Boron　and　nitrogen　co-doped　Titanium　dioxide　(TiO2)　nanosheets　(BNT)　with　high　surface　area　of　136.5　m2　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　H2　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.　©　2020