In this section, we reported the synthesis of nearly aligned VOx NWs with precursor of V2O5 NWs on substrate. Products of VO2(R), VO2(B), and V2O3 NWs were successfully obtained via controlled concentration of reducing gas flow and reduced period. We also demonstrate the single crystalline properties of reduced vanadium oxides NWs with specific growth direction. On the basis of the growth direction of each product, possible mechanisms of conversion during reduction reaction are proposed. All as-obtained VOx NWs possess interesting field emission properties with linear F-N property, which are influenced by morphology of NWs and the nature of material. Among these NWs, V2O3 NWS shows the best FE properties with a low turn-on field of 5.3 V/μm and a maximum current density of 8.3 mA/cm2at the applied field of 11.0 V/μm. The feature of vanadium oxide shows excellent FE properties with low turn field and high maximum current density, which might be used as field emission emitter.
Chapter 3
Controlled Synthesis of Nearly Vertical-Aligned Na
0.24V
2O
5Nanowire Thin Films
3.1 Introduction
As a member of vanadium oxide derivative compounds, β-NaxV2O5 (x=0.23-0.41) have
been synthesized with 1D nanostructure by CVD and hydrothermal route, which require long reaction time and complicated procedures.51-52, 58, 60
Moreover, grown NWs of Na1+xV3O8 and β-NaxV2O5 on glass substrates were reported.60, 73 The source of Na+ ions was from the
substrate that diffused into the vanadium oxide precursor.
In this section, a ternary phase of bronze vanadium oxide β-NaxV2O5 NWs were
successfully deposited on the substrate via a modified procedure that combined the original procedures with an additional treatment on the surface of substrate. The source of the Na+ is from sodium silicate, which was first coated on the substrates and the amount of precursor was carefully controlled. Several parameters, including reaction temperature and the concentration of reactants, were found to play important roles in controlling morphology of
the final products. Field emission measurements were carried out and the results show small actuation voltages and a large current density of β-NaxV2O5 NWs arrays, which properties
are prospectively useful in an optoelectronic nanodevice.
3.2 Experimental Section
3.2.1 Synthesis
Thin films of β-NaxV2O5 NWs were grown on a substrate with a two-steps synthetic
process including substrate treatment and thermal evaporation.
3.2.1.1 Substrate treatment.
Sodium metasilicate solutions (Na-solution) were prepared by dissolving Sodium metasilicate (Na2SiO3．9H2O) in deionized water with concentrations ranging from 0.0125M to 0.05M. Prior to the coating treatment, a sodium-free glass substrate (2cm×1cm) was first cleaned by ethanol and deionized water, followed by dropping 0.2 ml of as-prepared solution onto substrate, and finally dried overnight at 40°C to form a thin film of sodium metasilicate salt on the glass.
3.2.1.2. Thermal evaporation.
The solution for each precursor was prepared as following: V2O5 powder (0.1g, 0.55mmole) and NH2OH⋅HCl(aq)(3M, 2mL) were well mixed in a glass vial, and stirred at 50°C. The concentration of precursor solution was ranged from 0.15M to 0.45M with fixed ratio of V2O5 and NH2OH⋅HCl. The color of mixture finally turned from orange to blue, indicating the reduction of V2O5. Thereafter, a glass substrate as prepared was placed on the top of the vial, and this installation was transferred into a programming furnace. The
temperature was raised to 300°C -400°C with a constant rate of 100°C /hr and kept for 1 hour.
Finally, well-aligned β-NaxV2O5 NWs could be found on the lower side of glass.
3.2.2 Characterization.
The as-prepared thin film products were characterized with several analytical techniques.
The crystallinity of products were confirmed by powder X-ray diffraction (XRD, Bruker AXS D8 Advance, Leipzig Germany) with Cu-Kα radiation (λ = 1.54060 Å) operating at 40kV, 40
mA. Cell parameters were refined with the program CELREF.74 Chemical composition of the β-NaxV2O5 NWs was determined with an inductively coupled plasma-atomic emission
spectrometer (ICP-AES, Jarrell-Ash ICAP 9000). The crystal morphology and dimension of NWs were determined from the micrograph analyses of scanning electron microscope (SEM,
Hitachi, S-4700I, operated at 15kV) and transmission electron microscope (TEM, JEOL, JEM-3000F, operated at 200kV). To prepare the sample for TEM experiment, the β-NaxV2O5
NWs were scraped from glass by ultrasonic dispersion of the thin-film in ethanol for 5 minutes. The resulting solution was then dropped onto a copper grid (holey carbon-coated 100 mesh), and dried in air to spread NWs on the carbon film. X-ray photoelectron spectroscopic (XPS) analysis was made with a PHI Quantera SXM spectrometer; the binding energy was
performed near 25°C with a UV–visible spectrophotometer (Hitachi/U-3010) and an
integrating sphere was used to measure the diffuse reflectance spectra over a range 400–800 nm. For field emission measurement, the β-NaxV2O5 thin films were placed in a vacuum chamber with a pressure less than 5×10-6 Pa at room temperature. The distance between the sample and electrode was adjusted to 100µm and the current-to-voltage characteristics were
recorded by high-voltage source meter (Keithly 2410).
3.3 Results and Discussion
3.3.1 Structural and Composition Characterization
The crystallinity and purity of as-prepared β-NaxV2O5 NWs were confirmed by using powder XRD. Figure 3.1 shows XRD patterns of products as deposited on a glass substrate synthesized at various temperature. The patterns can be indexed on the basis of a monoclinic
unit cell with refined lattice parameters of a = 15.40 (3) Å, b = 3.612 (3) Å, c = 10.05 (3) Å, β
= 109.5° (2), which is close to the calculated XRD pattern for Na0.76V6O15 (JCPDS number:
75-1653, space group: C2/m(12) ). As shown in Figure 3.1, the sample synthesized at lower temperature exhibits broad XRD signal, indicative of poor crystallinity. With raising reaction temperature, sharp XRD signal with high signal-to-noise ratio are observed due to improved
crystallinity. Moreover, reaction temperature also affects the intensity of Bragg peaks. For the sample prepared at 400°C, the strongest Bragg peak set is located at (1 1 -1), whereas the
samples prepared at low temperature exhibit the same maximum diffraction peak (2 0 0) as the calculated pattern. The results indicate that NaxV2O5 NWs synthesized at 400°C exhibit preferential orientation with (1 1 -1) facet. No obvious preferential orientation is observed for
samples synthesized at lower temperature. According to these results, we speculate that the NWs observed at T < 400°C were randomly oriented on the substrate without any preferential
orientation. This assumption will be demonstrated from cross-section view of SEM images in
Figure 3.1 The calculated XRD pattern and the XRD profiles of typical product synthesized at the temperature of (a) 300°C, (b) 350°C and (c) 400°C.
Chemical composition of typical product was examined with ICP-AES, which shows
atomic ratio of Na/V ~ 0.12. Combining with XRD result, it is concluded that the NWs deposited on the substrate via a thermal evaporation route are β phase Na0.24V2O5.
3.3.2 The Morphology and Structure of As-Obtained Monoclinic Na0.24V2O5 NWs
The size and morphology of the Na0.24V2O5 NWs as synthesized were first examined with SEM and TEM, as shown in Figure 3.2a and Figure 3.2b, respectively. It is clear that the Na0.24V2O5 product contains long and uniform NWs with average length of 35µm micrometers. Furthermore, the side view (inset of Figure 3.2a) reveals nearly vertically
aligned Na0.24V2O5 NWs on the substrate. The TEM image shows a single NW with the width estimated to be 80-100nm. The selected area electronic diffraction (SAED) pattern of a single wire recorded from [-1 3 1] zone axis reveals sharp and clean diffraction spots, indicative of single crystalline property (inset of Figure 3.2b). HRTEM image recorded from [-1 3 1] zone axis indicates lattice fringe of 3.48Å and 2.93Å, corresponding to (2 0 2) and (1
1 -2) crystal planes for Na0.24V2O5, respectively. The growth direction along the a-axis is deduced from the angle of 155° between the a-axis and the normal vector of plane (1 1 -2),
consistent with the previous studies.51, 59
Figure 3.2 (a) Top view SEM image of Na0.24V2O5 hin-film and side view in inset (b) TEM (left), SAED (right top), and HRTEM (right bottom) images of as-obtained Na0.24V2O5 NWs.
3.3.3 X-Ray Photoelectron Spectroscopic Analyses
XPS measurements have been performed to study the information of oxidation states in the Na0.24V2O5 NWs. As shown in Figure 3.3a, the spectrum demonstrates the presence of the elements of Na, V, and O. The spectrum for V-2p region (Figure 3.3b) shows binding energies (BE) of V 2p3/2 (516.15 eV) and V 2p1/2 (523.71 eV). The peak in V 2p3/2 spectrum exhibits a shoulder, indicative of mixed oxidation states of vanadium ion. After peak modeling, the V 2p3/2 spectrum contains two contributions at 516.9 eV and 515.4 eV corresponding to V5+ and V4+ ions; similar results can also be found in various ternary vanadium oxide bronze compounds.46-47
Figure 3.3 (a) Overall XPS spectrum and (b) high-magnification XPS spectrum for V-2p region of the NWs of Na0.24V2O5
3.3.4 Optimization of Synthetic Condition
The coverage density and the average length of β-Na0.24V2O5 NWs can be controlled by
reaction temperature, the concentration of precursor solution and the concentration of sodium metasilicate during the thermal evaporation. To optimize the synthetic process, we tested several conditions to understand what factors affect the growth of Na0.24V2O5 NWs, and the results are summarizes in Table 3.1 and Figure 3.4-3.6. Figure 3.4a-3.4c shows the effect of temperature to Na0.24V2O5 NWs that were carried out at 300°C, 350°C, and 400°C, respectively. The average length of Na0.24V2O5 wires increase with arising reaction temperature, and the film with the longest average length NWs was obtained at 400°C, similar to our previous study on the V2O5 and MoO3 wires.62-63 Notably, the coverage density of NWs increased as temperature arising, indicating that the temperature of deposition affect the amount of Na0.24V2O5 nanocrystals created on the substrate. The spaces of NWs growth will be limited when the coverage density of NWs is high, resulting in the growth of nearly vertical NWs with no space to grow NWs along the horizontal orientation. For substrate with low coverage density of NWs, we observe randomly oriented NWs lying on the substrate. The results indicate that the length and amount of Na0.24V2O5 NWs can be controlled by reaction temperatures. The substrates with preferred orientation of NWs can be observed in the samples synthesized at high temperature.
Figure 3.4 The side view images of as prepared Na0.24V2O5 NWs thin-films synthesized at (a) 300°C, (b) 350°C and (c) 400°C, respectively.
The concentration of precursor solution and Na-solution are two additional factors that may affect the yield and morphology of the final product. For comparison, experiments with different concentration of the precursor solutions or sodium metasilicate solutions were prepared to synthesize Na0.24V2O5 NWs while keeping other synthetic parameters unchanged.
Figure 3.5 and 3.6 show the side view of NWs on substrates using different amount of precursor and sodium metasilicate. In general, all reactions yield wired shape of Na0.24V2O5
NWs that are essentially independent of reaction temperature. For the effect of vanadium precursor, the average lengths of Na0.24V2O5 NWs are 5.5, 23.1, and 30.5μm for concentration of 0.15, 0.3, and 0.45 M, respectively. For reactions with controlled amount of sodium metasilicate, the average lengths of Na0.24V2O5 NWs are 12.2, 15.7, and 23.1μm for concentration of 0.0125, 0.025, and 0.05 M, respectively. The results indicate that the
coverage density and length of NWs are affected by the concentration of the reactants (either precursor solution or sodium metasilicate solution). The reason for the evolution of NWs might be derived from the supply of reactants. The more the precursors are provided, the longer the growth of Na0.24V2O5 NWs.
Figure 3.5 The side view images of as prepared Na0.24V2O5 NWs thin-films synthesized by using precursor solution of concentration (a) 0.15M, (b) 0.30M and (c) 0.45M, respectively.
Figure 3.6 The side view images of as prepared Na V O NWs thin-films synthesized by
3.3.5 Formation Mechanism of As-Obtained Na0.24V2O5 NWs
To understand the formation mechanism of as-prepared NWs, the evolution process was analyzed with XRD. The temperature of reaction was kept at 300 °C, which were terminated
at definite reaction periods of 15, 30, and 45 minutes. The products were collected for XRD studies and the results are shown in Figure 3.7. For reaction terminated at 15 minutes, the XRD diffraction peaks can be indexed on the basis of the layered phase V2O5·xH2O, consistent with the previous studies.21 For reaction ceased at 30 minutes, the product contains mixtures of NaVO3 and Na0.24V2O5. Finally, the product obtained from reaction period of 45 minutes could be identified as pure single phase of Na0.24V2O5. On the basis of these results, we propose a reaction mechanism about the growth of Na0.24V2O5 NWs, as shown in Figure 3.8.
Table 3.1 Reaction conditions and morphology properties of Na0.24V2O5 NWs identified with SEM
[Na- Solution] [Precursor Solution]
During the initial stage of reaction, polyvanadate species (VOx) were deposited on the substrate with the assistance of vapor transportation.62 The polyvanadate species (VOx) can be considered as mixtures of amorphous and crystalline phase of partially reduced vanadium oxide. At the initial stage, crystalline phase of V2O5·xH2O appeared. Thereafter, the sodium ions from sodium metasilicate were diffused and reacted to polyvanadate species (VOx and
V2O5·xH2O) to form NaVO3, which was further reacted to form the final product β-NaxV2O5.55 The β-NaxV2O5 NWs will continue to grow as polyvanaate species and Na+
ion gradually delivered to the seed crystals, resulting in the formation of β-NaxV2O5 NWs.
The crystal growth along [100] is affected by stacking of VOx and the nature of β-NaxV2O5.
Figure 3.7 The XRD pattern of typical intermediate product synthesized at during period of (a)
Figure 3.8 Schematic illustration of reaction mechanism to deposit β-NaxV2O5 NWs on the
surface of a substrate
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,
42. Takahashi, K.; Limmer, S. J.; Wang, Y., et al., Japanese Journal of Applied Physics 2005,