Synthesis of zinc oxide semiconductors-graphene nanocomposites by microwave irradiation for application to gas sensors

Accepted Manuscript Title: Synthesis of zinc oxide semiconductors-graphene nanocomposites by microwave irradiation for application to gas sensors Author: Hyoun Woo Kim Yong Jung Kwon Ali Mirzaei Sung Yong Kang Myung Sik Choi Jae Hoon Bang Sang Sub Kim PII: DOI: Reference:

S0925-4005(17)30576-2 http://dx.doi.org/doi:10.1016/j.snb.2017.03.149 SNB 22061

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

14-10-2016 17-3-2017 28-3-2017

Please cite this article as: H.W. Kim, Y.J. Kwon, A. Mirzaei, S.Y. Kang, M.S. Choi, J.H. Bang, S.S. Kim, Synthesis of zinc oxide semiconductors-graphene nanocomposites by microwave irradiation for application to gas sensors, Sensors and Actuators B: Chemical (2017), http://dx.doi.org/10.1016/j.snb.2017.03.149 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ip t

Synthesis of zinc oxide semiconductors-graphene nanocomposites by microwave irradiation for application to gas sensors

Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea

The Research Institute of Industrial Science, Hanyang University, Seoul 133-791, Republic of Korea Department of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea

d

M

c

an

b

us

a

cr

Hyoun Woo Kima,b,* , Yong Jung Kwona, Ali Mirzaeib, Sung Yong Kanga, Myung Sik Choia, Jae Hoon Banga, Sang Sub Kimc,**

te

Key words: Microwave irradiation, Gas sensor, ZnO, Graphene, Nanocomposite.

Ac ce p

* Corresponding author at: 222 Wangsimni-ro, Seongdong-gu, Seoul 133-791, Korea. Tel: +82 10 8428 0883; fax: +82 2 2220 0389. ** Corresponding author at: 100 Inha-ro, Nam-gu, Incheon 402-751, Korea. Tel: +82 32 860 7546; fax: +82 32 862 5546 E-mail: [email protected] (H.W. Kim), [email protected](S.S. Kim).

1

Page 1 of 40

Abstract Microwave (MW) irradiation has obtained extensive importance in the field of synthesis and treatment of nanoparticles, because of its faster, cleaner and cost effectiveness than the other

ip t

conventional and wet chemical methods. In this study, ZnO/graphene nanocomposites were prepared and subsequently post-treated by MW irradiation. Responses of the MW irradiated ZnO/graphene nanocomposites sensors were tested towards various gases including NO2,

cr

ethanol, acetone, toluene and CO and the results were compared with those of pristine ZnO and ZnO/graphene sensors without MW irradiation. It was demonstrated that the MW

us

irradiated sensor had much higher response particularly to NO2 gas along with superior selectivity and shorter response/recovery times in comparison to unirradiated ZnO/graphene

an

and pristine ZnO sensors. The possible underlying mechanism of this behavior is discussed in detail, mainly in terms of the MW-induced surface defects and the generation of finer ZnO nanoparticles. The results obtained demonstrated the beneficial effect of MW irradiation for

M

enhancing the NO2-gas sensing behavior of ZnO/graphene nanocomposites, opening a new door not only to a novel synthesis of semiconductors/graphene nanocomposites, but also to a

1. Introduction

te

d

cost-effective way of improving their sensing capabilities.

Ac ce p

Zin oxide (ZnO) with a band gap (Eg) of 3.37 eV, is a well-known semiconductor, which is very promising for the application in sensor devices due to its good electrical, structural and environment-friendly properties as well as high response to different reducing and oxidizing gases [1,2]. On the other hand, high response to different gases, is responsible for the lack of selectivity in the ZnO based gas sensors and this problem has motivated researchers to improve selectivity of ZnO-based gas sensors [3]. Among different strategies, one possible solution is to make composites with p-type materials such as CuO [4], Cr2O3 [5] or graphene [6,7]. The incorporation of p-type materials will inevitably introduce the p-n heterojunctions. This heterojunctions will contribute to the additional modulation of resistance and furthermore, without conducting electrons in ZnO being smeared into the p-type nanomaterials. Among the p-type materials as sensing material, graphene has unique and promising 2

Page 2 of 40

characteristics. This is due to its low cost, outstanding electrical conductivity (as result of remarkable electron mobility (200 000 cm2 V-1 s-1 [8])), high density of electronics states (~1012 cm-2 [8]), good thermal/chemical/physical stability, low Johnson noise, hydrophilicity and optical transparency. More importantly, it has a huge theoretical surface area (2630 m2 g), in which all atoms of a single-layer graphene sheet are exposed to the air. Accordingly,

ip t

1

they can adsorb gas molecules and provide the extraordinary sensing sites per unit volume [9].

cr

Therefore, ZnO (n-type)/graphene (p-type) composite can be a novel sensor material that can show outstanding sensing properties to a specific gas.

us

Recently, MW-assisted irradiation has been known as a novel and popular way for the synthesis of nanomaterials, mainly due to its unique advantages for reduction of synthesis

an

time. Indeed, due to the penetration of MWs into the material, heat is throughout fully generated in the material`s volume leading in a so-called volumetric heating. In other words, in conventional heating, there is always a temperature gradient between surface and bulk

M

owing to the transmission of heat through convection, conduction and radiation to the material. Therefore edges and corners being much hotter than the inside of the material. On

d

the other hand, MW irradiation results in fast synthesis of nanomaterials without temperature gradient [10]. In addition, in comparison with the traditional heating, MW heating offers

te

advantages like rapid and uniform heating, high reaction rates (short reaction times), increased reaction selectivity, and energy saving [11]. Moreover, the method can lead to the

Ac ce p

synthesis of materials with smaller and narrow particle size distribution, high purity, and enhanced physicochemical properties. Finally, MWs processes are able to synthesize nanomaterials in a large-scale, which is very important from an industrial point of view. [12,13].

Several researchers report the effect of MW irradiation on the gas sensing properties of metal oxides nanoparticles (NPs) [14]. In particular for ZnO, Qurashi et al. [15] reported enhanced H2 gas sensing of MW irradiated ZnO nanowires and similarly Rai et al. [16] reported increase of CO response of ZnO nanorods after irradiation with MWs. Tan et al. reported synthesis of ZnO nanorod arrays using a microwave-assisted hydrolysis, revealing that ZnO nanorods, composing of a higher (002) plane and defects-rich, showed the highest response to CO gas [17]. Liang et al. reported MW-assisted hydrothermal synthesis of ZnO structures 3

Page 3 of 40

with flower-like and rod-like morphologies. The flower-like ZnO possessed a higher sensor response than the rod-like samples, originated from the plenty of point-contacts and low stacking density, where they facilitated the surface-potential-barrier-governing processes and percolation of target gas molecules [18]. Li et al. prepared a ZnO yolk-shell structure by an

ip t

efficient MW hydrothermal route. They reported that the unique porous yolk-shell nanostructure which had a large numbers of well-defined surface pores was the main reason

cr

of the observed high response [19]. Ridha et al. fabricated nanoporous ZnO directly on the substrate surface by MW-assisted method. The high response was related to the porous

us

structure of nanoporous ZnO with a large surface area [20]. Hamedani et al. synthesized CeO2-doped ZnO nanostructures via a MW-assisted method. They reported that the high concentration of oxygen vacancies along with improved surface basicity induced by the

an

presence of CeO2 were the main reasons of a high response in these nanostructures [21]. Bagheri et al. reported synthesis of Sm-doped ZnO flower-like nanostructures for selective

M

detection of C2H5OH. By changing of the Sm amount and the sensing temperature, a highly selective sensor to C2H5OH was obtained [22]. Accordingly, we realize that surface defects, vacancies, porous structures, and potential barriers generated or increased by MW irradiation

d

will certainly enhance the sensing performances.

te

These particular, defective, and heterogeneous structures will be further enhanced by forming nanocomposites. For example, the sensitivity of 1D α-MoO3/5 wt% rGO

Ac ce p

nanocomposite prepared by MW-assisted method was higher than that of pure α-MoO3, owing to the formation of p-n nano-junctions and large surface area provided by the rGO substrates which facilitated gas adsorption and diffusion and finally mass transport [23]. Accordingly, the sensing properties of pristine ZnO structures will be further enhanced not only by incorporation of graphene but also by MW irradiation. Furthermore, the synergistic effects of both factors will additionally enhance the sensing capabilities. However, as far as the authors know, there is no published study about gas sensing properties of ZnO/graphene nanocomposites irradiated with MW irradiation. Therefore, herein, we report for the first time ZnO/graphene nanocomposites synthesized by utilizing the MW irradiation. This MW irradiated sensor demonstrates superior selectivity and sensitivity to NO2 gas, which is a highly toxic gas [24], compared to unirradiated ZnO/graphene nanocomposites and ZnO sensors. Since NO2 is one of the most harmful gases, it is urgently required to develop highly 4

Page 4 of 40

sensitive and fast sensors for detecting NO2 at low concentrations (Table S1, Supplementary Material). 2. Experimentation

ip t

2.1 Synthesis and MW irradiation

Commercial ZnO nanopowders (Daejung Chemicals & Metals Co., LTD) and expandable

cr

graphite (Hyundai Coma Industry) were used as starting materials. 1g of expandable graphite was put on the alumina crucible and irradiated in microwave oven (LG, Model LGMM-

us

M301) with a 2.45 GHz frequency and 1 kW power for 1 min. After irradiation, it was dispersed in ethanol and sonicated for 10 min to exfoliate the graphite. Subsequently exfoliated graphite was dried in a vacuum oven. After three times repetition of this process

an

thin layers of graphene flakes were obtained. Then, ZnO and graphene powders (0.01 wt%) were sonicated in ethanol and uniform powders were collected on filter paper with an

M

aspirator. The obtained powders were heated by a microwave oven at a power of 1 kW for 5 minutes with a frequency of 2.45 GHz, in alumina crucible. MW irradiated ZnO-graphene nanocomposite powders were dispersed in ethanol and using a spray gun were spray-coated

d

on a heated SiO2 substrate (140-160°C). Fig. 1 schematically shows the synthesis steps for

te

preparation of MW irradiated ZnO/graphene nanocomposites. 2.2 Material characterization

Ac ce p

The morphology of products were studied using scanning electron microscopy (SEM Hitachi S-4200), and transmission electron microscopy (TEM-Philips CM 200) operating at 200 kV. Photoluminescence (PL) was recorded on a Hitachi F-7000 FL Spectrophotometer by a 325 nm excitation from Xe lamp at room temperature. 2.3 Procedure for gas sensing tests The present gas dilution and sensing system was explained in the previous papers of authors (Fig. 2) [25-29]. For gas sensing tests, top-top electrode configuration was applied. For this purpose a double layer electrode comprising an Au layer (300 nm thick) and a Ti layer (50 nm thick) were sequentially deposited on the specimens. By changing the mixing ratio of dry air to the target gas (NO2), the gas flow was precisely controlled. Final concentration of target gas was set to be 1, 3 and 5 and 10 ppm using mass flow controllers (MFCs) with total stream 5

Page 5 of 40

of 500 sccm. All measurements were performed in a special gas chamber at the optimum temperature of 300oC and the sensors’ resistance data were collected by means of a multimeter (Keithley 2400). The operating temperature of 300oC used for the ZnO-graphene nanocomposite sensors of this study is certainly high, when the developed sensor material is

ip t

fabricated in the form of the conventional device structure. However, the sensor material can be integrated into a sensor device with the micro-electro mechanical system architecture, only

cr

a small amount of power will be consumed during the sensor operation at 300oC. As the sensors showed n-type behavior, the sensor response (R) was calculated as R = Rg/Ra, (for

us

NO2) and R = Ra/Rg (for reductive gas), where Ra is the resistance in the presence of air and Rg is the resistance in presence of the target gas. The response and recovery times were defined as the time to reach 90% of the final resistance after injection of the target gas (tres90)

an

and the time to recover 90% of the initial resistance after injection of air (trec90) respectively. To study the effects of humidity, we studied the sensing behaviors of MW irradiated

M

ZnO/graphene nanocomposites in dry and humid air (RH 19.1-79.4%, measured at 25°C) at the sensor operating temperature of 300°C. The schematic outline of sensing apparatus is shown in Fig. S1 in Supplementary Material. The actual temperature and RH value of the

d

mixed humid gas will be measured before being loaded into the chamber by the RH probe.

te

The temperature of the mixed humid gas was measured to about 25°C. The RH values were measured to be 19.1, 36.2, 55.7, 65.4, and 79.4%, respectively. By the way, during the

Ac ce p

sensing tests, the heater temperature under the sensor samples was set to 300°C.

3. Results and discussions

To study morphology of synthesized powders, SEM analyses were performed and the results are presented in Fig. 3. Fig. 3(a) indicates the SEM image of pristine ZnO NPs, consisting ZnO NPs with nearly spherical shape, narrow particle size distribution, and approximate average diameter of below 100 nm. In the Figs. 3(b) and 3(c) the SEM images of MW irradiated and unirradiated ZnO/graphene nanocomposites, are shown respectively. The main difference is that in irradiated sample, very fine secondary ZnO NPs, in addition to primary ZnO NPs exist, as a result of MW irradiation on the sample. These fine powders with high surface area can affect the sensing behavior of resultant product. To have a better insight to 6

Page 6 of 40

the sample morphology, TEM analyses were performed (Fig. 4). The low magnification TEM image of ZnO NPs/graphene irradiated with MW is shown in Fig. 4(a). It consist of primary ZnO particles, graphene sheets and the secondary ZnO NPs. High magnification TEM image of ZnO NPs/graphene irradiated with MWs are shown in Fig. 4(b), where the ZnO NPs with

ip t

hexagonal shapes are visible along with graphene sheets. Lattice-resolved TEM images are presented in Figs. 4(c) and 4(d). In Fig. 4(c), the resolved spacing of neighboring parallel

cr

fringes were approximately 0.281 nm and 0.260 nm, respectively, which were indexed to (100) and (002) lattice planes of simple hexagonal ZnO respectively. In Fig. 4(d), the

us

spacings of two neighboring fringes were approximately 0.281 nm and 0.21 nm, respectively, corresponding to the (100) lattice plane of simple hexagonal ZnO and the (100) lattice plane of carbon. Fig. 4(c) indicates the polycrystalline nature of ZnO structures. The corresponding

an

SAED pattern (Fig. 4(e)) shows the ring patterns belong to (101), (100) and (002) planes of polycrystalline hexagonal ZnO and (100) plane of carbon. To sum up, SEM and TEM

M

analyses approve successful synthesis of ZnO/graphene nanocomposites. Therefore, the gas

te

3.1. Gas sensing studies

d

sensing tests were performed on the synthesized powders.

The preliminary gas sensing tests showed that the optimum working temperature was 300°C.

Ac ce p

In fact in this temperature, thermal energy is higher than activation energy barrier of chemisorption, and therefore all gas sensing tests were performed at this temperature. Fig. 5 shows the dynamic resistance changes of all sensors towards 1, 3, 5 and 10 ppm NO2 gas at 300°C for (a) ZnO, (b) unirradiated ZnO/graphene nanocomposites and (c) MW irradiated ZnO/graphene nanocomposites. After exposure to NO2 gas, the resistance of sensors increased, which approved n-type behavior of all sensors, originating from intrinsic n-type behavior of ZnO, as the ZnO was the main material for sensing. ZnO (99.9 wt%) and graphene powders (0.01 wt%) were used as original source materials. Accordingly, the amount of graphene is very small compared to ZnO. Actually, abundant ZnO structures are connected, whereas small amount of graphene is dispersed and disconnected. Furthermore, all sensors showed the reversible behavior, where after stopping of NO2 gas, the resistance comes back to its initial value. The initial resistance of pristine ZnO was 120 kΩ, 7

Page 7 of 40

and the initial resistance of ZnO NPs/graphene and MW irradiated ZnO NPs/graphene was ~3 MΩ. The corresponding graphs with x-axis prolonged have been shown in Supplementary Material (Fig. S2), exhibiting that the sensing times have been sufficiently given to obtain the steady-state response values. Fig. 6(a) compares the response of all sensors at different NO2

ip t

concentrations. It is seen that the response of MW irradiated ZnO/graphene composite sensor is much higher than those of other sensors in all NO2 concentrations. In more details, the

cr

response to 1 ppm NO2 gas for pristine ZnO sensor, ZnO/graphene nanocomposite sensor and MW irradiated ZnO/graphene nanocomposite sensor were 1.34, 1.39, and 12.57, respectively

us

and for 10 ppm NO2 gas, the response of mentioned sensors were 6.76, 9.54 and 46.42, respectively. It can be seen that the order of increase in the response to all concentrations of NO2 is as follows: pristine ZnO sensor < ZnO/graphene nanocomposite sensor < MW

an

irradiated ZnO/graphene nanocomposite sensor. The reason for such trend will be discussed in the sensing mechanism part. Figs. 6(b) and 6(c) compare responses and recovery times of

M

all sensors towards different NO2 concentrations. Obviously, for all NO2 concentrations, the response and recovery times are shortest for MW irradiated ZnO/graphene nanocomposite sensor. One possibility is that this results from the creation of more complicated and defective

te

NO2 gas molecules.

d

structures as a result of MW irradiation, which facilitate the adsorption and desorption for

To evaluate the selectivity of sensors, they were exposed to the interfering gases. Figs. S3(a)-

Ac ce p

3(e) present the dynamic responses of all sensors to 10 ppm of several possible interfering gases i.e. ethanol, acetone, toluene, benzene, and CO gases at 300°C. Table 1 and Fig. 7 show the selectivity patterns for all of the sensors. If we define Q=RNO2/Ri where “i” represents an interfering gas at the same concentration of NO2 gas (10 ppm), it can be seen that the Qvalues for MW irradiated ZnO/graphene nanocomposite sensor is much higher than other sensors. For example, Q-values for MW irradiated ZnO/graphene nanocomposite sensor for ethanol, acetone, toluene, benzene, and CO gases are 30.14, 25.50, 26.26, 29.81, and 20.81, respectively. In contrast, the Q-values are much lower for unirradiated sensors, for example the Q-values for ZnO/graphene nanocomposite sensor are 3.96, 5.05, 6.87, 7.02 and 7.64 for the same above mentioned gases respectively and those for pristine ZnO sensor are 4.57, 3.74, 5.04, 5.83 and 5.28, respectively. Higher Q-value shows the higher selectivity and it can be easily seen that with MW irradiation, the selectivity can be significantly increased, 8

Page 8 of 40

demonstrating that MW irradiation is an efficient strategy to increase the sensitivity and selectivity of NO2 gas sensors. Table 2 compares the NO2 response of other MW irradiated sensors with the present sensor [30-34]. It is noteworthy that the present ZnO/graphene nanocomposite gas sensor in our work exhibits the significantly higher sensor response, in

cr

ip t

comparison to those reported previously in the literature.

3.2. Gas sensing mechanism

us

The gas sensing mechanism of metal oxide semiconductors has been well documented. The response of metal oxide gas sensors is owing to the variations of the resistances as a result of

an

chemisorption of oxygen and the subsequent reactions between adsorbed oxygen and target gas [35]. ZnO is an n-type semiconductor because the amount of free electrons is higher than that of trapped electrons [33]. In air, molecular oxygen abstract electrons from the conduction

M

band of ZnO and become chemisorbed on the surface and on the grain boundaries in the form

Ac ce p

te

d

of O2−, O−, or O2- species (depend on the temperatures) [36,37]:

(1)

RT
(2)

100°C
(3)

T>300°C

(4)

As a result of capturing of electrons on the surface of ZnO NPs, a depletion layer will be formed (Fig. S4(a)), which has a higher resistance than bulk of ZnO due to decrease of electrons. When the pristine ZnO sensor is exposed to NO2, the electrophilic NO2 molecules can either directly abstract electrons from the conductance band of ZnO, or react with the absorbed oxygen ions, resulting in a decrease of the electron concentration and increase of depletion layer width (Fig. S4(b)), which eventually increase the sensor resistance [38]. During the recovery period, the absorbed NO2 species are desorbed, and the resistance comes back to its initial value [39].

9

Page 9 of 40

For ZnO/graphene nanocomposite, enhancement of NO2 response can be related to the several factors. Firstly, the homojunctions between ZnO NPs which provide a source for modulation of the resistance like pristine ZnO gas sensor. Second, it is possible that the graphene sheets with many overlapped and crumpled regions provide high surface areas, thus

ip t

can facilitate the effective access of NO2 gas to the ZnO surfaces. In other words, graphene nanosheets will provide a spillover effect, being similar to metal nanoparticles. Being similar

cr

to the present work, in our previous paper on the H2 gas sensor using reduced graphene oxide (rGO)-loaded ZnO nanofibers, the rGO nanosheets were supposed to exert a spillover effect,

us

playing a catalytic function in the adsorption, dissociation, and migration of H2 gas molecules [40]. Third, the active defects at the graphene/ZnO heterointerfaces will further enhance the sensitivity. The heterointerfaces will have a lot of defects. The adsorption of NO2 species will

an

significantly increase the sensitivity, because the active defects at the graphene/ZnO heterointerfaces will considerably facilitate the adsorption of NO2 species, which will

M

increase the depletion region. Similar suggestion was made in case of graphene/SnO2 heterointerfaces [41]. Fourth, a lot of graphene-ZnO heterojunctions exist on the grapheneZnO, which are expected to affect the response of the sensor. Based on work function

d

differences, there are two possibilities. One possibility is that work function of ZnO is larger

te

than that of graphene. Based on work function values in Table S2, it is likely that work function of ZnO is larger than that of graphene. Upon the generation of graphene/ZnO

Ac ce p

heterointerfaces, the electron accumulation region is developed on the ZnO surface. We surmise that charge transfer at heterojunctions will occur between graphene and ZnO due to difference between their work functions. It has been accepted as a general rule in the semiconductor devices society that the charge transfer will occur when two materials with two different work functions are contacted. Since electrons and holes are accumulated on the ZnO and graphene sides of the heterojunctions, it is possible that holes have been transferred from ZnO to graphene and/or or electrons have been transferred from graphene to ZnO. Although it may be inappropriate to exactly define the way how the equilibrium has been attained, several researchers describe that electrons flow from graphene (or reduced graphene oxide) to ZnO, if the work function of ZnO is larger than that of graphene or rGO [42-45]. This flow will also be regarded as the hole flow from ZnO to graphene. Graphene is generally regarded as a p-type material [46] and if graphene used in the present work is p-type, electron 10

Page 10 of 40

as a minority carrier will flow from graphene to ZnO and/or hole as a majority carrier (in graphene) will flow from ZnO to graphene. With the generation of electron accumulation layer, the initial resistance of ZnO will be decreased. By the adsorption of NO 2 gas, electrons will be transferred from ZnO to gas molecules and the resistance will be

ip t

increased. With the larger amount of electrons initially, the reduction of electrons in ZnO by the NO 2 adsorption will turn out to be a smaller response, in which the sensor

cr

response will be defined as R = Rg/Ra, (for NO2), where Ra and Rg are the resistances of the sensor in the absence and presence of the target gas, respectively. The other possibility is that

us

the work function of graphene is larger than that of ZnO. Upon the generation of graphene/ZnO heterointerfaces, to equate the Fermi level, ultimately the surface depletion region is developed on the ZnO surface. With the smaller amount of electrons initially,

an

the reduction of electrons in ZnO by the NO 2 adsorption will turn out to be a larger sensor response. Accordingly, the presence of graphene/ZnO heterointerfaces will enhance

M

the sensitivity by the increase of initial resistance, when the work function of ZnO is smaller than that of graphene. Although the initial resistance values are changeable depending on a variety of factors, our data on dynamic resistance curves, which indicate that the

d

incorporation of graphene increases the initial resistance of ZnO, are favorable for the

te

possibility that the work function of graphene is larger than that of ZnO in the present work. In addition, active defects at the graphene/ZnO heterointerfaces will enhance the sensitivity,

Ac ce p

by the enhanced adsorption of NO2 gas.

In the present case, due to high energy, MW irradiation can lead to partial evaporation of surface ZnO, forming the ZnO-related vapor and the subsequent deposition of finer ZnO NPs. The MW energy absorbed per unit volume Pabs by non-magnetic materials such as ZnO depends on dielectric properties as follow [47]:

(5)

Where f is the operating frequency of microwave radiation, ε 0 is the dielectric permittivity of free space, εeff is the effective relative dielectric loss factor, Erms is root mean square of the internal electric field,

is relative dielectric constant and tan δ is the loss tangent. In a short

11

Page 11 of 40

interval time of

, the absorbed MW power per unit volume is mainly converted into

thermal energy within the ZnO (assuming negligible diffusion and heat losses) [47]:

ip t

(6)

cr

Where ρ is the density, Cp is the specific heat capacity, ∆T is the temperature rise and ∆t is the time. While conventionally heated ZnO/graphene sensor has bigger ZnO NPs due to

us

agglomeration at high temperature during the long-time synthetic process, in MW irradiated ZnO/graphene nanocomposite sensor, much finer ZnO NPs have been deposited not only on the graphene sheets but also on the original ZnO NPs. Accordingly, a lot of homojunctions

an

are generated between newly formed finer ZnO NPs and original source ZnO NPs, which will facilitate the further modulation of the resistance (Fig. 8(a)). Also, a lot of heterojunctions

M

can be generated between newly formed finer ZnO NPs and original graphene nanosheets. Since most fine ZnO NPs are supposed to be discretely deposited on the graphene sheets, the generation of heterojunction will not directly contribute to the enhancement of sensing

d

behavior. However, there is a weak possibility that finer ZnO NPs are connected in some

te

parts, forming the path for electrical flow. In this case, a marginal contribution by the finer ZnO NPs to the enhanced resistance modulation is expected. In addition, the latter sensor,

Ac ce p

which is comprised of finer ZnO NPs, can provide much higher surface area relative to the former sensor, resulting in higher response to gases (Fig. 8(b)). Furthermore, it has been demonstrated that the response to NO2 gas was particularly enhanced by the MW irradiation. For example, by the MW irradiation, the sensor responses to NO2, ethanol, acetone, toluene, benzene, and CO were increased by 386.0, -36.1, -3.7, 27.3, 15.6, and 78.4%, respectively. While the responses to ethanol and acetone gases were decreased by the MW irradiation, it is noteworthy that the MW irradiation is especially effective to enhance the sensing behavior of NO2 gas. In our previous study, we reported the selective enhancement of NO2 gas sensing behavior in SnO2 nanowires by ion-beam irradiation [48]. In regard to the effect of different irradiations on the gas sensing characteristics, Sagade et al.[49] reported that ion-beam irradiation significantly increases the surface roughness of and increases the defect concentration and the sensing enhancement was attributed to the 12

Page 12 of 40

irradiation-induced defect sites, which facilitate the adsorption of gas species. Also Merdrignac et al. reported that ion irradiation changed the bulk and/or surface structures, resulting in improvements of gas sensing characteristics [50]. Kwon et al. showed that SnO2 nanowires irradiated with He ions (45 MeV) lead to the formation of surface Sn interstitials,

ip t

which eventually increase the response to NO2 [48]. The experimental results were approved by DFT calculations.

cr

Therefore, it can be inferred that surface defects has an undeniable role on the sensing enhancement of gas sensors. In fact, a lot of defects generated on the nanostructure increase

us

the surface area for effective gas adsorption on the surface, resulting in high response to gas species. For example, Phan et al. reported gas adsorption enhancement due to the existence of

an

the high concentration of defects [51]. The same results were reported by Liao et al.,where they demonstrated that higher oxygen vacancies will result in the increase of the gas

M

adsorption [52].

In the present case, due to high vapor pressure of ZnO, MW irradiation can lead to partial evaporation of surface ZnO, presumably leaving

and

defects. However, some

d

evaporated ZnO can be condensed to generate very fine ZnO NPs on the surfaces. Therefore

te

it can be supposed that MW irradiation generates zinc vacancies and oxygen vacancies in addition to increase of surface area by formation very fine ZnO NPs.

Ac ce p

It is known that such defects produce a donor level inside the conduction band. Creation of a donor level in the MW irradiated sensor will cause the adsorption of more NO2 gas molecules relative to unirradiated sensor. Moreover, electron affinity of NO2 (2.28 eV) is higher in comparison to adsorbed oxygen (0.43 eV). In NO2, nitrogen (N) contains one unpaired electron, which explain why chemisorption of NO2 gas is more likely in comparison to other gases. It forms bond with the surface of sensor and consequently promote the chemisorption [53] (Fig. 8(c)).

To study the possibility of appearance of defects in the MW irradiated ZnO/graphene nanocomposites, we compared PL and XPS data of MW irradiated ZnO/graphene nanocomposites with those of pristine ZnO NPs. Fig. 9 shows PL spectra of pristine ZnO NPs, ZnO NPs/graphene sensor without and with the MW irradiation. The PL spectra are 13

Page 13 of 40

comprised of a broad visible emission band and a sharp ultraviolet (UV) band. The former is due to the recombination related to structural defects, such as oxygen/zinc vacancies and zinc interstitials and the latter is due to the excitonic recombination [54-57]. It is noteworthy that the relative intensity of visible to UV emission has been increased by applying the MW

ip t

irradiation. For investigating the nature of defects, we carried out the XPS analyses. Fig. 10 shows the O1s spectra of pristine ZnO NPs and ZnO NPs/graphene with the MW irradiation.

cr

The O1s peaks can be deconvoluted into three sub-peaks, which are centered at 530, 531, and 532 eV, respectively. The sub-peaks at 530, 531, and 532 eV are associated with lattice

us

oxygen, oxygen vacancy, and reactive O-H molecules, respectively [58]. By the MW irradiation, the ratio of 530 eV-peak has been decreased. However, the ratios of 531eV- and 532 eV-peaks increased by 51.0 and 24.3%, respectively. Accordingly, it is more probable

an

that the oxygen vacancies have been generated by the MW irradiation. In addition, Fig. 11 shows the Zn2p spectra of pristine ZnO NPs and ZnO NPs/graphene with the MW irradiation.

M

The Zn 2p1/2 peaks can be deconvoluted into two sub-peaks, centered at 1045.6 and 1047.0 eV, respectively. The Zn 2p3/2 peaks can be deconvoluted into two sub-peaks, centered at 1022.51022.6 and 1024.0 eV, respectively. It is worthy to note that the ratios of 1024.0 eV- and

d

1047.0 eV-shoulders have been increased by the MW irradiation. These shoulders are known

te

to be related to Zn-associated defects, including zinc interstitials and zinc vacancies [59]. Accordingly, it is probable that the zinc vacancies and/or zinc interstitials have been

Ac ce p

generated by the MW irradiation. PL and XPS data coincidentally suggest that the MW irradiation will increase the defects, including oxygen/ zinc vacancies and zinc interstitials. The effect of humidity on the sensing behaviors of the MW irradiated ZnO/graphene nanocomposites sensors was studied. Figs. 12a and 12b show response curves and values of the MW irradiated ZnO/graphene nanocomposites, respectively. In humid air, the RH values were set to 19.1, 36.2, 55.7, 65.4, and 79.4%, respectively. The sensor responses at RH of 0, 19.1, 36.2, 55.7, 65.4, and 79.4%, are 46.42, 45.21, 42.97, 39.42, 35.66, and 32.27, respectively. The sensor responses at RH of 19.1, 36.2, 55.7, 65.4, and 79.4% are decreased by 2.6, 7.4, 15.1, 23.2, and 30.5%, in comparison with those in dry air, respectively. Although the recovery times vary in the rage of 40-66 s in an irregular manner, the response times tend to increase with increasing the RH value. The water molecules or species will be adsorbed in the humid environment. The adsorbed water molecules will prevent the chemisorption of 14

Page 14 of 40

oxygen species and target gas species. Accordingly, the incoming NO2 gas molecules will have a less chance to take a lot of electrons from SnO2 surfaces. Thus, it will result in a lower

ip t

sensor response and longer response time.

4. Conclusions

cr

We successfully synthesized ZnO/graphene nanocomposites with extraordinarily selective NO2 sensing properties, by MW irradiation. TEM/SEM analyses confirmed the formation of

us

the fine ZnO NPs on the ZnO/graphene nanocomposites. Based on the sensing studies with NO2, ethanol, acetone, toluene and CO gases, MW irradiation most significantly enhanced

an

the sensitivity of ZnO/graphene nanocomposites to NO2 gas. In terms of sensor response, response and recovery times, the MW irradiation considerably increased the sensing performances of NO2 gas. The significant improvement of NO2 sensing behavior by facile

M

and inexpensive MW irradiation of ZnO/graphene will be attributed to more creation of defects due to MW irradiation, increased surface area and additional generation of ZnOgraphene heterojunctions and ZnO-ZnO homojunctions by the formation of finer ZnO NPs.

d

Present study shows the importance of MW irradiation as a cheap, available, easy to use,

Ac ce p

te

clean and efficient strategy to enhance gas sensing properties of nanocomposites.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03013422). This work was supported by LG Display under LGD-(HYU) Incubation Program.

15

Page 15 of 40

References A. Mirzaei, S. Park, H. Kheel, G.J. Sun, S. Lee, C. Lee, ZnO-capped nanorod gas

ip t

[1]

sensors, Ceram. Int. 42 (2016) 6187-6197.

S.G. Chatterjee, S. Chatterjee, A.K. Ray, A.K. Chakraborty, Graphene-metal oxide

cr

[2]

nanohybrids for toxic gas sensor: A review, Sens. Actuators B 221 (2015) 1170-1181. T.J. Hsueh, S.J. Chang, C.L. Hsu, Y.R. Lin, I.C. Chen, ZnO nanotube ethanol gas

us

[3]

sensors, J. Electrochem. Soc. 155 (2008) K152–K155. [4]

Y. Hu, X. Zhou, Q. Han, Q. Cao, Y. Huang, Sensing properties of CuO-ZnO

an

heterojunction gas sensors, Mater. Sci. Eng. B 39 (2003) 41-43. [5]

S. Park, G.J. Sun, C. Jin, H.W. Kim, S. Lee, C. Lee, Synergistic effects of a combination of Cr2O3-functionalization and UV-irradiation techniques on the ethanol

M

gas sensing performance of ZnO nanorod gas sensors, ACS Appl. Mater. Interfaces 8 (2016) 2805-2811.

G. Singh, A. Choudhary, D. Haranath, A.G. Joshi, N. Singh, S. Singh, R. Pasrich, ZnO

d

[6]

50 (2012) 385-394.

J. Yi, J.M. Lee, W.I. Park, Vertically aligned ZnO nanorods and graphene hybrid

Ac ce p

[7]

te

decorated luminescent graphene as a potential gas sensor at room temperature, Carbon

architectures for high-sensitive flexible gas sensors, Sens. Actuators B 155 (2011) 264-269.

[8]

W. Yuan, G. Shi, Graphene-based gas sensors, J. Mater. Chem. A 1 (2013) 1007810091.

[9]

C.N.R. Rao, A.K. Sood, K.S. Subrahmanyam, A. Govindaraj, Graphene: the newtwodimensional nanomaterial, Angew. Chem. Int. Ed. 48 (2009) 7752-7777.

[10]

N.F. Hamedani, A.R. Mahjoub, A.A. Khodadadi, Y. Mortazavi, Microwave assisted fast synthesis of various ZnO morphologies for selective detection of CO, CH4 and

ethanol, Sens. Actuators B 156 (2011) 737- 742. [11]

X. Chu, T. Chen, W. Zhang, B. Zheng, H. Shui, Investigation on formaldehyde gas sensor with ZnO thick film prepared through microwave heating method, Sens. Actuators B 142 (2009) 49-54. 16

Page 16 of 40

[12]

A. Mirzaei, G. Neri, Microwave-assisted synthesis of metal oxide nanostructures for gas sensing application: A review, Sens. Actuators B 237 (2016) 749-775.

[13]

A. Srivastava, S.T. Lakshmikumar, A.K. Srivastava, K. Jain, Gas sensing properties of nanocrystalline SnO2 prepared in solvent media using a microwave assisted technique,

[14]

ip t

Sens. Actuators B 126 (2007) 583-587.

Z. Ai, K. Deng, Q. Wan, L. Zhang, S. Lee, Facile microwave-assisted synthesis and

cr

magnetic and gas sensing properties of Fe3O4 nanoroses, J. Phys. Chem. C 114 (2010) 6237-6242.

A. Qurashi, N. Tabet, M. Faiz, T. Yamzaki, Ultra-fast microwave synthesis of ZnO

us

[15]

nanowires and their dynamic response toward hydrogen gas, Nanoscale Res. Lett. 4 (2009) 948-954.

P. Rai, H.M. Song, Y.S. Kim, M.K. Song, P.R. Oh, J.M. Yoon, Y.T. Yu, Microwave

an

[16]

assisted hydrothermal synthesis of single crystalline ZnO nanorods for gassensor

[17]

M

application, Mater. Lett. 68 (2012) 90-93.

S.T. Tan, C.H. Tan, Y. Chong, C.C. Yap, A.A. Umar, R.T. Ginting, H.B. Lee, K.S. Lim, M. Yahayaa, M.M. Salleh, Microwave-assisted hydrolysis preparation of highly

d

crystalline ZnO nanorod array for room temperature photoluminescence-based CO

[18]

te

gas sensor Sens. Actuators B 227 (2016) 304-312. S. Liang, L. Zhu, G. Gai, Y. Yao, J. Huang, X. Ji, X. Zhou, D. Zhang, P. Zhang,

Ac ce p

Synthesis of morphology-controlled ZnO microstructures via a microwave-assisted hydrothermal method and their gas-sensing property, Ultrason. Sonochem. 21 (2014) 1335-1342.

[19]

X. Li, X. Zhou, Y. Liu, P. Sun, K. Shimanoe, N. Yamazoe, G. Lu, Microwave hydrothermal synthesis and gas sensing application of porous ZnO core-shell microstructures, RSC Adv. 4 (2014) 32538–32543.

[20]

N.J. Ridha, M.H.H. Jumali, A.A. Umar, F.K. Mohamad, Ethanol sensor based on ZnO nanostructures prepared via microwave oven, IEEE- Seventh international conference on sensing technology (2013) 121-126.

[21]

N. Faal Hamedania, A.R. Mahjouba, A.A. Khodadadib, Y. Mortazavi, CeO2 doped ZnO flower-like nanostructure sensor selective to ethanol in presence of CO and CH4, Sens. Actuators B 169 (2012) 67- 73. 17

Page 17 of 40

[22]

M. Bagheri, N. Faal Hamedania, A.R. Mahjoub, A.A. Khodadadi, Y. Mortazavi, Highly sensitive and selective ethanol sensor based on Sm2O3-loaded flower-like ZnO nanostructure, Sens. Actuators B 191 (2014) 283-290.

[23]

S. Bai, C. Chen, R. Luo, A. Chen, D. Lia, Synthesis of MoO3/reduced graphene oxide

ip t

hybrids and mechanism of enhancing H2S sensing performances, Sens. Actuators B 216 (2015) 113-120.

C. Jin, S. Park, C.W. Kim, C. Lee, S.W. Choi, K.H. Shin, D. Lee, Characterization and

cr

[24]

gas sensing properties of bead-like ZnO using multi-walled carbon nanotube

[25]

us

templates, Ceram. Int. 41 (2015) 7729-7734.

J.Y. Park, K. Asokan, S.W. Choi, S.S. Kim, Growth kinetics of nanograins in SnO2 fibers and size dependent sensing properties, Sens. Actuators B 152 (2011) 254-260. S.W. Choi, J.Y. Park, S.S. Kim, Synthesis of SnO2–ZnO core–shell nanofibers via a

an

[26]

novel two-step process and their gas sensing properties, Nanotechnology 20 (2009)

[27]

M

465603.

J.Y. Park, S.W. Choi, J.W. Lee, C. Lee, S.S. Kim, Synthesis and Gas Sensing

2554.

H.W. Kim, S.H. Shim, J.W. Lee, J.Y. Park, S.S. Kim, Bi2Sn2O7 nanoparticles attached

te

[28]

d

Properties of TiO2–ZnO Core-Shell Nanofibers, J. Am. Ceram. Soc. 92 (2009) 2551-

to SnO2 nanowires and used as catalysts, Chem. Phys. Lett. 456 (2008) 193-197. J. Y. Park, S. W. Choi, S. S. Kim, Fabrication of a highly sensitive chemical sensor

Ac ce p

[29]

based on ZnO nanorod arrays, Nanoscale Res. Lett. 5 (2010) 353-359.

[30]

Z. Wang, P. Sun, T. Yanga, Y. Gaoa, X. Li, G. Lu, Y. Du, Flower-like WO3

architectures synthesized via a microwave-assisted method and their gas sensing properties, Sens. Actuators B-Chem. 186 (2013) 734-740.

[31]

D.P. Volanti, A.A. Felix, M.O. Orlandi, G. Whitfield, D.J. Yang, E. Longo, H.L. Tuller, J.A. Varela, The role of hierarchical morphologies in the superior gas sensing performance of CuO-based chemiresistors, Adv. Funct. Mater. 23 (2013) 1759-1766.

[32]

P. Wang, W. Yang, X. Wanga, J. Hu, H. Zhang, Reduced graphene oxide modified with hierarchical flower-like In(OH)3 for NO2 room-temperature sensing, Sens. Actuators B-Chem. 214 (2015) 36-42.

[33]

M. Chen, Z. Wang, D. Han, F. Gu, G. Guo, Porous ZnO polygonal nanoflakes: 18

Page 18 of 40

synthesis, use in high-sensitivity NO2 gas sensor, and proposed mechanism of gas sensing, J. Phys. Chem. C 115 (2011) 12763-12773. [34]

W. Yang, P. Wan, X. Zhou, J. Hu, Y. Guan, L. Feng, An additive-free synthesis of In2O3 cubes embedded into graphene sheets and their enhanced NO2 sensing

[35]

ip t

performance at room temperature, ACS Appl. Mater. Interfaces 6 (2014) 21093-21100. T. Krishnakumar, R. Jayaprakash, N. Pinna, N. Donato, A. Bonavita, G. Micali, G.

wet chemical route, Sens. Actuators B 143 (2009) 198-204.

A. Mirzaei, S. Park, G.J. Sun, H. Kheel, C. Lee, CO gas sensing properties of

us

[36]

cr

Neri, CO gas sensing of ZnO nanostructures synthesized by an assisted microwave

In4Sn3O12 and TeO2 composite nanoparticle sensors, J. Hazard. Mater. 305 (2016) 130-138.

A. Mirzaei, M. Bonyani, S.G. Leonardi, K. Janghorban, B. Hashemi, G. Neri, Highly

an

[37]

stable and selective ethanol sensor based on α-Fe2O3 nanoparticles prepared by [38]

M

Pechini sol-gel method, Ceram. Int. 42 (2016) 6136-614416. S.S. Kim, S.W. Choi, H.G. Na, D.S. Kwak, Y.J. Kwon, H.W. Kim, ZnO-SnO2 branchestem nanowires based on a two-step process: Synthesis and sensing capability,

W. Yang, P. Wan, X. Zhou, J. Hu, Y. Guan, L. Feng, Additive-free synthesis of In2O3

te

[39]

d

Curr. Appl. Phys. 13 (2013) 526-532.

cubes embedded into graphene sheets and their enhanced NO2 sensing performance at

Ac ce p

room temperature, ACS Appl. Mater. Interfaces 6 (2014) 21093-21100. [40]

Z. U. Abideen, H. W. Kim, S. S. Kim, An ulatra-sensitive hydrogen gas sensor using reduced graphene oxide-loaded ZnO nanofibers, Chem. Commun. 51 (2015) 1541815421.

[41]

N. Tammanoon, A. Wisitsoraaat, C. Sriprachuabwong, D. Phokharatkul, A.

Tuantranont, S. Phanichphant, C. Lieshiran, Ultrasensitive NO2 sensor based on

ohmic metal-semiconductor interfaces graphene/flame-spray-made

SnO2

of

electrolytically

exfoliated

nanoparticles composite operating at low

temperature, ACS Appl. Mater. Interfaces 7 (2015) 24338-24352. [42]

K. Anand, O. Singh, M. P. Singh, J. Kaur, R. C. Singh, Hydrogen sensor based on graphene/ZnO nanocomposite, Sens. Actuators B 195 (2014) 409-415.

[43] R. Zou, G. He, K. Xu, Q. Liu, Z. Zhang, J. Hu, ZnO nanorods on reduced graphene 19

Page 19 of 40

sheets with excellent filed emission, gas sensor and photocatalytic properties, J. Mater. Chem. A 1 (2013) 8445-8452. [44]

Z. U. Abideen, A. Katoch, J.-H. Kim, Y. J. Kwon, H. W. Kim, S. S. Kim, Excellent gas detection of ZnO nanofibers by loading with reduced graphene oxide nanosheets,

ip t

Sens. Actuators B 221 (2015) 1499-1507.

[45] J. O. Hwang, D. H. Lee, J. Y. Kim, T. H. Han, B. H. Kim, M. Park, K. No, S. O. Kim,

cr

Vertical ZnO nanowires/graphene hybrids for transparent and flexible field emission, J. Mater. Chem. 21 (2011) 3432-3437.

M. Gautam, A. H. Jayatissa, Gas sensing properties of graphene synthesized by

us

[46]

chemical vapor deposition, Mater. Sci. Eng. C 31 (2011) 1405. [47]

R. Al-Gaashania, S. Radimana, N. Tabet, A.R. Dauda, Effect of microwave power on

an

the morphology and optical property of zinc oxide nano-structures prepared via a microwave-assisted aqueous solution method, Mater. Chem. Phys, 125 (2011) 846-

[48]

M

852.

Y.J. Kwon, S.Y. Kang, P. Wu, Y. Peng, S.S. Kim, H.W. Kim, Selective improvement of NO2 gas sensing behavior in SnO2 nanowires by ion-beam irradiation, ACS Appl. A.A. Sagade, R. Sharma, I. Sulaniya, Enhancement in sensitivity of copper sulfide

te

[49]

d

Mater. Interfaces 8 (2016) 13646-13658.

thin film ammonia gas sensor: effect of swift heavy ion irradiation, J. Appl. Phys. 105

Ac ce p

(2009) 043701. [50]

O.M. Merdrignac, P.T. Moseley, R. Rpeat, C.J. Sofield, S. Sugden, The modification of gas-sensing properties of semiconducting oxides by treatment with ionizing radiation, Sens. Actuators B 7 (1992) 651-655.

[51]

D.-T. Phan, G.-S. Chung, Effects of defects in Ga-doped ZnO nanorods formed by a hydrothermal method on CO sensing properties, Sens. Actuators B 187 (2013) 191-

199.

[52]

L. Liao, H. Lu, J. Li, H. He, D. Wang, D. Fu, C. Liu, W. Zhang, Size dependence of gas sensitivity of ZnO nanorods, J. Phys. Chem. C 111 (2007) 1900-1903.

[53]

A.A. Manea, M.P. Suryawanshi, J.H. Kim, A.V. Moholkar, Highly selective and sensitive response of 30.5 % of sprayed molybdenum trioxide (MoO3) nanobelts for nitrogen dioxide (NO2) gas detection, J. Colloid Interface Sci. 483 (2016) 220-231. 20

Page 20 of 40

[54]

H. W. Kim, S. H. Shim, J. W. Lee, Variation of ZnO shell thickness and its effects on the characteristics of coaxial nanowires, Nanotechnology 19 (2008) 145601.

[55]

B. Lin, Z. Fu, Y. Jia, Green luminescent center in undoped zinc oxide deposited on silicon substrates, Appl. Phys. Lett. 79 (2001) 943-945. P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J. Pham, R.

ip t

[56]

He, H.-J. Choi, Controlled growth of ZnO nanowires and their optical properties, Adv.

[57]

cr

Funct. Mater. 12 (2002) 323-331.

D.C. Reynolds, D.C. Look, B. Jogai, C.W. Litton, T.C. Collins, W. Harsch, G.

us

Cantwell, Neutral-donor-bound-exciton complexes in ZnO crystals, Phys. Rev. B 57 (1998) 12151-12155. [58]

H.B. Lee, R.T. Ginting, S.T. Tan, C.H. Tan, A. Alshanableh, H.F. Oleiwi, C.C. Yap,

an

M.H. Hj.Jumali, M. Yahaya, Controlled defects of fluorine-incorporated ZnO nanorods for photovoltaic enhancement, Sci. Rep. 6 (2016) 32645. F. Kayaci, S. Vempati, I. Donmez, N. Biyikli, T. Uyar, Role of zinc interstitials and

M

[59]

oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control defect

Ac ce p

te

d

density, Nanoscale 6 (2014) 10224-10234.

Biographies

Hyoun Woo Kim joined the Division of Materials Science and Engineering at Hanyang University as a full professor in 2011. He received his B.S. and M. S. degrees from Seoul National University and his Ph.D. degree from Massachusetts Institute of Technology (MIT) in Electronic Materials in 1986, 1988, and 1994, respectively. He was a senior researcher in the Samsung Electronics Co. Ltd. from 1994 to 2000. He has been a

21

Page 21 of 40

professor of materials science and engineering at Inha University from 2000 to 2010. He was a visiting professor at the Department of Chemistry of the Michigan State University, in 2009. His research interests include the one-dimensional nanostructures,

cr

ip t

nanosheets, and gas sensors.

us

Yong Jung Kwon received his B. Eng degree in materials science and engineering from Hanyang University, Korea in 2012. He is currently a graduate student in Hanyang

an

University. His research work is to focus on nanomaterials and their application to gas

M

sensors.

d

Ali Mirzaei received his B.S. degree in materials science and engineering from Isfahan

te

University, Iran in 2006. He received M.S. and PhD degree in materials science and engineering from Shiraz University, Iran in 2009 and 2016, respectively. He is currently

Ac ce p

a postdoctoral fellow in Hanyang University. His research work is to focus on the development of new gas sensors.

Sung Yong Kang received his B. Eng degree in materials science and engineering from Hanyang University, Korea in 2015. He is currently a graduate student in Hanyang University. His research work is to focus on nanomaterials and their application to gas sensors.

22

Page 22 of 40

Myung Sik Choi received his B. Eng degree in materials science and engineering from Inha University, Korea in 2014. He received his M. S. degrees from Inha University in

cr

focus on nanomaterials and their application to gas sensors.

ip t

2016. He is currently a graduate student in Hanyang University. His research work is to

us

Jae Hoon Bang received his B. Eng degree in materials science and engineering from Hanyang University, Korea in 2016. He is currently a graduate student in Hanyang

an

University. His research work is to focus on nanomaterials and their application to gas

M

sensors.

d

Sang Sub Kim joined the School of Materials Science and Engineering, Inha University,

te

in 2007 as a full professor. He received his B.S. degree from Seoul National University and his M.S and Ph.D. degrees from Pohang University of Science and Technology

Ac ce p

(POSTECH) in Material Science and Engineering in 1987, 1990, and 1994, respectively. He was a visiting researcher at National Research in Inorganic Materials (currently NIMS), Japan for 2 years each in 1995 and in 2000. In 2006, he was a visiting professor at Department of Chemistry, University of Alberta, Canada. In 2010, he also served as a cooperative professor at Nagaoka University of Technology, Japan. His research interests include the synthesis and applications of nanomaterials such as nanowires and nanofibers, functional thin films, and surface and interfacial characterizations.

23

Page 23 of 40

ip t cr us an M

Figure Captions

d

Fig. 1. Schematic illustration of different steps of synthesis procedure.

te

Fig. 2. Schematic outline of sensor apparatus.

Ac ce p

Fig. 3. SEM images of (a) pristine ZnO NPs, (b,c) ZnO NPs/graphene, (b) without and (c) with the MW irradiation.

Fig. 4. TEM analyses for ZnO/graphene irradiated with MW. (a) low magnification TEM image, (b) HRTEM image, (c) lattice-resolved image, and (d) SAED pattern. Fig. 5. Dynamic resistance curves of (a) pristine ZnO NPs sensor and (b,c) ZnO NPs/graphene sensor (b) without and (c) with the MW irradiation, towards 1, 2, 5 and 10 ppm of NO2 at 300°C.

Fig. 6. (a) Sensor responses of pristine ZnO NPs sensor and ZnO NPs/graphene sensor without and with the MW irradiation. Variation of (b) response times and (c) recovery times with varying the NO2 concentration in the range of 1-10 ppm.

24

Page 24 of 40

Fig. 7. Selectivity histogram of different sensors towards 10 ppm of various gases at 300°C. The gases include NO2, ethanol, acetone, toluene, benzene, and CO. Fig. 8. (a) Formation of finer ZnO NPs on the ZnO NPs/graphene nanocomposites. (b) NO2

ip t

adsorption on the surface of ZnO/graphene nanocomposites. (c) Incorporation of defects in ZnO by MW irradiation, promoting the chemisorption of NO2 gas.

cr

Fig. 9. (a) Overall PL spectra and (b) enlarged image of visible light emissions of pristine ZnO NPs, ZnO NPs/graphene, without and with the MW irradiation.

us

Fig. 10. O1s XPS spectra of (a) pristine ZnO NPs and (b) ZnO NPs/graphene with the MW irradiation.

an

Fig. 11. Zn2p XPS spectra of (a) pristine ZnO NPs and (b) ZnO NPs/graphene with the MW irradiation.

M

Fig. 12. Effect of humidity in sensing test on the sensing behaviors of the ZnO NPs/graphene sensor with the MW irradiation. The NO2 concentration was set to 10 ppm. The RH values were set to 0, 19.1, 36.2, 55.7, 65.4, and 79.4%. (a) Dynamic response values in humid air at

d

various RH values. Variation of (b) sensor responses, (c) response times, and (d) recovery

Ac ce p

te

times, with varying the RH values.

25

Page 25 of 40

ip t cr us an M d te Ac ce p Fig. 1

26

Page 26 of 40

ip t cr us an M d te Ac ce p Fig. 2

27

Page 27 of 40

ip t cr us an M d te Ac ce p

Fig. 3

28

Page 28 of 40

ip t cr us an M d Ac ce p

te

Fig. 4

29

Page 29 of 40

ip t cr us an M d te Ac ce p Fig. 5 30

Page 30 of 40

ip t cr us an M d te Ac ce p

Fig. 6

31

Page 31 of 40

ip t cr us an M d te Ac ce p

Fig. 7

32

Page 32 of 40

ip t cr us an M d Ac ce p

te

Fig. 8

33

Page 33 of 40

ip t cr us an M d te Ac ce p

Fig. 9

34

Page 34 of 40

ip t cr us an M d te Ac ce p

Fig. 10

35

Page 35 of 40

ip t cr us an M d te Ac ce p

Fig. 11

36

Page 36 of 40

ip t cr us an M d te Ac ce p RH (%)

0

19.1

36.2

55.7

65.4

79.4

Gas response (Rg/Ra)

46.42

45.21

42.97

39.42

35.66

32.27

Response time (sec)

99

132

173

160

176

180

Recovery time (sec)

66

43

40

64

41

41

Fig. 12

Table 1. Sensor responses and Q-values of different sensors towards 10 ppm of NO2, ethanol, acetone, toluene, benzene, and CO gases. 37

Page 37 of 40

Sensor response (Ra/Rg or Rg/Ra)

Q-value (RNO2/Ri)

ZnO-G

ZnO-G MW

ZnO

ZnO-G

NO2

6.76

9.55

46.41

-

-

-

Ethanol

1.15

2.41

1.54

4.57

3.96

30.14

Acetone

1.81

1.89

1.82

3.74

5.05

25.50

Toluene

1.34

1.39

1.77

5.04

Benzene

1.16

1.36

1.56

5.83

CO

1.28

1.25

2.23

cr

26.26

7.02

29.81

us

6.87

7.64

20.81

Ac ce p

te

d

M

an

5.28

ZnO-G MW

ip t

ZnO

38

Page 38 of 40

cr

ip t

Table 2. NO2 gas sensitivity of MW irradiated gas sensors in comparison with the present study.

Response

Conc. (ppm)

T (°C)

us

Composition/microstructure

Ref.

Ra/Rg

0.005

90

3

[30]

CuO NPs

50

250

2.75

[31]

rGO/ hierarchical flower In(OH)3

an

Flower-like WO3 architectures

1

25

1.19

[32]

0.5

170

120

[33]

5

RT

1.6

[34]

1

300

12.57

Present Work

M

porous ZnO polygonal nanoflakes In2O3 cubes into graphene sheets

Ac ce p

te

d

ZnO/graphene nanocomposites

39

Page 39 of 40

ip t

Highlights

us

cr

ZnO/graphene nanocomposites were prepared by microwave (MW) irradiation.

an

MW irradiated sensor had much higher sensing performances particularly to NO2 gas, in comparison to unirradiated ZnO/graphene and pristine ZnO sensors.

Ac ce p

te

d

M

The possible underlying mechanism is explained in terms of the MW-induced surface defects and the generation of finer ZnO nanoparticles.

40

Page 40 of 40