One embodiment of this invention relates to a substrate (3) having a crystalline tin (IV) oxide film grown thereon, the film comprising a plurality of nanorods each of hick includes crystals grown along the c-axis of the tin (IV) oxide crystal. Another embodiment of the invention relates to a method of forming a crystalline tin (IV) oxide film on a substrate, the film comprising a plurality of nanorods each of which includes c stals grown along the c-axis of the tin (IV) oxide crystal, the method comprising: at 1 ast partly immersing a substrate (3) in an aqueous solution (1) of a tin salt in an acid medium with a second substance, the second substance being chosen to provide a slow-release source of a basic medium; said slow-release source decomposing and reacting with said tin salt to thereby effect growth of tin oxide crystals along said crystal c-axis to form said crystalline tin (IV) oxide nanorods on said substrate (3). Applications described herein include photoelectrodes, photoelectrochemical systems, and gas sensors.