Novel synthesis of rutile titanium dioxide–polypyrrole nano composites and their application in hydrogen generation

Synthetic Metals 189 (2014) 77–85

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Novel synthesis of rutile titanium dioxide–polypyrrole nano composites and their application in hydrogen generation Yang Tan a , Yanggang Chen a , Zahid Mahimwalla a , Michel B. Johnson b , Tanu Sharma c , Ralf Brüning c , Khashayar Ghandi a,∗ a

Department of Chemistry and Biochemistry, Mount Allison University, Sackville, NB, Canada E4L 1G8 Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4R2 c Department of Physics, Mount Allison University, NB, Canada E4L 1E6 b

a r t i c l e

i n f o

Article history: Received 12 September 2013 Received in revised form 16 December 2013 Accepted 23 December 2013 Available online 28 January 2014

a b s t r a c t This work presents the first report of rutile titanium dioxide (TiO2 )–polypyrrole (PPy) nanocomposites prepared by chemical polymerization of pyrrole in situ in the presence of pure rutile phase TiO2 nanoparticles. The TiO2 rutile nanoparticles were made in water while the composite was made in a biphasic ionic liquid–water system. We attempted for the first time to split water under sunlight with this recoverable photocatalyst. © 2014 Elsevier B.V. All rights reserved.

Keywords: Light radiation Hydrogen generation Water splitting Conductive polymer metal nanoparticle composites

1. Introduction The combination of conducting polymer polypyrrole (PPy) and inorganic titanium dioxide (TiO2 ) produces material that is appearing in many applications such as photocatalysts [1], sensors [2], and conductive paints [3], and as nanomaterials in photovoltaic cells, sensors, actuators, and electromagnetic interference. In this work we report for the first time a method to make pure rutile nanoparticles in aqueous phase and synthesis of inexpensive PPy/TiO2 nanocomposites in situ on pure rutile phase TiO2 . The composites showed promising potential for photocatalytic transformation of water to generate H2 under solar light (full spectrum). To our knowledge this is the first reported transformation of water to hydrogen by PPy/TiO2 nanocomposites. PPy/TiO2 composites were previously prepared by vapour phase polymerization, electrochemical polymerization and photo polymerization [4–12]. Chemical polymerization is preferred for large scale production and industrial applications. Wang et al. [13] prepared PPy/TiO2 composites by in situ deposition oxidative polymerization of pyrrole using ferric chloride (FeCl3 ) as an oxidant in the presence of anatase TiO2 nanoparticles in an HCl solution.

∗ Corresponding author. Tel.: +1 506 961 0802. E-mail address: [email protected] (K. Ghandi). 0379-6779/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.12.025

Li et al. [14] reported a mixed phase PPy/TiO2 nanocomposite synthesized by the chemical polymerization of pyrrole with oxidant ammonium persulfate ((NH4 )2 S2 O8 ) in water dispersed with sodium dodecylbenzene sulfonate (SDBS; CH3 (CH2 )11 C6 H4 SO3 Na). The synthesis in these reports generally used strong acidic conditions, which could have negative environmental impacts. In our work, a “greener” method for PPy/TiO2 composites synthesis was used that minimizes the energy usage and reduces the toxicity of synthesis by using an ionic liquid/water two phase solvent system (the chemical polymerization of pyrrole is significantly affected by solvent [15]). Our synthetic methodology can generate composites possessing different structures and morphologies to optimize the material for various applications, such as photovoltaic materials and photocatalysts. One important application of these photocatalysts is the generation of hydrogen from water under visible light. An ideal solar hydrogen production catalyst would involve materials that decompose water using visible light, are stable in water, are nontoxic and abundant, are synthesized by green chemistry, and are amenable to further processing. Over the last 30 years, catalysts based on inorganic semiconductors have generated considerable interest [16–31]. However, many of these catalysts show limited catalytic activity outside of the UV spectrum. Sacrificial reagents and precious metal co-catalysts have been employed to decompose water to attempt bandgap tuning for improved efficiency [16–18,21–23].

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Fig. 1. Scheme of the synthesis of PPy–TiO2 nanoparticles. The IL is trihexyl(tetradecyl)phosphonium dicyanamide (IL105).

Here, we show rutile TiO2 and PPy composites with tunable bandgaps. Although TiO2 in the anatase phase has good photocatalytic activity, in the rutile phase it has the best thermal stability among the common forms of TiO2 . Therefore we focused on rutile PPy nanocomposites. The composites are prepared by an interfacial polymerization of pyrrole and rutile TiO2 nanoparticles (NPs) in an ionic liquid (IL)-water solvent system. We have also prepared the composites by an interfacial polymerization of pyrrole and rutile TiO2 NPs in a toluene–water solvent system. The yield is larger when using the IL. In these composites, the low bandgap of PPy allows the absorption of visible light and the transfer of an excited electron into the conduction band of TiO2 . The resulting electron-hole pairs split water and produce hydrogen gas. To our knowledge, this is the first time that the experimental potential of photocatalytic hydrogen generation from water by visible light with PPy/TiO2 nanocomposites is demonstrated.

dissolved in the aqueous solution for Method A (or in 20 mL of distilled water for Method B). Then 0.3 mL of pyrrole was added to the 20 mL IL phase. A PPy/TiO2 composite (Sample C) was prepared according to the following method: 0.01 g rutile TiO2 nanoparticles and 0.3 mL pyrrole were added to 40 mL aqueous FeCl3 (0.7 g) solution. This sample was used to compare the thermal stability with the two composites synthesized in a biphasic system. The samples were dried at 70 ◦ C overnight. We also used a conventional organic solvent, toluene, instead of ionic liquids to compare the products prepared in these two solvent systems. The synthesis methods were exactly the same. However, we only obtained films with Method A and Method B when we used toluene. The yield is about 50%; smaller than Sample A and Sample B with the IL (around 70%). 2.3. Characterization

2. Experimental 2.1. Chemicals Anhydrous FeCl3 (Sigma–Aldrich), trihexyl (tetradecyl) phosphonium dicyanamide (IL 105) (CYTEC Canada), TiCl4 (99.9%, Sigma–Aldrich), nitric acid (68–70%, ACP Chemical Inc.), toluene (Fisher Scientific Company) and acetone (99.5%, Caledon Laboratories Ltd.) were used as received. The H2 O was deionized (resistivity ≥18.2 M  cm, pH 7). Pyrrole (98%, Sigma–Aldrich) was purified by reduced-pressure distillation and kept in a refrigerator prior to use. 2.2. Synthesis The synthesis of pure rutile TiO2 nanoparticles was achieved by modification of a reported method to make TiO2 nanoparticles [32]. Cassaignon et al. [32] reported a mixture of three polymorphs of Titania (anatase, rutile and brookite) obtained by thermohydrolysis of TiCl4 in concentrated aqueous nitric acid; however, we modified the conditions (used pH 1, 95 ◦ C) to make pure rutile phase [33]. Pyrrole is polymerized in situ with TiO2 nanoparticles using a water–trihexyl(tetradecyl)phosphonium dicyanamide IL biphasic system (Fig. 1). FeCl3 was used as oxidant, and Cl− were doped into the produced PPy. The composites were synthesized using the methodologies schematically depicted in Fig. 1. More specifically, 0.01 g of TiO2 nanoparticles was suspended in 20 mL of deionized water for Method A (or in 20 mL of IL 105 for Method B). The solution (water or IL) was sonicated for 30 min and 0.7 g of FeCl3 was

2.3.1. SEM and TEM A JEOL JSM-5600 SEM was used to obtain the images of composites. A JEOL 2011 TEM was used to obtain the micrographs of nanoparticles. 2.3.2. FTIR spectroscopy IR spectra were recorded using a Nicolet FT-Infrared 200 Spectrometer with KBr pellets. The Fourier transform infrared spectrometer was used to characterize the functional groups in the samples over the 4000 to 400 cm−1 range. 2.3.3. X-ray diffraction XRD measurements were carried out with Cu–K␣ radiation. Data were collected at room temperature for scattering angles in the range from 4 ◦ C to 100 ◦ C. The XRD spectra show the scattered intensity (in arbitrary units) as a function of the scattering vector, q = 4␲ sin /, where 2 is the scattering angle. 2.3.4. UV–vis spectroscopy UV–vis spectra were recorded using a Cary 100 UV–vis spectrophotometer over the range 200 to 900 nm. All spectra were recorded using deionized water as solvent which was also used as reference (in the reference cell). 2.3.5. Thermal analysis Thermogravimetric analysis (TGA, SDT Q600 from TA Instruments) was performed from room temperature to 800 ◦ C with a

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heating rate of 15 ◦ C min−1 and an argon flow of 50 mL min−1 . We tared an Al2 O3 crucible, then added the samples and recorded the mass before increasing the temperature to 800 ◦ C. Differential scanning calorimetry (DSC, TA Q200 from TA Instruments) thermograms were recorded over temperature ranges from 20 ◦ C to 160 ◦ C, using a heating and cooling rate of 20 ◦ C min−1 under helium flow at a rate of 25 mL min−1 . The melting point of indium and the freezing point of water were used as standards to calibrate the DSC temperature. Indium was also used for the enthalpy calibration. The samples were hermetically sealed in aluminium pans, heated and then cooled to 20 ◦ C. Sample masses ranged from 2.15 to 5 mg. 3. Results and discussion The reactants and the dopants are transported from the ionic liquid phase to the interface for polymerization and doping. The synthetic method using IL enables morphological control of the final composite in either particle or film form based upon the phase and interfacial layers in which the composite is formed. The solvents in which the pyrrole and TiO2 NPs are dispersed prior to mixing can change the morphology of the composites. Thus, our materials are available as a film at the solution interface or as particles in the aqueous phase as shown in scheme A (Fig. 1), or as films formed at the interface and in the aqueous phase as shown in scheme B (Fig. 1). For clarity, the composites are hereafter referred to as Sample A or B depending upon the synthetic scheme used. The term film or particle refers to the morphology of the products. Our modified synthesis conditions helped us to obtain the pure rutile phase, as confirmed by XRD (Fig. 2). The XRD spectra show the relative scattered intensity as a function of the modulus of the scattering vector, |q| = 4␲ sin/, where  = 0.154 nm is the wavelength of the Cu K␣ radiation used in the experiment and 2 is the scattering angle. The results depicted in Fig. 2 for the precursor TiO2 NPs match the joint committee on powder diffraction standards (JCPDS) index entry for rutile TiO2 . Pure PPy, synthesized without TiO2 by Method A, exhibits a weak and broad diffraction peak at 17.5 nm−1 , which indicates that the PPy is amorphous. The XRD pattern for the film by Method A suggests a low TiO2 content within the film, but a much greater presence within the particle form of the composite by Method A. Selecting the appropriate acidity, [Ti4+ ] and temperature are the main factors to obtain single phase rutile TiO2 nanoparticles. Nevertheless, the composites prepared in toluene–water solvent system are films, which can be called Sample T. We have recently showed that polymerization of pyrrole is via addition of free radical cations to the monomer and oligomers [15]. Based on that work and our FTIR spectra (Fig. S9 and S10 in Supplementary Information) we believe the process of formation of our

Fig. 2. XRD spectra of Sample A film, Sample A bottom layer, PPy, TiO2 nanoparticles, and another Sample A bottom layer that contains more TiO2 . The JCPDS index peak positions and relative intensities of rutile TiO2 (JCPDS 21-1276) are shown for comparison.

nano-composites is first binding of pyrrole from its nitrogen site to the rutile TiO2 nanoparticles followed by addition of pyrrole radical cation to the bound pyrrole or pyrrole oligomer. A composite film was produced at the interface of the two solvents and a composite powder was produced in the aqueous solution by Method A. Composite films were generated also in the aqueous phase by Method B. Micrographs of the TiO2 NPs and composite Sample B PPy/TiO2 particles (Fig. 3) show the encapsulation of spherical TiO2 NPs of size 4 ± 2 nm (Fig. 3a) by the PPy polymer to yield composite particles with a mean size of 121 ± 59 nm (Fig. 3b). Histograms of TiO2 nanoparticles and PPy/TiO2 particle size distribution from the TEM analysis are presented in the Supplementary information (Figs. S7 and S8). SEM images of the Sample A particles indicate the presence of spheres of polymer in a three-dimensional porous structure within the composite (Fig. 4). The average dimension of the pores in our nanostructures is around 1 ␮m, and most particles are in the range of 90 nm to 210 nm. The porous structure enhances photocatalysis by increasing the surface area. The EDS

Fig. 3. TEM micrographs of TiO2 nanoparticles (a) and PPy–TiO2 nanoparticles (b).

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Fig. 4. SEM micrograph of the PPy–TiO2 nanocomposite (Sample A particles) showing a three-dimensional network. Micrographs were collected using an accelerating voltage of 15 kV.

image (see Supplementary Information) shows that Ti is present in the composites. An SEM of the films of Sample B (Fig. 5) indicates the presence of some TiO2 NPs embedded onto the surface of the PPy film. This is in contrast to the particles in Sample A, where the particles are encapsulated by PPy, and demonstrates the morphological tuning enabled by the synthetic methodology. To our knowledge this is the first report of film morphology for PPy/TiO2 nanoparticles synthesized by chemical methods (as opposed to electrochemical methods). All reported chemical synthesis of PPy/TiO2 NPs led to particle morphologies [14] or nanorods [7]. To obtain nanorods, ␤naphthalene sulfonic acid was used. It can lead to the overlapping of micelles, resulting in surface roughness and producing nano-rods. The composites prepared in the toluene–water solvent system are films called Sample T. They are composed of embedded nanoparticles ranging in size from tens to hundreds of nanometres of PPy/TiO2 composite particles as seen through its SEM image (Fig. 6). The SEM image of Sample T shows a uniform and smooth flat surface that is nearly free of cracks (Fig. 6). The film is dense and more regular in appearance than the loosely bound particles or film sheet shown in Fig. 4 and Fig. 5. A significant difference between toluene and ionic liquid solvents is that toluene can dissolve pyrrole very well but ionic liquids can only slightly dissolve pyrrole (like water). This means there is less contact at the interface of the two solvents when toluene is used, while in the IL phase in Sample A and B, due to its low solubility, the pyrrole falls from the ionic liquid phase into the water phase. Though some pyrroles polymerize at the interface, there are

Fig. 5. SEM micrograph of the PPy–TiO2 composite film (Sample B film).

Fig. 6. SEM image of Sample T film.

still pyrroles that react in the water phase, forming the particles that precipitate (in the case of the IL/water biphasic system). This is also the reason for the smaller yield of Sample T. The thermal stabilities of PPy and PPy–TiO2 composites were determined by TGA. Fig. 7 shows the TGA thermograms for pure PPy, the PPy–TiO2 particles in Sample A, and the PPy–TiO2 film in Sample B. The comparison of pure PPy and PPy–TiO2 composites shows a greater mass loss for the PPy polymer than for the composites. The composites also exhibit a very substantial enhanced thermal stability. Curves PPy/TiO2 composite with 3% TiO2 and the composite with 8% TiO2 represent the two samples synthesized with the same method, but with different amounts of TiO2 added. The degradation of PPy shows a three-stage decomposition pattern as three major slope changes are observed. At T < 100 ◦ C, the mass loss is caused by the presence of residual water in the sample. The next stage of the mass loss is by degradation that lasts until 225 ◦ C, and is attributed to the loss of dopant ions that are weakly (electrostatically) bound, from the inter-chain sites of the polymer. With further increases in temperature, material degradation and decomposition of the polymer backbone occurs. Unlike pure PPy, the degradation processes of PPy/TiO2 composites show two stages. In the composites, the mass loss of the PPy/TiO2 composite observed at T < 125 ◦ C is partly because of the evaporation of residual water in the sample. The binding of PPy to TiO2 , and possible crosslinking of PPy chains onto the nanoparticle surface, may affect the amount of water adsorbed by the composites. This may account for the reduced weight loss in the PPy/TiO2 composite at T < 125 ◦ C compared to the pure polymer. The second stage from 125 ◦ C to 800 ◦ C is due to the degradation and decomposition of part of the PPy backbone and the loss of dopant.

Fig. 7. TGA traces of (a) PPy/TiO2 composite with 3% TiO2 (red curve), (b) sample with 8% TiO2 (green curve), and (c) pure PPy (blue curve). The derivative plot is shown in the inset.

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Fig. 8. TGA curves of the PPy/TiO2 composites prepared using different methods. Red curve represents the PPy/TiO2 nanoparticles of Sample A. Blue curve is the TGA trace of the PPy/TiO2 film obtained at the interface of Sample B. Green curve represents the PPy/TiO2 particles prepared in deionized water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

At 800 ◦ C, the composites only loose approximately 50% of their mass, while pure PPy is completely degraded. The result is consistent with a previous report (although for a different phase of TiO2 ).[6] This data coupled with DSC data (vide infra) shows that pyrrole polymerized on the surface of TiO2 nanoparticles would undergo crosslinking that shifts the decomposition to higher temperature. Polymer cross-linking could increase the energy needed for composite decomposition, resulting in a better thermal stability. It is interesting that the decomposition of the composite with 3% TiO2 nanoparticles has the same mass loss with the composite with 8% TiO2 nanoparticles, indicating that in a range of TiO2 content, the extra addition of TiO2 nanoparticles does not enhance the thermal stability. A TGA comparison of the composites prepared by synthetic Methods A and B, as well as a third method which prepared PPy/TiO2 composites (C particles) in water, is shown in Fig. 8. The red curve represents the PPy/TiO2 nanoparticles formed by Method A. The blue curve is a plot for the composite film obtained at the interface in Method B. The green curve is the TGA curve of the PPy/TiO2 particles prepared in deionized water (sample C). As evident by the TGA curves, at 800 ◦ C, 94% of Sample C is decomposed in contrast with a 61% decomposition of Sample B and 51% decomposition of Sample A. The thermal stability of Sample T is similar to the thermal stability of Samples A and B, as shown in supporting information Fig. S17. The analysis highlights a higher thermal stability due to improved TiO2 –PPy interactions for materials synthesized by an ionic liquid–water interface than for the composites synthesized in water. This result shows the advantage of having the second phase (biphasic system) in the composite synthesis. Fig. 9 shows the UV–vis spectrum of the PPy/TiO2 nanocomposites (Sample A). This spectrum shows two bands at 348 nm and around 465 nm. These two bands can be attributed to pyrrole oligomer and polymer, respectively. ␲-Electrons absorb light as they become excited to anti-bonding molecular orbitals, with a lower energy gap between the HOMO and the LUMO moving absorption to a longer wavelength. The relation between energy and wavelength is governed by the equation: E = hc/ [14], so the energy of the bandgap of PPy is 2.6 eV, which is lower than the bandgap energy of rutile TiO2 (3.02 eV). Fig. 10 shows the UV–vis spectrum of the PPy/TiO2 film (Sample B). The two broad peaks ranging from 360 nm to 380 nm, and 650 nm to 700 nm, may be related to the pyrrole oligomer and PPy. As the PPy composite is difficult to dissolve, the UV–vis peak of PPy is very weak.

Particles from Method A have shorter absorption wavelengths, demonstrating that the PPy synthesized in our composites have shorter polymer chains. This could be due to the synthetic method and structure of the composites. For Composite A, the PPy grows around the core of TiO2 nanoparticles. The presence of the core may hinder the propagation of PPy chain. On the other hand, Composite B consists of a “free” growing PPy film which has a high molecular weight and TiO2 nanoparticles on the surface of the PPy film. The UV–vis spectra of Sample T shows similar peaks (supporting information Fig. S18). The bandgap of pure PPy is approximately 2.6 eV and is dependent on the chain length of the PPy polymer. Thus, particles prepared by Method A (Fig. 1) possess a larger bandgap than the films

Fig. 9. UV–vis absorption spectrum of the PPy/TiO2 composite (Sample A particles). Two bands at 348 nm and a range from 400 nm to 480 nm are assigned to pyrrole oligomer and PPy, respectively.

Fig. 10. UV–vis absorption spectrum of the PPy/TiO2 film (Sample B film). Two bands at 375 nm and 685 nm are assigned to pyrrole oligomer and PPy, respectively.

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Fig. 11. The energy-level diagram for TiO2 and PPy and the electron-injection process. CB is conduction band; VB is valence band; h+ is the hole in the valence band. It should be noted that the CB in the PPy component is actually an excited electronic state (LUMO).

prepared by Method B because of the shorter PPy chain length in the particle form of the composite as compared with the film. The ability to tune the bandgap of the conducting polymer has important implications for the creation of composites with photocatalytic activities that can be optimized for specific applications and wavelengths of absorption. We studied the photocatalytic reactions by splitting distilled water in the presence of the PPy–TiO2 composites under sunlight.

These composites showed absorption peaks in the visible light region. Our proposed mechanism is as follows: When the composites were exposed to visible light, the PPy component absorbed photons, exciting an electron from the valence band (VB) to the excited state. This excited electron is then transferred into the conduction band (CB) of TiO2 , which is lower in energy than the excited state of PPy. The resulting charge separation reduces the electron-hole pair recombination in PPy and improves the migration of electrons onto the surface of TiO2 particles. The electrons on the surface of TiO2 can reduce water to produce hydrogen. Thus, the composites increase the absorption of light energy for photocatalysis from the UV to the visible spectrum owing to the bandgap of the PPy polymer. The process is schematically depicted in Fig. 11. The photogenerated electron–hole pairs that migrate to the surface can oxidize and reduce water to produce oxygen and hydrogen according to the following reactions. 4h+ + 2H2 O → O2 + 4H+

(1)

2e– + 2H2 O → H2 + 2OH–

(2)

The composites were placed in a quartz cell, immersed in water, and exposed to sunlight. We measured the photocatalysis-produced gas in the sample cell with a custom-built measurement system comprised of sample and reference quartz chambers connected to a pressure transducer to allow accurate differential pressure measurements between the two chambers (Fig. 12) after calibration. This in situ measurement method does not disturb the system. A calibration of the transducer output to moles of gas and gas

Fig. 12. The structure diagram of the digital capture and release measurement used in this work.

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Fig. 14. The amount of H2 produced in the sample with the PPy–TiO2 film composite for the first and second trials. Fig. 13. The amount of H2 produced for the Sample B film (triangles) and Sample A particle (diamonds) composites at the light intensity of 460 lx.

pressure is done and this is used to measure the gas evolved over time by the photocatalytic composite materials. Using this system, the H2 produced by the photocatalytic water splitting by both particles synthesized using Method A, and film synthesized using Method B, are shown in Fig. 13. The PPy/TiO2 composites were dispersed in an ultrasonic bath for 10 min prior to use. Over a period of 10 min, particles formed by Method A produced 0.1 ␮mole of H2 . The film produced by Method B produced 0.4 ␮mole of H2 gas over the same time frame. The light intensity was around 460 lx in the above tests. These efficiency differences can be ascribed to the bandgap of PPy and the morphology of the composites. We used GC/MS to confirm that there was no CO2 gas from the photocatalysis reaction or any other gas but H2 and O2 released from this photoreaction. A Saturn 2000 GC–MS with a CP-3800 GC (Varian) fitted with a split/split less injector port was used. The analytical column was a Zabron capillary GC column ZB-5 (crosslinked 5% phenyl methyl polysiloxane, 30 m × 0.25 mm ID, 0.25 ␮m film thickness). Temperature program was as follows: initial temperature of 50 ◦ C for 1 min, increased to 100 ◦ C at 20 ◦ C/min, and held for 2 min giving a total run time of 5.5 min. The carrier gas is helium that was maintained at 1.0 mL/min in constant flow mode. The GC–MS was programmed to perform a 2 ␮L split-less injection. Data acquisition was performed using a Dell computer fitted with Saturn GC–MS workstation. Our palladium test confirms the existence of H2 (see SI). It is likely that the increased efficiency is a function of a more optimized bandgap in the films owing to a larger PPy chain length (according to the UV–vis spectra shown above). The TiO2 in the film morphology also has greater contact area with water molecules because the TiO2 particles are embedded on the surface of the film instead of being encapsulated by the PPy polymer in particle form. Most of the TiO2 nanoparticles are trapped by the PPy in the particulate form, therefore with no chance to donate electrons to water. The rate of hydrogen generation decreases after a certain time because the dispersed catalyst precipitates to the bottom of the flask and reduces the surface area available to catalyze the decomposition reaction. The activity and stability of the PPy–TiO2 composite was confirmed by repeat experiments using the same photocatalyst sample. This was done several times and we never observed a significant decrease in hydrogen generation with time; one example is demonstrated in Fig. 14. In the repeat experiments, we used films from Sample B to decompose water under sunlight for 10 min. We stopped the experiment for 20 min, during which time the film was dispersed in solution via an ultrasonic bath. Then we ran the test again under similar sunlight conditions. The

Fig. 15. The amount of H2 produced for the sample TiO2 nanoparticles.

photocatalytic performance for H2 evolution did not decrease with usage. This was repeated several times and therefore we are confident that the nanocomposites are recyclable photocatalysts for hydrogen production from water. Considering the reusability of the nanocomposites, this might be the least expensive present method for the photochemical transformation of water to hydrogen. We further studied the H2 generation from rutile TiO2 nanoparticles and PPy in the same light intensity. As shown in Fig. 15, only 0.04 ␮mol H2 is produced in the presence of TiO2 nanoparticles, and no gas was produced from the PPy sample. Although the TiO2 nanoparticles have a large band gap, it still can absorb the UV light from sunlight and oxidize and reduce water from the valence band and conduction band (although at a much lower level). For comparison, we also measured the amount of H2 generation in the presence of Sample T with the light intensity of around 450 lx (Fig. 16). The H2 generation is less than the amount generated with

Fig. 16. The amount of H2 generation from the Sample T in the light intensity of 450 lx.

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Sample A and Sample B. It is possible that because Sample T is a thicker film, the electron migration is impaired, resulting in a lower efficiency of photoactivity. Among these photocatalysts, InTaO4 [26] and YBiWO6 [34] have been reported to split water under visible light. Ni-doped InTaO4 –In0.9 Ni0.1 TaO4 shows a narrow band gap (2.25 eV) and good H2 generation rates (0.05 mmol min−1 ) under a light wavelength longer than 420 nm. YBiWO6 also has a band gap of 2.71 eV and produces 0.07 mmol min−1 of H2 when the wavelength of light is longer than 420 nm. However, the raw materials for these photocatalysts are very toxic and would cause environmental pollution. In contrast, PPy/TiO2 has a combined band gap (3.2 eV/2.6 eV), and produces H2 at a rate of 0.04 mmol min−1 .This photocatalyst is almost nontoxic, and is less expensive. Our H2 generation process is in sunlight, which covers the full range of wavelength of visible light and part of UV light. As the absorption wavelength of absorption was not selected, the efficiency of H2 generation might be increased by optimising that factor. For the two photocatalyst systems, the following catalysts were investigated: a mixture of Pt–WO3 and Pt–SrTiO3 (Cr–Ta doped) photocatalysts and an IO3 − /I− shuttle redox mediator [35], a mixture of Pt–TaON for H2 evolution and a Pt–WO3 catalyst for O2 evolution in an IO3 − /I− shuttle redox-mediated system [36], and a (Pt/SrTiO3 :Rh)–(BiVO4 ) system with Fe3+ /2+ redox mediator [37]. These systems are all more toxic and/or expensive than ours.

4. Conclusion We successfully performed novel simple and “green” processes to synthesize PPy/Rutile TiO2 nanocomposites by in situ polymerization. By using the interface of two solvents we obtained two different morphologies, particles and film. We use a recyclable solvent in the organic phase. This method offers a more economical, environmentally friendly, and easy method to produce and control the morphology of nanoparticles as compared to use of strong acids or templates for PPy/TiO2 nanocomposites with single phase TiO2 nanoparticles in either laboratory or industry. The PPy/TiO2 nanoparticles have a core-shell structure as PPy coats the surface of TiO2 nanoparticles. In the PPy/TiO2 film the TiO2 nanoparticles coat the surface of the PPy film. The process uses recyclable solvents with lower energy waste and toxicity than currently reported synthetic methodologies. It has good potential for large-scale industrial production compared with other synthesis methods, such as electrochemical polymerization, and vapour polymerization. To our knowledge, this is the first rutile TiO2 /PPy nanocomposite to be prepared. It could have important applications in the photosplitting of water and in photovoltaic cells. In this work, The TiO2 nanoparticles used in the nanomaterials are only in one phase (rutile). This decreases the number of traps, and hence enhances the efficiency of migration. We also used toluene to replace the ionic liquid to compare the products. We observed that only the film can be generated in the toluene–water solvent system. This sample has a smooth flat surface that is different than the composites prepared in the IL-water solvent. In the application of the water-splitting experiment, the gas produced from this sample is less than the products of the IL/water biphasic system. Although this is the first reported paper on applications of PPy/TiO2 rutile nanomaterials for visible-light-based solar hydrogen generation (to our knowledge), we have applied several concepts from the works on bulk heterojunction (BHJ) organic solar cells [38–40]. In particular, ultrafast photoinduced charge transfer occurs at the interface of TiO2 nanoparticles (acceptor) and PPy (donor). However, the current composites have significant

room to increase the efficiency of hydrogen production through the optimization and tunability of the conducting polymer, composite morphology, and the semiconductor components of the composite. We have highlighted a new green synthesis route to design and optimize bandgap-tunable, low-cost, efficient, and processable photocatalytic PPy/TiO2 nanomaterials for visible-light-based solar hydrogen generation. The PPy/TiO2 nanocomposites have greater thermal stability than the pure PPy. We studied the thermal stability of the composites in both air and argon atmospheres, and compared the thermal stability of PPy/TiO2 composites in inert gas between different preparation methods and different amounts of TiO2 nanoparticles. The composites prepared in the solvent with two phases are more stable thermally than the composites prepared in water. In an air atmosphere (see supplementary information), the composites decomposed faster due to the oxygen in the air. Moreover, synthesis of PPy/TiO2 composites in an IL/water solution can increase the content ratio of TiO2 in PPy/TiO2 particles. An ultrasonic bath applied during the suspension of TiO2 nanoparticles inhibits the agglomeration of nanoparticles and reduces the size of the composite particles. Acknowledgements This research was financially supported by the Natural Sciences and Engineering Research Council of Canada, the New Brunswick Innovation Foundation, and Canada Foundation for Innovation. K.G. also acknowledges the Canada Foundation for Innovation, Atlantic Innovation Fund and other partners that fund the Facilities for Materials Characterization managed by the Department of Chemistry and the Institute for Research in Materials at Dalhousie University. We thank James M. Ehrman from Mount Allison University for his kind help with the SEM images and Professor Ralf Brüning for discussion. 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.synthmet.2013. 12.025. References [1] F. Deng, Y. Li, X. Luo, L. Yang, X. Tu, Colloid. Surf. A: Physicochem. Eng. Asp. 395 (2012) 183–189. [2] R.N. Bulakhe, S.V. Patil, P.R. Deshmukh, N.M. Shinde, C.D. Lokhande, Sens. Actuators B: Chem. 181 (2013) 417–423. [3] E. Armelin, M. Martí, F. Liesa, J.I. Iribarren, C. Alemán, Prog. Org. Coat. 69 (2010) 26–30. [4] J. Wang, X. Ni, Solid State Commun. 146 (2008) 239–244. [5] Y. Jia, P. Xiao, H. He, J. Yao, F. Liu, Z. Wang, Y. Li, Appl. Surf. Sci. 258 (2012) 6627–6631. [6] M. Babazadeh, F. Rezazad Gohari, A. Olad, J. Appl. Polym. Sci. 123 (2012) 1922–1927. [7] M. Luo, Y. He, Q. Cheng, C. Li, J. Macromol. Sci. Part B 49 (2010) 419–428. [8] J. Li, Q. Zhang, J. Feng, W. Yan, Chem. Eng. J. 225 (2013) 766–775. [9] J. Li, J. Feng, W. Yan, Appl. Surf. Sci. 279 (2013) 400–408. [10] L. Sun, Y. Shi, B. Li, X. Li, Y. Wang, Polym. Compos. 34 (2013) 1076–1080. [11] S. Takagi, S. Makuta, A. Veamatahau, Y. Otsuka, Y. Tachibana, J. Mater. Chem. 22 (2012) 22181–22189. [12] Y. Xie, H. Du, J. Solid State Electrochem. 16 (2012) 2683–2689. [13] D. Wang, Y. Wang, X. Li, Q. Luo, J. An, J. Yue, Catal. Commun. 9 (2008) 1162–1166. [14] S. Li, M. Chen, L. He, F. Xu, G. Zhao, J. Mater. Res. 24 (2009) 2547–2554. [15] Y. Tan, K. Ghandi, Synth. Met. 175 (2013) 183–191. [16] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, Renew. Sust. Energy Rev. 11 (2007) 401–425. [17] Z. Yi, J. Ye, N. Kikugawa, T. Kako, S. Ouyang, H. Stuart-Williams, H. Yang, J. Cao, W. Luo, Z. Li, Y. Liu, R.L. Withers, Nat. Mater. 9 (2010) 559–564. [18] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746–750. [19] S.Y. Reece, J.A. Hamel, K. Sung, T.D. Jarvi, A.J. Esswein, J.J.H. Pijpers, D.G. Nocera, Science 334 (2011) 645–648. [20] Y. Sasaki, H. Kato, A. Kudo, J. Am. Chem. Soc. 135 (2013) 5441–5449.

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