DOI: 10.1002/adma.200701297 Galvanic Replacement Reaction on Metal Films: A One-Step Approach to Create Nanoporous Surfaces for Catalysis** By Vipul Bansal, Harit Jani, Johan Du Plessis , Peter J. Coloe, and Suresh K. Bhargava* Porous materials are of great interest because of their inter- esting structural, optical and surface properties, leading to a wide range of applications. [1,2] Among the various properties of porous materials, their high specific surface area, low den- sity, and cost-effectiveness are particularly notable and make them attractive candidates for catalysis, [3] sensing [4] and actua- tion [5] applications. Nanoporous particles in particular are at- tractive candidates for these applications, because of their higher surface-to-volume ratio in comparison with bulk coun- terparts. There have been numerous efforts towards the solu- tion-based synthesis of porous and hollow metal nanoparti- cles, [6–10] however their applicability is limited as the assembly of individual nanoparticles present in solution is still a major challenge. There have also been limited efforts towards the creation of porosity in thin films. [11] Most of the efforts in this direction have been limited to the de-alloying process, in which one of the components of an alloy is selectively dis- solved using an acid, leaving behind a porous network/chan- nel of the more noble metal component. [11,12] However, the acid-mediated de-alloying process requires the cumbersome task of alloy formation and has limited applicability to those metal alloy systems wherein both of the metal components dissolve into the de-alloying solution (acid). This therefore ne- cessitates the use of a different approach for the creation of porosity in supported metal thin films and/or metal foils, which can be universally applied to all the metallic systems. Recently, galvanic replacement reactions (transmetallation reactions) involving sacrificial metal nanoparticles and suit- able metal ions have been employed by various groups [13–18] for the synthesis of hollow/porous metal [13,14] and metal al- loy [15,16] nanostructures in aqueous [14–17] and organic environ- ments. [18] Galvanic replacement reactions are single-step reac- tions that utilize the differences in the standard electrode potentials of various elements, leading to deposition of the more noble element and dissolution of the less noble compo- nent. The electroless nature of galvanic replacement reactions provides them the unique and significant advantage of sim- plicity. Very recently, the potential of galvanic replacement reactions was also explored for the epitaxial growth of thin films. [19] In this study, we have investigated the scope of galvanic re- placement reactions as a versatile tool for the creation of nanoscale porosity in metal foils as a representative of bulk metal surfaces. More specifically, the reaction of Cu 2+ ions with nickel foil results in a Cu-Ni nanoporous surface. The Cu-Ni nanoporous material has been characterized by using various microscopy tools including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Auger microscopy in addition to X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) analysis. Recognizing that such nanoporous surfaces would have a significantly enhanced surface area and surface defect sites relative to their solid counterparts, we show that these nanoporous surfaces are ex- cellent candidates for catalysis applications. The applicability of highly porous Cu on nickel foil has been demonstrated in the catalytic wet air oxidation (CWAO) of ferulic acid (4-hy- droxy-3-methoxycinnamic acid), a model organic compound obtained by the degradation of lignin in pulp mill effluents. [20] Ferulic acid contains the guaiacol moiety prominent in both hardwood and softwood lignin as well as a propenyl substitu- ent similar to that found in various linkages between lignin constituents, which are extremely difficult to oxidize under ambient conditions and pose a major threat to water reser- voirs. [21] Our studies indicate that a nanoporous Cu-Ni catalyst is able to oxidize ferulic acid under significantly milder condi- tions than conventional wet air oxidation processes. The Cu-Ni nanocatalyst prepared using transmetallation re- action was analyzed using SEM. Figure 1A shows the repre- sentative SEM image recorded from nickel foil before its reaction with Cu 2+ ions. The SEM image clearly indicates a relatively flat surface of the nickel foil used to prepare the cat- COMMUNICATION Adv. Mater. 2008, 20, 717–723 © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 717 – [*] Dr. S. K. Bhargava, V. Bansal, H. Jani, J. Du Plessis, P. J. Coloe School of Applied Sciences RMIT University GPO Box 2476V, Melbourne, VIC 3001 (Australia) E-mail: suresh.bhargava@rmit.edu.au [**] V.B. thanks the Australian Research Council for ARC-International Visiting Research Fellowship. The authors thank Mr. Desmond Lau for assistance with EDX and EELS measurements. Supporting Infor- mation is available online from Wiley InterScience or from the authors. Figure 1. SEM images showing the surface of the nickel foil A) before, and B) after its reaction with Cu 2+ ions for 30 min.