Morphology-Controlled Fabrication of Large-Scale Dendritic Silver Nanostructures for Catalysis and SERS Applications
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Highly branched metallic nanostructures, which possess a large amount of catalyst active sites and surface-enhanced Raman scattering (SERS) hot spots owing to their large surface areas, multi-level branches, corners, and edges, have shown potential in various applications including catalysis and SERS. In this study, well-defined dendritic silver (Ag) nanostructures were prepared by a facile and controllable electrochemical deposition strategy. The morphology of Ag nanostructures is controlled by regulating electrodeposition time and concentration of AgNO3 in the electrolyte solution. Compared to conventional Ag nanoparticle films, dendritic Ag nanostructures exhibited larger SERS enhancement ascribed to the numerous hot spots exist in the nanogaps of parallel and vertically stacked multilayer Ag dendrites. In addition, the prepared dendritic Ag nanostructures show 3.2-fold higher catalytic activity towards the reduction of 4-nitrophenol (4-NP) by NaBH4 than the Ag nanoparticle films. The results indicate that the dendritic Ag nanostructures represent a unique bifunctional nanostructure that serves as both efficient catalysts and excellent SERS substrates, which may be further employed as a nanoreactor for in situ investigation and real-time monitoring of catalytic reactions by SERS technique.
KeywordsDendritic silver nanostructures Catalysis Surface-enhanced Raman scattering Electrochemical deposition
Energy-dispersive X-ray spectroscopy
Indium tin oxide
Scanning electron microscope
Surface-enhanced Raman scattering
Transmission electron microscopy
X-ray powder diffraction
Noble metal micro/nanostructures have attracted great attention due to their potential applications in optics , catalysis [2, 3, 4], surface-enhanced Raman scattering (SERS) [5, 6, 7], and solar energy harvesting . The physical and chemical properties of metal micro/nanostructures are mainly determined by their size, shape, and composition [9, 10]. The controlled fabrication of metal micro/nanostructures with tunable size and morphology provides great opportunities to systematically investigate their properties and practical applications. Recently, due to the progress in nanofabrication techniques, metal nanostructures with different sizes and morphologies have been successfully prepared by using various fabrication approaches [2, 9, 10, 11, 12, 13].
The applications based on the substrates with plasmonic nanostructures have been extensively explored [5, 7]. Most of the fabrication strategies, such as focused ion beams lithography , nanoimprint lithography , electron beam lithography , nanosphere lithography , and self-assembly , are used to fabricate large-scale and uniform-sized metallic nanostructure substrates. However, these fabrication strategies are still characterized by high cost, long time, and complex processes. Therefore, it is necessary to develop a simple and efficient synthesis route of large-area and shape-controlled metal micro/nanostructures. Electrochemical deposition is a simple, powerful, and convenient technique to one-step synthesize and immobilize large-area metal micro/nanostructures onto substrates simultaneously [7, 18, 19, 20, 21, 22, 23, 24, 25, 26]. The morphology and size of the electrodeposited metal products can be controlled by tuning the deposition conditions, such as the concentration and proportion of electrolyte solution, electrodeposition current density, and electrodeposition time. Generally, in the growth process of nanocrystals, the final morphology depends on the formation conditions departing from thermodynamic equilibrium [18, 25, 26, 27, 28, 29]. Electrochemistry is widely used to study morphological transitions of nanocrystals in non-equilibrium growth processes. Due to the fast nucleation and growth of nanocrystals, non-equilibrium processes are important for synthesizing interesting structures with hierarchical morphologies [18, 22, 23, 24, 25]. Recently, electrochemical deposition methods have been used to fabricate various metal structures, including pyramids , flower-like mesoparticles , nanosheets , nanorods [20, 21], dendrites [22, 23, 24, 25], and concave hexoctahedral nanocrystals .
In this work, dendritic fractal nanostructures on indium tin oxide (ITO) glass substrates were fabricated by a facile and controllable electrochemical deposition strategy. The shape evolution induced by the AgNO3 concentration, deposition time, deposition current density, and citric acid concentration were systematically investigated to reveal the influences of AgNO3 concentration and deposition time on final morphologies. The prepared dendritic Ag nanostructures exhibited larger SERS enhancement and catalytic activity compared to the Ag nanoparticle films prepared by magnetron sputtering method.
Fabrication of Dendritic Ag Fractal Nanostructures
Dendritic Ag fractal nanostructures were prepared by an electrochemical deposition process, which is described in our previous work [18, 25]. The electrochemical deposition process was conducted with a two-electrode system consisting of a ITO glass (1.5 × 1 cm2, 17 Ω/square) cathode and a platinum (Pt) plate anode. ITO glasses were cleaned by ultrasonication in acetone, distilled water, and ethanol for 15 min, respectively. The distance between the two electrodes was set to be 3 cm. The electrolyte solution contained AgNO3 (2 g/L) aqueous solution and citric acid (40 g/L). In the electrochemical deposition process, a constant current density of 1 mA cm−2 was applied. After the electrodeposition process was completed, the samples were rinsed with ultrapure water for several times and then dried with high-purity flowing nitrogen. The as-electrodeposited dendritic Ag fractal nanostructure samples were then submerged into 10−5 M 3,3′-diethylthiatricarbocyanine iodide (DTTCI) ethanol solution for 4 h to adsorb a self-assembled monolayer of molecules. The SERS samples were carefully rinsed with ethanol to remove the weakly bound molecules and then dried under N2 before analysis.
In a typical 4-nitrophenol (4-NP) reduction reaction, 1 mL of 4-NP (2 × 10−5 M) aqueous solution was mixed with 1 mL of ice-cold NaBH4 (6 × 10−2 M) aqueous solution under magnetic stirring conditions. A piece of catalyst (the obtained dendritic Ag nanostructure sample and Ag nanoparticle films) with the size of 5 × 10 mm2 was added into the reaction mixture. The reducing process of 4-NP was monitored by measuring the absorption spectra of the reaction solution at regular intervals.
The structure of the electrodeposited Ag products was characterized by using transmission electron microscope (TEM, JEOL 2010 HT) and scanning electron microscope (SEM, FEG Sirion 200) equipped with an energy-dispersive X-ray spectrometer (EDX). X-ray diffraction (XRD) measurements were performed on a Bruker D8-advance X-ray diffractometer with Cu Kα1 irradiation (λ = 1.5406 Å). The time-dependent absorption spectra of the reaction solution were measured using an UV-Vis spectrophotometer (TU-1810). SERS spectra were measured by using a micro-Raman spectrometer (HORIBA Jobin Yvon LabRAM HR800). The SERS samples were excited by focusing a 488-nm laser beam onto the sample through a × 50 objective.
Results and Discussion
Fabrication of Dendritic Ag Fractal Nanostructures and Effect of Reaction Conditions
According to the above results, the formation of dendritic Ag fractal nanostructures with the uniform size and morphology could be obtained by adjusting AgNO3 concentration, deposition time, deposition current density, and citric acid concentration. Obviously, the whole growth process is a non-equilibrium state as the fast nucleation and growth contribute to the formation of more complicated structures [18, 25, 26, 27, 28, 29, 30]. With the departure from thermodynamic equilibrium, the diverse morphologies of final products were obtained [18, 25, 26, 27, 28, 29, 30]. The diffusion-limited aggregation model can be used to interpret the non-equilibrium fractal growth process [31, 32]. In the formation process of dendritic Ag fractal nanostructures, numerous nanoparticles were firstly formed and then assembled as dendrites through oriented attachment [23, 24, 27]. The anisotropic crystal growth is ascribed to citric acid as the functional capping agent and the selective adhesion to a certain plane of Ag nanoparticles [18, 33, 34, 35].
Catalytic Activities of Dendritic Ag Nanostructures for the Reduction of 4-Nitrophenol
SERS Activities of Dendritic Ag Nanostructures
In conclusion, we have prepared the dendritic Ag nanostructures by a facile and controllable electrochemical deposition method. AgNO3 concentration and electrodeposition time were the key parameters of the formation of well-defined dendritic Ag nanostructures. Dendritic Ag nanostructures exhibited larger SERS enhancement and higher catalytic activity than Ag nanoparticle films. The excellent SERS performance and high catalytic activity should be ascribed to the high-density SERS hot spots and catalyst active sites provided by the large surface area, numerous branches, tips, edges, and gaps of dendritic Ag nanostructures. This work provides a simple route for large-area and shape-controlled synthesis of dendritic Ag nanostructures as an effective catalyst and excellent SERS substrate, which may have great potential in in situ SERS investigation and monitoring of catalytic reactions.
Special thanks to Mr. Qiang Fu from Wuhan University for providing support of SEM characterizations.
This work was supported by the National Natural Science Foundation of China (Nos. 11804093, 11504105, 11565013 and 61764005).
Availability of Data and Materials
All data generated or analysed during this study are included in this published article.
ZQC and JZ designed the experiments and drafted this manuscript. ZQC, ZWL, JHX, and RY performed the experiments. ZLL and SL perform the structural characterization of samples. GLC, YHZ, and XL helped in the data analysis and manuscript modification. All authors contributed to the data analysis and scientific discussion. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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