New Compounds and Phase Selection of Nickel Sulfides via Oxidation State Control in Molten Hydroxides

Abstract

Molten salts are promising reaction media candidates for the discovery of novel materials; however, they offer little control over oxidation state compared to aqueous solutions. Here, we demonstrated that when two hydroxides are mixed, their melts become fluxes with tunable solubility, which are surprisingly powerful solvents for ternary chalcogenides and offer effective paths for crystal growth to new compounds. We report that precise control of the oxidation state of Ni is achievable in mixed molten LiOH/KOH to grow single crystals of all known ternary K–Ni–S compounds. It is also possible to access several new phases, including a new polytope of β-K2Ni3S4, as well as low-valence KNi4S2 and K4Ni9S11. KNi4S2 is a two-dimensional low-valence nickel-rich sulfide, and β-K2Ni3S4 has a hexagonal lattice. Moreover, using KNi4S2 as a template, we obtained a new layered binary Ni2S by topotactic deintercalation of K. The new binary Ni2S has a van der Waals gap and can function as a new host layer for intercalation chemistry, as demonstrated by the intercalation of LiOH between its layers. The oxidation states of low-valence KNi4S2 and Ni2S were studied using X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Density functional theory calculations showed large antibonding interactions at the Fermi level for both KNi4S2 and Ni2S, corresponding to the flat-bands with large Ni-d x 2–y 2 character.

Sr(Ag1−xLix)2Se2 and [Sr3Se2][(Ag1−xLix)2Se2] Tunable Direct Band Gap Semiconductors

“…[22] For example, the tetrahedral AgÀ Se and LiÀ Se bond lengths are approximately 2.771(1) Å [23] and 2.6002 Å [24] in Ba 2 Ag 2 Se 2 (Se 2 ) and Li 2 Se, respectively. In comparison, the average bond lengths between the Ag L À 1 i and Se sites for Sr(Ag 1À x Li x ) 2 Se 2 were 2.6937(9) Å and 2.7401(6) Å for x = 0.438(6) and 0.077 (6), respectively. The AgÀ Se bond length for SrAg 2 Se 2 was 2.7329(4) Å at 100 K. Although the AgÀ Se bond lengths were the same within the error for each Sr(Ag 1À x Li x ) 2 Se 2 , the tetrahedral angles were different, with θ 1 smaller than θ 2 (Figure 2d).…”

“…It is likely that Ag + can be reduced to Ag 0 by Se 2À when the basicity is high enough, similar to our observations of reactions between Ni 2 + and S 2À in mixed KOH/LiOH, in which high basicity results in metallic Ni. [6] These findings described above suggest that reactions in LiOH/LiCl fluxes exhibit solution-based reaction characteristics, such as pH-dependent oxidation states in aqueous solutions. [17] We conducted differential thermal analysis (DTA) to study the stability of Sr(Ag 1À x Li x ) 2 Se 2 and explain why direct combination reactions were unsuccessful.…”

“…Our recent work on ternary K−Ni−S using mixed KOH/LiOH fluxes demonstrates mixed hydroxide/halide fluxes as promising candidates for exploratory chalcogenide synthesis. The mixed flux can stabilize all known ternaries of K−Ni−S, α‐K 2 Ni 3 S 4 , and KNi 2 S 2 and novel metastable phases such as KNi 4 S 2 , K 4 Ni 9 S 11 and β‐K 2 Ni 3 S 4 [6] . In addition, we reported that phase selection is controlled by only two parameters: basicity (solubility) and sulfur concentration, regardless of the ratios of the other starting materials, allowing precise and facile product selection [6] .…”

“…The mixed flux can stabilize all known ternaries of K−Ni−S, α‐K 2 Ni 3 S 4 , and KNi 2 S 2 and novel metastable phases such as KNi 4 S 2 , K 4 Ni 9 S 11 and β‐K 2 Ni 3 S 4 [6] . In addition, we reported that phase selection is controlled by only two parameters: basicity (solubility) and sulfur concentration, regardless of the ratios of the other starting materials, allowing precise and facile product selection [6] . The flux system also enables the use of versatile and easily accessible precursors such as NiO, Ni(OH) 2 , or NiCl 2 and elemental sulfur as Ni and S sources, respectively.…”

Amorphous–crystalline FeNi2S4@NiFe–LDH nanograsses with molten salt as an industrially promising electrocatalyst for oxygen evolution

“…17,18 In the recent literature, the molten salt method (MSM) is a relatively simple and economical method, in which molten salt (MS) is considered as a promising reaction medium candidate for preparing various inorganic materials. [19][20][21] Recently, various highcrystallinity TMS nanosheets were produced using the LiCl/KCl molten salt; however, the MS with high melting point leads to poor electrochemical performance and limits the large-scale industrial application of the catalyst because of its high energy consumption. 22 Compared with other high melting point salts, KSCN, with a low melting point of 175 °C, can exist stably in air.…”