Communication

DOI: 10.1002/cvde.201104307

Chemical Vapor Synthesis and Physico-chemicalProperties of V2O5 Nanoparticles**

By Hoang Anh Le, Sungmin Chin, Eunseuk Park,

Gwinam Bae, and Jongsoo Jurng*

transition metal oxides are of great interest. In particular,

vanadium oxide-based materials, including vanadium pent-

oxide (V O ) and vanadium phosphates, are catalysts for the

In the field of heterogeneous catalysts, nanometer-size

2 5

mild oxidation of hydrocarbons and alcohols.[1] To date,

V2O5 has attracted increasing interest for its potential

applications as catalysts, sensors, and electrodes. V2O5

powders can be obtained by various physical and chemical

methods, such as CVD,[2–4] hydrothermal method,[1,3,5]

spray pyrolysis,[6] sol-gel,[7–9] co-precipitation,[10] and

microwave plasma-torch method,[11] etc. Chemical vapor

synthesis, also known as CVD or chemical vapor condensa-

tion (CVC),[12–14] is an alternative method for the direct

synthesis of nanoparticles. The principle advantages of the

reactions in the gas phase are, very short process times, and

nanometer-scale powders of high purity with a narrow

particle size distribution. The particle morphology, crystal-

line phase, and surface chemistry of thermally decomposed

particles can be controlled by regulating the precursor

composition, reaction temperature, pressure, solvent prop-

erty, and aging time.[15,16] In a typical CVC process, the

precursor solution is atomized into an aerosol reactor

(electric furnace) where droplets undergo solvent evapora-

tion and solute precipitation within the droplets, which are

then dried. In a typical thermal decomposition process, the

precursor solution was atomized into an aerosol reactor,

where droplets underwent solvent evaporation and solute

precipitation within the droplets, which were then dried,

followed by thermolysis of the precipitates at higher

temperatures, and finally sintering to form the final

particles.[15]

In the CVC process, the synthesis temperature is one of

the most important factors for determining the particle

[*] Dr. J.-S. Jurng, H. A. Le, Dr. S.-M. Chin, E.-S. Park, Dr. G.-N. BaeKorea Institute of Science and Technology (KIST)39-1 Hawolgok, Seongbuk, Seoul 136-791 (Republic of Korea)E-mail: jongsoo@kist.re.kr

H. A. LeUniversity of Science and Technology (UST)176 Gajung-dong, Yuseong-gu, Daejeon (Republic of Korea)

E.-S. ParkUniversity of Seoul, 90 Jeonnong-dongDongdaemun-gu, Seoul 130-743 (Republic of Korea)

[**] This work has been supported by theMinistry of Environment (192-091-001), the Ministry of Education, Science and Technology(2011K000750).

6 wileyonlinelibrary.com � 2012 WILEY-VCH Verlag Gmb

morphology, such as the crystal phase, size, specific surface

area, surface composition, and chemical state.[15,17] In this

study, V2O5 nanoparticles were obtained by the thermal

decomposition of a vanadium oxytriethoxide (VOTE)

solution. The detailed microstructural characteristics of

the thermally decomposed nanoparticles were examined as

a function of the synthesis temperatures (500–1300 8C).Figure 1a shows X-ray diffraction (XRD) patterns of the

V2O5 nanoparticles prepared at each synthesis temperature.

The XRD patterns of the V2O5 samples synthesized at 900,

1100, and 1300 8C showed similar patterns with different

peak intensities, and indicated a single-Shcherbinaite phase

with an orthorhombic structure (JCPDS Card No. 41-1426)

and lattice parameters of a¼ 1.1516 nm, b¼ 0.332 nm, and

c¼ 0.43727 nm.[8,10] In addition, the peak intensities of the

V2O5 samples increased with increasing synthesis tempera-

ture to 900 and 1300 8C, indicating enhanced crystallization.

The peaks (001) intensity became higher and the peak shape

narrower, indicating an increase in V2O5 crystallite or grain

size, which was calculated from the line broadening of (001)

diffraction peak using the Debye-Scherrer formula

(r¼ 3.4 g cm�3) as listed in Table 1. No peaks of other

phases were observed in the XRD pattern, suggesting that

theV2O5 nanoparticles prepared byCVC contained a single,

highly crystalline phase without impurities. On the other

hand, no detectable peaks were observed at synthesis

temperatures of 500 and 700 8C, indicating the presence of

an amorphous structure.

N2 adsorption/desorption isothermal tests were carried

out using the Brunauer-Emmett-Teller (BET) and Barrett-

Joyner-Halenda (BJH) methods to estimate the pore size

distribution and adsorption properties of the prepared

samples, the isotherm curves of which are presented in

Figure 1b. The isotherm of the prepared samples showed

typical Type IV (BDDT classification) curves, and their

narrow hysteresis loops exhibited a typical Type H3

pattern,[8] indicating the presence of mesopores (2–50 nm).

In thermal decomposition synthesis of the precursor

solution, the particle morphology depends strongly on the

synthesis conditions, such as the synthesis temperature,

precursor concentration, and precursor aerosol residence

time in the heating zone. In particular, the synthesis

temperature is an important factor determining the resulting

particle size, crystallinity, and phase.[15] Figures 2 a to e

present transmission electron microscopy (TEM) images of

the V2O5 samples produced by CVC at various synthesis

temperatures. Figures 2a and b indicate that the samples

prepared at 500 and 700 8C have an amorphous structure.

Meanwhile, the particle structure was nanorods,[18] ribbon-

like shape,[9] or strip-like shape[19] at higher temperatures

(900–1300 8C). The particle size, dBET in Table 1, gradually

increased with increasing synthesis temperature from 900 to

H & Co. KGaA, Weinheim Chem. Vap. Deposition 2012, 18, 6–9

Communication

Fig. 1. a) XRD patterns, and b) hysteresis loop, of V2O5 nanoparticles synthesized at various synthesis temperatures.

Table 1. Physico-chemical properties of the prepared samples at various temperatures. The values in parentheses are the area ratio (%).

Synthesis

temperature

[8C]

SBET[m2 g�1]

dBET[a]

[nm]

dXRD[b]

[nm]

Vp

[cm3 g�1]

dpore[nm]

V 2p3/2 O 1s

V1 V2 O1 O2

500 37.1 � � 0.116 12.6 517 (70.9) 516.2 (29.1) 529.9 (70.1) 530.7 (29.9)

700 48.5 � � 0.341 28.1 517 (76.6) 515.8 (23.4) 529.9 (74.6) 530.7 (25.4)

900 46.8 37.7 25.1 0.269 29.6 517 (82.4) 516 (17.6) 529.9 (100) �1100 39.3 44.9 23.7 0.179 18.2 517 (89.4) 516 (10.6) 529.9 (100) �1300 36.3 48.6 33.8 0.223 19.1 517 (83.1) 516.3 (16.9) 529.9 (100) �[a]Particle size by equation using BET surface area.[b]Crystallite size calculated using Debye-Scherrer formula after analysis of XRD patterns using the PowderX program.

1300 8C. In addition, the resulting particles had various

colors according to the synthesis temperature, as shown in

the second row of Figure 2. Black powders at the 500 and

700 8C samples could be assigned toV4þ, while dark-green at

900 8C and completely bright yellow-orange at 1100 and

Fig. 2. PSD (top), TEM images (middle) and photographs (bottom) of the prepare

Chem. Vap. Deposition 2012, 18, 6–9 � 2012 WILEY-VCH Verlag Gm

1300 8C (which is the color of pure V2O5,[6,20,21]) correspond

to V5þ. Figure 2 (top row) also shows the particle size

distribution (PSD) of the samples synthesized at 900, 1100,

and 1300 8C with modal sizes at 15, 60, and 30 nm,

respectively. There are significant gaps in the particles sizes

d samples at various synthesis temperatures.

bH & Co. KGaA, Weinheim www.cvd-journal.de 7

Communication

that were calculated, based on the XRD and BET results, at

synthesis temperatures of 900 to 1300 8C, showing differ-

ences (dXRD/dBET) of a factor of 0.53 to 0.7. The possible

reason for this difference is polydispersity since dBET is a

surface-weighted particle property, whereas crystallite size

(dXRD) is a mass-weighted particle property. The TEM

images in Figure 2 show that the morphology of the samples

is not uniform, which means that it is difficult to estimate the

diameter of this material by one value.

Figure 3 shows the X-ray photoelectron spectroscopy

(XPS) spectra of theO 1s and V 2p3/2 regions. Generally, the

photoelectron peaks of O 1s could be resolved into two

components. The dominant peak at about 529.9 eV was

characteristic of metallic oxides, which was in agreement

with O 1s electron binding energy (BE) arising from

vanadium.[22] The O 1s excitation spectrum of the vanadyl

oxygen is characterized by a large peak centered at 530.7 eV

which could be assigned to V-O(2)-V bridging oxygen.[23]

Values for the V 2p3/2 BE for the various oxidation states of

Fig. 3. XPS spectra of theO 1s andV 2p3/2 regions for the prepared samples at

various synthesis temperatures.

8 www.cvd-journal.de � 2012 WILEY-VCH Verlag GmbH

vanadium are presented in Table 1. The main peak position,

517.0 eV, is lower than the literature datum (517.2 eV),[3]

corresponding to V2O5 stoichiometry (V5þ), however all

samples also have a ‘‘shoulder’’ peak in the V 2p3/2 at 515.8–

516.3 eV which could also be assigned to V4þ ions.[24] It can

be concluded that the oxidation of the vanadium on the

surface of nanorods is between þ4 and þ5. The mixed

valence, V4þ/5þ, was said to be crucial to the higher electric

conductivity.[25] Thus, the XPS results are in good agree-

ment with the XRD results.

V2O5 nanoparticles were successfully synthesized using

VOTE aerosol as the vanadium precursor, by thermal

decomposition in an electric furnace without additional

calcination. At lower temperatures (500–700 8C), the

obtained product had an amorphous structure. At higher

temperatures (900–1300 8C), single, highly crystalline V2O5

nanoparticles, without impurities, were synthesized. The

particle size and crystallinity increased with increasing

synthesis temperature. In addition, the colors of the

resulting particles differed according to the synthesis

temperatures (black, dark-green, and bright yellow-orange).

Experimental

Synthesis of Vanadium: In this study, the V2O5 powders were preparedby CVC. Detailed CVC process and preparation of V2O5 nanoparticles byCVC are described elsewhere [1,17]. The VOTE (VO(OC2H5)3, Aldrich,>97%) solution was used as the V2O5 precursor and was stored in a bubblerthat was placed in an oil bath at 95 -C. The bubbler was wrapped with heatingtape (90 -C) to prevent any loss due to condensation. The argon (the precursorcarrier gas) and air (the diluent gas) flow rates were fixed at 0.7 and7.0 Lmin�1, respectively. An alumina tube (0.8m in length and 0.01m indiameter) was placed in an electric furnace controlled by a temperaturecontroller. The precursor aerosol residence time in the heating zone wasapproximately 0.47 s. The VOTE vapor concentration was calculated as6.74T 10�5mol L�1 by assuming that the carrier gas through the bubbler wascompletely saturated. The synthesis temperature was varied from 500 to1300 -C at 200 -C intervals.

Characterization: The crystal phases of the prepared samples wereexamined by XRD (Rigaku D/Max 2500) using Cu Ka radiation. The XRDpattern and data were also analyzed using the PowderX program. TEManalysis was carried out using a CM-30 microscope (Philips; operated at300 kV, image resolution <0.23 nm). The TEM images were analyzed usingflexible plugin architecture of the public domain image processing programImageJ to determine precisely the primary particle size (dTEM) and PSD. Thepowder specific surface area (SSA, m2 g�1) was determined by nitrogenadsorption (>99.999%) at 77K on a Micromeritics Tristar 3000 apparatususing the BET method. The pore volume distribution was determined fromthe desorption isotherms (Micromeritics ASAP 2010 Multigas system) usingthe BJH method. Assuming monodispersity and spherical primary particles,the BET-equivalent particle diameter (dBET) was calculated using theformula, dBET¼ 6/(rT SSA), where r is the particle density. XPS (VGscientific ESCA Lab II) with a resolution of 0.1 eV was performed with MgKa (1253.6 eV) radiation as the excitation source. All BEs were referenced tothe C 1s peak at 285.0 eV for adventitious carbon. After subtracting theShirley-type background, the core-level spectra were decomposed into theircomponents with mixed Gaussian-Lorentzian lines using a non-linear, leastsquares, curve-fitting procedure on the public software package XPSPEAKv.4.1.

Received: August 1, 2011

Revised: October 26, 2011

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