However, the wide band gap (3.3 eV) and the high recombi- nation rate of ZnO constrain it only into the U-V region, due to lack of its response in the visible region and impede to photog[r]
Trang 1Original Article
Facile sol-gel synthesis and enhanced photocatalytic activity of the
a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247667, India
b Sarla Dwivedi Mahavidyalaya, Chhatrapati Shahu Ji Maharaj University, Kanpur, 209101, India
a r t i c l e i n f o
Article history:
Received 1 August 2018
Received in revised form
19 September 2018
Accepted 22 September 2018
Available online 29 September 2018
Keywords:
Sol-gel preparation
FTIR
FESEM
Nanocomposites
Photocatalytic activity
a b s t r a c t The structural, optical and photocatalytic properties of V2O5-ZnO nanoflakes are reported A facile sol-gel method was employed for the synthesis of ZnO and V2O5-ZnO nanostructures Structural characteriza-tions revealed aflake-type structure of V2O5-ZnO obtained from ZnO nanorods A decrease in the band gap from 3.28 eV for ZnO to 2.64 eV for V2O5-ZnO was observed by Ultraviolet (UV)-Visible spectroscopy The V2O5-ZnO based photodegradation of methylene blue (MB) dye indicated that the anchoring of V2O5
in the ZnO composite improved the photocatalytic efficiency of the composite under irradiation of the visible light
© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Nowadays, the progress in industrialization and urbanization
comes with a great problem, i.e environmental pollution
Espe-cially, the industrial leavings are a major source of water
contam-ination which affected livelihood needs In this direction, different
methods have been used for wastewater treatment Among them,
photocatalytic degradation is a popular and effective technique,
which is effectual for eliminating harmful contaminated elements,
for wastewater contamination [1e3] For this purpose,
semiconductor-based photocatalysts such as TiO2, ZnO, and SnO2
have been widely developed and reported in the literature[4] ZnO
is one of the most extensively studied materials due to its facile
synthesis, widely tunable morphologies and high carrier mobility
[5] However, the wide band gap (3.3 eV) and the high
recombi-nation rate of ZnO constrain it only into the U-V region, due to lack
of its response in the visible region and impede to photogenerated
carriers to taking part into photocatalytic reaction respectively
[6,7] Therefore, doping of nanosized metal or metal oxide with ZnO
gives a low recombination rate of photogenerated electronehole
pairs and increases the photocatalytic activity Among the oxides
based dopants, a narrow band gap (~2.2 eV) vanadium pentoxide (V2O5) semiconductor is broadly explored as an active catalyst in the visible region[8] Additionally, morphologies of nanostructures have a great impact on their widely varying properties; for example, ZnO nanoflowers showed a stronger photocatalytic ac-tivity than ZnO nanorods[9] The good photocatalyic response of a
V2O5-ZnO nanostructure has recently been reported[8], however, the focus was not on effect of various morphologies In this study, ZnO and V2O5-ZnO nanostructures were prepared by the sol-gel method, and their photocatalytic responses to methylene blue (MB) dye were investigated High resolution X-ray diffractometer (HRXRD), Fourier-transform infrared spectroscopy (FTIR) andfield emission scanning electron microscopy (FESEM) techniques were used for structural characterizations Optical properties and pho-tocatalytic activities were analyzed by UV-Vis spectrometer
2 Synthesis of V2O5-ZnO nanoflakes ZnO nanostructure was prepared by a sol-gel process as we re-ported earlier[7] In brief, 5 g zinc chloride with 50 ml Millipore water were kept in continuous stirring After 20 min, 2.4 g potas-sium hydroxide (KOH) dissolved in 20 ml distilled water was added
in the above solution, and at that time color changed from white to milky white, indicating the formation of the ZnO structure After this, the above milky solution was heated for 3 h at temperatures
* Corresponding author.
E-mail address: jitendrashkl9@gmail.com (J.K Shukla).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
https://doi.org/10.1016/j.jsamd.2018.09.005
2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Trang 2varying from 80 to 90C After reaching at room temperature, the
above solution was washed with distilled water and ethanol and
dried at 50e60C for 12 h A similar method was adopted to prepare
V2O5-ZnO nanoflakes Initially, in two separate beakers, 2.85 g ZnO
nanopowders with 30 ml distilled water and 0.18 g V2O5precursor
in 2 M HCl were stirred After 15 min, both solutions were mixed
together giving a yellow color in the process of continuous stirring
After dropping the diethanolamine (DEA), the color appeared slay
blue After this, 5 ml hydrazine hydrate (HH) was added The above
mixture was heated from 80 C to 90 C without stirring This
mixture was washed with distilled water Finally, the above mixture
was dried at 50C for overnight A synthesis procedure of V2O5-ZnO
is described inFig 1 The details of the instrumentation are available
in thesupplementary information (S1)
3 Results and discussion
The HRXRD patterns of ZnO and V2O5-ZnO are depicted in
Fig 2a Pure ZnO possesses a hexagonal wurtzite structure with
space group of P63mc Two kinds of phases in V2O5-ZnO were
observed and well indexed to the hexagonal ZnO (JCPDS
no.-00-009-0387) and the orthorhombic V2O5(JCPDS no.-00-004-0831)
No other impurity peaks such as VO, V2O3and VO3were
detec-ted, confirming that the acquired product comprises only
charac-teristic diffraction peaks of V2O5and ZnO only The lattice constants
a, b and c of ZnO and V2O5-ZnO were calculated using the following
equation:[10]
Sin2q¼l
2
4
h2
a2þk2
b2þl2
c2
(1)
The lattice constants obtained for V2O5-ZnO are a¼ 11.55 Å,
b¼ 4.41 Å and c ¼ 3.59 Å for the orthorhombic V2O5phase and
a¼ b ¼ 3.27 Å and c ¼ 5.28 Å for the hexagonal ZnO phase A
significant variation of the lattice parameters in V2O5-ZnO relative
to ZnO (a¼ b ¼ 3.24 Å and c ¼ 5.21 Å) is found, due to the larger ionic radii of V compared to Zn
The FTIR spectra of ZnO and V2O5-ZnO are shown inFig 2b The
V2O5-ZnO spectrum consists of the peaks at 467 cm1 (dV-O),
712 cm1(the asymmetric stretching mode of V-O-V), 924 cm1 (the symmetric stretching mode of V-O), 1162 cm1(the symmetric stretching mode of V¼O), 1627 cm1(C¼C), 3269 cm1(the O-H
bending vibration modes) and 3466 cm1 (the O-H symmetric mode) The peak at 924 cm1(the vibration of V¼O) gives infor-mation about the structural quality of the product; similarly, the presence of the absorption peak of V-O-V reveals the formation of the V2O5phase In case of pure ZnO, the peaks were observed at
552 cm1 (Zn-O), 1398 cm1 (the C¼O stretching vibration),
1639 cm1 (the C¼O symmetric stretching vibration) and
3401 cm1(the O-H symmetric mode), respectively
Fig 3(a and b) shows the surface morphologies of the ZnO and
V2O5-ZnO nanostructures Pure ZnO structure appears in the rod morphology along with a few bulk parts As reported in the liter-ature, the ZnO morphologies are highly dependent on the con-centration of KOH or NaOH Some changes in the basicity in the solution could yield ZnO with different morphologies That might
be a region for ZnO nanorods along with some bulky parts [9]
Fig 2b shows the FESEM image of V2O5-ZnO nanoflakes It is well known that tiny porous nanostructures play an important role in optoelectronic device applications In the V2O5-ZnO product, one can apparently see it appearing in the flake-type structure with small pores that nucleated directly on the ZnO structure and raised
in random directions These monodisperse nanoflakes were observed in samples with diameter of less than 1mm and thickness
of a few nanometers Such an aggregation could be the result of the DEA surfactant mediated synthesis that allows dispersing V2O5
with ZnO The energy-dispersive X-ray spectroscopy (EDS) map-pings confirmed the presence of all elements (V, O and Zn)
Fig 1 Synthesis procedure of V 2 O 5 -ZnO nanoflakes.
Trang 3Fig 3 SEM images of (a) ZnO, (b) V 2 O 5 -ZnO, (c) the total elemental mapping of V 2 O 5 -ZnO, (d) Zn mapping, (e) O mapping, and (f) V mapping images.
Fig 4 (a) UV-Vis absorption spectra and (bec) absorption spectra of MB using ZnO and V 2 O 5 -ZnO, (d) the degradation vs time plots, and (e) schematic diagram of
photo ZnO nanoflakes.
Trang 4distributed uniformly throughout the surfaces (Fig 3cef, S2),
which implied the successful anchoring of the V2O5on the surface
of the ZnO structure The observed elemental atomic ratios of the
elements V, O and Zn were 14.40%, 28.50% and 57.20% (Figure S3),
respectively
The UV-Vis absorption spectra of the ZnO and V2O5-ZnO
nano-structures, depicted in Fig 4a, clearly show that the absorption
peaks of ZnO and V2O5-ZnO at 374 and 389 nm, respectively The
optical band gaps of ZnO and V2O5-ZnO were estimated by the
following relation:[7]
ðahnÞ1¼ C hn Eg
(2)
whereais the absorption coefficient, h is Planck's constant, C is a
constant,nis the frequency of light, Egis the band gap energy, and
n¼ 1/2 and 2 for direct and indirect types of materials, respectively
The tau plots for ZnO and V2O5-ZnO are shown inFigure S4 The
band gap determined to be 3.28 eV and 2.64 eV for ZnO and V2O5
-ZnO, respectively A slight decrease in the band gap of V2O5-ZnO
with the anchoring of V2O5could be due to the atomic
hybridiza-tion between the Zn, V and O atoms, giving rise to the splitting of
the energy levels around the Fermi level
The photocatalytic performance of as-synthesized ZnO and
V2O5-ZnO structures was estimated via photodegradation of
methylene blue (MB) solution under visible light irradiation as
depicted in Fig 4b, c Note that the absorption peak occurs at
664 nm further as time increases, the intensity of the absorption
peak gradually decreases and after 80 min, it has completely
dis-appeared for V2O5-ZnO as compared to ZnO This indicates that the
MB has been photodegraded by the V2O5-ZnO catalyst The
degradation rate was calculated using the following equation:
Degradation¼ðA0 AÞ
where A0 is the initial absorbance of MB solution after the
ab-sorption without visible light irradiation, A is the absorbance of the
MB solution measured after the photocatalytic degradation for
80 min It can be seen fromFig 4d that for 20 min the degradation
rate was found to be same for both and after this V2O5-ZnO exhibits
an enhanced photodegradation of 97% while that of pure ZnO is 48%
in the corresponding intervals The photocatalytic process works
based on the principle of electronehole pair generation via band
gap excitation So that the response of ZnO of the MB solution
under the visible light irradiation could be attributed to the
pres-ence of defects/vacancies e.g oxygen vacancies/zinc interstitials
within ZnO that activates the energy levels within the wide band
gap (3.28 eV) The generated electrons and holes provide the free
radicals for degrading the MB solution While the improved
pho-tocatalytic activity of V2O5-ZnO could be the result of open
nano-structured surfaces, the interaction with vanadium, oxygen and
zinc (Zn-O-V) atoms and the presence of native defects/vacancies
within the V2O5-ZnO composite
The V2O5-ZnO composite's photocatalytic mechanism has been
proposed inFig 4e Based on the currently available studies on the
V2O5-ZnO nanostructures, the improved photodegradation
perfor-mance of V2O5-ZnO over pure ZnO could be ascribed to synergistic
effects and consequence of charge-transfer kinetics between V2O5
and ZnO When the photocatalytic V2O5/ZnO nanostructure is
irradiated under visible light, the excitons (electronehole pairs) are
generated in the V2O5by absorbing the photon energy and hence
electrons excited from valence band move to conduction band,
leaving holes in the valence band These conduction band electrons
of V2O5are injected to the conduction band of ZnO due to potential
difference of ZnO (5.3 eV) and V O (5.57 eV) In V O, the valence
band and conduction band lie below the energy band of ZnO so that the excited electrons from ZnO can easily cross the interface and reach to the conduction band of V2O5 Similarly, the excited holes from V2O5 reach to the valence band of ZnO Hence, the charge separation between photogenerated electrons and holes at the interface could be useful to impede the recombination of electron and hole pairs[11] As the result, V2O5-ZnO absorbed the visible light effectively and induce the free radicals e.g oxygen radicals (o2,) and hydroxyl radicals (OH,) and these radicals react with the
MB molecules and hence improve the photocatalytic ability
4 Conclusion
We have prepared the V2O5-ZnO nanoflakes using the facile simple sol-gel method The structural, bonding interaction, optical and photocatalytic responses of V2O5-ZnO have been studied by XRD, FESEM, FTIR, UV-Vis spectrometer UV-Vis analysis showed a decrease in the band gap from 3.28 eV for ZnO to 2.64 eV for V2O5 -ZnO The photocatalytic activity results indicate that the anchoring
of V2O5in the ZnO composite can improve the photocatalytic ef fi-ciency of the composite under visible light irradiation
Author's contributions
P Shukla has done the work J Shukla helped draft the manu-script In writing and reviewing, equal contributions have been made
to this manuscript Both the authors approved thefinal manuscript Acknowledgements
PS would like to thank Government of India Ministry of Human Resource Development, India forfinancial support
Appendix A Supplementary data Supplementary data to this article can be found online at
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