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Nanoscale Res Lett (2009) 4:97–105 DOI 10.1007/s11671-008-9208-3 NANO EXPRESS Photodegradation of Pollutants in Air: Enhanced Properties of Nano-TiO2 Prepared by Ultrasound Giuseppe Cappelletti Æ Silvia Ardizzone Æ Claudia L. Bianchi Æ Stefano Gialanella Æ Alberto Naldoni Æ Carlo Pirola Æ Vittorio Ragaini Received: 29 July 2008 / Accepted: 11 November 2008 / Published online: 25 November 2008 Ó to the authors 2008 Abstract Nanocrystalline TiO2 samples were prepared by promoting the growth of a sol–gel precursor, in the presence of water, under continuous (CW), or pulsed (PW) ultrasound. All the samples turned out to be made of both anatase and brookite polymorphs. Pulsed US treatments determine an increase in the sample surface area and a decrease of the crystallite size, that is also accompanied by a more ordered crystalline structure and the samples appear to be more regular and can be considered to contain a relatively low concentration of lattice defects. These features result in a lower recombination rate between electrons and holes and, therefore, in a good photocatalytic performance toward the degradation of NOx in air. The continuous mode induces, instead, the formation of surface defects (two components are present in XPS Ti 2p3/2 region) and consequently yields the best photocatalyst. The analysis of all the characterization data seems to suggest that the relevant parameter imposing the final features of the oxides is the ultrasound total energy per volume (Etot/V) and not the acoustic intensity or the pulsed/continuous mode. G. Cappelletti (&)
S. Ardizzone
C. L. Bianchi
A. Naldoni
C. Pirola
V. Ragaini Department of Physical Chemistry and Electrochemistry, University of Milan, Via Golgi 19, 20133 Milan, Italy e-mail: giuseppe.cappelletti@unimi.it G. Cappelletti
S. Ardizzone
C. L. Bianchi
A. Naldoni
C. Pirola
V. Ragaini Consorzio INSTM, Via Giusti 9, 50121 Florence, Italy S. Gialanella Department of Materials Engineering and Industrial Technologies, University of Trento, Via Mesiano 77, 38050 Trento, Italy Keywords US-assisted synthesis
Nanostructured TiO2
Photocatalysis
NOx degradation
Microstructural characterization Introduction The use of ultrasonic sources in environmental remediation has been extensively studied, both in combination with other processes and also by itself [1–4]. The principal mode of action of continuous ultrasound is the production of hydroxyl radicals from water sonolysis, that can promote the degradation process of pollutants. Sonochemical processing to obtain materials with improved or unusual properties is, instead, relatively recent [5–11]. The chemical effect of ultrasound arises from acoustic cavitation, that is, the formation, growth, and implosive collapse of bubbles in a liquid. The implosive collapse produces high temperatures and pressures with localized hot spots, characterized by transient temperatures up to about 5000 K and pressures up to 1800 atm. The involved heating and cooling rates may be larger than 108 K s-1. Nanoparticles showing a more uniform size distribution, higher surface area, a more controlled phase composition are some of the interesting features resulting from the application of sonication as a synthetic method. The synthesis of TiO2 with tailored features widens the field of applications of the semiconductor oxide, among which the photocatalytic processes aimed at degrading environmental pollutants [12–19]. It is well known that the morphology and structure of a nanosized material can deeply affect the semiconductor performance [14, 20]. Previously reported results [15, 16] have shown that the photocatalytic performance of TiO2, in reactions performed both in water solution and in gas phase, is the result of a 123 98 complex balance between diverging effects (e.g., crystallinity, surface area) and that the surface state plays a key role in determining the kinetics of the relevant reactions. Results present in the literature concerning the synthesis of TiO2 assisted by US are rather difficult to rationalize since very different experimental conditions are adopted and the specific power of the US source is not reported in all cases. Meskin et al. [10] report the hydrothermal synthesis of TiO2 in an autoclave, coupled with ultrasonic activation. The results demonstrate that ultrasonic activation markedly accelerates the crystallization rates and raises the rutile content with respect to the values obtained in synthesis carried out under identical conditions, but without sonication. Similarly, Arami et al. [8] produced nanostructured rutile with 15–20 nm crystallite size, which is usually difficult to be obtained at low temperatures, by simply treating in ultrasonic bath the product of dissolution of TiO2 pellets in 10 M NaOH. Other authors, instead, observe opposite effects. Kim et al. [11] combine a short aging of a TiCl4 hydro-alcoholic solution with a sonication treatment performed in a conventional low power sonifier. In this case US depresses rutile formation, induces smaller particle size and larger surface areas. Yu et al. [6] by a similar procedure based on the combination of a sol–gel synthesis with a treatment in a conventional ultrasonic bath, observe that ultrasonic irradiation enhances the crystallization rate of the TiO2 gel. Gedanken et al. [7, 9] report the beneficial effects of ultrasonic irradiation on the template synthesis of wormlike TiO2 in presence of a long chain amine. The authors measured very high surface areas (853 m2 g-1) for un-calcined products and a higher thermal stability of the mesoporous structures as compared to those samples that were not ultrasonically treated. To gain some new evidence on the key role played by US in the formation and growth of nanocrystalline TiO2, in the present work US treatments are applied during the aging of sol–gel precursors. Both continuous and pulsed US treatments are used. In the case of these latter treatments, both the on/off time and the power were modulated. To the authors best knowledge, no data are present in the literature so far concerning the use of pulsed US on the growth of TiO2 particles. The structural and morphological features of the synthesized samples are investigated using different experimental techniques. The surface state of both Ti 2p and O 1s are analysed by XPS and the value of the band gap of the semiconductor oxide estimated for all samples. The photocatalytic activity in the gas phase degradation of NOx is also investigated. Various processes are studied in the literature to abate the emissions of NO into the environment. The photocatalytic oxidation by a semiconductor, mainly TiO2, is very promising and the tailoring of its features may promote the efficiency of the process. 123 Nanoscale Res Lett (2009) 4:97–105 Experimental All the chemicals were of reagent grade purity and were used without further purification; bi-distilled water passed through a Milli-Q apparatus was used to prepare solutions and suspensions. Sample Preparation The sol–gel precursor (T), obtained by the hydrolysis (t = 90 min, T = 65 °C) of a solution of Ti(OC3H7)4 and 2-propanol (water/alkoxide molar ratio = 49 and water/ propanol molar ratio = 15) was dried as a xerogel [21]. A fraction of precursor was treated at 300 °C for 5 h in O2 flux (T_300); the remaining part of the precursor was submitted to either pulsed (Bandelin, Ti horn, 20 kHz) for 1 h, or continuous (NTS Italia, Ti horn, 20 kHz) ultrasound water treatment for 30 min. The acoustic intensity, as determined calorimetrically, is 9 W cm-2 for continuous and 140 W cm-2 (maximum value) for pulsed ultrasound. In the case of PW treatment, two acoustic intensities were used: 49 and 84 W cm-2 (35% and 60% of the maximum intensity, respectively). See Table 1 for the experimental parameters of the acoustic treatment. Three samples were obtained during the growth assisted by pulsed ultrasound, by varying both the ‘‘time on’’ and the acoustic intensity (TP0.5on49W, TP0.9on49W, and TP0.5on84W). The sample obtained using continuous US treatment is labeled TC9W. Sample Characterization Room-temperature X-ray powder diffraction (XRPD) patterns were collected with a Siemens D500 diffractometer over the 2h range 10°–80°, with a step scan of D2h = 0.02°, and a Cu Ka radiation. Rietveld refinement has been performed using the GSAS software suite and its graphical interface EXPGUI [15]. Specific surface areas were determined by the classical BET procedure using a Coulter SA 3100 apparatus. Scanning electron microscopy (SEM) photographs are acquired with a LEO 1430. TEM samples were prepared by spreading a suspension of powder in ethylic alcohol onto a carbon coated copper grid. A Philips 400T electron microscope operated at 120 keV was used for imaging and for acquiring selected area electron diffraction (SAED) patterns. Diffuse reflectance spectra of the powders were measured on UV–vis scanning spectrophotometer (Perkin Elmer, Lambda 35), which was equipped with a diffuse reflectance accessory. A TiO2 thin film was placed into the sample holder on integrated sphere for the reflectance measurements. A ‘‘total white’’ Perkin Elmer reference Nanoscale Res Lett (2009) 4:97–105 99 Table 1 General conditions adopted for the US treatment in suspension Instrumental features US source dhorn (cm) Experimental conditions 2 Shorn (cm ) -2 Acoustic intensity (W cm ) t (s) Etot (kJ) Etot/V (kJ L-1) TP0.5on49W Pulsed 1.3 1.3 49 3600 117 470 TP0.5on84W Pulsed 1.3 1.3 84 3600 202 808 TP0.9on49W Pulsed 1.3 1.3 49 3600 210 840 TC9W Continuous 5.6 24.6 9 1800 414 414 material was used as an internal reference. The experimental absorption versus lambda plot was elaborated by differentiation to better highlight the different optical features of the catalyst spectra. X-ray photoelectron spectra were acquired in an M-probe apparatus (Surface Science Instruments). The source was a monochromatic Al Ka radiation (1486.6 eV). The binding energies (BEs) were corrected for specimen charging by referencing the C 1s peak to 284.6 eV. For the background subtraction the Shirley’s method was used [16]. The relevant fittings were performed using only Gaussian line shapes, without BE or FWHM (full width at half maximum) constraints. The accuracy of the reported BE can be estimated to be ±0.1 eV. Photocatalytic Experiments In the photocatalytic oxidation of nitrogen, oxide immobilized particulate TiO2 layers (ca. 0.1 g) were prepared on glass sheets (7 cm2) by deposition from a suspension of the oxide in isopropanol. The immobilized photocatalyst was placed into a pyrex glass reactor (with a volume of 20 L) and irradiated with an halogenide lamp (Jelosil, model HG500) emitting in the 340–400 nm wavelength range, with a nominal power of 500 W, at room temperature. The relative humidity was kept constant in all the runs (50%). Air, NOx, and N2 gas streams were mixed to obtain the desired NOx concentration (400 ppb), inside the photoreactor. The photodegradation products concentrations (NO and NO2) were continuously monitored by an online chemiluminescent analyzer (Teledyne Instruments M200E). The NOx adsorption onto the TiO2 layer was determined through dark experiments. Degradation time was limited to 120 min. processes: (1) formation of additional nucleation centers in the vicinity of bubbles; (2) increased growth rate of the particles owing to accelerated mass transport; and (3) disintegration of aggregates and agglomerates of primary crystallites by the shock waves resulting from bubble collapse. To evaluate separately the different effects provoked by US, in the present case both continuous and pulsed ultrasound were used during the aging of a TiO2 precursor obtained by sol–gel reaction. Table 2 reports data concerning the synthesized samples. The specific surface area ranges from 215 to 280 m2 g-1. Sample T_300, not submitted to US treatment and calcined at 300 °C shows the lowest surface area. Variable effects are introduced by the different US treatments. Pulsed US, with the highest total energy per volume (TP0.9on49W and TP0.5on84W, see Table 1), produce an increase in surface area even with respect to the untreated precursor. This suggests that, in these cases, the prevailing effect of the US treatment may be the disintegration of aggregates. The continuous US treatment, instead, provokes a slight reduction in the surface area, with respect to the precursor, which suggests either a more pronounced grain growth or a less dispersed sample. The total pore volume and size are not significantly affected by the US treatment (Fig. 1a). The sample showing the largest pore volume is the pulsed one (TP0.5on84W), the increase concerning the largest pore sizes. All the samples are mesoporous; the shape of the N2 adsorption hysteresis loops (Fig. 1b) changes from H2 (prevailing ‘‘bottleneck’’ shape) in the precursor sample (T) to a Table 2 Quantitative phase composition (A = anatase, B = brookite) and BET surface area Results and Discussion Acoustic waves are known to cause the following effects in liquids [10]: (1) activation of mass transport, (2) heating, and (3) cavitation, i.e., generation of bubbles, which then collapse, giving rise to high local temperatures and pressures. Consequently, the growth of an oxide assisted by ultrasound can be differently affected by diverging Sample US source %A %B SBET, m2 g-1 T T_300 – – 60.2 67.0 39.8 33.0 254 215 TP0.5on49W Pulsed 55.6 44.4 231 TP0.5on84W Pulsed 53.0 47.0 268 TP0.9on49W Pulsed 53.5 46.5 278 TC9W Continuous 63.0 37.0 242 123 100 Nanoscale Res Lett (2009) 4:97–105 d>80 nm 6

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