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One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation
Journal of Alloys and Compounds xxx (xxxx) xxx
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One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation Yanyan Cui a, *, Yan Cheng a, Yurong Jiang a, Jian Yang a, Yaling Wang b, ** a Beijing Engineering Research Center of Mixed Reality and Advanced Display, School of Optoelectronics, Beijing Institute of Technology, Beijing, 100081, China b CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety and CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, 11 Beiyitiao Zhongguancun, Beijing, 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2019 Received in revised form 18 October 2019 Accepted 20 October 2019 Available online xxx
To address the issues of water pollution and energy shortage, significant research has gone into the design of nanostructures with enhanced photocatalytic activity. In this paper, the photocatalyst Bi2S3/Bi/ BiOI (BIS) was designed with both Z-scheme and heterojunction structures and successfully fabricated via a facile one-step solvothermal method. The as-synthesized BIS sample could quickly decompose the Rhodamine B (RhB) under visible light irradiation within 30 min and demonstrated good reproducibility and cyclability. The low bandgap energy, large specific surface area, and the high oxidation/reduction potential of holes/electrons were key factors for the improved photocatalytic performance. The photogenerated hþ and $O 2 species played important roles in the degradation of RhB. The photocatalyst showed excellent, stable photocatalytic activity, and could be a candidate for environmental pollution remediation applications. © 2019 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis Heterojunction Z-Scheme Water purification
1. Introduction Significant efforts have been made to develop effective environmentally friendly methods to deal with water pollution and energy shortages [1e5]. Since the first application of TiO2 nanostructure to assist with water splitting [6], photocatalysts have gained marked interest for various applications. Generally, the photocatalytic efficiency depends on the redox capacity and lifetime of the photogenerated electron-hole pairs. Numerous nanostructures have been designed to improve the photocatalytic efficiency [7], and a longer electron-holes pair lifetime leads to a higher catalytic efficiency [8,9]. The development of the heterojunction structure, which is composed of two different kinds of semiconductors, enabled the formation of a built-in electric field that could separate photogenerated electrons and holes to different two semiconductors. The spatial separation of electrons and holes into the different semiconductors inhibits their recombination. Layered BiOI is a typical photocatalyst with a narrow bandgap (1.77e1.92 eV), which can be
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Cui), [email protected] (Y. Wang).
excited by visible light. Heterojunctions with other semiconductor structures, such as Bi2S3 [10], AgI [11], BiOBr [12], BiOCl [13], BiVO4 [7], and Bi3FeO4 [14] have also been reported. These heterojunction photocatalysts all exhibited good performance for the photocatalytic decomposition of pollutants. However, the separation of excited charges in a heterojunction comes at the cost of a reduced redox potential [15]. In recent years, Z-scheme photocatalysts have gained significant interest. When the two semiconductors in a heterojunction structure are bridged by noble metal nanoparticles (a so called mediator), such as Ag, Pt, and Au, a Z-scheme structure is formed; this results in the formation of a Schottky barrier at the interface between the semiconductor and the metal nanoparticles. Light absorption and interfacial charge transfer are enhanced and the photocatalytic efficiency is promoted [16,17]. The nanometallic mediator with a good conductivity links the two semiconductors together in the Z-scheme structure, and it provides sites for the recombination of electrons and holes that come from the lower redox energy bands. The electrons and holes in the higher redox potential bands are not affected, thus improving the subsequent photodecomposition [18,19]. To develop facile methods for the fabrication of a Z-scheme photocatalyst, one-step synthesis methods are desired to accelerate the development of catalysts
https://doi.org/10.1016/j.jallcom.2019.152752 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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with enhanced pollutant degradation efficiency and for decreased costs. In this work, a heterogeneous BIS nanostructure was synthesized via a one-step solvothermal method using ethylene glycol as a solvent. In the presence of glycol, the Bi3þ of the raw material was reduced to metallic Bi. Transmission electron microscopy (TEM) showed a flower-like nanoparticle composed of many twisted nanosheets with a thickness between 6 and 33 nm; the Bi2S3 and BiOI were bridged by metallic Bi nanocrystals. The surface area was calculated to be 43.0189 m2/g. The synthesized BIS photocatalyst had a good, stable photocatalytic performance, and it completely decomposed Rhodamine B (denoted as RhB) within 30 min under visible light irradiation, with a synergistic photodegradation mechanism. This study demonstrates a viable alternate route for environmental pollution remediation.
h¼
RhBini RhBfin ; RhBini
(1)
where RhBini and RhBfin were the initial and the final RhB concentrations, respectively. The reaction was carried out in 100 ml of a 10 mg/L RhB solution with 20 mg catalyst, and the solution was stirred for 0.5 h in the dark to equilibrate the adsorption-desorption prior to irradiation. The light source was a 300 W Xenon lamp with a 420 nm cut-off filter. Four ml of the mixed solution was collected at selected irradiation time points and analyzed by measuring the UVeVis absorption spectra.
3. Results and discussion
2. Experimental section
3.1. Characterization
2.1. Chemical reagents
3.1.1. Morphology and crystal structure analysis SEM was used to investigate the morphology of the BIS nanocrystals (referred to as BIS), as shown in Fig. 1. Fig. 1(a) shows an overall view of the nanoparticle adhesion. From Fig. 1(b), it can be seen that the photocatalyst had a flower-like nanoparticle structure composed of many twisted nanosheets with a thickness between 6 and 33 nm. Comparing the samples of BiOI (Fig. S1a) and Bi2S3 (Fig. S1b), the formed BIS nanosheets were much larger. These crossed nanosheets had a large surface area that could provide many active sites for the adsorption of organic pollutants, which could promote the photocatalytic efficiency. TEM was used to further explore the detailed structure of the samples. For the as-prepared sample, the TEM image exhibited a polycrystalline and amorphous phase, as shown in Fig. 1 (c, d). The lattice fringe spacings of 0.253 nm, 0.311 nm, and 0.194 nm were well matched with the (1 0 2), (2 1 1), and (5 0 1) lattice planes of orthorhombic Bi2S3 (ICSD: 17e0320), respectively; the lattice spacing of ~0.301 nm corresponded to the (1 0 2) plane of tetragonal BiOI (ICSD: 10e0445) and the 0.165 nm lattice spacing can be attributed to the (0 2 4) crystal plane of the hexagonal metallic Bi (ICSD: 44e1246), which may originate from Bi3þ reduced by ethylene glycol [20e23]. These results demonstrate that the fabricated photocatalyst was comprised of a ternary nanostructure, that included pure Bi, BiOI, and Bi2S3. To further explore their spatial distribution, energy dispersive spectrometry (EDS) was used to obtain elemental mapping images of Bi, S, and I. Fig. 1(f-h) shows a homogeneous distribution of Bi, S, and I in the sheet-like BIS nanoparticles, confirming the presence of a hybrid structure.
All the used reagents were purchased from Sinopharm Chemical Reagent Co., Ltd (China) and used without further purification. Deionized water was used for all experiments in the study. 2.2. Fabrication of catalyst Typically, 0.5 mmol of Bi(NO)3 was added into 10 ml of ethylene glycol with stirring for 1 h at room temperature until completely dispersed and the solution was marked as solution A. Next, 0.3 mmol of KI was added into 10 ml of ethylene glycol with stirring for 1 h until completely dissolved, then 0.3 mmol of 3mercaptopropionic acid was added to the solution with stirring for 0.5 h, and the mixture was marked as solution B. Then, solution B was added dropwise added into solution An under stirring. The mixed solution was stirred at room temperature for 1 h and then transferred into a 50 ml stainless-steel Teflon lined autoclave and heated at 160 C for 15 h. Then, the reactor was cooled down to room temperature. The precipitate was then separated via centrifugation, washed with DI water and absolute alcohol 5 times, and then dried at 80 C for 6 h. 2.3. Characterization Scanning electron microscopy (SEM) images were taken using a JSM-6380 operating at 20 KV. The microstructure of the prepared photocatalyst was characterized via transmission electron microscopy (TEM) (JEM-1200EX). The X-Ray diffraction (XRD) pattern of the dried sample was measured using an XPERT-PRO diffractometer with Cu K-alpha radiation (1.5406 Å). The Brunauer-EmmettTeller (BET) specific surface areas were calculated via the N2 adsorptiondesorption isotherms at 77.35 K with a MicroActive for ASAP2460 2.01. XPS measurements were taken to determine the surface elemental composition using a PerkinElmer PHI 1257 electron spectrometer. The FT-IR spectra were recorded on a Nicolet Nexus spectrometer. UVeVis DRS was performed with a PerkinElmer Lambda 35 UV/Vis spectrophotometer. UVeVis adsorption spectra were obtained using a UV 1750 spectrometer. A fluorescence spectrometer (PL, F-2700, Hitachi, Japan) was used to obtain the photoluminescence (PL) spectra. 2.4. Photocatalytic activity RhB was used to evaluate the photocatalytic activities of the synthesized BIS by calculating the RhB photodegradation rate based on the formula:
3.1.2. X-ray diffraction (XRD) analysis The composition and crystallinity of the materials were determined via XRD spectra. Fig. 2 and Fig. S2 show the XRD patterns of BIS, pure BiOI, and Bi2S3 materials with 2q values ranging from 10 to 80 . The distinct peaks indicated that the sample was highly crystalline and contained multiple crystal structures. The peaks at 2q values of 28.50 , 38.03 , and 39.80 were well matched with the (0 1 2), (1 0 4) and (1 1 0) crystal planes of hexagonal metallic Bi (JCPDS File No. 44e1246), respectively. The peaks at 2q value of 24.93 and 28.50 could be assigned to the (1 3 0) and (2 1 1) crystallographic planes of orthorhombic Bi2S3 (JCPDS File No. 17e0320), respectively. Finally, the peaks at 2q value of 29.73 , 31.70 , 45.38 , and 55.19 were well indexed to the (1 0 2), (1 1 0), (2 0 0), and (2 1 2) crystallographic planes of tetragonal BiOI (JCPDS File No. 10e0445), respectively. These results indicated that the as-synthesized materials were composed of three structures, Bi, Bi2S3, and BiOI, which was consistent with the TEM results.
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 1. SEM (a, b), TEM (c, d) and STEM (e) images of BIS, and the EDS elemental mapping of f) Bi, g) S and h) I.
3.1.3. Brunauer-Emmett-Teller analysis The increased surface area of the catalyst could provide more locations for the redox reaction, which could enhance the photocatalytic efficiency [24]. The BET specific area of the BIS nanocrystals was measured via nitrogen adsorption-desorption measurements and was calculated to be 43.0189 m2/g. The large surface area of the as-prepared sample could provide an increased number of active sites to adsorb the reactant substances resulting in improved photocatalytic performance.
Fig. 2. Experimental XRD pattern of BIS and the theoretical XRD of Bi (black), Bi2S3 (blue), BiOI (brown). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3.1.4. XPS analysis Photocatalytic redox processes take place at the surface of the photocatalyst; therefore, the surface elemental information and ion oxidation were characterized via XPS, and the results are shown in Fig. 3. The full survey spectrum in Fig. 3 (a) shows that carbon, bismuth, iodine, oxygen, and sulfur coexisted in the as-prepared BIS specimens. The C1s spectrum derived from adventitious hydrocarbon in the spectrometer. The high-resolution Bi4f spectrum in Fig. 3 (b) showed peaks for Bi 4f7/2 and 4f5/2. The Gaussian fitted peaks at 159.34 eV and 164.76 eV corresponded to Bi3þ in Bi2S3 [10], while the peaks at 158.62 eV and 164.17 eV belonged to Bi3þ in BiOI [20]. The peaks at 159.08 eV and 162.2 eV in the inset figure demonstrate the formation of Bi0, which agreed with XRD results [21]. The two peaks at 618.71 eV and 630.14 eV present in the I3d spectrum (Fig. 3 (c)) could be assigned to 3d5/2 and 3d3/2 of I in BiOI [20], respectively, indicating that the valence state of element I
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 3. XPS spectra of BIS: survey (a), I 3 d (b), O1s (c), and Bi 4f (d).
was 1 in the composite. The two peaks at 158.94 eV and 164.30 eV in the S2p spectrum (Fig. S3) could be assigned to Bi2S3. The O1s spectrum could be fitted into three peaks, as shown in Fig. 3 (d), and the peaks at 529.9 eV, 531.4 eV, and 532.8 eV corresponded to the BieO, eOH, and C]O bonds, respectively [25].
3.1.5. UVevis diffuse reflectance spectrum (DRS) analysis The UVeVis DRS spectra show the optical properties of the obtained BIS nanocrystals, and the spectra of the BiOI and Bi2S3 nanocrystals synthesized via a similar method were obtained for contrast. As shown in Fig. 4(a), the Bi2S3 nanocrystals could absorb light at wavelengths >360 nm, and the absorption of BiOI was in the wavelength range of 200e650 nm, while BIS had absorption in the region of 200e800 nm. The bandgap was calculated using the Kubelka-Munk equation:
a ¼ A(hv-Eg)2/n,
(2)
where a, h, and v are the absorption coefficient, Planck’s constant, and the light source frequency, respectively, A is the proportionality constant, and Eg represents bandgap energy. The constant n is dependent on the optical transition type of the photocatalyst; for a direct optical transition, the value of n is 1, while for an indirect transition, n is 4. The reported n values for BiOI and Bi2S3 were 4 and 1, respectively [10,26,27]. The plot of (ahv)1/2 vs. (hv) and the plot of (ahv)2 vs. (hv) are shown in Fig. 4 (b). The bandgap of BIS, BiOI, and Bi2S3 were calculated to be 0.51, 1.53, and 1.67 eV, respectively, suggesting that these three nanocrystals could all be excited by visible light. Moreover, the valence band (VB) edge and conduction band (CB) edge were calculated using the following equations [28,29]: EVB ¼ X - EC þ 0.5 Eg,
(3)
ECB ¼ EVB e Eg,
(4)
where EVB and ECB are the VB and CB edge potential, respectively, X is the electronegativity of the semiconductor, EC is the energy of free electrons on the hydrogen scale (~4.5 eV), and Eg is the bandgap energy. The X values for Bi2S3 and BiOI are 5.276 and 5.943 eV, respectively [10,26]. Thus, the VB and CB potential for BiOI were calculated to be 2.208 eV and 0.678 eV, and for Bi2S3, they were 1.541 eV and 0.129 eV, respectively. 3.1.6. Fourier transform infrared spectroscopy (FT-IR) analysis The composition and chemical interaction of the as-prepared nanocrystals were further confirmed via Fourier Transform Infrared spectroscopy (FT-IR), as shown in Fig. 4 (c). The broad bands with peaks at 3405 cm1 and 1618 cm1 belong to OeH vibrations, which can be ascribed to surface-adsorbed water molecules and the COOH group of mercaptopropionic acid. The small peak at 532 cm1 can be attributed to the BieO stretching vibrations, which is due to the formation of Bi2O2 2 . The absorption at 765 cm1 can be ascribed to CeS bonds, while the peak at 1079 cm1 is assigned to the C]S bond, suggesting that some other groups were reduced. By combining the previous results, a possible explanation for this is that the Bi3þ ion was reduced to metallic Bi. The FT-IR spectra of pure BiOI and Bi2S3 are shown in Fig. S4, compared with the BIS sample, the BieO stretching vibrations peak of BiOI emerged at ~500 cm1, suggesting that in the BIS composite, some other structures were present besides the much smaller BiOI crystal, such as Bi2S3. 3.1.7. Electrochemical properties The photodecomposition of pollutants is based on oxidationreduction reactions via photogenerated charge carriers. The degradation efficiency depends on the probability of photogenerated electron (e) and hole (hþ) recombination. The
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 4. (a) DRS results and (b) energy gap of BiOI, Bi2S3, and BIS, (c) FT-IR spectra and (d) photocurrent response of BIS, BiOI, and Bi2S3.
photocurrent response was used to characterize the electrochemical properties, and it can be used to describe the separation, migration, and trapping of photoinduced charge carriers. For every turn-on and turn-off event over multiple cycles (Fig. 4 (d)), the photocurrent rapidly increased when the visible light was switched on and dropped quickly when the light was switched off [30]. The rapid increase in the photocurrent indicated that a large amount of photo-excited carriers formed quickly because of the narrow energy gap of the photocatalyst during the turn-on period, and the recombination of e-hþ was markedly reduced because the photocatalyst efficiently separated and trapped the e-hþ [31]. Because carrier recombination could result in luminance, the photoluminescence (PL) spectrum was obtained to further study the trapping, migration, and separation efficiency of photogenerated electrons and holes. Increased recombination of the ehþ would result in a stronger luminance intensity. The PL spectra (Fig. S5) show that the as-prepared BIS group exhibits a weaker PL intensity than pure BiOI and Bi2S3, which can be attributed to the formation of a Schottky barrier at the interface of the semiconductor (BiOI and Bi2S3) and the nanometallic Bi. The PL results indicate that the synergistic effects of the BIS nanocrystals result in a more efficient separation of photogenerated charge carriers than the BiOI and Bi2S3 heterojunction structures. 3.1.8. Photocatalytic performance for RhB degradation Rhodamine B, with a maximum absorption at 554 nm, was used as a model organic pollutant to estimate the photocatalytic activity of BIS under visible light irradiation [32]. The UVeVis absorption spectra of the nanocrystals demonstrated that the peaks at 554 nm decreased with time and indicated the degradation of the RhB chemical structure, as shown in Fig. 5 (a). A continuous blue shift of the absorption maximum was recorded during the degradation
process, suggesting that the molecular structure of RhB was gradually destroyed. The degradation rate of RhB under visible light irradiation with samples that had varying ratios of Bi2S3 to BiOI was studied. As shown in Fig. 5(b), the RhB photocatalytic degradation activity varied with composition, and a 1:1 ratio of Bi2S3 to BiOI was determined to be optimal for the photodegradation of RhB. The BiOI sample could also degrade RhB under visible light, while the Bi2S3 sample showed minimal degradation of RhB under visible light after more than 60 min. The standard kinetic data curves obtained by fitting the plot of ln (C/C0) vs. illumination time (t) (Fig. S6) was used to quantitatively analyze the photocatalytic efficiency. According to the first-order kinetics, the discoloration reaction can be described by: ln(C0/C) ¼ kt,
(5)
where C0 represents initial concentration of RhB, C represents the concentration at time t, and the slope k denotes the reduced rate constant [33]. The photo degradation constant of the as-prepared catalyst was calculated to be 0.108, indicating good catalytic performance. Cycling experiments were performed to evaluate the recyclability of the nanocrystals under visible light irradiation. As displayed in Fig. 5 (c), the stability of the catalyst after four cycles indicated that the BIS photocatalytic nanocrystals were not damaged by photocorrosion. This confirmed the recycling stability of BIS. The degradation process was further investigated via liquid chromatography-mass spectrometry (LC-MS), and the HPLC spectra are shown in Fig. 5 (d). The changes in the HPLC spectra indicate that different degradation products form with different illumination durations. There was an obvious RhB peak in the pristine
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 5. (a) Temporal UVevis adsorption spectra changes of RhB under visible light illumination, (b) degradation rate of RhB with the samples that have various amount ratio of Bi2S3 to BiOI (the numbers in the figure were the amount ratio of Bi2S3 to BiOI), (c) cycling experiments, and (d) the HPLC spectra changes of RhB under different illumination timing.
solution, and the intensity decreased with increasing irradiation time. Additionally, more peaks associated with the degradation product of RhB appeared and they subsequently all disappeared in the following order: the peaks of DER (diethyl-N-ethyl rhodamine), EER (N-ethyl-N-ethyl rhodamine), DR (N, N-diethyl rhodamine), ER (N-ethyl rhodamine), and R (Rhodamine) are the photodecomposition products of RhB, which can be ascribed to the degradation of the chromophore structure because of deethylation. The detailed intermediates for the degradation products of RhB are shown in Fig. 6. After 30 min, the rhodamine had almost entirely lost its ethyl group(s) and formed DER. Because the second and third bonds are extremely unstable, they also break rapidly and EER and DR formed after 40 min of reaction. After 60 min of catalytic activity, only peaks associated with ER could be observed. Finally, all the species had decomposed into small non-toxic molecules after 90 min. 3.1.9. Photocatalytic mechanism The photocatalytic degradation of RhB using a photocatalyst depends on a series of active radicals generated on the basis of the photogenerated electrons and holes, including active holes, superoxide radicals, and hydroxyl radicals. To explore the photocatalytic mechanism of BIS, active radical trapping experiments were carried out to identify the dominant active species. In this experiment, ethylenediamine tetra acetic acid disodium (EDTA-2Na, the scavenger of hþ), p-benzoquinone (BQ, $O 2 scavenger), and 2-Propanol (IPA, the scavenger of $OH) was used as a scavenger species to trap the corresponding radical species [34,35]. When EDTA-2Na, BQ, and IPA were introduced into the reaction system, the RhB removal efficiency declined by 0.90, 0.32, and 0.06,
respectively, as shown in Fig. 7 (a). When the EDTA-2Na was introduced, the degradation of RhB was almost completely inhibited; suggesting that hþ was the main species in the RhB decomposition process. The results for BQ addition indicate that O 2 also played an important role in the degradation process. The presence of 2-Propanol had minimal impact on the degradation of RhB, indicating that $OH only played a secondary role in the degradation þ process. The results indicate that $O 2 and h were the dominant active radials for the photocatalytic decomposition of RhB by BIS. Experiments investigating the effects of metal ions were also carried out and the results are shown in Fig. 7(b). The results demonstrate that Hg2þ had the strongest inhibition on the catalytic process, followed by Agþ, Zn2þ, and Cu2þ. Among these ions, Agþ could easily trap photogenerated electrons, imposing restrictions on the formation of $O 2 and thus inhibiting the decomposition. Zn2þ and Cu2þ are metal ions than could easily be reduced. The addition of Hg2þ not only inhibited the photocatalysis but also caused a red-shift of the absorption peak, which indicated that mercury ions could form complexes with RhB and inhibit its further degradation. To investigate the active radical generation mechanism, the probable structural composition of the nanocrystals BIS was further analyzed. From the HRTEM image of BIS (Fig. 1(d)), it can be seen that Bi2S3 and BiOI are joined together, forming a heterojunction structure. The electrons on the CB of Bi2S3 could move to the CB of BiOI because it was more negative than that of BiOI. Additionally, because the CB potential of BiOI (0.678) was more positive than E0 (O2/$O 2 ) (0.046 eV), the electrons on the CB of BiOI were not able to reduce O2 to $O 2 . However, the results of the trapping experiment showed that $O 2 played an important role on RhB
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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degradation. From Fig. 1(d), the metallic Bi nanostructure can be observed to link the Bi2S3 and BiOI structures, and the three together could form a Z-Scheme structure. In the Z-scheme structure, metallic Bi would serve as a bridge, and the electrons from the CB of BiOI would combine with holes from the VB of Bi2S3. The electrons on the CB of Bi2S3 with a potential of 0.129 were negative enough to react with O2 to form $O 2 . The potential of the holes remaining on the VB of BiOI was 2.208, which was more positive than H2O/$OH (2.38 eV). A small amount of $OH radicals would be produced, which is consistent with the previously detailed trapping experiments. Therefore, the BIS catalyst had a Z-scheme structure for the degradation for RhB. Additionally, the holes on the valence band of BiOI could react with the pollutants. In brief, two kinds of composite structures were present, the heterojunction structure of Bi2S3 and BiOI and the ZScheme structure of BIS; the holes in both these structures could participate in the reaction, while the $O 2 active species was only present in the Z-Scheme. The quantity of holes was more than $O 2, resulting in a larger contribution to the photodegradation of RhB, which is consistent with previous results. Based on the above discussion, a possible photodegradation mechanism for RhB by the prepared catalysts under visible light irradiation was proposed. Fig. 8 shows the electron pathway. The electrons at the top of the VB of Bi2S3 and BiOI were excited to their respective CBs, leaving holes on the VB. Then, the electrons (CB) of Bi2S3 and the holes (VB) of BiOI transfer to Bi and recombine there in a Z-scheme (Fig. 8(a)). In the heterostructure (Fig. 8(b)), the excited electrons of Bi2S3 transferred to BiOI. As a result, the generated electrons and holes were effectively separated, inhibiting their recombination. The stabilized electrons on the CB of Bi2S3 in the Z-scheme would combine with O2 to form $O 2. þ Finally, under attack by the active radicals ($O 2 and h ), the Nethyl group of RhB split into unstable ethyl groups (DER, EER, DR, ER, and R). Finally, Rhodamine B was completely decomposed to CO2 and H2O.
4. Conclusion
Fig. 6. Detailed formations of different molecular groups during RhB degradation.
In summary, an efficient photocatalyst, Bi2S3/Bi/BiOI, composed of both Z-scheme and heterojunction structures was successfully prepared via a one-step solvothermal method using ethylene glycol as the solvent. This composite demonstrated excellent photocatalytic performance for RhB degradation under visible light irradiation. Radical trapping experiments indicated that hþ and $O 2 were the main species involved in the degradation of organic pollutants. As a highly efficient sunlight driven photocatalyst, BIS can potentially be used for the treatment of water pollution for other photocatalytic applications.
Fig. 7. Active radical trapping experiments (a), and effects of different metal ions on photodegradation (b).
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 8. Schematic illustration of charge transfer pathway during RhB degradation process over BIS under visible light irradiation.
Declaration of competing interest The authors declare no competing financial interest. Acknowledgments This work was financially supported by the National Key Research and Development Program in China (2017YFC0112000), National Major Science and Technology Project of China (2018ZX10723-204-008) and National Natural Science Foundation Program of China (21505137, 21675157, 61527827, 11535015), the Beijing Natural Science Foundation (7164271). We thank Beijing Zhongkebaice Technology Service Co., Ltd. for the characterization data. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152752.
[11]
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation Yanyan Cui a, *, Yan Cheng a, Yurong Jiang a, Jian Yang a, Yaling Wang b, ** a Beijing Engineering Research Center of Mixed Reality and Advanced Display, School of Optoelectronics, Beijing Institute of Technology, Beijing, 100081, China b CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety and CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, 11 Beiyitiao Zhongguancun, Beijing, 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2019 Received in revised form 18 October 2019 Accepted 20 October 2019 Available online xxx
To address the issues of water pollution and energy shortage, significant research has gone into the design of nanostructures with enhanced photocatalytic activity. In this paper, the photocatalyst Bi2S3/Bi/ BiOI (BIS) was designed with both Z-scheme and heterojunction structures and successfully fabricated via a facile one-step solvothermal method. The as-synthesized BIS sample could quickly decompose the Rhodamine B (RhB) under visible light irradiation within 30 min and demonstrated good reproducibility and cyclability. The low bandgap energy, large specific surface area, and the high oxidation/reduction potential of holes/electrons were key factors for the improved photocatalytic performance. The photogenerated hþ and $O 2 species played important roles in the degradation of RhB. The photocatalyst showed excellent, stable photocatalytic activity, and could be a candidate for environmental pollution remediation applications. © 2019 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis Heterojunction Z-Scheme Water purification
1. Introduction Significant efforts have been made to develop effective environmentally friendly methods to deal with water pollution and energy shortages [1e5]. Since the first application of TiO2 nanostructure to assist with water splitting [6], photocatalysts have gained marked interest for various applications. Generally, the photocatalytic efficiency depends on the redox capacity and lifetime of the photogenerated electron-hole pairs. Numerous nanostructures have been designed to improve the photocatalytic efficiency [7], and a longer electron-holes pair lifetime leads to a higher catalytic efficiency [8,9]. The development of the heterojunction structure, which is composed of two different kinds of semiconductors, enabled the formation of a built-in electric field that could separate photogenerated electrons and holes to different two semiconductors. The spatial separation of electrons and holes into the different semiconductors inhibits their recombination. Layered BiOI is a typical photocatalyst with a narrow bandgap (1.77e1.92 eV), which can be
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Cui), [email protected] (Y. Wang).
excited by visible light. Heterojunctions with other semiconductor structures, such as Bi2S3 [10], AgI [11], BiOBr [12], BiOCl [13], BiVO4 [7], and Bi3FeO4 [14] have also been reported. These heterojunction photocatalysts all exhibited good performance for the photocatalytic decomposition of pollutants. However, the separation of excited charges in a heterojunction comes at the cost of a reduced redox potential [15]. In recent years, Z-scheme photocatalysts have gained significant interest. When the two semiconductors in a heterojunction structure are bridged by noble metal nanoparticles (a so called mediator), such as Ag, Pt, and Au, a Z-scheme structure is formed; this results in the formation of a Schottky barrier at the interface between the semiconductor and the metal nanoparticles. Light absorption and interfacial charge transfer are enhanced and the photocatalytic efficiency is promoted [16,17]. The nanometallic mediator with a good conductivity links the two semiconductors together in the Z-scheme structure, and it provides sites for the recombination of electrons and holes that come from the lower redox energy bands. The electrons and holes in the higher redox potential bands are not affected, thus improving the subsequent photodecomposition [18,19]. To develop facile methods for the fabrication of a Z-scheme photocatalyst, one-step synthesis methods are desired to accelerate the development of catalysts
https://doi.org/10.1016/j.jallcom.2019.152752 0925-8388/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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with enhanced pollutant degradation efficiency and for decreased costs. In this work, a heterogeneous BIS nanostructure was synthesized via a one-step solvothermal method using ethylene glycol as a solvent. In the presence of glycol, the Bi3þ of the raw material was reduced to metallic Bi. Transmission electron microscopy (TEM) showed a flower-like nanoparticle composed of many twisted nanosheets with a thickness between 6 and 33 nm; the Bi2S3 and BiOI were bridged by metallic Bi nanocrystals. The surface area was calculated to be 43.0189 m2/g. The synthesized BIS photocatalyst had a good, stable photocatalytic performance, and it completely decomposed Rhodamine B (denoted as RhB) within 30 min under visible light irradiation, with a synergistic photodegradation mechanism. This study demonstrates a viable alternate route for environmental pollution remediation.
h¼
RhBini RhBfin ; RhBini
(1)
where RhBini and RhBfin were the initial and the final RhB concentrations, respectively. The reaction was carried out in 100 ml of a 10 mg/L RhB solution with 20 mg catalyst, and the solution was stirred for 0.5 h in the dark to equilibrate the adsorption-desorption prior to irradiation. The light source was a 300 W Xenon lamp with a 420 nm cut-off filter. Four ml of the mixed solution was collected at selected irradiation time points and analyzed by measuring the UVeVis absorption spectra.
3. Results and discussion
2. Experimental section
3.1. Characterization
2.1. Chemical reagents
3.1.1. Morphology and crystal structure analysis SEM was used to investigate the morphology of the BIS nanocrystals (referred to as BIS), as shown in Fig. 1. Fig. 1(a) shows an overall view of the nanoparticle adhesion. From Fig. 1(b), it can be seen that the photocatalyst had a flower-like nanoparticle structure composed of many twisted nanosheets with a thickness between 6 and 33 nm. Comparing the samples of BiOI (Fig. S1a) and Bi2S3 (Fig. S1b), the formed BIS nanosheets were much larger. These crossed nanosheets had a large surface area that could provide many active sites for the adsorption of organic pollutants, which could promote the photocatalytic efficiency. TEM was used to further explore the detailed structure of the samples. For the as-prepared sample, the TEM image exhibited a polycrystalline and amorphous phase, as shown in Fig. 1 (c, d). The lattice fringe spacings of 0.253 nm, 0.311 nm, and 0.194 nm were well matched with the (1 0 2), (2 1 1), and (5 0 1) lattice planes of orthorhombic Bi2S3 (ICSD: 17e0320), respectively; the lattice spacing of ~0.301 nm corresponded to the (1 0 2) plane of tetragonal BiOI (ICSD: 10e0445) and the 0.165 nm lattice spacing can be attributed to the (0 2 4) crystal plane of the hexagonal metallic Bi (ICSD: 44e1246), which may originate from Bi3þ reduced by ethylene glycol [20e23]. These results demonstrate that the fabricated photocatalyst was comprised of a ternary nanostructure, that included pure Bi, BiOI, and Bi2S3. To further explore their spatial distribution, energy dispersive spectrometry (EDS) was used to obtain elemental mapping images of Bi, S, and I. Fig. 1(f-h) shows a homogeneous distribution of Bi, S, and I in the sheet-like BIS nanoparticles, confirming the presence of a hybrid structure.
All the used reagents were purchased from Sinopharm Chemical Reagent Co., Ltd (China) and used without further purification. Deionized water was used for all experiments in the study. 2.2. Fabrication of catalyst Typically, 0.5 mmol of Bi(NO)3 was added into 10 ml of ethylene glycol with stirring for 1 h at room temperature until completely dispersed and the solution was marked as solution A. Next, 0.3 mmol of KI was added into 10 ml of ethylene glycol with stirring for 1 h until completely dissolved, then 0.3 mmol of 3mercaptopropionic acid was added to the solution with stirring for 0.5 h, and the mixture was marked as solution B. Then, solution B was added dropwise added into solution An under stirring. The mixed solution was stirred at room temperature for 1 h and then transferred into a 50 ml stainless-steel Teflon lined autoclave and heated at 160 C for 15 h. Then, the reactor was cooled down to room temperature. The precipitate was then separated via centrifugation, washed with DI water and absolute alcohol 5 times, and then dried at 80 C for 6 h. 2.3. Characterization Scanning electron microscopy (SEM) images were taken using a JSM-6380 operating at 20 KV. The microstructure of the prepared photocatalyst was characterized via transmission electron microscopy (TEM) (JEM-1200EX). The X-Ray diffraction (XRD) pattern of the dried sample was measured using an XPERT-PRO diffractometer with Cu K-alpha radiation (1.5406 Å). The Brunauer-EmmettTeller (BET) specific surface areas were calculated via the N2 adsorptiondesorption isotherms at 77.35 K with a MicroActive for ASAP2460 2.01. XPS measurements were taken to determine the surface elemental composition using a PerkinElmer PHI 1257 electron spectrometer. The FT-IR spectra were recorded on a Nicolet Nexus spectrometer. UVeVis DRS was performed with a PerkinElmer Lambda 35 UV/Vis spectrophotometer. UVeVis adsorption spectra were obtained using a UV 1750 spectrometer. A fluorescence spectrometer (PL, F-2700, Hitachi, Japan) was used to obtain the photoluminescence (PL) spectra. 2.4. Photocatalytic activity RhB was used to evaluate the photocatalytic activities of the synthesized BIS by calculating the RhB photodegradation rate based on the formula:
3.1.2. X-ray diffraction (XRD) analysis The composition and crystallinity of the materials were determined via XRD spectra. Fig. 2 and Fig. S2 show the XRD patterns of BIS, pure BiOI, and Bi2S3 materials with 2q values ranging from 10 to 80 . The distinct peaks indicated that the sample was highly crystalline and contained multiple crystal structures. The peaks at 2q values of 28.50 , 38.03 , and 39.80 were well matched with the (0 1 2), (1 0 4) and (1 1 0) crystal planes of hexagonal metallic Bi (JCPDS File No. 44e1246), respectively. The peaks at 2q value of 24.93 and 28.50 could be assigned to the (1 3 0) and (2 1 1) crystallographic planes of orthorhombic Bi2S3 (JCPDS File No. 17e0320), respectively. Finally, the peaks at 2q value of 29.73 , 31.70 , 45.38 , and 55.19 were well indexed to the (1 0 2), (1 1 0), (2 0 0), and (2 1 2) crystallographic planes of tetragonal BiOI (JCPDS File No. 10e0445), respectively. These results indicated that the as-synthesized materials were composed of three structures, Bi, Bi2S3, and BiOI, which was consistent with the TEM results.
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 1. SEM (a, b), TEM (c, d) and STEM (e) images of BIS, and the EDS elemental mapping of f) Bi, g) S and h) I.
3.1.3. Brunauer-Emmett-Teller analysis The increased surface area of the catalyst could provide more locations for the redox reaction, which could enhance the photocatalytic efficiency [24]. The BET specific area of the BIS nanocrystals was measured via nitrogen adsorption-desorption measurements and was calculated to be 43.0189 m2/g. The large surface area of the as-prepared sample could provide an increased number of active sites to adsorb the reactant substances resulting in improved photocatalytic performance.
Fig. 2. Experimental XRD pattern of BIS and the theoretical XRD of Bi (black), Bi2S3 (blue), BiOI (brown). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3.1.4. XPS analysis Photocatalytic redox processes take place at the surface of the photocatalyst; therefore, the surface elemental information and ion oxidation were characterized via XPS, and the results are shown in Fig. 3. The full survey spectrum in Fig. 3 (a) shows that carbon, bismuth, iodine, oxygen, and sulfur coexisted in the as-prepared BIS specimens. The C1s spectrum derived from adventitious hydrocarbon in the spectrometer. The high-resolution Bi4f spectrum in Fig. 3 (b) showed peaks for Bi 4f7/2 and 4f5/2. The Gaussian fitted peaks at 159.34 eV and 164.76 eV corresponded to Bi3þ in Bi2S3 [10], while the peaks at 158.62 eV and 164.17 eV belonged to Bi3þ in BiOI [20]. The peaks at 159.08 eV and 162.2 eV in the inset figure demonstrate the formation of Bi0, which agreed with XRD results [21]. The two peaks at 618.71 eV and 630.14 eV present in the I3d spectrum (Fig. 3 (c)) could be assigned to 3d5/2 and 3d3/2 of I in BiOI [20], respectively, indicating that the valence state of element I
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 3. XPS spectra of BIS: survey (a), I 3 d (b), O1s (c), and Bi 4f (d).
was 1 in the composite. The two peaks at 158.94 eV and 164.30 eV in the S2p spectrum (Fig. S3) could be assigned to Bi2S3. The O1s spectrum could be fitted into three peaks, as shown in Fig. 3 (d), and the peaks at 529.9 eV, 531.4 eV, and 532.8 eV corresponded to the BieO, eOH, and C]O bonds, respectively [25].
3.1.5. UVevis diffuse reflectance spectrum (DRS) analysis The UVeVis DRS spectra show the optical properties of the obtained BIS nanocrystals, and the spectra of the BiOI and Bi2S3 nanocrystals synthesized via a similar method were obtained for contrast. As shown in Fig. 4(a), the Bi2S3 nanocrystals could absorb light at wavelengths >360 nm, and the absorption of BiOI was in the wavelength range of 200e650 nm, while BIS had absorption in the region of 200e800 nm. The bandgap was calculated using the Kubelka-Munk equation:
a ¼ A(hv-Eg)2/n,
(2)
where a, h, and v are the absorption coefficient, Planck’s constant, and the light source frequency, respectively, A is the proportionality constant, and Eg represents bandgap energy. The constant n is dependent on the optical transition type of the photocatalyst; for a direct optical transition, the value of n is 1, while for an indirect transition, n is 4. The reported n values for BiOI and Bi2S3 were 4 and 1, respectively [10,26,27]. The plot of (ahv)1/2 vs. (hv) and the plot of (ahv)2 vs. (hv) are shown in Fig. 4 (b). The bandgap of BIS, BiOI, and Bi2S3 were calculated to be 0.51, 1.53, and 1.67 eV, respectively, suggesting that these three nanocrystals could all be excited by visible light. Moreover, the valence band (VB) edge and conduction band (CB) edge were calculated using the following equations [28,29]: EVB ¼ X - EC þ 0.5 Eg,
(3)
ECB ¼ EVB e Eg,
(4)
where EVB and ECB are the VB and CB edge potential, respectively, X is the electronegativity of the semiconductor, EC is the energy of free electrons on the hydrogen scale (~4.5 eV), and Eg is the bandgap energy. The X values for Bi2S3 and BiOI are 5.276 and 5.943 eV, respectively [10,26]. Thus, the VB and CB potential for BiOI were calculated to be 2.208 eV and 0.678 eV, and for Bi2S3, they were 1.541 eV and 0.129 eV, respectively. 3.1.6. Fourier transform infrared spectroscopy (FT-IR) analysis The composition and chemical interaction of the as-prepared nanocrystals were further confirmed via Fourier Transform Infrared spectroscopy (FT-IR), as shown in Fig. 4 (c). The broad bands with peaks at 3405 cm1 and 1618 cm1 belong to OeH vibrations, which can be ascribed to surface-adsorbed water molecules and the COOH group of mercaptopropionic acid. The small peak at 532 cm1 can be attributed to the BieO stretching vibrations, which is due to the formation of Bi2O2 2 . The absorption at 765 cm1 can be ascribed to CeS bonds, while the peak at 1079 cm1 is assigned to the C]S bond, suggesting that some other groups were reduced. By combining the previous results, a possible explanation for this is that the Bi3þ ion was reduced to metallic Bi. The FT-IR spectra of pure BiOI and Bi2S3 are shown in Fig. S4, compared with the BIS sample, the BieO stretching vibrations peak of BiOI emerged at ~500 cm1, suggesting that in the BIS composite, some other structures were present besides the much smaller BiOI crystal, such as Bi2S3. 3.1.7. Electrochemical properties The photodecomposition of pollutants is based on oxidationreduction reactions via photogenerated charge carriers. The degradation efficiency depends on the probability of photogenerated electron (e) and hole (hþ) recombination. The
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 4. (a) DRS results and (b) energy gap of BiOI, Bi2S3, and BIS, (c) FT-IR spectra and (d) photocurrent response of BIS, BiOI, and Bi2S3.
photocurrent response was used to characterize the electrochemical properties, and it can be used to describe the separation, migration, and trapping of photoinduced charge carriers. For every turn-on and turn-off event over multiple cycles (Fig. 4 (d)), the photocurrent rapidly increased when the visible light was switched on and dropped quickly when the light was switched off [30]. The rapid increase in the photocurrent indicated that a large amount of photo-excited carriers formed quickly because of the narrow energy gap of the photocatalyst during the turn-on period, and the recombination of e-hþ was markedly reduced because the photocatalyst efficiently separated and trapped the e-hþ [31]. Because carrier recombination could result in luminance, the photoluminescence (PL) spectrum was obtained to further study the trapping, migration, and separation efficiency of photogenerated electrons and holes. Increased recombination of the ehþ would result in a stronger luminance intensity. The PL spectra (Fig. S5) show that the as-prepared BIS group exhibits a weaker PL intensity than pure BiOI and Bi2S3, which can be attributed to the formation of a Schottky barrier at the interface of the semiconductor (BiOI and Bi2S3) and the nanometallic Bi. The PL results indicate that the synergistic effects of the BIS nanocrystals result in a more efficient separation of photogenerated charge carriers than the BiOI and Bi2S3 heterojunction structures. 3.1.8. Photocatalytic performance for RhB degradation Rhodamine B, with a maximum absorption at 554 nm, was used as a model organic pollutant to estimate the photocatalytic activity of BIS under visible light irradiation [32]. The UVeVis absorption spectra of the nanocrystals demonstrated that the peaks at 554 nm decreased with time and indicated the degradation of the RhB chemical structure, as shown in Fig. 5 (a). A continuous blue shift of the absorption maximum was recorded during the degradation
process, suggesting that the molecular structure of RhB was gradually destroyed. The degradation rate of RhB under visible light irradiation with samples that had varying ratios of Bi2S3 to BiOI was studied. As shown in Fig. 5(b), the RhB photocatalytic degradation activity varied with composition, and a 1:1 ratio of Bi2S3 to BiOI was determined to be optimal for the photodegradation of RhB. The BiOI sample could also degrade RhB under visible light, while the Bi2S3 sample showed minimal degradation of RhB under visible light after more than 60 min. The standard kinetic data curves obtained by fitting the plot of ln (C/C0) vs. illumination time (t) (Fig. S6) was used to quantitatively analyze the photocatalytic efficiency. According to the first-order kinetics, the discoloration reaction can be described by: ln(C0/C) ¼ kt,
(5)
where C0 represents initial concentration of RhB, C represents the concentration at time t, and the slope k denotes the reduced rate constant [33]. The photo degradation constant of the as-prepared catalyst was calculated to be 0.108, indicating good catalytic performance. Cycling experiments were performed to evaluate the recyclability of the nanocrystals under visible light irradiation. As displayed in Fig. 5 (c), the stability of the catalyst after four cycles indicated that the BIS photocatalytic nanocrystals were not damaged by photocorrosion. This confirmed the recycling stability of BIS. The degradation process was further investigated via liquid chromatography-mass spectrometry (LC-MS), and the HPLC spectra are shown in Fig. 5 (d). The changes in the HPLC spectra indicate that different degradation products form with different illumination durations. There was an obvious RhB peak in the pristine
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 5. (a) Temporal UVevis adsorption spectra changes of RhB under visible light illumination, (b) degradation rate of RhB with the samples that have various amount ratio of Bi2S3 to BiOI (the numbers in the figure were the amount ratio of Bi2S3 to BiOI), (c) cycling experiments, and (d) the HPLC spectra changes of RhB under different illumination timing.
solution, and the intensity decreased with increasing irradiation time. Additionally, more peaks associated with the degradation product of RhB appeared and they subsequently all disappeared in the following order: the peaks of DER (diethyl-N-ethyl rhodamine), EER (N-ethyl-N-ethyl rhodamine), DR (N, N-diethyl rhodamine), ER (N-ethyl rhodamine), and R (Rhodamine) are the photodecomposition products of RhB, which can be ascribed to the degradation of the chromophore structure because of deethylation. The detailed intermediates for the degradation products of RhB are shown in Fig. 6. After 30 min, the rhodamine had almost entirely lost its ethyl group(s) and formed DER. Because the second and third bonds are extremely unstable, they also break rapidly and EER and DR formed after 40 min of reaction. After 60 min of catalytic activity, only peaks associated with ER could be observed. Finally, all the species had decomposed into small non-toxic molecules after 90 min. 3.1.9. Photocatalytic mechanism The photocatalytic degradation of RhB using a photocatalyst depends on a series of active radicals generated on the basis of the photogenerated electrons and holes, including active holes, superoxide radicals, and hydroxyl radicals. To explore the photocatalytic mechanism of BIS, active radical trapping experiments were carried out to identify the dominant active species. In this experiment, ethylenediamine tetra acetic acid disodium (EDTA-2Na, the scavenger of hþ), p-benzoquinone (BQ, $O 2 scavenger), and 2-Propanol (IPA, the scavenger of $OH) was used as a scavenger species to trap the corresponding radical species [34,35]. When EDTA-2Na, BQ, and IPA were introduced into the reaction system, the RhB removal efficiency declined by 0.90, 0.32, and 0.06,
respectively, as shown in Fig. 7 (a). When the EDTA-2Na was introduced, the degradation of RhB was almost completely inhibited; suggesting that hþ was the main species in the RhB decomposition process. The results for BQ addition indicate that O 2 also played an important role in the degradation process. The presence of 2-Propanol had minimal impact on the degradation of RhB, indicating that $OH only played a secondary role in the degradation þ process. The results indicate that $O 2 and h were the dominant active radials for the photocatalytic decomposition of RhB by BIS. Experiments investigating the effects of metal ions were also carried out and the results are shown in Fig. 7(b). The results demonstrate that Hg2þ had the strongest inhibition on the catalytic process, followed by Agþ, Zn2þ, and Cu2þ. Among these ions, Agþ could easily trap photogenerated electrons, imposing restrictions on the formation of $O 2 and thus inhibiting the decomposition. Zn2þ and Cu2þ are metal ions than could easily be reduced. The addition of Hg2þ not only inhibited the photocatalysis but also caused a red-shift of the absorption peak, which indicated that mercury ions could form complexes with RhB and inhibit its further degradation. To investigate the active radical generation mechanism, the probable structural composition of the nanocrystals BIS was further analyzed. From the HRTEM image of BIS (Fig. 1(d)), it can be seen that Bi2S3 and BiOI are joined together, forming a heterojunction structure. The electrons on the CB of Bi2S3 could move to the CB of BiOI because it was more negative than that of BiOI. Additionally, because the CB potential of BiOI (0.678) was more positive than E0 (O2/$O 2 ) (0.046 eV), the electrons on the CB of BiOI were not able to reduce O2 to $O 2 . However, the results of the trapping experiment showed that $O 2 played an important role on RhB
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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degradation. From Fig. 1(d), the metallic Bi nanostructure can be observed to link the Bi2S3 and BiOI structures, and the three together could form a Z-Scheme structure. In the Z-scheme structure, metallic Bi would serve as a bridge, and the electrons from the CB of BiOI would combine with holes from the VB of Bi2S3. The electrons on the CB of Bi2S3 with a potential of 0.129 were negative enough to react with O2 to form $O 2 . The potential of the holes remaining on the VB of BiOI was 2.208, which was more positive than H2O/$OH (2.38 eV). A small amount of $OH radicals would be produced, which is consistent with the previously detailed trapping experiments. Therefore, the BIS catalyst had a Z-scheme structure for the degradation for RhB. Additionally, the holes on the valence band of BiOI could react with the pollutants. In brief, two kinds of composite structures were present, the heterojunction structure of Bi2S3 and BiOI and the ZScheme structure of BIS; the holes in both these structures could participate in the reaction, while the $O 2 active species was only present in the Z-Scheme. The quantity of holes was more than $O 2, resulting in a larger contribution to the photodegradation of RhB, which is consistent with previous results. Based on the above discussion, a possible photodegradation mechanism for RhB by the prepared catalysts under visible light irradiation was proposed. Fig. 8 shows the electron pathway. The electrons at the top of the VB of Bi2S3 and BiOI were excited to their respective CBs, leaving holes on the VB. Then, the electrons (CB) of Bi2S3 and the holes (VB) of BiOI transfer to Bi and recombine there in a Z-scheme (Fig. 8(a)). In the heterostructure (Fig. 8(b)), the excited electrons of Bi2S3 transferred to BiOI. As a result, the generated electrons and holes were effectively separated, inhibiting their recombination. The stabilized electrons on the CB of Bi2S3 in the Z-scheme would combine with O2 to form $O 2. þ Finally, under attack by the active radicals ($O 2 and h ), the Nethyl group of RhB split into unstable ethyl groups (DER, EER, DR, ER, and R). Finally, Rhodamine B was completely decomposed to CO2 and H2O.
4. Conclusion
Fig. 6. Detailed formations of different molecular groups during RhB degradation.
In summary, an efficient photocatalyst, Bi2S3/Bi/BiOI, composed of both Z-scheme and heterojunction structures was successfully prepared via a one-step solvothermal method using ethylene glycol as the solvent. This composite demonstrated excellent photocatalytic performance for RhB degradation under visible light irradiation. Radical trapping experiments indicated that hþ and $O 2 were the main species involved in the degradation of organic pollutants. As a highly efficient sunlight driven photocatalyst, BIS can potentially be used for the treatment of water pollution for other photocatalytic applications.
Fig. 7. Active radical trapping experiments (a), and effects of different metal ions on photodegradation (b).
Please cite this article as: Y. Cui et al., One step approach for hybrid photocatalyst synthesis: Synergetic photocatalytic water pollutant degradation, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152752
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Fig. 8. Schematic illustration of charge transfer pathway during RhB degradation process over BIS under visible light irradiation.
Declaration of competing interest The authors declare no competing financial interest. Acknowledgments This work was financially supported by the National Key Research and Development Program in China (2017YFC0112000), National Major Science and Technology Project of China (2018ZX10723-204-008) and National Natural Science Foundation Program of China (21505137, 21675157, 61527827, 11535015), the Beijing Natural Science Foundation (7164271). We thank Beijing Zhongkebaice Technology Service Co., Ltd. for the characterization data. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152752.
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