ECS Transactions [ECS 220th ECS Meeting - Boston, MA (October 9 - October 14, 2011)] - ALD Applied to Conformal Coating of Nanoporous γ-Alumina: Spinel Formation and Luminescence

ALD Applied to Conformal Coating of Nanoporous γ-Alumina: Spinel Formation and Luminescence Induced by Europium Doping

E. Rauwela,c, O. Nilsenb,c, A. Galeckasc, J. C. Walmsleyd, E. Ryttere and H. Fjellvågb,c

aUniversity of Oslo, Dpt. of Chemistry & inGAP, Oslo, Norway

bUniversity of Oslo, Dpt. of Chemistry, Oslo, Norway cUniversity of Oslo, SMN, Oslo, Norway

dSINTEF, Materials and Chemistry, Trondheim, Norway eStatoil Technology Centre Trondheim, Trondheim, Norway

Contact: [email protected]

We report on conversion of nanoporous γ-alumina particles into the spinel type ZnAl2O4 without deteriorating porosity of the host material notably. The material was converted by depositing thin layers of ZnO by the atomic layer deposition (ALD) technique and annealing the sample at 800°C to facilitate a solid state reaction for formation of the spinel phase. Such particles represent robust and inert structures most suitable as host materials for catalysis and other processes where large surface area is desired. These samples were used as host materials for a red-emitting phosphor by depositing a thin layer of europium on the surface of the particles, also by the ALD technique. The europium coating was limited to the particle surface due to reaction kinetics, however a strong luminescence at 620 nm was observed. The synthesis and characterization of all these structures will be described.

Introduction

There is presently a real challenge to build nanostructured materials on a large variety of supports and atomic layer deposition (ALD) stands out as the most promising method for coating nanomaterials and more specifically nanoporous materials (1). In fact, ALD appears to be the most appropriate method to coat extremely complex shapes with high conformality and high aspect ratio features (2). ALD is a form of chemical vapour deposition (CVD) based on successive complementary surface-controlled reactions using two or more vapour-phase precursors to produce high quality thin films or coatings at the nanometric scale with perfect conformality along with a high controllability (3).

ALD offers accurate control of the thickness of the deposited film at the atomic level simply by counting the number of deposition cycles. We report on controlled deposition of ZnO by the ALD technique on porous γ-alumina with the aim for formation of the more robust ZnAl2O4 spinel. There is currently an increased interest in nanoporous materials of complex oxides such as the ZnO-based spinel type compounds (4). ZnAl2O4, belongs to the family of metal aluminates with spinel structure and is widely used as ceramic, electronic and catalytic materials (5), such as support in Fischer-Tropsch catalysis (6). ZnAl2O4 is a semiconductor with an optical band gap of 3.8eV, which makes it transparent to visible light and therefore useful for UV photoelectronic devices (7). ZnAl2O4 doped with rare earth metal ions has been mostly studied for its unique luminescent properties resulting from its stability and high emission quantum yields (8).

ECS Transactions, 41 (2) 123-130 (2011)10.1149/1.3633660 © The Electrochemical Society

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We have developed a new route towards synthesis of nanoporous spinel type materials using nanoporous γ-alumina particles (γ-ANPs) (Fig1a) as a support and reacting it with thin oxide coatings deposited by ALD. The challenge lies in the capability to coat the interior of the particles while maintaining the conformity and uniformity characteristics offered by the ALD technique. The current γ-alumina particles consist typically of pores which are 16 nm wide. When this is combined with the size of the particles in the micrometer range, rather long pulse times may be necessary to completely saturate the interior (9). A recent report from H.-Y. Lee et al. showed that pulse times of up to 90 min may be necessary for coating of aspect ratios higher than 105 for nanometeric pores due to Knudsen diffusion (10).

Figure 1: (a) SEM image of γ-alumina particles of diameter ranging from 20μm to 60μm, (b) Picture of the powder cell.

Experimental

The films were grown in a commercial F-120 Sat reactor (ASM Microchemistry Ltd.) using water and ZnEt2 as precursors for deposition of thin films of ZnO, and ozone and Eu(thd)3 as precursors for deposition of thin films of europium oxide. The ozone was generated by feeding O2 (99.999% AGA) into an OT-020 ozone generator providing an ozone concentration of about 15 vol% according to the specifications. The reactor pressure was kept at ca. 3 mbar by employing a N2 carrier flow (99.9995%, Schmidlin***) of 300 cm3min-1. The thin films of zinc oxide were deposited 175ºC by using a homemade powder cell (Fig. 1b) rather than the typical reaction chamber of the F-120 Sat configuration. The powder cell ensures passage of reactants around all particles and some movement due to a semi fluid bed operation. The thin films of europium oxide were deposited at 200°C using a conventional reaction chamber.

The films were characterized by X-ray diffraction (XRD), X-ray Reflectometry (XRR), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and photoluminescence (PL).

The films were analysed by XRD using a Philips X’Pert MPD diffractometer using Cu Kα radiation. The XRR measurements were performed using a Siemens D5000

ECS Transactions, 41 (2) 123-130 (2011)



diffractometer equipped with a Göbel mirror producing parallel Cu Kα. An environmental scanning electron microscope (SEM) Quanta 200F equipped with a field emission gun was used to study the morphology and perform elemental mapping using an Energy Dispersive x-ray Spectroscopy (EDS) system. High resolution transmission electron microscopy (HRTEM) studies were carried out on a JEOL 2010F operating at 200 kV and disposing which has a point to point resolution of 2Å. TEM samples were prepared by ultramicrotomy. Selected materials were annealed at 800ºC using a tubular furnace.

Optical absorption properties were derived from the diffuse reflectance measurements

performed at room temperature using ThermoScientific EVO-600 UV-VIS spectrophotometer. PL was investigated at a room temperature by employing 325 nm wavelength of cw He-Cd laser with an output power of 10mW as an excitation source. The emission was collected by a microscope and directed to fiber optic spectrometer (Ocean Optics USB4000, spectral resolution 2nm).

Results and discussion Structural characterizations

Formation of the spinel structure was achieved by first depositing a homogeneous and conformal thin film of ZnO on the surface of the nanoporous γ-alumina sample. A total of 25 cycles of ZnEt2 + H2O were used using a pulsing scheme of 1.4s/120s/2.5s/120s, providing a typical thickness of ca. 5 nm. The γ-ANPs were applied in sizes between 20 to ca. 100μm.

Analysis by SEM demonstrates that the particles are homogeneously coated on their

surface (Not shown) (11) and Zn was detected by EDS on the whole surface. SEM analysis of cross section of coated γ-ANPs, prepared by embedding particles in resin followed by cutting and polishing, showed that the particles are coated till a depth of ca. 14μm (Fig. 2b). This allows for complete coverage of the interior of particles of 28 μm, and partly coverage of a 65μm particle (Fig. 2b). A particle was analyzed by TEM with the aim to characterize conformal coating of the nanoporous structure. Figure 3 shows a high magnification TEM micrograph of the nanostructure of the γ-ANPs near the surface at the nanometric scale. The HRTEM picture corresponds to a coated area, showing that the nanostructure is well preserved. Contrast around the particles is due to the embedding resin for sample preparation. All the surface of the nanostructure is coated without sealing the pores of the γ-ANPs. The detailed study of the coating of the nanostructure is presented in a forthcoming paper (11).




Figure 2: SEM image of (a) a cross section embedded in resin of γ-alumina particles coated with ZnO, (b) Zn mapping of (a) (inset: Al mapping of the same area), (c) γ-alumina coated with ZnO (cross section embedded in resin), (d) Zn mapping of (c).

Figure 3: High resolution TEM micrographs of a ZnO coated area of a γ-alumina particle.

In order to form the ZnAl2O4 spinel phase, the ZnO-coated γ-ANPs were annealed

under air at 800ºC for 3 hours. The goal was to exploit the Kirkendall (12) effect to maintain an open porous structure of the γ-ANP. This effect is possible in the γ-Al2O3 / ZnO system due to the high concentration of vacancies in γ-alumina and a close resemblance between the tetragonal deformed spinel phase of the γ-alumina structure and the desired spinel (13). In fact, the Kirkendall effect is vacancy mediated mechanism and can only occur in matrix that harbors vacancies (14). This allows for formation of the




ZnAl2O4 spinel without deteriorating the porosity of the nanoporous structure (11). XRD measurements performed on the annealed films illustrated the ZnAl2O4 spinel structure formation occurring above 800ºC (Fig. 4). The diffraction reflections of the ZnAl2O4 spinel phase are clearly visible without any traces of secondary phases such as ZnO.

Figure 4: XRD diffractograms of (a) uncoated γ-alumina and (b) γ-alumina coated with ZnO and annealed under air at 800ºC for 3 hours.

Analysis of the surface area by BET proved a limited reduction in specific surface area subsequent to the annealing process. Uncoated γ-ANPs was measured to have a specific surface area of about 178m2.g-1 which decreases to 147m2.g-1 after annealing at 800ºC. The as-coated γ-ANPs had a specific surface area of 150cm2.g-1 which reduces to only 140cm2.g-1 after annealing at 800ºC, which is comparable to the surface area of the uncoated γ-ANPs after annealing. The annealing process did not result in diffusion of zinc in the depth direction of the particle. Figure 2d shows the distribution of Zn on a cross section ZnO-coated γ-ANPs after annealing, where the coating depth appears unaffected by the treatment. Europium deposition:

The converted porous particles of the spinel phase were tested as support for the red-emitting phosphors europium by coating with a very thin film of europium oxide by ALD at 200 °C using Eu(thd)3 and ozone as precursors. A total of 25 cycles were used with a pulsing scheme of 6s/120s/6s/120s in a conventional reaction chamber which should provide a typical film thickness of ca. 0.5 nm. The particles were spread on a glass substrate to a thickness of ca. 0.1 mm using pure ethanol as a dispersant before coating with europium oxide. Only the outer surface of the particles was coated, probably due to degradation of ozone as it enters the pores of the structure. EDS analysis affirms the presence of europium and a low content of carbon (Figure 5). Analysis of the cross section of spinel particles coated with europium (Fig. 6) shows that the depth profile of the europium coating into the particle is much shallower than for the zinc oxide coating. We attribute this to the degradation of ozone as it enters into the nanoporous structure

20 30 40 50 60 70

(a)

(440

)

(511

)

(331

) (422

)

(400

)

(311

) alumina ZnAl2O4

In

tens

ity (a

.u.)

2θ (degrees)

(220

)(b)




due to the limited stability of ozone. However, some differences should also be expected due to the different precursor chambers, although not to this extent. A limitation by using the conventional reaction chamber is that there may also be inhomogeneities in coating depth between different particles since they are stagnant during the deposition. Such effects may be observed in Fig. 6. More over an inhonomogeneous coating of the surface was observed due to the fact that in that configuration particle are stacked.

Figure 5: EDS analysis performed on the surface of a γ-alumina particle coated with zinc oxide and europium oxide (inset: SEM image of the γ-alumina particle analyzed).

Figure 6: (a) SEM image of a cross section embedded in resin of annealed γ-alumina coated with zinc oxide and Europium oxide. (b) Eu mapping of (a) (inset: Al mapping of the same area). Optical characterization

The spinel particles coated with europium oxide were annealed at 1000ºC in air for 3 hours to promote diffusion of europium into the spinel matrix. After such treatment a

Eu50μm

C

Zn Si Al O




strong red luminescence could be observed from the particles at room temperature. It is then possible to visually differentiate the Eu-coated spinel nanoporous particles (as deposited ones with a very weak red luminescence and annealed ones with a strong red luminescence) from the uncoated γ-ANPs under UV illumination from the characteristic red luminescence of the former (Inset fig. 7). Figure 7 shows PL spectra of the ZnAl2O4:Eu nanoporous particles annealed at 1000ºC measured at 300K (grey line). One can observe a characteristic set of sharp PL peaks in the spectral range from 550 to 730nm associated with the intra-shell transitions in Eu3+ ions. These spectral features are surrounded and partially superimposed with two broad emission bands originating from the nanoporous ZnAl2O4 support (dashed line in figure 7). Several most prominent emissions from the embedded Eu3+ ions correspond to 5D0→7FJ transitions with J = 0-5, among which the intensity of the 5D0→7F2 transition is found to be the strongest. Taking into account that the latter electric dipolar emission (5D0→7F2) is very sensitive to ligand field (15), the stronger intensity is most likely a result of increased covalency and/or a decrease of the symmetry in the matrix. It should be noted at this point that the above PL features are solely attributed to europium oxide coating of the particles. The fact carbon was not detected on the surface in our experiments rules out the alternative possibility of indirect excitation of europium, commonly termed as sensitization or “antenna” effect (16). Finally, our results demonstrate the possibility of inducing intense red emission in the Eu-doped spinel matrix. Such strongly luminescent and robust particles have several potential applications, e.g. as a promising tracer, can be used as a component in optoelectronics or as catalyst support.

Figure 7: PL spectrum obtained at 300K performed on from ZnAl2O4:Eu annealed at 1000ºC (gray curve) along with deconvolution components (black curves), black arrows indicate characteristic Eu transitions (Dashed line simulates the ZnAl2O4 support contribution). Inset shows (a) γ-alumina particles and (b) annealed γ-alumina particles coated with ZnO and europium oxide and then annealed under visible-light and under UV-light illumination (upper and lower rows, respectively).




Conclusions

We demonstrate the possibility to use ALD to convert γ-alumina particles to nanoporous particles with spinel structure by exploiting the Kirkendall effect to induce diffusion of Zn into the γ-alumina. This approach allows the formation of a nanoporous spinel structure without deteriorating the porosity of the structure. We also demonstrate europium coating of the spinel particles previously synthesized to form a highly luminescent nanoporous spinel structure. The characteristic red luminescence from Eu3+ ions was observed in europium-doped ZnAl2O4 nanoporous particles. An intense emission from the electric-dipole (5D0→7F2) transition is a necessary feature for display applications. The luminescent properties of Eu3+ ions coupled with the intrinsic photochemical proprieties of ZnAl2O4 make these compounds plausible promising candidates for optoelectronic devices.

Acknowledgments

Authors thank Dr. Maria Rosarìo from University of Aveiro, CICECO for XRD

measurements and Dr. Protima Rauwel for careful reading of the manuscript. Financial support from the Norwegian Research Council (Project 176740) and Statoil through the inGAP project (Innovative Natural Gas Processes and Products) and Marie Curie (PERG05-GA-2009-249243) is acknowledged.

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ECS Transactions [ECS 220th ECS Meeting - Boston, MA (October 9 - October 14, 2011)] - ALD Applied to Conformal Coating of Nanoporous γ-Alumina: Spinel Formation and Luminescence