Design and evaluation of SnO₂-Pt/MWCNTs hybrid system as room temperature-methane sensor

Highlights

• Preparing highly sensitive SnO₂-Pt/MWCNTs hybrid methane sensor using precipitation methods.

• High sensing response at room temperature to a wide range of methane concentrations.

• Excellent sensitivity and selectivity toward methane with high reproducibility toward methane gas.

Abstract

Herein, methane-sensing materials, including SnO₂, Pt/MWCNTs, and SnO₂-Pt/MWCNTs were synthesized and used to measure the sensing responses to 100−10000 ppm methane at room temperature. However, results showed presence of Pt and MWCNTs has significant effect on increasing the response of SnO₂ sensors toward methane gas. Exposed to 100 ppm methane, SnO₂, Pt/MWCNTs and SnO₂-Pt/MWCNTs demonstrated 1 %, 19.95 % and 28.25 % sensing response at room temperature, respectively. Using Pt/MWCNTs, enhanced sensing features of the bare SnO₂ and the obtained hybrid sensor indicated more stability at continuous working cycles. At a lower detected concentration of methane, time of response and recovery of SnO₂-Pt/MWCNTs nanohybrid sensor were 176 s and 763 s, respectively. A methane sensing mechanism was also suggested based on the experiments.

Introduction

Releasing volatile organic compounds (VOCs) including methane, CO_x, SO_x, NO_x, and H₂S in developed countries and especially the countries facing the beginning of industrialization with the most polluted cities in the world leads to raised environmental concerns about the biophysical issues which have already crossed the threshold and prescribed limit values [1].

Detecting methane is a permanent concern for mines, and chemical industries with a regard to safety of production processes, equipment, human resources, training, and capacity. In recent years, public awareness of environmental issues, like greenhouse effect has attracted growing attention due to greater demand for natural gas in houses as well as the fuel for motor vehicles, and, therefore, measurement of emissions and monitoring of methane is of utmost significance. As a result of this awareness, the robust demand has initiated for consistent, low-cost, convenient, easily-handled, portable, highly sensitive, and selective methane sensors [1,2].

Special geometry and amazing feature of being a material with all-surface reacting capacity, carbon nanotubes (CNTs) deal with great potential to be utilized as gas sensor devices in mild condition. As reported in [3], CNTs are very sensitive to their surrounding environments. CNTs-based Gas sensors have been exposed to sense nitrogen [4], hydrogen [5], ammonia [3,6], and other gas and vapors [7] even in their bare forms (without using dopants or hybrid materials).

Conversely, the CNTs have definite restrictions for application of gas sensor containing long time duration for recovery, limited detection capacity in terms of the gas type, and strong affinity to environmental conditions such as the presence of humidity and other gases. On the other hand, the sensitivity of CNTs can be enhanced using activated materials [8].

As a more serious issue related to CNTs, agglomerations of CNTs are inferable from high degrees of surface area and solid Van der Waals forces of fascination among the tubes [8,9]. Accordingly, a fundamental necessity of the uniform scattering of CNTs in a dispersion matrix, for their potential application is limited. Manufacturing nanocomposites, Pt or Pd nanoparticles can keep this agglomeration on the outside of CNT. Moreover, full preferred standpoint of the comparative properties of these materials can be taken [10,11]. Additionally, it has been recommended that nanocomposite of metal-doped multi-walled carbon nanotube (MWCNT) has practical applications for detection of inert gas. Sensing property of metal-doped MWCNT can be achieved by loading proper catalytic metals that have a good interaction with methane or carbon dioxide [12].

Recently, Pd-single-walled carbon nanotubes were coated using the sputtering method to detect methane at room temperature by Lu et al. [13]. In another study, carbon nanotubes and nanofibers were synthesized by Roy et al. using electrodeposition process to examine their affectability to methane [14]. Besides, Pd-MWCNT nanocomposites were prepared for sensing CH₄ [12]. Besides, Jesus et al. [15] have announced a high affectability with direct reaction extent and detection limit for methane, using Pt/CNTs based sensors [15].

As an n-type semiconductor, metal oxide, tin dioxide (SnO₂) has a band-gap of 3.6 eV [16], which makes it a great sensing material [17,18]. Besides, SnO₂ has been considered as high-performance gas sensor by various research groups thanks to the ability to adsorb gaseous phase atoms [[18], [19], [20]]. New researches by Xue et.al have introduced WO₃ [21] and Pt [22] doped SnO₂ as high response methane sensors in moderate temperatures.

Nonetheless, detecting properties of SnO₂ frequently experienced a degradation coming about the growth of aggregations among the nanoparticles [23]. Furthermore, as other gas-type sensing semiconductors, a high operating temperature is required for SnO₂ sensors which carries much inconvenience for commonsense applications and at times of detecting combustion gases, even leads to unsafe conditions [24,25]. SnO₂ and metal-doped SnO₂ nanocomposites are currently commercially available as respectable sensors [3]. Much exertion has been made to improve gas-sensitivity just as to lessen working temperature by presenting dopants or diminishing SnO₂ molecule size to the nanoscale (<10 nm) [[26], [27], [28], [29]].

With this respect, combination of SnO₂ and carbon nanotubes with metals nanoparticles appears to be encouraging for applications in gas sensors [30]. Along these lines, expansive surface to volume proportion, nearness of imperfections, impact on the charge exchange, and porous structure of CNTs present effective active sites for interaction with gas molecules [31], while some imperative impediments, like poor gas detecting response, high working temperature, low selectivity, and stability are identified with the unblemished SnO₂ nanomaterials in gas detecting applications and can be limited by joining metals nanoparticles and CNTs [1,32]. Moreover, the problem of aggregations of CNTs in dispersion medium could be resolved.

Although there are several reports about hybrids such as SnO₂-CNT [[33], [34], [35]], SnO₂-Pt [36] and ternary hybrid of SnO₂-Pd and graphene [37] as methane sensors, to the best of our knowledge, there is no report on ternary hybrids of SnO₂ Pt and CNT acting as methane sensors.

Herein, we reported the synthesis of a uniform, reversible sensing system based on hybrid nanostructures containing SnO₂ nanocrystals distributed on the surface of Pt-doped multi-walled CNTs (Pt/MWCNTs). The application of SnO₂–Pt/MWCNT hybrid was evaluated combining high-performance MWCNTs with popular SnO₂ and Pt sensing material as an outstanding catalytic system. It was observed that SnO₂–Pt/MWCNT hybrid structure exhibits high sensitivity toward low-concentration of CH₄ gas at room temperature which leads to a more preferable operating temperature for hybrid sensing platforms as low as room temperature, in contrast to essentially high temperature (200 °C or more) for conventional SnO₂ sensors.