Synthesis of nanocrystalline Y 2 O 3 :Eu phosphor through different chemical methods: studies on the chromaticity dependence and phase conversion A. Mohammadi 1,2 , Y. Ganjkhanlou 3 , A.B. Moghaddam 4 , M. Kazemzad 3 , F.Al. Hessari 3 , R. Dinarvand 5 1 Department of Drug and Food Control, Faculty of Pharmacy, Tehran University of Medical Sciences, P.O. Box 14155-6451, Tehran 14174, Iran 2 Pharmaceutical Sciences Research Center, Tehran University of Medical Sciences, Tehran, Iran 3 Materials and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran 4 Department of Engineering Science, College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran 5 Nanotechnology Research Centre, Faculty of Pharmacy, Tehran University of Medical Sciences, Tehran, Iran E-mail: bayandori@ut.ac.ir Published in Micro & Nano Letters; Received on 10th March 2012; Revised on 17th May 2012 In this study, Y 2 O 3 :Eu nanocrystals (NCs) were prepared through the combustion, Pechini and hydrothermal methods followed by the heat treatment of samples at 873 K. The resultant NCs were investigated by X-ray diffractometery, scanning electron microscopy, transmission elec- tron microscopy, selected area electron diffractometery and photoluminescence (PL) spectroscopy. The obtained sample by the Pechini method presents cubic phase, whereas the hydrothermal sample consists of different phases (cubic, monoclinic and hexagonal phases of Y 2 O 3 besides yttrium nitrate hydrate phase). Cubic and monoclinic phases of Y 2 O 3 as well as tetragonal phase of yttrium oxide nitride (YO(NO 3 ) 3 ) phases were observed in combustion sample before heat treatment, whereas just the cubic phase stayed after heat treatment of this sample. The mor- phological investigations demonstrated that the hydrothermal sample has flower-like morphology with micron-size petals, whereas other samples have rounded particles. The PL intensity and chromaticity of Pechini sample are not as good as combustion and hydrothermal samples, but low agglomeration of the Pechini sample makes it good candidate for biological applications as a luminescence sensors and tags. 1. Introduction: Rare-earth-doped luminescent materials are well- known phosphors with high colour quality and brightness [1, 2]. Among them, europium-doped yttrium oxide (Y 2 O 3 :Eu) is one of the most practical phosphors. This compound has high chemical and thermal stability and good solidity against high-current density. It is known as the best red colour for fluorescent lamps. It has been used in various applications such as field emission displays, cathode ray tube, three-colour fluorescent lamps, traffic colours and bio-tagging applications [2]. Recently, the utilisation of nano-sized phosphor has been increased by developing various displays with high resolution, quantum effi- ciency and response time [3]. The nano-sized phosphors are important in bio-labelling applications because of their comparable size with biomolecules [4, 5]. Therefore development of nanophosphor syn- thesis methods is compulsory. Many chemical synthesis methods have been reported for preparing doped and undoped yttrium oxide nanopowders [6–8]. Among them, combustion is a promising method because of its simplicity, rapidity, homogeneity and high crystallinity of its products [9]. The synthesis of highly agglomerated nanopowders is a disadvantage of this method, which limits its applications [10]. On the other hand, nanophosphors with low agglomeration can be synthesised by the Pechini method [11]. However, low agglomerated nanophosphors may be diminished its luminescence properties. In this work, this probability is investigated. Although the Y 2 O 3 :Eu compound is a candidate for biological application [4] but its unfavourable isoelectric point (pH ≏ 8) [12] decreases its application for biological application as it precipi- tate in the normal pH of human body. In accordance to the Stokes law (1), with reduction of size, the rate of particle settling is reduced. Therefore the stability of nano-sized particles in a suspen- sion is higher than the larger particles (the rate of sedimentation for small particles is too low). Therefore preparation of low agglomer- ated nanoparticles of Y 2 O 3 :Eu phosphor is appealing for biological application. In this work, we attempt to find an optimised method for synthesis of high chromatic Y 2 O 3 :Eu nanophosphors, which can be suitable for biological applications: w = 2(r p − r f )gr 2 9m (1) In (1), w is the settling velocity, r is the density (the subscripts ‘p’ and ‘f’ indicate the particle and the fluid, respectively), g is the acceleration because of gravity, r is the radius of the particle, and m is the dynamic viscosity of the fluid. 2. Experiment 2.1. Combustion method: Europium and yttrium nitrates were freshly prepared by reaction of europium and yttrium oxides (Eu 2 O 3 ,Y 2 O 3 ) with nitric acid. The excess acid was eliminated by slow warming. The stock nitrate solutions were prepared by addition of distilled water to nitrate salts in a volumetric flask. Aqueous solutions of europium (0.02 mol/l) and yttrium nitrates (0.5 mol/l) were mixed together to obtain the desired stoichiometry (3 mol% of Eu). Then, urea was added as a fuel to the mixture, the molar ratio of urea to the cations ion is about 2.5, which is stoichio- metric value in combustion reaction. The clear solution was heated in a muffle furnace until the free water evaporated and suddenly combustion of the mixture occurred. The combustion powder was then thermally treated at 873 K for 1 h. 2.2. Pechini method: In the Pechini method [11], the nitrate solution was prepared using the same procedure. The second solution was prepared by adding citric acid and ethylene glycol into distilled water. The molar ratios of citric acid and ethylene glycol to the cations were 2.5 and 1, respectively. Prepared solutions were mixed together and stirred for 24 h at 353 K. The polyesterification occurred and citric acid acts as a chelator to form polychelate. In this method, nanopowders are confined in polychelate and their growth was limited and therefore the nano-sized sample can be prepared. The solution was dried and the powder was heat treated at 873 K for 1 h. 2.3. Hydrothermal method: The as-prepared combustion sample Y 2 O 3 :Eu (3%), before heat treatment, was dispersed in distilled water. Stoicheiometric amount of the hexamethylenetetramine was added into the solution. The solutions were transferred into a Teflon-lined stainless steel autoclave with 70 ml capacity. It was sealed and kept in an oven at 423 K for 16 h. After cooling to room temperature naturally, the resulting white solid product was Micro & Nano Letters, 2012, Vol. 7, Iss. 6, pp. 515–518 515 doi: 10.1049/mnl.2012.0153 & The Institution of Engineering and Technology 2012