TiO2–ZnO Composite Sphere Decorated with ZnO Clusters for

Jul 3, 2015 - TiO2–ZnO Composite Sphere Decorated with ZnO Clusters for ...... De Angelis , F.; Di Valentin , C.; Fantacci , S.; Vittadini , A.; Sel...
0 downloads 0 Views 14MB Size
Subscriber access provided by UNIV OF MISSISSIPPI

Article

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

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

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)

ACS Paragon Plus Environment

1

Industrial & Engineering Chemistry Research

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

Page 2 of 28

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

ACS Paragon Plus Environment

2

Page 3 of 28

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

Industrial & Engineering Chemistry Research

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.

ACS Paragon Plus Environment

3

Industrial & Engineering Chemistry Research

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

Page 4 of 28

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

ACS Paragon Plus Environment

4

Page 5 of 28

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

Industrial & Engineering Chemistry Research

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.

ACS Paragon Plus Environment

5

Industrial & Engineering Chemistry Research

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

Page 6 of 28

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

ACS Paragon Plus Environment

6

Page 7 of 28

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

Industrial & Engineering Chemistry Research

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)

ACS Paragon Plus Environment

7

Industrial & Engineering Chemistry Research

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

Page 8 of 28

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

ACS Paragon Plus Environment

8

Page 9 of 28

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

Industrial & Engineering Chemistry Research

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

ACS Paragon Plus Environment

9

Industrial & Engineering Chemistry Research

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

Page 10 of 28

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.

ACS Paragon Plus Environment

10

Page 11 of 28

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

Industrial & Engineering Chemistry Research

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).

ACS Paragon Plus Environment

11

Industrial & Engineering Chemistry Research

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

Page 12 of 28

REFERENCES (1) Dahl, M.; Liu Y.; Yin Y. Composite titanium dioxide nanomaterials. Chem. Rev. 2014, 114, 9853. (2) Schmidt-Mende, L.; MacManus-Driscoll J. L. ZnO-nanostructures, defects, and devices. Mater. Today 2007, 10, 40. (3) Pal, S.; Leara, A. M.; Licciulli, A.; Catalano, M.; Taurino, A. Biphase TiO2 microspheres with enhanced photocatalytic activity. Ind. Eng. Chem. Res. 2014, 53, 7931. (4) Xu, S.; Wang Z. L. One-dimensional ZnO nanostructures: solution growth and functional properties. Nano Res. 2011, 4, 1013. (5) Xu, J.; Chen, Z.; Zapien, J. A.; Lee, C.-S.; Zhang, W. Surface engineering of ZnO nanostructures for semiconductor-sensitized solar cells. Adv. Mater. 2014, 26, 5337. (6) Chandiran, A. K.; Abdi-Jalebi, M.; Nazeeruddin, M. K.; Grätzel, M. Analysis of electron transfer properties of ZnO and TiO2 photoanodes for dye-sensitized solar cells. ACS Nano 2014, 8, 2261. (7) Wang, S.; Pan, L.; Song, J.-J.; Mi, W.; Zou, J.-J.; Wang, L; Zhang, X. Titanium-defected undoped anatase TiO2 with p-type conductivity, room-temperature ferromagnetism, and remarkable photocatalytic performance. J. Am. Chem. Soc. 2015, 137, 2975. (8) Pan, L.; Wang, S.; Mi, W.; Song, J.; Zou, J.-J.; Wang, L.; Zhang, X. Undoped ZnO abundant with metal vacancies. Nano Energy 2014, 9, 71.

ACS Paragon Plus Environment

12

Page 13 of 28

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

Industrial & Engineering Chemistry Research

(9) Matthews, R. W. Photocatalytic oxidation of chlorobenzene in aqueous suspensions of titanium dioxide. J. Catal. 1986, 97, 565. (10) Izumi, I.; Dunn, W. W.; Wilbourn, K. O.; Fan, F.-R. F.; Bard, A. J. Heterogeneous photocatalytic oxidation of hydrocarbons on platinized titanium dioxide powders. J. Phys. Chem. 1980, 84, 3207. (11) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental applications of semiconductor photocatalysis. Chem. Rev. 1995, 95, 69. (12) Yu, J.; Ran, J. Facile preparation and enhanced photocatalytic H2-production activity of Cu(OH)2 cluster modified TiO2. Energy Environ. Sci. 2011, 4, 1364. (13) Bian, Z.; Tachikawa, T.; Zhang, P.; Fujitsuka, M.; Majima, T. Au/TiO2 superstructure-based plasmonic photocatalysts exhibiting efficient charge separation and unprecedented activity. J. Am. Chem. Soc. 2014, 136, 458. (14) Eskandarloo, H.; Badiei, A.; Behnajady, M. A. TiO2/CeO2 hybrid photocatalyst with enhanced photocatalytic activity: optimization of synthesis variables. Ind. Eng. Chem. Res. 2014, 53, 7847. (15) Xiao, F.-X.; Hung, S.-F.; Tao, H. B.; Miao, J.; Yang, H. B.; 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. (16) Yu, J.; Qi, L.; Jaroniec, M. Hydrogen production by photocatalytic water splitting over Pt/TiO2 nanosheets with exposed (001) facets. J. Phys. Chem. C 2010, 114, 13118.

ACS Paragon Plus Environment

13

Industrial & Engineering Chemistry Research

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

Page 14 of 28

(17) Ulusoy, T. G.; Ghobadi, A.; Okyay, A. K. Surface engineered angstrom thick ZnO-sheathed TiO2 nanowires as photoanodes for performance enhanced dye-sensitized solar cells. J. Mater. Chem. A 2014, 2, 16867. (18) Wang, R.; Tan, H.; Zhao, Z.; Zhang, G.; Song, L.; Dong, W.; Sun, Z. Stable ZnO@TiO2 core/shell nanorod arrays with exposed high energy facets for self-cleaning coatings with antireflective properties. J. Mater. Chem. A 2014, 2, 7313. (19) Liu, R.; Wang, P.; Wang, X.; Yu, H.; Yu, J. UV- and visible-light photocatalytic activity of simultaneously deposited and doped Ag/Ag(I)-TiO2 photocatalyst. J. Phys. Chem. C 2012, 116, 17721. (20) Pan, L.; Zou, J.-J.; Wang, S.; Huang, Z.-F.; Yu, A.; Wang, L.; Zhang, X. Quantum dot selfdecorated TiO2 nanosheets. Chem. Commun. 2013, 49, 6593. (21) Park, K.; Zhang, Q.; Garcia, B. B.; Zhou, X.; Jeong Y.-H.; Cao, G. Effect of an ultrathin TiO2 layer coated on submicrometer-sized ZnO nanocrystallite aggregates by atomic layer deposition on the performance of dye-sensitized solar cells. Adv. Mater. 2010, 22, 2329. (22) Yu, H.; Liu, R.; Wang, X.; Wang, P.; Yu, J. Enhanced visible-light photocatalytic activity of Bi2WO6 nanoparticles by Ag2O cocatalyst. Appl. Catal. B: Environ. 2012, 111-112, 326. (23) Pan, L.; Wang, S.; Zou, J.-J.; Huang, Z.-F.; Wang, L.; Zhang, X. Ti3+-defected and V-doped TiO2 quantum dots loaded on MCM-41. Chem. Commun. 2014, 50, 988. (24) Pan, K.; Dong, Y.; Zhou, W.; Pan, Q.; Xie, Y.; Xie, T.; Tian, G.; Wang, G. Facile fabrication of hierarchical TiO2 nanobelt/ZnO nanorod heterogeneous nanostructure: an efficient photoanode for water splitting. ACS Appl. Mater. Interfaces 2013, 5, 8314.

ACS Paragon Plus Environment

14

Page 15 of 28

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

Industrial & Engineering Chemistry Research

(25) Hernández, S.; Cauda, V.; Chiodoni, A.; Dallorto, S.; Sacco, A.; Hidalgo, D.; Celasco, E.; Pirri, C. F. Optimization of 1D ZnO@TiO2 core-shell nanostructures for enhanced photoelectrochemical water splitting under solar light illumination. ACS Appl. Mater. Interfaces 2014, 6, 12153. (26) Kayaci, F.; Vempati, S.; Ozgit-Akgun, C.; Donmez, I.; Biyikli, N.; Uyar, T. Selective isolation of the electron or hole in photocatalysis: ZnO-TiO2 and TiO2-ZnO core-shell structured heterojunction nanofibers via electrospinning and atomic layer deposition. Nanoscale, 2014, 6, 5735. (27) Kayaci, F.; Vempati, S.; Donmez, I.; Biyikli, N.; Uyar, T. Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control defect density. Nanoscale 2014, 6, 10224. (28) Huang, Z.-F.; Zou, J.-J.; Pan, L.; Wang, S.; Zhang, X.; Wang, L. Synergetic promotion on photoactivity and stability of W18O49/TiO2 hybrid. Appl. Catal. B, Environ. 2014, 147, 167. (29) Yang, Y.-F.; Sangeetha, P.; Chen, Y.-W. Au/TiO2 catalysts prepared by photo-deposition method for selective CO oxidation in H2 stream. Int. J. Hydrogen Energy 2009, 34, 8912. (30) Zhang, Y. X.; Zeng, H. C. Gold sponges prepared via hydrothermally activated selfassembly of Au nanoparticles. J. Phys. Chem. C 2007, 111, 6970. (31) Huang, Z.-F.; Pan, L.; Zou, J.-J.; Zhang, X.; Wang, L. Nanostructured bismuth vanadatebased materials for solar-energy-driven water oxidation: a review on recent progress. Nanoscale 2014, 6, 14044.

ACS Paragon Plus Environment

15

Industrial & Engineering Chemistry Research

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

Page 16 of 28

(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.

ACS Paragon Plus Environment

16

Page 17 of 28

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

Industrial & Engineering Chemistry Research

(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.

ACS Paragon Plus Environment

17

Industrial & Engineering Chemistry Research

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

Page 18 of 28

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.

ACS Paragon Plus Environment

18

Page 19 of 28

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

Industrial & Engineering Chemistry Research

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

ACS Paragon Plus Environment

19

Industrial & Engineering Chemistry Research

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

Page 20 of 28

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.

ACS Paragon Plus Environment

20

Page 21 of 28

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

Industrial & Engineering Chemistry Research

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.

ACS Paragon Plus Environment

21

Industrial & Engineering Chemistry Research

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

Page 22 of 28

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.

ACS Paragon Plus Environment

22

Page 23 of 28

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

Industrial & Engineering Chemistry Research

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.

ACS Paragon Plus Environment

23

Industrial & Engineering Chemistry Research

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

Page 24 of 28

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.

ACS Paragon Plus Environment

24

Page 25 of 28

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

Industrial & Engineering Chemistry Research

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.

ACS Paragon Plus Environment

25

Industrial & Engineering Chemistry Research

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

Page 26 of 28

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

ACS Paragon Plus Environment

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

Industrial & Engineering Chemistry Research

=0 .

Page 27 of 28

-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.

ACS Paragon Plus Environment

27

Industrial & Engineering Chemistry Research

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

Page 28 of 28

For Table of Contents Only:

ACS Paragon Plus Environment

28