Semiconductor nanocrystal photocatalysis for the production of solar fuels

Reversible Ligand Detachment from CdSe Quantum Dots Following Photoexcitation

The nanocrystal-ligand boundaries of colloidal quantum dots (QDs) mediate charge and energy transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We used time-resolved infrared spectroscopy combined with transient electronic spectroscopy to probe vibrational modes of the carboxylate anchoring groups of stearate ligands attached to cadmium selenide (CdSe) QDs that were optically excited in solid nanocrystal films. The vibrational frequencies of surface-bonded carboxylate groups revealed their interactions with surface-localized holes in the excited states of the QDs. We also observed transient and reversible photoinduced ligand detachment from CdSe nanocrystals within their excited state lifetime. By probing both surface charge distributions and ligand dynamics on QDs in their excited states, we open a pathway to explore how the nanocrystal-ligand boundary can be understood and controlled for the design of QD architectures that most effectively drive charge transfer processes in solar energy harvesting and photoredox catalysis applications.

Interfacial Charge Transfer Regulates Photoredox Catalysis

Photoredox catalytic processes offer the potential for precise chemical reactions using light and materials. The central determinant is identified as interfacial charge transfer, which simultaneously engenders distinctive behavior in the overall reaction. An in-depth elucidation of the main mechanism and highlighting of the complexity of interfacial charge transfer can occur through both diffusive and direct transfer models, revealing its potential for sophisticated design in complex transformations. The fundamental photophysics uncover these comprehensive applications and offer a clue for future development. This research contributes to the growing body of knowledge on interfacial charge transfer in photoredox catalysis and sets the stage for further exploration of this fascinating area of research.

A tight-binding model for illustrating exciton confinement in semiconductor nanocrystals

The Brus equation describes the relation between the lowest energy of an electron-hole pair and the size of a semiconductor crystallite. However, taking the strong confinement regime as a starting point, the equation does not cover the transition from weak to strong confinement, the accompanying phenomenon of charge-carrier delocalization, or the change in the transition dipole moment of the electron-hole pair state. Here, we use a one-dimensional, two-particle Hubbard model for interacting electron-hole pairs that extends the well-known tight-binding approach through a point-like electron-hole interaction. On infinite chains, the resulting exciton states exhibit the known relation between the Bohr radius, the exciton binding energy, and the effective mass of the charge carriers. Moreover, by introducing infinite-well boundary conditions, the model enables the transition of the exciton states from weak to strong confinement to be tracked, while straightforward adaptations provide insights into the relation between defects, exciton localization, and confinement. In addition, by introducing the dipole operator, the variation of the transition dipole moment can be mapped when shifting from electron-hole pairs in strong confinement to delocalized and localized excitons in weak confinement. The proposed model system can be readily implemented and extended to different multi-carrier states, thus providing researchers a tool for exploring, understanding, and teaching confinement effects in semiconductor nanocrystals under different conditions.

The role of surface functionalization in quantum dot-based photocatalytic CO2 reduction: balancing efficiency and stability

Photocatalytic CO2 reduction offers a promising strategy to produce hydrocarbons without reliance on fossil fuels. Visible light-absorbing colloidal nanomaterials composed of earth-abundant metals suspended in aqueous media are particularly attractive owing to their low-cost, ease of separation, and highly modifiable surfaces. The current study explores such a system by employing water-soluble ZnSe quantum dots and a Co-based molecular catalyst. Water solubilization of the quantum dots is achieved with either carboxylate (3-mercaptopropionic acid) or ammonium (2-aminoethanethiol) functionalized ligands to produce nanoparticles with either negatively or positively-charged surfaces. Photocatalysis experiments are performed to compare the effectiveness of these two surface functionalization strategies on CO2 reduction and ultrafast spectroscopy is used to reveal the underlying photoexcited charge dynamics. We find that the positively-charged quantum dots can support sub-picosecond electron transfer to the carboxylate-based molecular catalyst and also produce >30% selectivity for CO and >170 mmolCO gZnSe-1. However, aggregation reduces activity in approximately one day. In contrast, the negatively-charged quantum dots exhibit >10 ps electron transfer and substantially lower CO selectivity, but they are colloidally stable for days. These results highlight the importance of the quantum dot-catalyst interaction for CO2 reduction. Furthermore, multi-dentate catalyst molecules create a trade-off between photocatalytic efficiency from strong interactions and deleterious aggregation of quantum dot-catalyst assemblies.

Extending Infrared Emission via Energy Transfer in a CsPbI3-Cyanine Dye Hybrid

Directing energy flow in light harvesting assemblies of nanocrystal-chromophore hybrid systems requires a better understanding of factors that dictate excited-state processes. In this study, we explore excited-state interactions within the CsPbI3-cyanine dye (IR125) hybrid assembly through a comprehensive set of steady-state and time-resolved absorption and photoluminescence (PL) experiments. Our photoluminescence investigations reveal the quenching of CsPbI3 emission alongside the simultaneous enhancement of IR125 fluorescence, providing evidence for a singlet energy transfer. The evaluation of both photoluminescence (PL) quenching and PL decay measurements yield ∼94% energy transfer efficiency for the CsPbI3-IR125 hybrid assembly. Transient absorption spectroscopy further unveils that this singlet energy transfer process operates on an ultrafast time scale, occurring within 400 ps with a rate constant of energy transfer of 1.4 × 1010 s-1. Our findings highlight the potential of the CsPbI3-IR125 hybrid assembly to extend the emission of halide perovskites into the infrared region, paving the way for light energy harvesting and display applications.

Photo/electrocatalytic hydrogen evolution using Type-II Cu2O/g-C3N4 Heterostructure: Density functional theory addresses the improved charge transport efficiency

One of the most efficient ways for the photogenerated charge carriers is by the development of heterojunction between p-type and n-type semiconductors, which creates an interfacial charge transfer between two semiconductors. By enhancing the bifunctional characteristics for hydrogen generation via photocatalytic and electrocatalytic water splitting reaction, we report the type-II Cu2O/g-C3N4 heterostructure in this article. Due to significantly increased catalytically active sites for the hydrogen evolution reaction (HER) reaction during electrocatalysis and decreased charge transfer resistance, the as-prepared heterostructure exhibits a lower overpotential of 47 and 72 mVdec-1 for the HER and oxygen evolution reactions (OER), respectively, when compared to alone g-C3N4. In addition, Cu2O/g-C3N4 heterostructures have a higher photocatalytic hydrogen evolution of 3492 µmol gcat-1 in the presence of Triethanolamine as a sacrificial agent, which is nearly 2-fold times greater than g-C3N4 (1818 µmol gcat-1) after 5 h of continuous light-irradiation. Moreover, produced heterostructure exhibits 81% of Faradaic efficiency and 18% of apparent quantum yield. This work successfully explains how the rise in water splitting is induced by the transfer of photogenerated electrons in a cascade way from p-type Cu2O to the n-type g-C3N4 using density functional theory (DFT) calculations.

Efficient Hole Transfer from CdSe Quantum Dots Enabled by Oxygen-Deficient Polyoxovanadate-Alkoxide Clusters

A limitation of the implementation of cadmium chalcogenide quantum dots (QDs) in charge transfer systems is the efficient removal of photogenerated holes. Rapid hole transfer has typically required the ex situ functionalization of hole acceptors with groups that can coordinate to the surface of the QD. In addition to being synthetically limiting, this strategy also necessitates a competitive binding equilibrium between the hole acceptor and native, solubilizing ligands on the nanocrystal. Here we show that the incorporation of oxygen vacancies into polyoxovanadate-alkoxide clusters improves hole transfer kinetics by promoting surface interactions between the metal oxide assembly and the QD. Investigating the reactivity of oxygen-deficient clusters with phosphonate-capped QDs reveals reversible complexation of the POV-alkoxide with a phosphonate ligand at the nanocrystal surface. These findings reveal a new method of facilitating QD-hole acceptor association that bypasses the restrictions of exchange interactions.

Light-Driven Nitrogen Fixation to Ammonia over Aqueous-Dispersed Mo-Doped TiO2 Colloidal Nanocrystals

Photocatalytic nitrogen fixation to ammonia and nitrates holds great promise as a sustainable route powered by solar energy and fed with renewable energy resources (N2 and H2O). This technology is currently under deep investigation to overcome the limited efficiency of the process. The rational design of efficient and robust photocatalysts is crucial to boost the photocatalytic performance. Widely used bulk materials generally suffer from charge recombination due to poor interfacial charge transfer and difficult surface diffusion. To overcome this limitation, this work explores the use of aqueous-dispersed colloidal semiconductor nanocrystals (NCs) with precise morphological control, better carrier mobility, and stronger redox ability. Here, the TiO2 framework has been modified via aliovalent molybdenum doping, and resulting Mo-TiO2 NCs have been functionalized with charged terminating hydroxyl groups (OH-) for the simultaneous production of ammonia, nitrites, and nitrates via photocatalytic nitrogen reduction in water, which has not been previously found in the literature. Our results demonstrate the positive effect of Mo-doping and nanostructuration on the overall N2 fixation performance. Ammonia production rates are found to be dependent on the Mo-doping loading. 5Mo-TiO2 delivers the highest NH4+ yield rate (ca. 105.3 μmol g-1 L-1 h-1) with an outstanding 90% selectivity, which is almost four times higher than that obtained over bare TiO2. The wide range of advance characterization techniques used in this work reveals that Mo-doping enhances charge-transfer processes and carriers lifetime as a consequence of the creation of new intra band gap states in Mo-doped TiO2 NCs.

Linker-Assisted Assembly of Ligand-Bridged CdS/MoS2 Heterostructures: Tunable Light-Harvesting Properties and Ligand-Dependent Control of Charge-Transfer Dynamics and Photocatalytic Hydrogen Evolution

We used linker-assisted assembly (LAA) to tether CdS quantum dots (QDs) to MoS2 nanosheets via L-cysteine (cys) or mercaptoalkanoic acids (MAAs) of varying lengths, yielding ligand-bridged CdS/MoS2 heterostructures for redox photocatalysis. LAA afforded precise control over the light-harvesting properties of QDs within heterostructures. Photoexcited CdS QDs transferred electrons to molecularly linked MoS2 nanosheets from both band-edge and trap states; the electron-transfer dynamics was tunable with the properties of bridging ligands. Rate constants of electron transfer, estimated from time-correlated single photon counting (TCSPC) measurements, ranged from (9.8 ± 3.8) × 106 s-1 for the extraction of electrons from trap states within heterostructures incorporating the longest MAAs to >5 × 109 s-1 for the extraction of electrons from band-edge or trap states in heterostructures with cys or 3-mercaptopropionic acid (3MPA) linkers. Ultrafast transient absorption measurements revealed that electrons were transferred within 0.5-2 ps or less for CdS-cys-MoS2 and CdS-3MPA-MoS2 heterostructures, corresponding to rate constants ≥5 × 109 s-1. Photoinduced CdS-to-MoS2 electron transfer could be exploited in photocatalytic hydrogen evolution reaction (HER) via the reduction of H+ to H2 in concert with the oxidation of lactic acid. CdS-L-MoS2-functionalized FTO electrodes promoted HER under oxidative conditions wherein H2 was evolved at a Pt counter electrode with Faradaic efficiencies of 90% or higher and under reductive conditions wherein H2 was evolved at the CdS-L-MoS2-heterostructure-functionalized working electrode with Faradaic efficiencies of 25-40%. Dispersed CdS-L-MoS2 heterostructures promoted photocatalytic HER (15.1 μmol h-1) under white-light illumination, whereas free cys-capped CdS QDs produced threefold less H2 and unfunctionalized MoS2 nanosheets produced no measurable H2. Charge separation across the CdS/MoS2 interface is thus pivotal for redox photocatalysis. Our results reveal that LAA affords tunability of the properties of constituent CdS QDs and MoS2 nanosheets and precise, programmable, ligand-dependent control over the assembly, interfacial structure, charge-transfer dynamics, and photocatalytic reactivity of CdS-L-MoS2 heterostructures.

Shewanella oneidensis MR-1 respires CdSe quantum dots for photocatalytic hydrogen evolution

Living bio-nano systems for artificial photosynthesis are of growing interest. Typically, these systems use photoinduced charge transfer to provide electrons for microbial metabolic processes, yielding a biosynthetic solar fuel. Here, we demonstrate an entirely different approach to constructing a living bio-nano system, in which electrogenic bacteria respire semiconductor nanoparticles to support nanoparticle photocatalysis. Semiconductor nanocrystals are highly active and robust photocatalysts for hydrogen (H2) evolution, but their use is hindered by the oxidative side of the reaction. In this system, Shewanella oneidensis MR-1 provides electrons to a CdSe nanocrystalline photocatalyst, enabling visible light-driven H2 production. Unlike microbial electrolysis cells, this system requires no external potential. Illuminating this system at 530 nm yields continuous H2 generation for 168 h, which can be lengthened further by replenishing bacterial nutrients.

Lateral charge migration in 1D semiconductor-metal hybrid photocatalytic systems

Colloidal nanorods based on CdS or CdSe, functionalized with metal particles, have proven to be efficient catalysts for light-driven hydrogen evolution. Seeded CdSe@CdS nanorods have shown increasing performance with increasing rod length. This observation was rationalized by the increasing lifetime of the separated charges, as a large distance between holes localized in the CdSe seed and electrons localized at the metal tip decreases their recombination rate. However, the impact of nanorod length on the electron-to-tip localization efficiency or pathway remained an open question. Therefore, we investigated the photo-induced electron transfer to the metal in a series of Ni-tipped CdSe@CdS nanorods with varying length. We find that the transfer processes occurring from the region close to the semiconductor-metal interface, the rod region, and the CdSe seed region depend in different ways on the rods' length. The rate of the fastest process from excitonic states generated directly at the interface is independent of the rod length, but the relative amplitude decreases with increasing rod length, as the weight of the interface region is decreasing. The transfer of electrons to the metal tip from excitons generated in the CdS rod region depends strongly on the length of the nanorods, which indicates an electron transport-limited process, i.e., electron diffusion toward the interface region, followed by fast interface crossing. The transfer originating from the CdSe excitonic states again shows no significant length dependence in its time constant, as it is probably limited by the rate of overcoming the shallow confinement in the CdSe seed.

Covalent Functionalization of CdSe Quantum Dot Films with Molecular [FeFe] Hydrogenase Mimics for Light-Driven Hydrogen Evolution

CdSe quantum dots (QDs) combined with [FeFe] hydrogenase mimics as molecular catalytic reaction centers based on earth-abundant elements have demonstrated promising activity for photocatalytic hydrogen generation. Direct linking of the [FeFe] hydrogenase mimics to the QD surface is expected to establish a close contact between the [FeFe] hydrogenase mimics and the light-harvesting QDs, supporting the transfer and accumulation of several electrons needed to drive hydrogen evolution. In this work, we report on the functionalization of QDs immobilized in a thin-film architecture on a substrate with [FeFe] hydrogenase mimics by covalent linking via carboxylate groups as the anchoring functionality. The functionalization was monitored via UV/vis, photoluminescence, IR, and X-ray photoelectron spectroscopy and quantified via micro-X-ray fluorescence spectrometry. The activity of the functionalized thin film was demonstrated, and turn-over numbers in the range of 360-580 (short linkers) and 130-160 (long linkers) were achieved. This work presents a proof-of-concept study, showing the potential of thin-film architectures of immobilized QDs as a platform for light-driven hydrogen evolution without the need for intricate surface modifications to ensure colloidal stability in aqueous environments.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Bioinspired and biomolecular catalysts for energy conversion and storage

Metalloenzymes are remarkable for facilitating challenging redox transformations with high efficiency and selectivity. In the area of alternative energy, scientists aim to capture these properties in bioinspired and engineered biomolecular catalysts for the efficient and fast production of fuels from low-energy feedstocks such as water and carbon dioxide. In this short review, efforts to mimic biological catalysts for proton reduction and carbon dioxide reduction are highlighted. Two important recurring themes are the importance of the microenvironment of the catalyst active site and the key role of proton delivery to the active site in achieving desired reactivity. Perspectives on ongoing and future challenges are also provided.

A Critical Review of the Use of Bismuth Halide Perovskites for CO2 Photoreduction: Stability Challenges and Strategies Implemented

Inspired by natural photosynthesis, the photocatalytic CO2 reduction reaction (CO2RR) stands as a viable strategy for the production of solar fuels to mitigate the high dependence on highly polluting fossil fuels, as well as to decrease the CO2 concentration in the atmosphere. The design of photocatalytic materials is crucial to ensure high efficiency of the CO2RR process. So far, perovskite materials have shown high efficiency and selectivity in CO2RR to generate different solar fuels. Particularly, bismuth halide perovskites have gained much attention due to their higher absorption coefficients, their more efficient charge transfer (compared to oxide perovskites), and their required thermodynamic potential for CO2RR. Moreover, these materials represent a promising alternative to the highly polluting lead halide perovskites. However, despite all the remarkable advantages of bismuth halide perovskites, their use has been limited, owing to instability concerns. As a consequence, recent reports have offered solutions to obtain structures highly stable against oxygen, water, and light, promoting the formation of solar fuels with promising efficiency for CO2RR. Thus, this review analyzes the current state of the art in this field, particularly studies about stability strategies from intrinsic and extrinsic standpoints. Lastly, we discuss the challenges and opportunities in designing stable bismuth halide perovskites, which open new opportunities for scaling up the CO2RR.

In Situ Imaging of Catalytic Reactions on Tungsten Oxide Nanowires Connects Surface-Ligand Redox Chemistry with Photocatalytic Activity

Semiconductor nanocrystals are promising candidates for generating chemical feedstocks through photocatalysis. Understanding the role of ligands used to prepare colloidal nanocrystals in catalysis is challenging due to the complexity and heterogeneity of nanocrystal surfaces. We use in situ single-molecule fluorescence imaging to map the spatial distribution of active regions along individual tungsten oxide nanowires before and after functionalizing them with ascorbic acid. Rather than blocking active sites, we observed a significant enhancement in activity for photocatalytic water oxidation after treatment with ascorbic acid. While the initial nanowires contain inactive regions dispersed along their length, the functionalized nanowires show high uniformity in their photocatalytic activity. Spatial colocalization of the active regions with their surface chemical properties shows that oxidation of ascorbic acid during photocatalysis generates new oxygen vacancies along the nanowire surface. We demonstrate that controlling surface-ligand redox chemistry during photocatalysis can enhance the active site concentration on nanocrystal catalysts.

Synthesis, properties and catalysis of quantum dots in C–C and C-heteroatom bond formations

Luminescent quantum dots (QDs) represent a new form of carbon nanomaterials which have gained widespread attention in recent years, especially in the area of chemical sensing, bioimaging, nanomedicine, solar cells, light-emitting diode (LED), and electrocatalysis. Their extremely small size renders some unusual properties such as quantum confinement effects, good surface binding properties, high surface‐to‐volume ratios, broad and intense absorption spectra in the visible region, optical and electronic properties different from those of bulk materials. Apart from, during the past few years, QDs offer new and versatile ways to serve as photocatalysts in organic synthesis. Quantum dots (QD) have band gaps that could be nicely controlled by a number of factors in a complicated way, mentioned in the article. Processing, structure, properties and applications are also reviewed for semiconducting quantum dots. Overall, this review aims to summarize the recent innovative applications of QD or its modified nanohybrid as efficient, robust, photoassisted redox catalysts in C–C and C-heteroatom bond forming reactions. The recent structural modifications of QD or its core structure in the development of new synthetic methodologies are also highlighted. Following a primer on the structure, properties, and bio-functionalization of QDs, herein selected examples of QD as a recoverable sustainable nanocatalyst in various green media are embodied for future reference.

Quantum Dot-Sensitized Photoreduction of CO2 in Water with Turnover Number > 80,000

Climate change and global energy demands motivate the search for sustainable transformations of carbon dioxide (CO₂) to storable liquid fuels. Photocatalysis is a pathway for direct conversion of CO₂ to CO, one step within light-powered reaction networks that could, if efficient enough, transform the solar energy conversion landscape. To date, the best performing photocatalytic CO₂ reduction systems operate in nonaqueous solvents, but technologically viable solar fuels networks will likely operate in water. Here we demonstrate catalytic photoreduction of CO₂ to CO in pure water at pH 6-7 with an unprecedented combination of performance parameters: turnover number (TON(CO)) = 72,484-84,101, quantum yield (QY) = 0.96-3.39%, and selectivity (S_CO) > 99%, using CuInS₂ colloidal quantum dots (QDs) as photosensitizers and a Co-porphyrin catalyst. At higher catalyst concentration, the system reaches QY = 3.53-5.23%. The performance of the QD-driven system greatly exceeds that of the benchmark aqueous system (926 turnovers with a quantum yield of 0.81% and selectivity of 82%), due primarily to (i) electrostatic attraction of the QD to the catalyst, which promotes fast multielectron delivery and colocalization of protons, CO₂, and catalyst at the source of photoelectrons, and (ii) termination of the QD's ligand shell with free amines, which capture CO₂ as carbamic acid that serves as a reservoir for CO₂, effectively increasing its solubility in water, and lowers the onset potential for catalytic CO₂ reduction by the Co-porphyrin. The breakthrough efficiency achieved in this work represents a nonincremental step in the realization of reaction networks for direct solar-to-fuel conversion.

Optical properties of fully inorganic core/gradient-shell CdSe/CdZnS nanocrystals at the ensemble and single-nanocrystal levels

We report the synthesis and optical characterization of fully inorganic gradient-shell CdSe/CdZnS nanocrystals (NCs) with high luminescence quantum yield (QY, 50%), which were obtained by replacing native oleic-acid (OA) ligands with halide ions (Cl^-and Br^-). Absorption, photoluminescence excitation (PLE) and photoluminescence (PL) spectra in solution were unaffected by the ligand-exchange procedure. The halide-capped NCs were stable in solution for several weeks without modification of their PL spectra; once deposited as unprotected thin films and exposed to air, however, they did show signs of aging which we attribute to increasing heterogeneity of (effective) NC size. Time-resolved PL measurements point to the existence of four distinct emissive states, which we attribute to neutral, singly-charged and multi-excitonic entities. We found that the relative contribution of these four components to the overall PL decay is modified by the OA-to-halide ligand exchange, while the excited-state lifetimes themselves, surprisingly, remain largely unaffected. The high PL quantum yield of the halide-capped NCs allowed observation of single particle blinking and photon-antibunching; one surprising result was that aging processes that occurs during the first few days after deposition on glass seemed to offer a certain increased protection against photobleaching. These results suggest that halide-capped CdSe/CdZnS NCs are promising candidates for incorporation into opto-electronic devices, based on, for example, hybrid perovskite matrices, which require eliminating the steric hindrance and electronic barrier of bulky organic ligands to ensure efficient coupling.

Influence of Ligand Structure on Excited State Surface Chemistry of Lead Sulfide Quantum Dots

The ligand-nanocrystal boundaries of colloidal quantum dots (QDs) mediate the primary energy and electron transfer processes that underpin photochemical and photocatalytic transformations at their surfaces. We use mid-infrared transient absorption spectroscopy to reveal the influence that ligand structure and bonding to nanocrystal surfaces have on the changes of the excited state surface chemistry of this boundary in PbS QDs and the corresponding impact on charge transfer processes between nanocrystals. We demonstrate that oleate ligands undergo marked changes in their bonding to surfaces in the excitonic excited states of the nanocrystals, indicating that oleate passivated PbS surfaces undergo significant structural changes following photoexcitation. These changes can impact the surface mobility of the ligands and the ability of redox shuttles to approach the nanocrystal surfaces to undergo charge transfer in photocatalytic reactions. In contrast, markedly different transient vibrational features are observed in iodide/mercaptoproprionic acid passivated PbS QD films that result from charge transfer between neighboring nanocrystals and localization of holes at the nanocrystal surfaces near MPA ligands. This ability to distinguish the influence that excitonic excited states vs charge transfer processes have on the surface chemistry of the ligand-nanocrystal boundary lays the groundwork for exploration of how this boundary can be understood and controlled for the design of nanocrystalline materials tailored for specific applications in solar energy harvesting and photocatalytic reactions.

Ultrafast Electron Transfer from CdSe Quantum Dots to an [FeFe]-Hydrogenase Mimic

The combination of CdSe nanoparticles as photosensitizers with [FeFe]-hydrogenase mimics is known to result in efficient systems for light-driven hydrogen generation with reported turnover numbers in the order of 10⁴-10⁶. Nevertheless, little is known about the details of the light-induced charge-transfer processes. Here, we investigate the time scale of light-induced electron transfer kinetics for a simple model system consisting of CdSe quantum dots (QDs) of 2.0 nm diameter and a simple [FeFe]-hydrogenase mimic adsorbed to the QD surface under noncatalytic conditions. Our (time-resolved) spectroscopic investigation shows that both hot electron transfer on a sub-ps time scale and band-edge electron transfer on a sub-10 ps time scale from photoexcited QDs to adsorbed [FeFe]-hydrogenase mimics occur. Fast recombination via back electron transfer is observed in the absence of a sacrificial agent or protons which, under real catalytic conditions, would quench remaining holes or could stabilize the charge separation, respectively.