Potential Modulated Infrared Reflectance Spectroscopy of Pt

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J. Phys. Chem. B 2001, 105, 941-947

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Potential Modulated Infrared Reflectance Spectroscopy of Pt-Diisocyanide Nanostructured Electrodes Sarah L. Horswell, Ian A. O’Neil, and David J. Schiffrin* Department of Chemistry, The UniVersity of LiVerpool, LiVerpool L69 7ZD, United Kingdom ReceiVed: July 26, 2000; In Final Form: October 21, 2000

The properties of nanostructured electrodes prepared by the attachment of platinum nanoparticles to a gold electrode surface functionalized with a self-assembled monolayer of 1,8-diisocyanooctane have been investigated by reflectance infrared spectroscopy using the SNIFTIRS technique. The characteristic vibrations of the NC group have been observed both in the absence and in the presence of an attached nanoparticle layer. The bipolar bands observed indicate a potential dependent vibration, and this has been discussed in terms of the Stark effect and of the coordination chemistry of the isocyanide group attached to metal surfaces. The potential dependence of the NC vibration is similar to that observed for adsorbed CO. Coadsorption experiments using CO confirm unequivocally the presence of Pt on the film. Similar results have been obtained with palladium nanoparticles.

Introduction The self-assembly of thiols on Au surfaces and the derivatization of nanosized metal particles with thiols have received much attention during recent years.1 Self-organization of nanoparticles onto substrates has been achieved by the assembly of bifunctional molecules on a surface followed by their reaction with metal particles,2 and this has led to the synthesis of nanostructured arrays, in some cases containing switchable redox groups.3 Most of this research has been limited to the coinage metals although the attachment of Pt particles to a Au surface by dithiols has been reported.4 It would be of obvious advantage to produce such arrays using Pt-group metals for electrocatalytic applications in order to study size effects on chemical reactivity in a controlled manner. In addition, Pt particles show unusual structures that can be controlled by the preparation technique employed. For instance, instead of the classical cubooctahedral structures commonly observed in gold nanocrystals, cubic shapes have been produced with surprising ease.5 Although many methods of producing Pt nanoparticles have been reported, relatively few preparations have been carried out in nonpolar solvents.6 Such solutions are preferable for the attachment of particles to a hydrophobic organic monolayer and, therefore, the two-phase chemical reduction method described previously7 used in the present work. These particles are stabilized by tetraalkylammonium salts; they are stable in solution but sufficiently reactive to allow further reaction with functional tail groups of a self-assembled monolayer on a substrate.2 Isocyanides have extensive coordination chemistry due to their strong binding to metal ions and the wide variety of functional groups that are available.8 Although isocyanide bonding has been compared to that of CO, it differs in that the σ-donation to the metal is stronger and the π-back-bonding is weaker,9 despite which strong binding to metal surfaces has been observed.10 An advantage of this ligand for reacting with metal surfaces and small metal particles is that attachment can be * Corresponding author. E-mail: [email protected].

easily studied by vibrational spectroscopies. The bond formed with Pt is particularly strong,10a,b,11 which makes it a viable alternative to thiols for the attachment of Pt clusters to substrates. Self-assembly of isocyanides onto Au and Pt surfaces is wellknown,11,12 and electron transfer through the resulting layers has been reported.11b However, the only example of the use of a diisocyanide to bind clusters to surfaces is the attachment of small Ni clusters to a Au substrate.12 Previous work in this laboratory demonstrated that alkyl isocyanides can strongly bind to Pt nanoparticles in a toluene solution7 and the reactivity of the NC group can therefore be used to attach Pt nanoparticles to a surface derivatized with diisocyanides. Isocyanides, in general, have very low mammalian toxicity.8 Although low molecular weight isocyanides have unpleasant odors, higher molecular weight isocyanides do not have this drawback and this family of compounds may be easily prepared by any of a wide variety of methods.8 The purpose of the present work was to investigate using IR reflectance spectroscopy the preparation and properties of Pt nanoparticle films attached to a Au substrate by means of diisocyanide tethers. In particular, the influence of electrochemical charge injection to the nanoparticle layer on the NC vibration, as a probe for binding characteristics, was investigated. Experimental Section The working electrodes (Au, 99.9985%, and Pt, 99.998%, Johnson Matthey) were disks of 12 mm diameter and 2 mm thickness. These had grooves cut into their sides around which a thin connecting wire of the corresponding metal was wound. The electrode was introduced into the cell through an entrance at the top, and a glass rod, of the same diameter as the electrode and attached to a syringe barrel, was used to push it against the infrared window. The thin layer of electrolyte thus formed usually has a thickness of the order of a few micrometers.13 Electrical connection to the working electrode was made by means of the attached wire. This was threaded through one of the entry ports of the cell, which was sealed with a rubber septum after the electrode was positioned against the window.

10.1021/jp002661o CCC: $20.00 © 2001 American Chemical Society Published on Web 01/12/2001

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Figure 1. SNIFTIRS spectra of a 1,8-diisoyanide derivatized Au electrode in contact with 0.1 M K2SO4. The potentials are indicated in the figure for a reference potential of 0 V. The scans shown correspond to (a) positive and (b) negative potentials.

The electrodes were prepared by polishing with successively finer grades of alumina powder (1 µm, 0.3 µm and 0.05 µm, Buehler), rinsing, and placing in pure water in an ultrasonic bath for several minutes. The Pt electrode was additionally flame annealed until red-hot using a microburner. The Au electrode was derivatized with diisocyanide and Pt nanoparticles as follows. The electrode was first derivatized by immersion in a toluene (Fluka, puriss) solution of 1,8-diisocyanooctane for 18 h. It was subsequently rinsed thoroughly with toluene and then immersed for 18 h in a toluene solution of Pt nanoparticles. Immediately prior to use, the electrode was rinsed with toluene, dried in a stream of nitrogen and rinsed with water. A three-electrode cell was employed. The counter electrode was a large-area platinum gauze, and the reference electrode was a saturated calomel electrode (SCE). All the potentials are quoted with respect to this electrode. A CaF2 infrared window was used. FTIR measurements were performed on a Bio-Rad FTS-40 spectrometer equipped with a liquid nitrogen cooled MCT detector. The beam path was modified by the addition of an extra mirror to deflect the infrared beam onto the electrode surface.14 A polarizer (Graseby Specac) was used to select por s-radiation. Normalized spectra were obtained by subtracting the reflectance spectrum obtained at a base potential of 0 V, where the diisocyanide film is stable, from that at the potential of interest and then dividing it by the first. The potential was modulated by using a trigger facility in the potentiostat (Hi-Tek DT2101) controlled by a TTL pulse from the PC using an in-house written program. The program also provided a time delay of 5 s to allow stabilization of the electrochemical system before spectral collection. The signal-to-noise ratio was improved by signal averaging. A total of 1000 interferograms were collected in 10 blocks of 100 scans in order to minimize problems arising from drifts in the electrochemical system.14 The blocks were averaged to give a final spectrum. The individual blocks were also viewed in

order to check for any ghost bands that can arise from absorptions due to the window or due to the electrolyte layer between the electrode and the window.15 All the spectra shown were acquired with p-polarized radiation. All glassware was cleaned by soaking in a 1:1 mixture of concentrated nitric and sulfuric acids (Caution! This mixture causes seVere burns) followed by thorough rinsing with pure water. Water used throughout was purified with a Millipore system (resistivity 18.2 MΩ). K2SO4 (0.1 M, AnalaR, BDH) solution was used for all IR experiments and was deoxygenated with OFN nitrogen (BOC Ltd.) before use. Carbon monoxide (99+%, Aldrich), when required, was bubbled through the deoxygenated solution for 15 min prior to measurements. 1,8-Diisocyanooctane was prepared as follows. 1,8-Diaminooctane (5.22 g, 3.6 × 10-2 mol) was stirred overnight at room temperature in methyl formate (60 mL) to yield 1,8-octyldiformamide, which was crystallized, filtered, and washed. 1,8Octyldiformamide (1.97 g, 1.2 × 10-2 mol) was dissolved in dry dichloromethane (30 mL), and then distilled triethylamine (4.86 g, 4.8 × 10-2 mol) was added first followed by diphosgene (2.37 g, 1.2 × 10-2 mol) (Caution! Diphosgene is extremely toxic and a well-Ventilated fume cupboard must be used) were added at 0 °C while stirring. On completion of the reaction, as determined by TLC, the solution was washed with sodium bicarbonate (5% aqueous) and then with water and finally dried with MgSO4. The product was purified by flash column chromatography on silica gel, eluting with 40% diethyl ether/ 60% petroleum ether. The toluene solutions of Pt nanoparticles stabilized with tetraoctylammonium bromide were prepared as described elsewhere.7 Results and Discussion Au Derivatized with Diisocyanide. Figure 1 shows the SNIFTIRS spectra for a Au electrode derivatized with 1,8-

Pt-Diisocyanide Nanostructured Electrodes diisocyanooctane. The region of interest, 1800-2400 cm-1, is displayed. Outside this region, only bands corresponding to the sulfate ion or to water are observed. Bipolar bands are evident between +0.3 V and +0.9 V (Figure 1a). The positive going lobe remains at 2213 cm_1 for each spectrum while the negative lobe shifts slightly with potential, from 2229 cm-1 at +0.3 to 2242 cm-1 at +0.9 V. When the potential is stepped in the negative direction (Figure 1b), bipolar bands are present between -0.2 and -1.0 V in a similar frequency range. Again, the positive lobe remains constant since this corresponds to a constant base potential while the negative lobe shifts from 2208 cm-1 at -0.2 V to 2186 cm-1 at -1.0 V. The bands are not observed in the corresponding spectra acquired with s-radiation or for a bare Au electrode, indicating that they originate from the potential dependence of the absorbance of adsorbed diisocyanide. The stretching frequency of the NC bond of the free diisocyanide, measured by transmission FTIR, was 2147 cm-1, in agreement with literature data for alkyl monoisocyanides.8,16 As shown in Figure 1, this frequency increases on adsorption to Au, a result in agreement with observations by Angelici et al.16 Vibrations in the region 2207-2223 cm-1 were observed for alkyl isocyanides adsorbed on Au powder, and these were assigned to stretching modes of NC adsorbed on on-top sites of the metal. The vibrations of NC adsorbed in a bridge-bonded configuration were expected to lie in the range 1570-1800 cm-1.16 Other studies of isocyanides adsorbed onto metal surfaces10a,17 also show that the NC stretching frequency increases on adsorption. This increase is due to coordination of the isocyanide group to the metal, leading to a greater participation of electrons from the N atom in the NC bond, resulting in the corresponding increase in bond order, to compensate for loss of electron density on the C atom caused by its σ-donation to the metal. Similar considerations also apply to coordination by RNC to inorganic complexes.9d This behavior is in contrast with that of carbon monoxide on metal surfaces, where the frequency of the CO stretch is considerably lower than that of unbound CO due to strong π-back-bonding to the adsorbed molecule.18 The potential dependence of the adsorbed isocyanide stretching frequency (Figure 1) can be related to the bonding mechanism. As the potential is made more positive, there is increased σ-donation from the C atom to the metal, resulting in a greater shift of electron density from the N atom into the NC bond, and as a consequence, the bond order and the vibrational frequency are increased. The opposite takes place as the potential is made more negative. Some back-donation from the metal to the NC π* orbitals may occur at negative potentials, decreasing the stretching frequency, as has been observed for some coordination compounds. For instance, the stretching frequency of Pt(0) complexes is lower than that of the free isocyanide19 due to back-donation from the metal center; this is in contrast with Pt(II) complexes, which show the opposite effect.20 For isocyanide adsorbed on Au at negative potentials (Figure 1b), the frequencies observed are higher than those of the free isocyanide, thus indicating that, even at negative potentials, back-bonding, if present, makes a minor contribution to the spectral features observed in the present experiments. The bonding of RNC to other metal substrates displays a different behavior that is dependent on the metal.10,17 The potential dependence of the NC stretching frequency of adsorbed 1,8-diisocyanooctane is shown in Figure 2. This dependence was calculated from the position of the bipolar band

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Figure 2. Dependence of the vibrational frequency of NC on electrode potential for a 1,8-diisocyanooctane derivatized Au electrode.

lobe. Although some distortion can occur as a result of the subtraction of one spectrum from another, the data shown in Figure 2 are sufficiently accurate to establish the dependence of band frequency and potential. A linear dependence with a slope of 30 cm-1 V-1 was observed, similar to the value for CO adsorbed on Pt in acidic electrolytes at constant coverage.21,22 For CO adsorbed on metals, this shift has been explained by either the electrochemical Stark effect18,23 or by the bonding characteristics of CO as derived from molecular orbital theory.24 Stark tuning originates from the interaction of the polar adsorbate with the interfacial electric field.23a A molecular orbital approach can be used to describe the effect of potential on the metal-isocyanide bonding, as shown by Anderson24a for cyanide ions adsorbed on Ag and for CO on Pt. The cyanide ion, similarly to RNC, has an acceptor orbital of high energy, which is unfavorable for π-back-bonding. An increase in vibrational frequency with increasing positive potential results from a greater donation from the cyanide ligand to the metal, whereas for CO, changes in the extent of π-backbonding have a greater influence. The model described by Anderson24a showed that cyanide adsorbed on Ag has a potential dependence of the vibration frequency similar to that of CO on Pt, despite the differences in bonding. This approach could also be used to analyze the behavior of adsorbed RNC. It is interesting to note that only one bipolar band is observed, although the two NC groups are present in different chemical environments, attached to the metal and in contact with the solution. This is centered around the vibration of adsorbed NC at 2213 cm-1, which is similar to literature values for isocyanide attached to Au,16,17a and sufficiently different from unbound NC to conclude that only vibrations of the former can be affected by the applied interfacial potential. This may result from the disordered nature of the film, for which the terminal NC groups are shielded by the electrolyte, or by the simultaneous binding of both NC groups to the metal surface.12,17c The latter is very unlikely since, as is shown below, Pt particles can be strongly bound at the adsorbed film. It is proposed that the electrolyte shielding model appears to be more applicable to the present system. The IR spectra also give the potential range of stability of the diisocyanide layer. The NC bands disappear at potentials more positive than +0.9 V and negative of -1.0 V, indicating that the isocyanide layer is no longer present at the surface. No new peaks are observed to identify products. The most likely are the corresponding formamide (RNHCHO) or carbon dioxide at positive potentials, and the primary amine (RNH2) at negative potentials. It is possible that if the formamide or the amine are formed, the peaks corresponding to the carbonyl stretch and

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Figure 3. SNIFTIRS spectra of a Au electrode first derivatized with 1,8-diisocyanooctane and then with Pt nanoparticles. Scans were obtained at (a) positive and (b) negative potentials.

N-H stretches are obscured by the bending and vibrational modes of water. Au Derivatized with Diisocyanide and Pt Nanoparticles. The SNIFTIRS spectra of a Au electrode derivatized with diisocyanide and Pt nanoparticles are shown in Figure 3. The bipolar bands observed are similar to those for diisocyanide on Au. Positive (2213 cm-1) and negative (2237 cm-1 at +0.2 V to 2247 cm-1 at +0.9 V, respectively) lobes are observed (Figure 3a). In Figure 3b, the positive lobe is present at 2223 cm-1 and negative lobes are observed varying from 2199 cm-1 at -0.5 V to 2191 cm-1 at -0.9 V. Spectra acquired with s-polarized radiation showed no absorbances in this region, supporting the assignment of the spectra to an adsorbed species. The frequency of the NC vibration for Pt nanoparticles functionalized with an alkyl isocyanide is 2218 cm-1,7 a value very close to that obtained in the present work at 0 V for diisocyanide on Au (Figure 1). For the instrumental resolution used in these experiments (4 cm-1), it was not expected that absorbances of NC groups adsorbed on the Au surface and on the Pt particles could be resolved. The dependence of the NC vibrational frequency on potential for the nanostructured electrode (Figure 4) is similar to that of a RNC monolayer, although the fit is not so good and the peaks are a little broader, smaller, and less well-defined. This may be a consequence of the convolution of two peaks of similar frequency, or the particles may screen the surface. It is interesting to note that the attachment of the particles does not completely shield the NC groups from the IR radiation but their presence could result in a decreased reflected intensity. Although there may be an incomplete coverage by Pt, the observation of the NC bands indicates that the overlaying particles do not shield the linker molecules completely. The structure of the interface, however, is more complex than that of a flat surface where the boundary conditions are well-defined. The intensity of electro-

Figure 4. Dependence of the NC vibrational frequency on potential for the spectra shown in Figure 3.

magnetic radiation that penetrates condensed matter (I) is proportional to |E|2 (E ) electric field). The dependence on distance from the interface, z, is given by25

I(z) ) I(0)e-Rz

(1)

with the attenuation coefficient R given by

R ) 4πk/λ

(2)

where k is the imaginary component of the refractive index and λ is the wavelength. In the 2200 cm-1 region, the k for Pt is 19.4,26 which gives a penetration depth of radiation (I(z)/ I(0) ) e-1) of 19 nm. This value is nearly an order of magnitude greater than the dimensions of the nanoparticles, and therefore, no major energy loss should occur within the nanoparticle layer. Since the bulk metal substrate represents, therefore, the dielectric termination for IR reflection, the NC groups within the linker

Pt-Diisocyanide Nanostructured Electrodes

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Figure 5. SNIFTIRS spectra of the Pt nanostructured film (Figure 3) in the presence of adsorbed CO.

layer will still act as absorbing centers although present between bulk metal and the nanoparticles. Although these are qualitative considerations since the electromagnetic field distribution of the interfacial structure will be very complex and include size effects in the dielectric function of Pt,27 the presence of a nanoparticle film does not necessarily result in complete quenching of the absorbance bands. As discussed before, the potential dependence of the vibrational frequency can be due to either Stark tuning or to bonding effects. If the Stark effect alone is responsible, the presence of a bipolar band must necessarily mean that a potential drop (i.e., an electric field) can be induced across the isocyanide linkers. The two possible cases discussed are schematically shown in Figure 5. Previous work with Au-dithiol nanostructured films28 indicated very little potential drop across the linker film; electronic overlap between the particles and the metal seems to be present. For bipolar bands to be observed in this case (Figure 3), the particles should not cover all the NC groups on the surface (Figure 5a). The molecular orbital description of the changes in vibration with applied potential provides an alternative explanation. If there is little potential drop across the linker molecules,28 the charge density on the Pt particles must be determined by the applied potential. The NC groups attached to the particles would then experience a change in electronic environment when the potential is stepped due to the change in occupancy of metal orbitals in the nanoparticles. For a positive potential step, the decrease in electronic density on the particles will increase σ-bonding to the NC group and hence increase the NC vibrational frequency. While an incomplete coverage of Pt on the adsorbed layer could explain the appearance of bipolar NC bands by the Stark effect, it is likely that the bipolar bands reflect the effect of potential on the bonding between the adsorbate and the metal. It is not possible at present to distinguish clearly between these two effects. Adsorption of CO. Since CO adsorbed on Pt has a characteristic infrared signature, its adsorption on nanostructures can give information on free electrode area, effective interfacial electric fields, and/or charge effects on bonding. Figure 6 shows SNIFTIRS spectra of CO adsorbed on the nanostructured Pt electrode. The NC stretching bands are present but a more intense bipolar band (2072 cm-1 and approximately 2053 cm-1) is also observed. This frequency corresponds to carbon monoxide adsorbed on the on-top Pt sites.22,29,30 The band is present between 0.1 and 0.5 V; at more positive potentials, a band at 2344 cm-1 corresponding to CO2 in solution is evident, indicating oxidation of the adsorbed CO. The CO band was absent for a bare Au electrode, for a Au electrode only derivatized with diisocyanide, and for a Au electrode immersed in the Pt particles solution and then rinsed with toluene, without being treated previously with diisocyanide. The absence of this band in these cases identifies the CO vibration in Figure 6 as

Figure 6. Schematic representation of possible structures of the nanostructured film. (a) The Pt layer does not cover all the available NC groups, and the interfacial field present between the substrate and the aqueous electrolyte leads to Stark tuning. (b) The Pt nanoparticles shield the NC groups, and the potential dependence of the NC vibration is due to charge density effects on the NC bond.

being related to CO adsorbed onto the Pt particles. These results provide unequivocal evidence that Pt particles are present on the electrode surface, chemically bound to the free end of the diisocyanide molecules attached to the Au surface, as shown in the images in Figure 5. Influence of Particle Size. SNIFTIRS spectra were also obtained using Pt particles prepared at different temperatures: 0, 20, and 50 °C. Particles prepared at 50 °C were of the same average size as those prepared at 20 °C. The solutions prepared at 0 °C contained a large number of smaller particles (diameter ) 1.2 ( 0.4 nm). Some of these spectra are presented in Figure 7. The CO spectra of adsorbed particles prepared at 20 and 50 °C are very similar, whereas those for particles prepared at 0 °C show lower stretching frequencies. The NC stretching frequencies are similar for all particles, and therefore, the differences observed for CO are likely to arise from dipoledipole coupling considerations rather than from chemical bonding effects related to particle size. Experiments and calculations performed for Pt particles supported on silica show that smaller particles exhibit CO bands of lower frequency than those for larger particles.31,32a Comparison with results obtained for CO adsorbed on step, kink, and terrace sites on Pt singlecrystal stepped surfaces32 leads also to the conclusion that there is a lower degree of dipole-dipole coupling on smaller particles due to the relatively high proportion of edge to terrace sites on the facets. The present results are in agreement with these observations;31,32 while they may appear to contrast with the conclusions of Mucalo et al.,33 who investigated the IR spectra of CO adsorbed on Pt particles as hydrosols and alcoholic solutions and reported no particle size effects on CO vibration frequency, the particles used in their studies had a wide size

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Figure 7. Comparison of the SNIFTIRS spectra of Pt nanostructured electrodes (Figure 3) in the presence of adsorbed CO for Pt nanoparticles prepared at different temperatures: (a) 0, (b) 20, and (c) 50 °C. The potentials are indicated in the figure.

distribution, which renders size effects difficult to observe. In addition, Bradley et al. also observed a considerably lower vibrational frequency for CO on small Pt clusters than bulk Pt (2040-2067 cm-1, compared to 2063-2100 cm-1 for the bulk).6b These particles had a diameter less than 0.8 nm, smaller than the preparations described in the present work and it was also reported that the frequency decreased as the particle size decreased, in agreement with the results reported by Greenler et al.31,32 and in the present work. Au Derivatized with Diisocyanide and Pd Nanoparticles. Figure 8 shows SNIFTIRS spectra for a Au electrode with a layer of 1,8-diisocyanoctane and palladium nanoparticles in a 0.1 M K2SO4 electrolyte containing CO. Bands corresponding to adsorbed NC are visible, along with a small band at approximately 1950 cm-1 corresponding to the adsorption of CO on bridge sites on the Pd surface.34 These results show, therefore, that it is possible to attach Pd particles to a layer of diisocyanide. It was more difficult to obtain spectra for Pd than for the Pt-functionalized electrode and more scans had to be taken to resolve the CO bands. It is possible that the coverage of Pd on the surface was lower or that the coverage of CO on the Pd particles was lower. Nevertheless, these preliminary results indicate that Pd can also be attached to diisocyanide layers. Conclusions The attachment of diisocyanide to a Au surface has been investigated, and the binding of Pt and Pd nanoparticles to this layer has been demonstrated. CO adsorbs onto the Pt particles and exhibits behavior similar to that of a polycrystalline Pt electrode. Particles prepared at low temperature have properties different from those obtained at higher temperatures, as demonstrated by a lower CO stretching frequency. SNIFTIRS

Horswell et al.

Figure 8. SNIFTIRS spectra of a Au electrode first derivatized with 1,8-diisocyanooctane and then with Pd nanoparticles. The reference spectra were taken at -0.3 V. The spectra labeled 0.2 and 0.3 V were acquired by taking 3000 scans at each potential, the other spectra were acquired by taking 1500 scans.

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