Unique hole-accepting carbon-dots promoting selective carbon dioxide reduction nearly 100% to methanol by pure water

The study addresses the challenge of photocatalytically converting CO₂ to energy-dense liquid fuels using water as the sole electron donor. In semiconductor-driven CO₂ reduction, photogenerated electron–hole pairs compete with fast charge recombination (< μs), while water oxidation proceeds much more slowly (~s), making selective, efficient conversion difficult without sacrificial agents. Carbon nitride (CN) is an attractive organic semiconductor, yet its valence band position and the kinetics of water oxidation limit performance. Carbon dots (CDs) have been explored to improve photocatalysis, typically as electron-accepting co-catalysts that enhance electron lifetimes but do not accelerate water oxidation. Achieving selective methanol formation is particularly demanding as it requires a six-electron transfer and long-lived charge carriers; moreover, methanol is readily oxidised on many photocatalysts, often outcompeting water oxidation. The hypothesis here is that a hole-accepting co-catalyst could prolong charge carrier lifetimes while preserving strong electron reduction potentials, enabling selective CO₂-to-methanol conversion with water oxidation proceeding efficiently.

Prior photocatalytic CO₂ reduction systems commonly rely on sacrificial hole scavengers to suppress recombination and enhance rates, which limits sustainability and scalability. CN-based photocatalysts have shown promise but face intrinsic limitations for water oxidation due to band energetics and kinetics. CDs have been widely reported as electron-accepting co-catalysts that can extend electron lifetimes and facilitate CO₂ reduction to two-electron products like CO; however, experimental evidence for CDs acting as selective hole-acceptors to promote water oxidation is scarce. Existing CD/CN composites often yield CO with limited O₂ evolution, consistent with electron-accepting behaviour of CDs. The literature therefore indicates a gap: developing a co-catalyst that selectively accepts holes, accelerates water oxidation, and enables accumulation of long-lived electrons for multi-electron CO₂ reduction to methanol.

Materials and synthesis: CN nanosheets were prepared by thermal treatment of a nitrogen-rich precursor (dicyandiamide) with subsequent oxidation/exfoliation steps to yield heptazine-based CN. Microwave carbon dots (mCDs) were synthesised from citric acid and urea via a rapid (~10 min) microwave method, purified, and then combined with a CN precursor (dicyandiamide) in DMF, followed by thermal polymerisation/annealing to form mCD/CN composites with varied mCD loadings (1.5–4.5 wt%; optimal ~3.5 wt%). Sonication-derived CDs (sCDs) were prepared by alkaline-assisted sonication of glucose, purified, and physically loaded onto preformed CN (optimal ~3 wt%) followed by brief heat treatment to form sCD/CN. Characterisation: Structural and optical properties were probed by PXRD (graphitic-like features in mCD; CN (002) and (100) peaks at ~27.4° and 13.0° 2θ), TEM/HRTEM (mCDs <10 nm decorating CN; sCDs 15–20 nm, amorphous), Raman (D and G bands; mCD more graphitic than sCD), FTIR (functional groups; mCD with N/O dopants; CN heptazine signatures), XPS (N and O functionalities in mCD/mCD-CN), and UV–Vis (enhanced visible absorption of CDs vs CN). Transient absorption spectroscopy (TAS): Diffuse reflectance TAS (μs–ms) under visible excitation probed charge carrier dynamics. Electron and hole spectral signatures were assigned around 700 nm and ~510–550 nm, respectively, aided by Ag⁺ as an electron scavenger. TAS compared CN, mCD/CN, and sCD/CN to resolve directionality of interfacial charge transfer and quantify electron half-lifetimes. Photocatalysis: CO₂ reduction with water was conducted in a glass reactor under visible light (λ > 420 nm) using a 300 W Xe lamp. Gas and liquid products were quantified by GC/GC–MS; ¹³CO₂ isotopic labelling verified product origins (¹³CH₃OH and ¹³CO). Control experiments included (i) no CO₂, (ii) no photocatalyst, (iii) dark conditions. Methanol oxidation tests on CN and mCD/CN under visible light assessed product selectivity mechanisms. Electrochemical measurements: Films on conductive substrates were used with Ag/AgCl reference and Pt counter in 1 M Na₂SO₄ to assess charge transfer characteristics. Quantum efficiency: Internal quantum yields (IQY) were measured under bandpass filters (420, 500, 600 nm) using the same reactor setup; photon flux was quantified to calculate IQY = (reacted electrons)/(absorbed photons) × 100.

- mCD acts as a hole-accepting co-catalyst in mCD/CN, while sCD behaves as an electron-accepting co-catalyst in sCD/CN, as evidenced by TAS. - TAS showed a substantial increase in long-lived electron population in CN when coupled with mCD, indicating efficient hole extraction by mCD. The 700 nm electron signal amplitude increased (CN to mCD/CN) and was insensitive to Ag⁺ addition for sCD/CN, consistent with electron transfer to sCD in that case. - Electron half-lifetime at 700 nm increased from ~25 μs (CN) to ~160 μs (mCD/CN), a ~6-fold enhancement, whereas sCD/CN showed only a modest increase to ~40 μs. - Photocatalytic performance: mCD/CN selectively produced methanol (six-electron product) with nearly 100% selectivity under visible light (λ > 420 nm), accompanied by stoichiometric O₂ evolution from water. The O₂:CH₃OH molar ratio reached ~1.451, close to the theoretical 1.5 for water oxidation coupled to CO₂-to-methanol reduction. - Average production rates for an optimised mCD/CN (ca. 3.5 wt% mCD): methanol ~1.39 μmol min⁻¹ and CO ~0.05 μmol min⁻¹, indicating near-unity selectivity to methanol over CO. mCD/CN activity was roughly twice that of sCD/CN under identical conditions. - sCD/CN predominantly yielded CO (two-electron product) with negligible O₂ evolution, consistent with sCD acting as an electron acceptor and not promoting water oxidation. - Isotopic labelling with ¹³CO₂ confirmed methanol (¹³CH₃OH) and CO (¹³CO) originate from CO₂ reduction, ruling out contamination. - IQY for mCD/CN reached ~2.1% at 420 nm and remained measurable at longer wavelengths (~0.7% at 500 nm; ~0.4% at 600 nm) under near one-sun conditions. - DFT adsorption energy analysis indicated CO₂ and CH₃OH adsorb more favourably on CN, while H₂O preferentially adsorbs on mCD. This favours electron accumulation/reduction of CO₂ on CN, with holes transferred to mCD to oxidize water. Weak methanol adsorption on mCD suppresses its undesired oxidation, reinforcing selectivity.

The results demonstrate that engineering the CD/CN interface to favour hole acceptance by CDs enables water to serve as the sole electron donor while achieving selective CO₂ reduction to methanol. Hole extraction by mCD suppresses electron–hole recombination in CN, substantially prolonging electron lifetimes to support the multi-electron steps required for methanol formation. Concurrently, preferential adsorption of water on mCD directs photoholes to oxidize water rather than methanol, mitigating the common issue of methanol oxidation which often outcompetes water oxidation on many photocatalysts. In contrast, sCDs, acting as electron acceptors, divert electrons from CN and favour two-electron CO formation, with minimal O₂ evolution, underscoring the critical role of hole-accepting behaviour for selective alcohol production. The combined TAS, isotopic labelling, product stoichiometry, and adsorption energy calculations provide a coherent mechanistic picture: electrons remain on CN for CO₂ reduction to CH₃OH, while holes transfer to mCD, where water oxidation to O₂ proceeds efficiently, delivering high selectivity and improved activity.

This work introduces microwave-synthesised carbon dots (mCDs) as effective hole-accepting co-catalysts for CN, enabling selective CO₂ reduction to methanol using water as the only oxidant. By prolonging electron lifetimes and steering holes to water oxidation sites, the mCD/CN system achieves near-unity selectivity to methanol with stoichiometric O₂ evolution and measurable internal quantum yields under visible light. The approach provides a scalable route to tailor charge separation and surface chemistry in metal-free photocatalysts for multi-electron CO₂ conversions. Future work could optimise mCD loading and composition, integrate with co-catalysts for further kinetic enhancement, improve light harvesting, and explore broader classes of organic semiconductors and carbon dots to boost efficiency and stability toward practical solar fuel generation.

Although selectivity is near-unity, overall efficiencies (IQY ~2.1% at 420 nm) remain modest, indicating room for further optimisation. The study focuses on controlled laboratory conditions (λ > 420 nm, Xe lamp) and specific mCD/sCD syntheses; generalisation to solar spectrum operation, long-term stability under continuous operation, and scalability need further evaluation. Structural variations between differently prepared CDs may affect reproducibility, and deeper mechanistic resolution of active interfacial sites would benefit from additional operando spectroscopies.