TiO2 nanocomposite for effective photocatalytic degradation of methylene blue under sunlight

TiO2 nanocomposite for effective photocatalytic degradation of methylene blue under sunlight

Nano-Structures & Nano-Objects 21 (2020) 100407 Contents lists available at ScienceDirect Nano-Structures & Nano-Objects journal homepage: www.elsev...

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Nano-Structures & Nano-Objects 21 (2020) 100407

Contents lists available at ScienceDirect

Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso

Ultrasound assisted preparation of rGO/TiO2 nanocomposite for effective photocatalytic degradation of methylene blue under sunlight Sayali P. Deshmukh a , Devyani P. Kale a , Shashwati Kar a , Sachin R. Shirsath b , ∗ Bharat A. Bhanvase a , , Virendra Kumar Saharan c , Shirish H. Sonawane d a

Department of Chemical Engineering, Laxminarayan Institute of Technology, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur 440033, MS, India b Chemical Engineering Department, Sinhgad College of Engineering, Pune 411041, MS, India c Chemical Engineering Department, Malaviya National Institute of Technology, Jaipur 302017, Rajasthan, India d Department of Chemical Engineering, National Institute of Technology, Warangal 506004, Telangana State, India

article

info

Article history: Received 26 May 2019 Received in revised form 17 October 2019 Accepted 19 November 2019 Keywords: Ultrasound rGO/TiO2 nanocomposite TEM XPS Methylene blue

a b s t r a c t In the present work, well known Hummers–Offeman method was employed for production of graphene oxide (GO) and in-situ deposition of TiO2 nanoparticles on prepared GO in presence of ultrasonic irradiations to get reduced graphene oxide-TiO2 (rGO/TiO2 ) nanocomposite. The structural and morphological analysis of synthesized photocatalysts was accomplished with UV–Vis, XRD, FT-IR, Raman Spectra, EDAX, XPS and TEM analysis. The absorption peak at 234 nm shows redshift to 285 nm which confirms the reduction of GO to rGO. XRD analysis of the prepared composite confirmed the presence of the combination of anatase and rutile phases of TiO2 . TEM images revealed that large amount of round-shaped TiO2 nanoparticles in size range of 3 to 5 nm were consistently deposited on the rGO sheet due to ultrasonic irradiations. Further, effectiveness of the prepared nanocomposite as a photocatalyst was examined with the decolourization of methylene blue (MB) dye in sun light. The effect of catalyst loading and pH on MB dye degradation was examined. The results indicated that the percent degradation of selected MB dye enhanced at higher catalyst loading and also a higher pH favoured the degradation. The maximum MB dye degradation was observed to be 91.3% within 30 min for pH value of 13.2 and photocatalyst dosage of 2 g/L. Further, the kinetic studies established the pseudo first-order reaction kinetics for the photocatalytic decolourization/degradation of MB dye. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Graphene is a hexagonal lattice having sp2-bonded carbon atoms and is stable under ambient conditions. Graphene has got exceptional thermal, electrical, mechanical and optical properties and is particularly important for applications in electronics, materials science and condensed-matter physics [1]. Additionally, graphene exhibits semimetal behaviour because of small overlay between the valence and conduction bands. The composite materials based on graphene find uses in varied fields such as drug delivery, nanoelectronic devices, catalysis, biomaterials etc. [2]. The production of graphene is easier and it can be produced from natural graphite which is inexpensive through GO [3,4]. Titanium dioxide (TiO2 ), a semiconductor photocatalyst, is an important material in material chemistry. Owing to the properties like suitable band gap energy, strong oxidizing power, durability against photo and chemical corrosion, chemical inertness, electronic and ∗ Corresponding author. E-mail address: [email protected] (B.A. Bhanvase). https://doi.org/10.1016/j.nanoso.2019.100407 2352-507X/© 2019 Elsevier B.V. All rights reserved.

optical properties TiO2 has been extensively examined [5–7]. On account of its high efficiency and high stability it is extensively used in various applications like hydrogen production [8], gas sensors [9], heterogeneous photocatalysis [10] and dye-sensitized solar cells [11]. Apart from this, it is also used for paints, filter materials, cosmetics, anti-reflection films, electronic paper etc. [12,13]. Moreover, titanium dioxide can be easily produced and used. It efficiently catalyses reactions, is cheap and does not pose any risk to the environment or humans [14]. Further, it has been extensively used in wastewater treatment as a photocatalyst. However, TiO2 has the disadvantage that it is active in UV irradiation, whereas the quantity of UV radiations is only 5% of the solar spectrum. In addition, the recombination rate of electrons and holes is higher in case of TiO2 . Therefore, it has less photocatalytic activity under visible sunlight, which necessitates the development of new photocatalyst materials delivering high reactivity in visible light (λ > 400 nm) that covers maximum percentage of the solar spectrum [15]. Further, the photoreactivity of TiO2 is extremely labile and blending with other mineral and organic materials, modifications

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in crystal structure, use of coatings can increase or decrease activation energies and lifetimes [16]. Recently, to intensify the photodegradation capability of semiconductor oxide photocatalysts, coupling with other semiconductor, doping with metal ions and anchoring TiO2 particles onto materials having enormous surface area, like zeolites, carbon-based materials or mesoporous materials is being done [17]. Nanocarbon materials such as C60 and carbon nanotubes have encouraging effects on the photocatalytic activity of prepared homogeneous and heterogeneous semiconductors by the virtue of better electron mobilization and interaction effects [18,19]. Thus, one of the way to overcome the limitations of TiO2 is by preparing graphene-TiO2 nanocomposite photocatalyst. Since the electron mobility in case of graphene materials is outstanding, it significantly improves the transference of the formed electrons during activation in the presence of light and suppresses the electron and hole recombination [20–23]. Graphene has a honeycomb structure with higher transparency and electric charge carrier mobility and also larger surface area. These photonic and electronic properties [24] make it a suitable material for enrichment of TiO2 photoreactivity [25]. During the production of graphene-TiO2 nanocomposite, the existence of oxygen-related functional groups like -OH and -COOH in GO act as ideal supporters to load TiO2 nanocrystals [26]. Furthermore, the combination of graphene with inorganic metal oxide nanoparticles allows one to tailor the properties of the formed nanocomposite for precise applications and is gaining lot of interest as the nanocomposites exhibiting properties that are rarely observed in the counterpart components [27]. Many researchers have worked on the development of graphene-TiO2 composites, but most of the work involves the preparation of composite using conventional methods. Different methods such as sol–gel [28], hydrothermal reduction [27], ion exchange, intercalation [29], chemical vapour deposition [30], electrochemical method [31], inner modification then in situ reduction are practised to incorporate nanoparticles into graphene sheets [32,33]. Chang et al. [34] have used hydrothermal method with various reaction temperatures for the preparation of rGO/TiO2 nanocomposite and the prepared rGO/TiO2 nanocomposite was used for the degradation of methylene blue in the presence of sunlight that showed better performance towards photocatalytic degradation. Siong et al. [35] reported one step solvothermal technique for the preparation of rGO/TiO2 nanocomposite for efficient degradation of the MB dye under simulated solar light and showed better degradation of MB dye with 0.15 g GO loading in the final nanocomposite. Loryuenyong et al. [36] reported the fabrication of rGO/TiO2 nanocomposite with the use of UV-assisted photocatalytic reduction of GO with TiO2 nanoparticles in ethanol and reported 500% enhancement in the results with the use of prepared nanocomposite compared to TiO2 alone. Benjwal et al. [37] have used one step solvothermal process for the preparation of binary rGO–TiO2 /rGO–Fe3 O4 and ternary rGO–Fe3 O4 –TiO2 nanocomposites which reported higher degradation efficiency of MB dyes. Raghavan et al. [38] further used two step solvothermal process for the preparation of rGO/TiO2 /ZnO ternary photocatalyst and reported 92% and 68% degradation of MB dye with rGO/TiO2 /ZnO ternary and rGO/TiO2 binary photocatalyst in 120 min. respectively. Ali et al. [39] have demonstrated the application of rGO–TiO2 nanocomposite for the degradation of Rhodamine-B dye. MB dye is a coloured cationic dye and it has wide applications in various textile industries. The health hazards associated with MB dye are breathe hazards. Its direct exposure creates several health problems like harm to eyes, burning, sickness, vomiting, mental disorders etc. [40,41]. Further, in case of nanocomposites, incorporated nanoparticles must get distributed uniformly on the surface of the graphene so that they act as effective spacers. Hence, precise control on

the formation and spreading of nanoparticles on the graphene is crucial. Therefore, the application of ultrasound assisted method for the preparation of rGO–TiO2 nanocomposite is crucial and expected to give better distribution of the nanoparticles on rGO nanosheets. The ultrasound has played a vital role in the adequate dispersion, size reduction of particles, de-agglomeration and even distribution of nanoparticles on the support during the preparation of nanomaterials such as alloys, high-surface area transition metal oxides, colloids and carbides [42,43]. Due to ultrasonic irradiations, the cavities are generated in the irradiated solution. The cavitation is the formation, growth and consequent collapse of microbubbles or cavities occurring in milliseconds releasing tremendous amount of energy in the reaction medium. Very high local temperature (>5000 K), pressure (>20 MPa) and extremely high cooling rates (>1010 K/s) are the prime advantageous parameters in the self-assembly and crystallization of nanostructured materials. Various physical changes and chemical reactions occur due to extreme conditions and nano-structured materials of several types can be successfully produced with desired particle size and shape. High-velocity inter-particle collisions occur leading to the breaking up of the particles to smaller size [44–46]. To exploit these advantages of ultrasound, rGO/TiO2 nanocomposite particles were synthesized with the use of ultrasound assisted approach. The prepared nanocomposite was thoroughly examined by using UV–Vis spectroscopy, FTIR, XRD, Raman Spectra, EDAX, XPS, and TEM analysis. Methylene blue dye was considered as a model pollutant to study the photocatalytic degradation capability of the synthesized nanocomposite under sunlight. Also, effect of catalyst loading and pH on percent decolourization/degradation of MB dye was investigated. 2. Experimental 2.1. Materials Firstly, sonochemical production of GO and then rGO/TiO2 nanocomposite was accomplished using 98% sulphuric acid, sodium nitrate, 30% hydrogen peroxide, HCl, potassium permanganate, NaOH and graphite powder, which were obtained from Loba Chemie Pvt. Ltd., India. Iso-propanol and titanium (IV) isopropoxide was obtained from Sisco Research Laboratories, India. Methylene blue was purchased from Loba Chemie. Pvt. Ltd., India and was used to prepare dye stock solution. All the A.R. chemicals were utilized as received and distilled water was used for all experiments. 2.2. Sonochemical preparation of GO The reported modified Hummers’ method by Deosarkar et al. [47,48] in the presence of ultrasound (Make: Dakshin, 22 KHz, 240 W, Probe diameter 20 mm) was used for the production of GO from graphite powder. In order to prepare graphene oxide in presence of ultrasound, graphite powder and sodium nitrate 1 g each were added in 46 mL of concentrated sulphuric acid and sonicated for 10 min at 4 ◦ C. Afterwards, steady addition of 5 g KMnO4 was accomplished to the above sonicated solution at temperature around 4 ◦ C. The obtained suspension was further ultrasonicated for 30 min at room temperature. To this sonicated suspension, addition of 500 mL distilled water was accomplished and it was further sonicated for 5 min. The obtained mixture was then treated with 8 mL hydrogen peroxide (30%) solution. The separation of the product i.e. graphene oxide was carried out by centrifugation (REMI PR-24) after multiple washing with 150 mL of 10% HCl solution and 100 mL distilled water and was dried in oven. Synthesized graphene oxide was further utilized for the sonochemical preparation of rGO/TiO2 nanocomposite.

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2.3. Sonochemical production of rGO/TiO2 nanocomposite Sonochemical production of finely dispersed rGO/TiO2 nanocomposite was accomplished with the use of above mentioned probe type ultrasonicator. For the same, initially 1 g of ultrasonically prepared graphene-oxide was dispersed in 50 ml of iso-propanol and subsequent addition of 5 ml of titanium (IV)isopropoxide was accomplished. The resulting reaction mixture was sonicated for 5 min. To this mixture, prepared NaOH solution (2.7 g of NaOH in 50 ml of distilled water) was added slowly in first 15 min in the presence of ultrasonication and then this mixture was again sonicated further for 30 min. The product separation was carried out with the use of centrifugation after distilled water washing. The resultant rGO/TiO2 nanocomposite was air dried for 24 h and then in oven at 70 ◦ C for 1 h. The obtained rGO/TiO2 nanocomposite was analysed with the use of UV/Vis, TEM, XRD, XPS, Raman and FTIR analysis. 2.4. Characterization UV/Vis spectrum of ultrasonically prepared rGO/TiO2 nanocomposite was obtained with UV/Vis Spectrophotometer (LABINDIA UV3200 model). XRD analysis of ultrasonically prepared rGO/TiO2 nanocomposite was accomplished by using powder Rigaku Mini-Flex X-ray diffractometer. FTIR spectrum of sonochemically prepared rGO/TiO2 nanocomposite was obtained using Fourier Transform Infrared Spectrophotometer (Shimadzu-IR Affinity-1, Japan). Raman spectrum was obtained on STR-500 Confocal Micro Raman Spectrometer (AIRIX Corporation). Elemental Map images and EDAX analysis of rGO/TiO2 nanocomposite were obtained using Transmission Electron Microscopy (Tecnai G2 20, FEI Company). The TEM images were obtained with the use of Transmission Electron Microscope (Make: Hitachi, Japan, Model: H-7500, 40–120 kV, magnification 6,00,000X). XPS analysis was done using an Omicron ESCA instrument (Electron Spectroscope for Chemical Analysis), Germany. The concentration of methylene blue in the solution was quantified by a UV–Vis spectrophotometer (LABINDIA UV3200 model).

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HCl and 0.1 N NaOH solutions. The % dye removal was obtained from Eq. (1). % Dye MB Removal =

C0 − C C0

× 100

(1)

where, C0 = Initial concentration of MB dye, C = Concentration of MB dye at given time. 3. Results and discussion 3.1. Formation mechanism of rGO/TiO2 nanocomposite with ultrasonication The formation mechanism of rGO/TiO2 nanocomposite under the influence of ultrasonic irradiations is depicted in Fig. 1. Firstly, pristine graphite was used and was treated with oxidizing agents in the presence of ultrasound, which is called as modified Hummers’ method. Due to this treatment, the oxidized graphene is formed which has epoxide, hydroxyl and carboxyl groups decorated in it. The available carboxylic moieties forms bonding with metal cations by various phenomena like physical adsorption, electrostatic binding or charge transfer [49–51]. As depicted in Fig. 1, Ti ion source (titanium (IV)-isopropoxide) was added in 2-propanol in the presence of ultrasound and GO. There is a possibility of interaction of Ti ion with carboxylic groups present in graphene oxide and can react to form TiO2 which is attached on the exfoliated surface of rGO. Further, the use of ultrasonication facilitates the exfoliation to form graphene oxide and allows even and fine distribution of TiO2 nanoparticles on the surface of reduced graphene oxide. Also the ultrasonication facilitates homogeneous dispersion of the solutes and enhances the diffusion of Ti ions in the reaction medium which leads to establishment of Ti–O–C bonds on surface of rGO. The aggregation of Ti ions on rGO surface is opposed by the physical and chemical effects of cavitation which leads to uniform distribution of TiO2 in formed rGO/TiO2 nanocomposite.

2.5. Photocatalytic degradation of MB dye

3.2. Physicochemical properties of ultrasonically prepared rGO/TiO2 nanocomposite

The photocatalytic degradation of MB dye using as synthesized rGO/TiO2 nanocomposite photocatalysts was performed in batch mode. In all the experiments, 100 ml of MB dye solution of predetermined concentration was taken in a reactor and calculated quantity of rGO/TiO2 nanocomposite photocatalyst was added to it. The solution of MB dye and rGO/TiO2 nanocomposite photocatalyst was subjected to vigorous stirring in a reactor under darkness for 15 min to attain the equilibrium and then transferred to the photocatalytic reactor. The MB dye degradation experiments were carried out in the presence of sunlight during daytime between 11 am to 3 pm when solar intensity fluctuations were minimum. The samples were collected with respect to time, immediately centrifuged and were analysed with the use of UV– Vis spectrophotometer to get the concentration values of MB dye solution. The maximum wavelength selected for MB dye concentration measurement was 670 nm. In this case, distilled water was used as the reference. The study of effect of catalyst loading (1–3 g/L) and pH on the decolourization/degradation of MB dye was accomplished. The effect of catalyst loading (1–3 g/L) was examined at fixed temperature 35 ◦ C, initial dye concentration of 20 ppm and at 6.44 pH. The effect of pH on decolourization/degradation of MB dye was investigated at various pH values such as 2.3, 4, 11.9 and 13.2. The pH of the dye solution was varied using 0.1 N

3.2.1. UV-visible analysis The light-absorbance properties of the prepared TiO2 , GO and rGO/TiO2 nanocomposite samples were analysed by UV–Vis spectroscopy and resulted spectrums are depicted in Fig. 2. The characteristics peaks at 234 and 301 nm are caused due to π →π ∗ and n→π ∗ transitions of aromatic C–C bonds and C==C bonds, respectively [52], which approves the formation of GO in the presence of ultrasound. Further, for rGO/TiO2 nanocomposite, the absorption peak at 234 nm show redshift to 285 nm and characteristics peak at 301 nm gets disappeared which is a characteristic of rGO and is caused due to the aromatic C==C bonds and removal of oxygen functionalities present in form of –OH and –COOH functional groups on the surface of GO. Further, the sharp absorption edge was observed at around 360 nm for rGO/TiO2 nanocomposite, which is for TiO2 loaded on rGO nanosheets. It can be observed that the nanocomposite particles reveal a strong absorption in the visible light range. The better light-harvesting ability of rGO–TiO2 nanocomposite can be caused due to the existence of chemical bonding between TiO2 and rGO, i.e. Ti–O–C, that is responsible for the charge transfer under visible light excitation [17]. Reduced graphene oxide-TiO2 composite sample showed an absorption edge around 375 nm which is typical absorption of TiO2 .

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Fig. 1. The formation mechanism of rGO/TiO2 nanocomposite by ultrasound assisted method.

Fig. 2. UV–visible absorption spectrum of ultrasonically prepared TiO2 , graphene oxide and rGO/TiO2 nanocomposite photocatalyst.

Fig. 3. XRD patterns of ultrasonically prepared TiO2 , graphene oxide and rGO/TiO2 nanocomposite photocatalyst.

3.2.2. XRD analysis To gain a better insight into the intrinsic properties of the nanocomposite, detailed characterization of rGO/TiO2 nanocomposite was accomplished. The XRD is considered to perform the structural analysis of the synthesized nanocomposite. Fig. 3 depicts the XRD patterns of the GO, TiO2 and sonochemically prepared rGO/TiO2 nanocomposite photocatalyst. The peak at 10.2o confirms the formation GO which is attributed to oxidation of graphite. The observed characteristics peaks at 2θ = 25.90, 47.81, 52.85, 56.07 and 61.99◦ could be attributed to the anatase phase. Whereas the characteristics peaks at 2θ = 27.35, 36.27, 39.41, 44.03 and 57.32◦ are conforming the rutile phase [49]. Thus, the XRD analysis of the prepared composite confirmed the presence of the combination of anatase and rutile phases of TiO2 . Moreover, a peak at 10.2o was found to be disappeared and broad diffraction peak at 2θ = 24.31◦ reveals the effective reduction of GO to rGO during the ultrasound assisted preparation. This peak confirms the formation of rGO with uniformly dispersed TiO2 demonstrating effective construction of rGO/TiO2 nanocomposite photocatalyst [20]. Further, the lower crystallinity i.e. less intensities of characteristic peaks is attributed to the use of ultrasonication during the preparation of the nanocomposite [53].

3.2.3. FTIR analysis Fig. 4 displays the FT-IR spectra of ultrasonically prepared TiO2 , graphene oxide (GO) and rGO/TiO2 nanocomposite. The FTIR spectrum of prepared neat TiO2 showed the broad band around 610 cm−1 , which is attributed to the presence of Ti–O–Ti stretching and bending vibrational modes. Also the presence of the broad band around 3400 cm−1 is due to O–H stretching. Further, FTIR spectrum of GO is also depicted in Fig. 4. The representative peaks around 3400, 1716, 1586, 1358, 1158 and 1041 cm−1 are related to O–H stretch, C==O stretch, C==C, C–OH bending, alkoxy C–O stretch and C–O–C stretch. The presence of these peaks confirms the oxidation of graphite and leading to formation of graphene oxide in the presence of ultrasound. IR spectrum of rGO/TiO2 nanocomposite exhibits a broad characteristics peak at 3367 cm−1 corresponding to the O–H stretching vibration of C–OH groups. The presence of characteristic peak at 1650 cm−1 is caused due to C==O which is present in COOH groups [16]. Further the band between 1480 and 1600 cm−1 is attributed to presence of Ti–O–C bonding, which approves bonding among rGO nanosheets and formed TiO2 nanoparticles [16]. This is also confirmed from UV/vis analysis of rGO/TiO2 nanocomposite. The band at 1380 cm−1 is caused due to C–OH stretching. One more characteristic peak at 1060 cm−1 is attributed to

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Fig. 4. FTIR spectra of ultrasonically prepared TiO2 , graphene oxide (GO), rGO/TiO2 nanocomposite photocatalyst. Fig. 5. Raman Spectrum of ultrasonically prepared rGO/TiO2 nanocomposite photocatalyst.

presence of C–O functionality. Additionally, these results demonstrate that the presence of oxygen-containing functionalities in GO were helpful for even loading of TiO2 nanoparticles on the rGO nanosheet [50–52,54]. FTIR spectra also confirmed the presence of Ti–O–Ti functionality in the range of 450–610 cm−1 . 3.2.4. Raman spectrum study Raman spectroscopy was used to investigate the structural properties of rGO/TiO2 nanocomposite, as shown in Fig. 5. In Fig. 5 peaks at 145 (B1g ), 277 (SOE), 455 and 655 cm−1 are attributed to presence of rutile TiO2 decorated on rGO nanosheets. Further, the presence of peaks at 197 (Eg ), 400 (B1g ), 513 (A1g ) and 639(Eg ) cm−1 confirms presence of anatase TiO2 bonded with rGO nanosheets. Also, other two peaks of rGO are observed at 1348 and 1587 cm−1 [55,56]. The two major characteristic peaks at 1348 and 1587 cm−1 represent the D-band and G-band of rGO. The G-band confirms the presence of sp2 bonded carbon atoms and gives insight into in-plane vibration. The D-band specifies the existence of defects in the symmetrical hexagonal graphitic lattice and also contains sp3 defects [17,50,57,58]. A small characteristics peak at 2729 cm−1 is also seen corresponding to the secondorder D band (2D). The second-order D band (2D) depicts stacking order of the rGO sheets with the c-axis and the number of layers. Further, it becomes more structured as the number of graphene layers gets increased [17]. 3.2.5. Elemental map images and EDAX for C, Ti and O Fig. 6 depicts elemental map images of C, Ti and O of reduced graphene oxide nanocomposite photocatalyst prepared with ultrasound assisted method. It has been observed that these elements are properly dispersed due to ultrasonic irradiations. This proper dispersion of elements i.e. TiO2 nanoparticles on rGO sheets is attributed to physical effects of cavitational events due to ultrasound. Fig. 7 shows elemental contents of sonochemically prepared rGO/TiO2 nanocomposite photocatalyst. EDAX showed the presence of elements in rGO/TiO2 nanocomposite which composed mainly of C, Ti and O elements with their content as 74.22, 5.80 and 16.54 weight%, respectively. In the sample, the higher quantity of carbon is shown by the spectrum. The signal for O and Ti are from the TiO2 nanoparticles while the signal for C mainly originates from the rGO sheets [17]. 3.2.6. TEM analysis TEM images of rGO/TiO2 nanocomposite photocatalyst are depicted in Fig. 7 (inset) at two different levels of magnification. In

the figure, TiO2 particles are shown by black spots whereas rGO sheet is shown by grey colour. TEM image indicates the smoothness of the surface and shows 2D structure of the rGO sheets. TEM images of rGO/TiO2 nanocomposite confirm that under sonochemical conditions even loading of TiO2 nanoparticles on the rGOnanosheets has been accomplished. The cavitational effects of ultrasound generate extreme environment in the reaction mixture, which is responsible for fine loading of nanoparticles with lesser size [51]. Large amount of round-shaped TiO2 nanoparticles approximately ranging in size from 3 to 5 nm are uniformly immobilized on the rGO sheet leading to successful formation of uniformly dispersed rGO/TiO2 nanocomposite. 3.2.7. XPS analysis Complicated titanium coordination states in sonochemically prepared rGO/TiO2 nanocomposite, were investigated through XPS analysis. Fig. 8(A) depicts the XPS survey spectrum of sonochemically prepared rGO/TiO2 nanocomposite. Qualitative evidence about atoms/molecules available in the sampling depth is given by survey scans. In the survey spectrum the presence of C (1s), O (1s) and Ti (2p) are originated from rGO and TiO2 nanoparticles which are the constituents of rGO/TiO2 nanocomposite. The C 1s XPS spectra of rGO/TiO2 nanocomposite is depicted in Fig. 8(B). The presence of peak at 284.7 eV is caused due to the sp2 carbon species. Additionally, the characteristic peaks detected at binding energies in between 285 and 290 eV are ascribed to oxygen-containing functionalities like hydroxyl, epoxy and carbonyl species located on the rGO surface [51]. The two peaks around 285.8 and 287.2 eV of rGO specify a significant degree of reduction from GO to rGO. The presence of peak at 288.6 cm−1 is an indication of formation of Ti–O–C functionality. Fig. 8(C) depicts the O 1s XPS spectrum of rGO/TiO2 nanocomposite. The O 1s spectrum is broad and consists of contributions from chemisorbed oxygen. The spectrum was deconvoluted into four peaks; to TiO2 (529.7 eV), C==O (531.6 eV), H2 O (533.5 eV), C–O (533.0 eV) and carboxylic group (535.7 eV). Fig. 8(D) depicts the Ti 2p XPS spectrum of rGO/TiO2 nanocomposite. In this spectrum, the characteristic peaks located at 464.2 eV (Ti 2p1/2 ) and 458.4 eV (Ti 2p3/2 ) are assigned to the Ti4+ state [50]. Overall, the XPS analysis confirms successful formation of rGO/TiO2 nanocomposite using sonochemical method.

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Fig. 6. Elemental Map images for C, Ti and O of rGO/TiO2 nanocomposite photocatalyst prepared by ultrasound-assisted method.

3.3. Effect of rGO/TiO2 nanocomposite photocatalyst loading The quantity of the catalyst added is one of the vital factors that have an effect on the degradation efficiency of the photocatalytic process. To avoid use of additional catalyst which may cause aggregation of catalyst particles and also to confirm maximum absorption of light photons for effective photomineralization, utilization of the optimal catalyst quantity is crucial [59]. Though there are many reports on the studies related to catalyst loading, a straightforward comparison between these investigations is challenging due to the varied working geometry, radiation fluxes and selected wavelengths. In addition, the optimal quantities of photocatalyst loading are dependent on the dimensions of the reactor as well. In this work, the influence of catalyst loading on the degradation rate of MB was investigated where photocatalyst quantities were changed from 1 to 3 g/L, at dye concentration of 20 ppm, temperature of 35 ◦ C and at pH 6.44 (natural pH of MB dye solution). The degradation curves of methylene blue are presented in Fig. 9. From the figure, it can be deduced that the degree of decolourization/degradation was improved from 18.7 to 52.2% with rise in the catalyst loading from 1 to 3 g/L, which is a characteristic of heterogeneous photocatalysis. It also increased with the irradiation time. The number of active sites on the surface of photocatalyst increases as the catalyst amount in the reaction medium gets increased, which in turn increases the decolourization/degradation of MB dye. It can be understood that the reduction in the MB dye concentration in the reaction medium is significant with increased catalyst loading from 1 and

3 g/L. However, 1 g/L catalyst dosage leads to lesser extent of decolourization/degradation when compared to catalyst dosage of 2 g/L. There is around 10% difference in the photocatalytic degradation of MB dye with the use of 2 and 3 g/L catalyst loadings. However, in the present study, 2 g/L of rGO/TiO2 nanocomposite catalyst was adopted to study the effect of pH on degradation of MB dye. 3.4. Effect of pH The adsorption of reactant on the surface of a photocatalyst is an important factor in photocatalytic oxidation processes which decides the degradation rate. The rate of decolourization/decomposition of pollutant in the existence of photocatalyst is enhanced if large numbers of pollutant molecules are adsorbed on the surface of photocatalyst. The attachment of those molecules on the surface of photocatalyst depends on the nature of the surface (acidic or basic) of the photocatalyst or due to the surface modifications with alteration in the pH of the system [60]. The pH of the reaction mixture is one of the influencing parameters that alter the surface charge of the photocatalyst. Further, in acidic or alkaline conditions the surface of the photocatalyst gets protonated or deprotonated. An influence of pH on decolourization/degradation of MB dye was investigated at four different values viz. 2.3, 4.0, 12.1 and 13.2. The other conditions were maintained constant at initial dye concentration 40 ppm, catalyst dosage of 2 g/L and temperature of 35 ◦ C and the experimental results were plotted as shown in Fig. 10.

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Fig. 7. EDAX analysis for C, Ti and O and TEM image (inset) of rGO/TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method.

As depicted in Fig. 10, as the pH increased from 2.3 to 13.2, the amount of decolourization/degradation of MB dye was also found to have enhanced from 17.4 to 91.3% at the end of 30 min. The possible reasons for these results may be explained as follows. At lower pH, acidic conditions prevail and the sonochemically prepared nanocomposite surface gets positively charged leading to the protonation of active sites. MB dye is a cationic dye hence this generates the electrostatic repulsion with the positively charged MB dye molecules and the catalyst surface. On the other hand, at higher pH the surface of the nanocomposite becomes alkaline in nature leading to deprotonation. At this condition, there are higher number of negative sites on the exterior surface of the photocatalyst and complex formation takes place due to the interaction of dye cation with one of these negative sites of nanocomposite. Hence, electrostatic attractions of the dye molecules with the photocatalyst are significant at higher value of pH leading to higher degradation rate. The point of zero charge (PZC) of ultrasonically prepared rGO/TiO2 nanocomposite suspension was measured with the help of zeta potential values at different pH and is depicted in Fig. 10 (Inset). The point of zero charge was found to be 2.62 for the nanocomposite. This confirms the presence of negative charge on the rGO/TiO2 nanocomposite at higher pH and therefore these negative charged species i.e. hydroxide ions interact with hole resulting in the increased formation of radicals by reducing the electron–hole combination [61]. Due to these reasons, the degradation of MB dye is more in case of higher pH values. Therefore, ultrasonically prepared rGO/TiO2 nanocomposite was observed to be very efficient for the decolourization/degradation of MB dye at basic pH and it shows 91.3% degradation of MB dye in 30 min at 13.2 pH and catalyst loading of 2 g/L.

3.5. Kinetics of degradation Investigation of the photocatalytic degradation kinetics of MB dye has been accomplished and the pseudo-first order MB dye degradation kinetics is signified with following equation.

( ln

C0 C

) = kt

(2)

Where C = MB dye concentration (ppm) at time t, C0 = The concentration (ppm) of MB after accomplishment of the adsorption equilibrium but before irradiation, t = time (min) and k = the observed reaction rate constant (min−1 ). The investigation of kinetics of degradation of MB dye has been accomplished for the data on different catalyst loadings hence photocatalytic degradation data presented in Fig. 9 was re-plotted in Fig. 11 [ln(C0 /C ) vs. t coordinates]. ln(C0 /C ) versus time plot depicts straight lines, indicating the first-order reaction mechanism. The observed rate constant values were estimated from the slope of the line. The calculated observed rate constant values are depicted in Table 1. It can be seen that the rate constants for 2 g/L and 3 g/L catalyst are substantially higher compared to that for 1 g/L. Thus, the kinetic study revealed that the rGO/TiO2 nanocomposite prepared using ultrasound is a proficient photo-catalyst for degradation of MB dye. 3.6. Plausible photocatalytic degradation mechanism of rGO/TiO2 nanocomposite The plausible mechanism for the photocatalytic degradation of the MB dye is depicted in Fig. 12 with related electron transfer mechanism in the presence of rGO/TiO2 nanocomposite. In

8

S.P. Deshmukh, D.P. Kale, S. Kar et al. / Nano-Structures & Nano-Objects 21 (2020) 100407

Fig. 8. XPS survey spectrum of rGO/TiO2 nanocomposite and resolved fitting signal of C 1s, O 1s and Ti 2p. Table 1 Kinetic parameters obtained by first order kinetic fitting. Sr. No.

Catalyst loading (g/L)

Initial dye concentration (ppm)

Temperature (◦ C)

pH

k (min−1 )

1 2 3

1.0 2.0 3.0

20 20 20

35 35 35

6.44 6.44 6.44

0.0068 0.0188 0.0273

presence of sunlight, rGO/TiO2 nanocomposite photocatalyst gets activated and formation of electron–hole occurs. Formed electron gets transferred to conduction band and hole is generated in the valance band. However, it has been reported that in presence of only TiO2 , recombination of electron and hole is substantial, which drastically reduces the photocatalytic activity. Uniform deposition of TiO2 nanoparticles on rGO nanosheets with the aid of ultrasonic irradiations enhances the trapping of electron to conduction band in rGO nanosheets which substantially reduces

the recombination rate of electron–hole pair. This is again attributed to higher electron mobility of the rGO. In addition, higher surface area of rGO enhances the deposition of MB dye on formed rGO/TiO2 nanocomposite, which in turn improves the degradation rate of MB dye in presence of sunlight. During this process, the formation of super oxide O2 •− and hydroxyl (OH• ) radicals takes place, which are responsible for the efficient degradation of MB dye as depicted in Fig. 12.

S.P. Deshmukh, D.P. Kale, S. Kar et al. / Nano-Structures & Nano-Objects 21 (2020) 100407

Fig. 9. Effect of catalyst loading on degradation of MB dye using rGO/TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method (Batch volume = 100 ml, Temperature = 35 ◦ C, Initial dye concentration = 20 ppm, pH = 6.44).

9

Fig. 12. Photocatalytic degradation of MB dye and electron transfer mechanism in presence of ultrasonically prepared rGO/TiO2 nanocomposite photocatalyst.

during synthesis resulted in uniform distribution of TiO2 nanoparticles with size between 3 and 5 nm on the rGO nanosheets as demonstrated by TEM images. The XRD analysis of the sonochemically prepared composite confirmed a mixture of anatase and rutile phases of TiO2 nanoparticles. The photocatalytic activity of the rGO/TiO2 nanocomposite is studied by monitoring the decolourization/degradation of MB dye in reaction medium under sun light. The results indicated that the percent decolourization/degradation gets enhanced at higher loading of photocatalyst and the degradation is prominent in the basic environment as against acidic ones. The maximum degradation of MB dye was found to be 91.3% at the end of 30 min for pH equal to 13.2 and photocatalyst dosage of 2 g/L. Higher decolourization/degradation of MB dye in presence of sun light is attributed to Ti–O–C bonding in ultrasonically formed rGO/TiO2 nanocomposite. Further, the photocatalytic decolourization/degradation observed the firstorder kinetics and MB dye was effectively degraded within 30 min. Fig. 10. Effect of pH on degradation of MB dye using rGO/TiO2 nanocomposite photocatalyst prepared by ultrasound assisted method (Batch volume = 100 ml, Temperature = 35 ◦ C, Initial dye concentration = 40 ppm, Catalyst Loading = 2 g/L).

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by the Rashtrasant Tukadoji Maharaj Nagpur University, India, Nagpur under Innovative Research Activities [Sanction order no. vikas/javak/2277 dated 7/10/2015]. References

Fig. 11. Kinetic plot based on the data of Fig. 9.

4. Conclusions

Sonochemical preparation of rGO/TiO2 nanocomposite was successfully accomplished. The sonochemical synthesis process is straightforward and effective. The application of ultrasound

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