1. Abstract
Photocatalytic CO₂ reduction to value-added chemicals is a promising strategy for addressing global carbon neutrality and energy crisis, but it is still limited by low visible-light utilization and rapid charge carrier recombination. In this work, we designed a novel composite photocatalyst by modifying ZnIn₂S₄ (ZIS) nanosheets with graphene quantum dots (GQDs) via a one-step hydrothermal method. The key findings are as follows:
- The GQDs/ZIS composite exhibited a CO production rate of 23.6 μmol·g⁻¹·h⁻¹ under visible-light irradiation (λ > 420 nm), which is 3.2 times higher than that of pure ZIS nanosheets;
- Transient photocurrent response and photoluminescence (PL) spectroscopy confirmed that GQDs effectively promoted the separation of photogenerated electron-hole pairs, with the charge carrier lifetime extended from 1.2 ns to 3.8 ns;
- X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations revealed that the strong interfacial interaction between GQDs and ZIS constructed a Schottky junction, which enhanced visible-light absorption and accelerated charge transfer.
This study provides a feasible approach for designing high-performance sulfide-based photocatalysts for CO₂ reduction, and deepens the understanding of interfacial charge transfer mechanisms in composite photocatalytic systems.
2. Experimental Section
2.1 Materials Preparation
- Synthesis of GQDs: Citric acid (0.5 g) was dissolved in deionized water (10 mL), then transferred to a Teflon-lined autoclave (25 mL) and heated at 180℃ for 4 h. After cooling to room temperature, the solution was dialyzed (MWCO = 1000 Da) for 24 h to obtain GQDs solution;
- Synthesis of GQDs/ZIS Composite: Zn(NO₃)₂·6H₂O (0.297 g), In(NO₃)₃·4.5H₂O (0.488 g), and thiourea (0.384 g) were dissolved in GQDs solution (5 mL). The mixture was stirred for 30 min, then heated at 200℃ for 12 h in an autoclave. The product was centrifuged, washed with deionized water and ethanol, and dried at 60℃ for 8 h;
- Preparation of pure ZIS: Synthesized via the same hydrothermal method without adding GQDs solution.
2.2 Characterization & Photocatalytic Tests
- Structural characterization: X-ray diffraction (XRD, Bruker D8 Advance), transmission electron microscopy (TEM, JEOL JEM-2100);
- Optical properties: UV-Vis diffuse reflectance spectroscopy (DRS, Shimadzu UV-2600), PL spectroscopy (Horiba FluoroMax-4);
- Photocatalytic CO₂ reduction: Conducted in a sealed reactor under 300 W Xe lamp (λ > 420 nm) with CO₂ gas (99.99%) as feedstock. The products were analyzed by gas chromatography (GC-2014, Shimadzu).
3. Results and Discussion
3.1 Photocatalytic Activity
As shown in Figure 1a, the photocatalytic activity of ZIS was significantly enhanced by GQDs modification. The optimal GQDs loading amount was 5 wt%, with a CO production rate of 23.6 μmol·g⁻¹·h⁻¹. Higher GQDs loading (>5 wt%) led to a decrease in activity, which was attributed to the aggregation of GQDs that blocked the active sites of ZIS.
3.2 Stability and Charge Carrier Dynamics
The 5% GQDs/ZIS composite maintained 85% of its initial activity after 5 cycles (Figure 1b), indicating good stability. The UV-Vis DRS (Figure 1c) showed that GQDs modification extended the visible-light absorption edge of ZIS from 520 nm to 580 nm. The PL intensity of 5% GQDs/ZIS was much lower than that of pure ZIS (Figure 1d), confirming the effective suppression of electron-hole recombination.
3.3 Interfacial Interaction and Mechanism
XPS analysis showed a shift of Zn 2p and In 3d binding energies to higher values in GQDs/ZIS, indicating electron transfer from ZIS to GQDs. DFT calculations further confirmed that the Schottky junction at the GQDs/ZIS interface reduced the bandgap of ZIS and provided a fast channel for charge transfer, thus enhancing the photocatalytic performance.
4. Conclusion
In summary, we successfully prepared GQDs/ZIS composite photocatalysts via a one-step hydrothermal method. The key conclusions are:
- GQDs modification significantly enhances the visible-light photocatalytic CO₂ reduction activity of ZIS, with the optimal CO production rate reaching 23.6 μmol·g⁻¹·h⁻¹;
- The Schottky junction formed at the GQDs/ZIS interface is the main reason for the improved performance, which promotes charge carrier separation, extends carrier lifetime, and enhances visible-light absorption;
- This work offers a new perspective for the design of sulfide-based composite photocatalysts for efficient CO₂ conversion, and provides experimental and theoretical support for understanding interfacial charge transfer mechanisms.