NOVELTY - A carbon dioxide reducing method involves forming photocatalytic capped colloidal nanocrystals. Plasmonic nanoparticles are formed. The formed photocatalytic capped colloidal nanocrystals are deposited on substrate between nanoparticles. The substrate, the photocatalytic capped colloidal nanocrystals, and the plasmonic nanoparticles are thermally treated, followed by absorbing light, reacting carbon dioxide and hydrogen with the photocatalytic capped colloidal nanocrystals so that the charge carriers in conduction band reduce carbon dioxide into methane and collecting the methane and water.    USE - Method for reducing carbon dioxide.    ADVANTAGE - The reduction system operates with low energy conversion efficiency for producing photoactive materials.    DETAILED DESCRIPTION - A carbon dioxide reducing method involves forming photocatalytic capped colloidal nanocrystals, where each photocatalytic capped colloidal nanocrystal includes a first semiconductor nanocrystal capped with a first inorganic capping agent. Plasmonic nanoparticles are formed, where the plasmonic nanoparticles include noble metal nanoparticles, and the formed plasmonic nanoparticles are deposited onto a substrate. The formed photocatalytic capped colloidal nanocrystals are deposited on the substrate between the plasmonic nanoparticles, where each photocatalytic capped colloidal nanocrystal is deposited between greater than or equal to two plasmonic nanoparticles. The substrate, the photocatalytic capped colloidal nanocrystals, and the plasmonic nanoparticles are thermally treated, followed by absorbing light with a frequency equal to or greater than a frequency of electrons oscillating against the restoring force of positive nuclei within the plasmonic nanopartides to cause localized surface plasmon resonance, where the localized surface plasmon resonance creates an electric field between two adjacent plasmonic nanoparticles, absorbing irradiated light with an energy equal to or greater than the band gap of the photocatalytic capped colloidal nanocrystals that causes electrons of the photocatalytic capped colloidal nanocrystals to migrate from the valance band of the photocatalytic capped colloidal nanocrystals into the conduction band of the photocatalytic capped colloidal nanocrystals in a reduction reaction, where the electric field prevents the electrons from recombining into the valence band of the plasmonic nanoparticles, reacting carbon dioxide and hydrogen with the photocatalytic capped colloidal nanocrystals so that the charge carriers in the conduction band reduce carbon dioxide into methane and holes in the valence band of the photocatalytic capped colloidal nanocrystals oxidize the hydrogen into water vapor and collecting the methane and water using a methane permeable membrane and a water vapor-permeable membrane. An INDEPENDENT CLAIM is also included for a carbon dioxide reduction system.