Cu doped TiO2-Bi2O3 nanocomposite for degradation of azo dye in aqueous solution: Process modeling and optimization using central composite design

•The nano-catalyst of Cu-doped TiO2/Bi2O3 was synthesized for the photocatalytic degradation of Methylene Orange (MO) under visible-light.•A statistical analysis based on the response surface methodology (RSM) was employed to optimize the Cu and Bi2O3 loadings.•The Cu-TiO2-Bi2O3 nanocomposite with a weight fraction of 1 and 8% for Cu and Bi2O3 shows the best MO photodegradation efficiency.•The influence of amount of catalyst, MO concentration and pH of the solution on the MO photodegradation was modeled and optimized using CCD.•The photodegradation kinetic result indicates the Cu-TiO2/Bi2O3 has a higher MO photodegradation rate compared to the Cu-TiO2 and pure TiO2.