A perfectly linear trinuclear zinc-Schiff base complex: Synthesis, luminescence property and photocatalytic activity of zinc oxide nanoparticle

Article abstract of DOI:10.1016/j.ica.2014.06.014

A perfectly linear trinuclear zinc(II) complex [Zn₃L ₂(μ-O₂CCH₃)₂] (1) containing a (N,O)-donor Schiff base ligand, (H₂L = N,N′- bis(salicyaldehydene)-1,3-diaminopropan-2-ol) has been synth

Full text of DOI:10.1016/j.ica.2014.06.014

Inorganica Chimica Acta 421 (2014) 335–341
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
A perfectly linear trinuclear zinc–Schiff base complex: Synthesis,
luminescence property and photocatalytic activity of zinc oxide
nanoparticle
a
b
c
b
a
Dhananjay Dey , Gurpreet Kaur , Moumita Patra , Angshuman Roy Choudhury , Niranjan Kole ,
Bhaskar Biswas
a
a,
⇑
Department of Chemistry, Raghunathpur College, Purulia 723133, India
Department of Chemical Sciences, Indian Institute of Science Education and Research, Mohali, Mohali 140 306, India
Department of Physics, Raghunathpur College, Purulia 723133, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 May 2014
Received in revised form 16 June 2014
Accepted 17 June 2014
Available online 2 July 2014
2 2 3 2
A perfectly linear trinuclear zinc(II) complex [Zn L (l-O CCH ) ] (1) containing a (N,O)-donor Schiff base
3
0
ligand, (H
ized by single crystal X-ray diffraction study. The X-ray crystal structure of 1 contains three zinc(II)
centers which are inter-connected through -phenolato and -acetato bridges. The terminal zinc centers
are in square pyramid geometry and central zinc ion is in distorted octahedral coordination geometry.
Both H L and 1 exhibit good fluorescence properties in solution. 1 has been used as a precursor to
2
L = N,N -bis(salicyaldehydene)-1,3-diaminopropan-2-ol) has been synthesized and character-
l
2
l
2
Keywords:
Zinc(II)
X-ray structure
Fluorescence property
ZnO nanoparticle
Dye degradation
fabricate zinc oxide nanoparticles (ZnONPs) by pyrolytic method. ZnONP has been characterized by pow-
der X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM), FT-IR spectroscopy
and UV–Vis spectroscopy techniques. ZnONP has been employed as photocatalytic agent to degrade
the organic dye, viz. Methylene blue under visible light and by exposing to visible light for 1 h, ZnONP
degraded methylene blue dye nearly 80%.
Ó 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Transition metal complexes with oxygen and nitrogen donor
coagulation/flocculation [10,11], electrocoagulation [12], coagula-
tion/carbon adsorption process [13] and so on. However, these
methods barely transfer the pollutants from one phase to another
without destruction or have the other limitations. In recent years,
as a promising tool to substitute the traditional wastewater
treatment, semiconductor-assisted photocatalysis among the
advanced oxide processes (AOP) has attracted the public concern
for its ability to convert the pollutants into the harmless sub-
stances directly in the waste water. Till now, many kinds of semi-
Schiff bases are of particular interest because of their unusual con-
figurations, structural lability and sensitivity to molecular environ-
ments as functional materials [1–3]. Schiff bases can accommodate
different metal centres involving various coordination modes
allowing successful synthesis of homo- and heterometallic com-
plexes with varied stereochemistry. Zinc being an essential and
one of the most bio-relevant transition metal ions; next to iron
conductors have been studied as photocatalysts including TiO
2
,
(
human beings contain an average of ꢀ2–3 g of zinc). It is no sur-
ZnO, CdS, WO and so on [14–17]. TiO is the most widely used
effective photocatalyst for its high efficiency, photochemical stabil-
ity, non-toxic nature and low cost. As a contrast, ZnO, a kind of
semiconductor that has the similar band gap as TiO , is not thor-
2
oughly investigated. However, the greatest advantage of ZnO is
that it absorbs large fraction of the solar spectrum and more light
3
2
II
prise that di- and trinuclear Zn –Schiff base complexes have
attracted particular interest as synthetic structural mimics of the
active site of a range of metalloenzymes [4,5], new cancer thera-
peutic agents and their photochemical materials [6–9].
Excess apply of various dyes in the textile industry has led to
the severe surface water and groundwater contamination by
releasing the toxic and coloured effluents, which are usually
disposed by various physical and chemical methods, such as
2
quanta than TiO [18].
In this present work, we have synthesized and crystallographi-
cally characterized a perfectly linear trinuclear Zn(II)–Schiff base
complex (1). Fluorescence studies for both the Schiff base ligand
and complex exhibits strong emission at room temperature. We
have also prepared zinc oxide nanoparticle from this trinuclear
complex as a precursor in the dimension of ꢀ98 nm and the
⇑
Corresponding author. Tel./fax: +91 3251 255235.
E-mail addresses: mr.bbiswas@rediffmail.com, bhaskarbiswas557@gmail.com
(
B. Biswas).
http://dx.doi.org/10.1016/j.ica.2014.06.014
020-1693/Ó 2014 Elsevier B.V. All rights reserved.
0

3
36
D. Dey et al. / Inorganica Chimica Acta 421 (2014) 335–341
nanoparticles have been characterized by powder X-ray diffraction
study and scanning electron microscope imaging. The visible light
driven photocatalytic activity of synthesized ZnO nanoparticle was
demonstrated using methylene blue (MB) as a representative dye
and the catalytic efficiency is optimized with respect to pH of the
solution.
2.3. X-ray diffraction study
Single crystal X-ray diffraction data were collected using a
Rigaku XtaLAB mini diffractometer equipped with Merury CCD
detector. The data were collected with graphite monochromated
Mo Ka radiation (k = 0.71073 Å) at 295(2) K using x scans. The
data were reduced using Crystal Clear suite and the space group
determination was done using Olex2. The structure was solved
by direct method and refined by full-matrix least-squares proce-
dures using the SHELXL-97 software package using Olex2 suite
2
. Experimental
2
2
2
.1. Preparation of the complex
[
20,21].
.1.1. Chemicals, solvents and starting materials
High purity Salisaldehyde (E. Merck, India), 1,3-diaminopropan-
-ol (Lancaster, UK), methylene blue (Aldrich, UK), zinc(II) acetate
2.4. Physicochemical characterization of ZnO nanopartcle
Size and morphology of the 98 nm ZnO nanoparticle was char-
dihydrate (E. Merck, India) were purchased from respective con-
cerns and used as received. All the other reagents and solvents
are of Analytical grade (A.R. grade) and were purchased from com-
mercial sources and used as received.
acterized by powder X-ray diffraction (PXRD) pattern and Scanning
Electron Microscopy (SEM) to examine their structure, shape and
surface morphology. The XRD pattern was measured by X-ray dif-
fractometer (Bruker D8 advance) in the range from 25° to 80° using
Cu K
a radiation. The SEM image has been obtained using a micro-
2
.1.2. General synthesis of the Schiff base ligand (H
2
L) and zinc
scope (FESEM, JEOL, and JSM-6700F).
compound (1)
0
The Schiff base ligand, H
2
L, N,N -bis(salicyaldehydene)-1,3-dia-
2.5. Photocatalytic experiments
minopropan-2-ol was synthesized in a reported literature [19].
Salisaldehyde (0.244 g, 2 mmol) was heated under reflux with
1
alcohol. After 10 h the reaction solution was evaporated under
reduced pressure to yield a gummy mass, which was dried under
The photocatalytic activity of ZnO nanoparticles was evaluated
,3-diaminopropan-2-ol (0.089 g, 1 mmol) in 30 ml dehydrated
by degradation of methylene blue (MB) dye solution. All the exper-
iments were carried out in presence of visible light. A 250 ml Boro-
sil beaker with outside water circulation was placed on a magnetic
stirrer, above which a high pressure mercury vapour lamp (125 W,
Philips) emitting visible light was placed. ZnO nanoparticle at a
vacuum and stored over CaCl
82.8%). Anal. Calc. for C17
3
.39. Found: C, 68.40; H, 6.02; N, 9.35%. H NMR (CDCl ) d = 3.68
2
for subsequent use. Yield, 0.278 g
(
9
H
18
N
2
O
3
(H L): C, 68.48; H, 6.08; N,
2
1
dose of 100 mg (solid) was added to 100 ml MB dye solution
(
(
dd, J = 12.4, 6.8 Hz, 2H), 3.84 (dd, J = 12.4, 4.0 Hz, 2H), 4.23–4.25
m, 1H), 6.88 (t, J = 7.2 Hz, 2H), 6.96 (d, J = 8.4 Hz, 2H), 7.25 (dd,
ꢁ4
(
1.2 ꢃ 10 M) in the beaker. The distance of the light source from
the upper level of dye solution is 18 cm for maximum utilization of
light. The solution was stirred in dark for 10 min to establish the
adsorption equilibrium. The zero time reading was taken and the
solution was then irradiated with visible light. Aliquots of 5 ml
samples were taken at regular time interval (10 min) and centri-
fuged to analyse the percent degradation of the MB dye. The per-
centage dye degradation was calculated using formula:
J = 7.6, 1.6 Hz, 2H), 7.32 (td, J = 8.8, 1.6 Hz, 2H), 8.36 (s, 2H) ppm.
1
3
C NMR d, 62.9, 70.2, 117.0, 118.5, 118.6, 131.5, 132.5, 161.1,
ꢁ1
1
67.3 ppm. IR (KBr, cm ): 1634, 1611 (vC@N), 3412 (vOH), UV–Vis
max, nm): ꢀ221, 267, 316, 410 nm.
(k
A methanolic solution of Zn(OAc)
2
ꢂ2H
2
O (0.657 g, 3 mmol)
L (0.596 g,
mmol) in the same solvent (15 cm ). The yellow solution of the
3
(
10 cm ) was added dropwise to a solution of H
2
3
2
Degradation = [(A
absorbance; A is the dye absorbance at time (t).
To study the effect of pH on degradation efficiency, the pH of
the MB dye solution was systematically adjusted by adding
o
ꢁ A
t
)/A
o
o
] ꢃ 100%, where, A is the initial dye
ligand immediately became colourless with quick precipitation of
white crystalline compound. It was filtered and recrystallized from
methanol–dimethyl formamide solvent mixture. The solvent mix-
ture produced colourless crystals. Yield: 0.453 g (69% based on
t
0
.1 (M) HCl or NaOH.
metal salt). Anal. Calc. for C38
N, 6.18. Found: C, 50.40; H, 4.27; N, 6.27%. IR (KBr, cm ): 3436
OH),1634, 1605 (vC@N), 1497, 1460, 1420 (vOAc), 1276 (vPhO);
UV–Vis (kmax, nm): 266, 361.
38 4 3
H N O10Zn (1): C, 50.32; H, 4.22;
ꢁ1
2.6. Thermogravimetric analysis of 1
(
v
The thermal behaviour of the trinuclear zinc complex (1) was
followed up to 700 °C in a static nitrogen atmosphere with a heat-
ing rate of 10 °C per minute.
2.2. Physical measurements
Infrared spectrum (KBr) was recorded with a FTIR-8400S SHI-
3. Results and discussion
ꢁ1 1
MADZU spectrophotometer in the range 400–3600 cm . H NMR
spectrum in DMSO-d was obtained on a Bruker Avance 300 MHz
spectrometer at 25 °C and was recorded at 299.948 MHz. Chemical
shifts are reported with reference to SiMe . Ground state absorp-
6
3.1. Syntheses and formulation
0
4
The Schiff base ligand, H
2
L, N,N -bis(salicyaldehydene)-1,3-
tion was measured with a JASCO V-530 UV–vis spectrophotometer.
Fluorescence spectra were recorded on a Hitachi F-4500 fluores-
cence spectrophotometer. Thermal analysis was carried out on a
PerkinElmer Diamond TG/DTA system up to 800 °C in a static
nitrogen atmosphere with a heating rate of 10 °C/min. Elemental
analyses were performed on a Perkin Elmer 2400 CHN microanaly-
ser. Electrospray ionization (ESI) mass spectrum was recorded
using a Q-tof-micro quadruple mass spectrometer. The pH value
of the solutions was measured by Systronics pH meter at room
temperature.
diaminopropan-2-ol was synthesized by condensing 1,3-diamino-
propan-2-ol with 2-salisaldehyde in 1:2 M ratio in dry ethanol.
The trinuclear zinc(II) complex 1 was prepared by mixing zinc(II)
acetate and the ligand in methanol–dimethyl formamide medium.
The coordination geometry of 1 was determined by mainly single
crystal X-ray diffraction study along with different spectroscopic
and analytical techniques. The colourless crystals suitable for
X-ray data collection were obtained by slow evaporation of resul-
tant reaction mixture (Scheme 1). The formulation was confirmed
1
by elemental analysis, IR, UV–Vis, H NMR, mass spectral analysis

D. Dey et al. / Inorganica Chimica Acta 421 (2014) 335–341
337
and X-ray structural analysis of the zinc compound. The schematic
presentation of syntheses is given below:
spectrum of H
2
L ligand at ꢀ13.2 ppm (Fig. S2). The characteristic
signals at 8.4 ppm indicate the proton of imine-CH and all aro-
matic-CH proton signals produce in the range of 7.3–6.8 ppm.
The proton signal at 4.2 ppm assigns aliphatic alcoholic protons.
There are no signals at ꢀ13.2 ppm in the Zn(II) complex which
assigns the deprotonated phenolic-OH groups during chelation
with Zn(II) ion (Fig. S3). Slight shift of the signals ꢀ8.3 ppm reflects
the coordination of imine-N with zinc ions. The characteristic sig-
nals at 4.1 are assignable to non-involvement of the aliphatic alco-
holic-OH group in chelation with Zn(II) ions.
3.2. Description of crystal structure
The X-ray structural determination of compound 1 reveals a tri-
ꢀ
nuclear neutral zinc(II) complex with the space group P1. The com-
pound 1 has been found to have a trinuclear molecule with a
central Zn(II) ion lying on a center of inversion. An ORTEP view
[
21] of l-acetato-l-phenoxo-trizinc(II) complex of 1 with an atom
labelling scheme is shown in Fig. 1. The trinuclear complex is built
up of two mononuclear ZnL moieties linked through bridging ace-
The complex is stable in the solid state as well as in the solution
phase. The UV–Vis spectra for ligand and trinuclear zinc(II) com-
plex show high intensity transitions in the range of 200–470 nm.
tate and
2
l -phenolato groups to the central Zn atom. The coordina-
i
tion geometry around the terminal Zn centers (Zn1 and Zn1 ) may
2
The Schiff base ligand H L shows the characteristic absorption
be regarded as square pyramidal geometry where the equatorial
bands at 221, 267, 316 and 411 nm in dimethyl sulphoxide (DMSO)
i
plane of a terminal Zn1/Zn1 atom is formed by phenoxo-bridged
solvent. The highly intense bands at 221 and 267 nm are assigned
i
i
⁄
oxygen (O2/O2 ), phenoxo oxygen (O1/O1 ), imine nitrogen (N3/
to
p–p
transition of the C@N chromophore, whereas the band
i
i
⁄
N3 ), imine nitrogen (N4/N4 ) and the axial positions are occupied
by acetato bridged oxygen (O3/O3 ). However, the coordination
with low intensity at 316 nm is due to n–
p
transition and a broad
i
band ꢀ411 nm is for intraligand charge transfer [22] (Fig. S4). On
complexation with zinc metal ion, these bands were shifted to
lower wavelength (bathochromic shift) region (361 and 266 nm)
in DMSO, suggesting the coordination of imine nitrogen and phe-
nolate oxygen with Zn(II) ion (Fig. S4).
geometry of the central zinc ion (Zn2) may also be best described
as distorted octahedral geometry, formed by six oxygen atoms
from the same Schiff base ligands and bridging acetate ions which
coordinate the terminal zinc ions. Thus four phenoxo-oxygen (O1/
i
i
O1 , O2/O2 ) from two Schiff base ligands and two acetate ions act
In order to probe the solution stability of the complex, we have
performed UV–Vis spectral measurements for the zinc complex in
i
as bridges between the two terminal zinc ions (Zn1 and Zn1 ) and
the central zinc ion (Zn2). The four equatorial positions for termi-
nal zinc ions (Zn1 and Zn1 ) are occupied by two phenoxo oxygen
3
CH OH–DMSO solution (95/5; v/v) at room temperature at various
i
time intervals. The LF band for the complex at 361 nm remained
unaffected over a period of at least 5 days revealing that the com-
plex is stable in solution at room temperature. The ESI mass spec-
i
i
i
atoms (O2/O2 , O1/O1 ) and two imine nitrogen atoms (N3/N4 , N4/
i
N4 ) where as the central zinc ion is surrounded by four phenoxo-
bridged oxygen atom (O1, O1 , O2, O2 ) from the two Schiff base
ligands. Axial positions for all three zinc ions are coordinated by
acetate bridged oxygen atom (O3, O4, O4 , O3 ). The three zinc ions
are in a perfect linear arrangement (\Zn1–Zn2–Zn1 = 180.0°). The
distances between the central zinc ion (Zn2) and the two terminal
i
i
tral study of 1 further consolidates the solution stability. [Zn
3 2
L (l-
O
2
CCH ] (1) exhibits the molecular ion peak, at m/z 929.9327 in
3 2
)
i
i
+
methanol medium (Fig. S5). ESI-MS (MeOH) m/z 929.93 [1 + Na ]
i
(calc. 929.03).
i
zinc ions (Zn1/Zn1 ) are 3.061(1) Å. In the crystalline state of 1,
3.4. Fluorescence properties
neighbouring zinc complexes form one dimensional chain struc-
ture through parallel H-bonding interaction along b axis (Fig. S1).
In fact, free aliphatic alcoholic oxygen atoms of one zinc complex
act as hosts to form strong intermolecular H-bonds towards C–H
bonds of another zinc complex (C11–H11ꢂ ꢂ ꢂO5). The structural
parameters are listed in Table 1. Selected bond lengths and angles
are presented in Table S1, and the relevant hydrogen bonding
parameters are summarized in Table S2.
It is well known that, d10 transition metal (such as Zn(II), Cd(II)
and Hg(II)) complexes usually exhibit diverse luminescent proper-
ties [23]. Here, steady-state fluorescence studies have been
employed as independent evidence of complexation between the
ligand and zinc ion. The solution state luminescent spectra of
H L and 1 in CH OH–DMSO (95/5; v/v) at room temperature are
depicted in Fig. 2. The Schiff base ligand exhibits luminescent
emission band at 352 and 441 nm upon excitation at 267 nm.
The zinc complex displays luminescent emission bands at
2
3
3.3. Solution properties
4
55 nm. (kex = 266 nm). Compared to the emission peak of the
The structural integrity in solution state for both the Schiff base
2
H L ligand, the luminescent behaviour of zinc complex 1 is red
ligand (H
2
L) and trinuclear zinc(II) complex have been determined
shift which tentatively ascribed to ligand-to-metal charge transfer
(LMCT) [23]. Fluorescence properties of the ligand and complex
(Fig. S2) indicate that it can exhibit strong fluorescence property.
Quenching of fluorescence of a ligand by transition metal ions
during complexation is a rather common phenomenon which is
explained by processes such as magnetic perturbation, redox-
activity, electronic energy transfer, etc. [24]. Enhancement of
fluorescence through complexation is, however, of much interest
as it opens up the window for photochemical applications of the
metal complexes [25,26]. Factors like a simple binding of ligand
by proton NMR, UV–Vis spectroscopy and ESI mass spectral
studies. Though the ligand is soluble in common organic solvents
like methanol, acetonitrile, dichloromethane etc but the zinc com-
plex is soluble in methanol–dimethyl sulphoxide (CH
5/5 (v/v)) and dimethyl formamide (DMF). The NMR spectra of
Schiff base and its Zn(II) complex are recorded in chloroform
CDCl ) and dimethylsulphoxide (DMSO-d6) solution respectively
3
OH–DMSO;
9
(
3
using tetramethylsilane (TMS) as an internal standard. Upon exam-
inations it is found the phenolic-OH signals appeared in the
OH
H2N
N H2
O H
N
N
C HO
CH
HC
Zn(OA c)2
OH
[
Zn (L) (OAc) ]
3 2 2
CH 3OH-DM F
OH
HO
Scheme 1. Preparative procedure of the ligand and zinc complex (1).

3
38
D. Dey et al. / Inorganica Chimica Acta 421 (2014) 335–341
Fig. 1. ORTEP Plot of [Zn
3
L (l
2
-O
CCH )
2 3 2
] (1) (30% ellipsoid probability) with atom numbering scheme.
500
1
Table 1
3 2 2 3 2
Crystallographic data of [Zn L (l-O CCH ) ] (1).
Crystal parameters
1
1200
900
_
___
Zn-complex
Ligand
Empirical formula
Formula weight
T (K)
C
38
H
38
4
N O
10Zn
3
____
906.83
293(2)
0.71073
k (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
triclinic
P1ꢀ
6
00
9.5022(12)
10.5453(15)
13.0143(8)
66.820(18)
78.47(3)
89.67(3)
1170.8(3)
1
300
0
a
(°)
b (°)
(°)
c
3
V (Å )
400
500
600
Z
λ, nm
D
calc (Mg/m3)
1.286
1.575
464
Absorption coefficient (mmꢁ1)
Fig. 2. Luminescence spectra of H
2
L and 1, recorded in CH OH–DMF solutions.
3
F(000)
Theta range for data collection (°)
Index ranges
1.7–27.50
ꢁ12 6 h 6 12,
3
.5. Thermogravimetric analysis
ꢁ
ꢁ
13 6 k 6 13,
16 6 l 6 16
The thermal behaviour of the trinuclear zinc-Schiff base com-
Reflections collected
Independent reflections
10199
5285
0.120
0.91
plex (1) was followed up to 800 °C in a static nitrogen atmosphere
with a heating rate of 10 °C per minute. Thermal analysis of 1
shows that it decomposes in three steps. In the first step, release
of two bridging acetate molecules in the temperature range
100–275 °C is occurred with a mass loss 13.18% (calc. 13.08%)
R
int
Goodness-of-fit GooF on F2
Final R indices [I > 2 (I)]
Largest diff. peak and hole (e Åꢁ3)
r
R
1
= 0.0894, wR
2
= 0.2812
0.94 and ꢁ0.63
(Fig. S6). In the second and third step, the mass corresponding to
ligand part is lost. The residual part of zinc complex remains as
zinc oxide powder which is further consolidated by the observa-
tion of straight line at 405 °C.
to the d10 metal ions [26], an increased rigidity in structure of the
complexes [27], a restriction in the photoinduced electron transfer
(
PET) [25,28], etc. are assigned to the increase in the
photoluminescence. In the present case, the enhancement of the
fluorescence of 1 [kabs (nm), 266; kem (nm); 455] compared to
the ligand [kabs (nm), 267; kem(I) nm; 352, 436] seem to be respon-
sible for the first two factors.
3.6. XRD patterns of ZnO nanoparticle
XRD patterns of ZnO compound reflect that all the diffraction
peaks of ZnO match the standard data for a ZnO nanoparticle. All

D. Dey et al. / Inorganica Chimica Acta 421 (2014) 335–341
339
the diffraction peaks (Fig. 3) can be well indexed to the hexagonal
ZnO wurtzite structure (JCPDS No. 36–1451). The sharp and
intense peaks in Fig. 3 indicate that the samples are highly crystal-
line. The lattice parameters obtained by CELSIZ programme are
a = 3.25191 and c = 5.21034 which gives the ratio c/a = 1.6 (almost
an ideal wurtzite structure). No other diffraction peaks corre-
sponding to impurity is found confirming the high purity of the
synthesized products. The mean crystalline sizes of ZnO nanostruc-
tures were calculated using the Scherrar’s formula:
1.0
0.8
0.6
0.4
0.2
0.0
D ¼ 0:9k=bCos£
where, k is the wavelength of X-rays (1.540 for Cu Ka), £ is the
Bragg’s angle, b is the full width at half maximum. The calculated
mean crystalline size of ZnO nanoparticles synthesized by pyrolytic
method is 98 nm.
3.7. Scanning electron microscopy image (SEM)
To obtain detailed information about the microstructure and
morphology of the synthesized ZnO nanoparticle, scanning elec-
tron microscopy (SEM) was carried out. Fig. 4 represents the image
of pure ZnO nanoparticle powders. It can be seen clearly that the
powders are rods and bulks which have a diameter of about
9
8 nm. As shown in the inset of the figure, the shape of the grain
is almost cylindrical with average grain size ꢀ98 nm.
20
30
40
50
60
70
80
3.8. UV–Vis spectrum of ZnONp
2
Theta
It is well known that the optical absorption behaviour of photo-
catalyst could significantly affect the photocatalytic activity. For
the characterization of the fabricated ZnONPs, in the present study,
an UV–Vis spectrophotometer was employed to record the absor-
bance spectra in the wavelength range of 220–500 nm for dis-
persed ZnONPs in distilled water. The absorbance spectrum of
ZnONPs is shown in Fig. 5. A strong absorption band ranging from
Fig. 3. PXRD plot of ZnO nanoparticle.
2
00 to 450 nm was observed for ZnONPs due to the metal ion [29].
⁄
The n?
and the n?
r
absorption bands appeared at approximately 269 nm
⁄
p
absorption band was observed at 335 nm. This
UV–Vis spectrum is consisted with previously reported data [29].
The band gap energy of ZnO nanoparticles was calculated based
on the absorption spectrum of the ZnONp according to the equa-
tion, Ebg = 1240/k (eV), where, Ebg is the band gap energy of the
photocatalyst, k is the wavelength in nm. The calculated band
gap of the ZnONp is 3.7 eV. This indicates that synthesized ZnO
has a suitable band gap for photocatalytic degradation of dyes.
3.9. Photoluminescence (PL) spectrum of ZnONp
The PL spectra are useful to disclose the efficiency of charge car-
rier trapping, immigration, and transfer and to understand the fate
Fig. 4. SEM image of ZnO nanoparticle.
0.3
0.2
0.1
0.0
3
3
2
2
1
1
500
000
500
000
500
000
5
00
0
3
00
400
500
300
400
Wavelength (nm)
λ, nm
Fig. 5. UV–Vis spectrum (left) and steady-state fluorescence spectrum (right) of ZnO nanoparticle in water at room temperature.

3
40
D. Dey et al. / Inorganica Chimica Acta 421 (2014) 335–341
of electron hole pairs in semiconductor particles since PL emission
results from the recombination of free carriers [30]. The PL spectra
of ZnO photocatalyst was examined using 269 nm excitation wave-
length and are depicted in Fig. 5. ZnONp showed a strong emission
peak 367 nm which can be attributed to the defect-related emis-
sions and is in accordance with the reported values for ZnO, which
possesses an exciton-related violet emission [31].
these photogenerated carriers recombine in the bulk of the nano-
particle, while the rest migrate to the surface of the catalyst, where
the holes act as powerful oxidants and electrons as powerful reluc-
tant. The high oxidative potential of the hole causes the direct oxi-
dation of the dye to reactive intermediates [32]. In the other hand
hydroxyl radicals are formed by the decomposition of water or by
ꢁ
the reaction of hole with OH , behave as reactive intermediate for
degradation. The hydroxyl radical is an extremely non-selective
oxidant which leads to partial or complete mineralization of the
dye.
4
. Photocatalytic activity of ZnONp under visible light
Methylene blue is a heterocyclic aromatic chemical compound
with the molecular formula C16H N SCl. The spectrum of the MB
18 3
! eꢁ þ h ; ðiiÞO
þ
þ eꢁ ! O ;
ꢀꢁ
ðiÞ ZnO þ h
c
2
2
solution (Fig. 6) in aqueous medium shows characteristics peaks
at 246, 292, 600 and 664 nm (Scheme 2).
ðiiiÞ hþ þ H
O ! OH þ H ; ðivÞh þ OHꢁ ! OH ;
ꢀ
þ
þ
ꢀ
2
The decrease in absorbance of the aqueous solution at 664 nm
with visible light illumination in the ZnONP suspension is due to
the breakdown of the chromophore. The photodegradation of
methylene blue was employed to evaluate the photocatalytic activ-
ities of the ZnO nanoparticles. The photocatalytic properties of
ZnONP are known to depend on several factors like size, morphol-
ogy, surface area and electronic state. The self degradation of MB in
visible light was negligible in the absence of photocatalyst. It is
proved that when the particle sizes of nanostructures increase
beyond the optimum size, the photocatalytic activity decreases
due to decrease in surface area. The ZnONP synthesized by pyro-
lytic method took 60 min, for maximum MB degradation.
ꢀ
ꢀ
ðvÞ Dye þ OH ! RCOO ! CO
2
þ H O þ Inorganic ions
2
Alternatively, the electron in the conduction band can be picked up
by the adsorbed dye molecules, leading to the formation of the dye
radical anion which is subsequently degraded.
In such photocatalytic process, the separation and recombina-
tion of photogenerated charge carriers are competitive pathways
and photocatalytic activity is effective when recombination
between them is prevented.
4.2. Effect of pH
The effect of pH on the photocatalytic activity was studied by
varying the pH of the dye solution. It can be seen from Fig. 7 that
pH has a significant effect on the photodegradation rate. From
the figure it is seen that the rate of degradation is directly
proportional to solution pH. At normal condition ZnONp degraded
4
.1. Mechanistic pathway for photocatalytic degradation by
nanoparticle
The photocatalytic degradation of a dye in the presence of metal
7
8% MB dye at pH ꢀ 7.2 under visible light. When we increased the
oxide nanoparticle is initiated by photoexcitation of the nanoparti-
cle which leads to the formation of electron–hole pair [32]. Part of
pH up to 12 for MB in aqueous solution, the% degradation increases
up to 99%, but with decrease in pH of the MB, degradation percent-
age decreased in a greater extent (ꢀ60%). It can be rationalised
with the fact that there exists an electrostatic interaction between
the catalyst surface and the dye molecules which consequently
enhances or inhibits the photodegradation rate [33]. The effect of
pH value on photocatalysis is generally due to surface charge of
the catalyst and the charge on dye molecules [34]. The change in
pH shifts the redox potentials of the valence and conduction bands,
which might affect the interfacial charge transfer [35]. The surface
of ZnO is positively charged at low pH whereas with rise in pH the
surface becomes negatively charged. As MB is a cationic dye, high
pH favours the adsorption of dye molecule on the catalyst surface
Fig. 6. Adsorption changes of MB aqueous solution at room temperature in the
presence of ZnO nanoparticles obtained under sunlight irradiation.
N
H C
3
CH3
+
N
S
N
Cl-
CH3
CH3
Fig. 7. Change of % degradation of MB in aqueous solution at different pH in the
Scheme 2. Structure of methylene blue dye.
presence of ZnO nanoparticles under visible light irradiation at room temperature.

D. Dey et al. / Inorganica Chimica Acta 421 (2014) 335–341
341
which results in high degradation efficiency. The degradation rate
was found to be enhanced in alkaline conditions and maximum
activity was obtained at pH 12. At pH 12, 99% dye degradation is
observed for ZnONP (Fig. 7).
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(
Herein we report the synthesis and single crystal isolation of a
new acetato- and phenoxo-bridged trinuclear zinc(II) complex of
the type [Zn -O CCH ], where H L is (NO)-donor compart-
(
L
3 2
(l
2
3
)
2
2
[
mental Schiff base ligand. Both ligand and zinc complex exhibits
strong fluorescence property in MeOH–DMSO solvent mixture
and fluorescence intensity enhances for trinuclear zinc(II)–Schiff
base complex compared to ligand. In the presence of visible light,
ZnONP can efficiently catalyze the decolorization and degradation
of MB in the aqueous suspension. Photodegradation of MB is accel-
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prepared ZnONP has been found to be quite stable under visible
light illumination at a pH of approximately 7.2, and there is some
activation of the surfaces of ZnO particles by this light.
(
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