Highly catalytically active CeO2-x based heterojunction nanostructures with mixed micro/meso-porous architectures Sajjad S. Mofarah,* Luisa Schreck, Claudio Cazorla, Xiaoran Zheng, Esmaeil Adabifiroozjaei, Constantine Tsounis, Jason Scott, Reza Shahmiri, Yin Yao, Roozbeh Abbasi, Kourosh Kalantar-Zadeh, Yuan Wang, Hamidreza Arandiyan, Leigh Sheppard, Esmail Doustkhah, Pramod Koshy, and Charles C. Sorrell* ABSTRACT: Achieving high densities of accessible active sites in catalysts, which depend principally on the architecture of nanostructures, is critical to obtain enhanced performance. The present work introduces a template-free, high-yield, and flexible approach to fabricate 3D, mesoporous, CeO 2-x nanostructures in centimeter-scale that are comprised of extremely thin holey 2D nanosheets. The method involves conversion of a stacked, 2D, Ce-based coordination polymer by controlling the removal kinetics of organic species. The resultant polycrystalline 2D-3D CeO 2-x exhibits a large density of defects as well as outstanding surface areas of 251 m 2 g -1 . This mesoporous nanomaterial yields superior CO conversion performance (T 90% = 148˚C). Further improvements in catalysis were attained by synthesis CeO 2-x -based transition metal oxides (TMOs) hetero-nanostructures, for which structural analyses and first principles simulations revealed active sites associated with the TMOs. This versatile fabrication technique delivers new pathways to engineer nanostructures and advance their functionalities for catalysis. KEYWORDS: Holey Two-Dimensional, Defect-rich CeO 2 , Porous Heterojunction Nanostructure, Ce-based Coordination Polymers ■ INTRODUCTION Three dimensional (3D) mesoporous metal oxides (MOs) hold great promise in catalysis through increasing accessibility of the active sites relative to bulk materials. Nonetheless, the use of sacrificial templates, which generally is employed to introduce the pores and allow the formation of the 3D structures, makes the fabrication process complex and of low efficiency. Certain shortcomings also apply to various etching techniques for creating pores in 3D scaffolds 1,2 . In porous structures, the walls of the 3D scaffolds limit the number of active sites to only the surface and subsurface of these 3D units. The recently developed techniques involving the creation of nanoholes in two dimensional (2D) MOs has facilitated the accessibility of active sites by exposing greater surface area, 3-5 thus making these materials attractive for surface-sensitive applications in energy, sensing, and heterogeneous catalysis. 6-8 Further, the intrinsic polycrystallinity of these holey 2D structures can overcome the critical shortcoming of irreversible restacking of pristine 2D nanosheets. 9, 10 Recent advances in the synthesis of polycrystalline holey 2D MOs include the heterogeneous deposition of mixed transition metal oxides (TMOs) on graphene nanosheets as a sacrificial template 11 and the synthesis of holey TMOs by etching pristine 2D TMOs. 12-14 Despite considerable improvements in the performances of these nanostructures, challenges, such as complex synthesis procedures, use of