Hydrogen treatment-improved uniform deposition of Ag nanoparticles on ZnO nanorod arrays and their visible-light photocatalytic and surface-enhanced Raman scattering properties
© Lin et al.; licensee Springer. 2013
Received: 5 June 2013
Accepted: 10 July 2013
Published: 16 July 2013
ZnO nanorod arrays were synthesized by chemical bath deposition. After heat treatment in hydrogen or air, Ag nanoparticles were deposited on ZnO nanorod arrays by photo-reduction method. The size of Ag nanoparticles as well as the surface morphology, structure, composition, and optical property of ZnO nanorod arrays before and after the deposition of Ag nanoparticles were characterized by SEM, XRD, EDS, and UV/VIS/NIR spectrophotometer. As compared to the samples with heat treatment in air or without heat treatment, the ZnO nanorod arrays after heat treatment in hydrogen allowed Ag nanoparticles to be deposited more uniformly, densely, and numerously. Also, they exhibited higher efficiency for the visible light-driven photocatalytic degradation of Rhodamine 6G (R6G) dye. The effects of the amount of Ag nanoparticles, initial dye concentration, and temperature on the photocatalytic degradation efficiency were investigated. Furthermore, they also exhibited better surface-enhanced Raman scattering property for the detection of R6G dyes.
Nowadays, environmental problems relating to wastewaters are becoming much more serious than ever, and the photocatalytic technique with metal oxide semiconductors has become one of the most promising methods for wastewater treatment [1–6]. Among various metal oxide semiconductors, ZnO has gained pretty much attention with respect to the degradation of various pollutants owning to its high photosensitivity, high catalytic efficiency, low cost, non-toxicity, environmental sustainment stability, and wide band gap [7, 8]. However, due to its wide band gap, ZnO can only be activated by ultraviolet light of wavelength below 385 nm, only accounting for less than 5% of the solar energy, which practically limits the use of solar light or visible light. Furthermore, energy saving consideration is now being more regarded. How to extend the photo response of ZnO toward the visible spectral region is now being an important issue . To solve this tough problem, ZnO modification has been extensively explored, such as combining with other semiconductors, doping and coating with noble metals, and modifying with organic polymers [9–17]. Many researchers have reported the synthesis of Ag/ZnO composites and their applications in various fields, especially in photocatalytic degradation of organic dyes [18–34] and surface-enhanced Raman scattering (SERS) [18, 35–37]. Silver metal exhibits plasmon resonances under visible light; moreover, it is stable, non-toxic, easy to synthesize, and relatively cheap compared to other noble metals. Therefore, combining silver metals with ZnO can effectively help the use of visible light.
In this work, we presented a method to synthesize silver-coated ZnO nanorod arrays with silver nanoparticles depositing uniformly onto top, side, and bottom of nanorods, which offered much more active sites to take part in photocatalysis. The effect of heat treatment in hydrogen or air on the deposition of Ag nanoparticles on ZnO nanorod arrays was examined. After the photocatalysts were successfully obtained, we used Rhodamine 6G (R6G) as the target containment and visible light as the light source to investigate the photocatalytic activity of silver-coated ZnO nanorod arrays. The effects of the amount of Ag nanoparticles, initial R6G concentration, and temperature on the photocatalytic degradation efficiency were investigated.
In addition to photocatalysis, Ag/ZnO can also be used in SERS, which is an extraordinary analytical tool for determining chemical and biological information of molecules on solid substrates and can provide unique fingerprints of analytes, making its rapid development since it appeared [38–47]. So, in this work, R6G was also used as the detection target for the study on the SERS property of silver-coated ZnO nanorod arrays. The effect of heat treatment in hydrogen or air on the influence of SERS performance was investigated. The detection limit of R6G was also determined.
Sodium hydroxide and 2-methoxyethanol were obtained from Fluka (Fluka Chemical Corporation, St. Louis, Milwaukee, WI, USA). Zinc acetate and zinc nitrate were purchased from J.T. Baker Chemical Company (Phillipsburg, NJ, USA). Diethylenetriamine (DETA) was the product of Riedel-DeHaen (Honeywell International, Inc., Morristown, NJ, USA). Silver nitrate 99.9% was the product of Alfa Aesar (Ward Hill, MA, USA). Rhodamine 6G and monoethanolamine (MEA; 99.5%) was obtained from Sigma-Aldrich Corporation (St. Louis, MO, USA). The water used throughout this work was the reagent grade water produced by a Milli-Q SP ultra-pure-water purification system of Nihon Millipore Ltd., Tokyo, Japan.
ZnO nanorod arrays were prepared according to our previous work on the synthesis of Al-doped ZnO nanorod arrays but without Al doping [48, 49]. Firstly, 0.5 ml MEA was added to a solution containing 11 ml 2-methoxyethanol and 1.8 g zinc acetate, which formed the ZnO sol–gel solution. ZnO seed layer was prepared by spin coating the sol–gel solution (0.1 ml) on a glass substrate (2.5 cm × 2.5 cm) at a rotation speed of 3,000 rpm for 30 s, and the films were then annealed at 350°C for 10 min. The step mentioned above was repeated eight times, and the acquired ZnO thin films were then annealed to 550°C for 2 h to get the final ZnO seed layer. Secondly, the ZnO seed layer was placed in an autoclave containing growth solution consisting of 30 ml water, 1.32 g zinc nitrate, 0.46 ml DETA, and 0.8 ml NaOH. After that, the growth solution was heated to 95°C for 6 h to get the ZnO nanorod arrays, which was noted as ZnO. The ZnO nanorod arrays were annealed in Ar/H2(97/3) or air atmosphere at 400°C for 2 h to get ZnO-H and ZnO-A, respectively.
For the deposition of Ag nanoparticles on ZnO, ZnO-H, and ZnO-A, the resultant ZnO, ZnO-H, and ZnO-A were immersed in an aqueous solution of AgNO3 (5 ml, 0.01 M) and were illuminated under UV light (λ = 254 nm) for 10 min. This step was repeated three times to get ZnO@Ag, ZnO-H@Ag, and ZnO-A@Ag. For the investigation on the effect of Ag content on the photocatalytic activity of ZnO-H@Ag, the deposition step was conducted for 4 min × 1, 7 min × 1, 10 min × 1, 10 min × 2, 10 min × 3, or 10 min × 4 (here × denotes the repeating time).
The surface morphology and energy dispersive X-ray (EDX) spectroscopy were observed by a high-resolution field emission scanning electron microscopy (HR-FESEM, JEOL SEM 6700F; JEOL Ltd., Tokyo, Japan). The crystalline structure was characterized by X-ray diffraction (XRD) analysis on a Rigaku D/max-ga X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) at 40 kV with Cu Kα radiation (λ = 0.1542 nm).
For the photocatalytic degradation of R6G, the photocatalysts (1.25 cm × 1.25 cm, ZnO, ZnO-H, ZnO-A, ZnO@Ag, ZnO-H@Ag, or ZnO-A@Ag) were placed into 5 ml of R6G solution, allowed to equilibrate for 30 min in the darkness, and then followed by the lamp’s (200 W, λ > 400 nm, manufactured by Oriel Instruments Corporation, Stratford, CT, USA) switching up to initiate the reaction. During the reaction, a certain amount of solution was withdrawn at certain reaction intervals to determine the remaining concentration of R6G by a JASCO model V-570 ultraviolet–visible near-infrared (UV/VIS/NIR) spectrophotometer (JASCO Analytical Instruments, Easton, MD, USA) at 527 nm.
For the study on the SERS property, the substrates (ZnO, ZnO-H, ZnO@Ag, ZnO-H@Ag, and ZnO-A@Ag) were immersed in the 40 ml of R6G solutions with different concentrations for 40 min and were then analyzed by the micro-Raman spectrometer (Scinco, 532 nm; Scinco Co., Ltd. Kangnam-Gu, Seoul, South Korea) to get the SERS spectra of R6G.
Results and discussion
In this work we have successfully synthesized Ag-coated ZnO nanorod arrays for the photocatalytic degradation and SERS analysis of R6G. Hydrogen treatment of ZnO nanorod arrays was demonstrated to be useful for the uniform deposition of Ag nanoparticles on the top, side, and bottom of ZnO nanorods. As compared to ZnO@Ag and ZnO-A@Ag, the ZnO-H@Ag showed the better photocatalytic activity for the degradation of R6G in the visible light region. Also, the photocatalytic degradation of R6G obeyed the pseudo-first-order kinetics, and the optimal atomic percentage of silver in ZnO-H@Ag was 3.37. With decreasing the initial R6G concentration or increasing the temperature, the corresponding rate constant increased slightly. The activation energy was 1.37 kJ/mol. In addition, the ZnO-H@Ag with an Ag atomic percentage of 3.37 was also demonstrated to be the best one for the SERS analysis of R6G as compared to ZnO@Ag, ZnO-A@Ag, and the ZnO-H@Ag with other Ag contents. The detection limit of R6G was 10−9M. The whole result revealed that hydrogen treatment of ZnO nanorod arrays was useful in improving the uniform deposition of Ag nanoparticles on ZnO nanorod arrays, which led to better visible-light photocatalytic and SERS properties.
SLL received his master degree in Chemical Engineering at National Cheng Kung University (Taiwan) in 2012 and now is in the army. KCH is currently a PhD student of the National Cheng Kung University (Taiwan). CHH received his PhD degree in Chemical Engineering at National Cheng Kung University (Taiwan) in 2011 and now works as a researcher in United Microelectronics Corporation (Taiwan). DHC is a distinguished professor of Chemical Engineering Department at National Cheng Kung University (Taiwan).
We are grateful to Taiwan Textile Research Institute and National Science Council (NSC 100-2221-E-006-164-MY2) for the support of this research.
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