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The ZnO nanowires were prepared by thermal evaporation via vapor-liquid-solid (VLS) mech- anism originally developed by Wagner and Ellis [98]. Ana-plane sapphire single crystalline sub-

1_{Adapted in part by permission from J. Chenet al. IEEE Tran. Nanotech. 9(2010)634. Please check Appendix} D. cCopyright [2010] IEEE.http://dx.doi.org/10.1109/tnano.2010.2052629

Figure 3.2: Schematic diagrams of nanowire cross-sections showing the growth processes of poly- crystalline Pd/SnO2 nanowire using a ZnO nanowire template: (i) VLS growth of pristine ZnO nanowire; (ii) PLD deposition of a polycrystalline layer of SnO2; (iii) Sputtering coating of Pd catalysts. [136]

strate was first cleaned and coated by a 3-nm gold layer with sputtering. In the thermal evaporation process, 1 g ZnO (Alfa Aesar, CAS# 131413-2) and graphite powder (Alfa Ae- sar, CAS# 231955- 3) mixture (mass ratio 1:1) worked as the source materials and was heated to 1000◦C through the whole growth process. The processing gas was a mixture of 100 sccm Argon and 10 sccm Oxygen, which carried the reduced Zn vapor from the source material to downstream low temperature area (800◦C) where the sapphire wafer was located. The sputtered gold layer melted and agglomerated into nanoparticles under the high growth temperature. The gold nanoparticles worked as the cat- alysts in the VLS growth and determined the diameter of the ZnO nanowires [98]. The as-grown ZnO nanowires were then loaded in a PLD system for SnO2nanocrystal coating. The background Argon pressure was maintained relatively high (10 torr) to promote the diffusion transport of depo- sition species, ensuring a homogeneous SnO2coating. A layer of 5 nm SnO2was coated at a tem- perature of 400◦C. Finally, the nanowires were loaded into a DC-sputtering system. A layer of Pd particles (0.2 nm normalized thickness) was sputtered onto the nanowire surfaces. The schematic diagrams of these processes are shown in Figure 3.2. All the as-synthesized nanowires were first

characterized by a Carl Zeiss 1530 VP field emission scanning electron microscope (FESEM) and a JEOL 2010 conventional transmission electron microscope (TEM). The initial composition anal- yses were conducted by the energy dispersive X-ray spectroscopy (EDS) equipped on the FESEM and TEM. Detailed structural characterization of Pd nanoparticle coatings was achieved by high- angle annual dark-field (HAADF) scanning TEM (STEM) by a JEM-2100 F-field emission TEM with CEOSCS-corrector and the HAADF detection angle was 73∼194 mrad.

Figure 3.3: (a) and (b) FESEM images of a ZnO nanowire and a Pd/SnO2nanocrystal coated ZnO nanowire. (c) and (d) Conventional bright field and dark field images of a Pd/SnO2 coated ZnO nanowire. [136]

VLS process is the most successful and controllable method for the growth of high-quality single crystalline ZnO nanowires. Figure 3.3(a) shows the FESEM image of a ZnO nanowire with a diameter of about 80 nm. The lengths of the nanowires are about 20 µm. The pristine ZnO nanowires show smooth surfaces without any undulations. The surface roughness increased after the SnO2 and Pd nanocrystal coatings, shown in Figure 3.3(b). Figure 3.3(c) and (d) are low-magnification bright field and dark field images taken by a JEOL 2010 conventional TEM. The coated SnO2 nanocrystals were highlighted in the dark-field image.The thickness of SnO2

coating is about∼4 nm with polycrystalline structure. Though the SnO2 nanocrystals formed a continuous film on the ZnO-nanowire surface, no specific epitaxial relation was found. Due to the different crystal structure and large lattice mismatch that induces high lattice stress, the SnO2was grown with Volmer-Weber mode on the ZnO surfaces [137]. Unlike many applications that require epitaxial growth to reduce density of grain boundaries (GB) to avoid carriers scattering [138, 139], the large amount of GB in these nanowires is beneficial for gas-sensing properties since the energy barriers formed at the GB are very sensitive to the surface reaction and may result in significant change of device conductivity. Detailed structural and compositional characterizations are needed to evaluate the Pd coating.

Figure 3.4(a) is a high-resolution TEM (HRTEM) image enlarged from the edge of a Pd/SnO2 nanocrystals-coated ZnO nanowire. The core ZnO nanowire was identified as single crystalline wurtzite structure with growth direction along [0001] by using electron diffraction, while SnO2 was identified as a rutile structure. A HAADF Z-contrast image was obtained under aC_S-corrected TEM to characterize the distribution of Pd-nanocrystal coating, as shown in Figure 3.4(b). Because Pd has much larger atomic mass than other elements (Zn, Sn, and O) in the nanowires, the Pd nanocrystals can be highlighted by the contrast difference. The Pd nanocrystals have an average diameter of about 2 nm with an interparticle distance of 2 nm. The interspaces of Pd nanocrystals that allow targeting species to reach SnO2layer and avoid current paths through the Pd layer are extremely critical for the sensing performance and require well-controlled deposition process. The inset of Figure 3.4(b) gives a high-resolution HAADF image of a single Pd nanocrystal sitting on the nanowire surface. The Pd nanocrystal has irregular surface facets, which contribute to the high-catalytical activity [140]. A nanoprobe EDS linescan is depicted in Figure 3.4(c) to confirm the element distributions and the EDS spectra at three typical spots are also given in Figure 3.4(d). From the nanoprobe linescan, it can be confirmed that the SnO2 nanocrystals are coated on the surface of ZnO nanowire and the Pd nanocrystals were deposited on the outermost layer of the nanowire.

Figure 3.4: (a) A high resolution TEM image showing the ZnO nanowire with polycrystalline SnO2 and Pd nanocrystal coating. (b) A HAADF Z-contrast image taken under aCS-corrected field emission TEM highlighting the Pd nanocrystal distribution. The inset shows the lattice image of a Pd nanocrystal taken by HAADF STEM through [110] zone. (c) Composition distribution obtained by nanoprobe EDS linescan across the nanowire. The sampling points are indicated in Figure (b). (d) Three EDS spectra obtained at the ZnO core (i), SnO2 layer (ii), and outmost Pd coating (iii). [136]