Ag decorated 3D urchin-like TiO2 microstructures synthesized via a one-step solvothermal method and their photocatalytic activity

Journal of Alloys and Compounds 648 (2015) 22e28

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Ag decorated 3D urchin-like TiO2 microstructures synthesized via a one-step solvothermal method and their photocatalytic activity Peng Wang, Chaohao Peng, Mu Yang* Beijing Key Laboratory of Function Materials for Molecule & Structure Construction, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 March 2015 Received in revised form 17 June 2015 Accepted 27 June 2015 Available online 2 July 2015

Novel 3D urchin-like TiO2 microstructures decorated with Ag nanoparticles were successfully fabricated by one-step solvothermal method with the mixed solution of ethylene glycol, AgNO3 and tetrabutyl titanate. The morphology, microstructure, crystalline and composition of the composite materials could be adjusted through changing the content of AgNO3, solvothermal temperature and time. The obtained samples were characterized by TEM, XRD and XPS. The formation mechanism of these microstructures was proposed. The Ag nanoparticle-decorated 3D urchin-like TiO2 microstructures showed excellent photocatalytic activity for the degradation of Rhodamine B under UVeVis irradiation, which was attributed to their special 3D superstructure, crystalline composition and Ag nanoparticle decorated as well. © 2015 Elsevier B.V. All rights reserved.

Keywords: Urchin-like Solvothermal TiO2 Ag decorated Photocatalyst

1. Introduction In recent years, titania (TiO2) nanomaterials with special structure and morphology, such as nanoparticles, nanofibers, nanotubes and flower-like structure, etc. [1e4], have raised extensive concerns. These nanostructures show excellent performance and broad applications in photodegradation of organic pollutants, storage and utilization of solar energy, photochromism, and so on [4,5]. Low-dimensional TiO2 nanomaterials, including zerodimensional nanoparticles and one-dimensional nanomaterials (nanofibers, nanotubes), show better activity than their corresponding bulk materials due to large specific surface areas. However, large specific surface area also makes these nanomaterials incline to aggregate. It is difficult to disperse the low-dimensional TiO2 nanomaterials into aqueous solution and their applications are restricted, especially in the water purification [6]. Assemble low-dimensional TiO2 nanomaterials into 3D microstructure is an effective approach to solve the dispersion of low-dimensional nanomaterials [7e10]. For example, one-dimensional nanofibers/ nanotubes can assemble to formation of urchin-like structure, which not only maintains the advantages of low-dimensional

* Corresponding author. E-mail address: [email protected] (M. Yang). http://dx.doi.org/10.1016/j.jallcom.2015.06.244 0925-8388/© 2015 Elsevier B.V. All rights reserved.

materials, but also shows good dispersion and can be easily separated from aqueous solution. Therefore, 3D structure TiO2 became a focus of current research. A large number of 3D urchin-like TiO2 nanostructures have been synthesized through hydrothermal synthesis [2,11], vapor deposition [12], self-assembly [13], and template-sacrificial techniques [14]. Some studies also show that the performance of the urchin-like structure is superior to lowdimensional nanomaterials [15e17]. In other way, pure TiO2 shows low photocatalytic efficiency due to the high recombination rate of charge carriers. In order to improve the photocatalytic activity of TiO2, metal nanoparticles are deposited onto the TiO2 surface [18e25]. Photo-generated electrons move to metals nanoparticles, and the metal nanoparticles become effective capture traps of photo-generated electron to suppress the recombine of electrons and holes. Among these decorated metals, Ag has been widely used to modify TiO2 material [10,21e29]. Yi [28] synthesized Ag modified TiO2 nanorods via solegel methods, and found that surface modification reduced the e
/hþ recombination and increased the charge transfer rate. Zhou [21] obtained one-dimensional anatase TiO2/Ag heterojunction plasmonic photocatalyst through a wet impregnation method. Zhao [29] prepared 3D urchin-like hierarchical TiO2 nanostructures decorated with Au or Ag nanoparticles by wet-chemical process. These synthetic processes include multi-steps and are complex. Beside, most products need calcination at high temperature to

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obtain good photocatalytic activity, which easily lead to morphology change. In this paper, a simple and one-step solvothermal method was used to synthesize Ag nanoparticles decorated 3D urchin-like TiO2 microstructures. The structure, morphology and crystalline of the product were adjusted through changing the reaction conditions, such as the content of AgNO3, solvothermal temperature and time. The formation mechanism of the novel 3D microstructure was proposed. The photocatalytic activity of the Ag nanoparticle-doped 3D urchin-like TiO2 microstructures was evaluated by photocatalytic degradation of Rhodamine B aqueous solution under UVeVis irradiation. 2. Experimental section 2.1. Materials Ethylene glycol (EG), silver nitrate (AgNO3) and tetrabutyl titanate (TBT) were purchased from Guangdong Chemical Reagent Company, Beijing Chemical Works and Tianjin Guangfu Fine Chemical Research Institute, respectively. All the chemicals were analytical grade and used without further purification. 2.2. Preparation of Ag nanoparticle-decorated 3D urchin-like TiO2 microstructures Ag nanoparticle-decorated 3D urchin-like TiO2 microstructures were synthesized by a simple solvothermal reaction of the EGAgNO3-TBT mixed solution at a high temperature. In a typical synthesis, 0.1125 g AgNO3 was dissolved into 15 mL EG under stirred, and then 0.6 mL TBT was added dropwise into the solution. The mixed solution was stirred for 2.5 h, and transferred to a 50 mL polyphenyl ester-lined stainless-steel autoclave. The reaction maintained at 240  C for 14 h. After the autoclave cooled to room temperature, the product was collected by centrifugated, washed with ethanol and then water several times, and dried at 60  C for 12 h. In order to study the effect of different reaction conditions and the formation mechanism of Ag nanoparticle-decorated 3D urchinlike TiO2 microstructures, other samples were prepared under the same procedure with different concentration of AgNO3, solvothermal time and temperature. 2.3. Characterization The structure and morphology of the obtained products were observed by transmission electron microscopy (TEM, JEOL JEM100CX II) and high resolution transmission electron microscopy (HRTEM, FEI TECNAI 20). The crystal structure of samples was characterized by a powder X-ray diffraction (XRD, M21X) analysis using Cu Ka radiation wavelength of 0.154 nm. Nitrogen adsorptionedesorption isotherms were obtained using an AUTOSORB-1C analyzer (USA Quantachrome Instruments) at 77 K. The chemical states and concentration of the doping elements were determinated by X-ray photoelectron spectroscopy (XPS, Thermo EscaLab 250Xi with a monochromatic Al Ka X-ray source). 2.4. Photocatalytic activity Analytical-grade Rhodamine B (RhB, Shenyang Reagent Factory) was served as the target organic pollutant for photocatalytic experiments. The photocatalytic activity measurement was performed at room temperature. In a typical process, 30 mg of Ag nanoparticles doped TiO2 sample was added into a 100 mL 5 mg/L RhB solution. The above mixture was ultrasonically vibrate for

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10 min, and then magnetically stirred under complete darkness condition for 50 min to reach absorption balance. The solution was sealed in the reaction vessel with recirculating cooling water and exposed to UV light irradiation using a 300 W SolarEdge700 xenon lamp, whose rated current was set to 20 A. During the degraded reaction, the solution was continuously stirred. Intermediate solutions were taken every 20 min from the reaction vessel and were separated by centrifugation to test the residual concentration of RhB using a UVeVis spectrophotometer (Shimadzu UV-2550) at l 553 nm. 3. Results and discussion 3.1. Structure characteristic Fig. 1 shows TEM images and XRD result of the as-prepared sample obtained from a one-step solvothermal synthesis. The sample displays 3D urchin-like microstructure with total size of about 800 nm. Each “urchin” is composed of nanofibers with 5e30 nm in diameter and 200e500 nm in length and nanoparticles are adhered on the surface of the nanofibers (Fig. 1a, b). Fig. 1eeg shows high-angle annular dark field STEM images of Ag decorated 3D urchin-like TiO2 microstructures and shows their morphology (Fig. 1e), their elemental maps for Ti (Fig. 1f) and Ag (Fig. 1g). Fig. 1g shows that large Ag nanoparticles are even distributed on/in the surfaces of the urchin-like structure. Fig. 1c presents the HRTEM image of the end of nanofibers. The lattice fringes with interplanar distances of 0.35 nm and 0.30 nm correspond to the anatase (101) and rutile (110) planes, respectively. This indicates the high crystallinity and mixed crystal phases of the urchin-like TiO2 microstructures. The XRD pattern of the sample is shown in Fig. 1d. It also assures that TiO2 in the sample contains two types of crystal: anatase and rutile. The diffraction peaks at 2q values of 25.3 , 48.2 and 63.4 , correspond to (101), (200) and (002) crystal planes of anatase TiO2 (JCPDS card no. 89-4921), respectively. While 27.5 , 36.0 , 41.3 , 54.2 , 56.1 and 68.7 correspond to (110), (101), (111), (211), (220) and (301) crystal planes of rutile TiO2 (JCPDS card no. 21-1276), respectively. Besides these diffraction peaks of TiO2, five strong diffraction peaks at 38.1, 44.3 , 64.4 , 77.4 and 81.5 corresponding to (111), (200), (220), (311) and (222) planes of facecentered cubic Ag (JCPDS card no. 4-783) are also observed in the XRD pattern. The crystallite size of anatase, rutile and Ag is 13.6, 24.5 and 64.7 nm, respectively, according to Scherrer's formula with their corresponding strongest peaks. Nitrogen adsorption data show BET specific surface area of Ag decorated 3D urchin-like TiO2 microstructures is 20.5 m2/g. Fig. 2 shows XPS survey scans of Ag decorated 3D urchin-like TiO2 microstructures. In the full-spectrum (Fig. 2a), the signal peaks at 367.8 eV, 460.2 eV and 529.4 eV correspond to the characteristic peaks of Ag 3d5/2, Ti 2p3/2, and O 1s, respectively. The mole ratio of Ag/Ti/O obtained from the peak area was approximately 2.9/32.0/65.1. Fig. 2b is high-resolution Ag 3d spectrum. The binding energy of Ag 3d5/2 and Ag 3d3/2 is 367.8 eV and 373.9 eV, respectively. The energy band between the two peaks is 6.1 eV, indicating silver species decorated on the surfaces of the urchinlike TiO2 microstructures is in the form of metal crystallite. 3.2. Effects of synthesis parameters on the morphology and crystal phase of TiO2/Ag composite material The morphology and crystal phase of the products can be adjusted by means of changing the amount of AgNO3, solvothermal temperature and time.

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Fig. 1. TEM images (a, b, c), XRD (d), high-angle annular dark field STEM (HAADF STEM) image (e) and the elemental maps for Ti (f), Ag (g) of Ag decorated 3D urchin-like TiO2 microstructures.

Fig. 2. XPS survey scans of Ag decorated 3D urchin-like TiO2 microstructures: a) full-spectrum; b) high-resolution Ag 3d spectrum.

3.2.1. Effect of amount of AgNO3 TEM images of the samples obtained at different amounts of AgNO3 are shown in Fig. 3. When no AgNO3 is added, the product presents aggregated short rod-like structure (Fig. 3a). Urchin-like microstructures begin to emerge while AgNO3 is added into the reaction solution (Fig. 3b). When the added quantity of AgNO3 is further elevated to 0.1125 g and 0.225 g, samples exhibit welldefined 3D urchin-like microstructures and show good dispersion (Fig. 1a and Fig. 3c). If the added amount of AgNO3 reaches 0.45 g (the molar ratio of Ag/Ti was 3/2), large globular aggregates of Ag

(100e200 nm in diameter) begin to appear around the urchin-like TiO2 microstructures (Fig. 3d). This is because excessive AgNO3 form large Ag particles through silver mirror reaction. The XRD patterns of these samples obtained at different amounts of AgNO3 is shown in Fig. 4. When no AgNO3 is added, the sample is amorphous (Fig. 4a). After AgNO3 is added, the XRD patterns indicate that TiO2 is a mixture of anatase and rutile (Fig. 4b). As the amount of AgNO3 increases, the content of anatase gradually decreases until it completely disappears (Fig. 4bee). The change of crystal phase should be attributed to the by-product of

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Fig. 3. TEM images of the samples obtained at different amounts of added AgNO3: a) 0 g; b) 0.0563 g; c) 0.225 g; d) 0.45 g.

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reaction temperature is raised to 210  C, the formation of nanocrystals grow preferentially in some directions, and have a tendency to form rod-like structure (Fig. 5b). At 240  C, well-defined 3D urchin-like nanostructures are synthesized (Fig. 1a). At 270  C, Ag particles (100 nm in size) disperse in thin TiO2 film (Fig. 5c). Fig. 6 shows XRD patterns of the samples obtained at different hydrothermal temperatures. At low reaction temperature (180  C, 210  C), samples are mainly composed of anatase phase (Fig. 6a, b).

Fig. 4. XRD patterns of the samples obtained at different amounts of added AgNO3: a) 0 g; b) 0.0563 g; c) 0.1125 g; d) 0.225 g; e) 0.45 g.

silver mirror reactiondnitric acid and acetic acid [30]. These results indicate that the amount of AgNO3 has a significant influence on the morphology and crystal phase of TiO2 samples. 3.2.2. Effect of temperature Samples obtained from different solvothermal temperatures are shown in Fig. 5. At 180  C, the sample shows small and thin nanosheets which are stacked disorderly (Fig. 5a). When the

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2 Theta/Degree Fig. 6. XRD patterns of the samples obtained at different solvothermal temperatures: a) 180  C; b) 210  C; c) 240  C; d) 270  C.

Fig. 5. TEM images of the samples obtained at different solvothermal temperatures: a) 180  C; b) 210  C; c) 270  C.

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As the temperature rises, part of anatase phase TiO2 transforms to rutile phase (Fig. 6c, d), which accord with their thermal stability. These results indicate that solvothermal temperature has important effect on the morphology and crystalline composition of the products. 3.2.3. Morphological evolution as the reaction time To study the formation process of urchin-like microstructure, the products at different reaction times were collected and characterized by TEM (Fig. 7). The morphological evolution of the desired products can be clearly observed with the reaction time (Fig. 7). When the reaction time is 2 h, the products begin to aggregate and grow into small curved nanosheets with 150 nm and 10 nm in length and width, respectively (Fig. 7a). With the reaction time extended to 6 h, the urchin-like structures about 300 nm in diameter are formed, and a lot of scattered fragments still exist in their surroundings (Fig. 7b). The urchin-like microstructures grow as reaction time prolongs. When the reaction time is 10 h, the fragments disappear, and the urchin-like structure forms with 500 nm in diameter and is composed of nanofibers with 10e30 nm in diameter and 100e300 nm in length (Fig. 7c). When the reaction time reaches 14 h, the diameter of the product is increased to about 800 nm (Fig. 1a). When the reaction time increases to 18 h and 22 h, many nanosheets and nanoparticles begin to cluster in the center of the urchin-like microspheres (Fig. 7d, e). Throughout the forming process, these urchin-like structures show good dispersion, and their diameters keep increasing until the reaction time reaches14 h. Fig. 8 shows the XRD patterns of the samples obtained at different solvothermal times. Titania contains three crystal types: anatase, rutile, brookite. At short reaction time (2 h), the three crystal types coexist (Fig. 8a). At 6 h, brookite crystallite vanishes, and the product is composed of anatase and rutile crystalline (Fig. 8b). When the reaction time is more than 6 h, the crystal components have no obvious change (Fig. 8cef). 3.2.4. Formation mechanism of Ag decorated 3D urchin-like TiO2 microstructures Based on these above studies, a formation mechanism for the Ag decorated 3D urchin-like TiO2 microstructures is proposed. During the one-step solvothermal synthesis, ethylene glycol reversibly converts into acetaldehyde and H2O. The produced acetaldehyde

Ag

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and H2O react with AgNO3 to generate Ag, nitric acid and acetic acid, and meanwhile H2O reacts with TBT to form titania [30]. Due to low solubility of titania, the early aggregation of TiO2 begins to precipitate and grows into small nanosheets which are mainly brookite (Fig. 7a). As the reaction proceeds, pH of the system decreases, and brookite begins to convert into anatase and rutile (Fig. 8). The small nanosheets undergo oriented growth to form nanofibers along the [101] plane of anatase and the [110] plane of rutile. These nanofibers with different growth directions result in the formation of 3D urchin-like microstructures (Fig. 7b). Then the nanofibers further increase in diameter and length (Fig. 7c and Fig. 1). The initiated Ag nanoparticles are even dispersed and closely adhered to the surfaces of TiO2 nanofibers. When no AgNO3, pH value nears neutrality and formed H2O is less, therefore no crystal produces and only small amount of amorphous TiO2 are obtained. As added AgNO3 reacts, acids are produced and pH value changes, which affect the final crystal phase of TiO2 and the morphology of samples as well. A large amount of AgNO3 (0.45 g, 0.176 mol/L) leads to the TiO2 sample with only rutile phase (Fig. 4e) and large Ag particles (sub-micrometer in size, Fig. 3d). Further, solvothermal temperature affects the TiO2 crystal formation, crystal growth and the crystal pattern transition of the products, which lead to their different morphologies and photocatalytic performances.

Fig. 7. TEM images of the samples obtained at different solvothermal times: a) 2 h; b) 6 h; c) 10 h; d) 18 h; e) 22 h.

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Fig. 9. Photodegradation of RhB under UVeVis light for samples synthesized with different amounts of AgNO3.

3.3. Photocatalytic properties The photocatalytic efficiency of Ag nanoparticle-decorated 3D urchin-like TiO2 microstructures is evaluated by RhB degradation under UVeVis light irradiation. The RhB degradation is monitored by the time-evolution of absorbance spectra of RhB aqueous solutions. The effect of synthesized parameters has been investigated in detail. 3.3.1. The influences of the amounts of Ag The degeneration efficiency of Rhodamine B for the samples obtained at different amounts of added AgNO3 is studied, as shown in Fig. 9. The catalytic activity of sample is very low when no AgNO3 is added into the reaction mixture. With AgNO3 added, the photocatalytic activity of samples increase rapidly. When the amount of AgNO3 is 0.1125 g, the photocatalytic activity reaches the highest. After the reaction for 140 min, the degradation rate reached 100%. If the addition quantity of AgNO3 is further elevated, the photocatalytic activity shows the trend of gradual reduction. The reason is that the TiO2 content and the anatase content in the product decreases, and that an excess of Ag nanoparticles become the photo-generated electronehole recombination centers and reduce their residence time [31]. 3.3.2. The influences of synthesized parameter the hydrothermal temperature Fig. 10 shows degradation curves of the RhB catalyzed by the samples obtained at different solvothermal temperatures under UVeVis irradiation. Only when the solvothermal reaction occurs at 240  C, Ag decorated 3D urchin-like TiO2 nanostructures can be

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Fig. 11. Photodegradation of RhB under UVeVis light for samples obtained at different solvothermal time.

synthesized, and it shows the highest photocatalytic activity among these samples. When solvothermal temperature is 270  C, the product has similar crystalline (Fig. 6c and d) and different morphology (Figs. 5 and 1) with the sample produced at 240  C, and it show low photocatalytic activity possible due to low specific surface area (15.1 m2/g). The samples obtained at solvothermal temperatures 180  C and 210  C are mainly nanosheets and nanoparticles, and their specific surface areas are 64.0 m2/g and 41.2 m2/ g, respectively. The crystalline of the samples are mainly anatase. These samples show a low photocatalytic activity. 3.3.3. The influences of synthesized parameter the hydrothermal time Fig. 11 compares the photocatalytic activity of the samples obtained at different solvothermal time. When the reaction time is 2 h, this sample shows low catalytic activity due to three crystal phases coexistence. The urchin-like structure form at the reaction time 6 h, and many scattered fragments still exist. Its photocatalytic activity is relatively weak, possibly because the fragments are low crystallinity or amorphous. When the reaction time is extended to 10 h and 14 h, the urchin-like microstructures further grow in size, and provide high catalytic activity. When the reaction time is further extended to 18 h or 22 h, the photocatalytic activity begins to decrease rapidly. The reason is, excessive agglomeration of nanoparticles in the core of the urchin-like structure makes smaller specific surface area and less exposed crystal planes for the samples. According to these experiments, the obtained Ag decorated 3D urchin-like TiO2 nanostructures with good morphology and crystal possess excellent photocatalytic properties for the degradation of RhB under UVeVis irradiation. Their photocatalytic properties are mainly affected by the following factors: the synthetic 3D urchinlike TiO2 microstructures have large specific surface area and good dispersion, and can adsorb more substrates in the catalytic process; the coexisted anatase and rutile crystals are closely linked and a cooperative phenomenon for photocatalytic activity is produced [10,26]; Ag nanoparticles are used as metal surface plasmon resonance and electron capture traps, which can expand and enhance the absorption of UVeVis spectrum [32], reduce the recombination of electrons and holes. All the three factors contribute altogether to the photocatalytic activity of Ag decorated 3D urchin-like TiO2 microstructures. 4. Conclusions

Fig. 10. Photodegradation of RhB under UVeVis light for samples obtained at different solvothermal temperatures.

Ag decorated 3D urchin-like TiO2 microstructures with average diameters of about 800 nm have been successfully synthesized by a

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one-step solvothermal method. The content of AgNO3, solvothermal temperature and time have an important influence on the morphology, structure and crystalline of the products. The effect of these factors on the photocatalytic activities is investigated in detail. The formed mechanism of 3D urchin-like microstructures is proposed. Photodegradation experiments show that the obtained Ag decorated 3D urchin-like TiO2 microstructures have superior photocatalytic activity for the degradation of RhB aqueous solution under UVeVis irradiation. The special morphology, crystalline composition and Ag nanoparticles doped result in improved photocatalytic activity. Acknowledgments We thank the National Science Foundation of China (grant no. 50934007, 51328202) and Co-building Special Project of Beijing Municipal Education for their support. References [1] K. Nakata, A. Fujishima, J. Photochem. Photobio. C 13 (2012) 169e189. [2] H.X. Li, Z.F. Bian, J. Zhu, D.Q. Zhang, G.S. Li, Y.N. Huo, H. Li, Y.F. Lu, J. Am. Chem. Soc. 129 (2007) 8406e8407. [3] I. Paramasivam, H. Jha, N. Liu, P. Schmuki, Small 8 (2012) 3073e3103. [4] J.M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer, P. Schmuki, Curr. Opin. Solid State Mater. Sci. 11 (2007) 3e18. [5] T. Tachikawa, M. Fujisuka, T. Majima, J. Phys. Chem. C 111 (2007) 5259e5275. [6] C.A. Coutinho, V.K. Gupta, J. Colloid Inferface Sci. 333 (2009) 457e464. [7] Y. Zhao, L. Jiang, Adv. Mater. 21 (2009) 3621e3638. [8] M.H. Bartl, S.W. Boettcher, K.L. Frindell, G.D. Stucky, Acc. Chem. Res. 38 (2005) 263e271. [9] R. Zhang, A.A. Elzatahry, S.S. Al-Deyab, D. Zhao, Nano Today 7 (2012)

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