Structure Dependence of CO2 Reduction Electrocatalyzed by Metal-Nanographene Complexes: A Computational Study.

Improving the energy efficiency of electrocatalytic reduction of CO2 requires tuning of redox properties of electrocatalysts to match redox potentials of the substrate. Recently, we introduced nanographenes as ligands for metal complexes for such purposes by taking advantage of size-dependent properties of the conjugated systems. Here, we use computations to investigate the structure dependence of the electrocatalysis at Re(diimine)(CO)3Cl complexes with nanographene ligands that contain a polycyclic aromatic hydrocarbon moiety through a pyrazinyl linkage. We show that the reduction potentials of the complexes depend not only on conjugation size but also on shape and geometry of the ligands, revealing another parameter in tuning the redox properties of the electrocatalysts. In addition, our work reveals a compromise between reduction potentials and activation of this class of electrocatalysts.

Proton-Coupled, Low-Energy Pathway for Electrocatalytic CO2 Reduction at Re(Diimine) Complexes with a Conjugated Pyrazinyl Moiety.

Large conjugated carbon framework has been incorporated as the diimine ligand for Re(α-diimine)(CO)3Cl complexes with a pyrazinyl linkage, either to increase energy efficiency or to turn them into heterogeneous catalysts for selective electrocatalytic CO2 reduction. However, there exists a nonmonotonic dependence of CO2 reduction overpotential on the conjugation size of the ligands. Understanding its origin could facilitate heterogenization of molecular catalysts with improved energy efficiency. Here, we show that the conjugated pyrazinyl moiety plays a crucial role in catalysis by enabling a proton-coupled, lower-energy pathway for CO2 reduction. With ligands of moderate size, the pathway leads to previously unknown intermediates and decreases CO2 reduction overpotential. Because the pathway hinges on the basicity of the pyrazinyl nitrogen, we propose that it imposes a limit on the conjugation size of the ligand for the pathway to be effective.

Redox "Innocence" of Re(I) in Electrochemical CO2 Reduction Catalyzed by Nanographene-Re Complexes.

Improving energy efficiency of electrocatalytic CO2 conversion to useful chemicals poses a significant scientific challenge. Recently we reported on using a colloidal nanographene as the diimine ligand to form a molecular complex Re(diimine)(CO)3Cl to tackle this challenge, leading to significantly improved CO2 reduction potential. In this work, we use theoretical computations to investigate the roles of the nanographene ligand in the reduction and the reaction pathways. Remarkably, our results show that the metal center merely provides a binding site for CO2 and a conduit for electron transfer between the nanographene ligand and the substrate instead of changing its own oxidation state in the processes. Thus, despite its multiple oxidation states, the Re is redox "innocent" in the CO2 reduction catalyzed by the nanographene complex.

Reductive defluorination of graphite monofluoride by weak, non-nucleophilic reductants reveals low-lying electron-accepting sites.

Graphite monofluoride (GF) can undergo reductive defluorination in the presence of weak, non-nucleophilic reductants. This leads to a new approach to GF-polyaniline composites as cathode materials for significantly improving the discharge capacity of primary lithium batteries. We postulate that the reduction is mediated by residual π-bonds in GF.

Well-Defined Nanographene-Rhenium Complex as an Efficient Electrocatalyst and Photocatalyst for Selective CO2 Reduction.

Improving energy efficiency of electrocatalytic and photocatalytic CO2 conversion to useful chemicals poses a significant scientific challenge. We report on using a colloidal nanographene to form a molecular complex with a metal ion to tackle this challenge. In this work, a well-defined nanographene-Re complex was synthesized, in which electron delocalization over the nanographene and the metal ion significantly decreases the electrical potential needed to drive the chemical reduction. We show the complex can selectively electrocatalyze CO2 reduction to CO in tetrahydrofuran at -0.48 V vs NHE, the least negative potential reported for a molecular catalyst. In addition, the complex can absorb a significant spectrum of visible light to photocatalyze the chemical transformation without the need for a photosensitizer.

A Model for the pH-Dependent Selectivity of the Oxygen Reduction Reaction Electrocatalyzed by N-Doped Graphitic Carbon.

Nitrogen-doped graphitic carbon materials have been extensively studied as potential replacements for Pt-based electrocatalysts for the oxygen reduction reaction (ORR). However, little is known about the catalytic mechanisms, including the parameters that determine the selectivity of the reaction. By comparing theoretical calculations of the ORR selectivity at a well-defined graphene nanostructure with experimental results, we propose a model based on interfacial solvation to explain the observed preference for the four-electron pathway in alkaline electrolytes. The hydrophobic environment around the active sites, as in enzymatic catalysis, restricts the access of water and destabilizes small ionic species such as peroxide, the product of the two-electron pathway. This model, when applied to acidic electrolytes, shows the ORR preferring the two-electron pathway, consistent with the well-known pH-dependent ORR selectivity catalyzed by graphitic carbon materials. Because of the similarity between more complex N-doped graphitic carbon materials and our model system, we can extend this model to the former and rationalize nearly all of the previously reported experimental results on the selectivity of ORR catalyzed by these materials.

Biexciton Binding of Dirac fermions Confined in Colloidal Graphene Quantum Dots.

We present transient absorption measurements and microscopic theory of biexciton binding in triangular colloidal graphene quantum dots consisting of 168 sp(2)-hybridized C atoms. We observe optical transitions from the lowest orbitally dark singlet exciton states to states below the energy of an unbound dark+bright singlet-exciton pair. Through microscopic calculations of the low-energy exciton and biexciton states via tight-binding, Hartree-Fock, and configuration interaction methods, the spectra reveal a biexciton consisting primarily of a dark-bright singlet-pair bound by ∼0.14 eV.

Electrocatalytic oxygen activation by carbanion intermediates of nitrogen-doped graphitic carbon.

Nitrogen-doped graphitic carbon has been intensively studied for potential use as an electrocatalyst in fuel cells for the oxygen reduction reaction (ORR). However, the lack of a mechanistic understanding on the carbon catalysis has severely hindered the progress of the catalyst development. Herein we use a well-defined graphene nanostructure as a model system and, for the first time, reveal an oxygen activation mechanism that involves carbanion intermediates in these materials. Our work shows that the overpotential of the electrocatalytic ORR is determined by the generation of the carbanion intermediates, and the current by the rate the intermediates activate oxygen.

Colloidal graphene quantum dots with well-defined structures.

When the size of a semiconductor crystal is reduced to the nanometer scale, the crystal boundary significantly modifies electron distribution, making properties such as bandgap and energy relaxation dynamics size dependent. This phenomenon, known as quantum confinement, has been demonstrated in many semiconductor materials, leading to practical applications in areas such as bioimaging, photovoltaics, and light-emitting diodes. Graphene, a unique type of semiconductor, is a two-dimensional crystal with a zero bandgap and a zero effective mass of charge carriers. Consequently, we expect new phenomena from nanometer-sized graphene, or graphene quantum dots (QDs), because the energy of charge carriers in graphene follows size-scaling laws that differ from those in other semiconductors. From a chemistry point of view, graphene is made of carbon, an element for which researchers have developed a whole branch of chemistry. Thus, it is possible to synthesize graphene QDs through stepwise, well-controlled organic chemistry, achieving structures with an atomic precision that has not been possible for any other semiconductor materials. Recently, we developed a new solubilizing strategy that led to synthesis of stable colloidal graphene QDs with more than 100 conjugated carbon atoms, allowing us to study their properties in a new size regime. In this Account, we review our recent progress working with the colloidal graphene QDs, including their synthesis and stabilization, tuning of their properties, and new phenomena in energy relaxation dynamics. In particular, we have observed extraordinarily slow "electron cooling"--the relaxation of electrons from high excited states to lower ones. With further investigation, these high-energy electrons could potentially be harvested in solar energy applications, for example, creating more efficient photovoltaic cells. We discuss additional emerging opportunities with these new materials and current challenges, hoping to draw the interest of researchers in various fields to overcome these obstacles.

Biexciton auger recombination in colloidal graphene quantum dots.

We measure biexciton Auger recombination (AR) in colloidal graphene quantum dots (GQDs) by transient absorption spectroscopy. AR is reflected in GQDs with 132 and 168 sp2-hybridized C atoms as a decay with a ∼0.3 ps time constant and an amplitude depending superlinearly on pump fluence. Despite the orders-of-magnitude difference in size between GQDs and carbon nanotubes, their biexciton AR rates are similar. This similarity is a result of strong carrier interactions in sp2-hybridized carbon nanostructures and suggests that GQDs may hold promise in the context of carrier multiplication.

Nitrogen-doped colloidal graphene quantum dots and their size-dependent electrocatalytic activity for the oxygen reduction reaction.

Nitrogen doping has been a powerful way to modify the properties of carbon materials ranging from activated carbon to graphene. Here we report on a solution chemistry approach to nitrogen-doped colloidal graphene quantum dots with well-defined structures. N-doping was demonstrated to significantly affect the properties of the quantum dots, including the emergence of size-dependent electrocatalytic activity for the oxygen reduction reaction.

Formation and stabilization of palladium nanoparticles on colloidal graphene quantum dots.

Metal particles supported by carbon materials are important for various technologies yet not well understood. Here, we report on the use of well-defined colloidal graphene quantum dots as a model system for the carbon materials to study metal-carbon interaction. In the case of palladium, our studies show high affinity between the metal nanoparticles with the graphene. IR spectroscopy reveals covalent nature of the interaction between the two, which had been predicted by theoretical calculations yet never directly proven before.

Slow hot-carrier relaxation in colloidal graphene quantum dots.

Reducing hot-carrier relaxation rates is of great significance in overcoming energy loss that fundamentally limits the efficiency of solar energy utilization. Semiconductor quantum dots are expected to have much slower carrier cooling because the spacing between their discrete electronic levels is much larger than phonon energy. However, the slower carrier cooling is difficult to observe due to the existence of many competing relaxation pathways. Here we show that carrier cooling in colloidal graphene quantum dots can be 2 orders of magnitude slower than in bulk materials, which could enable harvesting of hot charge carriers to improve the efficiency of solar energy conversion.

Alignment of colloidal graphene quantum dots on polar surfaces.

Controlling the orientation of nanostructures with anisotropic shapes is essential for taking advantage of their anisotropic electrical, optical, and transport properties in electro-optical devices. For large-area alignment of nanocrystals, so far orientations are mostly induced and controlled by external physical parameters, such as applied fields or changes in concentration. Herein we report on assemblies of colloidal graphene quantum dots, a new type of disk-shaped nanostructures, on polar surfaces and the control of their orientations. We show that the orientations of the graphene quantum dots can be determined, either in- or out-of-plane with the substrate, by chemical functionalization that introduces orientation-dependent interactions between the quantum dots and the surfaces.

Large, solution-processable graphene quantum dots as light absorbers for photovoltaics.

Graphenes have very attractive properties for photovoltaics. Their tunable bandgap and large optical absorptivity are desirable for efficient light harvesting. Their electronic levels and interfacing with other materials for charge transfer processes can both be tuned with well-developed carbon chemistry. Graphenes have also been shown to have very large charge mobilities, which could be useful for charge collection in solar cells. In addition, they consist of elements abundant on Earth and are environmentally friendly. However, these important properties have not been taken advantage of because graphenes that are large enough to be useful for photovoltaics have extremely poor solubility and have a strong tendency to aggregate into graphite. Here we present a novel solubilization strategy for large graphene nanostructures. It has enabled us to synthesize solution-processable, black graphene quantum dots with uniform size through solution chemistry, and we show that they can be used as sensitizers for solar cells.

Triplet States and electronic relaxation in photoexcited graphene quantum dots.

Electronic relaxation in photoexcited graphenes is central to their photoreactivity and their optoelectrical applications such as photodetectors and solar cells. Herein we report on the first ensemble studies of electronic energy relaxation pathways in colloidal graphene quantum dots with uniform size. We show that the photoexcited graphene quantum dots have a significant probability of relaxing into triplet states and emit both phosphorescence and fluorescence at room temperature, with relative intensities depending on the excitation energy. Because of the long lifetime and reactivity of triplet electronic states, our results could have significant implications for applications of graphenes.

Synthesis of large, stable colloidal graphene quantum dots with tunable size.

We report a solution-chemistry-based approach to large, stable colloidal graphene quantum dots with uniform size and shape. The versatility of solution chemistry allows us to tune the structures of the graphenes and thus their properties.

Aromatic Fragmentation Based on a Ring Overlap Scheme: An Algorithm for Large Polycyclic Aromatic Hydrocarbons Using the Molecules-in-Molecules Fragmentation-Based Method.

We present a novel and systematic fragmentation scheme to treat polycyclic aromatic hydrocarbons (PAHs) built off the molecules-in-molecules composite method. Our algorithm generates a set of biphenyl and naphthalene subsystems overlapping by whole sextet rings, ensuring all calculations are performed on aromatic molecules. Hence, our method is called Aromatic Fragmentation Based on a Ring Overlap Scheme (AroBOROS), and the generated fragments may be combined to form a hierarchy of subsystems to reduce errors for more complex PAHs. Errors are reduced to below chemical accuracy by combining subsystems that reflect the lowest energy structures determined by Clar's rule of aromatic sextets, and this is shown on two diverse test sets of PAHs ranging from 18 to 84 carbon atoms. Additionally, evaluations are performed for larger PAHs, as well as a nanotube fragment, containing up to 132 carbon atoms, and it is shown that good results may be achieved even with fragments representing an appreciably small portion of the full system.

Basal Plane Fluorination of Graphene by XeF2 via a Radical Cation Mechanism.

Graphene fluorination with XeF2 is an attractive method to introduce a nonzero bandgap to graphene under mild conditions for potential electro-optical applications. Herein, we use well-defined graphene nanostructures as a model system to study the reaction mechanism of graphene fluorination by XeF2. Our combined experimental and theoretical studies show that the reaction can proceed through a radical cation mechanism, leading to fluorination and sp(3)-hybridized carbon in the basal plane.

Hot electron injection from graphene quantum dots to TiO2.

The Shockley-Queisser limit is the maximum power conversion efficiency of a conventional solar cell based on a single semiconductor junction. One approach to exceed this limit is to harvest hot electrons/holes that have achieved quasi-equilibrium in the light absorbing material with electronic temperatures higher than the phonon temperature. We argue that graphene based materials are viable candidates for hot carrier chromophores. Here we probe hot electron injection and charge recombination dynamics for graphene quantum dots (QDs, each containing 48 fused benzene rings) anchored to the TiO2(110) surface via carboxyl linkers. We find ultrafast electron injection from photoexcited graphene QDs to the TiO2 conduction band with time constant τ(i) < 15 fs and charge recombination dynamics characterized by a fast channel (τ(r1) = 80-130 fs) and a slow one (τ(r2) = 0.5-2 ps). The fast decay channel is attributed to the prompt recombination of the bound electron-hole pair across the interface. The slow channel depends strongly on excitation photon energy or sample temperature and can be explained by a "boomerang" mechanism, in which hot electrons are injected into bulk TiO2, cooled down due to electron-phonon scattering, drifted back to the interface under the transient electric field, and recombine with the hole on graphene QDs. We discuss feasibilities of implementing the hot carrier solar cell using graphene nanomaterials.