Artificial Photosynthesis Using Semiconductor Nanomaterials

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Shining different lights on photocatalysis

The mitigation of steadily increasing atmospheric CO2 might well be the major global scientific challenge of the twenty-first century. Carbon capture is an important issue in Alberta, a jurisdiction with a large carbon footprint. The use of CO2 as a feedstock for energy storage is also of interest. An efficient sunlight-driven CO2 reduction process that produces alcohols or hydrocarbons from H2O+CO2 can store energy in the form of chemical bonds - an alternative to hydrogen-based energy storage. Natural photosynthesis is the chemical reaction on which life is based, but is inefficient in terms of conversion of light into chemical energy (global efficiency ~0.2%). Semiconductor nanomaterial-based artificial photosynthesis will enable efficiencies several times higher than natural photosynthesis. Artificial photosynthesis is complex with eight electron transfer steps, thus rendering the reaction vulnerable to energetic losses at every step. The mechanistic routes governing the formation of desirable end-products are not fully established. And yet, there is huge potential; the combination of government levied carbon taxes (ALBERTA: currently $20/tonne of CO2, set to increase to $30/tonne by 2018 and CANADA: $50/tonne of CO2 by 2020) and highly efficient catalysts can render CO2 reduction profitable in the not-too-distant future. Companies that package and sell carbon offsets to big and small CO2 emitters are very interested in this technology.

In the last four years of research, binary and ternary heterojunctions of the following four components have been shown by us to be particularly effective photocatalysts for artificial photosynthesis:

(i) Carbon nitrides: These are pi-conjugated polymers with a graphitic framework. They are easily exfoliated to form two-dimensional sheets. The leading member of this family is g-C3N4 whose bandgap of 2.6 eV is not optimal for solar energy harvesting. The Shankar Lab synthesized a new graphitic polymer with the chemical formula C3N5 which had a more optimal electronic bandgap of 1.7 eV. Information about the properties and performance of C3N5 was disseminated in a 2019 article in the prestigious Journal of the American Chemical Society in 2019 (https://doi.org/10.1021/jacs.9b00144) where it has since been cited over 200 times to date.

(ii) Plasmonic nanomaterials: Nanostructured coinage metals (Au, Ag, Cu) generate hot carriers that can be used to drive chemical reactions. However, hot carriers represent a short-lived exotic species and our research has advanced knowledge and understanding in this area. By controlling the shape, size and dielectric environment of coinage metal nanoparticles, we have shown an ability to increase the selectivity for a C2+ product (https://doi.org/10.1021/acsami.0c21067) and harvest hot holes (harder to do) as well as hot electrons in plasmonic heterojunctions (https://doi.org/10.1021/acsami.1c10698). Our work on tuning the reaction pathway in high rate CO2 photoreduction was published in 2020 in the high impact journal Applied Catalysis B: Environmental (https://doi.org/10.1016/j.apcatb.2020.118644) and has been cited 55 times to date. We have also shown a way to decrease dependence on noble metals for plasmonics by highlighting hot carrier-driven photochemistry in transition metal nitrides (https://doi.org/10.3390/catal11111374).

(iii) Metal oxides: We started the FES project with TiO2, an old workhorse of solar energy research which is limited by its large electronic bandgap of 3.0-3.2 eV. Recently, we have moved on to multi-element quinary and senary perovskite oxides with bandgaps as low as 1.5 eV (https://doi.org/10.1088/1361-6528/abedec). The concept of oxide perovskites is being pursued collaboratively with the University of Calgary and machine learning based methods are being used to find optimal compositions.

(iv) Halide perovskites: CsPbBr3 nanocrystals coated with a protective layer of carbon nitride gave us high performance in both photoelectrochemical water splitting and CO2 photoreduction. This work was disseminated in a 2022 article in Applied Surface Science (https://doi.org/10.1016/j.apsusc.2022.153276).

We have a wide network of local and international research collaborators. We are now seeking to partner with Canadian SMEs to increase the technology readiness level of our catalysts. It is noteworthy that three of our four platform material systems namely carbon nitrides, transition metal oxides and transition metal nitrides are made of earth-abundant elements, are photochemically stable, are relatively inexpensive to manufacture and lend themselves to "scale up".