КОЛЛОИДНЫЙ ЖУРНАЛ, 2009, том 71, № 2, с. 233-242
УДК 541.18
ZnO MICRORODS PHOTODEPOSITED WITH Au-Ag NANOPARTICLES: SYNTHESIS, CHARACTERIZATION AND APPLICATION IN SERS
© 2009 Yonglan Luo
School of Chemistry and Chemical Industry, China West Normal University, Nanchong 637002, Sichuan, P. R. China E-mail: luo.yonglan@hotmail.com Поступила в редакцию: 15.01.2008 г.
Decorating one-dimensional micromaterials with nanoparticles provides an effective method to manipulate and integrate their physicochemical and optoelectronic properties. Here, we demonstrate an example of photochemical deposition of Au core/Ag shell nanoparticles onto ZnO microrods (ZnO/Au-Ag). The as-prepared micromaterials were characterized and confirmed by UV-vis spectra, XRD, XPS, FE-SEM and EDX analysis. Raman spectra revealed that the vibration energy of oxygen sublattice decreased after deposition of metallic nanoparticles. Their apparent Fermi level was calculated to be 156.9 mV from a Nernstian equilibrium with a redox couple in solution and primary investigation indicated their potential application as an active surface enhanced Raman spectroscopy substrate. The proposed method could extend to synthesize a series of core-shell nanopar-ticles decorated one-dimensional composite micromaterials and the as-prepared multifunctional micromaterials may also find their application in photocatalysis, gas sensor and so on.
1. INTRODUCTION
Micromaterials possess many unusual physicochemical and optoelectronic properties compared with their bulk counterparts, thus holding tremendous potential of application in many fields [1-3]. In particular, one-dimensional micromaterials, not only provide a good system to investigate the dimension- and size-dependant electrical and thermal transports or mechanical properties, but also play an important role as both interconnects and functional units in constructing nanoscale electronic and photonic devices [4-7]. Consequently, various one-dimensional micromaterials, such as nanowires [8-10], nanotubes [11, 12] and nanobelts [13, 14] have been successfully synthesized by chemical or physical methods and showed size and/or shape dependant properties. Their properties could be further manipulated by combination of different components, named as, surface coating, and give more options in the micromaterial toolbox for their application. Nanorod decorated by nanoparti-cles is one important member in one-dimensional composite material family and has attracted some attention recently [15-23]. They often show better desired properties, such as enhanced gas sensitivity [15, 18, 19] and better electron-hole separation [22, 23], owning to the designed and controlled modification. However, compared with single-component nanorods, their preparation, properties and potential application are not well-developed and more effort in this field is highly desired.
Core-shell nanoparticles are also of significance in materials chemistry since they exhibit better properties or multifunctionalities in comparison with their individual counterparts. Gerritsen et al reported fluorescence enhancement of metal-core/silica shell nanoparticles [24]
and Eichhorn et al reported anomalous stability and reactivity of Pt core/Cu shell nanoparticles for heterogeneous NOX reduction [25]. In addition, Shi et al. had synthesized magnetic core/mesoporous shell nanoparticles which might find their application in drug delivery [26] and Richtering et al. proposed multiresponsive "intelligent" core-shell microgels [27]. While multiple functionalities derived from both core and shell components enable them to accomplish versatile roles, incorporation them with one-dimensional micromaterials will further expand manipulation of their properties and extend their potential application.
In this work, we demonstrated the photochemical construction of gold core/silver shell nanoparticles decorated zinc oxide microrods (ZnO/Au-Ag). Zinc oxide microrods were selected since ZnO is an important multifunctional semiconductor and its one-dimensional materials have many practical applications [28-30]. Au-Ag nanoparticles were chosen as the core-shell nanoparticle example on account of their enhanced and tunable optical and catalytic properties [31, 32]. The as-synthesized ZnO/Au-Ag composite micromaterials were analyzed and confirmed by UV-visible spectrum, Raman, XRD, XPS, FE-SEM and EDX characterization. Moreover, apparent Fermi level distribution has been investigated and their application as an active SERS substrate was also been presented. These composite micromaterials may find their application in gas sensors [18], photocatalysis [33, 34], electron storage [35], directional photoemitter [20], SERS substrate and so on.
2. EXPERIMENTAL
Zinc acetate anhydrous (Zn(OOCCH3)?, 99.98%) was purchased from Alfa Aesar, poly acrylic acid (sodium salt, Mw ~ 5100) was purchased from Aldrich, hydrogen tetra-chloroaurate(III) trihydrate (HAuCl4 ■ 3H2O), silver nitrate (AgNO3) and 4-aminothiophenol (4-ATP) were obtained from Acros. Hexamethylenetetramine (C6H12N4), sodium hydroxide (NaOH) and methylene blue (MB) were bought from Yili Refined Chemical Reagent, Beijing, China. All chemicals were used as received without further purification. Water used in all experiments was purified with Milli-Q (18.4 MQ) water system.
ZnO microrods (ZnO) were synthesized following the method proposed by Vayssieres and other authors [3638]. In a typical synthesis, 50.0 ml 0.10 M aqueous solution of C6H12N4 was added to 50.0 ml 0.10 M aqueous solution of Zn(OOCH3)2 under mild magnetic stirring and kept stirring for 10 minutes. Then the mixture was applied to a water bath at 90°C for 5 h. The white sediments at the bottom were collected, rinsed with water and ethanol for three times and dried in oven at 80°C for 2 h.
ZnO/Au-Ag was prepared by first decorated zinc oxide microrods with Au nanoparticles, followed by photo-depositing Ag shell onto Au nanoparticles. At first, 0.4 ml 0.06 M HAuCl4 solution was neutralized by 2.4 ml 0.001 M NaOH solution and diluted to 32.0 ml in a 50 ml beaker. The solution must be neutral otherwise ZnO would be corrupted by the acidity of HAuCl4. Then 48.96 mg poly(acrylic acid, sodium salt) and 40.0 mg ZnO microrods were dispersed in this solution and kept under magnetic stirring for about 20 min. A hand portable ultraviolet lamp (Beijing BaYi Apparatus, 254 nm 15 W) was horizontally laid on the beaker (the distance from the solution to the lamp was about 3 cm) and the solution was irradiated for 4 h under magnetic stirring. The size of gold nanoparticles on ZnO microrods could be controlled by the concentration of HAuCl4 in solution. After irradiation, the product ZnO/Au was colleted by centrifuga-tion, rinsed with water and ethanol several times and dried in the oven at 80°C for 2 h. ZnO/Au-Ag micromaterials were obtained in a similar procedure. In detail, 1.0 ml 0.01 M AgNO3 aqueous solution, 2.0 ml absolute ethanol, 24.48 mg poly(acrylic acid, sodium salt) and 20.0 mg ZnO/Au powder were dispersed in 20.0 ml double distilled water in a 50 ml beaker. The solution was subjected to UV irradiation in the air for 4 h (the procedure and apparatus, emission wavelength and power were exactly the same with that described above, the distance between the solution and the lamp was about 4 cm). The thickness of silver shell could also be manipulated by varying the amount of silver nitrate added. After preparation, products were colleted by centrifugation, washed with water and ethanol and dried in oven at 80°C for 2 h for further characterization.
We determined the apparent Fermi level of ZnO/Au-Ag composite systems by attaining a Nernstian equilibrium with a known redox couple (MB2+/MB). At first, 2.0 mg ZnO/Au-Ag powder was dispersed in a
mixture of 4.0 ml double distilled water and 1.0 ml absolute ethanol in a glass vial. The suspension was purged with nitrogen (99.99%) for 30 min and sealed immediately. Then the suspension was subjected to UV irradiation (WFH-201B, UV Reflecting and Transmitting Apparatus, Shanghai Jingke Co. Ltd. 315 nm, 45 W) for 1 h to accumulate electrons in the Au-Ag nanoparticles. After irradiation, 1.0 ml 0.05 mM methylene blue solution (pre-deaerated by nitrogen for 30 min) was swiftly injected into the vial, sealed again and stored for about 10 min. Then the suspension was centrifugated at a speed of 15000 rpm to completely discard the ZnO/Au-Ag sediments. Clear solution was obtained and subjected to UV-vis spectrum analysis. In order to eliminate the experimental error brought out by the physical ab-sorbance of methylene blue onto ZnO/Au-Ag micromaterials, control experiment had also been done during which all the procedure were exactly the same except for the omission of 1 h UV irradiation.
The ZnO/Au and ZnO/Au-Ag powder were first dispersed in absolute ethanol and 50 ^l of the suspension was dropped onto clean ITO slices which were directly imaged by FE-SEM without metallic coating. As for the SERS measurements, such ITO slices were first immersed into 10-6 M 4-ATP solution for 2 h. After carefully rinsed with water and dried naturally in the open air, they were subjected to Raman characterization.
The UV-vis spectra were recorded in a Varian Cary 50 spectrophotometer with a scanning rate of 600 nm per minute. Field emission scanning electron microscope (FE-SEM) analysis and X-ray energy dispersion spectrum (EDX) were conducted on a Philips XL-30 field-emission scanning electron microscope operated at 15 kV. X-ray photoelectron spectroscopy (XPS) was conducted using a VG ESCAlAb MK II spectrometer (VG Scientific, UK) employing a monochromatic Mg Ka X-ray source (hv = 1253.6 eV). Peak positions were internally referenced to the C1 s peak at 284.6 eV. X-ray powder diffraction (XRD) analysis was conducted on a Rigaku D/max-2500 X-ray diffractometer with graphite monochromated Cu Ka radiation (k = 1.5418 A). The Raman instrument include
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