Synthesis and characterization of nanoscale composite particles formed by 2D layers of Cu-Fe sulfide and Mg-based hydroxide

Abstract

We introduce here a multifunctional material composed of alternating atomic sulfide sheets close to CuFeS2 and Mg-based hydroxide ones (valleriite), which are assembled due to their electric charges of opposite sign. Valleriite particles of 50-200 nm in the lateral size and 10-20 nm thick were synthesized via a simple hydrothermal pathway using various concentrations of precursors and dopants, and examined with XRD, TEM, EDS, X-ray photoelectron spectroscopy, reflection electron energy loss spectroscopy (REELS), Mössbauer, Raman and UV-vis-NIR spectroscopies, magnetic, dynamic light scattering, zeta potential measurements. The electronic, magnetic and optical characteristics are found to be critically dependent of the charge (electron density) at the narrow-gap sulfide layers containing Cu+ and Fe3+ cations, and can be tuned via the composition of hydroxide part. Particularly, substitution of Mg2+ with Al3+ increases the negative charge of the hydroxide layers and reduces the content of Fe3+-OH centers (10-45% of total iron); the effects of Cr and Co dopants entering both layers are more complicated. Mössbauer doublets of paramagnetic Fe3+ detected at room temperature transform to several Zeeman sextets at 4.2 K; the hyperfine fields up to 500 kOe and complex magnetic behavior, but not pure paramagnetism or antiferromagnetism, were observed for valleriites with the higher positive charge of the sulfide sheets, probably due to the depopulation of the minority-spin 3d states of S-bonded Fe3+ ions. Aqueous colloids of valleriite show optical absorption at 500 - 750 nm, which, along with the peaks at the same energies in REELS, may arise due to quasi-static dielectric resonance involving the vacant Fe 3d band and being dependent on the composition of both layers too. These and other findings call attention to the of valleriites as a new rich family of 2D materials for a variety of potential applications.

Valleriite-containing ore from Kingash deposit (Siberia, Russia): Mössbauer and X-ray photoelectron spectroscopy characterization, thermal and interfacial properties

“…The differences observed between the valleriite samples depended on their origin in case of natural valleriites and the proportions of precursors used in the synthesis. It was established, among other things, that Mössbauer signals of paramagnetic Fe 3+ almost disappeared and the internal hyperfine magnetic fields arose due to magnetic ordering at cryogenic temperatures; no clear signatures of ferrous iron were found in both layers [33,34], in contrast to previous studies [12,[35][36][37].…”

“…Chistyakova et al [35] have detected such small feature for synthetic valleriite-containing products and assigned it to di-and trivalent iron in the hydroxide layers but we didn't observe this doublet in pure valleriites [34]. The second doublet with δ of 0.32 mm/s and  = 0.5 mm/s is attributed to Fe 3+ cations in tetrahedral coordination with sulfide anions in valleriite [12,[33][34][35][36][37] as it is close to the species in synthetic valleriite (Table 2) and clearly differs from Fe 3+ tetrahedrally bounded to oxygen in serpentines, for which δ of about 0.16 mm/s and  of 0.25-0.42 mm/s have been reported [39][40][41][42]. A smaller paramagnetic doublet with the isomer shift δ  0.4 mm/s and quadrupole splitting of 0.96 mm/s observed both in synthetic and natural valleriites [12,[33][34][35][36][37] can be assigned to octahedral Fe 3+ -OH sites in the hydroxide layers and partially in serpentines [39][40][41][42].…”

“…57 Fe Mössbauer spectroscopy is a powerful element-specific method that allows discriminating the chemical states of Fe atoms and magnetic ordering in various phases [12,[33][34][35][36][37][42][43][44]. Typical Mössbauer spectrum of the valleriite-bearing samples (Figure 4; Table 2 represents fitting parameters) consists of two intense Zeeman sextets with the ratio close to 1:2 and hyperfine parameters well agreed with the ones of magnetite Fe3O4 [42][43][44].…”

“…The second doublet with δ of 0.32 mm/s and  = 0.5 mm/s is attributed to Fe 3+ cations in tetrahedral coordination with sulfide anions in valleriite [12,[33][34][35][36][37] as it is close to the species in synthetic valleriite (Table 2) and clearly differs from Fe 3+ tetrahedrally bounded to oxygen in serpentines, for which δ of about 0.16 mm/s and  of 0.25-0.42 mm/s have been reported [39][40][41][42]. A smaller paramagnetic doublet with the isomer shift δ  0.4 mm/s and quadrupole splitting of 0.96 mm/s observed both in synthetic and natural valleriites [12,[33][34][35][36][37] can be assigned to octahedral Fe 3+ -OH sites in the hydroxide layers and partially in serpentines [39][40][41][42]. The excess of Fe found in spots 1 and 2 (Figure 2) suggests that up to 30-40% of the iron in valleriite occurs in the hydroxide layers.…”

“…Recently, we explored two valleriite samples from Noril'sk ores [32,33] associated either with pyrrhotite or serpentine and chalcopyrite using X-ray absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), Mössbauer spectroscopy and other techniques. Also, we managed to perform a successful hydrothermal synthesis of single-phase valleriite nanoflakes [34].…”