1. Introduction
WO3 is an N-type semiconductor functional material, which has a strong absorption effect on the solar spectrum compared with the widely studied wide-band gap TiO2 (Eg ≈ 3.2 eV) material. WO3 also has the advantages of non-toxicity, stable physical and chemical properties, good photochromism, gas sensitivity and photocorrosion resistance, so it is used as a photocatalyst for solar cells [1] , gas sensitive elements [2] , visible light decomposition of water [3] and degradation of organic matter [4] .
NanoWO3 with various micro morphology has been successfully prepared. Soultanidis [5] synthesized ultrafine tungsten oxide nanoparticles by solvothermal method, generating small tungsten oxide nanoparticles in the presence of organic oxidant trimethylamine N-oxide, and generating tungsten oxide nanorods in the presence of reducing agent 1, 12-dodecarbon diol. Kim [6] synthesized nano-sea urchin-shaped tungsten oxide containing W18O49 and WO3 and studied their electrochromic properties. Li [7] synthesized WO3 nanorods by simple microwave-assisted hydrothermal method using Na2SO4 as structural guide agent and investigated its excellent ethanol sensing characteristics. WO3 is an N-type semiconductor, and its optical absorption band edge is in the visible light region (Eg = 2.5~2.8 eV). It has many advantages suitable for visible light catalysis, such as deep valence band position (+3.1 eV), strong absorption ability of solar spectrum, stable physical and chemical properties, strong resistance to photocorrosion, etc. Kim [8] used self-assembled polystyrene (PS) colloidal as organic template and polyethylene glycol (PEG) as surfactant to prepare WO3 film and studied its excellent photoelectric catalytic ability under ultraviolet visible sunlight irradiation.
Toluene is one of the most common volatile organic compounds (VOCs) found in industrial emissions, indoor air and exhaust from motor vehicles. The adverse health effects of toluene on the human body depend on the conditions of exposure, and serious cases may cause nerve damage and sensory disorders [9] . Heterogeneous photocatalytic methods can treat environmental pollutants under mild conditions, and different catalysts have been used to study the photocatalytic degradation and removal of toluene [10] [11] [12] [13] . In-situ Fourier transform infrared spectroscopy (FTIR) technology can monitor the surface adsorbents, transition states and intermediates, further speculate the possible reaction pathways and mechanisms. Among them, transmission infrared spectroscopy is a commonly used method to study photocatalytic and thermocatalytic adsorption and reaction [14] [15] [16] , and in situ diffuse reflection infrared spectroscopy (DRIFT) and attenuated total reflection infrared spectroscopy (ATR-IR) are two effective methods to detect species on the catalyst surface and in the gas phase [17] [18] [19] . In this paper, maize cob WO3 nanomaterial was synthesized by simple hydrothermal method, and the photocatalytic degradation of toluene under visible light irradiation was studied by in situ infrared spectroscopy.
2. Experimental Part
2.1. Reagents and Instruments
Na2WO4, Na2SO4, hydrochloric acid (all analytically pure, Sinopharm Chemical Reagent Co., LTD.), toluene (Analytically pure, Tianjin Kemeiou Chemical Reagent Co., LTD.), and deionized water were used for the experiment.
Quanta 200 FEG Field Emission Environmental Scanning Electron Microscope (ESEM, FEI Company, USA); D/MAX-IIIA X-ray diffractometer (XRD, Shimadzu Company, Japan); VERTEX 70 Fourier Transform infrared spectrometer (BRUKER, Germany); UV550 UV-visible diffuse reflection spectrometer (DRS, JASCO, Japan); DF-101S collector thermostatic heating magnetic stirrer (Gongyi Yuhua Instrument Co., LTD.); TG-WS top high-speed centrifuge (Hunan Xiangyi Experimental Instrument Development Co., LTD.); XQ500W adjustable xenon lamp source (Shanghai Lansheng Electronics Co., LTD.).
2.2. Experimental Process
2.2.1. Preparation of WO3 Nanomaterials
Add Na2WO4 and Na2SO4 to 40 mL deionized water, stir them magnetically for 30 min, then drop 3 mol/L hydrochloric acid, and adjust the pH to 2. The mixed solution was poured into the reaction kettle and heated to 190˚C for 24 h. The solution was cooled to room temperature, separated by centrifugation, and washed with deionized water, and dried at 60˚C for 6 h. The light green WO3 nano powder was obtained by full grinding in an agate mortar.
2.2.2. Characterization of WO3 Nanomaterials
ESEM was used to characterize the surface morphology of WO3 nanomaterials. XRD, DRS and FTIR were used to determine the crystallinity, optical absorption characteristics and molecular structure of the prepared samples.
2.2.3. Photocatalytic Performance Test of WO3 Nanomaterials
The homemade infrared light reaction cell (diameter 4 cm, length 10 cm) consists of two sodium chloride windows and a sample holder (diameter 13 mm). WO3 nano-powder of 0.05 g was pressed into circular plates and placed on the sample shelf, and 2 μL toluene was injected into the reactor with a microsyringe. After 30 minutes, toluene vapor reaches adsorption equilibrium in the reactor, and Xenon lamp (λ > 400 nm) light source with light intensity of about 50 mW∙cm−2 is turned on. In situ FTIR was used to continuously collect infrared spectra with a resolution of 1 cm−1 and a scanning range of 4000 - 400 cm−1 during the photocatalytic reaction.
3. Results and Discussion
3.1. Morphology and Structure of WO3 Nanorods
Figure 1 shows the SEM photo of WO3. According to Figure 1(a), the microstructure of WO3 is corn-cob shape with good dispersion. It can be observed from Figure 1(b) that the length of nanorods is about 800nm, and the diameter is about 150 nm. The surface of the corn cob is uniformly covered with “corn kernels” with a diameter of about 20 nm.
Figure 2 is the X-ray diffraction pattern of WO3 nanomaterials, from which
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Figure 1. SEM images of WO3 nanorods (a) and its local magnification (b).
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Figure 2. Typical XRD pattern of the prepared WO3 nanorods.
the crystal planes of (020), (200), (120), (112), (202), (122), (132), (004), (040), (114), (240) can be clearly seen, and each characteristic diffraction peak of the WO3 standard card (JCPDS No. 20-1323) corresponded one to one, indicating that WO3 nanomaterial was successfully synthesized. The shape of each diffraction peak is sharp, which indicates that the crystallinity of the prepared sample is good.
3.2. Infrared and UV-Visible Spectral Characterization of WO3 Nanorods
Figure 3 shows the infrared representation of WO3 sample. The absorption peaks at 3443 and 1631 cm−1 are attributed to surface hydroxyl and adsorbed water molecules [20] , respectively, while the infrared peaks at 2362 cm−1 correspond to the adsorbed or trace CO2 in the atmosphere [21] . It should be particularly pointed out that the peak at 856 cm−1 is the characteristic peak of W-O bond [22] , indicating that the prepared sample is WO3 nanomaterial. The upper part of the figure shows the infrared spectrum of the newly prepared WO3 catalyst, and the lower part shows the infrared spectrum of the catalyst after the photocatalytic reaction for 8 h. It can be seen the position and intensity of each characteristic peak of the two spectral lines almost do not change, indicating that the WO3 nanocatalyzer is relatively stable.
Figure 4 shows the UV-VIS diffuse absorption spectra of WO3 sample. It can be seen from the figure that the samples have certain absorption intensity in the UV-visible region. And the absorption band edge is about 480 nm, indicating that the WO3 sample has potential visible light catalytic activity.
3.3. Visible Light Catalytic Performance of WO3 Nanorods
Figure 5 shows the infrared spectra of toluene adsorbed on WO3 catalyst every 5 min from toluene injection into the reactor to 30 min. As shown in the figure, in the full-band mid-infrared spectral region from 4000 to 400 cm−1, toluene has characteristic infrared absorption in the regions of 3250 - 2750 cm−1, 2000 - 1250 cm−1, and 750 - 400 cm−1, and the specific attribution is detailed below. According to the peak heights of each characteristic infrared peak in the figure, toluene
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Figure 3. FTIR spectra of the prepared WO3 nanorods before and after using.
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Figure 4. UV-vis diffuse reflection spectrum of the prepared WO3 nanorods.
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Figure 5. Full-band infrared spectra of toluene adsorption on WO3 nanorods.
could reach the adsorption-desorption equilibrium on WO3 catalyst within 30 min.
Figure 6 shows the infrared absorption spectra of toluene in various regions at different times after visible light catalytic degradation on WO3 nanorods for 8 h with the extension of illumination time. In Figure 6(a), the peaks at 3073, 3043 and 3032 cm−1 are attributed to the stretching vibration of C-H bond in benzene ring [23] , and the peaks at 2936 and 2880 cm−1 are attributed to the stretching vibration peak of C-H bond in toluene methyl group [24] . In Figure 6(c), the peaks at 1948, 1860 and 1801 cm−1 are attributed to the out-of-plane deformation vibration peaks of C-H bond on the benzene ring [25] , and the peaks at 1610 and 1498 cm−1 are attributed to the stretching vibration of the benzene ring skeleton C=C [26] . In Figure 6(d), the infrared peak between 1089 and 1025 cm−1 is attributed to the in-plane deformation vibration peak of C-H bond on the benzene ring [27] , and the peak at 729 and 695 cm−1 is attributed to the characteristic absorption peak of benzene ring mono-substitution [24] . As can be seen from the above three figures, with the increase of the photocatalytic reaction time, the peak heights of each characteristic peak of toluene gradually
decreased, indicating that toluene was effectively degraded on WO3 nanorods catalyst. In addition, it is worth mentioning that as shown in Figure 6(b), the peaks at 2360 and 2340 cm−1 are the characteristic peaks of CO2 [21] . It can be clearly seen that with the prolongation of the reaction time, the peak height of the characteristic peaks of CO2 increases significantly, indicating that CO2 is the final product of toluene degradation catalyzed by visible light.
In addition, it should be particularly pointed out that several new infrared peaks appeared in the reaction process. The peak at 951 cm−1 was attributed to the out-of-plane deformation vibration of O-H bond in carboxylic acid [28] , and the peak at 792 cm−1 was attributed to the C-H deformation vibration of aldehyde [29] . According to previous reports [29] and the results of this paper, benzaldehyde and benzoic acid are intermediates of toluene photocatalytic reaction.
4. Conclusions
1) One-step synthesis of corncob WO3 nanomaterial by hydrothermal method. The rod length is about 800 nm, the diameter is about 150 nm and the surface size of cornlike particles is about 20 nm.
2) The nanomaterials have a certain absorption intensity in the UV-Vis spectral region, and the absorption band edge is around 480 nm, which can effectively use solar energy.
3) In situ infrared spectroscopy showed that the adsorption equilibrium of toluene in gas phase was reached on the corncob WO3 nano-catalyst for 30 min. It could be seen that after 8 h irradiation, each characteristic peak of toluene was significantly weakened, indicating that it was photocatalytically degraded.
4) In situ infrared spectrum, the characteristic peaks of reaction intermediates such as carboxylic acid and aldehyde appear simultaneously, and the characteristic peak intensity of the final product CO2 is significantly increased, indicating that corncob WO3 nanomaterial can effectively degrade toluene under visible light irradiation.