**2. Experimental**

efficiency and service life [1–4]. This is because inorganic nanoparticles embedded in conducting polymers can improve the mechanical, electrical, and optical properties such as nonlinear optical behavior, photoluminescence, electroluminescence, and photoconductivity [5–7]. Nanostructured composites or nanohybrid layers containing numerous heterojunctions can be utilized for optoelectronics, organic light emitting diodes (OLEDs), organic solar flexible cells (OSC) [8, 9], etc. Among conducting polymers, polyethylenedioxythiophene:poly(4-styrenesulfonate) (abbreviated to PEDOT:PSS) as a p-type organic semiconductor is well used for the hole transport layer in OLED [10] and OSC [4] as well as for the matrix materials in various sensors [11]. Various nanocomposite films consisting of conducting polymers mixed with carbon nanotubes (CNT) as an active material have been prepared for application in gas thin-film sensors. Recently, Olenych et al. [12] used hybrid composites based on PEDOT:PSSporous silicon-CNT for preparation and characterization of humidity sensors. The value of the resistance of the hybrid films was as large as 10 MΩ that may have caused a reduced

It is known that graphene possesses many excellent electrical properties, since it is an allotrope

atoms. Graphene quantum dots (GQDs), as seen in [13, 14], are a kind of 0D material made from small pieces of graphene. GQDs exhibit new phenomena due to quantum confinement and edge effects, which are similar to semiconducting QDs [15]. Graphene and related materials like graphene oxide (GO) or reduced graphene oxide (rGO) as materials used for chemical sensing have significant application potential. This is due to the two-dimensional structure that results in a high sensing area per unit volume and a low noise compared to other solidstate sensors. There were many works reporting on the use of graphene or graphene-related materials for monitoring gases and vapors [16, 17]. Especially, some of the works attempted to connect the advantages of nanoscale metals with that of graphene for the improvement of gas sensor applications [18, 19]. GQDs were mainly used in a single-electron transistor (SET). Besides detecting charge in SETs, GQDs have also been recruited to build electronic sensors

hybridized carbon

; it contributes

, etc., have large sensitiv-

accuracy in monitoring the resistance change versus humidity.

82 Nanocomposites - Recent Evolutions

for the detection of humidity and pressure [20].

based on nanostructured inorganic structures like SnO2

of carbon with a structure of a single two-dimensional (2D) layer of sp2

Ammonia is a compound of nitrogen and hydrogen with the formula NH3

significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizer. With the development of the chemical industry, more and more generation of ammonia gas is brought into the environment. It is known that ammonia gas is a toxic compound; consequently, it is harmful to human health when a large enough concentration of this compound is attained [21]. Thus, production of devices (or sensors) to detect ammonia gas with a large sensitivity and selective property is very important. Many scientific groups have researched and developed gas ammonia sensors for applications. Sensors

ity and response time, but the technology for producing both the materials and devices for gas sensors usually requests vacuum and high temperature that results in considerably large expenses [22]. With the aim to reduce these costs, many scientific groups have developed gas sensors based on conducting polymers [23–25]. The advantage of the polymer-based sensors consists of easy fabrication, low power consumption, room temperature operation, etc. [26]. Among the conducting polymers, polyethylenedioxythiophene + poly(4-styrenesulfonate)

, WO<sup>3</sup>

, TiO<sup>2</sup>

#### **2.1. Samples preparation**

## *2.1.1. Preparation of PEDOT-PSS + GQD + CNT (GPC) and PEDOT-PSS + GQD + AgNW (GPA) humidity sensors*

Firstly, GQDs were prepared; for this, a solution of graphite flake (GF), KMnO<sup>4</sup> , and HNO3 with a weight ratio of 0.2 g:0.2 g:0.4 ml was prepared and put in a Pt crucible. This solution was then put in a microwave heaven for heating in 1 min to separate GF into laminar form (EG). The second solution was made from 0.2 g NaNO3 + 9.6 ml H2 SO4 (98%) + 1.2 g KMnO<sup>4</sup> (called as NKH). EG was mixed with NKH solution and carefully stirred by use of a magnetic device for 2 h to have a GO solution. Adding to the GO solution 30 ml distilled water, and then 10 ml H2 O2 allowed us to get a dark-yellow solution. By spinning with a rate of 7000 rpm for 5 min, a GO powder was obtained and it was diluted in deionized water. In the next step, NH3 was added in the solution and stirred at 100°C for 5 h until a solution with a uniform dispersion of GQDs was reached. Finally, the GQDs dispersed solution was filtrated by using the "Dialysis" funnel to collect GQD powder with a volume of 0.2 g. This powder then was dissolved in 20 ml of twicedistilled water to get GQD-dispersion solution of 10 wt.% GQDs (abbreviated to GQD10).

Next, GQD + PEDOT:PSS + CNT composite solutions were prepared. Firstly, a powder of multiple wall carbonate tubes (shortly abbreviated to CNT) with an average size of 30 nm in diameter and 2 μm in length was embedded in 10 ml of the GQD10 solution without CNT and with three contents of CNT, respectively, 0.5, 1.0, and 1.5 mg. All of the solutions obtained are called GQC solutions. These solutions were treated by plasma in a microwave oven. Then, 2 ml of polyethylene-dioxythiophene + PEDOT:PSS (1.25 wt.% in H2 O) was poured into each GQC solution. The solutions of GQDs-PEDOT:PSS without and with CNT of the three abovementioned volumes of CNT were stirred by ultrasonic wave for 1 h. From all the volumes of chemicals such as GQDs, PEDOT:PSS, and CNT used for the film preparation, the CNT weight contents (wt.%) in the GQDs-PEDOT:PSS matrix have been calculated. It is seen that the samples embedded with the CNT volume of 0.5, 1.0, and 1.5 mg consist of 0.4, 0.8, and 1.2 wt.%, respectively.

Synthesis of Ag nanowires (AgNW) was carried out as follows. Firstly, 20 ml of ethylene glycol was heated within stirring in a 250ml Corning-0215 glass at 70°C for 15 min; then, 17 mg of NaCl was added. Raising temperature up to 100°C, 20 mg of AgNO3 was filled into the glass. The reaction between NaCl and AgNO3 occurred, resulting in formation of opaque AgCl solution. Ethylene glycol was decomposed in aldehyde that played a role of a catalyst for creating Ag nuclei. The next step, 5 mg of KBr was added to the glass and heated up to 140°C for 10 min, following 300 mg of PVP was filled and raising temperature to 160°C. The solution temperature was maintained for 15 min. Finally, 250mg of AgNO3 was added into the solution. The last solution was kept at 160°C for 30 min for growing silver nanowires. In the duration of this time, one can observe the change of the solution color from opaque to bright-gray, proving the formation of AgNWs in the solution. After the solution was cooled automatically to room temperature (in ~90 min), the solution was diluted by 80 ml of ethanol and kept for 10 h to deposit an AgNWs paste. This paste was put into a glass with 350 ml of distilled water for spinning with 6000 rpm for 30 min to get silver nanowires adhering to the glass walls. This AgNWs paste was removed from the glass and put into other glass with 200 ml of ethanol. By ultrasonic stirring, the AgNWs paste was dispersed completely in 2 h. Finally, 100 ml of distilled water was added into the AgNWs + ethanol solution; totally, 300 ml of the AgNWs solution was prepared for further studies.

*2.1.2. Preparation of P3HT + rGO + CNT film sensors*

heating solution of graphite filled in KMnO<sup>4</sup>

<sup>−</sup> and MnO2

P3HT, rGO, and CNTs were abbreviated to P3GC.

into Mn<sup>+</sup>

the glass beaker top was taken and kept in another glass beaker for further use.

SO4

Dc-bias applied to the two electrodes.

and 9.6 ml H2

O2

to separate MnO<sup>4</sup>

10 ml H2

All chemicals like PEDOT:PSS, P3HT, and multiple wall CNTs, with purity of ≥ 99.9% were purchased from Sigma-Aldrich Corporation. To prepare reduced graphene oxide (rGO), graphite flakes (GF) were taken off from graphite pieces with fewer layers by microwave

**Figure 1.** Image of a humidity sensor made from a single layer of composite films (a) and the schematic drawing of the device with the two planar electrodes (b). Humidity change is detected by the change in the current with a constant

and 28 ml of distilled water were poured into the glass beaker to get a liquid solution. Next,

The obtained solution was unmoved for 24 h, and at the glass beaker bottom, a paste-like layer with dark-yellow color was deposited, constituting the rGO paste. By slowly sucking, the solution above the rGO paste was completely taken from the glass beaker. Finally, 0.2 g of rGO paste was diluted in 40 ml of N,N-dimethylformamide (DMF) solvent in 50 ml-volume glass beaker and ultrasonically stirred for 900 s to get completely dispersive rGO in DMF (rGO-DMF). After 24 h waiting for the solution stabilization, 30 ml of rGO-DMF solution from

P3HT powder with a volume of 6 mg was mixed in 0.6 ml of rGO-DMF solution. This solution was ultrasonically stirred for 2 h at room temperature. At the same time, 1 mg of multiwalled carbon nanotubes (shortly called CNTs) was embedded in 0.5 ml of DMF (CNT-DMF) and also stirred by ultrasonic machine for 1 h. Finally, mixtures of the rGO-DMF and CNT-DMF solutions were put in a small glass beaker and carefully stirred for 5 h at 80°C by using a magnetic stirrer. For all the volumes of chemicals of P3HT, rGO, and CNTs used for further solid film preparation, the weight ratio of P3HT:rGO:CNTs was 100:20:10 (namely, the volume content of rGO and CNTs embedded in P3HT matrix was chosen to be, respectively, 20 and 10 wt.%. For simplicity in further analysis, the composite samples with such contents of

and HNO3

was added to this solution and ultrasonically stirred at room temperature for 8 h

were put in a 200 mL volume Corning-247 glass beaker, then 1.2 g KMnO<sup>4</sup>

Conducting Polymers Incorporated with Related Graphene Compound Films for Use…

. Mixtures of 0.2 g GF, 0.2 g NaNO3

http://dx.doi.org/10.5772/intechopen.79060

ions, yielding a solution with a bright-yellow color.

,

85

Next, to prepare PEDOT-PSS + GQD + AgNW solutions, we used GQDs+PEDOT:PSS mixture with a volume ratio 2/1 of 10 wt.% GQDs solution/PEDOT:PSS, further this solution is called as GPA. Next step, to the GPA solution, a small amount of the AgNWs paste was added. The AgNWs pastes were dispersed in the GPA solutions by ultrasonic wave for at 65°C 1 h.

Using spin coating, GQC and GPA solutions were deposited onto glass substrates which were coated by two silver planar electrode arrays with a length (L) of 10 mm and separated each one from the other by a distance (l) of 5 mm, as shown in **Figure 1**. In the spin-coating technique used for preparing composite films, the following parameters were chosen: a delay time of 100 s, a rest time of 45 s, a spin speed of 1500–1800 rpm, an acceleration of 500 rpm, and finally, a drying time of 3 min. To dry the composite films, a flow of dried gaseous nitrogen was used for 10 h. For a solidification avoiding the use of solvents, the film samples were annealed at 120°C for 8 h in a "SPT-200" vacuum drier.

For simplicity in further analysis, the GPC sensor samples without and with CNT of 0.4, 0.8, and 1.2 wt.% were abbreviated to GPC-0, GPC-1, GPC-2, and GPC-3, respectively, whereas GPA sensors with AgNW of 0.2, 0.4, and 0.6 wt.% are called GPA1, GPA2, and GPA3, respectively.

Conducting Polymers Incorporated with Related Graphene Compound Films for Use… http://dx.doi.org/10.5772/intechopen.79060 85

**Figure 1.** Image of a humidity sensor made from a single layer of composite films (a) and the schematic drawing of the device with the two planar electrodes (b). Humidity change is detected by the change in the current with a constant Dc-bias applied to the two electrodes.

#### *2.1.2. Preparation of P3HT + rGO + CNT film sensors*

and 2 μm in length was embedded in 10 ml of the GQD10 solution without CNT and with three contents of CNT, respectively, 0.5, 1.0, and 1.5 mg. All of the solutions obtained are called GQC solutions. These solutions were treated by plasma in a microwave oven. Then, 2 ml of polyethyl-

solutions of GQDs-PEDOT:PSS without and with CNT of the three abovementioned volumes of CNT were stirred by ultrasonic wave for 1 h. From all the volumes of chemicals such as GQDs, PEDOT:PSS, and CNT used for the film preparation, the CNT weight contents (wt.%) in the GQDs-PEDOT:PSS matrix have been calculated. It is seen that the samples embedded with the

Synthesis of Ag nanowires (AgNW) was carried out as follows. Firstly, 20 ml of ethylene glycol was heated within stirring in a 250ml Corning-0215 glass at 70°C for 15 min; then,

AgCl solution. Ethylene glycol was decomposed in aldehyde that played a role of a catalyst for creating Ag nuclei. The next step, 5 mg of KBr was added to the glass and heated up to 140°C for 10 min, following 300 mg of PVP was filled and raising temperature to 160°C. The

the solution. The last solution was kept at 160°C for 30 min for growing silver nanowires. In the duration of this time, one can observe the change of the solution color from opaque to bright-gray, proving the formation of AgNWs in the solution. After the solution was cooled automatically to room temperature (in ~90 min), the solution was diluted by 80 ml of ethanol and kept for 10 h to deposit an AgNWs paste. This paste was put into a glass with 350 ml of distilled water for spinning with 6000 rpm for 30 min to get silver nanowires adhering to the glass walls. This AgNWs paste was removed from the glass and put into other glass with 200 ml of ethanol. By ultrasonic stirring, the AgNWs paste was dispersed completely in 2 h. Finally, 100 ml of distilled water was added into the AgNWs + ethanol solution; totally, 300 ml

Next, to prepare PEDOT-PSS + GQD + AgNW solutions, we used GQDs+PEDOT:PSS mixture with a volume ratio 2/1 of 10 wt.% GQDs solution/PEDOT:PSS, further this solution is called as GPA. Next step, to the GPA solution, a small amount of the AgNWs paste was added. The AgNWs pastes were dispersed in the GPA solutions by ultrasonic wave for at 65°C 1 h.

Using spin coating, GQC and GPA solutions were deposited onto glass substrates which were coated by two silver planar electrode arrays with a length (L) of 10 mm and separated each one from the other by a distance (l) of 5 mm, as shown in **Figure 1**. In the spin-coating technique used for preparing composite films, the following parameters were chosen: a delay time of 100 s, a rest time of 45 s, a spin speed of 1500–1800 rpm, an acceleration of 500 rpm, and finally, a drying time of 3 min. To dry the composite films, a flow of dried gaseous nitrogen was used for 10 h. For a solidification avoiding the use of solvents, the film samples were

For simplicity in further analysis, the GPC sensor samples without and with CNT of 0.4, 0.8, and 1.2 wt.% were abbreviated to GPC-0, GPC-1, GPC-2, and GPC-3, respectively, whereas GPA sensors with AgNW of 0.2, 0.4, and 0.6 wt.% are called GPA1, GPA2, and GPA3, respectively.

CNT volume of 0.5, 1.0, and 1.5 mg consist of 0.4, 0.8, and 1.2 wt.%, respectively.

17 mg of NaCl was added. Raising temperature up to 100°C, 20 mg of AgNO3

solution temperature was maintained for 15 min. Finally, 250mg of AgNO3

O) was poured into each GQC solution. The

occurred, resulting in formation of opaque

was filled into

was added into

ene-dioxythiophene + PEDOT:PSS (1.25 wt.% in H2

84 Nanocomposites - Recent Evolutions

the glass. The reaction between NaCl and AgNO3

of the AgNWs solution was prepared for further studies.

annealed at 120°C for 8 h in a "SPT-200" vacuum drier.

All chemicals like PEDOT:PSS, P3HT, and multiple wall CNTs, with purity of ≥ 99.9% were purchased from Sigma-Aldrich Corporation. To prepare reduced graphene oxide (rGO), graphite flakes (GF) were taken off from graphite pieces with fewer layers by microwave heating solution of graphite filled in KMnO<sup>4</sup> and HNO3 . Mixtures of 0.2 g GF, 0.2 g NaNO3 , and 9.6 ml H2 SO4 were put in a 200 mL volume Corning-247 glass beaker, then 1.2 g KMnO<sup>4</sup> and 28 ml of distilled water were poured into the glass beaker to get a liquid solution. Next, 10 ml H2 O2 was added to this solution and ultrasonically stirred at room temperature for 8 h to separate MnO<sup>4</sup> <sup>−</sup> and MnO2 into Mn<sup>+</sup> ions, yielding a solution with a bright-yellow color. The obtained solution was unmoved for 24 h, and at the glass beaker bottom, a paste-like layer with dark-yellow color was deposited, constituting the rGO paste. By slowly sucking, the solution above the rGO paste was completely taken from the glass beaker. Finally, 0.2 g of rGO paste was diluted in 40 ml of N,N-dimethylformamide (DMF) solvent in 50 ml-volume glass beaker and ultrasonically stirred for 900 s to get completely dispersive rGO in DMF (rGO-DMF). After 24 h waiting for the solution stabilization, 30 ml of rGO-DMF solution from the glass beaker top was taken and kept in another glass beaker for further use.

P3HT powder with a volume of 6 mg was mixed in 0.6 ml of rGO-DMF solution. This solution was ultrasonically stirred for 2 h at room temperature. At the same time, 1 mg of multiwalled carbon nanotubes (shortly called CNTs) was embedded in 0.5 ml of DMF (CNT-DMF) and also stirred by ultrasonic machine for 1 h. Finally, mixtures of the rGO-DMF and CNT-DMF solutions were put in a small glass beaker and carefully stirred for 5 h at 80°C by using a magnetic stirrer. For all the volumes of chemicals of P3HT, rGO, and CNTs used for further solid film preparation, the weight ratio of P3HT:rGO:CNTs was 100:20:10 (namely, the volume content of rGO and CNTs embedded in P3HT matrix was chosen to be, respectively, 20 and 10 wt.%. For simplicity in further analysis, the composite samples with such contents of P3HT, rGO, and CNTs were abbreviated to P3GC.

Using spin coating, P3GC solutions were deposited onto glass substrates which were coated by two silver planar electrode arrays with a length of 4 mm and separated from each other by a distance of 2 mm, which is similar to the image in **Figure 1**. The following parameters were used for spin coating: a delay time of 100 s, a rest time of 45 s, a spin speed of 1500–1800 rpm, an acceleration of 500 rpm, and finally a drying time of 300 s. To dry the composite films, a flow of dried gaseous nitrogen was used for 10 h. For a solidification avoiding the use of solvents, the film samples were annealed at 120°C for 2 h in a "SPT-200" vacuum drier. To compare the performance efficiency of P3HT + rGO + CNT with the one for PEDOT:PSS + rGO + CNTbased sensors, PEDOT:PSS + rGO + CNT composites (shortly called PEGC) were prepared by the abovementioned procedure with replacement of P3HT by PEDOT:PSS polymer.

The P3GC film sensors were exposed to NH3

extraction of NH3

**3. Results and discussion**

*PSS + GQD + AgNW composite films*

tions are rarely formed.

steps.

gas with concentration (*Cgas*) that was controlled

http://dx.doi.org/10.5772/intechopen.79060

gas detecting to

87

with step decreases from 50 to 40, 30, 20, and 10 ppm. The repeatability in the resistance change of P3GC sensor was studied by measuring the resistance of sensor as a function of both the insertion/extraction of ammonia gas in/out chamber and measurement time. Each

saturation state, 50 s for maintaining exposed ammonia gas in chamber with abovementioned concentrations (namely *Cgas* = 50, 40, 30, 20, and 10 ppm), 30 s for extraction of Ar and ammonia gas out from chamber; then, 20 s for heating samples at 70°C for completing

and finishing the prior cycle. The next cycle was repeated by the same

Conducting Polymers Incorporated with Related Graphene Compound Films for Use…

measurement cycle consists of the following time durations: 20 s for NH3

**3.1. PEDOT-PSS + GQD + CNT and PEDOT-PSS + GQD + AgNW**

*3.1.1. Electrical property and morphology of PEDOT-PSS + GQD + CNT and PEDOT-*

**Figure 2.** A TEM micrograph of GQDs sample (a) and FE-SEM of GPC-3 composite film (b) [29].

From a TEM micrograph of a GQDs sample (**Figure 2a**), it is seen that the size distribution of the dots is considerably homogenous, as evaluated in this micrograph, the dots size ranged from 10 to 15 nm. **Figure 2b** is an FE-SEM micrograph of the GPC-3 sample where the CNT and GQDs clearly appeared while the conjugate polymer PEDOT:PSS exhibited a transparent matrix. This SEM micrograph also shows that in the GPC composite film, there are mainly heterojunctions of the GQD/PEDOT-PSS and CNT/PEDOT:PSS, whereas CNT/GQD junc-

From the thickness measurements, it can be seen that embedding CNT made the GPC samples considerably thicker. However, for the CNT-embedded GPC films, the CNT concentration was not much affected by the film thickness, so that the change in the thickness versus CNT

## **2.2. Characterization techniques**

The thickness of the films was measured on a "Veeco Dektak 6M" stylus profilometer. The size of GQDs and the surface morphology of the films were characterized by using "Hitachi" Transmission Electron Microscopy (TEM), Emission Scanning Electron Microscopy (FE-SEM), and NT-MDT atomic force microscope operating in a tunnel current mode. Crystalline structures were investigated by X-ray diffraction (XRD) with a Philips D-5005 diffractometer using filtered Cu-K<sup>α</sup> radiation (λ = 0.15406 nm). The ultraviolet–visible absorption spectra were carried out on a Jasco UV–VIS–NIR V570.

For humidity sensing measurements, the samples were put in a 10 dm3 -volume chamber; a humidity value could be fixed in a range from 20 to 80% by the use of an "EPA-2TH" moisture profilometer (USA). The adsorption process is controlled by insertion of water vapor, while desorption process was done by extraction of the vapor followed by insertion of dry gaseous Ar. The measurement system that was described in [30] consisting of an Ar gas tank, gas/ vapor hoses and solenoids system, two flow meters, a bubbler with vapor solution, and an airtight test chamber connected with collect-store data DAQ component. The Ar gas played a role as carrier gas, dilution gas, and purge gas.

For each sample, the number of measuring cycles was chosen to be at least 10 cycles. The humidity flow taken for measurements was of ~60 sccm ml/min. The sheet resistance of the samples were measured on a "KEITHLEY 2602" system source meter. To characterize humidity sensitivity of the composite samples, the devices were placed in a test chamber and device electrodes were connected to electrical feedthroughs.

For monitoring gases, the prepared sensing samples were put in a testing chamber of 10 dm3 in volume. The gases value can be fixed in a rage from 10 to 1000 ppm by use of an "EPA-2TH" profilometer (USA). To characterize the gas sensitivity of the samples, the devices were placed in a test chamber at the room temperature (namely 300 K) and the Ar gas pressure of 101.325 kPa (or 1 Atm); the device electrodes were connected to electrical feedthroughs. The measurements that were carried out included two processes: adsorption and desorption. In the adsorption process, the gas (or vapor) flow consisting of Ar carrier and measuring vapor from a bubbler was introduced into the test chamber for an interval of time, following which the change in resistance of the sensors was recorded. In the desorption process, a dried Ar gas flow was inserted in the chamber in order to recover the initial resistance of the sensors. Through the recovering time dependence of the resistance, one can obtain information on the desorption ability of the sensor in the desorption process.

The P3GC film sensors were exposed to NH3 gas with concentration (*Cgas*) that was controlled with step decreases from 50 to 40, 30, 20, and 10 ppm. The repeatability in the resistance change of P3GC sensor was studied by measuring the resistance of sensor as a function of both the insertion/extraction of ammonia gas in/out chamber and measurement time. Each measurement cycle consists of the following time durations: 20 s for NH3 gas detecting to saturation state, 50 s for maintaining exposed ammonia gas in chamber with abovementioned concentrations (namely *Cgas* = 50, 40, 30, 20, and 10 ppm), 30 s for extraction of Ar and ammonia gas out from chamber; then, 20 s for heating samples at 70°C for completing extraction of NH3 and finishing the prior cycle. The next cycle was repeated by the same steps.
