Correlation between Particle Size and Raman Vibrations in WO3

Article pubs.acs.org/JPCC

Correlation between Particle Size and Raman Vibrations in WO3 Powders C. Ingram Vargas-Consuelos,† Kyungah Seo,‡ Marco Camacho-López,*,† and Olivia A. Graeve*,‡,§ †

Laboratorio de Investigación y Desarrollo de Materiales Avanzados, Universidad Autónoma del Estado de México, Km 14.5 Carretera Toluca-Atlacomulco Toluca, Edo. de México, 50925 México ‡ Department of Mechanical and Aerospace Engineering, University of California, San Diego, 9500 Gilman Drive − MC 0411, La Jolla, California 92093-0411, United States § Kazuo Inamori School of Engineering, Alfred University, 2 Pine Street, Alfred, New York 14802, United States ABSTRACT: Transition-metal oxides such as WO3 are of interest because of their photochromic, electrochromic, and photocatatytic properties, which are promising for a variety of applications in mirrors, windows, and gas sensor technologies. These applications require a detailed understanding of the morphology, particle size, and other material characteristics for effective utilization and implementation. We present a correlation between powder particle size, determined from dynamic light scattering, and the bonding characteristics of WO3 powders, showing that the W−O−W/WO integrated intensity ratio is directly related to the particle size of our powders and not just to the grain size of WO3 films, as has previously been shown. This correlation can serve as a complementary technique to gauge particle size as well as crystallinity in WO3 powders. When the WO signal is high and the W−O−W is low, the powders will be of small particle size and/or of lower crystallinity. Thus, this analysis provides a useful approach for obtaining powder particle size in WO3 powders. The analysis might also prove useful for powders that exhibit Raman behavior similar to that of WO3. the optical band gap of WO3 films decreases as the WO/W− O−W Raman integrated intensity ratio decreases.14 This can be correlated both to an increase in crystallinity and an increase in the grain size of the WO3 films because W−O−W bonds become predominant when crystallinity is high and grain boundary area is low (i.e., the grain size is large). The WO signal is predominant when there is a small grain size and low levels of crystallinity. In this study, we present a correlation between powder particle size, determined from dynamic light scattering,15−26 and the bonding characteristics of tungsten oxide powders, showing that the W−O−W/WO integrated intensity ratio is directly related to the particle size of our free-flowing powders and not just to the grain size of WO3 films, as has previously been shown. This correlation can serve as a complementary technique to gauge particle size as well as crystallinity in WO3 powders. When the WO signal is high and the W−O−W is low, the powders will be of small particle size and/or of lower crystallinity.

1. INTRODUCTION Transition-metal oxides such as WO3 and MoO3 are of interest because of their photochromic,1 electrochromic,2 and photocatatytic3 properties, which are promising for a variety of applications in mirrors, windows, and gas sensor technologies.4,5 The preparation of these materials can be accomplished in a variety of ways. One particular dissolution−precipitation technique was reported by Mürau6 and consisted of the dissolution of tungsten metal in hydrogen peroxide. The technique usually requires a postsynthesis calcination step to obtain crystalline powders. Other known techniques for preparing tungsten oxide powders include reverse micelle synthesis,7 electrical arc discharge,8 and solvothermal synthesis.9 Using a peroxo tungstic acid (PTA) can also result in the preparation of crystalline WO3.10 As a first step, a PTA solution is obtained by using a dissolution technique.11 Calcination then results in the removal of peroxo-groups,12 allowing one to obtain pure WO3 crystals at moderate temperatures. Films of monoclinic WO3 and WO3·H2O have been previously studied by Raman spectroscopy, with particular attention given to the bands at 807 and 948 cm−1 , corresponding to the ν(W−O−W) and ν(WO) vibrations, respectively.13 The W−O−W vibration corresponds to the bridging bonds between tungsten and oxygen atoms in the bulk material, and the WO vibration corresponds to the terminal oxygen double bonds present on the surfaces and/or grain boundaries of the material. In addition, it has been shown that © 2014 American Chemical Society

2. EXPERIMENTAL METHODOLOGY A peroxo tungstate solution was prepared by dissolving 6 g of 0.1 mm metallic tungsten wire (99.95%, Alfa Aesar) in 400 mL of 30% hydrogen peroxide. The solution was heated on a hot Received: January 30, 2014 Revised: April 13, 2014 Published: April 15, 2014 9531

dx.doi.org/10.1021/jp501095y | J. Phys. Chem. C 2014, 118, 9531−9537

The Journal of Physical Chemistry C

Article

plate at a controlled temperature of 60 °C to dissolve the tungsten. After complete dissolution of the tungsten wire (about 13 h), the solution continued to be heated at the same temperature for about 12 h to evaporate the excess hydrogen peroxide. After evaporation, 6 g of white powder were obtained. Subsequently, the powders were annealed in a Lindberg furnace at 100, 200, 300, 400, and 500 °C, for 45 min under ambient air. Powders were yellow in color after heat treatment. The powders were characterized by X-ray diffraction (XRD) using Cu Kα radiation (40 kV, 30 mA). Scans were carried out over the range of 20 to 80° 2θ with step size of 0.04° and a dwell time of 5 s. Raman spectra were obtained using a LabRaman HR-800 (Jobin-Yvon-Horiba) with a He−Ne laser (λ = 632.8 nm) and a 50× microscope objective lens (Olympus BX-41 optical microscope) by placing 1 g of powder on a glass slide. Particle size distributions were obtained by dynamic light scattering (DLS) using a Microtrac Nanotrac Ultra instrument. To measure particle size, 0.1 g of powder was dispersed in 25 mL of ethanol. After 5 min of stirring, the powder was further dispersed using an ultrasonic mixer for 25 min. This solution was then stirred five additional minutes before testing. The particle size distributions were calculated from an average of 5 runs with 30 s runtimes. High-resolution images of powders were acquired from a scanning electron microscope (FEI Inc., QUANTA 200F) operated at 10 kV. Each powder was dispersed in ethanol, drip-coated in graphite, and dried in air. The detailed morphology and structural characterization was studied by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) using a JEOL JEM-ARM 200F instrument operated at 200 kV. The samples were prepared by dispersing a small amount of powder in 5 mL of ethanol and then drop-coating the dispersion onto a copper grid.

tetraperoxoditungstate ([W2O3(O2)4·(H2O)2]2−), which can lead to the formation of a layered amorphous tungsten trioxide (a-WO3) or crystalline WO3·2H2O and WO3·H2O.11 On the basis of the white color (not pale yellow) of the as-synthesized powder and an applied evaporation temperature of 60 °C, our as-synthesized powder is considered to be mainly composed of amorphous tungsten trioxide hydrate (WO3·nH2O). XRD measurements were completed (Figure 1) to determine the progression of crystallinity on the powders. The as-

3. RESULTS AND DISCUSSION During the dissolution of the tungsten wire, hydrogen peroxide can act as both an oxidizer and a complexing agent resulting in species such as tetraperoxoditungstate ([W 2 O 3 (O 2 ) 4 · (H2O)2]2−) and tungstic acid (H2WO4), which can be formed in acid solutions (pH < 5), as expressed by the following reactions:10,11,27,28

Figure 1. X-ray diffraction profiles for the (a) as-synthesized powders and the powders after calcination at (b) 100 °C, (c) 200 °C, (d) 300 °C, (e) 400 °C, and (f) 500 °C.

synthesized and 100 °C powders are amorphous. Crystallization is initiated as the calcination temperature is increased to 200 °C. Powders annealed above 300 °C show sharp diffraction peaks reflecting an increasing degree of crystallinity. The three peaks between 20° and 30° 2θ are a product of splitting of the main reflection of the ideal cubic cell because of the slight distortion of the lattice; thus, these peaks can be found in monoclinic, triclinic, or orthorhombic phases of WO3. By comparing with JCPDS card no. 01-083-0950, we can confirm that the powder has likely crystallized into monoclinic WO3. The Raman spectra of the powders are illustrated in Figure 2. The 100 °C powders (Figure 2a) show a broad and weak signal spanning the approximate range between 600 and 760 cm−1, corresponding to the ν(W−O−W) stretching vibration. There is also a sharp signal at ∼945 cm−1, corresponding to the ν(WO) stretching mode of the symmetric terminal WO bond. Both the internal W−O−W bridging oxygen bonds and the WO surface terminal bonds can be seen in the crystal structures illustrated in Figure 2, formulated using the atom positions reported by Indrea et al.29 These two vibrations are commonly found in all types of tungsten trioxide hydrates (WO3·nH2O).12,13,30 Signals at 570, 840, and 880 cm−1, which

2W + 10H 2O2 → [W2O3(O2 )4 ·(H 2O)2 ]2 − + 2H3O+ + 5H 2O (1) W + 2H 2O2 → WO2 + 2H 2O ⇒ WO2 + H 2O2 → H 2WO4 (2)

Species such as tungstate (WO42−) and tetraperoxotetratungstate ([W4(O9)(O2)4]2−) might also be present in our solutions. Higher-order polymeric peroxo species such as [W7O23(O2)]6− and [W7O22(O2)2]6− can also exist in acidic tungstate solutions (pH < 4), but the amounts of these species are likely minimal.27 When the solution is heated to evaporate the excess H2O2, a peroxotungstic acid (W-PTA, WO3·xH2O2· yH2O), tungsten trioxide hydrate (WO3·nH2O), amorphous tungsten trioxide (a-WO3), and crystalline WO3·2H2O and WO 3·H2 O can be obtained.10 The W-PTA is usually synthesized at controlled evaporation temperatures between 60 and 80 °C and is of a pale yellow or orange color;10,12 this temperature is higher than the evaporation temperatures applied to our solutions. At lower evaporation temperature ranges, the tungstic acid (H2WO4) can be precipitated and transformed into tungsten trioxide hydrate (WO3·nH2O) and 9532



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Figure 2. (left panels) Raman spectra for the (a) as-synthesized powders and the powders after calcination at (b) 100 °C, (c) 200 °C, (d) 300 °C, (e) 400 °C and (f) 500 °C. (right panels) Ball-and-stick model and polyhedral representation of monoclinic WO3.

are generally attributed to H2O2,10 do not appear in the powders, indicating that the compound W-PTA (WO3·xH2O2· yH2O) is not present. When calcination temperature increases to 200 and 300 °C (Figure 2b,c), two peaks at ∼715 and ∼807 cm−1 are now evident. The decrease of the full width at halfmaximum in the 807 cm−1 signal as the calcination temperature is increased to 400 and 500 °C (Figure 2d,e) reflects a considerable improvement in the structural order of the powders in terms of bond length and angle of the W−O−W bonding.31 It can also be seen that the WO peak at ∼945 cm−1 for the 100 °C sample is shifted toward 972 cm−1 when the annealing temperature increases to 200 °C. This shift could be attributed to the loss of waters of hydration and the beginning of crystallization of the powders. Indeed, a shift of the peaks is evident at each temperature interval by following the deconvoluted peaks for the WO vibrations and the higher-wavenumber W−O−W vibrations at the five calcination temperatures. The deconvolution fits for these two vibrations are marked using dashed lines in Figure 2. The deconvolution fits for the other peaks in the spectra are marked with solid lines. On the basis of both XRD and Raman results, the crystallization process for the powders can be summarized as follows:

[WO3 ·nH 2O] (n > 1) → [WO3 ·nH 2O] (n < 1) and a‐WO3 → m‐WO3

Namely, the as-synthesized powders are composed of WO3· nH2O (n > 1); they subsequently decompose to WO3·nH2O (n < 1) and amorphous WO3 (a-WO3) as H2O and O2 are removed at calcination temperatures greater than 200 °C, and they are finally crystallized into monoclinic WO3 (m-WO3) at higher calcination temperatures. Tungsten oxide usually consists of networks of WO6 octahedra. As the tungsten oxide crystallizes, the WO6 octahedral units form clusters that are connected to each other with W−O−W bonds,14,32 evident in the crystal structures of Figure 2. Powder morphology is depicted in Figure 3 for the 100 and 500 °C powders. The powders calcined at 100 °C (Figure 3a) consist of large granules that are close to 100 μm in size. Calcination at 500 °C dramatically modifies powder morphology (Figure 3b), forming smaller and somewhat spherical particles. One of the smallest particles of the 500 °C powders is illustrated in Figure 4a. It is clear from the image that this particle is not a monocrystal, but composed of at least 3−4 crystals, and it is dense. The powders have a well-ordered and crystalline atomic arrangement and a lattice d-spacing of 0.38 nm (Figure 4b), which corresponds to the {001} planes of mWO3.33−35 9533



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Figure 5. Particle size distribution of the powders calcined at (a) 100 °C, (b) 200 °C, (c) 300 °C, (d) 400 °C, and (e) 500 °C obtained from dynamic light scattering. Figure 3. Scanning electron micrographs of the powders calcined at (a) 100 °C and (b) 500 °C.

smallest particles began to sinter, forming hard agglomerates; thus, this temperature is the beginning of neck formation among particles. Further calcination to 500 °C dramatically increases the particle size, resulting in a weighted average of 2.9 μm. This data can be correlated to the Raman results presented earlier. Figure 6a shows the W−O−W to WO integrated intensity ratio and average particle size of the powders at all calcination temperatures. It has been reported that the integrated intensity ratio of the W−O−W bridging oxygen signal to the WO surface oxygen signal can be interpreted to reflect the ratio of the surface area to the volume of grains (or clusters) in polycrystalline films.14,30,31,36 The W−O−W vibration represents the bulk of the film, whereas the WO vibration represents the surface of the film. Thus, as the size of the grains in the film increases, the grain boundary area decreases and the signal for the WO vibration is diminished. This certainly is consistent from an energy perspective. Nonetheless, these correlations have always been done on films. We propose that particle size of free-flowing powders can also be correlated to the W−O−W/WO ratio and confirm that this applies to our powders, as seen in Figure 6a. The integrated intensities for this figure were calculated from the deconvoluted peaks marked with dashed lines in Figure 2. Thus, the particle size from DLS can be correlated to the ratio between the W−O−W and W O bands irrespective of the crystallinity of the powders because the WO band is present in both the crystalline and amorphous powders, whereas the W−O−W band is present in only the crystalline powders. Note that the location of the peaks is not fixed at one wavenumber but varies according to Figure 6b. The peak position decreases in wavenumber for the WO vibration with respect to calcination temperature, whereas it increases for the W−O−W vibration because of the change in bond lengths as crystallinity improves. The relationship between the integrated intensity ratio (IW−O−W/IWO) and the average particle size (Daverage) in nanometers is illustrated in Figure 7a and can be expressed by

Figure 4. Transmission electron micrographs of the powders calcined at 500 °C at (a) lower magnification and (b) higher magnification of the same particle.

DLS measurements (Figure 5) were completed to determine particle size of all samples. It is evident from the particle size distributions that the powders from the 100 °C sample consisted of loose agglomerates that were easily dispersed into dense particles less than 500 nm in size (with a weighted average at ∼300 nm). Heat treatments at 200 and 300 °C had similar particle size distributions and weighted averages of ∼300 nm; thus, there is no change in particle size at temperatures between 100 and 300 °C. The 400 °C sample exhibited a higher weighted average of 431 nm because the 9534



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Figure 7. (a) Relationship between average particle size and integrated intensity ratio. (b) Approximation of grain boundary area versus calcination temperature of the WO3 powders.

Figure 6. (a) Change in the ratio of the Raman spectra intensity between WO bonds (962 cm−1) and W−O−W bonds (807 cm−1) [●] and average particle size [■] with respect to calcination temperature. (b) Change in deconvoluted peak positions for the WO and W−O−W vibrations with respect to calcination temperature.

⎛ ⎞ I Daverage = (193 nm)exp⎜0.1272 W−O−W ⎟ IWO ⎠ ⎝

where γ is a fitting constant of reciprocal nanometer units, Vparticle the volume of the powder particles, Agrain boundary the grain boundary area, and Asurface the surface area. Equation 4 assumes that the Raman integrated intensity of the W−O−W signal is proportional to the bulk volume of the particles, whereas the integrated intensity of WO signal is proportional to the grain boundary and surface areas. Assuming spherical particles, this equation can be modified as follows:

(3)

On the basis of the Raman spectra of Figure 2 and the particle size correlation expressed in Figure 6a and Figure 7a, we propose the following model. The powders calcined at lower temperatures (100 and 200 °C) are amorphous or nearly amorphous and have a signal corresponding to surface WO bonding but no internal W−O−W bonding that is associated with a crystalline material. As calcination temperature is increased and crystals form inside the amorphous particles (increasing crystallite size) and as the particles grow (decreasing surface area and grain boundary area), the WO vibration diminishes and bulk W−O−W becomes dominant. This correlation can serve as a complementary technique to gauge particle size as well as crystallinity in WO3 powders. When the WO signal is high and the W−O−W is low, the powders will be of small particle size and/or of low crystallinity. The contribution of particle size can be decoupled from the effect of crystallinity, resulting in a measure of surface and grain boundary area that can be expressed using the following relationship: Daverage

⎡ Daverage 3 4 π 2 ⎢ ⎛ Daverage ⎞ 3 ln⎜ ⎟ = 0.1272γ ·⎢ Daverage ⎝ 193 nm ⎠ ⎢A ⎣ grain boundary + 4π 2

(

) (

⎤ ⎥ ⎥ 2 ⎥ ⎦

)

(5)

which can be simplified further to Agrain boundary =

0.0212γπDaverage 3

(

ln

Daverage 193 nm

)

− πDaverage 2 (6)

For the powders calcined at 500 °C, the grain boundary area per unit volume assuming γ = 1 nm−1 is 0.0212π(1 nm−1)(2912 nm)3

Agrain boundary unit volume

=

nm ( 2912 193 nm )

ln

= 0.023 nm−1

⎛ ⎞ Vparticle ⎟ ∝ (193 nm)exp⎜⎜0.1272γ · Agrain boundary + A surface ⎟⎠ ⎝

− π (2912 nm)2

(2912 nm)3 (7)

The grain boundary areas for the additional calcination temperatures are illustrated in Figure 7b, where only the values for calcination temperatures of 300, 400, and 500 °C have the

(4) 9535



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average of 2.9 μm. This particle size was correlated to the Raman W−O−W/WO ratio, and this correlation can serve as a complementary technique to gauge particle size as well as crystallinity in WO3 powders. When the WO signal is high and the W−O−W is low, the powders will be of small particle size and/or of low crystallinity. Thus, this analysis provides a useful approach for obtaining powder particle size in WO3 powders. The analysis might also prove useful for powders that exhibit Raman behavior similar to that of WO3.

validity one expects if a grain boundary is assumed to occur only in crystalline materials. In our case, we propose that the values found at calcination temperatures of 100 and 200 °C (marked with white crosses) are also valid because the boundaries and surfaces of the amorphous precipitates still contain WO bands (see Figure 2). What we see from Figure 7b at the lower calcination temperatures is no change in grain boundary area upon heating at 100 and 200 °C. This would be expected on the basis of the negligible change in crystallinity seen from XRD analysis (Figure 1). Calcination at 300 °C results in powders that contain grain boundaries surrounding grains that are now beginning to crystallize. As calcination temperature increases, the grain boundary area per unit volume decreases, as one would expect for crystals that are experiencing grain growth during calcination. This provides an approximation of the grain growth behavior in these materials, showing that while crystallite size is increasing, particle size remains constant,21,23 at least until a critical temperature is reached (in our case 500 °C) that promotes significant sintering among the powders. Beyond 500 °C, both crystallite size and particle size increase because of Ostwald ripening and powder sintering, respectively. In summary, we present a detailed analysis of the morphology, composition, particle size, and bonding characteristics of WO3 powders. We have determined that the W−O−W and WO Raman vibrations in WO3 can be correlated to the degree of crystallinity and particle size of the powders. As the powders crystallize, forming W−O−W bonds, the W−O−W/ WO Raman integrated intensity ratio increases. Furthermore, as the calcination temperature increases, the grain boundary area of the powders decreases, reducing the signal from the WO terminal bonds. The average particle size of the powders can be approximated from eq 3 simply by knowing the ratio of the two Raman peaks in question. Thus, this analysis provides a useful approach for obtaining powder particle size in WO3 powders. The analysis might also prove useful for powders that exhibit Raman behavior similar to that of WO3.

■

AUTHOR INFORMATION

Corresponding Authors

*Tel: +52 (722) 180-6320. E-mail: [email protected]. *Tel: (858) 246-0146. E-mail: [email protected]. URL: http://graeve.ucsd.edu/. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This project was partially funded by a grant from the Defense Threat Reduction Agency under Contract HDTRA1-11-10067.

■

REFERENCES

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4. CONCLUSIONS We present an analysis of the effect of particle size on the Raman signal in WO3 powders. Material preparation was achieved by dissolving metallic tungsten wire in hydrogen peroxide and subsequently calcining the resulting powders at a variety of temperatures from 100 to 500 °C. Calcination temperature had an effect on the crystallization of the powders, eventually forming monoclinic WO3, and on the particle size of the powders. The as-synthesized and 100 °C powders were amorphous. Crystallization was initiated as the calcination temperature was increased to 200 °C. Powders annealed above 300 °C showed sharp diffraction peaks reflecting an increasing degree of crystallinity. Particle size distributions of the powders were determined from dynamic light scattering. The powders from the 100 °C sample consisted of loose agglomerates that were easily dispersed into dense particles less than 500 nm in size. Heat treatments at 200 and 300 °C had similar particle size distributions; thus, there was no change in particle size at temperatures between 100 and 300 °C. The 400 °C sample exhibited a higher weighted average of 431 nm because the smallest particles initiated the sintering process, forming hard agglomerates; thus, this temperature is the beginning of neck formation among particles. Further calcination to 500 °C dramatically increased the particle size, resulting in a weighted 9536



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Correlation between Particle Size and Raman Vibrations in WO3

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