Highly electro-conductive graphene-decorated PANI-BiVO4 polymer-semiconductor nanocomposite with outstanding photocatalytic performance

Materials Science & Engineering B 251 (2019) 114469

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Highly electro-conductive graphene-decorated PANI-BiVO4 polymersemiconductor nanocomposite with outstanding photocatalytic performance

T

Md Rokon Ud Dowla Biswasa, Kwang Youn Chob, Jae Doc Naa, Won-Chun Oha,

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a b

Department of Advanced Materials Science & Engineering, Hanseo University, Seosan-si, Chungnam 356-706, Republic of Korea Korea Institutes of Ceramic Engineering and Technology, Soho-ro, Jinju-Si, Gyeongsangnam-do, Republic of Korea

ARTICLE INFO

ABSTRACT

Keywords: Safranin O Polyaniline Heterojunction Photocatalytic stability Visible light

Polyaniline/Bismuth Vanadate/Graphene Oxide (PANI-BiVO4-GO or BGPA) composite was prepared by sonochemical deposition of bismuth vanadate-graphene oxide (BiVO4) nanoparticles on the surface of polyaniline (PANI). The 5 wt% BGPA composite had the best photocatalytic degradation performances for MB, RhB, and SO dyes, about 1.56, 1.81, and 1.68 times more, respectively, than 1% BGPA composite. Meanwhile, the photocatalytic stability of BiVO4 was significantly improved by introducing PANI into the PANI-BiVO4-GO composite. These improvements in photocatalytic degradation performance and photocatalytic stability can be attributed to the formation of a heterojunction free electron between PANI and BiVO4-GO. The existence of these extra free electrons can dramatically enhance the efficiency of photogenerated electrons, thus accelerating the transfer of photogenerated holes from BiVO4-GO to PANI and constraining the self-oxidation of BiVO4.

1. Introduction Solar energy is used directly to address issues of energy shortage and environmental pollution. It is one of the most important technologies [1,2]. In the last 40 years, many studies have been performed in the area of solar energy. These studies have greatly promoted the development of this field. However, the low quantum yield and weak response of photocatalyst in observable light region are still problems that limit its wide application. Thus, developing highly effective visible light receptive photocatalysts with high quantum yields is needed to break the bottleneck of this technology [3–6]. BiVO4 has been reported to possess good visible-light photocatalytic property. Bi et al. [7] have verified that BiVO4 with monoclinic structure shows super high photocatalytic property. Afterward, many studies have been systematically performed to investigate the photocatalytic property of BiVO4. It has been demonstrated that the photocatalytic property of BiVO4 is clearly better than those of other currently used visible-light responsive photocatalysts [8–11]. Recently, Ansari et al. [12] have developed Ag/ TiO2@Polyaniline nanocomposite film and found that this composite

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material possesses very strong visible light-induced photocatalytic capability. Preparing graphene on the surface of BiVO4 can also effectively enhance the photocatalytic performance of BiVO4 because of the super high electron migration rate of graphene and the heterojunction electric field formed on the interface of graphene and BiVO4 [13–15]. Huang et al. [16] have prepared series Ag@silver salt photocatalysts and observed that photocatalytic performances of these photocatalysts are associated with the stability and charge of the anion in silver salts. Yu et al. [17] have prepared BiVO4 nanoparticles on PANI fiber by ion conversation process and found that the heterojunction system could significantly increase the photocatalytic performance of the PAN/ BiVO4 composite. However, BiVO4 itself has a relatively weak photocatalytic stability. Photogenerated holes by BiVO4 possess very high oxidation capability which plays a major role in the photocatalytic degradation process of BiVO4. If these photoinduced holes generated by BiVO4 cannot swiftly be consumed by reacting with the surrounding electrolyte or transferring to other materials, they will quickly oxidize BiVO4, leading to a rapid photoinduced corrosion of BiVO4 and a fast decay of the photocatalytic property of BiVO4. Therefore, the photo-

Corresponding author. E-mail address: [email protected] (W.-C. Oh).

https://doi.org/10.1016/j.mseb.2019.114469 Received 6 July 2018; Received in revised form 5 October 2019; Accepted 9 November 2019 Available online 14 November 2019 0921-5107/ © 2019 Published by Elsevier B.V.

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Fig. 1. Sheet type sample for electrical properties measurement (a) BGPA-1 (b) BGPA-2 (c) BGPA-3 & (d), (e) measurement device.

graphene sheet with carboxylic, phenol hydroxyl, and epoxide groups on its edges and basal planes. It can be produced by the chemical oxidation of graphite and subsequent exfoliation [26,27]. Based on its aqueous stability, low production cost, and amphiphilic behavior [28–30], GO is a promising material as a building block for graphenebased nanomaterials and various applications such as conductive thin film, biosensors, and biomedical devices [31–33]. Additionally, it is worth noting that there has been no report on the synthesis or photoactivity of a BiVO4-GO-PANI system. However, it is very important to perform systemic studies on the pathway and mechanism of photoinduced electron-hole pairs under visible-light irradiation in order to design more efficient visible-light-driven photocatalytic composite materials that can meet the requirement of practical environmental application. Polyaniline (PANI) is the most intensively studied conductive polymer material. It has many applications in catalytic and photocatalytic areas [34–38]. PANI is a kind of π-conjugate long-chain polymer. Protonic acid doped PANI possesses very high conductivity. Meanwhile, PANI has three redox states: leucoemeraldine base, emeraldine base, and pernigraniline base. PANI with leucoemeraldine base possesses a weak reduction capability due to the existence of –NH– bonds for connecting benzene rings [39,40]. A p-n heterojunction electric field will be made at the interface if PANI forms a composite with an n-type semiconductor material. Under the influence of this electric field, photogenerated holes will be swiftly transferred to PANI and oxidize surrounding materials while photogenerated electrons will be rapidly transferred to the composited n-type semiconductor material which causes reduction reactions on the surface of this n-type

Fig. 2. XRD patterns of (a) BiVO4; (b) BiVO4-GO; (c) BGPA-1; (d) BGPA-2; (e) BGPA-3; samples (Produced by Ultrasonic process).

catalytic stability of BiVO4 needs to be further improved [18–23]. Graphene, a two-dimensional (2D) single atomic sheet of sp2-hybridized carbon atoms, has attracted great interest over the past decade due to its extraordinary electrical properties, optical transparency, and biocompatibility [24,25]. Graphene oxide (GO) is a waterdispersible

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Fig. 3. SEM image of different kind of sample, (a) BiVO4-GO; (b) BGPA-1; (c) BGPA-2 & (d) BGPA-3 (Produce by Ultrasonic process).

Fig. 4. TEM image of different kind of sample, (a) BGPA-1; (b) HRTEM of BGPA-2; (c) BGPA-2 & (d) BGPA-3 (Produce by Ultrasonic process).

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semiconductor material. Zhang et al. [41] have prepared a PANI@ BiVO4composite with quasi-shell-core structure by covering a PANI layer with thickness of approximately 0.717 nm on the surface of BiVO4. They reported that PANI coating could enhance the photocatalytic property of BiVO4 and inhibit the photocorrosion of BiVO4. They attributed these advantages to the strong photogenerated hole trapping capability of PANI. Photogenerated holes can be rapidly transferred to π-conjugated PANI, leading to the reduction of photocorrosion of BiVO4. Therefore, PANI-BiVO4-GO or BGPA composite is predicted to exhibit improved performance as a highly efficient visible light responsive photocatalyst. In the present study, PANI with branch-like structure was created followed by sonochemical growth of BiVO4-GO nanoparticles on the surface of PANI. During the sonochemical growth of BiVO4-GO nanoparticles, metallic Bi (Bi0) was created on the surface of PANI, resulting in the formation of PANI-BiVO4-GO) or BGPA composite. Subsequently, the photocatalytic degradation and stability performance of these prepared PANI-BiVO4-GO or BGPA composites were studied. 2. Experimental section Fig. 5. FTIR result of Bismuth Vanadate Graphene Oxide Polyaniline, Prepared by Ultrasonic process BGPA-3; BGPA-2; BGPA-1& BiVO4-GO etc Composites.

Polyaniline (molecular weight ~105) was purchased from Jilin Zhengji Corp. All other substances used in this study were of analytical purity. They were used as received from Dejone Chemical Co, Korea. 2.1. Synthesis of BiVO4-graphene oxide composite photocatalyst The typical preparation of BiVO4-GO was as follows. Graphene oxide (GO) was synthesized from purified natural graphite with a mean particle size of 44 nm (Qingdao Zhongtian Company) as reported previously [44]. BiVO4-GO was synthesized with a facile hydrothermal method. A typical experiment for the synthesis of BiVO4-graphene composite was as follows. First, 40 mg of GO was dispersed into 10 mL of absolute ethanol and 10 mL of de-ionized (DI) water followed by sonication for 1 h. Then 0.1796 g of Bi(NO3)3·5H2O and 0.0430 g of NH4VO3 were separately added to two solutions of absolute ethanol (10 mL) followed by stirring for over 30 min at room temperature. These three systems were then combined, adjusted to a pH of 8.0 with an ammonia solution, and mixed for 30 min, yielding a stable bottlegreen colored slurry. The resulting mixture was transferred to a 50 mL Teflon-lined stainless-steel autoclave and heated to 180 °C for 6 h under autogenous pressure. The reaction mixture was cooled to lukewarm. The precipitate was filtered, washed with distilled water five times, and then dried in a vacuum oven at 60 °C for 12 h to yield greenish color powder type sample. The product was labeled as BiVO4-GO.

Fig. 6. DRS spectra of BGPA-1; BGPA-2; BGPA-3.

2.2. Preparation of composite catalysts The typical preparation of BiVO4-GO-PANI photocatalyst was as follows. PANI was dissolved in tetrahydrofuran (THF) to obtain a concentration of 0.081 g·L−1 solution. A certain amount of BiVO4-GO powder was then added into 100 mL of the above solution. The suspension was ultrasonicated for 2 h, stirred for 6 h, and then filtered. The as-produced precipitate was washed with water three times and then dried at 343 K for 10 h. BiVO4-GO-PANI photocatalysts with different mass ratios (1, 3, and 5%) were synthesized by this method. We marked 1 wt%, 3 wt%, and 5 wt% samples as BGPA-1, BGPA-2, and BGPA-3, respectively. 2.3. Characterization X-ray diffraction (XRD) patterns of samples were measured by D/ MAX 2250 V diffractometry (Rigaku) using monochromatized Cu Kα (λ = 0.15418 nm) radiation under 40 kV and 100 mA with the following scanning range: 10° ≤ 2θ ≤ 70°. Morphologies and microstructures of as-prepared samples were analyzed by scanning electron

Fig. 7. Raman result of BGPA-1; BGPA-2 and BGPA-3 sample. 4

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Fig. 8. XPS results of BGPA-3 sample (a–f).

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obtained with a UV–vis spectrophotometer (Hitachi U-3010) using H2O as the reference. 2.4. Measurements of photocatalytic activities Photocatalytic activities of samples were evaluated by photocatalytic degradation of rhodamine-B (RhB), methylene blue (MB), and Safrarine O under visible light. A 500 W Xe lamp with 420 nm cut off filter was used as the light source to provide visible-light irradiation. For the degradation of RhB, MB, or SO, 0.1 g of photocatalyst was added to 100 mL of RhB, MB, or SO solution (1 × 10−5 to 1 × 10−4 M), respectively. Dye solutions were made under visible light (λ ≥ 420 nm) in a domestic reactor, with a cooling water circulator accumulated to keep the reactor at constant temperature. Before illumination, the solution was stirred for 120 min in the dark in order to reach adsorptiondesorption equilibrium between the photocatalyst and RhB, MB, or SO dye. At 20 min intervals, a 7 mL solution was sampled. The UV–visible absorption spectrum of the centrifugated solution was then recorded by Hitachi U-3010 UV–visible spectrophotometry.

Fig. 9. Graph of Current Vs specific resistance of (a) BGPA-1 (b) BGPA-2 (c) BGPA-3 sample.

2.5. Measurements of electrical properties Electrical properties of different samples were measured using a self-produced method (KERI in Korea). Before measuring, all samples had been pressed with a piston to make uniform surface and thickness. By calculating the thickness and length, we could easily calculate the area of the sample. The voltage-current was flowed through the sample. By applying the following equation [(Resistivity) (ρ) [Ω cm] = S/l × V/ l], we could calculate the resistivity of the sample. Fig. 1 shows the selfproduced vertical resistivity measurement technique. 3. Results and discussion Fig. 2 shows XRD patterns of prepared BiVO4-GO particles and BGPA composites with different weight percentages of PANI. Curve (a) is the XRD pattern of the prepared BiVO4-GO, from which the crystal structure of the prepared BiVO4-GO is indexed as a cubic structure (JCPDS No. 06-0505). Curve (b) is the XRD pattern of the prepared 1 wt % PANI-BiVO4-GO composite. There were no alterations for locations of diffraction peaks between BiVO4-GO and the 1 wt% PANI-BiVO4-GO composite, indicating that the introduction of a small amount of PANI did not move the crystal form or the composition of BiVO4-GO. Curve (c) is the XRD pattern of the prepared 3 wt% PANI-BiVO4-GO composite. In addition to diffraction peaks from BiVO4-GO, diffraction peaks at 38.1°, 44.1°, and 64.4° marked as “○” in curve c could be readily indexed as (1 1 1), (2 0 0), and (2 2 0) crystallographic planes of metallic Bi, respectively (JCPDS No. 04-0783). Curves (d) and (e) showed that intensities of diffraction peaks from Bi0 gradually increased with further increase of PANI weight percent, illustrating that to a certain extent, PANI could help the reduction of Bi3+ to Bi0. The prepared PANI in this work might have a leucoemeraldine base with weak reducibility. When Bi3+ chains with –NH– groups in the PANI molecular chain, Bi3+ can oxidize the –NH– group to –N] group. Bi3+ itself is reduced to Bi0 on the PANI surface, resulting in the formation of Bi0 [42,43]. Curves (d) and (e) showed the same diffraction peaks of BiVO4-GO as those in curves (a–c). These XRD results demonstrated that the composite existed as BGPA when the quantity of PANI in this composite was equivalent to or larger than 5 wt%. No typical diffraction peaks corresponding to those of PANI were detected in curves (b–d), demonstrating that the PANI in these composites was present in an amorphous form.

Fig. 10. Electrical property (voltage Vs current graph) of (a) BGPA-1 (b) BGPA2 (c) BGPA-3 sample.

microscopy (SEM; JEOL JSM-6700F) and transmission electron microscopy (TEM; JEOL JEM-2100F; accelerating voltage, 200 kV). Fourier transform infrared (FTIR) spectra were recorded with a Bruker VECTOR 22 spectrometer using the KBr pellet technique. Raman spectra were acquired on a Renishaw inVia™ Reflex Raman Microprobe. UV–vis diffuse reflectance spectra (DRS) of as-prepared samples were obtained with a Shimadzu UV-2550 spectrophotometer prepared with an integrating sphere using BaSO4 as the reflectance standard. X-ray photoelectron spectroscopy (XPS) examinations of samples were carried out on a Physical Electronics PHI 1600 ESCA system operating at a pass energy of 187.85 eV with an Al Kα X-ray source (E = 1486.6 eV). All binding energies of composing elements were referenced to the C1s peak at 284.6 eV. UV–vis diffuse reflectance spectra of samples were

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Fig. 11. The absorption change of MB in the presence of a) BGPA-3 b) BGPA-2 c) BGPA-1 sample.

Fig. 3 shows typical SEM images of as-prepared (a) BiVO4-GO; (b) BGPA-1; (c) BGPA-2 & (d) BGPA-3 photocatalyst, respectively. Fig. 3 shows that the product has a layered structure with platelet-like morphology. A close-up view of Fig. 3d showed that the majority of crystals possessed a non-uniform disordered shape. The GO particle intercalated non-homogenously with polymer. However, BiVO4 was staggered on the surface. Such amorphous structure was also revealed by TEM investigation as shown in Fig. 4((a) BGPA-1; (b) HRTEM of BGPA-2; (c) BGPA-2 & (d) BGPA-3). This BGPA was found to actually consist of many smaller nanoparticles with different sizes. TEM image (Fig. 4) was recorded on the edge of the nanoparticle. Fig. 4(b) HRTEM of BGPA-2 shows clear lattice fringe, specifying the high-crystallinity and single-crystalline behavior of nanoparticles. The interplanar spacing was 0.309 nm, corresponding to the (1 2 1) plane of monoclinic BiVO4. It has been reported that small grain size and high crystallinity can increase photocatalytic activity due to increased reactive sites, thus promoting electron-hole separation efficiency [44,45]. Therefore, the as-prepared nano-BGPA is expected to show enhanced photocatalytic performance. To confirm the effective transfer of GO and characterize carbon

species, FTIR was used to obtain more insights into the combination of GO, BiVO4, and PANI. Fig. 5 shows FTIR spectra of BGPA-1, BGPA-2, and BGPA-3, respectively. A strong absorption band of GO at 3010 cm−1 owing to O–H elongating vibration was expected. However, when GO was combined with BiVO4 and PANI, the peak intensity decreased. The characteristic peak at 1567 cm−1 could be ascribed to oxidized carbon backbone [46,47]. Peak intensities of various oxygencontaining groups at 800–1900 cm−1 in BiVO4-GO and PANI-BiVO4-GO significantly decreased or even disappeared, indicating that the sonochemical synthesis method was an effective method for synthesizing BGPA. In the case of BiVO4-GO, typical absorption peaks of GO dramatically weakened or even disappeared compared to those of pure GO [48]. Broad absorption peaks at frequency of lower range (less than 1000 cm−1) were associated with (VO4) and (VO4) [49]. Taken together, these results indicate that we could successfully prepare a composite catalyst by incorporating GO as a platform. Optical absorption properties of as-prepared BGPA-1, BGPA-2 and BGPA-3 samples were determined by UV–visible DRS spectrometry. Fig. 6 shows that the BiVO4-GO-PANI sample has photoabsorption from the range of UV light to visible light, with wavelength of the absorption

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Fig. 12. (a) MB photodegradation as a function of illumination time for BiVO4; BiVO4-GO; BGPA-1; BGPA-2; BGPA-3 under visible light. (b) The plots of Ct/C0 vs. t for BGPA-1; BGPA-2; BGPA-3 C0 is the initial concentration of MB and Ct is the concentration at irradiation time t.

edge at 525 nm [53]. The absorption of the 5% PANI-modified BiVO4GO sample increased over the whole range of the spectrum. Using the follwoing equation: ahν = A(hν − Eg)n/2 [50], band gaps of BGPA-3, BGPA-2, and BGPA-1 samples were projected to be 2.21, 2.3, and 2.42 eV, respectively, based on the onset of absorption edges. This shows that the band gap energy of the BGPA-3 sample is lower than that of the BGPA-1 sample. Therefore, the PANI-modified BGPA-3 sample can be excited to yield more electron-hole pairs under the same visiblelight illumination, which can ensure higher photocatalytic activity. Raman spectroscopy is one of the most helpful tools to characterize carbon-based materials. Fig. 7 shows Raman spectra of selective PANIBiVO4-GO composites (BGPA-1, BGPA-2, and BGPA-3). Consistent with XRD results, the Raman spectra showed that BiVO4 had a monoclinic phase based on characteristic stretching vibrations and bending vibrations of the VO43− tetrahedron. The Raman spectrum of PANI-BiVO4GO (BGPA) demonstrated two characteristic bands at 1355 and 1597 cm−1, corresponding to the D band and G band of GO, respectively [51]. In comparison, BGPA-1, BGPA-2, and BGPA-3 composites showed that the D band and G band were slightly blue and red shifted to 1346 and 1606 cm−1, respectively. This might be caused by the changed surface strain due to contact between GO and BiVO4. This phenomenon was consistent with what was observed in the hydrothermal in situ preparation of PANI-BiVO4-GO (BGPA-1, BGPA-2, and BGPA-3) composites [52], where the D/G ratio close to zero suggested an effective combination of BiVO4-GO with PANI polymer. Compositions of PANI-BiVO4-GO heterogeneous nanostructures

were further investigated using X-ray photoelectron spectroscopy (XPS). Fig. 8 shows high-resolution XPS spectra of as-prepared PANIBiVO4-GO heterogeneous nanostructures. Fig. 8(a) shows binding energies located at about 160.1 and 166.5 eV, corresponding to Bi4f5/2 and Bi4f7/2 bands, respectively. Fig. 8(b) shows the XPS spectrum of the O1s band, indicating that different oxygen species exist on the surface of PANI-BiVO4-GO heterogeneous nanostructures. Binding energies located at about 532.2 and 533.1 eV are ascribed to the O1s band of the lattice oxygen of GO crystallites [53] and the O1s band of the lattice oxygen of BiVO4 crystallites [54], respectively. In the high-resolution XPS spectrum of the V2p band shown in Fig. 8(c), peaks at about 527 and 518.5 eV correspond to V2p1/2 and V2p3/2 bands, respectively. The XPS spectrum of the C1s band presented in Fig. 8(d) clearly shows one peak located at 287.3 eV, corresponding to C1s bands of GO and PANI crystallites, respectively. The C1s peak was accompanied by two satellites that were evident on the high-binding-energy side (denoted as peaks I and II located at about 287.3 and 287.5 eV, respectively). The main peak in the XPS spectrum of the N1s band is known to be characteristic of N2−. Shake-up satellite peaks are evident and diagnostic of an open 3p4 shell of N2− state [55], indicating the presence of PANI at the surface. The fact that XRD did not show evidence of a PANI phase while XPS showed the surface presence of N2− ions suggested that –NH2 was present only on the surface of PANI nanocrystals, forming a very thin amorphous outer shell. Based on these XRD, TEM, and XPS results, it can be deduced that the N element exists in the form of NH− on the surface of PANI-BiVO4-GO heterogeneous nanostructures.

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Fig. 13. The absorption changes of RhB in the presence of a) BGPA-3 b) BGPA-2 c) BGPA-1 sample.

Figs. 9 and 10 show electrical properties of BGPA-1, BGPA-2 and BGPA-3 samples. Fig. 9 shows specific resistance versus voltage curve. We applied voltage to different types of samples separately. Different types of curves were obtained for different samples. The specific resistance of the BGPA-1 sample was higher than that of sample BGPA-2, & BGPA-2 was higher than that of BGPA-3. For BGPA-3, the specific resistance was very low, demonstrating a high electron flow through the sample. For this reason, it may show high photocatalytic activity. Fig. 10 shows voltage versus current curve. It revealed that the flow of electricity and its efficiency for BGPA-3 were higher than those of other samples. Conductivity of BGPA-3 sample was also increased.

light illumination (λ > 420 nm) with otherwise identical conditions. First, it was demonstrated that the photolysis of RhB, MB, or SO was slow for BGPA-1 photocatalyst under visible-light illumination. The adsorption of RhB, MB, or SO on the BGPA sample with different ratios in the dark was also checked. After 120 min, concentrations of RhB, MB, and SO were decreased by only 10%, suggesting that the decolorizing of RhB, MB, and SO by spherical-like BGPA was mainly caused by photodegradation, but not by adsorption. Moreover, only 30% RhB, 25% MB, and 17% SO could be photodegraded by BiVO4 under visible light in 180 min. However, all 5% H-BGA samples exhibited higher photocatalytic activities than 1% BiVO4. Among them, the 5% H-BGA photocatalyst showed the highest activity, which photodegraded 62% RhB, 73% MB, and 82% SO after only 180 min under the same condition. To quantitatively understand the reaction kinetics of RhB, MB, and SO degradation in our experiments, the Langmuir-Hinshelwood model was applied as expressed by Eq. (1). This model is well-established for photocatalytic experiments when the pollutant is in millimolar concentration range [56,57]:

3.1. Visible-light-induced photocatalytic performance and mechanism on BGPA 3.1.1. Photocatalytic degradation of dye RhB, MB, and SO were chosen as representative hazardous dyes to analyse photocatalytic properties of as-prepared PANI-BiVO4-GO first, which showed a major absorption band at 553 nm. Figs. 11, 13, and 15 show photodegradation efficiencies of RhB, MB, and SO, respectively, mediated by different mass ratios of BGPA photocatalysts under visible-

ln

9

C = kt C0

(1)

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Fig. 14. (a) RhB photodegradation as a function of illumination time for BiVO4; BiVO4-GO; BGPA-1; BGPA-2; BGPA-3 under visible light. (b) The plots of Ct/C0 vs. t for BGPA-1; BGPA-2; BGPA-3 C0 is the initial concentration of RhB and Ct is the concentration at irradiation time t.

where C0 and C represent concentrations of dye in solution at times 0 and t, respectively, and k represent apparent first-order rate constant. Figs. 12, 14, and 16 clearly show that in the plot of C/C0 and time t, PANI has a great influence on the photodegraded rate (k) of the asprepared samples. Table 1 shows rate constant values. The sample with BGPA-3 exhibited the highest photodegraded efficiency, which was about 3-fold than that of the pure BGPA-1 sample. Fig. 16 clearly and directly shows that BGPA-3 has great influence on photodegraded rates (k) of as-prepared samples (Table 2). Besides improved photocatalytic activity resulting from PANI, photostability of the photocatalyst was also retained [15]. Circulating runs in photocatalytic degradation of RhB in the presence of BGPA-3 under visible light (λ > 420 nm) were checked (Fig. 17). Subsequently, after five recycles for the photodegradation of RhB, the catalyst did not display any significant loss of activity. This indicates that the BGPA-3 photocatalyst has greater stability than BGPA-1 and BGPA-2. It does not photocorrode throughout the photocatalytic oxidation of model pollutant molecules. In addition, PANI is inexpensive than noble metals. Thus, the BGPA-3 photocatalyst is promising for practical application in water purification.

3.1.2. Photocatalytic mechanism Experiments shown above displayed an outstanding photocatalytic performance of the as-prepared spindle-like BGPA on the degradation of commonly used dye and phenol, indicating that the PANI modified BiVO4-GO photocatalyst might have high potential applications in the conservation of the environment. In addition, the photodegraded mechanism of phenol in visible light PANI-BiVO4-GO system might guide further development of its photocatalytic performance. The possible photocatalytic mechanism (Scheme 2) is proposed as follows:

BiVO4

GO

PANI

h

h+ + e

(2)

On the basis of the relative energy level of PANI (π-orbital and π*orbital) and BiVO4-GO (conduction band, CB, and valence band, VB) [13,29] which result in synergetic effect, the photogenerated gap in VB can directly transfer to the π-orbital of PANI. Here, GO acts as an electron transferring medium. It helps electron-hole recombination. Simultaneously, photogenerated electrons can be transferred to the CB of BiVO4 which produces charge separation and stabilization, thus hindering the recombination process. PANI is an excellent material for

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Fig. 15. The absorption change of SO in the presence of a) BGPA-3 b) BGPA-2 c) BGPA-1 sample.

transporting holes. In addition, the grain size of the photocatalyst is comparatively small [12]. Therefore, photogenerated charges can easily migrate to the surface of photocatalysts and photodegrade adsorbed contaminations (Fig. 18).

PANI-BiVO4-GO composite are mostly due to the formation of the heterojunction electric field at the interface of BiVO4-GO and PANI. On the one hand, the formation of the heterojunction electric field at the interface of BiVO4-GO and PANI dramatically increased the separation efficiency of photogenerated electron-hole pairs and the lifetime of photogenerated electrons. On the other hand, the formed electric field at the interface of BiVO4-GO and PANI swiftly transferred these photogenerated holes produced by BiVO4-GO to PANI, therefore very effectively slowing down the photocorrosion of BiVO4-GO.

4. Conclusions PANI-BiVO4-GO composites with very sturdy photocatalytic capability were effectively prepared by sonochemical formation of BiVO4GO particles on PANI. The introduction of PANI in the PANI-BiVO4-GO composite can significantly increase both the photocatalytic degradation performance and the photocatalytic degradation stability of BiVO4GO. The photocatalytic degradation performance of the PANI-BiVO4GO composite is strictly related to the weight percent of PANI in the composite. The 5 wt% PANI-BiVO4-GO composite possessed optimal photocatalytic degradation efficiency of more than 83% for RhB dye. However, too much PANI will decrease the light absorption intensity of BiVO4-GO, thus decreasing its photocatalytic degradation ability. Increases of both the photocatalytic degradation performance and the photocatalytic stability of BiVO4-GO after introducing PANI into the

5. Compliance with ethical standards Funding: All authors gave a statement that research was self-funded. We certify that the research is conducted independently with optimal value to science. All authors are responsible for disclosing their financial relationship with a grant provider to their own research organization and the public, if necessary.

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Fig. 16. (a) SO photodegradation as a function of illumination time for BiVO4; BiVO4-GO; BGPA-1; BGPA-2; BGPA-3 under visible light. (b) The plots of Ct/C0 vs. t for BGPA-1; BGPA-2; BGPA-3 C0 is the initial concentration of SO and Ct is the concentration at irradiation time t.

Table 1 The apparent rate constant of different dyes. Sample

The apparent rate constant of different dyes MB

BGPA-1 BGPA-2 BGPA-3

Table 2 Nomenclature of all samples.

RhB

SO

kapp (min−1)

R2

kapp (min−1)

R2

kapp (min−1)

R2

3.0 × 10−3 4.5 × 10−3 5.0 × 10−3

0.9812 0.9955 0.9964

2.5 × 10−3 3.2 × 10−3 4.3 × 10−3

0.9968 0.998 0.9961

4.3 × 10−3 6.5 × 10−3 8.2 × 10−3

0.9934 0.9941 0.9972

Sr.

Nomenclature

Sample name

1. 2. 3.

BGPA-1 BGPA-2 BGPA-3

BiVO4-GO-PANI-1% BiVO4-GO-PANI-3% BiVO4-GO-PANI-5%

Declaration of Competing Interest The authors declare that they have no conflicts of interest relevant to this study to disclose.

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Fig. 17. Cycling runs in the photocatalytic degradation of RhB, MB, SO in the presence of (a) BiVO4 (b) BiVO4-GO and (c) 5% BiVO4-GO-PANI (BGPA-3) under visible light.

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Fig. 18. Dye degradation mechanism with BiVO4-GO-PANI nano composite.

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