1. Thermal Behavior of Metal Nanoparticles in Geologic Materials
Michelle Heredia

2. Introduction:
Know a lot about the behavior of natural nanoparticles in Earth’s surface and near-surface environment.
Need to find out about nanoparticle stability in higher-temperature environments where they are increasingly being found.

3. Introduction cont.:
Behavior of mineral nanoparticles in natural systems is still not well understood
Information is needed on how nanoscale effects affect the occurrence of nanoparticles at temperatures typical of near-surface to deep-crustal conditions

4. Background on Species:
Study done on the thermal stability of nanoparticles of native Au in refractory sulfides
Provide insights into conditions that limit the formation of nanoparticles in high temperature geological materials.
Observations made on Au nanoparticles (AuNPs) in arsenian pyrite [Fe(SAs)2].
Au speciation in arsenian pyrite is controlled by the Au/As molar ratio.
At Au/As<.02 Au+1 is the dominant species (solid-solution Au)
When this limit is exceeded then the nanoparticle Au0(zero) occurs.
Can precipitate these nanoparticles out in slightly supersaturated hydrothermal solutions at temperatures below 250 C
So very useful!

5. Heating Experiment Procedure:
Wanted to test the stability of AuNPs as a function of temperature and particle size
Carried out heating experiments in arsenian pyrite samples from Carlin-type deposits (characterized by invisible-typically microscopic/dissolved gold in pyrite and arsenian pyrite) in Nevada
The analyzed samples contained highly dispersed native AuNPs having an average size of 4 nm. Secondary ion mass spectrometry analysis and electron probe microanalysis of these pyrites reveal that Au is contained in both Au0 (zero) and Au+1.
Arsenian pyrite grains were cut ultrasonically from 30 micrometer polished sections, mounted on 3mm Cu grids, and milled with an Ar ion beam. Samples were placed on a regular double-tilt heating holder connected to a Gatan Smart Set Model 901 Hot Stage Controller (allows controlled heating between 25 C and 800 C).
The temperature was raised manually

6. Heating Experiment Procedure cont.:
Examination was carried out using a high resolution transmission electron microscopy (HRTEM) coupled with a high angle annular dark field detector with scanning transmission electron microscopy (HAADF-STEM)
allows average atomic mass (Z) contrast imaging at nanoscale.
Sample changes during heating were recorded and the size was processed

7. Results (HAADF-STEM):
It is likely that further heating could have caused these particles to merge into a single one, but they stopped at 550 C.
Why not go to 800 C?
Upon cooling the larger particles remained unchanged-in other words- coarsening (in this case gaining other atoms and growing) is irreversible.
The discrepancy between the Au budgets of large particles with respect to the small particles from which they grew (more Au contained in the large particles than in the initial small particle distribution) is the result of solid-solution Au remobilization during the heating experiment,
Some notes: AuNPs remained immobile during heating and no coalescence (two larger particles combining) occurred

8. Second Heating Experiment Procedure:
Next experiment was to gain insight into the mechanisms by which the nanoparticles dissolve into pyrite
Used HRTEM on a faceted 15 nm AuNP with a pronounced crystallographic relationship between particle faces and the host mineral .

9. Results of Second Procedure (HRTEM):
1.During heating at 450 C it starts to decrease in size and crystal facets become irregular (B).
2. Between C the particle continues to shrink and its edges are no longer aligned parallel to specific crystallographic directions (C-D).
3. At 550 C it is dramatically reduced in size (D-H) and finally dissolves completely (I).

10. Heating Experiment with Two AuNPs:
Similar experiment
Two AuNPs of different size in close proximity are heated above 600 C.
Arrow points to the NP decreasing in size.

11. Thermal Stability of AuNP:
Melting temperature of bulk Au decreases significantly when the particle dimensions are reduced to the nanoscale.
The particle size versus temperature data from the heating experiments show that larger AuNPs grow at the expense of smaller ones (Ostwald-ripening) (lower curve) before they reach the size dependant melting points of isolated AuNPs (upper curve).
The lower (coarsening) curve sets an upper temperature limit for the stability field of AuNPs in arsenian pyrite.

12. Thermal Stability of AuNPs (Basic Overview of Graph)

13. Thermal Stability of AuNP cont:
The difference between the melting temperature of isolated AuNPs (upper curve) and the temperature at which complete dissolution of AuNPs into the surrounding pyrite occurs (lower curve) is influenced by:
1. The energy gain due to dissolution of AuNPs into the surrounding matrix (mostly this)
2. The energy loss of disrupting the former AuNP-host interface
3. The loss of intra-AuNP interactions
The stability of AuNP also reflects the physico-chemistry of the AuNP-host interactions (surroundings have an impact!)
Can conclude that the surrounding sulfide host plays an important role in promoting solid-state dissolution of AuNPs before a solid to liquid phase transition occurs.

14. Thermal Stability of AuNP cont:
There were two features observed during the dissolution of AuNPs:
1.The faster disappearance of the (100) Au faces compared to the (111) faces as the AuNP shrinks (A-C).
2. The development of a dark contrast halo around the AuNP (B-H).
(111) faces are more stable than (100) faces upon heating (molecular dynamics simulation).
This causes preferential detachment of Au atoms from the (100) faces over (111) faces followed by diffusion through the matrix and reattachment to larger particles (Ostwald- ripening)
Supported by dark halos of diffraction contrast that develop around the AuNP that may be the result of highly concentrated Au atoms dissolved in the pyrite matrix.

15. Constraints on the Natural Occurrence of Nanoparticulate Metals:
AuNPs in arsenian pyrite remain stable until they start to coarsen into larger particles above 370 C.
The observations set a constraint on the upper temperature limit of occurrence of AuNPs in arsenian pyrite in hydrothermal systems.

16. Constraints on the Natural Occurrence of Nanoparticulate Metals cont.:
For any AuNP-bearing mineral, the position of the coarsening curve of AuNPs in temperature-particle size space will depend on the solubility of Au within its host. Therefore, as a first approximation, the coarsening curve in Figure 4 will shift upwards if AuNPs are incorporated in a mineral host where Au is less soluble than arsenian pyrite.
In the limiting case, if Au is insoluble in a certain mineral host AuNPs will respond to increased temperature by melting instead of coarsening. However, AuNPs will easily dissolve within host phases where Au miscibility is high. In this case, the coarsening curve will shift downward.

17. Future Works:
Need to evaluate Au stability under other variables such as heating rate, pressure, As content of pyrite and diffusion time scale because temperature isn’t the only variable affecting AuNP in nature, and this would give a more complete description of their behavior.

18. Conclusions/Applications:
The results show that nanoparticulate metals can also occur and remain stable at high temperatures in well-defined temperature-particle size limits.
Previously thought to only be prevalent in low-temperature aqueous environments
Used as a tool to evaluate the thermal history of nanoparticle-bearing geological materials
As the mean particle size of a particular NP distribution sets an upper limit of stability, T vs. particle size stability diagrams can be used to estimate the maximum temperature of the host rock.
Impact on the metallurgical recovery of Au and other noble metals in refractory sulfide ores. AuNPs constitute a significant fraction of the invisible Au in refractory pyrite and arsenopyrite in these ores.
Currently, refractory Au ores are oxidized first in order to render the Au accessible, but a better understanding may lead to more cost-effective methods.

19. Analysis of Paper: Some criticism:
Inconsistency- for the first experiment they stopped heating it up at 550 C because they stated due to the limits of their instruments, but they also stated they had instruments that could make it go to 800 C, but extrapolated their data anyway
For Figure 3, they did not state what happened to the bigger nanoparticle
Does that mean it was not affected because it was still in the stability field? Does Ostwald ripening apply?
They also could have gone into more detail into this experiment
They also heated this one up to 650 C but stated they couldn’t go over 550 C in previous experiments

20. Analysis of Paper cont.:
Connections to class:
Ostwald ripening: when smaller particles are sacrificed for the growth of bigger ones, illustrated in the first experiment
Instrument Techniques:
They used HRTEM in the experiment to examine what happened to the particles- TEM is a microscopy technique in which a beam of electrons in transmitted through an ultra thin specimen, forming an image. It measure the lower range of sizes.
Used STEM as well
Different because it forces the electron beam into a narrow spot, which is scanned over the sample
Suitable for mapping by ADF, which allows direct correlation of image and quantitative data

21. Analysis of Paper cont.:
Connections to Readings:
Nanoparticles: Strained and Stiff:
Related to this paper referenced in class
Thermal Behavior suggested that the nature of nanoparticle surroundings can have a profound impact on nanoparticle stability
shown through Figure 4
Nanoparticles: Strained and Stiff also found this correlation
Shown though internal disorder being responsible for stiff nanoparticles and unsatisfied bonding environments at the surface driving strain

22. Any Questions?

23. Discussion Questions:
Are there any interesting applications you can think of for the findings of this paper?
Any other connections to the material discussed in class?