The increasing consumption of fossil fuels elevates atmospheric CO₂, contributing to climate change. Reproducing natural photosynthesis to convert CO₂ into usable fuels is a key goal. Photocatalytic CO₂ conversion systems utilize semiconductors excited by photons to generate electron-hole pairs, which reduce CO₂ and oxidize water. However, fast charge recombination hinders efficiency and selectivity. Most systems rely on sacrificial hole acceptors rather than water. While CO₂ reduction efficiency has improved, these systems remain uncommercial and unsustainable. To utilize abundant water, suitable co-catalysts are needed to promote water oxidation over recombination. Carbon nitrides (CN) are promising organic semiconductors with long-lived charge carriers, but their valence band limits water oxidation. Carbon dots (CDs) have shown potential as co-catalysts, but electron-accepting CDs mainly boost electron potential without accelerating water oxidation. A hole-accepting co-catalyst is needed to improve water oxidation and achieve selective CO₂ reduction to high-value products like methanol, a desirable liquid hydrogen source.

The literature extensively covers electron-accepting co-catalysts that enhance the reductive potential of photogenerated electrons in CN but do not address the sluggish water oxidation reaction. While some studies explore CDs for enhancing various photocatalytic reactions, their role in selective charge carrier acceptance is limited. The use of hole-accepting CD co-catalysts for improved water oxidation has been rarely reported. Methanol is a preferred product from CO₂ compared to CO, methane, and formic acid, but its generation requires exceptionally long-lived charge carriers to allow for electron accumulation. Methanol's rapid oxidation on TiO₂, for instance, favors water oxidation.

This study synthesized two types of carbon dots (CDs): mCDs via a microwave method and sCDs via a sonication method. Both were loaded onto CN at their optimal concentrations. mCDs were synthesized from urea and citric acid precursors using a 10-minute microwave method. sCDs were fabricated via alkaline carbonitrile treatment of glucose. Pristine CN was synthesized from dicyandiamide. Characterization techniques included Powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and UV-Vis spectroscopy. Transient absorption spectroscopy (TAS) was used to investigate charge carrier dynamics. Photocatalytic CO₂ conversion was evaluated under visible light irradiation (λ > 420 nm), and products were analyzed by gas chromatography (GC) and GC-mass spectrometry (GC-MS). Density functional theory (DFT) calculations were performed to investigate adsorption energies.

mCDs exhibit a unique hole-accepting nature, significantly prolonging the electron lifetime of CN (sixfold increase). The mCD/CN nanocomposite achieved nearly 100% selectivity towards methanol production from CO₂ and water under visible light irradiation, with an internal quantum efficiency of 2.1% at 420 nm. Stoichiometric oxygen evolution was observed, confirming water oxidation. In contrast, sCDs acted as electron acceptors, leading to CO production. TAS measurements revealed that mCDs extract holes from CN, preventing methanol oxidation and promoting water oxidation. DFT calculations supported these findings, showing that mCDs favor water adsorption while having unfavorable methanol adsorption, facilitating selective water oxidation. Control experiments confirmed the necessity of light, CO₂, and the photocatalyst for methanol production.

The superior performance of mCD/CN stems from the unique hole-accepting nature of mCDs, which promotes efficient charge separation, prolongs charge carrier lifetimes, and selectively oxidizes water over methanol. The nearly 100% selectivity towards methanol production and the stoichiometric O₂ evolution highlight the effectiveness of this strategy. The differences in the photocatalytic behavior of mCD/CN and sCD/CN emphasize the crucial role of the hole-accepting co-catalyst in achieving high selectivity and efficiency in CO₂ reduction. This approach offers a sustainable alternative for methanol production.

This study successfully demonstrated a highly selective and efficient photocatalytic system for CO₂ reduction to methanol using pure water as the electron donor. The key to this success is the use of uniquely synthesized mCDs as hole-accepting co-catalysts. Future research could explore optimizing mCD synthesis, investigating different semiconductor materials, and scaling up the system for practical applications. The unique functionality of hole-accepting carbon dots could potentially impact other areas such as photovoltaics and photoelectrochemical devices.

The current study focuses on laboratory-scale experiments. Further research is needed to scale up the process for industrial applications. While the internal quantum efficiency is 2.1% at 420 nm, further optimization of the photocatalyst and reaction conditions could potentially improve this efficiency. The long-term stability of the mCD/CN composite under continuous operation needs further investigation.