A high-sensitivity hydrogen gas sensor based on carbon nanotubes fabricated on SiO2 substrate

Abstract In this study, an inexpensive simple method for the fabrication of efficient hydrogen (H2) gas sensor based on carbon nanotubes (CNTs) was presented. The CNTs were synthesized using microwave oven and deposited onto SiO2 substrate by a dielectrophoretic method. The as-grown CNTs showed an n-type behavior because CNTs possess the characters of both metallic and semiconductor when placed between the two electrodes, meanwhile, the current was directed mostly by metallic tubes. Upon exposure to H2 gas at room temperature, the CNTs exhibited high sensitivity up to 315% at 140 ppm H2, and relatively good sensitivity of 40% at a very low H2 gas concentration of 20 ppm. To the best of our knowledge, this is the first work involving the fabrication of CNTs for detecting a low H2 gas concentration of 20 ppm at RT with high sensitivity comparing with other previous studies. Graphical Abstract


Introduction
Great efforts have been devoted to the development of novel nanostructured materials with specific properties for gas sensors with high performances such as selectivity and sensitivity. Since their discovery in 1991, carbon nanotubes (CNTs) have attracted scientific interest due to their remarkable mechanical, chemical, and electronic properties in addition to their semiconductive/metallic character depending on their diameter [1,2]. These unique characteristics make them a promising material for various applications including nanoelectronics, field emission devices, and multifunctional composite materials [3].
The application of CNTs for gas sensing has been widely investigated because of their large surface area due to their nanoscale regime and hollow geometry [4,5]. Some previous studies confirmed that CNTs-based gas sensors could be fabricated by electrophoresis [6,7].
Surface functionalization of CNTs via reflux treatment using some extreme acids, such as HNO 3 , resulted in the enhancement of surface area and modified morphological characteristics and thereby increased the ability to adsorb target gases [5,8].
Hydrogen (H 2 ) is an essential component in numerous applications such as hydrogenation processes, petroleum transformation, welding, chemical production of substances, cryogenic cooling, rocket engines. Nowadays, fossil fuels are considered pollutant agents, therefore utilization of H 2 as a clean energy career in fuel cells has gained great importance. It is a colorless and odorless gas. The studies have shown that if the concentration of H 2 in the air is higher than 4% it will be highly explosive and easily flammable [9].
The fabrication of sensors with high sensitivity for H 2 detection represents a major challenge in the development of H 2 -based technology. CNTs are successfully being employed as active sensing materials for the detection of H 2 [10][11][12][13][14]. Moreover, the human senses do not detect H 2 gas because it is colorless and odorless. Therefore, early detection of H 2 is very important to ensure the safe operation of the H 2 -based energy equipment or fuel cell vehicles, since H 2 leaks may cause deadly explosive accidents [9].
Most of the recent studies on composite sensors were based on their response towards NO 2 and NH 3 gases [6] However, there are few reports on the response of such CNT composite sensors towards H 2 . Furthermore, metals (Pd and Pt) and metals oxides (TiO 2 , SnO 2 , WO 2 ) were added to CNTs in order to improve their sensitivity for H 2 detection [4,11,12,15]. However, the fabrication of a gas detector by using multi-walled carbon nanotube was reported by Jung et al. [16] The results revealed a sensitivity of about 13% under 18,000 ppm of H 2 gas at room temperature, and the response time was 20 s.
Moreover, functionalized multiwall carbon nanotubes were used to fabricate H 2 gas sensors by Dhell et al. [8] The sensitivity of the sensors after 7 min of exposure were approximately 0.9, 0.13, and 0.14% for 500, 3500, and 5000 ppm of H 2 gas respectively, and the recovery time of these sensors was 95 s.
In this paper, a simple method for the synthesis of CNTs using a conventional microwave oven and a novel nanoelectronic gas sensor fabricated from CNTs for H 2 detection, are reported. The as-fabricated sensor was tested in the open air and at the presence of H 2 with several operating temperatures.
The novelty of this research work consists of a cost-effective dielectrophoretic DEP deposition method for CNTs exhibiting good gas sensing characteristics at lower operating temperature, low recovery time, higher sensitivity at low gas concentration with a much faster response rate in an open environment.
The as-fabricated CNTs-based sensor showed good reversible and reproducible responses towards H 2 in the temperatures range of 25-100 C over a period of 55 min. This high performance was attributed mainly to the high surface to volume ratio of CNTs and increased H 2 adsorption onto functionalization CNTs surface. The developed facile fabrication method in addition to the highly sensitive CNTs-based H 2 gas sensor would reduce the cost and the operating power consumption as well.

Materials and samples preparation procedure
Pristine multi-walled carbon nanotubes (MWCNTs) were produced by a new method using a microwave oven. Different ratios (30:70), (20:80), (80:20), and (50:50) of ferrocene and graphite flattened in a ceramic boat, then irradiated using microwave oven for 5 s; CNTs with an average outer diameter of 55 nm were obtained [16].
The CNTs as-produced by microwave oven were treated in nitric acid (HNO 3 , 2.6 M) and stirred at RT for 14 h followed by ultrasonication for 1 h. After that, centrifugation at 10,000 rpm for 1 h was performed to obtain a supernatant, then resuspension the solution in deionized water. The last process was repeated several times until the pH of the solution reaches 7. The as-obtained CNTs were dried in an electric furnace at 70 C for 24 h. The acid treatment can extremely facilitate the CNT by insertion of chemical functional groups (-COOH and -OH) to the ends and sidewalls of CNTs resulting in the enhancement of their hydrophilicity [17]

Characterization techniques
The microstructure of the as-grown CNTs was characterized by field emission scanning electron microscope (FESEM) using Carl Zeiss Leo-Supra 50VP. Raman spectra were recorded at RT in the range 1100-1800 cm À1 using an excitation wavelength of 633 nm of He-Ne laser to check the graphitic crystalline quality.

Fabrication of sensor
In order to fabricate a metal-semiconductor-metal (MSM) gas sensor, a 300 nm layer of SiO 2 was deposited onto Si wafer by wet oxidation ( Figure  1(a)). Palladium (Pd) contact finger of thickness 100 nm was deposited via a metal mask on SiO 2 substrate using RF magnetron sputtering system (A500 Edwards), see Figure 1(b). The electrode fingers are 0.35 mm wide and 3.4 mm long with a gap of 0.40 mm in space. These electrodes have been electrically contacted to AC power supply by copper wires using silver paste.
For the preparation of CNTs solution, 3 mg of multi-wall CNTs synthesised by microwave oven and functionalized using HNO 3 acid were dispersed in 20 ml of Ethanol solution. Then followed by sonication for 60 min to disperse and stabilize the CNTs dispersion in solution. The concentration of CNTs solution was about 150 mg ml À1 . The CNTs solution of 0.5 ll was dropped between Pd electrodes under an AC electric field of 10 V at 50 Hz. The solution that was dropped on the substrate between electrodes was approximately 10 ll of carbon-ethanol solution using a micropipette. The schematic diagram of CNTs deposition on Pd electrode using AC dielectrophoresis is shown in Figure 1(c). The inset of Figure 1(c) shows the micro-image of the as-fabricated sensing device.

Experimental sensing set-up
The gas-sensing properties of CNT were performed for the detection of H 2 , using a commercial sensing system equipped with a Keithley 2400 source meter. The sensor response under a constant applied voltage of 0.05 V was tested by measuring current-voltage curves, while the total flow rate was kept constant at 1000 sccm. The sensor was put in a homemade chamber, and the two Pd-electrodes of the sensor were connected to a computer-controlled by Keithley 2400 source-meter, as shown in Figure  2. Then the sensor was exposed to dilute H 2 gas concentration in the range 20-140 ppm at various operating temperatures. The mixture ratio of the main gasses (H 2 /N 2 ) and dry air, were varied in order to control the gas concentration through the two gas flow controllers. Figure 3 shows FESEM images of CNTs synthesized using different graphite/ferrocene mixture ratios. The tendency of agglomeration and the appearance of stacked bundles confirm the synthesis of CNTs, having diameters of about 55 nm as shown in Figure 3(a)-(d). Figure 4 shows Raman spectra for as-grown CNTs. Two distinctive peaks are observed; the (D) band at 1330 corresponding to defects present in the hexagonal sp 2 carbon lattice of graphene layer and (G) band at 1580 cm À1 ascribed to graphitic well-ordered carbon atoms. Meanwhile, the G-to-D peaks intensity ratio (I G /I D ) is usually considered as a measure of the well-ordered or relative amount of defects present in CNTs.
In order to obtain structural properties of the asgrown CNTs, the evaluated I D /I G values from Raman spectra are summarized in Table 1. The lowest I D /I G value that appears for CNTs obtained with 70:30 graphite/ferrocene ratio, This value indicates a high crystalline quality with fewer defects, thus, the highest sensitivity for hydrogen gas.
The D-to-G peaks intensity ratio (I D /I G ) has often been linked to the structural purity of graphitic materials with the crystal domain size. When the intensity of G band is higher than D band, this implies an improvement of crystal quality. CNTs usually contain a graphene layer formed at the edges with a higher quantity of structural defects. The intensity of D band suggests the presence of defects in the graphene structure. By referring to the double resonance theory, the excited electrons scattered by crystal defects are responsible for the wave generation [18]

Results and discussion
The CNT based on H 2 gas sensor was introduced into the chamber at RT and to evaluate the gassensing characteristics, the H 2 concentration was controlled by the MFC (mass flow controller), meanwhile, the flow rate of the gas mixture was set at 1000 ppm.
The sensitivity of the sensor is defined as the relative variation of the electrical resistance using the following equation [19]: where R air and R gas are the maximum resistances of CNTs when exposed to H 2 ambiance and air ambiance, respectively. The above expression, can also be expressed in terms of electrical current produced in CNTs under the operation of a DC bias voltage [20]: herein, I gas and I air are the maximum current recorded after the H 2 exposure and the current measured in pure air, respectively. From the above equations, it is clear that the sensitivity can be associated with the quantity of electron charge transfer, the nature of the surface interaction of the gas and CNTs. Furthermore, it is important to highlight that CNTs have four possible positions whereby H 2 gas molecules can be adsorbed: internally, in external grooves, in interstitial channels, and on external surfaces [21]. Figure 5 illustrates the variation of CNTs-based sensor sensitivity when exposed to consecutive pulses of 140 ppm H 2 gas at different operating temperatures (RT, 50, 75, and 100 C). This figure displays the actual time response of the sensor over a period of 55 min with excellent repeatability, especially at high temperatures. The repeatability improvement with operating temperature rising can be attributed to readily desorb of the H 2 molecules by lowering the binding energy of CNTs.
On the other hand, the current of the sensor returns to its original value after exposure to air at RT. This is a prominent feature for this device compared to previous studies which demonstrated an inability to return to the original value at RT [22,23].
The sensitivity of the sensor based on CNTs as defined by Eq. (1), at RT under an applied voltage of 0.05 V is shown in Figure 5(a). It is clear that at room temperature and under the H 2 /balance N 2 of 140 ppm exposure, the device displays a very high sensitivity of 315% ( Figure 5(a)), which is much higher than previously reported values from the literature [4,5,8,[10][11][12][13]15,17,[24][25][26][27][28][29]. Figure 6 illustrates the responses of the sensor when exposed to various H 2 concentrations in the range 20-140 ppm at different operating temperatures. It can be observed that the CNTsbased sensor displays a fast response even when exposed to a low H 2 concentration of 20 ppm and achieved a sensitivity of about 40% at RT ( Figure  6(a)). Besides, it was also found that the sensor response was repeatable and stable regarding its response changes without recording any major signal variations. Recently, I. Sayago et al. [28] reported that H 2 gas can be detected using MWCNTs functionalized with Pd exhibited a sensitivity of only 20% for H 2 concentration of 30,000 ppm at temperature 250 C with response and recovery time of 1800 and 3600s, respectively.
It is obvious that the present CNTs-based sensor exhibited much better sensitivity and more ability to detect several ppm of H 2 gas at RT comparing with previous studies (Table 2) It was observed that sensor response to H 2 has a rise time of about 179, 169, 112, and 70 s at an operating temperature of 25, 50, 75 and 100 C, respectively, indicating that this sensor enables fast response to H 2 gas within a few tens of seconds of response time.
The fast response of the as-prepared CNTs-based H 2 gas sensor can be attributed to the large surfaceto-volume ratio CNTs, which increases H 2 adsorption sites and facilitates the diffusion of H 2 molecules through CNT. After replacing the test gas with air, the gas sensor returns back rapidly to its baseline. The recovery time of the present CNT-based gas sensor is 19 s at RT, which is very small compared to the significant recovery time reported for some other CNTs-based sensors in some previous studies [5,12,13].
Furthermore, a relationship between the sensitivity and H 2 concentrations was also obtained, as shown in Figure 7(a). It was noticed that the sensitivity depended on the concentration of H 2 molecules, which gradually increases, though not totally linear in response with the increase in gas concentration, reaching saturation around 90 ppm, especially at 100 C. The maximum sensitivity detected with 140 ppm H 2 , was 415% at 100 C.
Moreover, Figure 7(b) exhibits a linear relationship between sensitivity and operating temperatures in the presence of 140 ppm of H 2 gas, where the increase in the operating temperature from RT to 100 C leads to an increase in the sensitivity. This is because higher temperatures facilitate the dissociation of H 2 molecules absorbed and thus enhance the adsorption of gas molecules on CNTs gas sensor, which decreases the response time [20,22].
The repeatability of this sensor for low H 2 concentration (140 ppm) indicates that, the sensitivity of the sensor does not weaken after repeated exposure to H 2 , which proves that the sensing characteristics are reproducible.
The dependence of response time, which is another important characteristic of the sensor, and the temperature is shown in Figure 7(c). The results indicated that higher temperature favors faster response; when the sensor was heated to 100 C, the response time became shorter as 70 s.
In general, the relationship between the electrical conductivity of semiconductors and the ambient temperature is expressed by the following equation [23]: where is K B is the Boltzmann constant, E a is the activation energy and T is the ambient temperature in Kelvin. Figure 8 shows a logarithmic plot of the sensitivity versus reciprocal of operating temperature (1/T), the activation energy of the CNTs-based sensor can be evaluated from the slope of the Arrhenius plot.    The obtained value was about 16 meV, which was much smaller compared to the activation energy value of 230 meV obtained for Pd-functionalized multi-layer graphene nanoribbons (MLGNs) network as reported by Jason et al. [31] and also smaller than other of some previous CNTs-based sensors research works [32,33]. However, low activation energy is an indication that the present CNTs-based gas sensor has a high response for H 2 gas with superior sensitivity.
It is important to highlight that the conductivity was increased with H 2 adsorption then decreased upon turning off H 2 gas flow meanwhile exposed to air. This behavior can only occurs when charge carriers in CNTs are of n-type, which is not in agreement with some previous studies, where CNTs were reported to be p-type.
This can be clarified as follows. CNTs deposited between the two electrodes were a mixture of 2 types of CNTs, metallic and semiconductor, forming junctions and crossings within the channel. Since the resistance of the semiconducting tubes is large, it can be stated that the measured current was directed mostly by the metallic tubes [34,35].
Notably, CNTs network in the gas sensor had a prevalent semiconductor behavior, although the network was possibly composed of a mixture of both semiconductor to metallic characters with different CNTs diameter.
It was observed that the present CNTs-based sensor had much higher sensitivity, and was operated at low applied voltage of 0.05 V with a total power consumption of around 3.5 mW at RT, which was the lowest as compared with other values of previously reported for other sensors [10,22,36,37]. This value was very much smaller than the lowest ideal value of 100 mW, hence this sensor with ultra-low power consumption is suitable for use in remote areas where the power sources might be very limited.
The mechanisms of sensing behavior of CNTs depend on the chemisorption and physisorption of H 2 . Chemisorption involves atomic H 2 adsorption on CNTs surface while physisorption involves the adsorption of molecular H 2 on CNTs surface.
In the meantime, Pd electrode plays a catalytic function during the chemisorption process to dissociate H 2 molecules to H atoms [13,14,24,36].
The increased conductivity of CNTs-based gas sensor during exposure to H 2 dilute gas can be explained by the following: H 2 molecules are considered as a reducing agent that delivers an electron to CNTs. In the present gas sensor, H 2 can play a role as an electron donor to CNT. Consequently, the electrons will be transported from H 2 to CNTs during exposure to H 2 gas. This process results in increasing the number of electrons leading to the reduction of CNTs resistivity meanwhile increase conductivity [13,14,24].
In the reverse process, when oxygen is present, the adsorbed atomic H 2 combines with O 2 to form hydroxyl groups (reaction 2), which will further combine with adsorbed H 2 to form water that can depart the CNTsbased sensor (reaction 3). Thus, the oxygen in the air plays the role as a recovery agent, on Pd electrodes and CNTs surfaces. When H 2 gas flow is switched off, this process leads to sensor reversibility and thus recovering CNTs electrical properties [20,22,38].
The main possible reactions that can describe H 2 absorption and recovery stages are [5,17,39]: Response: Recovery: It is worth noting that the group of hydroxides resulting from the reaction of oxygen with absorbed

Conclusion
In this study, we demonstrated that high sensitive, efficient, and inexpensive CNTs-based H 2 gas sensor can be easily fabricated by a simple dielectrophoresis deposition of CNTs onto SiO 2 substrates. The CNTs synthesis can be achieved by a one-step and simple method under microwave irradiation and using a graphite/ferrocene mixture. The n-type behavior of CNTs was attributed to the presence of a metallic CNTs network which directs the current in a CNTsbased gas sensor. Notably, the sensor offers an excellent response toward H 2 in the range 20-140 ppm level comparable with the highest values reported in some previous studies in the literature (Table 2). It was found that CNTs-based senor achieved high sensitivity of about 315% at 140 ppm level of H 2 concentration at RT with faster response/recovery times of 19/70 s. Furthermore, it is also shown that the CNTs-based gas sensor has a good sensitivity of 40% for the low concentration of 20 ppm level of H 2 at RT. On the other hand, successive thermal treatments significantly improve the sensor sensitivity and the sensor response was reproducible without supplementary assistance such as an additional metal decoration or a vacuum system. The successful utilization of CNTs in gas sensing based on novel nanostructure and its capability to detect a very low concentration of H 2 in the range of 20-140 ppm favors the development of enhanced H 2 gas sensors with high performance. Also, the present sensor operates at RT with ultra-low power consumption in the range of 3.5 mW, hence it is suitable for use in remote areas with limited power sources.  characterization techniques such as XRD, FESEM, TEM, PL, Raman, Hall effect, and FTIR. During my Ph.D. journey, I published few articles in ISI journals. My research interests are to employ different materials to prepare various nanostructures for potential applications like UVphotodetectors, solar cells and sensing applications.