Recent Research

I. Mn oxides

Mn oxides are a major player in controlling the geochemical cycling of many metals. In general, layer-type phyllomanganates and tunnel-type tectomanganates are dominant Mn oxides found in nature and easy to synthesize in experiment. However, these minerals are difficult to fully characterize due to the nanoparticle size,  low crystallinity,  mixed oxidation states of Mn (II, III, IV), and high defect content. Computational mineralogy becomes critical to resolving the complexity of Mn oxide structures and their reactivity to metal cations.    

Free energy perturbation (FEP) calcualtions

Todorokite is a tectomanganate with a 3×3 tunnel structure and contains various metal cations inside the nanopores. We performed free energy perturbation (FEP) MD simulation using the MnFn force field to reveal the disordered tunnel structure in terms of the hydration state, cation species, and average oxidation state (AOS) of Mn. We estiamted the optimum water content in the tunnel as a function of humidity and found that Ni2+, Zn2+, Mg2+, Ca2+ or Na+ prefer inner-sphere (IS) complexes in a low hydration state, but outer-sphere (OS) complexes in a high hydration state, whereas K+ or Cs+ always form IS complexes regardless of tunnel hydration state. Tunnel cation positions can be also dependent on the AOS of Mn, indicating various metal speciation in seafloor Mn oxides.

II. Carbonates

Metal speciation in CaCO3

Metals found in calcium carbonate minerals (e.g., Mg, Mn etc..) are often used as proxies that can be used to infer geochemical conditions of the past environment. However, the molecular speciation of metal cations in aragonite (orthorhombic CaCO3) has not been yet clearly defined in experiment, particularly for cations whose radii are smaller than Mg2+. Metal incorporation mechanisms to calcium carbonates are still in debate;  thus a question is raised as to the reliability of using metal cations associated with calcium carbonates as the paleo-environmental proxy.    

Equilibrium fractionation of non-traditional stable isotopes 

Our recent work investigates the speciation of divalent cations when they substitute the Ca sites of aragonite, a fundamental knowledge required in developing and validating geochemical proxies.  We calculated theoretical X-ray emission or absorption spectra for Mg2+- or Mn2+-substituted aragonite to guide interpretation of the experimental XES and XANES spectra (Son et al., 2019; Son et al., 2020).  In the case of Mg2+, theory has not been able to explain the enrichment of 26Mg in aragonite relative to calcite. For the first time, our DFT calculation results explain the experimental observation (Son et al., 2020); Mg2+ in aragonite is five-fold coordinated, while that in calcite is six-fold coordinated.  

III. Fe sulfides

Size-dependent thermodynamics

Iron sulfides such as FeS (mackinawite) and Fe3S4 (greigite) commonly found as nanoparticles are well known to be metastable compared to FeS2 (pyrite). However, the dominant formation of metastable phases in nature remains difficult to understand based on conventional thermodynamics alone. Our recent ab-initio thermodynamic work based on DFT/SCAN functional revealed that the lower  surface energies of FeS and Fe3S4 than that of FeS2 drive a stability reversal among iron sulfides at nanoparticle sizes (i.e., FeS  and Fe3S4 are mores stable than FeS2 at the nano scale) and lead to a faster nucleation of FeS and Fe3S4 than FeS2.

Nickelian and cobaltian mackinawite

In sulfide ore deposits, mackinawite possesses transition metals such as Co, Ni and Cu. The transition-metal-incorporated mackinawite may transform into various metal sulfide assemblages.  Unfortunately, the structure and stability about the metal-rich mackinawite are little examined in experiments.  We first examined systematically the relationships among the chemical composition, structure, and thermodynamic stability of transition-metal-incorporated mackinawite by application of DFT with dispersion correction and DFT atomistic thermodynamics methods.  Our results showed that transition metals tend to incorporate into mackinawite by substitution of the Fe sites within the FeS4 tetrahedral sheets.  Metal substitution tends to enhance the stability of mackinawite. Our findings are useful to understand the formation paths of metal sulfides in ore deposits and develop enhanced metal-sulfide catalysts.

FeS electronic structure

Mackinawite (tetragonal FeS)  comprises edge-sharing sheets of FeS4 tetrahedra stacked along the c direction with van der Waals (vdW) interactions. The sulfide mineral is very reactive in redox chemistry such reduction of uranium, chromium, and mercury. The high reactivity in the electron transfer is attributed to the fact that Fe in FeS is metallic.  Fe L-edge X-ray absorption spectra (XAS) and DFT-calculated density of states (DOS) of the FeS electronic states all indicate that Fe of FeS is metallic.  

IV. Clay minerals

Edge structures

Compared to the basal plane, edge surfaces of 2:1 clay minerals are not clearly characterized.  We have examined the edge surface structure and stability of pyrophyllite, which has no or few structural charges, by application of DFT atomistic thermodynamics.  We found that the stability of the AC and B edge surfaces greatly changes at different temperatures and humid conditions. We also examined defects occurring at the edge surface . The defect structure and stability investigation suggests that the edge defects may impact the metal binding activity and the equilibrium morphology of pyrophyllite crystals.   

V. Mineral/ nanoplastics interactions 

The transport of nanoplastics in soil and aquifers is most likely to be controlled by interactions with mineral grains. We investigate the interaction of nanoplastics with mineral surfaces by application of quartz crystal microbalance with dissipation (QCM-D).  In general, carboxylated polystyrene nanoplastics (~100 nm) prefer deposition to Al2O3 surface, and amine-functionalized polystyrene nanoplastics (~100 nm) prefer deposition to SiO2 surface. The preference can be understood by simple electronstic interactions beween surface charges. Even under electrostatically unfavorable conditions, we found significant deposition can occur at high ionic strengths. Mineral surface and solution chemistry should be key parameters in developing the transport model of nanoplastics in subsurface environment.

VI. Energy materials

Two dimensional MoS2-graphene hybrids are promising energy materials for ultrafast pseudocapacitance.  Our collaborator, Prof. Ho Seok Park (Sungkyunkwan U.), observed a phase transition from 2H MoS2 to 1T MoS2 at the interface with reduced graphene oxide.  We performed DFT calculations for 1H-MoS2 nanosheet/graphene and 1T-MoS2 nanosheet/graphene systems in order to obtain insights into the phase transform.  Isolated 1T-MoS2 showed a higher total energy than isolated 1H-MoS2, implying 1T type is more stable than 1H type.  However, the binding energy of 1T-MoS2/graphene was greater than that of 1H-MoS2/graphene.  This greater binding energy may explain the phase shift at the interface. Because triangular clustering among Mo atoms was found in the 1T-MoS2/graphene hybrid with slight rumpling of the S atomic plane, the structural distortion may play a role in the transformation from 1T to 1H phase. 

Phosphorus (P) incorporated graphene exhibits reversible and fast pseudocapacitance (Yu et al., 2015).  We performed DFT calculations to obtain insights in the structure of P-doped graphene (P-GRP).  Geometry optimization found that P-GRP is more stable when P is outside the graphene sheet than when it is within the sheet plane.  The former induced spin polarization on P and neighboring C, but the latter did not.  In situ FT-IR spectroscopy identified PO sites from partially oxidized P-GRP (PO-GRP).  Our PO-GRP model showed a greater rumpling than P-GRP, but it did not induce any spin polarization.  Adsorption of proton was much greater at the PO site than at C site in PO-GRP.