Vacuum 82 (2008) 8

ro

.

Tec

iver

27

gste

Aft

ers

(SEM), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and optical transmission/reection measurements were

unique properties such as the successive phase transitionover the range of temperatures, high electrical conductivity

n-type semiconducting for xo0.25 but metallic for

to the synthesis and characterization of a class of quasi-one-dimensional (Q1D) structures. Nanowires, nanorods,

have been produced by employing various strategies [5,6].Among these various classes of materials, tungsten oxideshave been applied to photocatalysis, photochromic and

ARTICLE IN PRESS

Corresponding author at: Department of Physics, Sharif University of

electrochromic devices, as well as gas sensors for manyyears. For improvement in the device performance, thenanostructured tungsten oxides may also potentially be

0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.vacuum.2007.11.012

Technology, P.O. Box 11155-9161, Tehran, Iran. Tel.: +98 21 6616 4516;

fax: +9821 6607 2636.

E-mail address: moshfegh@sharif.edu (A.Z. Moshfegh).and some interesting magnetic properties [1,2]. Tungstenbronzes have found technological applications in electro-chromic devices, humidity sensors, solid fuel cells, second-ary batteries, ion sensitive electrodes, etc. Among differentknown tungsten bronzes, the sodium tungsten bronzeshave been of great interest ever since their discovery byWohler [3], due to variation of their electronic and opticalproperties with composition. It is known that NaxWO3 are

nanowhiskers and nanobelts constitute an importantclass of Q1D nanostructures, which provide models tostudy the relationship between electrical transport, opticaland other properties with dimensionality and size conne-ment. In recent years, a variety of inorganic materialsnanowhiskers has been synthesized and characterized asactive components in future devices. In this regard,nanowhiskers of elements, oxides, nitrides, and carbidesnanowhiskers were dependent on the annealing time and found that the 80-min heat treatment was a proper time for the growth of

sodium-doped tungsten oxide nanowhiskers, in our experimental conditions. According to SEM observations, the synthesized

nanowhiskers have 70300 nm in width and 110 mm in length. It was also observed that by increasing the heating time to 180minresulted in diffusion of the nanowhiskers into the substrate.

r 2007 Elsevier Ltd. All rights reserved.

PACS: 81.07.b; 81.16.Hc

Keywords: NaxWO3; Nanowhiskers; SEM; XRD

1. Introduction

As a fully oxidized tungsten oxide compounds withdifferent structures, tungsten bronzes (MxWO3) havedrawn considerable attention in recent decades for their

x40.25; meanwhile, colors range from blue to violet tocoppery then to yellow-gold as x changes from 0.4 to 0.98.In addition, the crystalline structure of NaxWO3 is stronglydependent on the x values [4].Recent advances in the eld of nanotechnology have ledemployed to determine various properties of the grown nanowhiskers. Experimental results revealed that size and density ofThe effect of heating time on g

R. Azimirada, M. Goudarzia, OaDepartment of Physics, Sharif University of

bInstitute for Nanoscience and Nanotechnology, Sharif Un

Received 8 July 2007; received in revised form

Abstract

A simple method for synthesis of NaxWO3 nanowhiskers on tun

as a source of sodium atoms has been reported for the rst time.

different times (15, 80 and 180min), crystalline NaxWO3 nanowhisk21826

wth of NaxWO3 nanowhiskers

Akhavana, A.Z. Moshfegha,b,

hnology, P.O. Box 11155-9161, Tehran, Iran

sity of Technology, P.O. Box 14588-89694, Tehran, Iran

November 2007; accepted 27 November 2007

n thin lms with 40 nm thickness sputtered on soda lime substrate

er heat treatment of the W thin lms at 650 1C in N2 ambient forwith [0 0 1] direction were obtained. scanning electron microscopy

www.elsevier.com/locate/vacuum

substrate in up position was 40mm. Some details of the

ARTICLE IN PRESScuusputtering system arrangement were schematically reportedelsewhere [17]. The reaction of nanowhisker growth wascarried out in a furnace with horizontal quartz tube at650 1C for different heating times namely: 15, 80 and180min. The deposited samples were placed on an aluminaboat located in the center of the furnace. After heating thesamples in N2 (99.999%) environment with a constant owrate of 400 standard cubic centimeters per minute (sccm)for different times, the furnace was cooled down to roomtemperature rapidly.Surface morphological characteristics of the lms were

observed by SEM. Phase formation and crystallographicorientation of the samples were investigated by XRD usingCu Ka radiation source. XPS with Al Ka anode and X-rayincident energy of 1486.6 eV was employed to characterizethe atomic composition and chemical state of the samplesat the surface. All binding energy values were determinedby calibrating the C(1s) core level to 285.0 eV as a referencepoint. Optical transmission and reection measurements ofthe prepared samples were performed in a range ofused for these technological applications [79]. Recently,different methods for growing Q1D nanostructure oftungsten oxide doped with potassium [1012], lead [13],molybdenum [14] and boron [15] were reported. More-over, Cao et al. [16] recently published a hydro-thermal synthesis of sodium tungstate nanorods andnanobundles. To the best of our knowledge, there was noreport about growing of Q1D nanostructure of sodium-doped tungsten oxide with vaporliquidsolid (VLS)method, yet.In this paper, for the rst time, we have studied the

effects of heating time on growth of sodium-dopedtungsten oxide nanowhiskers from sputtered W thin lmsby using the sodium existing in soda lime substrates. Thegrown samples containing sodium-doped tungsten oxidenanostructures were characterized and analyzed by scan-ning electron microscopy (SEM), X-ray diffractometry(XRD), X-ray photoelectron spectroscopy (XPS) andUVvis spectrophotometry.

2. Experimental

Initially, thin lms of tungsten were deposited on sodalime substrates by using DC magnetron sputteringtechnique. The base pressure and Ar (99.999%) sputteringpressure were 2 106 and 5 103 Torr, respectively.Before the deposition process, a pre-sputtering wasperformed for about 2min. The discharge power to growW thin lms was about 22W which resulted in a depositionrate of 8.5 nm/min. Thickness of the deposited lms waskept about 40 nm monitored in situ by a quartz crystaloscillator and measured by an optical technique. Thedistance between the target in down position and the

R. Azimirad et al. / Va8223001100 nm wavelength using an UVvisible spectro-photometer with resolution of 1 nm.3. Results and discussion

The SEM images of the W lms after the thermalannealing at 650 1C for different times (15, 80 and 180min)are shown in Fig. 1 as compared to the as-deposited lm.As can be seen in Fig. 1(a), the as-deposited W thin lmhas a smooth surface without any nanostructures. Afterannealing the W lm for 15min, low-density nanowhiskerswith dimensions of 70 nm in width and 1 mm in lengthgrew horizontally on the surface (Fig. 1(b)). As illustratedin Fig. 1(c), density of the grown whiskers in differentorientations as well as their width and length increasedafter heat treatment for 80min. Meantime, as shown inFig. 1(d), there are still the nanowhiskers with dimensionssimilar to those that were observed for the 15-minannealing time. However, by further increasing the heatingtime to 180min (Fig. 1(e)), these characteristics were notmainly changed and only the whiskers diffused intosubstrate due to its softening (softening point of soda limesubstrate is about 726 1C).To determine the surface chemical composition of

the synthesized nanostructures, XPS technique was used.Fig. 2(a) shows XPS survey scans of the samples annealedat 650 1C for different times as compared to the spectrumof the as-deposited W lm (0min). It can be seen fromthe gure that only tungsten and oxygen peaks with a traceamount of surface physisorbed carbon and nitrogen arepresent on the surface. No other impurities exist on thesurface for the as-deposited lm. After the annealingprocess for 15min, nitrogen peak, N(1s), disappeared(because WO3 is not a nitrogen getter as compared to Wmetal) while sodium peak, Na(1s), appeared. The latterpeak is related to sodium existing in the substrate whichout-diffused to the surface after annealing the sample. Inaddition, after the heat treatment, silicon peaks from thesubstrate was also observed in the XPS spectra. The molarconcentration of O, W, Na and Si for the samples annealedfor the different times was measured from O(1s), W(4f),Na(1s) and Si(2p) XPS core level peaks and that they havebeen presented in Fig. 2(b). It is seen that by increasing theheat treatment time, the molar concentration of sodiumand silicon increased on the surface which was related tosoftening soda lime substrate and diffusion of theseelements onto surface.Fig. 2(c) shows W(4f) core level of XPS spectrum of the

thin lm after the annealing process for different times inthe binding energy range of 4130 eV. It is well establishedthat the binding energy of W(4f7/2) core level for metallicW0, W4+, W5+ and W6+ states are 31.4, 32.7, 34.1 and35.8 eV, respectively. In this range, there are also W(4f5/2)peaks for all these chemical states with 2.15 eV spinorbitseparation and 0.75 area ratio, relative to the correspond-ing W(4f7/2) peaks [18]. As can be seen from Fig. 2(c), forthe as-deposited thin lm (0min), the tungsten is mainlyin the metallic state, and there are some different oxides

m 82 (2008) 821826states related to formation of native oxide on the surfaceafter the deposition. By increasing the heating time, the

ARTICLE IN PRESScuuR. Azimirad et al. / Vametallic state disappeared and the amount of the surfacetungsten in the W6+ chemical state increased.XRD technique was also performed to analyze and

identify phase formation and crystallographic structure ofthe samples. Fig. 3 illustrates XRD spectra of the samplesafter the heat treatment at 650 1C for different annealingtimes as compared to the as-deposited sample (0min). Inour case, due to presence of (2 1 1) plane, the crystallinestructure of the as-deposited lm is identied as b-Wphase with the equilibrium bcc structure with 0.52 nmlattice parameter. It is important to note that residualstresses in monolithic W lm are sensitive and exhibitthickness dependence, especially for tungsten thickness lessthan 260 nm and in the low lm thicknesses, the b-W phaseis favorable state [19]. In addition, Chen et al. [20] showedthat, based on the experimental conditions, the latticeparameter of b-W varies from 0.496 to 0.518 nm, so thatour obtained value is in agreement with their results.According to the Scherrer equation, the crystalline size ofthe tungsten lm was estimated about 20 nm. After theannealing process and out-diffusion of sodium, theobserved diffraction peaks are attributed to the cubicstructure of NaxWO3 phase. The Na content (x) in thecubic NaxWO3 can be calculated from the experimentallymeasured lattice parameter a0 using the following reported

Fig. 1. SEM images of sodium-doped tungsten oxide nanowhiskers aft

(a) 0 (as-deposited), (b) 15, (c) 80, (d) 80 with a higher magnication and (m 82 (2008) 821826 823relation [21,22]:

a0 A 3:7845 0:0820x. (1)The values of x in the NaxWO3 nanostructures for thesamples annealed at different temperatures, calculatedfrom the above equation, have been listed in Table 1. Itshould be noted that this equation is correct in the range of0.3pxp0.85.As can be seen from Fig. 3, (0 0 1) is the intensive peak

for the annealed samples because [0 0 1] is the major growthorientation for Q1D nanostructures [10,23,]. Moreover,remaining b-W (2 1 1) phase after 15min heat treatmentshows that the lm was partially oxidized and the metaltungsten still exists. But, this peak disappeared after 80minheat treatment indicating the tungsten lm was completelyoxidized under these conditions.The optical transmission and reection spectra of the

as-deposited (0min) and annealed lms for differenttimes in a range of 3001100 nm wavelength have beenshown in Fig. 4. It is seen that, the transmittance andreectance of the as-deposited lm is lower than 8% andaround 30%, respectively. In addition, the transmittancedecreases linearly in visible range and becomes zero at348 nm. These optical properties are in agreement withmetallic property of tungsten thin lms [24,25]. By

er heat treatment in N2 atmosphere at 650 1C for different times:e) 180min.

ARTICLE IN PRESScuu.) W(4

d)

C(1

s)

W(4

p3/2

)

O(1

s)

O(A

uger)

Na(1

s)

Na(A

uger)

180

R. Azimirad et al. / Va824increasing the heating time, the transmittance increasedwhile the reectance decreased. Moreover, the absorptionedge is shifted toward blue and it was determined at 320 nmfor the annealed lms. The increase of transmittance andthe occurrence of the blue-shift for the absorption edge areassociated to surface oxidation of the lms and tungstenoxide formation. It is to note that a pure WO3 thin lm is

15 20 25 30 35 40 45 50

Inte

nsity (

a.u

.)

(001)

(002)

(111)

-W(2

11)

(011)

a)

c)

b)

x20d)

2 (deg.)

Fig. 3. XRD spectra of tungsten thin lms after heat treatment at 650 1Cfor different times: (a) 0 (as-deposited), (b) 15, (c) 80 and (d) 180min.

0

10

20

30

40

50

60

70

80

90

0

Mola

r concentr

ation (

%)

Inte

nsity (

a.u

0 min

15

80

1200 1000 800 600 400

Binding energy (eV)

200

Time (min.)

50 100 150

Fig. 2. (a) XPS survey spectra, (b) molar concentration of O, W, Na and Si at t

after heat treatment at 650 1C for different times.W(4

f)

Si(2p)

Si(2s)

180

W6+

W5+

W4+

W0

m 82 (2008) 821826highly transparent with 470% transmittance [26,27]. Thelow transmittance of the annealed samples even for 180minones is related to presence of sodium [21]. However, thereduction of the reectance is attributed to rise of surfaceroughness after growth of the nanowhiskers.Finally, to understand the role of sodium in growth of

tungsten oxide nanowhiskers, a sample was prepared underthe similar growth conditions and parameters, but on adifferent substrate. SiO2/Si(1 0 0) system (the oxidized Silayer was prepared in O2 ambient at 900 1C for 120min)was used as a substrate instead of the soda lime for growingtungsten layer by using the same sputtering conditions.After the annealing process for 80min, only nanoparticlesand microparticles were observed by using SEM, without

O

W

Na

Si

0

0 min

15

80

40 37 34 31

Binding Energy (eV)200

he surface and (c) W(4f) core level XPS spectrum of the tungsten thin lms

Table 1

XRD peak position, lattice parameter and Na content in the NaxWO3(0 0 1) phase synthesized at 650 1C for different heating times

Annealing time (min) 2y (1) a0 (A) x

15 23.177 3.8350 0.61

80 23.042 3.8572 0.89

180 23.179 3.8347 0.61

ARTICLE IN PRESScuu0

5

10

15

20

25

300 1100

Tra

nsm

itta

nce (

%)

0

5

10

15

20

25

30

35

40

400 800 1000

Re

fle

cta

nce

(%

)

0 min

15

80

180

0 min

15

80180

500 700 900

Wavelength (nm)

600

Fig. 4. (a) Optical transmittance and (b) optical reectance of the tungsten

R. Azimirad et al. / Vaany nanowhiskers on the surface for this sample. Inaddition, XPS survey spectrum of this sample determinedno sodium peaks on the surface. Therefore, it can beconcluded that diffusion of sodium from soda limesubstrate toward the surface of the lm and so its presenceon the surface plays a key role for the growth of tungstenoxide with Q1D nanostructures. This is similar to the roleof potassium in formation of Q1D nanostructure oftungsten oxide as reported recently [1012].It is known that melting point of pure WO3 is 1473 1C.

Thus, for the growth of Q1D nanostructure of WO3 byvaporsolid (VS) method in atmospheric pressure a hightemperature (T41000 1C) is usually needed. But, additionof sodium as an impurity to tungsten oxide decreases themelting point to a lower value of 698 1C for Na2WO4. Wepropose a growing procedure based on the VLS mechanismthat agrees well with our experimental observations. TheVLS crystal growth mechanism was proposed by Wagnerand Ellis in 1964 for silicon whisker growth [28] and hasbeen widely used to guide the growth of various kinds ofone-dimensional nanostructures [29]. In our system, thesource of W is clearly the W lm, and the source of Na isfrom the soda-lime substrate. It is reasonable to expect thatat the growth temperature, tiny droplets of low meltingpoint liquid containing Na, W and O can be produced fromthe diffusion of sodium on surface of W metal andoxidation from residual O2 in the ambient. These tinydroplets act as the seeds or templates for nanowhisker

thin lms after heat treatment at 650 1C for different times.growth. If more WO3 were dissolved in the droplet to reachthe supersaturating state, solid NaxWO3 would precipitatefrom the droplet in the form of nanowhiskers. Continuousfeeding of WO3 and Na into the liquid droplet sustains thegrowth of the nanowhiskers. Finally, when the temperatureof the system is slowly lowered to room temperature, thegrowth process was stopped. Recently, we have shown theheat treatment of the W thin lm at 750 1C for 15minresulted in growth of sodium-tungsten oxide nanobeltswith U-shape cross section on the surface [30].

4. Conclusions

In conclusion, we have reported here a simple and novelmethod to synthesize horizontal sodium-doped tungstenoxide nanowhiskers on soda lime substrates at a relativelylow temperature (650 1C). This method is much simplerthan other methods reported before and can be easilyscaled up to prepare a larger amount of the material.Annealing process for 80min was determined as the propertime for growing NaxWO3 nanowhiskers on the surface.Furthermore, it is determined that sodium from the sodalime substrate diffuses toward the surface of the annealedsamples and this diffusion plays an important role ingrowth and formation of the nanowhiskers.

Acknowledgments

The authors wish to thank Research Council ofSharif University of Technology for nancial support ofthe project. The assistance of Dr. M. Fathipour andMr. Sohrabi for SEM imaging is greatly acknowledged.

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ARTICLE IN PRESSR. Azimirad et al. / Vacuum 82 (2008) 821826826

The effect of heating time on growth of NaxWO3 nanowhiskersIntroductionExperimentalResults and discussionConclusionsAcknowledgmentsReferences