RESUMO
Engendering electrical conductivity in otherwise insulating metal-organic framework (MOF) materials is key to rendering these materials fully functional for a range of potential applications, including electrochemical and photo-electrochemical catalysis. Here we report that the platform MOF, NU-1000, can be made electrically conductive via reversible electrochemical oxidation of a fraction of the framework's tetraphenylpyrene linkers, where the basis for conduction is redox hopping. At a microscopic level, redox hopping is akin to electron self-exchange and is describable by Marcus' well-known theory of electron transfer. At a macroscopic level, the hopping behavior leads to diffusive charge transport and is quantifiable as an apparent diffusion coefficient, Dhopping. Theory suggests that the csq topology of NU-1000, together with its characteristic one-dimensional mesopores, will result in direction-dependent, that is, anisotropic, electrical conductivity. Detailed computations suggest that the governing factor is the strength of electronic coupling between pairs of linkers sited in the a,b plane of the MOF versus the mesopore-aligned c axis of the crystal. The notion has been put to the test experimentally by configuring the MOF as an array of selectively oriented, electrode-supported crystallites, where the rodlike crystallites are either oriented largely normal to the electrode (requiring redox hopping along the c direction) or mainly parallel (requiring redox hopping mainly through the a,b plane). The orientations are preselected by preparing MOF films either via interfacial solvothermal synthesis or via electrophoretic deposition. In semiquantitative accord with computational predictions, Dhopping is up to â¼3500 times larger in the c direction than through the a,b plane. In addition to their fundamental significance, the findings have clear implications for the design and optimization of MOFs for electrocatalysis and for other applications that rely upon electrical conductivity.
RESUMO
Atomic layer deposition is employed to install nickel oxide into NU-1000. Upon heating to 900 °C under nitrogen, a carbon material containing ZrO2 and Ni is formed. In notable contrast to the parent metal-organic framework, the pyrolyzed material is: (a) stable in highly alkaline solutions (typical conditions for water electro-oxidation) and (b) electrically conductive and thus able to deliver oxidizing equivalents (holes) to catalytic sites located far from the underlying conductive-glass electrode. The pyrolysis-derived material was characterized and its electrocatalytic activity for oxygen evolution was investigated.
RESUMO
Here, we describe the installation of a ferrocene derivative on and within the archetypal metal-organic framework (MOF), UiO-66, by solvent-assisted ligand incorporation. Thin films of the resulting material show a redox peak characteristic of the Fc/Fc+ couple, as measured by cyclic voltammetry. Consistent with restriction of redox reactivity solely to Fc molecules sited at or near the external surfaces of MOF crystallites, chronoamperometry measurements indicate that less than 20% of the installed Fc molecules are electrochemically active. Charge-transport diffusion coefficients, DCT, of 6.1 ± 0.8 × 10-11 and 2.6 ± 0.2 × 10-9 cm2/s were determined from potential step measurements, stepping oxidatively and reductively, respectively. The 40-fold difference in DCT values contrasts with the expectation, for simple systems, of identical values for oxidation-driven versus reduction-driven charge transport. The findings have implications for the design of MOFs suitable for delivery of redox equivalents to framework-immobilized electrocatalysts and/or delivery of charges from a chromophoric MOF film to an underlying electrode, processes that may be central to MOF-facilitated conversion of solar energy to chemical or electrical energy.