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1 Introduction ZnO is a wide band gap semiconductor with a high exciton binding energy which attracts considerable scientific and applied interest with respect to exploiting its optical properties [ 1 – 5 ]. For example, high thermal and chemical stability, simple tunability of the optical and electrical properties are widely applicable in optoelectronic devices [ 6 , 7 ]. Moreover, while exhibiting an excitonic emission peak in the ultraviolet (UV) region, ZnO typically shows strong visible luminescence bands which are due to numerous point defects present in

Solvothermal Synthesis of Co-doped ZnO Nanopowders Janusz Fidelusa, Radu Robert Piticescub, Roxana Mioara Piticescub, Witold Lojkowskia, and Liviu Giurgiuc a High Pressure Physics Centre of the Polish Academy of Science, Sokolowska 29/37, Warsaw, Poland b National Research & Development Institute for Non-ferrous and Rare Metals, 102 Biruintei Blvd, Pantelimon, Ilfov, Romania c National Institute for the Research & Development of Isotopic and Molecular Technology, 65-103 Donath Str. Cluj-Napoca, Romania Reprint requests to R. R. Piticescu. E-mail: rpiticescu

nanoparticles modified ZnO nanorods with improved photocatalytic activity. J. Colloid Interface Sci ., 363 , 175-181. 5. Zhao, X., Yao, W., Wu, Y., Zhang, Sh., Yang, H., & Zhu, Y. (2006). Fabrication and photochemical properties of porous ZnWO4 film. J. of Solid State Chem ., 179 , 2562-2570. 6. Grigorjeva, L., Millers, D., Grabis, J., & Jankovica, Dz. (2011). Photoluminescence and photocatalytic activity of zinc tungsten powders. Cent. Eur. J. Phys ., 9 (2), 510-514. 7. Kamei, M. (2012). Sense and reproducible photocatalytic activity evaluation instrument for

drinking water is 50 mg/l [ 6 ]. Hence, there are various methods for reducing nitrate from drinking water, such as filtration, ion exchange, and biological and photocatalytic processes [ 7 ], [ 8 ], [ 9 ], [ 10 ]. The photocatalytic process, an economic and simple method compared with other methods, has been the subject of many research projects. Various metal oxide semiconductors, such as TiO 2 , ZnO, MoO 3 , CeO 2 , ZrO 2 , WO 3 , α-Fe 2 O 3 and SnO 2 , are used as catalysts in the photocatalytic reactions [ 11 ], [ 12 ], [ 13 ], [ 14 ]. Mechanical stability, non

REFERENCES 1. Ozgur, U., Alivov, Ya. I., Liu, C., Teke, A., Reshchikov, M. A., Dogan, S., Avrutin, V., Cho, S.-J., and Morkoc, H. (2005). A comprehensive review of ZnO materials and devices. J. Appl. Phys. 98 , 041301. 2. Ma, D.W., Ye, Z.Z., Lu, H.M., Huang, J.Y., Zhao, B.H., Zhu, L.P., and Zhang, H.J. (2004). Sputtering deposited ternary Zn 1−x Cd x O crystal films on Si(111) substrates. Thin Solid Films. 461 , 250–255. 3. Wei, M., Boutwell, R.C., Mares, J.W., Scheurer, A., and Schoenfeld, W.V. (2011). Bandgap engineering of sol-gel synthesized amorphous Zn (1

nanomaterials as sorbents for the removal of radioactive elements and heavy metal ions has been observed [ 4 – 10 ]. In many cases, the modification of their surface functionality and their high surface area to volume ratio increase their sorption capacity and efficiency [ 11 ]. Nanosized metal oxides, including nanosized ferric, copper, aluminium, magnesium and cerium oxides are classified as the promising ones for radionuclides and heavy metals removal from aqueous systems [ 9 , 12 – 14 ]. Among other metal oxides, nano-zinc oxide (ZnO) as a semiconductor is an appreciable

References Martin, A. Green, Keith Emery, Yoshihiro Hishikawa & Wilhelm Warta, (2009). Solar Cell Efficiency Tables (Version 33). Prog. Photovolt.: Res. Appl., 17 , 85-94. Nagare , B.J, Sajeev Chacko & Kanhere , D. G. (2010) Ferromagnetism in carbon doped zinc oxide systems. J. Phys. Chem. A., 114 (7), 2689-2696. DOI: 10.1021/jp910594m. Huihui Huang, Guojia Fang, Xiaoming Mo, Hao Long, Longyan Yuan, Binzhong Dong, Xianquan Meng, & Xingzhong Zhao. (2009). ZnO-based fairly pure ultraviolet light-emitting diodes with a low operation voltage. Electron

. However, some drawbacks seem to exist in the original method, especially the use of toxic surfactants [ 6 – 8 ]. Moreover, these surfactants are difficult to be removed, and usually appropriate solvents are needed to wash them off. Thus, the morphology-controlled synthesis of the well aligned hierarchical structures via a simple, surfactant-free method is of great importance. Zinc oxide (ZnO), as an important semiconductor material has been extensively investigated because of its great potential applications in electronics, photoelectronics, optics, sensors and

References 1. Tang, W., and Wang, J. (2015). Mechanism for toluene detection of flower-like ZnO sensors prepared by hydrothermal approach: Charge transfer. Sensors and Actuators B, 207, 66-73. 2. Wei, A., Pan, L., and Huang, W. (2011). Recent progress in the ZnO nanostructure-based sensors. Materials Science and Engineering B, 176, 1409-1421. 3. Arya, S.K., Saha, S., Ramirez-Vick, J. E., Gupta, V., Bhansali, S., and Singh, S.P. (2012). Recent advances in ZnO nanostructures and thin films for biosensor applications: Review. Analytica Chimica Acta, 737, 1-21. 4

[1] Ang, Y., Li, Z., Xu, S., Han, D., & Lu, D. (2013). Optical properties and photocatalytic activities of spherical ZnO and flower-like ZnO structures synthesized by facile hydrothermal method. Journal of Alloys and Compounds, 575, 359–363. DOI: 10.1016/j.jallcom.2013.05.183. [2] Bamoharram, F. F. (2011). Preparation of ZnO nanorods in the presence of nano preyssler as agreen and eco-friendly polyoxometalate and its photocatalytic activity in the photodegradation of methyl orange. Synthesis and Reactivity in