nanoma e ials
A icle
Inco po a ion o NiO in o SiO2, TiO2, Al2O3,
and Na4.2Ca2.8(Si6O18) Ma ices: Medium E ec on
he Op ical P ope ies and Ca aly ic Deg ada ion o
Me hylene Blue
Ca los Diaz 1,*, Ma ía L. Valenzuela 2, Olga Ci uen es-Vaca 3, Ma jo ie Sego ia 1and
Miguel A. Laguna-Be ce o 4,*
1Depa amen o de Química, Facul ad de Química, Uni e sidad de Chile, La Palme as 3425, Nuñoa,
Casilla 653, 7800003 San iago de Chile, Chile; [email p o ec ed]
2
Ino ganic Chemis y and Molecula Ma e ial Cen e , Facul ad de Ingenie
í
a, Ins i u o de Ciencias Qu
í
micas
Aplicadas, Uni e sidad Au ónoma de Chile, A . El Llano Sube caseaux 2801, San Miguel,
8910060 San iago de Chile, Chile; [email p o ec ed]
3Depa amen o Ciencias Químicas, Facul ad de Ciencias Exac as, Uni e sidad And es Bello,
Sede Concepción, Au opis a Concepción-Talcahuano, 7100 Talcahuano, Chile; [email p o ec ed]
4Ins i u o de Nanociencia y Ma e iales de A agón (INMA), CSIC-Uni e sidad de Za agoza,
50009 Za agoza, Spain
*Co espondence: [email p o ec ed] (C.D.); malaguna@uniza .es (M.A.L.-B.)
Recei ed: 13 No embe 2020; Accep ed: 3 Decembe 2020; Published: 10 Decembe 2020


Abs ac :
The medium e ec o he op ical and ca aly ic deg ada ion o me hylene blue was s udied in
he NiO/SiO
2
, NiO/TiO
2
, NiO/Al
2
O
3
, and NiO/Na
4.2
Ca
2.8
(Si
6
O
18
) composi es, which we e p epa ed
by a solid-s a e me hod. The new composi es we e cha ac e ized by XRD (X- ay di ac ion o powde ),
SEM/EDS, TEM, and HR-TEM. The size o he NiO nanopa icles ob ained om he PSP-4-PVP
(poly inylpy olidone) p ecu so s inside he di e en ma ices ollow he o de o SiO
2
>TiO
2
>Al
2
O
3
. Howe e , NiO nanopa icles ob ained om he chi osan p ecu so does no p esen an
e ec on he pa icle size. I was ound ha he medium e ec o he ma ices (SiO
2
, TiO
2
, Al
2
O
3
,
and Na
4.2
Ca
2.8
(Si
6
O
18
)) on he pho oca aly ic me hylene blue deg ada ion, can be desc ibed as a
speci ic in e ac ion o he NiO ma e ial ac ing as a semiconduc o wi h he MxOyma e ials h ough
a possible p-n junc ion. The highes ca aly ic ac i i y was ound o he TiO
2
and glass composi es
whe e a a o able p-n junc ion was o med. The isola ing cha ac e o Al
2
O
3
and SiO
2
and hei
non-semiconduc o beha io p eclude his in e ac ion o o m a p-n junc ion, and hus a lowe ca aly ic
ac i i y. NiO/SiO
2
and NiO/Na
4.2
Ca
2.8
(Si
6
O
18
) showed a simila pho oca aly ic beha io . On he
o he hand, he e ec o he ma ix on he op ical p ope ies o he NiO/SiO
2
, NiO/TiO
2
, NiO/Al
2
O
3
,
and NiO/Na
4.2
Ca
2.8
(Si
6
O
18
) composi es can be desc ibed by he di e en dielec ic cons an s o he
SiO
2
, TiO
2
, Al
2
O
3
, Na
4.2
Ca
2.8
(Si
6
O
18
) ma ices. The maxima abso p ion o he composi es (
λmax
)
exhibi a di ec ela ionship wi h he dielec ic cons an s, while hei semiconduc o bandgap (E
g
)
p esen an in e se ela ionship wi h he dielec ic cons an s. A di ec ela ionship be ween
λmax
and E
g
was ound om hese co ela ions. The e ec o he polyme p ecu so on he pa icle size
can explain some de ia ions om his ela ionship, as he co ela ion be ween he pa icle size and
abso p ion is well known. Finally, he NiO/Na
4.2
Ca
2.8
(Si
6
O
18
) composi e was epo ed in his wo k
o he i s ime.
Keywo ds: nickel oxide; pho oca alysis; chi osan; poly inylpy olidone; op ical p ope ies
Nanoma e ials 2020,10, 2470; doi:10.3390/nano10122470 www.mdpi.com/jou nal/nanoma e ials
Nanoma e ials 2020,10, 2470 2 o 17
1. In oduc ion
Me al oxide nanopa icles a e widely used in many applica ions such as coa ings, ca alysis,
elec ode ma e ials, o senso s [
1
]. I is impo an o ema k ha hei physical and chemical p ope ies
a e s ongly in luenced by hei agglome a ion [
2
]. In his sense, i is well known ha he inco po a ion
o me al oxides on o ine suppo ma e ials wi h high su ace a eas could help p e en pa icle
agglome a ion and also imp o e hei eac i i y and s abili y [3,4].
NiO is a p- ype semiconduc o wi h E
G
=3.5 eV p esen ing mul iple p ac ical applica ions [
4
–
6
].
Howe e , hei band gap can be modi ied by doping wi h o he me al oxide semiconduc o s, and hus
changing hei pho oca aly ic p ope ies [
5
,
6
]. NiO has been widely used in ca alysis, ba e y ca hodes,
uel cell elec odes, elec och omic ilms, elec ochemical supe capaci o s, o magne ic ma e ials [
4
–
6
].
In his sense, Bonomo e al. [
7
] ecen ly epo ed on he elec ochemical and op o-elec ochemical
p ope ies o nanos uc u ed NiO o pho ocon e sion applica ions. Al hough hese applica ions a e
de e mined by hei band-gap, which depend on he en i onmen [
8
,
9
], no sys ema ic s udies ha e
been epo ed ega ding he e ec o he medium on he band-gap beha io [
10
–
12
]. In his sense, i is
well known ha he dielec ic medium a ec s he op ical p ope ies o nanopa icles, as p e iously
obse ed o Au and Ag sys ems [
10
]. The op ical p ope ies o Au nanopa icles embedded in o
TiO
2
, Z O
2
, and Al
2
O
3
ha e been also s udied quali a i ely [
10
]. In addi ion, he e ec o SiO
2
, TiO
2
,
and Z O
2
suppo s was ecen ly analyzed showing ha MoO
3
/SiO
2
is he mos e icien epoxida ion
ca alys [12].
The Na
4.2
Ca
2.8
(Si
6
O
18
) compound (combei e) is a c ys alline phase no mally ob ained om he
usion o p ecu so Na
2
O
·
CaO
·
SiO
2
glasses [
13
–
15
]. In his sense, he e a e no epo ed me al oxides
using Na4.2Ca2.8(Si6O18) as a solid ma ix.
In p e ious wo ks, we ha e epo ed a me hod o p epa e me al and me al oxide nanos uc u ed
ma e ials om a he mal ea men o he Chi osan (MLn)xand PS-co-4-PVP (MLn)xmac omolecula
complexes [
16
–
18
]. The me hod consis s o wo s eps: (1) Fo ma ion o bo h mac omolecula complexes
by a sol en assis ed eac ion be ween he espec i e polyme and he me allic sal ; and (2) a he mal
p ocess o he solid unde ai a mosphe e.
The M
◦
and M
x
O
y
nanos uc u es can be easily inco po a ed in o SiO
2
ma ices using a simila
app oach by di e en he mal ea men s o he solid-s a e p ecu so s: Chi osan (ML
n
)
x/
/SiO
2
and
PS-co-4-PVP (ML
n
)
x/
/SiO
2
a o ding M
x
O
Y
//SiO
2
composi es [
19
,
20
]. This me hod can be also used o
p epa e NiO//M
x
O
y
composi es using SiO
2
, TiO
2
, Al
2
O
3
, o Na
4.2
Ca
2.8
(Si
6
O
18
) ma ices. Al hough a
ew me hods we e p oposed o p epa e NiO/SiO
2
[
4
,
21
,
22
], NiO/TiO
2
[
6
,
23
–
25
], NiO/Al
2
O
3
[
26
–
29
]
composi es, none o hem is as gene al and simple as he one desc ibed he e. As o he Na
4.2
Ca
2.8
(Si
6
O
18
)
case, al hough his pa icula composi ion has no been epo ed, simila nickel oxide doped wi h silica
ma ices ha e been success ully syn hesized ia a sol–gel p ocess [
30
]. Fu he mo e, his solid s a e
me hod has been used o o he sys ems [
31
]. A summa y o he p oposed ab ica ion ou e is shown
in Figu e 1[13].
Nanoma e ials 2020, 10, x FOR PEER REVIEW 2 o 17
1. In oduc ion
Me al oxide nanopa icles a e widely used in many applica ions such as coa ings, ca alysis,
elec ode ma e ials, o senso s [1]. I is impo an o ema k ha hei physical and chemical
p ope ies a e s ongly in luenced by hei agglome a ion [2]. In his sense, i is well known ha he
inco po a ion o me al oxides on o ine suppo ma e ials wi h high su ace a eas could help
p e en pa icle agglome a ion and also imp o e hei eac i i y and s abili y [3,4].
NiO is a p- ype semiconduc o wi h E
G
= 3.5 eV p esen ing mul iple p ac ical applica ions [4–6].
Howe e , hei band gap can be modi ied by doping wi h o he me al oxide semiconduc o s, and
hus changing hei pho oca aly ic p ope ies [5,6]. NiO has been widely used in ca alysis, ba e y
ca hodes, uel cell elec odes, elec och omic ilms, elec ochemical supe capaci o s, o magne ic
ma e ials [4–6]. In his sense, Bonomo e al. [7] ecen ly epo ed on he elec ochemical and
op o-elec ochemical p ope ies o nanos uc u ed NiO o pho ocon e sion applica ions. Al hough
hese applica ions a e de e mined by hei band-gap, which depend on he en i onmen [8,9], no
sys ema ic s udies ha e been epo ed ega ding he e ec o he medium on he band-gap beha io
[10–12]. In his sense, i is well known ha he dielec ic medium a ec s he op ical p ope ies o
nanopa icles, as p e iously obse ed o Au and Ag sys ems [10]. The op ical p ope ies o Au
nanopa icles embedded in o TiO
2
, Z O
2
,
and Al
2
O
3
ha e been also s udied quali a i ely [10]. In
addi ion, he e ec o SiO
2
, TiO
2
, and Z O
2
suppo s was ecen ly analyzed showing ha MoO
3
/SiO
2
is he mos e icien epoxida ion ca alys [12].
The Na
4.2
Ca
2.8
(Si
6
O
18
) compound (combei e) is a c ys alline phase no mally ob ained om he
usion o p ecu so Na
2
O⋅CaO⋅SiO
2
glasses [13–15]. In his sense, he e a e no epo ed me al oxides
using Na
4.2
Ca
2.8
(Si
6
O
18
) as a solid ma ix.
In p e ious wo ks, we ha e epo ed a me hod o p epa e me al and me al oxide
nanos uc u ed ma e ials om a he mal ea men o he Chi osan (ML
n
)
x
and PS-co-4-PVP (ML
n
)
x
mac omolecula complexes [16–18]. The me hod consis s o wo s eps: (1) Fo ma ion o bo h
mac omolecula complexes by a sol en assis ed eac ion be ween he espec i e polyme and he
me allic sal ; and (2) a he mal p ocess o he solid unde ai a mosphe e.
The M°
and M
x
O
y
nanos uc u es can be easily inco po a ed in o SiO
2
ma ices using a simila
app oach by di e en he mal ea men s o he solid-s a e p ecu so s: Chi osan (ML
n
)
x/
/SiO
2
and
PS-co-4-PVP (ML
n
)
x/
/SiO
2
a o ding M
x
O
Y
//SiO
2
composi es [19,20]. This me hod can be also used o
p epa e NiO//M
x
O
y
composi es using SiO
2
, TiO
2
, Al
2
O
3
, o Na
4.2
Ca
2.8
(Si
6
O
18
) ma ices. Al hough a ew
me hods we e p oposed o p epa e NiO/SiO
2
[4,21,22], NiO/TiO
2
[6,23–25], NiO/Al
2
O
3
[26–29]
composi es, none o hem is as gene al and simple as he one desc ibed he e. As o he
Na
4.2
Ca
2.8
(Si
6
O
18
) case, al hough his pa icula composi ion has no been epo ed, simila nickel
oxide doped wi h silica ma ices ha e been success ully syn hesized ia a sol–gel p ocess [30].
Fu he mo e, his solid s a e me hod has been used o o he sys ems [31]. A summa y o he
p oposed ab ica ion ou e is shown in Figu e 1 [13].
In addi ion, he e ec o he di e en ma ices on he op ical p ope ies will also be s udied and
discussed.
Figu e 1. Schema ic ep esen a ion o he p epa a ion me hod o me allic M° and me al oxides M
x
O
y
nanopa icles inside M’
x
O´
y
ma ices.
Figu e 1.
Schema ic ep esen a ion o he p epa a ion me hod o me allic M
◦
and me al oxides M
x
O
y
nanopa icles inside M0xO0yma ices.
Nanoma e ials 2020,10, 2470 3 o 17
In addi ion, he e ec o he di e en ma ices on he op ical p ope ies will also be s udied
and discussed.
2. Ma e ials and Me hods
NiCl
2·
6H
2
O, e ae hyl o hosilica e (TEOS), chi osan, poly(s y ene-co-4- inilpy idine) PS-co-4-PVP,
e hyl alcohol, ace ic acid, and dichlo ome hane we e supplied om Sigma-Ald ich and we e used
as ecei ed.
2.1. P epa a ion o he NiO/SiO2, NiO/TiO2, NiO/Al2O3Composi es
SiO
2
was p epa ed acco ding o he li e a u e p ocedu es [
19
,
20
]. B ie ly, e ae hoxysilane (TEOS),
e hanol, and ace ic acid we e mixed in a mola a io o 1:4:4 wi h wa e (nanopu e milli-Q), and added
o e he dichlo ome hane solu ion o he p e iously p epa ed chi osan (NiCl
2·
6H
2
O)
x
and PS-co-4-PVP
(NiCl
2·
6H
2
O)
x
. The mix u e was s i ed o 3 days. The ob ained gel was d ied a 100
◦
C unde a
acuum. The chi osan (NiCl
2·
6H
2
O)
x
//SiO
2
and PS-co-4-PVP (NiCl
2·
6H
2
O)
x
//SiO
2
p ecu so s we e
inally calcined a 800 ◦C o 2 h unde ai .
2.2. P epa a ion o he Chi osan (NiCl2·6H2O)x//TiO2and PS-co-4-PVP (NiCl2)x//TiO2P ecu so s
TiO
2
was p epa ed acco ding o he li e a u e p ocedu es [
19
,
20
]. B ie ly, i anium e a-isop opoxide
(Ti(OC
3
H
7
)
4
, TTIP) e hanol and ace ic acid we e mixed in a mola a io o 1:4:4 wi h wa e
(nanopu e milli-Q), and added o e he dichlo ome hane solu ion o he p e iously p epa ed chi osan
(NiCl
2·
6H
2
O)
x
and PS-co-4-PVP (NiCl
2·
6H
2
O)
x
. The mix u e was s i ed o 3 days. The ob ained
gel was d ied a 100
◦
C unde a acuum. The solid chi osan (NiCl
2·
6H
2
O)
x/
/TiO
2
and PS-co-4-PVP
(NiCl2·6H2O)x//TiO2p ecu so s we e calcined a 800 ◦C o 2 h unde ai .
2.3. P epa a ion o he Chi osan (NiCl2·6H2O)x//Al2O3and PS-co-4-PVP (NiCl2)x//Al2O3P ecu so s
Al
2
O
3
was p epa ed acco ding o he li e a u e p ocedu es [
27
–
30
]. B ie ly, AlCl
3,
e hanol,
and ace ic acid we e mixed in a mola a io o 1:4:4 wi h wa e (nanopu e milli-Q), and added o e
he dichlo ome hane solu ion o he p e iously p epa ed chi osan (NiCl
2·
6H
2
O)
x
and PS-co-4-PVP
(NiCl
2·
6H
2
O)
x
. The mix u e was s i ed o 3 days. The ob ained gel was d ied a 100
◦
C unde a
acuum. The solid chi osan (NiCl
2·
6H
2
O)
x
//Al
2
O
3
and PS-co-4-PVP (NiCl
2·
6H
2
O)
x
//Al
2
O
3
p ecu so s
we e calcined a 800 ◦C o 2 h unde ai .
2.4. P epa a ion o he P ecu so s: Chi osan (NiCl2·6H2O)x//NiO/Na4.2Ca2.8(Si6O18) and PS-co-4-PVP
(NiCl2)x//NiO/Na4.2Ca2.8(Si6O18)
The compounds we e p epa ed acco ding o he li e a u e p ocedu es [
28
]. B ie ly, e ae hoxysilane
(TEOS), e hanol, and ace ic acid we e mixed in a mola a io o 1:4:4 wi h wa e (nanopu e milli-Q),
hen Na
2
O, CaO, and SiO
2
solids (in mol% o 14:1.5:73) we e added o e he dichlo ome hane solu ion
o he p e iously p epa ed chi osan (NiCl
2·
6H
2
O)
x
and PS-co-4-PVP (NiCl
2·
6H
2
O)
x
. The mix u e
was s i ed o 3 days. The ob ained gel was d ied a 100
◦
C unde a acuum. The solid chi osan
(NiCl
2·
6H
2
O)
x
//Na
2
O CaO SiO
2
and PS-co-4-PVP (NiCl
2·
6H
2
O)
x
//Na
2
O CaO SiO
2
p ecu so s we e
calcined a 800 ◦C o 2 h unde ai .
The coo dina ion o he polyme was con i med by IR analysis, as he b oad
ν
(OH)+
ν
(NH)
band obse ed a 3448 cm
−1
o ee chi osan becomes un olded upon coo dina ion, shi ing in he
ange o 3345–3393 cm
−1
. On he o he hand, he
ν
(py) band is shi ing o high equencies upon
coo dina ion [16–18].
Finally, polyme -me al complexes we e placed in o a box u nace (lab ech) using a py olysis
empe a u e o 180
◦
C o he p ecu so complexes and 800
◦
C o he polyme complexes. Addi ional
expe imen al condi ions a e summa ized in Table 1.
Nanoma e ials 2020,10, 2470 4 o 17
Table 1. Composi ion o he py oly ic p oduc s om he espec i e p ecu so s.
P ecu so P ecu so Fo mula Ma ix Composi e Composi e
Numbe
(1) Chi osan·NiCl2(chi osan) - NiO C1
(2) PSP-4-PVP·NiCl2(PVP) - NiO C2
(3) Chi osan·NiCl2SiO2NiO/SiO2C3
(4) PSP-4-PVP·NiCl2SiO2NiO/SiO2C4
(5) Chi osan·NiCl2TiO2NiO/TiO2C5
(6) PSP-4-PVP·NiCl2TiO2NiO/TiO2C6
(7) Chi osan·NiCl2Al2O3NiO/Al2O3C7
(8) Chi osan·NiCl2Na4.2Ca2.8(Si6O18)
NiO/Na
4.2
Ca
2.8
(Si
6
O
18
)
C8
2.5. Cha ac e iza ion
IR spec a we e eco ded wi h a FT-IR Jasco 4600 spec opho ome e (Jasco Inc., Eas on, MD, USA).
Scanning elec on mic oscopy (SEM) was pe o med on a JEOL 5410 scanning elec on mic oscope
(JEOL L d., Tokyo, Japan). Elemen al mic oanalysis was pe o med by ene gy dispe si e X- ay (EDS)
analysis using a NORAN Ins umen mic o-p obe a ached o he SEM (The mo Scien i ic, Wal ham,
MA, USA). High- esolu ion ansmission elec on mic oscopy (HR-TEM) was pe o med using a
JEOL 2000FX TEM mic oscope (JEOL L d., Tokyo, Japan)a 200 kV o cha ac e ize he a e age pa icle
size, dis ibu ion, and elemen al and c ys al composi ion. EDS analysis was pe o med in indi idual
pa icles in o de o disc imina e NiO om he ma ix. A e age pa icle sizes we e calcula ed using he
Digi al Mic og aph so wa e (Ga an, Inc., Pleasan on, CA, US). Me hylene blue (MB) was used as a
model compound o es he pho oca aly ic p ope ies a 655 nm unde UV-Vis illumina ion (Shimadzu
UV-2600 spec opho ome e , Shimadzu Coo po a ion, Kyo o, Japan) using a xenon lamp (150 W)
posi ioned 20 cm away om he pho o eac o in a 330–680 nm ange a oom empe a u e, o a oid he
sel -deg ada ion and he mal ca aly ic e ec s o ca ionic dye. Suspensions we e s i ed in he da k o
60 min o es ablish an adso p ion/deso p ion equilib ium, a e which he pho oca aly ic discolo a ion
o MB was ini ia ed.
3. Resul s and Discussion
3.1. Composi e NiO/SiO2
The X- ay di ac ion pa e n o he as-syn hesized NiO/SiO
2
composi e o he ma e ial om he
chi osan p ecu so is shown in Figu e 2a. All he e lec ion peaks o he XRD pa e n can be indexed o
NiO and SiO
2
phases [
19
] (JPDS no. 03-065-2901 o NiO and JPDS no. 01-088-1535 o SiO
2
). The b oad
ea u e appea ing a 22
◦
co esponds o amo phous silica [
19
]. Simila X- ay di ac ion pa e ns o
NiO om he PVP p ecu so we e ob ained.
The SEM analysis (Figu e 2b) shows i egula pa icle agglome a es, as ypically obse ed om
he p epa a ion o nanopa icles using he solid-s a e he mal ou e [
30
]. F om he TEM analysis,
he agglome a ion o NiO nanopa icles embedded in o a mesh o SiO
2
can be obse ed in Figu e 2c,
whe e hese agglome a es a e composed o used NiO nanopa icles. The size o hese nanopa icles
a e in he ange o 14 nm wi h a mean size o 25 nm (Figu e 2c). De ailed HR-TEM images in Figu e 2e,
show a homogeneous dispe sion o NiO o e he silica ne wo k. Howe e , i was no possible o
acqui e high esolu ion images in o de o s udy he in e aces be ween NiO and he di e en ma ices.
In any case, as also con i med by SEM-EDS mapping (Figu e 2g), he e is a uni o m dis ibu ion o
NiO and SiO
2
pa icles. Simila esul s we e obse ed o NiO ob ained om he PVP p ecu so
(see Supplemen a y Ma e ials, Figu e S1). The only di e ence is ha NiO pa icles a e bigge in size ca.
100 nm.
Nanoma e ials 2020,10, 2470 5 o 17
Nanoma e ials 2020, 10, x FOR PEER REVIEW 5 o 17
Figu e 2. (a) XRD pa e n; (b) SEM image; (c) TEM image; (d) pa icle his og am; (e, ) HRTEM
images; and (g) SEM elemen mapping o he py oly ic NiO compound ob ained using he chi osan
p ecu so .
The SEM analysis (Figu e 2b) shows i egula pa icle agglome a es, as ypically obse ed om
he p epa a ion o nanopa icles using he solid-s a e he mal ou e [30]. F om he TEM analysis, he
agglome a ion o NiO nanopa icles embedded in o a mesh o SiO
2
can be obse ed in Figu e 2c,
whe e hese agglome a es a e composed o used NiO nanopa icles. The size o hese nanopa icles
a e in he ange o 14 nm wi h a mean size o 25 nm (Figu e 2c). De ailed HR-TEM images in Figu e
2e, show a homogeneous dispe sion o NiO o e he silica ne wo k. Howe e , i was no possible o
acqui e high esolu ion images in o de o s udy he in e aces be ween NiO and he di e en
ma ices. In any case, as also con i med by SEM-EDS mapping (Figu e 2g), he e is a uni o m
dis ibu ion o NiO and SiO
2
pa icles. Simila esul s we e obse ed o NiO ob ained om he PVP
Figu e 2.
(
a
) XRD pa e n; (
b
) SEM image; (
c
) TEM image; (
d
) pa icle his og am; (
e
,
) HRTEM images;
and (
g
) SEM elemen mapping o he py oly ic NiO compound ob ained using he chi osan p ecu so .
3.2. NiO/TiO2
Figu e 3shows he XRD pa e n o he NiO/TiO
2
nanocomposi e om he chi osan p ecu so ,
whe e he ana ase phase and NiO a e obse ed as single phases. Using his me hod, he pu e TiO
2
ana ase phase was ob ained, in con as wi h o he solu ion me hods, whe e a mix u e o ana ase
and u ile in he NiO/TiO
2
composi e was ob ained [
22
]. The NiO/TiO
2
composi e shows a “co on”
ype mo phology om he chi osan p ecu so (Figu e 3b), whe eas he mo phology om he PVP
p ecu so p esen s a mo e densi ied s uc u e, as shown in Figu e 3c. The SEM-EDS mapping, shown in

Nanoma e ials 2020,10, 2470 6 o 17
Figu e 2g, indica es an homogeneous dis ibu ion o NiO and TiO
2
. Simila esul s we e ob ained o
he NiO/TiO2 om he PVP p ecu so (see Supplemen a y Ma e ials, Figu e S2).
Nanoma e ials 2020, 10, x FOR PEER REVIEW 6 o 17
p ecu so (see Supplemen a y Ma e ials, Figu e S1). The only di e ence is ha NiO pa icles a e
bigge in size ca. 100 nm.
3.2. NiO/TiO
2
Figu e 3 shows he XRD pa e n o he NiO/TiO
2
nanocomposi e om he chi osan p ecu so ,
whe e he ana ase phase and NiO a e obse ed as single phases. Using his me hod, he pu e TiO
2
ana ase phase was ob ained, in con as wi h o he solu ion me hods,
whe e a mix u e o ana ase and
u ile in he NiO/TiO
2
composi e was ob ained [22]. The NiO/TiO
2
composi e shows a “co on” ype
mo phology om he chi osan p ecu so (Figu e 3b), whe eas he mo phology om he PVP
p ecu so p esen s a mo e densi ied s uc u e, as shown in Figu e 3c. The SEM-EDS mapping,
shown in Figu e 2g, indica es an homogeneous dis ibu ion o NiO and TiO
2
. Simila esul s we e
ob ained o he NiO/TiO
2
om he PVP p ecu so (see Supplemen a y Ma e ials, Figu e S2).
The TEM analysis (Figu e 3d,e) p esen s a “spide web” TiO
2
ne wo k whe e he NiO nuclea es
o ming agglome a ed nanopa icles. They p esen a mean pa icle size o 25 nm (Figu e 2 ). A
simila TEM analysis was obse ed o NiO/TiO
2
ob ained om he PVP p ecu so (Figu e S1b and
Supplemen a y Ma e ials, Figu e S2).
Figu e 3. (a) XRD pa e n; (b) SEM image o NiO om chi osan and (c) om PVP; (d,e) TEM images
o NiO om chi osan and ( ) hei his og am; and (g) SEM mapping elemen o NiO om he
chi osan p ecu so .
Figu e 3.
(
a
) XRD pa e n; (
b
) SEM image o NiO om chi osan and (
c
) om PVP; (
d
,
e
) TEM images
o NiO om chi osan and (
) hei his og am; and (
g
) SEM mapping elemen o NiO om he
chi osan p ecu so .
The TEM analysis (Figu e 3d,e) p esen s a “spide web” TiO
2
ne wo k whe e he NiO nuclea es
o ming agglome a ed nanopa icles. They p esen a mean pa icle size o 25 nm (Figu e 2 ).
A simila TEM analysis was obse ed o NiO/TiO
2
ob ained om he PVP p ecu so (Figu e 3b and
Supplemen a y Ma e ials, Figu e S2).
3.3. NiO/Al2O3
Figu e 4a shows he XRD pa e n o he NiO/Al
2
O
3
composi e om he chi osan p ecu so whe e
he co esponding peaks o γ-Al2O3and NiO can be obse ed.
Nanoma e ials 2020,10, 2470 7 o 17
Nanoma e ials 2020, 10, x FOR PEER REVIEW 7 o 17
3.3. NiO/Al
2
O
3
Figu e 4a shows he XRD pa e n o he NiO/Al
2
O
3
composi e om he chi osan
p ecu so
whe e he co esponding peaks o γ-Al
2
O
3
and NiO can be obse ed.
Figu e 4. (a) XRD pa e n o NiO/Al
2
O
3
om he chi osan p ecu so ; (b) SEM image o NiO om
chi osan and (c) om PVP; (d) TEM image o NiO om chi osan and (e) om PVP; ( ) EDS mapping
o NiO om chi osan.
Figu e 4.
(
a
) XRD pa e n o NiO/Al
2
O
3
om he chi osan p ecu so ; (
b
) SEM image o NiO om
chi osan and (
c
) om PVP; (
d
) TEM image o NiO om chi osan and (
e
) om PVP; (
) EDS mapping o
NiO om chi osan.
The e ec o he polyme empla e on he mo phology can be obse ed in Figu e 4b,c. The chi osan
p ecu so induces a “co on” ype mo phology, while he PVP p ecu so also combines dense and
Nanoma e ials 2020,10, 2470 8 o 17
i egula zones. Figu e 4 shows an elemen al mapping image demons a ing ha NiO is well dispe sed
inside Al2O3. A comple e cha ac e iza ion is shown in Supplemen a y Ma e ials, Figu e S3.
As obse ed o he NiO/TiO
2
sys em, he TEM analysis (Figu e 4e) shows a “spide web” ne wo k
o Al
2
O
3
whe e he NiO nuclea es o m agglome a es. The his og am (Supplemen a y Ma e ials,
Figu e S3) shows a pa icle mean size o 17 nm. The HRTEM image o he NiO/Al
2
O
3
om he PVP
p ecu so is shown in Supplemen a y Ma e ials, Figu e 3c, whe e i can be obse ed ha he medium
pa icle size is 32 nm.
3.4. NiO/Na4.2Ca2.8(Si6O18)
The XRD pa e n o he NiO/Na
4.2
Ca
2.8
(Si
6
O
18
) composi e p epa ed om he chi osan p ecu so
indica es he o ma ion o NiO inside he glass Na
4.2
Ca
2.8
(Si
6
O
18
) (see Figu e 5a). The XRD pa e n
is in ag eemen wi h hose epo ed in he li e a u e [
13
–
15
]. The obse ed mo phology is simila o
he one p e iously epo ed [
13
–
15
] (see Figu e 5b,c), also p esen ing a uni o m dis ibu ion o NiO
inside he Na
4.2
Ca
2.8
(Si
6
O
18
) (Figu e 5d). Simila conclusions can be deduced o he PVP p ecu so
(see Supplemen a y Ma e ials, Figu e S4).
Nanoma e ials 2020, 10, x FOR PEER REVIEW 8 o 17
The e ec o he polyme empla e on he mo phology can be obse ed in Figu e 4b,c. The
chi osan p ecu so induces a “co on” ype mo phology, while he PVP p ecu so also combines
dense and i egula zones. Figu e 4 shows an elemen al mapping image demons a ing ha NiO is
well dispe sed inside Al
2
O
3
. A comple e cha ac e iza ion is shown in Supplemen a y Ma e ials,
Figu e S3.
As obse ed o he NiO/TiO
2
sys em, he TEM analysis (Figu e 4e) shows a “spide web”
ne wo k o Al
2
O
3
whe e he NiO nuclea es o m agglome a es. The his og am (Supplemen a y
Ma e ial, Figu e S3) shows a pa icle mean size o 17 nm. The HRTEM image o he NiO/Al
2
O
3
om
he PVP p ecu so is shown in Supplemen a y Ma e ials, Figu e 3c, whe e i can be obse ed ha he
medium pa icle size is 32 nm.
3.4. NiO/Na
4.2
Ca
2.8
(Si
6
O
18
)
The XRD pa e n o he NiO/Na
4.2
Ca
2.8
(Si
6
O
18
) composi e p epa ed om he chi osan p ecu so
indica es he o ma ion o NiO inside he glass Na
4.2
Ca
2.8
(Si
6
O
18
) (see Figu e 5a). The XRD pa e n is
in ag eemen wi h hose epo ed in he li e a u e [13–15]. The obse ed mo phology is simila o he
one p e iously epo ed [13–15] (see Figu e 5b,c), also p esen ing a uni o m dis ibu ion o NiO
inside he Na
4.2
Ca
2.8
(Si
6
O
18
) (Figu e 5d). Simila conclusions can be deduced o he PVP p ecu so
(see Supplemen a y Ma e ials, Figu e S4).
Figu e 5. (a) XRD pa e n o NiO inside Na
4.2
Ca
2.8
(Si
6
O
18
); (b) and (c) SEM images; and (d) EDS
mapping by an elemen o he composi e NiO/Na
4.2
Ca
2.8
(Si
6
O
18
).
Figu e 5.
(
a
) XRD pa e n o NiO inside Na
4.2
Ca
2.8
(Si
6
O
18
); (
b
) and (
c
) SEM images; and (
d
) EDS
mapping by an elemen o he composi e NiO/Na4.2Ca2.8(Si6O18).
Nanoma e ials 2020,10, 2470 9 o 17
A summa y o he medium pa icle sizes o NiO included in o he di e en ma ices is p esen ed
in Table 2, whe e he e ec o he ma ix and ha o he polyme p ecu so s on he inal pa icle sizes
can be obse ed.
Table 2. Nanopa icle size o he composi es.
Composi e P ecu so Fo mula Pa icle Size (nm) Re e ence
NiO Chi osan·NiCl2>50 [17]
NiO PSP-4-PVP·NiCl2>50 [17]
NiO/SiO2Chi osan·NiCl225 This wo k
NiO/SiO2PSP-4-PVP·NiCl2100 This wo k
NiO/TiO2Chi osan·NiCl225 This wo k
NiO/TiO2PSP-4-PVP·NiCl263 This wo k
NiO/Al2O3Chi osan·NiCl230 This wo k
NiO/Al2O3Chi osan·NiCl217 This wo k
NiO/Na4.2Ca2.8(Si6O18) Chi osan·NiCl2No measu ed This wo k
The nanopa icle size o NiO ob ained om he PVP p ecu so inside he ma ices ollow he
o de o SiO
2
>TiO
2
>Al
2
O
3
, while ha o he NiO om he chi osan p ecu so does no p esen a
signi ican e ec on he nanopa icle size.
3.5. Pho oca aly ic Beha io
Al hough he main applied p ope y o NiO is in he ield o elec ochemis y as Li-ion ba e ies [
32
]
and supe capaci o s applica ions, [
33
] i s applica ion as a pho oca aly ic ac i i y owa d o ganic dyes
ha e also been sugges ed [
34
]. In any case, epo s on he pho oca aly ic ac i i y owa d o ganic dyes
using NiO/ma ices a e sca ce. Yu e al. [
6
] ound a highe pho oca aly ic ac i i y o NiO/TiO
2
han o
pu e NiO, owa ds he pho odeg ada ion o p-chlo ophenol. Rega ding he pho oca aly ic e iciency
when using composi es, impo an pa ame e s o be conside ed include he o ma ion o hie a chical
po ous s uc u es, he dispe sion o he ca aly ic semiconduc o on he ma ix su ace, and he p-n
junc ion in a NiO/M
x
O
y
composi e, whe e a new band gap will be o med wi h a mos a o able alue
o he pho odeg ada ion chemical p ocesses.
3.6. NiO
Me hylene blue (MB) is ex ensi ely used as an o ganic dye in colo ing pape , empo a y hai
colo an , dyeing co ons, and coa ing o pape s ock [
35
]. The emo al o his haza dous dye is
conside ed as one o he g owing equi emen s in ecen yea s. The pho oca aly ic expe imen s we e
ca ied on he sample wi h de ini e dye concen a ion unde da k condi ions and UV i adia ion.
The band-gap o he NiO is 5.0 and 5.2 eV, when i is p epa ed om chi osan and PVP p ecu so s,
espec i ely. Fo he semiconduc o me al oxides, hei band gap alue dic a es hei pho oca aly ic
ac i i y [
35
,
36
]. Fo his eason, he band gap o he C
3
–C
8
composi es was de e mined. These alues a e:
5.0, 5.2, and 5.4 eV o he NiO/SiO
2
, NiO/TiO
2
, NiO/Al
2
O
3
composi es, espec i ely, all ob ained om
he chi osan p ecu so s. The alues o he PVP p ecu so a e: 5.5 eV, 5.2 eV o he NiO/SiO
2
, NiO/TiO
2
composi es, espec i ely. Those alues do no change signi ican ly, and a e sligh ly highe han hose
epo ed p e iously, which can be due o hei bigge pa icle sizes [
34
] (see Supplemen a y Ma e ials,
Figu e S5).
The changes in he abso p ion spec a o he MB aqueous solu ion exposed o UV ligh o
a ious imes in he p esence o NiO a e shown in Supplemen a y Ma e ials, Figu e S6. The peak a
655 nm is cha ac e is ic o me hylene blue and dec eases wi h he i adia ion ime. Figu e 6shows
he plo o ime s. concen a ion o me hylene blue measu ed as C/C
o
o NiO a ising om bo h
Nanoma e ials 2020,10, 2470 16 o 17
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