Nanoparticle-host interactions in natural systems

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Three-dimensional nanoparticle-host interactions: nanoparticles in a crystalline host

As temperature plays an important role in many geological and environmental processes, the role of this intensive variable on model nanoparticulate systems needs to be addressed. In addition, special attention to nanoparticle-crystalline host interactions is of critical importance, as very limited information is available about the role of the interfacial energy on nanoparticle stability in natural systems.

The thermodynamics of materials at small dimensions has been intensively studied in the last decades due to the technological applications of nanomaterials as, e.g., transistors and metal interconnects, catalysts, optical devices, and drug delivery agents, among many others (Daniel & Astruc, 2004).

In the geological sciences, the occurrence of native metal, oxide, sulphide, and silicate nanoparticles has been documented in a variety of terrestrial and extraterrestrial materials, such as soils and sediments (Waychunas et al., 2005), acid mine drainage (Hochella et al., 2005), groundwaters (Novikov et al., 2006), refractory sulphide ores (Palenik et al., 2004), atmospheric particulates (Anastasio & Martin, 2001), and meteorites (Lewis et al., 1987). The study of these particles at the nanoscale is necessary to better understand first-order geochemical and environmental processes such as, e.g., precious metal enrichment in hydrothermal systems and heavy metal dispersion into soils and groundwater. However, the behaviour of mineral nanoparticles in natural systems is still not understood, due to the small number of experiments describing the significant changes in physicochemical properties that arise when mineral dimensions are reduced to the nanoscale (Halperin, 1986; Hochella, 2002a, b). Although it is known that a decrease in particle size to the nanoscale can promote phase instability due to the increase in surface energy (McHale et al., 1997; Navrotsky, 2001), there is only little knowledge on how size-confinement effects control the occurrence of nanoparticles at temperatures typical of near-surface to deep-crustal conditions.

In the next sections, we will review the effects of nanoparticle-host interactions on the thermal and structural stability of nanoscale minerals as a function of internal (e.g. particle/cluster size) and external (e.g., nature of host phase, temperature) variables.

The fate of “invisible” gold at high temperatures

Reich et al. (2006) document dramatic changes in the thermodynamic behaviour of native Au nanoparticles incorporated in refractory sulphide ores, confirming a relevant role for temperature-dependent nanoscale phenomena in geologic systems.

Sample characteristics and in-situ TEM heating

Native Au nanoparticles (AuNPs) in As-rich pyrite were used as a model system to investigate the behaviour of metal nanoparticles as a function of temperature. Both the particles and the host in this system have been well characterized (Reich et al., 2005; Reich et al., 2006), as AuNPs were recently discovered to be an important part of “invisible” (refractory) Au in many Carlin-type and epithermal ore deposits (Palenik et al., 2004), which are by far the largest source of the world’s currently mined Au (Hofstra & Cline, 2000; Cooke & Simmons, 2000). The samples analyzed in this study contain highly dispersed native AuNPs (average size ~4 nm, bright spots in Fig. a), incorporated in arsenian pyrite (Fe(S2-xAsx), x = 0.15), which was deposited at ~150-200 °C from hydrothermal solutions (Reich et al., 2005). Using high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Utsunomiya & Ewing, 2003), the dynamic behaviour of this system at the nanoscale during heating was monitored. TEM samples were heated between 25 °C and ~800 °C. In-situ imaging was performed using a field-emission TEM (200 keV) and videos captured with an attached digital camera.

Ostwald ripening of Au nanoparticles

The evolution of the AuNP size-distribution between 25-550 °C is shown in the HAADF-STEM images (Fig. a-d) and Movie 1 in Reich (2006). All AuNPs described in this section are natural gold nanoparticles embedded in bulk pyrite some 40 million years ago. In-situ heating reveals that AuNPs remain unchanged from room temperature (Fig. a) until ~370 °C, where the smallest Au particles (<2 nm) start to dissolve into the pyrite matrix. At 390 °C (Fig. b), coarsening of some larger particles (~8 nm) is noticeable, although the most evident changes in nanoparticle size and distribution occur at >400 °C. Above this temperature, larger AuNPs grow at the expense of the smaller ones in an Ostwald-type ripening process (Fig. c). AuNPs remain immobile during heating and no coalescence ripening (where two larger particles combine) is observed throughout the duration of the experiment, which lasted ~45 minutes in total. At 550 °C, the upper temperature limit of this experiment, three nanoparticles (~30-35 nm) have survived in the area of observation, replacing the initial 115 particles of average size ~4 nm (Fig. d). Upon cooling from 550 °C back to room temperature, the particle growth process is not reversed. This experiment shows that complete ripening of AuNPs in arsenian pyrite (i.e. the final formation of a single large Au particle from a starting array of AuNPs) requires temperatures higher than 550-600 °C for completion for the heating rates used in this experiment (from 370 °C to 600 °C in approximately 4 hrs.).

Single particle dissolution and growth

Further insight into the temperature-dependent nanoparticle behaviour was obtained using HRTEM on a faceted ~15 nm AuNP with a more pronounced crystallographic relationship between particle faces and host mineral (Fig. a). These observations show no apparent variations in size or shape of this AuNP until ~440-450 °C, when the particle starts to decrease in size and crystal facets become more irregular (Fig. b, Movie 2). Between 480 and 550 °C, the AuNP continues to shrink, and its edges are no longer aligned parallel to specific crystallographic directions (Fig. c-d, Movie 3). At 550 °C, the Au particle dramatically reduces its size (Fig. d-h) and finally dissolves completely into the matrix (Fig. i, Movie 4). When two AuNPs of different sizes (~50 nm and ~25 nm) in close vicinity are heated to above 600 °C (under HRTEM mode), the smaller AuNP progressively decreases in size with increasing temperature until completely dissolved at 650 °C (Fig. a-d, Movie 5).

Two distinctive features are observed during these experiments: (1) the faster disappearance of the (100) Au faces (apparent as (200) diffraction maxima in Fig. a) than the (111) as the Au nanoparticle shrinks (Fig. a-c) and (2) the development of a dark-contrast halo around the AuNP (Fig. b-h). Both observations support the important role of diffusion during the heating experiment. Molecular dynamics (MD) simulations on pyrite-encapsulated Au clusters (see below) show that (111) faces are more stable than (100) faces upon heating, consistent with previous reports (Carnevali et al., 1987). Thus, the preferential detachment of Au atoms from (100) faces over (111) faces, followed by diffusion throughout the matrix accounts for the observed changes in nanoparticle shape and size during heating experiments (Fig. a-i). This interpretation is supported by the presence of dark halos of diffraction contrast that develop around the AuNP upon heating, which may be the result of highly concentrated single Au atoms dissolved in the pyrite host or pyrite lattice strain effects.

Nanoscale size effects

Heating experiments indicate that in the lower temperature range (25-370 °C), AuNPs do not undergo any visible structural and/or physico-chemical modification. In contrast, thermal effects have a significant impact on nanoparticle stability at temperatures above 370 °C. Previous studies by Ercolessi et al. (1991) and Lewis et al. (1997) have documented that size effects have a dramatic impact on the melting temperature when compared to bulk Au (1064 °C). Therefore, a ~3 nm diameter Au particle can melt at temperatures as low as ~500 °C, due to the increase in the surface/volume ratio when the particle size is decreased to the nanoscale (Buffat & Borel, 1976). Before the AuNPs reach the temperature for size-dependent melting isolated AuNPs as seen in Fig. and in the upper curve of Fig. , the embedded AuNPs are subject to solid-state diffusion into the arsenian pyrite matrix. Instead of melting, AuNPs arrays undergo Ostwald ripening as the larger nanoparticles grow at the expense of the smaller ones.

The stability of AuNPs as a function of size and temperature does not only reflect the thermo-physical properties of the AuNPs by themselves, but also the physico-chemistry of the particle-host interaction (Zhang et al., 2003). From the particle size-temperature data in Fig. , we conclude that the surrounding pyrite host plays a major role in promoting solid-state dissolution of AuNPs before a solidliquid phase transition occurs. As the temperature increases, Au atoms detach from small nanoparticles and diffuse throughout the pyrite matrix, attaching to larger particles that grow as a result of Au mass-transfer. The difference between the melting temperature of isolated AuNPs (Buffat & Borel, 1976) and the temperature at which complete dissolution of AuNPs into the surrounding matrix is observed (Fig. ) is influenced by (1) the energy gain due to dissolution of Au into the surrounding matrix, (2) the energy loss of disrupting the former AuNP-host interface and (3) the loss of intra-AuNP interactions.

Molecular dynamics simulations

Further insight into the mechanisms of Au nanoparticle dissolution and Au diffusion in pyrite can be obtained by means of atomistic computational techniques. Molecular dynamics (MD) simulations of gold nanoparticle dissolution can be performed as a function of particle distribution and temperature. These unpublished MD simulations are conducted in a cluster of perfectly crystalline pyrite containing one or two embedded Au cubic clusters (Fig. and Fig. , respectively). Since these calculations are conducted using a large atom cluster (as opposed to a large unit cell for which NPT would be used), a constant NVT ensemble (Number of atoms, Volume, and Temperature, respectively) was applied. The time step in these MD calculations, for which the equations of motions are recalculated, is 1 fs (10-12 s). The motion of the atoms in the system was equilibrated for a given temperature for a time period of 1 ps. During this time, the system disorders as a function of the temperature applied. Subsequently, the system is “observed” (so-called production time) for another 1000 time steps (1 ps). Interatomic potentials for pyrite and gold were taken from Rosso & Becker (2000) and Becker et al. (2003), respectively. It has to be noted that the original empirical force fields in Becker et al. (2003) were derived for Ag and only small modifications were applied for gold. However, the thermodynamic and kinetic behaviour of Ag and Au NPs is similar, such that, at this semi-quantitative level, force-field accuracy is sufficient.

For a single 63-atom Au cluster in pyrite (2559 atoms total), simulations at 300 K show detachments of a few Au atoms from the corners of the cubic Au cluster. Although relaxation of the Au atoms is observed during the simulation time, no disruption of the Au cluster is observed at this temperature (Fig. b). However, at 900 K, the Au nanoparticle disrupts and the Au atoms diffuse throughout the pyrite matrix. When the MD simulation is run for two neighbouring Au particles (Fig. c, each Au cluster containing 63 atoms, for a whole setting of 1950 atoms), the Au clusters remain coherent at room temperature (Fig. b), but get dissolved at higher temperatures (900 K, Fig. c), leading to Au diffusion. These results support the diffusional nature of the Au nanoparticle dissolution process observed by Reich et al. (2006).

In summary, while we learn from the heating experiments using TEM about temperature-dependent solid-state dissolution and about the thermal history of ore deposits, MD calculations give us an atomistic picture of these diffusion processes. These results give us significant insight into the behaviour of embedded metal nanoparticles during natural processes and into reaction mechanisms of noble metal recovery in mineral processing plants.

Geologic implications

Naturally formed nanoparticles are subject to size/host effects that alter their thermal stability and therefore their occurrence in the geologic record. Our results indicate that small AuNPs (<4 nm) in arsenian pyrite can survive up to ~350 °C, usually considered the upper boundary for hydrothermal deposition of Au in a variety of Au deposits (Benning & Seward, 1996; Phillips & Evans, 2004). Hence, our data confirm that AuNP occurrence in ore-forming environments is enhanced below these temperatures, an observation consistent with empirical data on Carlin-type and epithermal Au deposits by Reich et al. (2005). Therefore, the presence of AuNPs incorporated in sulphide phases in hydrothermal Au deposits must be carefully evaluated in order to gain insight into the saturation state of Au in mineralizing solutions.

Our results show that nanoparticulate metals, usually thought to be prevalent in low-temperature (T<100 °C) aqueous environments, can also occur and survive at higher temperatures when incorporated into refractory host phases. Future evidence of metal nanoparticles in continental/oceanic hydrothermal systems, deep sedimentary basins and planetary surfaces and interiors will bring new insights on nanoscale phenomena, influencing geochemical and thermodynamic modelling. Therefore, the mineralogical setting and chemical state of Au and other noble metals (e.g., Pt group elements, Momme et al., 2002) incorporated in refractory minerals in hydrothermal/magmatic environments must be carefully re-evaluated in order to understand their saturation state in their parent ore-forming aqueous solutions and/or silicate liquids.

These observations not only help us understand the behaviour of metal nanoparticles in geochemical systems (Penn et al., 2001; Hochella, 2002a, 2002b; Wang et al., 2004), but may also have an impact on the metallurgical recovery of Au and other noble metals locked in refractory sulphidic ores. As recently discovered (Reich et al., 2005), AuNPs constitute a significant fraction of the “invisible” Au locked within refractory pyrite and arsenopyrite. Currently, refractory Au ores such as arsenian pyrite are oxidized first in order to render the Au accessible to chemical leach (La Brooy et al., 1994). A better understanding of the temperature-size relationship of the nanoparticulate Au encapsulated within the refractory sulphides fraction during pressure oxidation in autoclaves (~170-225 °C) or during oxidizing roasting (~650-700 °C) may eventually lead to more cost-effective methods for metal recovery. Finally, this research is a good example how the thermal behaviour of natural nanoparticles may serve as a model for technical applications, e.g., the usage of gold nanoparticles and their stability in oxide-supported Au particles as catalytic converters in low-temperature exhausts.
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