St?ber synthesis of salen-formaldehyde resin polymer- And carbon spheres with high nitrogen content and application of the corresponding Mn-containing carbon spheres as efficient electrocatalysts for the oxygen reduction reaction

Article abstract of DOI:10.1039/d0ra03815f

Salen-formaldehyde (SF) resin polymer spheres were synthesized by the St?ber method from 4,4′-dihydroxysalen (N,N′-bis-(4-hydroxysalicylidene)-ethylenediamine; a tetradentate N2O2 Schiff base ligand) and formaldehyde. The salen precursor was prepared by condensation of ethylenediamine with 2,4-dihydroxybenzaldehyde in methanol. The SF resin colloidal spheres were also prepared by using Pluronic F127 and ammonia as a porogenic agent and catalyst, respectively (SF-P). In addition, corresponding Mn(ii)-coordinated polymer spheres of the SF-P were synthesized (SF-P-Mn(ii)). Corresponding monodispersed carbon spheres of all of the abovementioned samples were also obtained by pyrolysis technique. All of the products were characterized with conventional microscopic and spectroscopic techniques, as well as other physical methods such as BET analysis. It was found that carbonization of the SF resin spheres results in carbon spheres with specific surface areas in the range of 499-528 m2 g-1 and average pore sizes in the range of 2.58-3.08 nm. Nitrogen content of the SF-MWHT (obtained hydrothermally in a methanol/water mixture), and SF-P-C&at;Mn (obtained from carbonization of SF-P-Mn(ii)) samples were as high as 27.5 wtpercent and 35.02 wtpercent, respectively. Finally, a glassy carbon electrode (GCE) modified with SF-P-C&at;Mn (SF-P-C&at;Mn/GCE) was prepared and its electrocatalytic activity was evaluated for oxygen reduction reaction (ORR) by linear sweep voltammetry (LSV). The LSV results showed that the SF-P-C&at;Mn/GCE has a higher current density and a lower negative potential in the ORR compared to GCE. This journal is

Full text of DOI:10.1039/d0ra03815f

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Stober synthesis of salen-formaldehyde resin
¨
polymer- and carbon spheres with high nitrogen
Cite this: RSC Adv., 2020, 10, 27575
content and application of the corresponding Mn-
containing carbon spheres as efficient
electrocatalysts for the oxygen reduction reaction†
a
Tahereh Hosseinzadeh Sanatkar,a Alireza Khorshidi,
Rouhollah Yaghoubi,b
*
Esmail Sohoulic and Jamaladin Shakerid
Salen-formaldehyde (SF) resin polymer spheres were synthesized by the Stober method from 4,40-
¨
dihydroxysalen (N,N0-bis-(4-hydroxysalicylidene)-ethylenediamine;
a tetradentate N2O2 Schiff base
ligand) and formaldehyde. The salen precursor was prepared by condensation of ethylenediamine with
2,4-dihydroxybenzaldehyde in methanol. The SF resin colloidal spheres were also prepared by using
Pluronic F127 and ammonia as a porogenic agent and catalyst, respectively (SF-P). In addition,
corresponding Mn(II)-coordinated polymer spheres of the SF-P were synthesized (SF-P-Mn(II)).
Corresponding monodispersed carbon spheres of all of the abovementioned samples were also
obtained by pyrolysis technique. All of the products were characterized with conventional microscopic
and spectroscopic techniques, as well as other physical methods such as BET analysis. It was found that
carbonization of the SF resin spheres results in carbon spheres with specific surface areas in the range of
499–528 m2 gꢀ1 and average pore sizes in the range of 2.58–3.08 nm. Nitrogen content of the SF-
MWHT (obtained hydrothermally in
a methanol/water mixture), and SF-P-C@Mn (obtained from
carbonization of SF-P-Mn(II)) samples were as high as 27.5 wt% and 35.02 wt%, respectively. Finally,
a glassy carbon electrode (GCE) modified with SF-P-C@Mn (SF-P-C@Mn/GCE) was prepared and its
electrocatalytic activity was evaluated for oxygen reduction reaction (ORR) by linear sweep voltammetry
(LSV). The LSV results showed that the SF-P-C@Mn/GCE has a higher current density and a lower
negative potential in the ORR compared to GCE.
Received 28th April 2020
Accepted 7th July 2020
DOI: 10.1039/d0ra03815f
rsc.li/rsc-advances
features.7 Various strategies have been reported for the prepa-
ration of polymer colloids and their derived carbon-based
nanomaterials.8–10
Carbon nanospheres have been synthesized by several
methods such as chemical vapor deposition (CVD), nanocasting
with silica spheres as hard templates, hydrothermal carbon-
Introduction
Polymer and carbon spheres have attracted a myriad of research
interest due to their interesting properties including high
surface areas, electric conductivity, thermal stability, and
biocompatibility. Their application has been extended to
various elds such as catalyst preparation, separation, adsorp-
tion, energy storage/conversion, and biomedical engineering.1–6
In the past years, spherical carbon micro- and nanoparticles
have been at the center of interest because of their unique
¨
ization of sugars, Stober synthesis, so-templating strategies via
organic–organic self-assembly, etc.11 The well-known Stober
¨
method was rst reported in 1968 for the synthesis of colloidal
silica spheres via controlled hydrolysis and condensation of Si-
containing compounds such as tetraethyl orthosilicate (TEOS)
with ammonia as a catalyst in aqueous alcoholic solution.12
aDepartment of Chemistry, Faculty of Sciences, University of Guilan, Rasht, Guilan,
¨
Later, Liu et al. reported a modication to the Stober synthesis
41335-1914, Iran. E-mail: khorshidi@guilan.ac.ir
bDepartment of Chemistry, Faculty of Science, Tarbiat Modares University, Tehran,
for preparation of resorcinol formaldehyde (RF) polymer
spheres with uniform and controllable particle size on sub-
micrometer scale, and subsequent conversion of these spheres
into carbon spheres via pyrolysis.13 Particle size of the resultant
polymer- or carbon spheres can be controlled by varying the
experimental parameters, including the alkyl chain of the
alcohol, alcohol/water ratio, the amount of ammonia,
Iran
cYoung Researchers and Elites Club, Science and Research Branch, Islamic Azad
University, Tehran, Iran
dDepartment of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111,
Iran
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra03815f
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formaldehyde, RF precursor, and so on.13 Resorcinol formal- the chelating effect. Moreover, pyrolysis technique was used to
dehyde (RF) resins are interesting precursors which can be used prepare carbon spheres of the aforementioned resin polymers.
to prepare highly porous carbon materials.14,15 On the other Finally, Mn nanoparticles-loaded carbon spheres were
hand, various polymeric precursors, such as glucose,16 dopa- successfully used as an electrocatalyst for the ORR process.
mine,17 tannic acid,18 phenol-formaldehyde resin19 and con-
ducting polymers20 etc., have been successfully used to
synthesize new uniform nanospheres, and provided new
Experimental
Materials and methods
opportunities in the research area of colloidal and carbon
spheres. The scope of applications of carbon materials have
been made even more extensive by heteroatom-doping.21,22
Doping heteroatoms such as nitrogen, sulfur and phosphorus,
even in small quantities in the carbon framework, can lead to
signicant enhancements in the performance of polymer
colloids and their derived carbon-based nanomaterials.23,24
Nitrogen is the most widely used heteroatom that can be doped
directly into the polymer colloids.
All of the chemicals including 2,4-dihydroxybenzaldehyde and
ethylenediamine were purchased from Merck (http://
www.merck.com)
www.sigmaaldrich.com) Companies.
All solvents and reagents were of analytical grade and used
without further purication. Heraeus CHN–O-Rapid
or
Sigma-Aldrich
(http://
A
elemental analyzer was used for elemental analyses. FT-IR
spectra were recorded on a Bruker Alpha spectrophotometer
using KBr pellets over the range of 4000–400 cmꢀ1. UV-Vis
spectra were recorded on a Rayleigh UV-1800 spectrophotom-
Schiff bases are a group of compounds that contain nitrogen
atoms in their structure. They are characterized by an imine or
azomethine group (–RC]N–), and are generally synthesized by
condensation of primary amines with carbonyl compounds.25
They form an important class of ligands and have gained
considerable attention due to their diverse applications such as
catalysis,26 electrochemistry,27 bioscience,28 optics,29 host–guest
chemistry,30 and molecular recognition.31 Moreover, Schiff
bases have been successfully used to remove heavy metals
selectively from aqueous media.32 Among Schiff bases, salen-
type compounds are an interesting class of the tetradentate
ligands with four donor sites (N2O2).33 As shown in Scheme 1,
the 4,40-dihydroxysalen ligand provides an ideal environment
for equatorial coordination of various transition metals. While
synthesis of polymer spheres with high nitrogen content is
highly desirable for specic applications, there are few reports
on preparation of such monodisperse polymer spheres.22,34
Carbon nanospheres have been recognized as promising
electrocatalysts for the oxygen reduction reaction (ORR).13 N-
doped mesoporous carbon nanospheres exhibit unique
features, including enhanced dissociation of O2 within the
pores, and improved wettability and charge/spin density on the
carbon, to act as an electrode for the ORR.35 Moreover, deco-
ration of the carbon nanospheres with electroactive metal
nanoparticles can obviously enhance the ORR capacity.18
1
eter. H NMR spectra were recorded in DMSO-d6 on a Bruker
Avance III 300 MHz spectrometer at 298 K using tetrame-
thylsilane as an internal standard. X-ray powder diffraction
(XRD) data were acquired on a Philips PW1730 diffractometer
with Cu Ka radiation (l ¼ 1.5406 A). Field emission scanning
˚
electron microscopy (FESEM) images of the synthesized
samples were obtained using a Tescan Mira III scanning elec-
tron microscope (Tescan, Czech Republic). A layer of Au was
used for all samples before imaging. Energy dispersive X-ray
technique (EDX) was used for surface elemental analysis. TEM
imaging was performed on a Zeiss-EM10C microscope at an
accelerating voltage of 100 kV. Brunauer–Emmett–Teller (BET)
analyses were performed at 77 K on a Belsorp mini II gas
adsorption analyzer, Japan. All of the samples were degassed at
200 ꢁC for 2 h before BET analysis. X-ray photoelectron spec-
troscopy (XPS) analysis was performed by Gamma data-scienta
ESCA 200 hemispherical analyzer equipped with an Al Ka
(1486.6 eV) X-ray source.
Electrochemical measurements
All of the electrochemical measurements were carried out on
a potentiostat/galvanostat of the type Autolab PGSTAT302N (Eco
Chemie B. V., Switzerland) driven by the Nova 2.1.2 soware. A
conventional three-electrode setup including a modied or
unmodied GCE working electrode, an Ag/AgCl (3.00 M KCl)
reference electrode, and a Pt wire auxiliary electrode (Azar
Electrode, Iran) was used. Some electrochemical techniques
such as cyclic voltammetry (CV), electrochemical impedance
spectroscopy (EIS) and amperometry were used in the experi-
ments. The EIS measurements were performed at a frequency
range of 10.0 kHz to 10 MHz, and 0.1 M KOH was utilized as
a redox probe.
Herein, we report the synthesis of monodisperse salen-
0
¨
formaldehyde (SF) resin spheres by Stober method using 4,4 -
dihydroxysalen and formaldehyde as precursors, and Pluronic
F127 as a template and porogenic agent. In addition, the cor-
responding polymeric metal complex spheres of the SF resin
were obtained by deprotonation of the tetradentate (N2O2) salen
moieties and coordination with Mn(II) by taking advantage of
Syntheses
Synthesis of the salen compound
The
4,40-dihydroxysalen
(N,N0-bis-(4-hydroxysalicylidene)-
Scheme 1 Molecular structure of 4,40-dihydroxysalen.
27576 | RSC Adv., 2020, 10, 27575–27584
ethylenediamine) was synthesized by condensation of
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ethylenediamine with 2,4-dihydroxybenzaldehyde in methanol. process was similar to that described above section. First,
Briey, ethylenediamine (0.33 mL, 5.0 mmol) was added drop- ammonia solution (25 wt%, 0.1 mL) was mixed with 16 mL of
wise to a solution of 2,4-dihydroxybenzaldehyde (1.38 g, 10.0 absolute methanol and 12 mL of deionized water and stirred for
ꢁ
mmol) in 100 mL of methanol at room temperature. The reaction 1 h at 30 C. Then, 4,40-dihydroxysalen (0.55 g) was added and
mixture was then reuxed for 3 h with continuous stirring. Aer magnetically stirred for 30 min. Aerwards, 0.27 g of triblock
cooling down to room temperature, a yellow precipitate was copolymer Pluronic F127 was added and stirred for 30 min. In
ltered off and washed with methanol. Yield: 88%. Anal. calcd for the next step, 0.28 mL of 37% formaldehyde solution was added
C16H16N2O4 (MW ¼ 300.31 g molꢀ1): C, 63.99; H, 5.37; N, 9.33. dropwise. Aer few minutes, the solution became cloudy and
Found: C, 63.87; H, 5.45; N, 9.41%. IR (KBr pellet, cmꢀ1): 3416 (s, stirring was continued for 24 h at the same temperature. The
nOH), 1639 (s, nC]N), 1234 (m, nPh–O). UV-Vis: lmax (nm) light-yellow reaction mixture was then transferred into a Teon-
(3, Mꢀ1 cmꢀ1) (DMSO): ꢂ280 (25 750), 308 (21 100), 390 (2100). lined autoclave and heated at 100 ꢁC for 24 h. Aer cooling
1H NMR (DMSO-d6, 300 MHz, ppm); d: 13.58 (br, 2H7), 9.80 (br, down to room temperature, a dark-yellow solid product was
2H5), 8.37 (s, 2H2), 7.18 (d, 2H3, J ¼ 8.4 Hz), 6.28 (dd, 2H4, J ¼ 8.4 obtained aer centrifugation followed by air-drying at 100 C,
ꢁ
and 2.4 Hz), 6.27 (d, 2H6, J ¼ 2.4 Hz), 3.79 (s, 4H1).
which was denoted as SF-P (68% yield).
Synthesis of the Mn(II)-coordinated SF-P spheres (SF-P-Mn(II))
Synthesis of the SF resin polymer spheres
The overall process for the synthesis of SF-P-Mn(II) was similar
to that described in above sections. First, ammonia solution
(25 wt%, 0.1 mL) was mixed with 16 mL of absolute methanol
and 12 mL of deionized water and stirred for 1 h at 30 ꢁC. Then,
4,40-dihydroxysalen (0.55 g) was added and magnetically stirred
for 30 min. Aerwards, 0.27 g of triblock copolymer Pluronic
F127 was added and stirred for 30 min. In the next step, 0.28 mL
of 37% formaldehyde solution was added dropwise. Aer few
minutes, the solution became cloudy and stirring was
continued for 24 h at the same temperature. Then a methanolic
solution of Mn(NO3)2$4H2O (2 mL, 0.1 M) was added dropwise
to the reaction mixture and stirred for 8 h. It should be
mentioned that aer addition of the Mn(II) salt, color of the
solution became dark red. Then the reaction mixture was
Monodisperse SF resin spheres were synthesized by using 4,40-
dihydroxysalen and formaldehyde as precursors. In a typical
synthesis, ammonia solution (25 wt%, 0.1 mL) was mixed with
16 mL of absolute methanol and 12 mL of deionized water and
ꢁ
stirred for 1 h at 30 C. Then, 4,40-dihydroxysalen (0.55 g) was
added and magnetically stirred for 30 min. Aerwards, 0.28 mL
of 37% formaldehyde solution was added dropwise to the above
solution. Aer few minutes, the solution became cloudy and
stirring was continued for 24 h at the same temperature. A light-
yellow solid product was obtained aer centrifugation followed
by air-drying at 100 ꢁC, which was denoted as SF-MW (73% yield).
The same procedure was repeated and right before separation
of the product, a hydrothermal process was applied in a Teon-
ꢁ
lined autoclave at 100 C for 24 h under a static condition. Aer
ꢁ
transferred into a Teon-lined autoclave and heated at 100 C
for 24 h. Aer cooling down to room temperature, the polymeric
cooling down to room temperature, the dark-yellow SF resin
polymer spheres were separated by centrifugation and air-dried at
100 ꢁC for 24 h. The product was denoted as SF-MWHT (70% yield).
In addition, hydrothermally processed SF resin polymer
spheres were obtained in the same manner in 28 mL of absolute
methanol (instead of a 16 : 12 mixture of MeOH : H2O mixture),
which were denoted as SF-MHT (76% yield).
Mn(II) complex was collected by centrifugation and dried at
ꢁ
100 C for 24 h (67% yield).
Carbonization of the SF resin polymer spheres
For carbonization, the precursors were transferred in a quartz
boat into a tube furnace and heated under N2 atmosphere. As
Pluronic F127-directed synthesis of the SF resin polymer
spheres (SF-P)
The SF-P spheres were synthesized by the abovementioned
precursors and triblock copolymer Pluronic F127. The overall
Fig. 2 FT-IR spectra of (a) 4,40-dihydroxysalen, (b) SF-MWHT, (c) SF-P-
Fig. 1 The heating program used for carbonization.
C, and (d) SF-P-C@Mn.
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with a 5 ꢁC minꢀ1 heating rate and maintained at 600 ꢁC for 4 h.
Finally, the samples were cooled down to room temperature.
The obtained samples from SF-MWHT, SF-P, and SF-P-Mn(II)
were denoted as SF-C, SF-P-C, and SF-P-C@Mn, respectively.
Preparation of the modied carbon paste electrode
To prepare the SF-P-C@Mn-modied glassy carbon electrode,
surface of the electrode was polished by alumina powder,
washed with water, and sonicated in deionized water to elimi-
nate any surface contamination. Then, 1.0 mg of the SF-P-
C@Mn was mixed with 1.0 mL of water in a test tube and irra-
diated in an ultrasonic bath for 1 h. Then, 5 mL of this mixture
Fig. 3 UV-Vis spectrum of 4,40-dihydroxysalen in DMSO (2 ꢃ 10ꢀ5 M)
at room temperature.
ꢁ
was placed on the electrode in an oven at 50 C to prepare the
SF-P-C@Mn-modied glassy carbon electrode which was
denoted as SF-P-C@Mn/GCE. The SF-C/GCE and SF-P-C/GCE
were prepared in the same way.
Results and discussion
Spectral characterization of the salen compound
In the FT-IR spectrum of 4,40-dihydroxysalen (Fig. 2a), broad
band of the phenolic O–H stretching (nO–H) was appeared at
3416 cmꢀ1
.
Strong band of the azomethine moiety (nC]N) was observed
at 1639 cmꢀ1, and that of the C–O stretching (nPh–O) was
observed at 1234 cmꢀ1
.
Electronic absorption spectrum of the 4,40-dihydroxysalen
was recorded at room temperature in DMSO (2 ꢃ 10ꢀ5 M). As
shown in Fig. 3, the UV-Vis spectrum of 4,40-dihydroxysalen has
three absorption bands at 280, 308 and 390 nm. The highly
intense bands at 280 and 308 nm are assigned to p / p* and n
/ p* transitions of the salen, and the broad band at about
390 nm is attributed to intraligand charge-transfer.36
1
Fig. 4 H NMR spectrum of 4,40-dihydroxysalen in DMSO-d6.
shown in Fig. 1, rst, the temperature was raised from room
temperature to 350 ꢁC with a heating rate of 5 ꢁC minꢀ1 and
kept at 350 ꢁC for 2 h. Next, the temperature was raised to 600 ꢁC
1H NMR spectra of the salen Schiff base showed broad
signals for phenolic OH7 and OH5 moieties at 13.58 and
9.80 ppm, respectively (Fig. 4). The aromatic protons resonated
Fig. 5 A schematic illustration for preparation of the SF, SF-P and SF-P-Mn(II) samples and their corresponding carbonization products.
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Fig. 6 SEM images of (a) SF-MW, (b) SF-MWHT, and (c) SF-MHT resin polymer spheres.
Table
polymers
1 The parameters of nitrogen adsorption of the SF resin
in the range of 6.18–7.18 ppm and unambiguously assigned
according to their coupling constants (see Experimental). The
expected singlet for the proton of the imine moiety (H2) was
observed at 8.37 ppm, and the aliphatic protons (H1) were
observed at 3.79 ppm as a singlet.
Sample
SBET (m2 gꢀ1
)
VTP (cm3 gꢀ1
)
DMP (nm)
SF-MW
SF-MWHT
SF-MHT
10.88
15.78
4.08
0.11
0.18
0.01
41.21
46.72
11.74
Characterization of the prepared resin polymer- and carbon
spheres
A schematic illustration for preparation of the SF, SF-P and SF-
P-Mn(II) resin polymer spheres and their corresponding carbon
It is clear that the colloidal spheres had irregular shapes
before hydrothermal treatment (Fig. 6a) and they became more
spherical with smooth surfaces and reduced roughness aer
hydrothermal processing (Fig. 6b).37 It was also found that the
solvent has an impact on the size of the obtained spheres. From
Fig. 6c it is clear that in the case of the SF-MHT colloidal spheres,
although absolute methanol as a solvent leads to the production
of smoother spherical particles, instead it results in an increase
in their size.
¨
spheres is shown in Fig. 5. In a typical Stober method, stable
emulsion droplets are formed without any surfactant through
hydrogen bonding interactions between water, methanol and
4,40-dihydroxysalen Schiff base. Then, polymerization of 4,40-
dihydroxysalen and formaldehyde takes place inside the drop-
lets by ammonia catalysis. During this process, 4,40-dihydrox-
ysalen, formaldehyde and ammonia react to form numerous
condensation compounds such as hydroxymethyl substituted
species that cross-link together and result in the formation of
uniform colloidal SF polymer spheres. The ammonium cations
not only accelerate the polymerization reaction between salen
and formaldehyde in the emulsion, but also supply the positive
charges distributing on the outer surface of polymer spheres
that prevent the aggregation of the resin spheres.13
SEM images of the SF resin spheres before and aer hydro-
thermal treatment (SF-MW and SF-MWHT, respectively) are
shown in Fig. 6a and b.
Fig. 7 N2 adsorption–desorption isotherms of (A) SF-MW, (B) SF- Fig. 8 (a) EDX analysis for the SF-MWHT resin polymer spheres, and (b)
MWHT, and (C) SF-MHT samples. The insets show the corresponding the corresponding elemental mapping. The inset shows the elemental
BJH curves.
analysis data.
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Fig. 9 SEM images of (a and d) SF-C, (b and e) SF-P-C, and (c and f) SF-P-C@Mn. The insets show the corresponding particle size distribution
curves.
N2 adsorption–desorption isotherms of the SF samples are SF-MWHT samples, and micro- and mesopores in the SF-MHT
shown in Fig. 7. This study provided valuable information on sample.
the specic surface area and porosity of the prepared resin
Table 1 shows specic surface area (SBET), total pore volume
polymers. The SF resin spheres before and aer hydrothermal (VTP) and mean pore diameter (DMP) values for each of the
treatment showed a type IV N2-sorption isotherm with distinct samples.
hysteresis loops in the range of p/p0 from 0.85 to 1.0, indicative
It is clear that the solvent system and hydrothermal treatment
of porous adsorbents with pores in the range of 1.5–100 nm. have signicant positive effects on the morphology and physi-
The SF-MHT sample, on the other hand, showed a type II N2- cochemical properties of the SF resin polymer spheres, which in
sorption isotherm, characteristic of powders with diameters turn may appear in their corresponding carbon spheres.
exceeding micropores. The corresponding Barrett–Joyner–
Based on the abovementioned results, the SF-MWHT sample
Halenda (BJH) curves (insets of Fig. 7), clearly indicate the was selected for further studies.
presence of micro-, meso, and macropores in the SF-MW and
Fig. 11 (a) EDX spectrum of the SF-P-C@Mn carbon spheres, and (b–f)
Fig. 10 TEM images of (a, d) SF-C, (b, e) SF-P-C, and (c, f) SF-P-C@Mn. the corresponding mapping images. The inset shows the elemental
analysis data.
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Fig. 12 N2 adsorption–desorption isotherms of (A) SF-C, (B) SF-P-C, and (C) SF-P-C@Mn carbon spheres. The insets show the corresponding
BJH curves.
Nitrogen content of the SF-MWHT resin polymer spheres was inset of Fig. 11a). Also, homogeneous distribution of the C, N, O
determined by energy dispersive X-ray (EDX) analysis (Fig. 8). As and Mn constituents is evident from Fig. 11b–f.
the inset of Fig. 8a shows, nitrogen content of the SF-MWHT
Porosity of the carbonized samples was also studied. N2
sample was about 27.5 wt%. EDX-mapping (Fig. 8b), on the adsorption–desorption isotherms of the carbonized samples are
other hand, revealed homogeneous distribution of the nitrogen shown in Fig. 12. While the SF-C sample exhibited a type II
atoms throughout the sample.
isotherm, the other two showed a type IV isotherm with distinct
FT-IR spectrum of the SF-MWHT sample is shown in Fig. 2b hysteresis loops in the range of p/p0 from 0.3 to 1.0. Physico-
and exhibits the anticipated bands for nO–H, nC]N, and nPh–O at chemical properties of the carbonized samples are listed in
3420, 1635, and 1234 cmꢀ1, respectively.
Table 2. Clearly, specic surface area (SBET), total pore volume
Fig. 9 shows the SEM images of the carbonization products (VTP), and mean pore diameter (DMP) of each sample was
SF-C, SF-P-C and SF-P-C@Mn. It is clearly seen that the obtained improved aer calcination.
carbon spheres have regular spherical shapes with smooth
surfaces.
In the case of the SF-C sample, a signicant improvement in
the BET surface area from 15.78 to 499 m2 gꢀ1 was observed
+
Aer carbonization, however, some defects were observed on which can be attributed to the loss of water, NH4 and other
the surface of carbon particles as a result of shrinkage during functional groups during the carbonization process.38
the calcination process.18 It is noteworthy that the use of Plur-
Total pore volume was also increased accordingly from 0.18
onic F127 as a template resulted in an increase in particle size to 0.33 cm3 gꢀ1. In the case of the SF-P-C and SF-P-C@Mn
from about 140 nm for the SF-C to about 188 nm for the SF-P-C samples, on the other hand, removal of the template and
samples, respectively. The decrease in particle size to about complex formation effects may contribute to further enhance
106 nm in the SF-P-C@Mn sample may be related to the pres- the above mentioned properties.11 The corresponding BJH
ence of manganese species.
curves (insets of Fig. 12), clearly indicate the presence of micro-
TEM imaging further demonstrated that the carbonization and mesopores in the structure of SF, and micro-, meso-, and
products were monodispersed and homogeneously spherical macropores in the SF-P and SF-P-C@Mn carbon spheres.
(Fig. 10).
XRD patterns for the SF-C, SF-P-C and SF-P-C@Mn samples
To further characterize the obtained carbon spheres, EDX are shown in Fig. 13. As shown, the obtained carbon spheres are
analysis was performed. The results for the SF-C and SF-P-C
samples are shown in Fig. S1 and S2,† and clearly indicate
homogeneous distribution of the C, N, and O atoms in their
structure. EDX spectrum of the SF-P-C@Mn sample is shown in
Fig. 11a and conrms the presence of manganese in the
carbonized spheres.
Nitrogen and manganese content of the SF-P-C@Mn carbon
spheres were found to be 35 and 0.18 wt%, respectively (table
Table 2 Physico-chemical properties of the carbonized samples
Sample
SBET (m2 gꢀ1
)
VTP (cm3 gꢀ1
)
DMP (nm)
SF-C
SF-P-C
SF-P-C@Mn
499
516.83
528.49
0.33
0.35
0.41
2.54
2.78
3.08
Fig. 13 X-ray diffraction patterns for the (a) SF-C, (b) SF-P-C, and (c)
SF-P-C@Mn samples.
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Fig. 14 XPS survey spectra of (a) SF-C, (b) SF-P-C and (c) SF-P-C@Mn carbon spheres.
amorphous. It can be concluded that the use of Pluronic F127 or
In contrast, the ORR for SF-C/GCE, SF-P-C/GCE, and SF-P-
complexation with Mn(II) does not lead to crystallinity of the C@Mn/GCE was performed at ꢀ0.24, ꢀ0.18, and ꢀ0.11 V,
obtained carbon spheres. Broad diffraction peaks at 2 theta respectively. The results show that SF-P-C@Mn/GCE has the
angles about 23ꢁ, can be ascribed to the (002) plane of amor- higher potential for the reaction.
phous carbon.22
The Tafel slope and the exchange current density (J0) (Fig. 16)
Fig. 2c and d show the FT-IR spectra of the SF-P-C and SF-P- were calculated according to the following equation:
C@Mn samples. Although most of the characteristic peaks of
E ¼ RT/anF ln J0 ꢀ RT/anF ln J
the parent scaffolds were present, band intensities were
decreased to some extent. Also, in the FT-IR spectrum of SF-P-
where a, F, R, J0, J, and T are the transfer coefficient, the Faraday
constant, the gas constant, exchange current density, and the
absolute temperature, respectively. As listed in Table 3, the Tafel
slope and the current density of the modied electrode with SF-
P-C@Mn nanocomposite is higher than the electrode modied
with SF-C, SF-P-C.
C@Mn (Fig. 2d), a new peak was appeared at 608 cmꢀ1 which
can be attributed to the Mn–O vibrations.39
XPS spectra in Fig. 14a–c show that C 1s, N 1s and O 1s
characteristic peaks exist in SF-C, SF-P-C and SF-P-C@Mn, and
Mn 2p is present in SF-P-C@Mn, which is in good agreement
with EDX-mapping results.
Electrocatalytic performance
Eventually, catalytic activity of the SF-P-C@Mn sample was
evaluated in the oxygen reduction reaction. LSVs of the GCE and
MGCE in 0.1 M KOH are compared in Fig. 15. As shown, the
ORR was carried out very late and very slowly at the surface of
GCE, which indicates the lack of electrocatalytic performance.
Fig. 16 Tafel plot of GCE, SF-C/GCE, SF-P-C/GCE, and SF-P-C@Mn/
GCE in 0.1 M KOH at a scan rate of 5 mV sꢀ1
.
Table 3 The Tafel slope and current density of difference electrodes
Tafel slope
Current density
Electrode
(mV decꢀ1
)
(mA cmꢀ2
)
GCE
SF-C/GCE
SF-P-C/GCE
SF-P-C@Mn/GCE
41.23
73.41
101.38
138.12
20.51
33.29
45.83
70.32
Fig. 15 LSVs of GCE, SF-C/GCE, SF-P-C/GCE, and SF-P-C@Mn/GCE
in 0.1 M KOH at a scan rate of 0.05 mV sꢀ1
.
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the 4th week, only a 4.89% signal decrease was observed
compared with the rst week.
Conclusion
¨
An extension of the Stober method was used to prepare mono-
dispersed salen-formaldehyde resin polymer- and carbon
spheres with high nitrogen content. The results showed that the
solvent system used has an impact on the properties of the
obtained SF resin spheres. Pluronic F127 as a template and
porogenic agent signicantly improved the quality of the ob-
tained resin spheres and their descendant carbon analogues.
Mn(II)-coordinated polymer spheres of the SF resin were also
synthesized.
The SF-P-Mn(II) complex was then converted to SF-P-C@Mn
spheres aer carbonization under an inert atmosphere. Inter-
esting physico-chemical properties of the obtained SF-P-C@Mn
carbon particles, including a BET surface area of 528 m2 gꢀ1 and
a high nitrogen content of about 35 wt%, provided a suitable
basis for modication of a glassy carbon electrode to be used in
the oxygen reduction reaction. SF-P-C@Mn/GCE was success-
fully used as an electrocatalyst in the ORR process. The
improved electrocatalytic activity may be attributed to the
presence of Mn nanoparticles in the SF-P-C@Mn structure as
was evident from the FT-IR and TEM analyses.
Fig. 17 Nyquist curves obtained from electrochemical impedance
spectroscopy measurements in 0.1 M KOH.
In order to examine the resistance to charge transfer at the
electrode surface during the ORR process, electrochemical imped-
ance spectroscopy in 0.1 M KOH with a potential of ꢀ0.11 V within
a frequency range of 10.0 kHz to 10 MHz with an amplitude of
10 mV AC was used. The corresponding Nyquist curves are shown in
Fig. 17. As shown, resistance to charge transfer (RCT) of the SF-P-
C@Mn/GCE is much lower than the SF-P-C/GCE, SF-C/GCE, and
GCE which indicates high electrocatalytic performance of the
modied electrode in the ORR process.
Conflicts of interest
In order to determine the stability of the modied electrode
in the ORR process, chronoamperometry technique was used.
For this purpose, the SF-P-C@Mn/GCE was placed in 0.1 M KOH
at a constant potential of ꢀ0.11 V for 3600 s, and the chro-
noamperogram was recorded (Fig. 18). Clearly, a constant
current was obtained over the specied time period, which
indicates durability and stability of the modied electrode.
Also, the stability of the SF-P-C@Mn/GCE was checked in the
ORR process once a week consecutively for 4 weeks (with
chronoamperometry technique). The results indicate that the
desired SF-P-C@Mn/GCE surface has long-term stability and in
There are no conicts to declare.
Acknowledgements
We gratefully acknowledge the support of the Research Council
of University of Guilan.
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