Introduction

In recent years, ultrathin two-dimensional (2D) materials have garnered immense attention due to their excellent chemical and physical properties [1,13]. When the same semiconductors are reduced to ultralow thickness, photogenerated carriers can be quickly transferred from the inside to the surface, thus resulting in faster charge carrier transfer. Moreover, ultrathin structure materials expose more catalytically active sites, which facilitate surface reactions [12]. Thus, constructing ultrathin 2D materials with suitable band gaps is a desirable approach for designing efficient photocatalysts.

Bismuth oxychloride (BiOCl), as a layered material with good prospects for photocatalytic energy conversion and environmental remediation, has attracted extensive attention in recent years [14,13, 27, 28]. Among them, the synthesis of ultrathin nanosheets is an effective approach. According to the formula t = d2/k2D (where d, k, and D are the particle size, constant, and diffusion coefficient, respectively) [29], the ultrathin thickness of BiOCl allows a reduced d value, whereas self-built internal electric fields lead to an increased D value [2g–i).

Fig. 2
figure 2

a, b TEM images of BiOCl and 5 wt% CQDs/BiOCl. c High-magnification TEM image of 5 wt% CQDs/BiOCl. d–f HAADF-STEM images of 5 wt% CQDs/BiOCl. g–i Elemental map** of 5 wt% CQDs/BiOCl

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The photocatalytic performance of the obtained samples was then assessed for RhB degradation under visible-light irradiation. Before the photocatalytic degradation, a blank experiment was conducted (Fig. S4). From Fig. 3a, 80.6% of RhB could be removed using bare BiOCl material after 30 min of irradiation. With the introduction of CQDs to BiOCl, the photocatalytic performance of BiOCl was significantly improved. The 5 wt% CQDs/BiOCl composite exhibited the highest photocatalytic performance with a degradation efficiency of 94.5% after 30 min of irradiation. However, a further increase in the CQD content beyond 5 wt% resulted in a decrease in photocatalytic performance. Although the modification of BiOCl with CQDs can facilitate charge transfer, excessive CQDs covering the BiOCl surface may limit light absorption [54]. As shown in Fig. 4g, the superoxide radical (·O2−) was observed for the as-prepared materials under visible-light irradiation. The ·O2− intensity of the CQDs/BiOCl composite was significantly higher than that of bare BiOCl (Fig. 4g). As the generation of ·O2− originates from O2 reduction through one-electron transfer, the higher ·O2− intensity of the CQDs/BiOCl composites confirms that CQDs allow more photogenerated electrons to reduce O2. In fact, the delocalized conjugated structure of CQDs allows them to transfer photogenerated electrons easily [55]. From Fig. 4h, the hydroxyl radical (·OH) was also detected under visible-light irradiation, and the generated amounts of ·OH were increased with increasing CQD content. Moreover, Fig. 4i displays the singlet oxygen spectra of the as-prepared samples under UV irradiation. The generated amounts of singlet oxygen by CQDs/BiOCl were increased with increasing in CQD content, which is beneficial for the removal of pollutants. Catalytic measurements have revealed that the CQDs/BiOCl material can be utilized as an efficient photocatalytic degradation system. However, it is still unclear how the photocatalytic process works. To decode the mechanism, a series of control experiments were conducted to analyze the process (Fig. S11). Only 8.7% and 10.5% of TC degradation occurred in the absence of the catalyst and light, respectively, indicating that light and catalyst are indeed required for the reaction. When the reaction was conducted under an argon atmosphere, 56.3% of TC was degraded, which should be driven by photogenerated holes. To determine the types of reactive oxygen species responsible for photocatalytic degradation in our system, we performed characterization using different scavengers, confirming that photogenerated holes are indispensable for the photocatalytic degradation of TC. The control experiment also showed that the reaction activity is largely suppressed using Na2S as a hole scavenger. Moreover, ·O2− was confirmed to play a significant role in the photocatalytic oxidation reaction by ESR analysis (Fig. 4g) and free radical trap** experiments (Fig. S11).

Moreover, the valence band (VB) and conduction band (CB) potentials were calculated using ECB = EVB − Eg, where EVB is the VB edge potential, ECB is the CB edge potential, and Eg is the band gap energy. As observed in Fig. S12, the Eg value of 5 wt% CQDs/BiOCl was 2.92 eV. Figure S13 shows that the EVB value of 5 wt% CQDs/BiOCl was 1.21 eV. From ECB = EVB − Eg, the ECB value of 5 wt% CQDs/BiOCl was calculated to be − 1.71 eV. Here, electrons with energy above ECB (− 1.71 eV) can reduce O2 to generate ·O2− because E0(O2/·O2−) is − 0.046 eV (vs. NHE) [56, 57]. From the above discussion and results, the mechanism of organic pollutant degradation was proposed, as illustrated in Fig. S14. CQDs with remarkable electrical conductivity were introduced into the BiOCl materials as charge mediators. Due to the bridge effect between the two substances, the separation efficiency of the change carriers was significantly enhanced, which offers more electrons to generate the ·O2− active species. Moreover, CQDs can also absorb light at longer wavelengths than BiOCl [58], extending the range of light absorption. Thus, the photocatalytic activity of CQDs/BiOCl was significantly improved after the introduction of CQDs.

Conclusions

In this work, a novel CQDs/BiOCl composite photocatalyst was prepared by using a facile hydrothermal method. The CQDs were integrated on the surface of the BiOCl ultrathin nanosheets to form a tight junction. After introducing CQDs, the photocatalytic activity of the CQDs/BiOCl composites in RhB and TC degradation was significantly enhanced under visible-light irradiation. The 5 wt% CQDs/BiOCl material exhibited the highest photocatalytic performance with a degradation efficiency of 94.5% after 30 min of irradiation. Moreover, the N2 photoreduction performance was significantly improved after introducing CQDs. The 5 wt% CQDs/BiOCl demonstrated a nitrogen photoreduction performance to yield NH3 of 346.25 μmol/(g h), which is significantly higher than those of 3 wt% CQDs/BiOCl (256.04 μmol/(g h)), 7 wt% CQDs/BiOCl (254.07 μmol/(g h)), and bare BiOCl (240.19 μmol/(g h)). The key role of CQDs in improving photocatalytic performance was ascribed to their increased light harvesting capacity, outstanding electron transfer ability, and higher photocatalytic active sites. From the ESR and free radical trap** analysis results, holes and ·O2− were the main active species. This study provides insights into the design of composite photocatalysts by integrating 2D materials and quantum dots.