moreover, Au2Pt1 nanoparticles (corresponding to Au70Pt30 composition) present the highest catalytic efficiency toward MeOH oxidation and the highest surface coverage. The results demonstrate the promising potential of the one-step electrosynthesis of bimetallic Au-PtNPs for application in alkaline fuel cells. Acknowledgment. The authors thank the University of Bologna and the Ministero della Istruzione, Universita` e Ricerca (MIUR), for financial support. The authors thank Dr. S. Stipa and Dr. E. Boanini for help in acquiring and for discussion of SEM images and EDS mapping. References and Notes (1) Guo, S.; Wang, E. Anal. Chim. Acta 2007, 598, 181–192. (2) Daniel, M.; Astruc, D. Chem. ReV. 2004, 104, 293–346. (3) Compton, R. G.; Wildgoose, G. G.; Rees, N. V.; Streeter, I.; Baro, R. Chem. Phys. Lett. 2008, 459, 1–17. (4) Cheng, W.; Han, X.; Wang, E.; Dong, S. Electroanalysis 2004, 16, 127–131.
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