Synthesis, Characterization and Gas Sensor Application of New Composite Based on MWCNTs:CoPc:Metal Oxide

The synthesis of new substituted cobalt Phthalocyanine (CoPc) was carried out using starting materials Naphthalene-1,4,5, tetracarbonic acid dianhydride (NDI) employing dry process method. Metal oxides (MO) alloy of ( 60 % Ni3O4 40 % -Co3O4 ) have been functionalized with multiwall carbon nanotubes (FMWCNTs) to produce (F-MWCNTs/MO) nanocomposite (E2) and mixed with CoPc to yield (FMWCNT/CoPc/MO) (E3). These composites were investigated using different analytical and spectrophotometric methods such as H-NMR (0-18 ppm), FTIR spectroscopy in the range of (400-4000cm1), powder X-rays diffraction (PXRD, 2θ o = 10-80), Raman spectroscopy (0-4000 cm), and UV-Visible spectrophotometry (0-800 nm). Then the activity of these materials was investigated as a gas sensing of (Ammonia, Methanol and Acetone). For each case, 0.2 mg/.mL of the prepared Copc, Copc/MWCNT, Copc/MWCNTs–MO was dispersed in 1m of ammonia, methanol and acetone at 298K. The surface morphology of the prepared materials was heterogeneous.


Introduction:
Multiwall carbon nanotubes (MWCNTs) were discovered in 1991 as a secondary product of fullerene preparation (1). Graphene is the simplest carbon nanotube formed of a single sheet of honeycomb arranged of carbon particles (2) Graphene is rolled up consistently into a tubular shape (3). The tube walls are made up of a hexagonal grid of carbon molecules resembling closely to the atomic planes of graphite (4). Carbon nanotubes (CNTs) are characterized as a cylinder in three dimensions content of carbon atoms, and have hybridization sp 2 which is stronger than diamond with hybridization sp 3 (5). CNTs are an example of nanotechnology with dimensions less than 100 nm (5,6).
Generally, Carbon Nano Tubes are insoluble in all solvents due to solid Van-Der-Waals intelligent that firmly holds them, but chemical functionalization of CNTs may improve dissolvability in different solvents and to create novel hybrid materials possibly appropriate for diverse applications such as Catalysts, Sensors, Transistors, Membranes, Electrodes, Solar cells and Fuel cells (7). Combination of metal phthalocyanine with CNTs will have potential electrocatalytic properties (8). Metal Free Phthalocyanines (H 2 Pc) is an organic compound with the formula (C 8 H 4 N 2 )4H 2 . It was discovered in 1907 by Braun and Tcherniac (9). Phthalocyanine (PC,1) (tetrabenzo tetraazo porphyrin) compounds are thermally stable and has good catalytic performance (10).
Pc is an aromatic macrocycle compound; the structure of Pc is very similar to porphyrins with four meso-carbons substitution in position α-γ and β-δ axis with four addition benzene rings and content eight nitrogen atoms (11). PCs are heterocyclic aromatic compounds, used as colorant material (12)(13)(14)(15). They are important materials with a wide range of applications on the basis of their optical and electrical properties, as well as thermal stability (16). Metal phthalocyanines are prepared via the reaction of urea, metal salts, ammonium molybdate, with either NDI or phthalonitriles (phthalimide, phthalic anhydride, Phthalic acid) by different methods (8)(9)(10)(11)(12)(13)(14)(15). Mono or binuclear metal phthalocyanines are widely used in solar cell, sensor, and light-emitting device (17). More than 70 different metal ions into the central cavity can be introduced to improve physical properties of the phthalocyanine which can be with different electrical, optical, and their thermal stability (12,18). Phthalocyanines are the second most important class of colorant compounds (14,15).
Recently, the use of Pcs has been varied which can make this material an excellent choice for several applications such as gas sensors, organic solar cells (19) and oxidative degradation of pollutants or catalysts for photo and as photosensitizers (20).
The present study describes the preparation of cobalt phthalocyanine (CoPc) and its combination as nanocomposites materials to yield (Copc/F-MWCNTs/ Ni 3 O 4 -Co 3 O 4 ). Then, the synthesized composite would be investigated using different spectroscopic and analytical techniques to confirm its formation. Then the activity of this prepared composite would be investigated as a sensor for probing each of ammonia (NH 3 ), methanol (CH 3 OH) and, and acetone (CH 3 ) 2 CO. To the best of the author's knowledge, this is the first attempt to produce such composite and use it in gas sensor applications.

Synthesis of Copc-MWCNTs (E2)
In this part, 75mg of activated MWCNTs was dissolved in 3.5 mL of DMF under stirring, heating at 76°C and N 2 flush. After that, 1ml of SoCl 2 was added to the mixture followed by adding 0.06 g. of CoPc after 1hr. Finally, 1mL of triethylamine was added, the final mixture was kept under reflux for 4hr at 110°C as shown in Fig.2. Then, the mixture was filtered and dried for 48hr at 100°C (24).

Synthesis of Compound Copc-MWCNTs/MO nanocomposite (E3)
Nanocomposite compound was synthesized by combining E1 compound to co-metal oxide

Fourier Transform Infrared spectroscopy (FTIR)
FTIR was used to investigate functional groups on the surface of the prepared composite in the range from 400-4000 cm -1 using FTIR 8400S Shimadzu Japan.

Raman spectroscopy
Raman spectroscopy was used to characterize the properties or the diameter of the tubes. Measurements were carried out at room temperature using SENTERRA, BRUKER-Germany, with high spatial & spectral resolution (Spectral Resolution: < 3cm -1 ), using laser wavenumber of 785 nm.

Gas Sensor model
The performed studies showed using a stainless steel test chamber supported with sealed output wires for connection and a thermocouple to monitor the chamber temperature. 0.2 mg/mL of the prepared Copc, Copc/MWCNT, and Copc/MWCNTs-MO was dispersed in 1mL of acetone and sonicated for 15 min. in order to obtain a homogenous solution. Using casting method, thin films of the above solution were dried at ambient temperature. A desired concentration of ammonia, methanol and acetone were used in the chamber to determine sensing properties. Sensor resistance was recouped by opening the top of the test chamber (25,26). The response of the gas sensor was calculated using the equation (1): Ra and Rg represent the sensor resistance in air and gas environment respectively.

Results and Discussions:
FTIR Spectra for the prepared materials Figure 4 shows FTIR spectra of the materials under study. The starting compound NDI-NH 2 has demonstrated the peak (v,cm -1 ): the peaks around 3545-3439 are assigned to (NH aliph ). The peaks around 3080-3207 are assigned to (C-HAr.), the peaks around 2926-2854 can be assigned to (C-H aliph ), the peak at 1707 is attributed to (C=O carboxylic acid ). The peak at 1666 is related to (C=O amide ), and that at 1577.7 is related to (C=C Ar). The peaks at 1320-1210 is assigned to (C-O), and the peak at 1055 is assigned to (C-N).

Raman Spectra
To investigate the interaction between CoPc molecules and F-MWCNTs/MO, Raman spectra have been used to evaluate the effect of adding MO to the Copc/F-MWCNTs composite as shown in Fig.6. The vibrations of isoindole moieties (27) caused the peaks at (324, 459, 757, 850, 902 cm -1 ) in CoPc sample to appear. Pyrrole groups peaks appear between 1200 -1600 cm -1 , while cobalt ion at 1587 cm -1 ,and 1571 corresponds with the previous studies (28,29). Raman spectra show change in their positions and intensities. This is due to the change in the ratio of sp 3 hybridized carbon atom relative to sp 2 is the intensity ratio of D band to the G band (ID/IG) valued 1.07. The changes of ID/IG value indicates that CoPc was linked through a non-covalent on the surface of F-MWCNTs (29). Copc/F-MWCNTs/MO composite has exhibited G-band (C-C vibration with a sp 2 orbital) at 1587, 1571 cm -1 and D band related to sp 3 C with flaws at 1366 cm -1 G᷄ 2984, 2976 cm -1 (29). Figure 7, shows Uv-Visible spectrum of compound NDI-NH 2 , bands in UV region at 445, 379, and 360 nm were appeared ( B band=2.78 ev), while E1compound shows two important bands (Q band reaches to near IR region) at 669 nm(Q band=1.85 ev), and (B band=3.28 ev in Uv region of the spectrum) at 357-377 nm (30,31).

X-rays Diffraction Patterns
XRD patterns of CoPc/ F-MWCNTs are shown in Fig. 8 (33,34).From these patterns, it is clear that both materials show crystalline structure. The broad peak that can be seen in the XRD pattern indicates the presence of graphitic structure of CNTs, which is related to the ordered arrangement of the concentric cylinders of graphitic carbon. So, this confirms the presence of the hexagonal structure of MWCNTs, which means that doping MO with Copc /F-MWCNTs does not affect significantly its crystal structure. XRD data were employed to calculate the average crystalline size (D) of the prepared material via applying Scherrers equation as shown below: D= kʎ/ВcosӨ, whereas, D is the average crystal size, k is the shape factor depends on the shape of the crystal and it is equal to 0.94 for homogeneous shape and 0.89 for the heterogeneous shape.

Figure 8. P XRD patterns for Copc /F-MWCNTs and the inserted represents the Copc/F-MWCNTs/MO Nano composites Gas Sensing:
CNTs are vital materials that can be utilized in sensors due to their special and curious properties, such as enormous particular surface area, gas adsorption capability and electrical conductivity. This behaviour can be shown by the sensing process that occurs on the different sites of the sensing material and thus, the performance is related to the morphology. At lower gas concentrations, the gas extends slightly on the sensor's surface zone and thus leads to a decreasing in reaction. The higher concentration of gas covers generally bigger surface area and interacts with bigger number of active destinations guiding to higher sensor reaction (34).
Reduced gasses act as electron donor when linked with metal oxide surface. Due to this interaction, these gasses desorb or evacuate the chemisorbed oxygen ions and physisorbed -OH ions from the MO surface. Reduced gases such as NH 3 , Acetone and methanol. NH 3 , Acetone and methanol are the foremost critical organic molecule. NH 3 lone pair electrons supply strong electron acceptor conduct. It can be an electron donor to the metal oxide, when reacting with the adsorbed oxygen ions on the surface by returning the trapped electrons. Proposing free electrons mechanism was achieved via the number of oxygen ions which they react with NH 3 molecules as shown in Fig. 9 (33,34).  The lone pair of electrons of NH 3 provide strong electron acceptor behavior. However, it acts as an electron donor to the surface, when reacting with the adsorbed oxygen ions on the surface by reverting the trapped electrons (35). The proposed mechanism of ammonia that generates free electrons accomplished by the number of oxygen ions reacted with NH 3 molecules as shown in the equations (2-5). The time dependency of the prepared thin films based devices exhibited a reasonable stability in regard to time, upon the exposure of gas (Fig. 9). The response and recovery time, which are defined as the time it takes to reach close to the steady state and the time required to reach close to the base line respectively, were found to be different based on the gas type (with the same concentration ≈100ppm for all the different gasses) and the material under room temperature..
Generally, the gas molecules interacting with the central metal ions inside the phthalocyanine ring at the air/phthalocyanine interface leads to the formation of oxidized MPc + and O 2 species and injection of hole charge carriers into the thin film . This process is quite possible at the normal conditions because the change in the free Gibbs energy is negative and as a result of oxygen desorption and the released electrons change the resistance of the phthalocyanine based film (35).

Conclusion:
The nanocomposites preparation is accomplished by functionalization of CNTs with Cobalt-Phathalocyanine (CoPc) and co-oxide. The nano-sized tertiary system (Copc/F-MWCNTs/MO) nano composites are obtained. The avarege crystal size of the prepared materials ranges between (78-102) nm. This is evaluated using Scherres equation. These composites are investigated with XRD patterns, Raman spectroscopy, FTIR, and Uv-Vis spectroscopy. From Uv-Visible spectra, it is found that, these new compounds show strong absorptions peaks towards the NIR region between 646 and 720 nm. Therefore, this device shows differential resistance changes, response and recovery times through exposition to NB, CB and some other chemical vapours. The fabricated device contains transducing part from the CoPc functionalized MWNT-based nanocomposites. The tertiary system is employed to fabricate sensitive sensors for detection of vapour of NH 3 , methanol, and acetone.