3.2 Experimental Section
3.3.6 Diffuse Reflectance Measurements
We measured the optical absorption to deduce the intrinsic optical properties of the
Na-0.24V2O5 NWs aligned on glass substrate. Figure 3.9 shows the UV–visible absorption spectrum of the Na0.24V2O5 thin-film that exhibits an onset of absorption near 500 nm, and
the optical band gap (Eg) for the typical product is calculated from the absorption coefficient, α, using the relation αhυ=A(hυ-Eg)1/2 (A: constant; hυ: energy of incident photon).75-76 The inset of Figure 3.9 shows the plot of (αhυ)2 versus photon energy hυ, and the band gap of 2.18eV can be obtained by extrapolating the linear part of the graph to αhυ=0.
Figure 3.9. UV–visible absorption spectrum and the plot of (αhυ)2 versus incident photon energy hυ (inset) for the Na0.24V2O5 thin-film as prepared.
3.3.7 Electronic Field Emission Property.
The as-prepared films with nearly aligned Na0.24V2O5 NWs may exhibit interesting field emission effect, which was measured with a parallel-plate configuration of electrodes near 295 K with a separation 100µm between the anode and an emitting surface of area 0.785mm2. Figure 3.10 depicts the emission current density (J) versus an applied macroscopic field (E)
within a ~0-1100V bias voltage range between the anode and samples. The turn-on field (Eto) about 7.8V/µm is defined as the macroscopic field required producing a current density of 10μA/cm2. The FE current density can reach 4.66µA/cm2 when the applied field increases to 11V/µm. The FE properties between thin-films of Na0.24V2O5 and V2O5 NWs are
summarized in Table 3.2. The value of the turn-on field in this study is higher than the results previously reported for vanadium oxide NWs.62 The variations of the turn-on fields may be attributed to their crystal structure and chemical composition. A Fowler–Nordheim (F–N) plot of (ln I/E2) versus (1/E) appears in the inset of Figure 3.10; a linear relation indicates that the field emission from the film of Na0.24V2O5 NWs conforms to the F–N theory and the emitted current is caused by quantum tunneling at the surface. 77
Figure 3.10 Field emission current density as a function of an applied electric field of as-prepared Na0.24V2O5 NWs thin-film. Inset shows its corresponding Fowler-Nordheim plots.
Table 3.2 Comparison of FE properties between as-prepared product and V2O5 NWs Eto(V/µm) Imax(mA/cm2)
V2O5 NWs 8.3 1.8 at an applied field of 18V/µm NaxV2O5 NWs 7.8 4.66 at an applied field of 11V/µm
3.4 Summary
In this study, high quality and nearly aligned Na0.24V2O5 NWs were fabricated via a simple, economical, mild, and template-free thermal evaporation method. The diameter and average length of NWs are 80-100nm and tens of micrometers, which grew along the [100]
direction and tilted out of the glass substrate surface. A possible mechanism of crystal growth is proposed, and we also demonstrated that the distribution and length of NWs on thin film could be controlled by reaction temperature, concentration of precursor solution and sodium metasilicate. The as-obtained Na0.24V2O5 NWs exhibit excellent field emission properties with a low turn-on field of 7.8 V/μm and a maximum current density of 4.66 mA/cm2 at the applied field of 11.0 V/μm with linear F-N property, which might be used as field emission emitter. These results provide a new strategy to synthesize ternary inorganic NWs with great flexibilities in controlling the sizes, shapes, and coverage density of the NWs on different substrates. This unique synthetic route is expected to be applied to other aligned vanadium oxide bronze NWs, such as MxV2O5 (M = K, Cu, Ag).
Chapter 4 Conclusions
In summary, binary phase (VO2(R), VO2(B), and V2O3) and ternary phase (β-NaxV2O5) vanadium oxides nanowires have been successfully deposited and nearly vertically-aligned on the surface of substrate via two systematic procedure based on thermal evaporation. These as-prepared 1D nanomaterials possess diameter of 70-150nm and average length of tens micrometer, which grew and tilted on the surface of glass substrate surface with growth direction along specific direction of each product. The growth direction of the reduced products synthesized by post-reduction treatment, including VO2(R), VO2(B), and V2O3
nanowires, are affected by the growth of the original phase, V2O5 nanowires. Based on this
concept, we propose the possible transformation mechanisms during the reduction treatment.
On the other hand, the growth direction of β-NaxV2O5 nanowires were examined along [1 0 0]
direction, and the possible formation mechanism has also been discussed in detail.
The field emission properties of these binary and ternary vanadium oxides nanowires have been investigated. The as-prepared 1D nanomaterials exhibited excellent field emission performances which are highly dependent on their nature properties of crystal. Field emission properties of these as-prepared nanowires possessed low turn-on field, high emission current density and linear Fowler-Nordheim behaviors. Among these typical products, the lowest
with Eto of 5.2 V/µm and Jmax of 8.3 mA/cm2 at the field of 8.3V/um. These remarkable results suggest that these 1D nanomaterials can be served as promising candidates for future field emission devices.
In this thesis, our study provided a novel synthetic procedure to deposit binary and ternary phase of vanadium oxide with nanostructure on the substrates, which are excellent for preparing metal oxide with 1D nanostructure owing to its utility advantages of simple, economic, time-saving and convenient for depositing thin-film on substrates. This unique synthetic route is anticipated to be applied to fabricate various binary and ternary phase metal-oxide nanocrystals well aligned on substrates with special crystal morphologies in choosing suitable precursors.
Reference
1. Ercolessi, F.; Andreoni, W.; Tosatti, E., Phys.l Rev. Lett. 1991, 66, 911-914.
2. Takagahara, T.; Takeda, K., Phys. Rev. B 1992, 46, 15578-15581.
3. Zhou, C.; Mai, L.; Liu, Y., et al., J. Phys. Chem. C 2007, 111, 8202-8205.
4. Chen, W.; Zhou, C.; Mai, L., et al., J. Phys. Chem. C 2008, 112, 2262-2265.
5. Melosh, N. A.; Boukai, A.; Diana, F., et al., Science 2003, 300, 112-115.
6. Santangelo, S.; Messina, G.; Faggio, G., et al., Diam. Relat. Mater. 2010, 19, 590-594.
7. Jiang, Y.; Zhang, W. J.; Jie, J. S., et al., Adv. Mater. 2006, 18, 1527-1532.
8. Li, X. L.; Cai, K. F.; Li, H., et al., Superlattice. Microst. 2010, 47, 710-713.
9. Sanmathi, C. S.; Takahashi, Y.; Sawaki, D., et al., Mater. Res. Bull. 2010, 45, 558-563.
10. Liu, A.; Ichihara, M.; Honma, I., et al., Electrochem. Commun. 2007, 9, 1766-1771.
11. Jiao, L.; Yuan, H.; Si, Y., et al., Electrochem. Commun. 2006, 8, 1041-1044.
12. Wang, Y.; Cao, G., Electrochim. Acta 2006, 51, 4865-4872.
13. Zhang, K.; Bao, S.; Liu, X., et al., Mater. Res. Bull. 2006, 41, 1985-1989.
20. Isabelle, R.; Marko, B.; Ulrich, S., et al., Sensors Actuat. B- Chem. 2005, 106, 730-735.
21. Liu, J. F.; Wang, X.; Peng, Q., et al., Adv. Mater. 2005, 17, 764.
22. Bakhteeva, Y. A.; Podval’naya, N. V.; Volkov, V. L., Inorg. Mater. 2010, 46, 1112-1114.
24. Xiong, C. R.; Aliev, A. E.; Gnade, B., et al., Acs Nano 2008, 2, 293-301.
25. Ji, S.; Zhao, Y.; Zhang, F., et al., J. Cryst. Growth 2010, 312, 282-286.
26. Wang, Y.; Cao, G. Z., Chem. Mater. 2006, 18, 2787-2804.
27. Leger, C.; Bach, S.; Pereira-Ramos, J. P., J. Solid State Electr. 2007, 11, 71-76.
28. Ramana, C. V.; Smith, R. J.; Hussain, O. M., et al., Surf. Interface Anal. 2005, 37, 406-411.
29. Menezes, W. G.; Reis, D. M.; Benedetti, T. M., et al., J. Colloid Interf. Sci. 2009, 337, 586-593.
30. Leroux, C.; Nihoul, G.; Van Tendeloo, G., Phys. Rev. B 1998, 57, 5111-5121.
31. Morin, F., Phys.l Rev. Lett. 1959, 3, 34-36.
32. Richardson, M. A.; Coath, J. A., Opt. Laser Technol. 1998, 30, 137-140.
33. Nandakumar, N. K.; Seebauer, E. G., Thin Solid Films 2011, 519, 3663-3668.
34. Mathur, S.; Ruegamer, T.; Grobelsek, I., Chem. Vapor Depos. 2007, 13, 42-47.
35. Zhou, F.; Zhao, X. M.; Yuan, C. G., et al., Cryst. Growth Des. 2008, 8, 723-727.
36. Zhai, T.; Liu, H.; Li, H., et al., Adv. Mater. 2010, 22, 2547-2552.
37. Sediri, F.; Touati, F.; Gharbi, N., Mater. Sci. Eng. B-Adv. 2006, 129, 251-255.
38. Guiton, B. S.; Gu, Q.; Prieto, A. L., et al., J. Am. Chem. Soc. 2005, 127, 498-499.
39. Santulli, A. C.; Xu, W. Q.; Parise, J. B., et al., Phys. Chem. Chem. Phys. 2009, 11, 3718-3726.
40. Corr, S. A.; Grossman, M.; Furman, J. D., et al., Chem. Mater. 2008, 20, 6396-6404.
41. Zhou, F.; Zhao, X. M.; Yuan, C. G., et al., Chem. Lett. 2007, 36, 310-311.
42. Takahashi, K.; Limmer, S. J.; Wang, Y., et al., Japanese Journal of Applied Physics 2005, 44, 662-668.
43. Wu, X.; Tao, Y.; Dong, L., et al., Mater. Res. Bull. 2005, 40, 315-321.
44. Ji, S.; Zhang, F.; Jin, P., Mater. Lett. 2011, 65, 708-711.
46. Liu, Y.; Zhang, Y.; Zhang, M., et al., J. Cryst. Growth 2006, 289, 197-201.
47. Souza, E.; Lourenco, A.; Gorenstein, A., Solid State Ionics 2007, 178, 381-385.
48. An, J. N.; Xu, C. Y.; Zhen, L., et al., Ceram. Int. 2010, 36, 1825-1829.
54. Onoda, M.; Takahashi, T.; Nagasawa, H., J. Phys. Soc. Jpn. 1982, 51, 3868-3875.
55. Yamada, H.; Ueda, Y., J. Phys. Soc. Jpn. 1999, 68, 2735-2740.
61. Sahana, M. B.; Dharmaprakash, M. S.; Shivashankar, S. A., J. Mater. Chem. 2002, 12, 333-338.
71. Jeon, S.; Kim, H.; Yong, K., Journal of Vacuum Science & Technology B:
Microelectronics and Nanometer Structures 2009, 27, 671.
72. Wang, S.; He, Y.; Fang, X., et al., Adv. Mater. 2009, 21, 2387-2392.
73. Xu, J. J.; Zhang, X. T.; Wang, D. J., et al., Chem. Lett. 2005, 34, 838-839.
74. B. Laugier, J. B. Celref, http://www.inpg.fr/LMGPS; Laboratoire des Materiaux et du Ge ́nie Physique de l’Ecole Supe ́rieure de Physique de Grenoble.
75. Han, J.; Spanheimer, C.; Haindl, G., Sol. Energ. Mat. Sol. C. 2011, 95, 816-820.
76. Saipriya, S.; Sultan, M.; Singh, R., Physica B 2011, 406, 812-817.
77. Tang, Q.; Li, T.; Chen, X., et al., Solid State Commun. 2005, 134, 229-231.