(T2) Oxygen-evolving electrocatalysts and photoelectrocatalysts immobilized onto lightabsorbing perovskite metal oxides
CSULA Faculty Participants: Feimeng Zhou, Matthias Selke, Radi Jishi
Penn State Faculty Participants: Tom Mallouk, Venkatraman Gopalan
The depletion of and environmental pollution by fossil energy require the development of clean, renewable and sustainable energy sources. Among the various renewable energy sources, hydrogen generated from electrocatalytic or photoelectrocatalytic water splitting is arguably the cleanest . The water-splitting reaction constitutes the hydrogen- and oxygen-evolving processes, the latter of which is sluggish and rate-limiting [142, 143]. Nature through millions of years of evolution has developed the best oxygen-evolving catalyst, the Mn4O5Ca cluster in the photosystem II of plants [143—145].
The small Mn4O5Ca catalytic cluster is stabilized and buried in a large protein ensemble of about one million Dalton [146, 147]. Such a bulky protein complicates structural elucidation and renders significant challenges to the study of the catalytic cluster. Moreover, the low percentage of the size of the active center relative to the entire protein and the long-term photoinstability of the protein shell restrict the practical application. The lack of a thorough understanding of the catalytic mechanism impedes the development of synthetic catalysts through bio-mimicry. We propose to develop oxygen-evolving electrocatalysts and photocatalysts by extracting and stabilizing the cluster with small ligands, studying the structure and catalytic mechanism of the cluster, immobilizing the clusters with a high load onto perovskite metal oxides that efficiently absorb light in the solar spectrum, and synthesizing clusters of similar structure and activity.
The key step in our overall research plan is to obtain the Mn4O5Ca cluster stabilized by small coordinating ligands of choice through ligand exchange. The ligands of interest include the peptides with the same binding residues in the PSII particles, as well as “unnatural” ligands such as trioctylphosphine oxide (TOPO), which has been widely used to cap quantum dots , and inorganic polyoxometalate ions, which are known to stabilize metal oxide clusters [141, 149, 150]. The Zhou group has confirmed that the cluster can be stabilized and capped with phenylphosphonate ions . The next step will be to separate and purify milligram quantities of the cluster for characterization and catalytic water oxidation study. Because the PS II ensemble is not well characterized, information about the Mn4O5Ca cluster is limited. Even the valence states of the Mn center are not clear. Electrochemical investigation of the cluster will offer information about the electron transfer and catalytic mechanisms. The mechanistic characterization of the catalytic activity will be conducted in close collaboration with Mallouk who is the leader in the study of synthetic oxygen evolving catalyst (e.g., iridium oxide based materials) [142, 152]. Structural investigation of the clusters will guide the synthesis of analogs that mimic the Mn4O5Ca cluster. Based on the results from this fundamental study, we will search for methods to immobilize the cluster onto conductive substrates so that catalytic oxygen-evolving electrodes can be prepared. We will also explore the use of light-harvesting perovskite metal oxides, in collaboration with Gopalan at Penn State, as substrates for fabricating photoelectrocatalytic oxygen evolving electrodes. In a subsequent stage, the Selke group [153, 154] will synthesize clusters containing additional chromophores that absorb across the solar spectrum and study the photochemistry and photophysics of these materials, i.e. triplet lifetimes under aerobic and anaerobic conditions as well as electron transfer rates.
To ultimately achieve photocatalytic water splitting, immobilization of the interfacial electron transfer catalyst (the cluster) onto the light-harvesting semiconductors will be carried out. The ideal light harvesting semiconductors need to have proper bandgaps of 1.6 - 2.2 eV, a conduction band edge positioned above that of hydrogen evolution, long charge carrier lifetime and fast carrier mobility, and long term photochemical stability. Thus far, no semiconductors meet all these criteria. A key objective of our proposal will be to screen for and synthesize such semiconductor materials as the cluster carrier to make photocatalysts.
Wide bandgap metal oxides or nitrides are photochemically stable, yet possess a narrow solar spectrum coverage, slow interfacial electron transfer kinetics and short charge carrier life time all of which limits their photocatalytic efficiency. The recent significant success in developing solar cells with organic lead halide perovskite is attributed to the exceptional long carrier lifetime inherent in the ferroelectricity of certain semiconducting materials [155, 156]. The inherent electric field as a consequence of the strong polarization of the perovskite lattice enhances the charge separation and hence the charge carrier lifetime. Recently, ferroelectricity of tetragonal BaTiO3 is believed to be responsible for its higher photocatalytic activity of adsorbed dye degradation in comparison with non-ferroelectric cubic counterpart . Ferroelectric semiconductor materials of suitable bandgaps and band edge position are therefore potential candidates to use for water splitting photocatalysts. The advance in condensed matter physics of ferroic materials allows for readily tailoring physical properties of materials through structural and composition variations [158—160]. We propose to use ferroelectricity as a guiding parameter to search and screen for suitable semiconductors.
Specifically, we will choose stable wide bandgap mixed metal perovskite oxides ABO3 (e.g. SrTiO3, SrNbO3 or SrTaO3) as the base materials, and tune bandgaps, ferroelectricity and band edge positions, and ultimately
photocatalytic water splitting activities. Just like the successful bandgap tuned for perovskite lead halide by testing halides of different electronegtivities [156, 161], replacing oxygen in the oxides with N, C and Se could narrow the bandgap. Replacing the Ti center with Nb, Ta, and other transition metals can adjust the conduction band edge position . Changing the transition metals based on their ionic radii, deviation from ideal compositions and Jahn-Teller distortion from octahedral geometry are expected to lead to electric polarization and hence the ferroelectricity of the materials. The Jishi group has recently demonstrated that density functional theory (DFT), when used in conjunction with a reparametrized modified Becke-Johnson potential, can model accurately the electronic properties of lead halide perovskite materials such as CH3NH3PbI3.. Gopalan at Penn State is the leading expert in modeling, synthesis and characterization of perovskite ferroic materials of novel and desired properties [164, 165]. Therefore, in collaboration these two groups will use DFT and generalized gradient approximation (GGA) to screen for the right perovskite-based new materials. The Zhou group  will conduct the synthesis and characterization in collaboration with Gopalan at Penn State. Once the semiconductor materials are synthesized, we will measure their ferroelectric properties and immobilize the abovementioned catalytic cluster onto these light-harvesting perovskite metal oxides to test the overall photocatalytic activity. Correlation of the ferroelectric properties with their photocatalytic activity will be investigated and new insights into the photocatalytic activities of such semiconductors should be gained. Other existing semiconductor materials such as BiVO4 and hematites will be prepared, tested, and compared to the new perovskite metal oxides.