& Mesoporous Materials | Hot Paper | Controllable Synthesis of Mesoporous Iron Oxide Nanoparticle Assemblies for Chemoselective Catalytic Reduction of Nitroarenes Ioannis T. Papadas, [a] Stella Fountoulaki, [b] Ioannis N. Lykakis,* [b] and Gerasimos S. Armatas* [a] Abstract: Iron(III) oxide is a low-cost material with applica- tions ranging from electronics to magnetism, and catalysis. Recent efforts have targeted new nanostructured forms of Fe 2 O 3 with high surface area-to-volume ratio and large pore volume. Herein, the synthesis of 3D mesoporous networks consisting of 4–5 nm g-Fe 2 O 3 nanoparticles by a polymer-as- sisted aggregating self-assembly method is reported. Iron oxide assemblies obtained from the hybrid networks after heat treatment have an open-pore structure with high sur- face area (up to 167 m 2 g 1 ) and uniform pores (ca. 6.3 nm). The constituent iron oxide nanocrystals can undergo con- trollable phase transition from g-Fe 2 O 3 to a-Fe 2 O 3 and to Fe 3 O 4 under different annealing conditions while maintain- ing the 3D structure and open porosity. These new ensem- ble structures exhibit high catalytic activity and stability for the selective reduction of aryl and alkyl nitro compounds to the corresponding aryl amines and oximes, even in large- scale synthesis. Introduction The application of transition metal oxide nanoparticles (NPs) in catalysis has attracted a great deal of attention in the last years. [1] The use of these nanomaterials offers several advan- tages, such as unusual catalytic properties and high surface area-to-volume ratio. Among them, iron(III) oxide in particular is becoming very popular in organic synthesis due to its high chemical stability, low cost, and magnetic separability. [2] Exam- ples include borylation of arenes, [3] Sonogashira cross-cou- pling, [4] oxidation of styrene [5] and olefins, [6] and hydrogenation of nitroarenes. [7] More recently, Fe 2 O 3 has been regarded as a potential photoanode material in visible-light-driven water splitting owing to its visible-light response (bandgap energy  2–2.2 eV) and appropriate valence band edge position (  2.5 V vs. NHE at pH 1). [8] In this area, continuous research ef- forts have been made to synthesize well-defined iron oxide nanostructures, and so far a large variety of Fe 2 O 3 NPs with finely controlled size (from 2 to 40 nm) and shape (cubes, spheres, ellipsoids, and sheets) have been prepared. [9] Unfortu- nately, implementation of such nanomaterials, especially in cat- alysis and absorption, is not easy, as it often entails a strong tendency of NPs to form large agglomerates in solution. In ad- dition, the morphology of the aggregated assemblies is irregu- lar with limited porosity. In this context, new strategies to access NP-based architectures with large and accessible porosi- ty at the nanometer scale are necessary. One interesting approach that has been used with signifi- cant success in assembling porous networks from colloidal NPs is polymer templating. [10] This method involves assembly of soluble inorganic nano building blocks into various mesoscop- ic structures with the aid of amphiphilic surfactants or block copolymers. Porous networks of connected NPs hold promise for applications in catalysis, solar-energy conversion, and size- selective adsorption and separation. [11] Compared to individual NPs, 3D porous ensembles of NPs are expected to have advan- tageous characteristics such as rapid mass transport in the pore channels of the assembled structure and interfacial trans- fer of electrons along the framework. Furthermore, aligned nanostructures of semiconductor nanocrystals may exhibit new collective properties between adjacent particles that are not present in the single-component materials. [12] However, control over the morphology and porosity of NP aggregates at the mesoscale is not trivial, as more often rapid precipitation of randomly agglomerated NPs occurs. We recently demonstrated that ordered mesoporous net- works of interconnected bismuth ferrite (BiFeO 3 ) NPs can be prepared. [13] To produce this material, we used polymer-direct- ed self-assembly of ligand-stabilized BiFeO 3 nanocrystals. As ligand we used 3-aminopropanoic acid, the carboxyl group of which coordinated to the NP surface while the amino end group interacted with the polar fragment of the polymer tem- plate. Herein, we report on a new strategy that affords 3D highly porous iron oxide architectures through the aggregat- ing self-assembly (ASA) of ligand-stripped g-Fe 2 O 3 NPs. Com- pared to previous methods, this route does not require an or- ganic capping agent for the g-Fe 2 O 3 NPs and occurs in the [a] Dr. I. T. Papadas, Prof. Dr. G. S. Armatas Department of Materials Science and Technology University of Crete Heraklion 71003 (Greece) E-mail : garmatas@materials.uoc.gr [b] S. Fountoulaki, Prof. Dr. I. N. Lykakis Department of Chemistry Aristotle University of Thessaloniki Thessaloniki 54124 (Greece) E-mail : lykakis@chem.auth.gr Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201504685. Chem. Eur. J. 2016, 22, 4600 – 4607 # 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 4600 Full Paper DOI: 10.1002/chem.201504685