On the analysis of diffuse reflectance measurements to estimate the optical properties of amorphous porous carbons and semiconductor/carbon catalysts

This study provides a critical analysis on the use of diffuse reflectance spectroscopy and Tauc equation to estimate the optical energy band gap of semiconductor/carbon composites and amorphous porous carbons. Determination of the energy gap from diffuse reflectance is strongly dependent on the analyst's experience, due to uncertainties related to establish the adequate range to fit the experimental data to Tauc equation, and to identify the type of electronic transitions. Furthermore, its application to strong light absorbing or multiphase materials with several absorbing components is not straightforward, due to the appearance of various curvatures/linear ranges. For such, reporting the linear fitting range used in the diffuse reflectance spectra is recommended to avoid miscalculation of a gap value. For materials absorbing in the visible range (e.g., displaced onset in Tauc representation), a double-linear fitting must be used in the extrapolation of [F(R∞)hv]^1/n to avoid underestimation of E_g values. For amorphous porous carbons, different optical responses were obtained from the diffuse reflectance spectra recorded upon dilution with a non-absorbing matrix. The application of Tauc equation (indirect transitions) to data rendered band gap values ranging between 1.5-2.3 eV for fourteen carbons, which are in agreement with those reported for these materials.