Metrics & MoreArticle Recommendations * sı Supporting Information ABSTRACT: "Open-framework chalcogenides" are an important class of materials that combine porosity with semiconductor behavior, and yet fundamental aspects of their conductivity remain unexplored. Here, we report a combined experimental−computational approach to the iconic subclass of materials TMA 2 MGe 4 Q 10 (TMA = tetramethyl ammonium; M = Mn, Fe, Co, Ni, Zn; Q = S, Se). Direct current (DC) conductivity measurements and density functional theory (DFT) modeling reveal that metal ion and chalcogenide identities dominate key properties of the band structures, while impedance spectroscopy reveals purely electronic band-type transport in the Fe frameworks and redox-type mixed ion−electron conductivity in the others. Redox chemistry and computation suggest that the unique conductivity of Fe arises from its propensity toward Fe 2+ /Fe 3+ mixed valency as a source of ptype doping and from its highly covalent bonds that ensure high carrier mobilities. Taken together, these results demonstrate openframework chalcogenides as a well-defined platform for understanding porous semiconductors and for achieving highly tunable electronic performance.

Redox intercalation involves coupled ion-electron motion within host materials, finding extensive application in energy storage, electrocatalysis, sensing, and optoelectronics. Monodisperse MOF nanocrystals, compared to their bulk phases, exhibit accelerated mass transport kinetics that promote redox intercalation inside nanoconfined pores. However, nanosizing MOFs significantly increases their external surface-to-volume ratios, making the intercalation redox chemistry into MOF nanocrystals difficult to understand due to the challenge of differentiating redox sites at the exterior of MOF particles from the internal nanoconfined pores. Here, we report that Fe(1,2,3triazolate) 2 possesses an intercalation-based redox process shifted ca. 1.2 V from redox at the particle surface. Such distinct chemical environments do not appear in idealized MOF crystal structures but become magnified in MOF nanoparticles. Quartz crystal microbalance and time-of-flight secondary ion mass spectrometry combined with electrochemical studies identify the existence of a distinct and highly reversible Fe 2+ /Fe 3+ redox event occurring within the MOF interior. Systematic manipulation of experimental parameters (e.g., film thickness, electrolyte species, solvent, and reaction temperature) reveals that this feature arises from the nanoconfined (4.54 Å) pores gating the entry of charge-compensating anions. Due to the requirement for full desolvation and reorganization of electrolyte outside the MOF particle, the anion-coupled oxidation of internal Fe 2+ sites involves a giant redox entropy change (i.e., 164 J K −1 mol −1 ). Taken together, this study establishes a microscopic picture of ion-intercalation redox chemistry in nanoconfined environments and demonstrates the synthetic possibility of tuning electrode potentials by over a volt, with profound implications for energy capture and storage technologies.

Interactions between ions and itinerant charges govern electronic processes ranging from the redox chemistry of molecules to the conductivity of organic semiconductors, but remain an open frontier in the study of microporous materials.

Redox intercalation involves coupled ion-electron motion within host materials, finding extensive application in energy storage, electrocatalysis, sensing, and optoelectronics. While metal-organic frameworks (MOFs) comprise a diverse class of porous electrochemical materials, the intercalation redox chemistry of MOFs remains poorly understood due to the presence of redox sites at the exterior of MOF particles and in the internal pores. Here, we report that Fe(1,2,3-triazolate)2 possesses an intercalation-based redox process shifted ca. 1.2 V from redox at the particle surface. Such distinct chemical environments do not appear in idealized MOF crystal structures but become magnified in MOF nanoparticles. Quartz crystal microbalance and time-of-flight secondary ion mass spectrometry combined with electrochemical studies identify the existence of a distinct and highly reversible Fe2+/Fe3+ redox event occurring within the MOF interior. Systematic manipulation of experimental parameters (e.g., film thickness, electrolyte species, solvent, and reaction temperature) reveals that this feature arises from the nanoconfined (4.54 Å) pores gating the entry of charge-compensating anions. Due to the requirement for full desolvation and reorganization of electrolyte outside the MOF particle, the anion-coupled oxidation of internal Fe2+ sites involves a giant redox entropy change (i.e., 164 J K-1 mol-1). Taken together, this study establishes a microscopic picture of ion intercalation redox chemistry in nanoconfined environments and demonstrates the synthetic possibility of tuning electrode potentials by over a volt, with profound implications for energy capture and storage technologies.