Mode-selective Raman imaging of metal-organic frameworks reveals surface heterogeneities of single HKUST-1 crystals

TL;DR

This study uses Raman micro-spectroscopy to map and analyze surface heterogeneities in single HKUST-1 MOF crystals, revealing spatial spectral variations and defect-related features with statistical and multivariate analysis.

Contribution

It introduces mode-selective Raman imaging of single MOF crystals, revealing surface heterogeneities and defect features, validated by DFT simulations and supported by multivariate analysis.

Findings

Identified a new vibrational feature in HKUST-1

Mapped spectral heterogeneity across single crystals

Quantified intra- and inter-crystal variability

Abstract

Metal organic frameworks (MOFs) are nanoporous materials with high surface-to-volume ratio that have potential applications as gas sorbents. Sample quality is, however, often compromised and it is unclear how defects and surface contaminants affect the spectral properties of single MOF crystals. Raman micro-spectroscopy is a powerful tool for characterizing MOFs, yet spatial spectral heterogeneity distributions of single MOF crystals have not been reported so far. In this work, we use Raman micro-spectroscopy to characterize spatially isolated, single crystals of the MOF species HKUST-1. In a first step, we validate HKUST-1's Raman spectrum based on DFT simulations and we identify a previously unreported vibrational feature. In a second step, we acquire diffraction-limited, mode-selective Raman images of a single HKUST-1 crystals that reveal how the spectral variations are distributed across the crystal surface. In a third step, we statistically analyze the measured spectral peak positions and line widths for quantifying the variability occurring within the same crystal as well as between different crystals taken from the same batch. Finally, we explore how multivariate data analysis can aid feature identification in Raman images of single MOF crystals. For enabling validation and reuse, we have made the spectroscopic data and simulation code publicly available.