This journal is © The Royal Society of Chemistry 2012A convenient method is described that uses a quartz wool substrate to immobilise nanometre sized phosphor precursor particles enabling them to be fired at high temperature without sintering/agglomeration. The nanometre sized phosphor particles are easily removed from the substrate by re-dispersion into liquid for subsequent use.Biotechnology and biological sciences research council (BBSRC

Alireza Salimian

Brown

Buissette

Byrappa

Chai

Chawala

Feldmann

Fletcher

Flint

Fujishiro

Gedanken

George Robert Fern

Hasse

Heer

Jack Silver

Kang

Kijikowska

Lucas

Revaux

Robert Withnall

Suslick

Withnall

Yang

Zhang

Journal of Materials Chemistry

English

Salimian, A

Fern, GR

Withnall, R

Silver, J

Brunel University Research Archive

nspallatemprestability, high melting temperature7 and efficient light output18etching.27vel alternative route for thecrystalline LAP:Tb,Ce. Thisr report of the synthesis ofO4:Eu and CePO4:Tb with amean particle size of 5 nm and a narrow size distribution.28 OurDynamic Article LinksC<Journal ofMaterials ChemistryCite this: J. Mater. Chem., 2012, 22, 21529www.rsc.org/materialsPublished on 31 August 2012. Downloaded by BRUNEL UNIVERSITY on 06/11/2013 14:24:01. View Article Online / Journal Homepage / Table of Contents for this issuemer. An interesting hydrothermal synthesis route has beendemonstrated as an effective approach for the preparation ofnanocrystalline LAP where either rhabdophane or monazitecrystalline phases of LAP are produced depending on thetemperature of synthesis.19–21 However to produce LAP with anarrow particle size distribution by hydrothermal synthesis,complexing agents have to be introduced during the synthesiswhich subsequently need removing.22Synthesis by ultrasonication proceeds via the formation andcollapse of bubbles, referred to as acoustic cavitation, enablingnew route utilises part of the previously published syntheticprocedure,28 whereby the nanometre sized materials are synthe-sized within an organic matrix comprising of stearic and citricacids. Our modification, reported herein, involves the calcinationof the organic–inorganic mixture of materials on a quartz woolsubstrate.ExperimentalSamples of lanthanum phosphate doped with terbium andunder 254 nm excitation when doped with cerium and terbiumco-activators.8 Various methods of synthesising LAP dopedwith a number of elements have been investigated includingsol–gel,9–12 hydrothermal synthesis,13,14 wet chemical precipita-tion,15,16 synthesis via sonication,17 using microemulsion/poly-In this work, we report a nosynthesis of nanometre sizedmethod was based on an earlienanometre sized crystals of LaPA novel method for the preparation ofparticles of lanthanum phosphate phosupport in the firing processAlireza Salimian, George Robert Fern,* Robert WithnReceived 18th June 2012, Accepted 31st August 2012DOI: 10.1039/c2jm33931eA convenient method is described that uses a quartz wool substrphosphor precursor particles enabling them to be fired at high teagglomeration. The nanometre sized phosphor particles are easilyre-dispersion into liquid for subsequent use.IntroductionThe study of nanometre sized materials is one of the areas ofresearch attracting a great deal of interest amongst materialscientists, chemists, physicists, biomaterial scientists, and indeedbiologists as well as scientists working in various areas of envi-ronmental safety. In the field of molecular tagging of biologicalmolecules a number of nanometre sized particles of light emittingphosphors have been studied.1Lanthanum phosphate (LAP) phosphors have been intenselystudied because of their diverse applications in backlit LCDdisplays,2 emissive displays such as plasma display panels andfield emission displays,3,4 general lighting,5 luminescent markersfor imaging,6 and bio-imaging.1 LAP has been demonstrated tobe a suitable host lattice for rare earth elements due to itsthe formation of nanoparticles in the hot spots formed.Centre for Phosphor and Display Materials, Wolfson Centre for Materialsprocessing, Brunel University, Uxbridge, Middlesex UB8 3PH, UK.E-mail: jack.silver@brunel.ac.uk; george.fern@brunel.ac.uk; Fax: +44(0)1895 269737; Tel: +44 (0)1895 266116This journal is ª The Royal Society of Chemistry 2012on-agglomerated nanometre sizedhors utilising a high surface areaand Jack Silver*to immobilise nanometre sizederature without sintering/moved from the substrate byTemperatures of ca. 4700 C and pressures of ca. 1800 atmreached in these hot spots23,24 create a suitable nanometre sizedenvironment for the formation of the particles. However therapid cooling rate associated with this technique hinders crys-tallisation of the final product and the nanometre sized materialsproduced are amorphous.25 It has been reported that the crys-tallinity can be improved by using non-volatile compounds forthe synthesis, thereby enabling the reaction to occur at closerproximity to hot spots where both amorphous and crystallinenanomaterials are produced.16,26 During the synthesis of nano-phosphors it is well known that the light emission can beenhanced by a subsequent annealing process. However this willlead to sintering unless the particles are kept apart during thisprocess, consequently nano-phosphors have been bound inside asilica matrix which must subsequently be removed by HFPAPERcerium were prepared in an organic matrix by dissolving 0.0333moles of lanthanum nitrate (99.99%, Alfa Aesar), 0.0037 molesof terbium chloride (99.99%, Aldrich), 0.0024 moles of ceriumnitrate (99.99%, Aldrich) and 0.033 moles of citric acid (99.5%,Alfa Aesar) in 50 mL of methanol followed by the addition of 0.1moles of stearic acid (Fisher). This solution was then heatedJ. Mater. Chem., 2012, 22, 21529–21532 | 21529with a size distribution within the range 11–41 nm were observedin both GA and GP samples and a histogram for GA is shown inFig. 1 XRD pattern of the LAP particles and associated crystal param-eters for GA sample. Average coherent scattering domain size¼ 18.1 nm,   Fig. 2 Histogram of the GA particles demonstrating the narrow sizedistribution.Published on 31 August 2012. Downloaded by BRUNEL UNIVERSITY on 06/11/2013 14:24:01. View Article Onlineunder reflux while stirring for 2 hours. Then 2 mL of phosphoricacid were added to the solution and allowed to react for 2 hours.The same procedure was repeated, however instead of usingphosphoric acid, 3 mL of a 5 M solution of ammonium phos-phate (99.9% Fluka) was used. These two sample batches weresubsequently named GP and GA, respectively. The hot solutionwas then gently poured over 20 g of quartz wool, QW (which hadbeen pre-treated on its own in a furnace at approximately 1000C), on a Buchner funnel so that the wool was covered uniformlyby the solution and it was then allowed to cool. The wool wasdried under vacuum and 8 g from each QW sample (GP & GA)was fired at 1050 C for 10 minutes in air. Higher firingtemperatures than 1050 C and longer firing times than 10minutes gave stronger luminescence but larger particles, andconversely lower firing temperatures and shorter firing times gaveweaker luminescence and smaller particles. The Ce and Tbprecursor concentrations do not significantly affect the quality ofthe nanoparticles, although they do influence the PL intensity, asis well known for these co-activators.Each QW sample was then placed in 300 mL of iso-propanolfollowed by sonication and stirring for 30 minutes to disperse theparticles. The QW was subsequently removed and the solutionwas centrifuged at 3000 rpm to remove any residual piecesof QW.Each iso-propanol solution was used for electrodeposition ofLAP nanoparticles onto aluminium stubs for subsequent CLemission analysis at accelerating voltages of 1–5 kV. The CLspectra were collected with a TEL 301 Photometer Telescope,with an aperture mirror viewing system to focus precisely on thearea of measurement. The photometer telescope collects the lightoutput through the front viewing port of the low voltagechamber. The collected light passes to the M3000 mono-chromator through the integral flexible fibre optic bundle. Theseresults were compared to data obtained using micrometre sizedcommercial green-emitting LAP phosphor from Nichia. Using2 mL specimens from each sample batch the photoluminescentproperties of the material were investigated under 254 nm exci-tation using a Bentham Instruments photoluminescent (PL)analyser.The PL excitation and emission spectra were obtained using aBentham (Reading, UK) M300 programmable grating mono-chromator photometer system with computer controlled wave-length scanning and intensity data collection, using in the visibleregion an 1800 lines per mm grating. The stepping motor and sinedrive allows wavelength scanning to be completely controlledfrom a remote stepping drive unit (SMD3B). The SMD3B in turnis controlled by a PC via its IEEE 488 interface. Sample excita-tion and collection was collected inside an in-built sealedchamber connected to the M300 monochromator via a fibre-optic bundle.The crystalline phases of the products were determined byX-ray powder diffraction (XRPD) using a Bruker D8 AdvanceX-ray powder diffractometer fitted with a nickel-filtered coppersource and a LynxEye silicon strip detector. Data were recor-ded from 5 to 100 2q degrees at 293 K. The diffractometer waspreviously calibrated using an aluminium oxide line positionstandard from Bruker and LaB6 NIST SRM 660a line profilestandard. The emission of the nickel filtered Cu source and hencethe instrumental line broadening was determined by fitting the21530 | J. Mater. Chem., 2012, 22, 21529–21532NIST standard using Bruker Topas version 3 software. Theanalysis was conducted at ambient temperatures and the sampleswere prepared as powders (dried powders extracted from theQW) and rotated during the collection of the data to maximisethe randomisation. Phases were identified from the XRPD dif-fractograms by peak search matching using the ICSD PDF-2data files and CIF files retrieved using the Chemical DatabaseService.29 Rietveld refinement of the diffractograms was carriedout to determine the lattice parameters and crystallite sizes. Theidentifiable phases were refined using Bruker Topas version 3software.The morphological properties and particle dimensions of thenanomaterial products were investigated by scanning electronmicroscopy (Zeiss, Supra 35VP) and transmission electronmicroscopy (Jeol 2000FX).Results and discussionThe particles were characterised by XRPD. The data fits to theknown pattern of the pure Monazite phase of LAP (Fig. 1) (PDFnumber 83-0651) and no other phases are present. The deter-mination of lattice parameters and crystallite sizes by standardRietveld refinement indicate (Fig. 1) an average coherent scat-tering domain size of 18.1 nm which is consistent with theaverage particle size seen in the TEM study.Particle sizes and morphologies of the as synthesizedLAP:Tb,Ce particles were analysed by scanning (SEM) andtransmission electron microscopy (TEM). The particle size andmorphology were both independent of the phosphate sourceused (phosphoric acid or ammonium phosphate). Nanoparticlesa¼ 6.8181A, b¼ 7.0521A, c¼ 6.4910A, b¼ 103.42 . Red line¼ fit, greenline ¼ raw data, grey line ¼ difference line.This journal is ª The Royal Society of Chemistry 2012Fig. 4 SEM analysis of the quartz wool samples; A (top) and B (middle):QW fibres loaded with the GP sample and fired at 950 C. C (bottom):untreated QW fired at 950 C. (Note impurity ‘‘particle’’ is rare in Ccompared with the voluminous covering in A and B.) Scale bars¼ 10 mm.Fig. 3 TEM images of the LAP nanoparticles; (left): phosphoric acidsource (GP), (right): ammonium phosphate source (GA). Scale bars ¼50 nm.This journal is ª The Royal Society of Chemistry 2012Published on 31 August 2012. Downloaded by BRUNEL UNIVERSITY on 06/11/2013 14:24:01. View Article OnlineFig. 2. TEM images of the particles are presented in Fig. 3.Whilst some larger crystals are present in Fig. 3, the majority ofthe particles lie within the lower limit of this range. Fig. 3demonstrates that pure nano-crystals have been isolated after thecombustion firing which has been effective at removing the excessstearic acid used during synthesis and the clarity of the carbonTEM support shows the ease with which the particles can beisolated for further use.In the SEM micrographs taken of the quartz wool fibres afterfiring, the formation of a thin layer of LAP on the surface of thefibre is clearly visible (Fig. 4A and B). Higher resolution SEMreveals the particles on the surface (Fig. 4B). An uncoated fibre ispresented in Fig. 4C. Note the impurity ‘‘particle’’ is rare inFig. 4C compared with the voluminous covering in Fig. 4A andB. The particles were then separated from the fibre and dispersedin isopropanol by sonication.The photoluminescent spectra of the materials dispersed inisopropanol (see Fig. 5) clearly manifest an emission peak at544 nm. Even though the scale for the brightness of the samples isrecorded in arbitrary units, visual inspection of the emission ofthe samples under 254 nm excitation (see Fig. 6) indicated thatthe GA sample is the brighter phosphor. PL excitation spectraFig. 5 Photoluminescence spectra of the nanoparticles from each batchdispersed in iso-propanol and excited at 254 nm compared to the pho-toluminescence spectrum of a commercial bulk phosphor sample powder.GP (top), GA (middle) and a commercial large particle material(bottom).have previously been reported for LAP:Ce,Tb nanophosphors,and the excitation mechanism has been demonstrated to involveenergy transfer between Ce3+ and Tb3+.30The cathodoluminescent (CL) data were collected from thesamples plated onto a finely polished SEM stub. The sampleFig. 6 Photoluminescent emission observation of the nano-particlesdispersed in isopropanol under white light (left) and 254 nm (right)excitation. (GA on the left and GP on the right in each image.)J. Mater. Chem., 2012, 22, 21529–21532 | 21531biological submicroscopic multicolour imaging). We wish toacknowledge the use of the EPSRC funded Chemical DatabaseService at Daresbury.Notes and references1 K. Byrappa, M. K. Devaraju, J. R. Paramesh and K. Soga, J. Mater.Sci., 2008, 43, 2229.2 R. Withnall, J. Silver, C. Catherall and G. R. Fern, SID Int. Symp.Dig. Tech. Pap., 2008, 39, 1663.Fig. 7 Cathodoluminescent properties of the nanometre sized phosphorparticles. Dashed line is GA, solid line is GP.Published on 31 August 2012. Downloaded by BRUNEL UNIVERSITY on 06/11/2013 14:24:01. View Article Onlinecoating was a thin layer equivalent to 0.0008 g of the nano-material. The stub was then exposed to an electron beam with aradius that matched that of the stub. The CL emission intensitywas then measured at 3, 4 and 5 kV from the CL emissionspectra, which were in good agreement with those that havepreviously been reported from LAP:Ce,Tb nanophosphors.30 Aplot of the luminance against the accelerating voltage for thenanometre sized phosphor particles is presented in Fig. 7. Bothsamples exhibited luminescence but the GA sample was clearlybrighter.ConclusionsHerein we have demonstrated that:The quartz wool substrate was a useful material that offers ahigh surface to volume ratio during the combustion firings of thematerials, allowing the nanometre sized particles to crystallisewith minimal sintering.The nanometre sized particles are easily separated from thequartz wool via gentle sonication and can be dispersed easily inany medium for further application.The nanometre sized particles of lanthanum phosphate wereproduced in the monazite phase having a particle size distribu-tion in the range of 11–41 nm.The source of the phosphate ion did not affect the size orcrystal morphology of the particles, however it may affect thecathodoluminescent and photoluminescent properties of theparticles produced.This simple technique of using the large surface area of quartzwool as a substrate when firing the nanometre sized particles hasbeen demonstrated herein. The method offers potential for scaleup, hence offering an alternative to the hydrothermal and soni-cation techniques for producing nanometre sized particles, wherelarge scale production can be expensive and complicated.AcknowledgementsWe wish to thank the BBSRC for grant funding this work(BBF0050161, NanoProbes; Development of novel probes for21532 | J. Mater. Chem., 2012, 22, 21529–215323 Y. C. Kang, E. J. Kim, D. Y. Lee and H. D. Park, J. Alloys Compd.,2002, 347, 266.4 R. P. Rao, J. Lumin., 2005, 113, 271.5 C. Feldmann, T. Justel, C. R. Ronda and P. J. Schmidt, Adv. Funct.Mater., 2003, 13, 511.6 S. Heer, O. Lehman, M. Hasse and H. U. Gudel, Angew. Chem., Int.Ed., 2003, 42, 3179.7 S. Lucas, E. Champion, D. Bernache-Assollant and G. Leroy, J. SolidState Chem., 2004, 177, 1312.8 G. Blasse and B. C. Grabmeier, Luminescent Materials, Springer-Verlag, New York, 1994.9 R. P. Rao, J. Electrochem. Soc., 2003, 150, H165.10 K. K. Chawala, H. Liu, J. Janczak-Rusch and S. Sambasivan, J. Eur.Ceram. Soc., 2000, 20, 551.11 M. Yu, H. Wang, C. K. Lin, G. Z. Li and J. Lin, Nanotechnology,2006, 17, 3245.12 M. Yu, J. Lin, J. Fu, H. J. Zhang and Y. C. Han, J. Mater. Chem.,2003, 13, 1413.13 Y. Fujishiro, H. Ito, T. Sato and A. Okuwaki, J. Alloys Compd., 1997,252, 103.14 P. Yang, Z. Quan, C. Li, Z. Hou, W. Wang and J. Lin, J. Solid StateChem., 2009, 182, 1045.15 S. Lucas, E. Champion, D. Bregioux, D. Bernache-Assollant andF. Audubert, J. Solid State Chem., 2004, 177, 1302.16 R. Kijikowska, J. Mater. Sci., 2003, 38, 229.17 S. S. Brown, H. Im, A. J. Rondinone and S. Dai, J. Colloid InterfaceSci., 2005, 292, 127.18 R. Chai, H. Lian, P. Yang, Y. Fan, Z. Hou, X. Kang and J. Lin, J.Colloid Interface Sci., 2009, 336, 46.19 Y. W. Zhang, Z. G. Yan, L. P. You, R. Si and C. H. Yan, Eur. J.Inorg. Chem., 2003, 4099.20 M. Hasse, K. Riwotski, H. Meysssamy and A. Kornowski, J. AlloysCompd., 2000, 303–304, 191.21 W. B. Bu, Z. L. Hua, L. X. Zhang, H. R. Chen, W. M. Huang andJ. L. Shi, J. Mater. Res., 2004, 19, 2807.22 V. Buissette, M. Moreau, T. Gacoin, J. P. Boilot, J. Y. Chanechingand T. Le Merceir, Chem. Mater., 2004, 16, 3767.23 K. S. Suslick, Science, 1990, 247, 1439.24 E. B. Flint and K. S. Suslick, Science, 1991, 253, 1397.25 A. Gedanken, Ultrason. Sonochem., 2004, 11, 47.26 K. S. Suslick, D. A. Hammerton and R. E. Cline, J. Am. Chem. Soc.,1986, 108, 5641.27 A. Revaux, G. Dantelle, N. George, R. Seshadri, T. Gacoin andJ.-P. Boilot, Nanoscale, 2011, 3, 2015.28 L. N. Ho, H. Nishiguchi, K. Nagaoka and Y. Takita, J. Rare Earths,2007, 25, 684.29 D. A. Fletcher, R. F. McMeeking and D. Parkin, The UnitedKingdom chemical database service, J. Chem. Inf. Comput. Sci.,1996, 36, 746.30 Z. Hou, L.Wang, H. Lian, R. Chai, C. Zhang, Z. Cheng and J. Lin, J.Solid State Chem., 2009, 182, 698.This journal is ª The Royal Society of Chemistry 2012

A novel method for the preparation of non-agglomerated nanometre sized particles of lanthanum phosphate phosphors utilising a high surface area support in the firing process

Crossref

http://bura.brunel.ac.uk/bitstream/2438/7703/2/Fulltext.pdf

A novel method for the preparation of non-agglomerated nanometre sized particles of lanthanum phosphate phosphors utilising a high surface area support in the firing process

Abstract

Similar works

Full text

Available Versions

Brunel University Research Archive

Crossref