Aqueous Photogeneration of H2 with CdSe Nanocrystals and Nickel Catalysts: Electron Transfer Dynamics

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

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.

Theoretical Advances in Polariton Chemistry and Molecular Cavity Quantum Electrodynamics

When molecules are coupled to an optical cavity, new light-matter hybrid states, so-called polaritons, are formed due to quantum light-matter interactions. With the experimental demonstrations of modifying chemical reactivities by forming polaritons under strong light-matter interactions, theorists have been encouraged to develop new methods to simulate these systems and discover new strategies to tune and control reactions. This review summarizes some of these exciting theoretical advances in polariton chemistry, in methods ranging from the fundamental framework to computational techniques and applications spanning from photochemistry to vibrational strong coupling. Even though the theory of quantum light-matter interactions goes back to the midtwentieth century, the gaps in the knowledge of molecular quantum electrodynamics (QED) have only recently been filled. We review recent advances made in resolving gauge ambiguities, the correct form of different QED Hamiltonians under different gauges, and their connections to various quantum optics models. Then, we review recently developed ab initio QED approaches which can accurately describe polariton states in a realistic molecule-cavity hybrid system. We then discuss applications using these method advancements. We review advancements in polariton photochemistry where the cavity is made resonant to electronic transitions to control molecular nonadiabatic excited state dynamics and enable new photochemical reactivities. When the cavity resonance is tuned to the molecular vibrations instead, ground-state chemical reaction modifications have been demonstrated experimentally, though its mechanistic principle remains unclear. We present some recent theoretical progress in resolving this mystery. Finally, we review the recent advances in understanding the collective coupling regime between light and matter, where many molecules can collectively couple to a single cavity mode or many cavity modes. We also lay out the current challenges in theory to explain the observed experimental results. We hope that this review will serve as a useful document for anyone who wants to become familiar with the context of polariton chemistry and molecular cavity QED and thus significantly benefit the entire community.

Assembling CdSe Quantum Dots into Polymeric Micelles Formed by a Polyethylenimine-Based Amphiphilic Polymer to Enhance Efficiency and Selectivity of CO2-to-CO Photoreduction in Water

Colloidal quantum dots (QDs) as photocatalysts enable catalysis of CO₂-to-CO conversion in the presence of electron donors. The surface and/or interfacial chemical environment of the QDs is essential for the activity and selectivity of the CO₂ photoreduction. Various strategies, including exposing active metal sites or anchoring functional organic ligands, have been applied to tune the QDs' surface chemical environment and thus to improve both activity and selectivity of CO₂ photoreduction, which occurs at surface of the QDs. However, the efficient and selective photocatalytic CO₂ reduction with QD photocatalysts in water is still a challenging task due to low CO₂ solubility and robust competing reaction of proton reduction in water. Different from state-of-the-art QDs' surface manipulation, we proposed to ameliorate the interfacial chemical environment of CdSe QDs via assembling the QDs into functional polymeric micelles in water. Herein, CdSe@PEI-LA assemblies were constructed by loading CdSe QDs into polymeric micelles formed by PEI-LA, a polyethylenimine (PEI)-based functional amphiphilic polymer. Due to self-assembly and high CO₂ adsorption capacity of PEI-LA in water, the photocatalytic CO₂-to-CO conversion efficiency and selectivity of the CdSe@PEI-LA assemblies in water were dramatically improved to 28.0 mmol g^-1 and 87.5%, respectively. These two values increased 57 times and 1.5 times, respectively, compared with those of the pristine CdSe QDs. Mechanism studies revealed that CdSe QDs locate in polymeric micelles of high CO₂ local concentration and the photoinduced electron transfer from the conduction band of CdSe QDs to Cd-CO₂* species is thermodynamically and kinetically improved in the presence of PEI-LA. The CdSe@PEI-LA system represents a successful example of using a functionalized amphiphilic polymer to ameliorate interfacial microenvironments of nanocrystal photocatalysts and realizing efficient and selective CO₂ photoreduction in water.

Exploring the Potential of Water-Soluble Cu(II) Complexes with MPA–CdTe Quantum Dots for Photoinduced Electron Transfer

Three water-soluble copper complexes based on the amine/pyridine functionalities were investigated, along with quantum dots, as a catalyst–photosensitizer assembly, respectively, for fundamental understanding of photoinduced electron transfer. Luminescence quenching and lifetime measurements were performed to try and establish the actual process that leads to the quenching, such as electron transfer, energy transfer, or complex formation (static quenching). Cyclic voltammetry and dynamic light scattering experiments were also performed. Irrespective of the similar reduction potentials of the three complexes, very different photoluminescence properties were observed.

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.

Influence of Surface Ligands on Charge-Carrier Trapping and Relaxation in Water-Soluble CdSe@CdS Nanorods

In this study, the impact of the type of ligand at the surface of colloidal CdSe@CdS dot-in-rod nanostructures on the basic exciton relaxation and charge localization processes is closely examined. These systems have been introduced into the field of artificial photosynthesis as potent photosensitizers in assemblies for light driven hydrogen generation. Following photoinduced exciton generation, electrons can be transferred to catalytic reaction centers while holes localize into the CdSe seed, which can prevent charge recombination and lead to the formation of long-lived charge separation in assemblies containing catalytic reaction centers. These processes are in competition with trapping processes of charges at surface defect sites. The density and type of surface defects strongly depend on the type of ligand used. Here we report on a systematic steady-state and time-resolved spectroscopic investigation of the impact of the type of anchoring group (phosphine oxide, thiols, dithiols, amines) and the bulkiness of the ligand (alkyl chains vs. poly(ethylene glycol) (PEG)) to unravel trapping pathways and localization efficiencies. We show that the introduction of the widely used thiol ligands leads to an increase of hole traps at the surface compared to trioctylphosphine oxide (TOPO) capped rods, which prevent hole localization in the CdSe core. On the other hand, steric restrictions, e.g., in dithiolates or with bulky side chains (PEG), decrease the surface coverage, and increase the density of electron trap states, impacting the recombination dynamics at the ns timescale. The amines in poly(ethylene imine) (PEI) on the other hand can saturate and remove surface traps to a wide extent. Implications for catalysis are discussed.

Polariton-Mediated Electron Transfer via Cavity Quantum Electrodynamics

We investigate the polariton-mediated electron transfer reaction in a model system with analytic rate constant theory and direct quantum dynamical simulations. We demonstrate that the photoinduced charge transfer reaction between a bright donor state and dark acceptor state can be significantly enhanced or suppressed by coupling the molecular system to the quantized radiation field inside an optical cavity. This is because the quantum light-matter interaction can influence the effective driving force and electronic couplings between the donor state, which is the hybrid light-matter excitation, and the molecular acceptor state. Under the resonance condition between the photonic and electronic excitations, the effective driving force can be tuned by changing the light-matter coupling strength; for an off-resonant condition, the same effect can be accomplished by changing the molecule-cavity detuning. We further demonstrate that using both the electronic coupling and light-matter coupling helps to extend the effective couplings across the entire system, even for the dark state that carries a zero transition dipole. Theoretically, we find that both the counter-rotating terms and the dipole self-energy in the quantum electrodynamics Hamiltonian are important for obtaining an accurate polariton eigenspectrum as well as the polariton-mediated charge transfer rate constant, especially in the ultrastrong coupling regime.

Synthesis of water-soluble Ni(II) complexes and their role in photo-induced electron transfer with MPA-CdTe quantum dots

Photocatalytic water splitting using solar energy for hydrogen production offers a promising alternative form of storable and clean energy for the future. To design an artificial photosynthesis system that is cost-effective and scalable, earth abundant elements must be used to develop each of the components of the assembly. To develop artificial photosynthetic systems, we need to couple a catalyst for proton reduction to a photosensitizer and understand the mechanism of photo-induced electron transfer from the photosensitizer to the catalyst that serves as the fundamental step for photocatalysis. Therefore, our work is focused on the study of light driven electron transfer kinetics from the quantum dot systems made with inorganic chalcogenides in the presence of Ni-based reduction catalysts. Herein, we report the synthesis and characterization of four Ni(II) complexes of tetradentate ligands with amine and pyridine functionalities (N2/Py2) and their interactions with CdTe quantum dots stabilized by 3-mercaptopropionic acid. The lifetime of the quantum dots was investigated in the presence of the Ni complexes and absorbance, emission and electrochemical measurements were performed to gain a deeper understanding of the photo-induced electron transfer process.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

0D-2D Quantum Dot: Metal Dichalcogenide Nanocomposite Photocatalyst Achieves Efficient Hydrogen Generation

Hydrogen generation via photocatalysis-driven water splitting provides a convenient approach to turn solar energy into chemical fuel. The development of photocatalysis system that can effectively harvest visible light for hydrogen generation is an essential task in order to utilize this technology. Herein, a kind of cadmium free Zn-Ag-In-S (ZAIS) colloidal quantum dots (CQDs) that shows remarkably photocatalytic efficiency in the visible region is developed. More importantly, a nanocomposite based on the combination of 0D ZAIS CQDs and 2D MoS₂ nanosheet is developed. This can leverage the strong light harvesting capability of CQDs and catalytic performance of MoS₂ simultaneously. As a result, an excellent external quantum efficiency of 40.8% at 400 nm is achieved for CQD-based hydrogen generation catalyst. This work presents a new platform for the development of high-efficiency photocatalyst based on 0D-2D nanocomposite.

Design of a Hole Trapping Ligand

A new ligand that covalently attaches to the surface of colloidal CdSe/CdS nanorods and can simultaneously chelate a molecular metal center is described. The dithiocarbamate-bipyridine ligand system facilitates hole transfer through energetic overlap at the inorganic-organic interface and conjugation through the organic ligand to a chelated metal center. Density functional theory calculations show that the coordination of the free ligand to a CdS surface causes the formation of two hybridized molecular states that lie in the band gap of CdS. The further chelation of Fe(II) to the bipyridine moiety causes the presence of seven midgap states. Hole transfer from the CdS valence band to the midgap states is dipole allowed and occurs at a faster rate than what is experimentally known for the CdSe/CdS band-edge radiative recombination. In the case of the ligand bound with iron, a two-step process emerges that places the hole on the iron, again at rates much faster than band gap recombination. The system was experimentally assembled and characterized via UV-vis absorbance spectroscopy, fluorescence spectroscopy, time-resolved photoluminescence spectroscopy, and energy dispersive X-ray spectroscopy. Theoretically predicted red shifts in absorbance were observed experimentally, as well as the expected quench in photoluminescence and lifetimes in time-resolved photoluminescence.

Light-driven hydrogen production from aqueous solutions based on a new Dubois-type nickel catalyst

We developed a new water soluble CdSe/Ni hybrid, which yields remarkable photon-to-H₂ efficiency among all noble-metal free systems based on synthetic Ni molecular catalysts.

Photocatalytic Hydrogen Generation by CdSe/CdS Nanoparticles

The photocatalytic hydrogen (H2) production activity of various CdSe semiconductor nanoparticles was compared including CdSe and CdSe/CdS quantum dots (QDs), CdSe quantum rods (QRs), and CdSe/CdS dot-in-rods (DIRs). With equivalent photons absorbed, the H2 generation activity orders as CdSe QDs ≫ CdSe QRs > CdSe/CdS QDs > CdSe/CdS DIRs, which is surprisingly the opposite of the electron-hole separation efficiency. Calculations of photoexcited surface charge densities are positively correlated with the H2 production rate and suggest the size of the nanoparticle plays a critical role in determining the relative efficiency of H2 production.

Electronic Processes within Quantum Dot-Molecule Complexes

The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.

Photocatalytic Conversion of Nitrobenzene to Aniline through Sequential Proton-Coupled One-Electron Transfers from a Cadmium Sulfide Quantum Dot

This paper describes the use of cadmium sulfide quantum dots (CdS QDs) as visible-light photocatalysts for the reduction of nitrobenzene to aniline through six sequential photoinduced, proton-coupled electron transfers. At pH 3.6-4.3, the internal quantum yield of photons-to-reducing electrons is 37.1% over 54 h of illumination, with no apparent decrease in catalyst activity. Monitoring of the QD exciton by transient absorption reveals that, for each step in the catalytic cycle, the sacrificial reductant, 3-mercaptopropionic acid, scavenges the excitonic hole in ∼5 ps to form QD(•-); electron transfer to nitrobenzene or the intermediates nitrosobenzene and phenylhydroxylamine then occurs on the nanosecond time scale. The rate constants for the single-electron transfer reactions are correlated with the driving forces for the corresponding proton-coupled electron transfers. This result suggests, but does not prove, that electron transfer, not proton transfer, is rate-limiting for these reactions. Nuclear magnetic resonance analysis of the QD-molecule systems shows that the photoproduct aniline, left unprotonated, serves as a poison for the QD catalyst by adsorbing to its surface. Performing the reaction at an acidic pH not only encourages aniline to desorb but also increases the probability of protonated intermediates; the latter effect probably ensures that recruitment of protons is not rate-limiting.

Plasmon resonance energy transfer and hot electron injection induced high photocurrent density in liquid junction Ag@Ag2S sensitized solar cells

Due to plasmon induced absorption enhancement and direct hot electron injection, a high photocurrent density of ∼25.6 mA cm⁻² was demonstrated in an Ag@Ag₂S co-sensitized solar energy conversion device.

Photocatalytic Applications of Colloidal Heterostructured Nanocrystals: What's Next?

Recent progress in the colloidal synthesis of inorganic nanocrystals has led to the realization of complex, multidomain nanoparticle morphologies that give rise to advanced optoelectronic properties. Such nanocomposites are particularly appealing for photocatalytic applications where tunable absorption, extensive charge separation, and large surface-to-volume ratios are important. To date, heterostructured nanocrystals featuring a metal catalyst and a semiconductor "chromophore" component have shown compelling efficiencies in photoreduction reactions, including sacrificial hydrogen production. Time-resolved optical studies have attributed their success to a near-complete separation of photoinduced charges across dissimilar nanoparticle domains. The spectroscopy approach has also identified the key performance-limiting factors of nanocrystal catalysts that arise from inefficient extraction of photoinduced charges to catalytic sites. Along these lines, the main scope of present-day efforts targets the improvement of interstitial charge transfer pathways across the chromophore-catalyst assembly through the design of high-quality stoichiometric interfaces.