Effect of Cobalt Doping on the Phase

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tures: anatase, rutile, and brookite. Different structures lead to different physical properties, which, in turn, are the basis of various applications of TiO2.
Copyright © 2005 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. X, 1–7, 2005

Effect of Cobalt Doping on the Phase Transformation of TiO2 Nanoparticles M. A. Barakat,1 3 G. Hayes,1 and S. Ismat Shah1 2 ∗ Department of Materials Science and Engineering University of Delaware, Newark, Delaware 19716, USA 2 Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA 3 Central Metallurgical R & D Institute (CMRDI), P.O. Box 87, Helwan 11421, Cairo, Egypt

RESEARCH ARTICLE

1

Co-doped TiO2 nanoparticles containing 0.0085, 0.017, 0.0255, 0.034, and 0.085 mol % Co(III) ion dopant were synthesized via sol–gel and dip-coating techniques. The effects of metal ion doping on the transformation of anatase to the rutile phase have been investigated. Several analytical tools, such as X-ray diffraction (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray analysis (EDAX) were used to investigate the nanoparticle structure, size distribution, and composition. Results obtained revealed that the rutile to anatase concentration ratio increases with increase of the cobalt dopant concentration and annealing temperature. The typical composition of Co-doped TiO2 was Ti1−x Cox O2 , where x values ranged from 0.0085 to 0.085. The activation energy for the phase transformation from anatase to rutile was measured to be 229, 222, 211, and 195 kJ/mole for 0.0085, 0.017, 0.0255, and 0.034 mol % Co in TiO2 , respectively.

Keywords:

1. INTRODUCTION Titanium dioxide, TiO2 , occurs in three main crystal structures: anatase, rutile, and brookite. Different structures lead to different physical properties, which, in turn, are the basis of various applications of TiO2 . For instance, anatase is commonly used for photocatalysis because of its high photoreactivity,1 and rutile is largely used for pigments due to its effective light scattering properties.2 Therefore, understanding of the mechanism and the factors affecting phase stability and phase transformation is important. This will help in designing and controllably manipulating relative phase types and their concentrations for more efficient uses. With the increase of temperature, the possible phase transformation sequences among three TiO2 main structures can be described as anatase to rutile, anatase to brookite to rutile, brookite to rutile, and ∗

Author to whom correspondence should be addressed.

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brookite to anatase to rutile. The transformation behavior depends on several parameters such as the initial particle size,3–5 impurity (doping) concentration,6 7 starting phase,8 9 reaction atmosphere,10 etc. Rutile is the stable phase in bulk at room temperature, while phase stability of nanoparticles depends on the particle size. Zhang and Banfield3 demonstrated that the macrocrystalline rutile is more stable than macrocrystalline anatase, but the stability reverses when particle size becomes less than 14 nm. Nanoparticles are distinguished from bulk due to their high surface to volume ratio that causes the structural and electronic changes. These changes induce other properties to become different from that of the bulk. Unique photophysical, photochemical, photoelectronic, and photocatalytic properties can occur in semiconductor nanoparticle systems.11 12 It has been reported that many metal oxides can be easily formulated in the thin film form by using sol– gel and dip coating techniques.13–15 Titania films prepared

1533-4880/2005/X/001/007/$17.00+.25

doi:10.1166/jnn.2005.087

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by this method can be of high purity and the process is relatively cheap. The latter advantage results from the availability of high-purity chemical precursor materials in conjunction with the simplicity of sol–gel preparation at room temperature. Metal doping can alter the phase transformation and, consequently, the photocatalytic activity of TiO2 . One dopant of interest for photocatalysis and other electromagnetic applications is cobalt. Several techniques have been used for the preparation of Co-doped TiO2 . The effects of Co on the photocatalytic properties of TiO2 have been investigated by several authors.16–19 The effective photoexcitation of TiO2 semiconductor particles requires application of light with energy equal to or higher than its band-gap energy (Eg ). For anatase the band gap, Eganatase , is 3.2 eV and for rutile, Egrutile , it is 3.02 eV, which correspond to the absorption thresholds of 380 and 410 nm for the two polymorphs of titania, anatase, and rutile, respectively.20 Consequently, only the UV portion of the solar radiation is useful in the photoexcitation processes involving pure TiO2 . The incorporation of metal ions into the titania crystal lattice can significantly extend the absorption edge into the visible region.21–27 However, the photoactivity of the doped TiO2 photocatalysts depends substantially on the nature of the dopant ion and its concentration, in addition to the method of preparation and the thermal and reductive treatments involved in the synthesis.21–23 Recently, Co-doped TiO2 films have also attracted considerable attention due to their potential application as diluted magnetic semiconductors (DMS). Anatase structure Co-doped TiO2 films were reported to have room temperature ferromagnetism (FM).28–30 It has also been observed that cobalt-doped TiO2 growth conditions are more likely to be O-poor.28 29 O-poor conditions lead to roughly equal concentrations of substitutional and interstitial Co, n-type behavior resulting from thermal excitation of electrons from interstitial Co into the conduction band, and an average magnetic moment of 1.26 b . The anatase phase in metal-doped TiO2 is most important for the magnetic and photocatalytic properties. It is, therefore, desirable to stabilize the anatase phase of TiO2 . Therefore, a thorough understanding of the effect of the dopant on the phase transition is desirable. The aim of this work is to study the effect of Co doping on the phase transformation of TiO2 from anatase to rutile. Several parameters affecting the transformation process, such as Co dopant concentration and annealing temperature have been investigated.

2. EXPERIMENTAL SECTION Co-doped TiO2 nanoparticles containing 0.0085, 0.017, 0.0255, 0.034, and 0.085 mol % (corresponding to 0.5, 1.0, 1.5, 2.0, and 5.0 wt % respectively) Co(III) ion dopant were synthesized via a sol–gel technique. The doped titania nanoparticles were synthesized from titanium tetrachloride, TiCl4 (Fluka 98%), and cobalt(III) 2

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2,4-pentanedionate, Co[CH3 COCH C(O )CH3 ]3 (Alfa Aesar). The dopant stoichiometry was controlled by dissolving the Co(III) precursor in ethanol (Pharmco, 200 proof) prior to the dropwise addition of TiCl4 . The reaction was performed at room temperature while stirring in a fume hood to avoid exposure to the Cl2 and HCl gases that evolve during the reaction. The solution was allowed to rest and cool back to room temperature as the gas evolution ceased. Samples were dip-coated onto quartz and borondoped (p-type) silicon substrates at a withdrawal rate of 3 cm/min. The coated substrates were allowed to dry in a desiccator followed by calcinations for 30 min in a box furnace operating between 200 and 900  C in an ambient atmosphere. For X-ray diffracton analysis (XRD), several layers of TiO2 were deposited with a calcination step following each dipping cycle. Powder samples were also synthesized. These samples were dried in a desiccator and then annealed for 1 h, during which the anatase to rutile phase transformation took place. The calcined samples were pulverized using a mortar and pestle. To reduce agglomeration, samples were dissolved in DI water, sonicated, and then placed in a desiccator to dry. Both thin film and powder samples were analyzed by similar methods. Structural characterizations of the doped and undoped TiO2 samples were carried out by XRD. The –2 scans were recorded at several resolutions using Cu K radiation in a Rigaku D-Max B diffractometer equipped with a graphite crystal monochromator. Analysis of the resulting data was completed using Jade software to determine peak position, width, and intensity. The dopant concentration was verified by energy dispersive X-ray analysis (EDAX) and X-ray photoelectron spectroscopy (XPS). XPS was also employed to characterize the oxidation state of the cobalt dopant. XRD analysis was performed to determine the effects of the Co doping on the rutile and anatase concentrations. The mass fraction of rutile (Xr ) in the samples was calculated, based on the relationship between the integrated intensities of anatase (101) and rutile (110) peaks, following the formula developed by Spurr et al.31 Xr =

1 1 + KIa /Ir 

(1)

where Ia and Ir are the integrated peak intensities of the anatase (101) and rutile (110) peaks, respectively. The constant K was determined by XRD analysis of standard mixture of TiO2 powder (Degussa P-25) of known proportions of anatase and rutile (80:20) and was measured to be equal to 0.79. Particle size calculations were carried out using Scherrer’s equation: t=

09  cos

(2)

where  is the X-ray wavelength,  is the peak width, and  is the Bragg angle. The peak positions (2) of the

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Barakat et al./Effect of Cobalt Doping on the Phase Transformation

(3)

 lnXr  ·R  T1

(4)

where Ea is the anatase to rutile transformation activation energy, T is temperature in Kelvin, R is the universal gas constant (8.314 kJ/mol.), and Xr is the corrected weight fraction of rutile, as previously defined.

3. RESULTS AND DISCUSSION The particle sizes of the prepared samples were calculated from both XRD data using Scherrer’s formula and TEM analyses. Table I shows the values of TiO2 particle sizes as a function of the Co doping concentration. The average particle size increased slightly from 22 to 27 nm as the Co doping concentration was increased from 0.0085– 0.034 mol %, respectively. Figure 1 shows representative TEM micrographs of 0.0085 and 0.034 mol % Co-doped TiO2 samples. The Co doping concentrations measured by EDAX and XPS were similar and were close to the initial values used at the preparation step of the samples, indicating that all the cobalt was incorporated in TiO2 . Figure 2 shows the XPS survey spectra of 0.0085 and 0.034 mol % Co-doped TiO2 samples. The carbon peak is from surface Table I. Particle size values for the as-prepared Co-doped TiO2 samples. Co doping concentration, mol % Undoped TiO2 0.0085 0.017 0.0255 0.034

contamination due to exposure to air. A rough estimation of Co concentration from the XPS data matched well with that measured by EDAX. Figure 3 shows a highresolution XPS spectrum of the Co 2p region of a TiO2 sample containing 0.034 mol % Co. Table II compares the measured XPS peak positions to the reported values.

particle size, nm 15 22 24 25 27

1200

(a)

O

1000

Intensity (Arb. Units)

Ea = −

Ea 1 · R T

20 nm X200 K (b)

Fig. 1. TEM of (a) 0.0085 mol % and (b) 0.034 mol % Co-doped TiO2 samples.

Ti 800 Co 600

400 C

Ti

200

0 1100 1000 900

800

700

600

500

400

300

200

100

Binding Energy (eV)

1000

(b)

O

800

Intensity (Arb. Units)

ln Xr = −

20 nm X200 K (a)

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anatase (101) and (200) reflections were used to calculate the lattice parameters. Particle size and distribution measurements were also performed using transmission electron Microscopy (TEM) bright field micrograph analyses. An average of 20 particles were measured in each TEM picture. Such a comparison is necessary since Scherrer’s analyses do not take into account the effect of stress in the films or particles. From TEM, it was observed that the nanoparticles are good quality single crystals. The measured size of the crystals matched closely the size calculated by analyzing XRD data. Therefore, it can be safely presumed that there is little or no contribution to peak broadening due to stress. The activation energy, Ea , for the anatase to rutile phase transformations were calculated from the slope of the plot of the corrected rutile weight fractions, Xr , as a function of the reciprocal of annealing temperature. The relationship is given by an Arrhenius equation as follows:

Co

Ti

600

400 Ti

C 200

0 1100 1000 900

800

700

600

500

400

300

200

100

Binding Energy (eV)

Fig. 2. XPS spectra for Co-doped TiO2 samples: (a) 0.0085 mol % (b) 0.034 mol %.

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500

4000

350 300 250 200

100 810

800

790

780

770

760

750

740

R

of

Co

2p

region

for

Table II. Measured and previously reported 2p3/2 photoelectron peak positions of Co and various Co oxides.

Co Co3 O4 Co2 O3 CoO CoO multiplet Splitting

Reported binding energy (eV) 778 7793 7799 780 7863

A

A

2000

30

40

50

900 °C 850 °C 800 °C 750 °C 700 °C 650 °C 600 °C 550 °C 500 °C 450 °C 400 °C 350 °C 300 °C 250 °C 60

Bragg Angle (2θ)

The metallic Co 2p3/2 peak occurs at 778 eV.32 33 No such peak was observed in the sample. This is in agreement with the XRD result, which did not show the existence of any pure Co phase. The reported value of Co(II) in CoO is 780 eV.34 35 As shown in Table II, there is a peak at the Co(II) reported peak position. CoO is paramagnetic and, as such, shows a satellite peak. The reported peak position for this satellite peak in paramagnetic CoO is 786.3 eV.34 The measured peak position in our sample is 786.6 eV. This confirms the presence of paramagnetic CoO. The peaks due to the presence of Co2 O3 or mixedvalent Co3 O4 reportedly occur at 779.936 and 779.3 eV,33 respectively. Although the presence of these higher cobalt oxides cannot be explicitly ruled out, the presence of a strong paramagnetic peak suggests that the divalent cobalt is present as CoO and CoTiO3 . James and Steven37 explained the presence of Co(II) based on the correspondence between formal oxidation state and charge state as follows: a substitutional CoTi in the −1 charge has a formal oxidation state of III, and a substitutional CoTi in the −2 charge has an oxidation state of II. According to the local-density approximation (LDA) calculations, there is no stable −2 charge state of substitutional Co. Upon adding an additional electron to the stable −1 charge (Co(III)), an accompanying exchange splitting occurs and there is a downward shift of the fermi level between the −1 and −2 charge state. On the other hand, in the limit of low or poor oxygen, interstitial Co becomes energetically possible compared with the substitutional Co. At 1.5 eV below the O-rich limit, the oxidation

Species

A

3000

20

Measured binding energy (eV)

Ref.

786.9 786.6

32–33 33 36 34–35 34

Fig. 4. XRD Patterns for 0.017 mol % Co-doped TiO2 after annealing at various temperatures.

state III is dominant, while between 3 and 4 eV below the O-rich limit, the predominant oxidation state of Co is II. The presence of CoTiO3 in our samples, as described later, confirms that some Co is indeed −2 valance state. Figure 4 shows XRD patterns obtained from samples synthesized with 0.017 mol % Co-doped TiO2 . These samples were calcined at temperatures ranging from 250 to 900  C for 1 h at each temperature. The XRD patterns show that the transformation from amorphous to anatase phase and from anatase to rutile phase takes place as the calcination temperature increases. The phase transformation from amorphous to anatase takes place at temperature above 400  C. The pure anatase phase persisted up to 650  C. Rutile-related peaks began to appear at temperatures higher than 650  C. The sample converted completely to rutile between 800 and 900  C, and no anatase-related peak was detected beyond these temperatures. The phase change from anatase to rutile, however, depends not only on the calcinations temperature but also on Co doping concentration. Figure 5 shows 6500 6000

5000

CoTiO3

R

5500

Intensity (Arb. Units)

Fig. 3. High-resolution XPS spectrum 0.034 mol % Co-doped TiO2 samples.

R

A

1000

Binding Energy (eV)

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R

R

400

150

4

R CoTiO 3

5000

Intensity (Arb. Units)

Intensity (Arb. Units)

2p3/2

2p1/2

450

0.085 mol% Cobalt

R (101)

(110)

4500

0.034 mol% Cobalt

4000 3500

0.0255 mol% Cobalt

3000 2500

0.017 mol% Cobalt

2000 0.0085 mol% Cobalt

1500 1000

A

0 20

Undoped TiO2

A

500 30

A 40

A 50

60

Angle (2θ)

Fig. 5. XRD patterns for samples with various Co doping concentrations after annealing at 700  C.

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Barakat et al./Effect of Cobalt Doping on the Phase Transformation

1

2

0.8

–2

0.6

–6

0.5

–8 0.96

0.4 0.3

ln(xr)

0.1 675

725

1.00

1.04

1.08

1.12

–8 0.96

1.16

1.00

775

Temperature (°C)

0.0255 mol%

0.2 0.18 0.16

1.16

–4

–2 –4 –6

1.00

1.04

1.08

1.12

1.16

1000/ T (°K)

–8 0.96

1.00

1.04

1.08

1.12

1.16

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the XRD patterns of cobalt-doped TiO2 samples with various cobalt doping concentrations after the samples were annealed for 1 h at 700  C. The figure shows that with 0.017 mol % cobalt, the anatase-related peak is the major peak compared to that for rutile. However, with increasing Co concentration, the relative intensities of the rutile-related peaks increase gradually, reaching a maximum value for the 0.085 mol % Co-doped TiO2 sample. Additionally, new peaks start to appear at higher Co concentrations. These peaks have been identified as peaks related to the CoTiO3 phase, which confirmed the oxidation state II of Co. The peaks intensities related to the CoTiO3 phase increased with increasing Co concentration. From the XRD data of Figures 4 and 5, it can be seen that the crystallite size increases with the increase of both the calcination temperature and Co doping concentration. Figure 6 shows the effect of the calcination temperature on the anatase to rutile phase transformation for samples with the Co doping concentrations of 0.017, 0.0255, and 0.034 mol %. The figure plots Xr , mass fraction of rutile phase according to Eq. 1, based on the integrated intensity ratio (Ir /Ia ) of rutile and anatase XRD peaks.

1.12

0.034 mol%

0

–6

Fig. 6. Effect of annealing temperature on the rutile to anatase phase transformation at several Co doping concentrations.

1.08

1000/ T (°K)

–2

–8 0.96

1.04

2

0

0.034 mol%

625

–4 –6

2

0.0255 mol%

0 575

–2

1000/ T (°K)

0.017 mol%

0.2

Relative Peak Intensity of CoTiO3

–4

ln(xr)

Rutile fraction, xr

0.7

0.017 mol%

0

ln(xr)

0

ln(xr)

0.9

2 0.0085 mol%

1000/ T (°K)

Fig. 8. Variation of lnXr  as a function of the inverse of the calcination temperature for various Co-doped TiO2 samples.

The peaks chosen for the calculation of this ratio were (101) and (110) peaks for the anatase and rutile phases, respectively. It can be seen that Xr increases with increase in both the temperature and cobalt doping concentration. The anatase–rutile conversion occurs at lower temperatures as the cobalt concentration in TiO2 increases. Figure 7 shows the effect of Co doping concentration on the formation of CoTiO3 at 700  C, based on the integrated intensity ratio (Ir /Ia ) of rutile and anatase XRD peaks. It can be seen that the peak intensity of CoTiO3 increases gradually with the increase in the Co doping concentration. Figure 8 shows the Arhenius plot of ln Xr as a function of the 1/T , the inverse of the calcination temperature. According to the Eq. 4, the slope of this curve is the activation energy (Ea ). The calculated Ea values for the anatase to rutile transformation are listed in Table III. Ea values ranged between 195 and 229 kJ/mol for the Codoped TiO2 samples, whereas Ea of undoped TiO2 was 154 kJ/mol. All these valves are much lower than the reported activation energy value for the bulk, which is 350–700 kJ/mol.38 This is expected as the nanosize grains cause higher surface area to volume ratios compared to the bulk, which increases the total surface energy and

0.14 0.12

Table III. Activation energy calculations for Co-doped TiO2 samples after annealing at different temperatures.

0.1 0.08 0.06

Co doping concentration, mol %

0.04 0.02 0 0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Co3+ doping mol %

Fig. 7. Effect of Co doping concentration on the formation of CoTiO3 at 700  C.

Bulk TiO2 Undoped TiO2 0.0085 0.017 0.0255 0.034

Ea , kJ/mol 350–700 154 (ref. 39) 229 222 211 195

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1.002

Normalized Lattice Constant

c/c0 1.0015

1.001

1.0005

1

0.9995 0.005

a/a0

0.01

0.015

0.02

0.025

0.03

0.035

0.04

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Co doping (mol%)

Fig. 9. Lattice constants a and c of TiO2 (tetragonal) as a function of Co-doping concentration.

effectively reduces the Ea for anatase to rutile transformation. The lowering of the activation energy with the increase in the Co concentration leads to a decrease in the transformation temperature of anatase to rutile. The lower activation energy of the undoped TiO2 is due to its smaller particle size (15 nm). The anatase–rutile phase transformation is known to be a nucleation and growth process during which rutile nuclei form within the anatase phase and grow in size with increasing temperature, eventually consuming the surrounding anatase.4 The cobalt doping enhances this transformation. This can be explained as follows: In rutile, Ti+4 cations are octahedrally coordinated to O−2 anions with axial and equatorial bond lengths of 0.198 and 0.195 nm, respectively. In anatase, the octahedral coordination shell is significantly distorted, although the bond lengths are close to that of rutile (0.196 nm and 0.194 nm). The “a” and “c” lattice parameters in anatase are 0.378 and 0.952 nm, respectively, whereas those in rutile are 0.459 and 0.296 nm, respectively.39 From the data collected, it was determined that the lattice constant c increases gradually with Co-dopant addition, while the value of a remains essentially unchanged, as shown in Figure 9. The effective ionic radii of Ti4+ and Co3+ are 0.605 and 0.685 Å, respectively. This is a 13% difference in the lattice constant, which causes some distortion. The strain energy associated with the incorporation of Co results in lower activation energies, which facilitate the anatase-to-rutile phase transformation.

4. CONCLUSIONS Cobalt-doped TiO2 nanoparticles were synthesized via sol–gel and dip-coating techniques. The effect of metal ion doping on the transformation of anatase to rutile phase has been investigated. Results obtained can be summarized as follows: the presence of Co(III) ion doping in the TiO2 nanostructure has a significant effect on 6

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the transformation of anatase to rutile phase. The mass fraction of rutile (Xr ) increases with the increase of the Co-doping concentration and temperature. From XRD analysis, the peaks for CoTiO3 compound appear and increase in intensity with increasing doping level, XPS also confirmed the existence of some Co with oxidation state II in Co-doped TiO2 . Co substitution of Ti in and on both rutile and anatase lattices forming the electronic structure Ti1–x Cox O2 where x values ranged from 0.0085 to 0.085. The averages particle sizes of the prepared samples varied from 22 to 27 nm with increase in Co concentration from 0.0085 to 0.034 mol %, respectively. The activation energy of such process showed values of 209, 222, 211, and 195 kJ/mol with 0.0085, 0.017, 0.0255, and 0.034 mol % Co-doping, respectively. From the results of this study, the photocatalytic activities and magnetic properties of the prepared samples (anatase form) with different amounts of Co doping can be tested and evaluated. Acknowledgment: This research was funded by Grant No. NSF NIRT DMR-0210284. We acknowledge NSF INT-031664 for partial support of this work.

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Barakat et al./Effect of Cobalt Doping on the Phase Transformation

Received: 22 March 2004. Accepted: 18 October 2004.

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