Visible light Bi2S3/BiFeO3 photocatalyst for effective removal of Rhodamine B

Bi2S3-sensitized BiFO3 (BFO) photocatalyst (Bi2S3/BFO) was successfully synthesized through a facile and environmental ion exchange method between BFO and Thiourea (H2NCSNH2, TU). The photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and UV-vis diffuse reflection spectroscopy (DRS). The obtained Bi2S3/BFO composites showed excellent photocatalytic performance for decomposing Rhodamine B (RhB) compared with pure BFO under visible light irradiation (λ>400nm). 5% Bi2S3/BFO exhibited the highest photocatalytic activity and excessive amount of Bi2S3 would result in the decrease of photocatalytic activity of BFO. The mechanism of enhanced photocatalytic activity was proposed on the basis of the calculated energy band positions.


Introduction
Photocatalysis has received much attention owing to current environment concerns and energy demand [1]. Visible-light-responding photocatalysts have attracted increasing interest in the past years due to their potential applications in splitting of water and degradation of organic pollutants under visible light irradiation. Recently, a growing number of investigations on exploitation of Bicontaining compounds, such as BiVO4 [2,3], Bi2MoO6 [4], Bi2WO6 [5,6], have been undertaken. A series of researches have revealed that whether Bi 3+ -containing or Bi 5+containing compounds have exhibited high photocatalytic ability in visible light range, which is ascribed to the hybridized valence band (VB) of O 2p and Bi 6s that narrows the band gap [7]. As a member of Bi-based multicomponent oxides, typical Rhombohedral structure BFO has been widely studied because of its special properties, such as narrow band gap (2.0-2.7eV), ferroelectric and ferromagnetic properties [8]. BFO had been considered as one of the third-generation visible-light responsive photocatalyst. Gao et al. [9] synthesized perovskite-type BFO photocatalysts via a facile hydrothermal method and used it to degradat Methyl Orange under visible-light irradiation. Gao et al. [10] also synthesized perovskite phase material BFO by simple sol-gel route and evaluated the photocatalytic performance on visible-light degradation of Methyl Orange. However, research concerning the improvement of visible light photocatalytic properties of BFO is still scarce.
Up to now, previous studies had showed that semiconductor combine with other semiconductors could extend the spectral responsive range, separate the charge carriers effectively and further enhance photocatalytic activity of single-component materials dramatically [11,12]. Especially the construction of Bi2S3 heterostructured composites has considered as an attractive and promising method. Bismuth sulfide (Bi2S3) is a typical layered bandgap semiconductor with a narrow band gap (1.3-1.7eV) and large absorption coefficient [13], which has been used for electrochemical hydrogen storage [14], hydrogen sensor [15], biomolecule detection [16], X-ray computed tomography imaging [17], and photo-responsive materials [18]. Because of its narrow band-gap, it was used as a potential visible light photocatalyst through combine with Bi2O3 [19,20], BiVO4 [21], Bi2Sn2O7 [22], Bi4Ti3O12 [23] and BiOCl [24,25]. However, Bi2S3/BFO composite photocatalysts have never been reported.
Herein, we mention a new combination route preparation of sol-gel and solvothermal process for Bi2S3/BFO composites. Rhodamine B (RhB) as a model pollutant under visible light (lambda >400nm) evaluated the photocatalytic performance, results showed that the photocatalytic activity of Bi2S3/BFO composite is better than pure BFO. In addition, also studied the mechanism of Bi2S3/BFO composites.

Preparation of BFO nanoparticles
The BFO nanoparticles sample was prepared through a solgel method according to previous studies [10]. In a typical synthesis, BFO powders were synthesized by a simple solgel route. Bi(NO3)3· 5H2O and Fe(NO3)3· 9H2O of 0.02 mol (1:1 molar ratio) were dissolved in 50 mL Ethylene Glycol as precursor solution, and the final concentration was 0.4 mol/L. The mixture was stirred for about 1.5 h at 80°C to obtain the sol. The gel was kept at 120°C for 4 days to form a xerogel powder. Then the powder was heated at 300°C for 3 h as a pretreatment. The final powder was calcined at the temperatures of 500°C for 2 h under air condition.

Synthesis of Bi2S3/BFO composites
In order to synthesize Bi2S3/BFO composites, 0.1564 g of as-prepared BFO was dispersed in 20mL absolute Ethyl Alcohol under ultrasonic.A certain amount of TU dissolved in absolute Ethyl Alcohol was added dropwise to the BFO solution.Finally, transferred the appropriate absolute Ethyl Alcohol to the mixture to make the volume reach 24mL.After sonication for 30 min, put 30mL of solution into a Teflon-lined stainless steel autoclave and heated at 180°C for 6 h. Finally, the precipitates of Bi2S3/BFO with different theoretical molar percentage of Bi2S3 and initial BFO (from 2.5 to 50%) were collected, washed and dried at 80°C .
For comparison, the bulk Bi2S3 sample was prepared through a hydrothermal process according to Ref [26]. Bi(NO3)3· 5H2O and TU were mixed in deionized water (1:2.5 molar ratio) at 140°C for 12 h.

Characterization of catalyst
The phases of the samples were examined by X-ray diffraction (XRD) on a Philips X'pert PRO diffractometer with Cu Kα radiation. The size and morphology of the prepared samples were observed by scanning electronmicroscope (SEM, Carl Zeiss EVO LS-15). Chemical analysis of the photocatalyst was performed by X-ray analysis (EDS) in a CarlZeiss EVO LS-15 scanning electronmicroscopy (SEM). The UV-Vis diffuse reflectance spectrum (UV-DRS) of the powders were measured by a UV-Vis spectrophometer (Gucun ZF-I) equipped.

Degradation experiment
The photocatalytic performance of the photocatalysts were evaluated by the degradation of RhB under visible light irradiation. The visible light source was a 150 W halogen tungsten lamp (λmax= 588 nm) with the combination of a cut-off filter (λ> 400 nm) to eliminate UV radiation during visible light experiments. The system was cooled by wind and water and maintained at room temperature. In every run, 100mg Bi2S3/BFO was added to 100mL RhB solution (10mg/L) in a Pyrex vessel. Before the experiment, the suspension was magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption-desorption equilibrium of the dye on the catalysts' surfaces. Under visible light irradiation, samples were regularly extracted and the catalyst powders were centrifugally removed. The residual RhB concentration was monitored by measuring the absorbance of the solution at the characteristic absorption wavelength of 554nm for RhB with a UV-Vis spectrophotometer (UV-5100, Metash) at room temperature. The photocatalytic efficiency was calculated by the decolouration percentage: η= C/C0× 100%, where the C is the final dye concentration, C0 is the initial concentration of dye.

Crystalline property
In this study, BFO nano-materials was prepared by a solgel process with Bi(NO3)3· 5H2O and Fe(NO3)3· 9H2O in ethylene glycol solvent followed by heating, calcining and collecting in the first place. Then, the process of product synthesis is as follows.
The composition and phase structures of the asprepared samples were examined by XRD. Fig. 1 presents the XRD patterns of pure BFO, Bi2S3 and Bi2S3/BFO with different Bi2S3 contents. It can be observed that all the samples are well crystallized. The diffraction peaks of pure BFO could be indexed to the standard PDF card of rhombohedral BFO (JCPDS No.86-1518) , as well as the feature peaks of the pure Bi2S3 are clearly corresponding to the orthorhombic structure of Bi2S3 (JCPDS No.17-0320). No impurity peaks were detected, implying that the final products of Bi2S3 and BFO are pure phases.
Furthermore, when the amount of Bi2S3 is below 10%, there are no diffraction peaks of Bi2S3 that can be observed, which may be due to the low content of Bi2S3 below the XRD detection limit. However, when the amount of Bi2S3 reaches 25%, the peaks of (130) and (211) planes of orthorhombic Bi2S3 appear. Moreover, when the amount of Bi2S3 reaches 50%, the peak of (240) planes of orthorhombic Bi2S3 is appearance. In addition, with the molar percentage of Bi2S3 increasing, the intensities of diffraction peaks of Bi2S3 increase whereas those of BFO decrease. Therefore, the above results suggest that Bi2S3/BFO composites can be successfully synthesized by a solvothermal method.   fig. (a), revealing that the size of BFO materials pose irregular morphology with mean sizes below 100 nm. The Bi2S3/BFO samples exhibit two structures: a nanorod shape and a irregular shape particles, as shown in Fig. 2(b), (c), and (e-h). According to previous studies and these images, we can deduce that nanorod shape particles and polygonal shape particles are Bi2S3 and BFO respectively. At the same time, EDS analysis of the nanorods (Fig. 2d) suggests that the nanorods are composed of Bi and S elements, further confirming the result of XRD. All these above confirm the formation of Bi2S3 on BFO. For a semiconductor, theoretically, the optical absorption near the band edge follows the formula: αhν=A(hν−Eg) n/2 [27], where α, ν, Eg, and A are the absorption coefficient, light frequency, band-gap energy, and a constant, respectively. The value of n depends on whether the transition is direct (n = 1) or indirect (n = 4) in a semiconductor. For BFO and Bi2S3, they pertain to direct transition. Thus, n is equal to 1. Fig. 6b shows the curve of (αhν) 2

Photocatalytic activity test
The photocatalytic activities of the prepared products were evaluated by photodegradation of RhB under visible light irradiation (λ>400nm). Fig. 4a shows the change of absorption spectra of RhB aqueous when the Bi2S3/BFO content is 5% after 3 h. It can be seen from the spectra that the major absorption peaks of RhB at 554 nm gradually decreased as the irradiation time increases and blue shifted step by step. The results show that the ethyl groups are removed one by one which is consistent with the literature description. And compared with pure BFO and Bi2S3, 5% Bi2S3/BFO shows the excellent photocatalytic activity for the RhB degradation. Furthermore, the stepwise blue shift of the main peak can be attributed to the formation of the de-methylated of RhB [28]. Fig. 4b shows the photocatalytic activities of different catalysts. For comparison, carried out the same degradation experiment in the absence of photocatalyst which shows no appreciable degradation of RhB after irradiating for 3 h. Also, in the presence of pure BFO and Bi2S3 only about 62.6% and 11.1% of RhB concentration were decomposed after irradiation for 3 h, which could attributed to the high stability and negligible self-photolysis of RhB. The dark adsorption abilities for RhB were 14.4%, 12.1%, 32.5% and 19.6% for 2.5%, 5%, 10%, 25% Bi2S3/BFO samples, respectively. The activities of the four types of Bi2S3/BFO photocatalysts were better than that of BFO, arising from their visible light responsive behaviors. The highest  activity was obtained over the 5% Bi2S3/BFO sample, on which more than 96.3% of RhB was degraded within 3 h. Clearly, the content of Bi2S3 dramatically affect the photocatalytic activities of Bi2S3/BFO catalysists even though the content was very low. RhB was decomposed into 66.8%, 80.3% and 72.1% after 3 h irradiation with 2.5% Bi2S3/BFO, 10% Bi2S3/BFO and 25% Bi2S3/BFO respectively. Therefore, it can be observed that the photocatalytic activities of the Bi2S3/BFO composites increase and then decline with the increase of irradiate time. According to the Langmuir-Hinshelwood reaction kinetics model, the photocatalytic oxidation of Bi2S3/BFO is a firstorder reaction.

The energy band structure and possible photocatalytic mechanism
Whether Bi2S3/BFO heterostructure is conducive to the separation of photo-generated carriers or not is closely related to their band-edge positions. The valence band (VB) edge and the conduction band (CB) edge positions of Bi2S3/BFO were calculated empirically according to formula [27]: EVB=χ-E e +0.5Eg (4)

ECB=EVB-Eg
where EVB is the valence band-edge potential, χ is the electronegativity of the semiconductor, which is defined as the geometric mean of the absolute electronegativity of the constituent atoms, Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), Eg is the band gap energy of the semiconductor. The χ values of BFO and Bi2S3 were 5.74 eV and 5.28 eV [22,29], respectively. The band-gap energies of BFO and Bi2S3 are 2.18 eV and 1.3 eV. Given the equation above, the top of the valence band (VB) and the bottom of the conduction band (CB) of BFO and Bi2S3 are calculated to be 2.33, 0.14 and 1.48, 0.09 eV. Both the CB and the VB of Bi2S3 are higher than those of pure BFO. Obviously, the difference between energy bands of BFO and Bi2S3 demonstrated the heterostructure of Bi2S3/BFO is propitious to the separation and transportation of charge carriers.
Based on the above results, the energy band levels of Bi2S3/BFO and the possible charge-separation process are shown in Fig. 5 Under visible-light irradiation, Bi2S3 and BFO are both easily excited and photo-induced electrons and holes are generated correspondingly. Due to the higher CB position of Bi2S3 than that of BFO photo-generated electrons in Bi2S3 could transfer to the CB of BFO and the photo-generated holes in the VB of BFO migrate to Bi2S3 because of the less positive VB of Bi2S3 than that of BFO. Thus, the photo-induced electrons and holes can be efficiently separated and RhB will be oxidized to the final products. The redox potential of O2/OH¯ is 0.401 eV, the electrons located on the CB of BFO (ECB=0.14 eV) can reduce O2 to OH¯. However, the photo-generated holes at the VB of Bi2S3 with potential of 1.48 eV, which is more negative than the standard reduction potential of OH¯/· OH (1.99 eV) [22], cant oxidize OH¯ to · OH. Therefore, this result indicated that the active species are photogenerated holes that play more important role in the photodegradation process of RhB rather than · OH.

Conclusions
In summary, Bi2S3/BFO composites have been successfully synthesized through a facile and economical ion exchange method. The prepared Bi2S3/BFO photocatalyst with high photocatalytic activity of RhB under visible light irradiation, is about two times higher than the pure BFO photocatalyst. The content of Bi2S3 played an important role to the photocatalytic activities and the optimal content of Bi2S3 was 5% with maximal photocatalytic degradation efficiency of 96.3%. This result could be attributed to the good visible light absorption of Bi2S3 and the effective separation of electron-holes pairs due to the formation of heterojunction between the two semiconductors. The composites displayed much higher activity than the individual components towards the degradation of RhB under visible-light irradiation, suggesting that they are new types of visible-light-driven photocatalysts for water purification application and environmental remediation. Therefore, this study provides a new and simple way to prepare composite nanomaterials.