Ce doped RGO-TiO2 composite films on ITO substrates were prepared by sol-gel process using tetrabutyl titanate and reduced graphene oxide (RGO) as the starting materials. The sample was designed for the photoelectrocatalytic applications. The obtained samples were characterized by X-ray diffraction, UV-vis absorption spectroscopy, scanning electron microscopy, and Fourier transformed infrared spectroscopy. The results showed that doping of Ce on RGO-TiO2 composite film inhibited the TiO2 anatase-rutile phase transformation. In this case, Ce atoms could serve as dispersion oxide and suppress the recombination of photoinduced electron-hole pairs. Besides, the change in absorbance from UV to visible region was observed in Ce doped RGO-TiO2 nanocomposite films. The Ce doped RGO-TiO2 composite film showed higher photoelectrochemical performance than that of RGO-TiO2 composite and pure TiO2 under solar simulator irradiation. The main reason might be attributed to the optimum content of Ce that could act as electrons acceptor to hinder the recombination loss and facilitate the better transportation for photoinduced charge carriers.
Today, high performance photoactive electrode is considered to be one of the probable solutions for utilization of solar energy. Efficient photoelectrocatalysis (PEC) process is greatly dependent on such an electrode surface which shows high photon absorption, less photocorrosion, mesoporous surface structure, and highly dispersed uniform active sites [
After the invention of graphene in 2004, this two-dimensional sp2 bonded honeycomb-like carbon nanomaterial brought a massive change in electronics and solar and energy management devices. Reduced graphene oxide (RGO) support on photoelectrodes showed high electron mobility and high thermal resistivity and stability [
Nevertheless, the difficulties of catalyst recovery, low visible light absorption, and fast recombination of electron-hole pairs are still the main pitfalls of using powder photocatalyst in aqueous media [
The photocatalytic activity of RGO-TiO2 composite can be more enhanced either by promoting the light absorption or by suppressing the electron-hole recombination rate through the incorporation of other species to the binary composites [
Schematic diagram of the experimental procedure.
Tetra-n-butyl orthotitanate (TBOT) was procured from Sigma-Aldrich and all other chemicals used in this work were of analytical grade. The indium tin oxide (ITO) coated conducting glass plates (0.7 mm thickness) were procured from Osaka, Japan. Graphene oxide was prepared via modified Hummer’s method [
ITO coated glass substrate was cleaned with acetone, dehydrated alcohol, and ultrapure water, respectively. The powder photocatalyst was deposited on the ITO coated substrate via electrophoretic deposition method [
XRD spectra were recorded with a powder X-ray diffractometer (type Bruker D8 Advance equipped with EVA diffract software, Germany) over the range
All photoelectrochemical experiments were carried out with an Autolab PGSTAT302N potentiostat/galvanostat (EcoChemie, The Netherlands) with a three-electrode quartz cell. Ce-RGO-TiO2/ITO, RGO-TiO2/ITO, and TiO2/ITO modified electrodes were used as the working electrode. A saturated calomel electrode (SCE) was used as the reference and a platinum wire was used as the counter electrode. A 0.1 M Na2SO4 solution was used as a supporting electrolyte. An active area of 1.0 cm2 working electrodes for each sample was used in the photocurrent experiments. A 150 W solar simulator was used as a light source to study the photoelectrochemical response (Figure
Quartz made photoelectrocatalytic (PEC) reactor with solar simulator.
The samples were collected by scraping off the pure TiO2, RGO-TiO2, and Ce doped RGO-TiO2 from the ITO coated electrodes for XRD measurements to confirm that the samples are well crystallized. According to JCPDS data (reference code 832243), the peaks at 25.4° (1 0 1), 38.5° (1 1 2), and 48.6° (2 0 0) are the characteristic diffraction pattern of the anatase phase. The diffraction patterns are shown in Figure
XRD pattern of pure TiO2, RGO-TiO2, and Ce-RGO-TiO2.
The crystallite size for the RGO-TiO2 was between 40 and 62 nm which is similar to the pure TiO2 nanoparticles. So, it can be concluded that RGO has no effect on the crystal structure of TiO2 [
Figure
SEM image of (a) RGO-TiO2 and (b) Ce-RGO-TiO2 coated ITO substrate.
The morphology of Ce-RGO-TiO2 was also observed with TEM. Figure
TEM image of Ce-RGO-TiO2.
The optical absorption spectra of the samples are shown in Figure
Effect of Ce loading on UV absorbance in RGO-TiO2 photocatalysts.
A reduced band gap value was expected but no significant change in band gap energy was observed. This could be due to the aggregation of RGO material and the electron trap effect of ceria nanoparticles. The band gap energy was not much reduced and maximum absorption was still in the UV region for the modified TiO2 samples. Nevertheless, more photoelectrons can be produced using UV light source and it will ease the photocatalytic reduction process due to available electrons on the catalyst surface. It is clear that both RGO and Ce particles showed an enhanced photon absorption property in this respect.
The FTIR spectra of the samples were presented in Figure
FTIR spectra of Ce doped RGO-TiO2 composites.
The cyclic voltammetry for RGO-TiO2/ITO and Ce-RGO-TiO2/ITO modified electrodes are shown in Figure
Peak current table for RGO-TiO2 and Ce doped RGO-TiO2 thin films (values in mA/cm2).
Peak current | RGO-TiO2 thin film | Ce doped RGO-TiO2 thin film | ||
---|---|---|---|---|
Dark | Light | Dark | Light | |
+ve extraction peak current | 0.3 | 0.3 | 0.2 | 0.7 |
−ve injection peak current | 0.4 | 0.4 | 0.4 | 0.6 |
Cyclic voltammogram of catalytic activity with RGO-TiO2/ITO and Ce-RGO-TiO2/ITO modified electrodes in both dark and visible light irradiation.
It can be concluded that Ce atoms can absorb photons and transfer to the electrons that gain more energy to produce photocurrent. 1% Ce doping showed an optimal photocurrent value of 0.74 mA/cm2. The RGO-TiO2 sample showed another minor peak at 0.58 V both in dark and under light illumination. But that peak vanished in Ce-RGO-TiO2 sample. The reason behind this could be due to the defect formation in octahedral TiO2 structure from the doping with Ce. The regular TiO2 octahedron structure became distorted and highly amorphous. This structure provided more open spaces in the crystal lattice and facilitated electron trapping. That could hide the minor peak current formation.
The photoelectrochemical performance is largely dependent on charge transfer and recombination properties of a photocatalyst material. In this case, electrochemical impedance spectrum (EIS) is very useful tool to investigate the charge carriers transfer and recombination processes at semiconductor/electrolyte interfaces. The samples were investigated for EIS responses and the components of complex impedance
The EIS responses (Nyquist plots) of the RGO-TiO2 and Ce doped RGO-TiO2 film electrodes with and without light irradiation at open circuit potential (in dark) and at open circuit potential under visible light irradiation.
The values were measured at open circuit potential for both dark and light irradiation periods. It can be observed that the samples formed pseudoarcs in the Nyquist plot for both dark and light irradiation periods. Under light irradiation, the photogenerated electrons moved from the electrode surface to the outer circuit. This phenomenon helped to reduce the interface resistance. Thus, the diameter of the loops was also reduced under light irradiation. The diameter of the loops was reduced for Ce doped RGO-TiO2 samples as well. In fact, graphene can also show synergistic effect with doped metal atoms (here Ce) in terms of reducing charge transfer resistance [
Warburg impedance simulation cell (equivalent circuit with mixed kinetic and charge transfer control).
It can be simulated with Warburg semi-infinite diffusion model where a double layer capacitance and a charge transfer impedance are added with Warburg diffusion impedance. Here, polarization that occurred was influenced by the kinetic and diffusion processes and it can be interpreted by the Warburg model simulation. Here,
A simple mechanism of photoelectron excitation was illustrated in Figure
A schematic diagram of Ce doped RGO-TiO2 composite and a rough presentation of photoelectron release mechanism.
The Ce atoms also act as a charge carrier trap. And finally, with the help of a little outer circuit bias, the electrons move steadily towards counter electrode. RGO
When a semiconductor is dipped into an electrolyte containing redox species, the chemical potentials of electrons on both semiconductor and redox species will try to be in equilibrium. The charge transfer across the semiconductor surface will generate a space charge layer and band bending will occur. This is to minimize the effect of space charge layer in the semiconductor. Eventually, a potential barrier is established so that further electron transfer could not occur. Under illumination, the space charge layer is weakened due to electron-hole separation [
Ce doped RGO-TiO2 composite thin films were prepared by sol-gel and electrophoretic deposition method. The sample showed enhanced photoelectrocatalytic activity compared to pure TiO2 and RGO-TiO2 composite films. The experiment was conducted under visible light irradiation. ITO immobilized Ce doped RGO-TiO2 composite film can be an effective photocatalyst material with the assistance of electrochemical activity. Cerium atom absorbs photon at a higher degree and active photogenerated holes and electrons can be produced by applying an external bias. The excitons can be rectified towards efficient oxidation-reduction reaction. ITO glass substrate can provide a better route for the redox reactions to take place. Above all, the sample showed better stability even after 5-hour irradiation period. Further study is needed to understand the enhanced material efficiency.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank University of Malaya for sponsoring this work under National Nanotechnology Directorate (NND-53-02031090), High Impact Research (HIR-F-000032) and IPPP grant (PG043-2012B) for their cordial support to complete this work.