Heterometallic Metal−Organic Frameworks(MOFs): The Advent of Improving the EnergyLandscapeAllison M. Rice, Gabrielle A. Leith, Otega A. Ejegbavwo, Ekaterina A. Dolgopolova,and Natalia B. Shustova*Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States

ABSTRACT: In this Perspective, we highlight how recent studies ofheterometallic metal−organic frameworks (MOFs) could lead toadvances to the energy landscape. The ability to merge the inherentproperties of MOFs, including their modularity, porosity, versatility,high surface area, and structural tunability, with the ability toengineer metal nodes could benefit both the classical realm of MOFapplications as well as the recent shift toward electronic structurestudies. This Perspective is intended to provide a glimpse into theadvances, challenges, and future pathways for the uses ofheterometallic MOFs in sectors ranging from the oxygen evolutionreaction to nuclear waste administration in order to ultimatelyprovide valuable potential materials to the ever-expanding techno-logical landscape.

Construction of novel motifs and architectures withspecified properties has become a pressing objectivefor scientists as it could lead to great benefits to the

current energy landscape. Metal−organic frameworks (MOFs)have emerged as a potential solution to this problem as theirmodularity, porosity, versatility, high surface area, andstructural tunability have portended their use in the renewableand clean energy sectors.1−4 Initial development of MOFs wasprimarily based on their high surface area that is inherentlyconnected to gas storage and separation, and these areas haverecently been regarded as “classic MOF applications”.5−7

Recently, directions beyond the “smart sponge” approach forMOFs have been pursued with increased fundamentalunderstanding of MOF-based photophysics, electronic struc-ture, catalytic transformations, and actinide integration.8−14

Although there has been an insurgence of MOFs in the pastdecade, heterometallic MOFs (i.e., the addition of a second ormore metal ions to the structure) represent a less studied

subsection, which could result in unlocking novel intrinsicproperties. In this Perspective, for the first time, we intend tosurvey heterometallic MOFs with an emphasis on theirpotential benefits for applications in both the “classic” realmas well as the shift toward directions in electronics andcatalysis, thus highlighting their broad applicability to theoverall energy landscape. Comprehensive MOF reviews can befound elsewhere15,16 as it is not our goal to provide an all-inclusive review17,18 but rather insights on the use ofheterometallic MOFs for specific application-based subsectionssuch as oxygen evolution reaction (OER), hydrogen evolutionreaction (HER), catalysis, tailoring electronic structure, nuclearwaste administration, as well as separation, storage, and sensing(Scheme 1). As a pressing goal is to find more efficientsolutions with industrial promise, it is crucial to investigatechallenges, perspectives, and future directions for the increasedsuccess of heterometallic MOFs in the energy sector.Oxygen and Hydrogen Evolution Reactions. Current trends in

the technological landscape are leading to an increasingdemand for more renewable sources of energy, bringingelectrocatalytic materials capable of electrochemical energyconversion to the forefront. Electrocatalytic water splitting, theprocess in which hydrogen is generated at the cathode andoxygen is evolved at the anode, is a clean energy method toproduce one of the possible future fuels, hydrogen.19−22

Received: April 23, 2019Accepted: July 5, 2019Published: July 5, 2019

Although there has been an insurgenceof MOFs in the past decade, hetero-metallic MOFs (i.e., the addition of asecond or more metal ions to thestructure) represent a less studiedsubsection, which could result inunlocking novel intrinsic properties.

Perspectiv

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Electrocatalytic reactions such as the OER, the oxygenreduction reaction (ORR), and the HER are key for notonly water splitting technologies but also rechargeable metal−air batteries and regenerative fuel cells.23,24 As OER involvesthe removal of four electrons from two water molecules tomake one molecule of dioxygen, a large overpotential is usuallynecessary to attain enough current density. Current materialsused for OER are mainly precious metal oxides;25,26 however,their low reserves, high cost, and harsh environmental effects ofproduction render the need for improved options with lowercosts and higher durability. Although a large number ofnonprecious metal catalysts have been developed for HERsuch as transition metal phosphides (e.g., CoP and NiP),19,20

transition metal sulfides (e.g., MoS2 and CoS), and metaloxides (e.g., Co3O4), no catalyst is able to fully meet thedemand for practical application. Thus, it is important todevelop more economically viable electrocatalysts with highercatalytic performance for HER and/or OER, as well as tospearhead the use of these materials in energy storageapplications. MOFs can be considered as a class of promisingelectrocatalytic materials as their advantages range from highsurface area and uniform porosity to a large number of openmetal sites for adsorption (Figure 1). Even with these

advantages, the pursuit of MOFs as direct electrocatalystshas only taken off recently due to their typically low electricalconductivity, blockage of active metal centers by organicligands, and chemical stability.25,27 In attempts to overcomechallenges of OER and HER, solutions such as control of thethickness of the MOF layer on the electrode as a way tominimize the electron transport distance as well as the

development of conductive MOFs25 are in progress. At thesame time, heterometallic Ni−Co MOF nanosheets, consistingof Ni2+ and Co2+ connected by the benzendicarboxylate(BDC2−) linkers, have recently been prepared, allowing forbetter electron transfer as well as possessing a high number ofunsaturated reactive metal sites for high catalytic activity(Figure 2a).27 These heterometallic nanosheets have a muchlower overpotential (∼189 mV at 10 mA cm−2 in alkalineconditions) than both the monometallic framework andcommercially available RuO2. Moreover, the coupling effectbetween the Ni and Co metal centers allows for tunability ofthe electrocatalytic activity of the material. Another example ofa heterometallic MOF demonstrating a similar trend is Fe2Ni-MOF (NNU-23) in which Fe2M(μ3-O)(CH3COO)6(H2O)3(Fe2M, M = Ni) clusters are bridged by six biphenyl-3,4′,5-tricarboxylic acid ligands, resulting in formation of a three-dimensional framework (Figure 2b).28 This Fe2Ni-MOFexhibits efficient OER performance with an overpotential of365 mV at 10 mA cm−2 in 0.1 M KOH, which alsooutperforms monometallic MOF counterparts.A strategy to improve the stability of MOF-based materials

could be realized through the inclusion of high-valent metalions (e.g., Fe3+, Al3+, or Zr4+) in order to strengthen theelectrostatic interaction between metal nodes and organiclinkers.29 In particular, Fe-based MOFs ([{Fe3(μ3-O)-(BDC)3}4{Fe(NA)4(LT)}3] (where HNA = nicotinic acid, LT

= monodentate terminal ligand) with bowl-shaped tetradentatemetalloligands consisting of four nicotinate (NA−) ligands witha paddle-wheel-type metal carboxylate core) have been foundto be stable and active for OER.30 The performance can beincreased through the addition of a second metal (cobalt) toform the alkaline-stable MOF [{Fe3(μ3-O)(BDC)3}4-{Co2(NA)4(LT)2}3] with an OER activity and an overpotentialas low as 225 mV at 10.0 mA cm−2. In a very recent report, aseries of stable MOFs were first synthesized based ontrinuclear metal carboxylate clusters (M = Co and M′ = Ni)and a hexadentate carboxylate ligand with a (6,6)-connectednia net (CTGU-10a1-d1), followed by the ability forconstruction of corresponding hierarchical heterometallicnanostructures (CTGU-10a2-d2) through controlling themetal ratio during the solvothermal synthesis.31 The OERperformances of CTGU-10c2 nanobelts constructed fromnanosheets exhibited superior electrocatalytic OER perform-ances with a low overpotential of 240 mV at 10 mA cm−2, aswell as long-term stability of over 50 h in alkaline conditions.31

An alternative direction reported for utilization ofheterometallic MOFs toward OER/HER material develop-ment is their use as a template for preparation of porouscarbon composites. For instance, a porous N-doped carbon-encapsulated CoNi nanoparticle composite (CoNi@N−C) hasbeen prepared using a heterometallic MOF as a precursorconsisting of Co2+ and Ni2+ cations connected by 2-methylimidazole (2-MeIM) ligands.32 It is important to notethat, despite a number of reported advantages of these porouscarbon hybrid materials including enhanced OER/HERperformance, this method does not utilize the full advantageof the intrinsic benefits that MOFs have to offer, includingtheir high surface area, modularity, or structural tunability.Ultimately, the push toward ultrathin MOF nanosheets or

films could alleviate the current issues plaguing their success(e.g., low conductivity, small mass permeability, and stabilityproblems).33−37 As highlighted in this section, examples of this

Scheme 1. Heterometallic MOFs for Energy-RelatedApplications

Figure 1. Schematic representation of the use of heterometallicMOFs for OER and HER. Reprinted with permission from ref 25.Copyright 2016 Springer Nature.

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kind are on the rise for the development of materials withenhanced OER performance.To summarize, although there has been a recent surge of

MOFs for OER and/or HER with high catalytic activity, thereare still many fundamental and technical issues that need to besolved to bring these materials to the forefront of industrialcommercialization. In order to establish a structure−functionrelationship, it is sought after to understand the role of eachcomponent of a heterometallic MOF in regard to OER/HER,which could lead to maximum tunability and performance.There is also a need for more studies of reaction mechanismsbehind these processes, such as the effect of an electrolyte oncatalytic performance and diffusion properties of the startingmaterials and products. The major issues, however, in regard to

achieving greater efficiency of OER/HER are related to thenecessity to improve the electronic conductivity of MOFs, aswell as the ability to enlarge the pore channels of MOFs toaccommodate electrolytes while maintaining stability and highsurface area. It is also crucial for charge diffusion lengths to besystematically shortened to facilitate charge transfer for OERactivity. The potential benefits of heterometallic MOFs forOER/HER place paramount emphasis on studies that provideincreased fundamental understanding to address thesechallenges in order to unlock their full potential for energyconversion technologies.Heterogeneous Catalysis. MOFs have received an upsurge of

attention for catalysis as they efficiently combine the benefitsof heterogeneous (e.g., high surface area, recyclability, and

Figure 2. X-ray crystal structures, metal nodes, and linkers of heterometallic MOFs discussed in the presented overview. The plum, blue,green, light blue, lavender, pink, orange, dark purple, teal, gray, red, and dark blue spheres represent cobalt, nickel, iron, zirconium,vanadium, copper, manganese, uranium, zinc, carbon, oxygen, and nitrogen atoms, respectively.

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facile postreaction separation) and homogeneous (e.g.,structural information about size and geometry of the activesites) catalysts. The porous nature of MOFs coupled with theirstructural tunability could also facilitate diffusion of reactantsand/or products.38 Another unique advantage of MOFs stemsfrom the fact that a MOF consists of uniformly distributedcatalytically active centers with identical environmentsthroughout the scaffold, which can lead to higher selectivityand reactivity when compared to traditional heterogeneouscatalysts.39 Due to the unique combination of tailorableporosity, “on-demand” topology, rationally engineered catalyticcenters, and high surface area, MOFs have been applied tocatalyze a variety of organic reactions such as oxidations,condensations, and coupling reactions including but notlimited to the Michael additions, Knoevenagel condensations,or transesterifications.40,41

Heterometallic frameworks consisting of more than one typeof metal in their lattice could expand MOF opportunities inheterogeneous catalysis, for instance, to pursue tandemcatalysis or catalyze two independent processes within onematrix, as shown in Figure 3. There are a number of ways tointegrate a second (or third) metal in the scaffold, includingdirect synthesis, postsynthetic modification of metal nodes,coordination to organic linkers, or through impregnation ofmetal nanoparticles (MNs). In the latter case, MOFs can aid inovercoming the challenge of MN sintering, and the preparedcomposites, such as Au−Pd@MIL-10142 or Ag−Pd@MIL-101,43 demonstrate superior activity in the dehydrogenation offormic acid and a one-pot multistep cascade reaction for thesynthesis of secondary arylamines, respectively.42−44

Because chemical and thermal stability are among the keyfactors for development of heterogeneous catalysts, a numberof research groups have utilized robust Zr frameworks. One ofthe commonly used zirconium MOFs belongs to the UiOfamily, which consists of hexameric Zr metal nodes connectedby organic linkers. The redox-inactive nature of Zr4+ has led toa limitation of potential catalytic processes to Lewis acidcatalysis.45 However, a wide breadth of catalytic reactions canbe targeted through extension of Zr nodes with morecatalytically active metal centers, for example, late transitionmetals.46−49 For instance, a MOF (UiO-66) withZr6(O)4(OH)4 nodes connected by terephthalate ligands wasextended with vanadium (Figure 2c), which not only preservesstability over a wide temperature range (up to ∼540 °C)typical for Zr frameworks but also exhibits catalytic activity toperform selective gas-phase oxidative dehydrogenation ofcyclohexane to benzene.49

Another purpose for integration of a second metal inside ofcatalytically active zirconium-free MOFs is chemical stabilityenhancement. For instance, increased copper content in the

heterometallic Cu/Co-MOFs possessing the HKUST-1 top-ology (i.e., paddle wheel metal nodes connected by phenyl-tricarboxylate linkers)50 resulted in enhancement of theirstability on air (Figure 2d).51,52 Another example demonstrat-ing the same principle is a Mn-based MOF (STU-2, consistingof imidazole-containing ligands connected to unsaturated Mn2+

ion sites affording a 3D framework with a gyroidal surface),where Zn2+ and Cu2+ were immobilized into the structure(Figure 2e) to increase the stability as well as the catalyticactivity in the cyanosilylation reaction of aromatic aldehydes.53

Even though heterogeneous catalysis is one of the moreexplored areas for applications of heterometallic MOFs,challenges remain that would need to be overcome for theirfull potential to be maximized. For instance, the majority ofcatalytic studies on MOFs have been performed in the liquidphase, and only recently reported was a gas-phase continuousflow synthesis of β-lactones from epoxides using a chromium-based MOF (Cr-MIL-101, constructed from corner-sharedtetrahedra made from the linkage of chromium(III) trimersand BDC2− anions to afford a 3D MOF structure) withunsaturated metal sites based on some chromium clusters notbeing bound to the linkers, which provides the opportunity topostsynthetically integrate Co(CO)4.

54 This can lead touncovering the great potential to apply heterometallicframeworks for gas-phase reactions. To pursue this direction,it is vital to continue to explore systems, both computationallyand experimentally, with high thermal stability and structuralstability to activation. Another promising direction, in itsinfancy stage, is MOF-based tandem catalysis. There are veryfew monometallic examples including the isostructural MOFcatalysts of AlPF-1, [Al(OH)(hfipbb)] ((H2hfipbb = 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)), GaPF-1, [Ga-(OH)(hfipbb)], and InPF-11β, [In(O2C2H4)0.5(hfipbb)]utilized for a solvent-free, one-pot Strecker reaction as wellas a heterometallic example utilizing MIL-100 (Sc, M; M = Al,Cr, Fe) for tandem C−C bond formation and alcoholoxidation, that demonstrate the vast potential for thisdirection.55,56 As a long-term goal, the advantages ofheterometallic MOF-based catalysts could be an attractive

Figure 3. Homometallic (left) and heterometallic (right) MOFs as heterogeneous catalysts.

Heterometallic frameworks consistingof more than one type of metal in theirlattice could expand MOF opportuni-ties in heterogeneous catalysis, forinstance, to pursue tandem catalysis orcatalyze two independent processeswithin one matrix.

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alternative to the homogeneous catalysts currently used inindustry.Tailoring Electronic Structure. Attention to conductive porous

MOFs is driven by current demands in new materials forthermoelectrics, electrodes, batteries, capacitors, or smartmembranes.57 The main challenge in this direction is usuallypoor conductivity of MOFs that is related to fundamentals offramework construction, i.e., typically hard metal ionsconnected by redox-inactive organic ligands bound throughhard nitrogen or oxygen atoms. Metal node engineering couldbe a potential solution to surpass this challenge as it does notdestroy the porous architecture of a framework in contrast toredox-active guest incorporation.57 One of the routes for suchnode engineering is incorporation of a secondary metal (M′) inthe lattice through substitution of the primary metal (M) andformation of Mx-yM′y-MOFs (where M and M′ have the samecoordination environment) or metal node extension resultingin MxM′y-MOFs (where M and M′ have different coordinationenvironments, Figure 4).

In addition to metal pair variation, MOF tunability providesan opportunity for tailoring electronic properties of aframework as a function of metal node nuclearity, the presenceof unsaturated metal sites, the M to M′ ratio, or metal node

geometry for a desirable application. Recent studies demon-strate the support of this statement on the example of a Cu-based framework (HKUST-1)50 that underwent cobaltimmobilization as part of the paddle wheel metal node. Theheterometallic Cu/Co-MOF exhibits enhanced conductivity incomparison with the monometallic Cu analogue (Figure 2d).51

However, the choice of metal is a crucial parameter becausethe related isostructural Cu/Zn-MOF, containing redox-inactive zinc, has properties similar to the parent scaffold.51

The presence or absence of unsaturated metal centers alsoseverely affects the electronic properties as well as frameworktopology.51 Very recently, a heterometallic Ni−Co MOF([CoNi(μ3-BDC)2(μ2-PYZ)2] (PYZ = pyrazine) utilized insupercapacitor development has been reported (Figure 2f).58

This MOF exhibited excellent pseudocapacitive performanceand long-term cycling stability, rendering this class of MOF asa promising option in this realm.58 In attempts to increase thecontent of active species as well as enhance structuralrobustness for better cycling stability, which are necessaryfactors for more widespread use of heterometallic MOFs assupercapacitors, double-shelled zinc−cobalt sulfide rhombicdodecahedral cages using a heterometallic ZIF MOF as aprecursor have been recently prepared.59 As a result of bothstructural and compositional benefits, the resultant materialhad enhanced performance with high specific capacitance andlong-term cycling stability (91% retention over 10 000cycles).59 Recently, a series of heterometallic MOFs (Fe4[Fe-(CN)6]3/Mx[Fe(CN)6], M = Cu, Ni, Co) were constructedthrough a cation exchange approach followed by theirutilization for microcube preparation as an anode materialfor lithium ion batteries, which demonstrated a goodelectrochemical performance of 774 mAh/g after 120 cyclesat 500 mA/g.60

Overall, while mixed transition metal hybrid materials (e.g.,nanocubes61 and nanosheets62) have been known to offer richredox activity and higher electrical conductivity than singletransition metal oxides, which are commonly pursued in thissector of energy-related applications, the advent of hetero-metallic MOFs as materials, for instance, for supercapacitors,still must overcome several drawbacks for their full potential tobe reached. One of the directions to close these gaps is toutilize theoretical calculations to guide predictions of the mostefficient choices in metal pairing to obtain the desiredtunability in an electronic structure. Metal node geometryand nuclearity, framework topology, the nature of the metal,and the presence of unsaturated metal sites are also important

Figure 4. Schematic representation of (top) metal node nuclearityin heterometallic MOFs and (bottom) pathways for integration ofthe second metal into a framework.

Figure 5. (left) Schematic representation of an actinide-containing MOF demonstrating different ways (red) for actinide integration: as apart of the metal node, through cation exchange or metal node extension, coordination to the organic linker, and as a guest molecule. (right)X-ray crystal structure and secondary building block of Th6U4-Me2BPDC-8.

64 The red, dark blue, pink, and gray spheres represent thorium,uranium, oxygen, and carbon atoms, respectively. Hydrogen atoms have been omitted for clarity.

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factors to consider in reference to conductivity enhancement.Depending on the desired application at hand, it is sought afterto tune the electronic structure accordingly, and fundamentalknowledge of mechanisms responsible for the tunability ofelectronic structure could pave the way for this to turn into areality.Nuclear Waste Administration. Due to growing demands for

significant changes in the nuclear waste administration sector,the quest for fundamental understanding of actinide-containingmaterials has been deemed a top priority. Hierarchicalmaterials, such as MOFs, have the ability to be tailored atthe molecular level toward selective actinide separation,detection, or sequestration, thus providing pathways to presentwarnings of possible waste leaching or to speed up wastereprocessing through removal of volatile or other “problem-atic” components from a mixture.63 Due to exceptionalmodularity and tunability, MOFs can also be used as amodel system for engineering radionuclide-containing materi-als, in which hierarchical complexity can be built stepwise. Forinstance, MOFs are not only able to accommodate radio-nuclide species in the pores (red icosahedra, Figure 5) but arealso able to integrate actinides through a cation exchangeprocedure inside of the metal nodes (red striped spheres,Figure 5), chelation to organic linkers (gray T-shaped rods,Figure 5), metal node extensions (red figure eight, Figure 5),or coordination to anchors attached to capping linkers (redsprings, Figure 5).64

This stepwise approach will lead to the development ofmaterials with homogeneous actinide distribution possessinghigh-actinide content with minimal structural density.64 One ofthe first examples of heterometallic actinide-containing MOFshas been prepared through metal node extension or trans-metalation occurring through single-crystal-to-single-crystaltransformations and allowing for the opportunity to shedlight on mechanistic insights of such a process using X-raycrystallography.64 Preparation of heterometallic MOFs(Zr6U0.87-Me2BPDC-8 (Figure 2g), Zr6U0.87-Me2BPDC(SDC),Th6U4-Me2BPDC-8, Th5.65U0.35-Me2BPDC-8, and Th5.65U0.35-Me2BPDC(SDC), where H2Me2BPDC = 2,2′-dimethylbiphen-yl-4,4′-dicarboxylic acid and H2SDC = 4,4′-stilbenecarboxylicacid) has been achieved through integration of different metalsinto the pores or coordination to an organic linker (in additionto An-containing nodes).64

As the field of actinide-containing heterometallic MOFs isjust beginning to gain esteem, multiple avenues for theirconstruction have been recently developed, which sets thestage as both an intriguing and open research area for furtherinvestigations.Recently, the number of actinide-containing MOFs has

escalated;65 however, this structural information is only thefirst step toward understanding the main principles of applyingMOF versatility toward material design and, therefore, utilizingMOFs as a convenient tool for nuclear waste management. Inaddition to different mechanistic aspects, the ability tounderstand and isolate components that are necessary for asuccessful synthetic journey are based on insights onthermodynamic, photophysical, and electronic properties,which are crucial facets to facilitate applications of hetero-metallic MOFs in this sector. Moreover, the unrevealedpotential of theoretical modeling should be brought to theforefront for the prediction of, for instance, phase stability,radionuclide diffusion, cationic exchange, and thermodynamicparameters. Another aspect such as MOF packaging is also

waiting to be addressed. Currently, we have grazed the surfaceof the possible options of synthetic methodologies for

preparation of heterometallic actinide-containing MOFs;however, crucial cornerstones such as thermodynamics,electronics, photophysics, or theoretical modeling have notyet been touched.Separations/Storage/Sensing. Gas storage, separation, and

sensing are considered “classical” applications supported by thefact that companies such as BASF, NuMat, or TruPick havealready presented MOF-based products as efficient adsorbentsto the market. For instance, TruPick utilized MOFs to storeand release a plant growth regulator (e.g., 1-methylcyclopro-pene) to prevent fruits and vegetables from ripening. However,heterometallic MOFs can still bring a new twist in this“established” area of research. For instance, an early studyundertaken on the hydrogen adsorption capacity in a cation-exchanged microporous tetrazolate-based MOF demonstrateda large cation-dependent variation of H2 adsorption enthalpy,which shows a correlation between the exchanged cation andthe strength of H2 binding.

6

In terms of sensing, MOFs have stood out as an efficient andunique platform due to their combinatorial benefits of highsurface area, structural tunability, rapid response, and highsensitivity, which portends this application as one of the mostexplored directions to date. The sector of heterometallic MOFs

for sensing applications is less explored, although manybenefits, such as enhanced sensitivity and selectivity, couldresult from integration of the second metal inside of theframework matrix. For instance, in a recent study, carbonnanotube networks based on a zinc-doped Co-MOF (ZnZIF-67, constructed from the combination of Co2+ and Zn2+

connected to four 2-methylimidazole linkers, Figure 2h) wereused as a gas sensing material for sulfur dioxide.66 Thismaterial possesses remarkable selectivity, fast response, and alow detection limit at room temperature. In another example, aNi−Fe heterometallic MOF was utilized as a template forpreparation of hollow NiFe2O4 microspindles for acetonesensing.67 The developed sensor possessed good selectivity andsensitivity, cycling stability, and fast response.67 Utilizing a

Currently, we have grazed the surfaceof the possible options of syntheticmethodologies for preparation of het-erometallic actinide-containing MOFs;however, crucial cornerstones such asthermodynamics, electronics, photo-physics, or theoretical modeling havenot yet been touched.

The sector of heterometallic MOFs forsensing applications is less explored,although many benefits, such asenhanced sensitivity and selectivity,could result from integration of thesecond metal inside of the frameworkmatrix.

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different sensing mechanism, 3D Zn−Ln heterometallic MOFs([Ln2Zn(ABTC)2(H2O)4]·2H2O; Ln = Sm, Tb; ABTC =3,3′,5,5′-azobenzenetetracarboxylic acid) were used as sensorsfor the identification of benzaldehyde and NO2

− through afluorescence quenching process.68 In another similar study, aluminescent MOF based on Ln−TM (TM = transition metal)heterometallic clusters connected to 1,4-naphthalenedicarbox-ylic acid (H2NDC) ligands, Eu6Zn(μ3-OH)8(NDC)6(H2O)6]n,was studied for the highly sensitive, selective, and reversibleluminescent sensing of the antibiotic ronidazole.69

Despite great interest in MOF-based sensors, one of themost understudied and underestimated areas that can beachieved with heterometallic MOFs is tandem sensing as thetunability of MOFs could allow for enhanced recognition withthe ability to detect several molecules simultaneously. Currentstudies tend to be focused on a singular MOF “receptor”, suchas an unsaturated metal site to bind only one type of analyte,but it is possible for heterometallic sites to exhibit dualsensitivity. Another challenge that must be addressed is devicedevelopment. As heterometallic MOFs are typically grown assingle crystals or polycrystalline powders, thin-film productionis always an ongoing effort in the MOF area, with the goal toincrease processability.Perspectives and Summary. Since the inception of hetero-

metallic MOFs, there have been significant strides in noveltopologies and intricate metal node engineering resulting inenhanced intrinsic properties, but their widespread implemen-tation into device components on an industrial scale wouldrequire increased fundamental understanding of the impact ofthe presence of additional metals in the MOF. As specificchallenges for each application were highlighted in eachsection, it is also important to consider broad shifts inperspective that are necessary for next-generation hetero-metallic MOF-based energy advancements. This encompassesovercoming challenges to achieve low-cost and high-throughput preparation of heterometallic MOFs for massproduction. It is also crucial for these MOFs to be processableas thin films, which is both a main and challenging pursuit inthe MOF sector. This is the first Perspective that highlights themultifaceted nature of heterometallic MOFs for a wide rangeof applications that not only span the “classical” realm but alsoshift toward the less studied areas of electronics and nuclearwaste administration. As evident in this Perspective, thesematerials show copious potential as the move toward tailoredMOF design for specific applications has been an aspect ofutmost attention recently. Ultimately, the potential andchallenges of heterometallic MOFs for a variety of energy-based applications exemplify the direction in which the field isheaded in order to benefit the ever-expanding technologicallandscape with next-generation materials.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: shustova@mailbox.sc.edu. Website: https://www.shustovalab.org.ORCIDNatalia B. Shustova: 0000-0003-3952-1949NotesThe authors declare no competing financial interest.BiographiesAllison M. Rice received her B.S. degree in chemistry fromWestminster College in 2014 and obtained her Ph.D. in 2018 from

the University of South Carolina. Her research at USC was focused onnovel fulleretic materials.

Gabrielle A. Leith received her B.A. degree in chemistry fromHanover College in 2017 and is currently pursuing her Ph.D. at theUniversity of South Carolina. Her research at USC is focused ongraphitic hybrid materials.

Otega A. Ejegbavwo received her B.S. degree in chemistry fromUniversity of Ibadan in 2014 and is currently pursuing her Ph.D. atthe University of South Carolina. Her research at USC is focused onheterometallic MOFs.

Ekaterina A. Dolgopolova received her B.S. degree in materialsscience from Moscow State University in 2013 and obtained herPh.D. in 2019 from the University of South Carolina. Her research atUSC was focused on energy transfer in MOFs.

Natalia B. Shustova received two Ph.D. degrees in PhysicalChemistry (Moscow State University) and Inorganic Chemistry(Colorado State University). After postdoctoral research at theMassachusetts Institute of Technology, she joined the faculty at theUniversity of South Carolina. Her research interests are graphitichybrid materials, sensors, and artificial biomimetic systems.

■ ACKNOWLEDGMENTSThis work was supported by the Center for HierarchicalWasteform Materials (CHWM), the Cottrell Scholar Awardfrom the Research Corporation for Science Advancement, andthe Alfred P. Sloan Research Fellowship from Aflred P. SloanFoundation.

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