Nanoparticle-host interactions in natural systems




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НазваниеNanoparticle-host interactions in natural systems
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Figure captions


Fig. 1. Overview of some of the methods to analyze specific properties of nanomaterials. Theoretical and experimental approaches can be applied to design model systems and their natural equivalents.

Fig. 2. Melting behaviour of isolated Au particles as a function of size. The curve asymptotically approaches the melting point of bulk Au (1337 K, plot modified from Buffat & Borel, 1976).

Fig. 3. Experimental STM image of a pyrrhotite (001) surface after 6000 L of O2 exposure. The terraces (bright spots represent S atoms) are unaltered by oxygen exposure, whereas adsorption features can be observed at the corners of terraces where Fe is exposed. The step heights shown are between  3 Å (one quarter of a unit cell) and  17 Å (1.5 unit cells). The image was taken at a sample bias voltage of - 0.4 V. (image modified from Becker et al., 1997a)

Fig. 4. STM image of gold islands on PbS surface after exposure to AuCl4- solution. (image modified from Becker et al., 1997b, image taken by Carrick Eggleston)

Fig. 5. STM image of gold islands on PbS surface after extended exposure to AuCl4- solution. (image modified from Eggleston and Hochella, 1991)

Fig. 6. The crystal structure of molybdenite. The structure consists of MoS2 sandwich-like layers ||(001) with van der Waals forces between the layers. The figure shows the Mulliken charges and spin densities of the species as calculated using Crystal98 with three-dimensional periodic boundary conditions. (figure modified from Becker et al., 2003)

Fig. 7. STM image of silver islands on molybdenite taken at a sample bias voltage of +0.1 V and a setpoint current of 1 nA. (a) Arrangement and coverage of the deposited islands. In addition, the image shows the relatively uniform diameter of these islands of  2 nm (20 Å = 7 Ag atoms in diameter  37 Ag atoms per island) (b) One of the islands at a higher magnification (8.59.5 nm2). Outside the island, one can see a hexagonal pattern of the molybdenite structure and even within the island, one can roughly recognize the atomic resolution of the silver island (or a convolution of the electronic structure of the Ag island with the electronic structure of the underlying MoS2 substrate). (figure modified from Becker et al., 2003)

Fig. 8. Calculated STM image for negative bias voltages using Crystal98 and our own software (Becker, 1995). The image shows that the bright spots located above Mo atoms are due to the overlap of Mo 4dz2 orbitals and S 3p orbitals that point towards the threefold axis that goes through the Mo atoms and is parallel to the (001) direction. The orientation of the image is rotated by 8 with respect to the experimental image shown in Fig. 7. (Figure modified from Becker et al., 2003).

Fig. 9. Experimental STS spectrum taken above a bright spot of an STM image as shown in Fig. 7. The width of the low base in the centre of the spectrum ( 0.3 eV left and right of the Fermi level) can be roughly interpreted as the band gap if tunnelling specific processes such as band bending are ignored. (figure modified from Becker et al., 2003)

Fig. 10. UPS spectrum of (a) bulk silver, (b) pristine molybdenite, and a molybdenite surface partially covered with silver nanoparticles (c, ≈ 8 % coverage). “EF” denotes the Fermi level that is set to be 0 for the scale of the electron binding energy. “UPS” denotes the workfunction of the UPS analyzer. (figure modified from Becker et al., 2003)

Fig. 11. Model for the quantum-mechanical calculations using Crystal98. An Ag monolayer is adsorbed to two MoS2 sandwich layers. The energetically most favourable Ag position is located 3.79 Å above Mo atoms which results in a Mo-S bond distance of 2.81 Å. Due to the polarization of the Ag atoms on the surface by the uppermost molybdenite layer, the Ag atoms become positive (+0.51 Mulliken charges). This polarization effect is compensated by a more negative charge of the uppermost sulphur layer. The Mulliken charges of the Mo and S atoms in the uppermost MoS2 sandwich are more positive (less negative) than in bulk MoS2. The most energetically favourable spin configuration is antiferromagnetic in the uppermost two layers. (figure modified from Becker et al., 2003)

Fig. 12. Quantum-mechanical calculation of a 1/4 monolayer of Ag atoms on MoS2 using a 22 surface unit cell in Crystal98 with just one Ag atom per supercell. In this setup, two different surface S atoms have to be distinguished. The ones that are bonded to Ag atoms with a bond distance of 2.71 Å (3 per surface unit cell) are denoted “S1” and the ones that are not bonded to Ag are denoted “S2” (1 per surface unit cell). Similarly, there are two types of Mo atoms, “Mo1” are the ones directly underneath the Ag atom (1 / s.u.c.), and “Mo2” are the remaining ones. Accordingly, the two types have different charges and spin densities. The ones with index “1” are one the left of the “/” and the ones with index “2” are one the right of the “/”. The parallelogram denotes the 22 surface unit cell. (figure modified from Becker et al., 2003)

Fig. 13. Energetics of the adsorption of different Ag islands (with 1, 3, 7, 19, 37, 61 and an infinite number of Ag atoms adsorbed). (a) The clusters are shown with their respective adsorption energy. (b) Adsorption energy as a function of cluster size. In addition to the clusters shown in a, the graph contains the adsorption energy of a single Ag atom (denoted “Ag”) and the adsorption of a flat monolayer (see Fig. 11). The triangular cluster (“A”) is the least stable whereas the cluster with 37 Ag (7 atoms in diameter) is the most stable. (figure modified from Becker et al., 2003)

Fig. 14. Model of an Ag7 cluster (not adsorbed to MoS2) using different spin configurations. (a) The model with all spins up leads to a cluster with D6h symmetry. The bond distance between the central Ag atom (“A”, Mulliken charge = 0.048, spin = 0.95) and the six symmetry equivalent Ag atoms around (“B”, Mulliken charge = -0.008, spin = 1.01) is 2.995 Å. For the configuration with just one total spin (b), the symmetry is lowered to D2h and the partial spin density of  ¼ is located on the four symmetry equivalent Ag atoms denoted “C”. In this spin configuration, the bond distances are lowered to 2.796 Å and 2.855 Å. (figure modified from Becker et al., 2003)

Fig. 15. Calculated densities of states (DOS) for MoS2, Ag-MoS2, Ag in a hexagonal layer and bulk fcc Ag, with energies relative to the Fermi energy EF. Labels give the dominant contributions from atomic states, obtained by projecting individual wavefunctions at different energies and wavevectors onto valence electron orbitals from calculations of isolated atoms. The DOS is scaled such that integrals are proportional to the number of electrons in each system. (figure modified from Becker et al., 2003)

Fig. 16. Experimental STM image on the adsorption of copper onto a molybdenite (001) surface. (a) After the initial deposition, the surface is uniformly covered with copper islands with a uniform diameter of  8 nm and a height of  1.5 nm. (b) Due to surface diffusion, the copper islands move to a different place where the adsorption energy is higher. In this case, they get finally attached to “folds” of the uppermost MoS2 layer. (c) Even after  1 day after the deposition, a small number of islands can be found on flat terraces with a tendency to bind to other islands (see the pair of islands near the centre of the image). Note the smaller scale of image a compared to b and c. (figure modified from Becker et al., 2003)

Fig. 17. Experimental STM image of a molybdenite (001) surface after the adsorption of gold. (a) Small islands undergo more surface diffusion than larger islands and get eventually attached to these (process “B”, islands at the base of the arrow in a end up to be part of the larger islands in b). Also for the larger islands, some surface diffusion can be observed when images a and b are compared (process “A”, larger islans at the base of the arrow in a surface diffuse to a position at the arrowhead in b). (figure modified from Becker et al., 2003)

Fig. 18. Ostwald-type ripening of natural Au nanoparticles in arsenian pyrite host. HAADF-STEM images (Au particles as bright spots on a darker matrix) show no visible changes from room temperature (a) until ~390 °C (b), where smaller nanoparticles dissolve into the host and larger ones coarsen. At 450 °C (c), larger particles have grown at expenses of the smaller ones. At 550 °C (d), only three particles of >20 nm diameter survive in the view of 0.2 microns square (Movie 1 in Reich, 2006). Yellow arrows indicate particles that have been dissolved into the matrix with respect to the previous frame, while red arrows show some particles that have coarsened with respect to the previous temperature step. No sample alteration due to electron beam irradiation was noted during TEM examination (irradiation tests were undertaken according to Palenik et al., 2004). Scale bar is 100 nm. (figure modified from Reich et al., 2006)

Fig. 19. Dissolution of a single Au nanoparticle into its pyrite host. Selected HRTEM images show the evolution of a single AuNP during heating starting at 400 °C (a). Size/shape changes start at ~450 °C (b-c) (Movie 2, 3 in Reich 2006), although most significant decrease in size occur at 550 °C (d-h), until final dissolution (i) (Movie 4, in Reich 2006). Scale bar is 5 nm and inset in (a) is the Fast Fourier Transform of the HRTEM image of the AuNP. (figure modified from Reich et al., 2006)

Fig. 20. Sequence of selected HRTEM images during heating of two AuNPs of different sizes (~ 50 and 25 nm). Starting at 600 °C (a), the smaller AuNP progressively decreases in size (b-c) until it dissolves at 650 °C, 75 seconds after the initiation of the experiment (Movie 5 in Reich, 2006). Scale bar is 50 nm. Arrows indicate the dissolving AuNP. (figure modified from Reich et al., 2006)

Fig. 21. The effect of nanoparticle-host interaction during the thermal evolution of Au nanoparticles. The lower curve shows the temperature of complete dissolution of Au nanoparticles in arsenian pyrite, plotted as the average diameter of the particle-size distribution (circles with standard error bars) for a given temperature (upper curve, see Fig. 2). The plot shows how Au nanoparticles dissolve into the matrix instead of melting (which would require for the AuNPs to reach the upper curve) when incorporated in a sulphide host. The difference between the two curves, in terms of thermodynamic energy (see ordinate on the right), reflects the interplay of energy gain due dissolution and interface energy loss. (figure modified from Reich et al., 2006)

Fig. 22. Molecular dynamics (MD) simulations of a gold cluster incorporated in a pyrite cluster. (A) Initial geometry setup for MD simulation, after 1 ps of equilibration time; (B) MD simulation at 300 K, after 1 ps of observation/production time; (C) MD simulation at 900 K, after 1 ps of observation/production time. White and red balls are gold and iron atoms, respectively, and yellow sticks represent the sulphur-sulphur dimers. The true size of atoms (balls) has been modified for viewing purposes.

Fig. 23. Molecular dynamics (MD) simulations of two gold clusters incorporated in a pyrite cluster. (A) Initial geometry setup for MD simulation, after 1 ps of equilibration time; (B) MD simulation at 300 K, after 1 ps of observation/production time; (C) MD simulation at 900 K, after 1 ps of observation/production time. White and red balls are gold and iron atoms, respectively, and yellow sticks represent the sulphur-sulphur dimers. The true size of atoms (balls) has been modified for viewing purposes.

Fig. 24. (A) 12 amino-acid residue long peptide chain along a polar step on a calcite (10-14) surface: Ca2+ at step edge, total charge of the peptide chain is –13 (alkaline environment). (B) 12 amino-acid (alternating glycine and alanine) residue long peptide chain on polar, CO3- -bounded step edge, peptide is neutral (acidic environment). (figure modified from Biswas and Becker, submitted)

Fig. 25. Building block of Langmuir film: Amide containing polar head group (phospholipid) and long hydrocarbon tail. (figure modified from Biswas and Becker, submitted)

Fig. 26. Periodic film of molecule from Fig. 25 at its maximum pressure of 25.4 N/m. At higher piston pressures, the film disrupts and molecules are not aligned in a parallel fashion any more. (figure modified from Biswas and Becker, submitted)

Fig. 27. Organic Molecule network-calcite interfaces: calcite (A) (001), (B) (10-14), and (C) (100) faces, respectively, from left to right. The calcite (100) interface has the most favourable interface energy (-0.05eV/Å2) at the conditions from Fig. 26. (figure modified from Biswas and Becker, submitted)

Fig. 28. Organic molecule network-calcite (100) interface subsequent to molecular dynamics simulations. While with the setup of Fig. 27, the static interface and interface energy is calculated, this figure depicts the dynamic formation of nanoscopic calcite seeds with Ca2+ and CO32- in a supersaturated aqueous solution. (figure modified from Biswas and Becker, submitted)

Fig. 29. High resolution transmission electron micrographs showing nanocrystalline gold particles found in nature and made in the laboratory: (a) in natural pyrite (FeS2), (b) from the ground water near the Nevada Test Site, (c) in Au ion implanted rutile (TiO2) (unpublished micrographs of L.M. Wang).

Fig. 30. High resolution transmission electron micrographs showing nanocrystalline zirconia (ZrO2) in both naturally occurring zircon (ZrSiO4) which suffered -decay damage in geological times (~500 million years) (a) and in synthetic zircon after 800 keV krypton ion irradiation at 775 C to 3 dpa (b) (Meldrum et al., 1998).

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