A Versatile Surface Modification Scheme for Attaching Metal ...

8668 J. Phys. Chem. B, Vol. 106, No. 34, 2002 Park and WeaverFigure 1. Surface-modification scheme described here.boiled. A 10 mL solution of 1% sodium citrate was added tothe above solution and then boiling for 30 min. (The sols arestored at 4°) The average particle diameters were determinedin each case from transmission electron microscopy. Mostexperimental details of the EC-IRAS measurements are availableelsewhere. 20 The FTIR spectrometer was a Mattson RS-2instrument, with a custom-built external reflection compartmentcontaining the narrow-band MCT detector. The metal gold disksubstrate, (ca. 0.9 cm diameter) mounted on a glass plunger bywrapping with Teflon tape, was pressed against the CaF 2window forming the base of the spectroelectrochemical cell soas to create the optical thin layer.The steps involved in preparing the nanoparticle films areshown schematically in Figure 1. The polished gold substrate,after EC cleaning by several potential cycling between -0.4and 1.3 V vs SCE in 0.05 M H 2 SO 4 , was immersed in a 40mM solution of 3-mercaptopropyltrimethoxysilane (3-MPTS)in methanol for 3 h, so to produce a self-assembled monolayer. 12After thorough rinsing, the silane units were polymerized intoa 2d-network by dipping into aqueous 0.01 M NaOH for 2 h.A second silane layer was then formed by immersion back intothe 3-MPTS solution overnight. 12 The surface, now containingexposed thiol moieties (Figure 1), was immersed overnight intoa suspension (or sol) of the desired metal nanoparticles, so toachieve near-monolayer attachment levels.Results and DiscussionFigure 2A-C shows illustrative examples of voltammetricresponses obtained for the resulting nanoparticle array electrodes.Part A (solid trace) shows a cathodic-anodic cyclicvoltammogram (at 5 mV s -1 ) for gold nanoparticle array(diameter d ≈ 13 nm) in 0.1 M H 2 SO 4 containing 1 mM CuSO 4 ,showing the formation/removal of an underpotential deposited(upd) copper monolayer (a/a′) and subsequent multilayers (“bulkphase” copper) (b/b′). Significantly, this deposition occursentirely on the Au nanoparticle layers, rather than also on theunderlying Au substrate, as confirmed by the complete lack offaradaic current in the absence of the former (dashed trace,Figure 2A) [cf., point (b) above]. The formation of a upd Culayer, by holding at 0VvsSCE, was also used as a means ofcoating the Au nanoparticles with a Pt monolayer: this wasachieved by subsequent immersion into deaerated 5 mM K 2 -PtCl 4 , whereupon the Cu layer spontaneously undergoes redoxinducedreplacement with Pt. 7,13,14 (The epitaxial nature of thePt layer formed in this manner on Au nanoparticle films as wellas conventional Au electrodes is clearly evident from SERFigure 2. Illustrative voltammograms for nanoparticle films attachedas in Figure 1. (A) Solid trace: voltammogram (5 mV s -1 ) for copperdeposition on Au (d ≈ 13 nm) film in 0.1 M H 2SO 4 + 1 mM CuSO 4.Dashed (baseline) trace was obtained in the absence of attached Auparticles. (B) Solid trace: cyclic voltammogram (50 mV s -1 ) for PtmodifiedAu nanoparticle film in 0.05 M H 2SO 4; dashed trace is forelectrooxidation of irreversibly adsorbed CO. (C) Similarly to (B), butfor “Pt black” (d ≈ 6 nm) film in absence and presence of CO adlayer.(The current density scales refer to geometric area of the planar goldsubstrates.).spectra for suitable probe adsorbates as well as the voltammetricresponses. 7,14 ).Figure 2B (solid trace) shows a resulting voltammogram forsuch a Pt-modified Au nanoparticle film in 0.05 M H 2 SO 4 ,featuring the well-known hydrogen adsorption/desorption profilebelow 0 V, characteristic of Pt electrodes. 7,14 The chargeunderneath these features, ca. 100 µC cm -2 (based on thegeometric electrode area) indicates that the effective surface areafor the Au/Pt nanoparticle film is about half of that for aconventional planar Pt electrode (240 µC cm -2 ). The dashedtrace in Figure 2Bsthe anodic voltammogram for adsorbed COelectrooxidation (the adlayer being formed by CO and then N 2solution sparging)sis similar to that obtained on conventionalPt surfaces. Figure 2C shows corresponding voltammogramsobtained in the absence (solid trace) and presence (dashed trace)of adsorbed CO, for a film of “platinum black,” consisting ofca. 6 nm diameter Pt nanoparticles.Even though the voltammetric data demonstrate the abilityof the modified surfaces to provide acceptable electron transportbetween the nanoparticles and the underlying substrates, thisproperty (important for electrocatalytic and sensor applications)was checked further by examining the electrode kinetics forCo(NH 3 )3+ 6 reduction and Ru(NH 3 )3+/2+ 6 exchange on thesefilms. (The irreversible chemical nature of the former outersphereprocess, yielding ca. 10 5 -fold slower electroreduction