DOI: 10.1002/asia.201000550 Synthesis and Enhanced Photocatalytic Activity of a Hierarchical Porous Flowerlike p–n Junction NiO/TiO 2 Photocatalyst Jiaguo Yu,* Wenguang Wang, and Bei Cheng [a] Introduction In recent years, considerable interest has been focused on the synthesis of oxide semiconductors with controllable mor- phology, crystalline orientations, grain size, and surface ar- chitectures, which usually exhibit promising functions. [1, 2] Particularly, the self-assembly of low-dimensional nanostruc- tures (0D, 1D, 2D) into three-dimensional (3D) superstruc- tures has been a research hotspot owing to their unusual physical and chemical properties compared with those of the nanoparticles. [3, 4] The morphology and structure of the materials could greatly influence their optical, electronic, magnetic, and catalytic performance. A variety of morpholo- gies have been fabricated such as nanosheets, [5] nanotubes, [6] nanorods, [7] nanowires, [8] nanorings, [9] hollow spheres, [10] and so forth. These materials have potential applications such as catalysts, photonic materials, and battery materials. [11] Nickel oxide (NiO), as a p-type wide-bandgap semicon- ductor, has received increasing attention owing to its variety of specialized applications such as catalysis, [12] battery catho- des, [13] fuel-cell electrodes, [14] electrochromic films, [15] electro- chemical supercapacitors, [16] magnetic materials, [17] and gas sensors, [18] and it has an indirect band gap of about 3.55 eV. [15b] To date, nickel oxide superstructures with vari- ous morphologies have been achieved. For example, Song and Gao [19] reported the preparation of NiO hierarchical ar- chitectures with controllable morphologies and sizes, and the as-prepared hollow NiO hierarchical architectures showed a high photocatalytic property to decompose acid red pollutant. Lin et al. [20] prepared ordered NiO nanowire arrays embedded in anodic alumina membranes by using an electrochemical deposition method. Shi et al. [21] prepared or- dered NiO nanotube arrays embedded in porous anodic alu- mina templates employing a novel chemical transition method, while Beach et al. [22] synthesized NiO microspheres composed of relatively large, thin, and loosely associated Abstract: Hierarchical flowerlike b- Ni(OH) 2 superstructures composed of intermeshed nanoflakes are synthesized by hydrothermal treatment with a mixed solution of C 2 H 4 ACHTUNGTRENNUNG(NH 2 ) 2 , NaOH, and NiACHTUNGTRENNUNG(NO 3 ) 2 . The as-prepared b- Ni(OH) 2 superstructures could be easily changed into NiO superstruc- tures without great morphology change by calcination at 400 8C for 5 h. Fur- thermore, the TiO 2 nanoparticles can be homogeneously deposited on the surface of NiO superstructures by dis- persing b-Ni(OH) 2 powders in Ti- ACHTUNGTRENNUNG(OC 4 H 9 ) 4 –C 2 H 5 OH mixed solution and then vaporizing to remove the ethanol at 100 8C, and finally calcination at 400 8C for 5 h. The prepared NiO/TiO 2 p–n junction superstructures show much higher photocatalytic activity for photocatalytic degradation of p-chloro- phenol aqueous solution than conven- tional TiO 2 powders and NiO super- structures prepared under the same ex- perimental conditions. An obvious en- hancement in the photocatalytic activi- ty can be related to several factors, including formation of hierarchical porous structures, dispersion of TiO 2 particles on the surface of NiO super- structures, and production of a p–n junction. Further results show that NiO/TiO 2 composite superstructures can be more readily separated from the slurry system by filtration or sedimen- tation after photocatalytic reaction and re-used, compared with conventional powder photocatalysts. After many re- cycling experiments for the photode- gradation of p-chlorophenol, the NiO/ TiO 2 composite sample does not exhib- it any great activity loss, confirming that NiO/TiO 2 sample is stable and not photocorroded. Keywords: heterogeneous catalysis · interfaces · mesoporous materials · p–n junctions · photochemistry [a] Prof. J. Yu, Dr. W. Wang, Dr. B. Cheng State Key Laboratory of Advanced Technology for Material Synthesis and processing Wuhan University of Technology Luoshi Road 122#, Wuhan 430070 (P.R. China) Fax: (+ 86) 27-87879468 E-mail: jiaguoyu@yahoo.com Chem. Asian J. 2010, 5, 2499 – 2506  2010 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 2499