Artificial Photosynthesis Helps Address Environmental Pollution and Global Warming

Scientists at the Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS, South Korea), have developed a novel method to convert carbon dioxide (CO₂) into pure carbon monoxide (CO) and oxygen (O₂) by simply harvesting sunlight.

This process does not leave behind any byproducts in water. The study has been published in the Materials Today journal.

While photosynthesis in green plants converts CO₂ into sugars, the artificial photosynthesis described in the latest study can transform CO₂ into O₂ and pure CO as output. The pure CO can be subsequently employed for a wide range of applications in the pharmaceutical, electronics, chemical, and semiconductor industries.

It is essential to identify an optimal high-performance photocatalyst to promote photosynthesis by absorbing light, transforming CO₂, and ensuring an efficient electron flow, which is important for the entire system.

Titanium oxide (TiO₂) is a popular photocatalyst and has already attracted a great deal of attention in the areas of environmental protection and solar energy conversion fields because of its low cost, low toxicity, chemical stability, and high reactivity.

Although traditional TiO₂ is capable of absorbing only UV light, the researchers at IBS had earlier reported two different types of blue-colored TiO₂, or “blue titania,” nanoparticles that can absorb visible light because of a reduced bandgap of approximately 2.7 eV.

The nanoparticles were composed of ordered anatase/disordered rutile (A_o/R_d) TiO₂ (known as HYL’s blue TiO₂-I, as described in Energy & Environmental Science, 2016). They are also made of disordered anatase/ordered rutile (A_d/R_o) TiO₂ (known as HYL’s blue TiO₂-II, as described in ACS Applied Materials & Interfaces, 2019).

Here, rutile and anatase refer to two crystalline forms of TiO₂, and the addition of irregularities, or disorder, in the crystal improves the absorption of both infrared light and visible light.

To efficiently convert CO₂ into O₂ and pure CO through artificial photosynthesis, the IBS team aimed to enhance the nanoparticles’ performance by integrating blue (A_o/R_d) TiO₂ with other metals and semiconductors. Both metals and semiconductors can improve the oxidation of water to oxygen and simultaneously the reduction of CO₂ into CO alone.

The researchers achieved optimal results with hybrid nanoparticles composed of tungsten trioxide (WO₃), 1% silver (TiO₂/WO₃–Ag), and blue titania.

WO₃ was selected due to its low cost, high stability, and the low valence band position with its 2.6 eV narrow bandgap. Silver was also added since it improves the absorption of visible light, by producing a collective oscillation of free electrons activated by light. Silver also provides high CO selectivity

The hybrid nanoparticles displayed around 200 times greater performance when compared to nanoparticles made of TiO₂/WO₃ without silver and TiO₂ alone.

Using CO₂ and water, the new hybrid catalyst generated pure CO as well as O₂, without any byproducts such as methane (CH₄) and hydrogen gas (H₂). The evident quantum yield, that is, the ratio of the number of reacted electrons to the number of incident photons was 34.8%, and the speed of the reacted electrons was 2333.44 μmol g⁻¹h⁻¹.

The same kind of measurement was found to be lower for nanoparticles that lacked silver (2053.2 μmol g⁻¹h⁻¹), as well as for nanoparticles that have only blue TiO₂ (912.4 μmol g⁻¹h⁻¹).