第一原理計算探討凡得瓦爾作用力誘發之二維二硒化鉑原子層數相依之產氫催化活性

Abstract

近年來單層二硫化鉬分別在理論及實驗被證實擁有高效率的產氫催化活性；單層及多層二硒化鉑於近三年也在實驗中被發現可做為產氫的催化劑。對於二維二硫化鉬，隨著層數增加，其產氫催化活性降低；然而，二維二硒化鉑卻會因為層數的增加而有催化活性變好的現象。對於二維材料，維持其層狀堆疊結構的是層間的凡得瓦爾作用力，從單層到雙層乃至多層原子層，該作用力是主導並構成不同層數之二維系統的物理特徵，因此會主導整個系統的電子結構。由於產氫催化攸關氫原子與材料的交互作用，也就是該材料是否具備觸媒活性，而電子結構直接影響觸媒跟氫原子的反應活性，因此凡得瓦爾作用力很有可能是一個貢獻催化活性的因子；然而，目前的研究關於凡得瓦爾作用力如何影響產氫催化活性仍沒有著墨，若能了解此於催化反應所扮演的角色，將有助於尋找其它潛在的層狀材料做為產氫的活性基座。因此，本研究以二維二硒化鉑為研究標的，探究其隨層數增加而增加的產氫催化活性來源，並歸納凡得瓦爾作用力如何改變催化活性。 我們探討二維二硒化鉑在單、雙及三層原子層，透過完整表面與硒原子空缺來產氫的催化活性。對於單層的二硒化鉑，僅有硒空缺位置有活性，是主要的活性位置；然而雙層及三層的二硒化鉑不僅在硒空缺位置有活性，原本於單層沒有活性的完整表面亦出現活性。探究其原因後我們發現原子層間的凡得瓦爾作用力會增加氫原子與二維二硒化鉑完整表面的交互作用。對於完整表面，單層的二硒化鉑和氫的作用力為排斥力，氫原子不易吸附其上，活性低；然而於雙層及三層的二硒化鉑，凡得瓦爾作用力幫助消弭氫原子與完整表面的排斥力，使得氫吸附變得容易進行，因此完整表面出現高活性。對於硒空缺位置，凡得瓦爾作用力亦會增加氫原子與二維二硒化鉑表面的作用力，提升硒空缺位置產氫的催化活性。 另外，產氫的主導機制亦受到凡得瓦爾作用力的影響而出現機制隨著二硒化鉑原子層數的增加而改變的現象，此一機制轉換可以被氫與二維二硒化鉑間隨著層數增加而改變的作用力解釋。於完整表面上，氫和單層二硒化鉑的排斥力大，氫吸附不容易進行，因此由包含氫吸附步驟較少的Volmer-Heyrovsky機制主導；當層數增加，排斥力因為凡得瓦爾作用力而減輕，氫吸附反應變得容易發生，故包含較多氫吸附步驟的Volmer-Volmer-Tafel機制會在多層二硒化鉑上主導，反應機制由原本單層的Volmer-Heyrovsky轉成Volmer-Volmer-Tafel。至於在硒空缺上，氫原子與單層二硒化鉑的作用力已有一定強度，因此氫吸附反應容易發生，是Volmer-Volmer-Tafel反應主導；然而，在雙層及三層的二硒化鉑上，凡得瓦爾作用力對氫原子與催化表面作用力的增加使該作用力過強而讓氫脫附反應變得難發生，因此包含兩個氫原子由催化表面脫附之Tafel步驟的Volmer-Volmer-Tafel機制不利進行，由Volmer-Heyrovsky機制主導。

It is both experimentally and theoretically discovered that molybdenum disulfide (MoS2) single atomic layer has high catalytic activity for hydrogen evolution reaction (HER); besides, platinum diselenide (PtSe2) single and multiple atomic layers are confirmed to be efficient catalysts for HER. While MoS2 has declining catalytic activity for HER as its layer number increases, PtSe2 demonstrates enhanced catalytic activity from single layer to multiple layers. There is van der Waals (vdW) interaction between layers which consist of a two-dimensional (2D) material. In such a material, the vdW interlayer interaction is the physical feature that dominates and would alter the electronic structures of systems with different atomic layers. The pivot of HER lies in the interaction between hydrogen atoms and a given catalyst; or whether the catalyst has catalytic activity for hydrogen molecular formation. Since the interaction between hydrogen adatoms and a catalyst is affected by the electronic structure of the catalyst, the vdW interlayer interaction is potentially a critical factor to contribute the layer-dependent catalytic activity. However, current research has not covered how vdW interlayer dispersion affect the catalytic activity for HER on a 2D catalyst and the origin of it is still shrouded in mystery. If this issue is explored and one unearths the correlation between layer-dependent catalytic activity and vdW interlayer dispersion, it will facilitate the manipulation of the entirely new vdW interaction induced catalytic activity and open up an avenue of searching other potential 2D material systems as catalysts for HER. Indeed, we did find that the vdW interlayer interaction has a constructive effect on the catalytic activity for HER on both basal plane and selenide vacancy of 2D-PtSe2. The hydrogen molecular formation is greatly promoted on two and three layered PtSe2, indicating that vdW interaction induced catalytic activity on 2D-PtSe2 would help to harvest much hydrogen molecular as an alternative energy resource. In this research, we aimed to unveil the origin of the layer-dependent catalytic activity on 2D-PtSe2. We did this by answering the question of whether the vdW interlayer interaction contribute to the layer-dependent catalytic activity and if yes, by what way. We study the catalytic activity for HER on the basal plane and selenide vacancy of single, double and triple layered PtSe2 via first principle calculation and analyze the bonding between hydrogen adatoms and 2D-PtSe2 catalytic surface through projected crystal orbital Hamilton population (p-COHP). On single layered PtSe2, the basal plane is catalytically inert and selenide vacancy catalytically active; while on double and triple layered PtSe2, not only the selenide vacancy but also the basal plane is catalytically active. This novel phenomenon results from vdW interlayer interaction which facilitates HER in a consecutive way. When another PtSe2 atomic layer is introduced, it alters the electronic structure of the uppermost PtSe2 layer that adsorbs hydrogen atom via vdW interlayer dispersion; this varying electronic structure subsequently weakens the antibonding states between the hydrogen adatoms and 2D-PtSe2 basal plane, or strengthens the bonding states between hydrogen adatoms and 2D-PtSe2 surface of one selenide vacancy, leading to enhanced interaction between hydrogen atoms and 2D-PtSe2, and yielding promoted catalytic activity for HER. Furthermore, the dominant HER mechanism shows conversion as PtSe2 layer number increases owing to vdW interlayer dispersion, and which mechanism dominates on each hydrogen adsorption site of PtSe2 with different layer number is consistent with the layer-dependent interaction between hydrogen atoms and 2D-PtSe2. As the layer number increases, PtSe2 changes from single atomic layer to vdW homogeneous structure, the repulsion between hydrogen adatoms and 2D-PtSe2 basal plane resulting from antibonding states near EF is alleviated. This facilitates hydrogen adsorption and the dominant HER mechanism converts from volmer-Heyrovsky to Volmer-Volmer-Tafel. On the other hand, hydrogen adatoms has stronger interaction with the catalytic surface of selenide vacancy than that with intact basal plane on single layered PtSe2. As the layer number increases, vdW interlayer dispersion intensifies the interaction between hydrogen adatoms and the catalytic surface. It over strengthens the interaction and impedes hydrogen desorption; thus, the dominates HER mechanism converts from Volmer-Volmer-Tafel to Volmer-Heyrovsky.