Silica cubes with tunable coating thickness and porosity: From hematite filled silica boxes to hollow silica bubbles

• Preparation of colloidal silica cubes with tunable coating thickness and porosity.
• The thickness is governed by the amount of silica precursor.
• The silica etching process for larger pores is monitored by IR spectroscopy.
• A higher amount of the protective polymer PVP leads to a lower etching rate.
• The molar mass of the protective polymer PVP influences the porosity of the interior.

We investigate the material properties of micron-sized silica coated cubic colloids, focusing on the coating thickness and porosity. The thickness of the silica coating of core–shell α-Fe2O3@SiO2 cubes and their corresponding hollow cubes can be tuned between 20 and 80 nm, spanning the range of silica bubbles to silica boxes. The porosity of the silica cubes can be increased controllably by surface-protected etching using hot water as mild etchant and polyvinylpyrrolidone (PVP) as protecting polymer. We introduce infrared spectroscopy as a quantitative tool to monitor the extent of etching over time and to evaluate the influence of PVP on the etching process. The molar mass of PVP does not affect the etching rate, whereas an increased amount of PVP leads to enhanced protection against etching. Silica etching is found to be a two-step process, comprising a fast initial etching followed by a slower continuation. Hollow, porous silica cubes maintain their shape after extensive thermal treatment, demonstrating their mechanical stability.

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