Experimental and DFT Assessments of Nitrogen-Doped Carbon Dots with Distinct Nitrogen Configurations for Photothermal Conversion

Nitrogen-doped carbon dots (CDs) exhibit favorable optical properties, positioning them as promising zero-dimensional nanoparticulate additives for solar-thermal nanofluids aimed at alleviating global energy challenges. However, systematic elucidation of how different nitrogen configurations regulate photothermal performance remains limited. In this work, density functional theory (DFT) calculations were employed to delineate the structure–property relationships of nitrogen-doped CDs in solar photothermal applications. To facilitate a more consistent and quantitative assessment of the light-harvesting performance of nitrogen-doped CDs, the solar-weighted absorption intensity was introduced as a practical evaluation metric that consolidates the spectral coverage and absorption intensity into a single descriptor of solar-light absorption capability, thereby enabling quantitative comparison across different N-doped configurations. Graphite nitrogen was identified by DFT analyses as a key structural motif that optimizes photon capture, increasing spectral utilization and photothermal conversion efficiency (PCE) through extended near-infrared absorption and suppressed radiative decay. Guided by these predictions, CDs enriched in graphite nitrogen were synthesized, and experimental validation demonstrated 33.2% and 77.1% enhancements in photothermal conversion relative to the native N-CDs and base fluid, respectively. Moreover, compositional and performance variations of CDs were correlated with their synthetic reaction environments. Collectively, the integration of calculation modeling and experimental validation establishes a transferable design principle for engineering high-efficiency photothermal nanomaterials and is expected to advance solar photothermal technologies at the nanoscale.