Three-motif molecular junction photocatalysts with long-lived charge carriers for H2O2 production

Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been thoroughly investigated as the components of various photovoltaic cells due to several advantages such as spectral tunability, absence of charge-transfer (CT) states, giant aspect ratio, chemical robustness, and hydrophobicity. In the previous study, it was reported that the heterojunctions between s-SWCNTs and perylene diimide (PDI)-based electron acceptors yield long-lived charge separated states whose lifetimes are more than 1.5 µs. Besides the potential of PDI-based electron acceptors to substitute fullerene-based electron acceptors, this work noted the significance of the molecular geometries of PDI-based electron acceptors which result in molecular aggregation and the associated charge delocalization in the acceptor phase. However, the true characteristics of the long-lived charge carriers for these heterojunctions have not been revealed yet. For instance, the degree to which the charge carriers generated at these heterojunctions are free or trapped was not yet clear. Moreover, it was not determined whether the charge recombination process from these heterojunctions is monomolecular-like or bimolecular-like. In this study, we explored the nature of charge carriers for the heterojunctions between (6,5) s-SWCNTs and two different PDI-based electron acceptors by combining two effective spectroscopic techniques: transient absorption (TA) and time-resolved microwave conductivity (TRMC). Two PDI-based electron acceptors, hPDI2-pyr-hPDI2 and Trip-hPDI2, were synthesized and coated on (6,5) s-SWCNT films to form donor-acceptor heterojunctions. TA and TRMC studies reveal that the dynamics of the charge-separated states across the (6,5) s-SWCNT/PDI-based acceptor heterojunctions remain similar over three orders of magnitude in absorbed photon flux. This fluence independence of the heterojunctions indicates that the charge carriers recombine ‘pseudo’-monomolecularly. Moreover, the charge recombination kinetics from TA and TRMC studies are well-matched, indicating that most of the generated charge carriers are free, not trapped. The unconventionally strong suppression of bimolecular charge recombination from these heterojunctions, supported by fluence independence of charge recombination dynamics, may be attributed to the high carrier mobility and good charge delocalization in both (6,5) s-SWCNTs and PDI-based acceptors. These factors can also be regarded as the origin of high free charge carrier generation in these heterojunctions. These photophysical studies provide the fundamental understandings of the charge generation process in s-SWCNT-based heterojunctions and how different electron acceptor materials can impact the nature of charge generation with respect to the heterojunction energetics and molecular orientations. The results can inform rational design strategies for s-SWCNT-based optoelectronic applications.

We present the femtosecond spectroscopic investigation of a covalently linked dyad, PCB-P3HT, formed by a segment of the conjugated polymer P3HT (regioregular poly(3-hexylthiophene)) that is end capped with the fullerene derivative PCB ([6,6]-phenyl-C-61-butyric acid ester), adapted from PCBM. The fluorescence of the P3HT segment in tetrahydrofuran (THF) solution is reduced by 64% in the dyad compared to a control compound without attached fullerene (P3HT-OH). Fluorescence upconversion measurements reveal that the partial fluorescence quenching of PCB-P3HT in THF is multiphasic and occurs on an average time scale of 100 ps, in parallel to excited-state relaxation processes. Judging from ultrafast transient absorption experiments, the origin of the quenching is excitation energy transfer from the P3HT donor to the PCB acceptor. Due to the much higher solubility of P3HT compared to PCB in THF, the PCB-P3HT dyad molecules self-assemble into micelles. When pure C-60 is added to the solution, it is incorporated into the fullerene-rich center of the micelles. This dramatically increases the solubility of C-60 but does not lead to significant additional quenching of the P3HT fluorescence by the C-60 contained in the micelles. In PCB-P3HT thin films drop-cast from THF, the micelle structure is conserved. In contrast to solution, quantitative and ultrafast (<150 fs) charge separation occurs in the solid-state films and leads to the formation of long-lived mobile charge carriers with characteristic transient absorption signatures similar to those that have been observed in P3HT: PCBM bulk heterojunction blends. While p-stacking interactions between neighboring P3HT chains are weak in the micelles, they are strong in thin films drop-cast from ortho-dichlorobenzene. Here, PCB-P3HT self-assembles into a network of long fibers, clearly seen in atomic force microscopy images. Ultrafast charge separation occurs also for the fibrous morphology, but the transient absorption experiments show fast loss of part of the charge carriers due to intensity-induced recombination and annihilation processes and monomolecular interfacial trap-mediated or geminate recombination. The yield of the long-lived charge carriers in the highly organized fibers is however comparable to that obtained with annealed P3HT: PCBM blends. PCB-P3HT can therefore be considered as an active material in organic photovoltaic devices.

Understanding the dissociation of excitons into long-lived free charge carriers is a crucial issue when considering the applications of transition metal dichalcogenides (excitonic semiconductors) oriented toward the use of solar energy (such as photovoltaics or photocatalysis). In our work, long-lived carriers have been observed by time-resolved microwave photoconductivity (TRMC) for the first time in both atomically thin and bulk MoS2, MoSe2, WS2, and WSe2 crystals. The lifetime of majority carriers is close to microseconds and can even reach several microseconds due to different contribution of surface and defect states, as well as surface band bending (bulk). The three components depend on the material and vary from sample to sample, therefore determining the dynamics of the TRMC signal. Therise time of TRMC signal was found to be in the range of 0.1-0.2μs and as it depends on the studied material it can be speculated that it is related to the dissociation time of excitons captured by traps.

Photosynthesis of hydrogen peroxide (H2O2) from air and water has emerged as one of promising alternative strategies to the conventional anthraquinone process. Nevertheless, its practical development is impeded by limited charge separation efficiency and rapid charge-carrier recombination. In this study, a covalently connected molecular junction is synthesized via sequential Schiff and Knoevenagel polymerization reactions for visible-light-driven and sacrificial-agent-free H2O2 synthesis. The molecular junction effectively promotes charge separation and enhances photocatalytic efficiency. Femtosecond transient absorption (fs-TAS) spectra reveals that construction of the three-motif molecular junction dramatically extends carrier lifetimes up to 12ns, which is about 100 times longer than the two-motif junction. As expected, the three-motif molecular junction (TAS4) exhibits an impressive photocatalytic H2O2 production rate of 4302µmol g-1 h-1 under AM 1.5G irradiation without any sacrificial agent in air atmosphere, which is 2.4 and 2 times higher than that of the two-motif junctions (TA and TS). Density functional theory (DFT) calculations and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirm that H2O2 production on three-motif molecular junction TAS4 occurs via a stepwise one-electron oxygen reduction reaction (ORR). This work demonstrates the potential of molecular junction for efficient solar-driven H2O2 production.

Bismuth-based perovskites are attracting intense scientific interest due to low toxicity and excellent moisture stability compared to lead-based analogues. However, high exciton binding energy, poor charge carrier separation, and transport efficiencies lower their optoelectronic performances. To address these issues, we have integrated an electronically active organic cation, naphthalimide ethylammonium, between the [BiI52-]n chains via crystal engineering to form a novel perovskite-like material (naphthalimide ethylammonium)2BiI5 (NBI). Single crystal analysis revealed a one-dimensional quantum-well structure for NBI in which inter-inorganic well electronic coupling is screened by organic layers. It exhibited anisotropic photoconductivity and long-lived charge carriers with milliseconds lifetime, which is higher than that of CH3NH3PbI3. Density functional theory calculations confirmed type-IIa band alignment between organic cations and inorganic chains, allowing the former to electronically contribute to the overall charge transport properties of the material.

At the monolayer level, many layered 2D transition metal dichalcogenides (TMDCs) are direct bandgap semiconductors in which photo-excitation creates strongly bound excitons. To incorporate these excitonic semiconductors into solar energy harvesting and conversion devices, it is critical to dissociate excitons to produce long-lived free charge carriers. There are still many open questions regarding the fundamental steps of exciton dissociation and subsequent recombination of charge carriers within 2D TMDCs. These include the roles of defects and traps in localizing charges, the types of interfaces that facilitate fast charge separation and slow recombination, and the role of charge diffusion/delocalization in overcoming the Coulomb binding energy following charge separation. In this talk, I will discuss our efforts at answering some of these questions using a number of 2D TMDC model systems and interfaces. We observe long-lived charge generation in certain neat 2D TMDCs that can be related to specific growth-related defects. Charge generation can also be enhanced by creating interfaces with the appropriate energetics for separating electrons and holes, and the lifetime of the charge-separated state is tied to the degree to which the carrier(s) can escape the interface through diffusion and/or delocalization. These studies provide insight into harnessing the potential of monolayer 2D TMDCs in solar energy harvesting and solar fuels applications.

Converting CO2 into high-value-added chemicals has been recognized as a promising way to tackle the fossil fuel crisis. Quantum dots (QDs) have been extensively studied for photocatalytic CO2 reduction due to their excellent optoelectronic properties. However, most of the photogenerated charge carriers recombine before they participate in the photocatalytic reaction. It is crucial to regulate the charge carriers to minimize undesired charge recombination, thus, promoting surface photocatalysis. Herein, we report a copper-doped CdS (Cu:CdS) QD photocatalyst for CO2 reduction. Density functional theory simulations and experimental results demonstrate that Cu dopants create intermediate energy levels in CdS QDs that can extend the lifetime of exciton charge carriers. Furthermore, the long-lived charge carriers can be harnessed for the photocatalytic reaction on Cu:CdS QDs. The resultant Cu:CdS QDs exhibited a significantly enhanced photocatalytic activity toward CO2 reduction compared to the pristine CdS QDs. This work highlights the importance of charge regulation in photocatalysts and opens new pathways for the exploration of efficient QD photocatalysts.

The recombination kinetics of long-lived photoexcited charge carriers in a composite of $\mathrm{poly}[2\ensuremath{-}\mathrm{methoxy}{\ensuremath{-}5\ensuremath{-}(3}^{\ensuremath{'}}{,7}^{\ensuremath{'}}\ensuremath{-}\mathrm{dimethyloctyloxy})\ensuremath{-}1,4\ensuremath{-}\mathrm{phenylene}$ vinylene] (MDMO-PPV) and $1\ensuremath{-}(3\ensuremath{-}\mathrm{methoxycarbonyl})\ensuremath{-}\mathrm{propyl}\ensuremath{-}1\ensuremath{-}\mathrm{phenyl}\ensuremath{-}(6,6){\mathrm{C}}_{61}$ (PCBM) at low temperatures $(T=40\mathrm{K})$ are investigated by light induced electron-spin resonance (LESR). These long-lived (persistent) photoinduced charge carriers exhibit recombination times that extend over several hours after cessation of the photoexcitation. These long relaxation times can be explained by nongeminate recombination of randomly distributed carriers assuming charge neutrality. The decay curves fit well to a model in which the recombination mechanism of photoexcited carriers consists of tunneling processes and in which the recombination rate only depends on the intrapair distance between the photoexcited carriers. It is shown that the residual photoexcited carrier concentration after long times tends to be independent of the generation rate. The presented model has already been successful in describing the recombination kinetics of photoexcited carriers in inorganic, amorphous semiconductors, which indicates that the presented recombination mechanism is common to disordered organic and inorganic materials.

One-pot ultrasonic assisted sol-gel synthesis of spindle-like Nd and V codoped ZnO for efficient photocatalytic degradation of organic pollutants

Delayed high-energy fluorescence observed experimentally in methylammonium lead bromine CH3NH3PbBr3 (MAPbBr3) demonstrates long-lived energetic charge carriers with extremely high mobilities that can be used to enhance photon-to-electron conversion efficiency of perovskite solar cells. It has been suggested that hot fluorescence is associated with reorientational motions of the MA molecules. We support this hypothesis by time-domain ab initio quantum dynamics calculations showing that reorientation of the MA molecules can affect strongly the perovskite emission energy and lifetime. We demonstrate MAPbBr3 structures differing in the MA orientations and exhibiting the same emission properties as in the experiments. The higher bandgap structures responsible for hot fluorescence support delocalized wave functions that can be interpreted as free charge carriers. The lower energy structures exhibit localized polaron-like electrons and holes, and a significantly longer electron-hole recombination time, in agreement with experiment. The fluorescence lifetimes differ owing to variation in the nonadiabatic coupling between the emitting and ground states, stemming from charge carrier localization. Loss of coherence due to elastic electron-phonon scattering is similar in the two cases. The simulations provide a detailed atomistic understanding of excited-state dynamics in MAPbBr3 and show how structural transformations can rationalize the experimentally reported hot fluorescence in MAPbBr3. Other localized structures involving inorganic lattice distortions, defects, domain boundaries, ion diffusion, electric ordering, etc., can be invoked with the proposed two-emitter interpretation of hot and regular luminescence.

The ever-increasing demand for sustainable solutions for eliminating environmental pollutants, solar energy harvesting, water splitting, etc. have led to the design and development of novel materials to achieve the desired result. In this regard, structurally and electronically integrated (SEI) BiVO4-TiO2 (SEI-BVT) with abundant heterojunctions has emerged as a promising entity for efficient charge separation, which in turn enhances artificial photosynthesis (APS) activity. The present work adopted a unique synthetic strategy using SILAR to fabricate SEI-BVT from ionic precursors (Bi3+ and VO43-) into the pores of TiO2, exhibiting benchmark APS efficiency compared to the individual components. This preparation results in approximately 180 trillion uniformly distributed heterojunctions in 1 mg cm-2 of the SEI-BVT photoanode material. Charge carriers in SEI-BVT and BiVO4 are similar; however, the recombination is highly hindered when SEI-BVT heterojunctions are formed in the former. Our earlier work demonstrated 31-38% solar-to-fuel efficiency (STFE) with BiVO4-TiO2 for APS in the presence of the Pd-nanocube co-catalyst. The emphasis of the current work is to explore the dynamics of the light-induced processes in these heterojunctions to understand the interfacial charge transfer process. Femtosecond transient absorption (TA) spectroscopy has been employed to monitor the excited state dynamics. Our results show that new trap states have evolved under light illumination, which are significantly long-lived and hinder charge recombination, and consequently enhance STFE. A significantly large number of charge carriers exhibit a lifetime of ≫6 ns with visible light photons, at least up to 720 nm, which is higher than the band-gap absorption onset at 490 nm for SEI-BVT compared to bulk BiVO4. The rate of formation of charge carriers is significantly affected in the heterojunctions.

Photoinduced charge transfer (PCT) is a key step in the light-harvesting (LH) process producing the redox equivalents for energy conversion. However, like traditional macromolecular donor-acceptor assemblies, most MOF-derived LH systems are designed with a large ΔG0 to drive PCT. To emulate the functionality of the reaction center of the natural LH complex that drives PCT within a pair of identical chromophores producing charge carriers with maximum potentials, we prepared two electronically diverse carboxy-terminated zinc porphyrins, BFBP(Zn)-COOH and TFP(Zn)-COOH, and installed them into the hexagonal pores of NU-1000 via solvent-assisted ligand incorporation (SALI), resulting in BFBP(Zn)@NU-1000 and TFP(Zn)@NU-1000 compositions. Varying the number of trifluoromethyl groups at the porphyrin core, we tuned the ground-state redox potentials of the porphyrins within ca. 0.1 V relative to that of NU-1000, defining a small ΔG0 for PCT. For BFBP(Zn)@NU-1000, the relative ground- and excited-state redox potentials of the components facilitate an energy transfer (EnT) from NU-1000* to BFBP(Zn), forming BFBP(Zn)S1* which entails a long-lived charge-separated complex formed through an exciplex-like [BFBP(Zn)S1*-TBAPy] intermediate. Various time-resolved spectroscopic data suggest that EnT from NU-1000* may not involve a fast Förster-like resonance energy transfer (FRET) but rather through a slow [NU-1000*-BFBP(Zn)] intermediate formation. In contrast, TFP(Zn)@NU-1000 displays an efficient EnT from NU-1000* to [TFP(Zn)-TBAPy], a complex that formed at the ground state through electronic interaction, and thereon showed the excited-state feature of [TFP(Zn)-TBAPy]*. The results will help to develop synthetic LHC systems that can produce long-lived photogenerated charge carriers with high potentials, i.e., high open-circuit voltage in photoelectrochemical setups.

Combination of the strong light-absorbing power of plasmonic metals with the superior charge carrier dynamics of halide perovskites is appealing for bio-inspired solar-energy conversion due to the potential to acquire long-lived plasmon-induced hot electrons. However, the direct coupling of these two materials, with Au/CsPbBr3 heteronanocrystals (HNCs) as a prototype, results in severe suppression of plasmon resonances. The present work shows that interfacial engineering is a key knob for overcoming this impediment, based on the creation of a CdS mediate layer between Au and CsPbBr3 forming atomically organized Au-CdS and CdS-CsPbBr3 interfaces by nonepitaxial/epitaxial combined strategy. Transient spectroscopy studies demonstrate that the resulting Au@CdS/CsPbBr3 HNCs generate remarkably long-lived plasmon-induced charge carriers with lifetime up to nanosecond timescale, which is several orders of magnitude longer than those reported for colloidal plasmonic metal-semiconductor systems. Such long-lived carriers extracted from plasmonic antennas enable to drive CO2 photoreduction with efficiency outperforming previously reported CsPbBr3 -based photocatalysts. The findings disclose a new paradigm for achieving much elongated time windows to harness the substantial energy of transient plasmons through realization of synergistic coupling of plasmonic metals and halide perovskites.

We use photoinduced absorption spectroscopy to measure long-lived photogenerated charge carriers in optically thin donor/acceptor conjugated polymer blend films near plasmon-resonant silver nanoprisms. We measure up to 3 times more charge generation, as judged by the magnitude of the polaron absorption signal, in 35 nm thin blend films of poly(3-hexylthiophene)/phenyl-C(61)-butyric acid methyl ester on top of films of silver nanoprisms (approximately 40-100 nm edge length). We find that the polaron yields increase linearly with the total sample extinction. These excitation enhancements could in principle be used to increase photocurrents in thin organic solar cells.

Two-dimensional (2D) Ruddlesden-Popper perovskites (RPPs) are a class of quantum well (QW) materials showing large exciton binding energy owing to quantum confinement. The existence of localized edge states was proposed to accelerate exciton dissociation into long-lived charge carriers in 2D RPPs, but recent experimental reports suggested that highly efficient internal exciton dissociation is achievable in 2D RPPs despite the absence of edge states. Herein, we adopt first-principles calculations to unveil the physical origin of the high internal quantum efficiency in the bulk region of widely familiar (BA)2(MA)n-1PbnI3n+1 (BA = butylammonium; MA = methylammonium) materials. We discover that the dipolar nature of MA cations provides the driving force for the separation of photoexcited electron-hole pairs inside QWs as the inorganic layer thickens from n = 1 to n = 3. Concurrently, electronic coupling between organic spacer layers and QWs is enhanced in the energetically favorable configurations where MA cations orient with their CH3 groups towards the exterior PbI2 layers of QWs in the n = 3 structure. Consequently, hole delocalization is promoted along the out-of-plane direction of QWs, which in turn facilitates exciton dissociation into free charge carriers despite large exciton binding energy. Our simulations reveal that the hydrogen bonding between organic species (including both MA and BA cations) and iodine atoms, which is subtly interconnected, engineers the response of morphology in QWs and electronic interactions at organic-inorganic interfaces, providing novel insights for the exciton-free carrier behavior in the bulk area of 2D RPPs.