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Crystal Research and Technology Journal of Experimental and Industrial Crystallography Zeitschrift für experimentelle und technische Kristallographie
Established by W. Kleber and H. Neels Editor-in-Chief W. Neumann, Berlin Consulting Editor K.-W. Benz, Freiburg Editor’s Assistant H. Kleessen, Berlin
Editorial Board R. Fornari, Berlin P. Görnert, Jena M. Watanabe, Tokyo K. Sangwal, Lublin
T N I R P RE
Cryst. Res. Technol. 46, No. 8, 885 – 890 (2011) / DOI 10.1002/crat.201000594
Preparation of tetrapod-like ZnO/TiO2 core-shell nanostructures as photocatalytic powder A. Sartori*1, F. Visentin1, N. El Habra1,2, C. De Zorzi1, M. Natali1, D. Garoli3,4, R. Gerbasi1, M. Casarin2, and G. Rossetto1 1 2 3 4
ICIS-DPM-CNR, Padova, Italy Dipartimento di Scienze Chimiche, Padova, Italy Dipartimento di Fisica, Padova, Italy LANN, Padova, Italy
Received 19 November 2010, revised 14 December 2010, accepted 16 December 2010 Published online 7 January 2011 Key words core-shell, ZnO tetrapod, CVD, photocatalysis. The coupling of zinc oxide tetrapods (t-ZnO) with anatase TiO2 in the form of CVD coatings on ZnO nanotetrapods was investigated. t-ZnO/TiO2 core-shell structures, consisting of uniformly and completely TiO2 covered ZnO nanotetrapods, were characterized by scanning electron microscopy, X-ray diffraction and UV-Vis spectra. Photocatalytic activity, determined by degradation of a sodium methyl red solution, was found to be comparable to pure t-ZnO, while improved separation easiness was verified that makes the presented powders promising for wastewater treatment. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
In the past few years, many studies have been developed, searching for effective photocatalysts to decompose organic pollutants in wastewaters [1-3]. The photocatalyst effectiveness is based on the generation of electronhole pairs by means of band gap radiation which can give rise to redox reaction with pollutants species [4]. From this point of view, broadband semiconductors used as photocatalysts are of great interest because of their appropriate energy potentials of charge carriers: among other semiconductors, ZnO and especially TiO2 have been widely studied for their photocatalytic properties. ZnO is a n-type metal oxide semiconductor with wide band gap (3.37 eV) and high exciton binding energy (60 meV) [5]. It is employed in a wide range of technological application such as photocatalysis, watersplitting, optoeletronic, photovoltaic conversion and gas sensing [6]. Different morphologies of ZnO are reported to be function of the adopted synthesis method. More specifically, it is possible to obtain porous membrane, nanowires, nanobelt, nanoflower, and nanotetrapods [7,8]. Among them, ZnO with tetrapod shape (t-ZnO) have extensive applications in shock resistance, sound insulation, photosensitization, fluorescence, gas sensing and catalysis. Despite the wide band gap, the exceptionally long exciton lifetime and the good results obtained in photooxidation of organic compound (sometimes more efficient than TiO2) [9], t-ZnO is seldom used in photo-oxidation process of water soluble pollutant due to photocorrosion phenomena in acid solution [10]. On the other hand, TiO2 is the most common used catalyst thanks to its excellent photocatalytic properties and insolubility in water. The conventional photocatalytic processes employed nanometric suspended TiO2 powder in order to exploit its high superficial area. However, powdered photocatalysts have a serious limitation that is the need of post-treatment separation in a slurry system after photocatalytic reaction. To overcome this problem, many articles reported the attempt to anchor TiO2 on various rigid supports but, unfortunately, these systems are usually much less efficient than the corresponding powder [11-13], probably ____________________
* Corresponding author: e-mail:
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because of the low surface area available for the reacting species and of the existence of mass transfer resistance [14]. For geometrical consideration [6], tetrapod-like nanostructures may act as ideal support. In fact, in a liquid medium, this kind of nanostructure provides all-contactable surface, the spatial steric effect against dense agglomeration, and many pathways for the diffusion of reactants and products. Moreover, tetrapods may be assembled into three-dimensional networks, which is the geometrically ideal photocatalyst for better light harvesting in the suspension of powdered photocatalyst. Ideally, the prepared t-ZnO/TiO2 core-shell, obtained by a uniform deposition of anatase titania on the surface of t-ZnO, could be used as photocatalyst exploiting at the same time the synergic activity of two semiconductor (that should mantain high photocatalytic activity), a wide surface area, a good acid media tolerance and the possibility to easily recover the photocatalyst. The coupling of ZnO with anatase TiO2 have already been studied in different articles with significant results regarding photocatalysis [4,15-18]. The exploitation of tetrapod shape in order to obtained t-ZnO/TiO2 core-shell heterostructure has also been investigated [6,10]. Differently from these authors, to cover t-ZnO we used a CVD process, which has the advantages of uniform deposition even on complex surface profiles, high film densities and deposition rates, simplicity in process control and the possibility to operate at relatively low deposition temperature. Moreover, thanks to the wide choice of different parameters that control a CVD process, it is possible to deposit the desired TiO2 phase (i.e. anatase, for its photocatalytic properties). To our knowledge, such a method was used only in the case of ZnO/SnO2 core-shell [19].
2
Experimental
The nanocomposite material synthesis involved two steps: (1) the preparation of t-ZnO from the direct thermal sublimation of zinc powder under oxidant atmosphere and (2) the growth of anatase TiO2 shell via a Low Pressure-Metal Organic Chemical Vapor Deposition (LP- MOCVD). ZnO nanostructures were prepared by thermal evaporating pure 99.9% Zn powders (300 mesh) in air without the presence of carrier gas and catalyst. The system consisted of a horizontal tube furnace and quartz tube reactor. Zn powders were placed in an alumina crucible that was inserted in the center of the quartz tube reactor pre-heated at 730 °C; a Si(100) substrate was collocated over the reaction vessel in order to collect the product. After five minute of thermal evaporation and oxidation of Zn vapors, the crucibile was rapidly taken off from the furnace: a white wool-like product was obtained on the surface of the Si(100) substrate. Thin films of TiO2 were grown in a hot-wall, low-pressure MOCVD reactor. The precursor vaporization temperature was of 50 °C for the titanium source (Ti(OiPr)4). The gas carrier was N2 with a flow rate of 50 sccm. The reaction chamber was heated at 400 °C and the total working pressure was kept at 1.2 mbar. Tetrapods were collected in an quartz crucible placed in the center of the reactor. Deposition time to realize ca. 50 nm thick film was about 7 min, calculated according to previous deposition proofs. The phase characterization was done by X-ray diffraction (XRD) measurements, performed on as-deposited nanocomposites, by using a Philips PW3020 powder diffractometer in Bragg-Brentano θ-2θ geometry by using Cu Kα radiation (40 kV, 30 mA, λ = 1.54056 Å). The patterns were collected in the 20° – 85° 2θ range. Phase identification was performed with the support of the standard ICDD files. Reflection positions were determined with a statistical error of d(2θ) = 0.02°. The surface morphology of the samples was investigated via Scanning Electron Microscopy (SEM) by using a FEI Quanta 600i SEM/FIB instrument. It is equipped with a field emission gun, operating in high vacuum condition at an accelerating voltage variable from 5 to 30 keV, depending on the observation needs. Moreover, it is possible to better observe the 3D structure of the samples by tilting the stage. For the analyses tilt angles of 45° and 50° were used. The photocatalytic activity of prepared t-ZnO/TiO2 core-shell nanoparticles was evaluated in a batch reactor analysing the photo-degradation of a sodium methyl red (Sigma-Aldrich product) solution (MR) maintained at 19 °C. More specifically, t-ZnO/TiO2 nanoparticles were added (0.5 g/L) into 190 ml of MR (50 mg/L of sodium methyl red salt) and dispersed under ultrasonic vibrations (Sonorex Bandelin RK 255 S, 35 kHz, © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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100/200 W) for 30 min. At definite irradiation time intervals with UV light at 254 nm, about 4 ml of the mixture was withdrawn, and the nanopowder was separated from the suspension depending on the used catalyst (centrifugation and subsequent filtration for TiO2; only filtration for t-ZnO and t-ZnO/TiO2). Quantitative measurements of the MR concentration were performed by measuring the absorbance peak of MR centered at 428 nm. Absorbance and reflectance measurements were carried out by using a UV-Vis spectrophotometer (Unicam UV500). The UV-Vis spectra were recorded in the wavelength range 200-700 nm.
3
Results and discussion
XRD and SEM have been used to evaluate the resulting two-steps nanocomposite material synthesis. More specifically, these analyses provide a deep insight on both the complete reaction of Zn powder and the deposition of TiO2 anatase phase as well as on tetrapod and TiO2 shell morphology.
Fig. 1 XRD patterns of (a) pure t-ZnO, and (b) t-ZnO/TiO2 core-shell. In the box the region from 23° to 27° is evidenced to show the reflection relative to anatase TiO2 (101) plane.
Fig. 2 t-ZnO collected on Si(100) substrate after Zn thermal evaporation, dispersed in isopropylic alcohol by sonication and deposited onto a new Si substrate.
In figure 1 XRD patterns of tetrapods before and after TiO2 coating are reported. Pure t-ZnO (Fig. 1a) shows reflections corresponding to crystalline zinc oxide in the hexagonal wurtzite phase (ICCD 01-079-1169). No diffraction signal of the starting Zn powder or other impurities are detected, confirming that the product is pure ZnO. In figure 1b and in the evidenced box, the XRD pattern of t-ZnO covered with TiO2 shows, besides the ZnO signals, a reflection at 25.14° that is ascribable to the (101) plane of TiO2 anatase (ICDD 01-071-1169). The (101) Bragg reflection is more intense than all anatase peaks: its low intensity with respect to ZnO ones is ascribable to the low thickness of the deposited film. These analyses confirm the proper choice of the CVD www.crt-journal.org
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parameters to obtain anatase TiO2, which, as previously reported, is preferable than rutile phase for its claimed superior photocatalytic properties. As cited above, after Zn thermal evaporation a white wool-like product was obtained; in order to get an overall view on these nanostructures, fluffy ZnO was dispersed in isopropylic alcohol by sonication and deposited onto a new Si substrate. SEM image (Fig. 2) shows the result of this dispersion. All particles have a tetrahedral shape with four legs and smooth surfaces and the tetrapod are almost uniform in size and shapes. These images (Fig. 3-4) confirm that, in the adopted growth conditions, only tetrapods were produced. This demonstrates that no variations in the synthesis conditions occurred: in fact, it is well known that little variations in parameters such as temperature, heating rate, etc. [7,8] may produce unwanted ZnO morphologies (nanowires, nanobelt, nanoflower, etc.). The same tetrapod before and after TiO2 coating is illustrated in figure 3. With regard to the uncoated tetrapod, the average dimension of diameter and length of pods is in the order of 200-300 and 700-900 nm, respectively (Fig. 3a). Moreover, it is possible to observe a well definite hexagonal prism morphology of each arm of tetrapods with angles of about 109° between the axes of adjacent arms. The same tetrapod coated with titania is shown in figure 3b and it is worth to emphasize that the film covers uniformly the zinc oxide tetrapodal nanostructures. This is the proof of the good uniform coating that can be achieved by using a CVD process. The fact that no area without TiO2 coating is evident is of fundamental importance to prevent ZnO photocorrosion. With regard to the film thickness, it is difficult to establish it from this image, even after FIB cutting process. However, no measurable increase in the average diameter of the pods is found; thus the coating thickness is most likely below ca. 100 nm.
Fig. 3 Single tetrapod (a) before and (b) after TiO2 coating.
Fig. 4 UV-Vis light reflectance spectra of (a) TiO2 P25 Degussa, (b) t-ZnO, and (c) t-ZnO/TiO2 coreshell.
In order to evaluate the photocatalytic characteristics of the synthesized t-ZnO/TiO2 core-shell, it is important to compare some peculiar properties (such as reflectance, absorbance, photocatalytic activity, recycling easiness) with pure t-ZnO and TiO2 P25 Degussa. In this regard, the UV-Vis reflectance spectra of the TiO2 P25 Degussa, t-ZnO and t-ZnO/TiO2 core-shell are shown in figure 4. A clear red shift of UV-Vis reflectance spectrum of t-ZnO/TiO2 nanostructure was © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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observed when compared with the one of only TiO2 P25 Degussa, whereas a minor red shift beside t-ZnO is detected. In addition, as can be observed, the reflectance of the t-ZnO/TiO2 core-shell composite decreases to about 70% in the visible range.
Fig. 5 Photocatalytic decomposition of sodium methyl red solution by using (b) t-ZnO, (c) tZnO/TiO2 core-shell, and (d) TiO2 P25 Degussa. Curve (a) shows the effect of UV lamp exposition on MR without catalysts.
Photocatalytic decomposition of sodium methyl red solution was used to preliminary evaluate the nanocomposites photocatalytic activities. The MR was chosen because its solution was stable under UV illumination (Fig. 5 curve a). Figure 5 shows the relationship between (C/Co) and reaction time. The curve related to t-ZnO/TiO2 core-shell photocatalytic activity, is included between curves referred to pure t-ZnO and TiO2 P25 Degussa. These outcomes are in qualitative agreement with other results recently reported in the literature [6]. Besides this, it is worth to compare the easiness of separation among the used catalysts: t-ZnO and t-ZnO/TiO2 were separated by simple filtration while it was possible to recover TiO2 P25 Degussa only by successive solution centrifugations and filtrations.
4
Conclusion
Tetrapods of zinc oxide have been coated with titania by using a CVD process. As shown by XRD and SEM analyses, the TiO2 film covers uniformly and completely the complex shape of nanotetrapods evidencing the controlled coating that can be achieved by using a CVD process. Compared to t-ZnO and TiO2 P25 Degussa the TiO2 coated ZnO nanotetrapods maintain a significant photocatalytic activity for wastewater treatment, improving at the same time the separation easiness. Acknowledgements Skillful cooperation of Mr. Valerio Corrado and Mr. Franco De Zuane is gratefully acknowledged.
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