Microwave-induced fast crystallization of amorphous hierarchical anatase microspheres
© Calatayud et al.; licensee Springer. 2014
Received: 27 February 2014
Accepted: 21 May 2014
Published: 29 May 2014
The fabrication of hierarchical anatase microspheres with potential photocatalytic properties eventually comprises a consolidation step in which a high degree of crystalline order is typically achieved through conventional electric heating treatments. This however entails a substantial reduction in the specific surface area and porosity of the powders, with the consequent deterioration in their photocatalytic response. Here, we have tested the employ of microwave heating as an alternative energy-saving sintering method to promote fast crystallization. The results obtained suggest that under the microwave radiation, the TiO2 hierarchical structures can effectively crystallize in a drastically reduced heating time, allowing the specific surface area and the porosity to be kept in the high values required for an improved photocatalytic performance.
The unique physicochemical properties of TiO2 nanoparticles have lately attracted a tremendous interest in a wide range of scientific and technological fields[1–5]. Of particular interest for its potential photocatalytic applications to environmental purification, hydrogen generation and/or solar energy conversion is the preparation of hierarchical structures in which TiO2 anatase nanoparticles are assembled into organized configurations at a microscopic level[6–11]. On one hand, hierarchical structures may attain low density, high crystallinity and a large specific surface area, structural parameters all required to improve the photocatalytic performance. On the other hand, the micrometric size of the organized assemblies will allow an easy recovery of the photocatalyst from the working suspension after use. In this context, different synthesis strategies have been recently tested to prepare TiO2 hierarchical structures. For example, using templates and/or applying hydro(solvo)thermal conditions, anatase nanostructures assembled onto micron-sized spherical units have been synthesized initially showing a high stability and a monodisperse nature that can satisfy the abovementioned characteristics[12–15]. The main problem with all these methods is the subsequent thermal treatment at mild/high temperatures, which, being necessary to increase the crystallinity of the samples, also reduces their porosity and specific surface area. Eventually, this provokes a severe devaluation of their photocatalytic performance which hampers the practical application of these powders. Bearing this in mind, in this contribution, we propose to replace the conventional thermal treatment by a microwave heating process, an alternative and energy-saving sintering technique which has been successfully employed for the consolidation of some ceramic systems[16–19]. Microwave radiation may induce a fast crystallization of the amorphous hierarchical anatase microspheres, simultaneously keeping the constituent nanoparticles with a high specific surface area and a high porosity level necessary for a good photocatalytic activity.
The chemicals titanium (IV) tetrabutoxide (Ti(OBut)4, 98%, Fluka, St. Louis, MO, USA) and anhydrous ethanol (EtOH, analytically pure, Merck, Whitehouse Station, NJ, USA) were used without further purification. TiO2 microspheres have been obtained following a facile methodology previously developed by our group. In essence, a solution of Ti(OBut)4 in 1 L of absolute ethanol is stirred at room temperature, and after 6.5 h, it is evaporated to dryness under atmospheric conditions. The evolved white precipitate is washed with water and ethanol thoroughly and dried at room temperature. The obtained powder is spread on a high-density alumina crucible placed on the top of a microwave susceptor element, and microwave heating is finally applied at 700 W for different time intervals using a commercial Tesco microwave oven (Chestnut, England, UK). For comparison, a small fraction of the as-precipitated powder is subjected to a conventional heating at 400°C/1 h on electric furnace.
The analyses of the crystalline structure and the phase identification were performed by X-ray diffraction (XRD Bruker D8 ADVANCE, Madison, WI, USA) with a monochromatized source of Cu-Kα1 radiation (λ = 1.5406 nm) at 1.6 kW (40 KV, 40 mA); samples were prepared by placing a drop of a concentrated ethanol dispersion of particles onto a single crystal silicon plate. Powder samples were initially characterized using a Hitachi TM1000 tabletop scanning electron microscope (Chiyoda-ku, Japan) working on backscattered mode. Field-emission scanning electron microscopy (FESEM) images were obtained with a Hitachi S-4700 working at 20 kV. The specific surface area was determined by the Brunauer-Emmett-Telle (BET) method in a Monosorb Analyzer MS-13 QuantaChrome (Boca Raton, FL, USA). Nitrogen adsorption/desorption isotherms were carried out on an ASAP 2020-Micromeritics (Norcross, GA, USA) at 77 K. Samples were degassed at 30°C during 48 h before analysis. Transmission electron microscopy (TEM) images were obtained on a JEOL 2100 F TEM/STEM (Tokyo, Japan) operating at 200 kV and equipped with a field emission electron gun providing a point resolution of 0.19 nm; samples were prepared by placing a drop of a dilute ethanol dispersion of nanoparticles onto a 300-mesh carbon-coated copper grid and evaporated immediately at 60°C.
Testing of photocatalytic activity
The photocatalytic performance of the powders prepared in this study was evaluated in the following way: 50 mg of powder were initially suspended in an aqueous solution of methyl orange (10-5 M, 100 mL) using a quartz reactor. The suspension, kept under magnetic stirring, was then irradiated using a high-pressure mercury vapour lamp (250 W, HPL-N Philips, Amsterdam, The Netherlands) and 4 ml aliquots were taken progressively from the suspension after different irradiation times. The supernatant and the solid particles were separated by centrifugation at 6,000 rpm. The absorption spectrum of the supernatant solution was measured on a Perkin Elmer Lambda 950 UV/vis spectrometer (Waltham, MA, USA), and the concentration (degradation) of methyl orange was determined monitoring the changes in the absorbance at 465 nm. On collecting these data, two side effects must be considered which may lead to a misinterpreted decreased value in the methyl orange concentration: the self-degradation of the methyl orange molecule under the irradiation, as well as its incidental (partial) absorption to the surface of the TiO2 particles. In this contribution, both scenarios were contemplated as follows: on one hand, a blank solution of methyl orange with no TiO2 powder was irradiated under the same experimental conditions; as it was observed, in the absence of our anatase particles, no degradation of methyl orange was indeed produced. On the other hand, suspensions with methyl orange and the different TiO2 powders were prepared as described before but they were not subjected to irradiation: in such dark conditions, no changes in the methyl orange concentration were observed for these suspensions all along the test, so absorption to the TiO2 surface was discarded in all cases.
Results and discussion
Specific surface area of the prepared samples
Specific surface area (±1 m2/g)
As-synthesized Tisph powder
7 min MW heating
10 min MW heating
15 min MW heating
30 min MW heating
400°C/1 h conventional heating
When conventional electric heating is applied to consolidate an amorphous powder of hierarchically nanostructured anatase microspheres, an increase in the crystal order is inescapably accompanied by a deleterious decrease in the specific surface and the porosity which dramatically reduces the photoactivity of this TiO2-based material. To avoid this scenario, microwave sintering has been successfully applied as an eco-friendly (energy saving) consolidation alternative: by reducing the heating time to just a few minutes, microwave radiation promotes the fast crystallization of the nanostructured microspheres, allowing the starting anatase powder to achieve a high crystallinity while keeping a high specific surface area and low density. As a straight consequence, the hunting of photons, the absorption of guest species and the photo-induced charge separation is fostered, eventually harvesting an improved photocatalytic performance.
This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) through the projects IPT-120000-2010-033 (GESHTOS), IPT-2011-1113-310000 (NANOBAC), CICYTMAT 2010-16614, MAT2010-18432 and CSD2008-00023. Dr T. Jardiel also acknowledges the JAE-Doc contract of the Spanish National Research Council (CSIC) and the European Science Foundation (ESF). Dr M. Peiteado acknowledges the Ramon y Cajal Program of MINECO for the financial support.
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