GaN nano-pyramid arrays as an efficient photoelectrode for solar water splitting

Improvement in Photoelectrochemical Water Splitting Performance of GaN-nanowire Photoanode Using MXene

The photoelectrochemical water splitting (PEC-WS) performance of a photoanode consisting of GaN nanowires (NWs) is significantly improved using a Ti3C2-MXene coating as an intermediate layer to promote carrier transfer toward the electrolyte. The maximum current density and applied-bias photon-to-current efficiency of the photoanode comprising GaN NWs coated with Ti3C2-MXene (MGNWs) are measured to be 34.24 mA/cm2 and 14.47% at 1.2 and 0.4 V versus a reversible hydrogen electrode (RHE), respectively. These values are much higher than those of the GaN-NW photoanode without Ti3C2-MXene (4.04 mA/cm2 and 1.95%) and also markedly exceed those of previously reported photoanodes. After 8 days of PEC-WS, the current density was measured to be 31.07 mA/cm2, which corresponds to 97.58% of that measured immediately after the reaction started. Based on the time dependence of the current density, the hydrogen evolution rate over the reaction time is calculated to be 0.58 mmol/cm2·h. The results confirm that the PEC-WS performance of the optimized MGNW photoanode is superior to and more stable than those of previously reported photoanodes.

Selective area grown AlInGaN nanowire arrays with core-shell structures for photovoltaics on silicon

To pave the way for InGaN-on-Si integrated photovoltaics, uniform and close-packed n-GaN/(Al)InGaN/p-GaN nanowire (NW) arrays with a ∼0.29 μm thick absorption segment of ∼2.35 eV energy bandgap were fabricated on a Si substrate using Ti-mask selective area growth (SAG) in a molecular beam epitaxy (MBE) chamber. Instead of using thick and insulting buffer layers, this SAG process was realized by employing a 3 nm AlN/GaN: Ge buffer layer to facilitate electrical and thermal conduction between NWs and Si. Scanning transmission electron microscopy and high-resolution electron energy loss spectroscopy mapping revealed the discontinuities of AlN and the embedments of GaN:Ge which contribute to a negligible resistance of the NWs-on-Si interface. AlInGaN active segment exhibits core-shell structures, which suppress nonradiative surface recombination at NW surfaces. Working of AlInGaN core-shell NW solar cells was demonstrated with improved open-circuit voltage (Voc) and higher energy conversion efficiency (η) than those reported for InGaN NW solar cells. Stable output characteristics including the Voc of 1.41 V and η of 2.46% were obtained under 30-Sun illuminations. Such NWs-on-Si devices use Si substrate as the bottom electrode. With a low series resistance of ∼1 Ω, this work paves the way to monolithically integrate MBE-SAG III-nitride devices and Si-based electronics, such as Si solar cells and CMOS devices.

Fabrication of GaN truncated nanocone array using a pre-deposited metallic nano-hemispheres template for efficient solar water splitting

The GaN truncated nanocone is an excellent candidate for better photoelectrochemical efficiency than other GaN nanostructures. Here the highly ordered GaN truncated nanocone array was fabricated using a pre-deposited metallic nano-hemispheres template on a wafer scale. The highly ordered profiles of pre-deposited metallic nano-hemispheres template were defined by anodic aluminum oxide (AAO) masks through electron beam evaporation. The formation mechanism for the profiles of nano-hemispheres and GaN truncated nanocones were investigated. The results elucidate that proper selection of AAO parameters enables controllability of desired profiles and depth of Cr nano-hemispheres template, further controllability of desired profiles and depth of the GaN truncated nanocones. The optical and photoelectrochemical characterizations show the substantial improvements in ultraviolet light absorption and photoelectrochemical efficiency with photocurrent density by 300% times with respect to planar counterpart. The presented synthetic strategy will pave the way towards low-cost and mass production of GaN truncated nanocone photoelectrode for efficient photocatalysis.

Enhanced water splitting performance of GaN nanowires fabricated using anode aluminum oxide templates

Highly ordered GaN nanowires were fabricated using an anodic aluminum oxide (AAO) template. Compared to planar GaN, the GaN nanowires significantly increased the light absorption, and the saturated photocurrent increased by a factor of 5 from 0.075 to 0.38 mA cm⁻². The photocurrent increase with the GaN nanowires is not only due to their increased surface to volume ratio and reduction in the distance for photo-generated carriers to reach the electrolyte, but also the built-in electric field, which mainly contribute to the enhancement in their water splitting ability. The GaN nanowires can lead to band bending due to their surface states and the formation of a polarized electric field to accelerate the separation of photo-generated carriers. We also established a theoretic model to simulate the band bending in the nanowires. The results showed that when the nanowire diameters are equal or bigger than the full width of depletion region, the nanowires have the maximum electric field, which improves their water splitting performance significantly. These results provide a cost-effective way for highly efficient water splitting.

Highly ordered GaN nanowires were fabricated using an anodic aluminum oxide (AAO) template.

Correlations among morphology, composition, and photoelectrochemical water splitting properties of InGaN nanorods grown by molecular beam epitaxy

The mechanism underlying the effect of growth condition on the morphology evolution of InGaN nanorods (NRs) has been systematically investigated. The increased Ga flux enhances both the axial and the radial growth at the growth stage. However, the changed Ga flux influences not only the growth but also the nucleation of InGaN NRs. At the nucleation stage, the increased Ga flux shortens the delay time for NR formation, and prolongs the growth stage for a fixed total growth time. Those two aspects result in the increase of NR diameter and height with the supplied Ga flux. In addition, the continuous nucleation is ended much earlier due to the accelerated saturation of substrate area with the increased Ga flux, resulting in a decreased final NR density. In addition to the morphology evolution with the Ga flux, the composition characteristic of InGaN NRs has been also studied. The In distribution of InGaN NRs depends critically on the NR diameter along the NR growth direction, and the NRs show a morphology-dependent In incorporation. Interestingly, the InGaN NRs discussed here show a radial Stark effect induced by the pinned Fermi level. The radial Stark effect shifts the absorption edge of the InGaN NRs toward longer wavelengths, makes the InGaN NRs attractive for photoelectrochemical water splitting applications. The photoelectrochemical measurements present a significant increase in the photocurrent with the increased total surface area of the InGaN NRs, which is due to the enhanced light absorption effects and the enlarged interfacial area of the semiconductor/electrolyte.

Light Modulation and Water Splitting Enhancement Using a Composite Porous GaN Structure

On the basis of the laterally porous GaN, we designed and fabricated a composite porous GaN structure with both well-ordered lateral and vertical holes. Compared to the plane GaN, the composite porous GaN structure with the combination of the vertical holes can help to reduce UV reflectance and increase the saturation photocurrent during water splitting by a factor of ∼4.5. Furthermore, we investigated the underlying mechanism for the enhancement of the water splitting performance using a finite-difference time-domain method. The results show that the well-ordered vertical holes can not only help to open the embedded pore channels to the electrolyte at both sides and reduce the migration distance of the gas bubbles during the water splitting reactions but also help to modulate the light field. Using this composite porous GaN structure, most of the incident light can be modulated and trapped into the nanoholes, and thus the electric fields localized in the lateral pores can increase dramatically as a result of the strong optical coupling. Our findings pave a new way to develop GaN photoelectrodes for highly efficient solar water splitting.