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 str ong crystal-chemical dependence on the mode of dissolution of synthetic het erogenite (CoOOH) particles via ligand-assisted dissolution using iminodiac etic acid (IDA) and reductive dissolution using hydroquinone (H2O) Dissolut ion, using H(2)Q (10 muM to 2 mM) and IDA (10 muM to 2 mM), of synthesized heterogenite particles (37 mg/L) was examined in order to evaluate morpholo gy evolution as a function of dissolution agent concentration. No evidence for redox reactions was observed in experiments using IDA, and no aqueous c omplexes of Co(II) or Co(III) with H(2)Q or benzoquione, the oxidation prod uct of H(2)Q, in experiments using H(2)Q were detected. As-synthesized hete rogenite particles are micron-size hexagonal plates (aspect ratio, similar to1/30) constructed of crystallographically oriented similar to5 nm primary particles, or they are single similar to 21 nm or similar to 10 nm unattac hed heterogenite platelets (aspect ratio, similar to1/7 and similar to1/3, respectively). In experiments using the micron-sized hexagonal plates, two dominant modes of dissolution were observed: nonspecific dissolution that d issolved primary building blocks at all locations equally and pathway speci fic dissolution that occurred along boundaries of misorientation between pr imary building blocks. Both mechanisms occurred independent of the dissolut ion agent used. In comparison, TEM results show that dissolution of the una ttached heterogenite particles occurs primarily at the {101} and {10 (2) ov er bar}, or "edge," crystal faces and that no significant dissolution occur s at the (001), or "basal," crystal faces. This suggests that the reactive surface area is dominated by edge faces and further suggests that basal,fac es are essentially nonreactive under these conditions. Finally, dissolution by IDA produced two dissolved isomers, u-fac Co[IDA](2)(-) and s-fac Co[ID A](2)(-). Experiments using identical solution conditions show that dissolu tion of the micron-sized plates favors the production of the u-fac isomer w hile dissolution of the 21 nm particles favors the production of the s-fac isomer.