(353a) Laser-Made Nanocatalysts for Photoelectrochemical Water Oxidation

Sunlight-driven water oxidation catalysis is essential to produce our future clean fuels, value-added commodity chemicals, pharmaceuticals, and fertilizers [1]. Sustainability demands that water oxidation catalysts are efficient, robust, and exclusively made of non-precious elements [2]. We used pulsed-laser in liquids synthesis to prepare novel earth-abundant nanosized electrocatalysts for water oxidation and photoanode materials with controlled defect densities. The pulsed-laser in liquids technique is a flexible synthetic strategy for surfactant-free nanomaterials with controlled compositional, morphological, and structural properties, and defect densities. It permits rapid preparation or modification of tailored, complex nanostructures in sufficiently large quantities to study them in bulk [3,4].

We rationally designed our laser-made nanocatalysts by the following criteria: (1) they could consist only of earth-abundant elements, limiting the periodic table to main-group, first-row, and early transition metals, (2) metals must form oxides or hydroxides that are stable under anodic polarization in aqueous electrolyte, and (3) multiple oxidation states had to be easily accessible in the metals. Therefore, we chose cobalt, iron, and nickel as targets; we also explored doping with non-redox-active metals for improved carrier mobility in nickel-iron layered double hydroxide catalysts.

Laser-made Co₃O₄ nanocrystals exhibited a very high mass activity of more than 10 A m^−2 g^−1 at 500 mV overpotential [5]. Advancing traditional pulsed-laser ablation in liquids into a reactive method by simply adding metal ions into the aqueous liquid allowed us to synthesize multi-metal nanomaterials with tailored compositions; rapid optimization of highly active, robust nickel–iron layered double hydroxide nanocatalysts for water oxidation in base became possible this way [6]. The best material, a nickel–iron layered double hydroxide doped with small amounts of La³⁺ and Ti⁴⁺, catalyzed oxygen evolution at pH 14 at low overpotential (260 mV at 10 mA cm^−2) and with 100% faradaic efficiency, and it was stable for hours under turnover [6]. Structural analysis of these mixed-metal catalysts provided experimental evidence that water oxidation occurred at edge-site iron centers [7]. Interlayer anions played key roles during turnover; overpotentials depended sigmoidally on anion basicity, suggesting a base-assisted water-oxidation mechanism. Our nanocatalysts were regenerated and most active in alkaline electrolyte in ambient air, where ubiquitous carbonate rapidly replaced other interlayer anions [7]. We gained structural and mechanistic insights from operando spectroscopy data and identified a cis-dioxo iron(VI) reactive intermediate as the lowest-energy species before O–O bond formation during water oxidation catalysis [8].

Further, we prepared 70 ZnO and TiO₂ materials to systematically study the effects of laser parameters and adsorbed gold nanoparticles and concomitant changes to bandgap, crystal structure, and bulk and surface defect densities on photocurrent generation of ZnO and TiO₂ photoanodes under simulated sunlight irradiation in neutral aqueous electrolytes. We observed unambiguous color changes of the ZnO and TiO₂ nanoparticles as a function of laser irradiation and/or AuNP functionalization and were interested in (1) the physical origin of the individual colors, and (2) a correlation with photoelectrochemical performance. Two-dimensional photoluminescence data allowed us to extract the effects of surface versus bulk defects. We obtained key insights about the defect types and mechanisms that laser processing generated and how they affect photoelectrochemical performance. Controlled defect generation and healing enhanced photocurrent production in TiO₂ photoanodes [9].

Finally, as the next step towards an integrated solar water splitting device, we immobilized the laser-made nanocatalysts on photoanodes and optimized photocurrent generation by rational design [1]. Photoelectrochemical performance of integrated nanocatalyst–photoanode assemblies is highly dependent on how nanocatalysts are spatially distributed on the surface of the light absorber. Surfactant-free cobalt oxide and nickel–iron layered double hydroxide water oxidation nanocatalysts aggregate on bismuth vanadate surfaces. We added citrate to aqueous nanocatalyst drop-cast suspensions, and the citrate surfactant selectively ligated the dipositive metals of the catalyst nanoparticles, importantly, without blocking the catalytically active sites that we established through our mechanistic work. We demonstrated that even nanoparticle distributions led to enhanced photoelectrochemical performance.

References

1. Sinclair, T. S.; Gray, H. B.; Müller, A. M. Eur. J. Inorg. Chem. 2018, 1060 (2018).
2. Hunter, B. M.; Gray, H. B.; Müller, A. M. Chem. Rev. 116, 14120 (2016).
3. Roske, C. W.; Lefler, J. W.; Müller, A. M. J. Colloid Interface Sci. 489, 68 (2017).
4. Blumenfeld, C. M.; Lau, M.; Gray, H. B.; Müller, A. M. ChemPhysChem 18, 1091 (2017).
5. Blakemore, J. D.; Gray, H. B.; Winkler, J. R.; Müller, A. M. ACS Catal. 3, 2497 (2013).
6. Hunter, B. M.; Blakemore, J. D.; Deimund, M.; Gray, H. B.; Winkler, J. R.; Müller, A. M. J. Am. Chem. Soc. 136, 13118 (2014).
7. Hunter, B. M.; Hieringer, W.; Winkler, J. R.; Gray, H. B.; Müller, A. M. Energy Environ. Sci. 9, 1734 (2016).
8. Hunter, B. M.; Thompson, N. B.; Müller, A. M.; Rossman, G. R.; Hill, M. G.; Winkler, J. R.; Gray, H. B. Joule 2, 1 (2018).
9. M. Lau, S. Reichenberger, I. Haxhiaj, S. Barcikowski, and A. M. Müller, ACS Appl. Energy Mater. 1, 5366 (2018).