AVS2013 Session NS+AS+EM-MoA: Nanowires and Nanotubes

Ordered arrays of Fe-doped TiO₂ nanotubes are semiconductors with potential enhanced catalytic properties and have prospective applications in photocatalytic devices, sensors and dye-sensitized solar cells. Unified understanding of the interrelations between the band structure, crystal structure and response in presence of dopants is fundamental for tailoring functionality. In this study, Fe (1.8 at%)-doped TiO₂ nanotubes are synthesized via electrochemical anodization followed by annealing in an oxygen-rich environment at 450 °C to induce crystallization. The morphology, crystal structure and electronic structure of nominally-pure and Fe-doped titania nanotubes are investigated in the as-anodized and annealed states. Regardless of precise composition, scanning electron micrographs show that the nanotubes are 50 microns in length with 100-nm pore size, arranged in a closed-packed architecture. In the as-anodized state the nanostructures are amorphous; upon annealing in oxygen the anatase structure forms, with a slight expansion (~0.3% increase) in the unit cell volume and larger calculated crystallite size in presence of iron. Near-edge x-ray absorption fine structure spectroscopy (NEXAFS) carried out at the U7A beamline of the National Synchrotron Light Source at Brookhaven National Laboratory clarifies the electronic structure and chemical environment in pure and Fe-doped titania nanotubes. Preliminary results obtained from the oxygen K-edge and the Ti and Fe L-edge data of the nanotube surface confirm formation of the anatase structure upon annealing, with a lower oxidation state of titanium (lower than Ti⁴⁺) noted in presence of iron. Furthermore, the surface anatase formation was more significant (higher surface crystallinity) in the pure nanotubes versus the iron-doped nanotubes. Annealing the nanotubes in an oxygen-rich environment changes the oxidation state of iron. Correlations between the surface and bulk characteristics of the pure and Fe-doped titania nanotubes confirm incorporation of the iron into the lattice structure which modifies the electronic structure by changing the bonding and oxidation state of titanium. These results highlight potential pathways towards further optimization of these nanostructures in order to enhance catalytic functionality.

Funding: National Science Foundation (Grants No. DMR-0906608 and DMR-0908767), U.S. Department of Energy, Division of Materials Science, Office of Basic Energy Sciences (Contract No. DE-SC0005250). Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences (Contract No. DE-AC02-98CH10886).

[1] Paramasivam, H. Jha, N. Liu and P. Schmuki, Small (2012), 8, 3073-3013.

Research supported by the National Science Foundation under Grants No. DMR-0906608 and DMR-0908767, and by the U.S. Department of Energy, Division of Materials Science, Office of Basic Energy Sciences under Contract No. DE-SC0005250. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. This work was also financed by the U.S. Department of Energy, Office of Basic Energy Sciences under Contract DE-AC02-98CH10086.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 10974155 and 10774121), the Natural Science Foundation of Gansu Province of China (Grant No. 1208RJZA197).

References

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Huge efforts are nowadays dedicated to the development of relevant methods to fabricate hollow nanostructures of a wide variety of materials. Among hollow nanostructures, oxide nanotubes appear to be an interesting building block for nanoelectronics. Until today, the oxide nanotubes suffer of their short length and poor organization. In this contribution, we report on highly organized ultra-long metal oxide nanotube arrays (length-up to several centimeters) fabricated by thermal oxidation of metal nanowire arrays. The metal nanowires were grown on nano-grated surface by the plasma-assisted inverse-template method that we have developed recently. Based on the extensive structural study, performed by transmission electron microscopy, the fundamental mechanisms occurring during the formation of such oxide nanotubes will be discussed and explained according to the nanoscale Kirkendall effect. We further show that such method can be extended to the fabrication of periodic zero-dimensional hollow nano-objects.

In addition, we present an extensively structural study of the morphological transformation of oxide nanotubes upon in situ annealing in a transmission electron microscope. Based on this, the role of oxygen on the fundamental mechanisms occurring during the formation of such oxide nanotubes will be discussed. These results show the structural transformation and copper ions diffusion inside an oxide nanotube due to the effect of heating.

Precise control of twin boundaries and stacking faults in semiconductor nanowires provides a number of exciting opportunities to fundamentally manipulate their optical, electrical, and thermal properties. Group IV nanowires, as opposed to their III-V counterparts, rarely exhibit planar defects perpendicular to the {111} orientation and rationally engineering their insertion remains challenging. Here we extend our recent demonstration of defect introduction in vapor-liquid-solid (VLS) synthesized, {111} oriented Si nanowires¹ and report on the appearance of consecutive, geometry-dependent twin boundaries for the first time. Si nanowires with double twins were grown with a Au catalyst by initiating growth at T = 490 ^oC and P = 2×10^-4 Torr for 10 min before changing to T = 410 ^oC and P = 5×10^-4 Torr for another 10 min. While the position of the first twin boundary (TB₁) corresponds to the point where growth conditions were changed, and was the same for all nanowires, the second twin boundary (TB₂) exhibits a diameter-dependent axial position. Detailed electron microscopy analysis shows that thin {111} sidewall facets, which elongate following TB₁ and deform the triple-phase line, are responsible for the formation of TB₂. We hypothesize that an increased amount of surface hydrogen, present as a result of the condition change, favors the thin {111} facets. Our findings help to elucidate the mechanisms that underlie defect introduction in semiconductor nanowires and represent an important step toward the creation of complex defect superstructures in Si.

(1) Shin, N.; Chi, M.; Howe, J. Y.; Filler, M. A. Nano Lett., 2013, in press

As toxic gases like NO_x, benzene, and formaldehyde continue to be emitted globally, their negative impacts on people’s health persist. With gases capable of causing harm on the part per billion level, it is critical to accurately sense toxic gases in real time. This requires creating sensors that operate at lower temperatures, are more sensitive, and are more selective for sensing a desired gas than what is commercially available. SnO₂ is commonly used to create sensing materials because it has enhanced gas-surface interactions as a result of its variable oxidation states, namely Sn²⁺ and Sn⁴⁺. Plasma modification is one method of altering the SnO₂ surface to enhance sensitivity and selectivity. The plasma treatments create more surface oxygen vacancies in the SnO₂, which allows for increased sites for atmospheric oxygen to adsorb. The sorption of gases is highly dependent on the predominant oxygen species on the sensor surface at a given plasma treatment and temperature. Our results demonstrate that SnO₂ nanowires grown by chemical vapor deposition (CVD) and commercial SnO₂ nanoparticles treated in Ar/O₂ plasmas have lattice oxygen removed from the surface. Removal of surface oxygen is corroborated through X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). XPS data show that low plasma power treatments (10-60 W) alter the surface oxygen composition of SnO₂ nanowires and nanoparticles, with the maximum change in composition peaking at 30 W. More importantly, the oxygen vacancies in the treated nanowires are significantly higher than those observed for treated nanoparticles. In addition to surface analysis results, resistance of SnO₂ nanowire and nanoparticle sensors when exposed to a range of gases (i.e., methanol, ethanol, propanol, benzene, and formaldehyde) is used to explore specific gas-surface interactions. By relating plasma treatment conditions to the resulting surface composition and the sensor’s response upon exposure to gases, insight into how surface modification affects gas-surface interactions in sensors will be presented.

Keywords:

Plasma modification, Gas sensor, Tin oxide, Nanowire, Nanoparticle

We report on the controlled vapor transport growth of bismuth chalcogenide nanowires by the vapor-liquid-solid (VLS) mechanism, both in the form of ternary alloys as well as axial variation of the binary compounds bismuth telluride and bismuth selenide. The nanowires exhibiting axial heterojunctions are of specific interest as they promise to reduce a device's effective thermal conductivity through the incorporation of boundary impendences at each junction. These nanowires are measured by suspending a single nanowire between contacts and joule heating the nanowires in a four-probe configuration. The change in resistivity due to the joule heating, in combination with numerical modeling of the axially varying nanowire system, allows for the calculation of the effective thermal conductivity of the nanowires, useful for their exploration as a more efficient low temperature thermoelectric nanostructured material.

Work was supported by the U.S. Army Research Office (W911NF-08-1-0067) and C.J.H. was supported by the GAANN-RETAIN program supported by the U. S. Dept. of Education (P200A100117).