Harvesting sunlight, researchers of the Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS, South Korea) published in Materials Today a new strategy to transform carbon dioxide (CO₂) into oxygen (O₂) and pure carbon monoxide (CO) without side-products in water. This artificial photosynthesis method could bring new solutions to environmental pollution and global warming.

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.

▲ Figure 1. Proposed energy diagram representing the electron transfer mechanism in TiO₂/WO₃-Ag hybrid nanoparticles. This so-called Z-scheme shows the flow of charged particles (electrons, e^- and holes, ,h⁺) through the different components of the nanoparticles. Blue TiO₂ and WO₃’s e^- can occupy lower (valence band, VB) and higher (conductive band, CB) energy levels. Photons from sunlight (thunders) provide the energy for the e- to jump up from the VB to the CB (black arrows pointing upwards), leaving h+ behind. TiO₂’s lower band is close, just a bit lower than WO₃’s higher band level, so e^- from the high band of WO₃ can migrate to the VB of blue TiO₂ to trap its holes. After separation, the excited e^- jump from the CB of TiO₂ onto silver nanoparticles allowing the conversion of CO₂ into CO, while the photogenerated h⁺ in the WO₃ site oxidize water (H₂O) to form oxygen (O₂).

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).

▲ Figure 2. Efficient and selective production of CO with different nanoparticles. (a) The graph shows that hybrid TiO₂/WO₃-Ag (7BT/W1-A1) nanoparticles are the best at selectively producing pure CO, without H₂ and CH₄ side products within a 7-hour timeframe. These can be compared with nanoparticles made of blue TiO₂, WO₃, hybrid TiO₂/WO₃ (7BT/W1) and hybrid TiO₂/Ag (W1-A1). (b) CO production using different hybrid nanoparticles made of TiO₂/WO₃-Ag (red lines), TiO₂/WO₃ (green lines) and TiO₂-only nanoparticles (blue lines) within 9 hours. 7BT/W1-A1 with a concentration of 1% silver has the best performance.