Synthesis of WO₃ Nanorods and Their Excellent Ethanol Gas-Sensing Performance

Abstract

WO₃ nanorods were synthesized via a simple hydrothermal approach. Their microstructure and morphology were analyzed with x-ray diffraction, scanning electron microscope and x-ray photoelectron spectroscopy. The effects of reaction temperature, reaction time, and citric acid concentration, on the gas-sensing performance of WO₃ nanorods were investigated. The optimized response value of WO₃ sensor to ethanol gas (100 ppm) was 26.48, with a response time of 1 s under a low operating temperature (160 °C). The recovery capability and stability of the gas sensor were tested and discussed. Additionally, the working principle of WO₃ sensor was proposed. In comparison to the sensors published by previous researchers, the WO₃ sensor has shown great potential.

Keywords:  WO₃; gas sensor; response; nanorods; ethanol

1. Introduction

Along with the development of human society, air pollution has become a serious environmental problem. Ethanol gas is a representative of volatile organic compounds (VOCs), and its prevention and control standards have been continuously improved in recent years¹. Large and continuous expose to ethanol gas can cause serious health problems (to human beings), like dyspnea, internal organ damage, etc². In order to prevent economic loss and environmental pollution, there is an imminent need for highly sensitive and practical gas sensor.

In recent years, some types of gas sensors have been developed. Among them, the most attractive type of gas sensors is the metal oxide semiconductor (MOS). ZnO³, CoO⁴, NiO⁵, and other metal oxides have shown great performance in gas-sensing applications. WO₃ is a metal oxide of n-type semiconductor, with superior performance in various fields. At present, WO₃ has been widely used in catalysts⁶, solar cells⁷, gas sensors⁸, etc. In gas sensors, WO₃ also shows high performance, fast response, and outstanding stability. Up to now, WO₃ has been widely investigated, owing to its easy structure control and numerous synthetic methods. For gas-sensitive materials, structural difference will affect the gas-sensing performance, and the structure of the material is closely related to the synthesis method.

Nowadays, the methods used to synthesize WO₃ have been widely developed, such as sol-gel, template, thermal decomposition, etc. Additionally, the developed WO₃ structures are nanotubes^9-11, nano films¹², nano tablets¹³, nano microspheres¹⁴, etc. Among them, one-dimensional materials were widely explored on account of their simple structure and easy preparation. Cai et al. directly grew single crystal WO₃ nanowires on FTO substrate by hydrothermal method. The response of nanowires towards 500 ppm NO at 300 °C was 37¹⁵. Leng et al. synthesized WO₃ nanofibers by electrospinning. The response of WO₃ nanofibers to 100 ppm NH₃ reached 5.5¹⁶. An et al.¹⁷ used TeO₂ nanowire templates to fabricate WO₃ nanotubes, which showed a response of 676.57 to 5 ppm NO₂ at 300 °C. These works have proved that WO₃ was an excellent gas-sensitive material, but several essential issues still required to be solved. For instance, the response time of the gas sensor to the target gas was long, and the working principle of the gas sensor was still unclear. As a representative of one-dimensional materials, compared with other one-dimensional structures, nanorods have a rough surface, more surface-active vacancies, which can speed up the adsorption and diffusion of gas molecules on the surface of the material, thereby reducing the response time. Considering the unique porous morphology of WO₃ nanorods, changing the synthesis conditions of the reaction and further ameliorating the morphology of the material can improve the gas sensitivity of the WO₃ sensor.

In this work, WO₃ nanorods were synthesized by hydrothermal method. Meanwhile, their microstructure and composition were analyzed. The gas-sensitive performance of WO₃ was investigated by controlling reaction temperature, reaction time and citric acid concentration. Furthermore, the selectivity and the response recovery ability of WO₃ were systematically investigated, and the sensing mechanism of WO₃ was analyzed.

2. Experimental

2.1. Chemicals and materials

Ammonium tungstate (NH₄)₁₀(H₂W₁₂O₄₂) ·16H₂O, citric acid (C₆H₈O₇·H₂O) and anhydrous ethanol (CH₃CH₂OH) were supplied by Sinopharm Chemical Reagent Co., Ltd. (China). The water used in the experiment was deionized water.

2.2. Preparation of WO₃ nanorods

2.0 g of (NH₄)₁₀(H₂W₁₂O₄₂) ·16H₂O and 0.5 g of C₆H₈O₇·H₂O were added to 80 ml deionized water under stirring. Furthermore, the solution was stirred for more than 30 min at 25 °C until the drug was completely dissolved. The mixture was sealed into Teflon-lined autoclave of 100 ml and heated at 160 °C for 24 h. Afterwards, the Teflon-lined autoclave was placed in a dry and ventilated place, to allow the solution to be cooled naturally. The solid matter was collected by suction filtration. The collected solid matter was consecutively washed three times with deionized water and with absolute ethanol. Ultimately, the collected WO₃ solid matter was dried at 60 °C for 10 h. The WO₃ nanorods synthesis process is shown in Figure 1.

Figure 1
Schematic illustration of the synthesis process of WO₃.

In order to investigate the influence of synthesis conditions on the nanostructures of WO₃, the synthesis conditions were changed by controlling three variables: the reaction temperature, the reaction time and the citric acid concentration. The summary of synthesis conditions of WO₃ was shown in Table 1.

Table 1
Summary of synthesis conditions of WO_3.

Sample	Reaction temperature	Reaction time	Citric acid concentration
WO3-1	120 °C	24 h	0.03 mol/L
WO3-2	160 °C	24 h	0.03 mol/L
WO3-3	200 °C	24 h	0.03 mol/L
WO3-4	160 °C	12 h	0.03 mol/L
WO3-5	160 °C	36 h	0.03 mol/L
WO3-6	160 °C	48 h	0.03 mol/L
WO3-7	160 °C	36 h	0.02 mol/L
WO3-8	160 °C	36 h	0.04 mol/L
WO3-9	160 °C	36 h	0.05 mol/L

2.3. Characterization

The crystallization behavior of these samples was analyzed by X-ray diffraction (XRD, D/max-2400) with Cu-Kα radiation at the scanning range of 2θ from 10° to 80°. The morphologies of WO₃ nanomaterials were investigated by the scanning electron microscope (SEM) (Philips XL30 FEG). The X-ray photoelectron spectroscopy (XPS) of the WO₃ nanomaterials was analyzed by Thermo SCIENTIFIC ESCALAB 250 spectrometer using an Al-Ka source.

The lattice parameters, average grain size and unit cell volume of different WO₃ nanostructures were investigated. The formula is as follows, where λ is the X-ray wavelength - 0.1546 nm, θ is the diffraction peak angle, β is the half height width of the diffraction peak.

$Dhkl=\frac{0.89\lambda }{\beta \cos \theta }$ (1)

2.4. Fabrication and measurement of the gas sensor

A small amount of WO₃ nanomaterials was grounded in an agate mortar. The WO₃ nanomaterials powder was mixed thoroughly with ethanol, and the mixture was spread onto the external surface of Al₂O₃ tube, composed of gold electrode and four Pt wires. A Ni–Cr heating wire was inserted into the Al₂O₃ tube to constitute a small gas-sensing device. To complete the gas sensor, the four Pt wires and the Ni-Cr heating wire were welded on the metal pedestal with a welding machine. Afterwards, the gas sensor was inserted into the aging plate and then aged under a low heating voltage. The aging time exceeded 24 h. The gas sensing performance was measured by the intelligent gas sensing and measuring system of WS-30A (Zhengzhou Weisheng Electronic Technology Co., Ltd., China). The schematic diagram is shown in Figure 2. The sensing response of the gas sensor (s) was defined as the relative change of resistance. R_a and R_g are the resistances measured by WO₃ sensor in air and target gas, respectively. R_L is the load resistance. The formulas used to calculate the different resistances and the sensing response are presented as follows, where V_c is the circuit voltage (5 V), V_a and V_g are the voltages measured by WO₃ sensor in air and target gas. The sensor resistance can be calculated by changing the output voltage (V_OUT):

Figure 2
Schematic diagram of the WS-30A gas-sensing measurement system.

$$R_{\text{a}}\text{=}\frac{(V_{\text{c}}/V_a-1)}{R_L}$$ (2)

$$R_g\text{=}\frac{(V_{\text{c}}/V_g-1)}{R_L}$$ (3)

$$S\text{=}\frac{R_{\text{a}}}{R_g}$$ (4)

3. Results and Discussion

3.1. Morphology and structure of WO₃

Figure 3 shows the representative XRD of the WO₃ nanomaterials synthesized at 160 °C, with reaction time 36 h and 0.03 mol/L citric acid. All the samples show the WO₃ phase. As observed, the characteristic diffraction peaks at 2θ are 14.0°, 26.8°, 28.2°, 33.6°, 36.6°, 50.0°, 55.3° and 63.5°, which are assigned to (100), (001), (200), (111), (201), (220), (202) and (401) crystal faces, and are basically consistent with PDF card (JCPDS No. 33-1387). In addition, no other diffraction peaks are found, thus the samples are all hexagonal phase crystals WO₃. The lattice parameters, unit cell volume and average grain size of WO₃ are shown in Table 2.

Figure 3
XRD patterns of WO₃ samples.

Table 2
Lattice parameters, unit cell volume and average grain size of WO_3.

Sample	Lattice parameters (Å)			Unit cell volume (Å 3)	Average crystallite size(nm)
	a	b	c
WO3	6.3249	4.4969	3.7720	107.28	33.2

Figure 4 depicts the SEM micrographs of WO₃ at 160 °C, 36 h and 0.03 mol/L citric acid. The results demonstrate that the synthesized WO₃ nanomaterials are short rods. The low-magnification SEM is performed to show the existence of WO₃ nanorods. Apparently, it can be observed that the nanorods are obtained and with rough surfaces. The shape of nanorods can be clearly observed by the magnified SEM of Figure 4a and Figure 4c. We can see that nanorods formed complete and distributed uniformly on a large scale. These results correspond to the average grain size calculated in XRD.

Figure 4
SEM images of WO₃ synthesized at 160 °C, 36 h and 0.03 mol/L citric acid.

X-ray photoelectron spectroscopy (XPS) clarify the chemical binding state of the sample. X-ray photoelectron spectroscopy is shown in Figure 5. The results presented in Figure 5 show that the sample is composed of three elements: C, W and O. Among these elements, the element C is the impurity introduced by the target gas. Because of the asymmetry of the peak at a low binding energy, another bimodal is used to clearly present the W4f peak. It means that tungsten trioxide is deficient in oxygen, which supports the electronic transport properties of the material by the introduction of donor electrons^18-23. The spectrum of O(1s) shows an asymmetrical line. The main peak in the binding energy is about 531.1 eV. This is due to the single bond existing between tungsten and oxygen¹⁸. In Figure 5b, the high binding energy peaks at 36.08 eV and 38.12 eV correspond to W4f_{7 / 2} and W4f_{5 / 2} respectively. The peak value of W4f_{7 / 2} at 36.08 eV is higher than that of W4f_{5 / 2} at 38.12 eV, and the peak area ratio is about 4:3, which confirms that the element exists as WO₃^24,25. Based on the analysis results, the molecular formula of the sample can be determined as WO₃.

Figure 5
XPS spectrum of WO₃. (a) wide scan spectrum, (b) W4f spectrum, and (c) O1s spectrum.

3.2. Gas sensing properties of WO₃ nanomaterials

The performance of gas sensors with metal oxide nanomaterials depends largely on the operating temperature. Consequently, the sensing performance of WO₃ exposed to 100 ppm ethanol was investigated at a given operating temperature range (100-260 °C). Figure 6a reveals the response of the gas sensor to the reaction temperature, taken as a variable. At first, the sensitivity of the three materials (WO₃-1, WO₃-2, WO₃-3) increased. Afterwards, their sensitivity decreased. Among the three materials, WO₃-1 reached the maximum response when the operating temperature was 200 °C. However, WO₃-1 and WO₃-3 did not show excellent gas-sensing performance. The results imply that WO₃-1 failed to form a complete nanorod during the preparation process, which resulted in the material being unable to deoxidize. On the contrary, WO₃-3 has shown a bad performance. This may be because the high synthesis temperature destroys the formed WO₃ crystal nucleus²⁶, which weakens its ability to combine with adsorbed oxygen. As a result, comparing the images and data, the optimal reaction temperature was determined as 160 °C. Figure 6b illustrates the comparative sensing response of the gas sensors at different reaction time. The sensitivity of the three samples of gas sensors first increased, then decreased rapidly. The sample WO₃-5 shows the highest response with 26.48. The responses ​​of WO₃-4 and WO₃-2 both exceeded 8, but WO₃-6 showed almost no response. The reason behind those results may be that WO₃ gradually formed nanorod-like structures with the increase in reaction time, and the rod-like structures were most likely completely formed at 36 h. However, at 48 h, the original structures were broken and recombined into a mass, which reduced the active vacancies on the surface of the material. Figure 6c shows the response of the material to the citric acid concentration, taken as a variable. From the figure the optimum citric acid concentration is 0.03 mol/L, and the optimum operating temperature is 160 °C. Figure 7 reveals the response of WO₃ nanorods towards different ethanol gas concentrations. The responses are 1.08, 6.8, 18.2, 22.2, and 26.52, respectively. It can be observed from the figure that when the concentration of ethanol gas is lower than 100 ppm, the response value of the gas sensors increases rapidly. As the concentration continues to increase to 300 ppm, the response of the gas sensors increases gently. These trends are caused by the saturation of gas molecules and surface-active vacancy, and the stable equilibrium of adsorption rate and desorption rate.

Figure 6
Responses of the WO₃ sensors to ethanol (100 ppm) under different reaction conditions: (a) reaction temperatures, (b) reaction time, and (c) concentrations of the citric acid addition.

Figure 7
Sensitivity plot of WO₃ gas sensor under different ethanol concentration.

Under the optimum operating temperature of 160 °C, the response and recovery times of WO₃-5 were investigated. The response and recovery curve according to the time is shown in Figure 8, corresponding to 20, 50, 100, 200 and 300 ppm ethanol gas, respectively. When the ethanol gas was injected, the sensor response increased rapidly, with a response time of about 1 s. After the sensor was placed in contact with the air, the sensor response decreased gradually, with a recovery time of about 30 s. The response of the device nearly returned to its initial state after detecting the ethanol gas and being in contact with the air, which indicates that the WO₃ sensor has a superior repeatability.

Figure 8
Response and recovery curve of WO₃-5 gas sensor at optimum operating temperature (160 °C).

The selectivity of WO₃ nanorods to ethanol, benzene, dichloromethane, formaldehyde, and n-butanol at a concentration of 100 ppm was respectively investigated. The results are shown in Figure 9. The sample WO₃ - 5 shows an excellent response to ethanol, followed by benzene and acetone. These results show that the sample WO₃-5 has an excellent selectivity towards reducing gas in a gas environment.

Figure 9
Sensitivity histogram of WO₃-5 gas sensor under different gases.

Table 3 mainly introduces the response of WO₃ with different morphologies to 100 ppm ethanol gas. According to the published data, the WO₃ nanorods prepared in this work demonstrate high sensitivity than most of nanomaterials^27-29, and they still possess great advantages compared to WO₃ nanofilms³⁰. Although the response of the WO₃ nanorods prepared in this work is lower than the urchin-like WO₃¹ and Spider web WO₃³¹, the response and recovery times of these two materials are low. WO₃ nanorods can respond to the target gas rapidly and have a fast recovery time, which is very important for sensor applications. Compared to all the examples given, WO₃ nanorods did not only show a high response, but also worked at a low optimal operating temperature, which is suitable for low temperature operations.

Table 3
Gas sensors for ethanol detection reported in literatures and in this work.

Sensing materials	Concentration (ppm)	Operating temperature (°C)	Response (S)	Response time (s)	Recovery time (s)	References
WO3 nanorod	100	160	26.48	1	30	In this work
WO3 nanospheres	100	250	16.2	34.2	35.6	27
WO3 nanoscale	100	-	15.6	10-15	30-35	28
Urchin-like WO3	100	350	68.56	28	12	1
WO3 nanorod	100	275	6-7	141	148	29
WO3 deposition film	100	250	15.2	-	-	30
Spider web WO3	100	300	72	-	-	31

3.3. Gas sensing mechanism of WO₃ sensors

In most cases, when the metal oxide gas sensing materials are in contact with dry air, only physical adsorption occurs on their surface. However, when they are placed in contact with reducing gases such as ethanol vapor, both physical adsorption and chemical adsorption occur. The principle of chemical adsorption is presented as follows:

At first, the gas-sensitive material physically adsorbs the oxygen present in the air. After the oxygen in the air is adsorbed on the surface of the gas-sensitive material, chemical adsorption occurs, and electrons obtained from the gas-sensitive material form the oxygen anions (O₂^-2, O₂^-, O^2-), as shown by the following equations:

$$O_{2(gas)}\to O_{2(ads)}$$ (5)

$$O_{2(ads)}+e^{-}\to O_2{{}^{-}}_{(ads)}$$ (6)

$$O_2{{}^{-}}_{(ads)}+e^{-}\to 2O^{-}{}_{(ads)}$$ (7)

$$O^{-}{}_{(ads)}+e^{-}\to O^{2-}{}_{(ads)}$$ (8)

At that moment, the conductivity of WO₃ is weak. When the gas sensing materials are placed in contact with the reducing gas, the physical adsorption and the chemical adsorption occur at the same time, which is accompanied by the ionization of the reducing gas, as presented in the following equations:

$$C_2{H}_{5}O{H}_{(gas)}\to C_2{H}_{5}O{H}_{(ads)}$$ (9)

$$C_2{H}_{5}O{H}_{(ads)}+6O^{2-}{}_{(ads)}\to 2C{O}_2+3H_{2}O+12e^{-}$$ (10)

When the concentration of the gas continues to increase, the chemical ionization of the reducing gas on the surface of the material also increases. This phenomenon provides more conductive particles to the sensing material, which in turn improves the conductivity of the material, then ultimately reflects on the change in the response of material towards the reducing gas.

A graphical diagram of the ethanol-sensing mechanism on the surface of the WO₃ sensor is shown in Figure 10. WO₃ is an n-type semiconductor material. When the ethanol gas (C₂H₅OH) passes into the test environment, the anions O^2- adsorbed on the surface of WO₃ react with the C₂H₅OH. During the reaction, C₂H₅OH loses electrons, and the electrons obtained from the oxygen atoms return to WO₃. The electron transfer during the adsorption process increases the carrier concentration in WO₃, leading to an increase in conductivity of the sensor and a reduction of the load voltage of the sensor. Therefore, by changing the type and concentration of the adsorbed gas, the load voltage of the sensor can be adjusted.

Figure 10
Schematic diagram of WO₃ sensor sensing mechanism.

4. Conclusions

We reported our work on the preparation of WO₃ nanorods using the hydrothermal method. According to the response of the material to ethanol gas under different synthesis conditions, the influence of the reaction conditions on the gas sensing performance of the sensors was analyzed. The plausible formation mechanism of WO₃ nanorods was discussed in the light of the SEM figures of WO₃ synthesized by different reaction temperatures, reaction time and citric acid concentrations. The results showed that the morphology of WO₃ has a significant impact on the sensitivity of the gas sensor. The gas sensitivity of materials under different synthesis conditions were tested. In the presence of 100 ppm ethanol, the response of the WO₃-5(reaction temperature 160 °C, reaction time 36 h and citric acid concentration 0.03 mol/L) sensor was 26.48. WO₃ could recognize many reducing gases, which showed its excellent selectivity. WO₃ had a favorable recovery ability, which allowed the material to quickly identify the target gas in a transitory time and to quickly return to the original state of its response. In addition, the sensing mechanism of WO₃ sensor for ethanol was explained. Compared with the previous literatures, WO₃ showed a great potential in the rapid detection of ethanol.

5. Acknowledgements

This work was supported by Program for Liaoning Innovative Talents in University, and Key Research & Development Project of Liaoning Province (2017308005).

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Publication Dates

• Publication in this collection
  29 Mar 2021
• Date of issue
  2021

History

• Received
  22 Sept 2020
• Reviewed
  05 Jan 2021
• Accepted
  21 Feb 2021