Addressing global warming with new nanoparticles and sunshine

While, in green plants, photosynthesis fixes CO₂ into sugars, the artificial photosynthesis reported in this study can convert CO₂ into oxygen and pure CO as output. The latter can then be employed for a broad range of applications in electronics, semiconductor, pharmaceutical, and chemical industries. The key is to find the right high-performance photocatalyst to help the photosynthesis take place by absorbing light, convert CO₂, and ensuring an efficient flow of electrons, which is essential for the entire system.

Titanium oxide (TiO₂) is a well-known photocatalyst. It has already attracted significant attention in the fields of solar energy conversion and environmental protection due to its high reactivity, low toxicity, chemical stability, and low cost. While conventional TiO₂ can absorb only UV light, the IBS research team reported previously two different types of blue-colored TiO₂ (or "blue titania") nanoparticles that could absorb visible light thanks to a reduced bandgap of about 2.7 eV. They were made of ordered anatase/disordered rutile (A_o/R_d) TiO₂ (called, HYL's blue TiO₂-I) (Energy & Environmental Science, 2016), and disordered anatase/ordered rutile (A_d/R_o) TiO₂ (called, HYL's blue TiO₂-II) (ACS Applied Materials & Interfaces, 2019), where anatase and rutile refer to two crystalline forms of TiO₂ and the introduction of irregularities (disorder) in the crystal enhances the absorption of visible and infra-red light.

For the efficient artificial photosynthesis for the conversion of CO₂ into oxygen and pure CO, IBS researchers aimed to improve the performance of these nanoparticles by combining blue (A_o/R_d) TiO₂ with other semiconductors and metals that can enhance water oxidation to oxygen, in parallel to CO₂ reduction into CO only. The research team obtained the best results with hybrid nanoparticles made of blue titania, tungsten trioxide (WO₃), and 1% silver (TiO₂/WO₃-Ag). WO₃ was chosen because of the low valence band position with its narrow bandgap of 2.6 eV, high stability, and low cost. Silver was added because it enhances visible light absorption, by creating a collective oscillation of free electrons excited by light, and also gives high CO selectivity. The hybrid nanoparticles showed about 200 times higher performance than nanoparticles made of TiO₂ alone and TiO₂/WO₃ without silver.

Starting from water and CO₂, this novel hybrid catalyst produced O₂ and pure CO, without any side products, such as hydrogen gas (H₂) and metane (CH₄). The apparent quantum yield that is the ratio of several reacted electrons to the number of incident photons was 34.8 %, and the rate of reacted electrons 2333.44 μmol g⁻¹h⁻¹. The same measurement was lower for nanoparticles without silver (2053.2 μmol g⁻¹h⁻¹), and for nanoparticles with only blue TiO₂ (912.4 μmol g⁻¹h⁻¹).