The selective oxidation of styrene at its side chains to yield value-added C7 or C8 aromatic compounds represents a crucial reaction within the chemical industry. However, achieving precise control over product formation selectivity poses a significant challenge. In this study, we demonstrate the effectiveness of ultra-small nanosized TiO2-based catalysts in facilitating the selective oxidation of styrene through a mild, eco-friendly photocatalytic approach. Following modification with 0.4% NiO, a notable transformation in product selectivity is observed. The primary products shift from C8 with 96% selectivity to C7 with 92% selectivity under UV, and even sunlight-type excitation. Detailed structural and electronic analyses reveal that the incorporation of NiO onto the TiO2 surface induces the formation of a P-N heterojunction and additional active sites, crucial for promoting C-C cleavage, leading to a selectivity shift from C8 to C7 products. This research provides valuable insights into the design of simple TiO2-based catalysts capable of achieving high selectivity in the oxidation of styrene, highlighting the potential of green photocatalysis in industrial applications.