A two-step process, consisting of a photoelectrosynthetic process combined with a thermochemical process, is configured to produce a reduction product (e.g., methane gas, methanol, or carbon monoxide) from carbon dioxide and liquid waste streams. In a first step, photoelectrosynthetically active heterostructures (PAHs) and sunlight are used to drive oxidation/reduction reactions in which one primary product is hydrogen gas. In the second step, hydrogen generated in the first step is thermally catalytically reacted with carbon dioxide to form a reduction product from carbon dioxide (e.g., CO, formaldehyde, methane, or methanol). Synthesis gas (CO and H2) can be further reacted to form alkanes. The methods and systems may employ PAHs known in the art or improved PAHs having lower costs, improved stability, solar energy conversion efficiency, and/or other desired attributes as disclosed herein.