Impact of Nanostructuring on the Photoelectrochemical Performance of Si/Ta₃N₅ Nanowire Photoanodes

The nanostructuring of light-absorbing materials in photoelectrochemical applications can potentially improve the performance of charge transport limited semiconductors by increasing incident light absorption as well as the electrochemically active surface area. However, a drawback associated with an increase in electrode surface area is the increased effect of surface recombination on device performance. To understand the interplay of the positive and negative impacts of nanostructuring, we studied these effects by varying the nanowire length and thereby surface area on the photoelectrochemical performance of tandem core–shell Si/Ta₃N₅ photoanodes. Si/Ta₃N₅ nanowires of different lengths, 1.2–3.3 μm, were fabricated by changing the reactive ion etch duration by which the Si nanowires are formed and subsequently characterized by optical UV–vis reflectance measurements, effective charge carrier lifetime measurements, and photoelectrochemical ferrocyanide oxidation. Overall, we show that as the nanowire length is increased, the photovoltage decreases due to decreasing effective carrier lifetimes that arise from higher surface recombination. On the other hand, the device photocurrent increases as the nanowires become longer due to increasing electrochemically active surface area and decreased light reflection, which in turn increases absorption due to light trapping within the nanowires. Balancing these effects is crucial toward developing high performance devices

Transition Metal-Modified Zirconium Phosphate Electrocatalysts for the Oxygen Evolution Reaction

Zirconium phosphate (ZrP), an inorganic layered nanomaterial, is currently being investigated as a catalyst support for transition metal-based electrocatalysts for the oxygen evolution reaction (OER). Two metal-modified ZrP catalyst systems were synthesized: metal-intercalated ZrP and metal-adsorbed ZrP, each involving Fe(II), Fe(III), Co(II), and Ni(II) cations. Fourier transform infrared spectroscopy, X-ray powder diffraction, thermogravimetric analysis, and X-ray photoelectron spectroscopy were used to characterize the composite materials and confirm the incorporation of the metal cations either between the layers or on the surface of ZrP. Both types of metal-modified systems were examined for their catalytic activity for the OER in 0.1 M KOH solution. All metal-modified ZrP systems were active for the OER. Trends in activity are discussed as a function of the molar ratio in relation to the two types of catalyst systems, resulting in overpotentials for metal-adsorbed ZrP catalysts that were less than, or equal to, their metal-intercalated counterparts

Tandem core-shell Si-Ta3-N5 photoanodes for photoelectrochemical water splitting

Nanostructured core–shell Si–Ta3N5 photoanodes were designed and synthesized to overcome charge transport limitations of Ta3N5 for photoelectrochemical water splitting. The core–shell devices were fabricated by atomic layer deposition of amorphous Ta2O5 onto nanostructured Si and subsequent nitridation to crystalline Ta3N5. Nanostructuring with a thin shell of Ta3N5 results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the Ta3N5 shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner Ta3N5 shells, indicating minority carrier diffusion lengths on the order of tens of nanometers. The fabrication of a heterostructure based on a semiconducting, n-type Si core produced a tandem photoanode with a photocurrent onset shifted to lower potentials by 200 mV. CoTiOx and NiOx water oxidation cocatalysts were deposited onto the Si–Ta3N5 to yield active photoanodes that with NiOx retained 50–60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable Ta3N5 photoanodes reported to date