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Abdoulaye Djire1 Hanyu Zhang1 Elisa Miller-Link1 Nathan Neale1

1, National Renewable Energy Laboratory, Denver, Colorado, United States

Research highlighted in this abstract aims to provide a clean, cost-effective, and locally produced solar fuel by using electroactive, two-dimensional (2D) layered transition metal nitrides (MXenes) photoelectrodes. Theoretical reports have shown that these materials, although inherently metallic, can become semiconductors via surface functionalization, and can be used as photoelectrodes for solar fuel production. Results highlighted in this presentation revealed the physical and photoelectrochemical properties of Ti4N3Tx and Ti2NTx nitrides with surface functional groups (Tx = O, OH, F). The materials showed excellent photoelectrochemical properties (i.e. solar-to-hydrogen efficiency) and good stability in aqueous media. Ti4N3Tx and Ti2NTx nitrides were synthesized via molten salt fluoride treatment of their corresponding Ti4AlN3 and Ti2AlN precursors. The phase purity, crystal structure, and removal of Al from the precursors were confirmed by X-ray diffraction and X-ray photoelectron analyses. Scanning electron microscopy and high-resolution transmission electron microscopy were used to access the morphologies of the layers (multilayers and single layers). Both materials showed an order of magnitude increase in surface area, and more importantly, exhibited semiconductor behavior as indicated by the absorption and emission data. Ti4N3Tx exhibited two bang gaps located at ~1.9 eV and ~3.1 eV. These were attributed to different layer thicknesses. We hypothesized that the higher energy band gap derived from single or few layers, while the lower band gap is due to multiple layers. This hypothesis was further validated with Ti2NTx material. When delaminated with dimethyl sulfoxide for half and hour, the energy band gap in Ti2NTx was increased from ~3.1 eV to ~3.6 eV as corroborated by a blue shift in the absorption spectrum to lower wavelength. The change in the properties of Ti4N3Tx and Ti2NTx from metal to semiconductor was attributed to surface functional groups (Tx = O, OH, F) as supported by the Fourier-transform infrared spectroscopy results.

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