以理論計算研究二氧化碳分子與水分子在二硫化鉬與二硫化錫上的吸附行為

二硫化鉬(MoS2)與二硫化錫(SnS2)為現今熱門的二維材料，因其適當的能隙，與容易調整的性質，廣泛被研究與應用在光催化、燃料電池、氣體偵測等領域。摻雜、表面缺陷、張力、層數、邊界等都被認為是調整二維材料活性的方法。利用密度泛函的理論，我們可以探討上列情況對二維材料的影響，在此我們研究了三個主題：一、在單層二硫化鉬表面摻雜非金屬原子時的水分解反應。二、在單層二硫化錫中鎳取代錫與表面缺陷時，對水吸附的影響。三、單層二硫化鉬表面形成缺陷後對二氧化碳吸附的影響。這些研究與實驗上的水分解反應和光催化反應中的轉化二氧化碳成可利用的碳氫化合物相關。於第一部分，我們計算了由表面單原子取代硫後的二硫化鉬模型電子組態，並且利用晶體軌域漢米頓分佈(crystal orbital Hamilton population)的計算來了解這些摻雜後所產生的缺陷狀態(defect states)的特性，我們發現這些狀態主要是來自摻雜原子與正下方硫原子的反鍵結狀態(antibonding states) 。此外，我們分類了三種水在這些含有摻雜物時的二硫化鉬表面之吸附現象，分別是物理性吸附、經由氫鍵吸附與化學性吸附。在化學性吸附的情況，水分子於矽摻雜的二硫化鉬表面具有最低的分解活化能，其數值為0.306電子伏特。於第二部分，我們計算了由鎳原子取代二硫化錫中心錫原子後的模型，利用生成能的計算，我們發現了當硫缺陷存在於表面時，最穩定的硫缺陷是位於在摻雜的鎳周遭。此外，相較於完美的二硫化錫與無缺陷的鎳摻雜二硫化錫，具有缺陷的鎳摻雜二硫化錫具有最優異的水吸附活性。於第三部分，我們探討了二硫化鉬表面硫原子缺陷對二氧化碳分子吸附的影響，當表面聚集的硫原子缺陷到達三個以上時，二氧化碳分子開始以化學吸附的形式吸附於表面，狀態密度(density of states)的分析顯示，在具有三個硫原子缺陷的二硫化鉬模型中，缺陷下層中心鉬原子的dz2軌域對二氧化碳分子具有強的鍵結作用，造成線性的二氧化碳分子吸附﹔而在五個硫原子缺陷的二硫化鉬模型中，缺陷下層中心的兩個鉬原子，他們的dz2軌域與dxz軌域對二氧化碳分子具有強的鍵結作用，造成彎曲的二氧化碳分子吸附，此時，將近一個電子已經傳遞到二氧化碳分子上，弱化了二氧化碳分子的碳氧鍵結，進而可能進行之後的二氧化碳分子轉換反應。

關鍵字

二硫化鉬；二硫化錫；二維材料；密度泛函方法；水分子吸附

並列摘要

Two dimensional materials, Molybdenum disulfide (MoS2) and Tin disulfide (SnS2) have been extensively investigated recently. They are recognized as a promising candidate for various applications such as photocatalysis, fuel cells and gas sensor. Doping, surface defects, strain, number of layers and edge side are used to tune the activity of these two dimensional materials. Depending on the density functional theory (DFT), we can investigate that how these conditions affect two dimensional materials. Our study includes three parts: (1) water decomposition on nonmetal doped MoS2 monolayers, (2) water adsorption on defective Ni-doped SnS2 monolayer and (3) CO2 adsorption on defective MoS2. They are related to the water decomposition and photocatalytic conversion of CO2 into fuels in experiment. In our first part, we calculated the electronic structures of X-MoS2 monolayer models (X=B, C, N, O, Al, Si and P) by replacing an S atom with an X atom. The properties of these produced defect states after doping are analyzed by COHP (crystal orbital Hamilton population) calculations. These defect states come from the antibonding states of the dopant and the sulfur atom which is below the dopant. Besides, we classified the results of water adsorption on X-MoS2 monolayers into three kinds of interaction: physisorption, adsorption via hydrogen bond and chemisorption. In the chemisorption case, the lowest activation energy for OH bond cleavage is found when H2O molecule chemisorbed on Si-MoS2 monolayer and the Ea = 0.306 eV. In our second part, we optimized the Ni-SnS2 model which is replaced a Sn atom with a Ni atom. By calculating the formation energy, we found that the sulfur vacancy is preferred to locate near the doped Ni atom since sulfur vacancies are evitable during some synthesis processes. Furthermore, with comparing to pristine SnS2 and Ni-doped SnS2 monolayers, this defective Ni-doped SnS2 monolayer is more active for the adsorption of H2O molecule. In our third part, we investigated the effect on adsorption of CO2 molecule on MoS2 monolayer during different numbers of sulfur defect. We found from density of states (DOS) and COHP analysis that when the number of sulfur vacancies increases to three, the Mo atom in the center of sulfur vacancies becomes active and performs a chemisorption of linear CO2 via its dz2 orbital. In the same manner, a chemisorption of bending CO2 is found on the MoS2 monolayer with five sulfur vacancies. The interaction is due to the hybridization of orbitals of CO2 and dz2 and dxz orbitals of the Mo atoms which are in the center of defects. At this time, about one electron has been transferred from surface to CO2 molecule and the bond length of CO bonds increased about 0.08 Å. This result gives the possibility of further CO2 conversion reactions.