NOVELTY - Water splitting and carbon dioxide reduction involves: forming photocatalytic capped colloidal nanocrystals including semiconductor nanocrystal capped with inorganic capping agent; depositing the nanocrystals onto substrates, thus creating two photoactive materials; orienting the photocatalytic capped colloidal nanocrystals of the photoactive material; absorbing irradiated light; passing water; separating hydrogen from oxygen; passing the separated hydrogen; passing carbon dioxide; absorbing irradiated light; reacting the carbon dioxide and the hydrogen; and collecting the methane.    USE - For water splitting and carbon dioxide reduction (claimed), useful for light harvesting and converting water and carbon dioxide (CO2) into methane fuel.    ADVANTAGE - By employing the oriented photoactive material in combination with a light polarization system, achieves greater sunlight energy extraction. The semiconductor nanocrystals provide for higher surface area available for the absorption of light. The seeded growth method has the benefits of lowering the activation energy required for crystal growth to occur, and other reaction parameters, such as monomer concentration and reaction temperature; and allows a degree of control over deposition density, growth rate, and orientation dispersion to yield a highly uniform and oriented nanocrystal surface with 2D/3D orientation.    DETAILED DESCRIPTION - A method for water splitting and carbon dioxide reduction involves: forming photocatalytic capped colloidal nanocrystals, where each photocatalytic capped colloidal nanocrystal includes a first semiconductor nanocrystal capped with a first inorganic capping agent; depositing the formed photocatalytic capped colloidal nanocrystals onto a first substrate and a second substrate, thus creating first and second photoactive materials; orienting the photocatalytic capped colloidal nanocrystals of the first photoactive material; orienting the photocatalytic capped colloidal nanocrystals of the second photoactive material; absorbing irradiated light with an energy equal to or greater than the band gap of the semiconductor nanocrystals by the first photoactive material to create charge carriers in a conduction band and holes in a valence band of the photocatalytic capped colloidal nanocrystals of the first photoactive material; passing water through a first reaction vessel so that the water reacts with the first photoactive material to form hydrogen and oxygen, where the charge carriers in the conduction band reduce hydrogen molecules from the water and the holes in the valence band oxidize oxygen molecules from the water; separating the hydrogen from the oxygen using a hydrogen permeable membrane and an oxygen permeable membrane; passing the separated hydrogen from the first reaction vessel into a second reaction vessel; passing carbon dioxide into the second reaction vessel; absorbing irradiated light with an energy equal to or greater than the band gap of the semiconductor nanocrystals by the second photoactive material to create charge carriers in a conduction band and holes in a valence band of the photocatalytic capped colloidal nanocrystals of the second photoactive material; reacting the carbon dioxide and the hydrogen with the second photoactive material in the second reaction vessel so that the charge carriers in the conduction band reduce carbon dioxide into methane and the holes in the valence band oxidize the hydrogen into water vapor; and collecting the methane using a methane permeable membrane. An INDEPENDENT CLAIM is included for a photosynthetic system comprising: first and second oriented photoactive materials, where the first and second oriented photoactive materials include oriented photocatalytic capped colloidal nanocrystals; a first reaction vessel housing the first oriented photoactive material and configured to receive water through an inlet and facilitate a water splitting reaction that produces hydrogen and oxygen when the water reacts with the photocatalytic capped colloidal nanocrystals, where the water splitting reaction occurs when the photocatalytic capped colloidal nanocrystals absorb irradiated light to separate charge carriers of the first oriented photoactive material; and a second reaction vessel housing the second oriented photoactive material and configured to receive carbon dioxide through a first inlet, receive hydrogen from the first reaction vessel, and facilitate a carbon dioxide reduction reaction and a hydrogen oxidization reaction that produces methane and water vapor, where the reaction begins when the photocatalytic capped colloidal nanocrystals of the second photoactive material absorb polarized light to separate charge carriers of the second oriented photoactive material.