English

• Due to the massive discharge of industrial effluents and urban domestic wastewater, water pollution has become a serious threat to human health and the ecological environment^[1]. Photocatalytic water splitting for hydrogen production is considered a highly viable strategy because it directly converts solar energy into chemical energy^[2-4]. In this context, photocatalytic technology—offering unique advantages such as solar-driven catalytic activity and high cost-effectiveness—has attracted extensive attention for both organic pollutant degradation and solar hydrogen production^[5-6].

  As a key element of photocatalytic reactions, the performance of semiconductor photocatalysts directly determines the practical application value of the technology. Therefore, the search for efficient and stable semiconductor photocatalysts has become a major research focus^[7]. Rational design and structural optimization aimed at enhancing light-harvesting capacity and charge-separation efficiency have become central to overcoming current technological bottlenecks^[8-9]. Among various semiconductor photocatalysts, cadmium sulfide (CdS), as a typical Ⅱ-Ⅵ group semiconductor, has attracted extensive attention due to its unique properties. It has an appropriate band gap of approximately 2.4 eV, which enables it to effectively absorb visible light (λ < 520 nm)^[10-11]. Nevertheless, the short lifetime of photogenerated charge carriers and severe photocorrosion arising from surface agglomeration markedly reduce its activity and long-term stability. Polyoxometalates (POMs) are metal oxide inorganic nanoclusters with unique structural characteristics and adjustable bandgap structures^[12]. Keggin-type POMs show great potential in photocatalysis due to their strong electron storage capacity and rapid ionic conversion mechanism^[13-14]. Carbon quantum dots (CQDs), an emerging class of zero-dimensional carbon nanomaterial, have demonstrated considerable potential in photocatalysis, bio-imaging, and drug-delivery applications owing to their low toxicity, excellent biocompatibility, readily tunable optical properties, and low cost^[15]. However, their practical application still faces many challenges. Current research primarily focuses on surface engineering and heteroatom doping to boost their performance and advance practical implementation. Microwave-assisted synthesis delivers volumetric, uniform heating and precise temperature control through dielectric relaxation or dipole resonance, offering a highly efficient and environmentally benign route to materials fabrication^[16].

  In this work, a CQDs/PW₁₂/CdS Z-scheme heterojunction was proposed to be constructed using CQDs, preformed CdS, and Keggin-type H₃PW₁₂O₄₀·xH₂O (PW₁₂), with Keggin-type POMs regarded as the most stable structures in POM chemistry. The influence of different mass ratios of CQDs/PW₁₂/CdS on the removal of rhodamine B (RhB) was systematically studied, and hydrogen evolution was evaluated using lactic acid as the sacrificial agent. The ternary composite exhibited markedly higher activity than either bare PW₁₂ or CdS. The origin of the enhanced performance was elucidated, and the probable photocatalytic mechanism was proposed based on radical-trapping experiments.

  1.   Experimental

  1.1   Materials and reagents

  Information on reagents and materials is provided in the Supporting information.

  1.2   Preparation of photocatalytic reagents

  1.2.1   Preparation of CQDs and CdS composites

  Details of the preparation of CQDs and CdS are provided in the Supporting information.

  1.2.2   Preparation of PW₁₂/CdS

  The PW₁₂/CdS composite was prepared by a one-pot hydrothermal method. In a typical procedure, an amount of CdS and PW₁₂ was added to deionized water at predetermined mass ratios. The suspension was magnetically stirred until homogeneous, transferred into a Teflon-lined stainless-steel autoclave, and heated at 180 ℃ for 12 h. After cooling, the yellow precipitate was collected by centrifugation, washed thoroughly with water and ethanol, and dried at 60 ℃ overnight. Binary composites with PW₁₂ mass fractions of 10%, 20%, 30%, 40%, and 50% were fabricated and denoted as xPW₁₂/CdS (x=10%, 20%, 30%, 40%, and 50%)。

  1.2.3   Preparation of CQDs/PW₁₂/CdS

  The CQDs/PW₁₂/CdS composite was prepared via a hydrothermal method. Typically, predetermined amounts of PW₁₂, CdS, and CQDs powders were weighed and dispersed in deionized water under magnetic stirring. The resulting mixture was transferred into a Teflon-lined stainless-steel autoclave, which was then heated at 180 ℃ for 12 h. After naturally cooling to room temperature, the precipitate was collected by centrifugation, filtered, and thoroughly washed to afford the CQDs/PW₁₂/CdS composite, which was stored for future use. CQDs with mass fractions of 1%, 5%, and 10% were separately introduced into the 20%PW₁₂/CdS binary matrix to fabricate a set of samples denoted as yCQDs/PW₁₂/CdS (y=1%, 5%, and 10%). For comparison, simply physically mix CQDs, PW₁₂, and CdS to obtain 5%CQDs+PW₁₂+CdS.

  1.3   Characterization

  The physicochemical characterization of the prepared samples is supplied in the Supporting information.

  1.4   Photocatalytic activity and electrochemical experiments

  The detailed experiments are displayed in the Supporting information.

  2.   Results and discussion

  2.1   X-ray diffraction analysis

  To better study the chemical composition and crystal structure of the synthesized materials, X-ray diffraction (XRD) was used to analyze CdS, PW₁₂, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS. The results are shown in Fig. 1a and S1.

  Figure 1

  

  Figure 1.  XRD patterns (a) and FTIR spectra (b) of different samples

  下载: 全尺寸图片 幻灯片

  CdS exhibited multiple diffraction peaks at 24.9°, 26.5°, 28.2°, 43.8°, 47.9°, and 51.9°. The diffraction peaks matched the standard pattern (PDF No.04-001-7806) and could be indexed to the (100), (002), (101), (110), (103), and (112) planes of hexagonal wurtzite CdS. The composite exhibited broadened visible-light absorption compared with bare CdS. As can be seen from Fig.S1, PW₁₂ exhibited multiple diffraction peaks at 10.3°, 20.7°, 25.4°, 29.4°, and 34.6°. The diffraction peaks correspond to cubic-phase H₃PW₁₂O₄₀ and match the standard pattern (PDF No.04-009-5178).

  To investigate the crystal structure of PW₁₂/CdS composite materials with different multi-salt contents, XRD analysis was conducted. The results are shown in Fig.S2. As can be seen from Fig.S2, after loading different amounts of PW₁₂ onto CdS, a series of new peaks appeared near the characteristic peaks of CdS in the XRD spectrum. The positions of these new peaks correspond to the characteristic peaks of PW₁₂, indicating that PW₁₂ was successfully fixed on the surface of CdS. Moreover, as the loading amount of PW₁₂ increased, the intensity of the corresponding diffraction peaks gradually strengthened, further confirming the effective loading of PW₁₂ on the surface of CdS. It was worth noting that the position of the PW₁₂ diffraction peaks slightly shifted with the increase in loading amount, which might be due to the change in crystallinity during crystal growth or the lattice fine-tuning caused by interface interactions. This trend was more obvious at higher loading amounts.

  Furthermore, from the XRD diffraction patterns of the 20%PW₁₂/CdS and 5%CQDs/PW₁₂/CdS composite materials, the diffraction peaks belonging to CdS and PW₁₂, respectively, could be observed, confirming the coexistence of these two phases. In the XRD pattern of Fig. 1a, no characteristic peaks of CQDs were detected. Compared with 20%PW₁₂/CdS, 5%CQDs/PW₁₂/CdS showed no significant changes. This might be due to the extremely low addition amount of CQDs during the synthesis process, and no peaks were detected^[17]. Based on the XRD patterns of the materials, a further analysis of 5%CQDs/PW₁₂/CdS was conducted, and the results of the grain size calculation are shown in Table 1.

  Table 1

  

  Table 1.  Grain size, specific surface area, pore volume, and average pore diameter of different samples

  下载: 导出CSV

  Sample	D / nm	SBET / (m2·g-1)	VP / (cm3·g-1)	DP / nm
  CdS	29.99	13	0.12	37.73
  20%PW12/CdS	41.12	11	0.08	27.88
  5%CQDs/PW12/CdS	33.36	17	0.08	18.86

  2.2   FTIR analysis

  To further confirm the composition of the sample and analyze the interactions between the substances, the sample was tested using Fourier transform infrared (FTIR) spectroscopy. Fig. 1b shows the FTIR spectra of the 5%CQDs/PW₁₂/CdS, 20%PW₁₂/CdS samples, as well as bare CdS, PW₁₂, and CQDs. As shown in Fig. 1b, for pure PW₁₂, these peaks are considered to be the backbone vibrations of the fixed POMs crystals, and the stretching bands at 1 078 and 985 cm^-1 are attributed to the characteristic stretching modes of P—O and W=O stretching, respectively. The two peaks at 910 and 803 cm^-1 are assigned to different types of W—O—W stretching bands. Compared with pure PW₁₂ and CdS, the 20%PW₁₂/CdS composite material retained the Cd—S peak (from CdS) and the three characteristic peaks of PW₁₂ (P—O, W=O, W—O—W). This confirmed that PW₁₂ was successfully loaded onto CdS, and the Keggin structure of PW₁₂ was retained in the composite material.

  Pure CdS exhibited several strong vibration peaks, which were also observed in the composite material, excluding the PW₁₂-related peaks with identical or slightly shifted wavenumbers. The interfacial interaction between PW₁₂ and the CdS substrate is mainly attributed to hydrogen bonding. The slightly broadened stretching band at 803 cm^-1 indicates dipole-dipole interaction, and the stretching vibration was perturbed after the interaction between PW₁₂ and the CdS carrier. CQDs displayed distinct characteristic absorption bands: the peak in the 2 960 and 2 865 cm^-1 region is attributed to C—H vibration; a carbonyl band appeared around 1 710 cm^-1; and strong C=O/C—O—C features were observed between 1 170 and 1 040 cm^-1. These FTIR results were consistent with recent research reports on biomass-derived CQDs^[18] and provide supportive evidence for the surface functional groups determined by X-ray photoelectron spectroscopy (XPS) analysis. The FTIR spectra of 5%CQDs/PW₁₂/CdS inherit all the characteristic peaks of 20%PW₁₂/CdS (including Cd—S, P—O, W=O, W—O—W), indicating that the introduction of CQDs did not damage the intrinsic crystal and molecular structures of PW₁₂ or CdS. The PW₁₂-related peaks (which may represent a slight displacement/extension, possibly due to the interfacial interactions between CQDs, PW₁₂, and CdS (such as charge transfer or hydrogen bonding), are consistent with the successful formation of a ternary complex. The FTIR analysis results validated the structural integrity of each component and the effective integration of the three functional materials of CQDs, PW₁₂, and CdS.

  2.3   XPS analysis

  To systematically investigate the chemical states and valence distributions of each element in the 5%CQDs/PW₁₂/CdS composite, XPS analysis was performed on this sample. The results are displayed in Fig. 2.

  Figure 2

  

  Figure 2.  XPS spectra of 5%CQDs/PW₁₂/CdS: survey (a), Cd3d (b), W4f (c), S2p (d), and C1s (e)

  下载: 全尺寸图片 幻灯片

  Fig. 2a shows the survey spectrum, in which distinct photoelectron peaks for C, O, W, Cd, and S were observed. Fig. 2b presents the high-resolution Cd3d region. The binding energies at 405.1 and 411.8 eV correspond to Cd3d_5/2 and Cd3d_3/2, respectively, with a spin-orbit splitting of 6.7 eV. These values agreed well with the reported data for Cd²⁺ in CdS^[19], confirming both the successful incorporation of CdS and the chemical state of cadmium in the composite. Furthermore, the characteristic peaks at 405.5 and 412.3 eV are associated with Cd—O and Cd—O—S. However, no impurity phases (e.g., Cd(OH)₂ and CdO) were detected in the XRD patterns. This phenomenon is attributed to NaOH, which, during the synthesis of CQDs, provides an alkaline environment, causing the chemical adsorption of Cd²⁺ in the composite material, thereby forming Cd—O^[20] at the interface of the composite material. It is proposed that the peaks at 405.5 and 412.3 eV mainly originated from the interface Cd—O bonds formed through the chemical adsorption of Cd²⁺. However, considering the surface sensitivity of XPS and the inevitable surface oxidation of CdS nanocrystals under air exposure, these peaks might also result from the superposition of Cd—S bonds with oxygen adsorption at the surface sulfur sites forming Cd—S—O interface species^[21].

  As shown in Fig. 2c, the high-resolution XPS spectrum of W4f showed binding energy positions consistent with the reported values. These binding energy positions were consistent with the reported values of the W⁶⁺ oxidation state in the literature^[22], indicating that tungsten in the composite mainly existed in the W⁶⁺ oxidation state. In Fig. 2d, the S2p peaks at 162.8 and 161.6 eV correspond to S2p_1/2 and S2p_2/3 orbitals, respectively, indicating that sulfur existed in the S^2- oxidation state^[23]. Additionally, two characteristic peaks at 165.1 and 159.7 eV were found, which accords with prior studies^[24].

  Fig. 2e presents the high-resolution C1s XPS spectrum in the composite. Three distinct characteristic peaks were observed in the spectrum, respectively, with binding energies centered at 284.7, 285.7, and 286.8 eV, respectively. Among them, the peak at 284.7 eV is assigned to C—C/C—H bonds; the peak at 285.7 eV is assigned to C—O bonds; and the peak at 286.8 eV is assigned to the O=C—O functional group in the carboxyl or ester group^[25]. The existence of these oxygen-containing functional groups (C—O and O=C—O) was consistent with the literature reports, which are mainly attributed to the inherent hydrophilic functional groups of CQDs in the composite^[26].

  2.4   Morphological characterization

  The morphological structures of CdS, PW₁₂, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS were observed using a scanning electron microscope (SEM), and the results are shown in Fig. 3.

  Figure 3

  

  Figure 3.  SEM images of CdS (a, b), PW₁₂ (c, d), and 20%PW₁₂/CdS (e, f); SEM images (g, h) and corresponding EDS elemental mappings (i-o) of 5%CQDs/PW₁₂/CdS

  下载: 全尺寸图片 幻灯片

  From Fig. 3a and 3b, it can be seen that CdS exhibited a nanorod-like structure with a smooth surface. From Fig. 3c and 3d, it can be observed that PW₁₂ clusters were aggregated from numerous amorphous nanoparticles, from which it can be determined that pure PW₁₂ presented an irregular aggregated morphology. From Fig. 3e and 3f, it can be observed that numerous rod-like CdS structures were uniformly loaded on the rough surface of PW₁₂, confirming the effective composite. The morphology of the 5%CQDs/PW₁₂/CdS composite material is shown in Fig. 3g and 3h. With the introduction of PW₁₂ and CQDs, the intrinsic rod-like structure of CdS remained unchanged. This phenomenon is mainly attributed to the tiny particle size and sparse distribution of CQDs and their loose distribution. To further demonstrate the distribution of surface elements in 5%CQDs/PW₁₂/CdS, energy dispersive X-ray spectroscopy (EDS) was carried out. As shown in Fig. 3i-3o, elements such as Cd, S, P, W, O, and C were present in 5%CQDs/PW₁₂/CdS. Crucially, Fig. 3i-3o confirms the spatially uniform dispersion of CQDs within the composite. Such uniform element distribution favors efficient interfacial charge transfer and improved photocatalytic performance within the composite material, thereby enhancing the photocatalytic activity.

  The crystal structure characteristics of the 5%CQDs/PW₁₂/CdS composite material and the interactions between the materials were analyzed by transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM). The results are shown in Fig. 4 and S3. Fig. 4a shows the TEM image of 5%CQDs/PW₁₂/CdS. The image presented a relatively smooth morphology. The difference in transparency between the two phases indicates that PW₁₂ and rod-shaped CdS exhibit a strong interfacial interaction, further confirming the successful loading of PW₁₂ on CdS. Fig. 4b and 4d are high-resolution TEM images of 5%CQDs/PW₁₂/CdS, showing the 0.316 nm lattice fringes that correspond to the (101) crystal plane of CdS^[27]. Fast Fourier transform (FFT) (Fig. 4c) showed the 0.337 nm lattice fringes that correspond to the (002) crystal plane of CQDs^[28], further verifying the presence of CQDs. The above lattice spacing analysis results revealed that both CdS and CQDs existed in the composite, and the crystal planes of each component were consistent with the XRD analysis results. The close contact between CQDs, PW₁₂, and CdS in the HRTEM images verified the successful construction of the heterostructure, which facilitates the efficient migration and separation of photogenerated carriers and serves as the structural basis for the excellent photocatalytic performance of this material. As shown in Fig.S3, the particle size distribution of CQDs fitted the Gaussian function well. The average particle size of CQDs was approximately 6.2 nm, and the proportion of nanoparticles ranging from 6 to 6.4 nm reached the highest value of approximately 30%.

  Figure 4

  

  Figure 4.  TEM (a) and HRTEM (b) images of 5%CQDs/PW₁₂/CdS; FFT images of CQDs (c) and CdS (d)

  下载: 全尺寸图片 幻灯片

  2.5   N₂ adsorption-desorption analysis

  To investigate the surface physical and chemical properties of the prepared materials, N₂ adsorption-desorption tests were conducted on the 5%CQDs/PW₁₂/CdS composite materials. The results are shown in Fig. 5. The isotherms of all samples belong to type Ⅳ and are accompanied by H3-type hysteresis loops, which conform to the classification of mesoporous materials by the International Union of Pure and Applied Chemistry (IUPAC). Compared with CdS and 20%PW₁₂/CdS, the isotherms of 5%CQDs/PW₁₂/CdS shifted significantly upwards, indicating a higher specific surface area. The pore size distribution diagram (inset in Fig. 5) further showed that the composite possessed a mesopore-dominated structure, which might result from the aggregation of nanorods and nanoparticles.

  Figure 5

  

  Figure 5.  N₂ adsorption-desorption isotherms and pore-size distributions (inset) of CdS (a), 20%PW₁₂/CdS (b), and 5%CQDs/PW₁₂/CdS (c)

  下载: 全尺寸图片 幻灯片

  Table 1 lists the grain size (D), specific surface area (S_BET), pore volume (V_P), and average pore diameter (D_P) of each catalyst. Compared with CdS and 20%PW₁₂/CdS, the 5%CQDs/PW₁₂/CdS ternary composite exhibited the largest specific surface area and the smallest average pore diameter. Meanwhile, the grain size and pore volume increased slightly. This variation indicated that the introduction of PW₁₂ partially filled the pores of CdS. In contrast, the loading of CQDs significantly optimized the composite′s pore structure, increasing the specific surface area and forming uniform mesopores. The enhanced specific surface area is attributed to the surface effect of CQDs^[29]. The abundant pore structure provides more active sites and enhances the adsorption capacity for dye molecules, thereby improving the photocatalytic performance of the composite.

  2.6   Optical properties

  To study the light absorption characteristics of the catalysts, ultraviolet-visible diffuse reflection spectra (UV-Vis DRS) were conducted on different samples. The results are shown in Fig. 6.

  Figure 6

  

  Figure 6.  UV-Vis DRS (a) and Kubelka-Munk curves (b) of different samples

  下载: 全尺寸图片 幻灯片

  Among them, Fig. 6a shows the UV-Vis DRS of different materials, and Fig. 6b displays the corresponding Kubelka-Munk energy curves. As observed from Fig. 6a, the absorption edges of PW₁₂ and CdS were 440 and 532 nm, respectively. This indicates that PW₁₂ exhibits strong UV absorption, while CdS possesses superior light absorption performance in the UV-Vis regions. The absorption edge of the 20%PW₁₂/CdS composite red-shifted to the long-wavelength region, and compared with CdS and PW₁₂, 20%PW₁₂/CdS had a significant red shift. The enhanced light absorption ability of the PW₁₂/CdS composite material is attributed to the fact that the light response of some polyoxynitride ions themselves broadened the light absorption edge of the composite material, enabling it to utilize more visible light. POM ions temporarily store electrons, which facilitates the improvement of the photocatalytic performance of the composite^[30]. After introducing CQDs, the absorption edge of the composite was further red-shifted, indicating that CQD decoration improves the visible-light absorption capability of the composite for visible light.

  As shown in Fig. 6b, the band gap energies of CdS and PW₁₂ were 2.40 and 3.07 eV, respectively, which were consistent with previously reported values^[31-32]. The introduction of PW₁₂ and CQDs led to a reduced band gap for 5%CQDs/PW₁₂/CdS, because CQDs and PW₁₂ were loaded on the CdS surface, which broadened the visible-light absorption range. This demonstrates that the CdS-based composite with a broad spectral response is successfully fabricated. The optimized composite improves solar light utilization and further enhances photocatalytic activity.

  2.7   Photoluminescence (PL) spectra

  Generally, stronger PL indicates a higher electron (e^-)-hole (h⁺) recombination rate^[33]. To assess the recombination and migration behaviour of photogenerated charge carriers, PL spectra of CdS, PW₁₂, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS were recorded at an excitation wavelength of 325 nm. The results are shown in Fig. 7.

  Figure 7

  

  Figure 7.  PL spectra of CdS, PW₁₂, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS

  下载: 全尺寸图片 幻灯片

  As shown in Fig. 7, the PL intensity of the samples follows the order: PW₁₂ > CdS > 20%PW₁₂/CdS > 5%CQDs/PW₁₂/CdS. The results indicate that after the introduction of PW₁₂, the PL intensity of the 20%PW₁₂/CdS composite significantly decreased, suggesting that PW₁₂, as an efficient electron acceptor, inhibited the carrier recombination of CdS. With the introduction of CQDs, the PL spectral intensity of 5%CQDs/PW₁₂/CdS further decreased, suggesting that CQDs further promoted charge separation and transfer, demonstrating that the composite had higher photogenerated charge separation efficiency than bare CdS. The CQDs/PW₁₂/CdS heterojunction structure effectively inhibited the recombination of photogenerated e^--h⁺. This can be attributed to the unsaturated and dangling bonds on the CQDs′ surface, which generate surface defects, thereby improving photogenerated e^--h⁺ pairs separation efficiency and prolonging carrier lifetime. The experimental results further indicate that the 5%CQDs/PW₁₂/CdS composite may exhibit enhanced photocatalytic performance.

  2.8   Electrochemical analysis

  To further explore the e^--h⁺ transfer and separation efficiency of the samples, transient photocurrent response tests were also conducted on the samples. The results are shown in Fig. 8. As observed from Fig. 8a, five on-off switching cycles were performed, and the photocurrent density of different catalysts followed the order: 5%CQDs/PW₁₂/CdS > 20%PW₁₂/CdS > CdS > PW₁₂. The 5%CQDs/PW₁₂/CdS composite exhibited the highest photocurrent density among all samples, verifying its superior photogenerated carrier separation efficiency.

  Figure 8

  

  Figure 8.  Transient photocurrent response curves (a) and Nyquist plots (b) of CdS, PW₁₂, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS

  下载: 全尺寸图片 幻灯片

  Compared with pure CdS, the introduction of CQDs promoted the generation and separation of photogenerated charges, thereby improving the photocurrent intensity of the CQDs/PW₁₂/CdS composite. As shown in Fig. 8b, the Nyquist semicircle radii of different samples followed the sequence: 5%CQDs/PW₁₂/CdS < 20%PW₁₂/CdS < CdS < PW₁₂. In general, a smaller Nyquist radius corresponds to lower charge transfer resistance. Among all samples, the 5%CQDs/PW₁₂/CdS composite possessed the smallest Nyquist radius, indicating that the catalyst exhibited faster carrier transfer efficiency.

  Compared with other samples, 5%CQDs/PW₁₂/CdS exhibited the lowest PL intensity, the highest photocurrent density, and the lowest charge transfer resistance.

  2.9   Mott-Schottky analysis

  To further study the electronic band structure of the samples, Mott-Schottky tests were conducted on CdS and PW₁₂, and the results are shown in Fig. 9. The Mott-Schottky curves of both samples in Fig. 9 all exhibited positive slopes, reflecting the n-type semiconductor characteristics of CdS and PW₁₂. In this study, Ag/AgCl was used as the reference electrode. At 500, 1 000, and 1 500 Hz, from the x-axis intercepts, the flat band potentials of CdS and PW₁₂ were -0.83 and -0.75 V (vs Ag/AgCl), respectively.

  Figure 9

  

  Figure 9.  Mott-Schottky diagrams of CdS (a) and PW₁₂ (b)

  下载: 全尺寸图片 幻灯片

  According to the Nernst equation^[34], for n-type semiconductors, the conduction band (CB) energy (E_CB) is generally close to the flat band energy^[11]. Compared to the normal hydrogen electrode (E_NHE), the E_CB of CdS and PW₁₂ were -0.63 and -0.55 eV.

  $ E_{{\rm{NHE}}}=E_{{\rm{Ag}}/{\rm{AgCl}}}+0.20 $

  (1)

  $ E_{{\rm{CB}}}=E_{{\rm{VB}}}-E_{{\rm{g}}} $

  (2)

  where E_VB is the valence band (VB) energy, and E_g is the band gap. From the Kubelka-Munk function and Eq.1 and 2, the E_VB of CdS and PW₁₂ were calculated. The calculated results are shown in Table 2.

  Table 2

  

  Table 2.  E_CB, E_g, and E_VB values of CdS and PW₁₂

  下载: 导出CSV

  Sample	ECB / eV	Eg / eV	EVB / eV
  CdS	-0.63	2.40	1.77
  PW12	-0.55	3.07	2.52

  2.10   Photocatalytic degradation

  To evaluate the photocatalytic performance of the composites for degrading wastewater pollutants, RhB was selected as the model pollutant, and degradation tests were performed under simulated sunlight. The results are shown in Fig. 10.

  Figure 10

  

  Figure 10.  Photodegradation curves (a, b) and the corresponding kinetic curves (c) for different catalysts

  下载: 全尺寸图片 幻灯片

  Fig. 10a shows the degradation effects using xPW₁₂/CdS samples. The xPW₁₂/CdS was generated by adjusting the mass ratio of PW₁₂ and CdS by adding different amounts of PW₁₂. Without the catalyst, after 120 min of simulated sunlight exposure, the degradation rate of RhB was 10.2%. With the addition of the xPW₁₂/CdS catalyst, the degradation rate of RhB increased. It was found that the 20%PW₁₂/CdS composite had the optimal RhB degradation effect. Fig. 10b shows the degradation effect of different yCQDs/PW₁₂/CdS samples. It was found that the 5%CQDs/PW₁₂/CdS composite exhibited a degradation rate of up to 93.01%, showing the optimal RhB degradation effect. The degradation rate of pure CQDs within 120 min was only 7.77%, indicating relatively low photocatalytic activity. This confirms that while CQDs contribute to the overall performance, the significantly enhanced performance of the CQDs/PW₁₂/CdS composite mainly arises from a synergistic effect rather than from the properties of the quantum dots alone. Fig. 10c shows the corresponding first-order kinetic curves of the composite material, which indicates that -ln(ρ_t/ρ₀′) has a linear relationship with time.

  $-\ln \left(\rho_t / \rho_0{ }^{\prime}\right)=k t$

  (3)

  Here, t represents the reaction time, ρ₀′ and ρ_t are the initial concentration of RhB at the time when the adsorption-desorption equilibrium is reached, and the concentration of RhB at time t, respectively, and k is the pseudo-first-order rate constant. According to Eq.3, the degradation of RhB by the composite material followed pseudo-first-order reaction kinetics. Table S1 shows the rate constants and corresponding R² values for each sample.

  To ensure that the composite reached an adsorption-desorption equilibrium with the pollutants, a dark adsorption experiment was conducted for 120 min. The results are shown in Fig.S4. As shown in Fig.S4, after 30 min of adsorption, the catalysts and RhB nearly reached adsorption-desorption equilibrium. Additionally, a physical mixture of PW₁₂, CdS, and CQDs was used as a control to verify the formation of the heterojunction. The results are shown in Fig.S5. The experimental results indicate that, compared with the hydrothermally synthesized 5%CQDs/PW₁₂/CdS composite, the physical mixture exhibited a degradation efficiency of only 41.80%, which was significantly lower. This difference demonstrates that the enhanced photocatalytic activity originates from the heterojunction interface formed during hydrothermal synthesis, rather than from the simple physical coexistence of the three components. To evaluate the mineralization capacity of 5%CQDs/PW₁₂/CdS for RhB degradation, a total organic carbon (TOC) analysis was performed. The TOC removal results are shown in Fig.S6. After 90 and 180 min of irradiation, the mineralization rates of RhB by 5%CQDs/PW₁₂/CdS were 1.49% and 13.55%, respectively.

  2.11   Degradation of different dyes and antibiotics

  To explore the broad applicability of the as-prepared 5%CQDs/PW₁₂/CdS photocatalyst, photocatalytic degradation tests were carried out on various common dyes and antibiotics. The corresponding results are presented in Fig. 11. Fig. 11a reveals that the 5%CQDs/PW₁₂/CdS composite achieved prominent degradation activity against RhB, crystal violet (CV), methyl orange (MO), and methylene blue (MB), with CV showing the highest degradation efficiency. This suggests that the material can effectively degrade dyes with distinct molecular structures. As can be seen from Fig. 11b, the composite also displayed superior performance in degrading tetracycline hydrochloride (TC), chlortetracycline hydrochloride (CTC), and metronidazole (MTZ). These results confirm its versatility in removing various aqueous contaminants. Overall, the 5%CQDs/PW₁₂/CdS composite exhibited excellent photocatalytic activity toward diverse water pollutants, showing promising potential for practical wastewater remediation.

  Figure 11

  

  Figure 11.  Degradation curves of different pollutants (a), different antibiotics (b), and RhB in different water qualities (c) by 5%CQDs/PW₁₂/CdS

  下载: 全尺寸图片 幻灯片

  To study the degradation effect of the complex in real water environments, appropriate volumes of water were collected from household sources (domestic water) and natural sources (rainwater and lake water), and RhB solutions were prepared to simulate the presence of dyes in various aquatic environments. Photocatalytic degradation of dyes in different water qualities was carried out using 5%CQDs/PW₁₂/CdS composite as catalysts, and the results are shown in Fig. 11c.

  As can be seen from Fig. 11c, compared with deionized water, the degradation efficiency of RhB in rainwater, lake water, and domestic water decreased successively. This phenomenon can be attributed to the adsorption of other ions existing in lake water and domestic water onto the surface of the catalyst. These ions consume some free radicals and reduce the separation ability of e^- and h^+[35], resulting in a decrease in the degradation performance of 5%CQDs/PW₁₂/CdS. Domestic water has the strongest interference due to its complex components, such as disinfection by-products and additives. The key point is that in rain and lake water, the catalyst still maintained an efficient degradation rate of over 80%, fully demonstrating its strong adaptability and practical potential in natural water environments, and providing a promising solution for the treatment of dye pollution in rivers and lakes. However, in other water sources, the composite maintained a catalytic efficiency of over 80%, indicating that the synthesized catalyst holds significant promise in real-world water environments. The good anti-interference property of composite materials may stem from the charge separation promoted by the heterojunction structure (Fig. 4d).

  Fig.S7 presents the results of a series of four consecutive photocatalytic experiments conducted under the same reaction conditions and simulated sunlight exposure. These experiments were aimed at further evaluating the stability of the optimized composite material. As shown in Fig.S7, the 5%CQDs/PW₁₂/CdS photocatalyst maintained 89.47% of its initial activity after four consecutive cycles, with a degradation efficiency that decreased by only approximately 3.5%. This indicated that the 5%CQD/PW₁₂/CdS composite has excellent reusability and stability. To investigate whether the homogeneous photocatalytic reaction during the reaction process of 5%CQDs/PW₁₂/CdS photocatalyst is caused by the Cd²⁺ generated by the photoinfiltration of the CdS component, this study used inductively coupled plasma atomic emission spectroscopy (ICP-OES) to test the solution after the photocatalytic cycle reaction. The detection and calculation results showed that the average concentration of Cd in the solution after the reaction was 3.96 mg·L^-1, and the leaching rate of Cd in the catalyst was calculated to be approximately 0.066%. These results indicate that no obvious metal ion leaching phenomenon occurs in the photocatalytic reaction of this catalyst, demonstrating good stability. As shown in Table S2, the 5%CQDs/PW₁₂/CdS composite material exhibited excellent degradation performance under simulated sunlight.

  2.12   Photocatalytic hydrogen production performance

  To investigate the hydrogen production capacity of the prepared materials through photolysis, an 8 h hydrogen production experiment was conducted on the 5%CQDs/PW₁₂/CdS composite material using different sacrificial agents under the irradiation of a 300 W Xe lamp.

  When using lactic acid as the sacrificial agent, the hydrogen production effect of the 5%CQDs/PW₁₂/CdS composite was the best, and lactic acid was determined to be the most suitable sacrificial agent. The results of the photocatalytic hydrogen production of different samples using lactic acid as the sacrificial agent are shown in Fig. 12a. It was found that the hydrogen production rate of 5%CQDs/PW₁₂/CdS was approximately three times that of 20%PW₁₂/CdS and six times that of the monomer CdS, which further indicates that 5%CQDs/PW₁₂/CdS has a high photocatalytic hydrogen production activity. In addition, to study the cycling stability of the 5%CQDs/PW₁₂/CdS composite in the photocatalytic hydrogen production process using lactic acid as the sacrificial agent, the experimental results are shown in Fig. 12b. Although the hydrogen production rate of 5%CQDs/PW₁₂/CdS slightly decreased, it still maintained a high hydrogen production volume. After four cycles, it was further confirmed that the composite had good hydrogen evolution performance and stability. In contrast to the photocatalytic degradation reactions, photocatalytic hydrogen evolution is more sensitive to the separation and migration efficiency of photogenerated electrons. Therefore, the 5%CQDs/PW₁₂/CdS composite exhibited superior hydrogen production performance compared with the 20%PW₁₂/CdS composite. As shown in Table S3, in the absence of precious metals, the 5%CQDs/PW₁₂/CdS composite material also exhibited hydrogen production performance.

  Figure 12

  

  Figure 12.  Hydrogen production under simulated sunlight for CdS, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS (a); Measurement results of the photocatalytic hydrogen production cycle of 5%CQDs/PW₁₂/CdS (b)

  下载: 全尺寸图片 幻灯片

  2.13   Capture experiment

  To explore the potential photocatalytic mechanism of the 5%CQDs/PW₁₂/CdS composite material, a capture experiment was conducted to capture the active substances existing during the degradation process of the 5%CQDs/PW₁₂/CdS composite material. Different scavengers, namely para-phenylenediamine (BQ), isopropanol (IPA), and disodium ethylenediaminetetraacetate (EDTA), were selected to perform the capture experiments for oxygen ion (·O₂^-), hydroxyl radical(·OH), and h⁺, respectively. The results are shown in Fig. 13. From Fig. 13, it could be seen that after adding BQ, the degradation effect of RhB by the 5%CQDs/PW₁₂/CdS composite material was significantly reduced, with the degradation rate dropping to 21.87%. After adding EDTA, the degradation rate dropped to 70.42%. After adding IPA, the degradation rate was 80.55%, and no significant inhibition was observed. The research results indicate that in the photocatalytic degradation process of RhB by the 5%CQDs/PW₁₂/CdS composite material, ·O₂^- and ·OH play the main role, while h⁺ plays the auxiliary role.

  Figure 13

  

  Figure 13.  Photocatalytic activities of 5%CQDs/PW₁₂/CdS for RhB degradation with different scavengers

  下载: 全尺寸图片 幻灯片

  Based on the above experimental results, a possible traditional type Ⅱ photocatalytic reaction mechanism for the CQDs/PW₁₂/CdS system was proposed, as shown in Fig. 14a. When the CQDs/PW₁₂/CdS composite is illuminated, electrons are excited from the VB to the CB of the semiconductor. Photogenerated electrons accumulate on the CB, while an equal number of holes accumulate in the VB. Due to the close contact between the components, the CB of CdS is more negative than that of PW₁₂. Therefore, the photogenerated electrons can be effectively transferred to the CB of PW₁₂. The photogenerated electrons on the semiconductor CB react with dissolved oxygen on the catalyst surface to form strong oxidizing ·O₂^-, which converts RhB into CO₂ and H₂O. In this case, the VB potential of CdS (1.77 V) does not reach the standard oxidation potential of OH⁻/·OH (2.40 V). Obviously, in the assumed type Ⅱ heterojunction system, ·OH can not be generated. Considering the presence of ·OH radicals in the CQDs/PW₁₂/CdS photocatalytic system, the type Ⅱ charge transfer mechanism is not applicable.

  Figure 14

  

  Figure 14.  Possible mechanism of photocatalytic reaction of the CQDs/PW₁₂/CdS composite: the traditional Ⅱ-type heterojunction electron transfer pathway (a) and the Z-scheme heterojunction electron transfer pathway (b)

  下载: 全尺寸图片 幻灯片

  We are more inclined to believe that the photocatalytic degradation of the CQDs/PW₁₂/CdS composite follows the Z-scheme electron transfer mechanism, as shown in Fig. 14b. This mechanism not only closely matches the results of the free radical capture experiment but also can reasonably explain its excellent photocatalytic performance. Firstly, under simulated sunlight irradiation, CdS and PW₁₂ can be fully excited, generating abundant photogenerated e^--h⁺ pairs. Subsequently, the photogenerated carriers undergo efficient Z-scheme transfer at the composite interface: the photogenerated electrons on the CB of PW₁₂ and the photogenerated holes on the VB of CdS undergo efficient recombination through CQDs as the electron mediator. CQDs act as an electron transmission medium, accelerating the electron transfer rate. Through this selective interface recombination, the highly reducing electrons are retained on the CB of CdS, while the highly oxidizing holes are gathered on the VB of PW₁₂, achieving the spatial separation of photogenerated carriers and significantly improving the charge separation efficiency. Moreover, PW₁₂ not only serves as a key component of the Z-scheme junction but also acts as an "electron buffer" via its exceptional multi-electron redox capability, facilitating electron capture and storage while suppressing CdS photocorrosion.

  The electrons enriched in the CB of CdS [-0.63 V (vs NHE)] have strong reducing properties and can convert O₂ into ·O₂^-, for which the standard electrode potential was -0.33 V (vs NHE). Meanwhile, the holes enriched in the VB of PW₁₂ possess strong oxidizing properties and can directly oxidize pollutants or react with H₂O/OH^- to generate ·OH. These active species ·O₂^-, ·OH, and h⁺ work together to drive the efficient degradation of RhB. In this study, a direct Z-scheme heterojunction bridged by CQDs is successfully constructed. By ingeniously integrating the electron buffering and storage capabilities of CQDs and PW₁₂, efficient separation and utilization of photogenerated carriers are achieved.

  3.   Conclusions

  In this work, a broad-spectrum responsive 5%CQDs/PW₁₂/CdS composite modified with CQDs was fabricated via a microwave-assisted hydrothermal method. Under simulated solar irradiation, the composite degraded 93.01% of RhB, which was 1.27 times higher than that of pure CdS and twice that of PW₁₂. Moreover, its photocatalytic H₂ evolution rate reached 12.34 mmol·g^-1 over 8 h, which was six times higher than that of pristine CdS. The improved photocatalytic activity is attributed to the introduction of CQDs, which increase the specific surface area and broaden the light absorption range of the composite. Radical trapping experiments identify ·O₂^- and h⁺ as the dominant active species during the photocatalytic reaction. Based on the band structure analysis and radical trapping results, a Z-scheme charge transfer pathway was proposed for the CQDs/PW₁₂/CdS heterojunction, offering a simple and economical route to efficient CdS-based photocatalysts. This work not only constructs a high-efficiency Z-scheme heterojunction photocatalytic system but also provides a novel design strategy and experimental reference for the precise regulation of photogenerated carrier dynamics. Furthermore, this study offers a feasible route for the development of advanced solar energy conversion materials via the synergistic coupling of POMs and CQDs.

  
  Acknowledgements: This study is supported by the Heilongjiang Provincial Natural Science Foundation of China (Grant No.LH2021B031), the Fundamental Research Funds in Heilongjiang Provincial Universities of China (Grant No.135509104), and the College Students′ Innovative Entrepreneurial Training Program Funded Projects of Qiqihar University (Grant No.X202510232038). Supporting information is available at http://www.wjhxxb.cn
  
• 1. [1]

     QUSTI A H, MALKHASIAN A Y S, SALAM M A. Enhancement of CdS nanoparticles photocatalytic activity by Pt and In₂O₃ doping for the degradation of malachite green dye in water[J]. J. Mol. Liq., 2018, 255: 364-369 doi: 10.1016/j.molliq.2018.01.127

  2. [2]

     ZHAO T W, XIONG J Y, LI W, CHENG G, DOU S X. Dual-cocatalysts-embedded hollow photocatalysis system drives solar fuel production[J]. Chem. Eng. J., 2025, 506: 160313 doi: 10.1016/j.cej.2025.160313

  3. [3]

     DUAN X Y, YU F C, JIANG R B, REN J L, ZHANG J L, FENG C C, LI C X, HU K C, HOU X G. Study on the photocatalytic properties of the ternary ZnO/MgAl-LDH/FeOOH composite photocatalyst with a type-Ⅱ and S-scheme linked carrier migration mechanism in degrading TC solution[J]. Compos. Commun., 2024, 52: 102156 doi: 10.1016/j.coco.2024.102156

  4. [4]

     XU Y, LIU J X, CAO H Q, ZHANG Q L. CdS nanocrystal/NiO nanosheet heterojunction for enhanced visible-light photocatalytic H₂ production[J]. Compos. Commun., 2025, 55: 102323 doi: 10.1016/j.coco.2025.102323

  5. [5]

     ZHAI B Y, CHEN Y, LIANG Y N, GAO Y H, SHI J L, ZHANG H L, LI Y. Modifying Ag₃VO₄ with metal-organic frameworks for enhanced photocatalytic activity under visible light[J]. Mater. Chem. Phys., 2020, 239: 122078 doi: 10.1016/j.matchemphys.2019.122078

  6. [6]

     LIU L Z, HU T P, DAI K, ZHANG J F, LIANG C H. A novel step-scheme BiVO₄/Ag₃VO₄ photocatalyst for enhanced photocatalytic degradation activity under visible light irradiation[J]. Chin. J. Catal., 2021, 42(1): 46-55 doi: 10.1016/S1872-2067(20)63560-4

  7. [7]

     TANG Y, TAI R, SONG X J, GAO S, WU R J, CHEN S W, WANG Q. Zinc-doped BiOBr hollow microspheres for enhanced visible light photocatalytic degradation of antibiotic residues[J]. Langmuir, 2024, 40(12): 6515-6523 doi: 10.1021/acs.langmuir.4c00155

  8. [8]

     NISHIOKA S, SHIBATA K, MISEKI Y, SAYAMA K, MAEDA K. Visible-light-driven nonsacrificial hydrogen evolution by modified carbon nitride photocatalysts[J]. Chin. J. Catal., 2022, 43(9): 2316-2320 doi: 10.1016/S1872-2067(21)64015-9

  9. [9]

     LIU Y M, ZENG Q R, WEI Y Z, GAO R C, ZENG D Q. Boosting photocatalytic H₂ generation via interfacial charge transfer in CuInS₂ nanoflower-decorated CdS nanosheets[J]. Int. J. Hydrogen Energy, 2025, 138: 286-295 doi: 10.1016/j.ijhydene.2025.05.180

  10. [10]

      WEN X R, FENG J P, ZHANG J, FU H Q, GAO H J, WANG J H, LIAO Y W. Enhanced visible‐light photocatalytic dye degradation ability of CdS/O‐CNTs nanocomposites[J]. ChemistrySelect, 2022, 7(35): e202202576 doi: 10.1002/slct.202202576

  11. [11]

      CHEN H Y, YE K, ZHANG W Y, FAN L L, KONG F J, ZHAO R F, CHEN M. Nickel-based bifunctional cocatalyst to enhance CdS photocatalytic hydrogen production[J]. Langmuir, 2023, 39(51): 18935-18945 doi: 10.1021/acs.langmuir.3c02855

  12. [12]

      MARTIN-SABI M, SORIANO-LÓPEZ J, WINTER R S, CHEN J J, VILÀ-NADAL L, LONG D L, GALÁN-MASCARÓS J R, CRONIN L. Redox tuning the Weakley-type polyoxometalate archetype for the oxygen evolution reaction[J]. Nat. Catal., 2018, 1(3): 208-213 doi: 10.1038/s41929-018-0037-1

  13. [13]

      SUN H L, HU L F, LI Z Y, LANG J Y, WANG C L, LIU X H, HU Y H, JIN F M. Ultra‐stable molecular interface SiW₁₂O_x/TiO₂ catalyst derived from keggin‐type polyoxometalates for photocatalytic conversion of methane to oxygenates[J]. ChemCatChem, 2022, 14(10): e202200001 doi: 10.1002/cctc.202200001

  14. [14]

      HOU Y S, CUI L P, WANG Q, WANG M L, LI G, YU K. The ternary composite formed by intercalation of porous Keggin-type POM and lamellar MoS₂ into accordion-like MXene for enhanced electrochemical properties of the LIBs[J]. Chem. Eng. J., 2025, 507: 160503 doi: 10.1016/j.cej.2025.160503

  15. [15]

      LIU S D, WANG Q G, WANG K, YAO Y P, ZHANG H Y, REN T Q, YIN Z M, DU F L, ZHANG B T, HE J L. Two-photon saturable absorption properties and laser Q-switch application of carbon quantum dots[J]. Opt. Lett., 2017, 42(19): 3972-3975 doi: 10.1364/OL.42.003972

  16. [16]

      MISHRA S, PRIYADARSHINEE M, DEBNATH A K, MUTHE K P, MALLICK B C, DAS N, PARHI P. Rapid microwave assisted hydrothermal synthesis cerium vanadate nanoparticle and its photocatalytic and antibacterial studies[J]. J. Phys. Chem. Solids, 2020, 137: 109211 doi: 10.1016/j.jpcs.2019.109211

  17. [17]

      YAN M, HUA Y Q, ZHU F F, SUN L, GU W, SHI W D. Constructing nitrogen doped graphene quantum dots-ZnNb₂O₆/g-C₃N₄ catalysts for hydrogen production under visible light[J]. Appl. Catal. B‒Environ., 2017, 206: 531-537 doi: 10.1016/j.apcatb.2017.01.070

  18. [18]

      MOHAMMED H A, AZIZ D M, AZIZ S B, NOFAL M M, HAMASALIH G H, HUSSEIN I I, KAYANI K F, MOHAMMED P A, SANGARAJU S. Eco-friendly fabrication of flag blue flower-derived CQD-doped chitosan films with tunable optical and photoluminescent properties[J]. Sci. Rep., 2025, 16: 3030 doi: 10.1038/s41598-025-33019-1

  19. [19]

      SHI Z, LIU J N, LAN H X, LI X Y, ZHU B Y, YANG J H. Effect of CdS shell thickness on the photocatalytic properties of TiO₂@CdS core-shell nanorod arrays[J]. J. Mater. Sci‒Mater. Electron., 2019, 30(19): 17682-17692 doi: 10.1007/s10854-019-02118-x

  20. [20]

      LIU Y T, LI L, HU T Y, ZHU X Y, WANG H X, ZHANG W J, CHEN S T, CHENG S T, LI Y X. Enhanced photocatalytic performance of multifunctional composite Ag-Ag₂Se@CdSe/3DOM TiO₂ with dual Z-scheme heterostructures coupling Ag NPs: Degradation, hydrogen production and antibacterial activity[J]. J. Alloy. Compd., 2025, 1021: 179571 doi: 10.1016/j.jallcom.2025.179571

  21. [21]

      LOEZA-POOT M, MIS-FERNÁNDEZ R, CAMACHO-ESPINOSA E, PEÑA J L. Physico-chemical analysis of the properties and stability of CdS and CdS: O to be applied in thin-film solar cells[J]. Appl. Surf. Sci., 2024, 662: 160125 doi: 10.1016/j.apsusc.2024.160125

  22. [22]

      LI R N, CHEN R, TANG L X, LI Q, CHEN Y X X, LIAO J S, WANG W. Constructing a P₂W₁₈O₆₂^6--containing hybrid photocatalyst via noncovalent interactions for enhanced H₂ production[J]. ACS Omega, 2024, 9(16): 18556-18565 doi: 10.1021/acsomega.4c01179

  23. [23]

      LIU H M, PENG J Y, ZHANG X, ZHENG K, ZHENG L R, LV K L, LI Q, ZHOU P. Engineering atomic Pt-N₃ sites on CdS nanorods for overcoming the rate-determining organic dehydrogenation in photocatalytic coproduction of H₂ and value-added chemicals[J]. Chem. Eng. J., 2025, 504: 158618 doi: 10.1016/j.cej.2024.158618

  24. [24]

      YOU J H, WANG L, BAO W T, YAN A G, GUO R. Synthesis and visible-light photocatalytic properties of BiOBr/CdS nanomaterials[J]. J. Mater. Sci., 2021, 56(11): 6732-6744 doi: 10.1007/s10853-020-05721-0

  25. [25]

      DING Y, GAO Y H, LI Z H. Carbon quantum dots (CQDs) and Co(dmgH)₂PyCl synergistically promote photocatalytic hydrogen evolution over hexagonal ZnIn₂S₄[J]. Appl. Surf. Sci., 2018, 462: 255-262 doi: 10.1016/j.apsusc.2018.08.006

  26. [26]

      WANG B Q, DENG Z R, FU X Z, LI Z H. MoS₂/CQDs obtained by photoreduction for assembly of a ternary MoS₂/CQDs/ZnIn₂S₄ nanocomposite for efficient photocatalytic hydrogen evolution under visible light[J]. J. Mater. Chem. A, 2018, 6: 19735-19742 doi: 10.1039/C8TA07797E

  27. [27]

      JIANG H Q, LI X H, ZHENG W B, XU H S, HOU S, ZHENG L X, ZHENG H J, MAO L, SHI X W. Atomically dispersed Pt on CdS nanosheets for photocatalytic evolution of H₂ and 1, 1-diethoxyethane from ethanol[J]. ACS Appl. Nano Mater., 2023, 6(18): 17161-17170 doi: 10.1021/acsanm.3c03418

  28. [28]

      CHENG M X, LI H L, WU Z Y, YU Z X, TAO X M, HUANG L. Synergistic effects of CQDs and oxygen vacancies on CeO₂ photocatalyst for efficient photocatalytic nitrogen fixation[J]. Sep. Purif. Technol., 2025, 354: 129299 doi: 10.1016/j.seppur.2024.129299

  29. [29]

      DOU F Y, NISHIWAKI E, LARSON H, HOMER M K, ZION T, NGUYEN H A, COSSAIRT B M. Pathways of CdS quantum dot degradation during photocatalysis: Implications for enhancing stability and efficiency for organic synthesis[J]. ACS Appl. Nano Mater., 2024, 7(13): 15781-15785 doi: 10.1021/acsanm.4c02976

  30. [30]

      KHAN A, LI W J, MA X H, DONG M, LIU X T, REN C J. Novel binary photocatalysts containing polyoxometalate immobilized on CdS nanorods surface for enhance photocatalytic performance[J]. J. Photochem. Photobiol. A‒Chem., 2023, 443: 114854

  31. [31]

      HUANG J R, WANG M Y, ZHANG X Z, TAO J N, LU L, QIAO G J, LIU G W. Anchoring of 2D CdS on Nb₂CT_X MXene nanosheets for boosting photocatalytic H₂ evolution[J]. J. Alloy. Compd., 2022, 923: 166256 doi: 10.1016/j.jallcom.2022.166256

  32. [32]

      TONG H J, SHI B F, ZHAO S L. Facile synthesis of a direct Z-scheme BiOCl-phosphotungstic acid heterojunction for the improved photodegradation of tetracycline[J]. RSC Adv., 2020, 10(29): 17369-17376 doi: 10.1039/D0RA02396E

  33. [33]

      ZHANG K F, LIU Y X, DENG J G, XIE S H, LIN H X, ZHAO X T, YANG J, HAN Z, DAI H X. Fe₂O₃/3DOM BiVO₄: High-performance photocatalysts for the visible light-driven degradation of 4-nitrophenol[J]. Appl. Catal. B‒Environ., 2017, 202: 569-579 doi: 10.1016/j.apcatb.2016.09.069

  34. [34]

      WU Q, LU D Z, KONDAMAREDDY K K, HO W K, WANG Q P, ZHANG Y H, ZENG Y M, ZHANG B Y, XIE L H, ZHAO B, WANG Z N, HAO H J, FAN H Q, WANG H M. In-situ synthesis of ternary heterojunctions via g-C₃N₄ coupling with noble-metal-free NiS and CdS with efficient visible-light-induced photocatalytic H₂ evolution and mechanism insight[J]. Int. J. Hydrog. Energy, 2022, 47(30): 14063-14076 doi: 10.1016/j.ijhydene.2022.02.153

  35. [35]

      CUI Y L, ZHENG J, ZHU Z J, HU C Y, LIU B J. Preparation and application of Bi₄O₇/Cu-BiOCl heterojunction photocatalyst for photocatalytic degradation of tetracycline under visible light[J]. J. Mol. Struct., 2023, 1274: 134486 doi: 10.1016/j.molstruc.2022.134486

• 

  Figure 1  XRD patterns (a) and FTIR spectra (b) of different samples

  下载: 全尺寸图片 幻灯片
  

  Figure 2  XPS spectra of 5%CQDs/PW₁₂/CdS: survey (a), Cd3d (b), W4f (c), S2p (d), and C1s (e)

  下载: 全尺寸图片 幻灯片
  

  Figure 3  SEM images of CdS (a, b), PW₁₂ (c, d), and 20%PW₁₂/CdS (e, f); SEM images (g, h) and corresponding EDS elemental mappings (i-o) of 5%CQDs/PW₁₂/CdS

  下载: 全尺寸图片 幻灯片
  

  Figure 4  TEM (a) and HRTEM (b) images of 5%CQDs/PW₁₂/CdS; FFT images of CQDs (c) and CdS (d)

  下载: 全尺寸图片 幻灯片
  

  Figure 5  N₂ adsorption-desorption isotherms and pore-size distributions (inset) of CdS (a), 20%PW₁₂/CdS (b), and 5%CQDs/PW₁₂/CdS (c)

  下载: 全尺寸图片 幻灯片
  

  Figure 6  UV-Vis DRS (a) and Kubelka-Munk curves (b) of different samples

  下载: 全尺寸图片 幻灯片
  

  Figure 7  PL spectra of CdS, PW₁₂, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS

  下载: 全尺寸图片 幻灯片
  

  Figure 8  Transient photocurrent response curves (a) and Nyquist plots (b) of CdS, PW₁₂, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS

  下载: 全尺寸图片 幻灯片
  

  Figure 9  Mott-Schottky diagrams of CdS (a) and PW₁₂ (b)

  下载: 全尺寸图片 幻灯片
  

  Figure 10  Photodegradation curves (a, b) and the corresponding kinetic curves (c) for different catalysts

  下载: 全尺寸图片 幻灯片
  

  Figure 11  Degradation curves of different pollutants (a), different antibiotics (b), and RhB in different water qualities (c) by 5%CQDs/PW₁₂/CdS

  下载: 全尺寸图片 幻灯片
  

  Figure 12  Hydrogen production under simulated sunlight for CdS, 20%PW₁₂/CdS, and 5%CQDs/PW₁₂/CdS (a); Measurement results of the photocatalytic hydrogen production cycle of 5%CQDs/PW₁₂/CdS (b)

  下载: 全尺寸图片 幻灯片
  

  Figure 13  Photocatalytic activities of 5%CQDs/PW₁₂/CdS for RhB degradation with different scavengers

  下载: 全尺寸图片 幻灯片
  

  Figure 14  Possible mechanism of photocatalytic reaction of the CQDs/PW₁₂/CdS composite: the traditional Ⅱ-type heterojunction electron transfer pathway (a) and the Z-scheme heterojunction electron transfer pathway (b)

  下载: 全尺寸图片 幻灯片

  Table 1.  Grain size, specific surface area, pore volume, and average pore diameter of different samples

  Sample	D / nm	SBET / (m2·g-1)	VP / (cm3·g-1)	DP / nm
  CdS	29.99	13	0.12	37.73
  20%PW12/CdS	41.12	11	0.08	27.88
  5%CQDs/PW12/CdS	33.36	17	0.08	18.86

  下载: 导出CSV

  Table 2.  E_CB, E_g, and E_VB values of CdS and PW₁₂

  Sample	ECB / eV	Eg / eV	EVB / eV
  CdS	-0.63	2.40	1.77
  PW12	-0.55	3.07	2.52

  下载: 导出CSV
•