Synthesis and properties of Ag/ZnO core/shell nanostructures prepared by excimer laser ablation in liquid Yan Zhao, Shuanghao Li, Yong Zeng, and Yijian Jiang Citation: APL Mater 3, 086103 (2015); doi: 10.1063/1.4928287 View online: http://dx.doi.org/10.1063/1.4928287 View Table of Contents: http://aip.scitation.org/toc/apm/3/8 Published by the American Institute of Physics APL MATERIALS 3, 086103 (2015) Synthesis and properties of Ag/ZnO core/shell nanostructures prepared by excimer laser ablation in liquid Yan Zhao,a Shuanghao Li, Yong Zeng, and Yijian Jiang Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, People’s Republic of China (Received 15 June 2015; accepted 28 July 2015; published online 20 August 2015) Ag/ZnO core/shell nanostructure was synthesised by a 248-nm KrF excimer pulsed laser ablation in a liquid solution for the first time It was found that the surface plasma resonance absorption of the Ag/ZnO core/shell nanostructures can be tuned by the thickness of the ZnO shell, which is in agreement with the finite difference in the time domain simulation Furthermore, the ultraviolet emission spectrum of the Ag/ZnO core/shell nanostructures was stronger and blue-shifted compared with that of pure ZnO nanoparticles This interesting photoluminescent phenomenon is analysed in detail and a possible explanation is proposed C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4928287] Core/shell composite nanostructures have attracted much attention in recent years,1–4 which exhibit many unique properties,1 including monodispersion, core/shell operability, stability, and selfassembly Moreover, because of the strong coupling exciton effect between the surface plasmon resonance (SPR) of the noble metal and the excitons of the semiconductors,3,5 the noble metal/semiconductor core/shell composite nanostructure has been one of the most promising composite nanostructures of the 21st century In particular, Ag/ZnO core/shell nanostructures, which have unique properties in optics,3 electronics, and catalysis, have been widely used in micro-nano optoelectronic devices Typically, the microstructure and properties of a nanomaterial largely depend on the method of its synthesis.6 Methods of controlling the synthesis of noble metal/semiconductor core/shell composite nanostructures are therefore becoming an increasingly important area in material sciences The conventional synthesis of the Ag/ZnO core/shell nanostructure is based on chemical methods.7–10 These methods have the advantages of high combining efficiency; however, the processes are very complicated and it is difficult to avoid the introduction of impurities Pulsed laser ablation in liquid solutions (PLAL) is a simple,6,11 rapid, and environmentfriendly method of synthesising nanomaterials The PLAL method requires simple devices.12–14 More importantly, the greatest advantage of PLAL is that their productions are chemically pure A large number of products have been reported to have been synthesised by PLAL, such as Ag, Au, and Cu nanoparticles,15,16 alloy nanoparticles,17,18 heterostructures,19 nanocubes,20 nano-spindles, noble metal core/shell nanostructures,2,21 and metal/metal oxide semiconductor core/shell nanostructures.5,22 While PLAL is of great value in the synthesis of nanostructures, it must be integrated with chemical processes for the synthesis of metal and oxide core/shell nanostructures containing two different elements23 such as Ag/SiO2 or Au/SiO2 Metal and metal oxide core/shell nanostructures with two different metal elements synthesised by PLAL directly have not yet been reported Here, we report a two-step PLAL method for the synthesis of Ag/ZnO core/shell nanostructures It was found that the SPR absorption of the Ag/ZnO core/shell nanostructures can be tuned by the thickness of the ZnO shell, which is in agreement with simulated results It was also found that the ultraviolet (UV) emission of the core/shell nanostructure is stronger than the individual ZnO a Author to whom correspondence should be addressed Electronic mail: zhaoyan@bjut.edu.cn 2166-532X/2015/3(8)/086103/7 3, 086103-1 © Author(s) 2015 086103-2 Zhao et al APL Mater 3, 086103 (2015) FIG Synthesis method of the Ag/ZnO core/shell nanostructures nanoparticles synthesised by PLAL, and this photoluminescent spectrum is blue-shifted compared with that of the ZnO nanoparticle This phenomenon was analysed in detail and a possible explanation is proposed The Ag/ZnO core/shell structures were synthesised by following a two-step method; the experimental setup is shown in Fig First, a 1-mm-thick silver plate of 99.99% purity placed on the bottom of a rotating beaker at 10 rpm with 10 ml deionized water A 10-Hz KrF excimer laser with wavelength 248 nm, 400 mJ in energy, and pulse duration 20 ns was focused on the silver plate to × mm2 The silver plate was ablated by this laser for 30 Second, a zinc plate was ablated by the same laser with the same pulse energy in the Ag colloid synthesised by the first step for 5, 10, 15, 20, and 30 with the aim of producing the Ag/ZnO core/shell nanoparticles The ZnO nanoparticles used for comparison were synthesised by the same laser at the same energy by ablation of a zinc plate in deionized water for 30 Transmission electron microscopy (TEM), the x-ray diffraction spectrum (XRD), an UV-visible spectrophotometer, and a photoluminescence (PL) spectrometer were used to identify the morphology, structure, and properties of the particles produced by the “two-step” ablation process Figure shows the TEM image of the ZnO nanoparticles (Fig 2(a)) and the Ag/ZnO core/shell nanostructures (Figs 2(b) and 2(c)) The diameter of the ZnO nanoparticles is approximately 50–60 nm For the Ag/ZnO nanostructures, there is an obvious shell with thickness approximately nm coated on a core 15 nm in diameter With the aid of a high-resolution transmission electron microscope (HRTEM), the Ag core and ZnO shell was confirmed This suggests that the products of the two-step laser ablation method are indeed Ag/ZnO core/shell structures The XRD pattern of the Ag/ZnO core/shell nanostructures is shown in Fig There are two sets of strong diffraction peaks corresponding to the expected products (ZnO peaks are marked by “#” and Ag peaks are marked by “*”) All peaks are in good agreement with the powder diffraction file (PDF) of Ag and ZnO (PFD#03-065-3411 and PDF#03-065-2871) Notably, there is no evidence of a significant shift, nor any other peaks in the XRD pattern of the Ag/ZnO core/shell nanostructure, implying that there are no other elements or other crystal lattice arrangements present in the nanostructures To further verify the structure of the Ag/ZnO core/shell nanoparticles, the UV-visible spectrum of the Ag/ZnO core/shell nanostructure was examined (Fig 4) Figure 4(a) shows the SPR absorption spectra of the nanostructures produced by different ZnO ablation times: 5, 10, 15, 20, and 30 (the absorption below 300 nm in Fig 4(a) is from the ultraviolet absorption of water) The SPR peak of the nanostructure for 30 ablation is the same as that of 20 It is evident that for longer ablation times the SPR absorption is red-shifted from 394 to 441 nm FIG (a) TEM image of the Ag/ZnO core/shell nanostructures; (b) and (c) HRTEM images of the Ag/ZnO core/shell nanostructures 086103-3 Zhao et al APL Mater 3, 086103 (2015) FIG XRD spectra of the ZnO nanoparticles and the Ag/ZnO core/shell nanostructures The ZnO peaks are marked with “#” and the Ag peaks are marked with “*.” All peaks are in good agreement with the powder diffraction file (PDF) (PFD#03-065-3411 and PDF#03-065-2871) According to Mie theory, consider a homogeneous, isotropic sphere located at the origin in a uniform, static electric field The surrounding medium is isotropic and non-absorbing with dielectric constant εm , and the dielectric response of the sphere is described by the dielectric function ε = εr + iεi When the diameter 2r of the sphere is much less than the wavelength of the incident light, the distribution of the electric field E can be evaluated from the potentials to be24 ⃗Ein = 3ε m ⃗E0, ε + 2ε m ⃗Eout = ⃗E0 − αE0 ⃗z − 3z (x⃗x + y ⃗y + z⃗z ) , 4π r r (1a) (1b) ε−ε m where α = 4πa3 ε+2ε is the polarization While |Re (ε) + 2ε m | = 0, the corresponding cross secm tions for scattering and absorption Csca and Cabs can be calculated via the Poynting vector determined from 8π ε − ε m , k a ε + 2ε m ε − εm Cabs = 4πka Im ε + 2ε m Csca = (2a) (2b) Thus, the absorption of the nanoparticles is not only simply determined by the nature of the metal itself but also affected by the surrounding dielectric properties Generally, the SPR absorption of a pure Ag colloid is centred on approximately 390 nm, and as the ZnO shell is introduced, the surrounding dielectric properties of the Ag core change, leading to the observed changes in the SPR frequency of Ag FIG (a) The absorption spectra of the Ag/ZnO core/shell with different ablation times; (b) the simulation result of the Ag/ZnO core/shell nanostructures with a 15 nm Ag core and different ZnO shell thickness; (c) the absorption spectra of ZnO nanoparticles, Ag nanoparticles, and 1:1 mixed ZnO and Ag nanoparticles 086103-4 Zhao et al APL Mater 3, 086103 (2015) The software, FDTD Solutions, was used to simulate the red-shift result Figure 4(b) shows the simulated results for the SPR absorption spectrum of the Ag/ZnO core/shell nanostructure with different thicknesses of the ZnO shell In this simulation, the diameter of the core is 15 nm and the shell is varied between and nm at nm intervals Comparing the results of Figs 4(a) and 4(b) makes it clear that the simulated results are in good agreement with the experimental results It indicates that during the course of the two-step ablation process, the ZnO is coated on the Ag core layer by layer It should also be noted that the absorption peak does not appear to move in the solution in which Ag colloids and ZnO colloids are mixed directly (see Fig 4(c)), indicating that a simple mixture of Ag and ZnO nanoparticles cannot directly change the dielectric properties of the Ag surface The above analysis shows that the nanostructure synthesised by the two-step laser ablation method is indeed the desired Ag/ZnO core/shell nanostructure In general, the SPR of the Ag core could affect the photoluminescent properties of the ZnO shell Figure shows the photoluminescence spectrum of the ZnO nanoparticles and the Ag/ZnO FIG (a) Photoluminescence (PL) spectra of ZnO nanoparticles and Ag/ZnO core/shell nanostructures; (b) peak fitting of the visible part in photoluminescence of ZnO nanoparticles and Ag/ZnO core/shell nanostructures 086103-5 Zhao et al APL Mater 3, 086103 (2015) core/shell nanostructures As shown in Fig 5(a), the UV emission of the core/shell nanostructure is 2.9 times stronger (by area integration) than that of the ZnO nanoparticle In Fig 5(b), the visible part in photoluminescence has been divided into parts by curve fitting The parts which is centered at 1.84, 2.08, 2.25, and 2.46 eV are caused by the Oi, Vo++, VZn, and Vo+, respectively It is obviously that the peaks centered 1.84 and 2.08 eV have no obvious change in the ZnO nanoparticles and the Ag/ZnO core/shell nanostructures Moreover, the emission peaks centered at 2.25 and 2.46 eV in Ag/ZnO core/shell nanostructures have slightly weakened compared with those in the ZnO nanostructures In general, there are three main mechanisms that may contribute to the enhanced photoluminescence of the Ag/ZnO composite nanostructures: the increased local electric field, the increased recombination rate, and the electron transfer.25 The following discusses the possible contributions of these three mechanisms to the observed increased photoluminescence While Ag nanoparticles are located in an electromagnetic field, the local electric field is increased by adjacent Ag nanoparticles, and the excitation rate of the ZnO is stimulated by this enhanced local electric field This may lead to observations of increased photoluminescence CST software was used to simulate this increase in the local electric field in the experimental situation Figure 6(a) shows the calculated results from two Ag/ZnO core/shell nanostructures (core 15 nm and shell nm) located in an electromagnetic field As can be seen in this figure, there is no obvious enhancement in the local electric field The main mechanism of the enhanced photoluminescence is therefore not an enhanced local electric field By surface plasmon (SP) theory, excited ZnO can transfer energy to Ag and excite the SP of Ag, causing it to radiate photons This increases the recombination rate of electron-hole pairs To enhance the photoluminescence, these energies need to be scattered According to Mie theory, the scattering ability is proportional to the 6th power of the particle size In this model, the Ag core is 15 nm, meaning it is unable to so Therefore, the observed enhancement in photoluminescence is likely not due to increased recombination rate In the Ag/ZnO nanostructure, the Fermi level of Ag is near some defect levels (2.25 eV and 2.46 eV) of ZnO26,27 (Fig 6(b)); therefore, electrons can be transferred from the ZnO defect levels to the Fermi level of Ag, where these electrons are excited by incident light The energy level of these excited electrons is near the conduction band of ZnO; therefore, these excited electrons are transferred to the conduction band of ZnO where they become a part of the electron-hole recombination process, increasing the near band edge emission In this model, as a consequence of the FIG (a) The simulation results of the Ag/ZnO core/shell nanostructures with a 15 nm core and nm shell; (b) diagram of the electron transfer in Ag and ZnO 086103-6 Zhao et al APL Mater 3, 086103 (2015) electrons transferring, the visible emission will be reduced and the UV emission will be enhanced This is just in accord with the phenomenon in our experiment It is this electron transfer mechanism that is the main contributor to the observed increase in photoluminescence Another interesting phenomenon which is worth mentioning is that the UV emission peak of the ZnO nanoparticles is centred on 3.23 eV, whereas the UV emission of the Ag/ZnO core/shell nanostructure is centred on 3.31 eV This is likely caused by the difference in diameter of the ZnO nanoparticles compared to the Ag/ZnO core/shell nanostructures It is known that when the size of a semiconductor nanostructure decreases, the band emission shifts to a shorter wavelength, indicating an increase in the band gap Brus28 was one of the first to theoretically consider this problem Brus’ approach was to solve the Schrödinger equation for the first exited state using an effective mass approximation for the kinetic energy In the first approximation, the expression for the shift with respect to the bulk band gap is given by Equation (3), where d is the particle diameter, me is effective mass of the electron, mh is the effective mass of the hole, e is the elementary charge, ε2 is the dielectric constant of the particle, α is the polarisability, and S is the spatial position,26 ( ) ( s )2 ~2π 1 1.8e2 e2 ∞ ∆E = + − + a (3) n n=1 ε 2d d d 2d me mh The most useful result from this equation is that there is a physical motivation for a functional dependence between the band gap (Eg ) and the particle size (d) as in Equation (4) where C1, C2, and C3 are coefficients which are to be determined, Eg = C1 − C2 C3 + d d (4) As Jacobsson shows,29 these coefficients are given by 0.896 2.86 (5) + d d Using this equation, the diameter of the nanoparticles produced by the method detailed herein was calculated, with the following results For the ZnO nanoparticles of approximately 50 nm in diameter, the band gap width was determined to be 3.24 eV, which is in good agreement with experimental results of 3.23 eV For the ZnO nanoparticles of approximately 20 nm in diameter, the band gap width is approximately 3.28 eV, slightly less than the experimental result of 3.31 eV This small difference can be clarified by taking into account that the Ag/ZnO core/shell nanostructure is not a pure ZnO nanoparticle and that there is small natural variation in the diameters of individual nanostructures In conclusion, an Ag/ZnO core/shell nanostructure was synthesised by a two-step PLAL method for the first time The composition and structure of the Ag/ZnO core/shell nanostructure was verified by TEM and the XRD spectrum The SPR absorption of the core/shell nanostructures could be tuned by varying the thickness of the ZnO shell, in agreement with simulated results The UV emission of the core/shell nanostructure is stronger than that of pure ZnO nanoparticles synthesised by PLAL and a blue-shift was observed Such nanoparticles may have excellent 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(2015) Synthesis and properties of Ag/ ZnO core/ shell nanostructures prepared by excimer laser ablation in liquid Yan Zhao,a Shuanghao Li, Yong Zeng, and Yijian Jiang Institute of Laser Engineering,... photoluminescence spectrum of the ZnO nanoparticles and the Ag/ ZnO FIG (a) Photoluminescence (PL) spectra of ZnO nanoparticles and Ag/ ZnO core/ shell nanostructures; (b) peak fitting of the visible... nanostructures; (b) and (c) HRTEM images of the Ag/ ZnO core/ shell nanostructures 086103-3 Zhao et al APL Mater 3, 086103 (2015) FIG XRD spectra of the ZnO nanoparticles and the Ag/ ZnO core/ shell nanostructures