Plasmonic Catalytic Layer for Visible-Light Enhanced Methanol Oxidation Reaction

Abstract

Methanol oxidation reaction (MOR) has been an important topic in electrochemical fuel cells [1]. However, the slow kinetics of MOR brings the bottleneck that directly limits the efficiency of direct methanol fuel cells (DMFCs). Traditionally, platinum (Pt) based alloys, e.g. PtRu, PtAu, PtCu, and etc., are used as the catalysts for MOR [2]. Aiming at enhancing the MOR, titanium dioxide and other semiconductors have also been introduced to the anodic design of DMFCs [3]. Many nano-composite have been proposed as the catalysts for MOR, including Pt-TiO2 [4], Pt-ZnO [5], Pt-WO3 [6] and etc.. In another field of physical chemistry, plasmonic catalysis has been a rising field in photochemistry for more than one decade [7]. Noble metals, e.g., gold (Au), silver (Ag), Pt are common candidates for plasmonic catalysis. Besides, nano-composites , e.g., Ag-SiO2, Au-TiO2, Au-SiO2, and etc., have also attracted enormous attentions. By utilizing metallic nanostructures and/or metal-semiconductor nano-composites, visible light can be harvested and transformed into chemical energy to trigger or enhance chemical reactions, e.g. water splitting [8], hydrogen dissociation [9], steam reforming of ethanol [10], degradation of ammonium [11]. In this paper, self-supported metallic nano-dendrites are proposed as plasmonc catalytic layer for visible-light enhanced methanol oxidation reaction. Besides, an extra facile fluoride assisted galvanic replacement was proposed to prepare self-supported Ag and Ag nano-dendrites on Si base. These structures can be achieved by simply immersing Si wafers into the mixture of AgNO3 (or HAuCl4) and buffered oxide etchant (BOE) under ambient conditions within 5 minutes [12, 13]. The prepared Au (or Ag) nano-dendrites were used to boost methanol oxidation reaction under visible-light illumination (AM 1.5G). Obvious photo-enhancement on the oxidation current (28%) was achieved on Au nano-dendrites. A comparative study was conducted by measuring the photo-enhancement under various illumination wavelengths. The best photo-response was found at 700 nm. Moreover, the self-supported Au nano-dendrites with various pore sizes and branch sizes not only provide high aspect surface area but also enable a broadband plasmonic enhancement. In summary, we proposed a cost-effective and facile approach to prepare Ag and Au dendrites on Si base under ambient conditions. Both Ag and Au nano-dendrites show promising potential in plasmon enhanced methanol oxidation reaction. We expect these structures may soon find application in photo-electrochemical cells.