Rapid crystallization and proton conductivity of copper (II)-l-tartrate

A modified approach for the synthesis and crystallization of copper (II)-l-tartrate was developed. The cell parameters of the crystal were found to match with those of the earlier crystallographic report on this material and the bulk purity of the sample was ascertained by elemental analysis. The complex possesses inherent structural features such as Lewis acidic coordinated water and hydrogen bonded networks that are relevant for proton conducting materials. The PXRD results indicated the stability of the overall structural topology of the complex during dehydration despite some structural reorganization. The anhydrous phase, as revealed by the PXRD analysis, reabsorbed moisture with small reductions in the lattice constants, but did not fully return to its original hydrated state. The FT-IR spectrum of the rehydrated phase is mostly similar to that of the original crystalline complex, except in the hydrogen bonding level. Paralleling these observations, the complex showed better proton conductivity of 9 × 10−5 S cm−1 at 100% relative humidity (RH) at 25 °C. Upon drying the sample, the conductivity was found to decrease several orders of magnitude (5 × 10−11 S cm−1) owing to the loss of water molecules from the network. However, when the sample was exposed again to the humid environment, the conductivity bounced back to 5 × 10−5 S cm−1, which is very close to that of initial value observed in the original material. The results underscore the relevance of the material for moisture sensing applications through its proton conducting ability.

► A modified crystallization procedure for copper (II)-l-tartrate was developed.
► The structural topology of the network is maintained during dehydration. The anhydrous phase reabsorbs moisture but the original hydrated phase is not fully reversed.
► Copper (II)-l-tartrate is an efficient proton conductor at ambient temperatures and at high relative humidities.
► The dried sample is poorly conducting, but reestablishes high proton conduction when exposed again to moisture.