Nanocrystalline cobalt titanate (CoTiO3) has been synthesized from a well-mixed precursor. The precursor was prepared by a homogeneous precipitation method, in which urea was used as the precipitator. The as-synthesized sample is a mixture of CoTiO3 and rutile nanocrystallites with mean sizes of around 50 nm. It strongly absorbs visible light in the wavelength range of 500-690 nm besides ultraviolet light (wavelength < 370 nm). We found the synthetic CoTiO3 is a semiconducting material with a direct band gap of 2.53 eV. The green CoTiO3 has two absorbance peaks at the wavelengths of 537 and 606 nm, which correspond to the photon energies of 2.31 and 2.05 eV, respectively.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3195

Gao, Xiao-Mei

Hou, Yin-Ling

Li, Ming-Wei

Wang, Cheng-Yang

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 5 No 3, 03022(3pp) (2013) Том 5 № 3, 03022(3cc) (2013)   2077-6772/2013/5(3)03022(3) 03022-1  2013 Sumy State University Characterization of CoTiO3 Nanocrystallites Prepared  by Homogeneous Precipitation Method  Ming-Wei Li1,*, Xiao-Mei Gao1, Yin-Ling Hou1, Cheng-Yang Wang2  1 School of Science, Tianjin University, Tianjin 300072, China 2 School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China  (Received Received 12 May 2013; revised manuscript received 01 July 2013; published online 12 July 2013)  Nanocrystalline cobalt titanate (CoTiO3) has been synthesized from a well-mixed precursor. The pre-cursor was prepared by a homogeneous precipitation method, in which urea was used as the precipitator. The as-synthesized sample is a mixture of CoTiO3 and rutile nanocrystallites with mean sizes of around 50 nm. It strongly absorbs visible light in the wavelength range of 500-690 nm besides ultraviolet light (wavelength < 370 nm). We found the synthetic CoTiO3 is a semiconducting material with a direct band gap of 2.53 eV. The green CoTiO3 has two absorbance peaks at the wavelengths of 537 and 606 nm, which correspond to the photon energies of 2.31 and 2.05 eV, respectively.  Keywords: Cobalt titanate, Nanocrystalline materials, Microstructure, Optical property.   PACS numbers: 61.46. ± w, 78.67. ± n, 71.20.Nr                                                                   * mingweili@tju.edu.cn 1. INTRODUCTION  Titania (TiO2) and metal titanates MTiO3 (M  Fe, Co, Ni, Mn, Zn, et al) are semiconducting materials with unique properties. They have potential applica-tions as photocatalysts [1-4], gas sensors [5-7], electron-ic components [8-10], and pigments [11, 12]. CoTiO3 exhibits some excellent properties among them. CoTiO3 is stable and can be applied in high-temperature envi-ronments [9, 13]. Moreover, the band gap of CoTiO3 (~ 2.25 eV [14]) is narrower than that of TiO2 (~ 3.3 eV for anatase-phase TiO2). Therefore, CoTiO3 possibly utilizes visible light with less photon energy. CoTiO3 differs from the white TiO2, it is green and has strong optical absorption in visible light region [3, 12, 15]. The colored semiconducting materials often indicate that they selectively absorb visible light. The absorbed pho-ton energy can promote the electron transition and the formation of electron-hole pairs. This phenomenon has recently been reported when a red metallic oxide was used as a photocatalyst [16]. In this work, we synthesized CoTiO3 with green col-or and studied its band gap, optical property, and pho-tocatalytic activity. Actually, CoTiO3 could been facilely synthesized by solid-state reaction between Co3O4 and TiO2 powders at high temperatures [10], but it is diffi-cult to control its microstructure because the raw mate-rials are in different phases. This process often leads to heterogeneous powders, and has worse reproducibility. To synthesize well-crystallized CoTiO3 powders, most researchers firstly prepared precursors, and then  heated them at high temperatures (> 500 °C). If Co2 + and Ti4 + ions in the precursors were well mixed, gene-rally CoTiO3 crystals are easily formed. Up to now, the precursors had been prepared by varied methods, which include chemical co-precipitation [3, 5, 11, 17], aerogel method [4], polyol method [6, 7], chemical vapor deposition [8], sol-gel method [18, 19], and Pechini method [15]. Herein, we used a homogeneous precipitation me-thod to prepare the precursor, and heated it to synthe-size well-ordered CoTiO3 nanocrystallites. The homo-geneous precipitation method had ever been successful-ly used to prepare uniform nanoparticles [20]. The re-sults reveal CoTiO3 is a direct band gap semiconductor, which can strongly absorb visible light.  2. EXPERIMENTAL  7.5 mL hydrochloric acid solution of TiCl4 (contain-ing 0.02 mol of TiCl4), 0.02 mol Co(NO3)2·6H2O, and 18.18 g urea were dissolved in 600 mL deionized water under vigorous stirring. Urea was used as the precipi-tator to control the gradual formation of precipitate. The forming suspension was stirred for 6 h at refluxing temperature, and then was filtered. After the grey filter cake being washed with deionized water and anhydrous ethanol, it was dried in air overnight at 130 °C. After being grounded, the precursor was heated at 800 °C for 6 h and formed dark green product. A commercial P-25 sample purchased from Degussa Corporation was used as the reference sample, which consists of anatase (~ 80 %) and rutile (~ 20 %) nanoparticles.  Thermal behavior of the precursor was analyzed by thermogravimetric and differential scanning calorime-try (TG-DSC) using a NETZSCH STA 409 PC/PG si-multaneous thermal analyzer from room temperature to 1100 °C in air. The as-synthesized product was characterized by a Rigaku D/max 2500 V/PC powder X-ray diffractometer (XRD) using CuKα radiation (   0.15406 nm). The mean sizes of the crystallites were calculated by Scher-rer formula using 0.90 as the shape factor. We found the sample consists of CoTiO3 and rutile. The weight fraction of CoTiO3 (xi) was estimated by the equation [21]: xi  [1 + (Ir / Ii)(Ki / Kr)] – 1, where Ii and Ir are the intensities of main diffraction peaks of CoTiO3 and ru-tile, respectively. Ki (  2.81) and Kr (  3.68) are their reference intensity ratios values, which are selected from JCPDS 77-1373 and 84-1284, respectively. The sample’s microstructures were studied using an FEI Tec-nai G2 F20 field emission high resolution transmission  M.-W. LI, X.-M. GAO, Y.-L. HOU, C.-Y. WANG J. NANO- ELECTRON. PHYS. 5, 03022 (2013)   03022-2    Fig. 1 – TG-DSC curves of the precursor  electron microscope (HRTEM). The ultraviolet-visible light diffuse reflectance spectra (UV-Vis DRS) were measured at room temperature with a JASCO V-570 UV-Vis spectrophotometer. The photocatalytic activity of the samples was eval-uated by using them degrading methylene blue in aqueous solution (100 mg L – 1) under the irradiation of a 300 W high pressure Hg lamp.  3. RESULTS AND DISCUSSION  Fig. 1 exhibits TG-DSC curves of the precursor. The sample totally lost about 11.1 % weight below 770 °C. The most weight loss, ~ 9.3 %, occurred below 430 °C, and probably resulted from the escape of surface water and crystal water. The exothermic process below 680 °C was mainly caused by the formation of anatase and CoO crystallites. Additionally, the growth of crystallites also is an exothermic process. About 1.3 % weight loss occurred between 680 and 770 °C with an endothermic process. It probably resulted from the escape of the residual crystal water, the transformation of anatase to rutile, and the formation of CoTiO3 crystals.  Fig. 2 shows the XRD pattern of the as-synthesized sample. It is a mixture of CoTiO3 and rutile with a weight ratio of 53 / 47 or a molar ratio of 37 / 63. No oth-er phase, such as anatase, CoO or Co3O4, was detected.    Fig. 2 – XRD pattern of the as-synthesized CoTiO3/ rutile sample  The lattice parameters of CoTiO3 are: R–, 3 (148) a  5.0649 (1) Ǻ, c  13.9226 (9) Ǻ, V  309.31 Å3, Z  6, and Dx  4.9872 g cm – 3. The data are in good agree-ment with literature [19, 22]. The mean sizes of the CoTiO3 and rutile crystallites are 49 and 51 nm, respectively. We think almost all Co2 + ions had been utilized to build the CoTiO3 crys-tals, and the residual TiO2 exists in rutile phase.  Although the raw materials contain equimolar Ti4 + and Co2 + ions, the precursor could not keep it due to the different solubilities of Ti4 +- and Co2 +-containing pre-cipitates and the loss of soluble [Co(NH3)6]2 + during the washing process. HRTEM images of the synthetic sample are shown in Fig. 3. Although the powders easily agglomerated as it is shown in Fig. 3a, an enlarged image (Fig. 3b)  exhibits the microstructure of crystalline CoTiO3. The spacing 0.27 nm corresponds to the main lattice planes (104) of CoTiO3. The crystallite has a layered micro-structure, which is clearer than a sample synthesized via sol-gel process [18]. Fig. 4 shows the samples’ UV-Vis DRS. Commercial P-25 strongly absorbs ultraviolet light (wavelength < 320 nm), but hardly absorbs any visible light. It has an absorption threshold at 410 nm. However, the CoTiO3 / rutile sample strongly absorbs visible light in    Fig. 3 – HRTEM images of the CoTiO3 / rutile sample  CHARACTERIZATION OF COTIO3 NANOCRYSTALLITES… J. NANO- ELECTRON. PHYS. 6, 03022 (2013)   03022-3    Fig. 4 – UV-Vis DRS of the as-synthesized CoTiO3 / utile sample and the P-25 sample. Tauc plots are shown in the inset  the wavelength range of 500-690 nm besides ultraviolet light (wavelength < 370 nm). Its optical absorption edge also shifts to 500 nm in visible light region. We propose the absorbance in visible light region is mainly caused by CoTiO3 because TiO2 hardly absorb visible light. The samples’ band gaps can be estimated by Tauc plots, i.e., the curves of ( hv)2 / n vs hv, where  and hv are the absorbance and photon energy, respectively [2, 23, 24]. The n value indicates the optical transition is direct (n  1) or indirect (n  4). The hv value at the intersection point of the tangent and the horizontal axis is the band gap. As shown in the inset, the band gap of P-25 is about 3.32 eV, which coincides with the band gap of anatase and reflects the O2 – → Ti4 + charge-transfer transition. The synthetic CoTiO3 has a band gap of 2.53 eV, which originates from the Co2 + → Ti4 + charge-transfer transition. Both the sam-ples are direct band gap semiconductors because n  1. We found the UV-Vis DRS of CoTiO3 well coincides with literature [3, 4, 11]: There exist a small absorb-ance peak at 537 nm and a big one at 606 nm, which correspond to the photon energies of 2.31 and 2.05 eV, respectively. The selective optical absorbance in visible light region makes CoTiO3 being green color. The two absorbance peaks should be defined by the crystal structure of CoTiO3, which is an ilmenite structure be-ing constructed by octahedral CoO6 and TiO6. The de-tailed mechanism needs further investigation. The samples’ photocatalytic activities were evalua-ted. Experimentally, almost all methylene blue degra-ded in 20 min when P-25 was used as a photocatalyst. However, the CoTiO3 / rutile sample does not show ob-vious photocatalytic activity. Only less than 2 % meth-ylene blue degraded. This result is agreement with previous reports [4, 25]. Possibly, the low quantum effi-ciency and the poor oscillator strength for the Co2 + → Ti4 + charge-transfer transition weakened the performance of CoTiO3 as a photocatalyst.  4. CONCLUSIONS  The homogeneous precipitation method can be used to synthesize CoTiO3 nanocrystallites. The synthetic CoTiO3 is a semiconducting material with a narrower direct band gap of 2.53 eV. The sample is dark green and strongly absorbs visible light in the wavelength range of 500-690 nm. It also has two optical absorbance peaks at 537 and 606 nm. However, the sample does not exhibit obvious photocatalytic activity.  ACKNOWLEDGEMENTS   This work was supported by the Special Funds for Major State Basic Research Program of China under grant number 2012CB720300.  REFERENCES  1. X. Chen, S.S. Mao, Chem. Rev. 107, 2891 (2007). 2. D.S. Ginley, M.A. Butler, J. Appl. Phys. 48, 2019 (1977). 3. M. Iwasaki, M. Hara, H. Kawada, H. Tada, S. Ito, J. Col-loid Interface Sci. 224, 202 (2000). 4. P.N. Kapoor, S. Uma, S. Rodriguez, K.J. Klabunde, J. Mol. Catal. A Chem. 229, 145 (2005).  5. X. Chu, X. Liu, G. Wang, G. Meng, Mater. Res. Bull. 34, 1789 (1999). 6. M. Siemons, U. Simon, Sensor. Actuat. B Chem. 120, 110 (2006). 7. M. Siemons, U. Simon, Sensor. Actuat. B Chem. 126, 595 (2007). 8. T.M. Pan, T.F. Lei, T.S. Chao, J. Appl. Phys. 89, 3447 (2001). 9. H.T. Kim, M.T. Lanagan, J. Am. Ceram. Soc. 86, 1874 (2003).  10. P.S. Anjana, M.T. Sebastian, J. Am. Ceram. Soc. 89, 2114 (2006).  11. L. Zhou, S.-Y. Zhang, J.-C. Cheng, Zhang L.-D., Z. Zeng, Mater. Sci. Eng. B 49, 117 (1997). 12. R. Levinson, P. Berdahl, H. Akbari, Sol. Energ. Mater. Sol. C. 89, 351 (2005). 13. X. Liu, R. Hong, C. Tian, J. Mater. Sci. Mater. El. 20, 323 (2009). 14. Y. Xu, M.A.A. Schoonen, Am. Mineral. 85, 543 (2000). 15. Y.-J. Lin, Y.-H. Chang, W.-D. Yang, B.-S. Tsai, J. Non-Cryst. Solids 352, 789 (2006). 16. X. Xu, C. Randorn, P. Efstathiou, J.T.S. Irvine, Nat. Mater. 11, 595 (2012). 17. G.-W. Zhou, D.K. Lee, Y.H. Kim, C.W. Kim, Y.S. Kang, Bull. Korean Chem. Soc. 27, 368 (2006).  18. F. Pacheco, M. González, A. Medina, S. Velumani, J.A. Ascencio, Appl. Phys. A 78, 531 (2004).  19. H.Y. He, Powder Metall. 51, 224 (2008). 20. E. Matijević ,Chem. Mater. 5, 412 (1993). 21. C.R. Hubbard, E.H. Evans, D. Smith, J. Appl. Cryst. 9, 169 (1976). 22. Kidoh K., Tanaka K., Marumo F., H. Takei, Acta Cryst. B 40, 92 (1984). 23. J. Tauc, R. Grigorovici, A. Vancu, phys. status solidi 15, 627 (1966). 24. M.A. Butler, J. Appl. Phys. 48, 1914 (1977).  25. P. Salvador, C. Gutiérrez, J.B. Goodenough, J. Appl. Phys. 53, 7003 (1982).  

Characterization of CoTiO3 Nanocrystallites Prepared by Homogeneous Precipitation Method

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 5 No 3, 03022(3pp) (2013) Том 5 № 3, 03022(3cc) (2013) 
 
 
2077-6772/2013/5(3)03022(3) 03022-1  2013 Sumy State University 
Characterization of CoTiO3 Nanocrystallites Prepared  
by Homogeneous Precipitation Method 
 
Ming-Wei Li1,*, Xiao-Mei Gao1, Yin-Ling Hou1, Cheng-Yang Wang2 
 
1 School of Science, Tianjin University, Tianjin 300072, China 
2 School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China 
 
(Received Received 12 May 2013; revised manuscript received 01 July 2013; published online 12 July 2013) 
 
Nanocrystalline cobalt titanate (CoTiO3) has been synthesized from a well-mixed precursor. The pre-
cursor was prepared by a homogeneous precipitation method, in which urea was used as the precipitator. 
The as-synthesized sample is a mixture of CoTiO3 and rutile nanocrystallites with mean sizes of around 
50 nm. It strongly absorbs visible light in the wavelength range of 500-690 nm besides ultraviolet light 
(wavelength < 370 nm). We found the synthetic CoTiO3 is a semiconducting material with a direct band 
gap of 2.53 eV. The green CoTiO3 has two absorbance peaks at the wavelengths of 537 and 606 nm, which 
correspond to the photon energies of 2.31 and 2.05 eV, respectively. 
 
Keywords: Cobalt titanate, Nanocrystalline materials, Microstructure, Optical property. 
 
 PACS numbers: 61.46. ± w, 78.67. ± n, 71.20.Nr 
 
 
                                                                
* mingweili@tju.edu.cn 
1. INTRODUCTION 
 
Titania (TiO2) and metal titanates MTiO3 (M  Fe, 
Co, Ni, Mn, Zn, et al) are semiconducting materials 
with unique properties. They have potential applica-
tions as photocatalysts [1-4], gas sensors [5-7], electron-
ic components [8-10], and pigments [11, 12]. CoTiO3 
exhibits some excellent properties among them. CoTiO3 
is stable and can be applied in high-temperature envi-
ronments [9, 13]. Moreover, the band gap of CoTiO3 
(~ 2.25 eV [14]) is narrower than that of TiO2 (~ 3.3 eV 
for anatase-phase TiO2). Therefore, CoTiO3 possibly 
utilizes visible light with less photon energy. CoTiO3 
differs from the white TiO2, it is green and has strong 
optical absorption in visible light region [3, 12, 15]. The 
colored semiconducting materials often indicate that 
they selectively absorb visible light. The absorbed pho-
ton energy can promote the electron transition and the 
formation of electron-hole pairs. This phenomenon has 
recently been reported when a red metallic oxide was 
used as a photocatalyst [16]. 
In this work, we synthesized CoTiO3 with green col-
or and studied its band gap, optical property, and pho-
tocatalytic activity. Actually, CoTiO3 could been facilely 
synthesized by solid-state reaction between Co3O4 and 
TiO2 powders at high temperatures [10], but it is diffi-
cult to control its microstructure because the raw mate-
rials are in different phases. This process often leads to 
heterogeneous powders, and has worse reproducibility. 
To synthesize well-crystallized CoTiO3 powders, most 
researchers firstly prepared precursors, and then  
heated them at high temperatures (> 500 °C). If Co2 + 
and Ti4 + ions in the precursors were well mixed, gene-
rally CoTiO3 crystals are easily formed. Up to now, the 
precursors had been prepared by varied methods, 
which include chemical co-precipitation [3, 5, 11, 17], 
aerogel method [4], polyol method [6, 7], chemical vapor 
deposition [8], sol-gel method [18, 19], and Pechini 
method [15]. 
Herein, we used a homogeneous precipitation me-
thod to prepare the precursor, and heated it to synthe-
size well-ordered CoTiO3 nanocrystallites. The homo-
geneous precipitation method had ever been successful-
ly used to prepare uniform nanoparticles [20]. The re-
sults reveal CoTiO3 is a direct band gap semiconductor, 
which can strongly absorb visible light. 
 
2. EXPERIMENTAL 
 
7.5 mL hydrochloric acid solution of TiCl4 (contain-
ing 0.02 mol of TiCl4), 0.02 mol Co(NO3)2·6H2O, and 
18.18 g urea were dissolved in 600 mL deionized water 
under vigorous stirring. Urea was used as the precipi-
tator to control the gradual formation of precipitate. 
The forming suspension was stirred for 6 h at refluxing 
temperature, and then was filtered. After the grey filter 
cake being washed with deionized water and anhydrous 
ethanol, it was dried in air overnight at 130 °C. After 
being grounded, the precursor was heated at 800 °C for 
6 h and formed dark green product. A commercial P-25 
sample purchased from Degussa Corporation was used 
as the reference sample, which consists of anatase 
(~ 80 %) and rutile (~ 20 %) nanoparticles.  
Thermal behavior of the precursor was analyzed by 
thermogravimetric and differential scanning calorime-
try (TG-DSC) using a NETZSCH STA 409 PC/PG si-
multaneous thermal analyzer from room temperature 
to 1100 °C in air. 
The as-synthesized product was characterized by a 
Rigaku D/max 2500 V/PC powder X-ray diffractometer 
(XRD) using CuKα radiation (   0.15406 nm). The 
mean sizes of the crystallites were calculated by Scher-
rer formula using 0.90 as the shape factor. We found 
the sample consists of CoTiO3 and rutile. The weight 
fraction of CoTiO3 (xi) was estimated by the equation 
[21]: xi  [1 + (Ir / Ii)(Ki / Kr)] – 1, where Ii and Ir are the 
intensities of main diffraction peaks of CoTiO3 and ru-
tile, respectively. Ki (  2.81) and Kr (  3.68) are their 
reference intensity ratios values, which are selected 
from JCPDS 77-1373 and 84-1284, respectively. The 
sample’s microstructures were studied using an FEI Tec-
nai G2 F20 field emission high resolution transmission 
 M.-W. LI, X.-M. GAO, Y.-L. HOU, C.-Y. WANG J. NANO- ELECTRON. PHYS. 5, 03022 (2013) 
 
 
03022-2 
 
 
 
Fig. 1 – TG-DSC curves of the precursor 
 
electron microscope (HRTEM). The ultraviolet-visible 
light diffuse reflectance spectra (UV-Vis DRS) were 
measured at room temperature with a JASCO V-570 
UV-Vis spectrophotometer. 
The photocatalytic activity of the samples was eval-
uated by using them degrading methylene blue in 
aqueous solution (100 mg L – 1) under the irradiation of 
a 300 W high pressure Hg lamp. 
 
3. RESULTS AND DISCUSSION 
 
Fig. 1 exhibits TG-DSC curves of the precursor. The 
sample totally lost about 11.1 % weight below 770 °C. 
The most weight loss, ~ 9.3 %, occurred below 430 °C, 
and probably resulted from the escape of surface water 
and crystal water. The exothermic process below 680 °C 
was mainly caused by the formation of anatase and 
CoO crystallites. Additionally, the growth of crystallites 
also is an exothermic process. About 1.3 % weight loss 
occurred between 680 and 770 °C with an endothermic 
process. It probably resulted from the escape of the 
residual crystal water, the transformation of anatase to 
rutile, and the formation of CoTiO3 crystals.  
Fig. 2 shows the XRD pattern of the as-synthesized 
sample. It is a mixture of CoTiO3 and rutile with a 
weight ratio of 53 / 47 or a molar ratio of 37 / 63. No oth-
er phase, such as anatase, CoO or Co3O4, was detected. 
 
 
 
Fig. 2 – XRD pattern of the as-synthesized CoTiO3/ rutile 
sample 
 
The lattice parameters of CoTiO3 are: R
–
, 3 (148) 
a  5.0649 (1) Ǻ, c  13.9226 (9) Ǻ, V  309.31 Å3, Z  6, 
and Dx  4.9872 g cm – 3. The data are in good agree-
ment with literature [19, 22]. 
The mean sizes of the CoTiO3 and rutile crystallites 
are 49 and 51 nm, respectively. We think almost all 
Co2 + ions had been utilized to build the CoTiO3 crys-
tals, and the residual TiO2 exists in rutile phase.  
Although the raw materials contain equimolar Ti4 + and 
Co2 + ions, the precursor could not keep it due to the 
different solubilities of Ti4 +- and Co2 +-containing pre-
cipitates and the loss of soluble [Co(NH3)6]2 + during the 
washing process. 
HRTEM images of the synthetic sample are shown 
in Fig. 3. Although the powders easily agglomerated as 
it is shown in Fig. 3a, an enlarged image (Fig. 3b)  
exhibits the microstructure of crystalline CoTiO3. The 
spacing 0.27 nm corresponds to the main lattice planes 
(104) of CoTiO3. The crystallite has a layered micro-
structure, which is clearer than a sample synthesized 
via sol-gel process [18]. 
Fig. 4 shows the samples’ UV-Vis DRS. Commercial 
P-25 strongly absorbs ultraviolet light (wavelength 
< 320 nm), but hardly absorbs any visible light. It has 
an absorption threshold at 410 nm. However, the 
CoTiO3 / rutile sample strongly absorbs visible light in 
 
 
 
Fig. 3 – HRTEM images of the CoTiO3 / rutile sample 
 CHARACTERIZATION OF COTIO3 NANOCRYSTALLITES… J. NANO- ELECTRON. PHYS. 6, 03022 (2013) 
 
 
03022-3 
  
 
Fig. 4 – UV-Vis DRS of the as-synthesized CoTiO3 / utile 
sample and the P-25 sample. Tauc plots are shown in the inset 
 
the wavelength range of 500-690 nm besides ultraviolet 
light (wavelength < 370 nm). Its optical absorption 
edge also shifts to 500 nm in visible light region. We 
propose the absorbance in visible light region is mainly 
caused by CoTiO3 because TiO2 hardly absorb visible 
light. The samples’ band gaps can be estimated by Tauc 
plots, i.e., the curves of ( hv)2 / n vs hv, where  and hv 
are the absorbance and photon energy, respectively 
[2, 23, 24]. The n value indicates the optical transition 
is direct (n  1) or indirect (n  4). The hv value at the 
intersection point of the tangent and the horizontal 
axis is the band gap. As shown in the inset, the band 
gap of P-25 is about 3.32 eV, which coincides with the 
band gap of anatase and reflects the O2 – → Ti4 + 
charge-transfer transition. The synthetic CoTiO3 has a 
band gap of 2.53 eV, which originates from the 
Co2 + → Ti4 + charge-transfer transition. Both the sam-
ples are direct band gap semiconductors because n  1. 
We found the UV-Vis DRS of CoTiO3 well coincides 
with literature [3, 4, 11]: There exist a small absorb-
ance peak at 537 nm and a big one at 606 nm, which 
correspond to the photon energies of 2.31 and 2.05 eV, 
respectively. The selective optical absorbance in visible 
light region makes CoTiO3 being green color. The two 
absorbance peaks should be defined by the crystal 
structure of CoTiO3, which is an ilmenite structure be-
ing constructed by octahedral CoO6 and TiO6. The de-
tailed mechanism needs further investigation. 
The samples’ photocatalytic activities were evalua-
ted. Experimentally, almost all methylene blue degra-
ded in 20 min when P-25 was used as a photocatalyst. 
However, the CoTiO3 / rutile sample does not show ob-
vious photocatalytic activity. Only less than 2 % meth-
ylene blue degraded. This result is agreement with 
previous reports [4, 25]. Possibly, the low quantum effi-
ciency and the poor oscillator strength for the 
Co2 + → Ti4 + charge-transfer transition weakened the 
performance of CoTiO3 as a photocatalyst. 
 
4. CONCLUSIONS 
 
The homogeneous precipitation method can be used 
to synthesize CoTiO3 nanocrystallites. The synthetic 
CoTiO3 is a semiconducting material with a narrower 
direct band gap of 2.53 eV. The sample is dark green 
and strongly absorbs visible light in the wavelength 
range of 500-690 nm. It also has two optical absorbance 
peaks at 537 and 606 nm. However, the sample does 
not exhibit obvious photocatalytic activity. 
 
ACKNOWLEDGEMENTS  
 
This work was supported by the Special Funds for 
Major State Basic Research Program of China under 
grant number 2012CB720300. 
 
REFERENCES 
 
1. X. Chen, S.S. Mao, Chem. Rev. 107, 2891 (2007). 
2. D.S. Ginley, M.A. Butler, J. Appl. Phys. 48, 2019 (1977). 
3. M. Iwasaki, M. Hara, H. Kawada, H. Tada, S. Ito, J. Col-
loid Interface Sci. 224, 202 (2000). 
4. P.N. Kapoor, S. Uma, S. Rodriguez, K.J. Klabunde, J. Mol. 
Catal. A Chem. 229, 145 (2005).  
5. X. Chu, X. Liu, G. Wang, G. Meng, Mater. Res. Bull. 34, 
1789 (1999). 
6. M. Siemons, U. Simon, Sensor. Actuat. B Chem. 120, 110 
(2006). 
7. M. Siemons, U. Simon, Sensor. Actuat. B Chem. 126, 595 
(2007). 
8. T.M. Pan, T.F. Lei, T.S. Chao, J. Appl. Phys. 89, 3447 
(2001). 
9. H.T. Kim, M.T. Lanagan, J. Am. Ceram. Soc. 86, 1874 
(2003).  
10. P.S. Anjana, M.T. Sebastian, J. Am. Ceram. Soc. 89, 2114 
(2006).  
11. L. Zhou, S.-Y. Zhang, J.-C. Cheng, Zhang L.-D., Z. Zeng, 
Mater. Sci. Eng. B 49, 117 (1997). 
12. R. Levinson, P. Berdahl, H. Akbari, Sol. Energ. Mater. Sol. 
C. 89, 351 (2005). 
13. X. Liu, R. Hong, C. Tian, J. Mater. Sci. Mater. El. 20, 323 
(2009). 
14. Y. Xu, M.A.A. Schoonen, Am. Mineral. 85, 543 (2000). 
15. Y.-J. Lin, Y.-H. Chang, W.-D. Yang, B.-S. Tsai, J. Non-
Cryst. Solids 352, 789 (2006). 
16. X. Xu, C. Randorn, P. Efstathiou, J.T.S. Irvine, Nat. Mater. 
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Characterization of CoTiO3 Nanocrystallites Prepared by Homogeneous Precipitation Method

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