A　new　photocatalyst　Sr0.25H1.5Ta2O　6·H2O　(HST)　with　high　surface　area　was　successfully　prepared　by　a　facile　and　mild　hydrothermal　reaction　using　Ta2O　5·nH2O　as　a　precursor.　TEM　images　revealed　that　their　different　morphologies,　from　nanoplate　to　nanopolyhedron,　were　formed　under　different　pH　values　of　the　reactive　solutions.　The　formation　mechanism　of　HST　was　also　studied　and　proposed.　Growth　of　HST　nanocrystallites　followed　a　reaction-crystallization　model.　By　analyzing　the　results　of　XRD,　DRS,　XPS,　electrochemistry　and　theoretical　calculation,　the　crystal　and　electronic　structural　characterizations　of　HST　was　established.　Compared　with　isostructural　Sr0.4H1.2Nb2O6·H　2O　(HSN),　the　Ta　5d　orbitals　were　the　main　contribution　to　the　bottom　of　the　conduction　band　of　HST,　inducing　the　energy　level　more　negative　while　the　top　of　valence　band,　which　was　dominated　by　O　2p　states,　remained　almost　unchanged.　Due　to　the　more　suitable　electronic　band　structure　and　various　morphologies,　the　photocatalyst　showed　superior　photocatalytic　activities　for　water　splitting　to　generate　H2　and　for　degrading　benzene　as　compared　with　HSN.　The　rate　of　H2　evolution　and　the　conversion　ratio　of　benzene　were　81　times　and　4　times　higher　than　that　of　TiO2　(Degussa　P25),　respectively.　The　proposed　mechanisms　for　photocatalytic　reactions　based　on　the　experimental　results　were　discussed.　©　The　Royal　Society　of　Chemistry　2011.