硝酸在二氧化鈦表面的吸附及反應情形

Abstract

本論文藉由VASP(Vienna Ab-initio Simulation Package) 計算軟體中的理論計算方法，電子密度泛函理論(Density Functional Theory)及超軟贋位勢近似法(Ultrasoft pseudopotential approximation, US-PP)來探討單分子硝酸及雙分子硝酸分別在二氧化鈦金紅石(rutile)(110)表面及銳鈦礦(anatase)(101)表面吸附結構及可能的反應路徑。 從我們的計算結果中可知，單分子硝酸中的氧原子和二氧化鈦金紅石表面上的Ti5c形成單一鍵結，而且硝酸分子中的H原子與表面的O2c形成H鍵是最穩定的結構，其吸附能為6.7 kcal/mol。此硝酸分子的H原子可以解離至表面上最鄰近的O2c上，此反應步驟幾乎不需要任何的能障就可以生成NO3(a) + H(a)的結構 。然後，NO3分子可以旋轉Ti5c-O鍵而形成Ti5c-ON(O)-Ti5c,H-O2c(a)的結構，但需要跨越12.2 kcal/mol 的能障， Ti5c-ON(O)-Ti5c,H-O2c(a)的吸附能為16.5 kcal/mol。 在雙分子硝酸吸附的型態中，最穩定的結構是以兩個最穩定單分子的結構組成2(Ti¬5c_ON(O)OH…O2c(a))，其吸附能為12.8 kcal/mol。從 2(Ti¬5c_ON(O)OH…O2c(a)) 形成 N2O5 分子需要跨越46.2 kcal/mol的能障，由此可知聚合反應難以發生。單分子及雙分子硝酸在二氧化鈦銳鈦礦表面的吸附及反應路徑和在金紅石表面相似。 另外， 由上述反應中，我們發現H原子吸附在O2c上在吸附能上扮演一個重要的角色，特別是對NO3自由基。NO3可能成為一個有效的連接分子以連接半導體量子點系統，例如:氮化銦(InN)半導體量子點，和二氧化鈦表面。此外，我們也進一步計算以NO3連接氮化銦團簇(InN)X及二氧化鈦表面的電子分佈狀況。

The adsorption and reactions of the monomer and dimer of nitric acid on TiO2 rutile (110) and anatase (101) surfaces have been studied by first-principles calculations based on the density functional theory in conjunction with ultrasoft pseudopotential approximation implemented in the Vienna Ab-initio Simulation Package (VASP). The most stable configuration of HNO3 on the rutile surface is a molecular monodentate adsorbed on the 5-fold coordinated Ti atom with the hydrogen bonded to a neighboring surface bridging oxygen with the adsorption energy of 6.7 kcal/mol. It can dissociate its H atom to a nearest bridged oxygen with approximately no barrier to produce NO3(a) + H(a). The rotation of NO3 requires a barrier of 12.2 kcal/mol to form the by didentate configuration Ti5c-ON(O)-Ti5c,H-O2c(a), which adsorbs on two 5-fold coordinated Ti atoms with the adsorption energy of 16.5 kcal/mol. In the case of the adsorption of 2HNO3 molecules, the most stable configuration, 2(Ti¬5c_ON(O)OH…O2c(a)), has a structure similar to two single HNO3 adsorbates on two 5-fold coordinated Ti atoms with the adsorption energy of 12.8 kcal/mol, which is about twice that of the single HNO3 molecule. The result suggests that the interaction of the two planar HNO¬3 adsorbates is negligible. The dehydration from 2(Ti¬5c_ON(O)OH…O2c(a)) forming N2O5 requires an energy barrier of 46.2 kcal/mol, indicating that the dimerization of the two HNO3(a) is hard to occur. Similar adsorption phenomena appear on the anatase (101) surface. The result of our calculations shows that the co-adsorption of hydrogen plays a significant role in the adsorption energies of adsorbates, especially for the NO3 radical, which may be employed as a linker between semiconductor quantum dots such as InN with the TiO2 surface. Furthermore, we calculate the charge distribution of the NO3 group connecting (InN)x clusters with TiO2.