14221
2008, 112, 14221–14224 Published on Web 08/21/2008
Conversion of Anionic [Au25(SCH2CH2Ph)18]- Cluster to Charge Neutral Cluster via Air Oxidation Manzhou Zhu,† William T. Eckenhoff,‡ Tomislav Pintauer,‡ and Rongchao Jin*,† Department of Chemistry, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15213, and Department of Chemistry and Biochemistry, Duquesne UniVersity, Pittsburgh, PennsylVania 15282 ReceiVed: July 1, 2008; ReVised Manuscript ReceiVed: July 25, 2008
We report a solution-phase conversion process of the [Au25(SCH2CH2Ph)18]- anionic cluster into a charge neutral cluster [Au25(SCH2CH2Ph)18]0 via air oxidation. The one-electron loss of the Au25- cluster and conversion to Au250 is a surprise in light of the chemical inertness of gold nanoparticles. In contrast with the crystal structure of the anion cluster Au25-, which exhibits apparent structural distortions in the Au25S18 framework, such distortions are not observed in the neutral cluster. The cluster charge effect is also manifested in the optical absorption spectra of the clusters. Given the only recently reported structure of the parent Au25- cluster, it is of substantial interest whether its one-electron oxidized product is similar to that of Au25-. This work unambiguously determined the structure of the one-electron oxidized product and correlates the structure with the optical properties. When the size of gold nanoparticles approaches the de Broglie wavelength of the conduction electrons (∼1 nm), the quasi-continuous electronic bands evolve to discrete levels, and thus drastic changes occur in the electronic structure of the particle relative to larger crystalline nanoparticles (i.e., nanocrystals, typically >2 nm diameter).1 The pursuit of very small gold nanoparticles (size ranging from subnanometer to ∼2 nm, also called nanoclusters or clusters in short) and their physical and chemical properties constitutes an important forefront in current nanoscience research.2-13 Very recently, there has been remarkable progress in the synthesis and characterization of solution phase gold thiolate clusters.14-16 A high-yield synthesis of thiol-stabilized Au25 clusters has been achieved with a kinetic control approach15a and the X-ray crystal structure of the as-prepared [Au25(SCH2CH2Ph)18]- cluster (counterion: tetraoctylammonium TOA+), abbreviated as Au25- below for simplicity, has been successfully determined.15b,16a The availability of the Au25crystal structure permits systematic theoretical and experimental investigations of the Au25- cluster’s electronic and optical properties and correlating the cluster structure with the physical and chemical properties.15b Among them, the chemical reactivity and the charge states of these welldefined, atomically monodisperse 25-gold-atom clusters as well as the charge effects on the cluster’s optical and structural properties are of particular interest. Herein we report a solution-phase conversion of the Au25cluster into a charge neutral cluster [Au25(SCH2CH2Ph)18]0 (vide infra, abbreviated as Au250 below for convenience). The spontaneous one-electron loss of the Au25- cluster via oxidation with air (O2 source) and conversion to Au250 is a surprise, given * To whom correspondence should be addressed. E-mail: rongchao@ andrew.cmu.edu. † Carnegie Mellon University. ‡ Duquesne University.
10.1021/jp805786p CCC: $40.75
Figure 1. The optical absorption spectra of anionic (blue profile) and neutral (red profile) 25-gold-atom clusters in solution.
the well-known chemical inertness of gold nanoparticles. The crystal structure of the air oxidized cluster has been determined by X-ray crystallography. The anionic Au25- clusters (counterion: TOA+) were synthesized via a kinetic control approach.15a The pure Au25clusters were obtained by crystallization from a toluene-ethanol mixed solvent.15b X-ray crystallographic analysis has determined the Au25- structure to be a two-shell structure, which is comprised of an icosahedral core (Au13) with twelve of its triangular Au3 faces capped by twelve gold atoms and leaving eight triangular faces uncapped. The optical absorption spectrum of the Au25- clusters in solution (Au25- single crystal redissolved in CH2Cl2) is shown in Figure 1 (blue profile). Apart from the three main absorption bands at 400, 450, and 670 nm, there are additional fine spectral features, including a broad shoulder at ∼800 nm, and another small shoulder at ∼550 nm. Density functional theory (DFT) calculations15b,16b have identified the 2008 American Chemical Society
14222 J. Phys. Chem. C, Vol. 112, No. 37, 2008 670 nm band as the LUMOrHOMO dipole allowed transition. The 800 nm band is believed to be associated with a spinforbidden transition. The Au25- single crystal was used as the starting material for all the experiments in this work; this avoids potential interference and ambiguity from impurities. When a solution of pure Au25-TOA+ clusters (single crystal redissolved in CH2Cl2) was allowed to stand in air for several hours, interestingly we found that the optical absorption spectrum shows distinct changes (Figure 1, red profile) compared to the starting Au25- anion’s absorption spectrum. The 400 nm peak becomes more prominent while the 450 nm peak becomes less so. Concurrently, the 800 nm shoulder that is characteristic of the Au25- cluster disappears and a new, small shoulder at 630 nm emerges. These results indicate that the 400 and 450 nm states are likely associated with one another, and the intensity is transferred from the 450 nm state to the 400 nm state upon oxidation; similarly the 800 and 630 nm bands form another correlated pair. Control experiment showed that when the Au25- solution is bubbled with N2 (99.995%) to remove dissolved O2 in solution and blanketed with N2 (sealed in a cuvette), no spectral changes are observed within 24 h. When the air layer above the solution is displaced with N2 but no bubbling of the solution, the conversion reaction occurs but with a significantly slower rate. These results confirm that molecular oxygen (from air) is involved in the conversion process, in which O2 first diffuses into solution and withdraws an electron from the Au25- core, resulting in oxidation of the cluster. Of note, there is naturally a trace amount (∼ppm) of O2 dissolved in solvent or solution; thus, when the solution is blanketed in N2 but not bubbled with N2, the dissolved O2 can still effect conversion of the Au25cluster into an oxidized product but with a much slower rate since O2 is not continuously supplied from air into the solution. Interestingly, such a conversion does not occur in the solid state within a two-month period investigated (Au25-TOA+ single crystal exposed to air for two months under ambient conditions). This indicates that the complexation of TOA+ with the anion cluster in the unit cell helps preserve the negative charge of the cluster. Compared to the solid state result, the electron transfer in solution phase seems critical for air oxidation of the clusters in solution. The air oxidation process can be expedited when water was added externally. The oxidation process can also be effected by other oxidizing agents, such as H2O2. With respect to the reduced product of oxygen, the one-electron reduction presumably generates transient superoxide O2-, peroxide O22or their protonated forms since trace amounts of water (∼ppm) always exist in solvent (CH2Cl2 HPLC grade was used in this work, water
