Fabrication, characterization and photocatalytic activity of g-C₃N₄ coupled with FeVO₄ nanorods

Abstract

A novel FeVO₄/g-C₃N₄ composite photocatalyst was synthesized via a simple mixing-calcination method and characterized by various techniques including the Brunauer–Emmett–Teller method, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, and an electrochemical method. The photocatalytic activity was evaluated by degradation of rhodamine B (RhB). Results indicated that the FeVO₄/g-C₃N₄ composite exhibited a much higher photocatalytic activity than pure g-C₃N₄ under visible light illumination. The rate constant of RhB degradation for the optimal FeVO₄/g-C₃N₄ composite (5.0% FeVO₄/g-C₃N₄) is approximately 2.2 times that of pure g-C₃N₄. The formation of a heterojunction structure between FeVO₄ and g-C₃N₄ which efficiently promoted the separation of electron–hole pairs was believed to be the origin of the enhanced photoactivity.

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1. Introduction

Photocatalytic degradation of organic pollutants by a semiconductor as photocatalyst has attracted considerable interest in the past decades.^1,2 The development of an efficient photocatalyst under visible light is considered as the key factor for its practical application, which has triggered scientists to make great effort to design novel visible-light-driven photocatalysts. A great number of semiconductor photocatalysts, such as Ag₃PO₄,³ BiVO₄,⁴ g-C₃N₄,⁵ and CaBi₂O₄,⁶ were thus been reported. Of these well-known semiconductors, g-C₃N₄ has been proven to be the most representative and extensively used material owing to the following merits.^7,8 First, it has a band gap of approximately 2.70 eV, which fulfills the basic requirements as a photocatalyst for water splitting, CO₂ photoreduction, and organic pollutant decomposition under visible light irradiation. Second, g-C₃N₄ has high thermal and chemical stability. Finally, g-C₃N₄ is a polymeric metal-free semiconductor and can be prepared easily, indicating it is low cost and abundant. Nevertheless, the low surface area, the fast recombination of photoexcited electron–hole pairs, and the lack of absorption above 460 nm still limit the photocatalytic activity of g-C₃N₄. More and more researcher recognized that pure g-C₃N₄ is hardly competent for efficient solar energy conversion.

Meanwhile, heterojunctions that are formed between different semiconductor materials could provide great potential driving force to improve the separation of electron–hole pairs. This finding inspires many scientists to enhance the photocatalytic activity of g-C₃N₄ via the approach of heterojunction construction. Up to date, a great variety of g-C₃N₄ based composite photocatalysts have been reported, such as LnVO₄ (Ln = Sm, Dy, La)/g-C₃N₄,^9–11 TaON/g-C₃N₄,¹² CdS/g-C₃N₄,¹³ AgX (X = Cl, Br, I)/g-C₃N₄,^14,15 ZnO/g-C₃N₄ (ref. 16) and BiOX (X = Cl, Br, I)/g-C₃N₄.^17–19 Among them, metal vanadate semiconductors show great promotion effect due to their suitable band potential and visible light responsibility. For example, Li et al. reported that the doping of SmVO₄ on g-C₃N₄ increased the photocatalytic activity by a factor of two.⁹ Wang et al. used Ag₃VO₄ to modify g-C₃N₄, yielding an enhanced photoactivity on the composite that was eleven that of pure g-C₃N₄ and nearly six times higher than pure Ag₃VO₄.²⁰ Qu et al. synthesized BiVO₄/g-C₃N₄ composite by a simple mixing and calcination method. The prepared composite was proved to be extraordinary more active than that of pure g-C₃N₄.²¹ These results indicate that combing metal vanadate with g-C₃N₄ may provide a feasible way to synthesize an efficient photocatalyst.

In this work, FeVO₄ was chosen as a modifier to promote the photocatalytic activity of g-C₃N₄. FeVO₄ is a narrow band gap semiconductor and shows inherent visible light absorption onset at about 600 nm, which facilitates to collect around 45% of the incident solar spectrum energy. Meanwhile, FeVO₄ has been reported to show good photocatalytic activity under visible light irradiation.^22–24 Zhao et al. synthesized ultrathin FeVO₄ nanosheets and applied it in photodegradation of methyl orange.²² Ozturk et al. synthesized surfactant-assisted FeVO₄ nanostructure and found that the prepared sample exhibited good activity in phenol degradation.²³ Min et al. reported that the coupling of FeVO₄ with TiO₂ could result in an enhanced photoactivity in degradation of RhB.²⁴ However, to the best of our knowledge, no research focused on the photocatalytic activity of FeVO₄/g-C₃N₄ composite has been reported. This paper found that the coupling of FeVO₄ markedly increased the activity of g-C₃N₄ in RhB degradation. With the help of a thorough investigation of the composite's structure, surface area and optical properties, the origin of the higher photoactivity of the FeVO₄/g-C₃N₄ composite was documented.

2. Experimental section

2.1 Catalysts preparation

All these reagents were analytical pure grade and used without further purification. Pure g-C₃N₄ powders were prepared by heating melamine at 520 °C for 4 h at a heating rate of 3 °C min⁻¹ in a semi-closed alumina crucible with a cover. Pure FeVO₄ nanorod was prepared by a hydrothermal method.²² In a typical synthesis, 2 mmol FeCl₃ and 2 mmol NH₄VO₃ were dissolved in 80 mL deionized water under magnetic stirring at room temperature. After stirring for 30 min, the pH value was adjusted to 3 using HCl solution. Then, the resulting mixture was transferred into a Teflon-lined stainless-steel autoclave and heated to 180 °C for 24 h. After the autoclave was cooled to room temperature, the resulting products were collected, washed with distilled water several times, and dried at 60 °C in air.

The FeVO₄/g-C₃N₄ composites were prepared according to the following procedure. A given amount of FeVO₄ and g-C₃N₄ were mixed and ground in an agate mortar for 20 min. Then, the mixture was calcined at 400 °C for 2 h to obtain the FeVO₄/g-C₃N₄ catalyst. By this means, the FeVO₄/g-C₃N₄ composites with different FeVO₄ content were prepared.

2.2 Photocatalytic reaction

The photocatalytic activity of FeVO₄/g-C₃N₄ composite for RhB decolorization was evaluated under visible-light irradiation using a 350 W Xe lamp with two cut-off filter (800 nm > λ > 420 nm) as the light source. Experiments were conducted at ambient temperature and procedures were as follows: 100 mL of RhB solution (20 mg L⁻¹) and 0.1 g of photocatalyst were added to a 250 mL Pyrex glass cell. Then, the suspension was agitated for an hour to ensure the adsorption–desorption equilibrium between the catalysts and the dye. After that, the light is turned on and motivates the photodecolorization process of RhB solution. At regular intervals, samples were withdrawn and centrifuged to remove photocatalyst for analysis. The concentration of aqueous RhB was determined by measuring its absorbance of the solution at 554 nm using a UV-vis spectrophotometer. The examination experiment of active species is similar to the photodegradation experiment. A quantity of scavengers was introduced into the RhB solution prior to addition of the catalyst.^25,26 Chemical oxygen demand (COD) was measured according to the standard dichromate titration method, using a dichromate solution as the oxidizing agent in a strong acid medium.

2.3 Characterizations

The Brunauer–Emmett–Teller (BET) specific surface areas of the catalysts were measured by nitrogen adsorption on Autosorb-1 (Quantachrome Instruments). The powder X-ray diffraction (XRD, Philips PW3040/60) was used to record the diffraction patterns of photocatalysts employing Cu Kα radiation (40 kV/40 mA). The thermogravimetric (TG, Netzsch STA449) analysis of samples were performed from 50 to 800 °C in a flow of air (10 mL min⁻¹) at a heating rate of 10 °C min⁻¹. Scanning electron microscopy (SEM) pictures were taken on a field emission scanning electron microscope (LEO-1530). Transmission electron microscope (TEM) observations were carried out on a JEM-2010F transmission electron microscope at an accelerating voltage of 200 kV. X-Ray photoelectron spectroscopy (XPS) was performed on a Quantum 2000 Scanning ESCA Microprobe instrument using Al Kα. The C 1s signal was set to a position of 284.6 eV. Diffuse reflectance spectroscopy (DRS) was performed on a UV-vis spectrometer (PerkinElmer Lambda900) equipped with an integrating sphere. Photoluminescence spectra (PL) were collected on FLS-920 spectrometer (Edinburgh Instrument), using a Xe lamp (excitation at 365 nm) as light source. The electrochemical impedance spectroscopy (EIS) responses measurements were performed by using a CHI 660B electrochemical workstation with a standard three-electrode cell at room temperature.

3. Results and discussion

3.1 Characterizations of FeVO₄/g-C₃N₄ composites

Although pure g-C₃N₄ exhibits well stability in air below 550 °C, the oxidation process of g-C₃N₄ is usually accelerated when the coupled semiconductor has the capability of activating oxygen.⁹ Therefore, it is necessary to detect the real g-C₃N₄ content for g-C₃N₄ based composite. Fig. 1 shows the TG profiles of g-C₃N₄ and FeVO₄/g-C₃N₄ composite with different FeVO₄ content. It can be observed that FeVO₄/g-C₃N₄ composite presents a sharp weight loss occurring from 480 to 600 °C, which is much lower than that of pure g-C₃N₄. This result accords well with the previous report.^9,10 The real concentration of FeVO₄ can be calculated from the residuals after the samples were heated over 600 °C. It can be concluded that the synthesized FeVO₄/g-C₃N₄ composites has the FeVO₄ concentrations of 3.0 wt%, 5.0 wt%, 7.8 wt%, and 9.8 wt%.

Fig. 1 TG profiles of g-C₃N₄ and FeVO₄/g-C₃N₄ composites.

XRD patterns of FeVO₄/g-C₃N₄ composite photocatalysts with different FeVO₄ concentration are shown in Fig. 2. The diffraction peaks at 2θ = 27.4° and 13.4° can be ascribed to (002) and (100) planes of g-C₃N₄ (JCPDS 87-1526), which correspond to the interplanar staking peaks of aromatic systems and the interlayer structural packing, respectively.²⁷ For FeVO₄, the diffraction peaks are identical with the reported data of a triclinic phase in the JCPDS (38-1372). The XRD patterns of FeVO₄/g-C₃N₄ reveal coexistence peaks of FeVO₄ and g-C₃N₄ in the composite. With the content of FeVO₄ increasing from 3.8 to 9.7 wt%, the diffraction peaks of FeVO₄ increase and the diffraction peaks of g-C₃N₄ decrease gradually. No other new crystal phases in the composite are found.

Fig. 2 XRD patterns of FeVO₄/g-C₃N₄ composites with different FeVO₄ concentration.

The optical properties of g-C₃N₄, FeVO₄ and FeVO₄/g-C₃N₄ were investigated by DRS, and the result is shown in Fig. 3. It can be observed that absorbance threshold of g-C₃N₄ is located at approximately 460 nm, whereas the FeVO₄ sample can absorb light with a wavelength lower than 620 nm. The band gaps of g-C₃N₄ and FeVO₄ can be estimated to be 2.7 eV and 1.98 eV, respectively, according to the Kubelka–Munk equation. Due to the narrow band gap, the introduction of FeVO₄ into g-C₃N₄ clearly improves the photoabsorption performance of the composite. With the FeVO₄ content increasing, the FeVO₄/g-C₃N₄ displays an increased property in visible light absorption, which is beneficial for the photocatalytic reaction.

Fig. 3 UV-vis spectra of g-C₃N₄, FeVO₄ and FeVO₄/g-C₃N₄ composite with different FeVO₄ concentrations (a), and the estimated band gap of FeVO₄ and g-C₃N₄ (b).

Fig. 4 shows the SEM images of pure g-C₃N₄, FeVO₄, and 5.0% FeVO₄/g-C₃N₄ composite. It is revealed that the appearance of FeVO₄ is regular nanorod structure with a uniform diameter of ∼100 nm and length of up to 1–2 micrometers. Pure g-C₃N₄ displays aggregated morphologies, which are comprised of block-based flakiness and particles. For the FeVO₄/g-C₃N₄ composite, the morphology shows the combination of FeVO₄ and g-C₃N₄. Due to their special morphologies of the two semiconductors, it is easily observed that the FeVO₄ nanorods have randomly deposited and distributed on the surface of g-C₃N₄, and formed the composite structures, which accords well the XRD result.

Fig. 4 SEM and TEM images of g-C₃N₄ (a), FeVO₄ (b) and 5.0% FeVO₄/g-C₃N₄ composite (c and d).

High-resolution transmission electron microscopy (HRTEM) analysis was performed to get information on the microstructure of FeVO₄/g-C₃N₄. Fig. 5 shows the low- and high-magnification TEM images of the 5.0% FeVO₄/g-C₃N₄. Given the different morphology and molecular weight of FeVO₄ and g-C₃N₄, the dark parts in the TEM image (Fig. 5a) should be FeVO₄, while the light parts correspond to g-C₃N₄. It can be observed that FeVO₄ nanorods disperse in g-C₃N₄, which agrees well with the SEM result. Meanwhile, considering that the FeVO₄/g-C₃N₄ hybrids were ultrasonicated for 20 min before TEM analysis, the result in Fig. 5 indicates the strong interaction between FeVO₄ and g-C₃N₄ in the hybrids. The clear lattice fringe of 0.3609 nm in the HRTEM image of FeVO₄/g-C₃N₄ should be ascribed to the (100) planes of FeVO₄. Smooth and intimate interfaces are clearly observed between the g-C₃N₄ and FeVO₄, which confirms the formation of FeVO₄/g-C₃N₄ heterojunction.^28,29 The observed interface is possible favorable for the transport of photogenerated carriers, and thereby promotes the separation of electron–hole pairs.

Fig. 5 TEM (a) and HRTEM (b) images of 5.0% FeVO₄/g-C₃N₄ composite.

The surface chemical compositions and states of the g-C₃N₄, FeVO₄, and FeVO₄/g-C₃N₄ were investigated by XPS. The survey XPS spectra (Fig. 6a) indicates that C 1s and N 1s peaks are detected for g-C₃N₄ and FeVO₄/g-C₃N₄, while Fe 2p, V 3d, and O 1s peaks are displayed for FeVO₄ and FeVO₄/g-C₃N₄. This result is consistent with the chemical composition of the photocatalysts. Fig. 6b shows the high-resolution X-ray photoelectron spectra (HRXPS) of C 1s. Pure g-C₃N₄ shows two peaks at 284.6 eV and 287.8 eV, which can be assigned to the adventitious carbon and the sp²-bonded carbon in N-containing aromatic rings (N–C=N), respectively.³⁰ The FeVO₄/g-C₃N₄ composite also display the two C 1s peaks. However, compared with pure g-C₃N₄, the peak corresponding to the g-C₃N₄ shows a slightly positive shift. This kind shift is also observed in the N 1s XPS spectra (Fig. 6c), and can be ascribed to the fact that FeVO₄ hybridized with g-C₃N₄. Indeed, the hybridization of FeVO₄ and g-C₃N₄ influence not only the C 1s and N 1s spectra, but also the Fe 2p and V 2p spectra. As shown in Fig. 6d, the V 2p_3/2 and 2p_1/2 peaks of the FeVO₄ are located at 516.9 eV and 525.2 eV, respectively, corresponding to V⁵⁺ in FeVO₄.³¹ The BE of Fe 2p_3/2 and 2p_1/2 of the FeVO₄ is determined to be 711.2 eV and 725.3 eV, respectively, indicating the 3+ valence of Fe.³² Both of them are slightly lower than those of FeVO₄/g-C₃N₄. The XPS results prove the strong interaction between FeVO₄ and g-C₃N₄, which accords well with the TEM result. Fig. 6f shows the VB XPS spectra of g-C₃N₄ and FeVO₄. The positions of the VB edges of g-C₃N₄ and FeVO₄ are determined to be 1.50 and 1.76 eV, respectively.

Fig. 6 XPS spectra of g-C₃N₄, FeVO₄ and 5.0% FeVO₄/g-C₃N₄ composite (a) survey spectra (b) C 1s, (c) N 1s (d) V 2p, (e) Fe 3p, (f) VB XPS spectra of FeVO₄ and g-C₃N₄.

The specific surface area of FeVO₄/g-C₃N₄ composite was investigated by N₂ adsorption method. Pure FeVO₄ exhibits a BET surface area of 13.3 m² g⁻¹, which is slightly higher than that of pure g-C₃N₄ (11.2 m² g⁻¹). For the samples of 3.0% FeVO₄/g-C₃N₄, 5.0% FeVO₄/g-C₃N₄, 7.8% FeVO₄/g-C₃N₄ and 9.8% FeVO₄/g-C₃N₄, the BET values are equal to 12.7, 17.1, 15.3, 19.0 m² g⁻¹, respectively. These values are in the same order of magnitude, indicating that the addition of FeVO₄ to g-C₃N₄ affects the BET area slightly. Meanwhile, no correlation between the FeVO₄ content and the BET value is observed.

3.2 Photocatalytic activities of FeVO₄/g-C₃N₄

Due to RhB is a common contaminant in industry wastewater, the photocatalytic oxidation of RhB was used to investigate the photocatalytic activity of FeVO₄/g-C₃N₄ composites. Fig. 7 shows the photocatalytic activity of g-C₃N₄, FeVO₄ and FeVO₄/g-C₃N₄ with different FeVO₄ concentration. The blank test indicates that the photolysis of RhB is negligible. Pure FeVO₄ shows a low photocatalytic activity. The reaction rate constant (k) is estimated to be 0.002 min⁻¹, which is only 1/5 of that of pure g-C₃N₄. The addition of FeVO₄ on g-C₃N₄ enhances the photocatalytic activity. The photocatalytic activity of FeVO₄/g-C₃N₄ increases with the increase in the FeVO₄ content up to 5.0%. When the FeVO₄ content is higher than 5.0%, the photoactivity of the sample decreases gradually. It is clear that the amount of FeVO₄ has an important influence on the photocatalytic activity of the samples. The reason may be that when the amount of FeVO₄ is higher than 5.0%, the excess FeVO₄ on the surface of g-C₃N₄ may hinder light absorption of g-C₃N₄ and cover the active sites in g-C₃N₄, thereby the photocatalytic activity of the samples is decreased. Under the experimental conditions, the optimal concentration of FeVO₄ in FeVO₄/g-C₃N₄ is 5.0 wt%. The 5.0% FeVO₄/g-C₃N₄ presents the highest photocatalytic activity with a k of 0.022 min⁻¹, which is 2.2 times higher than that of pure g-C₃N₄. For comparative purpose, the photocatalytic activity of FeVO₄/g-C₃N₄ mixture was also investigated, and the result is shown in Fig. 7. It can be observed that the photoactivity of 5.0% FeVO₄/g-C₃N₄–PM is much lower than that of 5.0% FeVO₄/g-C₃N₄, indicating the synthesized hybrid is different from a physical mixture, as proved by the XPS and TEM experiments.

Fig. 7 Photocatalytic activity of FeVO₄/g-C₃N₄ composite with different FeVO₄ concentration (a) and the corresponding first-order kinetics plot (b).

Although the data in Fig. 7 has proven the high photocatalytic activity of FeVO₄/g-C₃N₄ composite, it should be noted that the obtained photoactivity means the catalyst's ability in dye decolorization. In other words, it is unknown whether the decolorization RhB was completely degraded to inorganic compounds. In order to resolve this issue, COD measurements of the RhB solution were also performed. The result shows a COD removal of 68.4% over 5% FeVO₄/g-C₃N₄ sample, which is slightly lower than that of color removal. Considering that the color is removed faster that of COD over many photocatalysts,^33–35 the synthesized FeVO₄/g-C₃N₄ hybrid can be seen as an efficient photocatalyst.

In addition to the photocatalytic activity, the catalyst's lifetime is another important parameter. Therefore, it is essential to evaluate the stability of the catalyst for practical application. 5.0% FeVO₄/g-C₃N₄ photocatalyst was chosen to as a sample to evaluate the stability of FeVO₄/g-C₃N₄ composite. Fig. 8 shows the photocatalytic activity of 5.0% FeVO₄/g-C₃N₄ sample during the repetition tests. Result suggests that there is no obvious decrease in the photocatalytic efficiency of RhB after five times experiments, which indicates that FeVO₄/g-C₃N₄ has a good stability in the photocatalytic reaction process.

Fig. 8 Cycling runs of 5.0% FeVO₄/g-C₃N₄ composite.

3.3 Possible mechanism of FeVO₄/g-C₃N₄ composite

One can see that, although the photoabsorption performance of the FeVO₄/g-C₃N₄ composite increases with the FeVO₄ content increasing, the photocatalytic activity of the samples does not. The similar phenomenon occurs on the adsorption of RhB. In general, a high adsorption of RhB is desired for the photocatalytic reaction. In the case of FeVO₄/g-C₃N₄ composite, all samples present similar ability in RhB adsorption (not shown here), which may be attributed to their similar BET surface areas. The result implies that both the optical property and the adsorption capability of the samples are not the key factor that influences the photocatalytic activity. There should be another crucial factor. Indeed, besides the optical absorption and surface area, the efficient charge separation of a semiconductor usually plays a crucial role in determining its photocatalytic property.³⁶ Especially for the composite photocatalysts, the enhanced separation efficiency via the charge transfer between different semiconductors has been believed to be the most important reason for their high photocatalytic activity. Therefore, the band potentials of g-C₃N₄ and FeVO₄ were investigated in order to clarify the charge separation in FeVO₄/g-C₃N₄ composite. The VB edges of FeVO₄ and g-C₃N₄ have been determined to be 1.76 eV and 1.50 eV, respectively, via the VB XPS experiment. By the equation of E_CB = E_VB − E_g, the CB edge potentials of FeVO₄ and g-C₃N₄ can be calculated to be −0.22 eV and −1.2 eV, respectively. On the basis of the above results and previous reports,^37–39 a possible mechanism for the enhanced photocatalytic activity over the present FeVO₄/g-C₃N₄ is shown in Fig. 9. Under visible light irradiation, both FeVO₄ and g-C₃N₄ can be excited and generate electron–hole pairs. Since the CB edge potential of g-C₃N₄ is more negative than that of FeVO₄, the photoexcited electrons on g-C₃N₄ can transfer to the CB of FeVO₄. Meanwhile, the photogenerated holes on FeVO₄ can migrate to the VB of g-C₃N₄. In such a way, the photoexcited electron–hole pairs could be effectively separated, which suppresses the recombination of electron–hole pairs and subsequently results in an enhanced photocatalytic activity of the FeVO₄/g-C₃N₄ heterojunctions.

Fig. 9 Possible scheme for electron–hole separation and transport in the FeVO₄/g-C₃N₄ composite.

In order to support aforementioned mechanism, PL experiment was performed to analyze the electron–hole pairs' recombination in FeVO₄/g-C₃N₄ composite. Based on the reported literatures,^40,41 it could be known that the higher PL intensity usually means more recombination of electron–hole pairs and lower photocatalytic activity. As shown in Fig. 10, pure g-C₃N₄ exhibits a strong emission at about 460 nm, which is similar to the previous reports.^13,14 Once FeVO₄ was added, the emission intensity of PL spectra for the FeVO₄/g-C₃N₄ composite is greatly decreased. Actually, the weakened PL emission is also observed on the physical mixture of FeVO₄ and g-C₃N₄ (5.0% FeVO₄/g-C₃N₄–PM). However, the PL peak of the 5.0% FeVO₄/g-C₃N₄–PM is still much higher than that of 5.0% FeVO₄/g-C₃N₄ composite. For the FeVO₄/g-C₃N₄ physical mixture, the weaken PL emission can be attributed to that less light reaches g-C₃N₄ due to the decreased g-C₃N₄ content. For the FeVO₄/g-C₃N₄ composite, the weaker PL peak when compared to the 5.0% FeVO₄/g-C₃N₄–PM definitely indicates that the composite has a much lower recombination rate of electron–hole pairs, which is consistent with the aforementioned mechanism.

Fig. 10 PL spectra of g-C₃N₄, 5.0% FeVO₄/g-C₃N₄–PM and 5.0% FeVO₄/g-C₃N₄ composite.

In addition to PL analysis, the EIS experiment was also carried out to investigate the separation efficiency of electron–hole pairs of the FeVO₄/g-C₃N₄ composite. Fig. 11 shows the EIS changes of g-C₃N₄, FeVO₄ and 5.0% FeVO₄/g-C₃N₄ composite. The arc size of the three electrodes is 5.0% FeVO₄/g-C₃N₄ < FeVO₄ < g-C₃N₄. In general, the decreased semicircle diameter indicates the smaller charge-transfer resistance on the electrode surface which results in an effective electron–hole separation.^42,43 The result in Fig. 11 indicates that the FeVO₄/g-C₃N₄ composite exhibits lower interfacial charge-transfer resistance, which would greatly improve carrier transport efficiency and is beneficial for reducing the recombination rate of photogenerated electrons and holes. This result agrees well the PL analysis and the suggested mechanism in Fig. 9.

Fig. 11 EIS changes of g-C₃N₄, FeVO₄ and 5.0% FeVO₄/g-C₃N₄ composite.

The reactive species trapping experiment was performed to further investigate the active species in the photocatalytic reaction on FeVO₄/g-C₃N₄ hybrid. Fig. 12 shows the photocatalytic activity of 5% FeVO₄/g-C₃N₄ with different quenchers. It can be observed that the addition of benzoquinone (BQ, quencher of ˙O₂⁻) leads to a significantly inactivity of the composite. The reaction constant decreases from 0.022 to 0.002 min⁻¹, indicating ˙O₂⁻ is the main reactive species in the photocatalytic process.^25,26 The addition of kalium iodide (KI, quencher of h⁺ and ˙OH) can also dramatically decrease the photocatalytic activity, whereas isopropanol (IPA, quencher of ˙OH) just shows a slight effect on the reaction of RhB photodegradation.^25,26 This result implies that h⁺ plays an more important role than ˙OH species. The data in Fig. 12 indicates that ˙O₂⁻ and h⁺ are two main reactive species in the photocatalytic process of FeVO₄/g-C₃N₄ photocatalyst.

Fig. 12 Photodegradation of RhB over 5.0% FeVO₄/g-C₃N₄ composite with different quenchers.

4. Conclusion

A novel FeVO₄/g-C₃N₄ composite photocatalyst was prepared by a simple milling and calcination method. The synthesized FeVO₄/g-C₃N₄ composite show obviously enhanced photocatalytic activity in RhB degradation under visible light irradiation. Such a remarkable enhancement of photocatalytic efficiency is mainly ascribed to the match of conduction and valence band levels between the g-C₃N₄ and the FeVO₄, which can induce the high separation of photogenerated electron–hole pairs in the heterojunction system. This work demonstrates that the combination of FeVO₄ and g-C₃N₄ may be an ideal system for practical application in environmental purification.

Acknowledgements

We acknowledge Dr Xiaodong Yi in Xiamen University for his help in XPS analysis. This work was financially supported financially supported by Natural Science Foundation of Zhejiang Province in China (LY14B030002).

Notes and references

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