Local Ordering of Molten Salts at NiO Crystal Interfaces Promotes High‐Index Faceting

Abstract Given the strong influence of surface structure on the reactivity of heterogeneous catalysts, understanding the mechanisms that control crystal morphology is an important component of designing catalytic materials with targeted shape and functionality. Herein, we employ density functional theory to examine the impact of growth media on NiO crystal faceting in line with experimental findings, showing that molten‐salt synthesis in alkali chlorides (KCl, LiCl, and NaCl) imposes shape selectivity on NiO particles. We find that the production of NiO octahedra is attributed to the dissociative adsorption of H2O, whereas the formation of trapezohedral particles is associated with the control of the growth kinetics exerted by ordered salt structures on high‐index facets. To our knowledge, this is the first observation that growth inhibition of metal‐oxide facets occurs by a localized ordering of molten salts at the crystal–solvent interface. These findings provide new molecular‐level insight on kinetics and thermodynamics of molten‐salt synthesis as a predictive route to shape‐engineer metal‐oxide crystals.

Abstract: Given the strong influence of surface structure on the reactivity of heterogeneous catalysts,u nderstanding the mechanisms that control crystal morphology is an important component of designing catalytic materials with targeted shape and functionality.Herein, we employdensity functional theory to examine the impact of growth media on NiO crystal faceting in line with experimental findings,s howing that molten-salt synthesis in alkali chlorides (KCl, LiCl, and NaCl) imposes shape selectivity on NiO particles.W efind that the production of NiO octahedra is attributed to the dissociative adsorption of H 2 O, whereas the formation of trapezohedral particles is associated with the control of the growth kinetics exerted by ordered salt structures on high-index facets.T oour knowledge, this is the first observation that growth inhibition of metaloxide facets occurs by alocalized orderingofmolten salts at the crystal-solvent interface.T hese findings provide new molecular-level insight on kinetics and thermodynamics of moltensalt synthesis as ap redictive route to shape-engineer metaloxide crystals.
The catalytic activity of heterogeneous catalysts is strongly correlated to the composition and surface structure of the catalyst materials. [1][2][3][4][5][6][7] Several experimental and theoretical studies have reported that, for certain types of structuresensitive reactions,c atalytic activity is enhanced by undercoordinated active sites presented on high-index (HI) crystal facets; [4] therefore,t he synthesis of catalysts exposing HI facets is particularly appealing for heterogeneous catalysis. Indeed, such materials could improve the efficiencya nd economics of catalytic processes.W hile several methodologies for the synthesis of materials exposing HI facets have been reported in the literature, [8][9][10] the fundamental understanding of the phenomena that control the final catalyst morphologies remains elusive.F or metals,common strategies to control the catalyst final shape include the use of crystal growth modifiers,s uch as ambidentate solvents, [10] and electrochemical square-wave-potentials. [11,12] Thef inal shape of the materials is sometimes associated with alower surface free energy (g {hkl} )o ft he desired {hkl}f acets,i nduced by specific agents present in the growth environment. Alternatively,the particle morphology can be rationalized by evoking kinetic phenomena of crystal growth, [13] often related to capping agents that selectively reduce the growth rate of particular crystal facets. [14][15][16] Form etal oxides,f ew synthesis methods yielding HI particles have been reported. [17][18][19][20][21] One particularly effective strategy for the synthesis of HI metal oxides involves the use of non-aqueous media, such as ionic liquids or by molten salt synthesis (MSS). [22] High-index nickel(II) oxide (NiO) particles are particularly interesting in heterogeneous catalysis,a sNiO is al owcost industrial catalyst for oxidation reactions. [19,[23][24][25][26] Susman et al. [25] recently synthesized NiO particles by MSS in various salt media and observed that the final particle shape is strongly dependent on the growth environment. Forsyntheses in different alkali nitrates,t hey observed the production of cubes,o ctahedra, or cuboctahedra;h owever,aswitch to media containing alkali chlorides directed the formation of HI facets.T he synthesis comprised the calcination of Ni-(NO 3 ) 2 ·6 H 2 Oinanalkali salt at aheating rate of 2.5 8 8Cmin À1 to am aximum temperature of 550 8 8C. Thes amples were analyzed by scanning electron microscopy (SEM) after purification, showing significant differences in crystal shape. TheM SS in KCl produced trapezohedra exposing NiO{311} HI facets;LiCl yielded {h11} facets where h ' 5-6;and NaCl produced NiO particles with undefined shape.T he thermal decomposition of Ni(NO 3 ) 2 ·6 H 2 Ointhe absence of salts was also studied. In air,NiO octahedra exposing {111} facets were formed, with complete NiO formation observed at ca. 350 8 8C. Similar experiments under aH 2 O-free air flow,h owever, resulted in an irregular particle morphology.O nt his basis,i t was postulated that the formation of octahedra is associated with the presence of H 2 Ointhe growth environment. Susman et al. [25] also studied the NiO synthesis from NiCl 2 in presence of KCl (and atmospheric air) to investigate if nitrates are necessary for {311} faceting. Their findings revealed that trapezohedra can crystallize in absence of nitrates,indicating that Cl À and K + ions are necessary for the formation of {311} HI facets.A tt he highest MSS calcination temperature (550 8 8C), nitrate mixtures are fully molten given their lower melting points (MPs), which are 255, 307, and 333 8 8Cf or LiNO 3 ,N aNO 3 ,a nd KNO 3 ,r espectively.I nc ontrast, alkali chlorides are not fully molten as the MPs of LiCl, NaCl, and KCl (613, 801, and 771 8 8C, respectively) are well above the synthesis temperature;however,the presence Ni 2+ and NO 3 À species during crystallization can induce local eutectic mixtures with MP < 550 8 8Cwhere the liquid is in equilibrium with the non-melted salts.
Herein, we employ density functional theory (DFT) within the generalized gradient approximation and Hubbard Uc orrections (GGA + U) [27][28][29] to study the interfaces between NiO and the different growth environments of MSS experiments with the aim of unraveling the chemistry behind the NiO shape selectivity achieved in different alkali chlorides.W eu se Quantum Espresso [30] to perform the DFT calculations,w ith the Environ library [31,32] for the implicit solvation models,a nd Grimme-D3 [33,34] dispersion corrections.W ef ocus on KCl and LiCl salts,w hich give rise to HI NiO particles,a nd NaCl, which produces irregular NiO shapes.F or each alkali chloride,w eo ptimize as olvation model that reproduces the experimental formation enthalpies of the salt in the solid and liquid phase.T he crystal facets selected for the analysis resemble those observed experimentally,i .e., NiO{100}, {111}, {311}, and {511}. Thel atter is investigated in place of NiO{611} because it has similar morphology,b ut has higher symmetry.I no ur analysis,w e investigate two scenarios in which HI facets are produced in the presence of capping agents:(1) via thermodynamic-driven changes in particle shape,and (2) via kinetic hindering of the growth of specific crystal facets.The first scenario involves the attainment of the equilibrium shape wherein the adsorption of species available in the growth environment reduce the g {hkl} of certain {hkl}f acets.T his can result in different stable shapes at different experimental conditions,w hich can be represented using Wulff constructions. [35] Thesecond scenario consists of controlling the growth kinetics by capping agents that selectively bind to {hkl}s urfaces and slow down (or enhance) the anisotropic rates of crystal growth. Thef inal habit is comprised of facets characterized by the slowest growth rates (in the directions perpendicular to these facets) without reaching thermodynamic equilibrium.
We first investigated the thermodynamically stable structures of NiO in equilibrium with an oxidizing environment in the absence of any salt or water. Using ab initio thermodynamics, [36] we calculated the g {hkl} of NiO crystal facets as af unction of the chemical potential of O 2 in the gas phase and, by considering an O 2 partial pressure of 0.21 atm, as afunction of temperature (Figure 1a). Our calculations show that astoichiometric NiO{100} termination is preferred in the entire temperature range tested. In agreement with previous theoretical and experimental studies, [37][38][39] we find that NiO-{111} undergoes surface reconstruction, and it can potentially form two stoichiometric structures;h owever,t he oxygenterminated interface (O) is more stable than the Ni-terminated one (Ni). Thefacets NiO{311} and NiO{511} show over-stoichiometric oxygen surface terminations (indicated as NiO{hkl} + O*) at temperatures below ca. 150 and 350 8 8C, respectively.A th igher temperatures,t hey show stable stoichiometric surface terminations.T he calculated most stable structures are presented in Figure S4. In the full range of temperatures under O 2 ,N iO{100} is the facet with the lowest g {hkl} ,consistent with the Wulff construction prediction of ac ube morphology in the absence of adsorbates.T his agrees with gas-phase syntheses of NiO,which result in cubic particles at high temperatures where adsorption of molecules from the environment is typically disfavored. [40] TheN iO facet stability in the presence of H 2 Oi s investigated by analyzing hydroxylated surface structures, produced by H 2 Od issociation, which results in OH* species binding to Ni cations,a nd H* ions binding to Oa nions (and forming additional OH* species). In Figure 1b, g {hkl} values are calculated in the presence of adsorbed OH* species as afunction of temperature,considering aH 2 Opartial pressure of 0.04 atm assuming saturated atmospheric (lab) air. The dissociative adsorption of H 2 OonNiO{100} is not favored at any temperature.Conversely,H 2 Od issociation is favored on NiO{111}, {311}, and {511} facets at temperatures lower than ca. 750, 890, and 900 8 8C, respectively.B elow these temperatures,O H* adsorbates significantly reduce g {hkl} of corresponding facets due to their high surface concentration. These results are in good agreement with previous calculations of Marks and co-workers [37] who investigated the stability of OH* species on NiO{111} at different coverages with ameta-GGA hybrid functional. The g {hkl} values were evaluated at conditions closer to experiment, [25] i.e., ca. 350 8 8C, where octahedral NiO{111} faceting was observed (Figure 1b). At this temperature,hydroxylated NiO{111}, {311}, and {511} are more stable than NiO{100}. In particular, g NiO{111} is significantly lower than the others considered, and the Wulff construction results in an octahedron;t herefore,t he formation of octahedra in air can be explained by the decrease of g {hkl} induced by the presence of adsorbed OH* species on NiO{111}. Theo ptimized NiO surface structures at these conditions are represented in Figure 1c.
Asimilar approach was used to study the structure of lowand high-index NiO facets in the presence of alkali chlorides. In these systems,the cations (K + ,Na + or Li + )and anion (Cl À ) can adsorb on the NiO facets and modify their energies and growth rates.I naddition to studying the adsorption (and coadsorption) of salt ions at different coverages,weinvestigated the formation of solid salt structures at the salt-NiO interface, showing different {hkl}s urfaces in contact with the NiO facets.T he formation of such solid-like structures at the saltparticle interface is aprobable scenario,asthe salts can be in solid-liquid equilibrium at the experimental conditions. [25] Moreover,t his phenomenon has already been observed in some theoretical studies on the specific heat of molten salts containing metal oxide nanoparticles. [41,42] At this stage,w e did not consider the presence of H 2 Oornitrates in the system, as experiments revealed that trapezohedra are produced under dry air and in the absence of nitrate ions. [25] Forcharged adsorbates,t he charge-neutrality is maintained in the DFT calculations by adding counterions,w hich are stabilized by the solvation models (details in the Supporting Information).
Prior experiments reported the crystallization of NiO in the presence of alkali chlorides at 550 8 8C. [25] Here,the results of ab initio thermodynamics do not explain the formation of HI facets as their g {hkl} are always higher than that of NiO{100}, either in the presence of KCl, LiCl, or NaCl ( Figure S5). Interestingly,t he alkali chloride structures that form on the NiO surfaces have the potential to operate as capping agents and tailor the shape selectivity of NiO particles by controlling their growth rates-an effect that is associated with the Gibbs binding energy (G b )ofthe capping agents. [14][15][16] In Figure 2a,f or each NiO stable clean surface termination (Figure 1a), the G b values of the structures representing the adsorption of K + or Cl À ions,t he coadsorption of K + and Cl À ,a nd the formation of KCl solid structures in contact with the NiO facet are reported. Forall the structures investigated, the same reservoir of K + and Cl À ions (i.e., the solid alkali salts at the experimental conditions) is considered in the calculation of G b .F or each condition, we show the most stable structures for each potential NiO surface termination (Figure 2a,t op row). Thea dsorption of K + is favored on NiO{100} and NiO{311} + O*, whereas on NiO-{111} and NiO{511} terminations the co-adsorption of K + and Cl À is preferred. NiO{311} is the only facet on which local ordering of crystalline KCl is favored. Ther esulting solidsolid structure at the interface between KCl and NiO{311} shows av ery strong G b ,t hus densely ordered KCl at the crystal interface can act as ac apping agent hindering NiO growth in the [311] direction. Moreover,w hen we compare the G b on different NiO terminations,w ed on ot find any apparent correlation between the adsorption of K + or Cl À ions and the exposure of NiO{311} facets in the final particles. Instead, the G b between crystalline KCl structures and NiO facets indicated am uch stronger interaction with NiO{311} compared to other facets.Asaresult, we associate the shape selectivity of KCl (Figure 2b)t othe localized ordering of crystalline KCl at the NiO{311} surface,forming avery stable solid-solid interface,and hindering the growth of that specific facet. It should be noted that this solid-solid KCl-NiO interface can still be dynamic,a su pon NiO crystallization there is ar elease of KCl from the conversion of the KNiCl 3 intermediate, [25] which provides further mobility to the KCl phase to adapt to the evolving NiO surface.
Similar G b values were calculated for the LiCl system (Figure 3a)w here it was observed that Li + adsorption is preferred on NiO{311} + O*, Li + and Cl À co-adsorption is favored on all remaining facets except for NiO{511}, which favors the crystallization of LiCl. When the adsorption of the salt ions are analyzed, there is no appreciable correlation between G b and the experimental particle shape;however, for the LiCl crystal-NiO interface,t he adsorption is instead stronger for NiO{511} compared to the other facets.A s ar esult, we associate the shape selectivity of LiCl to the crystallization of LiCl on NiO{511} facets.Amodel particle exposing {511} facets as well as an SEM image of particles obtained experimentally are represented in Figure 3b.
Fort he case of NiO synthesis in NaCl, which yields particles with undefined shape, [25] we calculated that the formation of locally ordered structures is not favored on any of the investigated {hkl}f acets.T his confirms that the local ordering of the alkali chloride is needed for the formation of HI NiO particles.The results for NaCl media are presented in Figure S6.
Aschematic representation of the proposed mechanism of alkali chloride induced shape selectivity of NiO particles is shown in Figure 4, using the KCl system as an example.O n NiO{100} and {111} facets,the liquid-like K + and Cl À media do  not fully cover the surfaces;and the dynamic adsorption and desorption of ions allows for growth units to reach NiO surfaces and promote crystal growth along the [100] and [111] directions.However,onNiO{311} surfaces,anordered solidlike KCl structure preferentially forms at the crystal-salt interface owing to its high G b .This local ordered salt structure significantly reduces the probability that growth units would reach the surface,h indering NiO crystal growth in the [311] direction. As ar esult, the final shape of the particle will display {311} facets,which are protected by the adsorbed KCl ordered structure,even if its g {hkl} is higher than the low-index facets.
In summary,w ei nvestigated the use of MSS media to tailor NiO crystal shape under different experimental conditions involving alkali chloride salts that produce HI facets. Tw od ifferent scenarios were investigated:t he change of thermodynamically stable particle shape induced by the presence of g {hkl} modifiers,a nd the control of the crystal growth provoked by the adsorption of capping agents.W e showed that the formation of octahedral particles is driven by the adsorption of water molecules that form OH* groups and stabilize the NiO{111} facet. To interpret NiO faceting induced by the judicious selection of alkali chlorides,w e analyzed the adsorption of alkali metal salt ions and counterion (K + ,Li + ,Na + ,and Cl À )aswell as the induced ordering of ions in local proximity to crystal surfaces,thus forming solidsolid interfaces between NiO HI facets and alkali salts.T his allowed identifying the role of the local ordering of the alkali chlorides at the NiO surfaces,w hich is ap lausible scenario considering the salts are in solid-liquid equilibrium at experimentally relevant temperatures.T he formation of NiO{311} HI facets during MSS in KCl is associated with the formation of an interface involving crystalline KCl that selectively adsorbs on {311} surfaces.T he formation of NiO{511} facets,o bserved using LiCl, is associated with the crystallization of the LiCl salt on {511} surfaces.Collectively, these results explain our reported experimental observations, thus providing as tarting point for the fundamental understanding of shape control in MSS of metal oxide particle crystallization. Habit modification by altered g {hkl} of crystal facets (or Wulff constructions) is aw idely accepted explanation of crystal shape engineering;h owever, few studies provide amolecular-level description of the mechanisms and phenomena responsible for altering the thermodynamics of crystal-solvent interfaces.Here,weshow for the first time that alocally ordered salt in amolten salt medium can behave as ac apping agent regulating the anisotropic growth rates of ametal oxide.The development of theoretical models,such as those presented here,h as great potential in heterogeneous catalysis,a sm odels can guide the synthesis of new materials with desired shapes and targeted functionalities.