N,Cu-CD-Decorated Mesoporous WO3 for Enhanced Photocatalysis Under UV–Vis–NIR Light Irradiation

Research on the design of semiconductor photocatalysts with rapid electron transfer efficiencies and broad-spectrum responses for environmental remediation remains a pressing challenge. Herein, we described the fabrication of a novel broad-spectrum nitrogen and copper codoped carbon dots/mesoporous WO3 nanocomposite (N,Cu-CDs/m-WO3), which exhibited complete UV–vis–NIR spectrum response, light harvesting capabilities, rich oxygen vacancies, rapid electron-transfer ability, low electron–hole (e−/h+) pair recombination rate, and extensive specific surface area. After 2 h of photocatalytic reaction, it showed excellent photoactivities for the degradation of rhodamine B, methylene blue, tetracycline hydrochloride, oxytetracycline, ciprofloxacin, and bisphenol A. Moreover, we found that the conversion between Cu (II) and Cu (I) played a key role in accelerating electron transfer and inhibiting the recombination of e−/h+ pairs. This work provides an efficient strategy for the utilization of solar light and enhancing the charge-transfer capacity in the semiconductor photocatalysis field.


INTRODUCTION
As a novel solar-driven technology, semiconductor photocatalysis for environmental remediation has garnered increasing attention due to its ecogreenness, recyclability, and high efficiency (Daneshvar et al., 2007;Ong, 2017). Of the semiconductor materials (e.g., TiO 2 , WO 3 , and SnO 2 ) investigated thus far, WO 3 exhibits extraordinary characteristics including nontoxicity, stability, and favorable band gaps (2.6-3.0 eV) Carmona et al., 2016). However, several drawbacks such as low visiblelight utilization efficiency and the rapid recombination rate of photoinduced e − /h + pairs still limit its applicability (Zhang et al., 2017). Thus, considerable efforts have been devoted to improve its photocatalytic activity. Note that controlling the formation and structure of heterojunction are two strategies for enhancing the photocatalytic performance of WO 3 .
Structural modifications can provide special channels for electron transfer and differently exposed surface areas, thus altering the photocatalytic activity of WO 3 . Compared with traditional nanostructures, mesoporous structures with extensive specific surface areas, uniform and tunable pore sizes, and large pore volumes endow them with additional reactive reaction sites and unique pathways for the diffusion of molecules, which can better interact with guest molecules (Luo et al., 2013;Zhao et al., 2016;Lv et al., 2017). However, pure mesoporous WO 3 is primarily used in gas sensors, and there are a few studies about single mesoporous WO 3 employed for photocatalysis (Teoh et al., 2003;Li et al., 2010;Zhu et al., 2017;. Besides, the intrinsic absorption of WO 3 is restricted to a marginal portion of the visible-light spectrum. Therefore, the synthesis of heterojunction to facilitate the utilization of solar energy is necessary (Zhang et al., 2017;Ni et al., 2020).
Among various composites, carbon dots (CDs) have been widely applied in photocatalysis due to their excellent physicochemical properties including water solubility, nontoxicity, upconversion capacities, and optical absorption, which can facilitate the charge migration, resulting in enhancing their photocatalytic activity Zhang et al., 2017;. However, the slow electrontransfer/storage capacities and low light-absorption efficiencies of CDs also limit their practical application. Toward the potential resolution of these deficits, heteroatom doping has been considered as an efficient strategy for improving the physicochemical properties of CDs (Barman et al., 2014;Ma Y. et al., 2017;and Wu et al., 2017). In our previous work, we found that doping nitrogen into carbon dots (N-CDs) could enhance their optical absorption in the visible-light region Wang et al., 2019;. In addition, metal doping has also been shown to be an effective method for enhancing the electron-transfer properties of CDs Xue et al., 2017). Unlike noble metals, copper (Cu) doping has the advantage of low cost, safety, and inclination to coordinate with CDs chemical groups (Zong et al., 2014;Guo et al., 2015;Xu et al., 2015;Zhang et al., 2017;. Several studies have reported that Cu can serve as a multielectron redox reaction site, which can accelerate the electron transfer and broaden the optical absorption, thus improving the photocatalytic activity (Irie et al., 2008;Nakajima et al., 2011;and Ma et al., 2018). Therefore, if N and Cu exist simultaneously in CDs that act as electron donors and acceptors, respectively, which could significantly facilitate electron-transfer and light-absorption capacities. Up to now, there have been no reports about N and Cu codoped CD-decorated WO 3 for photocatalytic application.
Herein, we successfully synthesized a series of mesoporous WO 3 via a solvent evaporation-induced self-assembly (EISA) process using diblock or triblock copolymers PEO x -b-PS x (PbS) as the templates, respectively. After decorating with N,Cu-CDs, the N,Cu-CDs/m-WO 3 showed high electrontransfer/reservoir capacities and enhanced photocatalytic degradation activities against rhodamine B (RhB), methylene blue (MB), tetracycline hydrochloride (TCH), oxytetracycline (OTC), ciprofloxacin (CIP), and bisphenol A (BPA) under UV, vis, and NIR light irradiation.

Preparation of Mesoporous WO 3
All chemicals used were of analytical grade. A series of mesoporous WO 3 were synthesized via the EISA process using WCl 6 as a precursor and diblock copolymers PEO x -b-PS y as templates, which was similar to a previous work (Zhu et al., 2017). Typically, 0.1 g templates were dissolved into 5.0 g tetrahydrofuran (THF) to form solution A. Solution B was then prepared by adding 0.25 g WCl 6 into a mixed solution of 0.5 g ethanol and 0.25 g acetylacetone (AcAc). Solutions A and B were subsequently mixed under stirring for 2 h at room temperature. The obtained solution mentioned above was then placed in Petri dishes to evaporate the solvent at room temperature for 1 h and heated at 100°C for 24 h. Finally, the resulting transparent films were further calcined at 350°C in N 2 for 3 h and then 500°C in air for 2 h at a heating rate of 1°C/min.

Preparation of Samples
A one-step hydrothermal method was employed to synthesize the N,Cu-CDs. Typically, 0.68 mmol folic acid (FA) and 0.13 mmol copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O) were dissolved into 30 ml of deionized water under stirring for 20 min at room temperature. Afterward, the solution was transferred to a 100 ml Teflon-lined autoclave that was placed in an oven at 200°C for 4 h. Finally, the CDs solution mentioned above was filtered using 0.22 µm filtration membranes and dialyzed by using a 1000 Da dialysis bag for 8 h. The CD powders were then obtained by a freeze-dry process.
A sonication-assisted hydrothermal method was employed to synthesize the nanocomposites. Typically, 0.1 g of m-WO 3 was added into different CD solution concentrations (0.2-1.0 g/L, 20 ml) followed by sonication for 15 min to expel any pore resident bubbles to form a homogeneous suspension. Subsequently, the suspension was poured into a Teflon-lined autoclave (50 ml) and heated at 190°C for 8 h. Finally, the precipitates were obtained by centrifugation at 8000 r/min and dried in an oven at 70°C. A series of N,Cu-CDs/m-WO 3 nanocomposites were synthesized and designated as N,Cu-CD/ m-WO 3 -C (C 0.2-1.0). The N-CDs/m-WO 3 nanocomposites were obtained by the same method, except for the N-CDs prepared in our previous work (Ni et al., 2020).

Measurements
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) tests were performed with Quanta FEG 450 and Tecnai G220S-Twin microscopes, respectively. X-ray diffraction (XRD) measurements were obtained using a Bruker D8 Advance X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) was characterized by using the EscaLab 250Xi spectrometer. The UV-vis-NIR diffuse reflection spectra (DRS) were analyzed with a Shimadzu UV2600 spectrophotometer. Fourier-transform infrared (FT-IR) and photoluminescence (PL) spectra were investigated using Shimadzu IRTracer-100 and Edinburgh FLS1000 spectrometers, respectively. Nitrogen adsorption-desorption isotherms were measured with an Autosorb IQ analyzer. A Malvern Nano ZS laser particle-size analyzer was employed to investigate the particle diameters.
(λ ≥ 420 nm, the average light intensity was 30 mW·cm −2 ), and 500 W xenon lamp (λ ≥ 780 nm, the average light intensity was 25 mW·cm −2 ) were employed to provide UV, visible, and NIR light, respectively. For each experiment, 50 ml of RhB (10 mg/L), MB (10 mg/L), CIP (20 mg/L), OTC (20 mg/L), BPA (20 mg/L), and TCH (20 mg/L) solutions containing 50 mg of the photocatalysts were stirred in the dark for 30 min to achieve adsorption-desorption equilibrium. The reaction conditions were controlled at 25 ± 1°C. The concentrations of RhB and MB were detected using a UV-2600 spectrophotometer. The concentrations of CIP, OTC, BPA, and TCH were analyzed by high-performance liquid chromatography (see Supplementary Material).

Photoelectrochemical Tests
The PEC properties including the photocurrent response and electrochemical impedance spectroscopy (EIS) were performed on an electrochemical workstation (Princeton, VersaSTAT 3) with Na 2 SO 4 (0.5 M) solution as the electrolyte. The assynthesized samples, a Pt plate, and a saturated calomel electrode (SCE) electrode were employed as the working electrode, counter electrode, and reference electrode, respectively. The working electrode was prepared on FTO glass via a spin-coating method, and a 300 W xenon lamp was used to simulate the solar light.

RESULTS AND DISCUSSION
The Synthesis Route for the N,Cu-CDs/ M-WO 3 Nanocomposite As shown in Scheme 1, the WCl 6 and PEO x -b-PS y were chosen as the precursor and template, respectively. The m-WO 3 with a large specific surface area and highly ordered pore sizes was successfully synthesized via an EISA process and then calcining under an N 2 and air atmosphere. The N,Cu-CDs were prepared by a facile one-step hydrothermal method, using folic acid and copper nitrate hydrate as C and Cu sources. A sonication-assisted hydrothermal approach was used to synthesize the N,Cu-CDs/m-WO 3 nanocomposite, in which the N,Cu-CDs were well located at the pores of the m-WO 3 .

Micromorphology Analysis
The SEM and TEM images ( Figures 1A-C) indicate that the m-WO 3 possessed highly ordered and uniform pores with dimensions of ∼30 nm, which was in good accordance with the pore size of the Barrett-Joyner-Halenda (BJH) results. Moreover, the hollow pores could significantly enhance its multiple light-reflection capacities. N,Cu-CDs with average sizes of 3.2 nm could be easily found within the pores ( Figures 1D,E). The HRTEM patterns shown in Figure 1F reveal that the m-WO 3 exhibited a good crystalline structure with a lattice spacing of 0.365 nm, which corresponded well to the (200) lattice plane of WO 3 (Zhu et al., 2017). In addition, the N,Cu-CDs showed a lattice distance of 0.22 nm.

Brunner−Emmet−Teller and BJH Analysis
As shown in Figure 2, compared to the BET results of the three mesoporous WO 3 samples, the m-WO 3 with PEO x -b-PS y as the template possessed the largest specific surface area of 35.350 m 2 /g, which was much higher than that of commercial WO 3 (6.135 m 2 /g). After coupling the N,Cu-CDs and N-CDs, the specific surface area of m-WO 3 was reduced to 29.431 m 2 /g and 22.465 m 2 /g, respectively. Furthermore, the average pore size decreased from 30.566 to 17.083 nm and 17.361 nm within the pores (shown in Figure 2).

Crystal Structure and Element Analysis
The XRD patterns of as-prepared samples ( Figure 4A    N,Cu-CDs were successfully combined with the m-WO 3 . In the high-resolution W 4f spectra ( Figure 4B), the W of pure m-WO 3 existed in four different states: W 4f 5/2 of W 6+ , W 4f 7/2 of W 6+ , W 4f 5/2 of W 5+ , and W 4f 7/2 of W 5+ (at 37.80, 35.64, 36.80, and 34.50 eV, respectively), which suggested that the m-WO 3 contained rich oxygen vacancies . In the O 1s spectra ( Figure 4C), the O 1s peaks in the pure m-WO 3 were divided into two peaks at 530.36 and 531.25 eV, which were ascribed to the lattice oxygen (W-O) and the adsorbed oxygen, respectively (Zhu et al., 2017). As for the O 1s of N-CDs/m-WO 3 and N,Cu-CDs/m-WO 3 -0.8, the new peaks appearing at ∼533 eV belonged to the binding energy of C O in the CDs. It can be clearly observed that the W4f and O 1s peaks of the nanocomposites showed shifts to lower binding energies after decorating with N-CDs or N,Cu-CDs, which was due to the interactions between the CDs and m-WO 3 (Di et al., 2015). Moreover, the introduction of Cu could further enhance this effect. As shown in the C 1s spectra ( Figure 4D), the peaks at 284.80 and 287.12 eV in the pure m-WO 3 were attributed to the surface carbon from ambient air (Ni et al., 2020). After decorating with N,Cu-CDs, the new C 1s peaks appearing at 287.65 and 286.22 eV were attributed to the C O, C-O/C-N bands of the CDs, respectively (Peng et al., 2019;. According to the N 1s spectra of the N,Cu-CDs/m-WO 3 -0.8 nanocomposite ( Figure 4E), the peaks at ∼399.66 and 401.66 eV were assigned to C-N and N-H bonds, respectively . Compared with the N-CD-decorated m-WO 3 , the C 1s and N 1s peaks in the N,Cu-CDs/m-WO 3 -0.8 nanocomposite underwent shifts, which was caused by the chelation between the Cu and N-CDs (Orozco-Guareño et al., 2010). In addition, as shown in the Cu 2p spectra of the N,Cu-CDs/m-WO 3 -0.8 ( Figure 4F), the Cu components with their Cu 2p 3/2 and Cu 2p 1/2 binding energies at 932.56 and 952.34 eV were characteristic of Cu 1+ , while the shoulder peaks at 934.97 eV for Cu 2p 3/2 and 954.67 eV for Cu 2p 1/2 could be ascribed to Cu 2+ (Orozco-Guareño et al., 2010;Liu et al., 2015;and Li et al., 2014). The transformable valence states of Cu demonstrated that the N,Cu-CDs could serve as multielectron redox reaction sites for the efficient electron migration.
Optical Property Analysis Figure 5A shows the UV-vis-NIR absorption in the regions of 200-1400 nm with the inset for band gaps. Interestingly, beyond the intrinsic absorption edge of 480 nm, pure m-WO 3 showed the absorption ability in the NIR region, which could be attributed to the existence of oxygen vacancies (Liu L. et al., 2012;Wu et al., 2019) and multiple light-reflection within the pores (Yan et al., 2019;Ni et al., 2020). Compared to the pure m-WO 3 , the N-CDand N,Cu-CD-decorated m-WO 3 performed the reduced band gaps (m-WO 3 ) from 2.9 to 2.8 eV and 2.7 eV, respectively. In particular, the N,Cu-CDs/m-WO 3 -0.8 nanocomposite exhibited a broader optical absorption from the UV to NIR regions, suggesting that more solar energy could be utilized through N,Cu-CDs decorating. As shown in Figure 5B, the N,Cu-CD-decorated m-WO 3 showed a significantly decreased PL intensity, indicating the efficient electron transfer capacity of N,Cu-CDs/m-WO 3 -0.8, which could greatly accelerate the separation of e − /h + pairs (Liang et al., 2017).

PEC Properties
The photocurrent response and EIS tests were conducted to further investigate the migration and separation of photoinduced electrons and holes. As shown in Figure 6A, the N,Cu-CDs/m-WO 3 -0.8 nanocomposite showed the highest photocurrent intensity in contrast to the pure m-WO 3 and the N-CDs/m-WO 3 -0.8 under solar light irradiation, suggesting faster electron transfer and highly efficient separation of e − /h + pairs in the N,Cu-CDs/m-WO 3 -0.8. In the EIS spectrum ( Figure 6B), a smaller semicircle arc radius implied a smaller resistance at the interfacial region, which further demonstrated

Photocatalytic Performance
As shown in Figure 7A, among the m-WO 3 with different contents of N,Cu-CDs, the N,Cu-CDs/m-WO 3 -0.8 nanocomposite showed the highest photodegradation efficiency on RhB under visible-light irradiation. Compared with the pure m-WO 3 and N-CDs/m-WO 3 -0.8 nanocomposite, the N,Cu-CDs/m-WO 3 -0.8 nanocomposite exhibited higher photodegradation efficiencies of 81.5, 97.6, and 56.2%, respectively, after 2 h of UV-vis-NIR light irradiation ( Figures 7B-D), respectively. Furthermore, the kinetics of the RhB photodegradation was investigated. As displayed in Supplementary Figure S2, the highest photodegradation rate constant of 1.895 h −1 under visible light irradiation was found by the N,Cu-CDs/m-WO 3 -0.8 nanocomposite, whereas the photodegradation rate of commercial WO 3 was only 0.083 h − . This suggests that the photodegradation rate of N,Cu-CDs/m-WO 3 -0.8 was almost 23 times higher than that of commercial WO 3 . In addition, the N,Cu-CDs/m-WO 3 -0.8 nanocomposite showed excellent photocatalytic performance for the degradation of MB, TCH, OTC, CIP, and BPA under visible-light irradiation for 2 h ( Figure 7E). The stability of the N,Cu-CDs/m-WO 3 -0.8 nanocomposite was studied via the cycling tests under visiblelight irradiation for 2 h. Figure 7F shows that the nanocomposite maintained almost unchanged RhB photodegradation efficiencies varying from 97.6 to 90.2% after a five-cycle recycling, confirming the stable and reusable properties of the N,Cu-CDs/m-WO 3 -0.8 nanocomposite.

Photocatalytic Mechanism
Electron spin resonance (ESR) tests using DMSO were conducted to investigate the reactive oxygen species (ROSs) in the N,Cu-CDs/ m-WO 3 -0.8 nanocomposite under solar light irradiation. As shown in Figures 8A,B, four peaks with an intensity ratio of 1: 2:2:1 corresponded to the signal of DMPO-·OH, and the characteristic peaks of DMPO-·O 2 --could be clearly observed  Frontiers in Materials | www.frontiersin.org March 2021 | Volume 8 | Article 649411 8 (Xiao et al., 2018). Moreover, the signal intensity of DMPO-·OH was significantly stronger than that of DMPO-·O 2 -, suggesting that ·OH might be the primary reactive species in the photoreaction. To further examine the key reactive species in the N,Cu-CDs/m-WO 3 -0.8 composite, ROSs trapping experiments were conducted by adding various scavengers including AO (700 mg/L) for h + , IPA (600 mg/L) for ·OH, and BQ (80 mg/L) for ·O 2 - (Cai et al., 2019). As presented in Figures 8C,D, the photodegradation rate of RhB showed partial inhibition rates of 89.67, 51.19, and 25.95% after adding the scavengers of AO, IPA, and BQ, respectively, indicating that the photogenerated h + and OH were the main ROSs in the photocatalytic reaction.
Based on the abovementioned results, a possible photocatalytic mechanism for the N,Cu-CDs/m-WO 3 -0.8 nanocomposite was proposed as follows ( Figure 9). Under UV-vis-NIR light irradiation, both m-WO 3 and N,Cu-CDs were excited, thus forming the e − /h + pairs. The photoinduced electrons in the conduction band (CB) of m-WO 3 combined with the h + in the valence band (VB) of the N,Cu-CDs. Moreover, the photoinduced e − was directly transferred from the VB of the N,Cu-CDs to Cu(II), forming a multielectron reaction site, where Cu(II) was reduced to Cu(I). Subsequently, Cu(I) was oxidized to Cu(II) with the conversion of O 2 to ·O 2 − (Irie et al., 2008;Ma et al., 2018), whereas the remaining h + in the VB of m-WO 3 could directly photodegrade the contaminants or indirectly transfer H 2 O to ·OH Ni et al., 2020). Therefore, the introduction of N,Cu-CDs as multielectron reaction sites could greatly accelerate electron transfer and efficiently prevent the recombination of e − /h + pairs.

CONCLUSION
In summary, mesoporous WO 3 with a large specific surface area was successfully synthesized via the EISA process with PbS as the template. For the first time, the N,Cu-CDs/m-WO 3 -0.8 nanocomposite was prepared by a sonication-assisted hydrothermal method. The N,Cu-CDs/m-WO 3 -0.8 nanocomposite, with a large specific surface area, full-spectrum response, efficient electron-transfer capacity, and low e − /h + pair recombination rate, exhibited outstanding photocatalytic activities for the degradation of RhB, MB, TCH, OTC, CIP, and BPA. Characterization results demonstrate that the conversion between Cu (II) and Cu (I) played a key role in accelerating electron transfer and inhibiting the recombination of e − /h + pairs. Furthermore, the extended spectrum absorption was attributed to the ample oxygen vacancies, the introduction of N,Cu-CDs, and multiple light reflections within the pores of the mesoporous WO 3 .

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding authors.

AUTHOR CONTRIBUTIONS
TN and KC conceptualized the work; TN and QL curated data; QL conducted formal analysis; ZY acquired fund; TN, YY, KC, and GL conducted investigation; YY was responsible for the methodology; TN, ZY, and GL supervised the work; TN and QL wrote the original draft; and TN and GL reviewed and edited the manuscript.