Improved photocatalytic activity of RGO/MoS₂ nanosheets decorated on TiO₂ nanoparticles

Abstract

In this work we have synthesized a heterogeneous hybrid junction of reduced graphene oxide (RGO)/MoS₂ and hybrid composites of TiO₂–RGO/MoS₂ using a hydrothermal method. The hybrid composites were characterized using UV-visible spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electron paramagnetic resonance spectroscopy (EPR). Raman spectroscopy displays that the hybrid junction was formed between two dimensional RGO and few layer MoS₂ nanosheets. In the TiO₂–RGO/MoS₂ hybrid composite, EPR studies show that the lattice Ti³⁺ signal was decreased and at the same time equivalent amounts of the O˙⁻ signal increased under UV light irradiation. These results suggested that the interfacial charge transferred from the TiO₂ surface to the MoS₂ surface active sites directly or through the RGO nanosheets toward suppressing the recombination rate of the electron–hole pairs for an enhanced methylene blue degradation. RGO behaves like a conductor, it accepted only electrons and transported to the active sites through separating the potential level distance between the two semiconductors.

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1. Introduction

Nowadays, clean energy is in demand due to its low cost and abundant source. Solar hydrogen production is one of the best ways to get clean energy from available semiconductors by the reduction of water splitting.¹ Titanium dioxide (TiO₂) is the most explored and applied photocatalyst.^1,2 It has a strong oxidizing power, high stability, low-cost, an abundant source and is relatively non-toxic. TiO₂ exists in mainly three phases as anatase, rutile and brookite. The anatase phase has been shown to be the 100% metastable state with a band gap near 3.2 eV.³ Under UV light irradiation charge carriers are generated in the valence band (VB) edge and conduction band (CB) edge of TiO₂ towards the oxidation and reduction reactions.⁴ However, the electron–hole recombination rate is faster and mainly responsible for its limited application in catalysis. Therefore, it is necessary to overcome this problem by charge separation through suppressing the recombination rate of the electron–hole pairs.⁵

A series of strategies have been developed to synthesize TiO₂ based nanocomposites by doping metals (Ag,⁶ Au,⁵ Pt⁷) or non-metals (B,⁸ N,^8,9 P,¹⁰ F,¹¹ S,¹² C¹³) into the TiO₂ lattice to inhibit the recombination rate and narrow the band gap. It has been reported that TiO₂ nanoparticles coupled with narrow band gap semiconductors (CuO,¹⁴ MoS₂,¹⁵ WO₃,¹⁶ and Fe₂O₃ (ref. 17)) can improve the interfacial charge transfer (IFCT) and inhibit the recombination rate of charge carriers. In addition to this, TiO₂ heterostructures have gained much interest with two-dimensional materials for photocatalytic as well as electrocatalytic applications. Chang et al.¹⁸ reported that TiO₂ nanoparticles coupled with inorganic heterostructures of graphene and molybdenum disulfide (MoS₂) nanosheets were an effective way to increase the oxidation and reduction reactions.

Single layer two dimensional graphene nanosheets have the excellent ability of electron acceptance and transport to the semiconductor–graphene junction.^19,20 However, chemically exfoliated graphene oxide (GO) exhibits poor conduction and electrical properties. Pei et al.²¹ reported that reduced graphene oxide (RGO) shows better properties than GO through removing the surface attached oxygen-containing functional groups to restore the sp²-hybridized network. In particular, molybdenum disulfide (MoS₂) with a layered structure has been extensively used in electrocatalysis, fuel cells, lubricants, and optoelectronic devices as well as in lithium storage batteries.^18,22–26 Thurston et al.²³ reported that bulk MoS₂ was turned into monolayer or few layers, at the same time its band gap also changed from an indirect to direct band gap. Monolayer or few layer MoS₂ nanosheets exhibited an improved number of active sites and conductivity. Recently, Xie’s group demonstrated that a novel defect-rich structure introduced the additional active sites into the MoS₂ ultrathin nanosheets which significantly improved the electrocatalytic performance.^27,28 Some contradicting models were proposed for photogenerated charge transfer from the TiO₂ to the MoS₂ surface and vice versa under light irradiation to account for the enhanced catalytic activity.^15,23,25,29 No unified direct spectroscopic evidence has been reached to suggest the charge carriers direction, which pointed to the relative position of the CB and VB edges in the two semiconductors.

Herein, we report anatase phase TiO₂ nanoparticles coupled with few layers of MoS₂ nanosheets (TiO₂–MoS₂) and a RGO/MoS₂ hybrid junction (TiO₂–RGO/MoS₂) using hydrothermal processes under basic conditions. The EPR spectra illustrated that the interfacial charge transferred from the TiO₂ nanoparticles to the MoS₂ surface via the RGO nanosheets. RGO behaves like a conductor; it accepts only electrons and transports them to the active sites of the MoS₂ cocatalyst surface under UV light irradiation. The TiO₂–RGO/MoS₂ composite showed an enhanced charge carrier separation in order to improve the photocatalytic activity towards the degradation of methylene blue dye. Overall, the EPR studies provided us further understanding of the charge transportation and recombination pathways mechanism to design and synthesise graphene-based materials for energy applications.

2. Experimental

2.1 Materials

Anatase phase TiO₂ nanoparticles (Hombikat UV100) were obtained from World Chem. Industry, Taipei, Taiwan. Graphite powder (99.9995%), Na₂MoO₄·2H₂O and thiourea were purchased from Alfa Aesar. NaNO₃, H₂O₂ (30–31%), NaBH₄ and N₂H₄ (64–65%) were purchased from Sigma Aldrich. H₂SO₄ from Fluka and KMnO₄ obtained from J. T. Baker were used as oxidizing agents. Double distilled water was used in all the experiments. All the materials were used as received.

2.2 Characterizations

Scanning electron microscopy (SEM) images were obtained from a JEOL JSM7000F field emission scanning electron microscope. X-ray photoelectron spectroscopy (XPS) characterizations were performed using a Kα system (Thermo Scientific) with monochromatic Al Kα excitation and a charge neutralizer. Hybrid junction investigations were performed using Raman Spectroscopy (Renishaw). X-Band Electron Paramagnetic Resonance (EPR) spectra were recorded at 77 K using a Bruker EMX spectrometer with a TE102 cavity. The EPR sample tube was immersed in a liquid nitrogen-containing finger Dewar and a Newport 1000 W xenon lamp with an IR filter was used as an irradiation light source to transmit into the EPR cavity through an optical fiber of the same time rate irradiation. The UV-visible diffuse reflectance and photocatalytic activity were recorded on a Shimadzu (UV-2550) UV-vis spectrophotometer.

2.3 Experimental methods

2.3.1 Synthesis of the RGO/MoS₂ hybrid junction. A two-dimensional layered MoS₂ catalyst was synthesized according to the previous literature.²⁷ In brief, 1 mmol of Na₂MoO₄·2H₂O and 7.5 mmol of thiourea were dissolved in 60 ml of distilled water. The homogeneous solution was stirred for a few minutes and transferred into a 100 ml Teflon-lined autoclave at 210 °C for 24 h. A black precipitate was collected using centrifugation, washed with deionized water and ethanol, and dried at 80 °C overnight in a vacuum oven. In order to prepare the RGO/MoS₂ hybrid junction, 4 mg of the prepared MoS₂ catalyst was added to a water and ethanol mixture and exfoliated for 1 h using sonication. The obtained large size particles were removed through low speed centrifugation to obtain supernatant as few layers of MoS₂ nanosheets. To the above supernatant 1 mg of RGO was added and kept for 1 h under sonication and stirred for 5 h accordingly to make a homogeneous solution. The solution was transferred to an autoclave at 180 °C for 10 h and gradually cooled down to room temperature. The obtained suspension was used as the hybrid junction of few layers RGO/MoS₂ nanosheets.

2.3.2 Synthesis of the TiO₂–RGO/MoS₂ hybrid nanocomposite. All the hybrid nanocomposites were prepared using a hydrothermal method. In a typical procedure, 400 mg of anatase phase TiO₂ nanoparticles was dispersed in the above water and ethanol mixture containing the hybrid junction of RGO/MoS₂ nanosheets. The mixture was sonicated for a few minutes and stirred for 5 h at room temperature to prepare a homogeneous solution. After that the solution was transferred to an autoclave for hydrothermal treatment at 180 °C for 10 h. The autoclave was naturally cooled down to room temperature and washed repeatedly with distilled water and ethanol, then dried at 80 °C. Other samples were also synthesized using the same procedure without RGO and varying the (0, 1) wt% concentration of the MoS₂ nanosheets.

2.4 Photocatalytic degradation

The hybrid composites photocatalytic activities were evaluated using the degradation of aqueous solutions of methylene blue (MB) under natural sunlight irradiation. The catalyst (50 mg) was added into 100 ml of MB dye solution (10 mg l⁻¹ aqueous solution) under ambient conditions. The solution was stirred for 20 min in the dark to get the adsorption–desorption equilibrium between the catalyst and dye solution. There was not much difference observed due to the graphene or MoS₂ nanosheets. The solution was exposed to sunlight (10 am to 12 pm) under constant stirring. At a given 20 min time interval, 3 ml aliquots of the solution were withdrawn and centrifuged to remove the catalyst. The concentration of MB in the aqueous solution was determined with the help of a UV-vis spectrophotometer. All sample activity was done with the same time under sunlight irradiation. The experiments were carried out at the National Dong Hwa University, Hualien, Taiwan (23°53’38.06”N, 121°32’36.88”E). The surrounding temperature was 25 ± 2 °C while the solution temperature increased to 28 ± 5 °C during the irradiation time.

3. Results and discussion

3.1 UV-visible spectroscopy

UV-visible spectroscopy is employed to estimate the band gap of the as prepared semiconductors. Fig. 1(a) shows two band peaks at 627 nm and 672 nm that are attributed to the direct excitonic transitions of A₁ and B₁ in the dispersion of the exfoliated MoS₂ nanosheet and the RGO/MoS₂ hybrid junctions. These observed characteristic peaks suggest that the energy splitting in the MoS₂ valence band is due to spin–orbit coupling.^26,30 It has been stated that as MoS₂ turns from bulk to monolayer or few layers, its band gap also changes from an indirect to direct band gap transition.²³ However, monolayer MoS₂ exhibited an improved number of active sites and conductivity.²⁷ Another characteristic band appears at 230–300 nm, corresponding to a π–π* plasmon peak of the reduced graphene oxide. The diffuse reflectance spectra of TiO₂/MoS₂ and the TiO₂–RGO/MoS₂ hybrid composite are shown in Fig. 1(b). The optical absorbance increased with the increase in MoS₂ concentration, whereas the band gap is slightly narrowed in the RGO/MoS₂ hybrid junction. This could be attributed to chemical bonding between TiO₂–RGO due to the formation of Ti–O–C bonds.

Fig. 1 UV-visible spectra of the as-prepared (a) dispersion of the RGO/MoS₂ hybrid junction and (b) the TiO₂–RGO/MoS₂ heterogeneous hybrid composites.

3.2 Morphology analysis

The morphology and microscopic structure of the as-prepared MoS₂ catalyst are characterized using SEM as shown in Fig. 2(a). The as prepared MoS₂ catalyst shows a nanoflower like structure obtained with an average diameter size of 1–2 μm. The surface of these nanoflowers possesses massive petals, which are free and closely aggregated. It could be seen that the petals grown on the surface of the MoS₂ flowers are disorderly intersected together and point toward a common center of the sphere to form the spherical structure.¹⁸ These nanoflowers are separated using sonication and used as a few layers MoS₂ catalyst for the formation of the TiO₂–RGO/MoS₂ composite as shown in Fig. 2(b). In the hydrothermal method, these few layered MoS₂ and RGO nanosheets are easily decorated on TiO₂ nanoparticles which can be seen in the TEM and HR-TEM images (ESI Fig. 2(a) and (b)†).

Fig. 2 SEM images of (a) the MoS₂ nanoflower and (b) TiO₂–RGO/MoS₂ hybrid composite.

3.3 XPS spectroscopy

High resolution X-ray photoelectron spectroscopy (XPS) is performed to study the oxidation state of the prepared samples such as the RGO, MoS₂ and TiO₂–RGO/MoS₂ composites. Chemically exfoliated graphene oxide shows binding energies at 284.4 eV and 286.6 eV which correspond to the C–C and C–O bonds, respectively^21,31 as shown in Fig. 3(a). However, graphene oxide exhibited poor conduction and electrical properties due to the oxygen-containing functional groups present on the surface. Fig. 3(b) shows the binding energy at 284.4 eV denoted as C 1s which suggested that reduced graphene oxide is formed by a reduction process.²¹ The as prepared MoS₂ nanoflower shows two peaks at 228.9 and 232.1 eV assigned to the Mo 3d_5/2 and Mo 3d_3/2, respectively and the valence state was determined as Mo⁴⁺.²⁸ Moreover, the S 2p peaks at 161.5 and 162.7 eV are equivalent to the S²⁻ valence state of MoS₂ (ESI Fig. 4(a) and (b)†).

Fig. 3 XPS spectra of the as prepared (a) graphene oxide (b) reduced graphene oxide; and in the TiO₂–RGO/MoS₂ hybrid composite (c) C 1s of RGO and (d) Mo 3d.

In the hydrothermal method, TiO₂ nanoparticles are decorated with a RGO/MoS₂ nanosheet junction in order to form the TiO₂RGO/MoS₂ hybrid composites. The high surface area of RGO supported the Ti–O–C bond formation between the TiO₂ nanoparticles and graphene nanosheets.¹⁵ RGO exhibited two peaks at 284.4 eV and 289 eV which are ascribed to the C–C and O=C–O bonds as shown in Fig. 3(c). The O=C–O bonds appeared, mainly due to the formation of Ti–O–C bonds as well as the partially oxidized RGO nanosheets with the TiO₂ nanoparticles.¹⁵ On the other side, the small amounts of observed binding energy at 228.8 eV and 231.4 eV imply typical values for Mo⁴⁺ in MoS₂ as shown in Fig. 3(d). There is no evidence found of Mo⁶⁺ valence states in MoO₃, which proved the molybdenum atom does not partially oxidize in the TiO₂–RGO/MoS₂ hybrid composite.²⁵

3.4 Raman spectroscopy

Raman spectroscopy is a useful technique to analyse the vibrational mode and the formation of the RGO and RGO/MoS₂ nanosheets hybrid junction. Fig. 4(a) shows the strong Raman features of the D band at 1343 cm⁻¹ and the G band at 1596 cm⁻¹ originate with additional lower intensity bands at 2500 cm⁻¹ and 2700 cm⁻¹ related to RGO.^15,24,32,33 In the RGO/MoS₂ hybrid junction, two characteristic peaks are ascribed to the MoS₂ site at 381 cm⁻¹ of the lower energy E¹_2g (in plane vibration of two ‘S’ atom with respect to the Mo atom) and 407 cm⁻¹ of the higher energy A_1g (out of plane vibration of S atom) Raman modes.^22,26 Simultaneously lower intensity peaks of the D and G bands are displayed showing the existence of reduced graphene oxide with chemical bonding as shown in Fig. 4(b). Li et al.²² reported that the graphene/MoS₂ nanosheet junction exhibited excellent HER catalytic activity. It has been reported that Raman mode spacing between E¹_2g and A_1g showed the layer dependence of MoS₂ nanosheets. It is worth noting that two peaks in the monolayer or few layers are coming towards each other while in the bulk they are shifting away from each other. The observed shifting difference between the two peaks ∼26 cm⁻¹ shows the few layers of MoS₂, which is consistent with the reported literature.^26,34–36

Fig. 4 Raman spectra of the as prepared (a) RGO and (b) RGO/MoS₂ heterogeneous hybrid junction.

3.5 EPR spectroscopy

3.5.1 Photoinduced electron transfer. Electron paramagnetic resonance spectroscopy (EPR) is a pivotal tool for investigating the paramagnetic defect species, and study their dynamics.

To investigate the interfacial charge transfer mechanism, we compared all the recorded EPR spectra of (bare) TiO₂, TiO₂–MoS₂ and the TiO₂–RGO/MoS₂ composite at 77 K under UV-light irradiation as shown in Fig. 5. In the TiO₂ nanoparticles, under light irradiation, electrons are excited from the valence band (VB) edge to the conduction band (CB) edge leaving positively charged holes. The EPR spectra of the TiO₂ nanoparticles exhibited two major signals in the VB edge and CB edge due to the trapped holes and electrons respectively. Above the Fermi level these electrons are free and a few of them trapped in defect sites leading to originate the paramagnetic cations Ti³⁺ signal while the remaining electrons continued staying on the surface of the nanoparticles. The sharp and intense Ti³⁺ signal B at g_⊥ = 1.991 and g_∥ = 1.961 clearly suggested that electrons trapped in the lattice defect sites lie a few electron volts below the CB edge. On the other hand, the observed O˙⁻ signal A at g values (2.025, 2.016, and 2.003) are attributed to the photogenerated holes trapped on the surface or subsurface bridging oxygen atom sites of the TiO₂ nanoparticles. The observed values are in good agreement with previous literature values, which independently found for O˙⁻ and Ti³⁺ signals in anatase phase TiO₂.^4,9,10,37,38 Reduced graphene oxide does not show any paramagnetic impurity signal except the free electron signal at g = 2.003 (data not added here). These signals could have appeared due to the existence of a defect rich surface in the RGO nanosheets.³⁹ It could be possible that dangling bonds are created on the surface of the RGO, such types of bonds usually occur on the surface of carbon–carbon bonding.^39,40 However, the observed signal does not change which clearly suggesting that the RGO behaves like a conductor under light irradiation.

Fig. 5 EPR spectra were recorded after 5 min. UV light irradiation at 77 K TiO₂ (black color), TiO₂–MoS₂ (blue color) and TiO₂–RGO/MoS₂ (red color).

The EPR spectra clearly displayed that bare (uncoupled) TiO₂ (black-color) nanoparticles are created a large amount of charge carriers (electrons and holes) in their appropriate bands as shown in signals A and B. In the TiO₂–MoS₂ composites, the obtained signal intensity of B has noticeably decreased, and at the same time signal A intensity is increased (5-blue color). This implies that the few layered MoS₂ redox potential is slightly lower than the TiO₂ conduction band. Thermodynamically, the photogenerated electrons quickly migrated to the lower potential of the MoS₂ surface in order to trap in the TiO₂ defect sites. Later, these transferred electrons are stored on the MoS₂ surface in which a few of them could trap on defect sites. On the other hand holes are accumulated at the VB edge of TiO₂ by charge carriers separation. In this hybrid composite, the MoS₂ surface has low electron mobility, less electrical conductivity as well as less surface area which could affect the smaller amount of conduction electrons transferred to the MoS₂ active sites.

In the TiO₂–RGO/MoS₂ composite, the signal intensity of B is decreased as compared to the bare TiO₂ nanoparticles, where simultaneously the signal intensity of A is remarkably increased as shown in Fig. 5-(red color). It suggests that a high concentration of photoinduced electrons are transferred from the TiO₂ surface to the MoS₂ surface in order to improve charge carriers separation. On the RGO nanosheets, these transported electrons have very high mobility and shuttling ability, they quickly migrated on the MoS₂ surface active sites. However, a Mo⁵⁺ (nearly g = 1.93) signal could not be observed and ensure that molybdenum atoms do not partially oxidised³⁶ which is in accordance with the XPS data Fig. 3(d). There could be two pathways that the photogenerated electrons can be transferred from the TiO₂ surface to the few layer MoS₂ surface either directly or through the RGO nanosheets. Min et al.⁴¹ reported that RGO and MoS₂ greatly enhanced the electronic conductivity of the nanohybrid which is beneficial to the photoinduced electron transfer during the illumination. RGO also supports the formation of the maximum percentage of junction between the TiO₂ nanoparticles and MoS₂ nanosheets. In these heterogeneous junctions, the electron transfer driving force is mainly dependent on their energy difference and the coupling distance between the donor–acceptor semiconductors.

3.5.2 Charge carrier separation. In addition to this, we studied the emission of the decay process or the charge recombination pathways of all the prepared hybrid composites as shown in Fig. 6. After UV light irradiation was turned off, the charge transfer process suddenly stopped and the available CB photoinduced electrons quickly followed the recombination process with the VB holes through reduction of signal A and B. In the TiO₂–RGO/MoS₂ hybrid composite, large amounts of O˙⁻ signal A is detected which clearly suggests that the holes are still localized in the VB edge of the TiO₂ nanoparticles, whereas the photoinduced electrons and holes followed a much slower recombination rate than that of the other samples. It is obvious that once electrons transferred from the TiO₂ CB to the RGO or MoS₂ nanosheets surface, then thermodynamically it has very less probability to recombine with the VB holes in TiO₂. Here, the graphene sheet behaves like a conductor which played a crucial role for separating the potential level distance between the two semiconductors.

Fig. 6 EPR spectra were recorded after 10 min. UV light irradiation turned off at 77 K TiO₂ (black color), TiO₂–MoS₂ (blue color) and TiO₂–RGO/MoS₂ (red color).

From Fig. 5 and 6, TiO₂–MoS₂ and the TiO₂–RGO/MoS₂ hybrid composites displayed that the lattice trapped electrons Ti³⁺ signal B and the localized holes O˙⁻ signal A are consistently fluctuated upon the light irradiation being turned on or off. It warrants that “light screening or shading effect” does not occurred with a low concentration of the RGO and MoS₂ cocatalyst.¹⁴ However, the TiO₂ nanoparticles are covered by a high loading concentration of layered MoS₂ or the RGO/MoS₂ cocatalyst; in which they could absorb light and reduce the photo-excitation capacity of TiO₂ to occurred the “light screening effect or shading effect” in a smaller fraction.

3.6 Photocatalytic performance

The photocatalytic activity of all the prepared samples was evaluated using methylene blue (MB) degradation as shown in Fig. 7. Under sunlight irradiation all samples band gap are activated through the generation of electron–hole pairs for redox reactions. It is clear that the TiO₂–RGO/MoS₂ composite exhibits an excellent photocatalytic activity better than the bare TiO₂ nanoparticles and TiO₂–MoS₂ composite. We believe that the observed catalytic activity comes from the interfacial charge transferred to the MoS₂ surface active sites and simultaneous holes accumulating at the VB edge of TiO₂. These transferred electrons on the MoS₂ active sites and the accumulated holes in the TiO₂ valence band edge are mainly participating in degradation of MB molecules. In this composite RGO supports to keep away the potential level distance between two semiconductors of TiO₂ and MoS₂ towards inhibiting the recombination rate of the electron–hole pairs.

Fig. 7 Photocatalytic degradation of methylene blue on various catalysts under sunlight irradiation.

A plausible mechanism for the charge transfer in the solid–solid interface of TiO₂–MoS₂ and the TiO₂–RGO/MoS₂ hybrid composite is proposed in Scheme 1. Under UV light irradiation, both the TiO₂ and MoS₂ bands are activated through the generation of electrons and holes in their appropriate bands (eqn (1)). EPR data supports that the photoinduced electrons (decrease in trapped electrons Ti³⁺ signal) are transferred from the TiO₂ nanoparticles surface to the few layered MoS₂ nanosheets surface via either a direct route or through the RGO junction. On the other hand the increased O˙⁻ signal confirms that the holes are accumulated in the TiO₂ valence band edge by enhanced charge carrier separation (eqn (2)). In this hybrid composite, the RGO sheet separated the potential level distance between two semiconductors towards suppressing the recombination rate of electron–hole pairs. The injected electrons are stored or captured in the MoS₂ surface active sites. These separated charge carriers could migrate to the surface of the photocatalysts, where they rapidly react with H₂O, O₂ and hydroxide ions to generate ˙OH radicals and superoxide radical anions O₂˙⁻ as indicated in eqn (3) and (4). Finally, the created powerful oxidizing agents H₂O₂ and the hydroxyl radicals ˙OH could effectively decompose the methylene blue molecules.²⁹ Holes and radicals can decomposing pollutants via the released CO₂ (eqn (8)). The related chemical reactions can be expressed as:

TiO₂–RGO/MoS₂ + hν → TiO₂ (e⁻ + h⁺)–RGO/MoS₂ (e⁻ + h⁺) (1)

TiO₂ (h⁺)–RGO (e⁻)/MoS₂ (e⁻ + h⁺) → TiO₂ (h⁺)–RGO/MoS₂ (e⁻) (e⁻ + h⁺) (2)

h⁺ + H₂O → ˙OH + H⁺ (3)

e⁻ + O₂ → O₂⁻ (4)

O₂⁻ + H₂O → ˙HO₂ + OH⁻ (5)

˙HO₂ + H₂O → H₂O₂ + ˙OH (6)

H₂O₂ → 2˙OH (7)

˙OH + MB → CO₂↑ + H₂O (8)

Scheme 1 Schematic diagram shows the charge carriers transfer mechanism under UV light irradiation (a) TiO₂–MoS₂ and (b) TiO₂–RGO/MoS₂ hybrid composite.

4. Conclusions

In summary, we have successfully synthesized a RGO/MoS₂ heterogeneous junction and a TiO₂–RGO/MoS₂ composite using a hydrothermal method. The characterization techniques demonstrated that the heterogeneous hybrid junctions were formed between the RGO and the few layers of MoS₂ nanosheets. In the TiO₂–RGO/MoS₂ hybrid composite, EPR studies illustrated that the photoinduced electrons (decreased Ti³⁺ signal) were transferred from the TiO₂ nanoparticles to the few layered MoS₂ nanosheets surface either directly or through the RGO interface. At the same time the holes (increased O˙⁻ signal) were accumulated in the TiO₂ valence band edge, suggesting that a charge carrier separation process occurred. In this composite, RGO played key roles; it accepted only electrons and transported them to the MoS₂ surface catalytically active edge sites. It is believed that the enhanced photo activity of the MB degradation could come from the improved interfacial charge transfer, increased number of catalytically active edge sites, and the slower recombination rate of the electron–hole pairs.

Conflict of interest

The authors declare no competing financial interests.

Acknowledgements

The National Science Council of Taiwan (Grant NSC-104-2627-M-259-001) supported this research.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis, XRD patterns, FE-SEM images, XPS spectra and photocatalytic degradation of MB absorbance spectra. See DOI: 10.1039/c6ra01591c

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