Inducing differences in modification degree and binding mode of organophosphonic acid grafted titania by changing pH and hydrocarbon chain length

Abstract

A R T I C L E I N F O Keywords: TiO 2 surface modification (amino)alkylphosphonic acids Hydrocarbon chain length pH NH 2 /NH 3 + ratio A B S T R A C T Organically modified metal oxide surfaces are of interest in many applications since they combine the advantages of metal oxide supports (structural properties, chemical and mechanical stability) and organic functionalities (for specific surface interactions). Although surface modification with organophosphonic acids (PAs) with an alkyl or aminoalkyl functional group on TiO 2 has been investigated previously, knowledge of the synthesis-properties correlation (e.g. the binding mode of the PAs) is still lacking, especially for functional group-surface interactions. These are however important as they can influence functional group availability in applications such as (metal) sorption. In this work, the dependence of modification degree and phosphorus chemical environment on pH and chain length (C1 to C6) was investigated with TGA, ICP-OES, nitrogen/argon sorption, XPS, and solid-state 31 P NMR and DFT calculations. The TGA and the ICP-OES results showed a clear impact of pH on surface modification degrees, with a different response for modification with alkyl-and aminoalkylphosphonic acids, featuring a more rapid decrease in modification degrees from pH 2 upwards for the alkylphosphonic acids compared to the aminoalkylphosphonic acids. Moreover, a clear correlation can be found between the amino-alkyl chain length and the NH 2 /NH 3 + ratio. In addition, a positive correlation between the modification degree and the protonation degree of the amine group is observed for aminomethylphosphonic acid (AMPA) and 2-ami-noethylphosphonic acid (2AEPA) modified samples, while this is absent for longer alkyl chains. This is supported by DFT calculations that indicate that the most stable binding modes of AMPA and 2AEPA grafted on the anatase (1 0 1) surface include hydrogen bonds between NH 3 + and the surface oxygen atoms, in contrast, the most stable binding modes for 3-aminopropylphosphonic acid (3APPA), 4-aminobutylphosphonic acid (4ABPA), and 6-ami-nohexylphosphonic acid (6AHPA) grafted on the anatase (1 0 1) surface involve a Lewis acid-base interaction between NH 2 and a surface Ti site.