Synchrotron radiation photoemission spectroscopy of the oxygen modified CrCl₃ surface

Abstract

We investigate the experimentally challenging CrCl3 surface by photon energy dependent photoemission. The core and valence electrons after cleavage of the single crystal, either in ultra high vacuum (UHV) or...

Two-Dimensional CrCl3-Layered Trihalide Nanoflake Sensor for the Detection of Humidity, NO2, and H2

“…Like zeolites and metal–organic frameworks (MOFs), monolayered TMTHs such as CrCl 3 can be classified as 2D microporous materials ( D pore < 2 nm) because of the presence of intrinsic atomic pores in a size range of 1.53–1.94 Å, as indicated by the dotted-black squares in Figure a . These empty cages, located at the center of the honeycomb lattice formed by Cr atoms (Figure a), similar to the model of Li adsorption over CrI 3 , can be spontaneously filled by ambient oxygen (red spheres in Figure a), resulting in the formation of an ordered semioxidized O–CrCl 3 surface phase, with oxygen impurities intercalated in the Cr atomic layer under ambient conditions, as recently demonstrated by theoretical and experimental observations. ,, Moreover, the presence of Cl vacancies in 2D-CrCl 3 flakes must be also considered, as indicated by the dotted-black circles in Figure a, which are energetically favorable, as proven in refs , . Consequently, the presence of the O–CrCl 3 phase and Cl vacancies induces a partial electron charge transfer from O and Cl atoms to Cr, ,, eventually encouraging the electrostatic interactions of ambient molecules over preferential atomic polar sites.…”

“…In-house-grown CrCl 3 crystals were used instead of polycrystalline commercial CrCl 3 powders to avoid any effect caused by the impurities and limited crystallinities of powders. The high quality of the self-transport technique-grown CrCl 3 crystals was reported in our previous studies. − LPE was performed using NMP (one of the most widely used LPE solvent), IPA (a nontoxic solvent with a low boiling point), and 30 vol % H 2 O/urea (a possible ecofriendly alternative for LPE). The UV–vis spectra of LPE CrCl 3 exfoliated in different solvents (Supporting Information Figure S1) exhibit the same absorption bands, with peaks centered at approximately 530 and 729 nm, proving that there are no substantial absorption discrepancies between CrCl 3 flakes exfoliated using NMP, IPA, and 30 vol % H 2 O/urea.…”

“…The long-term stability of the electrical properties of the 2D-CrCl 3 sensor under humidity was studied by collecting random data measured over 36 weeks at OTs of 25 °C (Figure d) and 100 °C (Figure a), as well as the response to 1 ppm of NO 2 , 250 ppm of H 2 , and NH 3 in 40% RH air at 100 °C (Figure c), which shows excellent RRs and no appreciable variations during the investigated period. The chemical stability of the flake surface eventually drives the molecular adsorption mechanism with interfering species and the electrical response of the sensor in the long run; therefore, the morphology and chemistry of LPE CrCl 3 flakes were deeply investigated in our previous papers, − which reveal that the formed semioxidized O–CrCl 3 surface is stable up to 400 °C, and Cl vacancies, mostly generated via the exfoliation procedure, are stable up to 200 °C, possibly excluding any significant chemical deterioration of the CrCl 3 surface at an OT of 100 °C after long-term exposures.…”

“…Among the various vdW-layered semiconductors, transition-metal trihalides (TMTHs; MX 3 , where M = Ti, V, Cr, Mo, Fe, Ru, and X = Cl, Br, or I) have recently attracted scientific attention because of their layered nature and consequent potential as ultrathin magnetic layers in spintronic devices, batteries, improved catalysts, and molecular sieves with high surface-to-volume ratios. − While few-layer MX 3 TMTHs are used as cleavable 2D ferromagnetic semiconductors, their humidity- and gas-sensing responses are still unknown, probably because of the assumption that their environmental instability in dry/wet air prevents their utilization as reproducible gas sensor interfaces. , Indeed, most TMTHs, such as VI 3 and CrI 3 , immediately degrade or evaporate after being exposed to an oxidizing atmosphere or light, even in their bulk phase. , Conversely, CrCl 3 is only relatively stable under ambient laboratory conditions even after its isolation in reduced dimensionality via mechanical exfoliation, as reported in refs , Specifically, few-layer CrCl 3 does not undergo dramatic oxidation or evaporation up to 200 °C. , This is because CrCl 3 has a semioxidized stable O–CrCl 3 surface phase that is stable up to 400 °C with charge imbalance, and this phase protects the inner pristine material. This means that the surface of CrCl 3 has native polar atomic sites that prefer the adsorption of molecules, which is very helpful for gas-sensing applications where the detection mechanism is mainly based on charge transfer.…”

“…29,30 Conversely, CrCl 3 is only relatively stable under ambient laboratory conditions even after its isolation in reduced dimensionality via mechanical exfoliation, as reported in refs 31,32 Specifically, few-layer CrCl 3 does not undergo dramatic oxidation or evaporation up to 200 °C. 31,32 This is because CrCl 3 has a semioxidized stable O−CrCl 3 surface phase that is stable up to 400 °C with charge imbalance, 33 and this phase protects the inner pristine material. This means that the surface of CrCl 3 has native polar atomic sites that prefer the adsorption of molecules, which is very helpful for gas-sensing applications where the detection mechanism is mainly based on charge transfer.…”