# Lattice variation upon water adsorption in silica opals measured by in situ atomic force microscopy

## TL;DR

This study shows that water adsorption causes reversible expansion in silica opal structures, observed using atomic force microscopy.

## Contribution

The paper provides direct in situ measurements of lattice expansion in silica opals due to water adsorption.

## Key findings

- Water adsorption causes reversible lattice expansion of up to several nanometers in silica opals.
- The expansion is attributed to sphere swelling due to micropore filling in silica.
- In situ atomic force microscopy enables precise observation of these structural changes.

## Abstract

Colloidal photonic crystals, or artificial opals, derive their unique optical properties from the periodic arrangement of their constituent particles. Central to the functionality of these materials is the precise control over their lattice parameter, which directly determines the photonic bandgap. Adsorption of environmental vapors may have a significant impact on the photonic response, most of which has been attributed to lattice variations. Here we aim to directly measure the arrangement and lattice parameter in opals made of silica spheres by using in situ atomic force microscopy (AFM) under changing ambient conditions. The periodicity of the opal structure offers a decisive advantage for such study, enabling the use of correlation tools for image processing. Opals having high mechanical stability (avoiding lattice distortions or sphere detachment during scanning) allow reliable AFM data and good analytical resolution. We show direct evidence of reversible lattice increments upon water adsorption, up to several nanometres in agreement with prior indirect estimates, which is compatible with swelling of the spheres due to filling of silica micropores.

This study shows reversible lattice expansion in silica opals due to water adsorption, observed via in situ AFM. The nanometer-scale changes, linked to sphere swelling and silica micropore filling, affect their photonic properties.