Carbon nanodots (CNDs) were synthesized from hazelnut shells through a green and sustainable approach and loaded onto titanium dioxide (TiO₂) to prepare CND/TiO₂ nanocomposites. To determine the optimum formulation for enhancing photocatalytic performance, 17 different CND/TiO₂ catalyst series with varying volume/mass ratios were prepared. The materials were extensively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), particle size analysis (Mastersizer), and X-ray photoelectron spectroscopy (XPS) to elucidate their morphological, structural, and compositional features. XPS analyses revealed that the surface chemistry of the CNDs plays an important role in governing photocatalytic performance. Photocatalytic activity was evaluated through methylene blue (MB) degradation under low-energy visible light irradiation (λ ≥ 400 nm, 300 µW cm⁻²). Among the prepared materials, HS300–3(0.05–5) exhibited the highest activity, achieving approximately 53% MB degradation under visible light, which was attributed to enhanced visible-light utilization and modified surface properties compared with pure TiO₂. These findings provide insight into the composition–structure–activity relationship of biomass-derived CND/TiO₂ nanocomposites and contribute to the development of sustainable and energy-efficient photocatalytic systems.