Evaluate the Humidity Sensing Properties of a CNTs/Co3O4 Nanorods Composite

For the development of humidity sensors, MWCNTs/Co 3 O 4 nanocomposite has been prepared by precipitation method. in order to evaluate the prepared materials quality both of functionalized MWCNTs and decorated CNTs characterized by X- ray powder diffraction (XRD) ,High resolution transmission electron microscope (HRTEM) Thermal analysis (DTA and TGA) FTIR and nally Raman spectroscopy. Meanwhile the humidity sensing behavior has been also investigated. The proposed composite has been tested in the wide range in relative humidity and tesing frequency. The obtained results conrmed that the optimum testing frequency is 100 Hz. The MWCNTs/Co 3 O 4 nanocomposite exhibited a good sensitivity toward humidity from11% up to 97% RH with reasonable response and recovery time. Also the sensor revealed a low hysteresis and good repeatability with increasing and decreasing of humidity levels.


Introduction
Relative humidity is one of the most important physical parameters that must be measured accurately because of its important effects on many vital functions of living organisms as well as industry. The measurement of relative humidity depends mainly on the use of certain functional materials that suffer from change in their physical properties when the surrounding humidity changes [1]- [7]. Metal oxide Semiconductors (MOS) are one of the most important materials used in measuring humidity, due to the low price and ease of synthesizing them in different morphologies [8] [9]. A carbon nanotube is one of the most interesting materials that have been discovered yet. Since their discovery, it subjected to detailed studies to explore its fundamental properties and nding the most possible applications. CNTs have a good reputation in scienti c society due to its outstanding physical and chemical properties. These outstanding properties are attributed to their unique geometric structure beside electronic and phonon structure [10] [11] [12][13] [14]. CNTs can be better visualized as a rolled up graphene sheet in the form of a hollow nanometer tube. Two main types of CNTs can be recognized based on the numbers of concentric tube. Single wall carbon nanotubes (SWCNTs) consist of a single sheet of graphene rolled up into a single nano scale tube; while multi wall carbon nanotubes (MWCNTs) is consist of multiple concentric tubes [15] [16]. CNTs can be synthesized by various methods while the most well established methods comprise arc discharge, laser ablation and chemical vapor deposition (CVD). Among all of these methods, CVD is most suitable and forward technique. The CVD method involves growth of CNTs from volatile carbonaceous precursors (e.g., acetylene, ethylene, ethanol or methane) at 350 to 1150°C using either a nanoparticle as a catalyst. CVD has the advantage of being very exible for modi cations and easy to scale [17][18] [19] [20]. In spite of unique properties of CNTs, their surface needs to be modi ed to enhance its chemical functionality. The chemical and or physical modi cation generally called functionalization. This functionalization often required to enrich the performance of CNTs in different applications [21]. The functionalization of CNTs can be executed via attaching an inorganic, organic and biological functional molecule to the CNTs. Four main approaches are available for functionalizations of CNTs include endohedral lling, covalent functionalization, noncovalent functionalization and surface decoration with inorganic particles. CNTs can considered one of the best materials to use as a moisture sensor due to their high aspect ratio. However the hydrophobic natures of CNTs hinder its ability to sense humidity. To overcome these shortage inorganic nanoparticle/CNTs have been utilized as humidity sensor [22] due to the synergistic effect coming from the integration of individual constituents. Cobalt oxide has been used in a variety of applications, including Li ion battery, catalysis and sensing [23]. Din [24]. Amanulla et al. prepared the β-CoMoO 4 , Co 3 O 4 through co-precipitation and solid-state method. They found that The humidity sensing measurement of composite than has been prepared by simple co-precipitation method possess highest sensitivity factor Sf = 4851 with response time of 60 s and recovery time of 230 s respectively [25]. Dai et al. produced a chemiresistive humidity sensor based on chitosan (CS)/zinc oxide/SWCNT. They found that the response of SWCNT to humidity was signi cantly enhanced with the help of conjugate material in the composite, particularly the CS. Also they demonstrated that the composite displayed good reproducibility [22].
In this study the MWCNTs were synthesized by CVD, and then functionalized with COOH group to reduce its hydrophobicity. Fe/Co catalyst at 750 o C using acetylene and nitrogen as carbon source and carrier gas respectively. To purify MWCNTs, a proper amount of as synthesized MWCNTs was re uxed in a 3:1 mixture of concentrated H2 S O 4 and HNO 3 solution. More details regarding synthesizing, puri cation and functionalization of MWCNTs can be found in our previous reported work [26], [27].

Decorating MWCNTs with Co 3 O 4
Cobalt oxide (Co 3 O 4 ) nanoparticles were anchored onto the outer surface of MWCNTs using hydrothermal rout. rstly 40mg of MWCNTs were dispersed in 30ml of distilled water by ultrasonic homogenizer for 30min. 300 mg of cobalt nitrate hexa hydrate was added to MWCNTs suspension under continuous stirring, and then 300mg of urea was added, the mixtures were allowed to stir for additional 2 hr. The solution was transferred into a 50ml Te on-lined sealed stainless steel autoclave and maintained at 150 ο C under autogenous pressure for 4hr. the MWCNTs decorating process is illustrated in g. 1.

Characterization techniques
-The shape and size of puri ed MWCNTs and Co 3 O 4 /MWCNTs nanocomposite were examined by high resolution transmission electron microscope (HRTEM) JEM-2100. The crystalline structure was determined by XRD diffractometer with a secondary monochromatic wavelength of Cu (λ=1.542A ο ) at 45 K.V., 35mA, and scanning speed of 0.02/sec. Thermal gravimetric analysis (TGA) measurements were obtained by STDQ-600 thermal analyzer from room temperature up to 1000 ο C under air atmosphere. The FTIR spectroscopy has been utilized to diagnose the functional groups on the surface of puri ed MWCNTs and the functional groups of nanoparticles on the puri ed MWCNTs. the spectra were collected by (FTIR Vertex 70 Bruker optic model device). Raman spectroscopy has been utilized to observe the structure and SP 2 hybridization. The spectra were collected by (Bruker SENTERRA) with a (ND-YAG) laser source and with wavelength of 532nm.

Sensor fabrication:
A humidity sensor was developed over uorinated tin oxide (FTO) coated glass substrate. 5mg of Co 3 O 4 /MWCNTs and 5 ml of distilled water were mixed with agate pestle to form a paste. A small amount of a paste was applied on the surface of FTO and then, the device was dried at 60 °C until complete evaporation of water. Subsequently the sensor was aged at 1 V and 1 kHz for 36 h to enhance its stability as shown in g. Where λ is the wave length, θ is the diffraction angle, and β is the peak width at half maximum. The size of cubic cobalt oxide is determined to be about 50nm which con rm the result obtained from HRTEM.
In order to estimate the thermal stability and existance the Co 3 [29].
It was belvied that the thermal degradtaion of MWCNTs is directly related to defects, the less the defects, the higher the decomposition temperature [30]. Thermal analysis is one of the important tools that has been used in the characterization of CNTs, as well as estimating the amount of metal oxides loaded on their surface, but in spite of this it cannot distinguish between species of carbon nanotubes and knows any defects act on structure and therefore, it is necessary to characterize the prepared materials using another tool.
To evaluate the functional groups on the surface of MWCNTs and the composite Co3O4/MWCNTs; FTIR spectra was used with spectra region from (4000-400cm -1 ) as shown in gure 6. For puri ed MWCNTs the broud band at approximatly 3435cm -1 is assigend to stretching of -OH in hydroxyle and carboxyle group(C-OH,O=C-OH) [31]. The spectra bands at 2924-2858cm -1 are attributed to symmetric and assymmetric stretching vibration of C-H in CH 2 and -CH 3 group [32]. The band at 1714cm -1 arrtibuted to C=O present in carboxylic group [33]. the band at 1628cm -1 associated to carbonyle group present in ring structure and related to stretch vibration C=C of SP 2 hypridization in carbon nanotube backbone [34].the band at 1115-1060cm -1 refer to C-O stretching vibration in carboxylic group [32]. this results con rm on successful oxidation of MWCNTs and also for Co 3 O 4 /MWCNTs there are two additional sharp bands at 669and 560cm -1 corresponding to presence of Co +2 and Co +3 in spinal Co-O stretching vibrations where Co +2 related to tetrahedral coordinate and Co +3 to octahedral coordinate.FTIR spectra con rm presrence of Co 3 O 4 structure [29].
Raman spectrometer is one of the most important characterization tools used to distinguish between different nanocarbon structures, as well as to determine the presence of deformations and also to determine the type of metal oxides on their surface. The Raman shifts of both puri ed MWCNTs and Co 3 O 4 /MWCNTs are shown in g. 7. For puri ed MWCNTs three Raman shifts at 1566cm -1 , 1338cm -1 , 2677cm -1 corresponds to G (the graphite band), D (disorder band), and G/ (second order harmonic) bands were recognized. The intensity ratio I D /I G is an important indicator for the precence of functional group [30] [29], [35]. The I D /I G was estimated to be 0.6 indicating the sucsessfully functionalization of MWCNTs. The charactrisitc Raman bands of Co 3 O 4 /MWCNTs observed at, 527 cm -1 were attributed to the F 2g Raman active mode, while those located at 483 and 670 were pelonging to E g and A 1g modes, respectively. The highest In order to evaluate the response of the prepared sensor toward humidity, the impedance variation of the nanocomposite was recorded at different humidity levels (11% RH -97 % RH) and at different testing frequency (50 Hz -100kHz). The impedance variation versus the humidity at different testing frequency is illustrated in g. 7. It was noticed that at low frequency, the impedance decreases linearly with increasing the humidity level. With further increasing in the testing frequency, the impedance variation increases, while at higher frequency (100 kHz) the impedance variation becomes insigni cant with no obvious trend. This could be explained based on the water molecules are not able to be polarized at high frequencies [5], [30].
Hence for further evaluation the testing frequency was set at 50Hz. The hysteresis effects due to the adsorption and desorption of water molecule is illustrated in g. 8. It was observed that during the adsorption process, the impedance increases with the increase of humidity; while during desorption of the water molecule the response decreases with decrease of humidity. Since the water molecule is adsorbed physically over the surface of the sensor via hydrogen bond; so that it was expected that the hydrogen bond plays a vital role in hysteresis. In our case, there are two types of formed hydrogen bonds: bonds formed between water molecule and sensing material, and bonds between water molecules and other water molecules. During the adsorption process (humidity increment) the water molecules are physically adsorbed on the surface of the sensor via hydrogen bond. In case of desorption process (humidity decrement) the water molecules departed from the surface of the sensor which require more energy to overcome the hydrogen bonds. As was con rmed earlier, the desorption of water molecule is di cult than adsorption of water molecule, which in turn will results in hysteresis.
The impedance variation versus humidity can be tted in a linear correlation as depicted in g. 9. Figure  10 represents the adsorption and desorption response curves for the sensor at RH% of 75, and 97, respectively. The curves indicated excellent ability of the sensors to respond against increasing and reducing humidity levels with realistic response and recovery times.
The humidity sensing mechanism of the Co 3 O 4 /MWCNTs composite is explained based on the complex impedance spectroscopy (CIS) measurements. The relationship between real part of impedance (Z\) and imaginary part (Z\\) at relative humidity of 11%, 43% and 97% is illustrated in g 11. It was observed that all curves are semicircles; also the curvature of the semicircle decreases with increasing humidity level.
This means that the bulk resistance of the sensing materials decreases as humidity increases [36]. In the case of low humidity value, the concentration of the adsorbed water on the surface of the sensing material is weak. The water molecule will be absorbed on the active sites in order to form a hydroxyl group. Therefore, the protons formed from the hydroxyl group can be transferred to form H3O + . And although these charged ions are insu cient to cause electrical conduction, they may bend energy levels, and therefore the conduction is done through electrons. Due to the presence of MWCNTs, the transmission of electronics is easy. When the humidity increases, the conduction process takes place through the aforementioned mechanism, and this has been proven through CIS measurements.

Conclusion
In this study the possibility of using a CNTs/Co 3 O 4 nanocomposite as a humidity sensor was examined.
The HRTEM and XRD measurements con rmed that the Co 3 O 4 nano particles formed in a crystalized form and attached to the surface of the MWCNTs. The size of cubic cobalt oxide is about 50nm as con rm HRTEM and XRD measurements. The Raman analysis con rmed the presence of D, G and G \ band as well as the characteristic bands of Co 3 O 4 particles. The humidity sensing behavior con rmed that the optimum testing frequency is 100Hz. The obtained results emphasizes that the sensor has a reasonable sensitivity, low hysteresis, and good repeatability.    The hysteresis effects due to the adsorption and desorption of water molecule Page 18/19

Figure 10
The linear tting of Co3O4/MWCNTs sensor Figure 11