TiO2âZnO Composite Sphere Decorated with ZnO Clusters for

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TiO2-ZnO composite sphere decorated with ZnO clusters for effective charge isolation in photocatalysis Lun Pan, Guo-Qiang Shen, Jing-Wen Zhang, Xiao-Chu Wei, Li Wang, Ji-Jun Zou, and Xiangwen Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01471 • Publication Date (Web): 03 Jul 2015 Downloaded from http://pubs.acs.org on July 8, 2015

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Industrial & Engineering Chemistry Research

TiO2-ZnO composite sphere decorated with ZnO clusters for effective charge isolation in photocatalysis Lun Pan, Guo-Qiang Shen, Jing-Wen Zhang, Xiao-Chu Wei, Li Wang, Ji-Jun Zou*, and Xiangwen Zhang* Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University. Collaborative Innovative Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

AUTHOR INFORMATION Corresponding Author * [email protected] (J.-J. Zou); [email protected] (X. Zhang)

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ABSTRACT TiO2 and ZnO are extensively used photocatalyst but their activity needs improvement due to the rapid charge recombination. Herein, we designed and synthesized a novel structure of TiO2ZnO composite sphere decorated with ZnO clusters by a one-pot solvethermal method. TEM and EDX characterization show this structure contains TiO2 core, TiO2-ZnO composite (type II heterojunction) surface layer and surface c-axis ZnO clusters. The in-situ Au and PbO2 photodeposition shows the photoinduced electrons and holes are driven to ZnO clusters and TiO2 respectively, attributed to synergy of type II heterojunction of TiO2-ZnO and high electron mobility of ZnO. PL spectra confirm such structure is much more efficient in retarding the charge recombination than the sole TiO2-ZnO sphere. Importantly, this structure shows higher photoactivity in degradation of rhodamine B and isomerization of norbornadiene than pure TiO2, ZnO and TiO2-ZnO composite spheres.

KEYWORDS: TiO2; ZnO; composite material; charge isolation; photocatalysis


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1. Introduction TiO2 and ZnO are technologically important semiconductors and have been widely used in a variety of applications like solar cells, photocatalysis and environmental remediation.1-11 However, there are still some problems existed for these materials, among which the major one is the fast charge recombination. The utilization of surface electron trappers and/or construction of heterojunction are effective to solve this problem.1,6,12-16 The composite consisting of TiO2 and ZnO displays profoundly improved charge isolating capability compared with pure ZnO and TiO2, because their band levels match with each other to form type II heterojunction that makes photo-induced electrons enriched in ZnO and holes confined in TiO2,17,18 retarding the chargepairs recombination rate and increasing their lifetime. This increases the availability of charge pairs on the surface of photocatalyst and consequently an improvement of redox processe can be expected. Generally, noble metals like Pt and Ag are used as surface electron trappers,13,16,19 but they are cost-ineffective. Recently, we reported that TiO2 quantum dots (QDs) can work as electron trappers on TiO2 nanosheet to make electron isolated on QDs and holes on the nanosheet and significantly improve the photoactivity.20 Actually, ZnO has higher electron mobility than TiO2 (ZnO with mobility of 115-155 cm2V-1s-1, while TiO2 with that of 10-5 cm2V-1s-1),21 suggesting that ZnO particles or clusters may also serve as the good electron trappers. It is expected that the combination of type II heterojunction and surface electron trapper could produce more efficient charge transfer and isolation.1,22 Herein, we synthesized a novel TiO2ZnO composite sphere decorated with surface ZnO clusters. This structure not only possesses type II heterojunction (TiO2-ZnO composite) that can separate the electron and hole in ZnO and TiO2, but also has additional electron trapper (surface ZnO clusters) that can further isolate the photoinduced charges. As a result, the charge recombination is suppressed and the photoactivity is improved greatly.


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2. Experimental section 2.1. Materials Rhodamine B (RhB, 96%) and norbornadiene (NBD, 97%) were purchased from J&K Scientific Ltd. Ethanol (chromatographic grade), tetrabutyl titanate (TBT, 99%), zinc acetate (reagent grade), HAuCl4 (reagent grade), and Pb(NO3)2 (reagent grade) were purchased from Tianjin Guangfu Fine Chemical Research Institute. Deionized water (18.0 MΩ·cm) was used in all experiments. 2.2. Sample preparation For the preparation of TiO2-ZnO composite: 0.5 mL TBT (1.5 mmol), 0.327 g (1.8 mmol) or 0.164 g (0.9 mmol) zinc acetate and 79 mL ethanol were mixed and solvothermally treated in a 100 mL Teflon-lined autoclave at 150 °C for 24 h, and the produced white powders were calcined at 500 °C in air for 1 h. Pure ZnO or TiO2 was synthesized with the same procedure respectively using TBT and zinc acetate as precursors. 2.3. Characterization XRD characterization was conducted using a D/MAX-2500 X-ray diffractometer equipped with Cu Kα radiation. SEM images were observed using a field-emission scanning electron microscope (Hitachi S-4800). High-resolution TEM observations were carried out with a Tecnai G2 F-20 transmission electron microscope. Energy dispersive spectrum (EDS) characterization was performed with an EDX system attached to TEM. Steady-state photoluminescence (PL) spectra were measured by a Horiba JobinYvon Fluorolog3-21 with the excitation light at 325 nm. Surface composition and chemical states were analyzed with a PHI-5000 X-ray photoelectron spectroscope (XPS) equipped with Al Kα radiation, and the binding energy was calibrated by the C1s peak (284.6 eV) of the contamination carbon. UV-vis diffuse reflectance spectra (UV-vis


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DRS) were recorded with a Hitachi U-3010 spectrometer equipped with a 60 mm diameter integrating sphere using BaSO4 as the standard reflectance. 2.4. Photocatalytic reaction Photodegradation of organic dye was conducted in a closed quartz chamber (150 mL) vertically irradiated by a 300 W high-pressure xenon lamp (PLS-SXE300UV, Beijing Trusttech. Co. Ltd., wavelength ranges from 300 nm to 800 nm with total intensity of ca. 200 mW/cm2) that located on the upper position. The irradiation area was ca. 20 cm2. Reaction conditions: temperature, 25±0.2 ºC; C0(RhB)= 20 µmol·L–1, catalyst: 0.2 g·L–1; no acid or alkaline reagents were added. Reaction was conducted by magnetic stirring under atmosphere, after stirring for 20 minutes in dark to achieve adsorption equilibrium. Samples were withdrawn, centrifuged and analyzed using UV-vis spectrometer (U-3010, Hitachi Ltd.). Photocatalytic isomerization of norbornadiene (NBD) was evaluated in a closed cylindrical quartz vessel with inner irradiation. A high-pressure xenon lamp (Tianjinruisente UV Company, wavelength ranges from 300 nm to 800 nm with the total intensity of ca. 100 mW/cm2) was positioned inside the vessel and cooled by circulating water jacket. A mixture containing 5 mL NBD, 500 mL p-xylene and 0.1 g photocatalyst was suspended in the vessel under magnetic stirring. Samples were withdrawn at regular intervals and analyzed using an Agilent 7890A gas chromatography equipped with FID detector and AT-SE-54 capillary column (50 m× 0.32 mm). Photodeposition of Au and PbO2 were conducted using photo-reduction and photo-oxidation methods, respectively. Typically, 50 mg of catalyst was dispersed in 30 mL aqueous solution containing 0.78 mg·L−1 HAuCl4 or 2 mg·L−1 Pb(NO3)2 with magnetic stirring. After irradiation under UV-vis light for 30 min, the powders were separated, washed for five times and dried at room temperature.


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3. Results and discussion 3.1. Morphology and structure As shown in Figure 1a,b, the solvethermal treatment of TBT (1.5 mmol) in ethanol produces uniform TiO2 spheres in diameter of ca. 800 nm, which is abbreviated as TiO2-S. It has crystal structure of anatase (as confirmed by XRD pattern, Figure 1h). Similar treatment of zinc acetate (1.8 mmol) produces wurtzite ZnO nanoparticles in diameter of ca. 20 nm (ZnO-NPs, see Figure 1c,d,h, ). When both Ti and Zn precursors (1.5 and 0.9 mmol, respectively) are present, unique spheres in diameter of ca. 900 nm are formed (Figure 1e,f), named as TiO2-ZnO-S. Interestingly, further increasing the amount of Zn precursor (1.8 mmol) produces some clusters on the surface of spheres (namely TiO2-ZnO-SC, see Figure 1g). XRD patterns (Figure 1h) confirm the crystal mixture of anatase TiO2 and wurtzite ZnO in TiO2-ZnO-S and TiO2-ZnO-SC. And the peaks of ZnO are more intensive for TiO2-ZnO-SC than for TiO2-ZnO-S. (Figure 1) From Figure 1, rougher surface is observed with the increase of Zn precursor, from TiO2-S to TiO2-ZnO-S, and then to TiO2-ZnO-SC. HR-TEM (Figure 2a,b) indicates the surface of TiO2ZnO-S and TiO2-ZnO-SC is a mixture of TiO2 and ZnO NPs and the bulk is mainly TiO2, confirmed by the inter-plane spacing of of 0.35 nm and 0.26 nm, respectively.23,24 The particle size of TiO2 and ZnO are about 4 nm, and the thickness of surface compsite layer is about 5 nm (Figure 2b), which agrees with the grain size derived from XRD patterns (Table 1). Interestingly for TiO2-ZnO-SC, the clusters growing on the surface is pure ZnO along c-axis [0001] direction (inter-plane spacing of 0.26 nm,24 Figure 2c,d). EDX line scan (Figure 2e) and mapping images (Figure 3) also show the clusters contain Zn and O but no Ti elements. This result indicates the clusters on TiO2-ZnO-SC are pure ZnO whereas the surface is ZnO-TiO2 composite. Such structure should be resulted from the different hydrolysis rate of Ti and Zn precursors, with TBT


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first hydrolyzing to TiO2 core, then TiO2 and ZnO co-crystallizing to form surface layer, finally ZnO clusters growing on the surface. (Figure 2) (Figure 3) 3.2. Surface chemical status and band structure XPS characterization and Ar+ sputtering of TiO2-ZnO-SC were conducted to investigate its composition. As shown in Figure 4a, the O1s peak (ca. 530 eV) of Ar+ sputtered surface is intensified by ca. 2 times than that of fresh surface, because surface sputtering removes surface layer and makes inner TiO2 exposed. Ti4+ signals located at ca. 458.6 eV with relatively low intensity are detected for the fresh sample (Figure 4b), indicating the existence of surface TiO2 nanoparticles. The sputtering also causes the increase of Ti2p signals because the inner TiO2 cores are exposed. In contrast, the sputtering decreases the Zn2p signals (Figure 4c), so ZnO should be located on the surface. These results are consistent with TEM characterization. VB spectra were also conducted and shown in Figure 4d. The VB edge of surface is ca. 2.58 eV, which is low-energy shifted to 2.24 eV after Ar+ sputtering. Since the sputtering makes more inner TiO2 exposed, the VB maximum of ZnO must be lower than that of TiO2, agree with the previous work.17,18,25 Moreover, the band gap of ZnO and TiO2 is similar (about 3.1-3.2 eV).1-4 UV-vis DRS in Figure 5a indicates TiO2, ZnO and their composites possess the same optical absorption and band gap. Therefore, the TiO2-ZnO composite has type II heterostructure with well-matched band structure, that is, the CB minimum and VB maximum of TiO2 locate higher level than those of ZnO. (Figure 4) (Figure 5)


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3.3. Charge transfer and isolation PL spectra of the prepared samples are shown in Figure 5b-f. Since the majority of optical absorption and subsequent photoexcitation take place within the surface region of photocatalyst,26 the emission mainly reflects the surface charge recombination. For pure TiO2 and ZnO, two emission peaks appears (Figure 5c,d): CB-VB interband transition (3.1 eV and 3.2 eV, respectively)

and

oxygen-vacancy-related

visible

emission

(2.7

eV

and

2.4

eV,

respectively).8,20,26,27 TiO2-ZnO-S and TiO2-ZnO-SC show similar PL spectra (Figure 5b,e,f). The UV-emission occurred at ca. 3.2 eV is interband transition, and the emissions at ca. 2.8 eV and 2.4 eV originate from the shollow donor level of VO-TiO2 and VO-ZnO to valence band, respectively. In bulk grain region, VO+ captures a hole from VB and forms VO++ ca. 1.2 eV above the VB.26,27 Notably, the TiO2-ZnO composite exhibits much lower PL intensity than pure TiO2 and ZnO, especially in interband transition and VO-related transition, suggesting the high efficiency of charge separation via type II heterostructure. Notably, the PL intensity of TiO2-ZnO-SC is much lower than TiO2-ZnO-S (about 1:4, Figure 5b), indicating the presence of ZnO clusters can significantly enhance the charge transfer and separation. This result suggests ZnO clusters serve as very good electron trappers. Moreover, ZnO nanoparticles in the surface composite that connects TiO2 nanoparticles and ZnO clusters are very important to collect and then transfer electrons to ZnO clusters. So the synergetic function of heterojunction (TiO2-ZnO) and ZnO clusters facilitates the isolation of charge carriers and effectively suppresses the surface charge recombination. (Figure 6) To further demonstrate the charge-carrier isolation on TiO2-ZnO-SC, in-situ photodeposition of Au and PbO2 was conducted.28,29 Selective-area EDX and high-resolution TEM characterization (Figure 6a,c,d) verify the selective location of Au nanoparticles on c-axis ZnO


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clusters, see the crystal plane spacing of 0.21 nm (200) (Figure 6b).30 The optical absorption at ca. 550 nm also indicates the formation of Au nanoparticles on the surface (Figure 6e). In contrast, PbO2 only photo-deposites on the sphere surface, as confirmed by selective-area EDX spectra in Figure 7a,c,d. Also the PbO2 nanoparticles are verified by high-resolution TEM image, with crystal plane spacing of 0.25 nm (101) (Figure 7b).31 (Figure 7) Therefore, the photodeposition of Au and PbO2 separately occurs on clusters and sphere TiO2 grains. Since Au and PbO2 depositions are derived from photoinduced electron reduction and hole oxidation,28 the electron and hole must be isolated on ZnO clusters and TiO2 nanoparticles, respectively. Such kind spatial isolation of charges can greatly enhance the charge-separation efficiency. 3.4. Photocatalytic performance Two kind of model photocatalytic reactions, degradation of organic dye (rhodamine B, RhB) and isomerization of norbornadiene (NBD), were conducted to evaluate the photoactivity. During the photodegradation of RhB, the formation of hydroxyl radical (·OH) caused by both electrons and holes is crucial.32-34 Photoisomerization of NBD to quadricyclane (QC) is promising to convert solar energy into chemical energy stored in metastable isomers, and QC/N2O4 is promising high energy hypergolic bipropellant.23,35-38 This reaction proceeds through the exciplex (charge-transfer) intermediate of adsorbed NBD and surface trapped hole.39 For these two photoreactions, the rate-determining procedure is the charge transfer and isolation. (Figure 8) As shown in Figure 8, the photocatalyst shows similar trend in both reactions, and the activity is closely related to the charge-separation efficiency that shown in PL spectra. ZnO-NPs shows the lowest photoactivity, with k=0.005 min-1 and 0.357 h-1 for RhB degradation and NBD


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isomerization, respectively, due to the rapid recombination of photo-induced electron and hole. TiO2-S exhibits relatively higher photoactivity (0.006 min-1 and 0.447 h-1), attributed to the inherently higher photo-efficiency of indirect-band structure.40-42 Furthermore, TiO2-ZnO-S performs better activity (0.008 min-1 and 0.501 h-1) than TiO2-S and ZnO-NPs, owing to the formation of type II heterostructure layer that improves the charge separation. More importantly, TiO2-ZnO-SC (0.014 min-1 and 0.591 h-1) is more active than TiO2-ZnO-S, indicating ZnO clusters are vital in the photo-induced charge separation. For RhB photodegradation, the activity of TiO2-ZnO-SC is 2.3 and 2.8 times higher than pure TiO2-S and ZnO-NPs, respectively. As for NBD photoisomerization, the activity of TiO2-ZnO-SC is 1.3 and 1.7 times higher than pure TiO2-S and ZnO-NPs, respectively. Besides, the specific surface area of TiO2-ZnO-SC is lower than TiO2-S and ZnO NPs (Table 1), indicating the higher photoactivity of TiO2-ZnO-SC is not attributed to the surface area, but the higher charge-separation efficiency. 4. Conclusions We synthesized the TiO2-ZnO composite sphere deposited with novel c-axis ZnO clusters. In this structure, the type II heterojunction of TiO2-ZnO layer drives electron and hole respectively to ZnO and TiO2, and then electrons are transferred and isolated on ZnO clusters. The synergetic effect of type II heterojunction and ZnO clusters makes the charge-separation much more efficient than TiO2, ZnO and TiO2-ZnO composite. Therefore, it performs much higher activity in photodegradation of RhB and photoisomerization of norbornadiene.


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ACKNOWLEDGMENT The authors appreciate the supports from the National Natural Science Foundation of China (21222607), the Tianjin Municipal Natural Science Foundation (15JCZDJC37300), and the Program for New Century Excellent Talents in University (NCET-09-0594).


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(32) Chen, C. C.; Ma, W. H.; Zhao, J. C. Semiconductor-mediated photodegradation of pollutants under visible-light irradiation. Chem. Soc. Rev. 2010, 39, 4206. (33) Zhao, J.; Wu, T.; Wu, K.; Oikawa, K.; Hidaka H.; Serpone, N. Photoassisted degradation of dye pollutants. 3. Degradation of the cationic dye rhodamine B in aqueous anionic surfactant/TiO2 dispersions under visible light irradiation: evidence for the need of substrate adsorption on TiO2 particles. Environ. Sci. Technol. 1998, 32, 2394. (34) Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Water-mediated promotion of dye sensitization of TiO2 under visible light. J. Am. Chem. Soc. 2011, 133, 10000. (35) Zou, J.-J.; Liu, Y.; Pan, L.; Wang, L.; Zhang, X. Photocatalytic isomerization of norbornadiene to quadricyclane over metal (V, Fe and Cr)-incorporated Ti-MCM-41. Appl. Catal. B, Environ. 2010, 95, 439. (36) Dubonosov, A. D.; Bren, V. A.; Chernoivanov, V. A. Norbornadiene-quadricyclane as an abiotic system for the storage of solar energy. Russ. Chem. Rev. 2002, 71, 917. (37) Fan, H.-F.; Chin, T.-L.; Lin, K.-C. Kinetics of catalytic isomerization of quadricyclane to norbornadiene using near infrared absorption spectroscopy: conversion rate and diffusion motion in heterogeneous reaction. J. Phys. Chem. B 2004, 108, 9364. (38) Pan, L.; Feng, R.; Peng, H.; E, X.-t.-f.; Zou, J.-J.; Wang, L.; Zhang, X. A solar-energyderived strained hydrocarbon as an energetic hypergolic fuel. RSC Adv. 2014, 4, 50998. (39) Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Photoisomerization of norbornadiene to quadricyclane using transition metal doped TiO2. Ind. Eng. Chem. Res. 2010, 49, 8526.


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(40) Linsebigler, A. L.; Lu, G.; Yates, J. T. Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem. Rev. 1995, 95, 735. (41) Angelis, F. D.; Valentin, C. D.; Fantacci, S.; Vittadini, A.; Selloni, A. Theoretical studies on anatase and less common TiO2 phases: bulk, surfaces, and nanomaterials. Chem. Rev. 2014, 114, 9708. (42) Pan, L.; Zou, J.-J.; Wang, S.; Huang, Z.-F.; Zhang, X.; Wang, L. Enhancement of visiblelight-induced photodegradation over hierarchical porous TiO2 by nonmetal doping and watermediated dye sensitization. Appl. Surf. Sci. 2013, 268, 252.


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Table caption Table 1. Crystal size (via Scherrer’s equation) and special surface area of as-prepared samples. Figure captions Figure 1. SEM images of (a, b) TiO2-S, (c, d) ZnO-NPs, (e, f) TiO2-ZnO-S, and (g) TiO2-ZnOSC (inset is the magnified image); (h), XRD patterns of as-prepared samples. Figure 2. TEM images of (a, b) TiO2-ZnO-S, and (c-e) TiO2-ZnO-SC. Insets in (d) and (e) are FFT patterns of ZnO clusters in wurtzite phase and EDX line scan along the sphere, respectively. Figure 3. TEM-EDX mapping images of TiO2-ZnO-SC. (a) HAADF-STEM image, (b-d) are the mapping images of O, Ti and Zn elements in selected area of 1 in (a). The 2 in (a) is the reference site during EDX testing. Figure 4. XPS spectra of fresh and Ar+-sputtered (for 120 s) TiO2-ZnO-SC. (a) O1s, (b) Ti2p, (c) Zn2p, and (d) VB spectra. Figure 5. (a) UV-vis DRS and (b) PL spectra of ZnO-NPs, TiO2-S, TiO2-ZnO-S and TiO2-ZnOSC, and peak-fitting PL spectra of (c) TiO2-S, (d) ZnO-NPs, (e) TiO2/ZnO-SC and (f) TiO2/ZnOS. CB, conduction band; VB, valence band; VO, oxygen vacancy; VO++, hole-captured VO; hcptr, the hole capture. Figure 6. In-situ photo-deposition of Au nanoparticles on TiO2-ZnO-SC. (a, b) TEM images; (c, d) selected-area EDX spectra; (e) UV-vis DRS of TiO2-ZnO-SC with Au deposition after UV irradiation. Figure 7. In-situ photo-deposition of PbO2 nanoparticles on TiO2-ZnO-SC. (a, b) TEM images; (c, d) selected-area EDX spectra. Figure 8. Pseudo-first-order kinetics of (a) photodegradation of RhB and (b) photoisomerization of NBD.


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Table 1. Crystal size (via Scherrer’s equation) and special surface area of as-prepared samples.

Sample

TiO2-S

TiO2-ZnO-S

TiO2-ZnO-SC

ZnO-NPs

Crystal size/nm

6.1

5.2(TiO2)/6.2(ZnO)

4.7(TiO2)/8.5(ZnO)

13.2

SBET/(m2/g)

50.1

9.6

8.5

22.1


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Figure 1. SEM images of (a, b) TiO2-S, (c, d) ZnO-NPs, (e, f) TiO2-ZnO-S, and (g) TiO2-ZnOSC (inset is the magnified image); (h), XRD patterns of as-prepared samples.


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Figure 2. TEM images of (a, b) TiO2-ZnO-S, and (c-e) TiO2-ZnO-SC. Insets in (d) and (e) are FFT patterns of ZnO clusters in wurtzite phase and EDX line scan along the sphere, respectively.


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Figure 3. TEM-EDX mapping images of TiO2-ZnO-SC. (a) HAADF-STEM image, (b-d) are the mapping images of O, Ti and Zn elements in selected area of 1 in (a). The 2 in (a) is the reference site during EDX testing.


22

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a

b

Ti2p Fresh surface + After Ar sputtering for 120s

Intensity/a.u.

Intensity/a.u.

O1s Fresh surface + After Ar sputtering for 120s

520

525

530

535

450

540

1015

460

465

470

Fresh surface Zn2p + After Ar sputtering for 120s

d

VB Fresh surface + After Ar sputtering for 120s

Intensity/a.u.

c

455

Binding Energy/eV

Binding Energy/eV

Intensity/a.u.

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


1020

1025

Binding Energy/eV

1030

0

1

2

3

4

Binding energy/eV

Figure 4. XPS spectra of fresh and Ar+-sputtered (for 120 s) TiO2-ZnO-SC. (a) O1s, (b) Ti2p, (c) Zn2p, and (d) VB spectra.


23


1.0

a

TiO2-ZnO-S TiO2-ZnO-SC

0.6

ZnO-NPs TiO2-S TiO2-ZnO-S

Intensity/a.u.

Intensity/a.u.

b

ZnO-NPs TiO2-S

0.8

0.4

TiO2-ZnO-SC

0.2

0.0 350

400

450

500

550

350

600

400

450

500

550

600

650

c

d

CB

VO

Intensity

Intensity

CB

VO 3.1 eV

700

Wavelength/nm

Wavelength/nm

2.7 eV

3.2 eV

2.4 eV

VB VB

3.6

3.2

2.8

2.4

2.0

1.6

3.6

3.2

2.8

e

1.6

CB

VO -TiO2 2.09 eV

Intensity

VO-ZnO

++

2.76 eV

2.0

f

CB

VO -TiO2 3.20 eV

2.4

hv/eV

hv/eV

Intensity

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

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2.39 eV

VO hcptr

3.20 eV

VO-ZnO

2.07 eV ++

2.78 eV

VO

2.42 eV

hcptr VB

VB

3.6

3.2

2.8

2.4

2.0

1.6

3.6

3.2

hv /eV

2.8

2.4

2.0

1.6

hv /eV

Figure 5. (a) UV-vis DRS and (b) PL spectra of ZnO-NPs, TiO2-S, TiO2-ZnO-S and TiO2-ZnOSC, and peak-fitting PL spectra of (c) TiO2-S, (d) ZnO-NPs, (e) TiO2/ZnO-SC and (f) TiO2/ZnOS. CB, conduction band; VB, valence band; VO, oxygen vacancy; VO++, hole-captured VO; hcptr, the hole capture.


24

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Figure 6. In-situ photo-deposition of Au nanoparticles on TiO2-ZnO-SC. (a, b) TEM images; (c, d) selected-area EDX spectra; (e) UV-vis DRS of TiO2-ZnO-SC with Au deposition after UV irradiation.


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Figure 7. In-situ photo-deposition of PbO2 nanoparticles on TiO2-ZnO-SC. (a, b) TEM images; (c, d) selected-area EDX spectra.


26

b

TiO2-S TiO2-ZnO-S

0.8

1.4 TiO2-S 1.2

TiO2-ZnO-SC ZnO-NPs

1.0

TiO2-ZnO-S TiO2-ZnO-SC ZnO-NPs

-1

1.0

59 1h

a

k

0.6 -1

-1

k=0.014 min

0.4

k=

0.2

0 0.

in m

08

-1

n mi 06 0.0 = k

-ln(1-x)

-ln(C/C0)

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


=0 .

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-1

0.8

h 01 .5 =0

k

0.6

-1

44 0. k=

7h -1

7h .35 =0

0.4

k

-1

05 m

k=0.0

in

0.2

0.0 0

20

40

60

Reaction time/min

80

0.0 0.0

0.5

1.0 1.5 Time/h

2.0

Figure 8. Pseudo-first-order kinetics of (a) photodegradation of RhB and (b) photoisomerization of NBD.


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For Table of Contents Only：


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TiO2âZnO Composite Sphere Decorated with ZnO Clusters for

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