CdS quantum dots modified surface oxygen vacancy defect ZnO1-x-TiO2-x solid solution sphere as Z-Scheme heterojunctions for efficient visible light-driven photothermal-photocatalytic performance

Journal of Alloys and Compounds 826 (2020) 154218

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CdS quantum dots modified surface oxygen vacancy defect ZnO1-xTiO2-x solid solution sphere as Z-Scheme heterojunctions for efficient visible light-driven photothermal-photocatalytic performance Dandan Sun a, Dechao Chi a, Zekang Yang a, Zipeng Xing a, *, Peng Chen a, Zhenzi Li b, ***, Kai Pan a, Wei Zhou a, b, ** a Department of Environmental Science, School of Chemistry and Materials Science, Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin, 150080, PR China b Shandong Provincial Key Laboratory of Molecular Engineering, School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250353, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 November 2019 Received in revised form 2 February 2020 Accepted 5 February 2020 Available online xxx

CdS quantum dots (QDs) modified surface oxygen vacancy defect ZnO1-x-TiO2-x solid solution spheres were prepared by using a hydrothermal, chemical reduction and electroless plating strategy, which have sufficient negative conduction band potential while having a photoresponse in visible light region. According to the band energy alignment, Z-scheme structure is formed, which favors spatial charge separation. The obtained CdS QDs/ZnO1-x-TiO2-x heterojunctions with the gap of ~2.09 eV exhibit excellent photothermal performance and photocatalytic degradation of bisphenol A (~99.5%), 2,6-dichlorophenol (~99.1%), and 2,4,5-trichlorophenol (~98.9%). It can be attributed to the following reasons: (1) a solid solution strategy can be used to enhance the photocatalytic activity of a given semiconductor photocatalyst. (2) The presence of oxygen defects can extend the photoresponse to visible light region. (3) TiO2x-ZnO1-x and CdS QDs can form Z-Scheme heterojunctions to increase the spatial separation of photogenerated electron-holes, which can promote photocatalytic performance. After recycle test, the resultant catalysts show high stability, which has superiority in practical application. So this novel CdS QDs modified surface oxygen vacancy defect ZnO1-x-TiO2-x solid solution sphere will have potential applications in environmental fields. © 2020 Elsevier B.V. All rights reserved.

Keywords: Photothermal-photocatalytic Oxygen vacancy defect Solid solution Z-Scheme heterojunction Quantum dot

1. Introduction Water pollution has become increasingly serious, and recently industry has rapidly developed to combat this problem [1]. Traditional treatment methods are unable to easily dislodge durable organic pollutants such as phenols [2]. Hence, the use of the highly efficient semiconductor photocatalytic technology to protect the environment has attracted people’s attention as a potential method

* Corresponding author. ** Corresponding author. Department of Environmental Science, School of Chemistry and Materials Science, Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin, 150080, PR China. *** Corresponding author. E-mail addresses: [email protected] (Z. Xing), [email protected] (Z. Li), [email protected] (W. Zhou). https://doi.org/10.1016/j.jallcom.2020.154218 0925-8388/© 2020 Elsevier B.V. All rights reserved.

to solve the global energy shortage [3]. Semiconductor photocatalysis is deemed to be an efficient and environmentally friendly technology, and its solar energy conversion characteristic has attracted increasing attention [4]. TiO2 is the earliest and most widely studied photocatalytic material because of its lack of toxicity, relatively high activity [5], good physical and chemical stability and low cost [6]. However, the recombination of electronhole pairs stimulated by single-phase TiO2 limits its photocatalytic activity. This problem can be overcome by doping elements [7], depositing noble metals [8], manufacturing defects [9] and compounding with other photocatalysts [10]. Among these approaches, coupling with other semiconductors to form heterojunctions is the most attractive method to improve the photocatalytic activity [11]. The bandgap of TiO2 should be adjustable to extend the light absorption to visible light region. Recently, studies have focused on transforming the color of TiO2 to increase the light absorption [12]. Mao et al. proposed a pioneering study to synthesize black TiO2

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with high activity using hydrogenation in 2011. The narrowed band gap of hydrogenated black TiO2 results in remarkable photocatalytic activity due to the formation of oxygen vacancies and the introduction of Ti3þ. This effect also effectively reduces the bandwidth, thereby improving the visible light response [13e17]. There are many ways to create oxygen vacancies, (1) Mechanical synthesis: in high-energy ball milling (BM) process, the collision and friction of ball could create high temperature, which can cause the dislodging of surface oxygen and thus the formation of oxygen vacancy. (2) Hydrogenation reduction: hydrogen is strong reducing agent that can reduce metal oxides at high temperatures. The “hydrogenation” is initially demonstrated in black TiO2 nanomaterials, the same strategy of introducing oxygen vacancy has been successfully applied in many other transition metal oxides. (3) Reduction in solution: to avoid the high-temperature conditions in hydrogenation processes, some cost-effective solution reduction methods are also developed for the synthesis of oxygen vacancy. (4) Thermal annealing in oxygen-deficient environment: researchers found that thermal annealing in oxygen deficient conditions can also create oxygen vacancies in metal oxides. (5) Flame reduction: the flame reduction method has advantages of ultrafast heating rate, high temperature, tunable reduction environment and open atmosphere operation, which enables rapid formation of defects. (6) Electrochemical reduction: oxygen vacancies can be chemically created in a reducing environment; further demonstrated that such defects can also be prepared by electrochemical reduction. ZnO is another attractive n-type semiconductor with high photocatalytic activity comparable to TiO2. At the same time, ZnO is an important broad band gap (~3.37 eV) semiconductor [18,19]. And its photocatalytic activity in response to visible light can be improved by introducing oxygen defects [20]. The formation of isolated energy levels in the forbidden band after introducing oxygen defects is the key factor for the visible light response of the zinc oxide material [21]. After the introduction of oxygen vacancy defects in metal oxides, the energy levels can generate capture electrons, resulting in a large number of movable carriers and changes in the properties of the materials [22,23]. Beane et al. [24] verified the important role for oxygen vacancies in charge transfer between dyes and ZnO nanocrystals during photocatalysis. The concentration and type of oxygen defects altered the photocatalytic activity of ZnO in another study [25]. Further mechanical studies showed that an increase in the number of surface defects in ZnO nanocrystals significantly increases the separation of photogenerated electrons [26]. However, methods to produce different types of surface defects in ZnO remain a challenge [27]. Zheng et al. [28] and Wang et al. [29] created oxygen vacancies in ZnO nanocrystals. The photocatalytic activity is increased by inducing specific defects in the synthesis process. Zheng et al. [30] prepared Ag/ ZnO nanocomposites. The photocatalytic activity of the Ag/ZnO nanocomposites is closely related to the heterostructure and oxygen defects. In general, the band structure of a semiconductor affects its activity in different photocatalytic reactions by controlling the position of its conduction band (CB) and valence band (VB). It is worth noting that the higher reductive power of a semiconductor is usually a direct result of its negative CB position. In this regard, it is necessary to adjust the electronic band structure of the semiconductor photocatalyst such that its CB position is suitable for enhancing the photoreduction intensity, and therefore, the formation of solid solution is considered to be one of the most effective strategies [31]. In recent years, some solid solution photocatalysts have been reported to have excellent light energy conversion efficiency. The basic working mechanism of these solid solution systems is that they move the conduction band (CB) potential in a negative direction. Therefore, on the one hand, the light

absorption range of the solid solution can be expanded, and on the other hand, the photogenerated electrons have a good reduction ability, which can maintain a relatively high level to improve its photocatalytic performance [32]. In this paper, we use a hydrothermal method to react a zinc source with a titanium source to form precursor, and then an annealing treatment to obtain a ZnOeTiO2 solid solution having a sufficient negative CB potential and having a response in the visible region. A solid solution strategy can be used to enhance the photocatalytic activity of a given semiconductor photocatalyst [33]. Photocatalysts have been used in an increasing number of applications [34e36]. A series of materials have been used as photocatalysts, such as oxides [37], sulfides [38], nitrides [39], layered dihydroxides [40], MOF [41], and polyacids [42]. The electron-hole pairs of these single-phase photocatalysts are readily recombined, resulting in a low photocatalytic performance. Heterojunction photocatalysts (usually type II) separate photoelectrons and holes from two different semiconductors, thus improving the photocatalytic performance [43]. However, its ability to oxidize holes and reduce electrons is lower. Recently, Z-scheme heterojunction photocatalysts have attracted considerable interest because of their stronger oxidation and reduction capabilities. In Z-scheme system, electrons in the CB of a semiconductor and holes in the VB of another semiconductor retain the ability of reduction and oxidation, inhibit recombination of electrons and holes, and enhance photocatalytic activity [44e46]. The band gap of CdS is relatively narrow, approximately 2.3 eV, which favors the absorption of light [47]. Currently, quantum dots (QDs) have attracted the attention of many researchers because of their large specific surface area, small size and short charge transfer distance [48,49]. CdS QDs can be uniformly dispersed on ZnO1-x-TiO2-x, which provides more active sites and subsequently improves the photocatalytic property. Therefore, in the present study, the presence of CdS was shown to form a Z-scheme photocatalytic system between CdS and ZnO1-xTiO2-x. The new photocatalyst would have excellent photocatalytic performance. In this study, we have fabricated ZnOeTiO2 solid solution to enhance the photocatalytic activity. Surface oxygen vacancy defect ZnO1-x-TiO2-x has photothermal effect and can conduct photocatalytic reaction at low temperature, ZnO1-x-TiO2-x and CdS QDs induces the formation of Z-Scheme heterojunctions to increase the spatial separation of photogenerated electron-hole pairs, so CdS quantum dots modified surface oxygen vacancy defect ZnO1-x-TiO2x solid solution sphere exhibited excellent visible light-driven photocatalytic and electrochemical properties. Additionally, a reasonable photocatalytic mechanism was also proposed in this study.

2. Experimental procedures 2.1. Synthesis of ZnTiO3 sphere ZnTiO3 was synthesized by using a simple solvothermal approach. A typical synthetic procedure is described as below. First, 1.19 g of Zn(NO3)2$6H2O and 0.22 mL of TiCl4 were added to 100 mL of deionized water and mixed by magnetic stirring. After 10 min, 3.0 g of urea and 0.74 g of NH4F were dissolved in this homogenous solution, and the mixture was subjected to magnetic stirring for 30 min at room temperature. The mixture was hydrothermally treated in a 100 mL Teflon-lined autoclave at 130  C for 48 h. The precipitate was centrifuged, washed thoroughly with deionized water and ethanol, and finally dried overnight at 60  C.

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2.2. Synthesis of solid solution ZnOeTiO2 sphere ZnOeTiO2 was synthesized by using a simple thermal condensation method. Approximately 0.5 g of ZnTiO3 was placed in a muffle furnace and thermally treated at 650  C with a heating rate of 5  C min1 for 4 h. The obtained agglomerates were ground into a fine powder and collected for further use. 2.3. Synthesis of surface oxygen vacancy defect ZnO1-x-TiO2-x solid solution sphere First, 0.1 g of the samples prepared using the methods described above were mixed with 0.2 g of NaBH4 and thoroughly ground for 30 min at room temperature (20 ± 2  C). Then, the mixtures were transferred to porcelain boats and placed in a tubular furnace to heat at 350  C for 2 h under N2 atmosphere. After naturally cooling to room temperature, the mixtures were washed with distilled water and ethanol three times and dried at 60  C for 10 h. The asreceived samples were denoted as ZnO1-x-TiO2-x. 2.4. Synthesis of CdS QDs modified surface oxygen vacancy defect ZnO1-x-TiO2-x solid solution sphere The sample of CdS QDs/ZnO1-x-TiO2-x was fabricated by using the chemical bath deposition method. Briefly, 200 mg of ZnO1-xTiO2-x and 25 mg of Cd(NO3)2 were added to 50 mL of absolute ethanol, followed by stirring for 1 h and ultrasonication for 2 h. Then, 50 mL of 0.002 M Na2S solution were added and stirred at 25  C for 4 h. The obtained yellow powder was washed with deionized water several times and oven-dried at 60  C for 8 h. The as-received samples were denoted as CdS QDs/ZnO1-x-TiO2-x. For comparison, pure CdS was also synthesized under the same conditions without adding ZnO1-x-TiO2-x (Scheme 1). Characterization and performance testing were shown in Supplementary Material. 3. Results and discussion The XRD patterns of the different materials are shown in Fig. 1a. XRD patterns exhibit basal reflections 2q ¼ 31.8, 34.4, 36.2, 47.7, 56.6, 62.8, 66.4, 68.0 and 69.0 , which correspond to the (100), (002), (101), (102), (103), (200), (112) and (201) crystal planes of a wurtzite-type structure, respectively. The pattern coincides with the PDF card No. 36e1451 of ZnO, which is a typical wurtzite structure. Characteristic peaks at 2q ¼ 25.3, 38.6, 54.9, 62.7 and 68.9 are observed that correspond to the (101) (112), (211), (204) and (116) crystal planes of TiO2, respectively. The pattern coincides

Scheme 1. Schematic depicting the formation of CdS QDs/ZnO1-x-TiO2-x.

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with PDF card No. 21e1276. Characteristic peaks at 2q ¼ 35.1, 38.0, 60.6 and 62.8 are observed that correspond to the PDF card No. 39e0190 of ZnTiO3. Therefore, the ZnOeTiO2 has been successfully synthesized. The XRD pattern of CdS is shown in Fig. S1, the characteristic peaks at 2q ¼ 24.9, 26.5, 28.1, 36.7, 43.8, 47.9 and 51.9 are observed that correspond to the (100), (002), (101), (102), (110), (103), and (112) crystal planes of CdS QDs, respectively. The pattern coincides with PDF card No. 41e1049. But in CdS QDs/ZnO1-x-TiO2-x, no characteristic peak for CdS is detected in the composite material because the content of CdS is relatively low and the instrument cannot detect the characteristic peak of CdS. Therefore, we can prove the successful synthesis of CdS QDs/ZnO1-x-TiO2-x through the following characterization. Fig. 1b shows the FTIR spectra of the synthesized ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x, CdS QDs/ZnO1-x-TiO2-x and CdS. For ZnTiO3, the sharp and intense band at 3430 cm1 corresponded to the ZneOH bond, TieOH bond and OeH bond stretching vibration of water, the broad band at approximately 1630 cm1 corresponded to the bending vibration of water on the catalyst surface. Additional bands at 400-700 cm1 represent the translational motion of the metal-oxygen bond (ZneO and TieO). Thus, abundant hydroxyl functional groups are present on the surface of the catalyst, and the presence of surface hydroxyl groups is conducive to capturing photogenerated holes to form hydroxyl radicals with oxidation activity, which could improve the photocatalytic reaction. In the ZnOeTiO2 spectra, the strong wide absorption band between 1200 and 1900 cm1 is attributed to the TieOeZn/ZneOeTi stretching vibration. The strong and wide absorption band at 3000-3600 cm1 represents the overlap of the OeH group, providing evidence for the coordination of water molecule with the Ti4þ cation. No obvious differences were noted between ZnOeTiO2 and ZnO1-x-TiO2-x, which showed that the structure of its functional group would not be changed after calcination. For CdS QDs/ZnO1-x-TiO2-x, the strong wide absorption band between 1200 and 1700 cm1 was attributed to the CdeS bond. These results clearly confirmed the existence of CdS and ZnO1-x-TiO2-x. UVevis diffuse reflection spectra of the synthesized photocatalysts were characterized. As shown in Fig. 1c, we can see that the prepared photocatalysts have excellent responds in visible light range, the light absorption range of ZnTiO3 with a very weak absorption peak is observed in visible light region. However, the solid solution ZnOeTiO2 is formed after calcination, and its visible light response is improved because the formation of heterojunction between ZnO and TiO2, and recombination of electron-hole pairs can be suppressed by charges transfer in the heterojunction. The response of the obtained ZnO1-x-TiO2-x after the addition of sodium borohydride is excellent in visible light range, it is mainly due to the formation of oxygen defects, which can reduce the band gap and activate O2 to generate superoxide radicals by generating isolated energy levels, and it can enhance the response in visible light range. CdS QDs has a very narrow band gap and has good absorption in visible light, when CdS QDs is loaded on ZnO1-x-TiO2-x, CdS QDs/ ZnO1-x-TiO2-x exhibited stronger absorption of wavelengths in the near-infrared region (NIR), which indicates that CdS QDs/ZnO1-xTiO2-x can more efficient absorption utilizes visible light and the generation of more electron-hole pairs to increase the photocatalytic activity. The absorbance of the semiconductor after compositing is extended to the visible light and NIR regions. The absorption follows formula 1-1: ahy ¼ K(hy-Eg)x/2 1-1.where a is the absorption coefficient, n is the optical frequency, Eg is the energy of band gap, K is a constant, and x represents the optical conversion type of semiconductor, where x ¼ 1 is a direct transition and x ¼ 4 is an indirect transition. Fig. 1d shows the relationship between (ahy)1/2 and the photon energy. Through the Kubelka-Munk method, we calculated the band gap of the sample. The band

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Fig. 1. XRD patterns (a), FTIR spectra (b), and UVeVis diffuse reflectance absorption spectra (c), the calculated band gaps (d), adsorption and desorption isotherms (e) and the corresponding BJH pore size distributions (f) of ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x, CdS and CdS QDs/ZnO1-x-TiO2-x, respectively.

gaps of ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x, CdS and CdS QDs/ZnO1-xTiO2-x are estimated to be 3.12, 2.78, 2.53, 2.33 and 2.09 eV, respectively. Among them, CdS QDs/ZnO1-x-TiO2-x exhibits a narrow band gap and has exceedingly good photocatalytic properties due to the oxygen vacancy of ZnO and TiO2. The narrow band gap resulted in high visible light-driven photocatalytic activity, and a reduction in the band gap was concluded to improve the light absorption. The BET surface area and the pore size distributions of the ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x, CdS and CdS QDs/ZnO1-x-TiO2-x samples are used to characterize their microstructures, as shown in Fig. 1e and f. The BET surface areas of ZnTiO3, ZnOeTiO2, ZnO1-xTiO2-x, CdS and CdS QDs/ZnO1-x-TiO2-x are 9.97, 15.57, 40.46, 53.06 and 114.22 m2 g1, respectively, and the samples exhibited type IV isotherms, indicating mesoporous structure. In addition, the high catalytic activity of the prepared sample is due to its high specific surface area, resulting in a strong adsorption capacity for the target molecule and the transport of electrons and holes. The wide size

distribution range of the mesopores in the sample is conducive to charge transport and diffusion in the material. Therefore, the mesoporous structure and large specific surface area are favorable for improving the photocatalytic activity. The surface morphologies of ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x, and CdS QDs/ZnO1-x-TiO2-x were investigated by SEM and TEM. ZnTiO3 possessed a sphere-like wool ball structure (Fig. 2a). The shape of ZnOeTiO2 formed after calcining did not change, and remained spherical (Fig. 2b), the results showed that ZnOeTiO2 was solid solution. The shape of ZnO1-x-TiO2-x formed by reduction remained unchanged (Fig. 2c). The TEM micrographs (Fig. 2d) of CdS QDs/ZnO1-x-TiO2-x confirmed its spherical shape, the illustration showed that CdS quantum dots were already loaded on ZnO1-xTiO2-x, and the HRTEM micrographs of CdS QDs also revealed lattice fringes of 0.336 nm, corresponding to the (111) plane of CdS QDs (Fig. 2e). In the mapping diagram, CdS QDs decorated the surface of ZnO1-x-TiO2-x sphere (Fig. 2fek). According to the XPS spectra (Fig. 3b), the peaks of Zn 2p3/2 and

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Fig. 2. SEM images of ZnTiO3 (a), ZnOeTiO2 (b), and ZnO1-x-TiO2-x (c), TEM (s) and HRTEM (e) images of CdS QDs/ZnO1-x-TiO2-x, SEM (f) image of CdS QDs/ZnO1-x-TiO2-x and elemental maps of S, Cd, Zn Ti, and O (gek), respectively.

Zn 2p1/2 are located at approximately 1045.6 and 1022.4 eV, respectively. Meanwhile, the binding energies of Zn 2p1/2 and Zn 2p3/2 show a decreasing trend as the number of oxygen vacancies increased, which are located at approximately 1044.7 and 1021.6 eV, respectively. Fig. 3c shows the Ti 2p XPS of the CdS QDs/ ZnO1-x-TiO2-x, with peaks at 457.8 eV (Ti3þ 2p3/2), 463.2 eV (Ti3þ 2p1/2), 464.2 eV (Ti4þ 2p1/2) and 458.4 eV (Ti4þ 2p3/2). Clearly, the presence of Ti3þ is attributed to the reduction of Ti4þ by TiO2. The peaks at 531.7, 530.5, and 529.6 eV in the XPS spectrum of O 1s (Fig. 3d) are assigned to the O atoms in the vicinity of oxygen vacancies, O bond of ZneOeZn and TieOeTi bonds, respectively. EPR spectra of ZnO1-x-TiO2-x have been added in Fig. S4, this further proves the successful formation of Ti3þ and Ov. Peaks at 161.2 and 162.4 eV are observed, as shown in Fig. 3e, and assigned to characteristics of S2. Fig. 3f shows the XPS of Cd 3d. The peak at 411.2 eV represents Cd 3d3/2 and the peak at 404.9 eV represents Cd 3d5/2 of Cd2þ in CdS. Based on the XPS analysis, we can see that the composite sample contains CdS QDs, ZnO1-x and TiO2-x. Two main heat sources have been identified in the reaction

process: direct thermal radiation and the photothermal effect of samples. In the images, different colors designate temperature changes, and the information on the right indicates the relationship between temperature and color. The initial temperature of the photocatalyst was controlled at approximately 24  C before irradiation (Fig. 4a). After irradiation for 1 min, the temperature changes in different samples differed, and the corresponding photothermal effects also differed. When ZnOeTiO2 was formed, the temperature increased significantly (Fig. 4b). As shown in Fig. 4c, temperature of ZnO1-x-TiO2-x was further increased due to the formation of oxygen defects. Infrared images of CdS QDs/ZnO1 x-TiO2-x revealed the highest temperature (97 C), which was attributed to the oxygen vacancies that increase the NIR absorption (Fig. 4d). The photocatalytic degradation of BPA, 2,6-dichlorophenol, and 2,4,5-trichlorophenol is shown in Fig. 5a, d, and g, respectively. The degradation rates of the CdS QDs/ZnO1-x-TiO2-x composite were higher than other catalysts, indicating that the CdS QDs/ZnO1-xTiO2-x composite possesses high photocatalytic activity. The

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Fig. 3. The full XPS spectrum (a) and Zn 2p (b), Ti 2p (c), O 1s (d), S 2p (e) and Cd 3d (f) spectra of CdS QDs/ZnO1-x-TiO2-x.

Fig. 4. IR images of ZnTiO3 (a), ZnOeTiO2 (b), ZnO1-x-TiO2-x (c) and CdS QDs/ZnO1-x-TiO2-x (d), respectively.

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Fig. 5. The photocatalytic degradation rates of BPA (a), variation in -ln(C/C0) versus (b), and cycling test for the photocatalytic degradation (c). The photocatalytic degradation rate of 2,6-dichlorophenol (d), variation in -ln(C/C0) versus (e), and cycling test for the photocatalytic degradation (f). The photocatalytic degradation rate of 2,4,5-trichlorophenol (g), variation in -ln(C/C0) versus (h), and cycling test for the photocatalytic degradation (i).

degradation rates of the CdS QDs/ZnO1-x-TiO2-x composite were ~99% after irradiation for 210 min, and the degradation rates of the ZnTiO3, ZnOeTiO2 and ZnO1-x-TiO2-x were less than 80%. Comparison experiments of photodegradation CdS QDs/TiO2 (Fig. S5) and CdS QDs/ZnO (Fig. S6) were also performed. It helps to better proof ZnO1-x-TiO2-x solid solution with high photocatalytic activity. TOC has been provided in Figs. S7eS9. We can see from the TOC removal that bisphenol A, 2,6-dichlorophenol, and 2,4,5-trichlorophenol were totally mineralized into CO2 and H2O. This phenomenon was attributed to solid solution strategy, the presence of oxygen defects and TiO2-x-ZnO1-x and CdS QDs induces the formation of ZScheme heterojunctions. As shown in Fig. 5c, f, and i, the photocatalytic degradation rate of CdS QDs/ZnO1-x-TiO2-x is relatively stable over four cycles, confirming the high stability of the composite. Moreover, the first-order rate constants (k) for ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x and CdS QDs/ZnO1-x-TiO2-x are shown in Fig. 5b (BPA) and are estimated to be 0.0039, 0.00506, 0.00745, and 0.01664 min1, respectively.

The values of k for ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x and CdS QDs/ZnO1-x-TiO2-x are shown in Fig. 5e (2,6-dichlorophenol) and are estimated to be 0.00363, 0.00525, 0.00778, and 0.01592 min1, respectively. The values of k for ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x and CdS QDs/ZnO1-x-TiO2-x are shown in Fig. 5i (2,4,5trichlorophenol) and are estimated to be 0.00352, 0.00529, 0.00754, and 0.01673 min1, respectively. The value of k for CdS QDs/ZnO1-x-TiO2-x was several times higher than that of ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x. Thus, CdS QDs/ZnO1-x-TiO2-x exhibited increased photocatalytic activity, which was attributed to the effects of CdS QDs and surface hydrogenation. The photoelectrical properties of ZnTiO3, ZnOeTiO2, ZnO1-xTiO2-x, CdS QDs/ZnO1-x-TiO2-x and CdS were examined by measuring the electrochemical impedance and I-T curves. The I-T curves in Fig. 6a show the transient photocurrent response. The photocurrent density of CdS QDs/ZnO1-x-TiO2-x was exceptional, indicating that the photogenerated charge carriers in the CdS QDs/ ZnO1-x-TiO2-x exhibited high separation efficiency. A potential

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Fig. 6. The photocurrent responses (a) and the Nyquist plots (b) of ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x, CdS and CdS QDs/ZnO1-x-TiO2-x. The fluorescence intensity of the as-prepared samples under visible light irradiation (c). Scanning Kelvin probe microscopy maps (d) for (A) ZnTiO3, (B) ZnOeTiO2, (C) ZnO1-x-TiO2-x, (D) CdS and (E) CdS QDs/ZnO1-x-TiO2-x, respectively.

explanation is that the existence of oxygen vacancies and the synergistic effect of CdS in CdS QDs/ZnO1-x-TiO2-x accelerates charge transfer and narrows the band gap. In Fig. 6b, the EIS Nyquist plots mainly show the separation efficiency. Generally, the arc diameter determines the charge transfer resistance, and a smaller diameter indicates a lower resistance and higher separation rate of charge carriers. The sizes of the semicircles descended in order from ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x, CdS and CdS QDs/ ZnO1-x-TiO2-x. Therefore, the separation efficiency followed the order of CdS QDs/ZnO1-x-TiO2-x > CdS > ZnO1-x-TiO2x > ZnOeTiO2 > ZnTiO3. In other words, the photogenerated charge carriers in CdS QDs/ZnO1-x-TiO2-x possess the highest separation efficiency. As shown in Fig. 6c, the CdS QDs/ZnO1-x-TiO2-x fluorescence intensity was increased compared with the other carriers, implying it can generated a large amount of the $OH radical, which consistent with the photocatalytic degradation of phenols. The work functions of ZnTiO3, ZnOeTiO2, ZnO1-x-TiO2-x, CdS and CdS QDs/ZnO1-x-TiO2-x are shown in Fig. 6d. The work function represents the energy required for the electrons in the Fermi energy level to escape from the interior of the metal into the vacuum, which is measured by the scanning Kelvin probe. Obviously, the work function of CdS QDs/ZnO1-x-TiO2-x is the lowest one, the value of the work function represents the bonding strength of the electrons in the metal, and the smaller the work function, the easier it is for the electrons to leave the metal. The stronger their transition ability, and the better their escape ability, as electrons are more readily escaped from CdS QDs/ZnO1-x-TiO2-x, increasing

transfer efficiency of the photogenerated e-hþ pairs. Therefore, the CdS QDs/ZnO1-x-TiO2-x exhibited exceptional photocatalytic performance. Scheme 2 shows the mechanism of phenol photodegradation. ESR has been measured to identify the active species as Figs. S11e14. Also, the active species has been tested using capture experiments as Fig. S10. The flat-band potential positions of

Scheme 2. Schematic illustrating the proposed visible light-driven photocatalytic mechanism of CdS QDs/ZnO1-x-TiO2-x.

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samples were measured by the MottSchottky plot (Figs. S2 and S3). The conduction band (CB) of CdS QDs and ZnO1-x-TiO2-x were estimated to be approximately 0.84 eV and 0.08 eV compared with NHE. Therefore, the valence band (VB) of CdS QDs and ZnO1-xTiO2-x were 1.49 eV and 2.45 eV compared with NHE because their band gaps are estimated to be 2.33 and 2.53 eV (Fig. 1d). Generally, electrons are transferred from the CB of CdS to ZnO1-x-TiO2-x, and the holes migrate from VB of ZnO1-x-TiO2-x to VB of CdS. The VB potential of CdS is lower than OH/$OH and $OH does not form. The  CB of ZnO1-x-TiO2-x is higher than O2/$O 2 and $O2 does not form. Therefore, the Z-scheme mechanism is proposed. In this system, electrons from the CB of ZnO1-x-TiO2-x transfer to the VB of CdS, leading to the formation of the Z-scheme structure. In Z-scheme system, hþ, $O 2 and $OH are involved in the degradation, which has been proven to exist in trapping experiments. Thus, a visible-light photocatalyst based on the CdS QDs/ZnO1-x-TiO2-x follows a Zscheme photocatalytic mechanism, which effectively suppresses the recombination of electron-hole pairs and improves photocatalytic performance.

4. Conclusions In summary, Z-scheme heterojunctions of surface oxygen deficiency ZnOeTiO2 solid solution and CdS QDs were fabricated using simple hydrothermal and calcination methods combined with electroless plating and chemical reduction. Moreover, CdS QDs/ ZnO1-x-TiO2-x produced the highest (99%) rates of phenol photocatalytic degradation among the different samples. This increase in photocatalytic activity may be ascribed to the formation of oxygen vacancies and Z-scheme heterojunctions. Therefore, ternary Zscheme heterojunctions may have potential applications in energy and environmental fields. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement Dandan Sun: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Visualization, Investigation, Validation, Writing - review & editing. Dechao Chi: Visualization, Investigation. Zekang Yang: Software, Validation. Zipeng Xing: Conceptualization, Methodology, Software, Writing - review & editing, Supervision. Peng Chen: Supervision. Zhenzi Li: Supervision. Kai Pan: Supervision. Wei Zhou: Supervision.

Acknowledgments We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21871078 and 51672073), the Natural Science Foundation of Heilongjiang Province (JQ2019B001 and B2018010), the Heilongjiang Postdoctoral Startup Fund (LBH-Q14135), the Heilongjiang University Science Fund for Distinguished Young Scholars (JCL201802), the Heilongjiang Provincial Institutions of Higher Learning Basic Research Funds Basic Research Projects (KJCX201909), the Youth Science and Technology Innovation Team Project of Heilongjiang Province (2018-KYYWF-1593), and the Heilongjiang Touyan Innovation Team Program.

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