Strong Tribocatalytic Nitrogen Fixation of Graphite Carbon Nitride g-C3N4 through Harvesting Friction Energy

Mechanical energy derived from friction is a kind of clean energy which is ubiquitous in nature. In this research, two-dimensional graphite carbon nitride (g-C3N4) is successfully applied to the conversion of nitrogen (N2) fixation through collecting the mechanical energy generated from the friction between a g-C3N4 catalyst and a stirring rod. At the stirring speed of 1000 r/min, the tribocatalytic ammonia radical (NH4+) generation rate of g-C3N4 can achieve 100.56 μmol·L−1·g−1·h−1 using methanol as a positive charge scavenger, which is 3.91 times higher than that without any scavengers. Meanwhile, ammonia is not generated without a catalyst or contact between the g-C3N4 catalyst and the stirring rod. The tribocatalytic effect originates from the friction between the g-C3N4 catalyst and the stirring rod which results in the charges transfer crossing the contact interface, then the positive and negative charges remain on the catalyst and the stirring rod respectively, which can further react with the substance dissolved in the reaction solution to achieve the conversion of N2 to ammonia. The effects of number and stirring speed of the rods on the performance of g-C3N4 tribocatalytic N2 fixation are further investigated. This excellent and efficient tribocatalysis can provide a potential avenue towards harvesting the mechanical energy in a natural environment.


Introduction
Nowadays, due to the immoderate consumption and mining of fossil fuels, energy shortage and environment pollution have become critical issues which are a threat to the survival and development of society [1]. Accordingly, it is necessary to look for renewable and green energy to replace these fossil fuels. Ammonia has been regarded as a green energy source with some advantages such as no carbon dioxide emission, high energy density and convenient transportation [2,3]. Nevertheless, how to perform ammonia production is also a vital issue. At present, various methods have been reported to convert nitrogen (N 2 ) to ammonia (NH 3 ), such as thermocatalytic reduction, electrocatalytic reduction and photocatalytic reduction [4][5][6]. However, thermocatalytic and electrocatalytic reduction usually require high-pressure or high-temperature operating conditions, which limit the actual application of ammonia production [7,8]. Additionally, photocatalytic reduction of nitrogen can be performed at a mild condition, but low light utilization efficiency would seriously hinder the actual production of ammonia [9]. Therefore, it is crucial to seek mild and highly efficient approaches for nitrogen reduction.
In nature, mechanical energy is widespread distributed energy, which exists in wind, river flows and human movement [10]. If such abundant energy can be successfully har-vested for the reduction of nitrogen, it would be meaningful to propel the production of ammonia. Tribocatalysis, which can convert the friction of mechanical energy into the electric energy via persistent friction, has attracted the attention of researchers [11][12][13]. Under friction, when two different kinds of materials contact each other, chemical bonds will form through physical contact on the interface [14]. After being separated, two materials will carry the positive or negative charges respectively, due to breakage of the chemical bonds [15]. Then the free charges generated via the triboelectric process can be further applied to the catalytic reaction such as dye degradation, carbon dioxide or flammable gas production [16][17][18][19][20]. However, to date, there has been no report about harvesting the mechanical energy from friction via tribocatalysis for nitrogen reduction to produce ammonia.
Graphite carbon nitride (g-C 3 N 4 ) is an emerging semiconductor material with layered structure similar to graphite [21]. The C and N atoms inside it are arranged alternately through sp 2 hybridization [22]. g-C 3 N 4 is a candidate catalyst in several catalytic areas such as the decomposition of organic pollutants [23][24][25], hydrogen evolution [26,27] and carbon dioxide reduction [28][29][30] based on its advantages of being insoluble in water and having a large specific surface area and stable chemical properties. In 2019, Xia et al. achieved the photocatalytic synthesis of ammonia from nitrogen through using a defecting g-C 3 N 4 catalyst. After 100 min light irradiation, the yield of ammonia can be up to 54 µmol/L [31]. Dong et al. achieved highly efficient dichlorophenols decomposition via photocatalysis using g-C 3 N 4 [23]. Recent studies have found that mesoporous carbon nitride materials can exhibit excellent catalytic activity through introducing metal atoms inside them. Gianvito Vilé et al. have reported that Cu-based single-atom catalysts developed on a mesoporous carbon nitride carrier exhibited excellent catalytic activity during the synthesis of triazoles [32]. Liu et al. found that the photocatalytic decomposition activity of gemfibrozil can be significantly improved by introducing Ag or Cu atoms into the mesoporous carbon nitride skeleton, which is related to ligand-to-metal charge transfer (LMCT) or ligand-tometal-to-ligand charge transfer (LMLCT) [33]. Based on the above analysis, g-C 3 N 4 is hopeful for applications in highly efficient tribocatalytic nitrogen fixation, which is not reported at present.
In this work, the excellent tribocatalytic reduction of nitrogen to ammonia under stirring is achieved in g-C 3 N 4 which is fabricated via the chemical blowing method. After 10 h stirring at room temperature in the dark, the tribocatalytic ammonia radical (NH 4 + ) generation rate is up to about 100.56 µmol·L −1 ·g −1 ·h −1 using a positive charge scavenger (methanol), which is 3.91 times higher than that without any charge scavengers. In addition, the effects of a negative charge scavenger, the number and speed of stirring rods, and the contact area between the catalyst and stirring rods in tribocatalysis on the performance of g-C 3 N 4 tribocatalytic N 2 fixation are also investigated. The possible tribocatalytic mechanism of N 2 fixation has been also proposed. As the N 2 fixation research continues to thrive and expand, the finding in this work provides a great potential application to harvest the mechanical energy via tribocatalysis for clean energy production.

Materials and Methods
2.1. Preparation of g-C 3 N 4 Sample g-C 3 N 4 used in this research was prepared according to the reported chemical blowing method [34,35]. A certain amount of ammonium chloride (16 g) and melamine (4 g) were accurately weighed and then mixed together. The mixture was thoroughly ground in a mortar and placed in a crucible. Then, it was stuffed into the muffle furnace, heated from room temperature to 550 • C (6 • C/min), and the calcination time was set to 4 h. After the calcination, the faint yellow product in the crucible was collected and ground through an agate mortar to obtain g-C 3 N 4 .

Characterization
The X-ray diffraction pattern of the g-C 3 N 4 sample was tested on a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany). The micro morphology of the sample was examined with a scanning electron microscope (Gemini SEM 300, ZEISS, Oberkochen, Germany). The chemical properties of the sample were analyzed through using the X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Waltham, MA, USA). The infrared spectra of the sample prepared through the KBr tablet pressing method were characterized via the Fourier transform infrared spectrometer (FTIR, Nicolet NEXUS 670, Ramsey, MN, USA). The content of ammonia (NH 4 + ) was analyzed via a UV-Vis spectrophotometer (Ocean Optics QE65Pro, Dunedin, FL, USA).

Tribocatalytic Performance Measurements
To investigate g-C 3 N 4 tribocatalytic performance, N 2 fixation experiments were performed. A total of 50 mL methanol solution (containing 5 mL methanol and 45 mL DI water) mixed with 50 mg g-C 3 N 4 was contained in a brown bottle. The solution was then placed under shading conditions and stirred at 1000 rpm for 2 h through a polytetrafluoroethylene (PTFE) stirring rod with a specification of ∅ 6 × 20 mm 2 to achieve adsorption-desorption equilibrium. Then under continuous stirring, the suspension of 3 mL was collected through a rubber-tipped dropper every 2 h of stirring. The supernatan was obtained through a centrifuge (3 min, 6500 rpm). The generation of NH 4 + produced in the process of tribocatalysis was determined through the colorimetric method, and Nessler reagent was selected as the indicator [36,37]. Then 40 µL sodium tartrate solution (0.5 g·mL −1 ) and 60 µL Nessler's reagent were added dropwise into supernatant and left to stand for 12 min to react sufficiently. Finally, the content of ammonium was analyzed at the peak of~420 nm via a UV-Vis spectrophotometer.

Results and Discussion
SEM images of the g-C 3 N 4 sample before the tribocatalytic N 2 fixation reaction are depicted in Figure 1. g-C 3 N 4 samples show the agglomerate morphology composed of many irregular ultra-thin two-dimensional sheet-like structures. From Figure 1, g-C 3 N 4 catalyst material composed of many huge flakes has a large specific surface area.

Characterization
The X-ray diffraction pattern of the g-C3N4 sample was tested on a D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany). The micro morphology of the sample was examined with a scanning electron microscope (Gemini SEM 300, ZEISS, Oberkochen, Germany). The chemical properties of the sample were analyzed through using the X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Waltham, MA, USA). The infrared spectra of the sample prepared through the KBr tablet pressing method were characterized via the Fourier transform infrared spectrometer (FTIR, Nicolet NEXUS 670, Ramsey, MN, USA). The content of ammonia (NH4 + ) was analyzed via a UV-Vis spectrophotometer (Ocean Optics QE65Pro, Dunedin, FL, USA).

Tribocatalytic Performance Measurements
To investigate g-C3N4 tribocatalytic performance, N2 fixation experiments were performed. A total of 50 mL methanol solution (containing 5 mL methanol and 45 mL DI water) mixed with 50 mg g-C3N4 was contained in a brown bottle. The solution was then placed under shading conditions and stirred at 1000 rpm for 2 h through a polytetrafluoroethylene (PTFE) stirring rod with a specification of ∅ 6 × 20 mm 2 to achieve adsorptiondesorption equilibrium. Then under continuous stirring, the suspension of 3 mL was collected through a rubber-tipped dropper every 2 h of stirring. The supernatan was obtained through a centrifuge (3 min, 6500 rpm). The generation of NH4 + produced in the process of tribocatalysis was determined through the colorimetric method, and Nessler reagent was selected as the indicator [36,37]. Then 40 μL sodium tartrate solution (0.5 g·mL −1 ) and 60 μL Nessler's reagent were added dropwise into supernatant and left to stand for 12 min to react sufficiently. Finally, the content of ammonium was analyzed at the peak of ~420 nm via a UV-Vis spectrophotometer.

Results and Discussion
SEM images of the g-C3N4 sample before the tribocatalytic N2 fixation reaction are depicted in Figure 1. g-C3N4 samples show the agglomerate morphology composed of many irregular ultra-thin two-dimensional sheet-like structures. From Figure 1, g-C3N4 catalyst material composed of many huge flakes has a large specific surface area. The crystal diffraction patterns of g-C3N4 have been measured with XRD, as shown in Figure 2. The obvious peaks at 2θ value of 12.93° and 27.69° are corresponding to the (100) and (002) crystal plane of g-C3N4 through referring the standard card PDF#87-1526. It is ascribed to the orderly stacking of the conjugated carbon-nitrogen heterocycles in a planar and layered framework, respectively [38,39]. In addition, the high-intensity diffraction peaks and the absence of other impurity peaks indict the excellent synthesis of g-C3N4. The crystal diffraction patterns of g-C 3 N 4 have been measured with XRD, as shown in Figure 2. The obvious peaks at 2θ value of 12.93 • and 27.69 • are corresponding to the (100) and (002) crystal plane of g-C 3 N 4 through referring the standard card PDF#87-1526. It is ascribed to the orderly stacking of the conjugated carbon-nitrogen heterocycles in a planar and layered framework, respectively [38,39]. In addition, the high-intensity diffraction peaks and the absence of other impurity peaks indict the excellent synthesis of g-C 3 N 4 . FTIR spectra depicted in Figure 3 reveal the functional groups of g-C3N4 before the tribocatalytic N2 fixation. The peak around the wave number of 812 cm −1 is caused by the out-of-plane bending vibration of the triazine structure [40,41]. The peaks in the wave number range of 1240-1640 cm −1 are related to the stretching vibration mode of C-N heterocycle in the g-C3N4 sample [23]. The broad peaks in the range of 3080-3450 cm −1 are ascribed to the stretching vibrations of N−H and O−H groups [39,42]. The elemental states of the g-C3N4 sample have been performed with XPS measurement, as depicted in Figure 4. From Figure 4a, the measured survey spectrum confirms the inclusion of both C and N elements in g-C3N4. The high-resolution XPS spectrum of C 1s are presented in Figure 4b. The spectrum of C 1s has two peaks at 284.42 eV and 287.22 eV, which are assign to graphitic carbon adsorbed on the sample surface and sp 2 -bonded carbon in the triazine structure [39]. As shown in Figure 4c, the N 1s spectrum of g-C3N4 was deconvoluted into three peaks at binding energies of 397.72 eV, 398.87 eV and 400.07 eV, corresponding to C=N−C, N−(C)3 and N−H in the sample, respectively [38,43]. FTIR spectra depicted in Figure 3 reveal the functional groups of g-C 3 N 4 before the tribocatalytic N 2 fixation. The peak around the wave number of 812 cm −1 is caused by the out-of-plane bending vibration of the triazine structure [40,41]. The peaks in the wave number range of 1240-1640 cm −1 are related to the stretching vibration mode of C-N heterocycle in the g-C 3 N 4 sample [23]. The broad peaks in the range of 3080-3450 cm −1 are ascribed to the stretching vibrations of N−H and O−H groups [39,42]. FTIR spectra depicted in Figure 3 reveal the functional groups of g-C3N4 before the tribocatalytic N2 fixation. The peak around the wave number of 812 cm −1 is caused by the out-of-plane bending vibration of the triazine structure [40,41]. The peaks in the wave number range of 1240-1640 cm −1 are related to the stretching vibration mode of C-N heterocycle in the g-C3N4 sample [23]. The broad peaks in the range of 3080-3450 cm −1 are ascribed to the stretching vibrations of N−H and O−H groups [39,42]. The elemental states of the g-C3N4 sample have been performed with XPS measurement, as depicted in Figure 4. From Figure 4a, the measured survey spectrum confirms the inclusion of both C and N elements in g-C3N4. The high-resolution XPS spectrum of C 1s are presented in Figure 4b. The spectrum of C 1s has two peaks at 284.42 eV and 287.22 eV, which are assign to graphitic carbon adsorbed on the sample surface and sp 2 -bonded carbon in the triazine structure [39]. As shown in Figure 4c, the N 1s spectrum of g-C3N4 was deconvoluted into three peaks at binding energies of 397.72 eV, 398.87 eV and 400.07 eV, corresponding to C=N−C, N−(C)3 and N−H in the sample, respectively [38,43]. The elemental states of the g-C 3 N 4 sample have been performed with XPS measurement, as depicted in Figure 4. From Figure 4a, the measured survey spectrum confirms the inclusion of both C and N elements in g-C 3 N 4 . The high-resolution XPS spectrum of C 1s are presented in Figure 4b. The spectrum of C 1s has two peaks at 284.42 eV and 287.22 eV, which are assign to graphitic carbon adsorbed on the sample surface and sp 2 -bonded carbon in the triazine structure [39]. As shown in Figure 4c, the N 1s spectrum of g-C 3 N 4 was deconvoluted into three peaks at binding energies of 397.72 eV, 398.87 eV and 400.07 eV, corresponding to C=N−C, N−(C) 3 and N−H in the sample, respectively [38,43]. Nanomaterials 2022, 12, x FOR PEER REVIEW 5 of 11 To investigate the tribocatalytic activity of the g-C3N4 sample, N2 fixation experiments were performed under stirring. Figure 5 depicts the tribocatalytic N2 fixation performance with the different scavengers. When methanol is added as the positive charge scavenger, with the increase of the stirring time, the NH4 + content increases linearly [44]. The generation rate of NH4 + can reach 100.56 μmol·L −1 ·g −1 ·h −1 after 10 h stirring. It is 3.91 times higher than that without any scavengers. Since the methanol can consume the positive charges, the generation of the negative charges is promoted, which can enhance the tribocatalytic N2 fixation performance effectively. It is worth noting that the reductive active radicals, such as the negative charges, are necessary in the N2 fixation reaction [45]. TBA can react with hydroxyl radicals (OH) in solution as the radical scavenger [46,47]. Since the formation of ·OH requires the participation of positive charge (q + ) and hydroxide ion (OH − ), the addition of TBA promotes the consumption of q+ and effectively prolongs the lifetime of negative charge (q − ). Therefore, the addition of TBA is beneficial to the tribocatalytic nitrogen fixation of g-C3N4. Consequently, KBrO3 as the negative charge scavenger is used to investigate the important role of the negative charges in this catalytic process [48]. It can be observed that the NH4 + generation rate is about 0.26 μmol·L −1 ·g −1 ·h −1 . It can be concluded that the addition of KBrO3 is not beneficial for the N2 fixation reaction, which accords with the theoretic expectation.  To investigate the tribocatalytic activity of the g-C 3 N 4 sample, N 2 fixation experiments were performed under stirring. Figure 5 depicts the tribocatalytic N 2 fixation performance with the different scavengers. When methanol is added as the positive charge scavenger, with the increase of the stirring time, the NH 4 + content increases linearly [44]. The generation rate of NH 4 + can reach 100.56 µmol·L −1 ·g −1 ·h −1 after 10 h stirring. It is 3.91 times higher than that without any scavengers. Since the methanol can consume the positive charges, the generation of the negative charges is promoted, which can enhance the tribocatalytic N 2 fixation performance effectively. It is worth noting that the reductive active radicals, such as the negative charges, are necessary in the N 2 fixation reaction [45]. TBA can react with hydroxyl radicals (OH) in solution as the radical scavenger [46,47]. Since the formation of ·OH requires the participation of positive charge (q + ) and hydroxide ion (OH − ), the addition of TBA promotes the consumption of q+ and effectively prolongs the lifetime of negative charge (q − ). Therefore, the addition of TBA is beneficial to the tribocatalytic nitrogen fixation of g-C 3 N 4 . Consequently, KBrO 3 as the negative charge scavenger is used to investigate the important role of the negative charges in this catalytic process [48]. It can be observed that the NH 4 + generation rate is about 0.26 µmol·L −1 ·g −1 ·h −1 . It can be concluded that the addition of KBrO 3 is not beneficial for the N 2 fixation reaction, which accords with the theoretic expectation.  To investigate the tribocatalytic activity of the g-C3N4 sample, N2 fixation experiments were performed under stirring. Figure 5 depicts the tribocatalytic N2 fixation performance with the different scavengers. When methanol is added as the positive charge scavenger, with the increase of the stirring time, the NH4 + content increases linearly [44]. The generation rate of NH4 + can reach 100.56 μmol·L −1 ·g −1 ·h −1 after 10 h stirring. It is 3.91 times higher than that without any scavengers. Since the methanol can consume the positive charges, the generation of the negative charges is promoted, which can enhance the tribocatalytic N2 fixation performance effectively. It is worth noting that the reductive active radicals, such as the negative charges, are necessary in the N2 fixation reaction [45]. TBA can react with hydroxyl radicals (OH) in solution as the radical scavenger [46,47]. Since the formation of ·OH requires the participation of positive charge (q + ) and hydroxide ion (OH − ), the addition of TBA promotes the consumption of q+ and effectively prolongs the lifetime of negative charge (q − ). Therefore, the addition of TBA is beneficial to the tribocatalytic nitrogen fixation of g-C3N4. Consequently, KBrO3 as the negative charge scavenger is used to investigate the important role of the negative charges in this catalytic process [48]. It can be observed that the NH4 + generation rate is about 0.26 μmol·L −1 ·g −1 ·h −1 . It can be concluded that the addition of KBrO3 is not beneficial for the N2 fixation reaction, which accords with the theoretic expectation.   To further investigate the schematic mechanism of the tribocatalytic N 2 fixation reaction, the control experiments under the different addition have been performed as shown in Figure 6. There is scarcely any generation of NH 4 + without a catalyst, which indicates that the addition of a catalyst is essential for the tribocatalytic N 2 fixation reaction. Additionally, the tribocatalytic performance associates with the contact area between the g-C 3 N 4 catalyst and the stirring rod [49]. Therefore, control experiments with the different stirring rods were carried out. Rod I is the commercial PTFE-sealed rod. Two polyvinyl chloride (PVC) electrical tape rings with a width of 1mm were wound on the stirring rod to avoid contact between the catalyst and stirring rod as far as possible, and the stirring rod is named rod II. Obviously, with the decrease of the contact area, the NH 4 + generation rate reduces gradually, indicating that the contact area is an important influencing factor of the tribocatalytic performance. To further investigate the schematic mechanism of the tribocatalytic N2 fixation reaction, the control experiments under the different addition have been performed as shown in Figure 6. There is scarcely any generation of NH4 + without a catalyst, which indicates that the addition of a catalyst is essential for the tribocatalytic N2 fixation reaction. Additionally, the tribocatalytic performance associates with the contact area between the g-C3N4 catalyst and the stirring rod [49]. Therefore, control experiments with the different stirring rods were carried out. Rod I is the commercial PTFE-sealed rod. Two polyvinyl chloride (PVC) electrical tape rings with a width of 1mm were wound on the stirring rod to avoid contact between the catalyst and stirring rod as far as possible, and the stirring rod is named rod II. Obviously, with the decrease of the contact area, the NH4 + generation rate reduces gradually, indicating that the contact area is an important influencing factor of the tribocatalytic performance. The influence of the stirring speed on the tribocatalytic performance was also considered. The much faster stirring speed provides much more contact times per minute, that is to say, the contact area is also enlarged relatively per minute. Consequently, the tribocatalytic performance would be enhanced. As depicted in Figure 7, the NH4 + generation rate is improved significantly from 15.63 to 100.56 μmol·L −1 ·g −1 ·h −1 with the stirring speed increasing from 400 to 1000 rpm, and a linear relationship between g-C3N4 tribocatalytic nitrogen fixation rate and stirring speed is observed. As the stirring speed increases, the friction frequency increases, and more active substances can be produced to participate in the nitrogen fixation process, so an efficient nitrogen fixation can be obtained.  The influence of the stirring speed on the tribocatalytic performance was also considered. The much faster stirring speed provides much more contact times per minute, that is to say, the contact area is also enlarged relatively per minute. Consequently, the tribocatalytic performance would be enhanced. As depicted in Figure 7, the NH 4 + generation rate is improved significantly from 15.63 to 100.56 µmol·L −1 ·g −1 ·h −1 with the stirring speed increasing from 400 to 1000 rpm, and a linear relationship between g-C 3 N 4 tribocatalytic nitrogen fixation rate and stirring speed is observed. As the stirring speed increases, the friction frequency increases, and more active substances can be produced to participate in the nitrogen fixation process, so an efficient nitrogen fixation can be obtained. To further investigate the schematic mechanism of the tribocatalytic N2 fixation reaction, the control experiments under the different addition have been performed as shown in Figure 6. There is scarcely any generation of NH4 + without a catalyst, which indicates that the addition of a catalyst is essential for the tribocatalytic N2 fixation reaction. Additionally, the tribocatalytic performance associates with the contact area between the g-C3N4 catalyst and the stirring rod [49]. Therefore, control experiments with the different stirring rods were carried out. Rod I is the commercial PTFE-sealed rod. Two polyvinyl chloride (PVC) electrical tape rings with a width of 1mm were wound on the stirring rod to avoid contact between the catalyst and stirring rod as far as possible, and the stirring rod is named rod II. Obviously, with the decrease of the contact area, the NH4 + generation rate reduces gradually, indicating that the contact area is an important influencing factor of the tribocatalytic performance. The influence of the stirring speed on the tribocatalytic performance was also considered. The much faster stirring speed provides much more contact times per minute, that is to say, the contact area is also enlarged relatively per minute. Consequently, the tribocatalytic performance would be enhanced. As depicted in Figure 7, the NH4 + generation rate is improved significantly from 15.63 to 100.56 μmol·L −1 ·g −1 ·h −1 with the stirring speed increasing from 400 to 1000 rpm, and a linear relationship between g-C3N4 tribocatalytic nitrogen fixation rate and stirring speed is observed. As the stirring speed increases, the friction frequency increases, and more active substances can be produced to participate in the nitrogen fixation process, so an efficient nitrogen fixation can be obtained.  The effect of contact area on the performance of g-C 3 N 4 tribocatalytic N 2 fixation is investigated through adjusting the number of stirring rods. Figure 8 shows the nitrogen fixation effect with a different number of stirring rods. Obviously, the total contact area is strongly enlarged as the number of the stirring rods increases. As expected, the NH 4 + generation rate is about 244.02 µmol·L −1 ·g −1 ·h −1 using three stirring rods, which is 2.43 times that of only one stirring rod. Typically, the contact area is usually proportional to the number of rods, but the NH 4 + generation rate is not. For instance, the distribution of the catalyst is not uniform in suspension, which leads to inadequate contact between the rods and catalyst, that is to say, each stirring time may not necessarily induce the tribocatalytic reaction. Perhaps there are other influencing factors which influence the tribocatalytic performance. Consequently, the NH 4 + generation rate is not in linear correlation with the number of stirring rods. The effect of contact area on the performance of g-C3N4 tribocatalytic N2 fixation is investigated through adjusting the number of stirring rods. Figure 8 shows the nitrogen fixation effect with a different number of stirring rods. Obviously, the total contact area is strongly enlarged as the number of the stirring rods increases. As expected, the NH4 + generation rate is about 244.02 μmol·L −1 ·g −1 ·h −1 using three stirring rods, which is 2.43 times that of only one stirring rod. Typically, the contact area is usually proportional to the number of rods, but the NH4 + generation rate is not. For instance, the distribution of the catalyst is not uniform in suspension, which leads to inadequate contact between the rods and catalyst, that is to say, each stirring time may not necessarily induce the tribocatalytic reaction. Perhaps there are other influencing factors which influence the tribocatalytic performance. Consequently, the NH4 + generation rate is not in linear correlation with the number of stirring rods. Furthermore, the tribocatalytic nitrogen fixation performance of g-C3N4 was evaluated through comparison with other existing nitrogen fixation research. It can be seen from Table 1 that g-C3N4 can realize nitrogen fixation through both photocatalysis and tribocatalysis, and it has an excellent performance [50]. In addition, due to its large specific surface area, g-C3N4 has superior nitrogen fixation activity compared to other materials [51][52][53][54][55]. According to the previous analysis, the tribocatalytic mechanism of g-C3N4 is drawn in Figure 9. The friction between the g-C3N4 catalyst and the stirring rod is realized via mechanical stirring, which is accompanied by charge transfer. It is known from the empirical rule of triboelectrification, the g-C3N4 catalyst is more likely to lose electrons with positive charge than the stirring rod. Therefore, in the process of tribocatalysis, the stirring Furthermore, the tribocatalytic nitrogen fixation performance of g-C 3 N 4 was evaluated through comparison with other existing nitrogen fixation research. It can be seen from Table 1 that g-C 3 N 4 can realize nitrogen fixation through both photocatalysis and tribocatalysis, and it has an excellent performance [50]. In addition, due to its large specific surface area, g-C 3 N 4 has superior nitrogen fixation activity compared to other materials [51][52][53][54][55].  [55] According to the previous analysis, the tribocatalytic mechanism of g-C 3 N 4 is drawn in Figure 9. The friction between the g-C 3 N 4 catalyst and the stirring rod is realized via mechanical stirring, which is accompanied by charge transfer. It is known from the empirical rule of triboelectrification, the g-C 3 N 4 catalyst is more likely to lose electrons with positive charge than the stirring rod. Therefore, in the process of tribocatalysis, the Nanomaterials 2022, 12, 1981 8 of 11 stirring rod is negatively charged and the catalyst is positively charged. The above process can be represented by Equation (1) [56]: rod is negatively charged and the catalyst is positively charged. The above process can be represented by Equation (1) [56]: g C N stirring rod ⎯⎯⎯⎯ g C N q stirring rod q Then methanol as the positive charge scavenger will react with q+ to avoid NH4 + being oxidized [37]. In addition, q − will react with N2 to produce NH4 + , which is dissolved in DI water to form NH4 + . The main reaction process is described in Equations (2)-(4) [57,58]:

NH H O → NH • H O ⇌ NH OH
(4) Figure 9. The schematic diagram for the tribocatalytic mechanism of g-C3N4.
In this work, g-C3N4 exhibits excellent tribocatalytic performance in the N2 fixation process. Though, as a matter of fact, the applications of the tribocatalysis are not restricted in this field. In the past few years, He et al. have achieved excellent tribocatalytic performance on removal of the heavy metal ion Cr 6+ through using commercial iron turning with amorphous iron oxides. After 36 h stirring, the removal ratio of Cr 6+ can reach about 98% [59]. Additionally, Li et al. have successfully produced flammable gases such as CO, CH4 and H2 through harvesting mechanical energy with TiO2 nanoparticles [20]. In the future, g-C3N4 has potential applications in wastewater treatment and energy generation fields via tribocatalysis.

Conclusions
In summary, g-C3N4 has been fabricated successfully via the chemical blowing method and shows excellent tribocatalytic performance in the reduction of nitrogen to NH4 + . After 10 h of continuous stirring at 1000 rpm in the dark, the generation rate of NH4 + can reach 100.56 μmol·L −1 ·g −1 ·h −1 using methanol as a positive charges scavenger, which is 3.91 times higher than that without any scavengers. Furthermore, the performance of the tribocatalytic nitrogen fixation of g-C3N4 can be effectively optimized through increasing the stirring speed or number of stirring rods. Consequently, g-C3N4 has the remarkable potential application in the tribocatalytic N2 fixation reaction. Tribocatalysis has a bright application prospect in energy development fields such as nitrogen fixation, carbon dioxide reduction and water decomposition in the future.  Then methanol as the positive charge scavenger will react with q+ to avoid NH 4 + being oxidized [37]. In addition, q − will react with N 2 to produce NH 4 + , which is dissolved in DI water to form NH 4 + . The main reaction process is described in Equations (2)-(4) [57,58]: In this work, g-C 3 N 4 exhibits excellent tribocatalytic performance in the N 2 fixation process. Though, as a matter of fact, the applications of the tribocatalysis are not restricted in this field. In the past few years, He et al. have achieved excellent tribocatalytic performance on removal of the heavy metal ion Cr 6+ through using commercial iron turning with amorphous iron oxides. After 36 h stirring, the removal ratio of Cr 6+ can reach about 98% [59]. Additionally, Li et al. have successfully produced flammable gases such as CO, CH 4 and H 2 through harvesting mechanical energy with TiO 2 nanoparticles [20]. In the future, g-C 3 N 4 has potential applications in wastewater treatment and energy generation fields via tribocatalysis.

Conclusions
In summary, g-C 3 N 4 has been fabricated successfully via the chemical blowing method and shows excellent tribocatalytic performance in the reduction of nitrogen to NH 4 + . After 10 h of continuous stirring at 1000 rpm in the dark, the generation rate of NH 4 + can reach 100.56 µmol·L −1 ·g −1 ·h −1 using methanol as a positive charges scavenger, which is 3.91 times higher than that without any scavengers. Furthermore, the performance of the tribocatalytic nitrogen fixation of g-C 3 N 4 can be effectively optimized through increasing the stirring speed or number of stirring rods. Consequently, g-C 3 N 4 has the remarkable potential application in the tribocatalytic N 2 fixation reaction. Tribocatalysis has a bright application prospect in energy development fields such as nitrogen fixation, carbon dioxide reduction and water decomposition in the future.