Nanoparticle-host interactions in natural systems

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From dimers to nanoparticles: gold nanoparticle formation on sulphide surfaces

Geologic background

Adsorption/reduction reactions of heavy metals, in particular precious metals such as gold and silver, at sulphide surfaces have been found to be an important mechanism for the control of heavy metal concentrations in reducing aqueous environments (James & Parks, 1975; Bacon et al., 1980; Dyrssen et al., 1984; Morse et al., 1987; Bancroft & Hyland, 1990). In addition, it has been suggested that such surface reactions can significantly contribute to the formation of precious metal ore deposits such as Carlin-type Au deposits (Bakken et al., 1989; Starling et al., 1989).

In recent years, it has become possible to characterize many aspects of these sorption/redox reactions using surface sensitive techniques such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Raman spectroscopy, secondary ion mass spectroscopy (SIMS), and Mößbauer spectroscopy (e.g., Bancroft, 1973; Bancroft & Jean, 1982; Buckley et al., 1989; Hyland & Bancroft, 1989, 1990; Hyland et al., 1990; Mycroft et al., 1995; Scaini et al., 1995). These studies have made excellent contributions to our knowledge of adsorption/reduction reactions of metals on sulphide surfaces, and they have explained many details such as changes in oxidation states of the reactants or the reaction kinetics. However, these approaches are usually restricted to describing the initial and final states of surface reactions averaged over hundreds of µm2 or even mm2. In order to understand the initial step of the adsorption/reduction process and the formation and diffusion of nanoparticles, it is necessary to investigate the reaction process at an atomic level. With atomic-scale resolution and a detailed knowledge of electronic transfer processes, it is possible to understand the key controls of sulphide surface reactivity such as reactions at steps, vacancies, or defects and to understand why reactions occur at different rates on different sulphide minerals. Already at around the turn of the century, Skey (1871) and Palmer and Bastin (1913) had noticed that the nature of a precious metal ore may depend significantly on the sulphide with which its precipitation is associated. This observation was supported by later studies using electrochemical methods that stated that pyrite is a more efficient reductant for gold than either pyrrhotite or galena (Sakharova et al., 1975, 1981). Palenik et al. (2004), Reich et al. (2005) and Reich and Becker (2006) found that arsenian pyrite and especially arsenopyrite are even better reductant/hosts for gold. Thus, we have to ask “What are the specifics of nanoparticle-host interactions” and “What makes one mineral a better host than another one for a particular nanoparticle species?” Furthermore, can we deduce anything about the formation conditions, the thermal history, or even the best way to extract certain elements from their hosts by knowing more about the structural, electronic, and thermodynamic properties of the nanoparticle-host interface?

Observations of different stages of nanoparticle formation

In the context of spatial resolution, Eggleston & Hochella (1991, 1993) have contributed to the understanding of reaction mechanisms in an aqueous gold/sulphide system by using the STM to obtain images with close to atomic resolution. On some of these images (Fig. ), one can see a bright spot of high electron density that can be assigned to a gold species (of unknown oxidation state) on the galena surface. The identification of such spots as a gold species is very likely because the galena surface was only exposed to a gold chloride solution, and the number of such spots increased after exposure to an AuCl4- solution. Nevertheless, it was not clear if these spots of high valence-band electron density represent metallic gold on the surface or some intermediate gold complex containing another ligand such as chloride or polysulphide. Eggleston & Hochella (1993) also collected Scanning Tunnelling Spectroscopy (STS) spectra on gold islands only a few atoms in diameter. They showed that, once a few gold atoms form an island, the electronic structure near the Fermi level of such a cluster closely resembles the electronic structure of metallic sheets of gold. In the future, it may be possible to experimentally investigate the electronic structure of single atoms adsorbed to a surface and determine transitional stages of adsorption/redox reactions, e.g., with advances in scanning tunnelling spectroscopy. However, the interpretation of such experimental data will depend on the knowledge obtained from theoretical approaches.

Quantum-mechanical calculations to understand the nanoparticle formation and to interpret spectroscopic findings

First, one has to investigate possible reaction paths for the adsorption of Au(III)Cl4- on galena and the stepwise reduction of Au(III) to Au(I) and Au(0). Furthermore, we have to ask if intermediate and transition states for gold adsorption/reduction on galena along these reaction paths are thermodynamically metastable (some of which might only exist from minutes to microseconds), that is, if there are oxidation states of gold atoms with specific adsorbate structures that occupy local energy minima. In all of these calculations, one has to pay attention to the role that hydration processes may play in the release of dissolved species from the surface (Becker et al., 1997b). Even though significant amounts of energy are gained during adsorption, almost equal amounts of hydration energies are lost during the adsorption process. The difficulty in estimating this loss is that different amounts of hydration energy are preserved, depending on the adsorption site (terrace, step, kink) because part of the adsorbate is still exposed to the solution.

In order to be able to interpret spectroscopic XPS data of surface adsorbates and nanoparticles (for which no standard spectra are available), one can calculate XPS spectra of the proposed adsorbate structures and the associated substrate in order to compare these calculated XPS spectra with the experimental XPS data of other groups. XPS peak shifts, especially when compared to calculated ones, are a powerful indicator of electron transfer from one chemical species to another (Becker & Hochella, 1996; Becker et al., 1997b). These calculations were performed on a galena cluster using the quantum-mechanical nonperiodic program Gaussian (Frisch et al., 1998).

Finally, STS spectra (e.g., by Eggleston & Hochella, 1993) can be calculated in order to aid interpretation or prediction of experimental STS spectra. The calculation of spectra is also instructive to monitor how the adsorption of metal atoms to the surface changes the band gap of a semiconducting mineral surface and thereby its electronic properties.

Such an ab initio approach to the adsorption/reduction of gold on galena surfaces can, in the future, be extended to other metals with similar properties, e.g., copper and silver or other adsorbing sulphides such as pyrite, pyrrhotite, and sphalerite. This will be very important in helping us work out characteristic electronic structure differences between these metals and the minerals they are adsorbed to or precipitated on.

Thermodynamics of Au adsorption and reduction

Becker et al. (1997b) show that the adsorption of AuCl4- with the simultaneous dissolution of one of the four Cl- is energetically downhill. While the formal oxidation state of Au does not change in this reaction (neutral AuCl3 is adsorbed to neutral PbS), its Mulliken charge (a simplified way of extraction atomic charges from quantum-mechanical calculations) goes to almost zero because the Cl- forms bonds to surface Pb atoms than to the Au atom. This charge transfer to the Au atom is achieved by tens of S atoms contributing less than 10 % of a unit charge each to the reduction of gold. Such an electron density transfer over long distances is only possible due to the semiconducting nature of galena and is, thus, specific to some sulphides and very few oxides in nature. The AuCl30 adsorbate (after release of Cl- or lead chloride complexes) is further stabilized by Coulomb interactions between Cl and Pb atoms on the surface.

Further reduction of Au requires the release of PbCl+ or PbCl2 complexes (in the first case, more hydration energy is gained, whereas in the latter, a stronger complex is formed), both leading to the formation of Pb vacancies on the surface. The Pb vacancy does not need to be created in the direct vicinity of the adsorption site. This is again due to the semiconducting nature of the sulphide surface because the positive charge deficiency of the vacancy is easily facilitated by long-distance (up to tens of Å) electron transfer to the adsorption site (proximity effect). The structure of the –Au(I)-Cl adsorbate complex with Au and Cl stacked on top of each other is such that the Cl atom is not in contact with the galena surface or specifically with a Pb atom on the surface any more.

For further reduction, disproportionation and direct reduction were considered by Becker et al. (1997b) as well as the formation of elemental sulphur and polysulphides, both of which have been observed in experimental studies (Hyland and Bancroft, 1989; Scaini et al., 1995). For the formation of nanoparticles, it is important to note that the only exothermic reactions that were found to form metallic gold on the surface required the formation of at least Au dimers. The formation of an Au-Au bond delivers enough energy for the Au(I) reduction to Au(0) to become exothermic.

Initial nanoparticle formation, diffusion, and spectroscopic properties of metallic nanoparticles on sulphides

Thus, we can consider the Au dimer to be the smallest nanoparticle on the sulphide surface, but we also need a mechanism that transports Au atoms to other ones to start the nanoparticle formation process. The activation energy for an Au atom to diffuse from one S atom on the surface to another one is about 3-4 times the thermal energy at room temperature (kBT). In other words, at any given time, at least 1 2 % of all Au atoms are hopping from one adsorption site to another one. This finding helps to explain why extensive surface diffusion and thus, nanoparticle formation is observed (Fig. ). Becker et al. (1997b) describe how these elongated nanoparticles may be stabilized by polysulphides along the edge of the gold cluster.

Barnett et al. (1999) describe how gold nanoparticles can still have a discrete band gap once their size is less than 1-2 nm in diameter. Eggleston & Hochella (1993) took STS spectra showing that at the edge of their gold nanoparticles on galena, one can still measure a band gap of about 1 eV. Thus, the edges of noble metal clusters and extremely small nanoparticles may have some electronic characteristics of semiconductors rather than metals that would have no band gap between the valence and conduction band. Becker et al. (1997b) calculated the local density of states (LDOS), an approximation of STS spectra, for tip positions above gold dimers and their results confirm a band gap of  1 eV.

One method that is often used to determine the composition, oxidation state, and chemical nature of surface species is XPS, in particular the chemical shift of XPS peaks. Oxidation states and chemistry of adsorbates are often derived from comparison of peak chemical shifts with bulk standards or dissolved species that can be adsorbed to inert surfaces without changing their coordination environment or oxidation state. Consequently, since there is no bulk analogue and no corresponding dissolved species for most nanoparticles, peak chemical shifts of these specific surface-bound complexes can only be interpreted in combination with calculated XPS spectra for these surface species. Examples for such calculated peak chemical shifts for different adsorbed gold complexes are given in Becker et al. (1997b). Even if the adsorbate consists exclusively of metal atoms, there can still be a shift, which is due to polarization by the underlying substrate. For example, during the adsorption of silver onto molybdenite, which we discuss below, Ag charge- and spin-polarizes the MoS2 surface such that the charge of each silver atom of an adsorbed Ag nanoparticle is +½ to +1, depending on the size of the nanoparticle.
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