Synthesis of reduced graphene oxide-TiO2 ...

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Synthesis of reduced graphene oxideeTiO2 nanoparticle composite systems and its application in hydrogen production Pawan Kumar Dubey a,*, Prashant Tripathi a, R.S. Tiwari a, A.S.K. Sinha b, O.N. Srivastava a,* a

Department of Physics, Banaras Hindu University, Varanasi 221005, India Department of Chemical Engineering and Technology, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India

b

article info

abstract

Article history:

The utilization of solar energy for the conversion of water to hydrogen and oxygen has

Received 19 July 2013

been considered to be an efficient strategy to solve crisis of energy and environment. Here,

Received in revised form

we report the synthesis of reduced graphene oxideeTiO2 nanoparticle composite system

25 February 2014

through the photocatalytic reduction of graphite oxide using TiO2 nanoparticles. Photo-

Accepted 16 March 2014

electrochemical characterizations and hydrogen evolution measurements of these nanocomposites reveal that the presence of graphene enhances the photocurrent density and hydrogen generation rate. The optimum photocurrent density and hydrogen generation

Keywords:

rate has been found to be 3.4 mA cm2 and 127.5 mmole cm2h1 in 0.5 M Na2SO4 electrolyte

TiO2 nanoparticles

solution under 1.5AM solar irradiance of white light with illumination intensity of

Hydrogen production

100 mW cm2. In grapheneeTiO2 nanocomposite, photogenerated electrons in TiO2 are

Water electrolysis

scavenged by graphene sheets and percolate to counter electrode to reduce Hþ to molec-

Graphene

ular hydrogen thus increasing the performance of water-splitting reaction.

Nanocomposite

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The utilization of solar energy for the conversion of water to hydrogen is the crucial component for harvesting hydrogen energy. It is now generally agreed that hydrogen may be the best option for tackling the triple issues of, depletion, pollution and climate change effects. Photoelectrolysis of water is considered to be one of the most promising methods to generate hydrogen (and oxygen) through the cleavage of water utilizing solar energy. Many photocatalysts have been

reported to catalyse the evolution of hydrogen from aqueous solutions. Among these photocatalysts, TiO2 is one of the most promising because of its superior photocatalytic performance, easy availability, long-term stability, and nontoxicity [1e3] However, TiO2 suffers from the limited absorption of solar light due to its large bandgap (3 eV) and fast recombination of charge carriers [4]. Typically, photoexcited electronehole pairs can be generated under the irradiation with wavelength lower than that corresponding to the band gap energy of TiO2 (3.20 eV for anatase phase). The photogenerated electrons

* Corresponding authors. Tel.: þ91 542 2368468; fax: þ91 542 2369889. E-mail addresses: [email protected] (P.K. Dubey), [email protected] (O.N. Srivastava). http://dx.doi.org/10.1016/j.ijhydene.2014.03.104 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.


then drive the water-splitting reaction to produce hydrogen [5]. The recombination of the electrons and holes is one of the main reasons for the low efficiency of photocatalysis [6]. Therefore, one of the most challenging issues in photocatalysis is to overcome the quick recombination of photogenerated electrons and holes. Several strategies have been proposed to increase the efficiency of TiO2 photocatalyst. On the other hand, in the sunlight spectrum, the energy of UV light represents only 5% of the total spectrum energy. Thus, to inhibit the recombination of photo-induced electroneholes and extend the light absorption to the visible light region are the key factors for improving the photoconversion efficiency of TiO2. The approaches to enhance the photoconversion efficiency of TiO2 include doping with metallic particles such as Pt, Au, Ag, [7e9] nonmetal elements such as S, N, C, [10e13], and developing composite materials to suppress the ultra fast recombination of the photo-induced electronehole pairs [14,15]. The hybrid/composite form of TiO2 and carbon nanomaterials, particularly carbon nanotubes (CNTs), has attracted much attention in recent years [15]. It is reported in several literature that the photogenerated electrons in the spacecharge regions may be transferred into CNTs, and the holes remain on TiO2, this will retard the recombination of electrons and holes [16e21]. In addition, CNTs may also provide high surface areas or desired functional groups for the efficient adsorption of reactants and may act as photosensitizers. The possible formation of TieOeC bonds may also affect the photocatalytic behaviours of the TiO2eCNT composites. Furthermore, CNTs could function for controlling the morphology of TiO2 nanoparticles (TiO2 np) [22]. The latest carbon nanostructures, graphene with its unique structure of one-atom thick planar sheets of sp2 bonded carbon atoms closely packed in a honeycomb crystal lattice. It has several unique properties e.g. high thermal conductivity (5000 W m1 K1), excellent mobility of charge carriers (200,000 cm2 V1 s1), large surface area (calculated value 2630 m2 g1) and good mechanical stability [23e28]. The discovery of isolated graphene sheets (GSs) obtained from the simple mechanical cleavage [29] has found several interesting applications. More excitingly, the transfer printing of exfoliated graphene onto electrodes on different substrates for large-scale integration has been reported recently [30e32]. Solution-processable graphene can be derived from aqueous suspension of graphene oxide (GO). The latter can be readily obtained by the exfoliation of graphite through acid oxidation; [33,34]. Methods such as drop casting [35,36], rapid freezing by spraying [37] and dip coating (Wang et al., 2007) from GO suspension have been used to obtain isolated individual and multilayered sheets or thin films. Layer-by-layer (LBL) assembly techniques have also been applied to fabricate multilayer films of GO using polyelectrolyte as linkers for electrochemical and Li ion battery applications [38,39]. Graphene can be viewed as split carbon nanotubes (CNTs), suggesting that similar to CNTs it may enhance the photocatalytic activity of catalysts like TiO2 [21,40]. Compared with CNTs, graphene has many advantages including high surface [41] and good interfacial contact with adsorbates. Therefore, it is desirable to explore simple and effective approaches for synthesizing graphene based composites and explore their applications as photocatalyst. Very recently, there have been a

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few reports of the synthesis of TiO2/graphene composites and their use in hydrogen evolution [42e44]. Most of these studies are focused on photocatalytic hydrogen generation. Only sparse studies are available on Photoelectrochemical electrolysis of water. There seems to be no effort so far made for use of TiO2egraphene composites with enhanced light absorption and charge separation capacity as a photoelectrode for hydrogen production through photoelectrolysis of water. Keeping the aforesaid aspects in view we have in the present investigation carried out photoelectrolysis of water using photoelectrodes of reduced graphene oxide (rGO)eTiO2 composites. The rGO have been obtained through in-situ reduction of GO from electrons produced and stored into TiO2 by UV light illumination.

Experimental methods Synthesis of graphene oxide Graphite oxide was prepared by the oxidation of graphite powder employing modified Staudenmaier method [45]. In a typical synthesis process, sulphuric acid (18 ml) and nitric acid (9 ml) mixed together in a reaction beaker and cooled by immersing the beaker in an ice bath. Graphite powder (1 g) was added into the mixture under vigorous magnetic stirring and then potassium chlorate (11 g) was added slowly for 1 h. Sudden increase in temperature was avoided by ice cooling. For evolution of the gases from reaction mixture the reaction beaker was loosely capped and continuously stirred for 96 h at room temperature. After completion of the reaction the mixture was washed with deionized water for 5e6 times and then filtered. The filtered sample was dried under room temperature for overnight and further at 80 C under constant vacuum. The as obtained powder is designated as graphite oxide. The known amount of graphite oxide was dissolved in 10 ml absolute alcohol solution and then ultrasonicated for 2 h to disperse the sample. After ultrasonication colour of the solution becomes brownish which is known as graphene oxide (GO). The schematic of the process for the preparation of GO from graphite oxide is shown in Fig. 1.

Synthesis of TiO2 np TiO2 np of size ~5e7 nm used in this study were prepared by sol gel method through the hydrolysis of titanium tetraisopropoxide (TTIP). In a typical synthesis, 5 ml of TTIP was mixed drop wise in a vigorously stirred 6 ml isopropanol. The resulting mixture was stirred for 15 min and then 75 ml of deionised water was added drop wise. As the water is added in the stirred mixture, white precipitate is formed and in few seconds time a white solution is formed. Then the temperature knob was adjusted so that the temperature of the sol becomes 80 C. After stirring at 80 C for one hour 2.5 ml of acetic acid was added and the resulting solution was allowed to stir for 5 h. After 5 h 1 ml of Nitric acid was added for peptization and the resulting solution was further stirred for 2 h and then resulting solution was cooled naturally to room temperature. The as formed solution was vacuum dried and

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Fig. 1 e Schematics of the preparation of graphene oxide suspension from graphite oxide through ultrasonication in ethanol.

finally dried at 60 C in an electric oven. The dried powder was crushed in mortar to make fine powder and then this powder was finally annealed at 500 C for 3 h.

Synthesis of GrapheneeTiO2 np composite system rGOeTiO2 np composite systems with different mass ratios of GO were prepared through the reduction of GO by ultra violet radiation. In a typical synthesis process 500 mg of TiO2 np powder was dispersed in absolute ethanol solution by ultrasonication for 1 h and then exposed to UV radiation to store the electrons inside the TiO2 np. A 1000 W HgeXe arc lamp served as the source for ultraviolet radiation. Then known amount of GO suspensions was added to the TiO2 np solution to prepare GOeTiO2 np suspension with different mass ratios of GO (1, 2, 5 and 10 wt.%, respectively). The GO-TiO2 suspension was stirred for another 2 h to obtain a homogeneous suspension and then exposed to UV radiation for the preparation of rGOeTiO2 np composite through the reduction of GO to rGO. For this the GOeTiO2 np suspension was irradiated for 2 h by ultra violet radiation. On irradiation the electron out of electronehole pairs created in TiO2 np get transferred to GO. The electron reduces the oxygen functionalities in GO. This leads to the formation of rGO. The resulting composite was recovered by drying at 50 C for 6 h. A representative picture of the preparation of grapheneeTiO2 np composite through the photocatalytic reduction by the ultraviolet radiation of GOeTiO2 np suspension is shown in Fig. 2. It may be pointed out that rGO result from reduction of the source material GO. However, the exact concentration of rGO is not known. The concentration of rGO will vary with the concentration of GO in

the composite material TiO2 np e x wt.% GO. In order to specify quantitatively the concentration of rGO which originates from GO for the composite material we will hereafter write rGO(x%)eTiO2 np to represent the concentration of rGO evolving from reduction of x wt.% GO.

Synthesis of photoanode of rGOeTiO2 np composite The photoanodes were prepared by spraying the colloidal suspension of the rGOeTiO2 np on the fluorine doped tin oxide (FTO) glass substrate. For the preparation of colloidal solution rGOeTiO2 np composite system, 0.2 g of this composite was vigorously stirred with 0.5 ml of acetyl acetone and 2 ml of distilled water. After 30 min 2 ml of water and 0.1 ml of TritonX were added and stirred for further 30 min to obtain the colloidal solution. This colloidal solution was sprayed by a nebulizer on the conducting substrate FTO which was preheated to 300 C. The as deposited samples were sintered in argon gas ambient at 250 C for 30 min to remove the unwanted organic contaminants.

Characterization techniques The structural characterization of the samples were carried out using X-ray diffraction technique employing X’Pert PRO PANalytical diffractometer equipped with graphite monochromator with a Cu source (l ¼ 1.54 Å, CuKa operating at 45 KV and 40 mA). The morphological characteristics of the different samples were examined by scanning electron microscopy (SEM) using FEI quanta 200 instrument. Microstructural analysis was carried out using transmission electron

Fig. 2 e Schematics of the preparation of grapheneeTiO2 nanocomposites through the photocatalytic reduction by the ultraviolet radiation of GOeTiO2 nanoparticle suspension.


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microscopy (TEM) using TECNAI G2, operating at 200 KV in diffraction & imaging modes. The samples for TEM were prepared by ultrasonically dispersing the powder in a mixture of ethanol and deionized water and then coating the above dispersing solution onto copper grids. The optical characterizations were carried out by UVeVisible spectrophotometer (Pekin Elmer Lambda900). Raman spectrums were recorded on Horiba Jobin Yvon Raman spectrophotometer (Model no. 45517). The FTIR spectra of the materials were recorded from 4000 to 400 cm1, employing KBr pellet technique using PerkinElmer (Spectrum 100). X-ray photoelectron spectrometer AMICUS 3400 (Kratos Analytical Shimadzu Corporation), high performance analytical instrument using Mg target under 1.0 106 Pa pressure was used for elemental and quantitative analysis of samples.

Photoelectrochemical measurements The photoelectrodes were prepared through fixing the FTO based composites samples over a perspex sheet, having a central hole of predefined area, using a chemically inert epoxy resin. The ohmic contact was made on the back of sample by scraping the film through Cu wire glued with the help of silver paint and sealed using epoxy resin. This configuration formed the photoelectrode. The photoelectrochemical experiments were performed in a rectangular shaped perspex cell having quartz window on front using a three-electrode system with a platinum foil as counter electrode, a saturated calomel electrode as reference electrode and TiO2 np and rGO(x%)eTiO2 np as photoanode. The part of the cell between quartz window and the photoelectrode was filled with 0.5 M Na2SO4 electrolyte solution.

Measurement of photocurrent For photoelectrochemical IeV experiments a 1000 W HgeXe light source (Oriel Corporation, USA) has been used for the irradiation of photoanode. The distance of the photoanode from the light source was adjusted so that illumination intensity at photoanode was 100 mW/cm2. A Model PGSTAT III (autolab, Netherland) potentiostat/galvanostat was used to set the applied bias relative to the saturated calomel reference electrode (SCE) and measure the photocurrent response.

Measurement of hydrogen generation rate A 150 W HgeXe lamp (Oriel Corporation, USA) was used as light source. The hydrogen production measurement has been performed at 1 V (vs. SCE) by collecting gas liberated over cathode in a predefined duration with the help of an inverted burette.

Results and discussions The phase and structure analysis of the samples were investigated by X-ray diffraction pattern. Fig. 3 shows the XRD patterns of the graphite oxide, TiO2 np and rGOeTiO2 np composites with varying content of rGO. Fig. 3(a) shows the peak at 2q ¼ 11.35 with d ~7.78 Å which corresponds to graphite oxide. These reveal the formation of graphite oxide phase upon chemical treatment of graphite flakes. The

Fig. 3 e XRD pattern of (a) graphite oxide, (b) TiO2 nanoparticle and grapheneeTiO2 nanocomposite with (c) 1 wt.% (d)2 wt.% (e) 5 wt.% and (f) 10 wt.% of GO prepared by the photocatalytic reduction by the ultraviolet radiation of GOeTiO2 suspension.

d spacing gets increased from 3.36 to 7.78 Å, due to incorporation of various oxygen functional groups in between the 002 type basal planes of graphite. The presence of sharp peak shows the stacking of large number of rGO forming crystalline block. This is consistent with reported work of others [36,46]. The XRD of TiO2 np shown in Fig. 3(b) corresponds to anatase phase of TiO2 (JCPDS Card NO: 21-1272). XRD patterns of rGOe TiO2 np composites shown in Fig. 3(c)e(f) shows the peaks of TiO2 with broadening in FWHM with the exception for rGO(1%)eTiO2 np. The peak corresponding to graphite oxide was not observed in these patterns which may be due to the removal of oxygen functionalities. However, the main characteristic peak of few-layer rGO which appears at 24.5 was not present. This may be due to shielding by the strong peak of anatase TiO2 at 25.3 . Similar observations are reported by other workers [47e49]. The significant broadening in the peaks of composite above 1 wt.% GO the source material for rGO may be attributed to the strain caused by the addition of rGO. In order to confirm the reduction of GO after photocatalytic reduction employing TiO2 np and using ultra violet radiation, Fourier transform infrared spectra (FTIR) of GO and rGO(x%)e TiO2 np composites were recorded and analysed. It can be observed from Fig. 4(a) that GO exhibits several characteristic absorption bands of oxygen containing groups. The IR absorption at 1720 cm1 could be attributed to the C]O stretching vibration, and the broad peak in the range of 3000e3500 cm1 is attributed to the OeH stretching vibrations of the CeOH groups. The peaks at 1060 cm1 and 1250 cm1 are attributed to CeO and CeOeC stretching modes, respectively. The C]C skeleton vibration peak could be observed around 1620 cm1. The absorption band at lower frequency side at 600 and 836 cm1 appeared due to the unsaturated alkenes group (ReCeH). Similar FTIR pattern was obtained for GO by Park et al. [50]. FTIR spectrum of TiO2 np in Fig. 4(b) shows the broad IR band at low frequency (below 1000 cm1) which reflects the stretching vibration of TieOeTi bonds in

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Table 1 e Quantification of carbon atoms with different oxygen functionalities in GO and rGO. Bonding type of C

CeC/C]C CeO C]O O]CeOH

Fig. 4 e FTIR spectrum of the graphite oxide, TiO2 nanoparticle and grapheneeTiO2 nanoparticle composite with 2 wt.% of GO.

TiO2 np [44]. The FTIR spectrum of rGO(2%)eTiO2 np is shown in Fig. 4(c). From this figure it can be concluded that the intensities of absorption bands of oxygen containing functional groups (OeH, C]O, CeOeC, and CeO) are significantly decreased and carbonyl C]O band (1733 cm1) has even disappeared for the rGO(2%)eTiO2 np. This indicates reduction of GO to rGO through the UV photocatalytic reduction. The band at around 1593 cm1 can be attributed to the skeletal vibration of the rGO, due to the interactions between titanium dioxide and rGO. These present results of FTIR are nearly compatible with known results on rGOeTiO2 composites [44,51,52]. To further support our claim XPS of GO and rGOeTiO2 nanocomposite have been performed and analysed. High

GO

rGO(2%)eTiO2 np

Peak position

Atomic%

Peak position

Atomic%

284.7 286.1 288.0 289.6

27.14 39.82 30.52 2.52

284.7 286.3 288.4 289.8

60.66 26.69 10.18 2.47

resolution C1s XPS spectra of GO and rGO(2%)eTiO2 nanocomposite is shown in Fig. 5. C1s spectra of GO (Fig. 5(a)) shows the de-convoluted four different peaks centred at 284.7 eV, 286.1 eV, 287.9 eV, and 289.6 eV, corresponding to CeC/C]C in aromatic rings, CeO (epoxy and alkoxy), C]O, and COOH groups, respectively [33]. De-convoluted spectra of rGO(2%)eTiO2 nanocomposite (Fig. 5(b)), the intensities of all C1s peaks of the carbons binding to oxygen, especially the peak of CeO (epoxy and alkoxy), decreased dramatically, revealing that most oxygen containing functional groups were removed and GO could be reduced to rGO by the photocatalytic treatment using ultraviolet radiation. Table 1 summarises the percentages of oxygen functionalities before and after reduction. The approximate loading of rGO in the composite was calculated with the help of XPS quantification. The 1 wt.%, 2 wt.%, 5 wt.% and 10 wt.% of GO resulted into rGO loading of 0.6 wt.%, 1.3 wt.%, 3.2 wt.% and 6.4 wt.% respectively after reduction in the composite. Raman spectrum is considered as an efficient tool for the characterization of carbon nanomaterials particularly graphene. Fig. 6 shows the Raman spectrum of the graphite oxide, TiO2 np and rGOeTiO2 np composite photocatalytically reduced irradiation from ultra violet light. Raman spectrum of the TiO2 np shows peaks 115 cme1, which is attributed to the main Eg anatase vibration mode. Furthermore, vibration peaks at 364 cme1 (B1g), 484 cme1 (A1g), and 605 cme1 (Eg) are also characteristic of anatase TiO2 [53]. Intensity of these peaks suggests highly crystalline nature of the TiO2 np. Raman spectrum of the composite of rGOeTiO2 np shows the peaks at 1311 cme1 and 1570 cme1 which are typical D and G bands

Fig. 5 e High resolution XPS spectra of C1s of (a) GO and (b) rGO(2%)eTiO2 np composite.


Fig. 6 e Raman spectrum of (a) Graphite oxide, (b) TiO2 nanoparticle and grapheneeTiO2 nanocomposite with (c) 1 wt.% (d)2 wt.% (e) 5 wt.% and (f) 10 wt.% of GO.

respectively. The G band is the characteristics of all sp2 carbon forms [54] and provides the information on in plane vibration of the sp2 carbon atoms [55]. The D band suggested the presence of sp3 defects [56]. The intensity ratio of the D band to G band (ID/IG) shows an indication of disorder in carbon materials such as GO or rGO originating from the defects associated with the vacancies, grain boundaries and amorphous carbon

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[56]. The ID/IG for all the rGOeTiO2 np composite was found to be ~1 which shows that the composite contain the rGO with comparatively low density of defects. This suggests the presence of tiny reduced graphene oxide sheets in the sample. Apart from that hump like peaks also can be seen at 2700 cme1 and 2912 cme1 which are the second order feature of the D band and G band known as 2D and D þ G band. Hump like structure of the 2D-band confirms the formation of few-layer of rGO [57] through the reduction of GO by TiO2 np irradiated with UV. Fig. 7(a)e(d) shows the representative SEM image of the rGOeTiO2 np composite. SEM image of rGO(1%)eTiO2 composite do not show clear cut presence of the rGO flakes. However, it shows the agglomerated TiO2 np. This may be due to very small amount of rGO w.r.t. TiO2 np and intercalation of rGO inside the TiO2 matrix. The nanoparticles are shown by arrow in Fig. 7(a) SEM image of rGO(2%)eTiO2 np composite shows the flakes of rGO and very small TiO2 np on its surface apart from of the clusters of the TiO2 np. The presence of rGO is shown by arrows in Fig. 7(b). The SEM Images of rGOeTiO2 np composite with higher content of rGO originating from reduction of GO shows somewhat similar feature as that of rGO(2%)eTiO2 np composite with 2 wt.% of GO, however they contain higher density of rGO. TEM images of the rGOeTiO2 np composite are shown in Fig. 8(a)e(d). TEM images of the rGOeTiO2 np composite shows the presence of rGO layers (marked by arrow in each

Fig. 7 e SEM micrographs of grapheneeTiO2 nanocomposite with (a) 1 wt.% (b) 2 wt.% (c) 5 wt.% and (d) 10 wt.% of prepared by the photocatalytic reduction by the ultraviolet radiation of GOeTiO2 suspension.

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Fig. 8 e TEM image of the graphene TiO2 nanoparticle composite with different wt.% of graphene originating from the reduction of GO. Black arrow shows the graphene intercalated in to the TiO2 nanoparticle matrix.

figure). The number of rGO layers were found to be get increased with increase in GO concentration. It was also found that the rGO are intercalated in the TiO2 matrix. Fig. 8(a) and (b) showed the presence of 2e3 layers with interplaner spacing of d ¼ 5Å. rGO(5%)eTiO2 np shows ~5 layers. However rGO(10%)eTiO2 np shows that TiO2 np gets wrapped by rGO.

The UVeVisible studies of the TiO2 np and rGOeTiO2 np composite have been recorded by the UVeVisible spectrophotometer (Perkin Elmer Lambda 750S) in diffuse reflectance mode. Fig. 9(a) shows the diffuse reflectance spectrum TiO2 np and rGOeTiO2 np composite. From diffuse reflectance spectrum, it is clear that the absorption in the rGOeTiO2 np composite increases with increase in the density of rGO

Fig. 9 e Diffuse relectance spectrum and transformed Kubelka-Munk function versus the photon energy for TiO2 nanoparticles and grapheneeTiO2 nanocomposite with 1 wt.%, 2 wt.%, 5 wt.% and 10 wt.% of GO.


Fig. 10 e Photoluminiscence spectrum of TiO2 nanoparticles and grapheneeTiO2 nanocomposite with 1 wt.%, 2 wt.%, 5 wt.% and 10 wt.% of GO.

originating from the reduction of GO. This may be due to the absorption of radiation by rGO. Diffuse reflectance spectra revealed that the there is no noticeable shift in the absorption. The only exception is the case of rGO(2%)eTiO2 np composite. For this shift in the visible region employing narrowing of band gap can be inferred. Since rGO do not get substituted in TiO2 np, narrowing of band gap for a specific concentration of rGO loading to a particular composite material is expected. The band gap for the TiO2 np and rGOeTiO2 np composite have been calculated from the plots of transformed KubelkaMunk function versus the energy of the light absorbed. The x intercept of the extrapolation of the linear region gives the value of bandgap. From Tauc plot the band gaps were calculated to be 2.90 eV, 3.0 eV 2.79 eV, 2.90 eV and 2.82 eV for TiO2 np, rGO(1%)eTiO2 np, rGO(2%)eTiO2 np, rGO(5%)eTiO2 np, and rGO(10%)eTiO2 np, respectively. It can be noted that the lowest band gap of 2.79 eV has been obtained for rGO(2%)e TiO2 np. Fig. 10 shows photoluminescence spectra of the TiO2 np and rGOeTiO2 np composite. The PL spectrum of TiO2 np

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shows the emission peak at 527 nm, 542 nm and 612 nm. All these peaks can be attributed to the radiative recombination of the photogenerated electron and hole pair. An abrupt emission quenching of TiO2 is observed in the PL spectra of all rGOeTiO2 np composite. The quenching of photoluminescence is apparently due to the transfer of photogenerated electrons from excited TiO2 to grapheme and effective separation of the charge carriers. It was also observed that further increase in the rGO originating from the reduction of 5 wt.% GO in rGOeTiO2 np composite gave almost similar photoluminescence. A feasible schematic diagram elucidating the charge behaviour at the interface of rGO (concentrations varying with the source material GO concentration) and TiO2 is shown in Fig. 11. Electrons are photoexcited to the conduction band of TiO2 under irradiation, leaving positive charged holes in the valance band. Without the presence of other materials, electrons will undergo a quick transition to the VB owing to the instability of excited states, resulting in the emission of fluorescence resulting in the poor photoelectrolysis efficiency. In the case of rGOeTiO2 np composite with different wt.% of GO, a heterojunction forms at the interface, where there is a space-charge separation region. Electrons tend to flow from the higher to lower Fermi level to adjust the Fermi energy levels [21]. As the calculated work function of graphene is 4.42 eV [58] and the CB position of anatase is about 4.21 eV with a bandgap of about 3.20 eV (using vacuum level as a reference) [59]. rGO can accept the photoexcited electrons from TiO2. This hinders the electronehole recombination. A typical variation of the photocurrent density vs. applied potential (E) for the photoelectrochemical cell with the electrode area of 0.5 cm2 in 0.5 M Na2SO4 has been shown by the representative IeV characteristics as in Fig. 12. The photocurrent densities were found to get increased with increase in the rGO concentration coming from 2 wt.% of GO. This elucidates the higher separation efficiency of photo-induced electrons and holes in the case of rGOeTiO2 np electrode. The enhanced photocurrent in the case of rGOeTiO2 np films can be explained in the following way. Upon UV illumination, photo-generated electrons and holes within the TiO2

Fig. 11 e Electron transfer mechanism in TiO2 nanoparticles and grapheneeTiO2 nanocomposite on irradiation.

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Fig. 12 e IeV characteristics graph of TiO2 nanoparticles anode and grapheneeTiO2 nanocomposite anode with 1 wt.%, 2 wt.%, 5 wt.% and 10 wt.% of GO.

np take part in redox reactions at the surface or recombine. The recombination process has a faster kinetics than the redox reactions and therefore controls the efficiency of the photoelectrolysis process. In the case of UV reduced rGO, photo-generated electrons are scavenged by the rGO and percolates to the FTO glass substrate, giving rise to larger photocurrent. The schematics are shown in Fig. 13. However, when rGO content was increased beyond 2% then photocurrent densities decreases. This is due to the mass absorption of incident radiation by rGO which limits the absorption of radiation by TiO2 leading to decrease in photocurrent density. The hydrogen generation rate was measured at an applied potential of 1 V vs. SCE. Fig. 14 shows the graph plotted between volumes of hydrogen produced vs. time. The hydrogen rate was calculated by the volume generated in one hour from the graph. From graph the hydrogen generation rate was found to be 39.5 mmol cm2 h1, 60 mmol cm2 h1, 127.5 mmol cm2 h1, 123.5 mmol cm2 h1 and 118 mmol cm2 h-1for TiO2 np, rGO(1%)eTiO2 np, rGO(2%)eTiO2 np, rGO(5%)eTiO2 np and (e)rGO(10%)eTiO2 np respectively.

Conclusion In summary, it can be said that rGOeTiO2 np composite were successfully prepared by the photo assisted reduction of

Fig. 14 e Volume of hydrogen evolved versus time for TiO2 nanoparticles anode and grapheneeTiO2 nanocomposite anode with 1 wt.%, 2 wt.%, 5 wt.% and 10 wt.% of GO.

GOeTiO2 np mixture using ultra violet radiation and employing ethanol as a solvent. The characterizations of the composite suggested that GO can be reduced to rGO by UV radiation. The photoluminescence characterizations of these composite reveal that rGO acts as an electron scavenger and inhibits the recombination of photoelectrons. The photoelectrochemical characterizations and hydrogen generation studies employing rGOeTiO2 np anodes with 0.5 M Na2SO4 electrolyte under 1000 W HgeXe lamp illumination show the increase in photocurrent up to ~3.5 mA cm2 for rGO concentration originating from the source material GO. The hydrogen generation rate from the above photoanode has been found to be ~127.5 mmol cm2 h1. Further increase in rGO content decreases the photocurrent and hydrogen generation rate because of increased absorption of light in rGO.

Acknowledgements The authors are grateful to Prof. C. N. R. Rao (F.R.S.), Prof. Lalji Singh (Vice Chancellor BHU) and Prof. S. B, Rai for their encouragements, support and fruitful discussions. The authors acknowledge with gratitude the financial supports from the Department of Science and Technology, Ministry of Science and Technology (UNANST: BHU), Ministry of New and Renewable Energy India Govt. of India, University Grants

Fig. 13 e Proposed mechanism for the charge transfer from TiO2 nanoparticles electrode and grapheneeTiO2 nanocomposite electrode.


Commission India and Council for Scientific and Industrial Research New Delhi. Author PKD acknowledges CSIR New Delhi for providing senior research fellowship (SRF).

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