Differential Diffuse Reflectance Spectral Calculation of Crystalline Composition and Bandgap Energy in Metal Oxides Mixtures

Mixtures of metal oxides pose unique challenges for independent measurement of bandgap energies of each phase present. A facile technique with such a capability that can also determine the composition of the mixture would be of interest in industrial applications and academic research. UV–vis spectroscopy has several benefits among characterization techniques, including fast data collection, short training time for users, including safety training, and the ability to perform consistent quantitative analysis in continuous operation. Here we modify the derivative peak fitting of the diffuse reflectance UV–vis spectroscopy (DPR) technique by implementing the exponentially modified Gaussian (EMG) as a fitting model. With EMG, the applicability of the DPR method was extended to additional metal oxides, including all three common phases of titania and other semiconducting metal oxides commonly used in catalysis and photocatalysis: CeO₂, ZnO, SnO₂, V₂O₅, MoO₃, Y₂O₃, Ta₂O₅, and Nb₂O₅. This is due to the similarity between the distribution of electronic excitation by the UV–vis photon and the EMG function. Using EMG, the DPR method was used to calculate the “effective” bandgap energy of each metal oxide phase in binary and ternary mixtures of powders and to quantify phase composition. A response factor for each metal oxide studied relative to anatase TiO₂ is reported as a calibration method and demonstrated to be reusable, even in ternary mixtures.