Activity of ZnO polar surfaces: an insight from surface energies. 
The calculation of the accurate surface energies for (0001) surfaces of wurtzite ZnO is difficult because it is impossible to decouple the two inequivalent (0001)-Zn and (0001¯)-O surfaces.  By using a heterojunction model we have transformed the uncertainty of the surface energies into that of interface energies which is much smaller than the former and hence estimated the surface energies to a high degree of accuracy.  It is found that the oxygen terminated (0001¯)-O face of the wurtzite phase and (1¯1¯1¯O of the zinc blende phase are more stable than their Zn-terminated counterparts within the major temperature and oxygen partial pressure range accessible to experiment.  The instability of Zn-terminated polar surfaces explains the experimentally observed high activity of these surfaces.  The effects of native surface vacancies on the surface energies have also been discussed.  These results provide insights into the modification of the surface stability and activity of ZnO nanoparticles.