Photochemical versus Thermal
Synthesis of Cobalt Oxyhydroxide
Nanocrystals
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Abstract
Photochemical methods facilitate the generation, isolation,
and
study of metastable nanomaterials having unusual size, composition,
and morphology. These harder-to-isolate and highly reactive phases,
inaccessible using conventional
high-temperature pyrolysis, are likely to possess enhanced and unprecedented
chemical, electromagnetic, and catalytic properties. We report a fast,
low-temperature and scalable photochemical route to synthesize very
small (∼3 nm) monodisperse cobalt oxyhydroxide (Co(O)OH) nanocrystals.
This method uses readily and commercially available pentaamminechlorocobalt(III)
chloride, [Co(NH<sub>3</sub>)<sub>5</sub>Cl]Cl<sub>2</sub>, under
acidic or neutral pH and proceeds under either near-UV (350 nm) or
Vis (575 nm) illumination. Control experiments showed that the reaction
proceeds at competent rates only in the presence of light, does not
involve a free radical mechanism, is insensitive to O<sub>2</sub>,
and proceeds in two steps: (1) Aquation of [Co(NH<sub>3</sub>)<sub>5</sub>Cl]<sup>2+</sup> to yield [Co(NH<sub>3</sub>)<sub>5</sub>(H<sub>2</sub>O)]<sup>3+</sup>, followed by (2) slow photoinduced release
of NH<sub>3</sub> from the aqua complex. This reaction is slow enough
for Co(O)OH to form but fast enough so that nanocrystals are small
(ca. 3 nm). The alternative dark thermal reaction proceeds much more
slowly and produces much larger (∼250 nm) polydisperse Co(O)OH
aggregates. UV–Vis absorption measurements and ab initio calculations
yield a Co(O)OH band gap of 1.7 eV. Fast thermal annealing of Co(O)OH
nanocrystals leads to Co<sub>3</sub>O<sub>4</sub> nanocrystals with
overall retention of nanoparticle size and morphology. Thermogravimetric
analysis shows that oxyhydroxide to mixed-oxide phase transition occurs
at significantly lower temperatures (up to Δ<i>T</i> = 64 °C) for small nanocrystals compared with the bulk