Abstract
Thin TiO2 layers grown over few-layers graphene were prepared in order to evaluate the photoinduced chemical response of this composite. Graphene was grown over copper foils by decomposition of acetylene in a standard chemical vapor deposition apparatus. Graphene was subsequently transferred to a silicon substrate, on which the titanium dioxide was grown to form a TiO2/FLG/SiO2/Si composite. The formation of each layered material was verified by Raman spectroscopy and the morphology was characterized by scanning electron microscopy. The photoelectrochemical evaluation of the resulting composite, using it as a photoanode, was accomplished with a potentiostat, a solar simulator, and a three-electrode configuration. The electrochemical response indicates that the new composite preserves the average photoactive properties of the base material and at the same time shows a singular transient response where explicit benefits seem to be derived from the FLG/TiO2 combination.
1. Introduction
The basis of photocatalytic systems, supported mainly by semiconductor materials, is that they can promote certain chemical reactions in the presence of light without being consumed in the process [1]. Probably the most noteworthy example in this field is the generation of molecular hydrogen (H2) from the dissociation of water molecules. Some studies indicate [2] that this mechanism could be the best solution to the problems arising from the use of fossil fuels as energy sources. Although it is now possible to obtain H2 through electrolysis [3], this process is highly energy consuming and hence costly to produce. An alternative and very attractive method is to obtain H2 by the photocatalytic splitting of water using solar radiation to reduce the energy requirements. H2 obtained through this process may then be used as a clean and efficient source of energy for a variety of applications.
Water splitting, using electromagnetic radiation and semiconductors as catalysts, has been studied for at least four decades [4, 5]. Among the many well-known semiconductors used in this field, titanium dioxide (TiO2) is one of the most promising catalysts for the production of H2 from the water photoelectrolysis due to its high chemical stability, low toxicity, and relatively low production cost. It does however exhibit a rapid recombination of photogenerated charge carriers, which decreases substantially the quantum efficiency of photoinduced processes. Some strategies used to improve the photocatalytic performance consist of doping TiO2 with transition metal elements such as silver (Ag) [6] or nonmetallic elements like carbon (C), nitrogen (N), and also sulfur (S) [7]. Another option to improve the efficiency of the photocatalytic process or even diminish production costs has recently been proposed and consists of locating TiO2 nanoparticles in close contact with carbon nanostructured materials to form a hybrid nanostructure, in which the photocatalytic properties of TiO2 are combined with complementary properties of the host nanostructure [8–10].
Graphene is an allotrope of carbon, whose unusual transport properties make it a very attractive material for the development of new technologies, particularly when the large carrier mobilities represent a benefit. Graphene has indeed high electron mobility at room temperature, high elasticity, and resistance, among other singular physical properties [11]. With the purpose of examining new materials to be used in the photoelectrolysis of water, in this paper, we examine the preparation method of a TiO2/graphene composite and evaluate its photoelectrochemical activity considering graphene as the conducting material and TiO2 as the photoactive component.
2. Materials and Methods
2.1. Synthesis and Transfer of Graphene
Graphene was synthesized by atmospheric pressure chemical vapor deposition (CVD) at 1000°C using acetylene as carbon source [12]. High purity copper foils were used as substrates (1.5 × 1.5 cm2, 25 μm, purity > 99.99% MTI). In the CVD furnace, the Cu substrates were heated at a rate of 20°C/min in a flow of an argon/hydrogen mixture (100 sccm each) up to 1000°C. This temperature was maintained for 20 min in order to remove any residual oxide and improve the nucleation density [13]. After this process, the flow of argon and hydrogen was set to 500 and 5 sccm, respectively, and acetylene was admitted into the reactor at a rate of 2 sccm during 1 min. Subsequently, the furnace was cooled down to room temperature at a rate of 20°C/min in an Ar/H2 atmosphere.
Graphene was then transferred onto silicon substrates following Suk’s recipe [14]. A poly-methyl methacrylate (PMMA) layer was deposited over the graphene/Cu substrate by spin-coating; 100 μL of a PMMA/chlorobenzene (20 mg/mL) dissolution was spread over the substrate (10 s/500 rpm + 50 s/1500 rpm). The PMMA layer was cured at 120°C for 5 minutes. The PMMA/graphene/Cu material was then floated on a 1.0 M Fe(NO3)3/water dissolution in order to dissolve the copper layer. The remaining PMMA/graphene stamp was thoroughly washed in deionized water and carefully placed onto clean silicon chips for the photoelectrode fabrication and SEM characterization [Si(100), n-type, P-doped] or SiO2/Si(100) for Raman characterization (~100 nm thermal oxide grown by Si dry oxidation at 1100°C for 36 min). The PMMA/graphene/substrate was then dried in vacuum and subsequently heated up to 180°C for 30 min to improve the graphene adhesion. PMMA was later dissolved and washed away with acetone and 2-propanol, respectively, to obtain a graphene/Si chip.
2.2. Synthesis of TiO2
TiO2 layers were deposited on bare Si and on graphene covered Si substrates. Titanium (IV) tetraisopropoxide (TTIP) was used as a precursor, which was evaporated in a two-zone CVD tube furnace [15]. The TTIP introduced to the first zone of the furnace was evaporated at a temperature of 130°C in a low pressure environment (~3.5 torr). The collecting substrates were located in the second heating zone at 500°C. The characteristic time of synthesis was 15 min. As expected, the availability of TiO2 obtained through decomposition of TTIP is inhomogeneous along the reactor, since the density of material is gradually rarified by collection on the furnace walls along the second zone. Hence, the amount of material deposited on the substrates depends on the relative position of the substrates with respect to the separation of the two heating zones. We have used a fixed distance (6 cm) to prepare the photoelectrode materials.
2.3. Characterization
The morphological and structural characterization was made by using several techniques. Samples and substrates characterization through Raman spectroscopy was performed using a Renishaw in-Via, with a 532 nm laser. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) characterization was performed in a Thermo-Nicolet IS10 apparatus. Scanning electron microscopy (SEM) characterization was done using a Carl-Zeiss, EVO MA10.
The photoelectrochemical response was evaluated by measuring linear sweep voltammetry and chronoamperometry. A 3-electrode arrangement and a potentiostat (Gamry Interface 1000) were used to register the response of samples when they were illuminated with a 300 W solar simulator (Sciencetech SF300A with an AM1.5 filter). Standard Ag/AgCl electrodes and a platinum wire counter electrode were used to characterize the photoanodes (working electrode), with the samples prepared as described in Section 2.2. The measurements were done using KCl 0.1 M as electrolyte.
3. Results and Discussion
Figure 1(a) shows a typical Raman spectrum of a sample prepared by the decomposition of acetylene on copper, as previously described. The red curve in Figure 1 (lower trace) corresponds to the sample attached to the copper substrate, while the black curve corresponds to the same sample after being transferred onto a thermal silicon dioxide substrate. The characteristic ratio of intensities (after transfer) was 0.8 indicating that the synthesized material corresponds to few-layers graphene (FLG). In agreement with the criteria presented by Song et al. [16] and Hao et al. [17], our material corresponds to 2 to 3 layers of graphene. Figure 1(b) shows a SEM image of a sample transferred onto a monocrystalline silicon substrate. The image shows the wrinkled FLG in the left side and the flat surface of silicon in the right side.
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The TiO2 layer was deposited on graphene transferred onto silicon to form a TiO2/FLG/Si composite. For comparative purposes TiO2 was also deposited over bare silicon to form a binary TiO2/Si composite. As mentioned in the experimental section, the deposition rate of TiO2 depends on the position inside the furnace. We have chosen distances between 5 and 6 cm from the furnace zone boundary, where evident accumulation of material was observed and the samples have a uniform optical appearance. In order to verify the presence of TiO2 we have performed ATR-FTIR and Raman spectroscopy. Figure 2(a) shows the formation of TiO2 that was identified by the infrared absorption band close to 850 cm−1. This frequency corresponds to a characteristic vibrational mode, related to the bending of the Ti-O-Ti bond [18]. Figure 2(b) shows the Raman spectra of TiO2/Si and TiO2/FLG/Si samples in the low frequency range. The Raman signals revealed that the TiO2 is mainly in the anatase (A) phase as indicated by the characteristic peaks at 145 cm−1, 396 cm−1, and 636 cm−1 associated with , , and active modes, respectively [19]. In the TiO2/FLG/Si samples it is possible to observe the Raman signals corresponding to graphene with a similar ratio but with a higher band intensity; this is due probably to the introduction of edge defects in the FLG during the TiO2 growth process.
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Figures 3(b) and 3(c) show SEM images of the surface of TiO2/Si and TiO2/FLG/Si, respectively, prepared under identical growth conditions. The TiO2 grown over bare silicon showed a granulated surface, whereas those on FLG/Si exhibited a similar morphology but with larger grain sizes. This difference could be explained by the stronger interaction of TiO2 with silicon compared to FLG, thus resulting in a higher surface density of nucleation sites and as a consequence a larger density of TiO2 grains. Both samples exhibited a golden color (Figure 3(a)) due to the TiO2 film, thus indicating a similar thickness. This color corresponds to a TiO2 layer with a thickness in the range of 20–25 nm [20].
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Photoelectrochemical measurements were performed to evaluate the response of the individual materials and the hybrid system. The linear sweep voltammetry of TiO2 (6 cm)/Si sample showed the behavior expected for a semiconductor [21] (figure not shown here). The current density increased almost linearly as the voltage is raised, only under illumination, reaching a value close to 80 μA/cm2, for a potential difference of 1.0 V, while in darkness it showed no significant response. On the other hand, FLG supported on silicon presented a much higher current response, which also increased almost linearly, reaching a value close to 3 mA/cm2 for an applied voltage of 1.0 V. No significant differences were observed for the FLG/Si materials, between light and dark conditions, in either the general behavior or the magnitude of the current in the linear sweep voltammograms.
Figure 4(a) shows a linear sweep voltammetry for the TiO2/FLG/Si composite. The current was measured under both dark and illumination conditions. The optical response is roughly similar to that of FLG/Si. In order to observe the contribution of TiO2 in the composed material, we performed chronoamperometric measurements, alternating dark and light cycles for different constant voltages. Figure 4(b) shows the photocurrent density for TiO2/Si for different voltages. The current density follows the cyclic nature of the light intensity, with almost no response in the dark and a current density that scales with the applied voltage under illumination. Figure 4(c) shows the current response of the TiO2/FLG/Si composite for different voltages. In order to compare with the TiO2/Si sample we have subtracted the FLG background contribution and normalized it to the electrode area. Figure 4(d) shows the photocurrent for the TiO2/FLG/Si, after subtracting the graphene background contribution. The average value displayed during illumination is quite similar in magnitude to that observed in TiO2/Si. Nevertheless spike-like features were present at the photocurrent rise, upon switching on the light, followed by a rapid exponential decay. At the end of the cycle a cathodic current overshoot occurred when the light was switched off.
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This latter phenomenon has been reported for TiO2 by several authors as a typical fingerprint of intense surface charge recombination processes [22–24]. Its occurrence requires the existence of electronic surface states, above the valence band edge, which allows the production of extra photoinduced charge carriers during irradiation, even with wavelengths larger than that of the TiO2 band gap. These electronic states can also act as recombination centers, inducing a rapid photocurrent decay. When the light is switched off, the unoccupied states remain active as recombination centers, inducing a characteristic cathodic overshoot of current as displayed in our data in Figure 4. What is the exact origin of this effect in the TiO2/FLG/Si system remains to be explored: if it is really a surface effect at the TiO2/FLG interface or a bulk characteristic of the complete TiO2 film grown on FLG. If for a certain application a transient operation mode is a requirement and the “on” cycle is kept short with respect to the time constant for current decay, the use of the TiO2/FLG/Si composite is indeed more efficient than the TiO2/Si. In this case the use of the new composite becomes an advantageous option.
The results presented here indicate that the photoelectrochemical behavior is similar to the combined effect of both materials, with no synergistic effects in terms of the average photocurrent. This property opens the possibility of using the layered composite without a conducting substrate, for example, a low cost support, such as glass, with an electrode design that removes the charge directly from the graphene. On the other hand, this new layered material presents an interesting transient photoresponse where explicit benefits seem to be derived from the FLG/TiO2 combination.
4. Conclusions
The experimental evidence shows that TiO2 films grown on FLG/Si present similar morphological characteristics as those grown directly on Si. FTIR and Raman spectroscopy, together with SEM and optical images, show no significant changes between both systems. The average photoelectrochemical response of TiO2/FLG/Si to a sun simulator showed the expected behavior based on the combination of the base materials, which is an advantage for pursuing further uses of this particular composite. A singular effect is nonetheless detected in the transient response of this composite, in which the interaction TiO2/FLG seems to play a significant role. Future research is needed to optimize this system such as the direct contact of graphene with the external circuit for measuring their efficiency for H2 generation.
Competing Interests
The authors declare that there are no competing interests related to this paper.
Acknowledgments
The authors want to acknowledge the financial support of the following grants: Fondecyt 1121203 and Anillo ACT1108.