F S c a S N a b c a A R R A A K P R S M N 1 h [ f o i t [ n g h 0 Applied Surface Science 394 (2017) 87–97 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsev ier .com/ locate /apsusc ull Length Article tructural and surface functionality changes in reticulated vitreous arbon produced from poly(furfuryl alcohol) with sodium hydroxide dditions ilvia Sizuka Oishia,∗, Edson Cocchieri Botelhob, Mirabel Cerqueira Rezendec, eidenêi Gomes Ferreiraa LAS, Instituto Nacional de Pesquisas Espaciais (INPE), Av. dos Astronautas 1758, São José dos Campos, SP 12227-010, Brazil Departamento de Materiais e Tecnologia, Univ Estadual Paulista (UNESP), Av. Doutor Ariberto Pereira da Cunha 333, Guaratinguetá, SP 12516-410, Brazil Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo (UNIFESP), Rua Talim 330, São José dos Campos, SP 12231-280, Brazil r t i c l e i n f o rticle history: eceived 2 August 2016 eceived in revised form 14 October 2016 ccepted 17 October 2016 vailable online 18 October 2016 eywords: oly(furfuryl alcohol) eticulated vitreous carbon urface functionalities icrostructure aOH oxidation a b s t r a c t The use of sodium hydroxide to neutralize the acid catalyst increases the storage life of poly(furfuryl alco- hol) (PFA) resin avoiding its continuous polymerization. In this work, a concentrated sodium hydroxide solution (NaOH) was added directly to the PFA resin in order to minimize the production of wastes gen- erated when PFA is washed with diluted basic solution. Thus, different amounts of this concentrated basic solution were added to the resin up to reaching pH values of around 3, 5, 7, and 9. From these four types of modified PFA two sample sets of reticulated vitreous carbon (RVC) were processed and heat treated at two different temperatures (1000 and 1700 ◦C). A correlation among cross-link density of PFA and RVC morphology, structural ordering and surface functionalities was systematically studied using Fourier transform infrared spectroscopy, scanning electron microscopy, Raman spectroscopy, X- ray diffraction, and X-ray photoelectron spectroscopy techniques. The PFA neutralization (pH 7) led to its higher polymerization degree, promoting a crystallinity decrease on RVC treated at 1000 ◦C as well as its highest percentages of carboxylic groups on surface. A NaOH excess (pH 9) substantially increased the RVC oxygen content, but its crystallinity remained similar to those for samples from pH 3 and 5 treated at 1000 ◦C, probably due to the reduced presence of carboxylic group and the lower polymerization degree of its cured resin. Samples with pH 3 and 5 heat treated at 1000 and 1700 ◦C can be considered the most ordered which indicated that small quantities of NaOH may be advantageous to minimize continuous polymerization of PFA resin increasing its storage life and improving RVC microstructure. © 2016 Elsevier B.V. All rights reserved. . Introduction The preparation of carbon materials using poly(furfuryl alco- ol) (PFA) has been extensively explored in many research papers 1–3]. With a growing concern about using materials obtained rom renewable resources of low cost and accessibility, furan ligomers are an alternative to obtain polymers that present broad ndustrial application. Thus, furfuryl alcohol is the most impor- ant furan monomer used as precursor to different resin types 4,5] and carbon materials such as nanostructured carbons and anocomposites [6–9], micro and nanoporous carbon [10–14], lassy carbon [3,15–17], among others. PFA can be obtained from a ∗ Corresponding author. E-mail address: silviaoishi@uol.com.br (S.S. Oishi). ttp://dx.doi.org/10.1016/j.apsusc.2016.10.112 169-4332/© 2016 Elsevier B.V. All rights reserved. polycondensation reaction of furfuryl alcohol catalyzed by acids. Due to the furan ring great reactivity, curing and carbonization speed of furan resins are always faster compared to those of aro- matic rings [18]. To avoid a violent reaction, the synthesis of PFA resin is carried out at low temperature (from 0 to 25 ◦C) and reac- tion time of up to 24 h [19–22]. The synthesis reaction can be finalized by cooling and neutralization to obtain the appropriate viscosity for the chosen application. In order to neutralize the acid catalyst, a solution of sodium hydroxide may be used to avoid con- tinuous polymerization of PFA, increasing its storage life [20,22]. Some authors report that the neutralization process is slow and the PFA resin is usually washed several times with a diluted basic solution to avoid the emulsification [19,20]. However, this pro- cess generates toxic wastes due to the presence of monomers and PFA chains with low molecular weight. As a novelty of this work, it is proposed the neutralization of PFA with a minimum amount dx.doi.org/10.1016/j.apsusc.2016.10.112 http://www.sciencedirect.com/science/journal/01694332 http://www.elsevier.com/locate/apsusc http://crossmark.crossref.org/dialog/?doi=10.1016/j.apsusc.2016.10.112&domain=pdf mailto:silviaoishi@uol.com.br dx.doi.org/10.1016/j.apsusc.2016.10.112 8 rface o t s m b a f u p s b N c i a q f e e s c c t R a p o s a o w p a h o t R c 5 t t a t t m 2 2 a c a ( r b p t h o d L 8 S.S. Oishi et al. / Applied Su f concentrated NaOH solution which would avoid waste produc- ion. Also, the influence of this base on the crystalline structure and urface functionalities on vitreous carbon processed from this PFA ust be investigated. Vitreous carbon (or glassy carbon) is a form of carbon produced y the pyrolysis of an aromatic polymer, generally a phenolic or PFA resin [16,23]. Reticulated vitreous carbon (RVC) is a porous oam-like structure and one of the most commonly carbon material sed as electrode due mainly to the presence of many accessible ores with controlled sizes and high surface area, in addition to everal other characteristics such as high chemical and thermal sta- ilities, catalytic properties, light weight, and low cost [2,23–27]. ormally, metals and alkali metals are added to non-graphitizing arbons in order to promote catalytic graphitization upon anneal- ng [14,28–31]. The presence of sodium hydroxide can be evaluated s filler, which will affect the cross-link density of PFA and, conse- uently, the carbon properties and structural ordering processed rom it. The carbon surface chemistry also influences many prop- rties of carbon materials such as wetting, adsorption, catalysis, and lectrochemical response [32–34]. As reported by Collins et al. [34], urface oxygen groups (SOG) are responsible for structure change, ontributing to the partial graphitization process. However, SOG oncentration in excess promotes graphite-like disorder. The struc- ural influence of surface functionalities is rarely correlated with aman or XRD. In this sense, the connection between Raman, XRD nd surface functionalities results as well as among carbon material rocessing parameters is necessary for a complete understanding f their crystallite dimensions, graphitic-order, reactivity, and the tructural influence of edge-site composition [34]. In this work the PFA resin acid catalyst was neutralized by dding different amounts of concentrated NaOH solution. Thus, ur objective was to evaluate how this basic solution addition, hich influences the cross-link density of cured PFA, can affect the rocessed RVC morphology, microstructure, and surface function- lities. To our knowledge, there are no papers in the literature that ave tried to neutralize the PFA resin with a concentrated solution f NaOH and have showed a relationship between NaOH propor- ion in the PFA and the properties of processed carbon material. VC was prepared using PFA resin with different amounts of con- entrated sodium hydroxide solution until reaching pH of around 3, , 7 and 9. These samples were cured and subsequently submitted o heat treatment temperature (HTT) of 1000 and 1700 ◦C in order o obtain two sample sets with different graphitization indexes as function of the PFA pH. All samples were characterized by Fourier ransform infrared spectroscopy (FTIR), X-ray photoelectron spec- roscopy (XPS), Raman spectroscopy, XRD, and scanning electron icroscopy (SEM). . Experimental .1. Poly(furfuryl alcohol) with different sodium hydroxide mounts PFA was synthesized according to the previously described pro- edure [35], using furfuryl alcohol (Fluka) and diluted sulfuric cid solution (Fmaia) (0.5 mol L−1) as catalyst. The acid PFA resin pH ∼ 3) obtained after partial polymerization was distilled in a otary evaporator at a reduced pressure until reaching moisture elow 2 wt%. The resin was then separated in four portions to pre- are resins with different pH values (pH 3, 5, 7 and 9). The pH of he resin was varied by adding a concentrated solution of sodium ydroxide (2.0 mol L−1) until reaching the desired values without ccurs emulsification. The resin pH control was accomplished by irectly measuring the resin with a pH meter from Methrom 827 pH ab, with a glass pH combination electrode (Unitrode). PFA viscos- Science 394 (2017) 87–97 ity varied from 12.9 Pas (pH 9 resin) to 21.0 Pas (pH 3 resin) at 25 ◦C. These measurements were performed in a Brookfield viscometer, model RV DV-II + Pro, with a SC4-34 spindle, using a shear rate of 8 s−1, torque at 60.3% for pH 9 resin and shear rate of 6 s−1, torque at 65.6% for pH 3 resin. 2.2. Reticulated vitreous carbon processing Polyurethane foams with 70 pores per inch (ppi) were kindly donated by Sanko Espumas. The foams were cut into dimensions of 18 cm x 8 cm, followed by impregnation of about 20 g of PFA resins containing different quantities of sodium hydroxide. The resin was catalyzed with 3 w/w% of diluted p-toluenesulfonic acid (60 w/v%). Impregnated foams were cured in an oven for 1 h in the following temperatures: 50, 70, 90, 110, 130 ◦C. HTT of 1000 ◦C was carried out in a tube furnace from room temperature at heating rate of 1 ◦C/min in N2 atmosphere reaching the maximum for 1 h up to its cooling down to room temperature. HTT of 1700 ◦C was car- ried out in similar way, except for using a heating rate of 5 ◦C/min. Characterizations are included in the Supplementary material. 3. Results 3.1. FTIR analysis Fig. 1a shows the cured PFA FTIR spectra with different sodium hydroxide amounts. The band at 3400 cm−1 increased as a func- tion of NaOH amount increase related to OH stretching [36]. Other characteristic bands of PFA are [19,37–40]: the bands at 2920 and 1430 cm−1 related to the aliphatic segments presence; the band at 1715 cm−1 due to the occurrence of some ring open- ing of furan ring; the band at 1560 cm−1 assigned to conjugated C C species; the bands at 1506, 1157 and 1010 cm−1 attributed to furan ring; the band at 1360 cm−1 due to C C or CO stretch- ing; the band at 1220 cm−1 related to C O from the alcohol or C O C of furan ring; and the band at 780 cm−1 characteristic of 2,5-disubstituted furan ring. The reaction progress (�) can be evalu- ated by the quotient among integrated intensity of primitive bands, i.e., � = I736 + I750/(I782 + I790 + I803) [40]. The smaller this quotient, the farther the reaction has progressed. The decomposed bands at 736, 751, 782, 790, and 803 cm−1 were determined by spectra deconvolution in the interval from 700 to 850 cm−1 (Fig. 1b), using Gaussian shaped bands for the spectrum in absorbance. The calcu- lated � values for pH 3, 5, 7, and 9 are 0.82, 0.79, 0.72, and 0.86, respectively. From these results, it can be inferred that sample pH7 presented the highest polymerization degree, since it had the low- est � value, followed by sample pH 5, 3 and 9 in descending order of polymerization degree. 3.2. Morphological and surface characterizations RVC obtained from PFA resin with different NaOH additions for HTT at 1000 and 1700 ◦C were characterized by FEG-SEM images of their stems surfaces in two different magnifications (2000x and 10000×). Fig. 2 shows representative micrographs of RVC in which the nomenclature was created concerning the adjusted pH for each resin followed by its related RVC HTT values. RVC images heat treated at 1000 ◦C (RVC1000), indicate that pH3 1000 sample present a uniform texture resulted from good polyurethane foam impregnations. Sample pH5 1000 present a similar texture but at higher magnification it is possible to notice some irregular surface. On the other hand, pH7 1000 and pH9 1000 present morpholo- gies significantly modified with higher roughness, attributed to the higher inclusions of sodium hydroxide. Some regions with agglom- erates of NaOH and small pores can be seen in pH7 1000 sample and small crystals are visible over the whole pH9 1000 surface. S.S. Oishi et al. / Applied Surface Science 394 (2017) 87–97 89 Fig. 1. FTIR spectra of cured PFA: a) FTIR spectra in absorbance ranging from 4000–600 cm−1; b) FTIR spectra in absorbance in the interval 700–850 cm−1 used for the determination of � value for different pHs. ples v e N t b r Fig. 2. FEG-SEM images of RVC1000 and RVC1700 sam The morphology of pH3 1700 and pH5 1700 samples remains isibly unchanged. However, small grains and/or pores with diam- ter around 0.7 �m are noted in the sample pH7 1700 attributed to aOH removal. Furthermore, for sample pH9 1700, which presents he highest NaOH amount, the sodium hydroxide crystals seem to e completely volatilized leaving pores in the RVC matrix with very ough surface. in two different magnifications (2000× and 10000×). The presence of sodium compounds were confirmed by XPS analyses, which provide quantitative information of the sample surface compositions. The carbon, oxygen, and sodium atomic per- centage, which are the possible elements found in the analyzed samples, are presented in Table 1. The O1s/C1s ratio, which indi- cates the degree of surface oxidation, is also included. 90 S.S. Oishi et al. / Applied Surface Table 1 Atomic percentage of carbon, oxygen and sodium and the O1s/C1s ratio of RVC1000 and RVC1700 samples processed from PFA with different pH values adjusted with sodium hydroxide. Composition (atomic%) Sample C1s O1s Na1s O1s/C1s ratio (%) pH3 1000 95.37 4.65 – 4.88 pH5 1000 93.59 6.41 – 6.85 pH7 1000 92.04 7.85 0.10 8.53 pH9 1000 74.00 21.45 4.55 28.99 pH3 1700 97.84 2.16 – 2.21 pH5 1700 98.81 1.19 – 1.20 b i O b H o r e o p t S ( b a t o f d t f r t o t f n w a p v T o i s c p a t p 9 s s a i a F pH7 1700 98.18 1.82 – 1.85 pH9 1700 98.60 1.40 – 1.42 The NaOH addition increase on PFA tends to decrease the car- on amount on the respective processed RVC1000 samples, which s associated to their oxygen concentration increase. Thus, the 1s/C1s ratio reflects exactly the expected behavior. Sodium could e identified only on samples pH7 1000 and pH9 1000. For 1700 ◦C TT, the carbon concentration increases for all samples, while the xygen loss occurs and the sodium compounds disappear. These esults confirm that the volatilization of sodium compounds gen- rate a rough surface. Fig. 3 shows high-resolution XPS spectra and the curve fitting f C1s region for samples pH3 and 9 at 1000 and 1700 ◦C. The peak osition and FWHM (full width at half maximum) constraints for he curve fitting were assigned according to the literature [41–45]. ix peaks were considered: graphitic (∼284.4 eV, peak I), �-carbon ∼285.2 eV, peak II), hydroxide or ether (∼286.1 eV, peak III), car- onyl (∼287.7 eV, peak IV), carboxyl or ester (∼288.8 eV, peak V) nd �-�* shake-up (∼290.6 eV, peak VI). Some authors correlate he 290.6 eV position to CO, CO2 and carbonate groups [45,46] r also as aromatic structure [47,48]. The percentages of each unctional group determined by XPS for RVC1000 and 1700 with ifferent pHs are shown in Fig. 4. There is a decrease in the rela- ive content of graphitic carbon (C C) with NaOH addition, mainly or samples pH7 1000 and pH9 1000. For these samples the peak elated to C OH decreases, whereas an increase occurs mostly in he peak assigned to a second graphitic peak (carbons bonded to xidized carbons) [41,44] and C O, respectively. This fact shows hat NaOH acts as an oxidant, increasing the degree of oxidation or samples pH7 1000 and pH9 1000, where the carbon compo- ents bonded with oxygen are more pronounced in C1s spectrum hen compared to those for pH3 1000. Sample ph9 1000 exhibits prominent bump at 290.6 eV, related to �-�* shake-up, which is robably induced by the NaOH excess. For RVC treated at 1700 ◦C, FWHM of C1s spectra (Fig. 3b and d) isually decreases due to the oxidize carbon components decrease. he removal of some surface functionalities is expected as it can be bserved by the C OH, C O and COOH groups decrease and by an ncrease of C C and �-carbon for sample pH3 and pH5. However, amples pH7 1700 and pH9 1700 showed an increase in C OH oming from a more oxidative state similarly to those for samples H7 1000 and pH9 1000. In addition, sample pH9 1700 present n increase in COOH group due to a decrease in the peak related o �-�* shake-up, while samples in other pH values at this HTT resented a decrease in COOH group. Fig. 5 shows the O1s spectra fitted for both samples pH 3 and heat treated at 1000 and 1700 ◦C. For the curve fitting of O1s pectra, four dominant components of oxidized carbon were con- idered [49,50], named C O at ∼531.1 eV (peak I); hydroxyls, ethers nd C O in esters, amides, anhydrides at 532.3 eV (peak II); C O n esters and anhydrides at 533.3 eV (peak III); carboxylic groups t 534.2 eV (peak IV); and adsorbed water at 536.0 eV (peak V). or sample pH9 1000 (Fig. 5c), the presence of sodium hydrox- Science 394 (2017) 87–97 ide at ∼532.8 eV was considered. Fig. 6 presents the percentage of surface oxygen functional group. The content of oxygen atoms of peak I for RVC1000 samples varies from 10.2% (pH5 1000) to 17.4% (pH9 1000) while for RVC1700 samples, peak I decreases with percentages from 4.0% (pH5 1700) to 7.92% (pH7 1700). Peak II is predominant in relation to the other peaks for both RVC1000 and 1700 samples with the exception of pH3 1000, pH9 1000 and pH7 1700 samples that present values of 40.4, 19.8 and 31.7%, respectively. These same set of samples pH3 1000, pH9 1000 and pH7 1700 show the largest percentage of peak III in relation to the other components, with values of 40.8, 29.1, and 39.6%, respec- tively. For peak IV, the percentages from 2.75% (pH9 1000) to 20.0% (pH7 1000) were obtained. However, for RVC1700 samples, peak IV increases varying from 11.7% (pH5 1700) to 24.9% (pH9 1700). The oxygen atoms from NaOH appeared only on sample pH9 1000 and were estimated at 16.2%. In addition, this sample also presented a prominent band at 536 eV, which is probably related to Na KLL and not to adsorbed water. 3.3. XRD analysis Carbon materials quality is usually assessed by X-ray diffraction (XRD) and Raman spectroscopy. Both techniques are complemen- tary and can provide information about the carbon crystallinity. The determination of carbon crystallinity by XRD has been well established with good reproducibility [51–53]. XRD may provide information from the bulk due to its penetration depth of around 500 �m in carbon material [54], while Raman spectroscopy has as advantage a higher surface selectivity thus providing less averaged information with an estimated sampling depth of about 100 nm [16,53]. Fig. 7 presents the diffraction patterns of RVC1000 and RVC1700 for all pH variations. The investigated RVC samples show carbon bands at 2� around 24.5◦, 43.6◦ and 80.0◦ for RVC1000 and 24.8◦, 43.5◦ and 79◦ for RVC1700. These bands correspond to the reflections (002), (10) and (11), respectively. Comparing RVC1000 and 1700 XRD patterns, all the bands show an intensity and nar- rowing increase after 1700 ◦C HTT, indicating an improvement of graphitization index. Comparing the pH variations, the highest change in intensity and width occurs for (002) band that is related to the graphitic stacking structure, while small variation occurs for (10) band which is associated with the in-plane structure. Fig. 8 shows the effects of pH and HTT for the interlayer spacing (d002), crystallite height (Lc), and crystallite width (La). The interlayer spacing (Fig. 8a) of RVC1000 is of about 0.366 nm for samples pH3, pH5 and pH7 showing slight increase for pH9 1000 of 0.367 nm, which may be associated to defects on graphitic sheets due to the NaOH contribution. For RVC treated at 1700 ◦C, there is a modest decrease in d002 for all samples in comparison to those of RVC1000 and one can highlight pH5 1700 sample as the highest interlayer ordering. Analyzing Lc values, pH5 1000 and pH5 1700 samples exhibit the highest crystallite height among samples with differ- ent pH in both HTT. A discrete variation occurs among La values of samples with different pHs for both set of samples (RVC1000 and RVC1700). One can also highlight samples pH7 heat treated at 1000 and 1700 ◦C, which present the lowest crystallite width. 3.4. Raman spectroscopy results Raman spectroscopy results were discussed in order to evaluate the possible structural changes with sodium hydroxide addition at different RVC HTT. The D band (∼1350 cm−1) represents disordered structure of carbon and reflects sp2 vibration of the rings, which is caused by defects such as impurities, edges and finite-size effects that destruct the translational symmetry of A1g. D-band intensity is proportional to the presence of six fold aromatic rings. The G band, on the other hand, corresponds to graphite in-plane vibrations with S.S. Oishi et al. / Applied Surface Science 394 (2017) 87–97 91 Fig. 3. Curve fitting of C1s spectra of: a) pH3 1000; b) pH3 1700; c) pH9 1000; and d) pH9 1700. Fig. 4. Percentages of each surface functional group from C1s spectra for RVC1000 and 1700 with different pH values. 92 S.S. Oishi et al. / Applied Surface Science 394 (2017) 87–97 Fig. 5. Curve fitting of O1s spectra of: a) pH3 1000; b) pH3 1700; c) pH9 1000; and d) pH9 1700. Fig. 6. Percentages of each surface functional groups from C1s spectra for RVC1000 and 1700 with different pHs. S.S. Oishi et al. / Applied Surface Science 394 (2017) 87–97 93 Fig. 7. X-ray diffractograms of: a) RVC1000 and b) RVC1700 for all pH variations. cryst E i t t t 1 f t ∼ R t n t m t i 1 ( m u I a t s b w ( t o s Fig. 8. Interlayer spacing, d002 (a), crystallite height, Lc (b), and 2g symmetry which reflects the degree of graphitization. G-band ntensity is proportional to the presence of any sp2 bonds, not only o those in the rings [55–57]. The G band in glassy carbon is presented as a superposition of he G and D’ (∼1620 cm−1) bands [16]. The D’ band also corresponds o E2g symmetry of graphitic structure. A band between 1165 and 200 cm−1, named I, is assigned to disordered structures formed rom the original polymeric structure, polyenes and ionic impuri- ies [58,59]. Another first-order band, denominated D”, is located at 1500 cm−1 and is related to sp2 amorphous carbon band [58,59]. Fig. 9 presents the first and second order Raman spectra of VC1000 and 1700 with different pH values. The Raman spec- ra of carbonaceous materials have been continuously studied and ew visions and interpretations have been published recently in he literature [13,60,61]. In this work, the analyses and deter- ination of spectral parameters by curve fitting were based on he best bands combination as presented by Sadezky et al. [59], .e., Lorentzian-shaped bands G, D, D’, and I bands (1580, 1350, 620, and 1200 cm−1, respectively), and Gaussian-shaped D” band ∼1500 cm−1). The D and G band parameters like full width at half aximum (FWHM) and intensity ratio of D and G bands (ID/IG), are seful to estimate the degree of ordering in carbonaceous materials. n case of low crystallinity samples, the use of second order bands is solution when ID/IG ratio shows a conflicting behavior, but only if he second order is clear enough to be analyzed [60]. Therefore, the econd order bands were fitted only for RVC1700 samples due to the road bands of RVC1000 samples. Four Lorentzian-shaped bands ith their initial positions at 2450, 2700, 2900, and 3100 cm−1 named 2I, G’, D + G, and 2D’, respectively) were used according o Sadezky et al. [59]. The D, G and G’ are the three main bands f graphite and their intensity ratios combinations and FWHM are hown in Fig. 10. allite width, La (c), derived from Bragg and Scherrer equations. ID/IG ratio is associated with in-plane defects, such as point defects, dislocations and finite size boundaries [62]. Thus, ID/IG ratio decreases as sample disorder decreases. Several works have demonstrated the PFA behavior when subjected to heat treatment at different temperatures [13,54,63–67]. In this work, comparing ID/IG ratio for samples heat treated at 1000 and 1700 ◦C (Fig. 10a), ID/IG ratio decreases after 1700 ◦C HTT, as expected due mainly to an increase in the sample graphitization degree. For RVC1000, pH7 sample presents the highest disorder and there are no significant changes among the samples with other pH values. For RVC1700, pH 5 presents ID/IG ratio slightly lower than other samples. The narrowing of D and G band widths are related to the release of heteroatoms from the polymeric precursor and is also associ- ated to the decrease of structural defects [54]. A decline for both D and G band after 1700 ◦C HTT is evident from FWHM of D and G bands (wD and wG, respectively) in Fig. 10b and c, due to the oxy- gen release. An increase in wD and wG values as a function of pH increase for RVC1000 samples is expected since the sodium hydrox- ide addition increases the oxygen concentration in the matrix, as already previously shown by XPS analyses. However, among sam- ples with different pH values of RVC1000 and 1700 there is a slightly difference at wD and wG and no trend is observed. For samples with short La (between 2 and 3 nm), the sec- ond order band intensity ratios display a clear behavior [60]. For example, an increase in IG′ /IG is associated with an increase in crystallinity, while ID/IG′ increase is assigned to a decrease in crys- tallinity [60]. From the analyses of IG′ /IG and ID/IG′ ratios (Fig. 10d and e) for RVC1700, the differences among different pH samples become more evident, i.e., pH 3 sample can be highlighted as the most ordered sample while pH 7 as the most disordered. The G’ is sensitive to structure changes and wG’ must decrease as the crys- tallinity increase [51]. According to Antunes et al. [68], wG’ is of 94 S.S. Oishi et al. / Applied Surface Science 394 (2017) 87–97 Fig. 9. Raman spectra of: a) RVC1000 and b) RVC1700 processed from PFA with different pH variations. IG’ ratio and f) wG’ for RVC1000 and/or RVC1700 with different pH values. a w r b p e [ g g f t c c T d s t Table 2 La values obtained by Raman and XRD. Sample La (nm) from Raman La (nm) from XRD pH3 1000 3.41 3.56 pH5 1000 3.32 3.52 pH7 1000 3.12 3.47 pH9 1000 3.44 3.56 pH3 1700 8.52 4.88 pH5 1700 8.35 4.84 Fig. 10. Graphics of: a) ID/IG ratio, b) wD, c) wG, d) IG’/IG ratio, e) ID/ bout twice wG and should exhibit the same trend. For this work, G’ shows a different trend of wG, which can be related to inaccu- acy in the G band fitting. Therefore, in case of uncertainty in the G and fitting, the ID/IG′ ratio is a more reliable alternative [60]. A higher standard deviation can be observed for both samples H9 1000 and pH9 1700 that can be assigned to their higher het- rogeneity caused by the excess of NaOH addition. Barros et al. 62,69] noted that in the case of graphitic foams, both 2D and 3D raphite regions were found due to the foam three-dimensional eometry that present spherical cells intersecting with each other, orming pores and junctions between them. The region of the junc- ion, where the ligaments meet, have shown the presence of highly urved and folded graphitic planes [69]. Moreover, the region of urvature of cell walls also naturally originates 2D graphite regions. herefore, the anisotropic nature of RVC foams also justifies the ispersion of intensity ratios and FWHM standard deviation for all amples. The crystallite size were also estimated by Raman spectra using he general equation proposed by Canç ado et al. [70], i.e., La = pH7 1700 8.21 4.72 pH9 1700 8.56 4.83 ( 2.4x10−10 ) �laser 4( ID IG )−1 , where ID/IG is the integrated intensity ratio of the D and G bands. Table 2 shows a comparison between La results obtained by Raman and XRD. The La calculated for samples with HTT at 1000 ◦C present approximated values for both Raman and XRD. On the other hand, samples with HTT at 1700 ◦C present higher La when estimated by Raman than those of XRD. Baldan et al. [54] has also observed this discrepancy at higher HTT that can be rface m b t c t o A f c d t h s a s i 4 e i c c s i i r f r s a R p a w t i g t a m [ s i t b c i t w i o I i t l s a a t b a S.S. Oishi et al. / Applied Su ainly related to the use of the area ratio where the effect of line roadening is included in the calculation. However, it is important o note that La calculated by Raman has the same tendency and dis- rete variation in different pH values for both HTT when compared o La calculated by XRD. This fact indicates that the crystallinity f the bulk and the carbon surface are quite similar. According to rzani et al. [71], La can be considered an indicator for degree of unctionalization on the surface and edges of graphene nanoparti- les. Taking into account that the lower La value the higher is the ensity of defects on the surface due to the higher degree of func- ionalization [71], samples pH7 1000 and pH7 1700 present the igher density of defects. However, in this case, the higher den- ity of defects is not only from the degree of functionalization but lso probably from the presence of some specific functional groups, ince sample pH9 1000 present the highest degree of functional- zation with oxygen functional groups. . Discussion Polymer modification may provide desired or undesired prop- rties for the final carbon material. This research aimed at nvestigating how the NaOH addition used to neutralize the acid atalyst of PFA resin affected its degree of polymerization and, onsequently, the processed RVC crystallinity, morphology, and urface functionalities. The sodium hydroxide addition seemed to ncrease the degree of polymerization of PFA resin up to pH 7 while t decreased for resin with pH 9 as shown by FTIR analyses. This esult showed that only an excess of NaOH hinders the cross-link ormation in PFA samples. For higher levels of cross-linking, the eorganization is difficult at high temperature, then a less ordered ample is expected. On the other hand, more ordering may occur t lower levels of cross-linking for the same HTT [72]. In this case, aman results showed a good agreement for RVC1000 samples, i.e., H7 1000 showed the highest disorder considering ID/IG ratio, wD, nd wG while for the other RVC1000 samples ID/IG ratio and wD ere quite close. Carboxylic acid is one of oxygen-containing groups responsible o transform aromatic �-density into olefin �-density by break- ng aromatic bonds resulting from the attachment of carboxylic roup, which enables the extended conjugation of the carbon sys- em increasing crystallite dimension (La) [73]. On the other hand, high yield in carboxylic groups is associated with an intense aro- atic ring cleavage and, consequently, a damage of aromatic sheets 48]. The NaOH oxidation effects were confirmed by XPS analy- es, which showed an increase in surface oxidation degree with ncreasing the NaOH amount. An increase in oxygen content led o an increase in disorder mainly for pH7 1000 sample as shown y Raman analysis. In this case, the highest disorder of pH7 1000 an be related to its highest percentages of carboxylic acid verified n C1s and O1s spectra, besides the highest degree of polymeriza- ion in its related PFA resin. The largest content of oxygen (21.45%) as observed for pH9 1000 sample, which should be associated to ts high level of defects. However, this sample presented a similar rdering as those for pH3 1000 and pH5 1000 samples considering D/IG ratio and FWHM. The graphitic ordering of sample pH9 1000 s being influenced by the reduced presence of carboxylic group hat is an indicative of lower damage of aromatic sheets and by the owest degree of polymerization of its cured PFA resin. XRD analysis howed a d002 slightly higher for pH9 1000 sample, probably gener- ted by stacking faults resulting from the presence of heteroatoms nd also steric hindrance between surface functionalities. The annealing under inert atmosphere until 1700 ◦C removed he NaOH and most of surface oxygen groups, increasing the car- on composition to approximated values (ranging from 98 to 99%, s shown in Table 1) among different pH values. The heteroatoms Science 394 (2017) 87–97 95 released increased the crystallinity leading to a better organization of graphene sheets comparing to RVC1000 samples, as shown by XRD and Raman analyses. For RVC1700 samples, the use of sec- ond order Raman bands was possible due to a more defined bands. Moreover, the variation among different pH was more evident on intensity ratio (IG/IG′ and ID/IG′ ) and FWHM of G’ band (Fig. 10d–f) in comparison to the results of first order Raman bands that showed little variation (Fig. 10a–c). From the second order Raman G’ band results, pH7 1700 sample continued presenting the highest disor- der as well as pH7 1000 sample, probably due to the predominant effect of higher degree of polymerization of its cured PFA resin which hinders the structure reorganization. A good structural orga- nization of pH3 1700 sample was expected owing to the purity of its precursor resin. On the other hand, sample pH9 1700 has presented the average values of intensity ratio (IG′ /IG and ID/IG′ ) and FWHM (wG’) close to those of pH3 1700, but it also presented a high stan- dard deviation. The higher standard deviation of pH9 1700 sample may be derived from some regions with higher defects that are con- centrated in the unaligned regions of the material confirmed by the strong D-band in Fig. 9b, as reported by Barros et al. [69]. Besides, the damage of aromatic sheets can be confirmed by an increase in carboxylic groups (Figs. 4 and 6) of pH9 1700 compared to other RVC1700 samples. Some regions with increased graphitic ordering on pH9 1700 sample may have resulted from the high oxygen con- tent (around 21%) observed on pH9 1000 sample that was removed with the smallest graphitic crystallites during heat treatment at 1700 ◦C leaving the larger crystallites, as well as may have con- sumed the disordered carbon increasing the degree of organization of pH9 1700 sample. Considering XRD results (d002, Lc, and La), pH5 1000 and pH5 1700 samples presented the best graphitic ordering, which was an indicative that little amount of NaOH can be beneficial to minimize continuous polymerization of PFA resin and improve RVC microstructure of carbon bulk. 5. Conclusions FTIR analyses showed that cured PFA resin with pH 7 pre- sented the highest degree of polymerization while the lowest was observed for pH 9 resin. This means that only an excess of NaOH reduced the cross-link formation. The O1s/C1s ratio obtained by XPS indicated that NaOH addition increased the oxygen concen- tration for RVC1000 samples, consequently, an increase in RVC disorder would be observed. However, sample pH9 1000, which presented the highest oxygen content, had a similar ordering to those for samples pH3 1000 and pH5 1000 when Raman param- eters were analyzed. This behavior was attributed to the reduced presence of carboxylic group and also by its processing from PFA with the lowest degree of polymerization. On the other hand, sam- ple pH7 1000 presented the highest disorder due to the highest carboxylic acid percentage among RVC1000 samples, associated to its processing from the cured PFA with the highest degree of poly- merization. After 1700 ◦C HTT, the removal of oxygen and NaOH led the carbon composition to close values considering the differ- ent pH samples. Sample pH7 1700 showed the highest disorder probably due to the predominant effect of its cured PFA resin with the highest degree of polymerization. The most ordered sample was exhibited for pH3 1700. Although sample pH9 1700 showed the average values of ID/IG intensity ratio and FWHM close to those of pH3 1700, its high standard deviation demonstrated the pres- ence of regions with high defect concentrations. XRD analyses, which provide information from the carbon bulk, showed samples pH5 1000 and pH5 1700 as the most ordered, which means that the addition of small quantities of NaOH in the PFA resin may be advantageous to minimize continuous polymerization of PFA resin 9 rface a w p A # S t A t 1 R [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ [ 6 S.S. Oishi et al. / Applied Su nd improve RVC microstructure. In general, the structural ordering as mainly influenced by the predominant effect of the cured PFA olymerization degree and carboxylic acid content on RVC surface. cknowledgements The authors acknowledge the financial support from grant 162683/2013-8 and 303287/2013-6 CNPq, grant #2014/27164-6 ão Paulo Research Foundation (FAPESP) and CAPES/PVNS. Special hanks to Dr. M.R. Baldan by XPS measurements. ppendix A. Supplementary data Supplementary data associated with this article can be found, in he online version, at http://dx.doi.org/10.1016/j.apsusc.2016.10. 12. eferences [1] B.A. Samuel, R. Rajagopalan, H.C. Foley, M.A. Haque, Effect of pyrolysis temperature on the microstructure of disordered carbon nanowires, Thin Solid Films 519 (2010) 91–95, http://dx.doi.org/10.1016/j.tsf.2010.07.066. [2] N. Amini, K.F. Aguey-Zinsou, Z.X. Guo, Processing of strong and highly conductive carbon foams as electrode, Carbon N.Y. 49 (2011) 3857–3864, http://dx.doi.org/10.1016/j.carbon.2011.05.022. 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3 Results 3.1 FTIR analysis 3.2 Morphological and surface characterizations 3.3 XRD analysis 3.4 Raman spectroscopy results 4 Discussion 5 Conclusions Acknowledgements Appendix A Supplementary data References