In recent decades, organic dyes have been widely used in many industrial production process, such as clothing, food processing, medicine, spray and rubber manufacturing industries^[1,2]. The emissions of organic dye wastewater from these industries are growing and become a challenge to human. Photocatalytic decomposition of organic dyes in aquatic environment on semiconductor catalysts, such as TiO₂^[3], WO₃^[4], ZnO^[5], MoS₂^[6,7], is a potential technique to tackle the organic pollution. However, the wide band gap and poor quantum yield of these traditional semiconductor catalysts significantly limit their efficient absorption of visible-light, which is essential for photocatalysts, and result in low photocatalytic efficiency.

Graphitic carbon nitride (g-C₃N₄), a non-metal polymeric visible-light driven photocatalyst with suitable band gap (around 2.7 eV) and valence band (around 1.5 eV) has received intense interest in photocatalysis community owing to its excellent chemical stability, relatively low cost and non-toxicity^[8,9]. Nevertheless, pure g-C₃N₄ faces high recombination rate of photogenerated carriers, inadequate visible-light absorption range and small specific surface area^[10]. Tremendous attempts have been made to make up for these shortcomings, and a Z-scheme photocatalytic system would be a better approach to tackle the problems. An electron mediator or redox shuttle was applied in Z-scheme to regulate charge transfer, creating spatial isolation of photogenerated carriers, more positive valence band potential and more negative conduction band^[11]. The system can inhibit the recombination of electrons-holes pairs, and subsequently improve visible-light induced photocatalytic performance. Recently, many Z-scheme photocatalysts have been prepared to degrade hazardous organic compounds, such as g-C₃N₄/TiO₂^[12], MoO₃/g-C₃N₄^[13], g-C₃N₄/ZnO^[14], SnO₂/Zn₂SnO₄^[15].

Bismuth vanadate (BiVO₄) possesses relatively narrow bandgap (~2.43 eV) and valence band (~2.75 eV), high chemical stability and remarkable energy conversion, and can act as a photocatalyst to degrade organic pollutants^[16,17]. However, for pure BiVO₄, there are problems such as large particle size, small specific surface area, low visible light absorption, and high electron-hole recombination rate, which subsequently leads to low photocatalytic activity^[18]. Therefore, it is necessary to select suitable semiconductor and recombine BiVO₄ into a Z-scheme heterojunction, which could promote the separation of photogenerated carriers to improve the photocatalytic efficiency. Moreover, GO (graphene oxide) owns outstanding electrical conductivity, excellent mechanical properties and high surface area, and could be applied in Z-scheme system as a cocatalyst to improve electron transfer rates and photocatalytic activity^[11,19].

In this work, a new photocatalyst with a ternary heterojunction, BiVO₄/GO/g-C₃N₄, was prepared by a two-step method. The binary composite GO/g-C₃N₄ was synthesized by impregnation-calcination method and combined with BiVO₄ to form a ternary heterojunction. The photocatalytic performance of the ternary system (BiVO₄/ GO/g-C₃N₄), the binary system (GO/g-C₃N₄) and single systems (g-C₃N₄ and BiVO₄) under visible-light radiation, was systematically evaluated by Rhdamine B degradation, and the mechanism of photocatalytic reaction of the Z-scheme heterojunction BiVO₄/GO/g-C₃N₄ was discussed in detail.

Dicyandiamide, bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), ammonium metavanadate (NH₄VO₃), were purchased from Aladdin Chemical Reagent Co. Ltd (Shanghai, China). Graphene oxide (GO) powder was purchased from Suzhou Hengqiu Graphene Co., Ltd., China.

1.1.1 Preparation of the GO/g-C₃N₄

GO/g-C₃N₄ binary photocatalytic heterojunction was prepared by an impregnation-calcination method according to previous report with some modifications^[20]. The GO/g-C₃N₄ samples were synthesized as follows: 36 mg GO was put into 300 mL water, stirred and ultrasound for 0.5 h. Then 120 mg dicyandiamide was dispersed in mixture solution. The mixture solution was ceaselessly stirred at 80 ℃ for 4 h, and then dried in an oven at 80 ℃ for 8 h. In this process, dicyandiamide could deposit on the GO surface via electrostatic interactions. The as-prepared gray solid samples was put into crucibles with the covers, and then heated to 520 ℃ for 2.5 h at a heating rate of 5 ℃·min^-1 and kept at 520 ℃ for 2 h in a quartz tube under flowing pure argon. The calcined product was ground to powder, washed with deionized water for three times and then dried at 60 ℃. The ultimate pale-yellowish green GO/g-C₃N₄ powder was prepared.

1.1.2 Preparation of the BiVO₄/GO/g-C₃N₄ ternary photocatalyst

Firstly, 0.7 mmol Bi(NO₃)₃·5H₂O and 0.7 mmol NH₄VO₄ were completely dissolved in 10 mL 2 mol/L HNO₃ aqueous solution and 10 mL deionized water, respectively. Secondly, different amounts of GO/g-C₃N₄ were added to Bi(NO₃)₃ mixture solution and dispersed by 0.5 h stirring and ultrasound, then NH₄VO₄ aqueous solution was put into the above mixture solution. In order to get the precursor, the resulting pale yellow suspension was vigorous stirred for 1 h at room temperature. Then it was poured into a 50 mL Teflon-lined autoclave, heated at 180 ℃ for 7 h, and naturally cooled to room temperature. The obtained precipitate was washed with deionized water and absolute ethanol for 3 times, respectively. Finally, the samples were dried at 60 ℃ for 4 h. The composite photocatalysts having different mass ratios of BiVO₄ to GO/g-C₃N₄ were recorded as 8 : 2, 6 : 4, 4 : 6 and 2 : 8 (the former is BiVO₄ occupancy). Here the photocatalytic heterojunctions were denoted as B8GC2, B6GC4, B4GC6, B2GC8, respectively according to these different mass ratios. Meanwhile, for comparison, pure BiVO₄ was also prepared in the same manner without the addition of GO/g-C₃N₄.

X-ray diffraction (XRD) patterns were characterized on a D/Max-2500 X-ray diffractometer (Rigaku, Japan) with Cu-Kα radiation at a scan rate (2θ) of 0.05 (°)/s in the range of 3°-70°. The microstructures were examined by a transmission electron microscope (TEM) (Hitachi H7650, HITACHI, Japan) and high-resolution transmission electron micrographs (HRTEM) (TecnaiG2 F20, FEI, America). The sizes and morphologies were collected with a field emission scanning electron microscope (Gemini SEM500, ZEISS, Germany). UV-visible absorbance spectra of the dry-pressed disk samples were obtained by means of a UV-visible spectrophotometer (UV-2600, Shimadzu, Japan) in the range of 200-800 nm with BaSO₄ as a reflectance standard. The chemical bonding status of the samples were analyzed on an FT-IR spectro-meter (FTIR-650, Gangdong Technology, China). The specific surface area of the samples were measured through a mesoporous surface physical adsorber (NOVA4200e, Quantachrome, America). Photoluminescence (PL) spectra were measured under 315 nm excitation wavelength by a Fluorescence Spectrophotometer (F380, Gangdong Technology, China).

All photoelectrochemical studies were carried out by an electrochemical workstation (CHI760E, Chenhua, China) in a standard three-electrode system which used the as-prepared samples as the working electrodes, the platinum wire as the counter electrode, and Ag/AgCl electrode as the reference electrode. 0.1 mol/L Na₂SO₄ aqueous solution was used as the electrolyte. The working electrodes were prepared as follows: 0.2 g photocatalyst was ground and 0.5 mL anhydrous ethanol with the ultrasonic dispersion to make uniform suspension slurry. Then, the obtained slurry was smeared onto a 2 cm×2 cm F-doped SnO₂- coated conducting glass (FTO glass) using spincoating strategy for twice. Photocurrent curves were measured using an Amperometric I-t curves method under a 30 s intermittent irradiation, and the initial voltage was 1.2 V. Electrochemical impedance spectra (EIS) were obtained in the frequency range of 0.01-100,000 Hz at initial voltage of 0 V.

A 500 W Xe lamp (Beijing AuLight, China) was utilized as a visible-light source with 400 nm cut-off filters. The degradation of RhB was used as the evaluation standard for the photocatalytic ability of the sample. The photocatalytic degradation products of RhB were analyzed using an UV-Vis spectrophotometer (UV-2600, SHIMADZU, Japan) at 553 nm.

In the photocatalytic performance evaluation experiment, 10 mg catalysts was added into the RhB (20 mL, 20 mg·L^-1) each time. In order to reach adsorption-desorption equilibrium, the solution was stirred for 30 min in the dark before the irradiation. Suspensions with dosage of 4.0 mL were withdrawn and analyzed regularly. After suspensions were centrifuged (8000 r/min, 10 min) for photocatalyst removal, detected at 553 nm absorbance by UV-Vis spectrophotometer.

The photocatalytic degradation efficiency (E) was obtained for measurement and calculation according to Eq. (1):

Where C is the concentration of RhB at different time, C₀ is the initial concentration at the RhB adsorption equilibrium, A and A₀ are the corresponding absorption values.

The reflective patterns of as-prepared composite samples were studied by powder XRD to infer the phase structures. As shown in Fig. 1(a), the strongest diffraction (002) peak near 2θ=27.4° corresponds to an interplanar distance of the conjugated aromatic system of 0.326 nm, which reflects the interlayer tight accumulation of aromatic rings. Another apparent additional (001) peak relates to an in-plane structural packing motif which interlayer space is 0.672 nm^[21]. In addition, due to incomplete pyrolysis of reactant, the weak diffraction peaks of intermediates were observed in the XRD patterns.

In addition, compared to the crystalline structures of pure BiVO₄, no matter whether it is the shape or position, the added compound has little effect on the characteristic peak of pure BiVO₄. Therefore, BiVO₄ in heterojunction maintains a stable crystal structure, which isn’t affected by the introduction of GO/g-C₃N₄. Due to no introduction of other new peaks, the combination of BiVO₄ and GO/g-C₃N₄ don’t generate impurities, revealing that GO, g-C₃N₄ and BiVO₄ coexist in the ternary nanocomposite photocatalysts.

FT-IR spectra (Fig. 1(b)) show that graphite oxide exhibits strong hydroxyl stretching vibration absorption peak near 3477 cm^-1. The C=O stretching vibration peak on the carboxyl group of graphene oxide appears at 1790 cm^-1. The absorption peak at 1685 cm^-1 may be the absorption peak due to the bending vibration of C-OH^[22]. The peak at 1116 cm^-1 derives from the vibration of graphene oxide C-O-C. Therefore under the experimental conditions, at least 4 functional groups of -OH, -COOH, C-O-C and -C=O exist in the graphite oxide^[23]. The infrared spectrum of GO/g-C₃N₄ shows that the region of 1200- 1650 cm^-1 contains several strong peaks with peaks at 1232, 1326, 1415, 1567 and 1642 cm^-1, which are typical CN heterocycles. The characteristic peak caused by tensile vibration and the peak at 806 cm^-1 is the characteristic breathing pattern of the triazine unit.

No other peaks were observed in BiVO₄/GO/g-C₃N₄ spectrum, probably due to the reduction of graphene oxide during the calcination. Vibration band at 739 cm^-1 is generated by ѵ(VO₄^3-) stretching vibration^[24]. The part of primary bands of g-C₃N₄ and BiVO₄ are included in the ternary nanocomposite, indicating that the BiVO₄/ GO/g-C₃N₄ ternary photocatalyst was synthesized successfully.

To examine chemical composition and states of samples, the prepared photocatalysts were analyzed by XPS characterization (Fig. 2). It is obvious that the detection spectrum of the BiVO₄/GO/g-C₃N₄ can determine that the sample contains only Bi, V and O elements for BiVO₄, and C, N and O elements for GO/g-C₃N₄, and no impurities (Fig. 2(a)). According to the Gaussian curve-fitted signal deconvolution, the high-resolution N1s spectrum of BiVO₄/GO/g-C₃N₄ has N species in different chemical environments (Fig. 2(b)). The highest N1s peak at 398.6 eV corresponds to sp2-hybridized nitrogen (C=N-C). Two other weak N1s peaks locating at 399.8 and 401.5 eV can be assigned to tertiary nitrogen (N-(C)₃) and amino functional groups (C-N-H), respectively. The weak C1s peak (284.6 eV) in the BiVO₄ sample spectrum was produced by the XPS instrument itself contaminating hydrocarbons, whereas the peak of the GO/g-C₃N₄ sample at 288.08 eV was attributed to the sp2-hybridized carbon in N-containing aromatic ring (N-C=N), which positively shifted to 288.38 eV for the BiVO₄/GO/g-C₃N₄ sample in Fig. 2(c)^[25]. The Bi4f spectrum (Fig. 2(d)) of pristine BiVO₄ at 159.08 and 164.48 eV are respectively corresponding to Bi4f7/2 and Bi4f5/2 of Bi³⁺ induced, while these two peaks in the spectrum of BiVO₄/GO/g-C₃N₄ shifted to 158.98 and 164.38 eV, respectively. Compared to Bi4f of pure BiVO₄, B2GC8 exhibits a weak blue shift effect, demonstrating that GO/g-C₃N₄ acts as an electron acceptor and promotes the carrier separation.

TEM images (Fig. 3(a, b) show that the layered g-C₃N₄ tightly anchors on the wrinkled surface of the GO to form a stable heterojunction in the GO/g-C₃N₄ composite. After hydrothermal reaction, 1-5 µm BiVO₄ crystal particles uniformly distributed on the surface of GO/g-C₃N₄, and a ternary photocatalyst was synthesized. In Fig. 3(c), the pure BiVO₄ crystal in the shape of decagon and polyhedron is monoclinic, which is consistent with the results of XRD. It can be observed that the pure BiVO₄ particles own average size of 1-5 µm, while due to the introduction of GO/g-C₃N₄ in the system, BiVO₄ crystals become more delicate and reduce to the size of 100- 500 nm (Fig. 3(d)), which may be affected by the chemical environment on the surface or edge of GO/g-C₃N₄, such as hydroxyl and carbonyl that could easily limit the nucleation and growth process of the BiVO₄ crystal. The layered structure of GO/g-C₃N₄ may include stacking layers, which provides a site for the production and growth of BiVO₄ particles, and are in accordance with the XRD results.

The optical property of semiconductor photocatalysts plays a significant role in investigating the energy band structure for the photocatalytic activity. The UV-Vis diffuse reflectance spectra (DRS) of the obtained samples were revealed in Fig. 4(a). The absorption edge of the as-synthesized pure g-C₃N₄ is at about 556 nm, while a broader absorption edge is at about 550 nm, suggesting that BiVO₄ can effectively utilize visible light for photocatalysis. The addition of GO to the GO/g-C₃N₄ composite increases the background absorption of the sample, allowing the sample to more fully utilize visible-light as compared with pure g-C₃N₄. Moreover, B2GC8 exhibits similar absorption characteristics to pure BiVO₄, indicating that the binding of GO/g-C₃N₄ only on the surface of BiVO₄ did not change the original lattice structure, which is consistent with XRD and SEM results. The widest visible light absorption of the ternary BiVO₄/GO/ g-C₃N₄ composites show that BiVO₄, g-C₃N₄ and GO generate effective synergy.

The band gaps were elevated by the following Eq. (2):

Where α, h, ν, and A are absorption coefficient, Planck constant, light frequency, and a constant, respectively. The n value is determined by the type of optical transition of semiconductors (n=1 for direct transition and n=4 for indirect transition). BiVO₄ belongs to the direct transition semiconductors but g-C₃N₄ direct transition semiconductors. Through data processing and analysis, the E_g values of BiVO₄ (2.43 eV) and g-C₃N₄ (2.7 eV) can be obtained respectively (Fig. 4(b)).

The visible light photocatalytic degradation spectra of the RhB with obtained photocatalysts is showed in Fig. 5(a). It is apparent that g-C₃N₄ and BiVO₄ exhibit poor photocatalytic degradation of RhB solution activity under visible light owing to faster photogenerated electron/hole pair recombination and relatively narrow absorption of visible light regions, which is the same as the UV-Vis diffuse reflectance spectra.

Although g-C₃N₄ and BiVO₄ respond to visible light, the effect is not significant, and the degradation efficiencies after irradiation for 120 min under visible light are 17% and 22%. As contrasted with g-C₃N₄ and BiVO₄, the photocatalytic degradation efficiency of RhB by GO/g-C₃N₄ and B2GC8 composites is significantly improved, reaching 51% and 85%, respectively. The specific surface areas of g-C₃N₄, BiVO₄, GO/g-C₃N₄ and B2CG8 are 7.6, 1.7, 5.7, 3.2 m²·g^-1, respectively. The calculated reaction rates per unit area were 0.197, 1.17, 0.68, 3.29 mg·h^-1·m^-2, respectively. The degradation efficiency of the ternary BiVO₄/GO/g-C₃N₄ composite is higher than those of other catalysts, and the photoluminescence (PL) intensity of the ternary heterojunction was very low. The above results indicate that the Z scheme ternary photocatalyst has an effective redox capability and an excellent charge separation driving force to degrade RhB. The comparison results show that the Z-scheme ternary heterojunction has significant photocatalytic activity under visible light in terms of degradation of RhB.

Fig. 5(b) shows that the photocatalytic decomposition of RhB is supposed to follow a pseudo-first-order kinetics reaction, and the evaluating formula is expressed as Eq. (3):

Where K_app is the apparent rate constant (min^-1), C₀ and C_t is the RhB concentration at reaction time 0 and t, respectively.

In order to obtain more evidence that Z-scheme heterojunction promotes photocatalytic degradation of dyes, samples electrodes are recorded for several on-off cycles of irradiation. As can be seen from Fig. 6(a), the I-t curve of the synthetic sample was analyzed by a 30 s interval dark and visible light illumination cycle. When visible light is irradiated, the photocurrent value is gradually stabilized, and when the light is turned off, the photocurrent rapidly drops to zero, and this process can be cyclically reproduced. The performance indicates that the generated electrons move in the direction of the visible light, thereby generating a certain intensity of photocurrent. In the present case, the photocurrent diagram of B2GC8 ternary heterojunction and GO/g-C₃N₄ binary composites reveals an improved photocurrent response than that of the single BiVO₄, which indicates excellent charge separation efficiency. This apparent enhancement of photocurrent also demonstrates that due to spatial isolation of photogenerated carriers at the interface between BiVO₄, GO and g-C₃N₄, photogenerated electron-hole pairs achieves lower recombination and more efficient separation for the GO/g-C₃N₄ composites. The reduced species in the electrolyte are trapped or captured the hole on the BiVO₄ surface, while the electrons are effectively transported to g-C₃N₄ by GO sheets.

Photoluminescence (PL) analysis was carried out to reveal the intensity of the fluorescence emitted by the recombination of photogenerated electrons and the holes, thus inferring the process of migration, separation and recombination of photogenerated electrons and holes in catalyst. It is obvious that at an excitation wavelength of 315 nm, BiVO₄ is excited to a very strong steady-state fluorescence intensity, and the GO/g-C₃N₄ composite is weaker than that of BiVO₄, which is attributed to the fact that GO can act as an electronic repository to facilitate charge transfer and inhibit charge recombination in Fig. 6(b). The B2GC8 composite was excited to have the weakest fluorescence intensity, indicating that it has a lowest recombination rate of photogenerated carriers under visible-light irradiation. This can be mainly attributed to the transfer of electrons excited on BiVO₄ to the valence band of g-C₃N₄ through GO, while the holes on g-C₃N₄ are transferred to the conduction band of BiVO₄ through GO, thereby achieving spatial isolation of the two carriers. As a result to some extent, direct recombination of electrons and holes was prevented.

Electrochemical impedance (EIS) is an effective method for analyzing the improved charge separation efficiency, which is also conducted in Fig. 6(c). It depicted the magnitude of the impedance arc diameter under sample illumination, g-C₃N₄>BiVO₄>GO/g-C₃N₄> B2GC8, indicating that the B2GC8 ternary coupled photocatalyst has the ability to rapidly transfer and separate photogenerated carriers. This result is consistent with the effect of the RhB degradation experiment.

Through the above characterization results and analysis, the potential photocatalytic mechanism of the Z-scheme BiVO₄/GO/g-C₃N₄ ternary photocatalyst was tentatively proposed, and the reaction process was illustrated in Fig. 7. Due to the energy band gap of 2.43 and 2.7 eV, BiVO₄ and g-C₃N₄ can be excited even under visible light irradiation, resulting in electron hole separation. According to the different conduction band and valence band position of the BiVO₄ and g-C₃N₄, g-C₃N₄ and BiVO₄ anchored on GO by strong interfacial electrostatic interaction to form a Z-scheme heterojunction. Under the interface Z-scheme heterostructure, the photogenerated electrons in the CB of BiVO₄ spontaneously move to the VB g-C₃N₄ and BiVO₄. The interface Z-scheme heterostructure results in spatial segregation of photogenerated carriers and then accumulates electrons and holes. The electrons in the CB of g-C₃N₄ respectively generate ·OH radicals, and ·O^2- has strong oxidation, and then induce RhB degradation together with the holes. Therefore, the photocatalytic performance of the g-C₃N₄/GO/BiVO₄ ternary heterostructure can be significantly enhanced.

A new Z-scheme photocatalyst of ternary BiVO₄/GO/g-C₃N₄ composites was successfully synthesized and employed for the photocatalytic degradation of RhB. In the photocatalytic process of BiVO₄/GO/g-C₃N₄, GO nanosheet act as fast transmission channels between BiVO₄ and g-C₃N₄ and can suppress electron-hole recombination, which significantly promotes the charge separation and improves the redox ability of the ternary heterojunction. As a result, about 85% of RhB dye was degraded by the ternary composites after 120 min visible light irradiation, superior to that of the single or binary system. In addition, the enhanced degradation efficiency could be ascribed to the active h⁺ species in degradation progress in trapping experiment. Overall, the Z-scheme BiVO₄/GO/g-C₃N₄ is an excellent photocatalyst with high-efficiency and has good potential for further application to organic-pollutants decomposition.