The Facile Preparation of Partially Reduced V2O5 Nanowire Sheets

Chapter 12

The Facile Preparation of Partially Reduced V O Nanowire Sheets 2

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Chunrong Xiong , Ali E . Aliev , Gaurang Pant , Bruce Gnade , and Kenneth J. Balkus, Downloaded by HARVARD UNIV on October 1, 2013 | http://pubs.acs.org Publication Date: September 19, 2008 | doi: 10.1021/bk-2008-0996.ch012

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Department of Chemistry, UTD Nanotech Institute, and Department of Electrical Engineering, The University of Texas at Dallas, Richardson, TX 75083 3

Hydrated and partially reduced V O nanowires with high aspect ratios have been synthesized by the hydrothermal condensation of ammonium metavanadate. The V O nanowires can be fabricated into free standing and flexible sheets. The V /V ratio in the nanowires can be increased with the hydrothermal synthesis temperature. The extremely low heat capacitance of thin V O sheets and the high temperature gradient of resistance make them attractive materials for bolometric (IR) sensing. The current responsivity of a 30μm sheet is S = d V / d Q =178 V/W. 2

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Introduction V 0 exhibits a layered structure, which allows the intercalation of a variety of ions (1-8), organic compounds (9), and polymeric species (10-11) to form functional materials that are promising for lithium batteries (3-8), catalysis (7219), electrochromic devices (20-24), and supercapacitors (25-27). Additionally, fibrous V 0 has shown some electrical properties as nanolithography templates (28-29), sensors for ethanol and amine (30-33), electric field-effect transistors (34-36), crossed junctions and rotational actuators (37). Several methods have been developed to prepare vanadium oxide 1-D nanostructures. Nesper and co workers (38-40) synthesized vanadium oxide nanotubes (VO -NTs) by a sol-gel reaction of vanadium alkoxides with a primary amine, followed by hydrothermal treatment. Stucky and co-workers (41) synthesized mixed-valence vanadium 2

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© 2008 American Chemical Society

In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

153 oxide nanorods via a nonaqueous low-temperature procedure. V 0 nanorods and nanowires have been synthesized in reverse micelles (42-43). Cao et al. (44) have also developed a polyol process to self-assemble V 0 • x H 0 nanorods into hollow microspheres. Takahashi et al (22) reported the growth of V 0 nanorod arrays using electrophoretic deposition combined with templating. For these syntheses, the length of the nanowires was usually less than 10 jim. Long nanowires are expected to facilitate formation of free standing sheets. V 0 sheets comprising entangled nanowires have been used as actuators (45), which were demonstrated to provide a high Young's modulus, high actuator-generated stress, and high actuator stroke at low applied voltage. In this case, the hydrated V 0 nanowires were 10 \im long and synthesized by the polycondensation of vanadic acid in water at room temperature (45,46). This process requires several weeks due to the slow ion exchange between Na and H ions in a resin from sodium metavanadate solutions at room temperature. In this paper, we introduce a facile method to make high aspect ratio V 0 nanowires within one day, and these nanowires were partially reduced. Presence of V in nanowires may increase electrical conductivity. Furthermore, V 0 undergoes an abrupt semiconductor-metal transition near 68°C, accompanied by dramatic changes in electrical and optical properties. These dramatic changes and the near room temperature phase transition make V 0 an attractive material for a wide variety of applications such as optical switching and detection (47,48), optical storage (49), laser protection (50), smart window coatings (57) and bolometers (52). 2

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Downloaded by HARVARD UNIV on October 1, 2013 | http://pubs.acs.org Publication Date: September 19, 2008 | doi: 10.1021/bk-2008-0996.ch012

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Experimental V 0 nanowires were prepared by the hydrothermal polycondensation of ammonium metavanadate (Aldrich) in autoclave. In a typical synthesis, 0.3 g ammonium metavanadate and 0.20 g P123 (E0 oP0 E0 o) were dissolved in 30 ml water containing 1.5 ml 2M HC1. This mixture was stirred at room temperature for 7 hours, and then transferred to 50 ml Teflon lined autoclave and heated to 130 °C and kept at this temperature for 24 hours. The resulting precipitate was dispersed in 50 ml deionized water under vigorous stirring. Subsequently, the product was suction filtered, rinsed with deionized water, and acetone. Nanowires were dried at 80 °C for 12 hours. The V 0 can be peeled off the filter paper as a sheet. To increase The V / V ratio of V 0 nanowires, the hydrothermal polycondensation of ammonium metavanadate was conducted at 160 °C and 190°C. X ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X ray photoelectron spectroscopy (XPS) were used to characterize the materials. 2

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154 Results and Discussion 1.

Preparation and Characterization of Nanowires

Figure la displays a digital image of a free standing V 0 nanowire sheet. As can be seen in the image, the sheet can be flexed multiple times without breaking. The paper can also be cut with scissors. The resulting sheet comprises entangled nanowires as shown in the Figure lb. The as made nanowires have high aspect ratios (> 4000). The nanowires are over 120 \im long and 29 nm wide as seen in the Figure lb and lc respectively. The HRTEM images in the Figure Id shows the crystalline layered structure with a lattice spacing of 9.7 A. The nanowires grow along the [010] direction. The green-brown color of the nanowires suggests partial reduction of the V ions. PI23 behaved as reducing agent. Without PI23, the as-made wires were pure V 0 wires, but with -20 nm long and ~ 130 nm wide. The X-ray diffraction pattern of the as-synthesized nanowires shows a characteristic peak at 20 = 8.8° as seen in Figure 2, consistent with hydrated V 0 (7,11,26). The 00/ reflections are consistent with the layered structure of hydrated V 0 . The interlayer spacing of the V 0 x H 0 nanowires was 10.1 A, which is larger than that of V 0 - 0.3 H 0 (8.9 A) and smaller than that of V 0 1.7 H 0 (11.5 A) (53,54). Therefore, the value of x could be between 0.3 and 1.7. To determine the content of water in the hydrated V 0 layers, thermogravimetric analysis (TGA) was carried out on the nanowires. The derivative thermogravimetric curve shown in Figure 3 a reveals three peaks due to removal of adsorbed and chemically coordinated water molecules, as previously reported (26). The low-temperature weight loss peak at 130 °C may be attributed to the loss of free water. The second weight loss located at ~ 270 °C is due to the loss of physically adsorbed water. For these two low temperature peaks, the weight loss percent were 2.0 % and 4.5 % respectively as seen in Figure 3b. The maximum peak temperature for weight loss was centered at 420 °C, and the weight loss was 12.3 %, which was ascribed to the loss of coordinated water. Thus, the molar ratio of the coordinated water to V 0 in the hydrated V 0 nanowires may be - 1.25. This is also consistent with the dspacing of 10.1 A. The XPS spectrum of the as-made nanowires is shown in Figure 5a. The main peak (2p ) located at 517.2 eV, suggests a +5 oxidation state for most of the vanadium ions. In addition, a small peak at 515.85 eV indicates that partial reduction of the V ions to V occurred during the hydrothermal condensation. The molar ratio of V to V was 0.17. The green-brown color of the nanofibers before heating is consistent with partial reduction of the V . If the hydrothermal condensation temperature was increased to 160 °C, the nanowires displayed an XPS spectrum with peaks at 515.85 eV and 517.2 eV as


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shown in the Figure 5b, indicating that more V ions were reduced compared to the 130 °C synthesis. The molar ratio of V to V was 0.38 for the 160 °C synthesis. The ratio of V to V was further increased to 0.49 as the hydrothermal condensation temperature was increased to 190 °C. For these two deeply reduced V 0 samples, they can also be made into flexible sheets. However, when the hydrothermal condensation temperature was 160 °C, the sheet was green as seen in figure 6a, and cyan for 190 °C as seen in figure 6c. The SEM images of the deeply reduced V 0 nanowires exhibited shorter length (10 -20 nm) and bigger diameter (~ 100 nm) as shown in figure 6b, 6d. 4 +

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Application of the Partially Reduced V 0 Nanowires in a Bolometer 2

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The high temperature gradient of resistivity of V 0 at dielectric-metal phase transition (T =67°C) appear to be very attractive for thermal sensing applications. However, the high heat capacity of bulk material or thin films deposited on substrates represents the main obstacle for the thermal sensitivity response time of these type devices. Free standing V 0 . nanowire paper can create new opportunities for the application of V 0 . nanowires in metric sensing. The V 0 . nanowire paper as a bolometric material has many advantages including extremely low heat capacity (low inertia), high absorbance coefficient in wide wave range from 0.2 to 20 ^im, emissivity coefficient of paper reduced in vacuum is close to unity (black as graphite), flexibility, resistance to hard radiation damage, high resistance gradient and absorption of radiation, all of these make nanowires very attractive for bolometric materials. While we have not made pure V 0 , the reduced V 0 may still provide interesting properties. The bolometric heat balance equation in constant current mode (CCM) can be written as 2

C

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TiP + I R ( T ) - ^ ( T - T ) h

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where P is the incident radiation power, r| is the absorbance of the bolometer, I is the constant bias current, X is the thermal conductivity to the heat sink, C is the heat capacity of the sensitive element, T is the heat sink temperature and R(T) is the temperature dependent sensor resistance. The output voltage of the bolometer operated in CCM is determined by the responsivity which is defined as the change of output voltage for a change in incident power. Neglecting electrothermal effects the responsivity can be written as, h

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Figure 1. Nanowires prepared with hydrothermal condensation of ammonium metavanadate at 130 °C, (a) a photo of as prepared V 0 x H 0 sheet, (b) and (c) SEM images ofV 0 x H 0 nanowires with difference magnifications, (d) TEM image ofV 0 x H 0 nanowires, and the inset is high resolution TEM image of lattice planes.



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Figure 2. XRD pattern of (a) V O x H 0 nanowires prepared at 130 °C, (b) after annealing at 350 °Cfor 5 hours.

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Figure 3. (a) Thermogravimetric analysis (TGA) curve for the as made V 0 x H 0 nanowires prepared at 130°C. Test conditions: N flow rate, 20ml min , heating rate, 4 Kmiri . (b) Weight loss curve with temperature for the as made V Oyx H 0 nanowires.

Temperature / Degree

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Figure 4. V 0 nanowire sheet after annealing in air at 350°C for 5 hours, (a) a photo ofV 0 nanowire sheet, (b) and (c) SEM images ofV 0 nanowires with difference magnifications, (d) TEM images ofV 0 nanowires.



5172 eV * 5159 eV

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Binding Energy (eV) Figure 5. XPS spectra of the as made V 0 nanowires, (a) hydrothermal condensation at 130 °C, (b) hydrothermal condensation at 160 °C, and (c) hydrothermal condensation at 190 °C. 2

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163 where, dV = 0.37 V is the potential change on the sample measured in chronopotentiometry mode with constant applied current of 8 mA, R^l.03 kOhm is the initial resistance of the sample and P=9.8 mW is the incident light power (halogen-tungsten lamp, 15 W). The obtained value of the responsivity for the nanowire paper prepared at 190°C is relatively low due to the low R(T) dependence at room temperature. This value can be significantly enhanced for V 0 with pronounced metaldielectric phase transition. This type transition at 67°C is well known for bulk and thin film V 0 . The as-made nanowire sheet has negative temperature gradient over the entire temperature range. However, if the nanowires are further reduced at 600°C in 5% hydrogen environment (or in argon atmosphere) for 4 hours, the negative slope at 240 K changed to positive. The absolute value of resistance at room temperature decreases more than one order of magnitude (see Figure 7). The appearance of samples also changes after each thermal treatment. The pristine sample had dark cyan color. Sintering at 500°C for 4 hours in air (oxidation) turns the sample color to light yellow. Sintering in hydrogen or argon atmosphere at the same conditions (reduction) turn the sample color to black. The elemental analysis by electron microscope (LEO 1530 VP equipped with Energy Dispersive Spectrometer (EDS) for X-ray microanalysis) shows significant reduction of V 0 . . It is obvious that the reflectance and transmittance spectra should also change. The oxidation substantially increases the transmittance and reflectance of the sample. At the same time the resistance of sample increased to two orders and reached of several MQ at room temperature. The temperature dependence of resistivity resembles the behavior of wide bandgap semiconductors. The reduction decreases the transmittance and reflectance of the sample (see curve 3 in Figure 8a, 8b) and brings it to metallic behavior of resistivity at room temperature. The obtained response time measured by electronically modulated laser beam is also relatively high (see Figure 9.). The sample thickness fabricated so far (d=15 nm) creates long thermal response and prevent the penetration of heat radiation into the sample (Eq.l). The further development of thinner free standing V 0 . film is in progress. In summary, high aspect ratio hydrated V 0 nanowires have been synthesized, which can be entangled into a flexible sheet. Hydrated and partially reduced V 0 nanowires exhibit a layered structure with -1 nm layer spacing. Upon loss of coordinated water by annealing, fibrous morphology is maintained although contraction of the lattice occurred due to dehydration, the orthorhombic V 0 phase formed. V 0 nanowires with a higher degree of reduction were produced by increasing the hydrothermal condensation temperature or by past synthesis of heating under H or Ar. The hydrothermal polycondensation procedure is very simple and highly reproducible, and it is readily applicable to large scale synthesis for potential industrial applications. 2


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In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008. 2

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Figure 6. (a) a photo and (b) SEM images ofpartially reduced V O x H 0 sheet prepared at 160 °C, (c) a photo and (d) SEM images ofpartially reduced V 0 x H 0 sheet prepared at 190 °C.


In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008. a

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Figure 7. Temperature dependence of resistivity of (a) pristinefree-standingpaper of V0 . nanowires, the inset shows the 4-probe resistance measurement design and (b) annealed at 400°C in air and then sintered at 600° C in 5% hydrogen environment.

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In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008. 2

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Figure 8. (a) transmittance and (b) reflectance spectra of curve 1 for pristine nanowire sheet, curve 2 for the oxidized nanowire sheet in air atmosphere at 500°Cfor 4 hours, curve 3 for the reduced nanowire sheet in argon atmosphere at 500°C for 4 hours, V 05. nanowire paper (d=15 jum).



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Figure 9. Potential modulation by tungsten-halogen lamp (10mW/cm ) for fixed applied current, 8 mA.

Acknowledgments We gratefully acknowledge the financial support from the R. A . Welch Foundation.

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The Facile Preparation of Partially Reduced V2O5 Nanowire Sheets

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