Hydrothermal synthesis of TiO₂–RGO composites and their improved photocatalytic activity in visible light

Abstract

Composites of titanium dioxide–reduced graphene oxide (TiO₂–RGO) have been prepared via an efficient and facile hydrothermal reaction of graphene oxide and Ti(OH)₄ in a ethanol–water solvent. The structure and composition of the composites have been characterized by X-ray diffraction, Scanning electron microscopy, Brunauer–Emmett–Teller specific area analysis, X-ray photoelectron spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy. The results showed that the TiO₂–RGO composites exhibited a layered structure with well-dispersed anatase TiO₂ on the surface of the reduced graphene oxide. Compared with the pure TiO₂ nanoparticles, the composites had larger surface area, and extended the photoresponse range to the visible light region. The photocatalytic activity of the composites was much higher than that of TiO₂ in the photodegradation of methyl orange (MO) under visible light irradiation.

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Introduction

Photodegradation processes of organic pollutants have attracted increasing attention during the past decades.¹ Among various oxide semiconductor photocatalysts, titanium dioxide (TiO₂) has been considered as the most suitable material for widespread environmental applications because of its long-term thermodynamic stability, strong oxidizing power, and relative nontoxicity. However, the need of ultraviolet (UV) light for activating the photocatalysts greatly limits the technology in practical applications because of the low content of UV light in the solar spectrum.^2,3 On the other hand, the photoproduced electrons and holes in TiO₂ may experience a rapid recombination, which diminishes the efficiency of the photocatalytic reaction significantly.⁴ Thus, to inhibit the recombination of photoinduced electron–holes and extend the light absorption of the visible light region are the key factors for improving the photocatalytic activity of TiO₂.

Since its discovery in 2004,⁵ graphene, a two dimensional, sp²-bonded carbon material, has attracted more and more attention due to its outstanding mechanical, electrical, thermal and optical properties.^6,7 Deposition other materials on graphene sheets is an efficient way to construct composite materials and to improve the properties of these materials by utilizing the unique properties of graphene. It has been shown previously that the composites of TiO₂ and carbon, including activated carbon, carbon nanotubes, and fullerenes, are able to exhibit enhanced photocatalytic performance than TiO₂ alone.^8–10 Recently, the composite of graphene and TiO₂ nanoparticles are being considered as potential photocatalyst in air and water purification. The composite simultaneously covered three excellent advantages: the increasing absorptivity of pollutants, extended light absorption range, and facile charge transportation and separation.^11,12 Sun et al.¹³ reported a comparative study in photocatalytic performances of reduced graphene oxide modified TiO₂, ZnO and Ta₂O₅ under UV and visible irradiations. However, only G–TiO₂ showed visible light photocatalytic activity. Yang et al.¹⁴ prepared heterostructured TiO₂–graphene porous microspheres by ultrasonic spray pyrolysis method. Due to the suppression of electron–hole pair recombination the grapheme–TiO₂ microspheres showed higher photocatalytic activity under solar irradiation. Many efforts also have been made to utilize the unique properties of graphene in order to increase the efficiency of photocatalysis.^13–18

Herein, we reported a facile method to obtain a chemically bonded anatase TiO₂–RGO composite via a one-step hydrothermal reaction. In the as prepared anatase TiO₂–RGO photocatalyst, TiO₂ nanoparticles were loaded on the platform of a graphene nanosheet. We proposed here the preparation of anatase TiO₂–RGO composite by forming anatase TiO₂ and reducing graphene oxide simultaneously, which could achieve stronger binding between TiO₂ and graphene. Then we used the highly efficient composite photocatalyst to remove pollutants by photocatalytic oxidation. Due to its low cost and convenient process, TiO₂–RGO composites can be produced in a large scale which may contribute to application to the disposal of industrial pollutants.

Experimental

Materials and reagent

Graphite (99.8%) with an average particle size of 45 μm was obtained from Alfa Aesar Co., Ltd. Sulfuric acid (H₂SO₄), hydrochloric acid (HCl), potassium permanganate (KMnO₄), sodium nitrate (NaNO₃), hydrogen peroxide (H₂O₂) were purchased from Sigma-Aldrich. Titanium oxysulfate-sulfuric acid hydrate (TiOSO₄·nH₂SO₄·nH₂O) was obtained from Aladdin chemistry Co., Ltd. Sodium hydroxide (NaOH), barium nitrate (Ba(NO₃)₂) and methyl orange were purchased from sinopharm chemical Co., Ltd.

Synthesis of the TiO₂–RGO composites

The graphene oxide was synthesized from natural graphite by the modified Hummers methods.¹⁹ To synthesize the Ti(OH)₄ precursor, 53.0 g TiOSO₄·nH₂SO₄·nH₂O was mixed with 150 mL deionized water in a 500 mL flask under continuously stirring for 30 min at 90 °C, then 3.0 mol L⁻¹ NaOH aqueous solution was added dropwise until the pH value reached 8.0. The white mixture was stirred for another 2 h, and the amorphous Ti(OH)₄ precipitate was collected by filtration and washed with deionized water until no SO₄²⁻ was detected by Ba(NO₃)₂ solution.²⁰

The TiO₂–RGO composite was obtained via a hydrothermal method. Briefly, graphene oxide was ultrasonicated in a 40 mL of deionized water and 20 mL of anhydrous ethanol to disperse it well. After that 0.58 g of Ti(OH)₄ was add to the calculated amount of the above GO solution to prepare 1, 2, 5 wt% TiO₂–RGO composites catalysts with different weight addition ratios of RGO in the composites of TiO₂–RGO. The mixing solution was aged with vigorous stirring for 1 h to obtain a homogeneous suspension. Then, this suspension was transferred to an 80 mL Teflon sealed autoclave and maintained at 180 °C for 6 h.²¹ By this hydrothermal treatment, the formation of Ti(OH)₄ to anatase TiO₂, the reduction of GO to RGO and the deposition of TiO₂ onto the RGO sheet can be simultaneously achieved. The resulting composite was recovered by filtration, washed by water, and fully dried at 60 °C in oven to get the final TiO₂–RGO composites with different weight addition ratios of RGO.

Characterization

The powder X-ray diffraction (XRD) pattern was carried out using a Rigaku X-ray generator (Cu Kα radiation with λ = 1.54 Å) at room temperature. The average crystallite size was calculated from Scherrer equation (d = 0.9λ/β_1/2 cos θ, where λ is the characteristic X-ray wavelength applied, β_1/2 is the half width of the peak at the 2θ value). The specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method (ASAP2020M+ C, MICROMERITICS). The X-ray photoelectron spectroscopy (XPS) measurements were conducted on a VG ESCALAB MARK II spectrometer, using Mg Kα X-ray as the excitation source. The UV-vis diffuse reflectance spectra of the TiO₂ and TiO₂–RGO powers were obtained using an UV-vis spectrophotometer (UV-2450, SHIMADZU, Japan) and BaSO₄ was used as a reference. Scanning electron microscopy (SEM) observation of the TiO₂–RGO nanoparticles were carried out on an S-4800 microscope (HITACHI, Japan) under an accelerating voltage of 3.0 kV. Transmission electron microscopy (TEM) images were obtained using a JEM-1230 instrument at an acceleration voltage of 120 kV.

Photocatalytic experiments

A 400w Metal Halide Lamp and a cutoff filter (λ > 400 nm) were used as the light source for photocatalytic reaction. The photocatalytic activity of the photocatalysts was evaluated by decomposition of methyl orange in an aqueous solution. 50 mg of photocatalysts was dispersed into 100 mL water solution containing 10 mg L⁻¹ MO, and the mixture was stirred incessantly under the light with a distance of 30 cm. Prior to irradiation, the suspension was sonicated for 15 min, followed by stirring for 1 h in the dark to favor the adsorption–desorption equilibration. The concentration of the substrate after equilibration was measured and taken as the initial concentration (c₀) to discount the adsorption in the dark. At given time intervals, 4 mL aliquots were sampled, centrifuged at 8000 rpm for 15 min to remove the TiO₂–RGO particles. The residual concentration of MO in the aliquots was analyzed using a UV-vis spectrophotometer.

Results and discussion

A series of composites of TiO₂ and reduced graphene oxide, denoted as TiO₂–RGO, had been obtained by a simple hydrothermal treatment of Ti(OH)₄ and graphene oxide in the solvent of ethanol–water. In the reaction process, graphene oxide was reduced to graphene sheet, the color of the suspension shifted from brown to grayish, which further indicates the change from GO to RGO. With the increase weight addition of GO, the color of TiO₂–RGO product changes from grayish to black. In the hydrothermal reaction, ethanol function as reducing agent and the autogenous pressure developed inside the sealed autoclave contributes to the reduction of GO to RGO.²²

The XRD patterns of the as-prepared TiO₂–RGO nanocomposites were shown in Fig. 1. It was obviously that the TiO₂–RGO nanocomposites with different weight addition ratios of GO exhibit similar XRD patterns. It could be observed that all the samples were primarily composed of the anatase TiO₂ phase and show no diffraction peaks of GO layered structure, which might be due to the layer-stacking regularity almost disappeared after reduction or the low amount and relatively low diffraction intensity of graphene.²³ With the GO content increased, the intensity of the diffraction peaks for the composites showed a little decrease. The average crystal size calculated by applying the Scherrer formula on the anatase diffraction peaks were 19.1 nm, 16.7 nm, 24.6 nm and 15.9 nm. The feed ratio had not obviously effect on the crystallite size.²⁰

Fig. 1 XRD patterns of TiO₂, TiO₂–1%RGO, TiO₂–2%RGO, TiO₂–5%RGO.

The morphology of TiO₂–2%RGO was observed with SEM. It was easy to find many platelets of TiO₂–RGO composites, along with large aggregated TiO₂ particles (Fig. 2a). For a single platelet, TiO₂ nanoparticles deposited on the graphene sheets (Fig. 2b). The TiO₂ sizes of the samples were around 20 nm, which was in agreement with the mean crystal size value of 20 nm estimated by the Scherrer equation from the X-ray diffractogram.

Fig. 2 SEM images of TiO₂–2%RGO (a) and (b).

Typical TEM image of TiO₂–2%RGO are shown in Fig. 3. The crumbled structure of RGO can be observed for the composite, Fig. 3a reveals a homogenous dispersion of TiO₂ on an individual RGO sheet and that are eager to accumulate along the wrinkles of RGO as can be seen at the middle of the RGO sheet. Fig. 3b shows that anatase TiO₂ nanoparticles are sphere-shaped for TiO₂–RGO composite while some of them are rod-shaped, with average crystal size of about 20 nm. The results are in consistent with the XRD results.

Fig. 3 TEM images of TiO₂–2%RGO composite (a) low and (b) high magnification.

Fig. 4 showed the N₂ adsorption–desorption isotherms and the BJH pore size distribution (PSD) curves of samples TiO₂ and TiO₂–2%RGO. The nonlimiting adsorption at high P/P₀ for the samples was characteristic of a type H3 loop, and the overall shapes of the samples indicated the presence of mesopores in the composites (Fig. 4a and b).²⁴ The BET analysis revealed that the TiO₂–RGO composite had a surface area of 222 m² g⁻¹, being much higher than 183 m² g⁻¹ of the pure TiO₂. The addition of RGO in TiO₂ do increases surface area, not only itself contribute to the BET surface area, but also the existed RGO reduces the crystal size of TiO₂, which will result in increasing surface area. Fig. 4c was the pore size distribution curve derived from the desorption branch, in comparison with the pure TiO₂, the TiO₂–2%RGO composite involved more mesopores with the pore sizes around 10 nm. The mesopores would be attributed to the interstitial space between the nanoparticles and the interlayers of TiO₂–2%RGO, which may account for the great BET specific surface area of the TiO₂–2%RGO composite.

Fig. 4 N₂ adsorption–desorption isotherms of (a) TiO₂ and (b) TiO₂–RGO, (c) the corresponding pore size distributions.

Fig. 5 showed the XPS spectra of GO and TiO₂–5%RGO. Fig. 5a showed the Ti2p core level photoelectron spectrum. Ti2p_3/2 centers at 458.5 eV and Ti2p_1/2 at 464.4 eV, respectively, in good agreement with the binding energy values of Ti⁴⁺ in pure anatase.²⁵ In addition, the results of the peak deconvolution of the Ti2p envelope of the composite showed no peaks centered at 465.8 and 460.2 eV, which were attributed to formation of a Ti–C bond in the composite.²⁶ In Fig. 5b, deconvolution of the C1s peaks, with a binding energy of 284.6 eV can be attributed to the C–C, C=C, and C–H bonds, while the deconvoluted peaks centered at the binding energies of 286.9 eV can be assigned to the C–O bond.²⁷ As shown in Fig. 5c (C1s spectra of TiO₂–5%RGO), three peaks located at 284.6, 286.5, 288.4 eV correspond to C–C (aromatic), C–O, C=O (Ti–O–C).^28,29 The presence of the Ti–O–C structure revealed that the C atoms have substituted some of the Ti atoms in the TiO₂ lattice during the composite preparation. The C1s peaks show that the relative ratio of the areas of the C–C and C–O peaks in the GO spectrum is 1 : 1.4 (C : O). The C : O ratio goes from 1 : 1.4 to 1 : 0.95 following reduction, and the C–O peak represents a substantial blue-shift from 286.9 to 286.5 when comparing to C1s spectra of TiO₂–5%RGO nanocomposites, which shows that the hydrothermal reduction seems to be effective.

Fig. 5 XPS spectra of (a) Ti2p of TiO₂–RGO and C1s of (b) GO, (c) TiO₂–RGO.

Fig. 6a showed the UV-vis diffuse reflectance spectra (DRS) of the TiO₂–RGO composites. The presence of different amounts of RGO affected the optical property of light absorption for TiO₂–RGO composites significantly. This could be attributed to the presence of carbon in the TiO₂–RGO composites. The addition of RGO induced the increased light absorption intensity in the UV and visible light regions. The absorption edge of TiO₂–RGO shifts to the visible light range comparing with TiO₂, this result indicated that a narrowing of the band gap of TiO₂.³⁰ The narrowing should be attributed to the chemical bonding between TiO₂ and RGO. That was, the formation of Ti–O–C bond, similar to the case of carbon doped TiO₂ composites.³¹ However, it was difficult to determine the value for such a red shift because the background absorption ranging from 400 to 800 nm was increased upon the incorporation of graphene into the matrix of TiO₂. A plot of the transformed Kubelka–Munk function as a function of energy of light was shown in Fig. 6b, by which the roughly estimated band gaps were 3.2, 3.04, 2.88 and 2.72 eV corresponding to TiO₂, TiO₂–1%RGO, TiO₂–2%RGO and TiO₂–5%RGO, respectively. The strong absorption intensity of light for the TiO₂–RGO composites suggested that they could have higher photocatalytic activity for a given reaction.

Fig. 6 UV-vis diffuse reflectance spectra (DRS) of (a) TiO₂–RGO composites, (b) the plot of transformed Kubelka–Munk function versus the energy of light.

The photocatalytic activities of TiO₂–RGO composites were measured by the photodegradation of methyl orange as model reaction under visible light (λ > 400 nm). Black experiments at the same conditions showed that no activity was observed in the absence of catalyst or light irradiation.

It was clearly seen that in the tested concentration range, the photocatalytic degradation ratio of MO under visible light followed the order TiO₂–2%RGO > TiO₂–1%RGO > TiO₂–5%RGO > TiO₂ at first, as shown in Fig. 7. When the suspension system was irradiated for 6 h, the degradation percentage of MO reached 87.4% by TiO₂–1%RGO, compared to 7.6% by pure TiO₂, respectively. TiO₂ could degrade a little MO because of the MO self sensitization. When the weight addition ratio of RGO increased to 5%, the activity of TiO₂–5%RGO would be lower than the other TiO₂–RGO composites. With the increase of graphene oxide content, the black reduced graphene oxide sheets in the composite would increase photo absorbing and scattering leading to a decrease of the photocatalytic activity of the composites. That is to say that the higher addition ratio of RGO into the matrix increases the adsorptivity of MO on one hand; however, on the other hand, this also lowers the contact surface of TiO₂ particles with the light irradiation, which would lead to a decreased photo catalytic activity. That is the reason why here 1% and 2% RGO show higher activity than 5% RGO material. The higher addition amount of RGO is not beneficial to enhance the photocatalytic performance of TiO₂ in comparison to the lower addition amount of RGO, and similar results have been found by Zhang et al.²¹ However, any addition of graphene oxide in TiO₂–RGO composites showed much better photocatalytic activity than the TiO₂ alone. The reasons for high photocatalytic activity of TiO₂–RGO composite were mainly attributed to three aspects: firstly, the formation of Ti–O–C bond resulted in the absorbance edge of TiO₂ shifting to the higher wavelength region. Secondly, the two dimensional nanostructure could enhance adsorption ability of the TiO₂–RGO composites. The third one was the remarkable electrical transport property. In the composite, the portion of graphene played a role for conducting electrons, which improved the separation of the electron–hole pairs and the photocatalytic efficiency.

Fig. 7 Photocatalytic degradation of 10 mg L⁻¹ methyl orange under the irradiation of visible light (λ > 400 nm) over the TiO₂–RGO nanocomposites.

Conclusions

In summary, we have prepared the nanocomposites of TiO₂–RGO with different weight addition ratios of GO by a facile hydrothermal treatment of GO and Ti(OH)₄ in a solvent of ethanol–water. The TiO₂–RGO composites exhibited a layered structure with well-dispersed nanoparticles on the surface of the reduced graphene oxide. The composites extended photoresponding range, possessed great adsorptivity of dyes and enhanced charge separation and transportation properties simultaneously. On the basis of these advantages, TiO₂–RGO exhibited much higher activity than that of pure TiO₂ in the photodegradation of MO under visible light irradiation. Thus, the composites may find promising applications in the field of environmental photocatalysis.

Acknowledgements

We appreciate the financial support from Doctoral Program Foundation of East China Institute of Technology (no. DHBK2013210).

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