Abstract
Producing crystals of the desired form (polymorph) is currently a challenge as nucleation is yet to be fully understood. Templated crystallization is an efficient approach to achieve polymorph selectivity; however, it is still unclear how to design the template to achieve selective crystallization of specific polymorphs. More insights into the nanoscale interactions happening during nucleation are needed. In this work, we investigate crystallization of glycine using graphene, with different surface chemistry, as a template. We show that graphene induces the preferential crystallization of the metastable α-polymorph compared to the unstable β-form at the contact region of an evaporating droplet. Computer modeling indicates the presence of a small amount of oxidized moieties on graphene to be responsible for the increased stabilization of the α-form. In conclusion, our work shows that graphene could become an attractive material for polymorph selectivity and screening by exploiting its tunable surface chemistry.
Highlights
Producing crystals of the desired form is currently a challenge as nucleation is yet to be fully understood
Two types of crystallization experiments are performed: (i) additive-templated crystallization, in which solution-processed graphene was directly added into the droplet containing glycine, which is drop-casted onto Si/ SiO2 (Figure 1a); and (ii) substrate-templated crystallization, Figure 1
Solution-processed graphene was produced by electrochemical exfoliation (ECE) of graphite[30] using two different salts: (NH4)2SO4 and KHSO4, as this allows for the production of graphene with different surface chemistry: graphene produced with (NH4)2SO4 (Gr ECE(NH4)) contains, on average, a lower C/O ratio compared to that of graphene produced with KHSO4 (Gr ECE(K)).[31]
Summary
Producing crystals of the desired form (polymorph) is currently a challenge as nucleation is yet to be fully understood. We use a particular type of nanomaterial as template: graphene, the most famous twodimensional crystal, characterized by unique properties.[17,18] This material is very attractive to study crystallization because, being entirely a surface, its surface properties can be tuned via covalent functionalization[19−22] and electrostatic doping[23] and it is solution processable.[24,25] This allows us to perform two types of crystallization experiments: one where graphene is used as a substrate, and another where graphene is used as an additive This is in contrast to previous works, where the template is tailored exclusively for being either a substrate[13] or an additive.[15] the tunable surface chemistry of graphene can be used to identify the specific intermolecular interactions responsible for the crystallization of a particular polymorph. Computer modeling indicates this selectivity to be related to the presence of hydroxyl groups allowing for hydrogen bonding interactions with the glycine molecules, thereby favoring the α-form more than the β-form once additional layers of the polymorphs are added during crystal growth
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