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 characterizations revealed a flake-type structure of V2O5-ZnO obtained from ZnO nanorods.
Journal of Science: Advanced Materials and Devices (2018) 452e455 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Facile sol-gel synthesis and enhanced photocatalytic activity of the V2O5-ZnO nanoflakes Prakhar Shukla a, Jitendra Kumar Shukla b, * a b Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, 247667, India Sarla Dwivedi Mahavidyalaya, Chhatrapati Shahu Ji Maharaj University, Kanpur, 209101, India a r t i c l e i n f o a b s t r a c t Article history: Received August 2018 Received in revised form 19 September 2018 Accepted 22 September 2018 Available online 29 September 2018 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 characterizations revealed a flake-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/) Keywords: Sol-gel preparation FTIR FESEM Nanocomposites Photocatalytic activity Introduction Nowadays, the progress in industrialization and urbanization comes with a great problem, i.e environmental pollution Especially, the industrial leavings are a major source of water contamination 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 recombination 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 * Corresponding author E-mail address: jitendrashkl9@gmail.com (J.K Shukla) Peer review under responsibility of Vietnam National University, Hanoi 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 activity 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) and field emission scanning electron microscopy (FESEM) techniques were used for structural characterizations Optical properties and photocatalytic activities were analyzed by UV-Vis spectrometer Synthesis of V2O5-ZnO nanoflakes ZnO nanostructure was prepared by a sol-gel process as we reported earlier [7] In brief, g zinc chloride with 50 ml Millipore water were kept in continuous stirring After 20 min, 2.4 g potassium 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 h at temperatures 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 (http://creativecommons.org/licenses/by/4.0/) P Shukla, J.K Shukla / Journal of Science: Advanced Materials and Devices (2018) 452e455 varying from 80 to 90 C After reaching at room temperature, the above solution was washed with distilled water and ethanol and dried at 50e60 C 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 V2O5 precursor in 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, 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 50 C for overnight A synthesis procedure of V2O5-ZnO is described in Fig The details of the instrumentation are available in the supplementary information (S1) 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.-00009-0387) and the orthorhombic V2O5 (JCPDS no.-00-004-0831) No other impurity peaks such as VO, V2O3 and VO3 were detected, confirming that the acquired product comprises only characteristic diffraction peaks of V2O5 and ZnO only The lattice constants a, b and c of ZnO and V2O5-ZnO were calculated using the following equation: [10] Sin2 q ¼ l2 h2 a2 ỵ k2 l2 ỵ b2 c ! (1) The lattice constants obtained for V2O5-ZnO are a ¼ 11.55 Å, b ¼ 4.41 Å and c ¼ 3.59 Å for the orthorhombic V2O5 phase and a ¼ b ¼ 3.27 Å and c ¼ 5.28 Å for the hexagonal ZnO phase A 453 significant variation of the lattice parameters in V2O5-ZnO relative to ZnO (a ¼ 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 in Fig 2b The V2O5-ZnO spectrum consists of the peaks at 467 cmÀ1 (dV-O), 712 cmÀ1 (the asymmetric stretching mode of V-O-V), 924 cmÀ1 (the symmetric stretching mode of V-O), 1162 cmÀ1 (the symmetric stretching mode of V¼O), 1627 cmÀ1 (C¼C), 3269 cmÀ1 (the O-H bending vibration modes) and 3466 cmÀ1 (the O-H symmetric mode) The peak at 924 cmÀ1 (the vibration of V¼O) gives information about the structural quality of the product; similarly, the presence of the absorption peak of V-O-V reveals the formation of the V2O5 phase In case of pure ZnO, the peaks were observed at 552 cmÀ1 (Zn-O), 1398 cmÀ1 (the C¼O stretching vibration), 1639 cmÀ1 (the C¼O symmetric stretching vibration) and 3401 cmÀ1 (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 literature, the ZnO morphologies are highly dependent on the concentration 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 mm 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) mappings confirmed the presence of all elements (V, O and Zn) Fig Synthesis procedure of V2O5-ZnO nanoflakes Fig (a) XRD patterns and (b) FTIR spectra of ZnO and V2O5-ZnO nanostructures 454 P Shukla, J.K Shukla / Journal of Science: Advanced Materials and Devices (2018) 452e455 Fig SEM images of (a) ZnO, (b) V2O5-ZnO, (c) the total elemental mapping of V2O5-ZnO, (d) Zn mapping, (e) O mapping, and (f) V mapping images Fig (a) UV-Vis absorption spectra and (bec) absorption spectra of MB using ZnO and V2O5-ZnO, (d) the degradation vs time plots, and (e) schematic diagram of photodegradation of MB dye molecule due to the V2O5-ZnO nanoflakes P Shukla, J.K Shukla / Journal of Science: Advanced Materials and Devices (2018) 452e455 455 distributed uniformly throughout the surfaces (Fig 3cef, S2), which implied the successful anchoring of the V2O5 on 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 nanostructures, 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] 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 À Á ahnịn ẳ C hn Eg Conclusion (2) where a is the absorption coefficient, h is Planck's constant, C is a constant, n is the frequency of light, Eg is the band gap energy, and n ¼ 1/2 and for direct and indirect types of materials, respectively The tau plots for ZnO and V2O5-ZnO are shown in Figure S4 The band gap determined to be 3.28 eV and 2.64 eV for ZnO and V2O5ZnO, respectively A slight decrease in the band gap of V2O5-ZnO with the anchoring of V2O5 could be due to the atomic hybridization 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 disappeared 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: Degradation ẳ A0 Aị *100% A0 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 V2O5ZnO The photocatalytic activity results indicate that the anchoring of V2O5 in the ZnO composite can improve the photocatalytic efficiency of the composite under visible light irradiation Author's contributions P Shukla has done the work J Shukla helped draft the manuscript In writing and reviewing, equal contributions have been made to this manuscript Both the authors approved the final manuscript Acknowledgements PS would like to thank Government of India Ministry of Human Resource Development, India for financial support (3) where A0 is the initial absorbance of MB solution after the absorption without visible light irradiation, A is the absorbance of the MB solution measured after the photocatalytic degradation for 80 It can be seen from Fig 4d that for 20 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 presence 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 photocatalytic activity of V2O5-ZnO could be the result of open nanostructured 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 in Fig 4e Based on the currently available studies on the V2O5-ZnO nanostructures, the improved photodegradation performance 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 V2O5 by 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 V2O5 are injected to the conduction band of ZnO due to potential difference of ZnO (5.3 eV) and V2O5 (5.57 eV) In V2O5, the valence Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2018.09.005 References [1] M Shanmugam, A Alsalme, A Alghamdi, R Jayavel, Enhanced photocatalytic performance of the graphene-v2o5 nanocomposite in the degradation of methylene blue dye under direct sunlight, ACS Appl Mater Interfaces (27) (2015) 14905e14911 [2] O Legrini, E Oliveros, A Braun, Photochemical processes for water treatment, Chem Rev 93 (2) (1993) 671e698 [3] A Mills, R.H Davies, D Worsley, Water purification by semiconductor photocatalysis, Chem Soc Rev 22 (6) (1993) 417e425 [4] Q Shi, S Wang, H Wu, M Yu, X Su, F Ma, J Jiang, Synthesis and characterizations of v2o5/zno nanocomposites and enhanced photocatalytic activity, Ferroelectrics 523 (1) (2018) 74e81 [5] Z.W Pan, Z.R Dai, Z.L Wang, Nanobelts of semiconducting oxides, Science 291 (5510) (2001) 1947e1949 [6] S Zhang, H.-S 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of au-decorated v2o5@ zno heteronanostructures and enhanced plasmonic photocatalytic activity, ACS Appl Mater Interfaces (17) (2014) 14851e14860 ... 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] band and conduction band lie... decrease in the band gap of V2O5-ZnO with the anchoring of V2O5 could be due to the atomic hybridization between the Zn, V and O atoms, giving rise to the splitting of the energy levels around the Fermi... 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 photocatalytic activity of V2O5-ZnO