nanoma e ials
A icle
Pos -Syn he ic De i a iza ion o G aphi ic Ca bon
Ni ide wi h Me hanesul onyl Chlo ide: Syn hesis,
Cha ac e iza ion and Pho oca alysis
Pe P aus 1,2,* , Ane a Smýkalo á1,2, K yš o Foniok 1,2, Pe Velíšek 3, Daniel C ejn 4,
Ja osla Žádný3,* and Jan S o ch 3
1Depa men o Chemis y, VŠB-Technical Uni e si y o Os a a, 700 80 Os a a, Czech Republic;
[email p o ec ed] (A.S.); [email p o ec ed] (K.F.)
2
Ins i u e o En i onmen al Technology, VŠB-Technical Uni e si y o Os a a, 708 00 Os a a, Czech Republic
3Depa men o Ad anced Nanoma e ials and O ganic Syn hesis,
Ins i u e o Chemical P ocess Fundamen als, . .i., Czech Academy o Sciences, Roz ojo á1/135, 165 02
P ague 6, Czech Republic; [email p o ec ed] (P.V.); s o [email p o ec ed] (J.S.)
4ENET Cen e, VŠB-Technical Uni e si y o Os a a, 708 00 Os a a, Czech Republic; [email p o ec ed]
*Co espondence: pe [email p o ec ed] (P.P.); [email p o ec ed] (J.Ž.);
Tel.: +420-59-732-1625 (P.P.); +420-220-390-236 (J.Ž.)
Recei ed: 5 Janua y 2020; Accep ed: 20 Janua y 2020; Published: 22 Janua y 2020


Abs ac :
Bulk g aphi ic ca bon ni ide (CN) was syn he ized by hea ing o melamine a 550
◦
C, and
he ex olia ed CN (ExCN) was p epa ed by hea ing o CN a 500
◦
C. Sul u -doped CN was syn hesized
by hea ing o hiou ea (S-CN) and by a no el p ocedu e based on he pos -syn he ic de i a iza ion o
CN wi h me hanesul onyl (CH
3
SO
2−
) chlo ide (Mes-CN and Mes-ExCN). The ob ained nanoma e ials
we e in es iga ed by common cha ac e iza ion me hods and hei pho oca aly ic ac i i y was es ed
by means o he decomposi ion o ace ic o ange 7 (AO7) unde ul a iole A (UVA) i adia ion. The
con en o sul u in he modi ied CN dec eased in he sequence o Mes-ExCN >Mes-CN >S-CN. The
abso p ion o ligh dec eased in he opposi e manne , bu no in luence on he band gap ene gies was
obse ed. The me hanesul onyl (mesyl) g oups connec ed o p ima y and seconda y amine g oups
we e con i med by high esolu ion mass spec ome y (HRMS). The pho oca aly ic ac i i y dec eased
in he sequence o Mes-ExCN >ExCN >CN
≈
Mes-CN >S-CN. The highes ac i i y o Mes-ExCN
and ExCN was explained by he highes amoun s o adso bed Ace ic O ange 7 (AO7). In addi ion,
in he case o Mes-ExCN, chlo ide ions inco po a ed in he CN la ice enhanced he pho oca aly ic
ac i i y as well.
Keywo ds: g aphi ic ca bon ni ide; de i a iza ion; sul u ; mesyl chlo ide; pho oca alysis
1. In oduc ion
G aphi ic ca bon ni ide is a semiconduc ing nanoma e ial ha has been in ensi ely s udied du ing
he las decade owing o i s in e es ing p ope ies such as high he mal, chemical, and pho ochemical
s abili y [
1
,
2
]. Ano he impo an ea u e o ca bon ni ide (CN) is abso p ion o isible ligh as a esul
o band gap ene gy o 2.7 eV (459 nm), which is p omising o a ious applica ions including sola
cells’ ab ica ion [
3
], imaging, bio he apy, sensing o some compounds [
4
–
7
], and so on. Howe e ,
mos o he applica ions ha e been di ec ed in he ield o pho oca alysis [8–13].
Besides hese posi i e and use ul p ope ies, he se ious sho coming o CN is as ecombina ion o
pho oinduced elec ons and holes, which has been sol ed by he o ma ion o he e ojunc ion composi es
wi h me al oxides, ino ganic sal s noble me als, and so on [
13
–
16
]. Recen ly, we ha e in es iga ed he
he e ojunc ion composi es o CN wi h TiO
2
[
17
,
18
], WO
3
[
19
,
20
], SnO
2
[
21
], BiVO
4
[
22
,
23
], BiOIO
3
[
24
],
Nanoma e ials 2020,10, 193; doi:10.3390/nano10020193 www.mdpi.com/jou nal/nanoma e ials
Nanoma e ials 2020,10, 193 2 o 18
ZnO [
25
], and g aphene oxide [
26
]. Ano he possibili y is he o ma ion o he e ojunc ion composi es
o pu e and doped CN.
The doping wi h me al and/o non-me al elemen s allows us o une band gap ene gy and o
enhance abso p ion o isible ligh , physico-chemical, and pho oca aly ic p ope ies. The opic o
CN doping has been well desc ibed in se e al e iew pape s [
6
,
27
,
28
]. Especially, doping wi h
en i onmen al- iendly non-me als, such as S, O, P, and N, is an in e es ing esea ch opic ha has
been in es iga ed a p esen [
6
,
27
,
28
]. The e a e a lo o pape s dealing wi h he S-doping o CN in
he li e a u e. The common way is he syn hesis o CN om sul u - ich o ganic compounds, such as
benzyl disulphide [
29
], hiou ea [
30
–
36
], i hiocyanu ic acid [
37
–
40
], elemen al sul u [
41
], H
2
S [
42
],
and sul u ic acid [43].
Unlike hese syn hesis p ocedu es om S-con aining p ecu so s, ou new app oach was based on
he pos -syn he ic de i a iza ion o al eady p epa ed bulk and ex olia ed CN [
44
] wi h sui able highly
eac i e chemical agen s ha ing sul u in hei s uc u e such as mesyl chlo ide. To he bes o ou
knowledge, his is he i s pos -syn he ic app oach o S-dope CN, and no such syn he ic s a egy has
been epo ed in he li e a u e ye . Some modi ica ions o CN wi h a ious o ganic compounds we e
e e ed o in he li e a u e, bu no in e ms o he mesyl de i a iza ion [45,46].
As al eady men ioned, hiou ea is one o he o en-used sul u p ecu so s and, he e o e, such
p epa ed S-doped CN was used as a compa a i e nanoma e ial. All new nanoma e ials we e p epa ed
and hei p ope ies we e s udied by means o common cha ac e iza ion and he pho oca aly ic
decomposi ion o he commonly u ilized dye Acid O ange (AO7).
2. Ma e ials and Me hods
2.1. Chemicals
All used chemicals we e o analy ical- eagen g ade. Melamine and Acid O ange 7 we e
pu chased om Sigma-Ald ich (Da ms ad , Ge many), and hiou ea was pu chased om Me ck
(Da ms ad , Ge many). T ie hylamine (Pen a, Czech Republic) and 1,4-dioxane (Lachne , Czech
Republic) we e d ied by molecula sie es p io o eac ion, and mesyl chlo id (Sigma-Ald ich) was
used wi hou addi ional pu i ica ion. Dis illed wa e was used o he p epa a ion o all solu ions
and pos -syn he ic washing. The s uc u es o melamine and AO7 a e p esen ed in Supplemen a y
Ma e ials (Figu es S1 and S2).
2.2. P epa a ion o Bulk and S-Doped CN
Bulk CN was p epa ed by hea ing melamine a 550
◦
C o 4 h wi h he hea ing a e o 3
◦
C min
−1
in a ce amic c ucible wi h a lid (diame e 5 cm, 30 mL) in a mu le u nace. The c ucible was cooled
down ou o he u nace o ambien empe a u e and hen g ounded in an aga e mo a o a ine
powde . S-doped bulk CN was p epa ed unde same condi ions as bulk CN. The only di e ence
was ha hiou ea was used as a p ecu so ins ead o melamine. The con en o sul u in S-CN was
de e mined by he X- ay luo escence spec oscopy (XRF) (SPECTRO Xepos, SPECTRO Analy ical
Ins umen s GmbH, Kle e, Ge many) and elemen al analysis; see Table 1.
Table 1. Elemen al composi ion o ca bon ni ide (CN)-based nanoma e ials. XRF, X- ay luo escence
spec oscopy; EA, elemen al analysis; CI, con idence in e al.
Nanoma e ials C (%) H (%) N (%) S (XRF/EA) (%) Cl (%)
CN 34.54 1.72 62.31 — —
ExCN 33.42 1.87 60.42 — —
S-CN 34.06 1.74 61.15 0.22/n.d. —
Mes-CN 35.43 2.20 56.36 0.26/0.34 1.43
Mes-ExCN 33.39 1.80 60.20 0.56/0.70 3.33
No e: n.d.—no de e mined.
Nanoma e ials 2020,10, 193 3 o 18
2.3. Ex olia ion o Bulk CN
Ex olia ed CN was pe o med by hea ing he bulk CN [
44
] in a hin laye on a ce amic pla e
(diame e 8 cm, 50 mL) a 500
◦
C in he mu le u nace o 2 h wi h he hea ing a e o 10
◦
C min
−1
. The
ce amic pla e wi h he p oduc was cooled down o ambien empe a u e ou o he o en.
2.4. De i a iza ion o Bulk and Ex olia ed CN
The bulk and ex olia ed CN (10 g) we e suspended in d y 1,4-dioxane (100 mL) unde ine
a mosphe e and ie hylamine was added, and he suspension was cooled down in a cooling ba h
(wa e /ice) o ca. 10
◦
C. Then, mesyl chlo ide (50 mL, 0.73 mol) was added sequen ially and he mix u e
was dilu ed by addi ion o 1,4-dioxane (50 mL). The cooling ba h was emo ed and he eac ion mix u e
was s i ed a oom empe a u e o e nigh . Then, he mix u es we e quenched by addi ion o dis illed
wa e (ca. 20 mL) and he p oduc s we e il e ed o using a pape il e and washed wi h wa e
(
ca. 1000 mL
) un il neu al pH (acco ding o pH- es s ips). The con en o sul u in S-CN, Mes-CN,
and Mes-ExCN was de e mined by he X- ay luo escence spec oscopy (SPECTRO Xepos, SPECTRO
Analy ical Ins umen s GmbH Kle e, Ge many) and elemen al analysis (EA); see Table 1.
2.5. Ion-Exchange o Mes-ExCN wi h Hyd oxide
Fi s , 0.15 g o Mes-ExCN was added in o 150 mL o NaOH (1 mmol/L) and s i ed on a magne ic
s i e o 4 h. Then, he suspension was il e ed h ough a memb ane il e (0.6
µ
m), washed wi h
200 mL o wa e , and d ied a 70 ◦C o e nigh . The solid p oduc was used o he pho oca alysis.
2.6. Elemen al Analysis
The elemen al analysis o C, N, and H in he p epa ed CN-based nanoma e ials was pe o med
using a Flash 2000 Elemen al analyse (The moFishe Scien i ic, Wal ham, MA, USA). The con en o
chlo ine was de e mined by he Schönige combus ion me hod ollowed by he a gen ome ic i a ion
o chlo ide.
2.7. UV/Vis Di use Re lec ance Spec oscopy
The UV/ is di use e lec ance spec a (DRS) in he ange o 220–1400 nm we e eco ded using a
spec opho ome e Shimadzu UV-2600 (IRS-2600Plus) a labo a o y empe a u e. Re lec ance da a
we e e-calcula ed o abso bance ones using Schus e –Kubelka–Munk’s equa ion F(R∞) as ollows:
F(R∞)=(1−R∞)2
2R∞
, (1)
whe e R
∞
is he di use e lec ance om a semi-in ini e laye . The ob ained DRS spec a we e
ans o med o he dependencies o (F(R
∞
)
·
h
ν
)
2
on h
ν
in o de o ob ain he op ical band gap ene gies
o he p epa ed nanoma e ials.
2.8. FTIR-ATR Spec oscopy
Fou ie ans o m in a ed spec oscopy wi h (FTIR) was measu ed using he Nicole iS50 de ice
(The mo Scien i ic, Wal ham, MA, USA). The spec a we e collec ed in he A enua ed o al e lec ion
(ATR) mode using a diamond ATR c ys al. The spec a we e collec ed in he wa enumbe ange o
400–4000 cm
−1
, and 32 scans we e a e aged. The ATR co ec ion ollowed by baseline sub ac ing was
applied on each spec um using he OMNIC so wa e (Wal ham, MA USA).
2.9. X-Ray Di ac ion
The phase composi ion and mic os uc u al p ope ies o he p epa ed nanoma e ials we e
de e mined using he X- ay powde di ac ion (XRD) echnique. XRD pa e ns we e ob ained using a
Nanoma e ials 2020,10, 193 4 o 18
(Rigaku Sma Lab di ac ome e Rigaku, Tokyo, Japan) wi h a de ec o D/ eX Ul a 250. A sou ce o
X- ay i adia ion was a Co ube (CoK
α
,
λ1
=0.178892 nm,
λ2
=0.179278 nm) ope a ed a 40 kV and
40 mA
. The XRD pa e ns we e collec ed in a 2
θ
ange o 5–90
◦
wi h a s ep size o 0.01
◦
and speed o
0.5 deg min−1
. The c ys alli e size Lwas calcula ed acco ding o Sche e
´
s equa ion o b oadening
B(2θ) (in adians) a a hal maximum in ensi y (FWHM) o a di ac ion peak as
B(2Θ)=Kλ
LcosΘ, (2)
whe e
λ
is he wa eleng h o X- ays,
θ
is B agg
´
s angle, and Kis he cons an equal o 0.94 o cube o
0.89 o sphe ical c ys alli es. In his s udy, Kwas ounded o 0.9.
2.10. TEM Analysis
T ansmission elec on mic oscopy (TEM) was pe o med wi h a JEOL 2100 mic oscope wi h
(
Jeol L d.,
Tokyo, Japan) a LaB6 elec on gun. The accele a ing ol age o 200 kV was applied.
Mic og aphs we e aken by a came a Teng a (EMSIS GmbH, Müns e , Ge many). Fo he TEM
analysis, he samples we e p epa ed by suspending in e hanol and we e hen sonica ed o 5 min.
One d op o his suspension was placed on a coppe g id wi h a holey ca bon ilm and was d ied a
oom empe a u e.
2.11. Speci ic Su ace A ea Measu emen s
The speci ic su ace a ea (SSA) o each nanoma e ial was measu ed by a de ice SORPTOMATIC
1990 se ies (The mo Scien i ic, Wal ham, MA, USA). SSA was de e mined by he analysis o N
2
adso p ion iso he m a −196 ◦C by means o he B unaue –Emme –Telle (BET) me hod.
2.12. XPS Analysis
The supe icial elemen al analyses o he samples we e ca ied ou by means o an X- ay
pho oelec on spec ome e (XPS) ESCA 3400 (K a os) wi h a base p essu e in he analysis chambe o
~5.0
×
10
−7
Pa. Elec ons we e exci ed wi h an Mg K
α
adia ion (h
ν
=1253.6 eV) gene a ed a 12 kV
and 10 mA. Fo all spec a, he Shi ley backg ound was sub ac ed. Peaks in all spec a asc ibed o he
sp2hyb idized ni ogen (C=N-C) we e se o 398.8 eV as a cha ge co ec ion.
2.13. High Resolu ion Mass Spec ome y (HRMS) Analysis
High esolu ion mass spec a we e ob ained by means o a Mic O o III spec ome e
(
B uke Dal onik,
B emen, Ge many) wi h A mosphe ic p essu e chemical ioniza ion (APCI) ioniza ion
in a posi i e mode. Be o e he measu emen , he samples we e we ed wi h me hanol and deli e ed
in o an APCI sou ce in he solid s a e using a di ec glass capilla y inle . The se ing o he ion sou ce
was as ollows: capilla y—4000 V, end pla e—500 V, co ona—3000 nA, nebulize (N
2
)—1 Ba , d y gas
(N
2
)—4 L/min, empe a u e—d ying 350
◦
C, and apo ize —400
◦
C. The HRMS spec a we e aken in
he m/z ange o 50 o 1550 Da wi h he low mass o 50 m/z and collision RF o 400 Vpp. The spec a
we e p ocessed using he Compass Da a Analysis 1.5 so wa e (B uke Dal onik, B emen, Ge many).
The accu a e mass scale was calib a ed using ESI-L Low Conc. Tuning Mix (Agilen echnologies,
San a Cla a, CA, USA).
2.14. Pho oca aly ic Expe imen s
The pho oca aly ic ac i i y was es ed by means o he decomposi ion o AO7 in he concen a ion
o 25 mg L
−1
and his solu ion (150 mL) was placed in o glass essels: olume o 25 mL, heigh o
49 mm, and diame e o 32 mm. Unde da k condi ions, 10 mg o each nanoma e ial was added.
These suspensions we e s i ed o 60 min o each adso p ion equilib ia and hen i adia ed unde a
UVA ube (368 nm, 0.94 mW cm
−2
) du ing imes up o 120 min. The samples o 2 mL we e aken and
Nanoma e ials 2020,10, 193 5 o 18
abso bances a 485 nm we e measu ed by a UV/ is spec ome e Helios (The mo Scien i ic, Wal ham,
MA, USA). The expe imen s e i ying he in luence o inco po a ed chlo ide ions we e pe o med in
glass essels o 250 mL wi h a heigh o 52 mm and diame e o 90 mm.
3. Resul s
In o de o inco po a e a sul u con aining moie y in o he s uc u e o g-C
3
N
4
, highly eac i e and
cheap mesyl chlo ide was used ins ead o osyl chlo ide, which is a he less eac i e, o i lic chlo ide,
which is mo e expensi e. The physico-chemical p ope ies o he CN-based nanoma e ials we e
s udied by means o se e al cha ac e iza ion me hods. Fi s , he nanoma e ial elemen al composi ions
we e de e mined a e hei syn hesis; see Table 1. Then, he ligh abso p ion was s udied by UV/ is
DRS; he s uc u e was s udied by XRD, FTIR-ATR, XPS, and HRMS. The ex u e p ope ies we e
in es iga ed by means o TEM and he physiso p ion o ni ogen. The pho oca aly ic p ope ies we e
s udied by means o he decomposi ion o AO7.
3.1. UV/Vis Di use Re lec ance Spec ome y
The UV/ is DRS spec a shown in Figu e 1we e eco ded o us o obse e ligh abso p ion
p ope ies and o de e mine he op ical band gap ene gies o he p epa ed nanoma e ials.
Fo compa ison
, images o he p epa ed nanoma e ials a e demons a ed in Supplemen a y Ma e ials
(Figu e S3) in o de o display hei eal colo s.
Nanoma e ials 2020, 10, 193 5 o 20
p ope ies we e in es iga ed by means o TEM and he physiso p ion o ni ogen. The pho oca aly ic
p ope ies we e s udied by means o he decomposi ion o AO7.
Table 1. Elemen al composi ion o ca bon ni ide (CN)-based nanoma e ials. XRF, X- ay luo escence
spec oscopy; EA, elemen al analysis; CI, con idence in e al.
Nanoma e ials C (%) H (%) N (%) S (XRF/EA) (%) Cl (%)
CN 34.54 1.72 62.31 --- ---
ExCN 33.42 1.87 60.42 --- ---
S-CN 34.06 1.74 61.15 0.22/n.d. ---
Mes-CN 35.43 2.20 56.36 0.26/0.34 1.43
Mes-ExCN 33.39 1.80 60.20 0.56/0.70 3.33
No e: n.d.—no de e mined.
3.1. UV/Vis Di use Re lec ance Spec ome y
The UV/ is DRS spec a shown in Figu e 1 we e eco ded o us o obse e ligh abso p ion
p ope ies and o de e mine he op ical band gap ene gies o he p epa ed nanoma e ials. Fo
compa ison, images o he p epa ed nanoma e ials a e demons a ed in Supplemen a y Ma e ials
(Figu e S3) in o de o display hei eal colo s.
Figu e 1. UV/ is di use e lec ance spec a o ca bon ni ide (CN)-based nanoma e ials.
Figu e 1 demons a es ha S-CN was he mos -ligh abso bing sample and he abso p ion
dec eased in he sequence o S-CN > Mes-CN ˃ Mes-ExCN > CN > ExCN, which ag ees wi h he
nanoma e ial colo s displayed in Figu e S3. The highe abso p ion o S-CN in compa ison wi h CN
was also e e ed o in he li e a u e, o example, in he wo ks o [32,35,36,39]. The de i a iza ion o
CN wi h mesyl chlo ide was supposed o inco po a e new a om g oups o hep azine uni s ac ing as
ch omopho es. The esul ing de i a ized CN s uc u es we e discussed below based on o he
expe imen al esul s.
The op ical band gap ene gies (he eina e , he band gap ene gy) we e e alua ed by means o
he commonly employed Tauc´s plo [47]
𝜀ℎ𝜈 = 𝐶(ℎ𝜈 − 𝐸), (3)
Figu e 1. UV/ is di use e lec ance spec a o ca bon ni ide (CN)-based nanoma e ials.
Figu e 1demons a es ha S-CN was he mos -ligh abso bing sample and he abso p ion
dec eased in he sequence o S-CN >Mes-CN >Mes-ExCN >CN >ExCN, which ag ees wi h he
nanoma e ial colo s displayed in Figu e S3. The highe abso p ion o S-CN in compa ison wi h CN
was also e e ed o in he li e a u e, o example, in he wo ks o [
32
,
35
,
36
,
39
]. The de i a iza ion
o CN wi h mesyl chlo ide was supposed o inco po a e new a om g oups o hep azine uni s ac ing
as ch omopho es. The esul ing de i a ized CN s uc u es we e discussed below based on o he
expe imen al esul s.

Nanoma e ials 2020,10, 193 6 o 18
The op ical band gap ene gies (he eina e , he band gap ene gy) we e e alua ed by means o he
commonly employed Tauc´s plo [47]
εhν=Chν−Egp, (3)
whe e
ε
is he mola ex inc ion coe icien ; h
ν
is he ene gy o inciden pho ons; E
g
is he band gap
ene gy, Cis a cons an ; and pis he powe depending on he ype o elec on ansi ion: p=2 and
p=1
2
o di ec and indi ec semiconduc o s, espec i ely. In his s udy, p=
1
2
was used [
48
–
52
]. The
e alua ed band gap ene gies a e summa ized in Table 2. Bo h he di ec syn hesis as well as he
de i a iza ion led o insigni ican changes o he band gap ene gies.
Table 2. E alua ed alues o band gap ene gy and c ys alli e size o p epa ed nanoma e ials.
Nanoma e ial Eg(eV) L002 (nm)
CN 2.69 6.6
ExCN 2.77 6.6
S-CN 2.63 6.6
Mes-CN 2.66 6.6
Mes-ExCN 2.73 6.7
3.2. FTIR-ATR Spec ome y
The p epa ed nanoma e ials we e s udied by FTIR-ATR spec ome y, as shown in Figu e 2. The
spec a con ained b oad spec al bands in he egions labelled as A and B. The spec al bands in
egion A can be a ibu ed o he s e ching ib a ions o N–H bonds. The spec al bands in egion
B, such as 1232 cm
−1
, 1318 cm
−1
, 1399 cm
−1
, 1541 cm
−1
, and 1630 cm
−1
, a e ypically asc ibed o he
s e ching ib a ions o C=N and C–N bonds o he e ocyclic ings. The medium band a 804 cm
−1
can
be a ibu ed o he b ea hing mode o iazine uni s. All hese spec a a e ypical o g aphi ic ca bon
ni ide and hei explana ion can be ound elsewhe e, o example, in he wo ks o [53–56].
Nanoma e ials 2020, 10, 193 6 o 20
whe e ε is he mola ex inc ion coe icien ; hν is he ene gy o inciden pho ons; Eg is he band gap
ene gy, C is a cons an ; and p is he powe depending on he ype o elec on ansi ion: p = 2 and p =
½ o di ec and indi ec semiconduc o s, espec i ely. In his s udy, p = ½ was used [48–52]. The
e alua ed band gap ene gies a e summa ized in Table 2. Bo h he di ec syn hesis as well as he
de i a iza ion led o insigni ican changes o he band gap ene gies.
Table 2. E alua ed alues o band gap ene gy and c ys alli e size o p epa ed nanoma e ials.
Nanoma e ial Eg (eV) L002 (nm)
CN 2.69 6.6
ExCN 2.77 6.6
S-CN 2.63 6.6
Mes-CN 2.66 6.6
Mes-ExCN 2.73 6.7
3.2. FTIR-ATR Spec ome y
The p epa ed nanoma e ials we e s udied by FTIR-ATR spec ome y, as shown in Figu e 2. The
spec a con ained b oad spec al bands in he egions labelled as A and B. The spec al bands in
egion A can be a ibu ed o he s e ching ib a ions o N–H bonds. The spec al bands in egion B,
such as 1232 cm−1, 1318 cm−1, 1399 cm−1, 1541 cm−1, and 1630 cm−1, a e ypically asc ibed o he
s e ching ib a ions o C=N and C–N bonds o he e ocyclic ings. The medium band a 804 cm−1 can
be a ibu ed o he b ea hing mode o iazine uni s. All hese spec a a e ypical o g aphi ic ca bon
ni ide and hei explana ion can be ound elsewhe e, o example, in he wo ks o [53–56].
Figu e 2. Fou ie ans o m in a ed spec oscopy (FTIR)-ATR spec a o CN-based nanoma e ials.
P obably because o he low deg ee o de i a iza ion, no signi ican di e ences be ween he
FTIR-ATR spec a o CN and ExCN and Mes-CN and Mes-ExCN we e obse ed. Thus, no e idence
o he de i a iza ion was b ough .
Figu e 2. Fou ie ans o m in a ed spec oscopy (FTIR)-ATR spec a o CN-based nanoma e ials.
Nanoma e ials 2020,10, 193 7 o 18
P obably because o he low deg ee o de i a iza ion, no signi ican di e ences be ween he
FTIR-ATR spec a o CN and ExCN and Mes-CN and Mes-ExCN we e obse ed. Thus, no e idence o
he de i a iza ion was b ough .
3.3. XRD and TEM Analysis
The p epa ed nanoma e ials we e cha ac e ized by XRD and he pa e ns a e shown in Figu e 3.
The wo low in ensi e di ac ion peaks a 2
Θ
=14.9
◦
(d
100
=0.690 nm) and 32.1
◦
(d
002
=0.324 nm)
co espond o (100) and (002) di ac ions, which can be a ibu ed o he hexagonal phase o CN
(JCPDS 87-1526). The s onge (002) di ac ion peak is ela ed o in e laye s acking o he (002) melem
planes. The weake (100) one is a ibu ed o he in-plane o de ing o ni ogen-linked hep azine
uni s [57].
Nanoma e ials 2020, 10, 193 7 o 20
3.3. XRD and TEM Analysis
The p epa ed nanoma e ials we e cha ac e ized by XRD and he pa e ns a e shown in Figu e 3.
The wo low in ensi e di ac ion peaks a 2Θ = 14.9° (d100 = 0.690 nm) and 32.1° (d002 = 0.324 nm)
co espond o (100) and (002) di ac ions, which can be a ibu ed o he hexagonal phase o CN
(JCPDS 87-1526). The s onge (002) di ac ion peak is ela ed o in e laye s acking o he (002)
melem planes. The weake (100) one is a ibu ed o he in-plane o de ing o ni ogen-linked
hep azine uni s [57].
Figu e 3. X- ay powde di ac ion (XRD) pa e ns o CN-based nanoma e ials.
The c ys alli e sizes we e calcula ed based on he (002) di ac ion peaks by means o Sche e ´s
Equa ion (2) and a e summa ized in Table 2. The L002 alues we e nea ly he same. This means ha
he sizes o di ac ing domains as well as hei in e laye dis ances we e changed nei he by he
ex olia ion no he de i a iza ion o CN. Unlike o he au ho s [32,37,58], no change in he XRD
pa e n o S-CN as a esul o he supposed subs i u ion o ni ogen wi h sul u in he CN la ice was
obse ed.
The mo phology o he nanoma e ials was s udied by means o ansmission elec on
mic oscopy (TEM). The TEM mic og aphs a e displayed in Figu e 4. The complex CN s uc u es we e
composed o lake-like shee s and snake-like shells. The lake-like s uc u e was ypical o bulk CN,
ega dless o he S-doping p ocedu e. The shell-like s uc u e as a esul o he he mal ex olia ion
was obse ed o he ExCN (Figu e 4a) as well as Mes-ExCN (Figu e 4b). Du ing he ex olia ion, he
la nanoshee s pa ially w apped hemsel es in o he shells and no de i a iza ion e ec was
obse ed in he TEM mic og aphs. An ene gy dispe si e X- ay (EDX) spec um o Mes-ExCN is
shown in Figu e 4c. The p esence o sul u , chlo ine, and oxygen is clea ly demons a ed.
Figu e 3. X- ay powde di ac ion (XRD) pa e ns o CN-based nanoma e ials.
The c ys alli e sizes we e calcula ed based on he (002) di ac ion peaks by means o Sche e
´
s
Equa ion (2) and a e summa ized in Table 2. The L
002
alues we e nea ly he same. This means ha he
sizes o di ac ing domains as well as hei in e laye dis ances we e changed nei he by he ex olia ion
no he de i a iza ion o CN. Unlike o he au ho s [
32
,
37
,
58
], no change in he XRD pa e n o S-CN as
a esul o he supposed subs i u ion o ni ogen wi h sul u in he CN la ice was obse ed.
The mo phology o he nanoma e ials was s udied by means o ansmission elec on mic oscopy
(TEM). The TEM mic og aphs a e displayed in Figu e 4. The complex CN s uc u es we e composed
o lake-like shee s and snake-like shells. The lake-like s uc u e was ypical o bulk CN, ega dless o
he S-doping p ocedu e. The shell-like s uc u e as a esul o he he mal ex olia ion was obse ed o
he ExCN (Figu e 4a) as well as Mes-ExCN (Figu e 4b). Du ing he ex olia ion, he la nanoshee s
pa ially w apped hemsel es in o he shells and no de i a iza ion e ec was obse ed in he TEM
mic og aphs. An ene gy dispe si e X- ay (EDX) spec um o Mes-ExCN is shown in Figu e 4c. The
p esence o sul u , chlo ine, and oxygen is clea ly demons a ed.
3.4. XPS Analysis
The XPS analysis con i med he p esence o ca bon, ni ogen, oxygen, and sul u . The ca bon
1s spec um in Figu e 5a shows wo dis inc peaks a 285.5 eV and 288.3 eV o binding ene gies.
While he peak a 288.3 eV is clea ly CN
3
ca bon ypical o CN [
59
], he peak o 285.5 eV belongs
o sp
2
hyb idized ca bon o C=C o CN
2
bonds [
60
,
61
]. Some po ion o he 285.5 eV signal migh
Nanoma e ials 2020,10, 193 8 o 18
be a ibu ed o he ca bon ape, which was used as a sample ca ie . The a io be ween he signals
was oughly 3:1 in a ou o 288.3 eV o CN and ExCN and 2:1 o S-CN. In he de i a ised samples,
he a io dec eased o 2:1 o Mes-ExCN and 1:1 o Mes-CN. This migh sugges he ex olia ion and
de i a iza ion e ec on o g-C
3
N
4
. The signal o CH
3
, which occu ed a abou 284 eV, could no be
equi ocally de ec ed by XPS owing o a low deg ee o de i a iza ion in Mes-CN and Mes-ExCN and
i s o e lap wi h he signal a 285.5 eV.
Nanoma e ials 2020, 10, 193 8 o 20
Figu e 4. T ansmission elec on mic oscopy (TEM) mic og aphs o Mes-CN (a), Mes-ExCN (b), and
EDX spec um o Mes-ExCN (c).
3.4. XPS Analysis
The XPS analysis con i med he p esence o ca bon, ni ogen, oxygen, and sul u . The ca bon 1s
spec um in Figu e 5a shows wo dis inc peaks a 285.5 eV and 288.3 eV o binding ene gies. While
he peak a 288.3 eV is clea ly CN3 ca bon ypical o CN [59], he peak o 285.5 eV belongs o sp2
hyb idized ca bon o C=C o CN2 bonds [60,61]. Some po ion o he 285.5 eV signal migh be
a ibu ed o he ca bon ape, which was used as a sample ca ie . The a io be ween he signals was
oughly 3:1 in a ou o 288.3 eV o CN and ExCN and 2:1 o S-CN. In he de i a ised samples, he
a io dec eased o 2:1 o Mes-ExCN and 1:1 o Mes-CN. This migh sugges he ex olia ion and
de i a iza ion e ec on o g-C3N4. The signal o CH3, which occu ed a abou 284 eV, could no be
equi ocally de ec ed by XPS owing o a low deg ee o de i a iza ion in Mes-CN and Mes-ExCN and
i s o e lap wi h he signal a 285.5 eV.
The shape o he ni ogen 1s peak (Figu e 5b) was o med by he supe posi ion o a leas ou
signals a 398.8 eV, 400.0 eV, 401.4 eV, and 404.2 eV [62]. The dominan pa o his spec um was he
py idinic ( iazinic) NC2 ni ogen signal, which co esponded o ni ogen a oms loca ed a he edges
o he melem subuni s o g-C3N4. The signals o 400.0 eV and 401.4 eV we e a ibu ed o he NC3
ni ogen. The lowe one was likely connec ed wi h b idging ni ogen a oms be ween melem s uc u e
co es (NC3B); he highe one was a ibu ed o NC3C ni ogen a oms in he cen e s o melem uni s. The
heo e ical a ios o NC2 and NC3 ni ogen 6:1 (in a o o N-C2 ni ogen o bo h NC3B and NC3C)
we e ound in no nanoma e ials. Tha was p obably because o he addi ional p esence o NH+–C2
p o ona ed ni ogen a oms (binding ene gy a ound 402 eV), b idging NH–C2 (binding ene gy a ound
399 eV), and ma ginal C–NH2 ni ogen (binding ene gy a ound 399 eV). Thei capabili y o in luence
he shape o he spec a has o be aken in o accoun [63].
Figu e 4.
T ansmission elec on mic oscopy (TEM) mic og aphs o Mes-CN (
a
), Mes-ExCN (
b
), and EDX
spec um o Mes-ExCN (c).
Nanoma e ials 2020, 10, 193 9 o 20
The ni ogen egion in he samples ExCN and Mes-ExCN showed he a io be ween NC2 and
NC3B o oughly 4.5:1, sugges ing he he mally ex olia ed samples bea ing mo e NH–C2 and
ma ginal C–NH2 g oups we e no in luenced by he de i a iza ion. In Mes-CN, his a io dec eased
o 3.3:1, while he pu e CN had a a io a ound 7.7:1, which sugges s ha , in he bulk nanoma e ial,
he de i a iza ion led o inc ease o he amoun o ni ogen a oms in NH–C2 o C–NH2. In
compa ison, he S-CN nanoma e ial had a ios be ween bo h NC2 and NC3 iden ically a ound 10:1.
Mo eo e , he o e all po ion in py idine N–C2 ni ogen a oms dec eased signi ican ly in he CN
nanoma e ial a e he de i a iza ion (77 a .% s. 66 a .% o all ni ogen a oms). This phenomenon
was no obse ed in ExCN (71 a .% o all ni ogen a oms in bo h ExCN and Mes-ExCN). The mos
p oli ic nanoma e ial in he o e all NC2 po ion is S-CN (81 a .% o all ni ogen a oms). All o he
spec a ha e dis inc b oad peaks ep esen ing he binding ene gy o 404.2 eV and can be asc ibed o
π–π* (HOMO–LUMO) ansi ion, ha is, a shake-up line, a sa elli e peak [64].
The oxygen 1s b oad peak (Figu e 5c) a ound he binding ene gy 533 eV only con i med i s
p esence in he g aphi ic ca bon s uc u e and did no p o e any u he s uc u e mo i e [65]. The
sul u 2p sec ion o he spec um in Figu e 5d p o ided unsu p isingly no signal in he CN and ExCN
samples. In e es ingly, no signi ican sul u 2p signal was eco ded o he S-CN nanoma e ial as
well. On he o he hand, in he cases o Mes-CN and Mes-ExCN, wo ypes o sul u inco po a ed in
hei s uc u es we e indica ed. The i s signal included a couple o 2p binding ene gies a ound 164
eV, sugges ing sul u in a low oxida ion s a e (sul ide o hiol mo i e); he second one included
ano he couple o 2p binding ene gies a ound 168 eV, sugges ing a mo e oxidized o m o sul u [66],
which could be a ibu ed o mesyl g oups. The sul ide XPS signal was explained by he p esence o
possible de ec s on he edge, which could p omo e elec on delocaliza ion, enabling he educ ion o
–SO2– g oups [67]. The a io be ween educed and oxidized sul u was in e se in he case o S-CN
(app oxima ely 1:3 in a o o oxidized sul u ) and S-ExCN (app oxima ely 2:1 in a o o educed
sul u ).
Figu e 5. Con .
Nanoma e ials 2020,10, 193 9 o 18
Nanoma e ials 2020, 10, 193 10 o 20
Figu e 5. X- ay pho oelec on spec ome e (XPS) spec a o Mes-ExCN. (a) C 1s, (b) N 1S, (c) O 1s,
(d) S 2p.
Figu e 5.
X- ay pho oelec on spec ome e (XPS) spec a o Mes-ExCN. (
a
) C 1s, (
b
) N 1S, (
c
) O 1s,
(d) S 2p.
The shape o he ni ogen 1s peak (Figu e 5b) was o med by he supe posi ion o a leas ou
signals a 398.8 eV, 400.0 eV, 401.4 eV, and 404.2 eV [
62
]. The dominan pa o his spec um was he
py idinic ( iazinic) NC
2
ni ogen signal, which co esponded o ni ogen a oms loca ed a he edges
o he melem subuni s o g-C
3
N
4
. The signals o 400.0 eV and 401.4 eV we e a ibu ed o he NC
3
ni ogen. The lowe one was likely connec ed wi h b idging ni ogen a oms be ween melem s uc u e
co es (NC
3B
); he highe one was a ibu ed o NC
3C
ni ogen a oms in he cen e s o melem uni s. The
Nanoma e ials 2020,10, 193 16 o 18
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