NOVELTY - The photocatalyst has a semiconductor thin film layer(s) applied on the surface undulations (102) of a substrate (101). The surface undulations having certain depth pitch alters the bandgap of the semiconductor thin film to better match the wavelength(s) in an illuminant, thus increases the absorption and conversion efficiency of the illuminant.    USE - For photoelectrolytic cell device (claimed), photovoltaic device for production of electricity, photocatalytic device for detoxification and disinfection, photoelectrolysis device for hydrogen based internal combustion engine, solar conversion detector, water desalination, continual tuning of stress and bandgap properties of telecom devices, alternation and improving magnetic properties of thin films applied to hard drive disks for data storage, and protective clothing.    ADVANTAGE - The photocatalyst is inexpensive since very thin semiconductor material is used without using more exotic and costly semiconductors. The substrate has larger effective area and corrugated focal surface. The film is in a highly stressed state and the corrugate substrate makes the film robust by providing mechanical locking, and prevents cracking, crazing and delamination of plastic. The photocatalyst avoids use of tracking mechanism because the nano-optics have a large angular field of view and keeps the sun focused. The materials used for the photocatalyst are non-toxic and have excellent durability. The absorption and conversion efficiency of the illuminant are increased by the photocatalyst. The photocatalyst has bandgap matching efficiently with solar spectrum and/or artificial illumination.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) photoelectrolytic cell device;    (2) manufacture of photocatalytic semiconductor working electrode; and    (3) stress-induced bandgap shifted semiconductor thin film.    DESCRIPTION OF DRAWING(S) - The figure shows photocatalytic surface.    photon (100)    substrate (101)    surface undulation (102)    titania (103)    harmful toxins or germs and bacteria (104)