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Article

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

by
Huajing Gao
1,3,
Chengxiang Zheng
1,
Hua Yang
1,*,
Xiaowei Niu
3 and
Shifa Wang
2,3,*
1
State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China
2
Chongqing Key Laboratory of Geological Environment Monitoring and Disaster Early-Warning in Three Gorges Reservoir Area, Chongqing Three Gorges University, Chongqing 404000, China
3
School of Electronic and Information Engineering, Chongqing Three Gorges University, Chongqing 404000, China
*
Authors to whom correspondence should be addressed.
Micromachines 2019, 10(9), 557; https://doi.org/10.3390/mi10090557
Submission received: 4 July 2019 / Revised: 13 August 2019 / Accepted: 21 August 2019 / Published: 23 August 2019
(This article belongs to the Special Issue Nanostructures for Photocatalysis)
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Abstract

:
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.

1. Introduction

The photocatalytic degradation of organic pollutants in wastewater is an attractive, environmentally friendly and green method that offers a way to harness solar power efficiently and convert them into non-toxic degradation products [1,2,3,4,5,6,7,8]. Recently, although great progress has been made in the field of photocatalysis, only few photocatalysts can effectively use visible light in the degradation of organic pollutions. Therefore, it is desirable to develop novel photocatalysts with high visible-light utilization for degradation of organic pollutions in wastewater. In recent years, silver phosphate (Ag3PO4) based composite photocatalysts, such as Bi4Ti3O12/Ag3PO4 [9], Ag3PO4/NaTaO3 [10], MoS2/Ag2S/Ag3PO4 [11], Ag3PO4/Bi2WO6 [12], Ag3PO4/Cu2O [13], TiO2/Ag3PO4/bentonite [14], Co3(PO4)2/Ag3PO4 [15], and Ag3PO4/BiFeO3 [16] have been extensively studied due to their excellent photocatalytic activity for photocatalytic degradation of the organic pollutions under visible light irradiation.
Bismuth phosphate (BiPO4) as a photocatalyst has been widely studied because of its good photoelectric performance, low cost, low toxicity, excellent photocatalytic activity, and high stability [1]. However, the large optical bandgap (Eg = 4.5 eV) of BiPO4 limits the transmission efficiency of photon-generated carriers and light-response range to sunlight [17,18]. To expand the photoresponse range of BiPO4, constructing composite photocatalysts with Ag3PO4 (Eg = 2.43 eV) can effectively improve the photocatalytic activity of the composite photocatalysts [19,20,21,22,23,24]. However, the Ag3PO4/BiPO4 photocatalysts have a high recombination rate of photogenerated electrons (e−) and holes (h+) in the degradation of organic pollutions in wastewater [25]. To achieve excellent photocatalytic performances of the photocatalysts, the photoexcited electrons and holes must be efficiently separated [26,27,28,29].
Noble metal nanoparticles (NPs) and carbon nanomaterials including carbon quantum dots (CQDs), carbon nanotubes (CNTs) and graphene manifest many intriguing physicochemical characteristics and offer a wide scope of technological applications in electronic devices, biomedicine, sensors, and wave absorption [30,31,32,33,34,35,36,37]. These nanomaterials are good carrier transport materials and also exhibit interesting localized surface plasmon resonance (LSPR) effect or photoluminescence (PL) up-conversion effect [38,39,40]. Due to these outstanding properties, noble metal NPs, CQDs, CNTs, and graphene have been demonstrated to be excellent modifiers or co-catalysts to enhance the photocatalytic performances of semiconductor photocatalysts [41,42,43,44,45,46].
Herein, we report a hydrothermal synthesis of unique CQDs/Ag3PO4/BiPO4 heterostructure photocatalyst. The composite photocatalyst with the CQDs, Ag3PO4, and BiPO4 three phase junction structure has not been reported previously and may be commonly applicable to other composite photocatalyst systems. The as-obtained CQDs/Ag3PO4/BiPO4 heterostructure photocatalyst possesses a high light absorption capacity, high utilization and separation efficiency of photogenerated carriers, and exhibits a high photocatalytic activity for photocatalytic degradation of the rhodamine B (RhB) dye. The present CQDs/Ag3PO4/BiPO4 heterostructure photocatalysts can be used for the design of micro/nano-photocatalytic devices for the wastewater treatment.

2. Materials and Methods

2.1. Synthesis of the Ag3PO4 Photocatalyst

According to the formula Ag3PO4, an amount of silver nitrate (AgNO3) was mixed with a stoichiometric amount of sodium dihydrogen phosphate (NaH2PO4) powder with Ag/P = 3:1 and added into 60 mL distilled water. After that, a stoichiometric amount of ammonium hydroxide (NH3·H2O) was added to the mixture. The whole process was accompanied by magnetic stirring. Subsequently, the above mixture was transferred to a 100 mL high-pressure autoclave and heated to 160 °C for 6 h. After finishing the hydrothermal reaction, the content was taken out and washed with distilled water several times to remove excess alkali ions. The slurry was centrifuged and dried for 12 h at 80 °C to obtain the Ag3PO4 photocatalyst. The flow-chart for the synthesis of Ag3PO4 photocatalyst via the hydrothermal synthesis method is shown schematically in Figure 1(I).

2.2. Synthesis of the BiPO4 Photocatalyst

According to the formula BiPO4, stoichiometric amounts of bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), NaH2PO4 and NH3·H2O were successively added in 20 mL of dilute nitric acid solution (2 mL HNO3 + 18 mL distilled water). The role of HNO3 is to dissolve Bi(NO3)3·5H2O. The mixture was filled up to 60 mL by adding distilled water. The remaining experimental steps are consistent with Section 2.1. The flow-chart for the synthesis of BiPO4 photocatalyst via the hydrothermal method is shown schematically in Figure 1(II).

2.3. Synthesis of Ag3PO4/BiPO4 Photocatalyst

To prepare Ag3PO4/BiPO4 photocatalyst, stoichiometric amounts of Bi(NO3)3·5H2O, AgNO3, NaH2PO4 and NH3·H2O (nAg3PO4:nBiPO4 = 1:0.11) were successively added in 20 mL of dilute HNO3 solution, and then filled up to 60 mL by adding distilled water. The assembly of Ag3PO4 on BiPO4 followed the procedure as described in Section 2.1. The flow-chart for the synthesis of the Ag3PO4/BiPO4 photocatalyst is schematically shown in Figure 1(III).

2.4. 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).

2.5. 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 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 cm2), and subjected to drying 60 °C for 5 h. The used electrolyte was 0.1 mol L−1 Na2SO4 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: 10−2–105 Hz).

2.6. 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 (Cphotocatalyst = 1 g L−1, CRhB = 5 mg L−1). Based on the initial RhB concentration (C0) and residual RhB concentration (Ct), the degradation percentage (DP) of RhB was given as: DP = (C0 − Ct)/C0 × 100%.

3. Results and Discussion

3.1. Phase Structural Analysis

Figure 2a,b show the XRD patterns of Ag3PO4 and BiPO4, respectively. For the Ag3PO4 and BiPO4 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 Ag3PO4 and BiPO4 can be ascribed to JCPDF#06-0505 and JCPDF#15-0767, respectively. Figure 2c shows the XRD patterns of Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4. The main XRD diffraction peaks of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites are similar to those of pure Ag3PO4, indicating that the host lattice of Ag3PO4 in these composites undergoes no change. In addition to the XRD characteristic peaks of the Ag3PO4 phase, the XRD characteristic peaks of BiPO4 are also observed in these composites. For the CQDs/Ag3PO4/BiPO4 composite, the intensity of the diffraction peaks is sharper than that for Ag3PO4/BiPO4. The structure analysis shows that the introduction of CQDs in the Ag3PO4/BiPO4 composites obviously accelerate the formation of Ag3PO4 and BiPO4. In our previous study, the carbon can suppress the formation of M-ferrite [47] and α-Al2O3 [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 BiPO4 and Ag3PO4, respectively. The BiPO4 and Ag3PO4 are monoclinic phase with space group P21/n (14) and cubic phase with space group P-43n (218), respectively. For the BiPO4, 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 BiPO4 exhibits a high photocatalytic activity for photocatalytic degradation of organic pollutants [49]. For the Ag3PO4, 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].

3.2. Surface Morphology and Elemental Component Analysis

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.
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.

3.3. 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 Bi3+ 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 those for the Ag3PO4/BiPO4 composite, which could be due to the fact that the CQDs facilitate the formation of CQDs/Ag3PO4/BiPO4 heterostructures.

3.4. 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 (Ho), 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 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):
E g ( eV ) = h c λ 0 ( nm ) ≈ 1240 λ 0 ( nm )
where λ0, h, and c is the maximum absorption wavelength, Plank constant, and velocity of light, respectively.

3.5. Photoelectrochemical Properties

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.

3.6. 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].
The first order kinetic rate of the dye degradation photocatalyzed by the samples can be evaluated by Equation (2) [66]:
Ln(Ct/C0) = −kt
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 = 0.02853, kAg3PO4/BiPO4 = 0.04489, and kCQDs/Ag3PO4/BiPO4 = 0.08259 min−1. The result further indicates that the CQDs/Ag3PO4/BiPO4 composite exhibits a photocatalytic activity for the degradation of RhB 31.6, 2.9, and 1.8 times higher than that of BiPO4, Ag3PO4 and Ag3PO4/BiPO4, respectively. We compare the photodegradation performance of CQDs/Ag3PO4/BiPO4 with that of other typical composite photocatalysts, as shown in Table 2. It is seen that the CQDs/Ag3PO4/BiPO4 composite photocatalyst prepared in this work manifests a photodegradation performance superior to most of other photocatalysts.
The stability and reusability of the CQDs/Ag3PO4/BiPO4 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/Ag3PO4/BiPO4 photocatalyst has a high stability and maintains a high photocatalytic activity for the degradation of RhB.

3.7. Photocatalytic Mechanism

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]:
ECB = X − Ee − 0.5Eg
EVB = X − Ee + 0.5Eg
where Ee is 4.5 eV, being the free electron energy on the hydrogen scale. XAg3PO4 and XBiPO4 are estimated as 5.959 and 6.633 eV, respectively, according to Equations (5) and (6):
X ( A g 3 P O 4 ) = X ( A g ) 3 X ( P ) X ( O ) 4 8
X ( B i P O 4 ) = X ( B i ) X ( P ) X ( O ) 4 6
where X(Ag) = 4.44, X(P) = 5.62, X(Bi) = 4.69, and X(O) = 7.54 eV. The CB potentials of BiPO4 and Ag3PO4 are estimated as −0.127, and +0.222 V, respectively, and their corresponding VB potentials are +4.434, and +2.691 V. For the Ag3PO4/BiPO4 composite, the energy band of Ag3PO4 is completely located within the energy band of BiPO4. Therefore, the Ag3PO4/BiPO4 composite obeys the type-I band alignment. When CQDs are introduced to the Ag3PO4/BiPO4 composite, it promotes the charge transfer between the two kinds of semiconductors. When the CQDs/Ag3PO4/BiPO4 photocatalyst is irradiated by simulated sunlight, the electron transition occurs from the VB to the CB of Ag3PO4, thus producing electron-hole pairs. Subsequently, the holes in the VB of Ag3PO4 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, BiPO4 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 Ag3PO4 will transfer to CQDs (π or σ orbitals), and the photoexcited electrons in CQDs will transfer to the CB of BiPO4. Due to this interesting electron transfer process, the recombination of the photoexchited electron-hole pairs in Ag3PO4 are efficiently suppressed. Furthermore, the up-conversion photoluminescence emitted from CQDs could excite Ag3PO4 to generate additional electron-hole pairs. The photoexcited electrons in the LUMO of CQDs and those relaxed to the CB of BiPO4 react with oxygen in the photocatalytic system to form superoxide (•O2−) radicals. The produced •O2− radicals react with dye molecules adsorbed on the surface of the photocatalyst to produce degradation products.

4. 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 three-phase junction structure results in an efficient charge separation and utilization, high light absorption capacity and low photoluminescence intensity. The CQDs/Ag3PO4/BiPO4 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.

Author Contributions

H.Y. conceived the idea of experiment; H.G. and C.Z. performed the experiments; H.Y., H.G., C.Z., X.N. and S.W. discussed the results; S.W. wrote the manuscript; and all authors read and approved the final manuscript.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (51662027), the Chongqing basic research and frontier exploration (general project) (cstc2019jcyj-msxm1327), the Major Cultivation Projects of Chongqing Three Gorges University (18ZDPY01), the University Scientific Research Project in Gansu Province (2018A-242), and the Study on the Detection of UWB High Range Resolution Radar Target project of the Science and Technology Research Program of the Chongqing Education Commission of China (KJ1601004).

Conflicts of Interest

The authors declare that they have no competing interests.

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Figure 1. Chemical route for the preparation of (I) Ag3PO4, (II) BiPO4, (III) Ag3PO4/BiPO4, and (IV) CQDs/Ag3PO4/BiPO4.
Figure 1. Chemical route for the preparation of (I) Ag3PO4, (II) BiPO4, (III) Ag3PO4/BiPO4, and (IV) CQDs/Ag3PO4/BiPO4.
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Figure 2. XRD patterns of (a) BiPO4, (b) Ag3PO4, (c) Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4, and crystal structures of (d) BiPO4 and (e) Ag3PO4.
Figure 2. XRD patterns of (a) BiPO4, (b) Ag3PO4, (c) Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4, and crystal structures of (d) BiPO4 and (e) Ag3PO4.
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Figure 3. SEM images of (a) Ag3PO4/BiPO4, (b) CQDs/Ag3PO4/BiPO4, and (c) pure CQDs. The insets represent the real photos of Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4.
Figure 3. SEM images of (a) Ag3PO4/BiPO4, (b) CQDs/Ag3PO4/BiPO4, and (c) pure CQDs. The insets represent the real photos of Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4.
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Figure 4. TEM image (a), HRTEM image (b), EDS spectrum (c), DF-STEM image (d), and elemental mapping images (e–i) of the CQDs/Ag3PO4/BiPO4 composite.
Figure 4. TEM image (a), HRTEM image (b), EDS spectrum (c), DF-STEM image (d), and elemental mapping images (e–i) of the CQDs/Ag3PO4/BiPO4 composite.
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Figure 5. XPS survey scan spectra (a), Bi 4f spectra (b), P 2p spectra (c), Ag 3d spectra (d), O 1s spectra (e), and C 1s spectra (f) of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites.
Figure 5. XPS survey scan spectra (a), Bi 4f spectra (b), P 2p spectra (c), Ag 3d spectra (d), O 1s spectra (e), and C 1s spectra (f) of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites.
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Figure 6. UV–VIS diffuse reflectance spectra (a) and the corresponding first derivative curves of the UV–VIS diffuse reflectance spectra (b) of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites.
Figure 6. UV–VIS diffuse reflectance spectra (a) and the corresponding first derivative curves of the UV–VIS diffuse reflectance spectra (b) of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites.
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Figure 7. EIS spectra (a) and photocurrent response curves (b) of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites.
Figure 7. EIS spectra (a) and photocurrent response curves (b) of the Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4 composites.
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Figure 8. (a) Time-dependent photocatalytic degradation of RhB dye over the BiPO4, Ag3PO4, Ag3PO4/BiPO4, and CQDs/Ag3PO4/BiPO4 photocatalysts under simulated sunlight irradiation. (b) Plots of Ln(C/C0) vs. irradiation time for the samples.
Figure 8. (a) Time-dependent photocatalytic degradation of RhB dye over the BiPO4, Ag3PO4, Ag3PO4/BiPO4, and CQDs/Ag3PO4/BiPO4 photocatalysts under simulated sunlight irradiation. (b) Plots of Ln(C/C0) vs. irradiation time for the samples.
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Figure 9. Recyclability of the CQDs/Ag3PO4/BiPO4 photocatalyst for the photocatalytic degradation of RhB under simulated sunlight irradiation.
Figure 9. Recyclability of the CQDs/Ag3PO4/BiPO4 photocatalyst for the photocatalytic degradation of RhB under simulated sunlight irradiation.
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Figure 10. Schematic illustration of the assembly structure (a) and a possible photodegradation mechanism (b) of the CQDs/Ag3PO4/BiPO4 composite.
Figure 10. Schematic illustration of the assembly structure (a) and a possible photodegradation mechanism (b) of the CQDs/Ag3PO4/BiPO4 composite.
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Table
Table 1. Color coordinates and Eg values of Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4.
Table 1. Color coordinates and Eg values of Ag3PO4/BiPO4 and CQDs/Ag3PO4/BiPO4.
SampleColor CoordinatesEg of Ag3PO4 (eV)Eg of BiPO4 (eV)
L*a*b*c*HoECIE*
Ag3PO4/BiPO487.110 −3.666 30.235 30.456−83.08792.2812.4694.491
CQDs/Ag3PO4/BiPO463.817 4.286 15.421 16.00674.46865.794-4.561
Table
Table 2. Comparison of the photocatalytic performance of CQDs/Ag3PO4/BiPO4 with that of previously reported Ag3PO4-based composite photocatalysts toward the degradation of RhB.
Table 2. Comparison of the photocatalytic performance of CQDs/Ag3PO4/BiPO4 with that of previously reported Ag3PO4-based composite photocatalysts toward the degradation of RhB.
SamplesLight SourceCphotocatalyst (g L−1)CRhB (mg L−1)Irradiation Time (min)D%Reference
CQDs/Ag3PO4/BiPO4200 W Xe lamp155098.7This work
20wt%Ag3PO4/Bi2WO6200 W Xe lamp0.5512094[9]
10% Bi4Ti3O12/Ag3PO4200 W Xe lamp0.253099.5[6]
Ag-Ag3PO430 W fluorescent light lamp (λ ≥ 420 nm)0.75106070[67]
Fe3O4/ZnO/Ag3PO450 W LED lamp 0.412 (10−5 mol/L)10075[68]
15 wt% Ag3PO4-Bi2MoO6300 W Xe lamp with a 400-nm cutoff filter11010039[69]
Ag3PO4-ZnO (1:40)300 W Xe lamp with a 400-nm cutoff filter0.6712 (10−5 mol/L)3093[70]
Ag3PO415 W four fluorescent lamp0.3156075[71]
AgI/BiPO4500 W Xe lamp with a 420-nm cutoff filter1.67106092.2[72]
Ag2S/CQDs/CuBi2O4200 W Xe lamp156099.3[45]
BiPO4/Ag/Ag3PO4150 W Xe lamp with a 420-nm cutoff filter0.12012065[73]

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MDPI and ACS Style

Gao, H.; Zheng, C.; Yang, H.; Niu, X.; Wang, S. Construction of a CQDs/Ag3PO4/BiPO4 Heterostructure Photocatalyst with Enhanced Photocatalytic Degradation of Rhodamine B under Simulated Solar Irradiation. Micromachines 2019, 10, 557. https://doi.org/10.3390/mi10090557

AMA Style

Gao H, Zheng C, Yang H, Niu X, Wang S. Construction of a CQDs/Ag3PO4/BiPO4 Heterostructure Photocatalyst with Enhanced Photocatalytic Degradation of Rhodamine B under Simulated Solar Irradiation. Micromachines. 2019; 10(9):557. https://doi.org/10.3390/mi10090557

Chicago/Turabian Style

Gao, Huajing, Chengxiang Zheng, Hua Yang, Xiaowei Niu, and Shifa Wang. 2019. "Construction of a CQDs/Ag3PO4/BiPO4 Heterostructure Photocatalyst with Enhanced Photocatalytic Degradation of Rhodamine B under Simulated Solar Irradiation" Micromachines 10, no. 9: 557. https://doi.org/10.3390/mi10090557

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