Biphasic TiO2 nanoparticles decorated graphene nanosheets for visible light driven photocatalytic degradation of organic dyes

Accepted Manuscript Title: Biphasic TiO2 nanoparticles decorated graphene nanosheets for visible light driven photocatalytic degradation of organic dyes Authors: K. Alamelu, V. Raja, L. Shiamala, B.M. Jaffar Ali PII: DOI: Reference:

S0169-4332(17)31366-1 http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.054 APSUSC 35989

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Received date: Revised date: Accepted date:

31-1-2017 2-5-2017 7-5-2017

Please cite this article as: K.Alamelu, V.Raja, L.Shiamala, B.M.Jaffar Ali, Biphasic TiO2 nanoparticles decorated graphene nanosheets for visible light driven photocatalytic degradation of organic dyes, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.054 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.

Biphasic TiO2 nanoparticles decorated graphene nanosheets for visible light driven photocatalytic degradation of organic dyes

K. Alamelu, V. Raja, L. Shiamala, B.M. Jaffar Ali*

Bioenergy and Biophotonics Laboratory, Centre for Green Energy Technology, Pondicherry University, Puducherry – 605014, India.  

 

   

Graphical Abstract  

   

Highlights  Biphasic TiO2-graphene rendered enhanced visible light photocatalytic activity.  Graphene nanosheets reduced rate of recombination of electron-hole pair.  Resulted fifteen-fold increase in photodegradation of Congo red compared to TiO2.  Graphene-TiO2 developed to degrade both cationic and anionic dyes.

Abstract We present characterization of biphasic TiO2 nanoparticles and its graphene nanocomposite synthesized by cost effective, hydrothermal method. The structural properties and morphology of the samples were characterized by series of spectroscopic and microscopic techniques. Introducing high surface area graphene could suppress the electron hole pair recombination rate in the nanocomposite. Further, the nanocomposite shows red-shift of the absorption edge and contract of the band gap from 2.98 eV to 2.85 eV. We have characterized its photocatalytic activity under natural sunlight and UV filtered sunlight irradiation. Data reveal graphene-TiO2 composite exhibit about 15 and 3.5 folds increase in degradability of Congo red and Methylene Blue dyes, respectively, comparison to pristine TiO2. This underscores the marginal effect of UV component of sunlight on the degradation ability of composite, implying its increased efficiency in harnessing visible region of solar spectrum. We have thus developed a visible light active graphene composite catalyst that can degrade both cationic and anionic dyes and making it potentially useful in environmental remediation and water splitting applications, under direct sunlight.

Key words: Biphasic TiO2 nanoparticles; Graphene-TiO2 composite; Hydrothermal process; Congo red; Methylene blue; Photocatalysis; Photoelectrochemical studies.

1. Introduction Worldwide environmental pollution issues can be effectively addressed by semiconductor photocatalysis by using photocatalysts for water spliting, organic pollutant degradation and antibacterial activity [1]. TiO2 is the most studied photocatalyst owing to its use in hydrogen generation and high efficiency in removal of pollutants in air and water [2,3]. However, the photocatalytic efficiency of the TiO2 is limited due to its absorption majorly confined to UV range, having larger band gap (~3.2eV) of the material and relatively faster electron-hole recombination process. This limits its usefulness in harnessing 2-5% of the energy in the solar spectrum. Among the three phases of TiO2 (anatase, rutile and brookite), anatase is believed to be the most photocatalytically effective phase [4]. It is pertinent to note that the biphasic TiO2 consisting of anatase and rutile has been reported to exhibit higher photocatalytic activity compared to single phase TiO2 anatase.

Synergetic effect between the biphasic particles

attributed to suppress the rate of recombination [5,6]. To further enhance the utility of TiO2 photocatalyst it necessary to extend its photoactivity in the visible range of the solar spectrum [7]. Several strategies have been adapted to enhance the visible light absorbing capacity of TiO2 viz., doping with metal/non-metal elements [8–10], coupling with semiconductors [11], with noble metals [12–14], polymers and carbon based material such as carbon black [15], CNTs [16], and graphene [17,18]. In all these efforts, emphasis was on to minimize the rate of recombination of photogenerated electron, and generate more charge carriers. Recent studies on graphene-TiO2 system demonstrate, highly conducting graphitic graphene is a good candidate to minimise the rate of charge recombination. Therefore, combining the synergetic effect of mixed phase titania with graphene holds a potential to develop better photocatalyst. Graphene, an inexpensive, two dimensional sp2-hybridized novel carbonaceous material, has received much attention owing to its high surface area (~2600 m2/g), electron mobility (~ 15,000 m2/V.s) of charge carriers, chemical stability, optical absorption and mechanical strength [19,20]. The high electron mobility and

conjugation structure of graphene composites render it as

an ideal electron acceptor and good electron transporting bridge [12]. The combination of graphene and TiO2 found to decrease the electron-hole pair recombination and increase the

absorptivity in the visible region thereby increasing the photocatalytic efficiency [21,22]. It is well known that carbon materials have astonishing adsorption properties and hence are used in various environmental applications [23]. For instance, the graphene-metal composites have shown to be enhancing catalysis efficiency and are able to degrade both cationic and anionic organic dyes [24]. The photocatalytic activity of the material remarkably depends upon the adsorbing ability of the surface and transfer of electron-hole pairs. Organic dyes are mostly used in cellulose industries, cotton textile industries, paper industries and extensively used in industrial production, which generally contaminates environment [25]. These dyes can be anionic and cationic in nature, or can contain both anionic and cationic functional group. Studies shows that the distribution of surface charge of catalyst is key to the specific adsorption of the dye and its consequent degradation. In this report, we describe the hydrothermal synthesis of bandgap tuned biphasic TiO2 photocatalyst composite with graphene. We study the phase composition, crystal structure, morphology and chemical composition of the material in detail and its photocatalytic activity on both cationic and anionic organic dyes under natural and UV- filtered sunlight irradiation.

2. Experimental details 2.1. Reagents and material Graphite flakes, titanium tetraisopropoxide, n-propanol, potassium permanganate, sodium nitrate, sodium borohydride from Spectrochem, hydrogen peroxide (30%), sulphuric acid (98%), hydrochloric acid (35%), nitric acid (70%), ethanol, congo red and methylene blue from Merck were used as received. 2.2. Synthesis of graphene oxide Graphene Oxide was synthesized following modified Hummer’s method [26]. Briefly, 1.25 g of graphite flakes was oxidized by a mixture of 58 mL of H2SO4, 1.5 g of NaNO3 and 7.5 g of KMnO4. KMnO4 was added gradually with vigorous stirring for 1 h. The temperature was maintained below 5 ºC during the reaction. It was followed by raising the temperature to 35 ºC and stirred vigorously for 6 h. 20 mL of distilled water was then added drop-wise to this mixture.

The reaction mixture was then heated at 98 ºC with stirring for 30 min. 50 mL water, 1.5 mL of 30% H2O2 solution was added to the above mixture to stop the reaction. Yellow coloured GO suspension obtained was washed with dilute hydrochloric acid to remove metal impurities and then washed copiously with distilled water until the pH of the supernatant reached ~7 [27]. The obtained GO solution was dried in vacuum at 40 ºC for 24 h. 2.3. Synthesis of TiO2 nanoparticles To a mixture of 2 mL titanium tetraisopropoxide and 8 mL n-propanol, dilute nitric acid was added until it reaches pH 1. The obtained transparent solution was then transferred to the stainless steel autoclave (Teflon lined) and heated to 175 ºC for 12h. Thus formed precipitate was centrifuged, washed copiously with distilled water and dried overnight at 80 ºC in air followed by calcination at 400 ºC for 3 h [4]. 2.4. Synthesis of G-TiO2 nanocomposite To a mixture of distilled water/ethanol taken in 5:3 ratios, 50 mg of graphene oxide was added and sonicated for 1 h. To this GO solution about 450 mg of prepared TiO2 was added and stirred for 3h to homogenize the suspension. Thus formed suspension was taken in a 100mL Teflon lined stainless steel autoclave and heated to 120 ºC for 24h. The resulting composite was recovered by centrifugation and washed copiously with distilled water, and dried at 70 ºC for 12 h. The above hydrothermal processing of GO in ethanol water mixture resulted in reduction to Graphene and facilitated deposition of TiO2 over the graphene nanosheets [28]. We have also independently verified the reduction of GO to graphene following hydrothermal processing (Fig.S1, Supplementary Material). 2.5. Characterization The crystallographic structure and phase composition of the prepared samples TiO2 and G-TiO2 were determined by X- ray diffraction studies (Rigaku, Ultima IV) operated in the reflection mode with Cu Kα radiation (λ = 1.5406 Å). The surface morphology of the sample was examined by using field emission gun-scanning electron microscopy (FEGSEM, JSM-7600F). The properties of carbonaceous materials were identified using a confocal micro-Raman spectrometer with a laser beam of 514 nm (Renishaw, RM 2000). Fourier transform infrared spectra of the samples were obtained (Thermo Nicolet, 6700) with scan range of 4000-400 cm-1.

The optical property of the sample was measured using UV-vis spectrophotometer (Lambda 650 Perkin Elmer Spectrometer) having integrating sphere attachment, BaSO4 was used as reference. Photoluminescence spectra (PL) were recorded using Fluorolog-FL3-11Spectrofluorometer with the 325 nm excitation (JobinYvon). Time resolved photoluminescence decay rate was recorded using Fluorolog-FL3-11 295 nm femtosecond laser. Chemical state of photocatalytic material was analyzed by X-ray Photo Electron Spectroscopy (Kratos AXIS ULTRA) with Al-kα line. The specific surface area, pore size and pore volume of the samples was measured with Brunauer-Emmett-Teller (BET) method using the Micro-meritics Gemini VII 2390 by using N2 adsorption at 77 K. HRTEM images were taken using JEM2100 electron microscope, with a 200 kV accelerating voltage to study the particle morphology. Dye degradation profile has been studied using Ocean Optics spectrophotometer with LS1 light source, USB 2000 detector and SpectraSuite data acquisition system.

2.6. Photoelectrochemical Measurements The electrochemical experiment was performed using an electrochemical analyzer (BioLogicSP150) in a standard three electrode system with Ag/AgCl (saturated with KCl) and synthesized sample, Pt wire, as reference electrode, working electrode and counter electrode respectively. For the working electrode preparation, 100 mg of the photocatalyst (TiO2 and G-TiO2) was mixed with 30 mg of Polyethylene glycol (PEG, M.W. 20,000) and 1 ml of ethanol to make slurry. Then the slurry was coated onto a (1 cm * 1 cm) FTO glass plate using doctor blade method. Then slide was calcinated at 450 ºC for 1h under argon atmosphere. Electrochemical impedance spectra was measured by applying sinusoidal perturbations of 10 mV under a bias of 0.5V, and the frequency ranges from 0.1 Hz to 100 kHz in 0.1M Na2S and 0.02M Na2SO3 electrolyte solution. 2.7. Photocatalytic experiment The photocatalytic activities of samples TiO2 and G-TiO2 were evaluated separately based upon the discoloration of anionic Congo red and cationic Methylene Blue dyes in aqueous solutions separately. Briefly, for photocatalytic reaction, 0.4 mg/mL photocatalyst was suspended in the aqueous solution of Congo red (50 µM, 100 mL), Methylene Blue (20 µM, 100 mL) which was irradiated with natural sunlight between (April month) 11.45 am to 12.45 pm at 11.93o North,

79.13o East on southern-east coast of India. Adsorption-desorption equilibrium was attained before the photocatalytic reaction by keeping the solution in dark for 30 min under magnetic stirring. The reactor was covered with an UV filter (400 nm long pass) in order to receive only visible light spectrum to study visible-light-driven photocatalytic reaction.

During the

photocatalysis process, 3 mL of the aliquot was drawn at intervals 10 min for 1h. The suspended photocatalyst was removed by centrifugation, and the resulting solution was analysed with the UV-Visible spectrometer by recording the absorption band maximum of Congo red and Methylene Blue. The amount of dyes adsorbed by the photocatalyst was determined from the difference in the absorbance before and after adsorption [19]. To evaluate the reusability of the G-TiO2 photocatalyst, experiments have been performed by reusing the photocatalyst after every 1h degradation. The retrieved samples were dried and carefully weighed. To the catalyst, calculated amount of fresh dye solution from same stock was added to maintain dye to catalyst ratio constant. The process of reusing repeated five times and the degradation was monitored at every 10 min interval.

3. Results and discussion 3.1. XRD The crystallographic structures and phase composition of the materials were characterized by Xray diffraction. Fig. 1 shows the X-ray diffraction pattern of GO, TiO2 and G-TiO2. The XRD pattern of GO exhibits major peak at 2θ value of 11.12 corresponds to (002) plane with d-spacing value of 0.795 nm that is much higher than the d-spacing value of graphite, which indicates the oxidation of graphite to graphene oxide (GO) [29]. Diffraction pattern of TiO2 shows the major peaks around the 2θ values 25.3 and 27.5 corresponding to anatase (101) and rutile (110) phases respectively, confirming the biphasic nature of the synthesized materials. It must be noted that in order to facilitate significant rutile phase formation, extreme acidic pH was maintained [30]. Further, absence of peak at 2θ value of 11.12 in G-TiO2 shows GO is completely reduced. It is likely to be converted into graphene. However, it noted that no characteristics peak of graphene could be discernable from the XRD data. It may be due to screening of major peak of graphene (2θ=24.5) by the major characteristic peak of anatase (2θ=25.3) [29]. Table 1 summarises the percentage of anatase and rutile phases. The relative composition of the anatase and rutile

phases in the sample was obtained by comparing the intensities Aanatase(101), and Arutile (110) reflection planes using the following equation [31].

 A anatase  χ rutile = 1+   1.26 A rutile 

-1

Though the phase transformation of anatase to rutile contracted by the high surface area of graphene is being attributed to such observed increase in anatase phase by Aief et al., [31], analysis of relative intensity of XRD peak in our case reveal the overlapping of anatase phase peak with graphene only contribute to this apparent increase. It is emphasized that existence of rutile phase suggests preserving of biphasic composite of TiO2. Scherrer’s formula was used to calculate the crystallite size of the samples. The average crystallite sizes are 8.36 nm, 8.40 nm for TiO2 and G-TiO2 respectively. 3.2. UV-DRS The UV-Vis diffuse reflectance spectra (UV-DRS) of TiO2 and G-TiO2 samples are given in (Fig. 2). It can be seen that pristine TiO2 nanoparticles have greater absorption in the ultraviolet region which exhibit a small red shift upon introduction of the graphene due to the formation of Ti-O-C bond, which reduces the bandgap energy of the composite material [32,33]. At the same time, it is noted that the yellow coloration of the TiO2 sample changes to grey due to the formation of graphene composite (G-TiO2). The grey coloration was reported to be responsible for the higher absorption in the composite sample [15,34] as exhibited by the broad absorption spectra in the visible region. In order to deduce the bandgap of the materials, a Tauc plot was generated from the transformed Kubelka-Munk function as a function of energy of photons as shown in (Fig. S2 Supplementary Material). The band gap was determined to be 2.98 and 2.85 eV for TiO2 and G-TiO2 samples respectively. 3.3. FT-IR FT-IR spectrum of TiO2, GO and G-TiO2 materials are shown in (Fig.S3 Supplementary Material). In the FTIR spectra, broad bands were observed in the range of 400-1000 cm-1 for TiO2 which can be related to the bending and stretching vibrational modes of Ti-O-Ti bonds [35]. GO exhibits peak at 1094.6 cm-1 (C-O alkoxy stretching), 1229.8 cm-1 (phenolic C-OH

stretching). The band appearing at 1398.4 cm-1, 1729.8 cm-1 were due to carboxyl O-H groups and C=O carbonyl groups, respectively. The peak at 1628.5 cm-1 was assigned to the bending mode of H-O-H bonds of the adsorbed H2O molecules or in-plane vibrations of sp2-hybridized C-C bonding [36]. In the FTIR spectrum of G-TiO2composite, the intensity of the peaks due to hydroxyl and carboxyl groups were decreased significantly, which clearly indicates the formation of graphene nanosheets after hydrothermal treatment.

This is evident from the

removal of oxygen containing group and reduction of GO to graphene. The reduced signal at 1628.5 cm-1 for the C-O group in G-TiO2 suggests the existence of both Ti-O-Ti and Ti-O-C bonds in the composite, revealing the chemical interaction between surface hydroxyl group of TiO2 and functional group of graphene oxide [37,38]. 3.4. Raman analysis Raman spectra of GO, TiO2 and G-TiO2 are shown in Fig.3. Graphene oxide shows peak at ~1352 and ~1607 cm-1 corresponds to D- and G- bands, respectively.

G-band represent

characteristic of sp2 hybridized carbon materials, which can provide the information on the inplane vibration of sp2 bonded carbon domains [39]. The D-band relates to the disorder band associated with structural defects in graphene [12]. The peak intensity ratio ID/IG for GO was found to be 0.91. The Raman spectrum of the TiO2 and G-TiO2 exhibit peaks at 147.3, 401, 520.5 and 638 cm-1, which were assigned to Eg, B1g, A1g+B1g and Eg of anatase phase titania. The peaks at 248 cm-1 were attributed to the second order effective (Eg) of rutile titania [40]. Therefore, the Raman spectra further confirmed that the synthesized titania was composed of biphasic nature. Further, the intensity ratio ID/IG for G-TiO2 composite was increased to 0.96, which proved the reduction of GO during hydrothermal process. We can therefore infer that the composite material consists of graphene nanosheets decorated with TiO2 particles. 3.5. Photoelectrochemical Studies Electrochemical impedance spectra (Nyquist plots) derived for TiO2 and G-TiO2 composite are given in Fig.4a. Nyquist plot used to identify the charge carrier migration in a three electrode system. The semicircle corresponds to the charge transfer resistance at the sample electrode interface [14,41]. The equivalent circuit used for this analysis is given in inset to Fig.4a. The different component in the circuit represents the different electrochemical processes that occur in

the electrode –electrolyte interface. RΩ is the solution resistance is determined by the x-intercept of Nyquist plot and describes the overall resistance between the electrode and electrolyte [42]. The RΩ value determined to be 52.5Ω and 108.3Ω for TiO2 and G-TiO2 composite, respectively. The semicircle found in a middle frequency region corresponds to the interfacial charge transfer resistance (Rct). The charge transfer resistance has been reported to be mostly due to intra particle and inter particle resistance [42]. The fastest interfacial electron transfer and efficient separation of electron-hole pairs was indicated by the smallest semicircle.

This could be

discerned from the fact that presence of graphene made the electron transfer easier owing to its good promotion of interfacial electron transfer between the electrode and electrolyte interface [43]. In G-TiO2 nanocomposite, graphene served as an acceptor of the generated electrons by TiO2 that effectively decrease the charge recombination, promoting more charge carrier to form reactive species and thereby enhance the degradation of dye. It must me noted that graphene has excellent conductivity due to its two dimensional planar structure. Therefore, the rapid transport of charge carriers could be achieved and an effective charge separation subsequently accomplished [44]. It is also noted that results of the fit to impedance spectra (Fig. 4a) reveal higher Rct values for TiO2 (1.038 kΩ) compared to G-TiO2 (0.5003 kΩ). G-TiO2 composite gives smallest semicircle implying existence of faster interfacial electron transfer compared to that of pure TiO2. This finding gives credence to the existence graphene nanosheets decorated by titania particles. This strongly retards the recombination of charge carriers and there by resulting in a higher rate of photocatalytic activity. Further, it is also evident from bode-phase plot (Fig.4b) that the characteristic peak of the composite is shifted to the lower frequency range indicating the reduced rate of recombination and longer electron life time [45]. 3.6. Photoluminescence analysis The Photoluminescence (PL) spectra of TiO2 and G-TiO2 were shown in (Fig.5). The emission signal in the PL spectra at 367 nm and 398 nm correspond to direct and indirect transitions of TiO2 [46]. The emission band at 425 nm was assigned to the excitonic PL peaks trapped by defect of the material [47]. The quenching effect of the PL spectrum for G-TiO2 is the direct measure of electron–hole pair recombination rate.

Expectedly, the PL intensity of the

nanocomposite was decreased compared to pristine TiO2, due to excited electrons that are

trapped and transferred to the graphene sheets, which prevent the electron-hole pair recombination. The time resolved photoluminescence decay profile for TiO2 and G-TiO2 measuring the interfacial charge transfer rate are given in Fig.6. The emission decay was analysed and fitted to triple exponential function, the average charge transfer life time is determined using the following expression [48].   

A1 12  A2 2 2  A3 32 A1 1  A2 2  A3 3

Where τ1, τ2 and τ3 are lifetimes, and A1, A2 and A3 are corresponding amplitudes. Triple exponential function fitted fluorescence data are shown in Table 2. Represent τ1 and τ2 faster decay lifetimes arising from the recombination of the surface trapped electron hole pair, τ3 usually represents the delayed decay lifetime involving the recombination of the interstitially trapped electrons with the holes in metal doped systems [49]. Though interstitial carbon as carbonate is a possibility that can contribute to the delayed lifetime [50], we observe that pristine TiO2 also exhibiting delayed lifetime of similar magnitude. We therefore infer that in our case, dominant mechanism may be the rutile phase in the mixed phase titania preventing the carrier recombination due to difference in band gap leading to delayed lifetime. 3.7. XPS analysis The wide range X-ray photoelectron spectrum of G-TiO2 composite and TiO2 have been obtained (Fig S4a,b Supplementary Material), which indicates the existence of C, Ti, and O. It represents chemical binding energies at 284.6, 459.1 and 530.5 eV for C 1s, Ti 2p3/2 and O 1s, respectively. High resolution C1s spectra of G-TiO2 nanocomposite was shown in (Fig.S4c), C1s spectra of GTiO2was de-convoluted to four different peak centered at 284.6, 286.2, 288.3 and 289.6 eV corresponds to C-C, C-O, C=O and O=C-OH, respectively [51,52]. The intensity of all C1s peaks of the carbon binding to oxygen decreased when compared to that of GO sample which reveals the removal of most of the oxygen containing groups, and successful transformation of GO to graphene nanosheets (reduced graphene oxide) by hydrothermal method. Fig. S4d shows the Ti 2p spectra of G-TiO2. The Ti2p3/2 and Ti 2p1/2 peaks were centered at 459.1 and 464.8 eV

respectively. Moreover, due to the formation of a higher valence state Ti5+ peak centered at 461 eV was observed [53]. 3.8. FESEM and TEM studies The surface morphology and distribution of TiO2 nanoparticles on graphene were investigated with FESEM (Fig.S5 Supplementary Material) spherically shaped nanoparticles. In G-TiO2 composite the image reveal the formation of TiO2 nanoparticles decorated on graphene nanosheets (Fig.7a&b).

Composite, shows uniform distribution of TiO2 nanoparticles over

graphene sheets. The corresponding HRTEM image showed clear lattice fringes (Fig.7b), which is used to identify the crystallographic spacing. Fig.7c Shows the SAED pattern of G-TiO2 composite.

The fringes spacing 0.35 nm derived (Fig.7b) is matched [54] with (101)

crystallographic plane of anatase TiO2 (Fig.7c). Fig.9d gives the distribution of TiO2 particle size which was found to average around 10 nm. 3.9. Surface area and porosity measurement High surface area and large pore volumes are the important factors that can enhance the photocatalytic performance of the material. They contribute to increased adsorption of reactant molecules and fast transportation of products, increases the efficiency of separation of electronhole pairs thereby giving rise to enhanced harvesting of light [14,55]. Fig. 8 shows nitrogen adsorption-desorption isotherms of photocatalytic materials carried out at 77 K. The isotherms exhibit H3 hysteresis loops at a low relative pressure range of 0.6 to 1.0, correlating well with the narrow pore size distribution shown in the inset to the Fig. 8. Size distribution suggests that TiO2 and G-TiO2 samples are mesoporous material. The specific Brunauer-Emmett-Teller (BET) surface areas of TiO2, G-TiO2 and the pore size distribution and pore volume of the samples were determined from the desorption branch of the nitrogen isotherms by the BJH method (Table S1supplementary material).

It is observed that with the inclusion of graphene sheets, the

effective surface area of the composite material has increased significantly compared to pristine TiO2 material. 3.10. Photodegradation of organic pollutants

Photocatalytic activities of synthesized TiO2 and G-TiO2 were evaluated by monitoring the degradation of Congo red and Methylene Blue dyes under natural sunlight and UV filtered sunlight (>400nm) exposure at atmospheric pressure and temperature. percentage of dye degradation in terms of C/C0 was reported.

Accordingly, the

The normalized temporal

concentration changes (C/C0) of Congo red and Methylene Blue during the photodegradation was taken to be directly proportional to the normalized characteristic absorbance (A/A0), of the dye. About 10% of the dye was adsorbed on TiO2 surface, whereas 50% of the dye was found to be adsorbed on G-TiO2 surface prior to photodegradation (Fig.S6 Supplementary Material). High surface area of G-TiO2 sample might have facilitated adsorption of larger number of dye molecules than pristine TiO2 sample. The absorption spectra of Congo red and Methylene Blue aqueous solution exposed to natural sunlight and UV filtered sunlight for various time periods in the presence of as prepared. G-TiO2 was also obtained and provided in supplementary material (Fig.S7a-d Supplementary Material).

The time dependent degradation of Congo red and

methylene Blue dye under exposure of natural sunlight and UV filtered sunlight were plotted in (Fig. 9a-b). It was observed from (Fig. 9a) that Congo red was degraded efficiently by G-TiO2 to an extent of above 90% with and without UV spectrum in the sunlight in 1 h duration, suggesting UV spectrum had marginal effect on the dye degradability. It is noted that TiO2 could degrade anionic Congo red to the extent of 34 and 16 % in natural sunlight and UV filtered sunlight, respectively. However, for the case of G-TiO2 composite, it was about 8.6 fold and 15 fold increase in degradability. This reiterates the increased photocatalytic activity of TiO2 upon composite formation with graphene. Similarly, relative increase in degradability for cationic Methylene Blue found to be of the order of 1.7 fold and 3.5 fold for G-TiO2 (Fig. 9b). Table 3 summarises the photocatalytic performance of TiO2 and G-TiO2 samples under natural sunlight and UV filtered sunlight. It is emphasized that the inability to efficiently degrade anionic Congo red by pristine TiO2 has been transformed into efficient Congo red degrading catalyst in the form of G-TiO2. It is noteworthy that the G-TiO2 composite is therefore capable of degrading both anionic and cationic dyes, albeit to a different extent. Importantly, it is accomplished in the visible region of the solar spectrum.

Graphene-TiO2 composite using pure anatase has been reported to enhance the catalysis [14] however, the aborption edge was reported to be around 400 nm. In the studies involving commercially procured P25 as well as synthesized mixed phase titania, importance of using mixed phase titania above P25 composition has been reiterated (7). In this work, a cost effective and simple synthesis procedure to obtain biphasic titania having ~30% rutile is being reported. Results on comparison to various G-TiO2 composites indicate enhanced photocatalytic property being observed (Table 4) for catalyst that is significantly different from P25. It must be noted that studies on pure rutile phase titania also been reported that yield better photocatalysis compared to P25, albeit under specific structural morphology [56]. In this work, by choosing optimal composition ratio of anatase to rutile, an attempt was made to enhance the utility bandwidth of solar radiation in the photocatalysis of composite. Expectedly, using UV filtered sunlight, 15 fold increases has been established. It is known that photocatalysis is sensitive to the pH of dye solution. Specially, pH plays a crucial role in the degradability of cationic and anionic dyes [57,58]. Typically, acidic pH degrades anionic dye and basic pH is shows to be induce for cationic dye degradation. In our studies, the pH of the solution was maintained at 7.2.

We have compared the relative

degradation of dye at this pH. This open up the possibility to study the photodegradability by the catalysts in wide range of pH to identify optimal pH for degradation, particularly in mixed dye solution. The results of reusability of photocatalyst in the degradation of Congo red and Methylene Blue dyes were summarized in (Fig.S8 Supplementary Material). In brief, over repeated use of phtocatalyst in five cycles of 1 h duration, we observe about 10% decrease is efficiency of degradation compared to the first cycle for Congo red. In the case of Methylene Blue it was about 3% decrease only. The data suggest satisfactory performance on the repeated use of the photocatalyst. The catalytic decomposition of the dye could be assigned to a pseudo-first order kinetics reaction with a simplified Langmuir-Hinshelwood model [59,60].

C  ln   Co

   kt   

Where C0 is the initial concentration of the dye before adsorption, C is the temporal concentration, and t is the corresponding reaction time, k is the apparent first-order kinetics rate constant, and was determined from a linear fit to the data as shown in (Fig.S9 a-b). The Congo red and Methylene Blue degradation rate constants for various photocatalytic material studied under natural and UV filtered sunlight was given in Table 3. It could be seen that rate of degradation of dyes are 2.5 fold more for TiO2 under UV-VIS irradiation in comparison to pure visible light. This reduced to about 1.5 fold for G-TiO2 composite, suggesting less activity under UV irradiation. Interestingly rate of degradation of Congo red increases to 15 fold more in composite material compared to pristine TiO2 under pure visible light. Similarly, in the case of Methylene Blue it is about 3.5 times. This establishes the formation of more efficient and stable composites for degradation of Congo red and Methylene Blue. 3.11. Mechanism of photocatalytic degradation of organic dye by graphene composite The schematic illustration of the mechanism of enhanced photocatalytic activity for GrapheneTiO2 composite is depicted in (Fig 10). Based on the experimental data, we propose the photodegradation mechanism of organic dye by Graphene composite as: [61–64].

Step:1 TiO 2 + h  e-CB + h +VB + Graphene(e- ) + h + Step:2 Graphene(e- ) + O 2  • O-2 + Graphene •

H2O O-2 + Graphene   • OH + Graphene

 

Step:3 TiO 2 e-CB + O 2  • O-2 + TiO 2 H2O • O 2 + TiO 2   • OH + TiO 2



TiO 2 h +VB

 + OH

Organic dye

ads

ads

 • OH + TiO 2

+ • OH  Degraded products

Step1: Under sunlight irradiation, electrons were excited from the valence band to conduction band of TiO2, creating holes on the valence band. Since the photocatalyst is biphasic TiO2 – graphene composite, in this biphasic condition the band gap of rutile is favorable for visible light excitation as the conduction band edge of rutile lies below 0.2 eV compared to the conduction band edge of anatase. Step 2: The excited electrons travels freely along the conducting network

of graphene sheets, and subsequently get transferred to the surface to react with water and oxygen to yield hydroxyl radicals that in turn can oxidize organic dye. The excited electrons are transferred to the graphene from the surface of TiO2, allowing charge separation and hindering the electron-hole pair recombination [61]. This is further aided by the antenna effect of rutile phase wherein visible light excited photogenerated electron from conduction band of rutile gets transferred to trapping sites of anatase phase. It is noted that the lattice trapping sites of anatase are located about 0.8 eV below the conduction band edge of anatase [65]. These trapping sites prevent recombination to a large extent, thus facilitating the charge separation thereby activating the catalyst [66]. Step 3: The electrons reduce the dissolved oxygen, leading to the formation of superoxide (O2-) ions, which reacts with water to give hydroxide radicals [12]. These hydroxide radicals degrade the Organic dye, which remains adsorbed on the surface of graphene composite in increasing amount due to active sites. On the other hand, holes on the valence band of TiO2 breaks water molecules to form hydroxyl radicals and degrade the dye molecules directly [12].

4. Conclusion In brief, we have reported a simple and efficient methodology to prepare biphasic TiO2 nanoparticles and G-TiO2nanocomposites following hydrothermal method. We gather evidence from photoluminescence and UV-DRS study that supports improvised optical properties in GTiO2 composite, facilitating better photocatalytic activity compared with pristine TiO2. Distinct shift in adsorption edge from 416 nm to 436 nm resulted in decreased bandgap of the material and increased utilization of the solar spectrum. Our studies show the enhanced photocatalytic performance of these materials stems from significant decrease in recombination of surface trapped electrons in the composite. It is emphasized that the G-TiO2 material is rendered as much as 15 fold increase in the performance and, it could be utilized for both cationic and anionic dyes. Various pieces of evidence in morphological analysis give credence to the existence of graphene nanosheets decorated by ~10nm TiO2 particles. We therefore conclude that the G-TiO2 composite developed here could cover broader spectrum of solar radiation and effectively degrade both anionic and cationic dyes, and thus holds great potential in harnessing sunlight for application related to environmental remediation of toxic and pollutant substances.

Acknowledgements 

The authors gratefully acknowledge Central Instrumentation Facility (CIF) of Pondicherry University for the instrumentation facilities availed. The authors also acknowledge Dr.K.Ramesh, Indian Institute of Science, Bangalore for his help with XPS measurement. The author KA acknowledges the MNRE-NREF, Government of India for Research Fellowship.

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Figure Captions: Fig. 1. XRD pattern of synthesized GO, TiO2 and G-TiO2 composite. Fig. 2.

Normalized UV-Visible diffuse reflectance spectra (DRS) of TiO2 and G-TiO2

composite. Fig. 3. Raman spectra of GO, TiO2 and G-TiO2 composite. Fig. 4. (a) Nyquist plots of TiO2 and G-TiO2 composite; (b) Bode-phase plot of TiO2 and G-TiO2 composite. Fig. 5. Photoluminescence spectra of TiO2 and G-TiO2 composite. Fig. 6. Time resolved photoluminescence spectra of TiO2 and G-TiO2 composite. Fig. 7. Microscopic characterization of G-TiO2 composite: (a) TEM image, (b) HRTEM image, (c) SAED pattern and (d) Particle size distribution. Fig. 8. Typical nitrogen adsorption-desorption isotherm of TiO2 and G-TiO2. Inset to the figure gives its pore size distribution. Fig. 9. Normalized rate of removal of dyes by TiO2 and G-TiO2 under natural and UV filtered sunlight irradiation for: (a) Congo red; (b) Methylene Blue. Fig. 10. Depiction of proposed mechanism of organic dye degradation by G-TiO2 composite.

   

FIGURE 1

FIGURE 2

FIGURE 3

(b)

FIGURE 4

FIGURE 5

FIGURE 6

FIGURE 7

FIGURE 8

FIGURE 9

FIGURE 10  

 

Table Captions:

Table.1. Estimated crystallite size and relative composition of anatase and rutile phases in the synthesized materials. Table.2. Kinetic parameter of time resolved Photoluminescence of TiO2 and G-TiO2 composite. Table.3. Derived rate constants for Congo red and Methylene Blue degradation by TiO2 and GTiO2, under natural and UV filtered sunlight. Table.4. Degradation percentage of dye obtained by different phase of Graphene-TiO2 composite.

Table 1 Photocatalyst

                           

Anatase phase (%)

Rutile phase (%)

Crystallite Size (nm)

TiO2

71.4

28.6

8.36

G-TiO2

73.1

26.9

8.40

             

Table 2

                 

χ2

Photocatalyst

τ1 (ns)

TiO2

0.39

32.92

2.79

8.74

41.64

58.34

41.05

1.59

G-TiO2

0.39

41.32

2.79

8.85

40.27

49.82

39.50

1.75

A1 (%)

τ2

(ns)

A2 (%)

τ3 (ns)

A3 (%)

τav (ns)

               

Table 3 Photocatalyst

Rate of degradation of Congo red k (min-1) kUV-Vis / K Vis

UV-Vis exposure

Pure visible light

light

exposure

TiO2 (A)

0.0063

0.0024

2.62

G-TiO2 (B)

0.0546

0.0361

1.51

B/A

8.6

15

Rate of degradation of Methylene Blue k (min-1)

Photocatalyst

kUV-Vis / K Vis

UV-Vis exposure

Pure visible light

light

exposure

TiO2 (A)

0.0214

0.0083

2.57

G-TiO2 (B)

0.0363

0.0294

1.23

B/A

1.7

3.5

Table 4

Photocatalyst

Lightsource

Catalyst Amount

Dye

% Time removal (hour)

Figure of Ref merit (Cat./Dye) X h

G-TiO2 (P25) G-TiO2 (Anatase) G-TiO2 (P25) G-TiO2 (Anatase) G-TiO2 Mixed phases G-TiO2 Mixed phases G-TiO2 Mixed phases

   

500W Xenon lamp 40 W UV light 500 W Xenon lamp 500 W Xenon lamp Sunlight

0.75g/L

Solar Simulator Sunlight

0.6g/L

1g/L 0.4g/L 0.5g/L 0.4g/L

0.4g/L

8.6mg/L MB 10mg/L MB 3.2mg/L MB 10mg/L MB 6.5mg/L MB 10mg/L RhB 35mg/L CR

65

1.00

87

67

85.2

1.00

100

1

100

2.00

250

7

82.5

1.00

50

17

93

1.00

62

Present work

98.8

1.33

80

31

97.5

1.00

12

Present work