A facile approach to prepare silicon-based Pt-Ag tubular dendritic nano-forests (tDNFs) for solar-light-enhanced methanol oxidation reaction
© Lin et al.; licensee Springer. 2015
Received: 28 November 2014
Accepted: 30 January 2015
Published: 18 February 2015
In this paper, a facile two-step Galvanic replacement reaction (GRR) is proposed to prepare Pt-Ag tubular dendritic nano-forests (tDNFs) in ambient condition for enhancing methanol oxidation reaction (MOR) under solar illumination. In the first GRR, a homogeneous layer of silver dendritic nano-forests (DNFs) with 10 μm in thickness was grown on Si wafer in 5 min in silver nitride (AgNO3) and buffer oxide etchant (BOE) solution. In the second GRR, we utilized chloroplatinic acid (H2PtCl6) as the precursor for platinum (Pt) deposition to further transform the prepared Ag DNFs into Pt-Ag tDNFs. The catalytic performance and solar response of the Pt-Ag tDNFs toward methanol electro-oxidation are also studied by cyclic voltammetry (CV) and chronoamperometry (CA). The methanol oxidation current was boosted by 6.4% under solar illumination on the Pt-Ag tDNFs due to the induced localized surface plasmon resonance (LSPR) on the dendritic structure. Current results provide a cost-effective and facile approach to prepare solar-driven metallic electrodes potentially applicable to photo-electro-chemical fuel cells.
Direct methanol fuel cell (DMFC) has been deemed as one of the important power suppliers for renewable power applications due to the high energy-conversion efficiency thereof [1,2]. One of the major issues of DMFCs is the slow process of methanol oxidation reaction (MOR), which directly limits the efficiency of DMFC . Traditionally, platinum (Pt)-based alloy has been used as common a catalyst in MOR. In the past two decades, many bimetallic catalysts have been proposed to enhance the efficiency of MOR, including Pt-Ru , Pt-Ag , Pt-Au , etc. Recently, metal-oxide-supported Pt catalysts, including Pt-TiO2 , Pt-ZnO , and PtRu-TiO2 , were proposed to boost methanol oxidation under ultraviolet (UV) illumination for photo-electrochemical fuel cells . Although over 60% of enhancement on MOR has been realized under UV illumination (365 nm, 100 W) , seldom, reports discussed the solar enhancement toward MOR, especially on pure metallic catalysts. In this paper, a facile two-step Galvanic replacement reaction (GRR) is proposed to prepare Pt-Ag tubular dendritic nano-forests (tDNFs) in ambient condition for enhancing MOR under solar illumination.
In preparation of the aforementioned bimetallic catalysts, GRR was widely employed to provide a simple and cost-effective fabrication approach [10,11]. By utilizing the difference in the standard reduction potentials, replacement between two metals can be easily achieved at ambient condition. Many metal composites prepared by GRR have been reported, including Ag-Au [12,13], Pt-Au [14,15], Pd-Pt [16,17], Ag-Pt [18-21], Pd-Ag [22,23], Cu-Pd , and Cu-Ag . However, most of the studies focused on the preparation of non-supported catalysts. The prepared catalysts suspended in the solution could be hardly collected and deposited on the electrodes in the electrochemical cells. Moreover, the effective electrochemical surface area of the non-supported catalysts could be greatly sacrificed due to the aggregation of nano-catalysts in brushing or printing process [4,26].
In order to prepare metallic nanostructures directly on supporting substrates, fluoride-assisted Galvanic replacement reaction (FAGRR) was proposed to synthesize three-dimensional metallic dendrites on silicon-based substrates [27-31]. Recently, Ye et al. reported a facile method for preparing self-assembled silver dendrites on silicon wafer in fluoride and silver nitride solution [27,29] for improving surface-enhanced Raman spectroscopy (SERS) [27-29,31]. However, the prepared silver dendrites could be easily contaminated by sulfur or oxygen to from Ag2O or Ag2S at ambient [32,33], which directly limits the applications for catalytic reactions.
In this paper, we propose the preparation of Si-based Pt-Ag tDNFs for solar-light-enhanced MOR by a two-step facile GRR at ambient without any energy input. This self-assembled Pt-Ag tDNFs not only benefit from the large aspect surface area provided by the Ag dendrites but also the localized surface plasmon resonance (LSPR) effect for the enhancement of methanol electrode-oxidation. Besides, the Pt outer shell of Pt-Ag tDNFs provides a protection to Ag and thus greatly enhances the stability of the prepared photo-electrodes.
Preparation of Ag DNFs and Pt-Ag tDNFs
Surface morphology and material characterization
Scanning electron microscope (SEM; Hitachi FE 4300 in Instrument Technology Research Center (ITRC); Hitachi, Tokyo, Japan) and field emission transmission electron microscope (TEM; JOEL JEM2100F in National Chung Hsing University (NCHU); JOEL Ltd., Tokyo, Japan) were employed to investigate the surface profile of the prepared samples. Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the elemental composition.
Investigation on photo-enhanced electrochemical reactions
Electrochemical reactions were measured by a potentiostat (Autolab PGSTAT302N in ITRC) in a rectangular three-electrode reaction tank (500 mL) made by quartz. The prepared samples with projection area of 1 cm2 were used as the working electrodes, Pt-coated titanium mesh (25 cm2) as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. Cyclic voltammetry (CV) and chronoamperometry (CA) were used to evaluate the catalytic capability for methanol electro-oxidation. A solar simulator (SADHUDESIGN; class B; 400 to 1,000 nm; 1,000 W m−2) was employed for the illumination experiments. All chemicals used in this experiment were reagent grade. The resistance of DI water was 18.2 MΩ. All experiments were conducted at 22°C at ambient pressure.
Results and discussion
Surface morphology of Ag DNFs
Surface morphology of Pt-Ag tDNFs
Photo-enhancement on methanol electro-oxidation
The mechanism of LSPR enhancement on electro-catalysis is a synergetic process, comprising plasmonic heating, magnification of local electromagnetic field, electron injection process, etc. [40-43]. Although more experiments are required to further elucidate the energy transfer process, current results provided a direct observation on the oxidation current enhancement and OCP variation under solar illumination in methanol electro-oxidation. The cost-effective and easily-prepared silicon-based Pt-Ag tDNFs are active to solar illumination and have high potential to serve as promising candidates for photo-electrochemical fuel cells.
In this letter, focus is placed on the facile two-step GRR to prepare silicon-based Pt-Ag tDNFs in ambient condition for enhancing MOR under solar illumination. The FAGRR enables the fast growth of Ag NDFs on the silicon wafer within 5 min. Following that, the chloroplatinic acid further transformed the surface of Ag NDFs into Pt nano-shells and emptied the structure simultaneously within another 5 min. The prepared Pt-Ag tDNFs showed solar response (6.4% of enhancement on oxidation current) toward methanol oxidation. The solar response is attributed to the strong LSPR provided by the Ag DNFs. This cost-effective Pt-Ag tDNFs could be a promising candidate for photo-electrochemical fuel cells.
The authors would like to express appreciation to the Ministry of Science and Technology of the Republic of China, Taiwan, for the financial supports under the following contract numbers: MOST103-2321-B-007-004, NSC102-2321-B-007-006, MOST103-3113-E-007-006, NSC102-2627-M-007-002, MOST104-2623-E-492-001-ET, and NSC102-2221-E-492-003. Also, we appreciate Mr. Chih-Jung Lu at the Instrument Center in National Chung Hsing University for his help in TEM operation.
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