Open top anodic Ta 3 N 5 nanotubes for higher solar water splitting efficiency

•High aspect ratio Ta2O5 nanotubes (NTs) with an open top are grown via anodization.•Key anodic growth parameters are temperature, double step anodization and ramping.•Ta2O5 is converted to Ta3N5 by nitridation in ammonia gas.•Ta3N5 photoelectrodes are further modified with cobalt co-catalysts.•Open Ta3N5 NTs modified with Co-Pi+Co(OH)x enhance PEC water splitting performance.