Highly Selective Gas Sensor Based on Litchi-like g-C₃N₄/In₂O₃ for Rapid Detection of H₂

Abstract

Hydrogen (H₂) has gradually become a substitute for traditional energy, but its potential danger cannot be ignored. In this study, litchi-like g-C₃N₄/In₂O₃ composites were synthesized by a hydrothermal method and used to develop H₂ sensors. The morphology characteristics and chemical composition of the samples were characterized to analyze the gas-sensing properties. Meanwhile, a series of sensors were tested to evaluate the gas-sensing performance. Among these sensors, the sensor based on the 3 wt% g-C₃N₄/In₂O₃ (the mass ratio of g-C₃N₄ to In₂O₃ is 3:100) showeds good response properties to H₂, exhibiting fast response/recovery time and excellent selectivity to H₂. The improvement in the gas-sensing performance may be related to the special morphology, the oxygen state and the g-C₃N₄/In₂O₃ heterojunction. To sum up, a sensor based on 3 wt% g-C₃N₄/In₂O₃ exhibits preeminent performance for H₂ with high sensitivity, fast response, and excellent selectivity.

1. Introduction

Hydrogen (H₂) is a potential clean energy that has rich application prospects in automobile, aerospace, and other fields [1]. However, due to its flammable and explosive characteristics, H₂ can very easily cause disaster during utilization and storage [2]. Therefore, an effective survey of H₂ has become an important problem to be solved. At present, detection methods of H₂ include gas chromatography, mass spectrometry, gas sensors, and so on [3]. In the above methods, gas sensors have become the best choice, with the advantages of low cost, simple preparation, and small size [4]. Furthermore, the type of gas sensor include semiconductor oxide [5], catalytic combustion [6], solid electrolyte [7,8], etc. Among them, a semiconductor oxide sensor is widely used in people’s lives because of the outstanding performance [9]. So far, many semiconductor oxides have been researched as sensing-materials in H₂ sensors, such as TiO₂, ZnO, SnO₂, NiO, and In₂O₃. Some reported H₂ sensors based on different semiconductor oxides are summarized in Table 1.

In₂O₃ is a common semiconductor with a band gap of 3.6 eV, which is broadly used in gas sensors [10]. Up to now, preparing In₂O₃ with distinct morphologies has proved to be an available measure to heighten the performance of gas sensors. However, a single In₂O₃-based sensor has the defects of low response and poor selectivity in practical applications [11]. Many researchers have made great efforts to enhance the gas-sensing performance, such as doping noble metals [12], fabricating heterojunctions [13], and constructing 2D materials or polymer composites [14,15]. Graphitic carbon nitride (g-C₃N₄) is a typical polymer semiconductor with a band gap of 2.76 eV, which has been used in gas sensors owing to high chemical stability, large surface area, and good catalytic function [16,17]. It is reported that In₂O₃-based sensors can elevate gas-sensing performance through the g-C₃N₄ composite. Liu et al. prepared In₂O₃, and g-C₃N₄ was compounded by way of MOF, and the response value of the sensor achieved 294 to 100 ppm NO_x at RT [18]. Sun et al. synthesized g-C₃N₄/In₂O₃ through a calcination annealing process, and the response value of the sensor reached 1405 at 119 °C to 100 ppm formaldehyde [19]. The enhanced performance of the In₂O₃ sensor can be attributed to the formation of g-C₃N₄/In₂O₃ heterojunction different oxygen species content [20].

Table 1. Gas-sensing properties of H₂ sensor made by some semiconductor oxides.

Sensing Materials	Conc. (ppm)	Res.	τres. (s)	τrec. (s)	Ref.
WO3-TiO2	10000	5.62 c	48	5	[21]
Ag/ZnO	300	479% b	175	655	[22]
Pd/SnO2	1000	1.2 a	214	51.5	[23]
Pt@NiO	5000	4.25 c	91	8	[24]
Pd-doped In2O3	100	3.6 a	4	7	[25]
3 wt% g-C3N4/In2O3	100	180% b	2	2.4	This work

Conc.: Concentration; Res.: Response; τ_res.: Response time; τ_rec.: Recovery time; Ref.: reference; ^a R = R_a/R_g; ^b R = (R_a − R_g)/R_g × 100%, ^c R = R_g/R_a.

Table 1. Gas-sensing properties of H₂ sensor made by some semiconductor oxides.

Sensing Materials	Conc. (ppm)	Res.	τres. (s)	τrec. (s)	Ref.
WO3-TiO2	10000	5.62 c	48	5	[21]
Ag/ZnO	300	479% b	175	655	[22]
Pd/SnO2	1000	1.2 a	214	51.5	[23]
Pt@NiO	5000	4.25 c	91	8	[24]
Pd-doped In2O3	100	3.6 a	4	7	[25]
3 wt% g-C3N4/In2O3	100	180% b	2	2.4	This work

Conc.: Concentration; Res.: Response; τ_res.: Response time; τ_rec.: Recovery time; Ref.: reference; ^a R = R_a/R_g; ^b R = (R_a − R_g)/R_g × 100%, ^c R = R_g/R_a.

In this paper, a litchi-like g-C₃N₄/In₂O₃ composite was successfully prepared by the hydrothermal method. Additionally, the morphology and composition of C₃N₄/In₂O₃ were characterized by XRD, SEM, TEM, and XPS. Sensors based on different amount of g-C₃N₄ composite were fabricated to investigate their gas-sensing specifics. Among them, the performance of the sensor based on 3 wt% g-C₃N₄/In₂O₃ was significantly improved for H₂, giving it the merits of a fast response and excellent selectivity.

2. Experiment

2.1. Synthesis of Peachcore-like Pure In₂O₃

All chemicals are purchased from Aladdin Reagent and are analytical grade without being further purified for use. In a typical process, 147.5 mg of InCl₃·4H₂O was dissolved in 15 mL of deionized water, then 15 mL of glycerol was added and stirred until it was homogeneous. Afterwards, 520 mg of Na₃C₆H₅O₇·2H₂O was added into the above solution and stirred vigorously for 20 min. Finally, 250 μL of NaOH (0.1 M) aqueous solution was added slowly to form a uniform solution. The obtained solution was transferred to an autoclave lined with 50 mL PTFE for a hydrothermal reaction at 190 °C for 16 h. After being cooled to room temperature, the precipitate was centrifuged with deionized water and absolute ethanol for several times. The powder was collected and dried at 80 °C overnight. The dried sample was placed in an Al₂O₃ boat and calcined in a muffle furnace at 400 °C for 2 h (2 °C/min) to obtain peachcore-like pure In₂O₃.

2.2. Synthesis of Litchi-like g-C₃N₄/In₂O₃

Firstly, 20 g of urea was added into a lidded crucible and heated in a muffle furnace at 550 °C for 2 h (2 °C/min) to prepare g-C₃N₄. Then, the prepared g-C₃N₄ was dispersed in 15 mL deionized water and sonicated for 2 h to ensure dispersion. Subsequently, 147.5 mg of InCl₃·4H₂O and 15 mL of glycerol was added the above solution in turn and stirred until homogeneous. Thereafter, 520 mg of Na₃C₆H₅O₇·2H₂O was added and stirred vigorously for 20 min. Finally, 250 μL of NaOH (0.1 M) aqueous solution was added to form a uniform solution. The subsequent process was the same as the preparation of pure In₂O₃. According to the different contents of g-C₃N₄, 1 wt% g-C₃N₄/In₂O₃, 3 wt% g-C₃N₄/In₂O₃, and 5 wt% g-C₃N₄/In₂O₃ composites were prepared, respectively.

2.3. Material Characterization

The crystal structures of as-samples were determined by an X-ray diffractometer (XRD, Rigaku Miniflex 600 X, Cu Kα1 radiation, λ = 1.5406 Å) operated at 40 kV and 15 mA. The morphology characteristics of samples were characterized by a scanning electron microscope (SEM, PHENOM SCIENTIFIC ProX G5, The Netherlands) and a transmission electron microscope (TEM, FEI Tecnai G2 F30, USA). X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi, USA) was used for the chemical composition analysis of samples.

2.4. Fabrication and Measurement of Gas Sensors

The device structure of the gas sensor is shown in Figure 1. As-prepared samples were mixed with deionized water to form a uniform slurry and coated on an alumina ceramic tube as the sensing layer. The coated sensing layer was baked under an infrared lamp for 15 min. Then, the ceramic tube was calcined at 300 °C for 1 h (2 °C/min). Finally, the heating wire was passed through the ceramic tube and welded to a six-legged base to make a gas sensor. In addition, the components of the test gas are the target gas and the component gas, wherein the standard value of the target gas is 1% mol/mol and that of the component gas is nitrogen.

The evaluation of gas-sensing performances was carried out in a static test system (50% RH, 25 °C), as shown in Figure 2. The heating current of the sensor was provided by the DC-regulated power supply, and the resistance value was recorded by the multimeter. The response value is defined as R = (R_a − R_g)/R_g × 100%, wherein R_a and R_g are the resistance value of the sensor respectively exposed to clean air and target gas. Additionally, the response/recovery time is defined as the time for 90% of the resistance change.

3. Results and Discussion

3.1. Characterization of Material Structure

The crystal phase of the as-prepared samples was obtained by XRD, as shown in Figure 3. The g-C₃N₄ has two peaks at 2θ angles of 13.36° and 27.38°, which are related to the tris-triazine units and aromatic systems, respectively [26]. The diffraction peaks of pure In₂O₃ at 2θ angles of 22.37°, 30.99°, 32.61°, 45.61°, 50.25°, 57.20° and 58.19° are index to the crystal planes (012), (104), (110), (024), (116), (214) and (300) of In₂O₃ (JCPDS 22-0366). However, the diffraction peaks of g-C₃N₄ are not obviously observed in the XRD pictures of g-C₃N₄/In₂O₃ composites, probably due to the low content of g-C₃N₄ [27].

The SEM images of pure In₂O₃ and 3 wt% g-C₃N₄/In₂O₃ are shown in Figure 4. Pure In₂O₃ exhibits a peachcore-like structure with a diameter of about 250 nm in Figure 4a,b. The morphology of 3 wt% g-C₃N₄/In₂O₃ possesses a distinctive litchi-like structure, and the diameter of 3 wt% g-C₃N₄/In₂O₃ is only 130 nm, as shown in Figure 4c,d. Apparently, the morphology of In₂O₃ was changed from peachcore-like to litchi-like, and the diameter of 3 wt% g-C₃N₄/In₂O₃ is smaller about 100 nm than that of pure In₂O₃. Moreover, the size of 3 wt% g-C₃N₄/In₂O₃ is more uniform than that of pure In₂O₃. The crystallite sizes of as-prepared samples were calculated by the Scherrer formula:

$$D=\frac{K\lambda }{\beta cos\theta }$$

(1)

where K is Scherrer constant of 0.9, $\lambda$ is the X-ray wavelength of 0.15406 nm, $\beta$ is the half-width of the diffraction peak, and $\theta$ is the Bragg diffraction angle. The average size of pure In₂O₃, 1 wt% g-C₃N₄/In₂O₃, 3 wt% g-C₃N₄/In₂O₃, and 3 wt% g-C₃N₄/In₂O₃ is about 15 nm, 14.4 nm, 13.9 nm and 12.6 nm, respectively. Based on the above results, it is inferred that the g-C₃N₄ may inhibit the growth of In₂O₃ crystal, thus leading to morphological changes [28].

The microstructure of 3 wt% g-C₃N₄/In₂O₃ was characterized by TEM and HRTEM, as shown in Figure 5. Through TEM in Figure 5a, we can clearly see that the 3 wt% g-C₃N₄/In₂O₃ presents a litchi-like structure with uniform size. Furthermore, HRTEM in Figure 5b is manifested by the different lattice spacings of In₂O₃, which plainly indicates In₂O₃ with a highly crystalline form. The three lattice spacings are 0.12 nm, 0.18 nm and 0.17 nm, which are assigned to the (012), (110) and (104) planes of In₂O₃, respectively. In addition, the element mapping is shown in Figure 5c–h. From element mapping, it can be explicitly noticed that the In, O, C, and N elements are evenly distributed; this can be evidence that the 3 wt% g-C₃N₄/In₂O₃ composite was successfully prepared.

To analyze the chemical composition of pure In₂O₃ and 3 wt% g-C₃N₄/In₂O₃, the XPS is shown in Figure 6. As shown in Figure 6a, the In 3d of pure In₂O₃ and 3 wt% g-C₃N₄/In₂O₃ have two strong peaks at 451.6 eV, 444.1 eV and 451.7 eV, 444.2 eV, which correspond to In 3d_5/2 and In 3d_3/2, respectively [29]. The O 1s peak spectrums in Figure 6b are decomposed into three fitting peaks around 532.1 eV, 531.2 eV, 529.5 eV and 532.2 eV, 531.2 eV, 529.6 eV, wherein the fitting peaks are assigned to the hydroxyl (OH) or chemisorbed oxygen (O_C), oxygen vacancy (O_V), and lattice oxygen (O_L) [30,31]. Furthermore, the O_C, O_V, and O_L content of pure In₂O₃ and 3 wt% g-C₃N₄/In₂O₃ are about 8.56%, 23.59%, 67.85% and 9.14%, 33.42%, 57.44%, respectively. It is worth noting that the content of O_C and O_V increases with the introduction of g-C₃N₄. It may be one of the reasons why the gas-sensing performance of 3wt% g-C₃N₄/In₂O₃-based sensors has improved [32,33]. The C 1s spectrum displays three peaks at 288.3 eV, 286.3 eV, and 284.8 eV, as shown in Figure 6c. These three peaks of C 1s respectively belong to the sp^2- bonded carbon (N–C=N) and the sp^3- fitted carbon bond from surface defects of g-C₃N₄ and carbon atoms (C-C) [34]. However, the N 1s peak appears to be a weak peak at 398.8eV, as shown in Figure 6d, which may be caused by the low content of g-C₃N₄ [35].

3.2. Gas-Sensing Properties

To determine the optimal operating temperature of the sensors, the different sensors based on g-C₃N₄/In₂O₃ composites with different proportions were evaluated from 225 °C to 300 °C, as shown in Figure 7. When the operating temperature reaches 275 °C, the response value is 180% to 100 ppm H₂ of 3 wt% g-C₃N₄/In₂O₃ sensor in Figure 7a, which is 3.5 times that of pure In₂O₃ sensor. The operating temperature can be explained according to the desorption equilibrium of the gas molecules and the chemical reaction kinetics. When the working temperature is too low, the gas molecules do not have enough heat energy and kinetic energy to react on the In₂O₃ surface, so the adsorption capacity of the gas is reduced. However, when the operating temperature is too high, the gas molecules adsorbed on the In₂O₃ surface will have a high activity and escape before the electron carrier transfer, resulting in a reduced response [3,36]. In addition, the response values of different sensors were tested with different concentrations of H₂ (10~1000 ppm) at 275 °C. As exhibited in Figure 7b, the response value of the 3 wt% g-C₃N₄/In₂O₃ sensor is greatly improved compared with other sensors within the whole range of H₂ concentrations. With the increase in the H₂ concentration, the rising trend of the response value curve is gradually stable, which may indicate that the sensor is gradually saturated.

The response/recovery time of the different sensors at 275 °C is shown in Figure 8a–d. It is not hard to notice that the increase of g-C₃N₄ content, which may have a positive action in the sensor. Although g-C₃N₄ is beneficial to the response and recovery characteristics of the sensor, excessive g-C₃N₄ may play a reverse role. Furthermore, the 3 wt% g-C₃N₄/In₂O₃ sensor exhibits fast response/recovery time (2 s/2.4 s) to 100 ppm H₂. From Figure 8e, even if it is in a high concentration H₂ atmosphere, the 3 wt% g-C₃N₄/In₂O₃ sensor also demonstrated extremely fast response/recovery time. The above results reveal that the 3 wt% g-C₃N₄/In₂O₃ sensor has great potential in practical application. The R_a of different sensors at different temperatures are shown in Figure 8f; the R_a decrease with the increase in temperature. In addition, the R_a of sensors based on g-C₃N₄/In₂O₃ is lower than that of the pure In₂O₃ and decreased with the increasing amount of g-C₃N₄ in the composites. The reason for the increase in conductivity is firstly due to the increase in oxygen vacancy in g-C₃N₄/In₂O₃. The second reason is attributed to the C–N bond breaking, which releases a lot of electrons [37]. Based on the above reasons, it can be proven that the sensor based on g-C₃N₄/In₂O₃ has high conductivity.

Stability and reproducibility are important indicators of a sensor’s performance, and the evaluation result of the 3 wt% g-C₃N₄/In₂O₃ sensor at 275 °C is shown in Figure 9. As shown in Figure 9a, the dynamic curves of the 3 wt% g-C₃N₄/In₂O₃ sensor is exhibited. Within the variation range of H₂ concentrations, the response and recovery characteristics of the 3 wt% g-C₃N₄/In₂O₃ sensor are stable. Meanwhile, the detection limit was tested to 1 ppm H₂, and the response value is 10%. Reproducibility was evaluated by continuous exposure to 50 ppm H₂ for five cycles, as shown in Figure 9b. In the five-cycle experiment, the response/recovery time and response value were basically stable without significant change. By assessing the gas-sensing properties of the 3 wt% g-C₃N₄/In₂O₃ sensor, it was sufficienly proven that the sensor has fine stability and reproducibility.

Selectivity is a momentous parameter for the practical application of the sensor. Various gases were tested at 275 °C for the appraisement of different sensors, as presented in Figure 10. Among them, 1 wt% g-C₃N₄/In₂O₃ and 3 wt% g-C₃N₄/In₂O₃ sensors exhibit similar selectivity characteristics, which possess a higher response value to 50 ppm H₂ than other gases (50 ppm SO₂, 50 ppm NH₃, 50 ppm CO₂, 50 ppm CO, 50 ppm CH₄, 100 ppm NO₂). Besides, the response value of the 3 wt% g-C₃N₄/In₂O₃ sensor is three times that of the 1 wt% g-C₃N₄/In₂O₃ sensor to 50 ppm H₂. Meanwhile, the response value of the 3 wt% g-C₃N₄/In₂O₃ sensor to 50 ppm H₂ is 7.5 times that of 50 ppm SO₂ (the secondary response gas). As we all know, CO, CH₄, and H₂ are common fuel gases, and the response values of the 3 wt% g-C₃N₄/In₂O₃ sensor to H₂ are 20 times and 40 times that of CO and CH₄ of 50 ppm, respectively. The improvement of H₂ selectivity is due to the better dispersion of litchi-like 3wt% g-C₃N₄/In₂O₃ compared with pure In₂O₃, which makes H₂ with the smallest molecular size easy to diffuse [38]. In addition, at 275 °C for the sensor based on the 3 wt% g-C₃N₄/In₂O₃ surface, the adsorption energy of H₂ gas is much higher than that of other gases [22]. From the above results, the 3 wt% g-C₃N₄/In₂O₃ sensor exhibits excellent selectivity in H₂ detection.

The long-term stability is shown in Figure 11, which was evaluated by continuous exposure to 50 ppm H₂ at 275 °C within 2 weeks. It is distinctly perceived from Figure 11a that the response value to 50 ppm H₂ is not significantly changed. Moreover, the transient curves for different days are shown in Figure 11b–d, and it is noteworthy that the air resistance (R_a) and response features of the sensor are basically stable. In view of the above assessment, it can be assured that the 3 wt% g-C₃N₄/In₂O₃ sensor has good long-term stability for H₂ detection.

3.3. Gas Sensing Mechanism

In order to understand the enhanced properties of the H₂ sensor based on g-C₃N₄/In₂O₃, the gas-sensing mechanism of the In₂O₃ sensor was analyzed. In air at different temperatures, the oxygen molecules on the surface of In₂O₃ materials will become O₂^- (<147 °C), O⁻ (147 °C–397 °C), O^2- (>397 °C), as follows in Equations (2)–(5) [29].

O₂ → O_2(ads)

(2)

O_2(ads) + e⁻ → O₂⁻_(ads)

(3)

O₂⁻_(ads) + e⁻ → 2O⁻_(ads)

(4)

O⁻_(ads) + e⁻ → O²⁻_(ads)

(5)

2 H + O⁻_(ads) → H₂O + e⁻

(6)

In this paper, the optimum operating temperature was determined to be 275°C, so most of the oxygen molecules are converted into O⁻. When the In₂O₃ sensor exposure to H₂, the H molecules are oxidized by O⁻ to form H₂O and free electrons as in Equation (6) [22]. The reaction process is shown in Figure 12a. The elevated gas-sensing performance of sensor based on 3 wt% g-C₃N₄/In₂O₃ may be attributed to the morphology characteristic, the oxygen state and the g-C₃N₄/In₂O₃ heterojunctions. The first reason is the distinctive litchi-like morphology of 3 wt% g-C₃N₄/In₂O₃. Through observing SEM and TEM photos, the 3 wt% g-C₃N₄/In₂O₃ has smaller size and better dispersion than pure In₂O₃. The performance of the sensor based on 3 wt% g-C₃N₄/In₂O₃ may be improved due to the unique morphology configuration.

The second reason can be attributed to the adjustment of the oxygen state. The XPS results show that the O_V and O_C contents in 3 wt% g-C₃N₄/In₂O₃ are higher than pure in In₂O₃. For the sensor based on 3 wt% g-C₃N₄/In₂O₃, a large amount of O_V provides more active sites for the adsorption of active oxygen, and the increase in O_C content indicates that more chemically adsorbed oxygen participates in the redox reaction [19]. Therefore the performance of the 3 wt% g-C₃N₄/In₂O₃ sensor may be improved. The last reason is the formation of the g-C₃N₄/In₂O₃ heterojunction [19]. The work functions and band gaps of g-C₃N₄ and In₂O₃ are W = 4.3 [39], E_g = 2.76 eV [16] and W = 5.0 [13], E_g = 3.6 eV [10], respectively, as exhibited in Figure 12b. The electrons flow from g-C₃N₄ to In₂O₃ to the new equilibrium of the Fermi level. Therefore, the sensor based on 3 wt% g-C₃N₄/In₂O₃ shows excellent response characteristics [40].

4. Conclusions

In this work, the g-C₃N₄/In₂O₃ composite was prepared by a hydrothermal method. The morphology features and chemical compositions of g-C₃N₄/In₂O₃ were characterized by XRD, SEM, TEM and XPS. The sensor based on 3 wt% g-C₃N₄/In₂O₃ perform excellent gas-sensing behavior. The response value of 3 wt% g-C₃N₄/In₂O₃ was 180% to 100 ppm H₂ at 275 °C, which is 3.5 times higher than that of the pure In₂O₃ sensor. Furthermore, the 3 wt% g-C₃N₄/In₂O₃ sensor exhibits fast response/recovery time (2 s/2.4 s) to 100 ppm H₂ and excellent selectivity (R_{50 ppm H2}/R_{50 ppm CH4} = 40, R_{50 ppm H2}/R_{50 ppm CO} = 20). The improvement of sensor performance based on 3 wt% g-C₃N₄/In₂O₃ can be attributed to the special morphology characteristic, the state of oxygen, and the g-C₃N₄/In₂O₃ heterojunctions. In conclusion, this work provides a useful composite for preparing highly efficient H₂ sensors and proves that this composite has certain application value.