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.

Facile synthesis and selected characteristics of two-dimensional material composed of iron sulfide and magnesium-based hydroxide layers (tochilinite)

“…The spectra are better fitted with additional doublets at 162.6 eV and near 163.5 eV, which arise from minor S-S bonding. The first, "disulfide" component is believed [50,[64][65][66][67][68] to originate from terminal atoms bearing higher negative charge in -S-S n -Sanions rather than central "polysulfide" atoms, and their relative intensities suggest that trisulfide anions form in the iron-deficient sulfidic layers. A wide maximum between 164 eV and 165 eV seems to be a satellite related to electron transfer onto empty Fe 3d states [50,[64][65][66][67][68].…”

“…Valleriite (CuFeS 2 )⋅(Mg,Al,Fe)(OH) 2 [19][20][21][22][23][24][25][26][27] and tochilinite mFeS⋅n(Mg,Fe)(OH) 2 [30][31][32][33][34][35][36] are the most known and important of them; particularly, valleriite ores of Noril'sk ore provenance (Russia) are of commercial importance but not involved in the exploitation because of the lack of processing technologies. Synthetic valleriite and tochilinite have been prepared and characterized too [37][38][39][40][41][42][43][44][45][46][47][48][49][50]; however, the yields of the target substances were lower than 50%, and those were substantially contaminated by the reaction by-products. Another group of layered iron chalcogenide-hydroxide materials with the composition Fe(Se,S)⋅(Li,Fe)OH and some their derivatives [51][52][53][54][55] synthesized in the recent years are attracting much attention because of the superconducting properties but little efforts have been made to study other characteristics and applications.…”

“…Further progress with the composites requires, first of all, a facile synthesis of pure materials. Recently [50], we proposed a hydrothermal synthesis of valleriite (CuFeS 2 )⋅(Mg,Al,Fe)(OH) 2 , examined the effect of dopants entered magnesium hydroxide (Al, Li) or both sulfide and hydroxide layers (Co, Ni, Cr), and studied the electronic structure, physical and chemical characteristics as a function of the composition. The aims of the current research were to develop a reliable preparation procedure and to elucidate specific features of the tochilinite-type materials.…”

“…The aims of the current research were to develop a reliable preparation procedure and to elucidate specific features of the tochilinite-type materials. We started from the protocol elaborated for valleriite [50], and modified it by varying the concentrations of precursors and dopants in order to reject the reaction by-products and to control the composition, electronic, magnetic and optical characteristics of tochilinite. In order to simplify the protocol, at this stage of research we didn't strictly deoxygenate the reaction media and sample handling environment so paramagnetic materials with high-spin Fe 3+ and Fe 2+ in the sulfide layers were obtained, which, nonetheless, exhibit a number of non-trivial promising properties.…”

“…Magnesium hydroxide or its mixture with aluminum hydroxide or/and lithium hydroxide were precipitated by adding 25% aqueous ammonia to the metal sulfate solutions. Then the dispersions were transferred to an in-home made stainless steel autoclave with Teflon liner [50]. Alternatively, all the reagents were simultaneously loaded into the autoclave, and pH was adjusted with aqueous ammonia to ∼11.…”