g-C3N4 photocatalyst via microwave heating for effective degradation of RhB under visible light

Journal of Physics and Chemistry of Solids 136 (2020) 109122

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One step and fast preparation of VOx/g-C3N4 photocatalyst via microwave heating for effective degradation of RhB under visible light

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Chunran Zhaoa, Yijing Chena, Chihao Lia, Qingle Zhanga, Pengfei Chena, Keli Shia,**, Ying Wub, Yiming Hea,b,* a b

Department of Materials Science and Engineering, Zhejiang Normal University, Jinhua, 321004, China Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, China

A R T I C LE I N FO

A B S T R A C T

Keywords: VOx/g-C3N4 Photocatalysis Microwave heating RhB degradation H2 production

This work reports a microwave heating method for synthesis of a VOx/g-C3N4 complex for applications in photocatalytic degradation of rhodamine B (RhB) under visible light illumination. The facile strategy allows the fabrication of the VOx/g-C3N4 composite in 40 min with NH4VO3 and melamine as precursors. A thorough investigation was performed to investigate the structure, morphology, photoabsorption performance and charge separation capability of the VOx/g-C3N4 composite. XRD, FT-IR, XPS and TEM experiments proved that VOx nanoparticles which have the mixed valence state of V5+ and V4+ were dispersed in the g-C3N4. DRS experiment demonstrated that these introduced VOx nanoparticles improved the light absorption property for visible light. N2-adsorption analysis showed that the surface area of VOx/g-C3N4 sample was enhanced with increasing VOx content. Although the improved photoabsorpion performance and surface area are beneficial to the photocatalytic RhB degradation, the mostly important reason of the enhanced photoactivity was ascribed to the formed heterojunction structure of g-C3N4 and VOx, which triggers electrons migration via a type-II mechanism and elevates the charge separation. This improved charge separation was confirmed by the PL, EIS, and PC experiments, while the type-II mechanism was verified by reactive species trapping experiment and photocatalytic H2 evolution test. Due to the improved efficiency in separating charge carriers, the VOx/g-C3N4 worked very well in RhB degradation via photocatalysis. The best VOx/g-C3N4 sample displayed a RhB degradation velocity of 0.079 min−1 under visible light, which is 3.6 folds faster than that of neat g-C3N4.

1. Introduction With the development of the economy and the deepening of the industrialization process, the problem of industrial effluents is getting more and more serious. Sewage treatment thus becomes an important part of improving environmental quality and protecting water resources. Among the reported technologies, photocatalysis can effectively realize the degradation of organic pollutants using solar energy, and hence attracts scientists’ attention. The development of a photocatalyst with the merits of low cost, good visible-light response, high activity and stability is the key to the practical application of the technique. Since 1972, varies semiconductor photocatalysts including TiO2 [1], ZnO [2–4], metal oxynitrides [5], BiOX (X = F, Cl, Br, I) [6], have been developed for water purification under solar light. Although some of them present high performance, the requirement of the practical application is still not reached. The development of novel

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photocatalysts with higher efficiency is thus desired. Graphite carbon nitride (g-C3N4) is a new type of polymer photocatalyst with many advantages including low cost, visible light response, easy to prepare, and high chemical stability. Thus, it has been widely applied in photocatalysis field, such as photocatalytic N2 fixation [7], CO2 reduction [8], H2 evolution [9], and organics degradation [10–12]. A great variety of g-C3N4 contained photocatalysts including porous g-C3N4 [13,14], P-g-C3N4 [15], Ag/g-C3N4 [16], B doped g-C3N4 [17,18], K-g-C3N4 [19], Au/g-C3N4 [20], ZnO/g-C3N4 [21], ANbO3/gC3N4 (A = K, Na, Ag) [22–24], AgX (X = I, Br, Cl)/g-C3N4 [25,26], LnVO4 (Ln = Gd, Sm, La, Dy)/g-C3N4 [27–29], CuS/g-C3N4 [30], gC3N4/Fe3O4/Ag2WO4/AgBr [31], CuWO4/Fe3O4/g-C3N4 [32], C/gC3N4 [33], C/BiOI/g-C3N4 [34], C/AgCl/g-C3N4 [35], CoP/g-C3N4 [36], Sn3O4/g-C3N4 [37], Fe2O3/g-C3N4 [38], and Ag0.68V2O5/g-C3N4 [39] have been reported. It can be noted that many methods including nanostructure fabrication, non-metal or metal ion doping, precious metal

Corresponding author. Department of Materials Science and Engineering, Zhejiang Normal University, Jinhua, 321004, China. Corresponding author. E-mail addresses: [email protected] (K. Shi), [email protected] (Y. He).

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https://doi.org/10.1016/j.jpcs.2019.109122 Received 12 May 2019; Received in revised form 20 June 2019; Accepted 1 August 2019 Available online 01 August 2019 0022-3697/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Scheme of synthesis of VOx/g-C3N4 via microwave heating method.

the VOx/g-C3N4 was also performed to disclose the cause of the enhanced catalytic activity.

modification, and semiconductor coupling are applied. Nevertheless, the heterojunction composite construction is the most popular because this approach allows the improvement in both charge separation and light absorption. As for the fabrication of g-C3N4 based composite, a suitable band potential of the coupling semiconductor is the prerequisite. Additionally, a strong interaction between g-C3N4 and the coupling semiconductor is required, which can reduce the interface energy, hasten the electron migration through interface, and ultimately allow the formed heterojunction to work well. In order to reach it, some scientists mixed the doped semiconductor with g-C3N4 precursor and heated them at high temperature. The in-situ formation of g-C3N4 on semiconductor surface endows the consisted phases intimate contact and high performance in photocatalytic reaction [40,41]. However, the preparation of g-C3N4 performs at high temperature and needs a long time (about 10 h including the heating and cooling time). For the semiconductor with the capability in activating oxygen, g-C3N4 would be catalytically decomposed to gas product. Therefore, this approach is only applicable for some semiconductors with weak redox ability, such as ZrO2 [42], Nb2O5 [43] and SiO2 [44]. Very recently, our group reported the work of preparation of two novel g-C3N4 based composites (AgNbO3/g-C3N4 and KTa0.75Nb0.25O3/g-C3N4) [22,45] via microwave heating method, which allows the fast preparation of g-C3N4 based composite in minutes and saves a lot of energy. More importantly, the fast synthesis significantly decreases the chance of the decomposition of g-C3N4 or the coupled semiconductor. Hence, the microwave heating method is better than the conventionally heating process, and is more suitable to prepare g-C3N4 based composite due to its advantages of energy-saving and high efficiency. Nevertheless, currently, there are few studies using this strategy. In this paper, we present a successful application of the microwave heating method in synthesizing VOx/g-C3N4 composite. Vanadium oxide is chosen as a modifier to construct a heterojunction with g-C3N4. Due to the smaller band gap, the coupling of vanadium oxide can promote the photoabsorption performance of g-C3N4, which benefits the photocatalytic reaction. Additionally, vanadium oxide is also a good co-catalyst in photocatalysis. A series of LnVO4-V2O5 composites have been developed and applied in catalytic degradation of volatile organic compounds under irradiation of visible light [46–51]. For g-C3N4, the promotion effect of V2O5 has also been reported. Liu et al. synthesized V2O5/g-C3N4 photocatalyst and applied it in dye photodegradation [52]. The enhanced photoactivity was ascribed to the improved separation of charge carriers via type-II mechanism. Hong et al. also reported the promotion effect of V2O5 to g-C3N4 in dyes degradation [53]. Nevertheless, they suggested that the V2O5/g-C3N4 photocatalyst performed a direct Z-scheme mechanism. Thus, although the promotion effect of V2O5 is no doubt, the nature behind the photocatalytic activity enhancement is still unclear and deserves to be investigated. Additionally, Song et al. reported that the introduction of V4+ into V2O5 would increase the electrical conductivity and boost the charge separation [54], which indicates that the V4+ self doped V2O5 (VOx) is a better modifier than V2O5. Based on the above discussion, the VOx/gC3N4 complex was synthesized via microwave heating in 40 min. The results of photocatalytic test indicated that the composite showed high activity in photocatalytic RhB degradation. A thorough investigation of

2. Experimental section 2.1. Photocatalysts preparation Melamine, ammonium metavanadate (NH4VO3), and oxalic acid (C2H2O4) were used as raw materials for preparation of VOx/g-C3N4 photocatalyst. The synthesis procedure is shown below. Take 1% VOx/ g-C3N4 as an example, 0.01 g NH4VO3 and 0.0216 g C2H2O4 were dissolved in 40 mL deionized water at 50 °C to form a clear solution. 10.0 g melamine was then added and impregnated for 2 h. After the water in the solution was evaporated, the obtained solids were further dried at 65 °C for 24 h. As the precursor of 0.1% VOx/g-C3N4, the dried solids were ground for 30 min, moved to a corundum crucible, and heated in a microwave for 40 min (Fig. 1). After the crucible was cooled naturally, the obtained product was ground into powders to get the 1%VOx/gC3N4. Other VOx/g-C3N4 composites with the V to melamine ratio of 0.05%, 0.2%, 1%, 5% and pure g-C3N4 were also prepared via the same procedure. The photoactivity of the synthesized VOx/g-C3N4 was examined via the RhB degradation (10 mg/L) and H2 generation. A thorough characterization of the photocatalysts was also carried out. The detailed information is provided in the supporting materials. 3. Results and discussion 3.1. Characterization of VOx/g-C3N4 The XRD patterns of g-C3N4 and VOx/g-C3N4 composites are presented in Fig. 2a. Two strong peaks at about 13.0° and 27.3° indexing to (100) and (002) diffraction planes of graphitic materials are distinctly observed for as-prepared g-C3N4 [15,55]. The result demonstrates that graphitic carbon nitride was successfully synthesized by microwave roasting in 40 min. After the introduction of VOx, the diffraction peak intensity of carbon nitride is decreased, while the peak position is not changed, indicating the introduced V is not doped into the carbon nitride lattice. As the VOx concentration increases, the g-C3N4 diffraction peak is weakened more obviously. In general, the decrease in peak intensity can be explained by the reduced concentration or the decreased crystallinity. For VOx/g-C3N4 complex, the VOx concentration is very low. No peak corresponding to VOx phase is observed. This result indicates that the weakened peak intensity should be mainly ascribed to the decreased crystallinity of g-C3N4. The added VOx may decrease the aggregation of g-C3N4 and endow it its nanostructure which results in the lower crystallinity. Fig. 2b shows the results obtained by the FT-IR experiment. The absorption peak of g-C3N4 mainly appears at 804 cm−1, 12501670 cm−1, and 3070-3280 cm−1, respectively [56,57]. The absorption peak of 804 cm−1 is generated by the bending vibration of triazine ring structure. The strong absorption peaks in the range of 1250-1670 cm−1 belong to the stretching vibration of CN heterocyclic compounds [58]. For the IR peak at 3070-3280 cm−1, the N–H species in the g-C3N4 is 2

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Just like g-C3N4, VOx/g-C3N4 complex also presents a sharp weight loss. Nevertheless, the weight loss temperature is lower than g-C3N4 because of the catalytic effect of VOx. Due to the capability in activating oxygen in air, the added VOx hastens the oxidation of g-C3N4 and decreases the thermal stability of VOx/g-C3N4 [27–29]. With the increase in VOx content, the g-C3N4 oxidation process is performed more easily. Hence, the weight loss temperature of g-C3N4 is decreased. Different from gC3N4, VOx phase cannot be converted to gas product in the TG analysis, indicating that the residue should be the VOx. Based on this deduction, the VOx weight contents in the composite are estimated to be 1.03%, 2.41%, 4.41%, and 8.96% for 0.05%VOx/g-C3N4, 0.1%VOx/g-C3N4, 0.2%VOx/g-C3N4, and 1%VOx/g-C3N4, respectively. The real VOx content in the VOx/g-C3N4 complex is still low. XPS spectra of g-C3N4 and VOx/g-C3N4 composites with VOx content of 0.1% and 1% are displayed in Fig. 4. Fig. 4a indicates that two peaks are observed in the C 1s XPS spectra of the three photocatalysts. The signal at 284.6eV originates from the sp2 C–C bond coordination, and the peak at 288.0 eV is attributed to the N–C]N coordination in g-C3N4 [59,60]. The N 1s signal can be divided into three peaks at 398.4, 399.2 and 400.6 eV (Fig. 4b), which are attributed to sp2-hybridized nitrogen (C]N–C), tertiary nitrogen (N-(C)3) and amino functional groups with a hydrogen atom (C–N–H), respectively [61–63]. No shift in the binding energy (BE) of C and N is detected, confirming that the atomic substitution of C or N by vanadium does not occur in the VOx/g-C3N4. Fig. 3 (c) exhibits the O 1s XPS spectra of g-C3N4 and VOx/g-C3N4. Neat g-C3N4 exhibits a O1s peak at 532.0 eV due to the oxygen signal coming from the surface hydroxyl group (OH) or H2O [64]. In addition to this O1s signal, 0.1%VOx/g-C3N4 sample presents a shoulder O1s peak at approximate 529.9 eV, corresponding to the lattice oxygen in VOx species. For 1%VOx/g-C3N4 photocatayst, the O1s signal of the lattice oxygen is more obvious because of the increased VOx content. The BE of the O1s main peak shifts to 530.3 eV. The variation in the O1s signal indirectly proves the introduction of VOx phase. The direct and powerful proof about the existence of VOx is supplied by the V2p XPS spectra. As Fig. 4d shows, both 0.1%VOx/g-C3N4 and 1%VOx/g-C3N4 photocatalysts display the V2p3/2 and 2p1/2 peaks. Based on the BE and the peak shape, the V2p3/2 peak of the two samples can be dissociated into two peaks. The peak at 516.0 eV can be ascribed to V4+ species, while the peak of 516.9 eV originates from V5+ species [54,64]. Clearly, due to the rapid preparation via microwave heating and the pretreatment on vanadium, the formed vanadium oxide presents a mixed valence state, which is different from the reported literatures [52,53]. Meanwhile, it is noted that the increase in V content improves the V5+ concentration. Based on the peak area, it can be estimated that 1% VOx/g-C3N4 sample presents a lower V4+ content (32%) than 0.1% VOx/g-C3N4 (49%). The phenomenon may be ascribed to the different disperse of VOx. A low V content usually represents the good dispersion of VOx on g-C3N4. The interaction between VOx and g-C3N4 may stabilize the low valence state. For the sample with high V content (1% VOx/g-C3N4), the aggregated VOx species cannot get the help of g-C3N4 and is easily oxidized to V5+ during the thermal treatment. Finally, the atomic content of C, N, V, and O elements was calculated based on the XPS peak area and the corresponding correction factors. The result shown in Table S1 indicates that the V contents of 0.1%VOx/g-C3N4 and 1%VOx/g-C3N4 are estimated to be 0.27% and 0.99%, respectively. This result is very close to the value obtained via TG analysis (0.35% and 0.96%), indicating that the VOx disperses finely in the g-C3N4. In order to analyze the microstructure and morphology of the samples, SEM experiment was carried out. Pure g-C3N4 shows the morphology of big irregular particles, indicating its serious agglomeration. The surface is relative smooth due to its layer structure (Fig. 5a). After the addition of vanadium, the surface of g-C3N4 becomes rough and loose. Some thin sheets are also observed (Fig. 5b). The morphology change may be related to that the loaded VOx on g-C3N4 surface prevents the aggregation of carbon nitride to some extent. Meanwhile, the result suggests that the VOx/g-C3N4 complex would

Fig. 2. XRD patterns (a) and FT-IR spectra (b) of VOx/g-C3N4 composites.

the origin [58]. In comparison to g-C3N4, VOx/g-C3N4 complexes with different VOx contents exhibit basically the same absorption peak, suggesting that the VOx doping does not change the original layered structure of g-C3N4. Notably, besides the peaks of g-C3N4, the 1%VOx/gC3N4 sample present an IR band at about 986 cm−1, which can be attributed to the stretching vibration modes of V]O species [47,48]. This peak definitely verifies that the VOx species is introduced and the VOxg-C3N4 composite is formed. TG analysis in air atmosphere was carried out to study the thermal stability of the synthesized VOx/g-C3N4 composite. The TG curve in Fig. 3 shows that the weight loss of g-C3N4 begins at about 550 °C. It can be fully converted to gas product when the temperature reaches 700 °C.

Fig. 3. TG profiles of VOx/g-C3N4 photocatalyst with different VOx concentrations. 3

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Fig. 4. XPS spectra of g-C3N4, 0.1%VOx/g-C3N4 and 1% VOx/g-C3N4 samples, (a) C1s; (b) N1s; (c) O1s; (d) V2p.

16.9 m2/g, respectively. Fig. 5c and d display the TEM image of 0.1% VOx/g-C3N4. The thin layer structure of g-C3N4 is observed. The black strip can be attributed to the g-C3N4 sheets facing the paper direction, which is consistent with the loose structure observed in SEM image. No

have higher surface area than g-C3N4, which has been verified by N2 adsorption analysis. The specific surface area of g-C3N4 is only 7.8 m2/ g. For the VOx/g-C3N4 with theoretical V contents of 0.05%, 0.1%, 0.2%, and 1%, the surface areas are estimated to be 11.1, 14.2, 16.3,

Fig. 5. SEM images of g-C3N4 (a), 0.1%VOx/g-C3N4 (b), and TEM images of 0.1%VOx/g-C3N4 photocatalyst (c,d). 4

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Fig. 6. DRS spectra of VOx/g-C3N4 composites (a) and their estimated band gaps (b).

Fig. 7. Transient photocurrent response (a) and EIS (b) of g-C3N4, 0.1%VOx/gC3N4 and 1.0% VOx/g-C3N4 photocatalysts.

big particles corresponding to vanadium oxide is detected, suggesting its good disperse. The HR-TEM image indicates that the particle size of the detected VOx is approximate 7–8 nm. The lattice fringe is estimated to be 0.2878 nm, which is close to the d value of (400) plane of V2O5 (d = 0.2882 nm). Hence, it is no doubt that VOx nanoparticles decorate on the surface of g-C3N4 and construct the VOx-g-C3N4 heterojunction composite. The absorption characteristics of VOx/g-C3N4 samples are characterized by UV–vis diffuse reflectance spectroscopy (DRS), and the data is displayed in Fig. 6. G-C3N4 presents an absorption band between 390 nm and 600 nm. VOx/g-C3N4 photocatalysts with different VOx contents show similar absorption characteristics to pure g-C3N4 since the main phase is carbon nitride. However, the red shift of the absorption edges still appears in the VOx/g-C3N4 complex, which is mainly ascribed that the added VOx presents much better visible-light response than g-C3N4. The band gap energies (Eg) of V2O5 and VO2 are about 2.2 and 1.7 eV, respectively [47,48,65]. Both are much smaller than g-C3N4 (2.7 eV). Thereby, as the VOx concentration increases, the visible-light absorption performance of the VOx/g-C3N4 complex is improved. Correspondingly, the color of the photocatalyst is gradually deepened from yellow to brown (Fig. 6b). To show the variation in light absorption more clearly, the Eg of the VOx/g-C3N4 photocatalysts are estimated via the K-M method [30,43,66]. The band gaps of 0.05% VOx/g-C3N4, 0.1% VOx/g-C3N4, 0.2% VOx/g-C3N4, and 1% VOx/g-C3N4 are estimated to be 2.66, 2.66, 2.60 and 2.32 eV, respectively. It is clear that the VOx/g-C3N4 hybrids have smaller forbidden band gap than gC3N4 and present better capability in absorbing visible light, suggesting their higher potential in photocatalytic reaction. Besides the optical property, the transfer and separation of photoinduced charge carriers are also very important for the photocatalytic reaction. Thus, the photoelectric chemical properties of the VOx/g-C3N4 catalysts were analyzed by transient photocurrent (PC) response.

Fig. 7a shows the PC response of g-C3N4, 0.1%VOx/g-C3N4 and 1% VOx/g-C3N4 with three on-off intermittent irradiation cycles. When the samples receive the irradiation of visible light, the PC intensity of 0.1% VOx/g-C3N4 was the highest, whereas pure g-C3N4 presents the lowest PC. Normally, a strong the photocurrent intensity means a high electron transfer rate, which usually leads to high efficiency in separating photogenerated charge carriers [67–69]. Therefore, the experimental result in Fig. 7a indicates that the addition of VOx to g-C3N4 elevates the charge separation efficiency. The 0.1%VOx/g-C3N4 composite material may present a suitable VOx content and show the most obvious promotion effect. Electrochemical impedance spectra (EIS) of VOx/g-C3N4 complex are displayed in Fig. 7b. The detected radii of the three samples present the following sequence: g-C3N4 > 1%VOx/g-C3N4 > 0.1% VOx/g-C3N4. According to the published literatures [67–72], it is known that a small radius of Nyquist plot demonstrates a low resistance of electron transfer and a better conductivity. The smallest radius of 0.1%VOx/g-C3N4 curve indicates that the sample holds the best capability in boosting the migration of electrons from the generated centers to the catalyst's surface, which results in the highest photocurrent and accords well with the data in Fig. 7a. The photoluminescence (PL) process contains abundant information of the transport and recombination of photogenic carriers in semiconductor material. The PL peak intensity can be applied to examine the charge separation efficiency since the PL signal comes from the annihilation of charge carriers. In other words, the higher the PL peak is, the lower efficiency of the charge separation is. Thus, herein, PL experiments of g-C3N4 and VOx/g-C3N4 were conducted to confirm the result in PC and EIS. It is shown that pure g-C3N4 presents strong PL signal at approximate 460 nm (Fig. 8), which is caused by the band gap recombination. The VOx/g-C3N4 samples also exhibit the PL peak of gC3N4. Nevertheless, the peak intensity is weakened significantly, 5

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composites present stronger ability in both RhB adsorption and degradation. It can be seen that the RhB adsorption in the dark is increased with increasing VOx content, which may be related to the enhanced specific surface area. Nevertheless, the photocatalytic behaviors under visible light are different. As the VOx content increases, the photoactivity of VOx/g-C3N4 is improved first and then reduced. The 0.1%VOx/g-C3N4 hybrid displays the best performance in degradation RhB solution. Under visible light irradiation for 40 min, 95% of RhB is decolored. The kinetic behavior of the VOx/g-C3N4 photocatalysts in RhB degradation was further studied. The data in Fig. 9b indicate that degradation processes of all samples fit well with a pseudo-first-order homogeneous reaction model. The apparent degradation constants of gC3N4, 0.05%VOx/g-C3N4, 0.1%VOx/g-C3N4, 0.2%VOx/g-C3N4, and 1% VOx/g-C3N4 samples are estimated to be 0.022, 0.027, 0.079, 0.054, and 0.033 min−1, respectively. The degradation rate of the best 0.1% VOx/g-C3N4 photocatalyst reaches 3.6 folds that of g-C3N4, confirming the great promotion effect of VOx. Fig. 9c shows the influence of solution pH on the photoactivity of 0.1% VOx/g-C3N4. Under neutral (pH = 7) and alkaline (pH = 9) environments, the degradation percentage of RhB solution are 69% and 63%, respectively. Under weak acidic environment (pH = 4 and 6), however, the corresponding degradation percentage reaches 99%. The result indicates that the VOx/g-C3N4 photocatalysts works better in acidic solution, which can be ascribed to the stronger RhB adsorption. The isoelectric point of g-C3N4 and V2O5 are reported to be about 4.0 and 2.0, respectively [75,76]. The introduction of VOx would most probably decrease the isoelectric point of g-C3N4 [77]. That is to say, the surface of VOx/g-C3N4 is negatively charged under the experimental conditions in Fig. 9c. The RhB molecules tend to adsorb on the VOx/gC3N4 surface through positively charged diethylamino group. When the pH of solution is 7 and 9, most of the carboxyl group of RhB would be dissociated and negatively charged at −COO− state [77], which would greatly decrease the electrostatic interaction between RhB and the

Fig. 8. PL spectra of g-C3N4, 0.1%VOx/g-C3N4 and 1% VOx/g-C3N4 photocatalysts.

indicating that the fast charge recombination in g-C3N4 is effectively retarded [73,74]. The 0.1%VOx/g-C3N4 hybrid exhibits the lowest PL intensity, confirming its best ability in charge separation. The data in Fig. 8 agrees well with the result in PC and EIS experiments.

3.2. Photocatalytic test The photoactivity of VOx/g-C3N4 hybrids was assessed by examining their performance in RhB degradation under visible light illumination. As Fig. 9 shows, without the presence of a photocatayst, the RhB content keeps stable, indicating that the contribution of RhB photolysis can be ignored. All the photocatalysts show the performance in degrading RhB solution. Compared with neat g-C3N4, VOx/g-C3N4

Fig. 9. Photocatalytic activity of VOx/g-C3N4 composites in RhB degradation (a) under visible light irradiation and their linear plot of ln(C0/C) versus irradiation time (b), the effect of solution pH (c) and cycle times (d) on the photocatalytic activity of 0.1% VOx/g-C3N4 composite. 6

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Fig. 10. Photocatalytic RhB degradation of g-C3N4 (a) and 0.1%VOx/g-C3N4 (b) in the presence of different scavengers.

the introduced VOx changes the RhB degradation process on g-C3N4. Besides the photocatalytic RhB degradation, photocatalytic H2 evolution test was also carried out to evaluate the photoactivity of the VOx/g-C3N4 complex. The reaction was carried out in a methanol solution (20 ml methanol and 80 ml deionized water). Methanol was used as holes scavenger to improve the charge separation. Additionally, 1 ml 1 wt% chloroplatinic acid solution was also added. When the Xe lamp is on, metal Pt (0.37 wt%) would be in-situ deposited on the catalyst surface to hasten the evolution of H2. The result in Fig. 10 displays that pure g-C3N4 presents the highest H2-production rate of 386.7 μmol g−1 h−1 under simulated sunlight. The addition of VOx is not beneficial to the H2 generation of g-C3N4. The detected H2-generation rate is decreased as the VOx content increases. Clearly, the synthesized VOx/g-C3N4 photocatalyst works well in dyes degradation, but is not suitable for photocatalytic H2 production.

photocatalyst. The RhB adsorption and degradation are thus retarded. For practical application purpose, besides the catalytic efficiency, the photocatalytic stability is another critical factor that needs to be considered. Thus, the cyclic experiment of RhB degradation over 0.1% VOx/g-C3N4 sample was conducted, and the result is shown in Fig. 9d. After five cycles, the photocatalytic degradation efficiency of RhB decreased from 99.3% to 80.1%. The decrease of photocatalytic activity is primarily due to the loss of photocatalyst during the recycle, which is inevitable. XRD and FT-IR analyses of the used sample were further performed to investigate the stability of the photocatalyst. The result in Fig. S1 shows that no new phase is detected, indicating the good stability of the composite in structure and composition. Hence, the synthesized VOx/g-C3N4 can be seen to have a good recyclability, indicating its great significance for practical applications. Fig. 10 displays the photoactivity of g-C3N4 and 0.1%VOx/g-C3N4 in the presence of benzoquinone (BQ), isopropanol (IPA), and potassium iodide (KI) which are used to catch superoxide radicals (•O2−), hydroxyl radicals (•OH), and holes (h+), respectively [78,79]. The result in Fig. 10a and b show that the addition of 2-propanol shows nearly no effect on the performance of pure g-C3N4, indicating that •OH is not the reactive species. In comparison, the photoactivity of the g-C3N4 sample is greatly decreased in the presence of KI and BQ scavengers, while the BQ is the stronger inactivating agent. This phenomenon reveals that •O2− species dominates the RhB degradation process on pure g-C3N4, while holes also play an important role. This conclusion is consistent with the previous literature [53]. For 0.1%VOx/g-C3N4 photocatalyst, similar result is obtained (Fig. 10 c and d). The •O2− and holes are still the primary reactive species. Nevertheless, the inactivation effect of KI is more obvious than BQ, indicating that hole species is the dominant reactive species. Additionally, the addition of IPA reduces the degradation velocity from 0.079 min−1 to 0.055min−1, suggesting that the hydroxyl radical species is not ignorable in the photocatalytic degradation process of RhB. The data in Fig. 10 clearly demonstrate that

3.3. Discussion The VOx/g-C3N4 composite prepared via microwave heating method has been verified to show high efficiency in RhB degradation under visible light. Various characterizations have been conducted to reveal the reason for the high photoactivity. It is found that the both the surface area and photoabsorption performance of g-C3N4 are enhanced with the introduction of VOx nanoparticles, which contributes to the RhB adsorption and the photocatalytic reaction. Nevertheless, the consistency between the specific surface area (or the capability in absorbing visible light) and the photoactivity is not observed. For instance, 1%VOx/g-C3N4 has the largest BET value (16.9 m2/g) and the smallest band gap (2.32eV), the RhB degradation is much slower than that of 0.1%VOx/g-C3N4. Therefore, combined with the reported work about composite photocatalysts, the excellent photoactivity of VOx/gC3N4 should be mostly attributed to the elevated separation efficiency of charge carriers, which has been verified via PL, EIS and PC 7

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experiments. However, the origin of the enhanced charge separation still needs to be discussed. Actually, just as said in the preface, some scientists have reported the similar works of V2O5/g-C3N4 photocatalyst [52,53]. The suitable band structures of V2O5 and g-C3N4 trigger the formation a heterojunction structure, which improves the charge separation and subsequently results in the enhanced photoactivity. In the current case, the coupled V2O5 semiconductor is substituted by VOx nanoparticles, which leads to the following question. Do the VOx nanoparticles still have a suitable band potential with g-C3N4 to fabricate the heterojunction? Because the VOx nanoparticles are in-situ formed in the composite and present mixed valence state of V5+ and V4+, it is difficult to synthesize pure VOx nanoparticles and investigate their band potentials. However, on the basis of the reported literatures [80–82], the relative CB and VB positions of the VOx can still be estimated. The electronic properties of V2O5 have been thoroughly investigated via density-functional theory. The result indicates that the VB is mainly composed of O2p, while the CB is dominated by V3d orbits [80]. The self-doping of V4+ into V2O5 would mainly affect the conduction band. An impurity band near the CB would be generated, which usually leads to lower the CB position and increases the light absorption performance. This variation in the V2O5 band structure caused by V4+ self-doping can also be observed in Ti3+ self-doped TiO2 [81,82]. The CB and VB positions of V2O5 have been determined to be 0.49 and 2.71 eV, respectively [53]. For VOx phase, based on the above discussion, it can be inferred that the VB top will remain unchanged, while CB bottom would be slightly lower than 0.49 eV. In other words, just like V2O5, the VOx nanoparticles also present a matched band potential with g-C3N4 and can construct a heterojunction structure which leads to the improved charge separation and photoactivity in RhB degradation. Nevertheless, another issue still exists. There is an inconsistency in explaining the high activity of V2O5/g-C3N4. Two different mechanisms of electron transfer (type II and Z-scheme) have been suggested. For the similar composite of VOx/g-C3N4, what kind of mechanism would work? In order to resolve it, the reactive trapping experiment has been performed. The result in Fig. 10 shows that •O2− and holes are the primary active species of VOx/g-C3N4 composite. Considering that the photogenerated electrons in the CB of VOx cannot reduce the adsorbed O2 to •O2− species (EO2/•O2- = −0.046 V vs NHE) [83], the photoinduced electrons seem to accumulate on the g-C3N4 CB. The Z-scheme mechanism works in the VOx/g-C3N4. However, notably, the VOx content in the composite is very small. It is believed that the photocatalytic reaction still mainly occurs in g-C3N4 phase. Only a portion of photogenerated charge carriers migrate between VOx and g-C3N4, escaping the recombination process, participating in the reaction, and ultimately resulting in the improved photocatalytic performance. It is not strange to find that the •O2− and holes species dominate the RhB degradation process in VOx/g-C3N4. The more important thing is the change induced by the added VOx. As shown in reactive trapping experiment (Fig. 10), •O2− is the dominant reactive species in pure g-C3N4. For VOx/g-C3N4 composite, it becomes the holes. If the VOx/g-C3N4 follows a direct Zscheme mechanism, the electrons would be enriched in the g-C3N4 CB. The •O2− would still be the dominant reactive species. Hence, the only reasonable explanation for the result in Fig. 10 is that the VOx/g-C3N4 catalyst follows the type-II mechanism. Some electrons migrate from gC3N4 to the CB of VOx, reducing the concentration of •O2− species. Meanwhile, the holes transfer from VOx to g-C3N4 strengthens the role of holes in the RhB degradation. Another powerful proof for the suggested type-II mechanism is displayed in the photocatalytic H2-evolution test. As Fig. 11 shows, the addition of VOx exhibits no promotion effect to the photocatalytic H2 production of g-C3N4. That is because the CB position of VOx is positive than the potential of H+/H2 (0 V vs NHE) [83]. The electrons migration from g-C3N4 to VOx can increase the charge separation efficiency. Nevertheless, it is useless for the H2-evolution. If the Z-scheme mechanism works in the VOx/g-C3N4, the enhanced photocatalytic H2 generation would be definitely observed (see

Fig. 11. Photocatalytic H2 production of VOx/g-C3N4 photocatalysts.

Fig. 12. Possible mechanism of VOx/g-C3N4 complex under visible light.

Fig. 12). In addition to the contribution of the electron transport, the presence of V4+ may also partially lead to the increased efficiency in charge separation. Song et al. reported that the self-doping of V4+ would generate oxygen vacancies in the V2O5. The intrinsic electrical conductivity is thus greatly increased. Chen and Lin suggested that the V4+ can act as both hole/electron traps to hamper the annihilation of charge carriers [84,85]. Therefore, it is inferred that the existence of V4+ favors the charge separation in the VOx. In order to confirm this deduction, 0.1%V2O5/g-C3N4 was prepared via the calcination in a muffle furnace at 520 °C. The photocatalytic test indicates that the synthesized 0.1%V2O5/g-C3N4 presents much lower activity than 0.1% VOx/g-C3N4, confirming the promotion effect of V4+ species (Fig. S2). Meanwhile, this result suggests that the decreased photocatalytic activity of the VOx/g-C3N4 with high VOx content may be not only due to that the high VOx content hastens the recombination of charge carriers. The decreased V4+ content caused by the increased V content may be another possible reason. 4. Conclusion VOx/g-C3N4 composite was rapidly prepared by heating the mixture of melamine and NH4VO3 in microwave oven for 40 min. The as-synthesized composite presents an close contact between g-C3N4 and VOx, which promoted the constitution of a heterojunction structure between them and induced the enhanced efficiency in charge separation via the type-II mechanism. Accordingly, the catalytic test demonstrated that the VOx/g-C3N4 complex showed more superior performance than gC3N4 in RhB degradation and photocatalytic H2 production. In addition to the elevated charge separation, the introduction of VOx also increased the specific surface area and photoabsorption performance. This change should be also partially responsible to the improved 8

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photocatalytic activity. This work not only synthesized an efficient photocatalytic for water purification, but also provided an energy-efficient and convenient method for preparation of g-C3N4 based composite.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2019.109122.

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