Ceria Foams DOI: 10.1002/anie.201108708 Ceria Foam with Atomically Thin Single-Crystal Walls** Jun Xing, HaiFeng Wang, Chen Yang, Dong Wang, Hui Jun Zhao, Guan Zhong Lu, P. Hu, and Hua Gui Yang* Porous solids are of scientific and technological interest because of a wide range of emerging applications. [1–5] Up to now, the soft or hard templating route is still the major synthetic strategy to create high-surface-area meso- or macro- porous inorganic materials, including carbon, simple metal oxides, multiple metal oxides, metal sulfides, and metal nitrides. [6, 7] For the soft template method to fabricate porous transition metal oxides in which organic structure-directing agents (SDA) are involved, the main problem is that the temperature required for the crystallization is normally high and the liquid crystals of organic SDA templating the metal oxide would decompose before the amorphous walls of metal oxides start to crystallize. Often such materials were not fully crystalline but instead composed of nanocrystals embedded in amorphous matrix to form a semicrystalline structure owing to the low treatment temperature (400–450 8C). [8] The hard templating method is an alternative approach developed to obtain high crystallinity without structural collapse. [9] More- over, even if rigid templates such as mesoporous carbon or silica materials are applied, this strategy suffers some disadvantages, such as tedious synthesis processes, the use of undesirable reagents (such as HF), and hardly any mesostructure produced with ultrathin walls. Thus, it is essential to develop new facile synthetic strategies to fabricate non-siliceous metal oxides in porous form with highly crystalline walls. Ceria (CeO 2 ) is one of the most studied functional metal oxides, and it is extensively used in clean energy and environmental protection areas, such as solar-driven thermo- chemical CO 2 reduction, solid oxide fuel cells, solar cells, CO oxidation, and biomass reforming. [10–16] For all of these applications, the efficacies largely depend on the specific surface area and degree of crystallinity of CeO 2 . The grain boundaries of primary CeO 2 building blocks also need to be minimized to facilitate the electron transfer for redox or photovoltaic processes. Thus, fabricating CeO 2 porous archi- tectures with high surface area and long-range single crystal- line walls is highly desired to enhance the performance of these catalytic or photovoltaic applications. However, owing to the cubic crystal structure, which lacks an intrinsic driving force for continuous anisotropic crystal growth, the synthesis of porous ceria with ultrathin single-crystalline walls has attained very limited success and still remains a major challenge. Herein, using scheelite-type CeGeO 4 as starting material, we report a new facile thermal decomposition process under an ammonia (NH 3 ) atmosphere to fabricate three-dimen- sional (3D) CeO 2 foams with long-range atomically thin single-crystalline walls. First-principles calculations were also performed to understand the feasibility and reaction path- ways of thermal decomposition of CeGeO 4 . To our knowl- edge, this is the first report of a 3D foam of a metal oxide with an ultrathin (4–8 ) single-crystalline wall. CeO 2 foam with atomically thin single-crystalline wall was synthesized by a two-step method. First, scheelite-type CeGeO 4 crystals prepared by a facile hydrothermal process were used as the solid precursor. The synthetic method involves keeping cerium (III) nitride aqueous solution con- taining GeO 2 powder and citric acid in a Teflon-lined stainless autoclave under 200 8C for 24 h. The as-prepared CeGeO 4 crystals were then treated under NH 3 atmosphere at (780  20) 8C (see the Experimental Section in the Supporting Information for details). After heat treatment in an NH 3 atmosphere, the color of the sample changed from white to yellow-green and the volume dramatically expanded, as also illustrated in a digital camera image (Figure 1a). The crystal- phase transformation during the entire synthesis was moni- tored by wide-angle X-ray diffraction (XRD) (Figure 1 b). The XRD pattern of the as-synthesized CeGeO 4 crystals matches well with the tetragonal structure of scheelite-type CeGeO 4 (141/a, a = b = 5.043 , c = 11.174 ; JCPDS Card [*] J. Xing, [+] Dr. C. Yang, [+] Prof. Dr. H. G. Yang Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology Shanghai, 200237 (China) E-mail: hgyang@ecust.edu.cn Prof. Dr. P. Hu School of Chemistry and Chemical Engineering The Queen’s University of Belfast, Belfast, BT9 5AG (UK) Dr. H. F. Wang, [+] D. Wang, Prof. G. Z. Lu Labs for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology Shanghai, 200237 (China) Prof. H. J. Zhao Centre for Clean Environment and Energy, Gold Coast Campus Griffith University, Queensland 4222 (Australia) [ + ] These authors contributed equally to this work. [**] This work was financially supported by Scientific Research Foun- dation of East China University of Science and Technology (YD0142125), Pujiang Talents Programme of Science and Technol- ogy Commission of Shanghai Municipality (09J1402800), Shuguang Talents Programme of Education Commission of Shanghai Munic- ipality (09SG27), National Natural Science Foundation of China (20973059, 91022023, 21076076), Fundamental Research Funds for the Central Universities (WJ0913001), Program for Prof. of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and Program for New Century Excellent Talents in University (NCET-09-0347) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201108708. A ngewandte Chemi e 3611 Angew. Chem. Int. Ed. 2012, 51, 3611 –3615  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim