Highly Dispersion Cu2O QDs Decorated Bi2WO6 S-Scheme Heterojunction for Enhanced Photocatalytic Water Oxidation

Developing suitable photocatalysts for the oxygen evolution reaction (OER) is still a challenging issue for efficient water splitting due to the high requirements to create a significant impact on water splitting reaction kinetics. Herein, n-type Bi2WO6 with flower-like hierarchical structure and p-type Cu2O quantum dots (QDs) are coupled together to construct an efficient S-scheme heterojunction, which could enhance the migration efficiency of photogenerated charge carriers. The electrochemical properties are investigated to explore the transportation features and donor density of charge carriers in the S-scheme heterojunction system. Meanwhile, the as-prepared S-scheme heterojunction presents improved photocatalytic activity towards water oxidation in comparison with the sole Bi2WO6 and Cu2O QDs systems under simulated solar light irradiation. Moreover, the initial O2 evolution rate of the Cu2O QDs/Bi2WO6 heterojunction system is 2.3 and 9.7 fold that of sole Bi2WO6 and Cu2O QDs systems, respectively.


Introduction
Sunlight provides an abundant renewable energy source to overcome the energy crisis that humans face in the future. Among all the strategies, solar energy conversion from sunlight into chemical energy has shown up as a sustainable and efficient route utilizing semiconductor photocatalysts [1,2]. As we know, water oxidation to dioxygen is a multielectron transfer reaction in a photocatalytic water splitting process, which is a critical step and involves the difficult breaking of the O-H bond as well as the formation of an O-O bond [3,4]. Continuous efforts have been dedicated to the development of efficient water oxidation catalysts (WOCs), consisting of desirable semiconductor photocatalysts and cocatalysts with proper band structure and electrophilic ability, which could improve the light absorption capability and charge transportation with overall promoted photocatalytic performance for water oxidation [5,6].
Among various semiconductor photocatalysts, ternary metal oxide, n-type Bi 2 WO 6 , as one of the simplest members of the Aurivillius family, is comprised of accumulated layers of perovskite-like [WO 4 ] 2− octahedral sheets and [Bi 2 O 2 ] 2+ sheets [7][8][9]. Density functional theory (DFT) calculations show that the conduction band (CB) of Bi 2 WO 6 is comprised of W 5d orbitals; the valence band (VB) mainly originates from hybridizing O 2p with Bi 6s orbitals, which not only enables the VB to be highly dispersed, but also facilitates the migration of photogenerated holes for specific oxidation reactions. In addition, the band gap of Bi 2 WO 6 is about 2.8 eV, and the valence band edge is at +2.95 V vs. NHE (normal hydrogen electrode), which is high enough to trigger the water oxidation reaction for oxygen production. These unique properties reveal that Bi 2 WO 6 can be utilized as a visible-light-driven photocatalyst for organic synthesis, CO 2 reduction,

Synthesis of Cu 2 O QDs/Bi 2 WO 6 Heterojunction
Firstly, 0.025 g of hexadecyl trimethyl ammonium bromide (CTAB) was dissolved into 20 mL of purified water to form transparent solution. Then, 0.1 g of the as-prepared Bi 2 WO 6 sample was added into the above CTAB solution with stirring for 30 min. Meanwhile, 0.008 g of copper acetate (Cu(Ac) 2 ) and 0.016 g of ethylenediaminetetraacetic acid disodium (EDTA-Na) were dissolved into 5 mL of purified water. Subsequently, the Cu solution was mixed with the Bi 2 WO 6 solution. Then, 10 mL of NaOH solution (0.05 M) was added dropwise into the mixed solution with stirring for 30 min. Afterwards, 10 mL of ascorbic acid (AA) solution (0.33 g) was dropped into the above solution with vigorous stirring for 1 h. The generated Cu 2 O/Bi 2 WO 6 was washed with absolute ethanol and distilled water several times to remove the surfactant, and dried overnight in a vacuum oven. The final products were named 1.5 wt% Cu 2 O/Bi 2 WO 6 , 3wt% Cu 2 O/Bi 2 WO 6 , and 6 wt% Cu 2 O/Bi 2 WO 6 , where the 1.5, 3 and 6 wt% were the mass ratios of Cu 2 O to Bi 2 WO 6 in the mixed solution according to the theoretical stoichiometric ratio of added copper and bismuth elements. For comparison, a control sample was prepared without the addition of Bi 2 WO 6 and labeled as Cu 2 O.

Characterizations
X-ray diffraction (XRD) patterns of the prepared heterojunctions were performed using a Bruker D8 diffractometer (Billerica, MA, USA). The morphology and microstructure of the obtained catalysts were observed using a JSM5510LV (Tokyo, Japan) field emission scanning electron microscopy (SEM) and a JEOL 2100 (Tokyo, Japan) transmission electron microscopy (TEM). Raman spectra were recorded on an ISA dispersive Raman spectroscopy at 514 nm. Fourier transform infrared spectra (FTIR) were determined using a Bruker spectrometer (Billerica, MA, USA) with an ATR correction mode. X-ray photoelectron spectroscopy (XPS) was examined by a Thermo Escalab 250 instrument (Waltham, MA, USA) with Al-Kα radiation to determine the surface chemical species. UV-vis absorption spectra were conducted by a Cary 4000 UV-vis spectrometer (Waltham, MA, USA). Electron paramagnetic resonance (EPR) analyses were carried out using a Bruker EMS-plus instrument (Billerica, MA, USA) to detect the free radicals by using 5,5-dimethyl-1-pyrroline (DMPO) as a spin-trapping agent.

Photoelectrochemical Tests
Photoelectrochemical measurements were conducted using a CHI660E electrochemical workstation (Shanghai, China) with a three-electrode system in 0.05 M Na 2 SO 4 electrolyte (20 mL, pH = 6.8). A catalyst deposited fluorine-doped tin oxide (FTO) electrode was served as a photoanode, while a Pt wire and a saturated calomel electrode (SCE) were applied as the counter electrode and reference electrode, respectively. For the photoanode preparation, 40 mg of the prepared photocatalysts were added into 2 mL of ethanol with 40 µL Nafion solution (5 wt%) and mixed homogeneously using a vortex oscillator. After that, the resulting mixture was dip-coated onto the prewashed FTO glass to obtain a film electrode with a controlled electrode area of 1 cm 2 . The solar light source (I 0 = 100 mW cm −2 ) was simulated using a 200 W Xenon lamp coupled with an AM 1.5G filter. Electrochemical impedance spectroscopy (EIS) tests were measured at a scan frequency range of 0.1 to 100 kHz under a voltage amplitude of 10 mV and a potential bias of 0.298 V vs. SCE.

Photocatalytic Activities
The photocatalytic reactions were performed in a Teflon lining reactor under the simulated solar light. 0.05 g of samples were added into 200 mL of the solution with La 2 O 3 (0.2 g) and AgNO 3 (0.03 M). Before irradiation, the mixture was stirred for 30 min in the dark and then purged with N 2 to removal O 2 . The concentration of O 2 in the reactor was measured by using gas chromatograph (Tet, GC-2030,Tokyo, Japan) with a thermal conductivity at an interval of 30 min.

Results and Discussion
Figure 1a displays a possible formation procedure of Cu 2 O QDs/Bi 2 WO 6 heterojunction through a facile hydrothermal and deposition route. Firstly, when the cationic surfactant CTAB is introduced, the CTAB can be adsorbed and anchored at the surface of Bi 2 WO 6 MFs. The characteristic flower-like hierarchical Bi 2 WO 6 with high SSA provides a structural framework for the uniform growth of nanoparticles on the sheets slowly with directed high-density. On the other hand, the EDTA and Cu(Ac) 2 are mixed with the purified water to form a blue Cu complex. Subsequently, the mixture is added dropwise into the Bi 2 WO 6 /CTAB solution. As a result, the Cu complex is deposited on the surface of flower-like hierarchical Bi 2 WO 6 . With the addition of NaOH, Cu(II) ions from the Cu complex are slowly released to generate Cu(OH) 2 nanoparticles. As expected, the negatively charged nanoparticles could be attracted and grafted by the positive CTAB to restrain the agglomeration effect. When the weak reductive AA is added, the formed Cu(OH) 2 nanoparticles can be reduced to Cu 2 O QDs on the surface of Bi 2 WO 6 MFs, which further maintains the stability of the nanosized Cu 2 O QDs without apparent aggregation. In Figure 1b, the XRD patterns of Bi 2 WO 6 with different contents of Cu 2 O QDs are present. As displayed, the XRD pattern of the as-prepared Bi 2 WO 6 is in good agreement with the standard diffraction pattern of orthorhombic Bi 2 WO 6 (JCPDS No. 73-2020) [29], where the obvious peaks at 28 dark and then purged with N2 to removal O2. The concentration of O2 in the reactor was measured by using gas chromatograph (Tet, GC-2030,Tokyo, Japan) with a thermal conductivity at an interval of 30 min.

Results and Discussion
Figure 1a displays a possible formation procedure of Cu2O QDs/Bi2WO6 heterojunction through a facile hydrothermal and deposition route. Firstly, when the cationic surfactant CTAB is introduced, the CTAB can be adsorbed and anchored at the surface of Bi2WO6 MFs. The characteristic flower-like hierarchical Bi2WO6 with high SSA provides a structural framework for the uniform growth of nanoparticles on the sheets slowly with directed high-density. On the other hand, the EDTA and Cu(Ac)2 are mixed with the purified water to form a blue Cu complex. Subsequently, the mixture is added dropwise into the Bi2WO6/CTAB solution. As a result, the Cu complex is deposited on the surface of flower-like hierarchical Bi2WO6. With the addition of NaOH, Cu(II) ions from the Cu complex are slowly released to generate Cu(OH)2 nanoparticles. As expected, the negatively charged nanoparticles could be attracted and grafted by the positive CTAB to restrain the agglomeration effect. When the weak reductive AA is added, the formed Cu(OH)2 nanoparticles can be reduced to Cu2O QDs on the surface of Bi2WO6 MFs, which further maintains the stability of the nanosized Cu2O QDs without apparent aggregation. In Figure 1b, the XRD patterns of Bi2WO6 with different contents of Cu2O QDs are present. As displayed, the XRD pattern of the as-prepared Bi2WO6 is in good agreement with the standard diffraction pattern of orthorhombic Bi2WO6 (JCPDS No. 73-2020) [29], where the obvious peaks at 28.   thickness, as present in Figure 2c,d, inferring high porosity and huge surface area, which benefits the exposure of more active sites. SEM images of the bare Bi2WO6 MFs are displayed in Figure 2a,b, where the uniform flower-like hierarchical Bi2WO6 with 2-3 μm diameter are observed clearly. It is found that the hierarchical structure of Bi2WO6 is assembled by ultrathin sheets with 40 nm of thickness, as present in Figure 2c,d, inferring high porosity and huge surface area, which benefits the exposure of more active sites. After introducing the Cu2O QDs, as shown in Figure 3a,b, it is clearly observed that the size of the Bi2WO6 hierarchical flowers displays a negligible change, while the nanosheets comprised of the flowers are mechanically exfoliated and the surface of the flower-like hierarchical structure becomes smoother, which is possibly due to the vigorous stirring during the Cu2O QDs deposition process. Meanwhile, with the increasing of Cu initial amount, the Cu2O nanoparticles are observed and anchored at the surface of the hierarchical Bi2WO6 MFs. As displayed in Figure 3c, the 3 wt% Cu2O QDs are uniformly deposited on the surface of Bi2WO6 MFs, while once the amount of Cu(II) precursor reaches to 6 wt%, large Cu2O nanoparticles are detected in Figure 3d,e, which indicates that the excess Cu(II) precursor is harmful for the dispersion of Cu2O QDs and causes the aggregation.
TEM and HRTEM images of the Cu2O QDs/Bi2WO6 heterojunction are presented in Figure 4. The micro-size Bi2WO6 MFs with 2-3 μm diameter is observed, which is agreement with the results of SEM, as displayed in Figure 4a, where the large thickness of the sample hampers the penetration of electron beams, leading to the black area. In general, quantum dots are defined as semiconductor nanocrystals with particle sizes ranging from 1 to 20 nm, which possess unique electronic properties owing to the apparent quantum confinement effect. It can be clearly observed that the Cu2O nanoparticles with ~20 nm of diameter are uniformly dispersed at the surface of Bi2WO6 MFs in Figure 4b,c. Owing to the smaller size, the Cu2O QDs can easily anchor at the surface of micro-sized Bi2WO6 to form micro-heterojunctions, which shorten the charge-carrier transfer pathways through the intimately contacted interface. The clear lattice fringe of 0.307 nm ascribed to the (110) crystal facet of Cu2O is detected in Figure 4d. These results demonstrate the successful construction of heterojunctions between Bi2WO6 and Cu2O. After introducing the Cu 2 O QDs, as shown in Figure 3a,b, it is clearly observed that the size of the Bi 2 WO 6 hierarchical flowers displays a negligible change, while the nanosheets comprised of the flowers are mechanically exfoliated and the surface of the flower-like hierarchical structure becomes smoother, which is possibly due to the vigorous stirring during the Cu 2 O QDs deposition process. Meanwhile, with the increasing of Cu initial amount, the Cu 2 O nanoparticles are observed and anchored at the surface of the hierarchical Bi 2 WO 6 MFs. As displayed in Figure 3c, the 3 wt% Cu 2 O QDs are uniformly deposited on the surface of Bi 2 WO 6 MFs, while once the amount of Cu(II) precursor reaches to 6 wt%, large Cu 2 O nanoparticles are detected in Figure 3d,e, which indicates that the excess Cu(II) precursor is harmful for the dispersion of Cu 2 O QDs and causes the aggregation.
TEM and HRTEM images of the Cu 2 O QDs/Bi 2 WO 6 heterojunction are presented in Figure 4. The micro-size Bi 2 WO 6 MFs with 2-3 µm diameter is observed, which is agreement with the results of SEM, as displayed in Figure 4a, where the large thickness of the sample hampers the penetration of electron beams, leading to the black area. In general, quantum dots are defined as semiconductor nanocrystals with particle sizes ranging from 1 to 20 nm, which possess unique electronic properties owing to the apparent quantum confinement effect. It can be clearly observed that the Cu 2 O nanoparticles with~20 nm of diameter are uniformly dispersed at the surface of Bi 2 WO 6 MFs in Figure 4b,c. Owing to the smaller size, the Cu 2 O QDs can easily anchor at the surface of micro-sized Bi 2 WO 6 to form micro-heterojunctions, which shorten the charge-carrier transfer pathways through the intimately contacted interface. The clear lattice fringe of 0.307 nm ascribed to the (110) crystal facet of Cu 2 O is detected in Figure 4d. These results demonstrate the successful construction of heterojunctions between Bi 2 WO 6 and Cu 2 O. Nanomaterials 2022, 12, x FOR PEER REVIEW 6 of 16  FTIR spectra of Bi2WO6 MFs, Cu2O, and Cu2O/Bi2WO6 are displayed in Figure 5a. The peaks at 818 and 703 cm −1 are attributed to the symmetric and asymmetric vibration of W-O, respectively [30]. The peaks centered at 1599, 2924 and 2845 cm −1 are due to the stretching vibration of O-H and C-H, respectively, which could be because of the usage of organic surfactants (CTAB, EDTA) during the synthesis procedure of the heterojunction system [31]. Besides, the characteristic peak of Cu2O is not found in the samples of Cu2O/Bi2WO6. To further investigate the composition of samples, Raman spectroscopy of the samples was performed, as shown in the Figure 5b. The characteristic peaks at 796 and   FTIR spectra of Bi2WO6 MFs, Cu2O, and Cu2O/Bi2WO6 are displayed in Figure 5a. The peaks at 818 and 703 cm −1 are attributed to the symmetric and asymmetric vibration of W-O, respectively [30]. The peaks centered at 1599, 2924 and 2845 cm −1 are due to the stretching vibration of O-H and C-H, respectively, which could be because of the usage of organic surfactants (CTAB, EDTA) during the synthesis procedure of the heterojunction system [31]. Besides, the characteristic peak of Cu2O is not found in the samples of Cu2O/Bi2WO6. To further investigate the composition of samples, Raman spectroscopy of the samples was performed, as shown in the Figure 5b. The characteristic peaks at 796 and FTIR spectra of Bi 2 WO 6 MFs, Cu 2 O, and Cu 2 O/Bi 2 WO 6 are displayed in Figure 5a. The peaks at 818 and 703 cm −1 are attributed to the symmetric and asymmetric vibration of W-O, respectively [30]. The peaks centered at 1599, 2924 and 2845 cm −1 are due to the stretching vibration of O-H and C-H, respectively, which could be because of the usage of organic surfactants (CTAB, EDTA) during the synthesis procedure of the heterojunction system [31]. Besides, the characteristic peak of Cu 2 O is not found in the samples of Cu 2 O/Bi 2 WO 6 . To further investigate the composition of samples, Raman spectroscopy of the samples was performed, as shown in the Figure 5b. The characteristic peaks at 796 and 827 cm −1 can be ascribed to the antisymmetric and symmetric A g stretch modes of the O-W-O band, respectively [32,33]. The peak at 714 cm −1 is associated with the antisymmetric bridging mode of the tungstate chain. In addition, the obvious vibration peak at 308 cm −1 is assigned to translational modes involving simultaneous motions of WO 6 6− and Bi 3+ [34]. For the pure Cu 2 O, the intense peaks at low frequencies of 213 and 260 cm −1 originate from the stretching vibration of Cu 2 O, which is consistent with the previous reports [35,36]. In the case of Cu 2 O/Bi 2 WO 6 , the characteristic peak at 308 cm −1 shifted to 296 cm −1 , and the two peaks at 796 and 827 cm −1 became a broad peak at 809 cm −1 due to the cover of Cu 2 O on the surface of the Bi 2 WO 6 MFs.  [32,33]. The peak at 714 cm −1 is associated with the antisymmetric bridging mode of the tungstate chain. In addition, the obvious vibration peak at 308 cm −1 is assigned to translational modes involving simultaneous motions of WO6 6− and Bi 3+ [34].
For the pure Cu2O, the intense peaks at low frequencies of 213 and 260 cm −1 originate from the stretching vibration of Cu2O, which is consistent with the previous reports [35,36]. In the case of Cu2O/Bi2WO6, the characteristic peak at 308 cm −1 shifted to 296 cm −1 , and the two peaks at 796 and 827 cm −1 became a broad peak at 809 cm −1 due to the cover of Cu2O on the surface of the Bi2WO6 MFs. The XPS spectra were conducted to detect the chemical environment of elements in the catalyst, and all characteristic peaks were calibrated using C 1s (binding energy at 284.6 eV) as a reference. In Figure 6a, elements of W 4f, Bi 4f, O 1s, and Cu 2p were detected in the full survey spectrum of the 3 wt% Cu2O/Bi2WO6, demonstrating the coexistence of these elements in the sample. As presented in Figure 6b, two distinct peaks located at 159.8 and 165.1 eV are assigned to the characteristic peaks of Bi 4f7/2 and Bi 4f5/2 in the trivalent oxidation state, respectively. In the previous report, the binding energy of Bi 4f7/2 in Bi2WO6 MFs locates in the range of 158 to 159 eV while that for Bi2O3 appears between 159 and 160 eV. Therefore, the peak located at 159.8 eV could be assigned to Bi 3+ in Bi2WO6 MFs [37,38]. In Figure 6c, the high resolution deconvoluted W 4f spectrum reveals two broad peaks at 38.2 and 36.0 eV corresponding to W 4f5/2 and W 4f7/2, respectively, suggesting the valence state of W element is +6 in the sample of Cu2O/Bi2WO6 heterojunction [39]. Moreover, as seen from Figure 6d, there are two obvious characteristic peaks at 953.3 and 933.5 eV, attributed to Cu 2p1/2 and Cu 2p3/2, respectively, revealing the feature of Cu + in Cu2O [40,41]. In contrast, the CuO state generally has a main characteristic peak locates at a binding energy of higher than 933 eV and characteristic shake-up satellite peaks at around 937-945 eV [42][43][44][45]. The shake-up peaks are often detected at around 9-10 eV higher than the main peaks, which results from the vigorous photoelectrons synchronously interacting with a valence electron and then being excited to a higher binding energy level [46]. However, in Figure 6d, the peak belonging to Cu 2+ at 933.7 eV with the shake-up peaks at 937-945 eV is not observed, revealing that the copper species in Cu2O/Bi2WO6 hybrids are mainly presented as Cu(I) [47][48][49]. The XPS spectra were conducted to detect the chemical environment of elements in the catalyst, and all characteristic peaks were calibrated using C 1s (binding energy at 284.6 eV) as a reference. In Figure 6a, elements of W 4f, Bi 4f, O 1s, and Cu 2p were detected in the full survey spectrum of the 3 wt% Cu 2 O/Bi 2 WO 6 , demonstrating the coexistence of these elements in the sample. As presented in Figure 6b, two distinct peaks located at 159.8 and 165.1 eV are assigned to the characteristic peaks of Bi 4f 7/2 and Bi 4f 5/2 in the trivalent oxidation state, respectively. In the previous report, the binding energy of Bi 4f 7/2 in Bi 2 WO 6 MFs locates in the range of 158 to 159 eV while that for Bi 2 O 3 appears between 159 and 160 eV. Therefore, the peak located at 159.8 eV could be assigned to Bi 3+ in Bi 2 WO 6 MFs [37,38]. In Figure 6c, the high resolution deconvoluted W 4f spectrum reveals two broad peaks at 38.2 and 36.0 eV corresponding to W 4f 5/2 and W 4f 7/2 , respectively, suggesting the valence state of W element is +6 in the sample of Cu 2 O/Bi 2 WO 6 heterojunction [39]. Moreover, as seen from Figure 6d, there are two obvious characteristic peaks at 953.3 and 933.5 eV, attributed to Cu 2p 1/2 and Cu 2p 3/2 , respectively, revealing the feature of Cu + in Cu 2 O [40,41]. In contrast, the CuO state generally has a main characteristic peak locates at a binding energy of higher than 933 eV and characteristic shake-up satellite peaks at around 937-945 eV [42][43][44][45]. The shake-up peaks are often detected at around 9-10 eV higher than the main peaks, which results from the vigorous photoelectrons synchronously interacting with a valence electron and then being excited to a higher binding energy level [46]. However, in Figure 6d, the peak belonging to Cu 2+ at 933.7 eV with the shake-up peaks at 937-945 eV is not observed, revealing that the copper species in Cu 2 O/Bi 2 WO 6 hybrids are mainly presented as Cu(I) [47][48][49]. UV-vis absorption spectra of various heterojunctions and the corresponding band gap energies calculated from the Tauc's plots by (αhν) = A(hν − Eg) 1/2 are presented in Figure 7, which reveals the sunlight response and absorption capability of Cu2O, Bi2WO6 MFs, and various Cu2O/Bi2WO6 hybrids. The absorption edge of Bi2WO6 MFs is about 460 nm, which suggests that the pure Bi2WO6 can only absorb UV and near-visible light. However, the absorption spectrum of Cu2O sharply rises at the beginning of 650 nm, displaying strong visible light response ability, which makes it a desirable candidate for utilization of solar energy. When depositing Cu2O QDs on the surface of Bi2WO6, the obtained Cu2O/Bi2WO6 hybrid system exhibits improved absorption ability for visible light, as displayed in Figure 7a. The corresponding band gap energies are calculated and displayed in Figure 7b, where the band gap energy of Cu2O/Bi2WO6 hybrids decreases with the introduction of Cu2O. Meanwhile, it is observed that the band gap of the 6 wt% Cu2O/Bi2WO6 hybrid is narrowed to 2.05 eV, which is obviously different from those of the 1.5 wt% and 3 wt% Cu2O/Bi2WO6 hybrids. This result suggests that the excess amount of Cu precursor did not result in the formation of Cu2O QDs but Cu2O microstructures on the surface of Bi2WO6. It demonstrates that the optimal amount of Cu precursor exists in the formation of QDs-MFs micro-heterojunction structure. On the other word, the excessive Cu precursor leads to the enhancement of sunlight response property. UV-vis absorption spectra of various heterojunctions and the corresponding band gap energies calculated from the Tauc's plots by (αhν) = A(hν − E g ) 1/2 are presented in Figure 7, which reveals the sunlight response and absorption capability of Cu 2 O, Bi 2 WO 6 MFs, and various Cu 2 O/Bi 2 WO 6 hybrids. The absorption edge of Bi 2 WO 6 MFs is about 460 nm, which suggests that the pure Bi 2 WO 6 can only absorb UV and near-visible light. However, the absorption spectrum of Cu 2 O sharply rises at the beginning of 650 nm, displaying strong visible light response ability, which makes it a desirable candidate for utilization of solar energy. When depositing Cu 2 O QDs on the surface of Bi 2 WO 6 , the obtained Cu 2 O/Bi 2 WO 6 hybrid system exhibits improved absorption ability for visible light, as displayed in Figure 7a. The corresponding band gap energies are calculated and displayed in Figure 7b, where the band gap energy of Cu 2 O/Bi 2 WO 6 hybrids decreases with the introduction of Cu 2 O. Meanwhile, it is observed that the band gap of the 6 wt% Cu 2 O/Bi 2 WO 6 hybrid is narrowed to 2.05 eV, which is obviously different from those of the 1.5 wt% and 3 wt% Cu 2 O/Bi 2 WO 6 hybrids. This result suggests that the excess amount of Cu precursor did not result in the formation of Cu 2 O QDs but Cu 2 O microstructures on the surface of Bi 2 WO 6 . It demonstrates that the optimal amount of Cu precursor exists in the formation of QDs-MFs micro-heterojunction structure. On the other word, the excessive Cu precursor leads to the enhancement of sunlight response property. Nanomaterials 2022, 12, x FOR PEER REVIEW 9 of 16 To investigate the transportation behavior and efficiency of photoinduced charge carriers at the heterojunction interface, the photoelectrochemical properties of these samples were investigated. In Figure 8a, electrochemical impedance spectroscopies (EIS) of these samples in the manner of a Nyquist diagram were recorded in the dark and under light irradiation. In general, the radius of each semicircle is correlated to charge-transfer resistance (Rct) at the interface of electrode/electrolyte; a smaller semicircle implies a lower Rct value [50][51][52]. As shown in Figure 8a, Cu2O exhibits significantly smaller Rct under light irradiation (l) in comparison with being in darkness (d), indicating that the electrical resistance at the electrode/electrolyte interface is decreased due to the production of photoinduced charge carriers. In the case of the flower-like Bi2WO6 MFs, a larger semicircle is recorded, suggesting that the Bi2WO6 possesses poor electrochemical performance in charge-transfer process [53,54]. With the formation of the Cu2O QDs/Bi2WO6 heterojunction, the Rct of Bi2WO6 is intensively reduced, which apparently improves the photoelectrochemical property of Bi2WO6 and is favorable for the transportation of the photogenerated charge carriers.  To investigate the transportation behavior and efficiency of photoinduced charge carriers at the heterojunction interface, the photoelectrochemical properties of these samples were investigated. In Figure 8a, electrochemical impedance spectroscopies (EIS) of these samples in the manner of a Nyquist diagram were recorded in the dark and under light irradiation. In general, the radius of each semicircle is correlated to charge-transfer resistance (R ct ) at the interface of electrode/electrolyte; a smaller semicircle implies a lower R ct value [50][51][52]. As shown in Figure 8a, Cu 2 O exhibits significantly smaller R ct under light irradiation (l) in comparison with being in darkness (d), indicating that the electrical resistance at the electrode/electrolyte interface is decreased due to the production of photoinduced charge carriers. In the case of the flower-like Bi 2 WO 6 MFs, a larger semicircle is recorded, suggesting that the Bi 2 WO 6 possesses poor electrochemical performance in charge-transfer process [53,54]. With the formation of the Cu 2 O QDs/Bi 2 WO 6 heterojunction, the R ct of Bi 2 WO 6 is intensively reduced, which apparently improves the photoelectrochemical property of Bi 2 WO 6 and is favorable for the transportation of the photogenerated charge carriers. To investigate the transportation behavior and efficiency of photoinduced charge carriers at the heterojunction interface, the photoelectrochemical properties of these samples were investigated. In Figure 8a, electrochemical impedance spectroscopies (EIS) of these samples in the manner of a Nyquist diagram were recorded in the dark and under light irradiation. In general, the radius of each semicircle is correlated to charge-transfer resistance (Rct) at the interface of electrode/electrolyte; a smaller semicircle implies a lower Rct value [50][51][52]. As shown in Figure 8a, Cu2O exhibits significantly smaller Rct under light irradiation (l) in comparison with being in darkness (d), indicating that the electrical resistance at the electrode/electrolyte interface is decreased due to the production of photoinduced charge carriers. In the case of the flower-like Bi2WO6 MFs, a larger semicircle is recorded, suggesting that the Bi2WO6 possesses poor electrochemical performance in charge-transfer process [53,54]. With the formation of the Cu2O QDs/Bi2WO6 heterojunction, the Rct of Bi2WO6 is intensively reduced, which apparently improves the photoelectrochemical property of Bi2WO6 and is favorable for the transportation of the photogenerated charge carriers.  To gain deeper insights into the characteristics of the prepared heterojunctions, flat band potential and carrier concentrations are deduced from the Mott-Schottky (M-S) curves [55,56]. The electrode potentials vs. SCE are converted to the reversible hydrogen electrode (RHE) potentials based on the following Nernst equation [57]: where V SCE is the experimental potential measured against the SCE, V RHE represents the converted potential vs. RHE, and V 0 SCE = 0.245 V at 25 • C. The Mott-Schottky (M-S) plots are depicted in Figure 8b-d, in which the flat band potentials at the electrode/electrolyte interface are calculated according to Equation (2) [36]: where C is the specific capacity, ε r and ε 0 are the dielectric constant of the samples and the electric permittivity of vacuum (8.85 × 10 −12 N −1 C 2 m −2 ), respectively; N d represents the carrier density of the catalysts, A is the efficient area of electrode, V and V fb are the applied working potential and the flat band potential, respectively; k is the Boltzmann constant, T donates the absolute temperature, and e is the electron charge (1.602 × 10 −19 C).
In Figure 8b, a positive slope of M-S plot is observed, inferring a n-type semiconductor of Bi 2 WO 6 . In contrast, the negative slope of the M-S plot indicates a p-type behavior of Cu 2 O in Figure 8c, which is consistent with the previous reports [36,58]. Meanwhile, the flat band potentials of Cu 2 O and Bi 2 WO 6 are calculated to be 0.74 and −0.18 V vs. RHE at pH = 6.8, respectively. In Figure 8d, an inverted "V-shape" curve is detected in the M-S plot of Cu 2 O/Bi 2 WO 6 , which is attributed to a characteristic curve of the p-n junction. It demonstrates that two distinct electronic behaviors (p-and n-type) are exhibited in the Cu 2 O/Bi 2 WO 6 photoelectrode. Moreover, a slight shift of x intercept in Cu 2 O/Bi 2 WO 6 occurs, implying the band realignment of Cu 2 O and Bi 2 WO 6 . The photocatalytic water oxidization performances of these prepared samples are presented in Figure 9. As shown in Figure 9a (Figure 8a), thereby resulting in the enhancement of photocatalytic activity towards water oxidation under solar light irradiation, as the 1.5 wt% Cu 2 O QDs/Bi 2 WO 6 heterojunction shown in Figure 9a. Meanwhile, the 3 wt% Cu 2 O QDs/Bi 2 WO 6 heterojunction exhibits the best photocatalytic water oxidation performance, up to 50 µmol/L within 3 h, which is 2.1 and 6.1 times higher than that of pure Bi 2 WO 6 and Cu 2 O QDs, respectively. Furthermore, the initial O 2 evolution rate of the 3 wt% Cu 2 O QDs/Bi 2 WO 6 heterojunction reaches 329 µmol h −1 g −1 , which is 2.3 and 9.7 fold that of sole Bi 2 WO 6 and Cu 2 O QDs system, respectively (Figure 9b), and is also superior to the reports in the literature (Table 1). However, excessive Cu(II) dosage (6wt%) is harmful for the dispersion of Cu 2 O QDs and causes the aggregation, leading to deteriorated catalytic performance. For the stability of the heterojunction system, as the recycling tests shown in Figure 9c, the photocatalytic performance of the 3 wt% Cu 2 O QDs/Bi 2 WO 6 hybrid fades to some extent due to the excess deposition of Ag + ions at the surface of heterojunction, but it still maintains good long-term stability and reuse potentiality. As a result, in Figure 9d, the 3 wt% Cu 2 O QDs/Bi 2 WO 6 hybrid exhibits a sustainable photocatalytic O 2 production capacity from water splitting. Nanomaterials 2022, 12, x FOR PEER REVIEW 11 of 16  For the 3 wt% Cu2O QDs/Bi2WO6 S-scheme heterojunction, the EPR results are displayed in Figure 10, where the signals attributed to the hydroxyl radicals (·OH) and superoxide radicals (·O2 − ) are detected. As shown in Figure 10a, the characteristic four peaks caused by the existence of DMPO-OH· adduct are observed, apparently, which demonstrates that water molecular adsorbed on the surface of photocatalyst could efficiently react with the photoinduced holes and form ·OH [69]. On the other hand, in Figure 10b, the characteristic six peaks are clearly found, which is ascribed to the superoxide radical [70]. It is demonstrated that both of ·OH and ·O2 − can be efficiently produced over the Cu2O QDs/Bi2WO6 hybrids under the solar light irradiation.   Figure 10, where the signals attributed to the hydroxyl radicals (·OH) and superoxide radicals (·O 2 − ) are detected. As shown in Figure 10a, the characteristic four peaks caused by the existence of DMPO-OH· adduct are observed, apparently, which demonstrates that water molecular adsorbed on the surface of photocatalyst could efficiently react with the photoinduced holes and form ·OH [69]. On the other hand, in Figure 10b, the characteristic six peaks are clearly found, which is ascribed to the superoxide radical [70]. It is demonstrated that both of ·OH and ·O 2 − can be efficiently produced over the Cu 2 O QDs/Bi 2 WO 6 hybrids under the solar light irradiation. Based on the above results, two types of II or S-scheme heterojunction can be built between Cu2O QDs and Bi2WO6. Once the type II heterojunction is constructed, the trend of photoinduced charge carriers is for photogenerated holes at the VB of Bi2WO6 to migrate to the VB of Cu2O; correspondingly, the photoinduced electrons at the CB of Cu2O transfer to the CB of Bi2WO6. Consequently, photoinduced holes and electrons gather at the CB of Bi2WO5 and VB of Cu2O, respectively. Unfortunately, the VB potential of Cu2O is situated at +0.83 eV, which is quite low and makes it hard to guarantee enough oxidative potential to oxidize water and produce gaseous O2 [71]. Therefore, it is concluded that the Cu2O QDs/Bi2WO6 hybrids might tend to construct a novel S-scheme band structure, as presented in Figure 11a. The photoinduced electrons at the CB of Bi2WO6 are likely to quench the holes at the VB of Cu2O. Subsequently, the stronger reductive electrons at the CB of Cu2O and oxidative holes at the VB of Bi2WO6 are efficiently retained simultaneously. As described in Figure 11b, the separated photoinduced holes at the VB of Bi2WO6 react with the adsorbed H2O at the surface of hybridized system to generate O2, and the retained electrons at the CB of Cu2O are quenched by Ag + ions. Therefore, the construction of an S-scheme heterojunction is conducive to inhibiting the recombination efficiency of the photoinduced charge carriers, giving rise to more photogenerated holes taking part in the photocatalytic reactions, thereby enhancing the photocatalytic efficiency towards O2 production.  Based on the above results, two types of II or S-scheme heterojunction can be built between Cu 2 O QDs and Bi 2 WO 6 . Once the type II heterojunction is constructed, the trend of photoinduced charge carriers is for photogenerated holes at the VB of Bi 2 WO 6 to migrate to the VB of Cu 2 O; correspondingly, the photoinduced electrons at the CB of Cu 2 O transfer to the CB of Bi 2 WO 6 . Consequently, photoinduced holes and electrons gather at the CB of Bi 2 WO 5 and VB of Cu 2 O, respectively. Unfortunately, the VB potential of Cu 2 O is situated at +0.83 eV, which is quite low and makes it hard to guarantee enough oxidative potential to oxidize water and produce gaseous O 2 [71]. Therefore, it is concluded that the Cu 2 O QDs/Bi 2 WO 6 hybrids might tend to construct a novel S-scheme band structure, as presented in Figure 11a. The photoinduced electrons at the CB of Bi 2 WO 6 are likely to quench the holes at the VB of Cu 2 O. Subsequently, the stronger reductive electrons at the CB of Cu 2 O and oxidative holes at the VB of Bi 2 WO 6 are efficiently retained simultaneously. As described in Figure 11b, the separated photoinduced holes at the VB of Bi 2 WO 6 react with the adsorbed H 2 O at the surface of hybridized system to generate O 2 , and the retained electrons at the CB of Cu 2 O are quenched by Ag + ions. Therefore, the construction of an S-scheme heterojunction is conducive to inhibiting the recombination efficiency of the photoinduced charge carriers, giving rise to more photogenerated holes taking part in the photocatalytic reactions, thereby enhancing the photocatalytic efficiency towards O 2 production. Based on the above results, two types of II or S-scheme heterojunction can be built between Cu2O QDs and Bi2WO6. Once the type II heterojunction is constructed, the trend of photoinduced charge carriers is for photogenerated holes at the VB of Bi2WO6 to migrate to the VB of Cu2O; correspondingly, the photoinduced electrons at the CB of Cu2O transfer to the CB of Bi2WO6. Consequently, photoinduced holes and electrons gather at the CB of Bi2WO5 and VB of Cu2O, respectively. Unfortunately, the VB potential of Cu2O is situated at +0.83 eV, which is quite low and makes it hard to guarantee enough oxidative potential to oxidize water and produce gaseous O2 [71]. Therefore, it is concluded that the Cu2O QDs/Bi2WO6 hybrids might tend to construct a novel S-scheme band structure, as presented in Figure 11a. The photoinduced electrons at the CB of Bi2WO6 are likely to quench the holes at the VB of Cu2O. Subsequently, the stronger reductive electrons at the CB of Cu2O and oxidative holes at the VB of Bi2WO6 are efficiently retained simultaneously. As described in Figure 11b, the separated photoinduced holes at the VB of Bi2WO6 react with the adsorbed H2O at the surface of hybridized system to generate O2, and the retained electrons at the CB of Cu2O are quenched by Ag + ions. Therefore, the construction of an S-scheme heterojunction is conducive to inhibiting the recombination efficiency of the photoinduced charge carriers, giving rise to more photogenerated holes taking part in the photocatalytic reactions, thereby enhancing the photocatalytic efficiency towards O2 production.

Conclusions
In summary, we successfully prepared Cu 2 O QDs/Bi 2 WO 6 heterojunctions by coupling hierarchical Bi 2 WO 6 MFs with Cu 2 O QDs to construct efficient S-scheme heterojunctions, which could facilitate the migration of photoinduced charge carriers. The electrochemical properties are investigated to explore the transportation performance and donor density of charge carriers in the S-scheme heterojunction system. The results indicate that the synthesized S-scheme heterojunction shows improved photocatalytic activity for water oxidation compared with the sole Bi 2 WO 6 and Cu 2 O QDs systems under simulated solar light illumination. The initial O 2 evolution rate of the heterojunction system is 2.3 and 9.7 fold that of sole Bi 2 WO 6 and Cu 2 O QDs system, respectively. Furthermore, it is evidently demonstrated that both of ·OH and ·O 2 − can be generated efficiently over the Cu 2 O QDs/Bi 2 WO 6 heterojunction under the simulated solar light illumination.

Data Availability Statement:
The data presented in this study is available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.