Influence of interface combination of reduced graphene oxide/P25 composites on their visible photocatalytic performance

Abstract

Reduced graphene oxide (RGO)/P25 composites are successfully synthesized by using a facile hydrothermal method, and Raman mapping images show that the P25 are very uniformly dispersed in the composite. The RGO/P25 composites are used in the photocatalysis of methyl orange under UV and visible light illumination. The degradation ratio for the 0.75 wt% RGO/P25 composite is the best with 100% degradation after 120 and 150 minutes of irradiation under UV and visible light, respectively, which is greatly enhanced compared with that of P25, especially in the visible light region. It is found that a good interface combination between RGO and P25 may improve the electric conductivity and lifetime of photo-induced electrons in the photodegradation process as well as enhance the light absorption, which may favor the efficient enhancement of the photodegradation rate of the RGO/P25 composite.

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1. Introduction

The photocatalytic performance of TiO₂ nanomaterials has attracted a lot of attention since it was discovered by Fujishima in 1971. Currently, more attention has been paid to semiconductor photocatalysis as a solution for environmental pollution, and making full use of the light energy is the key factor for an efficient and mild route to photodegrade the waste.^1–8 Considerable effort has been put into promoting this property by designing and modifying various TiO₂-based composites.

Among these composites, graphene/TiO₂ hybrid nanomaterials have been widely researched for their perfect graphene properties. Highly reactive crystal facets and highly dispersed TiO₂ nanocrystals are synthesized on the graphene surface by a hydrothermal process.^9,10 It is also reported that graphene between the semiconductors can lengthen the lifetime of photoexcited charge carriers in the semiconductor and improve the photoactivity.^11,12 Many kinds of methods have been proposed to prepare the graphene/TiO₂ hybrid composites in photocatalysis. For example, reduced graphene oxide (RGO) was applied to prepare various RGO/photocatalyst composites such as RGO/TiO₂, RGO/ZnO and RGO/Ta₂O₅, and the photocatalytic activities of the composites under UV and visible light were studied in the degradation of methylene blue.¹³ An electro-spinning method was used for the fabrication of one-dimensional graphene/TiO₂ composites, which were used in the photodegradation of methyl orange.¹⁴ TiO₂ nanoparticles on the surface of few layered graphene sheets were prepared by using a single step hydrothermal method, and their photocatalytic degradation abilities under visible light were also studied.¹⁵

Normally, the interface combination between the graphene and TiO₂ nanomaterials is quite important for understanding the enhancement of their photocatalytic performance.^16–18 An investigation of the interface combinations in the graphene/TiO₂ composites is quite important to reveal the influence of the resulting quality and structural properties of the composites on their photocatalytic performance.^19,20

In the present work, we synthesize reduced graphene oxide (RGO)/P25 composites by using a facile hydrothermal method, and use them in the photodegradation of methyl orange under UV and visible light irradiation. The influence of the interface combination of the composites on the photocatalytic performance is also studied.

2. Experimental

2.1 Synthesis of the RGO/P25 composites

Graphene oxide (GO) was prepared by Hummers method.⁴⁰ Firstly, 2 g graphite powder and 1.5 g NaNO₃ were mixed in an ice bath, and then placed into the concentrated H₂SO₄ solution (80 mL). With vigorous stirring, 10 g KMnO₄ was gradually added and the temperature was kept below 20 °C. Then, the mixture was stirred at 35 °C in a water bath for 3 h. After the reaction, the mixture became pasty with a brownish color. 100 mL H₂O was slowly added to the pasty mixture while the mixture was in an ice bath to keep the temperature below 35 °C. 10 mL of 30% H₂O₂ solvent was added to the mixture, and the color of this solution became a brilliant yellow. Then, the mixture was washed with 10% HCl (aq.) and deionized water in that order. Finally, the resulting sample was dried under vacuum conditions. Different weights of GO were dissolved in 60 mL glycol solution, sonicated for 2 h until the solution became clear, and then 3 g P25 powder was added into the solution. After vigorous stirring, the suspension was transferred into a Teflon-lined stainless steel autoclave (100 mL), and reacted at 180 °C for 6 h. Finally, the products were filtered with distilled water and dried in a vacuum drier.

2.2 Photocatalytic activity experiments

Before irradiation, 0.05 g RGO/P25 composites were placed into an aqueous methyl orange solution (50 mL, 32.73 ppm), and the suspension was stirred for 2 h in a dark environment to establish an adsorption/desorption equilibrium. The photogradation rates of the RGO/P25 composites were evaluated by examining the concentration variation of methyl orange under UV and visible light illumination from 125 W high-pressure Hg lamps every 30 min, which has UV (200–400 nm) and visible light (400–800 nm) spectra, with the main peak located at the 365 nm wavelength. For the visible light irradiation, the light source is equipped with an UV cut-off filter, which may remove 99% of the UV light with a wavelength between 320 nm and 400 nm. The light irradiation intensity on the samples under UV and visible light illumination was kept equal by adjusting the distance between the lamp and samples.

2.3 Measurements

The crystal structure and microstructure of the samples were characterized by using X-ray diffraction (XRD, PertPro, PANalytical, Netherlands), Fourier transform infrared spectroscopy (FTIR) (TENSOR27, Bruker, Germany) and scanning electron microscopy (FESEM, JSEM-5610LV, Japan). A 3D Raman mapping image was measured by Confocal Raman System (WITec alpha300R, Germany) equipped with a 532 nm laser and optical diffraction limit of ∼200 nm. The composite films were detected to obtain 3D information from an area of 100 μm × 100 μm with a mesh of 2 μm at the center of the RGO/P25 composite films. UV-vis absorption spectra (UV-2550, Shimadzu, Japan) were used to characterize the absorption properties of samples. The photoluminescence decay was measured using time-resolved fluorescence spectroscopy (HORIBA Fluoromax-4, France).

3. Results and discussion

XRD patterns of the composites in Fig. 1(a) show that the diffraction peaks originate from the anatase and rutile TiO₂ phases, and also identify that the relative content of anatase phase and rutile phase is about 4 : 1, which is in accordance with the P25 peaks. Moreover, after being decorated with RGO, there is no obvious peak change for the various RGO/P25 composites when compared with the P25 pattern, which is related to the fact that the RGO content is too small to be detected by XRD observation. However, there is a broadened peak in the range of 20–30 degrees for the various RGO/P25 composites. Fig. 1(b) shows the SEM image of the 0.75 wt% RGO/P25 composite, which shows that the TiO₂ nanoparticles form some clusters on the surface of the RGO. It also shows that the various RGO/P25 composites have similar morphologies, as shown in Fig. S1.†

Fig. 1 XRD patterns of the as-prepared RGO/P25 composites with various RGO contents (a); SEM image of the 0.75 wt% RGO/P25 composite (b); and FTIR spectra of P25 and the 0.75 wt% RGO/P25 composite (c).

Fig. 1(c) shows the FTIR spectra of the P25 and the as-prepared RGO/P25 (0.75 wt%). For P25, the absorption peaks at 3612 cm⁻¹ and 1621 cm⁻¹ come from the –OH stretching group, while the small peaks at 400–900 cm⁻¹ originate from the stretching vibration of the Ti–O–Ti bonds in crystalline TiO₂.^21,22 Compared with the P25 spectrum, the broad peaks around 1100 cm⁻¹ in the spectrum of the RGO/P25 composite originate from the stretching vibrations of C–O–C (1127 cm⁻¹) and C–OH (1048 cm⁻¹) bonds, which are caused by the partial existence of graphene oxide as it is impossible to be totally reduced in the process.²³ The broad absorption below 1000 cm⁻¹ is much decreased with a sharper peak than the corresponding peak in pure P25, which is attributed to be the presence of the Ti–O–Ti vibration and Ti–O–C vibration (815 cm⁻¹).^23,24 Moreover, it is clear that the FTIR spectrum of the RGO/P25 composite shows some strong absorption peaks that correspond to functional groups such as CH₂ (2925 cm⁻¹) and alcoholic C–OH stretching (1442 cm⁻¹).

The interface combination of the graphene/TiO₂ composites is quite important for their photocatalytic performance. To date, many kinds of microscopes have been used to evaluate the morphology and microstructure of the graphene hybrid film, such as high resolution transmission electron microscopy (HRTEM),^25,26 scanning probe microscopy (SPM),²⁷ atomic resolution scanning tunneling microscopy (STM),^28,29 low-energy electron microscopy (LEEM).^30,31 These techniques require cumbersome sample preparation and a high level of technical expertise, and they cannot give a macroscopic image of the graphene distribution. A Raman mapping image from Raman spectroscopy is a fast and useful tool for the distribution characterization of the composites containing carbon nanostructures.^16–18 Raman spectra of TiO₂ and graphene show non-overlapping well-defined features, which provide valuable distribution information for both components; thus, they can help to build a complete description of the RGO/P25 composites.

Fig. 2(a) shows that the RGO and P25 are uniformly dispersed in the formed composites. Fig. 2(b) shows the Raman spectrum obtained from the zone in Fig. 2(a), and the characteristic peaks at 148 cm⁻¹ (E_g(1)), 391 cm⁻¹ (B_1g), 516 cm⁻¹ (A_1g) and 637 cm⁻¹ (E_g(2)) reveal the existence of TiO₂.^32,33 The bands located at 1352, 1614 and 2671 cm⁻¹ correspond to the D (breathing mode of A_1g symmetry), G (E_2g symmetry, in-plane bond-stretching motion of pairs of sp² C atoms) and 2D bands, respectively, which are the typical two bonds of graphitic materials. The G band is a typical zone center vibration mode of crystalline graphite corresponding to sp² bonded carbon, whereas the D band is an edge vibration mode or disorder layer. The sharp peaks and narrow full width at half maximum of the G and D bands indicate a high degree of graphitization of the carbon coating layer.³⁴

Fig. 2 (a) Raman map; (b) Raman spectrum; (c) Raman mapping image of TiO₂ (E_g(1) mode) and (d) Raman mapping image of RGO (2D mode) for the 0.75 wt% RGO/P25 composite.

The Raman data are reorganized into intensity mapping, and Fig. 2(c) and (d) show the Raman mapping image of the RGO/P25 composites. Fig. 2(c) shows the image obtained from the 2D bands of the graphene. The ‘bright’ regions with high intensity show the existence of the graphene, while the ‘dark’ regions are related to the P25 information, which proves the uniform distribution of the graphene in the composites. The strongest band of anatase TiO₂ (E_g mode around 148 cm⁻¹) is selected as the P25 fingerprint, and the Raman mapping in Fig. 2(d) also shows that the P25 are quite uniformly dispersed in the composites. The Raman mapping images of the 2D bands of the graphene in the other RGO/P25 composites also prove the uniform dispersion, as shown in Fig. S2.† For comparison, the RGO/P25 composite is also prepared by mechanical mixing of the graphene and P25, and the corresponding SEM image and Raman mapping image are shown Fig. S3.† It is found that the graphene exists in clusters in this composite and does not have good dispersion. Therefore, the RGO may be fairly homogeneously formed in the composites by the facile hydrothermal process.

UV-vis spectra are used to characterize the optical absorption of all the above samples in the wavelength range of 300–800 nm as shown in Fig. 3. The absorption edges of the RGO/P25 composites have no obvious peak-shifting, which proves that there is no influence on the band gap of the P25 after the formation of the composites. Moreover, the RGO/P25 composites possess obviously enhanced UV and visible light absorption compared to P25, and the 0.75 wt% RGO/P25 composite has the highest optical absorption intensity. In addition, the well-combined RGO and P25 in the composites also favor the rapid transfer of the photo-generated electrons and reduce the recombination of the photo-generated electrons and holes in the RGO/P25 composites.

Fig. 3 UV-vis spectra of the as-prepared RGO/P25 composites with various RGO contents.

The photocatalytic efficiency of the various RGO/P25 composites is evaluated in terms of the degradation rate of methylene orange (MO) under UV light and visible light irradiation. The ratio of the intensity of the MO absorption bands before and after irradiation (I/I₀) is correlated with irradiation time in Fig. 4 by choosing the absorption peak at 586 nm. This shows that the photodegradation rates may be enhanced both under the irradiation of UV and visible light after the formation of the RGO/P25 composites. For example, the photodegradation rate of the pure P25 is about 60.2% after 120 minutes of UV light irradiation, and the 0.75 wt% RGO/P25 composite has the best photodegradation efficiency and may completely photodegrade the MO molecules, as shown in Fig. 4(a).

Fig. 4 Photodegradation curves of P25 and various RGO/P25 composites: (a) under UV light irradiation; (b) under visible light irradiation.

Pure P25 has almost no photodegradation effect on MO under visible light irradiation, while the 0.75 wt% RGO/P25 composite has the best photodegradation efficiency and may completely photodegrade the MO molecules after 150 minutes of visible light irradiation as shown in Fig. 4(b). This shows that the reaction rate increases by a factor of 1.67 under UV-light irradiation and may increase by several factors of a hundred under visible-light irradiation compared with that of the pure P25, which is quite good when compared with similar reports.^13–15 Therefore, the RGO existing inside the P25 nanoparticles can efficiently enhance the photodegradation ability, which results from the fact that the graphene may accelerate the separation of photogenerated electrons and holes on P25 nanocrystals in the composites.³⁵ On the other hand, the excess content of RGO (1.00 wt%) will decrease the optical absorption due to the shading effect,^5,36 which then decreases the photocatalytic properties, and is in agreement with the UV-vis spectra in Fig. 3.

It is well known that a typical Schottky junction barrier at the interface between graphene and P25 will be formed,^37,38 as the heterojunctions will suppress the recombination of electron–hole pairs in P25 nanoparticles, in which the RGO serves as an efficient electron trap aiding electron–hole separation. The possible mechanism of the RGO/P25 composites can be understood through their schematic diagram as shown in Fig. 5. In the UV region, both the P25 and RGO are photoactive components in the composites, which can generate electrons and holes under illumination from the valence band (VB) to the conduction band (CB) that may migrate to the surface of the composites. The Schottky junction barrier facilitates the electron capture, and it will increase the lifetime of the photo-excited electron–hole pairs and retard the electron–hole recombination to enhance the photocatalytic performance. Then, the photo-excited electrons migrate to O₂ molecules adsorbed on the surface of the P25 nanocrystals,^39,40 and subsequently reduce the recombination between electrons and holes. This allows more opportunities for the electrons to participate in the reduction reaction to form superoxide radicals (O₂⁻), which serve as a strong oxidant that can decompose MO molecules effectively. The holes generated under irradiation of P25 participate in the oxidation reaction, and then produce hydroxyl radicals (˙OH), which are a very strong oxidant and favor the decomposition of organic substances in parallel. Moreover, in the visible light region most of the photo-excited electrons are generated due to the RGO,^41,42 which may diffuse through the RGO/P25 interface into the CB of P25. The well-combined interface between the RGO and P25 may favor the migration of the photo-excited electrons and accelerate the production of superoxide and hydroxyl radicals.

Fig. 5 Schematic illustration of the photocatalytic process in the RGO/P25 composites.

4. Conclusions

Various RGO/P25 composites with well-combined interfaces are synthesized by using the hydrothermal method, and it is found that the RGO/P25 composites have an obviously enhanced photodegradation towards the methylene orange molecules. The 0.75 wt% RGO/P25 composite possesses the best photodegradation ability with complete degradation observed after 120 and 150 minutes of irradiation under the UV and visible light, respectively. The good interface combination between RGO and P25 may improve the electric conductivity and lifetime of photo-induced electrons in the photodegradation process as well as enhance light absorption, and cause an enhancement in the photodegradation rate of the RGO/P25 composites.

Acknowledgements

This work is supported by the International S&T Cooperation Program of China (ISTCP) (no. 2013DFR50710) and the Equipment pre-research project (no. 625010402).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05681g

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