DOI: 10.1002/adma.200702055 Titanate Nanofibers as Intelligent Absorbents for the Removal of Radioactive Ions from Water** By Dong Jiang Yang, Zhan Feng Zheng, Huai Yong Zhu, * Hong Wei Liu, and Xue Ping Gao* Environmental contamination with radioactive ions that ori- ginate from, e.g., tailings and heap leaching residues of uranium mining (such as 226 Ra ions), byproducts of nuclear fusions (such as 90 Sr), and leakage of nuclear reactors can cause long-term problems which may pose a serious threat to the health of a large part of the population. [1–6] In order to address this serious issue, techniques must be developed for the removal of radioactive ions from the environment (mainly from wastewater) and their subsequent safe disposal. The key issue in the development of such a technology is to devise materials which are able to absorb radioactive ions irreversibly, selectively, efficiently, and in large quantities from contami- nated water. An irreversible sorption assures that the radio- active ions will not be released from the sorbent and can not cause secondary pollution. Besides, the sorbent materials should be very stable to radiation, chemicals, and thermal as well as mechanical stress so that ions and sorbent material together can be safely disposed. Since the demand for nuclear material for energy production increases continually, we are urged to find effective solutions for the removal of highly hazardous, radioactive ions from waste water discharged from the nuclear power industry as well as the nuclear material mining industry. Advanced materials are of great interests for this purpose. [1–8] Natural inorganic cation exchange materials, such as clays and zeolites, have been extensively studied and used in the removal of radioactive ions from water via ion exchange and are subsequently disposed of in a safe way. [9,10] Synthetic inorganic cation exchange materials, such as synthetic micas, [2,4] g -zirconium phosphate, [1] niobate mole- cular sieves, [5,6] and titanate, [8,11] have also been studied for this particular purpose. Synthetic exchange materials are by far superior to natural materials in terms of selectivity for the removal of radioactive cations from water. [1–6,8] Radioactive cations are preferentially exchanged with sodium ions or pro- tons in the synthetic material. More importantly, a structural collapse of the exchange materials occurs after the ion exchange proceeds to a certain extent, [2–4] thereby forming a stable solid with the radioactive cations being permanently trapped inside. Hence, the immobilized radioactive cations can be disposed safely. This phenomenon that the uptake of large, radioactive cations eventually triggers the trapping of the cations themselves represents a desirable intelligent property for any material to be used in decontamination of water from radioactive cations. It requires a metastable structure of the inorganic ion exchange material, rather than a rigid one, in addition to the obviously required cation exchange ability. Generally, ion exchange materials exhibiting a layered structure are less stable than those with 3D crystal structures and the collapse of the layers can take place under moderate conditions. Then again, it has also been found that nanopar- ticles of inorganic solids readily react with other species or are quickly converted to other crystal phases under moderate conditions, [12,13] and thus are substantially less stable than the corresponding bulk material. Knowing this we focused our search for potential candidates for intelligent absorbents on nanoparticles of inorganic ion exchange materials with a layered structure. Recently, we synthesized nanofibers of trititanate (Na 2 Ti 3 O 7 , labeled as T3 in the present study) which exhibit a layered structure and the interlayer contains exchangeable sodium cations. In these nanofibers, octahedra of TiO 6 are the basic structural units and each TiO 6 octahedron shares edges with its neighbouring octahedra so as to form a zigzag chainlike structure. By sharing edges the chains link together as negatively charged layers, [14–16] and exchangeable sodium cations are located between the layers, which presents the fibers as an inorganic exchange materials similar to layered clays. Upon heating to 210 8C the trititanate is converted into a new phase, Na 1.5 H 0.5 Ti 3 O 7 (labeled as T3(H) hereafter), as obvious from the X-ray diffraction (XRD) pattern shown in Figure 1, but maintains a fibril morphology according to the transmission electron microscopy (TEM) images (Figs. 1A and 1B). The Na 1.5 H 0.5 Ti 3 O 7 phase can be converted into a Na 2 Ti 6 O 13 phase, [17,18] by heating to higher temperatures (>300 8C), hence it represents an intermediate of low structural stability. The cation exchange capacities (CECs) of T3 and T3(H) nanofibers were calculated from their respective chemical COMMUNICATION [*] Prof. H. Y. Zhu, D. J. Yang, Z. F. Zheng, H. W. Liu School of Physical and Chemical Sciences, Queensland University of Technology Brisbane QLD, 4001 (Australia) E-mail: hy.zhu@qut.edu.au Prof. X. P. Gao Institute of New Energy Material Chemistry, Nankai University Tianjin, 300071 (PR China) E-mail: xpgao@nankai.edu.cn [**] This research was supported by the Australian Research Council (ARC) and the National Natural Science Foundation of China (NSFC, 90206043). Supporting Information is available online from Wiley InterScience or from the author. Adv. Mater. 2008, 20, 2777–2781 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2777