CdS heterojunctions: A discussion on photocatalysis mechanism

Journal of Alloys and Compounds 817 (2020) 153246

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Effects of morphology on the visible-light-driven photocatalytic and bactericidal properties of BiVO4/CdS heterojunctions: A discussion on photocatalysis mechanism Rui Guo a, c, Aiguo Yan a, b, Juanjuan Xu a, b, Botao Xu b, c, Tingting Li b, c, Xuanwen Liu a, b, *, Tingfeng Yi a, b, **, Shaohua Luo a, b, *** a b c

School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, PR China School of Resources and Materials, Northeastern University at Qinhuangdao, 066004, PR China Key Laboratory of Nano-Materials and Photoelectric Catalysis of Qinhuangdao, 066004, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 August 2019 Received in revised form 5 November 2019 Accepted 1 December 2019 Available online 6 December 2019

BiVO4/CdS composites with different morphologies and mass ratios were successfully prepared via a facile deposition-precipitation method by adding different dispersants. Compared with spherical BiVO4 (BS), sheet-like BiVO4 nanoarrays (BA) showed higher (020) exposed areas, as confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Xray photoelectron spectroscopy (XPS) revealed further details on the microstructure of the composites namely, CdS was deposited on the surface of BiVO4 forming an S-(111) interface layer. The photocatalytic properties of the samples were tested by studying the degradation of Rhodamine B (RhB) and the sterilization of Escherichia coli (E. coli) under visible-light irradiation. The BAC-60 sample showed the best photocatalytic performance among all the catalysts owing to its: (i) improved high separation efficiency of the photo-induced charge carriers and (ii) high exposure activity of the (020) plane. Electrochemical tests with non-equilibrium carrier injection and free radical trapping experiments are introduced to analyze the mechanism, such as the effects of different crystal planes on the photocatalytic reaction and the flow direction of carriers at the CdSeBiVO4 interface. © 2019 Elsevier B.V. All rights reserved.

Keywords: BiVO4 CdS Photocatalysis Shape control Sterilization

1. Introduction Visible-light-driven photocatalysis has attracted considerable interest as a promising route to address the environmental crisis and energy shortage issues [1e3]. Therefore, various semiconductor materials (e.g., g-C3N4, BiOBr, and BiVO4) have been successively employed as photocatalysts [4e8]. The greatest challenge in these systems lies in reducing the recombination rate of the photogenerated electron-hole pairs and increasing the surface activity of the catalyst [9]. Among these photocatalysts, bismuth

* Corresponding author. School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, PR China. ** Corresponding author. School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, PR China. *** Corresponding author. School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, PR China. E-mail addresses: [email protected] (X. Liu), [email protected] (T. Yi), [email protected] (S. Luo). https://doi.org/10.1016/j.jallcom.2019.153246 0925-8388/© 2019 Elsevier B.V. All rights reserved.

vanadate (BiVO4) possesses particularly favorable optical absorption properties and a suitable band gap of ca. 2.4 eV, and has received intense attention in the past decade [10,11]. With the aim to enhance the separation efficiency of the photogenerated electron-hole pairs and enlarge the redox window under visiblelight irradiation, BiVO4 has been coupled with other semiconductor materials to form heterojunctions. This approach has been regarded as a promising strategy giving rise to several excellent works. For example, a Z-scheme CdS/BiVO4 photocatalyst fabricated with a low-temperature water bath exhibited significantly higher visible-light-driven photocatalytic activities compared with pure CdS and BiVO4 [12]. Cao et al. developed Ag3PO4/BiVO4 composite for degradation of norfloxacin with low bias potential for photocurrent, emphasizing the importance of interface control [13]. Li [14] prepared a novel ellipsoidal (t-s) BiVO4-modified g-C3N4 composite photocatalyst via a facile twostep synthesis strategy assisted by ultrasound and magnetic stirring. This composite showed RhB degradation efficiencies as high as 97.7% after 120 min. The Co3O4-modified BiVO4 p-n heterojunctions

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reported by Chang et al. exhibited greatly enhanced separation efficiencies for the photogenerated electron-hole pairs and high surface reactivity [15,16]. Although BiVO4 is usually prepared by precipitation or ultrasound-assisted precipitation, electrodeposition methods are often used to prepare BiVO4-containing composites. For example, Li et al. reported a novel type of semiconductor, FeF2/BiVO4, by electric deposition of FeF2 on the surface of BiVO4 for generating hydrogen [17]. Moreover, Wu and Bao introduced carbon dots (CDs) and Au as solid-state electron mediators to BiVO4/CdS photocatalytic system to increase the carrier transport capability between BiVO4 and CdS [11,18]. Besides the intrinsic modification on the electro n structure of BiVO4 by coupling with other semiconductors, extrinsic manipulation such as nanostructures (e.g., nanostructure adjustment) aimed at increasing the surface area and controlling the morphology of the composite can also improve the photocatalysis efficiency of BiVO4 [3,19,20]. As reported by Zhou, a novel CdS nanoparticles decorated BiVO4 nanowires without using an electron mediator showed two times higher photocatalytic H2 generation rate than bare CdS [21]. CdS quantum dots were also synthesized and used as sensitizers for mesoporous BiVO4 heterostructures to give an enhanced photo-electrical conversion efficiency [22]. In addition, the anisotropy of BiVO4 gives different crystal planes different reactivity. Thus, Bao et al. reported a facile method to enhance the photocatalytic activity of BiVO4/CdS heterojunction via the selective deposition of CdS and Au on the {010} faces of BiVO4 [18]. Furthermore, the theoretical study by Ullah et al. revealed that the photocatalytic activities of BiVO4 are greatly influenced by the exposed crystal face [23]. While photocatalytic dye degradation has been widely reported, algaecide and sterilization studies are scarce in the literature [24,25] despite the fact that surface water is increasingly contaminated by algae and bacteria. Thus, the demand for an integrated solution to water pollution has become increasingly pressing. Encouragingly, a multifunctional catalyst such as BiVO4/CdS can precisely provide this solution. In addition, for all photocatalysts, a discussion of the photocatalytic mechanism is necessary, which gives the routes to enhance photocatalytic performances. In the case of composite semiconductor photocatalysts, the carrier transport direction at the interface remains as one of the main issues when studying the catalytic mechanism. The above investigations inspired us that one of the best ways to fabricate a highly efficient photocatalyst may be to combine these approaches together, that is, CdS couples with Bi-based photocatalysts with a modified surface to achieve efficient charge separation and enhanced photocatalytic efficiency. In this study, to test this hypothesis, BiVO4 with different morphologies were prepared via a facile precipitation method by adding different dispersants. Then, a BiVO4/CdS heterojunction photocatalyst was synthesized by depositing CdS nanoparticles on the as-prepared BiVO4 surface by a two-step sonication-precipitation method with an S layer as the interface. The visible-light-driven photocatalytic properties of the as-prepared composite catalyst were detected by degradation of RhB and inactivation of E. coli, indicating that the as-prepared photocatalyst is a multifunctional catalyst. The optimal mass ratio of the two components of the BiVO4/CdS composite catalyst was also carefully studied herein [21]. Moreover, the catalytic activities of the different crystal surfaces of BiVO4 were compared by carrying out electrochemical tests and photocatalytic experiments. Finally, the interfacial current inversion mechanism of the composite photocatalyst BAC-60 was analyzed in detail by electrochemical testing with and without visible radiation.

2. Experimental 2.1. Materials Bismuth nitrate (Bi(NO3)3$5H2O), cetyltrimethylammonium bromide (CTAB), ammonium metavanadate (NH4VO3), silver nitrate (AN), isopropyl alcohol (IPA), and benzoquinone (BQ), were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). cadmium nitrate (Cd(NO3)2$4H2O), sodium sulfide (Na2S$9H2O), citric acid and RhB were provided by Aladdin Industrial Corporation (Shanghai, China). All reagents were analytical grade and used without further purification. Solutions were prepared with deionized water. 2.2. Synthesis 2.2.1. Synthesis of the CdS sample CdS was prepared following the procedure given in Ref. [11] with some modifications. In a typical experiment, a mixture of 5 mmol of cadmium nitrate (Cd(NO3)2$4H2O) and 5 mmol of citric acid was added into 50 mL of deionized water under magnetic stirring for ca. 30 min. Then, a solution containing 5 mmol of Na2S$9H2O was dropped into the above solution and left reacting under continuous magnetic stirring for 30 min. Subsequently, the as-prepared suspension was centrifuged, washed three times with deionized water and anhydrous ethanol and vacuum dried at 80
C for 24 h. 2.2.2. Synthesis of the sphere-like BiVO4 (BS) sample A mixture containing 10 mmol of Bi(NO3)3$5H2O and 20 mmol of citric acid was added to 200 mL of deionized water under magnetic stirring for ca. 10 min to prepare solution A. 10 mmol of ammonium metavanadate (NH4VO3) were added into the hot deionized water (70
C) until complete dissolution to obtain solution B. Then, the solution A was slowly added to the solution B under vigorous stirring, aged for 3 h and the resultant suspension was subsequently filtered, washed three times with deionized water and anhydrous ethanol, and vacuum-dried at 80
C for 24 h [26]. 2.2.3. Synthesis of sheet-like BiVO4 nanoarrays (BA) The BA sample was prepared by using the same precipitation method as BS but with CTAB as a dispersant instead of citric acid. 2.2.4. Synthesis of the CdS/BiVO4 samples BiVO4/CdS (BC) photocatalysts with different weight ratios were prepared by a simple two-step sonication-precipitation method at room temperature. Typically, a settled mass of the above prepared BiVO4 was dispersed in a Cd(NO3)2$4H2O aqueous solution with the corresponding stoichiometric ratio under ultrasound conditions for 1 h. Then, a Na2S$9H2O aqueous solution was added to the above suspension and left reacting under continuous ultrasound and vigorous stirring for 2 h. The as-prepared suspension was subsequently centrifuged, washed three times with water and anhydrous ethanol, and vacuum-dried at 80
C overnight. The as-prepared samples were labeled as BSC-x and BAC-x for CdS-modified BS and BA, respectively, with x indicating the mass ratio of CdS. 2.2.5. Synthesis of the working electrodes Typically, the catalyst film-coated fluorine-doped tin oxide (FTO) working electrodes were prepared as follow: Firstly, FTO glass was cleaned by ultrasonic in ethanol for 10 min and dried at 333 K. Secondly, 40 mg of catalyst particles and 35 mL Nafion solution (5 wt %) were dispersed in mixed solution with 500 mL DI water and 500 mL ethanol under the sonication for 1 h to form a homogeneous

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ink. Then 20 mL of the ink was coated on the surface of FTO electrode. Finally, the catalyst loading on the surface of the FTO is 0.773 mg/cm2. 2.3. Characterization X-ray diffraction (XRD) patterns were recorded on a D/Max-RB X-ray diffractometer (Rigaku) using Cu Ka irradiation at a scan rate (2q) of 0.05
/s from 10 to 90
. The morphology of the powder samples was characterized by scanning electron microscopy (SEM, Zeiss Supra 55) and high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai F30). X-ray photoelectron spectroscopy (XPS) measurements were carried out to study the composition and microstructure of the samples on a PHI 5000C ESCA system with an Mg Ka source operating at 14.0 kV and 25 mA. UVevis diffuse reflectance spectroscopy (DRS) measurements were carried out on a UVevisible spectrophotometer (PERSEE T9) with BaSO4 as the reference sample. The functional groups were analyzed by suing Fourier transformed infrared spectra (FT-IR) using a FTIR-8400S spectrometer. Photoluminescence emission spectra (PL) were recorded with the excitation wavelength of 270 nm on an F-7000FL spectrofluorophotometer with a spectral slit width of 5 nm over a range of 220e800 nm. Electrochemical measurements such as electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) tests were performed on a PARSTAT 2273 electrochemical workstation with a three-electrode system including a FTO glass electrode (working electrode), a platinum electrode (counter electrode), and a saturated calomel electrode (SCE, reference electrode) in 0.5 M Na2SO4 aqueous solution. MS plots was recorded using an alternating voltage of 10 mV amplitude at 1 kHz. EIS was performed in a frequency range of 0.01 Hze100 kHz. The magnitude of the modulation signal applied to the potential was 10 mV. 2.4. Photodegradation of RhB Photocatalytic kinetics were obtained from the variation of the dye concentration with the reaction time at 298 K. Typically, 50 mg of the as-prepared samples were added to 100 mL of 0.02 g L1 Rhodamine B (RhB) solution under stirring with/without visiblelight irradiation. Light source is obtained by a 300W Xe lamp with 420 nm filter. At different intervals, 5 mL suspension was extracted and the solid particles were removed by centrifugation. The concentration of RhB was determined by UVevis spectrophotometry (PERSEE T9) at the corresponding maximum absorbance wavelength [27].

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2.5. Photocatalytic sterilization E. coli (CICC 10389) was employed as the target bacteria to evaluate the antibacterial activity of the samples under visible light irradiation. All materials including solutions and vessels were sterilized in an autoclave at 120
C for 30 min before antibacterial examinations. In these experiments, the antibacterial property of the samples was widely measured by dropping the sample into the total colony test pieces (3 M) with an initial concentration of E. coli of ca. 1.0  106 CFU/mL. Suspension aliquots of 5 mL were extracted from the photoreactor every 30 min and E. coli bacteria were separated by centrifugation at 8000 rpm for 5 min and diluted with deionized water at a controlled pH of 7.2. The protein information of E. coli was analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS, Daltonics Bruker, Germany) with a 1.25 m flight tube operating under the positive linear mode [24].

3. Results and discussion 3.1. Characterizations The phase and crystal structures of the as-prepared samples were investigated by XRD. Fig. 1 shows the XRD patterns of the BiVO4, CdS, BSC -x, and BAC-x samples. As shown in Fig. 1, both BS and BA are highly crystalline and can be indexed to monoclinic BiVO4 structure (PDF# 75e1867) [11,28,29]. The main characteristic peaks of BiVO4 located at 18.67
, 28.94
, 30.53
, 35.21
and 40.25
are assigned to (101), (112), (020), (004), and (121) [30]. It shows that BiVO4 prepared by precipitation method has a higher purity than those by other methods such as electrodeposition [15]. For pure CdS, it reveals that the diffraction peaks at 26.5 5, 44.04
and 52.16
can be indexed to (111), (220), and (311) of the cubic CdS (PDF# 80e0019) [11,31]. The broadening of the diffraction peak suggests the nanocrystallization of CdS. Each of the characteristic diffraction peaks of BiVO4 and CdS is observed in the XRD patterns of BSC-x and BAC-x samples, indicating that CdS nanoparticles were successfully deposited on the surface of BiVO4 particles. No impurity peaks are observed in the composite catalysts, confirming the absence of the intermediate phases. Moreover, the attachment of CdS does not obviously affect the diffraction peak positions of BiVO4, implying that Cd2þ and S2 ions are not incorporated into the BiVO4 lattice. Fig. 2 shows the SEM images of BS, BA, CdS, BSC-60, and BAC-60. As shown in Fig. 2a, pure spherical BiVO4 (BS) consists of well-

Fig. 1. XRD patterns of BSC (a) and BAC (b).

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Fig. 2. SEM images of BS (a), BSC-80 (b), BA (c), and BAC-80 (d) and the schematic diagram of the preparation process.

dispersed irregular polyhedral particles with diameters ranging from 100 to 200 nm [32]. Under ultrasonic conditions, after the two-step deposition of S2 and Cd2þ respectively, CdS nanoparticles (ca. 30e60 nm in diameter) were successfully attached to the surface of BiVO4. The morphology and size of the as-prepared BSC-60 are similar to those of CdS modified BiVO4 photocatalysts via an impregnation method reported by Zeng [33]. While, the morphology of the as-prepared BA (in Fig. 2c) presents the aggregations of several sheet-like nanoarrays with a thickness of about 30e50 nm, in line with the morphology reported by Zhang et al. [34], but with more refined sheets. The formation of this sheet-like morphology should be related to the addition of CTAB during the preparation process. Obviously, the rod-like nuclei were formed first in the solution and sheet-like crystals subsequently grew along the normal to the surface direction of the rod, similarly to the growth of branch crystals during metal solidification. Line defects were clearly observed, inducing the crystals to crack into sheet-like

arrays during growth. Finally, a construction formed by a large number of sheet-like nanoarrays gathered on a rod was formed. Moreover, CdS nanoparticles and BiVO4 particles are attached closely to each other with BiVO4 nanosheets still retaining their original morphologies and sizes both in BSC and BAC. Compared to pure CdS prepared by precipitation method, the CdS deposited on the surface of BiVO4 has similar size (see Figs. S1, 2b and 2d) [11]. The particle size of CdS is so small that the X-ray diffraction intensity of BiVO4 particles is not significantly influenced in the composites, which is consistent with the observed XRD pattern. The detailed structures of BAC-60 and BSC-60 were further investigated by TEM. Fig. 3a and b shows TEM images of the synthesized BSC-60 sample. Well-dispersed polyhedral BiVO4 particles tightly surrounded by CdS nanoparticles were clearly observed. Lattice plane spacings of ca. 0.335 and 0.470 nm were indexed for the (111) and (101) planes to CdS [35,36] and BiVO4, respectively (Fig. 3b). The CdS sample prepared herein shows a structure similar

Fig. 3. TEM images of BSC-60 (a) and BAC-60 (c), HR-TEM images of BSC-60 (b) and BAC-60 (d), and schematic diagram of the corresponding crystal surface indexes (e).

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to those prepared by precipitation [11]. In BAC-60 (Fig. 3c and d), the sheet-like BiVO4 nanoarrays are also tightly surrounded by CdS, although the BiVO4 nanoarray showed a surface spacing of 0.293 nm corresponding to (004) planes, in line with the results reported by Zou for BiVO4 nanosheets [12]. The exposed (004) facet of BA also reveals that the dispersant CTAB hindered BiVO4 growth along the <010> direction. The electron diffraction pattern of CdS and BiVO4 (inserts in Fig. 3a and c) and showed ring pattern, indicating that CdS and BiVO4 nanoparticles were polycrystalline in nature. Considering the monoclinic symmetry of BiVO4, the approximate vertical relationship between (101) and (020), the angle of 65.7
between (101) and (004), and the corresponding diffraction peaks of BiVO4 shown in Fig. 1b, two reasonable spatial structures are proposed for the spherical BiVO4 and sheet-like BiVO4 arrays (Fig. 3e), respectively, where both spherical BiVO4 and BiVO4 arrays are enclosed by (101), (020), and (004). As a result, the BiVO4 array has a larger (020) exposed surface and a smaller (101) exposed surface than its spherical counterpart. Similarly, as revealed by XRD and TEM and considering that (111) and (100) planes of CdS are usually exposed due to the stability [35,37,38], it can be concluded that the CdS nanopolyhedron may be enclosed by {100} and {111} (Fig. 3e). As shown in Fig. 3aed, the CdS particles are translucent, allowing visible light to penetrate them and effectively excite BiVO4. Moreover, the tight contact between CdS and BiVO4 allows charge separation during the photocatalytic process. The surface chemical compositions of the as-prepared samples and the interior interaction between BiVO4 and CdS were investigated by XPS measurement. The full-range XPS spectrum (Fig. 4a) clearly shows the presences of Bi, V, O, Cd, and S elements in BAC and BSC, further confirming the coexistence of BiVO4 and CdS in these nanocomposites. Moreover, XPS studies reveal the absence of 3 þ 3 impurity ions such as NO ions after the 3 , PO4 , Na , and N deposition-precipitation processes. The high-resolution XPS spectra of Bi, V, O, Cd, and S are attentively deconvoluted considering the spin-orbit coupling [39]. As shown in Fig. 4b, the Bi 4f XPS spectra of pure BA shows one doublet at 164.48 (4f5/2) and 159.08 eV (4f7/2), which accordingly red-shift to 163.78eV and 158.48eV for BS [40]. In the case of BSC and BAC, the 4f doublet is found at an intermediate binding energy between BS and BA, suggesting that the valence state of Bi gradually increases with the trend BS/BSC/BAC/BA. A satellite peak at 161.48e161.58 eV is observed for BSC and BAC and assigned to S 2p. This band is discussed in Fig. 4f. As in the case of Bi, the V 2p XPS spectra (Fig. 4c) revealed a similar redshift. The V 2p XPS spectra of pure BA also involve one doublet centered at 516.78 (2p3/2) and 524.18 eV (2p1/ 2), which red-shifts to 516.18 and 523.58 eV for BS, respectively [26,41]. Notably, the O 1s spectra (Fig. 4d) are greatly different between pure BiVO4 samples and composite samples [42]. A new peak appears at 533.08 eV, indicating that the O atom donors electrons during the bonding process between BiVO4 and CdS. This can be reasonably explained by O atom forming a coordination bond with the Cd atom. As shown in Fig. 4e, the Cd 3d peaks at ca. 405 (3d5/2) and 412 eV (3d3/2) remains nearly similar during the formation of the composites, indicating that the amount of Cd deposited directly on BiVO4 is low [31,43]. In Fig. 4f, the S 2p peaks of pure CdS, BSA and BCA can be deconvoluted into two obvious bands centered at about 161.4 eV and about 162.5 eV for 2p3/2 and 2p1/2, respectively, which is assigned to S with a formal charge of þ2 [35]. However, in the composites, the presences of the XPS peak of S at 168.38 eV and 168.53 eV (for BSC-60 and BAC-60, respectively) indicate a formal charge higher than þ2 (þ2 þ d) for S, which is ascribed to S species located at the BiVO4/CdS interface. These interface S atom would be bound directly to O atoms on the surface of BiVO4 [35]. In summary, XPS spectra provide more information on the potential coupling processes taking

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place between BiVO4 and CdS. Thus, S atoms are adhered to the surface of BiVO4 extensively at the interfaces, affecting the coordination environment of S and O without altering those of Cd, Bi, and V. By combining SEM and TEM observations, the potential formation process as well as the structure of BiVO4/CdS can be tentatively proposed (Fig. 4g). First, a small number of Cd atoms were deposited on the surface of BiVO4, and S atoms were subsequently deposited around the Cd atoms to form the interface. Considering that there are two possible (111) planes for CdS (i.e., Scontaining or Cd-containing planes), the S-containing (111) surface was inferred to act as the interface during the deposition process according to the observation in Fig. 4f. UVevis DRS was used to determine the optical properties of the prepared samples. The DRS spectra of BiVO4, CdS, BAC-x, and BSC-x are displayed in Fig. 5. All samples show very similar absorption profiles. Compared to pure CdS, the BAC-x samples show lower absorption intensities for l > 500 nm, while the BSC-x samples show higher absorption intensities over the same range. Thus, the BSC-x samples show higher visible light absorptions ability to generate electronehole pairs under visible light irradiation. Compared to the BiVO4 nanowires [21], the absorption band edge of BA and BS redshifts to ca. 540 nm, revealing a stronger visible light absorption for the herein prepared bulk BiVO4 [16,44]. Fig. 5c and d shows the FT-IR spectra of CdS, BA, BS, BAC, and BSC. The band at 470 and 750 cm1 were assigned to the stretching vibrations of BieO and VeO, respectively [45]. For pure CdS, the typical characteristic absorption bands at 622, 1114, and 1380 cm1 were ascribed to the vibrations of CdeS bonds [46,47]. These results further confirmed that the CdSeBiVO4 composites were successfully synthesized. 3.2. Photocatalysis properties The photocatalytic performances of the as-prepared samples towards the photodegradation of RhB are shown in Fig. 6a and b. CdS, BiVO4, BSC-x, and BAC-x showed no obvious adsorption activities. As shown in Fig. 6a, the photocatalytic activity of CdS is significantly larger than that of BiVO4, revealing that the photogenerated electron-hole pairs of BA recombined faster than that of CdS [21,22]. BAC-20 shows poorer photocatalytic performance than CdS, although the photogenerated electrons and holes of CdS and BiVO4 can be separated due to carrier drift by built-in electric field upon the coupling of CdS and BiVO4. However, the photocatalytic performance of BAC-40 and BAC-60 are greatly improved (ca. 1.6 and 2.7 times, respectively) compared to bare CdS, suggesting that the optimal weight ratio of CdS/BiVO4 was 3:2. The photocatalytic activity of BAC-80 is lower again than that of CdS, which indicates that CdS content cannot effectively touch with BiVO4 in this material to form a heterojunction due to the decrease of BiVO4 content, so that the photogenerated electron/hole cannot be effectively separated. A similar trend is also observed for the BSC-x composites (Fig. 6b). The photocatalytic rates of both series of samples are compared in Fig. 6c. The photocatalytic activity of BAC-60 is about 1.5 times higher than that of BSC-60. Obviously, the photocatalytic activities of both BAC-x and BSC-x series of composites are affected by the morphology of BiVO4. As shown in Table 1, the CdSeBiVO4 catalysts prepared by different methods showed different catalytic activities for RhB degradation. Evaluations of catalytic activity depended on a combination of light source intensity, amount of catalyst, and concentration and volume of the dye being degraded. Note that a large volume, high concentration dye sample was used in this work, so a catalytic activity, 60% in 120 min, was acceptable. Fig. 6d shows the nitrogen adsorption-desorption isotherms and the relevant pore-size distributions (inset) of BAC-60 and BSC-60.

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The relevant specific surface areas and pore structures are calculated by the Brunauer-Emmett-Teller (BET) method. The adsorption-desorption isotherm of BSC-60 shows a type III profile (according to the IUPAC classification) with a typical H3 distinct hysteresis loop, while that of BAC-60 is a type V adsorption-

desorption isotherm with a typical H3 loop. These results reveal the existence of micropores and mesopores in both BAC-60 and BSC-60 samples. The specific surface area of BAC-60 and BSC-60 were calculated to be 164.37 and 138.55 m2 g1, respectively. BAC-60 shows a more uniform pore size distribution and a lower

Fig. 4. XPS spectra of the total survey (a), Bi (b), V (c), O (d), Cd (e) and S (f) of BiVO4, CdS, BSC-60 and BAC-60, and structure diagram of BiVO4/CdS (g).

R. Guo et al. / Journal of Alloys and Compounds 817 (2020) 153246

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Fig. 5. UVevis DRS (a, b) and FTIR (c, d) of CdS, BiVO4, BSC-x, and BAC-x.

Fig. 6. Photodegradation of RhB over blank BiVO4, pure CdS, BAC-x (a), and BSC-x (b). The comparison of photodegradation efficiencies of RhB over BAC-x and BSC-x after 120 min visible light irradiation (c). Nitrogen adsorption-desorption isotherms of BSC-60 and BAC-60 and the corresponding pore size distributions (inset) (d). RhB dye concentration: 0.02 g L1, pH ¼ 7. Catalyst suspended: 1 g L1.

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Table 1 A comparison of some selected catalysts for RhB degradation. No.

Catalyst

Light source (Xe lamp,420 nm filter)

Concentration/Volume

Dosage

Degradation rate/time

Ref.

1 2 3 4 5 6

CdSeBiVO4 CdSeBiVO4 CdSeBiVO4 CdSeBiVO4 CdSeBiVO4 This work

300 300 500 500 500 300

50 mL/10 mg L1 50 mL/10 mg L1 50 mL/10 mg L1 20 mL/5 mg L1 20 mL/5 mg L1 100 mL/20 mg L1

30 mg 100 mg 50 mg 20 mg 20 mg 50 mg

100%/30 min 30%/90 min 28.7% 89%/180 min 60%/180 min 60% 120 min

[48] [49] [50] [51] [52]

W W W W W W

mean pore size than the corresponding BSC counterpart. Therefore, once normalized with the specific area, the photocatalytic activity of BAC-60 is 1.8 times higher that of the BSC-60. We can conclude that the (020) plane of BiVO4 has a higher surface activity or electrical performance. E. coli, one of the most common bacterial, was selected to study the antibacterial ability of the prepared composites. 1  106 CFU/mL E. coli solution was selected as the optimum experimental concentration. The bactericidal efficiency was assessed by mixing the suspension of E. coli and the prepared photocatalysts under visible light irradiation with different time intervals (0, 30, 60, 90, and 120 min). The inactivation of E. coli under visible light irradiation alone (without photocatalysts) was also measured as a control experiment. As shown in Fig. 7a, the changes of E. coli concentration are negligible in the control experiment. When BA and BS were used as catalysts, the deactivation rate of E. coli is low, in line with the results reported by Regmi et al. [53]. CdS show a noticeable bactericidal activity (ca. 98% inactivation of E. coli after 60 min of irradiation) [54]. BAC-60 and BSC-60 show slightly lower bactericidal performance compared to CdS. The above results indicate that CdS possesses higher bactericidal activity than BiVO4, and, unlike RhB degradation, that the effects of the different crystal planes of BiVO4 on the bactericidal performance are not obvious. The variations of the E. coli protein concentration during the photocatalytic process on BAC-60 are shown in Fig. 7b. Prior to the photocatalytic process, the main characteristic peaks for E. coli are observed at m/z of 2835, 3638, 4365, 4871, 5382, 6256, 6412, 7274, 7871, 9064, and 9741 Da, in line with the results reported by Chang et al. for highly concentrated E. coli solutions [55]. After 1 h of visible light illumination, the protein with a mass range m/z of 2000e6000 Da disappeared almost completely. After 2 h of photocatalytic reaction, the content of proteins in the m/z range of 6e12 kDa decreased significantly. As can be seen in Fig. 7a, E. coli is almost completely inactivated at the same time. These experimental results indicate that the degradation of the low-molecularweight proteins results in the inactivation of E. coli. Subsequently, the degradation of large-molecular-weight proteins may lead to the death of E. coli by cell wall rupture [56e59].

3.3. Discussion on the mechanism To further investigate the photocatalytic mechanism of BAC-x, PL, DRS, MS, and EIS tests were carried out, and the results are summarized in Fig. 8. Fig. 8a shows the PL spectra of the BAC-x series of composites. BiVO4 shows a strong PL emission peak at ca. 525 nm, while CdS presents a PL emission peak at ca. 550 nm, which can be attributed to the recombination of the photogenerated electron-hole pairs [11,22]. BiVO4 shows the largest PL emission intensity, revealing that the low photocatalytic activity of BiVO4 is due to the high recombination rate of the photogenerated electron-hole pairs. In contrast, the BAC series show significantly lower PL emission intensities as a result of the reduced recombination rate of the photogenerated electronehole pairs upon combination with CdS. Among the composites of this series, BAC-60 shows the lowest PL emission intensity, indicating that the separation efficiency of the photogenerated electronehole pairs is maximum as a result of the migration of the photogenerated carriers between BiVO4 and CdS. As reported by Wu [11], BiVO4 and CdS are direct band gap semiconductors. Thus, the band gaps (Eg) of CdS and BA are respectively estimated to be 2.30 and 2.32 eV by fitting the (ahn)2 vs. (hn) plots (Fig. 8b). Fig. 8c shows the MottSchottky (MS) tests of CdS and BA. These results reveal that both CdS and BA are n-type semiconductors with flat-band potentials (approximately equal to the CB band in n-type semiconductors) of 0.59 and 0.68 V (vs. normal hydrogen electrode (RHE)), respectively [16,60]. Based on the above results, the diagrams of the electronic energy level structure for CdS and BA are plotted in Fig. 8d. To further analyze the flow direction of photogenerated electrons/hole at the CdS/BA interface, the Tafel curves of BA and CdS are depicted in Fig. 9a under visible light/no light conditions. Under visible light irradiation, the redox equilibrium potentials of BA and CdS shift towards the more negative voltage direction. Moreover, the redox properties of BA are more strongly affected by visible light irradiation. suggesting that the energy level structure of BA is more affected by minority carrier injection. In this case, the open circuit voltage of the BAS-60 is 0.79 V (vs. RHE) and 0.57 V (vs. RHE)

Fig. 7. Inactivation process of 106 CFU/mL E. coli over as-prepared catalysts under visible light irradiation (a) and MALDI-TOF-MS evaluation of the inactivation of E. coli by BAC-60 at 0 h, 1 h, and 2 h (b).

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Fig. 8. PL emission spectra of BAC-x at lex ¼ 270 nm (a), DRS spectra of BA and CdS (b), MS curves of BA and CdS (c), and schematic diagram of the energy band structure of BA and CdS with/without visible light irradiation (d).

Fig. 9. Tafel curves of BA and CdS with/without visible light irradiation and the corresponding open circuit voltage of BAC-60 electrode (the red dotted line) (a), EIS plots of BA, CdS and BAC-60 (b), and the corresponding fitting results (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 10. Photocatalytic degradation curves of RhB over BA (a), CdS (b) and BAC-60 (c) in the presence of different scavengers.

in dark or under visible light irradiation, respectively (the black/red dotted line in Fig. 9a). Therefore, in the absence of light, electrons will flow from CdS to BA according to the Tafel curve of CdS and BA, indicating that although the electron energy level structure of CdS is similar to that of BA, the Fermi level (EF) of CdS prepared under current conditions is higher than that of BA. As reported by Tan [61], for an n-type semiconductor, the built-in electric field near the surface drives electrons toward bulk and holes are directed to the catalyst-solution interface. Therefore, a few positive charges usually reside on the surface of the n-type BiVO4 particles. However, a well-developed built-in electric field is only present in the large crystals but absent in nanoparticles, such as the nanosized CdS in BAS-60 (as shown in Fig. 2). Therefore, in the absence of light, the majority of carriers (electron) on the surface of the CdS are more likely to flow from the CdS to the surface of the BA and the corresponding interfacial current is shown as route 1 in Fig. 8d, which is consistent with the results of the Tafel tests. Therefore, compared to pure CdS and BA, the electron energy level of the CdS component carrying a few positive charges in the BAC-60 is lower, while the electronic energy level of the BA component carrying a few negative charges is higher, resulting in an open circuit voltage of 0.79 V in dark. Under visible light irradiation, the open circuit voltage of BAC-60 decreased to 0.57 V, indicating that the photocatalyst improves significantly its reducing ability. Different from the results under dark condition, the photogenerated electrons flow from BiVO4 to CdS in BAC-60 under visible light irradiation, following a Z-scheme photocatalytic mechanism, as shown in route 2 in Fig. 2d. Since the open circuit voltage of BAC-60 shifts to a more negative potential under visible light irradiation, the photogenerated holes

of the catalyst possessed higher catalytic activity. This also implies that the reactive species may be photogenerated holes on the surface of CdS in BAC-60. For n-type semiconductors, electrons are majority carriers, so the injection of photogenerated electrons has little effect on the conduction band electron concentration and the electron quasi-Fermi level (E’Fn). However, the photogenerated nonequilibrium carriers affect significantly on the quasi-Fermi level (E’Fp) of the minority carriers namely, holes with a higher positive potential. Therefore, the reversal of this current transfer at the BiVO4/CdS interface under visible light irradiation reveals that the conduction band electrons of BiVO4 and the valence band holes of CdS are combined, confirming that the photocatalytic process followed a Z-scheme mechanism under visible light irradiation. The EIS plots of CdS, BA, and BAC-60 over the frequency range of 10 mHze100 kHz with an amplitude of 0.005 V are shown in Fig. 9b. These curves show a semicircular arc in the high-frequency region and a straight line in the low-frequency region. In these plots, the semicircular arc in the high-frequency region represents the conductivities (Rct) of the photocatalysts and the electrode, while the straight line that appears in the low-frequency region represents the “Warburg impedance" (Zw), which measures the difficulty of mass transport of the electroactive species [62,63]. BAS-60 shows slightly lower Rct values (45.40 U) compared to CdS (49.23 U), suggesting that nanosized CdS has lower charge conductivity (in Fig. 9c). The value of Y0 (1/Zw·ɷ) of BAS-60 is much smaller than that of BA, which indicates that the coupling of CdS and BA significantly enhances mass transport of the electroactive species possibly due to the effects of interface, the surface charge of the catalysts, or the concentration of an electrochemically active

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species on the surface of the electrode. To further investigate the main reactive species directly involved in the photodegradation process over the CdS/BiVO4, the radical trapping tests were performed. Silver nitrate (AN), isopropyl alcohol (IPA), ethanol (EtOH), and benzoquinone (BQ) were added To the reaction solution as electron, $OH, hole, and $O 2 radical scavengers, respectively [12]. As shown in Fig. 10a, the photocatalytic performance of BA towards the degradation of RhB is very poor. The effects of IPA, EtOH, and BQ as scavengers on the photocatalytic rate of BA are negligible. In contrast, AN significantly accelerates the photocatalytic activity of BA for RhB. The mechanism may be that after the photogenerated electrons are consumed by AN, the photogenerated electron/hole recombination rate is lowered, the photogenerated hole concentration is increased, and the oxidation speed of RhB over BA is accelerated. These results confirm the poor reduction ability of the conduction band electrons of BA and the strong oxidization ability of the valence band holes. Considering that BA is an n-type semiconductor with a low hole concentration, its photocatalytic activity is weaker compared to a p-type BiVO4, which is stem to low mobility of the photogenerated charge carriers, positive potential of CB (vs RHE) and high charge recombination rates [23]. As shown in Fig. 10b, AN significantly reduced the photocatalytic activity of CdS, and BQ also had a non-negligible effect on the photocatalytic process. While the impact of IPA and EtOH can be ignored. This indicates that the RhB molecule can be directly reduced on the surface of the catalyst followed after adsorption, or can be photodegraded by combining with $O 2 in the solution. However, IPA and EtOH do not significantly affect the photocatalytic process of CdS, which indicates that different from n-type BiVO4, the valence band electron of n-type CdS has a higher photocatalytic activity. In Fig. 10c, AN significantly affects the photocatalytic performance of BAC-60, while the effects of other scavengers are negligible, suggesting that photogenerated electrons, rather than photogenerated holes and free radicals, play a decisive role in the photocatalytic process. Considering the Z-scheme photocatalytic mechanism of BAC-60 and the higher photocatalytic activity of BAC-60 than CdS, the coupling of CdS and BA significantly decreases the potential of the conduction band of CdS content in BAC-60, resulting in a higher reduction activity. Another reason for the increase in the photocatalytic activity of BAC-60 may be due to the low recombination rate of photocatalytic electron-hole pairs resulting from the coupling of CdS and BA. In brief, XRD, SEM, and TEM were used to analyze the structural characteristics of the as-prepared catalysts. The results showed that in BAC and BSC, CdS particles maintain a similar structure, while BA and BS have a different combination of exposed faces. The photocatalytic and bactericidal activities measurements were carried out to experimentally compare the reactivity of different crystal faces. In addition, the inversion of the interface current between the different components of the composite catalysts was an interesting finding of this work. 4. Conclusion The growth of the (101) crystal plane of BiVO4 is effectively hindered by adding CTAB during precipitation process. As a result, BA samples have more exposed (020) plane compared to BS. BiVO4/ CdS composites were successfully prepared by a two-step sonication-precipitation method with S-(111) layer as an interface. The photocatalytic activity of the BAC-60 towards the photodegradation of RhB is enhanced by 1.5 times compared to BSC-60. Comparative studies reveal that the (020) plane of BiVO4 has a higher visiblelight-driven photocatalytic activity than the (101) plane. The initial inactivation of E. coli is due to the decomposition of

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macromolecules with a typical mass range m/z of 2e6 kDa. Visible light irradiation significantly affected the band structure of BiVO4 and CdS in BAC-60. Under visible light irradiation, the nonequilibrium carrier injection obviously increases the energy of the photogenerated holes of CdS, and also increases the recombination ability of the holes with the valence band electrons of BiVO4, resulting in the reversal of the interface current between CdS and BiVO4 and the conversion of photocatalytic mechanism from the traditional type II heterojunction to the Z-scheme mechanism. Authors’ contributions Rui Guo and Aiguo Yan carried out the laboratory experiment and drafted the manuscript. The other authors provided assistance with the experimental measurements and data analysis. All authors read and approved the final manuscript. Declaration of competing interest The authors declare that they have no competing interests. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant Nos. 51704064, 51874079), the Fundamental Research Funds for the Central Universities of China (Grant N162302001), Hebei Province higher education science and technology research project of China (Grant No. ZD2017309). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.153246. References [1] C.L. Yu, W.Q. Zhou, H. Liu, Y. Liu, D.D. Dionysiou, Design and fabrication of microsphere photocatalysts for environmental purification and energy conversion, Chem. Eng. J. 287 (2016) 117e129. [2] X.W. Liu, J.J. Xu, Z.Y. Ni, R.C. Wang, J.H. You, R. Guo, Adsorption and visiblelight-driven photocatalytic properties of Ag3PO4/WO3 composites: a discussion of the mechanism, Chem. Eng. J. 356 (2019) 22e33. [3] N.Y. Zhu, J. Tang, C.L. Tang, P.F. Duan, L.G. Yao, Y.H. Wu, D.D. Dionysiou, Combined CdS nanoparticles-assisted photocatalysis and periphytic biological processes for nitrate removal, Chem. Eng. J. 353 (2018) 237e245. [4] J. Liu, X.X. Zhao, P. Jing, W. Shi, P. Chen, A metal-organic-framework-derived g-C3N4/alpha-Fe2O3 hybrid for enhanced visible-light-driven photocatalytic hydrogen evolution, Chem. Eur J. 25 (2019) 2330e2336. [5] X. Tian, Y.J. Sun, J.Y. He, X.J. Wang, J. Zhao, S.Z. Qiao, F.T. Li, Surface P atom grafting of g-C3N4 for improved local spatial charge separation and enhanced photocatalytic H-2 production, J. Mater. Chem. 7 (2019) 7628e7635. [6] X.W. Liu, Z.Y. Ni, Y. He, N. Su, R. Guo, Q. Wang, T.F. Yi, Ultrasound-assisted twostep water-bath synthesis of g-C3N4/BiOBr composites: visible light-driven photocatalysis, sterilization, and reaction mechanism 43 (2019) 8711e8721. [7] Y.J. Yuan, J.R. Tu, Z.J. Ye, D.Q. Chen, B. Hu, Y.W. Huang, T.T. Chen, D.P. Cao, Z.T. Yu, Z.G. Zou, MoS2-graphene/ZnIn2S4 hierarchical microarchitectures with an electron transport bridge between light-harvesting semiconductor and cocatalyst: a highly efficient photocatalyst for solar hydrogen generation, Appl. Catal., B 188 (2016) 13e22. [8] X.C. Meng, Z.Z. Li, Z.S. Zhang, Palladium nanoparticles and rGO co-modified BiVO4 with greatly improved visible light-induced photocatalytic activity, Chemosphere 198 (2018) 1e12. [9] K. Zhang, J. Wang, W.J. Jiang, W.Q. Yao, H.P. Yang, Y.F. Zhu, Self-assembled perylene diimide based supramolecular heterojunction with Bi2WO6 for efficient visible-light-driven photocatalysis, Appl. Catal., B 232 (2018) 175e181. [10] D.L. Huang, C.J. Hu, G.M. Zeng, M. Cheng, P.A. Xu, X.M. Gong, R.Z. Wang, W.J. Xue, Combination of Fenton processes and biotreatment for wastewater treatment and soil remediation, Sci. Total Environ. 574 (2017) 1599e1610. [11] X.Q. Wu, J. Zhao, L. P Wang, M.M. Han, M.L. Zhang, H.B. Wang, H. Huang, Y. Liu, Z.H. Kang, Carbon dots as solid-state electron mediator for BiVO4/CDs/CdS Zscheme photocatalyst working under visible light, Appl. Catal., B 206 (2017) 501e509. [12] L. Zou, H.R. Wang, X. Wang, High efficient photodegradation and photocatalytic hydrogen production of CdS/BiVO4 heterostructure through Z-

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