The morphology of Bi2WO6 (Fig. 1a) was assembled by numerous nanosheets, which intercrossed each other and aggregated together, forming the flower-like microstructure with a diameter of 3–4 µm. As seen from Fig. 1b, CSs and Bi2WO6 were attached to each other. However, CSs were unevenly distributed on the surface of Bi2WO6 and the ends were more likely to be covered [29]. The main reason for this phenomenon was probably owing to the stacking of Bi2WO6 sheets in flower shape and the uneven distribution of the stress on the surface. The structure and morphology of the as-prepared CSs are displayed in Fig. 1c, from which we can see clearly that the shape of CSs was uniform and well-dispersed spheres with more regular structures and smooth surfaces, and the average diameters of the CSs was about 450 nm. The surface of CSs in Fig. 1b changed from smooth to rough, which may be affected by the growth of Bi2WO6 [30]. It could not prevent the CSs from forming close contact with Bi2WO6 to form a ‘flower-spheres’ structure, and will even be more conducive to the surface adsorption of organic pollutants. However, if the content of CSs increases, it is easy to form large clusters. This will affect the interaction between CSs and Bi2WO6. The EDS results in Fig. 1d-1g demonstrate the distribution of C, O, W, and Bi elements in the CSs/Bi2WO6 composites, indicating the successful production of the composite catalysts.
In Fig. 1h, it can be seen that the 2% CSs-Bi2WO6 composite structure exhibits the ‘flower-spheres’ morphology. The layered structure of Bi2WO6 can be clearly seen in the picture as consisting of a large number of nanosheets. The CSs are attached to the nanosheets. Further, The lattice spacings measured in HRTEM image (Fig. 1i) were approximately 0.27 nm and 0.32 nm, corresponding to the (200) and (131) crystal faces of Bi2WO6 [31]. The isotherm curves of Bi2WO6 belonged to type IV with H3 hysteresis loops, corresponding to slit-shaped pores consistent with the sheet-like morphology (displayed in Fig. 3) [32]. In Fig. 1j, we can still find the (020) and (131) crystal faces of Bi2WO6 clearly in the 2% CSs-Bi2WO6 catalyst.
The XRD patterns showed the phase structure of the CSs-Bi2WO6 complexes in different proportions (Fig. 2a). The characteristic diffraction peaks appeared at 2θ = 28.30°, 32.79°, 47.15°, 55.83°, 58.55°, 75.96°, and 78.35°, which were well-matched with the crystal planes (113), (200), (220), (313), (226), (139), and (420) of Bi2WO6 standard card (PDF#26-1044), demonstrating the successful preparation of Bi2WO6 [33, 34]. There were no other reflections associated of carbon phases in any with the XRD patterns of the composites. This was somehow expected due to the low amount of carbon additive used in the preparation of the composites and their high dispersion in samples.
The chemical states of Bi2WO6 and 2% CSs-Bi2WO6 were investigated by XPS analysis, and the results were shown in Fig. 2b-2d. From Fig. 2b, the presence of elements such as C, O, Bi, and W was clearly observed. Figure 2c shows the XPS spectra of C 1 s peak for pure Bi2WO6. and 2% CSs-Bi2WO6. The C 1 s peaks at around 284.6 eV and 288.3 eV were attributed to the carbon signal from carbon in the instrument for calibration and the adsorbed CO2 on the surface respectively [35]. It was worth noting that there was a new peak at 286.2 eV for 2% CSs-Bi2WO6, which could be ascribed to the formation of C - O - C (the O atoms in Bi2WO6) between Bi2WO6 and CSs. The above result indicated that most of C in 2% CSs-Bi2WO6 was sp2 hybridized. The presence of oxygen-containing functional groups such as C = O, C - O and C - O - C in the 2% CSs-Bi2WO6 system suggests that the strong interaction between Bi2WO6 and carbon nanospheres is formed during the hydrothermal reaction.
O 1 s spectra for the samples of pure Bi2WO6 and 2% CSs-Bi2WO6 are shown in Fig. 2d. The asymmetric peak centered at 530 eV was decomposed into two components at the binding energy of 529.74 eV and 531.33 eV for pure Bi2WO6, which were due to the surface lattice oxygen and the adsorbed oxygen species respectively [36, 37]. With the addition of the CSs, the oxygen peak becomes complex. The binding energy shifts from 529.75 eV, 531.33 eV (Bi2WO6) to 531.08 eV, 527.68 eV (2% CSs-Bi2WO6). The decrease in binding energy indicated the presence of complex oxygen groups in the composite. The electron cloud density and electronegativity around O decrease due to the addition of carbon spheres.
The BET and the pore structure of the prepared samples are shown in Fig. 3. The isotherm curves belonged to type IV with H3 hysteresis loops [32], which corresponds to slit-like pores, consistent with the morphology of the flakes as shown in the SEM.
In Fig. 3a, specific surface area of pure Bi2WO6, 0.5% CSs-Bi2WO6, 1% CSs-Bi2WO6, 2% CSs-Bi2WO6, 4% CSs-Bi2WO6, and 8% CSs-Bi2WO6 was 20.76, 86.34, 87.57, 88.05 87.04 and 85.92 m2 g− 1, respectively. As mentioned earlier, the growth of Bi2WO6 affects the surface morphology of the CSs. On one hand, the addition of surface area was mainly caused by the surface folds of CSs, which will increase the contact area of CSs with contaminants. On the other hand, the pore size and total pore volume of the sample increased somewhat compared to that of pure Bi2WO6, and the value of 2% CSs-Bi2WO6 reached its maximum. This proves that adsorption is strongly related to the specific surface area of the composite catalyst. The pore size distribution is shown in Fig. 3b, which indicated that the major pore sizes were in the range from 2 to 10 nm, belonging to mesopore scope.
Photoluminescence (PL) is regarded as a powerful tool to study the efficiency of electron-hole pair separation. The PL spectra clearly showed the strong emission peaks of the samples at 425 nm under excitation at 378 nm. Figure 4a shows that the emission intensity of CSs-Bi2WO6 was lower than that of pure Bi2WO6. In particular, the 2% CSs-Bi2WO6 showed the lowest emission intensity. This trend suggested that the separation efficiency of photogenerated electrons and holes can be effectively improved by introducing CSs, which in turn promoted the enhancement of photocatalytic activity.
Figure 4. PL spectra for CSs-Bi2WO6 (a), Mott-Schottky curve of Bi2WO6 and 2% CSs-Bi2WO6 (b), Transient photocurrent response of CSs-Bi2WO6 (c), electrochemical impedance spectroscopy (EIS) measurements of Bi2WO6 and 2% CSs-Bi2WO6 (d).
In Fig. 4b the x-intercept of the linear region was used to calculate the flat-band potential, and the obtained results are converted into NHE potentials to further calculate the ECB for Bi2WO6 and 2% CSs-Bi2WO6 Schottky contacts [40–42]. The straight upward curves demonstrated that Bi2WO6 was a n-type semiconductor. The flat-band potentials of both the Bi2WO6 and 2% CSs-Bi2WO6 composites was − 0.46 V vs. Ag/AgCl (− 0.26 eV vs. NHE (normal hydrogen electrode)), respectively. The ECB for both the Bi2WO6 and 2% CSs-Bi2WO6 Schottky contacts was approximate − 0.36 eV. Combined with the band gap values obtained from Tauc plots, the valence band potentials (VB) of 2% CSs-Bi2WO6 samples was calculated to be 2.17 eV [43].
The photocurrent responses of the pure Bi2WO6 and 2% CSs-Bi2WO6 were tested and the results are displayed in Fig. 4c. It can be found that both of the photoelectrodes exhibit stable photocurrent responses over several on-off cycles. When the photoelectrodes were exposed to the light, photocurrents were generated immediately for all the samples. Obviously, Bi2WO6 had the lower photocurrent responses due to the higher recombination rate of photogenerated electrons and holes in the Bi2WO6 crystal. The photocurrent response of the composite photocatalyst 2% CSs-Bi2WO6 was about 9 times higher than pure Bi2WO6, which effectively indicated more efficient photo-induced charge separation and faster electron transport. The EIS Nyquist plots for Bi2WO6 and 2% CSs-Bi2WO6 under dark and light conditions are shown in Fig. 4d, respectively. The graph clearly showed that the arc radius of 2% CSs-Bi2WO6 was smaller than Bi2WO6, indicated that the introduction of CSs enhanced the charge migration of Bi2WO6 and reduces the reaction resistance at the semiconductor interface. Furthermore, the arc radiation of Bi2WO6 and 2% CSs-Bi2WO6 was smaller under light than under dark conditions, indicated that more carriers were generated due to the excitation of light.
In order to test the photocatalytic property, TC was chosen as the target pollutants. To study the TC degradation kinetics in the presence of CSs-Bi2WO6, the apparent reaction rate constant (-k) was calculated by using the first-order reaction model as shown by Eq. (3) [44]:
$${ln}(\frac{C}{{C}_{0}})=-kt (3)$$
where C0 and C are the concentration of TC at time 0 and t min respectively. The normalized reaction rate constant was calculated by normalizing k with the total mass of catalysts. The photodegradation result of TC is shown in Fig. 6. Under visible light, 50 mg photocatalyst was added to 100 mL 50 mg/L TC solution for photocatalytic degradation. The experimental results showed that 2% CSs-Bi2WO6 has the best photocatalytic degradation performance for TC.
Figure 5a is a diagram of the degradation rate of TC by the catalyst. The composite photocatalyst reached adsorption-desorption equilibrium within 40 minutes of the dark reaction (detailed data of adsorption in support information). The degradation rate of TC by 2% CSs-Bi2WO6 was 84.6%, which is nearly 25% higher than pure Bi2WO6. This was because 2% CSs-Bi2WO6 had a large specific surface area, especially the folded structure of CSs, which effectively increased the adsorption of TC by the catalyst and promoted the photoreaction. In addition, the carbon component can effectively enhance the absorption of visible light and form a strong interface electronic effect with the semiconductor. Compared with the work reported in the literature on degradation of organic pollutants by carbon materials in Table S2.
In Fig. 5b, the degradation kinetics had a linear relationship between ln (C0/Ct) and irradiation time, which shows that the catalyst follows the pseudo-first order kinetics. The rate constant in the equation was calculated from the slope k of the kinetic curve. The rate constants for pure Bi2WO6, 0.5% CSs-Bi2WO6, 1% CSs-Bi2WO6, 2% CSs-Bi2WO6, 4% CSs-Bi2WO6, and the 8% CSs-Bi2WO6 were 0.01502 min− 1, 0.00257 min− 1, 0.02141 min− 1, 0.03929 min− 1, 0.02951 min− 1 and 0.0703 min− 1, respectively. In particular, 2% CSs-Bi2WO6 had the highest photocatalytic degradation rate, which should be attributed to more efficient charge separation by the adding CSs. The stability and reproducibility of photocatalysts are of great research value in practical applications. Figure 5c shows the photocatalytic cycling test plots of the prepared catalyst samples. After the 5th cycle, the final degradation rate was about 80.5%, which displayed a better photocatalytic stability of 2% CSs-Bi2WO6.
The main active species of different photocatalysts may differ due to their different energy band structures. In the present study: dimethyl sulfoxide (DMSO), ammonium oxalate (AMO), isopropyl alcohol (IPA) and 1,4-benzoquinone (BQ) were used as electron (e−), hole (h+), hydroxyl radical (·OH−) and superoxide radical (·O2−) scavengers, respectively [45]. The photocatalytic degradation of TC was carried out with different scavengers under visible light irradiation and the results are shown in Fig. 5d. The presence of BQ and AMO significantly inhibited the photodegradation of TC, indicated that ·O2− and h+ were the main active species (results were consistent with ESR in supporting information). Photodegradation was also somewhat inhibited when DMSO was added into the system, indicating that the e− are more active in the composite photocatalyst. In addition, the addition of IPA had little effect on the photocatalytic performance. Therefore, the order of role of the active species in the 2% CSs-Bi2WO6 photocatalytic degradation TC was ‧O2− > h+ > e− > ‧OH−.
Based on the above results, a schematic representation of the photocatalytic degradation mechanism of 2% CSs-Bi2WO6 under visible light irradiation is given in Fig. 6. It was generally accepted that, after photo illumination, the generation of the electron and hole in CB and VB is expected, respectively. According to Fig. 5b, the CB of 2% CSs-Bi2WO6 was − 0.36 eV vs. NHE, which was more negative than the redox potential (E O2/‧O2− = − 0.33 eV vs. NHE). Thus, the photogenerated electrons were transferred to O2, which was transformed to produce ‧O2−. In addition, combined with the band gap energy (Eg) of 2% CSs-Bi2WO6, the VB potential of 2% CSs-Bi2WO6 can be calculated as + 2.17 eV vs. NHE according to the formula (CB = VB - E g) [46]. Specific data are described in the supporting information. The VB level of 2% CSs-Bi2WO6 was lower than E ‧OH/OH− = + 2.31 V vs. NHE. Therefore, ·OH− did not play a major role in the photocatalytic reaction, which is in agreement with Fig. 6.
Upon visible light irradiation of the semiconductor, the electron-hole pairs were generated. The photogenerated electrons reacted with O2 (from the air) on the photocatalyst surface to form the ‧O2−, which finally can be converted to ‧OH−. In addition, the photogenerated holes in the VB of the photocatalyst could also oxidize the TC [27, 47]. To sum up, the 2% CSs-Bi2WO6 photocatalyst exhibited a greater electron-hole separation efficiency, compared to the bare Bi2WO6 photocatalyst. It should be noted that the photodegradation mechanism of the TC was proposed as Equations (4–8):
$${Bi}_{2}{WO}_{6}+hv\to {Bi}_{2}{WO}_{6}+{e}^{-}+{h}^{+} \left(4\right)$$
$${e}^{-}+{O}_{2}\to {·O}_{2}^{-} \left(5\right)$$
$${·O}_{2}^{-}+{2H}_{2}O+{e}^{-}\to {4·OH}^{-} \left(6\right)$$
$${·O}_{2}^{-}+TC\to products \left(7\right)$$
$${h}^{+}+TC\to products \left(8\right)$$
During the dark reaction, TC was adsorbed on the surface of CSs due to irregular folds on the surface and strong adsorption. It facilitated the transfer of pollutants from the environment to the surface of the catalyst, forming a high concentration TC environment on the surface. And finally adsorption - dissolution equilibrium was achieved. In addition, CSs have a good electrical conductivity, which is conducive to the separation of photogenerated electrons and holes, and prolongs the life of carriers, thus improving the photocatalytic performance.