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Porphyrins as Templates for Site-Selective Atomic Layer Deposition: Vapor Metalation and in Situ Monitoring of Island Growth Jason R. Avila,†,# Jonathan D. Emery,#,§ Michael J. Pellin,†,#,§ Alex B. F. Martinson,#,§ Omar K. Farha,*,†,#,‡ and Joseph T. Hupp*,†,#,§ †
Department of Chemistry and #Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, 2145 Sheridan Road Evanston, Illinois 60208, United States ‡ Faculty of Science, Department of Chemistry, King Abdulaziz University, Jeddah 21577, Saudi Arabia § Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Examinations of enzymatic catalysts suggest one key to efficient catalytic activity is discrete size metallo clusters. Mimicking enzymatic cluster systems is synthetically challenging because conventional solution methods are prone to aggregation or require capping of the cluster, thereby limiting its catalytic activity. We introduce site-selective atomic layer deposition (ALD) on porphyrins as an alternative approach to grow isolated metal oxide islands that are spatially separated. Surface-bound tetra-acid free base porphyrins (H2TCPP) may be metalated with Mn using conventional ALD precursor exposure to induce homogeneous hydroxide synthetic handles which acts as a nucleation point for subsequent ALD MnO island growth. Analytical fitting of in situ QCM mass uptake reveals island growth to be hemispherical with a convergence radius of 1.74 nm. This growth mode is confirmed with synchrotron grazing-incidence small-angle X-ray scattering (GISAXS) measurements. Finally, we extend this approach to other ALD chemistries to demonstrate the generality of this route to discrete metallo island materials. KEYWORDS: atomic layer deposition, porphyrin, manganese oxide, island nucleation, vapor metalation, quartz crystal microbalance, grazing incident small angle scattering
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difficulties, clusters have been used as a means of evaluating charge transfer mechanisms for photocatalysts18 and energy storage.22,23 In this study, we show that template-directed atomic layer deposition (ALD) can produce an assembly of islands that can mimic the structure of metallo clusters, which may lead to well-defined and facile catalysis. ALD is a low-vacuum vapor deposition method in which sequential exposures of a substrate to reactive precursors are separated in time by inert gas purging to induce self-limiting reaction at the substrate interface.24,25 Conventional ALD of oxides typically involves sequential dosing of a volatile inorganic metal compound (e.g., Mn, Fe, etc.) and an oxygen source (water or ozone) to deposit a thin film. As an example, MnO ALD starts by exposing Mn(II) bisethylcyclopentene (Mn(CpEt)2) to a hydroxo-terminated surface to produce a uniformly metal−ligand-terminated surface. A subsequent purge period removes unreacted and physisorbed precursor from the surface and gas phase. Next, water is dosed, cleaving
he ever-increasing concern over anthropogenic climate change has created a great demand for more effective materials for renewable energy production and energy storage. Materials utilizing cost-effective metals including Ni, Fe, and Mn show promise for electrochemical storage1,2 and photo/ electrochemical catalysis,3−9 but are limited largely because of low bulk conductivity, restricting activity to the surface of electrodes.1,6,10 To maximize the effectiveness of materials dominated by surface reactivity, researchers have been prioritizing either the synthesis of 2D materials, such nanowires or fibers, or the functionalization of 3D networks, such as inverse opals.9,11 Natural systems also reveal that metallo clusters are among the most effective means of maximizing performance of metal oxide materials.12−15 Nanocluster sites typically contain only a few atoms (
