Correlation between Raman spectra and color of tungsten trioxide (WO₃) thermally evaporated from a tungsten filament

Tungsten oxide, also known as tungsten trioxide (WO₃), is an n-type metal oxide semiconductor that exhibits an indirect bandgap of 2.7 eV, which can be tunable according to the regime size (bulk or nano), crystallinity, or stoichiometry.^1–3 WO₃ crystallizes at room temperature in the monoclinic structure and has the following lattice parameters—a = 7.30 Å, b = 7.53 Å, c = 7.68 Å, and β = 90°54′—containing eight molecules of WO₃.⁴ Today, there is renewed interest in studying properties and applications of WO₃, which is evidenced by several recently published reviews.^5–10 The most common use of WO₃ is in electrochromic devices due to its reversible and tunable coloration under excitation of an electric field.¹¹ However, there are some interesting properties of WO₃ beyond electrochromism such as visible light photoactivity, high surface sensitivity, and tunable bandgap, which enables its potential application in photocatalysis,¹² solar energy conversion,¹³ gas sensors,^14,15 and photodetectors,¹⁶ among others. Several published papers have shown that WO₃ can exhibit different colors according to the synthesis conditions. For example, Acosta et al. found a correlation between stoichiometry of WO₃ thin films obtained by sputtering and the color exhibited by such films.¹⁷ Sato et al. found that the color of WO₃ becomes denser when it exhibits an oxygen-deficit characteristic.¹⁸ In terms of stoichiometry, the range of colors exhibited by WO₃ begins from deep-blue under an oxygen-deficient condition and ends in semitransparent or white for stoichiometric WO₃. However, Mardare et al. reported that the color of stoichiometric WO₃ results in yellow powders and it turns to green when oxygen deficiency is present.⁵ The aim of this report is to correlate the color of WO₃ powders observed by the naked-eye with their structural properties characterized by Raman spectroscopy. In addition, the mechanism of deposition of the WO₃ powders was qualitatively discussed.

Tungsten trioxide (WO₃) powders were synthesized by the thermal evaporation method using a home-made hot filament chemical vapor deposition (HFCVD) system [Fig. 1(a)]. In a typical HFCVD experiment, a metal filament heated at temperatures around 2000 °C acts as a catalyzer in the forming of gas precursors by decomposition of reactant gases on its surface.¹⁹ In the experiment reported here, the tungsten filament (purity of 99.95%) itself acts as a precursor because the tungsten oxide formed on its surface is thermally evaporated. Before placing the substrates (glass sheets, size of 1 cm²) into the HFCVD system, they were washed in an ultrasonic cleaner for five minutes: first in xylene, then in acetone, and finally in isopropyl alcohol solutions. Preparation of the HFCVD system for evaporation of tungsten oxide included two stages: first, once the glass substrates were placed four mm above the tungsten filament [Figs. 1(b) and 1(c)], the HFCVD system was closed and pumped down to −30 InHg. Then, to purge it, high-purity 4.7 grade argon gas (99.997% purity) was flowed at a rate of 400 standard cubic centimeters per minute (sccm) for five minutes. Evaporation of tungsten trioxide was performed by applying an AC voltage of 15 V to the tungsten filament and flowing argon gas or a mixture of argon and water vapor through the HFCVD system. The temperature of the tungsten filament was estimated to be around 920 °C by performing current vs voltage measurements and comparing to electrical resistivity vs temperature tables reported by Desai et al.²⁰ Although thermal evaporation of tungsten oxide was performed at atmospheric pressure, a little increase to five-pound square inches (psi) was observed during all experiments. The vapor emitted by the tungsten filament was transported to the upper area of the HFCVD reactor and was deposited on the glass substrates [Fig. 1(d)]. Parameters for evaporation of tungsten oxide and the color exhibited by the deposited powders are shown in Table I. The WO₃ powders were thermally treated in a muffle furnace at 500 °C for 30 min in ambient atmosphere. The pressure in the muffle furnace increased from zero psi to five psi during the thermal treatment performed to the WO₃ powders. Photographs of the WO₃ powders are shown in Fig. 2. The powders deposited using only flow of argon exhibit a navy-blue color [Fig. 2(a)]. In experiments performed by flowing a mixture of argon and water vapor at a rate of 200 SCCM, the powders exhibited a royal-blue color [Fig. 2(b)]. When the argon gas flow rate was increased to 400 sccm, the color of the powders changed to sky-blue [Fig. 2(c)]. After thermal treatment, the navy-blue, royal-blue, or sky-blue powders transformed into white powders [Fig. 2(d)]. Raman measurements were recorded using a micro-Raman Horiba Jobin Yvon system (Xplora plus model). A green laser (λ = 532 nm) was used to induce scattering, using a maximum power of 10%. The laser beam was focused using a 100x lens, and it was also used to recollect scattered light. A 600 lines/mm grating was employed; 100 acquisitions were averaged with an exposure time of 5 s for each one. Scanning electron microscope images were obtained by scanning electron microscopy (SEM, JEOL JSM-6510LV).

As shown in photographs in Fig. 2, the WO₃ powders exhibit different colors when argon or a mixture of argon and water vapor is flowed through the HFCVD system and also after they were thermally treated. Raman spectra of WO₃ powders of different colors were divided into three zones: low frequency (45–450 cm⁻¹), medium frequency (451–1100 cm⁻¹), and high frequency (1101–1700 cm⁻¹). Figure 3 shows Raman spectra of navy-blue, royal-blue, and sky-blue powders. In the low frequency zone [Fig. 3(a)], the Raman spectrum of navy-blue powders shows seven peaks at 68, 129, 175, 269, 320, 430, and 444 cm⁻¹ [Fig. 3(a)]. The peaks at 68 and 129 cm⁻¹ can be assigned to vibrational modes of (W₂O₂)_n chains into the lattice of WO₃.^21,22 The peaks at 175, 430, and 444 cm⁻¹ have been previously observed in microcrystalline WO₃ grown under high-pressure conditions.²³ The peaks at 269 and 320 cm⁻¹ can be assigned to O–W–O bending modes of binding oxygen of WO₃.²⁴ Raman spectra of royal-blue powders exhibit four bands with peaks at 79, 133, 263, and 323 cm⁻¹. Although there is a shift in peak positions compared to that of navy-blue powders, such bands can also be assigned to vibrational modes of (W₂O₂)_n chains of WO₃ (79 and 133 cm⁻¹) and bending modes of WO₃ (263 and 323 cm⁻¹). The peaks at 175, 430, and 444 cm⁻¹ associated with high-pressure conditions are no longer observed in this spectrum. The Raman spectrum of sky-blue powders shows five peaks at 71, 90, 134, 267, and 325 cm⁻¹. In frequencies lower than 100 cm⁻¹, it is common to find several peaks of the different phases of WO₃ (monoclinic, orthorhombic, and hexagonal). However, the peak at 71 cm⁻¹ is a characteristic frequency of the monoclinic structure.²¹ The intense peak at 90 cm⁻¹ has arisen because of (W₂O₂)_n chain vibrations of sub-stoichiometric WO₃.¹ In the middle frequency zone, the Raman spectrum of navy-blue powders exhibits a broad band from 550 to around 960 cm⁻¹, in which two peaks are observed: the first at around 710 cm⁻¹ (in the shoulder of the band) and the second one at around 804 cm⁻¹[Fig. 3(b)]. These peaks correspond to stretching modes of WO₃.²³ The Raman spectrum of royal-blue powders shows two peaks at 701 and 805 cm⁻¹, which can be associated with stretching bands of WO₃. These bands are sharper than those of the navy-blue powders. However, the typical stretching band of the Raman spectra of WO₃, commonly centered around 715 cm⁻¹, exhibits a low-frequency shift to 701 cm⁻¹. Although the peak at 805 cm⁻¹ corresponds to a mode of the monoclinic structure of WO₃, this peak has been previously assigned to orthorhombic or hexagonal phases of WO₃.²¹ Royal-blue WO₃ films obtained by thermal oxidation of tungsten obtained by other workers have shown Raman peaks at 130, 270, 703, and 807 cm⁻¹, confirming the monoclinic structure of the WO₃.²⁵ In the Raman spectrum of sky-blue powders, the two Raman bands associated with stretching modes of WO₃ are the sharpest in comparison to those of the navy-blue and royal-blue powders. In addition, the peak at 714 cm⁻¹ does not show the low-frequency shift observed in the Raman spectrum corresponding to the royal-blue powders. In the high-frequency zone, only weak and broad bands from around 1475 to 1675 cm⁻¹ are observed [Fig. 3(c)]. In the close-up view of such Raman spectra (by reduction of the “y axis” scale), some Raman bands can be identified [Fig. 3(d)]. The Raman spectrum of navy-blue powders exhibits a broad band from 1120 to 1700 cm⁻¹. This band shows two peaks—one at 1440 cm⁻¹ in the shoulder of the band and the second one at 1565 cm⁻¹. These peaks can be associated with carbon modes.²⁴ The broad band disappeared in the Raman spectrum of royal-blue powders. However, an almost identical band is observed in the Raman spectrum of sky-blue powders. Raman spectra of thermally treated WO₃ powders are shown in Fig. 4. The Raman spectrum of thermally treated navy-blue powders (which turned to white powders) shows peaks at 68, 130, 175, 267, 321, 437, 713, 802, and 948 cm⁻¹. The peaks at 68, 130, 175, 267, 321, 437, 713, and 802 correspond to WO₃ modes. These bands are sharper than those of the untreated navy-blue powders. In whitey-yellow powders obtained by hydrothermal synthesis, sharp Raman bands were also observed at peak positions similar to those of the thermally treated navy-blue powders.²⁶ The additional peak at 948 cm⁻¹ corresponds to a stretching mode of the double bond of the tungsten atom with terminal oxygen atoms (W=O).²² Such a Raman band has been previously observed in white WO₃ hydrates.²⁷ Raman spectra of royal-blue and sky-blue WO₃ powders exhibit peaks at positions almost identical to those of navy-blue powders. These peaks match well with the Raman frequencies of WO₃, and their sharpness indicate a good crystalline characteristic, independent of the color that they exhibited before thermal treatment.

Figure 5 shows SEM images of WO₃ powders deposited on glass substrates. Figure 5(a) shows the SEM image of navy-blue WO₃ powders. The texture of the surface is observed to be irregular and relatively dense at the scale of the SEM measurement. Although the navy-blue WO₃ powders turned to white after thermal treatment at 500 °C for 30 min in ambient atmosphere, they remain almost identical to its original morphology, and only some dispersed fine grains that exhibit brighter contrast are additionally observed [Fig. 5(b)]. Figure 5(c) shows the SEM image of the royal-blue WO₃ powders. The morphology exhibits some agglomerates composed of needle-like particles of micrometer sizes. In contrast, the surface of the sky-blue WO₃ powders [Fig. 5(d)] is observed to be compact and relatively soft when compared to that of the WO₃ powders observed in Figs. 5(a)–5(c). Figures 5(e) and 5(f) show SEM images of the sky-blue WO₃ powders after they were thermally treated at 500 °C for 30 min in ambient atmosphere. Morphology of such powders is observed to be composed by agglomerated particles without a well-defined morphology.

Colors exhibited by WO₃ powders studied in this work are shown in Fig. 6. Their corresponding Raman modes are shown in Table II. It has been previously reported that the color of WO₃ powders can be tuned from light-blue to deep-blue by increasing the grinding time.²⁸ Raman spectra of such powders exhibit that for deep-blue WO₃, the Raman intensity and broadening of the bands are lower and higher, respectively, in comparison to those of light-blue powders. A higher broadening of the Raman bands was related to a smaller grain size in deep-blue WO₃ powders. For structural characterization, both the breadth and intensity of a Raman band can be used as an indirect measurement of crystalline quality of WO₃. For example, the decrease in the width of a Raman band reveals improvement of the structural order in terms of bond length and angle of W–O–W bonding.²⁹ On the other hand, the color exhibited by tungsten oxide can be associated with its stoichiometry. In sputtered WO_x (2 ≤ x ≤ 3) thin films, transition from x = 2 starts with deep (navy)-blue and ends with x = 3 for transparent films (white for powders). In addition, by XRD, the transparent thin films exhibit the crystalline structure of WO₃.³⁰ As was expected, the broad and weak bands of WO₃ and the carbon bands observed in the Raman spectrum of the navy-blue WO₃ powders (Fig. 3) indicate an amorphous and contaminated structure of WO₃, probably due to low-oxygen content in the atmosphere of the experiment performed to deposit such WO₃ powders. Carbon contamination in the WO₃ powders could have been caused by the relatively low purity of the tungsten filament (99.95%). Some workers have previously found that in colored royal-blue WO₃ with a monoclinic structure, the characteristic Raman modes appear at 130, 270, 718, and 809 cm⁻¹.³¹ Such characteristic frequencies of WO₃ were also found in Raman spectra of both royal-blue and sky-blue powders obtained in the present work. Although Raman bands of these royal-blue and sky-blue powders were sharper than those of the navy-blue powders, Raman bands corresponding to the royal-blue powders were observed to be broader than those of the sky-blue powders, which indicates better crystalline properties of the latter, which could have been favored by the increase in the argon-water vapor flow rate from 200 to 400 SCCM. The large shift from 715 to 701 cm⁻¹ in the stretching band of WO₃ observed in the Raman peak of the royal-blue powders can be associated with changes in the crystal symmetry as well as lattice distortion of WO₃, which has been previously found in heavily hydrated WO₃.³² Thermal treatment favored crystallization of navy-blue WO₃ powders as well as removal of carbon from its surface. The broad Raman bands became sharp bands with well-defined peaks, and the color of such samples changed from navy-blue to white, which is the characteristic optical appearance commonly exhibited by stoichiometric WO₃.³³ As commented above, Raman bands of thermally treated royal-blue and sky-blue WO₃ powders were found in peak positions almost identical to those of the thermally treated navy-blue powders. Only a peak at 948 cm⁻¹ was observed for the thermally treated navy-blue WO₃ powders. Although this peak was assigned to a W=O bond, such a Raman mode is commonly found in hydrated WO₃,³⁴ which suggests that these powders would need additional thermal treatment to remove all hydrates and impurities. Although the weak Raman bands of WO₃ associated with high-pressure experimental conditions were identified only in the spectrum of the navy-blue WO₃ powders, all thermally treated powders exhibited such peaks. These bands could have emerged due to the increase in the pressure observed in the thermal treatments performed to the WO₃ powders. However, further studies on this topic are needed due to the large difference in the pressure between the previously reported value and that measured in the present work.²³ Previous reports have suggested that the mechanism that governs the growth of WO₃ using a heated filament involves evaporation of metal tungsten and oxidation in the gas phase.^35,36 According to the high melting point of tungsten (3422 °C), evaporation of such metal requires that the process temperatures be near such a value. For example, it has been previously reported that no traces of tungsten or its oxide are found in experiments when a metal tungsten filament was heated to temperatures around 2000 °C.³⁷ Under atmospheric pressure and when heating the filament at temperatures around 920 °C, it is complicated to evaporate metal tungsten. In fact, a company focused on fabrication of evaporation systems suggests that due to the high power need, it is nearly impossible to evaporate tungsten by thermal evaporation.³⁸ On the other hand, it has been previously reported that oxidation of metal tungsten can occur at temperatures as low as 300 °C.³⁴ In fact, a pure monoclinic WO₃ structure can be formed by heating metal tungsten at 700 °C.³⁹ Thus, once the metal tungsten filament is oxidized during the HFCVD process, thermal evaporation of tungsten oxide could take place at 920 °C.^40,35 At a first stage, a tungsten oxide layer can be formed at the surface of the tungsten filament under an oxidant atmosphere. Then, tungsten trioxide sublimates according to the following reaction:⁵ 

According to the above-mentioned equation, some oxygen is released when evaporation takes place, and the deposited WO₃ can exhibit different stoichiometries. This process could justify both color and consequently structural properties of the WO₃ powders.

WO₃ powders have been synthesized by thermal evaporation of tungsten oxide from a metallic tungsten filament using an HFCVD system. It is concluded that the color of WO₃ observed by the naked-eye is an indirect and qualitative measurement of the crystalline quality and purity of WO₃. Further works performing field-emission SEM measurements could improve analysis between structure, color, and morphology of WO₃. According to the experiments reported here and the literature previously reported, it is suggested that the mechanism of depositing WO₃ powders using a heated metal tungsten filament follows two steps: first, a tungsten oxidation and then, thermal evaporation of tungsten oxide.

The data that support the findings of this study are available from the corresponding author upon reasonable request.