Protection Mechanism against Photocorrosion of GaN Photoanodes Provided by NiO Thin Layers

The photoelectrochemical properties of n-type Ga-polar GaN photoelectrodes covered with NiO layers of different thicknesses in the range 0–20 nm are investigated for aqueous solution. To obtain layers of well-defined thickness and high crystal quality, NiO is grown by plasma-assisted molecular-beam epitaxy. Stability tests reveal that the NiO layers suppress photocorrosion. With increasing NiO thickness, the onset of the photocurrent is shifted to more positive voltages and the photocurrent is reduced, especially for low bias potentials, indicating that hole transfer to the electrolyte interface is hindered by thicker NiO layers. Furthermore, cathodic transient spikes are observed under intermittent illumination, which hints at surface recombination processes. These results are inconsistent with the common explanation of the protection mechanism that the band alignment of GaN/NiO enables efficient hole-injection, thus preventing hole accumulation at the GaN surface that would lead to anodic photocorrosion. Interestingly, the morphology of the etch pits as well as further experiments involving the photodeposition of Ag indicate that photocorrosion of GaN photoanodes is related to reductive processes at threading dislocations. Therefore, it is concluded that the NiO layers block the transfer of photogenerated electrons from GaN to the electrolyte interface, which prevents the cathodic photocorrosion. © 2020 The Authors. Solar RRL published by Wiley-VCH Gmb

Photoelectrochemical Properties of In,Ga N Nanowires for Water Splitting Investigated by in Situ Electrochemical Mass Spectroscopy

We investigated the photoelectrochemical properties of both n and p type In,Ga N nanowires NWs for water splitting by in situ electrochemical mass spectroscopy EMS . All NWs were prepared by plasma assisted molecular beam epitaxy. Under illumination, the n In,Ga N NWs exhibited an anodic photocurrent, however, no O2 but only N2 evolution was detected by EMS, indicating that the photocurrent was related to photocorrosion rather than water oxidation. In contrast, the p In,Ga N NWs showed a cathodic photocurrent under illumination which was correlated with the evolution of H2. After photodeposition of Pt on such NWs, the photo current density was significantly enhanced to 5 mA cm2 at a potential of 0.5 V NHE under visible light irradiation of 40 mW cm2. Also, incident photon to current conversion efficiencies of around 40 were obtained at 0.45 V NHE across the entire visible spectral region. The stability of the NW photocathodes for at least 60 min was verified by EMS. These results suggest that p In,Ga N NWs are a promising basis for solar hydrogen productio

Photoelectrochemical Properties of (In,Ga)N Nanowires for Water Splitting Investigated by in Situ Electrochemical Mass Spectroscopy

We investigated the photoelectrochemical properties of both n- and p-type (In,Ga)N nanowires (NWs) for water splitting by in situ electrochemical mass spectroscopy (EMS). All NWs were prepared by plasma-assisted molecular beam epitaxy. Under illumination, the n-(In,Ga)­N NWs exhibited an anodic photocurrent, however, no O₂ but only N₂ evolution was detected by EMS, indicating that the photocurrent was related to photocorrosion rather than water oxidation. In contrast, the p-(In,Ga)N NWs showed a cathodic photocurrent under illumination which was correlated with the evolution of H₂. After photodeposition of Pt on such NWs, the photocurrent density was significantly enhanced to 5 mA/cm² at a potential of −0.5 V/NHE under visible light irradiation of ∼40 mW/cm². Also, incident photon-to-current conversion efficiencies of around 40% were obtained at −0.45 V/NHE across the entire visible spectral region. The stability of the NW photocathodes for at least 60 min was verified by EMS. These results suggest that p-(In,Ga)­N NWs are a promising basis for solar hydrogen production

Radial Stark Effect in (In,Ga)N Nanowires

We study the luminescence of unintentionally doped and Si-doped In_<i>x</i>Ga_1–<i>x</i>N nanowires with a low In content (<i>x</i> < 0.2) grown by molecular beam epitaxy on Si substrates. The emission band observed at 300 K from the unintentionally doped samples is centered at much lower energies (800 meV) than expected from the In content measured by X-ray diffractometry and energy dispersive X-ray spectroscopy. This discrepancy arises from the pinning of the Fermi level at the sidewalls of the nanowires, which gives rise to strong radial built-in electric fields. The combination of the built-in electric fields with the compositional fluctuations inherent to (In,Ga)N alloys induces a competition between spatially direct and indirect recombination channels. At elevated temperatures, electrons at the core of the nanowire recombine with holes close to the surface, and the emission from unintentionally doped nanowires exhibits a Stark shift of several hundreds of meV. The competition between spatially direct and indirect transitions is analyzed as a function of temperature for samples with various Si concentrations. We propose that the radial Stark effect is responsible for the broadband absorption of (In,Ga)N nanowires across the entire visible range, which makes these nanostructures a promising platform for solar energy applications