NOVELTY - Photocatalytic capped colloidal nanocrystals including semiconductor nanocrystals (a) capped with inorganic capping agent (b) are deposited and oriented on porous substrate. Irradiated light with energy equal to or greater than band gap of semiconductor nanocrystals are absorbed by photocatalytic capped colloidal nanocrystals, to form charge carriers in conduction band of photocatalytic capped colloidal nanocrystals. Carbon dioxide and hydrogen are reacted with photocatalytic capped colloidal nanocrystals such that charge carriers in conduction band reduce carbon dioxide into methane.    USE - Method of reducing carbon dioxide in production of fuel.    ADVANTAGE - The method efficiently reduces carbon dioxide using nanocrystals with high catalytic effect and light-harvesting efficiency.    DETAILED DESCRIPTION - Photocatalytic capped colloidal nanocrystals including semiconductor nanocrystals (a) capped with an inorganic capping agent (b) are deposited and oriented on a porous substrate. Irradiated light with an energy equal to or greater than the band gap of the semiconductor nanocrystals are absorbed by photocatalytic capped colloidal nanocrystals, to form charge carriers in a conduction band of the photocatalytic capped colloidal nanocrystals and holes in a valence band of the photocatalytic capped colloidal nanocrystals. Carbon dioxide and hydrogen are reacted with the photocatalytic capped colloidal nanocrystals such 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 methane and water are collected using a methane permeable membrane and a water vapor-permeable membrane. An INDEPENDENT CLAIM is included for carbon dioxide reduction system (1200).    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic structure of carbon dioxide reduction system.    Photoactive material (506)    Light source (804)    Carbon dioxide reduction system (1200)    Carbon dioxide (1202)    Boiler (1204)