A novel & effective visible light-driven TiO₂/magnetic porous graphene oxide nanocomposite for the degradation of dye pollutants

Abstract

Porous graphene oxide (pGO) was applied for the preparation of various nanocomposites, including TiO₂ (anatase)/pGO, TiO₂ (mix phase)/pGO, TiO₂ (anatase)/magnetic pGO and TiO₂ (mix)/magnetic pGO, using titanium(IV) chloride as the photocatalyst precursor. Field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, diffuse reflectance UV-vis spectroscopy (DR-UV-vis), and N₂ adsorption–desorption using Brunauer–Emmett–Teller (BET) analysis were employed to investigate the morphology, crystal structure, surface groups, and optical properties of the prepared nanocomposites. The photocatalytic performance of the synthesized nanocomposites under visible light was investigated in the degradation of Rhodamine B (RhB) as a model organic pollutant. The TiO₂ (mix)/magnetic pGO nanocomposite showed enhanced efficiency under visible light irradiation compared with the other catalysts. 100% degradation using the TiO₂ (mix)/magnetic pGO nanocomposite under visible light was achieved in less than 20 min in comparison with TiO₂ (mix, anatase) nanoparticles, and TiO₂ (anatase)/pGO and TiO₂ (mix)/pGO nanocomposites. The degradation efficiency of RhB using the stable nanocomposite remained at 85% after 10 times reuse. The highest photocatalyst performance was achieved with 0.01 g of nanocatalyst and 10 mg L⁻¹ of RhB at pH 9 under visible light irradiation in less than 20 minutes. Moreover, the effect of various scavengers, such as methanol (OH˙ radicals scavenger), disodium ethylenediaminetetraacetate (EDTA, holes scavenger), and p-benzoquinone (BQ, O₂˙⁻ radicals scavenger), on RhB degradation was investigated.

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1. Introduction

Environmental problems, such as the treatment of pollutants in the air and water, is an important challenge that we are faced with in the world.¹ A great issue over a last decade lies in the purification of waste water during industrial, agronomical, and domestic activities.² Semiconductor photocatalysis has been reported as an impressive method for solving worldwide environmental pollution issues.¹ Titanium dioxide as an important semiconductor has been widely studied in the photocatalytic field. It has attracted much attention due to its advantages such as wide compatibility, unique optical characteristics, chemical stability, long durability, nontoxicity, low cost, and electronic properties. These suitable properties also make TiO₂ a potential candidate for applications in Li-ion batteries, sensors, solar cells, UV lasers, field-effect transistors. Moreover, its advantages make TiO₂ suitable for the decomposition of organic pollutants in air and aqueous media (e.g., organic dyes, toxic micro pollutants and oils) into CO₂ and H₂O.^3–5 TiO₂ exists as two main polymorphs, anatase and rutile, in which the rutile phase is known as the “photocatalytically less” active form, while the anatase form is most widely used as a photocatalyst, due to its high surface area and slow charge carrier recombination.⁶ However, the mixture of anatase and rutile phase exhibits synergetic effects between both phases.⁶ Nonetheless, the main issue is that the band gap of TiO₂ photocatalysts lies in the UV region, which covers less than 5% of the solar spectrum and the photonic efficiency of unmodified monophasic TiO₂ is restricted owing to its large band gap (3.2 eV), high charge carrier recombination rate, and low surface area.^7,8 Some of the strategies to overcome these limitations and extend its light absorption range include metal (V, Co, Ag, and etc.) and non-metal doping (C, N). Among these dopants carbon has received eminent importance.^7,9

Within the past decade a new carbonaceous material, “graphene” has attracted much attention, due to its high carrier mobility, unique electronic property, high thermal conductivity, and extremely high theoretical specific surface area (∼2600 m² g⁻¹).¹⁰ Graphene has better conductivity in comparison to other types of carbon materials (e.g., carbon nanotubes, and activated carbon) due to its higher surface area.¹¹ One of the most important derivatives of graphene is graphene oxide (GO).¹¹ GO is a two dimensional sheet with sp² hybridized orbitals, which is characterized by a layered structure with hydrophilic oxygen functional groups located in the edge and basal plane.^3,11 GO can act as an acceptor/transporter of photogenerated electrons for TiO₂ nanoparticles (NPs) and reduce the recombination of photogenerated electron–holes, thus resulting in the enhancement of the photocatalytic activity.^3,5,12 It is possible to improve the properties of graphene and create new types of graphene-based materials by modifying the structure of graphene (edges or basal planes).¹³ Recently, porous graphene (pGO), which is a graphene-based material, has stimulated much interest of many researchers because of its potential applications in nanoelectronics, gas sensors, hydrogen storage, water and air purification and many other fields.¹³ pGO contains holes either with high or random regularity. pGO nanoparticles are the most employed porous materials due to their high surface areas and “space” for the transportation or storage of electrons/ions and gases or liquids with a hydrophobic nature, and their low cost to manufacture.

In the previous decades, Fe₃O₄ magnetic NPs have attracted great attention due to their excellent adsorption, low toxicity, catalytic properties, unusual structural, and biocompatibility.¹⁴ Decorating porous graphene oxide sheets with inorganic materials such as Fe₃O₄ could result in the formation of an extraordinary 2D nanocomposite structure.¹⁵ The combination of Fe₃O₄ with porous graphene oxide can effectively prevent the agglomeration of individual graphene sheets and it will have a very specific surface area and potential applications in waste water treatment and it can also be a convenient adsorbent for dye pollutants.¹⁴ Furthermore, another advantage of this combination is the complete removal of the sorbent by an external magnetic field.¹⁶

For the first time, the effectiveness of TiO₂ (mix)/magnetic pGO and TiO₂ (anatase)/magnetic pGO nanocomposites as photocatalysts for RhB degradation under visible light irradiation is compared. Using these magnetic nanocomposites the photocatalytic efficiency of neat TiO₂ nanoparticles was enhanced under visible light irradiation. Some experimental parameters, such as photocatalyst and RhB concentration and pH of the sample, were optimized. The structures of the prepared nanocomposites were characterized using various techniques, including UV-vis spectroscopy, FT-IR, SEM, EDX, XRD, TGA and BET.

2. Experimental

2.1. Reagents and materials

Titanium(IV) chloride (>99%), ammonium sulfate (>99%), ammonium hydroxide (25–30%), FeCl₃·6H₂O (>99%), FeCl₂·4H₂O (>99%), 3-aminopropyltrimethoxysilane (APTMS > 97%), oxalic acid dehydrate (>99%), KMnO₄, RhB and hydrazine hydrate of analytical grade were obtained from Merck Company (Whitehouse Station, NJ). Furthermore, graphite powder (99.95%) was purchased from Alfa Aesar.

2.2. Catalyst characterization

The morphology of the products was characterized using a Cambridge scanning electron microscope (Stereoscan 360). BET analysis was used to determine the specific surface areas of the prepared nanophotocatalysts using a BELSORP-18 Plus (BEL Japan) through N₂ adsorption (degassing temperature was at 120 °C for 12 h). XRD measurements were conducted on a XD-3A diffractometer with CuK radiation (λ = 1.5406 Å). Thermogravimetric analysis of the nanocomposites was conducted using a TGA/DTA (BAHR: STA 503) at a heating rate of 10 °C min⁻¹ in air. FT-IR measurements were carried out on a BOMEM MB-series FT-IR spectrometer in the form of dried samples using KBr pellets. A Jencons ultrasonic system was used to disperse the NPs or nanocomposites in an aqueous medium. The concentrations of dyes solutions were determined using a UV-S600 spectrophotometer (Analytikaljena). Samples were irradiated using visible light (180 power LED lamp in a circular manner, 3.2 V, 1 W, (79 000 Lux)). The intensity was determined using a light meter (MASTECH model MS 6612 digital Lux meter) at λ > 390 nm. UV-vis diffuse reflectance spectra of the samples were obtained on a Shimadzu-2100 UV-vis spectrophotometer in the range of 200–900 nm and BaSO₄ was used as a reference material. The pH of the solutions was measured using a Methrohm digital pH meter 827. An inductively coupled plasma optical emission spectrometer (ICP-OES; Varian Vista PRO Radical) was used to determine the content of Ti and Fe in the samples. The magnetic properties of the particles were analyzed using a vibrating sample magnetometer (DC magnetometer 1.5 Tesla) (I_max = 150 A, P ≤ 9 kW) at room temperature.

2.3. TiO₂ NPs synthesis

For the synthesis of the TiO₂ NPs, 0.75 mL of TiCl₄ was slowly added to a solution of 1.5 M ammonium sulfate. Then, the prepared solution was heated at 75 °C under stirring for 90 min. In the next step, an ammonium hydroxide solution (2.5 M) was added drop by drop under high speed stirring until the pH was 7.0. Then, the white solid precipitate was filtered and repeatedly washed with distilled water and ethanol and dried at 50 °C. Subsequently, the powder was calcined at temperatures of 450 °C and 750 °C for 4 h (for the synthesis of anatase and mix phase TiO₂, respectively).¹⁷

2.4. Synthesis of surface modified TiO₂

At first, 0.5 g of TiO₂ (anatase and mix phase) was added to 40 mL ethanol/water with a 7 : 1 ratio and dispersed in an ultrasonic bath. Then, 1 mL APTMS was added to the solution to form a homogeneous suspension under vigorous stirring, following by refluxing for 12 h at 85 °C. Subsequently, the product was washed with ethanol and then dried sufficiently.

2.5. Synthesis of porous GO

GO was prepared via a modified Hummers' method from graphite powder.¹⁸ In brief, 0.5 g GO was added to 100 mL deionized water, and dispersed using an ultrasonic bath for 3 h. Then, 1 g KMnO₄ was added to the solution, followed by stirring for 15 min. The resulting suspension was heated in household microwave oven for 5 min and then the black product was mixed with 100 mL water and 20 mL hydrazine solution. The reaction mixture was refluxed at 100 °C (24 h) for the reduction of GO. Finally, in the last step, the obtained product was washed several times with oxalic acid and hydrochloric acid (v/v 1 : 1), water and ethanol and then dried at 70 °C in a vacuum oven.¹⁹

2.6. Synthesis of magnetic pGO

To obtain the super paramagnetic pGO nanoscomposite, first 0.5 g pGO was added to 100 mL deionized water and then dispersed using an ultrasonic bath for 1 h. Then, under vigorous stirring 0.25 g FeCl₂·4H₂O and 0.5 g FeCl₃·6H₂O with the molar ratio of Fe³⁺ : Fe²⁺ = 2 : 1 was added to the solution under an N₂ atmosphere.²⁰ Subsequently, 15 mL of ammonia solution (% 25) was added to the suspension at 80 °C. After stirring for 30 min, the precipitates were separated with an external magnetic field and washed several times with water and ethanol, respectively. Then, the product was dried at 35 °C in a vacuum oven.

2.7. Synthesis of TiO₂/magnetic pGO

First, 0.5 g magnetic pGO and 0.5 g of TiO₂-APTMS (in a ratio of 1 : 1) were added to 100 mL water and dispersed for 30 min in an ultrasonic bath under vigorous stirring. The mixture was refluxed at 100 °C for 24 h. The obtained precipitates were washed with water and ethanol several times. The product was separated with an external magnetic field and dried at 50 °C. The amount of Ti and Fe in the TiO₂/magnetic pGO nanocomposite was determined to be 2.23 and 9.57 mmol g⁻¹, respectively, by inductively coupled plasma-optical emission spectroscopy (ICP-OES).

2.8. Photocatalytic degradation of RhB

The photocatalytic performance of the prepared TiO₂ (mix)/magnetic pGO nanocomposite was investigated in degradation of RhB dye under visible light (Fig. 1). A solution of RhB was prepared as an initial solution with a concentration of 10 mg L⁻¹. The 0.01 g photocatalyst was added to 100 mL of the initial RhB solution. The irradiation source was a visible light (LED lamp and λ > 390 nm). The mixture was sonicated in the dark for 5 min and stirred for 30 min to ensure the establishment of adsorption–desorption equilibrium. During the photocatalytic reaction, the mixture was exposed to visible light and stirred by a magnetic stirrer. Subsequently, the analytical samples were taken out from the reactor and immediately centrifuged or separated using an external magnet (for magnetic catalysts) or filtered through a nanofilter to remove the photocatalysts. The transparent solutions were analyzed by recording the variation in absorption in the UV-vis spectrum of RhB using a UV-vis spectrophotometer (λ = 560 nm).

Fig. 1 Photocatalytic activity test of TiO₂/magnetic pGO nanocomposite under visible light irradiation for RhB degradation.

2.9. Detection of reactive oxidative species

The reactive species in the oxidation were detected via in situ trapping experiments during the photodegradation tests. The process of tracing is similar to the photodegradation experimental process. Different scavengers, including methanol (OH˙ radicals scavenger), disodium ethylenediaminetetraacetate (EDTA, holes scavenger), and p-benzoquinone (BQ, O₂˙⁻ radicals scavenger), prior to the addition of the photocatalyst in three different photodegradation experiments, were added to the RhB solution and the concentration of the scavengers was 1.0 mM.²¹

3. Results and discussion

Scheme 1 shows the synthetic reactions for the TiO₂/magnetic pGO nanocomposite as a photocatalyst. The amine functionalized TiO₂ NPs react with the functional groups on the pGO sheets and electrons can be transported along the pGO sheets and then the generation of OH˙ from adsorbed O₂ and H₂O occurs. Accordingly, this effective charge transfer could decrease the charge recombination and increase the photocatalytic activity of TiO₂/magnetic pGO nanocomposites.²²

Scheme 1 Schematic of TiO₂/magnetic pGO nanocomposite synthetic stages as a photocatalyst.

3.1. Characterization of photocatalysts

3.1.1. SEM and EDX. The morphologies and size of the nanoparticles were demonstrated using SEM images. The elementary contents were evaluated by EDX. Fig. 2 shows the SEM images of (a) GO and (b) pGO, (c) magnetic pGO and (d) the TiO₂/magnetic pGO nanocomposite.

Fig. 2 SEM images of GO (a), pGO (b), magnetic pGO (c), TiO₂/magnetic pGO (d) nanocomposite and (e) EDX element analysis of TiO₂/magnetic pGO as the final nanophotocatalyst.

Fig. 2a and b show GO and porous GO, respectively. The graphene oxide layer is clear in these figures. The porous surface of pGO contains holes with random regularity. It can be observed that Fe₃O₄ NPs and TiO₂ NPs are uniformly distributed on the pGO sheets (Fig. 2c and d). In addition, TiO₂ and Fe₃O₄ NPs could sufficiently interact with the functional groups on the pGO sheets, which are well dispersed in an aqueous phase and lead to formation of a uniform mixture of pGO sheets and TiO₂ NPs. The micrographs of the nanocomposites Fig. 2c and d show that the average size of the nanoparticles is approximately 50 nm. The nanoparticles have a spherical structure with a smooth surface morphology. The EDX spectra (Fig. 2e) of the final nanocomposite confirmed the presence of expected elements in the obtained product on final stage.

3.1.2. XRD patterns. The nanoparticle and nanocomposite crystal structures were characterized using XRD patterns. Fig. 3 shows the XRD patterns of the TiO₂ (anatase), TiO₂ (mix), GO, pGO, magnetic pGO, and TiO₂ (mix)/magnetic pGO nanocomposites. The XRD pattern of anatase TiO₂ shows peaks at 2θ = 25.36°, 37.84°, 48.05°, 54.05°, 62.64° and 75.16°. This result of the anatase phase proves that the characteristics of the anatase phase of TiO₂ are in good agreement with reference data.²³ The XRD pattern of mix phase TiO₂ shows peaks at 2θ = 25.4°, 37.91°, 48.12°, 53.96°, and 62.76° which confirm the anatase phase of TiO₂ and the peaks at 2θ = 36.99°, 38.63°, 55.14°, 68.81°, 70.37°,75.10°, and 76.32° are proven to be the rutile phase of TiO₂.²⁴ The XRD patterns of mix phase TiO₂ display the presence of both anatase and rutile phases in its structure. The figure shows the (001) diffraction peak for GO at 2θ = 11.7°, which can be impute to the water trapped among the graphene oxide sheets.²⁵ The crystalline structure of graphene oxide is fragmented and there is no solidarity therefore it becomes an amorphous structure. There is no common peak between GO and porous graphene oxide. However, in the diffraction pattern of pGO there is a bump close to 24.22° and a peak arises at 18.16° in comparison with GO, which suggests that some functionalized groups are reduced.²⁶ The XRD patterns of magnetic pGO shows diffraction peaks at (111), (220), (311), (400), (422), (440) and (511). These are the characteristics peaks of Fe₃O₄ NPs with a cubic spinel structure.¹⁴

Fig. 3 XRD patterns of TiO₂ (mix), TiO₂ (anatase), GO, pGO, magnetic pGO and TiO₂ (mix)/magnetic pGO nanocomposites.

Diffraction lines of the TiO₂ (mix)/magnetic pGO nanocomposites were observed at 2θ = 30.10°, 35.52°, 48.06°, 53.90°, 56.97°, 62.76° and 75°, which can be assigned to the 220, 311, 400, 422, 511, 440 and 533 reflection, respectively, and these show standard Fe₃O₄ NPs crystals with a spinal structure. The peaks located at 2θ = 25.32°, 37.81° and 55° are due to TiO₂ and the absence of the typical peak of pGO is possibly due to the disruption and good exfoliation of pGO in the TiO₂ (mix)/magnetic pGO nanocomposites or because of the low intensity of the pGO pattern.²⁷

3.1.3. FT-IR analysis. Fig. 4 displays the FT-IR spectra of the synthesized TiO₂, GO, pGO, magnetic pGO, and TiO₂/magnetic pGO nanocomposites. In the spectrum of the pure TiO₂ NPs, the appearance of a peak below is 558.33 cm⁻¹ ascribed to the Ti–O bending vibration and stretching modes. In addition, there is a broad band peak at 3236.78 cm⁻¹, which is because of the O–H stretching frequency from the hydroxyl groups. The FT-IR spectra of the pGO sheets have various functional groups, such as the epoxy C–O group at 1310.52 cm⁻¹, alkoxy C–O stretching group at 1012.26 cm⁻¹, and carbonyl C=O stretching vibration at 1622.10 cm⁻¹, which affirm the presence of oxygen containing functional groups at the edge of the graphene oxide layers. The broad absorption band located in 3370.27 cm⁻¹ is imputed to the remaining –OH groups of pGO. In general, in the comparison of GO nanosheets there is no remarkable difference between these nanosheets and merely some of the functional groups on GO was reduced. In the magnetic pGO spectrum the additional peaks at 1403.24 cm⁻¹ and 1617.96 cm⁻¹ are due to the presence of Fe₃O₄ in the nanocomposites.^10–12 The spectrum of the TiO₂/magnetic pGO nanocomposite is similar to the magnetic pGO spectrum. The broad band peak at 3387 cm⁻¹ is assigned to O–H stretching from the hydroxyl groups. Moreover, the peaks at 1569 cm⁻¹, 1483 cm⁻¹, 1135 cm⁻¹ and 685 cm⁻¹ are related to carbonyl C=O stretching vibration of pGO, symmetric stretching vibrations of Fe₃O₄, alkoxy C–O stretching group of pGO, and bending vibration and stretching modes of the TiO₂ NPs, respectively.

Fig. 4 FT-IR spectra of TiO₂, GO, pGO, magnetic pGO and TiO₂/magnetic pGO nanocomposite.

3.1.4. UV-vis diffuse reflectance. The TiO₂/magnetic pGO nanocomposite was characterized using UV-vis diffuse reflectance spectroscopy. Fig. 5 shows the UV-vis DRS spectra of the TiO₂ and pGO nanoparticles, magnetic pGO and TiO₂/magnetic pGO nanocomposites. The band gap energy (E_g) of a photocatalyst usually has a major effect on its photocatalytic activity. The band gap energy is obtained from the Kubelka–Munk theory. In this study, TiO₂ displays a light response in the range of 200–400 nm (UV region), which is imputed to the electron transition from the valence band to the conduction band. As shown in Fig. 5, the pGO nanosheets exhibit a powerful absorption in the entire wavelength range.²⁸ There is no difference between the pGO nanosheets and magnetic pGO because of the Fe₃O₄ NPs presence, which act as a light harvesting material just like the pGO nanosheets.²⁹ In comparing the pure TiO₂ NPs, the absorption band of the TiO₂/magnetic pGO nanocomposites, shifted to the visible light region and this result is due to the hybridization of C_2p and O_2p atomic orbits to create a new valence band.¹² Therefore, the energy of band gap was decreased more in the TiO₂/magnetic pGO nanocomposite over the presence of Fe₃O₄ in the final catalyst, which caused it to absorb more visible light.

Fig. 5 UV-vis DRS of TiO₂, GO, magnetic pGO, and TiO₂/magnetic pGO nanocomposite.

3.1.5. Thermogravimetric analysis. To identify the thermal stability and weight loss of the nanocomposites, TGA studies were carried out for the TiO₂ and pGO nanoparticles, magnetic pGO and TiO₂/magnetic pGO nanocomposites (Fig. 6). The thermogram of pGO displays a weight loss of about 20% up to 0–200 °C due to the removal of moisture and decomposition of oxygen functional groups. After 200 °C, the skeleton of pGO is decomposed. The decrease in the weight loss of the magnetic pGO nanocomposite in comparison with pGO is due to the strong interaction between pGO and Fe₃O₄ NPs and participation of Fe₃O₄ NPs as a stable material. Moreover, TiO₂/magnetic pGO gradually lost about 28 wt% mass between 100 and 480 °C and 72 wt% mass of nanocomposites was retained at 800 °C. This result suggests that the TiO₂/magnetic pGO nanocomposite has higher thermal stability than the pGO nanoparticles. This happens because of the powerful interaction existing between the TiO₂ NPs and stable magnetic pGO nanocomposite.

Fig. 6 TGA diagram of TiO₂, pGO, magnetic pGO and TiO₂/magnetic pGO nanocomposites.

3.1.6. BET studies. The specific surface area of TiO₂/magnetic pGO was measured via the BET method using adsorption isotherms. Pore volume and pore size distribution were calculated using the BJH model based on the desorption isotherm. The specific surface area and pore volume are 9526 m² g⁻¹ and 10.21 nm, respectively, and the total pore volume is 0.2431 cm³ g⁻¹. The TiO₂/magnetic pGO nanocomposite has micropores and mesopores in its structure.

The large surface area (395.26 m² g⁻¹) is due to the porosity of pGO and its small pore size can make more active sites for nanocomposites. The large S_BET of the TiO₂/magnetic pGO nanocomposite indicates a good catalytic performance.

3.1.7. VSM analysis. The magnetization loop related to TiO₂/magnetic pGO at room temperature is presented in Fig. 7. As observed in this figure, the catalyst remains super paramagnetic because there is a minimal coercivity value and remanence on the magnetization loop. The M_s value for the composite (25 emu g⁻¹) is distinguished to be significantly lower than the Fe₃O₄ nanoparticles (62 emu g⁻¹)⁴⁵ as a result of the presence of a diamagnetic species in the nanocomposite structure that surrounds the Fe₃O₄ nanoparticles.

Fig. 7 VSM magnetization curve of the TiO₂ (mix)/magnetic pGO nanocomposite.

3.2. Photocatalytic activity

Fig. 8a shows the photocatalytic activity of TiO₂ (anatase), TiO₂ (mix) nanoparticles and TiO₂ (anatase)/pGO, TiO₂ (mix)/pGO, TiO₂ (anatase)/magnetic pGO and TiO₂ (mix)/magnetic pGO nanocomposites under visible light irradiation. In the presence of each pure TiO₂ (anatase and mix), there is a low degradation efficiency for RhB due to their wide band gap (3.2 eV for anatase and >3.2 eV for mix phase).²⁴ However, the efficiency of the mix phase is a little better than the monophasic TiO₂, because the presence of the rutile phase (in addition to the anatase phase) better results are obtained because the rutile phase may be activated by visible light.^6,30 Therefore, the degradation efficiency of the RhB solution (in 60 min) over pure TiO₂ NPs in anatase and mix phase is 72% and 75%, respectively. However, the efficiency of degradation was significantly recovered under the same conditions by the loaded pGO and magnetic pGO nanocomposites. The photocatalytic efficiencies of the TiO₂ (mix)/pGO, TiO₂ (anatase)/pGO, TiO₂ (anatase)/magnetic pGO and TiO₂ (mix)/magnetic pGO nanocomposites were increased to 93% and 89% in 60 min and 95% and 99% in 20 min, respectively. The degradation of RhB by TiO₂ (Degussa), ZnO and porous graphene oxide compared to TiO₂/magnetic porous graphene oxide confirmed the highest performance of the TiO₂/magnetic porous graphene oxide. Furthermore, the results of our investigation were compared with previous reports of RhB degradation from reaction times and performance viewpoints (Table 1).

Fig. 8 Photocatalytic degradation of RhB in the presence of the synthesized photocatalysts under visible light irradiation (a), the relationship between −ln(C/C₀) and reaction time (b) and the relationship between (C/C₀) and reaction time under dark conditions (c).

Table 1 Comparison of the results obtained from different photocatalysts for the degradation of RhB dye

Photocatalysts	Degradation time	Irradiation source	Efficiency	Ref.
Pt/TiO2	4 h	Visible	80%	41
Ag/AgBr/ZnO-2	3 h	Visible	100%	42
Amorphous carbon doped TiO2	60 min	Visible	95%	43
TiO2/CeO2/Bi2O3	2 h	Visible	55%	44
TiO2 (mix)/magnetic pGO	20 min	Visible	100%	This work

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The Fig. 8c shows the relationship between C/C₀ and reaction time in dark conditions and visible light irradiation for RhB. As observed in the graphs, there is no significant adsorption in the dark condition. However, it could be observed, for the TiO₂ (anatase)/magnetic pGO and TiO₂ (mix)/magnetic pGO nanocomposites, the amount of adsorption increased and this due to the effective presence of pGO and magnetic pGO.

The characterization results show that the optical response of TiO₂ shifted to the visible region from the UV region with the introduction of Fe₃O₄ and pGO nanoparticles. The reaction rate of different catalysts can be compared using the pseudo-first-order reaction equation:

−dC/dt = kC

The concentration of RhB at time “t” is “C” and the apparent rate reaction constant is k. Therefore, the equation can be integrated to give the following form:⁸

−ln(C/C₀) = kt

Therefore, Fig. 8b shows the relationships between in −ln(C/C₀) and irradiation time for RhB degradation. The linear relationships (≥99%) in the photocatalyst curves demonstrate that the degradation of RhB is well matched with first order kinetics. The TiO₂ (mix)/magnetic pGO nanocomposite displays the highest rate constant (k = 0.2), in all the nanocomposites, which was nearly 10 times larger than the pure anatase TiO₂ and even larger than the TiO₂ (anatase)/magnetic pGO nanocatalyst (k = 0.17).

3.3. Photocatalyst stability

An important factor for practical applications is the stability of photocatalysts. In the photodegradation of RhB over the TiO₂ (mix)/magnetic pGO photocatalyst, cycling runs were conducted, as shown in Fig. 9a. After 10 runs, the photocatalytic activity of the nanocomposite slightly decreased. The photodegradation efficiency of RhB is about 85%, which indicates that the photocatalyst is stable. The XRD pattern and IR analysis of nanocatalyst after 10 times usage shows the same peaks and only small decreases in the peak intensity are shown (Fig. 9b and c).

Fig. 9 Photocatalytic degradation of RhB with TiO₂ (anatase)/magnetic pGO and TiO₂ (mix)/magnetic pGO nanocomposites at different recycling times (a) and XRD pattern (b) and IR analysis (c) of TiO₂ (mix)/magnetic pGO catalyst before and after 10 cycles.

3.4. Photodegradation mechanism of TiO₂/magnetic pGO

To understand the active species in the degradation of RhB, the effect of several radical scavengers, such as methanol (OH˙ radicals scavenger), disodium methylenediaminetetraacetate (EDTA, holes scavenger) and p-benzoquinone (BQ, O₂˙⁻ radicals scavenger), was tested to determine the reaction mechanism.²¹ Fig. 10 shows the photocatalytic degradation of RhB using the TiO₂/magnetic pGO nanocomposite in the presence of different scavengers. As shown in Fig. 10 with the addition of O₂˙⁻ as the radical scavenger (p-benzoquinone), the RhB degradation was depressed evidently, which suggests that the O₂˙⁻ radical was main active species in the mechanism. Therefore, the rate of electron–hole pair recombination was mainly reduced in the nanocomposites and thus the photocatalytic activity was improved.

Fig. 10 Photocatalytic degradation of RhB dye in the presence of different scavengers.

The possible mechanism of TiO₂/magnetic pGO under visible light irradiation is proposed in Fig. 11. During the photocatalytic oxidation process, when visible light irradiated on the TiO₂ NPs surface, holes and electrons were generated. Furthermore, the presence of Fe₃O₄ NPs caused the conduction band of TiO₂ go to higher level, thus the gap energy decreased. In the pure TiO₂ NPs, the recombination of hole and electrons occurred quickly and therefore the activity of the photocatalyst decreased.^12,31

Fig. 11 Mechanism of the TiO₂/magnetic pGO nanocomposite to enhance photocatalytic activity under visible light irradiation.

In the TiO₂ (mix)/magnetic pGO nanocomposite, the electrons can transport along the pGO sheets and this is due to the fact that the potential of graphene oxide is below the conduction band of TiO₂.¹² The porous graphene oxide sheets can promote the effective charge separation of electron–hole pairs. The trapped electrons on the pGO sheets react with adsorbed O₂ to generate reactive oxygen species (O₂˙⁻), which react with water to produce hydroxyl radicals. Subsequently, the hydroxyl radicals degrade the dye (RhB). At this time, the holes on the valence band of TiO₂ combine with H₂O to generate active OH˙. In addition, reactive holes on TiO₂ react with adsorbed water or hydroxyl groups to produce surface hydroxyl radicals, which are the reactive species for degrading RhB dye and the holes can oxidize RhB molecules directly.^32–37

The main reactions are shown below:

TiO₂ + hν → TiO₂ (h⁺ + e⁻)

TiO₂ (h⁺) + H₂O → H⁺ + OH⁻

TiO₂ (e⁻) + GO → GO (e⁻)

GO (e⁻) + O_2adsorb → GO + O₂⁻˙

O₂⁻˙ + H₂O → OH˙

OH˙ + dye → degradation product

These very active species (OH˙, O₂˙⁻, e⁻, and h⁺) efficiently degrade RhB and turn RhB into the less harmful H₂O and CO₂. In the photocatalytic reaction process, charge recombination is suppressed in the TiO₂ NPs, resulting in the enhanced photocatalytic activity of the TiO₂/magnetic pGO nanocomposite.

3.5. Chemical oxygen demand (COD) analysis

COD analysis was carried out to confirm the mineralization of RhB dye and the COD value was analyzed. After 20 min irradiation time, the percentage of COD was about 97.4%. It can be observed that the degradation was slower than the decolorization of the dye, which may be due to the breaking the chromophoric group of the dye molecule by the photogenerated oxidants. These results indicate that mineralization of the dye required a little longer irradiation time. Therefore, the activity of the catalyst is convenient and organic pollutants are degraded in an aqueous medium.³⁸

3.6. Optimization of reaction conditions

3.6.1. Effect of catalyst dosage. For the optimization of operational conditions, catalyst dosage is an important parameter. Therefore, the concentration of catalyst was studied in catalyst dosage range from 0.005 to 0.05 g for the degradation of RhB and the results are shown in Fig. S1a.† The results show that the rate of degradation gradually increased with the increase in the catalyst concentration from 0.005 to 0.01 g; however, the efficiency of degradation slightly decreased with an increase in catalyst concentration from 0.01 to 0.05 g. This may be due to the fact that excessive catalyst can scatter the photons in a photocatalytic system.³⁹

3.6.2. Effect of pH. The most important parameter in photocatalyst experiments is the pH of the solution. Thus, the effect of pH on RhB degradation was investigated in the pH range from 2 to 11. As can be observed in the Fig. S1b,† the most effective pH condition is 9. The form of the RhB molecule at pH = 2 is the cationic form (RhB⁺).⁴⁰ The PZC (point of zero charge) of TiO₂ is 6.5 (ref. 41) and under this point the TiO₂ surface is positively charged. Therefore, the repulsion between RhB and TiO₂ is increased because in the acidic condition the dye exists in the cationic form. However, in a basic medium, the RhB becomes deprotonated, and the zwitterion is formed (Fig. S2†). Therefore, there is no repulsion between TiO₂ NPs and RhB, which results in increased degradation of RhB. Furthermore, under basic conditions, the number of OH⁻ groups increases, which results in enhanced degradation.⁴¹

3.6.3. Effect of the initial concentration of dye. To study the efficiency of initial concentration on degradation of RhB, the initial concentration of the dye was changed from 5 to 20 mg L⁻¹ while the other experimental conditions were fixed. As shown in Fig. S1c,† with an increase in the initial concentration of RhB, the rate of degradation decreased. This is due to the excessive amount of RhB that absorbed on the catalyst surface and therefore the efficiency of degradation was decreased. Through several experiments, the optimal concentration was determined to be 10 mg L⁻¹.³⁹

4. Conclusion

In summary, various nanocomposites of TiO₂ (mix)/magnetic pGO, TiO₂ (anatase)/magnetic pGO, TiO₂ (mix)/pGO and TiO₂ (anatase)/pGO were successfully prepared and their properties were characterized. After quantitative analysis, TiO₂ (mix)/magnetic pGO exhibited the best photocatalytic performance for the degradation of RhB, due to its high surface area and the pore volume of pGO in addition to the favorable TiO₂ (mix) nanoparticles in the presence of visible irradiation. The presence of pGO as an electron acceptor results an excellent optical property. Therefore, the lifetime of photo-generated electron–hole pairs was significantly increased. The aim is the effective oxidation of RhB under the visible light irradiation. The degradation efficiency of RhB after reusing of the nanocatalyst 10 times remained over 85%. Furthermore, the resulting photocatalyst is able to be separated with an external magnet easily and can be reused several times (because of the presence of Fe₃O₄ nanoparticles in the nanocomposite). All these advantages demonstrate that this nanocomposite can be a convenient photocatalyst in the purification of environmental organic pollutants.

References

1. S. W. Lee and W. M. Sigmund, Chem. Commun., 2003, 780–781.
2. Y. Ni, W. Wang, W. Huang, C. Lu and Z. Xu, J. Colloid Interface Sci., 2014, 428, 162–169.
3. P. T. Nguyen, C. Salim, W. Kurniawan and H. Hinode, Catal. Today, 2014, 230, 166–173.
4. K. R. Reddy, V. G. Gomes and M. Hassan, Mater. Res. Express, 2014, 1, 015012.
5. Y. Gao, M. Hu and B. Mi, J. Membr. Sci., 2014, 455, 349–356.
6. M. Boehme and W. Ensinger, Nano-Micro Lett., 2011, 3, 236–241.
7. S. Yu, H. J. Yun, Y. H. Kim and J. Yi, Appl. Catal., B, 2014, 144, 893–899.
8. R. Kavitha and L. G. Devi, J. Environ. Chem. Eng., 2014, 2, 857–867.
9. H. G. Lee, G. Sai-Anand, S. Komathi, A. I. Gopalan, S. W. Kang and K. P. Lee, J. Hazard. Mater., 2015, 283, 400–409.
10. H. Sun, S. Liu, S. Liu and S. Wang, Appl. Catal., B, 2014, 146, 162–168.
11. Y. Cong, M. Long, Z. Cui, X. Li, Z. Dong, G. Yuan and J. Zhang, Appl. Surf. Sci., 2013, 282, 400–407.
12. P. Muthirulan, C. N. Devi and M. M. Sundaram, Ceram. Int., 2014, 40, 5945–5957.
13. P. Russo, A. Hu and G. Compagnini, Nano-Micro Lett., 2013, 5, 260–273.
14. W. Lü, Y. Wu, J. Chen and Y. Yang, CrystEngComm, 2014, 16, 609–615.
15. X. Yan, Y. Li, F. Du, K. Zhu, Y. Zhang, A. Su, G. Chen and Y. Wei, Nanoscale, 2014, 6, 4108–4116.
16. G. Z. Kyzas, N. A. Travlou, O. Kalogirou and E. A. Deliyanni, Materials, 2013, 6, 1360–1376.
17. Z. Wang, C. Chen, F. Wu, B. Zou, M. Zhao, J. Wang and C. Feng, J. Hazard. Mater., 2009, 164, 615–620.
18. M. R. Nabid, M. Golbabaee, A. B. Moghaddam, R. Dinarvand and R. Sedghi, Int. J. Electrochem. Sci., 2008, 3, 1117–1126.
19. D. Fan, Y. Liu, J. He, Y. Zhou and Y. Yang, J. Mater. Chem., 2012, 22, 1396–1402.
20. Z. Yuanbi, Q. Zumin and J. Huang, Chin. J. Chem. Eng., 2008, 16, 451–455.
21. C. Cui, Y. Wang, D. Liang, W. Cui, H. Hu, B. Lu, S. Xu, X. Li, C. Wang and Y. Yang, Appl. Catal., B, 2014, 158, 150–160.
22. B. H. Nguyen, V. H. Nguyen and D. L. Vu, Adv. Nat. Sci.: Nanosci. Nanotechnol., 2015, 6, 033001.
23. R. J.-N. Z. Xiu-Jian and L. B.-S. H. Xin, Chin. J. Inorg. Chem., 2006, 3, 029.
24. A. Ramchiary and S. Samdarshi, Adv. Nat. Sci.: Nanosci. Nanotechnol., 2014, 305, 33–39.
25. P. Gao, K. Ng and D. D. Sun, J. Hazard. Mater., 2013, 262, 826–835.
26. Z. Fan, Q. Zhao, T. Li, J. Yan, Y. Ren, J. Feng and T. Wei, Carbon, 2012, 50, 1699–1703.
27. J. Jing, Y. Zhang, W. Li and W. Y. William, J. Catal., 2014, 316, 174–181.
28. Y. Shi, K. Zhou, B. Wang, S. Jiang, X. Qian, Z. Gui, R. K. Yuen and Y. Hu, J. Mater. Chem. A., 2014, 2, 535–544.
29. Z. Wang, L. Yin, Z. Chen, G. Zhou and H. Shi, J. Nanomater., 2014, 2014, 1–6.
30. S. Bakardjieva, J. Šubrt, V. Štengl, M. J. Dianez and M. J. Sayagues, Appl. Catal., B, 2005, 58, 193–202.
31. S. Morales-Torres, L. M. Pastrana-Martínez, J. L. Figueiredo, J. L. Faria and A. M. Silva, Environ. Sci. Pollut. Res., 2012, 19, 3676–3687.
32. U. I. Gaya and A. H. Abdullah, Progress and problems, J. Photochem. Photobiol., C, 2008, 9, 1–12.
33. K. Maeda, J. Photochem. Photobiol. C, 2011, 12, 237–268.
34. T. Ochiai and A. Fujishima, J. Photochem. Photobiol. C, 2012, 13, 247–262.
35. H. Park, Y. Park, W. Kim and W. Choi, J. Photochem. Photobiol. C, 2013, 15, 1–20.
36. B. T. Zhang, X. Zheng, H. F. Li and J. M. Lin, Anal. Chim. Acta, 2013, 784, 1–17.
37. X. An and C. Y. Jimmy, RSC Adv., 2011, 1, 1426–1434.
38. D. Maruthamani, D. Divakar and M. Kumaravel, J. Ind. Eng. Chem., 2015, 30, 33–43.
39. S. Yang, Y. Huang, Y. Wang, Y. Yang, M. Xu and G. Wang, Int. J. Photoenergy, 2012, 2012, 1–6.
40. L. You-ji and C. Wei, Catal. Sci. Technol., 2011, 1, 802–809.
41. N. Hariprasad, S. Anju and E. Yesodharan, Res. J. Mater. Sci., 2013, 2320, 6055.
42. S. Lei, L. Lin, M. Jun, M. Yanan, Z. Shifa, W. Fangxiao and S. Jianmin, Ceram. Int., 2014, 40, 3495–3502.
43. S. Penghui, T. Jiayu, Z. Zhiwei, S. Wenxin, G. Shanshan and C. Fuyi, Appl. Surf. Sci., 2015, 324, 35–43.
44. Z. Zhijun, X. Changsheng, Z. Shasha, Y. xueli, Z. Tao and L. Jie, J. Alloys Compd., 2013, 581, 385–391.
45. M. Kalantari, M. Kazemeini and A. Arpanaei, Mater. Res. Bull., 2013, 48, 2023–2028.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02827f

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