Institute of Condensed Matter and Nanosciences, Division MOlecules, Solids and reactiviTy, Université catholique de Louvain, Croix du Sud 2, L7.05.17, B-1348, Louvain-la-Neuve, Belgium ; CEA Grenoble, INAC, UMR 5819 SPRAM (CEA/CNRS/UJF-Grenoble 1), Laboratoire d'Electronique Moléculaire, Organique et Hybride, 17 rue des Martyrs, 38054 Grenoble cedex 9, France.
Institute of Condensed Matter and Nanosciences, Division MOlecules, Solids and reactiviTy, Université catholique de Louvain, Croix du Sud 2, L7.05.17, B-1348, Louvain-la-Neuve, Belgium.
Beilstein J Nanotechnol. 2014 Oct 10;5:1749-59. doi: 10.3762/bjnano.5.185. eCollection 2014.
The hybridization of polyoxometalates (POMs) through an organic-inorganic association offers several processing advantages in the design of heterogeneous catalysts. A clear understanding of the organization of these hybrid materials on solid surfaces is necessary to optimise their properties. Herein, we report for the first time the organization of Keggin phosphotungstic PW12O40 and Wells-Dawson (WD) phosphomolybdic P2Mo18O62 anions deposited on mica (hydrophilic), and highly oriented pyrolytic graphite (HOPG) (hydrophobic) surfaces. Next, the supramolecular organization of the organic-inorganic hybrid materials formed from the association of POM anions and dimethyldioctadecylammonium bromide (DODA) is investigated as a function of the hydrophilic or hydrophobic nature of the surfaces. The height of the Keggin-POM anions, measured with tapping mode (TM-AFM) is always in good agreement with the molecular dimension of symmetric Keggin-POM anions (ca. 1 nm). However, the asymmetric WD-POM anions form monolayer assemblies on the surfaces with the orientation of their long molecular axis (ca. 1.6 nm) depending on the hydrophilic or hydrophobic properties of the substrate. Namely, the long axis is parallel on mica, and perpendicular on HOPG. When hybridized with DODA, the organization of the hybrid material is dictated by the interaction of the alkyl side chains of DODA with the substrate surface. On HOPG, the DODA-POM hybrid forms small domains of epitaxially arranged straight nanorod structures with their orientation parallel to each other. Conversely, randomly distributed nanospheres are formed when the hybrid material is deposited on freshly cleaved mica. Finally, a UV-ozone treatment of the hybrid material allows one to obtain highly dispersed isolated POM entities on both hydrophilic and hydrophobic surfaces. The hybridization strategy to prevent the clustering of POMs on various supports would enable to develop highly dispersed POM-based heterogeneous catalysts with enhanced functionalities.
多金属氧酸盐（POMs）的有机-无机杂化通过杂交提供了在设计多相催化剂方面的几个加工优势。为了优化它们的性能，有必要清楚地了解这些杂化材料在固体表面上的组织。在这里，我们首次报道了 Keggin 磷钨酸[PW12O40]（3-）和 Wells-Dawson（WD）磷钼酸[P2Mo18O62]（6-）阴离子在云母（亲水）和高定向热解石墨（HOPG）（疏水）表面上的沉积。接下来，研究了由 POM 阴离子和二甲基十八烷基溴化铵（DODA）的缔合形成的有机-无机杂化材料的超分子组织，作为表面亲水性或疏水性的函数。使用敲击模式（TM-AFM）测量的 Keggin-POM 阴离子的高度始终与对称 Keggin-POM 阴离子的分子尺寸（约 1nm）非常吻合。然而，不对称 WD-POM 阴离子在表面上形成单层组装，其长分子轴（约 1.6nm）的取向取决于基底的亲水性或疏水性。也就是说，长轴在云母上是平行的，在 HOPG 上是垂直的。当与 DODA 杂交时，杂化材料的组织由 DODA 的烷基侧链与基底表面的相互作用决定。在 HOPG 上，DODA-POM 杂化形成具有彼此平行取向的直纳米棒结构的小有序排列的纳米域。相反，当将杂化材料沉积在新鲜劈开的云母上时，形成随机分布的纳米球。最后，对杂化材料进行 UV-臭氧处理可以在亲水和疏水表面上获得高度分散的孤立 POM 实体。在各种载体上防止 POM 聚集的杂化策略将能够开发具有增强功能的高度分散的基于 POM 的多相催化剂。
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