Multi-Walled Carbon Nanotube-Doped Tungsten Oxide Thin Films for Hydrogen Gas Sensing

In this work we have fabricated hydrogen gas sensors based on undoped and 1 wt% multi-walled carbon nanotube (MWCNT)-doped tungsten oxide (WO3) thin films by means of the powder mixing and electron beam (E-beam) evaporation technique. Hydrogen sensing properties of the thin films have been investigated at different operating temperatures and gas concentrations ranging from 100 ppm to 50,000 ppm. The results indicate that the MWCNT-doped WO3 thin film exhibits high sensitivity and selectivity to hydrogen. Thus, MWCNT doping based on E-beam co-evaporation was shown to be an effective means of preparing hydrogen gas sensors with enhanced sensing and reduced operating temperatures. Creation of nanochannels and formation of p-n heterojunctions were proposed as the sensing mechanism underlying the enhanced hydrogen sensitivity of this hybridized gas sensor. To our best knowledge, this is the first report on a MWCNT-doped WO3 hydrogen sensor prepared by the E-beam method.


Introduction
Hydrogen (H 2 ) is one of the most useful gases, being used in many chemical processes and various industries including aerospace, medical, petrochemical, transportation, and energy [1][2][3]. In recent years, H 2 has attracted a great deal of attention as a potential clean energy source for the next generation of automobiles and household appliances due to its perfectly clean combustion without any release of pollutants or greenhouse gases [4]. However, this low molecular weighted gas can easily leak out and may cause fires or explosions when its concentration in air is between 4% and 75% by volume [5]. Moreover, H 2 is a colorless, odorless and tasteless gas that cannot be detected by human senses. Therefore, it is very essential to develop the effective H 2 gas sensors for monitoring of H 2 leaks.
In the present work, multi-walled carbon nanotube (MWCNT)-doped WO 3 thin films fabricated by an electron beam (E-beam) evaporation process and their application for H 2 gas sensing are reported for the first time. The E-beam process offers extensive possibilities for controlling film structure and morphology with desired properties such as dense coating, high thermal efficiency, low contamination, high reliability and high productivity. MWCNTs were selected for doping because of their larger effective surface area, with many sites available to adsorb gas molecules, and their hollow geometry that may be helpful to enhance the sensitivity and reduce the operating temperature. Furthermore, MWCNTs were reported to be sensitive to H 2 , with good recovery times [27].

Preparation of Materials
Commercial WO 3 powder was obtained from Merck and used without further purification. MWCNTs were grown by the thermal chemical vapor deposition (CVD) process. The catalyst layer of aluminium oxide (10 nm) and stainless steel (5 nm) was deposited on the silicon (100) substrates (Semiconductor Wafer Inc.) using reactive sputtering apparatus. The synthesis of MWCNTs was performed under a flow of acetylene/hydrogen at a ratio of 3.6:1 at 700 °C for 3 min. To obtain highpurity MWCNTs, the water-assisted selective etching technique [28] was applied after each CNT's growth stage. Water vapor (300 ppm) was introduced into the system by bubbling argon gas through liquid water at room temperature for 3 min. The sequence of acetylene/hydrogen and water vapor flows was repeated for five cycles. Based on the scanning electron microscopic (SEM) image, as shown in Figure 1, the diameter and length of the MWCNTs are ~35 nm and ~26 μm, respectively. The electrical conductivity of MWCNTs was ~75 S/cm, as measured by a four-point probe method at room temperature. In addition, high-resolution transmission electron microscopic (HR-TEM) imaging, as shown in Figure 2, confirms that CNTs are multi-walled, with the width and number of walls being ~4.6 nm and 14, respectively. Thus, the spacing between two graphitic layers is ~0.33 nm, which is in good agreement with theoretical and experimental values.

Fabrication of MWCNTs-doped WO 3 Thin Film
MWCNT-doped WO 3 thin film was fabricated by the E-beam evaporation technique onto Cr/Au interdigitated electrodes on an alumina substrate [29]. The target was prepared by mixing 99 wt% of WO 3 powder with 1 wt% of MWCNT powder using a grinder in a mortar for 30 min and then pelletizing with a hydraulic compressor. Deposition was performed at a pressure of 5 × 10 −6 Torr in the evaporation chamber. The substrate was rotated and kept at 130 °C during the deposition in order to obtain a homogeneous thin film. The deposition rate was 2 Å/sec and the final film thickness was 150 nm, as controlled by a quartz crystal monitor. After E-beam evaporation, the film was annealed at 500 °C for 3 h in air to stabilize the crystalline structure. In addition, an undoped WO 3 thin film was also fabricated using the same conditions for comparison.

Measurement of Gas Sensing
To evaluate the gas sensing properties of the thus prepared thin films, MWCNT-doped WO 3 and undoped WO 3 gas sensors were placed inside a stainless steel chamber and the resistance measured using a 8846A Fluke multimeter with 6.5 digit resolution. The gas sensing measurements were made within a dynamic flow system with control of sensor operating temperatures (200-400 °C) under variable gas concentrations (100-50,000 ppm). Hydrogen (H 2 ), ethanol (C 2 H 5 OH), methane (CH 4 ), acetylene (C 2 H 2 ), and ethylene (C 2 H 4 ) were used to test the sensing properties and selectivity of the thin films. The sample gas flow time and the clean air reference flow time were fixed at 5 min and 15 min, respectively. It should be noted that these switching interval was selected so that the resistance change is at least 90% of the saturated value. The sensor resistances were sampled and recorded every second using LabVIEW with a USB DAQ device for subsequent analyses.

Characterization of Thin Films
Surface morphology, particle size and crystalline structure of the films were characterized by SEM and TEM. Figure 3 shows the SEM surface morphology of MWCNT-doped WO 3 thin film deposited on an alumina substrate. It was seen that the film coated on the rough alumina substrate has approximate grain sizes ranging from 40 to 80 nm. The nanometer grain size together with the roughness of the alumina substrate can enhance the gas sensitivity of thin films [30,31] because more gas adsorption sites are available due to the increased surface area and porosity. With the SEM resolution, CNT structure cannot be observed on the thin film surface. Therefore, TEM characterization was used to confirm CNT inclusion into the WO 3 film. It should be noted that copper TEM grid samples were loaded inside the evaporation chamber for sample deposition at the same time as coating on the Cr/Au interdigitated electrodes. TEM observation clearly shows CNT inclusion into the nanocrystalline WO 3 , while the electron diffraction pattern exhibits polycrystalline phase in the film, as shown in Figure 4a,b, respectively. The film morphology obtained in our study is in accordance with observations on nanocrystalline WO 3 films grown by other methods [32,33]. Doping of CNT does not change the phase or surface morphology of the film, but it may help form nanochannels in WO 3 films, leading to the enhancement of the sensitivity and reduction of the operating temperature.

Sensing Properties of Thin Films
The sensor response (S) of the thin films is defined as the percentage of resistance change: where R 0 and R are the resistance of the thin films in pure air and test gas, respectively. Figure 5 shows the response of the undoped WO 3 and MWCNT-doped WO 3 thin films to 1,000 ppm H 2 at varying operating temperatures. It can be seen that the response of the films increases as the operating temperature increases up to 350 °C, and then decreases. The gas-sensing response increases with temperature in the 200-350 °C range because thermal energy helps the reactions involved overcome their respective activation energy barriers [34,35]. However, if the operating temperature becomes too high (i.e., >350 °C), the adsorbed oxygen species at the sensing sites on the film surface will be diminished and less available to react with H 2 molecules [36], thereby limiting the film's response. At any operating temperature, the sensor response of the MWCNT-doped WO 3 thin film is higher than that of the undoped WO 3 thin film. Specifically, at the optimum operating temperature (350 °C), MWCNT-doped WO 3 thin film yields a 26.9 % higher response than the undoped one. The doped sensor prepared in this work also shows higher response than the WO 3 films prepared by the sol-gel process [25]. One major advantage of MWCNT-doped WO 3 thin film is that the sensor can be operated at a lower operating temperature (250 °C), especially if this sensor is used to measure the H 2 gas at higher concentrations (5,000-50,000 ppm). As shown in Figure 6, at such a concentration range, there are sufficient numbers of H 2 molecules available to react with the surface oxygen adsorption sites. It is also well-known that MWCNTs contribute to the reduction of sensor resistance of metal oxides [37] and the activation energy between the WO 3 surface and H 2 gas. The details of the sensing mechanisms of MWCNT-doped WO 3 thin films will be discussed in the next section.
To demonstrate the selectivity of the MWCNT-doped WO 3 thin film, its sensing response (at the operating temperature of 350 °C) to various gas vapors, namely H 2 , C 2 H 5 OH, CH 4 , and C 2 H 2 , was measured and plotted ( Figure 7). It can be seen that MWCNT-doped WO 3 thin film exhibits a strong response to H 2 , and much weaker responses to C 2 H 5 OH, CH 4 , and C 2 H 2 . In particular, this thin film was found to be insensitive to C 2 H 4 at the optimum operating temperature of 350 °C. It is therefore concluded that the MWCNT-doped WO 3 thin film exhibits high selectivity to H 2 .

Sensing Mechanism of MWCNTs-doped WO 3 Thin Film
It is well known that WO 3 is an n-type semiconductor while CNT is a p-type semiconductor. MWCNT-doped WO 3 thin film can be either p-type or n-type semiconductors depending on the quantity of MWCNTs and the operating temperature [38]. In this work, the produced MWCNTs-doped WO 3 thin film behaves as an n-type semiconductor since the electrical conductivity of the film increases when reducing gases, i.e., H 2 , are absorbed by its surface. Doping of MWCNTs into the WO 3 matrix can introduce nanochannels and form p-n heterojunctions in the thin film. These nanochannels play an important role for gas diffusion. The gas molecules can easily transport into the gas-sensing layers leading to increasing sensitivity [39,40]. In addition, MWCNT-doped WO 3 thin film p-n heterojunctions could be formed at the interface between WO 3 and the MWCNTs [38,41]. When H 2 gas is exposed to MWCNT-doped WO 3 thin film, the widths of the depletion layers at the p-n heterojunctions can be modulated. The potential barriers at the interfaces or inside the WO 3 may be changed. This change of the depletion layer in the p-n heterojunctions of MWCNT-doped WO 3 thin film may explain the enhanced response of the film at low operating temperatures. Various oxygen species chemisorbed at the thin film surface such as O 2-, O 2 -, and Oare available for catalytic reactions with H 2 , thus depending on the temperature at the metal oxide surface [42]. At the operating temperature range of 200-400 °C, Ois commonly chemisorbed. Consequently, the chemical reaction underlying the H 2 gas sensing in this study is given by [ The adsorbed Oon the thin film surface reacts with the H 2 gas yielding H 2 O and releasing electrons which contribute to the current increase through the thin film that causes the electrical conductivity to increase.

Conclusions
MWCNT-doped WO 3 thin film was successfully prepared by the E-beam evaporation technique. The 1 wt% MWCNT-doped WO 3 thin film exhibits n-type semiconductor behavior of the polycrystalline phase. Doping with MWCNTs does not significantly change any phase or surface morphology of the film, but it introduces nanochannels and form p-n heterojunctions in the WO 3 matrix. The MWCNT-doped WO 3 thin film exhibits high selectivity and sensitivity to H 2 over a relatively wide range of concentrations (100-50,000 ppm). Moreover, it can operate at a relatively low temperature. This should be useful for developing high performance H 2 gas sensors. To our best knowledge, this is the first report on MWCNT-doped WO 3 hydrogen sensors prepared by the E-beam method.