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Single-crystalline Ag2V4O11 nanobelts: hydrothermal synthesis, field emission, and magnetic

properties

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2005 Nanotechnology 16 2892

(http://iopscience.iop.org/0957-4484/16/12/027)

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INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY

Nanotechnology 16 (2005) 2892–2896 doi:10.1088/0957-4484/16/12/027

Single-crystalline Ag2V4O11 nanobelts:hydrothermal synthesis, field emission,and magnetic propertiesChangjie Mao, Xingcai Wu, Hongcheng Pan, Junjie Zhu1 andHongyuan Chen

Department of Chemistry, Key Lab of Analytical Chemistry for Life Science,Nanjing University, Nanjing 210093, People’s Republic of China

E-mail: jjzhu@mail.nju.edu.cn

Received 2 August 2005, in final form 15 September 2005Published 20 October 2005Online at stacks.iop.org/Nano/16/2892

AbstractA large number of single-crystalline Ag2V4O11 nanobelts with thickness of10–30 nm, width of 70–200 nm, and lengths of 2–5 µm have beensuccessfully synthesized by a hydrothermal method. The morphologies andstructures of the nanobelts were characterized by x-ray powder diffraction,x-ray photoelectron spectroscopy, transmission electron microscopy,high-resolution transmission electron microscopy, and thermal gravimetricanalysis. The magnetic measurement data showed that Ag2V4O11 nanobeltsare paramagnetic materials. The nanobelts exhibited an electronic emissionturn-on field of 28.9 V µm−1, which is the expected macroscopic fieldrequired to produce a current density of 10 µA cm−2. On the basis of theanalytical data, it was observed that Ag2V4O11 nanobelts could serve as anovel candidate for a future field emitter.

1. Introduction

Attention has been focused on both the preparationand the properties of one-dimensional (1D) nanostructurematerials (nanotubes, nanowires, nanobelts, and nanorods),due to their distinctive geometries, novel physical andchemical properties, and potential application in numerousareas such as single-electron transistors [1], field-effecttransistors [2], interconnections in nanoelectronics [3],lasers [4], photodetectors [5–7], and sensors [8–11].At present there are a number of methods for thepreparation of 1D nanomaterials, including the sonochemicalmethod [12], microwave irradiation [13], microemulsion-mediated systems [14], hydrothermal synthesis [15], andchemical vapour deposition [16, 17]. Due to the advancementin the synthetic methods, various nanowires and nanobelts havebeen obtained, ranging from elementary substances to binarycomponents. All these have resulted in limited types of single-crystal ternary component nanobelts being obtained so far,because the preparation of new ternary component nanobeltsis still a challenge.

1 Author to whom any correspondence should be addressed.

The derivatives of vanadium oxide compounds areimportant both in catalysis and cathode materials [18]. Forexample, ReVO4 (Re = Pr, Er, Gd, Dy, and Nd) showscatalytic performances in the oxidative dehydrogenation ofpropane [19], whereas vanadium oxide compounds containingsilver are ideal cathode material in rechargeable high-energy-density lithium batteries due to their high specific capacityand high rate capability [20]. Present results have shownthat Li-doped Ag2V4O11 is unique in commercialized batteriesfor powering the implantable cardiac defibrillator [21]. Suchmaterials would be of great significance because of the possiblenovel properties induced by their reduced dimensionality.Over the past few years, there have been some reports onthe synthesis of 1D nanomaterial of vanadium oxides andvanadates. For instance, Yu et al prepared single-crystallineNa2V6O16·3H2O nanobelts [22] by a hydrothermal methodusing V2O5 and NaF. Wu et al synthesized single-crystalline(NH4)0.5V2O5 nanowires [23] by a hydrothermal reactionusing NH4VO3 and prepared VO2 nanowires [24] and NdVO4

nanorods [25] using (NH4)0.5V2O5 nanowires as templates,respectively. Xie et al [26] synthesized Ag2V4O11 gel by anultrasonic sol–gel method using V2O5 sol and Ag2O. Despite

0957-4484/05/122892+05$30.00 © 2005 IOP Publishing Ltd Printed in the UK 2892

Single-crystalline Ag2V4O11 nanobelts

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811

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3

323

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702

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411

30231

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0

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Figure 1. Powder XRD pattern of the as-prepared Ag2V4O11

nanobelts.

all these, the preparation of high-quality Ag2V4O11 nanobeltsand their electronic field-emission properties have not beenreported in literature. In view of the above, we report hereina controllable and convenient approach to the manufacture ofsingle-crystalline Ag2V4O11 nanobelts, which requires neithersophisticated techniques nor catalysts.

2. Experimental section

The Ag2V4O11 nanobelts were synthesized by a hydrothermalmethod. All the reagents used in this experiments were ofanalytical purity and were used without further purification.In a typical procedure, a mixture of 2.5 mmol vanadiumpentoxide powders, 2.5 mmol silver nitrate and 2.5 mmol 1,6-hexanediamine (as organic template) was dissolved in 40 mldistilled water. This solution mixture was then placed into aTeflon-lined stainless steel autoclave, sealed and maintained at180 ◦C for 2 days, after which a grey precipitate was formed.On cooling the sample at room temperature, the precipitatewas separated by centrifuging at a rotation rate of 9000 rpm.It was then washed with distilled water and absolute ethanolin sequence, and dried in air at room temperature.

Powder x-ray diffraction (XRD) was carried out on aPhilip X’pert x-ray diffractometer with Cu Kα radiation (λ =1.5418 Å). X-ray photoelectron spectra (XPS) were recordedon an ESCALAB MK II x-ray photoelectron spectrometer,using nonmonochromatized Mg Kα x-ray as the excitationsource and choosing C 1s (284.6 eV) as the reference line. TheTEM images and SAED (selected area electron diffraction)images were recorded on a JEOL JEM-200CX transmissionelectron microscope, using an accelerating voltage of 200 kV.SEM images were taken by a LEO-1530VP scanning electronmicroscope. The HRTEM images were recorded on a PhilipsTecanai F20 transmission electron microscope, using anaccelerating voltage of 400 kV. Thermal gravity analysis wasperformed on a Pyris 1 TGA from 30 to 700 ◦C at 10 ◦C min−1.

3. Results and discussion

XRD measurement was carried out to determine the crystallinephase of the as-prepared powder. The XRD pattern of the as-synthesized Ag2V4O11 nanobelts is shown in figure 1. All

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Figure 2. XPS spectra of the as-prepared Ag2V4O11 nanobelts.(a) Wide scan spectrum. (b) High-resolution spectrum for O (1s)and V (2p). (c) High-resolution spectrum for Ag (3d).

diffraction peaks can be perfectly indexed to the monoclinicsystem with the lattice constants a = 15.3, b = 3.6, c = 7.6 Å,and β = 102◦ , which are in good agreement with the literaturevalues [27]. No peaks of any other phases were detected,indicating the high purity of the product.

XPS measurements provide further information for theevaluation of the composition and purity of the product. Thewide-scan XPS spectrum of the product is shown in figure 2(a).The C 1s binding energy in the XPS spectrum is located at285.4 eV, and it was standardized using C 1s as reference at284.6 eV. All other peaks were calibrated accordingly. Noobvious impurities could be detected. The two strong peaks atthe Ag region of 368.2 and 374.2 eV are respectively assignedto Ag 3d (5/2) and Ag 3d (3/2), whereas the two peaks located

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400nm 500nm

200nm 5 nm

Figure 3. Morphologies of as-prepared Ag2V4O11 nanobelts. (a) A typical SEM image; (b) a typical TEM image; (c) TEM image andSAED pattern of a single nanobelt, (arrows represent small Ag particles on the surface of as-prepared Ag2V4O11 nanobelts); (d) HRTEMimage of a Ag2V4O11 nanobelt taken from the highlighted area.

at 517.2 and 524.2 eV correspond to V 2p (3/2) and V 2p(1/2), respectively. The other peak located at 529.9 eV isattributed to O 1s. The atomic ratio of the Ag:V:O in Ag2V4O11

calculated from the peak area is approximately 1:2:6, whichis consistent with the given formula for Ag2V4O11 withinexperimental errors.

The size and morphology of the product were examinedby scanning electron microscopy (SEM) and transmissionelectron microscopy (TEM). It can be observed in figure 3(a)that the product represents a belt-like morphology withthickness of 10–30 nm and a rectangular cross-section. Arepresentative TEM image of the as-prepared Ag2V4O11

nanobelts (figure 3(b)) reveals that this sample is composed ofa large quantity of nanobelts with widths of 70–200 nm, andlengths of 2–5 µm. The TEM image (figure 3(c)) indicatesthe presence of a few Ag particles on the surfaces of thenanobelt, which is due to the loss of silver during the samplepreparation [27]. The SAED pattern (inset in figure 3(c))recorded on an individual nanobelt indicates that as-preparednanobelts exhibit a single-crystalline structure with a preferredgrowth oriented along the [302̄] crystalline plane. Figure 3(d)is a representative HRTEM image of a single-crystallinenanobelt, which exhibits good crystalline structure and clearlattice fringes. The interplanar spacing is measured to be about0.59 nm, which is in good agreement with the [201̄] crystallineplane of the monoclinic system of Ag2V4O11. These latticefringes are parallel to the belt axis of Ag2V4O11 nanobelts.

The influence of reaction conditions, such as temperature,reaction time, and organic template, on the morphologyhas been investigated. A series of experiments showedthat the lower temperature resulted in impurity phase dueto incomplete reaction, and shorter reaction times led to

poor crystallization. From our findings, pure Ag2V4O11

nanobelts could not have been obtained on addition of 1,6-hexanediamine or dodecylamine. The product obtained withdifferent amine molecule showed the same morphology andpure phase, because the amine reacted with Ag+ forming acomplex compound, and the remaining Ag+ and V2O5 can bedissolved by amines. Hence, the amine molecules play animportant role in the formation of pure Ag2V4O11 nanobelts.

The formation of nanobelts can be expressed by thefollowing equation:

2AgNO3 + 2V2O5 + 2OH− → Ag2V4O11 + 2NO−3 .

As the temperature increases, vanadium pentoxide isdissolved and hydrolysed under hydrothermal conditions toform various species such as VO−

3 , VO3−4 , and H2VO−

4 .When the reaction proceeds to a certain extent, these speciesand organic template co-condense and polymerize to forma distorted VO6 octahedron, further linking up throughsharing the edges or the corners to form a V4O2−

11 frameworkstructure [28, 29]. The silver ions can accommodate theseframework tunnels.

The thermal stability of as-prepared Ag2V4O11 nanobeltswas studied by thermal gravity analysis in N2 flowingatmosphere (including 5% oxygen). The thermal gravity curve(figure 4) shows only one minor lost weight between 19 and486 ◦C, which corresponds to the decomposition of residualorganic templates and the loss of water between vanadiumoxide layers respectively. With the increase in temperature,there was an increase in mass, which is related to the oxidationof vanadium in lower oxidations by oxygen [28].

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Figure 4. TGA curve of the Ag2V4O11 nanobelts.

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Figure 5. Current–voltage curves of the Ag2V4O11 nanobelts. Theinset is the corresponding FN plot.

Since the Ag2V4O11 nanobelts are vertically aligned withvery rough edges (figure 3(a)), they are potential candidates forfield-emission studies. Field-emission measurements of thenanobelts were conducted in a vacuum chamber at a pressureof 8.5×10−5 Pa. A stainless probe was used as the anode, andscreen-printed nanobelts with 1.76 cm2 cross section servedas the cathode. The distance between cathode and anode was0.1 mm. A DC voltage sweeping from 0 to 3000 V wasapplied in the measurement. Figure 5 is a typical plot of thefield-emission current density versus the applied electric fieldof Ag2V4O11 nanobelts. The Ag2V4O11 nanobelts exhibiteda turn-on field of 28.9 V µm−1, which is defined to bethe macroscopic field required to produce a current densityof 10 µA cm−2. The maximum current obtained with ournanobelts sample is 29.55 µA cm−2, which is lower thanthat of the carbon nanotube films reported recently [30],but is higher than that of NbS2 nanowires [31]. Therefore,Ag2V4O11 nanobelts can serve as a novel candidate for a futurefield emitter. The inset in figure 5 shows the correspondingFowler–Nordheim (FN) plot of the field-emission properties ofAg2V4O11 nanobelts. The FN plot exhibits two linear regionsdue to the space charge effect [32]. If they are fabricated into ananowire array on the conducting substrate, the field-emissionproperty is probably improved. This work is under way inour laboratory.

The magnetization of the nanobelts was measured witha superconducting quantum interference device (SQUID)

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30

M(1

0-3em

u/g)

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Figure 6. Magnetization curves of the Ag2V4O11 nanobelts.

magnetometer. Figure 6 shows the magnetization curves as afunction of temperature between 2 and 300 K in a 2000 Oe field,which indicates that Ag2V4O11 nanobelts are paramagneticmaterials. This paramagnetism is attributed to the oxygendeficiencies [28].

4. Conclusion

In summary, a monoclinic single-crystalline Ag2V4O11

nanobelt has been successfully synthesized by a hydrothermalmethod using V2O5 powders with 1,6-hexanediamine ordodecylamine. The magnetic measurement data show thatAg2V4O11 nanobelts are paramagnetic materials, and as-prepared nanobelts show an electronic emission turn-on fieldof 28.9 V µm−1 at a current density of 10 µA cm−2.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (No. 20325516). The authorswould like to thank Y M Hu and Z Hu of Nanjing Universityfor help with the field-emission measurements.

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