In this work, we demonstrated the successful construction of metal-free zero- dimensional/two-dimensional carbon nanodot (CND)-hybridized protonated g-C₃N₄ (pCN) (CND/pCN) heterojunction photocatalysts by means of electrostatic attraction. We experimentally found that CNDs with an average diameter of 4.4 nm were uniformly distributed on the surface of pCN using electron microscopy analysis. The CND/pCN-3 sample with a CND content of 3 wt.% showed the highest catalytic activity in the CO₂ photoreduction process under visible and simulated solar light. This process results in the evolution of CH₄ and CO. The total amounts of CH₄ and CO generated by the CND/pCN-3 photocatalyst after 10 h of visible-light activity were found to be 29.23 and 58.82 µmol·g_catalyst⁻¹, respectively. These values were 3.6 and 2.28 times higher, respectively, than the amounts generated when using pCN alone. The corresponding apparent quantum efficiency (AQE) was calculated to be 0.076%. Furthermore, the CND/pCN-3 sample demonstrated high stability and durability after four consecutive photoreaction cycles, with no significant decrease in the catalytic activity. The significant improvement in the photoactivity using CND/pCN-3 was attributed to the synergistic interaction between pCN and CNDs. This synergy allows the effective migration of photoexcited electrons from pCN to CNDs via well- contacted heterojunction interfaces, which retards the charge recombination. This was confirmed by photoelectrochemical measurements, and steady-state and time-resolved photoluminescence analyses. The first-principles density functional theory (DFT) calculations were consistent with our experimental results, and showed that the work function of CNDs (5.56 eV) was larger than that of pCN (4.66 eV). This suggests that the efficient shuttling of electrons from the conduction band of pCN to CNDs hampers the recombination of electron–hole pairs. This significantly increased the probability of free charge carriers reducing CO₂ to CH₄ and CO. Overall, this study underlines the importance of understanding the charge carrier dynamics of the CND/pCN hybrid nanocomposites, in order to enhance solar energy conversion.

The photocatalytic reduction of CO₂ to energy-rich hydrocarbon fuels is a promising and sustainable method of addressing global warming and the imminent energy crisis concomitantly. However, a vast majority of the existing photocatalysts are only capable of harnessing ultraviolet (UV) or/and visible light (Vis), whereas the near-infrared (NIR) region still remains unexplored. In this study, carbon quantum dots (CQDs)-decorated ultrathin Bi₂WO₆ nanosheets (UBW) were demonstrated to be an efficient photocatalyst for CO₂ photoreduction over the Vis–NIR broad spectrum. It is noteworthy that the synthesis procedure of the CQDs/UBW hybrid nanocomposites was highly facile, involving a one-pot hexadecyltrimethylammonium bromide (CTAB)-assisted hydrothermal process. Under visible light irradiation, the optimized 1CQDs/UBW (1 wt.% CQD content) exhibited a remarkable 9.5-fold and 3.1-fold enhancement of CH₄ production over pristine Bi₂WO₆ nanoplatelets (PBW) and bare UBW, respectively. More importantly, the photocatalytic responsiveness of CQDs/UBW was successfully extended to the NIR region, which was achieved without involving any rare earth or noble metals. The realization of NIR-driven CO₂ reduction could be attributed to the synergistic effects of (ⅰ) the ultrathin nanostructures and highly exposed {001} active facets of UBW, (ⅱ) the excellent spectral coupling of UBW and CQDs, where UBW could be excited by the up-converted photoluminescence of CQDs, and (ⅲ) the electron-withdrawing nature of the CQDs to trap the photogenerated electrons and retard the recombination of charge carriers.

Tailored synthesis of well-defined anatase TiO₂-based crystals with exposed {001} facets has stimulated incessant research interest worldwide due to their scientific and technological importance. Herein, anatase nitrogen-doped TiO₂ (N-TiO₂) nanoparticles with exposed {001} facets deposited on the graphene (GR) sheets (N-TiO₂-001/GR) were synthesized for the first time via a one-step solvothermal synthetic route using NH₄F as the morphology-controlling agent. The experimental results exemplified that GR was uniformly covered with anatase N-TiO₂ nanoparticles (10–17 nm), exposing the {001} facets. The percentage of exposed {001} facets in the N-TiO₂-001/GR nanocomposites was calculated to be ca. 35%. Also, a red shift in the absorption edge and a strong absorption in the visible light range were observed due to the formation of Ti-O-C bonds, resulting in the successful narrowing of the band gap from 3.23 to 2.9 eV. The photocatalytic activities of the as-prepared photocatalysts were evaluated for CO₂ reduction to produce CH₄ in the presence of water vapor under ambient temperature and atmospheric pressure using a low-power 15 W energy-saving daylight lamp as the visible light source—in contrast to the most commonly employed high-power xenon lamps—which rendered the process economically and practically feasible. Among all the studied photocatalysts, the N-TiO₂-001/GR nanocomposites exhibited the greatest CH₄ yield of 3.70 μmol·g_catalyst^-1, approximately 11-fold higher activity than the TiO₂-001. The enhancement of photocatalytic performance was ascribed to the effective charge anti-recombination of graphene, high absorption of visible light region and high catalytic activity of {001} facets relative to the {101} facets.