Modulating the immobilization process of Au nanoparticles on TiO2(110) by electrostatic interaction between the surface and ionic liquids

We found that the adsorption states of Au nanoparticles (NPs) on TiO2(110) varied with the ionic liquids (ILs) that encapsulate the NPs during the entire process from synthesis of NPs to immobilization of the NPs on the surface at 323 K. Five kinds of ILs, including three 1-butyl-3-methylimidazolium ([C(4)MIm]) cation-based ILs of tetrafluoroborate ([C(4)MIm][BF4]), hexafluorophosphate ([C(4)MIm][PF6]) and bis(trifluoromethylsulfonyl)amide ([C(4)MIm][NTf2]), and two [BF4] anion-based ILs of 1-(methoxymethyl)-1-methylpyrrolidinium ([PY1,1O1][BF4]) and N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium ([N122,1O2][BF4]), were used. They provided different adsorption states of the Au NPs on the surface. We considered the determinant factor for the adsorption states of NPs on the surface from the viewpoint of electrostatic interactions between IL and IL and between IL and the surface. We deduced the influence of these interactions from the increments of NP's diameter and the amount of NPs after the immobilization, and obtained the order of anions as [BF4] approximate to [NTf2] > [PF6] for the three [C(4)MIm] based ILs and the order of cations as [PY1,1O1] > [C(4)MIm] > [N122,1O2] for the three [BF4] based ILs. Interestingly, the above orders were no match with the conventional physicochemical and chemical parameters but beta of Kamlet-Taft solvent descriptors and Lewis basicity of anions in the case of anions of ILs. The results of anions indicate that the chemical natures of ILs might affect the immobilization of NPs through hydrogen bonding or coordination bonding with the surface. We suggested that such a trend in anions was 'local viscosity' of ILs in the vicinity of surface. Besides, layer formation of IL on the surface might be also an influential factor to control the size of NPs and the surface density of NPs. Our results show that the combination of a charged surface and charged molecules has the potential for controlling locations of the NPs on the surface preferentially with tuning the electrostatic interaction.