Interrogating Light-initiated Dynamics in Metal-Organic Frameworks with Time-resolved Spectroscopy.

Time-resolved spectroscopy is an essential part of both fundamental and applied chemical research. Such techniques access light-initiated dynamics on time scales ranging from femtosecond to microsecond. Many techniques falling under this description have been applied to gain significant insight into metal-organic frameworks (MOFs), a diverse class of porous coordination polymers. MOFs are highly tunable, both compositionally and structurally, and unique challenges are encountered in applying time-resolved spectroscopy to interrogate their light-initiated properties. These properties involve various excited state mechanisms such as crystallographically defined energy transfer, charge transfer, and localization within the framework, photoconductivity, and structural dynamics. The field of time-resolved MOF spectroscopic studies is quite nascent; each original report cited in this review was published within the past decade. As such, this review is a timely and comprehensive summary of the most significant contributions in this emerging field, with focuses on the overarching spectroscopic concepts applied and on identifying key challenges and future outlooks moving forward.

Zeolitic imidazolate frameworks as intrinsic light harvesting and charge separation materials for photocatalysis.

Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal organic frameworks that have attracted considerable attention in the past years and have found many applications including heterogeneous catalysis due to their highly ordered porous structure, large surface area, and structural flexibility. However, ZIFs are largely utilized as simple hosts or passive media for dispersing other catalytically active species, resembling the roles of zeolites in catalysis. In contrast, our recent findings show that ZIFs not only have broad absorption across the UV-visible and near IR spectral region but also have an exceptionally long-lived excited charge separated state, suggesting that ZIFs may be used as intrinsic light harvesting and photocatalytic materials rather than as inert hosts. This Perspective will focus on the recent progress on the fundamental studies of the intrinsic light absorption, charge separation, and photocatalytic properties of ZIFs and will discuss the outlook for future development.

Direct Evidence of Photoinduced Charge Transport Mechanism in 2D Conductive Metal Organic Frameworks.

Conductive metal organic frameworks (MOFs) represent a promising class of porous crystalline materials that have demonstrated potential in photo-electronics and photocatalytic applications. However, the lack of fundamental understanding on charge transport (CT) mechanism as well as the correlation of CT mechanism with their structure hampered their further development. Herein, we report the direct evidence of CT mechanism in 2D Cu-THQ MOFs and the correlation of temporal and spatial behaviors of charge carriers with their photoconductivity by combining three advanced spectroscopic methods, including time resolved optical and X-ray absorption spectroscopy and terahertz spectroscopy. In addition to Cu-THQ, the CT in Cu/Zn-THQ after incorporating Zn2+ guest metal was also examined to uncover the contribution of through space pathway, as the presence of the redox inactive 3d10 Zn2+ is expected to perturb the long range in-plane CT. We show that the hot carriers in Cu-THQ generated after photoexcitation are highly mobile and undergo fast localization to a lower energy state (cool carriers) with electrons occupying Cu center and holes in ligands. The cool carriers, which have super long lifetime (>17 ns), are responsible for the long-term photoconductivity in Cu-THQ and transport through the O-Cu-O motif with negligible contribution from interlayer ligand π-π stacking, as incorporation of Zn2+ in Cu-THQ significantly reduced photoconductivity. These unprecedented results not only demonstrate the capability to experimentally probe CT mechanism but also provide important insight in the rational design of 2D MOFs for photoelectronic and photocatalytic applications.

Metal-Organic Framework Photoconductivity via Time-Resolved Terahertz Spectroscopy.

While metal-organic frameworks (MOFs) have been under thorough investigation over the past two decades, photoconductive MOFs are an emerging class of materials with promising applications in light harvesting and photocatalysis. To date, there is not a general method to investigate the photoconductivity of polycrystalline MOF samples as-prepared. Herein, we utilize time-resolved terahertz spectroscopy along with a new sample preparation method to determine the photoconductivity of Zn2TTFTB, an archetypical conductive MOF, in a noncontact manner. Using this technique, we were able to gain insight into MOF photoconductivity dynamics with subpicosecond resolution, revealing two distinct carrier lifetimes of 0.6 and 31 ps and a long-lived component of several ns. Additionally, we determined the frequency dependent photoconductivity of Zn2TTFTB which was shown to follow Drude-Smith behavior. Such insights are crucially important with regard to developing the next generation of functional photoconductive MOF materials.

Direct Observation of Node-to-Node Communication in Zeolitic Imidazolate Frameworks.

Zeolitic imidazolate frameworks (ZIFs) with open-shell transition metal nodes represent a promising class of highly ordered light harvesting antennas for photoenergy applications. However, their charge transport properties within the framework, the key criterion to achieve efficient photoenergy conversion, are not yet explored. Herein, we report the first direct evidence of a charge transport pathway through node-to-node communication in both ground state and excited state ZIFs using the combination of paramagnetic susceptibility measurements and time-resolved optical and X-ray absorption spectroscopy. These findings provide unprecedented new insights into the photoactivity and charge transport nature of ZIF frameworks, paving the way for their novel application as light harvesting arrays in diverse photoenergy conversion devices.

Donor-Acceptor Fluorophores for Energy-Transfer-Mediated Photocatalysis.

Triplet-triplet energy transfer (EnT) is a fundamental activation pathway in photocatalysis. In this work, we report the mechanistic origins of the triplet excited state of carbazole-cyanobenzene donor-acceptor (D-A) fluorophores in EnT-based photocatalytic reactions and demonstrate the key factors that control the accessibility of the 3LE (locally excited triplet state) and 3CT (charge-transfer triplet state) via a combined photochemical and transient absorption spectroscopic study. We found that the energy order between 1CT (charge transfer singlet state) and 3LE dictates the accessibility of 3LE/3CT for EnT, which can be effectively engineered by varying solvent polarity and D-A character to depopulate 3LE and facilitate EnT from the chemically more tunable 3CT state for photosensitization. Following the above design principle, a new D-A fluorophore with strong D-A character and weak redox potential is identified, which exhibits high efficiency for Ni(II)-catalyzed cross-coupling of carboxylic acids and aryl halides with a wide substrate scope and high selectivity. Our results not only provide key fundamental insight on the EnT mechanism of D-A fluorophores but also establish its wide utility in EnT-mediated photocatalytic reactions.

2D Covalent Organic Frameworks as Intrinsic Photocatalysts for Visible Light-Driven CO2 Reduction.

Covalent organic framework (COF) represents an emerging class of porous materials that have exhibited great potential in various applications, particularly in catalysis. In this work, we report a newly designed 2D COF with incorporated Re complex, which exhibits intrinsic light absorption and charge separation (CS) properties. We show that this hybrid catalyst can efficiently reduce CO2 to form CO under visible light illumination with high electivity (98%) and better activity than its homogeneous Re counterpart. More importantly, using advanced transient optical and X-ray absorption spectroscopy and in situ diffuse reflectance spectroscopy, we unraveled three key intermediates that are responsible for CS, the induction period, and rate limiting step in catalysis. This work not only demonstrates the potential of COFs as next generation photocatalysts for solar fuel conversion but also provide unprecedented insight into the mechanistic origins for light-driven CO2 reduction.

Photoinduced interfacial charge separation dynamics in zeolitic imidazolate framework.

Owing to their porous structure and tunable framework, zeolitic imidazolate frameworks (ZIFs) have garnered considerable attention as promising photocatalytic materials. However, little is known regarding their photophysical properties. In this work, we report the photoinduced charge separation dynamics in a ZIF-67 thin film through interfacial electron transfer (ET) to methylene blue (MB+) via ultrafast transient absorption spectroscopy. We show that the ET process occurs through two distinct pathways, including an ultrafast (<200 fs) process from the [CoII(mim)2] units located on the surface of ZIF-67 film that are directly in contact with MB+ and a relatively slower ET process with a 101.4 ps time constant from the units in the bulk of the film that were isolated from MB+ by the surface units. This first direct evidence of the ET process from ZIF-67 to electron acceptor strongly suggests that ZIF materials may be used as intrinsic photocatalytic materials rather than inert hosts.

Conformational States of Cytochrome P450 Oxidoreductase Evaluated by Förster Resonance Energy Transfer Using Ultrafast Transient Absorption Spectroscopy.

NADPH-cytochrome P450 oxidoreductase (CYPOR) was shown to undergo large conformational rearrangements in its functional cycle. Using a new Förster resonance energy transfer (FRET) approach based on femtosecond transient absorption spectroscopy (TA), we determined the donor-acceptor distance distribution in the reduced and oxidized states of CYPOR. The unmatched time resolution of TA allowed the quantitative assessment of the donor-acceptor FRET, indicating that CYPOR assumes a closed conformation in both reduced and oxidized states in the absence of the redox partner. The described ultrafast TA measurements of FRET with readily available red-infrared fluorescent labels open new opportunities for structural studies in chromophore-rich proteins and their complexes.

Exceptionally Long-Lived Charge Separated State in Zeolitic Imidazolate Framework: Implication for Photocatalytic Applications.

Zeolitic imidazolate frameworks (ZIFs) have emerged as a novel class of porous metal-organic frameworks (MOFs) for catalysis application because of their exceptional thermal and chemical stability. Inspired by the broad absorption of ZIF-67 in UV-vis-near IR region, we explored its excited state and charge separation dynamics, properties essential for photocatalytic applications, using optical (OTA) and X-ray transient absorption (XTA) spectroscopy. OTA results show that an exceptionally long-lived excited state is formed after photoexcitation. This long-lived excited state was confirmed to be the charge-separated (CS) state with ligand-to-metal charge-transfer character using XTA. The surprisingly long-lived CS state, together with its intrinsic hybrid nature, all point to its potential application in heterogeneous photocatalysis and energy conversion.

The effect of Mo doping on the charge separation dynamics and photocurrent performance of BiVO4 photoanodes.

Doping with electron-rich elements in BiVO4 photoanodes has been demonstrated as a desirable approach for improving their carrier mobility and charge separation efficiency. However, the effect of doping and dopant concentration on the carrier dynamics and photoelectrochemical performance remains unclear. In this work, we examined the effects of Mo doping on the charge separation dynamics and photocurrent performance in BiVO4 photoanodes. We show that the photocurrent of BiVO4 photoanodes increases with increasing concentration of the Mo dopant, which can be attributed to both the improved carrier mobility resulting from increased electron density and charge separation efficiency due to the diminishing of trap states upon Mo doping. The effect of doping on the electronic structure, carrier dynamics and photocurrent performance of BiVO4 photoanodes resulting from W and Mo dopants was also compared and discussed in this study. The knowledge gained from this work will provide important insights into the optimization of the carrier mobility and charge separation efficiency of BiVO4 photoanodes by controlling the dopants and their concentrations.

Terahertz Conductivity of Semiconducting 2H and Metallic 1T Phases of Molybdenum Disulfide.

Molybdenum disulfide (MoS2) has been extensively studied in its commonly occurring semiconducting 2H phase. Recent synthetic advances have enabled the bulk synthesis of the catalytically promising metallic 1T phase. However, the conductivity of bulk 1T-MoS2 has not been well characterized to ascertain the carrier transport properties. Terahertz (THz) spectroscopy is an ideal technique for obtaining this crucial information because it is a noncontact method of measuring the conductivity of emerging materials with ultrafast time resolution. This work applies THz spectroscopy to bulk 2H-MoS2 and 1T-MoS2, representing the first application of the technique on the 1T phase, with measurements confirming the semiconducting character of 2H-MoS2 and the metallic character of 1T-MoS2. This study provides new insight into the metallic nature of bulk 1T-MoS2 and a direct comparison to the semiconducting 2H phase that, together with physical characterization to obtain material parameters, are important for optimizing applications in catalysis devices and beyond.

Single Copper Atoms Enhance Photoconductivity in g-C3N4.

Graphitic carbon nitride (g-C3N4) and its doped analogues have been studied over the past decade in part due to their promising applications in heterogeneous photocatalysis; however, the effect of doping on the photoconductivity is poorly understood. Herein, we investigate Cu doped g-C3N4 (Cu-g-C3N4) and demonstrate via extended X-ray absorption fine structure that Cu+ incorporates as an individual ion. Time-resolved optical pump terahertz probe spectroscopy was utilized to measure the ultrafast photoconductivity in response to a 400 nm pump pulse and showed that the Cu+ dopant significantly enhances photoconductivity of the as-prepared powdered sample, which decays within 10 ps. Furthermore, a film preparation technique was applied that further enhanced the photoconductivity and induced a longer-lived photoconductive state with a lifetime on the order of 100 ps. This study provides valuable insight into the ultrafast photoconductivity dynamics of g-C3N4 materials, which is essential toward developing efficient g-C3N4 photocatalysts.

A conductive metal-organic framework photoanode.

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.

Dynamic evolution and reversibility of single-atom Ni(II) active site in 1T-MoS2 electrocatalysts for hydrogen evolution.

1T-MoS2 and single-atom modified analogues represent a highly promising class of low-cost catalysts for hydrogen evolution reaction (HER). However, the role of single atoms, either as active species or promoters, remains vague despite its essentiality toward more efficient HER. In this work, we report the unambiguous identification of Ni single atom as key active sites in the basal plane of 1T-MoS2 (Ni@1T-MoS2) that result in efficient HER performance. The intermediate structure of this Ni active site under catalytic conditions was captured by in situ X-ray absorption spectroscopy, where a reversible metallic Ni species (Ni0) is observed in alkaline conditions whereas Ni remains in its local structure under acidic conditions. These insights provide crucial mechanistic understanding of Ni@1T-MoS2 HER electrocatalysts and suggest that the understanding gained from such in situ studies is necessary toward the development of highly efficient single-atom decorated 1T-MoS2 electrocatalysts.

Publisher Correction: Atomically engineering activation sites onto metallic 1T-MoS2 catalysts for enhanced electrochemical hydrogen evolution.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Atomically engineering activation sites onto metallic 1T-MoS2 catalysts for enhanced electrochemical hydrogen evolution.

Engineering catalytic sites at the atomic level provides an opportunity to understand the catalyst's active sites, which is vital to the development of improved catalysts. Here we show a reliable and tunable polyoxometalate template-based synthetic strategy to atomically engineer metal doping sites onto metallic 1T-MoS2, using Anderson-type polyoxometalates as precursors. Benefiting from engineering nickel and oxygen atoms, the optimized electrocatalyst shows great enhancement in the hydrogen evolution reaction with a positive onset potential of ~ 0 V and a low overpotential of -46 mV in alkaline electrolyte, comparable to platinum-based catalysts. First-principles calculations reveal co-doping nickel and oxygen into 1T-MoS2 assists the process of water dissociation and hydrogen generation from their intermediate states. This research will expand on the ability to improve the activities of various catalysts by precisely engineering atomic activation sites to achieve significant electronic modulations and improve atomic utilization efficiencies.

High-index faceted CuFeS2 nanosheets with enhanced behavior for boosting hydrogen evolution reaction.

A rational design of highly active and robust catalysts based on earth-abundant elements for hydrogen evolution reaction (HER) is essential for future renewable energy applications. Herein, we report the synthesis of a new class of ultrathin metallic CuFeS2 nanosheets (NSs) with abundant exposed high-index {02[combining macron]4} facets. They serve as a robust catalyst for the HER with a lower onset potential of 28.1 mV, an overpotential of only 88.7 mV (at j = 10 mA cm-2) and remarkable long-term stability in 0.5 M H2SO4, which make them the best system among all the reported non-noble metal catalysts. The theoretical calculations reveal that the mechanistic origin for such a high HER activity should be attributed to the excess S2- active sites on the exposed {02[combining macron]4} high-index facets of CuFeS2 NSs, which have a rather favorable Gibbs free energy for atomic hydrogen adsorption. The present work highlights the importance of designing ultrathin metallic chalcopyrite nanosheets with high-index facets in order to increase the number of active sites for boosting the HER performance.

Ultrafast Hole Trapping and Relaxation Dynamics in p-Type CuS Nanodisks.

CuS nanocrystals are potential materials for developing low-cost solar energy conversion devices. Understanding the underlying dynamics of photoinduced carriers in CuS nanocrystals is essential to improve their performance in these devices. In this work, we investigated the photoinduced hole dynamics in CuS nanodisks (NDs) using the combination of transient optical (OTA) and X-ray (XTA) absorption spectroscopy. OTA results show that the broad transient absorption in the visible region is attributed to the photoinduced hot and trapped holes. The hole trapping process occurs on a subpicosecond time scale, followed by carrier recombination (~100 ps). The nature of the hole trapping sites, revealed by XTA, is characteristic of S or organic ligands on the surface of CuS NDs. These results not only suggest the possibility to control the hole dynamics by tuning the surface chemistry of CuS but also represent the first time observation of hole dynamics in semiconductor nanocrystals using XTA.