Simple Method to Synthesize NaxWO3 Nanorods and Nanobelts

R. Azimirad,†,‡ O. Akhavan,† and A. Z. Moshfegh*,†,§

Department of Physics, Sharif UniVersity of Technology, P.O. Box 11155-9161, Tehran, Iran, Institute ofPhysics, Malek-Ashtar UniVersity of Technology, Tehran, Iran, and Institute for Nanoscience andNanotechnology, Sharif UniVersity of Technology, P.O. Box 14588-89694, Tehran, Iran

ReceiVed: March 11, 2009; ReVised Manuscript ReceiVed: May 27, 2009

A simple method for synthesis of NaxWO3 nanorods and nanobelts on sputtered tungsten films by usingsodium in soda lime substrate as the catalyst was reported for the first time. After thermally post annealingthin films in a temperature range of 600-750 °C in N2 ambient for 80 min, crystalline NaxWO3 nanorods andnanobelts with [001] direction were formed depending on the annealing temperature. Experimental resultsreveal that the annealing temperature at 700 °C is the optimum temperature for the growth of sodium-dopedtungsten oxide nanorods with maximum density on the surface. According to scanning electron microscopicobservations, the synthesized nanorods are ∼50 nm in width and a few micrometers in length at the optimumtemperature. It was also observed that increasing annealing temperature facilitates the growth of the NaxWO3

nanobelts. A solid-liquid-solid mechanism was proposed for describing the growth process of the sodium-doped tungsten oxide nanorods and nanobelts.

1. Introduction

Tungsten bronzes are a group of nonstoichiometric com-pounds with the general formula of MxWO3, where M is a metalelement and x is in the range of 0 < x < 1. Tungsten bronzeshave drawn considerable attention in recent decades for theirunique properties such as the successive phase transition overa range of temperatures, high electrical conductivity, and someinteresting magnetic properties.1,2 Tungsten bronzes have beenapplied in many technological applications such as electrochro-mic devices, humidity sensors, solid fuel cells, secondarybatteries, ion-sensitive electrodes, etc. Among all kinds oftungsten bronzes known, the sodium tungsten bronzes are themost studied ones ever since their discovery in 1823.3 It wasfound their physical properties and structures of the NaxWO3

are strongly dependent on their compositions. It is known thatNaxWO3 is n-type semiconducting for x < 0.25 but metallic forx > 0.25; meanwhile, their colors range from blue to violet tocoppery then to yellow-gold as x changes from 0.4 to 0.98. Inaddition, the crystalline structure of NaxWO3 is closely relatedto x values.4

There are various literatures that reported different synthetictechniques to prepare sodium tungsten bronzes with powder orthin film structure.5-7 Recently, we have introduced a newmethod for growth of NaxWO3 nanowhiskers8 and nanobeltswith a U-shaped cross section9 from sputtered W thin films byusing the existing sodium, as a catalyst, in soda lime substratesafter the heat treatment at 650 °C for different times (15, 80,and 180 min) and at 750 °C for 15 min, respectively. But, thesurface density of the NaxWO3 nanostructures (number of thenanostructures formed per unit area) grown under these experi-mental conditions was low.

In this work to determine the optimum annealing temperaturefor growth of NaxWO3 nanostructures with high density on thefilm surface, we have selected the optimum growth time (80min)8 and studied the effect of annealing temperature in a rangeof 550-750 °C on the growth process of the nanostructures.The synthesized samples containing the sodium-doped tungstenoxide nanostructures were characterized and analyzed byscanning electron microscopy (SEM), X-ray diffractometry(XRD), X-ray photoelectron spectroscopy (XPS), high-resolutiontransmission electron microscopy (HRTEM), and ultraviolet(UV)-visible spectrophotometry. In addition, a more detaileddescription for the growth process of the nanostructures has beenpresented based on the solid-liquid-solid (SLS) mechanism.

2. Experimental Section

Initially, thin films of tungsten were deposited on cleanedsoda lime substrates by using DC magnetron sputtering tech-nique. The base pressure and Ar sputtering pressure were 2.5× 10-6 and 5 × 10-3 Torr, respectively. Before the depositionprocess, a presputtering was performed for about 2 min to cleanthe target surface. The discharge power to grow W thin filmswas considered about 22 W that resulted in a deposition rate of∼8.5 nm/min. Thickness of the deposited films was consideredto be ∼40 nm monitored in situ by a quartz crystal oscillatorand measured by an optical technique. The distance betweenthe target in down and the substrate in up was 40 mm. Someother schematic arrangement of the sputtering system werereported elsewhere.10

The reaction for the growth of nanorods was carried out in ahorizontal quartz tube furnace at various temperatures rangingfrom 550 to 750 °C. For this step, the deposited samples wereplaced on an alumina boat located in the furnace. After heatingthe samples in N2 environment with a constant flow rate of 400standard cubic centimeters per minute (sccm) for 80 min (theoptimum time for growth of NaxWO3 1D nanostructure as hasbeen recently reported8), then the furnace was cooled down toroom temperature rapidly.

* To whom correspondence should be addressed. E-mail: moshfegh@sharif.edu. Tel: +98-21-6616-4516. Fax: +98-21-6601-2983.

† Department of Physics, Sharif University of Technology.‡ Malek-Ashtar University of Technology.§ Institute for Nanoscience and Nanotechnology, Sharif University of

Technology.

J. Phys. Chem. C 2009, 113, 13098–1310213098

10.1021/jp902189h CCC: $40.75 2009 American Chemical SocietyPublished on Web 07/01/2009

Surface morphological characteristics of the films wereobserved by SEM. XRD with a Cu KR radiation source wasused to determine phase formation and crystallographic orienta-tion of the samples. XPS with Al KR anode with X-ray incidentenergy of 1486.6 eV was employed to study the surface atomiccomposition and chemical state of the samples. All bindingenergy values were determined by calibrating the C(1s) corelevel at 285.0 eV. TEM operated at 200 kV was used for detailobservation of nanostructures and selected area electron dif-fraction (SAED) investigation was also performed during theTEM observation. Optical transmission and reflection measure-ments of the prepared samples were performed in a range of300-1100 nm wavelength using an UV-visible spectropho-tometer with resolution of 1 nm.

3. Results and Discussion

The SEM images of the W films after the thermal annealingat different temperatures from 600 to 750 °C have been shownin Figure 1. After annealing the W film up to 550 °C, no any1D features were observed on the surface. However, byincreasing the annealing temperature to 600 °C, low densitynanorods with dimensions <100 nm in width and <5 µm inlength grew horizontally on the surface as indicated by whitearrow in Figure 1a. As can be seen from Figure 1b, surfaceconcentration of nanorods, as well as their width and length,increased after heat treatment at 650 °C. The surface density ofnanorods was densified by increasing the annealing temperatureso that nanorods with ∼50 nm in width and a few micrometersin length covered all surface of the film at 700 °C (Figure 1c).Meanwhile, nanobelts several hundred nanometers wide andabout the same thickness and length as nanorods also grew atthis temperature. It is clear from the figure that the 1Dnanostructures grew in random directions on the surface. Afterannealing the film at 750 °C, it seems that nanorods have meltedand only some larger nanobelts and crystallized islands areobserved on the surface (Figure 1d).

X-ray diffraction technique was also performed to analyzeand identify phase formation and crystallographic structure ofthe samples. Figure 2 illustrates XRD spectra of the samplesafter the annealing process at different temperatures as comparedto the “as deposited” one. In our case, due to observation of(110) plane, the crystalline structure of the “as deposited” film

is identified as �-W phase with the equilibrium bcc structureand 0.52 nm in its lattice parameter. It is important to note thatresidual stresses in monolithic W film are sensitive and exhibitthickness dependence, especially for tungsten thickness less than260 nm, and in the low film thicknesses, the �-W phase isfavorable state.11 In addition, Chen et al. showed that, based onthe experimental conditions, the lattice parameter of �-W variesfrom 0.496 to 0.518 nm.12 These reported values are inagreement with our obtained results. After the annealing processand the migration of sodium toward the surface, the cubicstructure of NaxWO3 phase was identified according to analysisof the diffraction peaks. The Na content (x) in the cubic NaxWO3

could be calculated from the experimentally measured latticeparameter a0 using the following reported relation13,14

The values of x in the NaxWO3 nanostructures for the samplesannealed at different temperatures, calculated form eq 1, andthe results are listed in Table 1. It should be noted that theequation is only correct for x values cover in the range of 0.3e x e 0.85. Thus, the computed value of x for the sampleannealed at 750 °C is only estimation.

As can be seen from Figure 2, (001) is the most intensivepeak for the annealed samples because [001] is the major growthorientation for 1D nanostructures.15,16 Moreover, the remaining�-W(110) phase up to 700 °C shows that only surface of thefilm was oxidized and a metal tungsten film existed under thenanostructured sodium-doped tungsten oxides. However, thispeak disappeared after heat treatment at 750 °C, which showscomplete oxidation of the tungsten film at this temperature asalso supported by our XPS analysis (see below).

To determine the surface chemical composition of thesynthesized nanostructures, XPS analysis was also used. Figure3a shows XPS survey scans of the samples annealed at differenttemperatures as compared to the spectrum of the “as deposited”W film. It can be seen from the figure that only tungsten andoxygen peaks with a trace amount of surface physisorbed carbonand nitrogen and no other impurities exist on the surface of the“as deposited” film. After the annealing process at 550 °C,

Figure 1. SEM images of sodium-doped tungsten oxide nanorods andnanobelts after post annealing in N2 atmosphere at different tempera-tures: (a) 600, (b) 650, (c) 700, and (d) 750 °C (all scale bars are 5µm).

Figure 2. XRD spectra of the tungsten thin films after annealed atdifferent temperatures: (a) “as deposited”, (b) 600, (c) 700, and (d)750 °C.

TABLE 1: XRD Peak Position, Lattice Parameter andCalculated Na Content in the NaxWO3(001) PhaseSynthesized at Different Annealing Temperatures

annealing temperature (°C) 2θ (deg) a0 (Å) x

600 23.184 3.8339 0.60650 23.042 3.8572 0.89700 23.32 3.8118 0.33750 22.849 3.8893 1.28

a0 (Å) ) (3.7845 + 0.0820x) (1)

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nitrogen peak, N(1s), disappeared (because WO3 is not anitrogen getter as compared to W metal) while a sodium peak,Na(1s), appeared. The latter peak is related to existing sodiumin the substrate which out diffused into the surface afterannealing of the sample at this temperature. By further increas-ing the annealing temperature, silicon peaks from the substratewas also observed in the XPS spectra. The molar ratio of Na/Wand Si/W for the samples annealed at the different temperatureswas measured from the Na(1s), Si(2p), and W(4f) XPS peaksand have been illustrated in Figure 3b. As can be seen, the Na/Wratio increases by arising annealing temperature as was also seenfor the x value from the XRD analysis (Table 1). However, thisratio is higher than x value at the same temperature. This isbecause some diffused sodium atoms on the surface consumedin the growth process of NaxWO3 structures. It is seen that atannealing temperatures of 650 and 700 °C there is a reductionin the Na/W ratio. This reduction is in agreement with XRDresults (Table 1) that related to covering the film surface by alot of the grown NaxWO3 nanorods and nanobelts at thesetemperatures. Therefore, the diffused sodium atoms have beenconsumed and distributed in the structure of the 1D nanostruc-tures. The sharp increase of sodium and silicon on the surface,after heat treatment at 750 °C, was mainly associated withsoftening of the soda lime substrate at this high temperature, aswas also seen in deformation of the substrate from naked eye.

Figure 3c shows deconvoluted XPS spectrum of the nanorodsand nanobelts grown at 700 °C for the W(4f) core level bindingenergy in the range of 42-30 eV. As indicated in the figure,there are two W(4f7/2) core levels for W6+ and W5+ statescorresponding to 35.5 and 34.1 eV binding energies, respec-tively. In this range, there are also two W(4f5/2) peaks for thetwo states with 2.15 eV spin-orbit separation and 0.75 arearatio, relative to the corresponding W(4f7/2) peaks. As can beseen from Figure 3c, about 30% of the surface tungsten atomsare in W5+ oxidation state and other are in W6+ oxidation statewhich are in good agreement with the XRD result for theNa0.33WO3 sample (Table 1). It should be noted that theagreement between XPS data (surface analysis technique withdefocusing beam) and XRD analysis (diffraction peaks fromthe crystallized 1D nanostructures) is valid only for the sampleannealed at 700 °C because only all surface of this samplecovered with the NaxWO3 1D nanostructures.

The TEM image and the SAED pattern of the sampleannealed at 700 °C (the optimum conditions) can provide furtherdetail information on the morphology and microscopic insightof the prepared products shown in Figure 4. For TEM samplepreparation, the surface of the sample was scratched and thensolved in ethanol solution. Then, a drop of the final solutionwas dripped on the copper grid and after drying was used forTEM observation. From the TEM image, the nanorod is verystraight with a uniform diameter of about 100 nm along theentire length. The SAED pattern (Figure 4b) confirms that grownnanorods are single crystalline with cubic Na0.33WO3 structure.The growth of nanorods along the orientation [001], is inagreement with the XRD result showing the strongest peak inthe (001) orientation. In addition, the SAED pattern of thenanorod shows streaking around diffraction spots revealing theexistence of stacking faults formed in the direction normal tothe [001] direction. The HRTEM image in Figure 4c revealed

Figure 3. (a) XPS survey spectra of the tungsten thin films after heattreatment at different temperatures, (b) molar ratio of sodium and siliconrelative to tungsten on the surface of the thin films after heat treatmentat different temperatures, and (c) deconvoluted W(4f) core level XPSspectrum of the nanorods and nanobelts grown at 700 °C.

Figure 4. (a) TEM image, (b) SAED pattern, and (c) HRTEM of theNaxWO3 sample annealed at 700 °C.

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the single crystalline structure of the nanorods prepared underour experimental conditions. The fringe spacing is about 0.38nm, corresponding to the (001) crystal planes of the cubicNa0.33WO3 according to eq 1. It is also noted that the end ofthe nanorod is covered by a very thin amorphous shell (∼1 nm).Therefore, the TEM observations confirm both XRD and XPSresults concerning composition and structure of the synthesizedNaxWO3 nanorods.

The optical transmission and reflection spectra of the “asdeposited” and the films annealed at the different temperaturesin a range of 300-1100 nm wavelength have been shown inFigure 5. It is seen that, the transmittance and reflectance ofthe “as deposited” film is lower than 8% and around 30%,respectively. In addition, the transmittance decreases linearlyin visible range and becomes zero at 348 nm. These opticalproperties are in agreement with metallic property of tungstenthin films. By increasing the annealing temperature, the trans-mittance increased while the reflectance decreased. Moreover,the absorption edge is shifted toward blue and it was determinedat 320 nm for the annealed films. The increase of transmittanceand the occurrence of the blue-shift for the absorption edge areboth associated with surface oxidation of the films and tungstenoxide formation by increasing annealing temperature. It is tonote that a pure WO3 thin film is highly transparent with >70%transmittance.17 The low transmittance of the annealed sampleseven at 750 °C is related to presence of sodium.13 The reductionof the reflectance by increasing annealing temperature is relatedto rise of surface roughness after growth of 1D nanostructures.

Finally, to understand the role of sodium in growth of tungstenoxide nanorods, a sample was prepared under the similar growthconditions and parameters, but on a different substrate. SiO2/Si(100) system instead of soda lime was used as a substrate byoxidizing Si(100) wafer in O2 ambient at 900 °C for 120 min.Then, W layer was grown by sputtering technique on the SiO2/Si(100) substrate under similar conditions. After the annealingprocess at 700 °C, it was observed that only nanoparticles andmicroparticles were grown on the surface without any nanorodsor nanobelts, by using SEM (Figure 6a). XPS survey spectrumof this sample has been shown in Figure 6b. It is clear from thespectrum that no sodium peaks were seen on the surface.Therefore, it can be concluded that diffusion of sodium fromsoda lime substrate toward the surface of the film, and so itspresence on the surface plays a key role for the growth oftungsten oxide with 1D nanostructures. This mechanism is

similar to the role of potassium in formation of 1D nanostructureof tungsten oxide as reported recently.18,19

It is well esatablished that growth of nanowire with thepresence of catalysts is most likely controlled by the well-knownvapor-liquid-solid (VLS) growth mechanism.20 However, therewas no external W source supplied in our whole growth processand thus the conventional VLS mechanism could not explainthe growth of NaxWO3 nanorods. In our present case, the Wconcentration in the vapor phase is negligible at the growthtemperature and possible W source comes only from thetungsten thin film on the substrate. In addition, the source ofNa and O is originated from the diffusion of Na2O within thesoda lime substrate onto the surface of the W metal layer asverified from XPS spectra (Figure 3). It is because the sodalime glass is composed of SiO2 (71-75%), Na2O (12-16%),and CaO (10-15%), and it is well established that the Na2O inglass substrate can diffuse onto surface of a film after annealingprocess at >400 °C.21 It should be noted that the oxygen canalso be originated from residual O2 in the ambient and/or nativetungsten oxide on the surface. On the basis of the phase diagramof Na2WO4/WO3,22 the minimum melting point temperature ofsodium tungsten bronzes is at ∼600 °C. Therefore, it isreasonable to expect that tiny droplets of low-melting-pointliquid containing Na, W, and O form at the growth temperatureshigher than 600 °C. These tiny droplets act as the seeds for thegrowth of nanorods. If more WO3 was dissolved in the dropletto reach the supersaturated state, solid NaxWO3 would precipitatefrom the droplet in the form of nanorods. Continuous feedingof tungsten and sodium oxides into the liquid droplet sustainsthe growth of the nanorods. Finally, when the temperature ofthe system is slowly lowered to room temperature, the growthprocess was stopped. Figure 7 shows schematically the mech-anism for the growth process. In this proposed mechanism, thesize of the nanorods is directly related to the size of the initialdroplet of the liquid. As observed from Figure 1, the growth ofnanorods began at 600 °C and their surface density and width

Figure 5. (a) Optical transmittance and (b) optical reflectance of the“as deposited” tungsten thin film and the annealed samples containingtungsten oxide nanostructures.

Figure 6. (a) SEM image and (b) XPS survey spectrum of the tungstenfilm on the SiO2/Si(100) substrate annealed at 700 °C.

Synthetic Method for NaxWO3 Nanorods and Nanobelts J. Phys. Chem. C, Vol. 113, No. 30, 2009 13101

increased by increasing the annealing temperature. The increasein width of the nanostructures can be attributed to formation oflarger droplets on the surface. However, at 750 °C, due tosoftening of the glass and diffusion of the film into the substrate(Figure 1d), the density was decreased and the nanorods becamenanobelts. This deformation can be as a result of connectionand coalescence of the nanorods together due to sinteringprocess. A similar deformation process was also observed byY. Wu et al. for the growth of hexagonal tungsten trioxidetubes.23 Since this growth process involves SLS phases, it isnamed as a SLS growth, which is in fact an analogy of theVLS mechanism. It should be noted that there are many reportsthat used SLS growth mechanism for growth of other materialswith 1D nanostructures, such as Si nanowires,24 Ge nanowires,25

and In2O3 nanosheets.26

4. Conclusions

In conclusion, we report here a simple and novel method tosynthesize sodium-doped tungsten oxide nanorods and nanobeltson soda lime substrates at a relatively low temperature. Thisnew method is very simple and can be easily scaled up toprepare a larger amount of the material. Annealing process at700 °C was determined as the optimum temperature for growing1D nanostructure of the NaxWO3 with maximum density on thesurface. Furthermore, it is determined that the existing sodiumin the soda lime substrate diffuses toward the surface of theannealed samples and this diffusion plays an important role information of the 1D tungsten oxide nanostructure. A SLS growthmechanism of the nanorod formation was also proposed todescribe formation of NaxWO3 nanorods and nanobelts.

Acknowledgment. The authors wish to thank ResearchCouncil of Sharif University of Technology for financial support

of the project. The assistance of Mr. M. Goudarzi in the projectis greatly acknowledged.

References and Notes

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JP902189H

Figure 7. Schematic illustration of the mechanism for the NaxWO3

nanorod growth.

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