Nanostructured photocatalysts have become a subject of much interest, with most research focusing on the visible-light sensitization in order to effectively utilize incoming solar energy for environmental remediation. Herein, we design an acetic acid-assisted solvothermal strategy, enabling Nb3O7(OH) nanoaggregates with C=C and C=O functional groups on the surface and extending the absorption edge located at 550 nm approximately. The success extension of harvesting light from intrinsic UV region below 380 nm to visible region relies on effective surface modification by carbonyl groups originated from citric acid in acetic acid solution. Specifically, the unique C=O-rich surface state and advantageous structural features render them particularly attractive for Cr(VI) photoreduction applications with the assistance of hole scavenger tartaric acid. No phase or morphology transition occurs during Cr(VI) reduction. The reaction rate slows down and the photoresponse area turns back into intrinsic UV region of Nb3O7(OH) with the prolonging reaction time, originating from the exhaustion of surface C=O functional groups. This work is expected to help to elucidate the rational design and efficient synthesis of visible-light absorption catalyst materials for actual applications in the near future.