Abstract. We report the growth of well-aligned RuO
2/R-TiO2 heteronanostructures on sapphire (100) substrates by reactive magnetron sputtering using Ti and Ru metal targets under different conditions.The surface morphology and structural properties of the as-deposited heteronanostructures were characterized using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffractometry (SAED). The FESEM micrographs and XRD patterns indicated the growth of vertically aligned RuO2(001) nanotubes and twinned V-shaped RuO2(101) nanowedges (NWs) on top of R-TiO2 nanorods under different sputtering pressures. TEM and SAED characterizations of the V-shaped RuO2 NWs showed that the NWs are crystalline RuO2 with twin planes of (101) and twin direction of [
1
01] at the V-junction.Introduction
Recently, nanoscaled materials have become the focus of intensive research owing to the interests in fundamental science and potential in fabrication of nanodevices [1]. The development of nanodevices may also benefit from the unique morphology, large surface area and high aspect ratio of nanocrystals (NCs) [2,3]. More recently, the technology has advanced to deposit various materials using one-dimensional (1D) nanostructures as templates [4]. Such an approach not only provides a convenient way to grow 1D nanostructures from materials that are difficult to be fabricated into 1D form by themselves, but also paves the way to synthesize 1D heteronanostructure. The obtained 1D heteronanostructure from this approach has been shown to enhance functionality [5].
Recently, we have grown well-aligned TiO2 rutile (R) phase nanorods (NRs) [6] and nanostructural RuO2 [7] on sapphire substrates via radio frequency magnetron sputtering (RFMS).
Nanostructural TiO2 has been widely studied as a very promising material for applications in photocatalysis [8], sensors [9], solar energy conversion [10] and optical devices [11]. RuO2 is a good electric conductor with high thermal and chemical stability, and depositing RuO2 nanocrystals on TiO2 NRs to form 1D heteronanostructures may be useful in electrical and electrochemical applications. The RuO2 has the lattice parameters close to that of R-TiO2 (RuO2: a = b = 4.49Å and c
= 3.11Å [JCPDS No.88-0322]; TiO2: a = b = 4.59 Å and c = 2.96 Å [JCPDS No. 21-1276]). Owing to the close lattice parameters and same crystal structure, a heteroepitaxial growth of RuO2 on R-TiO2
NRs may readily be facilitated.
Solid State Phenomena Vol. 170 (2011) pp 78-82 Online available since 2011/Apr/19 at www.scientific.net
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In this work, we report the deposition of well-aligned RuO2/R-TiO2 heteronanostructures on sapphire (SA)(100) substrates by RFMS using Ti and Ru metal targets under different conditions. The surface morphology, structural and spectroscopic properties of the as-deposited heteronanostructures were characterized by using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected-area electron diffractometry (SAED).
The results revealed the growth of vertically aligned RuO2(001) nanotubes (NTs) and twinned V-shaped RuO2 nanowedges (NWs) on top of R-TiO2 NRs. The probable mechanisms for the formation of well-aligned RuO2 NCs on top of R-TiO2 NRs were discussed.
Experimental
The deposition of RuO2 NCs on TiO2 NCs template was carried out in a home-made high vacuum RFMS system. The sputtering gun has a standard circular planar magnetron. The sputtering targets were Ru (99.95%) and Ti (99.95%) metal. The sputtering chamber was evacuated with a turbo-molecular pump and had a base pressure of ~3×10−5 Pa. Reactive sputtering was carried out in a mixture of oxygen and argon. The sputtering parameters for TiO2 (RuO2) were: flux of oxygen and argon O2 : Ar = 10 : 10 sccm and 2 : 10 sccm corresponding to sputtering pressures of 14 and 8 Pa, denoted as Pa-I and Pa-II, respectively (O2 : Ar = 10 : 10 sccm); RF power 230 W (50W); substrate temperature Ts as 400°C (200°C).
A JEOL-JSM6500F FESEM was used to study the morphology of samples. Crystal structures were analyzed using a Rigaku D/Max-RC X-ray diffractometer equipped with Cu Kα radiation source.
TEM images and SAED patterns were recorded to characterize the structure of the individual RuO2
V-shaped RuO2 NW by a Phillips Tecnai G2 F20 FE-TEM at working voltage of 200 kV.
Results and discussion
Fig. 1(a) and (b) presented the 30° perspective view and cross-sectional view FESEM images of TiO2
NCs deposited on SA(100) substrate under Pa-I. Fig. 1(c) shows the XRD pattern of TiO2 NRs deposited on the SA(100) under Pa-I, where the two reflection peaks are identified to be due to the R-TiO2(002) and SA(300). The FESEM images and XRD pattern of the TiO2 NRs grown on SA(100) under Pa-I reveal the uniquely single-directional growth of rutile phase TiO2 NRs along [001] with smooth tops. FESEM micrographs depicted in Fig. 1(d) and (e), respectively, are 30° perspective view and cross-sectional view images of RuO2 NCs deposited on top of TiO2 NRs. As can be seen in Fig. 1(d) and (e), the vertically aligned RuO2 nanotubes (NTs) are grown on top of R-TiO2 NRs. The XRD pattern of RuO2 NTs grown on top of TiO2 NRs (see Fig. 1(f)) indicates that a single directional growth of RuO2(001) NTs has been deposited on the smooth top of TiO2 NRs.The main driving force for the formation of the vertical-aligned RuO2 NCs on top of R-TiO2 NRs is attributed to the close lattice parameters and same crystal structure with the c-direction growth mechanism.
As illustrated in Fig. 2, the FESEM images (a) 30° perspective view and (b) cross-sectional view show densely packed vertically aligned TiO2 NRs with sharp tips supported by octahedrally faceted rods grown on SA(100) substrate under Pa-II. Figure 2(c) shows the XRD pattern of TiO2 NRs deposited on the SA(100) under Pa-II, where the main reflection peaks are identified to be from the R-TiO2(002) and SA(300), indicating pure rutile phase with a (001) preferred orientation has been deposited. Figure 2(c) also indicates the presence of a weak R-TiO2 (101) feature located around 36°.
The FESEM images and XRD pattern of the TiO2 NRs grown on SA(100) under Pa-II reveal the uniquely single directional growth of rutile phase TiO2 NRs along [001] with (101) planes on the tips of NRs. FESEM micrographs depicted in Fig. 2(d) and (e), respectively, display 30◦ perspective view and cross-sectional view images of RuO2 NCs deposited on top of TiO2 NRs. As can be seen in Fig.
2(d) and (e), the well-aligned V-shaped RuO2 NWs with two wedge-like arms were grown on the so called octahedral faceted sharp tips of R-TiO2 NRs. Fig. 2(f) displays the XRD pattern of RuO2 NCs grown on top of TiO2 NRs. The XRD pattern indicates a preferred orientation of RuO2 (101) NCs has
Solid State Phenomena Vol. 170 79
been deposited.
Further structural characterization of the RuO2 NWs on octahedral sharp tips of R-TiO2 NRs was performed using TEM. Fig. 3(a) displays the TEM image of a free stand single V-shaped RuO2 nanocrystal. Fig. 3(b) shows the SAED pattern along the [010] direction corresponding to the twin boundary in Fig. 3(a). The SAED pattern of the junction depicted in Fig. 3(b) shows an overlap of the two series of doublet spots, a result of intersecting of the two arms of different orientations. Fig. 3(c) shows that the HRTEM image in the vicinity of the V-shaped junction regime with the orientation relationship of major axis [001] and plane (101). As shown in Fig. 3(c), the [001] and [101] direction can be identified in each of the arms. The experimentally measured value between these two planes (55°) is consistent with the calculated value (55.33°). It is clear that the preferential growth direction for the rods is along [001] direction in the (002) plane. From a detailed analysis of Fig. 3(b) and (c), it can be concluded that the 110° V-shaped NWs are crystalline RuO2 with a twin plane of (101) and
Fig. 1. FESEM images of rutile-TiO
2 NCs with smooth top (a) 30° perspective-view and (b) cross-sectional-view. (c) XRD pattern of corresponding rutile-TiO2 NCs on SA(100) substrate. FESEM images of vertically aligned RuO2 (001) on rutile-TiO2 NCs with smooth top (d) 30° perspective-view, (e) cross-sectional-view and (f) XRD pattern ofcorresponding RuO2/TiO2
heteronanostructures.
Fig. 2. FESEM images of rutile-TiO
2 NCs with octahedral sharp tips (a) 30°perspective-view and (b)
cross-sectional-view. (c) XRD pattern of corresponding rutile-TiO2 NCs on SA(100) substrate. FESEM images of twinned V-shaped RuO2 (101) on rutile-TiO2 NCs with octahedral sharp tips (d) 30°
perspective-view, (e) cross-sectional-view and (f) XRD pattern of the corresponding RuO2/TiO2 heteronanostructures.
80 Solid Compounds of Transition Elements
twin direction of [
1
01] at the V-junction.Summary
Well-aligned RuO2/R-TiO2 heteronanostructures on SA(100) substrates has been deposited by RFMS under different conditions. The FESEM images and XRD patterns indicated that the growth of vertical-aligned RuO2(001) NTs and twinned V-shaped RuO2(101) NWs on top of R-TiO2 nanorods under different sputtering pressures. The main driving force for the formation of the vertical-aligned RuO2 NCs on top of R-TiO2 NRs is attributed to the c-directional growth mechanism and the same crystal structure with close lattice parameters. TEM and SAED characterizations of the V-shaped RuO2 NWs showed that the NWs are crystalline RuO2 with twin planes of (101) and twin direction of [
1
01] at the V-junction.Acknowledgements
The authors would like to acknowledge the financial supports by National Science Council of Taiwan (Nos. NSC99-2628-E-131-008 and NSC97-2112-M-011-001-MY3) and Ming Chi University of Technology.
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Fig. 3. (a) The TEM image of a free-standing V-shaped RuO
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