Construction of a CQDs/Ag3PO4/BiPO4 Heterostructure Photocatalyst with Enhanced Photocatalytic Degradation of Rhodamine B under Simulated Solar Irradiation

A carbon quantum dot (CQDs)/Ag3PO4/BiPO4 heterostructure photocatalyst was constructed by a simple hydrothermal synthesis method. The as-prepared CQDs/Ag3PO4/BiPO4 photocatalyst has been characterized in detail by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet–visible spectroscopy, and photoelectrochemical measurements. It is demonstrated that the CQDs/Ag3PO4/BiPO4 composite is constructed by assembling Ag3PO4 fine particles and CQDs on the surface of rice-like BiPO4 granules. The CQDs/Ag3PO4/BiPO4 heterostructure photocatalyst exhibits a higher photocatalytic activity for the degradation of the rhodamine B dye than that of Ag3PO4, BiPO4, and Ag3PO4/BiPO4. The synergistic effects of light absorption capacity, band edge position, separation, and utilization efficiency of photogenerated carriers play the key role for the enhanced photodegradation of the rhodamine B dye.

Bismuth phosphate (BiPO 4 ) as a photocatalyst has been widely studied because of its good photoelectric performance, low cost, low toxicity, excellent photocatalytic activity, and high

Synthesis of CQDs/Ag3PO4/BiPO4 Photocatalyst
To obtain the CQDs/Ag3PO4/BiPO4 photocatalyst, stoichiometric amounts of Bi(NO3)3·5H2O, AgNO3, NaH2PO4 and NH3·H2O and 6 mL of the CQDs suspension derived according the literature [45] were successively 20 mL of dilute HNO3 solution, and then filled up to 60 mL by adding distilled water. The subsequent preparation process is consistent with Section 2.1. The flow-chart for the synthesis of the CQDs/Ag3PO4/BiPO4 photocatalyst is shown in Figure 1(IV).

Sample Characterization
The phase purity of the Ag3PO4, BiPO4, Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 photocatalysts was analyzed by means of D8 advanced X-ray diffractometer with Cu Kα radiation at a wavelength of 1.5406 Å. The surface morphology of the samples was characterized by JSM-6701F field-emission scanning electron microscopy (SEM, JEOL Ltd., Tokyo, Japan) and JEM-1200EX field-emission transmission electron microscopy (TEM, JEOL Ltd., Tokyo, Japan). Ultraviolet-visible (UV-VIS) diffuse reflectance spectra of the samples were examined on a UV-VIS spectrophotometer with an integrating sphere attachment using BaSO4 as the reference. To determine the bonding states, chemical composition, and electron levels of the samples, X-ray photoelectron spectroscopy (XPS) measurements were carried out by using a PHI-5702 X-ray photoelectron spectrometer (Physical Electronics, Hanhassen, MN, USA).
The electrochemical properties of the samples were investigated according to the method reported in the literature [45]. A CST 350 electrochemical workstation (Wuhan Corrtest Instruments Co. Ltd., Wuhan, China) equipped with a three-electrode cell configuration was used to study the electrochemical impedance spectroscopy (EIS) and photocurrent response of the samples. The working electrode was prepared as follows: 15 mg of the photocatalyst, 0.75 mg of polyvinylidene

Synthesis of Ag 3 PO 4 /BiPO 4 Photocatalyst
To prepare Ag 3 PO 4 /BiPO 4 photocatalyst, stoichiometric amounts of Bi(NO 3 ) 3 ·5H 2 O, AgNO 3 , NaH 2 PO 4 and NH 3 ·H 2 O (n Ag3PO4 :n BiPO4 = 1:0.11) were successively added in 20 mL of dilute HNO 3 solution, and then filled up to 60 mL by adding distilled water. The assembly of Ag 3 PO 4 on BiPO 4 followed the procedure as described in Section 2.1. The flow-chart for the synthesis of the Ag 3 PO 4 /BiPO 4 photocatalyst is schematically shown in Figure 1(III).

Synthesis of CQDs/Ag 3 PO 4 /BiPO 4 Photocatalyst
To obtain the CQDs/Ag 3 PO 4 /BiPO 4 photocatalyst, stoichiometric amounts of Bi(NO 3 ) 3 ·5H 2 O, AgNO 3 , NaH 2 PO 4 and NH 3 ·H 2 O and 6 mL of the CQDs suspension derived according the literature [45] were successively 20 mL of dilute HNO 3 solution, and then filled up to 60 mL by adding distilled water. The subsequent preparation process is consistent with Section 2.1. The flow-chart for the synthesis of the CQDs/Ag 3 PO 4 /BiPO 4 photocatalyst is shown in Figure 1(IV).

Sample Characterization
The phase purity of the Ag 3 PO 4 , BiPO 4 , Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 photocatalysts was analyzed by means of D8 advanced X-ray diffractometer with Cu Kα radiation at a wavelength of 1.5406 Å. The surface morphology of the samples was characterized by JSM-6701F field-emission scanning electron microscopy (SEM, JEOL Ltd., Tokyo, Japan) and JEM-1200EX field-emission transmission electron microscopy (TEM, JEOL Ltd., Tokyo, Japan). Ultraviolet-visible (UV-VIS) diffuse reflectance spectra of the samples were examined on a UV-VIS spectrophotometer with an integrating sphere attachment using BaSO 4 as the reference. To determine the bonding states, chemical composition, and electron levels of the samples, X-ray photoelectron spectroscopy (XPS) measurements were carried out by using a PHI-5702 X-ray photoelectron spectrometer (Physical Electronics, Hanhassen, MN, USA).
The electrochemical properties of the samples were investigated according to the method reported in the literature [45]. A CST 350 electrochemical workstation (Wuhan Corrtest Instruments Co., Ltd., Wuhan, China) equipped with a three-electrode cell configuration was used to study the electrochemical impedance spectroscopy (EIS) and photocurrent response of the samples. The working electrode was prepared as follows: 15 mg of the photocatalyst, 0.75 mg of polyvinylidene fluoride (PVDF), 0.75 mg of carbon black and 1 mL of 1-methyl-2-pyrrolidione (NMP) were mixed together to form uniform slurry. The slurry mixture was homogeneously coated on the surface of fluorine-doped tin oxide (FTO) thin film (effective area: 1 × 1 cm 2 ), and subjected to drying 60 • C for 5 h. The used electrolyte was 0.1 mol L −1 Na 2 SO 4 aqueous solution. The used light source was a 200 W xenon lamp emitting simulated sunlight. A 0.2 V bias voltage was used during the transient photocurrent measurement. The sinusoidal voltage pulse was used for the EIS measurement (amplitude: 5 mV; frequency range: 10 −2 -10 5 Hz).

Photocatalytic Testing
The photocatalytic activities of the samples were investigated by removing RhB from aqueous solution according to the procedure as described in the literature [45]. A 200-W xenon lamp (sunlight simulator) was used as the light source. The photocatalytic system was composed of 0.1 g photocatalyst and 100 mL RhB solution (C photocatalyst = 1 g L −1 , C RhB = 5 mg L −1 ). Based on the initial RhB concentration (C 0 ) and residual RhB concentration (C t ), the degradation percentage (DP) of RhB was given as: DP = (C 0 − C t )/C 0 × 100%.  4 and BiPO 4 samples, the XRD curves were fitted using the Jade 6.0 package. The black curves, red curves, vertical olive lines, and blue lines represent the observed XRD peaks, theoretically estimated curves, Bragg peaks, and difference between the observed values, and theoretically estimated values of XRD diffraction peaks, respectively. The result indicates that the theoretically simulated values are in good agreement with the observed XRD diffraction peaks. The XRD diffraction peaks of Ag 3 PO 4 and BiPO 4 can be ascribed to JCPDF#06-0505 and JCPDF#15-0767, respectively. Figure 2c shows the XRD patterns of Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 . The main XRD diffraction peaks of the Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 composites are similar to those of pure Ag 3 PO 4 , indicating that the host lattice of Ag 3 PO 4 in these composites undergoes no change. In addition to the XRD characteristic peaks of the Ag 3 PO 4 phase, the XRD characteristic peaks of BiPO 4 are also observed in these composites. For the CQDs/Ag 3 PO 4 /BiPO 4 composite, the intensity of the diffraction peaks is sharper than that for Ag 3 PO 4 /BiPO 4 . The structure analysis shows that the introduction of CQDs in the Ag 3 PO 4 /BiPO 4 composites obviously accelerate the formation of Ag 3 PO 4 and BiPO 4 . In our previous study, the carbon can suppress the formation of M-ferrite [47] and α-Al 2 O 3 [48] phase prepared by a polyacrylamide gel method. In this case, this phenomenon may be due to the fact that CQDs do not react with oxygen in the reactor to form carbon dioxide. Figure 2d,e show the crystal structures of BiPO 4 and Ag 3 PO 4 , respectively. The BiPO 4 and Ag 3 PO 4 are monoclinic phase with space group P21/n (14) and cubic phase with space group P-43n (218), respectively. For the BiPO 4 , the Bi atom and the P atom are surrounded by eight oxygen atoms and four oxygen atoms, respectively. The wide Bi-O and P-O bond length of BiPO 4 exhibits a high photocatalytic activity for photocatalytic degradation of organic pollutants [49]. For the Ag 3 PO 4 , the Ag atom, P atom and O atom experience four-fold coordination by four O atoms, four-fold coordination by four O atoms, and 4-fold coordination by one P atom and three Ag atoms, respectively [50].  Figure 3a,b show the SEM images of Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4, respectively. For the Ag3PO4/BiPO4 composite, the sample is composed of fine spherical particles and rice-like granules, as shown in Figure 3a. Figure 3b represents the SEM image of the CQDs/Ag3PO4/BiPO4 composite, revealing that its morphology is very similar to that of the Ag3PO4/BiPO4 composite. The insets in Figure 3a,b show the real photos of Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4, respectively. The results show that the introduction of CQDs to the Ag3PO4/BiPO4 composite deepens the color of the sample. The detailed analysis will be done in the optical properties section. Figure 3c shows the SEM image of pure CQDs, from which it is seen that the prepared CQDs have a narrow size distribution of 7-10 nm.   Figure 3b represents the SEM image of the CQDs/Ag 3 PO 4 /BiPO 4 composite, revealing that its morphology is very similar to that of the Ag 3 PO 4 /BiPO 4 composite. The insets in Figure 3a,b show the real photos of Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 , respectively. The results show that the introduction of CQDs to the Ag 3 PO 4 /BiPO 4 composite deepens the color of the sample. The detailed analysis will be done in the optical properties section. Figure 3c shows the SEM image of pure CQDs, from which it is seen that the prepared CQDs have a narrow size distribution of 7-10 nm.

Surface Morphology and Elemental Component Analysis
The microstructure and elemental composition of the CQDs/Ag 3 PO 4 /BiPO 4 composite was characterized by TEM, as shown in Figure 4. Figure 4a displays the TEM image of the composite. Spherical fine particles (Ag 3 PO 4 ) are seen to be assembled on the surface of rice-like granules (BiPO 4 ). The high-resolution TEM (HRTEM) image further confirms the assembly of Ag 3 PO 4 fine particles on the surface of BiPO 4 rice-like granules, as depicted in Figure 4b. The rice-like granules manifest obvious lattice fringes with an interlayer spacing of 0.347 nm, which correspond to the (222) facet of the cubic Ag 3 PO 4 phase. The attached spherical particles exhibit the lattice fringes with a d-spacing of 0.407 nm, which correspond to the (101) facet of the monoclinic Ag 3 PO 4 phase. The decorated ultrafine particles with no lattice fringes could be CQDs. The energy-dispersive X-ray spectroscopy (EDS) spectrum ( Figure 4c) demonstrates that the elemental composition of the CQDs/Ag 3 PO 4 /BiPO 4 composite is Ag, Bi, P, O, and C. Additional Cu signal observed on the EDS spectrum can be ascribed to the TEM microgrid holder [51]. To further elucidate the spatial distribution of elements, Figure 4b  The microstructure and elemental composition of the CQDs/Ag3PO4/BiPO4 composite was characterized by TEM, as shown in Figure 4. Figure 4a displays the TEM image of the composite. Spherical fine particles (Ag3PO4) are seen to be assembled on the surface of rice-like granules (BiPO4). The high-resolution TEM (HRTEM) image further confirms the assembly of Ag3PO4 fine particles on the surface of BiPO4 rice-like granules, as depicted in Figure 4b. The rice-like granules manifest obvious lattice fringes with an interlayer spacing of 0.347 nm, which correspond to the (222) facet of the cubic Ag3PO4 phase. The attached spherical particles exhibit the lattice fringes with a d-spacing of 0.407 nm, which correspond to the (101) facet of the monoclinic Ag3PO4 phase. The decorated ultrafine particles with no lattice fringes could be CQDs. The energy-dispersive X-ray spectroscopy (EDS) spectrum (Figure 4c) demonstrates that the elemental composition of the CQDs/Ag3PO4/BiPO4 composite is Ag, Bi, P, O, and C. Additional Cu signal observed on the EDS spectrum can be ascribed to the TEM microgrid holder [51]. To further elucidate the spatial distribution of elements, Figure 4b shows the dark-field scanning TEM (DF-STEM) image of the CQDs/Ag3PO4/BiPO4 composite and Figure 4e-i display the corresponding elemental maps. Ag, P, O, Bi, and C elementals are homogenously distributed throughout the rice-like granules, implying the uniform decoration of Ag3PO4 nanoparticles and CQDs on the surface of rice-like BiPO4 granules. The observed C element in the blank area without the sample could come from the TEM microgrid holder.

XPS Analysis
To understand the chemical composition and electronic core levels of the Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 composites, Figure 5 shows the XPS results of the two composites. In Figure 5a, the XPS survey scan spectra for the Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 composites clearly contain the P, Bi, Ag, O, and C elements. The electronic core levels of Bi 4f, P 2p, Ag 3d, O 1s, and C 1s in the composites are further characterized using the high-resolution XPS spectra. Figure 5b shows the Bi 4f core-level XPS spectra. Two obvious characteristic peaks at 161.02/160.29 and 166.26/165.63 eV are observed on the spectra, which are assigned to Bi 4f 7/2 and Bi 4f 5/2 binding energies of Bi 3+ in BiPO 4 , respectively [52].

XPS Analysis
To understand the chemical composition and electronic core levels of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites, Figure 5 shows the XPS results of the two composites. In Figure 5a, the XPS survey scan spectra for the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites clearly contain the P, Bi, Ag, O, and C elements. The electronic core levels of Bi 4f, P 2p, Ag 3d, O 1s, and C 1s in the composites are further characterized using the high-resolution XPS spectra. Figure 5b shows the Bi 4f core-level XPS spectra. Two obvious characteristic peaks at 161.02/160.29 and 166.26/165.63 eV are observed on the spectra, which are assigned to Bi 4f7/2 and Bi 4f5/2 binding energies of Bi 3+ in BiPO4, respectively [52].
The XPS spectra of P 2p core level shown in Figure 5c present a broad peak at 134.36 (or 133.69) eV, suggesting that P species exhibits +5 oxidation state [52]. Figure 5d shows the Ag 3d core level XPS spectra. The peaks at 369.59/368.86 and 375.58/374.79 eV can be assigned to Ag 3d5/2 and Ag 3d3/2 of Ag3PO4, respectively [53]. For the O 1s core-level XPS spectra, the peak at 531.63/532.28 eV can be ascribed to the lattice oxygen, while the peak at 532.92/533.53 eV is related to the adsorbed oxygen [54,55], as shown in Figure 5e. The C 1s core-level XPS spectra are shown in Figure 5f. For the Ag3PO4/BiPO4 composite, the peak at 284.77 eV can be assigned to the adventitious hydrocarbon for the XPS instruments [56]. For the CQDs/Ag3PO4/BiPO4 composite, the C 1s peak can be divided in to three separate peaks at 283.75, 284.77 and 286.38 eV, corresponding to CQDs [57], adventitious hydrocarbon [56] and impurity structure of carbon [58]. It is noted that the electronic core levels of Bi 4f, P 2p, Ag 3d and O 1s for the CQDs/Ag3PO4/BiPO4 composite are smaller (about 0.61-0.73 eV) than The XPS spectra of P 2p core level shown in Figure 5c present a broad peak at 134.36 (or 133.69) eV, suggesting that P species exhibits +5 oxidation state [52]. Figure 5d shows the Ag 3d core level XPS spectra. The peaks at 369.59/368.86 and 375.58/374.79 eV can be assigned to Ag 3d 5/2 and Ag 3d 3/2 of Ag 3 PO 4 , respectively [53]. For the O 1s core-level XPS spectra, the peak at 531.63/532.28 eV can be ascribed to the lattice oxygen, while the peak at 532.92/533.53 eV is related to the adsorbed oxygen [54,55], as shown in Figure 5e. The C 1s core-level XPS spectra are shown in Figure 5f. For the Ag 3 PO 4 /BiPO 4 composite, the peak at 284.77 eV can be assigned to the adventitious hydrocarbon for the XPS instruments [56]. For the CQDs/Ag 3 PO 4 /BiPO 4 composite, the C 1s peak can be divided in to three separate peaks at 283.75, 284.77 and 286.38 eV, corresponding to CQDs [57], adventitious hydrocarbon [56] and impurity structure of carbon [58]. It is noted that the electronic core levels of Bi 4f, P 2p, Ag 3d and O 1s for the CQDs/Ag 3 PO 4 /BiPO 4 composite are smaller (about 0.61-0.73 eV) than those for the Ag 3 PO 4 /BiPO 4 composite, which could be due to the fact that the CQDs facilitate the formation of CQDs/Ag 3 PO 4 /BiPO 4 heterostructures. those for the Ag3PO4/BiPO4 composite, which could be due to the fact that the CQDs facilitate the formation of CQDs/Ag3PO4/BiPO4 heterostructures.

Optical Properties
It is noted that the optical properties of semiconductors have an important effect on their photocatalytic performances, which can be determined by UV-vis DRS measurements [59]. Figure  6a shows the UV-VIS diffuse reflectance spectra of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 photocatalysts. For both the samples, the reflectance first increases and then decreases with the increase in the wavelength, and finally increases again. The two samples present higher reflectance in the wavelength range from 550 to 850 nm. When CQDs are introduced to Ag3PO4/BiPO4, a decrease in the reflectance of the resultant CQDs/Ag3PO4/BiPO4 composite in the wavelength range from 300 to 850 nm is observed. According to the literatures [60], the color parameters (L * , a * , b * ), chroma parameter (c * ), hue angle (H o ), and total color difference (ECIE * ) of Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 are evaluated, as shown in Table 1. The Ag3PO4/BiPO4 composite shows a negative a * value, indicating that it displays a reseda, as shown in the inset of Figure 3a. The CQDs/Ag3PO4/BiPO4 composite exhibits the smaller L * and b * values and positive a * value, which means it exhibits yellowish black, as shown in the inset of Figure 3b. The first derivative curves of UV-vis diffuse reflectance spectra are useful to determine the optical bandgaps (Eg) of

Optical Properties
It is noted that the optical properties of semiconductors have an important effect on their photocatalytic performances, which can be determined by UV-vis DRS measurements [59]. Figure 6a shows the UV-VIS diffuse reflectance spectra of the Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 photocatalysts. For both the samples, the reflectance first increases and then decreases with the increase in the wavelength, and finally increases again. The two samples present higher reflectance in the wavelength range from 550 to 850 nm. When CQDs are introduced to Ag 3 PO 4 /BiPO 4 , a decrease in the reflectance of the resultant CQDs/Ag 3 PO 4 /BiPO 4 composite in the wavelength range from 300 to 850 nm is observed. According to the literatures [60], the color parameters (L*, a*, b*), chroma parameter (c*), hue angle (H o ), and total color difference (E CIE *) of Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 are evaluated, as shown in Table 1. The Ag 3 PO 4 /BiPO 4 composite shows a negative a* value, indicating that it displays a reseda, as shown in the inset of Figure 3a. The CQDs/Ag 3 PO 4 /BiPO 4 composite exhibits the smaller L* and b* values and positive a* value, which means it exhibits yellowish black, as shown in the inset of Figure 3b. The first derivative curves of UV-vis diffuse reflectance spectra are useful to determine the optical bandgaps (E g ) of semiconductors [61]. As shown in Figure 6b, the Ag 3 PO 4 /BiPO 4 composite shows two absorption edges at 276.1 and 502.3 nm, whereas the CQDs/Ag 3 PO 4 /BiPO 4 composite exhibits an absorption edge at 271.9 nm. The absorption edges at 276.1/271.9 and 502.3 nm can be assigned to BiPO 4 and Ag 3 PO 4 , respectively. The disappearance of the Ag 3 PO 4 absorption peak on the spectrum of the CQDs/Ag 3 PO 4 /BiPO 4 composite is ascribed to the enhanced optical absorption caused by CQDs. The E g values of Ag 3 PO 4 and BiPO 4 in the samples (see Table 1) can be derived on the basis of Equation (1): where λ 0 , h, and c is the maximum absorption wavelength, Plank constant, and velocity of light, respectively.
Micromachines 2019, 10, x FOR PEER REVIEW 9 of 17 semiconductors [61]. As shown in Figure 6b, the Ag3PO4/BiPO4 composite shows two absorption edges at 276.1 and 502.3 nm, whereas the CQDs/Ag3PO4/BiPO4 composite exhibits an absorption edge at 271.9 nm. The absorption edges at 276.1/271.9 and 502.3 nm can be assigned to BiPO4 and Ag3PO4, respectively. The disappearance of the Ag3PO4 absorption peak on the spectrum of the CQDs/Ag3PO4/BiPO4 composite is ascribed to the enhanced optical absorption caused by CQDs. The Eg values of Ag3PO4 and BiPO4 in the samples (see Table 1) can be derived on the basis of Equation (1): where λ0, h, and c is the maximum absorption wavelength, Plank constant, and velocity of light, respectively.   Figure 7a shows the EIS spectra of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites. For the two samples, the EIS spectra show a semicircle and a straight line, which can be ascribed to the charge transfer and the Warburg impedance, respectively [62,63]. The CQDs/Ag3PO4/BiPO4 photocatalyst has a smaller semicircle than that for the Ag3PO4/BiPO4 photocatalyst, which means the former exhibits a higher photocatalytic activity. Photocurrent response curves can also be used to predict the photocatalytic activity of semiconductor materials [64]. Figure 7b shows the photocurrent response curves of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 photocatalysts. The photocurrent response of Ag3PO4/BiPO4 can be attributed to the electron transfer between Ag3PO4 and BiPO4. The CQDs/Ag3PO4/BiPO4 photocatalyst exhibits a higher photocurrent intensity than that of Ag3PO4/BiPO4, indicating that it possesses a higher photocatalytic activity because of its higher electron transfer and separation efficiency.   Figure 7a shows the EIS spectra of the Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 composites. For the two samples, the EIS spectra show a semicircle and a straight line, which can be ascribed to the charge transfer and the Warburg impedance, respectively [62,63]. The CQDs/Ag 3 PO 4 /BiPO 4 photocatalyst has a smaller semicircle than that for the Ag 3 PO 4 /BiPO 4 photocatalyst, which means the former exhibits a higher photocatalytic activity. Photocurrent response curves can also be used to predict the photocatalytic activity of semiconductor materials [64]. Figure 7b shows the photocurrent response curves of the Ag 3 PO 4 /BiPO 4 and CQDs/Ag 3 PO 4 /BiPO 4 photocatalysts. The photocurrent response of Ag 3 PO 4 /BiPO 4 can be attributed to the electron transfer between Ag 3 PO 4 and BiPO 4 . The CQDs/Ag 3 PO 4 /BiPO 4 photocatalyst exhibits a higher photocurrent intensity than that of Ag 3 PO 4 /BiPO 4 , indicating that it possesses a higher photocatalytic activity because of its higher electron transfer and separation efficiency.

Photocatalytic Activity
To study the photocatalytic activity of the BiPO4, Ag3PO4, Ag3PO4/BiPO4, and CQDs/Ag3PO4/BiPO4 photocatalysts, RhB dye was used as a degradation dye. Figure 8a shows the time-dependent photodegradation of RhB in the presence of the samples under simulated sunlight irradiation. Based on the blank experiment, the RhB dye exhibits a high stability and is non-biodegradable at ambient conditions. The dye degradation rate over the samples increases with increasing the irradiation time. The photocatalytic activity of these photocatalysts follows the order: CQDs/Ag3PO4/BiPO4 > Ag3PO4/BiPO4 > Ag3PO4 > BiPO4. The result indicates that the CQDs/Ag3PO4/BiPO4 composite has the highest photocatalytic activity. It should be noted out that photosensitized degradation of RhB could occur in the present photocatalytic system. However, the photosensitization effect is not the dominant degradation mechanism since Ag3PO4 based composite photocatalysts have also been demonstrated to exhibit pronounced degradation of colorless phenol [65].
where C0, Ct, k, and t is the initial concentration of RhB, apparent concentration of RhB after degradation, kinetic rate constant, and irradiation time, respectively. Figure 8b shows the plots of Ln(Ct/C0) vs t. The rate constant (k) for the photocatalysts is found to be kBiPO4 = 0.00261, kAg3PO4 =

Photocatalytic Activity
To study the photocatalytic activity of the BiPO 4 , Ag 3 PO 4 , Ag 3 PO 4 /BiPO 4 , and CQDs/Ag 3 PO 4 / BiPO 4 photocatalysts, RhB dye was used as a degradation dye. Figure 8a shows the time-dependent photodegradation of RhB in the presence of the samples under simulated sunlight irradiation. Based on the blank experiment, the RhB dye exhibits a high stability and is non-biodegradable at ambient conditions. The dye degradation rate over the samples increases with increasing the irradiation time. The photocatalytic activity of these photocatalysts follows the order: CQDs/Ag 3 PO 4 /BiPO 4 > Ag 3 PO 4 /BiPO 4 > Ag 3 PO 4 > BiPO 4 . The result indicates that the CQDs/Ag 3 PO 4 /BiPO 4 composite has the highest photocatalytic activity. It should be noted out that photosensitized degradation of RhB could occur in the present photocatalytic system. However, the photosensitization effect is not the dominant degradation mechanism since Ag 3 PO 4 based composite photocatalysts have also been demonstrated to exhibit pronounced degradation of colorless phenol [65].

Photocatalytic Activity
To study the photocatalytic activity of the BiPO4, Ag3PO4, Ag3PO4/BiPO4, and CQDs/Ag3PO4/BiPO4 photocatalysts, RhB dye was used as a degradation dye. Figure 8a shows the time-dependent photodegradation of RhB in the presence of the samples under simulated sunlight irradiation. Based on the blank experiment, the RhB dye exhibits a high stability and is non-biodegradable at ambient conditions. The dye degradation rate over the samples increases with increasing the irradiation time. The photocatalytic activity of these photocatalysts follows the order: CQDs/Ag3PO4/BiPO4 > Ag3PO4/BiPO4 > Ag3PO4 > BiPO4. The result indicates that the CQDs/Ag3PO4/BiPO4 composite has the highest photocatalytic activity. It should be noted out that photosensitized degradation of RhB could occur in the present photocatalytic system. However, the photosensitization effect is not the dominant degradation mechanism since Ag3PO4 based composite photocatalysts have also been demonstrated to exhibit pronounced degradation of colorless phenol [65].
where C0, Ct, k, and t is the initial concentration of RhB, apparent concentration of RhB after degradation, kinetic rate constant, and irradiation time, respectively. Figure 8b shows the plots of Ln(Ct/C0) vs t. The rate constant (k) for the photocatalysts is found to be kBiPO4 = 0.00261, kAg3PO4 = The first order kinetic rate of the dye degradation photocatalyzed by the samples can be evaluated by Equation (2) [66]: where C 0 , C t , k, and t is the initial concentration of RhB, apparent concentration of RhB after degradation, kinetic rate constant, and irradiation time, respectively. Figure 8b shows the plots of Ln(C t /C 0 ) vs. t. The rate constant (k) for the photocatalysts is found to be k BiPO4 = 0.00261, k Ag3PO4 =  The stability and reusability of the CQDs/Ag 3 PO 4 /BiPO 4 photocatalyst was performed by repeating the experiments for the degradation of the RhB dye under simulated sunlight irradiation, as shown in Figure 9. It is seen that, after five cycles, no obvious decrease in the dye degradation is observed, which indicates that the CQDs/Ag 3 PO 4 /BiPO 4 photocatalyst has a high stability and maintains a high photocatalytic activity for the degradation of RhB.  Figure 10a schematically shows the assembly structure of the CQDs/Ag3PO4/BiPO4 composite with Ag3PO4 fine particles and CQDs homogenously decorated on the surface of rice-like BiPO4 granules. A possible photocatalytic mechanism of the CQDs/Ag3PO4/BiPO4 composite toward the degradation of RhB under simulated sunlight irradiation is schematically depicted in Figure 10b. The conduction band (CB) and valence band (VB) potentials of BiPO4 and Ag3PO4 can be calculated by using Equations (3) and (4) [74,75]:   Figure 10b. The conduction band (CB) and valence band (VB) potentials of BiPO 4 and Ag 3 PO 4 can be calculated by using Equations (3) and (4) [74,75]:

Photocatalytic Mechanism
where E e is 4.5 eV, being the free electron energy on the hydrogen scale. X Ag3PO4 and X BiPO4 are estimated as 5.959 and 6.633 eV, respectively, according to Equations (5) and (6): where X(Ag) When the CQDs/Ag 3 PO 4 /BiPO 4 photocatalyst is irradiated by simulated sunlight, the electron transition occurs from the VB to the CB of Ag 3 PO 4 , thus producing electron-hole pairs. Subsequently, the holes in the VB of Ag 3 PO 4 react with the RhB dye to form degradation products. Simultaneously, CQDs can be also excited by absorbing visible light, i.e., the π electrons or σ electrons are excited to the lowest unoccupied molecular orbital (LUMO) [76,77]. The excited CQDs can be acted as excellent electron donors and acceptors. However, BiPO 4 could not be photoexcited to generate electron-hole pairs under simulated sunlight irradiation due to its large bandgap energy (4.561 eV). Consequently, the CB electrons in Ag 3 PO 4 will transfer to CQDs (π or σ orbitals), and the photoexcited electrons in CQDs will transfer to the CB of BiPO 4 . Due to this interesting electron transfer process, the recombination of the photoexchited electron-hole pairs in Ag 3 PO 4 are efficiently suppressed. Furthermore, the up-conversion photoluminescence emitted from CQDs could excite Ag 3 PO 4 to generate additional electron-hole pairs. The photoexcited electrons in the LUMO of CQDs and those relaxed to the CB of BiPO 4 react with oxygen in the photocatalytic system to form superoxide (•O 2 − ) radicals. The produced •O 2 − radicals react with dye molecules adsorbed on the surface of the photocatalyst to produce degradation products.
Micromachines 2019, 10, x FOR PEER REVIEW 13 of 17 Figure 10. Schematic illustration of the assembly structure (a) and a possible photodegradation mechanism (b) of the CQDs/Ag3PO4/BiPO4 composite.

Conclusions
A simple hydrothermal method has been used to synthesize the CQDs/Ag3PO4/BiPO4 heterostructure photocatalyst. The carbon quantum dots are anchored at the interfaces between Ag3PO4 and BiPO4, thus forming the CQDs/Ag3PO4/BiPO4 three-phase junction structure. The

Conclusions
A simple hydrothermal method has been used to synthesize the CQDs/Ag 3 PO 4 /BiPO 4 heterostructure photocatalyst. The carbon quantum dots are anchored at the interfaces between Ag 3 PO 4 and BiPO 4 , thus forming the CQDs/Ag 3 PO 4 /BiPO 4 three-phase junction structure. The three-phase junction structure results in an efficient charge separation and utilization, high light absorption capacity and low photoluminescence intensity. The CQDs/Ag 3 PO 4 /BiPO 4 composite exhibits significantly enhanced photocatalytic activity for the degradation of RhB, which can be explained as the result of efficient charge separation and increased visible-light absorption.