Interaction of Phosphate with Lithium Cobalt Oxide Nanoparticles: A Combined Spectroscopic and Calorimetric Study

Adsorption of small ions such as phosphates to the surfaces of metal oxides can significantly alter the behavior of these materials, especially when present in the nanoscale form. Lithium cobalt oxide is a good model system for understanding small-molecule interactions with emerging nanomaterials because of its widespread use in lithium ion batteries and its known activity as a water oxidation catalyst. Here, we present a thermodynamic analysis of phosphate adsorption to LiCoO₂ and corroborate the results with additional in situ techniques, including zeta potential measurements and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, at pH values relevant to potential environmental release scenarios. Flow microcalorimetry measurements of phosphate interaction with LiCoO₂ at pH 7.4 show that there are two distinct exothermic processes taking place. Time-sequence in situ ATR-FTIR with two-dimensional correlation analysis reveals the spectroscopic signatures of these processes. We interpret the data as an interaction of phosphate with LiCoO₂ that occurs through the release of two water molecules and is therefore, best described as a condensation process rather than a simple adsorption, consistent with prior studies, demonstrating that phosphate interaction with LiCoO₂ is highly irreversible. Additional measurements for over longer times of 5 months show that phosphate adsorption terminates with one surface layer and that continued transformation over longer periods of time arises from H⁺/Li⁺ exchange and slow transformation to a cobalt hydroxide, with phosphate adsorbed to the surface only. To the best of our knowledge, this is the first time that flow microcalorimetry and two-dimensional correlation spectroscopy have been applied in tandem to clarify the specific chemical reactions that occur at the interface of solids and adsorbates.