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

Abstract

The layer-by-layer epitaxial growth of HKUST-1 (Cu 3 (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 SiO 2 , 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 CO 2 adsorption isotherms, depends on film properties (morphology and grain size) as well as film age. For films deposited on hydroxylated SiO 2 surfaces, CO 2 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 CO 2 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.

Carbon dioxide measurements using long period grating optical fibre sensor coated with metal organic framework HKUST-1

“…HKUST-1 structure can collapse as a result of water adsorption. This effect was previously described for degradation with time of a HKUST-1 film stored under elevated relative humidity levels (49).…”

“…No changes in mass frequency during the deposition when the substrates were immersed in ethanol was observed (49).…”

“…Alternatively, the LPG was coated with HKUST-1 using LbL method described in details elsewhere (49). …”

“…The changes in the transmission spectrum of Array A as the response to CO2 levels in a range of 2,000 - The increase of the sensitivity could be achieved by increasing the thickness of the film however it was reported that film consisting of 80 layers lost the preferred {222} crystal orientation enabling the optimal conditions of CO2 adsorption (49). Higher sensitivity can be obtained by optimising the LPG period to operate at the PMTP described in the next section.…”

“…In particular, the LbL approach for HKUST-1 leads to the fabrication of films with higher crystal density, conforming into homogenous films with preferred {222} orientation (49).…”