Superior photocatalytic performance of reduced graphene oxide wrapped electrospun anatase mesoporous TiO2 nanofibers

Accepted Manuscript Superior photocatalytic performance of reduced graphene oxide wrapped electrospun anatase mesoporous TiO2 nanofibers Thirugnanam Lavanya, Kaveri Satheesh, Mrinal Dutta, N. Victor Jaya, Naoki Fukata PII: DOI: Reference:

S0925-8388(14)01163-3 http://dx.doi.org/10.1016/j.jallcom.2014.05.088 JALCOM 31277

To appear in: Received Date: Revised Date: Accepted Date:

21 January 2014 6 May 2014 7 May 2014

Please cite this article as: T. Lavanya, K. Satheesh, M. Dutta, N. Victor Jaya, N. Fukata, Superior photocatalytic performance of reduced graphene oxide wrapped electrospun anatase mesoporous TiO2 nanofibers, (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.05.088

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Superior photocatalytic performance of reduced graphene oxide wrapped electrospun anatase mesoporous TiO2 nanofibers Thirugnanam Lavanya,a, b Kaveri Satheesh c, MrinalDutta a*, N. Victor Jayab and Naoki Fukataa* a

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan b c

Department of Physics, Anna University, Chennai 600025, India

Centre for Nanoscience and Technology, Anna University, Chennai 600025, India

ABSTRACT Reduced graphene oxide (rGO) wrapped with anatase mesoporous TiO2 nanofibers (TNFs) were synthesized, by using the electrospinning technique along with easy chemical methods. The structural and morphological results demonstrate the success of wrapping of nanofibers with rGO. Wrapping with rGO leads to an efficient photogenerated charge carrier separation across the interface of rGO and TNFs. As a result, the photocatalytic activity of the composites is enhanced to 96 % compared to only 43% for TNFs alone, in the photo degradation of methyl orange. This simple synthetic method can be applied not only to wrap rGO over other nanostructures of different morphology, but also for enhancing the properties of multifunctional materials.

Keywords: Nanofiber; Mesoporous; TiO2; Wrapping; Composite; Photocatalysis.

Email: [email protected] and [email protected] 1. Introduction Semiconductor photocatalysis of organic compounds in water and air is a promising technology for solving worldwide environmental pollution. Among various semiconductor photocatalysts, titanium dioxide (TiO2) has been considered as the most suitable material for widespread environmental applications because of its strong photocatalytic performance, easy availability, long-term stability, and nontoxicity [1-3]. Compare to conventional nanopowders and thin film photocatalysts, TiO2 nanofibers (NFs) show higher surface active sites for adsorption and catalysis of reactants due to greater surface to volume ratio [4, 5]. However, the photocatalytic activity of TiO2 is limited by absorbing only in the UV region of the solar spectrum due to its wide bandgap. In addition to this, the recombination time of the electron-hole pairs is very short (~10-9 s) compared to the chemical interaction time (~ 10-3 to 10-8 s) on the surface of TiO2 with the adsorbed dyes or other molecules [6-8]. These limitations have been often overcome by doping with and without metal ions, loading of noble metals, or making composites of TiO2 with other metal oxides [9, 10]. However, the materials synthesized by the above methods show low stability against photo corrosion, and in some cases, suffer from low concentration of doped ions. Furthermore, in the case of composites with other metal oxides the partial loss of active surface sites often occurred, resulting in a decrease in the photocatalytic efficiency [11]. Nevertheless, the introduction of carbon materials, including activated carbon, carbon nanotubes, and fullerenes can effectively enhance the charge separation rate of TiO2-C composites [12, 13]. Among the carbon family, Graphene, a two-dimensional honeycomb lattice of carbon atoms, due to its unique properties such as excellent carrier mobility (20,000 cm2 v-1 s-1), high transmittance, large surface area (~2600 m2 g-1), and chemical stability, has become a rising star for future nanodevice applications [14-16]. Thus, graphene could be an ideal

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mechanical support and electric charge carrier transporter to construct nanocomposites with enhanced performances [16, 17]. Among the graphene-based composites, TiO2-graphene composites (TGCs) have been widely studied for various applications with enhanced performances in photocatalysis, solar cells, and lithium-ion batteries [19-21].

However, TGCs

showed the cytotoxic effect on minuscule animals under solar irradiation, which limits the extensive use of these highly efficient composite photocatalysts as reported by Akhavan et al. [22]. Among the different methods, such as hydrothermal, solvothermal, heterogeneous coagulation, electrospinning etc., so far used for the synthesis of TiO2-graphene composites, electrospinning is a simple and cost-effective technique for fabricating nanofibers in a core/shell, random and aligned, and hollow configurations [23-26]. In spite of these advantages, very few attempts have been made to synthesize TiO2 NFs (TNFs)/graphene composites by using a simple electrospinning technique to enhance the photocatalytic properties of TiO2 [27, 28]. Furthermore, previous research activities show difficulty to obtain uniform and homogeneous distribution of TiO2 nanostructures on graphene in composites, by overcoming the agglomeration among the nanostructures [16, 29]. The nanostructures are easily accumulated along the wrinkles of graphene sheets or other defects, instead of being uniformly distributed on the graphene. This agglomeration effect reduces the separation of photogenerated charge carriers across the interface, and as a result the synergetic catalytic effect of TiO2 nanostructures and graphene dramatically reduces. Another demerit of agglomeration is to reduce the benefit of the large surface area of graphene to adsorb dye molecules, as suggested by Pan et al. [16], whereas the synthesis of graphene wrapped semiconductor nanostructures is a novel and a new technique to obtain efficient photogenerated charge carrier separation across the interface, by overcoming the

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problem of agglomeration. This has been proved by the potential applications of these wrapped nanostructures in Li-ion battery, bio-imaging and gas sensing [30-32]. In the present work, we report on the fabrication of reduced graphene oxide (rGO) wrapped with mesoporous anatase TNFs composites and their performance to enhance the photocatalytic degradation of methyl orange (MO). The low cost and scalable electrospinning technique is used for the synthesis of TNFs. The most notable novel aspect of this approach is the fabrication of rGO wrapped with TiO2 nanofibers by a simple hot plate drying, which helped to overcome the problems of agglomeration among the nanofibers, as reported by previous researchers. Furthermore, the reduction of GO was performed by annealing without using any hazardous chemical, which is an established technique to rGO, as proposed by previous researchers [33-35]. The structural and morphological characterization provides the evidence of wrapping of rGO over the TNFs. The efficient separation of photogenerated electron-hole pairs in the composite, leads to 96% photocatalytic degradation of MO compared to only 43% degradation by bare TNFs. These results ensure the efficient photogenerated charge carrier separation across the interface of rGO and nanofibers, which is the bottleneck of the present research, for the synthesis of high performance composites beneficial for multifunctional applications.

2. Experimental 2.1. Chemicals and materials Commercial graphite powder (99.95%), titanium tetra isopropoxide Ti{COH(CH3)2}4 (97%), and polyvinylpyrrolidone (PVP) (Mw=13,00,000), N, N- dimethylformamide (DMF), sodium nitrate (NaNO3), potassium permanganate (KMnO4, 98%), sulfuric acid (H2SO4, 98%), methyl orange (MO), and absolute ethanol were purchased from Aldrich, and used without any further purification.

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2.2. Fabrication of TNFs The TNFs were synthesized by an electrospinning technique. Spun solution was prepared in two steps. In the first step, titanium tetra isopropoxide was dissolved in a mixture of 2 ml of ethanol and 2ml of acetic acid. The mixed solution was stirred for an hour to get a transparent yellow solution. The second solution was prepared using a mixture of 10 ml of ethanol and 2 ml of N,N-Dimethylformamide (DMF) mixed with 0.5 gm of polyvinylpyrrolidone (PVP), to control the viscosity under magnetic stirring for 30 min. The previously prepared solution was added to this solution and stirred for a further 15 min. The viscous transparent solution was loaded into a syringe having a stainless steel needle with a diameter of 0.6 mm at a constant flow rate of 0.3 ml/h. The feeding rate was controlled by the syringe pump. The electrically ground aluminum foil was used as the collector. The distance between the tip of the needle and the aluminium foil was maintained at 15 cm, and a DC voltage of 20 kV was applied. The charged NFs were deposited on the metal collector. After obtaining electrospinning, the fibers were peeled off and calcined in air at 450°C for 2 h, to obtain the anatase phase TNFs.

2.3. Fabrication of rGO wrapped TNFs Graphite oxide was synthesized from natural graphite powder, following modified Hummer’s method [36, 37]. For the preparation of TNFs/rGO composites, 1.25 mg, 2 mg and 5 mg of graphite oxide was added in 60 ml of deionized water under sonication for 1h, to obtain a homogeneous dispersion of GO. Then 5 mg of TNFs was added into each of these GO solutions under constant stirring in the dark, at room temperature to get homogeneous mixtures. Water was slowly evaporated from these mixtures by hot plate drying under continuous stirring in the dark. Finally, the brown colored samples were collected and annealed at 400°C for 2 h in an argon

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atmosphere in order to reduce the GO sheets to form rGO wrapped TNFs in ratios of 1:0.25, 1:0.5, 1:1 for TiO2:GO, which will be referred to as TNFG-1, TNFG-2 and TNFG-3 respectively, throughout the manuscript.

2.4. Dye adsorption measurements The adsorption of MO on the nanofiber surface was measured in the same way as reported by Acar et al. [38]. For the adsorption of MO on the nanofiber surface, the TNF and TNFG composites were soaked in MO solution for 2 h. For desorption of dye from the nanofiber surface, 1:1 ethanol- 0.1M NaOH solution was prepared. After adsorption of MO, The nanofibers were separated by centrifugation and immersed in a 1:1 ethanol-0.1M NaOH solution for the desorption of MO. The MO concentration was determined by measuring the solution absorbance at 462 nm.

2.5. Photocatalytic measurements Methyl Orange (MO) was used as an organic pollutant to evaluate the photocatalytic activity of the samples in an aqueous medium inside a quartz tube under UV irradiation. For UV irradiation, the quartz reaction tube was placed axially, and clamped in front of the UV lamp (power: 20 W, emission wavelength: 365 nm). The distance between the lamp and the quartz tube was maintained at 10 cm. 0.01 g of each sample was added to 25 ml of the MO solution (15 mg (MO)/L). Prior to illumination, the suspension was magnetically stirred and sonicated for 20 min in darkness, to achieve absorption–desorption equilibrium of the dye on the surface of the photocatalyst. It was then irradiated with ultraviolet light. At time intervals of 15 min, 3-ml aliquots were sampled and filtered through a 0.22-µm PTFE filter, to remove the catalyst. The filtered solutions were analyzed by a UV/Vis spectrometer and the absorbance spectra of MO

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(major absorption band around 462 nm) [39] were recorded to measure the change in the concentration of MO, since the intensity of the absorption band at ~462 nm is closely proportional to the concentration of MO. The crystalline structure was characterized using a Rigaku Rintz Ultima X-ray diffraction unit. The Raman spectra were recorded at room temperature, using a Horiba Jobin Yvon instrument, with a 514-nm green laser as the excitation light source. The morphology and microstructures of the samples were examined using a Hitachi SU8000 Field emission scanning electron microscope (FESEM) and a high resolution transmission electron microscope (JEOL JEM 2100) operating at 200 kV. The specific surface area and pore volumes were analyzed using N2 adsorption-desorption isotherms using Brunauer-Emmett-Teller method (BET, Quantachrome autosorb iQ2) automated gas sorption analyzer. Photoluminescence (PL) spectra were acquired using a Horiba Jobin-Yvon spectrometer with a He–Cd laser.

3. Results and discussion X-ray diffraction (XRD) was employed for analyzing the crystalline phase of GO, rGO, TNFs and TNFG-2 (as a representative composite) as shown in Fig. 1A. The XRD pattern of GO revealed (002) a diffraction peak at 2θ = 10.7° corresponding to the d-spacing of 0.83 nm [8, 37]. This implies the introduction of functional groups on the basal plane of graphite, during the oxidization of natural graphite, which increases the d spacing from 0.335 nm to 0.83 nm [37, 40]. After the reduction, this peak disappeared and a new peak centered at 2θ = 24.9° corresponding to the d-spacing 0.357 nm of graphene appeared, which indicates the removal of most of the functional groups during the reduction of GO [41]. The sharp peaks in the diffraction pattern of TNFs can be assigned to the (101), (001), (200), (105) and (211) crystal plane of pure anatase phase of TiO2 [42, 43]. All the identified peaks of the diffraction pattern of the composite

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exhibited a similar crystal structure to that of the pure anatase TiO2 phase [44]. It is worth noting that the diffraction peak of rGO was not separately seen in the composite, presumably due to the overlap or screened by (101) the diffraction peak of TiO2 [11, 13]. In order to further confirm the crystalline quality and the formation of the chemical bonds in the TNFs and TNFG, The Raman spectrum was recorded in the range of 200 to 2500 cm-1. Fig. 1B shows the Raman spectrum of GO, TNFs and TNFG-2. Only for the TNFs, the four peaks observed in the low frequency region were assigned to the E1g (148.5 cm-1), B1g (400 cm-1), A1g (517 cm-1) and Eg (638 cm-1) modes of the anatase phase respectively [45, 46]. GO exhibited two broad bands at around 1354 cm−1 and 1598 cm−1, which were assigned to disordered amorphous carbon (D band) and graphitic sp2 carbon (G band) respectively, with an ID/IG ratio of ~1 (ID and IG corresponds to the intensity of D band and G band respectively) [47, 48]. Especially, the D band in the samples can be resulted from the structural defects such as corrugation, twisting, and edges [49]. More importantly, the composite TNFG showed the presence of the G band corresponding to sp2 hybridized carbon, at around 1586 cm-1, and the D band at around 1350 cm-1 with a low ID/IG intensity ratio. This shifting of the G band after reduction can be attributed to the successful exfoliation and the presence of 2-4 layers of reduced graphene oxide on the surface of TNFs, as demonstrated by previous investigations [14, 50]. Similar observations of the reduction of the ID/IG ratio, after the reduction of GO in the composites have been reported by Shi et al. and other researchers [41, 47]. During the formation of the composite, carbon ions interact with the surface of the TNFs, and the low value of the ID/IG ratio demonstrated the strong interaction between Ti and C via Ti-O-C bonds on the surface. Moreover, after the removal of the oxygen containing functional groups, the defects were neutralized by making further bonds with Ti on the surfaces of the TNFs. For the composite, the vibration modes observed in the low frequency range were similar to the

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vibration modes observed in the Raman spectra of the TNFs, except for a shift that can be attributed to the strong interaction between the Ti atoms on the surface of NFs with C via Ti-O-C bonds [51]. As a result, the apparent formation of the crystal defects and lattice distortion were reduced, during the reduction of GO that helps in the restoration of sp2 hybridized domains. However, the Raman spectra of the rGO had similarity with those of the diamond like carbons (amorphous), as argued by Wong et al. [52]. The versatility of the physical properties of the carbon allotropes strongly depends on the ratio of the sp2 (graphite like) and sp3 (diamond like) bonds. In amorphous carbons, the sp3 bonds dominate over the sp2 bonds [53]. Nevertheless, the success of the rGO in enhancing the properties of different composites for various applications, has already been established its potential compared to amorphous carbons. The analysis of the Raman 2D band can provide information about the number of graphene layers. The 2D peak of the single layer graphene was observed around 2679 cm-1, while the 2D band of the multilayer shifts to higher wave numbers by 19 cm-1 [54, 55]. The single layer graphene shows a symmetric 2D peak, while with the increase of layers, the 2D band shows asymmetry, and for more than 10 layers it merges with that of graphite [50, 56]. For the composite, a nearly symmetric 2D band at 2681 cm-1 indicates presence of more than one layer of rGO on the surface of the TNFs [8, 54] Field emission scanning electron microscopy (FE-SEM) was used to investigate the morphological features of the TNFs and TNFG-2. The FE-SEM images of the as spun TNFs (Fig. 2A and inset) show a relatively smooth surface with the average diameter of 150 nm (Fig. 2D). After calcination (Fig.2B), the average diameter of the nanofibers shrank to 120 nm with a rough surface. Fig. 2C shows the FE-SEM image of the composite. The morphology of this composite is similar to that of pure TNFs, and no separate rGO sheets were observed. From the FE-SEM images it was clear that the morphology of the composites remains similar to that of the

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bare TNFs. Furthermore, the morphology of the composites was analyzed by Transmission electron microscopy (TEM). Fig. 3 summarizes all the TEM results. Fig. 3A shows the typical microstructure of a single NF with pores. Further analysis of these porous structures by studying N2 adsorption-desorption isotherms proved that these NFs are mesoporous. The HRTEM analysis of these NFs revealed the interplanar lattice spacing of 0.35 nm, which correspond to the (101) planes of anatase TiO2 (Fig. 3B) [57]. From the TEM images of the composite, the presence of a shell layer on the outer surface of the NFs was seen (Fig. 3C). The HRTEM observation (Fig. 3D) of this shell layer of ~ 2 nm width, shows the interlayer spacing of about 0.37 nm, which was slightly larger than that of the interplanar spacing of graphite (~0.34 nm) but smaller than that of graphene oxide (~0.83 nm), due to the removal of the oxygen containing functional groups present in the GO. This observation was further supported by the XRD result. In addition to this shell layer, interplanar lattice spacing of 0.35 nm corresponding to the (101) plane of anatase phase of TiO2 were also observed in the core region, which suggested that the TNFs in the composite were in the anatase phase. The selected area electron diffraction (SAED) pattern in Fig. 3E, shows reflections from the different planes of the anatase structure of TiO2, indicating random orientations, which are in agreement with the results of the powder X-ray diffraction (XRD) analysis of the TNFs. Fig. 3F confirmed the formation of the polycrystalline TiO2 anatase phase in the composite. Elemental mapping of electron energy loss spectroscopy (Fig. 4), was used to establish the distribution of rGO on the TNFs surface, by detecting carbon, oxygen and titanium signals from the composite nanofibers. The rather uniform distribution of carbon along with oxygen over the whole area of the nanofiber surface, confirmed the existence and homogeneous wrapping of rGO on the TNF surface [58].

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To investigate the chemical states of the elements and the interactions between TiO2 and rGO in the composite, X-ray Photoelectron spectroscopy (XPS) measurements were taken. The XPS analysis of the TNFs and the best performance of the composite TNFG-2, are shown in Fig. 5. The survey spectrum of the GO showed the peaks of C and O, whereas the survey spectrum of the composite exhibited the presence of Ti peaks together with the peaks for C and O (Fig. 5A and B). Fig. 5C shows the high resolution C1s XPS spectrum of the TNFs and composite. The deconvolution of the C1s XPS spectrum of GO showed a considerable degree of oxidation at the binding energy of 286.5, 287.6 eV, as observed in Fig. 5D; the peak at the binding energy of 286.5 eV is attributed to the C-O and C-O-C in the hydroxyl or epoxy groups; the peak at 287.6 eV was assigned to the O-C=O oxygen containing a carbonaceous band [41, 42]. In addition to these peaks, the peak at the binding energy of 284.4 eV was assigned to the C-C, C=C (sp2) and C-H bonds [40, 43, 44]. In the C1s XPS spectrum of the composite, the intensity of the peaks (286.5, 287.6 eV) reduced because of the oxygen containing functional groups pauperizing substantially, which suggests the successful reduction of GO in the composites. The O1s XPS spectrum of GO exhibited different shapes compared to the composite, as shown in Fig. 5F. The peak at the binding energy of 532.2 eV originated from the hydroxyl and epoxy groups present in GO. While for the TNFs and composite, the peak at 530.75 eV was attributed to the Ti-O bonds present in the NFs. The peak due to the oxygen of the oxygen containing functional groups reduced substantially in the composite. The deconvolution of the O1s XPS spectrum of the composite revealed another small peak (inset of Fig. 5F) at the binding energy of 536 eV, which can be assigned to the Ti-O-C bonds between the TNFs surface and rGO [43, 44, 59, 60]. An analysis of the C1s XPS spectra of GO and TNFG-2 showed, that the peak area ratio of oxygen-containing carbon to the nonoxygenated

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graphitic carbon (AC−O/AC−C) reduced from 0.77 to 0.27, due to the reduction of GO during the formation of the composite. This result implies that nearly 65% of the oxygen containing groups were removed from GO during reduction. This result was comparable with the TiO2 photocatalytic reduction of GO and hydrothermal reduction of GO [8, 61]. The atomic ratio of C:O:Ti in the composite was determined as 0.39:0.44:0.16 from the XPS data. The O/C and Ti/C ratios were around 1.12 and 0.39 respectively. The Ti/C ratio was less than the previously reported ratio in the composite [22], which can be understood by considering the rGO wrapped morphology of the TNFs. So, the contribution from the C atoms of rGO is much higher. During the photocatalytic performance of the composites, there is a probability of further reduction of GO on the surface of the TiO2 nanofibers, as previous reports showed the reduction of GO by semiconductors under UV light irradiation [52, 62-64]. The XPS analysis of the TNFG-2 composite after photocatalytic degradation of MO showed that the AC−O/AC−C ratio further reduced to 0.25. This reveals further reduction of GO in the presence of UV light [54, 15]. However, the composite irradiated by UV light for 6 h after thermal reduction, did not show any further reduction of GO, compared to the used composite (Fig. 5E). These results are consistent with the previous reports of Akhavan et al. [52]. The porous structure of the TNFs was evident from the analysis of the TEM image of the TNFs. Now, to get more information about the pore size distributions and specific surface area, the Brunauer–Emmett–Teller (BET) analysis was performed by studying N2 adsorption–desorption isotherms for the TNFs and composites, as shown in Fig. 6. The pore size distribution

of the

corresponding

samples

was

calculated,

on

the

basis

of

the

Barrett−Joyner−Halenda (BJH) method. Fig. 6B shows a broad distribution of pore sizes in the range of 2–20 nm, which clearly indicates that TNFs possess a mesoporous structure [57]. The

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BET study revealed that the synthesized TNFs have a specific surface area of 52 m2 g-1, which is higher than that of commercially available TiO2 (39m2g-1) [8]. With the increase of the rGO content in the composites, the surface area increases, presumably due to the high specific surface area of the rGO. The BET specific surface area of the TNFG-3 composite showed the highest specific surface area of about 102 m2g-1. The specific surface area of different samples is summarized in Table 1. When the surface area increases, the dye adsorption also increased, which was consistent with the specific surface area measurement (Table 1). These results were similar to the previous reports of dye adsorption on one dimensional TiO2 nanostructure surfaces [38, 65]. To probe the charge transfer behavior of the photogenerated carriers across the interface in the composite, photoluminescence measurements was carried out as presented in Fig. 7A. Two characteristic luminescence peaks for bare TNFs were observed around 420 nm and 547 nm. The narrow emission around ~ 420 nm is generally attributed to the band edge emission of TiO2 as reported previously by Akhavan et al. [66], and the broad, intense emission in the visible region can be assigned to a combination of different deep level emissions, which are believed to occur due to the recombination at different defect levels within the band gap of TiO2 [67]. The PL intensity for both types of emissions was found to be quenched in the composite, which indicated that the recombination of the photogenerated electrons and holes in the TNFs can be effectively inhibited by wrapping rGO on the surface of the NFs. A similar observation of the diminishing of electron-hole pairs in rGO nanoribbon/TiO2 nanoparticle composite had been reported by Akhavan et al. [66]. This quenching effect can be understood by considering the energy band alignment at the interface of the TNFs and rGO (inset of Fig. 7A). Since the electron affinity for anatase TiO2 is χTiO2 = 4.21 eV, and the work function of graphene is known to be 4.42 eV [45],

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direct electron transfer from the conduction band of the photoexcited TNFs to the rGO was therefore energetically favorable, when the TNFs were wrapped by rGO. Different amounts of rGO in the composites result in different amounts of quenching, and the TNFG-2 showed the maximum quenching. However, an increase of rGO content resulted in a decrease in the quenching of the PL peaks. Thus, there was an optimal value of the TNFs and GO ratio, which can produce rGO wrapped TNFs, to efficiently separate the photo generated charge carriers across the interface, and suppress the electron-hole recombination in TiO2. To test the photocatalytic activity of the TNFs and composites, photodegradation experiments were performed using MO as a pollutant under the irradiation of UV light as shown in Fig. 7B, because MO has a negligible absorption in the UV region, while TiO2 has a strong absorption in this region. This negligible absorption was verified by the significant stability of the MO solution under the exposure of UV light for different periods, in the absence of a catalyst. MO was degraded up to 43 % in the presence of TNFs, compared to 30% degradation by commercial TiO2-P25. While the degradation was remarkably accelerated to 96 % for TNFG-2 within the measured time interval, rGO itself degraded MO by 20%. This enhancement in the degradation of MO due to TNFG-2 also means that in the UV region MO can be used as an organic pollutant, to test the photocatalytic efficiency of TiO2, as previously suggested by Ohtani et al. [68, 69]. Photocatalytic activity depends mainly on the surface area of the catalyst, where the photodegradation occurred and also on the number of photo electrons available from the photocatalyst. For the composites TNFG-1 and TNFG-2, the photo electrons were quickly transferred to the rGO shell, which reduces the probability of the electron-hole recombination in TiO2. Thus, more photo electrons were available compared to only TNFs, to take part in the photodegradation process. Additionally, due to the high surface area of the rGO reaction area

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also increased as a result adsorption of MO also increases (Table 1). However, the use of an excessive amount of rGO in the TNFG-3 composite reduced the light absorption on the TiO2 surface, resulting in the decrease of photoexcited electrons [45]. This excess rGO also increased the opportunity for the recombination of the photo-generated electron-hole pairs, by increasing the collision among the photogenerated electrons and holes [41]. Thus, an optimal amount of GO was considered in the preparation of the composites. To test the effect of further reduction of GO during the photocatalytic process as revealed by the XPS analysis of the used composite, a photocatalytic experiment was conducted by reusing the used TNFG-2 composite. This used composite showed 99% degradation of MO within the measured time interval, which further supports the XPS measurement. However, the TNFG-2 composite prepared by 6 h of UV irradiation after thermal reduction, did not show any further improvement of photocatalytic efficiency, compared to the used composite (for clarity not shown in Fig. 7 B).

4. Conclusions In summary, rGO wrapped anatase TNFs were prepared by using the electrospinning technique, along with easy chemical methods. The advantage of this method is that, it does not require any toxic solvents or chemicals to reduce the graphene oxide. Fibrous morphology was clearly evident from the SEM observations. HR-TEM images and EELS spectroscopy confirmed that the surface of the TNFs was wrapped by rGO. The N2 adsorption-desorption isotherms proved the mesoporous nature of the NFs. For the composite, 96% photocatalytic degradation of MO was achieved, compared to only 43 % degradation by the bare TNFs.

Acknowledgment

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The first author wishes to express her gratitude to the National Institute for Materials Science (NIMS), Japan, for the IJCS fellowship, and also for providing the facilities needed to carry out the characterization.

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Table 1. The specific surface area of the TNF and TNFG composites

Samples

Surface area Adsorbed MO (m2 g-1) (mol/gm)

TNF

52.97

2.36

TNFG-1

88.51

5

TNFG-2

97.85

5.5

TNFG-3

102.94

5.7

21

Figure caption [1] Fig.1. (Color online) (A) XRD pattern GO, rGO, TNFs and TNFG, (B) Raman spectra of GO, TNFsand TNFG-2 composite.The inset in (B) shows the enlarge Raman spectra of GO, TNFand TNFG-2 composite in the range of 1000-2000 cm-1 and 2D peak of the composite. [2] Fig. 2. (Color online) SEM image of (A) as-spun NFs (inset shows high magnification image of smooth surface of a typical as-spun NF) (B) anatase TNF after calcination (inset shows high magnification image of rough surface of a typical anatase TNF), (C) TNFG-2. (D) Distribution of diameters of the as-spun nanofibers and anatase TNFs after calcinations. [3] Fig. 3. (Color online) TEM images of a typical (A) anatase TiO2, (C) composite NF. (B) and (D) HRTEM images corresponding to (A) and (C) respectively. [4] Fig. 4. (Color online) (A) Bright field TEM image of a typical composite nanofiber. (B), (C) and (D) elemental mapping of the composite nanofiber in (A). [5] The XPS survey spectrum of the (A) GO and (B) TNFG-2 composite. (C) C1s XPS spectra of GO and TNFG-2 composite. (D) Deconvoluted C 1s XPS spectra of the GO. (E) Deconvoluted C1s XPS spectra of TNFG-2, used TNFG-2 and 6h UV radiated TNFG-2 composite respectively. (F) O1s XPS spectra of the GO, TNF and TNFG-2 composite. Inset shows the Deconvoluted O [6] Fig. 6. (Color online) (A) N2 adsorption-desorption

isotherms at 77 K, (B) Differential

pore volume of TNFs and composites with various content rGO of as a function of the pore diameter. [7] Fig. 5. (Color online) (A) PL spectra of TNF and TNFG. (B) Photodegradation of MO by TNF and TNFG. Inset of (A) shows the band alignment at the interface of TNFs and rGO.

22

Intensity (a. u.)

Intensity (a.u.)

Intensity (a.u)

(220)

E1g

(215)

(204)

(105) (211)

(200)

(004)

Intensity (a.u) (001)

(B)

TNFG TNFs rGO GO

(101)

(A)

2D

2550

2700

1000

A1g

Eg

1250

1500

1750

2000 -1

Wavenumber (cm )

TNFG TNFs

(002)

GO

10

Fig. 1. Lavanya et al.

20

30

40

50

2 (deg)

60

70

2850 -1

Wavenumber (cm )

B1g

500

1000

1500

2000

-1

Wavenumber (cm )

2500

(D)

As-spun NFs Annealed NFs

30

Counts(%)

25 20 15 10 5 0 50

100

150

200

Diameter(nm)

Fig. 2. Lavanya et al.

250

Fig. 3. Lavanya et al.

(A)

(B)

(C)

(D)

Fig. 4. Lavanya et al.

(A)

(B)

O1s

O1s

Intensity (a.u.)

Intensity (a.u.)

C1s

0

200

400

600

800

1000 1200

Ti2p C1s

0

200

Binding energy (eV) (C)

(D)

C1s

400 600 800 1000 1200 Binding energy (eV)

C1s

C-C

Intensity (a.u.)

Intensity (a.u.)

GO GO

C-O

O-C=O

TNFG-2 285 290 Binding energy (eV)

295

282

C1s

(D)

(F) (O1s)

6 h UV-TNFG-2

C-C

TNFG-2

532.2 eV

530.75 eV

Intensity (a.u.)

Used TNFG-2

284 286 288 Binding energy (eV)

GOO1s CompositeO1s TiO2 O1s

290

Ti-O

Intensity (a.u.)

280

C-O Ti-O-C

528 531 534 537 Binding energy (eV) Binding energy (eV)

C-O O-C=O

280

Figure. 5 Lavanya et al.

285 290 Binding energy (eV)

295

525

530 535 Binding energy (eV)

540

(A)

120 100

0.45

TNF TNFG-1 TNFG-2 TNFG-3

(B)

0.40 0.35

TNF TNFG-1 TNFG-2 TNFG-3

0.30

0

Volume (STP(cc/g))

140

dV(r) (cc/A /g)

160

80 60 40 20 0

0.25 0.20 0.15 0.10 0.05

-20 0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

Figure. 6 Lavanya et al.

1.0

0.00 0

50

100

150

Pore diameter(nm)

200

250

(A)

3000 2000

E vacc

4.21 eV

TNF TNFG-1 TNFG-2 TNFG-3

1.0

3.2eV

TiO 2

MO dark

0.8

E CB E VB

(B)

rGO 4.42 eV

Graphene

C/C0

Intensity (a.u)

4000

P25 TNF

0.6

TNFG-3

0.4

1000

TNFG-1

0.2 TNFG-2

0

Used TNFG-2

0.0 200

400

600

800

Wavelength (nm)

Fig. 7. Lavanya et al.

1000

0

20

40

60

80

Time (min)

100

120

Graphical abstract

Reduced graphene oxide wrapped anatase mesoporous TiO2 nanofibers were synthesized to efficiently control the photogenerated charge carrier transfer across the interface which as a result enhanced the photocatalytic activity of the composite to a high extent.

23

Highlights: • • •

Synthesis of mesoporous TiO2 nanofibers by electrospinning technique. Wrapping by reduced graphene oxide over nanofibers were done by hot plate drying and annealing which is free from use of any toxic chemicals. Composite nanofibers showed 96% photocatalytic degradation of methyl orange compare to only 43% degradation by only nanofibers.

24