Photochemical versus Thermal Synthesis of Cobalt Oxyhydroxide Nanocrystals

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(NH₃)₅Cl]­Cl₂, 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 O₂, and proceeds in two steps: (1) Aquation of [Co­(NH₃)₅Cl]²⁺ to yield [Co­(NH₃)₅(H₂O)]³⁺, followed by (2) slow photoinduced release of NH₃ 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 Co₃O₄ 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