TiO2 nanostructures prepared by ferrocene/cobalt catalyst agents

Article abstract of DOI:10.1002/pssa.200723134

We present the growth and characterization of TiO₂ nano-crystals. Nanostructured growth is obtained in a low-pressure CVD system by using an organometallic precursor Ti(OC₃H₇)₄ as both the Ti and O source catalyzed by both ferrocene (an organometallic precursor) and cobalt metallic clusters prepared by the microwave-assisted polyol method. Two kinds of TiO₂ structures were obtained in the cobalt clusters: a) pine-tree like (with short-leaf structure) and b) long-leaf structures as large as a few micrometers in size and both under 10 nm in thickness. Long-leaf TiO₂ structures were mown at cobalt grain boundaries. For the growth conditions utilized, the TiO₂ structures are composed of both anatase and rutile crystallographic phases.

Full text of DOI:10.1002/pssa.200723134

phys. stat. sol. (a) 205, No. 2, 289–293 (2008) / DOI 10.1002/pssa.200723134
a
p s s
www.pss-a.com
applications and materials science
TiO nanostructures prepared
by ferrocene/cobalt catalyst agents
2
1
, 2
1
3
*, 4
4
4
3
A.-M. Lazar , D. Chaumont , Y. Lacroute , M. E. Gómez , J. C. Caicedo , G. Zambrano , and M. Sacilotti
1
LRRS and FR 2604 Université de Bourgogne 21078 Dijon, France
2
Faculté de Science et Ingénierie des Matériaux, Univ. Transilvania, Brasov, Roumanie
3
CMN and LPUB. UFR Sc. Techn. 2604 – Université de Bourgogne, 9 avenue A. Savary, BP 47870, 21078 Dijon Cedex, France
4
Thin Film Group, Department of Physics, Universidad del Valle, A. A. 25360 Cali, Colombia
Received 2 February 2007, revised 28 September 2007, accepted 31 October 2007
Published online 6 February 2008
PACS 61.46.–w, 62.23.St, 81.07.Bc, 81.15.Gh, 81.16.Hc
*
Corresponding author: e-mail megomez@calima.univalle.edu.co, Phone: +572 3307 627, Fax: +572 315 3564
We present the growth and characterization of TiO nano- clusters: a) pine-tree like (with short-leaf structure) and b)
2
crystals. Nanostructured growth is obtained in a low-pressure long-leaf structures as large as a few micrometers in size and
CVD system by using an organometallic precursor both under 10 nm in thickness. Long-leaf TiO2 structures
Ti(OC H ) as both the Ti and O source catalyzed by both were grown at cobalt grain boundaries. For the growth condi-
3
7 4
ferrocene (an organometallic precursor) and cobalt metallic tions utilized, the TiO structures are composed of both ana-
2
clusters prepared by the microwave-assisted polyol method. tase and rutile crystallographic phases.
Two kinds of TiO structures were obtained in the cobalt
2
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction Mesostructured materials have been various morphologies such as nanoparticles, nanorods,
the subject of intense scientific and technological interest nanowires, and nanotubes with the aim of significantly in-
because of their stable, controlled, nanoscale morphologies creasing its specific surface area and delocalization of car-
that promise enormous benefits in applications such as ca- riers, which may improve the performance of this material
talysis, electronic, optoelectronic and photovoltaic devices, in highly efficient sensors, photocatalysis, and photovoltaic
chemical and biological sensors, and functional materials cells [6, 7]. The proposal of other catalysts to grow nano-
[
1]. In particular, titanium dioxide has attracted much in- structured materials can be advantageous in improving fu-
terest due to its photochemical, optical, and electronic ture technological steps.
properties [2]. It is a wide-band-gap semiconductor TiO membranes have been prepared by various meth-
3.2 eV), which can be chemically activated by light and ods but only as thin planar films [7, 8]. Cobalt has been
2
(
the material tends to decompose organic materials as a re- utilized as a catalyst for the fabrication of carbon tubes [9].
sult of photo-catalytic processes. Titanium dioxide films Ferrocene has been used for the growth of carbon nano-
have great potential, since they have high surface areas and tubes [10]. Quite surprisingly, the use of ferrocene as a
controlled nanoscale morphologies coupled with relatively catalyst produces carbon nanotubes and multi-carpet layers
high refractive indices (1.5 to 1.6). Fabrication of 2D and between the substrate surface and the last carpet layer
3D assemblies with ordered nanostructures [3] are nowa- grown [10]. In other words, the ferrocene plus the silicon
days the trend in nanosciences and nanotechnology due to substrate surface (and its oxides) are the catalyst/base
their dimensions and very high surface areas. Nanocrystal- agents for carbon nanotube carpet multi-layer growth [10].
line titanium dioxide has been extensively investigated, It seems that substrate surface oxide or oxide post-
given that it plays a role in fundamental studies and appli- formation play important roles on the nanomaterial forma-
cations in, for example, photochromic devices [4], photo- tion [9, 10]. Oxide-assisted nanomaterial growth has yet to
catalysis [5], gas sensing [6], etc. Recently, there have be presented [11, 12]. It seems, also, that the role of oxides
been reports on the preparation of nanoscaled titania in (on the substrate surface or post-formed) should be taken
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

290
A.-M. Lazar et al.: TiO nanostructures prepared by ferrocene/cobalt catalyst agents
2
into account for an acceptable growth mechanism proposal previously dissolved in ethanol were deposited by spin-
13]. This could be useful for the real growth mechanism coating at 1800 rpm on silicon substrates. The silicon sub-
[
taking place when growing nano/micro-structured materi- strates were carefully cleaned through conventional means
als [11–19]. To our knowledge, the growth of TiO , three- to eliminate any trace of water, mineral deposits, or dust.
2
dimensional (3D) nanostructures catalyzed by ferro- Immediately after the Co deposit, the substrates were
cene/cobalt agents has yet to be published.
placed into the MOCVD chamber and TiO growth was
2
Herein, we report the growth of 3D TiO nanostruc- begun.
2
tures as short and long leaves that resemble palm tree
TiO structures were grown in a home-made MOCVD
2
leaves on silicon substrates via metal organic chemical va- system to produce TiO /TiNO superlattices and delta dop-
2
por deposition (MOCVD) technique by using ferro- ing structures [22, 23]. Briefly described, it is a horizontal
cene/cobalt as agents for the formation of these palm tree geometry MOCVD reactor operating at 76 Torr, the
leave-like structures. We discuss the growth procedure of growth temperature is between 550 °C and 650 °C, using
these nanostructures and their characterization via scan- N as carrier gas (0.6 slm), Ti(OC H ) as both the Ti and
2
3
7 4
ning electron microscopy (SEM), energy dispersive X-ray O source, Fe(C H ) and cobalt as catalyst agents. The
5 5 2
analysis (EDX), and X-ray diffraction analysis (XRD) MOCVD system is radio frequency heated (450 KHz). Be-
techniques. fore the injection of the Ti/O source within the MOCVD
reactor, the ferrocene flow is turned on for 1 min into the
Experimental Cobalt powder was obtained through reactor cell. Hence, the Si substrate and its surface (nor-
2
microwave-assisted polyol process [20, 21]. A first solu- mally Si oxide composed) are exposed to surface reac-
tion was prepared by dissolving the metal precursor cobalt tion/ferrocene at the growth temperature. The Ti/O source
acetate (Co(CH COO) ) in 6 ml of ethylene glycol (con- is held at 40 °C and the N bubbling/carrier gas through it
3
2
2
–4
centration of the precursor: 1.8 × 10 M). A second solu- is 0.6 slm. The ferrocene source is held at 50 °C and the N2
tion was prepared by dissolving sodium hydroxide in 6 ml carrier gas through it is 1 slm. All the MOCVD lines are
–3
of ethylene glycol (soda concentration is 1.8 × 10 M). heated to 80 °C to avoid line internal vapor gas condensa-
The two solutions were then mixed, stirred, and introduced tion during TiO growth.
2
quickly into the Pyrex reactor surmounted on a backward
flow system. The Pyrex reactor was placed inside the mi-
crowave cavity for the desired reaction. A microwave substrates by spin-coating, the “cobalt clusters on Si sub-
power of 600 W was then applied to the solution. Reaction strate” system is charged into the MOCVD reactor to grow
temperature was 197.5 °C, fixed by the boiling point of the TiO material by using Ti(OC as Ti and O chemi-
ethylene glycol. After 22 minutes of heating, power was cal elements, catalyzed by both the ferrocene source and
cut off and the reactor cooled down. The suspension was the cobalt clusters, as depicted in Fig. 2.
then recovered and washed with ethanol. The precipitate
TiO growth at 550 °C, 20 min duration, gives rise to
was dried in a Petri box at room temperature (approxi- pine-shaped trees composed of TiO short-leaf structures
mately 20 °C).
on solitary cobalt clusters, as illustrated in Fig. 2. We did
3 Results After Cobalt cluster deposition on (100)Si
H )
3 7 4
2
2
2
The dry, pure cobalt powder consists of nonporous not obtain leaf-like membrane growth at 650 °C to 750 °C
polycrystalline aggregates ranging in size between 500 nm
and 800 nm, as noted in the SEM micrograph displayed in
Fig. 1. Cobalt particles are metallic magnetic materials
with well-known agglomeration tendency, as described
elsewhere [20]. The Co metallic particles, in suspension,
Figure 2 Illustration depicting the growth of TiO nanostruc-
2
tures: pine-tree like on solitary cobalt clusters and long leaf-like
on cobalt agglomerate grain boundaries, assisted by ferrocene or-
Figure 1 SEM image of pure cobalt cluster deposited on Si sub- ganometallic precursor. The growth of TiO2 comes from the
strates by spinning. Average size 600 nm diameter.
Ti(OC H ) decomposition on the MOCVD reactor.
3 7 4
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com

Original
Paper
phys. stat. sol. (a) 205, No. 2 (2008)
291
on Co-clusters. So, by increasing the growth temperature and in Fig. 5 the long-leaf shaped, at low (a) and high (b)
membrane formation is no more observed. The growth of magnification. Leaf thickness is below 10 nm.
TiO at 550 °C on agglomerate cobalt clusters yields long- EDX analysis of the TiO long-leaf type shows that Ti
2
2
leaf structures at the grain boundaries, as depicted in Fig. 2. and O are the most detected elements. It is noteworthy to
In both cases (pine-shaped, short and long leaves), cobalt stress that EDX detection is always taking the silicon sub-
plays an important role in TiO nanostructure formation. strate and Co particles background information (Si and Co)
2
Without cobalt clusters, TiO growth has the shape of due to the membrane thickness. By increasing the distance
2
nanorods, as depicted in Fig. 3(a) and (b). Note that on from the cobalt clusters of the analyzed area, the Co EDX
both SEM micrographs there are surface starting wires and detected peak goes to zero (less than 1 at%), as shown in
grown wires under simultaneous injection of ferrocene and Table 1. Fe is not detected for membranes even if we have
Ti(OC H ) . The growth of TiO without ferrocene gives membrane growth in presence of ferrocene. Fe (1–2 at%)
3
7 4
2
rise to planar structures, as described elsewhere (22, 23). is detected when we grow TiO nanowire in presence of
2
Thus, the TiO structures grown are different by using: ferrocene. These 3D structures, displayed in SEM micro-
2
only Ti(OC H ) ; or Ti(OC H ) plus ferrocene; or graphs in Figs. 4 and 5, are based on TiO layers grown on
3 7 4 3 7 4 2
Ti(OC H ) plus ferrocene and Cobalt.
3 7 4
an Si substrate surface. Grazing angle X-ray analysis (not
SEM images of both TiO shapes on cobalt clusters are shown) of this TiO planar layer/substrate and on cobalt
2 2
presented in Figs. 4 and 5, respectively. In Fig. 4 is shown clusters shows that the TiO layer is composed of rutile and
2
the pine-tree shaped at low (a) and high (b) magnification; anatase crystallographic phases.
Figure 3 SEM micrographs of TiO
2
wire-like structure grown on Si sub-
strates by using only ferrocene as
catalyst. Note, magnification is dif-
ferent in each micrograph.
Figure 4 SEM micrograph of TiO
2
pine-tree like (a) and its leaf-like
nanostructures (b) grown on cobalt
solitary clusters at 550 °C, 20 min.
Figure 5 SEM micrographs of long
leaf-like TiO growth on cobalt clus-
2
ter grain boundaries (a), and its
leaves (long palm-tree-like leaves)
(b). Growth temperature 550 °C, dur-
ing 20 min.
www.pss-a.com
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

292
A.-M. Lazar et al.: TiO nanostructures prepared by ferrocene/cobalt catalyst agents
2
Table 1 EDX analysis content of TiO long-leaf nanostructure Co wires by using ferrocene, but 3D structures were not
2
with thickness under 10 nm, presented in Fig. 5b, showing Ti and observed.
O as the most important constituents. Growth temperature and
In summary, we have grown 3D-TiO nanostructures
2
time: 550 °C, 20 min.
from the Ti(OC H ) decomposition on a MOCVD reactor
3 7 4
in the presence of both Ti(OC H ) plus ferocene and Co-
3 7 4
element analysis
elemental %
O
Si
Ti
Co
total
100.00
balt. TiO growth at 550 °C, 20 min duration, gives rise to
2
23.82 72.92 3.26
pine-shaped trees composed of 10 nm thick-TiO short-leaf
2
structures on solitary cobalt clusters, and agglomerate co-
balt clusters yields 10 nm thick-long-leaf structures at the
4
Ti(OC H ) catalyzed by ferrocene and cobalt clusters the shape of nanorods, whereas growth of TiO
Discussion and conclusions The pyrolysis of grain boundaries. Without cobalt clusters, TiO
2
growth has
without
3
7 4
2
gives rise to TiO nanometric columnar structures (or tree ferrocene gives rise to planar structures.
2
leaves). To our knowledge, this is the first time that three-
Acknowledgements This work was accomplished under
contract project title FILIMON35 No. ANR-05-NANO-016-03
France, and the Excellence Center for Novel Materials – CENM
contract 043-2005 with COLCIENCIAS, Colombia.
dimensional TiO nanometric leaves are artificially pro-
2
duced. These TiO nanostructures are up to a few mi-
2
crometers long and less than 10 nm thick. The growth of
TiO wires or rods catalyzed by cobalt alloys by using the
2
MOCVD technique has been published [24]. The produc-
References
tion of TiO structures deposited as planar thin films has
2
also been published [7, 8]. Nevertheless, 3D structures as
[
1] P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, and
G. D. Stucky, Nature 396, 152 (1998).
tree leaves can be much more advantageous for TiO cata-
2
lytic applications due to their increased surface coverage
[
2] H. Yun, K. Miyazawa, H. Zhou, I. Honma, and M. Kuwa-
bara, Adv. Mater. 13, 1377 (2001).
[7, 8]. The growth mechanism for these leaf-like nano-
structures is not yet understood, i.e., nanomaterial growth
methods currently used in the literature [11–19] (VLS,
SLS, or oxide-assisted) can not be applied to the present
case. Studies are currently being undertaken to understand
the cobalt grain boundary effect on the long leaf growth.
Cobalt nanoclusters have the tendency to agglomerate as
[3] M. S. Gudiksen, L. J. Lauhon, J. Wang, D. C. Smith, and
C. M. Lieber, Nature 415, 617 (2002).
N. I. Kovtyukhova and T. E. Mallouk, Chem. Eur. J. 8, 4354
(2002).
[
4] Y. Ohko, T. Tatsuma, T. Fujii, K. Naoi, C. Niwa, Y. Kubota,
and A. Fujishima, Nature Mater. 2, 29 (2003).
[
5] M. R. Hoffmann, S. T. Martin, W. Choi, and D. W. Bahne-
mann, Chem. Rev. 95, 69 (1995).
explained by Shao et al. [20]. Since the TiO growth takes
2
place within a MOCVD system using a radio frequency
heating furnace, the cobalt clusters or (Co, Fe) alloy acti-
vate boundary grain, and the leaf-like growth could be un-
der its influence. Issues like cobalt oxide assisted at the
grain boundary can be raised on the present growth
mechanism [10–13]. The magnetic properties of nanopar-
ticles containing Fe, Co, and their alloys can be at the
origin of the leaf-like growth, according to Shao et al. [20].
In our case, it is the first time that cluster magnetic pro-
perties are associated with CVD material growth. The
effect of ferrocene as a catalyst is also debated for alloy
formation as for the present leaf-like formation [10]. We
are not as of yet certain of the specific role of iron in TiO2
leave formation; this merits further systematic research.
What we do know is that ferrocene helps to grow the
L. Gao and Q. H. Zhang, Scr. Mater. 44, 1195 (2001).
S. Y. Chae, M. K. Park, S. K. Lee, T. Y. Kim, S. K. Kim,
and W. I. Lee, Chem. Mater. 15, 3326 (2003).
[
6] Y. Zhu, J. Shi, Z. Zhang, C. Zhang, and X. Zhang, Anal.
Chem. 74, 120 (2002).
N. Wu, S. Wang, and I. A. Rusakova, Science 285, 1375
(1999).
[7] U. Diebold, Surf. Sci. Rep. 48, 53–229 (2003).
M. Adachi, Y. Murate, and S. Yoshikawa, Chem. Lett. 8,
942 (2000).
S. Uchida, R. Chiba, M. Tomiha, N. Masaki, and M. Shirai,
Electrochem. 70, 418 (2002).
J. A. Byrne and B. R. Eggins, J. Electroanal. Chem. 457, 61
(1998).
J. E. G. Wijinhoven and W. L. Vos, Science 281, 802 (1998).
8] H. Y. Ha, S. W. Nam, T. H. Lim, I. H. Oh, and S. A. Hong,
J. Membr. Sci. 111, 81 (1996).
[
3D structures as mentioned in reference [10]. Perhaps
ferrocene reacts with the oxide on the (Si) substrate surface
or with the Ti oxide source, thus forming 3D structures.
The growth mechanism for the present results is not under-
stood, nor is the VLS growth mechanism for the results
presented. It is not yet clear why the leave-like formation
occurs in the presence of Co nanoparticles. As explained in
reference 10, maybe iron oxides are responsible for the
T. Gestel, C. Vandecasteele, A. Buekenhoudt, Ch. Dotre-
mont, J. Luyten, R. Leysen, B. Bruggen, and G. Maes,
J. Membr. Sci. 209, 379–389 (2002).
J. Tilmant, C. Pommier, and K. Chhor, Mater. Chem. Phys.
6
4, 156 (2000).
9] J. S. Lee, G. H. Gu, H. Kim, J. S. Suh, I. Han, N. S. Lee,
J. M. Kim, and G. S. Park, Synth. Met. 124, 307 (2001).
D structure growth. We did not try to grow on other metal [10] M. Pinault, V. Pichot, H. Khodja, P. Launois, C. Reynaud,
clusters. We did try on other metal substrates (stainless
and M. L’Hermite, Nano Lett. 5, 2394 (2005).
steel, Pt, Au, Ni, without ferrocene) and no 3D structures [11] S. Lee, N. Wang, and C. Lee, Mater. Sci. Eng. A 286, 16
were obtained. We tried to grow TiO nanostructures on
(2000).
[
3
2
©
2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-a.com

Original
Paper
phys. stat. sol. (a) 205, No. 2 (2008)
293
[
[
[
12] W. Shi, Y. Zheng, N. Wang, C. Lee, and S. Lee, Adv. Mater. [20] H. Shao, Y. Huang, H. Lee, Y. J. Shue, and C. O. Kim,
3, 591 (2001). Curr. Appl. Phys. 6S1, e195 (2006).
13] Th. Chiaramonte, G. Patriarche, J. Decobert, L. P. Cardoso, [21] B. Blin, F. Fievet, D. Beaupère, and M. Figlarz, New J.
and M. Sacilotti, Nanotechnology 16, 2790 (2005). Chem. 13, 67 (1989).
14] M. T. Bjork, B. J. Ohlsson, T. Sass, A. I. Persson, C. The- [22] F. Fabreguette, J. Guillot, L. P. Cardoso, R. Marcon, L. Im-
1
lander, M. H. Magnusson, K. Deppert, L. R. Wallenberg,
and L. Samuelson, Nano Lett. 2, 87 (2002).
hoff, M. C. Marco de Lucas, P. Sibillot, S. Bourgeois,
P. Dufour, and M. Sacilotti, Thin Solid Films 400, 125
(2001).
[
15] J. Johansson, B. Wacaser, K. Dick, and W. Seifert, Nano-
technology 17, S355 (2006).
[23] Th. Chiaramonte, L. P. Cardoso, R. V. Gelamo, F. Fabre-
guette, M. Sacilotti, M. C. Marco de Lucas, L. Imhoff,
S. Bourgeois, Y. Kihn, and M.-J. Casanove, Appl. Surf. Sci.
9833, 1 (2003).
[
16] X. Duan and Ch. Lieber, Adv. Mater. 12, 298 (2000).
17] H. Yu, J. Li, R. A. Loomis, L. W. Wang, and W. E. Buhro,
Nature Mater. 2, 517 (2003).
[
[
18] R. Wagner and W. Ellis, Appl. Phys. Lett. 4, 89 (1964).
19] E. Givargizov, J. Cryst. Growth 20, 217 (1973); J. Cryst.
Growth 31, 20 (1975).
[24] S. K. Prachan, Ph. J. Reucroft, F. Yang, and A. Dozier,
J. Cryst. Growth 256, 83 (2003).
[
www.pss-a.com
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim