The Standard Formation Enthalpies of Spherical ZnO Nano-Particles: Size Matters

Abstract

Four kinds of spherical nanoZnO particles with diameter of 20 nm, 30 nm, 40 nm and 50 nm were characterized by XRD and SEM. Based on a thermodynamic cycle, the standard molar formation enthalpies (ΔfHm) were measured and calculated by the RD496-2000 micro-calorimeter. They are (-328.41±1.575) kJ·mol–1 (50 nm), (-323.11 ± 1.579) kJ·mol–1 (40 nm), (-313.47 ± 1.581) kJ·mol–1 (30 nm), (-307.66 ±1.616) kJ·mol–1 (20 nm). The results show that nanosized ZnO has higher standard molar formation enthalpy compared to that of bulk ZnO (–350.46 ± 0.27 kJ·mol–1), indicating that the stability of nanosized ZnO decreases. More interestingly, the results show that the smaller size of the ZnO nanoparticles, the higher standard molar formation enthalpy was obtained. This indicates that ZnO nanoparticles get destabilized when the size decreases. The value of the standard molar formation enthalpies y (kJ·mol–1) and size x (nm) can be approximately described by a linear function of y = –0.7189 x –293.

Structure and Photocatalytic Activity of Copper and Carbon-Doped Metallic Zn Phase-Rich ZnO Oxide Films

“…The formation enthalpy of ZnO nanoparticles is lower than the formation enthalpy of bulk ZnO [46]. For example, while the formation enthalpy value for bulk ZnO is −350.5 kJ/mol [43], the corresponding values for 50 nm size and 20 nm size nanoparticles are −328.4 kJ/mol and −307.7 kJ/mol, respectively [46]. Accordingly, it is more advantageous thermodynamically to sustain the growth of existing c-axis oriented crystals than to form new randomly oriented ZnO nucleates.…”

“…Naturally, ZnO films produced by reactive magnetron sputtering tend to form the most thermodynamically favourable orientation, which is (002) [31]. The formation enthalpy of ZnO nanoparticles is lower than the formation enthalpy of bulk ZnO [46]. For example, while the formation enthalpy value for bulk ZnO is −350.5 kJ/mol [43], the corresponding values for 50 nm size and 20 nm size nanoparticles are −328.4 kJ/mol and −307.7 kJ/mol, respectively [46].…”