Electrochemical nanostructuring of n-GaAs photoelectrodes

Reliable bi-functional nickel-phosphate /TiO2 integration enables stable n-GaAs photoanode for water oxidation under alkaline condition

Hydrogen is one of the most widely used essential chemicals worldwide, and it is also employed in the production of many other chemicals, especially carbon-free energy fuels produced via photoelectrochemical (PEC) water splitting. At present, gallium arsenide represents the most efficient photoanode material for PEC water oxidation, but it is known to either be anodically photocorroded or photopassivated by native metal oxides in the competitive reaction, limiting efficiency and stability. Here, we report chemically etched GaAs that is decorated with thin titanium dioxide (~30 nm-thick, crystalline) surface passivation layer along with nickel-phosphate (Ni-Pi) cocatalyst as a surface hole-sink layer. The integration of Ni-Pi bifunctional co-catalyst results in a highly efficient GaAs electrode with a ~ 100 mV cathodic shift of the onset potential. In this work, the electrode also has enhanced photostability under 110 h testing for PEC water oxidation at a steady current density Jph > 25 mA·cm-2. The Et-GaAs/TiO2/Ni-Pi║Ni-Pi tandem configuration results in the best unassisted bias-free water splitting device with the highest Jph (~7.6 mA·cm-2) and a stable solar-to-hydrogen conversion efficiency of 9.5%.

Primary Corrosion Processes for Polymer-Embedded Free-Standing or Substrate-Supported Silicon Microwire Arrays in Aqueous Alkaline Electrolytes

Solar fuel devices have shown promise as a sustainable source of chemical fuels. However, long-term stability of light absorbing materials remains a substantial barrier to practical devices. Herein, multiple corrosion pathways in 1 M KOH(aq) have been defined for TiO₂-protected Si microwire arrays in a polymer membrane either attached to a substrate or free-standing. Top-down corrosion was observed in both morphologies through defects in the TiO₂ coating. For the substrate-based samples, bottom-up corrosion was observed through the substrate and up the adjacent wires. In the free-standing samples, uniform bottom-up corrosion was observed through the membrane with all wire material corroded within 10 days of immersion in the dark in 1 M KOH(aq).

Tailored Nanopatterning by Controlled Continuous Nanoinscribing with Tunable Shape, Depth, and Dimension

We present that the tailored nanopatterning with tunable shape, depth, and dimension for diverse application-specific designs can be realized by utilizing controlled dynamic nanoinscribing (DNI), which can generate bur-free plastic deformation on various flexible substrates via continuous mechanical inscription of a small sliced edge of a nanopatterned mold in a compact and vacuum-free system. Systematic controlling of prime DNI processing parameters including inscribing force, temperature, and substrate feed rate can determine the nanopattern depths and their specific profiles from rounded to angular shapes as a summation of the force-driven plastic deformation and heat-driven thermal deformation. More complex nanopatterns with gradient depths and/or multidimensional profiles can also be readily created by modulating the horizontal mold edge alignment and/or combining sequential DNI strokes, which otherwise demand laborious and costly procedures. Many practical user-specific applications may benefit from this study by tailor-making the desired nanopattern structures within desired areas, including precision machine and optics components, transparent electronics and photonics, flexible sensors, and reattachable and wearable devices. We demonstrate one vivid example in which the light diffusion direction of a light-emitting diode can be tuned by application of specifically designed DNI nanopatterns.

Subwavelength photocathodes via metal-assisted chemical etching of GaAs for solar hydrogen generation

MacEtch allows subwavelength-structured (SWS) texturing on the GaAs surface without compromising crystallinity. The current density increases greatly, which is directly due to the reduction in the reflectance. Photons absorbed under reduced light reflectance are less affected by the charge recombination arising from crystal defects. The catalytic metal remaining after MacEtch serves as a catalyst for water splitting and increases the open-circuit potentials of the SWS GaAs photocathodes. The SWS GaAs not only amplifies the absorption of light, but also improves the collection of deeply generated photons at long wavelengths. The solar-weighted reflectance (SWR) of SWS GaAs is 6.6%, which was much lower than the 39.0% of bare GaAs. The light-limited photocurrent density (LLPC) increased by approximately 90% and the tafel slope improved as etching progressed. The external quantum efficiency was as high as 80%, especially at long wavelengths, after MacEtch. SWS GaAs photocathodes fabricated using MacEtch significantly reduce reflectance and recombination loss, thereby improving the key performance of PEC for hydrogen production. This technology can fully utilize the high absorption rate and carrier mobility of GaAs and is applicable to various photoelectric conversion device performance enhancements.

Interface Manipulation to Improve Plasmon-Coupled Photoelectrochemical Water Splitting on α-Fe2 O3 Photoanodes

The plasmon resonance effect of metal nanoparticles (NPs) offers a promising route to improve the solar energy conversion efficiency of semiconductors. In this study, it is revealed that hot electrons generated by the plasmon resonance effect of Au NPs tend to inject into the surface states instead of the conduction band of Fe₂ O₃ photoanodes, and then severe surface recombination occurs. Such an electron-transfer process seems to be independent of external applied potentials, but is sensitive to metal-semiconductor interface properties. Passivating the surface states of Fe₂ O₃ with a noncatalytic Al₂ O₃ layer can construct an effective resonant energy-transfer interface between Ti-doped Fe₂ O₃ (Ti-Fe₂ O₃ ) and Au NPs. In such a Ti-Fe₂ O₃ /Al₂ O₃ /Au electrode configuration, the enhanced photoelectrochemical (PEC) water-splitting performance can be attributed to the following two factors: 1) in the non-light-responsive wavelength range of Au NPs, both the relaxing Fermi pinning effect of the Al₂ O₃ passivation layer and the higher work function of Au enlarge band bending; thus promoting the charge separation; and 2) in the light-responsive wavelength range of Au NPs, the effective resonant energy transfer contributes to light harvesting and conversion. The interface manipulation proposed herein may provide a new route to design efficient plasmonic PEC devices for energy conversion.

Chemical Composition of Nanoporous Layer Formed by Electrochemical Etching of p-Type GaAs

We have performed a detailed characterization study of electrochemically etched p-type GaAs in a hydrofluoric acid-based electrolyte. The samples were investigated and characterized through cathodoluminescence (CL), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). It was found that after electrochemical etching, the porous layer showed a major decrease in the CL intensity and a change in chemical composition and in the crystalline phase. Contrary to previous reports on p-GaAs porosification, which stated that the formed layer is composed of porous GaAs, we report evidence that the porous layer is in fact mainly constituted of porous As₂O₃. Finally, a qualitative model is proposed to explain the porous As₂O₃ layer formation on p-GaAs substrate.

Investigation of Localized States in GaAsSb Epilayers Grown by Molecular Beam Epitaxy

We report the carrier dynamics in GaAsSb ternary alloy grown by molecular beam epitaxy through comprehensive spectroscopic characterization over a wide temperature range. A detailed analysis of the experimental data reveals a complex carrier relaxation process involving both localized and delocalized states. At low temperature, the localized degree shows linear relationship with the increase of Sb component. The existence of localized states is also confirmed by the temperature dependence of peak position and band width of the emission. At temperature higher than 60 K, emissions related to localized states are quenched while the band to band transition dominates the whole spectrum. This study indicates that the localized states are related to the Sb component in the GaAsSb alloy, while it leads to the poor crystal quality of the material, and the application of GaAsSb alloy would be limited by this deterioration.