ZnO ternary photocatalyst for synergetic removal of aqueous organic pollutants and Cr(VI) ions

Science of the Total Environment 706 (2020) 136026

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Fabrication and characterization of ZnTiO3/Zn2Ti3O8/ZnO ternary photocatalyst for synergetic removal of aqueous organic pollutants and Cr(VI) ions Fanyun Chen a,b, Changlin Yu a,b,⁎, Longfu Wei a, Qizhe Fan a, Fei Ma a, Julan Zeng c, Junhui Yi a, Kai Yang b, Hongbing Ji a,⁎⁎ a School of Chemical Engineering, Guangdong Provincial Key Laboratory of Petrochemical Pollution Processes and Control, Guangdong University of Petrochemical Technology, Maoming, Guangdong 525000, China b School of Chemistry and Chemical Engineering, Jiangxi University of Science and Technology, 86 Hongqi Road, Ganzhou 341000, China c Department of Chemistry, Changsha University of Science and Technology, Changsha 410114, Hunan, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• A two-step route for building ZnTiO3/ Zn2Ti3O8/ZnO nanostructure was developed. • Experiment parameters on the crystal structure and photocatalytic performance were optimized. • The excellent removal efficiency for organic pollutants and Cr(VI) ions were obtained simultaneously. • A unique Z-scheme could illuminate the high removal performance.

a r t i c l e

i n f o

Article history: Received 29 September 2019 Received in revised form 23 November 2019 Accepted 7 December 2019 Available online 09 December 2019 Editor: Baoliang Chen Keywords: Organic pollutants degradation Cr(VI) removal Photocatalysis Z-scheme Ternary photocatalyst

a b s t r a c t Highly efficient photocatalysts have great development prospects in wastewater treatment, especially in the degradation of organic pollutants and reduction of inorganic heavy metal ions. Herein, a Z-scheme ZnTiO3/Zn2Ti3O8/ ZnO ternary photocatalyst was prepared by the solvothermal-calcination method and the influence of the content of tetrabutyl titanate precursor and different reaction temperature on the crystal phase structures, photoelectrochemical properties and photocatalytic activities of the samples were investigated. Due to its unique Z-scheme structure and suitable band gap position, which is favorable for the efficient migration and separation of photo-generated electrons and holes and the improvement of photocatalytic redox reaction capability, the samples show excellent performance for the degradation of organic pollutants and reduction of heavy metal Cr (VI) ions. Based on a series of characterization analyses, a possible Z-scheme photocatalytic mechanism is proposed. This work provides a simple preparation method for fabrication of multivariate heterojunction photocatalyst for degradation of organic pollutants and removal of heavy metal ions. © 2019 Elsevier B.V. All rights reserved.

⁎ Correspondence to: C. Yu, School of Chemical Engineering, Guangdong Provincial Key Laboratory of Petrochemical Pollution Processes and Control, Guangdong University of Petrochemical Technology, Maoming, Guangdong 525000, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (C. Yu), [email protected] (H. Ji).

https://doi.org/10.1016/j.scitotenv.2019.136026 0048-9697/© 2019 Elsevier B.V. All rights reserved.

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1. Introduction Nowadays, environmental problems have become prominent, among which industrial wastewater and domestic wastewater are the most serious pollution to the environment (Sheng et al., 2017; H.H. Wang et al., 2019; Ye et al., 2017a). There are a variety of pollutants in the wastewater, including various organic dyes, phenolic organic substances, antibiotics, as well as inorganic heavy metal ions, such as Cr (VI), Pb(II) and Hg(II) (Shannon et al., 2008; Zhao et al., 2017). These pollutants directly damage the ecological environment or indirectly harm human health through the transmission of food chain (Dong et al., 2015). The existing wastewater treatment technologies manly include physical adsorption methods, biological treatment techniques and chemical methods. However, these traditional technologies are always not effective in removing these persistent organic pollutant or highly toxic metal ions. For example, traditional physical adsorption methods can effectively adsorb pollutants, but can't completely remove the pollutants, just transfer them to other places. Biological treatment techniques cannot be used for the treatment of pollutants that are difficult to decompose by microorganisms (Ye et al., 2019a; Ye et al., 2017b). Therefore, the development of green and efficient wastewater treatment technology has become a hot spot. Semiconductor photocatalysis, known as “green chemistry process”, has been attracted great attention. Semiconductor photocatalysis technology possesses some distinct advantages, e.g. mild reaction conditions, without secondary pollution and high efficiency, and it can utilize the synergistic reaction between some pollutants to simultaneously remove a variety of pollutants, such as the synergistic removal between Cr(VI) and organic pollutants (Zhang and Lu, 2018; Ye et al., 2017c; Lu et al., 2019). Based on the above advantages, photocatalytic technology has promising applications in the removal of aqueous organic pollutants and heavy metals (Gan et al., 2018; Ye et al., 2019b), water splitting (Zhang et al., 2013), selective oxidation of organic compounds (Si et al., 2017), carbon dioxide conversion (Jin et al., 2015), hydrogen peroxide production (Cai et al., 2019a) and so on, which can not only solve the problem of wastewater pollution, but also develop and utilize new energy and other high value-added products. So far, many semiconductor photocatalysts, e.g. oxide semiconductors (TiO2 (Wei et al., 2018; Cai et al., 2019b), ZnO (Le et al., 2017), WO3 (Zhang et al., 2018)), perovskite materials (CaTiO3 (Pei et al., 2019), ZnTiO3 (Perween and Ranjan, 2017)) and non-precious metal deposited semiconductor materials (Cu/ZnO (Thang et al., 2018), Fe/ TiO2 (Tahir, 2018), Ni/ZnTiO3 (Baamran and Tahir, 2019)) have been investigated for the photocatalytic. Among these photocatalysts, bare TiO2 and ZnO have obvious advantages such as economical, efficient, nontoxic and harmless. However, they also have some disadvantages, e.g. wide band gap (TiO2, Eg = 3.2 eV; ZnO, Eg = 3.4 eV) which can only absorb the ultraviolet light, as well as the high recombination rate of photogenerated carriers (Hernándezalonso et al., 2009; Vaianoa et al., 2018). The ZnO-TiO2 composite system has superior properties due to high separation rate of photogenerated carriers and wide optical response range (Ranjith and Uyar, 2018a). The ZnO-TiO2 composite system contains three crystal phases: zinc metatitanate ZnTiO3, zinc orthotitanate Zn2TiO4 and zinc polytitanate Zn2Ti3O8 (Bartram and Slepetys, 1961; Mohammadi and Fray, 2010). Furthermore, these three crystal phases can be converted to each other under some certain conditions (Kang et al., 2018). Recently, Z-scheme photocatalysts have been aroused much attention owing to high transfer efficiency of charge carriers and high quantum yield. For example, X.B. Li et al. (2019) designed a Z-scheme g-C3N4/ h'ZnTiO3-a'TiO2 heterojunction photocatalyst to degrade methylene blue (MB) under the visible light illumination. The photocatalytic performance of Z-scheme g-C3N4/h'ZnTiO3-a'TiO2 heterojunction was significantly higher than that of pure g-C3N4 and h'ZnTiO3-a'TiO2. The h'ZnTiO3 with high electron mobility played a role of electronic transfer station and the direct Z-scheme structure could effectively transfer the

photogenerated carriers. Besides, Wang et al. (2017) synthesized the Z-scheme photocatalyst that g-C3N4 nanosheets coupled with oxygendefective ZnO (OD-ZnO) nanorods for photocatalytic hydrogen evolution. The oxygen defects of OD-ZnO directly recombining with the holes in the valence band of g-C3N4 at the heterojunction interface. The generated reactive species including •O–2 and •OH clearly supported the Z-scheme mechanism. This strategy not only benefits the construction of novel visible-light-driven Z-scheme photocatalysts, but also avails the oxygen-defects of semiconductors mediating the Z-scheme charge separation. Many other Z-scheme photocatalysts such as ZnO/ CdS (S. Wang et al., 2019), BP/BiVO4 (Zhu et al., 2018), α-Fe2O3/gC3N4 (Jiang et al., 2018) and g-C3N4/TiO2 (Li et al., 2017) have been developed. Here, Z-scheme ZnTiO3/Zn2Ti3O8/ZnO ternary heterostructure was synthesized by solvothermal-calcination method and the influence of the volume of tetrabutyl titanate precursor and reaction temperatures on the crystal phase structure, photoelectrochemical properties and active species were also discussed. Due to its unique Z-scheme structure and suitable band gap position, the samples exhibited high performance for the degradation of organic pollutants and reduction of heavy metal Cr(VI) ions. And the possible Z-scheme photocatalytic mechanism was also proposed. 2. Experimental The detailed preparation, characterization and photocatalytic activity tests of the catalysts were shown in the supporting information. The samples were denoted as X-ZTO and Y-ZTO, where ZTO represented element composite of Zn, Ti and O, respectively; X expressed the addition amounts of tetrabutyl titanate (X = 2.8, 3.4, 4.0, 4.6 and 5.2 mL) and Y expressed the reaction temperatures (Y = 120, 140, 160, 180 and 200 °C). 3. Results and discussions 3.1. Crystal structure analysis The X-ray powder diffraction (XRD) is usually used to analyze the crystal structure and crystallinity. Fig. 1(a) shows the XRD pattern of the X-ZTO catalysts. It can be seen that when the volume of tetrabutyl titanate is 2.8 mL, the main component is the Zn2Ti3O8. The diffraction peaks at 2θ of 30.1°, 35.4°, 53.4°, 56.9° and 62.5° belong to the crystal planes of (220), (311), (422), (511) and (440) of cubic Zn2Ti3O8 (JCPDS NO.87-1781), respectively. When the volume of tetrabutyl titanate is 3.4 mL, another ZnTiO3 phase appears. The diffraction peaks of 32.8°, 35.3°, 40.5°, 49.0°, 53.4°, 61.8° and 63.4° are ascribed to (104), (110), (113), (024), (116), (214) and (300) crystal planes of hexagonal ZnTiO3 (JCPDS NO.85-0547), respectively. When the volume of tetrabutyl titanate is above 3.4 mL, ZnTiO3, Zn2Ti3O8 and ZnO coexist. The new characteristic diffraction peaks at 2θ of 35.4° and 62.1° are assigned to the crystal planes of (101) and (103) of hexagonal ZnO (JCPDS NO.75-1533), respectively. It can be seen that the volume of tetrabutyl titanate has great influence on the crystal phase structure of the samples. Fig. 1(b) shows the XRD pattern of the Y-ZTO catalysts and the diffraction peaks are similar to that of the sample 4.0-ZTO. It can be observed that different synthesis temperatures have little effect on the crystal phase structure of samples and the main characteristic diffraction peaks are almost unchanged. Overall, the samples contain three phases of ZnTiO3, Zn2Ti3O8 and ZnO, as well as have a good crystallinity. 3.2. Morphologic structure analysis The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of the catalysts are displayed in the Fig. 2. Fig. 2 (a) shows the SEM image of pure ZnTiO3. It can be seen that the sample

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Fig. 1. XRD patterns of the samples: (a) X-ZTO catalysts; (b) Y-ZTO catalysts.

has no special morphology and the agglomeration is serious. Fig. 2(b)– (d) show the SEM images of 3.4-ZTO, 4.0-ZTO (180-ZTO) and 4.6-ZTO, respectively. With the increase of tetrabutyl titanate volume, the lamellar structure is gradually obvious and the layers are distinct. Fig. 2 (e) and (f) show the SEM images of 160-ZTO and 200-ZTO, respectively. Compared with the Fig. 2(c), (e) and (f), it can be found that samples are agglomerated and the dispersions decreased with the increase of the reaction temperature. The sample of 200-ZTO obviously collapsed, which may be due to that the relatively high reaction temperature caused the structural damage. Fig. 2(g) and (h) show the TEM images of typical sample 4.0-ZTO (180-ZTO). Fig. 2(h) shows that there are three different lattice fringes in the sample: the lattice spacing of 0.254 nm corresponds to the (1−10) crystal plane of ZnTiO3 (JCPDS NO.85-0547), the lattice spacing of 0.297 nm belongs to the (220) crystal plane of Zn2Ti3O8 (JCPDS NO.87-1781) and the other lattice spacing of 0.261 nm corresponds to the (002) crystal plane of ZnO (JCPDS NO.75-1533). It can be further proved the existence of three crystal phases of ZnTiO3, Zn2Ti3O8 and ZnO, which is consistent with the XRD results.

peak of instrument itself (Pei et al., 2019). Fig. S1(b) shows that the binding energies at 1022.01 and 1045.00 eV are assigned to the Zn 2p3/2 and Zn 2p1/2, respectively, which are correspond to the Zn2+ of ZnTiO3, Zn2Ti3O8 and ZnO (Reddy et al., 2018; J. Wang et al., 2018). Fig. S1(c) reveals that the binding energies of Ti 2p at 457.52 and 463.43 eV belong to Ti 2p3/2 and Ti 2p1/2, respectively. Compared with the references (J.S. Li et al., 2019), the shift induced in the Ti 2p3/2 and Ti 2p1/2 spectrum represents the possible interaction with the Zn to form the Zn-O-Ti (Ranjith and Uyar, 2018b). The spectral separation between Ti 2p3/2 and Ti 2p1/2 is 5.9 eV, which represents the Ti4+ nature on surface of the sample. The valence state of Ti element in the sample of ZnTiO3 and Zn2Ti3O8 are +4, which is consistent with reference (Ranjith and Uyar, 2018b). The O 1s region in Fig. S1(d) can be fitted into two peaks at the binding energies of 531.61 and 529.88 eV, respectively. The binding energy at 529.88 eV is ascribed to the lattice oxygen in the sample (i.e., Zn\\O and Ti\\O), whereas the binding energy at 531.61 eV is classified as the oxygen in surface hydroxyl groups of the sample (Yang et al., 2018; Tang and Zhang, 2017). 3.4. Infrared spectroscopy analysis

3.3. Composition and valence state analysis The X-ray photoelectron spectroscopy (XPS) of the typical sample 4.0-ZTO (180-ZTO) is presented in Fig. S1. Fig. S1(a) indicates the XPS spectrum of Zn 2p, Ti 2p, O 1s and C 1s, showing that the existence of Zn, Ti, O, and C elements. About the C 1s at the binding energy of 284.1 eV, it can be attributed to the adventitious carbon source pollution

The infrared spectra (FT-IR) of the samples X-ZTO and Y-ZTO are shown in Fig. 3(a) and (b), respectively. In the Fig. 3(a), the absorption peak at 3432.88 cm−1 is relatively wide, which is attributed to the O\\H bond associated with the surface adsorption water (Huang et al., 2016). The absorption peak at 2359.94 cm−1 is attributed to the C_O asymmetric stretching vibration peak from CO2 in air (Agrawal et al., 2009).

Fig. 2. SEM and TEM images of the samples. (a)–(f) represent the SEM images of ZnTiO3, 3.4-ZTO, 4.0-ZTO (180-ZTO), 4.6-ZTO, 160-ZTO, 200-ZTO, respectively; (g) and (h) represent the TEM images of 4.0-ZTO (180-ZTO).

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Fig. 3. FT-IR spectra of the samples: (a) X-ZTO catalysts; (b) Y-ZTO catalysts.

The absorption peak at 1637.07 cm−1 belongs to the stretching vibration of absorbed water (Yadav et al., 2013). The absorption peak at 1059.94 cm−1 is most obvious in the sample 3.4-ZTO, which may be that 3.4-ZTO is in the coexistence state of ZnTiO3 and Zn2Ti3O8 and the corresponding peak at 1059.94 cm−1 is related to the stretching of ZnO-Ti and Ti\\O or Ti-O-C bonds (Mohammadi and Ghorbani, 2018). The absorption peaks at 500–800 cm−1 are assigned to the characteristic absorption peaks of Ti\\O, Ti-O-Ti and Zn-O-Ti, which may come from the molecular bonds in the ZnTiO3 or Zn2Ti3O8 (Wang et al., 2014; Kubiak et al., 2019). The absorption peak at 430.42 cm−1 belongs to Ti\\O bending vibration peak in TiO6 structure from perovskite type ZnTiO3, indicating the existence of ZnTiO3 (Wang et al., 2015). The samples fabricated at different temperatures have five characteristic absorption peaks at 2359.94, 1637.07, 728.91, 592.43 and 430.42 cm−1, respectively, which are similar to that of the samples X-ZTO (Fig. 3 (b)). However, there have no characteristic absorption peaks at 3432.88 and 1059.94 cm−1, which may be related to the preparation conditions of samples. 3.5. Surface area and pore structure Fig. S2 shows that the N2 absorption-desorption isotherms and corresponding BJH pore-size distribution curves (inset) of all the samples. According to the classification of the Brunauer-Deming-Deming-Teller (BDDT), the N2 absorption-desorption isotherms of all the samples are classified as the type IV, indicating that they are the mesoporous materials (Sing et al., 1985). In the relative high pressure region (0.7–0.995 P/ P0), the hysteresis loops are shown in all the samples, which are classified as H3 type (Hong et al., 2015). It can be found that the larger the hysteresis loop indicates the smaller size region (28–34 nm). Compared with the pore size distribution curves in the inset figures of the samples, they show a wide pore size distribution (28–48 nm). Meanwhile, Table S1 shows the specific surface areas and pore structures of all the samples. The pure ZnTiO3 displays the smallest surface area in all the samples. With the increase of tetrabutyl titanate volume, the specific surface area and pore volume decreased slightly, whereas the pore size first increased and then decreased. With increasing of the reaction temperature, the specific surface areas are increased, however, the pore volumes have a little change. And the 180-ZTO catalyst has the maximum pore size. 3.6. Optical absorption property The UV–visible diffuse reflectance spectra (UV–vis DRS) of all the samples are shown in the Fig. S3. It can be seen from Fig. S3(a) that the absorption edges of samples 4.0-ZTO, 4.6-ZTO and 5.2-ZTO are about 410 nm, which are similar to the absorption edge of pure ZnTiO3, while the absorption edges of samples 2.8-ZTO and 3.4-ZTO

have blue shift. According to the XRD analysis, the main component of 2.8-ZTO is Zn2Ti3O8, and the main component of 3.4-ZTO is ZnTiO3 and Zn2Ti3O8, therefore, the absorption edges are different from the pure ZnTiO3. In addition, the absorption peak has a bending change range from 300 to 410 nm over 3.4-ZTO catalyst, which may be caused by the coexistence of the two main components to bring about the p-n type transformation after the illumination. Fig. S3(b) shows the UV–vis DRS of Y-ZTO and pure ZnTiO3 catalysts, indicating that the absorption edges are all around 410 nm, which are consistent with the XRD results. Fig. S3(c) and (d) are the band gap energies for all the samples. According to the Tauc's law, the band gap of all the samples can be determined (Tian et al., 2017). Tangent is made to the curve of [F(Rα)hν]1/2 vs. hν, where F(Rα) is the function of Kubelka-Munk and hν is the energy of incident photon, and the position of intersection point with Xaxis is the band gap value (Long et al., 2014). The band gap energies of all the samples are listed in Table S2. It can be seen that the band gap energies are between 2.84 and 2.94 eV, excepting the 2.8-ZTO and 3.4-ZTO. The main bodies of 2.8-ZTO and 3.4-ZTO are Zn2Ti3O8, which is different from the absorption edge and band gap of the samples with ZnTiO3 as the main body. In photocatalytic reaction, the positions of conduction band (CB) and valence band (VB) in semiconductor can determine the redox ability of photogenerated electrons and holes. In order to further investigate the CB and VB positions of all the samples, the following formulas are used to calculate: EVB = X − Ee + 0.5Eg and ECB = EVB − Eg, where EVB and ECB represent the band gap energy of VB and CB, respectively; X is the absolute electronegativity of semiconductor; Ee is the energy of free electrons on the hydrogen scale (generally 4.5 eV); and Eg is the band gap energy of semiconductor (Y.L. Wang et al., 2018). According to the reference (Yu et al., 2019), the band gap energies of ZnTiO3, Zn2Ti3O8 and ZnO are 2.95, 2.66 and 3.43 eV, respectively. And the calculated EVB and ECB of ZnTiO3, Zn2Ti3O8 and ZnO are EVB,ZnTiO3 = +2.78 eV, ECB,ZnTiO3 = −0.17 eV, EVB,Zn2Ti3O8 = +2.63 eV, ECB, Zn2Ti3O8 = −0.03 eV, EVB,ZnO = +3.01 eV and ECB,ZnO = −0.42 eV, respectively. 3.7. Photoluminescence properties Typically, the low emission peak intensity of the photoluminescence (PL) spectrum means the low recombination amounts of photogenerated electrons and holes (Zeng et al., 2007). The PL spectra of all the samples at the excitation wavelength of 450 nm exhibit significant emission peaks around 520 nm. As shown in Fig. 4(a), the intensities of emission peak for the X-ZTO samples are stronger than that of pure ZnTiO3, which indicates that the X-ZTO samples have more recombination amounts of photogenerated electrons and holes than that of the pure ZnTiO3. But the excitation peak intensity of the X-ZTO samples do not enhance too much compared with that of pure ZnTiO3, indicating

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Fig. 4. Photoluminescence spectra of the fabricated samples: (a) X-ZTO catalysts; (b) Y-ZTO catalysts.

that the carriers in these composite samples may undergo single Zscheme transfer. While the emission peak intensity of the sample 4.0ZTO is the strongest, indicating that the transfer mechanism of photogenerated electrons and holes is similar to the Z-scheme transfer mechanism. The emission peaks of 2.8-ZTO and 3.4-ZTO are similar, which may be affected by the crystal structure. As shown in Fig. 4(b), the fluorescence intensities of Y-ZTO samples are stronger than that of the pure ZnTiO3. And the fluorescence intensity of 180-ZTO is the strongest, which can further confirm the Z-scheme mechanism. According to the above analysis, the photocatalytic reaction is assumed to be a single Z-scheme mechanism. 3.8. Photoelectrochemical property The dynamic characteristics of carriers are studied by transient photocurrent test and AC impedance measurement. The transient photocurrent test reflects the separation of photogenerated carriers of the catalyst: the stronger the photocurrent response capability, the higher the separation efficiency of electrons and holes. It can be seen that all the samples have a photocurrent response under alternating illumination conditions of 20 s in Fig. 5(a) and (b). And the photocurrent responses of all the composites are stronger than that of pure ZnTiO3, indicating that the photogenerated electrons and holes of the composites have higher separation efficiency. Furthermore, 4.0-ZTO (180ZTO) shows the strongest photocurrent response capability, indicating that it has the strongest separation ability of electron-hole pairs. The PL and photoelectrochemical results also indicate that the electrons and holes generated by 4.0-ZTO (180-ZTO) are more than other samples. Therefore, 4.0-ZTO (180-ZTO) has good photocatalytic performance, and the above results can further prove the single Z-scheme in photocatalytic reaction. The AC impedance measurement can further reflect the migration resistance of carriers. The greater the curvature of the curve in the AC impedance spectrum, the greater the migration resistance of the carriers (Wei et al., 2017). It can be seen from Fig. S4 that the curve radiuses of all composites are smaller than that of pure ZnTiO3, indicating that the charge of the composites transfer fast at the interface and the separation resistance of the electron-hole pairs is small, which is consistent with the analysis results of the photocurrent. Moreover, 4.0-ZTO (180ZTO) exhibits the smallest curve radius, indicating that the separation efficiency of electron-hole pairs is high and the photocatalytic performance could be expected. 3.9. Photocatalytic activity The photocatalytic performance of the synthesized samples was tested by reducing Cr(VI) (10 ppm) and degrading Rh B (10 ppm) under 400 W metal halide lamp (wave length: 300–800 nm)

illumination. The photocatalytic redox ability of all the composite samples is stronger than that of pure ZnTiO3. As shown in Fig. 6(a) and (b), 4.0-ZTO (180-ZTO) shows the best reduction ability compared with other samples. The reduction rate of Cr(VI) over 4.0-ZTO (180-ZTO) is about 47% after 150 min light illumination, which is 13 times higher than that over pure ZnTiO3 (3%). In addition, Fig. 6(c) and (d) exhibit the degradation of Rh B over samples of X-ZTO and Y-ZTO, respectively. And the 4.0-ZTO (180-ZTO) sample shows 67% degradation rate for RhB, which is about 4 times higher than that of pure ZnTiO3 (15%). And the dynamic simulation equation was added to the supporting information (Fig. S5(a–d), ESI). According to the results of linear fitting, photocatalytic degradation of Rh B and reduction of Cr(VI) are consistent with the first-order kinetic reaction equation. In order to further explore the photocatalytic performance of the samples, the synergistic reaction of Cr(VI) (10 ppm) reduction and phenol (10 ppm) degradation were carried out under 500 W xenon lamp (wave length: 200–2500 nm) illumination. Fig. 6(e) and (g) reflect the Cr(VI) reduction rate and phenol degradation rate over X-ZTO samples, respectively. The 4.0-ZTO sample shows the best Cr(VI) reduction capability and phenol degradation capability, with 33% reduction rate of Cr (VI) and 30% degradation rate of phenol in 150 min under the xenon lamp illumination, which is 11 times and 7 times higher than that over pure ZnTiO 3 (3%), respectively. Besides, Fig. 6(f) and (h) reveal the Cr( VI) reduction rate and phenol degradation rate over Y-ZTO samples, respectively. The 180-ZTO sample shows the best Cr(VI) reduction capability and phenol degradation capability, which has the same redox capacity as the sample 4.0-ZTO. In addition, the 4.0-ZTO (180-ZTO) catalyst almost has little photocatalytic performance for Cr( VI ) reduction or phenol degradation alone under the xenon lamp illumination. The dynamic simulation equations have been added to the supporting information (Fig. S5(e–h), ESI). According to the results of linear fitting, photocatalytic reduction of Cr(VI) and degradation of phenol are also consistent with the first-order kinetic reaction equation. Fig. 6(i) and (j) display that the degradation rate and total organic carbon (TOC) mineralization rate of phenol over X-ZTO and Y-ZTO samples, respectively. All the samples show a certain mineralization rate to phenol. Among them, the mineralization rate of phenol over 4.0-ZTO (180-ZTO) is as high as 20% (phenol degradation rate is 30%). The main reason is that under the illumination of xenon lamp, electrons and holes are generated on the surface of the catalyst. Some electrons reduced Cr (VI), at the same time, the holes reacted with phenol, which promote the separation of electrons and holes. Therefore, the relative high removal efficiency for organic pollutants and Cr(VI) ions were obtained simultaneously. The catalyst itself has a strong ability to absorb ultraviolet light, while the ultraviolet light of xenon lamp is weak, so the degradation rate of phenol and the reduction rate of hexavalent chromium are not high.

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Fig. 5. The photocurrent response of the synthesized samples X-ZTO (a) and Y-ZTO (b).

Moreover, the recycle test in Rh B degradation and Cr(VI) reduction were carried out to detect the stability of the typical 4.0-ZTO (180ZTO) sample. As shown in Fig. 6(k) and (l), respectively. The degradation rate of Rh B decreases from 63% to 48% and the reduction rate of Cr(VI) solution reduces from 41% to 29% after four cycles under the metal halide illumination for 150 min. The photocatalytic performance of the sample shows a slightly change, indicating its good stability.

3.10. Reaction mechanism In the experiment of the addition free radical scavengers, the radical + scavengers of superoxide radicals (•O− 2 ), photogenerated holes (h ) and hydroxyl radicals (•OH) are benzoquinone (p-BZQ, 1 mM), disodium ethylenediaminetetraacetate (Na2-EDTA, 1 mM) and tertbutyl alcohol (TBA, 1 mM) were used, respectively. Fig. S6(a) shows that after adding scavenger of p-BZQ the activity slightly decreased, but the addition of Na2-EDTA scavenger displayed strong inhibitory ability on catalytic activity, followed by the scavenger of TBA. In electron spin resonance (ESR) test, Fig. S6(b) and (c) show that, under dark condition, the catalysts have no ESR signals for DMPO-•OH and DMPO-•O− 2 , indicating the reaction process did not occur. The ESR signals of DMPO-•OH and DMPO-•O− 2 were observed under light illumination, and the •OH signal of 4.0-ZTO (180-ZTO) catalyst is significantly stronger than that of ZnTiO3, indicating that 4.0-ZTO (180-ZTO) catalyst can produce more •OH radicals. However, the ESR intensities of •O− 2 in ZnTiO3 and 4.0-ZTO (180-ZTO) catalysts are similar, indicating that both of them can produce •O− 2 radicals. But scavenger of p-BZQ did not significantly inhibit the photocatalytic degradation activity of 4.0ZTO (180-ZTO). It can be seen that •O− 2 radicals were produced in the photocatalytic reaction process, but they did not participate in the photocatalytic reaction. However, the conduction band of ZnTiO3 (ECB = −0.17 eV) is more positive than the standard reduction potential (E − (O2/∙O2) = −0.33 eV) (Chen et al., 2017), which indicates that the generation of •O− 2 is not due to the reduction of O2 by electrons on the CB of ZnTiO3 but due to local lattice defects. In addition, the ESR signal of •O− 2 in 4.0-ZTO (180-ZTO) is stronger than that of ZnTiO3, indicating that the electrons on the CB of ZnO were also partially reduced. In the conventional heterojunction system, the photogenerated electrons and holes transferred to the more positive conduction band (CB) or the more negative valence band (VB), respectively and the ESR signals of •OH and •O− 2 will be synchronously changed (Zeng et al., 2018). As can be seen from Fig. S6(b) and (c), the ESR signals of •OH and •O− 2 did not synchronously change. The signal of •OH increases more, while the signal of •O− 2 only increases slightly, which is contrary to the traditional heterojunction mechanism. Combined with the

former PL and photocurrent results, we inferred that the reaction mechanism is a single Z-scheme. According to Matsumoto's report in the literature, photocatalytic active sites can be detected by photo-deposition of metals or metal oxides (Matsumoto et al., 2018). In order to further verify the single Z-scheme photocatalytic reaction mechanism, Ag and Mn2O3 with mass fractions of 3% were deposited on the typical 4.0-ZTO (180-ZTO) sample and the active sites on the surface of the sample were detected by the positions of each crystal phase in TEM, which has been explained in our previous article (Yu et al., 2019). In the TEM of the samples after photodeposition of 3% Ag and 3% Mn2O3 (Fig. S7, ESI), it can be known that the crystal phase of Ag is next to the crystal phases of ZnTiO3 and ZnO, and the crystal phase of Mn2O3 is next to the crystal phases of Zn2Ti3O8 and ZnO. According to the formulas Ag+ + e− → Ag and 2Mn2+ + 3H2O + h+ → Mn2O3 + 5H+ (Matsumoto et al., 2018), it can be inferred that Ag+ reacted with e− on the CB of ZnTiO3 and ZnO, and Mn2+ reacted with h+ on the VB of Zn2Ti3O8 and ZnO. Therefore, it can be inferred that the photocatalytic active sites of the ZnTiO3/ Zn2Ti3O8/ZnO ternary heterojunction are the e− on the CB of ZnTiO3 and ZnO and the h+ on the VB of Zn2Ti3O8 and ZnO. And it further proved the previous speculation of single Z-scheme photocatalytic mechanism, in which, the e− on the CB of Zn2Ti3O8 recombines with the h+ on the VB of ZnTiO3, and other e− and h+ undergo migration and redox reactions. In addition, according to the position of the band gap, it can be further proved that the above-mentioned e− on the CB of Zn2Ti3O8 recombines with h+ on the VB of ZnTiO3. The possible single Z-scheme photocatalytic reaction mechanism of ZnTiO3/Zn2Ti3O8/ZnO ternary heterojunction was shown in Fig. S8 (Yu et al., 2019). The e− on the CB of Zn2Ti3O8 (ECB,Zn2Ti3O8 = −0.03 eV) recombines with the h+ on the VB of ZnTiO3 (EVB,ZnTiO3 = +2.78 eV), which is coincide with the PL detection. The e− on the CB of ZnO (ECB,ZnO = −0.42 eV) transfers to the CB of ZnTiO3 (ECB,ZnTiO3 = −0.17 eV). At the same time, a small part of e− reacts with O2 to − form •O− on the CB of ZnTiO3 reduces Cr 2 (Afroz et al., 2018). The e (VI) to Cr(III) (E(Cr(VI)/Cr(III)) = +0.98 eV) (Nanda et al., 2017). In addition, the h+ on the VB of Zn2Ti3O8 (EVB,Zn2Ti3O8 = +2.63 eV) and ZnO (EVB,ZnO = +3.01 eV) reacts with dye and phenol molecules to generate small molecular substances, and a part of h+ react with H2O to generate •OH (E(H2O/•OH) = +2.28 eV) (Li et al., 2018), which is also used to participate in the photocatalytic oxidation reaction. 4. Conclusions In conclusion, the effects of the content of tetrabutyl titanate precursor and reaction temperatures on the crystal phase structure, photoelectrochemical property and photocatalytic activity of the Zscheme ZnTiO3/Zn2Ti3O8/ZnO ternary heterojunction samples were

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Fig. 6. Photocatalytic performances of the prepared samples under different light sources: (a) and (b) Reduction of Cr(VI), (c) and (d) Degradation of Rh B (metal halide lamp irradiation); (e)–(h) Synergetic reaction for Cr(VI) reduction and phenol degradation (Xenon lamp irradiation), respectively; (i) and (j) The degradation and mineralization rate of phenol; (k) and (l) Photocatalytic stability of 4.0-ZTO (180-ZTO) in the cyclic reaction for the degradation of Rh B and the reduction of Cr(VI).

investigated in detail. The obtained Z-scheme ZnTiO3/Zn2Ti3O8/ZnO ternary heterojunction photocatalyst exhibited excellent performance for the degradation of Rh B and phenol, as well as the reduction of Cr(VI) ions. The improved photocatalytic performance could be ascribed to the effective migration and separation of photo-generated carriers and more active sites over the surface of the unique ternary photocatalyst. Declaration of competing interest We declared that there are no conflicts to declare.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21567008, 21707055, 21938001, 2191101377), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2019), Guangdong Basic and Applied Basic Research Foundation (2019A1515011249, 2019A1515012130), Key-Area Research and Development Program of GuangDong Province (2019B110206002), the program for Innovative Research Team of Guangdong University of Petrochemical Technology, National Natural

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Science Foundation of China-SINOPEC Joint fund (U1663220), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01C102), Academic and Technical Leaders of the Main Disciplines in Jiangxi Province (20172BCB22018), and Yangfan Talents Project of Guangdong Province. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2019.136026. References Afroz, K., Moniruddin, M., Bakranov, N., Nuraje, N., 2018. Heterojunction strategy to improve visible light sensitive water splitting performance of photocatalytic materials. J. Mater. Chem. A 6, 21696–21718. Agrawal, M., Gupta, S., Pich, A., Zafeiropoulos, N.E., Stamm, M., 2009. A facile approach to fabrication of ZnO-TiO2 hollow spheres. Chem. Mater. 21, 5343–5348. Baamran, K.S., Tahir, M., 2019. Thermodynamic investigation and experimental analysis on phenol steam reforming towards enhanced H2 production over structured Ni/ ZnTiO3 nanocatalyst. Energ. Convers. Manage. 180, 796–810. Bartram, S.F., Slepetys, R.A., 1961. Compound formation and crystal structure in the system ZnO-TiO2. J. Am. Ceram. Soc. 44, 493–499. Cai, J.S., Huang, J.Y., Wang, S.C., Iocozzia, J., Sun, Z.T., Sun, J.Y., Yang, Y.K., Lai, Y.K., Lin, Z.Q., 2019a. Crafting mussel-inspired metal nanoparticle-decorated ultrathin graphitic carbon nitride for the degradation of chemical pollutants and production of chemical resources. Adv. Mater. 31, 1806314. Cai, J.S., Shen, J.L., Zhang, X.N., Ng, Y.H., Huang, J.Y., Guo, W.X., Lin, C.J., Lai, Y.K., 2019b. Light-driven sustainable hydrogen production utilizing TiO2 nanostructures: a review. SamllMethods 3, 1800184. Chen, F., Yang, Q., Wang, S.N., Yao, F.B., Sun, J., Wang, Y.L., Zhang, C., Li, X.M., Niu, C.G., Wang, D.B., Zeng, G.M., 2017. Graphene oxide and carbon nitride nanosheets comodified silver chromate nanoparticles with enhanced visible-light photoactivity and anti-photocorrosion properties towards multiple refractory pollutants degradation. Appl. Catal. B Environ. 209, 493–505. Dong, S.Y., Feng, J.L., Fan, M.H., Pi, Y.Q., Hu, L.M., Han, X., Liu, M.L., Sun, J.Y., Sun, J.H., 2015. Recent developments in heterogeneous photocatalytic water treatment using visible light responsive photocatalysts: a review. RSC Adv. 5, 14610–14630. Gan, H.H., Liu, J., Zhang, H.N., Qian, Y.X., Jin, H.X., Zhang, K.F., 2018. Enhanced photocatalytic removal of hexavalent chromium and organic dye from aqueous solution by hybrid bismuth titanate Bi4Ti3O12/Bi2Ti2O7. Res. Chem. Intermed. 44, 2123–2138. Hernándezalonso, M.D., Fresno, F., Suárez, S., Coronado, J.M., 2009. Development of alternative photocatalysts to TiO2: challenges and opportunities. Energy Environ. Sci. 2, 1231–1257. Hong, Y.P., Zhang, J., Huang, F., Zhang, J.Y., Wang, X., Wu, Z.C., Lin, Z., Yu, J.G., 2015. Enhanced visible light photocatalytic hydrogen production activity of CuS/ZnS nanoflower spheres. J. Mater. Chem. A 3, 13913–13919. Huang, Y.C., Fan, W.J., Long, B., Li, H.B., Zhao, F.Y., Liu, Z.L., Tong, Y.X., Ji, H.B., 2016. Visible light Bi2S3/Bi2O3/Bi2O2CO3 photocatalyst for effective degradation of organic pollutions. Appl. Catal. B Environ. 185, 68–76. Jiang, Z.F., Wan, W.M., Li, H.M., Yuan, S.Q., Zhao, H.J., Wong, P.K., 2018. A hierarchical Zscheme α-Fe2O3/g-C3N4 hybrid for enhanced photocatalytic CO2 reduction. Adv. Mater. 10, 1706108. Jin, J., Yu, J., Guo, D.P., Cui, C., Ho, W.K., 2015. A hierarchical Z-scheme CdS-WO3 photocatalyst with enhanced CO2 reduction activity. Small 11, 5262–5271. Kang, C.L., Xiao, K.K., Yao, Z.F., Wang, Y.H., Huang, D.M., Zhu, L., Liu, F., Tian, T., 2018. Hydrothermal synthesis of graphene-ZnTiO3 nanocomposites with enhanced photocatalytic activities. Res. Chem. Intermediat 44, 6621–6636. Kubiak, A., Siwińska-Ciesielczyk, K., Bielan, Z., Zielińska-Jurek, A., Jesionowski, T., 2019. Synthesis of highly crystalline photocatalysts based on TiO2 and ZnO for the degradation of organic impurities under visible-light irradiation. J. Inter. Adsorp. Soc. 25, 1–17. Le, S.K., Jiang, T.S., Li, Y.W., Zhao, Q., Li, Y.Y., Fang, W.B., Gong, M., 2017. Highly efficient visible-light-driven mesoporous graphitic carbon nitride/ZnO nanocomposite photocatalysts. Appl. Catal. B Environ. 200, 601–610. Li, J., Zhang, M., Li, Q.Y., Yang, J.J., 2017. Enhanced visible light activity on direct contact Zscheme g-C3N4-TiO2 photocatalyst. Appl. Surf. Sci. 391, 184–193. Li, X., Xie, J., Jiang, C.J., Yu, J.G., Zhang, P.Y., 2018. Review on design and evaluation of environmental photocatalysts. Front. Environ. Sci. Eng. 12, 14–46. Li, X.B., Xiong, J., Huang, J.T., Feng, Z.J., Luo, J.M., 2019a. Novel g-C3N4/h’ZnTiO3-a’TiO2 direct Z-scheme heterojunction with significantly enhanced visible-light photocatalytic activity. Alloy. Compd. 774, 768–778. Li, J.S., Cui, H.T., Mu, D.Y., Liu, Y.L., Guan, T.Y., Xia, Z., Jiang, L.M., Zuo, J.L., Tan, C., You, H., 2019b. Synthesis and characterization of rGO decorated cubic ZnTiO3 rods for solar light-induced photodegradation of rhodamine B. New J. Chem. 43, 3374–3382. Long, J.L., Wang, S.C., Chang, H.J., Zhao, B.Z., Liu, B.T., Zhou, Y.G., Wei, W., Wang, X.X., Huang, L., Huang, W., 2014. Bi2MoO6 nanobelts for crystal facet-enhanced photocatalysis. Small 10, 2791–2795. Lu, D.Z., Zhang, X.Y., Wang, S., Peng, W.B., Wei, M.M., Neena, D., Fan, H.Q., Hao, H.J., 2019. Enhanced photocatalytic removal of Cr(VI) over 0D/2D anatase/graphene and its synergism with organic pollutants under visible light irradiation. Appl. Surf. Sci. 470, 368–375.

Matsumoto, Y., Ida, S., Inoue, T., 2018. Photodeposition of metal and metal oxide at the TiOx nanosheet to observe the photocatalytic active site. J. Phys. Chem. C 112, 11614–11616. Mohammadi, M.R., Fray, D.J., 2010. Low temperature nanostructured zinc titanate by an aqueous particulate sol-gel route: optimisation of heat treatment condition based on Zn: Ti molar ratio. J. Eur. Ceram. Soc. 30, 947–961. Mohammadi, H., Ghorbani, M., 2018. Synthesis photocatalytic TiO2/ZnO nanocomposite and investigation through anatase, wurtzite and ZnTiO3 phases antibacterial behaviors. J. Nano Res-SW 51, 69–77. Nanda, B., Pradhan, A.C., Parida, K.M., 2017. Fabrication of mesoporous CuO/ZrO2-MCM-41 nanocomposites for photocatalytic reduction of Cr(VI). Chem. Eng. J. 316, 1122–1135. Pei, J.Y., Meng, J., Wu, S.Y., Lin, Q.Y., Li, J.X., Wei, X., Han, G.R., Zhang, Z., 2019. Hierarchical CaTiO3 nanowire-network architectures for H2 evolution under visible-light irradiation. J. Alloy. Compd. 806, 889–896. Perween, S., Ranjan, A., 2017. Improved visible-light photocatalytic activity in ZnTiO3 nanopowder prepared by sol-electrospinning. Sol. Energ. Mat. Sol. C. 163, 148–156. Ranjith, K.S., Uyar, T., 2018a. ZnO-TiO2 composites and ternary ZnTiO3 electrospun nanofibers: the influence of annealing on the photocatalytic response and reusable functionality. Cryst. Eng. Comm. 20, 5801–5813. Ranjith, K.S., Uyar, T., 2018b. ZnO-TiO2 composites and ternary ZnTiO3 electrospun nanofibers: the influence of annealing on the photocatalytic response and reusable functionality. Cryst. Eng. Comm. 20, 5801–5813. Reddy, K.H., Martha, S., Parida, K.M., 2018. Erratic charge transfer dynamics of Au/ZnTiO3 nanocomposites under UV and visible light irradiation and their related photocatalytic activities. Nanoscale 10, 18540–18554. Shannon, M.A., Bohn, P.W., Elimelech, M., Georgiadis, J.G., Marinas, B.J., Mayes, A.M., 2008. Science and technology for water purification in the coming decades. Nature 452, 301–310. Sheng, X., Liu, Z., Zeng, R., Chen, L.P., Feng, X.J., Jiang, L., 2017. Enhanced photocatalytic reaction at air-liquid-solid joint interfaces. J. Am. Chem. Soc. 139, 12402–12405. Si, J.Y., Liu, Y., Chang, S.Z., Wu, D., Tian, B.Z., Zhang, J.L., 2017. AgBr@TiO2/GO ternary composites with enhanced photocatalytic activity for oxidation of benzyl alcohol to benzaldehyde. Res. Chem. Intermed. 43, 2067–2080. Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouquerol, J., Siemieniewska, T., 1985. Reporting physisorption data for gas/solid systems withspecial reference to the determination of surface area and porosity. Pure Appl. Chem. 57, 603–619. Tahir, M., 2018. Photocatalytic carbon dioxide reduction to fuels in continuous flow monolith photoreactor using montmorillonite dispersed Fe/TiO2 nanocatalyst. J. Clean. Prod. 170, 242–250. Tang, D.D., Zhang, G.K., 2017. Ultrasonic-assistant fabrication of cocoon-like Ag/AgFeO2 nanocatalyst with excellent plasmon enhanced visible-light photocatalytic activity. Ultrason. Sonochem. 37, 208–215. Thang, H.V., Tosoni, S., Pacchioni, G., 2018. Evidence of charge transfer to atomic and molecular adsorbates on ZnO/X(111) (X = Cu, Ag, Au) ultrathin films. Relevance for Cu/ ZnO catalysts. ACS Cataly 8, 4110–4119. Tian, J., Liu, R.Y., Liu, Z., Yu, C.L., Liu, M.C., 2017. Boosting the photocatalytic performance of Ag2CO3 crystals in phenol degradation via coupling with trace N-CQDs. Chin. J. Catal. 38, 1999–2008. Vaianoa, V., Matarangolo, M., Murcia, J.J., Rojas, H., Navío, J.A., Hidalgo, M.C., 2018. Enhanced photocatalytic removal of phenol from aqueous solutions using ZnO modified with Ag. Appl. Catal. B Environ. 225, 197–206. Wang, J.X., Huang, J., Xie, H.L., Qu, A.L., 2014. Synthesis of g-C3N4/TiO2 with enhanced photocatalytic activity for H2 evolution by a simple method. Int. J. Hydrog. Energy 39, 6354–6363. Wang, Y.L., Zhang, W.T., Zhang, P.C., Li, J.F., Long, J.P., 2015. Effect of replacement of Ca by Zn on the structure and optical property of CaTiO3: Eu3+ red phosphor prepared by sol-gel method. Luminescence 30, 533–537. Wang, J., Xia, Y., Zhao, H.Y., Wang, G.F., Xiang, L., Xu, J.L., Komarneni, S., 2017. Oxygen defects-mediated Z-scheme charge separation in g-C3N4/ZnO photocatalysts for enhanced visible-light degradation of 4-chlorophenol and hydrogen evolution. Appl. Catal. B Environ. 206, 406–416. Wang, J., Wang, G.H., Wei, X.H., Liu, G., Li, J., 2018a. ZnO nanoparticles implanted in TiO2 macrochannels as an effective direct Z-scheme heterojunction photocatalyst for degradation of Rh B. Appl. Surf. Sci. 456, 666–675. Wang, Y.L., Tian, Y., Lang, Z.L., Guan, W., Yan, L.K., 2018b. A highly efficient Z-scheme Bdoped g-C3N4/SnS2 photocatalyst for CO2 reduction reaction: a computational study. J. Mater. Chem. A 6, 21056–21063. Wang, H.H., Guo, H., Zhang, N., Chen, Z.S., Hu, B.W., Wang, X.K., 2019a. Enhanced photoreduction of U(VI) on C3N4 by Cr(VI) and bisphenol a: ESR, XPS, and EXAFS investigation. Environ. Sci. Technol. 53, 6454–6461. Wang, S., Zhu, B.C., Liu, M.J., Zhang, L.Y., Yu, J.G., Zhou, M.H., 2019b. Direct Z-scheme ZnO/ CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity. Appl. Catal. B Environ. 243, 19–26. Wei, R.B., Kuang, P.Y., Cheng, H., Chen, Y.B., Long, J.Y., Zhang, M.Y., Liu, Z.Q., 2017. Plasmon-enhanced photoelectrochemical water splitting on gold nanoparticle decorated ZnO/CdS nanotube arrays. ACS Sustain. Chem. Eng. 5, 4249–4257. Wei, L.F., Yu, C.L., Zhang, Q.H., Liu, H., Wang, Y., 2018. TiO2-based heterojunction photocatalysts for photocatalytic reduction of CO2 into solar fuels. J. Mater. Chem. A 6, 22411–22436. Yadav, B.C., Yadav, A., Singh, S., Singh, K., 2013. Nanocrystalline zinc titanate synthesized via physico-chemical route and its application as liquefied petroleum gas sensor. Sensors Actuators B Chem. 177, 605–611. Yang, T.T., Peng, J.M., Zheng, Y., He, X., Hou, Y.D., Wu, L., Fu, X.Z., 2018. Enhanced photocatalytic ozonation degradation of organic pollutants by ZnO modified TiO2 nanocomposites. Appl. Catal. B Environ. 221, 223–234.

F. Chen et al. / Science of the Total Environment 706 (2020) 136026 Ye, S.J., Zeng, G.M., Wu, H.P., Zhang, C., Liang, J., Dai, J., Liu, Z.F., Xiong, W.P., Wan, J., Xu, P., Cheng, M., 2017a. Co-occurrence and interactions of pollutants, and their impacts on soil remediation—a review. Crit. Rev. Env. Sci. Tec. 47, 1528–1553. Ye, S.J., Zeng, G.M., Wu, H.P., Zhang, C., Dai, J., Liang, J., Yu, J.F., Ren, X.Y., Yi, H., Cheng, M., Zhang, C., 2017b. Biological technologies for the remediation of co-contaminated soil. J. Crit. Rev. Biotechnol. 37 (8), 1062–1076. Ye, S.J., Zeng, G.M., Wu, H.P., Zhang, C., Liang, J., Dai, J., Liu, Z.F., Xiong, W.P., Wan, J., Xu, P., Cheng, M., 2017c. Co-occurrence and interactions of pollutants, and their impacts on soil remediation—a review. Crit. Rev. Env. Sci. Tec. 47 (16), 1528–1553. Ye, S.J., Zeng, G.M., Wu, H.P., Liang, J., Zhang, C., Dai, J., Xiong, W.P., Song, B., Wu, S.H., Yu, J.F., 2019a. The effects of activated biochar addition on remediation efficiency of cocomposting with contaminated wetland soil. Resour. Conserv. Recy. 140, 278–285. Ye, S.J., Yan, M., Tan, X.F., Liang, J., Zeng, G.M., Wu, H.P., Song, B., Zhou, C.Y., Yang, Y., Wang, H., 2019b. Facile assembled biochar-based nanocomposite with improved graphitization for efficient photocatalytic activity driven by visible light. Appl. Catal. B Environ. 250, 78–88. Yu, C.L., Chen, F.Y., Zhou, W.Q., Xie, Y., Zeng, D.B., Liu, Z., Wei, L.F., Yang, K., Li, D.H., 2019. A facile phase transformation strategy for fabrication of novel Z-scheme ternary heterojunctions with efficient photocatalytic properties. Nanoscale 11, 7720–7733. Zeng, H.B., Li, Z.G., Cai, W.P., Cao, B.Q., Liu, P.S., Yang, S.K., 2007. Microstructure control of Zn/ZnO core/shell nanoparticles and their temperature-dependent blue emissions. J. Phys. Chem. B 111, 14311–14317.

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Zeng, D.B., Yang, K., Yu, C.L., Chen, F.Y., Li, X.X., Wu, Z., Liu, H., 2018. Phase transformation and microwave hydrothermal guided a novel double Z-scheme ternary vanadate heterojunction with highly efficient photocatalytic performance. Appl. Catal. B Environ. 237, 449–463. Zhang, S.C., Lu, X.J., 2018. Treatment of wastewater containing Reactive Brilliant Blue KNR using TiO2/BC composite as heterogeneous photocatalyst and adsorbent. Chemosphere 206, 777–783. Zhang, L., Xi, Z.H., Xing, M.Y., Zhang, J.L., 2013. Effects of the preparation order of the ternary P25/GO/Pt hybrid photocatalysts on hydrogen production. Int. J. Hydrogen Energ. 38, 9169–9177. Zhang, S., Wu, S., Wang, J., Jin, J., Peng, T., 2018. Controllable syntheses of hierarchical WO3 films consisting of orientation-ordered nanorod bundles and their photocatalytic properties. Cryst. Growth Des. 18, 794–801. Zhao, L.M., Meng, Q.Y., Fan, X.B., Ye, C., Li, X.B., Chen, B., Wu, L.Z., 2017. Photocatalysis with quantum dots and visible light: selective and efficient oxidation of alcohols to carbonyl compounds through a radical relay process in water. Angew. Chem. 129, 3066–3070. Zhu, M.S., Sun, Z.C., Fujitsuka, M., Majima, T., 2018. Z-scheme photocatalytic water splitting on a 2D heterostructure of black phosphorus/bismuth vanadate using visible light. Angew. Chem. Int. Edit. 8, 2160–2164.