- Email: [email protected]
Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant
G Model
ARTICLE IN PRESS
CATTOD-10460; No. of Pages 7
Catalysis Today xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant Francesca Petronella a , Alessandra Truppi a,b , Teresa Sibillano c , Cinzia Giannini c , Marinella Striccoli a , Roberto Comparelli a,∗ , M. Lucia Curri a,∗ a
CNR-IPCF, Istituto Per i Processi Chimici e Fisici, U.O.S. Bari, c/o Dip. Chimica Via Orabona 4, 70126 Bari, Italy Università degli Studi di Bari “A.Moro”, Dip. Chimica, Via Orabona 4, 70126 Bari, Italy c CNR-IC, Istituto Di Cristallografia, Via Amendola 122/O, 70126 Bari, Italy b
a r t i c l e
i n f o
Article history: Received 4 July 2016 Received in revised form 20 October 2016 Accepted 6 November 2016 Available online xxx Keywords: Plasmonic photocatalyst Nanocrystalline heterostructure Magnetic nanocatalysts TiO2 based photocatalyst Visible light activity Nalidixic acid
a b s t r a c t The photocatalytic degradation of pollutants is a key technological application for nanomaterials. Our work aims at developing a multifunctional nanocrystalline heterostructure based on TiO2 nanorods, Fex Oy and Ag nanoparticles (NPs), TiO2 NRs/Fex Oy /Ag, integrating in one nanostructure a visible light photoactive moiety (TiO2 NRs/Ag) and a magnetic domain (Fex Oy ), in order to address the photoactivity under visible light and the possibility of recovery and reuse the photocatalyst. The synthesis was carried by preparing first the TiO2 NRs/Fex Oy based heterostructure and then growing Ag NPs with control on size. The resulting multidomain structures were characterized by FTIR and absorption spectroscopy, TEM and SEM microscopy, EDS and XRD analysis. The influence of the Ag NP domain and of its size on the photoactivity of the TiO2 NRs/Fex Oy /Ag nanostructures under visible light were investigated in the photocatalytic degradation of the Nalidixic Acid, an antibiotic used as a model compound representative of recalcitrant pollutants. In the presence of the Ag domain a significant increase of the photoactivity with respect to TiO2 NRs/Fex Oy heterostructures and to the commercially available TiO2 P25 was observed. Such an enhanced photocatalytic efficiency was found dependent on the size of the Ag domain and explained taking into account the plasmonic properties and the different possible photoactivation mechanisms. © 2016 Elsevier B.V. All rights reserved.
1. Introduction A topical challenge in the field of material chemistry is the real scale application of nanosized TiO2 in photocatalytic pollutant removal. In this perspective, a great deal of work has been devoted to extend the optical response of TiO2 in the visible range and for this purpose several different approaches are being investigated. The non-metal doping of TiO2 [1], the integration with short band gap semiconductors [2], the surface engineering [3] and the functionalization with suitable dyes as well as the integration of TiO2 nanocrystals with carbon based heterostructures, including carbon nanotubes [4], graphene [5] or fullerene [6], are relevant examples that highlight the intense research activities in this field. Promising outcomes are emerging from complex structures deriving from combination of TiO2 nanocrystals with noble metal nanoparticles (NPs) as Ag, Au [7,8]. Such class of heterostructures already demonstrated to significantly improve the photocatalytic activ-
∗ Corresponding authors. E-mail addresses: [email protected] (R. Comparelli), [email protected] (M.L. Curri).
ity of TiO2 under UV light because, due to their electronegativity, metal nanoparticles (NPs) can accumulate photogenerated electron from the conduction band (CB) of TiO2 , thus limiting the e− /h+ recombination rate [9]. However, the most remarkable properties of noble metal NPs rely on their ability to absorb visible light thanks to the phenomenon of Localized Surface Plasmon Resonance (LSPR). Under suitable conditions, which include proper size, shape and surrounding medium of noble metal NPs, visible light induces the resonant oscillation of surface free-electrons, resulting in an absorption band in the UV–vis spectrum of noble metal NPs. Thus, noble metal NPs can act as light-traps or electromagnetic field concentrators in rationally designed noble metal/TiO2 heterostructures [10]. Several papers recently proposed the preparation of a variety of noble metal/TiO2 based heterostructures with enhanced plasmonic photocatalytic properties. Wet chemistry approaches are very versatile and allowed to incorporate Au NPs in arrays of N-doped TiO2 bowl-shaped NPs [11] or enable the growth of TiO2 nanorods (TiO2 NRs) on the tip of Au NRs taking the advantage of the anisotropy of both the metal and the semiconductor components [12]. Beside Au NPs, also Ag NPs display similar plasmonic properties, being, in addition, less expensive [13], easy to synthesize and possessing antibacterial properties that extend their field of appli-
http://dx.doi.org/10.1016/j.cattod.2016.11.025 0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7 2
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
cation also for photocatalytic disinfection of water [14] and surfaces [15]. Among the main strategies to achieve the decoration of nanosized semiconductors with Ag NPs there are chemical reduction and photochemical methods. A one-pot solvothermal approach was employed for the fabrication of an Ag/Reduced Graphene Oxide/TiO2 heterostructure where Ag NPs were generated from the reduction of AgNO3 by dimethylacetamide [16]. In addition, a multi-step protocol was exploited to obtain more complex architecture as ␣-Fe2 O3 @Ag/AgCl, where an Ag shell was generated from the classic “mirror reaction”, consisting in the reduction of the diamine Ag(I) ions assisted by aldehyde groups, that functionalize the surface of Fe2 O3 short nanotubes [17]. The photochemical method for the functionalization of TiO2 NPs with Ag is also widely exploited. This approach was applied to grow Ag NPs onto TiO2 NRs [18] or on TiO2 supported on glass fibers [19] thus generating heterostructures with original architectures. However, a reliable use of nanomaterials for photocatalytic degradation of pollutants, especially for wastewater purification, poses important concerns on a safe recovery and reuse of NPs [20]. These issues can be addressed by rationally designing the photocatalyst surface. A possible strategy is based on the NP surface functionalization with molecules able to integrate NP in suitable host matrixes. As an alternative, photocatalytically active nanostructures can be combined with magnetic moieties that enable the magnetic recovery of the structures. An Ag decorated magnetic titania nanocomposite with a core- shell morphology was recently reported in literature [13] and its photocatalytic activity was successfully assessed under visible light by using organic dyes as target molecules. In this framework, the present work describes the synthesis and the characterization of a multifunctional nanosized photocatalytic heterostructure, formed of TiO2 NRs/Fex Oy /Ag with an anisotropic morphology. The aim was to merge distinct multiple functions in one nanostructure. In particular, the rod shaped titania nanocrystals (TiO2 NRs) were selected as basic structure to build such a multifunctional nanomaterial, taking advantage from their anisotropic shape, their high surface area, and, accordingly, the possible more effective delocalization of the photogenerated charge carriers along the longitudinal direction, thus limiting the detrimental e− /h+ recombination events. The direct growth of Fex Oy at the TiO2 NRs surface can effectively convey magnetic properties to the resulting heterostructure, thus potentially enabling the magnetic recovery of the photocatalyst. Finally, Ag NPs, thanks to their plasmonic properties, extend the photocatalytic activity of the system to the visible region. The integration of the distinct components in one nano-architecture was accomplished by following a multistep approach in order to specifically control the characteristics of each component in the structure. Therefore the synthesis of the TiO2 NRs/Fex Oy binary heterostructure was performed by following a hot injection seeded-growth approach [21]. Such a protocol was selected because it was demonstrated to lead to a TiO2 NRs/Fex Oy nanosized heterostructure with well-established magnetic properties [21], that are combined with the high photocatalytic activity and the anisotropy of TiO2 NRs [22]. The synthesis of the TiO2 NRs/Fex Oy was followed by the photochemical growth of Ag NPs with control on their size. In addition, the surface chemistry of the multifunctional heterostructures is characterized by the presence of organic molecules, which can be promptly exchanged and replaced by new functional agents able to integrate them in host polymers, substrates or painting which can convey photocatalytic properties to surfaces, thus allowing their potential use both in aqueous environment and at solid/air interface. The photocatalytic activity of the multifunctional photocatalyst was investigated by performing the degradation of a recalcitrant pollutant, the Nalidixic acid, under visible light.
2. Materials and methods 2.1. Materials All chemicals were of the highest purity available and were used as received without further purification. For the synthesis of TiO2 NRs, Titanium tetraisopropoxide (Ti(OPri)4 or TTIP, 99.999%), trimethylamino-N-oxide dihydrate ((CH3 )3 NO·2H2 O or TMAO, 98%), oleic acid (C18 H33 CO2 H or OLEA, 90%) were purchased from Aldrich. For the synthesis of TiO2 NRs/Fex Oy , oleic acid (C17 H33 CO2 H or OLEA, 90%), 1-octadecene (C18 H36 or ODE, 90%), oleyl amine (C17 H33 NH2 or OLEAM, 70%), iron pentacarbonyl (Fe(CO)5 , 98%) and dodecan-1,2-diol (C12 H24 (OH)2 or DDIOL, 90%) were purchased from Aldrich. For the synthesis of TiO2 NRs/Fex Oy /Ag the silver nitrate (AgNO3 , 99.998%), was purchased from Aldrich. All solvents used were of analytical grade and purchased from Aldrich. TiO2 “TiO2 P25” (TiO2 P25 Evonik) has-been selected as commercial standard material. Nalidixic Acid, NA (1-ethyl-1,4-dihydro-7-methyl-4-oxo1,8-naphthyridine-3-carboxylic acid) was purchased from Fluka. MilliQ water was employed for preparation of all aqueous solutions.
2.2. Synthesis of TiO2 NRs/Fex Oy /Ag The synthesis of the ternary heterostructure TiO2 NRs/Fex Oy /Ag was carried out, first, synthesizing the TiO2 NRs/FexOy heterostructure according a procedure described in [21] and reported in the Section 1 of Supplementary material (SM). Then a photochemical procedure developed for the preparation of the TiO2 NRs/Ag nanocomposite was suitably adapted, optimizing the relevant synthetic parameters [18]. A quartz cuvette containing a de-aerated CHCl3 :EtOH mixture (EtOH content 10%(v/v)) was filled with presynthesized colloidal TiO2 NRs/Fex Oy heterostructure and AgNO3 with a molar ratio TiO2 :Ag+ of 1:175. The mixture, purged for 30 min under nitrogen flow, was irradiated with UV light (light Source: high pressure 200 W mercury lamp; > 250 nm equipped with a neutral density filter in order to obtain a light intensity of 0.07 W/cm2 ). This mixture was irradiated for different time, namely 15 min, 60 and 120 min obtaining the TiO2 NRs/Fex Oy /Ag A, TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C samples, respectively.
2.3. Characterization 2.3.1. X-ray diffraction experiments and simulations X-ray diffraction data were collected at room temperature from samples of both TiO2 /Fex Oy and TiO2 /Fex Oy /Ag NPs deposited onto silicon substrates. Measurements were performed by using a Bruker D8 Discover diffractometer, equipped with a Göbel mirror, using Cu K␣ radiation (K␣1 = 1.54056 Å and K␣2 = 1.54439 Å), and an energy dispersive X-ray SolX-E detector. Data were collected at fixed incidence angle of 5◦ while moving the detector in the range 10–120◦ with a step size of 0.05◦ .
2.3.2. Transmission electron microscopy Transmission electron microscopy (TEM) analysis was performed by a JEOL JEM-1011 microscope operating at 100 kV. The TEM samples were prepared by casting a drop of TiO2 NRs·CHCl3 solution onto a carbon hollowed TEM grid.
2.3.3. UV–vis absorption spectroscopy UV–vis absorption spectra and reflectance spectra were recorded with a UV–vis-near IR Cary 5 (Varian spectrophotometer.).
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
2.3.4. Photocatalysis experiments In a typical experiment, 0.001 mol of TiO2 -based catalysts were cast onto a glass slide (with a surface of 3.6 cm2 ) by dropping the catalyst solution and letting the solvent to evaporate, thus leading to a nearly transparent film. The concentration of the catalyst solution was calculated by cyclic voltametry. Two sets of control experiments were carried out in presence of the same amount of TiO2 in TiO2 NRs, TiO2 NRs/Fex Oy and TiO2 P25, respectively. The transparent glass support, suitably shaped in order to fit a 1 cm × 1 cm quartz cell, was positioned against the far-off wall of the cell with respect to light beam. Photocatalytic experiments under visible light were performed by using a 250 W quartz tungsten halogen lamp, (QTH) filtered by band pass filter to transmit light from 430 nm to 3300 nm and a light intensity of 0.22 W/cm2 ∼ 2 SUN [23]. All experiments were performed in air, keeping the system under vigorous stirring. The NA solution was 10−4 M in MilliQ water at pH 7. The pH value was adjusted by adding the proper amount of 0.1 M HCl. Before irradiation the NA solution was left to stir in the dark for 30 min in order to promote the absorption/desorption equilibrium. At a fixed illumination time, samples were withdrawn from the solution reaction batch in order to monitor the NA degradation course by UV–vis absorption spectroscopy. A possible leaching of TiO2 in the aqueous solution during photocatalysis was investigated by ICP analyses on water treated with the tested catalysts and the Ti amount was found below the detection limit for all the investigated samples.
3. Results and discussion 3.1. Synthesis of TiO2 NRs/Fex Oy /Ag heterostructures The multi-step preparation of the TiO2 NRs/Fex Oy /Ag nanostructure was performed by adapting our previous reported procedure, based on the photochemical growth of Ag NPs on TiO2 NRs [18] leading to a UV photoactive nanocomposite [24]. In order to achieve a magnetically recoverable photocatalyst, that could be also simultaneously active under the visible light, in this work we aimed at growing Ag NPs on a magnetic TiO2 NRs/Fex Oy heterostructure. Firstly TiO2 NRs/Fex Oy nanostructures were synthesized by means of a previously reported colloidal route, that was purposely designed to achieve heterostructures endowed of the magnetic functionality of the Fex Oy domain along with the distinctive relevant features of the TiO2 NRs, including the semiconducting character, the anisotropic shape and the anatase crystalline phase [21], being the most photoactive phase of the TiO2 [25]. In addition, such a synthetic procedure allowed to obtain TiO2 NRs/Fex Oy heterostructures dispersible in organic apolar solvents, due to their specific surface chemistry. Such surface characteristics enable, in principle, the further processing of the heterostructures in organic solutions, including the fabrication of new domains, by exploiting the specific reactivity of the TiO2 NRs. Then, Ag domains were synthesized by performing the photochemical reduction of Ag+ ions under UV irradiation assisted by the TiO2 present in the TiO2 NRs/Fex Oy heterostructure as a catalyst. The photochemical growth of Ag NPs was carried out in a deaerated CHCl3 /EtOH mixture, under nitrogen atmosphere in order to avoid the scavenging of the conduction band (CB) electrons by oxygen, thus ensuring the availability of CB electrons (e− ) for the Ag+ photoreduction. Moreover the ethanol in the reaction mixture acts as an efficient hole (h+ ) scavenger, thus avoiding the detrimental e-/h+ recombination. The synthesis parameters, namely irradiation intensity, solvent polarity and molar ratio of the precursors, TiO2 NRs/Fex Oy and Ag+ , respectively, were optimized in order to achieve the ideal conditions for photoexcitation of the catalyst. The role played by the [TiO2 ]/[Ag+ ]
3
precursor molar ratio was investigated and a ratio of 1:175 was found suitable as it corresponds to the amount of TiO2 NRs/Fex Oy appropriate to safely accomplish the photocatalytic reaction. In fact, a higher amount of TiO2 NRs/Fex Oy , would result in a too dark solution, due to the high Fex Oy concentration, that would hamper the UV light absorption by TiO2 NRs and, consequently, limit the photoexcitation of the TiO2 NRs/Fex Oy for the photochemical synthesis of the Ag NPs. For the Ag growth step of the synthesis in the TiO2 NRs/Fex Oy /Ag nanostructures, three increasing irradiation times were investigated, namely 15 min (sample TiO2 NRs/Fex Oy /Ag A), 60 min (sample TiO2 NRs/Fex Oy /Ag B) and 120 min (sample TiO2 NRs/Fex Oy /Ag C) and the prepared samples were characterized by means of morphological, structural and spectroscopic investigation. The Ag NPs formation is due to the photoexcitation of TiO2 NRs in the TiO2 NRs/Fex Oy that photogenerate electrons in conduction band (CB). These CB electrons have the redox potential suited to promote the reduction of Ag+ ions at the TiO2 NRs surface, leading to the formation of Ag NPs [18]. In principle also the Fex Oy domain can be involved in the photoreduction of Ag+ ions. Indeed bulk Fe2 O3 is reported to have a band gap of 2.3 eV, therefore, under UV light irradiation, electrons can be photogenerated, inducing the photoreduction of Ag+ ions, considering the red-ox potential of the Ag/Ag+ , 0.799 V higher than that of Fe2 O3 , that is +0.18 V, both vs NHE at pH 0 [26]. However, the reducing ability of TiO2 CB vs NHE is higher than that reported for Fe2 O3 (bulk) (0.38 V for TiO2 vs +0.18 V for Fe2 O3 ), therefore the photochemical growth of Ag NPs is reasonable to occur preferentially at the surface of TiO2 NRs. Fig. 1 shows the morphological features of the different samples of TiO2 NRs/Fex Oy /Ag heterostructures obtained as a function of irradiation time. In the three samples, TiO2 NRs/Fex Oy /Ag A TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C, obtained upon irradiation for 15 min, 60 min and 120 min respectively, the TEM micrographs show dark spots in close proximity with the TiO2 NRs/Fex Oy units, which are, conversely, much brighter. In particular, the micrographs suggest that one Ag NP is in contact with several TiO2 NRs/Fex Oy heterostructures, thus leading to aggregates, formed of sparse networks of nanostructures TEM analysis points out that Ag NPs are basically isotropic in shape, being never isolated and positioned far from the Fex Oy domain in the structures, probably to minimize the steric hindrance in the overall heterostructure assembly. Moreover Ag NPs appear to be mainly positioned at the tip of TiO2 NRs (Fig. 1D). Such a feature can be explained by the likely Ag+ ions absorption, and their consequent photoreduction, preferentially occurring at the tips of TiO2 NRs, where the smaller curvature radius leads to a sparse coordination of the organic molecules, which are therefore less packed and make possible further adsorption of species. In addition, the probability of Ag nucleation at the tip of TiO2 NRs can be enhanced by the higher reactivity for the <004> exposed planes [18]. The results of statistical analysis of Ag NPs size in the three samples are reported in Fig. 2a–c. The Ag NPs average size was 11 ± 4 nm and 12 ± 2 nm for TiO2 NRs/Fex Oy /Ag A and TiO2 NRs/Fex Oy /Ag B, respectively. Accordingly, the formation of Ag NPs can be thought to occur during the first 15 min of photoreaction. In fact, during the subsequent 45 min of irradiation the average size stays almost constant, only a slight narrowing of size distribution can be noticed. For the sample TiO2 NRs/Fex Oy /Ag C, obtained upon 2 h irradiation, TEM micrographs clearly highlight the formation of large and close-packed aggregates, formed of heterostructures with large Ag domains and a broad size distribution. Therefore a longer irradiation time for TiO2 NRs/Fex Oy /Ag C can probably induce the coalescence of smaller AgNPs formed in earlier stages of irradiation [18].
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7 4
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
Fig. 1. TEM micrographs (A–D) and related size distribution (a–c) at different UV irradiation time: 15 min (A and a for, TiO2 NRs/Fex Oy /Ag A; 60 min (B and b for, TiO2 NRs/Fex Oy /Ag B) 120 min (C and c TiO2 NRs/Fex Oy /Ag B). TEM micrographs of TiO2 NRs/Fex Oy /Ag A. TiO2 NRs/Fex Oy /Ag B at higher magnification (D). The inset shows an Ag NP in direct contact with the TiO2 NRs/Fex Oy moiety. The scheme (E) reports the proposed mechanism for the Ag NPs photochemical growth on the tip of TiO2 NRs, in addition, a close-up of the TEM micrograph of a single ternary TiO2 NRs/Fex Oy /Ag structure has been added, highlighting the domains with different composition. The image F show the magnetically recoverable TiO2 NRs/Fex Oy /Ag nanostructure before and after the magnetic separation.
Interestingly the prepared heterostructures clearly showed a magnetic behaviour, conveyed to the system by the presence of iron oxide based domain. In Fig. 1F the TiO2 NRs/Fex Oy /Ag nanostructures under a magnetic field induced by a permanent magnet (Neodymium NdFeB, Flux density inside the magnet1.17 T, holding force on a steel plate, 59.84 N) can be seen effectively separated from the solution. XRD diffraction patterns of (a) TiO2 NRs/FexOy and (b–c) TiO2 NRs/Fex Oy /Ag B. Vertical bars: Bragg hkl reflections positions for anatase (cyan bars in panel a) and dotted grey bars in panel b-c-d), AgCl (blue bars), Ag (green bars), Fe2 O3 maghemite (black curvebars and Fe3 O4 magnetite (red bars) phases. Both Panels b and c report the same diffraction pattern of TiO2 NRs/Fex Oy /Ag to highlight the presence of anatase TiO2 NRs (dotted vertical bars), Ag (green markers) and AgCl (blue markers) (b) and Fe2 O3 (black markers) and Fe3 O4 (red markers) (c). Panel (d) shows a zoomed view of the panel (c) to highlight the small fraction of Ag phase. XRD analysis, reported in Fig. 2 for the TiO2 NRs/Fex Oy /Ag B as a representative sample of the nanostructures, allow to identify in all the investigated TiO2 NRs/Fex Oy /Ag samples, a mixture of TiO2 anatase, ␥-Fe2 O3 (maghemite), Fe3 O4 (magnetite), AgCl and a very small fraction of Ag. The diffraction peaks ascribed to the AgCl crystalline phase show a crystallite size larger than that of the metallic Ag NPs, as evidenced in panel (d) of Fig. 2. The diffraction pattern of the TiO2 NRs/Fex Oy /Ag sample demonstrates that the photochemical conditions to achieve Ag+ photoreduction did not changed the TiO2 polymorph maintaining the same structural features of the TiO2 NRs/Fex Oy reported in Fig. 2a [21]. The formation of AgCl species can be explained on the basis of the possible reaction between AgNO3 introduced in the mix-
ture as an alcoholic solution and the CHCl3 [27,28]. In fact, when the AgNO3 solution was added to the synthesis reaction mixture a white precipitate was observed to form, reasonably ascribable to the formation of AgCl, along with its rapidly re-dissolution, occurring as soon as a proper amount of ethanol was added to the mixture to adjust the polarity. Therefore, the most Ag species can be thought as available in the reaction mixture as Ag+ ion, being AgCl solubilized and available for the photoreduction, while only a small fraction of residual AgCl can be reasonably expected to stay, possibly, adsorbed onto the resulting TiO2 NRs/Fex Oy /Ag heterostructures. Such a hypothesis accounts for the results of the elemental analysis performed by EDS, reported in Figure SM2 of the SM, that shows as the amount of Ag is higher than that of Cl, being therefore not ascribable to the sole presence of AgCl. Moreover SEM micrographs of TiO2 NRs/Fex Oy /Ag, as reported in Figure SM3 of the SM, pointed out the presence of Ag NPs in close proximity of the TiO2 NRs/Fex Oy surface as well as on focal plane inside the investigated aggregates. The three synthesized samples resulted in optically clear dispersions that, thus, allowed a thorough optical characterization by UV–vis by absorption. The absorption spectrum of TiO2 NRs/Fex Oy /Ag A, B and C was recorded by using as a baseline the spectrum of the purged synthesis mixture before UV irradiation, in order to clearly point out possible feature due to the expected appearance of the Ag plasmon band. The spectra of TiO2 NRs/Fex Oy /Ag A, TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C in Fig. 3 show that the heterostructures present a clear absorption signal in the visible region, due to the Surface Plasmon band arising with the formation of AgNPs,
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
5
Fig. 2. XRD diffraction patterns of (a) TiO2 NRs/Fex Oy and (b–c) TiO2 NRs/Fex Oy /Ag B. Vertical bars: Bragg hkl reflections positions for anatase (cyan curve), AgCl (blue curve), Ag (green curve), Fe2 O3 maghemite (black curve) and Fe3 O4 magnetite (red curve) phases. Both Panels b and c report the same diffraction pattern of TiO2 NRs/Fex Oy /Ag B in order to highlight the presence of anatase TiO2 NRs (dotted vertical lines), Ag (green markers) and AgCl (blue markers) (b) and Fe2 O3 (black markers) and Fe3 O4 (red markers) (c). The panel (d) is a zoomed view of the panel (c) to highlight the small fraction of Ag phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
FTIR analysis, which are described in details in the Figure SM4 of the SM, highlights that the surface chemistry of the TiO2 NRs/Fex Oy /Ag nanostructures is mainly characterized by the presence of oleic acid, that is the capping agent used for the synthesis [29]. Such a surface chemistry makes the TiO2 NRs/Fex Oy /Ag structures stable in apolar solvents, including chloroform, toluene or hexane.
3.2. Photocatalytic activity under visible light irradiation
Fig. 3. Representative absorbance spectrum of as prepared TiO2 /Fex Oy /Ag heterostructures obtained at different irradiation times: 15 min (TiO2 NRs/Fex Oy /Ag A); 60 min (TiO2 NRs/Fex Oy /Ag B) 120 min (TiO2 NRs/Fex Oy /Ag C) in comparison with the absorbance spectrum of the TiO2 NRs/Fex Oy. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
that indicates a possible visible light photocatalytic activity of the obtained nanostructures. The position of the plasmon band of the Ag NPs red shifts with the irradiation time Indeed the position, the intensity and the full width at half maximum (FWHM) of the absorbance maximum in the plasmon band was found to change with the reaction conditions, that controlled the resulting size and distribution of the Ag NPs as demonstrated by TEM micrographs (Fig. 1). Notably the TiO2 NRs/Fex Oy /Ag C, presented an asymmetric plasmon band with a maximum at 480 nm, indicating a large size distribution for Ag NPs in agreement with the TEM results.
The photocatalytic activity of TiO2 NRs/Fex Oy /Ag A, TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C was investigated under visible light by using the Nalidixic Acid (NA) (10−4 M, pH 2.5) as target compound. The NA was selected as a model target molecule, being a real pollutant representative of fluoroquinolones, a class of antibiotics of environmental concern that is detected as a recalcitrant compound in different types of waste waters [30]. In addition, NA is a suitable model compound as it does not absorb in the visible range, thus ruling out possible sensitization effects. All experiments were performed in air, keeping the system under vigorous stirring. After a 30 min conditioning time in the dark, to promote adsorption/desorption equilibrium, the system was irradiated with a visible lamp. At a fixed illumination time, samples of the solution were withdrawn from the reaction batch in order to monitor the degradation course by UV–vis absorption spectroscopy. The absorption spectrum of the NA is reported in Figure SM5 of the SM. Fig. 4 reports the kinetic of NA degradation, under visible light irradiation, as ln(C/C0 ) vs. reaction time. The plot results in a straight line with the slope, k (Table 1) representing the apparent rate constant. Such k value was effectively used to compare the photocatalytic activity of the investigated heterostrucures. The comparison among the apparent kinetic con-
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
6
Fig. 4. Nalidixic Acid degradation rates in presence of TiO2 NRs/Fex Oy /Ag A, TiO2 Nrs/Fex Oy /Ag B and TiO2 Nrs/Fex Oy /Ag C, TiO2 NRs/Fex Oy , TiO2 NRs and TiO2 P25. Experiments were carried out at pH 2.5 under visible light irradiation. Nalidixic Acid concentration was evaluated by monitoring the absorbance intensity at 316 nm (pH 2.5). Experimental data are reported as mean values of 5 replicates ± standard deviation. (B and C) Scheme of possible photoactivation mechanism of the TiO2 NRs/Fex Oy /Ag under visible light: hot electrons effect (B) and near field effect (C). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 1 Summary of kinetic constants calculated for the photocatalytic degradation of NA assisted by TiO2 /Fex Oy /Ag based heterostructure. Sample name
k (min−1 )
TiO2 /Fex Oy /Ag A TiO2 /Fex Oy /Ag B TiO2 /Fex Oy /AgC TiO2 NRs TiO2 NRs/Fex Oy TiO2 P25
14 · 10−3 ±10−3 15 · 10−3 ±10−4 7 · 10−3 ±10−4 10 · 10−3 ±10−3 12 · 10−3 ±10−3 8 · 10−3 ± 0.5 · 10−3
stants revealed that the degradation of NA was faster when assisted by the prepared photocatalysts, with respect to that found for TiO2 P25 and for the TiO2 NRs. In particular, the apparent kinetic constants of the reaction carried out by using TiO2 NRs/Fex Oy /Ag A, and TiO2 /Fex Oy /Ag B was 1.8 and 1.9 times, respectively, higher than the apparent kinetic constant obtained in the case of use of TiO2 P25, while TiO2 NRs/Fex Oy /Ag C showed a degradation rate comparable with the TiO2 P25. Interestingly, the three samples TiO2 NRs/Fex Oy /Ag A, TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C, reported apparent kinetic constant values higher than those determined both for TiO2 NRs/Fex Oy and TiO2 NRs, as reported in Table 1. Therefore, the increase of the photocatalytic efficiency observed for TiO2 NRs/Fex Oy /Ag is ascribable to the presence of Ag species, that, thanks to their plasmonic properties, give rise to an intense and highly localized electromagnetic field. The visible lightinduced photoactivation of the ternary system can take place according two possible mechanisms, both related to the physicalchemical features of the plasmonic heterostructures. Indeed when LSPR (Localized Surface Plasmon Resonance) conditions are satisfied in metal NPs, the confined free electrons oscillate with the same frequency as the incident radiation and eventually enter in resonance. Their decay takes place on a femtosecond timescale, either radiatively or non-radiatively. The non-radiative relaxation of the surface plasmons can induces the generation of the so-called “hot electrons”, transferring the accumulated energy to electrons in the conduction band of the material. When the noble metal NPs are in direct contact with the semiconductor, such “hot electrons” can be injected into the conduction band of the semiconductor, provided that they have enough energy to overcome the Schottky barrier [9]. Conversely, when the semiconductor and the plasmonic metal are not in contact with each other but isolated by a thin, nonconductive spacer, preventing any possible charge transfer process,
under light excitation a near field effect can take place, thus representing a second possible mechanism [35]. Indeed, under LSPR conditions, the high intensity of the electric field, in the proximity of noble metal NPs, can increases the rate of electrons and holes formation. In the TiO2 NRs/Fex Oy /Ag A and TiO2 NRs/Fex Oy /Ag B the size of Ag NPs is less to 50 nm, therefore the “efficient scattering” mechanism can be considered negligible. Both the “hot electrons” mechanism and the “near field effect” mechanism can safely account for the photoactivation of the systems. Moreover, remarkably the apparent kinetic constants were affected by the size of Ag species. In particular TiO2 NRs/Fex Oy /Ag A and TiO2 NRs/Fex Oy /Ag B heterostructures, which possess Ag species with comparable dimensions, exhibited comparable apparent kinetic constants. In contrast, the TiO2 NRs/Fex Oy /Ag C sample, characterized by larger Ag domains of 51 nm, demonstrated a degradation rate two times slower with respect to TiO2 /Fex Oy /Ag A, and TiO2 /Fex Oy /Ag B samples. Such an evidence is in agreement with the report by Yu et al. that observed a higher efficiency of the photocatalytic process in presence of smaller Ag NPs respect to larger Ag NPs, as “hot electron” injection from noble metal to semiconductor has been demonstrated to occur more effectively for smaller metallic NPs. [31]. Remarkably XRD analysis revealed the presence of AgCl phase, that although not photoactive under visible light due to its band gap of 3.03 eV [32], could however contribute to the enhancement of the overall photocatalytic activity of the system. Indeed, in an Ag/AgCl based system, the surface of AgCl particles, terminating with Cl- ions, is negatively charged, while the AgCl bulk is positively charged. When metallic Ag NPs are under LSPR conditions, the dipolar character of surface plasmon induces a separation of electrons and holes. Under these conditions, the region of positive charges in the metallic Ag NPs is close to the Ag/AgCl interface, as electrons are polarized by AgCl bulk, while the region of negative charges is at the surface of the Ag NPs which is outermost from Ag/AgCl interface. Therefore, the reduction of Ag+ in AgCl is hindered, while a larger amount of electrons is made available for formation of oxyradical species [33,34]. Holes are in principle free to react with surface terminating Cl− anions, possibly generating strong oxidizing agents as Cl• [35]. In addition, when TiO2 NRs/Fex Oy /Ag nanostructures were cast from solution onto TEM grid, they were found to form, upon solvent evaporation, a three-dimensional network. A similar assembly of the nanostructures is expected to occur also when they are cast onto the glass slide employed for photocatalysis experiment, with the possible occurrence of an “antenna effect” resulting from the interparticle contact and able to enhance the photocatalytic activity of the system. The “antenna effect” takes place when photogenerated charge carrier, formed in a NP, can be rapidly transferred to their neighbours in the same nanoparticles assembly, until reaching a suitable trap site where it can be involved in an oxidation or reduction reaction with the absorbate, respectively [36,37]. Therefore the “antenna effect” can improve the photocatalytic activity increasing the formation of reactive radicals and preventing the e− /h+ recombination as demonstrated by a similar TiO2 NRs/Ag system [24]. The improved photocatalytic performances of TiO2 NRs compared to TiO2 P25 were already observed in the case of an organic dye as target molecule, under irradiation of a both QTH lamp [4] or an UV light source and explained in the light of the higher surface area, morphology and unique surface chemistry of TiO2 NRs [38,39]. However, the binary heterostructure TiO2 NRs/Fex Oy , under the investigated experimental condition, did not reveal an increase of the photocatalytic efficiency compared to TiO2 /NRs into the visible wavelength (in fact the logarithmic plots of TiO2 NRs/Fex Oy and TiO2 NRs in
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
Fig. 4 are overlapping). Probably the presence of Fex Oy induced defects or energy levels able to promote the e− /h+ recombination. 4. Conclusions The present work describes the synthesis and the photocatalytic activity of the multifunctional nanocrystalline heterostructure based on TiO2 NRs/Fex Oy /Ag. Such a photocatalyst was specifically designed to combine, in one structure, domains with different composition and distinct functions, namely the UV-photoactivity of semiconductive TiO2 , the visible light activity, enhanced by the Ag NPs, and, finally, magnetic properties of iron oxide domain. The presence of visible photoactive Ag domains in the photocatalyst demonstrated to enable the visible light photodegradation of the model, while the Fex Oy structure, The prepared multifunctional catalyst showed to be more effective than commercial catalysts TiO2 P25 Evonik in the removal of the NA, selected as model pollutant. Remarkably, the assessment of the photocatalytic efficiency based on the evaluation of the apparent rate constants, demonstrated that the TiO2 NRs/Fex Oy /Ag nanostructure, with Ag NPs of 12 nm in size, resulted 1.9 times faster than the commercial TiO2 P25 and 1.5 times faster than the TiO2 NRs. Such an enhancement has been accounted for by the presence of Ag NPs that enhance visible light photoactivity due to their peculiar plasmonic properties. In particular, the visible light-induced photoactivation of the ternary heterostructure has been discussed on the basis of two possible mechanisms, namely the generation of “hot electrons” and “near field” effect, that can both take place and result in an overall increases of electrons and holes available for the catalytic processes in proximity of the metal NPs in the heterostructures. The results highlighted the efficiency of TiO2 NRs/Fex Oy /Ag in the photocatalytic degradation of NA under visible light, suggesting that the heterostructure can be considered as promising candidate for pilot scale applications, as the magnetic properties of the iron oxide domain allow a facile photocatalyst recovery by means of a magnetic field, that could be further exploited in suitably designed photocatalytic reactors. Acknowledgments The authors wish to express their deepest and sincerest recognition to Prof. Andràs Dombi a key figure in the topic of photocatalytic materials for degradation of contaminants of environmental concern. This work was partially supported by the EC-funded 7th FP LIMPID Project (Grant No. 310177) and by Apulia Region funded NanoApulia (MDI6SR) and “Tecnologie Abilitanti per Produzioni Agroalimentari Sicure e Sostenibili” (T.A.P.A.S.S.) − PELM994” (CUP B38C14002040008) projects, by Italian PRIN 2012 (prot. 20128ZZS2H) project and by the Italian Regional Network of Laboratories “Sens&Micro” and “VALBIOR” projects(POFESR 20072013).
7
References [1] Y. Zhang, C. Han, M.N. Nadagouda, D.D. Dionysiou, Appl. Catal. B Environ. 168–169 (2015) 550. [2] B. Liu, X. Li, Q. Zhao, J. Ke, M. Tadé, S. Liu, Appl. Catal. B Environ. 185 (2016) 1. [3] T.R. Gordon, M. Cargnello, T. Paik, F. Mangolini, R.T. Weber, P. Fornasiero, C.B. Murray, J. Am. Chem. Soc. 134 (2012) 6751. [4] F. Petronella, M.L. Curri, M. Striccoli, E. Fanizza, C. Mateo-Mateo, R.A. Alvarez-Puebla, T. Sibillano, C. Giannini, M.A. Correa-Duarte, R. Comparelli, Appl. Catal. B Environ. 178 (2015) 91. [5] K. Dai, L. Lu, Q. Liu, G. Zhu, Q. Liu, Z. Liu, Dalton Trans. 43 (2014) 2202. [6] M.-Q. Yang, N. Zhang, Y.-J. Xu, ACS Appl. Mater. Interfaces 5 (2013) 1156. [7] J.C. Durán-Álvarez, E. Avella, R.M. Ramírez-Zamora, R. Zanella, Catal.Today 266 (2016) 175. [8] L. Pinho, M. Rojas, M.J. Mosquera, Appl. Catal. B Environ. 178 (2015) 144. [9] V. Subramanian, E.E. Wolf, P.V. Kamat, J. Am. Chem. Soc. 126 (2004) 4943. [10] C. Clavero, Nat. Photonics 8 (2014) 95. [11] X. Wang, R. Long, D. Liu, D. Yang, C. Wang, Y. Xiong, Nano Energy 24 (2016) 87. [12] B. Wu, D. Liu, S. Mubeen, T.T. Chuong, M. Moskovits, G.D. Stucky, J. Am. Chem. Soc. 138 (2016) 1114. [13] Y. Wang, F. Pan, W. Dong, L. Xu, K. Wu, G. Xu, W. Chen, Appl. Catal. B Environ. 189 (2016) 192. [14] D.M. Tobaldi, C. Piccirillo, R.C. Pullar, A.F. Gualtieri, M.P. Seabra, P.M.L. Castro, J.A. Labrincha, J. Phys. Chem. C 118 (2014) 4751. [15] S. Rtimi, M. Pascu, R. Sanjines, C. Pulgarin, M. Ben-Simon, A. Houas, J.C. Lavanchy, J. Kiwi, Appl. Catal. B Environ. 138–139 (2013) 113. [16] W. Gao, M. Wang, C. Ran, X. Yao, H. Yang, J. Liu, D. H, J. Bai, Nanoscale 6 (2014) 5498. [17] J. Liu, W. Wu, Q. Tian, S. Yang, L. Sun, X. Xiao, F. Ren, C. Jiang, V.A.L. Roy, R.S.C Adv. 5 (2015) 61239. [18] P.D. Cozzoli, R. Comparelli, E. Fanizza, M.L. Curri, A. Agostiano, D.l. Laub, J. Am. Chem. Soc. 126 (2004) 3868. [19] L. Chen, S. Yang, B. Hao, J. Ruan, P.-C. Ma, Appl. Catal. B Environ. 166–167 (2015) 287. [20] A. Panniello, M.L. Curri, D. Diso, A. Licciulli, V. Locaputo, A. Agostiano, R. Comparelli, G. Mascolo, Appl. Catal. B Environ. 121–122 (2012) 190. [21] R. Buonsanti, V. Grillo, E. Carlino, C. Giannini, M.L. Curri, C. Innocenti, C. Sangregorio, K. Achterhold, F.G.n. Parak, A. Agostiano, P.D. Cozzoli, J. Am. Chem. Soc. 128 (2006) 16953. [22] P.D. Cozzoli, R. Comparelli, E. Fanizza, M.L. Curri, A. Agostiano, Mater. Sci. Eng. C 23 (2003) 707. [23] F. Milano, L. Gerencsér, A. Agostiano, L. Nagy, M. Trotta, P. Maróti, J. Phys. Chem. B 111 (2007) 4261. [24] F. Petronella, S. Diomede, E. Fanizza, G. Mascolo, T. Sibillano, A. Agostiano, M.L. Curri, R. Comparelli, Chemosphere 91 (2013) 941. [25] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891. [26] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [27] J.A. Vona, J. Steigman, J. Am. Chem. Soc. 81 (1959) 1095. [28] R.T. Morrison, R.N. Boyd, Organic Chemistry, Prentice Hall, 1992. [29] P.D. Cozzoli, A. Kornowski, H. Weller, J. Am. Chem. Soc. 125 (2003) 14539. [30] L. Ge, J. Chen, X. Wei, S. Zhang, X. Qiao, X. Cai, Q. Xie, Environ. Sci. Technol. 44 (2010) 2400. [31] K. Yu, Y. Tian, T. Tatsuma, Phys. Chem. Chem. Phys. 8 (2006) 5417. [32] D. Chen, T. Li, Q. Chen, J. Gao, B. Fan, J. Li, X. Li, R. Zhang, J. Sun, L. Gao, Nanoscale 4 (2012) 5431. [33] G. Li, Y. Wang, L. Mao, R.S.C Adv. 4 (2014) 53649. [34] A. Ziylan, N.H. Ince, J. Hazard. Mater. 187 (2011) 24. [35] J. Yu, G. Dai, B. Huang, J. Phys. Chem. C 113 (2009) 16394. [36] A.A. Ismail, D.W. Bahnemann, I. Bannat, M. Wark, J. Phys. Chem. C 113 (2009) 7429. [37] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Chem. Rev. 114 (2014) 9919. [38] M. Fittipaldi, M.L. Curri, R. Comparelli, M. Striccoli, A. Agostiano, N. Grassi, C. Sangregorio, D. Gatteschi, J. Phys. Chem. C 113 (2009) 6221. [39] F. Petronella, E. Fanizza, G. Mascolo, V. Locaputo, L. Bertinetti, G. Martra, S. Coluccia, A. Agostiano, M.L. Curri, R. Comparelli, J. Phys. Chem. C 115 (2011) 12033.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.11. 025.
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
ARTICLE IN PRESS
CATTOD-10460; No. of Pages 7
Catalysis Today xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant Francesca Petronella a , Alessandra Truppi a,b , Teresa Sibillano c , Cinzia Giannini c , Marinella Striccoli a , Roberto Comparelli a,∗ , M. Lucia Curri a,∗ a
CNR-IPCF, Istituto Per i Processi Chimici e Fisici, U.O.S. Bari, c/o Dip. Chimica Via Orabona 4, 70126 Bari, Italy Università degli Studi di Bari “A.Moro”, Dip. Chimica, Via Orabona 4, 70126 Bari, Italy c CNR-IC, Istituto Di Cristallografia, Via Amendola 122/O, 70126 Bari, Italy b
a r t i c l e
i n f o
Article history: Received 4 July 2016 Received in revised form 20 October 2016 Accepted 6 November 2016 Available online xxx Keywords: Plasmonic photocatalyst Nanocrystalline heterostructure Magnetic nanocatalysts TiO2 based photocatalyst Visible light activity Nalidixic acid
a b s t r a c t The photocatalytic degradation of pollutants is a key technological application for nanomaterials. Our work aims at developing a multifunctional nanocrystalline heterostructure based on TiO2 nanorods, Fex Oy and Ag nanoparticles (NPs), TiO2 NRs/Fex Oy /Ag, integrating in one nanostructure a visible light photoactive moiety (TiO2 NRs/Ag) and a magnetic domain (Fex Oy ), in order to address the photoactivity under visible light and the possibility of recovery and reuse the photocatalyst. The synthesis was carried by preparing first the TiO2 NRs/Fex Oy based heterostructure and then growing Ag NPs with control on size. The resulting multidomain structures were characterized by FTIR and absorption spectroscopy, TEM and SEM microscopy, EDS and XRD analysis. The influence of the Ag NP domain and of its size on the photoactivity of the TiO2 NRs/Fex Oy /Ag nanostructures under visible light were investigated in the photocatalytic degradation of the Nalidixic Acid, an antibiotic used as a model compound representative of recalcitrant pollutants. In the presence of the Ag domain a significant increase of the photoactivity with respect to TiO2 NRs/Fex Oy heterostructures and to the commercially available TiO2 P25 was observed. Such an enhanced photocatalytic efficiency was found dependent on the size of the Ag domain and explained taking into account the plasmonic properties and the different possible photoactivation mechanisms. © 2016 Elsevier B.V. All rights reserved.
1. Introduction A topical challenge in the field of material chemistry is the real scale application of nanosized TiO2 in photocatalytic pollutant removal. In this perspective, a great deal of work has been devoted to extend the optical response of TiO2 in the visible range and for this purpose several different approaches are being investigated. The non-metal doping of TiO2 [1], the integration with short band gap semiconductors [2], the surface engineering [3] and the functionalization with suitable dyes as well as the integration of TiO2 nanocrystals with carbon based heterostructures, including carbon nanotubes [4], graphene [5] or fullerene [6], are relevant examples that highlight the intense research activities in this field. Promising outcomes are emerging from complex structures deriving from combination of TiO2 nanocrystals with noble metal nanoparticles (NPs) as Ag, Au [7,8]. Such class of heterostructures already demonstrated to significantly improve the photocatalytic activ-
∗ Corresponding authors. E-mail addresses: [email protected] (R. Comparelli), [email protected] (M.L. Curri).
ity of TiO2 under UV light because, due to their electronegativity, metal nanoparticles (NPs) can accumulate photogenerated electron from the conduction band (CB) of TiO2 , thus limiting the e− /h+ recombination rate [9]. However, the most remarkable properties of noble metal NPs rely on their ability to absorb visible light thanks to the phenomenon of Localized Surface Plasmon Resonance (LSPR). Under suitable conditions, which include proper size, shape and surrounding medium of noble metal NPs, visible light induces the resonant oscillation of surface free-electrons, resulting in an absorption band in the UV–vis spectrum of noble metal NPs. Thus, noble metal NPs can act as light-traps or electromagnetic field concentrators in rationally designed noble metal/TiO2 heterostructures [10]. Several papers recently proposed the preparation of a variety of noble metal/TiO2 based heterostructures with enhanced plasmonic photocatalytic properties. Wet chemistry approaches are very versatile and allowed to incorporate Au NPs in arrays of N-doped TiO2 bowl-shaped NPs [11] or enable the growth of TiO2 nanorods (TiO2 NRs) on the tip of Au NRs taking the advantage of the anisotropy of both the metal and the semiconductor components [12]. Beside Au NPs, also Ag NPs display similar plasmonic properties, being, in addition, less expensive [13], easy to synthesize and possessing antibacterial properties that extend their field of appli-
http://dx.doi.org/10.1016/j.cattod.2016.11.025 0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7 2
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
cation also for photocatalytic disinfection of water [14] and surfaces [15]. Among the main strategies to achieve the decoration of nanosized semiconductors with Ag NPs there are chemical reduction and photochemical methods. A one-pot solvothermal approach was employed for the fabrication of an Ag/Reduced Graphene Oxide/TiO2 heterostructure where Ag NPs were generated from the reduction of AgNO3 by dimethylacetamide [16]. In addition, a multi-step protocol was exploited to obtain more complex architecture as ␣-Fe2 O3 @Ag/AgCl, where an Ag shell was generated from the classic “mirror reaction”, consisting in the reduction of the diamine Ag(I) ions assisted by aldehyde groups, that functionalize the surface of Fe2 O3 short nanotubes [17]. The photochemical method for the functionalization of TiO2 NPs with Ag is also widely exploited. This approach was applied to grow Ag NPs onto TiO2 NRs [18] or on TiO2 supported on glass fibers [19] thus generating heterostructures with original architectures. However, a reliable use of nanomaterials for photocatalytic degradation of pollutants, especially for wastewater purification, poses important concerns on a safe recovery and reuse of NPs [20]. These issues can be addressed by rationally designing the photocatalyst surface. A possible strategy is based on the NP surface functionalization with molecules able to integrate NP in suitable host matrixes. As an alternative, photocatalytically active nanostructures can be combined with magnetic moieties that enable the magnetic recovery of the structures. An Ag decorated magnetic titania nanocomposite with a core- shell morphology was recently reported in literature [13] and its photocatalytic activity was successfully assessed under visible light by using organic dyes as target molecules. In this framework, the present work describes the synthesis and the characterization of a multifunctional nanosized photocatalytic heterostructure, formed of TiO2 NRs/Fex Oy /Ag with an anisotropic morphology. The aim was to merge distinct multiple functions in one nanostructure. In particular, the rod shaped titania nanocrystals (TiO2 NRs) were selected as basic structure to build such a multifunctional nanomaterial, taking advantage from their anisotropic shape, their high surface area, and, accordingly, the possible more effective delocalization of the photogenerated charge carriers along the longitudinal direction, thus limiting the detrimental e− /h+ recombination events. The direct growth of Fex Oy at the TiO2 NRs surface can effectively convey magnetic properties to the resulting heterostructure, thus potentially enabling the magnetic recovery of the photocatalyst. Finally, Ag NPs, thanks to their plasmonic properties, extend the photocatalytic activity of the system to the visible region. The integration of the distinct components in one nano-architecture was accomplished by following a multistep approach in order to specifically control the characteristics of each component in the structure. Therefore the synthesis of the TiO2 NRs/Fex Oy binary heterostructure was performed by following a hot injection seeded-growth approach [21]. Such a protocol was selected because it was demonstrated to lead to a TiO2 NRs/Fex Oy nanosized heterostructure with well-established magnetic properties [21], that are combined with the high photocatalytic activity and the anisotropy of TiO2 NRs [22]. The synthesis of the TiO2 NRs/Fex Oy was followed by the photochemical growth of Ag NPs with control on their size. In addition, the surface chemistry of the multifunctional heterostructures is characterized by the presence of organic molecules, which can be promptly exchanged and replaced by new functional agents able to integrate them in host polymers, substrates or painting which can convey photocatalytic properties to surfaces, thus allowing their potential use both in aqueous environment and at solid/air interface. The photocatalytic activity of the multifunctional photocatalyst was investigated by performing the degradation of a recalcitrant pollutant, the Nalidixic acid, under visible light.
2. Materials and methods 2.1. Materials All chemicals were of the highest purity available and were used as received without further purification. For the synthesis of TiO2 NRs, Titanium tetraisopropoxide (Ti(OPri)4 or TTIP, 99.999%), trimethylamino-N-oxide dihydrate ((CH3 )3 NO·2H2 O or TMAO, 98%), oleic acid (C18 H33 CO2 H or OLEA, 90%) were purchased from Aldrich. For the synthesis of TiO2 NRs/Fex Oy , oleic acid (C17 H33 CO2 H or OLEA, 90%), 1-octadecene (C18 H36 or ODE, 90%), oleyl amine (C17 H33 NH2 or OLEAM, 70%), iron pentacarbonyl (Fe(CO)5 , 98%) and dodecan-1,2-diol (C12 H24 (OH)2 or DDIOL, 90%) were purchased from Aldrich. For the synthesis of TiO2 NRs/Fex Oy /Ag the silver nitrate (AgNO3 , 99.998%), was purchased from Aldrich. All solvents used were of analytical grade and purchased from Aldrich. TiO2 “TiO2 P25” (TiO2 P25 Evonik) has-been selected as commercial standard material. Nalidixic Acid, NA (1-ethyl-1,4-dihydro-7-methyl-4-oxo1,8-naphthyridine-3-carboxylic acid) was purchased from Fluka. MilliQ water was employed for preparation of all aqueous solutions.
2.2. Synthesis of TiO2 NRs/Fex Oy /Ag The synthesis of the ternary heterostructure TiO2 NRs/Fex Oy /Ag was carried out, first, synthesizing the TiO2 NRs/FexOy heterostructure according a procedure described in [21] and reported in the Section 1 of Supplementary material (SM). Then a photochemical procedure developed for the preparation of the TiO2 NRs/Ag nanocomposite was suitably adapted, optimizing the relevant synthetic parameters [18]. A quartz cuvette containing a de-aerated CHCl3 :EtOH mixture (EtOH content 10%(v/v)) was filled with presynthesized colloidal TiO2 NRs/Fex Oy heterostructure and AgNO3 with a molar ratio TiO2 :Ag+ of 1:175. The mixture, purged for 30 min under nitrogen flow, was irradiated with UV light (light Source: high pressure 200 W mercury lamp; > 250 nm equipped with a neutral density filter in order to obtain a light intensity of 0.07 W/cm2 ). This mixture was irradiated for different time, namely 15 min, 60 and 120 min obtaining the TiO2 NRs/Fex Oy /Ag A, TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C samples, respectively.
2.3. Characterization 2.3.1. X-ray diffraction experiments and simulations X-ray diffraction data were collected at room temperature from samples of both TiO2 /Fex Oy and TiO2 /Fex Oy /Ag NPs deposited onto silicon substrates. Measurements were performed by using a Bruker D8 Discover diffractometer, equipped with a Göbel mirror, using Cu K␣ radiation (K␣1 = 1.54056 Å and K␣2 = 1.54439 Å), and an energy dispersive X-ray SolX-E detector. Data were collected at fixed incidence angle of 5◦ while moving the detector in the range 10–120◦ with a step size of 0.05◦ .
2.3.2. Transmission electron microscopy Transmission electron microscopy (TEM) analysis was performed by a JEOL JEM-1011 microscope operating at 100 kV. The TEM samples were prepared by casting a drop of TiO2 NRs·CHCl3 solution onto a carbon hollowed TEM grid.
2.3.3. UV–vis absorption spectroscopy UV–vis absorption spectra and reflectance spectra were recorded with a UV–vis-near IR Cary 5 (Varian spectrophotometer.).
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
2.3.4. Photocatalysis experiments In a typical experiment, 0.001 mol of TiO2 -based catalysts were cast onto a glass slide (with a surface of 3.6 cm2 ) by dropping the catalyst solution and letting the solvent to evaporate, thus leading to a nearly transparent film. The concentration of the catalyst solution was calculated by cyclic voltametry. Two sets of control experiments were carried out in presence of the same amount of TiO2 in TiO2 NRs, TiO2 NRs/Fex Oy and TiO2 P25, respectively. The transparent glass support, suitably shaped in order to fit a 1 cm × 1 cm quartz cell, was positioned against the far-off wall of the cell with respect to light beam. Photocatalytic experiments under visible light were performed by using a 250 W quartz tungsten halogen lamp, (QTH) filtered by band pass filter to transmit light from 430 nm to 3300 nm and a light intensity of 0.22 W/cm2 ∼ 2 SUN [23]. All experiments were performed in air, keeping the system under vigorous stirring. The NA solution was 10−4 M in MilliQ water at pH 7. The pH value was adjusted by adding the proper amount of 0.1 M HCl. Before irradiation the NA solution was left to stir in the dark for 30 min in order to promote the absorption/desorption equilibrium. At a fixed illumination time, samples were withdrawn from the solution reaction batch in order to monitor the NA degradation course by UV–vis absorption spectroscopy. A possible leaching of TiO2 in the aqueous solution during photocatalysis was investigated by ICP analyses on water treated with the tested catalysts and the Ti amount was found below the detection limit for all the investigated samples.
3. Results and discussion 3.1. Synthesis of TiO2 NRs/Fex Oy /Ag heterostructures The multi-step preparation of the TiO2 NRs/Fex Oy /Ag nanostructure was performed by adapting our previous reported procedure, based on the photochemical growth of Ag NPs on TiO2 NRs [18] leading to a UV photoactive nanocomposite [24]. In order to achieve a magnetically recoverable photocatalyst, that could be also simultaneously active under the visible light, in this work we aimed at growing Ag NPs on a magnetic TiO2 NRs/Fex Oy heterostructure. Firstly TiO2 NRs/Fex Oy nanostructures were synthesized by means of a previously reported colloidal route, that was purposely designed to achieve heterostructures endowed of the magnetic functionality of the Fex Oy domain along with the distinctive relevant features of the TiO2 NRs, including the semiconducting character, the anisotropic shape and the anatase crystalline phase [21], being the most photoactive phase of the TiO2 [25]. In addition, such a synthetic procedure allowed to obtain TiO2 NRs/Fex Oy heterostructures dispersible in organic apolar solvents, due to their specific surface chemistry. Such surface characteristics enable, in principle, the further processing of the heterostructures in organic solutions, including the fabrication of new domains, by exploiting the specific reactivity of the TiO2 NRs. Then, Ag domains were synthesized by performing the photochemical reduction of Ag+ ions under UV irradiation assisted by the TiO2 present in the TiO2 NRs/Fex Oy heterostructure as a catalyst. The photochemical growth of Ag NPs was carried out in a deaerated CHCl3 /EtOH mixture, under nitrogen atmosphere in order to avoid the scavenging of the conduction band (CB) electrons by oxygen, thus ensuring the availability of CB electrons (e− ) for the Ag+ photoreduction. Moreover the ethanol in the reaction mixture acts as an efficient hole (h+ ) scavenger, thus avoiding the detrimental e-/h+ recombination. The synthesis parameters, namely irradiation intensity, solvent polarity and molar ratio of the precursors, TiO2 NRs/Fex Oy and Ag+ , respectively, were optimized in order to achieve the ideal conditions for photoexcitation of the catalyst. The role played by the [TiO2 ]/[Ag+ ]
3
precursor molar ratio was investigated and a ratio of 1:175 was found suitable as it corresponds to the amount of TiO2 NRs/Fex Oy appropriate to safely accomplish the photocatalytic reaction. In fact, a higher amount of TiO2 NRs/Fex Oy , would result in a too dark solution, due to the high Fex Oy concentration, that would hamper the UV light absorption by TiO2 NRs and, consequently, limit the photoexcitation of the TiO2 NRs/Fex Oy for the photochemical synthesis of the Ag NPs. For the Ag growth step of the synthesis in the TiO2 NRs/Fex Oy /Ag nanostructures, three increasing irradiation times were investigated, namely 15 min (sample TiO2 NRs/Fex Oy /Ag A), 60 min (sample TiO2 NRs/Fex Oy /Ag B) and 120 min (sample TiO2 NRs/Fex Oy /Ag C) and the prepared samples were characterized by means of morphological, structural and spectroscopic investigation. The Ag NPs formation is due to the photoexcitation of TiO2 NRs in the TiO2 NRs/Fex Oy that photogenerate electrons in conduction band (CB). These CB electrons have the redox potential suited to promote the reduction of Ag+ ions at the TiO2 NRs surface, leading to the formation of Ag NPs [18]. In principle also the Fex Oy domain can be involved in the photoreduction of Ag+ ions. Indeed bulk Fe2 O3 is reported to have a band gap of 2.3 eV, therefore, under UV light irradiation, electrons can be photogenerated, inducing the photoreduction of Ag+ ions, considering the red-ox potential of the Ag/Ag+ , 0.799 V higher than that of Fe2 O3 , that is +0.18 V, both vs NHE at pH 0 [26]. However, the reducing ability of TiO2 CB vs NHE is higher than that reported for Fe2 O3 (bulk) (0.38 V for TiO2 vs +0.18 V for Fe2 O3 ), therefore the photochemical growth of Ag NPs is reasonable to occur preferentially at the surface of TiO2 NRs. Fig. 1 shows the morphological features of the different samples of TiO2 NRs/Fex Oy /Ag heterostructures obtained as a function of irradiation time. In the three samples, TiO2 NRs/Fex Oy /Ag A TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C, obtained upon irradiation for 15 min, 60 min and 120 min respectively, the TEM micrographs show dark spots in close proximity with the TiO2 NRs/Fex Oy units, which are, conversely, much brighter. In particular, the micrographs suggest that one Ag NP is in contact with several TiO2 NRs/Fex Oy heterostructures, thus leading to aggregates, formed of sparse networks of nanostructures TEM analysis points out that Ag NPs are basically isotropic in shape, being never isolated and positioned far from the Fex Oy domain in the structures, probably to minimize the steric hindrance in the overall heterostructure assembly. Moreover Ag NPs appear to be mainly positioned at the tip of TiO2 NRs (Fig. 1D). Such a feature can be explained by the likely Ag+ ions absorption, and their consequent photoreduction, preferentially occurring at the tips of TiO2 NRs, where the smaller curvature radius leads to a sparse coordination of the organic molecules, which are therefore less packed and make possible further adsorption of species. In addition, the probability of Ag nucleation at the tip of TiO2 NRs can be enhanced by the higher reactivity for the <004> exposed planes [18]. The results of statistical analysis of Ag NPs size in the three samples are reported in Fig. 2a–c. The Ag NPs average size was 11 ± 4 nm and 12 ± 2 nm for TiO2 NRs/Fex Oy /Ag A and TiO2 NRs/Fex Oy /Ag B, respectively. Accordingly, the formation of Ag NPs can be thought to occur during the first 15 min of photoreaction. In fact, during the subsequent 45 min of irradiation the average size stays almost constant, only a slight narrowing of size distribution can be noticed. For the sample TiO2 NRs/Fex Oy /Ag C, obtained upon 2 h irradiation, TEM micrographs clearly highlight the formation of large and close-packed aggregates, formed of heterostructures with large Ag domains and a broad size distribution. Therefore a longer irradiation time for TiO2 NRs/Fex Oy /Ag C can probably induce the coalescence of smaller AgNPs formed in earlier stages of irradiation [18].
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7 4
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
Fig. 1. TEM micrographs (A–D) and related size distribution (a–c) at different UV irradiation time: 15 min (A and a for, TiO2 NRs/Fex Oy /Ag A; 60 min (B and b for, TiO2 NRs/Fex Oy /Ag B) 120 min (C and c TiO2 NRs/Fex Oy /Ag B). TEM micrographs of TiO2 NRs/Fex Oy /Ag A. TiO2 NRs/Fex Oy /Ag B at higher magnification (D). The inset shows an Ag NP in direct contact with the TiO2 NRs/Fex Oy moiety. The scheme (E) reports the proposed mechanism for the Ag NPs photochemical growth on the tip of TiO2 NRs, in addition, a close-up of the TEM micrograph of a single ternary TiO2 NRs/Fex Oy /Ag structure has been added, highlighting the domains with different composition. The image F show the magnetically recoverable TiO2 NRs/Fex Oy /Ag nanostructure before and after the magnetic separation.
Interestingly the prepared heterostructures clearly showed a magnetic behaviour, conveyed to the system by the presence of iron oxide based domain. In Fig. 1F the TiO2 NRs/Fex Oy /Ag nanostructures under a magnetic field induced by a permanent magnet (Neodymium NdFeB, Flux density inside the magnet1.17 T, holding force on a steel plate, 59.84 N) can be seen effectively separated from the solution. XRD diffraction patterns of (a) TiO2 NRs/FexOy and (b–c) TiO2 NRs/Fex Oy /Ag B. Vertical bars: Bragg hkl reflections positions for anatase (cyan bars in panel a) and dotted grey bars in panel b-c-d), AgCl (blue bars), Ag (green bars), Fe2 O3 maghemite (black curvebars and Fe3 O4 magnetite (red bars) phases. Both Panels b and c report the same diffraction pattern of TiO2 NRs/Fex Oy /Ag to highlight the presence of anatase TiO2 NRs (dotted vertical bars), Ag (green markers) and AgCl (blue markers) (b) and Fe2 O3 (black markers) and Fe3 O4 (red markers) (c). Panel (d) shows a zoomed view of the panel (c) to highlight the small fraction of Ag phase. XRD analysis, reported in Fig. 2 for the TiO2 NRs/Fex Oy /Ag B as a representative sample of the nanostructures, allow to identify in all the investigated TiO2 NRs/Fex Oy /Ag samples, a mixture of TiO2 anatase, ␥-Fe2 O3 (maghemite), Fe3 O4 (magnetite), AgCl and a very small fraction of Ag. The diffraction peaks ascribed to the AgCl crystalline phase show a crystallite size larger than that of the metallic Ag NPs, as evidenced in panel (d) of Fig. 2. The diffraction pattern of the TiO2 NRs/Fex Oy /Ag sample demonstrates that the photochemical conditions to achieve Ag+ photoreduction did not changed the TiO2 polymorph maintaining the same structural features of the TiO2 NRs/Fex Oy reported in Fig. 2a [21]. The formation of AgCl species can be explained on the basis of the possible reaction between AgNO3 introduced in the mix-
ture as an alcoholic solution and the CHCl3 [27,28]. In fact, when the AgNO3 solution was added to the synthesis reaction mixture a white precipitate was observed to form, reasonably ascribable to the formation of AgCl, along with its rapidly re-dissolution, occurring as soon as a proper amount of ethanol was added to the mixture to adjust the polarity. Therefore, the most Ag species can be thought as available in the reaction mixture as Ag+ ion, being AgCl solubilized and available for the photoreduction, while only a small fraction of residual AgCl can be reasonably expected to stay, possibly, adsorbed onto the resulting TiO2 NRs/Fex Oy /Ag heterostructures. Such a hypothesis accounts for the results of the elemental analysis performed by EDS, reported in Figure SM2 of the SM, that shows as the amount of Ag is higher than that of Cl, being therefore not ascribable to the sole presence of AgCl. Moreover SEM micrographs of TiO2 NRs/Fex Oy /Ag, as reported in Figure SM3 of the SM, pointed out the presence of Ag NPs in close proximity of the TiO2 NRs/Fex Oy surface as well as on focal plane inside the investigated aggregates. The three synthesized samples resulted in optically clear dispersions that, thus, allowed a thorough optical characterization by UV–vis by absorption. The absorption spectrum of TiO2 NRs/Fex Oy /Ag A, B and C was recorded by using as a baseline the spectrum of the purged synthesis mixture before UV irradiation, in order to clearly point out possible feature due to the expected appearance of the Ag plasmon band. The spectra of TiO2 NRs/Fex Oy /Ag A, TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C in Fig. 3 show that the heterostructures present a clear absorption signal in the visible region, due to the Surface Plasmon band arising with the formation of AgNPs,
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
5
Fig. 2. XRD diffraction patterns of (a) TiO2 NRs/Fex Oy and (b–c) TiO2 NRs/Fex Oy /Ag B. Vertical bars: Bragg hkl reflections positions for anatase (cyan curve), AgCl (blue curve), Ag (green curve), Fe2 O3 maghemite (black curve) and Fe3 O4 magnetite (red curve) phases. Both Panels b and c report the same diffraction pattern of TiO2 NRs/Fex Oy /Ag B in order to highlight the presence of anatase TiO2 NRs (dotted vertical lines), Ag (green markers) and AgCl (blue markers) (b) and Fe2 O3 (black markers) and Fe3 O4 (red markers) (c). The panel (d) is a zoomed view of the panel (c) to highlight the small fraction of Ag phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
FTIR analysis, which are described in details in the Figure SM4 of the SM, highlights that the surface chemistry of the TiO2 NRs/Fex Oy /Ag nanostructures is mainly characterized by the presence of oleic acid, that is the capping agent used for the synthesis [29]. Such a surface chemistry makes the TiO2 NRs/Fex Oy /Ag structures stable in apolar solvents, including chloroform, toluene or hexane.
3.2. Photocatalytic activity under visible light irradiation
Fig. 3. Representative absorbance spectrum of as prepared TiO2 /Fex Oy /Ag heterostructures obtained at different irradiation times: 15 min (TiO2 NRs/Fex Oy /Ag A); 60 min (TiO2 NRs/Fex Oy /Ag B) 120 min (TiO2 NRs/Fex Oy /Ag C) in comparison with the absorbance spectrum of the TiO2 NRs/Fex Oy. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
that indicates a possible visible light photocatalytic activity of the obtained nanostructures. The position of the plasmon band of the Ag NPs red shifts with the irradiation time Indeed the position, the intensity and the full width at half maximum (FWHM) of the absorbance maximum in the plasmon band was found to change with the reaction conditions, that controlled the resulting size and distribution of the Ag NPs as demonstrated by TEM micrographs (Fig. 1). Notably the TiO2 NRs/Fex Oy /Ag C, presented an asymmetric plasmon band with a maximum at 480 nm, indicating a large size distribution for Ag NPs in agreement with the TEM results.
The photocatalytic activity of TiO2 NRs/Fex Oy /Ag A, TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C was investigated under visible light by using the Nalidixic Acid (NA) (10−4 M, pH 2.5) as target compound. The NA was selected as a model target molecule, being a real pollutant representative of fluoroquinolones, a class of antibiotics of environmental concern that is detected as a recalcitrant compound in different types of waste waters [30]. In addition, NA is a suitable model compound as it does not absorb in the visible range, thus ruling out possible sensitization effects. All experiments were performed in air, keeping the system under vigorous stirring. After a 30 min conditioning time in the dark, to promote adsorption/desorption equilibrium, the system was irradiated with a visible lamp. At a fixed illumination time, samples of the solution were withdrawn from the reaction batch in order to monitor the degradation course by UV–vis absorption spectroscopy. The absorption spectrum of the NA is reported in Figure SM5 of the SM. Fig. 4 reports the kinetic of NA degradation, under visible light irradiation, as ln(C/C0 ) vs. reaction time. The plot results in a straight line with the slope, k (Table 1) representing the apparent rate constant. Such k value was effectively used to compare the photocatalytic activity of the investigated heterostrucures. The comparison among the apparent kinetic con-
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
6
Fig. 4. Nalidixic Acid degradation rates in presence of TiO2 NRs/Fex Oy /Ag A, TiO2 Nrs/Fex Oy /Ag B and TiO2 Nrs/Fex Oy /Ag C, TiO2 NRs/Fex Oy , TiO2 NRs and TiO2 P25. Experiments were carried out at pH 2.5 under visible light irradiation. Nalidixic Acid concentration was evaluated by monitoring the absorbance intensity at 316 nm (pH 2.5). Experimental data are reported as mean values of 5 replicates ± standard deviation. (B and C) Scheme of possible photoactivation mechanism of the TiO2 NRs/Fex Oy /Ag under visible light: hot electrons effect (B) and near field effect (C). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 1 Summary of kinetic constants calculated for the photocatalytic degradation of NA assisted by TiO2 /Fex Oy /Ag based heterostructure. Sample name
k (min−1 )
TiO2 /Fex Oy /Ag A TiO2 /Fex Oy /Ag B TiO2 /Fex Oy /AgC TiO2 NRs TiO2 NRs/Fex Oy TiO2 P25
14 · 10−3 ±10−3 15 · 10−3 ±10−4 7 · 10−3 ±10−4 10 · 10−3 ±10−3 12 · 10−3 ±10−3 8 · 10−3 ± 0.5 · 10−3
stants revealed that the degradation of NA was faster when assisted by the prepared photocatalysts, with respect to that found for TiO2 P25 and for the TiO2 NRs. In particular, the apparent kinetic constants of the reaction carried out by using TiO2 NRs/Fex Oy /Ag A, and TiO2 /Fex Oy /Ag B was 1.8 and 1.9 times, respectively, higher than the apparent kinetic constant obtained in the case of use of TiO2 P25, while TiO2 NRs/Fex Oy /Ag C showed a degradation rate comparable with the TiO2 P25. Interestingly, the three samples TiO2 NRs/Fex Oy /Ag A, TiO2 NRs/Fex Oy /Ag B and TiO2 NRs/Fex Oy /Ag C, reported apparent kinetic constant values higher than those determined both for TiO2 NRs/Fex Oy and TiO2 NRs, as reported in Table 1. Therefore, the increase of the photocatalytic efficiency observed for TiO2 NRs/Fex Oy /Ag is ascribable to the presence of Ag species, that, thanks to their plasmonic properties, give rise to an intense and highly localized electromagnetic field. The visible lightinduced photoactivation of the ternary system can take place according two possible mechanisms, both related to the physicalchemical features of the plasmonic heterostructures. Indeed when LSPR (Localized Surface Plasmon Resonance) conditions are satisfied in metal NPs, the confined free electrons oscillate with the same frequency as the incident radiation and eventually enter in resonance. Their decay takes place on a femtosecond timescale, either radiatively or non-radiatively. The non-radiative relaxation of the surface plasmons can induces the generation of the so-called “hot electrons”, transferring the accumulated energy to electrons in the conduction band of the material. When the noble metal NPs are in direct contact with the semiconductor, such “hot electrons” can be injected into the conduction band of the semiconductor, provided that they have enough energy to overcome the Schottky barrier [9]. Conversely, when the semiconductor and the plasmonic metal are not in contact with each other but isolated by a thin, nonconductive spacer, preventing any possible charge transfer process,
under light excitation a near field effect can take place, thus representing a second possible mechanism [35]. Indeed, under LSPR conditions, the high intensity of the electric field, in the proximity of noble metal NPs, can increases the rate of electrons and holes formation. In the TiO2 NRs/Fex Oy /Ag A and TiO2 NRs/Fex Oy /Ag B the size of Ag NPs is less to 50 nm, therefore the “efficient scattering” mechanism can be considered negligible. Both the “hot electrons” mechanism and the “near field effect” mechanism can safely account for the photoactivation of the systems. Moreover, remarkably the apparent kinetic constants were affected by the size of Ag species. In particular TiO2 NRs/Fex Oy /Ag A and TiO2 NRs/Fex Oy /Ag B heterostructures, which possess Ag species with comparable dimensions, exhibited comparable apparent kinetic constants. In contrast, the TiO2 NRs/Fex Oy /Ag C sample, characterized by larger Ag domains of 51 nm, demonstrated a degradation rate two times slower with respect to TiO2 /Fex Oy /Ag A, and TiO2 /Fex Oy /Ag B samples. Such an evidence is in agreement with the report by Yu et al. that observed a higher efficiency of the photocatalytic process in presence of smaller Ag NPs respect to larger Ag NPs, as “hot electron” injection from noble metal to semiconductor has been demonstrated to occur more effectively for smaller metallic NPs. [31]. Remarkably XRD analysis revealed the presence of AgCl phase, that although not photoactive under visible light due to its band gap of 3.03 eV [32], could however contribute to the enhancement of the overall photocatalytic activity of the system. Indeed, in an Ag/AgCl based system, the surface of AgCl particles, terminating with Cl- ions, is negatively charged, while the AgCl bulk is positively charged. When metallic Ag NPs are under LSPR conditions, the dipolar character of surface plasmon induces a separation of electrons and holes. Under these conditions, the region of positive charges in the metallic Ag NPs is close to the Ag/AgCl interface, as electrons are polarized by AgCl bulk, while the region of negative charges is at the surface of the Ag NPs which is outermost from Ag/AgCl interface. Therefore, the reduction of Ag+ in AgCl is hindered, while a larger amount of electrons is made available for formation of oxyradical species [33,34]. Holes are in principle free to react with surface terminating Cl− anions, possibly generating strong oxidizing agents as Cl• [35]. In addition, when TiO2 NRs/Fex Oy /Ag nanostructures were cast from solution onto TEM grid, they were found to form, upon solvent evaporation, a three-dimensional network. A similar assembly of the nanostructures is expected to occur also when they are cast onto the glass slide employed for photocatalysis experiment, with the possible occurrence of an “antenna effect” resulting from the interparticle contact and able to enhance the photocatalytic activity of the system. The “antenna effect” takes place when photogenerated charge carrier, formed in a NP, can be rapidly transferred to their neighbours in the same nanoparticles assembly, until reaching a suitable trap site where it can be involved in an oxidation or reduction reaction with the absorbate, respectively [36,37]. Therefore the “antenna effect” can improve the photocatalytic activity increasing the formation of reactive radicals and preventing the e− /h+ recombination as demonstrated by a similar TiO2 NRs/Ag system [24]. The improved photocatalytic performances of TiO2 NRs compared to TiO2 P25 were already observed in the case of an organic dye as target molecule, under irradiation of a both QTH lamp [4] or an UV light source and explained in the light of the higher surface area, morphology and unique surface chemistry of TiO2 NRs [38,39]. However, the binary heterostructure TiO2 NRs/Fex Oy , under the investigated experimental condition, did not reveal an increase of the photocatalytic efficiency compared to TiO2 /NRs into the visible wavelength (in fact the logarithmic plots of TiO2 NRs/Fex Oy and TiO2 NRs in
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025
G Model CATTOD-10460; No. of Pages 7
ARTICLE IN PRESS F. Petronella et al. / Catalysis Today xxx (2016) xxx–xxx
Fig. 4 are overlapping). Probably the presence of Fex Oy induced defects or energy levels able to promote the e− /h+ recombination. 4. Conclusions The present work describes the synthesis and the photocatalytic activity of the multifunctional nanocrystalline heterostructure based on TiO2 NRs/Fex Oy /Ag. Such a photocatalyst was specifically designed to combine, in one structure, domains with different composition and distinct functions, namely the UV-photoactivity of semiconductive TiO2 , the visible light activity, enhanced by the Ag NPs, and, finally, magnetic properties of iron oxide domain. The presence of visible photoactive Ag domains in the photocatalyst demonstrated to enable the visible light photodegradation of the model, while the Fex Oy structure, The prepared multifunctional catalyst showed to be more effective than commercial catalysts TiO2 P25 Evonik in the removal of the NA, selected as model pollutant. Remarkably, the assessment of the photocatalytic efficiency based on the evaluation of the apparent rate constants, demonstrated that the TiO2 NRs/Fex Oy /Ag nanostructure, with Ag NPs of 12 nm in size, resulted 1.9 times faster than the commercial TiO2 P25 and 1.5 times faster than the TiO2 NRs. Such an enhancement has been accounted for by the presence of Ag NPs that enhance visible light photoactivity due to their peculiar plasmonic properties. In particular, the visible light-induced photoactivation of the ternary heterostructure has been discussed on the basis of two possible mechanisms, namely the generation of “hot electrons” and “near field” effect, that can both take place and result in an overall increases of electrons and holes available for the catalytic processes in proximity of the metal NPs in the heterostructures. The results highlighted the efficiency of TiO2 NRs/Fex Oy /Ag in the photocatalytic degradation of NA under visible light, suggesting that the heterostructure can be considered as promising candidate for pilot scale applications, as the magnetic properties of the iron oxide domain allow a facile photocatalyst recovery by means of a magnetic field, that could be further exploited in suitably designed photocatalytic reactors. Acknowledgments The authors wish to express their deepest and sincerest recognition to Prof. Andràs Dombi a key figure in the topic of photocatalytic materials for degradation of contaminants of environmental concern. This work was partially supported by the EC-funded 7th FP LIMPID Project (Grant No. 310177) and by Apulia Region funded NanoApulia (MDI6SR) and “Tecnologie Abilitanti per Produzioni Agroalimentari Sicure e Sostenibili” (T.A.P.A.S.S.) − PELM994” (CUP B38C14002040008) projects, by Italian PRIN 2012 (prot. 20128ZZS2H) project and by the Italian Regional Network of Laboratories “Sens&Micro” and “VALBIOR” projects(POFESR 20072013).
7
References [1] Y. Zhang, C. Han, M.N. Nadagouda, D.D. Dionysiou, Appl. Catal. B Environ. 168–169 (2015) 550. [2] B. Liu, X. Li, Q. Zhao, J. Ke, M. Tadé, S. Liu, Appl. Catal. B Environ. 185 (2016) 1. [3] T.R. Gordon, M. Cargnello, T. Paik, F. Mangolini, R.T. Weber, P. Fornasiero, C.B. Murray, J. Am. Chem. Soc. 134 (2012) 6751. [4] F. Petronella, M.L. Curri, M. Striccoli, E. Fanizza, C. Mateo-Mateo, R.A. Alvarez-Puebla, T. Sibillano, C. Giannini, M.A. Correa-Duarte, R. Comparelli, Appl. Catal. B Environ. 178 (2015) 91. [5] K. Dai, L. Lu, Q. Liu, G. Zhu, Q. Liu, Z. Liu, Dalton Trans. 43 (2014) 2202. [6] M.-Q. Yang, N. Zhang, Y.-J. Xu, ACS Appl. Mater. Interfaces 5 (2013) 1156. [7] J.C. Durán-Álvarez, E. Avella, R.M. Ramírez-Zamora, R. Zanella, Catal.Today 266 (2016) 175. [8] L. Pinho, M. Rojas, M.J. Mosquera, Appl. Catal. B Environ. 178 (2015) 144. [9] V. Subramanian, E.E. Wolf, P.V. Kamat, J. Am. Chem. Soc. 126 (2004) 4943. [10] C. Clavero, Nat. Photonics 8 (2014) 95. [11] X. Wang, R. Long, D. Liu, D. Yang, C. Wang, Y. Xiong, Nano Energy 24 (2016) 87. [12] B. Wu, D. Liu, S. Mubeen, T.T. Chuong, M. Moskovits, G.D. Stucky, J. Am. Chem. Soc. 138 (2016) 1114. [13] Y. Wang, F. Pan, W. Dong, L. Xu, K. Wu, G. Xu, W. Chen, Appl. Catal. B Environ. 189 (2016) 192. [14] D.M. Tobaldi, C. Piccirillo, R.C. Pullar, A.F. Gualtieri, M.P. Seabra, P.M.L. Castro, J.A. Labrincha, J. Phys. Chem. C 118 (2014) 4751. [15] S. Rtimi, M. Pascu, R. Sanjines, C. Pulgarin, M. Ben-Simon, A. Houas, J.C. Lavanchy, J. Kiwi, Appl. Catal. B Environ. 138–139 (2013) 113. [16] W. Gao, M. Wang, C. Ran, X. Yao, H. Yang, J. Liu, D. H, J. Bai, Nanoscale 6 (2014) 5498. [17] J. Liu, W. Wu, Q. Tian, S. Yang, L. Sun, X. Xiao, F. Ren, C. Jiang, V.A.L. Roy, R.S.C Adv. 5 (2015) 61239. [18] P.D. Cozzoli, R. Comparelli, E. Fanizza, M.L. Curri, A. Agostiano, D.l. Laub, J. Am. Chem. Soc. 126 (2004) 3868. [19] L. Chen, S. Yang, B. Hao, J. Ruan, P.-C. Ma, Appl. Catal. B Environ. 166–167 (2015) 287. [20] A. Panniello, M.L. Curri, D. Diso, A. Licciulli, V. Locaputo, A. Agostiano, R. Comparelli, G. Mascolo, Appl. Catal. B Environ. 121–122 (2012) 190. [21] R. Buonsanti, V. Grillo, E. Carlino, C. Giannini, M.L. Curri, C. Innocenti, C. Sangregorio, K. Achterhold, F.G.n. Parak, A. Agostiano, P.D. Cozzoli, J. Am. Chem. Soc. 128 (2006) 16953. [22] P.D. Cozzoli, R. Comparelli, E. Fanizza, M.L. Curri, A. Agostiano, Mater. Sci. Eng. C 23 (2003) 707. [23] F. Milano, L. Gerencsér, A. Agostiano, L. Nagy, M. Trotta, P. Maróti, J. Phys. Chem. B 111 (2007) 4261. [24] F. Petronella, S. Diomede, E. Fanizza, G. Mascolo, T. Sibillano, A. Agostiano, M.L. Curri, R. Comparelli, Chemosphere 91 (2013) 941. [25] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891. [26] A.L. Linsebigler, G. Lu, J.T. Yates, Chem. Rev. 95 (1995) 735. [27] J.A. Vona, J. Steigman, J. Am. Chem. Soc. 81 (1959) 1095. [28] R.T. Morrison, R.N. Boyd, Organic Chemistry, Prentice Hall, 1992. [29] P.D. Cozzoli, A. Kornowski, H. Weller, J. Am. Chem. Soc. 125 (2003) 14539. [30] L. Ge, J. Chen, X. Wei, S. Zhang, X. Qiao, X. Cai, Q. Xie, Environ. Sci. Technol. 44 (2010) 2400. [31] K. Yu, Y. Tian, T. Tatsuma, Phys. Chem. Chem. Phys. 8 (2006) 5417. [32] D. Chen, T. Li, Q. Chen, J. Gao, B. Fan, J. Li, X. Li, R. Zhang, J. Sun, L. Gao, Nanoscale 4 (2012) 5431. [33] G. Li, Y. Wang, L. Mao, R.S.C Adv. 4 (2014) 53649. [34] A. Ziylan, N.H. Ince, J. Hazard. Mater. 187 (2011) 24. [35] J. Yu, G. Dai, B. Huang, J. Phys. Chem. C 113 (2009) 16394. [36] A.A. Ismail, D.W. Bahnemann, I. Bannat, M. Wark, J. Phys. Chem. C 113 (2009) 7429. [37] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Chem. Rev. 114 (2014) 9919. [38] M. Fittipaldi, M.L. Curri, R. Comparelli, M. Striccoli, A. Agostiano, N. Grassi, C. Sangregorio, D. Gatteschi, J. Phys. Chem. C 113 (2009) 6221. [39] F. Petronella, E. Fanizza, G. Mascolo, V. Locaputo, L. Bertinetti, G. Martra, S. Coluccia, A. Agostiano, M.L. Curri, R. Comparelli, J. Phys. Chem. C 115 (2011) 12033.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.11. 025.
Please cite this article in press as: F. Petronella, et al., Multifunctional TiO2 /Fex Oy /Ag based nanocrystalline heterostructures for photocatalytic degradation of a recalcitrant pollutant, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.11.025