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

We introduce here a multifunctional material composed of alternating atomic sulfide sheets close to the composition of CuFeS₂ and Mg-based hydroxide ones (valleriite), which are assembled due to their electric charges of opposite sign. Valleriite particles 50–200 nm in lateral size and 10–20 nm in thickness 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, and magnetization, dynamic light scattering, and zeta potential measurements. The electronic, magnetic and optical characteristics are found to be critically dependent on the charge (electron density) at the narrow-gap sulfide layers containing Cu⁺ and Fe³⁺ cations, and can be tuned via the composition of the 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 into 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 valleriites as a new rich family of 2D materials for a variety of potential applications.