3.3. FTIR Analysis
FTIR analysis of graphene powder and untreated and treated cotton threads with graphene dispersion was studied in the wavenumber range from 400 to 4000 cm
−1, as
Figure 4 shows. The main purpose of FTIR analysis of the treated cotton thread is to investigate the effects of solvents, graphene concentration, and annealing temperature (see
Table 2). In the FTIR spectra of graphene powder, the presence of strong characteristic peaks at 672 cm
−1 and 950 cm
−1 corresponds to the bending vibrations of the C-H and C-O bonds, respectively. These peaks offer evidence of the chemical composition and functional groups present in the graphene material. For the untreated cotton thread, the FTIR spectra revealed characteristic peaks at 2900 cm
−1, indicative of the C-H stretching vibrations within the ß-glucose unit of cellulose, and at 1750 cm
−1 and 1019 cm
−1, associated with the C=O stretching of carboxylic acid and C-O stretching, respectively. Additionally, the bands observed at 1800 cm
−1 and 2250 cm
−1 represent C=C vibrational modes. These findings provide valuable information regarding the chemical constituents and structural attributes of the untreated cotton thread [
30]. The indication of the FT-IR spectrum lies in its ability to detect and differentiate these characteristic peaks, which can be attributed to specific chemical bonds and molecular groups. The FTIR results of graphene powder and the untreated cotton thread are in good agreement with the literature [
33,
34]. Moreover, through the FTIR analysis of the treated cotton threads, we aimed to investigate the impact of various factors, including solvents, graphene concentration, and annealing temperature, on the FTIR spectra. The absence or alterations of certain peaks under different experimental conditions offer insights into the interactions and modifications occurring within the treated cotton threads.
3.5. Electrical Conductivity Measurements
In this experiment, the effect of three independent variables, namely, the amount of graphene dispersion dispersed in different solvents, the annealing temperature, and temperature on the electrical conductivity, which is the dependent variable, is investigated. The electrical conductivity was calculated for the four solvents at graphene dispersion amounts of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 6.0 mL. This calculation was performed at two annealing temperatures: far from the solvent boiling point and close to it. It is worth noting that the total number of samples in this study was 56 samples.
First, the effect of the amount of graphene dispersion in different solvents on the electrical conductivity of the conductive threads is discussed. As
Figure 6 shows, the electrical behavior of all threads is the same at the two annealing temperatures. The conductivity increases with the increasing amount of graphene dispersion in the thread which is due to the increase in graphene concentration in the thread.
Figure 6 shows an exponential curve, and with a small amount of graphene dispersion, the value of conductivity is not clear. Moreover, some values of conductivity in this range also overlap. To solve this problem and find a relationship between electrical conductivity (
) and the amount of graphene dispersion (
), we plotted the natural logarithm of conductivity against the natural logarithm of graphene dispersion (see
Figure 7). The experimental data of these curves were fitted to the model:
where
and
are the fitting parameters (see
Table 4) which are calculated using the least square method. All conducting threads give well-fitting straight lines. From Equation (1), we can conclude that
is proportional to
according to the following equation:
where
and
Table 5 shows its values.
Secondly, we want to investigate whether or not the annealing temperature affects the electrical conductivity of the conductive thread. For this purpose, the conductivity was studied as a function of solvent type for a fixed amount of graphene dispersion. As
Figure 8 and
Table 5 show, the conductivity of the conductive thread depends strongly on the annealing temperature. For the conductive thread prepared with graphene dispersed in DMSO, the conductivity of the thread prepared at a temperature far from the boiling point (T = 100 °C) is higher than the conductivity of the thread prepared at a temperature close to the boiling point (T = 180 °C) for both the low and high graphene dispersion samples. The maximum conductivity was obtained as 702.717 S cm
−1 at an annealing temperature of 100 °C and 6 mL of graphene dispersion. In contrast, the conductivity of the conductive thread prepared with graphene dispersed in DI at an annealing temperature far from the boiling point (T = 85 °C) is lower than the conductivity at an annealing temperature near the boiling point (T = 100 °C). The maximum conductivity was determined to be 863.768 S cm
−1 at an annealing temperature of 100 °C and 6 mL of graphene dispersion. For the conductive thread prepared with graphene dispersed in DMF, the conductivity of the conductive thread at a graphene dispersion amount of 1 mL and 3 mL is approximately the same at both annealing temperatures. In contrast, the conductivity of the conductive thread at an annealing temperature far from the boiling point (T = 100 °C) is higher than the conductivity at an annealing temperature near the boiling point (T = 150 °C) for the conductive thread prepared with 2 mL and 6 mL of graphene dispersion, and the maximum conductivity of 2416.323 S cm
−1 was obtained at an annealing temperature of 100 °C for 6 mL of graphene dispersion. For the conductive thread prepared with graphene dispersed in ethanol, the conductivity of the thread prepared at an annealing temperature near the boiling point is higher than the conductivity of the thread prepared at an annealing temperature far from the boiling point for all samples except the thread prepared with a small amount of graphene dispersion. The maximum conductivity of 2505.675 S cm
−1 was obtained at an annealing temperature of 78 °C and a high amount of graphene dispersion.
The influence of the amount of graphene dispersion and annealing temperature on the electrical conductivity of the thread can be explained through the complex interplay of several factors at the nanoscale. Firstly, the amount of graphene dispersion directly impacts the concentration of graphene within the composite material. Graphene, being an excellent conductor of electricity, introduces more conductive pathways within the material. This increase in the number of conductive paths enhances the overall electrical conductivity of the thread. Secondly, the annealing temperature plays a crucial role in the structural arrangement of graphene within the thread. At elevated temperatures, graphene sheets tend to align more uniformly, reducing defects and discontinuities in the conductive pathways. This alignment enhances the charge transport efficiency, further contributing to increased electrical conductivity. Additionally, annealing can remove residual solvents or binders, which may hinder electron movement within the composite. By eliminating these impediments, annealing promotes better electrical connectivity between graphene sheets and the surrounding matrix. In summary, the amount of graphene dispersion and annealing temperature affect the electrical conductivity of the thread by influencing the concentration of graphene, the structural arrangement of graphene sheets, and the removal of hindrances to electron movement. These factors collectively contribute to the observed variations in electrical conductivity, making them critical parameters to optimize for tailored electrical performance in the composite material.
Finally, we have successfully fabricated highly conductive cotton threads through a dip-and-dry method using graphene as the substrate material. Our research has yielded remarkable results, with the cotton threads exhibiting an impressive maximum electrical conductivity of 2505.675 S cm
−1. This achievement places our work at the forefront of conductivity enhancement for cotton-based materials when compared to previous studies. Specifically, Yang et al. (2019) achieved conductivity of approximately 1.0 S m
−1 using graphene oxide and a dip-coating and chemical reduction method, while Yun et al. (2017) reported a conductivity of 286 S cm
−1 through an immersion process with gold/graphene. Maneval et al. (2021) demonstrated a conductivity of 1.1 S cm
−1 with graphene using a dip-and-dry technique. Our results represent a significant advancement in the field of conductive cotton thread fabrication, emphasizing the potential for various practical applications, see
Table 6.
Third, the effect of temperature on the electrical conductivity of the graphene-based conductive thread was investigated by placing the prepared thread in the oven and changing the temperature from 30 °C to 130 °C. This study was performed for four conductive threads (see
Table 7). The main purpose of this investigation was to study the electrical behavior of the conductive thread under the influence of temperature: is it semiconductor behavior, metallic behavior, or both? As
Figure 9 shows, the four conductive threads show the same trend where electrical conductivity decreases with increasing temperature, indicating metallic behavior. The changes in electrical conductivity are from 68.26 to 23.79 S cm
−1 (44.47 S cm
−1) for thread I, from 40.87 to 26.16 S cm
−1 (14.71 S cm
−1) for thread II, from 69.09 to 33.14 S cm
−1 (35.95 S cm
−1) for thread III, and from 118.39 to 45.08 S cm
−1 (73.31 S cm
−1) for thread 4. These results show that the electrical conductivities of threads I, II, III, and IV decrease by 65.15, 35.99, 52.03, and 61.92%, respectively.
The decrease in electrical conductivity with increasing temperature can be explained by the following factors:
Thermal Expansion of Cotton Fibers: Cotton fibers themselves undergo thermal expansion as the temperature rises. This expansion leads to increased spacing between individual cotton fibers in the thread. As the distance between conductive pathways (graphene-coated cotton fibers) increases, it becomes more difficult for electrons to traverse the larger gaps, resulting in higher electrical resistance and reduced conductivity.
Graphene–Cotton Interaction: The interaction between graphene and cotton fibers can be temperature-sensitive. At higher temperatures, the adhesive or binding properties of the graphene coating on the cotton fibers may weaken or become less effective. This can result in the detachment or reorientation of graphene particles on the cotton surface, leading to interruptions in the conductive pathways and a decrease in electrical conductivity.
Thermal Vibrations of Cotton: Elevated temperatures cause increased thermal vibrations of the cotton fibers. These vibrations can disrupt the alignment and orderliness of the graphene-coated cotton fibers, leading to a less organized and less conductive structure.
Cotton Degradation: Cotton fibers may undergo thermal degradation or decomposition at elevated temperatures. This can lead to changes in the cotton’s physical and chemical properties, potentially affecting the conductivity of the graphene-coated cotton thread.
In summary, the decrease in electrical conductivity of the cotton thread treated with graphene at higher temperatures can be attributed to factors such as thermal expansion of cotton fibers, changes in the graphene–cotton interaction, thermal vibrations of cotton, and potential cotton degradation. These factors collectively contribute to the observed reduction in conductivity as temperature increases.