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Graphene oxide weight ratio
Accepted Manuscript Enhancement of visible and UV light photocatalytic activity of rGO-TiO2 nanocomposites: The effect of TiO2/Graphene oxide weight ratio Mojtaba Mohammadi, Mahmoud Rezaee Roknabadi, Mohammad Behdani, Ahmad Kompany PII:
S0272-8842(19)30430-4
DOI:
https://doi.org/10.1016/j.ceramint.2019.02.129
Reference:
CERI 20852
To appear in:
Ceramics International
Received Date: 21 December 2018 Revised Date:
3 February 2019
Accepted Date: 18 February 2019
Please cite this article as: M. Mohammadi, M.R. Roknabadi, M. Behdani, A. Kompany, Enhancement of visible and UV light photocatalytic activity of rGO-TiO2 nanocomposites: The effect of TiO2/Graphene oxide weight ratio, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.02.129. 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.
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Enhancement of Visible and UV light
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photocatalytic activity of rGO TiO2
nanocomposites : The effect of TiO2/Graphene
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oxide weight ratio
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Mojtaba Mohammadi,†,‡ Mahmoud Rezaee Roknabadi,∗,†,‡ Mohammad Behdani,† and Ahmad Kompany† †Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
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‡Angstrom Thin Film Research Laboratory, Faculty of Science, Ferdowsi University of
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Mashhad, Mashhad, Iran
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E-mail: [email protected] Abstract
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In this study, rGO TiO2 nanocomposites were prepared by means of a simple hydrothermal approach using TiO2 powder and GO nanosheets as starting materials. The TiO2/GO weight ratio varied from 0.25 to 2 wt.%. The prepared samples were characterized using AFM, SEM and TEM microscopy as well as XRD, EDX, RAMAN, FTIR and UV-Vis spectroscopies. BET and BJH measurements were also performed to obtain the specific surface area and pore diameter size of the synthesized samples. It was found that the samples with TiO2/GO weight ratio below one have high effective surface area about 124 g/m2 and pore size of 6 nm. Photocatalyst evaluation revealed that only 0.06 mg/ml of prepared composite degraded 90 % of 20 ppm Methylene blue
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(MB) in 15 min under 50 mW/cm2 UV light intensity. The linear kinetic rate constant (k) of the samples promoted from 22 × 10−3 min−1 for TiO2 under UV exposure up to 153 × 10−3 min−1 and 65 × 10−3 min−1 for composites under UV and visible irradiation, respectively. More detailed analyses showed that the photocatalytic activity
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of samples obtained figure of merit below unity. To the best of our knowledge this seems to be an impressive result. Impedance spectroscopy analysis revealed that the enhancement of charge transfer in composites as well as high effective surface area and
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their mesoporous structure are mainly responsible for the significant enhancement of MB photodegradation. The reactive species trapping experiments were performed by a series of radical scavengers. On the basis of the experimental results, the electrons
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and superoxide ions species play the main role in degradation of MB.
Keywords: rGO TiO2 nanocomposites, hydrothermal approach, photocatalyst, MB pho-
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Graphical abstract
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todegradation, radical scavenger
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1
Introduction
Removal of hazardous organics and dyes from the environment has been a great concern in both research and industrial sectors. Upon this increasing demand, new nanostructured
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photocatalysts have been fabricated and developed for environment friendly applications. Nanostructured titanium dioxide (TiO2), which is well known as a classical photocatalyst, has been studied vastly for such purposes since 1972. 1 However, because of its large energy
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band gap and short life-time of the carriers, its application as a visible light photocatalyst has not been very promising. On the other hand, graphene and reduced graphene oxide nanosheets, which present a 2D structure with a very high surface area and high mobility
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of the charge transfer, have been proposed as scaffold for novel photocatalysts. Thus, composites of mesoporous TiO2 structures and graphene are suggested as promising solutions for advanced visible light photocatalyst. 2–5 To have an efficient photocatalyst, one has to deal with several issues such as band gap mismatch of graphene and TiO2, efficient charge transfer from TiO2 to graphene layers, electron-hole recombination hindering, high surface
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area for dye adsorption, light trapping and the interface of TiO2 graphene. 6 Specially, the
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interface plays an important role as Schottky barrier for electron transfer and charge separation which are very crucial for dye adsorption. See Liu et al. for example. 7 Therefore, the
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preparation method of TiO2- rGO (rGO: reduced graphene oxide) heteronanostructures play the essential role in functionality of the products for photocatalysis applications. In this re-
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gard, many researchers have studied the degradation of different pollutants using TiO2 rGO nanostructures. 8–18 The key parameters from the experimental point of view could be named as: dye concentration, initial amount of catalyst, light intensity, light wavelength range and dye adsorption in darkness condition. Li and co-workers 18 fabricated TiO2−graphene hydrogel with 3D network structure, and investigated the removal performance for Cr(VI) from aqueous solutions. Their results showed that 100% Cr(VI) (with concentration of 5 mg/L in the solution) could be removed within 30 min by the synergy performance of adsorption and photocatalysis. This improvment is due to non-porous surface adsorption and π − π 3
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interaction adsorption for graphene, and enhancment in photo-induced charge transport and separation by 3D network structure of TiO2−graphene hydrogel. Zhu et al. 19 prepared heterostructres nanofibrous TiO2 graphene PVAc materials by electrospinning route. The
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targeted polutant was 10 ppm methylene orange (MO) and the amount of catalyst was 0.1 mg/ml. As a result, 10% of the dye was adsorbed in darkness in 10 min following by 82% and 45% degradation under UV and visible light, respectively. Also, Baojiang et al. 20 synthesized TiO2 rGO nanocomposites by solvothermal approach and used 10 ppm phenol and
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10 ppm Methylene Blue (MB) as the dye and 0.2 mg/ml of catalyst under both UV and visible light sources with 60 and 50 mW/cm2 intensity, respectively. These authors reported
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30% dye adsoprtion in the darkness and 70% degradation after 60 min under visible light exposure. In addition, their impedence specteoscopy showed a reduction of charge transfer resistance (semicircular radii) after the adding graphene oxide. Kangfu et al. 21 also prepared this composite using solvothermal method and reported an enhancement in 3.2 ppm MB degradation using 1 mg/ml of TiO2-rGO composite under exposure of 50 mW/cm2 vis-
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ible Xenon lamp for 3 h with 75% dye destruction. Moreover, Yaxin et al. 22 reported a high
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efficient photocatalysis activity of graphene TiO2 with 97% degradation of 50 mg/L dye pollutant using 0.2 mg/ml of catalyst under mercury lamp for 2 h. As a sign of clear charge
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recombination hindering effect, the last report showed a significant increase of first order rate constant, i.e. k-value, from 1.81 × 10−3 min−1 to 33.19 × 10−3 min−1 . Recently, Alamelu
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and coleagues 23 also investigated the photodegradation of 6.5 mg/L MB under natural sunlight using 0.4 mg/ml G TiO2 catalyst. Its darkness adsorption was 50% and dye removal was 93% after 1h of light exposure. In addition, the obtained k-value was calculated to be 36 × 10−3 min−1 . In 2016, Morawski et al. 11 investigated a visible light-active TiO2 rGO photocatalysis of acetic acid 5% volume by a concentration of 5 mg/ml catalyst in which their highest degradation was obtained for a weight ratio of 0.5 wt.% reduced graphene oxide to TiO2 with surface area of 232 m2 g−1 . Yu et al. 12 used graphene oxide quantum dots-sensitized porous TiO2 microsphere for visible light photodegradation of Rhodamine B
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(RhB). Here, 1.25 mg/ml of catalyst with an effective surface area of 197 m2 g−1 was used for 90% degradation of 20 ppm RhB after 4 h. Recent studies investigated the role of active species for different dye degradation. 17,24,25 For example, Zhang and co-authors 24 prepared
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3D network structure of TiO2 rGH for removal of bisphenol A (BPA). Their results showed that superoxide radical ion (• O2− ) and hydroxyl radical (• OH) are the main active species involved in the degradation of BPA and its oxidized intermediates.
A key role which could be responsible for the low rate of degradation is
TiO2 GO
initial weight
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ratio. This ratio has been frequently considered high (>> 1) in prior studies.
In this work, rGO TiO2 nanocomposites are synthesized using a facile, cheap and scalable
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method for the enhancement of interface effects by implementation of raw powders of TiO2 and graphene oxide in synthesis procedure. More importantly, for the first time
TiO2 GO
weight
ratio varied below one, meaning more graphene oxide powder is used compared to TiO2 powder. This is the main novelty of the current research which results in higher k-values and
Synthesis of graphene oxide
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2.1
Materials and methods
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2
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consequently better photocatalytic degradation of MB under UV and visible light exposure.
For preparation of graphene oxide (GO), a modified improved Hummer method was used.
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Briefly, 1 g of graphite flakes was added into a 100 ml mixture of H2SO4/H3PO4/HNO3 with 67.5:10:22.5 in volumetric ratio. Then 6 g of KMnO4 was added slowly to the solution maintaining the temperature below 20 ◦ C using an ice bath. The resulting mixture was stirred at 45 − 50 ◦ C for 2 h. Then, 100 mL of de-ionized (DI) water was added to the solution and the solution was stirred for 15 min while maintaining it at 85 ◦ C for about 1 h. Additional 120 mL DI water was added and followed by a slow addition of 15 mL H2O2 (30%), turning the color of the solution from dark brown to yellow. The mixture was first filtered and washed with 1:10 HCl aqueous solution (100 mL) to remove metal ions. Thereafter, it 5
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was centrifuged at 750 rpm and the black unexfoliated precipitation was separated. Then, the remained yellow solution was centrifuged at 8000 rpm and the precipitation was washed several times until the pH reached the value of 6-7. The remaining solid was filtered over a
2.2
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PTFE membrane (0.45 µm pore size) and vacuum dried overnight at room temperature.
Synthesis of TiO2 powders
Two separate solutions, containing 1 g Polyvinylpyrrolidone (PVP 40000, Sigma Aldrich)
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and 0.5 g TTIP (Merck 99%) dissolved in 50 ml absolute ethanol, were prepared and mixed together for 10 min. Then, NaOH 1 M was injected to the solution drop wise until it
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becomes slurry opaque. Then it was centrifuged and the white powder was dried at 50 ◦ C and calcinated at 400 ◦ C for 2 h.
2.3
Synthesis of rGO TiO2 powders
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Hereafter, the index S refers to Supporting Information (Figures and Tables) attached to this work. Figure S1a depicts the synthesis procedure. At first, TiO2 powder and GO were
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separately dispersed in 75 ml of DI water using a horn sonicator at 25 kHz. Then, 0.1 g NaOH was dissolved in the dispersed TiO2 solution and the resultant solution was added
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drop wise to the dispersed GO suspension while stirring and sonicating simultaneously. The final 150 ml solution containing dispersed TiO2 and GO was put in a 200 ml home-made
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teflon lined stanless steel autoclave at 120 ◦ C for 12 h. See Figure S1b for a schematic view of this device. The resulted powder was centrifuged and washed several times with DI water until the pH reached 7 and dried at room temprature for further analyses. The amount of raw powders were used such that the initial weight ratio of
TiO2 GO
were 2, 1, 12 ,
1 4
named
respectively as B: TG 21, C: TG 11, D: TG 12, E: TG 14 in this report. Also, TiO2 powder, called A here, was used as bench mark for the sake of comparison throughout this study.
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2.4
Characterizations and measurements
X-ray diffraction was performed using a GNR Explorer. The data was simulated using the Rietveld refinement approach implemented in Fullprof V6 software. Raman spectroscopy
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was carried out using a visible (VIS) diode laser, emitting at 532 nm, included in a commercial Renishaw InVia Raman microscope in a back scattering configuration, including a monochromator and notch filters system. Samples were placed on the stage of a Leica microscope, equipped with 50× and 100× short and long working distance objectives. In all
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measurements, laser power intensity on the sample was kept at values lower than 0.03 mW to avoid laser-induced sample degradation. Atomic force microscopy was employed using a Full
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0101-A Ara Research Company model. Scanning electron microscopy together with energy dispersive x-ray spectroscopy were conducted using a Leo 1450VP. Transmission electron microscopy captured images using a Leo 912AB system. UV-Vis spectrscopy data was collected by means of UVD2950 system. Furrier transformed Infrared spectroscopy data was obtained using a AVATAR-370 Thermonicolet FTIR spectormeter. Brunauer-Emmett-Teller (BET)
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Surface Area Analysis and Barrett-Joyner-Halenda (BJH) Pore Size and Volume Analysis
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from N2 adsorption-desorption isotherms were obtained using a Belsorp miniII system. All electrochemical measurement were performed using Autolab PGSTAT302N with NOVA 1.11
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software (Eco Chemie, The Netherlands). Electrochemical impedence spectroscopy (EIS) data from 0.01 Hz to 100 kHz was collected using glassy carbon as working electrode, Pt as
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counter and Ag/AgCl as reference electrodes in 0.1 M KCl solution. Particle size distribution was analyzed using a particle size analyzer model Vasco3, Cordouan, France.
2.5
Photocatalysis characterization
To prepare the solution for photocatalysis characterization, 3.2 mg of catalyst was dispersed in 50 ml (0.06 mg/ml) of DI water presolved by an amount of 20 ppm MB. Then the resultant solution was stirred in darkness for 30 min for adsorption-desorption equilibrium at 25 ◦ C. Then the solution was exposed to 200 W HBO Mercury short arc with 50 mW/cm2 7
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Intensity (UV light source) and Osram 500 W Xenon lamp with a cut off UV filter at 400 nm as visible light source in a distance with 50 mW/cm2 intensity. Aliquests were taken in regular time intervals, centrifued at 10000 rpm for 5 min and then absorption specrrum were
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recorded accordingly. To elucidate which active species paly a key role in photodegradation of MB by rGO-TiO2 under visible light irradiation, different radical scavengers with the same concentration of 3 mmol/L were used. All experiment parameters including light intensity, the amounts of catalyst and dye, and time intervals were chosen the same as the
Results and discussions
3.1
Verification of catalyst structure
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3
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best performance of photocatalytic experiment of sample D.
Several characterizations were hired to verify the quality of synthesized graphene oxide. Figure S2 shows the AFM image of single sheet graphene oxide with vertical height of
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roughly 0.9 nm. Also, Figure S3 demonstrates strcuturally well-formed GO showing a sharp
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peak located around 2θ = 10.90◦ with atomic distance of d=0.8 nm for (001) plane of GO, which is in a good agreement with AFM analysis. In addition, Figure S3-inset presents
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the photograph of GO sheets in large scale indicating high quality of GO. Moreover, to investigate the molecular structure of GO, FTIR spectroscopy was employed (Figure S4).
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The strong absorption peaks at 1622 and 1733 cm−1 are attributed to C=C and C=O bonds that confirm the well oxidized graphetic sheets with aromatic chains. 12,26–29 Peaks at 1055 cm−1 and 1229 cm−1 are attributed to C-O stretching 12,21,26 and C-OH stretching, respectively. 12,26,30
Figure 1 demonstrates XRD plots of samples A to E. All the samples show the presence of Anatase phase of TiO2 (JCPDS-No.21-1272) with preferential growth plane of (101). The absence of the main diffraction peak of graphene at 24.5◦ could be due to shielding effect of the main diffraction peak of Anatase at 25.4◦ . 31 Although the overal spectrum of the samples 8
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seems quite similar to one another, there are some delicate differences in the spectrum which should be considered in details. To display this point, we have plotted (101) plane for all the samples in figure 1-inset. In this plot, a clear systematic shift with the decrease of TiO2/GO
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weight ratio to lower angles indicates an increase of atomic plane distances. This increase could be attributed to the relaxation of TiO2 structure due to its attachment to reduced GO structure. There is also a visible broadening of this plane showing the existence of nonuniform strain imposed on Anatase structure. To track the structural changes quantitavely, table S1
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presents XRD data of the samples. As illustrated in this table, FWHM (full width half maximum) increases as a sign of decrease in mean crystallite size from 28 nm for sample A
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up to 20 nm for sample E. Moreover, the lattice parameter changes as well. Also, figure S5 shows the simulation of XRD patterns of A, D and E using standard Reitvield refinement approach. 32 The extracted lattice parameters, i.e. a and c axes of the unit cell, derived from these simulations revealed a decrease in parameter a (from a = 3.8027 ˚ A for sample A to a = 3.7830 ˚ A for sample E) and an increase in parameter c (
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from c = 9.4983 ˚ A for A to c = 9.5025 ˚ A for E). Raman scattering was employed to prove
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the succeful reduction of graphene oxide while attaching TiO2 nanoparticles on its surface. Here, Figure 2 shows the Raman shift of Anatase phase for sample A, i.e. TiO2, with famous
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phonon bands of Eg at 143 cm−1 , B1g at 394 cm−1 , A1g + B1g at 512 cm−1 and Eg2 at 637 cm−1 . See similar reports. 8,15,33 The anatase vibration mode usually appears as a sharp
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peak at 143 cm−1 , independent of the nanoparticles. 18 As figure 2 shows, this peak has been appeared for all samples.
However, samples C, D and E show quite different patterns in
figure 2 compared to TiO2 which could be due to the increase in the share of graphene in these products. The only remained phonon band belonging to TiO2 structure is Eg which is weakened considerabley while the apparaent presence of D and G bands is the signature of the presence of reduced graphene oxide. The G band, which is located at about 1595 cm−1 , is related to the inplane steretching motion of symmetric sp2 C-C bonds. 34 In addition, the D band is attributed to the defects and disoreders on rGO sheets associated with introduction
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of oxygen containing functional groups to the graphitic layer. 23,29,35 To have a more accurate view of the
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ratio, we have also plotted the D and G bands, after removing the background
intensity, in figure 2-inset. The obtained ID IG
are 0.9, 1.3 and 1.5 for C, D and E samples,
ratio is a sign of increasing defects and disorderes on the
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respectively. The increment in
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surface of graphene oxide which comes from strong interactions between TiO2 and graphene oxide sheets 19,29,33,36,37 as well as successful reduction of GO sheets due to the decrease of sp2 domain size, 21,38 which in general suggest that the oxygen-containing groups are reduced. 18
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To indicate the elemental distribution as well as investigating the correctness of
TiO2 GO
weight ratio, EDX spectroscopy was performed. See figure 3. The results show qualitatively
Table S2 obtains
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the increase in C element from sample A to sample E while
TiO2 GO
weight ratio decreases.
weight ratio of 2.53, 1.27, 0.43 and 0.25 for B, C, D and E samples
respectively, which are very close to their nominal wieght ratio of initial powders of TiO2 and GO. However, it should be noted that the share of Oxygen has not been taken into account here because during the reduction of GO it is not clear how much Oxygen gets
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involved in the final composition. Also, SEM images in figure 4 show clear presence of rGO
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sheets and TiO2 NPs (nanoparticles) in both large scales (100 µm /20 µm) and smaller scales (1 µm/2nm). The SEM images of the sample B show almost complete coverage of reduced
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graphene sheet with TiO2 particles. By continuing to samples C, D and E, one could see distinct graphene sheets with less densities of particles which indicates succeful experimental TiO2 GO
variation in the final composite only by changing the initial amounts of
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verification of
raw powders. Also, to elucidate the true attachment of TiO2 NPs on rGO sheets, TEM images have been obtained and presented in figure 5. TEM image of sample A(TiO2 raw powder) shows particles with avarage size of 40nm. The histogram plot of size distribution of the corresponding TEM images are also presented in figure S6. Although, TEM could be used as a selective technique for the estimation of particles size, particle size analyser (P.S.A) as a conventional technique was aslo employed to obtain statistical size distribution of TiO2 particles. The result of this technique is shown in figure S7. This plot shows an avarage
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size of 40nm which is in complete agreement with TEM image of sample A. In addition, TEM images of samples C, D and E show the presence of particles together with graphene sheets in the background. Moreover, these images indicate that as
TiO2 GO
ratio decreases,
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the concentration of TiO2 particles decreases such that graphene sheets appear clearly in the background of the pictures. Also, the histogram size distribution of TiO2 NPs for samples C, D and E are obtained as 30, 25 and 25 nm, respectively (see figure S6). Due to the presence of some oxygen functional groups on the surfaces and the edges of rGO, a bond between these
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groups and the initial nuclei of TiO2 is formed and prevents further nanoparticles growth. In other words, rGO acts as a robust support for TiO2 NPs and hence decreases their size
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when they are embedded on rGO sheets. 39,40
Photodegradation of MB
Absorption spectrum of the catalyst is a key role in determining its photocatalysis activity. Figure S8 illustrates the absorption of catalysts dispersed in DI water. As it could be seen
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from figure S8, there is a considerable red-shift of absorption edge toward visible range as
TiO2 GO
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ratio decreases. The band tail extension in the visible range, specially for D and E samples, sheds light on the capability of prepared catalyst in the visible range. 3,19,26 Fig. S9 shows
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the absorption evolution of MB solution and their corresponding inset photographses under UV light exposure for all the samples. Moreover, figure 6a demonstrates the corresponding
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relative concentration changes versus UV illumination time. It is seen in this figure that all the rGO TiO2 samples enhance the photodegradation of MB significantly, compared to TiO2 particles as control sample. The highest rate of degradation belongs to sample D, i.e. TG 12, in which the degradation is above 90% of MB in only 15 min (see figure 6a inset). Also, to scrutinize the capability of catalyst, logarithmic change of relative concentration were plotted againest irradiation time shown in figure 6b. They all show a first ordered
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kinetic law in MB degradation with the following equation: 41,42
− ln
C C0
! = kt
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Where C is the MB concentration at a particular time, C0 is the initial MB concentration, k is the pseudo- linear first-order kinetic constant (min−1 ), and t is the reaction time in min. The rate constant of every sample is tabulated in table 1. Furthermore, the results show
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that the rate constant of degradation changes from 22 × 10−3 min−1 for pure TiO2 up to 153 × 10−3 min−1 for sample D. In addition, figures 7a and b also show the enhancement of
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MB degradation under visible light for rGO TiO2 nanocomposites. The highest degradation belongs to sample D which in 15 min could degrade over 70% of MB (figure 7a:inset). The samples show a pseudo- linear first-order kinetic law of degradation rate constant which are summed up in table 1. The relatively high rate of 65 × 10−3 min−1 driven from visible light activity for sample D is noticeable in comparison to values which have been reported for the
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similar catalysts. 22,23,41 To examine the reason behind this successful enhancement of photo
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degradation, we first insert the dye adsorption-desorption equilibrium plot in darkness. The result is shown in figure 8. As it can be seen in this figure, the dye adsorption has been 0%, 20%, 60%, 80% and 90% for A to E samples, respectively, placed only 30 min in darkness.
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This could happen when the effective surface area and pore size diameters have undergone a significant enhancement. This enhancement is not only caused by TiO2 NPs deposited on
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rGO surface but also from the π − π stacking between MB and aromatic regions of rGO which has a giant π conjugation. 24,43–45 The adsorption of dye on the surface os rGO-TiO2 maybe due to the noncovalent interactions or π − π stacking between the electron rich π system of rGO and cationic dye, namely MB. 37 Since the specific surface area (SSA) of TiO2 NPs increased by embedding on the rGO sheets (SSA for TiO2 and sample E are 62 and 148 m2 g−1 , respectively), the adsorption amount of dye in dark for sample E, increases compared with that of TiO2. 17
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Thus, BET and BJH techniques were employed to determine the effective surface area and pore size diameters. Figure 9 shows the adsorption-desorption isotherm of the samples obtained from BET experiment. The samples show type IV isotherm with H3 hysteresis
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which is a characteristic of mesoporous materials. 46,47 Moreover, pore size distributions of the samples have been obtained from BJH technique and shown in figure 10. Also, table 2 demonstrates the surface area extracted from BET isotherm plot. In addition, pore volume and pore size are extracted from BJH experiment. The results show a significant increase in
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surface area from 62 m2 g−1 for sample A, i.e. TiO2 NPs, to 148 m2 g−1 for sample E. More importantly, the pore size diameter undergoes a considerable decrease from 12 nm up to
in darkness can be understood partly.
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6 nm for samples A to E, respectively. Thus, the enhancement of dye adsorption of catalysts
To study the effect of interface of TiO2 rGO and enhancement of rate constant, impedance spectroscopy as a strong tool for determination of charge transfer mechanism has been hired. Figure 11a shows the Nyquist plots of the samples and Figure 11a-inset shows the equivalent
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circuit of simulated electrodes for the composites. The simulated circuit shows the presence
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of two RC elements representing two semiconductors rGO and TiO2. Figure S10 depicts the simulation of impedance measurements for samples A and C for more clarity. This intrigu-
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ing result shows a significant decrease of charge carrier transfer resistance from sample A to sample E. This obvious decrement suggests that the charge transfer properties and the
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charge separation effinciencies of the nonocomposites are improved. 25 The samples show a semicircular behavior that corresponds to charge recombination resistance at the interface of TiO2 rGO/dye/electrolyte, 48 resulting from inherent resistance and capacitance of semiconductor composites, together with an ascending line showing the inevitable diffusive behavior of powders. 49,50 The reduced semicircles are suggesting a decrease in the resistance of charge transfer by forming hybrid structure of TiO2 with reduced graphene oxide, 51 and prove that the rGO-TiO2 nanocomposites, from sample B to E, promote the rapid separation between electrons and holes. 17 The π − π conjugated structure of rGO is another factor which could
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promote the separation efficiency of photogenerated e− and h+ . 37 Also, the phase angle diagram (bode-phase plot) (figure 11b) shows a clear shift to lower frequencies as a sign of enhancement of charge transfer from catalyst to electrode. 52 This question might arise that
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although E shows the highest surface area and charge transfer rate, why the sample D has the highest value of degradation? To address this query, one has to take into account that the surface area is the sum of effective surface exposed by both graphene sheets and TiO2 nanoparticles. Although sample E has a higher surface area, which is due to more GO con-
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tent, but rGO only acts as a platform for charge transfer feasibility while dye degradation occurs on TiO2 surfaces.
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In fact, TEM and SEM images reveal the existence of a better surface coverage of rGO sheets by TiO2 particles in sample D compared to sample E. Therefore, one could conclude that sample D should have a higher photocatalyst activity in comparison with sample E. To examine the reliability of our results, the reusability of our best photocatalyst, i.e. sample D, was checked by drying the retrieved sample and under the same conditions
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(0.06 mg/ml of catalyst, 20 ppm of MB, placed in darkness for 30 min at 25◦ C and ex-
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posed to UV and visible light sources with 50 mW/cm2 intensity separately). The results are provided in figure 12 which show that after five times of reusing, 95 % (under UV light)
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and 93 % (under visible light) of the initial performance is still maintained. For a comparison, we have listed previous works on TiO2 rGO photocatalysts in table S3.
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Table S3 compares photodegradation parameters and figure of merit as an indication of photocatalytic activity between our results and recent reports. 8–10,12,19–23 Figure of merit obtained by this report is below one which is at least ten times smaller than other reports. 8–10,12,19–23 This significant enhancement of figure of merit could be explained by possible degradation mechanism as following: To clarify the photodegradation mechanism of MB by rGO TiO2 under visible light, several scavengers were used. Generally, during the photodegradation of dyes, diffrent reactive species such as • OH and • O2− are generated besides of e− /h+ pair. For example, the free
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electrons reduce the dissolved oxygen, resulting in formation of superoxide ions while the holes may react with H2O and OH− to produce hydroxyl radicals. 39 The scavengers used in this work are EDTA for holes, 25,53 p- benzoquinone for • O2− , 25,53 K2S2O8, 40 and AgNO3 54
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as electron scavengers, sodium azide (NaN3), for singlet oxygen, 1 O2, 55 DMSO for • OHbulk , 55 sodium iodide (NaI) for • OHads , 56 And tert- butanol as a free OH radical scavenger. 56 If the photodegradation of MB by the catalyst is performed because of any of the reactive species, the reaction is slowed down or inhibited in the presence of the corresponding scavenger. 39
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For the sake of comparison, the MB degradation in the absence of catalyst under light irradiation, namely photolysis of dye, was investigated for possible self- degradation of MB. Also
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the MB degradation by sample D without using scavenger was carried out. Fig 13 shows the variation of C/Co for MB as a function of illumination time by adding different scavengers into the photocatalytic system. As this figure shows, no degradation in the absence of the photocatalyst for MB under light irradiation was observed. It means that MB is a photo-stable dye during our experiments. Also, this figure shows that the dye was degraded
D
within 60 min in the absence of any scavenger. Adding NaI, tert-Butanol, DMSO, EDTA
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and NaN3 had no considerable influence on photodegradation procces. Thus OH radicals, holes and 1 O2 are not the main active species in MB photodegradation. As figure 13 shows,
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the degradation efficiency of MB over sample D dramatically decreases with the addition of AgNO3, indicating that e− is the main active species during the photocatalytic degra-
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dation process. Since the photocatalytic degradation efficiency decreases in the presents of p-banzoquinone, superoxide radical ion plays a suplimentary role. To ensure that e− has the key role in photodegradation of MB by the catalyst, K2S2O8 as a second electron scavenger was used. As a result, the photodegradation efficiency of MB decreses sharply by pottasiume persulfate, which confirms that the electrons are the main active species in the degradation. The general mechanism of the degradation is explained elsewhere. 13,19,22 Figure 14 shows schematically a possible mechanism for MB photodegradation by rGO-TiO2 photocatalyst under visible light irradiation based on our radical scavenger results. Under light irradiation,
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electrons are excited from valance band (VB) to conduction band (CB) of TiO2 nanoparticles. Thereafter, electrons transfer freely alongside of graphene network and react with dye and dissolved oxygen molecules. The excited electrons reduce the dissolved oxygen, resulting in
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formation of superoxide ions. 13,26,38,53 The electrons and superoxide radicals, degrade MB dye, which was adsorbed on the surface of rGO-TiO2 nanocomposite.
The weight ratio has a crucial effect such that results in a mesoporous structure (pores below 10 nm) with high surface area leading to effective dye adsorption. In the next step, well
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TiO2-decorated reduced graphene sheets can effectively absorb irradiated light and converts to electron and hole pairs. The sample with the most proper of TiO2 nanoparticles distributed
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on rGO sheets have the highest of charge transfer rate and consequently effective charge separation and MB degradation. As it is shown in the graphical abstract the optimization of TiO2/GO weight ratio gives rise to well dye adsorption, excellent charge transfer and spectacular dye degradation as indicated by low values of figure of merit in Table S3.
Conclusion
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3.3
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The rGO TiO2 samples with enhanced visible photocatalyst activity were fabricated via a thermochemical approach using TiO2 and GO raw powders. TiO2/GO weight ratios were
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chosen to be less than one to expose more rGO than TiO2 in composite. EDX, SEM and TEM characterizations testified the variations of TiO2/GO weight ratios in prepared composites
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which were in agreement with the initial values of powder used for synthesis. This confirms the simple method used in this work as efficient tool to control correctly the composition of final products as well. It was also found that TiO2/GO weight ratio reduction enhances the charge transfer through composites significantly which results in a noticeable promotion of its photocatalytic activity. The large linear kinetic constant of photodegradation obtained in this study was confirmed also by impedance spectroscopy. The radical scavenger results illustrate that the electrons are the main active species in the photodegradation of MB by rGO-TiO2 and •O2− palys a supplementary role in the photocatalytic process. 16
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Acknowledgement The authors acknowledge the financial support of Ferdosi University of Mashhad, Iran (grant
Mr. M. Komeili for their technical and instrumental support.
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List of Figures Figure 1: XRD patterns of the samples A: (TiO2NPs), B: TG 21, C: TG 11, D: TG 12 and E: TG 14. Inset: the comparison of (101) Bragg plane of the
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samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Figure 2: Raman spectra of samples A, C, D and E. Inset: Comparison of magnified 28
Figure 3: EDX-ray spectroscopy of the samples. . . . . . . . . . . . . . . . . . . .
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D and G bands of graphene structure in the same samples. . . . . . . .
Figure 4: SEM images of the samples with two different magnifications. . . . . . .
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Figure 5: TEM images of the samples. . . . . . . . . . . . . . . . . . . . . . . . .
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Figure 6: (a) Time evolution of relative MB concentration due to photocatalyst activity of the samples under UV irradiation (inset: percentile degradation of MB after 15 min of UV irradiation for different samples). (b) Its corresponding logarithm of the relative concentration of MB vs. ir-
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radiation time for the determination of degradation rate of the samples during photocatalytic activity. . . . . . . . . . . . . . . . . . . . . . . .
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Figure 7: (a) Time evolution of relative MB concentration due to photocatalyst activity of the samples under visible irradiation (inset: percentile degra-
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dation of MB after 15 min under visible irradiation for different samples). (b) Its corresponding logarithm of the relative concentration of MB vs.
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irradiation time for the determination of degradation rate of the samples during photocatalytic activity. . . . . . . . . . . . . . . . . . . . . . . .
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Figure 8: The relative concentration of remained MB due to adsorption of dye in the samples (placed in darkness for 30 min). . . . . . . . . . . . . . . .
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Figure 9: Nitrogen absorption-desorption isotherm and BET (inset) plots of the samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Figure 10:Pore size distribution curve of the samples extracted from BJH method.
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Figure 11:Nyquist (a) and phase angle (bode-phase) (b) plots of samples. . . . . .
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Figure 12:Reusability degradation result under UV and visible light for sample D.
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Figure 13:Effect of different radical scavengers (3 mmol/L) on the photodegradation of MB by rGO-TiO2. C/Co ratio (Co is the initial concentration) vs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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irradiation time.
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Figure 14:Schematic of photocatalytic degradation of MB by rGO-TiO2 under light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Figure 1: XRD patterns of the samples A: (TiO2NPs), B: TG 21, C: TG 11, D: TG 12 and E: TG 14. Inset: the comparison of (101) Bragg plane of the samples.
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Figure 2: Raman spectra of samples A, C, D and E. Inset: Comparison of magnified D and G bands of graphene structure in the same samples.
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Figure 3: EDX-ray spectroscopy of the samples.
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Figure 4: SEM images of the samples with two different magnifications.
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Figure 5: TEM images of the samples.
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Figure 6: (a) Time evolution of relative MB concentration due to photocatalyst activity of the samples under UV irradiation (inset: percentile degradation of MB after 15 min of UV irradiation for different samples). (b) Its corresponding logarithm of the relative concentration of MB vs. irradiation time for the determination of degradation rate of the samples during photocatalytic activity.
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Figure 7: (a) Time evolution of relative MB concentration due to photocatalyst activity of the samples under visible irradiation (inset: percentile degradation of MB after 15 min under visible irradiation for different samples). (b) Its corresponding logarithm of the relative concentration of MB vs. irradiation time for the determination of degradation rate of the samples during photocatalytic activity.
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Figure 8: The relative concentration of remained MB due to adsorption of dye in the samples (placed in darkness for 30 min).
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Figure 12: Reusability degradation result under UV and visible light for sample D.
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Figure 13: Effect of different radical scavengers (3 mmol/L) on the photodegradation of MB by rGO-TiO2. C/Co ratio (Co is the initial concentration) vs. irradiation time.
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Figure 14: Schematic of photocatalytic degradation of MB by rGO-TiO2 under light exposure.
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List of Tables 1
First-order rate constant k values of samples for the photocatalytic degradation of MB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Surface area, pore volume and pore size of samples extracted from BET
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B
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Slope
Vm [cm3 g−1 ]
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Pore diameter [nm]
TiO2
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12.24
TG 2-1
0.057
17.22
75
10.65
TG 1-1
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22.04
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TG 1-2
0.035
28.47
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6.05
TG 1-4
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33.98
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Sample
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S0272-8842(19)30430-4
DOI:
https://doi.org/10.1016/j.ceramint.2019.02.129
Reference:
CERI 20852
To appear in:
Ceramics International
Received Date: 21 December 2018 Revised Date:
3 February 2019
Accepted Date: 18 February 2019
Please cite this article as: M. Mohammadi, M.R. Roknabadi, M. Behdani, A. Kompany, Enhancement of visible and UV light photocatalytic activity of rGO-TiO2 nanocomposites: The effect of TiO2/Graphene oxide weight ratio, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.02.129. 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.
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Enhancement of Visible and UV light
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photocatalytic activity of rGO TiO2
nanocomposites : The effect of TiO2/Graphene
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oxide weight ratio
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Mojtaba Mohammadi,†,‡ Mahmoud Rezaee Roknabadi,∗,†,‡ Mohammad Behdani,† and Ahmad Kompany† †Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
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‡Angstrom Thin Film Research Laboratory, Faculty of Science, Ferdowsi University of
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Mashhad, Mashhad, Iran
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E-mail: [email protected] Abstract
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In this study, rGO TiO2 nanocomposites were prepared by means of a simple hydrothermal approach using TiO2 powder and GO nanosheets as starting materials. The TiO2/GO weight ratio varied from 0.25 to 2 wt.%. The prepared samples were characterized using AFM, SEM and TEM microscopy as well as XRD, EDX, RAMAN, FTIR and UV-Vis spectroscopies. BET and BJH measurements were also performed to obtain the specific surface area and pore diameter size of the synthesized samples. It was found that the samples with TiO2/GO weight ratio below one have high effective surface area about 124 g/m2 and pore size of 6 nm. Photocatalyst evaluation revealed that only 0.06 mg/ml of prepared composite degraded 90 % of 20 ppm Methylene blue
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(MB) in 15 min under 50 mW/cm2 UV light intensity. The linear kinetic rate constant (k) of the samples promoted from 22 × 10−3 min−1 for TiO2 under UV exposure up to 153 × 10−3 min−1 and 65 × 10−3 min−1 for composites under UV and visible irradiation, respectively. More detailed analyses showed that the photocatalytic activity
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of samples obtained figure of merit below unity. To the best of our knowledge this seems to be an impressive result. Impedance spectroscopy analysis revealed that the enhancement of charge transfer in composites as well as high effective surface area and
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their mesoporous structure are mainly responsible for the significant enhancement of MB photodegradation. The reactive species trapping experiments were performed by a series of radical scavengers. On the basis of the experimental results, the electrons
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and superoxide ions species play the main role in degradation of MB.
Keywords: rGO TiO2 nanocomposites, hydrothermal approach, photocatalyst, MB pho-
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Graphical abstract
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todegradation, radical scavenger
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1
Introduction
Removal of hazardous organics and dyes from the environment has been a great concern in both research and industrial sectors. Upon this increasing demand, new nanostructured
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photocatalysts have been fabricated and developed for environment friendly applications. Nanostructured titanium dioxide (TiO2), which is well known as a classical photocatalyst, has been studied vastly for such purposes since 1972. 1 However, because of its large energy
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band gap and short life-time of the carriers, its application as a visible light photocatalyst has not been very promising. On the other hand, graphene and reduced graphene oxide nanosheets, which present a 2D structure with a very high surface area and high mobility
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of the charge transfer, have been proposed as scaffold for novel photocatalysts. Thus, composites of mesoporous TiO2 structures and graphene are suggested as promising solutions for advanced visible light photocatalyst. 2–5 To have an efficient photocatalyst, one has to deal with several issues such as band gap mismatch of graphene and TiO2, efficient charge transfer from TiO2 to graphene layers, electron-hole recombination hindering, high surface
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area for dye adsorption, light trapping and the interface of TiO2 graphene. 6 Specially, the
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interface plays an important role as Schottky barrier for electron transfer and charge separation which are very crucial for dye adsorption. See Liu et al. for example. 7 Therefore, the
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preparation method of TiO2- rGO (rGO: reduced graphene oxide) heteronanostructures play the essential role in functionality of the products for photocatalysis applications. In this re-
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gard, many researchers have studied the degradation of different pollutants using TiO2 rGO nanostructures. 8–18 The key parameters from the experimental point of view could be named as: dye concentration, initial amount of catalyst, light intensity, light wavelength range and dye adsorption in darkness condition. Li and co-workers 18 fabricated TiO2−graphene hydrogel with 3D network structure, and investigated the removal performance for Cr(VI) from aqueous solutions. Their results showed that 100% Cr(VI) (with concentration of 5 mg/L in the solution) could be removed within 30 min by the synergy performance of adsorption and photocatalysis. This improvment is due to non-porous surface adsorption and π − π 3
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interaction adsorption for graphene, and enhancment in photo-induced charge transport and separation by 3D network structure of TiO2−graphene hydrogel. Zhu et al. 19 prepared heterostructres nanofibrous TiO2 graphene PVAc materials by electrospinning route. The
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targeted polutant was 10 ppm methylene orange (MO) and the amount of catalyst was 0.1 mg/ml. As a result, 10% of the dye was adsorbed in darkness in 10 min following by 82% and 45% degradation under UV and visible light, respectively. Also, Baojiang et al. 20 synthesized TiO2 rGO nanocomposites by solvothermal approach and used 10 ppm phenol and
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10 ppm Methylene Blue (MB) as the dye and 0.2 mg/ml of catalyst under both UV and visible light sources with 60 and 50 mW/cm2 intensity, respectively. These authors reported
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30% dye adsoprtion in the darkness and 70% degradation after 60 min under visible light exposure. In addition, their impedence specteoscopy showed a reduction of charge transfer resistance (semicircular radii) after the adding graphene oxide. Kangfu et al. 21 also prepared this composite using solvothermal method and reported an enhancement in 3.2 ppm MB degradation using 1 mg/ml of TiO2-rGO composite under exposure of 50 mW/cm2 vis-
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ible Xenon lamp for 3 h with 75% dye destruction. Moreover, Yaxin et al. 22 reported a high
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efficient photocatalysis activity of graphene TiO2 with 97% degradation of 50 mg/L dye pollutant using 0.2 mg/ml of catalyst under mercury lamp for 2 h. As a sign of clear charge
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recombination hindering effect, the last report showed a significant increase of first order rate constant, i.e. k-value, from 1.81 × 10−3 min−1 to 33.19 × 10−3 min−1 . Recently, Alamelu
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and coleagues 23 also investigated the photodegradation of 6.5 mg/L MB under natural sunlight using 0.4 mg/ml G TiO2 catalyst. Its darkness adsorption was 50% and dye removal was 93% after 1h of light exposure. In addition, the obtained k-value was calculated to be 36 × 10−3 min−1 . In 2016, Morawski et al. 11 investigated a visible light-active TiO2 rGO photocatalysis of acetic acid 5% volume by a concentration of 5 mg/ml catalyst in which their highest degradation was obtained for a weight ratio of 0.5 wt.% reduced graphene oxide to TiO2 with surface area of 232 m2 g−1 . Yu et al. 12 used graphene oxide quantum dots-sensitized porous TiO2 microsphere for visible light photodegradation of Rhodamine B
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(RhB). Here, 1.25 mg/ml of catalyst with an effective surface area of 197 m2 g−1 was used for 90% degradation of 20 ppm RhB after 4 h. Recent studies investigated the role of active species for different dye degradation. 17,24,25 For example, Zhang and co-authors 24 prepared
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3D network structure of TiO2 rGH for removal of bisphenol A (BPA). Their results showed that superoxide radical ion (• O2− ) and hydroxyl radical (• OH) are the main active species involved in the degradation of BPA and its oxidized intermediates.
A key role which could be responsible for the low rate of degradation is
TiO2 GO
initial weight
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ratio. This ratio has been frequently considered high (>> 1) in prior studies.
In this work, rGO TiO2 nanocomposites are synthesized using a facile, cheap and scalable
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method for the enhancement of interface effects by implementation of raw powders of TiO2 and graphene oxide in synthesis procedure. More importantly, for the first time
TiO2 GO
weight
ratio varied below one, meaning more graphene oxide powder is used compared to TiO2 powder. This is the main novelty of the current research which results in higher k-values and
Synthesis of graphene oxide
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2.1
Materials and methods
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2
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consequently better photocatalytic degradation of MB under UV and visible light exposure.
For preparation of graphene oxide (GO), a modified improved Hummer method was used.
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Briefly, 1 g of graphite flakes was added into a 100 ml mixture of H2SO4/H3PO4/HNO3 with 67.5:10:22.5 in volumetric ratio. Then 6 g of KMnO4 was added slowly to the solution maintaining the temperature below 20 ◦ C using an ice bath. The resulting mixture was stirred at 45 − 50 ◦ C for 2 h. Then, 100 mL of de-ionized (DI) water was added to the solution and the solution was stirred for 15 min while maintaining it at 85 ◦ C for about 1 h. Additional 120 mL DI water was added and followed by a slow addition of 15 mL H2O2 (30%), turning the color of the solution from dark brown to yellow. The mixture was first filtered and washed with 1:10 HCl aqueous solution (100 mL) to remove metal ions. Thereafter, it 5
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was centrifuged at 750 rpm and the black unexfoliated precipitation was separated. Then, the remained yellow solution was centrifuged at 8000 rpm and the precipitation was washed several times until the pH reached the value of 6-7. The remaining solid was filtered over a
2.2
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PTFE membrane (0.45 µm pore size) and vacuum dried overnight at room temperature.
Synthesis of TiO2 powders
Two separate solutions, containing 1 g Polyvinylpyrrolidone (PVP 40000, Sigma Aldrich)
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and 0.5 g TTIP (Merck 99%) dissolved in 50 ml absolute ethanol, were prepared and mixed together for 10 min. Then, NaOH 1 M was injected to the solution drop wise until it
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becomes slurry opaque. Then it was centrifuged and the white powder was dried at 50 ◦ C and calcinated at 400 ◦ C for 2 h.
2.3
Synthesis of rGO TiO2 powders
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Hereafter, the index S refers to Supporting Information (Figures and Tables) attached to this work. Figure S1a depicts the synthesis procedure. At first, TiO2 powder and GO were
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separately dispersed in 75 ml of DI water using a horn sonicator at 25 kHz. Then, 0.1 g NaOH was dissolved in the dispersed TiO2 solution and the resultant solution was added
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drop wise to the dispersed GO suspension while stirring and sonicating simultaneously. The final 150 ml solution containing dispersed TiO2 and GO was put in a 200 ml home-made
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teflon lined stanless steel autoclave at 120 ◦ C for 12 h. See Figure S1b for a schematic view of this device. The resulted powder was centrifuged and washed several times with DI water until the pH reached 7 and dried at room temprature for further analyses. The amount of raw powders were used such that the initial weight ratio of
TiO2 GO
were 2, 1, 12 ,
1 4
named
respectively as B: TG 21, C: TG 11, D: TG 12, E: TG 14 in this report. Also, TiO2 powder, called A here, was used as bench mark for the sake of comparison throughout this study.
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2.4
Characterizations and measurements
X-ray diffraction was performed using a GNR Explorer. The data was simulated using the Rietveld refinement approach implemented in Fullprof V6 software. Raman spectroscopy
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was carried out using a visible (VIS) diode laser, emitting at 532 nm, included in a commercial Renishaw InVia Raman microscope in a back scattering configuration, including a monochromator and notch filters system. Samples were placed on the stage of a Leica microscope, equipped with 50× and 100× short and long working distance objectives. In all
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measurements, laser power intensity on the sample was kept at values lower than 0.03 mW to avoid laser-induced sample degradation. Atomic force microscopy was employed using a Full
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0101-A Ara Research Company model. Scanning electron microscopy together with energy dispersive x-ray spectroscopy were conducted using a Leo 1450VP. Transmission electron microscopy captured images using a Leo 912AB system. UV-Vis spectrscopy data was collected by means of UVD2950 system. Furrier transformed Infrared spectroscopy data was obtained using a AVATAR-370 Thermonicolet FTIR spectormeter. Brunauer-Emmett-Teller (BET)
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Surface Area Analysis and Barrett-Joyner-Halenda (BJH) Pore Size and Volume Analysis
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from N2 adsorption-desorption isotherms were obtained using a Belsorp miniII system. All electrochemical measurement were performed using Autolab PGSTAT302N with NOVA 1.11
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software (Eco Chemie, The Netherlands). Electrochemical impedence spectroscopy (EIS) data from 0.01 Hz to 100 kHz was collected using glassy carbon as working electrode, Pt as
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counter and Ag/AgCl as reference electrodes in 0.1 M KCl solution. Particle size distribution was analyzed using a particle size analyzer model Vasco3, Cordouan, France.
2.5
Photocatalysis characterization
To prepare the solution for photocatalysis characterization, 3.2 mg of catalyst was dispersed in 50 ml (0.06 mg/ml) of DI water presolved by an amount of 20 ppm MB. Then the resultant solution was stirred in darkness for 30 min for adsorption-desorption equilibrium at 25 ◦ C. Then the solution was exposed to 200 W HBO Mercury short arc with 50 mW/cm2 7
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Intensity (UV light source) and Osram 500 W Xenon lamp with a cut off UV filter at 400 nm as visible light source in a distance with 50 mW/cm2 intensity. Aliquests were taken in regular time intervals, centrifued at 10000 rpm for 5 min and then absorption specrrum were
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recorded accordingly. To elucidate which active species paly a key role in photodegradation of MB by rGO-TiO2 under visible light irradiation, different radical scavengers with the same concentration of 3 mmol/L were used. All experiment parameters including light intensity, the amounts of catalyst and dye, and time intervals were chosen the same as the
Results and discussions
3.1
Verification of catalyst structure
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3
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best performance of photocatalytic experiment of sample D.
Several characterizations were hired to verify the quality of synthesized graphene oxide. Figure S2 shows the AFM image of single sheet graphene oxide with vertical height of
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roughly 0.9 nm. Also, Figure S3 demonstrates strcuturally well-formed GO showing a sharp
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peak located around 2θ = 10.90◦ with atomic distance of d=0.8 nm for (001) plane of GO, which is in a good agreement with AFM analysis. In addition, Figure S3-inset presents
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the photograph of GO sheets in large scale indicating high quality of GO. Moreover, to investigate the molecular structure of GO, FTIR spectroscopy was employed (Figure S4).
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The strong absorption peaks at 1622 and 1733 cm−1 are attributed to C=C and C=O bonds that confirm the well oxidized graphetic sheets with aromatic chains. 12,26–29 Peaks at 1055 cm−1 and 1229 cm−1 are attributed to C-O stretching 12,21,26 and C-OH stretching, respectively. 12,26,30
Figure 1 demonstrates XRD plots of samples A to E. All the samples show the presence of Anatase phase of TiO2 (JCPDS-No.21-1272) with preferential growth plane of (101). The absence of the main diffraction peak of graphene at 24.5◦ could be due to shielding effect of the main diffraction peak of Anatase at 25.4◦ . 31 Although the overal spectrum of the samples 8
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seems quite similar to one another, there are some delicate differences in the spectrum which should be considered in details. To display this point, we have plotted (101) plane for all the samples in figure 1-inset. In this plot, a clear systematic shift with the decrease of TiO2/GO
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weight ratio to lower angles indicates an increase of atomic plane distances. This increase could be attributed to the relaxation of TiO2 structure due to its attachment to reduced GO structure. There is also a visible broadening of this plane showing the existence of nonuniform strain imposed on Anatase structure. To track the structural changes quantitavely, table S1
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presents XRD data of the samples. As illustrated in this table, FWHM (full width half maximum) increases as a sign of decrease in mean crystallite size from 28 nm for sample A
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up to 20 nm for sample E. Moreover, the lattice parameter changes as well. Also, figure S5 shows the simulation of XRD patterns of A, D and E using standard Reitvield refinement approach. 32 The extracted lattice parameters, i.e. a and c axes of the unit cell, derived from these simulations revealed a decrease in parameter a (from a = 3.8027 ˚ A for sample A to a = 3.7830 ˚ A for sample E) and an increase in parameter c (
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from c = 9.4983 ˚ A for A to c = 9.5025 ˚ A for E). Raman scattering was employed to prove
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the succeful reduction of graphene oxide while attaching TiO2 nanoparticles on its surface. Here, Figure 2 shows the Raman shift of Anatase phase for sample A, i.e. TiO2, with famous
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phonon bands of Eg at 143 cm−1 , B1g at 394 cm−1 , A1g + B1g at 512 cm−1 and Eg2 at 637 cm−1 . See similar reports. 8,15,33 The anatase vibration mode usually appears as a sharp
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peak at 143 cm−1 , independent of the nanoparticles. 18 As figure 2 shows, this peak has been appeared for all samples.
However, samples C, D and E show quite different patterns in
figure 2 compared to TiO2 which could be due to the increase in the share of graphene in these products. The only remained phonon band belonging to TiO2 structure is Eg which is weakened considerabley while the apparaent presence of D and G bands is the signature of the presence of reduced graphene oxide. The G band, which is located at about 1595 cm−1 , is related to the inplane steretching motion of symmetric sp2 C-C bonds. 34 In addition, the D band is attributed to the defects and disoreders on rGO sheets associated with introduction
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of oxygen containing functional groups to the graphitic layer. 23,29,35 To have a more accurate view of the
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ratio, we have also plotted the D and G bands, after removing the background
intensity, in figure 2-inset. The obtained ID IG
are 0.9, 1.3 and 1.5 for C, D and E samples,
ratio is a sign of increasing defects and disorderes on the
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respectively. The increment in
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surface of graphene oxide which comes from strong interactions between TiO2 and graphene oxide sheets 19,29,33,36,37 as well as successful reduction of GO sheets due to the decrease of sp2 domain size, 21,38 which in general suggest that the oxygen-containing groups are reduced. 18
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To indicate the elemental distribution as well as investigating the correctness of
TiO2 GO
weight ratio, EDX spectroscopy was performed. See figure 3. The results show qualitatively
Table S2 obtains
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the increase in C element from sample A to sample E while
TiO2 GO
weight ratio decreases.
weight ratio of 2.53, 1.27, 0.43 and 0.25 for B, C, D and E samples
respectively, which are very close to their nominal wieght ratio of initial powders of TiO2 and GO. However, it should be noted that the share of Oxygen has not been taken into account here because during the reduction of GO it is not clear how much Oxygen gets
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involved in the final composition. Also, SEM images in figure 4 show clear presence of rGO
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sheets and TiO2 NPs (nanoparticles) in both large scales (100 µm /20 µm) and smaller scales (1 µm/2nm). The SEM images of the sample B show almost complete coverage of reduced
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graphene sheet with TiO2 particles. By continuing to samples C, D and E, one could see distinct graphene sheets with less densities of particles which indicates succeful experimental TiO2 GO
variation in the final composite only by changing the initial amounts of
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verification of
raw powders. Also, to elucidate the true attachment of TiO2 NPs on rGO sheets, TEM images have been obtained and presented in figure 5. TEM image of sample A(TiO2 raw powder) shows particles with avarage size of 40nm. The histogram plot of size distribution of the corresponding TEM images are also presented in figure S6. Although, TEM could be used as a selective technique for the estimation of particles size, particle size analyser (P.S.A) as a conventional technique was aslo employed to obtain statistical size distribution of TiO2 particles. The result of this technique is shown in figure S7. This plot shows an avarage
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size of 40nm which is in complete agreement with TEM image of sample A. In addition, TEM images of samples C, D and E show the presence of particles together with graphene sheets in the background. Moreover, these images indicate that as
TiO2 GO
ratio decreases,
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the concentration of TiO2 particles decreases such that graphene sheets appear clearly in the background of the pictures. Also, the histogram size distribution of TiO2 NPs for samples C, D and E are obtained as 30, 25 and 25 nm, respectively (see figure S6). Due to the presence of some oxygen functional groups on the surfaces and the edges of rGO, a bond between these
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groups and the initial nuclei of TiO2 is formed and prevents further nanoparticles growth. In other words, rGO acts as a robust support for TiO2 NPs and hence decreases their size
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when they are embedded on rGO sheets. 39,40
Photodegradation of MB
Absorption spectrum of the catalyst is a key role in determining its photocatalysis activity. Figure S8 illustrates the absorption of catalysts dispersed in DI water. As it could be seen
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from figure S8, there is a considerable red-shift of absorption edge toward visible range as
TiO2 GO
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ratio decreases. The band tail extension in the visible range, specially for D and E samples, sheds light on the capability of prepared catalyst in the visible range. 3,19,26 Fig. S9 shows
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the absorption evolution of MB solution and their corresponding inset photographses under UV light exposure for all the samples. Moreover, figure 6a demonstrates the corresponding
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relative concentration changes versus UV illumination time. It is seen in this figure that all the rGO TiO2 samples enhance the photodegradation of MB significantly, compared to TiO2 particles as control sample. The highest rate of degradation belongs to sample D, i.e. TG 12, in which the degradation is above 90% of MB in only 15 min (see figure 6a inset). Also, to scrutinize the capability of catalyst, logarithmic change of relative concentration were plotted againest irradiation time shown in figure 6b. They all show a first ordered
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kinetic law in MB degradation with the following equation: 41,42
− ln
C C0
! = kt
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Where C is the MB concentration at a particular time, C0 is the initial MB concentration, k is the pseudo- linear first-order kinetic constant (min−1 ), and t is the reaction time in min. The rate constant of every sample is tabulated in table 1. Furthermore, the results show
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that the rate constant of degradation changes from 22 × 10−3 min−1 for pure TiO2 up to 153 × 10−3 min−1 for sample D. In addition, figures 7a and b also show the enhancement of
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MB degradation under visible light for rGO TiO2 nanocomposites. The highest degradation belongs to sample D which in 15 min could degrade over 70% of MB (figure 7a:inset). The samples show a pseudo- linear first-order kinetic law of degradation rate constant which are summed up in table 1. The relatively high rate of 65 × 10−3 min−1 driven from visible light activity for sample D is noticeable in comparison to values which have been reported for the
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similar catalysts. 22,23,41 To examine the reason behind this successful enhancement of photo
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degradation, we first insert the dye adsorption-desorption equilibrium plot in darkness. The result is shown in figure 8. As it can be seen in this figure, the dye adsorption has been 0%, 20%, 60%, 80% and 90% for A to E samples, respectively, placed only 30 min in darkness.
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This could happen when the effective surface area and pore size diameters have undergone a significant enhancement. This enhancement is not only caused by TiO2 NPs deposited on
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rGO surface but also from the π − π stacking between MB and aromatic regions of rGO which has a giant π conjugation. 24,43–45 The adsorption of dye on the surface os rGO-TiO2 maybe due to the noncovalent interactions or π − π stacking between the electron rich π system of rGO and cationic dye, namely MB. 37 Since the specific surface area (SSA) of TiO2 NPs increased by embedding on the rGO sheets (SSA for TiO2 and sample E are 62 and 148 m2 g−1 , respectively), the adsorption amount of dye in dark for sample E, increases compared with that of TiO2. 17
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Thus, BET and BJH techniques were employed to determine the effective surface area and pore size diameters. Figure 9 shows the adsorption-desorption isotherm of the samples obtained from BET experiment. The samples show type IV isotherm with H3 hysteresis
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which is a characteristic of mesoporous materials. 46,47 Moreover, pore size distributions of the samples have been obtained from BJH technique and shown in figure 10. Also, table 2 demonstrates the surface area extracted from BET isotherm plot. In addition, pore volume and pore size are extracted from BJH experiment. The results show a significant increase in
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surface area from 62 m2 g−1 for sample A, i.e. TiO2 NPs, to 148 m2 g−1 for sample E. More importantly, the pore size diameter undergoes a considerable decrease from 12 nm up to
in darkness can be understood partly.
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6 nm for samples A to E, respectively. Thus, the enhancement of dye adsorption of catalysts
To study the effect of interface of TiO2 rGO and enhancement of rate constant, impedance spectroscopy as a strong tool for determination of charge transfer mechanism has been hired. Figure 11a shows the Nyquist plots of the samples and Figure 11a-inset shows the equivalent
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circuit of simulated electrodes for the composites. The simulated circuit shows the presence
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of two RC elements representing two semiconductors rGO and TiO2. Figure S10 depicts the simulation of impedance measurements for samples A and C for more clarity. This intrigu-
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ing result shows a significant decrease of charge carrier transfer resistance from sample A to sample E. This obvious decrement suggests that the charge transfer properties and the
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charge separation effinciencies of the nonocomposites are improved. 25 The samples show a semicircular behavior that corresponds to charge recombination resistance at the interface of TiO2 rGO/dye/electrolyte, 48 resulting from inherent resistance and capacitance of semiconductor composites, together with an ascending line showing the inevitable diffusive behavior of powders. 49,50 The reduced semicircles are suggesting a decrease in the resistance of charge transfer by forming hybrid structure of TiO2 with reduced graphene oxide, 51 and prove that the rGO-TiO2 nanocomposites, from sample B to E, promote the rapid separation between electrons and holes. 17 The π − π conjugated structure of rGO is another factor which could
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promote the separation efficiency of photogenerated e− and h+ . 37 Also, the phase angle diagram (bode-phase plot) (figure 11b) shows a clear shift to lower frequencies as a sign of enhancement of charge transfer from catalyst to electrode. 52 This question might arise that
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although E shows the highest surface area and charge transfer rate, why the sample D has the highest value of degradation? To address this query, one has to take into account that the surface area is the sum of effective surface exposed by both graphene sheets and TiO2 nanoparticles. Although sample E has a higher surface area, which is due to more GO con-
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tent, but rGO only acts as a platform for charge transfer feasibility while dye degradation occurs on TiO2 surfaces.
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In fact, TEM and SEM images reveal the existence of a better surface coverage of rGO sheets by TiO2 particles in sample D compared to sample E. Therefore, one could conclude that sample D should have a higher photocatalyst activity in comparison with sample E. To examine the reliability of our results, the reusability of our best photocatalyst, i.e. sample D, was checked by drying the retrieved sample and under the same conditions
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(0.06 mg/ml of catalyst, 20 ppm of MB, placed in darkness for 30 min at 25◦ C and ex-
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posed to UV and visible light sources with 50 mW/cm2 intensity separately). The results are provided in figure 12 which show that after five times of reusing, 95 % (under UV light)
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and 93 % (under visible light) of the initial performance is still maintained. For a comparison, we have listed previous works on TiO2 rGO photocatalysts in table S3.
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Table S3 compares photodegradation parameters and figure of merit as an indication of photocatalytic activity between our results and recent reports. 8–10,12,19–23 Figure of merit obtained by this report is below one which is at least ten times smaller than other reports. 8–10,12,19–23 This significant enhancement of figure of merit could be explained by possible degradation mechanism as following: To clarify the photodegradation mechanism of MB by rGO TiO2 under visible light, several scavengers were used. Generally, during the photodegradation of dyes, diffrent reactive species such as • OH and • O2− are generated besides of e− /h+ pair. For example, the free
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electrons reduce the dissolved oxygen, resulting in formation of superoxide ions while the holes may react with H2O and OH− to produce hydroxyl radicals. 39 The scavengers used in this work are EDTA for holes, 25,53 p- benzoquinone for • O2− , 25,53 K2S2O8, 40 and AgNO3 54
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as electron scavengers, sodium azide (NaN3), for singlet oxygen, 1 O2, 55 DMSO for • OHbulk , 55 sodium iodide (NaI) for • OHads , 56 And tert- butanol as a free OH radical scavenger. 56 If the photodegradation of MB by the catalyst is performed because of any of the reactive species, the reaction is slowed down or inhibited in the presence of the corresponding scavenger. 39
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For the sake of comparison, the MB degradation in the absence of catalyst under light irradiation, namely photolysis of dye, was investigated for possible self- degradation of MB. Also
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the MB degradation by sample D without using scavenger was carried out. Fig 13 shows the variation of C/Co for MB as a function of illumination time by adding different scavengers into the photocatalytic system. As this figure shows, no degradation in the absence of the photocatalyst for MB under light irradiation was observed. It means that MB is a photo-stable dye during our experiments. Also, this figure shows that the dye was degraded
D
within 60 min in the absence of any scavenger. Adding NaI, tert-Butanol, DMSO, EDTA
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and NaN3 had no considerable influence on photodegradation procces. Thus OH radicals, holes and 1 O2 are not the main active species in MB photodegradation. As figure 13 shows,
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the degradation efficiency of MB over sample D dramatically decreases with the addition of AgNO3, indicating that e− is the main active species during the photocatalytic degra-
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dation process. Since the photocatalytic degradation efficiency decreases in the presents of p-banzoquinone, superoxide radical ion plays a suplimentary role. To ensure that e− has the key role in photodegradation of MB by the catalyst, K2S2O8 as a second electron scavenger was used. As a result, the photodegradation efficiency of MB decreses sharply by pottasiume persulfate, which confirms that the electrons are the main active species in the degradation. The general mechanism of the degradation is explained elsewhere. 13,19,22 Figure 14 shows schematically a possible mechanism for MB photodegradation by rGO-TiO2 photocatalyst under visible light irradiation based on our radical scavenger results. Under light irradiation,
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electrons are excited from valance band (VB) to conduction band (CB) of TiO2 nanoparticles. Thereafter, electrons transfer freely alongside of graphene network and react with dye and dissolved oxygen molecules. The excited electrons reduce the dissolved oxygen, resulting in
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formation of superoxide ions. 13,26,38,53 The electrons and superoxide radicals, degrade MB dye, which was adsorbed on the surface of rGO-TiO2 nanocomposite.
The weight ratio has a crucial effect such that results in a mesoporous structure (pores below 10 nm) with high surface area leading to effective dye adsorption. In the next step, well
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TiO2-decorated reduced graphene sheets can effectively absorb irradiated light and converts to electron and hole pairs. The sample with the most proper of TiO2 nanoparticles distributed
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on rGO sheets have the highest of charge transfer rate and consequently effective charge separation and MB degradation. As it is shown in the graphical abstract the optimization of TiO2/GO weight ratio gives rise to well dye adsorption, excellent charge transfer and spectacular dye degradation as indicated by low values of figure of merit in Table S3.
Conclusion
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3.3
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The rGO TiO2 samples with enhanced visible photocatalyst activity were fabricated via a thermochemical approach using TiO2 and GO raw powders. TiO2/GO weight ratios were
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chosen to be less than one to expose more rGO than TiO2 in composite. EDX, SEM and TEM characterizations testified the variations of TiO2/GO weight ratios in prepared composites
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which were in agreement with the initial values of powder used for synthesis. This confirms the simple method used in this work as efficient tool to control correctly the composition of final products as well. It was also found that TiO2/GO weight ratio reduction enhances the charge transfer through composites significantly which results in a noticeable promotion of its photocatalytic activity. The large linear kinetic constant of photodegradation obtained in this study was confirmed also by impedance spectroscopy. The radical scavenger results illustrate that the electrons are the main active species in the photodegradation of MB by rGO-TiO2 and •O2− palys a supplementary role in the photocatalytic process. 16
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Acknowledgement The authors acknowledge the financial support of Ferdosi University of Mashhad, Iran (grant
Mr. M. Komeili for their technical and instrumental support.
References
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no:3/33186). Also, we would like to thank Prof. E. K. Goharshadi, Dr. M. Karimipour and
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List of Figures Figure 1: XRD patterns of the samples A: (TiO2NPs), B: TG 21, C: TG 11, D: TG 12 and E: TG 14. Inset: the comparison of (101) Bragg plane of the
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samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Figure 2: Raman spectra of samples A, C, D and E. Inset: Comparison of magnified 28
Figure 3: EDX-ray spectroscopy of the samples. . . . . . . . . . . . . . . . . . . .
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D and G bands of graphene structure in the same samples. . . . . . . .
Figure 4: SEM images of the samples with two different magnifications. . . . . . .
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Figure 5: TEM images of the samples. . . . . . . . . . . . . . . . . . . . . . . . .
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Figure 6: (a) Time evolution of relative MB concentration due to photocatalyst activity of the samples under UV irradiation (inset: percentile degradation of MB after 15 min of UV irradiation for different samples). (b) Its corresponding logarithm of the relative concentration of MB vs. ir-
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Figure 7: (a) Time evolution of relative MB concentration due to photocatalyst activity of the samples under visible irradiation (inset: percentile degra-
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irradiation time for the determination of degradation rate of the samples during photocatalytic activity. . . . . . . . . . . . . . . . . . . . . . . .
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Figure 8: The relative concentration of remained MB due to adsorption of dye in the samples (placed in darkness for 30 min). . . . . . . . . . . . . . . .
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Figure 9: Nitrogen absorption-desorption isotherm and BET (inset) plots of the samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Figure 10:Pore size distribution curve of the samples extracted from BJH method.
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Figure 11:Nyquist (a) and phase angle (bode-phase) (b) plots of samples. . . . . .
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Figure 12:Reusability degradation result under UV and visible light for sample D.
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Figure 13:Effect of different radical scavengers (3 mmol/L) on the photodegradation of MB by rGO-TiO2. C/Co ratio (Co is the initial concentration) vs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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irradiation time.
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Figure 14:Schematic of photocatalytic degradation of MB by rGO-TiO2 under light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Figure 1: XRD patterns of the samples A: (TiO2NPs), B: TG 21, C: TG 11, D: TG 12 and E: TG 14. Inset: the comparison of (101) Bragg plane of the samples.
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Figure 2: Raman spectra of samples A, C, D and E. Inset: Comparison of magnified D and G bands of graphene structure in the same samples.
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Figure 3: EDX-ray spectroscopy of the samples.
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Figure 4: SEM images of the samples with two different magnifications.
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Figure 5: TEM images of the samples.
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Figure 6: (a) Time evolution of relative MB concentration due to photocatalyst activity of the samples under UV irradiation (inset: percentile degradation of MB after 15 min of UV irradiation for different samples). (b) Its corresponding logarithm of the relative concentration of MB vs. irradiation time for the determination of degradation rate of the samples during photocatalytic activity.
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Figure 7: (a) Time evolution of relative MB concentration due to photocatalyst activity of the samples under visible irradiation (inset: percentile degradation of MB after 15 min under visible irradiation for different samples). (b) Its corresponding logarithm of the relative concentration of MB vs. irradiation time for the determination of degradation rate of the samples during photocatalytic activity.
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Figure 8: The relative concentration of remained MB due to adsorption of dye in the samples (placed in darkness for 30 min).
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Figure 10: Pore size distribution curve of the samples extracted from BJH method.
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Figure 12: Reusability degradation result under UV and visible light for sample D.
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Figure 13: Effect of different radical scavengers (3 mmol/L) on the photodegradation of MB by rGO-TiO2. C/Co ratio (Co is the initial concentration) vs. irradiation time.
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Figure 14: Schematic of photocatalytic degradation of MB by rGO-TiO2 under light exposure.
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List of Tables 1
First-order rate constant k values of samples for the photocatalytic degradation of MB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Surface area, pore volume and pore size of samples extracted from BET
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B
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C
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33
D
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65
E
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Slope
Vm [cm3 g−1 ]
as(BET) [m2 g−1 ]
Pore diameter [nm]
TiO2
0.070
14.29
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12.24
TG 2-1
0.057
17.22
75
10.65
TG 1-1
0.043
22.04
96
7.05
TG 1-2
0.035
28.47
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6.05
TG 1-4
0.030
33.98
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Sample
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