Tuning the surface Fermi level on p-type gallium nitride nanowires for efficient overall water splitting

Abstract

Solar water splitting is one of the key steps in artificial photosynthesis for future carbonneutral, storable and sustainable source of energy. Here we show that one of the major obstacles for achieving efficient and stable overall water splitting over the emerging nanostructured photocatalyst is directly related to the uncontrolled surface charge properties. By tuning the Fermi level on the nonpolar surfaces of gallium nitride nanowire arrays, we demonstrate that the quantum efficiency can be enhanced by more than two orders of magnitude. The internal quantum efficiency and activity on p-type gallium nitride nanowires can reach B51% and B4.0 mol hydrogen h À 1 g À 1 , respectively. The nanowires remain virtually unchanged after over 50,000 mmol gas (hydrogen and oxygen) is produced, which is more than 10,000 times the amount of photocatalyst itself (B4.6 mmol). The essential role of Fermi-level tuning in balancing redox reactions and in enhancing the efficiency and stability is also elucidated.

Group III-nitride nanowire structures for photocatalytic hydrogen evolution under visible light irradiation

“…An apparent quantum efficiency of 17.1% and 12.3% was measured under visible light irradiation (400 nm-490 nm) from the half and full reactions for InGaN nanowire photocatalysts with an optimum Mg dopant incorporation, which is explained by the efficient electron and hole migration for balanced surface redox reactions, due to the optimized surface band bending. 33,34 In this work, multi-stacked GaN:Mg/InGaN:Mg nanowire photocatalysts were grown on a Si substrate without any external catalyst by plasma-assisted molecular beam epitaxy (MBE). Results presented in this work are based on extensive studies on the growth and characterization of Mg doped InGaN nanostructures, which include careful optimizations of the III/V flux ratio, doping concentration, growth temperature, and growth duration for achieving high photocatalytic activities.…”

“…3(a)) by photodeposition from the liquid precursor (Na 3 RhCl 6 ). 33,34 Subsequently, the H 2 evolution reaction using aqueous CH 3 OH solution was performed under 300 W xenon lamp irradiation with different long-pass filters. The water (H 2 O) to methanol volume ratio of ∼5:1 was maintained in order to ensure direct oxidation of methanol rather than the indirect oxidation of interfacially formed * OH radicals.…”

“…Detailed studies of the near-surface band structure properties and their correlation with p-type dopant incorporation can be found elsewhere. 33,40,43 The relative AQE of overall water splitting (red solid line) from Rh/Cr 2 O 3 co-catalyst decorated Mg-doped In 0.26 Ga 0.74 N nanowires 40 is also shown in Fig. 3(d) for comparison.…”

Photoelectrochemical reduction of carbon dioxide using Ge doped GaN nanowire photoanodes

“…Moreover, electrolyte can readily diffuse in the space among nanowires, thus leading to improved photoelectrocatalytic performance. 18,28 Recent studies have revealed the extraordinary potential of Ga(In)N nanowires for solar-to-hydrogen conversion 20,[29][30][31][32] and for photo induced C-H bond activation. 33,34 To date, however, there have been no reports on the use of GaN-nanowire based PEC system for CO 2 reduction to the best of our knowledge.…”

“…When used as a photoanode, n-type GaN nanowires are desired because of the upward surface band bending, which can promote photo-generated holes to the surface to take part in the oxidation reaction. 30 The increased conductivity with n-type dopant incorporation also facilitates the transport of carriers and enhances the redox reactions. 35 Herein, we report on the direct CO 2 conversion with the PEC reduction strategy, which involves the use of germanium doped gallium nitride nanowire arrays grown on Si substrate and copper (Cu) plate as anodic and cathodic electrodes, respectively.…”