Mohammad Chowdhury1 Hong Guo3 Zetian Mi1 2

1, Electrical and Computer Engineering, McGill University, Montreal, Quebec, Canada
3, Department of Physics, McGill University, Montreal, Quebec, Canada
2, Electrical Engineering and Computer Science, University of Michigan–Ann Arbor, Ann Arbor, Michigan, United States

Hydrogen generation from the two of the most abundant free resources available on earth, i.e. sunlight and water via photocatalytic water splitting, is a very appealing approach for the crucial societal transition to a clean and sustainable energy resource future. High-efficiency devices for photovoltaic-assisted photoelectrochemical (PEC) water splitting and electrolysers are reported in conductive electrolytes with selective pH adjustments. However, direct splitting of pure or sea water with significantly enhanced device-longevity at concentrated sunlight holds enormous promise for hydrogen generation at pH-neutral condition without any external bias, sacrificial reagent or conductive electrolytes. Progress in this field has been limited by the low photocatalytic efficiency of conventional metal-oxide materials. We have recently demonstrated that nearly defect-free GaN-based nanostructures can meet the thermodynamics for overall water splitting (OWS) [1]; and by tuning the surface Fermi-level through controlled Mg-dopant incorporation, the apparent quantum yield for solar-to-hydrogen conversion can be enhanced by nearly two orders of magnitude under UV [2] and visible light illumination [3-4]. In this work, we demonstrate multi-band InGaN nanosheet photochemical diode (PCD) structures, which can induce spontaneous charge carrier separation and steer them toward distinct redox sites for water oxidation and proton reduction reaction. During the synthesis of InGaN PCD nanosheets, p-type dopant (Mg) concentrations are rationally tailored, which induces a large built-in electric field between the two parallel surfaces. Due to the presence of a net built-in potential (~300 meV) along the lateral dimension, the two surfaces are enriched with photo-generated holes and electrons to perform water oxidation and proton reduction reactions, respectively [5]. With spatially separated catalytic sites and reduced carrier recombination, the nanoscale PCDs exhibit stoichiometric H2 and O2 evolution, with a production rate of ~1.62 mmol h-1cm-2 and ~0.784 mmol h-1cm-2, respectively, which is equivalent to a solar-to-hydrogen efficiency over ~3%. We are currently devoloping a novel III-Nitride nanostructured device on Si wafer which, when decorated with suitable co-catalyst nanoparticles on surfaces, can demonstrate unprecedented performance-stability in photochemical water splitting reaction - the longest ever measured for any semiconductor photocatalysts or photoelectrodes without protection/passivation layers in unassisted solar water splitting with an STH >1%. With further structural and surface engineering of the nanowires, we aim to enhance the solar-to-hydrogen efficiency in the range of 5-10%.

[1] D. Wang et al., Nano Lett. 11 (6), 2353 (2011).
[2] M. G. Kibria et al., Nat. Commun. 5, 3825 (2014).
[3] F. A. Chowdhury et al., APL Mater. 3, 104408 (2015).
[4] M. G. Kibria et al., Nat. Commun. 6, 6797 (2015).
[5] F. A. Chowdhury et al., Nat. Commun. 9:1707 (2018).