Direct growth of shape controlled TiO2 nanocrystals onto SWCNTs for highly active photocatalytic materials in the visible
Graphical abstract
Introduction
Photoactive nanomaterials have got paramount relevance during the last 20 years due to their ability to generate electron–hole pairs, under appropriate irradiation. Such a property can be exploited for potential application in photocatalysis, environmental remediation, self-cleaning of surfaces, new generation solar cells, hydrogen production, sensing [1], [2], [3], [4], [5], [6]. One of the most promising photoactive nanomaterials is TiO2 because of its high quantum yield, chemical stability, commercial availability and low cost. The two most photoactive crystalline phases of TiO2 are anatase and rutile, being the former generally regarded as the most suitable for photocatalytic applications [7]. Nonetheless, the large scale application of TiO2 nanomaterials is still prevented due to several problems related to their photocatalytic efficiency, their low absorption in the visible range and the technological issue of their immobilization and/or recovery for photocatalytic application [8].
The photocatalytic efficiency is linked to competition between life-time of photogenerated species and of their recombination which could occur either in the bulk of the material and/or at the surface sites. In addition, the presence of defects in TiO2 lattice could promote charge carriers trapping thus preventing the interfacial charge transfer which is the conditio sine qua non to exploit photogenerated charges [9]. The photocatalytic efficiency of TiO2 is affected not only by surface atomic structure, but also by size, shape, crystallinity, degree of exposure, and hence availability, of reactive crystal facets [10], [11]. Indeed, the average surface energy of anatase TiO2 facets is 0.90 J m−2 for{0 0 1}, 0.53 J m−2 for {1 0 0}, 0.44 J m−2 for {1 0 1} [10], [12] and considering that highly reactive facets are expected to effectively enhance surface properties, enormous effort has been devoted to the preparation of (0 0 1) facet-dominated single-crystal anatase TiO2 and to its application to photocatalysis [11]. Moreover only 4% of solar light can be effective in generating electron–hole pairs in TiO2 [13] due to its wide band gap which strongly affects the efficiency of solar light activated processes.
Therefore, the main current open questions for such a class of photoactive materials are: (i) reducing electron–hole pair recombination, (ii) increasing visible light absorption and (iii) controlling size, shape and crystalline phase of TiO2 in order to enhance the surface-to-volume ratio and suitably tailor TiO2 (photo)catalytic properties. Indeed, the ability in tuning size and shape can turn in the capacity of tailoring e−/h+ redox potential and exposed crystalline plane [14].
Several strategies have been proposed to cope with the first two issues including surface modification of the semiconductor particles with red-ox couples or noble metals and/or doping or coupling with narrower band gap semiconductors [13], [15], [16]. Particularly interesting is the recent work by Bian and co-workers, where they synthesized TiO2 mesocrystals modified with plasmonic Au nanoparticles with improved photocatalytic activity in the visible for organic pollutant degradation. They showed an evident enhancement in photocatalytic performances of one order of magnitude higher than that found for common Au/TiO2 nanoparticle systems. Such an excellent behaviour can be explained in terms of an effective slowdown of the charge recombination process of electrons and holes in the Au nanoparticles [17]. An emerging alternative relies on the coupling of TiO2 nanocrystals with carbon nanotubes (CNTs) either multi- or single-walled [7], [18]. Indeed, CNTs can potentially contribute to increase photocatalytic activity by acting on three distinct features: (i) providing high-surface area and, hence a high density of active sites for adsorption and catalysis, (ii) retarding electron–hole recombination and (iii) inducing visible light catalysis by modification of band-gap and/or sensitization [19]. Several studies on the development of CNT–TiO2 hybrids for enhancing photocatalytic activity have been reported over the past decade [19], [20], [21]; however, a strong effort is still needed to design and control size, shape and crystalline phase of TiO2 synthesized in presence of CNTs. It has been reported that a close contact between titania and CNTs is required in order to fully promote their interaction, and thus effectively exploit their synergic behaviour [22], [23]. So far, only a few reports have accounted an effective control of TiO2 morphological features in presence of CNTs, e.g. shell thickness of TiO2 or nanocrystal shape of the titania grown on CNTs [21], [24]. Indeed, up to now the most effective approaches for the control over the nanocrystal size and shape have been demonstrated based on binding of pre-synthesized nanoparticles onto the nanotube walls. Therefore, several steps are then generally required for fabricating CNT/inorganic-nanoparticle hybrids, and especially when use of anisotropic nanoparticles is planned [25], [26].
In this work we report on a very effective colloidal synthesis to obtain just in one step in situ growth of anatase TiO2 at the surface of single walled carbon nanotubes (SWCNTs), controlling the shape of TiO2 as TiO2 nanorods (NRs) or, alternatively, nanospheres (dots by simply tuning the ratio between reactants. Interestingly, the obtained SWCNTs/TiO2 heterostructures resulted dispersible in apolar organic solvents as chloroform, hexane and toluene. They have been characterized by TEM, SEM, and Raman spectroscopy and their photocatalytic properties have been tested in the photocatalytic discolouration of a water soluble model compound (the azo-dye methyl red; MR) under UV and visible irradiation and compared with bare TiO2 NRs, dots and with a commercially available standard TiO2 powder (TiO2 P25). Experimental results demonstrated that (i) the proposed synthetic procedure allows to obtain a high control over TiO2 size and shape in the heterostructures; (ii) and that SWCNTs/TiO2 hybrids show a significant enhancement of photocatalytic properties in the UV and visible range. In particular, SWCNT/TiO2 shows a three-fold enhancement of kinetic constant with respect to standard commercially available TiO2 under UV excitation and a two-fold enhancement under visible light irradiation.
Section snippets
Materials
All chemicals were of the highest purity available and were used as received without further purification. (Titanium tetraisopropoxide (TTIP, 99.999%), trimethylamino-N-oxide anhydrous and dihydrate (TMAO, 98%), anhydrous ethylene glycol (EG, 98%) and oleic acid (OLEA, 90%) were purchased from Sigma-Aldrich. All solvents used were of analytical grade and purchased from Aldrich. Commercial TiO2 is TiO2 “Degussa P25” (TiO2 P25), and Methyl Red (MR) is 2-(4-dimethylamino-pheny-lazo)-benzoic acid,
Synthesis of SWCNTs/TiO2 heterostructures
The proposed synthetic procedure is based on a careful adjustment of reported synthetic protocols for the one-pot synthesis of rod-like or spherical anatase TiO2 nanocrystals [27]. The synthesis is promoted by the hydrolysis of titanium isopropoxide (TTIP) catalysed by trimethylamino-N-oxide dehydrate (TMAO) in the presence of oleic acid (OLEA) as capping agent and carried out at relatively low temperature (100 °C) under standard air-free conditions. Here the synthetic conditions have been
Conclusion
In summary, a colloidal synthetic route for the in situ growth of anisotropic (nanorods) and isotropic (dots) anatase TiO2 nanocrystals onto the surface of SWCNTs has been proposed, allowing the fine tuning of the nanocrystal geometry just in one step, without any post-synthesis thermal treatment. Remarkably, the obtained heterostructures are dispersible in organic solvents, providing a promptly processable material, which can be easily recovered by centrifugation, without adding any
Acknowledgements
The authors thank B. Rodríguez-González for their help on the HRTEM characterization. This work was partially funded by the EC-funded 7th FP projects LIMPID (Grant n. 310177) and ORION (CP-IP 229036-2), by Apulia Region funded Project RELA-VALBIOR, Network of Laboratories for Scientific Research (Italy), by the Spanish Ministerio de Economía y Competitividad (CTQ2011-23167), and the FIRB 2009/2010 project “Rete integrata per la Nano Medicina (RINAME)”–RBAP114AMK_006. Rocco Lassandro and
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