Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf Nanopatterned films of Co 3 O 4 nanopetals C. Maurizio ⁎ , N. Michieli, B. Kalinic, A. Marafon, C. Scian, G. Mattei University of Padova, Physics and Astronomy Department, via Marzolo 8, Padova 35131, Italy ARTICLE INFO Keywords: Hierarchiacal nanostructures Co 3 O 4 nanopetals Transition metal oxide nanostructures Nanosphere lithography Physical vapor deposition Nanostructured photoanodes ABSTRACT Hierarchical transition metal oxide nanostructures are promising materials in the field of catalysis. Physical vapor deposition of a Co film on a mask of self-ordered nanospheres has been used to obtain a 2D-ordered array of Co metallic nanostructures. Then, oxidizing annealing in specific conditions resulted in the progressive oxi- dation of Co to Co 3 O 4 , with the concomitant growth of nanopetals out of the free surface of the deposited film. In this way hexagonal arrays of nanotriangles or of nanodomes decorated with nanopetals have been obtained. X- ray Absorption Spectroscopy shows that the extent of Co oxidation depends on nanostructuring, being the oxidation of nanotriangle and nanodome arrays more rapid than the one of a flat film produced in the same deposition. X-ray diffraction and Scanning Electron Microscopy show that for long annealing time the sample morphology is preserved, and the only crystalline phase is Co 3 O 4 . 1. Introduction Nanostructuring is widely used as an effective way to boost the (electro)photocatalytic performance of transition metal oxides (TMOs), and to increase the sensitivity of TMO-based sensors [1–5]. Indeed, it promotes a huge increase of the active surface of the material. In ad- dition, a proper manipulation of the system at the nanoscale can pre- ferentially expose to the catalytic process those crystallographic planes that are more effective [6]. In the field of photo(electro)catalysis, supported TMO nanostructures are very interesting, since they do not remain dispersed into the solution at the end of the catalytic cycle, can easily be re-used [3] and can constitute effective nanostructured pho- toanodes if a bias is needed for the reaction of interest [7]. It has been shown by different research groups that TMO nanopetals and nanowires can grow out of a metal surface by specific oxidizing annealing condi- tions. Nevertheless, at present, the mechanism of the nanostructure formation is controversial [8–14]. It is known that the TMO catalytic properties can be greatly boosted by coupling with noble metal clusters, whose plasmonic properties can enhance locally the electromagnetic field in the proximity of the oxide, or can be a source of electrons that help the catalytic reaction [15–17]. In this framework, TMO nanostructuring into 2D periodic arrays, for example using simple fabrication techniques as nanosphere lithography [18], could widen the possibilities of coupling with plasmonic arrays that could be fabricated using the same mask. In this paper we have used Co physical vapor deposition coupled with nanosphere litho- graphy to produce 2D-ordered arrays of Co 3 O 4 nanostructures. Using suitable annealing conditions, the growth of Co-oxide nanopetals out of the deposited layer has been induced, so obtaining a system of hier- archical nanostructures, in which each structural unit of the array is further nanostructured at a smaller length scale. It is shown that this process is successful despite the annealing temperature used is higher than the glass transition temperature of the mask. The obtained hier- archical nanostructures are characterized by Scanning Electron Micro- scopy, X-ray diffraction and X-ray Absorption Spectroscopy. It is shown that the film nanostructuring has an effect on the oxidation kinetics of the deposited Co film. These results are promising for further coupling with plasmonic metal nanostructures for (electro)photocatalytic appli- cations. 2. Experimental Co-based 2D-ordered arrays of nanostructures have been obtained by coupling Physical Vapor Deposition (PVD) of a Co film with nano- sphere lithography. A sketch of the process is shown in Fig. 1(a). An array of self-assembled polystyrene (PS) nanospheres (diameter = 1030 nm) has been deposited on a silica or silicon substrate approx- imate size 2 × 2 cm 2 [18]. Then, a Co film, of 200 nm of equivalent thickness, was deposited on top of the nanosphere array by PVD. Starting from a base pressure of 1 × 10 −4 Pa in the sputtering chamber, Ar was used as working gas ( = p 0.5 Pa). The 2-inch Co target was placed on a DC source (power = 70 W). The target to sample distance was about 7 cm and the deposition rate was about 5 nm/min. The same deposition was also performed on a simple flat substrate. The https://doi.org/10.1016/j.tsf.2019.137628 Received 25 June 2019; Received in revised form 9 October 2019; Accepted 9 October 2019 ⁎ Corresponding author. E-mail address: chiara.maurizio@unipd.it (C. Maurizio). Thin Solid Films 691 (2019) 137628 Available online 10 October 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved. T