Environmentally friendly synthesis of carbon quantum dots (CQDs) to enhance photocatalytic activity for the photodegradation of various organic dyes and the evolution of hydrogen and oxygen.

This review delivers a focused and critical evaluation of recent progress in the green synthesis of carbon quantum dots (CQDs), with particular attention to state-of-the-art approaches utilizing renewable biomass as precursors. The main objective is to systematically examine innovative, environmentally friendly methods and clarify their direct influence on the core properties and photocatalytic performance of CQDs. The novelty of this review stems from its comprehensive comparison of green synthetic pathways, revealing how specific processes determine key structural, optical, and electronic attributes of the resulting CQDs. These intrinsic properties, in turn, enable significantly higher photocatalytic efficiency. Notable advancements highlighted include: (i) the enhanced stability and broadened visible light absorption achieved through green synthesis; (ii) improved photogenerated charge carrier separation; and (iii) superior rates of organic pollutant degradation (e.g., methylene blue, rhodamine B, phenol) and effective solar-driven hydrogen and oxygen evolution. Additionally, the review addresses how fine-tuning surface functionalities and synthesis parameters at the molecular level can optimize both catalytic reactivity and selectivity. By integrating and critically synthesizing the latest research, this review not only underscores the pivotal role of sustainable CQD fabrication for environmental remediation and clean energy generation, but also identifies major challenges and outlines future research directions. These insights are intended to guide the rational design of high-performance, eco-friendly photocatalytic nanomaterials in line with green chemistry principles.