Improve optical band gaps of metal oxides by a suitable doping is very interesting for optoelectronic application. However, the effect of doping is usually affected by the synthesis method. Among the existing synthesis routes, coprecipitation is cheap and offers a possibility to strictly control the stoichiometry, depending on the precipitating agents. Herein cobalt doped manganese oxides were prepared via coprecipitation using succinate ion as precipitating agent and their optical properties have been investigated. The results obtained from a set of instrumental techniques revealed that the main phase formed is Mn3O4 hausmatite with cobalt substituting manganese in the structure. The UV–visible diffuse reflectance results demonstrated that the band gap increases from 1.97 for undoped Mn3O4, to 2.91 eV for 5% Co doping, and therefore decreases to 2.34 eV for 6% Co. This trend can be explained by the difference in the substitution site of cobalt which is either tetrahedral or octahedral.
