Insight into the Transfer Mechanisms of Photogenerated Carriers for

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Insight Into the Transfer Mechanisms of Photogenerated Carriers for Heterojunction Photocatalysts with Analogous Positions of Valence Band and Conduction Band: a Case Study of ZnO/TiO 2

Wenting Sun, Sugang Meng, Sujuan Zhang, Xiuzhen Zheng, Xiangju Ye, Xianliang Fu, and Shifu Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03753 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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The Journal of Physical Chemistry

Insight into the Transfer Mechanisms of Photogenerated Carriers for Heterojunction Photocatalysts with Analogous Positions of Valence Band and Conduction Band: a Case Study of ZnO/TiO2 Wenting Suna, Sugang Meng*a, Sujuan Zhanga, Xiuzhen Zhenga, Xiangju Yeb, Xianliang Fua, Shifu Chen*a,b a

Department of Chemistry, Huaibei Normal University, Anhui Huaibei, 235000,

People’s Republic of China. b

Department of Chemistry, University of Science and Technology of Anhui, Anhui

Fengyang, 233100, People’s Republic of China. *Corresponding author, Tel: +86-561-3806611, Fax: +86-561-3806611. E-mail: [email protected], [email protected]

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Abstract: In the paper, the ZnO/TiO2 nanocomposites with different main parts (TiO2 or ZnO) were synthesized. When TiO2 is the main part of ZnO/TiO2 heterojunction photocatalysts (ZnO/TiO2), the photocatalytic activity is decreased rapidly with the increase of the amount of ZnO. The reason may be attributed to the relative p-n junction (p-ZnO/n-TiO2) produced between ZnO and TiO2. The migration directions of electrons and holes in the relative p-n junction are opposite to the transfer directions of the photogenerated electrons and holes in the VB and CB. However, when ZnO is the primary part of the heterojunction photocatalysts (TiO2/ZnO), the photocatalytic activity of the samples increases with the increase of TiO2 amount up to 5% (95% ZnO/TiO2). The reason may be that the migration directions of electrons and holes in the relative p-n junction (p-TiO2/n-ZnO) are the same as the transfer directions of the photoexcited electrons and holes in the VB and CB between the two semiconductors. It is proposed that the conductivity of the heterojunction photocatalyst will be changed with the difference of content for two semiconductors, which in turn affects the migration directions of the electrons and holes in the heterojunctions and their photocatalytic activity.

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1. Introduction Photocatalysis as a renewable, green and environmentally friendly technology has attracted a considerable attention among researchers in the past decades, because of its applications in environmental remediation, photolysis of water into hydrogen and oxygen, organics transformation, and CO2 reduction using sunlight.1-9 The key problem of the technology for the practical application is photocatalytic materials, which can be excited by long wave light, and have high quantum efficiency and long life. After more than 30 years of research, a series of important results have been achieved in the design, preparation and related basic theoretical research for the semiconductor photocatalysts.10-17 However, the photocatalytic activity of the single photocatalyst was not satisfactory, and could not meet people's expectations. Therefore, the development of composite photocatalysts has become an inevitable choice, among which the heterojunction photocatalyst is widely investigated due to the high separation efficiency of the photogenerated electrons and holes.18-22 From the reported results, it can be seen that the photocatalytic activity of these heterojunction photocatalysts is improved to some extent compared with a single photocatalyst. The reason for the increased activity is attributed to the different positions of CB and VB between the semiconductors, which make the effective separation of the photoexcited electron-hole pairs possible, and the quantum efficiency is increased.23-25 At the same time, it has been reported that for the two-phase A-B heterojunction photocatalyst, when the amount of a semiconductor exceeds a certain percentage, the photocatalytic activity of the heterojunction photocatalyst will be significantly reduced. The explanation for the decreased activity is attributed to the reduction of the light absorption for the main semiconductor in the heterojunction photocatalyst.26-27 Theoretically, for a two-phase A-B heterojunction photocatalyst, when the A and B semiconductors are combined with particle ratio of 1:1, the heterojunction photocatalyst should exhibit the highest photocatalytic activity. However, the actual results are not the same, so the above explanation is not sufficient to explain the results of the experiment, and the underlying reasons should be further studied. In 3



recent years, the composite photocatalysts with a p-n junction structure have been extensively studied.10, 15, 28-29 The results showed that the photoexcited electrons and holes are separated effectively due to the effect of the internal electric field in the p-n junction, and the photocatalytic activity is improved significantly. At the same time, the experimental results also showed that when the amount of p-type semiconductor exceeds a certain amount, the activity of the photocatalyst will be greatly reduced. The general explanation is that when the amount of a semiconductor reaches a certain amount, the light absorption of the primary photocatalyst is reduced,26-27 and its underlying reasons have not been further explained. As we all know, titanium dioxide (TiO2) and zinc oxide (ZnO) are two important semiconductors because of their applications in different systems and devices.30-37 The positions of VB and CB of TiO2 are 2.94 and -0.30 eV,38 and the VB and CB positions of ZnO are 2.88 and -0.32 eV, respectively. It is clear that the positions of VB and CB for TiO2 and ZnO are close to each other.38,39 In theory, a good heterojunction pohotocatalyst will be formed when the two semiconductors are combined. In this paper, the composite ZnO/TiO2 photocatalysts with different main parts of TiO2 or ZnO were prepared by mechanical mixing method. The photocatalytic activities of the ZnO/TiO2 photocalysts were evaluated by the degradation of Rhodamine B, Methyl Orange and Bisphenol A under UV-light illumination. The ZnO/TiO2 composites were characterized in detail. The migration processes of photogenerated electrons and holes of the ZnO/TiO2 photocatalysts were investigated by the photoluminescence technique, the electron spin resonance technology and the determination of reactive species in the photocatalytic reactions. Some interesting results were obtained. The transfer mechanisms of photoexcited carriers for ZnO/TiO2 photocatalysts were proposed and verified. It is expected that the work could give inspiration to find the migration process of the photogenerated carriers for other composite photocatalysts and open a pathway for constructing excellent heterojunction photocatalysts.

2. Experimental 2.1. Materials 4


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Zinc oxide (ZnO), titanium oxide (TiO2), methanol, and potassium hydroxide were used

as

received

and

supplied

by

Aladdin

Chemistry

Co.

Ltd.

5,5-dimethyl-1-pyrroline-N-oxide (DMPO), Rhodamine B (RhB), Methyl Orange (MO) and Bisphenol A (BPA), and other chemicals used in the experiments were purchased from China chemical reagent Ltd. They are of analytically pure grade and used without further purification. Deionized water was used throughout this study. 2.2. Preparation of ZnO/TiO2 heterojunctions The ZnO/TiO2 samples with different weight ratios of ZnO (1%, 3%, 5%, 10%, 15%, 30%, 50%, 70%, 90%, 93%, 95%, 97% and 99%) were prepared by mechanical mixing method. The process is as follows: The above samples of ZnO/TiO2 were dispersed into 50 mL methanol, and stirred for 30 min at 60 °C, respectively. In order to separate the samples efficiently, 10 mL of KOH (0.03 M) solution was added dropwise into the suspension, and continued stirring for 2 h. Finally, the mixed samples were collected by centrifugation, rinsed thoroughly with deionized water, and dried at 80 °C for 3 h. Pure TiO2 and ZnO samples were prepared with the same method. 2.3. Characterization The phase structures and the crystallite size of the photocatalysts were identified by powder X-ray diffraction (XRD) on a Bruker D 8 advance X-ray powder diffractometer with Cu Ka radiation at a scanning rate of 3˚ min-1. The accelerating voltage and emission current were 40 KV and 30 mA, respectively. Scanning electron microscopy (SEM) was detected on a Zeiss Merlin compact scanning electron microscope with 20 KV scanning voltage. Transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HRTEM) images were collected on a Tecnai G2 F20 S-Twin transmission electron microscope, using an accelerating voltage of 200 KV. UV-vis diffuse reflectance spectra (DRS) measurements were analyzed by a Shimadzu UV 3600 UV-vis-NIR spectrometer, using BaSO4 as a reflectance standard. Photoluminescence (PL) emission spectra were recorded on a JASCO FP-6500 type fluorescence spectrophotometer. X-ray 5



photoelectron spectroscopy (XPS) measurement was carried out on a Thermo Scientific ESCA LAB 250 photoelectron spectrometer with a monochromatic Al Ka ray beam (1486.6 eV) as the X-ray source. The binding energy values were calibrated using the contaminant carbon (C 1s = 284.6 eV). The Brunauer-Emment-Teller (BET) surface areas were carried out by N2 adsorption-desorption at 77.3 K using a Micromeritics ASAP 2460 instrument. Electron spin resonance (ESR) signals of spin-trapped paramagnetic species with 5, 5-dinethyl-1-pyrroline-N-oxide (DMPO) were detected using a Bruker E500 spectrometer. 2.4. Photoreaction apparatus and procedure Experiments were carried out in a photoreaction apparatus. The photoreaction apparatus consists of two parts.40-41 The first part is a circular quartz tube. A 375 W medium pressure mercury lamp with a maximum emission wavelength of 365 nm is used as the UV light source. The lamp is placed in a circular hollow chamber, and the water passes through the annular casing. As the water continues to cool, the temperature of reaction solution is kept at about 30 °C. The second part is a 250 mL unsealed beaker with a diameter of about 12 cm. Photocatalytic activities of the samples were evaluated by degradation of RhB, MO and BPA under the UV light. Before each experiment, the reaction suspension solution containing 0.05 g photocatalyst and 100 mL 80 ppm RhB (or 40 ppm MO or 10 ppm BPA) was stirred in the dark for 30 min to reach adsorption-desorption equilibrium between the photocatalyst and the solution. After irradiation time intervals of 20 min (for the BPA solution, illumination time is 10 min), about 5mL of the suspension was taken out and then centrifuged to remove the photocatalyst particles. Subsequently, the absorbance spectra of RhB (or MO or BPA) solutions were analyzed by a Shimadzu UV 3600 UV-vis-NIR spectrometer using deionized water as a reference sample. The photocatalytic efficiencies of RhB, MO and BPA were calculated from the following expression: Degradation efficiency = (1 – Ct/C0) × 100% Where C0 is the concentration of reactant before illumination; Ct is the concentration 6


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of reactant after illumination time t. In order to check the repeatability of the results, three batches of samples were prepared. And the average values of the data were used.

3. Results and discussion 3.1. Catalyst characterization The crystal structure properties and phase compositions of the samples were investigated by XRD analysis. The XRD patterns of TiO2, ZnO and ZnO/TiO2 samples are displayed in Fig.1. From Fig. 1, it is clear that the XRD characteristic peaks of TiO2 are in high accordance with standard tetragonal TiO2 according to XRD JCPDS card (NO.65-5714). While the peaks are located at about 31.8, 34.4, 36.3, 47.5, 56.6, 62.9, 66.4, 67.9 and 69.1°, which distinctly are indexed to the (100), (002), (101), (102), (110), (103), (200), (112) and (201) crystal planes of pure wurtzite ZnO phase (JCPDS NO.36-1451),39, 42 respectively. It is worth noting that no obvious diffraction peaks of ZnO can be found in the ZnO/TiO2 samples when the weight ratio of ZnO is lower than 10%. It may be due to the low amount of ZnO and high coverage of TiO2 nanoparticles in the ZnO/TiO2 composite. From Fig. 1, it is clear that when the weight ratio of ZnO is higher than 10%, the peaks of ZnO located at 31.8, 34.4, 36.3, and 56.6° can be observed. Meanwhile, the characteristic peaks of TiO2 are weakened gradually. When the amount of ZnO is higher than 99%, the characteristic peaks of TiO2 are disappeared because of the low amount of TiO2 in the samples. Meanwhile, no other peaks were detected in the XRD patterns, demonstrating that no new species or impurities were formed in ZnO/TiO2 composites.

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Fig. 1. XRD patterns of ZnO, TiO2 and ZnO(wt.%)/TiO2 photocatalysts. The UV-vis absorption spectra of samples are depicted in Fig. 2a. It is known that the optical absorption of a semiconductor is closely related to its electronic structure. It can be seen that the absorption edges of the TiO2 and ZnO are very similar, and it is about 382 nm and 388 nm, respectively. This can be assigned to the intrinsic band gap of TiO2 (3.24 eV) and ZnO (3.20 eV).38,39,43,44 It is clear that the absorption of ZnO/TiO2 samples is in the UV light range, and it is redshift with the addition of ZnO. The absorption intensities of ZnO/TiO2 composites are higher than that of TiO2. The UV light absorption spectra of the samples are getting closer to pure ZnO with the increase of the amount of ZnO. Meanwhile, when ZnO is the main part of the ZnO/TiO2 composites, the light absorption performance of the sample is similar. It is known that the band gap energy (Eg) of a photocatalyst can be calculated by the following equation: 45 ahv = A ( hv – Eg )n/2 In this equation, a, h, v, and A are the absorption coefficient, Planck constant, light frequency and proportionality, respectively.45 In addition, the n value is determined by 8


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the type of optical transition of a semiconductor ( n = 1 for a direct transition and n = 4 for an indirect transition ). TiO2 and ZnO are direct band semiconductors, and the value of n is 1.42-43 As shown in Fig. 2b, the band gaps of TiO2 and ZnO are about 3.24 and 3.2 eV, respectively. The results are in accordance with the previous reports.36-37 Furthermore, the band edge positions of the CB and VB of a semiconductor can be determined by the following equation:40-41 EVB = X – Ee + 0.5Eg ECB = EVB – Eg Where EVB, X, Ee and ECB are the VB edge, the absolute electronegativity of the semiconductor, the energy of free electrons on the hydrogen scale (likely 4.5 eV) and the CB edge, respectively. The X values for TiO2 and ZnO are 5.82 and 5.78 eV, respectively. From the calculation, the ECB of TiO2 and ZnO are -0.30 and -0.32 eV, and the EVB of TiO2 and ZnO are estimated to be 2.94 and 2.88 eV, respectively. It agrees well with the previous reports.38,39

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Fig. 2. a. UV-vis absorption of TiO2, ZnO (wt.%)/TiO2 and ZnO; b. Band gap energies of ZnO and TiO2. SEM was used to investigate the morphology of the photocatalysts. Fig. 3 shows the SEM photographs of TiO2, ZnO, 5% ZnO/TiO2 and 95% ZnO/TiO2, respectively. The dispersed TiO2 nanoparticles were negatively charged,44 and the Zeta potential is about -21.4 mV. However, the ZnO nanorods showed positively charged,46-47 and the Zeta potential value is about +16.7 mV. Thus, the positively charged ZnO and negatively charged TiO2 can construct a heterojunction photocatalyst ZnO/TiO2 by the electrostatic attraction. From Fig. 3a, it is revealed that pure TiO2 sample is composed of irregular nanoparticles with particle size of about 0.1 - 0.3 µm. However, the morphology of the pure ZnO was quite different from that of pure TiO2 sample. As shown in Fig. 3b, the appearance of pure ZnO is a rodlike structure with length of about 0.3 - 1.5 µm, and the diameter size is about 0.1 - 0.3 µm. From Fig. 3c and d, it can be seen that when the main part of the sample is TiO2, TiO2 nanoparticles cover on the surface of ZnO nanorods. However, when the main part is ZnO, the sample exhibits that the TiO2 nanoparticles disperse on the surface of ZnO nanorods. In addition, the selected energy dispersive spectra (EDS) of the samples are shown in Fig. 3e and f. It is obvious that the sample is composed of Ti, Zn, and O elements. The 10


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result is in accordance with the result of XRD.

Fig. 3. SEM images of (a) TiO2, (b) ZnO, (c) 5% ZnO/TiO2 and (d) 95% ZnO/TiO2. EDS of (e) 5% ZnO/TiO2 and (f) 95% ZnO/TiO2. TEM was used to investigate the morphology and microstructure of the sample. The TEM and HRTEM images of TiO2, ZnO, 5% ZnO/TiO2 and 95% ZnO/TiO2 photocatalysts were shown in Fig. 4. From Fig. 4a and c, it is clear that pure TiO 2 is composed of nanoparticles with particle size of about 100 nm. And pure ZnO exhibites a rodlike structure with length of about 200-500 nm and the diameter size is about 50-200 nm, which is consistent with the information obtained by the SEM. From Fig. 4e and k, it can be seen that the 5% ZnO/TiO2 and 95% ZnO/TiO2 photocatalysts show the excellent combination between ZnO and TiO2. The HRTEM images in Fig. 4f and l display the existence of heterojunctions between TiO2 nanoparticles and ZnO nanorods. From Fig. 4, it is clear that the TiO2 and ZnO display different orientations and lattice spacing. Two lattice fringes are clearly 11



observed with interlayer distances of 0.24 and 0.28 nm, corresponding to the (100) plane of ZnO and the (103) plane of TiO2, respectively. Based on the results, it is concluded that the ZnO/TiO2 heterojuction were successfully constructed between TiO2 and ZnO in 5% ZnO/TiO2 and 95% ZnO/TiO2 samples, respectively. The mapping of the 5% ZnO/TiO2 and 95% ZnO/TiO2 are shown in Fig. 4g, h, I, j and 4m, n, o, and p. It is obvious that the sample is composed of Ti, Zn, and O elements. Meanwhile, the distribution of the constituted elements in the samples is well-proportioned.

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Fig. 4. TEM and HRTEM images of (a), (b) TiO2, (c), (d) ZnO, (e), (f) 5% ZnO/TiO2, (k), (l) 95% ZnO/TiO2. Mapping of 5% ZnO/TiO2 composite: (g), (h), (i), and (j). Mapping of 95% ZnO/TiO2 composite: (m), (n), (o), and (p). The Raman spectra of TiO2, ZnO and ZnO/TiO2 heterojunction are shown in Fig. 5. The Raman peak at 438 cm-1 is the characteristic peak of ZnO, which meant that ZnO had a wurtzite structure and a high degree of crystallization. This peak is attributed to the Ehigh2 mode of hexagonal ZnO due to the vibrations of oxygen atoms in the ZnO 13



lattice. The typical Raman spectra of TiO2 show that the peaks at 143, 396, 517, and 638cm-1 are attributed to the anatase phase. All of these characteristic bands are composed of the four Raman activity modes of the anatase phase and the symmetry of Eg, B1g, B1g and Eg, respectively.39,48,49

Fig. 5. Raman spectra of TiO2, ZnO, 1% ZnO/TiO2, 5% ZnO/TiO2, 95% ZnO/TiO2 and 99% ZnO/TiO2. XPS analysis techniques were used to detect the chemical composition of the catalyst surface and the valence of various elements. The full spectra of the TiO2, ZnO and 5% ZnO/TiO2 and 95% ZnO/TiO2 photocatalysts are shown in Fig. 6a. It can be seen that the samples are comprised of Zn, Ti, O and C elements, and no other elements are detected. C element is derived from the background elements of the instrument. The high resolution XPS spectra of the Ti 2p and Zn 2p regions for TiO2, ZnO and 5%ZnO/TiO2 and 95%ZnO/TiO2 heterojunctions are given in Fig. 6b and c. As shown in Fig. 6b, the binding energies of the Ti 2p region exhibit two peaks at 458.4 and 464.2 eV, corresponding to Ti 2p3/2 and Ti 2p1/2 of Ti4+, respectively.38,39 The spectra of Zn 2p depicted in Fig. 6c show two peaks at 1021.2 eV (Zn 2p3/2) and 1044.2 eV (Zn 2p1/2) of Zn2+.42,50 The high resolution of O 1s of TiO2, ZnO, 5% ZnO/TiO2 and 95% ZnO/TiO2 heterojunctions are shown in Fig. 6d. The O 1s spectrum is composed of two peaks with binding energies of 529.8 and 531.9 eV, 14


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corresponding to the O of O-Ti and O-Zn bondings and the hydroxyl group (-OH) on the surface of TiO2/ZnO. The full width of the O 1s peak at 529.8 eV for TiO2/ZnO is found to be slightly increased due to the overlapping of O–Zn and O–Ti bondings. This phenomenon also indicates that the coupled Zn ions exist as ZnO on the TiO2 surface. Meanwhile, -OH presented on the surface of ZnO/TiO2 with O1s at 531.9 eV peak is considered to be the chemically absorbed H2O, because the physically adsorbed H2O should be easily desorbed in the super high vacuum surrounding of the XPS system.38,51

Fig. 6. XPS spectra of TiO2, 5% ZnO/TiO2, 95% ZnO/TiO2 and ZnO samples: (a) survey spectra, (b) Ti 2p, (c) Zn 2p, (d) O 1s. The nitrogen adsorption-desorption isotherms of TiO2, ZnO and ZnO/TiO2 photocatalysts are shown in Fig.7. It is clear that the samples show type IV isotherms. The BET surface areas of the samples were tested and calculated to be 14.69, 14.08, 12.45, 5.57, 9.33 and 7.14 m2/g for TiO2, 1% ZnO/TiO2, 5% ZnO/TiO2, ZnO, 95% ZnO/TiO2 and 99% ZnO/TiO2 samples, respectively. It is clear that the BET surface areas of the samples should be the plus of ZnO and TiO2 photocatalyst. The results 15



indicate that BET surface area will not be the main factor for affecting the photocatalytic performance of the samples.

Fig. 7. Nitrogen adsorption-desorption isotherms of TiO2, ZnO and ZnO/TiO2 samples.

3.2. Photocatalytic activities The photocatalytic activities of ZnO/TiO2 heterojunctions were evaluated by degradation of RhB, MO and BPA. The blank test shows that there is no obvious photodegradation of RhB (or MO or BPA) without photocatalysts under the UV-light irradiation. The results are shown in Fig. 8 and Table S1-S3, it is clear that the photocatalytic efficiencies of TiO2 for RhB, MO and BPA are 34.96 %, 37.41 %, and 26.30 % respectively, and the photocatalytic efficiencies of ZnO for RhB, MO and BPA are 36.41 %, 40.19 % and 31.09 % respectively. From Fig. 8a, b and c, it can be seen that when the main part of ZnO/TiO2 heterojunction is TiO2, the photocatalytic performance of the photocatalysts decreases rapidly with the increase of the ZnO weight ratio up to 5 %. The photocatalytic efficiencies of ZnO (5 %)/TiO2 for RhB, MO and BPA are 11.56 %, 8.9 %, and 10.2 %, respectively. When the amount of ZnO is higher than 5.0%, the photocatalytic activities of the ZnO/TiO2 heterojunctions increase gradually. However, when ZnO is the main part of ZnO/TiO2 heterojunctions (90, 93, 95, 97, and 99%), compared with the pure TiO2 and ZnO, the ZnO/TiO2 16


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photocatalysts show excellent photocatalytic performance. With the increase of the TiO2 weight ratio, the photocatalytic activity increases greatly up to 5% of TiO2 (95% ZnO/TiO2). When the amount of TiO2 is higher than 10%, the photocatalytic activity of ZnO/TiO2 heterojunctions are lower than that of pure ZnO and TiO2. Based on the results, it is clear that the amount of TiO2 and ZnO exhibit different effects on the photocatalytic activity of the ZnO/TiO2 heterojunction photocatalyst. When TiO2 is the main part of ZnO/TiO2 heterojunction (ZnO/TiO2), the photocatalytic activity is decreased rapidly with the increase of the amount of ZnO. However, when ZnO is the primary part of the heterojunction photocatalysts (TiO2/ZnO), the photocatalytic activity of the samples increase with the amount of TiO2. This phenomenon may be caused by the different migration processes and separation mechanisms of photogenerated electrons and holes for the ZnO/TiO2 heterojunction photocatalysts with different main parts.

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Fig. 8. a. The degradation of RhB with different photocatalysts under UV-light irradiation for 20 min. b. The degradation of MO with different photocatalysts under UV-light irradiation for 20 min. c. The degradation of BPA with different photocatalysts under UV-light irradiation for 10 min.

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3.3. Discussion on photocatalytic mechanism 3.3.1. Validation of the reactive species It is known that the CB and VB potentials of TiO2 are -0.30 eV and 2.94 eV, and the CB and VB of ZnO are -0.32 and 2.88 eV, respectively. The photoexcited electrons in the CB of TiO2 and ZnO can reduce O2 to give •O2-, because the CB edge potential of TiO2 and ZnO are more negative than standard redox potentials of Eɵ( O2/•O2-) (-0.05eV vs. NHE).52,53 Meanwhile, the VB potentials of ZnO and TiO2 are more positive than the standard redox potential of Eɵ(OH-/ •OH ) (2.4 eV vs. NHE), so the VB potentials of TiO2 and ZnO can oxidize OH- to give •OH. In order to determine the existence of the active species •O2- and •OH in the photocatalytic reaction, ESR technique was carried out. The results are shown in Fig. 9. Fig. 9a shows the ESR signals of DMPO-•O2- for TiO2, 1% ZnO/TiO2, 5% ZnO/TiO2, 95% ZnO/TiO2, 99% ZnO/TiO2 and ZnO, respectively. It is clear that the six characteristic peaks of the DMPO-•O2- adducts54,55 are observed over TiO2, ZnO and ZnO/TiO2 heterojunctions under UV-light irradiation. For comparison, blank test (no photocatalyst) shows that there is no information of •O2- for DMPO methanol dispersion. It demonstrates that •O2- radicals are indeed generated in the reaction system of TiO2, ZnO and ZnO/TiO2 heterojunctions under UV-light irradiation. From Fig. 9a, it is clear that the peaks intensity of the DMPO-•O2- adduct for the photocatalysts is as follows: TiO2 > 1% ZnOTiO2 > 5% ZnO/TiO2. That is to say, when the main part of ZnO/TiO2 heterojunction photocatalyst is TiO2 (ZnO/TiO2), the peaks intensity is decreased rapidly with the increase of the amount of ZnO. However, when ZnO is the primary part of the heterojunction photocatalysts (TiO2/ZnO), the peaks intensity of the sample increases with the increase of amount of TiO2 up to 5% (95% ZnO/TiO2). It is worth noting that the peaks intensity of the DMPO-•O2- adduct for the photocatalysts is as follows: 95% ZnO/TiO2 > 99% ZnO/TiO2 > ZnO > TiO2 > 1% ZnO/TiO2 > 5% ZnO/TiO2. The results are in accordance with the results of the photocatalytic performance of the samples. From Fig. 9b, it is clear that four characteristic peaks of DMPO-•OH56 are observed 19



in the curves of TiO2, ZnO and ZnO/TiO2 heterojunctions. There is no signal of DMPO-•OH adducts in blank test conditions. It is indicated that •OH radicals are indeed produced in the reaction condition. From Fig. 9b, it can be seen that the intensities of the DMPO-•OH peaks for TiO2 are higher than that of 1% ZnO/TiO2 and 5% ZnO/TiO2. However, the intensity of the DMPO-•OH peaks for 1% ZnO/TiO2 is the same as that of 5% ZnO/TiO2. It is clear that when ZnO is the primary part of the heterojunction photocatalysts (TiO2/ZnO), the intensity of peak of the sample increases with the increase of amount of TiO2 up to 5% (95% ZnO/TiO2). Meanwhile, it is obvious that the intensities of the DMPO-•OH peaks of 95% ZnO/TiO2 and 99% ZnO/TiO2 are stronger than that of ZnO. It is clear that the intensity of the DMPO-•OH peaks of 95% ZnO/TiO2 is the strongest, indicating that more •OH are generated over the 95% ZnO/TiO2 than other heterojunction photocatalysts. These results are also in accordance with the results of the photocatalytic activity test. It is worth noting that from Fig.9a and b, the intensities of the DMPO-•OH peaks of 1% ZnO/TiO2 and 5% ZnO/TiO2 are the same. However, the intensity of the DMPO-•O2- peaks of 5% ZnO/TiO2 is much smaller than that of 1% ZnO/TiO2. The reason may be that the recombination of the photogenerated electrons and holes exist a Z-scheme in the ZnO/TiO2 heterojunctions. Namely, the photoexcited electrons in the CB of TiO2 and photoexcited holes in the VB of ZnO are recombined.

20


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Fig. 9. ESR signals of DMPO-•O2- (a) and DMPO-•OH (b) with irradiation for 60 s. To further determine the •O2- and •OH radicals generated in the photocatalytic process, NBT (Nitroblue tetrazolium) and TA (Terephthalic acid-photoluminescence) experiments were performed. The amount of •O2- generated during the photocatalytic reaction was monitored through the evolution of NBT (nitroblue tetrazolium of 5×10-5 M with an absorption maximum at 259 nm).57 NBT can be specifically reduced by 21



photogenerated •O2-, forming the insoluble purple formazan forms in the aqueous solutions. The production of •O2- was quantitatively analyzed by detecting the decrease of the concentration of NBT in the supernatant solutions with UV–vis spectrophotometer. From Fig. S1, it can be seen that the UV-vis absorption spectra at 259 nm of the samples after irradiation for 5 min are lower than that of the NBT. It is confirmed that •O2- radicals are indeed produced for ZnO, TiO2 and ZnO/TiO2 samples in the reaction system. As shown in Fig. S1a, the generation rate of •O2radicals on the TiO2 surface is higher than that of other samples. The amount of •O2radicals generated on the ZnO/TiO2 heterojunctions is decreased with the increase of ZnO weight ratio up to 5% ZnO. From Fig. S1b, it is clear that when the amount of ZnO is higher than 5%, the amount of •O2- radicals generated on the surface of ZnO/TiO2 heterojunctions is increased gradually. However, Fig. S1c shows when ZnO is the primary part of the heterojunction photocatalysts (TiO2/ZnO), the amount of •O2- radicals generated on the TiO2/ZnO surface is higher than that of ZnO, and the amount of •O2- radicals generated on the surface of heterojunctions increases with the increase of TiO2 weight ratio up to 5% (95% ZnO/TiO2). From Fig. S1d, it is obvious that the amount of •O2- radicals produced for 95%ZnO/TiO2 is higher than that of other photocatalysts. The results are in accordance with the results of the photocatalytic activity test and ESR experiments. Moreover, the result also proved that the intensity of the DMPO-•O2- peaks of 5% ZnO/TiO2 is lower than that of TiO2 and 1% ZnO/TiO2. TA can react with free radical •OH to produce a fluorescence material, 2-hydroxyterephthalic acid. And its PL intensity is proportional to the amount of •OH radicals. Therefore, TA can be applied to detect whether •OH is produced in the reaction process.58,59 From Fig. S2a, it is clear that when the main part of the samples is TiO2, the amount of •OH produced on the surface of the ZnO/TiO2 heterojunctions is lowerer than that of TiO2. The peak intensities of 2-hydroxyterephthalic acid for the ZnO/TiO2 heterojunctions are decreased with the increase in the content of ZnO up to 5%. However, as shown in Fig. S2b, when the amount of ZnO is higher than 5%, the 22


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generation rate of •OH radicals is increased gradually. From Fig. S2c, it can be seen that when the main part of the ZnO/TiO2 heterojunctions is ZnO, the peak intensities of 2-hydroxyterephthalic acid for the ZnO/TiO2 samples are almost the same. However, the amount of •OH generated on the surface of ZnO/TiO2 heterojunctions are a little higher than that of ZnO. From Fig. S2d, it is obvious that the peak intensity of 2-hydroxyterephthalic acid for the 95% ZnO/TiO2 sample is the strongest. That is to say, the amount of •OH produced on the 95% ZnO/TiO2 heterojunctions is the highest. The results are also in accordance with the results of the photocatalytic activity test and ESR techniques. Simultaneously, it shows that the intensities of the DMPO-•OH peaks of TiO2, 1% ZnO/TiO2, and 5% ZnO/TiO2 are similar, and there are no change much.

3.3.2. Photoelectrochemical properties Photoelectrochemical experiments were carried out to explore the electronic interaction between TiO2 and ZnO. In consideration of the intensive relation between the separation of photoexcited charges and transient photocurrent intensity, the photocurrent responses of the samples were detected using electrochemical workstation under the UV-light irradiation.60 From Fig. 10a, it can be seen that compared with pure TiO2, when the main part of the ZnO/TiO2 heterojunction photocatalyst is TiO2, the photocurrent density of the ZnO/TiO2 heterojunctions is decreased rapidly with the increase of ZnO weight ratio up to 5%. However, from Fig. 10b, when the amount of ZnO is higher than 5%, the photocurrent density of the ZnO/TiO2 composites increases gradually. From Fig. 10c, when ZnO is the primary part of the heterojuction photocatalysts (TiO2/ZnO), the photocurrent density of the TiO2/ZnO heterojunctions increases with the increase of the TiO2 weight ratio up to 5% (95% ZnO/TiO2). Generally, the photocurrent intensity was determined by the separation efficiency of photogenerated charges, and the higher photocurrent intensity corresponds to higher separation efficiency of photogenerated charges, which means that photoexcited electro-hole pairs have a long life time. Moreover, from Fig. 10d, it can be seen that the 95% ZnO/TiO2 sample exhibits the strongest photocurrent 23



responses, indicating that the sample has the strongest separation efficiency for the photogenerated charges. The 5% ZnO/TiO2 sample exhibits the lowest photocurrent responses, showing that the addition of a small of ZnO will accelerate the recombination rate of photogenerated electrons and holes for the ZnO/TiO2 heterojunction photocatalysts with TiO2 as the main part. However, when the main part of ZnO/TiO2 heterojunction photocatalysts is ZnO, the ZnO/TiO2 photocatalysts demonstrate excellent separation efficiency for the photogenerated charges with the addition of a small amount of TiO2. The results of the photocurrent experiments are consistent with the results of the photocatalytic activity test. Electrochemical impedance spectroscopy (EIS) was further performed to investigate the separation efficiency of the photogenerated charge carriers.61 EIS Nyquist plots of the samples were carried out in a cycled 0.1 M KCl aqueous solution containing 0.1M K3[Fe(CN)6]–K4[Fe(CN)6] (1:1). As displayed in Fig. 11, only one semicircular arc is observed at high frequency on the EIS spectra, indicating that only surface charge-transfer is involved in the photocatalytic reaction system.62 The smaller arc radius in the EIS spectrum reveals the faster interfacial charge transfer and the separation of the photogenerated carriers. From Fig. 11a, it is clear that when the main part of the ZnO/TiO2 heterojunction photocatalyst is TiO2, the arc radius of the ZnO/TiO2 heterojunctions enlarge with the increase of the ZnO weight ratio up to 5%. However, from Fig. 11b, when the amount of ZnO is higher than 5%, the arc radiuses of the ZnO/TiO2 heterojunctions reduce gradually. From Fig. 11c, it can be seen that when ZnO is the primary part of the heterojuction photocatalysts (TiO2/ZnO), the arc radiuses of the TiO2/ZnO samples reduce with the increase of the TiO2 up to 5% (95% ZnO/TiO2). As shown in Fig. 11d, the 5% ZnO/TiO2 sample exhibits the biggest arc radius. It means that the separation efficiency of the photogenerated electrons and holes and the charge transfer are the lowest. Meanwhile, the 95% ZnO/TiO2 sample shows the smallest arc radius, suggesting that the separation efficiency of the photogenerated electrons and holes and the charge transfer are the highest. Based on the results, it is clear that the EIS results are in accordance with the results of 24


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photocatalytic activity test.

Fig. 10. Photocurrent transient response of the samples in a 0.2M Na2SO4 aqueous solution without bias versus Ag/AgCl electrode under UV-light irradiation.

25



Fig. 11. EIS Nyquist plots of the samples in 0.1 M KCl solution containing 0.1 M K3[Fe(CN)6]/K4[Fe(CN)6].

3.3.3. Speculated mechanisms of the ZnO/TiO2 heterojunctions Based on the results, it is known that the band gap of TiO2 and ZnO are 3.24 and 3.20 eV, respectively. The ECB of TiO2 and ZnO are -0.30 and -0.32 eV, and the EVB of TiO2 and ZnO are estimated to be 2.94 and 2.88 eV, respectively. In theory, when TiO2 and ZnO are combined, a heterojunction photocatalyst will be formed. Because of the difference in the positions of CB and VB between TiO2 and ZnO, the photoexcited electrons in the CB of ZnO will transfer into CB of TiO2, and the photoexcited holes in the VB of TiO2 will migrate in the VB of ZnO. The photoexcited carriers are separated effectively and the photocatalytic efficiency is improved. However, the experimental results did not fully support the above conclusions. The reason for the above results is that when TiO2 and ZnO are combined, a relative p-n junction between the TiO2 and ZnO will be formed due to their differences in conductivity. Therefore, the separation of photogenerated electrons and holes will be influenced by 26


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the internal electric field in the p-n junction, and thus affect the photocatalytic quantum efficiency. Meanwhile, it is proposed that when the amount of a semiconductor for a heterojunction photocatalyst exceeds a certain value, the conductivity of the relative p-n junction will be changed. This should affect the migration direction of the charge carriers in the p-n junction, and then influence the separation of the photogenerated electrons and holes, resulting in a change in the photocatalytic activity of the heterojunction photocatalysts.

3.3.4. Conductivity of the heterojunctions In order to prove the above speculation, the Motto-Schottky curves were measured and the results are shown in Fig. S3. The linear slopes of the samples are shown in Table 1. It is clear that the linear slopes of the M-S plots for the TiO2 and ZnO are positive, suggesting that the samples belong to typical n-type semiconductors.63 Theoretically, when the slope is larger, the sample is partial to n-type; and the slope is smaller, the sample is partial to p-type. Because the slope of ZnO is 3.57E9 and the TiO2 slope is 5.59E9, when TiO2 and ZnO are combined, a relative p-n junction photocatalyst (p-ZnO/n-TiO2) will be formed. In the relative p-n junction, TiO2 is predominant in the form of n-type, and ZnO is p-type. From Table 1, it is clear that when the main part of the samples is TiO2, the slopes of the p-ZnO/n-TiO2 samples decrease with the increase of the amount of ZnO. However, the slopes of the samples are between TiO2 and ZnO, indicating that the conductivity of the two semiconductors in the p-n junction photocatalyst does not change. When the amount of ZnO exceeds 5%, the slope of the sample increases gradually until the slope of the sample exceeds the slope of TiO2, which indicates that the conductivity of the p-n junction is changed with the increase of the amount of ZnO. From Table 1, it can be seen that when ZnO is the primary part of the heterojunctions (TiO2/ZnO), the slope of the heterojunction photocatalyst undergoes a qualitative change. When the TiO2 content is 1%, the slope of the sample is 8.63E9. It outdistances the slopes of ZnO and TiO2, indicating that the conductivity of TiO2 and ZnO in the heterojunction has changed qualitatively. At this point, the ZnO appears to be n-type and TiO2 is p-type 27



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in the relative p-n junction photocatalyst (p-TiO2/n-ZnO). With the increase of the amount of TiO2, the slope of the sample decreases gradually. However, the slope of the sample is larger than that of TiO2 (p-TiO2/n-ZnO), indicating that the conductivity of the TiO2 and ZnO in the relative p-n heterojunction has no change. Table 1. The linear slopes of the M-S plots for the samples. Sample

Slope

Sample

Slope

TiO2

5.59E9

70% ZnO/TiO2

5.57E9

1% ZnO/TiO2

5.10E9

90% ZnO/TiO2

5.84E9

3% ZnO/TiO2

4.65E9

93% ZnO/TiO2

6.66E9

5% ZnO/TiO2

4.49E9

95% ZnO/TiO2

7.38E9

10% ZnO/TiO2

4.38E9

97% ZnO/TiO2

8.45E9

15% ZnO/TiO2

4.41E9

99% ZnO/TiO2

8.63E9

30% ZnO/TiO2

4.52E9

ZnO

3.57E9

50% ZnO/TiO2

5.22E9

3.3.5. Proposed mechanisms of the ZnO/TiO2 heterojunctions Based on the results of photocatalytic performance of the samples and the characterization, the transfer mechanisms of photogenerated electrons and holes for ZnO/TiO2 heterojunction photocatalysts are proposed as follows: It is known by calculation that the work function of (103) plane of TiO2 is 4.38 eV, and the work function of the (100) plane of ZnO is 4.85 eV. It is clear that the work function of TiO2 is lower than that of ZnO. As shown in Fig. 12a, the photogenerated electrons in the TiO2 will flow into the ZnO, which levels up the potential energy surface at the interface of ZnO and TiO2 and eventually forms a heterojunction barrier (Fig. 12b and Fig. 12c). When the main part of the ZnO/TiO2 heterojunction is TiO2, a relative p-n junction photocatalyst (p-ZnO/n-TiO2) will be formed. At the equilibrium, the internal electric field formed between p-ZnO and n-TiO2 makes p-ZnO region have the negative charge, while n-TiO2 region have the positive charge. Under the effect of the internal electric field, the photoexcited holes flow into the negative field (p-ZnO) and the 28


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photogenerated electrons move into the positive field (n-TiO2). However, according to the work functions of TiO2 {103} and ZnO {100}, the photogenerated electrons in the TiO2 will flow into the ZnO. Therefore, the migration directions of the photogenerated electrons and holes in the CB and VB of ZnO and TiO2 are opposite to the transfer directions of electrons and holes in the relative p-n junction. It inevitably leads to the recombination of the photogenerated electron-hole pairs between the CB and VB of ZnO and TiO2, respectively. At the same time, the Z-scheme recombination of the photogenerated electrons and holes may occur in the CB of TiO2 and VB of ZnO. Therefore, it is not favorable for the formation of active species •OH and •O2-, and leads to lower photocatalytic performance. The schematic diagram is shown in Fig. 12b. When ZnO is the primary part of the heterojunction photocatalysts (TiO2/ZnO), the ZnO appears to be n-type and TiO2 is p-type in the relative p-n junction (p-TiO2/n-ZnO). Under the effect of the internal electric field, the photoexcited holes will flow into the negative field (p-TiO2) and the photogenerated electrons will move into the positive field (n-ZnO). Meanwhile, the photogenerated electrons in the TiO2 will flow into the ZnO due to the difference of the work function. Because the migration directions of the photoexcited electrons and holes in the CB and VB between the two semiconductors are the same as the transfer direction of electrons and holes in the relative p-n junction (p-TiO2/n-ZnO), the photogenerated electrons and holes of ZnO and TiO2 were separated effectively. It is favorable for the formation of active species •O2- and •OH in the p-TiO2/n-ZnO heterojunctions. Therefore, the p-TiO2/n-ZnO heterojunction exhibits excellent photocatalytic performance. The schematic diagram is shown in Fig. 12c.

29



Fig. 12. (a) Energy level lineup diagrams for the (103) plane of TiO2 and the (100) plane of ZnO. (b) Schematic diagrams of the photoexcited electron-hole transfer process over p-ZnO/n-TiO2 (the main part of the ZnO/TiO2 composite is TiO2) and (c) p-TiO2/n-ZnO (the main part of the TiO2/ZnO composite is ZnO) heterojunctions (CB, VB, e-, h+, Evac and EF are conduction band, valance band, photogenerated electrons, holes, vacuum level and Fermi level, respectively.).

4. Conclusions In summary, ZnO/TiO2 heterojunction photocatalysts with different weight ratio were successfully synthesized by mechanical mixing method. The conductivity of the samples is changed with the different amount of ZnO and TiO2 in the heterojunctions. When the main part of ZnO/TiO2 heterojunctions is TiO2, the photocatalytic performance is decreased rapidly with the increase of a small amount of ZnO. The reason may be attributed to the relative p-n junction (p-ZnO/n-TiO2) produced between ZnO and TiO2. The transfer directions of the electrons and holes in the CB and VB of TiO2 and ZnO are opposite to the migration directions of the photogenerated electrons and holes in the p-ZnO/n-TiO2 heterojunction due to the effect of internal electric field. However, when ZnO is the primary part of the heterojunction (p-TiO2/n-ZnO), the photocatalytic activity of the sample increases with the increase of amount of TiO2 up to 5% (95% ZnO/TiO2). The reason is that the transfer directions of electrons and holes in the CB and VB of TiO2 and ZnO are the same as the migration directions of the photogenerated electrons and holes in the 30


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p-TiO2/n-ZnO heterojunctions, which accelerates the separation of photogenerated electrons and holes. As a result, the photocatalytic performance of the heterojunction photocatalysts is increased.

ASSOCIATED CONTENT Supporting Information The degradation of RhB (Table S1), MO (Table S2) and BPA (Table S3) with three batches of photocatalysts, the absorbance of the NBT solution contained different photocatalysts (Fig. S1), photoluminescence (PL) emission spectra of the TA-•OH adducts formed under UV-light irradiation for different samples in TA solution (Fig. S2), and Mott-Schottky plots of the different samples (Fig. S3) (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (S. Meng), [email protected] (S. Chen). ORCID Sugang Meng: 0000-0002-2626-2637, Shifu Chen:0000-0002-0660-7773. Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by the Natural Science Foundation of China (NSFC, grant Nos. 51472005, 51772118, 21473066 and 21603002), the Natural Science Foundation of Anhui Province (grant No. 1608085QB37) and the Natural Science Foundation of Educational Committee of Anhui Province (grant No. KJ2018A0387).

References (1) Liu, G.; Yin, L.; Niu, P.; Jiao, W.; Cheng, H. Visible-Light-Responsive β-Rhombohedral Boron Photocatalysts. Angew. Chem. Int. Ed. 2013, 52, 6242-6245. (2) Grosst, M. A.; Reynal, A.; Durrant, J. R.; Reisner, E. Versatile Photocatalytic 31



Systems for H2 Generation in Water Based on an Efficient DuBois-Type Nickel Catalyst. J. Am. Chem. Soc. 2014, 136, 356-366. (3) Mamaghani, A. H.; Haghighat, F.; Lee, C.-S. Photocatalytic Oxidation Technology for Indoor Environment Air Purification: the State-of-the-Art. Appl. Catal. B: Environ. 2017, 203, 247-269. (4) Wang, S.; Guan, B. Y.; Lu, Y.; Lou, X. W. Formation of Hierarchical In2S3-CdIn2S4 Heterostructured Nanotubes for Efficient and Stable Visible Light CO2 Reduction. J. Am. Chem. Soc. 2017, 139, 17305-17308. (5) Yu, J.; Low, X.; Xiao, W.; Zhou, P.; Jaroniec, M. Enhanced Photocatalytic CO2-Reduction Activity of Anatase TiO2 by Coexposed {001} and {101} Facets. J. Am. Chem. Soc. 2014, 136, 8839-8842. (6) Paola, A. D.; García-López, E.; Marcì, G.; Palmisano, L. A Survey of Photocatalytic Materials for Environmental Remediation. J. Hazard. Mater. 2012, 211-212, 3-29. (7) Chu, S.; Wang, Y.; Guo, Y.; Feng, J.; Wang, C.; Low, W.; Fan, X.; Zou, Z. Band Structure Engineering of Carbon Nitride: in Search of a Polymer Photocatalyst with High Photooxidation Property. ACS Catal. 2013, 3, 912-919. (8) Enache, D. I.; Edwards, J. K.; Landon, P.; Espriu, B. S.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Solvent-Free Oxidation of Primary Alcohols to Aldehydes Using Au-Pd/TiO2 Catalysts. Science 2006, 311, 362-365. (9) Jiang, X.; Fu, X.; Zhang, L.; Meng, S.; Chen, S. Photocatalytic Reforming of Glycerol for H2 Evolution on Pt/TiO2: Fundamental Understanding the Effect of Co-Catalyst Pt and the Pt Deposition Route. J. Mater. Chem. A 2015, 3, 2271-2282. (10) He, Z.; Shi, Y.; Gao, C.; Wen, L.; Chen, J.; Song, S. BiOCl/BiVO4 p-n Heterojunction with Enhanced Photocatalytic Activity under Visible-Light Irradiation. J. Phys. Chem. C 2014, 118, 389-398. (11) Apgar, B. A.; Lee, S.; Schroeder, L. E.; Mratin, L. W. Enhanced Photoelectrochemical Activity in All-Oxide Heterojunction Devices Based on 32


Page 32 of 39

Page 33 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Correlated “Metallic” Oxides. Adv. Mater. 2013, 25, 6201-6206. (12) Kamat, P. V.; Jin, S. Semiconductor Photocatalysis: “Tell Us the Complete Story!”. ACS Energy Lett. 2018, 3, 622-623. (13) Chen, D.; Carus, R. A. Recent Progress in the Synthesis of Spherical Titania Nanostructures and their Applications. Adv. Funct. Mater. 2013, 23, 1356-1374. (14) Zahran, E. M.; Bedford, N. M.; Nguyen, M. A.; Chang, Y. J.; Guiton, B. S.; Nalk, R. R.; Bachas, L. G.; Knecht, M. R. Light-Activated Tandem Catalysis Driven by Multicomponent Nanomaterials. J. Am. Chem. Soc. 2014, 136, 32-35. (15) Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X. Semiconductor

Heterojunction

Photocatalysts:

Design,

Construction,

and

Photocatalytic Performances. Chem. Soc. Rev. 2014, 43, 5234-5244. (16) Zhang, J.; Xu, Q.; Feng, Z.; Li, M.; Li, C. Importance of the Relationship Between Surface Phases and Photocatalytic Activity of TiO2. Angew. Chem. Int. Ed. 2008, 47, 1766-1769. (17) Meng, S.; Ning, X.; Chang, S.; Fu, X.; Ye, X.; Chen, S. Simultaneous Dehydrogenation and Hydrogenolysis of Aromatic Alcohols in One Reaction System via Visible-Light-Driven Heterogeneous Photocatalysis. J. Catal. 2018, 357, 247-256. (18) Xiang, Q.; Yu, J. G.; Jaroniec, M. J. Synergetic Effect of MoS2 and Graphene as Cocatalysts for Enhanced Photocatalytic H2 Production Activity of TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134, 6575-6578. (19) Tang, H.; Ouyang, S.; Bi, Y.; Umeazwa, N.; Oshikiri, M.; Ye, J. Nano-Photocatalytic Materials: Possibilities and Challenges. Adv. Mater. 2012, 24, 229-251. (20) Wang, S.; Guan, B. Y.; Lou, X. W. Construction of ZnIn2S4-In2O3 Hierarchical Tubular Heterostructures for Efficient CO2 Photoreduction. J. Am. Chem. Soc. 2018, 140, 5037-5040. (21) Li, Y. F.; Liu, Z. P. Particle Size, Shape and Activity for Photocatalysis on Titania Anatase Nanoparticles in Aqueous Surroundings. J. Am. Chem. Soc. 2011, 133, 15743-15752. 33



Page 34 of 39

(22) Xia, J.; Di, J.; Li, H.; Xu, H.; Li, H.; Guo, S. Ionic Liquid-Induced Strategy for Carbon Quantum Dots/BiOX (X= Br, Cl) Hybrid Nanosheets with Superior Visible Light-Driven Photocatalysis. Appl. Catal. B: Environ. 2016, 181, 260-269. (23) Huang, H.; Li, D.; Lin, Q.; Zhang, W.; Shao, Y.; Chen, Y.; Sun, M.; Fu, X. Efficient Degradation of Benzene over LaVO4/TiO2 Nanocrystalline Heterojunction Photocatalyst under Visible Light Irradiation. Environ. Sci. Technol. 2009, 43, 4164-4168. (24) Yu, Y.; Cao, C.; Liu, H.; Li, P.; Wei, F.; Jiang, Y.; Song, W. A Bi/BiOCl Heterojunction Photocatalyst with Enhanced Electron-Hole Separation and Excellent Visible Light Photodegrading Activity. J. Mater. Chem. A 2014, 2, 1677-1681. (25) Zhou, J.; Zhang, M.; Zhu, Y. Photocatalytic Enhancement of Hybrid C3N4/TiO2 Prepared via Ball Milling Method. Phys. Chem. Chem. Phys. 2015, 17, 3647-3652. (26) Chen, S.; Zhao, W.; Liu, W.; Zhang, H.; Yu, X.; Chen, Y. Preparation, Characterization

and

Activity

Evaluation

of

pn

Junction

Photocatalyst

p-CaFe2O4/n-Ag3VO4 under Visible Light Irradiation. J. Hazard. Mater. 2009, 172, 1415-1423. (27) Lin, Y.; Li, D.; H, J.; Xiao, G.; Wang, J.; Li, W.; Fu, X. Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite. J. Phys. Chem. C 2012, 116, 5764-5772. (28) Huang, H.; He, Y.; Du, X.; Chu, P. K.; Zhang, Y. A General and Facile Approach to Heterostructured Core/Shell BiVO4/BiOI p-n Junction: Room-Temperature in Situ Assembly and Highly Boosted Visible-Light Photocatalysis. ACS Sustainable Chem. Eng. 2015, 3, 3262-3273. (29)

Dai,

G.;

Yu,

J.;

Liu,

G.

Synthesis

and

Enhanced

Visible-Light

Photoelectrocatalytic Activity of p-n Junction BiOI/TiO2 Nanotube Arrays. J. Phys. Chem. C 2011, 115, 7339-7346. (30) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Use of Highly-Ordered TiO2 Nanotube Arrays in Dye-Sensitized Solar Cells. Nano Lett. 2006, 6, 215-218. 34


Page 35 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(31) Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. (32) Zhang, N.; Zhang, Y.; Yang, M.; Tang, Z.; Xu, Y. A Critical and Benchmark Comparison on Graphene-, Carbon Nanotube-, and Fullerene-Semiconductor Nanocomposites as Visible Light Photocatalysts for Selective Oxidation. J. Catal. 2013, 299, 210-221. (33) Morales, M. V.; Asedegbega-Nieto, E.; Iglesias-Juez, A.; Rodrí guez-Ramos, I.; Guerrero-Ruiz, A. Role of Exposed Surfaces on Zinc Oxide Nanostructures in the Catalytic Ethanol Transformation. ChemSusChem 2015, 8, 2223-2230. (34) Wang, Y.; Shi, R.; Lin, J.; Zhu, Y. Enhancement of Photocurrent and Photocatalytic Activity of ZnO Hybridized with Graphite-Like C3N4. Energy Environ. Sci. 2011, 4, 2922-2929. (35) Chakrabarti, S.; Dutta, B. K. Photocatalytic Degradation of Model Textile Dyes in Wastewater Using ZnO as Semiconductor Catalyst. J. Hazard. Mater. 2014, 112, 269-278. (36) Wan, L.; Li, X.; Qu, Z.; Shi, Y.; Li, H.; Zhao, Q.; Chen, G. Facile Synthesis of ZnO/Zn2TiO4 Core/Shell Nanowires for Photocatalytic Oxidation of Acetone. J. Hazard. Mater. 2010, 184, 864-868. (37) Qu, Y.; Zhou, W.; Ren, Z.; Wang, G.; Jiang, B.; Fu, H. Facile Synthesis of Porous Zn2Ti3O8 Nanorods for Photocatalytic Overall Water Splitting. ChemCatChem 2014, 6, 2258-2262. (38) Zha, R.; Nadimicherla, R.; Guo, X. Ultraviolet Photocatalytic Degradation of Methyl Orange by Nanostructured TiO2/ZnO Heterojunctions. J. Mater. Chem. A 2015, 3, 6565-6574. (39) Zheng, X.; Li, D.; Li, X.; Chen, J.; Cao, C.; Fang, J.; Wang, J.; He, Y.; Zheng, Y. Construction of ZnO/TiO2 Photonic Crystal Heterostructures for Enhanced Photocatalytic Properties. Appl. Catal. B: Environ. 2015, 168, 408-415. (40) Chen, S.; Yang, Y.; Liu, W. Preparation, Characterization and Activity Evaluation of TiN/F-TiO2 Photocatalyst. J. Hazard. Mater. 2011, 186, 1560-1567. 35



Page 36 of 39

(41) Chen, S.; Ji, M.; Liu, W. Preparation, Characterization and Activity Evaluation of CaZrTi2O7 Photocatalyst. Mater. Chem. Phys. 2012, 134, 951-957. (42) Zhang, J.; Meng, S.; Ye, X.; Ling, C.; Zhang, S.; Fu, X.; Chen, S. Synergistic Effect of Photocatalysis and Thermocatalysis for Selective Oxidation of Aromatic Alcohols to Aromatic Aldehydes Using Zn3In2S6@ ZnO Composite. Appl. Catal. B: Environ. 2017, 218, 420-429. (43) Meng, S.; Li, D.; Fu, X.; Fu, X. Integrating Photonic Bandgaps with Surface Plasmon Resonance for the Enhancement of Visible-Light Photocatalytic Performance. J. Mater. Chem. A 2015, 3, 23501-23511. (44) Reddy, K. M.; Manorama, S. V.; Reddy, A. R. Bandgap Studies on Anatase Titanium Dioxide Nanoparticles. Mater. Chem. Phys. 2003, 78, 239-245. (45) Sakthivel, S.; Kisch, H. Daylight Photocatalysis by Carbon-Modified Titanium Dioxide. Angew. Chem. Int. Ed. 2003, 42, 4908-4911. (46) Guo, M.; Diao, P.; Cai, S. Hydrothermal Growth of Well-Aligned ZnO Nanorod Arrays: Dependence of Morphology and Alignment Ordering upon Preparing Conditions. J. Solid State Chem. 2005, 178, 1864-1873. (47) Hartanto, A. B.; Ning, X.; Nakata, Y.; Okada, T. Growth Mechanism of ZnO Nanorods from Nanoparticles Formed in a Laser Ablation Plume. Appl. Phys. A 2004, 78, 299-301. (48) Zhang, J.; Li, M. J.; Feng, Z. C.; Chen, J.; Li, C. UV Raman Spectroscopic Study on TiO2. I. Phase Transformation at the Surface and in the Bulk. J. Phys. Chem. B 2006, 110, 927-935. (49) Su, C.; Wang, C.; Hsueh, Y.; Gurylev, V.; Kei, C.; Perng, T. Enabling High Solubility of ZnO in TiO2 by Nanolamination of Atomic Layer Deposition. Nanoscale 2015, 7, 19222-19230. (50) Liu, Q.; Lu, H.; Shi, Z. W.; Wu, F. L.; Guo, J.; Deng, K. M.; Li, L. 2D ZnIn2S4 Nanosheet/1D

TiO2

Nanorod

Heterostructure

Arrays

for

Improved

Photoelectrochemical Water Splitting. ACS Appl. Mater. Interfaces 2014, 6, 17200-17207. 36


Page 37 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51) Fu, X.; Clark, L. A.; Yong, Q.; Anderson, M. A. Enhanced Photocatalytic Performance of Titania-Based Binary Metal Oxides: TiO2/SiO2 and TiO2/ZrO2. Environ. Sci. Technol. 1996, 30, 647-653. (52) Yu, J.; Wang, S.; Low, J.; Xiao, W. Enhanced Photocatalytic Performance of Direct Z-scheme g-C3N4-TiO2 Photocatalysts for the Decomposition of Formaldehyde in Air. Phys. Chem. Chem. Phys. 2013, 15, 16883-16890. (53) Dai, G.; Yu, J.; Liu, G. A New Approach for Photocorrosion Inhibition of Ag2CO3 Photocatalyst with Highly Visible-Light-Responsive Reactivity. J. Phys. Chem. C 2012, 116, 15519-15524. (54) Brezova, V.; Tarabek, P.; Dvoranova, D.; Stasko, A.; Biskupic, S. EPR Study of Photoinduced Reduction of Nitroso Compounds inTitanium Dioxide Suspensions. J. Photochem. Photobiol. A 2003, 155, 179-198. (55) Chen, C.; Ma, W.; Zhao, J. Semiconductor-Mediated Photodegradation of Pollutants under Visible-Light Irradiation. Chem. Soc. Rev. 2010, 39, 4206-4219. (56) Li, Y.; Wen, B.; Yu, C.; Chen, C.; Ji, H.; Ma, W.; Zhao, J. Pathway of Oxygen Incorporation from O2 in TiO2 Photocatalytic Hydroxylation of Aromatics: Oxygen Isotope Labeling Studies. Chem.-Eur. J. 2012, 18, 2030-2039. (57) Obregón, S.; Zhang, Y.; Colón, G. Cascade Charge Separation Mechanism by Ternary Heterostructured BiPO4/TiO2/g-C3N4 Photocatalyst. Appl. Catal. B: Environ. 2016, 184, 96-103. (58) Meng, S.; Li, D.; Sun, M.; Li, W.; Wang, J.; Chen, J.; Fu, X.; Xiao, G. Sonochemical Synthesis, Characterization and Photocatalytic Properties of a Novel Cube-Shaped CaSn(OH)6. Catal. Comm. 2011, 12, 972-975. (59) Pan, C.; Yu, J.; Chen, Y.; Zhu, Y. Influence of OH-related Defects on the Performances of BiPO4 Photocatalyst for the Degradation of Rhodamine B. Appl. Catal. B: Environ. 2012, 115-116, 314-319. (60) Soedergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S. E. Theoretical Models for the Action Spectrum and the Current-Voltage Characteristics of Microporous Semiconductor Films in Photoelectrochemical Cells. J. Phys. Chem. 1994, 98, 37



5552-5556. (61) Wang, D.; Choi, D.; Li, J.; Yang, Z.; Nie, Z.; Kou, R.; Hu, D.; Wang, C.; Saraf, L. V.; Zhang, J.; Aksay, I. A.; Liu, J. Self-Assembled TiO2-Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion. ACS Nano 2009, 3, 907-914. (62) Xiao, F.; Huang, S.; Tao, H.; Miao, J.; Yang, H.; Liu, B. Spatially Branched Hierarchical ZnO Nanorod-TiO2 Nanotube Array Heterostructures for Versatile Photocatalytic and Photoelectrocatalytic Applications: Towards Intimate Integration of 1D–1D Hybrid Nanostructures. Nanoscale 2014, 6, 14950-14961. (63) Kong, D. S. The Influence of Fluoride on the Physicochemical Properties of Anodic Oxide Films Formed on Titanium Surfaces. Langmuir 2008, 24, 5324-5331.

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