International Journal of Photoenergy

Volume 2013 (2013), Article ID 768348, 9 pages

http://dx.doi.org/10.1155/2013/768348

Research Article

Influence of Ce Doping on the Electrical and Optical Properties of TiO₂ and Its Photocatalytic Activity for the Degradation of Remazol Brilliant Blue R

Aisha Malik,¹ S. Hameed,¹ M. J. Siddiqui,² M. M. Haque,³ and M. Muneer³

¹Department of Electrical Engineering, Aligarh Muslim University, Aligarh 202002, India

²Department of Electronics Engineering, Aligarh Muslim University, Aligarh 202002, India

³Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India

Received 31 May 2013; Revised 16 September 2013; Accepted 18 September 2013

Academic Editor: Jimmy Yu

Copyright © 2013 Aisha Malik et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

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.

1. Introduction

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 [1]. 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 [14]. 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 [15]. 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 [16]. 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 [21]. 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.

2.2. Apparatus

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).

Figure 1: Typical procedure of Ce-TiO₂ preparation by sol-gel method.

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 [22] 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 [23].

Table 1: Crystallite size of Ce-doped TiO₂ with different concentration of Ce. Calcination temp: 400°C and calcination time: 4 h.

Sample number
Ce concentration (%)
Crystallite size (nm)

1
0
8.2

2
1.0
7.2

3
3.0
6.8

4
5.0
6.4

5
7.0
6.0

Figure 2: XRD patterns of undoped TiO₂ and Ce-TiO₂ with different concentrations of Ce. Calcination temp: 400°C and calcination time: 4 h.

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

Figure 3: SEM micrographs of undoped TiO₂ and Ce-doped TiO₂.

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.

Figure 4: TEM micrographs of 7% Ce-TiO₂, Calcination temp: 400°C and calcination time: 4 h.

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.

Figure 5: FTIR of undoped and Ce-doped TiO₂.

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 [28]

Figure 6: Absorption spectra of undoped and Ce-doped TiO₂.

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 [31].

Table 2: Band gap energy of Ce-doped TiO₂ with different concentration of Ce. Calcination temp: 400°C and calcination time: 4 h.

S. no.
Ce concentration (%)
Band gap (eV)

1
0.0
3.07

2
1.0
3.04

3
3.0
3.01

4
5.0
2.87

5
7.0
2.72

6
8.0
2.70

7
9.0
2.66

8
10
2.69

3.2. Electrical Properties

3.2.1. Impedance Analysis

The real part (Z′) and imaginary part (Z′′) of impedances with frequency at different concentrations of doped TiO₂ are plotted in Figures 7 and 8.

Figure 7: Variation in real impedence Z′ as a function of frequency and compositions.

Figure 8: Variation in imaginary impedence Z′′ with frequency.

The Z′ and Z′′ decrease with the increase in frequency because of the space charge polarization [32]. 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.

Figure 9: Variation in dielectric loss with frequency at different compositions.

Figure 10: Variation in dielectric constant with frequency for different compositions.

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 [33]. 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 [16].

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.

Figure 11: Variation of a.c. conductivity with frequency for different compositions.

The overall conductivity is the sum of a.c. conductivity and d.c. conductivity which is given by

where ω: angular frequency, ε o: permittivity of free space, and σ o(T) is the d.c. conductivity and independent of frequency, whereas ω εoε r 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 [15]. 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 [34].

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.

Figure 12: Change in absorbance as a function of time on irradiation of an aqueous solution of Remazol Brilliant Blue R in the presence of Ce-doped TiO₂. Experimental conditions. Reaction vessel: immersion well photochemical reactor made up of Pyrex glass, light source: visible light halogen linear lamp (500 W, 9500 Lumens), photocatalyst: Ce-doped TiO₂ (1 g L⁻¹), dopant conc. (Ce) 9% (w/v), Dye (0.05 mM), volume (200 mL), continuous stirring, and air purging. Calcination temperature: 400°C, calcination time: 4 h, and irradiation time: 90 min.

Figure 13: Change in concentration as a function of time on irradiation of an aqueous solution of Remazol Brilliant Blue R in the presence and absence of Ce-doped TiO₂. Experimental conditions. Reaction vessel: immersion well photochemical reactor made up of Pyrex glass, light source: visible light halogen linear lamp (500 W, 9500 Lumens), photocatalyst: Ce-doped TiO₂ (1 gL⁻¹), dopant conc. (Ce) 0, 1, 3, 5, 7, 8, 9, and 10% (w/v), Dye (0.05 mM), volume (200 mL), continuous stirring, and air purging. Calcination temperature: 400°C, calcination time: 4 h, and irradiation time: 90 min.

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 [37]. 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.

4. Conclusion

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.

Acknowledgments

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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