Porous ZnO/Co3O4 heteronanostructures derived from nano coordination polymers for enhanced gas sorption and visible light photocatalytic applications

Article abstract of DOI:10.1039/c5ra07799k

Porous ZnO/Co₃O₄ heteronanostructures are successfully fabricated by a one-step solid state conversion of a novel [(Zn)_x-(Co)_7-x(BDC)(DHS)]·nH₂O (x = 1, 3, 4, 6)] nano coordination polymers (NCPs). Th

Full text of DOI:10.1039/c5ra07799k

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ARTICLE TYPE
Porous ZnO/Co O Heteronanostructures Derived from Nano
3
4
Coordination Polymers for Enhanced Gas Sorption and Visible Light
Photocatalytic Applications
Md. Rakibuddin and Rajakumar Ananthakrishnan
*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Porous ZnO/Co O4 heteronanostructures are successfully fabricated by a novel oneꢀstep solid state
3
conversion of a novel [(Zn) −(Co) (BDC) (DHS)].nH O (x = 1, 3, 4, 6)] nano coordination polymers
x
7ꢀx
2
(NCPs). The precursor is prepared by oneꢀpot wet chemical approach at room temperature without any
1
1
2
0
5
0
surfactant using dihydroxy salophen ligand and 1,4ꢀbenzenedicarboxylic acid as the linkers in presence of
zinc (II) acetate and cobalt (II) acetate in DMF solvent. Different weight ratios of the Zn/Co are
fabricated inꢀsitu into the heteroꢀstructures during the preparation of the NCPs to tune their
physicochemical properties. The produced ZnO/Co O nanostructure possesses relatively higher surface
3
4
2
area (80ꢀ140 m /g) and having significant N gas sorption capacity in comparison with reported materials.
2
It is suggested that synergistic effect of each component, higher separation of electron and holes, high
surface area and porous structure led to the promising visible light photocatalytic properties. The prepared
mixed metal oxides show significant reusability in photocatalysis more than five times without losing
their properties. Importantly, the mixedꢀmetal NCPs could be applied as an effective route to generate not
only binary oxide, but also ternary or quaternary oxide nanostructures with desired morphology and
enhanced physicochemical properties.
†
Electronic supplementary information (ESI) available: Supporting
Introduction
information is available free of charge via internet at hppt://pubs.rsc.org/.
Now–aꢀdays, the synthesis of heteronanostructures by mixing
oxides of dissimilar materials is a centre of attention to improve
the physicochemical properties of functional materials. The
A coordination polymer (CP) is a common term widely used in
50 chemistry and material science, which deals with metal ions
linked by coordinated ligands into an infinite array. The
formation of these polymers is an automatic selfꢀassemble
1
2
3
3
5
0
5
partial reaction of two oxides creates an interface, which exhibit
some new interesting properties due to proximity and diffusion
12,13
2,3
process.
There are various types of architectures of CPs that
phenomena dominating at the nanoscale range.
Earlier
range from simple oneꢀdimensional chains to large crystalline
metal–organic frameworks (MOFs). Infinite nano coordination
polymers (NCPs) are one type of CPs having dynamic nature and
greater level of structural tailorability. These porous NCP
materials possess some new and interesting size, shapeꢀdependent
physical and chemical properties that are not generally observed
literature reports the synthesis of various heteronanostructures
based on this idea. For example, Fe O /ZnO hollow
5
5
2
3
nanostructures have been synthesized by Lou et al. to enhance
4
photocatalytic degradation of organic pollutants. In addition,
Fe O /CdS, Mn O /ZnO, ZnO/SnO, ZnO/CuO, and ZnO/ZnS
3
4
2
3
nanostructures have also been fabricated for different attractive
14
5
–7
60 in MOF. For this reason, NCPs has been extensively used in
many fields, including catalysis, separation, gas storage, drug
properties. Among various metal oxides, nꢀtype ZnO has been
8
recognized as versatile material for various application, while pꢀ
15,16
+
1
delivery and energy storage etc.
Recently, thermal
type Co O is well known for high catalytic activity and Li
3
4
9,10
decomposition of the NCPs has become a growing field for
preparation of metal oxides with improved properties for various
storage capacity. It is expected the synergistic combination of
these two materials will show the way to enhance
17,18
65
applications.
Nanomaterials with desired morphologies can be
physicochemical properties. Previously, ZnO/Co O4 heteroꢀ
3
achieved by choosing appropriate NCP precursors with special
morphologies and under suitable experimental conditions.
Calcinations of these materials produce a large amount of gas due
to the decomposition of the organic ligands, and a porous
structure is formed, which retains the shape of the precursor.
Binary, ternary or quaternary transition metal oxides are
traditionally synthesized in laboratory using highꢀtemperature
4
4
0
5
nanostructures or composites exhibited promising application in
gas sensing and catalytic behavior.
8
,11
Department of Chemistry, Environmental Materials
Chemistry Laboratory, Indian Institute of Technology,
& Analytical
19ꢀ21
70
Kharagpur 721302, India. E-mail: raja.iitchem@yahoo.com;
Fax: +91 3222-282252; Tel: +91 3222-282322
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reactions that often require several days of heating. This problem
can be overcome by using NCP as a precursor. In recent decades,
a number of single metal oxides have been synthesized using
NCP as the precursors. However, there are very few reports on
Preparation of porous hexagonal lump shaped Zn-Co NCPs
and corresponding ZnO-Co O heteronanostructures
3
4
1,22
5
0
5
0
5
0
5
the synthesis of mixed metal oxides from the NCP precursor.
Hence, synthesis of ZnOꢀCo O heteroꢀnanostructures through
3
4
60
65
70
this facile thermal treatment is demandable.
In the present work, porous ZnOꢀCo O heteronanostructures are
3
4
synthesized by in situ addition of different molar ratios of Zn/Co
into a novel mixedꢀmetal NCP as shown in scheme 1. In first
step, [(Zn) −(Co) (BDC) (DHS)].nH O (x = 1, 3, 4, 6)] NCPs
1
1
2
2
3
3
x
7ꢀx
2
or preciously ZnꢀCo NCP are obtained by oneꢀpot wet chemical
approach at room temperature mixing 2,5ꢀdihydroxy salophen
(
2,5ꢀDHS) ligand with Zn(OAc) . 2 H O, Co(OAc) .4 H O and
2 2 2 2
1,4ꢀbenzene dicarboxylic acid (1,4ꢀH BDC) in DMF solvent.
2
Then, the asꢀprepared mixed metal NCPs are calcined in air to
obtain ZnOꢀCo O nanostructures. The NCP has been chosen as a
3
4
precursor because the facile solidꢀstate diffusion and interfacial
contact are improved during calcinations introducing the
formation of uniform heterojunction at the nanoscale. Especially,
C and N can easily be oxidized into gases and release during
calcinations producing a porous nanostructure. Besides, this
process is easy for work up, very fast, reaction occurs at a mild
temperature and environmentally friendly. More attractively,
surface area, gas sorption and photocatalytic properties can easily
be tailored by changing metal ions and their stoichiometries. It is
important to mention that this method is totally distinct from the
other common strategies such as vaporꢀphase, solutionꢀbased and
photochemical coating method used for synthesis of ZnO/Co O
Scheme 1. Schematic representation of the formation of the hexagonal
shaped heteronanostructures from the mixedꢀmetal NCP precursors.
7
8
8
9
9
5
0
5
0
5
The asꢀprepared 2,5ꢀDHS isomer (23.8 mg, 0.007 mmol) was
dissolved in 1 mL of DMF at room temperature. Another
solution of Zn(OAc) . 2H O and Co(OAc) .4H O dissolving in a
2
2
2
2
minimum amount of DMF, was added successively to the above
prepared solution. Finally, 3 mL DMF solution of H BDC (60
2
mg, 0.035 mmol) was added to the ultimate prepared solution.
Within seconds, hexagonal lumps NCPs began to form. Products
were centrifuged and washed with acetonitrile two times. The
mixedꢀmetal NCPs were then dried at open air. The proportions
3
4
8
ꢀ11
heteronanostructure.
Although ZnO/Co O nanostructures
3 4
between Zn(OAc) and Co(OAc)2 were varied 6:1, 4:3, 3:4 and
2
have been widely applied, there is no report for the visible light
photocatalytic activity of the material. Interestingly, it is noted
that the asꢀprepared ZnO/Co O heterostructures in our method
1:6. For 6:1 ratio the concentration of Zn(OAc) and Co(OAc)
2
2
used were 0.042 mmol (92 mg) and 0.007 mmol (13.9 mg ),
respectively keeping the total salt concentration 0.049 mmol, and
the same was calculated for other ratios also. The NCP formed
3
4
exhibited high surface area, good gas sorption capacity and high
photocatalytic activity under visible light.
from Zn(OAc) and Co(OAc)2 was named as ZnꢀCo NCP (yield
2
~50%ꢀ70%). The ZnꢀCo NCPs formed from different ratios is
named like ZnꢀCo NCP 61, ZnꢀCo NCP 43, ZnꢀCo NCP 34 and
ZnꢀCo NCP 16 for the Zn/Co ratio 6:1, 4:3, 3:4 and 1:6,
respectively.
The asꢀprepared hexagonal lump mixedꢀmetal NCPs were then
placed in a conventional furnace and heated at 500 °C for 90 min.
Experimental
Reagents and materials
The 2,5ꢀdihydroxybenzaldehyde and Zinc acetate dihydrate were
obtained from SigmaꢀAldrich and SISCO Research Laboratories
Pvt. Ltd. (India), respectively and used as received. Cobalt
acetate tetra hydrate was obtained from Loba Chemie Pvt. Ltd
40
The hexagonal shaped ZnOꢀCo O4 heteronanostructures were
3
generated, and cooled to room temperature. In every case, the
loss of weight was ~ 51ꢀ57%. The heteronanostructure obtained
from corresponding ZnꢀCo NCP 61, ZnꢀCo NCP 43, ZnꢀCo NCP
(India). All other chemicals (solvents) used in this investigation
were analytical grade (99.9%). All the solutions were prepared in
Distilled/ Millipore water (MilliꢀQ system).
1
00 34 and ZnꢀCo NCP 16 was named as ZnOꢀCo O 61, ZnOꢀCo O
3 4 3 4
45
Preparation of 2,5- dihydroxylated salophen ligands (DHS)
43, ZnOꢀCo O 34 and ZnOꢀCo O 16, respectively.
3 4 3 4
The 2,5ꢀDHS isomer were synthesized according to the literature
Photocatalytic Experiments
2
2b
procedure
with
slight
modifications.
2,5ꢀdihydroxy
To measure the photocatalytic activities of the asꢀsynthesized
ZnOꢀCo O heteronanostructures, 15 mL of reaction mixture in
benzaldehyde (0.276 g, 2 mmol) was dissolve in ethanol (20 mL),
and was added dropꢀwise to 20 mL ethanol solution of oꢀ
phenylenediamine (0.108 g, 1 mmol) and was refluxed at 60 ⁰C
for 30 min. The red colour solution was then evaporated to get
dry mass. The mass was dissolved with a small amount of
acetone, and then petroleum ether was added to get the product.
After filtration of the product, it was dried under open air (yieldꢀ
0.324 g, 93%).
3
4
−5
1
05 an aqueous medium containing methylene blue (MB) (1.2×10
50
M
) and catalytic amount (0.2 g/L) of the samples were taken in a
Pyrex glass vessel, and kept in the dark under stirring for 30 min,
to maintain adsorptionꢀdesorption equilibrium between the dye
and the catalyst. After that, the mixture was irradiated under
10 visible light (300 W halogen lamp that contained the aqueous
1
55
NaNO solution (10% w/w) as a light filter through which only
2
2
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visible light, λ≥420 nm could pass). After a certain regular
interval of irradiation time, 2 mL of an aliquot was taken from the
reaction mixture, and 2 mL of deionized water was added to the
Firstly, the formation of coordination polymers was verified by
infrared (IR) and H NMR spectroscopic techniques (Supporting
Information, Fig.S1). As shown in Figure 1a, the nature of the IR
1
aliquot each time to make the centrifuge easy. The transparent 60 spectra of all the NCPs is nearly identical. The coordination of
2
+
+2
5
solution thus obtained after centrifuge was preserved in the dark
for spectral analysis.
the carboxylate groups of 1,4ꢀH BDC to Zn /Co ions was
2
confirmed by the infrared spectra of the NCPs. The CO stretching
frequency shifted to 1575ꢀ1579 cm from 1686 cm for the unꢀ
coordinated ligand H BDC after the formation of coordination
ꢀ
1
ꢀ1
Instrumentation
2
23
Fourier Transform InfraRed spectra (FTꢀIR) were measured with 65 polymers. In all the NCPs, a strong and broad peak at around
ꢀ
1
a PerkinꢀElmer FTꢀIR spectrophotometer RXI. Powdered Xꢀray
Diffraction was performed by Bruker APEXꢀ2 diffractometer.
Transmission Electron Microscopy (TEM) and High Resolution
TEM (HRTEM) were carried out with JEOL JEM2010 electron
3384 cm is observed (Supporting Information, Fig. S2) typical
for H O molecules with hydrogen bonds indicating that H O
1
1
2
2
0
5
0
5
2
2
+
2
+2
molecules participate in the coordination of Zn /Co of the NCP
ꢀ
1
structures. The peak of higher intensities at 1371ꢀ1385 cm in all
microscope operating at 200 kV. Energy Dispersive Xꢀray (EDX) 70 the ZnꢀCo NCPs is due to carboxylate stretching frequencies of
2ꢀ
measurement was performed with FEI TECNAIꢀG2ꢀ20SꢀTWIN
(USA). Field Emission Scanning Electron Microscopic (FESEM)
image and Elemental Mapping were obtained from a Supra 40,
Carl Zeiss instrument. Thermogravimetric Analysis (TGA) was
the ligand, BDC . The aromatic ꢀCꢀH in plane and out of plane
bending vibration in all the NCPs are appeared between 950ꢀ1248
cm and 670ꢀ885 cm , respectively. Interestingly, all these
strong and weak peaks are vanished after calcinations of the
ꢀ
1
ꢀ1
performed on PerkinꢀElmer instrument, Pyris Diamond TG/DTA 75 NCPs at 500 ⁰C for 90 min suggesting the formation of metal
with Al O crucible. BETꢀN2 sorption isotherms (77K) were
oxide heteronanostructures (Figure 1b). A weak band at 1630
2
3
ꢀ
1
performed by using Quantachrome Autosorbꢀ1. The Zn/Co
weight percentage in the NCPs was determined by inductively
coupled plasma atomic mass spectroscopy (ICPꢀMS) with a
ThermoFisher Scientific ICAPꢀQ instrument. Prior to analysis,
the samples were digested in hot concentrated HNO and H O
cm is observed in the ZnOꢀCo O samples, which is due to O–H
3 4
bending vibration of adsorbed water molecules on the surface of
the heteronanostructures.
8
0
3
2
2
mixture. The Xꢀray Photoelectron Spectroscopy was carried out
by Specs (German). UVꢀVisible and UVꢀVisible Diffuse
Reflectance Spectra (DRS) was recorded using UVꢀ 1601,
Shimadzu and CARY 5000 UVꢀVisꢀNIR spectrophotometer,
respectively.
85
3
0
Results and Discussion
Characterization of the Zn-Co NCP derived ZnO-Co O4
3
heteronanostructure
90
ZnꢀCo NCPs were synthesized at room temperature by mixing
the corresponding salophen ligand (2,5ꢀDHS) in DMF solution.
When the solution of Zn(OAc) , 2H O in DMF was added to the
previously prepared ligand solution, Zn is getting coordinated to
the salen pocket to form a light red coloured Znꢀmetalated salen
3
4
4
5
5
5
0
5
0
5
2
2
2
+
95
complex in the reaction.
Another DMF solution of
Co(OAc) .4H O when added to the above solution, one
2
2
+2
2+
equivalent of Co replaces the Zn from the salen pocket, and
another Co occupies the carboxylate node position after the
addition of H BDC to the final solution. This process also has 100
+2
1
4
2
been supported by immediate color change from red to black in
the final solution, where hexagonal lump NCPs began to form
within seconds in every case. Interestingly, instead of M(OAc)2
II
when MCl , M(NO3)2 and M SO were used as metal salts,
2
4
formation of the NCPs did not occur. And when 4ꢀamino benzoic 105
acid was used as the linker instead of 1,4ꢀH BDC, NCP
2
formation also did not occur. The above results prove the fact that
the formation of the NCP is highly salt and ligand dependent. The
ZnꢀCo NCPs were found to be stable in several solvents such as
DMF, DMSO, methanol, and nonꢀpolar solvents. However, we 110
have tried to get the crystal of the synthesized polymers
dissolving in a suitable solvent by several techniques such as,
solvent evaporation, solvent diffusion, slow cooling of the
solution, but unfortunately no crystals are obtained.
Figure1. FTꢀIR spectra of the a) ZnꢀCo NCPs and b) ZnOꢀCo O heteroꢀ
3
4
nanostructures after calcinations of the ZnꢀCo NCPs at 500 ºC.
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ꢀ
1
As shown in Figure 1b, the two sharp peaks at 666 cm and 575
ꢀ
1
ꢀ1
ꢀ1
cm in the ZnOꢀCo O 16 are shifted from 678 cm and 602 cm
3
4
+2
+3
of the ZnOꢀCo O 61 are assigned to Co ‒O and Co ‒O
3
4
2
4
vibration of spinal cobalt oxide, respectively. This type of shift
indicates an interaction between the two different proportions of
one metal oxide with another metal oxide. In addition, the peak at
5
0
5
0
ꢀ
1
4
91 cm of the ZnOꢀCo O is due to ZnꢀO vibration, and the
3 4
intensity of the peak is slowly decreasing with an increase of
Co O content in the sample.
3
4
1
1
2
The powder Xꢀray diffraction (PXRD) of the mixedꢀmetal NCPs
reveals sharp and intense peak; and hence, indicates crystalline
nature of all the NCPs (Figure 2). Though all the NCPs contain
same molecular building blocks and same elemental composition,
they all are not isoꢀstructural. The PXRD pattern of the NCPs
indicates the ZnꢀCo NCP 34 possesses some different structure
than the others. It is noticed that the structure of the NCPs are
not changed for all cases by changing the metal ratios, but it is
changed at some particular ratio which varied from metal to
metal. However, more research is needed to explain the exact
reason for this type of changing at that particular ratio of metals
as the exact structure of the NCPs is not known at this moment.
3 4
Figure 3. Powder Xꢀray diffraction pattern of the ZnOꢀCo O heteroꢀ
nanostructures synthesized from the ZnꢀCo NCPs after calcinations at 500
ºC for 90 min.
60
No peaks of impurities are observed in the XRD pattern, which
indicates the formation of singleꢀphase ZnOꢀCo O
3
4
heteronanostructures in all cases.
65
25
30
35
7
0
75
(degree)
Figure 2. Powder Xꢀray diffraction pattern of the asꢀsynthesized ZnꢀCo
NCPs: a) ZnꢀCo NCP 61, b) ZnꢀCo NCP 43, c) ZnꢀCo NCP 34, d) ZnꢀCo
NCP 16
80
40
45
50
55
Interestingly, an enormous change in the PXRD pattern is
observed after heating the NCPs at 500 ⁰C for 90 min. All the
peaks of the NCPs are disappeared and some new sharp and
intense diffraction peaks appeared due to the solid state
transformation of the NCPs to metal oxide heteronanostructures.
The appearance of new peak in the heteronanostructures
suggests that it is highly crystalline in nature (Figure 3). In all
heteronanostructures, there are three strong diffraction peaks of
the ZnO, which can be ascribed as the pure hexagonal wurtzite
phase of ZnO (JCPDS, 36–1451). In Figure 3, the dotted state
line shows that the peak intensity of ZnO in the ZnOꢀCo O is
8
5
Figure 4. TEM image of the synthesized ZnꢀCo NCP and ZnOꢀCo
heteronanostructures; a) ZnꢀCo NCP 61, b) ZnOꢀCo 61,c) ZnOꢀCo
3 and d) ZnOꢀCo
heteronanostructures).
3
O
4
3
O
4
3
O
4
4
3 4
O 16 (inset corresponding SAED pattern of the
90
In order to take insight into the morphology and size of the ZnꢀCo
NCPs and the asꢀsynthesized ZnOꢀCo O heteroꢀnanostructures,
3
4
Transmission Electron Microscopy (TEM) is studied. As shown
in Figure 4a, the TEM image of the ZnꢀCo NCPs reveals the
formation of uniform porous particles with a hexagonal lump
shape. The TEM image also shows that the average size of the
ZnꢀCo NCP is about 70ꢀ100 nm upon changing the Zn/Co ratios
3
4
decreased, whereas the peak intensity of the Co O is increased
3
4
gradually by changing their concentration. The peak of the Co O4
3
95
has been indexed by an asterisk mark in the figure (Fig. 3). The
measured peaks at 2θ = 31.13, 36.76, 38.5, 44.8, 55.5, 59.2, 65.26
and 77.4 correspond to (111), (220), (311), (222), (400), (422),
(Supporting Information, Fig.S3). However, after the calcinations
of the grown ZnꢀCo NCPs in a conventional furnace, the shape of
the produced nanostructures is found to be hexagonal, but the size
(
511), (440) and (533) planes of cubic phase of Co O crystals,
3 4
2
5
respectively (JCPDS, 42ꢀ1467).
4
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is reduced. As shown in Figure 4b, the TEM images clearly show 45 ZnOꢀCo O4 clearly exhibits the increase or decrease of peak
3
the presence of both individual ZnO and Co O in the ZnOꢀCo O
4
intensity of the metals (Zn or Co) with changing the proportion of
the corresponding salts at the time of preparation of NCPs.
Hence, all of these observations suggest successful formation of
heteronanostructures from the NCPs while maintaining proper
3
4
3
heteronanostructures having two different sizes. The TEM images
reveal uniform distribution of the particles in the
heteronanostructures, where the larger hexagonal shaped particles
5
0
5
0
5
0
are of the ZnOs and the smaller hexagonal particles correspond to 50 weight ratios.
Co O . The average size of the ZnOs in the ZnOꢀCo O is found
3
4
3
4
to be 60 nm, whereas the sizes of the Co O particles are found to
3
4
be ~30 nm, which is also supported by the FESEM images
(Supporting Information, Fig.S4). Interestingly, the size of the as
prepared ZnOꢀCo O nanostructures is found to be less than the
1
1
2
2
3
55
3
4
corresponding ZnꢀCo NCPs, which is due to ~50ꢀ60% removal of
the organic moiety of the NCPs, which is further confirmed by
the TGA curve.
When the weight ratio of metal salts (Zn/Co) is varied in the
preparation of the NCPs and then the heteronanostructures, there
is no significant change in shape of the particles. However, it is
noted that the size of the as synthesized nanomaterials is
depended on the ligand as well as the nature of metal salts and
their proportions used for the preparation of the NCPs.
60
65
The ZnꢀCo NCPs also have been characterized by Energy
Dispersive Xꢀray (EDX) (Supporting Information, Fig.S5). Also
the metal ion ratios are verified using Inductive Coupled Plasma
Atomic Mass Spectroscopy (ICPꢀMS) (Fig. 5). The EDX shows
the presence of elements C, N, O, Zn and Co in all the NCP
structures. The Zn/Co wt.% measured by ICPꢀMS analysis shows
the different loading of Zn and Co from 7.9% to 53.2% on the
NCP, depending on the different ratios. These values are well
agreed with the loading of metal wt.% measured by EDX analysis
and experimentally calculated value.
Figure 6 a) Low magnification HRTEM image of the synthesized ZnOꢀ
16 heteronanostructures, b) High magnification HRTEM image of
the ZnOꢀCo 16 showing lattice fringes; c) FESEM images of the ZnOꢀ
Co 16 and corresponding Elemental Mapping of the Zn, Co, O in the
3 4
70 Co O
3
O
4
3
O
4
heteronanostructures (dꢀf).
Figure 6a shows the low magnification High Resolution
75 Transmission Electron Microscopic (HRTEM) image of the
prepared heteronanostructures. Figure 6b is a highꢀresolution
image from the red circle containing both larger and smaller
particles in Figure 6a, which reveals the heterojunction of the
crystalline Co O and ZnO crystal lattices in the ZnOꢀCo O 16.
3
4
3
4
8
8
9
9
0
5
0
5
The HRTEM images clearly exhibits lattice fringes of the
prepared materials suggesting highly crystalline nature of the
samples, which is also confirmed by the corresponding SAED
pattern of the materials in Figure 4. As shown in Figure 6b, the
interꢀplanar distance of 0.286 nm and 0.31 nm is corresponded to
the (220) plane of the cubic Co O and (002) plane of the
3
4
26
hexagonal ZnO. Moreover, a distinguished interface and
continuity of lattice fringes between the Co O and ZnO, can be
3
4
observed in Figure 6b, suggesting that the pꢀn heterojunction is
formed between the Co O and ZnO in the heteronanostructures.
3
4
The Figure 6d also shows the elemental area mappings of Zn, O
Figure. 5 Metal loading (wt %) in different samples of NCP determined
by ICPꢀMS analysis.
and Co in the selected area of the ZnOꢀCo
heteronanostructures of Figure 6c.
3
O
4
61
The elemental mapping also reveals that the signals are very
sensitive to the different proportion of Zn/Co, and clearly show
the change (higher or lower) in distribution of the element
3
5
The DRS spectra of the NCPs show broad visible light
absorption, and hence the color of the NCPs appears to be black
from the two initial colorless precursors (Supporting Information,
Fig.S6). After the calcinations of the ZnꢀCo NCPs at 500 ⁰C, the
EDX shows only the presence of only Zn, O and Co of the ZnOꢀ
Co O (Supporting Information, Fig.S7). No peaks of C and N
(Zn/Co) in the sample (Supporting Information, Fig.S8). Hence,
the elemental area mapping of the materials indicates successful
incorporation of the elements into the heteronanostructures.
4
0
3
4
are observed in EDX analysis indicating successful removal of
the organic template. However, the carbon and copper atom
observed in the EDX analysis are due to carbon coated Cu grid
used during TEM sample preparation. The EDX analysis of the
1
00 Xꢀray Photoelectron Spectroscopy (XPS) is a powerful technique
to investigate the chemical composition, surface property and the
interaction of the nanomaterials.
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the interface between Co O and ZnO in the heterostructures,
3
4
which is further confirmed by the HRTEM results. Hence, there
might exists a strong interaction between the pꢀtype Co O and nꢀ
type ZnO nanoparticles, which enhances electron transfer in the
composites indicating the formation of pꢀn heterojunctions like
structure.
The ability of light absorption is important for the effective
excitation and separation of photo induced electron and holes
carriers, which are useful factors for the photocatalytic activity of
a material. The UVꢀVis diffuse reflectance spectra (DRS) of the
6
6
7
7
0
5
0
5
3
4
5
10
ZnOꢀCo O4 heteronanostructures displayed optical absorption
3
capabilities nearly in the entire visible region from 400 to 800 nm
(Supporting Information, Fig.S9). The broadꢀspectrum response
indicates that it can be used as good visible lightꢀresponsive
photocatalysts. The absorbance intensity in the range of 400 nm
15
+2
to 800 nm increases gradually with an increase of Co salt
+2
concentration with respect to the Zn during the preparation of
the ZnꢀCo NCPs. The absorption spectra of ZnO appeared at 365
nm, and the two distinct peaks at 250 to 350 nm and 400 to700
nm wavelength ranges in the ZnOꢀCo O can be assigned to the
3 4
Figure 7. XPS full survey of the synthesized ZnOꢀCo O
3
4
20
heteronanostructures, b) High resolution XPS measurement of the Co 2p,
c) Zn 2p and d) O 1s of the heterostructures.
2−
2+
2−
3+
O
→ Co
and O
→ Co
charge transfer process,
respectively. The peaks at 609 and 660 nm of the ZnOꢀCo O
33
3
4
Figure 7 shows the full scan and the high resolution XPS spectra 80 became red shifted to 672 and 740 nm with the higher Co ratio,
3
4
of the ZnOꢀCo O heteroꢀnanostructures. As observed in Figure
which are the characteristic peak of Co
3
O
4
.
As shown in the
3
4
+2
+3
7
a, the XPS survey indicates the presence of Zn, Co and O in the
Figure, the two absorption edges (Co and Co ) of the ZnOꢀ
Co shifted from 516 and 742 nm to 552 nm and 760 nm with
25
30
35
40
45
50
55
ZnOꢀCo O samples, and no peaks of other elements are
O
3 4
3
4
observed, indicating high purity of the synthesized
nanostructures, which are also well in agreement with the above
XRD and EDX results. The presence of C element could be
attributed to the adventitious carbonꢀbased contaminant, and the
binding energy at 284.6 eV of the C 1s (not labelled in the figure)
is used as the reference for calibration. Subsequently, the Zn 2p
highꢀresolution XPS spectrum of the nanostructures is also
analyzed (Figure 7c). For ZnOꢀCo O , the Zn 2p and Zn 2p1/2
the change of proportion of Zn/Co.
8
5
Thermal stabilities of the precursor Zn-Co NCPs and
synthesized ZnO-Co O heteronanostructures
3
4
The thermal stabilities of the ZnꢀCo NCPs and corresponding
derived heterostructures were performed by thermogravimetric
analysis (TGA) under N atmosphere at the heating rate of 10 ⁰C
2
ꢀ
1
90
min . As shown in Figure 8, the ZnꢀCo NCPs almost maintained
the same thermal stability around 500ꢀ513 ⁰C, and is not changed
so much upon changing the ratios of the Zn/Co salts. However,
different weight loss is observed for ZnꢀCo NCPs, which varied
from 50% to 64%.
3
4
3/2
peak appeared at 1020.6 eV and 1043.6 eV, respectively. The
observed spinꢀorbit splitting of Zn 2p (between Zn 2p3/2 and Zn
2
+
2
p1/2) is about 23 eV, indicating a normal form of Zn in the
27
samples. In comparison with pure ZnO (Zn 2p3/2 at 1020 eV and
Zn 2p1/2 at 1042.3 eV), the Zn 2p peaks of both materials shift to
higher energy confirming the interaction and electron transfer
95
2
8
between ZnO and Co O .
3
4
As shown in Figure 6b, the high resolution XPS fitting of Co
p of the ZnOꢀCo O nanostructures shows prominent peaks
2
3
4
3
+
around at 779.4 eV and 794.5 eV are attributed to Co 2p3/2 and
1
1
1
00
05
10
3
+
Co 2p1/2 configuration of the Co O , respectively and the fitting
3
4
2
+
peaks at 780.9, 789.2 and 796.2 eV are due to Co peaks of the
2
9
Co O . The energy difference between the peak of Co 2p3/2 and
3
4
the peak of Co 2p1 is 15.1 eV, which demonstrates the presence
/2
30
of a Co O phase. The high resolution XPS spectra of O 1s
3
4
consist of two peaks ( Figure 7 d). A peak at around 529.0 eV is
due to oxygen in the Co O crystal lattice, which is a
3
4
characteristic value of cobalt oxide networks, while another peak
at around 530.8 eV is due to surface hydroxyl of the obtained
31
nanostructure. The Co 2p peaks in the ZnOꢀCo O is shifted
3
4
comparatively to lower binding energy than pure Co O (781.2
3
4
3
+
32
eV for Co 2p3/2). Thus, shifting of the Zn 2p peaks toward the
higher binding energy, and the Co 2p peaks to the lower binding
energy suggests that there might be CoꢀOꢀZn bonds present at
Figure 8. Thermogravimetric Analysis (TGA) curve of the synthesized
ZnꢀCo NCPs and ZnOꢀCo
nanostructures.
3
O
4
61 and ZnOꢀCo
3
O
4
16 heteroꢀ
6
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All the ZnꢀCo NCPs shows its first weight loss around at 100ꢀ150
⁰
C due to removal of water molecules, and the second weight loss
60
65
70
75
around at 300ꢀ350 ⁰C due to removal of H BDC unit from the
NCPs and finally, the third weight loss around at 500 ⁰C due to
2
3
5
5
0
5
removal of salophen framework from the NCPs. After that, no
weight loss is observed indicating successful transformation of
the ZnꢀCo NCPs to ZnOꢀCo O heteronanostructures at that
temperature. From the different thermal stabilities and percentage
weight loss of the NCP materials, it is demonstrated that the
thermal property of NCP is highly dependent on its salt and its
proportions, and this is also due to different porosities of the
NCPs molecule. Similarly, to observe the thermal stabilities of
the synthesized heterostructures, TGA analyses of the samples
are carried out under N atmosphere at the same heating rate like
3
4
1
3
6
2
1
the NCPs. The prepared ZnOꢀCo O4 exhibits no significant
weight loss (~ 0.5%) up to 600 ⁰C indicating high thermal
stability of the heterostructures.
3
Gas sorption property of the as synthesized Zn-Co NCPs and
ZnO-Co O heteronanostructures
3
4
2
2
3
3
4
4
5
5
0
5
0
5
0
5
0
5
The pore volume, surface area and N gas uptake capacity of the
2
ZnꢀCo NCP materials and corresponding derived ZnOꢀCo O4
3
8
8
9
9
0
5
0
5
nanostructures are obtained by Brunauer–Emmett–Teller (BET)
measurement. As shown in Figure 9, the BET isotherms of the
prepared NCPs and heterostructures follow type III behaviour,
which is a weak van der Waals force of attraction between the
adsorbate (N gas) and adsorbents (NCPs/heteronanostructures).
2
Thus, the isotherms are reversible in nature as soon as the
pressure is released. All the curves do not have any flattish
portions, and thus explain the multilayer sorption property of the
37
materials. It is observed that the prepared ZnꢀCo NCPs exhibits
2
high surface area following the order ZnꢀCo NCP 61 (225.3 m /g)
2
2
>
ZnꢀCo NCP 43 (202.4 m /g) > ZnꢀCo NCP 16 (189.3 m /g)
(Supporting Information, Table S1). However, it is noted that the
ZnꢀCo NCP 16 (having highest cobalt ratio) shows the greater
pore volume (0.272 cc/g) and gas sorption capacity (295 cc/g)
than the other ZnꢀCo NCPs (Figure 9a). All the ZnꢀCo NCPs are
found to be permanently porous material. It is suggested that the
proportions containing higher Zn ratios exhibited higher surface
area; hence a particular ratio of Zn/Co is needed to get the
optimum surface area and pore size of the NCP material. The Znꢀ
Co NCP derived ZnOꢀCo O heteroꢀnanostructures also exhibited
3
4
high surface area, and maintains the previous order as expected
2
2
like ZnOꢀCo O 61 (140.4 m /g) > ZnOꢀCo O 43 (110.8 m /g) >
3
4
3
4
2
100
ZnOꢀCo O 16 (80.5 m /g).
All the prepared heteroꢀ
3
4
nanostructures are also found to be permanently porous, and
show significantly higher N gas sorption capacity.
Figure 9. BETꢀN gas sorption isotherm curve of the synthesized a) Znꢀ
2
2
The ZnOꢀCo O 61 possesses the highest porosity amongst the
Co NCPs, b) NCP derived ZnOꢀCo
3
O
4
heteronanostructures and c) BETꢀ
61.
3
4
N2 sorption isotherm of the ZnOꢀCo
3
O
4
heteronanostructures having pore volume 0.552 cc/g and N gas
2
sorption capacity (645 cc/g). Nevertheless, this type of higher
porosity of the metal oxide heteronanostructures is not generally
observed. Hence, experimental observations indicate that the
surface area and pore size of ZnꢀCo NCPs and synthesized
heterostructures are highly depended on salt and their
1
05
Photocatalytic activity of the synthesized ZnO-Co O
3
4
heteronanostructures
The photocatalytic activities of the asꢀprepared ZnOꢀCo O
3
4
proportions, and hence, the surface area, pore volume and N gas
heteronanostructures are evaluated by measuring the
2
sorption properties can easily be tailored by simply changing the 110 photodegradation of a model dye Methylene Blue (MB) under
proportions, which will help to choose the best system for desired
properties among the different NCPs as well as NCP derived
heteroꢀnanostructures.
visible light (λ ≥ 420 nm). MB has its characteristic absorbance
maxima at 663 nm, which is used to monitor the photocatalytic
−5
degradation. A 15 mL of reaction mixture of MB (1.2×10
M) in
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an aqueous medium containing catalytic amount (0.2 g/L) of the
particular ZnOꢀCo O heteronanostructures were taken in a glass
3
4
60
vessel, and kept in the dark for 30 min under magnetic stirring
condition for any adsorptionꢀdesorption equilibrium, and after
that it was irradiated under light. The photocatalytic activity is
checked for all the heteronanostructure materials.
5
0
5
0
5
0
5
0
5
0
5
Figure 10a shows UVꢀvisible absorption spectra of the
degradation of MB dye by the different ZnOꢀCo O4 catalyst
3
under visible light and the kinetic plot (C /C vs time) of the
65
t
0
1
1
2
2
3
3
4
4
5
5
degradation under various conditions is shown in Figure 10b. MB
itself does not decompose under the irradiation of visible light.
Similarly, ZnO also does not have any photocatalytic
performance under the visible light as it does not absorb visible
light. It is noted that when the photodegradation of MB dye is
carried out in presence of ZnOꢀCo O 61 catalyst under dark, no
70
3
4
significant photodegradation (3ꢀ4%) of MB is observed. Hence,
this control experiment proves the essentiality of the visible light
in the photocatalytic process. However, in presence of the ZnOꢀ
Co O 61 catalyst under visible light, the degradation of MB is
3
4
reached up to 85% in 5 h. Interestingly, the photocatalytic activity
is significantly decreased to 50% in 5 h when ZnOꢀCo O 16 is
3
4
75
employed as the photocatalyst, whereas the ZnOꢀCo O4 43
3
catalyst exhibit moderate photocatalytic activity (70%)
(
Supporting Information, Fig.S10). The ZnOꢀCo O 34 catalyst
3 4
exhibited poor photocatalytic activity (54%) similarly like ZnOꢀ
Co O 16 However, the huge change in photocatalytic activity
.
3
4
can be attributed by the different factors such as, surface area,
porosities and pꢀn heterojunction of the heteronanostructures. As
the BET surface area measurement shows the catalyst, ZnOꢀ
Co O 61 possessed relatively higher surface area and pore size
80
3
4
than that of the ZnOꢀCo O 43 and ZnOꢀCo O 16; hence, it
3
4
3
4
shows higher activity than the others. This is due to the
adsorption of the dye molecule on the porous catalyst surface, so
when the light is irradiated the adsorbed dye fully degraded. The
Fig ure 10 a) UVꢀVisible absorption spectra of the degradation of MB dye
in presence of ZnOꢀCo
visible light (λ ≥ 420 nm) photocatalytic decomposition of MB in the
presence of different heteroꢀnanostructures: i) ZnOꢀCo 61, ii) ZnOꢀ
Co 43, iii) ZnOꢀCo 16, iv) only MB dye under visible light
irradiation, v) only ZnO under visible light irradiation and vi) ZnOꢀCo
1catalyst under dark.
3 4
O 61 and b) kinetic plots (Ct/C0 vs time) for the
8
9
9
5
0
5
3
O
4
enhanced photocatalytic activity of the ZnOꢀCo O4
3
3
O
4
3 4
O
heteronanostructures for the degradation of MB could be ascribed
by also the formation of pꢀn heterojunctions like structure in the
heteroꢀnanostructures, which enhances the separation of electronꢀ
holes at the interface of the heterojunction of the ZnO and Co O .
3
O
4
6
can be recycled, the reusability of the prepared catalysts i.e.
heteronanostructures, haven been experimented in our study and
reported (Supporting Information, Fig. S13). It is noticed that the
prepared catalyst can be easily recycled several times without any
major loss of photocatalytic activity (2ꢀ3% loss of activity after
four cycles). However, more decrease in photocatalytic activity is
observed after fifth cycles due to photo corrosion of ZnO and
3
4
The formation of the heterojunction is also supported by the
HRTEM (Figure 6b) and XPS studies. All the ZnOꢀCo O4
3
photocatalyst follow pseudoꢀfirst order rate kinetics, and the
ZnOꢀCo O 61 exhibits 2.5 times higher rate constant (k = 5.8 ×
3
4
ꢀ
3
ꢀ1
ꢀ3
1
0 min ) value than the ZnOꢀCo O 16 catalyst (k = 2.2 × 10
min ) (Supporting Informations, Fig.S10c). It is noted that the
photocatalytic activity of the ZnOꢀCo O 61 is superior to some
other ZnOꢀbased nanostructures reported previously.
The effect of dye concentration on the photodegradation is also
3
4
ꢀ
1
Co O nanostructures. Overall, the present photocatalytic system
3
4
3
4
3
8 ꢀ42
is stable enough at room temperature.
Electron-hole scavenger study and probable mechanistic
examined. For this, we have chosen the ZnOꢀCo O 61 catalyst 100 pathways of the reaction
3
4
having highest photocatalytic activity, and the concentration of
ꢀ
5
ꢀ1
ꢀ5
ꢀ1
Inorder to gain more inꢀdepth knowledge about the photocatalytic
mechanism, an electron and hole scavenger study is carried out.
dye is varied from 1.2 x 10 molL to 3.6 x 10 molL . It is
ꢀ
5
observed that with increase in dye concentration from 1.2 x 10
molL to 3.6 x 10 molL the photodegradation is decreased to
0% from 90% for the ZnOꢀCo O 61 catalyst (supporting
˙ꢀ
ꢀ
1
ꢀ5
ꢀ1
The hydroxyl radical (OH˙) and superoxide radical ion (O ) are
2
reactive intermediates in the photocatalysis process, and are
6
3
4
1
05 generated during the reaction of electrons and holes with H O
2
information, Fig.S11). A comparative study also has been carried
out between the ZnꢀCo NCPs and ZnOꢀCo O , which reveals Znꢀ
separately. However, the main reactive species are electrons and
holes. To prove this, scavenging experiments were carried out in
presence of AgNO3 and ammonium oxalate, which are good
3
4
Co NCPs has very low activity in comparison with ZnOꢀCo O4
3
(supporting information, Fig.S12). As the heterogeneous catalyst
43
electron and hole scavengers. Electron and hole scavengers
8
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ꢀ
3
(
10 M) are introduced prior to the addition of the catalyst to the
solution of MB.
5
0
5
0
1
1
2
Figure 12. Schematic diagram showing the energy band structure and
Electron–hole pair separation in the ZnOꢀCo heterojunction.
3
O
4
The ZnOꢀCo O4 heteronanostructure upon irradiation of a
3
55
particular light generates electron–hole pairs on the surface of
Co O and absorbing the necessary energy, electrons from the
3
4
valence band (VB) of Co O excited across the conduction band
3
4
(CB). As the conduction band energy of ZnO is lower than
Figure 11. Bar diagram representation of electronꢀhole scavenger study
Co O , the electrons from the CB of Co O transfer to the CB of
showing the dye removal efficiency of the ZnOꢀCo
3
O
4
61in 3 h under
3
4
3
4
visible light in the presence of ammonium oxalate and silver nitrate.
60 ZnO through the heterojunction (Fig. 12).
The conduction band electrons of ZnO then react with
dissolved oxygen forming a superoxide radical, which again
reacts with water to give a hydroxyl radical. The produced
hydroxyl radical then degrades the organic dye molecule (eqn. 1).
Again, the holes of the VB of MO react with water molecules to
generate hydroxyl radicals, which can further decompose the
organic dye (eqn. 2).
Figure 11 shows a bar diagram to represent the degradation of
MB by the ZnOꢀCo O 61 in the presence of electron scavenger,
3
4
46
2
3
3
4
4
5
5
0
5
0
5
0
AgNO and hole scavenger, ammonium oxalate. The bare ZnOꢀ
3
Co O 61 shows 60% dye removal efficiency in 3 h under visible
3
4
65
+
light. The immobilization of the Ag ions on the catalyst surface
4
4
hinder the adsorption of cationic Methylene Blue as a result
only 36% of the dye is degraded under the same condition by the
ZnOꢀCo O 61. However, in the presence of ammonium oxalate
3
4
Conclusions
the rate of the degradation is increased to 85% at the same
condition for the ZnOꢀCo O 61. This is due to the scavenging of
In summary, porous ZnOꢀCo O heteronanostructures were
3 4
3
4
the holes, the probability of recombination of electron and holes 70 successfully prepared by thermal decomposition of novel [(Zn)
x
diminishes and the rate of reaction increases. Hence, electrons
and holes are the most powerful species that actively take part in
photocatalysis. Successive electron transfer from the conduction
band (CB) of Co O to the CB of ZnO leads to the higher
(Co) 7ꢀx [C60 H N O24].nH O (x = 1, 3, 4, 6)] NCPs precursor
32 2 2
template. The mixedꢀmetal NCPs have been utilized as a tool to
tune the surface area, porosity and size of the
heteronanostructures, which are useful factors for many
3
4
.
45
electronꢀhole separation Based on the results and literature 75 physicochemical properties. The ZnOꢀCo
report, plausible mechanistic pathway of the present
nanostructures exhibited relatively high surface area (140.4
O
4
61 heteroꢀ
3
a
2
photocatalytic system in our experiment is shown as follows:
m /g), pore volume (0.552 cc/g ) and N
gas sorption capacity
2
(
645 cc/g). Moreover, ZnOꢀCo O4 heteronanostructures show
3
high visible light photocatalytic ability, which can be attributed to
the high surface area, greater pore volume and higher separation
of electron and holes in the heterojunction.
8
8
9
0
5
0
The prepared heterogeneous ZnOꢀCo O heteronanostructures
3 4
photocatalyst is stable enough, which would greatly promote their
industrial application in eliminating organic pollutants from
wastewater. We believe that these active porous ZnOꢀCo O
3
4
heteroꢀnanostructures could serve as a potential candidate for
other applications, such as artificial photosynthesis and lithiumꢀ
ion batteries. Hence, it is expected that library of porous heteroꢀ
nanostructures with enhanced properties could be fabricated by
using these mixedꢀmetal NCPs as a precursor. Interestingly, it is
tested that the synthesis of heteronanostructures is not only
limited to binary, but also possible to fabricate ternary and
quaternary systems by the facile inꢀsitu addition of different salts
during the preparation of the NCP precursors.
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2
6 C. W. Na, H. S. Woo, I. D. Kim and J. H Lee, Chem. Commun. 2011,
7, 5148–5150.
27 P. Gao, Z. Liu and D. D. Sun, J. Mater. Chem. A 2013, 1, 14262ꢀ
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28 Q. Zhang, C. Tian, A. Wu, T. Tan, L. Sun, L. Wang and H. Fu, J.
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Acknowledgements
4
Authors are thankful to the CRFꢀIIT Kharagpur for TEM,
HRTEM, FESEM, EDX analysis and DSTꢀFIST for powder Xꢀ
ray (at Department of Chemistry) and XPS (at Department of
Physics). The author, MR, thanks the CSIR for research
fellowship and RA thanks SRIC, IIT Kharagpur for SDGRI
funding (NPA). Also authors are thankful to Prof. K. Biradha,
Department of Chemistry (for BET analysis) and Prof. D.
Upadhyay, Department of Geology and Geophysics (for ICPꢀMS
analysis), IIT Kharagpur.
1
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DOI: 10.1039/C5RA07799K
Graphical Abstract
Porous ZnO/Co O Heteronanostructures Derived from Nano
3
4
Coordination Polymers for Enhanced Gas Sorption and Visible
Light Photocatalytic Applications
Md.Rakibuddin and Rajakumar Ananthakrishnan*
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India.