Photoluminescence (PL) is the most significant feature of graphene quantum dots (GQDs). However, the PL mechanism in GQDs has been debated due to the fact that the microstructures, such as edge and in-plane defects that are critical for PL emission, have not been convincingly identified due to the lack of effective detection methods. Conventional measures such as high-resolution transmission electron microscopy and infrared spectroscopy only show some localized lattice fringes of GQDs and the structures of some substituents, which have little significance in terms of thoroughly understanding the PL effect. Here, surface-enhanced Raman spectroscopy (SERS) was introduced as a highly sensitive surface technique to study the microstructures of GQDs. Pure GQDs were prepared by laser ablating and cutting highly oriented pyrolytic graphite (HOPG) parallel to the graphite layers. Consequently, abundant SERS signals of the GQDs were obtained on an Ag electrode in an electrochemical environment for the first time. The results convincingly and experimentally characterized the typical and detailed features of GQDs, such as the crystallinity of sp2 hexagons, the quantum confinement effect, various defects on the edges, sp3-like defects and disorders on the basal planes, and passivated structures on the periphery and surface of the GQDs. This work demonstrates that SERS is thus by far the most effective technique for probing the microstructures of GQDs.
The electrocatalytic nitrogen reduction reaction (NRR) is a most attractive approach to ammonia synthesis, and the development of catalysts with excellent activity, high NRR selectivity, and long‐term durability is crucial but remains a great challenge. Herein, by means of density functional theory calculations, the stability and catalytic performance of anchored bimetals was systematically investigated by pairing different transition‐metal atoms (Mo, Cr, Ti, V, Ru, and W) on graphene with different coordination atoms (C, N, O, P, and S) for N2 fixation. By screening the stability, limiting potential, and selectivity of 105 candidates, carbon was found to be the optimal coordination atom for bimetallic pairs, whereas the other four coordination atoms were unsatisfactory owing to either thermodynamically unstable anchor sites for bimetallic pairs (O, P, and S atoms) or relatively low catalytic activity (N atom). Notably, the bimetallic compound of Mo and Ti supported on C‐coordinated graphene (MoTi‐CG) and TiV‐CG were predicted as effective NRR catalysts with the attractive limiting potentials of −0.34 and −0.30 V. Furthermore, the volcano curve between the limiting potential and the adsorption free energy of NH2* [ΔG(NH2*)] was revealed, in which a moderate ΔG(NH2*) was required for high‐activity NRR catalysts. This study not only provides a theoretical basis for the rational design of bimetallic compounds anchored on graphene as effective NRR catalysts under ambient conditions but also opens up a new way to accelerate the screening of NRR catalysts.
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