Stability of van der Waals FePX₃ materials (X: S, Se) for water-splitting applications

Abstract

The interaction of high-quality transition metal trichalcogenides (TMTs) single crystals FePX3 (X: S, Se) with water molecules is studied using near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) in a wide range of temperature and partial pressure of H2O. The physisorption nature of interaction between H2O and FePX3 is found at low temperatures and relatively small concentrations of water molecules, that is supported by the DFT results. When temperature of the FePX3 samples and partial pressure of H2O are increased, the interaction at the interface is defined by two competing processes -- adsorption of molecules at high partial pressure of H2O and desorption of molecules due to the increased surface mobility and physisorption nature of interaction. Our intensive XPS/NEXAFS experiments accompanied by DFT calculations bring new understanding on the interaction of H2O with surface of a new class of 2D materials, TMTs, pointing to their stability and reactivity, that is important for further applications in different areas, like sensing and catalysis.

Dual Character of the Insulating State in the van der Waals Fe1–xNixPS3 Alloyed Compounds

“…The 3D MPS 3 bulk crystals consist of single 2D layers separated by a relatively large van der Waals gaps between adjusting layers (Figure S1a,b), which are d 0 = 3.32 Å and d 0 = 3.17 Å as calculated by DFT for FePS 3 and NiPS 3 , respectively, which allows modeling the Fe 1– x Ni x PS 3 crystals through the 2D monolayers. Although in this case the calculated band gaps will be slightly larger than the values calculated for the bulk phases (due to the quantum confinement effects), the nature of the energy bands in the vicinity of the Fermi energy are expected to be the same independent of the system dimensionality. ,− …”

Probing Active Sites on Pristine and Defective MnPX3 (X: S and Se) Monolayers for Electrocatalytic Water Splitting

“…Overall, ΔE def are in the same range as those of other MPX 3 . 28,36,37 For the same defect type, the Se vacancy is more likely to occur compared to the S vacancy, which correlates with the respective electronegativity values. For the same X, the feasibility of defect formation is V X @1L < V X2b @2L < V X2a @2L < V X2 @1L.…”

“…The band gaps obtained in this work are E g = 2.47 and 1.81 eV for MnPS 3 (Figure a) and MnPSe 3 (Figure b), respectively, and they are in agreement with previously published results. , At that, the top of the valence band is mainly formed by S/Se p-states with a significant contribution form the Mn d-states, whereas the bottom of the conduction band is contributed by Mn, P, and S/Se states (Figure a,b). Thus, both systems under study can be assigned to the class of Mott–Hubbard-type insulators, similarly to FePX 3 . , …”

“…Thus, both systems under study can be assigned to the class of Mott−Hubbard-type insulators, similarly to FePX 3 . 28,30 The mechanical exfoliation used for monolayer production can lead to the formation of chalcogen vacancies. Therefore, we consider four different kinds of defects: one vacancy at the X-site, named as V X @1L (Figure 1c) with a defect concentration of about 2.5%, two vacancies at the neighboring X-sites of the same chalcogen sublayer named as V X2 @1L (Figure 1d) with a concentration of about 5%, and two vacancies at the X-sites of the different chalcogen sublayers named as V X2a @2L (Figure 1e) and V X2b @2L (Figure 1f) with a concentration of about 5%.…”

“…An additional motivation to employ 2D materials in electrocatalytic applications is that they are typically inexpensive. Following many experimental and theoretical works on 2D materials and the respective interest to renewable energy sources, in the past decade, significant attention is given to van der Waals layered materials known as transition metal trichalcogenides (MPX 3 with M: transition metal and X: S or Se). − From the crystallographic point of view, these materials are similar to transition metal dichalcogenides, MX 2 , where one-third of the metal cations with connected X-anions is substituted by an ethane-like [P 2 X 6 ] 4– unit (Figure a,b). The diversity in transition metals and two different chalcogens can lead to a wide variation in the electronic and magnetic properties of these materials, − which can be further tailored by applying stress, an external voltage gate, , or by alloying. , Due to the layered structure of MPX 3 , they can be easily prepared as two-dimensional nanostructures , with a large surface area and high number of active sites. ,− Consequently, the MPX 3 materials have been the subject of several application-related investigations, ,,,, and FePSe 3 and MnPSe 3 yielded the highest efficiency for HER and very good stability in a wide pH range.…”