Sc2BDC3 prepared by Miller et al. formed yellow crystals in excess of 160 µm along the z-axis.39 Figure 2.19 below shows SEM images of Sc2BDC3, synthesised simultaneously from the same procedure in one oven in three different reaction vessels, which show polydisperse crystallites within the range of 50–700 µm across the three batches.
Figure 2.19. SEM images at 100x magnification of Sc2BDC3 crystals from simultaneous synthesis in three different vessels.
The images in Figure 2.19 do not show reproducible production of consistently sized crystals, even under the same conditions. Ideal Sc2BDC3 crystals for subsequent analyses were 100–250 µm in length and ~50 µm deep, preferably with significantly reduced polydispersity than those shown on the previous page.
In order to control the range of sizes of Sc2BDC3 crystals, we employed a doping method, first described by Wang and co-workers to regulate crystal growth of ZIF-8.40 The doping method involves a variable R factor, described below in Equation 2.1, which is a molar ratio of metal salt that is pre-heated with the linker before combining it with the remaining metal salt. Here, the approach pre-binds the carboxylate groups of the terephthalic acid linker with a Sc3+ cation in order to control nucleation of the desired product. Equation 2.1 defines R, where n is number of moles and pre-M+ refers to the metal being pre-heated with the linker.
𝑅 = 𝑛(𝑝𝑟𝑒-𝑀+) 𝑛(𝑜𝑟𝑔𝑎𝑛𝑖𝑐 𝑙𝑖𝑔𝑎𝑛𝑑)
Equation 2.1. R factor for control of crystal polydispersity.
Experiments varying R (i.e. the mass of Sc(NO3)3 introduced prior to combination of linker and bulk metal salt) and final heating time were carried out to examine the effects of both factors on particle size. Variations in experimental parameters are shown below in Table 2.2.
Table 2.2. Reagent masses and associated R factors for doping experiments.
Reagents from the synthesis by Miller et al are highlighted in blue, and are taken from reference 39.
BDC
Chapter 2 - Tracking fluorescent guests in metal-organic frameworks
The doping experiments were performed by introducing a precise amount of Sc(NO3)2 (indicated in Table 2.2) to a slurry of terephthalic acid linker in water and heating at 70 °C with stirring for 2 hours. The remaining Sc(NO3)2 was heated in water at 70 °C with stirring for 2 hours. The solution and slurry were subsequently combined and heated at 220 °C for 48 hours.
Figure 2.20. SEM images at 59/60x magnification of Sc2BDC3, synthesised by the pre-doping method using the stated R factors. Reaction time was 48 hr in all cases.
Figure 2.20 shows that pre-doping the linker slurry with different amounts of Sc(NO3)3 heavily influences the size of crystals that grow. SEM images illustrate how lower R factors of 0.017 and 0.067 encourage the formation of cuboctahedra up to ca. 400 µm in size. The larger doping ratios, 0.12 and 0.01, promote the growth of hexagonal rods up to ca. 800 µm in length. Crystal size variation was qualitatively lowest in the sample with the smallest R factor of 0.017, where crystals range from 150–300 µm.
R = 0.017 R = 0.067
R = 0.12 R = 0.17
Optimum crystals for single-crystal infrared microscopy experiments, described in Section 1.7 on pp.15, are plates with a thickness of up to 20 µm. The cuboctahedra shown in Figure 2.20, synthesised using R = 0.017 and 0.067, are therefore too large in all directions. Similarly, even across the shorter axes, the hexagonal rods synthesised using R = 0.12 are also too thick. Smaller cuboctahedra in the region of 20 µm would be more likely to exhibit scattering effects, described in Section 1.7 on pp.16. Hexagonal rods minimise both saturation and scattering effects due to differing distances along the a, b and c crystal axes. Doping ratios R = 0.12 and 0.17 were therefore selected for experiments investigating the effect of reaction time on crystal formation.
SEM images of as-synthesised materials from 24- and 48-hour syntheses are shown in Figure 2.21.
Figure 2.21. SEM images at x100 magnification of Sc2BDC3 synthesised with R = 0.12 (top) and 0.17 (bottom) with indicated reaction times.
R = 0. 17 R = 0. 12
24 h 48 h
24 h 48 h
Chapter 2 - Tracking fluorescent guests in metal-organic frameworks
Figure 2.21 does not appear to indicate any trends in polydispersity and discrete crystals in these images are of widely varying sizes. Crystals grown with R = 0.17 show a higher degree of clustering, while crystals grown with R = 0.12 appear more discrete. All crystals shown are far too large for IR microscopy experiments.
In an attempt to decrease the overall average crystal size produced, the total metal and ligand concentration in the reaction mixture used for the aforementioned R = 0.12 synthesis was increased, from 0.00087 and 0.0013 mM to 0.0011 and 0.0017 mM, for Sc(NO3)3 and terephthalic acid respectively.
Figure 2.22. SEM images at 50x magnification of Sc2BDC3 synthesised from reaction solution at five-fold concentration, with indicated reaction times.
We also investigated the time evolution of crystal size at three time points during the synthesis. The SEM images in Figure 2.22 show Sc2BDC3 crystals of varying sizes and extents of aggregation. The reaction heated for 24 hr contains mainly smaller hexagonal rods, often in clusters. The 48 hr reaction also contains clusters of hexagonal rods, however the rods herein are significantly larger than the shorter timescale reaction. The longer 72 hr reaction time appears to fracture the longer rods, forming instead numerous clusters of smaller crystallites.
Given the outcomes of qualitative analysis of SEM images of the materials, a doping factor of R = 0.12 in the more concentrated reaction solution was used to synthesise size-optimised Sc2BDC3 crystals. An experimental procedure for the final synthesis is in Section 2.5.4 on pp.121.