Artificial photosynthesis:visible-light-driven photocatalysts convert CO 2 into oxygen and

pure CO under water

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 (CO 2 ) into oxygen (O 2 ) 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 CO 2 into sugars, the artificial photosynthesis reported in this study can convert CO 2 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 CO 2 , and ensuring an efficient flow of electrons, which is essential for the entire system.

Titanium oxide (TiO 2 ) 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 TiO 2 can absorb only UV light, the IBS research team reported previously two different types of blue-colored TiO 2 (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 ) TiO 2 (called, HYL’s blue TiO 2 -I) (Energy & Environmental Science, 2016), and disordered anatase/ordered rutile (A d /R o ) TiO 2 (called, HYL’s blue TiO 2 -II) (ACS Applied Materials & Interfaces, 2019), where anatase and rutile refer to two crystalline forms of TiO 2 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 TiO 2 /WO 3 -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 TiO 2 and WO 3 ’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. TiO 2 ’s lower band is close, just a bit lower than WO 3 ’s higher band level, so e- from the high band of WO 3 can migrate to the VB of blue TiO 2 to trap its holes. After separation, the excited e- jump from the CB of TiO 2 onto silver nanoparticles allowing the conversion of CO 2 into CO, while the photogenerated h+ in the WO 3 site oxidize water (H 2 O) to form oxygen (O 2 ).

For the efficient artificial photosynthesis for the conversion of CO 2 into oxygen and pure CO, IBS researchers aimed to improve the performance of these nanoparticles by combining blue (A o /R d ) TiO 2 with other semiconductors and metals that can enhance water oxidation to oxygen, in parallel to CO 2 reduction into CO only. The research team obtained the best results with hybrid nanoparticles made of blue titania, tungsten trioxide (WO 3 ), and 1% silver (TiO 2 /WO 3 –Ag). WO 3 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 TiO 2 alone and TiO 2 /WO 3 without silver.

Starting from water and CO 2 , this novel hybrid catalyst produced O 2 and pure CO, without any side products, such as hydrogen gas (H 2 ) and metane (CH 4 ). 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 TiO 2 (912.4 μmol g−1h−1).



▲ Figure 2. Efficient and selective production of CO with different nanoparticles. (a) The graph shows that hybrid TiO 2 /WO 3 -Ag (7BT/W1-A1) nanoparticles are the best at selectively producing pure CO, without H 2 and CH 4 side products within a 7-hour timeframe. These can be compared with nanoparticles made of blue TiO 2 , WO 3 , hybrid TiO 2 /WO 3 (7BT/W1) and hybrid TiO 2 /Ag (W1-A1). (b) CO production using different hybrid nanoparticles made of TiO 2 /WO 3 -Ag (red lines), TiO 2 /WO 3 (green lines) and TiO 2 -only nanoparticles (blue lines) within 9 hours. 7BT/W1-A1 with a concentration of 1% silver has the best performance.