Synergistically tuned noble metals and intentionally formed complex heterostructured nanomaterials can enhance the required application effectiveness but at the cost of tedious synthesis routes, expensive chemicals, and sophisticated instruments. To overcome such demerits, herein, we report on the oleic acid-mediated convenient co-precipitation route using water–hexane as a biphasic solvent for CuO synthesis in the form of nano feathers (CuO-NF), solid/hollow hexagonal thin sheets (CuO-HS), and mega sheets (CuO-MS) at room temperature. The exotic CuO nanoarchitectures achieved were tested and compared with control samples (CuO-IS) for CO2 sensing, natural sunlight induced dye degradation, and catalytic CO2 reduction. Among the various CuO nanostructures synthesized, CuO-HS depicted higher oxygen deficiency, electronic conductivity, and visible light absorption. Most of the solid/hollow hexagonal thin sheets depicted an edge length in the 50–350 nm range with an observed thickness as low as 5 nm. The CuO-HS microsensor demonstrated ultrasensitivity (Rg/Ra = ∼85), dominant selectivity (>6 gases), repeatability (98.7%), CoV (1.3%), and LoD (4.3 ppm) at 32 °C towards CO2 in 20–5000 ppm. The role of structural defects in sensing was confirmed from operando UV-Vis-DRS & PL. Rapid dye degradation in natural sunlight shown by CuO-HS was primarily attributed to the lower charge reunification. Additionally, CuO-HS facilitated methanol formation within 3 h at a rate of 53 and 18 μmol g−1 in the presence of artificial solar and natural sunlight, respectively. Dye degradation and CO2 photoreduction pathways were probed using HPLC and GC-MS, respectively.