Microwave Preparation of Porous Graphene from Wasted Tires and Its Pyrolysis Behavior

Nowadays, most waste tires are disposed of by direct combustion or via landfill, which inevitably creates a large volume of waste. However, the soft carbon found in tires can be an excellent precursor in the preparation of graphene. In this study, waste tires were mixed with potassium hydroxide and heated in a microwave environment to be converted rapidly into porous graphene, a process having no complicated pre-processing. Moreover, the processes, tools, and chemical reagents are distinct and inexpensive. This research innovatively uses the characteristics of microwave rapid and selective heating to study the pyrolysis behavior of waste tires after adding alkali, and realizes the conversion of soft carbon in tires to graphene. At the same time, the time required for the preparation process is greatly reduced which compared with conventional heating methods. Thermogravimetric analysis and dielectric property testing of the tires and mixtures were conducted to study their pyrolysis behavior and catalytic reactions. It was found that when the temperature reached more than 750 °C, potassium ions and carbon underwent a reduction reaction to generate potassium vapor, which catalyzed the carbon in the tires into porous graphene, and that microwave heating could shorten the heating time. Simultaneously, the carbon in the tire could be sheared smaller and thinner during the rapid pyrolysis and volatilization of the gas, benefitting the graphene production. The products of the mixed materials were then characterized and analyzed at different holding temperatures and times. As the holding time increased, the degree of graphitization of the products increased, the number of layers increased slightly, and the impurity elements were completely removed from the products.


Introduction
Millions of tons of used tires are thrown away annually around the world [1], and with the ongoing development of automobiles, this is constantly increasing [2].During tire production, the chemical and mechanical properties of tires are rigorously researched and optimized to withstand most environmental or mechanical stresses [3,4].Consequently, their disposal has become one of the most troublesome problems in the world, because of their huge storage requirements and non-biodegradable characteristics, most waste tires being incinerated or dumped into land lls [5].However, these methods are not particularly economical and cause secondary environmental problems [6].Nonetheless, it has been noticed that tires contain various long-chain organic polymers, which are excellent soft carbon precursors that can be converted into graphite or even graphene at high temperatures [7][8][9].However, the value of activated carbon obtained via the traditional pyrolysis of waste tires is relatively low.Consequently, the question of how to convert low-value activated carbon after pyrolysis into high-value graphite has become topical.Currently, there are few studies on the reuse of waste tires.In one such study, Wang C [10] et al. used one-step strong alkali etching of tires at high temperatures, heating them in a mu e furnace for 200 min to 1000°C, before holding them there for 8 h to obtain 3D porous graphene.Such research is time ine cient, as a short holding time can lead to the collapse of the graphene 3D structure.
Our research is devoted to nding a more e cient, large-scale method to transform tires.
Microwave high-temperature treatment has clean, fast [11], uneven heating [12] characteristics, which can achieve different heating e ciencies based on the different absorption properties of the matter inside the material, temperature gradients inside the material favoring the separation of the complexes.Yuwen C [13] used microwaves to achieve rapid separation of copper foil and graphite in waste batteries.
Compared with conventional high-temperature treatments, the heating process was greatly accelerated after the use of microwaves, the gas in the tire rapidly volatilizing, resulting in the carbon material being sheared smaller and thinner so that during catalytic graphitization the distance between the graphite layers expanded [14].Consequently, the force between the graphite layers was reduced, the rate of waste tire to graphene conversion being accelerated, making it a waste tire treatment method worthy of promotion.
This study is based on the concept of converting waste to a useful resource.Consequently, the pyrolysis behavior of waste tires using strong alkali etching under microwave heating was examined, providing a simple route to and abundant raw material for the large-scale production of graphene and high-value reuse of waste tires.

Preparation of graphene samples
The waste tire powder (particle size 150 mesh) used in the experiment came from Huayi Rubber Co., Ltd., China.The tire powder was mixed with potassium hydroxide (KOH) at a mass ratio of 1: 2. The mixture was then directly heated using a microwave-based heating system to perform a two-stage heating process.The temperature of the rst stage was raised to 150-250°C, and maintained for 5 min to remove any moisture adsorbed by the KOH.After the second stage had been heated to 800-1000°C, the temperature was maintained for several hours.During the entire process, the microwave power was xed, argon gas being introduced as a protective gas.Based on the different holding temperatures and times, four groups of experiments were conducted and four groups of products were obtained, recorded as MT800-3, MT1000-3, MT1000-4, and MT1000-7-that is, in terms of holding temperature (°C)-holding time (h).The black powder obtained in the experiment was thoroughly washed using a dilute hydrochloric acid solution to remove impurities, before being washed with distilled water for neutrality.The obtained product was then ltered, before adding absolute ethanol and ultrasonicating it for several hours, after which it was centrifuged and dried to obtain the nal graphene product.The microwave heating setup used in the experiment is shown in Fig. 1.When adding microwave heating to the process, an exhaust system can be used to extract the gas-that is, the gas generated in the experiment can be timeously extracted via the exhaust system, so as to avoid gas corrosion of the instrument and environmental pollution.

Measurements and characterizations
A thermogravimetric analyzer (STA499, Netzach, China) was used to conduct thermogravimetric analysis.
The phase composition of the samples was examined using an X-ray diffractometer (X'pert 3 Powder, Panaco, New Zealand) with Cu Kα radiation (λ = 1.5406Å) and a scanning rate of 8 °⋅min − 1 at 40 kV, 40mA.The chemical functional groups of the tire and products were determined using an infrared spectrometer (Thermo Scienti c K-Alpha, webinar, USA).Raman spectroscopy analyses were conducted using a continuous wave (CW) argon ion (Ar + ) laser (LabRAM HR Evolution, Horiba, France) with 532 nm laser excitation and a scanning range of 3500-500 cm − 1 .The microstructure was observed using a MIRA SEM (TESCAN, Czech Republic) and Talos F200S (FEI, USA).The pore volume and size distribution of the samples were measured by means of nitrogen adsorption-desorption experiments using a speci c surface area and pore size analyzer (ASAP 2460, Micromeritics Instrument Corp., USA).The dielectric properties were evaluated at 2.45 GHz by means of cavity perturbation using an Agilent E5071C vector network analyzer in single-resonance mode.This method measures the microwave cavity response between a cavity with an empty sample-holder and the same cavity with a sample-holder plus a sample, after which the quality factor and response value of the resonant cavity are inverted by the test system to obtain the dielectric properties of the tested material.The temperature change test system for the dielectric properties is shown in Fig. 2.

Thermogravimetric analysis of materials
Figure 3(a) shows the thermogravimetric/differential scanning calorimetry (TG/DSC) curve of waste tires under a heating rate of 10 K/min in an argon atmosphere, TG being the weight loss curve and DSC being the endothermic curve [15,16].It can be seen from the gure that the weight loss of the waste tires can be divided into three stages.The rst stage is from 0-300°C, when the material does not reach the temperature of gas decomposition; the second stage is from 300-450°C, in which the tire pyrolyzes a high volume of gas [17]; the third stage is from 450-1000°C, at which temperature the residual solid is stable and endothermic.After the pyrolysis process, the residual solid mass ratio is approximately 45%, comprising carbon black and a small volume of ash [18].Figure 3(b) shows the TG/DSC curve of waste tires with KOH at a heating rate of 10 K/min in an argon atmosphere.The weight loss of the material can be divided into four stages.The rst stage of weight loss occurs from 0-200 ℃.Based on the DSC curve, it can be seen that the material has a strong endothermic peak at approximately 100 ℃, which may be the evaporation of water adsorbed by the KOH; the second stage of weight loss occurs from 200-400 ℃, which may be caused by the volatilization of the gas and liquid produced by the tire pyrolysis; the third stage of weight loss occurs from 400-750 ℃, the weight loss in this stage being less-that is, combined with the DSC curve, there are only two endothermic peaks, perhaps because the mixed materials are reacting with each other; the temperature of the fourth stage weight loss occurs from 750-1000°C, the DSC curve exhibiting two endothermic peaks, which may be caused by the catalytic reaction of the mixture-that is, when the temperature rises, the functional groups in the graphite fall off, the quality decreasing during the exothermic process.

Analysis of dielectric properties of materials
Figure 4 shows the change curve of the dielectric properties of the waste tire raw material (Tire) and the mixed material (T + KOH) with temperature; ε′ is the dielectric constant, representing the ability of the material to absorb and store microwaves; ε″ is the loss factor, representing the ability of a substance to convert absorbed microwaves into heat energy; tan δ represents the ratio of the heat energy generated when the material is irradiated by microwaves to the stored microwave energy, and represents the e ciency of the substance in converting the absorbed microwave energy into heat energy [19].To a certain extent, the change of the dielectric properties re ects the pyrolysis behavior of the materials at high temperatures.
As shown in Fig. 4(a), the dielectric constant of the raw material is small and essentially unchanged from 0-300°C, indicating that the tire itself has poor absorption performance-that is, it shows that there are substances with better absorbing properties in the materials, which may be the result of the gradual pyrolysis and separation of carbon in the tires.The pyrolysis of raw materials can be divided into three stages, as shown in Fig. 4(b) and 4(c)-that is, 0-300 ℃ is the heating stage, the loss factor and loss tangent of the raw materials being essentially unchanged; 300-450 ℃ is the generation of pyrolysis gas stage, and the loss factor, the loss tangent increasing during this stage; 450-1000 ℃, the carbonaceous material is gradually generated by pyrolysis, its dielectric constant increasing with increasing temperature, the loss factor remaining unchanged, and the loss tangent gradually decreasing.From 0-200 ℃, the dielectric constant of the mixed material is essentially unchanged, the loss factor and loss tangent increasing, which may be due to the evaporation of water absorbed by the KOH; from 200-750 ℃, the mixed material exhibits three peaks in its dielectric properties, which may be caused by the constant compositional change of the material due to the gas being generated by pyrolysis; after the temperature is raised to 750°C, the dielectric properties of the material remain essentially unchanged, and it can be considered that the carbonaceous matter has been completely pyrolyzed out.This test result is consistent with the thermogravimetric test-that is, that the catalytic graphitization reaction only proceeds above 750 ℃.

Graphene formation under microwave
A schematic of the formation of the graphene structure from waste tires is shown in Fig. 5.The tire and KOH are rapidly heated in a microwave environment, the gas generated in the tire producing a strong force when it rapidly volatilizes, so that the bulk carbon is sheared into a large number of small pieces ( akes), increasing the graphite layer spacing and decreasing the graphite layers.
Above 750°C, the KOH reacts with carbon to generate a large number of active potassium atoms with strong reducibility.Under the action of the potassium atoms, the surface functional groups of carbon are removed-that is, the chemical bonds between carbon atoms and these functional groups are broken, a large number of dangling bonds being formed around the carbon.At high temperatures, the carbon dangling bonds repair each other, which also favors the random motion of K atoms (the formation of K vapor).These highly active K atoms (steam mechanisms) strongly promote carbon rearrangement when the pyrolysis time is gradually increased [20][21][22][23][24]. Consequently, the synergy of tires with active K atoms in a microwave environment converts thin carbon sheets into porous graphene.The reaction equation can be expressed as follows: 6KOH + 2C→2K + 3H 2 + 2K 2 CO 3 (1)

X-ray diffraction (XRD) and Raman analysis of raw materials and products
Figure 6(a) shows the X-ray diffraction pattern of the Tire, MT800-3, MT1000-3, MT1000-4, and MT1000-7 samples.It can be seen from the gure that the raw materials have peaks around 20°, which may be due to the soft carbon caused by the carbon-carbon peak shift; the four groups of product characterizations all exhibit obvious convex peaks around 26.5°, which is the characteristic peak of graphene 002, there also being peaks on its 100 and 101 planes.
The interlayer spacing of graphene can be calculated based on the Bragg equation 2dsinθ = nλ [25].The interlayer spacings of the MT800-3, MT1000-3, MT1000-4, and MT1000-7 samples are 0.332, 0.338, 0.343, and 0.342 nm, respectively.The intensity of their peaks increases with increasing holding time, the interlayer spacing also exhibiting the same pattern, but after the holding time exceeds 4 h, the MT1000-7 exhibits a slight decrease in the interlayer spacing; moreover, the holding temperature and the peak intensity also exhibit a comparative trend.The magnitude of the peak intensity re ects the degree of graphitization of the product to a certain extent [26].Figure 6(a) shows that the longer the holding time, the higher the holding temperature, the higher the degree of graphitization of the product, and the larger the graphene layer spacing of the product.
Raman spectroscopy is an important means to detect the number of graphite layers and the degree of graphitization [27].Graphene generally exhibits D peaks, G peaks, and 2D peaks in Raman analyses.The D peak is generally around 1350 cm − 1 , which represents the degree of disorder of the graphite-that is, the higher the D peak, the more defects in the graphite.The G peak is a typical graphite peak, usually around 1580 cm − 1 , which characterizes the strength and size of the graphite-that is, the G peak is an important peak that distinguishes graphite from graphene [28][29][30][31][32].
Figure 6(b) shows the results of Raman spectroscopy analyses of the Tire, MT800-3, MT1000-3, MT1000-4, and MT1000-7 samples.The raw material detection shows D and G peaks, indicating that the tire contains carbon; its 2D peak is 0, indicating that the carbon in the tire is not graphene.In the analyses of the four product groups, D, G, and 2D peaks appear, indicating that the products form graphene.The ID/IG ratio is usually used to characterize the degree of graphitization, and I represents the intensity [33][34][35][36].The lower the value, the higher the degree of graphitization.The ID/IG of the MT800-3, MT1000-3, MT1000-4, and MT1000-7 samples can be obtained by calculation, the values being 0.39, 0.47, 0.31, and 0.33, respectively; the I2D/IG ratio can be used to characterize the number of layers of graphene [37]-that is, the larger the value, the lower the number of layers.The I2D/IG of the MT800-3, MT1000-3, MT1000-4, and MT1000-7 samples can once again be obtained by calculation, the values being 0.47, 0.62, 0.54, and 0.66, respectively.Compared to the other three groups of products, MT800-3 exhibits the largest ID/IG value and the smallest I2D/IG value, while the G peak intensity is low, indicating that the degree of graphitization at 800°C is low, the graphene production being less.The MT1000-3, MT1000-4, and MT1000-7 samples exhibit a high degree of graphitization, and with increasing holding time, the D peak exhibits a decreasing trend-that is, the defects decrease.It may be that thermal insulation during the manufacturing process facilitates the separation of oxygen atoms in the tire.However, the I2D/IG values of the MT1000-3, MT1000-4, and MT1000-7 samples exhibit no obvious regularity, though the I2D/IG of the MT1000-4 is lower than that of the MT1000-3 and MT1000-7 samples.

X-ray photoelectron spectroscopy (XPS) of raw materials and products
The X-ray photoelectron spectroscopy analyses of the Tire, MT800-3, MT1000-3, MT1000-4, and MT1000-7 samples are shown in Fig. 7.This test focuses on the residual content of C and O elements and the migration of elements in the tire.XPS can characterize the element valence in graphene and can be used as an auxiliary means to determine the content of oxygen defects [38][39][40][41].
It can be seen from Fig. 7 that the XPS analysis of the raw tire contains only O and C elements, while the MT800-3 sample contains large amounts of O and C elements, as well as a small amount of F, Al, Si, Ca, Mg, and K and other impurities; as the holding temperature reaches 1000°C, as shown in the MT1000-3 sample, the F element disappears, the contents of elements such as Al, Si, Ca, Mg, and K decrease, the content of O decreases, and the proportion of C increases; when the time increases to 4 h or even 7 h, all the impurities disappear.
Throughout the four product groups, the content of O decreases with increasing holding temperature and holding time, with C increasing accordingly.This may be because the oxides of these minor elements and the oxygen produced by the reaction are not separated and volatilized at low temperature.In a hightemperature environment, these impurities are gradually separated and volatilized, the content of O also being reduced.Moreover, the carbon in the tire is continuously reduced, so the content of C increases accordingly.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of graphene
Figure 8 shows the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of the MT1000-3, MT1000-4, and MT1000-7 samples after dispersion in ethanol, TEM being a typical means to characterize the number of graphite layers and its stacking [42].
It can be seen from Fig. 8 that 5-10 layers of multi-layer graphene appear in MT1000-3, the graphene being well dispersed and presenting organized straight lines; most of the graphene layers in MT1000-4 are 3-6 layers, but the graphite stacking is disordered; much the same as MT1000-4, MT1000-7 is irregularly stacked, but the number of graphene layers has increased compared to MT1000-4, which may be the aggregation of graphene caused by too long a holding time.The three groups of products after strong alkali etching exhibit a porous structure under SEM analyses.We believe that the formation of the porous structure is the joint effect of the KOH etching and the gas generated by the tire pyrolysis.MT1000-3 exhibits some regular shapes; MT1000-4 and MT1000-7 exhibit disorderly stacking, which is consistent with the TEM analyses.

Adsorption study of porous graphene
Figure 9(a) shows the adsorption and desorption curve of MT1000-4, the pore volume being 0.825115 cm³/g; Fig. 9(b) shows the pore size distribution curve of MT1000-4, the pore size of most of its pores being between 0-10 nm, the average pore size being 6.0290 nm, which belongs to mesoporous materials, and the speci c surface area being 491.4570 m²/g.The existence of pores in porous graphene products can promote the improvement of material transport e ciency, and more importantly, the introduction of pores can also effectively open the energy band gap of graphene, which can promote the application of graphene in the eld of supercapacitors [43][44][45][46].

4.
In summary, a green and cost effective means of strong base-assisted catalytic graphitization as well as the effect of heat treatment under microwaves, was examined.The Thermal Behavior Study of Raw Materials and Mixtures showed that the catalytic reaction between waste tire powder and (strong alkali) KOH begins at 750°C.The mixture was kept at 1000°C for several hours before being ultrasonically washed to obtain graphene with a high degree of graphitization as demonstrated by the XRD patterns and Raman spectra obtained.It was established that with the holding time increasing, the graphene surface functional group exfoliated, as shown in its X-ray photoelectron spectroscopy analyses.The SEM and TEM results revealed graphene stacking and its layers.Moreover, if the holding time was too long, the agglomeration between graphene was not easy to peel off, and the number of layers was relatively large.BET analysis revealed the MT1000-4's pore volume to be 0.825115 cm³/g, the average pore size 6.0290 nm, and the speci c surface area 491.4570 m²/g.