Nanogeoscience is the study of nanoscale phenomena related to geological systems. Predominantly, this is interrogated by studying environmental nanoparticles between 1-100 nanometers in size. Other applicable fields of study include studying materials with at least one dimension restricted to the nanoscale (e.g. thin films, confined fluids) and the transfer of energy, electrons, protons, and matter across environmental interfaces.
The atmosphereAs more dust enters the atmosphere due to the consequences of human activity (from direct effects, such as clearing of land and desertification, versus indirect effects, such as global warming), it becomes more important to understand the effects of mineral dust on the gaseous composition of the atmosphere, cloud formation conditions, and global-mean radiative forcing (i.e., heating or cooling effects).
The oceanOceanographers generally study particles that measure 0.2 micrometres and larger, which means a lot of nanoscale particles are not examined, particularly with respect to formation mechanisms.
- Water–rock–bacteria nanoscience
- Metal transport nanoscience
Size-dependent stability and reactivity of nanoparticlesNanogeoscience deals with structures, properties and behaviors of nanoparticles in soils, aquatic systems and atmospheres. One of the key features of nanoparticles is the size-dependence of the nanoparticle stability and reactivity.Banfield, J. F.; Zhang, H. Nanoparticles in the environment. Rev. Mineral. & Geochem. 2001, 44, 1. This arises from the large specific surface area and differences in surface atomic structure of nanoparticles at small particle sizes. In general, the free energy of nanoparticles is inversely proportional to their particle size. For materials that can adopt two or more structures, size-dependent free energy may result in phase stability crossover at certain sizes.Ranade, M. R.; Navrotsky, A.; Zhang, H.; Banfield, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.; Doran, G. S.; Whitfield, H. J. Energetics of nanocrystalline TiO2. PNAS 2002, 99 (Suppl 2), 6476. Free energy reduction drives crystal growth (atom-by-atom or by oriented attachment Penn, R. L.; Banfield, J. F. Imperfect oriented attachment: dislocation generation in defect-free nanocrystals. Science 1998, 281, 969.Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Aggregation-based crystal growth and microstructure development in natural iron oxyhydroxide biomineralization products. Science 2000, 289, 751.), which may again drive the phase transformation due to the change of the relative phase stability at increasing sizes. These processes impact the surface reactivity and mobility of nanoparticles in natural systems. Well-identified size-dependent phenomena of nanoparticles include
- Phase stability reversal of bulk (macroscopic) particles at small sizes. Usually, a less stable bulk-phase at low temperature (and/or low pressure) becomes more stable than the bulk-stable phase as the particle size decreases below a certain critical size. For instance, bulk anatase (TiO2) is metastable with respect to bulk rutile (TiO2). However, in air, anatase becomes more stable than rutile at particle sizes below 14 nm.Zhang, H.; Banfield, J. F. Thermodynamic analysis of phase stability of nanocrystalline titania. J. Mater. Chem. 1998, 8, 2073. Similarly, below 1293 K, wurtzite (ZnS) is less stable than sphalerite (ZnS). In vacuum, wurtzite becomes more stable than sphalerite when the particle size is less than 7 nm at 300 K.Zhang, H.; Huang, F.; Gilbert, B.; Banfield, J. F. Molecular dynamics simulations, thermodynamics analysis and experimental study of phase stability of zinc sulfide nanoparticles. J. Phys. Chem. B 2003, 107, 13051. At very small particle sizes, the addition of water to the surface of ZnS nanoparticles can induce a change in nanoparticle structure Zhang, H; Gilbert, B.; Huang, F.; Banfield, J. F. Water-driven structure transformation in nanoparticles at room temperature. Nature 2003, 424, 1025. and surface-surface interactions can drive a reversible structural transformation upon aggregation/disaggregation.Huang, F.; Gilbert, B.; Zhang, H.; Banfield, J. F. Reversible, surface-controlled structure transformation in nanoparticles induced by an aggregation state. Phys. Rev. Lett. 2004, 92, 155501. Other examples of size-dependent phase stability include systems of Al2O3,McHale, J. M.; Auroux, A.; Perrotta, A. J.; Navrotsky, A. Surface energies and thermodynamic phase stability in nanocrystalline aluminas. Science 1997, 277, 788. ZrO2,Pitcher, M. W.; Ushakov, S. V.; Navrotsky, A.; Woodfield, B. F.; Li, G.; Boerio-Goates, J.; Tissue, B. M. Energy crossovers in nanocrystalline zirconia. J. Am. Ceramic Soc. 2005, 88, 160. C, CdS, BaTiO3, Fe2O3, Cr2O3, Mn2O3, Nb2O3, Y2O3, and Au-Sb.
- Phase transformation kinetics is size-dependent and transformations usually occur at low temperatures (less than several hundred degrees). Under such conditions, rates of surface nucleation and bulk nucleation are low due to their high activation energies. Thus, phase transformation occurs predominantly via interface nucleation Zhang, H.; Banfield, J. F. New kinetic model for the nanocrystalline anatase-to-rutile transformation revealing rate dependence on number of particles. Am. Mineral. 1999, 84, 528. that depends on contact between nanoparticles. As a consequence, the transformation rate is particle number (size)-dependent and it proceeds faster in densely packed (or highly aggregated) than in loosely packed nanoparticles.Zhang, H.; Banfield, J. F. Phase transformation of nanocrystalline anatase-to-rutile via combined interface and surface nucleation. J. Mater. Res. 2000, 15, 437 Complex concurrent phase transformation and particle coarsening often occur in nanoparticles.Zhang, H.; Banfield, J. F. Polymorphic transformations and particle coarsening in nanocrystalline titania ceramic powders and membranes. J. Phys. Chem. C 2007, 111, 6621.
- Size-dependent adsorption on nanoparticles Zhang, H.; Penn, R. L.; Hamers, R. J.; Banfield, J. F. Enhanced adsorption of molecules on surfaces of nanocrystalline particles. J. Phys. Chem. B 1999, 103, 4656.Madden, A. S.; Hochella, M. F.; Luxton, T. P. Insights for size -dependent reactivity of hematite nanomineral surfaces through Cu2+ sorption. Geochim. Cosmochim. Acta 2006, 70, 4095. and oxidation of nanominerals.Madden, A. S.; Hochella, M. F. A test of geochemical reactivity as a function of mineral size: manganese oxidation promoted by hematite nanoparticles. Geochim. Cosmoch. Acta 2005, 69, 389.
- The Case for Nanogeoscience, MICHAEL F. HOCHELLA, Jr., Annals of the New York Academy of Sciences 1093 (1), 108–122.
- Charting the future of nanogeoscience, August 26, 2002, Dan Krotz