N,Fe-Doped Carbon Dot Decorated Gear-Shaped WO 3 for Highly E ﬃ cient UV-Vis-NIR-Driven Photocatalytic Performance

: The development of e ﬃ cient and non-toxic photocatalysts with a full spectrum response is a primary strategy in the area of photocatalytically mediated pollutant elimination. Herein, we report the preparation of novel nitrogen and iron co-doped carbon dots / gear-shaped WO 3 (N,Fe-CDs / G-WO 3 ) with signiﬁcantly improved broad-spectrum utilization. Characterization results demonstrated that the gear-shaped G-WO 3 , decorated by N,Fe-CDs with excellent electron transfer / reservoir properties, possessed abundant oxygen vacancies, had large speciﬁc surface areas, had multiple light-reﬂections and had a narrow band gap. As a result, the N,Fe-CDs / G-WO 3 composite exhibited excellent photocatalytic activity towards the degradation of water contaminants under full spectrum irradiation. For example, the photodegradative e ﬃ ciencies of rhodamine B (RhB) reached 81.4%, 97.1%, and 75% in 2 h, under ultraviolet, visible, and near-infrared (UV, vis, and NIR) light irradiation, respectively. Moreover, the N,Fe-CDs / G-WO 3 composite also exhibited an outstanding photocatalytic degradation e ﬃ ciency for other dyes, pharmaceuticals, and personal care products (PPCPs) like methylene blue (MB), ciproﬂoxacin (CIP), tetracycline hydrochloride (TCH), and oxytetracycline (OTC) (91.1%, 70.5%, 54.5%, and 47.8% in 3 h, respectively). The radical trapping experiments indicated that h + and · OH were the main reactive oxidative species (ROS), and the conversion between Fe (III) and Fe (II) played a key role in the photocatalytic reactions. Such a N,Fe-CD decorated material with brilliant photocatalytic activity has tremendous potential for application in environmental remediation. -0.6


Introduction
With the continuous process of industrialization and modernization in recent years, more and more organic contaminants, such as antibiotics, have been discharged, which causes increasingly serious environmental problems. Hence, diverse sewage treatment technologies, including various Scheme 1. A schematic illustration of the fabrication of the gear-shaped WO3 (G-WO3) and N,Fe-CDs/G-WO3.

Morphological Analysis
As shown in Figure 1a,b,d, the SEM and TEM images of G-WO3 showed a 3-D gear-like structure consisting of a plate with many protruding nanorods on its surface, making it possible to possess high surface areas and reflect the incident light during irradiation. The particle diameters of the N,Fe-CDs ranged from 3.5 to 13.5 nm, with an average diameter of 6.5 nm (Figure 1c). N,Fe-CDs could be clearly observed on the surface of G-WO3, as shown in Figure 1e. As illustrated in Figure 1f, the high resolution transmission electron microscopy images (HRTEM) of G-WO3 displayed an obvious lattice fringe with a lattice distance of 0.39 nm corresponding to the (001) lattice plane of WO3 [30], and the N,Fe-CDs displayed an obvious lattice fringe with a lattice spacing of 0.32 nm.

Morphological Analysis
As shown in Figure 1a,b,d, the SEM and TEM images of G-WO 3 showed a 3-D gear-like structure consisting of a plate with many protruding nanorods on its surface, making it possible to possess high surface areas and reflect the incident light during irradiation. The particle diameters of the N,Fe-CDs ranged from 3.5 to 13.5 nm, with an average diameter of 6.5 nm (Figure 1c). N,Fe-CDs could be clearly observed on the surface of G-WO 3 , as shown in Figure 1e. As illustrated in Figure 1f, the high resolution transmission electron microscopy images (HRTEM) of G-WO 3 displayed an obvious lattice fringe with a lattice distance of 0.39 nm corresponding to the (001) lattice plane of WO 3 [30], and the N,Fe-CDs displayed an obvious lattice fringe with a lattice spacing of 0.32 nm.

FT-IR Analysis
As shown in the FT-IR spectrum of the N-CDs and N,Fe-CDs (Figure 2), the absorption bands at around 3400 cm −1 were ascribed to the O-H stretching vibration, the bands at 3210 cm −1 were due to N-H stretching mode, and the bands located at 2840 and 1290 cm −1 were assigned to the C-H vibration.
Catalysts 2020, 10, 416 4 of 14 In addition, the bands found at 1675, 1580, and 1400 cm −1 corresponded to v (C=O), δ (N-H), and v (C-N), respectively [31][32][33]. No obvious characteristic bands of Fe in N,Fe-CDs could be observed because of the low content. In the FT-IR spectrum of the pure WO 3 , the bands appearing at 3400 and 1650 cm −1 were assigned to the O-H stretching vibration of surface-adsorbed water. The set of bands between 600 and 1000 cm −1 can be ascribed to the W-O bending vibration of WO 3 [34]. The FT-IR spectra of N-CDs/G-WO 3 -0.6 and N,Fe-CDs/G-WO 3 -0.6 showed the same bands. It is worth noting that bands located at 1400 cm −1 can be observed, which could be associated with the stretching vibrations of C-N. In addition, the bands broadened in the region between 1500 and 1650 cm −1 , a consequence of the CDs' structures [11,31]. The above results indicate that CDs were successfully decorated onto the surfaces of WO 3 .

FT-IR Analysis
As shown in the FT-IR spectrum of the N-CDs and N,Fe-CDs (Figure 2), the absorption bands at around 3400 cm −1 were ascribed to the O-H stretching vibration, the bands at 3210 cm −1 were due to N-H stretching mode, and the bands located at 2840 and 1290 cm −1 were assigned to the C-H vibration. In addition, the bands found at 1675, 1580, and 1400 cm −1 corresponded to v (C=O), δ (N-H), and v (C-N), respectively [31][32][33]. No obvious characteristic bands of Fe in N,Fe-CDs could be observed because of the low content. In the FT-IR spectrum of the pure WO3, the bands appearing at 3400 and 1650 cm −1 were assigned to the O-H stretching vibration of surface-adsorbed water. The set of bands between 600 and 1000 cm −1 can be ascribed to the W-O bending vibration of WO3 [34]. The FT-IR spectra of N-CDs/G-WO3-0.6 and N,Fe-CDs/G-WO3-0.6 showed the same bands. It is worth noting that bands located at 1400 cm −1 can be observed, which could be associated with the stretching vibrations of C-N. In addition, the bands broadened in the region between 1500 and 1650 cm −1 , a consequence of the CDs' structures [11,31]. The above results indicate that CDs were successfully decorated onto the surfaces of WO3.

BET Analysis
To clearly investigate the surface adsorption properties of the as-synthesized samples, the BET test and Barrett-Joyner-Halenda (BJH) analysis were carried out ( Figure 3). The BET surface area of pure G-WO3 (36.039 m 2 /g STP) is nearly six-fold larger than that of commercial WO3 (6.135 m 2 /g STP), revealing that G-WO3 possesses more interfacial active sites. After doping with N-CDs or N,Fe-CDs, the surface area of G-WO3 decreased, which was attributable to the internal combination. The space

BET Analysis
To clearly investigate the surface adsorption properties of the as-synthesized samples, the BET test and Barrett-Joyner-Halenda (BJH) analysis were carried out ( Figure 3). The BET surface area of pure G-WO 3 (36.039 m 2 /g STP) is nearly six-fold larger than that of commercial WO 3 (6.135 m 2 /g STP), revealing that G-WO 3 possesses more interfacial active sites. After doping with N-CDs or N,Fe-CDs, the surface area of G-WO 3 decreased, which was attributable to the internal combination. The space between the interlayers of G-WO 3 could accommodate CDs, and the vacant spaces between these protruding nanorods on G-WO 3 made it more mesoporous, allowing it to better couple with CDs. Figure 2. The FT-IR spectra of N-CDs, N,Fe-CDs, pure G-WO3, N-CDs/G-WO3-0.6, and N,Fe-CDs/G-WO3-0.6.

BET Analysis
To clearly investigate the surface adsorption properties of the as-synthesized samples, the BET test and Barrett-Joyner-Halenda (BJH) analysis were carried out ( Figure 3). The BET surface area of pure G-WO3 (36.039 m 2 /g STP) is nearly six-fold larger than that of commercial WO3 (6.135 m 2 /g STP), revealing that G-WO3 possesses more interfacial active sites. After doping with N-CDs or N,Fe-CDs, the surface area of G-WO3 decreased, which was attributable to the internal combination. The space between the interlayers of G-WO3 could accommodate CDs, and the vacant spaces between these protruding nanorods on G-WO3 made it more mesoporous, allowing it to better couple with CDs.

Phase Structures and Elemental State Analysis
The XRD patterns of the as-synthesized samples are presented in Figure 4a. The main diffraction peaks of G-WO 3 appeared at 13.8 • , 22.6 • , 28.0 • , 36.3 • , and 49.7 • , demonstrating that the G-WO 3 was in the hexagonal crystalline phase (JCPDS No. 33-1387) [30]. No diffraction peaks for the CDs could be detected in the composites because of their low content. In addition, the XRD patterns of G-WO 3 and its composites showed no significant differences, indicating that the addition of CDs has almost no influence on the growth of G-WO 3 .
The surface chemical compositions of as-synthesized samples were further analyzed and confirmed using X-ray photoelectron spectroscopy (XPS). Compared with pure G-WO 3 , the co-existence of C, N, O, W, and Fe could be demonstrated in the full XPS spectrum of N,Fe-CDs/G-WO 3 -0.6 ( Figure S1). Two valence states of tungsten (W) in all three samples could be confirmed by the W 4f spectrum (Figure 4b).
The strong peaks at 38.12 eV for 4f 5/2 and 35.99 eV for 4f 7/2 were attributed to W 6+ , while the shoulder peaks at 37.37 eV for 4f 5/2 and 35.28 eV for 4f 7/2 were considered as W 5+ , demonstrating that G-WO 3 has abundant oxygen vacancies [35]. Compared with the pure G-WO 3 , the W 6+ and W 5+ peaks of the N-CDs/G-WO 3 -0.6 and N,Fe-CDs/G-WO 3 -0.6 composites all showed an obvious shift to lower binding energies. This suggested that the surface chemical environment of W in the WO 3 /CDs changed, which could be caused by the different chemical interactions between carbon dots and G-WO 3 [36]. For the O 1s spectra in Figure 4c, the strong peak of pure G-WO 3 at 530.61 eV was related to the binding energy of W-O and WO=, and G-WO 3 decorated with carbon dots exhibited shifts from 530.61 eV to 530.45 eV [37]. The peaks appearing at 532.16 eV and 532.98 eV corresponded to the oxygens of the hydroxyl groups or water bonded on the material's surface, while the new peak for N-CDs or N,Fe-CDs/G-WO 3 composites located at 531.95 eV was assigned to the C-O bond from CDs [21,38]. The C 1s spectra of G-WO 3 (Figure 4d) was resolved into two peaks at 284.80 eV and 286.56 eV, which could be attributed to the surface adsorbed carbon and the C-O bond, respectively [37]. Compared with the pure WO 3 (G), the C 1s peaks of N-CDs/G-WO 3 -0.6 or N,Fe-CDs/G-WO 3 -0.6 at about 286 eV and 288 eV were assigned to the binding energies of C-N and C=O from the CDs [29,39]. Besides, as shown in Figure 4e, compared to the pure G-WO 3 , the N 1s peaks at around 402 eV and 400 eV were obvious in the N-CDs/G-WO 3 -0.6 and N,Fe-CDs/G-WO 3 -0.6 composites, corresponding to the nitrogen atoms of N-H and C-N bonds, respectively [40]. The differences of C 1s and N 1s between N-CDs/G-WO 3 -0.6 and N,Fe-CDs/G-WO 3 -0.6 were attributed to the coordination of iron ions with the surface groups of N-CDs [26,27]. In the high-resolution Fe 2p spectrum (Figure 4f), there were two valence states of Fe, corresponding to Fe 2+ 2p 1/2 (721.29 eV), Fe 3+ 2p 1/2 (725.72 eV), Fe 2+ 2p 3/2 (708.61 eV), and Fe 3+ 2p 3/2 (713.86 eV), respectively. This indicated that iron in the CDs might act as a multi-electron transfer center [12,29].

The Optical Absorption and Photoluminescence Properties
The optical band gaps (Eg) of the pure WO3 and CDs/WO3 composites were calculated according to this equation: αhν = A(hν-Eg) n/2 , where α, h, and ν are the absorption coefficient, Planck constant, and light frequency, respectively. Here, n was equal to 1 for a direct-band-gap material [41]. The UV-vis-NIR diffuse reflectance spectra (DRS) are shown in Figure 5a. The G-WO3 exhibited a full spectrum absorption in the region from 200 to 1400 nm, which was primarily attributed to the oxygen vacancy [42]. Moreover, the special gear-shaped structure could enhance the UV-vis-NIR

The Optical Absorption and Photoluminescence Properties
The optical band gaps (E g ) of the pure WO 3 and CDs/WO 3 composites were calculated according to this equation: αhν = A(hν-E g ) n/2 , where α, h, and ν are the absorption coefficient, Planck constant, and light frequency, respectively. Here, n was equal to 1 for a direct-band-gap material [41]. The UV-vis-NIR diffuse reflectance spectra (DRS) are shown in Figure 5a. The G-WO 3 exhibited a full spectrum absorption in the region from 200 to 1400 nm, which was primarily attributed to the oxygen vacancy [42]. Moreover, the special gear-shaped structure could enhance the UV-vis-NIR light harnessing ability and photocatalytic activity due to the multiple reflections of the incident light during irradiation [43,44]. N-CDs/G-WO 3 -0.6, with an energy gap of 2.83 eV, performed a broader absorption than pure G-WO 3 , indicating that the decoration of N-CDs could improve the optical absorption capacity of WO 3 , which is in agreement with other reports [31,45]. Compared with N-CD decoration, N,Fe-CDs/G-WO 3 -0.6, with an energy gap of 2.50 eV, exhibited a narrower band gap and broader-solar-spectrum absorption in the UV-vis-NIR regions, which demonstrated that N,Fe-CD decorated G-WO 3 could use more solar energy and generate more e − /h + pairs under UV-vis-NIR light irradiation.
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 15 broader absorption than pure G-WO3, indicating that the decoration of N-CDs could improve the optical absorption capacity of WO3, which is in agreement with other reports [31,45]. Compared with N-CD decoration, N,Fe-CDs/G-WO3-0.6, with an energy gap of 2.50 eV, exhibited a narrower band gap and broader-solar-spectrum absorption in the UV-vis-NIR regions, which demonstrated that N,Fe-CD decorated G-WO3 could use more solar energy and generate more e -/h + pairs under UV-vis-NIR light irradiation. As shown in Figure 5b, the PL intensity of G-WO3 decreased significantly after decorating with CDs, especially with N,Fe-CDs, which indicated that the N,Fe-CDs on the G-WO3 surface acted as an electron acceptor and effectively inhibited the recombination of the photo-generated e-/h+ pairs [31].

Photocatalytic Performance
The photocatalytic performance of the WO3/CDs composites were evaluated via the photocatalytic degradation of RhB under UV-vis-NIR light irradiation. As shown in Figure 6a, under visible light irradiation, the G-WO3 with different amounts of N,Fe-CDs exhibited improved photocatalytic activity, and the RhB degradation rate initially increased and then decreased as the N,Fe-CDs amount was increased. The overloaded N,Fe-CDs increased the surface thickness, which hindered the transfer of photoinduced electrons, and also prevented the exposure of the active sites. The N,Fe-CDs/G-WO3-0.6 composite presented the maximum degradation efficiency of 97.1% under visible light irradiation for 2 h. In addition, the effects of different factors like catalyst dosage, RhB concentration, pH values, and ions on the photocatalytic degradation of RhB were also investigated. A catalyst dosage of 50 mg, RhB concentration of 10 mg/L, and pH of 3 were adopted as the optimal reaction conditions ( Figure S2a-f). As exhibited in Figure 6b, compared to that of pure G-WO3 and N-CDs/G-WO3-0.6, the photocatalytic activity of Fe-CDs/G-WO3-0.6 was significantly enhanced.
As illustrated in Figure S3, under visible light irradiation, the N,Fe-CDs/G-WO3-0.6 composite exhibited the best photocatalytic activity, and the RhB photodegradation kinetics can be well fitted by a pseudo-first-order kinetic model [46], where the RhB degradation rate constant over the N,Fe-CDs/G-WO3-0.6 was 1.72 h −1 , which is about 20 fold higher than for the commercial WO3. The photodegradation efficiencies of RhB under UV and NIR light irradiation for 2 h were 81.4% and 75.0%, respectively (Figure 6c,d). A stability test of N,Fe-CDs/G-WO3-0.6 was conducted under visible light irradiation (shown in Figure 7). After recycling five times, the photocatalytic activities of the material remained almost unchanged and the photodegradation efficiencies only decreased from 97.1% to 91.6% under visible light irradiation in 2 h, suggesting that the as-prepared composite was stable and reusable. As shown in Figure 5b, the PL intensity of G-WO 3 decreased significantly after decorating with CDs, especially with N,Fe-CDs, which indicated that the N,Fe-CDs on the G-WO 3 surface acted as an electron acceptor and effectively inhibited the recombination of the photo-generated e-/h+ pairs [31].

Photocatalytic Performance
The photocatalytic performance of the WO 3 /CDs composites were evaluated via the photocatalytic degradation of RhB under UV-vis-NIR light irradiation. As shown in Figure 6a, under visible light irradiation, the G-WO 3 with different amounts of N,Fe-CDs exhibited improved photocatalytic activity, and the RhB degradation rate initially increased and then decreased as the N,Fe-CDs amount was increased. The overloaded N,Fe-CDs increased the surface thickness, which hindered the transfer of photoinduced electrons, and also prevented the exposure of the active sites. The N,Fe-CDs/G-WO 3 -0.6 composite presented the maximum degradation efficiency of 97.1% under visible light irradiation for 2 h. In addition, the effects of different factors like catalyst dosage, RhB concentration, pH values, and ions on the photocatalytic degradation of RhB were also investigated. A catalyst dosage of 50 mg, RhB concentration of 10 mg/L, and pH of 3 were adopted as the optimal reaction conditions ( Figure  S2a-f). As exhibited in Figure 6b, compared to that of pure G-WO 3 and N-CDs/G-WO 3 -0.6, the photocatalytic activity of Fe-CDs/G-WO 3 -0.6 was significantly enhanced. and 0.0781 h −1 , respectively. This indicated that the percentages of inhibition of h + , ·OH, and ·O 2− , were 54.6%, 39.0%, and 28.5%, respectively, suggesting that ·O 2− , ·OH, and h + all participated in the reaction on different levels and that h + played a primary role in the reaction. The photogenerated h + could not only directly break down surface absorbed pollutants, but also convert H2O to ·OH for indirect degradation processes [49].  As illustrated in Figure S3, under visible light irradiation, the N,Fe-CDs/G-WO 3 -0.6 composite exhibited the best photocatalytic activity, and the RhB photodegradation kinetics can be well fitted by a pseudo-first-order kinetic model [46], where the RhB degradation rate constant over the N,Fe-CDs/G-WO 3 -0.6 was 1.72 h −1 , which is about 20 fold higher than for the commercial WO 3 . The photodegradation efficiencies of RhB under UV and NIR light irradiation for 2 h were 81.4% and 75.0%, respectively (Figure 6c,d). A stability test of N,Fe-CDs/G-WO 3 -0.6 was conducted under visible light irradiation (shown in Figure 7). After recycling five times, the photocatalytic activities of the material remained almost unchanged and the photodegradation efficiencies only decreased from 97.1% to 91.6% under visible light irradiation in 2 h, suggesting that the as-prepared composite was stable and reusable.
To investigate the universal application of N,Fe-CDs/G-WO 3 -0.6, four other organics, MB, CIP, TCH, and OTC, were selected as target contaminants. As shown in Figure 6e, the synthesized material exhibited good photodegradation efficiencies for MB, CIP, TCH, and OTC (91.1%, 70.5%, 54.5%, and 47.8% in 3 h, respectively. Apparently, the removal efficiencies for dyes (RhB and MB) were higher than those for colorless drugs, and one possible reason was the photosensitization effect of dye degradation. However, on the whole, the Fe-CDs/G-WO 3 -0.6 composite exhibited considerable activity for degrading different organic contaminants. The photogenerated ·OH, ·O 2− , and h + are three typical active species for the degradation of pollutants during the photocatalytic reactions [47,48]. To investigate the mechanisms of the photocatalytic process involving N,Fe-CDs/G-WO 3 -0.6, various scavengers were used to analyze the active species under visible light. In this experiment, in which isopropyl alcohol (IPA) was used as a ·OH scavenger to eliminate the ·OH, benzoquinone (BQ) and ammonium oxalate (AO) were applied to capture ·O 2− and h + , respectively [49,50]. As shown in Figure 6f and Figure S4, when IPA and BQ were added, the additions of IPA, BQ, and AO all provoked the partial inhibition of RhB photodegradation activity, and the degradation rate constants decreased from 1.72 h −1 to 1.23, 1.05, and 0.0781 h −1 , respectively. This indicated that the percentages of inhibition of h + , ·OH, and ·O 2− , were 54.6%, 39.0%, and 28.5%, respectively, suggesting that ·O 2− , ·OH, and h + all participated in the reaction on different levels and that h + played a primary role in the reaction. The photogenerated h + could not only directly break down surface absorbed pollutants, but also convert H 2 O to ·OH for indirect degradation processes [49].

Possible Mechanism of Photocatalysis
Based on the results of the active species trapping experiments, a possible photocatalytic mechanism for the N,Fe-CDs/G-WO3-0.6 composite is proposed in Figure 8. Under solar irradiation, both WO3 and N,Fe-CDs are excited and photogenerated electrons transition from the valence band (VB) to the conduction band (CB). Then, the electrons on the CB of WO3 transfer to the N,Fe-CDs and couple with holes on the VB of N,Fe-CDs via the hetero-interface, leaving the holes to directly degrade pollutants into CO2 and H2O or convert H2O to ·OH for indirect degradation processes, which significantly facilitates the separation of e -/h + pairs [22,31]. Finally, the photogenerated electrons from N,Fe-CDs can directly transfer from the VB to the Fe (III) as active sites for multi-electron reactions, which efficiently accelerates the transmission between WO3 and N,Fe-CDs. As eare transferred to Fe (III), Fe (III) can convert to Fe (II), while Fe (II) can easily revert to Fe (III) with the reduction of O2 to ·O 2− [12,51]. Thus, the N,Fe-CDs can not only act as oxygen reduction sites but also significantly inhibit electron-hole recombination.

Possible Mechanism of Photocatalysis
Based on the results of the active species trapping experiments, a possible photocatalytic mechanism for the N,Fe-CDs/G-WO 3 -0.6 composite is proposed in Figure 8. Under solar irradiation, both WO 3 and N,Fe-CDs are excited and photogenerated electrons transition from the valence band (VB) to the conduction band (CB). Then, the electrons on the CB of WO 3 transfer to the N,Fe-CDs and couple with holes on the VB of N,Fe-CDs via the hetero-interface, leaving the holes to directly degrade pollutants into CO 2 and H 2 O or convert H 2 O to ·OH for indirect degradation processes, which significantly facilitates the separation of e − /h + pairs [22,31]. Finally, the photogenerated electrons from N,Fe-CDs can directly transfer from the VB to the Fe (III) as active sites for multi-electron reactions, which efficiently accelerates the transmission between WO 3 and N,Fe-CDs. As e − are transferred to Fe (III), Fe (III) can convert to Fe (II), while Fe (II) can easily revert to Fe (III) with the reduction of O 2 to ·O 2− [12,51]. Thus, the N,Fe-CDs can not only act as oxygen reduction sites but also significantly inhibit electron-hole recombination. electrons from N,Fe-CDs can directly transfer from the VB to the Fe (III) as active sites for multi-electron reactions, which efficiently accelerates the transmission between WO3 and N,Fe-CDs. As eare transferred to Fe (III), Fe (III) can convert to Fe (II), while Fe (II) can easily revert to Fe (III) with the reduction of O2 to ·O 2− [12,51]. Thus, the N,Fe-CDs can not only act as oxygen reduction sites but also significantly inhibit electron-hole recombination.

Preparation of Gear-Shaped WO 3
WO 3 gears, abbreviated to G-WO 3 , were prepared by using a simple hydrothermal process. Typically, 1.0 g Na 2 WO 4 ·2H 2 O was dissolved in 80 mL of deionized water with ultrasound for 20 min. After complete dissolution, the pH values were adjusted to 2 using concentrated hydrochloric acid (HCl, 36%) with continuous stirring. Then, the solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave which was maintained at 180 • C for 12 h. After cooling down to room temperature, the precipitates were washed two times with deionized water and ethanol and then dried in an oven at 80 • C for 6 h.

Preparation of N-CDs and N,Fe-CDs
N-CDs were synthesized by a one-step hydrothermal method. In a typical experiment, 0.3 g Folic acid (FA)was dissolved in 30 mL deionized water under vigorous stirring for 20 min. The solution was then transferred to a 100 mL Teflon-lined stainless-steel autoclave, which was kept at 200 • C for 4 h.
N,Fe-CDs were prepared as follows: 0.3 g FA and 0.03 g FeCl 3 ·6H 2 O were mixed into 30 mL of deionized water. The mixed solution was continuously stirred for 20 min and then transferred into a 100 mL Teflon-lined autoclave, which was heated at 200 • C for 4 h.
After cooling down to room temperature, the solutions above were filtered with 0.22 µm aqueous microporous membranes and purified with dialysis membranes (MWCO of 1000) for 6 h. Finally, the solutions were freeze-dried to get powder for the further applications.

Preparation of N-CDs/WO 3 and N,Fe-CDs/WO 3 Composites
The composites were prepared via a hydrothermal method. Firstly, 0.1 g WO 3 was dispersed in the two kinds of CD solution (20 mL) respectively. The different concentrations of CD solution used were 0.2, 0.4, 0.6, 0.8, and 1.0 g/L, respectively. After sonication for 10 min, the suspensions were transferred to a 50 mL Teflon-lined autoclave, which was maintained at 190 • C for 8 h. Finally, the composites were obtained by centrifugation to remove supernatants and dried at 80 • C. A series of N-CDs/WO 3 or N,Fe-CDs/WO 3 composites were obtained by adding different concentrations of CDs, which were denoted as N-CDs/WO 3 -X (X = 0.2-1.0) or N,Fe-CDs/WO 3 -X (X = 0.2-1.0), respectively. In this paper, we used N,Fe-CDs/WO 3 (G)-0.6, due to it having the highest photocatalytic activity.

Photodegradation Experiments
The photocatalytic reactions were processed on a photoreactor (XPA-7, Nanjing XUJ Co. ltd., Nanjing, China) equipped with a 500 W gold halide lamp (coupled with a 420 nm cut-off filter) as the visible light source; a 300 W high pressure mercury lamp as the UV light source, with a 365 nm filter; and a 500 W xenon lamp as the NIR light source, with a 780 nm cut-off filter. The photocatalytic degradations of RhB, MB, TCH, CIP, and OTC were conducted to evaluate the activities of the synthesized photocatalysts. The temperature of the aqueous phase was accurately controlled at 25 ± 1 • C by the circulating cooling water system. Typically, 50 mg of photocatalysts were dispersed into 50 mL of RhB aqueous solution (10 mg/L) in a quartz tube, with pH values of about 3, and stirred in the dark for 30 min to ensure that adsorption/desorption equilibrium was reached. Then, 4 mL of the suspension were taken out every half hour, and 1 mL of sodium thiosulfate solution (1.24 g/mL) was added as a quenching agent, which was filtered using 0.22 µm aqueous microporous membranes. The concentration of RhB was measured using the UV-vis spectrophotometer (Shimadzu, UV-2600), according to its absorbance at 554 nm. The contents of MB (10 mg/L), TCH (20 mg/L), CIP (20 mg/L), and OTC (20 mg/L) were determined by the same method except for measuring the absorbances at 664, 354, 277, and 353 nm, respectively.

Conclusions
In summary, we successfully synthesized the N,Fe-CDs/G-WO 3 composite via a facile hydrothermal method for the first time. The N,Fe-CDs/G-WO 3 composite showed full spectrum absorption and photocatalytic activity superior to that of commercial WO 3 , pure G-WO 3 , and N-CDs/G-WO 3 composites, and could efficiently degrade RhB, MB, CIP, TCH, and OTC under visible light irradiation. The radical trapping experiments indicated that the conversion between Fe (III) and Fe (II) played a key role in the photocatalytic reactions. The enhanced photocatalytic activity was mainly attributed to an improved light harvesting capability due to the modification of N,Fe-CDs and the special gear-shaped structure of WO 3 , which caused multiple reflections of light. This work provides new insights for the efficient utilization of solar energy for environmental remediation.  Table S1: Molecular formulas and structures of several organic pollutants studied in this wok.