立方相Ag_3PO_4(111)面原子几何及弛豫结构的第一性原理计算

采用平面波超软赝势方法研究了立方相Ag_3PO_4(111)面的表面能和表面原子弛豫结构.首先对Ag_3PO_4(111)面的八种不同原子终止结构的体系总能量进行计算,结果表明B种表面模型被证实为最稳定的(111)面原子几何结构.针对该表面结构,探讨了表面能和原子弛豫与模型中原子层数和真空厚度的关系,当原子层数为24层,真空厚度为0.6 nm时,表面能收敛于1.41 J/m2(LDA-CAPZ)和1.39 J/m2(GGA-PBE).表面原子弛豫后,表面两个三配位的Ag原子均向里移动,超过0.06 nm,而表面次层的O原子则均向外移动约0.0042 nm,导致弛豫后暴露在最表面的是O原子,同时表面原子的核外电子向表面内部发生转移,结构趋于稳定.这些结果为进一步深入研究Ag_3PO_4表面的光催化活性起源提供理论支持.

First-principles calculations on the geometry and relaxation structure

First-principles calculations based on the plane-wave ultrasoft pseudopotential method have been taken to investigate the surface energy and structure of cubic Ag3PO4(111) surface. There are eight different structures of cubic Ag3PO4(111) surface because different atoms are terminated on the surface layers. The calculated results show that the B model is much more stable than other seven structures. By investigating the effect of variable vacuum width and slab thickness on the surface energies and surface atomic displacements, the surface energies are converged to 1.41 J/m2(LDA-CAPZ) and 1.39 J/m2(GGA-PBE) for slab thickness of 24-atom layers and vacuum width of 0.6 nm. At last, by the atomic relaxation of surface structure, the two threefold coordinated Ag atoms are inward relaxation of more 0.06 nm, and three O atoms in sublayer are outward relaxation of about 0.0042 nm. So O atoms are exposed to the outermost surface, while part of electrons upon O atoms transfer from the surface to internal atoms, the structure becomes more stable. This study serves as an initial and important step toward more in-depth analysis of the photocatalytic activity origin of Ag3PO4 surface.