Microwave-assisted synthesis of metal oxide nanostructures for gas sensing application: A review

Accepted Manuscript Title: Microwave-assisted synthesis of metal oxide nanostructures for gas sensing application: a review Author: A. Mirzaei G. Neri PII: DOI: Reference:

S0925-4005(16)30967-4 http://dx.doi.org/doi:10.1016/j.snb.2016.06.114 SNB 20440

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Sensors and Actuators B

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27-12-2015 25-5-2016 20-6-2016

Please cite this article as: A.Mirzaei, G.Neri, Microwave-assisted synthesis of metal oxide nanostructures for gas sensing application: a review, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2016.06.114 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.

Microwave-assisted synthesis of metal oxide nanostructures for gas sensing application: a review

A. Mirzaei1, G.Neri2 1 2

Department of Materials Science and Engineering, Shiraz University, Shiraz, Iran

Department of Engineering, University of Messina, Contrada di Dio, 98166 Messina,Italy Corresponding Author :E-mail: [email protected]

ABSTRACT This review gives a comprehensive report on the microwave-assisted synthesis of metal oxides for applications in the field of gas sensing. In recent years, microwave heating technology has gained importance in the synthesis of metal oxides because of its faster, cleaner and cost effectiveness than conventional thermal heating. Further, due to the peculiarity of microwave heating mechanism, the synthesis of metal oxides in different nanostructured forms by microwave-assisted methods has been widely pursued and the nanomaterials thus obtained have been applied as sensing elements in chemoresistive gas sensors. Their gas sensing performances are here described and discussed in detail, emphasizing the improved characteristics compared with materials produced by conventional synthesis procedures. Keywords: Microwave synthesis, Nanoparticles, Metal oxides, Gas sensors.

1. Metal oxide gas sensors: general overview Gas detection is of strategic importance in many fields, e.g., industry control, fuel emission control, automobile exhaust emission control, household security, and environmental pollution monitoring. Among the techniques for gas detection, solid state gas sensors based on semiconducting metal oxides are the most promising. By definition, metal oxide semiconductor gas sensors are electronic devices designed to monitor the concentration of a certain gas in surrounding medium. Nowadays, they are very popular and are utilized in factories, laboratories, hospitals, and almost all technical installations. The idea of using metal oxides as gas sensitive devices leads back to 1954 when Heiland used ZnO as sensing material [1], extending the original observation of Brattain and Barteen about gas effects on electrical conductivity of Ge-based devices[2]. In 1962, Seiyama et al., fabricated a simple chemoresistive device based on ZnO thin films for detection of propane (C3H8). This device displayed a response about 100 times higher compared to the thermal conductivity detector used at that time[3]. In the same year Taguchi, patented and subsequently marketed a simple SnO2-based semiconductor device capable of detecting low concentrations of combustible and reducing gases [4]. A schematic representation of a Taguchi-type gas sensor, along photos showing the internal and external features of a commercial device, are reported in Fig. 1.

Fig. 1. (a) Schematic view of a Taguchi type sensor (TGS) [5]. Photographs showing (b) the interior and (c) the external features of commercial Taguchi type devices.

The sensor is made by coating the as-prepared powders onto the ceramic tube to form a sensing film with a specific thickness. A heating wire such as Ni-Cr wire is also generally provided in order to rise the temperature of the semiconductor gas sensor sufficiently high to allow for fast and reproducible operation, generally between 150°C and 500 °C. A pair of conducting electrodes such as gold electrodes connected with Pt wire is installed at each end of the ceramic tube, between the two gold electrodes is the area which is used for coating sensing film [5]. Soon after first sensor devices based on SnO2, put to practical use in 1968 as domestic gas leak detectors, the research was expanded to other metal oxide candidates. The success of this type of devices is based on these specifics: (i) inexpensive; (ii) easy to use; (iii) sensitive (generally being able to measure down to a few ppm); (iv) very stable (with reported life times extending into decades); (v) easy to integrate in arrays for more ambitious analytical tasks and (vi) reasonably low power consumption[6]. Today, many companies such as Figaro commercialize this type of sensors[7]. Other typologies of metal oxide gas sensors comprise those based on a thin metal oxide layer, generally deposited on planar substrates or on the most advanced micro-electro-mechanical-systems (MEMs) devices[8], which accomplish the demand of miniaturized sensing devices. Generally, in metal oxide gas sensors, the sensing element is constituted by a metal oxide with a n-type behavior (SnO2, ZnO, In2O3,etc.), which are more largely with respect to p-type ones (CuO, CrTiO3, etc)[9]. The gas sensing mechanism of n-type metal oxides rely on the reactions occurring on their surface when they are exposed to gas. These materials, exposed to air, adsorbs oxygen on surface vacancies and form anionic oxygen (Fig. 2(a)) which will cause the formation of depletion region and band bending on the surface and determine their baseline resistance.

Metal oxide

Metal oxide

Fig. 2. Schematic view of gas sensing mechanism steps on n-type metal oxide: (a) oxygen adsorption; (b) surface reaction between the target gas and adsorbed oxygen ions.

When a reducing gas such as CO (Fig. 2(b)), ethanol or methane is adsorbed on their surface reacts with adsorbed oxygen species and releases electrons to the conduction band according the reactions:

(1) (2) (3)

This leads to a decrease in sensor`s resistance. On the contrary, when oxidative gases like NO2 are adsorbed on metal oxide surface, it will gain electrons from anionic adsorbed oxygen, which will increase the depletion region, leading to an increase of the resistance. The resistance change, which represents the signal response, can be easily read by conventional and low cost electronics [10, 11]. Despite many benefits of metal oxide gas sensors mentioned above, the need to operate at high temperatures (in some cases up to 600°C[12]) often limits their use. Moreover such gas sensors have generally the disadvantage of poor selectivity[13]. In order to overcome these main drawbacks, metal oxides having peculiar morphological, microstructural and electronic properties, have been proposed. For example, nanosized metal oxides favors the adsorption of gases and can increase the sensitivity of the device due to the higher interaction with the sensing material, so they can be used to increase the sensitivity, reduce working temperature, consume less power and operate safely [14, 15]. As regard the preparation of such nanosized metal oxides, chemical methods and physical methods can be used [6, 7, 14]. Among physical methods, chemical vapor deposition (CVD) and pulse laser deposition (PLD) are interesting because they are able to produce metal oxide nanoparticles in a simple way and offer a continuous way of nanoparticles production and are therefore easily scalable. On the other hand, chemical routes are simple, inexpensive and need low temperatures. These advantages make the wet approach dominant for metal oxide synthesis. Disadvantages include multistep processes, long reaction times and use of various hazardous chemicals. Wet processes such as hydrothermal[16], sol-gel[17], nonaqueous sol-gel process[18], coprecipitation[19], combustion route [20]and microemulsion [21] are among the most suitable

synthetic strategies for preparing metal oxides via chemical routes. In the following, these wet methods for metal oxide synthesis are briefly commented. Hydrothermal/solvothermal synthesis - Hydrothermal methods are applied to direct metal oxide formation. The hydrothermal method has been regarded as one of the most convenient and practical techniques, because it not only enables avoiding special instruments, complicated processes and severe preparation conditions, but also provides good control over homogeneity, particle size, chemical composition, phase and morphology of the resultant products. The method consists of dissolving precursor metal salts and complexes in water (hydrothermal) or a solvent (solvothermal) heated to high temperature and under pressure in sealed vessels (e.g., autoclaves). The chemical reaction is carried out for several (typically 648) hours leading to nucleation and growth of nanoparticles. These reactions generally produce nanoparticles with crystalline structure that are relatively uncontaminated and thus do not require purification or post treatment annealing, but may have a wider size distribution if special treatment to control size is not applied. A combination of methods can be used, for example, sol–gels, microemulsion, or colloid precipitates may be used as precursors for the solvothermal or hydrothermal processes. Also depending on hydrothermal

temperature,

synthesized powders can be anhydrous, crystalline or amorphous [22, 23]. Chemical precipitation/co-precipitation - This process comprises two main steps: (1) a chemical synthesis in the liquid phase that determines the chemical composition and (2) a thermal treatment that determines the crystal structure and morphology. The process consists of mixing precursor materials in aqueous or nonaqueous solutions (usually a chloride, oxychloride, or nitrate) at room to low temperature for several hours. The chemical reaction (reduction, oxidation, hydrolysis, etc.) allows precise control of the chemical composition. Then a concentrated base (e.g., KOH, NaOH , NH4OH, Na2CO3 ,urea, etc.) is added resulting in the precipitation of the corresponding metal hydroxides with low solubility in the solvent. Precipitation includes several processes that occur simultaneously including initial nucleation (formation of small crystallites), growth (aggregation), coarsening, and agglomeration. Then, particles are allowed to age for hours to days, which allows for particle growth to occur. Capping agents (hydroxypropyl cellulose, polyethylene glycol, cetyltrimethylammonium bromide, poly vinyl pyrrolidone, sodium dodecyl benzene sulfonate, etc.) or electrostatic stabilization is used to stabilize the nanoparticles and to control their crystal growth. The precursor powder undergoes thermal treatment at elevated temperature for several hours to determine the final crystalline structure and morphology of the nanomaterial.

Sol-gel - In this technique alkaline metal oxides and organic and inorganic salts are used as precursor materials in organic or aqueous solvents with catalysts, stabilizers, and other reactants such as gelling agents. The process has six steps: (1) formation of a stable metal precursor solution (sol), (2) formation of a gel by a polycondensation reaction (gel) to make the solution more viscous, (3) aging the gel for hours to days resulting in the expulsion of solvent, Ostwald ripening and formation of a solid mass, (4) drying the gel of any liquids, (5) dehydration and surface stabilization, and (6) heat treatment of the gels at high temperatures to generate crystalline nanoparticles. Different reactions can create the cross-linkages that result in the gelation of the solution. Nanoparticles in the gel are often amorphous, and the final step of thermal treatment imparts the desired crystalline structure to the particles, although it also leads to some agglomeration [24]. Sol-gel method presents various advantages. First of all, the mixing of starting reagents is at the atomic/molecular level, results in faster reaction times at lower temperatures, which helps to reduce the inter-diffusion from one phase into the other and the formation of parasitic phases. On the other hand, metallorganic precursors are more expensive and sol preparation requires sometimes the use of organic solvents, such as 2-methoxyethanol, that require careful handling and disposal because of their combined toxicity and high vapor pressure, so the use of water-based processes is therefore highly desirable [24]. Microemulsion - The microemulsion technique has been used for the synthesis of metal nanoparticles, metal oxides, semiconductor quantum dots, and polymeric nanoparticles in a range of structures through templating. Whenever two immiscible liquids are stirred together, they are known to form an ‘emulsion’. The tendency of the liquids is such that the liquid in smaller quantity tries to form small droplets, coagulated droplets or layers so that they are all separated from the rest of the liquid in large quantity. The droplet sizes in emulsions are usually larger than 100 nm up to even few millimeters. On the other hand there is another class of immiscible liquids, known as microemulsions which are transparent and the droplets are in the range of 1–100 nm [25]. A microemulsion is a thermodynamically stable dispersion of two immiscible fluids, different types of microemulsion such as, water-in-oil (w/o- which refers to spherical oil droplets dispersed in water), oil-in-water (o/w- which refers to spherical water droplets dispersed in oil or water), water-in-sc-CO2 (w/sc-CO2- which refers to water droplets dispersed in supercritical carbon dioxide ) have been widely studied. The system is stabilized by added surfactant (ionic or nonionic) and/or co-surfactants and under some critical concentration, ‘micelles’ or ‘inverse micelles’ are formed, depending upon the concentrations of water and organic liquid. Micelles are formed with excess water and inverse

micelles are instead formed in excess of organic liquid or oil. The micelles, or reverse micelles, collide resulting in formation of different shapes and the possibility of generating nanoparticles of different shapes [25, 26]. Each of these methods shows particular advantages for the synthesis of homogeneous, pure and small particles of metal oxides, however generally they need long reaction times and/or require high annealing temperatures to complete the formation of the metal oxide (Table 1).

Table 1. Comparison of main available methods for synthesis of metal oxide nanostructures. Synthesis

Reaction Tem. (°C)

Process Speed

Heating

Sol-gel

Simple

70-100°C

Slow

Non uniform

Size distribution/ Shape control Relatively narrow/Good

MW/Sol-gel

Simple

70-100°C

Fast

Uniform

Hydrothermal

Simple

160250°C

Slow

MW/Hydrothermal

Simple

70-100°C

Precipitation

Simple

MW/Precipitation

Meth

Yield

Cost

High

High

Narrow/Good

Very High

Higher

Non uniform

Narrow/Good

High

Medium

Fast

Uniform

Narrow/Good

Very High

High

50-80°C

Slow

Non uniform

Wide/Poor

High

Low

Simple

70-100°C

Fast

Uniform

Narrow/Good

Very High

Medium

Microemulsion

Complicate

20-30°C

Slow

Narrow/Good

Low

High

MW/Microemulsion

Complicate

70-100°C

Fast

Narrow/Good

High

Higher

od

Non uniform Uniform

Coupling these methods with microwave irradiation for synthesizing metal oxides (see Table 1), two main advantages can be easily reached: (i) extremely rapid synthesis, (ii) very rapid heating to the required temperature[27]. For example, the formation of SnO2 nanostructures by the hydrothermal synthesis process needs a few hours whereas the microwave synthesis leads to the desired reaction products in only 3 min [28]. Furthermore, the difference of reaction mechanisms may lead to different microstructure of the products, even if the same reactants are used [29]. Hydrothermal growth is a slow-heat process, thus the solution reaches super-saturation after a long time and the super-saturation is low leading to a wide size distribution of initial nuclei. While microwave-assisted synthesis raised the solution to a desired temperature in just a few minutes, it leads to the creation of abrupt super-saturation resulting in a very high nucleation density in a very short time. Because of the difference in heating mode between hydrothermal method and microwaveassisted synthesis, the microwave method can form more nuclei with a narrower size distribution due to very high heating rate and homogeneous temperature distribution compared to the hydrothermal method leading to the formation of fewer nuclei with broader

size distribution. Due to the wide use of metal oxides in virtually all technological fields, many excellent reviews appeared in recent years on this subject [27, 30-37]. However, to date, no review paper is available about the synthesis aided by microwave irradiation of metal oxides for their specific applications in gas sensing. This review aim to give a comprehensive view on the state of the art in this field.

2. Microwave-assisted synthesis of metal oxides As it stated above, in spite of benefits of wet chemical routes for synthesis of nano metal oxides, generally they need long time for synthesis. Microwave-assisted processing attracted a great deal of attention due to its advantages to supply higher synthesis rate, resulting superior to traditional heating. Indeed, since microwaves can penetrate the material and supply energy, heat can be generated throughout the volume of the material resulting in volumetric heating. The ability to elevate the temperature of a reaction well above the boiling point of the solvent increases the speed of reactions by a factor of 10-1000. Reactions are thus completed in minutes or even seconds[38]. Fig. 3 shows the difference in the very fast temperature profile under microwave heating and conventional heating.

Fig. 3. Difference in temperature profiles for a 5-ml sample of ethanol heated under single-mode, sealed-vessel microwave irradiation (maximum set temperature 160 °C) and open-vessel oil-bath conditions (oil-bath temperature 100 °C) for 3 minutes [39].

Yields are also generally higher and the technique may provide a means of synthesizing compounds that are not available conventionally [39]. Additionally, the method can leads to

the synthesis of materials with smaller particle size, narrow particle size distribution, high purity, and enhanced physicochemical properties [32]. Furthermore, microwave methods are unique in providing scaled-up processes without suffering thermal gradient effects, thus leading to a potentially industrially important advancement in the large-scale synthesis of nanomaterials [33]. It’s not then a surprise that microwave processes have been used to synthesize different inorganic materials such as metallic nanoparticles (Ag[40], Au[41], Pt[42]), amorphous and nanoporous materials[34, 43], core-shell particles (PbTe@Ag2Te[44]), semiconductors (CdSe[45], bioceramics[35], pure and mixed metal oxides (Fe3O4[46], ZnO [47, 48], TiO2 [49, 50], SnO2[51], NiO[52], PtO2[53], CeO2 [54] CuO [36], Zn2SnO4 [55](LaFe0.7Zn0.3O3[56], LiFePO4[41], NiFe2O4 [57]). For these latter metal oxide-based materials, the use in gas sensing started fifteen year ago when the fabrication of a gas sensor based on SnO2 nanoparticles prepared by a microwaveassisted synthesis has been reported for the first time by Morante and coworkers [58]. Since then, the use of microwave technology in this field has shown an impressive growth (Fig. 4).

Papers number

25

20

15

10

5

0 2000

2002

2004

2006

2008

2010

2012

2014

2016

Year

Fig. 4. Number of papers mentioning the use of metal oxides, synthesized by a microwave-assisted process, in chemoresitive gas sensors (Source: Scopus, December 2015).

In the next paragraph will be given briefly the principles of microwaves radiation and processes in use in the synthesis of inorganic materials, focusing the attention on the preparation of metal oxides with gas sensing characteristics.

2.1 Microwave principles 2.1.1 Microwave radiation

Microwave irradiation is electromagnetic irradiation in the frequency range 0.3 to 300 GHz, (wavelengths of 1mm to 1 m). Therefore, the microwave region of the electromagnetic spectrum lies between infrared and radio frequencies (Fig. 5). The major use of microwaves is either for transmission of information or for transmission of energy [59]. Another application, i.e. microwave heating, was discovered in 1946 by Spencer at the Raytheon Corp.[60]. By international agreement, all domestic microwave ovens and commercial microwave reactors for chemical synthesis operate at a frequency of 2.45 GHz (with a maximum output power of 1 kW), in order to avoid interference with telecommunication, wireless networks and cellular

Wavelength (mt)

phone frequencies [59]. At this frequency, oscillations occur 4.9 × 109 times per second. Consequently, molecules subjected to this electromagnetic radiation are extremely agitated as they align and realign themselves with the oscillating field, creating an intense internal heat that can escalate as quickly as 10°C per second.

Fig. 5. The electromagnetic spectrum.

2.1.2 Interaction of microwaves with different materials Heating in microwave cavities is based upon the ability of materials to absorb and transform electromagnetic energy into heat. Materials fall into three categories, with respect to their interaction with microwaves (Fig. 6a-c). (a) Microwave reflectors (metals); the material is not effectively heated by microwaves. (b) Microwave transmitters, which are transparent to microwaves (low-loss materials); microwaves can penetrate through the material without any absorption, losses or heat generation. Teflon and fused quartz are two examples, therefore they are employed for containers for carrying out synthesis and chemical reactions in microwaves reactors. (c) Microwave absorbers (high-loss materials) which constitute the most

important class of materials for microwave synthesis; they take up the energy from the microwave field and get heated up very rapidly [37, 61].

Fig. 6. Interaction of microwaves with different materials (a) electrical conductor, b) insulator, (c) Lossy dielectric [62].

Microwave dielectric heating is a bulk effect and the heating is a consequence of dielectric loss. The electric component of an electromagnetic field causes heating by two main mechanisms: dipolar polarization and ionic conduction. Irradiation of the sample at microwave frequencies results in the dipoles or ions aligning in the applied electric field. As the applied field oscillates, the dipole or ion field attempts to realign itself with the alternating electric field and, in the process, energy is lost in the form of heat through molecular friction and dielectric loss. Thus, if one or more species in the reaction mixture has a permanent dipole, dielectric heating by irradiation with microwave energy at 2.45 GHz, will be possible[63]. Hence, polar molecules such as DMF, acetonitrile, CH2Cl2, ethanol, and H2O are microwave-active, while nonpolar molecules such as toluene, carbon tetrachloride, diethyl ether, and benzene are microwave-inactive [64]. The ability of a specific substance to convert electromagnetic energy into heat at a given frequency and temperature is determined by the so-called loss factor tanδ. This loss factor is expressed as tanδ = εʹʹ/εʹ, where εʹʹ is the dielectric loss, which is indicative of the efficiency with which electromagnetic radiation is converted into heat, and εʹ is the dielectric constant describing the ability of molecules to be polarized by the electric field. A reaction medium with a high tanδ value is required for efficient absorption and, consequently, for rapid heating [65, 66]. The temperature profile within the material exposed to microwaves depends on several geometrical factors related to design of the microwave cavity. However, when microwaves are incident perpendicularly on the surface of the material, their intensity decreases

progressively inside the material in the direction of incidence as the microwave energy gets progressively dissipated. This is expressed through the parameter known as penetration depth, D(m) , which is the distance in the direction of penetration at which the incident power is reduced to half its initial value: (4) where λ0 is the wavelength of the microwaves,

is dielectric constant (tan ) is the loss

factor. The equation suggests that there is a slight advantage in working at lower frequencies when large samples are involved, but it is associated with a payoff in terms of power absorbed per unit volume [37]. In the case of dielectric material (solid or liquid) and assuming negligible diffusion and heat losses, the majority of the absorbed microwave power per unit volume is converted into thermal energy within the dielectric material, as shown in the following equation [67, 68]: | |

(5)

Where Pabs is microwave energy absorbed per unit volume, f is the operating frequency of microwave radiation, ε0 is the dielectric permittivity of free space, dielectric loss factor, f is applied frequency, E is electric field,

is the effective relative

is density of sample and Cp is

specific heat capacity of sample.

2.1.3 Microwave versus conventional thermal heating Traditionally, metal oxide synthesis is carried out by conductive heating with an external heat source (e.g. an oil-bath or heating mantle). This is a comparatively slow and inefficient method for transferring energy into the system since it depends on convection currents and on the thermal conductivity of the various materials that must be penetrated, and generally results in the temperature of the reaction vessel being higher than that of the reaction mixture (Fig. 7a). This is particularly true if reactions are performed under reflux conditions, whereby the temperature of the bath fluid is typically kept at 10–30 °C above the boiling point of the reaction mixture in order to ensure an efficient reflux. In addition, a temperature gradient can develop within the sample and local overheating can lead to product, substrate or reagent decomposition [59]. The inherent temperature gradients present during volumetric heating can lead to severe temperature non-uniformities, which at high heat-up rates may cause non-

uniform properties of the synthesized products. Also, in a conventional heating, as the direction of heating is from outside to inside of the powder resulting in higher temperature of the sample surface than the core, while for microwaves, the direction of heating is from inside to outside of the powder compact resulting in higher temperature of the sample core than the surface. The former mode of heating results in poor microstructural characteristics of the core of the powder compact while the latter results in poor microstructural characteristics of the surface [67]. In contrast, microwave irradiation produces efficient internal heating (in core volumetric heating) by direct coupling of microwave energy with the molecules (solvents, reagents, catalysts) that are present in the reaction mixture (Fig. 7b).

)c(

Fig. 7. (a) conventional heating; (b) microwave heating [59]; (c) Inverted temperature gradients in microwave versus oil-bath heating. Temperature profiles (modelling) 1 minute after heating by microwave irradiation (left) compared with treatment in an oil-bath (right) [39].

Microwave irradiation, therefore, raises the temperature of the whole volume simultaneously (bulk heating) whereas in the conventionally heated vessel, the reaction mixture in contact with the vessel wall is heated first (Fig. 7c). Since the reaction vessels employed in modern microwave reactors are typically made out of microwave transparent materials such as borosilicate glass, quartz or Teflon, the radiation passes through the walls of the vessel and an inverted temperature gradient as compared to conventional thermal heating results. If the microwave cavity is well designed, the temperature increase will be uniform throughout the sample. The very efficient internal heat transfer results in minimized wall effects (no hot vessel surface) which may lead to the observation of so-called specific microwave effects [59].

In brief, the advantages of microwave synthesis are the following [66, 69-71]: (i) Higher reaction temperatures by combining rapid microwave heating with sealed-vessel (autoclave) technology; (ii) significantly reduced reaction times and higher yields; (iii) use of solvents with lower boiling points under pressure (closed vessel conditions) and heated at temperatures considerably higher than their boiling point; (iv) ‘in core’ heating of the reaction mixture, with no wall or heat diffusion effects; (v) no direct contact between the energy source and the reacting chemicals; (vi) selective heating; (vii) direct molecular heating and inverted temperature gradients.

2.1.4 Microwave equipment A typical microwave instrument for materials synthesis consists of six main components: the microwave generator or magnetron, the waveguide, the microwave cavity, the mode stirrer, a circulator, and a turntable as shown in Fig. 8a.

(a)

(b) )

(c)

Fig. 8. (a) Schematic illustration of microwave equipment for the synthesis of inorganic materials [71]; (b) microwave oven (multimode); (c) monomode reactor [72].

The magnetron is a device for generating fixed frequency microwaves, which is principally a thermionic diode with heated cathode acting as sources of electrons. From the magnetrons the microwaves are commonly directed toward a target placed in microwave cavities with the use of waveguides, which are usually made of sheet metal, and where the mode stirrer homogeneously distributes the incoming energy in various directions. The materials used for construction of the primary components of microwave units are reflective materials, e.g., Teflon polymer. Microwave ovens for research can be designed for monitoring parameters such as temperature and pressure while heating a sample in the microwave. For monitoring systems, the only limitation is that the measuring probes must not perturb the microwave energy [71]. This is

more important because many materials could not couple with microwave directly at room temperature, so it is necessary to preheat them. In fact, since the dielectric loss properties increase with temperature, low dielectric loss materials can be made absorptive by raising their temperature [73]. Two types of microwave heating equipment (Fig. 8b-c) have been used for studies on synthesis and some specialized equipment has been constructed by individual research groups. Most commonly, a multimode oven (non-uniform electric field distribution) is employed – in some cases, a domestic oven, in others, a unit designed specifically for laboratory use. The former has the advantage of cheapness but lacks flexibility in control and reaction monitoring. The latter is more expensive but is purpose-built, has extensive facilities for programming and allows stirring of the reaction mixture and the continuous monitoring and control of temperature and pressure. Microwave ovens can accommodate large samples (e.g. 12 ×100 cm3). The second type of unit used in microwave synthesis is the single mode device in which microwave energy is piped (to some extent focused) into a reactor through waveguide. Such a reactor is usually small (1–2 cm3) but may be operable over a range of frequencies. Reaction vessels themselves are made of a material transparent to microwaves at the operating frequency, commonly borosilicate glass, PTFE etc. Temperature measurement is the biggest single problem in microwave synthesis. For microwave ovens, plated, earthed thermocouples or fibre-optic devices are used. For single-mode cavities, additional techniques include optical pyrometry and measurement of the temperature differential of a flowing gas [72].

3. Gas sensing applications of metal oxides synthesized by MW routes Since the 1960's, metal oxides have been largely investigated as gas sensing materials [74]. With the aim to improve their performances, researchers focused their attention mainly on the synthesis of nanostructured metal oxides with tailored grain size, shape, and morphology [75, 76]. A survey of the literature on metal oxides synthesized by methods assisted by microwave irradiation and applied as sensing layer in chemoresistive gas sensors, indicates that more than half of them is ZnO and SnO2 (Fig. 9).

Cu-Cu2O

WO3

TiO2 CdO Fe2O3

SnO2

In2O3 Other ZnO

Fig. 9. Distribution of various metal oxides, synthesized by microwave-assisted processes and applied as sensing layer in chemoresistive gas sensors (Source: Scopus, December 2015)

Table 2 reports a survey of metal oxides synthesized using MW-assisted methods and their gas sensing characteristics. The morphological and microstructural characteristics (i.e., shape of grains, dopant, etc.) and the sensing properties (target gas, operating temperature, response) are listed for a faster comparison.

Table 2. A survey of metal oxide synthesized using MW-assisted methods and their gas sensing characteristics. MW-assisted method Solvothermal Hydrothermal Hydrothermal Hydrothermal Solid state reaction Solvothermal Solvothermal Precipitation Solution method Solution method Hydrothermal Solution method

Shape/Microstucture

Target gas

Conc. (ppm)

Topt (°C)

Response (R/R0)

Ref.

NO2

0.005

90

3

[77]

Acetone

800

220

8

[78]

Ethanol

1000

220

7

[78]

Methanol

1000

220

5

[78]

ZnO NWs

H2

1000

200

1.5

[79]

Co3O4 nanocubes

Xylene

100

200

6.5

[80]

Co3O4 nanocubes

Xylene

100

200

5

[80]

SnO2 NPs

LPG

1000

350

85

[81]

ZnO NRs

H2

1000

250

12

[82]

Pd-doped SnO2

CO

500

250

21

[58]

ZnO NRs

CO

200

400

5.5

[83]

WO3 NRs

Ethanol

1000

500

5

[84]

Flower-like WO3 architectures Tadpole-shaped CuO NPs Tadpole-shaped CuO NPs Tadpole-shaped CuO NPs

Chemical process Chemical route Heating method Solution method Chemical bath deposition MW-assisted method MW assisted method Hydrothermal Hydrothermal MW synthesis MW assisted ethylene glycol approach MW assisted synthesis MW assisted synthesis MW method Microwave method MW-assisted synthesis MW-assisted synthesis MW-assisted hydrothermal MW assisted hydrothermal Hydrothermal Hydrothermal Hydrothermal MW assisted chemical Hydrothermal method Chemical synthesis MW assisted irradiation Hydrothermal Hydrothermal MW irradiation

ZnO nanostructures

CO

100

300

6

[85]

Porous ZnO Nanoplates

Ethanol

100

375

9

[86]

ZnO nanopowders

HCHO

0.001

210

7.4

[87]

SnO2 NRs

O2

10

RT

18

[88]

ZnO NRs

H2

1000

250

41

[89]

ZnO NPs

Ethanol

1000

350

250

[90]

SnO2 NPs

LPG

1000

450

60

[91]

CuO nanorods

Ethanol

1000

210

9.8

[92]

nanoporous cobalt oxides

Ethanol

300

200

3.2

[93]

SnO2

CO

1000

350

1.9

[94]

Fe3O4 NPs

Ethanol

200

RT

4.75

[95]

Fe2O3@WO3 nanostructures

H2S

10

150

198

[96]

Pd-SnO2 nanoflakes

CO

200

100

7

[97]

SnO2 NPs

Trimethylamine

1000

200

1000

[98]

nano crystalline (NiFe2O4)

H2

4500

175

4

[57]

CuO NPs

NO2

50

250

2.75

[99]

SnO2 NPs

Ethanol

200

275

50

[100]

ZnO NRs

Ethanol

150

300

19

[101]

MoO3/rGO

H2S

40

110

60

[102]

WO3/graphene nanocomposite

Triethylamine

100

RT

205

[103]

Flower-like 3D ZnO

Ethanol

100

420

10

[104]

CuO NPs

Ethanol

1000

240

9

[105]

ZnO NRs

H2

1000

RT

NO2

1

RT

1.19

[107]

HCHO

1

86

13

[108]

Ethanol

1000

270

2.4

[109]

Ethanol

1000

350

36

[110]

NO2

0.5

170

125

[111]

H2S

10

100

80

[112]

rGO/ hierarchical flower In(OH)3 0.75%CNTs-AgLaFeO3 Ag–Vanadium Oxide NTs Cerium-doped SnO2 porous ZnO polygonal nanoflakes Hierarchical SnO2@rGO

2.1

[106]

Hydrothermal synthesis MW assisted method Hydrothermal Hydrothermal Hydrothermal MW-assisted technique MW-Assisted Synthesis MW-assisted growth Hydrothermal Chemical route Hydrothermal Co-precipitation MW-assisted method Calcination MW-assisted method MW-assisted Solution method MW-assisted method hydrothermal MW-irradiation hydrothermal

Ni-doped ZnO NRs

Ethanol

100

370

14

[113]

ZnO NBs

CO

200

400

1.62

[114]

flower-like ZnO

Ethanol

100

260

13

[115]

Hierarchical WO3

Acetone

300

350

6

[116]

Mn-doped ZnO NRs

O2

15

RT

4

[117]

ZnO NPs

CO

500

300

12

[118]

WO3 Nanocrystals

Ethanol

100

300

16

[119]

In2O3@WO3 nanocomposites

H2S

10

150

143

[120]

ZnO core–shell

Ethanol

700

300

33

[121]

Cu2O microarchitectures Ag@SnO2 core–shell ZnO–In2O3 nanostructure

H2S

50

160

5.5

[122]

Propane

500

250

1.9

[123]

Ethanol

300

250

900

[124]

3D ZnO nanostructures

Ethanol

100

260

57

[125]

CO

30

450

2.6

[126]

Ethanol

500

300

2400

[127]

ZnO NRs

O2

20

200

2.5

[128]

Nanoporous ZnO

Ethanol

200

160

1.4

[129]

NH3

200

RT

2.4

[130]

CO

75

250

1.25

[131]

NO2

5

RT

1.6

[132]

Zn-LaFeO3

HCHO

100

250

1.38

[56]

CuO Nanoflowers

Ethanol

1000

260

4.7

[133]

SnO2 nanomaterials

Isopropyl alcohol

200

300

90

[134]

SnO2 acicular NPs Sm2O3-loaded flowerlike ZnO

Cu2O nanorods modified by rGO hexagonal-shaped CdO nanostructures In2O3 cubes into graphene sheets

Chemical synthesis Hydrothermal Microwave radiation

A comprehensive report on these materials with details on their preparation, morphological and microstructural characteristics is given in the following. Particular emphasis is given to the synthesis of particular morphologies and nanostructures, hybrid composites and metaldoped formulations. Their gas sensing performances are described and discussed in detail, emphasizing the improved characteristics compared with materials produced by conventional synthesis procedures.

3.1 ZnO Due to its excellent chemical stability and semiconducting properties, ZnO has been largely exploited on using ZnO-based chemoresistive sensors for a variety of gases such as NH3, formaldehyde, CO, H2S, ethanol, and NO2 [135]. Researches on ZnO materials for gas sensing are often aimed to optimize grain interconnections and densification because they are key factors for gas sensing. Extensive contact points, created by densification, are need for electrical conduction in the sensing layer. On the other hand, sensing layer should have a certain degree of porosity, to facilitate the diffusion of the gas to be detected. To obtain the optimal balance, annealing by conventional heating at a suitable (often high) temperature is generally performed, then this step results crucial in the synthesis of ZnO (and metal oxides in general) for applications in gas sensors. In this regard, Bai et al. [136] reported thick film gas sensors based on ZnO nanopowders fabricated by using microwave sintering for different irradiation time. Fig. 10a shows surface morphology for ZnO thick films by microwave sintering (c) 60 min) and by oven at 700 °C for 2.5 h (Fig. 10b). The dense thick films were achieved with the increase in microwave sintering time.

1 μm

1 μm

Fig. 10. The surface morphology of ZnO thick films by using microwave sintering for 60 min (a) and (b) by using muffle oven sintering at 700 °C for 2.5 h[136].

Fig. 11 shows the response (Ra/Rg) to formaldehyde as function of operating temperature. The responses were greatly affected by the operating temperature and the sintering time. The sensor by using microwave sintering for 20 min obtained the highest response and that for 60 min was the lowest. The optimum response of the sensor by using microwave sintering for 20 min were obtained at about 380 °C.

Fig. 11. Response vs. operating temperature of ZnO thick films by using microwave sintering to 100 ppm formaldehyde [136]

The effects of duration and heating microwave power, have been also evaluated [87]. The results revealed that the power and duration heating had little influence on the average crystallite sizes of the ZnO synthesized. The heating power has instead a great influence on the sensors response. The sensor based on ZnO powder obtained with low heating power exhibits highest response in all the sensors based on ZnO materials prepared with low power, middle power, middle-high power and high power. Appropriate power and duration heating maybe lead to the increase of number of active sites on the surface of materials on which reducing gas and oxygen can be adsorbed easily. Morphology-induced enhancements of the electrical and gas sensing performance of ZnO microstructures synthesized by MW were observed. Krishnakumar et al.[137], synthesized spherical, flower and star-like ZnO nanostructures, changing the precursor materials and/or microwave irradiated time. Making a comparison among them (Fig. 12) the flower-like ZnO-based sensor exhibits higher response to CO with respect to sensors based on other morphologies. Synergic effects between small crystallite size/high surface area and electron transport properties modification were responsible for the enhanced sensing properties of ZnO nanoflowers.

Fig. 12. Response to CO for the different ZnO nanostructures [137]

Controllable ZnO architectures with flower-like and rod-like morphologies were synthesized via hydrothermal method by adjusting the concentration of Zn2+ in the aqueous precursors under the same microwave irradiation conditions [101]. When the [Zn2+] increased, the morphologies occurred in the order as follows: seven-spine, flower-like, urchin shaped and rod-like architectures (Fig. 13a-d).

Current (A)

(e)

Fig. 13. Effect of [Zn2+] on the morphology of ZnO particles. The [Zn2+] in the initiated Zn(NO3)2 solutions were (a) 0.01, (b) 0.02, (c) 0.04, (d) 0.06 M, which correspond to seven spine, flower-like, sea urchin and rod-like architectures, respectively. Real-time response curves (e) of the ZnO microstructures exposed to different concentrations of ethanol at a working temperature of 300 °C [101].

The flower-like ZnO based gas sensor possessed higher resistivity than the rod-like samples. Additionally, the seven-spine ZnO sensor displayed the best gas sensing performances (Fig. 13e). Compared to the rod-like ZnO, the superior gas-sensing performances of the sevenspine ZnO most likely originate from the large number of point-contacts and low stacking

density, which facilitate the surface-potential-barrier-controlled processes and percolation of gas molecules. Flower-like nanostructures are commonly reported when preparing ZnO by means of MWassisted methods [104, 111, 138]. Gu et al. [125] synthesized 3D ZnO flower-like nanostructures by microwave-assisted method and successive calcination (Fig. 14a-f). The presence of numerous pores in the nanoflakes, which was accomplished by post-treatment of the flower-like ZnO nanostructures with different calcination temperatures and atmospheres, was used to further enhance the sensing responses.

Fig. 14. SEM images of the flower-like ZnO calcinated at different temperatures: 400 (a) and (d), 500 (b) and (e), 600 (c) and (f) [125].

Among the samples, that annealed at 500 °C in N2 displayed the highest response to ethanol, due to the most donor defects (zinc interstitial and oxygen vacancy) which were beneficial to the formation of active oxygen and adsorbed ethoxy species. Nanorods, nanoparticles and nano flowers of ZnO were synthesized via a fast and facile microwave assisted method using zinc acetate as starting material, guanidinium and acetyl acetone as structure directing agents, and water as solvent [90]. In all cases microwave irradiation was applied for 2 min. Flower-like ZnO structures are about 2 μm and consist of well-defined petals with average size of about 600-700 nm in length, 300–400 nm in width, and 50-70 nm in tip (Fig. 15a). No structural damage is evidenced after calcination at 600 °C (Fig. 15b). Rod-like ZnO nanostructures exhibit uniform morphology with 60-90 nm

diameters and a maximal length of 1.5 μm (Fig. 15c). Spherical-like ZnO is also synthesized with average diameter of 50 nm (Fig. 15d). Fig. 15e shows the possible mechanism of growth of these particles.

e

Fig. 15 SEM images of ZnO (a) flower-like (before calcination), (b) flower-like (after calcination), (c) nano rods and (d) nano particles. (e) Possible mechanism for the formation of different morphologies of ZnO by microwave irradiation [90].

ZnO nanorods were reported to exhibit higher sensor response to methane compared to flower-like and nano particle morphologies at an optimal temperature of 350 °C, explained by the modulation model of the depletion layer, where oxygen vacancies in ZnO nanorods act as electron donor species and a depletion layer is formed on the surface of nanorods. The controlled deposition of the metal oxide sensing layer on the sensor substrate is a critical step in the fabrication of thin films-based chemoresistive sensors. By adopting the unique performance of a microwave-assisted method, highly porous ZnO nanostructures were directly synthesized on the surface of an aluminum layer of 10 nm deposited on the substrate [129]. An aqueous growth solution of zinc nitrate hexahydrate and hexamethylenetetramine

was used for growth process carried in a commercial microwave oven under irradiation power of 110 W for 30 min. In Fig. 16a the 10-20 nm thickness nanoflakes connected with each other to form porous structure with size around 0.058 μm2 were observed. Although the nanoflakes were grown in a large size, its thickness was very fine which increase the surface to volume ratio of the nanoflakes and led to high surface area. By the same procedure, ZnO nanorods were also deposited on a bare substrate via conventional furnace heating (Fig. 16b).

1 μm

1 μm

Fig. 16. FESEM images of (a) ZnO nanoporous; (b) ZnO nanorods [129].

The higher response of ZnO nanoflakes-based sensors could be attributed to the sensing layer structure, where more porous and larger surface area was exhibited which lead to larger gas exposure. Interlinked ZnO tetrapod networks (ITN-ZnO) have been also realized by using microwaveassisted thermal oxidation [139]. With this leg-to-leg linking, for room-temperature gas sensors with improved performance are achieved. MW was a powerful synthesis approach for the preparation of mixed-metal oxides based on ZnO over a wide compositional range [127, 140, 141]. Bagheri et al. reported the response and selectivity of samaria-doped ZnO flower-like sensors for selective detection of C2H5OH in the presence of CH4, CO and toluene[127]. Samples of ZnO loaded with 2, 5 and 10 wt% Sm2O3 were prepared by employing a microwave (with 75% power) assisted method using aqueous solutions of zinc acetate, samarium nitrate, and guanidinium carbonate as a structure directing agent. By tuning the samaria content and the operating temperature, a highly selective sensor to ethanol in presence of toxic CO, combustible CH4 and toluene, as a typical representative of aromatic VOCs was obtained. As 5 wt% Sm2O3 was added to ZnO, significant enhancements in the response to ethanol at various temperatures are observed, while it showed negligible responses to CO, toluene and methane. At 300°C, the response of 5

wt% Sm2O3-ZnO sensor to ethanol was 60 times larger than that of pure ZnO. The Sm2O3ZnO sensors showed no or little response to methane, toluene and CO. CeO2-doped ZnO nanostructures, with different Ce/Zn ratios, were synthesized via a very fast microwave assisted method using zinc acetate and cerium nitrate as starting materials and water as solvent. Pure ZnO and the one doped with ceria, were both flower-like[141]. The size of flower-like ZnO structures was estimated to be about 2 μm with well-defined petals of about 700–800 nm in length, 200-300 nm in width, and 50-70 nm in tip. It was shown that addition of 5 wt% ceria to ZnO not only enhanced the response to ethanol significantly but also drastically reduced the recovery time. The large oxygen storage capacity of CeO2 is the main cause of the significant reduction in the recovery time. Due to its fluorite structure, the oxygen atoms in a ceria crystal are all in a plane with one another, allowing for rapid diffusion as a function of the number of oxygen vacancies. As the number of vacancies increases, oxygen can move easily around the crystal, allowing the ceria to reduce and oxidize the molecules on the surface. Thus, on exposure to air, CeO2 stores significant amount of oxygen, which in turn facilitates the oxidation reaction of ethanol and helps cerium containing ZnO to reach the initial resistance in a short period, i.e. shorter recovery time. Furthermore, this compound suppresses the response to CO and CH4 and therefore makes the sensor to selective for ethanol. The improved response of the doped sensors was mainly attributed to more oxygen vacancy and improved surface basicity induced by the presence of CeO2. The higher basicity of the CeO2 containing sensor favors oxidative dehydrogenation route, which result in larger response.

3.1 SnO2 Tin oxide (SnO2) is one of most important n-type oxide and wide band gap (3.6 eV) semiconductor, and its good electrical properties were largely exploited as sensing layer in solid-state chemical sensors [142]. Morante and his coworkers first demonstrated the superior gas sensing performance of SnO2 synthesized by using the microwave assisted route[58]. Further, different treatment procedures (OH-stimulated microwaves, and combined conventional-microwaves heating treatments) were also considered. Cho et al. confirmed that the rapid microwave heating of SnO2 is advantageous in enhancing the gas response [126]. Fig. 17 shows the dynamic CO sensing characteristics of the SnO2 prepared by conventional and microwave calcination at 500 °C and subsequent heat treatment at 700 °C for 1 h. More

significant changes by microwave calcination were observed in the response time, decreasing the response time to 10–12 s (Fig. 17a-c).

Fig. 17. Response transient of the SnO2 specimens. All the sensors were heat treated at 700 °C for 1 h after (a) slow conventional heating of SnC2O4 precursor to 500 °C (heating rate: 4.2 °C/min, CONV-700), (b) rapid microwave heating of SnC2O4 precursor to 500 °C (heating rate: 100 °C/min, MW-100-700), and (c) rapid microwave heating of SnC2O4 precursor to 500 °C (heating rate: 25 °C/min, MW-25-700). (e) Schematic diagram of the time-dependent gas sensing reaction for the SnO2 acicular particles prepared by slow conventional calcination and rapid microwave calcination [126].

Acicular SnO2 particles prepared by slow conventional calcination showed a dense structure. Therefore, the diffusion of CO gas occurs from the primary particles located on the outer part of the acicular particles (t2 in Fig. 17a) and requires a relatively long time (t3 in Fig. 17a). In contrast, the acicular particles prepared from rapid microwave calcination shows a

mesoporous structure. This enables the rapid CO diffusion along the mesopores, and occurs within a short reaction time (t2 in Fig. 17b). Solvent medium play a key role in MW synthesis. Water has a very high dipole moment, which makes it one of the best solvents for microwave assisted synthesis. Then, comparison between heating in organic and in aqueous medium for the preparation of SnO2 has been reported [91, 100]. Liu et al. synthesized Zn-doped SnO2 samples via a simple microwaveassisted wet route in aqueous solution. The response to ethanol (Fig. 18) is high and for ethanol concentrations between 20-350 ppm there is a linear relationship between response and concentration.

Fig. 18. Response versus ethanol concentration; inset is the calibration curve in the range of 10-350 ppm [100].

In another paper, SnO2 was prepared via the microwave induced heating in organic and in aqueous medium. Both samples showed spherical grain morphology and average grain size nearly 10 nm [91]. Sensing data pointed out that sensors fabricated with different routes behave in an opposite way for compressed natural gas (CNG) or liquefied petroleum gas (LPG). The exact reason is not known but may be due to differences in pore size or defects. The effect of different microwave irradiation times on the microstructural and sensing properties of SnO2 nanoparticles synthesized by microwave-assisted synthesis (800 W) was investigated by Neri et al.[143], XRD pattern of SnO2 without microwave irradiation indicated formation of a tin (oxy) hydroxide phase with composition Sn6O4(OH)4. On the contrary, microwave irradiated samples showed the formation of SnO2 nanostructures, matching well with SnO2 in the cassiterite-type tetragonal crystal structure. This confirmed that the microwave radiation caused the conversion of tin hydroxyl group into SnO2

nanostructures without necessity of post-synthesis heating. The increase of peak intensity suggested that the microwave treatment improved the crystalline structure. The characteristics of response and signal stability of chemoresistive sensors fabricated improved strongly increasing the irradiation time. They attributed this behavior to the improvement of structural stability caused by the increase of particle size with the increase of irradiation time. Surprisingly, the response of the sensor to different ethanol concentrations at the operating temperature of 100 °C, showed p-type behavior (i.e., the resistance increased during ethanol pulses). It was supposed that hydroxyl group present in the ethanol molecule, which may behaves as hydrogen-bond donors or proton donors (Brønsted acid), represents the key factor. Indeed, at low temperature, the direct interaction may take place via hydrogen bonding between the ethanol hydroxyl group and the sensing layer surface, leading to the inverted response. MW-assisted synthesis allows the preparation of very small SnO2 particles [144, 145]. Highly crystalline SnO2 quantum dots (QDs) were synthesized by dissolving Sn(OtBu)4 in dried 1butyl-3-methylimidazolium tetrafluoroborate ([BMIM] BF4) under argon atmosphere, followed by MW irradiation for 1 min[144]. The sensor exhibited a fast response towards ethanol. The selectivity studies among EtOH, CH4, NH3 and CO demonstrated a higher selectivity of SnO2 QDs towards ethanol compared to other gases. They reported that the higher selectivity of ethanol could be attributed to the strong interaction between the ethanol and the surface-adsorbed oxygen species and higher release of trapped electrons in SnO2 QDs. Nano-SnO2 powders were prepared using microwave heating in a domestic microwave oven (maximum power 800 W, multimode oven). Samples were heated with different heating power: 136, 264, 440, 616 and 800 W[145]. The average particle sizes of the sample obtained with 616 W (20 min) were about 5 nm. The sensor based on SnO2 heated with this microwave power exhibits the high response to trimethylamine. The maximum response of the sensor reaches 1374 when operating at 255 °C. Also the heating duration has a great influence on the sensitivities of sensors. When operating at 120-200 °C, the sensitivities to trimethylamine increase with increasing in the heating duration. In general, the particle sizes increase with increase of the heating temperature and the heating duration, but the sensitivities change in the reverse order. Pure SnO2 and cerium-doped SnO2 nanoparticles were synthesized by microwave (100°C, 10 min) hydrothermal method using SnCl4 and Ce(NO3)3 as raw material and urea as precipitant, respectively [110]. Compared with pure SnO2 thick film gas sensor, the intrinsic resistance of cerium-doped SnO2 thick film gas sensors decreased, and their sensor responses to acetone

vapor increased. Cerium doping restrained the growing process of SnO2 nanoparticles in sintering process. Moreover, there were large amount of pores in the surface of Ce-doped thick film, so the target gas molecules can rapidly and uniformly diffuse inside the sensing layer, thus surface reactions take place at higher speed. CuO/SnO2 composites for selectively sensing BTEX (benzene, toluene, ethylbenzene, and xylol) were synthesized by a microwaveassisted approach [130], Excellent gas sensitive performance of 3 mol% CuO/SnO2 sensor to BTEX belongs to the addition of catalyst CuO.

3.3 WO3 Tungsten trioxide (WO3) nanostructures are promising functional materials for gas sensors and many groups tried to prepare them by MW techniques. Yin et al. [119] compared gas sensing properties of WO3 hollow spheres samples obtained by conventionally heating (CWO3) and microwave-assisted heating (M-WO3). The average size (Fig. 19a) of M-WO3 hollow spheres were about 500 nm, larger than that of C-WO3. The shell thickness of M-WO3 hollow spheres is about 50 nm, much smaller than that (200 nm) of C-WO3. They attributed small sizes in shell thickness and grains of the M-WO3 hollow spheres to the shorter heating time (5 min) and the unique heating style in a microwave oven. Fig. 19b shows the response of both sensors as a function of ethanol concentration at 250 °C. The response of the M-WO3 sensor is higher than that of the C-WO3 sensor at all concentrations, probably due to the smaller particle-sizes and thinner shell of the hollow structure of the M-WO3 sample.

Fig. 19. (a) SEM images of M-WO3 samples (b) Plots of the response of the WO3-based sensors dependent on the concentration of ethanol vapors operating at 250 °C [119]

Monodisperse WO3 spheres via a microwave assisted hydrothermal method and a subsequent annealing process were successfully synthesized by Wang et al.[79], The synthesis was performed using peroxo-polytungstic acid as a precursor in the presence of sodium sulfate.

The products obtained were used as the sensing material for fabricating acetone sensors having good repeatability and stability. Gui et al. [146] prepared dispersed tungsten trioxide microsphere by chemical reduction with hydrazine hydrate in a glycol–water system. Composites of WO3/ SnO2 with different SnO2 weight fractions were also prepared by refluxing in presence of microwave irradiation (320 W). WO3/3% SnO2 composites showed the highest gas response (S > 85) to10 ppm H2S at 90 °C. This good response at low operating temperature has been attributed to the high active surface area and the heterojunction formed between WO3 and SnO2. Wang et al. [77], reported the synthesis of flower-like WO3 structures by simple calcination of W18O49 nanowires obtained by a microwave-assisted solvothermal method. The diameter of flowerlike particles were about 300- 400 nm and the building blocks of the flower-like WO3 architectures were a lot of intercrossing nanorods with diameters about 30-40 nm. The sensor using flower-like WO3 nanostructures has a relatively high response and a relatively low operating temperature, so it can be expected to serve as a promising functional material in NO2 gas sensors.

3.4 CuO and Cu2O Novel self-assembled quasi-spherical cuprous oxide (Cu2O) architectures with rough surface by a microwave chemical route without any template were produced by Liu et al.[122], Cu2O quasi-spherical architectures, showed high response to ethanol vapor and H2S gas at the working temperature of 160 °C, due to high specific surface area, which provide more interfaces for the detected gases. The quick response and recovery speed of sensor is attributed to the intrinsically rough surface, high surface-to-volume ratios and a large number of pores associated with the as-synthesized quasi-spherical Cu2O architectures. Yang et al. [133], synthesized 3D flower- and 2D branching sheet-like CuO nanostructures. CuO flowers exhibited an enhanced gas response to different VOCs gases tested (ethanol, ethylacetate, acetone, xylene, and toluene) at 260°C, compared to CuO nanosheets. In addition, the CuO flowers displayed a rapid response and recovery. They explained this as follows: first, the connection between separate individual nanosheets was lower, likely leading to a higher resistance of the CuO nanosheet sensor in air. Secondly, CuO nanosheets in the flowers were vertically aligned with high density and extended outward from the center of each flower. Open interspaces could be produced between vertically aligned nanosheets which was beneficial for the diffusion of target gas toward the surface of CuO nanosheets and

accompanying surface interaction. By contrast, separate individual CuO nanosheets may be stacked each other by face-to-face attraction and thus hinder the molecule diffusion. Volanti et al. [99], reported gas sensors, based on copper (II) oxide (CuO) with different morphologies (urchin-like, fiber-like, and nanorods), prepared by a microwave-assisted synthesis method, achieved by use of different bases and solvents. Fig 20 (left) shows H2 and NO2 response of different CuO morphologies. Figure 20 (right) shows overview SEM images of the samples as-prepared for gas sensor testing and schematic pictures of particle-particle

Response (Ra/Rg) Response (Ra/Rg)

Response (Rg/Ra) Response (Rg/Ra)

contacts and particle size for the different morphologies.

Fig. 20. (left) Dynamic response of different sensor morphologies to (a) H2 (b) NO2. (right) Schematic picture of particle-particle contacts and active sensing layer and for comparison the FESEM overview images of CuO gas sensor test devices as-prepared for (a) urchin-like, b) fiber-like, and (c) nanorods. The black and orange region on picture are corresponding to bulk resistance and sensing layer, respectively, and gray is electric contact [99].

Overall, the urchin-like CuO particles showed the highest response, with rods somewhat more sensitive than fibers. The authors explained these results according to a model for the gassensing response and conduction in p-type materials, which focuses on the relationship between the active sensing layer (Debye-layer) resistance and the grain-to-grain contact resistance. This model suggests that the sensor resistance changes are related to the

geometrical/morphological parameters of the samples and are proportional to the ratio of active sensing length (Debye-layer length) (LD) to effective contact area (DC) and to grain diameter (DG), respectively. Data reported suggest that the sensor response can be improved by changing the shape and/or dimensions of grains and sensing layers, with the highest changes in sensor signal coming from the effective contact area reduction. Therefore, the superior performance exhibited by the urchin-like structures is a consequence of the nature of the particle-particle contacts and large particle size. This unique morphology can be seen to enhance the importance of particleparticle contacts, due to the lower effective contact area and the multiple particle to particle contacts between the many spines, and the larger particle size.

3.5 CdO Neri’s group developed chemoresistive sensors based on cadmium oxides synthesized by MW [131, 147]. Highly crystalline CdO nanostructures with different morphology (spherical and rod-like) were prepared by a microwave-assisted procedure without any post-synthesis annealing treatment. Furthermore, the formation of these CdO nanostructures could be easily addressed within short times (5–15 min) by a fine tuning of the microwave irradiation. The CdO nanostructures have shown high response to NO2 down to 0.5 ppm at low operating temperature of 100 °C and high selectivity against CO. Two different preparation procedures were adopted by the same group [131] to synthesize hexagonal sheets of CdO, i.e. in the presence of urea as a directing agent or poly-vinylpyrrolidone (PVP) as a surfactant. CdO sheets prepared by using PVP have an average edge length similar to that obtained in the presence of urea. However, they are thicker (about 0.5-1 μm) than ones synthesized in the presence of urea. Sensors based on thinner CdO sheets have shown higher response and the maximum of response is shifted at lower temperature. They justified this behavior according to the fast CO diffusion toward the sheet surface and its reaction with surface oxygen. Recently, also Cd(OH)2 nanostructures with different morphologies synthesized by different techniques, including microwave-assisted methods, have attracted attention because of their distinct properties. Their applications in gas sensors have also briefly introduced in a review by Lu et al.[148].

3.6 Metal oxides miscellaneous A number of other metal oxides than those above reported have been synthesized by MWassisted routes. Nearly monodisperse Co3O4 nanocubes with a lateral size of ∼20 nm have been synthesized by a microwave-assisted solvothermal method at 180 °C. Co3O4 nanocubebased gas sensor elements were tested to 100 ppm of different organic gases [80]. The sensors have the highest response to xylene, moderate response to ethanol and toluene, and low response to benzene, acetone and cyclohexane. In the case of the aromatic compounds with the same structure, the responses of the sensors followed this order: xylene > toluene > benzene, revealing that the response increases with the number of methyl groups. Ai et al. [95], synthesized rose-like nanocrystalline Fe3O4 superstructures with ethylene glycol as the solvent by varying microwave irradiation time. Nanoroses were made up of a large number of "nanopetals" with tens of nanometers in thickness, and were reported to be sensitive to ethanol gas at ambient conditions. The high response of Fe3O4 nanoroses is associated with the small grain size, which resulted in a larger surface area and capacious interspaces. CoAl2O4 was synthesized by a non-aqueous solution method using aluminum chloride, cobalt nitrate and urea, dissolved in ethyl alcohol [149]. The solvent evaporation was made by applying microwave radiation at low power (∼160 W). The calcination of the precursor powder at 500 °C produced mesoporous CoAl2O4 having size in the range of 40-160 nm. The obtained mesoporous CoAl2O4 can detect variations in the concentration of CO and CO2. Using the same procedure, Michel et al. reported the synthesis of nanostructured CoSb2O6 microspheres with smooth surface [150]. The calcination at 700 °C produced single-phase nanostructured CoSb2O6 hollow microspheres having an average particle size of 38 nm. The gas sensing properties of CoSb2O6 thick films were investigated for detection of CO2 at 400 °C. CoSb2O6 behaved as a p-type semiconductor, reducing its resistance in the presence of target gas.

3.7 Noble metal-loaded metal oxides Metal-doped semiconductor oxides, with noble metals (Pt, Au, and Pd) playing the role of catalytic activators, have been investigated widely in chemical sensors because of their enhanced response to gases compared to pure metal oxide counterpart. Wang et al. presented a facile one-step microwave assisted hydrothermal route for the synthesis of a series of Pdloaded SnO2 nanoparticles [96]. The maximum heating power of the microwave system is

300W from the beginning to the end. The sensing performance was highly dependent upon the Pd-loaded amount. The sensor based on 3.0 wt% Pd-loaded SnO2 exhibited high response to carbon monoxide at various concentrations. This result has been interpreted in terms of the electric interaction between PdO and SnO2, for which PdO captured electrons from SnO2 and produced an electron depleted layer on the surface of SnO2. Also Cirera et al. reported a microwave-based procedure for synthesizing pure and Pd-doped SnO2 powders for gas sensing applications [58, 151]. Particles size of undoped SnO2 powders was 55 nm, whereas was 66 nm for doped-tin oxide was, so the higher response of this latter to NO2 was not due to finer particle size [58]. Electronic and chemical sensitization due to presence of Pd were the main reasons of high response of this sensor. The same authors tested gas sensing properties of Pd-doped SnO2 for detecting CO and CH4 [151]. SnO2 sensor has high response to both CO and CH4 gases, in other words it do not show selectivity to a specific gas. Despite lowering overall resistance, adding 1 wt% Pd had not significant effect in gas sensing properties, but adding 10 wt% Pd change the response behavior, showing high selectivity towards CH4 (Fig. 21). This was probably due to catalytic effect of Pd nanoparticles on SnO2 surface.

Fig. 21. 10 wt% Pd catalyzed sensor operated at 350°C [58].

Different Pd-loaded (0.03 and 0.1 wt%) SnO2 powders were prepared by template free conventional and microwave-assisted hydrothermal methods [152]. The results clearly confirmed that the presence of a low amount of Pd could effectively enhance the response value to 5 ppm NO2 gas and shorten the recovery time under the UV-light irradiation so that the sensor loaded with 0.1 wt% Pd showed the largest improvement in response value (almost 11 times) and recovery time (around 27 s) compared with the bare SnO2 sensor at an UV-light

intensity of 79 mW cm-2. This enhancement is most likely due to the role of Pd in facilitating the sensing reactions via producing additional NO2 adsorption sites on the SnO2 surface. One-step microwave assisted hydrothermal route for the synthesis of the unloaded and Pdloaded SnO2 nanostructures[97]. The Pd was grown in situ on the SnO2 nanostructure, constructing Pd/SnO2. A gas sensor based on the as-prepared Pd/SnO2 was fabricated and tested for response to carbon monoxide gas. Enhanced gas sensing performances could be attributed to both the contribution of Pd-loaded and the in situ method and, in addition, at the one-step in situ microwave assisted loading process.

4. Microwave-assisted synthesis of metal-oxide nanostructures 4.1 Low-dimensional metal-oxide nanostructures 1- or 2-dimensional (1D) nanostructures of metal-oxide semiconductors are currently the subject of intense researches for their potential in gas sensing applications. These metal-oxide nanostructures have large surface-to-volume ratio, and dimensions comparable to the extension of surface charge region. Further, they could not only provide more adsorption sites for the oxygen species and the tested gases but also facilitate the interaction between the oxide surfaces and gas molecules. This induces a more significant degree of electron transfer, and hence more pronounced output of electric signal, which is detected by the electric circuit. Ahmed et al. reported the growth of vertically aligned and single-crystalline zinc oxide (ZnO) nanorod arrays on the silicon (Si) substrate using a microwave-assisted solution method [128] The length and diameter of the well aligned rods were about ~500 nm and ~80 nm, respectively. The sensors have good sensing performance to oxygen concentrations ranging from 5 to 20 ppm, such as high response to oxygen and short response time. Layered basic zinc acetate nanosheets (LBZA NSs) from a zinc acetate, zinc nitrate and HMTA solution in only 2 min, were synthesized through a low-cost technique using a conventional microwave oven (800 W, 2.450 MHz) by Tarat et al. [153], The crystals have a rectangular shape with lateral dimensions between 1-5 μm and thickness in the nano range. The effect of CO exposure on the resistance of a film of ZnO NSs obtained by annealing LBZA NSs at 400°C has been evaluated. Sensor showed low response to CO gas, however the response time was under 30 s for 100 ppm CO, whilst the recovery time was 40 s that were relatively short times. Jin et al. [154] studied the effect of Pd and Ag decoration on the gas sensing properties of vanadium oxide nanotubes (VONTs). Pd nanoparticles of 2–5 nm were loaded on the surface

of VONTs via microwave irradiation. It has observed that with the increasing of microwave radiation power level, the loading amount of Pd nanoparticles increases. Exposing the PdVONTs to reducing gases an enhancement of the response was observed with respect to undoped VONTs. Qurashi et al. [79], reported the ultra-fast synthesis of ZnO nanowires in large-quantity by a novel microwave-assisted (2.45 GHz, 1250 W) method. The experimental set-up used is shown in Fig. 22a.

Fig. 22. (a) Schematic diagram of microwave-oven based reaction system used for the synthesis of ZnO nanowires. (b) FESEM image of ZnO nanowires (c) TEM image of ZnO nanowires [79].

High purity zinc metal was used as source material and placed on microwave absorber. As the temperature reached about 1200 °C, ZnO nanostructures (Fig. 22b,c) deposit on the inner surface of the glass container placed above the absorber. Nanowires are grown in high-density and large-scale with length of few micrometer and diameter in the range of 70–80 nm, and investigated as H2 gas sensors. Hassan et al. [82], reported arrays of vertical and oblique zinc oxide nanorods grown on a cplane sapphire substrate by microwave-assisted chemical bath deposition (Fig. 23). These nanorods provide a large surface-area-to-volume ratio to interact with the surrounding gas. The nanorods were not completely vertically aligned because ZnO nanorod growth depends on the amorphous SiO2 layer. The nanorods were linked with neighboring nanorods, which formed a nanorod network.

0.5 μm

1 μm

Fig. 23. FESEM images of ZnO nanorods arrays grown on c-plane sapphire substrate: (a) cross-section shown the oblique and vertical nanorods connections, (b) Top view[82].

The hydrogen sensing capabilities of the ZnO nanorod arrays, were investigated at room temperature. Gas sensors that are operated at room temperature have greater advantages, such as low power consumption, safe use in flammable environments, and long lifetime. The rods exhibited excellent response, of 500%, in the presence of 1000 ppm of H2. This was a result of the extremely high surface-area-to-volume ratio of nanorods that are equivalent to several thousand single nanorods connected to each other in series. The same procedure was used to grown ZnO nanorod arrays on a flexible Kapton tape[137]. Another room temperature sensor was reported by Azam et al.[88], using high-quality singlecrystalline SnO2 nanorods synthesized by a microwave-assisted solution method. At a power microwave of 300 W for 20 minutes, SnO2 nanorods with a uniform length of about 450–500 nm and a diameter of about 60–80 nm were obtained and used to detect very low oxygen concentrations, ranging from 1 to 10 ppm at room temperature. Ahmed et al. [117], reported another room temperature gas sensors based on undoped and Mn-doped ZnO nanorods prepared by microwave-hydrothermal method. Mn doped ZnO sensor showed higher response for different oxygen concentration at room temperature as compared with that of un-doped ZnO. The enhanced response at RT was attributed to the Mn which led to a reduction of the rod diameter and an increase of the surface to volume ratio. Tarat et al. [114], synthesized ZnO nanobelts (NBs) and nanosheets (NSs) using a microwave assisted (800W, 120s) chemical route. The thickness of the NBs and NSs ranged from 10 to 50 nm. PL shows a larger defect band for ZnO NBs, compared to NSs. The response at concentrations below 200 ppm is higher for the NSs sensor at 325°C than for the NBs sensor,

even though the NS particle size was larger and the temperature lower. This could be a consequence of the better crystalline quality of the ZnO particles. Yang et al.[92] prepared p-type CuO nanorods with the breadth of 15-20 nm and the length of 60–80 nm using a microwave-assisted hydrothermal (MH) method. Response of the CuObased sensor to different organic compound vapors was in the order of ethanol ∼ ethylacetate > acetone ∼ xylene ∼ toluene > cyclohexane. Compared with other gases, the as-synthesized CuO nanorod sensor has the best response to ethanol and ethyl-acetate at its optimum working temperature, whereas the lowest response was obtained upon exposure to cyclohexane. Dynamic response of the sensor to ethanol showed that the resistance increased drastically upon exposure to ethanol vapor and decreased rapidly when the gas was removed. CuO nanoparticles were synthesized with different morphologies by chemical precipitation adding a strong base to an aqueous solution of copper cations in the presence/absence of the polyethylene glycol and urea additives[105]. The modification of the nanoparticles was carried out by a microwave treatment of the precipitates. CuO sensors exhibit different gas response to ethanol, indicating their difference in gas sensing property. In the specific, gas responses indicate that there is no significant influence of microwave hydrothermal processing on the gas sensing property of the additive-free synthesized CuO nanoparticles. In contrast, the microwave hydrothermal processing has an apparent effect on the gas sensing property of the synthesized CuO nanoparticles with assistance of the additives. Co3O4 nanoparticles, behaving as p-type semiconductor[155], with different nanostructures were synthesized by microwave hydrothermal method using Co(NO3)2 as raw material, and CO(NH2)2 and KOH as precipitants[93]. For example, when using urea, Co3O4 nanochains were obtained (Fig. 24a) whose diameter of pore ranges from 17 nm to 32 nm. The response to alcohol of hollow Co3O4 nanoring is superior to nanochain-like, and that of nanosheet is the lowest, due to the larger specific surface area of Co3O4 nanoring (Fig. 24b).

100 nm

Fig. 24. (a) nanochain-like (b) corresponding nitrogen physisorption isotherms [93]

Single crystalline ZnO nanorods were prepared via microwave assisted hydrothermal method using zinc hydroxide as starting material, cetyltrimethylammonium bromide (CTAB) as structure directing agents, and water as solvent [83]. The length and width of these ZnO nanorods varied from 1–2 μm and 100–150 nm respectively (Fig. 25a,b). HRTEM image (Fig. 25c) revealed a spacing of 0.281 nm between two fringes, which corresponds to the (100) (100) crystal planes of the ZnO wurtzite. From SAED patterns, it is seen that these ZnO nanorods are single crystalline in nature, with a growth direction along the c-axis.

Fig. 25. (a) SEM, (b) TEM,and (c) HRTEM along with SAED patterns [83].

Gas sensors were prepared and tested for the detection of CO, ethanol and acetaldehyde in air. The response for ethanol is higher at every testing temperature as compared to CO and acetaldehyde. ZnO nanorods can be selectively used for the detection of ethanol. Recently, was reported a study on highly crystalline ZnO nanorods with different lengths and widths, grown using a microwave-assisted hydrothermal method at different irradiation times [156], The BET surface area increased while the lengths and widths of the ZnO nanorods were reduced with increasing irradiation time. Gas sensors based on these ZnO nanorods exhibited a high sensing response to CO at 350 °C., which originates from surface defects (Zni and VO) formed at the surfaces of ZnO nanorods. A mild template-free mixed solution medium with the assistant of microwave method (150°C, 10s) was established to synthesize well-aligned CuO nanostructures [104]. CuO with nanoflower structures exhibits superior gas sensing capabilities toward ethanol than the other samples. Li et al. [84], prepared WO3 nanorods by a simple microwave hydrothermal (MH) method via Na2SO4 as structure-directing agent. The obtained nanorods are about 20-50 nm in diameter

and several micrometers in length and exhibit excellent ethanol sensing properties. Jing et al. [86], synthesized porous ZnO nanoplates, from zinc acetate and urea as starting materials. The mixture was then kept in the microwave system under stirring at 95 °C and then annealed 400 °C. The obtained ZnO porous nanoplates provide abundant active sites to the environment, allowing the sensor to respond to ethanol (Fig. 26).

300 nm

Fig. 26. (a) Field emission scanning electron microscopy (SEM) image of porous ZnO nanoplates after annealing the precursor at 400 8C for 2 h. (b) Response curve of the porous ZnO nanoplate gas sensor to ethanol with increasing concentrations, at operating temperature of 380 °C [86].

4.2 Hierarchical metal oxide structures Hierarchical structures are higher dimensional structures composed of many, lower dimensional building blocks. These structures provide an effective gas diffusion path via well aligned mesoporous structures without sacrificing a high surface area. Consequently, they exhibit large response and fast gas response[157]. High-performance chemiresistive sensor for detection of VOCs vapors based on core-shell hybridized nanostructures of Fe3O4 magnetic nanoparticles (MNPs) and poly(3,4-ethylenedioxythiophene) (PEDOT)-conducting polymers, synthesized using microwave-assisted synthesis in the presence of polymerized ionic liquids

(PILs), which were used as a linker to couple the MNP and PEDOT, have been reported [158]. Among these complex nanostructures, core@shell nanomaterials, in which metal oxides are as core or as shell, are gaining much attention for gas sensing, as the physical properties of the core and shell can be easily and separately tuned [159, 160]. In the specific, metal@metal oxide core-shell systems, interface between metal and metal oxide is maximized, while still minimizing the amount of material acting as bulk. Au@SnO2 core-shell nanoparticles were synthesized by a precipitation method and a microwave hydrothermal synthesis method, and measured their CO responses [94, 161]. The average grain sizes of SnO2 NPs of precipitation method and microwave hydrothermal synthesis method were measured as 5.2 nm and 8.3 nm, respectively (Fig. 27).

Fig. 27. Bright-field low-magnified TEM image samples prepared by (a) precipitation method and (b) microwave hydrothermal method [161]

CO response of the sample prepared by the precipitation method was extremely low in comparison to the one by the microwave hydrothermal synthesis method. Authors attributed the higher gas response of microwave prepared samples to higher porosity within SnO2-shell layers prepared by the microwave hydrothermal synthesis method than the one prepared by the precipitation method, so surface area was higher and consequently gas adsorption and sensor response were higher in microwave prepared sample. This agree with the view that the formation of Schottky barriers at the interface between metal and metal oxide leads to improved charge carriers separation and transfer, with significant benefits on sensing [162]. Yin et al. [96], synthesized hierarchical Fe2O3@WO3 nanocomposites with ultrahigh specific areas, consisting of Fe2O3 nanoparticles (NPs) and single-crystal WO3 nanoplates, via a microwave-heating (MH) and water-bath heating (WH) in situ growth process. The

microwave-heating process is more favorable in forming Fe2O3@WO3 nanocomposites with higher H2S-sensing than the water heating process (Fig. 28).

Fig. 28. The plots of the [H2S]-dependent responses of the WO3, 5% Fe2O3@WO3-MH and 5%Fe2O3@WO3WH sensors operating at 150 °C upon exposure to H2S gases with various concentrations (0.5–10 ppm) [96].

The possible reason is its efficient control in more uniform distribution and smaller particle sizes of Fe2O3 NPs anchored on the surfaces of WO3 nanoplates, resulting in ultrahigh specific surface areas of the Fe2O3@WO3-MH samples. Au@TiO2 core-shell NPs by microwave assisted hydrothermal method in 1 h. Au NPs with 40 ± 5 nm size were synthesized by colloidal method [10]. TiO2 shell layer with 60 ± 10 nm shell thickness was deposited on Au by microwave assisted hydrothermal method. The high response of Au@TiO2 core-shell NPs compared to bare TiO2 NPs, was due to the catalytic activity of Au NPs which activate the dissociation of O2. Therefore, the response of Au@TiO2 core-shell NPs was higher than bare TiO2 NPs. However, for chemical sensitization, it is necessary that TiO2 shell is porous in nature and gas can easily diffuse to Au. Das et al. [123], reported a simple microwave assisted hydrothermal precipitation (M–H) technique for the synthesis of Ag@SnO2 core–shell structure nanoparticles. Ag NPs were synthesized via chemical reduction of metal salt followed by M–H deposition of tin dioxide shell for fabrication of monodispersed core–shell particles. Materials without a core–shell structure undergo a rapid grain growth and agglomeration and hence showed very poor response and their sensor response decreased after 10th cycles. Instead, core-shell materials remained active with minor change in its sensor response even after 15th cycles. ZnO core–shell structure with a movable core inside a hollow shell were rapidly synthesized by an efficient microwave hydrothermal method without any template[121]. The evolution of the sample microstructure is shown in Fig. 29a-e). It can be seen that a spherical shaped ZnO

with smooth surface had been formed, and the diameter of the ZnO microspheres was about 1.5 mm. When the ageing time increased to 10 min, a typical core-shell structure showed up, accompanying a little increase of external diameter. As the ageing time was prolonged to 20 min, these nanoparticles as well as the entire ZnO spherical shell continued to increase in size, while the internal ZnO core began to shrink gradually. For the sample having aged for half hour, the rattle-type structure evolved into hollow structure due to the excessive ripening.

Fig. 29 (a–d) SEM images of the products obtained at different reaction stage after heat treatment. (e) Schematic illustration of the evolution process of spherical architectures (f) Responses of the sensors as a function of ethanol concentrations ranging from 10 to 700 ppm at optimal operation temperatures [121].

The improvement of the sensing performance (Fig. 29f) of the ZnO core–shell structures were mainly ascribed to their unique configuration. Since the unique porous core-shell architecture possessed large numbers of well-defined pores on the surface domains, these pores in the structure facilitated the diffusion and adsorption of the gas molecules. Moreover, compared with hollow spheres, the core-shell structures possess more exposed surface, which can provide plenty of active sites and space for reaction between ethanol and oxygen. Yin et al. synthesized hierarchical In2O3@WO3 nanocomposites, consisting of discrete In2O3 nanoparticles (NPs) on single crystal WO3 nanoplates, via a novel microwave-assisted growth

of In2O3 NPs on the surfaces of WO3 nanoplates that were derived through an intercalation and topochemical-conversion route [120]. The responses of the In2O3@WO3 sensors increase with the increase in the In2O3 amounts from 0 to In/W = 0.8, and then decrease when the In2O3 NPs amount reaches to In/W = 1 (Fig. 30a). The enhancement of H2S-sensing performance was attributed to the synergistic effect of two-dimensional WO3 nanoplates and zero-dimensional In2O3 NPs. The hierarchical configuration of In2O3 NPs on WO3 nanoplates prevented the aggregation of the In2O3 NPs and increased efficient paths for diffusion and adsorption of H2S (Fig. 30b). On the other hand, the hetero-junctions at the interface of In2O3 and WO3 can generate a special electron donor–acceptor system. Upon exposure to H2S, the conductivity of the In2O3@WO3 sensor increases, and thus more electrons migrate to form electron depletion regions that improve the gas-sensing performance.

Fig. 30. (a) Plots of the response dependent on H2S concentration: (A) WO3, (B) In2O3@WO3 (In/W= 0.5), (C) In2O3@WO3 (In/W = 0.8), and (D) In2O3@WO3 (In/W = 1). (b) adsorption and reaction process of O2 and H2S molecules at the interface of the hierarchical In2O3@WO3 nanostructure[120].

Hu et al. [163], reported the MW-assisted synthesis of Fe2O3 nanorings. Figure 31a-d shows the morphological and microstructural characteristics of the as-prepared sample, consisting of single-crystalline α-Fe2O3 nanoring with a perfect circular shape.

Fig. 31. (a) Low-magnification bright-field TEM image of α-Fe2O3 nanorings. (b) High-magnification TEM image of a single nanoring. (c) ED pattern indicating the singe-crystal nature of the nanorings. (d) Typical HRTEM image taken from α-Fe2O3 nanoring and the corresponding fast Fourier-transform (FFT) pattern (inset). (e) Resistance changes for a sensor made of α-Fe2O3 nanorings switched between ethanol (∼ 6%) and air [163].

Average outer diameter of nanoring is about 100 nm and inner diameters ranging from 20 to 60 nm. Thin film sensor made of the α-Fe2O3 nanorings exhibits large response and good reversibility for gas-sensing of alcohol vapor under ambient conditions (Fig.40e). Thin-film sensor was composed the random-oriented α-Fe2O3 nanorings, resulting in a loose-film structure analogous to a highly porous architecture. Thus, film comprises a network of interconnected pores. The network of pores contributed to the high response, since it allowed the gas molecules more accessible to all the surfaces of α-Fe2O3 crystals included in the sensing unit. 4.2 Metal oxides-based composites with carbon nanostructures Composite structures of metal oxides with carbon nanostructures, such as carbon nanotubes (CNTs) and graphene, are of great scientific and applicative interests. Since their discovery in 1991, carbon nanotubes have drawn worldwide attentions because of their novel structural characteristics such as high surface area, high conductivity and high stability and nanorange hollow tubes. Combining CNT characteristics with metal oxides, superior sensing properties are expected to emerge. Potirak et al. [164], synthesized zinc oxide and multi-walled carbon nanotube (ZnO/MWCNT) hybrid nanocomposites by microwave-assisted method. The

syntheses were carried out at various microwave irradiation powers (300, 450 and 700 W), obtaining ZnO on CNTs walls with different particle size, ranging from 15 to 35 nm. The ethanol sensing performance of ZnO/MWCNT increased from 20% to 50% as the irradiation power elevated from 300 to 700 W, likely due to the better formation of nanocrystalline ZnO onto the CNT surface, providing greater specific surface area and more active sites for alcohol-sensing reaction. A very sensitive hybrid nanocomposite-based sensors (response to 1 ppm formaldehyde ~13) was developed, using the synthesized Ag-LaFeO3 modified by the CNTs using a sol-gel method combined with microwave chemical synthesis [108]. Graphene, a unique 2D carbon structure, has been considered as a promising candidate in the fabrication of gas sensors due to its ultra-high electron conductance and large surface area. In the attempt to improve the sensing performances of pure graphene, heterostructures made of graphene and semiconductor metal oxides have been explored. Metal oxide phases can help activating reactions occurring on the carbon surface by favouring the adsorption/desorption processes, leading in turn to higher responses and faster response/recovery times. Furthermore, decoration of graphene sheets with an n-type metal oxide can lead to the formation of an n-p junction and the resulting novel nanostructure can exhibit performances far better than those of the individual materials. Yin et al. [112], synthesized SnO2@rGO nanostructures with super high surface areas are via a simple redox reaction between Sn2+ ions and graphene oxide (GO) nanosheets under microwave irradiation (Fig. 32).

Fig. 32. A schematic demonstration of the normal-pressure microwave synthesis of hierarchical SnO2@rGO nanostructures[112].

SnO2 nanoparticles with particle sizes of 3-5 nm are uniformly anchored on the surfaces of reduced graphene oxide (rGO) nanosheets through a heteronucleation and growth process. The responses of the SnO2 sensor to H2S were relatively weak when compared with the SnO2@rGO sensor. A very short response time of 7 s for the SnO2@rGO sensor is achieved, whereas the SnO2 sensor has a long response time of 66 s. The enhanced H2S-sensing performance at lower operation temperature of the SnO2@rGO sensor were attributed to the unique hierarchical SnO2@rGO nanostructure with an ultrahigh surface area of larger than 2100 m2 g−1. Crystalline SnO2/r-GO nanocomposites were synthesized by a one-pot microwave-assisted non-aqueous sol-gel method, in which the partial reduction of graphene oxide and nanoparticle formation occurs simultaneously. In the specific, the combined properties of the microwave heating non-aqueous sol-gel method allowed to selectively grow, at 185 °C and in only few minutes, metal oxide nanoparticles on the surface of reduced graphene oxide sheets[165]. The materials prepared in such a way were found to have higher particle density and exhibit a more homogeneous coating of the r-GO sheets than the corresponding ones synthesized using traditional heating. The fabricated SnO2/r-GO

composites

showed

good

resistive

sensors,

based

on

the

sensing characteristics to NO2 [166]. The

response NO2 was found to be dependent on the SnO2/rGO ratio. Meng et al. synthesized Cu2O nanorods modified by reduced graphene oxide via a two-step synthesis method. CuO rods were firstly prepared in graphene oxide solution using cetyltrimethyl ammonium bromide (CTAB) as a soft template by the microwave-assisted hydrothermal method, accompanied with the reduction of GO. The complexes were subsequently annealed and Cu2O nanorods/r-GO composites were obtained. The resistance of the sensor increases significantly with the introduction of NH3, indicating the p-type response of the sensor based on the Cu2O/r-GO composites. The enhanced performance observed was the result of three factor: i) Cu2O nanorods prevents the GO sheets from restacking, thus leading to a good surface accessibility; ii) Cu2O/r-GO composites has excellent catalytic activity; iii) the effective electronic interaction between Cu2O and r-GO facilitates the gas molecule detection via the resistance change of the hybrid composites. Further, the improved selectivity of Cu2O nanorods/r-GO composites for NH3 could be explained by the selectivity adsorption of NH3 on r-GO. NH3 can react through hydrogen bonding with functional groups (carboxyl, carbonyl, epoxy, and hydroxyl) groups on GO in an ambient environment. Wan et al. [107], for the first time prepared graphene modified by hierarchical flower-like In(OH)3 by a one-step microwave-assisted hydrothermal method and fabricated a sensor with

an excellent selectivity towards NO2 gas. Also Yang et al. [132]proposed a selective NO2 sensor based on In2O3 on r-GO. In a typical preparation process, GO aqueous suspensions was added into distilled water, and was sonicated. Subsequently InN-NWs was added into the above GO solution, and a well-dispersed solution was obtained by ultrasonication. After that, the uniform dispersion was sealed in a Teflon container and transferred into a microwave digestion system. The reaction was then preformed at 150°C for 30 min under microwave irradiation. After cooling to room temperature naturally, the resulting products were cleaned by centrifugation/washing cycles, and then air-dried at 100 °C for 24 h. Fig. 33 shows a schematic picture of the formation process of In2O3/r-GO.

Fig. 33. Schematic illustration of the formation process of the In2O3/rGO composites [132]

The enhanced sensing performance of the In2O3 cubes/rGO composites is tentatively explained from the following aspects: uniform distribution of In2O3 in the graphene networks which guarantees the good gas penetration and transport in the composites, and the existence of graphene sheets in the hybrid architectures which overcomes the poor electrical conductivity of In2O3 at room temperature. Bai et al. [102]successfully synthesized the hetero-junction hybrids of 1-D molybdenum trioxide nanorods/reduced graphene oxide (MoO3/rGO) with different contents of r-GO by insitu one-step microwave hydro-thermal method. Remarkably, these reduced graphene oxides are decorated by numerous rod-like MoO3 structures with a typical length of about several micrometers and a diameter of about 100 nm. The sensor response values of

hybrid

composite to 5–80 ppm H2S gases are obviously higher than that of pure α-MoO3.The r-GO substrates offered a larger surface accessibility and fast carriers transport, which facilitated

molecular adsorption, gas diffusion and mass transport, especially, the planar 2D r-GO sheets created an extensive 3D network structures that enhances interconnectivity among the MoO 3 and r-GO. Further, the improvement of gas sensing for the hybrid can be attributed to the electrons migration at the interface between the MoO3 nanorods and the r-GO nanosheets due to difference of their work function. Similarly, Gui et al. [103], successfully synthesized hemispherical WO3/graphene hollow nanocomposite structures via a microwave-assistedhydrothermal method, which contribute to high gas-sensing performance to trimethylamine.

Conclusions and Outlook There is no doubt that the technique of microwave dielectric heating is becoming of increasing importance in inorganic chemical synthesis. Compared to the conventional heating process and unlike many new technologies, microwaves technology has become an acceptable and routinely applied method for the synthesis of metal oxides, particularly as it has the following advantages: (i) microwaves generate higher power densities, enabling increased production speeds and decreased production costs; (ii) microwave energy is precisely controllable and can be turned on and off instantly, eliminating the need for warm-up and cool-down and (iii) microwave energy is selectively absorbed by areas of greater moisture resulting in more uniform temperature and moisture profiles, improved yields and enhanced product performance. Presently, microwave heating seems the most promising way to achieve short synthesis times. Further, the MW technique appears very suitable in the synthesis of small sized particle/nanostructured metal oxide-based materials for applications in gas sensing devices, enhancing the material quality, suppressing side reactions, and thus improving the yield and reproducibility of a specific synthesis protocol. It is well known that the gas response for metal-oxide nanomaterials is mainly dependent on their surface structure. So, the difference of reaction mechanisms between conventional synthesis methods and MW-assisted processes may lead to a different microstructure of the metal oxide obtained and, of course, this may influence their gas sensing properties. For example, wet synthesis processes of metal oxides enable a fine tuning of the inorganic structural features, in particular, the crystal structure (phase, crystallinity degree), morphology (size, shape, anisotropy) and composition, in both the bulk and on the surface. These latter features, interplay a major role in determining the functional properties, such as gas sensing, of the synthesized nanomaterials. To be specific, the useful application of microwave heating

as a processing step in wet-chemistry synthetic protocols (such as hydrothermal, sol-gel, etc.) leads to the rapid nucleation of crystalline oxides as solid aggregates within the mixture and this allows a large number of nanosized crystalline particles with unusual surface characteristics to form at very low temperatures, as opposed to large crystalline growths or amorphous precipitates. The extent of such bulk and surface modifications depends on the nature of metal oxide, and also on the heating rate and heating mechanism. Generally, the short syntheses times achieved with the MW-assisted process, which could not be reproduced in any other way, resulted in nanomaterials with smaller oxide particles, narrower particle size distributions, more controlled and uniform particle morphology, compared to ones produced by slower heating of the reaction mixture and long reaction times. Limitations of microwave-assisted processes are instead due to the instrumental apparatus itself. The possibility of varying the reaction conditions by finely tuning/controlling the irradiation power and the temperature are the main drawbacks that may, in some cases, hinder reproducibility especially when not laboratory-designed microwave ovens are used. At last, it is noteworthy that papers so far reported in the literature regarding metal oxides synthesized by microwave results lacking of comprehensive investigations feels. Many researchers just synthesized the materials by microwave irradiation and did not focused their attention on the understanding of the effect of microwave irradiation parameters such as irradiation time, microwave power, etc., on their final properties. However, overcoming the fine tuning/control of microwave irradiation above reported, should give new impulse and lead to metal oxide materials with more controllable morphological and microstructural properties. With the significant interest and continued results inferring advantages of using microwaves rather than conventional processing, in the near future this technology could be adopted commercially for producing, mass quantities of nanostructured metal oxides for gas sensing applications. Indeed, although large scale application of microwave assisted techniques is restricted due to low penetration depth of the microwave radiations and scaling up of the Teflon autoclaves, a continuous microwave flow synthesis (CMFS) process can be used to mix desired reactants and provide appropriate residence heating times inside the microwave cavity at a larger scale while keeping actual reaction volumes small [167]. Such a system would circumvent a long standing challenge with large scale synthesis, and simultaneously maintain a high degree of control over reaction process parameters. Such modifications in the design of microwave-based reaction systems for the use in continuous-flow processes will have significant potentials for the production and commercialization of metal oxides with enhanced characteristics as sensing materials.

Acknowledgement The authors acknowledge the Iran nanotechnology council for the partial financial support.

References

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

[12]

[13]

[14] [15] [16] [17]

[18] [19]

[20] [21] [22]

G.Heiland, "Zum einfluss von wasserstoff auf die elektrische leitfähigkeit von ZnO-kristallen", Z. Phy., 1954, 138. J.Brattain, W.H. Bardeen, "Surface properties of germanium", Bel. Sys. Tech. J. , 1953, 32: 141. T. Seiyama, A.Kato "A new detector for gaseous components using semiconductor thin film", Anal. Chem, 1962, 34: 1502-1503. N.Taguchi, "Japenese Patent", 1962. A.Chiba, "Development of the TGS gas sensor," in Chemical Sensor Technology, Yamauchi S., Ed., ed Tokyo: Kodansha publication, 1992, p. p.2. M. Fleischer , M. Lehmann, "Solid State Gas Sensors - Industrial Application": Springer, 2012. N. Barsan, D. Koziej, U. Weimar, "Metal oxide-based gas sensor research: How to?", Sens. Actuators B, 2007, 121: 18-35. S. E. Moon, N.J. Choi, H.K. Lee, J. Lee, W. S. Yang, "Semiconductor-type MEMS gas sensor for real-time environmental monitoring applications", ETRI Journal, 2013, 35: 613-624. A. Mirzaei, S. Park, H. J. Kheel, G. J. Sun, S. Lee, C. Lee, "ZnO-capped nanorod gas sensors", Ceramics International, 2016, 42: 6187-6197. J. Bai, B. Zhou "Titanium dioxide nanomaterials for sensor applications", Chem. Rev., 2014, 114: 10131-10176. S.G.Leonardi1, A.Mirzaei, A Bonavita, S. Santangelo, P. Frontera, F. Pantò, P L. Antonucci, G.Neri, "A comparison of the ethanol sensing properties of α-iron oxide nanostructures repared via the sol–gel and electrospinning techniques", Nanotechnology 2016, 27: 075502075512. Y. S. Kim, P. Rai, Y.T. Yu, "Microwave assisted hydrothermal synthesis of Au@TiO2 core–shell nanoparticles for high temperature CO sensing applications", Sensors and Actuators B 2013, 186: 633- 639. B. T. Raut, P. R. Godse, S. G. Pawar, M. A. Chougule, D. K. Bandgar, V. B. Patil, "Novel method for fabrication of polyaniline-CdS sensor for H2S gas detection", Measurement 2012, 45: 94100. G. J.Cadena, J. Riu, F. X. Rius, "Gas sensors based on nanostructured materials", Analyst 2007, 132: 1083-1099. A. Mirzaei, S. Park, G.J. Sun, H. Kheel, C.Leeb, "CO gas sensing properties of In4Sn3O12band TeO2 compositenanoparticle sensors", Journal of Hazardous Materials, 2016, 305 30-138. N.S. Baick, G. Sakai, N. Miura, N. Yamazoe, "Hydrothermally treated sol solution of tin oxide for thin-film gas sensor", Sens. Actuators, B 2000, 63: 74-79. Ali Mirzaei, K.Janghorban, B. Hashemi, A. Bonavita, M. Bonyani, S. G.Leonardi, G. Neri, "Synthesis, Characterization and Gas Sensing Properties of Ag@α-Fe2O3 Core–Shell Nanocomposites", Nanomaterials, 2015, 5: 737-749. Niederberger, M., "Nonaqueous sol-gel routes to metal oxide nanoparticles", Acc. Chem. Res., 2007, 40: 793-800. S. Kato, H. Unuma, T. Ota, M. Takahashi, "Homogeneous precipitation of hydrous tin oxide powders at room temperature using enzymatically induced gluconic acid as a precipitant", J. Am. Ceram. Soc. , 2000, 83: 986-988. L.B. Fraigi, D.G. Lamas, N.E. Walsoe De Reca, "Comparison between two combustion routes for the synthesis of nanocrystalline SnO2 powders", Mater. Lett. , 2001, 47: 262-269. K.C. Song, J.H. Kim, "Synthesis of high surface area tin oxide powders via waterin- oil microemulsions", Powder Technol. , 2000, 107 268-272. B. Lee, S.Komarneni, "Chemical processing of ceramics", Second Edition ed.: CRC Press, 2005.

[23]

[24] [25] [26] [27]

[28]

[29] [30] [31] [32]

[33] [34] [35]

[36]

[37] [38] [39] [40] [41] [42] [43] [44]

[45]

E.Zhang, Y.Tang, K.Peng, C.Guo, Y. M.Zhang "Synthesis and magnetic properties of core–shell nanoparticles under hydrothermal conditions", Solid State Communications, 2008, 148: 496500. R. Corriu, N. T. Anh, "Molecular chemistry of sol-gel derived nanomaterials": John Wiley & Sons Ltd, 2009. Kulkarni, S. K., "Nanotechnology: Principles and Practices", Third Edition ed.: Springer, 2013. Eastoe, Julian, Hollamby Martin J.,Hudson Laura, "Recent advances in nanoparticle synthesis with reversed micelles", Advances in Colloid and Interface Science, 2006, 128–130: 5-15. S.Faraji, F. N. Ani, "Microwave-assisted synthesis of metal oxide/hydroxide composite electrodes for high power supercapacitors-A review", Journal of power sources, 2014, 263 338-360. Y. Wang, J. Tian, C. Fei, L. Lv, X. Liu, Z. Zhao, G. Cao, "Microwave-assisted synthesis of SnO2 nanosheets photoanodes for Dye-sensitized solar cells", J. Phys. Chem. C, 2014, 118: 25931−25938. S.Feng, G.Li, "Hydrothermal and solvothermal syntheses," in Modern Inorganic Synthetic Chemistry, ed Amsterdam, The Netherlands: Elsevier B.V, 2011, pp. pp. 63-93. J. Prado-Gonjal, R. Schmidt, E. Morán, "Microwave-Assisted Routes for the Synthesis of Complex Functional Oxides", Inorganics, 2015, 3: 101-117. I. Bilecka, M. Niederberger, "Microwave chemistry for inorganic nanomaterials synthesis", Nanoscale, 2010, 2: 1358-1374. S. C.Motshekga, S. K. Pillai, S.S. Ray,K. Jalama, R.W. M. Krause, "Recent trends in the microwave-assisted synthesis of metal oxide nanoparticles supported on carbon nanotubes and their applications", Journal of Nanomaterials, 2012. A. B. Panda, G. Glaspell, M. S. El-Shall, "Microwave synthesis of highly aligned ultra narrow semiconductor rods and wires", J. Am. Chem. Soc. , 2006, 128: 2790-2791. G. A. Tompsett, W. C. Conner, K. S.Yngvesson, "Microwave synthesis of nanoporous materials", Chem Phys Chem, 2006, 7: 296 - 319. M. P. Mahabole, R. C. Aiyer, C. V. Ramakrishna, B. Sreedhar,R.S.Khairnar, "Synthesis, characterization and gas sensing property of hydroxyapatite ceramic", Bull. Mater. Sci., 2005, 28: 535-545. A. Lagashettya, V. Havanoorb, S. Basavaraja, S.D. Balaji, A. Venkataraman, "Microwaveassisted route for synthesis of nanosized metal oxides", Science and technology of advanced materials, 2007, 8 484-493. K. J. Rao, B. Vaidhyanathan, M. Ganguli, P. A. Ramakrishnan, "Synthesis of inorganic solids using microwaves", Chem. Mater., 1999, 11: 882-895. B.A. Roberts, C R. Strauss, "Toward rapid, “green”, predictable microwave-assisted synthesis", Accounts of chemical research, 2005, 38: 653-661. S.Schanche, J., "Microwave synthesis solutions from personal chemistry", Molecular Diversity, 2003, 7: 293-300. M. N. Nadagouda, T. F. Speth, R. S. Varma, "Microwave-assisted green synthesis of silver nanostructures", Accounts of chemical research, 2010, 44: 469–478. K.S. Park, J.T. Son, H.T. Chung,S.J. Kim,C.H. Lee, H.G. Kim, "Synthesis of LiFePO4 by coprecipitation and microwave heating", Electrochem. Comm., 2003, 5: 839-842. S. Komarneni, D. Li, B. Newalkar, H. Katsuki, A. S. Bhalla, "Microwave-polyol process for Pt and Ag nanoparticles", Langmuir, 2002, 18: 5959-5962. O. Palchik, I, Felner, Gina Kataby,A.Gedanken, "Amorphous iron oxide prepared by microwave heating", J. Mater. Res., 2000, 15: 2176-2181. G. H. Dong, Y.J. Zhu, "One-step microwave-solvothermal rapid synthesis of Sb doped PbTe/Ag2Te core/shell composite nanocubes", Chemical Engineering Journal, 2012, 193-194: 227-233. J. A. Gerbec, D.Magana, A. Washington, G. F. Strouse, "Microwave-enhanced reaction rates for nanoparticle synthesis", J. Am. Chem. Soc., 2005, 127: 15791-15800.

[46]

[47] [48]

[49]

[50]

[51] [52]

[53]

[54] [55]

[56] [57] [58]

[59] [60] [61] [62] [63] [64]

[65] [66]

A. A. Al-Ghamdi, F. Al-Hazmi , R.M. Al-Tuwirqi ,F. Alnowaiser, O. A. Al-Hartomy, F. ElTantawyd, F. Yakuphanoglu, "Synthesis, magnetic and ethanol gas sensing properties of semiconducting magnetite nanoparticles", Solid state sciences 2013, 19: 111-116. M. G. Ma, Y.J. Zhu , G.F.Cheng, Y. H. Huang, "Microwave synthesis and characterization of ZnO with various morphologies", Mater. Lett. , 2008, 62: 507-510. M. Yahaya, S. T. Tan, A. A. Umar,C.C. Yap,M.M. Salleh, "Synthesis of ZnO nanorod arrays by chemical solution and microwave method for sensor application", Key Engineering Materials 2014, 605: 585-588. J.Rossignol, P.D. Stuerga, "Metal oxide nanoparticles obtained by microwave synthesis and application in gas sensing by microwave transduction", Key Engineering Materials 2014, 605: 299-302. V. Moghimifar A. Raisi, A. Aroujalian, N. B. Bandpey, "Preparation of nano crystalline titanium dioxide by microwave hydrothermal method", Advanced Materials Research 2014, 829: 846850. A. Cirera, A. Vila, A. Cornet, J.R. Morante, "Properties of nanocrystalline SnO2 obtained by means of a microwave process", Materials Science and Engineering C 2001, 15: 203-205. G.A. Babu, G. Ravi, Arivanandan, M. Navaneethan, Y. Hayakawa, "Facile synthesis of nickel oxide nanoparticles and their structural, optical and magnetic properties", Asian Journal of Chemistry, 2013, 25: S39-S41. R. Pana, Y. Wu, Q.Wang, Y. Hong, "Preparation and catalytic properties of platinum dioxide nanoparticles: A comparison between conventional heating and microwave-assisted method", Chemical Engineering Journal 2009, 153: 206-210. C.R. Michela, A. H. Martínez-Preciado, "CO sensor based on thick films of 3D hierarchical CeO2 architectures", Sensors and Actuators B, 2014, 197: 177-184. M. Parthibavarman, K.Vallalperuman, C.Sekar, G. Rajarajan, T. Logeswaran, "Microwave synthesis, characterization and humidity sensing properties of single crystalline Zn2SnO4 nanorods", Vacuum 2012, 86: 1488-1493. J. Zhang, Y. Zhang, C. Hu, Z. Zhu, Q.Liu, "A formaldehyde gas sensor based on zinc doped lanthanum ferrite", Advanced Materials Research 2014, 873: 304-310. S. Rane, S. Tatkare, S. Rane, S. Gosavi, "Thick Film Hydrogen sensor based on nanocrystalline nickel ferrite prepared using simple microwave Oven". A. Cirera, A. Vila, A. Die´Guez, A. Cabot, A. Cornet, J.R. Morante, "Microwave processing for the low cost, mass production of undoped and in situ catalytic doped nanosized SnO2 gas sensor powders", Sensors and Actuators B: Chemical 2000, 64: 65-69. C. O.Kappe, D. Dallinger, S. S. Murphree, "Practical microwave synthesis for organic chemists: strategies, instruments, and protocols". Germany: Wiley-VCH Verlag GmbH, 2009. J. Klinowski, F. A. Paz, P. Silvab, J. Rocha, "Microwave-assisted synthesis of metal–organic frameworks", Dalton Transactions, 2011, 40: 321–330. Y. Li, W. Yang, "Microwave synthesis of zeolite membranes: A review", Journal of Membrane Science, 2008, 316: 3-17. M. A. Surati, S.Jauhari, K. R. Desai, "A brief review: microwave assisted organic reaction", Archives of applied science research, 2012, 4 645-661. S. Caddick, R.Fitzmaurice, "Microwave enhanced synthesis", Tetrahedron 2009, 65 33253355. A. Lew, P. O. Krutzik, M. E. Hart, A. R.Chamberlin, "Increasing rates of reaction: microwaveassisted organic synthesis for combinatorial chemistry", Journal of combinatorial chemistry, 2002, 4: 95-106. Kappe, C. O., "Controlled microwave heating in modern organic synthesis", Angewandte chemie international edition, 2004, 43: 6250 –6284. J.P. Tierney, P. Lidstrom, "Microwave assisted organic synthesis". Australia: Blackwell Publishing Ltd, 2005

[67]

[68]

[69] [70]

[71] [72] [73] [74] [75]

[76] [77]

[78]

[79] [80]

[81]

[82]

[83]

[84] [85]

[86]

M. Oghbaei, O.Mirzaee, "Microwave versus conventional sintering: A review of fundamentals, advantages and applications", Journal of alloys and compounds, 2010, 494 175-189. R. Al-Gaashania, S. Radiman, N. Tabet, A. R. Daud, "Effect of microwave power on the morphology and optical property of zinc oxide nano-structures prepared via a microwaveassisted aqueous solution method", Materials Chemistry and Physics, 2011, 125: 846-852. C. O. Kappe, D. Dallinger, "The impact of microwave synthesis on drug discovery", Nature reviews, 2006, 5: 51-64. T. Krishnakumar, R. Jayaprakash, Nicola Pinna, V.N. Singh, B.R. Mehta, A.R. Phani, "Microwave-assisted synthesis and characterization of flower shaped zinc oxide nanostructures", Mater. Lett. , 2009, 63: 242-245. S. Das, A.K. Mukhopadhyay, S. Datta, D. Basu, "Prospects of microwave processing: An overview", Bull. Mater. Sci. , 2008, 31: 943-956. Cundy, C. S., "Microwave techniques in the synthesis and modification of zeolite catalysts: A review", Collect. czech. chem. commun., 1998, 63: 1699-1723. S. Balaji, D. Mutharasu, N. Sankara Subramanian, K. Ramanathan, "A review on microwave synthesis of electrode materials for lithium-ion batteries", Ionics, 2009, 15: 765-777. Neri, G., "First fifty years of chemoresistive gas sensors", Chemosensors, 2015, 3: 1-20. E. Comini, C. Baratto, G. Faglia, M. Ferroni, A. Vomiero, G. Sberveglieri, "Quasi-one dimensional metal oxide semiconductors:preparation, characterization and application as chemical sensors", Prog. Mater Sci, 2009, 54: 1-67. C. Nayral, E. Viala, V. Colliere, P. Fau, F. Senocq, A. Maisonnat, B. Chaudret "Synthesis and use of a novel SnO2 nanomaterial for gas sensing", Appl. Surf Sci, 2000, 164 219-226 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", Sensors and Actuators B, 2013, 186: 734- 740. C. Yang, X. Su, J. Wang, X. Cao, S. Wang, L. Zhang, "Facile microwave-assisted hydrothermal synthesis of varied-shaped CuO nanoparticles and their gas sensing properties", Sensors and Actuators B, 2013. A. Qurashi, N. Tabet, M. Faiz ,T. Yamzaki, "Ultra-fast microwave synthesis of ZnO nanowires and their dynamic response toward hydrogen gas", Nanoscale Res Lett, 2009, 4: 948-954. C. Sun, X.Sua, F. Xiao, C. Niua, J.Wang, "Synthesis of nearly monodisperse Co3O4 nanocubes via a microwave-assisted solvothermal process and their gas sensing properties", Sensors and Actuators B, 2011, 157: 681- 685. A. Srivastava, K. Jain, Rashmi , A.K. Srivastava , S.T. Lakshmikumar, "Study of structural and microstructural properties of SnO2 powder for LPG and CNG gas sensors", Materials Chemistry and Physics, 2006, 97: 85-90. J.J. Hassan, M.A. Mahdi, C.W. Chin, H. Abu-Hassan, Z. Hassan, "Room temperature hydrogen gas sensor based on ZnO nanorod arrays grown on a SiO2/Si substrate via a microwaveassisted chemical solution method", Journal of Alloys and Compounds, 2013, 546: 107-111. P.Rai, H.M.Song, Y.S.Kim, M.K. Song, P.R. Oh, J.M. Yoon, Y.T. Yu, "Microwave assisted hydrothermal synthesis of single crystalline ZnO nanorods for gas sensor application", Materials Letters, 2012, 68: 90-93. Y. Li, X. Su, J. Jian , J. Wang, "Ethanol sensing properties of tungsten oxide nanorods prepared by microwave hydrothermal method", Ceramics International 2010, 36: 1917-1920. T. Krishnakumar, R. Jayaprakash, N. Pinna,N. Donatod, A. Bonavita, G. Micali, G. Neri, "CO gas sensing of ZnO nanostructures synthesized by an assisted microwave wet chemical route", Sensors and Actuators B, 2009, 143: 198-204. Z. Jing , J.Zhan, "Fabrication and gas-sensing properties of porous ZnO nanoplates", Advanced Materials, 2008, 20: 4547-4551.

[87]

[88]

[89]

[90]

[91]

[92]

[93]

[94] [95] [96]

[97] [98] [99]

[100] [101]

[102]

[103]

[104]

[105]

X. Chu, T. Chen, W. Zhang, B. Zheng, H. Shui, "Investigation on formaldehyde gas sensor with ZnO thick film prepared through microwave heating method", Sensors and Actuators B 2009, 142: 49-54. A. Azam, S.S. Habib, N. A Salah, F.Ahmed, "Microwave-assisted synthesis of SnO2 nanorods for oxygen gas sensing at room temperature", International journal of nanomedicine, 2013, 8 3875-3882. J.J. Hassan, M.A. Mahdi, C.W. Chin, H. Abu-Hassan, Z. Hassan, "A high-sensitivity roomtemperature hydrogen gas sensor based on oblique and vertical ZnO nanorod arrays", Sensors and Actuators B: Chemical, 2012. N. Faal Hamedania, A. R.Mahjouba, A. A. Khodadadib,Y. Mortazavic, "Microwave assisted fast synthesis of various ZnO morphologies for selective detection of CO, CH4 and ethanol", Sensors and Actuators B 2011, 156: 737- 742. A.Srivastava, S.T. Lakshmikumar , A.K. Srivastava , R.K. Jain, "Gas sensing properties of nanocrystalline SnO2 prepared in solvent media using a microwave assisted technique", Sensors and Actuators B, 2007, 126: 583-587. C.Yang, X.Sua, F. Xiao, J.Jian, J. Wang, "Gas sensing properties of CuO nanorods synthesized by a microwave-assisted hydrothermal method", Sensors and Actuators B 2011, 158: 299303. L. Man, B. Niu, H. Xu, B. Cao, J. Wang, "Microwave hydrothermal synthesis of nanoporous cobalt oxides and their gas sensing properties", Materials Research Bulletin, 2011, 46: 10971101. Y. T.Yu, P. Dutta, "Examination of Au/SnO2 core-shell architecture nanoparticle for low temperature gas sensing applications", Sensors and Actuators B, 2011, 157: 444- 449. Z.Ai, K. Deng, Q. Wan, L.Zhang, S. Lee, "Facile microwave-assisted synthesis and magnetic and gas sensing properties of Fe3O4 nanoroses", J. Phys. Chem. C, 2010, 114: 6237-6242. L.Yin, D. Chen, M.Feng,L. Ge,D.Yang,Z. Song,B. Fan, R. Zhang,G. Shao, "Hierarchical Fe2O3@WO3 nanostructures with ultrahigh specific surface areas: microwaveassisted synthesis and enhanced H2S-sensing performance", RSC Adv., 2015, 5: 328-337. Q.Wang, C. Wang, H. Sun, P. Sun,Y. Wang, J. Lin,G. Lu, "Microwave assisted synthesis of hierarchical Pd/SnO2 nanostructures for CO gas sensor", Sensors and Actuators B, 2015. X. Chu, Y.Han, S. Zhou, H. Shui, "Trimethylamine sensing properties of nano-SnO2 prepared using microwave heating method", Ceramics International, 2010, 36: 2175-2180. 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. , 2013, 23: 1759-1766. Y. Liu, Q. Jiang, Y. Li, H. Zhao, "The microwave-assisted synthesis and gas sensing properties for ethanol of SnO2 nanomaterials", Advanced Materials Research 2013, 721: 237-240. S. Liang, L. Zhu, G. Gai, Y. Yao, J. Huang, X. Ji,X. Zhou, D. Zhang, P. Zhang, "Synthesis of morphology-controlled ZnO microstructures via a microwave-assisted hydrothermal method and their gas-sensing property", Ultrasonics Sonochemistry 2014, 21: 1335-1342. S.Bai, C.Chen, R.Luo, A. Chen, D. Lia, "Synthesis of MoO3/reduced graphene oxide hybrids and mechanism of enhancing H2S sensing performances", Sensors and Actuators B 2015, 216: 113-120. Y. Gui, J. Zhao, W. Wang, J. Tian, M.Zhao, "Synthesis of hemispherical WO3/graphene nanocomposite by a microwave-assisted hydrothermal method and the gas-sensing properties to triethylamine", Materials letters, 2015, 155: 4-7. X.Su, H. Zhao, F. Xiao,J. Jian, J. Wang, "Synthesis of flower-like 3D ZnO microstructures and their size-dependent ethanol sensing properties", Ceramics International, 2012, 38: 16431647. C. Yang, F. Xiao, J. Wang, X. Su, "Synthesis and microwave modification of CuO nanoparticles: crystallinity and morphological variations, catalysis, and gas sensing", Journal of colloid and interface science 2014, 435: 34-42.

[106]

[107]

[108]

[109]

[110] [111]

[112]

[113] [114]

[115]

[116]

[117] [118]

[119]

[120]

[121]

[122]

J.J. Hassan, M.A.Mahdi, C.W.Chin , H.Abu-Hassan, Z.Hassan, "Room-temperature hydrogen gas sensor with ZnO nanorod arrays grown on aquartz substrate", Physica E, 2012, 46: 254258. 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", Sensors and Actuators B 2015, 214 36-42. Y. Zhang, Y. Lin, J. Zhang, Z. Zhu,Q. Liu, Q.J. Liu, "Preparation and property of lanthanum ferrite formaldehyde gas sensor modified by carbon nano tubes and silver", Applied Mechanics and Materials, 2013, 320: 554-557. W. Jin, S. Yan, W. Chen, S. Yang, C. Zhao,Y. Dai, "Preparation and gas sensing property of Agsupported vanadium oxide nanotubes", Functional Materials Letters, 2014, 7: 14500311450035. H. Xu, X. Chen, L. Fang, B.Cao, "Preparation and characterization of cerium-doped tin oxide gas sensors", Advanced Materials Research 2011, 306-307: 1450-1455. M.Chen, Z.Wang, D. Han, F. Gu,G. Guo, "Porous ZnO polygonal nanoflakes: synthesis, use in high-sensitivity NO2 gas sensor, and proposed mechanism of gas sensing", J. Phys. Chem. C, 2011, 115: 12763-12773. L.Yin, D. Chen, X. Cui, L. Ge, J. Yang, L.Yu, B. Zhang, R. Zhang, G. Shao, "Normal-pressure microwave rapid synthesis of hierarchical SnO2@rGO nanostructures with superhigh surface areas as high-quality gas-sensing and electrochemical active materials", Nanoscale, 2014, 6: 13690-13700. M. Xu, Q. Li, Y. Ma, H. Fan, "Ni-doped ZnO nanorods gas sensor: Enhanced gas-sensing properties,AC and DC electrical behaviors", Sensors and Actuators B 2014, 199: 403-409. A. Tarat, R. Majithia, R, a, Brown, M. W. Penny,K. E. Meissner, T.G.G.Maffeis, "Nanocrystalline ZnO obtained from pyrolytic decomposition of layered basic zinc acetate: comparison between conventional and microwave oven growth", 12th IEEE International Conference on Nanotechnology (IEEE-NANO)-The International Conference Centre Birmingham, 2012, 1-5. X. Gao, H. Zhao, J.Wang, X. Su, F.Xiao, "Morphological evolution of flower-like ZnO microstructures and their gas sensing properties", Ceramics International, 2013, 39: 8629832. Z. Wang, X. Zhou, Z. Li,Y. Zhuo,Y. Gao, Q. Yang,X. Li, G. Lu, "Monodisperse WO3 hierarchical spheres synthesized via a microwave assisted hydrothermal method: time dependent morphologies and gas sensing characterization", RSC Adv., 2014, 4: 23281-23286. F. Ahmed, N. Arshi, M.S. Anwar, R. Danish, B. H. Koo, "Mn-doped ZnO nanorod gas sensor for oxygen detection", Current Applied Physics 2013, 1-5. T. Krishnakumar, N. Pinna, A. Bonavita, G. Micali, G. Rizzo, G. Neri, "Microwave-assisted synthesis of metal oxide nanostructures for sensing applications," in Sensors and Microsystems, Lecture Notes in Electrical Engineering. vol. 91, (eds.) G. Neri et al., Ed., ed: Springer Science+Business Media 2011. L.Yin Gang Shao, B. Fan, D. Chen, R. Zhang, "Microwave-assisted synthesis and gas-sensing performance of hollow-spherical WO3 nanocrystals", Key Engineering Materials 2014, 602603: 46-50. L.Yin, D. Chen, M. Hu, H. Shi,D. Yang,B. Fan,G.Shao,R. Zhang, G. Shao, "Microwave-assisted growth of In2O3 nanoparticles on WO3 nanoplates to improve H2S-sensing performance", J. Mater. Chem. A., 2014, 2: 18867-18874. 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., 2014, 4: 32538–32543. J. Liu, S.Wang, Q.Wang, B. Geng, "Microwave chemical route to self-assembled quasispherical Cu2O microarchitectures and their gas-sensing properties", Sensors and Actuators B 2009, 143: 253-260.

[123]

[124] [125]

[126] [127]

[128]

[129]

[130] [131]

[132]

[133]

[134] [135] [136] [137]

[138]

[139]

[140]

S. Das, S. Sinha, B. Das , S. K. Suar,S. K. S. Parashar, M. Mohapatra,A. Mishra, S. K. Tripathy, "Microwave assisted hydrothermal synthesis of well-dispersed and thermally stable Ag@SnO2 core-shell nanocomposites for propane sensing applications", J Mater Sci: Mater Electron 2014, 25: 217-223. M. Gholami, A. A. Khodadadi, A. A.Firooz,Y. Mortazavi, "In2O3-ZnO nanocomposites: High sensor response and selectivity to ethanol", Sens. Actuators, B, 2015. F. Gu, D. You, Z.Wang, D. Han, G. Guo, "Improvement of gas-sensing property by defect engineering inmicrowave-assisted synthesized 3D ZnO nanostructures", Sensors and Actuators B 2014, 204: 342-350. P.S. Cho, K.W. Kim, J.H. Lee, "Improvement of dynamic gas sensing behavior of SnO2 acicular particles by microwave calcination", Sensors and Actuators B 2007, 123: 1034-1039. M. Bagheri, N. Faal Hamedania, A. R. Mahjoub, A. A. Khodadadi,Y. Mortazavi, "Highly sensitive and selective ethanol sensor based on Sm2O3-loadedflower-like ZnO nanostructure", Sensors and Actuators B 2014, 191: 283- 290. F. Ahmed, N. Arshi, M.S. Anwar, R. Danish, B. H. Koo, "Facile growth of ZnO nanorod arrays by a microwave-assisted solution method for oxygen gas sensing", Thin Solid Films 2013, 547: 168-172. N. J. Ridha1, 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. H. Meng, W.Yang, K. Ding,L. Feng, Y. Guan, "Cu2O nanorods modified by reduced graphene oxide for NH3 sensing at room temperature", J. Mater. Chem. A, 2014, 1-8. D. S. Raja, T. Krishnakumar, R. Jayaprakasha, T. Prakasha, G. Leonardi, G. Neri, "CO sensing characteristics of hexagonal-shaped CdO nanostructures prepared by microwave irradiation", Sensors and Actuators B 2012, 171- 172: 853- 859. 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 performance at room temperature", ACS Appl. Mater. Interfaces, 2014, 1-27. C.Yang, F.Xiao, J. Wang, X.Su, "3D flower- and 2D sheet-like CuO nanostructures:Microwaveassisted synthesis and application in gas sensors", Sensors and Actuators B: Chemical, 2015, 207 177-185. H. Zhao, Q. Jiang, Y. Li, "Surfactant CTAB-assisted synthesis and gas-sensing characteristics of SnO2 nanomaterials", Advanced materials research, 2013, 750-752: 241-244. S. K. Gupta, A. Joshi, M. Kaur, "Development of gas sensors using ZnO nanostructures", J. Chem. Sci., 2010, 122: 57-62. Z. Bai, C. Xie, S.Zhang, W. Xu, J.Xu, "Microwave sintering of ZnO nanopowders and characterization for gas sensing", Materials Science and Engineering B, 2011, 176: 181-186. J.J. Hassan, M.A. Mahdi, S.J. Kasim, N. M. Ahmed,H. A. Hassan, Z. Hassan, "Fast UV detection and hydrogen sensing by ZnO nanorod arrays grown on a flexible Kapton tape", Materials Science-Poland, 2013, 31: 180-185. X. Gao, H. Zhao, J.Wang, X. Su, F.Xiao, "Morphological evolution of flower-like ZnO microstructures and their gas sensing properties", Ceramics International, 2013, 39: 86298632. M. Thepnurat, T. Chairuangsri, N. Hongsith, P. Ruankham, S. Choopun, "Realization of interlinked ZnO tetrapod networks for uv sensor and room-temperature gas sensor", ACS applied materials and interfaces, 2015, 7: 24177-27184. S. Trocino, T. Prakash, J. Jayaprakash, A. Donato, G. Neri, N. Donato, "Electrical characterization of nanostructured Sn-doped ZnO gas sensors," in Sensors, Lecture Notes in Electrical Engineering vol. 319, (eds.) D. Compagnone et al., Ed., ed Switzerland Springer International Publishing 2015, pp. 191-196.

[141]

[142] [143]

[144]

[145] [146]

[147]

[148] [149] [150]

[151]

[152]

[153]

[154]

[155]

[156]

[157] [158]

[159]

N. Faal Hamedania, A. R. Mahjouba, A. A. Khodadadib,Y.Mortazavi, "CeO2 doped ZnO flowerlike nanostructure sensor selective to ethanol in presence of CO and CH4", Sensors and Actuators B 2012, 169: 67- 73. S. Das, V. Jayaraman "SnO2: A comprehensive review on structures and gas sensors", Progress in materials science, 2014, 66: 112-255. N. Rajesha, J.C. Kannanb, T. Krishnakumar, S.G. Leonardid, G. Neri, "Sensing behavior to ethanol of tin oxide nanoparticles prepared by microwave synthesis with different irradiation time", Sensors and Actuators B, 2014, 194: 96- 104. L. Xiao, H. Shen, R. Von Hagen,J.Pan,L. Belkoura, S. Mathur, "Microwave assisted fast and facile synthesis of SnO2 quantum dots and their printing applications", Chem. Commun. , 2010, 46: 6509-6511. X. Chu, Y. Han, S. Zhou , H. Shui, "Trimethylamine sensing properties of nano-SnO2 prepared using microwave heating", Ceram. Int. , 2010, 36: 2175-2180. Y. Gui , F.Dong , Y. Zhang , Y. Zhang , J.Tian, "Preparation and gas sensitivity of WO3 hollow microspheres and SnO2 doped heterojunction sensors", Materials sciencein semiconductor processing, 2013, 16: 1531-1537. N. Rajesh , J.C. Kannan , S.G. Leonardi , G. Neri , T. Krishnakumar, "Investigation of CdO nanostructures synthesized by microwave assisted irradiation technique for NO2 gas detection", Journal of Alloys and Compounds 2014, 607: 54-60. Y. Lu, M. Ye, F. Liu, G. Chen, L. Xu, X. Kong, "Overview on the synthesis and applications of cadmium hydroxide nanomaterials", J. Iranian Chem. Soc., 2015, 12: 1829-1840. Michel, C. R., "CO and CO2 gas sensing properties of mesoporous CoAl2O4", Sensors and Actuators B 2010, 147: 635-641. C. R. Michel, H. Guillén-Bonilla, A. H. Martínez-Preciado, J. P. Morán-Lázaro, "Synthesis and gas sensing properties of nanostructured CoSb2O6 microspheres", Sensors and Actuators B 2009, 143: 278-285. A.Cirera, A. Cabot, A.Cornet, J.R. Morante, "CO-CH4 selectivity enhancement by in-situ Pdcatalyzed microwave SnO2 nanoparticles for gas detectors using active filter", Sensors and Actuators B: Chemical, 2001, 78: 151-160. F.H. Saboor, T. Ueda, K. Kamada, T. Hyodo, Y. Mortazavi, A. Khodadadi, Y. Shimizu, "Enhanced NO2 gas sensing performance of bare and Pd-loaded SnO2 thick film sensors under UV-light irradiation at room temperature", Sens. Actuators B, 2016, 223: 429-439. A. Tarat, Ch. J. Nettle, D.T. Bryant, D. R. Jones, M. W. Penny, R. A. Brown,R. Majitha, K. E. Meissner,T. G. Maffeis, "Microwave-assisted synthesis of layered basic zinc acetate nanosheets and their thermal decomposition into nanocrystalline ZnO", Nanoscale Research Letters ,:, 2014, 9: 1-8. W. Jin, S.Yan, W. Chen, S. Yang, C. Zhao, Yingdai, "Enhanced ethanol sensing characteristics by decorating dispersed Pd nanoparticles on vanadium oxide nanotubes", MaterialsLetters, 2014. B.Geng, F. Zhan, C. Fang,N. Yu, "A facile coordination compound precursor route to controlled synthesis of Co3O4 nanostructures and their room-temperature gas sensing properties", J. Mater. Chem., 2008, 18: 4977-4984. K. Shingange, G. H. Mhlongo, D. E. Motaung , O. M. Ntwaeaborwa, "Tailoring the sensing properties of microwave-assisted grown ZnO nanorods: Effect of irradiation time on luminescence and magnetic behaviour", J. Alloys Comp., 2016, 657: 917-926. Lee, J.H., "Gas sensors using hierarchical and hollow oxide nanostructures: overview", Sensors and Actuators B, 2009, 140: 319-336. T. T. Tung, D. Losic, S. J. Park, J.-F. Feller, T. Kim, "Core-shell nanostructured hybrid composites for volatile organic compound detection", Int. J. of Nanomedicine, 2015, 10: 203214. A. Mirzaei, K. Janghorban, B. Hashemi, G. Neri, "Metal-core@metal oxide-shell nanomaterials for gas-sensing applications: a review", Journal of nanoparticle research, 2015, 17: 1-36.

[160]

[161]

[162] [163] [164]

[165]

[166]

[167]

P. Rai, S. M. Majhi, Y.T. Yu, J.H. Lee, "Noble metal@metal oxide semiconductor core@shell nano-architectures as a new platform for gas sensor applications", RSC Advanced, 2015, 5: 76229-76248. T. Yanagimotoa, Y.T. Yu, K. Kaneko, "Microstructure and CO gas sensing property of Au/SnO2 core–shell structure nanoparticles synthesized by precipitation method and microwaveassisted hydrothermal synthesis method", Sensors and Actuators B, 2012, 166- 167: 31- 35. D. Barreca, A. Gasparotto, E.Tondello, "Metal/oxide interfaces in inorganic nanosystems: what's going on and what's next? ", J. Mater. Chem. , 2011, 21: 1648-1654. X. Hu, J. C. Yu, J. Gong, Q. Li, G. Li, "α-Fe2O3 nanorings prepared by a microwave-assisted hydrothermal process and their sensing properties", Adv. Mater. , 2007, 19: 2324-2329. P. Potiraka, W.Pecharapabc, W. Techitdheera, "Microwave-assisted synthesis of ZnO/MWCNT hybrid nanocomposites and their alcohol-sensing properties", Journal of experimental nanoscience, 2014, 9: 96–105. S. Baek, S.H. Yu, S.K. Park, A. Pucci, C. Marichy, D.C. Lee, Y.E. Sung, Y. Piao, N. Pinna, "A onepot microwave-assisted non-aqueous sol-gel approach to metal oxide/graphene nanocomposites for Li-ion batteries ", RSC Advanced, 2011, 1: 1687-1690. G. Neri, S. G. Leonardi, M. Latino, N. Donato, S. Baek, D. E. Conte, P. A. Russo, N. Pinna "Sensing behavior of SnO2/reduced graphene oxide nanocomposites toward NO2", Sens. and Actuators B, 2013, 179: 61-68. M. Akram, A. Z. Alshemary, F. K. Butt, Y.-Fan Goh, W. A. Wan Ibrahim, R. Hussain, "Continuous microwave flow synthesis and characterization of nanosized tin oxide", Materials Letters, 2015, 160: 146-149.

Ali Mirzaei was born in Dorood, Iran, in 1984. He received a BS. in Materials Science and Engineering from Isfahan University of Technology in 2006 and an M.Sc. degree in Materials Science and Engineering at Shiraz University in 2009. In 2016, he obtained his Ph.D. degree in Materials Science and Engineering at Shiraz University. He is interested in the synthesis, characterization and applications of composite nanomaterials, photocatalysts and semiconductor gas sensors.

Giovanni Neri was born in Reggio Calabria, Italy, in 1956. He received the M.S. degree in Chemistry from the University of Messina, in 1980. Since 2002 he is full professor of Chemistry at the University of Messina. From 2004 to 2007, he was Head of the Department of Industrial Chemistry and Materials Engineering, Univ. of Messina. He was visiting professor at the University of Michigan (USA) and University of Alagappa (India). His research interests include the synthesis and characterization of nanostructured materials for chemical sensors, applied in a wide range of sectors from medical diagnostics to automotive, industrial processes and environmental control.