Rationalizing molecular design in the electrodeposition of anisotropic lamellar nanostructures

Previous work has shown that nanoscale lamellar inorganic-organic hybrid materials can be synthesized on transparent conductive substrates via the electrodeposition of Zn(OH)₂ in the presence of conjugated surfactants. These surfactants introduce p-type semiconducting supramolecular phases; thus, following conversion of the Zn(OH)₂ phase to the n-type semiconductor ZnO, the lamellar hybrids exhibit high photoconductive gains and can exhibit photovoltaic activity. We report here on a family of carboxylated terthiophene-based surfactants designed with systematic modifications to molecular geometry, valency, and flexibility to investigate how these features affect the synthesis of the p-type/n-type semiconducting hybrid materials. We use scanning electron microscopy (SEM) and two-dimensional (2D) grazing-incidence X-ray diffraction (2D-GIXD) to correlate molecular features of the surfactants with growth and orientation of the nanoscale lamellae that form during electrodeposition on either hydrophilic or hydrophobic substrates. We find that molecularly flexible, monovalent terthiophene amphiphiles with linear geometries generate highly oriented and homogeneous films of the nanoscale hybrids, whereas T-shaped geometries, rigid molecules, or divalent surfactants tend to produce more heterogeneous and isotropically oriented lamellae under the same conditions. The critical aggregation concentrations (CAC) of the amphiphiles are higher than the concentrations used during electrodeposition, indicating that the growth and orientation of lamellar structures are mediated by surfactant-substrate interactions, rather than the assemblies they form in bulk solutions. Molecular design in these hybrid systems is a key factor in optimizing function, since dense and macroscopically oriented growth is necessary in both photoconductivity and photovoltaic efficiency of solar cells.