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Original Article
DOI: 10.1016/j.jmrt.2019.09.029
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Available online 28 September 2019
Application of nanostructured graphene oxide/titanium dioxide composites for photocatalytic degradation of rhodamine B and acid green 25 dyes
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M.S. Adly, Sh.M. El-Dafrawy
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shomirage@yahoo.com

Corresponding author.
, S.A. El-Hakam
Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt
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Table 1. Rate constants and R2 on photocatalytic degradation reaction using GO/TiO2 composites.
Table 2. Comparison of the previous studies of photocatalysts for degradation of Rh B and AG-25.
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Abstract

Graphene oxide/TiO2 (X wt% GO/TiO2, X=3, 7, 10, 12 and 15) composites were synthesized by a hydrothermal method followed by calcination treatment at 300, 400 and 550°C. These composites exhibit high efficiency for photocatalytic degradation of organic dyes such as rhodamine B (Rh B) and acid green 25 (AG-25) under UV-visible light irradiation. The photocatalytic degradation efficiency was influenced by the GO content, calcination temperature, dye concentration, photocatalyst dose, hydrogen peroxide concentration and initial pH. The samples were systematically evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (DRS), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). The results showed good distribution of TiO2 nanoparticles on the GO surface. A significant enhancement in the photocatalytic degradation of dyes was achieved using 10wt% GO/TiO2 treated at 400°C as a photocatalyst compared to pristine TiO2 and/or other prepared samples, i.e., complete degradation of Rh B after 75min and approximately 96% of acid green 25 after 180min. This enhancement in photocatalytic degradation efficiency is attributed to the GO sheets, which are characterized by a high surface area that increases the adsorption capacity, a strong electron transfer ability and 2D, thin layers.

Keywords:
Graphene oxide
TiO2
Photocatalytic degradation
Rhodamine B
Acid green 25
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1Introduction

In the last few decades, interest in water and air purification using heterogeneous photocatalysis with semiconductor materials has notably increased [1]. These photocatalysts utilize different semiconductor photocatalysts, such as TiO2, ZnO, Fe2O3, Bi2WO6, Bi20Ti20, Ag2CO3, Nb2O5, BiTiO3, SrTiO3, CuS/ZnS, ZnWO4, WO3, ZnS, and α-sulfur crystals, g-C3N4 and have established their effectiveness for pollutant degradation and complete mineralization [2–10]. TiO2 is one of the most effective and attractive photocatalysts that has been investigated due to its low toxicity, low cost, high photocatalytic activity and high thermal and chemical stability [11,12]. TiO2 exists as three polymorphs: rutile, anatase and brookite phases. The rutile phase is considered the most stable form of TiO2. Photocatalytic degradation remains a great challenge due to the large band gap of certain semiconductors that require UV radiation and have low absorption properties. The large band gaps limit the practical application of photocatalysis because only 4% of the sun’s energy that reaches Earth is UV radiation, while approximately 45% of the energy is visible light [13,14]. TiO2 has a large band gap (3.2–3.0eV) that is active under UV light [15]. To overcome this drawback, the band gap of TiO2 particles should be reduced and absorption in the visible region enhanced. Different strategies have been employed to enhance the photocatalytic efficiency of TiO2, including chemical modification by doping other components into TiO2 crystals and morphological modification by increasing the porosity and surface area. Modification of TiO2 for improved visible light photoactivity can be achieved by: (1) Metal doping that could introduce impurity energy levels near to valence band (VB) and conduction band (CB). In some cases, metal ions served as recombination centres due to formation of lattice defects. (2) Non-metal doping such as S, N, O, P and few others. After introducing non-metal to TiO2 lattice, it makes red shift towards visible light to form new VB over the original one. An upward shift of VB and decrease of the forbidden band observed as a result of hybridization of p orbital of non-metals with O 2p. (3) Noble metal doping [16].

Separation of TiO2 nanoparticles from aqueous media is difficult, so the nanoparticles are placed on supporting materials such as cellulose, zeolite, activated carbon, clay and ceramic, to allow easy separation and increase stability [17]. As a new carbonaceous material, graphene oxide (GO) has attracted significant attention in the past few decades for photocatalysis applications due to its unique properties, such as high surface area, flexible sheets, good thermal and electrical conductivity and excellent charge carrier mobility [18]. GO is an ideal material for use in optoelectronic devices, catalysis and energy conversion applications. This material is a novel catalyst promoter or carrier due to the abundance of reactive oxygen functional groups on the GO sheet surface [19]. Recently, attention has focused on the modification of TiO2 using GO derivatives [13,17,20,21]. The VB and CB of graphene consist of two types of orbital bonding, i.e., π and antibonding π (π*) orbitals, which produce a single sheet of graphene with a zero band gap semiconductor [22]. Due to the close distance between carbon atoms, electrons and holes act as similar mass-free charge carriers, leading to electronic overlap. Introducing TiO2 into nanostructured carbon materials, such as carbon nanotubes, fullerenes and GO sheets, can reduce charge recombination [23,24]. Here, we proposed a technique for preparing TiO2 on GO sheets to achieve better photocatalytic degradation of some organic dyes.

2Experimental2.1Materials

Chemicals were purchased and used without further purification. Flake graphite powder (99.95%) was purchased from Alpha Aesar. Concentrated sulfuric acid (H2SO4), hydrochloric acid (HCl), potassium permanganate (KMnO4), sodium nitrate (NaNO3), 30% hydrogen peroxide (H2O2), absolute ethanol, titanium (IV) isopropoxide, sodium hydroxide (NaOH), rhodamine B and acid green 25 dyes were purchased from Sigma–Aldrich.

2.2Preparation of GO

GO was synthesized according to the improved Hummer’s and Offeman method from graphite powder [19,25–29]. First, 2g NaNO3 was completely dissolved in 25mL of concentrated H2SO4, and 2g graphite powder (99.95%) was added to the solution to exfoliate the graphite flakes. Then, the mixture was kept in an ice bath at 0°C with vigorous stirring for approximately 2h. After that, 4g KMnO4 was slowly added over approximately 1.5h (maintaining the temperature below 20°C), and after cooling for 2h, the mixture was a green colour. Second, the mixture was allowed to stand overnight at room temperature with vigorous stirring. As the reaction occurred, the mixture gradually thickened. Then, 100mL of deionized H2O was slowly added to the paste, causing violent effervescence, brown vapour evolution, resulting from the oxidation of KMnO4, and an increase in temperature. The temperature was maintained at 98°C until the end of the effervescence, and the colour of the suspension changed to brown. After 15min, 200mL of hot deionized H2O was added to the mixture. Next, 30mL of H2O2 (30%) was added when the temperature had cooled to 60°C to eliminate the residual KMnO4, and the suspension became a golden yellow colour with bubbling. Finally, the solution was washed 3 times with HCl (10%) and then with deionized H2O to completely remove residual H2SO4, which was tested by BaCl2. Then, the samples were dried at 80°C under vacuum overnight to obtain GO sheets.

2.3Preparation of TiO2

0.5g of cetyltrimethylammonium bromide (CTAB) in an anhydrous system of 25mL absolute ethanol was stirred until completely dissolved. Then 12.02mmol (3.56mL) of titanium (IV) isopropoxide was added to the reaction mixture and stirred for 2h. Next, 10mL of deionized H2O was added dropwise to the solution until a white precipitate formed. The suspension was stirred for 6h, transferred into a Teflon-lined stainless-steel autoclave, heated at 180°C for 8h and then dried under vacuum at 90°C. The white precipitate was annealed in a muffle furnace at 300, 400 or 550°C at a rate of 10°C/minute and held at the required temperature for 2h.

2.4Preparation of GO/TiO2

GO/TiO2 composites were synthesized by a hydrothermal method. Different percentages (3, 7, 10, 12 and 15wt%) of GO and 0.5g of CTAB were added to an anhydrous system of 25mL absolute ethanol and stirred until the GO sheets were completely suspended to give a brown suspension. A calculated amount of titanium (IV) isopropoxide was added to the reaction mixture and stirred for 2h. Then, 10mL of deionized H2O was added dropwise to the mixture until a grey precipitate formed. The samples were stirred for 6h, transferred into a Teflon-lined stainless-steel autoclave, heated at 180°C for 8h, and then dried under vacuum at 90°C. The precipitate was annealed in a muffle furnace at 300, 400 and 550°C for 2h [30].

2.5Characterization

X-ray diffraction (XRD) patterns of all the prepared samples were obtained at a high angle using a PW 150 instrument (Philips) with a Cu Kα radiation source with a Ni filter. The instrument was operated at a current of 45mA and a voltage of 40kV. The IR spectra were measured using a Nicolet 550 FTIR spectrometer in the range of 400–4000cm−1. Scanning electron microscopy (SEM) images were used for surface morphological evaluation and were obtained by using a JEOL-JSM-6510LV scanning electron microscope, Japan. Transmission electron microscope (TEM) images and particle size were obtained using a JEOL-JEM-2100 transition electron microscope, Japan that operated at 200keV. The TEM samples were prepared by dropping an alcohol suspension of the fine sample powders onto a copper grid coated with holey carbon foil and then drying the grid at ambient temperature. UV-visible diffuse reflectance spectroscopy (UV-visible DRS) measurements of the samples were recorded on a UV/visible/NIR spectrophotometer (V-570, JASCO, Japan). The samples were analysed by a PHI 5800 ESCA system, Physical Electronics, USA, instrument at 45° in FAT mode with an X-ray gun and Mg target (1486.6eV) at a power of 12kV, an analytical background vacuum of 2×10−7 Pa, a channel energy of 93.90eV and a step length of 0.1eV/s. The surface of the samples was sputtered with Ar+ ions to eliminate interferences from contaminants and subsequently preserved under high vacuum. To determine the concentrations of the dye solutions, we used a Unicam 5625 UV/visible spectrophotometer (Perkin Elmer, USA).

2.6Photocatalytic activity

The photocatalytic activities of the samples were evaluated by monitoring the degradation of Rh B and AG-25 dyes under UV/visible-light irradiation. A Hg lamp was purchased from Westinghouse, UAE, and the reaction temperature was kept at 35°C. The degradation of dyes was performed in a system consisting of 50mL of dye with a concentration of 10mg/L for Rh B and 40mg/L for AG-25 using a 1.0g/L photocatalyst concentration. The suspension was magnetically stirred in the dark for 1h to reach adsorption/desorption equilibrium. At given time intervals, 1.0mL of sample was diluted 10 times with deionized H2O and then centrifuged until complete separation was achieved. The samples were analysed by UV-visible spectrophotometry at the wavelengths (λmax) 554 and 642nm for the Rh B and AG-25 dyes, respectively.

3Results and discussion3.1Characterization of GO/TiO2 photocatalysts

TiO2 particles were prepared by a hydrothermal method in the presence of the CTAB capping agent. The intense peak at 2θ=26.4° in the graphite flake XRD pattern is shown in Fig. 1a. During oxidation of the graphite powder, oxygen atoms were introduced as different functional groups, such as OH, COOH and epoxy groups, which attached to the graphite planner surface [27]. Fig. 1b shows a broad band at 2θ=10.7°, which corresponds to the [200] crystal plane of the GO sheets. The XRD patterns of the graphite flakes and GO showed that the inner spacing (d) increased from 0.34Å (graphite flakes) to 0.82Å (GO) due to the presence of a large number of oxygenated functional groups on the surface of the GO sheets and due to the increasing oxidation degree, the peak shifted to a lower angle (2θ) [18]. The characteristic peak of the graphite flakes was not observed in the pattern of GO, which can be explained by the small lateral size of the GO sheets, as shown in the SEM images. Although the GO sheets are stacked due to self-assembly through van der Waals forces and hydrogen bond and the GO sheets have enough disorder that they do not produce the signature π–π stacking diffraction peak [31]. Fig. 2 shows the XRD patterns of TiO2 at different calcination temperatures. As the calcination temperature increased from 300 to 400 and 550°C, the signals for anatase (tetragonal), polymorphous TiO2 increased. At 550°C, the XRD pattern shows a pure anatase structure with peaks at 2θ=25.28, 37, 37.8, 38.6, 48.1, 53.9, 55.1, 62.1, 62.7, 68.8, 71.3 and 75°, which can be indexed to the [101], [103], [004], [112], [200], [105], [211], [213], [204], [116], [220] and [215] crystal faces of TiO2, respectively [32]. The average crystal size of the TiO2 nanoparticles treated at 300, 400 and 550°C was estimated by Scherrer’s equation for the [101] crystal face. The crystallite size was 5.9, 10 and 18.1nm for TiO2 at 300, 400 and 550°C, respectively. When the composites were calcined above 400°C, the anatase crystals within the agglomerates were sintered, and the crystals grew through coalescence, which transformed the original agglomerate into a single anatase grain; thus, the crystal size of the prepared TiO2 samples increased [18]. This result indicated that a good distribution of TiO2 nanoparticles as shown in Figs. 3 and 4. No other peaks shown in the XRD patterns of prepared titania, which indicates that no other metal ions have entered into TiO2 lattice [33]. The lack of the characteristic peak for GO at 2θ=10.7° may be attributed to the low concentration of GO or the excellent distribution of TiO2 nanoparticles coating the surface leading to good exfoliation of the GO sheets in the nanocomposites [26,27,34,35].

Fig. 1.

XRD patterns for (a) graphite flakes and (b) graphene oxide sheets.

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Fig. 2.

XRD patterns for pure TiO2 calcined at (a) 300°C, (b) 400°C and (c) 550°C.

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Fig. 3.

XRD patterns for (a) pure TiO2, (b) 3wt% GO/TiO2, (c) 10wt% GO/TiO2 and (d) 15wt% GO/TiO2 calcined at 400°C.

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Fig. 4.

XRD patterns for (a) pure TiO2, (b) 3wt% GO/TiO2, (c) 10wt% GO/TiO2 and (d) 15wt% GO/TiO2 calcined at 550°C.

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Fig. 5a shows the FT-IR spectra for the graphite flakes and GO, which display a band at 3433cm−1 that is characteristic of the stretching vibration of the OH functional group that forms during the oxidation of graphite flakes. The bands at 1726cm−1 appeared due to the CO stretching vibration. Those at 1635cm−1 indicate the CC stretching vibration and 1600cm−1 represent unoxidized graphite domains. We also noted the characteristic bands for the COH and CO stretching vibrations at 1222 and 1051cm−1, respectively [36]. As shown in Fig. 6, the bands in the GO/TiO2 composites are the same as those of GO, as previously illustrated. Fig. 7 shows that the intensity of the bands decreased with increasing calcination temperature of 10wt% GO/TiO2.

Fig. 5.

FTIR spectrums of (a) graphene oxide (b) graphite flakes.

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Fig. 6.

FTIR spectrums of (a) pure TiO2, (b) 3wt% GO/TiO2, (c) 10wt% GO/TiO2 and (d) 15wt% GO/TiO2 calcined at 400°C.

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Fig. 7.

FTIR spectrums of 10wt% GO/TiO2 calcined at (a) 300°C, (b) 400°Cand (c) 550°C.

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SEM and TEM images for GO are displayed in Fig. 8a and b, and the images reveal thin, randomly aggregated and crumpled sheets close to each other, forming a disordered solid [37]. The aggregated and crumpled surfaces formed because of the high surface area of the GO sheets. Fig. 8c-k show the TEM images of pure TiO2, 3, 10 and 15wt% GO/TiO2 calcined at 400 and 550°C. The GO loaded with TiO2 presents a good distribution of TiO2 nanoparticles on the surface of the GO sheets at low temperature, and the TiO2 nanoparticles may self-assemble on the surface of GO during the preparation process [17]. The TiO2 nanoparticles were highly distributed upon increasing the GO concentration. In the case of increasing the calcination temperature to 550°C, the TiO2 nanoparticles tended to aggregate as large particles due to crystal growth, as shown in Fig. 8g, i and k, which matches the XRD results. Fig. 8l–n show pure TiO2 (300°C), pure TiO2 (400°C) and 10wt% GO/TiO2 (400°C). The anatase structure was confirmed by selected area electron diffraction (SAED), which proves polycrystalline nature of the polymorphous [38]. The bright diffraction rings of SAED, associated with anatase structure, was consistent with the results obtained from XRD [39,40]. The SAED pattern shows clear ring diffraction patterns and does not indicate dislocations in the lattice planes, which confirmed that the prepared TiO2 nanoparticles have a high crystallinity. A comparison study of the crystal sizes of the composites obtained from the XRD patterns and TEM images. As the calcination temperature increased, a larger crystal size was obtained because above 400°C the anatase crystals within the agglomerates are sintered, and the crystals grow through coalescence, transforming the original agglomerate into a single anatase grain and increasing the crystal size of the prepared TiO2 samples [18].

Fig. 8.

(a) Graphene oxide SEM image and TEM images for (b) Graphene oxide, (c) pure TiO2 (300°C), (d) pure TiO2 (400°C), (e) pure TiO2 (550°C), (f) 3wt% GO/TiO2 (400°C), (g) 3wt% GO/TiO2 (550°C), (h) 10wt% GO/TiO2 (400°C), (i) 10wt% GO/TiO2 (550°C), (j) 15wt% GO/TiO2 (400°C), (k) 15wt% GO/TiO2 (550°C), (l) SAED pure TiO2 (300°C), (m) SAED pure TiO2 (400°C), (n) SAED 10wt% GO/TiO2 (400°C).

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Fig. 9a shows DRS UV-visible spectra for modulation the band gap energy (Eg) of the prepared composites calculated from Kubelka-Munk transformation Eg=1240/λ, where λ is the cut off wavelength [41]. The term of the band gap is the energy from the top of the VB to the bottom of the CB. For the pure titania, the absorption threshold energy is about 393.5nm, which is corresponding to band gap about 3.16eV. The band gap energy for 3, 10 and 15wt% GO/TiO2 are 3.06, 2.98 and 2.3eV, respectively. The red shift of λ value to visible light is due to incorporation of graphene oxide into titania lattice [42]. The results obtained in case of composites calcined at 550°C but the band gap energy decreased by lower extent than in case of 400°C as shown in Fig. 9b.

Fig. 9.

Band gap modulation for pure (a) TiO2, 3, 10 and 15wt% GO/TiO2 calcined at 400°C. (b) TiO2, 3 and 10wt% GO/TiO2 calcined at 550°C.

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The GO was examined by XPS, and the obtained spectrum provides an elemental analysis of the GO surface. The XPS spectrum of the GO sheets includes C(1s) and O(1s) signals (Fig. 10a). The symmetrical broad peaks of C(1s) suggest the existence of distinguishable species. By analysing the peak in the spectrum, three bands at 284, 285.6 and 288.6eV were deconvoluted. The sharp peak at 284eV can be attributed to sp2 hybridized carbons (CC). The peaks at 285.6 and 288.6eV are due to the existence of COH and COOH bonds (Fig. 10b) [19,27,43–46]. The band at a high energy of 532eV may be ascribed to the CO bond originating from residual oxygen-containing groups and/or residual water (Fig. 10c) [18].

Fig. 10.

XPS analysis (a) Graphene oxide sheets, (b) Peak analysis of C (1s).

(c) Peak analysis of O (1s).

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3.2Photocatalytic degradation of Rh B and AG-25 using GO/TiO2 composites3.2.1Photocatalytic degradation of Rh B and AG-25

Photocatalytic degradation of Rh B and AG-25 dyes under UV-visible light in the presence of pure TiO2 particles resulted in nearly 75% degradation of the dyes after 60 and 90min, respectively. This percentage increased when GO was used as a dopant. GO allows the incident UV and/or visible radiation to penetrate the TiO2 nanoparticles and proceed with the photocatalytic degradation. As shown in Fig. 11a and b, increasing the GO content to 10wt% GO/TiO2 increased the photocatalytic degradation efficiency of Rh B and AG-25. With a high concentration of GO, the photocatalytic activity decreased, which can be explained by the increasing light absorption into the GO sheets and the lower relative content of TiO2 for the generation of radicals [47]. GO acts as an adsorbent for organic contaminants that diffuse to the phase boundary or the interphase to undergo effective degradation. A higher GO content results in more oxygen-containing groups, such as hydroxyl, epoxy and carboxyl groups, leading to high ionic-electronic interactions with contaminants. The GO sheets also act as an electron scavenger to increase the electron transfer rate of TiO2 particles, which enhances the photocatalytic properties and inhibits charge carrier recombination. Additionally, during the photocatalytic degradation process, the GO sheets may be reduced to graphene under UV radiation and regenerate electrons. This action can lead to an increase in the possibility of π - π interactions with organic contaminants to enhance charge separation [21].

Fig. 11.

Photocatalytic reaction using different contents of GO loaded on TiO2 nanoparticles used in degradation of (a) Rh B and (b) AG-25.

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In the present work, the kinetics of photocatalytic degradation of Rh B is fitted using Langmuir–Hinshelwood model as expressed in Eq. (1).

where r is the photocatalytic degradation rate (mg/(L minute)), t is the illumination time (minute), C is the dye concentration, k is the reaction rate constant (mg/(L minute)) and K is the adsorption coefficient of the dye (L/mg). Fig. 12a shows the photocatalytic degradation of Rh B under UV/visible light, which can be described as a first-order reaction with a rate constant of 0.051min−1 (R2=0.984). Fig. 12b shows the first-order reaction for the degradation of AG-25 with 10wt% GO/TiO2, which has a rate constant of 1.577×10−6min−1 (R2=0.984), as illustrated in Table 1. Table 2 shows a comparison studies of present and pervious works on degradation of Rh B and AG-25.

Fig. 12.

First-order kinetic model for differently prepared composites (a) Rh B and (b) AG-25.

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Table 1.

Rate constants and R2 on photocatalytic degradation reaction using GO/TiO2 composites.

Samples  Rh BAcid green 25
  k (min−1R2  k (min−1R2 
Pure TiO2 (400°C)  0.033  0.942  0.948×10-6  0.983 
3 wt% GO/TiO2 (400°C)  0.037  0.978  0.558×10-6  0.942 
7 wt% GO/TiO2 (400°C)  0.034  0.981  0.759×10-6  0.974 
10 wt% GO/TiO2 (400°C)  0.051  0.984  1.577×10-6  0.984 
12 wt% GO/TiO2 (400°C)  0.041  0.948  0.781×10-6  0.944 
15 wt% GO/TiO2 (400°C)  0.036  0.985  0.732×10-6  0.948 
Table 2.

Comparison of the previous studies of photocatalysts for degradation of Rh B and AG-25.

Photoactive materials  Dye concentration (mg/L)  Dose (g/L)  Duration (min.)  Degradation %  References 
Ag3PO4@GO  1.0  40  100  [48] 
Bi2S3/3DOM-TiO2  10  9.0  360  96  [49] 
TiO2@GO  20  0.6  210  100  [50] 
TiO2@GO (Rh B)  20  1.0  120  100  Our work 
TiO2@GO (AG-25)  40  1.0  180  97  Our work 
3.2.2Effect of the dye concentration

An experiment was conducted with different concentrations of the dyes, i.e., 10, 20, 30 and 50mg/L for Rh B and 20, 40, 70 and 100mg/L for AG-25, with 1.0g/L of 10wt% GO/TiO2 photocatalyst. Fig. 13a and b show that as the initial concentration of the dye increased, the photocatalytic degradation efficiency decreased because the amount of light absorbed by the photocatalyst decreased. Light radiation has difficulty penetrating a solution with a high concentration of dye. The concentration of organic pollutants such as dyes may affect the photocatalytic degradation mechanism. At low concentrations, both the photoinduced electron and hole participate in degradation by the typical mechanism. However, at high concentrations, the degradation mechanism depends on the contact angle of the photocatalyst, and the radical concentration is not as high as that in the previous mechanism due to pollutant or dye molecules blocking the active centres on the catalyst surface, resulting in a large amount of dye remaining [51].

Fig. 13.

Photocatalytic degradation of different concentrations of (a) Rh B and (b) AG-25 using 10wt% GO/TiO2.

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3.2.3Effect of the photocatalyst concentration

One of the most important factors influencing the kinetics of photocatalytic degradation is the catalyst concentration. An experiment was carried out using four different concentrations, 0.5, 1.0, 1.5 and 2.0g/L, of the photocatalyst. As shown in Fig. 14a and b upon increasing the concentration of photocatalyst from 0.5g/L to 2.0g/L, the photocatalytic degradation of dyes increased. Additionally, by increasing the photocatalyst concentration, the number of photons adsorbed on active sites increased, and the number of adsorbed dye molecules increased with the amount of photocatalyst [52–56].

Fig. 14.

Effect of photocatalyst dose on degradation of (a) Rh B (b) AG-25.

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3.2.3.1Effect of hydrogen peroxide (H2O2)

Fig. 15a shows the effect of 0.5, 1.0 and 2.0mL of H2O2 on the photocatalytic degradation of Rh B dye. Approximately 95.5% of the dye photodegraded after 20min using 2.0mL H2O2. Upon increasing the H2O2 concentration, the degradation and number of adsorbed dye molecules both increased. The addition of H2O2 has two important functions. First, it increases the amount of the hydroxyl radical (HO), which inhibits recombination of photoinduced holes and electrons, Eq. (2).

e‾ (TiO2) + H2O2 → TiO2 + ‾OH + OH

Fig. 15.

Effect of H2O2 on photocatalytic degradation of (a) Rh B and (b) AG-25 using 10wt% GO/TiO2.

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The second function of H2O2 is promoting charge separation by accepting the electron in the conduction band to form OH, as expressed in Eqs. (3) and (4)[46,57,58].

H2O2 + e‾ → ‾OH + OH
H2O2 +O2•− → ‾OH + OH + O2

In the case of AG-25, increasing the concentration of H2O2 resulted in a decrease in the photocatalytic degradation, as shown in Fig. 15b. A high concentration of H2O2 may inhibit the oxidation rate because of very reactive hydroxyl radicals, and H2O2 may consume the valance band holes (Eqs. 5–7). Additionally, a competitive reaction due to radical-radical recombination must be considered (Eq. 8) [59].

OH + H2O2 → HO2• + H2O
OH + HO2• → O2 + H2O
OH + OH → H2O2

3.2.3.2Effect of the initial pH of the dye solution

The effect of the solution pH on the degradation of Rh B was studied at pH values of 2, 4, 6, 8 and 10. Fig. 16a and b show the strong dependence of the photocatalyst performance on the initial pH for the photocatalytic degradation of Rh B dye. The optimum pH for Rh B was 4, and the optimum pH was 2 for AG-25 using 10wt% GO/TiO2, which proves that dyes tend to degrade in acidic media. Any change in the initial degradation rate with the varying initial pH can be attributed to the acidic/basic characteristics of the TiO2 nanoparticles on the surface [52]. The initial pH of the solution affects the photocatalytic degradation of dyes, but it is very difficult to isolate the pH effect because of its multiple roles. First, an ionized species forms on the surface of the photocatalyst, according to Eqs. (9) and (10).

TiOH + HO‾ → TiO‾ + H2O

Fig. 16.

Effect of pH change on degradation of (a) Rh B and (b) AG-25 using 10wt% GO/TiO2 (400°C).

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Second, hydroxide ions and positive holes interact with each other, producing hydroxyl radicals (HO). At low pH, positive holes are the major oxidant species, whereas HO is the predominate species at neutral and high pH values. It is easy to generate hydroxyl radicals by oxidizing hydroxide ions on the surface of TiO2 in alkaline media. At a high pH, there is repulsion between the negative charges on the photocatalyst surface and the hydroxyl radicals that inhibits photooxidation.

Third, in acidic media, TiO2 particles tend to agglomerate, which decreases the adsorption of dye molecules and photons. TiO2 particles tend to adsorb cationic dyes better than anionic dyes, where the pH depends on the charge [59].

3.2.3.3Reusability of photocatalysts

The catalyst with high performance in photocatalytic degradation of Rh B and AG-25 selected to measure the recyclability and stability. From the general trend of the curve, photocatalytic activity decreases from 100% to 91% and from 96.2% to 88% after the fourth run in case of Rh B and AG-25, respectively (Fig. 17a and b). These tests conducted after an hour of adsorption for each run. The loss of activity is small and can be neglected that may attributed to the diminution of adsorption property of composites. It can be concluded that, the prepared photocatalysts can reused many times with high stability and performance during degradation of different organic dyes.

Fig. 17.

Reusability of photocatalyst (a) Rh B (b) AG-25.

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3.2.3.4Photocatalytic degradation mechanism

Photocatalysis depends on the semiconductor materials which interact with certain wavelength (or sufficient energy) to generate reactive oxidizing species (ROS) which play the main role in degradation of organic pollutants. There are two simultaneous processes take place in order to produce a highly reactive oxidizing species. The first is oxidation of adsorbed H2O molecules by hole and the second involves the reduction of dissolved oxygen by exited electrons, these two processes generate hydroxyl and superoxide anion radical. In photocatalysis, the photons energy must equal or greater than the band gap energy (Eg) of the semiconductor. Certain energy required to shoot each photon of the outer occupied orbital of the valance band (VB) and excited to the unoccupied orbital of the conduction band (CB) in the semiconductor materials.

In case of anatase structure, the energy of light (λ387nm) is required due to the band gap is 3.2eV. The absorption of energy excites an electron from the valance band forming positively charged hole hVB+ to conduction band generating an electron eCB- Eq. (11).

Ti3+ and O defect sites trapped as charge carriers in the TiO2 lattice or they disperse the energy by recombination process. Positive holes oxidize H2O and HO at the surface producing hydroxyl radical (HO) Eq. (12) which is the strongest oxidants with oxidation potential equal 2.8V. The organic species can be oxidized by the HO producing CO2, H2O and mineral acids Eq. (15).

Titania particles adsorbed molecular oxygen on its surface which trap electrons in the conduction band producing superoxide radical anion by reduction reaction Eq. (14) that may react with H+ to produce hydroperoxyl radical followed by reduction reaction producing H2O2 Eqs. (16) and (17). The reactive oxygen species may pass another way for degradation of organic pollutants in the aqueous solution Eqs. (18) and (19)[60].

GO sheets may act as electron conductor that accelerate the electron-hole separation and prompt the rate of electron migration. Strong oxidizers like  O2-• and HO• produced from migration of electron to CB of TiO2 that reacted to oxygen species distributed along the GO surface. Photocatalytic activity enhanced as a result of GO sheets prolong the charge carrier separation, which generate more active oxidizing species [61].

To determine the photodegradation mechanism, different quenchers like EDTA, benzoquinone and methanol were used to trap holes, superoxide anion and hydroxyl radicals, respectively [62]. This experiment carried out at constant concentrations of inhibitors with fixed amount of catalyst that added to dye solutions. As shown in Fig. 18, additives inhibit the photocatalytic degradation of dyes. EDTA and methanol show the lowest degradation of dyes that proved that holes and hydroxyl radicals, respectively. The rate decreased in case of benzoquinone, which indicates superoxide anion radicals also necessary for photodegradation process [63].

Fig. 18.

Effect of scavengers on photocatalytic degradation of Rh B and AG-25.

(0.1MB).
4Conclusion

A facile technique was used for the synthesis of GO/TiO2 composites by hydrothermal methods. Different contents of GO were incorporated into TiO2 nanoparticles to enhance the photocatalytic degradation activity. The most effective sample was 10wt% GO/TiO2, which enhanced the degradation of Rh B and AG-25 dyes. Samples were calcined at 300, 400 and 550°C, and the optimal calcination temperature was 400°C. Many experiments were conducted to study the effect of the dye concentration, photocatalyst dosage, concentration of H2O2 and initial pH of the solution. By increasing the dye concentration, the photocatalytic degradation efficiency decreased, but the degradation efficiency was enhanced by increasing both the concentration of the photocatalyst and/or the H2O2 concentration for Rh B. The optimum pH for the degradation of the dye was 4 for Rh B and 6 for AG-25. The prepared samples were examined by X-ray diffraction, TEM, FT-IR, UV/visible DRS and XPS.

Conflicts of interest

The authors declare no conflicts of interest.

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