The State of Electrodeposited Sn Nanopillars within Porous Anodic Alumina from in Situ X‑ray Observations Weronica Linpe ́ ,* ,† Gary S. Harlow, † Jonas Evertsson, ‡ Uta Hejral, † Giuseppe Abbondanza, † Filip Lenrick, † Soenke Seifert, ∥ Roberto Felici, ⊥ Nikolay A. Vinogradov, § and Edvin Lundgren † † Division of Synchrotron Radiation Research, Lund University, SE-22100 Lund, Sweden ‡ Photon Science, DESY, Notkestrasse 85, Hamburg 22607, Germany § MAX IV Laboratory, SE-22594 Lund, Sweden ∥ Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439-4838, United States ⊥ SPIN-CNR c/o DICII, University of Rome Tor Vergata, Via del Politecnico 1, I-00133 Roma, Italy * S Supporting Information ABSTRACT: Porous anodic alumina (PAA) can be used as a template for controlled electrodeposition and growth of nanostruc- tures; it is also essential for long-lasting decorative coloring of aluminum. We have investigated the deposition of Sn nanopillars into PAA in situ, with grazing transmission small-angle X-ray scattering, X- ray fluorescence, and X-ray absorption near edge structure spectros- copy. An accumulation of Sn could be detected in the pores during the electrodeposition. From the X-ray absorption near edge structure spectroscopy measurements we could conclude that the deposited Sn was primarily in the metallic state. Ex situ scanning electron microscopy cross-section measurements show Sn nanopillars inside the PAA. KEYWORDS: porous anodic alumina, glossy coloring, GTSAXS, XANES, XRF ■ INTRODUCTION When aluminum is exposed to air, a thin native oxide layer will form at the aluminum−air interface. This oxide can be grown thicker through anodization. 1,2 Mainly depending on the electrolyte used for the anodization process, either a homogeneous barrier layer oxide or a porous oxide can be grown. A porous oxide is grown when aluminum is anodized in acidic electrolytes in which the oxide is soluble. 3 A field- assisted dissolution of the oxide has been suggested as the reason for pore formation, but nuclear-tracer studies have proved inconclusive. Other studies have suggested that the pores form due to plastic flow of the oxide caused by stress. 4 Depending on the acid used, certain voltages close to the breakdown potential 5 lead to a self-organization of the pores into a hexagonal arrangement. 6 The ordering of the pores improves the longer the sample is anodized, creating an oxide which will have a greater ordering closer to the pore bottoms than the top of the oxide. 7 A two-step anodization process can be used for growing an oxide with pores ordered throughout the whole oxide. This is achieved by removing the oxide grown in the first anodization step via a chemical etch, leaving the aluminum substrate with a patterned surface of nanoconcaves. This patterned surface will guide the growth of the oxide when reanodizing, creating well-aligned and ordered pores. 8 The PAA can be used as a template for creating and growing high aspect ratio nanowires, magnetic storage devices, solar cells, sensors, and decorative coloring of aluminum. 9−11 Electrochemical deposition into the PAA is a useful method for the growth of nanowires inside of the pores as well as for the coloring of aluminum products. Through this method metals can be deposited in a controlled manner into the PAA. There are many examples in the literature of the deposition of different metals, such as Fe, 12 Ni, 13 Cu, 14 Pd, 15,16 Ag, 17 and Au. 18 Because of cathodic side reactions in the oxide, a buildup of charge in the barrier layer (BL), and diffusion of metal ions within the pores, direct current (dc) deposition into on- substrate PAA is often ineffective, leading to no electro- deposition or only local electrodeposition in a few pores. 19 This has led to the development of alternating current (ac) 20−22 and pulsed electrodeposition (PED) techni- ques. 14,15,19,23 High potentials are required for electrons to tunnel through the barrier layer during electrodeposition. To overcome this problem, one can remove the PAA from the aluminum and attach or coat the base with another more conducting material after removal of the barrier layer. 12,16 Another method to improve the deposition into the PAA is to evaporate an aluminum film on a conducting material and then Received: March 4, 2019 Accepted: April 30, 2019 Published: April 30, 2019 Article www.acsanm.org Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acsanm.9b00408 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX Downloaded via 107.173.112.136 on May 9, 2019 at 16:07:18 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.