Improved Synthesis of Reduced Graphene Oxide-Titanium Dioxide Composite with Highly Exposed 001 Facets and Its Photoelectrochemical Response
Crystal facet engineering has attracted worldwide attention, particularly in facet manipulation of titanium dioxide (TiO2) surface properties. An improved synthesis by solvothermal route has been employed for the formation of TiO2 with highly exposed facets decorated on reduced graphene oxide (RGO) sheets. The RGO-TiO2 composite could be produced with high yield by following a stringently methodical yet simple approach. Field emission scanning electron microscope and high resolution transmission electron microscope imaging reveal that the structure consists of TiO2 nanoparticles covered with TiO2 nanosheets of exposed facets on a RGO sheet. The photocurrent response of the RGO-TiO2 composite was discovered to outperform that of pure TiO2, as a ~10-fold increase in photocurrent density was observed for the RGO-TiO2 electrodes. This may be attributed to rapid electron transport and the delayed recombination of electron-hole pairs due to improved ionic interaction between titanium and carbon.
Titanium dioxide (TiO2) has been widely studied owing to its nontoxic, chemically inert, photostable characteristics, and cheap production cost, which make it a promising candidate for energy and environmental applications [1–3]. Over the past decade, significant progress has been made in crystal facet engineering of TiO2 crystals, to enhance performance for applications, such as photocatalysis, lithium storage, dye sensitised solar cells, and gas sensors [4–7]. Generally, TiO2 crystals can be described in terms of three main structures: anatase, rutile, and brookite. In recent years, focus has been directed toward anatase TiO2 due to its higher activity, especially as a heterogeneous photocatalyst. The anatase phase of TiO2 has a truncated tetragonal bipyramid structure with two facets and eight facets . According to the Wulff construction method for determining equilibrium crystal shapes , anatase TiO2 crystals are usually dominated by facets (more than 90% of the total number of facets), which have a lower surface energy (0.44 J m−2) compared to the reactive facets (0.96 J m−2). Under equilibrium conditions, the lower surface energy plane is favourable during crystal growth to minimise the total surface energy, causing reactive facets to diminish quickly, while encouraging the dominance of the thermodynamically stable facets . Hence, the percentage of exposed reactive facets is greatly reduced, resulting in loss of performance in value-added applications . This field was started by Yang and coworkers, who produced crystals consisting of approximately 50% reactive facets by using hydrofluoric acid (HF) as a capping agent . Consequently, more effort is being directed toward improving the synthesis of TiO2 structures with a high percentage of exposed facets, including control of the concentration of precursor , selective use of solvents [14, 15], and use of fluorine-sourced chemicals (HF, NH4F), in which fluorine species were discovered to have the potential to stabilise facet growth [16, 17].
The performance efficiency of TiO2 crystals can also be enhanced by doping with metal ions or nonmetal ions, noble metal loading, or attaching the crystals onto large surface area materials such as graphene sheets [18–20]. Recently, graphene, a two-dimensional sheet of sp2 bonded carbon atoms densely packed in a honeycomb crystal lattice structure, has attracted much attention due to its exceptional electronic, biological, mechanical, and thermal properties [21–24]. The efficiency of graphene-based semiconductor photocatalysts is vastly increased compared to that of pure TiO2, which was evident in terms of extended light adsorption range, enhanced charge separation, and high dye adsorption . These beneficial characteristics of the composite materials would have great potential in enhancing photoelectrochemical performance applications. In addition, the role of the increased exposed facets of TiO2 on graphene could be demonstrated in its photoelectrochemical performance properties.
In this work, a fluorine-free solvothermal route is reported to produce highly exposed facets in anatase TiO2 nanosheets patterned on reduced graphene oxide (RGO) sheets. Photoelectrochemical behaviour was investigated based on the materials photocurrent response for various electrodes sintered at different temperatures.
2. Experimental Methods
Graphite flakes (3061, Asbury Graphite Mills Inc.), potassium permanganate (KMnO4, 98%), tetraisopropyl orthotitanate (TIPT) (98%), and diethylenetriamine (DETA) (98%) were supplied from Merck, USA. Sulphuric acid (H2SO4, 98%), phosphoric acid (H3PO4, 98%), hydrochloric acid (HCl), hydrogen peroxide (H2O2, 30%), potassium chloride (KCl), and isopropyl alcohol were purchased from Systerm (Malaysia). Absolute ethanol and ethanol (95%) were purchased from HmbG Chemical, Malaysia, and potassium hexa-cyanoferrate(III) was purchased from Sigma-Aldrich. Deionised water was used throughout sample preparation. All chemicals were used as received without further purification.
2.2. Preparation of Graphene Oxide (GO)
GO aqueous suspension was synthesized using a simplified Hummers method . Graphite flakes (3 g) were slowly added into a H2SO4 and H3PO4 (9 : 1) solution. Following this, 18 g of KMnO4 powder was gradually added into the mixture and stirred for 3 days to allow oxidation to occur. The obtained solution was transferred into a beaker containing ice, and 27 mL of 30% H2O2 was added. A yellowish-brown solution was observed and washed with 1 M HCl solution and deionised water.
2.3. Preparation of RGO-TiO2 Composite
The RGO-TiO2 composite was synthesized using a solvothermal route. GO aqueous suspension was freeze-dried for 24 h using a freeze dryer (Martin Christ, ALPHA 1-2/LD plus). Then, 40 mg of freeze-dried GO was dispersed in a solution containing 40 mL of isopropyl alcohol and gently sonicated for 1 hour. Following this, 0.824 mL of TIPT was added drop-wise into the GO solution (14 : 1), and then 30 μL of DETA was added and stirred for a few minutes. This step is crucial as it ensures the formation of the desired morphology. The solution was then transferred into a Teflon-lined autoclave and heated at 200°C for 24 h. The resulting black precipitate was washed with ethanol and left to dry in an oven at 60°C overnight. Finally, the obtained black powder was calcined at 400°C in air, with a heating rate of 1°C min−1 for 2 h. Pure TiO2 and RGO were synthesized for comparison purposes.
2.4. Fabrication of RGO-TiO2 Electrodes
The samples were ground and mixed with absolute ethanol to obtain a slurry paste. The paste was painted onto taped-down clean indium tin oxide (ITO) glass and then spread over using a glass rod. To study the effect of the sintering temperature on the photocurrent response, the ITO glass was sintered in a furnace under purified N2 gas flow at different temperatures: 300, 400, and 500°C. The fabricated electrodes were labeled as G-TiO2 300, G-TiO2 400, and G-TiO2 500, and a TiO2 500 electrode was fabricated for comparison purposes.
The crystallographic phases of the samples were analysed using X-ray diffraction (XRD, D5000, Siemens), with Cu Kα radiation ( Å) and a scan rate of 0.02° s−1. The morphology and structural properties were investigated using a field emission scanning electron microscope (FESEM, JEOL JSM-7600F) and a high resolution transmission electron microscope (HRTEM, JEOL JEM-2100F). X-ray photoelectron spectroscopy (XPS) measurements were performed using synchrotron radiation from beamline number 3.2 at the Siam Photon Laboratory in the Synchrotron Light Research Institute, Thailand. Raman and photoluminescence (PL) spectra were obtained using 514 nm and 325 nm laser beams, using a Renishaw inVia Raman microscope system. Electrochemical impedance spectra (EIS) measurements were carried out using a VersaSTAT 3 potentiostat (Princeton Applied Research) in a 0.1 M KCl solution containing 1 mM K4[Fe(CN)6].
2.6. Photocurrent Response Measurements
Photocurrent response measurements were carried out using a VersaSTAT 3 potentiostat and recorded in VersaStudio v2.2 data acquisition software. In a three-electrode cell configuration, the fabricated electrodes were employed as the working electrode a saturated calomel electrode was used as the reference electrode, and a Pt wire was used as the counter electrode. A 150 W Xenon lamp was used as a light source and all electrodes were dipped in 0.5 M KCl electrolyte. All the photoelectrochemical experiments were carried out in the presence of N2 atmosphere. The CA measurements were carried out at applied potential of 0.8 V under light “on-off” condition using Newport solar simulator with manual shutter.
3. Results and Discussion
The RGO-TiO2 composite was synthesized by mixing GO, TIPT, and DETA in a solution. Figure 1 illustrates two preparation routes to obtain RGO-TiO2 composites, using different mixing sequences. The mixing sequence of TIPT and DETA was found to influence the final product of synthesis. Figure 1(a) shows an initial synthesis route similar to Chen et al. , first mixing GO with DETA, and then adding TIPT. However, the stirred solution was found to be nonhomogenously dispersed and yielded irreversible agglomerations, as mixing GO with DETA causes flocculation of the GO sheets. Thus, after centrifugation, three different distinct layers were formed that can be attributed to TiO2, RGO-TiO2, and RGO. By altering the mixing sequence, using TIPT first instead of DETA, as shown in Figure 1(b), a homogeneously dispersed solution was observed, in which a single homogenous layer of RGO-TiO2 composite, without any evidence of pure TiO2 or RGO, was easily obtained. The distinguishable end-products of Figures 1(a) and 1(b) were clearly influenced by the sequence in which the chemicals were mixed. In Figure 1(a), GO could plausibly be reduced by the amine groups of DETA, not unlike the reduction of GO with NaOH. This gives rise to certain degree of aggregation caused by the π-π restacking interaction of RGO. When TIPT was added into the coagulate, only the exposed active sites of GO, which had not formed complexes with DETA, interacted with TIPT to form TiO2 on RGO during the solvothermal reaction. On the contrary, the order of addition of chemicals in Figure 1(b) allows complete reaction between GO and TIPT. After TIPT was introduced to GO, the network of TIPT molecules expanded, forming a cage-like continuous network structure that enabled maximum electrostatic interaction with GO. The addition of DETA stabilised the complex scaffold, leading to 100% formation of RGO-TiO2. Throughout this paper, the characterisation results are given for samples obtained using Figure 1(b) mixing sequence.
Figure 2 depicts the XRD patterns of graphite, GO, RGO, TiO2, and the RGO-TiO2 composite. The peak position at ° of the (002) graphite plane corresponds to a d-spacing value of 3.4 Å. It was observed that the graphite peak completely disappeared for GO, and a new peak position emerged at 9.7° with an increase of the d-spacing to 9.1 Å, which was thought to be due to exfoliation and oxidation of graphite to GO. On the other hand, a similar peak position to graphite was shown for RGO, but with a much broader peak, indicating the successful dispersion of GO sheets after the solvothermal reaction . Clearly, RGO was successfully synthesized through this route, by reduction and removal of the oxide functional groups from GO to RGO. Both TiO2 and RGO-TiO2 characteristic peaks at 25.2°, 37.8°, 48.1°, 53.9°, and 55.1° corresponded well to the (101), (004), (200), (105), and (211) crystal planes of anatase TiO2, with lattice constants of Å and Å (JCPDS 21-1272) . Therefore, no other TiO2 phases were detected. Additionally, the absence of RGO peak at 25.2° in the RGO-TiO2 composite was probably due to the relatively low diffraction intensity of RGO that was shielded by the (101) crystal peak of TiO2 .
Morphological features of the as-prepared samples were obtained using FESEM imaging. Pure TiO2 nanoparticles can be observed in Figure 3(a), agglomerating together, with a surface that appears rough. Increased magnification (Figure 3(b)) of the surface reveals the presence of randomly assembled TiO2 nanosheets, which will be discussed in the HRTEM section. Similar features were observed for the RGO-TiO2 composite (Figure 3(c)), as the agglomeration of TiO2 nanosheets was greatly reduced limiting them to the surface of RGO, indicating the successful anchoring of TiO2 nanosheets onto RGO sheets. With higher magnification, Figure 3(d) reveals that the surface of RGO-TiO2 is composed of two different types of TiO2 features: TiO2 nanoparticles and TiO2 nanosheets (white worm-like features due to folding of the TiO2 nanosheet) that cover the entire surface of the RGO.
HRTEM characterisation was employed to further determine the structure and planes of the samples. In Figure 4(a), the morphological similarities of agglomerated TiO2 crystals were consistent with the FESEM results; such a structure formed to minimise the surface energy . A magnified view, shown in Figure 4(b), reveals that the outer region was populated with thin layers of TiO2 that resemble sheet-like features. However, the entire inner sphere was populated with densely packed TiO2 nanoparticles. The output of these two different types of TiO2 nanostructures could be attributed to the Oswald ripening process and the use of DETA as the morphology controlling agent . At high magnification, the image of a TiO2 nanosheet edge (Figure 4(c)) reveals a lattice with fringe spacing of 0.19 nm that corresponds to either the or planes of anatase TiO2. According to Chen et al. , the TiO2 spheres formed have almost 100% of their planes exposed. It was noted that DETA played a key role, as the growth along the direction was retarded, stabilising the growth of the exposed facets and eventually leading to a higher amount of exposed facets. Similarly, the RGO-TiO2 composite (Figure 4(d)) reveals the same structure. Two different structures of TiO2 nanoparticles and nanosheets were wrapped around the RGO sheet (Figure 4(e)). Therefore, this exhibits two different fringe spacings, 0.19 nm and 0.351 nm in Figure 4(f), which correspond to the planes of TiO2 nanosheets and the thermodynamically stable planes of TiO2 nanoparticles [17, 33].
To investigate the chemical state of the as-prepared samples, XPS measurements was carried out; the results are shown in Figure 5. In Figure 5(a), a considerable degree of oxidation for the peaks at binding energies of 284.5 eV, 286.2 eV, 287.8 eV, and 289 eV in the C 1s of GO could be attributed to C–C, C–O, C=O, and O=C–O bonds [34, 35]. However, in comparison with RGO-TiO2, the oxide functional groups’ peak intensities were greatly reduced, which confirms that reduction from GO to RGO was successful. In addition, the small peak at 282.4 eV could be attributed to the Ti–C bond, which reflects the chemical bonding between titanium and carbon. For the Ti 2p peaks in Figure 5(b), two peaks centred at 453.5 eV and 459.2 eV were observed for TiO2, which correspond to the Ti 2p3/2 and Ti 2p1/2 spin-orbital splitting photoelectrons in the Ti4+ state . These peaks redshift to 453.8 eV and 459.6 eV for RGO-TiO2 due to the interaction between Ti and the oxygen groups of RGO. Since oxygen is much more electronegative than carbon, the TiO2 electron density is attracted more strongly, which then increases the binding energy of Ti in the composite . In addition, the calculated spin orbit splitting between Ti 2p3/2 and Ti 2p1/2 was 5.7 eV in both samples, which indicates the presence of normal states of Ti4+ .
Raman spectroscopy was employed for molecular morphology characterisation of carbonaceous materials. As shown in Figure 6, two peaks were located at 1353 cm−1 and 1597 cm−1 for the fabricated electrodes. These peaks correspond well to the documented D band, which is attributed to sp3 defects, and to the G band, which is due to in-plane vibrations of sp2 carbon atoms and a doubly-degenerated phonon mode ( symmetry) at the Brillouin zone centre . In addition, shifts at 142, 395, 514, and 637 cm−1 can be attributed to the , , , and modes of anatase TiO2 (inset) and are consistent with the XRD results . This also suggests that the fabricated G-TiO2 electrodes’ TiO2 anatase phase was retained, even with increased sintering temperature.
PL spectra of various samples were recorded, which reveal the surface structure and excited state of the semiconductor . In Figure 7, the PL intensity was greatly affected by the recombination rate of electron-hole pairs. A lower PL intensity indicates an enhanced charge separation of electron-hole pairs. A low PL emission intensity was depicted for the RGO-TiO2 composite, which was attributed to RGO sheets acting as an electron shuttle for TiO2 to hinder electron-hole recombination. Thus, the recombination rate was greatly suppressed. The inset of Figure 7 shows the PL spectra of the fabricated electrodes sintered at various temperatures. As shown, TiO2 500 has the highest emission intensity, greater than G-TiO2 500 or any of the composite electrodes, which is consistent with the previous explanation. The PL intensity decreases with increasing sintering temperatures, with G-TiO2 500 being the lowest. Wang et al. reported that thermal annealing of photocatalysts under N2 gas reduces the density of surface defects . Hence, the decrease of recombination points for the electron-hole pairs enhances charge separation efficiency.
In order to confirm the charge recombination kinetics of the TiO2 and RGO-TiO2 composite electrodes, EIS measurements were applied to understand the interfacial charge transfer process. The Nyquist plots of TiO2 and RGO-TiO2 composite electrodes were plotted in Figure 8 as the arc shape reveals information of the charge transport and recombination properties and a depressed semicircle was observed for RGO-TiO2 composite compared to TiO2. The charge transfer resistance () was determined by the diameters of the semicircles and RGO-TiO2 composite (48.244 ) has a lower value than TiO2 (83.192 ) indicating increase charge transport and reduced recombination rate that is in good agreement with the PL results [42, 43].
Photocurrent response measurements of the photo-generated electron-hole interaction of the fabricated electrodes were carried out under repeated ON-OFF cycles, and the results are shown in Figure 9. In this case, the photocurrent density is greatly affected by two significant factors: the time taken for the photo-induced electrons to withdraw from the TiO2 to the ITO substrate and the rate of electron-hole recombination within the film and the film/electrolyte interface . In the results shown, the photocurrent density under dark conditions is relatively small and negligible. A rapid current rise and decay was observed when the light was switched on and off. The effect of the sintering temperature of the RGO-TiO2 films on the photocurrent response was investigated. The output of the photocurrent density was observed to increase with higher sintering temperatures. The highest recorded photocurrent response was that of the G-TiO2 500 electrode (104.4 μA cm−2) which is around ten times higher than that of the G-TiO2 300 electrode (9.6 μA cm−2). The dramatic increase could be attributed to a delayed recombination rate and longer electron lifetime, as shown in the PL spectra [45, 46]. When comparing both the TiO2 500 and G-TiO2 500 electrodes, the photocurrent response of the G-TiO2 was observed to be better than that of the TiO2 500 electrode. This suggests that the presence of RGO sheets increases the photocurrent response due to their extensive two-dimensional π-π conjugation structure. Hence, photo-induced electrons from TiO2 can be accepted by RGO and transferred to the external circuit more quickly [47, 48]. Moreover, the charge separation efficiency increases due to the electronic interaction between TiO2 and RGO in the composite . The highly exposed facets of the TiO2 nanosheets in a hierarchical structure contribute to the photocurrent response by favouring electron transport, which minimises the grain interface effect and improves the light harvesting efficiency [50, 51].
RGO-TiO2 composite composed of highly exposed TiO2 facets was successfully synthesized through a solvothermal route with 100% yield of composite material. The advantage of this method harnesses the benefits of both the highly exposed facets for higher reactivity and RGO as an excellent platform with a large surface area, by using DETA instead of the commonly-used HF, which is hazardous and toxic to the environment. The role of the sintering temperature on the characteristics of the fabricated electrodes was remarkably interesting, as the highest photocurrent density, 104.4 μA cm−2, was observed for a sintering temperature of 500°C, which is attributed to the reduced recombination of electron-hole pairs and improved light harvesting efficiency. We anticipate that this study of the RGO-TiO2 composite with highly exposed facet features would aid in further improving photoelectrochemical performance applications.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The author would like to express his gratitude to the staff at photoemission spectroscopy (PES) synchrotron beamline 3.2a at the Synchrotron Light Research Institute, Thailand, for their assistance. This work was financially supported by the High Impact Research Grant of Ministry of Higher Education of Malaysia (UM.C/625/1/HIR/MOHE/05) and Postgraduate Research Grant (PPP) (PG107-2012B) of the University of Malaya.
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