1 Introduction

The electronic-nose for sensing toxic and inflammable gases has become very necessary due to the wide usage of these gases [1]. In the current global economy, hydrogen gas (H2) has become a main subject for the development of new sustainability due to its being considered a more efficient and clean energy source. As a result, it is used instead of a dependence on oil in automobiles, fuel cells, aircraft, and chemical industries applications [2, 3].

In the past decades, different kinds of gas sensors have been developed to detect H2 gas at different sensing temperatures [4]. A solid-state gas sensor based on metal oxide wide band gap semiconductor materials emerged with high robustness and reliability for gas detection. Tin dioxide SnO2 is the most interesting metal oxide semiconductor due to its large energy gap (3.6 eV) at 300 K [5]. Gas sensors that are operated at room temperature (RT) have many benefits, such as low power consumption [6], a long lifetime [7] and safe use in flammable environments [8]. Therefore, researchers have focused on developing gas sensor materials that employ functions at RT. However, the development of the sensitivity and selectivity of gas sensors is still a challenge in terms of their practical applications [9]. One of these challenges is the doping process, where different metals, including palladium (Pd) [10], Copper (Cu) [11] and antimony (Sb) [12], are doped into SnO2. This process is used to enhance the sensing-performance of a gas sensor. But doping into thin films can only be achieved via long and complicated preparation steps, which contribute towards the increase in cost of the sensing device [13]. Previous studies have attempted to increase the sensitivity of devices without doping. Cantalinia et al. [14] showed that the porosity in the thin film allows for increasing the adsorption and desorption of gas molecules on the surface of a device. Abdullah et al. [15] noted that the sensitivity increased at high operating temperatures because of an increase in the adsorption/desorption of gas molecules.

The sol–gel spin coating technique is widely used to synthesize SnO2 thin film, because of its many advantages as compared with other techniques, advantages such as ease of process, low cost and low reaction temperature [16]. Kadhim et al. [5] showed that the addition of glycerin to the sol solution when preparing SnO2 thin films using the sol–gel spin coating technique contributes to the increase in porosity and elimination of cracks for thin films. Usually, the gas sensing measurements were conducted at high operating temperature [17]. Hamaguchi et al. [18] reported that H2 gas sensing response and recovery times became much longer whenever the ambient temperature is decreased.

In this paper, a simple technique was used to synthesize NC SnO2 thin film on an insulating silicon dioxide (SiO2) layer of approximately 1 µm thick formed on silicon (Si) substrate. The main goal is to take advantage of the high porosity that is produced in NC SnO2 thin film by adding glycerin to the sol solution in order to fabricate a NC SnO2 thin film sensor, which has high robustness and reliability for the detection of H2 gas at different concentrations with high performance at RT.

2 Experimental details

A p-type Si (100) wafer (10 mm × 10 mm) was cleaned via the Radio Corporation of America (RCA) technique and heated to 900 °C for 7 h by a tube furnace to obtain a 1 µm thick layer of SiO2 by wet air flow. The NC SnO2 thin film was synthesized via the sol–gel spin coating technique [5, 16]. The sol solution was synthesized by dissolving 0.1 M of tin (II) chloride dihydrate (SnCl2·2H2O) into 70 mL of pure ethanol (C2H5OH). Furthermore, glycerin (C3H8O3) was added at a volume ratio of (1:12) to the sol solution for the elimination of cracks and enhancement of film porosity; then, the sol solution was put in a closed beaker [5]. The mixture in the beaker was stirred using a magnetic stirrer for 3 h and maintained at 70 °C for 8 h. The preparation of the sol solution was completed at RT for the remainder of the 24 h. Then, the resulted sol solution was spin-coated on a SiO2/Si substrate at a rotation speed of 3000 rpm for 30 s. Afterward the as-grown film was oven-dried at 100 °C for 10 min. In order to obtain a high thickness, the spin coating and drying process were repeated 10 times for the sample. Furthermore, to realize the crystallization of SnO2, the thin film was annealed at 500 °C in air for 2 h.

The fabrication of the metal–semiconductor-metal (MSM) device was implemented by radio frequency (RF) sputtering of a Pd grid by a shadow mask on top of the NC SnO2 thin film. NC SnO2 thin film characterization was achieved using X-ray diffraction analysis (XRD) PANalytical X’ pert Pro MRD equipped with a Cu Kα radiation of (λ = 0.154060 nm), and field-emission scanning electron microscopy (FESEM) (Leo-Supra 50VP, Carl Zeiss, Germany). Gas sensing was performed using a homebuilt gas chamber manufactured from an acrylic plastic box, and the heat was supplied using a small ceramic heater that was connected to a thermocouple temperature controller. The current source (2400 Source Meter, Keithley, Cleveland, OH, USA) is connected to a PC, which was utilized for measuring the electrical current passing through the sensing device, which was set to a bias voltage of 0.5 V. Furthermore, the total flow H2 gas rate was kept constant at 1000 sccm during all sensing measurements with each pulse equal to 5 min.

3 Results and discussion

3.1 Characterization of the nanocrystalline SnO2 thin film

The XRD patterns indicated different reflection peaks that agree with standard bulk SnO2 having a tetragonal rutile structure (JCPDS card No. 041-1445) with lattice constants of a = 4.737 Å and c = 3.185 Å [5]. Several diffraction peaks related to the (110), (101), (200), and (211) planes of the NC SnO2 thin film can be observed in Fig. 1. Moreover, the other peak related to the Si substrate emerged at 2θ = 69.2°. Small nanoparticles can be observed in polycrystalline thin film with uniform distribution the SnO2 thin film as shown in Fig. 2. This occurred due to the annealing of the thin film at 500 °C, which results in the improvement of the crystallinity [19, 20].

Fig. 1
figure 1

XRD patterns of NC SnO2 thin film annealed at 500 °C

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Fig. 2
figure 2

FESEM images of NC SnO2 thin film annealed at 500 °C

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The crystallite size was calculated using the Debye–Sheerer formula Eq. (1) [5].

$$D = \frac{0.9\lambda }{\beta \cos \theta }$$
(1)

where β the full width at half-maximum (FWHM) of (110) the main peak, θ presented Bragg angle. In addition, the lattice constants (a, c) were calculated using Eq. (2).

$$\frac{1}{{d^{2} }} = \frac{{h^{2} + k^{2} }}{{a^{2} }} + \frac{{l^{2} }}{{c^{2} }}$$
(2)

where d: is the lattice plane spacing of NC SnO2 that obtained from XRD measurement, h, k, and l: are the Miller indices. The strains (ɛ a , ɛ z ) of NC SnO2 was calculated along the (a -axis and c -axis) using Eq. (3) [21] as follows:

$$\varepsilon_{z} \left( \% \right) = \frac{{c - c_{o} }}{{c_{o} }} \times 100$$
(3)

where c: is the lattice constant of the strained atoms, and c o : is the standard lattice constant of the crystalline material (the same measurement for ɛ a ). The values for lattice constants, crystallite size, and strain are memorized in Table 1.

Table 1 Properties of the NC SnO2 thin film grown on SiO2/Si substrates after annealing at 500 °C
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The obtained strains were low-positive values (0.2714, 0.1679 %) that indicate to the tensile strain [22], and the high quality of the examined film.

Figure 3 shows that the average thickness of the as-grown SnO2 thin film was approximately 450 nm. In addition, approximately 1 µm thick layer of SiO2 is shown on the top of the silicon substrate. This high thickness of the thin film is attributed to the high viscosity of the sol solution which was generated by the 8 h aging heat time [23].

Fig. 3
figure 3

Cross-section FESEM image for as-grown SnO2 thin film synthesized from sol solution at 70 °C for 8 h aging heat time

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3.2 Hydrogen sensing characterization

The schematic diagram of the NC SnO2 thin film gas sensor is shown in Fig. 4, which shows the Pd finger contacts deposited on the top surface of the NC SnO2 thin film. The Pd mask includes two electrodes; each electrode contains four fingers, the distance between every two neighbouring fingers is 0.4 mm, and the width of each finger is 0.35 mm. The Pd contact has the ability to dissociate H2 molecules to H atoms that can diffuse on the NC SnO2 surface and react quickly with different adsorbed oxygen species via negative charges [15].

Fig. 4
figure 4

Image of Pd grid contact deposited on the NC SnO2 thin film

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In ambient air, the NC SnO2 thin film surface adsorbs oxygen (O2) molecules. This generates an increase in resistivity. This phenomenon is related to electron capture by O2 ions from the SnO2 conduction band at different operating temperatures according to the following reactions [24]:

$${O}_{2} \left( {gas} \right) \to {O}_{2} \left( {{ads}.} \right),$$
(4)
$${O}_{2} \left( {{ads}.} \right) + e^{ - } \to {O}_{2}^{ - } \quad \text{T less than } 100\, ^\circ{\text{C}}$$
(5)
$${O}_{2}^{ - } + e^{ - } \to 2{O}^{ - } \quad {\text{T between }} 100{\text{ to }}300 \, ^\circ{\text{C}}$$
(6)

When H2 molecules were dissociated to H atoms on the Pd contact, which diffused into the surface of NC SnO2 thin films, and reacts very rapidly with different adsorbed oxygen species by negative charges. This results in facilitating the interaction between H2 molecules and the chemisorbed O2 ions. This reaction will increase the conductivity of the NC SnO2 thin film by releasing electrons in chemisorbed O2 to the SnO2 conduction band as shown in the following reactions [15, 25]:

$$2{H}_{2} + {O}_{2}^{ - } \left( {ads} . \right) \to 2{H}_{2} {O}^{{}} + e^{ - }$$
(7)
$$4{H} + {O}_{2}^{ - } ({ads} .) \to 2{H}_{2} {O} + e^{ - }$$
(8)

The sensitivity of a gas sensor is defined as the change in the conductivity of the sensing material upon exposure to a reducing gas. It can also be defined as the electric current passing through a NC SnO2 thin film, which is computed by the following equation [15, 26]:

$$S\left( \% \right) = \frac{{\left( {I_{g} - I_{air} } \right)}}{{I_{air} }} \times 100$$
(9)

where S is the sensitivity, I g is the current measured in the presence H2 gas, and I air is the current measured in air.

Figure 5 shows the sensitivity of the NC SnO2 thin film grown on the SiO2/Si substrate at different operating temperatures in the presence of 1000 ppm of H2 for 5 min. The sensitivity of H2 gas detection at RT was 600 % with less repeatability for over 50 min as shown in Fig. 5a. The decreased repeatability and poor recovery over time happened because of the residual H2 gas on the NC SnO2 thin film surface due to the low operating temperature. Increasing operating temperature to 75 and 125 °C lead to improvement in sensitivity and repeatability of H2 gas detection as shown in Figs. 5b, c, where the sensitivity at 125 °C was increased to 1950 %. This enhancement in the sensitivity occurred because of a improvements in the adsorption and desorption of gas molecules, where at higher operating temperatures various types of oxygen species are presence [15]. Consequently, the removal process of H2 gas from the surface of NC SnO2 thin film sensors is increased. Thereby, it can be observed that the sensitivity in Fig. 5c did not drift from the baseline. These high sensitivities for detecting H2 gas at different operating temperatures were occurred by the high porosity for the NC SnO2 thin film that was produced through adding glycerin to the sol solution [5]. This allowed easy working to adsorption/desorption of gas molecules [14], thus enhancing the sensitivity of the H2 gas sensor [15].

Fig. 5
figure 5

The sensitivity and repeatability of NC SnO2 thin film sensor on SiO2/Si substrates upon exposure to successive pulses of 1000 ppm H2/N2 gas and dry air at different sensor temperatures: a RT, b 75 °C, and c 125 °C

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The variation in the sensitivity values of NC SnO2 thin film gas sensor in the presence of different concentrations of H2 (150–1000 ppm) at different operating temperatures (RT to 125 °C) is shown in Fig. 6. The variation in sensitivity was observed for the H2 gas sensor operated at a low temperature due to an insufficient adsorption and desorption processes of H2 and O2 molecules [13, 15]. A significant difference was observed in the sensitivity with increasing concentrations of H2 at RT due to the low sensitivity values at lower concentrations of H2. The variation in the sensitivity with increasing H2 concentrations was decreases with increasing the sensor temperature. These findings are attributed to the considerably improved sensitivity with increasing operating temperatures [13, 15, 26]. Moreover, Fig. 7 displays the linear correlation between sensitivity and operating temperatures in the presence 1000 ppm of H2, where the increase in the operating temperature from RT to 125 °C leads to increasing sensitivity. Response and recovery times are defined as the time needed for the target gas to access 90 % of the saturation state current and the time needed to recover 10 % of the initial value respectively [27]. Table 2 displays the response and recovery times of the NC SnO2 thin film H2 gas sensor and the relative sensitivities at different operating temperatures compared with previous studies.

Fig. 6
figure 6

The sensitivity and repeatability of NC SnO2 thin film sensor on SiO2/Si substrates under exposure to different concentrations of H2 (150–1000 ppm) at different sensor temperatures: a RT, b 75 °C, and c 125 °C

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Fig. 7
figure 7

The relationship between the sensitivity and operating temperatures for NC SnO2 thin film sensor

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Table 2 The comparison between the previous studies and the present work of the sensitivity, the response and recovery times for the NC SnO2 thin film sensor at various operating temperatures
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4 Conclusions

High-quality NC SnO2 thin film was grown on a SiO2/Si substrate by sol–gel spin coating technique. Good structural properties were obtained by adding glycerin to the sol solution, which allowed the increase of surface porosity for the NC SnO2 thin film. This results in improving the adsorption and desorption of the sensing gas on the NC SnO2 thin film surface, leading to an increase in sensitivity. The NC SnO2 thin film on SiO2/Si displayed good sensitivity at RT; hence, it is considered as a promising portable H2 gas sensor that can operate in a flammable environment. The repeatability of the NC SnO2 thin film sensor upon exposure to H2 gas for over 50 min showed good performance at RT and for different operating temperatures. The NC SnO2 thin film sensor also showed a good capability of H2 gas detection at different operating temperatures when exposed to different concentrations of H2.