Self-Assembled Monolayers As Nucleating Centers for the Preparation

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Self-Assembled Monolayers As Nucleating Centers for the Preparation of Multilayers of Catalytically Active Pt Films P. Arunkumar, Sheela Berchmans,* and V. Yegnaraman Central Electrochemical Research Institute, Karaikudi-630006, Tamilnadu, India ReceiVed: January 20, 2009; ReVised Manuscript ReceiVed: March 19, 2009

A soft route for the preparation of catalytically active multilayers of Pt films free from poisoning effects is reported. The SAMs (self-assembled monolayers) were initially formed on the gold substrates and they act as nucleating centers initiating the formation of multilayers of Pt on gold substrates. The nanoscale to mesoscale structures of Pt thus prepared were characterized by SEM and XPS. The SAM-based Pt structures exhibited well-defined hydrogen adsorption, desorption regions associated with very high charge indicating high surface area Pt. The electrocatalytic activity of the multilayers of Pt was characterized by methanol oxidation and a comparison was made with two commercial Pt catalyst preparations. Introduction Platinum occupies a unique position among the catalytic materials and is required for many technological applications1-4 including electrocatalysis in proton exchange membrane fuel cells,5-7 direct methanol fuel cells8 and catalysis in solar water splitting devices,9 wastewater treatment, and as an active material in many sensor and device applications.10-14 The prohibitive cost of Pt requires smaller concentrations of Pt to be used as catalyst. The manipulation of the size and shape of platinum materials at the nanoscale can significantly contribute to lowering the Pt usage and achieving cost reduction. As the catalytic and the device efficiency may depend upon the size and shape of the Pt structure, the preparation of nanostructured Pt with specific structural features is an area of considerable interest.15 Nanostructured Pt has been prepared by using a range of techniques. Previous efforts to produce Pt in various shapes have yielded nanoparticles, nanowires, single crystal nanowires, nanowire networks, nanotubes, nanosheets, hollow nanospheres, and nanocrystals of different shapes16-20 (viz., tetrahedra, octahedra, icosahedra, etc). Polypeptides,21 peptide nanotubes,22 and peptide-functionalized nanotubes23 have been used to template the growth of nanostructured Pt. Finally, hollow nanospheres24-26 have been prepared by the nanoparticle templating method, which involves a galvanic replacement reaction between sacrificial nanoelectrode and a metal complex. Pure Pt is, however, readily poisoned by the strongly adsorbed COlike intermediates and other products produced during oxidation. To improve methanol electrooxidation activity, much effort has been devoted to the synthesis of Pt-based alloy catalysts like Pt/Mn, Pt/Sn, Pt/Re, Pt/Os, Pt/Ir, and Pt/Ru.27-30 The addition of Ru to Pt greatly inhibits the CO poisoning via an effective oxygen transfer step from electrogenerated Ru-OH at lower onset potentials compared to pure Pt. The efficiency of the methanol oxidation reaction is further improved by impregnating the active catalyst materials on tungsten oxide31 and carbon supports like carbon powder, carbon black, carbon nanofibers, carbon nanotubes, and fullerenes.32,33 * To whom correspondence should be addressed. Phone: +91-4565227551. Fax: +91-4565-227779. E-mail:[email protected].

SCHEME 1: Schematic Diagram Explaining the Different Steps Involved in the Construction of the First Pt Layer with p-Mercaptobenzoic Acid As the Base Layera

a

Repetition of the steps yields a multilayer of Pt.

Recently, it has been shown that the Au electrode does not undergo poisoning during methanol oxidation by the electrogenerated CO. It has been demonstrated by the use of timeresolved electrochemical impedance and surface plasmon resonance techniques that the Au surface is free from site poisoning by chemisorbed CO during methanol oxidation.34-37 In this work, SAMs of an aliphatic short chain thiol (dithiodipropionic acid (DTDPA)) and an aromatic thiol (p-mercaptobenzoic acid (MBA)) were used as nucleating base films and we describe

10.1021/jp9006069 CCC: $40.75  2009 American Chemical Society Published on Web 04/16/2009

Preparation of Multilayers of Catalytically Active Pt Films

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Figure 1. Cyclic voltammograms showing the response of different Cu and Pt layers formed on gold substrate with DTDPA as the base layer in 0.5 M sulfuric acid; scan rate 50mV/s: (a, c, e) Cu layers 1, 4, and 8 and (b, d, f) Pt layers 1, 4, and 8 formed by galvanic replacement.

the formation of multilayers of catalytic metals like Pt by adopting a methodology that involves the formation of a Cu adlayer on the SAM layer by chemical preconcentration of Cu2+ ions, followed by the electrochemical reduction of Cu2+ ions

and then the galvanic replacement of the Cu adlayer by Pt. The second layer is initiated by further spontaneous deposition of Cu on Pt and subsequent galvanic replacement of Cu with Pt to obtain the second Pt layer (Scheme 1). Successive repetition

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TABLE 1: Hydrogen Adsorption-Desorption Charges Corresponding to Eight Layers of Pt layer Hydrogen adsorption/ Hydrogen desorption/ no. µC cm-2 µC cm-2 DTDPA SAM

MBA SAM

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

344 1779 1701 1913 2577 3576 4747 5221 4060 6365 11692 12021 17250 23559 29724 38808

508 3012 4233 5639 6536 7556 8107 10668 6353 8195 14526 17101 23092 27061 35292 44172

of these steps could lead to multilayers of Pt which exhibited superior electrocatalytic properties. Experimental Section Au (111) substrate is immersed in a saturated solution of DTDPA (Aldrich) or MBA (Aldrich) in ethanol for 30 to 36 h. It is withdrawn, washed with ethanol, air-dried, and then immersed in a 1 mM solution of CuSO4 [Central Drug House Ltd., Mumbai] in water for 20 min. (A time period of 20 min was sufficient to obtain the maximum loading of Cu2+ ions in the SAM-modified Au substrate. This time was fixed by doing an independent experiment measuring Cu2+ loading versus preconcentration time.) After preconcentration, the substrate is taken out of the solution, washed with water, and then introduced into a cell containing 0.5 M H2SO4 [Merck (India) Ltd.] for electrochemically reducing the Cu2+ ions trapped by the monolayer. The Pt and Hg/Hg2SO4 electrodes served as counter and reference electrodes, respectively. The working electrode is clamped at a potential of -600 mV sufficient to reduce the Cu2+ ions. The Cu adlayer thus formed was galvanically replaced by Pt by immersion in an aqueous solution of K2PtCl4 (1 mg/1 mL) for 20 min. The film thus prepared was cycled in the potential range of -0.6 to 0.8 V in 0.5 M H2SO4 again for adsorption of hydrogen to take place. Then the film was again dipped in a 1 mM CuSO4 solution to form the second layer of Cu through the reduction of Cu2+ ions by adsorbed hydrogen. The adsorbed hydrogen on the Pt overlayer aids in the deposition of the non-noble metal layer of Cu, which is again galvanically replaced by Pt. Repetition of the above steps leads to the formation of multilayers of Pt on Au surface. Cyclic voltammograms were recorded with use of PARSTAT 2263. A conventional three-electrode cell was used, with a gold slide, Hg|Hg2SO4|H2SO4 (0.5 M) (MSE), and a Pt wire as working, reference, and counter electrodes, respectively. Gold slides (1000 Å Au coating on Si wafers with an intermediate adhesion layer of 100 Å thick Ti, procured from Lance Goddard Associates, USA) of size 1 × 1 cm2 were used in XPS and SEM measurements for the characterization of the metallic films. The performance of the catalytic layers was compared with that of two commercial catalysts E Tek (10% Pt loading) and HiMedia (10% Pt loading). The amount of Pt loading comparable to that of the film prepared by our method was incorporated onto a similar gold substrate and the experiments were carried out under the same conditions for comparison.

MULTILAB 2000 was used for XPS analysis. MgAl X-ray with 300 W/400 W power was used as the exciting source. A vacuum was maintained during the experiment at 10-10 mbar. SEM images were taken with a Hitachi S3000-H model microscope. Results and Discussion Figure 1a corresponds to the first layer of Cu formed with the base layer of the SAM, DTDPA. The peak (anodic) corresponding to bulk copper appears at -0.22 and -0.048 V. These two peaks are not resolved completely. They overlap each other. The upd Cu peak appears at 0.399 V. The features of gold oxide formation/reduction are also seen in the figure. On the reduction side only one peak corresponding to Cu appears at around -0.3 V. The charge corresponding to bulk Cu turns out to be 838 µC/cm2 and that of upd Cu turns out to be 19.2 µC/cm2. The charge was calculated by integration of the area under the respective peak. Figure 1b corresponds to the first layer of Pt formed by galvanic replacement of the first Cu layer. The formation of Pt is clearly seen by the typical hydrogen/ adsorption regions lying between -0.3 and -0.6 V. Pt oxide formation and reduction occurs at 0.62 and 0.044 V, respectively. The charge corresponding to hydrogen adsorption/ desorption determines the surface coverage of Pt. In the case of the first Pt layer the charge corresponding to hydrogen adsorption and desorption comes out to be 344 and 508 µC/ cm2, respectively. (See Table 1.) Comparison of the charges corresponding to Cu and Pt suggest that only half of the Cu is replaced. Stoichiometrically one-to-one replacement of Pt is possible. However, complete replacement has not occurred as inferred from the charge calculations in the case of the first layer. In the case of the second layer of Cu, the peak corresponding to bulk Cu shifts toward -0.1 V and features of upd Cu are absent (see the Supporting Information). The charge corresponding to Cu is only around 230 µC/cm2. However, surface coverage of hydrogen adsorption and desorption corresponds to 1.779 and 3.012 mC/cm2 (Table 1). Hence from the second layer onward, it is not just replacement. The Pt centers initially formed act as nucleating centers and the Pt becomes aggregated at these centers. With the further increase in the number of layers (see Table 1) the charge corresponding to Pt shows an increase that is much greater than the charge corresponding to Cu. The observations are seen clearly in the case of the fourth and eighth layer of Cu and Pt given in Figure 1b-f. Figure 2 a corresponds to the first Cu layer formed on gold substrate with MBA as the base layer. The peak corresponding to bulk Cu appears at -0.05 V and the upd Cu appears at 0.45 V. The charges corresponding to bulk and upd Cu are 209 and 716 µC/cm2. Hence total charge corresponding to Cu comes out to be 925 µC/cm2. However, for the first Pt layer, the hydrogen adsorption and desorption charge correspond to 4.060 and 6.335 mC/cm2, respectively. Hence in the case of MBA, the nucleation starts in the first layer itself and the amount of Pt replaced is always greater than that of Cu in all the layers. In the case of the second Cu layer, the bulk Cu peak shifts to 0 V and the features of upd Cu are absent (see the Supporting Information) similar to the case of DTDPA. Figure 2C shows the fourth layer of Cu. The bulk Cu peak appears at 0.05 V and a small peak corresponding to Pt also appears closely. The features of Pt oxide are also seen in the Cu layer. The amount of Cu deposited is always higher in the case of MBA-based layers compared to DTDPA-based layers. The amount of Pt replaced was also very much higher in the case of MBA-based layers. From the sixth Cu layer onward, the bulk Cu peak splits



Figure 2. Cyclic voltammograms showing the response of different Cu and Pt layers formed on gold substrate with MBA as the base layer in 0.5 M sulfuric acid; scan rate 50 mV/s: (a, c, e) Cu layers1, 4, and 8 and (b, d, f) Pt layers 1, 4, and 8 formed by galvanic replacement.

into two peaks (around 0 and -0.3 V). This can be clearly seen in the case of the eighth layer of Cu shown in the Figure 2e. Figure 3a-c presents the SEM micrographs of the Pt film after the formation of the first, fourth, and eighth layers, respectively, on MBA SAM. Figure 3a shows that the Pt nanostructures are spherical and their size lies in the range of 70-90 nm. In the case of the film after the fourth layer (Figure 3b), the spherical nanoparticles have further grown to about 100 nm. Also, in the background we can see 70-90 nm size particles which have started nucleating. The SEM micrograph of the film after the formation of the eighth layer (Figure 3c) shows that the surface is fully covered with the spherical Pt structures and

the sizes of the structures vary from 70 to 100 nm. A few particles appear very big whose size varies between 500 and 1000 nm. The SEM micrographs in Figure 4a-c depict the Pt film after the formation of the first, fourth, and eighth layers, respectively, on DTDPA SAM. In the case of the first layer (Figure 4a), the size of the Pt nanostructures is mostly between 50 and 90 nm, with some particles lying in the range of 100 to 130 nm. The SEM micrograph of the Pt film after the formation of the fourth layer (Figure 4b) shows the entire surface to be covered by fine nanostructures of Pt. The Pt film after the formation of the eighth layer (Figure 4c) reveals the fractal growth of Pt.

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Figure 3. SEM images of the (a) first, (b) fourth, and (c) eighth layers of Pt formed with a base layer of MBA.

Figure 4. SEM images of the (a) first, (b) fourth, and (c) eighth layers of Pt formed with a base layer of DTDPA.

Figure 5. Cyclic voltammograms exhibiting the hydrogen adsorption-desorption and Pt oxide formation/reduction regions of eight layers of Pt in 0.5 M sulfuric acid formed on (a) DTDPA SAM (working electrode area ) 0.32 cm2) and (b) MBA SAM (working electrode area ) 0.26 cm2); scan rate 50 mV/s. The numbers on the voltammograms indicate the number of layers of Pt.

The formation of Cu adlayers and their subsequent galvanic replacement by Pt has been followed by XPS measurements and the results reveal the presence of Cu0 in the adlayer and Pt0 in the overlayers (see the Supporting Information). The SEM images of the Cu layers are also analyzed (see the Supporting Information). In the case of DTDPA SAM-based layers, the first layer consists of around 500 to 600 nm sized spherical particles. In the case of the MBA SAM-based layers, the first Cu layer consists of 800 to 900 nm sized particles. The growth of Pt layers has also been probed voltammetrically by recording the cyclic voltammograms (CV) of the successive layers of Pt film in 0.5 M H2SO4 between the

potential limits -0.6 and 0.8 V (Figure 5a,b). The Pt layers are characterized by the typical hydrogen adsorption/desorption regions and Pt oxide formation/reduction regions. The charges corresponding to hydrogen adsorption/desorption are given in Table 1. The high coverage values for hydrogen adsorption/ desorption resulting from the increasing number of Pt layers point to the high surface area of the Pt films formed in the above process. A monolayer coverage of H2 on a Pt surface corresponds to 210 µC/cm2. On the basis of this value, the surface area resulting from eight layers of Pt corresponds nearly to 25 monolayers on DTDPA SAM and to 185 monolayers on MBA SAM.



Figure 6. Cyclic voltammograms exhibiting methanol oxidation on different layers of Pt formed on (a) DTDPA SAM (working electrode area ) 0.32 cm2; concentration of methanol ) 1.176 M) and (b) MBA SAM (working electrode area ) 0.26 cm2; concentration of methanol ) 0.602 M in 0.5 M sulfuric acid); scan rate 50mV/s. The numbers on the voltammograms indicate the number of layers of Pt.

Figure 7. Chronoamperometric response for the catalysts: (1) 8 layers of Pt prepared from MBA; (2) 8 layers of Pt prepared using a base layer of DTDPA; (3) Hi media catalyst; and (4) E-TEK catalyst.

The efficiency of methanol oxidation on Pt films was evaluated by cyclic voltammetry, chronoamperometry, and impedance spectroscopy and the results obtained are presented in Figures 6, 7, and 8, respectively. In the case of Pt layers initiated from DTDPA SAM, the onset of oxidation initially occurred at 83.66 mV, which then shifted toward more negative values with the increase in the number of catalytic layers, and with eight layers of Pt, the onset was shifted to -102.7 mV. In the case of catalytic layers prepared from MBA SAM, the onset of oxidation initially occurred at -321 mV, which shifted in the positive direction with an increase in the number of layers, and for the eight layers of Pt the onset of oxidation occurred at -262.5 mV. In the case of commercial catalysts, the onset potential occurred at -265.5 mV in the case of Hi-Media catalyst, which is a higher value when compared to catalytic layers prepared from MBA and a lower value when compared to that of catalytic layers prepared from DTDPA (Figure 6). In the case of E-TEK catalyst, the onset potential of methanol oxidation was comparable with that of catalytic layers prepared from MBA SAM and far lower than that of catalytic layers prepared from DTDPA SAM. The methanol oxidation potential occurs at comparable potentials when compared to commercial catalysts, within (50mV in the case of the catalytic layers prepared from MBA and DTDPA. The variation of the methanol oxidation potential with the increase in the number of layers is

clearly depicted in Table 2. The interesting aspect observed in this work is that with the increase in the number of layers of Pt, the charge transfer resistance for methanol oxidation decreases indicating the formation of efficient catalytic films (Table 2). In the case of the Hi-Media catalysts, the Nyquist plot revealed an incomplete semicircle and in E-TEK catalyst an Rct value of 99 ohms is obtained. The efficiency of catalysis was further confirmed by the results of the chronoamperometric experiments. The steady state current derived at the methanol oxidation potential was observed to be 7.33 mA/cm2 for the Pt multilayer formed on DTDPA SAM and 15.70 mA/cm2 in the Pt multilayer formed on MBA SAM. The commercial catalysts showed a very poor performance: 2.35 mA/cm2 in the case of Hi-Media catalyst and 3.43 mA/cm2 in the case of E-TEK catalyst (Table 2). The results of the impedance measurements (Figure 8 and Table 3) clearly indicate that the charge transfer becomes kinetically very facile with the increase in the number of over layers of Pt. In the case of the DTDPA monolayer, the Rct value for the first Pt layer is high (7796 ohms) compared to that of MBA (430 ohms). This result suggests how the structural organization of the MBA SAM layer and the π electrons of the aromatic ring are favorable for facile charge transfer kinetics. With the increase in the number of Pt layers Rct values decrease and become as low as 27 ohms in the case of MBA SAMbased Pt structures and 43 ohms in the case of DTDPA SAMbased Pt structures. The poisoning effect was checked by holding the potential at the methanol oxidation potential and film stability was compared by cycling the potential between -0.6 and 0.8 V. It is observed that after being subjected to cycling (20-25 cycles) and to holding the potential at the methanol oxidation potential for 5 min, only an increase in the methanol oxidation current was observed in both the cases (see the Supporting Information). This is certainly a positive indicator of the performance of our catalysts and this shows the efficacy of the underlying gold substrates in reducing the poisoning effects. The anodic peak that appears in the reverse scan is attributed38 to the removal of the incompletely oxidized carbonaceous species formed in the forward scan. These carbonaceous species are mostly in the form of linearly bonded PtsCdO. Hence the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), If/Ib, can be used to describe the catalyst tolerance to carbonaceous species accumulation. A low If/Ib ratio indicates poor oxidation of methanol to carbon

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Figure 8. Impedance spectra for the eighth layers of Pt formed on (a) DTDPA SAM (working electrode area ) 0.32 cm2; concentration of methanol ) 1.176 M, Eapplied ) 335 mV) and (b) MBA SAM (working electrode area ) 0.26 cm2; concentration of methanol ) 0.602 M, Eapplied ) 495 mV in 0.5 M sulfuric acid).

Figure 9. Comparative performance of the commercial catalyst and the catalytic layers prepared with a base layer of (a) DTDPA and (b) MBA.

TABLE 2: Performance Indicators of the Catalytic Layers Vs. Commercial Catalysts

catalyst MBA (1 layer Pt) MBA (8 layers Pt) DTDPA (1 layer Pt) DTDPA (8 layers Pt) Hi-Media (10% Pt) E-TEK (10% Pt)

current output/mA cm-2

current density for methanol oxidation/mA cm-2

If/Ib

Rct/ohms

3.36 142.9 0.16 48.56 3.14 19.8

2.48 1.35 2.94 1.50 3.89 1.75

7796 27.1 430 43 93 110

15.70 7.33 2.35 3.43

dioxide during the anodic scan and excessive accumulation of carbonaceous residues on the catalyst surface. Table 2 presents the If/Ib values observed for the different catalytic layers. These values are quite high compared to the literature reports.39-43 The values are comparable to those of E-TEK catalyst for the first layer of Pt and for the other layers; the value is comparable with that of the Hi-Media catalyst. This indicates that the underlying Au substrates modified with SAMs present a relatively poison free surface for methanol oxidation, which is the biggest advantage of building multilayers through this route. While testing for the poisoning effects it was observed that the MBA SAM-based catalytic electrodes did not show any reverse current Ib and the Pt oxide peak is also completely absent

(see the Supporting Information). These experiments are conducted on already used electrodes which were tested for methanol oxidation. Hence we conclude that with repeated use, MBA SAM-based Pt structures exhibit infinitely high If/Ib values and hence these structures are highly suitable for practical applications On the basis of the above results, it can be inferred that four parameters essentially control the methanol oxidation efficiency, viz., onset of methanol oxidation, methanol oxidation potential, charge transfer resistance, and If/Ib values. With respect to all these parameters, the prepared catalytic layers perform well in comparison to the commercial catalyst. The final performance indicator is obtained from the chronoamperometric experiments



TABLE 3: Performance of the Catalytic Multilayers for Methanol Oxidation layer concn of onset peak no. methanol/M potential/mV DTDPA SAM

MBA SAM

Hi-Media 10% Pt E-TEK 10% Pt

1 3 4 5 7 8 1 3 4 5 7 8

1.176

0.602

0.602 0.602

83.66 -76.11 -70.19 -70.19 -73.15 -102.7 -321 -309.9 -298 -301 -274.4 -262.5 -265.5 -342.4

forward anodic forward anodic reverse anodic current peak current If /mA cm-2 potential/mV Ib /mA cm-2 If/Ib 279 264 279 302.6 355.9 335.2 202 264.2 279 317.4 527.5 495 237.5 320

which give us an idea about the current output expected from such fuel cell catalysts. The catalytic layers prepared by our new strategy perform outstandingly well compared with the commercial catalysts, which is depicted in Figure 9. The multilayers of Pt formed exhibited facile charge transfer characteristics as exhibited by a decrease in charge transfer resistance with an increase in the number of layers, which is another deciding parameter that determines the current output. Similar catalytic electrodes have been prepared by us recently44 using a base layer of fourth generation amine terminated PAMAM dendrimers on gold substrates and the catalytic multilayers of Pt exhibited very good electrocatalytic properties. The use of thiol-based SAMs as base layers for the construction of Pt multilayers has yielded further interesting results. The electrodes prepared with thiol-based SAM layers exhibited no poisoning effects and these films were found to be highly stable. With the increase in the number of catalytic layers, charge transfer resistance decreases, which indicates facile charge transfer kinetics. A transformation from the usually observed electron transfer blocking to electron transfer facilitation is observed in the case of thiol-based SAM layers. Furthermore the aromatic thiol MBA SAM-based Pt structures exhibited very high surface area (see Table 1) as discussed earlier, and with the repeated use of these electrodes, the reverse catalytic current completely disappeared, which will be highly useful for practical applications. Conclusion In this paper we have demonstrated that multilayers of Pt can be formed on Au substrates modified by SAMs through a procedure involving galvanic replacement of Cu adlayers by Pt. SAMs direct the growth of nanoscale to mesoscale structures of Pt which exhibited very high surface area, facile charge transfer kinetics, and relatively poison free catalytic films. These films possess superior catalytic properties and can find applications in microfuel cells, laboratory-on-a-chip devices, and as sensor materials. Acknowledgement. . The authors wish to thank Director, CECRI for his keen interest in this work. Supporting Information Available: Cyclic voltammograms for the eight Cu layers, SEM images of first and fourth Cu layers, XPS data for the Cu and Pt layer, and the effect of potential cycling and potential holding on the Pt films as shown

0.16 7.39 14.37 21.35 42.24 48.56 3.36 31.81 42.70 57.61 127.19 142.93 12.24 34.67

0.05 5.56 8.91 14.47 27.65 32.30 1.35 23.19 28.46 41.72 94.49 105.72 3.14 19.84

Rct/ ohms

2.94 7796 1.33 1120 1.61 560 1.47 440 1.53 108 1.50 43 2.48 430 1.37 128 1.50 127.4 1.41 130.7 1.34 44.2 1.35 27.1 3.89 93 1.75 99.4

mass activity/ ks/× mA µg-1 10-8 s-1 0.23 2.04 3.71 4.09 4.40 4.59 0.41 1.34 1.75 1.65 2.11 1.82 0.15 0.43

0.26 1.79 3.56 4.54 18.5 49.5 11.2 37.5 37.7 36.7 109 177

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Self-Assembled Monolayers As Nucleating Centers for the Preparation

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