A conductive metal-organic framework photoanode

Abstract

We report the development of photosensitizing arrays based on conductive metal-organic frameworks (MOFs) that enable light harvesting and efficient charge separation. Zn₂TTFTB (TTFTB = tetrathiafulvalene tetrabenzoate) MOFs are deposited directly onto TiO₂ photoanodes and structurally characterized by pXRD and EXAFS measurements. Photoinduced interfacial charge transfer dynamics are investigated by combining time-resolved THz spectroscopy and quantum dynamics simulations. Sub-600 fs electron injection into TiO₂ is observed for Zn₂TTFTB-TiO₂ and is compared to the corresponding dynamics for TTFTB-TiO₂ analogues that lack the extended MOF architecture. Rapid electron injection from the MOF into TiO₂ is enhanced by facile migration of the hole away from the interfacial region. Holes migrate through strongly coupled HOMO orbitals localized on the tetrathiafulvalene cores of the columnar stacks of the MOF, whereas electrons are less easily transferred through the spiral staircase arrangement of phenyl substituents of the MOF. The reported findings suggest that conductive MOFs could be exploited as novel photosensitizing arrays in applications to slow, and thereby make difficult, photocatalytic reactions such as those required for water-splitting in artificial photosynthesis.

Fig. 1. Depiction showing a top (a) and side (b) view of the 3-layer model of Zn₂TTFTB conductive MOF in this study. The 3-layer cutout shown is half an optimized unit cell where each layer rotates 60° about the center. The 1 layer cut from an optimized unit cell (c) and the protonated ligand H₄TTFTB (d) are shown.

Fig. 2. Low 2θ region of the pXRD pattern containing Zn₂TTFTB diffractions (a) and Zn K-edge XANES spectra (b). EXAFS spectra in k-space (c) and R-space (d) with data shown as open points and solid best fit lines. Inset of (b) shows a segment of the MOF structure depicting the two unique Zn centers repeating in a chain.

Fig. 3. SEM results for Zn₂TTFTB–TiO₂. Top-view images (a and b) and cross-section images (c and d) show the formation of a discontinuous monolayer of hemispherical flower-like MOF structures. EDS mapping (e), demonstrating localization of C, S, and Zn within the MOF structures. Scales: (a) 50 μm; (b) 2 μm; (c) 20 μm; (d) 100 μm; (e) 20 μm.

Fig. 4. (a) Spectroelectrochemistry of H₄TTFTB under oxidation and (b) reduction. (c) Spectroelectrochemistry of Zn₂TTFTB–FTO. (d) Nernst plot of Zn₂TTFTB–FTO spectroelectrochemistry data at 700 nm. Inset of (a) is the cyclic voltammetry of H₄TTFTB in 0.1 M TBAPF₆-DMF.

Fig. 5. Experimental (solid) and calculated (dashed) normalized UV-visible spectra. Stacked Natural Transition Orbitals (NTOs) show the initial (bottom) and final (top) orbitals for each transition. (a) UV-visible spectrum of H₄TTFTB in DMF compared to the diffuse reflectance spectrum of Zn₂TTFTB and the calculated neutral ligand in DMF with NTO's for the transition of interest. (b and c) Difference spectra comparison of the experimental and TDDFT calculated H₄TTFTB cationic-neutral/anion-neutral species spectra and the NTO's of important features. (d) Comparison of TDDFT calculated spectra for charge varied species and NTO's for the same transition in each species.

Fig. 6. The (a) calculated density of states (DOS) are shown for TTFTB lying (b) flat, long, or tall on the surface, each with 2 attachment points. The HOMO and LUMO positions of the isolated ligand are denoted in red. Full DOS for each joint system orientation are overlaid with the isolated ligand and isolated TiO₂ DOS. All DOS containing TiO₂ which have been vertically scaled to a maximum peak height of 10. (c) Subtraction of the joint systems from the summation of isolated systems shows the changes in the DOS when the ligand is bound to TiO₂. (a) and (c) are both centered on the Fermi level of the joint flat system.

Fig. 7. Interfacial electron transfer from H₄TTFTB on TiO₂ in a flat (top), long (middle), and tall (bottom) orientation.

Fig. 8. OPTP traces for TTFTB-sensitized TiO₂ (a) and Zn₂TTFTB (b) and their respective normalized traces (c and d). Inset of (d) compares the normalized TTFTB-sensitized trace to the Zn₂TTFTB–TiO₂ trace at 1.27 mJ cm⁻² excitation fluence.

Fig. 9. (a) ns-TA results showing the recombination lifetime and (b) proposed scheme.

Fig. 10. Spin densities of the cationic H₄TTFTB and 1 layer models. Spin density is represented in blue and is seen on the tetrathiafulvaline core only for an isovalue of 0.0008 for 1 layer Zn₂TTFTB (A) and H₄TTFTB (B).

Fig. 11. Proposed electron and hole transfer pathways. Calculations in this work suggest that electrons travel between phenyl stacks and holes through the S-rich TTF core.