Synthesis of visible light-responsive cobalt-doped TiO₂ nanoparticles with tunable optical band gap

Abstract: Visible light-responsive photocatalysts are the most promising candidates for green bioremediation processes that will degrade toxic organic industrial waste into harmless compounds. Among the photocatalysts, TiO₂ is best suited for large-scale photo-induced bioremediation processes mainly because of low cost and abundance. The major obstacle in its utilization as photocatalyst is its poor response to sunlight due to its wide energy band gap. This article reports sol–gel synthesis of pristine and cobalt-doped TiO₂ nanoparticles (TNPs). Titanium (IV) isopropoxide is hydrolyzed and condensed into amorphous titanium dioxide gel by water/ethanol under acidic conditions. Irrespective of the Co concentration, TNPs always crystallize into anatase phase when calcine at 500 °C. No signature of other isomorphous phases, i.e., rutile or brookite, is detected. The optical band gap of pristine (0 % Co doped) TNPs is 3.03 eV (λ = 409 nm), which decreases up to 1.93 eV (λ = 642 nm) when Co concentration in TiO₂ matrix increases from 0 to 2 %. Co(+2) substitution at Ti(+4) site generates additional oxygen vacancies in the TiO₂ unit cell, which introduces extra energy levels in the forbidden band that reduces the indirect energy band gap of TNPs. Co doping in TNPs makes them sensitive to visible radiation, and hence, their photoresponse is expected to be better under sunlight than pristine bulk titania, which is active only in the UV region of the electromagnetic spectrum. Graphical Abstract: [Figure not available: see fulltext.]