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Synthesis, characterization and photocatalytic properties of nanoscale pyrochlore type Bi2Zr2O7
Materials Science & Engineering B 240 (2019) 133–139
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Synthesis, characterization and photocatalytic properties of nanoscale pyrochlore type Bi2Zr2O7
T
Yijia Luoa, Liyun Caoa, , Liangliang Fenga, , Jianfeng Huanga, Liuqing Yanga, Chunyan Yaob, Yayi Chenga ⁎
a b
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School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, PR China School of Arts and Sciences, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, PR China
ARTICLE INFO
ABSTRACT
Keywords: Bi2Zr2O7 Photocatalytic properties Cr(VI) reduction Coprecipitation method
Developing efficient visible-light-driven photocatalysts for environmental decontamination is of considerable importance in solar energy conversion and utilization. Herein, nanoscale Bi2Zr2O7 pyrochlore structure materials was synthesized by a facile coprecipitation method and the influence of nano-size effects on its photocatalytic reduction of Cr(VI) under visible light was investigated. Results show that the resultant Bi2Zr2O7 prepared at 2 h optimum calcination time with uniform small crystal size leading to a high specific surface area exhibits the best photocatalytic performance. In addition, the possible mechanisms of the photocatalytic reduction of Cr(VI) over Bi2Zr2O7 were proposed.
1. Introduction Hexavalent Chromium (Cr(VI)), one of the most toxic and highly soluble heavy metal ion, discharged from leather tanning, mining, electroplating, chromate manufacturing, adversely affects the environment and human health [1–3]. Consequently, effective treatment for removing Cr(VI) from waste water is vital for human living environment. At present, techniques for removing Cr(VI) from waste water, including chemical precipitation [4], ion exchange [5], adsorption [6], membrane separation [7], and reduction [8] have been used. In the above methods, the reduction of Cr(VI)–Cr(III) is considered to be a preferred treatment for the reason that Cr(III) is environmental friendly and also an important trace element for human health [9]. Therefore, given the low cost, reusability and no production of secondary pollutants, photocatalytic reduction of Cr(VI) performed under solar light irradiation is regarded as an attractive technology for waste water treatment [8,10]. The study of photocatalytic reduction of Cr(VI) over TiO2 has been reported extensively [11,12]. Nevertheless, TiO2 presents photocatalytic activity mainly under UV light taking up less than 5% of solar radiation [13–15]. Therefore, the development of photocatalytic active materials which can effectively reduce Cr(VI) under visible light is very indispensable for solar energy utilization. As a visible-light-driven photocatalytic material, the Bi2Zr2O7 has been reported to be used in fields of degradation of Indigo carmine, Ramazoline brilliant blue and
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Methyl orange dyes [16–19]. Being a close family material of Bi2Zr2O7, La2Ti2O7 has a high photocatalytic reduction of Cr(VI) under mercury lamp irradiation in the literature report [20]. Therefore, the photocatalytic properties of Bi2Zr2O7 in reducing Cr(VI) under visible light irradiation also have potential for research. Before our work, Vignesh et al. reported the reduction of Cr(VI) with Bi2O3-ZrO2 under 300 W Xe arc lamp irradiation with a 400 nm cutoff filter, the activity of catalysts for the photoreduction of Cr(VI) was studied with an initial Cr(VI) concentration of 0.1 mM, catalyst concentration of 1.25 g/L and irradiation time of 180 min and the photocatalytic activity of the photocatalytic is 92.3% in 180 min under irradiation [21]. In this work, we synthesized nano-sized Bi2Zr2O7 via a facile coprecipitation method, and investigated the effects of nano-size on its morphology and the photocatalytic reduction of Cr(VI) under visible light irradiation. More specifically, the resulting Bi2Zr2O7 prepared in 2 h calcination showed a higher photocatalytic activity for Cr(VI) reduction under visible-light irradiation (λ > 420 nm) than other samples prepared in 4 h and 6 h calcination, for the reason that its uniform smaller nanoparticles lead to a higher specific surface area. Besides, the mechanism of photocatalytic Cr(VI) removal also have been studied. Compared with previous work, 1) Bi2Zr2O7 nanoparticles with pyrochlore type structure were synthesized by coprecipitation method in a rather lower temperature and shorter calcination time (600 °C, 2 h); 2) the effect of pH on Bi2Zr2O7 photocatalytic reduction of Cr(VI) was investigated, confirmed that the photocatalytic reduction of Cr(VI) is an
Corresponding authors. E-mail addresses: [email protected] (L. Cao), [email protected] (L. Feng).
https://doi.org/10.1016/j.mseb.2019.01.017 Received 18 September 2017; Received in revised form 20 December 2018; Accepted 19 January 2019 Available online 28 January 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
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acid catalyzed behavior; 3) the photocatalyst shows a good stability and reusability, by means of XRD, SEM and photo-reduction experiments, and there are no obvious changes for its phase, morphology and performance. This work expands the variety of materials that applied in photocatalytic reduction of Cr(VI) under visible light irradiation. 2. Experimental 2.1. Synthesis of Bi2Zr2O7 All the reagents used in the experiments were of analytical grade. The molar ratio of bismuth nitrate and zirconium nitrate was 1:1. In a typical synthetic procedure towards the resulting Bi2Zr2O7 could be illustrated clearly as follows. 3 mM Bi(NO3)3·5H2O was dispersed in 12 mL HNO3 solution (2 M), subsequently added with Zr(NO3)4 solution, which was prepared by dissolving 3 mM Zr(NO3)4·5H2O into 18 mL deionized water. The pH value was adjusted with NH3·H2O solution to 10 to obtain the precursor solution. After being continuously stirred for 2 h at room temperature, the precipitate was collected and washed with deionized water and absolute ethyl alcohol for three times, respectively. And then dried at the 60 °C for 4 h. Finally, the obtained powder was calcined in air at 600 °C for a certain time (1, 2, 4 and 6 h).
Fig. 1. XRD patterns of Bi2Zr2O7 prepared at various calcination time.
electrode, the saturated calomel electrode (SCE) is the reference electrode and the Bi2Zr2O7 film electrodes as working electrodes. The typical working electrode was prepared as follows: 2 mg sample was mixed with 1 mL absolute ethanol and 0.5 mL nafion to make slurry, and the slurry was then dispersed onto a 4 cm × 2 cm FTO glass and then dried overnight at room temperature to obtain the electrodes. The photoelectrochemical measurements were performed under a 300 W Xe light source with a 420 nm cutoff filter.
2.2. Characterizations The crystal structure of the as-prepared powders was characterized by a powder X-ray diffraction (XRD, Rigaku D/max-2000) with Cu Kα radiation (λ = 0.15406 nm). The morphology study of the products was performed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and a transmission electron microscope (TEM, Tecnai G2 F20, USA), The Brunauer-Emmet-Teller specific surface area was determined through N2 absorption at 77 K using an adsorption instrument (BET, ASAP2460, Micromeritics, USA). The surface analysis was studied by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). The binding energy data was calibrated with the C 1s signal at 284.6 eV. And UV–vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV/vis/NIR spectrophotometer (Agilent Cary 5000). BaSO4 was used as a reflectance standard. The photoluminescence spectra were recorded on a spectrofluorometer (PL, Edinburgh FS5, UK).
3. Results and discussion 3.1. Structure and morphology 3.1.1. XRD analysis Fig. 1 shows the XRD patterns of Bi2Zr2O7 samples prepared under different calcination time, in which it can be seen that a dominant Bi2Zr2O7 has been formed at 1 h, while ZrO2 impurity is also observed, this result indicates that the reaction is not complete at this calcination time. The calcination time increased to 2 h, pure Bi2Zr2O7 has been synthesized successfully, and its diffraction peaks are in agreement with the results earlier reported in literature [17], and no peaks of impurities are detected. On increasing calcination time, the diffraction peaks become stronger, which indicates the better crystallinity of samples. The average crystallite size of these samples is estimated by the Scherrer’s equation, D = Kλ/(βcosθ). Where θ is the Bragg angle, λ is X-ray wavelength (1.54 Ǻ), β is the FWHM of the diffraction peak, K is the constant (K = 0.9) and D is the average crystallite size [23]. The results showed that the crystallite size is 19.7 nm, 21.8 nm, and 23.4 nm (the error for average crystal size should be ± 1.5 nm) when the calcination time is 2 h, 4 h, and 6 h, respectively. It exhibits that the prepared samples are all in nanoscale, and the morphology of samples can be determined by FE-SEM and TEM analysis.
2.3. Photocatalytic reduction of Cr(VI) The photocatalytic activities of samples were evaluated by photocatalytic reduction of Cr(VI) under visible-light irradiation of a 500 W Xe lamp with a 420 nm cutoff filter. In a typical process, 16 mg powder was added to 40 mL of Cr(VI) solution (based on Cr in a dilute K2Cr2O7 solution) with the concentration of 10 ppm on BL-GHX-V photocatalytic reactor (Xi’an, BILON, Co. Ltd.). The pH of the solution was adjusted by using NaOH and H2SO4. After the photocatalyst was dispersed in the solution with an ultrasonic bath for 5 min, the suspensions were magnetically stirred in the dark for 2 h to reach adsorption-desorption equilibrium and then was exposed to visible-light irradiation. After a given irradiation time, 5 mL of the solution was collected by centrifugation (9000 rpm, 6 min), and the Cr(VI) reduction was determined colorimetrically by UV–vis spectrophotometry (Unico, UV-2600) at 540 nm using the diphenylcarbazide (DPC) method with deionized water as a reference sample [22]. The photocatalytic efficiency was determined by C/C0, where C is the remained Cr(VI) concentration and C0 is the concentration at the irradiation time of 0 min.
3.1.2. FE-SEM analysis Fig. 2 presents the FE-SEM spectra of as-prepared Bi2Zr2O7 samples at different calcination time. The particles are all at the nanoscale, which is consistent with the Scherrer’s equation results. In addition, it can be seen that the nanoparticles are not well-dispersed distribution, but gathered together, and there are different degrees of agglomeration. With the calcination time increased, the size of nanoparticles grew bigger and the agglomeration of that became more serious. Therefore, it is concluded that the Fig. 2(a) showed the smaller particle size and a better dispersity among these samples. The formation of larger specific surface area could benefit from the
2.4. Photoelectrochemical measurements Photoelectrochemical measurements were carried out on the electrochemical station (CHI-660B, China) using a three-electrode mode with 0.5 M Na2SO4 solution as the electrolyte. The Pt wire is the counter 134
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Fig. 2. FE-SEM images of Bi2Zr2O7 in (a) 2 h; (b) 4 h; (c) 6 h.
smaller nanoparticles size, which is one of the reasons contributing to high catalytic performance [24]. When the size of the semiconductor particles is smaller than the free path of the photogenerated carriers, the recombination rate of the electron-hole pairs on the surface will be greatly reduced, thereby improving the redox efficiency. Moreover, the smaller particle size also reduces the diffuse reflection of light and increases the absorption of light [25,26].
further confirm the formation of Bi2Zr2O7. 3.1.5. BET analysis The Brunauer-Emmett-Teller (BET) area and pore size distribution of as-prepared Bi2Zr2O7 were characterized by N2 adsorption-desorption analysis. The BET absorption-desorption isotherms of the Bi2Zr2O7 samples prepared in 2, 4, 6 h were obtained from the N2 adsorption-desorption measurements at 77 K, as shown in Fig. 5(a). The shape of the isotherms is a type II isotherm with a type H1 hysteresis loop at high relative pressures according to the IUPAC classification. The type II isotherm is the normal form of isotherm obtained with a nonporous or macroporous adsorbent, while the type H1 hysteresis loop is often associated with porous materials or the existence of agglomeration [33]. Besides, the existence of hysteresis loop in BET isotherm of the as-prepared Bi2Zr2O7 material may be attributed to the formation of mesoporous resulting from particle aggregation. The pore size distribution (Fig. 5(b)) of Bi2Zr2O7 were quite narrow and monomodal, implying that the prepared samples are composed of uniform particles [34]. As shown in Table 1, in which we can obtain the BET surface and detailed parameters of pore for all the samples. Obviously, the BET specific surface areas of the samples decreased with increasing calcination time. Compared to 4 h and 6 h, sample prepared in 2 h presented a slightly higher pore volume and pore size.
3.1.3. TEM analysis The crystal structure of samples was further elucidated by TEM analysis. When the calcination time was controlled at 2 h, it was observed that the diameter of nanoparticles is in the range of 12–22 nm as seen in Fig. 3(a) and Supplementary Information Fig. S1. That is consistent with the crystallite size determined by Scherrer’s method. The HRTEM image is shown in Fig. 3(b), clear lattice fringes reveal the good crystallinity of the resultant Bi2Zr2O7 materials. The observed interplanar spacing of 0.30 nm may correspond to (2 2 2) lattice plane of Bi2Zr2O7, as evidenced from the XRD pattern of it (Fig. 1). 3.1.4. XPS analysis To further confirm the surface compositions and chemical state of the as-prepared sample, XPS was carried out by us [27]. As shown in Fig. 4, the survey spectrum clearly indicates the presence of Bi, Zr and O elements in this Bi2Zr2O7 material (Fig. 4(a)). The C 1s peak appears due to the contamination of organic compounds in the investigation process [28]. Fig. 4(b) shows the high-resolution XPS spectrum of Bi 4f, in which fitting peaks at 158.6 eV and 163.9 eV are assigned to Bi 4f7/2 and Bi 4f5/2 of Bi3+, respectively [29]. The Zr 3d XPS spectrum only displays two peaks at 181.5 eV and 183.9 eV (Fig. 4(c)), which can be assigned to Zr 3d5/3 and Zr 3d3/2 [18], respectively. The separation between Zr 3d5/3 and Zr 3d3/2 peak is observed to be 2.4 eV and the area ratio of the two peaks is 3:2, which is typical of Zr4+ ions in a full oxidation state [30]. The O 1s XPS spectrum of the sample can be split into three peaks at 526.4 eV, 527.8 eV and 529.4 eV. The peak at 526.4 eV is assigned to oxygen ions at 8a sites and the peak at higher bind energy of 529.4 eV is assigned to O at 48f sites, and the peak at 527.8 eV is attributed to oxygen related defects such as oxygen vacancies and oxygen interstitials [31,32]. Consequently, XPS studies
3.2. Optical properties 3.2.1. UV–vis analysis In order to investigate the optical absorption property of the samples, the UV–vis DRS was introduced, and it is known that the optical absorption property is closely related to its electronic structure [35]. Fig. 6 reveals that all the samples have absorption in the visible light region, and the sample calcined for 2 h has the strongest absorption in the visible light region. The smaller crystallite size caused by shortening calcination time, the UV–vis absorption spectrum has a red shift and the absorption intensity of visible light become stronger. It can be concluded that the crystallite size has an effect on the absorbance, which can be ascribed to nano-size effect of particles [36,37]. In addition, the Fig. 3. (a) Transmission electron microscope (TEM) image of the product prepared at 2 h; (b) Corresponding high resolution transmission electron microscopy (HRTEM) image was obtained at the center of the red rectangles in the TEM image. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. XPS spectra of the Bi2Zr2O7 sample: (a) survey spectrum; (b) Bi 4f; (c) Zr 3d; (d) O 1s.
Fig. 5. The N2 adsorption-desorption isotherms (a) and pore size distribution (b) of the samples prepared at 600 °C when calcine time is 2 h, 4 h and 6 h, respectively. Table 1 BET surface area and pore parameters of the prepared samples. Sample
SBET/(m2/g)
Pore volume (cm3/g)
Pore size (nm)
2h 4h 6h
14 13 10
0.100 0.089 0.074
25.5 24.3 23.7
band gap energy (Eg) of the samples could be calculated by using the following Eq. (1):
F(R) E= A(E
Eg) n/2
(1)
where F(R) is the diffuse absorption coefficient, A is the constant, E is photon energy, Eg is the band gap, n is 4 for a direct transition, n is 1 for an indirect transition, and the value of n for Bi2Zr2O7 is 4 [18]. The band gap energy of the sample prepared in 2, 4, and 6 h are estimated to be 2.86, 2.88, and 2.94 eV, respectively, calculated from the intercept
Fig. 6. UV–vis diffuse reflectance spectra of samples prepared at 600 °C (Calcination time is 2 h, 4 h, and 6 h.).
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Fig. 7. Reduction profiles of photocatalytic reduction of Cr(VI) over Bi2Zr2O7 (t = 2 h) (a) at different pH values, (b) over various photocatalysts (pH = 2); (c) photocurrent response of the products prepared under different calcination time; (d) recycling test. Reaction condition: 16 mg photocatalyst, 40 mL of 10 ppm Cr(VI), reaction temperature is 20 °C, the solution is stirred for 2 h in the dark and then is exposed to visible light (λ > 420 nm) irradiation.
of the tangents to the plots [F(R)E]1/2 versus E curves showed in Fig. S2.
oxide semiconductor materials in the previous literature reported, on the condition that all the materials without further doping or composite modification (see Supplementary Table S1). Their apparent reaction rate constants based on the specific surface area can be estimated to be 2.36 × 10−4, 1.35 × 10−4 and 8.0 × 10−5 g·m−2·min−1, respectively (Fig. S3). It exhibited that the sample whose calcination time is 2 h shows the highest reaction rate, and the results probably contributed to the biggest surface area of this sample possess the most surface catalytic active sites compared with others [40]. With the calcination time decreasing, the particles size of the as-prepared Bi2Zr2O7 decreases, leading to a higher specific surface area and thus provide more active sites for photocatalytic reaction. Therefore, the apparent activity of the sample whose calcination time is 2 h is superior than that of 4 h and 6 h. The results revealed that the size of nanoparticles plays an important role in the property of the photocatalyst and the resultant Bi2Zr2O7 with different sizes can be prepared by tuning the calcination time. It is proposed that high-temperature calcination makes nanoparticles possess high surface energy and tend to slightly agglomerate, leading to the reduction of specific surface area [41,42]. Meanwhile, as the calcination time increases, the size of nanoparticles grows bigger, and the specific surface area of samples becomes smaller, which further results in the less active sites per unit weight of photocatalysts. Hence, the sample whose calcination time is 2 h under the same calcination temperature exhibits the best photocatalytic activity. Photocatalytic reaction using semiconductors as catalysts involved the generation, separation, transfer, and recombination of electron-hole pairs. It is a commonly held view that the separation efficiency of electron-hole pairs played a vital role in the photocatalytic reaction [43]. To further understand the charge separation and transfer dynamics of photocatalyst, the photocurrent-time responses of these samples were recorded for several on-off cycles. Fig. 7(c) shows the
3.3. Evaluation of photocatalytic activity Based on the results of the absorption experiment, the photocatalytic reduction of Cr(VI) over Bi2Zr2O7 in water was carried out under visible light irradiation. The photocatalytic reaction was evaluated by monitoring the decolorization of the UV–vis absorption spectra of diphenylcarbazide (DPC)-Cr(VI) complex solutions. To obtain the optimum reaction condition, as Fig. 7(a) displays, the effects of pH on reduction of Cr(VI) catalyzed by Bi2Zr2O7 were studied. The pH value of the K2Cr2O7 solution was adjusted in the range from 2 to 7 by the addition of 0.2 M H2SO4 and NaOH. The experimental results show that the sample displays the best photocatalytic performance at pH = 2, and then confirm that the photocatalytic reduction of Cr(VI) is an acid catalyzed behavior and the acid medium is beneficial to the photocatalytic reduction of Cr(VI) because of the existence of abundant H+ [38]. At the pH range from 2 to 6, the Cr(VI) reduction reaction could be expressed as follows [39]:
Cr2O27 + 14H+ + 6e 2H2 O+ 2h+
2Cr 3 + + 7H2 O
H2 O2 + 2H+
(2) (3)
Fig. 7(b) shows the photocatalytic reduction of Cr(VI) over Bi2Zr2O7 photocatalysts which calcined at different time. In addition, photocatalytic reduction of Cr(VI) without the photocatalyst as the blank contrast sample. It is obvious that the reduction of Cr(VI) hardly occurred in the absence of photocatalyst. Furthermore, sample that calcination time is 2 h exhibits highest activity compared with the other samples (efficiency of Cr(VI) removal achieved 38% for 150 min). Importantly, the performance for the photocatalytic reduction of Cr(VI) over the resulting Bi2Zr2O7 is outstanding compared with other metal 137
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Cr2O72− and H+ to Cr3+ and H2O, meanwhile, the hole oxidize water to H2O2 and H+ in single systems. Since there is a certain amount of oxygen vacancies existing in the pyrochlore structure, which is conducive to the activation of adsorbed oxygen on the surface of Bi2Zr2O7 [17,50]. This kind of active oxygen species can not only prevent the recombination of photogenerated electron hole pairs, but also produce high active hydroxyl radicals, which can promote the photocatalytic oxidation reaction rate [51]. 4. Conclusions In summary, nanoscale Bi2Zr2O7 were synthesized via a facile coprecipitation method, and the effects of nano-size on its photocatalytic performance for Cr(VI) reduction under visible light irradiation were investigated. The catalyst with smaller particle size, better dispersion, larger surface area shows the red-shift adsorption in the visible light region and lower recombination rate of photogenerated charge carriers, resulting in better performance for reduction of Cr(VI). And the possible mechanism of photocatalytic reduction of Cr(VI) is also proposed. Identifying Bi2Zr2O7 as a photocatalyst for reducing Cr(VI) under visible light irradiation is expected to provide a fresh impetus to explore new catalysts for pollution elimination.
Fig. 8. Proposed schematic diagram for photocatalytic reduction of aqueous Cr (VI) over Bi2Zi2O7 under visible-light (λ > 420 nm) irradiation.
transient photocurrent responses of Bi2Zr2O7 prepared at the different calcination time under intermittent visible light illumination. The photocurrent for Bi2Zr2O7 that calcination time is 2 h (0.040 μA/cm2) revealed to be much higher than those of 4 h (0.027 μA/cm2) and 6 h (0.020 μA/cm2) upon light irradiation. The enhanced photocurrent response of the as-prepared Bi2Zr2O7 that calcination time is 2 h indicates smaller nanoparticles has a positive effect on the higher separation efficiency and a lower recombination rate of the photoinduced electronhole pairs. This is probably because the shorter the calcination time is, the smaller the particles grow, the more defects are retained in the materials, which is beneficial to prevent the recombination of photogenerated electron holes [44,45]. The migration, transfer, and recombination of the photogenerated electron-hole pairs can be explored from photoluminescence spectra of the semiconductor and lower fluorescence emission intensity implies a lower recombination rate of electron-hole pairs [46–48]. Fig. S4 Shows the PL spectra of the synthesized with different calcination time range from 2 to 6 h excited by 352 nm. It could be found that the main emission peak was centered at about 483 nm for Bi2Zr2O7. From the figure, it can be observed that with the calcination time decreased the emission intensity decreased, which indicated that the Bi2Zr2O7 we synthesized with smaller nanoparticles had a much lower recombination rate of photogenerated charge carriers. Out of the economic point of view, the reusability and stability of the photocatalysts are very important. The reusability of the Bi2Zr2O7 has been studied by collecting and reusing the photocatalyst for three times, as shown in Fig. 7(d), the variation of photocatalytic activity in every cycle was similar. Only insignificant decrease compared to that for the first use is observed, which might be partly caused by loss of photocatalyst during each collection [49]. The used photocatalyst can be collected by washing with water and ethanol for several times. The XRD patterns of Bi2Zr2O7 before and after the photocatalytic reaction did not show obvious changes in the crystal structure (Fig. S5). SEM image after photocatalytic reaction at pH = 2 was measured and shown in Fig. S6. It shows that the morphology of Bi2Zr2O7 is maintained at pH = 2. The data above indicate the stability of the photocatalyst in this reaction conditions. As shown in Fig. S7, the XPS spectra of samples measured before and after photocatalytic reaction are almost the same, which further confirm the stability of the photocatalyst.
Acknowledgments The authors would like to thank the support from the National Natural Science Foundation of China (No. 51672165, No. 21701107, No. 61875231), the Support Program of Shaanxi Province Youth Science and Technology Leading Talents. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mseb.2019.01.017. References [1] J. Yu, S. Zhuang, X. Xu, W. Zhu, B. Feng, J. Hu, Photogenerated electron reservoir in hetero-p-n CuO-ZnO nanocomposite device for visible-light-driven photocatalytic reduction of aqueous Cr(VI), J. Mater. Chem. A 3 (2015) 1199–1207. [2] H. Wang, X. Yuan, Y. Wu, G. Zeng, X. Chen, L. Leng, Z. Wu, L. Jiang, H. Li, Facile synthesis of amino-functionalized titanium metal-organic frameworks and their superior visible-light photocatalytic activity for Cr(VI) reduction, J. Hazard. Mater. 286 (2015) 187–194. [3] S. Rajput, C.U. Pittman, D. Mohan, Magnetic magnetite (Fe3O4) nanoparticle synthesis and applications for lead (Pb2+) and chromium (Cr6+) removal from water, J. Colloid Interface Sci. 468 (2016) 334–346. [4] A. Salmani Abyaneh, M.H. Fazaelipoor, Evaluation of rhamnolipid (RL) as a biosurfactant for the removal of chromium from aqueous solutions by precipitate flotation, J. Environ. Manage. 165 (2016) 184–187. [5] K. Xiao, F. Xu, L. Jiang, N. Duan, S. Zheng, Resin oxidization phenomenon and its influence factor during chromium(VI) removal from wastewater using gel-type anion exchangers, Chem. Eng. J. 283 (2016) 1349–1356. [6] Y. Lu, B. Jiang, L. Fang, F. Ling, J. Gao, F. Wu, X. Zhang, High performance NiFe layered double hydroxide for methyl orange dye and Cr(VI) adsorption, Chemosphere 152 (2016) 415–422. [7] Y. Bao, X. Yan, W. Du, X. Xie, Z. Pan, J. Zhou, L. Li, Application of amine-functionalized MCM-41 modified ultrafiltration membrane to remove chromium (VI) and copper (II), Chem. Eng. J. 281 (2015) 460–467. [8] Y. Zhang, Q. Zhang, Q. Shi, Z. Cai, Z. Yang, Acid-treated g-C3N4 with improved photocatalytic performance in the reduction of aqueous Cr(VI) under visible-light, Sep. Purif. Technol. 142 (2015) 251–257. [9] A. Mohamed, T.A. Osman, M.S. Toprak, M. Muhammed, E. Yilmaz, A. Uheida, Visible light photocatalytic reduction of Cr(VI) by surface modified CNT/titanium dioxide composites nanofibers, J. Mol. Catal. A: Chem. 424 (2016) 45–53. [10] R. Nagarjuna, S. Challagulla, R. Ganesan, S. Roy, High rates of Cr(VI) photoreduction with magnetically recoverable nano-Fe3O4@Fe2O3/Al2O3 catalyst under visible light, Chem. Eng. J. 308 (2017) 59–66. [11] Y. Ku, I.-L. Jung, Photocatalytic reduction of Cr(VI) in aqueous solutions by UV irradiation with the presence of titanium dioxide, Water Res. 35 (2001) 135–142. [12] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, TiO2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants, Carbon 49 (2011) 2693–2701. [13] Z. Lin, J. Li, Z. Zheng, J. Yan, P. Liu, C. Wang, G. Yang, Electronic reconstruction of
3.4. Proposed mechanisms of Cr(VI) reduction The possible mechanisms involved in the photocatalytic reduction of aqueous Cr(VI) over Bi2Zr2O7 under visible-light (λ > 420 nm) irradiation were proposed in Fig. 8. The electrons in the valence band of Bi2Zr2O7 can be excited by visible light irradiation to its conduction band, and at the same time, an equal number of holes generated in its valence band. The photogenerated electrons reduce the adsorped 138
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[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
[27] [28] [29] [30] [31] [32]
α-Ag2WO4 nanorods for visible-light photocatalysis, ACS Nano 9 (2015) 7256–7265. Y. Yang, W. Que, X. Zhang, X. Yin, Y. Xing, M. Que, H. Zhao, Y. Du, High-quality Cu2ZnSnS4 and Cu2ZnSnSe4 nanocrystals hybrid with ZnO and NaYF4: Yb, Tm as efficient photocatalytic sensitizers, Appl. Catal., B 200 (2017) 402–411. A. Mukherji, B. Seger, G.Q. Lu, L. Wang, Nitrogen doped Sr2Ta2O7 coupled with graphene sheets as photocatalysts for increased photocatalytic hydrogen production, ACS Nano 5 (2011) 3483–3492. M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Oxide pyrochlores — a review, Prog. Solid State Chem. 15 (1983) 55–143. V.M. Sharma, D. Saha, G. Madras, T.N.G. Row, Synthesis, structure, characterization and photocatalytic activity of Bi2Zr2O7 under solar radiation, RSC Adv. 3 (2013) 18938–18943. D. Wu, T. He, J. Xia, Y. Tan, Preparation and photocatalytic properties of Bi2Zr2O7 photocatalyst, Mater. Lett. 156 (2015) 195–197. T. He, D. Wu, Y. Tan, H. Tan, Ag/AgI modified Bi2Zr2O7 nanosheets with excellent photocatalytic activity, Mater. Lett. 193 (2017) 210–212. Q. Yang, S. Kang, H. Chen, W. Bu, J. Mu, La2Ti2O7: An efficient and stable photocatalyst for the photoreduction of Cr(VI) ions in water, Desalination 266 (2011) 149–153. K. Vignesh, R. Priyanka, M. Rajarajan, A. Suganthi, Photoreduction of Cr(VI) in water using Bi2O3–ZrO2 nanocomposite under visible light irradiation, Mater. Sci. Eng. B 178 (2013) 149–157. W. Yang, L. Zhang, Y. Hu, Y. Zhong, H.B. Wu, X.W. Lou, Microwave-assisted synthesis of porous Ag2S–Ag hybrid nanotubes with high visible-light photocatalytic activity, Angew. Chem. Int. Ed. 51 (2012) 11501–11504. N. Elkhoshkhany, A. Hafnway, A. Khaled, Electrodeposition and corrosion behavior of nano-structured Ni-WC and Ni-Co-WC composite coating, J. Alloys Compd. 695 (2017) 1505–1514. H. Li, Z. Su, S. Hu, Y. Yan, Free-standing and flexible Cu/Cu2O/CuO heterojunction net: a novel material as cost-effective and easily recycled visible-light photocatalyst, Appl. Catal., B 207 (2017) 134–142. M. Li, Z. Qiang, C. Pulgarin, J. Kiwi, Accelerated methylene blue (MB) degradation by Fenton reagent exposed to UV or VUV/UV light in an innovative micro photoreactor, Appl. Catal., B 187 (2016) 83–89. D. Lu, S. Ouyang, H. Xu, D. Li, X. Zhang, Y. Li, J. Ye, Designing Au surface-modified nanoporous-single-crystalline SrTiO3 to optimize diffusion of surface plasmon resonance-induce photoelectron toward enhanced visible-light photoactivity, ACS Appl. Mater. Interfaces 8 (2016) 9506–9513. L. Zheng, S. Han, H. Liu, P. Yu, X. Fang, Hierarchical MoS2 nanosheet@TiO2 nanotube array composites with enhanced photocatalytic and photocurrent performances, Small 12 (2016) 1527–1536. B. Vijaya Kumar, R. Velchuri, G. Prasad, B. Sreedhar, K. Ravikumar, M. Vithal, Preparation, characterization, photoactivity and XPS studies of Ln2ZrTiO7 (Ln = Sm and Nd), Ceram. Int. 36 (2010) 1347–1355. X. Gao, G. Huang, H. Gao, C. Pan, H. Wang, J. Yan, Y. Liu, H. Qiu, N. Ma, J. Gao, Facile fabrication of Bi2S3/SnS2 heterojunction photocatalysts with efficient photocatalytic activity under visible light, J. Alloys Compd. 674 (2016) 98–108. Z.A. Tan, S. Li, F. Wang, D. Qian, J. Lin, J. Hou, Y. Li, High performance polymer solar cells with as-prepared zirconium acetylacetonate film as cathode buffer layer, Sci. Rep. 4 (2014) 4691. S.K. Gupta, C. Reghukumar, M. Keskar, R.M. Kadam, Revealing the oxidation number and local coordination of uranium in Nd2Zr2O7 pyrochlore: a photoluminescence study, J. Lumin. 177 (2016) 166–171. K. Vinod, K. Vijay, S. Som, J.H. Neethling, O. Ezra, O.M. Ntwaeaborwa, H.C. Swart, The role of surface and deep-level defects on the emission of tin oxide quantum dots, Nanotechnology 25 (2014) 135701.
[33] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting physisorption data for gas/solid systems, Handbook of Heterogeneous Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, 2008. [34] L. Li, B. Cheng, Y. Wang, J. Yu, Enhanced photocatalytic H2-production activity of bicomponent NiO/TiO2 composite nanofibers, J. Colloid Interface Sci. 449 (2015) 115–121. [35] S. Chen, Y. Hu, S. Meng, X. Fu, Study on the separation mechanisms of photogenerated electrons and holes for composite photocatalysts g-C3N4-WO3, Appl. Catal., B 150–151 (2014) 564–573. [36] Y. Tong, X. Chen, Q. Wang, H. Huo, Crystal growth dynamics and structural characterization of Re-doped Y2Zr2O7 nanocrystals, Mater. Lett. 157 (2015) 106–108. [37] R. Sun, Q. Shi, M. Zhang, L. Xie, J. Chen, X. Yang, M. Chen, W. Zhao, Enhanced photocatalytic oxidation of toluene with a coral-like direct Z-scheme BiVO4/g-C3N4 photocatalyst, J. Alloys Compd. 714 (2017) 619–626. [38] J. Liu, Y. Zhao, J. Ma, Y. Dai, J. Li, J. Zhang, Flower-like ZnO hollow microspheres on ceramic mesh substrate for photocatalytic reduction of Cr(VI) in tannery wastewater, Ceram. Int. 42 (2016) 15968–15974. [39] L. Shi, T. Wang, H. Zhang, K. Chang, X. Meng, H. Liu, J. Ye, An amine-functionalized iron(III) metal–organic framework as efficient visible-light photocatalyst for Cr(VI) reduction, Adv. Sci. 2 (2015) 1500006-n/a. [40] Y. Luo, L. Cao, J. Huang, L. Feng, C. Yao, A new approach to preparing Bi2Zr2O7 photocatalysts for dye degradation, Mater. Res. Express 5 (2018) 015039. [41] X. Gu, Y. Deng, C. Wang, Fabrication of anion-exchange polymer layered graphenemelamine electrodes for membrane capacitive deionization, ACS Sustain. Chem. Eng. 5 (2017) 325–333. [42] J. Melcher, N. Barth, C. Schilde, A. Kwade, D. Bahnemann, Influence of TiO2 agglomerate and aggregate sizes on photocatalytic activity, J. Mater. Sci. 52 (2017) 1047–1056. [43] H. Xu, J. Yan, Y. Xu, Y. Song, H. Li, J. Xia, C. Huang, H. Wan, Novel visible-lightdriven AgX/graphite-like C3N4 (X=Br, I) hybrid materials with synergistic photocatalytic activity, Appl. Catal., B 129 (2013) 182–193. [44] J. Fang, H. Fan, Y. Ma, Z. Wang, Q. Chang, Surface defects control for ZnO nanorods synthesized by quenching and their anti-recombination in photocatalysis, Appl. Surf. Sci. 332 (2015) 47–54. [45] A.A. Ismail, D.W. Bahnemann, Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms, J. Mater. Chem. 21 (2011) 11686–11707. [46] Y. Guo, J. Li, Z. Gao, X. Zhu, Y. Liu, Z. Wei, W. Zhao, C. Sun, A simple and effective method for fabricating novel p–n heterojunction photocatalyst g-C3N4/Bi4Ti3O12 and its photocatalytic performances, Appl. Catal., B 192 (2016) 57–71. [47] K. Li, S. Gao, Q. Wang, H. Xu, Z. Wang, B. Huang, Y. Dai, J. Lu, In-situ-reduced synthesis of Ti3+ self-doped TiO2/g-C3N4 heterojunctions with high photocatalytic performance under LED light irradiation, ACS Appl. Mater. Interfaces 7 (2015) 9023–9030. [48] J. Rong, T. Zhang, F. Qiu, X. Rong, X. Zhu, X. Zhang, Preparation of hierarchical micro/nanostructured Bi2S3-WO3 composites for enhanced photocatalytic performance, J. Alloys Compd. 685 (2016) 812–819. [49] X. Gao, H.B. Wu, L. Zheng, Y. Zhong, Y. Hu, X.W. Lou, Formation of mesoporous heterostructured BiVO4/Bi2S3 hollow discoids with enhanced photoactivity, Angew. Chem. Int. Ed. 53 (2014) 5917–5921. [50] J. Shamblin, M. Feygenson, J. Neuefeind, C.L. Tracy, F. Zhang, S. Finkeldei, D. Bosbach, H. Zhou, R.C. Ewing, M. Lang, Probing disorder in isometric pyrochlore and related complex oxides, Nat. Mater. 15 (2016) 507–511. [51] C. Wang, X. Zhang, Y. Liu, Promotion of multi-electron transfer for enhanced photocatalysis: a review focused on oxygen reduction reaction, Appl. Surf. Sci. 358 (Part A) (2015) 28–45.
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Synthesis, characterization and photocatalytic properties of nanoscale pyrochlore type Bi2Zr2O7
T
Yijia Luoa, Liyun Caoa, , Liangliang Fenga, , Jianfeng Huanga, Liuqing Yanga, Chunyan Yaob, Yayi Chenga ⁎
a b
⁎
School of Material Science and Engineering, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, PR China School of Arts and Sciences, Shaanxi University of Science and Technology, Xi’an, Shaanxi 710021, PR China
ARTICLE INFO
ABSTRACT
Keywords: Bi2Zr2O7 Photocatalytic properties Cr(VI) reduction Coprecipitation method
Developing efficient visible-light-driven photocatalysts for environmental decontamination is of considerable importance in solar energy conversion and utilization. Herein, nanoscale Bi2Zr2O7 pyrochlore structure materials was synthesized by a facile coprecipitation method and the influence of nano-size effects on its photocatalytic reduction of Cr(VI) under visible light was investigated. Results show that the resultant Bi2Zr2O7 prepared at 2 h optimum calcination time with uniform small crystal size leading to a high specific surface area exhibits the best photocatalytic performance. In addition, the possible mechanisms of the photocatalytic reduction of Cr(VI) over Bi2Zr2O7 were proposed.
1. Introduction Hexavalent Chromium (Cr(VI)), one of the most toxic and highly soluble heavy metal ion, discharged from leather tanning, mining, electroplating, chromate manufacturing, adversely affects the environment and human health [1–3]. Consequently, effective treatment for removing Cr(VI) from waste water is vital for human living environment. At present, techniques for removing Cr(VI) from waste water, including chemical precipitation [4], ion exchange [5], adsorption [6], membrane separation [7], and reduction [8] have been used. In the above methods, the reduction of Cr(VI)–Cr(III) is considered to be a preferred treatment for the reason that Cr(III) is environmental friendly and also an important trace element for human health [9]. Therefore, given the low cost, reusability and no production of secondary pollutants, photocatalytic reduction of Cr(VI) performed under solar light irradiation is regarded as an attractive technology for waste water treatment [8,10]. The study of photocatalytic reduction of Cr(VI) over TiO2 has been reported extensively [11,12]. Nevertheless, TiO2 presents photocatalytic activity mainly under UV light taking up less than 5% of solar radiation [13–15]. Therefore, the development of photocatalytic active materials which can effectively reduce Cr(VI) under visible light is very indispensable for solar energy utilization. As a visible-light-driven photocatalytic material, the Bi2Zr2O7 has been reported to be used in fields of degradation of Indigo carmine, Ramazoline brilliant blue and
⁎
Methyl orange dyes [16–19]. Being a close family material of Bi2Zr2O7, La2Ti2O7 has a high photocatalytic reduction of Cr(VI) under mercury lamp irradiation in the literature report [20]. Therefore, the photocatalytic properties of Bi2Zr2O7 in reducing Cr(VI) under visible light irradiation also have potential for research. Before our work, Vignesh et al. reported the reduction of Cr(VI) with Bi2O3-ZrO2 under 300 W Xe arc lamp irradiation with a 400 nm cutoff filter, the activity of catalysts for the photoreduction of Cr(VI) was studied with an initial Cr(VI) concentration of 0.1 mM, catalyst concentration of 1.25 g/L and irradiation time of 180 min and the photocatalytic activity of the photocatalytic is 92.3% in 180 min under irradiation [21]. In this work, we synthesized nano-sized Bi2Zr2O7 via a facile coprecipitation method, and investigated the effects of nano-size on its morphology and the photocatalytic reduction of Cr(VI) under visible light irradiation. More specifically, the resulting Bi2Zr2O7 prepared in 2 h calcination showed a higher photocatalytic activity for Cr(VI) reduction under visible-light irradiation (λ > 420 nm) than other samples prepared in 4 h and 6 h calcination, for the reason that its uniform smaller nanoparticles lead to a higher specific surface area. Besides, the mechanism of photocatalytic Cr(VI) removal also have been studied. Compared with previous work, 1) Bi2Zr2O7 nanoparticles with pyrochlore type structure were synthesized by coprecipitation method in a rather lower temperature and shorter calcination time (600 °C, 2 h); 2) the effect of pH on Bi2Zr2O7 photocatalytic reduction of Cr(VI) was investigated, confirmed that the photocatalytic reduction of Cr(VI) is an
Corresponding authors. E-mail addresses: [email protected] (L. Cao), [email protected] (L. Feng).
https://doi.org/10.1016/j.mseb.2019.01.017 Received 18 September 2017; Received in revised form 20 December 2018; Accepted 19 January 2019 Available online 28 January 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
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acid catalyzed behavior; 3) the photocatalyst shows a good stability and reusability, by means of XRD, SEM and photo-reduction experiments, and there are no obvious changes for its phase, morphology and performance. This work expands the variety of materials that applied in photocatalytic reduction of Cr(VI) under visible light irradiation. 2. Experimental 2.1. Synthesis of Bi2Zr2O7 All the reagents used in the experiments were of analytical grade. The molar ratio of bismuth nitrate and zirconium nitrate was 1:1. In a typical synthetic procedure towards the resulting Bi2Zr2O7 could be illustrated clearly as follows. 3 mM Bi(NO3)3·5H2O was dispersed in 12 mL HNO3 solution (2 M), subsequently added with Zr(NO3)4 solution, which was prepared by dissolving 3 mM Zr(NO3)4·5H2O into 18 mL deionized water. The pH value was adjusted with NH3·H2O solution to 10 to obtain the precursor solution. After being continuously stirred for 2 h at room temperature, the precipitate was collected and washed with deionized water and absolute ethyl alcohol for three times, respectively. And then dried at the 60 °C for 4 h. Finally, the obtained powder was calcined in air at 600 °C for a certain time (1, 2, 4 and 6 h).
Fig. 1. XRD patterns of Bi2Zr2O7 prepared at various calcination time.
electrode, the saturated calomel electrode (SCE) is the reference electrode and the Bi2Zr2O7 film electrodes as working electrodes. The typical working electrode was prepared as follows: 2 mg sample was mixed with 1 mL absolute ethanol and 0.5 mL nafion to make slurry, and the slurry was then dispersed onto a 4 cm × 2 cm FTO glass and then dried overnight at room temperature to obtain the electrodes. The photoelectrochemical measurements were performed under a 300 W Xe light source with a 420 nm cutoff filter.
2.2. Characterizations The crystal structure of the as-prepared powders was characterized by a powder X-ray diffraction (XRD, Rigaku D/max-2000) with Cu Kα radiation (λ = 0.15406 nm). The morphology study of the products was performed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and a transmission electron microscope (TEM, Tecnai G2 F20, USA), The Brunauer-Emmet-Teller specific surface area was determined through N2 absorption at 77 K using an adsorption instrument (BET, ASAP2460, Micromeritics, USA). The surface analysis was studied by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). The binding energy data was calibrated with the C 1s signal at 284.6 eV. And UV–vis diffuse reflectance spectra (DRS) of the samples were recorded on a UV/vis/NIR spectrophotometer (Agilent Cary 5000). BaSO4 was used as a reflectance standard. The photoluminescence spectra were recorded on a spectrofluorometer (PL, Edinburgh FS5, UK).
3. Results and discussion 3.1. Structure and morphology 3.1.1. XRD analysis Fig. 1 shows the XRD patterns of Bi2Zr2O7 samples prepared under different calcination time, in which it can be seen that a dominant Bi2Zr2O7 has been formed at 1 h, while ZrO2 impurity is also observed, this result indicates that the reaction is not complete at this calcination time. The calcination time increased to 2 h, pure Bi2Zr2O7 has been synthesized successfully, and its diffraction peaks are in agreement with the results earlier reported in literature [17], and no peaks of impurities are detected. On increasing calcination time, the diffraction peaks become stronger, which indicates the better crystallinity of samples. The average crystallite size of these samples is estimated by the Scherrer’s equation, D = Kλ/(βcosθ). Where θ is the Bragg angle, λ is X-ray wavelength (1.54 Ǻ), β is the FWHM of the diffraction peak, K is the constant (K = 0.9) and D is the average crystallite size [23]. The results showed that the crystallite size is 19.7 nm, 21.8 nm, and 23.4 nm (the error for average crystal size should be ± 1.5 nm) when the calcination time is 2 h, 4 h, and 6 h, respectively. It exhibits that the prepared samples are all in nanoscale, and the morphology of samples can be determined by FE-SEM and TEM analysis.
2.3. Photocatalytic reduction of Cr(VI) The photocatalytic activities of samples were evaluated by photocatalytic reduction of Cr(VI) under visible-light irradiation of a 500 W Xe lamp with a 420 nm cutoff filter. In a typical process, 16 mg powder was added to 40 mL of Cr(VI) solution (based on Cr in a dilute K2Cr2O7 solution) with the concentration of 10 ppm on BL-GHX-V photocatalytic reactor (Xi’an, BILON, Co. Ltd.). The pH of the solution was adjusted by using NaOH and H2SO4. After the photocatalyst was dispersed in the solution with an ultrasonic bath for 5 min, the suspensions were magnetically stirred in the dark for 2 h to reach adsorption-desorption equilibrium and then was exposed to visible-light irradiation. After a given irradiation time, 5 mL of the solution was collected by centrifugation (9000 rpm, 6 min), and the Cr(VI) reduction was determined colorimetrically by UV–vis spectrophotometry (Unico, UV-2600) at 540 nm using the diphenylcarbazide (DPC) method with deionized water as a reference sample [22]. The photocatalytic efficiency was determined by C/C0, where C is the remained Cr(VI) concentration and C0 is the concentration at the irradiation time of 0 min.
3.1.2. FE-SEM analysis Fig. 2 presents the FE-SEM spectra of as-prepared Bi2Zr2O7 samples at different calcination time. The particles are all at the nanoscale, which is consistent with the Scherrer’s equation results. In addition, it can be seen that the nanoparticles are not well-dispersed distribution, but gathered together, and there are different degrees of agglomeration. With the calcination time increased, the size of nanoparticles grew bigger and the agglomeration of that became more serious. Therefore, it is concluded that the Fig. 2(a) showed the smaller particle size and a better dispersity among these samples. The formation of larger specific surface area could benefit from the
2.4. Photoelectrochemical measurements Photoelectrochemical measurements were carried out on the electrochemical station (CHI-660B, China) using a three-electrode mode with 0.5 M Na2SO4 solution as the electrolyte. The Pt wire is the counter 134
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Fig. 2. FE-SEM images of Bi2Zr2O7 in (a) 2 h; (b) 4 h; (c) 6 h.
smaller nanoparticles size, which is one of the reasons contributing to high catalytic performance [24]. When the size of the semiconductor particles is smaller than the free path of the photogenerated carriers, the recombination rate of the electron-hole pairs on the surface will be greatly reduced, thereby improving the redox efficiency. Moreover, the smaller particle size also reduces the diffuse reflection of light and increases the absorption of light [25,26].
further confirm the formation of Bi2Zr2O7. 3.1.5. BET analysis The Brunauer-Emmett-Teller (BET) area and pore size distribution of as-prepared Bi2Zr2O7 were characterized by N2 adsorption-desorption analysis. The BET absorption-desorption isotherms of the Bi2Zr2O7 samples prepared in 2, 4, 6 h were obtained from the N2 adsorption-desorption measurements at 77 K, as shown in Fig. 5(a). The shape of the isotherms is a type II isotherm with a type H1 hysteresis loop at high relative pressures according to the IUPAC classification. The type II isotherm is the normal form of isotherm obtained with a nonporous or macroporous adsorbent, while the type H1 hysteresis loop is often associated with porous materials or the existence of agglomeration [33]. Besides, the existence of hysteresis loop in BET isotherm of the as-prepared Bi2Zr2O7 material may be attributed to the formation of mesoporous resulting from particle aggregation. The pore size distribution (Fig. 5(b)) of Bi2Zr2O7 were quite narrow and monomodal, implying that the prepared samples are composed of uniform particles [34]. As shown in Table 1, in which we can obtain the BET surface and detailed parameters of pore for all the samples. Obviously, the BET specific surface areas of the samples decreased with increasing calcination time. Compared to 4 h and 6 h, sample prepared in 2 h presented a slightly higher pore volume and pore size.
3.1.3. TEM analysis The crystal structure of samples was further elucidated by TEM analysis. When the calcination time was controlled at 2 h, it was observed that the diameter of nanoparticles is in the range of 12–22 nm as seen in Fig. 3(a) and Supplementary Information Fig. S1. That is consistent with the crystallite size determined by Scherrer’s method. The HRTEM image is shown in Fig. 3(b), clear lattice fringes reveal the good crystallinity of the resultant Bi2Zr2O7 materials. The observed interplanar spacing of 0.30 nm may correspond to (2 2 2) lattice plane of Bi2Zr2O7, as evidenced from the XRD pattern of it (Fig. 1). 3.1.4. XPS analysis To further confirm the surface compositions and chemical state of the as-prepared sample, XPS was carried out by us [27]. As shown in Fig. 4, the survey spectrum clearly indicates the presence of Bi, Zr and O elements in this Bi2Zr2O7 material (Fig. 4(a)). The C 1s peak appears due to the contamination of organic compounds in the investigation process [28]. Fig. 4(b) shows the high-resolution XPS spectrum of Bi 4f, in which fitting peaks at 158.6 eV and 163.9 eV are assigned to Bi 4f7/2 and Bi 4f5/2 of Bi3+, respectively [29]. The Zr 3d XPS spectrum only displays two peaks at 181.5 eV and 183.9 eV (Fig. 4(c)), which can be assigned to Zr 3d5/3 and Zr 3d3/2 [18], respectively. The separation between Zr 3d5/3 and Zr 3d3/2 peak is observed to be 2.4 eV and the area ratio of the two peaks is 3:2, which is typical of Zr4+ ions in a full oxidation state [30]. The O 1s XPS spectrum of the sample can be split into three peaks at 526.4 eV, 527.8 eV and 529.4 eV. The peak at 526.4 eV is assigned to oxygen ions at 8a sites and the peak at higher bind energy of 529.4 eV is assigned to O at 48f sites, and the peak at 527.8 eV is attributed to oxygen related defects such as oxygen vacancies and oxygen interstitials [31,32]. Consequently, XPS studies
3.2. Optical properties 3.2.1. UV–vis analysis In order to investigate the optical absorption property of the samples, the UV–vis DRS was introduced, and it is known that the optical absorption property is closely related to its electronic structure [35]. Fig. 6 reveals that all the samples have absorption in the visible light region, and the sample calcined for 2 h has the strongest absorption in the visible light region. The smaller crystallite size caused by shortening calcination time, the UV–vis absorption spectrum has a red shift and the absorption intensity of visible light become stronger. It can be concluded that the crystallite size has an effect on the absorbance, which can be ascribed to nano-size effect of particles [36,37]. In addition, the Fig. 3. (a) Transmission electron microscope (TEM) image of the product prepared at 2 h; (b) Corresponding high resolution transmission electron microscopy (HRTEM) image was obtained at the center of the red rectangles in the TEM image. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. XPS spectra of the Bi2Zr2O7 sample: (a) survey spectrum; (b) Bi 4f; (c) Zr 3d; (d) O 1s.
Fig. 5. The N2 adsorption-desorption isotherms (a) and pore size distribution (b) of the samples prepared at 600 °C when calcine time is 2 h, 4 h and 6 h, respectively. Table 1 BET surface area and pore parameters of the prepared samples. Sample
SBET/(m2/g)
Pore volume (cm3/g)
Pore size (nm)
2h 4h 6h
14 13 10
0.100 0.089 0.074
25.5 24.3 23.7
band gap energy (Eg) of the samples could be calculated by using the following Eq. (1):
F(R) E= A(E
Eg) n/2
(1)
where F(R) is the diffuse absorption coefficient, A is the constant, E is photon energy, Eg is the band gap, n is 4 for a direct transition, n is 1 for an indirect transition, and the value of n for Bi2Zr2O7 is 4 [18]. The band gap energy of the sample prepared in 2, 4, and 6 h are estimated to be 2.86, 2.88, and 2.94 eV, respectively, calculated from the intercept
Fig. 6. UV–vis diffuse reflectance spectra of samples prepared at 600 °C (Calcination time is 2 h, 4 h, and 6 h.).
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Fig. 7. Reduction profiles of photocatalytic reduction of Cr(VI) over Bi2Zr2O7 (t = 2 h) (a) at different pH values, (b) over various photocatalysts (pH = 2); (c) photocurrent response of the products prepared under different calcination time; (d) recycling test. Reaction condition: 16 mg photocatalyst, 40 mL of 10 ppm Cr(VI), reaction temperature is 20 °C, the solution is stirred for 2 h in the dark and then is exposed to visible light (λ > 420 nm) irradiation.
of the tangents to the plots [F(R)E]1/2 versus E curves showed in Fig. S2.
oxide semiconductor materials in the previous literature reported, on the condition that all the materials without further doping or composite modification (see Supplementary Table S1). Their apparent reaction rate constants based on the specific surface area can be estimated to be 2.36 × 10−4, 1.35 × 10−4 and 8.0 × 10−5 g·m−2·min−1, respectively (Fig. S3). It exhibited that the sample whose calcination time is 2 h shows the highest reaction rate, and the results probably contributed to the biggest surface area of this sample possess the most surface catalytic active sites compared with others [40]. With the calcination time decreasing, the particles size of the as-prepared Bi2Zr2O7 decreases, leading to a higher specific surface area and thus provide more active sites for photocatalytic reaction. Therefore, the apparent activity of the sample whose calcination time is 2 h is superior than that of 4 h and 6 h. The results revealed that the size of nanoparticles plays an important role in the property of the photocatalyst and the resultant Bi2Zr2O7 with different sizes can be prepared by tuning the calcination time. It is proposed that high-temperature calcination makes nanoparticles possess high surface energy and tend to slightly agglomerate, leading to the reduction of specific surface area [41,42]. Meanwhile, as the calcination time increases, the size of nanoparticles grows bigger, and the specific surface area of samples becomes smaller, which further results in the less active sites per unit weight of photocatalysts. Hence, the sample whose calcination time is 2 h under the same calcination temperature exhibits the best photocatalytic activity. Photocatalytic reaction using semiconductors as catalysts involved the generation, separation, transfer, and recombination of electron-hole pairs. It is a commonly held view that the separation efficiency of electron-hole pairs played a vital role in the photocatalytic reaction [43]. To further understand the charge separation and transfer dynamics of photocatalyst, the photocurrent-time responses of these samples were recorded for several on-off cycles. Fig. 7(c) shows the
3.3. Evaluation of photocatalytic activity Based on the results of the absorption experiment, the photocatalytic reduction of Cr(VI) over Bi2Zr2O7 in water was carried out under visible light irradiation. The photocatalytic reaction was evaluated by monitoring the decolorization of the UV–vis absorption spectra of diphenylcarbazide (DPC)-Cr(VI) complex solutions. To obtain the optimum reaction condition, as Fig. 7(a) displays, the effects of pH on reduction of Cr(VI) catalyzed by Bi2Zr2O7 were studied. The pH value of the K2Cr2O7 solution was adjusted in the range from 2 to 7 by the addition of 0.2 M H2SO4 and NaOH. The experimental results show that the sample displays the best photocatalytic performance at pH = 2, and then confirm that the photocatalytic reduction of Cr(VI) is an acid catalyzed behavior and the acid medium is beneficial to the photocatalytic reduction of Cr(VI) because of the existence of abundant H+ [38]. At the pH range from 2 to 6, the Cr(VI) reduction reaction could be expressed as follows [39]:
Cr2O27 + 14H+ + 6e 2H2 O+ 2h+
2Cr 3 + + 7H2 O
H2 O2 + 2H+
(2) (3)
Fig. 7(b) shows the photocatalytic reduction of Cr(VI) over Bi2Zr2O7 photocatalysts which calcined at different time. In addition, photocatalytic reduction of Cr(VI) without the photocatalyst as the blank contrast sample. It is obvious that the reduction of Cr(VI) hardly occurred in the absence of photocatalyst. Furthermore, sample that calcination time is 2 h exhibits highest activity compared with the other samples (efficiency of Cr(VI) removal achieved 38% for 150 min). Importantly, the performance for the photocatalytic reduction of Cr(VI) over the resulting Bi2Zr2O7 is outstanding compared with other metal 137
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Cr2O72− and H+ to Cr3+ and H2O, meanwhile, the hole oxidize water to H2O2 and H+ in single systems. Since there is a certain amount of oxygen vacancies existing in the pyrochlore structure, which is conducive to the activation of adsorbed oxygen on the surface of Bi2Zr2O7 [17,50]. This kind of active oxygen species can not only prevent the recombination of photogenerated electron hole pairs, but also produce high active hydroxyl radicals, which can promote the photocatalytic oxidation reaction rate [51]. 4. Conclusions In summary, nanoscale Bi2Zr2O7 were synthesized via a facile coprecipitation method, and the effects of nano-size on its photocatalytic performance for Cr(VI) reduction under visible light irradiation were investigated. The catalyst with smaller particle size, better dispersion, larger surface area shows the red-shift adsorption in the visible light region and lower recombination rate of photogenerated charge carriers, resulting in better performance for reduction of Cr(VI). And the possible mechanism of photocatalytic reduction of Cr(VI) is also proposed. Identifying Bi2Zr2O7 as a photocatalyst for reducing Cr(VI) under visible light irradiation is expected to provide a fresh impetus to explore new catalysts for pollution elimination.
Fig. 8. Proposed schematic diagram for photocatalytic reduction of aqueous Cr (VI) over Bi2Zi2O7 under visible-light (λ > 420 nm) irradiation.
transient photocurrent responses of Bi2Zr2O7 prepared at the different calcination time under intermittent visible light illumination. The photocurrent for Bi2Zr2O7 that calcination time is 2 h (0.040 μA/cm2) revealed to be much higher than those of 4 h (0.027 μA/cm2) and 6 h (0.020 μA/cm2) upon light irradiation. The enhanced photocurrent response of the as-prepared Bi2Zr2O7 that calcination time is 2 h indicates smaller nanoparticles has a positive effect on the higher separation efficiency and a lower recombination rate of the photoinduced electronhole pairs. This is probably because the shorter the calcination time is, the smaller the particles grow, the more defects are retained in the materials, which is beneficial to prevent the recombination of photogenerated electron holes [44,45]. The migration, transfer, and recombination of the photogenerated electron-hole pairs can be explored from photoluminescence spectra of the semiconductor and lower fluorescence emission intensity implies a lower recombination rate of electron-hole pairs [46–48]. Fig. S4 Shows the PL spectra of the synthesized with different calcination time range from 2 to 6 h excited by 352 nm. It could be found that the main emission peak was centered at about 483 nm for Bi2Zr2O7. From the figure, it can be observed that with the calcination time decreased the emission intensity decreased, which indicated that the Bi2Zr2O7 we synthesized with smaller nanoparticles had a much lower recombination rate of photogenerated charge carriers. Out of the economic point of view, the reusability and stability of the photocatalysts are very important. The reusability of the Bi2Zr2O7 has been studied by collecting and reusing the photocatalyst for three times, as shown in Fig. 7(d), the variation of photocatalytic activity in every cycle was similar. Only insignificant decrease compared to that for the first use is observed, which might be partly caused by loss of photocatalyst during each collection [49]. The used photocatalyst can be collected by washing with water and ethanol for several times. The XRD patterns of Bi2Zr2O7 before and after the photocatalytic reaction did not show obvious changes in the crystal structure (Fig. S5). SEM image after photocatalytic reaction at pH = 2 was measured and shown in Fig. S6. It shows that the morphology of Bi2Zr2O7 is maintained at pH = 2. The data above indicate the stability of the photocatalyst in this reaction conditions. As shown in Fig. S7, the XPS spectra of samples measured before and after photocatalytic reaction are almost the same, which further confirm the stability of the photocatalyst.
Acknowledgments The authors would like to thank the support from the National Natural Science Foundation of China (No. 51672165, No. 21701107, No. 61875231), the Support Program of Shaanxi Province Youth Science and Technology Leading Talents. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.mseb.2019.01.017. References [1] J. Yu, S. Zhuang, X. Xu, W. Zhu, B. Feng, J. Hu, Photogenerated electron reservoir in hetero-p-n CuO-ZnO nanocomposite device for visible-light-driven photocatalytic reduction of aqueous Cr(VI), J. Mater. Chem. A 3 (2015) 1199–1207. [2] H. Wang, X. Yuan, Y. Wu, G. Zeng, X. Chen, L. Leng, Z. Wu, L. Jiang, H. Li, Facile synthesis of amino-functionalized titanium metal-organic frameworks and their superior visible-light photocatalytic activity for Cr(VI) reduction, J. Hazard. Mater. 286 (2015) 187–194. [3] S. Rajput, C.U. Pittman, D. Mohan, Magnetic magnetite (Fe3O4) nanoparticle synthesis and applications for lead (Pb2+) and chromium (Cr6+) removal from water, J. Colloid Interface Sci. 468 (2016) 334–346. [4] A. Salmani Abyaneh, M.H. Fazaelipoor, Evaluation of rhamnolipid (RL) as a biosurfactant for the removal of chromium from aqueous solutions by precipitate flotation, J. Environ. Manage. 165 (2016) 184–187. [5] K. Xiao, F. Xu, L. Jiang, N. Duan, S. Zheng, Resin oxidization phenomenon and its influence factor during chromium(VI) removal from wastewater using gel-type anion exchangers, Chem. Eng. J. 283 (2016) 1349–1356. [6] Y. Lu, B. Jiang, L. Fang, F. Ling, J. Gao, F. Wu, X. Zhang, High performance NiFe layered double hydroxide for methyl orange dye and Cr(VI) adsorption, Chemosphere 152 (2016) 415–422. [7] Y. Bao, X. Yan, W. Du, X. Xie, Z. Pan, J. Zhou, L. Li, Application of amine-functionalized MCM-41 modified ultrafiltration membrane to remove chromium (VI) and copper (II), Chem. Eng. J. 281 (2015) 460–467. [8] Y. Zhang, Q. Zhang, Q. Shi, Z. Cai, Z. Yang, Acid-treated g-C3N4 with improved photocatalytic performance in the reduction of aqueous Cr(VI) under visible-light, Sep. Purif. Technol. 142 (2015) 251–257. [9] A. Mohamed, T.A. Osman, M.S. Toprak, M. Muhammed, E. Yilmaz, A. Uheida, Visible light photocatalytic reduction of Cr(VI) by surface modified CNT/titanium dioxide composites nanofibers, J. Mol. Catal. A: Chem. 424 (2016) 45–53. [10] R. Nagarjuna, S. Challagulla, R. Ganesan, S. Roy, High rates of Cr(VI) photoreduction with magnetically recoverable nano-Fe3O4@Fe2O3/Al2O3 catalyst under visible light, Chem. Eng. J. 308 (2017) 59–66. [11] Y. Ku, I.-L. Jung, Photocatalytic reduction of Cr(VI) in aqueous solutions by UV irradiation with the presence of titanium dioxide, Water Res. 35 (2001) 135–142. [12] G. Jiang, Z. Lin, C. Chen, L. Zhu, Q. Chang, N. Wang, W. Wei, H. Tang, TiO2 nanoparticles assembled on graphene oxide nanosheets with high photocatalytic activity for removal of pollutants, Carbon 49 (2011) 2693–2701. [13] Z. Lin, J. Li, Z. Zheng, J. Yan, P. Liu, C. Wang, G. Yang, Electronic reconstruction of
3.4. Proposed mechanisms of Cr(VI) reduction The possible mechanisms involved in the photocatalytic reduction of aqueous Cr(VI) over Bi2Zr2O7 under visible-light (λ > 420 nm) irradiation were proposed in Fig. 8. The electrons in the valence band of Bi2Zr2O7 can be excited by visible light irradiation to its conduction band, and at the same time, an equal number of holes generated in its valence band. The photogenerated electrons reduce the adsorped 138
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[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
[27] [28] [29] [30] [31] [32]
α-Ag2WO4 nanorods for visible-light photocatalysis, ACS Nano 9 (2015) 7256–7265. Y. Yang, W. Que, X. Zhang, X. Yin, Y. Xing, M. Que, H. Zhao, Y. Du, High-quality Cu2ZnSnS4 and Cu2ZnSnSe4 nanocrystals hybrid with ZnO and NaYF4: Yb, Tm as efficient photocatalytic sensitizers, Appl. Catal., B 200 (2017) 402–411. A. Mukherji, B. Seger, G.Q. Lu, L. Wang, Nitrogen doped Sr2Ta2O7 coupled with graphene sheets as photocatalysts for increased photocatalytic hydrogen production, ACS Nano 5 (2011) 3483–3492. M.A. Subramanian, G. Aravamudan, G.V. Subba Rao, Oxide pyrochlores — a review, Prog. Solid State Chem. 15 (1983) 55–143. V.M. Sharma, D. Saha, G. Madras, T.N.G. Row, Synthesis, structure, characterization and photocatalytic activity of Bi2Zr2O7 under solar radiation, RSC Adv. 3 (2013) 18938–18943. D. Wu, T. He, J. Xia, Y. Tan, Preparation and photocatalytic properties of Bi2Zr2O7 photocatalyst, Mater. Lett. 156 (2015) 195–197. T. He, D. Wu, Y. Tan, H. Tan, Ag/AgI modified Bi2Zr2O7 nanosheets with excellent photocatalytic activity, Mater. Lett. 193 (2017) 210–212. Q. Yang, S. Kang, H. Chen, W. Bu, J. Mu, La2Ti2O7: An efficient and stable photocatalyst for the photoreduction of Cr(VI) ions in water, Desalination 266 (2011) 149–153. K. Vignesh, R. Priyanka, M. Rajarajan, A. Suganthi, Photoreduction of Cr(VI) in water using Bi2O3–ZrO2 nanocomposite under visible light irradiation, Mater. Sci. Eng. B 178 (2013) 149–157. W. Yang, L. Zhang, Y. Hu, Y. Zhong, H.B. Wu, X.W. Lou, Microwave-assisted synthesis of porous Ag2S–Ag hybrid nanotubes with high visible-light photocatalytic activity, Angew. Chem. Int. Ed. 51 (2012) 11501–11504. N. Elkhoshkhany, A. Hafnway, A. Khaled, Electrodeposition and corrosion behavior of nano-structured Ni-WC and Ni-Co-WC composite coating, J. Alloys Compd. 695 (2017) 1505–1514. H. Li, Z. Su, S. Hu, Y. Yan, Free-standing and flexible Cu/Cu2O/CuO heterojunction net: a novel material as cost-effective and easily recycled visible-light photocatalyst, Appl. Catal., B 207 (2017) 134–142. M. Li, Z. Qiang, C. Pulgarin, J. Kiwi, Accelerated methylene blue (MB) degradation by Fenton reagent exposed to UV or VUV/UV light in an innovative micro photoreactor, Appl. Catal., B 187 (2016) 83–89. D. Lu, S. Ouyang, H. Xu, D. Li, X. Zhang, Y. Li, J. Ye, Designing Au surface-modified nanoporous-single-crystalline SrTiO3 to optimize diffusion of surface plasmon resonance-induce photoelectron toward enhanced visible-light photoactivity, ACS Appl. Mater. Interfaces 8 (2016) 9506–9513. L. Zheng, S. Han, H. Liu, P. Yu, X. Fang, Hierarchical MoS2 nanosheet@TiO2 nanotube array composites with enhanced photocatalytic and photocurrent performances, Small 12 (2016) 1527–1536. B. Vijaya Kumar, R. Velchuri, G. Prasad, B. Sreedhar, K. Ravikumar, M. Vithal, Preparation, characterization, photoactivity and XPS studies of Ln2ZrTiO7 (Ln = Sm and Nd), Ceram. Int. 36 (2010) 1347–1355. X. Gao, G. Huang, H. Gao, C. Pan, H. Wang, J. Yan, Y. Liu, H. Qiu, N. Ma, J. Gao, Facile fabrication of Bi2S3/SnS2 heterojunction photocatalysts with efficient photocatalytic activity under visible light, J. Alloys Compd. 674 (2016) 98–108. Z.A. Tan, S. Li, F. Wang, D. Qian, J. Lin, J. Hou, Y. Li, High performance polymer solar cells with as-prepared zirconium acetylacetonate film as cathode buffer layer, Sci. Rep. 4 (2014) 4691. S.K. Gupta, C. Reghukumar, M. Keskar, R.M. Kadam, Revealing the oxidation number and local coordination of uranium in Nd2Zr2O7 pyrochlore: a photoluminescence study, J. Lumin. 177 (2016) 166–171. K. Vinod, K. Vijay, S. Som, J.H. Neethling, O. Ezra, O.M. Ntwaeaborwa, H.C. Swart, The role of surface and deep-level defects on the emission of tin oxide quantum dots, Nanotechnology 25 (2014) 135701.
[33] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Reporting physisorption data for gas/solid systems, Handbook of Heterogeneous Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, 2008. [34] L. Li, B. Cheng, Y. Wang, J. Yu, Enhanced photocatalytic H2-production activity of bicomponent NiO/TiO2 composite nanofibers, J. Colloid Interface Sci. 449 (2015) 115–121. [35] S. Chen, Y. Hu, S. Meng, X. Fu, Study on the separation mechanisms of photogenerated electrons and holes for composite photocatalysts g-C3N4-WO3, Appl. Catal., B 150–151 (2014) 564–573. [36] Y. Tong, X. Chen, Q. Wang, H. Huo, Crystal growth dynamics and structural characterization of Re-doped Y2Zr2O7 nanocrystals, Mater. Lett. 157 (2015) 106–108. [37] R. Sun, Q. Shi, M. Zhang, L. Xie, J. Chen, X. Yang, M. Chen, W. Zhao, Enhanced photocatalytic oxidation of toluene with a coral-like direct Z-scheme BiVO4/g-C3N4 photocatalyst, J. Alloys Compd. 714 (2017) 619–626. [38] J. Liu, Y. Zhao, J. Ma, Y. Dai, J. Li, J. Zhang, Flower-like ZnO hollow microspheres on ceramic mesh substrate for photocatalytic reduction of Cr(VI) in tannery wastewater, Ceram. Int. 42 (2016) 15968–15974. [39] L. Shi, T. Wang, H. Zhang, K. Chang, X. Meng, H. Liu, J. Ye, An amine-functionalized iron(III) metal–organic framework as efficient visible-light photocatalyst for Cr(VI) reduction, Adv. Sci. 2 (2015) 1500006-n/a. [40] Y. Luo, L. Cao, J. Huang, L. Feng, C. Yao, A new approach to preparing Bi2Zr2O7 photocatalysts for dye degradation, Mater. Res. Express 5 (2018) 015039. [41] X. Gu, Y. Deng, C. Wang, Fabrication of anion-exchange polymer layered graphenemelamine electrodes for membrane capacitive deionization, ACS Sustain. Chem. Eng. 5 (2017) 325–333. [42] J. Melcher, N. Barth, C. Schilde, A. Kwade, D. Bahnemann, Influence of TiO2 agglomerate and aggregate sizes on photocatalytic activity, J. Mater. Sci. 52 (2017) 1047–1056. [43] H. Xu, J. Yan, Y. Xu, Y. Song, H. Li, J. Xia, C. Huang, H. Wan, Novel visible-lightdriven AgX/graphite-like C3N4 (X=Br, I) hybrid materials with synergistic photocatalytic activity, Appl. Catal., B 129 (2013) 182–193. [44] J. Fang, H. Fan, Y. Ma, Z. Wang, Q. Chang, Surface defects control for ZnO nanorods synthesized by quenching and their anti-recombination in photocatalysis, Appl. Surf. Sci. 332 (2015) 47–54. [45] A.A. Ismail, D.W. Bahnemann, Mesoporous titania photocatalysts: preparation, characterization and reaction mechanisms, J. Mater. Chem. 21 (2011) 11686–11707. [46] Y. Guo, J. Li, Z. Gao, X. Zhu, Y. Liu, Z. Wei, W. Zhao, C. Sun, A simple and effective method for fabricating novel p–n heterojunction photocatalyst g-C3N4/Bi4Ti3O12 and its photocatalytic performances, Appl. Catal., B 192 (2016) 57–71. [47] K. Li, S. Gao, Q. Wang, H. Xu, Z. Wang, B. Huang, Y. Dai, J. Lu, In-situ-reduced synthesis of Ti3+ self-doped TiO2/g-C3N4 heterojunctions with high photocatalytic performance under LED light irradiation, ACS Appl. Mater. Interfaces 7 (2015) 9023–9030. [48] J. Rong, T. Zhang, F. Qiu, X. Rong, X. Zhu, X. Zhang, Preparation of hierarchical micro/nanostructured Bi2S3-WO3 composites for enhanced photocatalytic performance, J. Alloys Compd. 685 (2016) 812–819. [49] X. Gao, H.B. Wu, L. Zheng, Y. Zhong, Y. Hu, X.W. Lou, Formation of mesoporous heterostructured BiVO4/Bi2S3 hollow discoids with enhanced photoactivity, Angew. Chem. Int. Ed. 53 (2014) 5917–5921. [50] J. Shamblin, M. Feygenson, J. Neuefeind, C.L. Tracy, F. Zhang, S. Finkeldei, D. Bosbach, H. Zhou, R.C. Ewing, M. Lang, Probing disorder in isometric pyrochlore and related complex oxides, Nat. Mater. 15 (2016) 507–511. [51] C. Wang, X. Zhang, Y. Liu, Promotion of multi-electron transfer for enhanced photocatalysis: a review focused on oxygen reduction reaction, Appl. Surf. Sci. 358 (Part A) (2015) 28–45.
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