In-situ formation of nitrogen doped microporous carbon nanospheres derived from polystyrene as lubricant additives for anti-wear and friction reduction

This study presents a nitrogen-doped microporous carbon nanospheres (N@MCNs) prepared by a facile polymerization–carbonization process using low-cost styrene. The N element in situ introduces polystyrene (PS) nanospheres via emulsion polymerization of styrene with cyanuric chloride as crosslinking agent, and then carbonization obtains N@MCNs. The as-prepared carbon nanospheres possess the complete spherical structure and adjustable nitrogen amount by controlling the relative proportion of tetrachloromethane and cyanuric chloride. The friction performance of N@MCNs as lubricating oil additives was surveyed utilizing the friction experiment of ball-disc structure. The results showed that N@MCNs exhibit superb reduction performance of friction and wear. When the addition of N@MCNs was 0.06 wt%, the friction coefficient of PAO-10 decreased from 0.188 to 0.105, and the wear volume reduced by 94.4%. The width and depth of wear marks of N@MCNs decreased by 49.2% and 94.5%, respectively. The carrying capacity of load was rocketed from 100 to 400 N concurrently. Through the analysis of the lubrication mechanism, the result manifested that the prepared N@MCNs enter clearance of the friction pair, transform the sliding friction into the mixed friction of sliding and rolling, and repair the contact surface through the repair effect. Furthermore, the tribochemical reaction between nanoparticles and friction pairs forms a protective film containing nitride and metal oxides, which can avert direct contact with the matrix and improve the tribological properties. This experiment showed that nitrogen-doped polystyrene-based carbon nanospheres prepared by in-situ doping are the promising materials for wear resistance and reducing friction. This preparing method can be ulteriorly expanded to multi-element co-permeable materials. Nitrogen and boron co-doped carbon nanospheres (B,N@MCNs) were prepared by mixed carbonization of N-enriched PS and boric acid, and exhibited high load carrying capacity and good tribological properties.


Introduction
In industrial production, friction and wear can lead to mechanical wear and energy consumption.Hence, lubricant is necessary to protect machinery from wear and maintain surface quality [1].It is well recognized that the base oil used alone is not able to meet the promoted requirements under extreme conditions.In this regard, adding functional additives is deemed as one of the most universal and facile means to enhance the comprehensive properties of base oil [2,3].Nanoparticles, such as metal [4,5], metal oxide [6,7], sulfide [8], carbon [9][10][11] and so on, can effectively improve properties of friction and enhance the Friction 12(3): 439-451 (2024) | https://mc03.manuscriptcentral.com/frictionload-carrying capacity of base oil due to size effect, colloidal effect, protective film, and third body effects [1,3].In previous studies, TiO 2 nanoparticles as lubricating oil additives can reduce the friction coefficient of base oil by 85% through bearing effect and repair effect [7].Cu as nano-additives greatly improved the friction properties of lubricating oil through the protective film effect, and the friction coefficient was reduced by 17.3%-23.6%[4].
Carbon ball has received much more attention in the field of lubrication.Due to the unique physical and chemical properties, carbon ball can significantly improve the friction properties of base oil [12].Panickar et al. have synthesized carbon spheres using noncatalytic CVD method, the as-prepared carbon spheres displayed good tribological properties due to rolling friction, and the friction coefficient of the neat oil reduced by 11% with addition of 0.3 wt% of carbon spheres [13].The carbon spheres with abundant functional groups were prepared by hydrothermal carbonization method, which as additives reduced the friction coefficient and wear rate by 33.2% and 24.2% respectively [14].
Carbon nanomaterials have gathered great interests in the past several decades as nano-lubricant additives, however, carbon nanomaterials always accumulate spontaneously in lubricating oil owing to their high surface energy, resulting in poor dispersion stability, which is fatal for lubrication effect and long-term usability of hybrid lubricants.According to the research, the lubrication performance of carbon spheres could be boosted by doping a variety of elements including sulfur, phosphorus, nitrogen, and boron [15][16][17] due to the high reactivity brought by heteroatom.Xiong et al. have prepared SiO 2 -reinforced B-N-co-doped reduced graphene oxide via microwave-synthesis method, which displayed eminent load-carrying, anti-wear, and anti-friction performance as additives [18].The S, N co-doped carbon nanoparticle (S,N-CN) were prepared via hydrothermal reaction, the as-obtained S,N-CN exhibited good lubricating performance compared to base oil PEG200 when playing the role as lubricating additives [17].
Polystyrene (PS) is one of the best known synthetic polymers, which is easily manufactured by the free radical polymerization of styrene.PS has been of great practical value in many diversified fields attributable to its enduring stability, low cost, and convenience of fabrication.Carbon nanomaterials prepared via carbonization of PS have been explored thoroughly to serve various application scenarios.Nie et al. synthesized carbon material by carbonizing waste polystyrene and CCl 4 as raw materials [19].Whereas, the common pyrolysis of polystyrene can only obtain undoped carbon nanomaterials, which limits extended properties and further functionalization.Consequently, doping active heteroatom by introducing foreign source during preparation, such as S, P, N, B, is usually used as an effective mean to alter the intrinsic properties of carbon materials.Chen et al. have prepared sulfur-doped hard carbon by carbonizing mixture of PS spheres and sulfur powders under nitrogen atmosphere [20].At present, tramp element doping of carbon materials can be well realized via various methods such as hydrothermal method, activation method, post-processing method, chemical vapor deposition, etc. [21].Herein, we have successfully prepared N@MCNs via a facile polymerizationcarbonization process using low-cost styrene.Abundant N atoms were in situ introduced into polystyrene (PS) nanospheres via emulsion polymerization of styrene with cyanuric chloride as crosslinking agent, and then the crosslinking PS nanospheres could be further carbonized to obtain N@MCNs.The as-prepared N@MCNs gained more regular spherical structure and adjustable nitrogen amount by controlling the relative proportion of tetrachloromethane and cyanuric chloride.The tribological performances of base oil PAO-10 abound with all kinds of N@MCNs, were assessed remarkably under extreme test conditions.As oil-based lubricating additives, the as-prepared N@MCNs displayed outstanding friction reduction and anti-wear abilities with a trace amount addition in PAO-10 through rolling effect, polishing effect, repairing effect, and protective film effect.The mean COF of PAO-10 containing 0.06 wt% N@MCNs decreased to 0.105 and the wear volume reduced by 94.4%.The load-carrying ability increased from 100 to 400 N. The preparation method provided can be further expanded to obtain multi-element co-permeable materials, the N and B co-doped carbon nanospheres were prepared triumphantly by mixed carbonization www.Springer.com/journal/40544| Friction of N-enriched PS and boric acid, the tribological performance of which were studied in detailed.

Preparation of hypercrosslinked polystyrene nanospheres (HPSs)
Polystyrene nanospheres were prepared by homopolymerization of styrene.Specific production process referred to the previous literature [22].For synthesis of HPSs, 0.5g PS and 0.37g cyanuric chloride was dispersed in 30 mL tetrachloromethane and 25 mlmethylene chloride, respectively, and the two solutions were mixed well together.1.4 g anhydrous aluminium chloride was added to initiate polymerization.The reaction was carried out at 75 °C for 24 h under 300 rpm.The relative content of tetrachloromethane and cyanuric chloride was changed for the preparation of CHPSs, HPSs-1, HPSs-2, HPSs-3, and THPSs.The specific content is shown in Table S1 in the Electronic Supplementary Material (ESM).

Preparation of nitrogen doped microporous carbon nanospheres (N@MCNs)
In the first place, 0.2 g CHPSs was added into 60 mL deionized water, which was heated to 185 °C in 100 mL Teflon valve sealing cartridge for 6 h in order to avoid serious agglomeration of particles during carbonization.CMCNs was synthesized by carbonizing the samples after hydrothermal pretreatment at 500 °C for 5 h under nitrogen atmosphere.The production process of N@MCNs-1, N@MCNs-2, N@MCNs-3, and TMCNs was the same as that of CMCNs, where HPSs-1, HPSs-2, HPSs-3, and THPSs were used as raw materials, respectively.

Preparation of boron, nitrogen co-doped microporous carbon nanospheres (B,N@MCNs)
100 mg hydrothermal-pretreated samples were mixed with 200 mg boric acid under uniform stirring for one hour.Above mixture were placed in the tubular furnace at 550 °C for 5 h at nitrogen atmosphere.B,N@MCNs were obtained by washing the carbonized samples with pure water at 70 °C and then drying under vacuum at 45 °C, in order to remove residual boron oxide.
The materials and characterization methods used in the experiment are provided in the ESM.

Results and discussions
Scheme 1 shows the production process of microporous carbon nanospheres.Carbon-crosslinked bridges and triazine-crosslinked bridges are formed between benzene rings by using tetrachloromethane and cyanuric chloride as crosslinking agents, respectively.The morphology characteristics of microporous carbon nanospheres were shown in Fig. 1.CMCNs, N@MCNs-1, N@MCNs-2, and N@MCNs-3 all showed perfect spherical structure, while the spherical structure of TMCNs collapsed obviously, indicating that carbon tetrachloride, as a crosslinking agent, can improve the structural stability of carbon nanospheres and inherit nanostructure during carbonization.EDS mappings of N@MCNs-2 (Figs. 1(f)-1(h)) exhibited a well-proportioned distribution of N elements, which is signifying that nitrogen was successfully doping The FT-IR spectrum was used for ulteriorly certifying the formation of carbon-crosslinked bridge and triazine-crosslinked bridges (Fig. S1 in the ESM).The peaks can be observed at 1,666 cm -1 in CMCNs, N@MCNs-1, N@MCNs-2, and N@MCNs-3 in comparison with the spectrum of TMCNs, which fully proved the triumphant formation of crosslinking C=O bridges [23].The characteristic peaks of 1,510 and 1,387 cm -1 appeared at N@MCNs-1, N@MCNs-2, N@MCNs-3, and TMCNs, corresponding to the stretching vibration peak of triazinyl, which are not detected in the CMCNs [24].The alteration of pore structure caused by crosslinking agent was assessed by N 2 adsorption-desorption isotherms.Nitrogen absorption of CMCNs, N@MCNs-1, N@MCNs-2 and N@MCNs-3 sharply increased at P/P 0 <0.1, indicating that abundant micropores existed in samples, which was not observed in TMCNs (Fig. S2 in the ESM).The Brunauer-Emmett-Teller surface area (S BET ) of CMCNs, N@MCNs-1, N@MCNs-2, and N@MCNs-3 were 374.5, 323.8, 416.4,and 389.5 m 2 •g −1 respectively, while the value of TMCNs decreased to 16.3 m 2 •g −1 , which is due to the collapse of TMCNs structure during carbonization (Table S2 in the ESM).The results further prove that carbon nanospheres possess the structural stability and inherit nanostructure due to formation of carbon-crosslinked bridges.For purpose of analyzing the surface chemical composition of the prepared samples, the XPS of the samples was carried out.Figure S3 in the ESM shows the full XPS survey spectra of CMCNs, N@MCNs-1, N@MCNs-2, N@MCNs-3, and TMCNs.Obvious nitrogen signal was observed in N@MCNs-1, N@MCNs-2, N@MCNs-3, and TMCNs, which is not detected in CMCNs.It can be concluded from XPS analysis that nitrogen was introduced in situ into carbon nanospheres by forming the triazine-crosslinked bridge with cyanuric chloride as a crosslinking agent.Table S3 in the ESM summarizes the surface chemical composition data of carbon nanospheres prepared with different adding amounts of tetrachloromethane and cyanuric chloride.The nitrogen content of carbon nanospheres increased with the augment of the addition of cyanuric chloride, which is due to the increasement of triazinecrosslinked bridges between benzene rings.As shown in Fig. 2(a), the binding energy of the C 1s of N@MCNs-2 at 284.7, 286.1, and 288.8 eV was attributed to the C-C/C=C, C-O/C-N, and C=O, which coincided with the O 1s peaks (Fig. 2(c)) of 533.5 eV (C-O) and 532.1 eV (C=O), respectively [25].The signal at 288.8 eV indicated the C=O crosslinking bridge between benzene rings, which is ascribed to CCl 4 acting as the crosslinking agent.The peaks at 400.4 eV (pyridinic N) and 399.2 eV(pyrrolic N) are deemed to deconvoluted N 1s peak (Fig. 2(b)) [22,26].
Graphitization and structural defects of the N@MCNs were analyzed by the Raman spectra.As shown in Fig. 2(d), the intensity of D (1,346 cm -1 ) and G (1,595 cm -1 ) peaks of N@MCNs decreased with the augment of nitrogen doping amount.The D band manifests the existence of amorphous carbon and surface defects of N@MCNs, and the G band www.Springer.com/journal/40544| Friction represents characteristic crystalline graphite which is corresponding to highly graphitized carbon with the high intension of the G band [27].In order to further research defect density of N@MCNs, the intensity ratio of D/G band was surveyed (Table S4 in the ESM), which implies the density of defects and the disorder extent in the structure [28].With the increase of nitrogen content, the I D /I G ratio decreased, indicating that the void defects of carbon nanospheres decreased and the graphitization degree increased, which may be attributed to the structural instability of the triazine-crosslinked bridge during the carbonization process.The peaks (Fig. 2(e)) could be fit into four peaks (D1, D2, D3, and G in the vicinity of 1,188, 1,246, 1,345, and 1,595 cm -1 , respectively).The D1 and D3 peaks originate from sp 3 -rich phase of disordered amorphous carbon and the A 1g symmetry mode of disordered graphitic lattice located at the edges of the graphene layer, respectively.Whereas the D2 band is related to the C-C and C=C stretching vibrations or the A 1g symmetry mode of the disordered graphitic lattice [27,29].The G band is attributed to sp 2 carbon atoms in a hexagonal lattice with E 2g symmetry [17].As shown in Fig. 2(f) and Table S5 in the ESM, thermogravimetric (TG) analysis showed that the weight loss rates of N@MCNs-1, N@MCNs-2, N@MCNs-3, and TMCNs were 17.6, 13.3, 12.7, and 8.6 wt%, respectively, whereas CMCNs established a 91.3 wt% weight loss within the limits of 30-800 °C.The result suggested that the thermal stability of carbon nanospheres was improved, which may be attributed to the formation of triazine-crosslinking bridges between benzene rings.Dispersive stability (Fig. S4 in the ESM) of nanoparticles in base oil is the critical factor impacting properties of tribology.With the increase of nitrogen content, the suspension of carbon nanomaterials seemed more stable gradually.The carbon nanomaterials (N@MCNs-2 and N@MCNs-3) remained stable in suspension for five days because the lipophilic functional groups in nitrogen-doped carbon nanospheres improved the dispersion stability.On the contrary, obvious reunion particles of TMCNs could be viewed flowing and precipitating in the bottom of bottle only after 1 days under the same ultrasonic time, which was caused by the collapse of TMCNs carbonization structure, thus resulting in serious particle aggregation.
The tribological properties of N@MCNs as oil-based lubricant additives were evaluated firstly under constant conditions.As shown in Fig. S5 in the ESM, sharp increase in the coefficient of friction took place under circumstance of lubricating by PAO-10 at about 50 s, causing violent seizure between two tribopairs.This phenomenon occurred because the volume of the friction pair is much larger than the contact peak point, the temperature of peak point will drop rapidly when | https://mc03.manuscriptcentral.com/frictionseparated from the contact point, as a consequence, the protective film like the lubricating oil film and adsorption film covering the worn surface will break instantly in this state, making the contacting peak point stick, and then the adhesion node will be destroyed in the sliding process, resulting in serious wear.Expectedly, the COF of lubricants containing N@MCNs decreased significantly compared with base oil, N@MCNs-2 and N@MCNs-3 (Figs.S5(a) and S5(b) in the ESM) showed eminent performance of reducing friction and anti-wear because of forming the protective film.However, the COF of TMCNs seemed to be unstable from the beginning of test, which may be caused by the poor dispersion induced stacking of nanospheres.It is well known that whether superfluous or inadequate nano-additives in PAO-10 will lead to worsen performance of lubrication.In order to improve its commercial value, N@MCNs-2 was selected as lubricating oil additive for further analysis and research.Therefore, the concentration effect of N@MCNs-2 (Fig. 3) was examined via tracing the COF versus time at different addition (0.1 to 0.02 wt%).The lubricating oil of 0.06 wt% N@MCNs-2 performed optimum.Comparing to base oil, the mean COF of PAO-10 containing 0.06 wt% N@MCNs-2 decreased from 0.188 to 0.105 (Fig. 3(a)), and the wear volume reduced from 9.12×10 5 to 0.51×10 5 μm 3 (Fig. 3(d)).The additive is insufficient to cover the contact surface when the additive amount was under 0.06 wt%, resulting in direct collisions of friction pairs.However, when the addition amount is greater than 1.0 wt%, nanoparticles were prone to aggregate and deposit in the base oil, which could lead to inferior friction performance.In order to further explore the performance improvement brought by carbon nanomaterials as additives, the tribological tests of HPSs-2 and N@MCNs-2 were conducted.In comparison with HPSs-2, the mean COF of PAO-10 containing 0.06 wt% N@MCNs-2 decreased from 0.15 to 0.11 (Fig. 3(b)), and the wear volume reduced from 3.34×10 5 to 0.51×10 5 μm 3 (Fig. 3(e)).The sectional profiles show the width and depth of the wear scars depicted in Fig. 3(c) and Fig. S (6) in the ESM.Compared with the base oil, the width and depth of wear scar decreased by 49.2% and 94.5 %, respectively, after adding 0.06 wt% of the N@MCNs-2 and decreased by 22.0% and 91.0%, respectively, compared with HPSs.Similar results were obtained from the 3D contour morphology results of worn surfaces (Fig. 3(f)).The lower disk lubricated by PAO-10 showed apparent scratches and grooves, mainly due to adhesive wear.The exterior morphology of wear scar was not obviously modified, www.Springer.com/journal/40544| Friction but the depth of the furrows could be mildly shallower after adding of HPSs.Scratches and furrows were dramatically lessened with the addition of N@MCNs-2 thanks to the forming of protective film, proving that the effects of N@MCNs as lubricant additives on reducing friction and wear resistance occured simultaneously.That also may be attributed to the additives of carbon nanomaterials bringing more solid compounds forming on the surface [3].
Figure 4 displays the lubrication properties of nanoadditives under different conditions (under variational loads and frequencies).Under the frequency gradient from 5 to 40 Hz, PAO-10 and PAO-10 containing 0.06 wt% HPSs (Fig. 4(a)) showed early lubrication failure, while the utilizing N@MCNs-2 as additives can maintain the correspondingly lower COF in the vicinity of 0.097.Figure 4(b) depicts that PAO-10 containing 0.06 wt% HPSs showed the load carrying capacity of 150 N, while that with N@MCNs-2 possessed the higher load capacity from 100 to 400 N compared to pure base oil.COF of base oil increases markedly along with the increase of load, when the load is lower than 400 N, the lubricant with nanoadditives always maintains a low friction coefficient.When greater than 400 N, the friction coefficient increases sharply.This is because COF of the base oil is related to the normal load.When the nanoparticles are added, COF of the lubricant becomes less dependent on the normal load [30].When the load is higher than 400 N, the friction coefficient increases sharply because the rolling friction of nanoparticles is limited [31].Under the synergism of ball bearing effect and protective film effect, the load resistance of N@MCNs-2 is obviously improved.
The formation mechanism of friction film on worn surface was discussed.Raman spectra of three points on the scar surface were taken for analysis to detect the chemical binding of the worn surface lubricated by PAO-10 with 0.06 wt% N@MCNs-2 as shown in Fig. S7 in the ESM.The characteristic D and G bands (1,346 and 1,595 cm -1 ) of the carbon material were required on the surface of the wear scar as similar with the as-prepared carbon nanospheres, indicating the formation of carbon film [32].As shown in Fig. S8 in the ESM, the uniform protective film took shape on the surface of the abrasion mark, and the element types and classification of the friction film are further observed by EDS (Fig. S8(c) in the ESM).The friction film mainly composed of metal oxides and carbonnitrogen compounds was formed on the surface of the steel matrix.At the same time, carbon signal is clearly observed in the friction film region, indicating the formation of carbon film during the friction process [32,33].The detailed composition of carbon film on wear surface was further evaluated by XPS spectrum in Fig. 5.The C 1s spectrum peaks at 284.8 and 288.1 eV put down to C−C/C=C and C=O, which rooted from the organic section in N@MCNs-2 adsorbed on the surface of wear scar.The peaks of N 1s at 400.1 eV and the peak of C 1s at 286.0 eV suggested the formation of carbonitride in the tribo-chemical reaction [34], which is consistent with the results of Raman and FIB-TEM.The peaks of Fe 2p at 712.6 and 710.3 eV can be identified as Fe 2p3/2 for Fe 2 O 3 , Fe 2p3/2 for Fe 3 O 4 respectively, which coincided with the O 1s peaks of 531.8 and 530.0 eV [17,35,36].Ultimately, the results show that under the effects of physical adsorption and tribo-chemical reaction, the composite  The experimental method can prepare multi-element co-doped carbon materials by mixed carbonization.B,N@MCNs were successfully prepared via mixed carbonization of HPSs-2 and boric acid.Faultless sphericity was exposed in SEM (Fig. S9(a) in the ESM) and TEM (Fig. S9(b) in the ESM) images of B,N@MCNs-2, indicating that the spherical structures of carbon nanospheres have been barely affected by mixing boric acid into carbonization.High-resolution XPS spectra of B,N@MCNs-2 (Fig. S10 in the ESM) emerged the undetected boron signal in N@MCNs-2, verifying that boron is doped into carbon nanospheres successfully (doping boron content is 0.83 at%) (Table S6 in the ESM).As shown in Fig. S11 and Table S7 in the ESM, the graphitization degree and structural defects of B,N@MCNs-2 are sightly changed comparing with N@MCNs-2.In order to further explore the tribological properties of B,N@MCNs-2, the COF curves of B,N@MCNs-2 at different concentrations of 0.1-0.02wt% were showed in Fig. 6.The result showed that PAO-10 with 0.04 wt% B,N@MCNs-2 possessed the best lubricating performance.The mean COF of PAO-10 containing 0.04 wt% B,N@MCNs-2 decreased from 0.188 to 0.100 (Fig. 6(a)) and wear volume reduced from 9.12×10 5 to 0.60×10 5 μm 3 (Fig. 6(d)) comparing with the base oil.To distinguish the improvement effect between B,N@MCNs-2 and N@MCNs-2, contrast tribological tests were carried out.Compared with PAO-10 containing 0.04 wt% N@MCNs-2, the COF of PAO-10 containing 0.04 wt% B,N@MCNs-2 decreased from 0.117 to 0.100 (Fig. 6(b)), and the wear volume reduced from 2.56×10 5 to 0.60×10 5 μm 3 (Fig. 6(e)).Similar results were shown in 3D contour morphology results of worn surfaces in Fig. 6(c) that there were fewer scratches and furrows with 0.04 wt% B,N@MCNs-2.Compared with the base oil, the width and depth of wear scar of B,N@MCNs-2 decreased by 44.4% and 91.9% respectively, while compared with N@MCNs-2, decreased by 6.0% and 85.1% respectively, which can be ascribed to the involvement of boron in forming the more effective protective film (Fig. S12 in the ESM).To extend its application, the tribological properties of B,N@MCNs-2 under varying loads are analyzed in detail.Figure 6(e) depicted that the B,N@MCNs-2 extends the load resistance from 100 to 600 N compared with PAO-10.
The chemical bonding of carbon film on B,N@MCNs-2-based wear surface was further evaluated by XPS spectrum.www.Springer.com/journal/40544| Friction with the N 1s peaks of 399.7 and 400.6 eV as well as the B 1s peak of 191.5 eV, which were derived from the organic section in B,N@MCNs-2 adsorbed on the wear scar and the formation of carbonic nitride compound and carbonic boron compound in the tribo-chemical reaction [16,25].The peaks of Fe 2p at 712.7 and 710.3 eV can be identified as Fe 2p3/2 for Fe 2 O 3 , Fe 2p3/2 for Fe 3 O 4 respectively, which coincided with the O 1s peaks of 531.8 and 530.2 eV [35,36].Finally, the results manifested that the complex protective film took shape with the synergetic effects of physical adsorption and tribo-chemical reaction on the worn surface, primarily composing of iron oxide, carbonic nitride compound, and carbonic boron compound [37,38].
To sum up, the lubrication mechanism of N@MCNs

Scheme 1
Scheme 1 Schematic diagram of preparation nitrogen doped microporous carbon nanomaterials as nanoadditives for PAO-10 lubricants.

Fig. 4
Fig. 4 (a) COF of different lubricanting oil under variable frequency conditions (150 N, 50 °C, and the frequency ramp test from 5 to 50 Hz at 5 Hz/5 min).(b) COF under extreme pressure (50 °C, 25 Hz, and the load ramp test from 50 to 500 N at 50 N/5 min).Concentrations of nano-additives were 0.06 wt%.
Figure 7(a) depicted that the peaks of the deconvolution C 1s spectrum at 288.5, 286.6, 285.3, and 284.6 eV are attributed to C=O, C=N, C-O/C-N/C-B and C=C, respectively, coinciding