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Effect of cobalt doping on the electronic, optical and photocatalytic properties of TiO2
Solid State Sciences 46 (2015) 27e32
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Effect of cobalt doping on the electronic, optical and photocatalytic properties of TiO2 Peng Jiang a, Wei Xiang a, Jianlei Kuang a, Wenxiu Liu a, Wenbin Cao a, b, * a
Department of Inorganic Nonmetallic Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China b Tianjin College, University of Science and Technology Beijing, Tianjin 301830, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2015 Received in revised form 21 May 2015 Accepted 22 May 2015 Available online 27 May 2015
To extend the optical response to visible light region, cobalt doped TiO2 (Co-doped TiO2) with dopant concentration of 0.1 %e3.0 at % was synthesized by one step hydrothermal method without any postheat treatment for crystallization. X-ray diffraction results confirm that all the doped and undoped TiO2 were composed of pure anatase phase with good crystallinity. The calculated grain size was ranged from 8.4 to 10.5 nm by Scherrer's method. The photocatalytic activities of the synthesized samples were evaluated by degradation of phenol under visible light irradiation. The Co-doped TiO2 sample with dopant concentration of 0.3 at% shows the highest degradation ratio. First principle calculations were performed and the results revealed that cobalt doping could realize its visible light response and enhance the photocatalytic performance of TiO2 by introducing impurity states in its band gap. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: TiO2 Cobalt doping Photocatalytic activity First principle calculation
1. Introduction Water and air pollution have become urgent challenges for many countries in recent years. Photocatalytic semiconductors, such as TiO2, ZnO and SnO2, have attracted more and more attention for their potentials in photodegradation of harmful contents. Especially, TiO2 is considered as one of the most promising materials due to its high stability and environmental safety. However, the application of TiO2 in water and air purification is restricted as its intrinsic wide band gap (3.2 eV) requires high energy excitation such as UV light, which only composes 5% of solar irradiation. Thus, the major portion of solar energy (visible light) could not be used for photocatalytic reaction. Besides, high recombination rate of photo excited carriers is another limitation for the applicable fields of TiO2. Due to the short life of photo excited carriers, only a small part of electrons and vacancies can move to its surface, which lead to further reduction of photocatalytic efficiency. Lots of attempts were tried to improve the photocatalytic performance of TiO2 and doping with transition metal ions, such as copper, vanadium and chromium, was found to be a useful method
* Corresponding author. Department of Inorganic Nonmetallic Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail address: [email protected] (W. Cao). http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.007 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
[1e6]. Among all the available transition metals, cobalt was proved to be one of the most effective dopants to enhance the light response and photoactivity of TiO2. Bryan [7] prepared colloidal cobalt doped TiO2 which shows a wide absorption range extended into the visible region. Iwasaki [8] synthesized cobalt doped TiO2 and found that the introduction of Co2þ could apparently shift the light absorption edge of anatase TiO2 to the visible region and enhance photoactivity under both UV and visible light irradiation. However, most cobalt doped TiO2 samples were synthesized from the organic titanium precursors, which are usually poisonous. Bryan [10] synthesized the cobalt doped TiO2 by titanium tetraisopropoxide. Manivannan [9] synthesized cobalt doped TiO2 films from titanium acetylacetonate followed with the heat treatment at 400 C; Titanium alkoxides [11,12] were also chosen as the raw material by researchers. Except for the use of harmful organic compounds, another disadvantage of the reported preparation technology is that high temperature heat treatment was prerequisite for crystallization. Sadanandam [13] synthesized cobalt doped TiO2 by impregnation method which requires calcinations at 400 C for 5 h. In this study, we successfully prepared cobalt doped TiO2 powders from TiOSO4 by one step hydrothermal method at moderate temperature (150 C) without any following heat treatment. Multiple characterization methods, such as XRD, XPS, TEM and UVevis absorption, were used to characterize the assynthesized samples [14]. First principle calculations based on
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P. Jiang et al. / Solid State Sciences 46 (2015) 27e32
density function theory (DFT) were carried out to investigate the effect of the introduced cobalt on band structure and density of state (DOS) of TiO2.
displacement within 0.001 Å.
3. Results and discussion 2. Experiments and calculations 3.1. XRD and TEM 2.1. Synthesis of cobalt doped TiO2 Both pure and cobalt doped TiO2 were synthesized via hydrothermal method. CoSO4 (analytic grade) was chosen as cobalt source. In the synthesis process, stoichiometric amount of TiOSO4 (technical grade), (NH2)2CO (technical grade) and CoSO4 were dissolved in deionized water and stirred for 5 min. Subsequently, the mixed solution was transferred into a 100 ml autoclave and heated at 150 C for 10 h. The obtained precipitates were repeatedly washed with deionized water until no SO2þ can be detected by 4 Ba2þ, then they were extracted and dried at 80 C without any following heat treatment. Cobalt doped TiO2 samples with different nominal doping concentrations (0.1%, 0.3%, 0.5%, 1%, and 2%) and un-doped TiO2 were prepared through the same method. 2.2. Characterization methods X-ray diffraction (XRD) was utilized to identify the phase composition and crystallinity using Cu Ka radiation at 40 KV and 130 mA. The patterns were recorded continuously with 2q range of 10e90 . Sherrer's equation was used to calculate the crystal size. Xray photoelectric spectrum (XPS, VG Escalab MK II) was carried to characterize surface chemical state. Transmission electron microscopy (TEM, JEOL-200CX; Hitachi) was used to observe the morphologies of prepared samples. The light adsorption spectra of cobalt doped TiO2 were obtained by a UVevis scanning spectrophotometer (TU-1901) using BaSO4 as a reference standard. The photocatalytic activity was evaluated by degradation of 100 ml phenol, of which the concentration is 25 mg/L under visible light irradiation. Prior to degradation, 0.25 g powder was added into the phenol solution, then the solution was continuously stirred for 1 h in dark to reach the adsorption/desorption equilibrium. After that, visible light source (15.4 mW/cm3, 400e700 nm) was turned on and the solution was sampled every 2 h. All samples were centrifuged to remove the residual particles, and then UVevisible spectra of each sample were measured and the results were utilized to describe concentration variation of phenol solution using LamberteBeer law.
The XRD patterns of pure and cobalt doped TiO2 with various concentrations are illustrated in Fig. 1. The patterns of all synthesized powders via hydrothermal method show sharp peaks indicating the particles are highly crystallized and all the peaks can be indexed as anatase TiO2 phase (JCPDS: no.21e1272) [16]. It is deserve to be mentioned that the color of pure TiO2 is white, however, cobalt doped samples are all pale yellow and the color is deepened when cobalt concentration is elevated. For no peaks corresponding to metallic cobalt or cobalt compound is observed and the color of TiO2 is changed with cobalt amount, it is reasonable to suppose that cobalt cations are successfully introduced into TiO2 and homogenously distributed in the lattice. The average particle size of each sample is calculated from the full width at halfmaximum (FWHM) of the (101) diffraction peak using Scherrer's equation [17]. The results are demonstrated in Table 1. All prepared samples are in nano-size range, from 8.4 to 10.5 nm, and cobalt doped samples show smaller or equal particle size compared to pure TiO2. TEM is used to describe the morphology of the powders and the images of pure and 0.3% CoeTiO2 is displayed in Fig. 2. All these particles are in nano-size range and the average diameter is about 10 nm which is in agreement with the XRD result. Photocatalytic reaction is occurred on the surface and the exposure of planes with high surface energy would apparently accelerate the reaction rate. But in the process of crystal growth, low energy planes would cover the surface particles spontaneously. Proved by thermodynamic calculation, the {101} facets of anatase TiO2 are the most stable planes and the steadiest shape of anatase TiO2 is slightly truncated diamond with more than 90% surface surrounded by {101} [18]. However, in our TEM images, both pure and cobalt doped TiO2 particles synthesized by hydrothermal method existed partly in the shape of sphere or polygon, thus the exposure of {101} facet is lower than diamond shape and other facets with high surface energy would be exposed. So, the photocatalytic ability of hydrothermal synthesized powders would be improved.
2.3. Calculation method First principle calculations were carried out based on spinpolarized density function theory (DFT) in the framework of generalized gradient approximation (GGA) [15]. For exchange and correlation interaction, Perdew-Purke-Ernzerhof (PBE) function was applied and all simulations were performed with ultrasoft pseuodopotential, which was adopted to describe the potential between ionic core and valence electrons by CASTEP code. Cut-off energy of wave function was set at 380 eV, 3 3 3 k mesh was used for integration in the first Brillion Zone (BZ) and all calculations were performed in reciprocal space. The primitive anatase lattice was introduced from Materials Studio library and then expanded to a 2 2 1 supercell with 96 atoms. In order to simulate cobalt doped TiO2, one and two Ti atoms were replaced by Co atoms in the supercell model marked as Co1Ti15O32 and Co2Ti14O32. Geometry optimization was implemented before calculating each property to get a stable configuration and the criterion for relaxation process was set as atom residual forces should be limited within 0.03 eV/Å and the maximum ionic
Fig. 1. XRD patterns of pure TiO2 and cobalt doped TiO2.
P. Jiang et al. / Solid State Sciences 46 (2015) 27e32
29
Table 1 Average particle sizes calculated from Scherrer's equation. Samples
Pure TiO2
0.1% CoeTiO2
0.3% CoeTiO2
0.5% CoeTiO2
1% CoeTiO2
2% CoeTiO2
3% CoeTiO2
Size (nm)
10.5
8.4
8.4
10.5
10.5
8.4
8.4
Fig. 2. TEM images of (a) pure TiO2 (b) 0.3% CoeTiO2.
3.2. X-ray photoelectron spectrum analysis
3.3. Electron structure calculations
Identification of chemical elements in cobalt doped TiO2 and their corresponding valence states are accomplished by XPS test, and the spectra (shown in Fig. 3) provide another evidence of the successful introduction of cobalt ions. It is found that 5 different chemical elements (Ti, O, C, N, Co) existed. Ti, O and Co originates from the precursor materials and C was used as calibration element. N came from two possible sources: (1) urea which is one of raw materials (2) nitrogen gas from air adsorbed on sample's surface. Strong peaks at about 464.08 eV and 458.08 eV are corresponding to Ti 2p1 and 2p3 states respectively. These peaks match well with characteristic peaks of Ti4þ in anatase prepared by Joen [19] and Manivannan [9], which implies the phase of the synthesized powder is anatase. The binding energy of Co 2p1 and Co 2p3 are 796.06 eV and 780.66 eV respectively, with an energy difference of 15.4 eV, shown in Fig. 3(b). Considering both XPS and XRD results, we can get a conclusion that Co ions are successfully introduced into TiO2 lattice. It is found that the binding energy of Co 2p3 in CoO at 780.4 eV is close to our results, and the shake up peaks indicating the existence of Co2þ [20], are also observed in high energy side of the spectrum. Moreover, the binding energy difference between 2p1 and 2p3 is consistence with XPS data of Co2þ [21,22]. Base on these evidences, cobalt ion in anatase lattice is at þ2 valence state.
First principle calculations were carried out to investigate the effect of cobalt on the electronic structure of anatase TiO2. Both pure and cobalt doped models with two different concentrations were built and their band structures are illustrated in Fig. 4. The dash line represents Fermi level that is set at 0 eV in all the calculations. The calculated band gap of pure TiO2 is 2.219 eV that is lower than experimental value of 3.2 eV, which is due to the drawback of DFT theory. As only relative change compared with pure TiO2 is discussed, this drawback would not have an effective influence on our results. Compared with the band structure of undoped TiO2, the Fermi level of doped one move towards the conduction band and lots of impurity states are introduced into its forbidden band. Widely distributed impurity states appeared in the band gap could act as ‘steps’ for photo carriers transition, and then promote the visible light absorption and photocatalytic reaction. With the increase of dopant concentration, the density of impurity state is enriched and it is reasonable to speculate that its light response would also be improved. Light absorption edge is closely related to the band gap, and the wide intrinsic band gap is one of the most important factors that restrict the photocatalytic applications of TiO2. In order to red (in the web version) shift the absorption edge of anatase TiO2, many attempts were tried to reduce the gap value. In our built models,
Fig. 3. XPS spectra of (a) 2% cobalt doped TiO2 (b) Co 2p core level.
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P. Jiang et al. / Solid State Sciences 46 (2015) 27e32
Fig. 4. Band structures of different Co-doped models.
the band gaps of cobalt doped TiO2 with different concentrations (Fig. 5(b) and (c)) shows similar width with the undoped one (Fig. 5(a)), so the introduction of cobalt doped would not greatly affect the absorption edge. However, it can be seen that impurity states are introduced into the forbidden band because of the cobalt doping, as shown in Fig. 5(b). And, the density of impurity state increased with the elevated dopant concentration and some impurity states overlap with valence band or conduction band (Fig. 5(c)). Therefore, these impurity states are beneficial to the enhancement of visible light absorption and the improvement of photo catalysis efficiency. Partial density of state (PDOS) of Co1Ti15O32 (Fig. 5(d)e(g))
Fig. 5. DOS patterns of: (a) pure TiO2 (Ti16O32) (b) CoTi15O32 and (c) Co2Ti15O32; PDOS patterns of (d) Co1Ti15O32 (e) Ti (f) Co (g) O in CoTi15O32.
Fig. 6. UVevis spectra of cobalt doped and undoped TiO2 samples.
P. Jiang et al. / Solid State Sciences 46 (2015) 27e32
31
Table 2 Edge wavelength and band gap of different samples. Sample
TiO2
0.1% CoeTiO2
0.3% CoeTiO2
0.5% CoeTiO2
1% CoeTiO2
2% CoeTiO2
3% CoeTiO2
Absorption edge (nm) Band gap (eV)
382 3.25
386 3.21
388 3.19
392 3.16
394 3.15
403 3.08
400 3.10
3.5. Photocatalytic activity
Fig. 7. Photocatalytic degradation of phenol under visible light by synthesized samples and P25.
indicates the imported impurity states come from Co 3d states, meanwhile, Ti 3d and O 2p states mostly consist of conduction band and valence band respectively. It can be seen that Co 3d state partially overlapped with O 2p state near valence band, and the low concentration of cobalt corresponds to the slight variation of band gap.
Photocatalytic activities of the prepared samples were evaluated by degrading phenol solution under visible light irradiation. Fig. 7 represents the photocatalytic reaction results compared with P25 (20% rutile and 80% anatase). C and Co stand for the concentration of phenol at a specific time after dark adsorption. All the tested samples show little adsorption, which can be neglected in the degradation. The photocatalytic reaction follows a zero-order rate law, the apparent rate constants (K/C0, Time: h) and degradation rate of all cobalt doped sample are demonstrated in Table 3. Moderate degradation efficiency (64.51%) exhibited by P25 is due to the well-known mixed phase effect which promote photo induced carriers separation. Pure TiO2 degrades 53% of phenol solution, which may be attributed to the existence of trace impurity doped into TiO2 from the raw material of industrial level TiOSO4. Degradation rate is enhanced to 64.21% by adding 0.1% cobalt and the highest degradation rate (81.72%) is demonstrated by 0.3% cobalt doped TiO2. Enhanced degradation rate is due to the doped cobalt ions which not only improve absorption rate of visible light but act as shallow traps to separate photo induced carriers efficiently as well. However, when the concentration of doping ions is too high, doping ions may act as recombination centers and decrease the photocatalytic activity. It is reported [23] that the optimal concentration of doping ions should make the thickness of the space charger layer substantially equal to the light penetration depth. So, further addition of cobalt leads to gradually reduction of catalytic efficiency as indicated in Table 3. The highest photocatalytic activity of 0.3% cobalt doped TiO2 attributes to its proper electron structure, improved visible absorption and appropriate doped concentration.
4. Conclusions 3.4. UVevis spectra As shown in Fig. 6 and Table 2, UVevis spectra of pure and doped samples are tested and the results are used to evaluate the shift of band gaps. Pure TiO2, with band gap equal to 3.25 eV and absorption edge at 382 nm, are synthesized with hydrothermal method and shows almost no visible light absorption. The band gap of all cobalt doped samples decreased slightly compared with pure TiO2. But for all the doped samples, the absorption efficiencies in the range of 400 nme700 nm are improved and the increment is proportional to the cobalt amount. Density of Co 3d impurity states in Fig. 5(f) show two peaks and these would contribute to the formation of ‘steps’ in the absorption spectra. So, widely distributed impurity states of Co 3d level in forbidden band would improve the photo response of TiO2, and then enhance its visible light photocatalytic activity.
Cobalt doped TiO2 powders are successfully synthesized using hydrothermal method without any high temperature heat treatment. Comprehensive characterization results confirm the successful introduction of Co2þ in anatase crystal structure and enhanced absorption in visible range for all doped samples. The photocatalytic activity is tested by the degradation of phenol under visible light irradiation and the highest degradation efficiency is performed by 0.3% cobalt doped TiO2. First principle calculation reveals lots of 3d impurity states are appeared in forbidden band after substantial introduction of cobalt and this would contribute to photocatalytic activity under visible light irradiation. Though we cannot confirm how impurity element is introduced into TiO2 lattice during hydrothermal process and many attempts are necessary for further investigation. We are focusing on elucidating mechanism about of doping and crystallization by simulation and experiment.
Table 3 Degradation rate of each cobalt doped sample. Sample
P25
Pure
0.1% CoeTiO2
0.3% CoeTiO2
0.5% CoeTiO2
1% CoeTiO2
2% CoeTiO2
3% CoeTiO2
Degradation rate (%) K/C0
64.51 0.055
53.00 0.040
64.21 0.054
81.72 0.070
70.43 0.060
56.13 0.048
44.66 0.030
35.15 0.031
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Acknowledgment This work is financially supported by National Natural Science Foundation of China under the grant No: 51402016, Fundamental Research Funds for the Central Universities under the grant No: FRF-TP-14-007A1, and project No: BZZ14J001. References [1] C. Lee, C.M. Aikens, Effects of Mn doping on (TiO2)n (n ¼ 2-5) complexes, Comput. Theor. Chem. 1013 (2013) 32e45. [2] L.G. Devi, et al., Enhanced photocatalytic activity of transition metal ions Mn2þ, Ni2þ and Zn2þ doped polycrystalline titania for the degradation of aniline blue under UV/solar light, J. Mol. Catal. A Chem. 328 (1) (2010) 44e52. [3] Y.Y. Gurkan, E. Kasapbasi, Z. Cinar, Enhanced solar photocatalytic activity of TiO2 by selenium (IV) ion-doping: characterization and DFT modeling of the surface, Chem. Eng. J. 214 (2013) 34e44. [4] X. Zou, et al., Heterometal alkoxides as precursors for the preparation of porous Fe- and Mn-TiO2 photocatalysts with high efficiencies, Chemistry A Eur. J. 14 (35) (2008) 11123e11131. [5] H. Feng, M. Zhang, L.E. Yu, Hydrothermal synthesis and photocatalytic performance of metal-ions doped TiO2, Appl. Catal. A: General 413e414 (2012) 238e244. [6] V.N. Nguyen, N.K.T. Nguyen, P.H. Nguyen, Hydrothermal synthesis of Fe-doped TiO2 nanostructure photocatalyst, Adv. Nat. Sci. Nanosci. Nanotechnol. 2 (2011) 035014e035018. [7] J.D. Bryan, et al., Strong room-temperature ferromagnetism in Co2þ-doped TiO2 made from colloidal nanocrystals, J. Am. Chem. Soc. 126 (37) (2004) 11640e11647. [8] M. Iwasaki, et al., Cobalt ion-doped TiO2 photocatalyst response to visible light, J. Colloid Interface Sci. 224 (1) (2000) 202e204. [9] A. Manivannan, et al., Magnetism of co-doped titania thin films prepared by spray pyrolysis, Appl. Phys. Lett. 83 (1) (2003) 111e113. [10] J.D. Bryan, et al., Activation of high-TC ferromagnetism in Co2þ: TiO2 and Cr3þ: TiO2 nanorods and nanocrystals by grain boundary defects, J. Am. Chem. Soc.
127 (44) (2005) 15568e15574. [11] J.T. Thienprasert, et al., Local structures of cobalt in co-doped TiO2 by synchrotron x-ray absorption near edge structures, Curr. Appl. Phys. 11 (3S) (2011) S279eS284. [12] B. Choudhury, A. Choudhury, Luminescence characteristics of cobalt doped TiO2 nanoparticles, J. Luminescence 132 (1) (2012) 178e184. [13] G. Sadanandam, et al., Cobalt doped TiO2: a stable and efficient photocatalyst for continuous hydrogen production from glycerol: water mixtures under solar light irradiation, Int. J. Hydrogen Energy 38 (23) (2013) 9655e9664. [14] M. Khan, W. Cao, Cationic (V, Y)-codoped TiO2 with enhanced visible light induced photocatalytic activity: a combined experimental and theoretical study, J. Appl. Phys. 114 (18) (2013) 183514. [15] M. Khan, W. Cao, Preparation of Y-doped TiO2 by hydrothermal method and investigation of its visible light photocatalytic activity by the degradation of methylene blue, J. Mol. Catal. A Chem. 376 (2013) 71e77. [16] M.K. Seery, et al., Silver doped titanium dioxide nanomaterials for enhanced visible light photocatalysis, J. Photochem. Photobiol. A Chem. 189 (2) (2007) 258e263. [17] M. Khan, W. Cao, Cationic (V, Y)-codoped TiO2 with enhanced visible light induced photocatalytic activity: a combined experimental and theoretical study, J. Appl. Phys. 114 (18) (2013) 183514. [18] X. Zhao, et al., Shape-and size-controlled synthesis of uniform anatase TiO2 nanocuboids enclosed by active {100} and {001} facets, Adv. Funct. Mater. 21 (18) (2011) 3554e3563. [19] B. Jeong, et al., Properties of anatase CoxTi1-xO2 thin films epitaxially grown by reactive sputtering, Thin Solid Films 488 (1) (2005) 194e199. [20] J. Li, et al., Cobalt-Doped TiO2 nanocrystallites: radio-frequency thermal plasma processing, phase structure, and magnetic properties, J. Phys. Chem. C 113 (19) (2009) 8009e8015. [21] B. Choudhury, et al., Effect of oxygen vacancy and dopant concentration on the magnetic properties of high spin Co2þ doped TiO2 nanoparticles, J. Magnetism Magnetic Mater. 323 (5) (2011) 440e446. [22] Y.B. Lin, et al., Ferromagnetism of co-doped TiO2 films prepared by plasma enhanced chemical vapour deposition (PECVD) method, J. Phys. D Appl. Phys. 41 (19) (2008) 195007. [23] W. Zhou, et al., Preparation and properties of vanadium-doped TiO2 photocatalysts, J. Phys. D Appl. Phys. 43 (3) (2010) 035301.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Effect of cobalt doping on the electronic, optical and photocatalytic properties of TiO2 Peng Jiang a, Wei Xiang a, Jianlei Kuang a, Wenxiu Liu a, Wenbin Cao a, b, * a
Department of Inorganic Nonmetallic Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China b Tianjin College, University of Science and Technology Beijing, Tianjin 301830, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2015 Received in revised form 21 May 2015 Accepted 22 May 2015 Available online 27 May 2015
To extend the optical response to visible light region, cobalt doped TiO2 (Co-doped TiO2) with dopant concentration of 0.1 %e3.0 at % was synthesized by one step hydrothermal method without any postheat treatment for crystallization. X-ray diffraction results confirm that all the doped and undoped TiO2 were composed of pure anatase phase with good crystallinity. The calculated grain size was ranged from 8.4 to 10.5 nm by Scherrer's method. The photocatalytic activities of the synthesized samples were evaluated by degradation of phenol under visible light irradiation. The Co-doped TiO2 sample with dopant concentration of 0.3 at% shows the highest degradation ratio. First principle calculations were performed and the results revealed that cobalt doping could realize its visible light response and enhance the photocatalytic performance of TiO2 by introducing impurity states in its band gap. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: TiO2 Cobalt doping Photocatalytic activity First principle calculation
1. Introduction Water and air pollution have become urgent challenges for many countries in recent years. Photocatalytic semiconductors, such as TiO2, ZnO and SnO2, have attracted more and more attention for their potentials in photodegradation of harmful contents. Especially, TiO2 is considered as one of the most promising materials due to its high stability and environmental safety. However, the application of TiO2 in water and air purification is restricted as its intrinsic wide band gap (3.2 eV) requires high energy excitation such as UV light, which only composes 5% of solar irradiation. Thus, the major portion of solar energy (visible light) could not be used for photocatalytic reaction. Besides, high recombination rate of photo excited carriers is another limitation for the applicable fields of TiO2. Due to the short life of photo excited carriers, only a small part of electrons and vacancies can move to its surface, which lead to further reduction of photocatalytic efficiency. Lots of attempts were tried to improve the photocatalytic performance of TiO2 and doping with transition metal ions, such as copper, vanadium and chromium, was found to be a useful method
* Corresponding author. Department of Inorganic Nonmetallic Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. E-mail address: [email protected] (W. Cao). http://dx.doi.org/10.1016/j.solidstatesciences.2015.05.007 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
[1e6]. Among all the available transition metals, cobalt was proved to be one of the most effective dopants to enhance the light response and photoactivity of TiO2. Bryan [7] prepared colloidal cobalt doped TiO2 which shows a wide absorption range extended into the visible region. Iwasaki [8] synthesized cobalt doped TiO2 and found that the introduction of Co2þ could apparently shift the light absorption edge of anatase TiO2 to the visible region and enhance photoactivity under both UV and visible light irradiation. However, most cobalt doped TiO2 samples were synthesized from the organic titanium precursors, which are usually poisonous. Bryan [10] synthesized the cobalt doped TiO2 by titanium tetraisopropoxide. Manivannan [9] synthesized cobalt doped TiO2 films from titanium acetylacetonate followed with the heat treatment at 400 C; Titanium alkoxides [11,12] were also chosen as the raw material by researchers. Except for the use of harmful organic compounds, another disadvantage of the reported preparation technology is that high temperature heat treatment was prerequisite for crystallization. Sadanandam [13] synthesized cobalt doped TiO2 by impregnation method which requires calcinations at 400 C for 5 h. In this study, we successfully prepared cobalt doped TiO2 powders from TiOSO4 by one step hydrothermal method at moderate temperature (150 C) without any following heat treatment. Multiple characterization methods, such as XRD, XPS, TEM and UVevis absorption, were used to characterize the assynthesized samples [14]. First principle calculations based on
28
P. Jiang et al. / Solid State Sciences 46 (2015) 27e32
density function theory (DFT) were carried out to investigate the effect of the introduced cobalt on band structure and density of state (DOS) of TiO2.
displacement within 0.001 Å.
3. Results and discussion 2. Experiments and calculations 3.1. XRD and TEM 2.1. Synthesis of cobalt doped TiO2 Both pure and cobalt doped TiO2 were synthesized via hydrothermal method. CoSO4 (analytic grade) was chosen as cobalt source. In the synthesis process, stoichiometric amount of TiOSO4 (technical grade), (NH2)2CO (technical grade) and CoSO4 were dissolved in deionized water and stirred for 5 min. Subsequently, the mixed solution was transferred into a 100 ml autoclave and heated at 150 C for 10 h. The obtained precipitates were repeatedly washed with deionized water until no SO2þ can be detected by 4 Ba2þ, then they were extracted and dried at 80 C without any following heat treatment. Cobalt doped TiO2 samples with different nominal doping concentrations (0.1%, 0.3%, 0.5%, 1%, and 2%) and un-doped TiO2 were prepared through the same method. 2.2. Characterization methods X-ray diffraction (XRD) was utilized to identify the phase composition and crystallinity using Cu Ka radiation at 40 KV and 130 mA. The patterns were recorded continuously with 2q range of 10e90 . Sherrer's equation was used to calculate the crystal size. Xray photoelectric spectrum (XPS, VG Escalab MK II) was carried to characterize surface chemical state. Transmission electron microscopy (TEM, JEOL-200CX; Hitachi) was used to observe the morphologies of prepared samples. The light adsorption spectra of cobalt doped TiO2 were obtained by a UVevis scanning spectrophotometer (TU-1901) using BaSO4 as a reference standard. The photocatalytic activity was evaluated by degradation of 100 ml phenol, of which the concentration is 25 mg/L under visible light irradiation. Prior to degradation, 0.25 g powder was added into the phenol solution, then the solution was continuously stirred for 1 h in dark to reach the adsorption/desorption equilibrium. After that, visible light source (15.4 mW/cm3, 400e700 nm) was turned on and the solution was sampled every 2 h. All samples were centrifuged to remove the residual particles, and then UVevisible spectra of each sample were measured and the results were utilized to describe concentration variation of phenol solution using LamberteBeer law.
The XRD patterns of pure and cobalt doped TiO2 with various concentrations are illustrated in Fig. 1. The patterns of all synthesized powders via hydrothermal method show sharp peaks indicating the particles are highly crystallized and all the peaks can be indexed as anatase TiO2 phase (JCPDS: no.21e1272) [16]. It is deserve to be mentioned that the color of pure TiO2 is white, however, cobalt doped samples are all pale yellow and the color is deepened when cobalt concentration is elevated. For no peaks corresponding to metallic cobalt or cobalt compound is observed and the color of TiO2 is changed with cobalt amount, it is reasonable to suppose that cobalt cations are successfully introduced into TiO2 and homogenously distributed in the lattice. The average particle size of each sample is calculated from the full width at halfmaximum (FWHM) of the (101) diffraction peak using Scherrer's equation [17]. The results are demonstrated in Table 1. All prepared samples are in nano-size range, from 8.4 to 10.5 nm, and cobalt doped samples show smaller or equal particle size compared to pure TiO2. TEM is used to describe the morphology of the powders and the images of pure and 0.3% CoeTiO2 is displayed in Fig. 2. All these particles are in nano-size range and the average diameter is about 10 nm which is in agreement with the XRD result. Photocatalytic reaction is occurred on the surface and the exposure of planes with high surface energy would apparently accelerate the reaction rate. But in the process of crystal growth, low energy planes would cover the surface particles spontaneously. Proved by thermodynamic calculation, the {101} facets of anatase TiO2 are the most stable planes and the steadiest shape of anatase TiO2 is slightly truncated diamond with more than 90% surface surrounded by {101} [18]. However, in our TEM images, both pure and cobalt doped TiO2 particles synthesized by hydrothermal method existed partly in the shape of sphere or polygon, thus the exposure of {101} facet is lower than diamond shape and other facets with high surface energy would be exposed. So, the photocatalytic ability of hydrothermal synthesized powders would be improved.
2.3. Calculation method First principle calculations were carried out based on spinpolarized density function theory (DFT) in the framework of generalized gradient approximation (GGA) [15]. For exchange and correlation interaction, Perdew-Purke-Ernzerhof (PBE) function was applied and all simulations were performed with ultrasoft pseuodopotential, which was adopted to describe the potential between ionic core and valence electrons by CASTEP code. Cut-off energy of wave function was set at 380 eV, 3 3 3 k mesh was used for integration in the first Brillion Zone (BZ) and all calculations were performed in reciprocal space. The primitive anatase lattice was introduced from Materials Studio library and then expanded to a 2 2 1 supercell with 96 atoms. In order to simulate cobalt doped TiO2, one and two Ti atoms were replaced by Co atoms in the supercell model marked as Co1Ti15O32 and Co2Ti14O32. Geometry optimization was implemented before calculating each property to get a stable configuration and the criterion for relaxation process was set as atom residual forces should be limited within 0.03 eV/Å and the maximum ionic
Fig. 1. XRD patterns of pure TiO2 and cobalt doped TiO2.
P. Jiang et al. / Solid State Sciences 46 (2015) 27e32
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Table 1 Average particle sizes calculated from Scherrer's equation. Samples
Pure TiO2
0.1% CoeTiO2
0.3% CoeTiO2
0.5% CoeTiO2
1% CoeTiO2
2% CoeTiO2
3% CoeTiO2
Size (nm)
10.5
8.4
8.4
10.5
10.5
8.4
8.4
Fig. 2. TEM images of (a) pure TiO2 (b) 0.3% CoeTiO2.
3.2. X-ray photoelectron spectrum analysis
3.3. Electron structure calculations
Identification of chemical elements in cobalt doped TiO2 and their corresponding valence states are accomplished by XPS test, and the spectra (shown in Fig. 3) provide another evidence of the successful introduction of cobalt ions. It is found that 5 different chemical elements (Ti, O, C, N, Co) existed. Ti, O and Co originates from the precursor materials and C was used as calibration element. N came from two possible sources: (1) urea which is one of raw materials (2) nitrogen gas from air adsorbed on sample's surface. Strong peaks at about 464.08 eV and 458.08 eV are corresponding to Ti 2p1 and 2p3 states respectively. These peaks match well with characteristic peaks of Ti4þ in anatase prepared by Joen [19] and Manivannan [9], which implies the phase of the synthesized powder is anatase. The binding energy of Co 2p1 and Co 2p3 are 796.06 eV and 780.66 eV respectively, with an energy difference of 15.4 eV, shown in Fig. 3(b). Considering both XPS and XRD results, we can get a conclusion that Co ions are successfully introduced into TiO2 lattice. It is found that the binding energy of Co 2p3 in CoO at 780.4 eV is close to our results, and the shake up peaks indicating the existence of Co2þ [20], are also observed in high energy side of the spectrum. Moreover, the binding energy difference between 2p1 and 2p3 is consistence with XPS data of Co2þ [21,22]. Base on these evidences, cobalt ion in anatase lattice is at þ2 valence state.
First principle calculations were carried out to investigate the effect of cobalt on the electronic structure of anatase TiO2. Both pure and cobalt doped models with two different concentrations were built and their band structures are illustrated in Fig. 4. The dash line represents Fermi level that is set at 0 eV in all the calculations. The calculated band gap of pure TiO2 is 2.219 eV that is lower than experimental value of 3.2 eV, which is due to the drawback of DFT theory. As only relative change compared with pure TiO2 is discussed, this drawback would not have an effective influence on our results. Compared with the band structure of undoped TiO2, the Fermi level of doped one move towards the conduction band and lots of impurity states are introduced into its forbidden band. Widely distributed impurity states appeared in the band gap could act as ‘steps’ for photo carriers transition, and then promote the visible light absorption and photocatalytic reaction. With the increase of dopant concentration, the density of impurity state is enriched and it is reasonable to speculate that its light response would also be improved. Light absorption edge is closely related to the band gap, and the wide intrinsic band gap is one of the most important factors that restrict the photocatalytic applications of TiO2. In order to red (in the web version) shift the absorption edge of anatase TiO2, many attempts were tried to reduce the gap value. In our built models,
Fig. 3. XPS spectra of (a) 2% cobalt doped TiO2 (b) Co 2p core level.
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P. Jiang et al. / Solid State Sciences 46 (2015) 27e32
Fig. 4. Band structures of different Co-doped models.
the band gaps of cobalt doped TiO2 with different concentrations (Fig. 5(b) and (c)) shows similar width with the undoped one (Fig. 5(a)), so the introduction of cobalt doped would not greatly affect the absorption edge. However, it can be seen that impurity states are introduced into the forbidden band because of the cobalt doping, as shown in Fig. 5(b). And, the density of impurity state increased with the elevated dopant concentration and some impurity states overlap with valence band or conduction band (Fig. 5(c)). Therefore, these impurity states are beneficial to the enhancement of visible light absorption and the improvement of photo catalysis efficiency. Partial density of state (PDOS) of Co1Ti15O32 (Fig. 5(d)e(g))
Fig. 5. DOS patterns of: (a) pure TiO2 (Ti16O32) (b) CoTi15O32 and (c) Co2Ti15O32; PDOS patterns of (d) Co1Ti15O32 (e) Ti (f) Co (g) O in CoTi15O32.
Fig. 6. UVevis spectra of cobalt doped and undoped TiO2 samples.
P. Jiang et al. / Solid State Sciences 46 (2015) 27e32
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Table 2 Edge wavelength and band gap of different samples. Sample
TiO2
0.1% CoeTiO2
0.3% CoeTiO2
0.5% CoeTiO2
1% CoeTiO2
2% CoeTiO2
3% CoeTiO2
Absorption edge (nm) Band gap (eV)
382 3.25
386 3.21
388 3.19
392 3.16
394 3.15
403 3.08
400 3.10
3.5. Photocatalytic activity
Fig. 7. Photocatalytic degradation of phenol under visible light by synthesized samples and P25.
indicates the imported impurity states come from Co 3d states, meanwhile, Ti 3d and O 2p states mostly consist of conduction band and valence band respectively. It can be seen that Co 3d state partially overlapped with O 2p state near valence band, and the low concentration of cobalt corresponds to the slight variation of band gap.
Photocatalytic activities of the prepared samples were evaluated by degrading phenol solution under visible light irradiation. Fig. 7 represents the photocatalytic reaction results compared with P25 (20% rutile and 80% anatase). C and Co stand for the concentration of phenol at a specific time after dark adsorption. All the tested samples show little adsorption, which can be neglected in the degradation. The photocatalytic reaction follows a zero-order rate law, the apparent rate constants (K/C0, Time: h) and degradation rate of all cobalt doped sample are demonstrated in Table 3. Moderate degradation efficiency (64.51%) exhibited by P25 is due to the well-known mixed phase effect which promote photo induced carriers separation. Pure TiO2 degrades 53% of phenol solution, which may be attributed to the existence of trace impurity doped into TiO2 from the raw material of industrial level TiOSO4. Degradation rate is enhanced to 64.21% by adding 0.1% cobalt and the highest degradation rate (81.72%) is demonstrated by 0.3% cobalt doped TiO2. Enhanced degradation rate is due to the doped cobalt ions which not only improve absorption rate of visible light but act as shallow traps to separate photo induced carriers efficiently as well. However, when the concentration of doping ions is too high, doping ions may act as recombination centers and decrease the photocatalytic activity. It is reported [23] that the optimal concentration of doping ions should make the thickness of the space charger layer substantially equal to the light penetration depth. So, further addition of cobalt leads to gradually reduction of catalytic efficiency as indicated in Table 3. The highest photocatalytic activity of 0.3% cobalt doped TiO2 attributes to its proper electron structure, improved visible absorption and appropriate doped concentration.
4. Conclusions 3.4. UVevis spectra As shown in Fig. 6 and Table 2, UVevis spectra of pure and doped samples are tested and the results are used to evaluate the shift of band gaps. Pure TiO2, with band gap equal to 3.25 eV and absorption edge at 382 nm, are synthesized with hydrothermal method and shows almost no visible light absorption. The band gap of all cobalt doped samples decreased slightly compared with pure TiO2. But for all the doped samples, the absorption efficiencies in the range of 400 nme700 nm are improved and the increment is proportional to the cobalt amount. Density of Co 3d impurity states in Fig. 5(f) show two peaks and these would contribute to the formation of ‘steps’ in the absorption spectra. So, widely distributed impurity states of Co 3d level in forbidden band would improve the photo response of TiO2, and then enhance its visible light photocatalytic activity.
Cobalt doped TiO2 powders are successfully synthesized using hydrothermal method without any high temperature heat treatment. Comprehensive characterization results confirm the successful introduction of Co2þ in anatase crystal structure and enhanced absorption in visible range for all doped samples. The photocatalytic activity is tested by the degradation of phenol under visible light irradiation and the highest degradation efficiency is performed by 0.3% cobalt doped TiO2. First principle calculation reveals lots of 3d impurity states are appeared in forbidden band after substantial introduction of cobalt and this would contribute to photocatalytic activity under visible light irradiation. Though we cannot confirm how impurity element is introduced into TiO2 lattice during hydrothermal process and many attempts are necessary for further investigation. We are focusing on elucidating mechanism about of doping and crystallization by simulation and experiment.
Table 3 Degradation rate of each cobalt doped sample. Sample
P25
Pure
0.1% CoeTiO2
0.3% CoeTiO2
0.5% CoeTiO2
1% CoeTiO2
2% CoeTiO2
3% CoeTiO2
Degradation rate (%) K/C0
64.51 0.055
53.00 0.040
64.21 0.054
81.72 0.070
70.43 0.060
56.13 0.048
44.66 0.030
35.15 0.031
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Acknowledgment This work is financially supported by National Natural Science Foundation of China under the grant No: 51402016, Fundamental Research Funds for the Central Universities under the grant No: FRF-TP-14-007A1, and project No: BZZ14J001. References [1] C. Lee, C.M. Aikens, Effects of Mn doping on (TiO2)n (n ¼ 2-5) complexes, Comput. Theor. Chem. 1013 (2013) 32e45. [2] L.G. Devi, et al., Enhanced photocatalytic activity of transition metal ions Mn2þ, Ni2þ and Zn2þ doped polycrystalline titania for the degradation of aniline blue under UV/solar light, J. Mol. Catal. A Chem. 328 (1) (2010) 44e52. [3] Y.Y. Gurkan, E. Kasapbasi, Z. Cinar, Enhanced solar photocatalytic activity of TiO2 by selenium (IV) ion-doping: characterization and DFT modeling of the surface, Chem. Eng. J. 214 (2013) 34e44. [4] X. Zou, et al., Heterometal alkoxides as precursors for the preparation of porous Fe- and Mn-TiO2 photocatalysts with high efficiencies, Chemistry A Eur. J. 14 (35) (2008) 11123e11131. [5] H. Feng, M. Zhang, L.E. Yu, Hydrothermal synthesis and photocatalytic performance of metal-ions doped TiO2, Appl. Catal. A: General 413e414 (2012) 238e244. [6] V.N. Nguyen, N.K.T. Nguyen, P.H. Nguyen, Hydrothermal synthesis of Fe-doped TiO2 nanostructure photocatalyst, Adv. Nat. Sci. Nanosci. Nanotechnol. 2 (2011) 035014e035018. [7] J.D. Bryan, et al., Strong room-temperature ferromagnetism in Co2þ-doped TiO2 made from colloidal nanocrystals, J. Am. Chem. Soc. 126 (37) (2004) 11640e11647. [8] M. Iwasaki, et al., Cobalt ion-doped TiO2 photocatalyst response to visible light, J. Colloid Interface Sci. 224 (1) (2000) 202e204. [9] A. Manivannan, et al., Magnetism of co-doped titania thin films prepared by spray pyrolysis, Appl. Phys. Lett. 83 (1) (2003) 111e113. [10] J.D. Bryan, et al., Activation of high-TC ferromagnetism in Co2þ: TiO2 and Cr3þ: TiO2 nanorods and nanocrystals by grain boundary defects, J. Am. Chem. Soc.
127 (44) (2005) 15568e15574. [11] J.T. Thienprasert, et al., Local structures of cobalt in co-doped TiO2 by synchrotron x-ray absorption near edge structures, Curr. Appl. Phys. 11 (3S) (2011) S279eS284. [12] B. Choudhury, A. Choudhury, Luminescence characteristics of cobalt doped TiO2 nanoparticles, J. Luminescence 132 (1) (2012) 178e184. [13] G. Sadanandam, et al., Cobalt doped TiO2: a stable and efficient photocatalyst for continuous hydrogen production from glycerol: water mixtures under solar light irradiation, Int. J. Hydrogen Energy 38 (23) (2013) 9655e9664. [14] M. Khan, W. Cao, Cationic (V, Y)-codoped TiO2 with enhanced visible light induced photocatalytic activity: a combined experimental and theoretical study, J. Appl. Phys. 114 (18) (2013) 183514. [15] M. Khan, W. Cao, Preparation of Y-doped TiO2 by hydrothermal method and investigation of its visible light photocatalytic activity by the degradation of methylene blue, J. Mol. Catal. A Chem. 376 (2013) 71e77. [16] M.K. Seery, et al., Silver doped titanium dioxide nanomaterials for enhanced visible light photocatalysis, J. Photochem. Photobiol. A Chem. 189 (2) (2007) 258e263. [17] M. Khan, W. Cao, Cationic (V, Y)-codoped TiO2 with enhanced visible light induced photocatalytic activity: a combined experimental and theoretical study, J. Appl. Phys. 114 (18) (2013) 183514. [18] X. Zhao, et al., Shape-and size-controlled synthesis of uniform anatase TiO2 nanocuboids enclosed by active {100} and {001} facets, Adv. Funct. Mater. 21 (18) (2011) 3554e3563. [19] B. Jeong, et al., Properties of anatase CoxTi1-xO2 thin films epitaxially grown by reactive sputtering, Thin Solid Films 488 (1) (2005) 194e199. [20] J. Li, et al., Cobalt-Doped TiO2 nanocrystallites: radio-frequency thermal plasma processing, phase structure, and magnetic properties, J. Phys. Chem. C 113 (19) (2009) 8009e8015. [21] B. Choudhury, et al., Effect of oxygen vacancy and dopant concentration on the magnetic properties of high spin Co2þ doped TiO2 nanoparticles, J. Magnetism Magnetic Mater. 323 (5) (2011) 440e446. [22] Y.B. Lin, et al., Ferromagnetism of co-doped TiO2 films prepared by plasma enhanced chemical vapour deposition (PECVD) method, J. Phys. D Appl. Phys. 41 (19) (2008) 195007. [23] W. Zhou, et al., Preparation and properties of vanadium-doped TiO2 photocatalysts, J. Phys. D Appl. Phys. 43 (3) (2010) 035301.