Novel lanthanide(III) 4-methylbenzoylhydrazide complexes as precursors for lanthanide oxide nanophotocatalysts

Article abstract of DOI:10.1039/c9ra08080e

A series of metal complexes were prepared from separate reactions of lanthanide nitrate salts (La(iii), Ce(iii), Sm(iii), Gd(iii) and Ho(iii)) with 4-methylbenzoylhydrazide. The structures of the complexes were confirmed by analytical studies, spectral measurements and thermal studies. Complexes were formed with different stoichiometries of 1 : 2 and 1 : 3 (M : L). The ligand chelates by the nitrogen and oxygen atoms of the amino and carbonyl groups of the hydrazide moiety in the neutral keto form. The coordination compounds were converted to metal oxide nanoparticles (MONPs) through solid state thermal decomposition as monocular source precursors. The obtained MONPs were investigated via XRD, TEM and UV-Vis spectra. As a representative, CeO₂ was utilized as a nanophotocatalyst to examine the photocatalytic activity of the MONPs for methylene blue (MB) photodegradation. CeO₂ showed high removal of MB dye by 90.1% after UV illumination for 220 min. The reported method provides a generalized and systematic method for the preparation of many metal oxide nanoparticles with manageable and reproducible features.

Full text of DOI:10.1039/c9ra08080e

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PAPER
Novel lanthanide(III) 4-methylbenzoylhydrazide
complexes as precursors for lanthanide oxide
nanophotocatalysts
Cite this: RSC Adv., 2019, 9, 42010
a
*
S. A. Abou El-Enein,
and Metwally Madkour
A. M. Ali,a Y. K. Abdel-Monem,a M. H. Sennab
*
c
A series of metal complexes were prepared from separate reactions of lanthanide nitrate salts (La(III), Ce(III),
Sm(III), Gd(III) and Ho(III)) with 4-methylbenzoylhydrazide. The structures of the complexes were confirmed
by analytical studies, spectral measurements and thermal studies. Complexes were formed with different
stoichiometries of 1 : 2 and 1 : 3 (M : L). The ligand chelates by the nitrogen and oxygen atoms of the
amino and carbonyl groups of the hydrazide moiety in the neutral keto form. The coordination
compounds were converted to metal oxide nanoparticles (MONPs) through solid state thermal
decomposition as monocular source precursors. The obtained MONPs were investigated via XRD, TEM
and UV-Vis spectra. As a representative, CeO2 was utilized as a nanophotocatalyst to examine the
photocatalytic activity of the MONPs for methylene blue (MB) photodegradation. CeO2 showed high
removal of MB dye by 90.1% after UV illumination for 220 min. The reported method provides
a generalized and systematic method for the preparation of many metal oxide nanoparticles with
manageable and reproducible features.
Received 5th October 2019
Accepted 14th November 2019
DOI: 10.1039/c9ra08080e
rsc.li/rsc-advances
Metal oxide nanoparticles show a wide range of applications,
including catalysis, sensing, optics, semiconductors, solar cells,
1. Introduction
batteries and biotechnology. Also, metal oxide nanoparticles
have been used as nanophotocatalysts for the degradation of
methylene blue and Safranin-O dyes.8–12 The synthesis of metal
oxide nanoparticles from their corresponding metal complexes
is of great interest. This technique offers several unique
advantages over other methods, including facility, reproduc-
ibility, short reaction times, short washing and drying dura-
tions, and production of inorganic nanomaterials with narrow
size distributions.9
In this work, we aimed to synthesize lanthanide metal
complexes of lanthanum(III), cerium(III), samarium(III), gadoli-
nium(III), and holmium(III) with 4-methylbenzoylhydrazide
ligand. Structural characterization of all the prepared
compounds was performed based on elemental analysis, spec-
tral methods (UV-Vis, IR, 1H NMR, and mass spectroscopy),
molar electrical conductivity, magnetic susceptibility and
thermal analyses. Also, nano-metal oxides were prepared by
thermal degradation of the metal complexes, and their appli-
cations in wastewater treatment were studied through photo-
catalytic degradation of methylene blue.
Hydrazide compounds contain terminal NH2 groups and are
precursors for numerous synthesized Schiff and azo compounds.
Hydrazides and their derivatives have potential active sites; C]O,
NH and NH2 have the ability to coordinate with different metal
ions.1 Hydrazides and their derivative metal complexes have
applications in biological elds as insecticidal, acaricidal, nemati-
cidal, and antileishmanial agents and inhibit protease enzymes in
some parasitic species and diamine oxidases.1 Their lanthanide
complexes have luminescent and magnetic properties; thus, they
can be used in electroluminescent devices.2,3 Also, due to their
exhibited spectroscopic, magnetic, and chemical properties, these
lanthanide complexes have many useful applications compared to
other metal complexes. This is due to the uxional nature of lan-
thanide(III) ion, which possesses high and variable coordination
numbers as well as a lack of angular preference.4 Hydrazide
lanthanide complexes are an ongoing area of active research due to
their good antioxidant activities and their biological activities, such
as anticoagulation, antiallergic, antibacterial, anti-inammatory
and anticancer. Also, they have been used for treatment of burns.5–7
aDepartment of Chemistry, Faculty of Science, Menoua University, Shibin El Kom,
Egypt. E-mail: dr.saeyda_elenein@yahoo.com
bRadiation Chemistry Department, National Center for Radiation Research and
2. Experimental
2.1 Materials and methods
Technology, Cairo, Egypt
cDepartment of Chemistry, Faculty of Science, Kuwait University, P. O. Box: 5969,
13060, Kuwait. E-mail: metwally.madkour@ku.edu.kw
All chemicals were of analytical grade (BDH, Sigma or Aldrich)
and were used as received without further purication.
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Elemental microanalysis (C, H, and N) was performed using crystals which were nally dried under vacuum over anhy-
a PerkinElmer-2400 elemental analyzer at Main Defense drous calcium chloride. The measured melting point was
ꢁ
Chemical Laboratory. Infrared spectra were registered on 164 C.
a JASCO FTIR-6300 spectrophotometer in the range of 4000 to
1
400 cmꢀ1 to investigate the infrared spectral bands. The H- 2.3 Synthesis of the lanthanide(III) complexes
NMR spectra were recorded in DMSO-d6 on a Bruker NMR
spectrometer at 600 MHz. Fast atom bombardment (FAB)
mass spectra for the ligand and its La(III) and Gd(III)
complexes were acquired using a spectrometer. The electronic
The complexes were synthesized by adding an appropriate
ethanolic solution of M(NO3)3$nH2O (M ¼ La(III), Ce(III), Sm(III),
Gd(III) and Ho(III), n ¼ 3 to 6) to an ethanolic solution of ligand
with a molar ratio of 1 : 3 (M : L). The mixture was stirred for
absorption spectral bands were revealed in Nujol mulls using
three hours at 50 ꢁC, then ltered, washed and completely dried
Varian Vary 5000 UV-Vis spectrophotometer. Molar conduc-
under vacuum over anhydrous CaCl2.
tance measurements of the complexes in 10ꢀ3 M DMSO were
performed using
magnetic susceptibility was measured at room temperature by
a Tacussel CD6N conductimeter. The
2.4 Preparation of metal oxide nanoparticles
In a typical procedure, certain amounts of La(III), Ce(III), Sm(III),
Gd(III) and Ho(III) complexes were maintained inside a crucible
and heated to 600 ꢁC under air in a muffle furnace for 60
a modied Gouy method using a Johnson Mathey magnetic
susceptibility balance.
A Shimadzu DAT/TG-50 thermal
analyzer was used to conduct the thermogravimetric analysis
(TG and DꢀT1G) under certain conditions (a heating rate of
minutes (heating rate: 10 C minꢀ1). The thermal degradation
ꢁ
of the coordinated precursors resulted in the formation of their
analogue metal oxides. The produced oxides were cooled to
room temperature and then collected for analysis.
10 ꢁC min
under nitrogen and oxygen atmosphere with
a owing rate of 20 ml minꢀ1 from room temperature to
ꢁ
900 C using platinum crucibles). Melting points were detec-
ted by a Stuart melting point apparatus. The X-ray diffraction
(XRD) measurements for the MONPs were performed using an
Bruker D8 Advance diffractometer with a copper target and
2.5 Photocatalytic activity
The photodegradation test for methylene blue dye (model
pollutant) was performed as follows: CeO2 nanoparticles (10
mg/100 ml) were dispersed in methylene blue aqueous solution
(10 ppm) using a sonicator for 20 minutes in the dark to exclude
the adsorption results. Aer that, the solution was irradiated
with a UV lamp (6 W at a wavelength of 254 nm). Aliquots of
5.0 ml of the tested solution were collected each 20 min,
centrifuged at 5000 rpm to monitor absorbance changes, then
analyzed spectrophotometrically to detect the absorbance
changes as a function of irradiation time.
nickel lter with CuKa radiation (k
¼
0.154056 nm).
Measurements were performed in the 2q range of 20ꢁ to 80ꢁ.
The morphology of the CeO2 nanoparticles was obtained by
transmission electron microscopy (TEM) with a JEOL JEM
1230 (JEOL Ltd., Japan) operating at 120 kV. The average
particle size was determined statistically by manually count-
ing 90 particles.
2.2 Synthesis of 4-methylbenzoylhydrazide ligand (HL)
The ligand was prepared as reported.13 The reaction was per-
formed by dissolving 4-toulic acid in ethanol in the presence of
conc. sulfuric acid as a catalyst to yield the ester derivative.
3. Results and discussion
3.1 Analytical data
Then, an excess of hydrazine hydrate was added. The reaction Table 1 presents the analytical data of the prepared lanthani-
mixture was reuxed for 4 h. The solid ligand was ltered, de(III) nitrate complexes. The prepared complexes have the
washed, and recrystallized from methanol to afford pale yellow chemical formulas [La(HL)2(NO3)2]$NO3$EtOH, [Ce(HL)3(NO3)2]$
Table 1 Analytical data and molar conductance values of 4-methylbenzoylhydrazide (HL) and its metal complexes
Elemental analysis, F (cal.)%
No.
Compound
Colour, F. W.
C
H
N
Mp
Aa
HL1$0.5H2O, C8H11N2O1.5
[La(HL)2(NO3)3]$EtOH, C18H26N7O9La
[Ce(HL)3(NO3)2]$NO3, C24H30N9O12Ce
[Sm(HL)3(NO3)2]$NO3$H2O,
Off-white, 159.19
White, 623.35
White, 812.67
White, 804.93
60.67, (60.36)
34.49, (34.40)
36.96, (37.11)
35.66, (35.81)
6.42, (6.96)
3.67, (3.95)
3.94, (3.89)
4.00, (4.01)
17.64, 17.59
15.05, (14.78)
16.22, (16.23)
16.09, (15.67)
132
222
238
246
—
42
52
56
1
2
3
C
24H32N9O13Sm
[Gd(HL)3(NO3)2]$NO3$0.5H2O,
24H31N9O12.5Gd
[Ho(HL)3(NO3)2]$NO3$0.5EtOH,
25H32N9O12.5Ho
4
5
White, 802.81
White, 854.52
36.06, (35.91)
35.99, (35.13)
3.82, (3.89)
3.82, (3.77)
15.69, (15.75)
15.68, (14.75)
248
250
55
54
C
C
a
(Uꢀ1 cm2 molꢀ1).
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Fig. 1 FAB mass spectrum of the ligand.
NO3, [Sm(HL)3(NO3)2]$NO3$H2O, [Gd(HL)3(NO3)2]$NO3$0.5H2O EtOH and [Gd(HL)3(NO3)2]$NO3$0.5H2O, giving parent
and [Ho(HL)3(NO3)2]$NO3$0.5EtOH. The complexes are stable in molecular ions at m/z 623 and 803, respectively; this conrms
air and non-hydroscopic. All the chelates showed solubility in the theoretical data.
polar and nonpolar solvents. The molar conductance values of
the complexes were in the range of 52 to 56 Uꢀ1 cm2 molꢀ1
3.2.2 1H-NMR. The 1H-NMR spectrum of the ligand (HL)
and its lanthanum(III) complex are compiled in Table 2 and
,
which demonstrates the uni-electrolytic properties of all the represented in Fig. 2. The spectrum of the ligand exhibits
complexes.14
signals at d, ppm: 2.32 (3H, s, CH3), 4.52 (2H, s, NH2), 7.22 to
7.82 (4H, s, Ar-CH) and 9.69 (1H, s, NH).15,16 In comparison, the
spectrum of the La(III) complex shows signals at 2.92, 4.80, and
9.77 ppm, which are assigned to CH3, NH2, and NH,
3.2 Spectral studies
3.2.1 Mass spectra. The FAB mass spectra of the ligand is
shown in Fig. 1. The FAB mass spectrum of the hydrazide
ligand (Fig. 1) contains a molecular ion peak at m/z 150.0
amu, which matches the theoretical molecular weight of the
ligand without solvent. This proved its formation. Also, the
spectrum exhibited other degradation peaks at 119, 91.05, 76,
and 55 amu, which are assignable to [C8H7O]+, [C7H7]+,
[C6H4]+ and 4.5C, respectively; this is compatible with
Scheme 1. The mass spectra of the La(III) and Gd(III)
complexes are consistent with the formulae [La(HL)2(NO3)3]$
Table 2 1H NMR spectra of hydrazide ligand (HL) and its La(III) complex
(HL)
La(III) complex
Assignment
2.32
4.52
7.22–7.82
9.69
2.33–2.49
4.80
7.23–7.72
9.77
3H of 2(CH3)
2H of (NH2)
H of (CH)
1H of (NH)
Scheme 1 Mass ion of the HL ligand and its fragmentation.
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Fig. 2 1H NMR spectra of the hydrazide ligand (HL) and its La(III)
complex.
respectively. The NH2 signal shied downeld relative to the
uncoordinated ligand, conrming its involvement in chela-
tion. The absence of a signal at 12 to 14 ppm assigned to the
enolic OH suggests that the ligand chelated with metal ion in
its neutral keto structure.
3.2.3 Infrared spectra of the ligand (HL) and its metal
complexes. The fundamental spectral bands and their
assignments of the ligand and its metal complexes are
tabulated in Table 3 and diagrammed in Fig. 3. The spectrum
of the ligand contains strong bands at 3306, 3222, 3027,
1616, and 1503 cmꢀ1; 1042 and 922 cmꢀ1; and 787, 719 and
446 cmꢀ1, which can be assigned to yasy(NH2), ysy(NH2),
y(NH), d(NH2), and d(NH)amide
g(NH)amide; and r(NH2), respectively.16 The carbonyl groups
are assigned to strong bands located at 1659 and 677 cmꢀ1
; y(N–N), g(NH2), and
,
633 cmꢀ1 and 522 cmꢀ1, attributed to y(C]O), d(C]O), and
g(C]O), respectively. The spectrum of the ligand exhibits
the presence of fundamental bands corresponding to NH
and C]O (y, d, g) with the absence of y(OH); this indicates
that the ligand has a keto structure. The spectrum of the
ligand shows additional fundamental bands at 2920 and
2878 cmꢀ1 due to the y(CH) aromatic and methyl groups,
respectively. Comparison of the spectral bands of the ligand
with those of its metal complexes led to the following
observations: yasy, ysy, and d of the NH2 and y(C]O) groups of
the ligand undergo blue shis at 10 to 13 cmꢀ1 upon
complexation. However, d(C]O) of the ligand at 617 cmꢀ1
disappeared, while the second one at 633 cmꢀ1 shied to
lower values by 5 to 17 cmꢀ1 upon complexation.
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bands at 522 and 446 cmꢀ1 due to g(C]O) and r(NH2) exhibit
shis in the ranges of 518 to 528 and 462 to 472 cmꢀ1 upon
complexation, which are attributed to y(M–O) and y(M–N),
respectively.17 The absorption bands appearing at around 1380
and 784 cmꢀ1 in the spectra of the investigated complexes are
assigned to the ionic nitrate group.14 The complexes exhibit
absorption bands in the regions of 1463 to 1466 cmꢀ1 and 1208
to 1212 cmꢀ1; these are not observed in the spectrum of the
ligand and can be assigned to y4 and y1, respectively. This
situation suggests that the nitrate ions are covalently bonded
and are located in the coordination sphere; this indicates that
the magnitude of separation Dy ¼ y1 ꢀ y4 is sufficiently greater
than 200 cmꢀ1, considering that the nitrate groups are cova-
lently bonded to the metal ion in bidentate fashion. The data
show that the lanthanide nitrate complexes contain both coor-
dinated and ionic nitrate groups, which conrms the chemical
formulae.
Fig. 3 IR spectra of the ligand and its metal complexes.
The above assignments indicate that the metal ions attach to
the ligand via carbonyl oxygen and amino nitrogen atoms,
leading to the construction of ve-membered rings. The ligand
Table 4 Electronic absorption spectra and magnetic moment values of the ligand and its complexes
Electronic spectral
No.
Compound
bands (nm)
Assignment
m/mB
(HL)0.5H2O
[La(HL)2(NO3)3]$EtOH
[Ce(HL)3(NO3)2]$NO3
[Sm(HL)3(NO3)2]$NO3$H2O
[Gd(HL)3(NO3)2]$NO3$0.5H2O
[Ho(HL)3(NO3)2]$NO3$0.5EtOH
296, 272, 240
308, 284, 242
297, 287, 242
313, 284, 242
312, 284, 246
312, 284, 242
N–p*, p–p*
n–p*, p–p*
n–p*, p–p*
n–p*, p–p*
n–p*, p–p*
n–p*, p–p*
—
1
2
3
4
5
Diam
3.09
3.69
7.91
10.78
Fig. 4 Electronic absorption spectra of the ligand and its complexes.
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Table 5 Thermal decomposition of the ligand and its metal complexes
Mass loss%
TG range (ꢁC) DTA peak (ꢁC) found (calcd.) Assignment
No. Compound
TS (ꢁC)
163
1
(HL)$0.5H2O
109–163
163–292
292–322
30–228
228–325
325–787
At 787
2.70 (2.83)
Loss of 0.25 mol of H2Oa
69.37 (70.14) Ligand pyrolysisc
26.13 (24.52) Complete ligand pyrolysisc
2
[La(HL)2(NO3)3]$EtOH
7.39 (8.18)
Loss of 1 mol of EtOHb
228
c
293
487
55.21 (55.90) Loss of 1 mol of L + 2 mol of NO3ꢀ + 1 mol of NO2
18.95 (17.73) Complete ligand pyrolysisc
d
18.44 (18.91) 0.5La2O3
ꢀb
3
4
5
6
[Ce(HL)3(NO3)2]$NO3
30–206
206–250
250–466
At 466
30–217
217–297
297–692
At 692
7.49 (7.63)
Loss of 1 mol of NO3
206
217
267
234
ꢀc
231
387
63.80 (63.56) Loss of 2 mol of L + 2 mol of NO3
7.06 (6.89)
Complete L pyrolysisc
d
20.65 (20.19) 0.5Ce2O3
2.60 (2.24)
[Sm(HL)3(NO3)2]$NO3$H2O
Loss of 1 mol of H2Oa
c
257
467
56.93 (58.43) Loss of 2 mol of L + 2 mol of NO3ꢀ + 1 mol of NO2
19.34 (17.92) Complete ligand pyrolysisc
d
21.41 (21.67) 0.5Sm2O3
[Gd(HL)3(NO3)2]$NO3$0.5H2O 30–267
1.36 (1.12)
Loss of 0.5 mol of H2Oa
267–334
334–893
At 893
290
453
64.81 (65.07) Loss of 2 mol of L + 1 mol of NO3ꢀ + 1 mol of NO2
11.55 (11.23) Complete ligand pyrolysisc
d
22.41 (22.58) 0.5Gd2O3
[Ho(HL)3(NO3)2]$NO3$0.5EtOH 30–234
2.70 (2.81)
Loss of 0.5EtOHb
c
234–300
300–575
At 575
174
264
452
64.42 (63.89) Loss of 2 mol of L + 2 mol of NO3ꢀ + 1 mol of NO2
11.01 (11.23) Complete ligand pyrolysisc
d
22.11 (22.08) 0.5Ho2O3
Dehydration. b Desolvation. Decomposition. d Remain.
a
c
corresponds with the measured value. The thermoanalytical
data in Table 5 combined with the TG and DTA patterns of the
La(III), Sm(III), Gd(III), and Ho(III) complexes show that these
complexes decompose in a similar way, indicating that they
are isothermal.
The TG curves showed weight losses in the temperature
range of 30 ꢁC to 234 ꢁC due to removal of the solvents of
crystallization. Aer that, the chelates decomposed in two
consecutive stages. The rst stage exhibited weight losses of
3.2.4 Electronic absorption spectra. The electronic spec-
tral data of the hydrazide organic compound (HL) and its
metal chelates in Nujol mulls are reported in Table 4 and
demonstrated in Fig. 4. The electronic spectrum of the ligand
displays spectral bands in the range of 272 to 240 nm and at
296 nm, assigned to the p–p* and n–p* electronic transi-
tions of the entire conjugate system of the ligand, respec-
tively. On the other hand, the electronic spectra of the metal
chelates (1–5) display p–p* and n–p* transitions of the
ligand, which are shied to higher values upon complexa-
tion; this indicates that chelation was achieved. None of the
spectra of the complexes showed bands in the visible region.
This is probably due the fact that the f–f bands are weak and
obscured by the intense charge transfer bands.18,19 The
magnetic moments of the lanthanide(III) complexes indicate
that only the La(III) complex is diamagnetic, and the other
tripositive lanthanide ion complexes are paramagnetic.
3.2.5 Thermal analysis of the metal complexes. The ther-
mograms (TG, DTA) gathered with thermoanalytical data and
their assignments for each decomposition step to follow the
decomposition pathways for the ligand and its metal
complexes are given in Table 5 and Fig. 5. The TGA curve of the
ligand shows thermal stability up to 109 ꢁC followed by a small
weight loss of 2.70% up to 163 ꢁC, corresponding to the release
of 0.25 mol of H2O. The pyrolytic process was achieved
through two steps by TG weight losses of 69.37% and 26.13%
and DTA peaks at Tmax ¼ 180 ꢁC and 292 ꢁC, respectively. The
DTA curve displays a sharp endothermic peak at 132 ꢁC, which
is assignable to the melting point of the compound and
ꢁ
ꢁ
55.21%, 56.90%, 64.80% and 64.42% from 2ꢁ28 C to 325 C,
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
217 C to 297 C, 214 C to 334 C and 234 C to 300 C for
La(III), Sm(III), Gd(III) and Ho(III), respectively. This process is
associated with sharp exothermic DTA peaks at Tmax
¼
293 ꢁC, 257 ꢁC, 290 ꢁC, and 264 ꢁC, respectively; this signies
that the oxidative process of the ligand involves the release of
two moles of organic ligand (one mole in the La(III) complex),
two moles of NO3 groups and one mole of NO2 gas. The last
ligand pyrolysis was characterized by TG weight losses of
18.93%, 19.34%, 11.55% and 11.01% up to 787 ꢁC, 692 ꢁC,
893 ꢁC and 575 ꢁC for the La(III), Sm(III), Gd(III) and Ho(III)
complexes, respectively. The thermal decomposition process
ended with the formation of 0.5 M2O3 (M ¼ La(III), Sm(III),
Gd(III) and Ho(III)). However, the TG curves of the Ce(III)
ꢁ
ꢁ
complex reveal a weight loss of 7.49% in the 30 C to 206 C
range due to elimination of the ionic nitrate group. This step
was followed by a TG weight loss of 63.8% up to 250 ꢁC. This
process was accompanied by two exothermic DTA peaks at
Tmax ¼ 231 ꢁC to 253 ꢁC, indicating the oxidation of the
ligands with removal of two moles of the ligand and two
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Fig. 5 Thermograms of the ligand and its metal complexes.
moles of the coordinated nitrate group. The last decompo- 31.21ꢁ, 43.28ꢁ, 47.86ꢁ, 59.29ꢁ, 64.97ꢁ and 70.12ꢁ are indexed to
sition step exerted a TG weight loss of 6.89% in the 250 ꢁC to the (002), (400), (125), (600), (127), (800) and (822) planes,
466 ꢁC range due to complete ligand pyrolysis, and the metal respectively, and assigned as per JCPDS no. 65-3185.20 For the
reacts with atmospheric oxygen to produce 0.5Ce2O3 as Sm2O3 NPs, the diffraction peaks located at 28.2ꢁ, 32.7ꢁ, 38.6ꢁ,
a product. This process is associated with the exothermic 45.4ꢁ, and 54.3ꢁ correspond to the (222), (400), (332), (521),
ꢁ
DTA peak at Tmax ¼ 387 C.
and (541) planes of cubic Sm2O3 (JCPDS no. 15-0813),
respectively.21 For the Ho2O3 NPs, diffraction lines of cubic
holmium oxide (JCPDS card 88-2163) were obtained.22 For the
Gd2O3 NPs, the pattern was well-indexed with the cubic
structure reported in the standard JCPDS card no. 43-1014.23
No diffraction peaks of any impurity were observed, indi-
cating the high purity of the nal products. These results
3.3 Characterization of nanoparticles
In Fig. 6, the diffraction peaks for CeO2 at 28.5ꢁ, 33.0ꢁ, 47.4ꢁ
and 56.3ꢁ can be indexed to the (111), (200), (220) and (311)
planes of the face-centered cubic structure of CeO2 (JCPDS no.
81-0792). For the La2O3 NPs, the peaks obtained at 28.67ꢁ,
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Fig. 6 X-ray diffraction patterns of the prepared lanthanide oxide
nanoparticles.
Fig. 8 TEM image of the CeO2 nanoparticles. The inset is the size
distribution for CeO2 nanoparticles.
suggest that the precursor samples crystallized into their
analogous oxide nanoparticles at a heating temperature of where h is Planck's constant, n is the photon frequency, a is
700 ꢁC. The crystallite sizes for the lanthanide oxide NPs were the absorption coefficient, Eg is the band gap, the exponent n
calculated using the Debye–Scherrer equation:24
denotes the nature of the electronic transition (direct or
indirect), and A is a proportionality constant. The calculated
values were found to be in the range of 4.64 to 4.85 eV. The
increase of the band gap can be attributed to the quantum
connement effect, which led to decreased charge carrier
recombination rates.20 The sizes and morphologies of the
CeO2 nanoparticles were investigated by TEM. The TEM
image of CeO2 is shown in Fig. 8. From the gure, the nano-
particles can be found to be monodisperse and spherical, with
an average particle size of 5.6 nm.
D ¼ kl/(b cos q)
(1)
where k is a shape factor (k ¼ 0.89), l is the wavelength of the
radiation, b is the full width at half maximum (FWHM) and q is
the angle corresponding to the most intense peak in the XRD
pattern. The crystallite sizes were found to be 2.9 nm, 3.7 nm,
4.2 nm, 3.8 nm and 3.3 nm for the CeO2, La2O3, Sm2O3, Ho2O3
and Gd2O3 NPs, respectively.
The absorption spectra for the prepared lanthanide oxide
nanoparticles were measured, as shown in Fig. 7. All the
nanoparticles showed similar absorption behavior, with
a strong absorption peak at ꢂ240 to 250 nm, which is
consistent with the literature. The band gap energy values for
the lanthanide oxide nanoparticles were calculated using
Tauc plots by applying the equation (ahn)1/n ¼ A(hn ꢀ Eg),
3.4 Photocatalytic activity
Because all the prepared NPs revealed the same absorption
spectra and band gap energy values, in this work, the CeO2
NPs were chosen for evaluation as a photocatalyst due to their
smaller particle size relative to the other metal oxides. Pho-
tocatalytic degradation of MB dye using the CeO2 NPs as
a model photocatalyst was performed to evaluate their pho-
tocatalytic activity. Fig. 9 shows the photocatalytic activity of
the CeO2 NPs. The photo degradation efficiency was calcu-
lated to be 90.1%, as shown in Fig. 9B. Fig. 9C showed that the
photodegradation
reaction
follows
pseudo-rst-order
kinetics. The photodegradation rate constant for the photo-
degradation reaction was determined from the equation:
ln(Co/C) ¼ kt
(2)
where Co and C are the initial concentration and the concen-
tration at time t, respectively, and k is the apparent rst-order
rate constant. The plot of ln(Co/C) versus time represents
a straight line, as shown in Fig. 9C; the slope upon linear
regression equals the apparent rst-order rate constant k. The
degradation rate obtained was 0.014 minꢀ1. To evaluate the
photostability of CeO2, four photodegradation experiments
(reusability tests) under identical conditions were performed.
Fig. 7 UV-Vis absorption spectra of the prepared lanthanide oxide
nanoparticles.
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Fig. 9 (A) Methylene blue dye absorption spectra at different times upon irradiation with CeO2. (B) Evaluation of MB conversion efficiency upon
irradiation. (C) Kinetics of MB photocatalytic degradation. (D) Reusability of CeO2 nanoparticles.
From Fig. 9D, it can be noted that CeO2 was almost completely
Conflicts of interest
stable and photoactive up to the 4th run, as its photocatalytic
activity decreased by only 15% aer the 4th run.
There are no conicts to declare.
4. Conclusion
Acknowledgements
In this work, a series of metal complexes were prepared by
separate reactions of lanthanide nitrate salts (La(III), Ce(III),
Sm(III), Gd(III) and Ho(III)) with 4-methylbenzoylhydrazide. The
structures of the complexes were conrmed by analytical
studies, different spectral measurements and thermal studies.
Complexes were formed with different stoichiometries of 1 : 2
and 1 : 3 (M : L). The coordination compounds were converted
to metal oxide nanoparticles (MONPs) through solid state
thermal decomposition as monocular source precursors. The
obtained MONPs were investigated via XRD, TEM and UV-Vis
spectral analyses. CeO2 was utilized as a representative nano-
photocatalyst to examine the photocatalytic activity of the
MONPs in methylene blue (MB) photodegradation. The CeO2
catalyst showed high MB removal of 90.1% aer UV illumina-
tion for 220 min. The reported method provides a generalized
and systematic method for the preparation of many nano-
particle metal oxides with manageable and reproducible
features.
The authors gratefully acknowledge RSPU at Kuwait University,
Facilities No. GS 01/01, GS 01/03, GS 01/05, GS 02/01, GS 02/08,
GS 03/01 and GE 03/08. The Nanoscopy Science Center is also
highly acknowledged.
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