Effect of lithium triflate on the pH-dependent ionization, surface morphology, and apparent electrical conductivity of spin-coated poly(acrylic acid) thin films on ITO

This is the authors' abstract. We don't add key points for this paper.

We examined how a small amount of lithium triflate (LiOTf; 6.4 mM, Li + :COOH = 0.046) added to the casting solution affects spin-coated, non-grafted poly(acrylic acid) (PAA) films on indium tin oxide. The pH was adjusted with NaOH, so the comparison is between PAA/Na + and PAA/Na + /Li + /OTf − films. The degree of dissociation was obtained from deconvoluted FTIR spectra at pH 2.5, 5.0, 7.5, and 11.0. A global charge-regulation fit gave effective pK a values of 5.55 ± 0.41 (PAA) and 5.33 ± 0.41 (PAA/LiOTf; 95 % confidence intervals). The difference (ΔpK a = −0.22 ± 0.58) is not significant, and alternative band-integration schemes give ΔpK a between −0.05 and 0.00. AFM height data show smooth films at all pH values (regional R q 0.32–0.70 nm), with a modest roughening at pH 11. The apparent electrical conductivity from four-point-probe measurements increases with pH and is 2.3–5.0 times higher for LiOTf-containing films at 298 K (up to 5.7 × 10 −4 S cm −1 at pH 11). At pH ≥ 7.5, LiOTf also lowers the apparent activation energy (e.g. 0.37 ± 0.04 vs 0.58 ± 0.10 eV at pH 11). TGA and DSC show that thermal behaviour is governed by the degree of neutralization: LiOTf does not raise decomposition temperatures, and the higher residue at pH 2.5 reflects the inorganic salt content. At this loading and in a Na + -rich background, LiOTf therefore affects charge transport rather than the acid–base equilibrium of PAA. Measurements that would separate ionic from electronic conduction and Li + -specific from ionic-strength effects are outlined. One independently prepared film was available per condition; therefore, the reported uncertainties represent fitting or within-film spatial uncertainty rather than film-to-film reproducibility.