Graphene–TiO₂ nanocomposite photocatalysts for selective organic synthesis in water under simulated solar light irradiation

Abstract

Heterogeneous photocatalysis offers a promising route to realize green oxidation processes in organic synthesis because such selective organic transformation processes can be enabled under solar light irradiation with ambient conditions. In this research, graphene–TiO₂ nanocomposites have been utilized for aerobic selective oxidation of benzylic alcohols to corresponding aldehydes and acids in water under simulated solar light irradiation. The photocatalytic performance and related properties of solvent exfoliated graphene–TiO₂ (SEG–TiO₂) and reduced graphene oxide–TiO₂ (RGO–TiO₂) have been comparatively studied. It has been found that decreasing the defect density of graphene, by using SEG instead of graphene oxide (GO) as the precursor of RGO, can enhance the photoactivity of graphene–TiO₂ nanocomposites due to the improved electron conductivity of SEG as compared to that of RGO. It is hoped that this work would stimulate further interest in utilizing graphene–semiconductor nanocomposite photocatalysts in the field of green-chemistry-oriented photocatalytic selective organic transformation.

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Introduction

Graphene, because of its unique planar structure, excellent transparency, superior electron conductivity and mobility, can be used as an ideal high-performance candidate for photocatalyst carriers or promoters.^1–5 In recent years, graphene-based composite photocatalysts have attracted a lot of attention because of their promising potential for conversion of solar energy to chemical energy in numerous fields.^3–12 Nevertheless, most research studies are focused on utilizing graphene-based composite photocatalysts in the fields of “non-selective” degradation of pollutants (dyes, bacteria, and volatile organic pollutant) and water splitting.^3–20 In contrast, research works on utilizing graphene-based semiconductor nanocomposites for photocatalytic “selective” photoredox reaction are relatively limited. Recently, our group has demonstrated that graphene–semiconductor (e.g., TiO₂, ZnS or CdS) nanocomposites can serve as a promising type of effective photocatalysts to promote aerobic selective oxidation of alcohols in the organic solvent of benzotrifluoride (BTF) under ambient conditions.^21–26 These works highlight that, in addition to the photocatalytic applications in environmental remediation and water splitting, there is a wide promising scope to exploit the potential applications of graphene-based nanocomposites in heterogeneous photocatalytic selective organic transformation under ambient conditions.

In comparison with photocatalytic selective organic transformation in the organic solvent of BTF, the organic synthesis reaction in the green solvent of water is more desirable because photocatalytic organic synthesis in water meets the typical tenet of green chemistry.^27,28 From a viewpoint of organic synthesis, water is a highly desirable and green solvent for chemical reactions, because of its environmental concerns, safety, and cost.^27–32 Thus, in recent years, increasing interests have been paid to photocatalytic selective oxidation of alcohols in water.^33–43 For example, Palmisano's group has carried out an aqueous-phase photocatalytic selective oxidation of alcohols to corresponding aldehydes over the home-prepared rutile TiO₂ under UV light irradiation.^39–42 Recently, our group has reported that the graphene–CdS nanocomposites can act as a visible-light-driven photocatalyst for the selective oxidation of alcohols to aldehydes and acids in water under visible light irradiation along with ambient conditions.⁴³ However, it is still unclear if graphene–TiO₂ semiconductor composites can be a good selective photocatalyst for aerobic oxidation of alcohols in water.

Herein, we report photocatalytic aerobic oxidation of alcohols to aldehydes and acids in water over graphene–TiO₂ nanocomposites under simulated solar light irradiation. The effect of defects density of graphene on its photocatalytic performance toward selective oxidation of alcohols in water has also been investigated by using the solvent exfoliated graphene (SEG) with lower defects density and reduced graphene oxide (RGO) with higher defects density. The results show that SEG–TiO₂ exhibits improved photoactivity as compared to RGO–TiO₂ toward oxidation of alcohols in water, similar to the case in the BTF solvent.²³ Importantly, the graphene–TiO₂ nanocomposites can still behave as a selective photocatalyst under simulated solar light irradiation. The primary products are dominated by corresponding aldehydes and acids. Thus, this work would enrich the applications of graphene–semiconductor nanocomposites in photocatalytic selective organic transformations in water. It further corroborates the fact that the proper choice of photocatalysts along with control of reaction conditions is able to promote photocatalytic reactions to occur in a selective way for synthesis of fine chemicals under the framework of green chemistry.

Experimental section

2.1. Preparation

Chemicals. Graphite powder, sulfuric acid (H₂SO₄), potassium persulfate (K₂S₂O₈), phosphorus pentoxide (P₂O₅), nitric acid (HNO₃), potassium permanganate (KMnO₄), hydrochloric acid (HCl), hydrogen peroxide, 30% (H₂O₂), N,N-dimethylformamide (DMF), polyvinylpyrrolidone (PVP) (average MW: 40 000), and ethanol (C₂H₅OH) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Titanium fluoride (TiF₄) was obtained from Alfa Aesar China Co., Ltd. (Tianjin, China). All of the materials were analytic grade and used as received without further purification. Deionized water and ultra-pure water used in the synthesis was from local sources.

Catalyst preparation.
(a) Synthesis of graphene oxide (GO) and solvent exfoliated graphene (SEG). Graphene oxide (GO), the precursor of reduced graphene oxide (RGO) in this work, was prepared from natural graphite powder by a modified Hummers’ method that involves a strong oxidation and exfoliation process in solution, which was also used in our previous studies.^20–23 Solvent exfoliated graphene (SEG) with lower defects density was synthesized from natural graphite powder by an exfoliation method in the organic solvent of DMF, as reported previously in the literature.^23,44
(b) Fabrication of graphene (SEG, RGO)–TiO₂ nanocomposites. Graphene–TiO₂ nanocomposites were synthesized by a facile two-step wet-chemistry approach as reported previously.^22,23 To modify the surface of SEG to make it well dispersed in the aqueous solution and able to provide adequate hydrophilic functional groups for the growth of semiconductor TiO₂, a certain amount of PVP as an interface linker was added to the SEG suspension in water and stirred for 24 h to functionalize the surface of SEG.²³ In a typical process, the given amount of the as-prepared GO or PVP-functionalized SEG was dispersed in 200 mL of deionized water by ultrasonication. Then, 4.0 g of TiF₄ was added to the solution. The mixture solution was ultrasonicated for 1 h and then heated to 333 K in an open oil-bath kept with magnetic stirring for 24 h. The mixture was separated by centrifugation, washed until the pH of rinse water became neutral, and fully dried at 333 K in oven. The as-obtained composites were aged in 26 mL of deionized water and 13 mL of anhydrous ethanol solution with vigorous stirring for 0.5 h to obtain a homogeneous suspension. After that, it was transferred to 50 mL Teflon-sealed autoclave and maintained at 393 K for 12 h. The products were cooled to room temperature, separated by centrifugation and washed by water. Followed by a dry process, a series of graphene (SEG, RGO)–TiO₂ nanocomposites with different weight addition ratios of graphene were obtained.

2.2. Characterization

The crystalline structure of the samples was determined by the powder X-ray diffraction (XRD, Philip X' Pert Pro MPP) using a Cu Kα radiation (λ = 1.5418 Å) in the 2θ ranging from 5° to 80° with a scan rate of 0.08° per second. The optical properties of the catalysts were analyzed by UV-vis diffuse reflectance spectroscopy (DRS) using a Cary-500 spectrophotometer over a wavelength range of 250–800 nm, during which BaSO₄ was employed as the internal reflectance standard. Field-emission scanning electron microscopy (FE-SEM) was used to characterize the morphology of the samples on a FEI Nova NANOSEM230 spectrophotometer. The Brunauer–Emmett–Teller (BET) specific surface area of the samples was analyzed by nitrogen (N₂) adsorption in a Micromeritics ASAP 2020 apparatus. Raman spectroscopic measurements were performed on a Renishaw inVia Raman System 1000 with a 532 nm Nd:YAG excitation source at room temperature. The photoluminescence (PL) spectra were obtained using an Edinburgh Analytical Instrument PLS920 system. For the PL analysis of solid samples of grapheme–TiO₂ nanocomposites, the excitation wavelength is 300 nm.

The electrochemical analysis was carried out in a conventional three-electrode cell using a Pt plate and an Ag/AgCl electrode as the counter electrode and reference electrode, respectively. The electrolyte was 0.2 M Na₂SO₄ aqueous solution without additive (pH = 6.8). The working electrode was prepared on indium-tin oxide (ITO) glass that was cleaned by sonication in ethanol for 30 min and dried at 353 K. The boundary of ITO glass was protected using scotch tape. The 10 mg sample was dispersed in 1 mL of ethanol by sonication to get slurry. The slurry was spread onto the pretreated ITO glass. The working electrode was dried overnight under ambient conditions. Then, the scotch tape was unstuck, and the uncoated part of the electrode was isolated with epoxy resin. The exposed area of the working electrode was 0.25 cm². The simulated solar light irradiation source was a 300 W Xe arc lamp system, which is the same light source as for the photoactivity test. The chopping photocurrent–voltage curves were measured on a BAS Epsilon workstation with applied potentials of 0 to +1.05 V. The electrochemical impedance spectroscopy (EIS) measurements were measured with a CHI-660D electrochemical workstation (CH instruments, USA) and the electrolyte contained 10 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] and 0.5 M KCl.

2.3. Photocatalytic activity measurement

The liquid-phase photocatalytic selective oxidation of benzylic alcohols in water was performed under ambient conditions, i.e., room temperature and atmospheric pressure.^37,38,45 Typically, alcohols (0.1 mmol) and 8 mg of the catalyst were added into the ultra-pure water (1.5 mL) saturated with pure molecular oxygen. Then, the above mixture was transferred into a 10 mL Pyrex glass bottle filled with molecular oxygen at a pressure of 0.1 MPa, and stirred at room temperature for half an hour to make the catalyst distribute evenly in the solution. The suspensions were irradiated at ambient conditions. A 300 W Xe arc lamp (PLS-SXE 300, Beijing Perfectlight, Co. Ltd.) was employed as the simulated solar light (320 nm–780 nm). After the reaction, the mixture was centrifuged at 12 000 rpm for 20 min to completely remove the catalyst particles. The remaining solution was analyzed using a Shimadzu High Performance Liquid Chromatograph (HPLC-LC20AT equipped with a C18 column and SPD-M20A photo diode array detector). Conversion of alcohol and selectivity for aldehyde were defined as follows:

Conversion(%) = [(C₀ − C_r)/C₀] × 100

Selectivity(%) = [C_p/(C₀ − C_r)] × 100

where C₀ is the initial concentration of alcohol, C_r and C_p are the concentrations of the alcohol substrate and the corresponding aldehyde, respectively, at a certain time after the photocatalytic reaction.

Controlled photoactivity experiments were performed similar to the above photocatalytic oxidation of benzyl alcohol except the addition of different radical scavengers (0.1 mmol): ammonium oxalate (AO) as scavenger for photogenerated holes, AgNO₃ as scavenger for electrons, tert-butyl alcohol (TBA) as scavenger for hydroxyl radicals, and benzoquinone (BQ) as scavenger for superoxide radical species.^37,38,45,46

Results and discussion

Fig. 1 shows the photocatalytic activity of the as-prepared solvent exfoliated graphene–TiO₂ (SEG–TiO₂) and reduced graphene oxide–TiO₂ (RGO–TiO₂) samples for selective oxidation of benzyl alcohol in water under simulated solar light irradiation. It can be seen that, among those SEG–TiO₂ nanocomposites, the 5%SEG–TiO₂ nanocomposite exhibits the best photocatalytic performance. Under 4 h simulated solar light irradiation, the conversion of benzyl alcohol and the selectivity for benzaldehyde over 5%SEG–TiO₂ are measured to be ca. 50% and ca. 90%, respectively, which are higher than the values obtained over the optimal 5%RGO–TiO₂ (ca. 37% conversion and 90% selectivity for benzaldehyde) among RGO–TiO₂ nanocomposites under identical reaction conditions. In addition, it is noted that, when SEG is used instead of RGO, the optimum addition ratio of graphene in the graphene–TiO₂ nanocomposites is unchanged, i.e., the optimum addition weight ratio of SEG or RGO is 5%. This also indicates the importance of controlling the addition ratios of graphene in order to achieve an optimal synergistic interaction between graphene and TiO₂ for achievement of the best photoactivity of graphene–TiO₂ nanocomposites. Excessive addition of SEG or RGO in graphene–TiO₂ nanocomposites means the significant amount decrease of primary photoactive ingredient TiO₂ and meanwhile lowers the light intensity through the depth of reaction solution.^22–25 In addition, it should be noted that the excessive addition of SEG or RGO in graphene–TiO₂ nanocomposites will lead to a decrease of selectivity for benzaldehyde, with the main byproduct of benzoic acid. Furthermore, we have also carried out the reaction for selective oxidation of benzyl alcohol over the optimal 5%SEG–TiO₂ and 5%RGO–TiO₂ by extending the reaction time, which is shown in Fig. 2. It can be seen that the conversion of benzyl alcohol is enhanced with the evolution of reaction time. After simulated solar light irradiation of 8 h, the conversion of benzyl alcohol over 5%SEG–TiO₂ is up to 90% while the selectivity to benzaldehyde is 85%, which are higher than the values obtained over 5%RGO–TiO₂ (ca. 65% conversion and 84% selectivity for benzaldehyde) under identical reaction conditions. Blank experiments performed in the absence of catalyst and/or visible light show that no conversion of alcohols is observed, therefore confirming the reaction is really enabled by a photocatalytic process. A controlled experiment in the presence of nitrogen shows trace conversion of alcohols (Fig. S1 in the ESI†), proving that oxygen is the primary oxidant that selectively oxidizes alcohols to corresponding aldehydes. In particular, our reactions with water as solvent, molecular oxygen as oxidant, simulated solar light as the driving energy source, and ambient conditions represent typical tenets of green chemistry.^43,45

Fig. 1 Photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over the nanocomposites of RGO–TiO₂ and SEG–TiO₂ with different weight addition ratios of graphene in water under simulated solar light irradiation for 4 h.

Fig. 2 Time-online photocatalytic selective oxidation of benzyl alcohol to benzaldehyde over 5%RGO–TiO₂ and 5%SEG–TiO₂ in water under the irradiation of simulated solar light.

To ensure if this photoactivity enhancement is general, we have also performed the oxidation of other alcohols. Table 1 lists the photocatalytic performance of selective oxidation of a range of benzylic alcohols over the photocatalyst of 5%SEG–TiO₂ and 5%RGO–TiO₂ under simulated solar light irradiation of 4 h. It can be seen that 5%SEG–TiO₂ nanocomposite exhibits a higher photoactivity in all selected reaction systems than 5%RGO–TiO₂ nanocomposite. The selectivity to the corresponding benzylic aldehydes is >85%, while the primary byproduct is acid. Therefore, it is clear that both 5%SEG–TiO₂ and 5%RGO–TiO₂ can be used as highly selective photocatalysts toward aerobic oxidation of alcohols to aldehydes and acids in the solvent of water under simulated solar light irradiation. The main difference between them is the higher photoactivity of 5%SEG–TiO₂ than 5%RGO–TiO₂. To understand the origin resulting in the photoactivity difference, we have performed comparative characterizations on the 5%SEG–TiO₂ and 5%RGO–TiO₂ photocatalysts in terms of joint analysis on the crystalline structure, optical properties, surface area, morphology, and lifetime and transfer of photogenerated electron–hole pairs.

Table 1 Selective oxidation of a range of alcohols to aldehydes over the 5%RGO–TiO₂ and 5%SEG–TiO₂ photocatalysts in water under the irradiation of simulated solar light for 4 h

Catalyst	Substrate	Product	Conv. (%)	Sel. (%)
5%RGO–TiO2			37	90
5%SEG–TiO2			50	90
5%RGO–TiO2			40	88
5%SEG–TiO2			57	89
5%RGO–TiO2			38	87
5%SEG–TiO2			55	88
5%RGO–TiO2			35	86
5%SEG–TiO2			49	88
5%RGO–TiO2			30	85
5%SEG–TiO2			46	87
5%RGO–TiO2			37	86
5%SEG–TiO2			52	87

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Fig. 3a shows the powder X-ray diffraction (XRD) patterns of the 5%SEG–TiO₂ and 5%RGO–TiO₂ nanocomposites. It is clear to see that the 5%SEG–TiO₂ and 5%RGO–TiO₂ have similar XRD patterns. The peaks of scattering angles 2θ values located at ca. 25.3°, 37.8°, 48.0°, 53.9°, 55.0°, 62.7°, 68.8°, 70.3°, and 75.0° can be indexed to (101), (004), (200), (105), (211), (204), (116), (220), and (215) facet crystal planes of anatase TiO₂ (JCPDS no. 21-1272), respectively.^22,23 Similar to the previous reports,^22,23 no typical diffraction peaks of the separate SEG or RGO are detected, which could be ascribed to its relatively low diffraction intensity of graphene and the disappearance of the layer-stacking regularity after redox of graphite. In addition, according to the Scherrer formula, the average crystallite sizes of TiO₂ particles are calculated from the (101), (004), (200), (105), (211), (204), (116), (220), and (215) peaks of the XRD patterns. The average crystallite size of TiO₂ particles is about 19.2 nm and 19.4 nm, corresponding to the 5%SEG–TiO₂ and 5%RGO–TiO₂ samples, respectively. These results indicate that the crystalline structure and size of TiO₂ are similar between the 5%SEG–TiO₂ and 5%RGO–TiO₂ samples. The UV-vis diffuse reflectance spectra (DRS) are used to determine the optical properties of the samples. It can be observed from Fig. 3b that the 5%SEG–TiO₂ nanocomposite has a nearly identical light absorption intensity compared to 5%RGO–TiO₂ from UV to visible light region. Fig. 3c shows the nitrogen (N₂) adsorption–desorption isotherms and the corresponding pore size distribution. The surface area of 5%SEG–TiO₂ and 5%RGO–TiO₂ is ca. 70 m² g⁻¹ and 69 m² g⁻¹, respectively. In addition, there is no significant difference between these two samples in both total pore volume and porosity. To obtain the microscopic morphology and structure information, the field-emission scanning electron microscopy (FE-SEM) analysis of 5%SEG–TiO₂ and 5%RGO–TiO₂ has been performed, as shown in Fig. 3d. It is clear that the SEG or RGO sheet and semiconductor TiO₂ ingredient have been integrated by a way of an intimate interfacial contact. The very similar XRD patterns and morphology of 5%SEG–TiO₂ and 5%RGO–TiO₂ suggest that the particle size, crystal phase and morphology of TiO₂ in these two samples are almost the same. Therefore, crystalline structure, optical properties, surface area and morphology should not be the primary reason, leading to the large photoactivity difference between 5%SEG–TiO₂ and 5%RGO–TiO₂.

Fig. 3 The XRD patterns (a), UV-vis DRS (b), N₂ adsorption–desorption isotherms (c), inset is the corresponding pore size distribution and SEM images (d) of the samples of 5%SEG–TiO₂ and 5%RGO–TiO₂ nanocomposites.

Next, we have used the photoelectrochemical and photoluminescence spectra to characterize the lifetime and transfer rate of photogenerated electron–hole pairs of SEG–TiO₂ and RGO–TiO₂ upon light irradiation. The chopping photocurrent–voltage plots under simulated solar light irradiation, photoluminescence (PL) spectra, and electrochemical impedance spectroscopy (EIS) are displayed in Fig. 4. It can be seen that the photocurrent density obtained on 5%SEG–TiO₂ under simulated solar light irradiation is obviously higher than that of 5%RGO–TiO₂, as shown in Fig. 4a, which suggests the longer lifetime of photogenerated electron–hole pairs achieved on the 5%SEG–TiO₂ nanocomposite. This prolonged lifetime of the photogenerated charge carriers can also be confirmed by the PL spectra. The PL spectra are often employed to study surface processes involving the electron–hole fate of the semiconductor TiO₂.^23,47 With the electron–hole pairs recombination after a photocatalyst is irradiated, photons are emitted, which results in PL. It can be seen from Fig. 4b that the PL intensity of 5%SEG–TiO₂ is lower than 5%RGO–TiO₂, thus indicating the more efficient inhibition of photogenerated charge carriers recombination.

Fig. 4 Chopping simulated solar light photocurrent–voltage curves in 0.2 M Na₂SO₄ (pH = 6.8) aqueous solution versus Ag/AgCl (a), photoluminesence (PL) spectra excited at 300 nm (b), and Nyquist impedance plots (c) of the samples of 5%SEG–TiO₂ and 5%RGO–TiO₂ nanocomposites.

In addition, other electrochemical analysis has also been carried out. It is well known that the electrochemical impedance technique is a reliable way to characterize the electrical conductivity of materials. Therefore, the electrochemical impedance spectra (EIS) data can be used to reflect the capability of graphene–TiO₂ to transfer the charge carriers. Since the preparation method of electrodes and the electrolyte used are alike, the high frequency semicircle is related to the resistance of sample electrodes.²³ It can be seen that the 5%SEG–TiO₂ has a smaller arc than that of 5%RGO–TiO₂, as shown in Fig. 4c, suggesting the more efficient interfacial charge carriers transfer over 5%SEG–TiO₂ than 5%RGO–TiO₂.

Obviously, the use of SEG instead of GO as the precursor of graphene contributes to the photoactivity enhancement of graphene–TiO₂ nanocomposites. This is reasonable, because it has been recognized that SEG has better electron conductivity than RGO,^23,43,44 and this can be reflected by Raman spectra, as displayed in Fig. S2 (in the ESI†). Of particular note is the intensity ratio of the D and G bands, I_D/I_G, which is a measure of the relative concentration of local defects or disorders (particularly the sp³ hybridized defects) compared to the sp² hybridized graphene domains. As can be seen clearly, the I_D/I_G ratio is 0.18 for SEG, which is much lower than 1.01 for GO. This indicates the much lower defects density of SEG as compared to GO, suggesting that SEG can improve electron conductivity as compared to RGO, which is also in agreement with the previous report.⁴⁴ In addition, from the photograph as shown in inset of Fig. S2 (in the ESI†), we can see that the colour of SEG and GO dispersions in water is remarkably different. In particular, the GO dispersion in water exhibits a brilliant yellow color, which indicates the richness of oxygen-containing functional groups of GO.^22,23

To further understand the role of active species involved for photocatalytic selective oxidation of alcohols over the graphene–TiO₂ nanocomposite in water under simulated solar light irradiation, we have further carried out controlled experiments with adding scavengers for ˙OH, O₂˙⁻, h⁺, and e⁻, respectively.^37,38,45,46 Fig. 5 shows the photocatalytic activities of 5%SEG–TiO₂ and 5%RGO–TiO₂ for selective oxidation of benzyl alcohol in the presence of different radical scavengers. From the data, we can see that when the scavenger AO for holes (h⁺) is added, conversion of benzyl alcohol is significantly prohibited. When the scavenger BQ for superoxide radicals (O₂˙⁻) or scavenger AgNO₃ for electrons (e⁻) is added, conversion of benzyl alcohol is also decreased remarkably. However, when the scavenger TBA for hydroxyl radicals (˙OH) is added into the reaction system, the conversion of benzyl alcohol is decreased moderately. Analogous phenomena are observed for both 5%SEG–TiO₂ and 5%RGO–TiO₂. In particular, it is worth noting that although photogenerated electrons can not directly participate in the oxidation process, they can activate molecular oxygen to generate superoxide radicals (O₂˙⁻). Consequently, this leads to the observation that the conversion of benzyl alcohol is markedly inhibited when adding AgNO₃ scavenger for electrons into the photocatalytic reaction system. The results from controlled experiments suggest that the order of active species on affecting the rate of oxidation of alcohol over 5%SEG–TiO₂ and 5%RGO–TiO₂ is quite similar, i.e., the h⁺, e⁻, and O₂˙⁻ play the more dominant role than ˙OH.

Fig. 5 Controlled experiments of photocatalytic selective oxidation of benzyl alcohol with the addition of different radical scavengers: tert-butyl alcohol (TBA) as scavenger for hydroxyl radicals (˙OH), benzoquinone as (BQ) scavenger for superoxide radicals (O₂˙⁻), ammonium oxalate (AO) as scavenger for holes (h⁺) and AgNO₃ as scavenger for electrons (e⁻) over the 5%SEG–TiO₂ (a) and 5%RGO–TiO₂ (b) under simulated solar light irradiation for 4 h.

Conclusions

In summary, we have reported that graphene (SEG, RGO)–TiO₂ nanocomposites prepared via a two-step wet-chemistry method can serve as a type of selective photocatalysts in green solvent, water. In order to investigate the effect of defects density in graphene on the photocatalytic performance of graphene–TiO₂ nanocomposites, GO with higher defects density and SEG with lower defects density are both employed as the precursor of graphene. It discloses that the as-prepared graphene (SEG, RGO)–TiO₂ nanocomposites are effective toward photocatalytic selective oxidation of alcohols to corresponding aldehydes and acids in water under simulated solar light irradiation. In particular, SEG with lower defects density exhibits higher photoactivity enhancement of the semiconductor TiO₂ due to its better electron conductivity as compared to RGO. It is hoped that this work can open up new frontiers for advancing the utilization of graphene-based nanocomposite photocatalysts in the potential field of diverse selective oxidation processes in water under the framework of green chemistry.

Acknowledgements

The support by the National Natural Science Foundation of China (NSFC) (21173045, 20903023), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), Program for Returned High-Level Overseas Chinese Scholars of Fujian Province, and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The results of photocatalytic selective oxidation of benzyl alcohol over the optimal 5%SEG–TiO₂ and 5%RGO–TiO₂ under O₂ and N₂ atmosphere. Raman spectra and photograph dispersions in water of SEG and GO. See DOI: 10.1039/c4ra01190b

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