Photocatalytic degradation of phenol using a new developed TiO₂/graphene/heteropoly acid nanocomposite: synthesis, characterization and process optimization

Abstract

A TiO₂/graphene (TiO₂/Gr) nanocomposite was synthesized using bottom-up assembly. The TiO₂/Gr nanocomposite was modified with 12-tungstophosphoric acid (H₃PW₁₂O₄₀, TiO₂/Gr/xPW). The structural properties of the nanocomposite have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), zeta potential, Brunauer–Emmett–Teller (BET), Fourier transform infrared (FTIR), and diffuse reflectance spectra (DRS). Results of DRS showed a visible shift when the TiO₂/Gr nanocomposite was modified with PW. The photocatalytic activity of the synthesized nanocomposite was measured by the degradation of phenol under visible and UV light irradiation. The effects of initial concentration of phenol, concentration of photocatalyst, reaction time, loading of PW and initial pH on phenol removal efficiency in batch experiments were investigated. The results showed that the degradation efficiency is decreased with an increase in initial concentration of phenol from 50 to 150 mg L⁻¹ while pH did not show a significant impact. As a result, TiO₂/Gr/xPW exhibited a higher visible-light photocatalytic activity in comparison with TiO₂/Gr and pure TiO₂ with the maximum degradation efficiency of 91, 68, and 15%, respectively. This clearly shows that among the modified composites, the catalyst with 20 wt% PW showed maximum visible shift. COD measurements under optimum conditions showed 78% phenol mineralization. And also, the reusability of the catalyst was proved when 80% degradation could be achieved after three consecutive batches.

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Introduction

Phenols and phenolic compounds are some of the major organic contaminants in industrial wastewater. Sources of phenol include discharges of chemical process industries such as pulp and paper, dyestuff, pharmaceutical, agrochemical, petrochemical, food-processing, petroleum refining, steel, tanning, fibre wood, preservatives of food stuffs, coal gasification, polymeric resin production, oil refining, coking plants, paper mills and pesticide production.^1,2 When phenol-containing water is chlorinated, toxic polychlorinated phenols can be formed. Hence, such effluents require proper treatment before being discharged into the environment. Since they are stable and soluble in water, their removal to reach safety levels in the range 0.1–1.0 mg L⁻¹ is not easy.³ Economical and effective degradation of the toxic compounds from wastewaters is one of the most urgent recent challenges in environmental chemistry and engineering. Conventional wastewater treatment systems (physical, chemical and/or biological) are non-destructive, since they only convey the pollutants to another phase or location and produce a potentially dangerous and toxic secondary effluent which will leave its own disposal requirements.^4,5 In recent years, an important research topic is using of nanocomposites as a photocatalyst in degradation of organic and inorganic pollutants in air and water.^6,7 Titanium dioxide (TiO₂) has been widely used in photocatalytic and photochemical processes due to its nontoxic, high chemical inertness, long-term stability, and relatively cheap.^8,9 However the fast process is the recombination of e⁻/h⁺ pairs which is one of the key factors limiting further improvement of its photocatalytic efficiency. Several strategies have been employed to avoid the recombination of e⁻/h⁺ pairs such as coupling TiO₂ with metals or other semiconductors.^10–12 Graphene is a very attractive material for producing inorganic composites due to its very high electric charge carrier mobility, optical transparency, mechanical flexibility and strength.^13–15 One of the important applications of graphene is it's use in the preparation of nanocomposite photocatalysts.^16,17 One procedure for the synthesis of TiO₂/graphene (TiO₂/Gr) is the functionalization of TiO₂ nanocrystals.¹⁸ An efficiency route for the functionalization of TiO₂ is emulsion-based bottom-up self-assembly of hydrophobic TiO₂ nanocrystals to positively charged colloidal spheres. The TiO₂ nanocrystal colloidal spheres are electrostatic assembly with negatively charged graphene in an acidic aqueous solution.¹⁸ The TiO₂/Gr nanocomposite exhibits higher photocatalytic activities than pure TiO₂. After TiO₂ activation, the electrons induced are easily transported to the graphene nanosheets. By this process, recombination of e⁻/h⁺ is strongly reduced.¹⁹ Although, the graphene is known to have high theoretical surface area,²⁰ but it is expected that the activity of graphene is reduced due to aggregation of its sheets. When metal oxygen clusters, like polyoxometalates (POM), are supported on graphene, the flaking of these aggregated sheets can be carry out.²¹ Recently, incorporation of POMs with TiO₂ is a promising way to improve the activity of photocatalyst. POMs share the same general photochemical characteristics to semiconductor photocatalysts. After introduction of POMs particles into nanocomposite, fast recombination of e⁻/h⁺ pairs on the surface of TiO₂ could be suppressed by transferring the electrons from the conduction band of photoexcited TiO₂ into empty d orbits of POMs as efficient electron acceptors, resulting in significant enhancement of the rate of the photoreaction. The energy band of POM particles are similar to the TiO₂, which can affect the characteristic of TiO₂ and therefore improving the photocatalytic activity of TiO₂.^22,23 In the present work, we introduce H₃PW₁₂O₄₀ (PW) in TiO₂/Gr to synthesize a TiO₂/Gr/PW nanocomposite, which utilized solar light and conductivity of graphene concurrently and promote the electron transfer from photocatalyst surfaces to reactants. The prepared photocatalyst was used for the degradation and mineralization of phenol under visible light irradiation. The effects of initial concentration of phenol, concentration of photocatalyst, reaction time and initial pH on phenol removal efficiency in batch experiments were investigated.

Experimental

General

The reagents and solvents used in this work were obtained from Fluka, Aldrich, or Merck and were used without further purification. For characterization of the products, X-ray diffraction (XRD) patterns were recorded with a diffract meter on Riguka, Japan, RINT 2500 V using Cu-Kα radiation. Transmission electron microscopy (TEM) image was obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were observed and estimated by means of a Philips XL30 microscope at an accelerating voltage of 10 kV. The optical absorption was investigated using a UV-vis diffuse reflectance spectrophotometer (U-41000, HITACHI). Brunauer–Emmett–Teller (BET) surface areas and pore volumes were measured on a sorptometer Kelvin 1042 using nitrogen adsorption at 77 K. The zeta potential of the samples were obtained using a laser particle size analyzer (HPPS5001, Malvern, UK). Fourier transform infrared (FT-IR) spectra were recorded in KBr wafers with Bruker FT-IR. Chemical oxygen demand (COD) was measured according to the Standard Methods for water and wastewater.²⁴ A colorimetric method with closed reflux method was developed. Spectrophotometer (DR 5000, Hach, Jenway, USA) at 600 nm was used to measure the absorbance of COD samples. A pH meter (JENWAY 3510) was used for pH measurements. The absorption spectra of irradiated samples were recorded by UV-vis spectrophotometer. The phenol removal efficiency (%) has been calculated as follows:

Removal of phenol (%) = (A₀ − A_t)/A₀ × 100

Catalyst preparation

(a) Synthesis of GO. GO was prepared by the modified Hummers' method.²⁵ First, graphite powder (1.00 g) was added into a mixture of H₂SO₄/H₃PO₄ (108 : 12 mL). When the mixture was cooled with an ice bath, KMnO₄ was added slowly and carefully into the above reaction mixture. The mixture was stirred at room temperature for 3 days and then H₂O₂ (10 mL, 30 wt%) was added to the mixture in order to stop the oxidation process. After one hour, 100 mL deionized water was added. The resultant GO was washed with HCl solution (1 M) to remove metallic impurities. Finally the produced GO was washed with deionized water until pH reach up to 5.¹⁸

(b) Synthesis of TiO₂ nanocrystals. Oleic acid (OA) capped TiO₂ nanocrystals (NCs), were synthesized through mixing OA (7 mL) and cyclohexane (20 mL) in a 50 mL flask and then stirred for 10 min at room temperature, Ti (OBu)₄ (1 mL) and triethylamine (5 mL) were added to the mixture, then the flask was transferred to the autoclave and held at 180 °C for 24 h, followed by cooling down to room temperature. Products were harvested by three successive wash cycles of dispersion in cyclohexane, precipitation with ethanol, centrifugation, and then dispersion in cyclohexane. In the next step, hexadecyltrimethylammonium chloride (CTAC, 320 g, 1 mmol) was dissolved in deionized water (100 mL) and cyclohexane dispersion (10 mL) containing NCs (ca. 200 mg) was added to the aqueous solution. This system was emulsified by ultrasonic treatment with magnetic stirring and was stirred at 60 °C for 6 h to evaporate the cyclohexane to form condense colloidal spheres. This mixture was washed by water for subsequent use.¹⁸

(c) Synthesis of TiO₂/Gr. GO (1 mL, 5 mg mL⁻¹) was added to a colloidal microsphere dispersion (200 mL, 1 mg mL⁻¹) and stirred for 30 min. The produced GO was obtained by addition of hydrazine hydrate (1 mL, 80 wt%). After being stirred for another 0.5 h, precipitates were obtained. The resulted nanocomposite was collected by centrifugation, washed repeatedly with water, dried at 70 °C overnight and calcined at 400 °C for 2 h at a heating rate of 5 °C per minute.¹⁸

(d) Synthesis of TiO₂/Gr/PW. TiO₂/Gr/PW nanocomposites with different loading of PW (5, 10, 20, 30 and 40 wt% named TiO₂/Gr/5PW, TiO₂/Gr/10PW, TiO₂/Gr/20PW, TiO₂/Gr/30PW, TiO₂/Gr/40PW respectively) were synthesized. First, the appropriate amount of PW was dissolved in 5 mL of dry methanol. Then, this solution was added dropwise to a suspension of 1.0 g of TiO₂/Gr in methanol (50 mL) while being dispersed by sonication. The mixture was heated at 70 °C for 3 h under vacuum and calcined at 150 °C for 2 h.

Photodegradation of phenol

The reaction was performed in a glass photoreactor system containing 200 mL of aqueous solution. In this work, the effects of phenol concentration, photocatalyst loading, reaction time and initial pH on phenol removal efficiency were investigated. The photocatalytic activity of the prepared nanocomposite was evaluated for the degradation of phenol under visible illumination. In the first stage, photocatalyst powder was added to phenol solution, and then the catalyst suspension was kept in the dark condition for 30 min to achieve adsorption equilibrium. Then, the aqueous solutions were transferred to the reactor and the container was put under light irradiation by a 100 W tungsten lamp at room temperature (average 25 °C). The effect of various parameters including irradiation time (1–6 h), initial concentration of phenol (0.5–1.5 g L⁻¹), catalyst concentration (0.5–1.5 g L⁻¹) and pH (3.2, 4.2 and 7.2) in the removal efficiency of phenol were studied. To carry out the experiments, the initial pH of phenol solution was adjusted by using 0.1 M HCl or NaOH. The photocatalyst particles were separated through centrifugation using HETTICH UNIVERSAL. Before samples analysis, all the samples were wrapped with aluminum foil and kept in the dark condition.

Results and discussion

Catalyst characterization

The FT-IR analysis of TiO₂/Gr, TiO₂/Gr/5PW, TiO₂/Gr/10PW, TiO₂/Gr/20PW, TiO₂/Gr/30PW and TiO₂/Gr/40PW nanocomposites are illustrated in Fig. 1. The broad absorption peaks about 3400 cm⁻¹ belongs to the bending and stretching mode of surface adsorbed water and O–H groups on the catalyst surface. The band at 465 cm⁻¹ (Fig. 1a), one strong band between 500–1000 and 1413 cm⁻¹ are most probably related to the Ti–O–Ti stretching vibration.^26–28 The spectra broadening around 798 cm⁻¹ was attributed to the formation of Ti–O–C bonds. The creation of Ti–O–C bonds confirms that the chemical bonds were firmly built between graphene and TiO₂ nanostructures.^29–31 The band at 1632 cm⁻¹ is assigned to the vibrations of the adsorbed water molecules and the contributions from the vibration of aromatic C=C indicates the skeletal vibration of the graphene sheets reduced from graphene oxide.³² All the characteristic peaks in TiO₂/Gr are present in the TiO₂/Gr/5PW, TiO₂/Gr/10PW, TiO₂/Gr/20PW, TiO₂/Gr/30PW and TiO₂/Gr/40PW nanocomposites as well. In addition, the peaks observed at 1080, 985 and 812 cm⁻¹ in TiO₂/Gr/20PW, TiO₂/Gr/30PW and TiO₂/Gr/40PW nanocomposites attributes to PW Keggin type structure. It should be noted that according to low loading of PW in TiO₂/Gr/5PW, TiO₂/Gr/10PW these characteristic peaks are not observed sharply. The shift in peak positions of Keggin unit in the nanocomposites with different PW loading compared with the vibrational frequencies of PW (1083, 991, 895, and 815 cm⁻¹) confirm to the interaction of PW with support.³³

Fig. 1 FT-IR spectra of (a) TiO₂/Gr (b) PW (c) TiO₂/Gr/40PW and (d) TiO₂/Gr/30PW (e) TiO₂/Gr/20PW (f) TiO₂/Gr/10PW (g) TiO₂/Gr/5PW (h) reused TiO₂/Gr/30PW nanocomposites.

Fig. 2 shows the XRD spectra of TiO₂/Gr, TiO₂/Gr/5PW, TiO₂/Gr/10PW, TiO₂/Gr/20PW, TiO₂/Gr/30PW and TiO₂/Gr/40PW nanocomposites to analyze their crystalline phase. The obtained XRD patterns indicated that the TiO₂/Gr nanocomposite is in the anatase phase structure with 2θ values of 25.3°, 36.9°, 37.8°, 48.0°, 55.1° and 68.8° attributed to (1 0 1), (1 0 3), (0 0 4), (2 0 0), (2 1 1), and (1 1 6) crystal planes, respectively (JCPDS 04-0477).

Fig. 2 XRD patterns of (a) TiO₂/Gr (b) PW (c) TiO₂/Gr/5PW (d) TiO₂/Gr/10PW (e) TiO₂/Gr/20PW (f) TiO₂/Gr/30PW (g) TiO₂/Gr/40PW (h) reused TiO₂/Gr/PW nanocomposites.

Also the peak around 10 was not observed related to GO which confirm production of Gr from GO. Notably, no typical diffraction peaks belonging to the separate graphene are observed in the TiO₂/Gr nanocomposite.

The reason can be ascribed to the fact that the main peak of graphene at 24.5° might be shielded by the main peak of anatase TiO₂ at 25.3°.³⁴ The similarity observed in the XRD patterns for pure TiO₂ and the prepared photocatalyst (TiO₂/Gr, TiO₂/Gr/5PW, TiO₂/Gr/10PW, TiO₂/Gr/20PW, TiO₂/Gr/30PW and TiO₂/Gr/40PW) indicated that the anatase phase of TiO₂ was conserved well in the nanocomposite photocatalyst. Additional less intense peaks in TiO₂/Gr/30PW observed at 9.7° and 26.3° are attributed to percent of Keggin unit which implies a good dispersion of the Keggin unit throughout TiO₂ matrix in nanocomposite.^33–35 It should be noted that according to low loading and high dispersion of PW in TiO₂/Gr/5PW, TiO₂/Gr/10PW these characteristic peaks are not observed.

Fig. 3a shows the representative SEM images of TiO₂/Gr nanocomposite in which spherical-like TiO₂ particles aggregated on the top of Gr layer. The different morphologies can be observed for TiO₂/Gr/30PW nanocomposite (Fig. 3b–d). The result indicated that the integration of PW changed the morphology of the TiO₂/Gr nanocomposite. Large and rigid agglomerates seem to be formed in this case with a porous surface. Morphology of the TiO₂/Gr/30PW composite was characterized by transmission electron microscope (TEM) (Fig. 4). The average particle size of the spherical nanoparticles found to be in the range of 9–12 nm. Moreover, some ellipsoids nanoparticles are also observed in the TEM image.

Fig. 3 SEM images of (a) TiO₂/Gr and (b–d) TiO₂/Gr/30PW nanocomposites.

Fig. 4 (a) TEM image, (b) size histogram of TiO₂/Gr/30PW nanocomposite.

In order to investigate the porous structure and surface area of TiO₂/Gr and all catalyst the N₂ adsorption–desorption isotherm at liquid nitrogen temperature were conducted and shown in Fig. 5a. The isotherm curves of TiO₂/Gr and TiO₂/Gr/30PW nanocomposites are matched to type IV with a weak hysteresis loop in the relative pressure, which shown the presence of porous structure of TiO₂/Gr and TiO₂/Gr/30PW nanocomposites. Isotherm curves of other catalysts are presented in ESI for comparison (ESI Fig. S1†). The BET specific surface areas of TiO₂/Gr and all nanocomposites are shown in Table 1. Modification of TiO₂/Gr with PW leads to reduce of specific surface area (Table 1). The strong reduction of BET for nanocomposites compare to the TiO₂/Gr indicates that some pores in the nanocomposite are blocked with PW. The same results were reported in literatures.³⁶ Increasing of pore size due to deposition of PW is may be because of different reasons as examples breaking of the pore walls, producing of new channels, releasing of some compound adsorbed or trapped in some pores or deposited on the top of the pores and etc. via preparation procedure of PW loading specially under sonication and calcination temperature.^37,38 Also pore size distributions of TiO₂/Gr/30PW (Fig. 5b) show that loading of PW make the pore size of TiO₂/Gr more uniform.

Fig. 5 (a) N₂ adsorption–desorption isotherm and (b) pore size distribution of TiO₂/Gr and TiO₂/Gr/30PW nanocomposites.

Table 1 The parameters of BET surface area and pore volume of TiO₂/Gr and TiO₂/Gr/30PW nanocomposites

Sample	BET surface area [m^2 g^−1]	Pore volume [cm^3 g^−1]	Pore size [nm]
TiO2/Gr	119	0.228	7.67
TiO2/Gr/40PW	22	0.094	16.68
TiO2/Gr/30PW	25	0.095	19.54
TiO2/Gr/20PW	28	0.160	22.89
TiO2/Gr/10PW	37	0.236	25.49
TiO2/Gr/5PW	43	0.253	23.40
PW	<10	—	—

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Photocatalyst performance

(a) Phenol removal efficiency. The effect of the catalyst concentration on phenol degradation was significant as shown in Fig. 6. The results clearly demonstrate that by increasing the amount of the catalyst from 0.5 to 1 g L⁻¹, the photocatalytic conversion is progressively increased. When only a small amount of photocatalyst (>1 g L⁻¹) was added, it may have proceeded principally to scatter light and the degradation efficiency decreased. On the other hand, the number of producing ˙OH radicals on the active sites will not be enough to degrade and mineralize the organic pollutant. Further increase in the catalyst dosage to 1.5 g L⁻¹ has also decreased the degradation reaction efficiency owing to light scattering on TiO₂ particles.³ Another reason for this issue is the agglomeration of the catalyst particles that decreases the effective surface area of the catalyst in the reactor. Therefore, the optimum catalyst loading for this study was determined 1 g L⁻¹.

Fig. 6 Effect of the catalyst loading on phenol degradation (phenol concentration 0.5 g L⁻¹ and pH = 4.2).

It is well known that one of the important parameter on photodegradation of organic compounds is the initial concentration of reactant. Fig. 7 illustrates the effect of the initial phenol concentration on photocatalytic degradation efficiency. It is clear that the degradation efficiency is decreased with an increase in initial concentration of phenol from 50 to 150 mg L⁻¹ while holding other parameters constant (catalyst loading 1 g L⁻¹ and pH = 4.2). This is because the possibility of interaction between phenol molecules with ˙OH decreases and hence, a lower degradation efficiency is observed.

Fig. 7 Effect of the initial phenol loading (catalyst loading 1 g L⁻¹ and pH = 4.2).

The pH value is one of the important parameter that influence the degradation efficiency of some organic compounds. Therefore, the experiments of phenol degradation at different pH values (3.2, 4.2 and 7.2) were performed in the presence of TiO₂/Gr/30PW catalyst under the optimized conditions. Fig. 8 depicts the effect of pH on the photocatalytic degradation of phenol. From the results, the pH in the range studied did not show significant impact on the response. One important parameter for interaction between both TiO₂ and phenol is their surface electric charge properties. In aqueous media, phenol has a pK_a of 9.9 (at 25 °C)³⁹ indicating that phenol is in the cationic structure (C₆H₅O⁺) when the solution pH < pK_a. The zeta potential measurements of TiO₂/Gr/30PW nanocomposites for various pH (3.2, 4.2, 7.2) are shown in Fig. 9a–c. The results showed that the electric charge for the TiO₂/Gr/30PW nanocomposites in all mentioned pH values were negative. Thus, in different pH nanocomposites with negative charge are approximately same (Fig. 9).

Fig. 8 Effect of the pH on the photocatalytic degradation efficiency (phenol concentration 0.5 g L⁻¹ and catalyst loading 1 g L⁻¹).

Fig. 9 Zeta potential distribution change of TiO₂/Gr/30PW nanocomposite for various pH (a) 3.2, (b) 4.2 and (c) 7.2.

Fig. 10 shows compares the photoactivity of TiO₂/Gr, TiO₂/Gr/30PW and pure TiO₂ under visible light irradiation at optimum conditions determined in this study (phenol concentration 0.5 g L⁻¹ and catalyst loading 1 g L⁻¹, pH = 3.2 and 6 h). As can be seen in the Fig. 10, the modified TiO₂ photocatalysts showed a higher visible-light photocatalytic activity than pure TiO₂. TiO₂/Gr/30PW exhibited a higher visible-light photocatalytic activity in compared with TiO₂/Gr and pure TiO₂ with the maximum degradation efficiency of 91, 68, and 15%, respectively. Graphene is the ideal nano structure compound to be paired with titanium dioxide, growing its ability to adsorb as well as its photocatalytic activity. The electrons freed after the titanium dioxide activation are easily transported to the graphene nano sheets and recombination of e⁻ and h⁺ is strongly reduced, which enhances the process efficiency.⁴⁰ The high photocatalytic activity of the TiO₂/Gr/30PW composites attributed to the following reasons. First, compared with pure TiO₂, as-prepared TiO₂/Gr/30PW composite showed stronger adsorption capacity to phenol molecules (at dark condition absorbance for TiO₂/Gr/30PW, TiO₂/Gr and pure TiO₂ is 12, 7 and 2% respectively) which played an important role to improve their photocatalytic activity since the absorbance is the first step of the photocatalytic reaction.

Fig. 10 Photoactivity of TiO₂/Gr/30PW, TiO₂/Gr and pure TiO₂ degrading phenol under visible light irradiation (phenol concentration 0.5 g L⁻¹, catalyst loading 1 g L⁻¹ and pH = 3.2).

Second, the synergistic effect between the Keggin unit and TiO₂ also contributed in enhancement of photocatalytic activity of the composite catalyst. After introducing PW particle into nanocomposite, fast recombination of e⁻/h⁺ pairs on the surface of TiO₂ could be suppressed by transferring the electrons into empty d orbits of PW as an efficient electron acceptors, resulting in significant enhancement of the rate of the photoreaction conversion. Third the energy bands of PW particles are similar to the TiO₂, which can affect the characteristic of TiO₂ and therefore improving the photocatalytic activity of TiO₂.^19,33 In order to compare of photo activity of TiO₂/Gr, TiO₂/Gr/30PW and pure TiO₂ under UV and vis light, degradation of phenol at optimum conditions was investigated under UV light. According to the obtained results (Fig. 11) degradation of phenol under UV light was occurred in shorter reaction time compared with visible light.

Fig. 11 Comparison of photoactivity of TiO₂/Gr/30PW, TiO₂/Gr and pure TiO₂ efficiency (phenol concentration 0.5 g L⁻¹ and catalyst loading 1 g L⁻¹ and pH = 3.2 under UV light).

The effect of the PW loading on the photocatalytic performance was studied (Fig. 12). The results showed that with increasing of PW loading from 5 to 20 wt% the photoactivity of nanocomposite is increased. But more increasing in PW loading decreased the activity of the catalyst. In order to convince the decreasing effect, optical absorption properties of the as-prepared samples, was carried out (Fig. 13). From the figure the PW-modified nanocomposites showed an extension to visible range. It clearly discloses that among the modified composites, the catalyst with 20 wt% PW appeared maximum visible shift.

Fig. 12 Effect of the catalyst loading on phenol degradation (phenol concentration 0.5 g L⁻¹ and catalyst loading 1 g L⁻¹ and pH = 3.2).

Fig. 13 UV-visible diffuse reflectance spectra of nanocomposites with different PW loading, results in parenthesis are wt% of PW to photocatalyst.

(b) COD removal. Fig. 14 shows the performance of TiO₂/Gr/30PW photocatalyst for phenol mineralization as COD/COD₀ versus reaction time at initial COD of 300 mg L⁻¹, catalyst loading of 1 g L⁻¹ and pH of 3.2. As reaction time progressed, COD/COD₀ showed to be decreased. The highest COD removal efficiency (78%) achieved after 6 h. As observed, the effect of reaction time is clearly shown as an increasing trend. Comparison of the TiO₂/Gr/30PW with other photocatalysts are reported in Table 1. The results show that the TiO₂/Gr/30PW photocatalyst is able to phenol degradation at suitable time (Table 2).

Fig. 14 Performance of the TiO₂/Gr/30PW photocatalyst for phenol mineralization (initial COD 300 mg L⁻¹, catalyst loading 1 g L⁻¹ and pH of 3.2).

Table 2 Comparison of the activity TiO₂/Gr/30PW photocatalysts with other photocatalysts in degradation of phenol

Entry	Experimental conditions	Degradation rate	Ref.
1	C0 phenol = 50 ppm, TiO2 = 2 g L^−1, under UV	92% degradation achieved in 6 h	41
2	C0 phenol = 100 ppm, TiO2 = 1 g L^−1, under UV irradiation	83% degradation of phenol occurs in 9 h	42
3	C0 phenol = 40 ppm, EY–TiO2/Pt = 0.8 g L^−1, under visible irradiation	93% degradation of phenols occurs in 90 min	43
4	C0 phenol = 200 ppm, Pr-doped TiO2 = 1 g L^−1 under UV-vis irradiation	About 50% degradation of phenol takes place in 2 h irradiation time	44
5	C0 phenol = 85 ppm, TiO2/Cu-loaded carbon = 0.5 g L^−1 under visible irradiation	73% degradation achieved in 6 h irradiation time	45
6	C0 phenol = 50 ppm, TiO2/Gr/30PW = 1 g L^−1, under visible irradiation	91% degradation achieved in 6 h	Present work

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(c) Recyclability of the catalyst. The reusability of the catalyst was examined under optimized conditions. After the first catalytic run, the catalyst was removed by centrifugation, and then washed with hot water and dried at 120 °C. The results show that TiO₂/Gr/30PW nanocomposite exhibits considerably high catalytic stability, and 80% degradation was achieved after third catalytic cycle. The FTIR spectrum and XRD pattern for the recycled TiO₂/Gr/30PW nanocomposite are shown in Fig. 1d and 2d respectively. The characteristic peaks of the Keggin anion were observed in the FTIR for the recycled TiO₂/Gr/30PW nanocomposite. The result of FTIR and XRD pattern of recycled TiO₂/Gr/30PW nanocomposite indicate good structural stability of the reused catalyst. The presence of the peaks in the XRD illustrates that there is no significant variation in the structure of TiO₂/Gr/30PW nanocomposite after reusing (Fig. 15).

Fig. 15 Recycling tests of the TiO₂/Gr/30PW nanocomposite in optimum reaction conditions.

Conclusions

TiO₂/Gr/30PW nanocomposite was successfully prepared using bottom-up assembly and the immobilization of PW on the surface TiO₂/Gr. The catalysis was characterized by XRD, FTIR, SEM, zeta potential, DRS, BET and TEM. The catalyst characterization showed visible shift when TiO₂/Gr was modified by PW. The photocatalyst TiO₂/Gr/20PW nanocomposite exhibited higher photoactivity relative to the other prepared photocatalysts. As a conclusion, the modified nanocomposite can be introduced as a promising efficient visible driven photocatalyst to reclaim polluted water.

Acknowledgements

The authors thank the Razi University Research Council and Iran National Science Foundation (INSF) for support of this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09897e

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