Detailed Study of the Nanocasting Process by in Situ X‑ray Scattering and Diffraction Motolani Sakeye, † Sebastian Ziller, ‡ Heinz Amenitsch, § Mika Linde ́ n, ‡ and Jan-Henrik Sma ̊ tt* ,† † Laboratory of Physical Chemistry and the Center for Functional Materials, Åbo Akademi University, Porthansgatan 3-5, 20500 Turku, Finland ‡ Inorganic Chemistry II, University of Ulm, Albert-Einstein-Allee 11, 89031 Ulm, Germany § Institute for Inorganic Chemistry, Graz University of Technology, Stremayrgasse 9/V, A-8010 Graz, Austria * S Supporting Information ABSTRACT: The nanocasting method is a valuable tool for producing metal oxides with a well-defined nanostructure. However, the precise details on how the metal oxide is developed inside the mesoporous silica template remain unclear. In this study, we clarify how nickel nitrate species are evolving to nickel oxide and how they are redistributed inside mesoporous SBA-15 particles as a function of heating temperature and surrounding gas atmosphere by a combina- tion of in situ small-angle X-ray scattering, X-ray diffraction and thermogravimetric techniques as well as ex situ transmission electron microscopy and nitrogen physisorption measure- ments. The SBA-15 template was initially impregnated with Ni(NO 3 ) 2 ·6H 2 O using the wet infiltration method. The results indicate an initial redistribution of the nickel nitrate salt located outside the pore system into the mesopores due to dissolution, while at temperatures of 110-150 °C (depending on which type of gas flow is used) the mobility of the salt is lost due to drying of the salt. Above 220 °C, the nickel nitrate decomposes, possibly via nickel hydroxynitrate, to NiO, forming nanoparticles inside the pore channels. The results shed light on the events occurring during the nanocasting process and can be used for further optimization of the fidelity of replication. 1. INTRODUCTION The nanocasting process has proven to be a powerful tool for producing a wide variety of nanostructured metal oxides materials with controlled porosity and morphology. 1-3 Such materials have been utilized in a range of application areas, including separation, heterogeneous catalysis, sensing, bio- materials, and renewable energy. 4 In the nanocasting process, a porous silica template with interconnected pore channels (e.g., SBA-15, KIT-6, or SBA-16) is first infiltrated with a metal oxide precursor, typically in the form of a metal salt. Highly concentrated aqueous or ethanolic solutions of the precursor salts are often used in the infiltration process, 5 but due to the low melting points of many hydrated metal nitrate salts, solvent-free impregnation methods of the support are also available. 6,7 Upon heat treatment, the metal salt is thermally converted to the corresponding metal oxide, and by subsequent etching in hydrofluoric acid or hydroxide solutions, the silica template can selectively be removed to produce a metal oxide replica material with an inverted pore structure but with a similar morphology as the starting template. Thus, the nanocasting approach also allows for a straightforward tuning of the morphology of the replicated material, including mesoporous powders, 8 monoliths, 9 and thin films. 10 However, in most cases, the structural evolution during the nanocasting process can be considered a “black box”, as most often only the final replica structure has been evaluated. Nevertheless, there are a number of reports attempting to address the effects the various processing steps have on the final replica structure. 5,10-16 For instance, Roggenbuck et al. investigated which effects surface modification and solvent polarity had on the infiltration process of SBA-15 silica. 5 However, the authors noticed that the number of free silanol groups on the silica surface did not have a dramatic impact on the final replica structure. A more pronounced influence on the replication fidelity is expected as a result of precursor mobility during the subsequent heating step. Importantly, the dehydration of the metal salts upon heating and also the decomposition of the metal salt to the oxide are associated with quite radical volume decreases (often in the range of 88- 94% 2 ). This is why several impregnation-thermal treatment cycles are often needed to fill up the porosity of the template to a level where a mechanically stable replica can be obtained after the removal of the template. Recently, Sun and co-workers observed that the replication fidelity is to a large extent influenced by the size and the shape of the container in which Received: August 17, 2015 Revised: January 5, 2016 Published: January 5, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 1854 DOI: 10.1021/acs.jpcc.5b07993 J. Phys. Chem. C 2016, 120, 1854-1862