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Functional Nanostructured Materials (including low-D carbon)

Graphene Decorated with Silver Nanoparticles as a Low-Temperature Methane Gas Sensor Rezvan Ghanbari, Rosa Safaiee, Mohammad Hossein Sheikhi, M.M. Golshan, and zahra karami horastani ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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ACS Applied Materials & Interfaces

Graphene Decorated with Silver Nanoparticles as a Low-Temperature Methane Gas Sensor Rezvan Ghanbari a, Rosa Safaiee* b, Mohammad H. Sheikhi c, Mohammad M. Golshan a, Z. Karami Horastani d a Physics

Department, College of Sciences, Shiraz University, Shiraz, Iran

b Faculty

c Department

of Advanced Technologies, Shiraz University, Shiraz, Iran

of Communication and Electronics Engineering, School of Electrical and Computer Engineering, Shiraz University, Shiraz, Iran

d Department

*

of Electrical Engineering, Shiraz Branch, Islamic Azad University, Shiraz, Iran

Corresponding author: [email protected]; Tel: +98-71-36139661; Fax: +98-71-36460839.

KEYWORDS: Decorated graphene, Silver nanoparticle, Methane sensor, Low temperatures, Humidity

1 Environment ACS Paragon Plus

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ABSTRACT This paper is devoted to an investigation on the methane sensing properties of graphene (G), decorated with silver nanoparticles (AgNPs), under ambient conditions. To do so, we first present an effective modification in the standard manner of decorating graphene by AgNPs. From Structural analysis of the product (AgNPs/G), it is concluded that graphene is indeed decorated by AgNPs of a mean size, 29.3 nm, aggregation-free with a uniform distribution. The so-produced material is then used, as a resistivity-based sensor, to examine its response to the presence of methane gas. Our measurements are performed at relatively low temperatures, for various silver-to-graphene mass ratios (SGMRs) and methane concentrations. To account for the effects of humidity, we have made the measurements, at room temperature, for different levels of humidity. Our results demonstrate that an increase in the SGMR enhances the response of AgNPs/G to methane with an optimum value of SGMR ≅ 12%. It is also illustrated that for methane concentrations less than 2000 ppm the maximal response increases linearly and rapidly, even at room temperature. Moreover, we demonstrate that AgNPs/G is of low limit of detection, highly stable, selective, reversible, repeatable and sensor-to-sensor reproducible, for methane sensing. The results thus promise a low-cost and simple-to-fabricate methane sensing device.

1. INTRODUCTION Development of efficient devices to sense methane gas (CH4), with wide industrial applications, has formed a challenging task for the past several decades.1-5 Interest in this task has much grown with the advances in petro-chemical industry, still being an active area of research.1,6,7 Recent studies, moreover, point to the fact that methane gas in the environment has reached an unacceptable level.1,8 Along these lines we may also mention recent uses of CH4, of


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specific doses, in medical diagnosis. 9,10 To this end, design of practical methane gas sensors of low production cost which readily operates at low temperatures is most desirable. Moreover, the sensor has to be of high sensitivity and miniature sizes. Owing to the fact that the current CH4 sensors suffer from, low sensitivity,2,7 highly power consuming,11,12 high cost,13 etc.,5,14,15,16 research on novel materials for compensating such short comes is of much interest. It is worth mentioning that the presently available inexpensive metal-oxide based CH4 sensors commonly operate at relatively high temperature and thus consume considerable power.3,4 As the present state of fabrication of graphene, a carbon single sheet of honey-comb structure, has made this material commercially available, in the present article we take advantage of the properties of graphene, when decorated by silver nanoparticles, to propose a novel methane gas sensor. The results of the present work, along with the properties of graphene, pave the way for the development of a nano-scale methane gas sensor which meets a reasonable portion of the desired demands. In the development of a sensing device, for any entity, comparison is normally made between the changes in physical properties of the main elements forming the device, in the presence and absence of the entity. Such changes may include variations of temperature,17-20 mechanical properties,21,22 electrical and/or optical properties.23-27 Amongst such physical properties, conductance (resistance) of the sensing material, because of accurate and ease of measurements, is of considerable importance. 28 To this end, the resistance (conductance) of the chosen material should notably and quickly vary when the entity is in contact with the device.29 A vivid candidate for applications of this sort, as is well known, is graphene.30-32 The fundamental principle for graphene based gas sensors is the change in their electrical conductivity due to adsorption of gas molecules on graphene surface, acting as a donor-acceptor system. 29,33,34 The



principal agent responsible for this behavior is the π-electrons. 33,34 In what follows we illustrate that such properties, arising from the π-electrons, are much enhanced when graphene sheet is decorated with silver nanoparticles. The behavior of the so-called π-electrons in graphene, as quasi-Dirac particles, has been the subject of immense investigation. 33-35 The vast interest in graphene stems from its very unusual electronic structure, which leads to high electronic mobility, essential for the development of sensing devices based on current measurements.36,37 To this end, expedition of resources that control the π-electronic states, in particular charge carriers, is of great importance. The controlling agents are normally divided into external and internal categories.38 The former may be generated by decorating graphene sheets with elements of specific characteristics. Although it is not of concern in the present report, we may mention that the internal ones is formed mostly by the well-known Rashba spin-orbit interaction.38-40 In graphene, moreover, the ratio of area to volume (due to negligible thickness) is very high and thus the probability of getting in contact with gas molecules becomes large. 34,35,41 It has also been well established that electric signals in graphene are almost noise free (because of almost perfect crystalline structure),33,42,43 so that signal-to-noise ratio in current measurements is high. We also note that at low temperatures, the gas sensing mechanism of graphene is in fact a physical adsorption type,12,33,44 making it a reversible and repeatable sensing device. Such extraordinary features are of course desirable for the design of new generations of solid-state electronic devices, with on-demand characteristics, to be employed as a methane gas sensor. 2,12,45,46 In spite of these benefits, however, pristine graphene has been demonstrated to be poor in responding to the presence of methane gas.12 This point is mainly due to the fact that interaction between pristine graphene and methane molecules, being very stable with C-H bond of 439


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kJ/mol,47 is too weak.44,48,49 To enhance the interaction, thus the sensitivity, with methane gas, in the present article we suggest the decoration of pristine graphene with a transition metal, namely silver, in the form of nanoparticles. To this end, decoration of plain graphene with silver nanoparticles, a noble metal with nearly free electrons that can easily be formed into crystalline nanoparticles and adsorbed onto plain graphene, has proven to be of great value for sensing ammonia and nitrogen dioxide.50-53 It is also worth mentioning that amongst transition metals, silver nanoparticles are of lower toxicity and thus of more bio-compatibility.54 Adsorption of nanoparticles with these properties also prevents the aggregation of graphene sheets.55,56 We shall make more comments on the role of silver-nanoparticle decoration in section 6. At this stage of the discussion, we may mention that decoration of graphene by other materials, in particular, polyaniline, 2,57 gold, 58 palladium, 59 etc.60-67 has also been used for sensing common gases. Such points, therefore, encourage us to experimentally investigate the sensitivity of graphene, decorated with silver nanoparticles, as a methane gas sensor. The present article is organized as follows. We devote section 2 to a discussion on the physical reasons leading to CH4 sensitivity of AgNPs/G. The section provides an insight view of why the compound changes its resistance ( ∝ response) as reported in the following sections. In section 3, the experimental procedure of forming silver nanoparticles and adsorption by graphene is presented. Structural properties of the product are discussed and verified, through inductivelycoupled-plasma (ICP), Raman analysis, X-Ray photoelectron spectroscopy (XPS), particle-sizeanalysis (PSA) and energy-dispersive-Xray-spectroscopy (EDS) tests along with scanningelectron-microscopy (SEM) and transmission-electron-microscopy (TEM) images, in section 4 and in the supporting file. The material presented in this section verifies that silver nanoparticles of specific sizes are uniformly adsorbed onto the graphene sheet. The experimental set-up for



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response (resistivity) measurements of the decorated graphene forms the subject of section 5. Details of response measurements, along with the justifications of the results, are presented in sections 6. In particular, we demonstrate in section 6 that our sensor is of low limit of detection, highly reversible, repeatable and sensor-to-sensor reproducible, as well as operational at room temperature. In this section we also point out the advantages of our methane gas sensing device compared with those of previous reports. We outline the more important results of the present investigation in the concluding section.

2. MECHANISM OF METHANE GAS SENSING BY THE PROPOSED MATERIAL In the present section we discuss the physical reasons for the response of AgNPs/G towards the presence of CH4, as predicted and characterized in the following sections. To begin with, we note that as ambient oxygen molecules are adsorbed by graphene, the π-electrons migrate from graphene to O2 and/or O, changing the hybridization of carbon atoms from sp2 to sp3. These processes are well known7,44,58 and can be summarized as, O2 (Adsorbed) + e (From sensing material) → O2- (Adsorbed)

(1)

O2- (Adsorbed) + e (From sensing material) → 2O- (Adsorbed).

(2)

Although both processes lead to ionized oxygen, the former is predominant at low temperatures (

Graphene Decorated with Silver Nanoparticles as ... - ACS Publications

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