Photocatalytic CO₂ reduction to valuable chemical compounds could be a promising approach for carbon-neutral practice. In this work, a simple and robust thermal decomposition process was developed with ammonium carbonate ((NH₄)₂CO₃) as both precipitation agent and sacrificial template to produce fine Nb₂O₅ nanoparticles with the rich existence of surface hydroxyl (–OH) groups. It was found by density functional theory (DFT) calculations and experiments that the rich existence of the surface –OH groups enhanced the adsorption of both reactants (CO₂ and H₂O molecules) for the photocatalytic CO₂ reduction on these fine Nb₂O₅ nanoparticles, and the highly selective conversion of CO₂ to the high-value chemical compound of ethylene (C₂H₄, ~68 μmol·g⁻¹·h⁻¹ with ~100% product selectivity) was achieved under simulated solar illumination without usage of any sacrificial agents or noble metal cocatalysts. This synthesis process may also be readily applied as a surface engineering method to enrich the existence of the surface –OH groups on various metal oxide-based photocatalysts for a broad range of technical applications.