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reduced graphene oxide nanocomposites for enhanced visible-light photocatalytic activity
Accepted Manuscript Title: Ag-loaded TiO2 /reduced graphene oxide nanocomposites for enhanced visible-light photocatalytic activity Author: E. Vasilaki I. Georgaki D. Vernardou M. Vamvakaki N. Katsarakis PII: DOI: Reference:
S0169-4332(15)01623-2 http://dx.doi.org/doi:10.1016/j.apsusc.2015.07.056 APSUSC 30783
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-3-2015 16-6-2015 7-7-2015
Please cite this article as: E. Vasilaki, I. Georgaki, D. Vernardou, M. Vamvakaki, N. Katsarakis, Ag-loaded TiO2 /reduced graphene oxide nanocomposites for enhanced visible-light photocatalytic activity, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.07.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ag-loaded TiO2 / reduced graphene oxide nanocomposites for enhanced visible-light photocatalytic activity E. Vasilakia,b*, I. Georgakib, D. Vernardoub, M. Vamvakakic,d and N. Katsarakisb,d Department of Chemistry, University of Crete, 71003, Heraklion, Crete, Greece
b
Center of Materials Technology and Photonics, School of Applied Technology,
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a
Technological Educational Institute of Crete, 710 04 Heraklion, Crete, Greece c
Department of Materials Science and Technology, University of Crete, 71003, Heraklion,
d
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Crete, Greece
Institute of Electronic Structure and Laser, Foundation for Research & Technology-Hellas,
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P.O. Box 1385, Vassilika Vouton, 711 10 Heraklion, Crete, Greece
Ag nanoparticles were loaded on TiO2 by chemical reduction. TiO2/Ag and TiO2 samples were deposited on reduced graphene oxide (rGO). Their performance was evaluated via methylene blue removal under visible-light. TiO2/Ag/rGO presented superior activity compared to TiO2, TiO2/Ag and TiO2/rGO.
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*corresponding author, e-mail: [email protected]
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Abstract
In this work, Ag nanoparticles were loaded by chemical reduction onto TiO2 P25 under
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different loadings ranging from 1 up to 4 wt % and hydrothermally deposited on reduced graphene oxide sheets. Chemical reduction was determined to be an effective preparation approach for Ag attachment to titania, leading to the formation of small silver nanoparticles with an average diameter of 4.2 nm. The photocatalytic performance of the hybrid
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nanocomposite materials was evaluated via methylene blue (MB) dye removal under visiblelight irradiation. The rate of dye decolorization was found to depend on the metal loading, showing an increase till a threshold value of 3 wt %, above which the rate drops. Next, the as prepared sample of TiO2/Ag of better photocatalytic response, i.e., at a 3 wt % loading value, was hydrothermally deposited on a platform of reduced graphene oxide (rGO) of tunable content (mass ratio). TiO2/Ag/rGO coupled nanocomposite presented significantly enhanced photocatalytic activity compared to the TiO2/Ag, TiO2/rGO composites and bare P25 titania semiconductor photocatalysts. In particular, after 45 min of irradiation almost complete decolorization of the dye was observed for the TiO2/Ag/rGO nanocatalyst, while the respective removal efficiency was 92% for TiO2/Ag, 93% for TiO2/rGO and only 80% for the 1
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bare TiO2 nanoparticles. This simple step by step preparation strategy allows for optimum exploitation of the advanced properties of metal plasmonic effect and reduced graphene oxide as the critical host for boosting the overall photocatalytic activity towards visible-light. Keywords: Titanium dioxide; Plasmonic photocatalysts; Silver loaded titanium dioxide; Reduced graphene oxide coupled titanium dioxide.
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1. Introduction
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Conventional treatment methods applied for the decomposition of organic and inorganic water contaminants have limited efficiency [1]. In the context of more effectual alternatives,
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heterogeneous photocatalysis is one of the most promising advanced oxidation processes (AOP’s), being capable of achieving complete oxidation of both organic and inorganic water contaminants [2]. Heterogeneous photocatalysis refers to the mineralization of the targeted
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pollutant by reactive species (mainly hydroxyl radicals, •OH) that are created on the surface of a semiconductor material by photonic activation.
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Titanium dioxide (TiO2) is the semiconductor material that has been extensively used for the photocatalytic degradation of numerous pollutants, because of its low cost, high efficiency and photochemical stability [3]. On the other hand, the main drawback to the maximization of
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TiO2’s photocatalytic performance is its wide band gap (~3.0-3.2 eV), that limits its activity only in the UV light region, thus excluding the utilization of the whole solar spectrum, as well
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as the high electron-hole recombination rate. Towards the direction of visible-light driven photocatalysis, many potential approaches are currently considered such as seeding with transition or noble metals and coupling with other semiconductors or carbonaceous materials [4]. The optimization of photocatalytic reactions by the modification of semiconductor materials by noble metals is ascribed to the increased electron-hole separation, the extension
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of the photoactivity towards the visible light and the surface plasmon resonance (SPR) of the metal nanoparticles [5]. Nanoparticles of noble metals (Au, Pt, Ag) strongly absorb visible light due to their SPR, in which their conducting electrons undergo a collective oscillation induced by the electric field of visible light [6]. By engineering the morphology and dielectric environment of the metallic nanoparticles, their absorption and scattering properties can be flexibly tuned. Localized SPR of gold and silver nanoparticles usually results in strong and broad absorption bands in the visible light region, and it is thus exploited to develop visiblelight-activated photocatalysts. In this concept, the size of the metal nanoparticles, the preparation method and the loading weight are of great importance. 2
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Likewise, deposition of TiO2 on carbon substrates has shown promising photocatalytic performance. Graphene is a unique material with a structure of atomic sheets of sp2-bonded carbon atoms and has attracted considerable attention because of its superior properties that include large surface area, thermal and chemical stability, good interfacial contact with adsorbents and excellent mobility of charge carriers [7]. However, the main disadvantage of graphene is that it exhibits very low solubility in common organic solvents [8]. On the other
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hand, exfoliated graphene oxide (GO) is an ideal alternative for the production of solution processable graphene, as it readily yields stable dispersions in various solvents [9]. In
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addition, GO can be partially reduced to conductive graphene-like sheets, generating reduced graphene oxide (rGO) with important electrical properties partially restored [10]. The
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combination of TiO2 with graphene-based materials can therefore provide reduced holeelectron recombination and a shift of TiO2’s response from UV to visible-light region can be achieved [11]. In addition, hybrid composites of nanoparticulate metals and rGO can exhibit
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advanced features, such as high adsorption of organic dyes, low recombination rate of photogenerated charge carriers and strong π-πstacking with dye chromophores [12]. Thus, the
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integration of TiO2, rGO and a noble metal into a multifunctional nanocomposite is expected to possess enhanced merits compared to bare TiO2.
Zhang et al. [13] firstly reported the photocatalytic activity of TiO2/rGO nanocomposites
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for hydrogen evolution under UV-Vis light irradiation. The results showed that the photocatalytic performance of the TiO2/rGO (5 wt %) nanocomposites was much higher than
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that of P25. In addition, the activities were higher for the samples calcined in nitrogen atmosphere than those in air. Fan et al. [14] studied the influence of different reduction approaches on the efficiency of P25/rGO nanocomposites. Their photocatalytic results showed that the application of the hydrothermal method leads to the best performance concerning hydrogen evolution from methanol aqueous solution under UV-Vis light
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irradiation, followed by P25/rGO via photoreduction and P25/rGO by hydrazine reduction. The effects of a uniform coating and strong coupling between TiO 2 and rGO on the degradation of Rhodamine B (RhB) have been studied by Liang and co-workers [15]. Moreover, some reports have been presented on the synthesis of composite materials consisting of TiO2, rGO and metal nanoparticles, i.e. Au, Pt or Ag, but focus mainly on the synthetic approach and do not examine their efficiency for the photocatalytic removal of a pollutant
[11, 16]. To the best of our knowledge, extremely limited studies have been
presented on the photocatalytic degradation of a pollutant by a composite catalyst comprised of TiO2, Ag and rGO. Towards this direction for instance, Wen et al. [17] prepared a 3
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nanocomposite powder by anchoring anatase TiO2 on rGO and afterwards loading with Ag nanoparticles. The assumption that the silver nanoparticles were loaded on TiO2 instead of rGO was made. The synthesized powder exhibited significantly increased absorption within the visible-light region and improved photocatalytic activity compared with the bare TiO2 and TiO2/rGO samples, but the performance of the TiO2/Ag composite was not evaluated. From the literature survey, it seems that there is an on-going research where great efforts
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are currently made to develop effective visible-light photocatalysts with multifunctional properties by different methods and material combinations, i.e., noble metal loading on
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different semiconductor supports, chemically bonded TiO2-carbon composites or rGO composites. The vast majority of these reports refers to nanocomposites such as TiO2/Ag [18-
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20], TiO2/rGO [13, 14, 21], Ag/rGO [22-24] and even Ag/TiO2/GO [18, 25], while Ag/TiO2/rGO has received significantly less attention. In all these studies, the two critical issues consist of reducing the photocatalyst absorption energy below 3 eV and separating
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photogenerated electrons and holes efficiently so that these photogenerated carriers can be used for reduction and/or oxidation reactions before they recombine.
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In this work, a simple strategy for synthesizing a hybrid nanocomposite of TiO2/Ag hydrothermally deposited on rGO is proposed, aiming to the utilization of the SPR effect of Ag nanoparticles reinforced by the advanced properties of rGO towards: (i) extension of the
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photocatalytic response of TiO2 to the visible-light range and (ii) acceleration of the separation of photogenerated charge carrier pairs. First, commercial TiO2 (Degussa P25) was
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loaded with Ag nanoparticles and the optimum loading weight of the metal nanoparticles was assessed. Then, TiO2/Ag was hydrothermally dispersed on the carbon substrate by a one-step hydrothermal reaction along with the simultaneous reduction of GO to rGO. Nanocomposites of TiO2/rGO were additionally hydrothermally prepared for comparison reasons. The photocatalytic performance of the obtained nanocomposite was evaluated, compared to its
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counterparts, through the decolorization of MB aqueous solutions under visible-light irradiation.
2. Experimental details 2.1. Materials Silver nitrate (AgNO3) was supplied by Alfa Aesar and used as received. Dimethylamine borane complex, methylene blue (MB) and absolute ethanol were received by Sigma Aldrich. 4
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Aeroxide TiO2 P25 (a non porous 75:25 (w/w) mixture of anatase:rutile) was supplied by Degussa AG. In addition, Milli-Q Direct water was used throughout this study and was obtained from a Millipore apparatus with a resistivity of 18.2 MΩ×cm at 298 K. Finally, an aqueous solution of graphene oxide of 0.5 mg/ml concentration was supplied by Graphene Supermarket. All photocatalytic experiments were carried out at the natural pH corresponding
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to aqueous solutions of MB, which did not change significantly during the process.
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2.2. Synthesis
As source for silver nanoparticles, AgNO3 was used at different loading weights and
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deposition was achieved by chemical reduction of metal ions into metallic nanoparticles [26]. In particular, TiO2 P25 was dispersed in H2O, the necessary quantity of AgNO3 was added and the dispersion was left under stirring overnight. Afterwards, the dispersion was filtrated,
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redispersed in a mixture of EtOH:H2O (1:4) and dimethylamine borane, at a number of moles 10 times higher than that of AgNO3, was added. Upon the addition of the reducing agent, the
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metal cations were reduced and nanoparticles were deposited on the surface of TiO2 P25. The metal-loaded catalyst was collected by filtration, washed repeatedly with water to remove non-attached metal nanoparticles and dried under vacuum.
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TiO2 or TiO2/Ag were dispersed on carbon substrates by a hydrothermal reaction, along with the simultaneous reduction of GO to rGO [27]. Briefly, an appropriate volume of an
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aqueous GO solution was poured into a mixture of H2O/EtOH (2:1), followed by the addition of TiO2 P25. The obtained suspension was stirred for 2 h in order to become homogeneous. It was then placed in a laboratory Pyrex glass bottle with polypropylene autoclavable screw cap and was maintained at 120 °C for 24 h. The resulting composite was recovered by filtration,
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washed with water and dried at 50 °C. 2.3. Characterization techniques
X-ray diffraction (XRD) patterns were collected on a Panalytical Expert Pro X-ray diffractometer, using Cu Kα radiation (λ = 1.5406 Å), while the samples were also characterized by Raman spectroscopy at room temperature on a Nicolet Almega XR Raman spectrometer (Thermo Scientific) with a 473 nm blue laser as an excitation source. Fourier transform infrared (FT-IR) spectra were measured on a BRUKER FT-IR spectrometer IFS 66v/F (MIR) and diffuse reflectance of the powder samples was monitored by a Perkin Elmer 5
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LAMBDA 950 UV-NIR spectrophotometer equipped with an integrating sphere with BaSO4 as a reference. The morphology of the surfaces was examined by field emission scanning electron microscopy (FE-SEM, JEOLJSM-7000F) and further analysis was carried out using transmission electron microscopy (TEM, JEOL JEM-2100).
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2.4. Photocatalytic activity study The evaluation of the photoactivity of the as-prepared samples was conducted via the
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quantification of MB dye removal under visible-light irradiation. An aqueous solution of 20 mg/L of MB was first prepared and used as the target pollutant. The photocatalyst loading in
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the MB aqueous suspensions was always kept at 200 ppm. Prior to the illumination, the suspension was first stirred in the dark for ca. 40 min, to ensure establishment of adsorption/desorption equilibrium. In the case of photolysis, no catalyst was added to the
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solution. The photocatalytic / photolytic removal of MB was carried out in a photoreactor provided by Heraeus (Noblelight GmbH, Hanau - Germany) [28]. The irradiation was
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provided by a high pressure mercury lamp (TQ 150) at a constant power of 150 W, with an emission spectrum of 200 - 600 nm and λmax at 365 nm. The lamp was mounted axially in the reactor inside a cylindrical, double-walled M380 glass jacket, which filtered out the UV lines
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at λexc< 390nm. The experiments were carried out at a constant stirring speed (600 rpm) insured by a magnetic stirrer at the reactor basis, at a constant temperature maintained by
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water circulating in the double-walled lamp jacket. Changes in the concentration (decolorization) of MB were monitored using a UV-Vis spectrophotometer (UV-2401 PC, Shimadzu). In order to estimate the rate of decolorization, aliquots of the suspension were withdrawn periodically from the photoreactor, the photocatalyst was removed by centrifugation and the absorbance spectrum of the sample was
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recorded. MB presents a characteristic absorbance peak at 664 nm and thus, quantification of dye removal was estimated by the calculation of the area below this peak from 540 to 700 nm and normalization to the corresponding MB peak area prior to the illumination.
3. Results and discussion
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3.1. Structure of the nanocomposite photocatalysts The XRD patterns of the synthesized samples are shown in Fig. 1. The XRD profile of TiO2 P25 indicates that it is composed of both anatase and rutile. In particular, diffraction peaks at 2θ values of 25.4, 36.9, 37.8, 48.1, 54.0 and 55.1º, can be indexed to the (101), (103), (004), (200), (105) and (211) crystal planes of anatase, while the characteristic peaks at 27.5,
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36.1, 41.0 and 56.7º are attributed to the (110), (101), (111) and (220) rutile phase [27]. No diffraction peaks of the noble metal were observed in the case of the TiO2/Ag composite,
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indicating that silver nanoparticles were merely deposited on the surface of the catalyst. Τhe XRD pattern of GO showed a strong and sharp diffraction peak at ~13º, which disappeared
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after the hydrothermal reduction and was replaced by a very broad peak at 24.5º, as presented in the inset of Fig. 1b. This observation points to the successful reduction of GO to rGO [14]. Furthermore, similar XRD patterns with pure TiO2 P25 and no diffraction peaks for carbon
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species were observed for TiO2/rGO and TiO2/Ag/rGO composites (Fig. 1b), which can be attributed to the overlap of the main peak of rGO at 24.5º with the strong anatase peak at 25.4º
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for TiO2 P25.
In order to further analyze the structure of the samples, Raman spectra were collected and are presented in Fig. 2. For TiO2/rGO (1 wt %) composites, apart from the anatase TiO2
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signals, two additional low and broad peaks are observed at 1349 cm-1 and 1594 cm-1 due to the D and G bands of rGO respectively [29, 30]. The D band is due to the modes of sp2 atoms
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in rings, while the G band is derived from the stretching of the sp2-hybridized carbon-carbon bonds [31]. The intensity ratio of D band to G band is proposed to be an indication of disorder in GO or rGO [32]. An increment in D/G intensity ratio is observed from 0.78 in the Raman spectrum of GO to 0.97 for rGO, suggesting a decrease in the average size of the sp2 domains. This phenomenon is expected, since reduction of GO causes fragmentation along the reactive
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sites, leading to numerous rGO layers which are smaller in size. In addition, the small size of the rGO layers would result in a large quantity of edges that act as defects, leading to an increase of the D peak. Furthermore, the narrow G peak indicates that the D peak originates from edges rather than structural defects [32]. After the loading of Ag nanoparticles, there is no discrepancy of the spectra as compared with the TiO2/rGO (1 wt %). However, the intensity of the Raman peaks of anatase TiO2 significantly increased compared with that of TiO2/rGO, which could be associated with the SPR effect of Ag nanoparticles in the TiO 2/Ag (3 wt %)/rGO (1 wt %) composite [17]. This happened probably because the plasmon resonance of Ag is located at ~440 nm, which is very close to the laser wavelength applied in 7
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the Raman experiments [33]. Since the increase of the intensity of the Raman peaks was observed only for TiO2 and not for rGO, this is an indication that Ag nanoparticles were indeed deposited on the surface of TiO2 and not on the rGO layers, as expected from the experimental process that was followed [17]. FT-IR measurements for GO and rGO samples are illustrated in Fig. 3a. The presence of oxygen functional groups in GO was confirmed by a number of characteristic bands at ~3400
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cm-1 (O-H stretching vibrations), 1728 cm-1 (stretching vibrations from C=O), 1620 cm-1 (skeletal vibrations from unoxidized graphitic domains), 1420 cm-1 (O–H deformation) and at
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1075 cm-1 (C-O stretching vibrations). A significant decrease in the intensities of those bands is observed for rGO, but the elimination of these bands is not complete, implying that a little
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fraction of oxygen functionalities still remains in the rGO sample. At the same time, a peak at ~1580 cm-1, which is attributed to the skeletal vibration of rGO evolves, pointing out to the reduction of GO and the formation of rGO during the hydrothermal reaction. It can be noticed
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that the same band at ∼1580 cm-1 is evident for TiO2/rGO (1 wt %) as well, as shown in Fig. 3b. Furthermore, a broad band below 1000 cm-1 is observed for the bare TiO2 P25 sample
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which is related to bending and stretching vibrational modes of Ti-O-Ti bonds [34], while, for the TiO2/rGO (1 wt %) sample two peaks are evident, which could be assigned to Ti-O-Ti (670 cm-1) and Ti-O-C (792 cm-1) vibrations [35]. As a result, the presence of Ti-O-C bonds
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indicates that during the hydrothermal reduction, graphene oxide with the residual carboxylic acid groups interacted with the surface hydroxyl groups of TiO2 nanoparticles and finally
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formed chemically bonded TiO2/rGO.
Fig. 4 depicts transformed Kubelka-Munk plots as a function of light energy for the synthesized samples. When Ag nanoparticles were loaded on TiO2, a broad absorption band between 400 to 700 nm appeared (see blue curve in Fig. 4b), which could be assigned to the SPR effect of the silver nanoparticles. This band is also observed for TiO2/Ag-loaded on rGO
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(see black curve in Fig. 4b), demonstrating the presence of Ag nanoparticles in this sample. Moreover, it can be noticed that loading TiO2 with Ag induces a red shift to higher wavelengths in the absorption edge of TiO2, from an energy band gap value of 3.15 eV for bare TiO2 to 2.85 eV for the TiO2/Ag (3 wt %) composite (see Fig. 4b). The same conclusion can apply to TiO2/rGO (1 wt %) (Fig. 4a), whose band gap value decreased to 2.80 eV, while when TiO2/Ag (3 wt %) was loaded on rGO (1 wt %), a further narrowing of the band gap to 2.75 eV occurred (black curve in Fig. 4b). The formation of TiO2/Ag and loading of TiO2 and TiO2/Ag samples on rGO were further confirmed by TEM analysis, as demonstrated in Fig. 5. TEM images indicate that the silver 8
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nanoparticles synthesized by chemical reduction are close to spherical, with an average diameter of 4.2 nm and a standard deviation of 1.4 nm (Figs. 5a, b). Typical sheet structures with micrometers-long wrinkles are observed for rGO, while TiO2 was dispersed on the rGO sheets, as illustrated by TEM and SEM images in Figs. 5c, d and Fig. 6b, respectively. TiO2 P25 nanoparticles tended to accumulate along the wrinkles and edges of rGO sheets, because
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3.2. Evaluation of the nanocomposites photocatalytic activity
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the remaining carboxylic acid groups are most likely situated at the edges [36].
First, the photocatalytic activity of TiO2 P25 modified with metallic Ag nanoparticles,
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embedded on its surface by chemical reduction, was studied regarding the decolorization of MB aqueous solutions under visible-light irradiation (see Fig. 7a). Metal (Ag) - support (TiO2 P25) interactions with regard to the photocatalytic activity are closely related to the size and
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morphology of the metal nanoparticles as well as electronic interactions between the nanoparticles and support [26]. In general, chemical reduction proceeds rapidly and in order
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to reduce all Ag ions involved, a few seconds were required. Consequently, small and close to spherical nanoparticles were obtained (see Figs. 5a and b), as fast reduction methods produce a larger number of metal atoms limiting their nucleation, thus leading to smaller particles [26,
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37].
Additional to the size of the metal nanoparticles, another critical parameter for the
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photocatalytic activity of the metal-nanocomposite materials was shown to be the metal load, as demonstrated in Figs. 7a and b. According to Fig. 7b, the percentage loading of silver to titania increased the photocatalytic performance of TiO2/Ag nanocomposites up to a threshold value of 3 wt % Ag, above which the rate of decolorization dropped (100% dye removal for 3 wt % Ag loading versus ~95% for 4 wt % loading and 93% for bare TiO2 after 75 min of
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irradiation). Sufficient amount of metal loaded on the catalyst surface will expand the catalytic response down to the visible-light region resulting in high photocatalytic rates. However, an excessive metal loading leads to the limitation of the amount of light reaching to the surface of the catalyst, and therefore to a reduction of the number of photogenerated charge carriers [38]. Moreover, the TiO2/rGO composite showed increased photocatalytic rates for the removal of MB in comparison with bare TiO2 P25 (see Fig. 8a). Titania/rGO composites present high rates in photocatalytic activities, thanks to the unique properties of rGO, being more conductive than GO [8] and less soluble while remaining photostable [14]. Fig. 8a shows that 9
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the removal (decolorization) rate of MB was further enhanced with increasing rGO content from 0.5 to 1 wt %. The optimum TiO2/rGO mass ratio in the composite is yet to be investigated. This effect is boosted, as it is clearly presented in Figs. 8a and b, in the case of the TiO2/Ag (3 wt %)/rGO (1 wt %) coupled nanocomposite photocatalyst, which exhibits superior performance for MB removal over conventional TiO2 P25, TiO2/Ag (3 wt %) and/or TiO2/rGO (1 wt %). In the first five minutes of visible-light irradiation, close to 50% of the
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initial dye was already removed, while within 45 minutes dye decolorization was almost completed.
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This phenomenon can be ascribed to the hybrid’s enhanced photocatalytic properties due to synergistic interactions between TiO2, rGO and Ag and a plausible mechanism is illustrated
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in Fig. 9. Since the Fermi level of TiO2 is higher than that of silver nanoparticles, a Schottky junction is formed between them upon contact, blocking the electron transfer from Ag to TiO2. However, upon visible-light absorption, the SPR photoexcited electrons in the Ag
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nanoparticulates of TiO2/Ag nanocomposites manage to overcome the Schottky barrier and move through the TiO2/Ag interface into the conduction band of TiO2 [39]. Especially for
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noble metal loadings, as in the case of silver, owing to SPR effect, this electron transfer is ultrafast, whereas the direct electron transfer from Ag nanoparticles to dissolved O2 is expected to be relatively slow [40]. By this way, the electron-hole recombination occurring in
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the process of photoexcitation is inhibited, allowing the steady formation of •OH radicals responsible for the decolorization of the dye solution, as complete oxidation of the MB dye
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requires preferentially reaction with the photogenerated holes, mainly involved in the decarboxylation reaction (‘photo-kolbe’) [41]. The formation of •OH radicals occurs because the electron transfer from the Ag nanoparticles to the TiO2 particle is compensated by a simultaneous electron transfer from H2O or OH- adsorbed on the surface of the catalyst or from the dye molecules that are present in the solution. This leads to the regeneration of the
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metal nanoparticles and the production of •OH radicals. Finally, the higher performance of the rGO composite materials in comparison to bare TiO2 could be attributed to the red shift in the absorption edge, as presented in Fig. 4, as well as to an enhanced migration efficiency of the photoinduced electrons. In particular, the band gap of TiO2 can be narrowed to the visible-light region due to chemical bonding between rGO and TiO2, resulting from the direct interaction between C and Ti atoms during the hydrothermal treatment and subsequently, higher absorption of visible light can be achieved. Moreover, the energy level differences between rGO, TiO2 and Ag nanoparticles are in favorable positions to promote an effective charge separation, retarding the charge 10
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recombination. As illustrated in Fig. 9, rGO probably functions as a better promoter for increasing the electron transport, since the SPR photogenerated electrons located at the conduction band of TiO2 can be finally transferred to the rGO sheets, further retarding charge recombination and resulting in even higher electron-hole separation efficiency.
We
hydrothermally
synthesized
and
photocatalytically
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4. Conclusions evaluated
TiO2/Ag/rGO
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nanocomposites. The proposed step by step strategy is simple and provides a good control over the material’s properties, i.e. via the utilization of the optimum loadings of every
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component, while it exploits the individual merits as well as the synergetic properties between the catalyst’s counterparts. The as-prepared nanocomposite semiconductor demonstrates superior photocatalytic response when exposed to visible-light irradiation, compared to
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conventional TiO2 P25 or other titania composites, such as TiO2/Ag and TiO2/rGO. The enhancement in the photocatalytic activity of TiO2/Ag/rGO nanocomposites is attributed to
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the synergetic effect between rGO and TiO2/Ag counterparts. It is first ascribed to the great ability of rGO to capture and shuttle electrons due to its high electrical conductivity. Furthermore, as an excellent photocatalytic substrate, rGO provides a high surface area
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support for the immobilization of TiO2/Ag, thus favoring dye adsorption. At the same time, together with Ag, rGO leads to the suppression of electron-hole recombination and an
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increase in the number of holes participating in the photooxidation process. Finally, an overall band-gap narrowing is observed due to the formation of Ti-O-C and Ti-O-metal bonds, further accomplishing the scope of developing photocatalysts with high carrier mobility, able to efficiently harvest solar light.
In few words, a very challenging strategy for the synthesis of novel and efficient visible-
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light-activated nanocomposite photocatalysts is suggested via the combination of the excellent electrical properties of reduced graphene oxide and the surface plasmon resonance effect of noble metallic (Ag) nanoparticles supported on the surface of TiO2 P25. Achieving further control of the synthesis process, i.e., investigation of the optimum mass ratio of rGO-titania, the photocatalytic activity of the designed TiO2/metal/rGO nanocomposite materials is expected to be remarkably enhanced. Acknowledgements
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This project is implemented through the Operational Program “Education and Lifelong Learning”, Action Archimedes III and is co-financed by the European Union (European Social Fund) and Greek national funds (National Strategic Reference Framework 2007 2013). We would also like to acknowledge Dr. Maria Kaliva for helping in synthesizing Ag-
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an
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concerning the hydrothermal growth of reduced graphene oxide layers.
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loaded titania and Mr. Dimitris Konios and Prof. Emmanuel Kymakis for valuable discussions
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[13] X.-Y. Zhang, H.-P. Li, X.-L. Cui, Y. Lin, J. Mater. Chem. 20 (2010) 2801-2806. [14] W. Fan, Q. Lai, Q. Zhang, Y. Wang, J. Phys. Chem. C 115 (2011) 10694-10701. [15] Y. Liang, H. Wang, H. Sanchez Casalongue, Z. Chen, H. Dai, Nano Res. 3 (2010) 701705.
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[23] X.-Z. Tang, Z. Cao, H.-B. Zhang, J. Liu, Z.-Z. Yu, Chem. Commun. 47 (2011) 30843086. [24] T. Wu, S. Liu, Y. Luo, W. Lu, L. Wang, X. Sun, Nanoscale 3 (2011) 2142-2144. [25] L. Liu, H. Bai, J. Liu, D.D. Sun, J. Hazard. Mater. 261 (2013) 214-223. [26] R. Isono, T. Yoshimura, K. Esumi, J. Colloid Interface Sci. 288 (2005) 177-183. [27] Y. Zhang, Z.-R. Tang, X. Fu, Y.-J. Xu, ACS Nano 4 (2010) 7303-7314.
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Figure captions
Figure 1: XRD patterns of (a) TiO2 P25 and TiO2/Ag (3 wt %) and (b) TiO2 P25, TiO2/rGO (1 wt %) and TiO2/Ag (3 wt %)/rGO (1 wt %). In the inset, the XRD patterns of GO and rGO are shown. Figure 2: Raman spectra of GO, rGO, TiO2 P25, TiO2/rGO (1 wt %) and TiO2/Ag (3 wt
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%)/rGO (1 wt %) samples.
Figure 3: FTIR spectra of (a) GO and rGO and (b) TiO2 P25 and TiO2 deposited on rGO (1
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wt %).
Figure 4: Transformed Kubelka-Munk function versus light energy of (a) TiO2 P25 and TiO2
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on rGO (1 wt %) and (b) TiO2 P25, TiO2/Ag (3 wt %) and TiO2/Ag (3 wt %) loaded on rGO (1 wt %).
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Figure 5: TEM images of (a), (b) Ag-loaded TiO2 (3 wt %) and (c), (d) TiO2 deposited on rGO layers (1 wt %).
Figure 6: SEM images of (a) bare TiO2 P25 and (b) TiO2/rGO (1 wt %). TiO2 with various Ag loading values.
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Figure 7: Photocatalytic decolorization of methylene blue aqueous solutions by Ag-loaded
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Figure 8: Photocatalytic decolorization of methylene blue aqueous solutions by (a) TiO2, TiO2/rGO with various rGO loading values and TiO2/Ag (3 wt %) deposited on rGO and (b) TiO2, TiO2/Ag (3 wt %) and TiO2/Ag (3 wt %) on rGO (1 wt %).
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Figure 9: Proposed mechanism illustrating the mobility of charge carriers in the case of
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TiO2/Ag coupled with rGO.
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S0169-4332(15)01623-2 http://dx.doi.org/doi:10.1016/j.apsusc.2015.07.056 APSUSC 30783
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-3-2015 16-6-2015 7-7-2015
Please cite this article as: E. Vasilaki, I. Georgaki, D. Vernardou, M. Vamvakaki, N. Katsarakis, Ag-loaded TiO2 /reduced graphene oxide nanocomposites for enhanced visible-light photocatalytic activity, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.07.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ag-loaded TiO2 / reduced graphene oxide nanocomposites for enhanced visible-light photocatalytic activity E. Vasilakia,b*, I. Georgakib, D. Vernardoub, M. Vamvakakic,d and N. Katsarakisb,d Department of Chemistry, University of Crete, 71003, Heraklion, Crete, Greece
b
Center of Materials Technology and Photonics, School of Applied Technology,
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a
Technological Educational Institute of Crete, 710 04 Heraklion, Crete, Greece c
Department of Materials Science and Technology, University of Crete, 71003, Heraklion,
d
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Crete, Greece
Institute of Electronic Structure and Laser, Foundation for Research & Technology-Hellas,
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P.O. Box 1385, Vassilika Vouton, 711 10 Heraklion, Crete, Greece
Ag nanoparticles were loaded on TiO2 by chemical reduction. TiO2/Ag and TiO2 samples were deposited on reduced graphene oxide (rGO). Their performance was evaluated via methylene blue removal under visible-light. TiO2/Ag/rGO presented superior activity compared to TiO2, TiO2/Ag and TiO2/rGO.
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*corresponding author, e-mail: [email protected]
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Abstract
In this work, Ag nanoparticles were loaded by chemical reduction onto TiO2 P25 under
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different loadings ranging from 1 up to 4 wt % and hydrothermally deposited on reduced graphene oxide sheets. Chemical reduction was determined to be an effective preparation approach for Ag attachment to titania, leading to the formation of small silver nanoparticles with an average diameter of 4.2 nm. The photocatalytic performance of the hybrid
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nanocomposite materials was evaluated via methylene blue (MB) dye removal under visiblelight irradiation. The rate of dye decolorization was found to depend on the metal loading, showing an increase till a threshold value of 3 wt %, above which the rate drops. Next, the as prepared sample of TiO2/Ag of better photocatalytic response, i.e., at a 3 wt % loading value, was hydrothermally deposited on a platform of reduced graphene oxide (rGO) of tunable content (mass ratio). TiO2/Ag/rGO coupled nanocomposite presented significantly enhanced photocatalytic activity compared to the TiO2/Ag, TiO2/rGO composites and bare P25 titania semiconductor photocatalysts. In particular, after 45 min of irradiation almost complete decolorization of the dye was observed for the TiO2/Ag/rGO nanocatalyst, while the respective removal efficiency was 92% for TiO2/Ag, 93% for TiO2/rGO and only 80% for the 1
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bare TiO2 nanoparticles. This simple step by step preparation strategy allows for optimum exploitation of the advanced properties of metal plasmonic effect and reduced graphene oxide as the critical host for boosting the overall photocatalytic activity towards visible-light. Keywords: Titanium dioxide; Plasmonic photocatalysts; Silver loaded titanium dioxide; Reduced graphene oxide coupled titanium dioxide.
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1. Introduction
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Conventional treatment methods applied for the decomposition of organic and inorganic water contaminants have limited efficiency [1]. In the context of more effectual alternatives,
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heterogeneous photocatalysis is one of the most promising advanced oxidation processes (AOP’s), being capable of achieving complete oxidation of both organic and inorganic water contaminants [2]. Heterogeneous photocatalysis refers to the mineralization of the targeted
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pollutant by reactive species (mainly hydroxyl radicals, •OH) that are created on the surface of a semiconductor material by photonic activation.
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Titanium dioxide (TiO2) is the semiconductor material that has been extensively used for the photocatalytic degradation of numerous pollutants, because of its low cost, high efficiency and photochemical stability [3]. On the other hand, the main drawback to the maximization of
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TiO2’s photocatalytic performance is its wide band gap (~3.0-3.2 eV), that limits its activity only in the UV light region, thus excluding the utilization of the whole solar spectrum, as well
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as the high electron-hole recombination rate. Towards the direction of visible-light driven photocatalysis, many potential approaches are currently considered such as seeding with transition or noble metals and coupling with other semiconductors or carbonaceous materials [4]. The optimization of photocatalytic reactions by the modification of semiconductor materials by noble metals is ascribed to the increased electron-hole separation, the extension
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of the photoactivity towards the visible light and the surface plasmon resonance (SPR) of the metal nanoparticles [5]. Nanoparticles of noble metals (Au, Pt, Ag) strongly absorb visible light due to their SPR, in which their conducting electrons undergo a collective oscillation induced by the electric field of visible light [6]. By engineering the morphology and dielectric environment of the metallic nanoparticles, their absorption and scattering properties can be flexibly tuned. Localized SPR of gold and silver nanoparticles usually results in strong and broad absorption bands in the visible light region, and it is thus exploited to develop visiblelight-activated photocatalysts. In this concept, the size of the metal nanoparticles, the preparation method and the loading weight are of great importance. 2
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Likewise, deposition of TiO2 on carbon substrates has shown promising photocatalytic performance. Graphene is a unique material with a structure of atomic sheets of sp2-bonded carbon atoms and has attracted considerable attention because of its superior properties that include large surface area, thermal and chemical stability, good interfacial contact with adsorbents and excellent mobility of charge carriers [7]. However, the main disadvantage of graphene is that it exhibits very low solubility in common organic solvents [8]. On the other
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hand, exfoliated graphene oxide (GO) is an ideal alternative for the production of solution processable graphene, as it readily yields stable dispersions in various solvents [9]. In
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addition, GO can be partially reduced to conductive graphene-like sheets, generating reduced graphene oxide (rGO) with important electrical properties partially restored [10]. The
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combination of TiO2 with graphene-based materials can therefore provide reduced holeelectron recombination and a shift of TiO2’s response from UV to visible-light region can be achieved [11]. In addition, hybrid composites of nanoparticulate metals and rGO can exhibit
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advanced features, such as high adsorption of organic dyes, low recombination rate of photogenerated charge carriers and strong π-πstacking with dye chromophores [12]. Thus, the
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integration of TiO2, rGO and a noble metal into a multifunctional nanocomposite is expected to possess enhanced merits compared to bare TiO2.
Zhang et al. [13] firstly reported the photocatalytic activity of TiO2/rGO nanocomposites
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for hydrogen evolution under UV-Vis light irradiation. The results showed that the photocatalytic performance of the TiO2/rGO (5 wt %) nanocomposites was much higher than
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that of P25. In addition, the activities were higher for the samples calcined in nitrogen atmosphere than those in air. Fan et al. [14] studied the influence of different reduction approaches on the efficiency of P25/rGO nanocomposites. Their photocatalytic results showed that the application of the hydrothermal method leads to the best performance concerning hydrogen evolution from methanol aqueous solution under UV-Vis light
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irradiation, followed by P25/rGO via photoreduction and P25/rGO by hydrazine reduction. The effects of a uniform coating and strong coupling between TiO 2 and rGO on the degradation of Rhodamine B (RhB) have been studied by Liang and co-workers [15]. Moreover, some reports have been presented on the synthesis of composite materials consisting of TiO2, rGO and metal nanoparticles, i.e. Au, Pt or Ag, but focus mainly on the synthetic approach and do not examine their efficiency for the photocatalytic removal of a pollutant
[11, 16]. To the best of our knowledge, extremely limited studies have been
presented on the photocatalytic degradation of a pollutant by a composite catalyst comprised of TiO2, Ag and rGO. Towards this direction for instance, Wen et al. [17] prepared a 3
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nanocomposite powder by anchoring anatase TiO2 on rGO and afterwards loading with Ag nanoparticles. The assumption that the silver nanoparticles were loaded on TiO2 instead of rGO was made. The synthesized powder exhibited significantly increased absorption within the visible-light region and improved photocatalytic activity compared with the bare TiO2 and TiO2/rGO samples, but the performance of the TiO2/Ag composite was not evaluated. From the literature survey, it seems that there is an on-going research where great efforts
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are currently made to develop effective visible-light photocatalysts with multifunctional properties by different methods and material combinations, i.e., noble metal loading on
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different semiconductor supports, chemically bonded TiO2-carbon composites or rGO composites. The vast majority of these reports refers to nanocomposites such as TiO2/Ag [18-
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20], TiO2/rGO [13, 14, 21], Ag/rGO [22-24] and even Ag/TiO2/GO [18, 25], while Ag/TiO2/rGO has received significantly less attention. In all these studies, the two critical issues consist of reducing the photocatalyst absorption energy below 3 eV and separating
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photogenerated electrons and holes efficiently so that these photogenerated carriers can be used for reduction and/or oxidation reactions before they recombine.
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In this work, a simple strategy for synthesizing a hybrid nanocomposite of TiO2/Ag hydrothermally deposited on rGO is proposed, aiming to the utilization of the SPR effect of Ag nanoparticles reinforced by the advanced properties of rGO towards: (i) extension of the
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photocatalytic response of TiO2 to the visible-light range and (ii) acceleration of the separation of photogenerated charge carrier pairs. First, commercial TiO2 (Degussa P25) was
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loaded with Ag nanoparticles and the optimum loading weight of the metal nanoparticles was assessed. Then, TiO2/Ag was hydrothermally dispersed on the carbon substrate by a one-step hydrothermal reaction along with the simultaneous reduction of GO to rGO. Nanocomposites of TiO2/rGO were additionally hydrothermally prepared for comparison reasons. The photocatalytic performance of the obtained nanocomposite was evaluated, compared to its
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counterparts, through the decolorization of MB aqueous solutions under visible-light irradiation.
2. Experimental details 2.1. Materials Silver nitrate (AgNO3) was supplied by Alfa Aesar and used as received. Dimethylamine borane complex, methylene blue (MB) and absolute ethanol were received by Sigma Aldrich. 4
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Aeroxide TiO2 P25 (a non porous 75:25 (w/w) mixture of anatase:rutile) was supplied by Degussa AG. In addition, Milli-Q Direct water was used throughout this study and was obtained from a Millipore apparatus with a resistivity of 18.2 MΩ×cm at 298 K. Finally, an aqueous solution of graphene oxide of 0.5 mg/ml concentration was supplied by Graphene Supermarket. All photocatalytic experiments were carried out at the natural pH corresponding
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to aqueous solutions of MB, which did not change significantly during the process.
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2.2. Synthesis
As source for silver nanoparticles, AgNO3 was used at different loading weights and
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deposition was achieved by chemical reduction of metal ions into metallic nanoparticles [26]. In particular, TiO2 P25 was dispersed in H2O, the necessary quantity of AgNO3 was added and the dispersion was left under stirring overnight. Afterwards, the dispersion was filtrated,
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redispersed in a mixture of EtOH:H2O (1:4) and dimethylamine borane, at a number of moles 10 times higher than that of AgNO3, was added. Upon the addition of the reducing agent, the
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metal cations were reduced and nanoparticles were deposited on the surface of TiO2 P25. The metal-loaded catalyst was collected by filtration, washed repeatedly with water to remove non-attached metal nanoparticles and dried under vacuum.
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TiO2 or TiO2/Ag were dispersed on carbon substrates by a hydrothermal reaction, along with the simultaneous reduction of GO to rGO [27]. Briefly, an appropriate volume of an
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aqueous GO solution was poured into a mixture of H2O/EtOH (2:1), followed by the addition of TiO2 P25. The obtained suspension was stirred for 2 h in order to become homogeneous. It was then placed in a laboratory Pyrex glass bottle with polypropylene autoclavable screw cap and was maintained at 120 °C for 24 h. The resulting composite was recovered by filtration,
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washed with water and dried at 50 °C. 2.3. Characterization techniques
X-ray diffraction (XRD) patterns were collected on a Panalytical Expert Pro X-ray diffractometer, using Cu Kα radiation (λ = 1.5406 Å), while the samples were also characterized by Raman spectroscopy at room temperature on a Nicolet Almega XR Raman spectrometer (Thermo Scientific) with a 473 nm blue laser as an excitation source. Fourier transform infrared (FT-IR) spectra were measured on a BRUKER FT-IR spectrometer IFS 66v/F (MIR) and diffuse reflectance of the powder samples was monitored by a Perkin Elmer 5
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LAMBDA 950 UV-NIR spectrophotometer equipped with an integrating sphere with BaSO4 as a reference. The morphology of the surfaces was examined by field emission scanning electron microscopy (FE-SEM, JEOLJSM-7000F) and further analysis was carried out using transmission electron microscopy (TEM, JEOL JEM-2100).
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2.4. Photocatalytic activity study The evaluation of the photoactivity of the as-prepared samples was conducted via the
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quantification of MB dye removal under visible-light irradiation. An aqueous solution of 20 mg/L of MB was first prepared and used as the target pollutant. The photocatalyst loading in
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the MB aqueous suspensions was always kept at 200 ppm. Prior to the illumination, the suspension was first stirred in the dark for ca. 40 min, to ensure establishment of adsorption/desorption equilibrium. In the case of photolysis, no catalyst was added to the
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solution. The photocatalytic / photolytic removal of MB was carried out in a photoreactor provided by Heraeus (Noblelight GmbH, Hanau - Germany) [28]. The irradiation was
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provided by a high pressure mercury lamp (TQ 150) at a constant power of 150 W, with an emission spectrum of 200 - 600 nm and λmax at 365 nm. The lamp was mounted axially in the reactor inside a cylindrical, double-walled M380 glass jacket, which filtered out the UV lines
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at λexc< 390nm. The experiments were carried out at a constant stirring speed (600 rpm) insured by a magnetic stirrer at the reactor basis, at a constant temperature maintained by
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water circulating in the double-walled lamp jacket. Changes in the concentration (decolorization) of MB were monitored using a UV-Vis spectrophotometer (UV-2401 PC, Shimadzu). In order to estimate the rate of decolorization, aliquots of the suspension were withdrawn periodically from the photoreactor, the photocatalyst was removed by centrifugation and the absorbance spectrum of the sample was
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recorded. MB presents a characteristic absorbance peak at 664 nm and thus, quantification of dye removal was estimated by the calculation of the area below this peak from 540 to 700 nm and normalization to the corresponding MB peak area prior to the illumination.
3. Results and discussion
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3.1. Structure of the nanocomposite photocatalysts The XRD patterns of the synthesized samples are shown in Fig. 1. The XRD profile of TiO2 P25 indicates that it is composed of both anatase and rutile. In particular, diffraction peaks at 2θ values of 25.4, 36.9, 37.8, 48.1, 54.0 and 55.1º, can be indexed to the (101), (103), (004), (200), (105) and (211) crystal planes of anatase, while the characteristic peaks at 27.5,
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36.1, 41.0 and 56.7º are attributed to the (110), (101), (111) and (220) rutile phase [27]. No diffraction peaks of the noble metal were observed in the case of the TiO2/Ag composite,
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indicating that silver nanoparticles were merely deposited on the surface of the catalyst. Τhe XRD pattern of GO showed a strong and sharp diffraction peak at ~13º, which disappeared
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after the hydrothermal reduction and was replaced by a very broad peak at 24.5º, as presented in the inset of Fig. 1b. This observation points to the successful reduction of GO to rGO [14]. Furthermore, similar XRD patterns with pure TiO2 P25 and no diffraction peaks for carbon
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species were observed for TiO2/rGO and TiO2/Ag/rGO composites (Fig. 1b), which can be attributed to the overlap of the main peak of rGO at 24.5º with the strong anatase peak at 25.4º
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for TiO2 P25.
In order to further analyze the structure of the samples, Raman spectra were collected and are presented in Fig. 2. For TiO2/rGO (1 wt %) composites, apart from the anatase TiO2
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signals, two additional low and broad peaks are observed at 1349 cm-1 and 1594 cm-1 due to the D and G bands of rGO respectively [29, 30]. The D band is due to the modes of sp2 atoms
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in rings, while the G band is derived from the stretching of the sp2-hybridized carbon-carbon bonds [31]. The intensity ratio of D band to G band is proposed to be an indication of disorder in GO or rGO [32]. An increment in D/G intensity ratio is observed from 0.78 in the Raman spectrum of GO to 0.97 for rGO, suggesting a decrease in the average size of the sp2 domains. This phenomenon is expected, since reduction of GO causes fragmentation along the reactive
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sites, leading to numerous rGO layers which are smaller in size. In addition, the small size of the rGO layers would result in a large quantity of edges that act as defects, leading to an increase of the D peak. Furthermore, the narrow G peak indicates that the D peak originates from edges rather than structural defects [32]. After the loading of Ag nanoparticles, there is no discrepancy of the spectra as compared with the TiO2/rGO (1 wt %). However, the intensity of the Raman peaks of anatase TiO2 significantly increased compared with that of TiO2/rGO, which could be associated with the SPR effect of Ag nanoparticles in the TiO 2/Ag (3 wt %)/rGO (1 wt %) composite [17]. This happened probably because the plasmon resonance of Ag is located at ~440 nm, which is very close to the laser wavelength applied in 7
Page 7 of 15
the Raman experiments [33]. Since the increase of the intensity of the Raman peaks was observed only for TiO2 and not for rGO, this is an indication that Ag nanoparticles were indeed deposited on the surface of TiO2 and not on the rGO layers, as expected from the experimental process that was followed [17]. FT-IR measurements for GO and rGO samples are illustrated in Fig. 3a. The presence of oxygen functional groups in GO was confirmed by a number of characteristic bands at ~3400
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cm-1 (O-H stretching vibrations), 1728 cm-1 (stretching vibrations from C=O), 1620 cm-1 (skeletal vibrations from unoxidized graphitic domains), 1420 cm-1 (O–H deformation) and at
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1075 cm-1 (C-O stretching vibrations). A significant decrease in the intensities of those bands is observed for rGO, but the elimination of these bands is not complete, implying that a little
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fraction of oxygen functionalities still remains in the rGO sample. At the same time, a peak at ~1580 cm-1, which is attributed to the skeletal vibration of rGO evolves, pointing out to the reduction of GO and the formation of rGO during the hydrothermal reaction. It can be noticed
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that the same band at ∼1580 cm-1 is evident for TiO2/rGO (1 wt %) as well, as shown in Fig. 3b. Furthermore, a broad band below 1000 cm-1 is observed for the bare TiO2 P25 sample
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which is related to bending and stretching vibrational modes of Ti-O-Ti bonds [34], while, for the TiO2/rGO (1 wt %) sample two peaks are evident, which could be assigned to Ti-O-Ti (670 cm-1) and Ti-O-C (792 cm-1) vibrations [35]. As a result, the presence of Ti-O-C bonds
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indicates that during the hydrothermal reduction, graphene oxide with the residual carboxylic acid groups interacted with the surface hydroxyl groups of TiO2 nanoparticles and finally
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formed chemically bonded TiO2/rGO.
Fig. 4 depicts transformed Kubelka-Munk plots as a function of light energy for the synthesized samples. When Ag nanoparticles were loaded on TiO2, a broad absorption band between 400 to 700 nm appeared (see blue curve in Fig. 4b), which could be assigned to the SPR effect of the silver nanoparticles. This band is also observed for TiO2/Ag-loaded on rGO
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(see black curve in Fig. 4b), demonstrating the presence of Ag nanoparticles in this sample. Moreover, it can be noticed that loading TiO2 with Ag induces a red shift to higher wavelengths in the absorption edge of TiO2, from an energy band gap value of 3.15 eV for bare TiO2 to 2.85 eV for the TiO2/Ag (3 wt %) composite (see Fig. 4b). The same conclusion can apply to TiO2/rGO (1 wt %) (Fig. 4a), whose band gap value decreased to 2.80 eV, while when TiO2/Ag (3 wt %) was loaded on rGO (1 wt %), a further narrowing of the band gap to 2.75 eV occurred (black curve in Fig. 4b). The formation of TiO2/Ag and loading of TiO2 and TiO2/Ag samples on rGO were further confirmed by TEM analysis, as demonstrated in Fig. 5. TEM images indicate that the silver 8
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nanoparticles synthesized by chemical reduction are close to spherical, with an average diameter of 4.2 nm and a standard deviation of 1.4 nm (Figs. 5a, b). Typical sheet structures with micrometers-long wrinkles are observed for rGO, while TiO2 was dispersed on the rGO sheets, as illustrated by TEM and SEM images in Figs. 5c, d and Fig. 6b, respectively. TiO2 P25 nanoparticles tended to accumulate along the wrinkles and edges of rGO sheets, because
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3.2. Evaluation of the nanocomposites photocatalytic activity
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the remaining carboxylic acid groups are most likely situated at the edges [36].
First, the photocatalytic activity of TiO2 P25 modified with metallic Ag nanoparticles,
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embedded on its surface by chemical reduction, was studied regarding the decolorization of MB aqueous solutions under visible-light irradiation (see Fig. 7a). Metal (Ag) - support (TiO2 P25) interactions with regard to the photocatalytic activity are closely related to the size and
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morphology of the metal nanoparticles as well as electronic interactions between the nanoparticles and support [26]. In general, chemical reduction proceeds rapidly and in order
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to reduce all Ag ions involved, a few seconds were required. Consequently, small and close to spherical nanoparticles were obtained (see Figs. 5a and b), as fast reduction methods produce a larger number of metal atoms limiting their nucleation, thus leading to smaller particles [26,
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37].
Additional to the size of the metal nanoparticles, another critical parameter for the
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photocatalytic activity of the metal-nanocomposite materials was shown to be the metal load, as demonstrated in Figs. 7a and b. According to Fig. 7b, the percentage loading of silver to titania increased the photocatalytic performance of TiO2/Ag nanocomposites up to a threshold value of 3 wt % Ag, above which the rate of decolorization dropped (100% dye removal for 3 wt % Ag loading versus ~95% for 4 wt % loading and 93% for bare TiO2 after 75 min of
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irradiation). Sufficient amount of metal loaded on the catalyst surface will expand the catalytic response down to the visible-light region resulting in high photocatalytic rates. However, an excessive metal loading leads to the limitation of the amount of light reaching to the surface of the catalyst, and therefore to a reduction of the number of photogenerated charge carriers [38]. Moreover, the TiO2/rGO composite showed increased photocatalytic rates for the removal of MB in comparison with bare TiO2 P25 (see Fig. 8a). Titania/rGO composites present high rates in photocatalytic activities, thanks to the unique properties of rGO, being more conductive than GO [8] and less soluble while remaining photostable [14]. Fig. 8a shows that 9
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the removal (decolorization) rate of MB was further enhanced with increasing rGO content from 0.5 to 1 wt %. The optimum TiO2/rGO mass ratio in the composite is yet to be investigated. This effect is boosted, as it is clearly presented in Figs. 8a and b, in the case of the TiO2/Ag (3 wt %)/rGO (1 wt %) coupled nanocomposite photocatalyst, which exhibits superior performance for MB removal over conventional TiO2 P25, TiO2/Ag (3 wt %) and/or TiO2/rGO (1 wt %). In the first five minutes of visible-light irradiation, close to 50% of the
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initial dye was already removed, while within 45 minutes dye decolorization was almost completed.
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This phenomenon can be ascribed to the hybrid’s enhanced photocatalytic properties due to synergistic interactions between TiO2, rGO and Ag and a plausible mechanism is illustrated
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in Fig. 9. Since the Fermi level of TiO2 is higher than that of silver nanoparticles, a Schottky junction is formed between them upon contact, blocking the electron transfer from Ag to TiO2. However, upon visible-light absorption, the SPR photoexcited electrons in the Ag
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nanoparticulates of TiO2/Ag nanocomposites manage to overcome the Schottky barrier and move through the TiO2/Ag interface into the conduction band of TiO2 [39]. Especially for
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noble metal loadings, as in the case of silver, owing to SPR effect, this electron transfer is ultrafast, whereas the direct electron transfer from Ag nanoparticles to dissolved O2 is expected to be relatively slow [40]. By this way, the electron-hole recombination occurring in
ed
the process of photoexcitation is inhibited, allowing the steady formation of •OH radicals responsible for the decolorization of the dye solution, as complete oxidation of the MB dye
ce pt
requires preferentially reaction with the photogenerated holes, mainly involved in the decarboxylation reaction (‘photo-kolbe’) [41]. The formation of •OH radicals occurs because the electron transfer from the Ag nanoparticles to the TiO2 particle is compensated by a simultaneous electron transfer from H2O or OH- adsorbed on the surface of the catalyst or from the dye molecules that are present in the solution. This leads to the regeneration of the
Ac
metal nanoparticles and the production of •OH radicals. Finally, the higher performance of the rGO composite materials in comparison to bare TiO2 could be attributed to the red shift in the absorption edge, as presented in Fig. 4, as well as to an enhanced migration efficiency of the photoinduced electrons. In particular, the band gap of TiO2 can be narrowed to the visible-light region due to chemical bonding between rGO and TiO2, resulting from the direct interaction between C and Ti atoms during the hydrothermal treatment and subsequently, higher absorption of visible light can be achieved. Moreover, the energy level differences between rGO, TiO2 and Ag nanoparticles are in favorable positions to promote an effective charge separation, retarding the charge 10
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recombination. As illustrated in Fig. 9, rGO probably functions as a better promoter for increasing the electron transport, since the SPR photogenerated electrons located at the conduction band of TiO2 can be finally transferred to the rGO sheets, further retarding charge recombination and resulting in even higher electron-hole separation efficiency.
We
hydrothermally
synthesized
and
photocatalytically
ip t
4. Conclusions evaluated
TiO2/Ag/rGO
cr
nanocomposites. The proposed step by step strategy is simple and provides a good control over the material’s properties, i.e. via the utilization of the optimum loadings of every
us
component, while it exploits the individual merits as well as the synergetic properties between the catalyst’s counterparts. The as-prepared nanocomposite semiconductor demonstrates superior photocatalytic response when exposed to visible-light irradiation, compared to
an
conventional TiO2 P25 or other titania composites, such as TiO2/Ag and TiO2/rGO. The enhancement in the photocatalytic activity of TiO2/Ag/rGO nanocomposites is attributed to
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the synergetic effect between rGO and TiO2/Ag counterparts. It is first ascribed to the great ability of rGO to capture and shuttle electrons due to its high electrical conductivity. Furthermore, as an excellent photocatalytic substrate, rGO provides a high surface area
ed
support for the immobilization of TiO2/Ag, thus favoring dye adsorption. At the same time, together with Ag, rGO leads to the suppression of electron-hole recombination and an
ce pt
increase in the number of holes participating in the photooxidation process. Finally, an overall band-gap narrowing is observed due to the formation of Ti-O-C and Ti-O-metal bonds, further accomplishing the scope of developing photocatalysts with high carrier mobility, able to efficiently harvest solar light.
In few words, a very challenging strategy for the synthesis of novel and efficient visible-
Ac
light-activated nanocomposite photocatalysts is suggested via the combination of the excellent electrical properties of reduced graphene oxide and the surface plasmon resonance effect of noble metallic (Ag) nanoparticles supported on the surface of TiO2 P25. Achieving further control of the synthesis process, i.e., investigation of the optimum mass ratio of rGO-titania, the photocatalytic activity of the designed TiO2/metal/rGO nanocomposite materials is expected to be remarkably enhanced. Acknowledgements
11
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This project is implemented through the Operational Program “Education and Lifelong Learning”, Action Archimedes III and is co-financed by the European Union (European Social Fund) and Greek national funds (National Strategic Reference Framework 2007 2013). We would also like to acknowledge Dr. Maria Kaliva for helping in synthesizing Ag-
Ac
ce pt
ed
M
an
us
cr
concerning the hydrothermal growth of reduced graphene oxide layers.
ip t
loaded titania and Mr. Dimitris Konios and Prof. Emmanuel Kymakis for valuable discussions
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Figure captions
Figure 1: XRD patterns of (a) TiO2 P25 and TiO2/Ag (3 wt %) and (b) TiO2 P25, TiO2/rGO (1 wt %) and TiO2/Ag (3 wt %)/rGO (1 wt %). In the inset, the XRD patterns of GO and rGO are shown. Figure 2: Raman spectra of GO, rGO, TiO2 P25, TiO2/rGO (1 wt %) and TiO2/Ag (3 wt
ip t
%)/rGO (1 wt %) samples.
Figure 3: FTIR spectra of (a) GO and rGO and (b) TiO2 P25 and TiO2 deposited on rGO (1
cr
wt %).
Figure 4: Transformed Kubelka-Munk function versus light energy of (a) TiO2 P25 and TiO2
us
on rGO (1 wt %) and (b) TiO2 P25, TiO2/Ag (3 wt %) and TiO2/Ag (3 wt %) loaded on rGO (1 wt %).
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Figure 5: TEM images of (a), (b) Ag-loaded TiO2 (3 wt %) and (c), (d) TiO2 deposited on rGO layers (1 wt %).
Figure 6: SEM images of (a) bare TiO2 P25 and (b) TiO2/rGO (1 wt %). TiO2 with various Ag loading values.
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Figure 7: Photocatalytic decolorization of methylene blue aqueous solutions by Ag-loaded
ed
Figure 8: Photocatalytic decolorization of methylene blue aqueous solutions by (a) TiO2, TiO2/rGO with various rGO loading values and TiO2/Ag (3 wt %) deposited on rGO and (b) TiO2, TiO2/Ag (3 wt %) and TiO2/Ag (3 wt %) on rGO (1 wt %).
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Figure 9: Proposed mechanism illustrating the mobility of charge carriers in the case of
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TiO2/Ag coupled with rGO.
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