Solar-driven overall water splitting on CoO nanoparticles : first-principles density functional theory studies
Massachusetts Institute of Technology. Department of Materials Science and Engineering.
Alexie M. Kolpak.
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Photoelectrochemical (PEC) water splitting has been suggested as a promising techinique for large-scale hydrogen fuel production. In particular, spontaneous photocatalytic overall water splitting on self-standing particles in water without external driving potential has been highlighted as a clean and economical energy generation method for the future. Among various photocatalytic materials, some cobalt-based materials including CoP, Co₂P, Co(OH)₂, CoO, have attained major interest because they exhibit improved catalytic activity for hydrogen evolution in the form of nanoparticles, unlike most cobalt-based materials which have been assessed as water oxidizing catalysts in the past decade. CoO nanoparticles have been observed to photocatalytically split water into H₂ and O₂ at room temperature without an externally applied potential or co-catalyst, with high photo-catalytic efficiency (solar-to-hydrogen efficiency of ~5%) which hits the record among single-material self-standing photocatalysts. The photocatalytic activity of CoO nanoparticles was experimentally shown to stem from the optimal conduction and valence band edge positions (Ec and Ev) relative to water reduction and oxidation potential levels (H+/H₂ and H₂O/O₂), such that the Ec and EV span the water redox potentials. The overall water splitting is not expected from CoO micropowder or bulk CoO because they have band edges far below the H+/H2 level, which are not optimal for overall water splitting. However, the origin of the shift in the band edges due to decrease in particle size (from bulk or micropowder to nanoparticle) was unknown. Moreover, the mechanism by which H₂ and O₂ simultaneously and spontaneously evolve on the nanoparticles, as well as how the CoO nanoparticles could exhibit a high photocatalytic efficiency even without a co-catalyst or an external driving potential have remained unanswered. In this work, we use first-principles density functional theory (DFT) calculations to explore thermodynamically stable surface configurations of CoO in an aqueous environment in which photocatalytic water splitting occurs. We also calculate the Ec and Ev of CoO surfaces relative to water redox potentials, showing that the band edge positions are sensitive to surface chemistry which is determined by surface orientation, adsorbates, and stoichiometry, and thus growth conditions and operating environment. In particular, we predict that CoO nanoparticles have fully hydroxylated CoO(111) facets (OH*-CoO(111)), with band edges spanning the water redox potentials, while larger CoO particles (such as CoO micropowders) have a full monolayer of hydrogen on the CoO(111) facets, with a band alignment that favors water oxidation but not water reduction. From these calculations, we demonstrate that explicit inclusion of liquid water is crucial for accurately predicting the band edge positions, and thus photocatalytic behavior of CoO in an aqueous solution. In order to find the origin of the high efficiency and spontaneous overall water splitting without an external bias or a co-catalyst, we also elucidate the mechanisms for charge separation and H₂ and O₂ evolution on CoO nanoparticles under illumination in an aqueous solution. We demonstrate that electrons are driven to CoO(100) facets and holes are driven to OH*-CoO(111) facets as a result of a built-in potential arising from the very different potential levels of the two facets. We show that H₂ evolution preferentially occurs on the CoO(100) facets, while O2 evolves on the OH*-CoO(111) surfaces, based on our new criteria. Importantly, we suggest that the conventional criterion for determining the feasibility of H₂ or O₂ generation from water splitting - i.e., EC < H+/H₂ level or Ev > H₂O/O₂ level - is insufficient. Instead, we suggest that a more appropriate set of criteria is whether the photo-excited electrons and holes have sufficient energy to overcome the kinetic barrier for the H₂ and O₂ evolution reaction, respectively, on the relevant surface facet. This work explains why and how photocatalytic overall water splitting has been observed only on CoO nanoparticles. Our understanding of the overall water splitting mechanism on CoO nanoparticles provides a general explanation of experimentally observed overall water splitting phenomena on a variety of self-standing photocatalysts as well as a new approach for screening novel photocatalytic materials for efficient water splitting and other reactions.
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged student-submitted from PDF version of thesis.Includes bibliographical references (pages 143-157).
DepartmentMassachusetts Institute of Technology. Department of Materials Science and Engineering.
Massachusetts Institute of Technology
Materials Science and Engineering.