3.1 Formation of C-GWA
The scheme of preparation process of C-GWA is shown in the Fig. 1. The shredded printing waste paper was changed into pulp after being oxidized by NaClO2. Fiber bundles in the pulp are opened under vigorous stirring. Lignin and impurities in waste paper fibers are removed by chlorine dioxide produced by NaClO2, which leads to a rough surface of the waste paper(Li et al. 2017b). Subsequently, waste paper fibers were dispersed in the ethanol dispersion of graphene to form a homogeneous suspension. The rough surface of the waste paper fiber provides adsorption sites which graphene in the ethanol is easily coated on. Afterward, the suspension was poured into a cylindraceous mould, filtered to yield the wet graphene coated waste paper aerogels, followed by drying directly at 60°C in an oven to obtain the graphene coated waste paper aerogel (GWA) under atmospheric pressure. The high aspect ratio and rough surface of waste paper fibers cause them to be easily entangled, forming a continuous 3D network. Graphene sheets are linked together to form a continuous conductive layer in the 3D network. Ethanol as a solvent in the suspension is essential for the drying process under the atmosphere because the surface tension of ethanol evaporation is too low to change the gap between the fibers. Therefore, the volume of the wet aerogel will not shrink and the aerogel structure will not collapse during drying(Li et al. 2017a). Compared with the vacuum freeze-drying and supercritical drying commonly used to prepare aerogels, drying under atmospheric condition only needs simple equipment and short time. For excellent compression space and mechanical behavior, cellulose in aerogels is generally annealed into carbon materials. Continuous porous structure of the aerogels is not only kept, but also the fiber diameter and the density of fiber entanglement points reduce which cause the decreased density and the increased compression space of the aerogel(Huang et al. 2021b; Sun et al. 2021a). Thus, GWA is annealed to the carbonized graphene coated waste paper aerogel (C-GWA) in a nitrogen atmosphere at 500°C to realize the characteristics of light weight and high compression. The carbon material converted from cellulose at 500°C has too low conductivity that cannot be measured (Shao et al. 2018), therefore, C-WA is not conductive. However, compared with C-WA, C-GWA has good conductivity provided by a continuous conductive layer formed by graphene.
3.2 SEM analysis
Surface morphology of C-WA, GWA and C-GWA is illustrated in Figs. 2a-2i. Figure 2a, 2d and 2g show that C-WA, GWA and C-GWA have porous and rough surface. Figure 2b, 2e and 2h show that C-WA, GWA and C-GWA have high aspect ratio of fibers and 3D structure of interpenetrated fibers. The rough surface is formed because the fibers inside the aerogel pass through the pores on the surface of the aerogel. It also can be seen from Fig. 2b, 2e and 2h that the diameter of fibers in C-WA and C-GWA is smaller than that of GWA because O, H and C elements escape from the fiber and fibers shrink during annealing. Figure 2c, 2f and 2i show the appearance of individual fiber in C-WA, GWA and C-GWA. The fiber in GWA exhibits rough surfaces due to the decomposition of lignin and other impurities by chlorine dioxide. The fiber in C-WA and C-GWA exhibits a regular wrinkle structure due to the escape of CO2 and H2O from the surface of the waste paper fiber during annealing. In addition, compared with the fiber in C-WA, graphene is evenly coated on the surface of the fiber in GWA and C-GWA as shown in Fig. 2f and 2i. Figure 2j presents the photos of GWA and C-GWA. The specific shrinkage value of the volume is illustrated in Fig. 2k. The volume shrinkage rate is 24.2% when GWA is annealed to C-GWA. The volume of the aerogel shrinks after annealing due to the shrinkage of the fibers, which results in the shrinkage of the 3D skeleton formed by interpenetrated fibers. The density of GWA is 68.9mg/cm3, however, the density of C-GWA is only 25mg/cm3. Lower density of the C-GWA than GWA can be explained by the fact that the pyrolysis of cellulose leads to a large loss of mass of aerogel during annealing. Although the volume of carbon aerogel decreases, the mass per unit volume reduces.
3.3 XRD, XPS and Raman analyses
Figure 3a shows in the XRD spectra of GWA, C-WA and C-GWA. The characteristic peaks of GWA located at 14.9°, 22.57°, and 34.28° represent the crystal planes d101, d002, and d040 of cellulose I in the diffraction patterns of GWA(Wu et al. 2020). The characteristic peak of graphene is covered with the characteristic peak of cellulose at 22.57°. For C-GWA and C-WA, the characteristic peaks of cellulose at 14.9° and 34.28° disappear, and the diffraction peak at 22° becomes wide, representing the characteristic peak of amorphous carbon(Bi et al. 2014). Intensity of characteristic peak of C-GWA is higher than that of C-WA because the coating of graphene increases the content of ordered carbon. The result shows that the cellulose in waste paper fibers is successfully transformed to amorphous carbon during annealing. Figure 3b shows the Raman spectra of GWA and C-GWA. A G-band at ≈ 1580 cm− 1 (related to crystalline sp2 carbon) and a D-band at ≈ 1350 cm− 1 (associated with defects or heteroatom doping) appear for GWA and C-GWA, indicating the successful coating of graphene on the fiber surface(Zhang et al. 2020b). The peak intensity of ID/IG of C-GWA is lower than that of GWA. This phenomenon can be explained by that the cellulose in waste paper fibers is transformed into amorphous carbon during annealing. Figures 3c shows XPS the spectra of GWA and C-GWA. The O/C atomic ratio of GWA is 0.315. However, the O/C atomic ratio of C-GWA is only 0.048, which suggests that most of the oxygen atoms in the GWA aerogel have been almost removed during annealing.
3.4 Mechanical performance of aerogels
Figure 4 shows the photos of C-GWA and GWA at different compression strains. GWA has obvious loss of height after recovery at different compression strains. However, C-GWA can return to the original height at low compressive strain, and height slightly reduces at high compressive strain. GWA returns to 95% of its original height at 10% compressive strain, however C-GWA completely returns to its original height. GWA and C-GWA return to 88% and 97% of its original height at a compressive strain of 30%, respectively. GWA and C-GWA returns to 88% and 91% of its original height at 50% compressive strain, respectively.
3.5 Electromechanical performance of the pressure Sensor
The schematic diagram of the wearable pressure sensor based on C-GWA is illustrated in Fig. 5a. The C-GWA are placed on an elastic tape with interdigitated electrodes. Figure 5b shows that the resistance of the C-GWA-based pressure sensor varies with the increasing external pressure. The circuit is connected when the GCA-based pressure sensor is used as a touch sensitive switch. The brightness of LED strongly depends upon the compressive strain amplitudes. The higher the compressive strain is, the stronger the brightness of the LED lamp will be. Figure 5c to 5e show the relative current changes of the C-GWA-based pressure sensor in the pressure range from 16 Pa to 132 kPa where the relative current increases with the rise of pressure, certifying that the real-time current is highly sensitive to external pressure, which is attributed to the increasing contact area between the rough surface of the aerogel and the interdigital electrode. Additionally, the contact mode of graphene sheets is transformed from the edge-to-edge into the surface-to-surface under external pressure, which also leads to increased conductivity. Figure 5f shows the ultra-low detection capability of the C-GWA-based pressure sensor. A significant relative current change is observed when a green bean with a mass of 53.4 mg (≈ 2.5 Pa) is placed on the upper surface of the sensor due to the increase in pressure. The ability to detect ultra-low pressure is resulted from the rise of contact area between rough surface of the aerogel and the interdigital electrode under a smaller pressure. In addition, the wrinkle structure on the surface of the carbon fiber is easily connected under a small pressure, which will cause a change in resistance.
Sensitivity is an important index to evaluate the performance of wearable pressure sensors. The sensitivity (S) is defined and calculated based on the following formulas: S = (ΔI/I0)/ΔP, where ΔI (A) is the relative change of current. I0 (A) is the current without loading and ΔP (kPa) is the change of the applied pressure. As shown in Fig. 6, the C-GWA-based pressure sensor possesses a satisfyingly linear relationship between current response and applied pressure and has the sensitivity factors of S1 = 31.6 kPa− 1 within 0.7 kPa stress, S2 = 7.3 kPa− 1 for stress range of 0.7–3.5 kPa, and S3 = 3.12 kPa− 1 over 3.5 kPa, respectively. It should be noted that the sensitivity of the low-pressure range is higher than that of the high-pressure range. In the low-pressure range, the increasing contact area between the aerogel and the electrode leads to a significantly reduced resistance. Simultaneously, the contact of wrinkle structures on the skeleton of the C-GWA also causes the resistance of the aerogel to decrease under small external pressure. As the pressure load rises, the sensitivity reduces because the rough surface of the aerogel fully contacts with the interdigital electrode and the aerogel is less and less susceptible to deformation due to the reduction in compressible space caused by increased pressure. Compared with other sensors based on 3D porous materials using graphene as a conductive filler reported in recent years, C-GWA-based pressure sensors have higher sensitivity and lower detection limit (Table S1), which is attributed to the changeable contact area between the C-GWA and the electrode and easily contacted wrinkle structures on the skeleton of the C-GWA. The reproducibility of the C-GWA-based sensor was evaluated through loading–unloading cyclic tests for 8000 cycles as shown in Fig. 6b, verifying the superior durability of the C-GWA-based pressure sensor. Figure 6c presents the relative current response of the C-GWA -based pressure sensor under 4538 Pa at various compression speeds of 0.5 mm/s, 1 mm/s and 2 mm/s, indicating the stability and repeatability of the pressure sensor under different compression speeds. Additionally, the sensor exhibits a fast response time of 176 ms and recovery time of 107 ms (Fig. 6d), ensuring its real-time feedback for external pressure. Figures 6e and 6f show the hydrophobic properties of C-GWA with a contact angle of up to 141° due to the removal of oxygen-containing functional groups in the fiber during carbonization. Hydrophobicity contributes to application of the sensor based on C-GWA in humid environments.
3.6 The working mechanism of the pressure sensor.
Figure 7a shows the equivalent circuit diagram of the pressure sensor based on C-GWA. The total resistance of the pressure sensor is defined as Rtotal = Re + Ra + Rb, where Re is the electrode resistance and wire resistance, Ra is the contact resistance between the C-GWA and the electrode, and Rb is the bulk resistance of C-GWA. Re remains unchanged with increasing external pressure, however, Ra, Rb and Rtotal is reduced. The variety of resistance and microstructural of the pressure sensor based on C-GWA under released and pressed states are further illustrated in Figs. 7b-d. For the sensor without pressure loading, there are many pores inside the aerogel and a gap between the aerogel and the electrode attributed to rough outer surface of the aerogel as shown in Fig. 7b. When the external pressure is applied, the deformed C-GWA achieved good contact to the electrode and pores within the aerogel are compressed, which causes fibers in C-GWA are compacted together. Additionally, the contact mode of the graphene sheets coated on the surface of the fibers is also transformed with the fibers in C-GWA aggregated (Fig. 7d). The edge-to-edge contact of the 2D graphene sheet is transformed into the surface-to-surface contact of the sheet. Figure 7c illustrates the process of contact between fibers in C-GWA. The wrinkle structure on the surface of the fiber comes into contact first in low-pressure stage. As the pressure enhances, the contact area between the fibers is enlarged. Resultly, the resistance of the C-GWA-based pressure sensor remarkably decreases. Pores inside C-GWA and the gap between C-GWA and the electrode are restored to their original size when the external pressure load is released, which cause the discontinuity of the conductive network and the resistance to return to its original value.