**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-PSS + GQD + AgNW composite films*

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 junctions are rarely formed.

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

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


**Table 1.** Thickness and resistance at room temperature of graphene quantum dots/CNT composite films [29].

concentration could be neglected. Indeed, for GPC-0 samples (i.e., the samples without CNT) the value of the film thickness was found to be ~5% smaller than that of the GPC + CNT samples (**Table 1**). This can be explained by the lower viscosity of GPC solution in comparison with the viscosity of GPC composite solutions. The results of measurements of the sheet resistance (R) of the samples are listed in **Table 1**.

under elevated operating temperatures. This thermal stability property is a desired factor for

**Film thickness, d** 

**Table 2.** The data of the AgNWs-doped GQDs+PEDOT:PSS composite films used for humidity sensors [28].

**Resistance at 50°C** 

**(MΩ)**

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**(nm)**

GPA1 0.2 450 4.56 0.024 GPA2 0.4 460 4.24 0.026 GPA3 0.6 480 3.88 0.027

The data of the samples including the AgNWs content, thickness, initial resistance, and conductivity are listed in **Table 2**. The value of the conductivity of the pure PEDOT:PSS film is ~80 S/cm as reported in [17] that is much larger than the one of the GPA composite films. This proves that the composite films possess a poor concentration of charge carriers. However, for materials used in gas sensing monitoring, this fact is an advantage in detecting a small

FE-SEM image of AgNWs solution (**Figure 4a**) shows clearly the shape and dimension of the stick-like Ag wires, as evaluated in this image, the wire size is of 70 nm. **Figure 4b** is an FE-SEM image of the GPA3 film where the AgNWs and GQDs clearly appeared while the conjugate polymer PEDOT:PSS exhibited a transparent matrix. This SEM micrograph also shows that in the composite film, there are mainly heterojunctions of the GQD/PEDOT-PSS

From our experiments, the temperature dependences of the resistance of AgNWs-doped GQDs+PEDOT:PSS composite films were found to be similar to those reported for CNTsdoped GQDs+PEDOT:PSS films [14]. With the increase of temperature, the AgNWs-doped composite exhibited the behavior of a heavily doped semiconductor: the resistance decreased

O vapor.

**Conductivity (S/m)**

89

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amount of charge carries generated from adsorbed molecules, for instance, H2

and AgNW/PEDOT:PSS, whereas AgNW/GQD junctions are rarely formed.

**Figure 4.** FE-SEM micrograph of an AgNWs containing solution (a) and surface of GPA3 film [28].

materials that are used in sensing applications.

**AgNW content (wt.%)**

**Samples abbreviation**

The conductivity of the GPC-3 film is the largest and can be compatible to the conductivity of a pure PEDOT-PSS film as reported in [31]. Embedding GQDs and CNT into PEDOT-PSS has made the conductivity of PEDOT-PSS to decrease, leading to the expectation that the sensitivity of the GPC composite films would be enhanced. The temperature dependence of the conductivity of GPC samples is shown in **Figure 3**. For GPC-1 sample, σ versus T curves exhibit a typical property of the inorganic semiconductors: with increase in temperature, the conductivity increases. With increases in the CNT content, the composite exhibited a clearer semiconductor behavior; and when it reached a value as large as 1.2 wt.% (namely in GPC-3 sample), the conductivity of the films maintained an almost unchanged value of 37.2 S/cm

**Figure 3.** Temperature dependence of the conductivity of GPC-1, GPC-2 and GPC-3 films [29].


**Table 2.** The data of the AgNWs-doped GQDs+PEDOT:PSS composite films used for humidity sensors [28].

concentration could be neglected. Indeed, for GPC-0 samples (i.e., the samples without CNT) the value of the film thickness was found to be ~5% smaller than that of the GPC + CNT samples (**Table 1**). This can be explained by the lower viscosity of GPC solution in comparison with the viscosity of GPC composite solutions. The results of measurements of the sheet

**Table 1.** Thickness and resistance at room temperature of graphene quantum dots/CNT composite films [29].

 **(kΩ) Conductivity, σ (S/cm)**

The conductivity of the GPC-3 film is the largest and can be compatible to the conductivity of a pure PEDOT-PSS film as reported in [31]. Embedding GQDs and CNT into PEDOT-PSS has made the conductivity of PEDOT-PSS to decrease, leading to the expectation that the sensitivity of the GPC composite films would be enhanced. The temperature dependence of the conductivity of GPC samples is shown in **Figure 3**. For GPC-1 sample, σ versus T curves exhibit a typical property of the inorganic semiconductors: with increase in temperature, the conductivity increases. With increases in the CNT content, the composite exhibited a clearer semiconductor behavior; and when it reached a value as large as 1.2 wt.% (namely in GPC-3 sample), the conductivity of the films maintained an almost unchanged value of 37.2 S/cm

**Figure 3.** Temperature dependence of the conductivity of GPC-1, GPC-2 and GPC-3 films [29].

resistance (R) of the samples are listed in **Table 1**.

**Samples CNT content (wt.%) Thickness, d (nm) Rs**

88 Nanocomposites - Recent Evolutions

GPC-0 0 460 2.180 4.98 GPC-1 0.4 485 2.160 4.76 GPC-2 0.8 487 0.814 7.93 GPC-3 1.2 490 0.356 27.52

> under elevated operating temperatures. This thermal stability property is a desired factor for materials that are used in sensing applications.

> The data of the samples including the AgNWs content, thickness, initial resistance, and conductivity are listed in **Table 2**. The value of the conductivity of the pure PEDOT:PSS film is ~80 S/cm as reported in [17] that is much larger than the one of the GPA composite films. This proves that the composite films possess a poor concentration of charge carriers. However, for materials used in gas sensing monitoring, this fact is an advantage in detecting a small amount of charge carries generated from adsorbed molecules, for instance, H2 O vapor.

> FE-SEM image of AgNWs solution (**Figure 4a**) shows clearly the shape and dimension of the stick-like Ag wires, as evaluated in this image, the wire size is of 70 nm. **Figure 4b** is an FE-SEM image of the GPA3 film where the AgNWs and GQDs clearly appeared while the conjugate polymer PEDOT:PSS exhibited a transparent matrix. This SEM micrograph also shows that in the composite film, there are mainly heterojunctions of the GQD/PEDOT-PSS and AgNW/PEDOT:PSS, whereas AgNW/GQD junctions are rarely formed.

> From our experiments, the temperature dependences of the resistance of AgNWs-doped GQDs+PEDOT:PSS composite films were found to be similar to those reported for CNTsdoped GQDs+PEDOT:PSS films [14]. With the increase of temperature, the AgNWs-doped composite exhibited the behavior of a heavily doped semiconductor: the resistance decreased

**Figure 4.** FE-SEM micrograph of an AgNWs containing solution (a) and surface of GPA3 film [28].

one order in magnitude from the initial values. Indeed, with the AgNWs content of 0.6 wt.% (GPA3), the resistance of the sensor lowered from 3.88 to 400 kΩ with increase of temperature from room temperature to 80°C and maintained a unchanged value of 350 kΩ under elevated (100–140°C) operating temperatures. This thermal stability is a desired factor for materials used in sensing applications.

more desorption/adsorption cycles, the more holes were eliminated in the deeper distances in the composite films. The similar feature in the sheet resistance change versus humidity was observed for the CNT-PEDOT:PSS, but the sensitivity of the last was much less than the one of the GQDs-PEDOT:PSS sensor. This proves the advantage of GQDs embedded in PEDOT:PSS

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To evaluate sensing performance, a sensitivity (η) of the devices was introduced. It is deter-

*R*0

The absolute magnitude of the sensitivity of the GPC-0 calculated by formula (3) is of ca. 2.5%. Plots of time dependence of the sensitivity of the CNT-doped GPC composite films are shown in **Figure 6**. From **Figure 6**, one can see that for the GPC samples, vice versa to the GQDs-

of the films. Moreover, the resistance increased at a much faster rate than when it decreased. Looking at the humidity sensing curves in **Figure 6**, one can distinguish two phenomena: the "rapid" (steep slope) and "slow" (shallow slope) response. The rapid response arises from

slow response arises from molecular interactions with higher energy binding sites, such as vacancies, structural defects, and other functional groups [32, 33]. For the next step, the sensitivity ability of GPC composite was studied and the whole experiment process as described above was repeated. The data in **Figure 6** show that the presence of CNT can improve the sensing properties of GPC sheets. With increase in the CNT content, the resistivity increased,

for all three GPC sheets is almost the same value of 20 s, whereas the recovery time (the

**Figure 6.** Comparison of the humidity sensing of the GPC composite-based sensors vs. CNT content; (a) GPC-1 (0.4 wt%),

O molecular adsorption onto low-energy binding sites, such as sp2

from 4.5% (for GPC-1) to 9.0% (for GPC-2) and 11.0% (for GPC-3).

The response time (i.e., the duration for R<sup>o</sup>

(b) GPC-2 (0.8 wt %) and (c) GPC-3 (1.2 wt.%) [29].

(%) (1)

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91

raising up to Rmax in the adsorption process)


O vapor) adsorption process led to increase in the resistance

polymer for the humidity sensing.

PEDOT:PSS, the humidity (i.e., H2

H2

<sup>η</sup> <sup>=</sup> *<sup>R</sup>* <sup>−</sup> *<sup>R</sup>* \_\_\_\_\_*<sup>o</sup>*

mined by following equation:

#### *3.1.2. Humidity sensing*

#### *3.1.2.1. For GPC*

**Figure 5** demonstrates the adsorption and desorption processes of the GQDs-PEDOT:PSS and CNT-PEDOT:PSS sensors. **Figure 5** shows that in the first 60 s, Ar gaseous flow eliminated the contamination agents from the GQDs-PEDOT:PSS surface, consequently the surface resistance increased. After the cleaning of the sensor surface during 30 s, the introduced humidity vapor was adsorbed onto the sensor surface, resulting in the decrease of the resistance. In the subsequent cycles, the humidity desorption/adsorption process led respectively to increase and decrease of the resistance of sensors, with results similar to those reported in [11]. However, through each cycle, the resistance of the GQDs-PEDOT:PSS film did not recover/restore to its initial value, but increased in 1–2 kΩ, to a final value of 235 kΩ after 1000 s from 220 kΩ. The increase in the initial resistance of the GQDs-PEDOT:PSS mainly related to the decrease of the major charge carriers in PEDOT:PSS. This is due to the elimination of holes (as the major carriers in PEDOT:PSS) by electrons that were generated from the H2 O adsorption. The

**Figure 5.** Sheet resistance change vs. humidity of GQDs-PEDOT:PSS and CNT-PEDOT:PSS composite films during adsorption/desorption processes [29].

more desorption/adsorption cycles, the more holes were eliminated in the deeper distances in the composite films. The similar feature in the sheet resistance change versus humidity was observed for the CNT-PEDOT:PSS, but the sensitivity of the last was much less than the one of the GQDs-PEDOT:PSS sensor. This proves the advantage of GQDs embedded in PEDOT:PSS polymer for the humidity sensing.

one order in magnitude from the initial values. Indeed, with the AgNWs content of 0.6 wt.% (GPA3), the resistance of the sensor lowered from 3.88 to 400 kΩ with increase of temperature from room temperature to 80°C and maintained a unchanged value of 350 kΩ under elevated (100–140°C) operating temperatures. This thermal stability is a desired factor for materials

**Figure 5** demonstrates the adsorption and desorption processes of the GQDs-PEDOT:PSS and CNT-PEDOT:PSS sensors. **Figure 5** shows that in the first 60 s, Ar gaseous flow eliminated the contamination agents from the GQDs-PEDOT:PSS surface, consequently the surface resistance increased. After the cleaning of the sensor surface during 30 s, the introduced humidity vapor was adsorbed onto the sensor surface, resulting in the decrease of the resistance. In the subsequent cycles, the humidity desorption/adsorption process led respectively to increase and decrease of the resistance of sensors, with results similar to those reported in [11]. However, through each cycle, the resistance of the GQDs-PEDOT:PSS film did not recover/restore to its initial value, but increased in 1–2 kΩ, to a final value of 235 kΩ after 1000 s from 220 kΩ. The increase in the initial resistance of the GQDs-PEDOT:PSS mainly related to the decrease of the major charge carriers in PEDOT:PSS. This is due to the elimination of holes (as the

**Figure 5.** Sheet resistance change vs. humidity of GQDs-PEDOT:PSS and CNT-PEDOT:PSS composite films during

O adsorption. The

major carriers in PEDOT:PSS) by electrons that were generated from the H2

used in sensing applications.

90 Nanocomposites - Recent Evolutions

adsorption/desorption processes [29].

*3.1.2. Humidity sensing*

*3.1.2.1. For GPC*

To evaluate sensing performance, a sensitivity (η) of the devices was introduced. It is determined by following equation:

$$
\eta = \frac{R - R\_s}{R\_0} (\%) \tag{1}
$$

The absolute magnitude of the sensitivity of the GPC-0 calculated by formula (3) is of ca. 2.5%.

Plots of time dependence of the sensitivity of the CNT-doped GPC composite films are shown in **Figure 6**. From **Figure 6**, one can see that for the GPC samples, vice versa to the GQDs-PEDOT:PSS, the humidity (i.e., H2 O vapor) adsorption process led to increase in the resistance of the films. Moreover, the resistance increased at a much faster rate than when it decreased.

Looking at the humidity sensing curves in **Figure 6**, one can distinguish two phenomena: the "rapid" (steep slope) and "slow" (shallow slope) response. The rapid response arises from H2 O molecular adsorption onto low-energy binding sites, such as sp2 -bonded carbon, and the slow response arises from molecular interactions with higher energy binding sites, such as vacancies, structural defects, and other functional groups [32, 33]. For the next step, the sensitivity ability of GPC composite was studied and the whole experiment process as described above was repeated. The data in **Figure 6** show that the presence of CNT can improve the sensing properties of GPC sheets. With increase in the CNT content, the resistivity increased, from 4.5% (for GPC-1) to 9.0% (for GPC-2) and 11.0% (for GPC-3).

The response time (i.e., the duration for R<sup>o</sup> raising up to Rmax in the adsorption process) for all three GPC sheets is almost the same value of 20 s, whereas the recovery time (the

**Figure 6.** Comparison of the humidity sensing of the GPC composite-based sensors vs. CNT content; (a) GPC-1 (0.4 wt%), (b) GPC-2 (0.8 wt %) and (c) GPC-3 (1.2 wt.%) [29].

duration for R<sup>o</sup> lowering to Rmax in the desorption process) decreased from 70 s (GPC-1, **Figure 6a**) to 60 s (GPC-2, **Figure 6b**) and 40 s (GPC-3, **Figure 6c**). In addition, the complete H2 O molecular desorption on the surface of GPC composites took place at room temperature and atmospheric pressure. One can guess that connecting together, individual GPC sheets by CNTs caused the increase of the mobility of carriers in GPC composite films, consequently leading to higher H2 O vapor sensing ability of the CNT-doped GQDs-PEDOT:PSS composites. Indeed, due to the appearance of CNTs bridges, the number of the sites with high binding energies in GPC sheets decreases, while the number of those with low binding energies increases. Since the H2 O molecules was mainly adsorbed at the sites with low binding energies, the appearance of CNTs bridges led to the complete desorption ability of GPC composites.

#### *3.1.2.2. For GPA*

From experimental measurements, we have found that the electrical characteristics of our thin-film sensor elements are strongly dependent on the surrounding atmosphere, on humidity in particular. The increase in relative humidity results in significant decrease of the electrical resistance of the GPA composite films, namely GPA1, GPA2, and GPA3 (see **Figure 7**). At the RH lower 30%, the resistance of the sensors intensively decreased and reached an almost the same value of 400 kΩ from RH larger 50%. This demonstrates that AgNWs-doped GQDs+PEDOT:PSS composite films can be used well for humidity sensing in a range from RH10% to RH40%. Moreover, in this RH range, GRA3 sensor is the most sensitive to humidity, comparing to GRA1 and GRA2.

The humidity dependence of the resistance of the hybrid (or composite) films can be explained by the interaction of water molecules with the surface of the composite, which leads to changing electric parameters of the GQDs. On the other hand, water impurities might induce additional or so called "secondary" doping of the conjugated polymer PEDOT:PSS. This manifests itself in change of the chain shape to an "unfolded spiral" and, therefore, stimulates increase

**Figure 8.** Responses of resistance of the sensors based on AgNWs-doped GQD/PEDOT:PSS films to the pulse of relative humidity (RH 30%) at room temperature for samples GPA1 (curve "1"), GPA2 (curve "2"), GPA3 (curve "3") and GPA1

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More detailed measurements of the time response of the sensors were carried out in the con-

tion processes. **Figure 8** demonstrates the results of the measurements for AgNWs-doped GQDs+PEDOT:PSS sensors, i.e., for GPA1, GPA2, and GPA3. From **Figure 4**, one can see that the best humidity sensitivity was obtained in the sensor made from GPA3 film where the AgNWs content is of 0.6 wt.%. The samples with larger AgNWs contents (namely 0.8– 1.2 wt.%) in the composites were also made; however, the sensing to humidity of these composite decreased rapidly. Indeed, in **Figure 8**, the adsorption and desorption processes of the 0.8 wt.% AgNWs-doped GQDs+PEDOT:PSS sensor (called as GPA4) were revealed worse than that of the GPA3 sensor (0.6 wt.% AgNWs). **Figure 8** shows that the humidity desorption/adsorption process led, respectively, to increase/decrease of the resistance of sensors,

**Figure 9** shows the sensitivity determined by Formula (1) for the GPA3 sensor during

sensitivity of the GPA3 calculated by formula (3) reached a value as large as 15.2%. The plots for GPA1 and GPA2 sensors have a shape similar to the one of GPA3 (here they are not presented); however, the sensitivity was smaller, namely 5*.*5 and 6.5%, respectively, for

O vapor. The absolute magnitude of the

O vapor insertion and extraction, respectively, to the adsorption and desorp-

in the conductivity [8].

with results similar to those reported in [18].

5 cycles of the adsorption and desorption of H2

ditions of H2

(curve "4") [28].

**Figure 7.** RH% dependence of the surface resistance of AgNWs-doped GQDs+PEDOT:PSS for three composite films with 0.2 wt.% (curve "1"), 0.4 wt.% (curve "2") and 0.6 wt.% of AgNWs (curve "3") [28].

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duration for R<sup>o</sup>

92 Nanocomposites - Recent Evolutions

GPC composites.

*3.1.2.2. For GPA*

sequently leading to higher H2

ing energies increases. Since the H2

ity, comparing to GRA1 and GRA2.

H2

lowering to Rmax in the desorption process) decreased from 70 s (GPC-1,

O vapor sensing ability of the CNT-doped GQDs-PEDOT:PSS

O molecules was mainly adsorbed at the sites with low

**Figure 6a**) to 60 s (GPC-2, **Figure 6b**) and 40 s (GPC-3, **Figure 6c**). In addition, the complete

composites. Indeed, due to the appearance of CNTs bridges, the number of the sites with high binding energies in GPC sheets decreases, while the number of those with low bind-

binding energies, the appearance of CNTs bridges led to the complete desorption ability of

From experimental measurements, we have found that the electrical characteristics of our thin-film sensor elements are strongly dependent on the surrounding atmosphere, on humidity in particular. The increase in relative humidity results in significant decrease of the electrical resistance of the GPA composite films, namely GPA1, GPA2, and GPA3 (see **Figure 7**). At the RH lower 30%, the resistance of the sensors intensively decreased and reached an almost the same value of 400 kΩ from RH larger 50%. This demonstrates that AgNWs-doped GQDs+PEDOT:PSS composite films can be used well for humidity sensing in a range from RH10% to RH40%. Moreover, in this RH range, GRA3 sensor is the most sensitive to humid-

**Figure 7.** RH% dependence of the surface resistance of AgNWs-doped GQDs+PEDOT:PSS for three composite films with

0.2 wt.% (curve "1"), 0.4 wt.% (curve "2") and 0.6 wt.% of AgNWs (curve "3") [28].

O molecular desorption on the surface of GPC composites took place at room temperature and atmospheric pressure. One can guess that connecting together, individual GPC sheets by CNTs caused the increase of the mobility of carriers in GPC composite films, con-

**Figure 8.** Responses of resistance of the sensors based on AgNWs-doped GQD/PEDOT:PSS films to the pulse of relative humidity (RH 30%) at room temperature for samples GPA1 (curve "1"), GPA2 (curve "2"), GPA3 (curve "3") and GPA1 (curve "4") [28].

The humidity dependence of the resistance of the hybrid (or composite) films can be explained by the interaction of water molecules with the surface of the composite, which leads to changing electric parameters of the GQDs. On the other hand, water impurities might induce additional or so called "secondary" doping of the conjugated polymer PEDOT:PSS. This manifests itself in change of the chain shape to an "unfolded spiral" and, therefore, stimulates increase in the conductivity [8].

More detailed measurements of the time response of the sensors were carried out in the conditions of H2 O vapor insertion and extraction, respectively, to the adsorption and desorption processes. **Figure 8** demonstrates the results of the measurements for AgNWs-doped GQDs+PEDOT:PSS sensors, i.e., for GPA1, GPA2, and GPA3. From **Figure 4**, one can see that the best humidity sensitivity was obtained in the sensor made from GPA3 film where the AgNWs content is of 0.6 wt.%. The samples with larger AgNWs contents (namely 0.8– 1.2 wt.%) in the composites were also made; however, the sensing to humidity of these composite decreased rapidly. Indeed, in **Figure 8**, the adsorption and desorption processes of the 0.8 wt.% AgNWs-doped GQDs+PEDOT:PSS sensor (called as GPA4) were revealed worse than that of the GPA3 sensor (0.6 wt.% AgNWs). **Figure 8** shows that the humidity desorption/adsorption process led, respectively, to increase/decrease of the resistance of sensors, with results similar to those reported in [18].

**Figure 9** shows the sensitivity determined by Formula (1) for the GPA3 sensor during 5 cycles of the adsorption and desorption of H2 O vapor. The absolute magnitude of the sensitivity of the GPA3 calculated by formula (3) reached a value as large as 15.2%. The plots for GPA1 and GPA2 sensors have a shape similar to the one of GPA3 (here they are not presented); however, the sensitivity was smaller, namely 5*.*5 and 6.5%, respectively, for

**Figure 9.** Responses of the sensitivity of the GPA3 sensor to the pulse of relative air humidity (RH30%) at room temperature [28].

GPA1 and GPA2. Comparing with the CNT-doped GQDs+PEDOT:PSS film sensor (η ~11%) as reported in [14], the humidity sensing of 0.6 wt.% AgNWs-doped composite is much larger.

In addition, the complete H2 O molecular desorption on the surface of GPA composites took place at room temperature and atmospheric pressure. One can guess that connecting together individual GPA sheets by AgNWs caused the increase of the mobility of carriers in composite films, consequently leading to higher H2 O vapor sensing ability of the AgNWs-doped GQDs+PEDOT:PSS composites. Similar to CNT-doped GQDs+PEDOT:PSS composites, due to the appearance of AgNWs bridges, the number of the sites with high binding energies in GPA sheets decreases, while the number of those with low binding energies increases. Since the H2 O molecules were mainly adsorbed at the sites with low binding energies, the appearance of AgNWs bridges led to the complete desorption ability of GPA composites.

(**Figure 11**) where P3HT polymer matrix is not revealed in the FE-SEM. **Figure 11** clearly

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The results for the measurements of 5 cycles according to the ammonia concentration from 50, 40, 30, 20, and 10 ppm are shown in **Figure 12**. The cyclic behavior of the sensor performance shows that the P3GC sensors exhibited a good reversible sensing property toward ammonia gas. With exposition of ammonia gas in chamber, the sensor resistance increased rapidly, reaching the saturation value in about 20 s; and recovering its initial value in about 30 s after

shows the presence of the multiwalled carbon nanotubes in the P3GC sample.

**Figure 10.** AFMs of a pure P3HT (a) and P3GC (b) film annealed at 120°C for 2 h.

*3.2.2. Ammonia gas sensing*

**Figure 11.** FE-SEM of a P3GC composite film.

#### **3.2. NH3 gas sensing**

#### *3.2.1. Film morphology and structures*

**Figure 10** shows AFM images of a pure P3HT and annealed P3GC composite films. The thickness of the film is 550 nm, the annealing temperature is 120°C and the annealing time is 2 h. **Figure 10a** shows that the pure P3HT film exhibited a smooth surface, whereas the roughness of the P3GC film surface was estimated as about 1.50 nm (**Figure 10b**). Thus, the roughness of the P3GC film can be attributed to the presence of both rGO and CNTs nanoparticles. The roughness and porosity of the composite sample were also observed by FE-SEM micrograph Conducting Polymers Incorporated with Related Graphene Compound Films for Use… http://dx.doi.org/10.5772/intechopen.79060 95

**Figure 10.** AFMs of a pure P3HT (a) and P3GC (b) film annealed at 120°C for 2 h.

**Figure 11.** FE-SEM of a P3GC composite film.

(**Figure 11**) where P3HT polymer matrix is not revealed in the FE-SEM. **Figure 11** clearly shows the presence of the multiwalled carbon nanotubes in the P3GC sample.

#### *3.2.2. Ammonia gas sensing*

GPA1 and GPA2. Comparing with the CNT-doped GQDs+PEDOT:PSS film sensor (η ~11%) as reported in [14], the humidity sensing of 0.6 wt.% AgNWs-doped composite is much

**Figure 9.** Responses of the sensitivity of the GPA3 sensor to the pulse of relative air humidity (RH30%) at room

place at room temperature and atmospheric pressure. One can guess that connecting together individual GPA sheets by AgNWs caused the increase of the mobility of carriers in com-

GQDs+PEDOT:PSS composites. Similar to CNT-doped GQDs+PEDOT:PSS composites, due to the appearance of AgNWs bridges, the number of the sites with high binding energies in GPA sheets decreases, while the number of those with low binding energies increases. Since

**Figure 10** shows AFM images of a pure P3HT and annealed P3GC composite films. The thickness of the film is 550 nm, the annealing temperature is 120°C and the annealing time is 2 h. **Figure 10a** shows that the pure P3HT film exhibited a smooth surface, whereas the roughness of the P3GC film surface was estimated as about 1.50 nm (**Figure 10b**). Thus, the roughness of the P3GC film can be attributed to the presence of both rGO and CNTs nanoparticles. The roughness and porosity of the composite sample were also observed by FE-SEM micrograph

ance of AgNWs bridges led to the complete desorption ability of GPA composites.

O molecules were mainly adsorbed at the sites with low binding energies, the appear-

O molecular desorption on the surface of GPA composites took

O vapor sensing ability of the AgNWs-doped

larger.

temperature [28].

94 Nanocomposites - Recent Evolutions

the H2

**3.2. NH3**

In addition, the complete H2

 **gas sensing**

*3.2.1. Film morphology and structures*

posite films, consequently leading to higher H2

The results for the measurements of 5 cycles according to the ammonia concentration from 50, 40, 30, 20, and 10 ppm are shown in **Figure 12**. The cyclic behavior of the sensor performance shows that the P3GC sensors exhibited a good reversible sensing property toward ammonia gas. With exposition of ammonia gas in chamber, the sensor resistance increased rapidly, reaching the saturation value in about 20 s; and recovering its initial value in about 30 s after

the presence of nanoheterojunctions of P3HT/rGO and P3HT/CNT that together reduced the

Conducting Polymers Incorporated with Related Graphene Compound Films for Use…

From the sensitivity (η) of the P3GC sensor determined by Formula (1), the *η*−*Cgas* dependence was plotted in **Figure 13**. The responding time of the sensor was about 30 s and the resistance of the P3GC composite films fast recovered to baseline when exposed to air. In the same

an accurate concentration in the range from 0 to 10 ppm. From **Figure 13**, one can see that the response of the sensor linearly decreases with decreases in ammonia gas concentration; and the slope of the linear plot reflects the relative sensitivity of the sensor. Thus, for the P3GC composite film sensor, the relative sensitivity was found to be of 0.05%/ppm. This value is still rather low, but it is about two times larger than the sensitivity of the ammonia gas sensor

Concerning the capacity for detecting ammonia gas in an environment that often has a relatively large humidity, we found that the P3HT-based sensor does not respond to humidity, whereas the PEDOT: PSS-based sensor is highly sensitive to this factor [29]. Indeed, this was confirmed by our results of the investigation on humidity (RH%) sensing of the two types of sensors, as a function of both the measurement time (**Figure 14**) and the relative humidity in

with *Cgas* lowering from 50 to 40, 30,

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gas can attain a value is lower than

gas with

97

charge traps, one can enhance the performance of the sensors made from P3GC films.

10 ppm. However, using the EPA-2TH gas profilometer, we could not introduce NH3

period of time (namely 30 s), the sensing response to NH3

**Figures 12** and **13** show that the detection limit for NH3

made from PEDOT: PSS [37].

the range from RH%20 to RH%65 (**Figure 15**).

**Figure 13.** Sensitivity of P3GC sensor vs. ammonia concentration.

20, and 10 ppm decreased from 2.9 to 2.4, 1.8, 1.3, and 0.8%, respectively.

**Figure 12.** Time dependence of resistance of P3GC film on repeated exposure and removal of NH3 gas.

the extraction of ammonia gas from the chamber. The increase in resistance of the P3GC sensor is closely related to a lowering of major charge carriers (namely holes) in P3HT polymer that is considered as a p-type organic semiconductor [34]. Whereas NH3 is a highly active and electron-donating free radical [24], electrons generated by the absorption of ammonia gas on the surface of P3GC film eliminated a part of holes by coupling with each other, resulting in an increase of the P3HT resistance. When the P3GC film was slightly heated, NH3 molecules in P3HT rapidly evaporated from the film surface, leaving holes along the backbone of the polymer. In such fashion, the concentration of major charge carriers rapidly increases while the resistance of the sensor decreases.

Embedding rGO in P3HT has enabled to enhance sensing properties of the P3GC films. This is similar to the results reported in [35] for polypyrrole (Py)-rGO-based sensors. Wang et al. explained the excellent sensing properties of Py-rGO-based sensors due to the parts of oxygen-based moieties and structure defects after chemical reduction process, resulting to the p-type semiconducting behavior of the resultant rGO. NH3 , as a reducing agent, has a lone electron pair that can be easily donated to the p-type rGO sheets, leading to the increase of the resistance of the rGO devices, whereas multiwalled CNTs have contributed to improve the adsorption efficiency of gas molecules (included NH3 ) due to larger effective surface areas with many sites, as suggested by Varghese et al. [36]. Moreover, the addition of rGO and CNTs together in P3GC composite films created not only numerous nanoheterojunctions of P3HT/ rGO and P3HT/CNT, but also nanotube "bridges" for electron transferring. These "bridges" are clearly revealed by the SEM micrograph, as shown in **Figure 11**.

In [4], we also demonstrated that inorganic nanoparticles embedded in polymers filled up most of the cracked spots in polymers that were often created during postannealing. By this way, the cracked spots served as charge traps were eliminated in nanocomposite films. With the presence of nanoheterojunctions of P3HT/rGO and P3HT/CNT that together reduced the charge traps, one can enhance the performance of the sensors made from P3GC films.

From the sensitivity (η) of the P3GC sensor determined by Formula (1), the *η*−*Cgas* dependence was plotted in **Figure 13**. The responding time of the sensor was about 30 s and the resistance of the P3GC composite films fast recovered to baseline when exposed to air. In the same period of time (namely 30 s), the sensing response to NH3 with *Cgas* lowering from 50 to 40, 30, 20, and 10 ppm decreased from 2.9 to 2.4, 1.8, 1.3, and 0.8%, respectively.

**Figures 12** and **13** show that the detection limit for NH3 gas can attain a value is lower than 10 ppm. However, using the EPA-2TH gas profilometer, we could not introduce NH3 gas with an accurate concentration in the range from 0 to 10 ppm. From **Figure 13**, one can see that the response of the sensor linearly decreases with decreases in ammonia gas concentration; and the slope of the linear plot reflects the relative sensitivity of the sensor. Thus, for the P3GC composite film sensor, the relative sensitivity was found to be of 0.05%/ppm. This value is still rather low, but it is about two times larger than the sensitivity of the ammonia gas sensor made from PEDOT: PSS [37].

Concerning the capacity for detecting ammonia gas in an environment that often has a relatively large humidity, we found that the P3HT-based sensor does not respond to humidity, whereas the PEDOT: PSS-based sensor is highly sensitive to this factor [29]. Indeed, this was confirmed by our results of the investigation on humidity (RH%) sensing of the two types of sensors, as a function of both the measurement time (**Figure 14**) and the relative humidity in the range from RH%20 to RH%65 (**Figure 15**).

**Figure 13.** Sensitivity of P3GC sensor vs. ammonia concentration.

the extraction of ammonia gas from the chamber. The increase in resistance of the P3GC sensor is closely related to a lowering of major charge carriers (namely holes) in P3HT polymer

electron-donating free radical [24], electrons generated by the absorption of ammonia gas on the surface of P3GC film eliminated a part of holes by coupling with each other, resulting in

in P3HT rapidly evaporated from the film surface, leaving holes along the backbone of the polymer. In such fashion, the concentration of major charge carriers rapidly increases while

Embedding rGO in P3HT has enabled to enhance sensing properties of the P3GC films. This is similar to the results reported in [35] for polypyrrole (Py)-rGO-based sensors. Wang et al. explained the excellent sensing properties of Py-rGO-based sensors due to the parts of oxygen-based moieties and structure defects after chemical reduction process, resulting to the

electron pair that can be easily donated to the p-type rGO sheets, leading to the increase of the resistance of the rGO devices, whereas multiwalled CNTs have contributed to improve

with many sites, as suggested by Varghese et al. [36]. Moreover, the addition of rGO and CNTs together in P3GC composite films created not only numerous nanoheterojunctions of P3HT/ rGO and P3HT/CNT, but also nanotube "bridges" for electron transferring. These "bridges"

In [4], we also demonstrated that inorganic nanoparticles embedded in polymers filled up most of the cracked spots in polymers that were often created during postannealing. By this way, the cracked spots served as charge traps were eliminated in nanocomposite films. With

an increase of the P3HT resistance. When the P3GC film was slightly heated, NH3

is a highly active and

gas.

, as a reducing agent, has a lone

) due to larger effective surface areas

molecules

that is considered as a p-type organic semiconductor [34]. Whereas NH3

**Figure 12.** Time dependence of resistance of P3GC film on repeated exposure and removal of NH3

p-type semiconducting behavior of the resultant rGO. NH3

the adsorption efficiency of gas molecules (included NH3

are clearly revealed by the SEM micrograph, as shown in **Figure 11**.

the resistance of the sensor decreases.

96 Nanocomposites - Recent Evolutions

**4. Conclusion**

GPC and GPA) and NH3

GQDs+PEDOT: PSS composite films.

University, Canada) for useful discussions.

Address all correspondence to: dinhnn@vnu.edu.vn

**Acknowledgements**

**Author details**

Nguyen Nang Dinh

Vietnam

• Using spin-coating technique, PEDOT: PSS + GQD + CNT (GPC), PEDOT: PSS + GQD +AgNW (GPA) films used for humidity sensors and P3HT + rGO + CNT (P3GC) films used for NH3 gas sensors were prepared at room temperature and atmospheric pressure, all the sensing devices have extremely simple structure and they respond well to the humidity change (for

Conducting Polymers Incorporated with Related Graphene Compound Films for Use…

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

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• With the CNT content increase, from 0% (GPC-0) to 0.4 wt.% (GPC-1), 0.8 wt.% (GPC-2), and 1.2 wt.% (GPC-3), the sensitivity of the humidity sensing devices based on CNTdoped graphene quantum dot-PEDOT: PSS composites improved from 2.5% (GPC-0) to 4*.*5% (GPC-1), 9.0% (GPC-1), and 11.0% (GPC-2), respectively The response time of the GPC sensors was as fast as 20 s; and the recovery time of the sensors lowered from 70 s (0.4 wt.% CNT) to 60 s (0.8 wt.% CNT) and 40 s (1.2 wt.% CNT). With the AgNWs content increase, from 0.2 wt.% (GPA1) to 0.4 wt.% (GPA2) and 0.6 wt.% (GPA3), the sensitivity of the humidity sensing devices based on AgNWs-doped graphene quantum dot-PEDOT: PSS composites improved from 5*.*5% (GPA1), 6.5% (GPA2) and 15.2% (GPA3), respectively The best response time (~30 s) was obtained for sensors made from 0.6 wt.% AgNWs-doped

• P3GC (namely P3HT embedded with a content of 20 wt.% of rGO and 10% of CNTs) film sensors possessed a responding time of ca. 30 s, a sensing response of 0.8% at ammonia gas concentration of 10 ppm and a relative sensitivity of 0.05%/ppm. Investigation of humidity sensing of both the PEDOT: PSS + rGO + CNT and P3HT + rGO + CNT film sensors has demonstrated that P3HT + rGO + CNT film does not respond to humidity as it is the case for PEDOT: PSS + rGO + CNT. Useful applications in gas thin-film sensors for selectively

This research was funded by the Vietnam National Foundation for Science and Technology (NAFOSTED). The authors express sincere thanks to Prof. Dr. Vo-Van Truong (Concordia

University of Engineering and Technology, Vietnam National University Hanoi, Hanoi,

sensing ammonia gas in a humid environment can thus be envisaged.

gas (for P3GC).

**Figure 14.** Comparison of the RH% sensitivity of PEGC (a) and P3GC sensors (b).

**Figure 15.** Humidity dependence of resistance of the PEGC (a) and P3GC films (b).

Although the ammonia gas response of P3GC sensors at 50 ppm (*η* = 2.9%) is lower compared to PEDOT: PSS-based sensors (*η* = 4.0% [28]), when one needs to monitor ammonia gas in a humid environment, P3HT/rGO/CNT (namely P3GC) sensors would be preferred. This is because signals obtained from measurements on PEDOT: PSS-based sensors would be undistinguishable between ammonia gas and water vapor.
