The　development　of　highly　active,　cost-effective,　environmentally　friendly　and　stable　g-C3N4　based　photocatalysts　for　H2　evolution　is　one　of　the　most　anticipated　potential　pathways　for　future　hydrogen　utilization.　Herein,　a　facile　gaseous　bubble　template　approach　was　designed　to　prepare　large-scale　thin　g-C3N4　nanosheets　(g-C3N4　NSs)　using　melamine　and　ammonium　sulphate　as　the　bubble　template.　Through　distinctive　structural　improvements　for　a　large　bandgap,　excellent　electron　mobility,　prolonged　lifetime　of　the　photogenerated　charge　carriers　and　a　high　specific　surface　area　with　highly　accessible　potential　reaction　sites,　the　as-synthesized　g-C3N4　NSs　demonstrated　a　high　photocatalytic　hydrogen　evolution　rate　of　9871　μmol　h-1　g-1　and　efficient　photocatalytic　degradation　of　Rhodamine　B　(RhB)　and　phenol　under　simulated　solar　light　irradiation.　©　2017　The　Royal　Society　of　Chemistry.