0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2008.05.021
Corresponding author. Tel.: +886 2 27376385; fax: +886 2 27376424.
E-mail address:[email protected] (Y.S. Huang).
Journal of Crystal Growth 310 (2008) 3663– 3667
stoichiometry, with an advantage of large-area deposition.
However, up-to-date only a few of the studies have been reported on the thermally activated anatase-to-rutile phase transition in MOCVD-grown TiO2nanocrystals (NCs).
In this article, we report a detailed study of thermal-annealing effects on TiO2 NCs via X-ray diffraction (XRD) and Raman scattering (RS) spectroscopy. Vertically aligned anatase TiO2(11 0) NCs were grown on the sapphire (SA)(1 0 0) substrates at 550 1C by MOCVD, using titanium-tetraisopropoxide (TTIP, Ti[OCH(CH3)2]4) as the source reagent. The effects of thermal annealing in oxygen atmosphere between 600 and 1000 1C were studied. A combined XRD and RS techniques were used to extract the information of phase transformation of the vertically aligned TiO2NCs.
2. Experimental details
2.1. Sample deposition and thermal annealing
A vertical-flow cold-wall MOCVD system, using the TTIP as a source reagent, was utilized for the growth of the anatase TiO2
NCs samples on SA(1 0 0) substrates. A schematic of the MOCVD apparatus is illustrated inFig. 1. There are two different flow paths connecting to the growth chamber designed for gas transport. The first one is a bypass flow path, which is designed for controlling the steady-state chamber pressure prior to the deposition. The second path was heated to the designated temperature to facilitate the transport of the source vapor to the growth chamber.
Three independent thermal couples were mounted on this source transport line to control and monitor the temperature of the shower head (Tsh), gas transfer line (Ttl) and the precursor reservoir (Tpr). During the source vapor transport, Tsh, Ttland Tpr
controlled by three independent controllers were kept at 50, 55 and 100 1C, respectively, to avoid the precursor condensation. The reaction chamber was equipped with a turbo molecular pump, and the chamber back pressure was 105mbar. Pure oxygen gas was used to convey the source vapor and to invoke the chemical reaction without adding any other inert gas. The oxygen flow rate (FO2) of the reactive carrier gas was kept at 30 sccm. The substrate temperature (Ts) and chamber pressure (Pc) were kept at 550 1C and 1.5 mbar, respectively, during the CVD process. As-deposited NCs were annealed at temperatures of 600, 700, 800, 900 and 1000 1C in O2atmosphere for 1 h, respectively.
2.2. Characterization of the TiO2NCs
The morphology of TiO2 NCs was studied using a JEOL-JSM6500F field-emission scanning electron microscopy (FESEM).
XRD patterns taken on a Rigaku RTP300RC X-ray diffractometer were used to examine the phases and orientations of the samples.
The rocking curve (y scan) of the growth plane was used to study the degree of crystal orientation. RS was used to characterize the structural phases of the as-deposited and annealed NCs samples.
The RS spectra were recorded at room temperature utilizing the backscattering mode on a Renishaw in Via micro-Raman system with 1800 grooves/mm grating and an optical microscope with a 50 objective. The Ar+laser beam of the 514.5 nm excitation line with a power of 1.5 mW was focused onto a spot size 5 mm in diameter. Prior to the measurement, the system was calibrated by means of the 520 cm1Raman peak of a polycrystalline Si.
3. Results and discussion
3.1. Morphology and structure of the as-deposited TiO2NCs
FESEM images illustrated inFig. 2(a) show the as-deposited TiO2
NCs on SA(10 0) substrate, which exhibit vertically aligned growth behavior. The densely populated TiO2NCs have an average edge size and length of about 200 nm and 3.5 mm, respectively. X-ray y– 2y scan data of the as-deposited sample are depicted inFig. 2(b). In addition to the strong substrate peak, a single diffraction peak with 2y 701 is clearly seen in the spectrum. The 2y value corresponds well to that of the 220 diffraction peak of anatase (JCPDS no. 83-2243). The single anatase TiO2220 diffraction peak denoted as in A-220 shows the uniquely single directional growth of anatase TiO2
NCs along [110], for the as-grown sample. This preferred oriented growth on SA(10 0) is designated as A-TiO2(110).
3.2. Phase conformation by XRD analysis
Fig. 3 depicts the XRD patterns of TiO2 NCs, as a function of annealing temperature. The XRD patterns at Ta¼600 and 700 1C are similar to that of the as-deposited sample, viz. the anatase phase with (110) preferential orientation. At Ta¼800 1C a weak rutile 002 peak, denoted as R-002 inFig. 3, at 2y61.81 began to appear. When the TiO2NCs were annealed at 900 and 1000 1C, pure rutile phase with a (0 0 1) preferred orientation, designated as R-TiO2(0 0 1), was observed.Fig. 4(a) and (b) show the rocking curves (y scan) of TiO2
NCs as a function of Ta for A-220 and R-002 diffraction peaks, respectively. As can be seen inFig. 4(a) for A-220 as Taincreases, the peak position shifts towards to that of the powder anatase TiO2
(JCPDS no. 83-2243) and the full-width at half-maximum (FWHM) decreases from 1.171 (as-deposited) to 0.541 (Ta¼800 1C). For R-002, seeFig. 4(b), the peak position shifts from 30.91 to 31.41 (a value close to that of the powder rutile–TiO2, JCPDS no. 77-0441) and FWHM decreases from 1.111 to 0.841 as Taincreased from 800 to 1000 1C. The obtained peak positions and the values of the FWHM for A-220 or/and R-002 for the investigated samples are listed in Table 1. The results indicate that the lattice parameters decrease toward the values of powder TiO2 and the degree of crystal orientations improved with a higher annealing temperature.
It has previously been reported that the anatase–rutile transformation temperature depends on experimental parameters [16,29–33]such as deposition methods, process parameters, and different substrates. In the present work, the anatase phase of TiO2(11 0) NCs on SA(1 0 0) deposited at 550 1C, remains up to the annealing temperature of 700 1C. The mixed phases of anatase and rutile appears at Ta¼800 1C, and pure rutile phase of TiO2(0 0 1) prevails at temperatures above 900 1C.
3.3. Lattice dynamics of anatase and rutile phases in TiO2
Titania crystallizes mainly in anatase or rutile phase. Anatase is tetragonal and belongs to the space group D4h19[34]. The primitive
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Fig. 1. A schematic representation of cold-wall MOCVD apparatus.
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unit cell contains two TiO2chemical units. According to the factor group analysis, there are six Raman active modes (1A1g+2B1g+3Eg) [35]. The Raman spectrum for anatase single crystal was investigated by Ohsaka et al.[36]and the six allowed bands in the first-order Raman spectrum were identified to be 144 cm1 (Eg), 197 cm1 (Eg), 399 cm1 (B1g), 516 cm1 (A1g+B1g), and 639 cm1 (Eg). The room temperature Raman band at 516 cm1
can be resolved into two peaks centered at 513 cm1 (A1g) and 519 cm1 (B1g) at 73 K [36]. Rutile phase is also tetragonal, but belongs to the space group D4h14with two TiO2molecules per unit cell. There are four Raman active modes with irreducible representations of A1g, B1g, B2g, and Eg [35]. These four Raman active modes of rutile TiO2 were detected at 143 cm1 (B1g), 447 cm1(Eg), 612 cm1(A1g), and 826 cm1(B2g) by Porto et al.
[37]. Additionally, the Raman spectra of anatase phase exhibit features different from those of rutile phase, namely, in rutile phase, second-order scattering is even more intense than one-phonon scattering [37], whereas for anatase phase only a few weak bands due to two-phonon scattering are observed[36].
3.4. Phase confirmation by RS analysis
Fig. 5shows the Raman spectra of TiO2NCs as a function of annealing temperature. As can be seen inFig. 5, the RS spectrum of the as-deposited TiO2NCs exhibits five distinct peaks located at 143, 196, 396, 514 and 637 cm1assigned as Eg, Eg, B1g, A1g+B1g, and Egmodes. These features are close to those in the bulk anatase phase[36], except for their lower intensity and broader linewidth.
Therefore, it is evident that the as-deposited TiO2NCs possesses a certain degree of long-range order of the anatase phase. For the Raman spectra at Ta¼600 and 700 1C, the peak positions are similar to that of the as-deposited one and the linewidths of the high wave number modes decrease, indicating the enhancement of long-range order for the higher annealing temperature. As annealing temperature increases to 800 1C, a small peak at around 448 cm1and a weak shoulder at around 608 cm1appeared. The features at 448 and 608 cm1are the Egmode and A1gmode of the rutile phase [37], respectively. The presence of rutile Raman
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Fig. 2. (a) FESEM images (301 perspective- and cross-sectional view) and (b) XRD pattern of the as-deposited A-TiO2(11 0) NCs grown on SA(1 0 0) substrate via MOCVD. The anatase TiO2220 diffraction peak is denoted as A-220.
Fig. 3. XRD patterns of TiO2NCs grown on SA(1 0 0) substrate as a function of the annealing temperature. The anatase TiO2220 and rutile TiO2002 diffraction peaks are denoted as A-220 and R-002, respectively.
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modes indicates that the high-temperature rutile phase starts to form at Ta¼800 1C. At Ta¼900 1C, the entire anatase features have been completely wiped out with only the presence of rutile
features. This event of full phase transition is illustrated inFig. 5 by the RS spectrum for sample annealed at 900 1C, where only broad rutile peaks stand out. The crystalline quality of rutile phase could be further improved upon annealing at an even higher temperature of Ta¼1000 1C. Raman spectra at Ta¼900 and 1000 1C show that the Eg and A1g modes, as well as the two-phonon bands at 242 and 515 cm1(marked as *) are the major features of rutile TiO2NCs. The B2gmode is extremely weak and B1gmode is almost absent.
3.5. Morphology and structure of the TiO2NCs annealed at Ta¼1000 1C
The formation of pure rutile phase of vertically aligned TiO2
NCs after annealing at 1000 1C is further confirmed by FESEM and XRD measurements. Fig. 6(a–c) show the FESEM images, XRD pattern and Raman spectrum of the TiO2 NCs on SA(1 0 0) annealed at 1000 1C. The FESEM images illustrated inFig. 6(a) reveal the vertically aligned growth behavior of the annealed TiO2
NCs. The well-aligned densely packed TiO2NCs have an average edge size and length of about 120 nm and 1.05 mm, respectively.
The size and length of the annealed NCs are smaller than that of the as-deposited anatase NCs is consistent with the well-known property of TiO2 that anatase phase is a low-temperature polymorph with a less-dense structure. The XRD pattern (see Fig. 6(b)) shows the uniquely single-directional growth of rutile phase TiO2NCs along [0 0 1], for the annealed grown on SA(1 0 0).
As depicted inFig. 6(c) Raman spectrum shows the major features of rutile TiO2NCs: the Eg, A1gand B2gmodes as well as the two-phonon bands at 242 and 515 cm1 (marked as *). The redshifts of the peak positions and broadening of linewidths of the rutile TiO2 Raman features could be related to the phonon confinement effect in the NCs and residual stress effects[38].
4. Summary
A detailed study of thermal annealing effects on TiO2NCs via XRD and Raman scattering spectroscopy has been carried out. The FESEM images and XRD pattern of the as-grown sample revealed that the vertically aligned anatase TiO2(11 0) NCs were grown on
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Fig. 4. X-ray rocking curves of TiO2NCs grown on SA(10 0) substrate as a function of the annealing temperature around (a) A-220 and (b) R-002 diffraction peaks, respectively. For comparison, the rocking curve around A-220 peak for the as-deposited sample is also included in (a).
Table 1
Summery of the y-scan results: peak positions and the full-width of half-maximum (FWHM) as a function of annealing temperature for TiO2NCs grown on SA(10 0) substrate
Temperature (1C) Diffraction peak Peak position (deg.) FWHM (deg.)
550 (as-deposited) A-220 34.6 1.17
Ta¼600 A-220 34.8 0.99
Ta¼700 A-220 35.0 0.75
Ta¼800 A-220 35.2 0.54
R-002 30.9 1.11
Ta¼900 R-002 31.2 0.92
Ta¼1000 R-002 31.4 0.84
For comparison, the value for the as-deposited 220 diffraction peak of anatase is also included.
Fig. 5. Raman scattering spectra of TiO2NCs grown on the SA(1 0 0) substrate as a function of the annealing temperature. The Raman features of anatase and rutile phases are denoted as A and R, respectively. The two-phonon bands of the rutile phase are marked as *.
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the sapphire (SA)(10 0) substrate at 550 1C. The XRD patterns of the TiO2NCs annealed at 900 and 1000 1C showed the pure rutile phase of TiO2 with a (0 0 1) preferred orientation. The rocking curves of TiO2NCs around A-220 or/and R-002 diffraction peaks indicated that the lattice parameters decrease toward the values of powder TiO2, and the degree of crystal orientations improved
with a higher annealing temperature. The XRD and RS spectra showed the onset of phase transformation process from the as-grown anatase TiO2(11 0) NCs into rutile TiO2(0 0 1) at Ta¼800 1C.
At Tahigher than 900 1C, pure rutile phase of TiO2(0 0 1) NCs were formed and the crystalline quality of TiO2 NCs could be further improved upon higher annealing temperature. The results demonstrate the usefulness of XRD and RS as nondestructive phase transformation characterization techniques for TiO2NCs.
Acknowledgment
The authors acknowledge the support of the National Science Council of Taiwan under Contract no. NSC 96-2112-M-011-001.
References
[1] L.E. Coulter, A.G. Sault, J. Catal. 154 (1995) 56.
[2] S.P. Albu, A. Ghicov, J.M. Macak, R. Hahn, P. Schmuki, Nano Lett. 7 (2007) 1286.
[3] X. Zhang, M. Jin, Z. Liu, D.A. Tryk, S. Nishimoto, T. Murakami, A. Fujishima, J. Phys. Chem. C 111 (2007) 14521.
[4] A.R. Armstrong, G. Armstrong, J. Canales, R. Garcia, P.G. Bruce, Adv. Mater. 17 (2005) 862.
[5] O.V. Sakhno, L.M. Goldenberg, J. Stumpe, T.N. Smirnova, Nanotechnology 18 (2007) 105704.
[6] R. Rabady, I. Avrutsky, Appl. Opt. 44 (2005) 378.
[7] A. Richel, N.P. Johnson, D.W. McComb, Appl. Phys. Lett. 76 (2000) 1816.
[8] A.L. Linsebigler, G. Lu, J.T. Yates Jr., Chem. Rev. 95 (1995) 735.
[9] N. Daude, C. Gout, C. Jouanin, Phys. Rev. B 15 (1977) 3229.
[10] H. Tang, K. Prasad, R. Sanjines, P.E. Schmid, F. Levy, J. Appl. Phys. 75 (1994) 2042.
[11] G.K. Bosschloo, A. Goossens, J. Schoonman, J. Electrochem. Soc. 144 (1997) 1311.
[12] X. Zhang, M. Jin, Z. Liu, D.A. Tryk, S. Nishimoto, T. Murakami, A. Fujishima, J. Phys. Chem. C 111 (2007) 14521.
[13] A.R. Armstrong, G. Armstrong, J. Canales, R. Garcia, P.G. Bruce, Adv. Mater. 17 (2005) 862.
[14] K. Hashimoto, H. Irie, A. Fujishima, Jpn. J. Appl. Phys. Part 1 44 (2005) 8269.
[15] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1.
[16] D.J. Won, C.H. Wang, H.K. Jang, D.J. Choi, Appl. Phys. A 73 (2001) 595.
[17] P.I. Gouma, P.K. Dutta, M.J. Mills, NanoStructured Mater. 11 (1999) 1231.
[18] P. Loeble, M. Huppertz, D. Mergel, Thin Solid Films 251 (1994) 72.
[19] J. Musil, D. Herman, J. Sicha, J. Vac. Sci. Technol. A 24 (2006) 521.
[20] J.M. Wu, W.T. Wu, H.C. Shih, J. Electrochem. Soc. 152 (2005) G613.
[21] J.M. Wu, H.C. Shih, W.T. Wu, J. Vac. Sci. Technol. B 23 (2005) 2122.
[22] S. Cerneaux, X. Xiong, G.P. Simon, Y.B. Cheng, L. Spiccia, Nanotechnology 18 (2007) 055708.
[23] D. Liu, M.Z. Yates, Langmuir 23 (2007) 10333.
[24] Z. Miao, D. Xu, J. Ouyang, G. Guo, X. Zhao, Y. Tang, Nano Lett. 2 (2002) 717.
[25] P. Evans, M.E. Pemble, D.W. Sheel, Chem. Mater. 18 (2006) 5750.
[26] L.A. Brook, P. Evans, H.A. Foster, M.E. Pemble, A. Steele, D.W. Sheel, H.M. Yates, J. PhotoChem. A 187 (2006) 53.
[27] C.A. Chen, Y.M. Chen, A. Korotcov, Y.S. Huang, D.S. Tsai, K.K. Tiong, Nanotechnology 19 (2008) 075611.
[28] J.J. Wu, C.C. Yu, J. Phys. Chem. B 108 (2004) 3377.
[29] C.S. Kim, K. Nakaso, B. Xia, K. Okuyama, M. Shimada, Aerosol Sci. Technol. 39 (2005) 104.
[30] P.I. Gouma, M.J. Mills, J. Am. Ceram. Soc. 84 (2001) 619.
[31] Y. Sun, T. Egawa, L. Zhang, X. Yao, Jpn. J. Appl. Phys. 41 (2002) L945.
[32] G.A. Battiston, R. Gerbasi, M. Porchia, Thin Solid Films 239 (1994) 186.
[33] M.D. Wiggins, M.C. Nelson, C.R. Aita, J. Vac. Sci. Technol. A 14 (1996) 772.
[34] R.W.G. Wyckoff, Crystal Structures, Vol. 1, Interscience Publishers, New York, 1965 (p. 253).
[35] R. Loudon, Adv. Phys. 13 (1964) 423.
[36] T. Ohsaka, F. Izumi, Y. Fujiki, J. Raman Spectrosc. 7 (1978) 321.
[37] S.P.S. Porto, P.A. Fleury, T.C. Damen, Phys. Rev. 154 (1967) 52.
[38] A.V. Korotcov, Y.S. Huang, K.K. Tiong, D.S. Tsai, J. Raman Spectrosc. 38 (2007) 737.
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Fig. 6. (a) FESEM images (301 perspective- and cross-sectional view), (b) XRD pattern, where rutile TiO2002 diffraction peak is denoted as R-002, and (c) Raman spectrum of TiO2NCs on SA(10 0) substrate after annealing at 1000 1C for 1 h. The Eg, A1gand B2gmodes as well as the two-phonon bands at 242 and 515 cm1 (marked as *) are the major features of rutile TiO2NCs.
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