HKUST‑1 Thin Film Layer-by-Layer Liquid Phase Epitaxial Growth: Film Properties and Stability Dependence on Layer Number

The layer-by-layer epitaxial growth of HKUST-1 (Cu₃(btc)₂ where btc = 1,3,5-benzenetricarboxylate) thin films is measured by quartz crystal microbalance with dissipation monitoring (QCM-D), X-ray diffraction (XRD), and scanning electron microscopy (SEM) as a function of the number of layers (20–80 layers) for −OH and −COOH functionalized surfaces. Up to approximately 40 layers, the film growth proceeds by a layer-by-layer mode controlled by the chemical functionalization of the surface. For example, on hydroxylated SiO₂, film growth is in the preferred [222] direction. Beyond 40 layers, for both −COOH and −OH functionalized surfaces, the crystallite grain size increases and ∼50–100 nm octahedral crystals are formed. Independent of the surface functional groups (−COOH and −OH), the octahedral crystals form with the {200} planes oriented parallel to the surface. By monitoring changes in mass and dissipation, the QCM data provides evidence for the change in growth behavior. The stability of the films, determined by measuring CO₂ adsorption isotherms, depends on film properties (morphology and grain size) as well as film age. For films deposited on hydroxylated SiO₂ surfaces, CO₂ uptake decreases rapidly within a few days after film synthesis with the 40 layer films ({222} planes) exhibiting a more pronounced decrease than the 80 layer films ({200} planes, octahedral crystals). The decrease in CO₂ uptake is attributed to the differing propensities for water uptake in thin films of different morphologies as evidenced by water vapor adsorption isotherms and Raman spectral changes.