Toward Reaction-by-Design: Achieving Kinetic Control of Solid State

Perspective pubs.acs.org/cm

Toward Reaction-by-Design: Achieving Kinetic Control of Solid State Chemistry with Metathesis† Andrew J. Martinolich and James R. Neilson*

Chem. Mater. 2017.29:479-489. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/22/18. For personal use only.

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States ABSTRACT: The control of solid state reaction pathways will enable the design and discovery of new functional inorganic materials. A range of synthetic approaches have been used to shift solid state chemistry away from thermodynamic control, in which the most energetically favorable product forms, toward a regime of kinetic control, so that metastable materials can be controllably produced. In this Perspective, we focus on the kinetic control of solid state metathesis reactions to alter solid state reaction pathways and products. We provide insight into the necessary components of a kinetically controlled solid state reaction and illustrate the utility of studying reactions in situ in order to observe the various intermediates and kinetic pathways that may extend synthetic solid state chemistry toward a paradigm of reaction-by-design.

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KINETIC CONTROL AND ITS ROLE IN SOLID STATE CHEMISTRY The products formed in a chemical reaction are dependent on the pathway through which the reaction proceeds. The two canonical regimes of reactivity throughout synthetic chemical disciplines are either thermodynamically or kinetically limited reactions. Reactions that occur under thermodynamic control often have higher activation barriers and in turn yield the products that are the most stable at thermodynamic equilibrium. Alternatively, if the activation energy can be lowered, the reaction may proceed under kinetic control, in which the products that form the fastest may be stabilized despite possibly not being the lowest-energy configuration in the system. While it is possible to access both kinetic and thermodynamic products in most synthetic molecular chemistry, solid state reactions yield a distinct challenge in dictating the reaction coordinate.1,2 Approaches that overcome solid state diffusion become paramount: when atomic-scale mixing is not facilitated by a solvent or by the solid nature of the reactants, other means must be used to promote reactivity.3,4 The application of heat will provide the requisite energy to overcome the activation energy of diffusion to aid in mixing of solid reactants; such heating often yields the product or products that are most stable since the system has sufficient thermal energy to relax into the lowest energy configuration. In practice, this does not realistically limit the number of feasible synthetic targets available to the solid state chemist.5 Although a large number of materials will invariably form at thermodynamic equilibrium, the formation of these materials occurs spontaneously without control (Figure 1a). If one wants to rationally direct a reaction to prepare a new material with a specific structural motif or directed functional properties, metallurgical reactions are inadequate if the phase diagram does not contain the desired phase. Some stoichiometries of

materials are metastable with respect to partial decomposition (e.g., MS2 → MS + S, M = Mn, Cu, Zn)6−8 while others are not (e.g., M = Fe, Co, Ni).9−11 Additionally, polymorphs of the same stoichiometry may have vastly different properties that originate from the minor structural differences. A key example of this is the various polymorphs of TiO2: slight changes in the connectivity of [TiO6] octahedra have dramatic effects on the electronic band structure and thus the properties and ranges of viable applications for the different polymorphs.12 Therefore, the synthetic pathway to form new materials must be altered in order to stabilize phases that are metastable under ambient conditions. In the case of MS2 (M = Mn, Cu, Zn), synthesis from the elements requires the application of high pressure (p > 10 kbar) at approximately 1000 °C to stabilize the disulfide rather than the monosulfide phase.13,14 Application of pressure and high temperature, followed by quenching, can be considered kinetic trapping (Figure 1b): at increased pressure, the ground-state energy of a given material can change with respect to other configurations of the same material in another form (e.g., ZnS2 vs ZnS + S). This can also be achieved with temperature: at high temperature, the hexagonal wurtzite polymorph of ZnS becomes thermodynamically favored over the cubic sphalerite phase.8 Alternatively, extrinsic material factors such as particle size can be used. The thermodynamic stability of the three naturally occurring TiO2 polymorphs depends on the particle size because of their variable surface energies: while rutile forms in the bulk, the polymorphs anatase and brookite become thermodynamically stable with increasing specific surface area.15 Therefore, forming metastable materials in the bulk without extreme applied conditions is a significant challenge. Achieving kinetic control, where the activation energies of solid state reaction processes are altered in such a way that metastable Received: November 14, 2016 Revised: December 13, 2016 Published: December 20, 2016

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This Perspective is part of the Up-and-Coming series. © 2016 American Chemical Society

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DOI: 10.1021/acs.chemmater.6b04861 Chem. Mater. 2017, 29, 479−489

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Chemistry of Materials

Figure 1. Schematic reaction coordinate diagrams illustrating (a) thermodynamic control, (b) kinetic trapping, and (c) kinetic control.

Ba1−xKxFe2As2 superconductors from alkali and alkaline earth hydrides reacted with iron arsenide, which significantly increases the reaction kinetics and yields materials comparable to those formed by slow elemental reactions.29 Another solid state synthetic approach that has the potential to provide kinetic control is the use of reactive alkali (poly)chalcogenide fluxes. Work in the late 1980s and early 1990s by Ibers and co-workers30−32 as well as Kanatzidis and co-workers33,34 was successful in producing kinetically stable materials using varieties of A2Qx (A = Na, K; Q = S, Se; x = 1− 5) with low melting temperatures. The fluxes are corrosive and oxidizing and often result in the inclusion of polychalcogenide anions in the crystal structures of the products. Concatenated polyanions are often only kinetically stable in complex transition-metal-containing compounds and disproportionate to the thermodynamically favored chalcogenide and the elemental chalcogen when metallurgical approaches are used.33,34 More recently, in situ spectroscopic35 and diffraction36 studies of flux mixtures and reactions have provided a deeper understanding of the structure and reactivity of these fluxes. Depending on the polysulfide composition in mixtures of K2S3 + K2S5, the local structure of the liquid flux is strongly altered. Additionally, the input stoichiometry, heating rate, and flux composition can yield a range of new materials that are transiently stable at various temperatures during heating and cooling in the flux. For example, the new phases K5Sn2S8 and K4Sn2S6 are formed when tin metal is reacted in excess K2S5, as discovered using in situ powder X-ray diffraction (PXRD). If instead K2S3 is used, another new phase, K6Sn2S7, is observed.36 These fundamental studies are integral in determining kinetic pathways to new functional materials. Another notable example of kinetic control in preparing new solid state materials is the use of the modulated elemental reactant method. In this method, precisely controlled thicknesses of various elemental reactants are deposited as ultrathin films onto a substrate sequentially.37−39 The layered elemental reactants are then annealed to yield a homogeneous amorphous intermediate; further annealing directly crystallizes the product, which is often metastable. The sequential deposition of very thin layers of the various elements is key in reducing the diffusion lengths in the films, which promotes mixing before crystalline phase nucleation. This, along with the low temperatures required to permit only nucleation rather than complete structural rearrangement,40 has allowed the preparation of many ternary and quaternary41,42 solid state materials without first nucleating binary phases,43,44 as well as the formation of many metastable phases.37,40,45

materials can be accessed (Figure 1c) requires alternative chemistries to traditional high-temperature solid state reactions; the key component of these alternative reactions is the facilitation of or lack of dependence upon diffusion between reactants. Chimie douce (i.e., “soft chemistry”) often uses redox activity to either reductively or oxidatively intercalate or deintercalate ions into or out of a solid state structure, in turn maintaining the bulk framework but altering the electronic properties. A canonical example of this is the oxidative deintercalation of Li from LiVS2 with I2 in acetonitrile, which affords the layered, metastable VS2 phase.16 Direct combination of the elements, V + 2S, does not result in this phase, as suggested by the phase diagram.17 In these oxidative deintercalation reactions, the rate-limiting step is often diffusion of the alkali ion.18 Additionally, complete or near-complete deintercalation can lead to structural instability, which causes rearrangement of a layered crystal structure (e.g., K1−xNi2Se2) into a 3D-interconnected crystal structure (e.g., K1−xNi2−ySe2).18 When the stability of the host lattice is maintained, multiple ion-exchange or (de)intercalation steps can be exploited to design new materials with predictable structure.19 This has been very fruitful in the preparation of perovskite-structured oxide ceramics, where the rigid oxide lattice allows multiple exchanges of the intercalated A-site cations, enabling the preparation of a wide range of novel and metastable perovskite derivatives.20,21 Anion deintercalation and exchange in extended inorganic solids also yields an electronically altered material with a crystal structure that is related to that of the precursor. Reaction of alkali hydrides with various metal oxides can yield 2D layered structures with unusually low oxidation state metals (e.g., Co(I) and Ni(I))22,23 as well as magnetic oxyhydride materials.24 Kageyama and co-workers have shown that hydride reduction of BaTiO3 yields an electronically conducting mixed-valence Ti3+/Ti4+ material and that the hydride anion in BaTiO3−xHx is surprisingly labile.25 This lability was then exploited as a facile route to oxynitride materials by then reacting the hydride under NH3(g)26 and surprisingly even under N2(g) at 400 °C.27 Additional investigation of the nitridation of EuTiO3 indicated that use of the hydride precursor allows nitrogen exchange at much lower temperatures and more complete exchange of nitrogen into the lattice via enhanced anion diffusion pathways, yielding a metastable oxynitride perovskite.28 These examples of topotactic modification to derive metastable phases have tremendous utility but are limited in the sense that a relationship to the original crystalline lattice of the precursor material is typically required. The lability of the hydride anion has also been exploited in the direct preparation of 480


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Figure 2. Two pathways hypothesized for solid state metathesis reactions. The reactants could either progress through (a) ionic intermediates, where the products would form from facile ion exchange, or (b) elemental intermediates, where the constituents are redox-active and the reaction likely is driven by the rapid heating of the reaction mixture upon the formation of NaCl. The observed intermediates indicate that a combination of these two pathways is at play in solid state metathesis reactions.

hypothetical transition state might be electrostatically disfavored. Alternatively, the elemental pathway would progress through elemental intermediates that are formally reduced or oxidized in situ before being reoxidized or rereduced to form the expected products. There, the multiple electron-transfer steps may be disfavored. These limiting extremes inspired our subsequent experiments to understand the reaction pathways of these ion-exchange reactions.

Gaining kinetic control in materials synthesis and studying the pathways of solid state reactivity have provided various materials chemistry breakthroughs. The intercalation of lithium, lithium amide, and ammonia between the layers of superconducting FeSe increases the transition temperature from 8 to 43 K.46 Observing kinetic processes in applied systems is very important in understanding their function, such as the cyclingrate-dependent formation of metastable LixFePO4 solid solution phases in battery cathodes47 as well as comprehensive studies of the reaction mechanisms in intercalation48 and conversion49,50 type cathodes. Further investigation of reaction pathways in both preparatory and functional reactions will allow breakthroughs in the discovery, design, and use of new functional materials.

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THE KEY TO KINETIC CONTROL: INTERMEDIATES IN SOLID STATE REACTIONS Solid state chemistry traditionally targets the preparation of equilibrium products; materials produced via solid state methods are normally produced at a global thermodynamic equilibrium. There, one must provide the requisite thermal energy to overcome diffusion, which in turn allows the system to relax to the equilibrium products irrespective of the reaction pathway. The role of kinetics is traditionally disadvantageous in solid state chemistry in that the reactions simply form inhomogeneous, diffusion-limited products if they are not allowed to equilibrate. Our research focuses on taking advantage of kinetic control of solid state reactions and on ways to understand and circumvent solid state diffusion. To address the barriers imposed by diffusion, we study lowtemperature solid state metathesis reactions, which appear to have a lower activation barrier for diffusion than ceramic or metallurgical reactions. These reactions progress on modest time scales and are well-suited to temperature- and timedependent studies that permit the detection of varied reaction intermediates. Knowing the intermediates through which chemical reactions proceed is of key importance in understanding why a product may or may not have formed and will allow the reaction pathway to be altered in the pursuit of designing new functional materials.59 As described above, highly exothermic solid state metathesis reactions that readily self-propagate are not well suited to the application of in-depth crystallography to detect kinetically stabilized intermediates and understand the reaction pathway. Instead, we pursue the syntheses of MQ2 phases (M = Mn−Zn; Q = S, Se), which are functionally useful, challenging to prepare from the elements, and encompass various thermodynamically and kinetically stable phases and polymorphs. FeS2 is an earthabundant semiconducting compound that has been under investigation for applications as a photovoltaic absorber60 as well as a cathode material in conversion batteries.61 Gaining

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KINETIC CONTROL VIA SOLID STATE METATHESIS We have recently shown the role of kinetic control in solid state metathesis (double-ion-exchange) reactions. These reactions can be thought of as being driven by the formation of stoichiometric equivalents of a thermodynamically stable byproduct (e.g., NaCl) along with the targeted material. Initial investigations into this field by Kaner and Parkin in the early 1990s yielded spectacular results. Combination of various transition metal halide salts with various alkali chalcogenide and alkali pnictide salts would rapidly self-propagate upon mixing, igniting even under inert atmospheres to yield refractory compounds such as transition metal borides and nitrides.51−55 The larger ΔHf° of these reactions compared with the analogous metallurgical reactions is due to the formation of multiple equivalents of a binary salt (e.g., NaCl). These selfpropagating reactions often heat the mixture to above the melting point of the salt byproduct to form a flux that allows the counterions to mix and form highly crystalline transition metal compounds. Metathesis assisted reactions can also reduce the reaction temperature to form perovskite oxides, as attributed to the co-formation of potassium sulfate and CO2(g).54 Thus, this approach is amenable for forming hard, refractory materials from constituent elements that are otherwise kinetically inert or sluggish when combined metallurgically.56,57 Initial hypotheses surrounding the reaction pathways of these solid state metathesis reactions suggested that they progress through either ionic or elemental intermediates (Figure 2).53,58 The ionic pathway would proceed through facile ion exchange and indicate that the reactions are not redox-active; however, a 481


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Chemistry of Materials greater understanding of the (de)formation of FeS2 could provide insight into the cycling mechanism of the conversion reaction FeS2 + 4A → 2A2S + Fe (A = Li, Na). CoS2 is a metallic ferromagnet,62,63 which can be made half-metallic via substitution of iron for cobalt (i.e., Co1−xFexS2) and is thus of interest for spintronic applications.65,64 FeS2, CoS2, and NiS2 are all viable electrocatalysts for the hydrogen evolution reaction, and their efficiencies can be altered via alloying.66 MnS2, CuS2, and ZnS2 are metastable phases that have only been synthesized from the elements at increased temperature and pressure, where the disulfides can be kinetically trapped (cf., Figure 1b).13,14,67 The reaction FeCl2 + Na2S2 → FeS2 + 2NaCl proceeds to completion at Trxn ≥ 250 °C and can also be quenched from intermediate reaction temperatures at incomplete stages. It was found that NaFeS2 and elemental sulfur were also present at Trxn ≤ 175 °C, along with NaCl.68 This was unexpected for a number of reasons. Most obviously, the formation of these intermediates indicates that a range of redox activity occurs in these reactions. The formal oxidation of iron from Fe2+ to Fe3+ and the reduction of S22− to S2− in NaFeS2 as well as the oxidation of S22− to elemental sulfur is not expected for a facile ion exchange [Figure 2, hypothesis (a)]. Additionally, the incomplete formation of NaCl (indicated by the formation of NaFeS2) is suggestive of a more complex reaction pathway than originally suggested for solid state metathesis reactions.53,58 The decoupled activation barriers of NaCl and FeS2 suggest that there are kinetically favored reaction pathways that can be exploited to produce metastable products. Simply examining the products that form upon stoichiometric completion with varied metal salt reactants is insightful. While the dichalcogenide forms in the cases of Fe, Co, and Ni, the monochalcogenide (along with elemental sulfur) forms in the cases of Mn, Cu, and Zn (Figure 3). Looking at the phase diagrams of these systems6−11 shows that FeS2, CoS2, and NiS2

are the only thermodynamically stable disulfides from this subset of transition metals. However, two polymorphs of ZnS are observed in the PXRD data. While the low-temperature polymorph (cubic sphalerite) is expected to form under the reaction conditions and is indeed formed, the high-temperature polymorph (hexagonal wurtzite, stable at T > 1020 °C)8 is also observed. This suggests that kinetic products may be synthetically tractable via solid state metathesis.

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IN SITU CRYSTALLOGRAPHY OF REACTION PATHWAYS Detecting intermediates of these solid state reactions as they form and react to yield terminal products is integral to understanding the reaction pathway and the isolation of metastable products. In situ crystallography with synchrotron powder X-ray diffraction and total scattering experiments correlated to thermal measurements from differential scanning calorimetry (DSC) allow the direct study of reaction pathways and elucidation of the major barriers to reactivity. Additional information about structural rearrangements and kinetic product formation is then derived to understand the origins of kinetic control in these systems. From these detailed studies, we gain insight regarding these solid state metathesis reactions to encompass a broader scope of solid state and materials chemistry to transition toward a paradigm of “reaction-bydesign”2 in the synthesis of new and metastable materials. To study these solid state metathesis reactions, high-flux, highenergy synchrotron X-rays were used to provide sufficient temporal resolution and penetrating depth to directly observe intermediate phases that cannot be detected ex situ. We specifically investigated the formation of FeS2, CoS2, and NiS2 via metathesis using an in situ experimental setup (Figure 4). Understanding the formation pathways of the thermodynamically stable products is important in designing new reactions that may target kinetically stable products instead.2,70 The synchrotron PXRD data collected continuously as the reaction mixtures are heated from room temperature to 400 °C demonstrate the evolution of the crystalline phases from reactants to intermediates to products. With a scattering geometry that permits access to high diffracted angle (i.e., Qmax, Figure 4), total scattering experiments with pair distribution function analysis can be used to study amorphous components not evaluated by powder X-ray diffraction. In all of the cases studied, the first major observation in diffraction that occurs is the gradual formation of NaCl at ca. T = 100 °C. The next and clearest set of changes that occur in these reactions is the formation of a range of metal sulfide and sodium sulfide intermediates. In each reaction, various MxSy (where y < 2x and M = Fe, Co, Ni) phases are observed, as well as Na2S4 and Na2S5. This suggests a dif f usion-limited reaction pathway in which NaCl formation displaces the metal and sulfide ions slowly as the reaction mixture is heated. Among the intermediates that form are a range of compounds shown to be thermodynamically stable (e.g., Fe7S8, Co3S4, Ni3S2) and metastable (e.g., cubic Fe3S4, CoS, high-temperature NiS) at the reaction temperature. Metastable phase formation has been observed previously in diffusion-limited metallurgical reactions of thin films, where the phase that yields the highest ΔG/Δt often forms first, irrespective of its overall thermodynamic stability.71−74 Eventually, as higher temperatures are reached and more thermal energy is provided to the reaction mixtures to promote diffusion, the targeted disulfides are formed. Interestingly, marcasite (orthorhombic, metastable) and pyrite

Figure 3. PXRD patterns and Rietveld refinements of the products of the reactions MCl2 + Na2S2 (M = Mn, Fe, Co, Ni, Cu, Zn). MS2 phases only form for the systems in which the dichalcogenide is a thermodynamically stable phase (e.g., M = Fe, Co, Ni); otherwise, the monochalcogenide and sulfur form. Reprinted from ref 68. Copyright 2014 American Chemical Society. 482


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Figure 4. In situ crystallography experiments allowing the collection of X-ray scattering data as a function of temperature and observation of the reaction pathway as it happens. At short sample-to-detector distances (l, ca. 15 cm) with shorter-wavelength X-rays (λ ≈ 0.2 Å), high Qmax (20−30 Å−1) data suitable for pair distribution function analysis are collected. At longer l with softer X-rays (λ ≈ 0.75 Å) the ΔQ/Q resolution increases, yielding high-quality powder diffraction data suitable for quantitative Rietveld analysis. Details regarding the heating elements and mounting specifications can be found in ref 69. The components are not shown to scale.

Figure 5. (a, c) DSC and (b, d) PXRD data for the reaction FeCl2 + Na2S2 between room temperature and 400 °C. The air-free reaction (a, b) is exothermic and progresses through a range of FeSx intermediates, where x < 2. The air-exposed reaction (c, d) is not exothermic and during heating directly nucleates the FeS2 phase as an amorphous phase disappears. (e, f) Photographs of the (e) air-free and (f) air-exposed reaction mixtures show a clear color change upon air exposure. Adapted from ref 70. Copyright 2016 American Chemical Society.

Comparison of the evolution of crystalline phases to the DSC data also clarifies the reaction pathway (Figure 5a,b). The majority of the heat released from these reactions occurs during the phase evolution of the intermediates rather than during the formation of NaCl, despite the fact that NaCl is the most energetically favored material to form (ΔH°f (NaCl) = −411 kJ mol−1 vs ΔH°f (FeS2) = −178 kJ mol−1).78 This, in conjunction with the formation of NaCl by itself at lower temperatures, suggests that any heat released from NaCl formation alone is spread out over a large temperature range and is not detected

(cubic, thermodynamically stable) FeS2 are observed simultaneously, with the pyrite phase becoming the majority phase as the temperature is increased. This polymorphism has been a challenge in the preparation of pyrite FeS2 nanostructures because of the smaller band gap of marcasite, which can lead to a smaller open-circuit voltage in photovoltaic devices,75−77 which suggests that the relative energy difference to nucleate one over the other in a diffusion-limited regime is small. This in turn leads to the conclusion that synthetic pathways that exclusively produce pyrite FeS2 are desirable. 483


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Figure 6. Pair distribution function analysis of total scattering data of the as-ground air-exposed reactions MCl2 + Na2S2, where M = Fe (a), Co (b), and Ni (c). Fits to NaCl (solid lines, top) are subtracted from the data (black circles) to yield the difference PDF (data − NaCl). Comparison of the difference PDF to simulated PDFs of various metal sulfides (labeled below) indicates the formation of M−S bonds with only short-range ordering. Reprinted from ref 70. Copyright 2016 American Chemical Society.

via DSC or that NaCl forms adiabatically. Once the reaction begins to take place more rapidly (i.e., more changes are observed over a shorter period of time and temperature), the heat produced becomes greater than can be dissipated and is then detected. The formation of a range of sulfur-poor metal sulfide intermediates suggests that the main barrier to product formation is diffusion. Even with a well-mixed matrix of precursor reactants, the reaction is dependent upon the interdiffusion of individual grains. At an interface, NaCl may begin to form and displace the respective counterions, which then mix less rapidly until further heated. This implies that the activation barriers for the formation of more covalent solids (FeSx) are higher. However, changing the way in which the reaction is prepared leads to an entirely different reaction pathway. Instead, the metathesis reaction can be initiated by grinding the premixed reactants in an ambient atmosphere. This is indicated by a rapid color change to black upon air exposure and the observation of crystalline NaCl by PXRD (Figure 5c−f). While the targeted transition metal dichalcogenides are expected to be black, no crystalline components corresponding to transition metal sulfides are detected via PXRD. Annealing the as-ground material under vacuum yields direct crystallization of the targeted metal dichalcogenides at T ≈ 100−200 °C (depending on the metal) with no observation of sulfurpoor intermediates or structural polymorphs. Structural characterization of the amorphous precursor at room temperature using pair distribution function analysis of total scattering data (Figure 6) reveals the formation of nearestneighbor correlations distinctive of metal−sulfur bonds, but without long-range crystallographic order. This, in concert with the PXRD data and small exotherm observed via DSC (Figure 5), indicates that the majority of the bonds have formed and only a small amount of energy is released upon nucleation of the dichalcogenide phase. Rather than the formation of a range of metal sulfide intermediates, the preparation of a homogeneous amorphous MS2 phase yields a reaction limited by nucleation rather by diffusion (Figure 7).

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Figure 7. Cartoon schematic illustrating the different pathways achievable via solid state metathesis reactions. Diffusion-limited reactivity occurs when the reactants are maintained in an air-free environment. Air exposure allows complete mixing of the reactants and the formation of an amorphous intermediate that directly nucleates the targeted MS2 product upon annealing. Reprinted from ref 70. Copyright 2016 American Chemical Society.

products, whether through a diffusion- or nucleation-limited pathway. CuSe2 has been reported in two distinct polymorphs (Figure 8). The pyrite phase was previously synthesized only via solid state reaction of the elements at elevated temperature (ca. 1000 °C) and pressure (ca. 60 kbar).13 The thermodynamically stable orthorhombic marcasite-type phase is otherwise stable below 370 °C at ambient pressure (Figure 8).80 Both polymorphs of CuSe2 are superconductors, pyrite below Tc = 2.4 K and marcasite below Tc = 0.7 K.81 Direct reaction of Na2Se2 and CuCl2 under vacuum without any exposure to air yields the expected marcasite phase after 24 h at Trxn = 300 °C; reactions do not achieve stoichiometric completion at lower temperatures.79 This is in good agreement with our initial report that controlled metathesis reactions yield the thermodynamically stable products when not performed in a way to induce kinetic control.68 Alternatively, mixing of Na2Se2 and CuCl2 in low-humidity air (10−15%) in a mortar and pestle yields the formation of NaCl and poorly crystalline CuSe and selenium. Annealing this mixture at 100 °C under vacuum for 24 h yields stoichiometric conversion to the metastable pyrite polymorph. At higher temperatures or longer reaction times (ca. 170 h), the pyrite begins to transform to the thermodynamically stable marcasite phase. The origin of this polymorph selectivity was studied in situ using synchrotron PXRD and pair distribution function analysis

POLYMORPH SELECTION VIA KINETIC CONTROL

Our previous studies described here demonstrated that solid state metathesis reactions can be performed in the limit of kinetic control and that their reaction pathways can be altered. However, the reactions discussed thus far form the same 484


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Figure 9. (a) Reaction coordinate indicating the steps to form the metastable pyrite polymorph of CuSe2 via initial air exposure of the reactants, which allows the formation of the CuSe and selenium intermediates. (b) Copper and (c) selenium sublattices of CuSe, pyrite CuSe2, and marcasite CuSe2 indicating the topotactic relationship among the three compounds. Adapted from ref 79. Copyright 2015 American Chemical Society.

Figure 8. (a) Pyrite and (b) marcasite structures of CuSe2. (c) DFTcalculated energies (points) as functions of unit cell volume with fits to the Burch−Murnaghan equation of state (lines) indicate that the pyrite is the metastable polymorph. (d) A portion of the Cu−Se phase diagram indicates that marcasite CuSe2 is the only thermodynamically stable phase, which decomposes to CuSe and Se above 334 °C. (a−c) Reproduced from ref 79. Copyright 2015 American Chemical Society. (d) Adapted with permission from ref 80. Copyright 1995 Elsevier.

atmospheric H2O promotes partial interfacial mixing of the reactants through solubilization or direct phase transfer. Tertiary phosphines (R3P) have been shown to form adducts with chalcogenides and can be chemically tuned by means of the functionality of the organic components.84−88 We proposed that if atom transfer is involved, addition of a molecular Lewis base, such as triphenylphosphine (Ph3P), to the reaction mixture may promote similar reactivity, and the results are consistent with this hypothesis, as described below. The addition of Ph3P to the anhydrous CuCl2 + Na2Se2 reaction mixture in small amounts (ca. 10 mol %) yields the formation of pyrite CuSe2 after 24 h at reaction temperatures between 100 and 150 °C.89 This encouraged us to understand the chemistry involved and how the phosphine additive allows the kinetically controlled formation of the metastable CuSe2. Increasing the amount of Ph3P added to the reaction mixture yields increased formation of CuSe and less CuSe2, suggesting that selenium is consumed by the Ph3P. Accordingly, 31P NMR analysis of benzene extracts of the product mixture confirmed the formation of Ph3PSe, and mass spectrometry of methanolsoluble fractions of quenched reactions revealed the formation of (Ph3P)xCuCl4−x (x = 2, 3) species. These observations, concomitant with the identification of CuCl, CuSe, and Se as crystalline intermediates, led to the conclusion that the smallmolecule intermediates are atom transfer agents that promote mixing and reactivity at low temperatures, thus allowing the formation of pyrite CuSe2. As the molecular intermediates are consumed, the formation of the metastable phase yields to the transition to the thermodynamically stable marcasite CuSe2. More definitively, the chemical and physical properties of the molecular additive have profound effects on the outcome of the reaction (Figure 10). Addition of nonbasic analogous molecules such as triphenylamine or triphenylmethane, which are liquid at Trxn = 150 °C, yields mixed pyrite and marcasite products under

of total scattering data. Unlike the previous study where an amorphous precursor formed upon grinding in air, CuSe and selenium form along with NaCl. As this mixture is heated, the pyrite CuSe2 forms and eventually transforms to the marcasite polymorph as the temperature is increased. While these changes are obvious in the diffraction profile, the PDF shows only minor changes across the entire temperature range, suggesting that while the bulk crystalline symmetry of the phases present is actively changing, the local structure does not change dramatically. The similarities in the PDF observed between the CuSe and selenium intermediates at low temperatures and the various phase transitions at higher temperatures indicate that pyrite is formed as a structural intermediate in this reaction coordinate. Indeed, comparing the copper and selenium sublattices of the CuSe, pyrite, and marcasite crystal structures allows the elucidation of a topochemical relationship. Rather than the direct formation of the thermodynamically stable marcasite phase, the pyrite phase is kinetically stable as an intermediate as selenium reacts with CuSe to form CuSe2 (Figure 9), akin to Ostwald’s step rule, which is based upon the concept that a less stable phase should have a lower surface energy and thus a lower barrier to nucleation.82,83

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CHEMICAL CONTROL OF THE REACTION PATHWAY The stabilization of the metastable pyrite CuSe2 polymorph via air exposure is encouraging, but it is still unclear why air exposure enables kinetic control. Simply grinding the reaction in air precludes the ability to study the preformation of NaCl in situ and is uncontrolled with respect to ambient humidity and minor temperature fluctuations. We hypothesized that 485


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• Versatile chemistry and enthalpic tunability: A wide range of materials (transition metal−group IV, V, VI binaries) have been produced via solid state metathesis,53,58 and the coforming alkali halide allows the enthalpy of the reaction to be tuned. Controlling both of these parameters under kinetic control will allow the formation of many more metastable materials. The key outcomes of kinetically controlled solid state metathesis reactions are the pathway and polymorph selectivity afforded by the technique. Selecting reactants that will not react violently but are also able to react at moderately low temperatures enables kinetic control of these reactions.68 Observation of the reaction coordinate as it progresses using in situ PXRD allows the pathway to be determined. The pathway can be changed via air exposure, which can yield amorphous intermediate phases that directly crystallize into the targeted products, in turn making the reaction progression nucleation-limited.70 Changing the reaction pathway can in turn enable polymorph selectivity (e.g., CuSe2) by formation of structural precursor intermediates79 or molecular intermediates that serve as atom transfer agents.89 The use of molecular additives to influence solid state reactions holds much promise in the further development of kinetic control in the field. Through the use of the organometallic and inorganic coordination chemistry literature, the atom transfer reagents can be selected on the basis of the targeted inorganic extended solid. These features will allow control to be exerted over solid state reactivity. Changing the activation barriers and reaction pathways via additives also provides the opportunity for reaction-by-design, where the reaction conditions can be tuned to the system and allow kinetic control of the formation of a range of new, metastable, functional materials.

Figure 10. Diffraction patterns of the products of the reactions CuCl2 + Na2Se2 + 10% X, where X is the molecule indicated on the right. Only the Lewis basic molecules that are melts at the reaction temperature (150 °C) show appreciable yield and polymorph selectivity (e.g., Ph3P, (o-CH3Ph)3P). Reproduced from ref 89. Copyright 2016 American Chemical Society.

identical reaction conditions. Addition of a nonmelting, nonbasic molecule such as tetraphenylmethane yields no conversion to marcasite or pyrite. A basic but nonmelting (at Trxn = 150 °C) molecule such as tris(o-methoxyphenyl)phosphine provides pyrite selectivity, but the reaction coordinate is retarded relative to that for the molten bases. These reactions indicate that Lewis bases promote polymorph selectivity; if the additive is a liquid at the reaction temperature, then diffusion of the reactants is enhanced.

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OUTLOOK The studies presented indicate that a wide range of kinetic control can be achieved over solid state reactions, with an emphasis on metathesis reactions. Key aspects in gaining kinetic control of solid state reactions are the following: • enhancing diffusion at low temperatures either via a reactive flux, solvent, or atom transfer reagent; • promoting reaction pathways that circumvent diffusion; • topotactically exchanging ions in a lattice to maintain a framework structure with altered properties. The advantages of using solid state metathesis to gain kinetic control are the following: • The chemical potential provided for stoichiometric completion of the reaction: The coformation of NaCl in these reactions provides an additional energy-lowering process to form the desired compositions. • The apparent “low” activation barrier, ΔG⧧: Solid state metathesis reactions have been shown to self-propagate under inert atmospheres as well as controllably at low temperatures (T < 300 °C). The apparent low activation energy indicates that kinetic control is more easily accessed than in traditional ceramic or metallurgical solid state reactions. • Internal standard of the reaction coordinate: The observation of NaCl (or any other salt byproduct) in these reactions aids in understanding the propagation of the reaction. That is, since NaCl is often observed first, its observation serves as an indicator of the reactivity and reaction progress.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

James R. Neilson: 0000-0001-9282-5752 Notes

The authors declare no competing financial interest. Biographies Andrew J. Martinolich received his B.S. in Chemistry in 2012 from Santa Clara University. Since 2013 he has been pursuing his Ph.D. in Chemistry at Colorado State University under the supervision of Prof. James R. Neilson. James R. Neilson received his B.S. in Materials Science and Engineering in 2006 from Lehigh University, where he was a Barry M. Goldwater Scholar. He then pursued his doctoral studies in Biomolecular Science and Engineering at the University of California at Santa Barbara (conferred in 2011), where he was a National Science Foundation Graduate Research Fellow, under the mentorship of Prof. Daniel E. Morse and in collaboration with Prof. Ram Seshadri. He then was a postdoctoral associate at Johns Hopkins University from 2011−2013 with Prof. Tyrel M. McQueen in the Institute for Quantum Matter and the Departments of Chemistry and Physics & Astronomy. In 2013, he began his independent career in the Department of Chemistry at Colorado State University. 486


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ACKNOWLEDGMENTS A.J.M. and J.R.N. acknowledge financial support from Colorado State University and the W. M. Keck Foundation.

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