Influence of Ce Doping on the Electrical and Optical Properties of TiO2 and Its Photocatalytic Activity for the Degradation of Remazol Brilliant Blue R
Nanocrystalline TiO2 particles doped with different concentrations of Cerium (Ce, 1–10%) have been synthesized using sol-gel method. The prepared particles were characterized by standard analytical techniques such as X-ray diffraction (XRD), FTIR and Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). The XRD analysis shows no change in crystal structure of TiO2 after doping with different concentrations of Ce, which indicates the single-phase polycrystalline material. The SEM analysis shows the partial crystalline nature of undoped, and doped TiO2 and TEM analysis shows the particle sizes were in the range of 9–14 nm in size. The a.c. analysis shows that the dielectric constant and dielectric loss tan decrease with the increase in frequency. The dielectric property decreases with the increase in dopant concentration. It is also observed that the impedance increases with an increase in dopant concentration. The photocatalytic activity of the synthesized particles (Ce-doped TiO2) with dopant concentration of 9% (Ce) showed the highest photocatalytic activity for the degradation of the dye derivative Remazol Brilliant Blue R in an immersion well photochemical reactor with 500 W halogen linear lamp in the presence of atmospheric oxygen.
Due to the large surface area for the interaction of visible light, the synthesis of nanostructured semiconductor particles attracted greater attention during last few years . TiO2, a semiconductor has received the greater attention due to nontoxic, easily available, resistant to photocorrosion, and high catalytic efficiency [2, 3]. Due to the large band gap of TiO2 (3.2 eV), the material limits the use of visible light. This property inhibits the use of solar spectrum as a light source. To enhance the catalytic property of TiO2 under the visible light many attempts have been made to change the physical and chemical compositions of TiO2 by doping with metals/nonmetals such as V, Cr, Mn, Fe, Ni, and Cu. In order to extend the optical absorption of the catalyst to the visible spectrum region many studies have been reported employing different methodologies [4–9]. The various methods used for doping of TiO2 involve ion implantation, sol-gel reaction, hydrothermal reaction, solid-state reaction, and so forth [10–13]. Out of which the sol-gel process is undoubtedly the simplest and the cheapest one and also it provides control on the size and shape of nanoparticles. As it is well known that UV irradiation requires high energy source and is costlier and hazardous . Hence, the use of visible light is preferred.
To characterize the electrical property impedance spectroscopy is an effective method. It can resolve the grain and grain boundary contribution to the system and calculate the conductivity and dielectric constant . Recently much more attention has been given to the TiO2 because of having stable dielectric properties which is characterized by high relative dielectric constant and low dielectric loss . It is having an important application such as high density, dynamic memory device, microelectronics device, and microcommunication system [17–20].
The present paper deals with the synthesis of Ce-doped TiO2 with different concentrations of metal ions using sol-gel method followed by characterization using standard analytical techniques such as X-ray diffraction (XRD), UV-Vis spectroscopy, Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). Also in this work, the computer control LCR meter analyser is used for Complex impedance spectroscopy for a wide range of frequencies. The impedance analysis gives the maximum possible information about the material . The activity of the synthesized photocatalyst was also tested by studying the degradation of a textile dye derivative Remazol Brilliant Blue R under visible light source.
2. Experimental Details
2.1. Reagents and Chemicals
High purity Titanium isopropoxide and Remazol Brilliant Blue R were purchased from Sigam-Aldrich; Ammonium ceric nitrate, 2-propanol and ethanol were purchased from Rankem, CDH, and SD Fine-Chem. Ltd. India, respectively.
The structural characterization of undoped TiO2 and doped TiO2 nanoparticles was performed by XRD in the 2θ range of 20–80° (RigakuMiniflex II) with Cu Kα radiations (λ = 1.5418 Å) operated at voltage of 30 kV andcurrent of 15 mA. The UV-Vis spectra were recorded at room temperature in the range of 300–800 nm using Shimadzu UV-Vis spectrophotometer (Model 1601). The morphology and size were observed by SEM (Leo 435 VP) and TEM (Jeol 2100F). Electrical properties were measured using computer control LCR meter analyser (Model Agilant 48).
2.3. Preparation of TiO2 and Ce-TiO2
Undoped TiO2 was prepared by sol-gel process with titanium isopropoxide as the titania precursor. For this, twenty five mL water was first dissolved in twenty five mL of 2-propanol. The second solution was prepared by dissolving 0.1 M titanium isopropoxide completely in 125 mL of 2-proponal. Both the solutions were sealed immediately and stirred rapidly using magnetic stirrer to obtain homogeneous solutions. The 2-propanol solutions containing water were then added dropwise to the alkoxide part under continuous stirring. This would result in the hydrolysis of titanium isopropoxide due to reaction with water by changing the colour of the solution from colourless to white. After the complete addition of the water part of the solution to that of the alkoxide part, the resulting solution was stirred overnight and then filtered under reduced pressure. For doping of TiO2 particle with Ce, a known concentration of (NH4)2Ce(NO3)6 (1–10%, w/v) was added to the solution containing water and 2-propanol which was then added to titanium isopropoxide solution. The residue was washed with double distilled water and ethanol several times and then kept in oven at 100°C for complete removal of water and solvent. The dried residue was manually grinded in agate mortar and then calcined at 400°C for 4 h (Figure 1).
The synthesized nanoparticles were regrind and mixed with binder (polyvinyl alcohol), the mixture was then heated at 300°C to burn out the binder. Further a pellet of 13 mm in diameter and of thickness 1.8 mm were made by applying pressure up to 7 tonn/cm2 on the powder sample by using hydraulic press. The pallet was then used for electrical measurement by coating the opposite faces with silver paste to form parallel plate capacitor.
2.4. Photocatalytic Experiments
The solutions of desired concentration of dye derivatives Remazol Brilliant Blue R (0.05 Mm) were prepared in double distilled water. An immersion well photochemical reactor made of Pyrex glass was used in this study. For irradiation experiments, an aqueous solution (200 mL) of dye was taken into the reactor and the required amount of photocatalyst (TiO2/doped TiO2, 1 gL−1) was added and the solution was stirred and bubbled with atmospheric oxygen for at least 15 minutes in the dark to allow equilibration of the system so that the loss of dye due to adsorption can be taken into account. The zero time reading was obtained from a blank solution kept in the dark but otherwise treated similarly to the irradiated solution. Irradiations were carried out using a visible light halogen linear lamp (500 W, 9500 Lumens). Samples (10 mL) were collected before and at regular intervals during the irradiation and centrifuged before analysis.
3. Results and Discussion
3.1. Optical Properties
3.1.1. X-Ray Diffraction
The structural characterization of pure and Ce-doped TiO2 nanoparticles was performed by X-ray diffraction (XRD) in the 2θ range of 20–80° (Rigaku Miniflex II) with Cu Kα radiations (λ = 1.5418 Å) operated at voltage of 30 kV and current of 15 mA. Figure 2 shows the crystal structure of undoped TiO2 and Ce-doped TiO2 as determined by X-ray diffraction. The diffraction patterns of doped powders were similar with those of pure anatase TiO2. The average anatase crystallite size was determined by Scherer formula  as given by where D = crystallite size, K = shape factor, λ = wavelength, θ = diffraction angle, and β = full width at half maximum. The mean size of the crystallites in samples was estimated by the FWHM of the XRD peak (101) using the Debye-Scherer equation, as given in Table 1. The data obtained from the Scherer equation shows decrease in crystallite size with an increase in Ce content. This trend can be explained on the basis of the fact that the addition of dopant may hinder the growth of TiO2 particle to some degree .
3.1.2. Scanning Electron Microscopy
The microstructural characterization of TiO2 and Ce-doped TO2 nanoparticles was carried out by Scanning Electron Microscopy. The morphology of undoped and 7% Ce-doped TiO2 particles calcined at 400°C for 4 h are presented in Figures 3(a) and 3(b), respectively. The SEM images of undoped and doped TiO2 at different magnifications show the partial crystalline nature. After doping the morphology of doped material remains unchanged, showing the amorphous nature with rough surfaces.
(a) Undoped TiO2 Calcination temp: 400°C and calcination time: 4 h
(b) 7% Ce-TiO2 Calcination temp: 400°C, calcination time: 4 h
3.1.3. Transmission Electron Microscopy
The morphology of 7%-doped TiO2 particles calcined at 400°C using Transmission Electron Microscopic technique (TEM) is shown in Figure 4. The TEM image of 7% Ce-doped TiO2 shows that particles are spherical and fully crystallized. The particle size is found to be 9–14 nm.
3.1.4. FTIR Absorption Analysis
The FTIR spectra of Ce-doped TiO2 (0–7%) powder calcined for 4 h at 400°C are presented in Figure 5. The presence of transmittance bands between 3400 and 3600 cm−1 are seen, which increased by doping Ce in TiO2 while transmittance bands at 1625 cm−1 decreases with increase in doping Ce content in TiO2. These bands are attributed to the stretching vibrations of the O–H groups and the bending vibrations of the adsorbed water molecules, respectively [24, 25]. A band between 650 and 830 cm−1 is seen which is attributed to different vibrational modes of TiO2. Anatase phases of TiO2 exhibit strong FT-IR absorption bands in the regions of 850–650 cm−1 [26, 27]. The band seen below 1200 cm−1 is due to Ti–O–Ti vibrations.
3.1.5. UV-Vis Absorption Spectra
The UV-Vis absorption spectra of undoped TiO2 and Ce-doped TiO2 with different dopant concentrations of Ce was studied and the results are shown in Figure 6. The band gap energies of undoped and Ce-doped TiO2 particles with the obtained wavelength from UV-Vis absorption spectra were calculated using the following 
The band gap energies of undoped and Ce-doped TiO2 with dopant concentrations (1–10%) are listed in Table 2. As expected, incorporation of dopant (Ce) into TiO2 lattice has been found to shift the fundamental absorption edge towards the longer wavelength, that is, red shift, which decreases the band gap energy upto 9% dopant concentration [29, 30]. Further increase in dopant concentration (10%) leads to increase in band gap energy. This may be due to the deposition of the metal on the photocatalyst which covered the surface of TiO2 and can reduce the effective surface area for absorbing light .
3.2. Electrical Properties
3.2.1. Impedance Analysis
The Z′ and Z′′ decrease with the increase in frequency because of the space charge polarization . At low frequency the complex impedance values are higher which indicate the larger polarization, whereas at higher frequency the complex impedance shows independent behaviour.
3.2.2. Dielectric Studies
The variation of dielectric constant (έ) and tangent loss (tanδ) with frequency for various concentration of doped TiO2 are plotted in Figures 9 and 10 at room temperature. The dielectric properties of a material depends upon different types of polarization, that is, dipolar, electronic, atomic, and space charge polarization.
The values of έ and tan δ decreases with an increase in frequency. At low frequency, the dielectric constant increases due to the accumulation of charge at grain boundary and the sample and electrode interface with each other also called space charge polarization . As the frequency increases the dielectric constant decreases due to the space charge polarization, which diminishes gradually; hence, the electronic and atomic contribution dominates. The dielectric constant is independent at higher frequency indicating the domination of electronic and atomic contribution .
3.2.3. a.c. Conductivity
The variation of a.c. conductivity with frequency for different concentration of doped TiO2 at room temperature are given in Figure 11.
The overall conductivity is the sum of a.c. conductivity and d.c. conductivity which is given by where ω: angular frequency, εo: permittivity of free space, and σo(T) is the d.c. conductivity and independent of frequency, whereas ωεoεr tan δ is the a.c. conductivity which is dependent of frequency.
The conductivity increases with the increase in frequency as the electron hopping frequency enhances . It also shows that the a.c. conductivity decreases with the increase in dopant concentration. This may be due to the decrease in the particle size, resulting in increase in the ratio of the surface volume, which indicate the corresponding occurrence of scattering .
3.3. Photocatalytic Activity
3.3.1. Effect of Different Percentage of Ce Doping on the Photocatalytic Activity of TiO2
An aqueous suspension of Remazol Brilliant Blue R (0.05 mM, 200 mL) in the presence of undoped TiO2 or Ce-TiO2 (with different concentrations of Ce varying from 1 to 10% calcined at 400°C) was irradiated with 500 W linear visible light halogen lamp at different time interval with constant bubbling of atmospheric oxygen. The degradation of the dye was monitored by measuring the change in absorbance as a function of irradiation time. As a representative example Figure 12 shows the change in absorption intensity of the dye as a function of irradiation time in the presence Ce-doped TiO2 (9% dopant). Figure 13 shows the change in concentration of the dye as a function of irradiation time in the presence of Ce-doped TiO2 (1–10%) and absence of Ce-doped TiO2. It could be seen from the figure that in the presence of undoped TiO2, very little change in concentration was observed. Whereas in the presence of doped TiO2 degradation of the dye increases with the increase in dopant concentration and the highest degradation was observed with 9% Ce-doped TiO2 and further increase in the dopant concentration leads to little decrease in the efficiency.
The increase in the photocatalytic activity by increasing the dopant concentration from 1 to 9% may be due to the shortening of band gap thereby effectively absorbing the light of longer wavelength. Another reason for the increase in the photocatalytic activity by increasing the dopant concentration could be attributed to the fact that the doping of TiO2 with Ce introduces new trapping sites which affects the life time of charge carriers by splitting the arrival time of photogenerated electrons and holes to reach the surface of photocatalyst and thus electron-hole recombination is reduced. At higher dopant concentration (10%) there is occurrence of multiple trapping of charge carriers and hence the possibility of electron-hole recombination increases [35, 36] and fewer charge carriers will reach the surface to initiate the degradation of the dye; hence, decrease in degradation efficiency at higher dopant concentration was observed.
The slow degradation of Remazol Brilliant Blue R in the presence of undoped TiO2 under visible light irradiation could be due to the direct absorption of light by the dye molecule and can lead to charge injection from the excited state of the dye to the conduction band of the semiconductor as summarized in the following equations:
The mechanism of TiO2 doped with Ce could be visualised as follows. Doping of TiO2 with Ce introduces a new energy level (Ce impurity level) by the dispersion of metal nanoparticles in the TiO2 matrix which acts as electron trap . The trap of electron can inhibit electron-hole recombination during irradiation thereby increasing the lifetime of charge carriers. The doping of TiO2 with Ce not only improves the separating efficiency of photoinduced electrons and holes, but it also increases the visible light absorption due to shortening of band gap. The mechanism of doped TiO2 can be represented by the following equations: M corresponds to doped Metal.
On absorption of photon of energy equal to or greater than its band gap by the TiO2 particle, an electron may be promoted from the valence band to the conduction band () leaving behind an electron vacancy or “hole” in the valence band . Similarly, an electron may be promoted from impurity level to conduction band of TiO2 by absorbing photon of energy equaling to or greater than its band gap. The vacancy created in the impurity band acts as electron trap. The electron generated in the valence band of TiO2 is trapped by the electron trap thereby reducing the electron-hole recombination. If charge separation is maintained, the electron and hole may migrate to the catalyst surface where they participate in redox reactions with sorbed species. Specially, may react with surface bound H2O or OH− to produce the hydroxyl radical and () is picked up by oxygen to generate superoxide radical anion.
The doping of Ce into TiO2 lattice shifts the position of its fundamental absorption edge towards the longer wavelength and reduces its band gap energy so that it can absorb energy from a major portion of visible light. The XRD analysis shows no change in crystal structure of TiO2 after doping with different concentration of Ce indicating single-phase polycrystalline material. The SEM and FTIR analysis shows the crystalline nature and anatase phase of undoped and doped TiO2, respectively. The TEM analysis shows that particles are in the range of 9–14 nm in size. In the low frequency region the dielectric constant decreases with increase in frequency, whereas in the high frequency region it shows the frequency independent behavior, and at high frequency dielectric loss is constant so it can be used for high frequency devices. Ce-TiO2 also shows that a high dielectric constant and low dielectric loss with frequency imply that the material is suitable for microelectronic device applications. As the frequency increases the magnitude of complex impedance decreases resulting in the increase in a.c. conductivity. It is also observed that the impedance increases with the increase in dopant concentration, resulting in the decrease in a.c. conductivity. The doped TiO2 was found to be more efficient for the degradation of dye under visible light source as compared to undoped TiO2.
Conflict of Interests
The authors declare that they do not have any financial relation with the commercial identities or associative interests that represents a conflict of interest in connection with the work submitted.
Financial Support by the Council of Science and Technology, U.P. for providing young scientist award under young scientist scheme to Dr. M. M. Haque (Ref no. CST/YSS/D-3916) and UGC New Delhi for granting Maulana Azad National Fellowship for Minority Student to Aisha Malik (no. F.40-3(M/S)/2009(SA-III/MANF)), and the Centre for Excellence in Nano-Technology, Department of Applied Physics and Department of Chemistry, Aligarh Muslim University, Aligarh for providing necessary facilities are gratefully acknowledged.
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