Optimization of Synthesis Parameters Employed during Pt

Aug 22, 2007 - H. A. Andreas, S. K. Y. Kung, E. J. McLeod, J. L. Young, and V. I. Birss*. Department of Chemistry, University of Calgary, Calgary, Alb...
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J. Phys. Chem. C 2007, 111, 13321-13330

13321

Optimization of Synthesis Parameters Employed during Pt Nanoparticle Formation by in situ Reduction H. A. Andreas,‡ S. K. Y. Kung,† E. J. McLeod,† J. L. Young,† and V. I. Birss*,† Department of Chemistry, UniVersity of Calgary, Calgary, Alberta, Canada T2N 1N4, and Department of Chemistry, Dalhousie UniVersity, Halifax, NS, Canada B3H 4J3 ReceiVed: April 23, 2007; In Final Form: June 29, 2007

This work is focused on the optimization of the synthesis conditions of a Pt sol phase containing suspended metallic Pt nanoparticles with the primary goal being to produce thin (ca. 1 monolayer) Pt films having the highest possible electroactive surface area per gram. This is gauged here by the surface roughness factor, determined from the magnitude of the Pt electrochemical response in sulfuric acid solution. Two Pt(IV) chloride compounds (H2PtCl6, Na2PtCl6) are shown to be the best Pt precursors, producing stable Pt nanoparticles with an average particle diameter of 1-3 nm. Sodium ethoxide and formic acid are found to be excellent reducing agents of the PtCl62- anion, although formaldehyde results in a lower yield of Pt nanoparticles. A ratio of sodium ethoxide to H2PtCl6 of 2:1 and a 72 °C reflux in ethanol between 30 min and 5 h resulted in the highest Pt roughness factor (ca. 8). Transmission electron microscopy analysis has verified that all of the reducing agents produce Pt particles of a similar size and that the higher roughness factors are the result of a higher yield of Pt nanoparticles. The effect of time of storage of Pt sols formed using sodium ethoxide showed that only a minor aging effect is observed over long periods of time, likely minimized by the stabilization offered by PtCl3(C2H4)-, a species formed as a byproduct during the synthesis.

1. Introduction Of the many methods used to form stable Pt nanoparticles for deposition on low cost, high surface area supports, one of the most discussed in the literature is the in situ reduction of Pt salts using a variety of reducing agents. These include formate,1,2 formaldehyde,2,3 hydrazine,2,4,5 borohydride,4,6-8 sodium citrate,9-12 and sodium thiosulfate.13 Generally, prior research has focused on the deposition of Pt nanoparticles directly onto high surface area carbon powder (for fuel cell applications), normally added to the synthesis mixture. The aim of the present in situ reduction work, however, has been to keep the Pt nanoparticles suspended in the solution phase and then to use this “sol” state to easily form thin, highly electroactive Pt nanoparticulate films that may be deposited on a range of substrates. Our previous work involved the in situ reduction of hexachloroplatinic acid (CPA) using sodium ethoxide (NaOC2H5) as the reducing agent.14,15 It was shown14 that this alkoxide synthesis route results in the formation of metallic Pt nanoparticles (ca. 1-3 nm in diameter), which undergo sintering at higher temperatures. Inductively coupled plasma atomic emission spectroscopy and electrochemistry were used to show that under the particular conditions used ∼75% of the Pt in the sol phase is in the form of metallic Pt nanoparticles.15,16 The remainder of the Pt is retained in the sol in the form of a residual partly oxidized Pt(II) species (NaPtCl3(C2H4)), a sodium analogue of Zeise’s salt.15,16 A mechanism for the in situ reduction of H2PtCl6 to Pt using NaOC2H5 has also been proposed15 (Reactions 1-6). In a * Corresponding author. E-mail: [email protected]. Phone: (403) 2206432. Fax: (403) 289-9488. † University of Calgary. ‡ Dalhousie University.

metathesis reaction, one Cl- ion in H2PtCl6 is exchanged with an OC2H5- ion from NaOC2H5, yielding an unstable Pt(IV)(OC2H5)Cl5-2 species and NaCl. This Pt(IV) species then undergoes a β-hydride elimination step, producing acetaldehyde and Pt(IV)HCl5-2. A further reductive elimination step produces HCl and Pt(II)Cl4-2. The process is repeated, reducing this Pt(II) species to Pt(0).15 NMR and X-ray Photoelectron Spectroscopy (XPS)14-15 data have provided evidence for the presence of only metallic Pt (the final product) and PtCl3(C2H4)(produced in a side reaction). There was no evidence for the formation of any other stable Pt intermediates, suggesting that this reaction goes to completion.

[Pt(IV)Cl6]2- + Na+ + OC2H5- f [Pt(IV)Cl5(OC2H5)]2- + NaCl (Reaction 1) [Pt(IV)Cl5(OC2H5)]2- f [Pt(IV)HCl5]2- + CH3COH (Reaction 2) [Pt(IV)Cl5H]2- f [Pt(II)Cl4]2- + HCl (Reaction 3) [Pt(II)Cl4]2- + Na+ + OC2H5- f [Pt(II)Cl3(OC2H5)]2- + NaCl (Reaction 4) [Pt(II)Cl3(OC2H5)]2- f [Pt(II)HCl3]2- + CH3COH (Reaction 5) [Pt(II)Cl3H]2- f Pt(0) + HCl + 2Cl(Reaction 6) The purpose of the present paper is to report on the optimization of the formation of Pt nanoparticles using this general approach by varying the synthesis conditions, including the nature of the reducing agent (RA), the Pt precursor (Ptpre), the RA to Ptpre ratio, the synthesis temperature (Ts), and the

10.1021/jp073111u CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007

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Andreas et al.

TABLE 1: The Synthesis Variables Used for the Formation of Pt-Containing Solsa

a

sol type

reducing agent (RA)

Pt precursor (Ptpre)

A B C D E Z

NaOC2H5 (yellow/white) HCOOH (clear liquid) HCHO (clear liquid) NaOC2H5 NaOC2H5 none

CPA (deep orange) CPA (deep orange) CPA (deep orange) Na2PtCl6 (orange) Pt(acac) (red/brown) CPA (deep orange)

sol color

precipitate (PPT) color

colors vary with RA:CPA ratio orange/yellow no PPT orange/yellow no PPT yellow black/dark gray red/brown dark green/black orange no PPT

All colors described are based on sols formed using 0.12 mol/L Ptpre in ethanol at a synthesis temperature of 72 °C and maintained for 2 h.

synthesis time. Mutations in these variables have been found to greatly affect the effectiveness of this synthetic route in forming metallic Pt, as determined by the electrochemically measured roughness factor (γ). The roughness factor is a measure of the electroactive surface area of the Pt nanoparticles after deposition on an electrode surface, as calculated from the hydrogen underpotential deposition (H upd) charge and comparison with the geometric area of the electrode.17,18 An increase in γ could be due to a decrease in particle size and a concomitant increase in the total surface area, or to the deposition of a greater number of nanoparticles on the electrode surface (due to a higher yield of particles in the synthesis and/or a more efficient transfer of nanoparticles from the sol phase to the substrate). In the present work, the Pt particle size is shown to be independent of the sol synthesis conditions and thus is related only to the number of nanoparticles transferred from the sol phase to the Au substrate. Here, all of the films were deposited by dip-coating. It has been previously shown19 that the thickness of the liquid film deposited by dip-coating from a sol is proportional to its viscosity, and therefore the sols formed here (given their similar viscosities) would be expected to deposit liquid films of very similar thickness (similar volume). Thus, the changes in γ are the result of the change in Pt nanoparticle concentration (Pt yield) in the sol synthesis. It should also be noted that because these Pt nanoparticles are designed to be deposited as thin films on a variety of substrates, our key focus is on developing approaches to maximize the deposited Pt electroactive area. 2. Experimental Methods 2.1. Synthesis of Pt Sols. The Pt sol synthesis employed in this work involved reacting a Pt precursor (0.12 mol/L of Pt) with a reducing agent in various molar ratios (Table 1) in 10 mL of absolute ethanol, using an appropriate temperature and length of time, all under Ar. The precipitate (PPT) was then filtered from the supernatant (the sol), and both the sol and PPT were kept in sealed containers for further testing. The sol types are classified here based on the RA and the Ptpre employed (Table 1). Four different types of sols were synthesized using a Ptpre of H2PtCl6‚H2O (CPA, Aldrich, 99.995% pure, >37.50% metal basis), but different RAs. Type A sols were formed using sodium ethoxide (NaOC2H5, Aldrich, 96% pure), Type B employed formic acid (HCOOH, Aldrich, 96%), Type C involved formaldehyde (HCHO, Aldrich, 37% in H2O), while Type Z sols involved no RA (serving as the blank). As well, two additional types of sols were synthesized using NaOC2H5 as the RA but using two different Pt precursors, that is, Type D sols employed sodium hexachloroplatinate (Na2PtCl6, Aldrich, 98% pure) as the precursor, while Type E sols used Pt acetylacetonate (Pt(acac), Aldrich, 99.99% pure). Different molar ratios of the Ptpre and RA were also examined and are denoted by the subscripts associated with the letter label. These numerical subscripts represent approximately the ratio of the number of moles of RA to Pt used in the synthesis.

2.2. Preparation of Pt Thin Films. The substrates for the Pt-based films were 0.5 to 1 cm2 glass slides onto which were sputtered a ca. 3 nm thick layer of Ti followed by a ca. 120 nm layer of Au. The Au substrate was chosen for this work as it does not contribute significantly to the cyclic voltammetric signal over the range of potential of interest and therefore does not interfere with the calculation of the Pt γ, as will be described in Section 2.3. These substrates were coated by their immersion in the sol and were then removed at a constant withdrawal rate (w/r) of 60 cm/min (shown previously to yield ca. one monolayer of Pt nanoparticles14), followed by drying in air at either 22 °C for at least 24 h or 200 °C for 15 min. Electrical connection to the working electrode (WE) was made via a Parafilm-wrapped alligator clip. The WE was rinsed with distilled water to remove any water-soluble contaminants before being placed in the electrochemical cell solution. 2.3. Electrochemical Instrumentation and Measurements. A Pt gauze counter electrode (CE) and a reversible hydrogen electrode (RHE) reference electrode were used in all of this work, and all electrochemical potentials are referenced to the RHE. The electrochemical measurements were performed in a three electrode, two compartment cell (with a Luggin capillary connecting the RE compartment to the main compartment containing the CE and WE) containing 35 mL of 0.5 M H2SO4. The solution in the main compartment was deaerated before and during electrochemical measurements by continuously bubbling N2 through or over the solution. All electrochemical measurements were carried out at room temperature (22 ( 2 °C). Electrochemical measurements were performed using a PARC EG&G 173 potentiostat and 175 programmer, and the results were recorded on an HP 7045B X/Y or an HP 7090A X/Y recorder. Computer-controlled cyclic voltammetry (CV) was performed using a PARC EG&G 271 potentiostat under Corrware control with data analysis carried out using Corrview software. The electrochemical cleaning regime used for the Pt nanoparticle thin films depended on the film-drying temperature. Films dried at 22 °C required the use of a less aggressive cleaning protocol due to their physical instability14 and were thus cleaned by cycling the potential between 0.05 and 1.6 V at 100 mV/s until a steady-state Pt signal was obtained. For films dried at 200 °C, the potential was cycled between -0.05 and 1.7 V for 3 min at a sweep rate of 1000 mV/s. Electrochemical quartz crystal microbalance (EQCM) experiments carried out in parallel14,20 confirmed that these potential cycling regimes are sufficient to remove organic contamination (e.g., ethanol solvent). Also, this procedure does not result in any Pt dissolution or electrochemically induced agglomeration or in the reduction/deposition (of Pt) of any dissolved Pt species. The determination of the electroactive Pt area was made using the measured hydrogen underpotential adsorption/desorption charge (H upd charge), assuming it to be 0.22 mC per real cm2 of electroactive area.21 The γ-value is then obtained from the

Pt Nanoparticle Formation by in situ Reduction

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ratio of the electroactive to geometric area of the Pt.17,18 All of the CVs are plotted in terms of current density, that is, as the current per geometric area. The error bars shown on the graphs are one standard deviation from the average, based on at least three measurements. 2.4. X-ray Diffraction Analysis (XRD). Thick films composed of Pt nanoparticles were formed by deposition of the Pt sol onto a glass XRD slide and drying at either 22 °C for 24 h or 200 °C for 15 min. XRD data were obtained using a Rigaku Multiflex powder diffractometer (Department of Geology and Geophysics, University of Calgary). The components were then compared with the JADE XRD pattern processing (Release 5.0.12) databases of d-spacings. 2.5. Transmission Electron Microscopy (TEM). The Pt nanoparticle-based films were examined using a TECNAI-F20 TEM (Microscopy and Imaging Facility, University of Calgary) in scanning transmission electron microscopy mode. The samples were made by depositing small aliquots of the sols (0.05-0.08 mL) onto carbon-supported Cu TEM grids (J.B. EM Services, Inc.) and then drying at 22 °C for several days.

The sols formed using HCOOH and HCHO as the RA (Type B and C sols, respectively) were synthesized in an identical manner to the Type A sols. As can be seen in Table 1, contrary to the Type A synthesis, neither the Type B or C syntheses yielded a PPT. This latter observation is unsurprising, considering the absence of Na+ in the Type B and C reactants and the proposed coprecipitation14,15 of Pt with NaCl. The mechanism of Pt reduction from Pt(IV) to Pt(0) by HCOOH is likely a fairly straightforward process (Reactions 7-9), involving the oxidative cleavage of the carboxylic acid (HCOOH) via the Kolbe Reaction,28 where the Pt(IV) center abstracts an electron from the ether oxygen of formate (reducing the Pt center to Pt(III)). The HCOO‚ radical then fragments to CO2 and a H-radical, which can further reduce the Pt(III) center to Pt(II). The process is then repeated to reduce the Pt(II) center to Pt(0).

3. Results and Discussion 3.1. Comparison of Pt Nanoparticles Formed Using Different Reducing Agents. To rigorously determine the effect of the reducing agent (RA) on the Pt sol characteristics, CPA was employed as the Pt precursor (Ptpre) in this part of the research. CPA was chosen as it is readily soluble in ethanol (our desired solvent, based on our previous research with Ir22,23) and, as will be shown below, it is the precursor leading to Pt films with the highest roughness factors. In general, only relatively weak RAs (NaOC2H5, HCHO, and HCOOH) were examined here to promote the relatively slow formation of small, suspended Pt nanoparticles, rather than accelerating the reduction process and precipitating large Pt particles out of solution. The relative strength of these reducing agents in anhydrous solvents is NaOC2H5 > HCOOH > HCHO. However, the presence of water in HCHO results in its hydration to form24 CH2(OH)2, which is a slightly stronger reducing agent than HCOOH. Indeed, CH2(OH)2 may be oxidized to formic acid25 during the reduction of Pt, as will be discussed below. The oxidation of primary alcohols to their respective carboxylic acids is well known when using Cr6+ and MnO3-,26,27 and it is likely that our Pt4+ system is undergoing the same type of reaction. Thus, the relative strengths of the reducing agents are more properly the following: NaOC2H5 > HCHO (in the form of CH2(OH)2) > HCOOH. 3.1.1. General Properties of Pt-Containing Sols and Precipitates Formed Using Sodium Ethoxide. The color of the various sols and of their associated PPTs is shown in Table 1. The Type A sol color varied from orange to light yellow, depending on the ratio of RA/Ptpre, with higher ratios resulting in lighter colored sols and darker PPTs, shown previously to indicate that a larger fraction of the Pt-containing anion had been reduced to Pt(0).15 It was also shown in our previous work14,15 that the Type A synthesis PPT consisted solely of NaCl and metallic Pt nanoparticles (6 nm average diameter) and that darker PPTs contained a significantly higher percentage of metallic Pt versus lighter ones with Pt coprecipitating with the relatively insoluble NaCl product. In general, as the goal of this work is to produce Pt nanoparticles that remain suspended in solution, syntheses giving lighter colored PPTs are preferred with up to 75% of the Pt in sol form.15,16 As the primary purpose of the present work is to maximize the extent of Pt(IV) reduction to form Pt nanoparticles, the effect of the RA/Ptpre ratio was examined more fully in Section 3.2.

H‚ + Pt(III)Cl41- f Pt(II)Cl42- + H+ (Reaction 9)

Pt(IV)Cl62- + HCOO- f Pt(III)Cl41- + HCOO‚ + 2Cl(Reaction 7) HCOO‚ f CO2 + H‚

(Reaction 8)

Hydrated formaldehyde (CH2(OH)2) can reduce the Pt(IV) complex by first undergoing an oxidation of one of the hydrate oxygens to a carbonyl group, giving the carboxylic acid (formic acid),26,27 after which the reduction may then follow Reactions 7-9. The primary alcohol oxidation (Reaction 10) is probably mediated by the Pt(IV) center, through the formation of a Ptester with one of the oxygens of CH2(OH)2 via a nucleophilic attack at Pt (Reaction 10) (releasing H+ from a OH group of CH2(OH)2). The electrons in the Pt-ester bond then serve to reduce the Pt(IV) center to Pt(II), releasing H+ from the hydrated formaldehyde (Reaction 11), resulting in the formation of formic acid. This process may be repeated with another hydrated formaldehyde to further reduce Pt(II) to Pt(0). Alternatively, because the RA has now been oxidized to formic acid, it may now follow Reactions 7-9 above to complete the reduction process.

CH2(OH)2 + Pt(IV)Cl62- f CH2(OH)O-Pt(IV)Cl4- + 2Cl- + H+ (Reaction 10) CH2(OH)O-Pt(IV)Cl4- f H+ + HCOOH + Pt(II)Cl42(Reaction 11) 3.1.2. Electrochemistry of Films Formed from Pt Sols Synthesized Using Different Reducing Agents. Thin Pt nanoparticle films were formed by the withdrawal (at a constant rate (w/r) of 60 cm/min) of Au sputter-coated glass electrodes from the Pt sol phase (of constant Pt content). Previous studies of sol behavior19 (and the Landau theory)19 have suggested that the same w/r for each film preparation ensures that the liquid films are of constant thickness, given similar solution characteristics (primarily viscosity). In our work, all sols have similar viscosities (being composed of mainly ethanol with small solute concentrations), and therefore the dip-coating method ensures that the same volume of sol is transferred to the electrode surface. Under the conditions of these experiments, our previous EQCM experiments have shown14 that these films consist of 1-2 monolayers of Pt nanoparticles, assuming tight packing. Figure 1a shows the typical CV responses of films formed from sols A2.5:1, B2.5:1, and C2.5:1 (all synthesized at a constant RA/ Ptpre ratio of 2.5:1). Upon comparison with the CV of a

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Figure 1. Steady-state CVs (in 0.5 M H2SO4, 100 mV/s) of (a) 22 °C dried films (deposited at 60 cm/min) formed from Type A (NaOC2H5, dotted line), Type B (HCOOH, dashed line) and Type C (HCHO, solid line) sols, and (b) CV of bulk Pt foil. The γ-value for each sol in (a) is shown in the legend.

polycrystalline Pt foil (Figure 1b), it can be readily seen that all of the films, and hence the sols, contain metallic Pt. This is evidenced by the presence of the H upd peaks seen in each CV between 0 and 0.3 V, the Pt oxidation plateau at 0.9 to 1.6 V, and the Pt oxide reduction peak centered at ca. 0.7 V. Given that each sol was synthesized from the Pt(IV) CPA precursor, these data confirm that all of the RAs successfully reduced CPA to metallic Pt. It has been previously shown using EQCM14 that the Pt signal seen in these experiments is not due to the electrochemical reduction of any dissolved Pt species, as this would be detected by a mass gain with electrochemical cycling, which is not seen. The small anodic wave at ca. 1.4 V and the small cathodic peak at 1.2 V (particularly obvious in the CV response of the sol B film in Figure 1a) are evidence of the formation/reduction of Au oxide, showing that the underlying Au substrate is still partially accessible to the acidic solution. Sol B-formed films gave the smallest Pt and largest Au CV features, indicating a low coverage by Pt, while sol A gave the largest (almost full) coverage of the Au substrate by Pt. A comparison of the CV responses of all of these Pt films, dried at ca. 22 °C (Figure 1a), indicates clearly that the film

Andreas et al. formed from sol A2.5:1 gives the highest roughness factor (highest electroactive Pt area, inset in Figure 1a) and hence the highest amount of usable Pt surface per geometric area of the electrode. This is followed by the HCHO- and then HCOOH-formed films. As will be shown in Sections 3.1.3 and 3.1.4, the enhanced γ for sol A (versus sols B and C) is not due to a decrease in the Pt particle size but rather arises from an increase in the amount of Pt deposited on the substrate and therefore is due to an increase in the yield of Pt nanoparticles formed during the synthesis. For sol B, it will be shown that less metallic Pt is formed in the synthesis, while for sol C, it is likely that a high degree of aggregation results in the smaller γ-values. Again, as our dip-coating procedure involved a constant withdrawal rate, resulting in a constant volume of solution being deposited,19 any differences in the γ-value will therefore be due either to differences in size or yield of Pt nanoparticles in that volume. The γ-values for these sols, all synthesized using the same RA/ Ptpre ratio, follow the trend expected with the stronger reducing agent (NaOC2H5) producing the highest γ (and therefore the largest amount of metallic Pt) and vice versa. 3.1.3. Transmission Electron Microscopy Study of Pt Nanoparticles. The TEM images of films formed from sols A2.5:1, B2.5:1, and C2.5:1 are shown in Figure 2 (panels a, b, and c, respectively). The sols formed using sodium ethoxide (Figure 2a), formic acid (Figure 2b), and formaldehyde (Figure 2c) all have approximately the same average particle diameter (1-2 nm). Given that each sol contains particles of the same size, the differences in γ are not due to different particle sizes. Indeed, as will be shown below, the smaller γ-values obtained from the Type B sol (Figure 1a) is due to less Pt(0) formed during that synthesis, while the smaller γ-values for the Type C sol (Figure 1a) reflect significant aggregation that occurs during nanoparticle deposition. At least one crystalline component is seen in the TEM images of both sol Types A- and B-formed films (Figure 2a,b insets, respectively). As discussed previously,14,15 XPS data coupled with NMR results suggest that one of the crystalline components in sol A (25% of the total Pt16) is the still partly oxidized NaPt(II)Cl3(C2H4) complex, a sodium analogue of Zeise’s salt (KPtCl3(C2H4)), formed in a side reaction. This result is unsurprising, as Anderson29 has shown that Zeise’s salt can be formed using a similar approach (refluxing of the potassium salt in ethanol). The greater prevalence of the crystalline species and larger crystal size seen by TEM in sol B-formed films suggests that a higher percentage of the Pt is in an oxidized form in this sol and is thus not seen in the CV response, explaining the significantly smaller γ-value of sol B versus sols A and C in Figure 1, and suggesting that the γ-values are tracking the yield of Pt(0) formed in the synthesis of sol B versus

Figure 2. TEM images of Pt sol-based films synthesized using CPA as the Ptpre and sodium ethoxide (a, sol A), formic acid (b, sol B), and formaldehyde (c, sol C), as the reducing agent. Insets show the crystalline species (a and b) formed and the aggregation (c) seen at low magnifications in these respective films.

Pt Nanoparticle Formation by in situ Reduction sol A. Again, this demonstrates that the stronger RAs are more successful in the reduction of Pt(IV) to Pt(0). In the case of sol C, the TEM images show evidence for significant aggregation of the Pt nanoparticles upon deposition and drying (inset, Figure 2c). Aggregation will serve to lower the fraction of the Pt, which is electrochemically active, as particles buried within the aggregate will not be in contact with the electrolyte and are therefore not electrochemically active. This is also consistent with the CV and γ-results shown in Figure 1a. As will be discussed below, the PtCl3(C2H4)- ion is believed to act as a sol stabilizer, keeping the Pt nanoparticles small and suspended in the liquid phase. This is particularly important in sol Type A, where the Pt nanoparticles coprecipitate with NaCl if the stabilizer is not present (as will be shown in Section 3.2 and discussed in a previous paper16). This stabilizer may also keep the nanoparticles spatially separated as they are deposited on the surface, preventing their aggregation during film formation. However, this mechanism fails in sol Type C, perhaps related to the presence of water in this synthesis. Indeed, after reaction completion, the only difference between the products in sols B and C (as per the mechanisms and XRD data, shown below) is the presence of water in sol C, which is introduced in the solvent of the HCHO reducing agent. This results in longer drying times for these films, allowing for a process similar to reaction-limited aggregation, where the particles have sufficient time to move to preferred sites before adhering.30 This leads to the formation of more compact aggregates,30 such as those seen in Figure 2c. 3.1.4. X-ray Diffraction Analysis of Pt Nanoparticle-Based Films. The XRD patterns of films formed from the Type A2.5:1, B2.5:1, C2.5:1, and Z sols exhibit no evidence for metallic Pt (at 2θ of 39.8 and 46.3) when the films were dried at 22 °C (Figure 3a-d), likely indicative of the very small Pt particle sizes (Figure 2) and resulting broad peaks. However, these patterns all have one crystalline component in common (peaks at 2θ of 11.26, 12.78, 14.34, 22.50, 26.94, 33.52, 35.2, and 43.9, denoted by a vertical line in Figure 3). This must be the result of refluxing CPA in ethanol, as evidenced by the presence of these peaks in the XRD pattern of the Type Z sol. Comparison with the XRD database31 showed that this material does not match any previously known Pt compound. As our previous thermal gravimetric analysis/differential scanning calorimetry studies15 showed that no starting material was present in sol Type A, this species must be a product of the reaction between CPA and ethanol. It is interesting that our previous NMR studies on the sol phase showed only one Pt signature: that of Zeise’s anion.15 This suggests that both of the crystalline species seen by XRD are analogues of each other with both containing Zeise’s anion. Thus, it is likely that this second crystalline species seen by XRD is HPtCl3(C2H4), commonly called Zeise’s acid,32 which may be formed by boiling Na2PtCl6 in ethanol, a process quite similar to the synthesis carried out here. A direct comparison cannot be made, as an extensive literature search has not revealed a previously published XRD pattern of HPtCl3(C2H4) or its degradation product, PtCl2(C2H4). Thus, the present data may represent the first published XRD pattern for this product. The Type A sol-formed film also shows XRD evidence for a second crystalline component (denoted by filled circles in Figure 3a) with peaks at 2θ values of 11.10, 17.00, 21.95, 44.6, 45.95, 52.05, and 52.65, which again are likely to be due to NaPtCl3(C2H4). Similarly, sol Types B and C, dried at 22 °C, also contain evidence for a second crystalline component in the XRD pattern (at 2θ of 18.02 and 36.56, denoted by a triangle),

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Figure 3. XRD patterns of various Pt sol films: (a) Type A2.5:1 dried at 22 °C, (b) Type B2.5:1 dried at 22 °C, (c) Type C2.5:1 dried at 22 °C, (d) Type Z dried at 22 °C, (e) Type A2.5:1 dried at 200 °C, (f) Type B2.5:1 dried at 200 °C, (g) Type C2.5:1 dried at 200 °C, and (h) Type Z dried at 200 °C (see Table 1 for explanation of sol descriptors).

although this component and its mechanism of formation are, as yet, unidentified. Again, comparison of the XRD patterns with the database confirms that this species is not a known Ptcontaining compound. When the sol-derived films were dried at 200 °C for 15 min, a signal for metallic Pt is seen in the small, broad peaks at 2θ of 39.7 (sols A-C, Figure 3e-g). When these were analyzed using the Scherrer equation, the average particle diameters were 6 nm for sol A and 8 nm for both sols B and C. As shown previously14 by TEM for sol A-formed films, the Pt nanoparticles can undergo thermally induced coalescence at this temperature, resulting in particles large enough to be seen easily by XRD. Although we have shown previously15,16 that the NaPtCl3(C2H4) species may thermally degrade to form Pt(0) at 200 °C, the Pt(0) signal seen in Figure 3e is not believed to have arisen from such a process. This is because a similar Pt peak is seen for sols that contained similarly sized nanoparticles but did not contain NaPtCl3(C2H4) (Figure 3f,g). It is likely that the Pt(0) formed in this degradation reaction is too low in yield to be detected by this technique (i.e., less than 5% of the film), and thus the presence of Pt(0) may have arisen from the aggregation of the nanoparticles seen in Figure 2. It was also shown previously14 that, for the Type A sol, the Zeise’s acid component undergoes thermal degradation when dried in air at 200 °C for 15 min. Comparison of the patterns of all of the sols dried at 200 °C with their 22 °C dried counterparts shows that the same degradation process occurs in all cases (sols A-C and Z) with a new set of peaks appearing (2θ of 21.88, 23.64, 27.50, 30.24, 33.74, 39.70, and 45.20, denoted with an “x” on Figure 3) along with a signal for crystalline PtCl2 (at 2θ of 12.50, 13.20, and 35.00, denoted with

13326 J. Phys. Chem. C, Vol. 111, No. 36, 2007 hollow square). This thermally formed crystalline species cannot be identified by comparison with the XRD database31 but is seen in all of the XRD patterns of sols dried to 200 °C, independent of the RA used. It has been previously shown that ethylene may be liberated from Zeise’s acid when heated,32 and that some versions of Zeise’s salt decompose to produce K2PtCl4 + PtCl2 (and other products).33 A similar decomposition may be occurring here (supported by the appearance of PtCl2), forming H2PtCl4 (or possibly a Na2PtCl4 species). Again, no XRD data can be found in the literature to make a direct comparison with the results shown here. The NaPtCl3(C2H4) component of Type A sol films dried at 22 °C (at 2θ of 11.10, 17.00, 21.95, and 44.60) also undergoes thermal degradation when dried at 200 °C, as this signal then disappears thus preventing its identification. It was suggested previously16 that some degradation, forming metallic Pt and possibly Cl2, may occur, which is a possible explanation for the disappearance of these peaks. The second crystalline species in evidence in the sol B and C patterns (at 2θ of 18.02 and 36.56, denoted with a hollow diamond in Figure 3) is still present at 200 °C and remains unidentified. 3.2. Effect of Reducing Agent to Pt Precursor Ratio on Pt Film Roughness Factor and Particle Size. It has been previously shown using Type A sols14,15 that the RA/Ptpre ratio can have a significant effect on the color and composition of the sol phase and PPT resulting from the synthesis. The mechanism (Reactions 1-6) of CPA reduction by sodium ethoxide predicts that 2 molar equiv of sodium ethoxide are needed to allow its full reduction to Pt(0); this will be tested in the present work. An additional goal of this work was to establish the optimum RA/Ptpre ratio for Pt nanoparticle formation for sols B and C and also to obtain insight into the sol formation mechanism. Again, the proposed mechanism for Pt reduction by HCOOH (Reactions 7-11) predicts that 2 mol of RA would be required to completely reduce Pt(IV) to Pt(0). The HCHO reducing agent, however, may require either 1 or 2 RA equivalents, based on how the reaction proceeds. If the reduction proceeds via one iteration of Reactions 10 and 11 followed by one iteration of Reactions 7-9, only one RA equivalent will be required to complete the reduction to Pt(0). However, if the reduction proceeds by two consecutive iterations of Reactions 10 and 11, two RA equivalents are require to complete this reduction. In all of these experiments, the Pt sol synthesis was carried out at a reflux temperature of 72 °C for 2 h, followed by stirring for ca. 18 h at 22 °C, all under Ar, while the Type Z procedure involved no reducing agent (RA/CPA ) 0:1). The effectiveness of the synthesis of Pt nanoparticles was gauged from the γ-values, obtained from the steady-state CV response of films formed on Au at 60 cm/min, and then dried at 22 °C for 24 h or 200 °C for 15 min. Figure 4a shows that the γ-values do indeed depend markedly on the NaOC2H5:CPA ratio for the Type A sol-formed films with the optimum RA/Ptpre ratio for both the 200 and 22 °C dried films being ∼2:1 (sol Type A2:1). This result agrees well with our proposed mechanism of Pt reduction (Reactions 1-6). The table in the inset of Figure 4a shows that an increase in the RA/CPA ratio results in a darker colored PPT (higher Pt content15). Taken together, these results demonstrate that the lower γ-values seen at higher RA/Ptpre ratios is indicative of less Pt in the sol phase and more in the PPT. Conversely, at low RA/Ptpre ratios, the PPT color, coupled with the low γ-values, demonstrate that there is insufficient RA available to allow the reaction to proceed to completion, resulting in a low yield of Pt(0).

Andreas et al.

Figure 4. Impact of RA/CPA ratio on γ for Pt sols Types (a) A, (b) B, and (c) C. The films were formed on Au at a withdrawal rate of 60 cm/min and dried at 22 (dashed lines) or 200 °C for 15 min (solid lines). Inset table: colors of the precipitate of Type A sols formed using different RA/CPA molar ratios.

It is also obvious from Figure 4a that the γ-values obtained for films dried at 200 °C are higher than at 22 °C, likely due to improved interconnectivity between particles and between the Pt film and substrate.14 Also, the NaPtCl3(C2H4) byproduct (XRD pattern in Figure 3a) has been shown to be thermally converted both to the second crystalline component (Figure 3b) and to Pt,15,16 which may also be in the form of nanoparticles, thus further increasing the roughness factor of the film. On the basis of the mechanism proposed, it was expected for sol B that 2 mol of HCOOH would be required to complete the reduction of Pt(IV) to Pt(II). In fact, Figure 4b shows a maximum γ-value at a RA/Ptpre ratio just below 2, likely due to a competitive reaction involving the very unstable H-radical, for example, the reaction of two H-radicals to form H2. At higher RA concentrations, more radicals will be present, and the reaction between two highly unstable H-radicals to form H2 is expected to be favored over a reaction between an H-radical and a relatively stable PtCl42- species, thus generating a γ-value profile similar to Figure 4b. The fact that the maximum γ-value for both Type A and B Pt sols is similar (at ca. 4 for films dried at 22 °C) argues that approximately the same extent of in

Pt Nanoparticle Formation by in situ Reduction

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13327

Figure 5. TEM images of Type A sols formed using a NaOC2H5:CPA ratio of (a) 2:1, (b) 3.5:1, and (c) 5:1. These films were formed by depositing 0.05-0.08 mL of sol onto a carbon-coated grid and then dried in air at 22 °C for several days.

situ reduction of CPA is occurring in the two syntheses, even though the RAs are quite different. Figure 4b shows a significant increase in the γ-values for Type B films dried at 200 versus 22 °C, perhaps due to improved electrical connectivity between the Pt nanoparticles.14,18 The γ-values for the Type C sol are seen in Figure 4c to be comparatively low and, unlike the Type A and B sols, they do not exhibit a maximum, likely due to the significant degree of aggregation seen in these films. This would argue for a Pt reduction mechanism that strongly depends on the molar ratio of HCHO to Ptpre and thus two iterations of Reactions 10 and 11 rather than one iteration of Reaction 10 and 11 followed by one iteration of Reactions 7-9 (using formic acid as the RA). If Pt reduction were strongly dependent on a second step involving formic acid as the RA, we would expect to see a drop in the amount of Pt produced, similar to what is seen with sol Type B. This suggests that the reduction of Pt occurs through the oxidation of two HCHO. A small enhancement of γ for films dried at 200 °C versus 22 °C is again seen. However, the maximum γ-value (Figure 4c) is still relatively low at all HCHO/ CPA ratios, compared to those achieved for Type A and B sols. Overall, the highest γ-values are achieved using the Type A2:1 Pt sols using NaOC2H5 as the RA, or the Type B1.25:1 sol using HCOOH. In contrast, HCHO (Type C sols) gives the lowest γ-values. This is contrary to what is expected based on the relative strengths of the RAs, as HCHO (in the form of CH2(OH)2) is a stronger reducing agent than is HCOOH. However, due to the significant degree of aggregation seen in the HCHO-formed sol, the effective γ-value would be expected to be lower. As discussed earlier,15 it is believed that the unreduced Pt(II) species in the sol (PtCl3(C2H4)-) probably serves as a nanoparticle stabilizer thus preventing significant aggregation and maintaining the size of the suspended Pt nanoparticles at ca. 2 nm. The addition of more reducing agent is expected to result in more complete reduction and thus insufficient stabilizer is likely to be present in the sol, causing precipitation. To test this hypothesis, the Pt nanoparticles in sols synthesized using a range of RA/Ptpre ratios were examined by TEM. Figure 5 shows that the Pt nanoparticles formed using sols A2:1 and A3.5:1 are very similar in size (ca. 1 nm). However, there is clearly a significantly lower yield of Pt particles in sol A3.5:1. Thus, the lower γ-values exhibited by sol A3.5:1 (Figure 4) can be explained by the lower concentration of Pt nanoparticles in the sol (and more Pt in the PPT, consistent with its darker color). It is also obvious from Figure 5 that there are significantly fewer Pt nanoparticles in the film formed using the highest RA/ Ptpre ratio (sol A5:1), serving to explain the lower γ-values obtained for the films formed from sol A5:1 (Figure 4). Additionally, Figure 5 shows that the particles are larger in the

Type A5:1 (ca. 2.5 nm) vs Type A2:1 sols (ca. 1 nm) and that the Type A5:1 sol particles are significantly more polydisperse, possibly the result of the lower concentration of the PtCl3(C2H4)stabilizer. This likely causes the suspended Pt particles to grow (and/or aggregate) and then precipitate, consistent with the darker color of the PPT. Indeed, it has been shown that the Pt nanoparticles found in the PPT are larger (ca. 6 nm)15 than those suspended in the sol phase (ca. 1-3 nm).14 As well, Pt is likely also being drawn out of the sol by coprecipitation with NaCl, which has a low solubility in ethanol (Reactions 1 and 4). This has been supported by SEM images showing Pt particles trapped in or on NaCl crystals.14 The TEM (Figure 5) and γ-data (Figure 4a) suggest that if the reaction is driven too far toward completion (lowering the PtCl3(C2H4)- stabilizer concentration and producing more NaCl), the concentration of Pt nanoparticles in the sol phase decreases, even though more metallic Pt is formed overall. On the basis of the fact that the highest γ-value is exhibited by sol A (using NaOC2H5 as a reducing agent) and that this sol likely contains a useful stabilizing agent NaPtCl3(C2H4),15 NaOC2H5 was used for further optimization of the Pt sol properties in the present work. Furthermore, the successful use of NaOC2H5 for Ir22,23 and Ru oxide nanoparticle formation34-36 and our interest in developing a one-pot synthesis of mixed noble metal nanoparticles supports our interest in further optimization of Pt nanoparticle formation using this reducing agent. 3.3. Effect of Pt Sol Synthesis Temperature and Time on Type A Pt Sol Properties. The effect of the sol synthesis temperature (TS, the maximum temperature of the synthesis) on Type A2:1 and A5:1 Pt sols was also examined to further optimize the yield of nanoparticulate Pt. This was considered to be particularly important for sols formed using high RA/CPA ratios, as by slowing down the synthesis process, the coprecipitation of Pt with NaCl could possibly be prevented. When TS was >22 °C, the temperature was kept constant for 2 h, followed by stirring at 22 °C for ca. 18 h (all under Ar). For TS ) 22 °C, the sol was stirred under Ar for a total of 20 h, while for still lower TS values (e.g., 2.5 °C), the sol was placed in an ice bath for a total of 20 h. The results (Figure 6a) show that high TS values of g72 °C are required to form significant amounts of metallic Pt from sol Type A2:1 (deposited as highly electroactive Pt films) and to overcome the energy barriers associated with the slow step of Reactions 1-6. Conversely, Figure 6b shows that for Type A5:1 sols, containing significantly more reducing agent versus Type A2:1, the optimum TS is ca. 25 °C. Above 25 °C, it can be seen from the inset table in Figure 6b that more of the Pt(0) produced in the reaction is found in the PPT. Thus, lowering the reaction rate by using lower synthesis temperatures allows

13328 J. Phys. Chem. C, Vol. 111, No. 36, 2007

Andreas et al.

Figure 6. Effect of synthesis temperature (TS) on γ-value of films composed of Type A2:1 (a) and A5:1 (b) Pt-derived sols (inset table: colors of the PPT phases formed at different synthesis temperatures). The Au substrate electrodes were coated at 60 cm/min and dried at 200 °C for 15 min.

more Pt(0) to remain suspended in the sol. Overall, the highest γ-value for the Type A5:1 sol, formed at 20 °C, is only about half that obtained for the Type A2:1 sol at 72 °C, and thus it is the latter synthesis conditions that are still considered optimum for Pt nanoparticle formation. An examination of the XRD patterns of films formed from Type A5:1 sols synthesized at different temperatures (Figure 7) supports the supposition that the reaction proceeds further toward completion as TS is increased, as expected. In Figure 7a, the XRD pattern of a film formed at 2.5 °C is seen to have three large peaks (at 2θ of 31.5, 38.5, and 49.5, peaks 1-3), and three very small peaks at 2θ of 27.5, 46.0, and 56.5 (peaks 4-6). Unfortunately, due to the hygroscopic nature of the CPA precursor and instrument limitations, an XRD pattern of the starting material itself could not be obtained. When compared to the 22 °C film (Figure 7b), the three large peaks are still in evidence, but now peaks 4-6 are much larger. These peaks are believed to be reflective of a reaction intermediate formed during the reduction of H2PtCl6 to Pt(0). This change in the XRD patterns and the increase in the γ-values (Figure 6a) show clearly that the production of Pt(0) has commenced at room temperature. In the XRD pattern of the 72 °C films (Figure 7c), the original peaks (peaks 1-3) have completely disappeared (arrows show the original positions), showing that the crystalline species has fully reacted and at least one new crystalline species has formed. The similarity between this XRD pattern and that for sol A2.5:1 dried at 22 °C (Figure 3a) suggests that the species formed in the A5:1 synthesis at 72 °C are again NaPtCl3(C2H4) and HPtCl3(C2H4), as discussed in Section 3.1.4. The time of reflux during Pt sol synthesis was also examined for a Type A2:1 Pt sol with all other conditions kept constant. After refluxing at 72 °C for a set time (30 min to 5 h), the sols were then stirred at 22 °C for 15 h, all under Ar. The γ-values show no obvious trends with synthesis time, and thus refluxing

Figure 7. XRD patterns of three Type A5:1 sols formed at different synthesis temperatures: (a) 2.5; (b) 22; and (c) 72 °C.

TABLE 2: Effect of Aging Time (Stored in a Sealed Glass Vial Filled with Air) on the Roughness Factor (γ) of a Type A5:1 Sol, Formed at 22 °C for 20 ha aging time (days)

g

0 15 45

3.2 ( 0.5 3.0 ( 0.3 2.7 ( 0.4

a Films were formed on Au at 60 cm/min and dried at 200 °C for 15 min.

time does not appear to be a variable in Pt nanoparticle formation using our approach. 3.4. Aging Characteristics of Pt Sols Formed Using Sodium Ethoxide as the Reducing Agent. Research on sols, in general, has shown that they often experience an aging effect, whereby the nanoparticles aggregate with time,37 resulting in a lowered surface area. This would be detected in our work with sol-formed thin films by a decrease in the Pt charge density (γ-values), as well as the continued formation of a black-colored PPT with time. In the case of Type A2.5 sols refluxed at 72 °C, no formation of PPT occurred during 45 days of storage in air (or, in some cases, even in 5 years of storage), suggesting that no significant sol aging is occurring. Table 2 shows that the γ-values of films formed from the aged Type A5:1 sol decrease by only ca. 15% after 45 days of storage in air, suggesting that a very small amount of aggregation may have occurred. As suggested in our earlier work,15 the

Pt Nanoparticle Formation by in situ Reduction PtCl3(C2H4)- anion byproduct likely serves as an excellent sol stabilizer, thus preventing nanoparticle aggregation. Even so, the small decrease in γ, shown in Table 2, suggests that these Pt sols should be used in their freshly formed state to achieve the smallest particle sizes and highest electroactive area. 3.5. Effect of Pt Precursor on Pt Sol Formation. We have shown previously15,16 that using CPA as the Ptpre for the synthesis of Pt nanoparticles yields a highly active Pt film, but that not all of the Pt(IV) is reduced fully to Pt(0), with ca. 25% undergoing a side reaction, forming the soluble NaPt(II)Cl3(C2H4) species. It was suspected that the acidity of the Pt precursor (H2PtCl6) may play a role in Pt sol formation, and thus sodium hexachloroplatinate (Na2PtCl6, Type D sol) was also examined as a Ptpre to determine if the cation (Na+ versus H+) would affect the observed γ-values. As well, a Pt(II) precursor (Pt acetylacetonate, Type E sol) was used to prevent the formation of NaPt(II)Cl3(C2H4) with the goal being to increase the yield of Pt nanoparticles and, in turn, to further enhance the γ-value associated with the resulting Pt sol-formed films. Because protons are produced when either HCOOH or HCHO are used as the RA (Reactions 7-11), the pH will not remain constant. Thus, to ensure that no H+ are produced during the synthesis, NaOC2H5 was chosen as the RA in these experiments. Each Ptpre was mixed with NaOC2H5 (the only RA used in these experiments) in various molar ratios, dissolved in 10 mL of ethanol, and refluxed at 72 °C for 2 h. The heat was then removed and stirring was continued for another 18 h, all under Ar. Any PPT formed was then filtered from the sol. Films formed from these sols were deposited on Au sputter-coated glass electrodes at 60 cm/min and dried at 200 °C to maximize the γ-values obtained (as per Figure 4). The dark gray color of the Type D PPT (Table 1) demonstrates that some reaction of the Ptpre has occurred, resulting in metallic Pt formation and, analogous to Type A sols, some coprecipitation with NaCl. The Type E synthesis also gave a dark PPT, again showing that some Pt had formed and precipitated during the synthesis. A full range of NaOC2H5/Na2PtCl6 ratios were examined, resulting in films exhibiting a similar γ-profile as seen with the Type A and B sols; however, the maximum γ-value (occurring at a NaO2H5/Na2PtCl6 ratio of 1:1) was a little lower (ca. 6) for the Type D versus Type A (ca. 8) films. This lower γ-value is again presumed to be due to the larger amount of Pt in the PPT, resulting in less metallic Pt suspended in the sol. This enhanced precipitation of Pt may be linked to the larger amount of NaCl produced in the Na2PtCl6 + NaOC2H5 (Type D) synthesis versus H2PtCl6 + NaOC2H5 (Type A). This suggests that changing the pH of the sol has little effect on Pt nanoparticle formation, arguing that despite the change in the Ptpre cation, the mechanism is still likely the in situ reduction reaction proposed in Reactions 1-6. The optimum NaOC2H5/Pt(II)(acac) ratio for the Type E sol was found to be approximately 1.25:1 NaOC2H5 per Pt(acac). However, even at this ratio, significantly smaller γ-values (ca. 1.1) were seen than when Pt(IV) precursors were employed (γvalues of 6 or 8). This may be due to a greater strength of interaction between Pt(II) and the acetylacetonate ligand, making it difficult for the ligand to be replaced by ethoxide in a metathesis reaction.15 Also, the Pt(IV) center is likely more reducible than Pt(II), allowing the relatively weak NaOC2H5 reducing agent to more easily reduce the Pt(IV) species fully to Pt(0). 4. Summary The optimization of various synthesis variables involved in the formation of a Pt sol phase, containing metallic Pt nano-

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13329 particles 1-3 nm in diameter, has been reported. Three different RAs were examined, NaOC2H5, HCOOH, and HCHO, and the optimum RA/CPA ratio was determined for each RA. The γ-value of thin films (∼ one monolayer) of Pt nanoparticles formed from these sols, coated using a constant withdrawal rate (constant solution volume transferred) on Au electrodes, were then compared. It was determined that although all three RAs were able to reduce the Pt(IV) precursor to Pt(0), NaOC2H5 and HCOOH are the most effective RAs, yielding the largest γ-values of 8 and 7, respectively, with an optimum RA/CPA ratio of 2:1 for NaOC2H5 and 1.25:1 for HCOOH. The sol formed using HCHO exhibited a very low γ-value, likely due to a significant amount of nanoparticle aggregation. TEM images of the sols formed using the different RAs showed that the particle size was the same in each case. Therefore, the roughness factor tracks the yield of Pt in these syntheses, and it is the high yield of the Pt nanoparticles that is crucial to the formation of highly electroactive Pt films. It was also shown that the incorporation of a drying step was beneficial for these Pt solformed films, and that drying at 200 °C results in better electrical connectivity within the film and thus in higher γ-values for films dried at this elevated temperature versus at 22 °C. The effect of Ptpre was also examined, using NaOC2H5 as the RA. The precursors examined were H2PtCl6 (CPA), Na2PtCl6, and Pt(acac). It was determined that CPA resulted in films (of ca. 1-2 monolayer thickness) exhibiting the highest γ-values (ca. 8). However, Na2PtCl6 also proved to be an excellent Ptpre, exhibiting maximum γ-values of ca. 6. Thus, the premium starting materials for Pt nanoparticle formation are NaOC2H5 and CPA with an optimum molar ratio of 2:1. This optimum ratio also supports the proposed reaction mechanism (Reactions 1-6),15 which suggests that two ethoxide ions are required to reduce the Pt(IV) center to metallic Pt. The synthesis temperature (TS) was also examined for films formed from NaOC2H5 and CPA sols with ratios of 2:1 and 5:1, respectively. It was found that as the RA/CPA ratio was increased, less heating was required to allow the formation of metallic Pt nanoparticles. Nevertheless, for the optimized Pt sol (Type A2:1), the ideal TS was 72 °C. Furthermore, the time of sol synthesis proved not to affect the Pt nanoparticle yield or time under any reaction conditions. Finally, it was determined that although Pt sol aging effects are minor, likely due to the stabilization effect of the PtCl3(C2H4)- byproduct, it is best to use the sol in a freshly formed state to achieve the highest electroactive area. Acknowledgment. The authors would like to acknowledge the financial support of the University of Calgary and the Natural Sciences and Engineering Research Council of Canada (NSERC) for the scholarship support of H.A.A. and for the overall support of this work. As well, the assistance of Dr. Jingbo (Louise) Liu in the collection of the XRD data is gratefully acknowledged. References and Notes (1) Wei, Z. D.; Zhang, S. T.; Tang, Z. Y.; Guo, H. T. J. Appl. Electrochem. 2000, 30, 723. (2) Goodenough, J. B.; Hamnett, A.; Kennedy, B. J.; Manoharan, R.; Weeks, S. A. Electrochim. Acta 1990, 35, 199. (3) Shan, J.; Pickup, P. G. Electrochim. Acta 2000, 46, 119. (4) Martin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2002, 106, 971. (5) Solla-Gullon, J.; Montiel, V.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 2000, 491, 69.

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Andreas et al. (22) Birss, V. I.; Andreas, H.; Serebrennikova, I.; Elzanowska, H. Electrochem. Solid-State Lett. 1999, 2 (7), 326. (23) Andreas, H.; Elzanowska, H; Serebrennikova, I.; Birss, V. J. Electrochem. Soc., 2000, 147 (2), 4598. (24) Sutton, H. C.; Downes, T. M. J. Chem. Soc., Chem. Comm. 1972, 1, 1. (25) Parker, V. D. In Organic Electrochemistry: An Introduction and a Guide; Baizer, M. M., Ed.; Marcel Dekker, Inc.: New York, 1973; p. 546. (26) Ege, S. Organic Chemistry, 2nd Ed.; D. C. Heath and Co.: Toronto, 1989; p. 440. (27) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms and Structure, 4th Ed.; John Wiley and Sons: Toronto, 1992; pp. 1160, 1168. (28) Eberson, L., as in reference 24, pg. 477. (29) Anderson, J. S. J. Chem. Soc. 1934, 971. (30) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd. ed.; Marcel Dekker, Inc.: New York, 1997. (31) JADE Software from Materials Data Incorporated (MDI), Release 5.0.12 ed. (32) Keller, R. N. Chem. ReV. 1941, 28, 229. (33) Anderson, J. S. J. Chem. Soc. 1936, 1042. (34) Murakami, Y.; Tsuchiay, S.; Yahikozawa, K.; Takasu, Y. J. Mater. Sci. Lett. 1994, 13, 1773. (35) Takasu, Y.; Onoue, S.; Kameyama, K.; Murakami, Y.; Yahikozawa, K. Electrochim. Acta 1994, 39, 1993. (36) Kameyama, K.; Shohji, S.; Onoue, S.; Nishimura, K.; Yahikozawa, K.; Takasu, Y. J. Electrochem. Soc. 1993, 140, 1034. (37) Seyferth, D. Organometallics 2001, 20, 2.