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Big-leaf hydrangea-like Bi2S3-BiOBr sensitized TiO2 nanotube arrays with enhanced photoelectrocatalytic performance
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 229 (2020) 117936
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Big-leaf hydrangea-like Bi2S3-BiOBr sensitized TiO2 nanotube arrays with enhanced photoelectrocatalytic performance Dawei Gao a,b, Lii Wang a, Qingyao Wang c,⁎, Zhenming Qi a, Yao Jia c, Chunxia Wang a a b c
College of Textile and Clothing, Yancheng Institute of Technology, Yancheng 224051, China Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA School of Chemistry and Materials Science, Ludong University, Yantai 264025, China
a r t i c l e
i n f o
Article history: Received 15 October 2019 Received in revised form 29 November 2019 Accepted 7 December 2019 Available online 12 December 2019 Keywords: Bi2S3 and BiOBr nanoparticles TiO2 nanotube arrays Solvothermal method Photocatalytic performance
a b s t r a c t TiO2 nanoparticles as solar cells and photocatalysts caused extensive attention in solar energy utilization and environment remediation due to the high photoelectrochemical performance. We demonstrated a novel approach to fabricate big-leaf hydrangea-like Bi2S3-BiOBr self-assembled by superthin nanosheets on TiO2 nanotube arrays (TiO2 NTs/B2S3-BiOBr). Results indicated that the Bi2S3-BiOBr co-sensitization showed higher photoelectric conversion efficiency than the single Bi2S3 or BiOBr sensitization. More remarkably, TiO2 NTs/B2S3-BiOBr showed excellent photoelectrocatalytic (PEC) removal of MB, MO, RhB and Cr(VI). The remarkable PEC performance could be attributed to the strong visible light absorption and effective electron transportation at the interface of TiO2/B2S3-BiOBr. The high photoelectrochemical performances indicate that the TiO2 NTs/B2S3-BiOBr could work as potential photoelectric materials for large-scale applications in the photoelectrochemical energy conversion and pollutant removal. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Novel nanoparticles and nanodevices with excellent photochemical or electrochemical properties caused enormous research interests [1–6], and it opened important possibilities for the design of novel optical and optoelectronics. Recently, the anodic TiO2 NTs have caused intense attention in the application of solar cell photoanode and photocatalyst owning to the uniform one dimensional nanotube microstructure, simple preparation condition, high chemical stability and surface area [7–9]. Many investigations about the photoelectrochemical activities indicated that the low visible light response was mainly dominated by the large band gap, which greatly influenced the applications of TiO2 NTs. Therefore, the sensitization coupled with another semiconductor with narrow band gap was an efficient way to extend the optical harvesting into visible light region. All kinds of semiconductors including metal sulfide [10], metal oxide [11], carbon group compound [12] and phosphate [13,14] were prepared to enhance the visible light photoelectrochemical performance. Currently, the co-sensitization by two or more materials was rapidly developed. Previous results [15,16] indicated that the multiple semiconductors deposited on the TiO2 NTs surface could form close connection that could further enhance the solar harvesting region and intensity, and the solar utilization was obviously improved compared with a single sensitization. Nevertheless, the defects formed at the interface of two semiconductor sensitizers would ⁎ Corresponding author. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.saa.2019.117936 1386-1425/© 2019 Elsevier B.V. All rights reserved.
trap electrons, which inversely reduce the final photoelectrochemical performance. Therefore, the matching of crystal structure and energy band location between two sensitizers and TiO2 NTs is the key point to influence the electron transportation and recombination. Two sulphides such as CdS, CdSe or CdTe with similar crystal structures and elemental compositions showed excellent electron transportation efficiency [17–20]. In addition, the two sensitizers with same cation or anion composition also provide unhindered electron transportation channel due to the sharing ions at the interface [21]. For examples, Prof. Eswar [22] prepared AgBr/Ag3PO4 photocatalysts with high photocatalytic activity, and the silver based semiconductors reduced the electron recombination at the interface. Jiang et al. [23] reported the preparation and photocatalytic performance of BiOBr/AgBr hybrids with the flower-like structure, and the photocatalyst showed an excellent photocatalytic degradation performance. As bismuth-based semiconductors, Bi2S3 and BiOBr are attractive photocatalysts with high visible light absorption and photocatalytic degradation of organic pollutants. Jiao [24] and Cui [25] prepared Bi2S3/BiOBr heterojunction photocatalyst with the enhanced photoelectrochemical and photocatalytic ability. The high photoelectrochemical performance is attributed to the convenient electron path transfer at the heterostructured Bi2S3/ BiOBr interface, which significantly reduced the possible recombination chance of charge carriers. In this paper, Bi2S3/BiOBr nanoparticles were deposited on the TiO2 NTs surface through a simple solvothermal strategy. The photoelectric conversion and PEC property of the prepared TiO2 NTs/Bi2S3-BiOBr photoelectrode were tested to obtain the optimal preparation condition.
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SEM (FEI SU 8010), TEM (FEI Tecnai F30), DRS (UV-2550) and XPS (KRATOS AXIS 165). The characterization procedures for photovoltage, photocurrent, interface resistance and PEC ability were described in our previous report [27]. The visible light and solar were provided by a 500 W Xe lamp (CEL-S500) with the visible light filter and AM 1.5 filter, respectively. The Pt counter electrode and commercial Ag/AgCl reference electrode (Schott, Germany) were used in the photoelectrochemical characterization, and the photoelectrochemical test was carried out in 0.1 M Na2SO4 electrolyte. 2.3. Measurement of the photoelectrocatalytic activities
Fig. 1. XRD patterns of (a) TiO2 NTs/Bi2S3, (b) TiO2 NTs/Bi2S3-BiOBr and (c) TiO2 NTs/BiOBr.
2. Experimental section 2.1. The synthesis of TiO2 NTs/Bi2S3-BiOBr TiO2 NTs grown on Ti foil surface were carried out by a two-step anodization progress with the anodic voltage of 60 V [26]. Bi2S3/BiOBr nanoparticles were deposited on TiO2 NTs by the one-pot solvothermal preparation. Briefly, 1 mmol of Bi(NO3)3·5H2O was slowly dissolved into 10 mL of methylglycol with intense magnetic stirring for 15 min, and 1 mmol of thiourea and 0.5 mmol of KBr were dispersed into 20 mL of methylglycol with sustained magnetic stirring. The two solutions were swiftly mixed together under magnetic stirring for 5 min and transferred into Teflon aligner of the autoclave. The autoclave was heated at 160 °C in a vacuum oven for 14 h. Then after the solvothermal reaction, the autoclave was cooled down to room temperature naturally, and samples were washed in ethanol for 1 min. For comparation, the single Bi2S3 or BiOBr sensitization was achieved by the reaction of 1 mmol of Bi(NO3)3·5H2O with 2 mmol of thiourea or 1 mmol of KBr, respectively. 2.2. Characterization The prepared products were further characterized for the elemental composition and morphology analysis through XRD (Siemens D-5000),
The photocatalytic performances of samples were investigated by the record of RhB, MO and MB dye concentration at their maximum absorption wavelength of 664, 464 and 552 nm, respectively. The samples set as photoelectrodes were immersed into dye solution with the concentration of 5 × 10−5 M, the external voltage was set at 1 V, and Na2SO4 was added into the solution as the electrolyte. After 20 or 30 min solar irradiation, the absorbance of dyes was recorded and calculated to obtain the degradation efficiency. Similarly, the Cr (VI) concentration was tested by the diphenylcarbazide spectrum method, and the external voltage was set to be 0.5 V. After 30 min solar irradiation, 1 mL of Cr (VI) was taken out and added into 50 mL of deionized water. Then 2 mL of diphenylcarbazide, 0.5 mL of H2SO4 and 0.5 mL of H3PO4 were added into the above solution, and the purple solution maintained the stirring state for 15 min. The absorbance was recorded at 540 nm, and the PEC efficiency was calculated. The OH• radical formation was confirmed by the electron spin resonance (ESR, A300) spin-trapping technique. The mineralization of dyes was evaluated by the total organic carbon (TOC-L CPH/CPN) of the residual dye solution after solar irradiation. 3. Results and discussion The XRD patterns of the as-prepared samples were displayed in Fig. 1. The typical diffraction patterns of anatase TiO2 (JCPDS no. 211272) could be obtained in all samples, and the diffraction peaks located at 25.28°, 36.95°, 37.80°, 38.57°, 48.05°, 53.89° and 55.06° correspond to the (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5) and (2 1 1) facets of anatase, respectively. After the solvothermal deposition, two new diffraction peaks at 28.61° and 39.89° can be clearly noticed, which can be indexed to (2 1 1) and (1 4 1) lattice planes of Bi2S3 (JCPDS no. 170320) in TiO2 NTs/Bi2S3 [28]. Furthermore, three peaks at 2θ values of 32.22°, 46.21° and 57.12° correspond to (1 1 0), (2 0 0) and (2 1 2) facets
Fig. 2. SEM images of (a) TiO2 NTs, (b) TiO2 NTs/Bi2S3, (c) TiO2 NTs/Bi2S3-BiOBr and (d) TiO2 NTs/BiOBr.
D. Gao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 229 (2020) 117936
Fig. 3. The high magnified SEM images and EDS of TiO2 NTs/Bi2S3-BiOBr.
Fig. 4. The TEM (a, c) and HRTEM (b, d) images of TiO2 NTs/Bi2S3-BiOBr.
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of BiOBr (JCPDS no. 09-0393) [29]. As shown in Fig. 1b, the diffraction peaks of Bi2S3 and BiOBr synchronously appear in TiO2 NTs/Bi2S3BiOBr, indicating the co-sensitization of Bi2S3 and BiOBr by the onepot solvothermal deposition. The morphology of TiO2 NTs was investigated by typical SEM images. The smooth tubular surface of TiO2 NTs is clearly observed in Fig. 2a, and the enlarged images indicate the detailed microstructures. The nanotube surface with the average diameter of 200 nm is composed by polygon including hexagon, pentagon even square, and the diverse morphologies could be attributed to the different internal stress in Ti foils. Compared with pure TiO2 NTs, the solvothermal deposition with thiourea or KBr solution causes completely different morphologies. The Bi2S3 nanoparticles cover the nanotube walls, making the coarseness of TiO2 NTs as shown in Fig. 2b. However, the BiOBr accumulation with large sizes (Fig. 2d)
deposits on TiO2 NTs surface, and the accumulations are self-assembled by BiOBr blocks with various sizes. When thiourea and KBr were dispersed into Bi(NO3)3 solution, the growth of Bi2S3-BiOBr hybrids is limited along specific orientation to form two dimensional nanosheets. The detailed morphology of TiO2 NTs/Bi2S3-BiOBr is exhibited in Figs. 2c and 3. The big-leaf hydrangea microstructure of Bi2S3-BiOBr is composed by the interspersed crosslinking of nanosheets, and the hierarchical structures could dramatically enhance the solar harvesting and pollutant adsorption, which would provide the guarantee for the excellent photoelectric performance. The chemical composition and element amount of the TiO2 NTs/Bi2S3-BiOBr were measured by EDS, the peaks of Ti, O, Bi, S and Br are clearly seen, which confirms the successful codeposition of Bi2S3 and BiOBr. In addition, the EDS result analysis reveals that the composition ratio of Bi2S3 and BiOBr is about 1:4.
Fig. 5. XPS spectra of TiO2 NTs/Bi2S3-BiOBr: (a) a survey spectrum; (b) Bi 4f; (c) S 2p; (d) O 1s and (e) Br 3d.
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The TEM and HRTEM images were provided in Fig. 4 to further investigate the microstructure of TiO2 NTs/Bi2S3-BiOBr. The low magnification TEM image in Fig. 4a reveals the nanotubular microstructure of TiO2 NTs, and the clear interplanar spacing of 0.243 nm of crystal lattice is consistent with the (1 0 3) plane of anatase. The TEM and HRTEM images of Bi2S3-BiOBr co-sensitizers are shown in Fig. 4c and d, and the big-leaf hydrangea-like sample is composed by nanosheets. The interplanar spacing of 0.226 nm of crystal lattices are fairly close to that of the (1 4 1) plane of Bi2S3, while the interplanar spacing of 0.278 and 0.161 nm is ascribed to the (1 1 0) and (2 1 2) planes of BiOBr. The TEM images further confirm the successful deposition of Bi2S3-BiOBr on TiO2 NTs. The element composition and chemical status of TiO2 NTs/Bi2S3BiOBr were studied by XPS in Fig. 5. The survey XPS spectrum indicates the Bi, S, O and C elements exist in the sample, and the C 1s peak at 284.60 eV could be attributed to the adventitious hydrocarbon, which is used to regulate the peak position of other elements. Fig. 5a shows the high-resolution XPS spectra of various elements in the TiO2 NTs/ Bi2S3-BiOBr. In the XRS spectrum of Fig. 5b, a strong peak at 160.73 eV could be assigned to the binding energy of Bi 4f7/2 (158.79 eV) and Bi 4f5/2 (164.37 eV), respectively, which is characteristic of Bi3+ in BiOBr and Bi2S3. S 2p XPS spectrum is shown in Fig. 5c, where the strong peak at 167.25 eV deconvoluted into two peaks centered at 168.56 and 169.63 eV are associated with core lines of S 2p1/2 and 2p3/2 in Bi2S3. XPS of O 2p in the sample can be resolved into three typical peaks, the peak at 530.15 eV can be assigned to the Bi\\O bonds in the (BiO)2+ slabs of BiOBr [30], the fitted peak dominating around 2 529.80 eV can be ascribed to the Ti\\O bonds in TiO2 NTs [31], while another peak at 532.33 eV could be ascribed to the hydroxyl group (–OH) in surface adsorbed components. As for the high-resolution XPS spectrum of Br 3d in Fig. 5d, the peaks at 68.30 and 69.62 eV are consistent with the Br 3d5/2 and Br 3d3/2, respectively. The XPS results further indicate the successful preparation of Bi2S3-BiOBr co-sensitizers. The visible light harvesting ability of the as-prepared photoelectrodes was investigated by DRS. As displayed in Fig. 6a, TiO2 NTs only absorb the UV light with the short wavelength below 400 nm, after Bi2S3 and BiOBr sensitization, all samples show broad visible light harvesting region and high harvesting intensity. Due to the inherent energy bands of Bi2S3 (1.3 eV) and BiOBr (2.7 eV), the TiO2 NTs/ Bi2S3 shows much stronger visible light absorption properties than that of TiO2 NTs/BiOBr. The Bi2S3-BiOBr synergistic sensitization significantly enhance the response into the whole visible light region, and the band gap could be calculated to be 2.79 eV which is much narrower than those of TiO2 NTs (3.21 eV). The high visible light absorption activity would generate multiple photogenerated electrons, which enhances the high photoelectrochemical performances of TiO2 NTs/Bi2S3-BiOBr.
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Fig. 7. Photoluminescent (PL) spectra of the as-prepared photoelectrodes.
In addition, the PL spectra of the prepared samples were tested to investigate the recombination of electron-holes. As shown in Fig. 7, the wide PL peaks range from 340 nm to 500 nm appear with the excitation wavelength at 325 nm, and the PL peak intensity is much weaker than that of TiO2 NTs. Among these samples, the TiO2 NTs/Bi2S3-BiOBr sample has the lowest PL intensity, indicating the effective separation of electron-hole pairs. The minimized recombination chance of charge carriers is mainly ascribed to the suitable energy band edges of Bi2S3 and BiOBr with TiO2 NTs, and detailed electron transfer path is discussed in Scheme 1. The photoelectrochemical performances were studied to evaluate the visible light absorption and electron transfer activity by the calculation and comparation of visible light transient photocurrent, I-V curves, OCP curves, EIS and Mott-Schottky plots of the as-prepared photoelectrodes. Comparing with the low photocurrent of TiO2 NTs [32], the photoelectrodes show high visible light photocurrent generation when the Xe lamp is switched on in Fig. 8a, and the photocurrent densities rapidly decrease into 0 mA/cm2 in dark. The regular photocurrent fluctuation indicates the semiconductor sensitization effect in the visible light absorption, photoelectric conversion and electron transfer. TiO2 NTs/Bi2S3-BiOBr photoelectrode shows the higher photocurrent (4.23 mA/cm2), and the value is 1.43 and 1.32 times of TiO2 NTs/Bi2S3 and TiO2 NTs/BiOBr. The photocurrent results have the same trend with I-V curves under different voltages in Fig. 8b, and the results indicate the high photoelectric conversion of TiO2 NTs/Bi2S3-BiOBr. The high photocurrent implies the high visible light response and effective electron transfer of TiO2 NTs/Bi2S3-BiOBr.
Fig. 6. Diffuse reflectance spectra (a) and Kubelka-Munk plots (b) of the as-prepared photoelectrodes. Sample: (a) TiO2 NTs, (b) TiO2 NTs/Bi2S3, (c) TiO2 NTs/Bi2S3-BiOBr and (d) TiO2 NTs/ BiOBr.
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Scheme 1. Schematic diagram of PEC degradation of dyes by TiO2 NTs/Bi2S3-BiOBr under solar irradiation.
Moreover, the surface voltage was tested to investigate the electron accumulation and transfer efficiency in Fig. 8c. When the Xe lamp is switched on, the photoelectrodes produce high photovoltages, and retain the stable values in the irradiation progress. The photovoltage is recognized as the balanced value between the electron accumulation and recombination. The TiO2 NTs/Bi2S3-BiOBr photoelectrode shows the biggest photovoltage, and the specific value (−0.28 V) is 2.62 and 2.21 times of TiO2 NTs/Bi2S3 and TiO2 NTs/BiOBr. The electron transfer performance of photoelectrodes was also studied by EIS in Fig. 9a. The semicircle diameter in the Nyquist plot of TiO2 NTs/ Bi2S3-BiOBr is much smaller than that of TiO2 NTs/Bi2S3 and TiO2 NTs/ BiOBr, indicating the low interface impedance of TiO2 NTs/Bi2S3-BiOBr in Na2SO4 electrolyte, and the photoelectrode has the optimal electron separation and transfer efficiency. Moreover, the applied voltages induce the energy band edge bending of photoelectrodes, which also significantly influences the transfer of photogenerated electrons. The MottSchottky plots of the photoelectrodes were displayed in Fig. 9b. All the photoelectrodes show positive slopes, indicating that the sensitized samples are still n-type semiconductors. Moreover, the flat bands could be calculated from the Mott-Schottky plots according to the horizontal-axis (1/C2 = 0), and the flat bands of TiO2 NTs/Bi2S3, TiO2 NTs/Bi2S3-BiOBr and TiO2 NTs/BiOBr are −0.69, −0.75 and −0.78 V, respectively. The carrier concentration was also calculated, and the specific values are 1.09 × 1020, 1.59 × 1020 and 1.03 × 1020 cm−3, respectively. Apparently, the high carrier concentration of TiO2 NTs/Bi2S3-BiOBr shows the excellent charge separation and transfer efficiency, which implies the high PEC performances in the removal of organic dyes and Cr(VI). The PEC properties of the photocatalysts were evaluated by the decomposition of organic pollutants simulated by RhB, MO and MB
under solar irradiation, and the PEC efficiency of pure TiO2 NTs was only 45.2% in our previous report [13]. The PEC degradation of RhB dyes is displayed in Fig. 10a, the TiO2 NTs/Bi2S3-BiOBr and TiO2 NTs/ BiOBr have the similar PEC performance, and more than 90% of RhB was removed after 180 min solar irradiation, which is much higher than that of TiO2 NTs/Bi2S3. The efficiency function (Ln(C0/C)) vs solar irradiation time (min) shows the linear relationship, and the PEC degradation rate k values are 0.0063, 0.01502 and 0.01692 min−1. Analogously, the TiO2 NTs/Bi2S3-BiOBr and TiO2 NTs/Bi2S3 show high PEC degradation of MO, and their PEC degradation rate constant k values are close to 5 folds of TiO2 NTs/BiOBr. Moreover, the TiO2 NTs/Bi2S3BiOBr photocatalyst shows the optimal PEC degradation efficiency of MB, and 90.53% of MB could be decomposed after 120 min solar irradiation. The high PEC rate constant (0.2084 min−1) is 2.16 and 1.31 times of TiO2 NTs/Bi2S3 and TiO2 NTs/BiOBr, respectively. As shown in Fig. 10c, the PEC degradation rate of MB is much higher than other dyes, and the reason could be ascribed to the compatibility with photocatalysts and structure stability. The high stability of azo structure in MO and triphenylmethane structure in RhB causes the low PEC efficiency [33], but the anilino and =S+— chromophores in MB dyes could be decomposed easily. The detailed photocatalytic degradation progress and intermediate product of these dye molecules are complex, and the corresponding results were investigated by Prof. Hisaindee [34]. The similar reports about photocatalytic degradation of organic pollutant by Bi2S3 and BiOBr sensitized TiO2 NTs as photocatalytsts were listed in Table 1. Though Prof. Tang's research [35] indicated that the photocatalytic efficiency reported by different investigation groups couldn't be simply contrasted due to the different light resource, light intensity, photocatalyst addition amount, dye concentration and so on,
Fig. 8. The transient photocurrent (a), I-V curves (b) and photovoltages (c) of the as-prepared photoelectrodes. Sample: (a) TiO2 NTs/Bi2S3, (b) TiO2 NTs/Bi2S3-BiOBr and (c) TiO2 NTs/ BiOBr.
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Fig. 9. The EIS (a) and Mott-Schottky plots (b) of the as-prepared photoelectrodes.
the results only directly reveal the excellent photocatalytic performance of the as-prepared TiO2 NTs/Bi2S3-BiOBr photocatalysts. The synergic sensitization of Bi2S3-BiOBr dramatically enhances the solar response, electron transfer and surface degradation, and the detailed mechanism would be discussed in Scheme 1. The photocatalytic degradation efficiency doesn't really reflect the decomposition degree of dye molecules, and the destruction of chromogenic groups also could significantly reduce the absorbance of dye
solution. Therefore, the TOC was used to evaluate the true mineralization degree of dyes, and the specific values are shown in Fig. 11a. The trend is similar with the PEC degradation of all dyes, and the TOC removal efficiencies of RhB and MB achieve 78.52% and 81.26%. However, the TOC of MO is still high after 3 h solar irradiation, and the prolonged irradiation further improve the mineralization. The mineralization efficiency after 5 h is enhanced up to 71.56%. The active species for the dye decomposition were explored by ESR spin-trap technique (with
Fig. 10. The PEC degradation efficiencies, pseudo-first-order kinetics and PEC rate constants for RhB (a), MO (b) and MB (c) by the photocatalysts.
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Table 1 Bi2S3 and BiOBr sensitized TiO2 NTs as photocatalytsts in the photocatalytic decoloration. Sensitizers
Preparation methods
Bi2S3-BiOBr Solvothermal method
Bi2S3-BiOI Bi2S3 Bi2S3
Solvothermal method SILAR deposition Photodeposition
Bi2S3 Bi2S3 BiOBr BiOBr
Electrodeposition Electrodeposition Chemical bath deposition SILAR deposition
BiOBr
SILAR deposition
Light sources
Pollutants Efficiency Refs.
Solar light RhB MO MB Solar light RhB Solar light MO Visible MO light Solar light 2,4-D Solar light MB Visible RhB light Visible RhB light Solar light RhB
93.68% 56.75% 90.53% 93.6% 44.05% 83.3%
This work [9] [36] [37]
81% 82.3% ~60%
[38] [39] [40]
97.7%
[41]
93%
[42]
DMPO) in Fig. 11b by the comparation with/without solar irradiation. We could obviously notice four typical characteristic peaks of DMPOOH• with intensity 1:2:2:1 under solar irradiation for 10 min, which is the signal of OH• radical generation. No ESR signals of O2• were observed during the photocatalytic progress, and the results reveal that OH• radicals play the main roles in the PEC degradation of dyes. To further confirm the photocatalytic roles of OH• radicals, the PEC degradation with quenching agent addition of benzoquinone (BQ), EDTA-2Na and tertiary butyl alcohol (t-BuOH) was carried out. As
exhibited in Fig. 12a, the final PEC efficiency after the t-BuOH addition nearly remain unchanged, and this result indicates that the OH• captured by t-BuOH plays the vital roles in the RhB degradation. The addition of hole quenching agent (EDTA-2Na) also causes the reduction of PEC efficiency, indicating that holes are also active groups for the RhB decomposition. The addition of BQ inversely induces the PEC degradation of dyes, and the reason could be attributed that separation of holes is effectively induced by the electron consumption [43]. Therefore, the more generated holes, the more OH• and higher PEC efficiency. Stability of photocatalysts is the important factor in the decontamination application. To investigate the stability of TiO2 NTs/Bi2S3-BiOBr photocatalysts, the PEC degradation of RhB for 6 repeated cycles were conducted in Fig. 12b, and only less than 7% of the PEC efficiency was reduced. Except for the PEC oxidation decomposition of organic dyes, the reduction of toxic Cr(VI) into Cr(III) by the as-prepared photocatalysts was investigated, and the reduction of Cr (VI) also illuminate the generation and separation efficiency of photoexcited electron [44]. As shown in Fig. 12c, the TiO2 NTs/Bi2S3-BiOBr photocatalyst shows the highest PEC reduction efficiency among all photocatalysts, and 67.12%% of Cr (VI) is PEC removed after 180 min solar irradiation. In summary, the TiO2 NTs/Bi2S3-BiOBr photocatalyst shows the excellent PEC removal efficiencies of organic dyes and Cr(VI), which could be used for the printing and electroplating waste-water treatment. In order to investigate the external voltage influences on the PEC removal performance, the simple photocatalysis without additional voltage was carried out for comparation with PEC progress. As exhibited in Fig. 12d, the photocatalytic (PC) removal efficiencies of RhB, MO, MB and Cr(VI) pollutants are lower than the corresponding PEC results, and the PEC removal efficiencies of RhB and Cr(VI) are even 2 times of PC data. The main reason is attributed to the reduced electron recombination chance by additional voltage, which significantly enhances the utilization of photogenerated charge carriers. Therefore, the TiO2 NTs/Bi2S3-BiOBr sample is a versatile photocatalyst for the environmental application in dyeing waste-water purification. The PEC mechanism of organic pollutant decomposition is tentatively proposed in Scheme 1. The big-leaf hydrangea-like TiO2 NTs/ Bi2S3-BiOBr photoelectrode provides large superficial area for the pollutant adsorption and solar harvesting. As calculated in Fig. 6b, the photogenerated electrons could be excited in Bi2S3, BiOBr and TiO2 NTs under solar irradiation, and the energy band edges decide the transportation of charge carriers. As reported in previous reports [45], the conduction band (CB) positions of TiO2, Bi2S3 and BiOBr are around −0.29 eV, 0 eV and 0.86 eV, and electrons could rapid transfer along the CB of TiO2 NTs into CB of Bi2S3 and BiOBr, which effectively achieves the separation of electron-hole pairs. Because of the more negative potential of O2• (EO2 •= −0.33 V) than VB values of semiconductors, the O2• formation is inhibited, which is consistent with the ESR and radical capture experiments. Inversely, the valence band (VB) positions of TiO2, Bi2S3 and BiOBr are around 2.91 eV, 1.54 eV and 3.44 eV. Therefore the VB of BiOBr is more positive than the standard redox potential of •OH/ OH−, and the massive •OH could be produced over the sensitizer surface. The redox potential (E·OH = 2.8 V) of •OH with high oxidation capacity is much higher than those of MB (φθ = 0.011 V), RhB (φθ = 0.95 V) and MO (φθ = 0.96 V), and the •OH radical groups could achieve the effective PEC decomposition of pollutants. In addition, •OH radicals could form single oxygen, which also shows significant roles in the PEC degradation of organic pollutants [46–48]. 4. Conclusions
Fig. 11. The TOC removal of different dyes (a) and ESR (b) of TiO2 NTs/Bi2S3-BiOBr.
In summary, the big-leaf hydrangea-like Bi2S3-BiOBr self-assembled by nanosheets were prepared on the TiO2 NTs surface via the solvothermal deposition. The TiO2 NTs/Bi2S3-BiOBr photoelectrodes exhibited excellent solar absorption and photoelectric performances. The TiO2 NTs/Bi2S3-BiOBr had the visible light photovoltage of −0.28 V, visible light photocurrent density of 4.23 mA/cm2 and carrier concentration
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Fig. 12. The effects of different scavengers on photocatalysis (a), contrastive curves of photocatalytic and photoelectrocataytic efficiencies (b), PEC removal of Cr(VI) (c) and successive PEC degradation of RhB (d) by TiO2 NTs/Bi2S3-BiOBr.
of 1.59 × 1020 cm−3. In addition, the TiO2 NTs/Bi2S3-BiOBr photoelectrode shows the high PEC removal efficiencies of RhB (93.68%), MO (56.75%), MB (90.53%) and Cr(VI) (67.12%), respectively, and the •OH radicals were the main species for the pollutant degradation. The outstanding photoelectrochemical performance of TiO2 NTs/Bi2S3-BiOBr drives the novel photoelectrode glorious prospect in the application of energy and environment materials. Author contributions Dawei Gao and Qingyao Wang contributed the central idea, analyzed most of the data, and wrote the initial draft of the paper. The remaining authors contributed to refining the ideas, carrying out additional analyses and finalizing this paper. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Big-leaf hydrangea-like Bi2S3-BiOBr sensitized TiO2 nanotube arrays with enhanced photoelectrocatalytic performance”. Acknowledgments This work was financially supported by Natural Science Foundation of Shandong Province (ZR2019QB023), National Natural Science Foundation
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Big-leaf hydrangea-like Bi2S3-BiOBr sensitized TiO2 nanotube arrays with enhanced photoelectrocatalytic performance Dawei Gao a,b, Lii Wang a, Qingyao Wang c,⁎, Zhenming Qi a, Yao Jia c, Chunxia Wang a a b c
College of Textile and Clothing, Yancheng Institute of Technology, Yancheng 224051, China Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA School of Chemistry and Materials Science, Ludong University, Yantai 264025, China
a r t i c l e
i n f o
Article history: Received 15 October 2019 Received in revised form 29 November 2019 Accepted 7 December 2019 Available online 12 December 2019 Keywords: Bi2S3 and BiOBr nanoparticles TiO2 nanotube arrays Solvothermal method Photocatalytic performance
a b s t r a c t TiO2 nanoparticles as solar cells and photocatalysts caused extensive attention in solar energy utilization and environment remediation due to the high photoelectrochemical performance. We demonstrated a novel approach to fabricate big-leaf hydrangea-like Bi2S3-BiOBr self-assembled by superthin nanosheets on TiO2 nanotube arrays (TiO2 NTs/B2S3-BiOBr). Results indicated that the Bi2S3-BiOBr co-sensitization showed higher photoelectric conversion efficiency than the single Bi2S3 or BiOBr sensitization. More remarkably, TiO2 NTs/B2S3-BiOBr showed excellent photoelectrocatalytic (PEC) removal of MB, MO, RhB and Cr(VI). The remarkable PEC performance could be attributed to the strong visible light absorption and effective electron transportation at the interface of TiO2/B2S3-BiOBr. The high photoelectrochemical performances indicate that the TiO2 NTs/B2S3-BiOBr could work as potential photoelectric materials for large-scale applications in the photoelectrochemical energy conversion and pollutant removal. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Novel nanoparticles and nanodevices with excellent photochemical or electrochemical properties caused enormous research interests [1–6], and it opened important possibilities for the design of novel optical and optoelectronics. Recently, the anodic TiO2 NTs have caused intense attention in the application of solar cell photoanode and photocatalyst owning to the uniform one dimensional nanotube microstructure, simple preparation condition, high chemical stability and surface area [7–9]. Many investigations about the photoelectrochemical activities indicated that the low visible light response was mainly dominated by the large band gap, which greatly influenced the applications of TiO2 NTs. Therefore, the sensitization coupled with another semiconductor with narrow band gap was an efficient way to extend the optical harvesting into visible light region. All kinds of semiconductors including metal sulfide [10], metal oxide [11], carbon group compound [12] and phosphate [13,14] were prepared to enhance the visible light photoelectrochemical performance. Currently, the co-sensitization by two or more materials was rapidly developed. Previous results [15,16] indicated that the multiple semiconductors deposited on the TiO2 NTs surface could form close connection that could further enhance the solar harvesting region and intensity, and the solar utilization was obviously improved compared with a single sensitization. Nevertheless, the defects formed at the interface of two semiconductor sensitizers would ⁎ Corresponding author. E-mail address: [email protected] (Q. Wang).
https://doi.org/10.1016/j.saa.2019.117936 1386-1425/© 2019 Elsevier B.V. All rights reserved.
trap electrons, which inversely reduce the final photoelectrochemical performance. Therefore, the matching of crystal structure and energy band location between two sensitizers and TiO2 NTs is the key point to influence the electron transportation and recombination. Two sulphides such as CdS, CdSe or CdTe with similar crystal structures and elemental compositions showed excellent electron transportation efficiency [17–20]. In addition, the two sensitizers with same cation or anion composition also provide unhindered electron transportation channel due to the sharing ions at the interface [21]. For examples, Prof. Eswar [22] prepared AgBr/Ag3PO4 photocatalysts with high photocatalytic activity, and the silver based semiconductors reduced the electron recombination at the interface. Jiang et al. [23] reported the preparation and photocatalytic performance of BiOBr/AgBr hybrids with the flower-like structure, and the photocatalyst showed an excellent photocatalytic degradation performance. As bismuth-based semiconductors, Bi2S3 and BiOBr are attractive photocatalysts with high visible light absorption and photocatalytic degradation of organic pollutants. Jiao [24] and Cui [25] prepared Bi2S3/BiOBr heterojunction photocatalyst with the enhanced photoelectrochemical and photocatalytic ability. The high photoelectrochemical performance is attributed to the convenient electron path transfer at the heterostructured Bi2S3/ BiOBr interface, which significantly reduced the possible recombination chance of charge carriers. In this paper, Bi2S3/BiOBr nanoparticles were deposited on the TiO2 NTs surface through a simple solvothermal strategy. The photoelectric conversion and PEC property of the prepared TiO2 NTs/Bi2S3-BiOBr photoelectrode were tested to obtain the optimal preparation condition.
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SEM (FEI SU 8010), TEM (FEI Tecnai F30), DRS (UV-2550) and XPS (KRATOS AXIS 165). The characterization procedures for photovoltage, photocurrent, interface resistance and PEC ability were described in our previous report [27]. The visible light and solar were provided by a 500 W Xe lamp (CEL-S500) with the visible light filter and AM 1.5 filter, respectively. The Pt counter electrode and commercial Ag/AgCl reference electrode (Schott, Germany) were used in the photoelectrochemical characterization, and the photoelectrochemical test was carried out in 0.1 M Na2SO4 electrolyte. 2.3. Measurement of the photoelectrocatalytic activities
Fig. 1. XRD patterns of (a) TiO2 NTs/Bi2S3, (b) TiO2 NTs/Bi2S3-BiOBr and (c) TiO2 NTs/BiOBr.
2. Experimental section 2.1. The synthesis of TiO2 NTs/Bi2S3-BiOBr TiO2 NTs grown on Ti foil surface were carried out by a two-step anodization progress with the anodic voltage of 60 V [26]. Bi2S3/BiOBr nanoparticles were deposited on TiO2 NTs by the one-pot solvothermal preparation. Briefly, 1 mmol of Bi(NO3)3·5H2O was slowly dissolved into 10 mL of methylglycol with intense magnetic stirring for 15 min, and 1 mmol of thiourea and 0.5 mmol of KBr were dispersed into 20 mL of methylglycol with sustained magnetic stirring. The two solutions were swiftly mixed together under magnetic stirring for 5 min and transferred into Teflon aligner of the autoclave. The autoclave was heated at 160 °C in a vacuum oven for 14 h. Then after the solvothermal reaction, the autoclave was cooled down to room temperature naturally, and samples were washed in ethanol for 1 min. For comparation, the single Bi2S3 or BiOBr sensitization was achieved by the reaction of 1 mmol of Bi(NO3)3·5H2O with 2 mmol of thiourea or 1 mmol of KBr, respectively. 2.2. Characterization The prepared products were further characterized for the elemental composition and morphology analysis through XRD (Siemens D-5000),
The photocatalytic performances of samples were investigated by the record of RhB, MO and MB dye concentration at their maximum absorption wavelength of 664, 464 and 552 nm, respectively. The samples set as photoelectrodes were immersed into dye solution with the concentration of 5 × 10−5 M, the external voltage was set at 1 V, and Na2SO4 was added into the solution as the electrolyte. After 20 or 30 min solar irradiation, the absorbance of dyes was recorded and calculated to obtain the degradation efficiency. Similarly, the Cr (VI) concentration was tested by the diphenylcarbazide spectrum method, and the external voltage was set to be 0.5 V. After 30 min solar irradiation, 1 mL of Cr (VI) was taken out and added into 50 mL of deionized water. Then 2 mL of diphenylcarbazide, 0.5 mL of H2SO4 and 0.5 mL of H3PO4 were added into the above solution, and the purple solution maintained the stirring state for 15 min. The absorbance was recorded at 540 nm, and the PEC efficiency was calculated. The OH• radical formation was confirmed by the electron spin resonance (ESR, A300) spin-trapping technique. The mineralization of dyes was evaluated by the total organic carbon (TOC-L CPH/CPN) of the residual dye solution after solar irradiation. 3. Results and discussion The XRD patterns of the as-prepared samples were displayed in Fig. 1. The typical diffraction patterns of anatase TiO2 (JCPDS no. 211272) could be obtained in all samples, and the diffraction peaks located at 25.28°, 36.95°, 37.80°, 38.57°, 48.05°, 53.89° and 55.06° correspond to the (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5) and (2 1 1) facets of anatase, respectively. After the solvothermal deposition, two new diffraction peaks at 28.61° and 39.89° can be clearly noticed, which can be indexed to (2 1 1) and (1 4 1) lattice planes of Bi2S3 (JCPDS no. 170320) in TiO2 NTs/Bi2S3 [28]. Furthermore, three peaks at 2θ values of 32.22°, 46.21° and 57.12° correspond to (1 1 0), (2 0 0) and (2 1 2) facets
Fig. 2. SEM images of (a) TiO2 NTs, (b) TiO2 NTs/Bi2S3, (c) TiO2 NTs/Bi2S3-BiOBr and (d) TiO2 NTs/BiOBr.
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Fig. 3. The high magnified SEM images and EDS of TiO2 NTs/Bi2S3-BiOBr.
Fig. 4. The TEM (a, c) and HRTEM (b, d) images of TiO2 NTs/Bi2S3-BiOBr.
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of BiOBr (JCPDS no. 09-0393) [29]. As shown in Fig. 1b, the diffraction peaks of Bi2S3 and BiOBr synchronously appear in TiO2 NTs/Bi2S3BiOBr, indicating the co-sensitization of Bi2S3 and BiOBr by the onepot solvothermal deposition. The morphology of TiO2 NTs was investigated by typical SEM images. The smooth tubular surface of TiO2 NTs is clearly observed in Fig. 2a, and the enlarged images indicate the detailed microstructures. The nanotube surface with the average diameter of 200 nm is composed by polygon including hexagon, pentagon even square, and the diverse morphologies could be attributed to the different internal stress in Ti foils. Compared with pure TiO2 NTs, the solvothermal deposition with thiourea or KBr solution causes completely different morphologies. The Bi2S3 nanoparticles cover the nanotube walls, making the coarseness of TiO2 NTs as shown in Fig. 2b. However, the BiOBr accumulation with large sizes (Fig. 2d)
deposits on TiO2 NTs surface, and the accumulations are self-assembled by BiOBr blocks with various sizes. When thiourea and KBr were dispersed into Bi(NO3)3 solution, the growth of Bi2S3-BiOBr hybrids is limited along specific orientation to form two dimensional nanosheets. The detailed morphology of TiO2 NTs/Bi2S3-BiOBr is exhibited in Figs. 2c and 3. The big-leaf hydrangea microstructure of Bi2S3-BiOBr is composed by the interspersed crosslinking of nanosheets, and the hierarchical structures could dramatically enhance the solar harvesting and pollutant adsorption, which would provide the guarantee for the excellent photoelectric performance. The chemical composition and element amount of the TiO2 NTs/Bi2S3-BiOBr were measured by EDS, the peaks of Ti, O, Bi, S and Br are clearly seen, which confirms the successful codeposition of Bi2S3 and BiOBr. In addition, the EDS result analysis reveals that the composition ratio of Bi2S3 and BiOBr is about 1:4.
Fig. 5. XPS spectra of TiO2 NTs/Bi2S3-BiOBr: (a) a survey spectrum; (b) Bi 4f; (c) S 2p; (d) O 1s and (e) Br 3d.
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The TEM and HRTEM images were provided in Fig. 4 to further investigate the microstructure of TiO2 NTs/Bi2S3-BiOBr. The low magnification TEM image in Fig. 4a reveals the nanotubular microstructure of TiO2 NTs, and the clear interplanar spacing of 0.243 nm of crystal lattice is consistent with the (1 0 3) plane of anatase. The TEM and HRTEM images of Bi2S3-BiOBr co-sensitizers are shown in Fig. 4c and d, and the big-leaf hydrangea-like sample is composed by nanosheets. The interplanar spacing of 0.226 nm of crystal lattices are fairly close to that of the (1 4 1) plane of Bi2S3, while the interplanar spacing of 0.278 and 0.161 nm is ascribed to the (1 1 0) and (2 1 2) planes of BiOBr. The TEM images further confirm the successful deposition of Bi2S3-BiOBr on TiO2 NTs. The element composition and chemical status of TiO2 NTs/Bi2S3BiOBr were studied by XPS in Fig. 5. The survey XPS spectrum indicates the Bi, S, O and C elements exist in the sample, and the C 1s peak at 284.60 eV could be attributed to the adventitious hydrocarbon, which is used to regulate the peak position of other elements. Fig. 5a shows the high-resolution XPS spectra of various elements in the TiO2 NTs/ Bi2S3-BiOBr. In the XRS spectrum of Fig. 5b, a strong peak at 160.73 eV could be assigned to the binding energy of Bi 4f7/2 (158.79 eV) and Bi 4f5/2 (164.37 eV), respectively, which is characteristic of Bi3+ in BiOBr and Bi2S3. S 2p XPS spectrum is shown in Fig. 5c, where the strong peak at 167.25 eV deconvoluted into two peaks centered at 168.56 and 169.63 eV are associated with core lines of S 2p1/2 and 2p3/2 in Bi2S3. XPS of O 2p in the sample can be resolved into three typical peaks, the peak at 530.15 eV can be assigned to the Bi\\O bonds in the (BiO)2+ slabs of BiOBr [30], the fitted peak dominating around 2 529.80 eV can be ascribed to the Ti\\O bonds in TiO2 NTs [31], while another peak at 532.33 eV could be ascribed to the hydroxyl group (–OH) in surface adsorbed components. As for the high-resolution XPS spectrum of Br 3d in Fig. 5d, the peaks at 68.30 and 69.62 eV are consistent with the Br 3d5/2 and Br 3d3/2, respectively. The XPS results further indicate the successful preparation of Bi2S3-BiOBr co-sensitizers. The visible light harvesting ability of the as-prepared photoelectrodes was investigated by DRS. As displayed in Fig. 6a, TiO2 NTs only absorb the UV light with the short wavelength below 400 nm, after Bi2S3 and BiOBr sensitization, all samples show broad visible light harvesting region and high harvesting intensity. Due to the inherent energy bands of Bi2S3 (1.3 eV) and BiOBr (2.7 eV), the TiO2 NTs/ Bi2S3 shows much stronger visible light absorption properties than that of TiO2 NTs/BiOBr. The Bi2S3-BiOBr synergistic sensitization significantly enhance the response into the whole visible light region, and the band gap could be calculated to be 2.79 eV which is much narrower than those of TiO2 NTs (3.21 eV). The high visible light absorption activity would generate multiple photogenerated electrons, which enhances the high photoelectrochemical performances of TiO2 NTs/Bi2S3-BiOBr.
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Fig. 7. Photoluminescent (PL) spectra of the as-prepared photoelectrodes.
In addition, the PL spectra of the prepared samples were tested to investigate the recombination of electron-holes. As shown in Fig. 7, the wide PL peaks range from 340 nm to 500 nm appear with the excitation wavelength at 325 nm, and the PL peak intensity is much weaker than that of TiO2 NTs. Among these samples, the TiO2 NTs/Bi2S3-BiOBr sample has the lowest PL intensity, indicating the effective separation of electron-hole pairs. The minimized recombination chance of charge carriers is mainly ascribed to the suitable energy band edges of Bi2S3 and BiOBr with TiO2 NTs, and detailed electron transfer path is discussed in Scheme 1. The photoelectrochemical performances were studied to evaluate the visible light absorption and electron transfer activity by the calculation and comparation of visible light transient photocurrent, I-V curves, OCP curves, EIS and Mott-Schottky plots of the as-prepared photoelectrodes. Comparing with the low photocurrent of TiO2 NTs [32], the photoelectrodes show high visible light photocurrent generation when the Xe lamp is switched on in Fig. 8a, and the photocurrent densities rapidly decrease into 0 mA/cm2 in dark. The regular photocurrent fluctuation indicates the semiconductor sensitization effect in the visible light absorption, photoelectric conversion and electron transfer. TiO2 NTs/Bi2S3-BiOBr photoelectrode shows the higher photocurrent (4.23 mA/cm2), and the value is 1.43 and 1.32 times of TiO2 NTs/Bi2S3 and TiO2 NTs/BiOBr. The photocurrent results have the same trend with I-V curves under different voltages in Fig. 8b, and the results indicate the high photoelectric conversion of TiO2 NTs/Bi2S3-BiOBr. The high photocurrent implies the high visible light response and effective electron transfer of TiO2 NTs/Bi2S3-BiOBr.
Fig. 6. Diffuse reflectance spectra (a) and Kubelka-Munk plots (b) of the as-prepared photoelectrodes. Sample: (a) TiO2 NTs, (b) TiO2 NTs/Bi2S3, (c) TiO2 NTs/Bi2S3-BiOBr and (d) TiO2 NTs/ BiOBr.
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Scheme 1. Schematic diagram of PEC degradation of dyes by TiO2 NTs/Bi2S3-BiOBr under solar irradiation.
Moreover, the surface voltage was tested to investigate the electron accumulation and transfer efficiency in Fig. 8c. When the Xe lamp is switched on, the photoelectrodes produce high photovoltages, and retain the stable values in the irradiation progress. The photovoltage is recognized as the balanced value between the electron accumulation and recombination. The TiO2 NTs/Bi2S3-BiOBr photoelectrode shows the biggest photovoltage, and the specific value (−0.28 V) is 2.62 and 2.21 times of TiO2 NTs/Bi2S3 and TiO2 NTs/BiOBr. The electron transfer performance of photoelectrodes was also studied by EIS in Fig. 9a. The semicircle diameter in the Nyquist plot of TiO2 NTs/ Bi2S3-BiOBr is much smaller than that of TiO2 NTs/Bi2S3 and TiO2 NTs/ BiOBr, indicating the low interface impedance of TiO2 NTs/Bi2S3-BiOBr in Na2SO4 electrolyte, and the photoelectrode has the optimal electron separation and transfer efficiency. Moreover, the applied voltages induce the energy band edge bending of photoelectrodes, which also significantly influences the transfer of photogenerated electrons. The MottSchottky plots of the photoelectrodes were displayed in Fig. 9b. All the photoelectrodes show positive slopes, indicating that the sensitized samples are still n-type semiconductors. Moreover, the flat bands could be calculated from the Mott-Schottky plots according to the horizontal-axis (1/C2 = 0), and the flat bands of TiO2 NTs/Bi2S3, TiO2 NTs/Bi2S3-BiOBr and TiO2 NTs/BiOBr are −0.69, −0.75 and −0.78 V, respectively. The carrier concentration was also calculated, and the specific values are 1.09 × 1020, 1.59 × 1020 and 1.03 × 1020 cm−3, respectively. Apparently, the high carrier concentration of TiO2 NTs/Bi2S3-BiOBr shows the excellent charge separation and transfer efficiency, which implies the high PEC performances in the removal of organic dyes and Cr(VI). The PEC properties of the photocatalysts were evaluated by the decomposition of organic pollutants simulated by RhB, MO and MB
under solar irradiation, and the PEC efficiency of pure TiO2 NTs was only 45.2% in our previous report [13]. The PEC degradation of RhB dyes is displayed in Fig. 10a, the TiO2 NTs/Bi2S3-BiOBr and TiO2 NTs/ BiOBr have the similar PEC performance, and more than 90% of RhB was removed after 180 min solar irradiation, which is much higher than that of TiO2 NTs/Bi2S3. The efficiency function (Ln(C0/C)) vs solar irradiation time (min) shows the linear relationship, and the PEC degradation rate k values are 0.0063, 0.01502 and 0.01692 min−1. Analogously, the TiO2 NTs/Bi2S3-BiOBr and TiO2 NTs/Bi2S3 show high PEC degradation of MO, and their PEC degradation rate constant k values are close to 5 folds of TiO2 NTs/BiOBr. Moreover, the TiO2 NTs/Bi2S3BiOBr photocatalyst shows the optimal PEC degradation efficiency of MB, and 90.53% of MB could be decomposed after 120 min solar irradiation. The high PEC rate constant (0.2084 min−1) is 2.16 and 1.31 times of TiO2 NTs/Bi2S3 and TiO2 NTs/BiOBr, respectively. As shown in Fig. 10c, the PEC degradation rate of MB is much higher than other dyes, and the reason could be ascribed to the compatibility with photocatalysts and structure stability. The high stability of azo structure in MO and triphenylmethane structure in RhB causes the low PEC efficiency [33], but the anilino and =S+— chromophores in MB dyes could be decomposed easily. The detailed photocatalytic degradation progress and intermediate product of these dye molecules are complex, and the corresponding results were investigated by Prof. Hisaindee [34]. The similar reports about photocatalytic degradation of organic pollutant by Bi2S3 and BiOBr sensitized TiO2 NTs as photocatalytsts were listed in Table 1. Though Prof. Tang's research [35] indicated that the photocatalytic efficiency reported by different investigation groups couldn't be simply contrasted due to the different light resource, light intensity, photocatalyst addition amount, dye concentration and so on,
Fig. 8. The transient photocurrent (a), I-V curves (b) and photovoltages (c) of the as-prepared photoelectrodes. Sample: (a) TiO2 NTs/Bi2S3, (b) TiO2 NTs/Bi2S3-BiOBr and (c) TiO2 NTs/ BiOBr.
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Fig. 9. The EIS (a) and Mott-Schottky plots (b) of the as-prepared photoelectrodes.
the results only directly reveal the excellent photocatalytic performance of the as-prepared TiO2 NTs/Bi2S3-BiOBr photocatalysts. The synergic sensitization of Bi2S3-BiOBr dramatically enhances the solar response, electron transfer and surface degradation, and the detailed mechanism would be discussed in Scheme 1. The photocatalytic degradation efficiency doesn't really reflect the decomposition degree of dye molecules, and the destruction of chromogenic groups also could significantly reduce the absorbance of dye
solution. Therefore, the TOC was used to evaluate the true mineralization degree of dyes, and the specific values are shown in Fig. 11a. The trend is similar with the PEC degradation of all dyes, and the TOC removal efficiencies of RhB and MB achieve 78.52% and 81.26%. However, the TOC of MO is still high after 3 h solar irradiation, and the prolonged irradiation further improve the mineralization. The mineralization efficiency after 5 h is enhanced up to 71.56%. The active species for the dye decomposition were explored by ESR spin-trap technique (with
Fig. 10. The PEC degradation efficiencies, pseudo-first-order kinetics and PEC rate constants for RhB (a), MO (b) and MB (c) by the photocatalysts.
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Table 1 Bi2S3 and BiOBr sensitized TiO2 NTs as photocatalytsts in the photocatalytic decoloration. Sensitizers
Preparation methods
Bi2S3-BiOBr Solvothermal method
Bi2S3-BiOI Bi2S3 Bi2S3
Solvothermal method SILAR deposition Photodeposition
Bi2S3 Bi2S3 BiOBr BiOBr
Electrodeposition Electrodeposition Chemical bath deposition SILAR deposition
BiOBr
SILAR deposition
Light sources
Pollutants Efficiency Refs.
Solar light RhB MO MB Solar light RhB Solar light MO Visible MO light Solar light 2,4-D Solar light MB Visible RhB light Visible RhB light Solar light RhB
93.68% 56.75% 90.53% 93.6% 44.05% 83.3%
This work [9] [36] [37]
81% 82.3% ~60%
[38] [39] [40]
97.7%
[41]
93%
[42]
DMPO) in Fig. 11b by the comparation with/without solar irradiation. We could obviously notice four typical characteristic peaks of DMPOOH• with intensity 1:2:2:1 under solar irradiation for 10 min, which is the signal of OH• radical generation. No ESR signals of O2• were observed during the photocatalytic progress, and the results reveal that OH• radicals play the main roles in the PEC degradation of dyes. To further confirm the photocatalytic roles of OH• radicals, the PEC degradation with quenching agent addition of benzoquinone (BQ), EDTA-2Na and tertiary butyl alcohol (t-BuOH) was carried out. As
exhibited in Fig. 12a, the final PEC efficiency after the t-BuOH addition nearly remain unchanged, and this result indicates that the OH• captured by t-BuOH plays the vital roles in the RhB degradation. The addition of hole quenching agent (EDTA-2Na) also causes the reduction of PEC efficiency, indicating that holes are also active groups for the RhB decomposition. The addition of BQ inversely induces the PEC degradation of dyes, and the reason could be attributed that separation of holes is effectively induced by the electron consumption [43]. Therefore, the more generated holes, the more OH• and higher PEC efficiency. Stability of photocatalysts is the important factor in the decontamination application. To investigate the stability of TiO2 NTs/Bi2S3-BiOBr photocatalysts, the PEC degradation of RhB for 6 repeated cycles were conducted in Fig. 12b, and only less than 7% of the PEC efficiency was reduced. Except for the PEC oxidation decomposition of organic dyes, the reduction of toxic Cr(VI) into Cr(III) by the as-prepared photocatalysts was investigated, and the reduction of Cr (VI) also illuminate the generation and separation efficiency of photoexcited electron [44]. As shown in Fig. 12c, the TiO2 NTs/Bi2S3-BiOBr photocatalyst shows the highest PEC reduction efficiency among all photocatalysts, and 67.12%% of Cr (VI) is PEC removed after 180 min solar irradiation. In summary, the TiO2 NTs/Bi2S3-BiOBr photocatalyst shows the excellent PEC removal efficiencies of organic dyes and Cr(VI), which could be used for the printing and electroplating waste-water treatment. In order to investigate the external voltage influences on the PEC removal performance, the simple photocatalysis without additional voltage was carried out for comparation with PEC progress. As exhibited in Fig. 12d, the photocatalytic (PC) removal efficiencies of RhB, MO, MB and Cr(VI) pollutants are lower than the corresponding PEC results, and the PEC removal efficiencies of RhB and Cr(VI) are even 2 times of PC data. The main reason is attributed to the reduced electron recombination chance by additional voltage, which significantly enhances the utilization of photogenerated charge carriers. Therefore, the TiO2 NTs/Bi2S3-BiOBr sample is a versatile photocatalyst for the environmental application in dyeing waste-water purification. The PEC mechanism of organic pollutant decomposition is tentatively proposed in Scheme 1. The big-leaf hydrangea-like TiO2 NTs/ Bi2S3-BiOBr photoelectrode provides large superficial area for the pollutant adsorption and solar harvesting. As calculated in Fig. 6b, the photogenerated electrons could be excited in Bi2S3, BiOBr and TiO2 NTs under solar irradiation, and the energy band edges decide the transportation of charge carriers. As reported in previous reports [45], the conduction band (CB) positions of TiO2, Bi2S3 and BiOBr are around −0.29 eV, 0 eV and 0.86 eV, and electrons could rapid transfer along the CB of TiO2 NTs into CB of Bi2S3 and BiOBr, which effectively achieves the separation of electron-hole pairs. Because of the more negative potential of O2• (EO2 •= −0.33 V) than VB values of semiconductors, the O2• formation is inhibited, which is consistent with the ESR and radical capture experiments. Inversely, the valence band (VB) positions of TiO2, Bi2S3 and BiOBr are around 2.91 eV, 1.54 eV and 3.44 eV. Therefore the VB of BiOBr is more positive than the standard redox potential of •OH/ OH−, and the massive •OH could be produced over the sensitizer surface. The redox potential (E·OH = 2.8 V) of •OH with high oxidation capacity is much higher than those of MB (φθ = 0.011 V), RhB (φθ = 0.95 V) and MO (φθ = 0.96 V), and the •OH radical groups could achieve the effective PEC decomposition of pollutants. In addition, •OH radicals could form single oxygen, which also shows significant roles in the PEC degradation of organic pollutants [46–48]. 4. Conclusions
Fig. 11. The TOC removal of different dyes (a) and ESR (b) of TiO2 NTs/Bi2S3-BiOBr.
In summary, the big-leaf hydrangea-like Bi2S3-BiOBr self-assembled by nanosheets were prepared on the TiO2 NTs surface via the solvothermal deposition. The TiO2 NTs/Bi2S3-BiOBr photoelectrodes exhibited excellent solar absorption and photoelectric performances. The TiO2 NTs/Bi2S3-BiOBr had the visible light photovoltage of −0.28 V, visible light photocurrent density of 4.23 mA/cm2 and carrier concentration
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Fig. 12. The effects of different scavengers on photocatalysis (a), contrastive curves of photocatalytic and photoelectrocataytic efficiencies (b), PEC removal of Cr(VI) (c) and successive PEC degradation of RhB (d) by TiO2 NTs/Bi2S3-BiOBr.
of 1.59 × 1020 cm−3. In addition, the TiO2 NTs/Bi2S3-BiOBr photoelectrode shows the high PEC removal efficiencies of RhB (93.68%), MO (56.75%), MB (90.53%) and Cr(VI) (67.12%), respectively, and the •OH radicals were the main species for the pollutant degradation. The outstanding photoelectrochemical performance of TiO2 NTs/Bi2S3-BiOBr drives the novel photoelectrode glorious prospect in the application of energy and environment materials. Author contributions Dawei Gao and Qingyao Wang contributed the central idea, analyzed most of the data, and wrote the initial draft of the paper. The remaining authors contributed to refining the ideas, carrying out additional analyses and finalizing this paper. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Big-leaf hydrangea-like Bi2S3-BiOBr sensitized TiO2 nanotube arrays with enhanced photoelectrocatalytic performance”. Acknowledgments This work was financially supported by Natural Science Foundation of Shandong Province (ZR2019QB023), National Natural Science Foundation
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