J. Phys. Chem. C 2010, 114, 14811–14818
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“Naked” Gold Nanoparticles: Synthesis, Characterization, Catalytic Hydrogen Evolution, and SERS Getahun Merga,†,‡ Nuvia Saucedo,†,‡ Laura C. Cass,† James Puthussery,† and Dan Meisel*,† Radiation Laboratory and Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry and Biochemistry, Andrews UniVersity, Berrien Springs, Michigan 49104 ReceiVed: May 28, 2010; ReVised Manuscript ReceiVed: July 16, 2010
We describe the synthesis of gold nanoparticles upon reduction of Au2O3 by molecular hydrogen. The reaction generates particles that contain no foreign stabilizer other than gold or water species. The reaction readily proceeds at slightly elevated temperatures and somewhat higher than atmospheric pressure of H2, and these two parameters control the size of the particles produced. The suspensions of particles were analyzed for particle size, size distribution, residual ions, and metal-atom concentrations using TEM, dynamic light scattering, electrophoretic mobility, pH, conductivity, ICP, and UV-vis spectra. The particles were shown to be highly active redox catalysts in the conversion of strongly reducing radicals to hydrogen from water in basic solutions. Surface-enhanced Raman scattering (SERS) spectra of a probe molecule, p-aminothiphenol, adsorbed on the particles surface was determined, and the effects of pH, electron injection, and Au(III) ions on the SERS spectra were measured. These effects are compared with similar results from previously prepared silver particles in an analogous procedure. Introduction Most synthetic methods for the production of noble metal nanoparticles in suspension utilize a precursor salt, often a complex, e.g., tetrachloroaurate for gold, a reductant, e.g., borohydride or citrate, and a stabilizer to provide electrostatic or steric stabilization to the particles.1-4 Most of these preparations, therefore, contain in addition to the metallic particle a counterion, the reductant and/or its product, and an organic stabilizer. For gold particles, citrate is commonly present because of its frequent use as both a stabilizer and reductant. However, the presence of these foreign adsorbates affects the optical characteristics of the plasmon band of the particles5,6 as well as their photochemical reactivity.7,8 A recent procedure by Chumanov and co-workers for metallic silver particles circumvents these interfering effects by reducing a silver oxide precursor with H2 gas.9-11 Whereas reduction of precursor salts by H2 to produce the corresponding metallic particles has been widely applied to many metals, Chumanov and co-workers have shown that at least for silver even the presence of counterions in the electrolyte significantly affects the particles produced.10 No counterion is present in the initial solution of this preparation, and the concentration of electrolyte is minimal. In the present report we utilize the same approach to obtain gold colloidal particles. Starting with Au(III) oxide as the precursor and H2 as the reductant, the metallic particles are produced without any addition of stabilizer or a counterion other than the metal ions and water dissociation ions (reaction 1).
Au2O3 + 3H2 f 2Au + 3H2O
(1)
Many of the attributes that were ascribed to the analogous * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Notre Dame. ‡ Andrews University.
procedure for silver particles, including the very high concentrations of particles achievable by this approach, are shown in the present study to be common to gold suspensions made by a similar reduction. The standard redox potential for the reduction of Au2O3 to Au is more positive than that of silver oxide (1.36 and 1.17 V vs NHE at 1 N acid, respectively).12 Reaction 1, thus, is expected to proceed at least as readily as the equivalent silver reduction. The high stability of the silver particles in suspension synthesized by this procedure is a result of electrostatic stabilization by hydroxide ions.13 The presence of Ag+ ions on the metallic particles in the latter case, as well as the appearance of mirrorimage charges upon adsorption of hydroxide ions, provides a stabilizing hydroxide layer that resembles the stabilization of most oxide particles in water. Furthermore, the absence of high concentrations of potentially screening ions in the starting solutions keeps the ionic strength to a minimum, thus enhancing the particles’ stability. A similar stabilization mechanism is expected in the present case. The presence of hydroxide ions on gold particles has been established by a variety of electrochemical techniques, and the dependence of the surface potential of gold surfaces on pH is commonly accepted to indicate the adsorption of hydroxide ions.14,15 Taken together, these observations suggest that preparation of gold particles by reaction 1 should be feasible, resulting in similarly stable particle suspensions. Indeed, the reaction proceeds smoothly and allows facile preparation of clean Au particles. Furthermore, similar to silver, we find that the as-prepared particles can be utilized as redox catalysts for hydrogen evolution from water using reducing radicals, following a short preconditioning period that is required for reducing residual remaining gold ions.13 On the other hand, speciation of various gold ions and their complexed hydroxides that remain in solution or at the particles surface at low concentrations is more ambiguous than for the simpler silver system. We suggest that they are of Au(I) oxidation state.
10.1021/jp104922a 2010 American Chemical Society Published on Web 08/13/2010
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Details of the synthesis of reaction 1, characterization of the particles and their temporal evolution, and information on the solution species are provided in this report. A primary motivation for studying these metallic particles is their potential application as redox catalysts, specifically for hydrogen evolution from water, for example in energy conversion applications. Other catalytic processes on suspensions of gold particles by a variety of radicals have been reported as well.16-20 The utility of surface-enhanced Raman scattering (SERS) in studying catalytic processes is often invoked.21-23 Probe molecules adsorbed on silver particles can shed light on the water-particle interface during catalytic H2 evolution.24 Here we test the presently prepared gold particles as catalysts in that context and perform SERS studies of the well-documented SERS-active probe molecule p-aminothiophenol (p-ATP).25 For comparison, we also test its p-nitro and p-hydroxy analogues. Basu et al. recently reported that o-ATP, the ortho-derivative, induces aggregation of gold particles prepared by reduction with citrate.26 We find that p-ATP similarly induces aggregation of the “naked” gold particles, and as commonly observed the junctions that form increase significantly the SERS enhancement. Nonetheless, the potential-sensitive enhancement of the charge-transfer component of the SERS mechanism makes the p-ATP on silver an ideal probe for these studies.24,27 The larger work function of gold than of silver, on the other hand, renders the SERS enhancement on gold less sensitive to the Fermi level position than silver. Experimental Section Chemicals. Au2O3 (Strem Chemicals), KCN, acetone, 2-propanol (Fisher Scientific), NaAuCl4 (Alfa Aesar), HAuCl4, and p-ATP (Aldrich) were of highest purity commercially available and were used as received. Hydrogen and argon gases were of highest purity from Mittler. Deionized ultrapure water from a Millipure Milli-Q system was used throughout this study. Synthesis and Characterization of Gold Particles. “Naked” gold colloidal dispersions, free of any foreign stabilizer or counterion, were prepared following the analogous procedure of Evanoff and Chumanov9,10 for silver. We have previously used that synthesis for the production of silver particles and tested their catalytic activity in H2 evolution.13,24 Briefly, roundbottom three-necked flasks of various sizes equipped with a thermometer, a condenser, and a spout for withdrawing samples were used for the synthesis. Each synthesis followed the same basic procedure: Au2O3 powder in proportion of 0.8 g per 1.0 L of deionized water was added to the flask. The content was first bubbled with argon gas through the spout for 30 min, and its temperature was raised to the desired level. Following equilibration for an hour the vessel was flushed for 10 min with ultrapure H2 and then pressurized above atmospheric levels. Unless otherwise specified, particles were synthesized at 65 ( 5 °C and 1.5 ( 0.1 atm of H2 gas. During the reaction, aliquots of the reaction mixture were taken through the spout at various time intervals, and the progress of the reaction was monitored via the UV-vis extinction spectra. All the spectra shown here were normalized to 1 cm optical path. A fine glass frit at the bottom of the spout was used to block transfer of solid Au2O3 particles into the samples. Releasing the excess H2 pressure and dropping the temperature to the ambient terminated the synthesis. All subsequent speciation measurements were conducted after cooling to room temperature. Instrumentation. Extinction spectra were measured on Varian Cary 50-Bio spectrophotometer. Total gold concentrations were measured by inductively coupled plasma-atomic
Merga et al. emission spectroscopy (ICP-AES) on an Optima 3300 XL Perkin-Elmer spectrometer. Gold particles were completely dissolved in air saturated 0.05 M KCN solutions prior to the ICP measurements. Conductivity measurements were done using a Chemtrix type 70-conductivity meter, and 0.001 or 0.01 M KCl solutions were used as standards. To obtain higher concentrations than those obtained directly from the assynthesized original suspensions, they were centrifuged and the nearly clear supernatant solution was decanted. It was verified that this procedure does not affect the size of the particles or their distribution. The pH was adjusted by HClO4 or NaOH solutions. Particle size and size distribution were routinely measured using dynamic light scattering (DLS) using a Beckman-Coulter N4 Plus instrument. To verify size and shape determinations, samples were examined using a JEOL 2010 high-resolution transmission electron microscope (TEM) operating at 200 kV. The latter instrument is outfitted with an X-ray detector that enables energy-dispersive X-ray spectroscopy (EDXS) measurements of the particles elemental composition. Samples for TEM analyses were prepared by dropping a dilute solution of the gold nanoparticles in water onto ultrathin carboncoated copper grids (from Ladd Research). Raman Microscopy. SERS and Raman spectra were collected with a Renishaw RM1000 Raman spectrometer equipped with CCD detector and a confocal Leica microscope. The spectrograph uses 1200 grooves/mm gratings and a linearly polarized diode laser (785 nm) or an Ar ion laser (514 nm), operating at 1-10 or 0.1-1 mW, respectively. Each spectrum of the probe molecule, p-ATP, was collected for ∼5 min. Spectral shift calibration of the instrument was performed frequently using the Raman lines of pure ethanol. Aliquots of freshly prepared p-ATP dissolved in 2-propanol were added to samples for SERS measurements. Unless otherwise stated, SERS experiments were done at 1 × 10-6 M p-ATP and 1.0 M 2-propanol solutions. Spectra were corrected for background by subtracting the spectra from cuvette filled with identical suspensions but in the absence of p-ATP. While the accuracy of the Raman shifts could be determined to better than 1 cm-1, these corrections increase the error significantly. Samples for γ-irradiation were degassed by bubbling Ar gas before the irradiation and again before the collection of the SERS spectra. All SERS spectra were collected in cylindrical, 1.0 cm diameter, septum-capped quartz cuvettes. Radiation Experiments. Details of the radiation experiments have been given earlier.24 Briefly, samples were irradiated with a 60Co γ-source at a dose rate of 72 ( 5 Gy min-1 H2 yields were determined by gas chromatography in irradiation-sample cell made of a 1 × 1 × 3 cm quartz cuvette. A 3-m MolecularSieve 5A chromatographic column held at 50 °C was used to separate the gaseous products and a thermal conductivity detector to quantify them. All irradiated solutions contained 0.1 M acetone and 1.0 M 2-propanol and were deaerated by bubbling Ar gas for 15 min prior to the irradiation. It is now well established that under these conditions the dominant relevant radicals are (CH3)2COH radicals, which are generated within a few microseconds following the absorption of the radiation. These radicals recombine in the absence of the particles but produce molecular hydrogen from water when the catalytic process dominates. Under these conditions and at the dose rate used ∼4.5 × 10-5 M min-1 of reducing equivalents (i.e., (CH3)2COH radicals in this case) and additional 8 × 10-6 M min-1 H2 are generated by the radiation. The maximum yield of H2 then is ∼4.2 molecules per 100 eV of absorbed radiation.13 When easily reducible species, such as gold ions, are present
“Naked” Gold Nanoparticles
Figure 1. Extinction spectra of Au particles during the reaction of gold(III) oxide with hydrogen. Particles were grown at t ) 70 °C and H2 pressure of 1.5 atm. All spectra were collected at ambient temperatures and pressures.
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Figure 2. Changes in pH (squares, left-hand side axis) and particle size (circle, RHS) measured by dynamic light scattering of the same synthesis shown in Figure 1.
in the solution, the reducing equivalents from the radicals are utilized to reduce the ions before catalytic hydrogen evolution takes over. SERS measurements of irradiated samples were performed in deaerated cylindrical quartz cuvettes at the same dose rate as the samples for H2 evolution. Results and Discussion Synthesis and Characterization of the Particles. Gold particles of sizes 20-100 nm are readily obtained by reduction of the oxide with H2. Whereas most of the experiments described here utilized particles prepared at somewhat elevated temperatures and pressures (see Experimental Section), reduction proceeds at lower temperatures as well, albeit at slower rates. Qualitatively, slower rates at the lower temperatures result in larger particles. A set of extinction spectra of the particles collected during a synthesis at 70 °C is shown in Figure 1. The familiar plasmon band at around 530 nm is evident in this figure. The position of λmax shifts during the synthesis from 520 to 535 nm as the particles grow, but it is clear that most of the additional metallic gold adds to the number of particles and not their size. New seeds are continuously formed or are perhaps present, e.g., from the dissolving gold oxide, as the reduction proceeds. The large driving force for reaction 1 apparently maintains relatively high superstauration levels of metallic gold. As can be seen in Figure 1, the absorbance (and λmax) at times g8 h after initiation of the reaction remains constant, indicating complete consumption of the precursor oxide. ICP measurements of total gold, metallic, and all ionic species concur with this observation. The measured total Au concentration remains unchanged after this time. Size measurements from dynamic light scattering of the same solutions of Figure 1 indicate that the size increases only slightly, 50-60 nm, at various times of synthesis following the initial nucleation period. Figure 2 shows the particle sizes measured by dynamic light scattering during this synthesis, and Figure 3 shows TEM images from samples withdrawn at the end of the reaction. The TEM images show that the particles are mostly polygonal, nearly spherical. Notably, rods that constitute ∼10% of the particles in similar silver
Figure 3. Top: TEM images of gold nanoparticles collected at the end of the synthesis of Figure 1. Bottom: size distribution of the particles leading to dave ) 50 ( 9 nm.
preparations are absent in the gold preparations. Sizes estimated by the TEM and DLS agree reasonably well, considering the different averages measured by these two techniques, number vs volume average, respectively. Very large sizes, in excess of 140 nm, were measured at the very early stages of the synthesis (Figure 2). These are attributed to agglomeration of small particles at the early stages of the reaction when the surface potential is still not fully developed. Similar agglomeration at the gold-aqueous interface is observed during the preparation of gold colloids using the citrate reduction/stabilization method.28,29
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Zeta potentials of the particles were determined by electrophoretic mobility measurements. The potentials fluctuate in the range of -20 ( 7 mV for all preparations. The only source of negative charges in this system is hydroxide ions. Furthermore, the pH of the solution is not expected to change during the reduction according to reaction 1. Yet, the pH remains on the basic side throughout the reaction and increases by more than two units in the synthesis corresponding to Figure 2. The changes in pH indicate an imbalance between the hydrolysis and reduction steps of the overall process. In the case of silver, this was attributed to the different rates of oxide dissolution and reduction.13 The present case is more complex because of the higher oxidation state of gold and the various related species that may exist in solution. Baes and Mesmer quote very low concentrations (
