Journal of Sensors
Volume 2019 (2019), Article ID 6074046, 8 pages
https://doi.org/10.1155/2019/6074046

Research Article

Facile Preparation of g-C₃N₄-WO₃ Composite Gas Sensing Materials with Enhanced Gas Sensing Selectivity to Acetone

Xiangfeng Chu,¹ Junsong Liu,¹ Shiming Liang,² Linshan Bai,¹ Yongping Dong,¹ and Mauro Epifani³

¹School of Chemistry and Chemical Engineering, Anhui University of Technology, Maanshan 243002, China
²School of Materials Science and Engineering, Linyi University, Linyi 276005, China
³Consiglio Nazionale delle Ricerche, Istituto per la Microelettronica ed i Microsistemi, Via Monteroni I-73100 Lecce, Italy

Correspondence should be addressed to Xiangfeng Chu; xfchu99@ahut.edu.cn and Shiming Liang; lsmwind@163.com

Received 23 May 2019; Accepted 30 August 2019; Published 4 December 2019

Academic Editor: Bruno C. Janegitz

Copyright © 2019 Xiangfeng Chu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

In this paper, g-C₃N₄-WO₃ composite materials were prepared by hydrothermal processing. The composites were characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and N₂ adsorption-desorption, respectively. The gas sensing properties of the composites were investigated. The results indicated that the addition of appropriate amount of g-C₃N₄ to WO₃ could improve the response and selectivity to acetone. The sensor based on 2 wt% g-C₃N₄-WO₃ composite showed the best gas sensing performances. When operating at optimum temperature of 310°C, the responses to 1000 ppm and 0.5 ppm acetone were 58.2 and 1.6, respectively, and the ratio of the 1000 ppm acetone to 1000 ppm ethanol reached 3.7.

1. Introduction

Graphitic carbon nitride (g-C₃N₄) nanomaterial exhibits a stable layered structure and п-conjugated s-triazine unit composed of sp² hybridized carbon atoms and sp² hybridized nitrogen atom. g-C₃N₄ nanosheets have attracted the attention of researchers in recent years for its peculiar properties as a semiconductor such as immense specific surface area [1]. Wang et al. [2] prepared g-C₃N₄ by thermal treatment of glucose and urea, and the p-type sensor based on g-C₃N₄ exhibited good response to NO₂ at room temperature.

As a gas sensing material, WO₃ has been paid much attention in the past decade. Cho et al. [3] used ultrasonic spray pyrolysis to prepare WO₃ hollow spheres using a citric acid-containing precursor solution; the WO₃ hollow spheres exhibited a high response and good gas sensing selectivity to trimethylamine, but the sensor exhibited a depressed response to NO₂. Kida et al. [4] used acidification of Na₂WO₄ with H₂SO₄ solution to prepare lamellar-structured WO₃ particles which had a high response (–280) even to dilute NO₂ (50–1000 ppb) in air at 200°C. A study by Ma et al. [5] showed that WO₃ nanoplates obtained through a topochemical transformation of the corresponding H₂WO₄ precursor exhibited high response to ethanol while operating at 300°C.

The photocatalytic activity of g-C₃N₄-WO₃ nanocomposites also has been reported by many researchers [6–9]; the photocatalytic performances of WO₃/g-C₃N₄ nanocomposites were higher than those of pure WO₃ and pure g-C₃N₄, which were attributed to the synergistic effect of WO₃ and pure g-C₃N₄. A study by Zhang et al. [10] showed that the ethanol sensing performance of α-Fe₂O₃/g-C₃N₄ nanocomposites was better than that of pure α-Fe₂O₃ and g-C₃N₄, which could be caused by porous α-Fe₂O₃ nanotubes wrapped by lamellar g-C₃N₄ nanostructures resulting in the formation of heterojunction. Cao et al. [11] reported that the gas sensing response and selectivity to ethanol could also be enhanced by modification of g-C₃N₄ nanosheets. The combination of WO₃ and g-C₃N₄ may exhibit good gas sensing properties, which to the best of our knowledge has not been reported to date.

In this paper, we report the preparation of g-C₃N₄-WO₃ nanocomposites through a hydrothermal method and the investigation of their gas sensing properties. Analysis showed that 2 wt% g-C₃N₄-WO₃ nanocomposite responded highly and selectively to acetone.

2. Materials and Methods

2.1. Material Preparation and Characterization

g-C₃N₄ was prepared by heating 2.0 g melamine in an oven at 520°C for 5 hours, while keeping the heating rate at 5°C/min, which was similar to that reported in the literature [12]. After cooling to room temperature naturally, the product was purified with ethanol, following which it was dried at 60°C for 24 h succeeded by milling.

For preparing the nanocomposites, a certain amount of as-prepared g-C₃N₄ was added to 40 mL deionized water and sonicated for 1 hour to obtain a g-C₃N₄ suspension. 0.0025 mol Na₂WO₄·2H₂O was dissolved in 20 mL deionized water, and 4 mL concentrated hydrochloric acid was added dropwise in the Na₂WO₄ solution slowly while stirring resulting in the formation of H₂WO₄; the g-C₃N₄ suspension was added slowly to H₂WO₄ while stirring. The mixture was sealed in a 100 mL Teflon-lined stainless steel autoclave and heated at 200°C for 24 h; the obtained precipitate was filtered and washed with distilled water and ethanol, followed by drying in air at 80°C for 24 hours; finally, the g-C₃N₄-WO₃ composite was obtained. The weight ratios of g-C₃N₄ powders/WO₃ (the weight of WO₃ was calculated according to the weight of Na₂WO₄·2H₂O) were 0 wt%, 1 wt%, 2 wt%, 3 wt%, and 4 wt% (the samples were labeled as S-0, S-1, S-2, S-3, and S-4, respectively).

X-ray diffraction (XRD, Bruker D8 Advance, Cu-Kα radiation:  nm), operating at 40 kV and 30 mA in a range from 10° to 70° at room temperature, was used to analyze the crystal structure of g-C₃N₄-WO₃ nanocomposites. A scanning electron microscope (SEM, Hitachi S-4800 microscope), with an accelerating voltage of 10 kV, was used to characterize the surface morphology of the samples. Fourier transform infrared spectroscopy (FTIR, Nicolet 6700 FTIR Spectrometer) spectra were recorded by the KBr pellet technique in the range 400–4000 cm⁻¹. The chemical species of elements were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250Xi) with all of the binding energies corrected according to contaminant carbon (). The specific surface areas were characterized by the Brunauer-Emmett-Teller (BET, ASAP2010C) method using N₂ adsorption-desorption measurement.

2.2. Gas Sensing Measurement

The sensor device preparation process and the gas sensing measurement have been explained in the previous work [13]. The gas sensing response of the gas sensor was defined through the ratio of the resistance of the gas sensor in air () to that in the test gases (). Figure S1, representing the gas sensor, is shown in supplementary materials.

3. Results and Discussion

3.1. Characterization

Figure 1 shows the XRD patterns of pure g-C₃N₄, g-C₃N₄-WO₃ (S-1, S-2, S-3, and S-4), and WO₃. Two significant diffraction peaks were observed at 13.23° and 27.86° in the XRD pattern of g-C₃N₄, indicating the (100) and (002) planes of layered g-C₃N₄; the weaker peak at 13.23° indicates the in-planar tris-s-triazine structural packing, and the stronger peak at 27.86° corresponds to the stacked of the aromatic systems between layers [6–8, 14]. All the diffraction peaks observed in XRD patterns of S-0, S-1, S-2, S-3, and S-4 could be indexed to the standard data of WO₃ (JCPDS 43-1035). The diffraction peaks of g-C₃N₄ could not be found because of the low concentration of g-C₃N₄ in the composites [14]. It was reported that the diffraction peaks of g-C₃N₄ did not appear in the XRD patterns of g-C₃N₄-WO₃ composites when the content of WO₃ was higher than 10 wt% [15]. The average crystallite sizes were calculated by the Scherrer formula:

where is the crystallite size, is the instrument correction factor, is the Cu-Kα wavelength (0.151418 nm), is the full width at half maximum of the peaks, and is the position of the peaks. The average crystallite sizes of WO₃ in S-0, S-1, S-2, S-3, and S-4 were 54.9, 62.6, 44.4, 44.3, and 50.2 nm, respectively. The calculated particle size of g-C₃N₄ was about 5.7 nm.

Figure 1: The XRD patterns of pure g-C₃N₄, g-C₃N₄-WO₃ (S-1, S-2, S-3, and S-4), and WO₃.

Figure 2 exhibits the SEM images of g-C₃N₄, pure WO₃ (S-0), and g-C₃N₄-WO₃ (S-2). The morphology of g-C₃N₄ was a sheet that consisted of small particles, and the particle sizes were about 200 nm. The WO₃ particle sizes in WO₃ and g-C₃N₄-WO₃ were between 100 and 400 nm; the addition of g-C₃N₄ in the composite had no obvious influence on the particle size of WO₃. The particle sizes obtained from SEM images were larger than those calculated from the Scherrer formula, which manifested that the particles observed by SEM in g-C₃N₄, pure WO₃, and g-C₃N₄-WO₃ were aggregates of smaller particles.

(a)

(b)

(c)

Figure 2: The SEM images of (a) g-C₃N₄, (b) pure WO₃ (S-0), and (c) g-C₃N₄-WO₃ (S-2).

The FTIR spectra of WO₃ and g-C₃N₄-WO₃ (S-2) are shown in Figure 3. The absorption peaks at 763, 822, and 935 cm⁻¹ in the spectra of two samples originated from stretching vibrations of O-W-O in WO₃ [9, 15, 16]; the peaks at 1632 and 3436 cm⁻¹ in the FTIR spectra of WO₃ and g-C₃N₄-WO₃ (S-2) resulted from the vibration of bended H-O-H and stretched O-H of absorbed H₂O on the material surface [17]; and absorption peaks in the FTIR spectrum of g-C₃N₄-WO₃ (S-2), other than those of WO₃, at 1244, 1321, 1411, 1567, and 1632 cm⁻¹ were ascribed to stretching vibration of C-NH-C and C=N of heterocycles [15], which proved the existence of g-C₃N₄ in the composite. The XPS results were discussed in the supplementary materials, which could prove the formation of g-C₃N₄-WO₃.

Figure 3: The FTIR spectra of WO₃ and g-C₃N₄-WO₃ (S-2).

3.2. Gas Sensing Characterization

The gas sensing responses of pure WO₃, S-1, S-2, S-3, and S-4 to 1000 ppm concentration of acetone at different operating temperatures are shown in Figure 4. An increase in the response with an increase in the amount of g-C₃N₄ in the g-C₃N₄-WO₃ composite was observed while the content of g-C₃N₄ was lower than 2 wt%; when the contents of g-C₃N₄ in the g-C₃N₄-WO₃ composite were 3 wt% and 4 wt%, the responses decreased significantly; the optimal operating temperatures for S-1 and S-2 were all 310°C, which were better than that for pure WO₃. The responses of pure WO₃, S-1, S-2, S-3, and S-4 to 1000 ppm acetone at 310°C were 3.7, 16.2, 58.2, 1.0, and 1.3, respectively. It has been reported that pure g-C₃N₄ exhibited very little response to acetone; the response improved significantly for a particular content of g-C₃N₄ in a series of g-C₃N₄-SnO₂ nanocomposites [18]. The response of α-Fe₂O₃/g-C₃N₄ to ethanol was also reported [11]; an appropriate amount of g-C₃N₄ in the composites was propitious to the dispersion of α-Fe₂O₃ in the composites and the formation of better heterojunctions; the reasons for the enhancement of gas sensing response were attributed to the larger specific surface area, better permeability, and heterojunction. A sensor based on the S-2 composite exhibited higher response to acetone compared with pure WO₃. The N₂ adsorption-desorption results are shown in Figure S3 of supplementary materials; the average pore size difference between S-0 and S-2 was not obvious, and the gas diffusion rates in the inner sections of S-0 and S-2 were approximate; the enhancement of gas sensing response of S-2 was probably attributable to the larger specific surface area of S-2 and heterojunction. Many literatures have reported the acetone sensing mechanism of WO₃ [19, 20], and the conductance of the WO₃ sensor is influenced by the changes in chemisorbed oxygen present on the surface of the gas sensing material; on exposure of a sensor to air, oxygen is adsorbed on the surface of WO₃, which in turn captured electrons from the conduction band of WO₃, resulting in the decrease in electron concentration in the conduction band; oxygen molecules changes into O₂⁻(ads), O⁻(ads), and O²⁻(ads) with variation of temperature. When the sensor was placed in an acetone vapor atmosphere, acetone reacted with O₂⁻(ads), O⁻(ads), and O²⁻(ads), releasing the electrons captured by oxygen molecules to the conduction band of WO₃ and decreasing the resistance of the sensor. The reaction is as follows:

Figure 4: The gas sensing responses of pure WO₃, S-1, S-2, S-3, and S-4 to 1000 ppm acetone at different temperatures.

Figure 5 depicts the responses of an S-2-based sensor to 1000 ppm acetic acid, acetone, formaldehyde, ethanol, acetaldehyde, and ammonia at different temperatures. The optimal operating temperatures for formaldehyde, ethanol, and acetaldehyde were 360°C, 240°C, and 360°C, respectively; the maximum responses for acetic acid, acetone, and ammonia all appeared at 310°C. At an operating temperature of 310°C, the responses to 1000 ppm acetic acid, acetone, formaldehyde, ethanol, acetaldehyde, and ammonia were 8.9, 58.2, 6.0, 15.4, 4.3, and 6.0, respectively; the S-2 sensor showed significant gas sensing selectivity to acetone, with the response ratio of 1000 ppm acetone/1000 ppm ethanol reaching 3.8.

Figure 5: The responses of an S-2-based sensor to 1000 ppm acetic acid, acetone, formaldehyde, ethanol, acetaldehyde, and ammonia at different operating temperatures.

Figure 6 shows the responses of S-0- and S-2-based sensors to six kinds of gases, while keeping the concentration 1000 ppm, at 310°C. The responses of the S-2 sensor to acetaldehyde, ethanol, and formaldehyde were lower than those of the pure WO₃ sensor. But the responses of the S-2 sensor to ammonia, acetone, and acetic acid were higher than those of the pure WO₃ gas sensor; especially, the response of the S-2 sensor to acetone was 15 times that of the pure WO₃ gas sensor, proving the role of g-C₃N₄ in improving the selectivity of the gas sensor.

Figure 6: The responses of sensors based on S-0 and S-2 to six kinds of gases (1000 ppm) at 310°C.

The response time and recovery time were calculated using the formula defined in a previous literature [21]. The response curve transients of the sensor based on the sample S-2 composite to acetone (1000 ppm, 500 ppm, 100 ppm, 10 ppm, 1 ppm, and 0.5 ppm) at 310°C are shown in Figure 7. The responses to 1000 ppm, 500 ppm, 100 ppm, 10 ppm, 1 ppm, and 0.5 ppm acetone were 58.2, 36.6, 17.2, 3.0, 1.8, and 1.6, respectively; the detection limit of the S-2 composite-based sensor to acetone was 0.5 ppm. The response times for 1000, 500, 100, 10, 1, and 0.5 ppm acetone were 53, 24, 10, 15, 7, and 5 s, respectively, while the recovery times for 1000, 500, 100, 10, 1, and 0.5 ppm acetone were 29, 25, 6, 7, 12, and 3 s, respectively. The acetone concentration in the breath varies from 0.3 to 0.9 ppm for healthy people, but the acetone concentration exceeds 1.8 ppm for diabetic patients [22]. The S-2 composite-based sensor had a response of 1.6 to 0.5 ppm acetone which meant that it has the potential for application in diabetes detection.

Figure 7: The response transients of the sensor based on the sample S-2 composite to acetone (1000 ppm, 500 ppm, 100 ppm, 10 ppm, 1 ppm, and 0.5 ppm) at 310°C.

The gas sensor stability is a significant parameter for a gas sensor, and the curve of gas sensing response versus time of the S-2 composite-based sensor is shown in Figure 8. The gas sensing response decreased significantly in seven days and then achieved stability between the seventh and thirtieth days. The stability of the sensor can be improved further. The g-C₃N₄-WO₃ composite (S-2) has proven itself to be a potential candidate for application as an acetone sensor, if the stability of the sensor can be improved.

Figure 8: The curve of gas sensing response versus time of the S-2 sensor.

Table 1 compares the gas sensing properties of different acetone sensors. Overall, the g-C₃N₄-WO₃ acetone gas sensor showed good sensitivity to acetone and also had significant selectivity. Besides, the detection limit is as low as 0.5 ppm which is the second lowest in the listed acetone sensor.

Table 1: Comparison of acetone gas sensing properties with different acetone gas sensors.

Material
Operating temperature (°C)
Detection limit (ppm)
Response (a/g) (concentration)
Selectivity
Stability (days)
Reference

Au/ZnO
365
20
2923 (100 ppm)
1.8
90
[23]

MgFe2-xCexO₄
700
100
1.91 (2000 ppm)
1.2
-
[24]

Co₃O₄ NWS-HCSs
150
1
23 (200 ppm)
4.6
-
[25]

PrFeO₃
180
10
141 (200 ppm)
2.8
75
[26]

Pt-Fe₂O₃
139
0.2
25.7 (100 ppm)
2.9
15
[27]

Rh- doped SnO₂
200
1
60.6 (50 ppm)
8.6
-
[28]

ZnFe₂O₄
260
10
52.8 (100 ppm)
3.2
30
[29]

g-C₃N₄-WO₃
310
0.5
58.2 (1000 ppm)
3.7
30
This work

“-” means no data available in the literature.

4. Conclusions

It can be observed that the content of g-C₃N₄ in g-C₃N₄-WO₃ composites influences the response and selectivity of g-C₃N₄-WO₃ composite-based sensors to acetone. 2 wt% g-C₃N₄-WO₃ composite (S-2) showed the best gas sensing performances in the series of g-C₃N₄-WO₃ composites, when operating at an optimum temperature of 310°C; the responses to 1000 ppm and 0.5 ppm acetone were 58.2 and 1.6, respectively, with the ratio of the 1000 ppm acetone to 1000 ppm ethanol reaching 3.7; the S-2 composite-based sensor was able to detect acetone at concentrations as low as 0.5 ppm. The sensor took 5 s and 3 s to respond to 0.5 ppm acetone and to recover; the g-C₃N₄-WO₃ composite (S-2) has proven to be a potential candidate for application as an acetone sensor if the stability of the sensor can be improved.

Data Availability

The underlying data related to this manuscript is available on request.

Conflicts of Interest

The authors declare no conflict of interest.

Acknowledgments

This research was funded by the National Natural Science Foundation of China (Nos. 61671019 and 61971003).

Supplementary Materials

Figure S1: gas sensor used for characterizing gas sensing behavior of samples. Figure S2: XPS spectra of g-C₃N₄-WO₃ (S-2): (a) full spectrum. (b) C1s. (c) N1s. (d) O1s. (e) W. Figure S3: (a) N₂ adsorption-desorption isotherm of WO₃. (b) Pore size distribution curve of WO₃. (c) N₂ adsorption-desorption isotherm of g-C₃N₄-WO₃ (S-2). (d) Pore size distribution curve of g-C₃N₄-WO₃ (S-2). (Supplementary Materials)

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