Defects and disorder: Probing the surface chemistry of heterogenite (CoOOH) by dissolution using hydroquinone and iminodiacetic acid

High-resolution transmission electron microscopy (HRTEM) results show a strong crystal-chemical dependence on the mode of dissolution of synthetic heterogenite (CoOOH) particles via ligand-assisted dissolution using iminodiacetic acid (IDA) and reductive dissolution using hydroquinone (H₂Q). Dissolution, using H₂Q (10 μM to 2 mM) and IDA (10 μM to 2 mM), of synthesized heterogenite particles (37 mg/L) was examined in order to evaluate morphology evolution as a function of dissolution agent concentration. No evidence for redox reactions was observed in experiments using IDA, and no aqueous complexes of Co(II) or Co(III) with H₂Q or benzoquinone, the oxidation product of H₂Q, in experiments using H₂Q were detected. As-synthesized heterogenite particles are micron-size hexagonal plates (aspect ratio, ∼1/30) constructed of crystallographically oriented ∼5 nm primary particles, or they are single ∼21 nm or ∼10 nm unattached heterogenite platelets (aspect ratio, ∼1/7 and ∼1/3, respectively). In experiments using the micron-sized hexagonal plates, two dominant modes of dissolution were observed: nonspecific dissolution that dissolved primary building blocks at all locations equally and pathway specific dissolution that occurred along boundaries of misorientation between primary building blocks. Both mechanisms occurred independent of the dissolution agent used. In comparison, TEM results show that dissolution of the unattached heterogenite particles occurs primarily at the {101} and {102̄}, or "edge," crystal faces and that no significant dissolution occurs at the (001), or "basal," crystal faces. This suggests that the reactive surface area is dominated by edge faces and further suggests that basal faces are essentially nonreactive under these conditions. Finally, dissolution by IDA produced two dissolved isomers, u-fac Co[IDA]₂^- and s-fac Co[IDA]₂^-. Experiments using identical solution conditions show that dissolution of the micron-sized plates favors the production of the u-fac isomer while dissolution of the 21 nm particles favors the production of the s-fac isomer.