Nanostructured core-shell Si-TaN photoanodes were designed and synthesized to overcome charge transport limitations of TaN for photoelectrochemical water splitting. The core-shell devices were fabricated by atomic layer deposition of amorphous TaO onto nanostructured Si and subsequent nitridation to crystalline TaN. Nanostructuring with a thin shell of TaN results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the TaN shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner TaN 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. CoTiO and NiO water oxidation cocatalysts were deposited onto the Si-TaN to yield active photoanodes that with NiO retained 50-60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable TaN photoanodes reported to date.
A joint theoretical and experimental study of the opto-electronic properties of Ta3N5 was conducted by means of ab initio calculations and ellipsometry measurements. Previous experimental work on Ta3N5 has not been conclusive regarding the direct or indirect nature of light absorption. Our work found excellent agreement between the optical spectrum computed using the BetheSalpeter Equation and the measured one, with two prominent features occurring at 2.1 and 2.5 eV assigned to direct transitions between N and Ta states. The computed optical gap, obtained from the G0W0 direct photoemission gap, including spin-orbit coupling, electron-phonon renormalization of the conduction band and exciton binding energy, was found to be in excellent agreement with measurements. Our results also showed that Ta3N5 is a highly anisotropic material with heavy holes in several directions, suggesting low hole mobilities, consistent with low measured photocurrents in the Ta3N5 literature.
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.
The use of thin Ta 3 N 5 films in tandem Si-Ta 3 N 5 photoelectrochemical (PEC) devices motivates understanding of the surface Ta 3 N 5 properties, as they may have a strong effect on the device performance. The bulk and surface properties can change as a function of nitridation temperature; thus its effect is studied, ranging from 700 to 1000 C, on the PEC performance, morphology, and composition of thin (10 nm) Ta 3 N 5 films deposited on planar and nanostructured Si substrates. Scanning electron microscopy (SEM), scanning Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) are employed to gain fundamental understanding in the differences of the Ta 3 N 5 films. By controlling Ta 3 N 5 morphology and composition with nitridation temperature, it is determined that Ta 3 N 5 with high crystallinity and surface N/Ta ratio, synthesized at 800 C, yields the highest PEC performance with the earliest photocurrent onset and highest photocurrent. Samples nitrided at 700 C have lower crystallinity and that likely leads to lower performance. For samples nitrided at temperatures above 800 C, the N/Ta ratio decreases forming chemically reduced tantalum nitride phases, as well as N-deficient and correspondingly O-rich morphological domains that can adversely affect the PEC performance as hole-blocking layers or O trap-mediated recombination centers at the surface of Ta 3 N 5 .
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.
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