- Nano Express
- Open Access
Reduced erbium-doped ceria nanoparticles: one nano-host applicable for simultaneous optical down- and up-conversions
© Shehata et al.; licensee Springer. 2014
- Received: 24 February 2014
- Accepted: 7 April 2014
- Published: 13 May 2014
This paper introduces a new synthesis procedure to form erbium-doped ceria nanoparticles (EDC NPs) that can act as an optical medium for both up-conversion and down-conversion in the same time. This synthesis process results qualitatively in a high concentration of Ce3+ ions required to obtain high fluorescence efficiency in the down-conversion process. Simultaneously, the synthesized nanoparticles contain the molecular energy levels of erbium that are required for up-conversion. Therefore, the synthesized EDC NPs can emit visible light when excited with either UV or IR photons. This opens new opportunities for applications where emission of light via both up- and down-conversions from a single nanomaterial is desired such as solar cells and bio-imaging.
- Ceria nanoparticles
- Erbium dopant
Optical nanostructures that emit visible light when excited by ultraviolet (UV) or infrared (IR) photons have been extensively studied for applications that include bioimaging [1, 2], solar energy [3, 4], and optical gas sensors [5, 6]. Research on one of these nanomaterials, cerium oxide (ceria) nanoparticles, has shown that its material properties are extremely well suited for a lot of applications; ceria can be employed as the optical active agent in UV absorbents and filters , gas sensors , and bioimaging media . Visible emission from either UV excitation (down-conversion) or IR excitation (up-conversion) can be obtained from ceria nanoparticles. However, both up- and down-conversion processes involve different physiochemical properties in ceria and optimization of each optical process via various nanoparticle synthesis and post-growth procedures tends to quench the efficiency of the other process.
For example, ceria nanoparticles synthesized at or near room temperature by a chemical precipitation method will fluoresce in the visible wavelength region, λpeak approximately 520 nm, when excited by near-UV photons (λexc approximately 430 nm) . The down-conversion process requires that the cerium ions are in the Ce3+ state and are associated with oxygen vacancies, which implies that ceria nanoparticles contain Ce2O3 is a direct semiconductor . To obtain visible light via up-conversion, ceria nanoparticles must be doped with certain lanthanides, such as erbium, then annealed at temperatures above 700°C . Ceria is a low-phonon host for the erbium ions, which act as optical centers that convert the energy from absorbed IR photons into visible light . However, the presence of the negative-association energy element, erbium, and the high temperature anneal causes the dominant ionization state of cerium ions to be in the Ce4+ state where Ce4+ ions bond with oxygen to form CeO2, an indirect semiconductor [10, 14, 15]. Hence, the down-conversion emission efficiency of the erbium-doped ceria nanoparticles (EDC NPs), particularly after the thermal anneal, is low . On the other hand, there is no observable up-conversion emission from undoped ceria nanoparticles or from ceria nanoparticles doped with positive association energy lanthanide. Thus, to optimize the properties of ceria nanoparticles for the two optical conversion processes, it has been required two different nanoparticle synthesis and post-processing procedures.
EDC NPs are prepared using the chemical precipitation technique which is relatively simple and inexpensive synthesis process [16, 17]. Cerium (III) chloride (0.475 g) and erbium (III) chloride (0.025 g) are dissolved in de-ionized (DI) water (40 mL) to obtain a 5% weigh ratio of erbium to cerium in the synthesized nanoparticles. This weight ratio is selected after a study by the authors of EDC NPs, synthesized using the same process, in which it was found that optimal concentration of erbium in ceria for up-conversion is 5 wt.% which is close to the quenching ratio mentioned by another research group . The solution is stirred constantly at 500 rpm in a water bath, while the temperature of the water bath is raised to 60°C, and ammonia (1.6 mL) is then added to the solution. The solution is kept at 60°C for 1.5 h and, then, the solution is stirred for another 22.5 h at room temperature. The colloidal solution is centrifuged and washed with DI water and ethanol to remove any unreacted cerium and ammonia. Then, the wet powder is dried on a hot plate. The thermal anneal of the dried nanoparticles is performed in a tube furnace (CM Furnace, Model 1730-20HT, Bloomfield, NJ, USA) with an atmosphere of hydrogen and nitrogen gases that are injected into the furnace at flow rates equal to 10 and 5 standard cubic feet per minute (scfm), respectively, for 2 h at temperatures of 700°C, 800°C, and 900°C. The gases during the anneal assist with the reduction of the cerium ions from the Ce4+ to Ce3+ ionization states and the creation of the oxygen vacancies , while the thermal energy available during the high temperature anneal promotes the formation of the molecular energy levels of erbium inside the ceria host .
The annealed EDC NPs are imaged using TEM and compared to that of the unannealed EDC NPs. A representative image is shown in Figure 3c; it is an image of the EDC NPs after an 800°C anneal. Following the anneal temperature range between 700°C to 900°C, the mean diameter is found to be in the range of 9 to 13 nm as compared to a mean diameter of 7 nm for the unannealed (as-synthesized) EDC NPs. The synthesized EDC NPs have mean diameter smaller than other optical nanoparticles that have been studied as an optical active medium for down- or up-conversion [22–25]. An X-ray diffraction (XRD) pattern is presented in Figure 3d, measured on a sample of the EDC NPs annealed at 800°C, to demonstrate that the predominant nanostructure of the EDC NPs is cerium dioxide [10, 26]. The diffraction peaks in the XRD patterns measured on the as-synthesized EDC NPs and the nanoparticles annealed at 700°C and 900°C also are characteristics of ceria.
When the EDC NPs are excited by near-IR (λ = 780 nm) photons, visible emission is observed at two regions in the visible wavelength range; the primary emission is between 520 to 560 nm and a much smaller emission is found at 660 to 680 nm, as shown in Figure 4b. We hypothesize that erbium ions form stable complexes with oxygen in the ceria host during the anneal and the crystalline structure of the nanoparticle improves, both of which increase the efficiency of Er+3 ions to act as optically active centers for up-conversion . The results include a slight improvement of the intensity of the up-conversion emission with increasing annealing temperature. A portion of the Dieke diagram is illustrated in Figure 4c, which shows that excited state absorption (ESA) is possible. First, the erbium ion is excited from 4I15/2 level to 4I9/2. From the 4I9/2 state, the excited Er+3 ion non-radiatively relaxes to the 4I11/2 state. If a second 780-nm photon interacts with the excited Er+3 ion, an ESA process occurs, which excites the erbium ion to the level of 4 F7/2. After a series of non-radiative relaxations to lower levels such as 2H11/2, 4S3/2, and 4 F9/2, radiative relaxation to the 4I15/2 state occurs and visible emission results; green photons are emitted during the transitions from 2H11/2 and 4S3/2 to 4I15/2 while red photons are emitted during the 4 F9/2 to 4I15/2 transition.
In conclusion, this paper presents a study on a new synthesized nanomaterial, EDC NPs, that emit photons in the visible wavelength range when illuminated by two different excitation sources: near-UV light (430 nm) and near-IR (780 nm) light. When the excitation source is near-UV light, a down-conversion process results in a broad emission peak centred at 520 nm. Up-conversion of the near-IR light is responsible for the narrower bands of green and red emission. Anneals at temperatures of 700°C and 800°C in a hydrogen-nitrogen atmosphere reduces the cerium ions from the Ce4+ to Ce3+ state. The reduced state (Ce3+) associated to oxygen vacancies form defect states that are responsible for the down-conversion emission. At the same time, the erbium ions form complexes with oxygen, which improves up-conversion efficiency. EDC NPs, with average diameter of 9 to 13 nm, may be employed in new applications in biomedicine, solar cell technology, and gas sensing, where an optical nanomaterial that can emit via either up- or down-conversion may be of value.
This work was funded partially by a NSF STTR Phase I grant with MW Photonics (award# 0930364). The authors would like to thank Dr. Michael Ellis and his assistant Eng. Jeremy Beach, both from the Institute for Critical Technology and Applied Science (ICTAS), for their assistance with the furnace annealing of the nanoparticles in Dr. Ellis' laboratory. Also, the authors are grateful to the financial support of the Bradley Department of Electrical and Computer Engineering in Virginia Tech, Virginia Tech Middle East and North Africa (VT-MENA) program in Egypt, and Center of Advanced Materials (CAM) in Qatar University. The authors appreciate the technical support of Mr. Don Leber, manager of the Micron Technology Semiconductor Processing Laboratory at Virginia Tech.
- Chandra S, Das P, Bag S, Laha D, Pramanik P: Synthesis, functionalization and bioimaging applications of highly fluorescent carbon nanoparticles. Nanoscale 2011, 3: 1533–1540.View ArticleGoogle Scholar
- Rijke F, Zijlmans H, Li S, Vail T, Raap AK, Niedbala RS, Tanke HJ: Up-converting phosphor reporters for nucleic acid microarrays. Nat Biotechnol 2001, 19: 273–276.View ArticleGoogle Scholar
- Maruyama T, Shinyashiki Y, Osako S: Energy conversion efficiency of solar cells coated with fluorescent coloring agent. Sol Energy Mater Sol Cells 1998, 56: 1–6.View ArticleGoogle Scholar
- Shan GB, Demopoulos GP: Near-infrared sunlight harvesting in dye-sensitized solar cells via the insertion of an upconverter-TiO2 nanocomposite layer. Adv Mater 2010, 22: 4373–4377.View ArticleGoogle Scholar
- Carmona N, Villegas MA, Navarro JM: Sol-gel coatings in the ZrO2-SiO2 system for protection of historical works of glass. Sens Actuators A 2004, 116: 398–404.View ArticleGoogle Scholar
- Cardenas-Valencia AM, Byrne RH, Calves M, Langebrake L, Fries DP, Steimle ET: Development of stripped-cladding optical fiber sensors for continuous monitoring: II: Referencing method for spectral sensing of environmental corrosion. Sens Actuators B 2007, 122: 410–418.View ArticleGoogle Scholar
- Tsunekawa S, Fukuda T, Kasuya AJ: Blue shift in ultraviolet absorption spectra of monodisperse nanoparticles. Appl Phys 2000, 87: 1318–1321.View ArticleGoogle Scholar
- Shehata N, Meehan K, Leber DJ: Fluorescence quenching in ceria nanoparticles: dissolved oxygen molecular probe with relatively temperature insensitive Stern–Volmer constant up to 50°C. Nanophotonics 2012, 6: 063529/1–11.Google Scholar
- Babu S, Cho JH, Dowding JM, Heckert E, Komanski C, Das S, Colon J, Baker CH, Bass M, Self WT, Seal S: Multicolored redox active upconverter cerium oxide nanoparticle for bio-imaging and therapeutics. Chem Commun 2010, 46: 6915–6917.View ArticleGoogle Scholar
- Shehata N, Meehan K, Hudait M, Jain NJ: Control of oxygen vacancies and Ce+3 concentrations in doped ceria nanoparticles via the selection of lanthanide element. Nanopart Res 2012, 14: 1173–1183.View ArticleGoogle Scholar
- Zholobak NM, Ivanov VK, Shcherbakov AB, Shaporev AS, Polezhaeva OS, Baranchikov AY, Spivak NY, Tretyakov YDJ: UV-shielding property, photocatalytic activity and photocytotoxicity of ceria colloid solutions. Photochem Photobiol B 2011, 102: 32–38.View ArticleGoogle Scholar
- Cho JH, Bass M, Babu S, Dowding JM, Self WT, Seal SJ: Up conversion luminescence of Yb+3–Er+3 codoped CeO2 nanocrystals with imaging applications. Lumin 2012, 132: 743–749.View ArticleGoogle Scholar
- Guo HJ: Green and red upconversion luminescence in CeO2:Er+3 powders produced by 785 nm laser. Solid State Chem 2007, 180: 127–131.View ArticleGoogle Scholar
- Damyanova S, Pawelec B, Arishtirova K, Huerta MV, Fierro JG: Study of the surface and redox properties of ceria-zirconia oxides. Appl Catal A 2008, 337: 86–96.View ArticleGoogle Scholar
- Pedrosa AMG, Silva JEC, Pimentel PM, Melo DMA, Silva FRG: Synthesis and optical investigation of systems involving mixed Ce and Er oxides. J Alloys Compd 2004, 374: 223–229.View ArticleGoogle Scholar
- Chen H, Chang H: Homogeneous precipitation of cerium dioxide nanoparticles in alcohol/water mixed solvents. Colloids Surf A 2004, 242: 61–69.View ArticleGoogle Scholar
- Dhannia T, Jayalekshmi S, Kumar MCS, Rao TP, Bose AC: Effect of iron doping and annealing on structural and optical properties of cerium oxide nanocrystals. J Phys Chem Solids 2009, 70: 1443–1447.View ArticleGoogle Scholar
- Perrichon V, Laachir A, Bergeret G, Frety R, Tournayan LJ: Reduction of cerias with different textures by hydrogen and their reoxidation by oxygen. Chem Soc Faraday Trans 1994, 90: 773–781.View ArticleGoogle Scholar
- Balda R, Garcia-Revilla S, Fernandez J, Seznec V, Nazabal V, Zhang XH, Adam JL, Allix M, Matzen G: Upconversion luminescence of transparent Er3+-doped chalcohalide glass-ceramics. Opt Mater 2009, 31: 760–764.View ArticleGoogle Scholar
- Pankove J: Optical Processes in Semiconductors. New York: Dover Publications Inc; 1971:34–36.Google Scholar
- Shmyreva AN, Borisov AV, Maksimchuk NV: Electronic sensors built on nanostructured cerium oxide films. Nanotech Russia 2010, 5: 382–389.View ArticleGoogle Scholar
- Lee YEK, Kopelman R: Optical nanoparticles sensors for quantitative intracellular imaging. WIREs Nanomed Nanobiotech 2009, 1: 98–110.View ArticleGoogle Scholar
- Chu CS, Lo YL: Optical fiber dissolved oxygen sensor based on Pt(II) complex and core-shell silica nanoparticles incorporated with sol–gel matrix. Sens Actuators B 2010, 151: 83–89.View ArticleGoogle Scholar
- Shehata N, Meehan K, Ashry I, Kandas I, Xu Y: Lanthanide-doped ceria nanoparticles as fluorescence-quenching probes for dissolved oxygen. Sens Actuators B 2013, 183: 179–186.View ArticleGoogle Scholar
- Wang M, Abbineni G, Clevenger A, Mao C, Xu S: Upconversion nanoparticles: synthesis, surface modification and biological applications. Nanomed Nanotechnol Biol Med 2011, 7: 710–729.View ArticleGoogle Scholar
- Deshpande S, Patil S, Kuchibhatla S, Seal S: Size dependency variation in lattice parameter and valency states in nanocrystalline cerium oxide. Appl Phys Lett 2005, 87: 133113/1–3.View ArticleGoogle Scholar
- Patsalas P, Logothetidis S, Sygellou L, Kennou S: Structure-dependent electronic properties of nanocrystalline cerium oxide films. Phys Rev B 2003, 68: 035104.View ArticleGoogle Scholar
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