Photochemical versus Thermal Synthesis of Cobalt Oxyhydroxide Nanocrystals

TitlePhotochemical versus Thermal Synthesis of Cobalt Oxyhydroxide Nanocrystals
Publication TypeJournal Article
Year of Publication2012
AuthorsAlvarado SR, Guo YJ, Ruberu TPA, Bakac A, Vela J
Journal TitleJournal of Physical Chemistry C
Date Published05
Type of ArticleArticle
ISBN Number1932-7447
Accession NumberWOS:000303848600066
Keywordscation-exchange, chemistry, hollow spheres, metal, NANOSTRUCTURES, OXIDE, phase-transition, protein cage, SEMICONDUCTOR NANOCRYSTALS, size dependence

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 (similar to 3 nm) monodisperse cobalt oxyhydroxide (Co(O)OH) nanocrystals. This method uses readily and commercially available pentaamminechlorocobalt(III) chloride, [Co(NH3)(5)Cl]Cl-2, 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-2, and proceeds in two steps: (1) Aquation of [Co(NH3)(5)Cl](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 (similar to 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 Delta T = 64 degrees C) for small nanocrystals compared with the bulk.

URL<Go to ISI>://WOS:000303848600066
Alternate JournalJ. Phys. Chem. C