Constructing BiOBr/g-C3N4/Bi2O2CO3 Z-scheme photocatalyst with enhanced photocatalytic activity

In this work, the novel camellia-structured double Z-scheme BiOBr/g-C3N4/Bi2O2CO3 was simply prepared by a hydrothermal method. XRD, FTIR, SEM, TEM, XPS, UV-DRS, and PL were used to investigate the composition, morphology, chemical condition, and optical properties of the prepared samples, respectively. The ternary heterojunction photodegraded Rhodamine B under visible light within 60 min with much higher degradation efficiency (98 %) comparing with pure BiOBr, g-C3N4, and Bi2O2CO3. Radical trapping experiments and ESR results exhibited the main reactive species (·O2− and ·OH) during the degradation process. The formation of dual Z-scheme heterojunction improved the rate of charge separation, enhanced the absorption of visible light, and thus promoted the photocatalytic activity. The BiOBr/g-C3N4/Bi2O2CO3 photocatalysts are a promising material for the removal of dye in effluents.


Introduction
Rhodamine B (Rh B), as a widely used bright red cationic dye, can be seriously harmful to environment and wildlife and mutagenic and cancer-causing to humans [1]. Developing high efficiency photocatalysis based on semiconductor has been a green technology to effectively degrade detrimental organic pollutants and solve environment issues [2,3]. Graphitic carbon nitride (g-C 3 N 4 ) with a metal-free, visible light response, and narrow band gap of 2.7 eV has been attractive as a photocatalyst [4,5]. In addition, the conduction band (CB) potential of g-C 3 N 4 is comparatively negative (-1.15 eV) [5], which can bring higher reduction capability to photoexcited electrons. However, high recombination of photon-generated carriers of pure g-C 3 N 4 restricted the further application. A promising method is constructing effective semiconductor Z-scheme heterojunctions to intensify photocatalytic activity by improving the efficiency of charge separation [6,7]. For instance, Zhang [8] has reported remarkable Z-scheme heterojunction Ag 3 PO 4 /g-C 3 N 4 /MoSe 2 , exhibiting high photocatalytic activity and visible light absorption.
Hitherto, due to the appropriate band gap, unique electronic properties, and the response to visible light, bismuth-based photocatalysts have also been attracted extensive attention [9]. All kinds of Bi-based photocatalysts have been investigated, such as BiO 2-CO 3 [10], BiVO 4 [11], Bi 2 WO 6 [12], and BiOX (X = Cl, Br, I) [9]. Among them, BiO 2 CO 3 as a member of Aurivillius-based oxide family, has a twisted layered structure with a strongly positive valence band (VB) value at 3.63 eV [13] for oxidation of organic contaminants, which can provide a smooth transfer path for photogenerated electrons and holes, exhibits excellent catalytic activity, good stability, and low toxicity, and is a class of photocatalysts with application potential [10,14]. Nevertheless, the wide band gap, low utilization of visible light, and low carrier separation efficiency [10] resulted in lower photocatalytic activity of BiO 2 CO 3 . Moreover, the BiOBr compound photocatalyst is easy to accumulate because of its lamellar structure, and can provide enough polarization space, suitable forbidden bandwidth, high specific surface area, and porosity, leading to good photocatalytic activity [9,15]. The layered structures and different solubility constants of BiO 2-CO 3 and BiOBr are easier to bind by simple ion exchange [16], thereby improving separation of photogenerated carriers and photocatalytic activity.
In this work, considering the band position, advantages and disadvantages of BiOBr, g-C 3 N 4 , and Bi 2 O 2 CO 3 , a Z-scheme BiOBr/g-C 3 N 4 /Bi 2 O 2 CO 3 ternary heterojunction composite was designed and synthesized by the simple hydrothermal method. The photocatalytic ability of Rh B degradation under visible light illumination was examined, and the mechanism of the Z-scheme structure was eventually discussed.

Synthesis of photocatalysts
The melamine was thermally polymerized at 550°C for 4 h at a heating rate of 2.5°C/min to prepare g-C 3 N 4 [17].
BiOBr/g-C 3 N 4 /Bi 2 O 2 CO 3 was prepared by simple hydrothermal method. Briefly, 0.485 g of Bi(NO 3 ) 3-5H 2 O, 0.119 g of KBr, and 0.06 g of urea were dissolved in 40 ml deionized water and stirred for 1 h (suspension A). g-C 3 N 4 (0.010 g, 0.025 g, 0.05, and 0.1 g) was dissolved in 30 ml deionized water and sonicated for 30 min (suspension B). The mixture of A and B was stirred for another 30 min and then transferred to autoclave and reacted at 160°C for 24 h. The precipitates were washed with distilled water for three times and dried at 80°C for 12 h. The as-synthesized BiOBr/g-C 3 N 4 /Bi 2 O 2 CO 3 with 0.025 g, 0.05, and 0.1 g of g-C 3 N 4 were named as 0.01CN, 0.025CN, 0.05CN, and 0.1CN, respectively. BiOBr (BOB) and Bi 2 O 2 CO 3 (BOC) were synthesized by hydrothermal method. The detail and other experimental sections are shown in supplementary information.
3 Results and discussion 3.1 Structural analysis XRD patterns in Fig. 1a displayed graphitic-like layered stacking g-C 3 N 4 (JCPDS No. 87-1526), tetragonal BiOBr (JCPDS NO. 78-0348), and tetragonal phase Bi 2 O 2 CO 3 (JCPDS No. 41-1488) [18][19][20]. Characteristic peaks of BOB and BOC were observed in ternary composite, while no peak of g-C 3 N 4 was found due to the low contents and low crystallinity of g-C 3 N 4 in the composites. FTIR spectra are displayed in Fig. 1b. As for g-C 3 N 4 , stretching vibrations of N-H were observed at 3000-3400 cm -1 [21] and stretching vibration of C-N heterocycles was significantly showed at 1200-1600 cm -1 [22]. The peak at 808 cm -1 was attributed to the typical breathing mode of the triazine units [23]. The ternary heterojunction showed no significant difference with the peak of g-C 3 N 4 , indicating that the structural integrity of carbon nitride in the ternary composites was stable. Figure 2 shows the SEM images of the prepared photocatalysts. Pure g-C 3 N 4 showed typically irregular topography (Fig. 2a), BOB exhibited layered rectangular plates (Fig. 2b) and BOC material represented irregular lamellar (Fig. 2c). However, BiOBr/ g-C 3 N 4 /Bi 2 O 2 CO 3 composite in Fig. 2d appeared with a totally different morphology with camellialike shape. This morphology was formed gradually with the content decrease of g-C 3 N 4 (Fig. S1). The possible reason was that the BOB, BOC, and g-C 3 N 4 can disperse well and assemble to form a camellia shape with more interface contact and specific surface area in the presence of small amount of g-C 3 N 4 , which may contribute to the enhancement of the effective electrons and holes separation. The EDS spectrum of 0.025 CN composite in Fig. 2e showed the coexistence and appropriate ratio of C, O, N, Br, and Bi.

Morphology and composition analysis
TEM and HRTEM were further used to investigate the morphology and microstructure of 0.025CN composite. TEM image in Fig. 3a showed a flowerlike outline. HRTEM image in Fig. 3b showed that the two lattice spacings were 0.21(BOC) and 0.28(BOB) nm, respectively, and a good interface contact with g-C 3 N 4 was formed. The coexistence of N, C, O, Br, and Bi elements was depicted in the element mapping (Fig. 3c). These results demonstrated the successful preparation of BiOBr/g-C 3 N 4 / Bi 2 O 2 CO 3 ternary heterojunction.

Element state analysis
X-ray photoelectron spectroscopy (XPS) was used to confirm the valence state and elemental compositions of the product. The characteristic signal of Br, Bi, C, N, and O can be easily observed from the full spectrum (Fig. 4a), which confirmed the presence of these elements in the product. In Fig. 4b, the binding energy of 69.0 eV and 69.9 eV belonged to Br 3d 5/2 and Br 3d 3/2 , indicating the negative univalent of Br [24]. The peaks at 160.0 eV and 165.3 eV in Fig. 4c matched well with photoemission from Bi 4f 7/2 and Bi 4f 5/2 , confirming the valence state of Bi 3? [19]. The C1s spectrum in Fig. 4d displayed three different peaks. These peaks at 284.8 eV, 288.7 eV, and 289.6 were corresponding to the adventitious carbon, N-C = N, and O = C-O, respectively [25]. In Fig. 4e, the peaks at 398.9 eV, 400.1 eV, and 404.3 eV for N 1 s were attributed to the (N-(C) 3 ), (C-N = C), and = NH groups, respectively [21]. The O1s spectra in Fig. 4f implied the existence of lattice oxygen of Bi-O and C-O at binding energy values at 530.4 eV and 531.5 eV, respectively [26].
It was found that the binding energies of Br3d, Bi4f, C1s, and O1s, of 0.025CN were slightly higher than BOB, BOC, and g-C 3 N 4 , while the peak of N1s was slightly lower, indicating a strong interaction between them and further confirming the successful fabrication of heterojunction [27].

Photocatalytic performance and active species
The photocatalytic properties of the as-prepared catalysts were evaluated by the degradation experiment of Rh B under visible light irradiation. As shown in Fig. 5a, the concentration of Rh B solution decreased gradually with the reaction time in the presence of catalysts. Enhanced photocatalytic activity and degradation of 98 % of Rh B dye were observed for 0.025CN within 60 min, which exhibited remarkable photocatalytic activity compared to other reported ternary catalysts (Table S1). The dynamics plots in Fig. 5b revealed that the corresponding rate constant (k) of BOC, g-C 3 N 4 , BOB, 0.1CN, 0.05CN, 0.025CN and 0.01CN was calculated to be 0.00115 min -1 , 0.00915 min -1 , 0.01548 min -1 , 0.02248 min -1 , 0.02669 min -1 , 0.06224 min -1 , and 0.03008 min -1 , respectively. The rate constant of 0.025CN was about 51.1, 6.8, and 4.0 times higher than that of BOC, Fig. 1 a XRD patterns of different samples, b FTIR spectra of 0.025CN, 0.05CN, 0.1CN, and g-C 3 N 4 g-C 3 N 4 , and BOB, implying the best photocatalytic property in the removal of Rh B. The photocatalytic stability test demonstrates the ternary composite's good photocatalytic stability (Fig. S3).
In order to elucidate the photocatalytic mechanism, the trapping experiment of active species was performed. Isopropyl alcohol (IPA), benzoquinone (BQ), and triethanolamine (EDTA-2Na) were used as trapping agents for ÁOH, ÁO 2 -, and h ? scavenger, respectively. The results were displayed in in Fig. 5c.
The photodegradation of Rh B was effectively inhibited by BQ and IPA, indicating that ÁO 2 and ÁOH were the main active species. The free radicals were further verified by electron spin resonance (ESR) spectroscopy. As seen in Fig. 6, no apparent peak was observed for DMPO-ÁO 2 or DMPO-ÁOH in the dark, while obvious strong peaks appeared for both of them under visible light. The signal amplitude ratios were 1:1:1:1 (Fig. 6a) and 1:2:2:1 (Fig. 6b), suggesting the formation of ÁO 2 and ÁOH radicals during the

Optical property and photocatalytic mechanism
Photoluminescence (PL) spectrum was one of the valid methods to evaluate photocatalytic mechanism. An efficiency increase of the photoinduced electronhole separation caused generally the low PL peak intensity [28]. In Fig. 5d, the obvious lower PL intensity of 0.025CN indicated the high catalytic activity. The optical absorption of the semiconductor materials was evaluated by UV-Vis DRS. Wider absorption spectrum range of 0.025CN was observed in Fig. 5e. In Fig. 5f, the band gap of pure g-C 3 N 4 , BiOBr, and Bi 2 O 2 CO 3 were 2.31, 2.67, and 3.38 eV, respectively. The valence band (VB) and conduction band (CB) potentials were calculated using the following equations [13]: E VB = v ? 0.5E g -E c , E CB = E VB -E g . E c represents energy of free electrons on the hydrogen scale (4.5 eV). v is absolute electronegativity of the semiconductor (4.67, 6.176 eV, and 6.54 eV for g-C 3 N 4 , BiOBr, and Bi 2 O 2 CO 3 , respectively) [13,18,29]. The calculated E VB of g-C 3 N 4 , BiOBr, and Bi 2 O 2 CO 3 were 1.33 eV, 3.02 eV, and 3.73 eV, respectively, and their E CB were -0.98 eV, 0.34 eV, and 0.35 eV, respectively.
Based on the characteristic and experimental data discussed above, the photocatalytic mechanism of BiOBr/g-C 3 N 4 /Bi 2 O 2 CO 3 was proposed and described in Fig. 7. There were two types of possible transfer pathway of photoinduced hole-electron pairs: type II and Z-scheme heterojunction. As shown in Fig. 7a, photoinduced electrons (e -) can transfer to CB of BOB and BOC. Simultaneously, the photogenerated holes (h ? ) can migrate to VB of g-C 3 N 4 in the opposite direction. Because of more positive CB of BOB and BOC than CB of O 2 /ÁO 2 -(-0.33 eV) [30] and more negative VB of g-C 3 N 4 than VB of H 2 O/ ÁOH(? 2.72 eV) [31], no ÁO 2 and ÁOH could be produced. This contradicted the free radical capture experiment and ESR results. Hence, the direction Z-scheme system was proposed. The eon the CB of BOB and BOC would migrate to the VB of g-C 3 N 4 , and then the equickly recombined with the h ? on the VB of g-C 3 N 4 inside the Z-scheme heterojunction so that the photogenerated electrons and holes of composite were efficiently separated [31,32]. The ein the CB of g-C 3 N 4 can react with O 2 to produce reactive ÁO 2 radicals due to the more negative CB of g-C 3 N 4 compared with the potential of O 2 /ÁO 2 -(-0.33 eV). Meanwhile, the h ? in the VB of BOB and BOC can react with H 2 O to produce reactive ÁOH radicals due to the more positive VB of BOB and BOC compared to the potential of H 2 O/ÁOH(? 2.72 eV) [33]. Consequently, the ternary heterojunction produced ÁO 2 and ÁOH by visible light illumination that can effectively degrade Rh B.

Conclusion
Double Z-scheme BiOBr/g-C 3 N 4 /Bi 2 O 2 CO 3 heterojunction photocatalyst was designed and successfully synthesized. The obtained sample exhibited a camellia shape and remarkable photocatalytic activity on Rh B degradation. By contrast with pristine BiOBr, g-C 3 N 4 , and Bi 2 O 2 CO 3 , 0.025CN photocatalyst displayed high photocatalytic activity due to the efficient charge separation through the dual Z-scheme heterostructure. The investigation of the scavenger experiments and ESR revealed that ÁO 2 and ÁOH were the main active species in the photocatalytic system. This work provided a potential strategy in the treatment of environmental pollution and remediation.