In-Situ Construction of Bi2O3/Bi2SiO5 Heterojunction Photocatalyst On The Exfoliated Bentonite With Ecient Photocatalytic Performance

In this study, Bi 2 O 3 /Bi 2 SiO 5 heterojunction were in-situ constructed on the exfoliated bentonite via a novel one-pot method. The crystal structure, morphology, and optical features for the as-synthesized Bi 2 O 3 /Bi 2 SiO 5 heterojunctions were systematically characterized by a series of characterization methods. During the preparation process, the exfoliated bentonite acted as the Si source and framework for the in-situ formation of Bi 2 O 3 /Bi 2 SiO 5 p-n heterogeneous junction on the bentonite interlayer. As a result, the Bi 2 O 3 /Bi 2 SiO 5 photocatalyst exhibited a superior photocatalytic performance than that of bare α-Bi 2 O 3 toward the decomposition of Rhodamine B (RhB) via the simulated solar light irradiation, which was due to the synergetic effects of large specic surface areas and p-n junction between Bi 2 SiO 5 and Bi 2 O 3 . Moreover, a probable photocatalytic mechanism for the as-prepared photocatalysts was explored. This work provides a new insight into building the cost-ecient photocatalysts for the contaminant degradation and a latent photocatalytic application of bentonite.


Introduction
Nowadays, energy crisis and environmental pollution including water contaminants with dyes has become a dramatic problem to the eco-environment and human beings (Keshavarzi et  α-Bi 2 O 3 is n-type semiconductor photocatalyst with a bandgap of 2.8 eV, while β-Bi 2 O 3 is p-type semiconductor photocatalyst with a bandgap of 2.4 eV, which can be acted as a good candidate for visible light induced photocatalyst , Qiu et al. 2011, Sang et al. 2020 (Ma et al. 2016b). As compared with other clays, montmorillonite has an outstanding adsorption performance and su cient adsorption/ exchange sites between the layers, on the outer surface and edges (Ma et al. 2016a). Due to these advantages, bentonite has been broadly employed in the wastewater puri cation. In addition, many previous studies have con rmed that bentonite can be served as an excellent substrate for the synthesis of high-performance photocatalysts, for instance, ZnO/bentonite (Selvakumar et al. In this work, we used Bi(NO 3 ) 2 and exfoliated bentonite (EB) as the precursors to synthesize Bi 2 O 3 /Bi 2 SiO 5 heterojunctions by one-step calcination. Interestingly, by adjustment the reaction conditions, bentonite not only served as the substrate, but also reasonably used its internal Si elements to assist the in-situ formation of Bi 2 O 3 /Bi 2 SiO 5 p-n heterojunction in the bentonite interlayer. Compared with α-Bi 2 O 3 , the Bi 2 O 3 /Bi 2 SiO 5 heterojunction photocatalyst not only solves the agglomeration problem of Bi 2 O 3 , but also forms β-Bi 2 O 3 and legitimately creates a p-n heterojunction with Bi 2 SiO 5 , thus exhibiting a good photocatalytic activity for RhB. Meanwhile, the environment friendly raw material in this research is easy to be obtained and the preparation method is simple with low energy consumption, which is expected to the mass production and application in industry.

Raw materials
All the chemical reagents used in this study were of analytical grade and used without any further puri cation. Bentonite clay, Bi(NO) 3 ·5H 2 O and NaF were purchased from Aladdin company.

Preparation of Bi 2 SiO 5 -Bi 2 O 3 -exfoliated bentonite (Bi 2 SiO 5 /Bi 2 O 3 /EB) photocatalysts
The liquid phase exfoliation approach was used to exfoliate the bentonite. In detail, 20 g bentonite clays were dispersed into 0.2 L puri ed water including 1g NaF and treated by ultrasonic processing for 7 days.
After that, the blended solution was treated with centrifugation at 9000 rpm for 5 min and rinsed several times with deionized water. The gray solids obtained by the centrifugation were named as exfoliated bentonite (EB), which was used in all the experiments. Bi(NO 3 ) 3 powder was obtained by treating Bi(NO 3 ) 3 ·5H 2 O at 70 ℃ for 12 h in a vacuum drying cabinet.
After that, Bi(NO 3 ) 3 powder with different masses was added into the quartz agate mortar, then 0.5 g EB was added. After being mixed for 20 min, the mixture was transferred into a crucible at air atmosphere and heated at 600 ℃ for 3 h with a ramp of 2 ℃/min in a mu e furnace. After cooling down to room temperature, the obtained lemon products were Bi 2 SiO 5 /Bi 2 O 3 /EB. The products with different Bi(NO 3 ) 3 of 1 g, 3 g, 5 g and 7 g were named as BiSiB-1, BiSiB-3, BiSiB-5 and BiSiB-7, respectively. The BiSiB-3 samples were calcined at different temperatures of 500 ℃, 600 ℃, 700 ℃, and were marked as BiSiB-500 ℃, BiSiB-600 ℃ and BiSiB-700 ℃, respectively. The schematic illustration for the preparation process was showed in scheme 1. Pure α-Bi 2 O 3 was obtained by the same method without the addition of EB.

Characterization
The crystal structure was investigated by X-ray diffraction (XRD) (D8-FOCUS, BrukerAXS, Panalytical) using Cu Kα radiation. XPS spectra and chemical status were measured by VG Scienti c ESCALAB Mark II spectrometer (Thermo ESCALAB 250XI). The morphology and composition of the as-synthesized samples were analyzed by scanning electronic microscopy (SEM, SU8010, Japan) and transmission electron microscope (TEM, TF20, JEOL 200F). N 2 adsorption/desorption of the as-prepared samples was analyzed using a Micromeritics ASAP2460 surface area analyzer. The optical absorbance spectra analysis was conducted by UV-vis diffuse re ectance spectrometer (Lambda35, Perkin Elemer).
Electrochemical workstation (Chenhua, CHI 660E, Na 2 SO 4 as electrolyte) was used to detect the electrochemical impedance spectra (EIS) and photocurrent experiments. Ag/AgCl electrode was employed as the reference electrode, Pt wire was used as the counter electrode, and the as-prepared catalyst loaded on FTO were used as the working electrode. The working electrode was prepared as follows: 5 mg catalyst was placed into a blended solution of 10 µL Na on solution (5wt%) and 0.3 mL anhydrous alcohol to form a uniform slurry under constant ultrasonic processing. After that, the 30 µL slurry was uniformly coated onto FTO glass with an area of approximately 1 cm 2 and then dried under a natural condition. The EIS Nyquist plots tests were conducted from 0.01 to 100000 Hz under open circuit potential conditions.

Photocatalytic activity test
The photodegradation of organic pollutant (RhB) under simulated solar light irradiation was carried out to evaluate the performance of photocatalysts. The light was provided by a 300 W Xeon-lamp. In all the tests, the photocatalyst sample (0.2 g) was dispersed into 100 mL RhB aqueous solution (20 mg/L) for the photocatalytic activity testing. Before the lighting irradiation, the solution was stirred in dark condition for 60 min to reach an adsorption/desorption equilibrium. Afterward, 3 mL aliquot was taken out and ltered at a regular interval. To determine the corresponding RhB concentration, the maximum absorption wavelength (554 nm) were measured by using the optical characteristic absorption (DH-mini Ocean Optics).

Active species trapping experiments
Disodium ethylenediaminetetraacetate (EDTA), tertiary butyl alcohol, and the ascorbic acid was added into the RhB solution to investigate the main active species, such as remove holes (h + ), hydroxyl radical

Results And Discussion
Structure and composition analysis The crystal information of α-Bi 2 O 3 , exfoliated bentonite and Bi 2 SiO 5 /Bi 2 O 3 /EB photocatalysts was described by using X-ray diffraction (XRD). The results were displayed in Fig. 1. As shown in Fig. 1a, the characteristic diffraction peak for EB sample at 2θ = 8.20° corresponds to (001)  The morphology and structure of the as-prepared photocatalyst as determined by scanning electron microscopy (SEM). As shown in Fig. 2a, the EB has a sheet structure with a thickness of about 1 µm. SEM image of α-Bi 2 O 3 (Fig. 2b) demonstrates that pure α-Bi 2 O 3 shows an agglomerated bulk structure.
However, after the formation of heterojunction, BiSiB-3 andBiSiB-7 samples exhibit a u er scattered structure compared with pure α-Bi 2 O 3 , which is bene cial for the photocatalytic activity (Fig. 2c, 2d). Due to the fact that Bi 2 O 3 /Bi 2 SiO 5 heterojunction was in-situ generated on EB, the sheet structure could also be observed (Fig. 2c).  respectively. Obviously, the speci c surface area of BiSiB-3 was larger than of α-Bi 2 O 3 , which was 8.5 times higher than that of α-Bi 2 O 3 . The increase of speci c area, attributing to the in-situ formation of Bi 2 O 3 /Bi 2 SiO 5 heterojunction on the bentonite surface, not only improves the adsorption capacity of the catalyst for pollutants, but also allows more O 2 to be adsorbed on its surface, which can produce more O 2− with intense oxidizing properties under light irradiation, thus improving the photocatalytic degradation reaction.

Optical and photoelectrical performance
To investigate the optical properties and energy band structure of the photocatalysts, UV-Vis spectroscopy and the Kubelka-Munk equation were used (Wang et al. 2013). As shown in Fig. 6a  Electrochemical impedance spectroscopy (EIS) and photocurrent response were used to characterize the separation and transfer e ciency of photogenerated electro-hole pairs. As we can see from Fig. 7a Among the as-prepared photocatalysts, BiSiB-3 sample shows the optimum photocatalytic activity with a removal e ciency of 96.89%. And the decomposition e ciency for the RhB is 0.0148 min -1 , which is 49 folds than that of α-Bi 2 O 3 (Fig. 8d). Furthermore, as shown in Fig. 6c, the decolourization proportion is about 94.92%, 68.35% and 61.61% for the BiSiB-1, BiSiB-5 and BiSiB-7 samples after the irradiation of 180 min, respectively. The results indicated that the combination Bi 2 O 3 /Bi 2 SiO 5 heterojunction not only increases the speci c surface area to enhance the adsorption capacity, but also improves the separation e ciency of the photogenerated carriers, thereby improving the photocatalytic property.
To further investigate the photocatalytic performance, the reaction kinetics of RhB removal were studied. The kinetic rate constant value were calculated by the pseudo-rst-order model (-ln(C t /C 0 ) = kt) (Xu et al. times than that of BiSiB-1, BiSiB-5 and BiSiB-7 samples, which indicated that BiSiB-3 sample demonstrated the maximum photocatalytic performance toward the RhB degradation. To investigate the stability and reusability of Bi 2 SiO 5 /Bi 2 O 3 /EB sample, the recycling experiments were performed. In this procedure, all the parameters remained the same. As represented in Fig. 9a, the photocatalytic e ciency of BiSiB-3 still achieved 91.0% after four cycles. Moreover, Fig. S4 showed the SEM image of BiSiB-3 after the removal of RhB. The morphology and the sample size did not change obviously. Additionally, after the four times photocatalytic tests, the XRD patterns of BiSiB-3 (Fig. 9d) also had no change. All the above-mentioned results indicated that the as-prepared sample had a good stability.
The capture experiments of the active species were used to prove the photocatalytic degradation of the main active species in RhB. Tert-butanol, ascorbic acid and disodium ethylenediaminetetraacetate (EDTA-2Na) were used as the sacri cial agents of hydroxyl radical (·OH), superoxide radical (·O 2 − ) and hole (h + ) in the photodegradation process of RhB, respectively. As shown in Fig. 9b and 9c, when tert-butanol, ascorbic acid and EDTA-2Na were added to the photocatalytic degradation system, the RhB degradation rate dropped from 79.29-73.30%, 37.25% and 5.05%, respectively. The results demonstrated that h + and ·O 2 − were the principal active species for the photocatalytic degradation of RhB system.
In brief, the improvement of photocatalytic performance is primarily owed to the following four aspects. First, Bi 2 SiO 5 /Bi 2 O 3 /EB has a higher speci c surface area than bare α-Bi 2 O 3 and has a stronger adsorption for RhB. Thus, the active species produced by light irradiation are more likely to oxidize the contaminants adsorbed on the catalyst surface. Second, the exfoliated bentonite was added to the reactants, and as a result, not only Bi 2 SiO 5 but also β-Bi 2 O 3 was in-situ formed at the same time. Due to the lager light response range of β-Bi 2 O 3 , the utilization of light was improved. Third, Bi 2 SiO 5 is a canonical n-type semiconductor and β-Bi 2 O 3 is a p-type semiconductor. Therefore, Bi 2 SiO 5 and β-Bi 2 O 3 can construct a p-n heterogeneous junction, which can effectively separate the electrons and holes, and inhibit the recombination of photon-generated carriers, thereby increasing the photocatalytic e ciency. Furthermore, the negative charge electrostatic interaction on the surface of EB can accelerate the separation of electrons and holes and inhibit the recombination of electron-hole pairs.
According to the above discussion, a possible mechanism of the RhB photodegradation by Bi 2 SiO 5 /Bi 2 O 3 /EB photocatalysts was proposed. As shown in Fig S5,   This study presented a principal route to fabricate the e cient bismuth-based photocatalysts for the environmental puri cation.

Declaration of Competing Interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.

Authors' contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Tao Nie, Ben Chen, Yuyang Huang. The rst draft of the manuscript was written by Tao Nie and all authors commented on previous versions of the manuscript. All authors read and approved the nal manuscript. Availability of data and materials All the data and tools/models used for this work are publicly available.
Ethics approval Not applicable.

Consent to participate
All of the authors consented to participate in the drafting of this manuscript.

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