The properties and photocatalytic performance of anatase nanoparticles of pure TiO2 and a core–shell structure of TiO2 on calcined vetiver grass leaves have been compared. Samples were fabricated by sol-gel and heating at 450 °C for 5 h. The comparison was based on data for X-ray diffraction (XRD), UV-Vis spectrophotometry, photoluminescence, transmission electron microscopy, specific surface area measurement, pore volume assessment, and methylene blue degradation testing. The results showed that the pure TiO2 consisted of agglomerated equiaxed nanoparticles of individual grain sizes in the range 10–20 nm. In contrast, the TiO2-vetiver composite exhibited a core–shell structure consisting of a carbonaceous core and TiO2 shell of thickness 10–15 nm. These features influenced the photocatalytic performance in such a way that the lower cross-sectional area, greater surface area, and higher pore volume of the TiO2 shell increased the number of active sites, reduced the charge carrier diffusion distance, and reduced the recombination rate, thereby improving the photocatalytic activity. This improvement derived from morphological characteristics rather than crystallographic, semiconducting, or optical properties. The improved performance of the TiO2-vetiver core–shell was unexpected since the X-ray diffraction data showed that the crystallinity of the TiO2 was lower than that of the pure TiO2. These outcomes are attributed to the reducing effect of the carbon on the TiO2 during heating, thereby facilitating the formation of oxygen vacancies, which enhance charge separation and hence photocatalysis by TiO2.