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Controlled growth of TiO2 and TiO2–RGO composite nanoparticles in ionic liquids for enhanced photocatalytic H2 generation
Journal of Molecular Catalysis A: Chemical 378 (2013) 213–220
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Controlled growth of TiO2 and TiO2 –RGO composite nanoparticles in ionic liquids for enhanced photocatalytic H2 generation Ganganagappa Nagaraju a,b , Gunter Ebeling a , Renato V. Gonc¸alves c , Sergio R. Teixeira c , Daniel E. Weibel d , Jairton Dupont a,∗ a Laboratory of Molecular Chemistry, Institute of Chemistry, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonc¸alves 9500, 91501-970 Porto Alegre, RS, Brazil b Center for Nano and Materials Science, Jain University, Jakkasandra, Bangalore Rural 562112, India c Laboratory of Thin Films and Nanostructure Fabrication (L3Fnano), Institute of Physics, Universidade Federal do Rio Grande do Sul, UFRGS, Avenida Bento Gonc¸alves 9500, P.O. Box 15051, 91501-970 Porto Alegre, RS, Brazil d Laboratory of Photochemistry and Surfaces (LAFOS), Institute of Chemistry, Universidade Federal do Rio Grande do Sul, UFRGS, Avenida Bento Gonc¸alves 9500, P.O. Box 15003, 91501-970 Porto Alegre, RS, Brazil
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Article history: Received 11 January 2013 Received in revised form 6 June 2013 Accepted 17 June 2013 Available online xxx Keywords: TiO2 –RGO Nanoparticles Ionic liquids Hydrogen generation
a b s t r a c t TiO2 nanoparticles and TiO2 –RGO (reduced graphene oxide) composite were synthesized by ionothermal method at 100 and 120 ◦ C for 24 h using two different ionic liquids (ILs), 1-(2-methoxyethyl)-3methylimidazolium methanesulfonate (IL1) and 1-(2-methoxyethyl)-3-methylimidazolium tetrafluoroborate (IL2). BET surface area analysis of the composites showed surface areas of 170 and 161 m2 g−1 for TiO2 and TiO2 –RGO composite respectively. TEM images show that the sizes of the particles are around 4 nm and in the composite clearly show that TiO2 NPs are present on the surface of the graphene sheet. The use of IL2 produces a high crystalline TiO2 NPs with pure anatase phase without need of thermal treatment (TT). Hydrogen generation by UV light on TiO2 NPs or TiO2 –RGO composite prepared in ILs without thermal treatment (TT) show lower rate than TiO2 P25. After TT at 400 ◦ C a higher H2 production rate (up to 0.76 mmol h−1 g−1 ) due to the increase in crystallinity was measured. The photocatalytic activity was further enhanced for TiO2 –RGO composite. The electron-accepting and electron-transporting properties of graphene in the composite could indeed suppress the charge recombination and improve their photocatalytic activities. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Among the various nanostructured transition metal oxides, TiO2 is one of the most commonly used oxide semiconductor material mainly due to its low cost, non-toxic nature, strong oxidizing power and high resistance to chemical or photo-induced corrosion. It has been widely used in photocatalysis, photovoltaics, solar energy conversion, sensors, textiles, paints, cosmetics, etc. [1–3]. The chemical–physics properties of TiO2 can be tuned by its particle size, morphology and crystalline phase. From this observation, synthesis of nanostructured TiO2 with ultrafine crystallite sizes and high surface area, unusual optical, electrical and catalytic properties has been widely studied [4–7]. TiO2 exists mainly in four different polymorphs and it is known that, in most of the cases, anatase phase with particle size of 10–20 nm is a thermodynamically stable
∗ Corresponding author. Tel.: +55 51 33086321; fax: +55 51 33087304. E-mail addresses: [email protected], [email protected] (J. Dupont). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.06.010
modification of TiO2 , when considering the contribution of surface energy [8,9]. Many methods are available to prepare TiO2 nanoparticles (NPs), such as sol–gel, hydrothermal, solvothermal, sonochemical, chemical/physical vapor deposition, electrodeposition [10]. In this respect, the use of ionic liquids (ILs) [11] in particular those based on the imidazolium cation [12] has proven to be an interesting alternative for the preparation of TiO2 nanomaterials [13,14]. Indeed, the use o IL as entropic driven agent [15] may allow the size, shape and composition control of the prepared TiO2 in view of the IL surfactant-like nature, which inhibits the aggregation of the resultant NPs [16]. The term ‘ionothermal’ has been used to describe reactions that are conducted in ILs at high temperature with ambient pressures. As a result, ionothermal reactions avoid high pressure of hydrothermal or solvothermal reactions and eliminate safety problems related to high pressure [13]. ILs are prone to form self-assembly templates, rather than ordered micelle structures in the process of chemical synthesis and the low vapor pressure of IL is helpful to improve the surface area of the NPs. Thus various TiO2 nanomaterials have been prepared in
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non-functionalized ILs such as TiO2 nanocrystals [3], hollow TiO2 microspheres [17], macroporous TiO2 architectures [18] and mesoporous TiO2 [19]. Nowadays increasing attention has been paid to the photocatalytic decomposition of water into H2 and O2 using semiconductor oxide materials, since the first discover of photochemical water splitting over TiO2 photoelectrode by Fujishima et al. [20–22]. Indeed, TiO2 is one of the most widely used photocatalysts in photocatalytic water splitting. However, the application of TiO2 toward H2 generation is still having some limitations. The rapid recombination rate of photogenerated electron–hole pairs within TiO2 results in its low efficiency of H2 generation. Therefore, the suppression of recombination of charge carriers is one of the key parameters for the enhancement of photocatalytic activity in TiO2 nanomaterials. In this scene multi-walled carbon nanotube–TiO2 composites have been studied extensively and have proved to be very effective in improving the photocatalytic activity of TiO2 [23]. Composites of TiO2 with carbon based materials photocatalysts could potentially offer desirable efficiency for separating electron–hole pairs. Compared with carbon nanotube, graphene has many advantages and good interfacial contact with adsorbates. Therefore, it is desirable to explore simple and effective approaches for synthesizing graphene based composites and expand their applications. Graphene, a 2-D monolayer of fused sp2 carbon bonds in a honeycomb-like network, has attracted a great deal of scientific interest due to its outstanding mechanical, electrical, thermal, and optical properties and theoretically high surface area of ∼2600 m2 g−1 . Graphene-based materials have been widely used as transparent conducting electrodes, super-capacitors, optoelectronic devices, composites and catalysts [23]. However, only a few reports have been published related to the synthesis and photocatalytic properties of TiO2 –graphene composites [24–31]. A recent survey on the preparation of graphene-based TiO2 photocatalysts shows that the degradation of prototypes dyes is the main application used to test the photocatalytic properties of those new photocatalysts [32]. The obtained results of graphenebased TiO2 composites on photocatalytic degradation of pollutants encouraged its application also in other fields, such as hydrogen production from water splitting. For example, a recent study shows that the TiO2 –RGO (reduced graphene oxide) composite prepared by hydrothermal method presents good H2 generation compared to other methods [23]. A more recent report shows that using a simple hydrothermal process a nanocomposite of N-doped TiO2 with graphene oxide can be prepared and tested as a photocatalyst for hydrogen evolution from methanol aqueous solutions [33]. The hydrogen production rate reached 716.0 mol h−1 g−1 under UV light irradiation which was about 9.2 times higher than P25 photocatalyst. The most widely used method to prepare RGO is chemical reduction using different reducing agents, such us hydrazine, sodium borohydride, hydroquinone, or strong alkaline solutions. In general, these preparation methods require complex chemical process with long reaction times and hazardous reagents. For these reason, Shen et al. have recently reported the preparation of graphene sheets by a simple concept that precipitates hydroxides onto GO using a hydrothermal method with ILs avoiding the difficulties of preparing oxide/RGO nanocomposites [24]. The graphene sheets acts as an electron transfer channel for reducing the recombination of the photogenerated electron–holes that leads to improved efficiency of the photocatalytic hydrogen production. The rate of hydrogen production using TiO2 –RGO (prepared in ionic liquid [BMIM][PF6]) under high UV intensity irradiation in the presence of Na2 S + Na2 SO3 solution exhibited good photocatalytic activity. Additionally, it was showed by the same authors that the use of ILs produced photocatalysts with higher efficiency for hydrogen generation than traditional solvents. The above results demonstrate
that ILs can be used as a good solvent for the synthesis of TiO2 –RGO composites with better morphology and enhanced properties. We present here the synthesis of TiO2 NPs and TiO2 –RGO composites NPs using functionalized ILs as a reaction media together with the study of the effect of variation of experimental parameter on the structure and morphology of the products. A main difference with respect to other reported works is that ILs used in the synthesis method act as a reducing agent of the EGO to obtain RGO by ionothermal treatment. We have also examined the photocatalytic activity of TiO2 NPs and TiO2 –RGO composites NPs for the water splitting reaction to produce hydrogen in the presence of ethanol as a sacrificial agent. 2. Experimental 2.1. General consideration The ionic liquids (ILs), 1-(2-methoxyethyl)-3-methylimidazolium methanesulfonate (IL1) and 1-(2-methoxyethyl)-3methylimidazolium tetrafluoroborate (IL2), have been prepared using the same method published earlier [34] and their purity was checked by NMR [35]. All reactions involving TiCl4 have been performed under argon in a dry-box. X-ray powder diffraction (XRD) patterns were obtained with an X-ray diffractometer D5000 Siemens-Bruker-AXS operating at 40 kV, 25 mA, using Cu-K␣ radiation. The absorption spectra of the samples were measured on a Perkin Elmer Lambda–750 UV–vis spectrometer. Renishaw Invia Raman system equipped with a Leica microscope and an air cooled CCD detector. The laser excitation was the 632.8 nm line of a HeNe laser from Reninshaw. The spectra were collected in the backscattering configuration using a 50× objective. BET surface and the N2 adsorption–desorption measurements of the samples were measured using Tristar II, Micrometrics surface area and porosity instrument. The morphology of the product was examined by JEOL JEM 1200Ex Transmission electron microscopy (TEM) operating at 100 kV (CME-UFRGS, Brazil) and HRTEM was performed on an FEI Tecnai20 equipment at 200 kV (CETENE, Brazil). 2.2. Preparation of TiO2 and TiO2 –RGO composite One milliliter of titanium tetrachloride was added to 5 mL of IL in a Teflon tube under stirring. After homogenization of the mixture, 1 mL of distilled water was added slowly to the above solution. Hydrolysis of TiCl4 occurs immediately, as indicated by the effervescence of HCl vapor. This solution was subjected to ionothermal treatment at 100 and 120 ◦ C for 1 day. When the reaction was complete, autoclave was cooled to room temperature naturally. The obtained dispersion was mixed with acetonitrile and stirred overnight to remove IL. Finally the TiO2 NPs were separated by centrifugation. The final product was dried under vacuum for further characterization. The TiO2 –RGO composite was prepared by adding known quantity of exfoliated graphene oxide (EGO-22 mg) to the IL and subjected it to the same experimental conditions. Here IL acts as a reducing agent of the EGO to obtain RGO (reduced graphene oxide) by ionothermal treatment. 2.3. Preparation of exfoliated graphene oxide (EGO) Graphene oxide (GO) was synthesized from exfoliated graphite by Ramesh et al. [36] using modified Hummers method. Briefly, the process involved treatment of cleaned graphite flakes of certain size (300 m) with sulphuric acid/nitric acid mixture to obtain intercalated graphite. Then the intercalated graphite was submitted to a thermal shock at 800 ◦ C for a minute to exfoliate the dense graphite
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layer forming a low density exfoliated graphite which was further oxidized using KMnO4 and H2 O2 to obtain the brown colored exfoliated graphene oxide (EGO) colloid. 2.4. Photocatalytic H2 generation Photocatalytic H2 production reaction was carried out in a double wall quartz reactor. TiO2 NPs (4 mg) were sonicated for 20 min to disperse them in 6 mL aqueous solution. After sonication, 2 mL ethanol was added to the solution as a sacrificial reagent. Prior to irradiation, the system was deaerated by bubbling argon for about 10–15 min to reduce the oxygen content. A 240 W high pressure Hg–Xe arc lamp (Cermax) was used as excitation source. Photocatalytic activities of the TiO2 NPs under all UV and visible light excitation were evaluated by measuring hydrogen production in water/ethanol solutions by gas chromatography at room temperature. Analyses were conducted on an Agilent 6820 GC Chromatograph equipped with a thermal conductivity detector and a 5 A˚ molecular sieve packed column with argon as the carrier gas. Using a gastight syringe with a maximum volume of 50 L, the amount of hydrogen produced was measured at 0.5 h intervals. During the entire experiment, the reaction temperature was kept at 25 ◦ C by eliminating the IR radiation emitted from the lamp with the circulation of water in the water jacket of the reactor. 3. Results and discussion 3.1. Preparation and characterization of TiO2 and TiO2 –RGO nanomaterials The TiO2 nanomaterials were prepared by water hydrolysis of TiCl4 dissolved in 1-(2-methoxyethyl)-3-methylimidazolium methanesulfonate (IL1) and 1-(2-methoxyethyl)-3-methylimidazolium tetrafluoroborate (IL2) at room temperature followed by ionothermal treatment at 100 and 120 ◦ C for 1 day. The TiO2 –RGO composite was prepared by adding known quantity of exfoliated graphene oxide (EGO-22 mg) to the IL and subjected it to the same experimental conditions used above. Here IL acts also as a reducing agent for the reduction of EGO to reduced graphene oxide (RGO) during ionothermal treatment. 3.2. X-ray diffraction The XRD patterns of TiO2 , TiO2 –RGO composite, EGO and RGO samples are shown in Fig. 1. All the diffraction peaks of the product can be indexed to anatase phase of TiO2 (JCPDS 21-1272). In the composite, the absence of the characteristic peak of GO at 2 = 10.9 indicates the reduction of GO to RGO. The (0 0 2) peak of graphene (at 2 ≈ 25) is overlapped with the (0 1 0) peak of anatase TiO2 , which originates from the stacked graphene sheets in TiO2 –RGO composite. The average size calculated from the most intense diffraction peak (010) of Fig. 1(a) using the Debye–Scherrer equation was 8 nm, which matches with the TEM observation. Pure anatase phase was obtained at 100 ◦ C of thermal treatment for 24 h, and rutile phase appeared with further increase in temperature to 120 ◦ C. Interestingly, when methane sulfonate anion in IL1 is replaced by BF4 − (IL2), high crystallinity of TiO2 with pure anatase phase is obtained. Fig. 2a shows the XRD pattern of as prepared EGO which corresponds to the interlayer spacing of 8.2 A˚ (2 = 10.7◦ ). The strong reflection peak that is located at around 10.7◦ of EGO disappeared in the 5 wt% composite, which means that the GO has been completely reduced. Ionothermal treatment of pure EGO in IL1 shows the peak at 2 = 25.8◦ indicating the reduction of EGO to RGO (Fig. 2b), which can be indexed to the disordered stacking of (0 0 2) layer of RGO.
Fig. 1. XRD pattern of (a) TiO2 prepared at 100 ◦ C using IL1, (b) TiO2 prepared at 120 ◦ C using IL1, (c) TiO2 –RGO composite prepared at 120 ◦ C using IL1, (d) TiO2 prepared at 120 ◦ C using IL2, (e) TiO2 –RGO composite prepared 120 ◦ C using IL2.
Fig. 2c also shows a typical result that ethanol can reduce exfoliated graphene oxide to reduced graphene oxide (Graphene) via solvothermal method [37]. Fig. 3 shows the effect of temperature on the crystalline phase of the TiO2 . When the temperature increases an increase in the rutile phase concentration is obtained. Further we have carried out the experiment by halving or doubling the amount of water added for hydrolysis of TiCl4 to know the effect on the crystalline phases of TiO2 . Pure anatase TiO2 phase can be obtained at 100 ◦ C using 0.5 ml water for 1 day of ionothermal treatment (see Fig. S1 of ESI). This indicates that controlled hydrolysis of TiCl4 gives only anatase TiO2 . To know the effect of calcination on the crystalline phases of TiO2 , we have calcined the sample prepared at 100 ◦ C at 400 and 800 ◦ C for 3 h. The increase in calcinations temperature produces the transformation of anatase to rutile phase as it would be expected (see Fig. S2 of ESI). 3.3. UV–vis spectra of TiO2 To evaluate the optical properties of the EGO, RGO, TiO2 and TiO2 –RGO composite, UV–vis spectra were recorded from 200 to 800 nm as shown in Fig. 4. The reduction of GO sheets into RGO with ionothermal treatment is evidenced by the red shift of the
Fig. 2. XRD pattern of (a) EGO and (b) RGO prepared at 120 ◦ C for 1 day using IL1 and (c) RGO prepared at 120 ◦ C for 1 day using ethanol.
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Fig. 3. XRD pattern of TiO2 prepared at (a) 100 ◦ C, (b) 120 ◦ C and (c) 140 ◦ C using IL1.
absorption peak from 206 nm to 218 nm in the UV–vis spectrum of Fig. 4A. The UV–vis absorption spectrum shows that the absorption peak at 206 nm of GO corresponding to → * transitions of aromatic C C bonds red shifts to 218 nm after hydrothermal reduction treatment at 120 ◦ C for 1 day using IL1. The absorption in the whole spectral region increases which is indicative of the restoration of the -conjugation network within the RGO nanosheets [38]. UV–vis spectra of the different TiO2 samples are shown in Fig. 4B. It shows a high intense maximum absorbance band at 212 nm and broad weak absorption band 244 nm. The absorption between 212 nm is assigned to the imidazolium cation, which dominates the spectrum, even after extensive washing [39]. The second, much weaker absorption maximum can be assigned to the TiO2 nanoparticles. The peak at 212 nm, may be due to the presence of imidazolium moiety, is confirmed by conducting the experiment in absence of IL that shows the disappearance of the highest intense peak. This clearly indicates that the peak at 212 nm is due to imidazolium ring. UV–vis spectra of TiO2 nanoparticles and TiO2 –RGO composite nanoparticles were blue-shifted compared to that of bulk TiO2 (P-25, 336 nm). This absorption is due to the transition from the O2− anti-bonding orbital to the lowest empty orbital of Ti4+ . The origin of the blue shift in the absorption band of both TiO2
Fig. 5. Raman spectra of (a) TiO2 prepared at 120 ◦ C using IL1, (b) TiO2 –RGO composite prepared at 120 ◦ C using IL1, (c) EGO and (d) RGO prepared at 120 ◦ C using IL1.
and TiO2 –graphene composite can be attributed to quantum size and confinement effects that it was already observed in a study of morphological transformation during the hydrothermal soft chemical transformation, in neutral solution, of titanate nanostructures into their anatase titania counterparts [40]. The enhanced absorption of the TiO2 –RGO composites in the whole visible region can be attributed to the presence of graphene. 3.4. Raman spectra Raman spectra of TiO2 and TiO2 –RGO composites, EGO and RGO prepared using IL1 at 120 ◦ C for 1 day are shown in Fig. 5. A well resolved TiO2 Raman peak is clearly seen at 147 cm−1 , which is attributed to the main Eg anatase vibration mode. Furthermore, vibration peaks at 444 cm−1 (B1g ), 517 cm−1 (A1g ), and 610 cm−1 (Eg) are also characteristic of anatase TiO2 , Fig. 5a. Additionally, composite shows two peaks at about 1337 cm−1 (D band) and 1598 cm−1 (G band) as observed in the spectra, which can be attributed to the graphene substrate, Fig. 5b. Comparing with GO (Fig. 5c), the TiO2 –RGO composites display an increased ID /IG intensity ratio, suggesting a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO [29]. 3.5. Thermogravimetric analysis and Surface area determination
Fig. 4. UV–vis spectra of (A): (1) EGO and (2) RGO; (B): (1) Commercial TiO2 P 25, (2) TiO2 prepared at 120 ◦ C using IL1, (3) TiO2 –RGO composite prepared at 120 ◦ C using IL1 and (4) TiO2 –RGO composite prepared in absence of IL.
From the DTA curve (see Fig. S3 in ESI), it can be seen that an endothermic peak below 100 ◦ C is due to the desorption of water and alcohol, whereas the first low intense exothermic peak at 332 ◦ C, and the very sharp and narrow second exothermic peak at 401 ◦ C are attributed to the thermal decomposition of the organic substance as the evidence of weight loss ended at 400 ◦ C. Since the TG curve shows that weight loss ended at 400 ◦ C and total weight loss until this temperature was about 35 wt%, 400 ◦ C was sufficient for complete removal of the organic substance from TiO2 nanoparticles. The N2 adsorption–desorption isotherms along with the Barret–Joyner–Halenda pore size distribution plot of the TiO2 and TiO2 –RGO exhibit typical IUPAC type IV pattern with the presence of hysteresis loop as exemplified in Fig. S4a of ESI. The surface area, determined from Brunauer–Emmett–Teller (BET) analysis, was found to be 170 and 161 m2 g−1 for TiO2 and TiO2 –RGO respectively. The surface area of all the samples prepared by this method was found to be 160–170 m2 g−1 . Based on the Barrett–Joyner–Halenda
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Fig. 6. TEM images of (A) and (B) TiO2 nanoparticles (C) and (D) TiO2 –RGO composite and (E) and (F) HRTEM images of TiO2 –RGO composite prepared at 120 ◦ C for 1 day using IL 1.
(BJH) equation, the main pore size in TiO2 and TiO2 –RGO are 3.3 nm and 1.5 nm respectively (see Fig. S4b in ESI). 3.6. Transmission electron microscopy TiO2 NPs prepared at 100 ◦ C using IL1 showed to be monodispersed with a mono-modal distribution of a mean diameter of 4.9 ± 1.7 nm (see TEM/HRTEM images and NPs histogram in Figs. S5 and S6A, respectively of ESI). Furthermore, high magnification and high-resolution TEM images of TiO2 NPs showed the planar defects and interplanar distances. Fig. 6A and B shows the TEM images of TiO2 NPs prepared at 120 ◦ C using IL 1. As the ionothermal temperature increases by 20 ◦ C, the shape of the particles also changes. Fig. 6C and D shows the TEM images of the TiO2 –RGO composite. It clearly shows that TiO2 NPs are embedded on the surface of graphene sheet.
The obtained composite retained the two-dimensional sheet structure with micrometers-long after the ionothermal reduction. It was already shown that hydrothermal/ solvothermal/ionothermal treatments of metal oxide precursors with graphene oxide sheets lead to the formation of metal oxide graphene composites. For example, Fu et al. have synthesized ZnFe2 O4 –graphene composite photocatalyst by a one-step hydrothermal method in ethanol-aqueous solution [41]. During the hydrothermal reaction process, graphene oxide was reduced to graphene, and simultaneously ZnFe2 O4 nanoparticles were formed on the surface of graphene sheets. In a similar way, here also TiO2 nanoparticles are uniformly dispersed on the sheet of graphene (Fig. 6C and D) with a mono-modal distribution of a mean diameter of 4.1 ± 1.1 nm (see NPs histogram in Fig. S6B of ESI). HRETM images of TiO2 –RGO composites (Fig. 6E and F) shows that particles are uniformly distributed on RGO sheet.
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Fig. 7. TEM images of (A) and (B) TiO2 and (C) and (D) TiO2 –RGO composite prepared at 120 ◦ C using IL2.
Interesting when the anion of the ILs was changed from methanesulfonate to tetrafluoroborate, TiO2 NPs were grouped in a chain-like structure as it is shown in the TEM images of Fig. 7. This shape difference is much likely related to the IL template effect promoted by BF4 − . The IL aggregates induces the formation of the thermodynamic product through the oriented attachment of the nanostructures. Indeed, it has been already demonstrated that the BF4 anionic aggregates induces the formation worm-like structures in opposition to other anions where spherical-like shapes are formed preferentially as the thermodynamic nanostructures [42,43]. Fig. 7 shows TiO2 and TiO2 –RGO composite NPs prepared at 120 ◦ C using IL2. The composite (Fig. 7A and B) shows that TiO2 NPs are embedded on the graphene sheet similar to the observation of Fig. 6C and D.
TiO2 and TiO2 –RGO prepared using IL1 at 100 and 120 ◦ C is shown in Fig. 8. When the UV illumination was turned off the evolution of H2 stopped indicating that the gas evolution was induced by the UV irradiation as in a typical photocatalytic reaction. Fig. 8 shows the H2 generation of as-prepared and calcined TiO2 nanoparticles using IL1. It was found that as-prepared samples showed much lower H2 generation than the calcinated samples
3.7. Hydrogen generation The prepared TiO2 and TiO2 –RGO photocatalysts were tested for their ability to photochemically produce hydrogen in a mixture of water and ethanol. Water-splitting reactions using metal oxide semiconductors are usually carried out in aqueous solutions with the addition of easily oxidizable reducing agents. The photogenerated holes irreversibly oxidize those agents, such as ethanol, instead of water [44]. This effect increases the electron concentration in the photocatalysts, and the hydrogen evolution reaction is enhanced. Reactions using sacrificial agents are regarded as half reactions and are often employed in tests of photocatalytic hydrogen or oxygen evolution. The rate of hydrogen production from UV photolysis of water/ethanol solutions in the presence of
Fig. 8. Photocatalytic generation of hydrogen by UV irradiation from water/ethanol solutions on commercially TiO2 P25, TiO2 as prepared at 100 ◦ C and 120 ◦ C using IL1, and after thermal treatment (TT) at 400 ◦ C for 3 h.
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Fig. 9. Hydrogen generation by UV irradiation from water/ethanol solutions on TiO2 prepared at 120 ◦ C and calcined to 400 ◦ C using IL1 and IL2.
at 400 ◦ C. In the case of the as-prepared samples the hydrogen production was even lower than the commercial P25 sample. Probably the presence of IL on the surface and of MP’s and the low degree of crystallinity can explain the lower hydrogen rate. As the samples were thermal treated at 400 ◦ C for 3 h it was observed an increase in the H2 generation rate (see Fig. 8). The catalytic performance is higher for the product prepared at 120 ◦ C rather than 100 ◦ C probably due to a presence of rutile phase (see Fig. 1) which has a lower band gap allowing absorption of a larger number of photons. We have selected this condition (120 ◦ C) as standard for further investigation of the photocatalytic properties to know the effect of experimental parameters on the H2 generation. Fig. 9 shows the effect of the anion on the hydrogen generation rate. When CH3 SO3 − is replaced by BF4 − , further increase in H2 generation takes place. When the TiO2 was prepared using IL2 and calcined at 800 ◦ C the hydrogen photocatalytic activity strongly decreased compared to that calcined at 400 ◦ C (see Fig. S7 in ESI). The decrease in photocatalytic activity was considered due to the presence of rutile phase which has lower catalytic activity than anatase phase [45] and also may due to a lower surface area. Interestingly, TiO2 –RGO composite prepared in all experimental conditions using any IL enhanced the H2 generation to a greater extent when compared to the bare TiO2 NPs as shown in Fig. 10. The reason for the increase in the photocatalytic activity for TiO2 –RGO composite is as follows. In the composite, graphene serves as an acceptor of the photo-generated electrons from titania and effectively suppresses the charge recombination, leaving more photo-generated holes to form reactive species and facilitate the photocatalytic activity. The hetero junction formed at the interface then separates the photo-excited electron–hole pairs, and thus hinders the charge recombination [24,46]. It has also been demonstrated that the photo-excited electrons in the anatase TiO2 conduction band can be effectively transferred to electronic states of graphene because the minimum of conduction band of anatase TiO2 is higher than the Fermi level of grapheme [47]. Literature survey shows that among all the carbon based materials-TiO2 composite, in situ synthesis of TiO2 –RGO from hydrothermal method shows better H2 generation than other methods [23]. Du et al. studied the electrochemical impedance spectra of macro-mesoporous titania films with and without graphene under UV light the illumination [46]. They observed that the semicircle of medium frequencies in the plot became shorter after the introduction of graphene, which indicates a decrease in the solid-state interface layer resistance and
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Fig. 10. Photocatalytic generation of hydrogen by UV irradiation from water/ethanol solutions on TiO2 or TiO2 –RGO composite prepared using IL1 and TiO2 or TiO2 –RGO composite prepared using IL2.
the charge transfer resistance on the surface. Therefore, the electron-accepting and electron-transporting properties of graphene in the composite could indeed suppress the charge recombination and improve their photocatalytic activities. 4. Conclusions We have successfully synthesized TiO2 nanoparticles and TiO2 –RGO composite by ionothermal method at 100 and 120 ◦ C using two different ILs. XRD pattern shows that the obtained product is composed of anatase and rutile TiO2 and by controlling the hydrolysis of TiCl4 at low temperature, pure anatase TiO2 can be obtained. BET surface area analysis of the composites showed surface areas of 170 and 161 m2 g−1 for TiO2 and TiO2 –RGO composite respectively with a BJH pore size distribution plot typical from IUPAC type IV pattern. TEM images show that the sizes of the NP’s are around 4 nm, and that they are distributed on the surface of the graphene sheet. Hydrogen generation by UV light on TiO2 NPs or TiO2 –RGO composite as prepared in ILs shows lower rate than TiO2 P25 due to the presence of IL. Thermal treatment at 400 ◦ C shows much better H2 production. The photocatalytic activity was further enhanced for TiO2 –RGO composite. The electron-accepting and electron-transporting properties of graphene in the composite could indeed suppress the charge recombination and improve their photocatalytic activities. Acknowledgements Thanks are due to CNPq, CAPES, FAPERGS and INCT-Catal for funding. Dr. Ganganagappa Nagaraju thanks to CNPq-TWAS for financial help to carry-out the research work and also UFRGS, Porto Alegre, for providing all facilities. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata. 2013.06.010. References [1] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [2] C. Wessel, L.A. Zhao, S. Urban, R. Ostermann, I. Djerdj, B.M. Smarsly, L.Q. Chen, Y.S. Hu, S. Sallard, Chem-Eur. J. 17 (2011) 775–779.
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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata
Controlled growth of TiO2 and TiO2 –RGO composite nanoparticles in ionic liquids for enhanced photocatalytic H2 generation Ganganagappa Nagaraju a,b , Gunter Ebeling a , Renato V. Gonc¸alves c , Sergio R. Teixeira c , Daniel E. Weibel d , Jairton Dupont a,∗ a Laboratory of Molecular Chemistry, Institute of Chemistry, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonc¸alves 9500, 91501-970 Porto Alegre, RS, Brazil b Center for Nano and Materials Science, Jain University, Jakkasandra, Bangalore Rural 562112, India c Laboratory of Thin Films and Nanostructure Fabrication (L3Fnano), Institute of Physics, Universidade Federal do Rio Grande do Sul, UFRGS, Avenida Bento Gonc¸alves 9500, P.O. Box 15051, 91501-970 Porto Alegre, RS, Brazil d Laboratory of Photochemistry and Surfaces (LAFOS), Institute of Chemistry, Universidade Federal do Rio Grande do Sul, UFRGS, Avenida Bento Gonc¸alves 9500, P.O. Box 15003, 91501-970 Porto Alegre, RS, Brazil
a r t i c l e
i n f o
Article history: Received 11 January 2013 Received in revised form 6 June 2013 Accepted 17 June 2013 Available online xxx Keywords: TiO2 –RGO Nanoparticles Ionic liquids Hydrogen generation
a b s t r a c t TiO2 nanoparticles and TiO2 –RGO (reduced graphene oxide) composite were synthesized by ionothermal method at 100 and 120 ◦ C for 24 h using two different ionic liquids (ILs), 1-(2-methoxyethyl)-3methylimidazolium methanesulfonate (IL1) and 1-(2-methoxyethyl)-3-methylimidazolium tetrafluoroborate (IL2). BET surface area analysis of the composites showed surface areas of 170 and 161 m2 g−1 for TiO2 and TiO2 –RGO composite respectively. TEM images show that the sizes of the particles are around 4 nm and in the composite clearly show that TiO2 NPs are present on the surface of the graphene sheet. The use of IL2 produces a high crystalline TiO2 NPs with pure anatase phase without need of thermal treatment (TT). Hydrogen generation by UV light on TiO2 NPs or TiO2 –RGO composite prepared in ILs without thermal treatment (TT) show lower rate than TiO2 P25. After TT at 400 ◦ C a higher H2 production rate (up to 0.76 mmol h−1 g−1 ) due to the increase in crystallinity was measured. The photocatalytic activity was further enhanced for TiO2 –RGO composite. The electron-accepting and electron-transporting properties of graphene in the composite could indeed suppress the charge recombination and improve their photocatalytic activities. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Among the various nanostructured transition metal oxides, TiO2 is one of the most commonly used oxide semiconductor material mainly due to its low cost, non-toxic nature, strong oxidizing power and high resistance to chemical or photo-induced corrosion. It has been widely used in photocatalysis, photovoltaics, solar energy conversion, sensors, textiles, paints, cosmetics, etc. [1–3]. The chemical–physics properties of TiO2 can be tuned by its particle size, morphology and crystalline phase. From this observation, synthesis of nanostructured TiO2 with ultrafine crystallite sizes and high surface area, unusual optical, electrical and catalytic properties has been widely studied [4–7]. TiO2 exists mainly in four different polymorphs and it is known that, in most of the cases, anatase phase with particle size of 10–20 nm is a thermodynamically stable
∗ Corresponding author. Tel.: +55 51 33086321; fax: +55 51 33087304. E-mail addresses: [email protected], [email protected] (J. Dupont). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.06.010
modification of TiO2 , when considering the contribution of surface energy [8,9]. Many methods are available to prepare TiO2 nanoparticles (NPs), such as sol–gel, hydrothermal, solvothermal, sonochemical, chemical/physical vapor deposition, electrodeposition [10]. In this respect, the use of ionic liquids (ILs) [11] in particular those based on the imidazolium cation [12] has proven to be an interesting alternative for the preparation of TiO2 nanomaterials [13,14]. Indeed, the use o IL as entropic driven agent [15] may allow the size, shape and composition control of the prepared TiO2 in view of the IL surfactant-like nature, which inhibits the aggregation of the resultant NPs [16]. The term ‘ionothermal’ has been used to describe reactions that are conducted in ILs at high temperature with ambient pressures. As a result, ionothermal reactions avoid high pressure of hydrothermal or solvothermal reactions and eliminate safety problems related to high pressure [13]. ILs are prone to form self-assembly templates, rather than ordered micelle structures in the process of chemical synthesis and the low vapor pressure of IL is helpful to improve the surface area of the NPs. Thus various TiO2 nanomaterials have been prepared in
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non-functionalized ILs such as TiO2 nanocrystals [3], hollow TiO2 microspheres [17], macroporous TiO2 architectures [18] and mesoporous TiO2 [19]. Nowadays increasing attention has been paid to the photocatalytic decomposition of water into H2 and O2 using semiconductor oxide materials, since the first discover of photochemical water splitting over TiO2 photoelectrode by Fujishima et al. [20–22]. Indeed, TiO2 is one of the most widely used photocatalysts in photocatalytic water splitting. However, the application of TiO2 toward H2 generation is still having some limitations. The rapid recombination rate of photogenerated electron–hole pairs within TiO2 results in its low efficiency of H2 generation. Therefore, the suppression of recombination of charge carriers is one of the key parameters for the enhancement of photocatalytic activity in TiO2 nanomaterials. In this scene multi-walled carbon nanotube–TiO2 composites have been studied extensively and have proved to be very effective in improving the photocatalytic activity of TiO2 [23]. Composites of TiO2 with carbon based materials photocatalysts could potentially offer desirable efficiency for separating electron–hole pairs. Compared with carbon nanotube, graphene has many advantages and good interfacial contact with adsorbates. Therefore, it is desirable to explore simple and effective approaches for synthesizing graphene based composites and expand their applications. Graphene, a 2-D monolayer of fused sp2 carbon bonds in a honeycomb-like network, has attracted a great deal of scientific interest due to its outstanding mechanical, electrical, thermal, and optical properties and theoretically high surface area of ∼2600 m2 g−1 . Graphene-based materials have been widely used as transparent conducting electrodes, super-capacitors, optoelectronic devices, composites and catalysts [23]. However, only a few reports have been published related to the synthesis and photocatalytic properties of TiO2 –graphene composites [24–31]. A recent survey on the preparation of graphene-based TiO2 photocatalysts shows that the degradation of prototypes dyes is the main application used to test the photocatalytic properties of those new photocatalysts [32]. The obtained results of graphenebased TiO2 composites on photocatalytic degradation of pollutants encouraged its application also in other fields, such as hydrogen production from water splitting. For example, a recent study shows that the TiO2 –RGO (reduced graphene oxide) composite prepared by hydrothermal method presents good H2 generation compared to other methods [23]. A more recent report shows that using a simple hydrothermal process a nanocomposite of N-doped TiO2 with graphene oxide can be prepared and tested as a photocatalyst for hydrogen evolution from methanol aqueous solutions [33]. The hydrogen production rate reached 716.0 mol h−1 g−1 under UV light irradiation which was about 9.2 times higher than P25 photocatalyst. The most widely used method to prepare RGO is chemical reduction using different reducing agents, such us hydrazine, sodium borohydride, hydroquinone, or strong alkaline solutions. In general, these preparation methods require complex chemical process with long reaction times and hazardous reagents. For these reason, Shen et al. have recently reported the preparation of graphene sheets by a simple concept that precipitates hydroxides onto GO using a hydrothermal method with ILs avoiding the difficulties of preparing oxide/RGO nanocomposites [24]. The graphene sheets acts as an electron transfer channel for reducing the recombination of the photogenerated electron–holes that leads to improved efficiency of the photocatalytic hydrogen production. The rate of hydrogen production using TiO2 –RGO (prepared in ionic liquid [BMIM][PF6]) under high UV intensity irradiation in the presence of Na2 S + Na2 SO3 solution exhibited good photocatalytic activity. Additionally, it was showed by the same authors that the use of ILs produced photocatalysts with higher efficiency for hydrogen generation than traditional solvents. The above results demonstrate
that ILs can be used as a good solvent for the synthesis of TiO2 –RGO composites with better morphology and enhanced properties. We present here the synthesis of TiO2 NPs and TiO2 –RGO composites NPs using functionalized ILs as a reaction media together with the study of the effect of variation of experimental parameter on the structure and morphology of the products. A main difference with respect to other reported works is that ILs used in the synthesis method act as a reducing agent of the EGO to obtain RGO by ionothermal treatment. We have also examined the photocatalytic activity of TiO2 NPs and TiO2 –RGO composites NPs for the water splitting reaction to produce hydrogen in the presence of ethanol as a sacrificial agent. 2. Experimental 2.1. General consideration The ionic liquids (ILs), 1-(2-methoxyethyl)-3-methylimidazolium methanesulfonate (IL1) and 1-(2-methoxyethyl)-3methylimidazolium tetrafluoroborate (IL2), have been prepared using the same method published earlier [34] and their purity was checked by NMR [35]. All reactions involving TiCl4 have been performed under argon in a dry-box. X-ray powder diffraction (XRD) patterns were obtained with an X-ray diffractometer D5000 Siemens-Bruker-AXS operating at 40 kV, 25 mA, using Cu-K␣ radiation. The absorption spectra of the samples were measured on a Perkin Elmer Lambda–750 UV–vis spectrometer. Renishaw Invia Raman system equipped with a Leica microscope and an air cooled CCD detector. The laser excitation was the 632.8 nm line of a HeNe laser from Reninshaw. The spectra were collected in the backscattering configuration using a 50× objective. BET surface and the N2 adsorption–desorption measurements of the samples were measured using Tristar II, Micrometrics surface area and porosity instrument. The morphology of the product was examined by JEOL JEM 1200Ex Transmission electron microscopy (TEM) operating at 100 kV (CME-UFRGS, Brazil) and HRTEM was performed on an FEI Tecnai20 equipment at 200 kV (CETENE, Brazil). 2.2. Preparation of TiO2 and TiO2 –RGO composite One milliliter of titanium tetrachloride was added to 5 mL of IL in a Teflon tube under stirring. After homogenization of the mixture, 1 mL of distilled water was added slowly to the above solution. Hydrolysis of TiCl4 occurs immediately, as indicated by the effervescence of HCl vapor. This solution was subjected to ionothermal treatment at 100 and 120 ◦ C for 1 day. When the reaction was complete, autoclave was cooled to room temperature naturally. The obtained dispersion was mixed with acetonitrile and stirred overnight to remove IL. Finally the TiO2 NPs were separated by centrifugation. The final product was dried under vacuum for further characterization. The TiO2 –RGO composite was prepared by adding known quantity of exfoliated graphene oxide (EGO-22 mg) to the IL and subjected it to the same experimental conditions. Here IL acts as a reducing agent of the EGO to obtain RGO (reduced graphene oxide) by ionothermal treatment. 2.3. Preparation of exfoliated graphene oxide (EGO) Graphene oxide (GO) was synthesized from exfoliated graphite by Ramesh et al. [36] using modified Hummers method. Briefly, the process involved treatment of cleaned graphite flakes of certain size (300 m) with sulphuric acid/nitric acid mixture to obtain intercalated graphite. Then the intercalated graphite was submitted to a thermal shock at 800 ◦ C for a minute to exfoliate the dense graphite
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layer forming a low density exfoliated graphite which was further oxidized using KMnO4 and H2 O2 to obtain the brown colored exfoliated graphene oxide (EGO) colloid. 2.4. Photocatalytic H2 generation Photocatalytic H2 production reaction was carried out in a double wall quartz reactor. TiO2 NPs (4 mg) were sonicated for 20 min to disperse them in 6 mL aqueous solution. After sonication, 2 mL ethanol was added to the solution as a sacrificial reagent. Prior to irradiation, the system was deaerated by bubbling argon for about 10–15 min to reduce the oxygen content. A 240 W high pressure Hg–Xe arc lamp (Cermax) was used as excitation source. Photocatalytic activities of the TiO2 NPs under all UV and visible light excitation were evaluated by measuring hydrogen production in water/ethanol solutions by gas chromatography at room temperature. Analyses were conducted on an Agilent 6820 GC Chromatograph equipped with a thermal conductivity detector and a 5 A˚ molecular sieve packed column with argon as the carrier gas. Using a gastight syringe with a maximum volume of 50 L, the amount of hydrogen produced was measured at 0.5 h intervals. During the entire experiment, the reaction temperature was kept at 25 ◦ C by eliminating the IR radiation emitted from the lamp with the circulation of water in the water jacket of the reactor. 3. Results and discussion 3.1. Preparation and characterization of TiO2 and TiO2 –RGO nanomaterials The TiO2 nanomaterials were prepared by water hydrolysis of TiCl4 dissolved in 1-(2-methoxyethyl)-3-methylimidazolium methanesulfonate (IL1) and 1-(2-methoxyethyl)-3-methylimidazolium tetrafluoroborate (IL2) at room temperature followed by ionothermal treatment at 100 and 120 ◦ C for 1 day. The TiO2 –RGO composite was prepared by adding known quantity of exfoliated graphene oxide (EGO-22 mg) to the IL and subjected it to the same experimental conditions used above. Here IL acts also as a reducing agent for the reduction of EGO to reduced graphene oxide (RGO) during ionothermal treatment. 3.2. X-ray diffraction The XRD patterns of TiO2 , TiO2 –RGO composite, EGO and RGO samples are shown in Fig. 1. All the diffraction peaks of the product can be indexed to anatase phase of TiO2 (JCPDS 21-1272). In the composite, the absence of the characteristic peak of GO at 2 = 10.9 indicates the reduction of GO to RGO. The (0 0 2) peak of graphene (at 2 ≈ 25) is overlapped with the (0 1 0) peak of anatase TiO2 , which originates from the stacked graphene sheets in TiO2 –RGO composite. The average size calculated from the most intense diffraction peak (010) of Fig. 1(a) using the Debye–Scherrer equation was 8 nm, which matches with the TEM observation. Pure anatase phase was obtained at 100 ◦ C of thermal treatment for 24 h, and rutile phase appeared with further increase in temperature to 120 ◦ C. Interestingly, when methane sulfonate anion in IL1 is replaced by BF4 − (IL2), high crystallinity of TiO2 with pure anatase phase is obtained. Fig. 2a shows the XRD pattern of as prepared EGO which corresponds to the interlayer spacing of 8.2 A˚ (2 = 10.7◦ ). The strong reflection peak that is located at around 10.7◦ of EGO disappeared in the 5 wt% composite, which means that the GO has been completely reduced. Ionothermal treatment of pure EGO in IL1 shows the peak at 2 = 25.8◦ indicating the reduction of EGO to RGO (Fig. 2b), which can be indexed to the disordered stacking of (0 0 2) layer of RGO.
Fig. 1. XRD pattern of (a) TiO2 prepared at 100 ◦ C using IL1, (b) TiO2 prepared at 120 ◦ C using IL1, (c) TiO2 –RGO composite prepared at 120 ◦ C using IL1, (d) TiO2 prepared at 120 ◦ C using IL2, (e) TiO2 –RGO composite prepared 120 ◦ C using IL2.
Fig. 2c also shows a typical result that ethanol can reduce exfoliated graphene oxide to reduced graphene oxide (Graphene) via solvothermal method [37]. Fig. 3 shows the effect of temperature on the crystalline phase of the TiO2 . When the temperature increases an increase in the rutile phase concentration is obtained. Further we have carried out the experiment by halving or doubling the amount of water added for hydrolysis of TiCl4 to know the effect on the crystalline phases of TiO2 . Pure anatase TiO2 phase can be obtained at 100 ◦ C using 0.5 ml water for 1 day of ionothermal treatment (see Fig. S1 of ESI). This indicates that controlled hydrolysis of TiCl4 gives only anatase TiO2 . To know the effect of calcination on the crystalline phases of TiO2 , we have calcined the sample prepared at 100 ◦ C at 400 and 800 ◦ C for 3 h. The increase in calcinations temperature produces the transformation of anatase to rutile phase as it would be expected (see Fig. S2 of ESI). 3.3. UV–vis spectra of TiO2 To evaluate the optical properties of the EGO, RGO, TiO2 and TiO2 –RGO composite, UV–vis spectra were recorded from 200 to 800 nm as shown in Fig. 4. The reduction of GO sheets into RGO with ionothermal treatment is evidenced by the red shift of the
Fig. 2. XRD pattern of (a) EGO and (b) RGO prepared at 120 ◦ C for 1 day using IL1 and (c) RGO prepared at 120 ◦ C for 1 day using ethanol.
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Fig. 3. XRD pattern of TiO2 prepared at (a) 100 ◦ C, (b) 120 ◦ C and (c) 140 ◦ C using IL1.
absorption peak from 206 nm to 218 nm in the UV–vis spectrum of Fig. 4A. The UV–vis absorption spectrum shows that the absorption peak at 206 nm of GO corresponding to → * transitions of aromatic C C bonds red shifts to 218 nm after hydrothermal reduction treatment at 120 ◦ C for 1 day using IL1. The absorption in the whole spectral region increases which is indicative of the restoration of the -conjugation network within the RGO nanosheets [38]. UV–vis spectra of the different TiO2 samples are shown in Fig. 4B. It shows a high intense maximum absorbance band at 212 nm and broad weak absorption band 244 nm. The absorption between 212 nm is assigned to the imidazolium cation, which dominates the spectrum, even after extensive washing [39]. The second, much weaker absorption maximum can be assigned to the TiO2 nanoparticles. The peak at 212 nm, may be due to the presence of imidazolium moiety, is confirmed by conducting the experiment in absence of IL that shows the disappearance of the highest intense peak. This clearly indicates that the peak at 212 nm is due to imidazolium ring. UV–vis spectra of TiO2 nanoparticles and TiO2 –RGO composite nanoparticles were blue-shifted compared to that of bulk TiO2 (P-25, 336 nm). This absorption is due to the transition from the O2− anti-bonding orbital to the lowest empty orbital of Ti4+ . The origin of the blue shift in the absorption band of both TiO2
Fig. 5. Raman spectra of (a) TiO2 prepared at 120 ◦ C using IL1, (b) TiO2 –RGO composite prepared at 120 ◦ C using IL1, (c) EGO and (d) RGO prepared at 120 ◦ C using IL1.
and TiO2 –graphene composite can be attributed to quantum size and confinement effects that it was already observed in a study of morphological transformation during the hydrothermal soft chemical transformation, in neutral solution, of titanate nanostructures into their anatase titania counterparts [40]. The enhanced absorption of the TiO2 –RGO composites in the whole visible region can be attributed to the presence of graphene. 3.4. Raman spectra Raman spectra of TiO2 and TiO2 –RGO composites, EGO and RGO prepared using IL1 at 120 ◦ C for 1 day are shown in Fig. 5. A well resolved TiO2 Raman peak is clearly seen at 147 cm−1 , which is attributed to the main Eg anatase vibration mode. Furthermore, vibration peaks at 444 cm−1 (B1g ), 517 cm−1 (A1g ), and 610 cm−1 (Eg) are also characteristic of anatase TiO2 , Fig. 5a. Additionally, composite shows two peaks at about 1337 cm−1 (D band) and 1598 cm−1 (G band) as observed in the spectra, which can be attributed to the graphene substrate, Fig. 5b. Comparing with GO (Fig. 5c), the TiO2 –RGO composites display an increased ID /IG intensity ratio, suggesting a decrease in the average size of the sp2 domains upon reduction of the exfoliated GO [29]. 3.5. Thermogravimetric analysis and Surface area determination
Fig. 4. UV–vis spectra of (A): (1) EGO and (2) RGO; (B): (1) Commercial TiO2 P 25, (2) TiO2 prepared at 120 ◦ C using IL1, (3) TiO2 –RGO composite prepared at 120 ◦ C using IL1 and (4) TiO2 –RGO composite prepared in absence of IL.
From the DTA curve (see Fig. S3 in ESI), it can be seen that an endothermic peak below 100 ◦ C is due to the desorption of water and alcohol, whereas the first low intense exothermic peak at 332 ◦ C, and the very sharp and narrow second exothermic peak at 401 ◦ C are attributed to the thermal decomposition of the organic substance as the evidence of weight loss ended at 400 ◦ C. Since the TG curve shows that weight loss ended at 400 ◦ C and total weight loss until this temperature was about 35 wt%, 400 ◦ C was sufficient for complete removal of the organic substance from TiO2 nanoparticles. The N2 adsorption–desorption isotherms along with the Barret–Joyner–Halenda pore size distribution plot of the TiO2 and TiO2 –RGO exhibit typical IUPAC type IV pattern with the presence of hysteresis loop as exemplified in Fig. S4a of ESI. The surface area, determined from Brunauer–Emmett–Teller (BET) analysis, was found to be 170 and 161 m2 g−1 for TiO2 and TiO2 –RGO respectively. The surface area of all the samples prepared by this method was found to be 160–170 m2 g−1 . Based on the Barrett–Joyner–Halenda
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Fig. 6. TEM images of (A) and (B) TiO2 nanoparticles (C) and (D) TiO2 –RGO composite and (E) and (F) HRTEM images of TiO2 –RGO composite prepared at 120 ◦ C for 1 day using IL 1.
(BJH) equation, the main pore size in TiO2 and TiO2 –RGO are 3.3 nm and 1.5 nm respectively (see Fig. S4b in ESI). 3.6. Transmission electron microscopy TiO2 NPs prepared at 100 ◦ C using IL1 showed to be monodispersed with a mono-modal distribution of a mean diameter of 4.9 ± 1.7 nm (see TEM/HRTEM images and NPs histogram in Figs. S5 and S6A, respectively of ESI). Furthermore, high magnification and high-resolution TEM images of TiO2 NPs showed the planar defects and interplanar distances. Fig. 6A and B shows the TEM images of TiO2 NPs prepared at 120 ◦ C using IL 1. As the ionothermal temperature increases by 20 ◦ C, the shape of the particles also changes. Fig. 6C and D shows the TEM images of the TiO2 –RGO composite. It clearly shows that TiO2 NPs are embedded on the surface of graphene sheet.
The obtained composite retained the two-dimensional sheet structure with micrometers-long after the ionothermal reduction. It was already shown that hydrothermal/ solvothermal/ionothermal treatments of metal oxide precursors with graphene oxide sheets lead to the formation of metal oxide graphene composites. For example, Fu et al. have synthesized ZnFe2 O4 –graphene composite photocatalyst by a one-step hydrothermal method in ethanol-aqueous solution [41]. During the hydrothermal reaction process, graphene oxide was reduced to graphene, and simultaneously ZnFe2 O4 nanoparticles were formed on the surface of graphene sheets. In a similar way, here also TiO2 nanoparticles are uniformly dispersed on the sheet of graphene (Fig. 6C and D) with a mono-modal distribution of a mean diameter of 4.1 ± 1.1 nm (see NPs histogram in Fig. S6B of ESI). HRETM images of TiO2 –RGO composites (Fig. 6E and F) shows that particles are uniformly distributed on RGO sheet.
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Fig. 7. TEM images of (A) and (B) TiO2 and (C) and (D) TiO2 –RGO composite prepared at 120 ◦ C using IL2.
Interesting when the anion of the ILs was changed from methanesulfonate to tetrafluoroborate, TiO2 NPs were grouped in a chain-like structure as it is shown in the TEM images of Fig. 7. This shape difference is much likely related to the IL template effect promoted by BF4 − . The IL aggregates induces the formation of the thermodynamic product through the oriented attachment of the nanostructures. Indeed, it has been already demonstrated that the BF4 anionic aggregates induces the formation worm-like structures in opposition to other anions where spherical-like shapes are formed preferentially as the thermodynamic nanostructures [42,43]. Fig. 7 shows TiO2 and TiO2 –RGO composite NPs prepared at 120 ◦ C using IL2. The composite (Fig. 7A and B) shows that TiO2 NPs are embedded on the graphene sheet similar to the observation of Fig. 6C and D.
TiO2 and TiO2 –RGO prepared using IL1 at 100 and 120 ◦ C is shown in Fig. 8. When the UV illumination was turned off the evolution of H2 stopped indicating that the gas evolution was induced by the UV irradiation as in a typical photocatalytic reaction. Fig. 8 shows the H2 generation of as-prepared and calcined TiO2 nanoparticles using IL1. It was found that as-prepared samples showed much lower H2 generation than the calcinated samples
3.7. Hydrogen generation The prepared TiO2 and TiO2 –RGO photocatalysts were tested for their ability to photochemically produce hydrogen in a mixture of water and ethanol. Water-splitting reactions using metal oxide semiconductors are usually carried out in aqueous solutions with the addition of easily oxidizable reducing agents. The photogenerated holes irreversibly oxidize those agents, such as ethanol, instead of water [44]. This effect increases the electron concentration in the photocatalysts, and the hydrogen evolution reaction is enhanced. Reactions using sacrificial agents are regarded as half reactions and are often employed in tests of photocatalytic hydrogen or oxygen evolution. The rate of hydrogen production from UV photolysis of water/ethanol solutions in the presence of
Fig. 8. Photocatalytic generation of hydrogen by UV irradiation from water/ethanol solutions on commercially TiO2 P25, TiO2 as prepared at 100 ◦ C and 120 ◦ C using IL1, and after thermal treatment (TT) at 400 ◦ C for 3 h.
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Fig. 9. Hydrogen generation by UV irradiation from water/ethanol solutions on TiO2 prepared at 120 ◦ C and calcined to 400 ◦ C using IL1 and IL2.
at 400 ◦ C. In the case of the as-prepared samples the hydrogen production was even lower than the commercial P25 sample. Probably the presence of IL on the surface and of MP’s and the low degree of crystallinity can explain the lower hydrogen rate. As the samples were thermal treated at 400 ◦ C for 3 h it was observed an increase in the H2 generation rate (see Fig. 8). The catalytic performance is higher for the product prepared at 120 ◦ C rather than 100 ◦ C probably due to a presence of rutile phase (see Fig. 1) which has a lower band gap allowing absorption of a larger number of photons. We have selected this condition (120 ◦ C) as standard for further investigation of the photocatalytic properties to know the effect of experimental parameters on the H2 generation. Fig. 9 shows the effect of the anion on the hydrogen generation rate. When CH3 SO3 − is replaced by BF4 − , further increase in H2 generation takes place. When the TiO2 was prepared using IL2 and calcined at 800 ◦ C the hydrogen photocatalytic activity strongly decreased compared to that calcined at 400 ◦ C (see Fig. S7 in ESI). The decrease in photocatalytic activity was considered due to the presence of rutile phase which has lower catalytic activity than anatase phase [45] and also may due to a lower surface area. Interestingly, TiO2 –RGO composite prepared in all experimental conditions using any IL enhanced the H2 generation to a greater extent when compared to the bare TiO2 NPs as shown in Fig. 10. The reason for the increase in the photocatalytic activity for TiO2 –RGO composite is as follows. In the composite, graphene serves as an acceptor of the photo-generated electrons from titania and effectively suppresses the charge recombination, leaving more photo-generated holes to form reactive species and facilitate the photocatalytic activity. The hetero junction formed at the interface then separates the photo-excited electron–hole pairs, and thus hinders the charge recombination [24,46]. It has also been demonstrated that the photo-excited electrons in the anatase TiO2 conduction band can be effectively transferred to electronic states of graphene because the minimum of conduction band of anatase TiO2 is higher than the Fermi level of grapheme [47]. Literature survey shows that among all the carbon based materials-TiO2 composite, in situ synthesis of TiO2 –RGO from hydrothermal method shows better H2 generation than other methods [23]. Du et al. studied the electrochemical impedance spectra of macro-mesoporous titania films with and without graphene under UV light the illumination [46]. They observed that the semicircle of medium frequencies in the plot became shorter after the introduction of graphene, which indicates a decrease in the solid-state interface layer resistance and
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Fig. 10. Photocatalytic generation of hydrogen by UV irradiation from water/ethanol solutions on TiO2 or TiO2 –RGO composite prepared using IL1 and TiO2 or TiO2 –RGO composite prepared using IL2.
the charge transfer resistance on the surface. Therefore, the electron-accepting and electron-transporting properties of graphene in the composite could indeed suppress the charge recombination and improve their photocatalytic activities. 4. Conclusions We have successfully synthesized TiO2 nanoparticles and TiO2 –RGO composite by ionothermal method at 100 and 120 ◦ C using two different ILs. XRD pattern shows that the obtained product is composed of anatase and rutile TiO2 and by controlling the hydrolysis of TiCl4 at low temperature, pure anatase TiO2 can be obtained. BET surface area analysis of the composites showed surface areas of 170 and 161 m2 g−1 for TiO2 and TiO2 –RGO composite respectively with a BJH pore size distribution plot typical from IUPAC type IV pattern. TEM images show that the sizes of the NP’s are around 4 nm, and that they are distributed on the surface of the graphene sheet. Hydrogen generation by UV light on TiO2 NPs or TiO2 –RGO composite as prepared in ILs shows lower rate than TiO2 P25 due to the presence of IL. Thermal treatment at 400 ◦ C shows much better H2 production. The photocatalytic activity was further enhanced for TiO2 –RGO composite. The electron-accepting and electron-transporting properties of graphene in the composite could indeed suppress the charge recombination and improve their photocatalytic activities. Acknowledgements Thanks are due to CNPq, CAPES, FAPERGS and INCT-Catal for funding. Dr. Ganganagappa Nagaraju thanks to CNPq-TWAS for financial help to carry-out the research work and also UFRGS, Porto Alegre, for providing all facilities. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molcata. 2013.06.010. References [1] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [2] C. Wessel, L.A. Zhao, S. Urban, R. Ostermann, I. Djerdj, B.M. Smarsly, L.Q. Chen, Y.S. Hu, S. Sallard, Chem-Eur. J. 17 (2011) 775–779.
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