Volatile organic compounds (VOCs) can pollute the environment and harm human health. Acetone is one of VOCs and one of the gas in human respiration. It is used as a respiratory marker for non-invasive diagnosis of diabetes mellitus. Therefore, the detection of acetone is important for environmental safety and human health. In recent decades, many metal oxides (ZnO, In₂O₃, Fe₂O₃, SnO₂, MnO₂, Co₃O₄) have been used in sensitive studies of acetone [1] [2] [3]. Among them, WO₃ crystal, as a typical n-semiconductor, has the advantages of high cost-effectiveness, low pollution and good stability. Many experimental and theoretical results prove that WO₃ has great potential to detect VOCs in the field of gas sensing [4] [5] [6] [7] [8].

Graphene oxide (GO) with different oxygen-containing groups such as carboxylndndnd, amino, carbonyl and other functions is extracted by gas. More adsorption sites are provided, thereby improving gas sensor sensitivity. Oxygen-containing groups in graphene oxide make it an insulating material. Graphene oxide is not a suitable gas-sensitive material because it is difficult to control the content of these groups during oxidation [9] [10] [11]. In this chapter, the effect of carboxyl (-COOH) functionalized carboxy graphene (GCOOH) on the sensing performance of WO₃ sensor is studied. Nano-composite materials with different GCOOH content, named GCOOH-WO₃, were prepared by hydrothermal method. The structure of the prepared GCOOH-WO₃ was characterized, and its gas sensitivity was tested and analyzed [12] - [19]. Therefore, this paper studies the influence of the guiding agent and graphene on the hexagonal WO₃, in order to optimize the sensing performance of the hexagonal WO₃ gas sensor, improve the response value of the hexagonal WO₃ sensor to acetone gas, reduce the operating temperature, and shorten the response and recovery time.

Tungsten oxide GCOOH-WO₃ was synthesized by simple hydrothermal method. The preparation process is as follows: three different amounts of carboxyl-functionalized graphene were extracted from carboxyl-functionalized graphene GCOOH and dripped into deionized water for ultrasonic dispersion. Dissolve a suitable amount of sodium tungstate dihydrate in the above solution and stir for 30 minutes. Drop 6 mol·L⁻¹ hydrochloric acid into the solution at a constant speed. Stir side-dropping until the pH value reaches 1.0 to 2.0. Set aside for 1 hour. Add ammonium acetate and oxalic acid to make the quality of GCOOH and WO₃ 0.05 wt%, 0.1 wt%, 0.2 wt%, respectively. Stir the configured solution on a magnetic stirrer. After 3 hours of vigorous stirring, the solution turns dark blue. GCOOH-WO₃-1, GCOOH-WO₃-2, and GCOOH-WO₃-3, respectively. The milky white column powders, namely GCOOH-WO₃-1, GCOOH-WO₃-2, GCOOH-WO₃-3, were obtained by hydrothermal treatment, centrifugation and drying at 180˚C for 24 h.

Figure 1 shows the morphology of GCOOH-WO₃-2 composite. According to Figure 1(a) and Figure 1(b), it can be found that the columnar WO₃ is covered with graphene in flakes and agglomerates, covering the material surface, preventing the reaction with the gas to be measured, reducing the surface active sites, and affecting the reaction with the gas to be measured.

Figure 2 shows the XRD pattern of GCOOH-WO₃ composite. The main characteristic peaks are (100), (001), (110), (101), (200), (201), (220), (204). Most of the diffraction peaks of GCOOH-WO₃ can be well indexed to the XRD spectra of hexagonal tungsten trioxide WO₃ (JCPDS:00-033-1387) GCOOH-WO₃, which

shows that it has good crystallinity. The graphene phase cannot be detected by XRD because the modification of WO₃ nanocrystals causes the flakes of graphene to fall off.

As shown in Figure 3, GCOOH-WO₃-2 composite can detect clear peaks at 265 cm⁻¹, 704 cm⁻¹ and 803 cm⁻¹. The Raman peak at 265 cm⁻¹ is the stretching vibration peak of O-W-O; We can also observe two peaks at 1369 and 1620 in GCOOH-WO₃-2 complex, which are the two characteristic peaks of graphene, D and G.

Figure 4(a) shows the full spectrum of GCOOH-WO₃-2. The characteristic peaks of C1s, O1s and W4f can be observed from the figure, which proves the existence of C, O and W. Figure 4(b) shows the full spectrum of W. W4f_7/2 and W4f_5/2 represent the non-stoichiometric tungsten oxide composition at the binding energy of 35.9 eV and 54.0 eV, respectively, which further proves the formation of WO₃.

Figure 5 shows that the temperature test range is 225˚C - 325˚C, and the sensitivity of all sensors increases first and then decreases with the increase of temperature. GCOOH-WO₃-2 gas sensor has the highest sensitivity of 5.5 at 260˚C; The working state of the sensor at lower than 225˚C is not shown in the figure,

because when a certain amount of GCOOH is added, the resistance of the composite material will rise, exceeding the instrument range, and can not be measured accurately at present.

The mechanism of GCOOH-WO₃ on acetone can be described as follows. When exposed to air, oxygen gets adsorbed on GCOOH-WO₃ surface, and O 2 − , O⁻ or O²⁻ ions will form by obtaining electrons from the conduction band, then resulting in the increased resistance. The related reactions could be expressed as Equation (1):

O 2 ( gas ) ↔ O 2 ( adsorbed ) + e − ↔ O 2 − + e − ↔ 2 O − (1)

CH 3 COCH 3 + 8 O − ( ads ) → 3 CO 2 + 3 H 2 O + 8 e − (2)

CH 3 COCH 3 + 6 O 2 − ( ads ) → 3 CO 2 + 3 H 2 O + 12 e − (3)

The introduction of acetone on GCOOH-WO₃ surface will trigger the reaction with adsorbed oxygen, and meanwhile, electrons are released back to conduction band, which causing the decrease in the resistance of GCOOH-WO₃. The relevant reaction can be proposed as Equations (2)-(3). As the products (CO₂ and H₂O) desorbed from the surface, the gas sensor recovers to the initial condition. Thus, one cycle of the acetone detection is over and the sensor is ready for the next cycle (Figure 6).

In this study, GCOOH-WO₃ with different mass fractions of GCOOH was prepared by hydrothermal method. Many round graphene sheets are attached to the nanorods. GCOOH-WO₃ only responds to acetone gas. The sensing performance depends on the mass fraction of GCOOH added. It is found that only a proper amount of GCOOH can have good sensing performance.

The authors declare no conflicts of interest.

Guo, Y.X. and Zhang, H.M. (2023) Hexagonal WO₃ Acetone Gas Sensor Modified by Graphene-COOH Functionalized (GCOOH). Open Access Library Journal, 10: e9971. https://doi.org/10.4236/oalib.1109971