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

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

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

Efficient Photocatalytic Cleavage of C–C Bonds in β‑1 Lignin Models and Aromatic Vicinal Diols by Combining Alkali Metal-Treated Graphitic Carbon Nitride and Persulfate

The cleavage of lignin C–C bonds is of great significance for the high-value utilization of lignin. However, it still faces great challenges due to the stubbornness and complexity of lignin C–C bonds. Herein, the cyano group with an electron-withdrawing effect is successfully introduced into the graphitic carbon nitride (BCN) photocatalyst by alkali metal molten salt methods, and it is labeled KLCN. Compared with pure BCN, the micromorphology and electronic structure of KLCN have undergone significant changes. Markedly, KLCN with a short rod-like micromorphology has excellent photogenerated electron–hole pair separation ability and stronger photogenerated electron reduction ability. By combining KLCN and persulfate, the C–C bond in β-1 lignin models and aromatic vicinal diols was efficiently broken. Mechanistic studies have shown that the active radicals in the photocatalytic reaction are regulated and that the hydroxyl radicals are the main active radicals. It promotes the participation of water in the photocatalytic reaction and provides H and O atoms for breaking of the lignin C–C bonds. The photocatalytic reaction mainly follows a single-electron-transfer mechanism, which is rare in breaking lignin C–C bonds by using a heterogeneous photocatalyst. The current work provides helpful guidelines for designing effective photocatalysts for lignin C–C bond cleavage

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 sp³-hybridized carbon in the basal plane

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