Synthesis and characterization of resorcinol–formaldehyde resin chars doped by zinc oxide

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Synthesis and characterization of resorcinol–formaldehyde resin chars doped by zinc oxide Vladimir M. Gun’ko a,∗ , Viktor M. Bogatyrov a , Olena I. Oranska a , Iliya V. Urubkov b , ˛ c Roman Leboda c , Barbara Charmas c , Jadwiga Skubiszewska-Zieba a

Chuiko Institute of Surface Chemistry, 17 General Naumov Street, 03164 Kyiv, Ukraine Kurdyumov Institute of Metal Physics, 36 Vernadsky Boulevard, 03142 Kyiv, Ukraine c Faculty of Chemistry, Maria Curie-Skłodowska University, 20031 Lublin, Poland b

a r t i c l e

i n f o

Article history: Received 9 January 2014 Received in revised form 24 February 2014 Accepted 26 February 2014 Available online xxx Keywords: Resorcinol–formaldehyde resin Zinc acetate Char Particle morphology Composite structure Texture

a b s t r a c t Polycondensation polymerization of resorcinol–formaldehyde (RF) mixtures in water with addition of different amounts of zinc acetate and then carbonization of dried gels are studied to prepare ZnO doped chars. Zinc acetate as a catalyst of resorcinol–formaldehyde polycondensation affects structural features of the RF resin (RFR) and, therefore, the texture of chars prepared from Zn-doped RFR. The ZnO doped chars are characterized using thermogravimetry, low temperature nitrogen adsorption/desorption, Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). At a relatively high content of zinc acetate (1 mol per 10–40 mol of resorcinol) in the reaction mixture, the formation of crystallites of ZnO (zincite) occurs in a shape of straight nanorods of 20–130 nm in diameter and 1–3 ␮m in length. At a small content of zinc acetate (1 mol per 100–500 mol of resorcinol), ZnO in composites is XRD amorphous and does not form individual particles. The ZnO doped chars are pure nanoporous at a minimal ZnO content and nano-mesoporous or nano-meso-macroporous at a higher ZnO content. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Resorcinol–formaldehyde polymeric (RFP) materials such as resins, gels, adhesives, glues, etc. are of importance for industrial applications [1,2]. Porous and light RFP gels possessing unique properties can be also used for preparation of porous chars as precursors of highly porous activated carbons [3,4]. It is important that resorcinol is of little toxicity and has high activity in reactions with formaldehyde in aqueous solutions free of organic solvents. Addition of small amounts of acidic or basic catalysts can strongly affect resorcinol–formaldehyde polycondensation processes and structural features of RFP. Variations in content of reaction components and water as a solvent and changes in a catalyst type and content allow significant and controlled changes in the structural, morphological, and textural characteristics of the RFP gels and related materials, e.g. the products of RFP carbonization. Frequently, carbonates or hydroxides of sodium or potassium are used as catalysts to prepare RFP. The amounts of these catalysts estimated as a

∗ Corresponding author. Tel.: +380 44 4229627; fax: +380 44 4243567. E-mail addresses: vlad [email protected], [email protected] (V.M. Gun’ko).

resorcinol/catalyst (R/C) ratio have been varied in a broad range from 10/1 to 2000/1 mol/mol [5]. Resorcinol–formaldehyde resins (RFR) are of interest as porous polymers or precursors of chars which can be used to prepare highly porous carbon materials [6–13]. The properties of the final carbon materials are mainly determined by the structure of polymeric precursors prepared by polycondensation of resorcinol with formaldehyde in the aqueous media. During RFR preparation, water can play a role of both a solvent and a template regulating the textural characteristics of RFR and chars [14]. On the basis of RFR, porous carbon composites containing different metals Fe, Co, Pt, Ag, Ni, Sn, Mn, etc. and their species have been synthesized to be used in electrochemical, catalytic, adsorption and other applications [15–19]. Metals and metal oxides in these composites can strongly affect the structural, textural and other characteristics of the materials [20]. Additionally, metal salts can play a role of a catalyst of the polycondensation processes on the RFR preparation, as well as the polymer carbonization. It is known that zinc acetate can be used as an ortho-orienting agent during the phenol-formaldehyde resin synthesis [21]. The aim of this paper is to study the influence of zinc acetate and its products (e.g. ZnO) on the morphological, structural, and textural

http://dx.doi.org/10.1016/j.apsusc.2014.02.164 0169-4332/© 2014 Elsevier B.V. All rights reserved.

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2 Table 1 Initial ratio of components used to prepare RFR. Series

Samples

Components ratio R/F (mol/mol)

R/W (w/w)

R/C (mol/mol)

1

RF1 RF2 RF3

1:2.8 1:2.8 1:2.8

1:3.26 1:4.00 1:4.00

10:1 20:1 40:1

2

RF4 RF5 RF6

1:2.0 1:2.0 1:2.0

1:5.94 1:5.94 1:5.94

100:1 200:1 500:1

Note: R/F and R/C are the mole ratio of resorcinol (R), formaldehyde (F) and catalyst (C, zinc acetate); and R/W is the weight ratio of resorcinol and water (W) in the reaction mixture. R/C = 10:1 (mol/mol) ≈ 5:1 (w/w).

features of char-ZnO composites prepared by carbonization of RFR synthesized in the presence of different amounts of zinc acetate. 2. Materials and methods 2.1. Materials Resorcinol (99.0%, pharm grade), 37% aqueous solution of formaldehyde, zinc acetate Zn(COOCH3 )2 ·2H2 O and distilled water were used in the synthesis of RFR. In the first series of RF samples (used then in preparation of ZnO doped RF chars RFC1, RFC2, and RFC3), the amounts of zinc acetate were greater by an order of magnitude than that in the second series of samples (RF4, RF5, and RF6) (Table 1) as precursors of chars RFC4, RFC5, and RFC6. Zinc acetate and ZnO formed during sample preparation can catalyze the polycondensation and carbonization reactions. Reaction compositions with zinc acetate were prepared by dissolution of a certain amount of resorcinol in distilled water with then added certain amount of zinc acetate dissolved. Then a certain amount of aqueous solution of formaldehyde was added and the mixtures were agitated for 2 min using a magnetic stirrer. The first series of samples heated at 90 ◦ C can fast transform into sol during 10–20 s. Samples of the second series were heated at 60 ◦ C for 18 h to form the gel. For sample 4 (Table 1), the gel formation was already observed in 2.5 h. After gelation, all the samples were dried at room temperature for 5 days. Then ground dried samples were heated at 100 ◦ C (first series) or 120 ◦ C (second series) for 2 h. Carbonization of the samples was carried out in an argon atmosphere with the gaseous products of the carbonization since a fixed bed reactor was used. The samples were heated at a rate of 5 ◦ C/min to 780 ◦ C (first series with RFC1, RFC2, and RFC3) or 800 ◦ C (second series with RFC4, RFC5, and RFC6), and then they were heated at this temperature for 2 h. 2.2. Methods ZnO doped chars were characterized using thermogravimetry (TG), low temperature nitrogen adsorption/desorption, Raman spectroscopy, XRD, scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM). TG measurements were carried out in air using a Derivatograph C (MOM, Budapest) apparatus with 18–20 mg of samples placed in a ceramic crucible treated at a heating rate of 10 ◦ C/min. The textural characteristics of ZnO doped chars were determined using low-temperature nitrogen adsorption–desorption isotherms recorded using a Micromeritics ASAP 2420 analyzer or a Quantachrome Autosorb analyzer. The pore size distributions were calculated using nonlocal (NLDFT) or quenched solid (QSDFT) density functional theory (Quantachrome Software, http://www.quantachrome.com/) with a slit/cylindrical pore

Fig. 1. XRD patterns of ZnO doped chars at the initial ratio R/C = 10/1 (curve 1), 20/1 (2), 40/1 (3) and 100/1 (4); i.e. for RFC1 (1), RFC2 (2), RFC3 (3), and RFC4 (4), and pure crystalline ZnO (zincite).

model. A complex model with slit/cylindrical pores and voids [22–25] between spherical nanoparticles (SCV) was used with a self-consistent regularization (SCR) procedure (SCV/SCR model) as described elsewhere [26–28]. The X-ray diffraction (XRD) patterns of ZnO doped chars were recorded at room temperature using a DRON-4-07 (PO “Burevestnik”, St. Petersburg) diffractometer with filtered Cu K␣ ( = 0.15418 nm) radiation in the 2 range from 10◦ to 70◦ with a step of 0.1◦ . The average size of ZnO crystallites was estimated according to the Scherrer equation. The Raman spectra of the studied materials were recorded using an inVia Reflex (Microscope DMLM Leica Research Grade, Reflex, Renishaw, UK) Raman microscope. The excitation was carried out by a laser at 514 nm. SEM images were recorded using a JEOL JSM-6700F scanning electron microscope. HRTEM images were recorded using a JEOL JEM-2100F microscope with an X-ray microanalyzer (Oxford). 3. Results and discussion XRD study of doped chars shows the presence of crystalline ZnO particles (zincite with hexagonal syngony according to JCPDS #79206 [29]) in samples RFC1-RFC3 and of amorphous carbon phase observed in the XRD patterns at 2 = 22◦ and 44◦ (Fig. 1). Clear, the content of crystalline ZnO (Fig. 1) decreases with decreasing amount of zinc acetate in the initial reaction mixture (Table 1). The ZnO lines in RFC1-RFC3 correspond to pure crystalline ZnO (zincite) shown in Fig. 1. The average size of ZnO crystallites (estimated using the Scherrer equation) is between 20 nm (RFC1) and 28 nm (RFC2, RFC3). These sizes correspond to minimal diameters of nanorods observed in SEM images of the composites (vide infra). For RFC4 (Fig. 1, curve 4), as well as RFC5 and RFC6 (not shown here), only two halos of amorphous carbon phase are observed at 2 = 22◦ and 44◦ . These amorphous carbon phase features, besides lines of crystalline ZnO, are also characteristic for samples RFC1RFC3. The crystallinity of RFC1 is 42% and an amorphous phase corresponds to 58% estimated from the ratio of integral intensity of all crystalline bands and two broad halos (at 2 = 22◦ and 44◦ ), respectively. The crystallinity of RFC2 and RFC3 is much lower and corresponds to 17% and 9%, respectively. It is known [30–32] that in hydrothermal synthesis or treatment, zinc oxide can form particles in the shapes of whiskers, tetrapods,

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Fig. 2. SEM images of samples (a) RFC1, (b) RFC2 and (c) RFC3 (magnification ×5000).

nanorods, etc. at relatively low (∼300 ◦ C) temperatures. However, heating of initial RFP/zinc acetate composites, corresponding to RF1 and RF2 (Table 1), at 370 ◦ C for 2 h in the argon atmosphere did not result in the formation of crystalline ZnO (according to XRD patterns not shown here) in contrast to samples RFC1 and RFC2 heated at 780 ◦ C (Fig. 1). According to SEM images of RFC1 and RFC2 (Figs. 2 and 3), ZnO particles have a whisker shape. These whiskers can be found as separated bunches or nanorods embedded into carbon particles. At a low content of ZnO (which is XRD-amorphous) in RFC4, RFC5, and RFC6, individual ZnO particles (such as nanorods in RFC1–RFC3) are not observed (Figs. 4 and 5). ZnO particles (Fig. 3a) correspond

Fig. 3. SEM images of nanorods in RFC2 at different magnification: (a) ×25,000 (scale bar 1 ␮m) and (b) ×100,000 (scale bar 100 nm), and (c) size distribution of nanorods shown in (a) calculated using Fiji software with local thickness plugin (http://pacific.mpi-cbg.de/wiki/index.php/Main Page).

mainly to straight nanorods of 20–130 nm in diameter (Fig. 3c) and 1–3 ␮m in length. A part of nanorods has a rough surface due to certain twisting along the main axis (Fig. 3b). According to HRTEM images, char particles have 20–50 nm in size in RFC4 (Fig. 4a) without visible ZnO structures due to its low content. At a higher ZnO content, e.g. in RFC3, crystalline ZnO nanoparticles can be embedded into larger carbon particles (Fig. 4b). The spacing between (1 0 0) planes in this ZnO particle

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Fig. 4. HRTEM images of samples (a) RFC4 (scale bar 100 nm) and (b) RFC3, labels correspond to d = 0.279 nm (1, ZnO according to X-ray microanalyzer data), 0.36 nm (2, pure carbon), and 0.59 nm (3, pure carbon) (scale bar 5 nm).

corresponds to 0.279 nm close to those in the hexagonal structure of zincite. In amorphous carbon particle (Fig. 4b), the spacing between small graphene sheets (several nm in size) is between 0.36 and 0.59 nm which correspond to nanopores (vide infra). Different particle morphology is characteristic for samples of the second series (Fig. 5). Contribution of nanoparticles characteristic for RFC4 (Figs. 4a and 5a) decreases in RFC5 (Fig. 5b) and particles in RFC6 are smooth and relatively large (Fig. 5c) corresponding to the 0.5–3 ␮m range in diameter (average d ≈ 1.2 ␮m, Fig. 6). The corresponding particle size distributions (PaSD) (Fig. 6) have relatively smooth curves, i.e. the particles are relatively uniform. Observed changes in the particle structure and morphology (Figs. 1–5) lead to significant changes in the textural characteristics of the materials (Figs. 7 and 8, Table 2). The total porosity (Table 2, Vp ) and contribution of mesopores (Table 2, Smeso , Vmeso ) increase in samples of the first series with increasing carbon content (Table 3). Changes in the carbon particle morphology for the second series lead to narrowing pores.

Fig. 5. SEM images of samples (a) RFC4, (b) RFC5 and (c) RFC6 (scale bar 1 ␮m).

Therefore, Vp decreases, the hysteresis loop becomes narrower (Fig. 7, curves 4–6), contributions of macropores (Table 2, Smacro , Vmacro ) and mesopores (Smeso , Vmeso ) decrease, but contribution of nanopores (Snano , Vnano ) increases. Additionally, contribution of slitshaped pores significantly increases in RFC6 in comparison with all other chars (Table 2, cslit ). All these textural changes are well visible as changes of the PSD (Fig. 8) and correlated to changes in the particle morphology from RFC1 to RFC6 (Figs. 2–6). The differential PSD (fV (R) ∼ dV/dR, ʃfV (R)dR ∼ Vp ) can be transformed into incremental PSD, IPSD (˚V,i (R) = Vp , ˚S,i (R) = S,

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Table 2 Textural characteristics of ZnO doped chars (SCV/SCR method). Sample

SBET (m2 /g)

Snano (m2 /g)

Smeso (m2 /g)

Smacro (m2 /g)

Vp (cm3 /g)

Vnano (cm3 /g)

Vmeso (cm3 /g)

Vmacro (cm3 /g)

w

cslit

ccyl

cvoid

RFC1 RFC2 RFC3 RFC4 RFC5 RFC6

479 528 541 556 525 484

272 296 301 357 347 443

207 232 240 187 172 42

0.3 1 0.4 13 7 0

0.592 0.975 0.918 0.675 0.470 0.249

0.128 0.139 0.138 0.151 0.143 0.221

0.454 0.815 0.767 0.270 0.178 0.025

0.010 0.020 0.013 0.254 0.149 0.003

0.125 0.151 0.120 0.210 0.188 0.311

0.377 0.369 0.287 0.242 0.142 0.806

0.536 0.531 0.586 0.533 0.538 0.134

0.087 0.100 0.127 0.225 0.320 0.060

Note: Nanopores (Snano , Vnano ) at radius or half-width R < 1 nm, mesopores (Smeso , Vmeso ) at 1 nm < R < 25 nm, and macropores (Smacro , Vmacro ) at 25 nm < R < 100 nm; w is a criterion showing the deviation of the pore model from the real pore in respect to the specific surface area [22–28]. Weight coefficients (cslit , ccyl and cvoid ) in the SCV/SCR model show contributions of different pores.

Table 3 Weight loss (−W) of composites in different temperature ranges and calculated carbon content. Sample

Weight loss −W (%) up to shown temperature ◦

RFC1 RFC2 RFC3 RFC4 RFC5 RFC6

◦

Carbon content (wt.%) ◦

◦

◦

200 C

400 C

600 C

800 C

1000 C

1.14 0.58 0.61 0.82 0.51 0.66

0.80 0.34 0.75 0.41 −0.47 0.00

8.67 8.35 8.30 23.28 25.16 31.05

27.26 36.88 35.94 77.42 84.79 97.43

44.45 60.83 57.66 96.36 98.37 98.48

Fig. 6. Particle size distributions (PaSD) for samples RFC2, RFC4, and RFC6 calculated from SEM images; and the average diameter values are shown.

Fig. 7. Nitrogen adsorption–desorption isotherms for ZnO doped chars RFC1 (curve 1), RFC2 (2), RFC3 (3), RFC4 (4), RFC5 (5), and RFC6 (6).

43.3 60.2 57.0 95.5 97.9 97.8

where i is the pore type model) which give clear PSD pictures at large R values [22–28]. The main changes in the PSD are better seen in the SCV/SCR IPSD curves (Fig. 8a and b) than in NLDFT PSD (differential) curves (Fig. 8c and d). For the samples of the first series, a PSD peak of nanopores (Fig. 8a and c) depends weakly on the carbon and ZnO contents; however, the main mesoporous peak shifts toward larger pore sizes with increasing carbon content (Table 3, Fig. 8). For the second series, a PSD peak of nanopores is practically the same for RFC4 and RFC5, but for RFC6, it shifts toward smaller pore sizes. RFC6 does not practically have mesopores and macropores (Fig. 8b and d), and it is characterized by a minimal porosity among all the chars studied (Table 2). The porosity of RFC5 is lower than that of RFC4 over practically the total pore size range. It should be noted that contribution of slitshaped pores decreases for RFC5 in comparison with RFC4, but contributions of cylindrical pores and voids between particles increase (Table 2, cslit , ccyl , and cvoid , respectively). The SCV/SCR model errors (Table 2, w) [33] are smaller for the first series samples than that for the second series. This result can be explained by changes in the particle morphology (Figs. 2–6) with a significant increase in contributions of macropores (RFC4, RFC5) or nanopores (RFC6). Notice that the NLDFT and QSDDFT PSDs are similar for the studied chars, e.g. for RFC1 (Fig. 8e). Therefore, mainly NLDFT PSDs were shown here. Comparison of the PSD NLDFT and PSD HRTEM/Fiji (Fig. 8f) for RFC3 shows that the HRTEM/Fiji gives interatomic distances in the range up to 0.2 nm (Zn-O distance) which is smaller than the N2 molecule size. However, there is a certain correspondence of these PSD at the pore width in the 0.5 nm < d < 1 nm range. The first HRTEM/Fiji peak (Fig. 8f) corresponds to Zn-O distances in zincite (0.197–0.199 nm), and next peaks correspond to O–O and Zn–Zn distances in ZnO crystallites. Larger distances can be assigned to pores in the carbon phase (this is confirmed by X-ray microanalyzer data for different particles of studied samples). As a whole, it is difficult to expect that the HRTEM/Fiji PSD and NLDFT PSD will totally coincide not only because of the difference in these methods but also because of the fact that the former was calculated using image of one particle (Fig. 4b) but the latter was calculated using nitrogen adsorption/desorption onto ∼0.2 g of the material which includes a huge number (∼1014 ) of nanoparticles.

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Fig. 8. Incremental (a, b) and differential (c-f) pore size distributions calculated for chars using (a, b) SCV/SCR, (a, c, d, e, f) NLDFT and (e) QSDFT, and PSD calculated using HRTEM image (Fig. 4b) and Fiji software (http://pacific.mpi-cbg.de/wiki/index.php/Main Page, local thickness plugin).

Porous phenol-formaldehyde resin beads and corresponding chars [34–36] have the PSDs similar to that of RFC1-RFC5. The burnoff degree for phenol-formaldehyde resin chars from 25 to 85% can give highly porous activated carbons (AC) with the specific surface area of 1000–3500 m2 /g and the pore volume of 1.0–2.5 cm3 /g, respectively [35–37]. Therefore, one could assume that AC based on RFR chars studied here can be highly porous with great SBET values. According to the thermogravimetric (TG) study at a heating rate of 10 ◦ C/min, the weight loss W (Fig. 9a) and the weight loss rate d(W)/dT (Fig. 9b) decrease with increasing ZnO content in composites. The decrease in d(W)/dT for the first series samples can be explained by a kinetic deceleration of oxidation processes due to formation of denser carbon structures around ZnO particles

(e.g. intensity of the G band in Raman spectra increases, Fig. 10a). Additionally, for the first series samples, the TG and differential TG (DTG) curves do not demonstrate a plateau at 1000 ◦ C. However, the total oxidation of the second series samples with a smaller content of ZnO occurs at 800–900 ◦ C (Fig. 9a) at the maximal oxidation rate at 620–650 ◦ C (Fig. 9b). In the 200–400 ◦ C range, the weight of samples can increase (by 0.14–0.98%) due to oxidation of a carbon surface by oxygen from the air. The TG data in the range 200–1000 ◦ C were used to estimate the carbon content in the samples (Table 3). Raman spectra (Fig. 10) were recorded from microscaled areas of samples using the Raman microscope. The spectra of RFC1 and RFC2 (Fig. 10a) are typical for chars and related activated carbons [38–41], carbon-ZnO [42–45] or ZnO structures

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Fig. 9. ZnO doped chars: (a) thermograms in the 25–1000 ◦ C range and (b) differential TG curves.

[46,47]. For the carbon phase, there are two most characteristic bands such as the D band (sp3 carbons in non-graphitic structures) and the G band (sp2 carbons in graphitic structures composed of small graphenes observed in HRTEM image in Fig. 4b). Similar bands with a similar D/G ratio were observed for activated carbons prepared from phenol-formaldehyde resin chars [28,34–36]. The D and G bands of RFR chars (Fig. 10a) include both narrow and broad components [14] related to more ordered and disordered carbon structures, respectively. The broad component of the G band can be assigned to amorphous graphite-like structures [48], which are out of the graphene planes (see, e.g., Fig. 4b). There is no a unique assignment of a weak component of the D band at ∼1100–1200 cm−1 [49–55]. It can be assigned to mixed sp2 –sp3 bonding or to the C–C and C C stretching vibration modes of polyene-like structures [55], or structures with sp3 C atoms in defects out of the planes. There is a certain difference in the D/G ratio for pure RFR chars [14] and ZnO doped RFR chars (Fig. 10), since the D band intensity increases for local structures with ZnO abundance (compare D band intensity for RFC2(1) and RFC2(2) in Fig. 10a). In the 200–700 cm−1 range of the Raman spectra of RFC1 and RFC2 (Fig. 10b), there are bands corresponding to ZnO structures [32,56]. Notice that the ZnO bands intensity in the Raman spectra (recorded using the Raman microscope) depends on local structure of samples (compare curves RFC2(1) and RFC2(2) in Fig. 10a) due to nonuniformity of the ZnO distribution in the composite materials as

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Fig. 10. Raman spectra of samples RFC1 and RFC2 in different shift ranges: (a) RFC2(1) and RFC2(2) correspond to different parts of the RFC2 sample; (b) spectra in the range corresponding to ZnO structures in samples RFC1 and RFC2.

it is observed in SEM and HRTEM images (Figs. 2–5). The difference between the positions of the Raman bands for pure crystalline ZnO and ZnO/char composites can be caused by carbon doping of the ZnO particles and the formation of some defects [57–59], e.g. caused by ZnO nanorod twisting (Fig. 3b). 4. Conclusion Polycondensation of resorcinol–formaldehyde mixtures in the presence of zinc acetate at its broad content range and then carbonization of RFR result in the formation of ZnO doped chars of different particle morphology and texture. Zinc acetate can play a role of a catalyst of polycondensation of resorcinol–formaldehyde mixtures. At relatively large content of zinc acetate, the formation of ZnO crystallites occurs in the shape of straight nanorods of 20–130 nm in diameter and 1–3 ␮m in length. These ZnO crystallites of hexagonal symmetry (zincite) can form separated structures or can be embedded into carbon particles. In carbon particles, ZnO particles can be of smaller sizes than that in separated ZnO structures. In char samples at small dopant content, ZnO is XRD amorphous. The texture of the doped chars depends strongly on the ZnO content. The chars can be pure nanoporous and composed of microsized smooth globules (minimal ZnO content), or composed of nano-mesoporous particles of 20–100 nm in size (at higher ZnO content) differently aggregated into secondary

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meso-macroporous structures. Thus, changes in the amounts of zinc acetate in the resorcinol–formaldehyde mixtures forming sol and gel at different temperatures, dried and carbonized at 780–800 ◦ C to form ZnO doped chars can cause controlled changes in the textural characteristics of the carbon materials.

[26]

[27]

[28]

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Please cite this article in press as: V.M. Gun’ko, et al., Synthesis and characterization of resorcinol–formaldehyde resin chars doped by zinc oxide, Appl. Surf. Sci. (2014), http://dx.doi.org/10.1016/j.apsusc.2014.02.164