NOVELTY - Photo-inducing charge separation and transfer of charge carriers to charge acceptors comprises contacting nanoparticle(s) having metal/metal alloy region(s) and semiconductor region(s) and having an absorption onset in the visible (400-700 nm) to near infrared (NIR) range (0.7-3 mu m) with electron acceptor(s) and electron donor(s) in a medium; and optionally irradiating the medium with a radiation in the visible and/or NIR range and optionally UV range to allow formation of an electron-hole pair in the metal/semiconductor interface of the nanoparticle(s).    USE - Photo-induction of charge separation and transfer of charge carriers to charge acceptors for the reduction and/or oxidation of organic or inorganic compounds, production of hydrogen from water splitting, and photodegradation of organic or inorganic compounds, preferably contaminants present in water and/or air (claimed).    ADVANTAGE - The nanoparticles or photocatalysts have applicability in various photocatalytic reactions and other applications employing light induced charge separation, e.g. electrochemical and photovoltaic cells. Photocatalysis may be achieved by simply exposing the systems to solar radiation.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a method for the photocatalytic production of hydrogen comprising irradiating an aqueous medium containing nanoparticle(s) with light in the visible and/or near infrared range and optionally UV range to obtain hydrogen following water splitting;    (2) a method of degrading contaminant(s) in water or air comprising introducing nanoparticle(s) into a medium containing contaminant(s) and irradiating the medium with light in the visible and/or near infrared range and optionally UV range to cause reduction or oxidation of the contaminant(s);    (3) use of nanoparticle(s) comprising metal/metal alloy region(s) and semiconductor region(s) having an absorption onset in the visible to near infrared range for the photocatalysis of a reaction;    (4) a solar cell device comprising nanoparticle(s);    (5) a device for photochemical treatment of contaminants comprising nanoparticle(s);    (6) a photovoltaic cell comprising nanoparticle(s);    (7) an array of cells including solar cell, photoelectrochemical cell or photovoltaic cell; and    (8) a nanoparticle having greater than or equal to 2 metal/metal alloy regions separated by greater than or equal to 2 semiconductor regions having a different energy gap and/or different energy band positions.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the absorption of light by a nanoparticle constructed as a nanodumbbell.