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Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
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Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis B. Archana a,d, K. Manjunath b, G. Nagaraju c, K.B. Chandra Sekhar d, Nagaraju Kottam a,* a
Department of Chemistry, M. S. Ramaiah Institute of Technology (Autonomous Institute, Affiliated to Visvesvaraya Technological University, Belgaum), Bengaluru, 560054, India b Centre for Nano and Material Sciences, Jain Global Campus, Jain University, Kanakapura Road, Karnataka, 562112, India c Department of Chemistry, Siddaganga Institute of Technology, Tumakuru, 572102, India d Department of Chemistry, R & D Cell, JNTUA, Anantapuramu, 515001, India
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abstract
Article history:
ZnO nanoparticles are prepared by green synthesis using Moringaoleifera natural extract.
Received 9 September 2016
XRD and Raman analysis show crystalline ZnO with wurtzite structure. SEM and TEM
Received in revised form
images show the average size of the nanoparticles to be 100e200 nm. Photocatalytic gen-
4 November 2016
eration of hydrogen by these nanoparticles has been investigated under UVeVisible light
Accepted 13 November 2016
irradiation. Na2S and Na2SO3 sacrificial agents dispersed with the photocatalyst are
Available online xxx
employed as hole scavengers. ZnO nanoparticles with smaller size shows better H2 evolution rates up to 360 mmol hg1. It is noteworthy that ZnO nanoparticles prepared via novel
Keywords:
green synthesis exhibits oxygen vacancies and registers enhanced photocatalytic activity
Zinc oxide
as well as good photostability.
Green synthesis
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Nanoparticles H2 generation Photostability
Introduction In the verge of searching green and sustainable alternatives for the global energy problem, the production chemical fuels by the application of solar energy conversion is very promising [1]. The green route for energy is very necessary especially when the cost of fossil fuels increases and the evidence for global climate change due to the release of toxic greenhouse gases [2e4]. Of the all major alternative energy sources,
the production of hydrogen photo-catalytically by utilizing solar spectrum has been a great strategy due to its clean, low cost, environmentally friendly aspects [5,6]. Semiconductor nanostructured materials with a wide tunable band gap and a large exciton binding energy are of interest due to their exceptionally good properties and auspicious technological applications [7e10]. Since the first demonstration of photoelectrochemical H2 generation by Fujishima and Honda [11], intensive research efforts have been devoted towards semiconductor photocatalytic generation of H2 [12]. The utilization
* Corresponding author. Fax: þ91 802 360 3124. E-mail address: [email protected] (N. Kottam). http://dx.doi.org/10.1016/j.ijhydene.2016.11.099 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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of metal oxide semiconductors for light harvesting and H2 generation by splitting of water through photolysis is very promising [13]. In recent past notable effort has been made toward making semiconductor nanostructures with suitable materials synergize their properties [14e16]. Anatase TiO2 has proven to be a suitable material for hydrogen generation among various semiconductor oxide photocatalyst due to its long-term stability for the photo- and chemical corrosion [15,17,18]. There is intense effort in last decade to produce the potential photocatalyst alternative to TiO2 because of its wide band gap limits photocatalyst is not very suitable for many applications [19]. In this contest, ZnO is a promising semiconductor with the large exciton binding energy (60 meV) at room temperature [20,21]. Also, oxygen vacancies in ZnO known to induce band gap and increase its visible light photocatalytic activity [22,23]. The various ZnO materials and its devices were thoroughly studied and reviewed extensively [24e27]. Several ZnO nanostructures have been investigated for photochemical and photoelectrochemical H2 generation [16,28e31]. The metalemetal oxide, oxide-sulphide, Oxidenitride heterostructures [32,33] and noble-metal/oxide nanocrystals show excellent photocatalytic performance [34e38]. Cheng et al., observed better photocatalytic properties due to enhanced charge transfer and separation process from ZnOeTiO2 hybrid structures [39]. The good H2 evolution and apparent quantum yield have been obtained with ZnO/Cd nanostructures in UV and visible light irradiation [40e43]. The major disadvantage of ZnO photocatalyst is that it suffers from photo-corrosion [44,45] and it has been suppressed by doping, decorating with the optimum amount of carbonaceous material or by modifying the synthetic protocol [46,47]. In this article, we demonstrate a new green strategy to synthesize ZnO nanoparticles using Moringaoleifera natural extract as a fuel. The obtained oxygen-deficient ZnO nanoparticles show enhanced H2 production as well as good photostability under UVeVis light irradiation.
diffractometer. The nanoparticles were imaged using a Nova NanoSEM 600 FESEM system (FEI Company) fitted with Energy Dispersive X-ray Analysis (EDX). Transmission electron microscopy (TEM) images were recorded on a JEOL TEM 3010 instrument fitted with a Gatan CCD camera operating at an accelerating voltage of 300 kV. Electronic absorption spectra were recorded with a PerkineElmer Lambda 900 UV/Vis/NIR spectrometer in the diffuse reflectance mode. Photoluminescence measurements were carried out with a Fluorolog-3 spectrophotometer using a Horiba JobinYvon Xe lamp as light source. Raman spectra were recorded in the backscattering geometry using a 632 nm HeNe laser with a JobinYvonLabRam HR spectrometer.
Photochemical H2 generation H2 evolution experiments of ZnO nanoparticle (ZnO-2, 6, 10, 14) were carried out via photochemical splitting water and measured by gas chromatography at room temperature (25 C). The quartz reactor of 130 mL capacity has been used for the purpose. IR radiations from the light source were removed by circulating water and also maintain the normal temperature. In the reactor, 20 mg catalyst and 75 mL water were added followed by sonication of about 20 min to get homogeneous dispersion of nanoparticles. Na2S and Na2SO3 have been used as sacrificial agents for hole scavenger (pH ~13). The reactor was then de-aerated by passing N2for 10 min to remove the dissolved oxygen. Thereafter, the reactor was irradiated with 400 W Xenon lamp (UVeVis light source) and hydrogen gas was quantified using a gas-tight syringe by injecting 5 mL at every 30 min interval by Perkin Elmer Clarus 580 GC chromatograph equipped with a thermal conductivity detector (TCD).
Results and discussion Experimental All the chemicals used in the experiment brought from commercial sources and used without further purification. In a typical synthesis, 2.97 g of zinc nitrate hexahydrate was dissolved in 2, 6, 10 and 14 mL of Moringaoleifera natural extract and 10 mL of water. The components were mixed thoroughly by stirring for 15 min. The obtained homogeneous solution is heated on a hot plate with a stirrer to form gel kind of product. The gel is then transferred into the alumina crucible and kept it in a preheated muffle furnace maintained at 400 C. Smouldering type of the reaction takes place with the liberation of gases and within 5 min, nanocrystalline ZnO is obtained. The obtained product was calcined at 500 C for 3 h to remove the impurities. The synthesized products were named as ZnO-2, ZnO-6, ZnO-10 and ZnO-14 in accordance with the fuel amount.
Material characterization Powder X-ray diffraction (PXRD) patterns were collected with a Bruker D8 Discover diffractometer and Rigaku-99
ZnO nanoparticles (NPs) were prepared using Moringaoleifera natural extract by solution combustion process. 2e14 ml of natural extract was employed for the synthesis, of the various proportions. We found that 10 ml is the optimum amount to get nanoparticles with definite size and enhanced properties. The amount of fuel plays a crucial role to get the proper nanostructures with definite shape and size. As obtained NPs were characterized using several techniques and compare properties with commercially available bulk ZnO. Powder X-ray diffraction was employed to confirm the phase and crystallinity of ZnO NPs. Fig. 1 show PXRD patterns of ZnO-2, ZnO-6, ZnO-10 and ZnO-14, all the samples exhibit intense diffraction signals indicate good crystallinity. The diffraction planes (100), (002), (101), (102), (110) and (103) were readily indexed to standard wurtzite structure (ICSD #290968) A). The XRD with P63mc space group (a ¼ 3.2527 and c ¼ 5.2016 reflections of ZnO-10 are quite broad and less intense compared to ZnO-2, 6 and 14. The calculated crystallite size by Scherrer equation using the full width half-maximum values found to be 21.6 nm. Raman analysis is an effective tool to investigate the structure, crystallinity and defects in nanomaterials. Optical phonon vibrations of bulk, as well as
Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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Fig. 1 e X-ray powder diffraction patterns of hexagonal ZnO synthesized under different ratio of precursor via green synthesis.
nanostructured ZnO, were well studied [48,49]. Fig. 2 shows the Raman spectra of ZnO nanoparticles recorded using 632 nm green laser in backscattering geometry. We have observed mainly three phonon modes which includes A1, E2 high and E1 (LO). The high intense band observed at 439 cm1 assigned to the ZnO nonpolar optical phonon (E2 high) mode. The weak signal centred at 583 cm1 assigned to E1 (LO) mode of originating from oxygen deficiency [22]. The peak appeared at 332 cm1 (3E2HeE2L) is a multi phonon Raman mode arising from zone boundary phonons. The A1 mode at 380 arises due to ZneO polar bond lattice vibrations. The asymmetry and broadening of Raman signals are characteristics of NPs [50]. We observed more broadening in the case ZnO-10 and consisted with XRD results. There is no peak shift observed for the samples hence it indicates all the samples having similar crystallization, defects, and phonon vibrations.
Fig. 2 e Raman spectra of ZnO-2, ZnO-6, ZnO-10 and ZnO14 obtained in backscattering geometry using 632 nm green laser.
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The surface morphology and the size of ZnO particles were imaged using Field emission scanning electron microscopic (FESEM) analysis. Fig. 3 shows SEM images with the surface morphology of highly crystalline ZnO-10 and ZnO-14. The nanoparticles appear to be spherical in shape with the average size of 20e150 nm. SEM images of all samples show an agglomeration of the particles and are much lower in ZnO-10. The energy dispersive X-ray spectra of the samples clearly show the presence of Zn and O. No other signal was observed within the detection limit of EDS (see SI, Fig. S1). SEM images of the ZnO-6 and ZnO-14 displayed in Fig. S2 (see SI) and show similar features. Further, the morphology was confirmed by High-resolution transmission electron microscopic (HRTEM) analysis. Fig. 4 show the TEM images of ZnO-10 and ZnO-14 nanoparticles, the individual NPs identified and the lattice fringes of the particle have been resolved in HRTEM. Selected area electron diffraction (SAED) images (Fig. 4d) show the hexagonal patterns confirm the wurtzite structure and in agreement with XRD results. The lattice fringes in HRTEM image reveals the d spacing of 0.28 and 0.19 nm which corresponds to (001) and (101) planes of wurtzite ZnO (Fig. S3). The spherical nanoparticles clustering into small heaps as displayed in SEM and TEM and appeared to be bigger in size compared to calculated crystallite size from Scherrer formula. The UVeVisible optical diffusion reflectance spectra were shown in Fig. 5a. All the samples show characteristic strong absorption maximum below 400 nm corresponding to the band gap of ZnO. The absorption maximum is red shifted compared to bulk and shift increases as the fuel proportion increases. The band gap of the samples was obtained by extrapolating the linear portion of the KubelkaeMunk function as shown in the Fig. 5b. The obtained band gap values are 2.92 (ZnO-2), 3.05 (ZnO-6) 3.12 (ZnO10) and 3.10 (ZnO-14). NPs have a wide range of size distribution; their absorption can be attributed to intrinsic band gap of ZnO due to the electron transitions from the valence band to the conduction band [51]. The obtained band gap values are in good agreement with the reported values for ZnO [52]. All the samples prepared here showed a red shift in the band gap compared to bulk which can be attributed to impurity levels i.e. oxygen vacancies present in the sample [22]. It is clear that as the amount of fuel increases the particle size decreases and band gap increases due to quantum size effects. Photoluminescence measurements are helpful to reveal charge transfer and carrier trapping in semiconducting materials [53]. The room temperature PL spectra of ZnO-2, 6, 10 and ZnO-14 were shown in Fig. 6a. A strong PL band at 380 nm and a broad-band around 580 nm were observed in both the samples. In general, the PL emissions in a semiconductor are originated from the radiative recombination of photogenerated electrons and holes, and two major photophysical processes. The narrow violet UV emission around 380 nm is a near band edge emission originates from excitonic recombination of photogenerated electrons [54]. The green-yellow emission around 580 nm in visible region may be due to oxygen defects or Zn interstitials [16]. To confirm the band edge emission and defect related emission we collected the excitation spectra of ZnO-10 (Fig. 6b). The strong absorption peak at 350 nm corresponds to band gap and the weak signal around 400 nm due to oxygen vacancies. The CIE diagram for
Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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Fig. 3 e FESEM images of ZnO-10 (a,b) and ZnO-14 (c, d) in different magnification and corresponding particle size.
Fig. 4 e TEM images with different magnifications of ZnO-10 (aec) and ZnO-14 (eeg), SAED patterns of ZnO-10 (d) and ZnO14 (h).
Fig. 5 e UVeVisible diffuse reflectance spectra (a) and the energy band gap (b) of the as-prepared of ZnO-2, ZnO-6, ZnO-10 and ZnO-14. Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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Fig. 6 e (a) Photoluminescence spectra of ZnO-2, ZnO-6, ZnO-10, ZnO-14 and (b) corresponding excitation spectra of ZnO-10 to confirm band edge emission.
Fig. 7 e Schematic representation of the mechanism of charge separation and photocatalytic hydrogen evolution in the presence of Na2S, Na2SO3 sacrificial agents under UVeVis light irradiation.
ZnO-10 emission has been constructed, the X and Y coordinates are found to be 0.170 and 0.005 for band edge emission and 0.072 and 0.0420 for defect related emission (Fig. S4). ZnO is known to be very good photocatalyst for degradation of dye and other organic pollutants under UVeVis light irradiation [31,38]. The photocatalytic activity of the semiconductor oxides depends on various factors like chemical composition, morphology, particles size, crystallinity, vacancies, surface groups and surface area. The effect of above parameters on the ZnO photocatalytic activity is well studied [55]. ZnO nanostructure based photochemical and photoelectrochemical generation of hydrogen from water splitting were known [28,41]. Photocatalytic H2 generation measurements were conducted in an aqueous solution containing Na2S and Na2SO3 as sacrificial agents under the UVeVis light irradiation. Fig. 7 shows photocatalytic hydrogen production mechanism for the designed photocatalyst in the presence of sacrificial agents. Fig. 8(a,b) shows the H2 production by splitting of water under UVeVis light for ZnO NPs. All the samples have clearly show H2 evolution and increases linearly
Fig. 8 e (a) Hydrogen generation as a function of time with ZnO nanoparticles under irradiation of UVeVisible light in the presence of Na2S and Na2SO3 were used as the hole scavengers (b) Performance of ZnO NPs for photochemical H2 generation by varying the amount of photocatalyst under irradiation of UVeVis light. Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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with respect to time. Amongst, we observed a higher rate of H2 evolution (341.9 mmolg1) for ZnO-10 compared to other samples which may be due to its smaller particle size. Moreover, the experiments were carried out for bulk ZnO show very poor activity compared to ZnO NPs. In addition, the H2 evolution experiments were performed by varying the amount of catalyst (10 mg, 20 mg, 30 mg) for which a catalyst showed higher catalytic activity in the above studies (ZnO-10). The 20 mg catalysts show higher H2 production of 341.9 mmolg1 compared to 10 and 30 mg (66.96 and 28.13 mmolg1) in 2 h duration. Hence, 20 mg is the optimum catalyst concentration for 75 mL water and the details are shown in Fig. 8b. The higher catalytic activity of as-prepared NPs is attributed to a large amount of oxygen vacancies present in the sample which consisted with the earlier reports [22]. ZnO NPs shows very good photostability without undergoing photo-corrosion during the reaction. We expect that the stability may due to residual carbon present on the NPs even after calcination.
Conclusions We have successfully synthesized ZnO nanoparticles via green route employing Moringaoleifera natural extract as a fuel. The optimum amount of fuel ratio is very important to get desired size, shape and properties of the NPs. XRD and TEM studies show that ZnO is crystallized with hexagonal structure. ZnO NPs show better photocatalytic H2 generation compare to bulk and other nanostructures with increased photostability. The better photocatalytic activity is attributed to presence of oxygen defects which were confirmed by optical characterization. The novel synthetic strategy may open a new way to obtain many other semiconductor nanoparticles with improved properties.
Acknowledgements G. Nagaraju thanks to DST-Nano Mission, (SR/NM/NS-1226/ 2013) Govt. of India, for funding.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.11.099.
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Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis B. Archana a,d, K. Manjunath b, G. Nagaraju c, K.B. Chandra Sekhar d, Nagaraju Kottam a,* a
Department of Chemistry, M. S. Ramaiah Institute of Technology (Autonomous Institute, Affiliated to Visvesvaraya Technological University, Belgaum), Bengaluru, 560054, India b Centre for Nano and Material Sciences, Jain Global Campus, Jain University, Kanakapura Road, Karnataka, 562112, India c Department of Chemistry, Siddaganga Institute of Technology, Tumakuru, 572102, India d Department of Chemistry, R & D Cell, JNTUA, Anantapuramu, 515001, India
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abstract
Article history:
ZnO nanoparticles are prepared by green synthesis using Moringaoleifera natural extract.
Received 9 September 2016
XRD and Raman analysis show crystalline ZnO with wurtzite structure. SEM and TEM
Received in revised form
images show the average size of the nanoparticles to be 100e200 nm. Photocatalytic gen-
4 November 2016
eration of hydrogen by these nanoparticles has been investigated under UVeVisible light
Accepted 13 November 2016
irradiation. Na2S and Na2SO3 sacrificial agents dispersed with the photocatalyst are
Available online xxx
employed as hole scavengers. ZnO nanoparticles with smaller size shows better H2 evolution rates up to 360 mmol hg1. It is noteworthy that ZnO nanoparticles prepared via novel
Keywords:
green synthesis exhibits oxygen vacancies and registers enhanced photocatalytic activity
Zinc oxide
as well as good photostability.
Green synthesis
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Nanoparticles H2 generation Photostability
Introduction In the verge of searching green and sustainable alternatives for the global energy problem, the production chemical fuels by the application of solar energy conversion is very promising [1]. The green route for energy is very necessary especially when the cost of fossil fuels increases and the evidence for global climate change due to the release of toxic greenhouse gases [2e4]. Of the all major alternative energy sources,
the production of hydrogen photo-catalytically by utilizing solar spectrum has been a great strategy due to its clean, low cost, environmentally friendly aspects [5,6]. Semiconductor nanostructured materials with a wide tunable band gap and a large exciton binding energy are of interest due to their exceptionally good properties and auspicious technological applications [7e10]. Since the first demonstration of photoelectrochemical H2 generation by Fujishima and Honda [11], intensive research efforts have been devoted towards semiconductor photocatalytic generation of H2 [12]. The utilization
* Corresponding author. Fax: þ91 802 360 3124. E-mail address: [email protected] (N. Kottam). http://dx.doi.org/10.1016/j.ijhydene.2016.11.099 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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of metal oxide semiconductors for light harvesting and H2 generation by splitting of water through photolysis is very promising [13]. In recent past notable effort has been made toward making semiconductor nanostructures with suitable materials synergize their properties [14e16]. Anatase TiO2 has proven to be a suitable material for hydrogen generation among various semiconductor oxide photocatalyst due to its long-term stability for the photo- and chemical corrosion [15,17,18]. There is intense effort in last decade to produce the potential photocatalyst alternative to TiO2 because of its wide band gap limits photocatalyst is not very suitable for many applications [19]. In this contest, ZnO is a promising semiconductor with the large exciton binding energy (60 meV) at room temperature [20,21]. Also, oxygen vacancies in ZnO known to induce band gap and increase its visible light photocatalytic activity [22,23]. The various ZnO materials and its devices were thoroughly studied and reviewed extensively [24e27]. Several ZnO nanostructures have been investigated for photochemical and photoelectrochemical H2 generation [16,28e31]. The metalemetal oxide, oxide-sulphide, Oxidenitride heterostructures [32,33] and noble-metal/oxide nanocrystals show excellent photocatalytic performance [34e38]. Cheng et al., observed better photocatalytic properties due to enhanced charge transfer and separation process from ZnOeTiO2 hybrid structures [39]. The good H2 evolution and apparent quantum yield have been obtained with ZnO/Cd nanostructures in UV and visible light irradiation [40e43]. The major disadvantage of ZnO photocatalyst is that it suffers from photo-corrosion [44,45] and it has been suppressed by doping, decorating with the optimum amount of carbonaceous material or by modifying the synthetic protocol [46,47]. In this article, we demonstrate a new green strategy to synthesize ZnO nanoparticles using Moringaoleifera natural extract as a fuel. The obtained oxygen-deficient ZnO nanoparticles show enhanced H2 production as well as good photostability under UVeVis light irradiation.
diffractometer. The nanoparticles were imaged using a Nova NanoSEM 600 FESEM system (FEI Company) fitted with Energy Dispersive X-ray Analysis (EDX). Transmission electron microscopy (TEM) images were recorded on a JEOL TEM 3010 instrument fitted with a Gatan CCD camera operating at an accelerating voltage of 300 kV. Electronic absorption spectra were recorded with a PerkineElmer Lambda 900 UV/Vis/NIR spectrometer in the diffuse reflectance mode. Photoluminescence measurements were carried out with a Fluorolog-3 spectrophotometer using a Horiba JobinYvon Xe lamp as light source. Raman spectra were recorded in the backscattering geometry using a 632 nm HeNe laser with a JobinYvonLabRam HR spectrometer.
Photochemical H2 generation H2 evolution experiments of ZnO nanoparticle (ZnO-2, 6, 10, 14) were carried out via photochemical splitting water and measured by gas chromatography at room temperature (25 C). The quartz reactor of 130 mL capacity has been used for the purpose. IR radiations from the light source were removed by circulating water and also maintain the normal temperature. In the reactor, 20 mg catalyst and 75 mL water were added followed by sonication of about 20 min to get homogeneous dispersion of nanoparticles. Na2S and Na2SO3 have been used as sacrificial agents for hole scavenger (pH ~13). The reactor was then de-aerated by passing N2for 10 min to remove the dissolved oxygen. Thereafter, the reactor was irradiated with 400 W Xenon lamp (UVeVis light source) and hydrogen gas was quantified using a gas-tight syringe by injecting 5 mL at every 30 min interval by Perkin Elmer Clarus 580 GC chromatograph equipped with a thermal conductivity detector (TCD).
Results and discussion Experimental All the chemicals used in the experiment brought from commercial sources and used without further purification. In a typical synthesis, 2.97 g of zinc nitrate hexahydrate was dissolved in 2, 6, 10 and 14 mL of Moringaoleifera natural extract and 10 mL of water. The components were mixed thoroughly by stirring for 15 min. The obtained homogeneous solution is heated on a hot plate with a stirrer to form gel kind of product. The gel is then transferred into the alumina crucible and kept it in a preheated muffle furnace maintained at 400 C. Smouldering type of the reaction takes place with the liberation of gases and within 5 min, nanocrystalline ZnO is obtained. The obtained product was calcined at 500 C for 3 h to remove the impurities. The synthesized products were named as ZnO-2, ZnO-6, ZnO-10 and ZnO-14 in accordance with the fuel amount.
Material characterization Powder X-ray diffraction (PXRD) patterns were collected with a Bruker D8 Discover diffractometer and Rigaku-99
ZnO nanoparticles (NPs) were prepared using Moringaoleifera natural extract by solution combustion process. 2e14 ml of natural extract was employed for the synthesis, of the various proportions. We found that 10 ml is the optimum amount to get nanoparticles with definite size and enhanced properties. The amount of fuel plays a crucial role to get the proper nanostructures with definite shape and size. As obtained NPs were characterized using several techniques and compare properties with commercially available bulk ZnO. Powder X-ray diffraction was employed to confirm the phase and crystallinity of ZnO NPs. Fig. 1 show PXRD patterns of ZnO-2, ZnO-6, ZnO-10 and ZnO-14, all the samples exhibit intense diffraction signals indicate good crystallinity. The diffraction planes (100), (002), (101), (102), (110) and (103) were readily indexed to standard wurtzite structure (ICSD #290968) A). The XRD with P63mc space group (a ¼ 3.2527 and c ¼ 5.2016 reflections of ZnO-10 are quite broad and less intense compared to ZnO-2, 6 and 14. The calculated crystallite size by Scherrer equation using the full width half-maximum values found to be 21.6 nm. Raman analysis is an effective tool to investigate the structure, crystallinity and defects in nanomaterials. Optical phonon vibrations of bulk, as well as
Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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Fig. 1 e X-ray powder diffraction patterns of hexagonal ZnO synthesized under different ratio of precursor via green synthesis.
nanostructured ZnO, were well studied [48,49]. Fig. 2 shows the Raman spectra of ZnO nanoparticles recorded using 632 nm green laser in backscattering geometry. We have observed mainly three phonon modes which includes A1, E2 high and E1 (LO). The high intense band observed at 439 cm1 assigned to the ZnO nonpolar optical phonon (E2 high) mode. The weak signal centred at 583 cm1 assigned to E1 (LO) mode of originating from oxygen deficiency [22]. The peak appeared at 332 cm1 (3E2HeE2L) is a multi phonon Raman mode arising from zone boundary phonons. The A1 mode at 380 arises due to ZneO polar bond lattice vibrations. The asymmetry and broadening of Raman signals are characteristics of NPs [50]. We observed more broadening in the case ZnO-10 and consisted with XRD results. There is no peak shift observed for the samples hence it indicates all the samples having similar crystallization, defects, and phonon vibrations.
Fig. 2 e Raman spectra of ZnO-2, ZnO-6, ZnO-10 and ZnO14 obtained in backscattering geometry using 632 nm green laser.
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The surface morphology and the size of ZnO particles were imaged using Field emission scanning electron microscopic (FESEM) analysis. Fig. 3 shows SEM images with the surface morphology of highly crystalline ZnO-10 and ZnO-14. The nanoparticles appear to be spherical in shape with the average size of 20e150 nm. SEM images of all samples show an agglomeration of the particles and are much lower in ZnO-10. The energy dispersive X-ray spectra of the samples clearly show the presence of Zn and O. No other signal was observed within the detection limit of EDS (see SI, Fig. S1). SEM images of the ZnO-6 and ZnO-14 displayed in Fig. S2 (see SI) and show similar features. Further, the morphology was confirmed by High-resolution transmission electron microscopic (HRTEM) analysis. Fig. 4 show the TEM images of ZnO-10 and ZnO-14 nanoparticles, the individual NPs identified and the lattice fringes of the particle have been resolved in HRTEM. Selected area electron diffraction (SAED) images (Fig. 4d) show the hexagonal patterns confirm the wurtzite structure and in agreement with XRD results. The lattice fringes in HRTEM image reveals the d spacing of 0.28 and 0.19 nm which corresponds to (001) and (101) planes of wurtzite ZnO (Fig. S3). The spherical nanoparticles clustering into small heaps as displayed in SEM and TEM and appeared to be bigger in size compared to calculated crystallite size from Scherrer formula. The UVeVisible optical diffusion reflectance spectra were shown in Fig. 5a. All the samples show characteristic strong absorption maximum below 400 nm corresponding to the band gap of ZnO. The absorption maximum is red shifted compared to bulk and shift increases as the fuel proportion increases. The band gap of the samples was obtained by extrapolating the linear portion of the KubelkaeMunk function as shown in the Fig. 5b. The obtained band gap values are 2.92 (ZnO-2), 3.05 (ZnO-6) 3.12 (ZnO10) and 3.10 (ZnO-14). NPs have a wide range of size distribution; their absorption can be attributed to intrinsic band gap of ZnO due to the electron transitions from the valence band to the conduction band [51]. The obtained band gap values are in good agreement with the reported values for ZnO [52]. All the samples prepared here showed a red shift in the band gap compared to bulk which can be attributed to impurity levels i.e. oxygen vacancies present in the sample [22]. It is clear that as the amount of fuel increases the particle size decreases and band gap increases due to quantum size effects. Photoluminescence measurements are helpful to reveal charge transfer and carrier trapping in semiconducting materials [53]. The room temperature PL spectra of ZnO-2, 6, 10 and ZnO-14 were shown in Fig. 6a. A strong PL band at 380 nm and a broad-band around 580 nm were observed in both the samples. In general, the PL emissions in a semiconductor are originated from the radiative recombination of photogenerated electrons and holes, and two major photophysical processes. The narrow violet UV emission around 380 nm is a near band edge emission originates from excitonic recombination of photogenerated electrons [54]. The green-yellow emission around 580 nm in visible region may be due to oxygen defects or Zn interstitials [16]. To confirm the band edge emission and defect related emission we collected the excitation spectra of ZnO-10 (Fig. 6b). The strong absorption peak at 350 nm corresponds to band gap and the weak signal around 400 nm due to oxygen vacancies. The CIE diagram for
Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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Fig. 3 e FESEM images of ZnO-10 (a,b) and ZnO-14 (c, d) in different magnification and corresponding particle size.
Fig. 4 e TEM images with different magnifications of ZnO-10 (aec) and ZnO-14 (eeg), SAED patterns of ZnO-10 (d) and ZnO14 (h).
Fig. 5 e UVeVisible diffuse reflectance spectra (a) and the energy band gap (b) of the as-prepared of ZnO-2, ZnO-6, ZnO-10 and ZnO-14. Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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Fig. 6 e (a) Photoluminescence spectra of ZnO-2, ZnO-6, ZnO-10, ZnO-14 and (b) corresponding excitation spectra of ZnO-10 to confirm band edge emission.
Fig. 7 e Schematic representation of the mechanism of charge separation and photocatalytic hydrogen evolution in the presence of Na2S, Na2SO3 sacrificial agents under UVeVis light irradiation.
ZnO-10 emission has been constructed, the X and Y coordinates are found to be 0.170 and 0.005 for band edge emission and 0.072 and 0.0420 for defect related emission (Fig. S4). ZnO is known to be very good photocatalyst for degradation of dye and other organic pollutants under UVeVis light irradiation [31,38]. The photocatalytic activity of the semiconductor oxides depends on various factors like chemical composition, morphology, particles size, crystallinity, vacancies, surface groups and surface area. The effect of above parameters on the ZnO photocatalytic activity is well studied [55]. ZnO nanostructure based photochemical and photoelectrochemical generation of hydrogen from water splitting were known [28,41]. Photocatalytic H2 generation measurements were conducted in an aqueous solution containing Na2S and Na2SO3 as sacrificial agents under the UVeVis light irradiation. Fig. 7 shows photocatalytic hydrogen production mechanism for the designed photocatalyst in the presence of sacrificial agents. Fig. 8(a,b) shows the H2 production by splitting of water under UVeVis light for ZnO NPs. All the samples have clearly show H2 evolution and increases linearly
Fig. 8 e (a) Hydrogen generation as a function of time with ZnO nanoparticles under irradiation of UVeVisible light in the presence of Na2S and Na2SO3 were used as the hole scavengers (b) Performance of ZnO NPs for photochemical H2 generation by varying the amount of photocatalyst under irradiation of UVeVis light. Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099
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with respect to time. Amongst, we observed a higher rate of H2 evolution (341.9 mmolg1) for ZnO-10 compared to other samples which may be due to its smaller particle size. Moreover, the experiments were carried out for bulk ZnO show very poor activity compared to ZnO NPs. In addition, the H2 evolution experiments were performed by varying the amount of catalyst (10 mg, 20 mg, 30 mg) for which a catalyst showed higher catalytic activity in the above studies (ZnO-10). The 20 mg catalysts show higher H2 production of 341.9 mmolg1 compared to 10 and 30 mg (66.96 and 28.13 mmolg1) in 2 h duration. Hence, 20 mg is the optimum catalyst concentration for 75 mL water and the details are shown in Fig. 8b. The higher catalytic activity of as-prepared NPs is attributed to a large amount of oxygen vacancies present in the sample which consisted with the earlier reports [22]. ZnO NPs shows very good photostability without undergoing photo-corrosion during the reaction. We expect that the stability may due to residual carbon present on the NPs even after calcination.
Conclusions We have successfully synthesized ZnO nanoparticles via green route employing Moringaoleifera natural extract as a fuel. The optimum amount of fuel ratio is very important to get desired size, shape and properties of the NPs. XRD and TEM studies show that ZnO is crystallized with hexagonal structure. ZnO NPs show better photocatalytic H2 generation compare to bulk and other nanostructures with increased photostability. The better photocatalytic activity is attributed to presence of oxygen defects which were confirmed by optical characterization. The novel synthetic strategy may open a new way to obtain many other semiconductor nanoparticles with improved properties.
Acknowledgements G. Nagaraju thanks to DST-Nano Mission, (SR/NM/NS-1226/ 2013) Govt. of India, for funding.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.11.099.
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Please cite this article in press as: Archana B, et al., Enhanced photocatalytic hydrogen generation and photostability of ZnO nanoparticles obtained via green synthesis, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.11.099