C.A. Chen
a, Y.M. Chen
a, Y.S. Huang
a,∗, D.S. Tsai
b, P.C. Liao
c, K.K. Tiong
daDepartment of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
bDepartment of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
cDepartment of Electronic Engineering, Technology and Science Institute of Northern Taiwan, Taipei 112, Taiwan
dDepartment of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan
a r t i c l e i n f o
IrO2nanocrystals (NCs) were grown on top of rutile (R) TiO2nanorods (NRs) sitting on sapphire (SA) (1 0 0) substrate via metal organic chemical vapor deposition by using (C6H7)(C8H12)Ir and Ti[OCH(CH3)2]4
as source reagents. The surface morphology and structural properties of the as-deposited NCs were characterized. The field-emission scanning electron microscopy images and X-ray diffraction patterns indicate growth of V-shaped IrO2(1 0 1) nanowedges (NWs) on top of R-TiO2NRs. Transmission electron microscopy and selected-area electron diffractometry characterizations of IrO2NCs showed that the NWs were crystalline IrO2with a twin plane of (1 0 1) and twin direction of [ ¯1 0 1] at the V-junction.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Fabrication of one-dimensional (1D) nano-scaled materials, such as nanotubes (NTs) and nanorods (NRs), has gained consid-erable attention owing to interests in fundamental science and the potential in developing nanodevices [1,2]. The development of nanodevices has benefitted from the distinct morphology, huge surface area and high aspect ratio of NTs and NRs. More recently, the technology has advanced to deposit various materials using 1D nanostructures as templates[3]. Such an approach not only provides a convenient way to grow 1D nanostructures from mate-rials that are difficult to be fabricated into 1D form by themselves, but also paves the way to synthesize 1D heteronanostructures.
The obtained 1D heteronanostructure from this approach has been shown to demonstrate enhanced functionality[4,5].
∗ Corresponding author. Tel.: +886 2 27376385; fax: +886 2 27376424.
E-mail address:[email protected](Y.S. Huang).
Recently, we have successfully grown well-aligned rutile (R) phase TiO2NRs[6]and nanostructural IrO2[7]on sapphire sub-strate via metal organic chemical vapor deposition (MOCVD).
Nanostructural TiO2 has been widely studied as a very promis-ing material for applications in photocatalysis [8], sensors [9], solar energy conversion[10]and optical devices [11]. IrO2 is a good electric conductor with high thermal and chemical stability, and depositing IrO2 nanocrystals on TiO2 NRs to form 1D het-eronanostructures may be useful in electrical and electrochemical applications. The IrO2has the lattice constant close to that of R-TiO2 (IrO2: a = b = 0.450 nm and c = 0.316 nm [JCPDS no. 15-0870]; TiO2: a = b = 0.459 nm and c = 0.296 nm [JCPDS no. 21-1276]). Owing to the proximate lattice constant and identical crystal structure, a het-eroepitaxial growth of IrO2on R-TiO2NRs may be readily facilitated [12].
In this work, we report the growth of V-shaped IrO2nanowedges (NWs) on top of R-TiO2NRs via MOCVD. The source reagents were (C6H7)(C8H12)Ir and titanium (IV) i-propoxide (Ti[OCH(CH3)2]4 (TTIP)). The surface morphology and structural properties of the as-deposited NCs were characterized by field-emission scanning
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doi:10.1016/j.jallcom.2008.09.173
108 C.A. Chen et al. / Journal of Alloys and Compounds 480 (2009) 107–110
Fig. 1. (a) FESEM images of R-TiO2NRs, (b) the cross-sectional TEM image of a single R-TiO2NR, and (c) the XRD pattern of TiO2NRs deposited on SA(1 0 0). FESEM images of IrO2NCs deposited on top of R-TiO2NRs: (d) 30◦perspective view and (e) cross-sectional view images. (f) The XRD pattern of IrO2NCs grown on top of TiO2NRs sitting on the SA(1 0 0) substrate.
Fig. 2. (a) The cross-sectional TEM image of a single V-shaped IrO2NW on top of R-TiO2NR, (b) the cross-sectional TEM image along zone axis [1 1 0] focused on one arm of the V-shaped IrO2NW, (c) the SAED pattern projection along [1 1 0] zone axis, and (d) high-resolution TEM taken from the wedge sidewall marked in (b).
C.A. Chen et al. / Journal of Alloys and Compounds 480 (2009) 107–110 109
Fig. 3. (a) The TEM image of a free standing V-shaped IrO2NW with SAED patterns, showing the junction and the arms (b) the SAED pattern along the [0 1 0] zone axis of the junction and (c) the HRTEM images in the vicinity of the V-shaped junction regime.
electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffrac-tometry (SAED). The formation of V-shaped IrO2 NWs with a twin structure at the V-junction sitting on top of R-TiO2 NRs is presented.
2. Experimental
The growth of IrO2NWs on TiO2NRs was carried out in a vertical-flow cold wall MOCVD system, using (C6H7)(C8H12)Ir and TTIP as source reagents. The substrate was sapphire (SA) (1 0 0). The details for TiO2NRs and IrO2nanocrystals growth were described in Refs.[6]and[7], respectively.
A JEOL-JSM6500F FESEM was used to study the morphology of IrO2NWs grown on R-TiO2NRs. The growth orientations were examined using a Rigaku D/Max-RC X-ray diffractometer (XRD) equipped with Cu K␣ radiation source and Ni filter at a wavelength of 1.5418 Å. TEM images and SAED patterns were recorded to charac-terize the structure of the individual V-shaped IrO2NC by a Phillips Tecnai G2 F20 FE-TEM at working voltage of 200 kV.
3. Results and discussion
As illustrated in Fig. 1(a), the FESEM images show densely packed vertically aligned TiO2 NRs with sharp tips grown on SA(1 0 0) substrate. The TEM image of a single TiO2NR shown in Fig. 1(b) indicating the striation growth along c axis.Fig. 1(c) shows the XRD pattern of TiO2NRs deposited on the SA(1 0 0), where the
main reflection peaks are identified to be from the R-TiO2(0 0 2) and SA (3 0 0), indicating pure rutile phase with a (0 0 1) preferred ori-entation has been deposited.Fig. 1(c) also indicates the presence of a weak R-TiO2(1 0 1) feature located around 36◦. The FESEM images and XRD pattern of the TiO2 NRs grown on SA(1 0 0) reveal the uniquely single-directional growth of rutile phase TiO2NRs along [0 0 1] with (1 0 1) planes on the tips of NRs. The R-TiO2NRs were then used as template for IrO2NCs deposition. FESEM micrographs depicted inFig. 1(d) and (e) respectively, display 30◦perspective view and cross-sectional view images of IrO2NCs deposited on top of TiO2NRs. As can be seen inFig. 1(d) and (e), the well-aligned V-shaped IrO2with two wedge-like arms were grown on top of R-TiO2NRs.Fig. 1(f) displays the XRD pattern of IrO2NCs grown on top of TiO2NRs. The XRD pattern indicates a preferred orientation of IrO2(1 0 1) NCs has been deposited.
Further structural characterization of the IrO2NWs on R-TiO2
NRs was performed using TEM.Fig. 2(a) displays the TEM image of a single V-shaped IrO2wedge-like nanocrystal on top of R-TiO2 nanorod.Fig. 2(b) shows the TEM image focused on one arm of the V-shaped IrO2 NW.Fig. 2(c) is the SAED pattern taken from the wedge sidewall. The SAED pattern has been identified to be the [1 1 0] zone pattern, indicating that the wedge walls belong to the{1 1 0} facets and the preferential growth direction of the IrO2wedge is along the [0 0 1] direction (c-axis).Fig. 2(d) displays
110 C.A. Chen et al. / Journal of Alloys and Compounds 480 (2009) 107–110
the high-resolution TEM image taken from the wedge marked in Fig. 2(b) exhibiting the clear lattice plane of the IrO2wedge. The lattice spacing between adjacent lattice planes for the (0 0 1) and (1 1 0) planes are 0.322 nm and 0.325 nm, respectively. The results also confirmed the tetragonal rutile structure and single-crystalline quality of the IrO2NCs.
Fig. 3(a) shows the TEM image of a free standing V-shaped IrO2 nanocrystal with two wedge-like arms. The SAED pattern along the [0 1 0] zone axis of the junction depicted inFig. 3(b) shows an over-lap of two series of doublet spots, a result of intersecting two arms of different orientations. The HRTEM images were processed by fast Fourier transform (FFT) to examine the V-shaped junction of IrO2
NWs in detail.Fig. 3(c) shows the HRTEM images in the vicinity of the V-shaped junction regime with the orientation relationship of major axes [0 0 1] and [ ¯1 0 1]. It is clear that the preferential growth direction for the wedges is the [0 0 1] direction. From a detailed analysis ofFig. 3(b) and (c), it can be concluded that the V-shaped NWs have a single twin structure at the junction. The twin plane is (1 0 1) and the twin direction is [ ¯1 0 1]. The twin boundary is coher-ent and the two near symmetric arms are separated by an angle of
∼110◦. 4. Summary
IrO2NCs were grown on top of the tips of R-TiO2NRs deposited on SA(1 0 0) substrate. FESEM micrographs and XRD patterns revealed the bifurcated growth of twinned V-shaped IrO2 (1 0 1) with two wedge-like arms sitting on top of R-TiO2 NRs. TEM,
SAED and HRTEM characterizations of the NWs confirmed that the wedges were crystalline IrO2with a single twin structure with (1 0 1) twin plane along [ ¯1 0 1] twin direction.
Acknowledgements
The authors acknowledge the support of the National Science Council of Taiwan under Contract Nos. NSC 96-2112-M-011-001 and NSC 97-2112-M-011-001-MY3.
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