Enhanced Thermocatalytic Activity of Porous Yellow ZnO Nanoflakes: Defect- and Morphology-Induced Perspectives

Article abstract of DOI:10.1002/asia.201801745

Herein, we report a simple and effective strategy for the synthesis of yellow ZnO (Y-ZnO) nanostructures with abundant oxygen vacancies on a large scale, through the sulfidation of ZnO followed by calcination. The developed strategy allows retention of the overall morphology of Y-ZnO compared with pristine ZnO and the extent of oxygen vacancies can be tuned. The influence of oxygen deficiencies, the extent of defect sites, and the morphology of ZnO on its solution-phase thermocatalytic activity has been evaluated in the synthesis of 5-substituted-1H-tetrazoles with different nitriles and sodium azide. A reasonable enhancement in the reaction rate was achieved by using Y-ZnO nanoflakes (Y-ZnO NFs) as a catalyst in place of pristine ZnO NFs. The reaction was complete within 6 h at 110 °C with Y-ZnO NFs, whereas it took 14 h at 120 °C with pristine ZnO NFs. The catalyst is easy to recycle without a significant loss in catalytic activity.

Full text of DOI:10.1002/asia.201801745

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Enhanced thermo catalytic activity of porous yellow ZnO
nanoflakes: A defect and morphology induced perspectives
Authors: Asit Baran Panda and Sunil Galani
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Enhanced thermo catalytic activity of porous yellow ZnO
nanoflakes: A defect and morphology induced perspectives
Sunil M Galani and Asit Baran Panda*[a]
Abstract: Herein, we report a simple and effective strategy for the
synthesis of yellow ZnO (Y-ZnO) nanostructures with abundant
oxygen vacancies in large scale, through sulfidation of ZnO followed
by calcination. The developed strategy provides the opportunity of the
retention of overall morphology in Y-ZnO to that of pristine ZnO and
to tune the extent of oxygen vacancies. The influence of oxygen
deficiency, extent of defect sites and morphology of ZnO on the
solution phase thermo-catalytic activity has also been evaluated for
the synthesis of 5-substituted-1H-tetrazoles with different nitriles and
sodium azide. Reasonable enhancement in the reaction rate is
achieved using Y-ZnO nanoflake (Y-ZnO NFs) as a catalyst than that
of pristine ZnO nanoflake. The reaction is completed within 6h at
110°C using Y-ZnO NFs, whereas it took 14h and 120°C with pristine
ZnO NFs. The catalyst is easily recyclable without a significant loss in
catalytic activity.
pristine ZnO is widely used as a photocatalyst. Although it is a
good photocatalyst but owing to its wide band-gap, its
photocatalytic activity is limited to only UV range and limits its
visible light applications.[7-12] To improve the activity, in both the
thermos-, and photo- catalysis, much effort has been made
towards size, shape, and morphology selective ZnO nanocrystals.
[13]
In general, the introduction of defect sites is one of the most
important ways to enhance the catalytic activity of metal oxides.
Self-doping, particularly the defect from oxygen vacancy, is
always advantageous regarding the improvement of property over
conventional doping system, where the introduction of impurity
(doped) elements may cause the reduction of catalytic activity.[2,14]
Oxygen defect induced improvement in thermo-,[15] as well as
photo-,[16] catalytic activity of ZnO have been reported. However,
in these samples the extent of oxygen deficiency is very limited,
owing to the instability of oxygen deficiency in ambient condition.
For this reason, ZnO turns to yellow from white in high
temperature, recognized to oxygen vacancy related disorders, but
gradually it turns yellow to white on cooling, due to the instability
of high temperature generated oxygen vacancies.[17]
To overcome the problem and stabilize maximum possible
oxygen vacancies in room temperature, several physical methods,
like vacuum deoxidation, ball milling, cold plasma treatment, and
electrospinning have been developed to generate stable oxygen
vacant sites in ZnO.[18] Although, these methods have their own
advantages, but high cost, complicated steps, and uncontrolled
oxygen deficiency are the main drawbacks of these methods. In
this regard, solution-based chemical methods are more attractive.
However, very few chemical methods have been developed for
stable oxygen vacant ZnO system.[19-25] The mostly followed
method for stable yellow ZnO with adequate oxygen vacant site
is the calcination of zinc peroxide (ZnO2).[19-21] Later on Xu and his
group realized oxygen deficient yellow ZnO on heat treatment of
Zn(OH)F precursor.[22] Ullattil et al.[23] synthesized yellowish ZnO
by calcination of zinc hydroxyl nitrate. However, the reported
methods used corrosive HF, H2O2, and morphology specific, not
possible to synthesize oxygen deficient ZnO with desired
morphology. But these strategies gave an idea that it is possible
to develop a simple protocol, to generate stable oxygen vacancy
in ZnO structure through calcination of a specific precursor having
an easy leaving group.
Introduction
Nanostructured metal oxides are highly important part in
heterogeneous catalysts, owing to their huge compositional,
structural, electronic variation and broad spectrum of properties
and behaviors.[1] In the very early days, it is thought that catalytic
activity of oxide based nanomaterials is mainly dependent on the
surface area but later on, it was confirmed that activity is also
dependent on corresponding size, shape, morphology, exposed
facet, and more importantly on crystal defects, both the functional
defect and oxygen vacancies, and extent of the defect.[2-3]
ZnO is one of the most important low cost, earth abundant,
sustainable wide band gap semiconducting metal oxide and finds
a wide range of application.[4] ZnO is a key catalyst component in
different gas phase reaction such as methanol synthesis, water
gas shift reaction, water and sulphur hydride dissociations,
oxidation of CO, desulfurization processes, CO2 activation
processes, and conversion of maleic anhydride into 1,4-
butanediol.[5] It has also been used in different liquid phase
reactions.[6-11] However, in most of the reactions ZnO is used as
co-catalyst or catalyst support, but the use of pristine ZnO as a
catalyst is rarely reported, most probably due to its lower
activity.[8-11] Particularly for catalytic application in solution phase,
Adequate oxygen vacancy also alters its band structure and
make it visible light active by narrowing the band-gap.[19-23] All the
synthesized yellow ZnO showed improved visible light induced
photocatalytic degradation of organic pollutant in the solution
phase. However, the photocatalytic reaction mechanism is totally
different to that of solution phase thermo-catalysis. The adequate
oxygen vacancy in yellow ZnO may have adverse or positive
[a]
Central Salt and Marine Chemicals Research Institute (CSIR-
CSMCRI) and CSMCRI-Academy of Scientific and Innovative
Research (AcSIR)
G. B. Marg, Bhavnagar-364002, Gujarat,
India.
E-mail: abpanda@csmcri.org, scghosh@csmcri.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200xxxxxx.
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effect in thermos-catalysis. Unfortunately, best of our knowledge
no one has studied the effect of yellow ZnO with varying degree
of oxygen vacancy in solution phase thermo-catalytic activity.
Thus, it is essential to develop a simple protocol for the synthesis
of size and shape selective stable yellow ZnO with varying degree
of oxygen vacancy and their corresponding effect in solution
phase thermos-catalysis.
ZnO/ZnS samples were calcined at 600°C. CHNS analysis of
ZnO/ZnS samples showed the gradual enhancement in the sulfur
incorporation with the enhancement in the molar ratio of Na2S.
Whereas, the corresponding calcined Y-ZnO samples showed the
absence of any sulphur. Thus, CHNS analysis specifies the
incorporation of sulphur during sulfidation, the degree of sulphur
incorporation was increased with the enhancement of amount of
Na2S in the reaction mixture and total decomposition of ZnS at
600 °C.
XRD pattern analysis. The powder XRD patterns were
recorded to evaluate the phase of synthesized materials in a
different stage of the reaction. Figure 1 represents the XRD
pattern of pristine ZnO NFs, corresponding ZnO/ZnS NFs-30
Herein we have developed a simple synthetic strategy for the
preparation of yellow ZnO nanostructures with abundant oxygen
vacancies through calcination of ZnO/ZnS heterostructure. The
ZnO/ZnS was prepared by sulfidation of respective ZnO through
substitution reaction using Na2S as sulphur source. After
substitution reaction followed by calcination, no such distinct
morphological change to that of pristine ZnO was observed. The
developed synthetic strategy not only offers the opportunity to
synthesize morphology selective oxygen deficient yellow ZnO, but
also it is possible to control the degree of oxygen deficiency
through controlling the extent of sulfidation, i.e., amount of Na2S,
and this is the prime novelty of this work. We have also utilized
these yellow ZnO for solution phase synthesis of 5-substituted-
1H-tetrazoles form nitriles and sodium azide and studied the
corresponding effect of defect and morphology on catalytic
activity. It is observed that yellow ZnO enhance the rate of
reaction reasonably and the reaction is completed in a short
period of time (6 h), less than half, to that of pristine ZnO (14 h)
and also reduce the reaction temperature.
Results and Discussion
Catalyst characterization
The defect incorporated yellow ZnO was synthesized through
the formation of ZnO/ZnS hetero-structure by sulfidation of
pristine ZnO using varying moles of Na2S as sulphur source in
ambient condition, followed by calcination of ZnO/ZnS hetero-
structure. Hereafter, the ZnO/ZnS hetero-structures, obtained
after sulfidation, with varying moles of Na2S, of pristine ZnO will
be referred as ZnO/ZnS structure-x, where x is molar % of Na2S
used and structure is NFs (nanoflakes), NPs (nanoparticles), NRs
(nano rods) and AGs (aggregates). As for example, the ZnO/ZnS
nanoflakes synthesized using 30 mole% Na2S will be termed as
“ZnO/ZnS NFs-30”. Similarly, the yellow ZnO NFs obtained after
calcination of respective ZnO-ZnS hetero-structure, will be
referred as Y-ZnO structure-x. As for example, calcined material
from “ZnO/ZnS NFs-30” will be referred as “Y-ZnO NFs-30”
Thermogravimetric analysis and sulphur estimation. In
the first sets of experiments, TGA analysis of ZnO/ZnS materials
were performed to identify the calcination temperature where the
formed ZnS will decompose totally to ZnO. Estimation of sulphur
in ZnO/ZnS and corresponding calcined Y-ZnO materials was
also performed, using CHNS analyser, to confirm the sulphur
incorporation and degree of incorporated sulphur during
sulfidation. TGA curve of ZnO/ZnS-10 evidenced nearly 1.5%
weight loss within 200 °C, ascribed to the removal of trapped
moisture and the 2nd weight loss of approximately 1.4 % in the
temperature range of 500-625 °C, ascribed to the decomposition
of ZnS to ZnO (Figure S1, Supporting Information). Thus, all the
Figure 1. XRD pattern of (a) pristine ZnO NFs, (b) ZnO/ZnS NFs-30, (c) Y-ZnO
NFs-10, (d) Y-ZnO NFs-20, (e) Y-ZnO NFs-30 and (f) Y-ZnO NFs-50. Inset. The
digital photograph of the corresponding ZnO samples.
heterostructure and all the Y ZnO NFs-X( X=10, 20, 30, 50). The
pristine ZnO NFs depict the presence of well-resolved X-ray
diffraction peaks indexed to the hexagonal wurtzite structure of
ZnO (JCPDS 36-1451) (Figure 1a). In the XRD pattern of
ZnO/ZnS heterostructure, in addition to that of all the diffraction
peaks of wurtzite ZnO, as obtained in pristine ZnO, one low
intense peak at 2θ= 28.48° was observed (Figure 1b). The
additional peak can be recognized as (111) plane of cubic zinc
blende ZnS phase and depict the formation of ZnS in the surface
of ZnO during sulfidation through ion-exchange by Na2S. Here it
is essential to mention that the peak intensity of (111) plane of
cubic zinc blende ZnS phase of ZnO/ZnS samples obtained after
sulfidation with varying amount of Na2S was varied. It gradually
increased with increase in the amount of Na2S, indicating
enhancement of the degree of sulfidation (Figure S2, Supporting
Information). Whereas the XRD pattern of the sample obtained
after calcination of ZnO/ZnS heterostructure showed the
presence of diffraction peaks only for wurtzite ZnO, similar to that
of pristine ZnO, the peak at 2θ= 28.48◦ for ZnS disappeared
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(Figure 1c-f). No such distinct difference in XRD pattern between
pristine ZnO and Na2S treated ZnO was observed. The only
identifiable difference is in their physical appearance. Pristine
ZnO is white, whereas Na2S treated ZnO is yellow, with varying
color intensity, and the color intensity is increased with increase
in the degree of sulfidation (inset Figure 1). The Y-ZnO NFs-10 is
light yellow, whereas Y-ZnO NFs-30 is quite brighter yellow.
However, no such distinct color change is identified in between Y-
ZnO NFs-30 and Y-ZnO NFs-50 and the color intensity of both the
sample is almost identical (inset Figure 1). ZnO with varying
morphology also showed an identical characteristic change in
XRD pattern and physical appearance to that of ZnO NFs (Figure
S3, Supporting Information). This phenomenon indicates some
surface structural change took place in the ZnO surface during
calcination of ZnO/ZnS hetero-structures, due to the
decomposition of surface ZnS. This surface structural change is
the prime responsible for the color change, keeping overall crystal
structure intact. However, it is not possible to identify the specific
surface structural change by XRD pattern.
2c). The SEM images of Y-ZnO with other morphology also
evidenced the retention of the overall morphology of respective
pristine ZnO (Figure S4-6, Supporting Information). TEM image of
Y-ZnO NFs-30 confirms the formation of porous flake structure
(Figure 2d). Corresponding magnified image evidenced the
presence of distinct pores with a broad pore size distribution in
the range of (20-60nm) (Figure 2e). Respective high-resolution
TEM (HR-TEM) image depict the presence of distinct lattice
fringes, confirmed high crystalline nature of the synthesized ZnO
materials. The lattice fringe spacing of 0.257 nm can be ascribed
to the d-spacing between the (002) plane of hexagonal wurtzite
ZnO and supports the XRD result (Figure 2f). EDX spectra of Y-
ZnO, from both the TEM and SEM, also evidenced the absence
of sulphur and confirmed the total decomposition of ZnS in
ZnO/ZnS hetero-structure during calcination (Figure S7,
Supporting Information).
Raman spectra analysis. In order to confirm the structure
and surface composition of the synthesize Y-ZnO, Raman
spectroscopic analysis was performed. Figure 3 represents the
respective Raman spectra of synthesized ZnO samples in the
range of 200-700 cm-1. Raman spectrum of pristine ZnO NFs
showed the presence of one intense peak at 435 cm-1, one low
intense peak at 330 cm-1, and a small and broad hump at around
577 cm-1 (Figure 3a). The intense peak at 435 cm-1 can be
assigned to the typical Raman vibration modes of wurtzite ZnO,
the peak at 330 cm-1 is responsible for the multiple phonon
scattering processes, and the broad hump at 577 cm-1 is
associated with the oxygen vacant sites.[22-23] Raman spectra of
Y-ZnO, obtained after calcination of respective sulfidation product
(ZnO/ZnS), evidenced the presence of all the respective peaks of
pristine ZnO. However, the peak intensity at 577 cm-1, responsible
for oxygen vacant sites, is varied and gradually increased with the
enhancement of the degree of sulfidation in ZnO-ZnS, i.e.,
Figure 2. SEM image of pristine ZnO NFs (a), ZnO/ZnS-30 NFs (b),
corresponding Y-ZnO NFs-30 (c) and the TEM (d,e) and HR-TEM (f) image of
Y-ZnO NFs-30.
SEM and TEM analysis. The SEM image of the pristine ZnO
NFs evidenced the formation of flake like structure, with an
average length of 2-3 µm, width of 400-600 nm and the flakes are
quite thin (Figure 2a). Flakes are random and no specific
arrangement of flakes was identified. Flakes are also porous.
After reaction of the pristine ZnO with Na2S (ZnO/ZnS), i.e.,
sulfide formation, the flake structure remained intact and resulted
in nearly identical flake structure with almost similar overall
morphology to that of pristine ZnO. However, flake surface
becomes smoother and maximum pores were disappeared
(Figure 2b). In the time of sulfidation through ion-exchange, the
hexagonal ZnO was transformed to cubic ZnS. The phase
transformation may generate strain in the system, resulted in the
destruction of the system, which is one of the probable reason of
pore blockage. But after calcination, i.e., decomposition of ZnS,
the pores were regenerated. The overall morphology of Y-ZnO
NFs is more or less identical to that of pristine ZnO NFs (Figure
Figure 3. Raman spectra of synthesized (a) pristine ZnO NFs, (b) Y-ZnO NFs-
10, (c) Y-ZnO NFs-20, (d) Y-ZnO NFs-30 and (e) Y-ZnO NFs-50.
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loosely bounded oxygen in ZnO surface. Whereas, the peak
situated in the medium binding energy, i.e., at 531.5 eV, can be
recognized to the O2- ions present in the oxygen deficient sites of
ZnO surface.[19, 22, 24] In all the three spectra’s, the ratio of peak
intensity and the corresponding area of the peaks associated to
that of structural O2- ions and loosely bounded oxygens are almost
identical. However, peak intensity and the corresponding area of
the peak at 531.5 associated to O2- ions present in the oxygen
deficient sites are varied reasonably and this is much high for Y-
ZnO NFs-30. The area ratio of the peak at 530 and 531.5 eV of
Y-ZnO NFs-30, Y-ZnO NFs-10 and pristine ZnO NFs are 0.56, 0.3
and 0.17, respectively. The XPS results confirmed that oxygen
vacant site has a direct relation to that of the degree of sulfidation
and Y-ZnO NFs-30 contain much high oxygen vacant sites with
respect to the pristine ZnO NFs. Thus, the XPS result supports
the result obtained in Raman spectra.
Textural and microstructural analysis. Now, it is imperative
to determine the probable overall textural and microstructural
change in bare ZnO flake and respective Y-ZnO flake, as these
are the curtail parameters of catalytic activity, which were
Figure 4. (a) Survey XPS spectrum of Y-ZnO NFs-30 (a). The O1s spectra of
Y-ZnO NFs-30 (b), Y-ZnO NFs-10 (c) and pristine ZnO NFs (d).
evaluated by nitrogen adsorption
– desorption analysis.
Respective nitrogen sorption isotherm of both the samples
correspond to type II and hysteresis loops are almost H3 type
ascribed to the aggregated particles (Figure S9, Supporting
Information). The pore size distribution curve, obtained from the
respective desorption part of the nitrogen sorption isotherm
confirm the presence of a broad pore size distribution, in the range
of 10-90 nm with an average pore size of 50 nm and supports the
result obtained in TEM analysis. Total BET surface area of bare
ZnO flake is 13 m2g-1 with total pore volume of 0.11 ccg-1, whereas
total BET surface area and total pore volume of Y-ZnO NFs-30
are 10 m2g-1 and 0.098 ccg-1, respectively. Thus, nitrogen sorption
analysis indicate that no such distinct textural and microstructural
change took place during sulfidation followed by calcination.
amount of Na2S used during sulfidation. This observation
evidenced that the extent of oxygen vacancy is directly related
with the degree of sulfidation. With the increase in the sulfidation,
the extent of oxygen vacancy is increased. For which, Y-ZnO
NFs-10 is light yellow in color due to the presence of
comparatively less oxygen vacant sites, whereas Y-ZnO NFs-30
is the intense yellow color for the presence of enhanced oxygen
vacant sites. However, the peak intensity at 577 cm-1 of Y-ZnO
NFs-30 is almost similar to that of Y-ZnO NFs-50, indicating the
identical surface saturation of oxygen vacant sites in Y-ZnO NFs-
30. ZnO with varying morphology obtained after sulfidation with
30% Na2S also showed similar Raman spectra.
XPS analysis. Further XPS analysis was performed to
confirm the composition and get more insight information about
oxygen vacancies. Before analysis, calibrations of all the spectra
were performed keeping C1s peak located at 284.50 eV. Figure
4a represents the corresponding wide survey elemental spectrum
of Y-ZnO NFs-30. The survey spectrum clearly evidenced the
presence of only oxygen and zinc, and further confirmed the total
decomposition of ZnS. The corresponding high-resolution core
level Zn 2p spectrum in the binding energy range of 1010−1055
eV evidence the presence of two distinct peaks at 1021.9 and
1044.8 eV indexed to the Zn 2p3/2 and Zn 2p1/2 and confirms the
presence of Zn2+ (Figure S8, Supporting Information). However,
high resolution O1s spectra are more important to get the detailed
information about the environment and status of surface oxygen
and respective vacancies. Figure 4b, c & d represents the high-
resolution O1s spectra of Y-ZnO NFs-30, Y-ZnO NFs-10, and
pristine ZnO NFs, respectively. Deconvoluted high-resolution O1s
spectra of all the samples showed the presence of distinctly
identifiable three peaks that appear at about 530, 531.5 and 532.3
eV. The high intense peak at lower energy level usually
recognized to that of O2- ions present with hexagonal Zn2+ ion
array in wurtzite ZnO structure (i.e., structural O2- ions). Low
intense peak at higher binding energy is associated to that of
pristine ZnO NFs
Y-ZnO NFs-10
Y-ZnO NFs-30
Y-ZnO NFs-50
300
400
500
600
700
Wavelength (nm)
Figure 5. UV-Vis. diffuse reflectance spectroscopy (DRS) of the synthesized
samples.
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DRS analysis. It is well established that oxygen vacant site in
ZnO has a direct effect on the corresponding band structure.[19-23]
Thus, the optical properties were measured by UV-Vis. diffuse
reflectance spectroscopy (DRS) for further confirmation of the
presence of oxygen vacancy and effect of degree of oxygen
vacancy in the band structure. From the Figure 5, it is evidenced
that the absorption edge of all the Y-ZnO NFs samples was
shifted towards longer wavelength (red shifting) with respect to
that of pristine ZnO NFs. Further, it is also evidenced that with the
increase in the degree of sulfidation, i.e. degree of oxygen vacant
site, the gradual red shifting of the absorption edge was observed.
However, the adsorption edge of Y-ZnO NFs-30 and Y-ZnO NFs-
50 appeared in the very closed position. This observation
evidenced that saturation of oxygen vacant site in Y-ZnO NFs-30
and support both the Raman and XPS analysis.
acids. Later on different metal complexes, inorganic salts and
Zn/Al HT (hydrotalcite), Zn hydroxyapatite, ZnO and ZnS has
been developed as a homogeneous and heterogeneous catalyst,
respectively.[30-31] However, homogeneous catalysts are not re-
usable and both the homogeneous and heterogeneous catalysts
needs longer reaction time, large excess sodium azide (3 : 1::
azide: nitrile), and high temperature. Even after all these most of
the times, the yields are not satisfactory. Recently, we have used
porous ZnO flake for the synthesis of 5-substituted-1H-tetrazoles,
which is capable to give satisfactory yield (90% for 5-p-
bromophenyl-1H-tetrazole) with a reduced amount of azide (1.2
mole).[10] Still, the reaction time and temperature is quite long (14h
& 120°C), which should be reduced. Study of reaction pathway
indicate that adsorption of nitrile on the Zn site at the surface of
ZnO facilitate the reaction progress.[10] It is expected that the
defect in ZnO may facilitate the nitrile adsorption on the ZnO
surface and accelerate the rate of the reaction.
Formation Mechanism
The overall strategy for the synthesis of morphology
selective oxygen deficient Y-ZnO is based on the synthesis of well
crystalline ZnO of desired morphology, formation of ZnO-ZnS
hetero-structure through sulfidation of the synthesized ZnO using
Na2S and calcination of resultant ZnO-ZnS heterostructure
resulted in the desired oxygen deficient Y-ZnO with more or less
identical morphology to that of pristine ZnO. Sulfidation took place
in the ZnO surface through ion exchange mechanism.[26] On
calcination in air, the ZnS formed in the surface of ZnO was
decomposed and resulted ZnO.[27] During calcination, the sulphur
of ZnS oxidized and removed from the surface as SO2. In the
same time, the zinc is oxidized to ZnO and the oxidation occurred
with the exchange reaction between oxygen and sulphur. The
reactions took part during the overall transformation of pristine
ZnO to oxygen deficient Y-ZnO is as follows:
Inspiring from this hypothesis, we have utilized the
synthesized Y-ZnO with different morphology having a varying
degree of the surface defect to understand the effect of the
surface defect, the corresponding degree of defect and respective
morphology on the rate of reaction. In the first set of reactions, all
the synthesized ZnO NF, includes pristine and Y-ZnO with varying
degree of oxygen deficient sites, were utilized for the synthesis of
5-phenyl 1H-tetrazole using 1.2 mole sodium azide with respect
to that of benzonitrile, DMF as solvent and the reactions were
performed at 120 °C for 14 h. All the reactions were ended with
100% selectivity of the desired product and more or less identical
isolated yield of 90%. The result confirmed that generation of
oxygen deficient sites did not reduce the catalytic activity to that
of pristine ZnO NF. Then, we have repeated the reactions for 6h
and showed a variation of yields with different samples. It is our
delight to report that both the Y-ZnO NFs-30 and Y-ZnO NFs-50
resulted in a maximum yield of 89%, almost closer to that of the
yield obtained after 14 h of reaction (Table 1). Whereas, Y-ZnO
NFs-20, Y-ZnO NFs-10, and pristine ZnO NFs ended with 84, 72
and 57 % isolated yield of desired product. The result confirmed
that there is a definite role of oxygen deficient sites on the rate of
reaction. For which, with an increase in the degree of defect sites
the yield increased after 6 h of reaction. The oxygen deficient Zn
sites of ZnO facilitate the chemisorption of nitrile on the surface
and accelerate the rate of reaction. With the increase in the
oxygen vacancy (the active sites), the nitrile adsorption increased,
in-turn the rate of reaction increased and completed the reaction
in a short period of time. Due to the presence of almost identical
oxygen deficient sites in Y-ZnO NFs-30 and Y-ZnO NFs-50, as
identified by Raman and DRS spectroscopic analysis, both the
sample showed similar catalytic activity. Hence, the results
support our hypothesis. Then we have repeated the reaction
using Y-ZnO NP/ NR/AG-30 as catalyst but resulted in a reduced
yield to that of Y-ZnO NFs-30 (Table 1). These results were
further evidenced the morphology induced catalytic activity of
oxygen deficient ZnO. Non porous nature of nanorod (NR), the
thick width of porous plates (NP) and aggregation of nanoparticles
reduced the available active sites of ZnO, which are the probable
prime origin of the reduced catalytic activity.
Na2S + H2O → NaOH + S2- + H+………………..……(1)
ZnO + 2OH- → ZnO22- + H2O ……………………..…(2)
2 ZnO22-+ S2- + 3H2O → ZnO/ZnS + 6OH- ………….(3)
∆
2 ZnS (ZnO/ZnS) + 3O2 → 2ZnO + 2SO2↑…..……(4)
However, the rate of de-sulfidation, i.e., removal of to SO2, is
much faster to that of formation of corresponding ZnO, i.e., re-
oxidation of zinc, and generates the oxygen deficient site in the
surface and turned a yellow color.[28] In the performed reaction
condition the sulfidation took place only on the surface. Thus, after
surface saturation of ZnO surface by ZnS further sulfidation in
inner core in negligible and degree of sulfidation remained
constant. As a result, the degree of oxygen vacancy also almost
the same.
Catalytic application: The nitrogen-rich tetrazoles are very
important hetero-aromatic compounds and used in a large
number of fields. These are used in different biomedical
applications, pharmaceuticals, agriculture, as anti-foggants in
photographic materials, information recording systems,
propellants, and explosives.[29] Traditionally, tetrazoles are
synthesized from the catalytic reaction of nitriles and azide
through [3 + 2] cycloaddition using corrosive concentrated Lewis
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Table 1. Optimization of the synthesis of 5-phenyl-1H-tetrazole using different
synthesized ZnO a
Table 2. Synthesis of 5-substituted 1H-tetrazole derivatives a
Temp.
(°C)
Time
(h)
Conv.
(%)
Isolated
Yield (%)
Entry
Catalyst
120
120
120
120
120
110
120
120
120
120
6
12
6
6
6
6
6
6
6
61
94
78
90
93
93
93
82
83
71
57
90
72
84
89
89
89
78
79
67
1
ZnO NFs
2
3
Y-ZnO NFs-10
Y-ZnO NFs-20
4
Y-ZnO NFs-30
5
6
7
8
Y-ZnO NFs-50
Y-ZnO NRs-30
Y-ZnO NPs-30
Y-ZnO AGs-30
6
a Reaction condition: Benzonitrile, 1.0 mmol (1 eq); sodium azide, 1.2
mmol (1.2 eq); DMF(solvent), 2 mL; catalyst amount, 30 mg.
a Reaction condition: Benzonitrile (1.0 mmol, 1 eq); sodium azide (1.2
mmol, 1.2 eq); catalyst Y-ZnO NFs-30 (30 mg); DMF (2 mL).
Then, we have selected Y-ZnO NFs-30 as an active catalyst
for further detailed investigation for the synthesis of tetrazole. To
understand the exact time required to complete the performed
reaction, the reaction was performed for different time durations
at 120 °C (Figure S10, Supporting Information). From the
corresponding reaction profile, it is evident that reactions proceed
at a faster rate in the initial stage; within 4 h 78% yield was
obtained. However, after that reaction becomes slightly slow, in 6
h 89% yield was observed and remain almost constant till 10 h.
The reaction temperature has also been optimized and performed
the reaction in varying temperature. It is our delight to report that
reaction also proceed smoothly even in 110 °C with an identical
yield and further reduction of temperature significantly affect the
yield (Figure S11, Supporting Information). No such distinct
change in yield was observed on the enhancement of the amount
of azide. DMF was found to be the best solvent for better yield
(Table S1, Supporting Information). Here it is essential to mention
that without catalyst no product formation was observed. 30mg of
the catalyst with respect to that of 1 mmol nitrile was found to be
the optimized amount of catalyst.
After getting an exciting result for the synthesis of 5-phenyl-
1H-tetrazoles with 90% yield at 110 °C within 6h of reaction, the
scope of the catalyst was explored for several structurally different
nitriles. Although, the activity is varied with the substrate and need
separate optimization for the individual substrate, but we were
able to get a good yield of different tetrazole successfully just by
optimizing the reaction time, which proved the broad applicability
of synthesized Y-ZnO NFs-30 as heterogeneous catalyst (Table
2). All the substituted aryl nitrile ended with excellent yield.
Halogen (Cl-, Br-), acid (-COOH) substituted aryl nitrile ended with
more than 90% isolated yield, whereas aldehyde (-CHO), cyanide
(-CN) and hydroxyl (-OH) substituted aryl nitrile resulted little
reduced yield (60-70%).
5-phenyl-1H-tetrazole for five consecutive runs. After completion
of the reaction in every cycle, the catalyst was separated by a
simple centrifugation, washed well with ethyl acetate, dried
overnight in the oven and used for next cycle. Figure S12
(Supporting Information) summarizes the conversion and isolated
yield after every cycle. More importantly to note that after the 5th
cycle negligible loss in the yield was observed (79% isolated yield).
Structural and morphological characterization evidenced that
catalyst is stable even after the 5th cycle and overall characteristic
remain intact (Figure S13-14, Supporting Information). Even the
physical appearance of yellow color also remains intact (Figure
S15, Supporting Information). However, in the IR spectrum of
used catalyst, the presence of surface adsorbed nitrile is identified.
This is the probable origin for the minor catalytic activity reduction
(Figure S16, Supporting Information).
Table S2 (Supporting Information) represent a catalytic
activity comparison of a different reported heterogeneous catalyst
including ZnO nanostructure with the synthesized Y-ZnO NFs-30
for the synthesis of 5-phenyl-1H-tetrazole using benzonitrile and
NaN3. From the table, it is evident that the synthesized Y-ZnO
NFs-30 showed superior catalytic activity with respect to that of
isolated yield, reaction time (less than half to that of reported
lowest reported time), amount of sodium azide used and
temperature, and thus advantageous to use.
It is difficult to study the reaction mechanism and the
corresponding pathway for heterogeneously catalyzed reaction.
However, there are reports on mechanistic study for the synthesis
of tetrazole using nitrile and azide by transition metal (Zn, Co)
based homogeneous catalyst. According to the reports, nitrile
coordinates with transition metal ion (Zn2+, Co2+) followed by
nucleophilic attack by the azide and subsequent cycloaddition.[10,
31]
For heterogeneous catalysts, recyclability is a crucial issue.
There is a high probability for the drop of catalytic activity of
synthesized y-ZnO originating from the reduction of oxygen
deficient sites in the reaction condition. Thus, recyclability of the
synthesized Y-ZnO NFs-30 catalyst is tested for the synthesis of
The co-ordination step reduced the active energy
(approximately 5-6 Kcal. mol.-1) and supposed to be the rate
determining step. As per our experimental finding, we believe that
the performed ZnO catalysed reaction also follow the same
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catalytic activity. Synthesized Y-ZnO with varying morphology
and degree of the defect can be utilized for a large variety of
reactions for improved results.
Experimental Section
Materials
Zinc nitrate hexahydrate [Zn(NO3)2·6H2O] was purchased from RanKem
(Ranbaxy), India; Ammonium carbonate (NH4HCO3
& NH2CO2NH4,
95.3%), triethanolamine (C6H15NO3) [TEA], Na2S·9H2O, bulk ZnO,
ammonium hydroxide (NH4OH) was purchased from S. D. Fine-Chem.
Limited, India. All purchased analytical grade chemicals were used without
further purification. All the organic reagents were purchased from Sigma
Aldrich, USA. Water from Millipore water purifier with a resistivity of 18 MΩ
cm was used.
Synthetic procedures of ZnO nanostructures with varying
morphology
Synthesis of porous ZnO nanoflakes (ZnO NFs). In a typical
process, 3 mL TEA was mixed with 15 mL deionized water. 10 mL aqueous
Zn(NO3)2.6H2O solution, containing 3.0 gm (10.07 mmol), was added
slowly to the TEA solution under a stirring condition at room temperature.
Followed by 15 mL aqueous solution of 4.5 gm ammonium carbonate was
added dropwise to the solution mixture. The final mixture was stirred 6 h
at room temperature. Then, 33 mL of that solution was transferred into a
50 mL Teflon-lined autoclave, sealed properly, kept in a pre-heated oven
at 150 oC for 6 h. The resultant white precipitate (obtained on cooling) was
collected, thoroughly washed several times with deionized water and dried
overnight at 70 °C in an oven. The dried product was calcined in air at
500 °C for 3 h.
Figure 5. Schematic representation of the reaction path likely to happen during
synthesis of 5-substituted-1H-tetreazole using Y-ZnO with abundant oxygen
vacancy as a heterogeneous catalyst.
pathway. The first step is the chemisorption of nitriles on the Zn
site of ZnO surface, then nucleophilic attack of the azide followed
by subsequent cycloaddition and desorption of desired product.
In pristine ZnO, the reaction proceeds slowly due to the slow
chemisorption step and took longer reaction time. Whereas, Zn in
oxygen deficient sites of Y-ZnO facilitate the chemisorption of
nitriles, assists the progress of reaction faster. Thus, the time for
the completion of the reaction is highly dependent on the degree
of oxygen deficient sites of Y-ZnO. We have proposed a probable
reaction pathway based on the experimental finding (Scheme 1).
Synthesis of aggregated ZnO nanoparticles (ZnO AGs). In a
typical process, dilute aqueous NaOH solution was added dropwise
to a 5.0 gm Zn(NO3)2.6H2O in 50 mL aqueous solution under a stirring
condition at room temperature till pH 10 and the stirring was
continued for another 30min. The obtained white precipitate was
collected through a centrifuge, washed thoroughly with deionized
water several times and dried overnight at 70 °C in an oven. The
obtained precursor material was calcined in air at 500 °C for 3 h.
Separately 3D assembled porous ZnO nanoplates (ZnO NPs)
and ZnO nanorods (ZnO NRs) were prepared following our
previously reported method.[9, 32]
Conclusions
Synthesis of yellow ZnO nanostructures
In summary, we have successfully synthesized yellow ZnO (Y-
ZnO) having abundant oxygen vacancies, by sulfidation followed
by calcination of a pristine ZnO having desired morphology and
resulted Y-ZnO with retention of overall morphology. The degree
of sulfidation can be tuned simply by varying the extent of
sulfidation. During calcination of formed ZnO/ZnS, the rate of de-
sulfidation is much faster to that of ZnO formation and generate
the oxygen deficient sites. Catalytic activity experiments
evidenced a reasonable influence of morphology and the
corresponding degree of oxygen vacancy of Y-ZnO on solution
phase thermo-catalytic activity. It is observed that oxygen
deficient sites enhanced the rate of the reaction of different nitriles
and sodium azide to 5-substituted-1H-tetrazoles sensibly. Y-ZnO
NFs-30 showed superior activity and ended the reaction in a very
short period of time (6h) and low temperature (110°C) with respect
to that of pristine ZnO NFs. The Y-ZnO is easily recyclable for at
least 5 consecutive catalytic cycles without a significant loss in
The defect incorporated yellow ZnO was synthesized through the
formation of ZnO/ZnS hetero-structure by sulfidation of pristine ZnO using
Na2S as sulphur source, followed by calcination of ZnO/ZnS hetero-
structure in ambient condition. In a typical reaction procedure, 200mg of
synthesized ZnO nanostructured were dispersed in 20 mL water and
separately varied amount of Na2S, (10, 20, 30, 50 mole% with respect to
that of ZnO) was dissolved in 40 mL water. Then, the two solutions were
missed together and heated at 60 °C for 2 h under constant stirring. Finally,
the product was washed 3-4 times with de-ionized water, followed by
methanol and dried for overnight at 70 °C. Finally, the dried ZnO/ZnS
hetero-structures were calcined at 600°C for 3h.
Detailed synthetic procedure for the synthesis of tetrazole.
In a typical synthesis of 5-phenyl 1H-tetrazole, 0.03g ZnO, 1.0 mmol
benzonitrile, 1.2 mmol sodium azide and 2 mL DMF (solvent) was taken in
a round bottom flask (RB). Then the RB was transferred to an oil bath,
fitted with a reflux condenser and heated at 110 °C for 6h with a stirring of
600 rpm. During the heat treatment, the progress of the reaction was
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monitored by thin-layer chromatography (TLC). After completion of the
reaction, 20 mL ethyl acetate was added to the reaction mixture and the
catalyst was separated by centrifuge. Then, in the reaction mixture 20 mL
3 N HCl and 20 mL ethyl acetate was added under vigorous stirring for 15
min and kept untouched for another 10 min. The resultant organic layer
was separated and the aqueous layer was again treated with ethyl acetate
(20 mL) and the process was repeated for another 3 times. The combined
extracted ethyl acetate was dried over anhydrous MgSO4, and further
concentrated to obtain the tetrazole products. All products were
characterized by 1H and 13C NMR, FT-IR analysis, which were in
agreement with literature reports. The separated catalyst was washed with
acetone and ethyl acetate and re-used.
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CSIR-CSMCRI Communication Number: 124/2018. Authors like
to acknowledge SERB, India (EMR/2014/001219) for financial
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Keywords: Zinc Oxide • Oxygen vacancy • Nanoflakes •
Tetrazole • Thermocatalysis
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Table of Contents
FULL PAPER
Simple and efficient synthetic protocol
for abundant oxygen deficient yellow
ZnO with desired morphology is
developed. The synthesized yellow
ZnO nanoflake showed excellent
solution phase thermocatalytic activity
for synthesis of 5-substituted-1H-
tetrazoles.
Sunil M Galani and Asit Baran Panda *
Page No. – 1-8.
Enhanced thermo catalytic activity of
porous yellow ZnO nanoflakes: A
defect and morphology induced
perspectives
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