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Ag nanowires nanocomposites as plasmonic photocatalysts and investigation of the effect of concentration and diameter size of Ag nanowires on their photocatalytic performance
Accepted Manuscript Preparation of ZnO nanoparticles/Ag nanowires nanocomposites as plasmonic photocatalysts and investigation of the effect of concentration and diameter size of Ag nanowires on their photocatalytic performance Marzieh Khademalrasool, Mansoor Farbod, Azam Iraji zad PII:
S0925-8388(16)30028-7
DOI:
10.1016/j.jallcom.2016.01.028
Reference:
JALCOM 36379
To appear in:
Journal of Alloys and Compounds
Received Date: 4 October 2015 Revised Date:
18 November 2015
Accepted Date: 4 January 2016
Please cite this article as: M. Khademalrasool, M. Farbod, A. Iraji zad, Preparation of ZnO nanoparticles/ Ag nanowires nanocomposites as plasmonic photocatalysts and investigation of the effect of concentration and diameter size of Ag nanowires on their photocatalytic performance, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.01.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preparation of ZnO nanoparticles/Ag nanowires nanocomposites as plasmonic photocatalysts and investigation of the effect of concentration and diameter
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size of Ag nanowires on their photocatalytic performance
a,b
Physics Department, Shahid Chamran University of Ahvaz, Ahvaz, I.R. Iran
Physics Department, Sharif University of Technology, Tehran, I.R. Iran
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c
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Marzieh Khademalrasool a,*, Mansoor Farbod b, Azam Iraji zad c
*Corresponding author e-mail: [email protected]
Abstract
In this work the plasmonic photocatalytic activity of ZnO/noble metals nanocomposites has been
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investigated. Ag nanowires and Au nanospheres were prepared and mixed with ZnO in order to prepare the nanocomposites. It was demonstrated that the composites containing the plasmonic Ag nanowires and ZnO nanoparticles showed a significant photocatalytic activity enhancement compared to the pure ZnO in decomposition of methylene blue (MB). By investigation of
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mechanisms governing the performance of plasmonic photocatalysts, it was found that the photocatalytic activity enhancement can be attributed to the energy transfer from Ag nanowires
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to the ZnO nanoparticles induced by the surface plasmonic resonance (SPR). Such energy transfer is due to increasing the average path length of the photons in the composite and the SPRinduced local electromagnetic field near the surface of Ag nanowires. This mechanism predicts an enhancement in the concentration of charge carriers at the semiconductor surface by which the photocatalytic activity improves. The results showed that the photocatalytic activity enhancement was depended on the diameter size and concentration of Ag nanowires in the composites. Our studies showed that the composite containing 4.5 wt % of Ag nanowires with mean diameter size of 280 nm exhibited the highest photocatalytic activity enhancement compared to the pure ZnO nanoparticles. Furthermore, ZnO/Au nanospheres composite
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presented a little enhancement in photocatalytic activity compared to the composites containing Ag nanowires.
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Keywords: Ag nanowires; plasmonic photocatalyst; ZnO nanoparticles; Au nanosphere; polyolmediated solvothermal; high-power LED photoreactor.
1. Introduction
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Zinc oxide (ZnO) is a metal oxide semiconductor that has been used widely for the
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different applications, such as photocatalytic degradation of organic pollutants in water and wastewater [1]. However, ZnO photocatalyst has a number of drawbacks including the large band gap, ~3.0 eV and fast recombination rates of electron-hole pairs. These drawbacks have led to low ability of ZnO photocatalyst in conversion of incident photons to appropriate charge carriers at the semiconductor surface and so low rates of its photocatalytic activity. Until now,
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many attempts such as loading or depositing metal and non-metal nanostructures in combination with ZnO photocatalyst have been carried out to improve its photocatalytic activity [2, 3]. However, compared with pure ZnO, the light absorbance of doped ZnO is less which limits its
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photochemical activity.
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It is now known that the existence of the noble metal nanostructures such as silver and gold in semiconductors improves their photocatalytic efficiency. Such composite semiconductors are called “plasmonic photocatalysts” [4]. Such efficient photocatalytic activity enhancement has been explained by a number of physical mechanisms including: (i) electron transfer from the semiconductor to the noble metal nanostructures through creating schottky junction between them leading to separation of e¯ /h+ pairs which increases their lifetime [5, 6] (ii) electron transfer from the noble metal nanostructures to the semiconductor
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induced by noble metal surface plasmonic resonance (SPR) which increases the electrons concentration in the semiconductor [7] (iii) the localized SPR-induced heating of noble metal nanostructures due to the non-radiative decay of noble metal surface plasmons into the lattice
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vibrations in the noble metal nanostructures [8] and (iv) radiative energy transfer from the noble metal nanostructures to the semiconductor via an increase in the average path length of photons and the SPR-induced intense electric fields near the surface of the noble metal nanostructures,
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leading to the excitation electrons in semiconductors and increase the e¯ /h+ pairs concentration [4, 9-10]. The intensity of the local electric field has been found to be dependent on several
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factors such as the size, shape and composition of the noble metal nanostructures and their surrounding medium [11-13]. In this work, a simple solvothermal route was used to synthesis high yield silver nanowires with different diameter sizes. Then ZnO nanoparticles/Ag nanowire composites were prepared to verify the elemental physical mechanisms which govern the
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photocatalytic performance of the composites. In following, the effects of the Ag nanowires concentration and diameter size on the photocatalytic degradation of MB dye under near-
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ultraviolet (UV) light has been investigated.
2. Materials and methods
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2.1. Materials
Starting materials in solvothermal synthesis of silver nanowires are ethylene glycol (EG)
99.5%, silver nitrate (AgNO3), polyvinylpyrrolidone (PVP, MW~55000) and sodium chloride (NaCl)
were
used
without
further
purification.
Moreover,
zinc
acetate
dehydrate
(Zn(O2CCH3)2(H2O)2), sodium hydroxide (NaOH) and deionized (DI) water were employed as the starting materials for synthesis of ZnO nanoparticles.
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2.2. ZnO nanoparticles preparation method ZnO nanoparticles were synthesized with a co-precipitation method. In this process,
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Zn(O2CCH3)2(H2O)2 (0.5 M, 25 mL) and NaOH (0.5 M, 25mL) solutions in DI water were prepared and transferred into two plastic syringes. Using a syringe pump, two solutions were simultaneously transferred to a 250 mL beaker under vigorous stirring at a rate of 30 mL/h. After
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injection of two solutions, the resultant solution appeared opaque and white. The solution was stirred at room temperature for 20 minutes afterward the resultant precipitate was filtered and
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washed with DI water. The precipitate was dried in an oven at 90 ºC and ground to powder by agate mortar. The resultant powder was calcined in air at a temperature of at least 250 ºC for 3 hours.
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2.3. Silver nanowires preparation method
In a solvothermal typical procedure, EG solution of NaCl (6 mM, 0.19 mL) was added to EG solution of PVP (0.15 M, 10.92 mL) and the mixture was stirred vigorously by
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ultrasonication. Then using a syringe pump the mixture was injected drop wised into a vial containing 11.11 mL EG solution of 0.1 M AgNO3 under vigorous stirring at a rate of 45 mL/h to
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form silver chloride colloidal solution. The resultant solution was transferred into a stainless steel autoclave with a Teflon liner of 50 mL capacity, and then heated in a furnace at 160 ºC for 2.5 h. Finally, the autoclave was allowed to cool naturally to room temperature and the resultant nanowires were separated by centrifugation at 3500 rpm for 15 min and thoroughly washed with acetone. In order to synthesis the Ag nanowires with different diameters, a series of experiments
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were performed with the PVP/AgNO3 ratios of 1:1, 1.3:1, and 1.5:1. In these experiments PVP
2.4. Plasmonic photocatalysts preparation method
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concentration was changed and the AgNO3 concentration was fixed.
In order to prepare substrates, glass disks with diameter of 1.5 cm were excellently washed with dishwasher and DI water. Then they were ultrasonicated in DI water and acetone
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each for 15 min respectively to remove all pollutions. After this process, they were dried and transferred into a 3-aminopropyltrimethoxysilane: DI water solution with volume ratio of 1:100.
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The glass disks were remained in this solution for 24 hours and finally dried. Next, the synthesized Ag nanowires (with the molar ratios of PVP/AgNO3 1:1, 1.3:1, and 1.5:1) and ZnO semiconductor suspensions were independently prepared in pure ethanol and sonicated. ZnO/Ag nanowires composite suspensions were prepared by combining the pure
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nanostructures suspensions and mixed using agitation and sonication for at least 1 hour. ZnO/Ag nanowires composite photocatalysts were prepared by the drop coating method onto preprepared glassy substrate using ZnO/Ag nanowires composite suspensions and dried in a
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stationary ambient atmosphere. During this process a physical mixture of the two types of nanostructures on the substrate was resulted; however very small direct contact between ZnO
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nanoparicles and Ag nanowires was existed within the composites, which was due to the coating of the Ag nanowires by PVP stabilizer. All composite suspensions used to prepare composite photocatalysts were contained constant weight of ZnO nanoparticles (0.023 mgr in 2mL pure ethanol) and the various amounts of Ag nanowires. In our work, the different percentages of Ag nanowires in composites (e.g. 1, 4.5 and 8 wt %) were referred to weight percent of the Ag nanowires with attention to the constant weight of ZnO nanoparticles. Morphology of the
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samples were determined by a field emission scanning electron microscope (FESEM, Mira Tescan) and also a scanning electron microscope (SEM, LEO 1455VP). The UV-Visible absorption spectra of the products were also taken by a UV-Vis Spectrophotometer (PG
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Instruments Ltd model T80). All the measurements were carried out at room temperature. Powder X-ray diffraction (XRD) measurements were also performed using a Philips diffractometer (PW1840 model, Philips, Germany) to determine the crystal structure and phase
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composition of the silver nanowires and ZnO nanoparticles. The Fourier transform infrared spectrum (FT-IR) was used to obtain more information about the interaction between the PVP
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and the Ag on the surface of the nanowires. This analysis has been carried out by Bomem MBseries 102 FT-IR spectrometer. The samples for XRD and SEM measurements were prepared by dropping the solutions onto glass substrates and dried at room temperature.
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3. Results and discussion
Fig. 1(a) shows the XRD pattern of the ZnO nanoparticles. Study of standard data JCPDS 76-0704 confirmed that the synthesized materials are hexagonal ZnO phase with Wurtzite
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Structure [14]. However, the strong and narrow diffraction peaks of XRD pattern of the ZnO nanoparticles indicated the good crystallinity of the product. Fig. 1(b) shows the FESEM image
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of ZnO nanoparticles synthesized with average size of 30-40nm.
The XRD pattern of Ag nanowires synthesized using 0.13 M PVP (PVP/AgNO3=1.3:1)
has been exhibited in Fig. 2. Five diffraction peaks observed at 2θ= 38.12°, 44.79°, 64.04°, 78.23°, and 81.53° were respectively corresponded to the (111), (200), (220), (311), and (222) reflections of pure silver metal with face centered cubic (fcc) crystal structure with attention to
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standard data JCPDS 04-0783 [15]. The lattice constant calculated from this pattern was 4.085 Å, which was close to the reported value of 4.086 Å [16]. This suggests that pure Ag nanowires were obtained under the present synthesis conditions. The intensity ratios of (111)/(200) and
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(111)/(220) peaks were 3.52 and 26.94 respectively, which were relatively higher than the conventional 2.5 and 4 values [16]. Therefore, this would demonstrate that the {111} planes of silver tend to be oriented in the solvothermal method. In fact, it has been proved that polymer
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PVP plays a significant role in final size and shape control of Ag nanostructures. PVP molecules through their carbonyl groups coordination with Ag+ ions (Ag-O coordination) could be firmly
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adsorbed on the {100} planes of pentagonal twinned seeds prepared by adding NaCl as intermediate in reaction and leaved the {111} planes uncovered. Thereupon, new silver atoms are deposited on {111} planes that are in the ends of the Ag nanowires, leading to the rapid anisotropic growth along the <110> direction of Ag nanowires with a pentagonal cross section
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[17-21]. It can be deduced that PVP is necessary for the nanowires synthesis. Fig. 3 illustrates the role of PVP in the preparation of Ag nanowires schematically.
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The Ag nanowires indicate interesting SPR-induced optical properties which strongly depend on the mean diameter of the nanowires. SPR is a collective oscillation of conduction
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electrons in the noble metal upon interaction with electromagnetic field of incident light that creates strong electric field near the interface of noble metal and semiconductor [22]. The UVVis absorption spectra of the Ag nanowires usually show two absorption peaks which the stronger one is corresponded to the transverse plasmon resonance of the silver nanowires, and the weaker one is attributed to the longitudinal mode of long silver nanowires similar to that of the bulk silver [18, 23]. Fig. 4 shows the UV-Vis absorption spectra taken from the Ag
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nanowires synthesized with different molar ratios of PVP/AgNO3. From this figure, it is clear that there are two peaks centered at 409-422 nm and at around 350 nm which are corresponded to the transverse and longitudinal plasmon resonance, respectively. As can be observed, a blue
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shift for the strong absorption peak occurs by increase of the molar ratio from 1:1 to 1.5:1. This phenomenon indicates that the mean diameter size of silver nanowires is decreased. Therefore the silver nanowires with different diameters can be synthesized by changing the PVP
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concentration.
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Fig. 5 shows the SEM images of synthesized Ag nanowires using NaCl (0.1 mM), AgNO3 (0.1 M) with different molar ratios of PVP/AgNO3 as 1, 1.3, and 1.5. The lengths of all samples were ranged from 3 to 40 µm with average size of 10 µm. As can be observed, by increasing the molar ratio of PVP from 1up to 1.5, the mean Ag nanowires diameter decreased from 445 to 155 nm. It
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seems at low PVP concentrations would allow the growth of the seeds leading to nanorods and nanowires with bigger mean diameters. In real, here the nanowires can grow along both (100) and (110) facets. This phenomenon can be attributed to the solution viscosity. Whereas by
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increasing the PVP concentration all (100) facets of Ag seeds were covered by PVP leading to
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the rapid anisotropic growth along the <110> direction of Ag nanowires.
Fourier transform infrared spectroscopy is used to investigate the surface chemical state
of the final Ag nanowires and possible interactions between silver and PVP. Fig. 6 presents the FT-IR spectra of the Ag nanowires synthesized with the molar ratio of PVP/AgNO3 of 1.3 (Ag nanowires 1.3) and the pure PVP. The peaks which are located around 1660 and 2954 cm-1 are the –C=O and –C–H stretching vibration, respectively and the one around 1290 cm-1 is due to the
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stretching vibration of –C–N–. Moreover, the peak around 3438 cm-1 represents the O–H stretching vibration. These observations suggest that a bonding between PVP and silver crystal exists. Compared to the pure PVP, the carbonyl absorption peak of Ag-PVP indicates a red-shift
are in agreement with findings of other groups [18].
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from 1660 cm-1 to 1648 cm-1 which can be due to interaction of silver with PVP. These results
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The UV–visible absorption spectra of the composites consist of ZnO and Ag nanowires synthesized with the PVP/AgNO3 molar ratios of 1.3 (ZnO/Ag nanowires 1.3 composites) with
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different weight percents of Ag nanowires are shown in Fig.7. The figure exhibits that the ZnO/Ag nanowires 1.3 composite has two important absorption peaks. The first peak around 375 nm is attributed to the excitonic absorption peak of ZnO nanoparticles and the second one which is weak and broad around 410-422 nm was appeared when the Ag nanowires concentration was
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high into composite. The significant broadening of SPR peak is due to deposition of Ag nanowires onto ZnO nanoparticles with higher refractive index and reduction of inter-particle spacing with increase of Ag concentration which is corresponded to stronger electromagnetic
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coupling within the Ag nanowires deposited into ZnO/Ag nanowires composite.
4. Testing the photocatalyst In order to examine the photocatalytic activity of pure ZnO and ZnO/Ag nanowire
composite, the photodecomposition rate of MB under near-UV illumination using a home-made photoreactor was measured. This photoreactor is consisted of 6 high power light emitting diodes (HP-LEDs) with a peak wavelength centered at around 365 nm. The safety of the UV-HP-LEDs is higher than the mercury vapor lamps that have been widely used in photocatalytic processes.
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These UV-HP-LEDs were anchored on a steel cylinder shield with equal distances. The system was approximately illuminated under the direction of 45° by UV-HP-LEDs sources. Reactions were carried out at room temperature in a liquid phase batch reaction vessel with an open top.
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Photocatalyst substrates were arranged on the bottom of the vessel containing 4 mL of 0.01 mM MB solution in DI water with pH of 6.6 -7. The system then was allowed to remain in the dark for 1 hour prior to illumination. The solution was continuously stirred and aerated by bubbling
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air into the photoreactor. Fig. 8 shows the first order kinetic data of photocatalytic performance of the ZnO/Ag nanowires 1.3 composites for MB decomposition. The weight percents of the Ag
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nanowires in composites are 1, 4.5 and 8 wt %. These data were monitored as a function of light exposure time using UV-Vis spectroscopy. As can be observed, by increasing the weight percent of Ag nanowires from 1 up to 4.5, the photodecomposition rate of MB improved. Whereas by increasing the Ag nanowires concentration in composite from 4.5 up to 8 wt % photocatalytic
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activity of composite decreased. From these results, it is clear that the 4.5 weight percent of Ag nanowires 1.3 is an optimum concentration to show the plasmonic photocatalytic activity of the composite. In order to investigate the effect of Ag nanowires diameter size on the plasmonic
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photocatalytic activity, composite photocatalysts consist of ZnO and Ag nanowires synthesized with the PVP/AgNO3 molar ratios of 1, 1.3, and 1.5 (ZnO/Ag nanowires 1, ZnO/Ag nanowires
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1.3, and ZnO/Ag nanowires 1.5) with optimum weight percent of 4.5% were prepared. First order kinetic data of their performance has been presented in Fig. 9. The results illustrate that the Ag nanowires 1.3 with diameter size of 280 nm are the ones with optimum diameter size by which the plasmonic photocatalytic activity improves. In fact the increase of weight percent and diameter size of Ag nanowires enhances the unabsorbed photons scattering efficiently, increases the average path length of photons, and causes more absorption near the semiconductor surface.
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Moreover the silver SPR-induced oscillating electromagnetic field can efficiently increase nearby the semiconductor by increasing the diameter size and concentration of Ag nanowires. However if their values increase inordinate they will prevent light from reaching to the
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semiconductor surface. Thereupon plasmonic photocatalytic activity reduces.
5. Study of mechanisms governing the plasmonic photocatalysts performance
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In order to investigate the electron transfer rate from ZnO nanoparticles to Ag nanowires, the MB decomposition using ZnO/Au nanospheres (4.5 wt %) composite was carried out. In fact,
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such electron transfer through creating schottky junction between semiconductor and metal is built and led to degrease the recombination rate of e¯ /h+ pairs in semiconductor [24-27]. Au nanospheres have an electronic structure similar to Ag and a larger work function which suggests that there should be a stronger driving force to transfer electrons from ZnO to Au than Ag [28-
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30]. Moreover the Au nanosphere SPR is red-shifted relative to that of Ag and does not overlap with source spectrum. So there could not be SPR-induced mechanisms for the composite containing Au. That is why we chose Au nanospheres and prospected more enhancement in
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photocatalytic activity of composite consisting of Au than Ag. In this study, Au nanospheres were synthesized via the citrate reduction method and then dispersed in a 10:1 ethanol: H2O
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solution prior to use. The synthesized Au nanospheres had diameters of about 30 nm and were stabilized and protected by citrate ligands. The investigation showed a very small enhancement in photocatalytic activity of this composite in decomposition of MB compared to the pure ZnO nanoparticles. Thereupon, this suggests that the electron transfer rate from the ZnO nanoparticles to the Ag nanowires is not the main factor for the observed Ag-induced enhancement of the ZnO photocatalytic activity. This consequence can also be confirmed by measurement of PL emission
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intensity at around near band edge (NBE) emission peak of ZnO for ZnO/Ag nanowires 1.3 composite and pure ZnO with excitation wavelength of 325 nm (see Fig. 12 (a)). Many researches demonstrated that the PL emission intensity reduces by creating schottky junction and
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also degreasing recombination rate of e¯ /h+ pair. Whereas, Fig.12 (a) shows negligible reduction in PL intensity at around NBE emission peak of ZnO of composite compared to that of pure ZnO, it can be demonstrated that electron transfer from ZnO nanoparticles to Ag nanowires do
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not play significant role on such improvement of photocatalytic activity [31-33]. Its reason could be due to the week direct contact between metallic nanostructures (Au or Ag) and ZnO owing to
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the presence of nonconductive polymeric stabilizers on the surface of metallic nanostructures [30, 34-35].
In order to test whether the direct injection of electrons from the excited Ag nanowires surface plasmon states into ZnO has a role in photocatalytic activity, we measured the absorption
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spectra for ZnO/Ag nanowires 1.3 (4.5 wt %) composite in a de-aerated ethanol solution illuminated by the 365 nm HP-LED source as a function of illumination time (see Fig. 10). The de-aerated solution was used only in this set of experiments to avoid any possible electrons
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transfer from ZnO conduction band to oxygen orbitals. Ethanol, which is a most effective electron donor, can also be created a surrounding medium with low probability to accept electron
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and in fact prepared conditions with a great probability to accept electron by ZnO [30, 36]. It has been shown previously that charged the ZnO nanoparticles and Ag nanostructures exhibit a significant change in the ZnO absorption band and a red-shifted in surface plasmon resonance of Ag, respectively [37-41]. No significant difference was observed between the absorbance spectra and also difference spectrum of ZnO/Ag nanowires 1.3 composite and pure ZnO spectra showed a broad degrease in absorption of ZnO (because low losing of electrons in ZnO) and the partial
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displacement of the plasmon resonance peak (see Fig.11). The resultant changes are very small to justify the observed rate enhancements in photocatalytic activities. In fact, during the course of the in-situ UV-vis experiments, meaningful change in the places of ZnO absorption band and the
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plasmon peak of Ag has not been observed [30, 35]. So it can be suggested that significant charge transfer from Ag nanowires to ZnO do not exist and by attention to result of photocatalytic activity related to ZnO/Au nanospheres (4.5 wt %) composite just a very small
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charge transfer from ZnO nanoparticles to the Ag nanowires could be existed. Therefore charge transfer cannot account as main reason for the magnitude of the photocatalytic activity
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enhancement.
Based on some mechanisms [8], the localized SPR-induced heating of Ag nanowires can lead to thermo-chemical degradation of MB. To evaluate such a hypothesis, the degradation of MB with illuminated samples containing only pure Ag nanowires was measured. The results for Ag
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nanowires 1.3 are shown in Figures 8 and 9. These figures show that the Ag nanowires alone cannot decompose MB. Therefore the local Ag heating cannot be responsible for the photocatalytic degradation of MB. In real, our findings are consistent with previous reports
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which showed that local heating of metallic particles is only important for fairly small particles (<30 nm diameter). Since the Ag nanowires used in our studies have significantly larger sizes it
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can be supposed that the local heating of Ag nanostructures is not important factor to enhance the existing reaction rate [42-43]. Above investigations show that the mentioned mechanisms cannot be the major reason for photocatalytic activity enhancement and in fact it should be explained by SPR-induced energy transfer from the Ag nanowires to the ZnO nanoparticles. This energy transfer is due to increasing the photons path length through the radiative Rayleigh scattering process in the
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composite and the SPR-induced local electromagnetic field near the surface of Ag nanowires [35]. In fact the one of main features of Ag nanowires leading to enhancement of photocatalytic activity rate of the composites is their large extinction cross section (more specifically, the
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scattering cross section) [35]. Based on this mechanism the additional e¯ /h+ pairs at ZnO nanoparticles surface are produced, where the recombination rate decreased. To verify the SPRinduced energy transfer mechanism, the PL emission of ZnO/Ag nanowires 1.3 (4.5 wt %) was
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measured at several excitation wavelengths (325, 345, and 365nm), normalized by the emission for pure ZnO nanoparticles excited at the same wavelengths. PL emission enhancements at
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around NBE emission peak of ZnO for composite are plotted as a function of the excitation wavelength in Fig. 12 (b). From the figure it is clear that the highest PL emission enhancement of composite is observed for excitation wavelengths close to the wavelength at which there is the largest overlap between the Ag surface plasmon resonance, the ZnO absorption spectrum and the
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source spectrum. These data are fully compatible with the UV-vis absorbance of existing Ag nanowires in the composite (see Fig.10). These results indicate that the observed PL emission enhancement from ZnO is due to the SPR-induced enhancement of the absorbance of the
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composite and also to e¯ /h+ pairs concentration at ZnO nanoparticles surface which improves their photocatalytic activity. These essentially require an overlap between the Ag SPR extinction
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spectrum, the semiconductor absorption spectrum and the source spectrum. This indicates that ZnO/Au nanosphere composite does not show any considerable photocatalytic performance enhancement due to the lack of such overlap for Au SPR extinction spectrum, the semiconductor absorption spectrum and the source spectrum during the experiment. Beside, the Fig. 13 shows a linear relationship between integrated areas under the extinction spectra in the region of the irradiation source, between 360 and 370 nm, for pure Ag nanowires
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with different concentrations (0, 1, and 4.5 wt%) and the measured MB decomposition rate for the ZnO/Ag nanowires composites consisted of these pure Ag nanowires. Since the Ag extinction is a consequence of the excitation of Ag surface plasmon states, such the qualitative
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planning between the rate enhancement and the Ag extinction and creating of linear relationship implies main role of the excitation of Ag surface plasmons for the enhancement of photocatalytic degradation rate in the composite systems. It also suggests that the enhanced photocatalytic
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reactivity of the composite systems can be directly corresponding to the Ag nanowires extinction cross section [30, 35]. However, plasmonic photocatalytic activity rate for the ZnO/Ag
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nanowires 1.3 (8 wt %) composite has been digressed from this linear relationship because light from reaching to the semiconductor surface was prevented by the increase of weight percent of Ag nanowires in this composite.
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6. Conclusions
It was shown that the presence of Ag nanowires in ZnO semiconductor/Ag nanowires composites called as “plasmonic photocatalyst” can enhance the e¯ /h+ pairs concentration of the
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photocatalyst and therefore improve their photocatalytic activity. The effects of the concentration and diameter size of Ag nanowires on the photocatalytic activity enhancement of the composite
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photocatalysts was investigated and the photocatalytic activity was optimized by controlling of them. The results showed that ZnO/Ag nanowires 1.3 (4.5 wt %) composite has the highest photocatalytic activity enhancement compared to the others. In this work, the physical mechanisms governing the plasmonic photocatalysts were also estimated. The results suggested that the efficient photocatalytic activity enhancement of the composite systems can be explained by SPR-induced energy transfer from the Ag nanowires to the ZnO nanoparticles. Such transfers
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are due to the increase in the photon path length in the composite and create the SPR-induced local electromagnetic fields near the surface of Ag nanowires. Based on this mechanism the e¯ /h+ pairs concentration increased at ZnO nanoparticles surface, where the recombination rates
is profitable for improvement of ZnO photocatalytic activity.
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Acknowledgement
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were significantly diminished. This work demonstrated that using the plasmonic nanostructures
The authors acknowledge Shahid-Chamran university of Ahvaz for financial support of
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this work and Jundi-Shapur University of Technology of Dezful for technical support of this work.
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Fig. 1. (a) XRD pattern, and (b) FESEM image of ZnO nanoparticles.
Fig. 2. XRD pattern of Ag nanowires synthesized with 0.13 M PVP.
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Fig. 3. Schematic illustration of the PVP role in the preparation of Ag nanowires.
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Fig. 4. The UV–Vis absorption spectra of the synthesized Ag nanowires.
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Fig. 5. SEM images of Ag nanowires prepared with different molar ratios of PVP/AgNO3 (a) 1, (b) 1.3, (c) 1.5.
Fig. 6. FT-IR spectra of Ag nanowires 1.3-PVP (■) and the pure PVP (●).
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Fig. 7. UV-vis absorbance spectra of the ZnO/Ag nanowires 1.3 composites with different weight percents of Ag
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nanowires.
Fig. 8. Kinetic data of photocatalytic performance for MB decomposition by pure ZnO, pure Ag nanowires, and ZnO/Ag nanowires 1.3 composites with different concentrations of Ag nanowires1, 4.5, and 8 wt %. Inset: top) UV– Vis absorption spectra for 0.01 mM MB decomposition in 20 min intervals by ZnO/Ag nanowires 1.3 (4.5 wt %).
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Fig. 9. Kinetic data of photocatalytic performance for MB decomposition by pure ZnO, pure Ag nanowires, ZnO/Au nanosphere, ZnO/Ag nanowires 1, ZnO/Ag nanowires 1.3, and ZnO/Ag nanowires 1.5 composites with
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concentration of 4.5 wt %. Inset: top) normalized dye concentration of MB versus reaction time for them.
Fig.10. Testing the charge transfer mechanism from Ag nanowires to ZnO.
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Fig. 11. Experimental absorbance UV-visible spectum for Ag nanowires 1.3 and a difference spectrum calculated by subtracting the contribution of ZnO from the composite spectrum.
Fig.12. (a) photoluminescence spectra of pure ZnO and ZnO/Ag nanowires 1.3 (4.5 wt %) with excitation wavelength of 325 nm, (b) photoluminescence emission enhancement near band edge (NBE) emission peak of ZnO
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nanoparticles as a function of excitation wavelength. Inset: top) photoluminescence spectra of pure ZnO and
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ZnO/Ag nanowires 1.3 (4.5 wt %) with excitation wavelength of 365nm.
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Fig. 13. (a) Rate constant measured for ZnO/Ag nanowires 1.3 composites is plotted as a function of the Ag plasmon intensity at the source wavelength for samples containing only Ag nanowires.
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Highlights:
We synthesized ZnO nanoparticles/Ag nanowires plasmonic photocatalysts. We studied the mechanisms governing the performance of plasmonic photocatalysts.
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Energy transfer from Ag nanowire to the ZnO nanoparticle was led to enhancement.
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We researched the effects of Ag nanowires size and concentration on photoactivity.
S0925-8388(16)30028-7
DOI:
10.1016/j.jallcom.2016.01.028
Reference:
JALCOM 36379
To appear in:
Journal of Alloys and Compounds
Received Date: 4 October 2015 Revised Date:
18 November 2015
Accepted Date: 4 January 2016
Please cite this article as: M. Khademalrasool, M. Farbod, A. Iraji zad, Preparation of ZnO nanoparticles/ Ag nanowires nanocomposites as plasmonic photocatalysts and investigation of the effect of concentration and diameter size of Ag nanowires on their photocatalytic performance, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.01.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preparation of ZnO nanoparticles/Ag nanowires nanocomposites as plasmonic photocatalysts and investigation of the effect of concentration and diameter
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size of Ag nanowires on their photocatalytic performance
a,b
Physics Department, Shahid Chamran University of Ahvaz, Ahvaz, I.R. Iran
Physics Department, Sharif University of Technology, Tehran, I.R. Iran
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c
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Marzieh Khademalrasool a,*, Mansoor Farbod b, Azam Iraji zad c
*Corresponding author e-mail: [email protected]
Abstract
In this work the plasmonic photocatalytic activity of ZnO/noble metals nanocomposites has been
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investigated. Ag nanowires and Au nanospheres were prepared and mixed with ZnO in order to prepare the nanocomposites. It was demonstrated that the composites containing the plasmonic Ag nanowires and ZnO nanoparticles showed a significant photocatalytic activity enhancement compared to the pure ZnO in decomposition of methylene blue (MB). By investigation of
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mechanisms governing the performance of plasmonic photocatalysts, it was found that the photocatalytic activity enhancement can be attributed to the energy transfer from Ag nanowires
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to the ZnO nanoparticles induced by the surface plasmonic resonance (SPR). Such energy transfer is due to increasing the average path length of the photons in the composite and the SPRinduced local electromagnetic field near the surface of Ag nanowires. This mechanism predicts an enhancement in the concentration of charge carriers at the semiconductor surface by which the photocatalytic activity improves. The results showed that the photocatalytic activity enhancement was depended on the diameter size and concentration of Ag nanowires in the composites. Our studies showed that the composite containing 4.5 wt % of Ag nanowires with mean diameter size of 280 nm exhibited the highest photocatalytic activity enhancement compared to the pure ZnO nanoparticles. Furthermore, ZnO/Au nanospheres composite
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presented a little enhancement in photocatalytic activity compared to the composites containing Ag nanowires.
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Keywords: Ag nanowires; plasmonic photocatalyst; ZnO nanoparticles; Au nanosphere; polyolmediated solvothermal; high-power LED photoreactor.
1. Introduction
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Zinc oxide (ZnO) is a metal oxide semiconductor that has been used widely for the
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different applications, such as photocatalytic degradation of organic pollutants in water and wastewater [1]. However, ZnO photocatalyst has a number of drawbacks including the large band gap, ~3.0 eV and fast recombination rates of electron-hole pairs. These drawbacks have led to low ability of ZnO photocatalyst in conversion of incident photons to appropriate charge carriers at the semiconductor surface and so low rates of its photocatalytic activity. Until now,
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many attempts such as loading or depositing metal and non-metal nanostructures in combination with ZnO photocatalyst have been carried out to improve its photocatalytic activity [2, 3]. However, compared with pure ZnO, the light absorbance of doped ZnO is less which limits its
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photochemical activity.
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It is now known that the existence of the noble metal nanostructures such as silver and gold in semiconductors improves their photocatalytic efficiency. Such composite semiconductors are called “plasmonic photocatalysts” [4]. Such efficient photocatalytic activity enhancement has been explained by a number of physical mechanisms including: (i) electron transfer from the semiconductor to the noble metal nanostructures through creating schottky junction between them leading to separation of e¯ /h+ pairs which increases their lifetime [5, 6] (ii) electron transfer from the noble metal nanostructures to the semiconductor
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induced by noble metal surface plasmonic resonance (SPR) which increases the electrons concentration in the semiconductor [7] (iii) the localized SPR-induced heating of noble metal nanostructures due to the non-radiative decay of noble metal surface plasmons into the lattice
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vibrations in the noble metal nanostructures [8] and (iv) radiative energy transfer from the noble metal nanostructures to the semiconductor via an increase in the average path length of photons and the SPR-induced intense electric fields near the surface of the noble metal nanostructures,
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leading to the excitation electrons in semiconductors and increase the e¯ /h+ pairs concentration [4, 9-10]. The intensity of the local electric field has been found to be dependent on several
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factors such as the size, shape and composition of the noble metal nanostructures and their surrounding medium [11-13]. In this work, a simple solvothermal route was used to synthesis high yield silver nanowires with different diameter sizes. Then ZnO nanoparticles/Ag nanowire composites were prepared to verify the elemental physical mechanisms which govern the
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photocatalytic performance of the composites. In following, the effects of the Ag nanowires concentration and diameter size on the photocatalytic degradation of MB dye under near-
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ultraviolet (UV) light has been investigated.
2. Materials and methods
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2.1. Materials
Starting materials in solvothermal synthesis of silver nanowires are ethylene glycol (EG)
99.5%, silver nitrate (AgNO3), polyvinylpyrrolidone (PVP, MW~55000) and sodium chloride (NaCl)
were
used
without
further
purification.
Moreover,
zinc
acetate
dehydrate
(Zn(O2CCH3)2(H2O)2), sodium hydroxide (NaOH) and deionized (DI) water were employed as the starting materials for synthesis of ZnO nanoparticles.
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2.2. ZnO nanoparticles preparation method ZnO nanoparticles were synthesized with a co-precipitation method. In this process,
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Zn(O2CCH3)2(H2O)2 (0.5 M, 25 mL) and NaOH (0.5 M, 25mL) solutions in DI water were prepared and transferred into two plastic syringes. Using a syringe pump, two solutions were simultaneously transferred to a 250 mL beaker under vigorous stirring at a rate of 30 mL/h. After
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injection of two solutions, the resultant solution appeared opaque and white. The solution was stirred at room temperature for 20 minutes afterward the resultant precipitate was filtered and
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washed with DI water. The precipitate was dried in an oven at 90 ºC and ground to powder by agate mortar. The resultant powder was calcined in air at a temperature of at least 250 ºC for 3 hours.
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2.3. Silver nanowires preparation method
In a solvothermal typical procedure, EG solution of NaCl (6 mM, 0.19 mL) was added to EG solution of PVP (0.15 M, 10.92 mL) and the mixture was stirred vigorously by
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ultrasonication. Then using a syringe pump the mixture was injected drop wised into a vial containing 11.11 mL EG solution of 0.1 M AgNO3 under vigorous stirring at a rate of 45 mL/h to
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form silver chloride colloidal solution. The resultant solution was transferred into a stainless steel autoclave with a Teflon liner of 50 mL capacity, and then heated in a furnace at 160 ºC for 2.5 h. Finally, the autoclave was allowed to cool naturally to room temperature and the resultant nanowires were separated by centrifugation at 3500 rpm for 15 min and thoroughly washed with acetone. In order to synthesis the Ag nanowires with different diameters, a series of experiments
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were performed with the PVP/AgNO3 ratios of 1:1, 1.3:1, and 1.5:1. In these experiments PVP
2.4. Plasmonic photocatalysts preparation method
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concentration was changed and the AgNO3 concentration was fixed.
In order to prepare substrates, glass disks with diameter of 1.5 cm were excellently washed with dishwasher and DI water. Then they were ultrasonicated in DI water and acetone
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each for 15 min respectively to remove all pollutions. After this process, they were dried and transferred into a 3-aminopropyltrimethoxysilane: DI water solution with volume ratio of 1:100.
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The glass disks were remained in this solution for 24 hours and finally dried. Next, the synthesized Ag nanowires (with the molar ratios of PVP/AgNO3 1:1, 1.3:1, and 1.5:1) and ZnO semiconductor suspensions were independently prepared in pure ethanol and sonicated. ZnO/Ag nanowires composite suspensions were prepared by combining the pure
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nanostructures suspensions and mixed using agitation and sonication for at least 1 hour. ZnO/Ag nanowires composite photocatalysts were prepared by the drop coating method onto preprepared glassy substrate using ZnO/Ag nanowires composite suspensions and dried in a
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stationary ambient atmosphere. During this process a physical mixture of the two types of nanostructures on the substrate was resulted; however very small direct contact between ZnO
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nanoparicles and Ag nanowires was existed within the composites, which was due to the coating of the Ag nanowires by PVP stabilizer. All composite suspensions used to prepare composite photocatalysts were contained constant weight of ZnO nanoparticles (0.023 mgr in 2mL pure ethanol) and the various amounts of Ag nanowires. In our work, the different percentages of Ag nanowires in composites (e.g. 1, 4.5 and 8 wt %) were referred to weight percent of the Ag nanowires with attention to the constant weight of ZnO nanoparticles. Morphology of the
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samples were determined by a field emission scanning electron microscope (FESEM, Mira Tescan) and also a scanning electron microscope (SEM, LEO 1455VP). The UV-Visible absorption spectra of the products were also taken by a UV-Vis Spectrophotometer (PG
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Instruments Ltd model T80). All the measurements were carried out at room temperature. Powder X-ray diffraction (XRD) measurements were also performed using a Philips diffractometer (PW1840 model, Philips, Germany) to determine the crystal structure and phase
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composition of the silver nanowires and ZnO nanoparticles. The Fourier transform infrared spectrum (FT-IR) was used to obtain more information about the interaction between the PVP
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and the Ag on the surface of the nanowires. This analysis has been carried out by Bomem MBseries 102 FT-IR spectrometer. The samples for XRD and SEM measurements were prepared by dropping the solutions onto glass substrates and dried at room temperature.
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3. Results and discussion
Fig. 1(a) shows the XRD pattern of the ZnO nanoparticles. Study of standard data JCPDS 76-0704 confirmed that the synthesized materials are hexagonal ZnO phase with Wurtzite
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Structure [14]. However, the strong and narrow diffraction peaks of XRD pattern of the ZnO nanoparticles indicated the good crystallinity of the product. Fig. 1(b) shows the FESEM image
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of ZnO nanoparticles synthesized with average size of 30-40nm.
The XRD pattern of Ag nanowires synthesized using 0.13 M PVP (PVP/AgNO3=1.3:1)
has been exhibited in Fig. 2. Five diffraction peaks observed at 2θ= 38.12°, 44.79°, 64.04°, 78.23°, and 81.53° were respectively corresponded to the (111), (200), (220), (311), and (222) reflections of pure silver metal with face centered cubic (fcc) crystal structure with attention to
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standard data JCPDS 04-0783 [15]. The lattice constant calculated from this pattern was 4.085 Å, which was close to the reported value of 4.086 Å [16]. This suggests that pure Ag nanowires were obtained under the present synthesis conditions. The intensity ratios of (111)/(200) and
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(111)/(220) peaks were 3.52 and 26.94 respectively, which were relatively higher than the conventional 2.5 and 4 values [16]. Therefore, this would demonstrate that the {111} planes of silver tend to be oriented in the solvothermal method. In fact, it has been proved that polymer
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PVP plays a significant role in final size and shape control of Ag nanostructures. PVP molecules through their carbonyl groups coordination with Ag+ ions (Ag-O coordination) could be firmly
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adsorbed on the {100} planes of pentagonal twinned seeds prepared by adding NaCl as intermediate in reaction and leaved the {111} planes uncovered. Thereupon, new silver atoms are deposited on {111} planes that are in the ends of the Ag nanowires, leading to the rapid anisotropic growth along the <110> direction of Ag nanowires with a pentagonal cross section
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[17-21]. It can be deduced that PVP is necessary for the nanowires synthesis. Fig. 3 illustrates the role of PVP in the preparation of Ag nanowires schematically.
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The Ag nanowires indicate interesting SPR-induced optical properties which strongly depend on the mean diameter of the nanowires. SPR is a collective oscillation of conduction
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electrons in the noble metal upon interaction with electromagnetic field of incident light that creates strong electric field near the interface of noble metal and semiconductor [22]. The UVVis absorption spectra of the Ag nanowires usually show two absorption peaks which the stronger one is corresponded to the transverse plasmon resonance of the silver nanowires, and the weaker one is attributed to the longitudinal mode of long silver nanowires similar to that of the bulk silver [18, 23]. Fig. 4 shows the UV-Vis absorption spectra taken from the Ag
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nanowires synthesized with different molar ratios of PVP/AgNO3. From this figure, it is clear that there are two peaks centered at 409-422 nm and at around 350 nm which are corresponded to the transverse and longitudinal plasmon resonance, respectively. As can be observed, a blue
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shift for the strong absorption peak occurs by increase of the molar ratio from 1:1 to 1.5:1. This phenomenon indicates that the mean diameter size of silver nanowires is decreased. Therefore the silver nanowires with different diameters can be synthesized by changing the PVP
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concentration.
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Fig. 5 shows the SEM images of synthesized Ag nanowires using NaCl (0.1 mM), AgNO3 (0.1 M) with different molar ratios of PVP/AgNO3 as 1, 1.3, and 1.5. The lengths of all samples were ranged from 3 to 40 µm with average size of 10 µm. As can be observed, by increasing the molar ratio of PVP from 1up to 1.5, the mean Ag nanowires diameter decreased from 445 to 155 nm. It
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seems at low PVP concentrations would allow the growth of the seeds leading to nanorods and nanowires with bigger mean diameters. In real, here the nanowires can grow along both (100) and (110) facets. This phenomenon can be attributed to the solution viscosity. Whereas by
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increasing the PVP concentration all (100) facets of Ag seeds were covered by PVP leading to
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the rapid anisotropic growth along the <110> direction of Ag nanowires.
Fourier transform infrared spectroscopy is used to investigate the surface chemical state
of the final Ag nanowires and possible interactions between silver and PVP. Fig. 6 presents the FT-IR spectra of the Ag nanowires synthesized with the molar ratio of PVP/AgNO3 of 1.3 (Ag nanowires 1.3) and the pure PVP. The peaks which are located around 1660 and 2954 cm-1 are the –C=O and –C–H stretching vibration, respectively and the one around 1290 cm-1 is due to the
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stretching vibration of –C–N–. Moreover, the peak around 3438 cm-1 represents the O–H stretching vibration. These observations suggest that a bonding between PVP and silver crystal exists. Compared to the pure PVP, the carbonyl absorption peak of Ag-PVP indicates a red-shift
are in agreement with findings of other groups [18].
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from 1660 cm-1 to 1648 cm-1 which can be due to interaction of silver with PVP. These results
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The UV–visible absorption spectra of the composites consist of ZnO and Ag nanowires synthesized with the PVP/AgNO3 molar ratios of 1.3 (ZnO/Ag nanowires 1.3 composites) with
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different weight percents of Ag nanowires are shown in Fig.7. The figure exhibits that the ZnO/Ag nanowires 1.3 composite has two important absorption peaks. The first peak around 375 nm is attributed to the excitonic absorption peak of ZnO nanoparticles and the second one which is weak and broad around 410-422 nm was appeared when the Ag nanowires concentration was
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high into composite. The significant broadening of SPR peak is due to deposition of Ag nanowires onto ZnO nanoparticles with higher refractive index and reduction of inter-particle spacing with increase of Ag concentration which is corresponded to stronger electromagnetic
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coupling within the Ag nanowires deposited into ZnO/Ag nanowires composite.
4. Testing the photocatalyst In order to examine the photocatalytic activity of pure ZnO and ZnO/Ag nanowire
composite, the photodecomposition rate of MB under near-UV illumination using a home-made photoreactor was measured. This photoreactor is consisted of 6 high power light emitting diodes (HP-LEDs) with a peak wavelength centered at around 365 nm. The safety of the UV-HP-LEDs is higher than the mercury vapor lamps that have been widely used in photocatalytic processes.
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These UV-HP-LEDs were anchored on a steel cylinder shield with equal distances. The system was approximately illuminated under the direction of 45° by UV-HP-LEDs sources. Reactions were carried out at room temperature in a liquid phase batch reaction vessel with an open top.
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Photocatalyst substrates were arranged on the bottom of the vessel containing 4 mL of 0.01 mM MB solution in DI water with pH of 6.6 -7. The system then was allowed to remain in the dark for 1 hour prior to illumination. The solution was continuously stirred and aerated by bubbling
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air into the photoreactor. Fig. 8 shows the first order kinetic data of photocatalytic performance of the ZnO/Ag nanowires 1.3 composites for MB decomposition. The weight percents of the Ag
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nanowires in composites are 1, 4.5 and 8 wt %. These data were monitored as a function of light exposure time using UV-Vis spectroscopy. As can be observed, by increasing the weight percent of Ag nanowires from 1 up to 4.5, the photodecomposition rate of MB improved. Whereas by increasing the Ag nanowires concentration in composite from 4.5 up to 8 wt % photocatalytic
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activity of composite decreased. From these results, it is clear that the 4.5 weight percent of Ag nanowires 1.3 is an optimum concentration to show the plasmonic photocatalytic activity of the composite. In order to investigate the effect of Ag nanowires diameter size on the plasmonic
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photocatalytic activity, composite photocatalysts consist of ZnO and Ag nanowires synthesized with the PVP/AgNO3 molar ratios of 1, 1.3, and 1.5 (ZnO/Ag nanowires 1, ZnO/Ag nanowires
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1.3, and ZnO/Ag nanowires 1.5) with optimum weight percent of 4.5% were prepared. First order kinetic data of their performance has been presented in Fig. 9. The results illustrate that the Ag nanowires 1.3 with diameter size of 280 nm are the ones with optimum diameter size by which the plasmonic photocatalytic activity improves. In fact the increase of weight percent and diameter size of Ag nanowires enhances the unabsorbed photons scattering efficiently, increases the average path length of photons, and causes more absorption near the semiconductor surface.
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Moreover the silver SPR-induced oscillating electromagnetic field can efficiently increase nearby the semiconductor by increasing the diameter size and concentration of Ag nanowires. However if their values increase inordinate they will prevent light from reaching to the
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semiconductor surface. Thereupon plasmonic photocatalytic activity reduces.
5. Study of mechanisms governing the plasmonic photocatalysts performance
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In order to investigate the electron transfer rate from ZnO nanoparticles to Ag nanowires, the MB decomposition using ZnO/Au nanospheres (4.5 wt %) composite was carried out. In fact,
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such electron transfer through creating schottky junction between semiconductor and metal is built and led to degrease the recombination rate of e¯ /h+ pairs in semiconductor [24-27]. Au nanospheres have an electronic structure similar to Ag and a larger work function which suggests that there should be a stronger driving force to transfer electrons from ZnO to Au than Ag [28-
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30]. Moreover the Au nanosphere SPR is red-shifted relative to that of Ag and does not overlap with source spectrum. So there could not be SPR-induced mechanisms for the composite containing Au. That is why we chose Au nanospheres and prospected more enhancement in
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photocatalytic activity of composite consisting of Au than Ag. In this study, Au nanospheres were synthesized via the citrate reduction method and then dispersed in a 10:1 ethanol: H2O
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solution prior to use. The synthesized Au nanospheres had diameters of about 30 nm and were stabilized and protected by citrate ligands. The investigation showed a very small enhancement in photocatalytic activity of this composite in decomposition of MB compared to the pure ZnO nanoparticles. Thereupon, this suggests that the electron transfer rate from the ZnO nanoparticles to the Ag nanowires is not the main factor for the observed Ag-induced enhancement of the ZnO photocatalytic activity. This consequence can also be confirmed by measurement of PL emission
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intensity at around near band edge (NBE) emission peak of ZnO for ZnO/Ag nanowires 1.3 composite and pure ZnO with excitation wavelength of 325 nm (see Fig. 12 (a)). Many researches demonstrated that the PL emission intensity reduces by creating schottky junction and
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also degreasing recombination rate of e¯ /h+ pair. Whereas, Fig.12 (a) shows negligible reduction in PL intensity at around NBE emission peak of ZnO of composite compared to that of pure ZnO, it can be demonstrated that electron transfer from ZnO nanoparticles to Ag nanowires do
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not play significant role on such improvement of photocatalytic activity [31-33]. Its reason could be due to the week direct contact between metallic nanostructures (Au or Ag) and ZnO owing to
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the presence of nonconductive polymeric stabilizers on the surface of metallic nanostructures [30, 34-35].
In order to test whether the direct injection of electrons from the excited Ag nanowires surface plasmon states into ZnO has a role in photocatalytic activity, we measured the absorption
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spectra for ZnO/Ag nanowires 1.3 (4.5 wt %) composite in a de-aerated ethanol solution illuminated by the 365 nm HP-LED source as a function of illumination time (see Fig. 10). The de-aerated solution was used only in this set of experiments to avoid any possible electrons
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transfer from ZnO conduction band to oxygen orbitals. Ethanol, which is a most effective electron donor, can also be created a surrounding medium with low probability to accept electron
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and in fact prepared conditions with a great probability to accept electron by ZnO [30, 36]. It has been shown previously that charged the ZnO nanoparticles and Ag nanostructures exhibit a significant change in the ZnO absorption band and a red-shifted in surface plasmon resonance of Ag, respectively [37-41]. No significant difference was observed between the absorbance spectra and also difference spectrum of ZnO/Ag nanowires 1.3 composite and pure ZnO spectra showed a broad degrease in absorption of ZnO (because low losing of electrons in ZnO) and the partial
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displacement of the plasmon resonance peak (see Fig.11). The resultant changes are very small to justify the observed rate enhancements in photocatalytic activities. In fact, during the course of the in-situ UV-vis experiments, meaningful change in the places of ZnO absorption band and the
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plasmon peak of Ag has not been observed [30, 35]. So it can be suggested that significant charge transfer from Ag nanowires to ZnO do not exist and by attention to result of photocatalytic activity related to ZnO/Au nanospheres (4.5 wt %) composite just a very small
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charge transfer from ZnO nanoparticles to the Ag nanowires could be existed. Therefore charge transfer cannot account as main reason for the magnitude of the photocatalytic activity
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enhancement.
Based on some mechanisms [8], the localized SPR-induced heating of Ag nanowires can lead to thermo-chemical degradation of MB. To evaluate such a hypothesis, the degradation of MB with illuminated samples containing only pure Ag nanowires was measured. The results for Ag
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nanowires 1.3 are shown in Figures 8 and 9. These figures show that the Ag nanowires alone cannot decompose MB. Therefore the local Ag heating cannot be responsible for the photocatalytic degradation of MB. In real, our findings are consistent with previous reports
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which showed that local heating of metallic particles is only important for fairly small particles (<30 nm diameter). Since the Ag nanowires used in our studies have significantly larger sizes it
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can be supposed that the local heating of Ag nanostructures is not important factor to enhance the existing reaction rate [42-43]. Above investigations show that the mentioned mechanisms cannot be the major reason for photocatalytic activity enhancement and in fact it should be explained by SPR-induced energy transfer from the Ag nanowires to the ZnO nanoparticles. This energy transfer is due to increasing the photons path length through the radiative Rayleigh scattering process in the
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composite and the SPR-induced local electromagnetic field near the surface of Ag nanowires [35]. In fact the one of main features of Ag nanowires leading to enhancement of photocatalytic activity rate of the composites is their large extinction cross section (more specifically, the
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scattering cross section) [35]. Based on this mechanism the additional e¯ /h+ pairs at ZnO nanoparticles surface are produced, where the recombination rate decreased. To verify the SPRinduced energy transfer mechanism, the PL emission of ZnO/Ag nanowires 1.3 (4.5 wt %) was
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measured at several excitation wavelengths (325, 345, and 365nm), normalized by the emission for pure ZnO nanoparticles excited at the same wavelengths. PL emission enhancements at
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around NBE emission peak of ZnO for composite are plotted as a function of the excitation wavelength in Fig. 12 (b). From the figure it is clear that the highest PL emission enhancement of composite is observed for excitation wavelengths close to the wavelength at which there is the largest overlap between the Ag surface plasmon resonance, the ZnO absorption spectrum and the
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source spectrum. These data are fully compatible with the UV-vis absorbance of existing Ag nanowires in the composite (see Fig.10). These results indicate that the observed PL emission enhancement from ZnO is due to the SPR-induced enhancement of the absorbance of the
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composite and also to e¯ /h+ pairs concentration at ZnO nanoparticles surface which improves their photocatalytic activity. These essentially require an overlap between the Ag SPR extinction
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spectrum, the semiconductor absorption spectrum and the source spectrum. This indicates that ZnO/Au nanosphere composite does not show any considerable photocatalytic performance enhancement due to the lack of such overlap for Au SPR extinction spectrum, the semiconductor absorption spectrum and the source spectrum during the experiment. Beside, the Fig. 13 shows a linear relationship between integrated areas under the extinction spectra in the region of the irradiation source, between 360 and 370 nm, for pure Ag nanowires
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with different concentrations (0, 1, and 4.5 wt%) and the measured MB decomposition rate for the ZnO/Ag nanowires composites consisted of these pure Ag nanowires. Since the Ag extinction is a consequence of the excitation of Ag surface plasmon states, such the qualitative
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planning between the rate enhancement and the Ag extinction and creating of linear relationship implies main role of the excitation of Ag surface plasmons for the enhancement of photocatalytic degradation rate in the composite systems. It also suggests that the enhanced photocatalytic
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reactivity of the composite systems can be directly corresponding to the Ag nanowires extinction cross section [30, 35]. However, plasmonic photocatalytic activity rate for the ZnO/Ag
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nanowires 1.3 (8 wt %) composite has been digressed from this linear relationship because light from reaching to the semiconductor surface was prevented by the increase of weight percent of Ag nanowires in this composite.
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6. Conclusions
It was shown that the presence of Ag nanowires in ZnO semiconductor/Ag nanowires composites called as “plasmonic photocatalyst” can enhance the e¯ /h+ pairs concentration of the
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photocatalyst and therefore improve their photocatalytic activity. The effects of the concentration and diameter size of Ag nanowires on the photocatalytic activity enhancement of the composite
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photocatalysts was investigated and the photocatalytic activity was optimized by controlling of them. The results showed that ZnO/Ag nanowires 1.3 (4.5 wt %) composite has the highest photocatalytic activity enhancement compared to the others. In this work, the physical mechanisms governing the plasmonic photocatalysts were also estimated. The results suggested that the efficient photocatalytic activity enhancement of the composite systems can be explained by SPR-induced energy transfer from the Ag nanowires to the ZnO nanoparticles. Such transfers
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are due to the increase in the photon path length in the composite and create the SPR-induced local electromagnetic fields near the surface of Ag nanowires. Based on this mechanism the e¯ /h+ pairs concentration increased at ZnO nanoparticles surface, where the recombination rates
is profitable for improvement of ZnO photocatalytic activity.
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Acknowledgement
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were significantly diminished. This work demonstrated that using the plasmonic nanostructures
The authors acknowledge Shahid-Chamran university of Ahvaz for financial support of
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this work and Jundi-Shapur University of Technology of Dezful for technical support of this work.
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Fig. 1. (a) XRD pattern, and (b) FESEM image of ZnO nanoparticles.
Fig. 2. XRD pattern of Ag nanowires synthesized with 0.13 M PVP.
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Fig. 3. Schematic illustration of the PVP role in the preparation of Ag nanowires.
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Fig. 4. The UV–Vis absorption spectra of the synthesized Ag nanowires.
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Fig. 5. SEM images of Ag nanowires prepared with different molar ratios of PVP/AgNO3 (a) 1, (b) 1.3, (c) 1.5.
Fig. 6. FT-IR spectra of Ag nanowires 1.3-PVP (■) and the pure PVP (●).
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Fig. 7. UV-vis absorbance spectra of the ZnO/Ag nanowires 1.3 composites with different weight percents of Ag
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nanowires.
Fig. 8. Kinetic data of photocatalytic performance for MB decomposition by pure ZnO, pure Ag nanowires, and ZnO/Ag nanowires 1.3 composites with different concentrations of Ag nanowires1, 4.5, and 8 wt %. Inset: top) UV– Vis absorption spectra for 0.01 mM MB decomposition in 20 min intervals by ZnO/Ag nanowires 1.3 (4.5 wt %).
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Fig. 9. Kinetic data of photocatalytic performance for MB decomposition by pure ZnO, pure Ag nanowires, ZnO/Au nanosphere, ZnO/Ag nanowires 1, ZnO/Ag nanowires 1.3, and ZnO/Ag nanowires 1.5 composites with
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concentration of 4.5 wt %. Inset: top) normalized dye concentration of MB versus reaction time for them.
Fig.10. Testing the charge transfer mechanism from Ag nanowires to ZnO.
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Fig. 11. Experimental absorbance UV-visible spectum for Ag nanowires 1.3 and a difference spectrum calculated by subtracting the contribution of ZnO from the composite spectrum.
Fig.12. (a) photoluminescence spectra of pure ZnO and ZnO/Ag nanowires 1.3 (4.5 wt %) with excitation wavelength of 325 nm, (b) photoluminescence emission enhancement near band edge (NBE) emission peak of ZnO
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nanoparticles as a function of excitation wavelength. Inset: top) photoluminescence spectra of pure ZnO and
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ZnO/Ag nanowires 1.3 (4.5 wt %) with excitation wavelength of 365nm.
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Fig. 13. (a) Rate constant measured for ZnO/Ag nanowires 1.3 composites is plotted as a function of the Ag plasmon intensity at the source wavelength for samples containing only Ag nanowires.
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Highlights:
We synthesized ZnO nanoparticles/Ag nanowires plasmonic photocatalysts. We studied the mechanisms governing the performance of plasmonic photocatalysts.
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Energy transfer from Ag nanowire to the ZnO nanoparticle was led to enhancement.
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We researched the effects of Ag nanowires size and concentration on photoactivity.