X-ray measurements reveal an unexpected role for copper in photocatalysts

Copper is a promising catalyst for sustainably converting carbon dioxide into substances with more electrons (called reduced species). This is an important step in converting carbon dioxide into fuels. This reaction is often initiated by electrical energy, but it can also be achieved using solar energy to produce solar fuels.

However, scientists do not fully understand the chemical nature of the copper catalyst during the solar reaction. In this work, scientists used X-rays to investigate how copper catalysts change when operating only with light and no applied electricity.

Changes to the copper composition during the reaction indicate that it plays an unexpected role. Instead of forming a more reduced species, the copper produces a more oxidized chemical species.

Understanding how copper catalysts operate under solar irradiation is key to using them to capture carbon and convert it for use in other applications. Recent research, published in the journal ACS Nano, discovered that copper forms more oxidized species when combined with a common plasmonic light absorbing material.

Plasmonics involves the synchronized vibration of electrons in metals. This new work provides important guidance on how to design and optimize future light-based catalytic systems. Researchers demonstrated photocatalytic carbon dioxide reduction to carbon monoxide under unassisted (electrically unbiased) conditions.

The study used catalysts that combine a p-type gallium-nitrogen (GaN) semiconductor, plasmonic gold light absorbers, and copper co-catalysts (p-GaN/Al₂O₃/Au/Cu). Scientists at the Liquid Sunlight Alliance (LiSA) Fuels from Sunlight Energy Innovation Hub investigated the mechanistic role of the Cu catalyst in this system.

This investigation used Cu K-edge X-ray absorption spectroscopy measurements at the Stanford Synchrotron Radiation Lightsource (SSRL). This light source is a Department of Energy Office of Science user facility.

Upon exposure to gas-phase carbon dioxide and water vapor reaction conditions, the researchers identified the composition of copper as a mixture of Cu(I) and Cu(II) oxide, hydroxide, and carbonate compounds without metallic Cu. Under photocatalytic operating conditions with visible light irradiation, the team observed further oxidation of Cu.

This indicates that light initiates hole transfer from Au-to-Cu. This observation was supported by first principles calculations of the band alignments of the oxidized Cu compositions with plasmonic Au particles, where light-driven hole transfer from Au-to-Cu is found to be thermodynamically favored.

These findings demonstrated that under unassisted gas-phase reaction conditions, Cu plays an oxidative rather than a reductive role in photocatalysis when coupled with plasmonic Au particles for light absorption.