Theoretical and Experimental Studies on the Critical Size Effect on Surface Heat Capacity Using In Situ Microcalorimetry: During the Solid-Liquid Reactions of Cu2O Nanocubes

Phase transition, energy storage, and other activities depend critically on the size variations of nanomaterials for their heat capacity. The size variation of the nanomaterial is also equally important to the thermodynamic properties for chemical reaction processes. Based on this, it will be fascinating to examine how the surface heat capacity of nanomaterials changes with size. Herein, we used an in situ microcalorimeter to report a general approach to obtaining the regular patterns of numerical changes in the surface heat capacity at different sizes and temperatures when the chemical reactions of cuprous oxide (Cu2O) take place in the liquid phase. We synthesized four Cu2O nanocubes with uniform size distributions, which range from 40 to 120 nm, by using the liquid-phase reduction method. Combined with the principle of thermodynamic cycle and transition state theory, the effects of size and temperature changes on the standard molar formation thermodynamic properties, surface thermodynamic properties, and specific surface thermodynamic properties of Cu2O nanocubes were calculated and analyzed. The calculated results are in agreement with the established thermodynamic model. It is inspiring that we find that Cu2O nanocubes also have a critical size in the range of 55-67 nm, and their molar surface heat capacity and specific surface heat capacity are 59.35 and 59.31 nm, respectively. This property allows them to exhibit differences in thermal resistance from bulk materials within an acceptable range of error. This work has important scientific significance and values for enriching and developing the surface physical chemistry and understanding the energy storage and other processes of nanomaterials.