Novel synthesis of WO3 nanocrystals through pyrolytic decomposition of tungstate-based inorganic–organic hybrid nanobelts

Materials Chemistry and Physics 116 (2009) 507–513

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Novel synthesis of WO3 nanocrystals through pyrolytic decomposition of tungstate-based inorganic–organic hybrid nanobelts Deliang Chen ∗ , Hejing Wen, Huimin Chen, Hailong Wang, Rui Zhang, Hongliang Xu, Daoyuan Yang, Hongxia Lu School of Materials Science and Engineering, Zhengzhou University, 100 Science Road, Zhengzhou 450001, PR China

a r t i c l e

i n f o

Article history: Received 21 December 2008 Received in revised form 16 March 2009 Accepted 13 April 2009 Keywords: Nanostructures Semiconductors Chemical synthesis Crystal growth

a b s t r a c t The paper described a novel approach toward WO3 nanocrystals by pyrolytically decomposing tungstatebased inorganic–organic hybrid nanobelts in air at 500–600 ◦ C for 2 h. The above-mentioned hybrid nanobelts were derived via a reaction of layered H2 W2 O7 ·xH2 O and n-octylamine in a reverse-micelle-like medium (H2 W2 O7 ·xH2 O/n-octylamine/heptane). The as-obtained WO3 nanocrystals and their intermediate products were characterized by the techniques of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM & FE-SEM), thermoanalysis (TG–DSC), Fouriertransform infrared spectra (FT/IR), UV–vis absorption and X-ray photoelectron spectroscopy (XPS). The as-obtained WO3 nanocrystals had an apparent size of 20–50 nm, and took on a loose-aggregate-like morphology. The WO3 nanocrystals derived via the pyrolytic decomposition process were almost separated from each other and could be redispersed readily, while the WO3 nanocrystals obtained by the conventional acid-precipitation process tightly agglomerated into large particles with apparent sizes of several micrometers, without redispersibility even under an intense sonication treatment. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Synthesis and characterization of low-dimensional nanocrystals have been intensively investigated in recent decades, because of their extensive applications in photocatalysis, sensors, display devices, energy transformation and storage, and environmental decontamination [1–3]. Various synthetic techniques, from wet-chemical methods, vapor–liquid–solid growth methods, to self-assembly routes, have been established to grow zerodimensional quantum dots [4–6], one-dimensional nanowires (or nanorods) [7,8], two-dimensional nanodisks (or nanoplates) [9,10], mesoporous nanostructures and nanoparticulate films [11,12]. Most of the routes have shown high efficiencies in sophisticated control over sizes and morphologies of nanostructures, but the requirements in repeatability and stability of the nanosized products in a large-scale synthetic process have still challenged chemists very much till now [4–12]. It is, therefore, urgent to develop novel and robust strategies to synthesize nanocrystals in a cost-effective manner and with a prospective of large-scale production. Nanostructured tungsten oxides and related materials possess unique properties and have important applications in modern

∗ Corresponding author. Tel.: +86 371 67781046; fax: +86 371 63818662. E-mail addresses: [email protected], [email protected] (D. Chen). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.04.024

industries [13,14]. For example, the reversible color change between blue (reduced state, W4+/5+ ) and transparent (oxidized state, W6+ ) under an external electrical field can be exploited as electrochromic layers in display devices and smart windows [12]. The ability to absorb visible light makes tungsten oxide materials be excellent photocatalysts in solar utilization and water decontamination [15–17]. In addition, tungsten oxide nanocrystals are gas-sensory semiconductors and can be used as the active species in chemical sensors to monitor toxic and explosive gases (e.g. NO2 , H2 S, CO and NH3 ) [18–23]. Tungsten oxide nanostructures also have unique field-emission properties [24]. There have been a great number of reports on the growth of tungsten oxide nanostructures. Thermal evaporation of metal W, WS2 or WO3 powders has been widely used to grow one-dimensional tungsten oxide nanocrystals, including W18 O49 nanoneedles [25], W18 O49 nanotubes [26], WO3 nanowires [27] and WO2.9 nanowire networks [28]. The microemulsion routes to WO3 nanoparticles [29], soft-chemistry routes to tungsten oxide nanowires [23,30], sol–gel routes to WO3 nanodisks [31] and WO3 nanofilms [12,32], and flame routes to WO2.9 nanowires [33] have also been developed in the last decade. For the thermal evaporation and flame routes, a high temperature process (usually higher than 800 ◦ C) is necessary [25–28]. For the soft-chemistry and sol–gel routes, special starting materials (e.g. WCl6 , W(CO)6 and tungsten isopropoxide) are usually indispensable [23–32]. On the other hand, although the acid-decomposition process uses water-soluble tungstate salts as the starting materials,

508

D. Chen et al. / Materials Chemistry and Physics 116 (2009) 507–513

Scheme 1. Schematic representation of the pyrolysis process for synthesizing WO3 nanocrystals using tungstate-based inorganic–organic hybrid nanobelts as the precursor.

the serious agglomerating phenomenon of the final WO3 nanocrystals has been a big challenge. We recently developed a novel strategy, involving a topochemical transformation process, to achieve two-dimensional WO3 nanoplates by using mesoporous lamellar inorganic–organic hybrid nanobelts as the precursors [34–37]. The process consisted of three steps: the preparation of inorganic–organic hybrid nanobelts, the removal of organic species by nitric-acid oxidation treatment and the subsequent dehydration process. The as-obtained WO3 nanoplates have high specific surface areas (higher than 180 m2 g−1 ) and can be easily redispersed in water and other solvents [34,35]. In this work, we extend the above-mentioned strategy to synthesize WO3 nanocrystals by directly pyrolyzing the mesoporous lamellar tungstate-based inorganic–organic hybrid nanobelts at an elevated temperature in air. The basic process is shown in Scheme 1. The remarkable merit of the present method is its ability to produce WO3 nanocrystals with controlled particle sizes and without hard aggregates. Also, the method is suitable for large-scale synthesis of WO3 nanocrystals in a cost-effective manner. For purposes of comparison, a conventional acid-precipitation process is also used to synthesize WO3 nanocrystals using water-soluble Na2 WO4 ·2H2 O as the starting material.

for 24 h at room temperature. Then the solid was collected by centrifugation and washed with distilled water and ethanol. After dried at 80 ◦ C for 20 h, the obtained solid was ground and calcined at 450 ◦ C for 2 h with a heat rate of 2 ◦ C min−1 . The phases of the products and their precursors were determined by X-ray diffraction (XRD, Rigaku RINT-2500, Cu K␣). The morphologies and microstructures of the products were observed using scanning electron microscopy (SEM, JEOL JSM-5600), field-emission scanning electron microscopy (FE-SEM, Hitachi S-4500S), transmission electron microscopy (TEM, JEOL JEM-100CX), and selected-area electron diffraction (SAED, JEOL JEM-100CX). Thermogravimetry (TG, PerkinElmer TGA-7) was used to characterize the inorganic–organic hybrids with a heating rate of 10 ◦ C min−1 in an air flow. Fourier-transform infrared spectrum (FT/IR, Jasco FT/IR460 Plus) was applied to characterize the inorganic–organic precursor using the KBr disk technique. The UV–vis absorption spectra (SHIMADZU, UV-3101PC, BaSO4 as a reference) were used to characterize the WO3 samples. The X-ray photoelectron spectroscopy (XPS) spectra were performed on a JEOL JPS90-MX spectrometer with Mg K␣ radiation. Elemental binding energies were calibrated by the C1s peak at 285 eV.

3. Results and discussion The phase of Bi2 W2 O9 with an alternate stacking structure of double W–O layers and [Bi2 O2 ] layers was synthesized through a reaction between Bi2 O3 and WO3 , as shown in Eq. (1) [39,40]. The as-obtained Bi2 W2 O9 powders take on an irregular morphology and their sizes range from 5 to 20 ␮m [39]: 800 ◦ C

Bi2 O3 + 2WO3 −→ Bi2 W2 O9

(1)

2. Experimental procedure The synthetic process of granular porous WO3 nanocrystals included the following steps: (a) the synthesis of Bi2 W2 O9 powders via a solid phase reaction of Bi2 O3 and WO3 , (b) the protonation of Bi2 W2 O9 into H2 W2 O7 ·xH2 O, (c) the formation of tungstate-based inorganic–organic hybrid nanobelts by reactions of H2 W2 O7 ·xH2 O and n-octylamine, and (d) the pyrolysis of the tungstate-based inorganic–organic hybrid nanobelts to be granular porous WO3 nanocrystals. Typically, a mixture of Bi2 O3 and WO3 with a molar ratio of 1:2 was calcined in an electric muffle furnace at ∼800 ◦ C for 2 days, and a layered Bi2 W2 O9 phase was obtained [36,38]. Selective leaching of the Bi2 O2 layers from Bi2 W2 O9 using a HCl aqueous solution (∼6 mol L−1 ) led to the formation of its protonated phase, H2 W2 O7 ·xH2 O. The value of x was controlled to be ∼2 by adjusting the dying conditions [36]. Tungstate-based inorganic–organic hybrid nanobelts were synthesized by the reactions of the as-obtained H2 W2 O7 ·xH2 O and n-octylamine at room temperature under normal pressure according to our previous report with some modification [36,37]. Typically, H2 W2 O7 ·xH2 O (5 g, ∼10 mmol, x ≈ 2.0) was dispersed in a mixture of n-octylamine (30 mL) and heptane (150 mL) under a magnetic stirring condition. The color of the reaction system was changed from yellow to white in several hours, and a gelatinous white suspension was finally obtained after a reaction time of more than 24 h. The resulting white solid was collected by centrifugation and washed with ethanol. After the washed product was dried under reduced pressure at room temperature for 10 h, the tungstate-based inorganic–organic hybrid nanobelts were obtained. The synthesis of WO3 nanocrystals was performed by pyrolyzing the as-obtained tungstate-based inorganic–organic hybrid nanobelts using an electric muffle furnace in air at 500–600 ◦ C for 2 h. The obtained pale-yellow product was porous and friable, and it could be easily ground into fine powders, which were used for characterization. For purposes of comparison, a conventional acid-precipitation process was applied to synthesize WO3 nanocrystals by a reaction of Na2 WO4 ·2H2 O and HCl, followed by a calcination treatment. Typically, Na2 WO4 ·2H2 O (0.1 M, 100 mL) aqueous solution was added dropwise into an aqueous solution (300 mL), containing concentrated HCl (35 wt.%, 5 mL) and PEG (0.1 g) under magnetic stirring at room temperature. After a 30-min reaction, the suspension became turbid and white, and the pH value of the reaction system was ca. 1.5. The suspension was kept stirring

The [Bi2 O2 ] layers of Bi2 W2 O9 were selectively removed by an HCl aqueous solution and the corresponding protonated phase of H2 W2 O7 ·xH2 O was formed. The reaction process can be expressed as Eq. (2) and the phase of BiOCl is water-soluble [41]: Bi2 W2 O9 + 2HCl + xH2 O → H2 W2 O7 ·xH2 O + 2BiOCl

(2)

In addition, the value of x completely depends on the drying conditions, and it was controlled at about 2 in the present work. The morphology and sizes of the H2 W2 O7 ·xH2 O powders obtained are similar to those of Bi2 W2 O9 powders [39–41]. The similarity in morphology and size between H2 W2 O7 ·xH2 O and Bi2 W2 O9 also indicates that the transformation from Bi2 W2 O9 to H2 W2 O7 ·xH2 O is a topochemical process. The reaction of H2 W2 O7 ·xH2 O and n-octylamine occurred in reverse-micelle-like media that consist of a surfactant (noctylamine), an oil phase (heptane) and a polar solid particulate phase (H2 W2 O7 ·xH2 O). Our previous study suggested that there were two steps in the reaction between H2 W2 O7 ·xH2 O and noctylamine [36]. The first step was the intercalation of n-octylamine molecules into interlayer spaces of H2 W2 O7 , and the resulting product kept the double W–O octahedral layers. The second step was the dissolution of double W–O octahedral layers and the sequent recrystallization of the dissolved species to form a wellordered lamellar structure with single W–O octahedral layers. The hydrolysis of excess n-octylamine molecules in the released crystal water led to the formation of a numerous alkali aqueous solutions confined in the reverse-micelle-like media. The above-mentioned alkali aqueous solutions played a pivotal role for the formation of tungstate-based inorganic–organic hybrids. The reactions can be

D. Chen et al. / Materials Chemistry and Physics 116 (2009) 507–513

509

Fig. 2. XRD patterns of (a) the tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts and (b) WO3 nanocrystals obtained by pyrolyzing the C8 NH3 @HWO nanobelts at 550 ◦ C in air for 2 h.

Fig. 1. (a and b) SEM images of the tungstate-based inorganic–organic hybrid nanobelts (C8 NH3 @HWO) derived from the reaction of H2 W2 O7 ·xH2 O and noctylamine in a nonpolar solvent of heptane. The nanobelts are 10–50 ␮m in length and 30–80 nm in thickness.

described as follow (Eqs. (3)–(5)) [36]: C8 H17 NH2

H2 W2 O7 · xH2 O

−→

(C8 H17 NH3 )a H2−a W2 O7 + H2 O

(3)

−

(4)

+

C8 H17 NH2 + H2 O → C8 H17 NH3 + OH H2

O,OH−

(C8 H17 NH3 )a H2−a W2 O7 −→ 2(C8 H17 NH3 )b H2−b WO4

(5)

Fig. 1 shows the SEM images of a typical sample (C8 NH3 @HWO) derived by the reaction of H2 W2 O7 ·xH2 O and n-octylamine in heptane. As Fig. 1(a) shows, they are of a unitary one-dimensional morphology in a large field of view, and the lengths of the onedimensional structures are 10–50 ␮m. Fig. 1(b) shows an enlarged SEM image of the above one-dimensional structures. It is clear that the one-dimensional structures are nanobelts or partly rolled nanotubes. The FE-SEM image indicated that the thicknesses of the nanobelts ranged 30–80 nm. The XRD pattern of the as-obtained C8 NH3 @HWO nanobelts is shown in Fig. 2(a). There are a series of diffraction peaks in the low 2-angle range of 2–25◦ . These peaks should belong to (0 0 l) reflections (l = 1, 2, 3, . . .), which indicate that the sample of C8 NH3 @HWO possesses a well-ordered lamellar microstructure [35–37]. The interlayer distance of the sample was calculated to be ca. 2.6 nm, very close to the literature data [36]. Fig. 3 shows a typical FT/IR spectrum of the as-obtained C8 NH3 @HWO nanobelts. The intense adsorption band at around 905 cm−1 is assignable to the stretching mode of terminal W O [42]. The bands located below 820 cm−1 can be assigned to the stretching and bending modes of the bridging oxygen atoms (O–W–O) [42]. The bands at 3100–3310 cm−1 should be assigned to the stretching vibration of the N–H groups [43]. Bands due to

both the scissoring mode of the –NH2 groups and the bending mode of the –NH3 + groups appear between 1635 and 1574 cm−1 [42]. The bands at 1394, 1468 and 869 cm−1 can be assigned to the deformation models of –CH2 – groups. Symmetrical stretching bands of –NH3 + appear at around 2771, 2653 and 2546 cm−1 [42]. The bands in the region of 2800–3000 cm−1 are assigned to the C–H stretching modes of the polymethylene [–(CH2 )n –] chains [(2852 cm−1 , s (CH2 ); 2923 cm−1 , as (CH2 )] and end-methyl (–CH3 ) groups [2870 cm−1 , s (CH3 ); 2956 cm−1 , as (CH3 )] [44]. The broad band centering at 2107 cm−1 is due to a combination of the asymmetrical bending vibration and torsional oscillation of the–NH3 + groups interacting with the apical oxygen of the W–O framework (C8 H17 –NH3 + · · ·− O–W) [36]. The FT/IR analysis indicates that the as-obtained product derived by a reaction of H2 W2 O7 ·xH2 O and n-octylamine is a hybrid system containing inorganic W–O units and organic species. TG–DSC curves of the as-obtained tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts are shown in Fig. 4. As the TG curve shows, the mass losses can be roughly bracketed into four steps: (i) less than 180 ◦ C, (ii) 180–250 ◦ C, (iii) 250–420 ◦ C, and (iv) 420–600 ◦ C. At less than 180 ◦ C, there is a remarkable mass loss of 38% and two corresponding endothermal peaks centering at 80 and 138 ◦ C, respectively, which should be ascribed to the desorption of n-alkylamine molecules or ions

Fig. 3. FT/IR spectrum of tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts.

510

D. Chen et al. / Materials Chemistry and Physics 116 (2009) 507–513

Fig. 4. TG–DSC curves of tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts measured in air with a heating rate of 10 ◦ C min−1 .

physically and chemically absorbed on the surfaces of the hybrid nanobelts. The endothermal peak at around 210 ◦ C and a mass loss of 6.0% between 180 and 250 ◦ C are probably due to the removal of n-alkylamine molecules or ions from the interlayer spaces. In the range of 250–420 ◦ C, there are a weak endothermal peak at 310 ◦ C and a very strong endothermal peak at 360 ◦ C, accompanying a mass loss of about 7.0%. The endothermal peaks and the corresponding mass loss should be assigned to the decomposition (or carbonization) of the organic species remaining and the removal of crystal water of the inorganic frameworks. The broad exothermal peak at 440–580 ◦ C and the corresponding mass loss of 5.0% should be due to the oxidation of carbonized organic species derived at 250–420 ◦ C as gas species. At high than 600 ◦ C, there is no change in either TG or DSC curve, suggesting that the crystal water and organic species can be completely removed below 600 ◦ C. In fact, it is no obvious change in the TG curve at a higher temperature than 550 ◦ C. The total mass loss between room temperature and 600 ◦ C is about 60%. The CHN analysis suggested that the atomic ratio of C:H:N is very close to 8:20:1. When taking the results of FT/IR, XRD and TG–DSC into account, the composition of the as-obtained tungstate-based inorganic–organic hybrid nanobelts can be described as (C8 H17 NH3 )2 WO4 [36]. The organic species of the tungstate-based inorganic–organic hybrid nanobelts can be removed by calcining at 550 ◦ C for 2 h in air, resulting in inorganic tungsten oxide nanocrystals. The XRD pattern of the as-obtained tungsten oxide nanocrystals is shown in Fig. 2(b). The phase of the sample can be readily indexed to be monoclinic WO3 (JCPDS card No. 43-1035). It is interesting that there is a wide reflection peak located at a very low 2-angle range of less than 8◦ , which suggests that the as-obtained WO3 possesses a kind of a porous structure. Fig. 5 shows some typical SEM images of the as-obtained WO3 nanocrystals. The low-magnification SEM image (Fig. 5(a)) indicates that the sample takes on a morphology of loose, cotton-like aggregates with a number of irregular cavities. The enlarged FE-SEM images (Fig. 5(b) and (c)) show that the above-mentioned cottonlike aggregates are constructed by particulate WO3 nanocrystals, with a size range of 20–50 nm. The cavities are, in fact, the interspaces of the aggregates derived from the particulate WO3 nanocrystals. It should be noted that the WO3 nanoparticles are aggregated very loosely and full of cavities, which is very important for the applications in catalysis and sensors. The morphology and microstructures of the sample obtained from SEM observations are consistent with the XRD results, and the porous characteristic of the WO3 nanocrystals obtained is due to the interspaces of

Fig. 5. (a) A conventional SEM image and (b and c) FE-SEM images of WO3 nanocrystals derived by pyrolyzing the as-obtained tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts at 550 ◦ C in air for 2 h.

the overlapped nanoparticles. The TEM images of the as-obtained WO3 nanocrystals are shown in Fig. 6. The apparent sizes of the WO3 nanocrystals are 20–50 nm, similar to the SEM observations. The nanoparticles should be nanodisks with thin thicknesses of 10–30 nm, due to their thin layered structures in the precursors [34]. In fact, the interfaces of overlapped nanodisks can be clearly identified, as shown in Fig. 6(b). Because the diameters of WO3 nanodisks are very close to their thicknesses, their disk-like morphology is similar to an equiaxed grain in the observation of FE-SEM images (Fig. 5). The separate diffractional dots in the SAED patterns (insets in Fig. 6) indicate that the individual WO3 nanodisks should be single-crystal. The TEM images also suggest that the WO3 nanocrystals derived from the tungstate-based inorganic–organic hybrid nanobelts are easily to be redispersed in the solvents including ethanol.

D. Chen et al. / Materials Chemistry and Physics 116 (2009) 507–513

511

Fig. 6. (a and b) TEM images of the WO3 nanocrystals obtained by pyrolyzing the tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts at 550 ◦ C in air for 2 h. The insets show their corresponding SAED patterns.

XPS spectra of the as-obtained WO3 nanocrystals from inorganic–organic hybrids are shown in Fig. 7. The wide scanning spectrum (Fig. 7(a)) indicates that the elements of W, O and C occur in the sample and no other impurities are found. The elements of W and O should belong to WO3 nanocrystals, and the C element is probably due to the adventitious carbon contamination of hydrocarbons [45]. Fig. 7(b) shows the core-level XPS spectrum of W4f. There appear two peaks with binding energies of 35.4 and 37.5 eV, which should belong to W4f7/2 and W4f5/2 of W6+ , respectively [27]. The spin–orbit separation energy between W4f5/2 and W4f7/2 is ca. 2.1 eV, which is consistent with the literature data [46]. The XPS analysis indicates that W6+ is the

major chemical state in the as-obtained WO3 nanocrystals. Fig. 7(c) shows the core-level XPS spectrum of O1 s, where there two peaks at 530.5 and 532.2 eV, respectively. The peak at 530.5 eV should correspond to the valence of the tungsten equal to +6 and the other peak at 532.2 eV ought to be assigned to residual water adsorbed on the surface of the as-obtained WO3 nanocrystals [27]. The UV–vis spectrum of the WO3 nanocrystals obtained by calcining the tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts at 550 ◦ C in air for 2 h are shown in Fig. 8(a). It is characteristic of a semiconductor with an absorption onset value (onset ) of 462 nm. The relationship between energy

Fig. 7. XPS spectra of the WO3 nanocrystals obtained by pyrolyzing the tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts at 550 ◦ C in air for 2 h: (a) XPS survey spectrum, (b) W4f core-level spectrum and (c) O1s core-level spectrum.

512

D. Chen et al. / Materials Chemistry and Physics 116 (2009) 507–513

The energy band gap of the as-obtained WO3 nanocrystals can be estimated to be 2.684 eV according to Eq. (6). One the other hand, considering that WO3 is an indirect n-type semiconductor, its energy band gap can also be determined by the following relationship (Eq. (7)), 1/2

(˛h)

= C(h − Eg ),

(7)

where ˛ is the absorbance, h is the incident photon energy and C is a constant [34]. Fig. 8(b) shows the plot of (˛h)1/2 as a function of h. There is a clear linear relationship in an energy range of 2.73–2.77 eV, with a linearly dependent coefficient of R2 = 0.998. According to the linear fit result, (˛h)

Fig. 8. (a) UV–vis spectra of the WO3 nanocrystals obtained by pyrolyzing the tungstate-based inorganic–organic hybrid (C8 NH3 @HWO) nanobelts at 550 ◦ C in air and (b) a plot of (˛h)1/2 as a function of h.

band gap (Eg ) and onset can be expressed as following [47]: Eg =

1240 , onset

(6)

1/2

= 5.333(h − 2.685),

(8)

the energy band gap can be readily determined to be Eg = 2.685 eV, which is close to the above-mentioned value (Eg = 2.684 eV) derived from its absorption onset value. The energy band gap of the commercially available WO3 micropowders was 2.58 eV according to our previous result [34]. The obviously broadened energy band gap indicates the dimensions of the as-obtained WO3 nanocrystals from inorganic–organic hybrid nanobelts should be within the size range with obvious quantum confinement effect. However, as Figs. 5 and 6 show, the apparent particle sizes observed by SEM and TEM are 20–50 nm, not small enough to generate obvious quantum confinement effect. This conflict corroborates that the as-obtained WO3 nanocrystals are not spherical but disk-like particles with small thicknesses of 10–30 nm. Fig. 9 shows typical SEM and TEM images of the WO3 nanocrystals synthesized by a conventional acid-precipitation reaction of Na2 WO4 ·2H2 O with HCl aqueous solutions, followed by calcining at 550 ◦ C. The low-magnification SEM image (Fig. 9(a)) shows that the sample mainly consists of large particles with sizes of 1–3 ␮m. The partly enlarged FE-SEM image (Fig. 9(b)) indicates the large particles are aggregates of small nanoparticles with sizes of 15–30 nm. Fig. 9(c) shows a TEM image of a closely packed aggregate with a diameter of 2.5 ␮m. Fig. 9(d) indicates that the large aggregate

Fig. 9. Morphology of WO3 nanocrystals obtained through an acid-deposition reaction followed by calcination at 550 ◦ C for 2 h: (a and b) typical FE-SEM images and (c and d) TEM images.

D. Chen et al. / Materials Chemistry and Physics 116 (2009) 507–513

consists of small nanoparticles. It should be noted that the sample was intensively sonicated in ethanol before TEM observation. However, the aggregates of WO3 nanocrystals formed through the conventional acid-precipitation cannot be redispersed, as shown in Fig. 9(c) and (d). The obvious contrast of the WO3 nanocrystals obtained by the conventional acid-decomposition method and the hybrid pyrolysis method, respectively, confirm that the latter has the potential to synthesize WO3 nanocrystals with controlled sizes and improved dispersibility on a large scale and in a cost-effective manner. 4. Conclusions We have developed a novel process to synthesize WO3 nanocrystals via pyrolyzing tungstate-based inorganic–organic hybrid nanobelts, which were derived by intercalating n-octylamine molecules into the interlayer spaces of the layered compound of H2 W2 O7 ·xH2 O in heptane. The WO3 nanocrystals obtained by the pyrolytic process probably take on a disk-like shape with a small thickness, and shows an obvious broadened energy gap (2.68 eV) when compared with the value (2.58 eV) of commercially available WO3 powders. The merits of the process proposed here can be mainly described in the following two aspects: one is that the particle sizes of WO3 nanocrystals can be controlled and the redispersibility of samples can be improved due to the in situ isolation of the organic species in the interlayer spaces of the C8 NH3 @HWO precursors; the other is that this process does not seriously depend on the experimental parameters, and can be suitable for the large-scale synthesis of WO3 nanocrystals in a cost-effective manner. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 50802090) and by Introduced Talent Project of Zhengzhou University. D. Chen thanks Professor Dr. Yoshiyuki Sugahara (Waseda University) for his valuable discussion on the formation mechanism of the tungstate-based inorganic–organic hybrid nanobelts. References [1] [2] [3] [4] [5] [6] [7]

J. Jortner, C.N.R. Rao, Pure Appl. Chem. 74 (2002) 1491. I. Willner, B. Willner, Pure Appl. Chem. 74 (2002) 1773. N. Wang, Y. Cai, R.Q. Zhang, Mater. Sci. Eng. R 60 (2008) 1. D. Chen, L. Gao, J. Colloid Interf. Sci. 279 (2004) 137. D. Chen, L. Gao, Solid State Commun. 133 (2005) 145. D. Chen, L. Gao, J. Inorg. Mater. 19 (2004) 58. D. Chen, L. Gao, Chem. Phys. Lett. 398 (2004) 201.

[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47]

513

D. Chen, L. Gao, J. Cryst. Growth 264 (2004) 216. D. Chen, L. Gao, Chem. Phys. Lett. 395 (2004) 316. D. Chen, L. Gao, Chem. Phys. Lett. 405 (2005) 159. M.A. Carreon, V.V. Guliants, Eur. J. Inorg. Chem. 2005 (2005) 27. T. Brezesinki, D.F. Rohlfing, S. Sallard, M. Antonietti, B.M. Smarsly, Small 2 (2006) 1203. C.G. Granqvist, Solar Energy Mater. Solar Cells 60 (2000) 201. G.A. Niklasson, C.G. Granqvist, J. Mater. Chem. 17 (2007) 127. S.-H. Baeck, K.-S. Choi, T.F. Jaramillo, G.D. Stucky, E.W. McFarland, Adv. Mater. 15 (2003) 1269. D. Monllor-Satoca, L. Borja, A. Rodes, R. Gómez, P. Salvador, Chem. Phys. Chem. 7 (2006) 2540. R. Abe, T. Takata, H. Sugihara, K. Domen, Chem. Commun. (2005) 3829. R. Ionescu, E. Llobet, X. Vilanova, J. Brezmes, J.E. Sueiras, J. Calderer, X. Correig, Analyst 127 (2002) 1237. W.-H. Tao, C.-H. Tsai, Sens. Actuators B: Chem. 81 (2002) 237. I. Jiménez, M.A. Centeno, R. Scotti, F. Morazzoni, J. Arbiol, A. Cornet, J.R. Morante, J. Mater. Chem. 14 (2004) 2412. E. Llobet, G. Molas, P. Molinàs, J. Calderer, X. Vilanova, J. Brezmes, J.E. Sueiras, X. Correig, J. Electrochem. Soc. 147 (2000) 776. A. Ponzoni, E. Comini, G. Sberveglieri, J. Zhou, S.Z. Deng, N.S. Xu, Y. Ding, Z.L. Wang, Appl. Phys. Lett. 88 (2006) 203101. J. Polleux, A. Gurlo, N. Barsan, U. Weimar, M. Antonietti, M. Niederberger, Angew. Chem. Int. Ed. 45 (2005) 261. J. Liu, Z. Zhang, Y. Zhao, X. Su, S. Liu, E. Wang, Small 1 (2005) 310. Y.Z. Jin, Y.Q. Zhu, R.L.D. Whitby, N. Yao, R. Ma, P.C.P. Watts, H.W. Kroto, D.R.M. Walton, J. Phys. Chem. B 108 (2004) 15572. Y. Li, Y. Bando, D. Golberg, Adv. Mater. 15 (2003) 1294. Y. Baek, K. Yong, J. Phys. Chem. C 111 (2007) 1213. Y.M. Zhao, Y.H. Li, I. Ahmad, D.G. McCartney, Y.Q. Zhu, W.B. Hu, Appl. Phys. Lett. 89 (2006) 133116. Z. Lu, S.M. Kanan, C.P. Tripp, J. Mater. Chem. 12 (2002) 983. J. Polleux, N. Pinna, M. Antonietti, M. Niederberger, J. Am. Chem. Soc. 127 (2005) 15595. A. Wolcott, T.R. Kuykendall, W. Chen, S. Chen, J.Z. Zhang, J. Phys. Chem. B 110 (2006) 25288. C. Santato, M. Odziemkowski, M. Ulmann, J. Augustynski, J. Am. Chem. Soc. 123 (2001) 10639. F. Xu, S.D. Tse, J.F. Al-Sharab, B.H. Kear, Appl. Phys. Lett. 88 (2006) 243115. D. Chen, L. Gao, A. Yasumori, K. Kuroda, Y. Sugahara, Small 4 (2008) 1813. D. Chen, H. Wang, R. Zhang, L. Gao, Y. Sugahara, A. Yasumori, J. Ceram. Process. Res. 9 (2008) 596. D. Chen, Y. Sugahara, Chem. Mater. 19 (2007) 1808. D. Chen, Y. Sugahara, Key Eng. Mater. 352 (2007) 85. J.C. Champarnaut-Mesjard, B. Frit, A. Watanabe, J. Mater. Chem. 9 (1999) 1319. M. Kudo, H. Ohkawa, W. Sugimoto, N. Kumada, Z. Liu, O. Terasaki, Y. Sugahara, Inorg. Chem. 42 (2003) 4479. D. Chen, H. Wang, R. Zhang, S. Guan, H. Lu, H. Xu, D. Yang, Y. Sugahara, L. Gao, Chem. J. Chin. Univ. 29 (2008) 1325. R.E. Schaak, T.E. Mallouk, Chem. Commun. (2002) 706. B. Ingham, S.V. Chong, J.L. Tallon, J. Phys. Chem. B 109 (2005) 4936. I.V. Chernyshova, K.H. Rao, A. Vidyadhar, A.V. Shchukarev, Langmuir 16 (2000) 8071. S.H. Park, C.E. Lee, Chem. Mater. 18 (2006) 981. J.F. Watts, J. Wolstenholme (Eds.), An Introduction to Surface Analysis by XPS and AES, John Wiley & Sons, Chichester, 2003. R. Azimirad, M. Goudarzi, O. Akhavana, A.Z. Moshfegh, M. Fathipour, J. Cryst. Growth 310 (2008) 824. D.S. Bhatkhande, V.G. Pangarkar, A.A.C.M. Beenackers, J. Chem. Technol. Biotechnol. 77 (2001) 102.