Oxygen　vacancies　in　photocatalysts　are　an　important　parameter　for　improving　their　catalytic　activities.　However,　this　study　found　that　oxygen　vacancies　do　not　always　help　with　photocatalytic　properties.　MXene　nanosheets　were　annealed　at　different　temperatures　to　prepare　nanosheets　of　titanium　dioxide　(TiO2)　with　anatase　or　rutile　phase.　Photo-induced　self-healing　of　oxygen　vacancies　in　MXene　nanosheets　annealed　at　400　degrees　C　promoted　the　efficient　photocatalytic　evolution　of　hydrogen　(H2)　with　a　-　6.8　mmol/g　optimized　yield.　In　contrast,　the　photoinduced　increase　of　oxygen　vacancies　in　MXene　nanosheets　annealed　at　500　degrees　C　corresponded　to　an　optimized　1.8　mmol/g/h　yield　of　hydrogen　peroxide　(H2O2).　Spectroscopic　and　electrochemical　methods　revealed　that　the　built-in　electric　field　between　the　anatase　and　rutile　TiO2　facilitated　the　fast　charge　separation　from　the　anatase　to　rutile　TiO2.　The　photo-induced　self-healing　of　oxygen　vacancies　and　the　phase　transition-induced　built-in　electric　field　regulated　the　photocatalytic　selectivity　of　H2　and　H2O2　production.