CO2-to-CO conversion efficiency. Photocatalytic CO2-to-CO conversion efficiency (%) of TiO2/CotpyP was calculated by dividing the mol of CO produced after 24 h by the mol of CO2 in the photoreactor headspace and multiplying the product by 100. The mol of CO2 was obtained using the ideal gas law equation (pV = nRT), where p is 1 atm, V is the volume of the reactor headspace in L multiplied by the molar fraction of CO2, R is the ideal gas constant (0.082 atm L mol -1 K -1 ) and T is 298.15 K.