Boron and nitrogen co-doped Titanium dioxide (TiO2) nanosheets (BNT) with high surface area of 136.5 m(2) g(-1) were synthesized using ammonia borane as the green and triple-functional regent, which avoids the harmful and explosive reducing regents commonly used to create surface defects on TiO2. The decomposition of ammonia borane could incorporate reactive Lewis acid-base (B, N) pairs, together with the as-generated H-2 to create mesoporous structure and rich oxygen vacancies in pristine TiO2. The BNTs prepared from various ammonia borane loading are evaluated in photoreduction of carbon dioxide (CO2) with steam under simulated sunlight, achieving about 3.5 times higher carbon monoxide (CO) production than pristine TiO2 under the same conditions. Steady state and transient optical measurements indicated BNT with reduced band gap, rich defect states and elevated conduction band position could enhance the light harvesting efficiency and promote the charge transfer at the catalyst/CO2 interface. Density functional theory simulation and in situ FTIR suggest that the Lewis acid-base (B, N) pairs on BNT may very substantially increase the activation of inert CO2 which facilitates their photoreduction with the hydrogen from the water splitting at the surface defects on TiO2. Finally, a reaction mechanism of Lewis acid-base assisted CO2 photoreduction leading to substantially improved performance is proposed. (C) 2020 Published by Elsevier Inc.