Advances in porous and high-energy (001)-faceted anatase TiO 2 nanostructures Akrajas Ali Umar a, * , Siti Khatijah Md Saad a , Marjoni Imamora Ali Umar b , Mohd Yusri Abd Rahman a , Munetaka Oyama c a Institute of Microengineering and Nanoelectronics, Universiti Kebangsaan Malaysia (UKM), 43600 Bangi, Selangor, Malaysia b Department of Physics Education, Facultyof Tarbiyah and Education, Institut Agama Islam Negeri (IAIN), Batusangkar, 27213, West Sumatera, Indonesia c Nanomaterials Chemistry Laboratory, Department of Materials Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto, 615- 8520, Japan article info Article history: Received 8 August 2017 Received in revised form 28 September 2017 Accepted 2 October 2017 abstract In this review, we present a summary of research to date on the anatase polymorph of TiO 2 nano- structures containing high-energy facet, particularly (001) plane, with porous structure, covering their synthesis and their application in photocatalysis as well as a review of any attempts to modify their electrical, optical and photocatalytic properties via doping. After giving a brief introduction on the role of crystalline facet on the physico-chemical properties of the anatase TiO 2 , we discuss the electrical and optical properties of pristine anatase TiO 2 and after being doped with both metal and non-metals dopants. We then continue to the discussion of the electrical properties of (001) faceted anatase TiO 2 and their modification upon being prepared in the form of porous morphology. Before coming to the review of the photocatalytic properties of the (001) faceted anatase and (001) with porous morphology in selected photocatalysis application, such as photodegradation of organic pollutant, hydrogenation reaction, water splitting, etc., we discuss the synthetic strategy for the preparation of them. We then end our discussion by giving an outlook on future strategy for development of research related to high- energy faceted and porous anatase TiO 2 . © 2017 Elsevier B.V. All rights reserved. 1. Introduction TiO 2 is a wide band gap semiconducting materials that has been for decades attracting a wide interest for broad range of applica- tions ranging from cosmetic to photocatalysis [1e3], solar cell [4], sensor [5,6], hydrogen energy harvesting from water dissociation [7e10], fuel cell [11e 14], application related to environment ben- efits [15e17] and optoelectronics [18e21] due to its unique physico-chemical properties, such as band gap energy level that enables for a facile charge injection or extraction, high-chemical stability that resists to photo-bleaching and non-toxicity and etc [19,22e25]. TiO 2 can be mostly found in the form of three poly- morph, i.e. rutile, anatase and brookite and its electrical, optical and other physicochemical properties depend on its crystalline phase state. Unique to anatase, it demonstrates superior properties in many aspect compared to others two phases, particularly its photoactivity and surface physico-chemical properties [26]. These crucial feature has made anatase polymorph become the center of interest, particularly for an application in the field of photocatalytic and energy conversion. Up to this stage, there is still a continuous interest in the synthesis of anatase polymorph of TiO 2 with peculiar morphology that may facilitate enhanced photophysical processes in this material, such as wide surface area high-energy facets dominant structure. In anatase phase, the order of the surface en- ergies follows the sequences of (110) > (001) > (100) > (101) with surface energy (g) of 1.09, 0.90, 0.53 and 0.44 J m 2 , respectively [27]. The high-energy facet are thermodynamically un-stable and rapidly annealed into a highly-stable (101) plane. For that reason, preserving the high energy facet during the nanocrystal growth in order to obtaining a reactive TiO 2 nanocrystals is a challenging task. Considering the role of TiO 2 surface in many photophysical pro- cesses, to develop methods to realize anatase TiO 2 with a wide area of high-energy facet is highly demanded for further augmenting their performance in applications. We have recently witnessed a number of attempts in realizing the nanostructured anatase TiO 2 * Corresponding author. E-mail address: akrajas@ukm.edu.my (A.A. Umar). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2017.10.002 0925-3467/© 2017 Elsevier B.V. All rights reserved. Optical Materials 75 (2018) 390e430