Nanoparticles as a Support: CaO Deposits on MgO Cubes - The

May 31, 2008 - MgO nanocubes with an average particle size of 8 nm were used to support thermally stable CaO deposits. Energy-filtered transmission el...
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2008, 112, 9120–9123 Published on Web 05/31/2008

Nanoparticles as a Support: CaO Deposits on MgO Cubes Markus Mu¨ller,† Andreas Sternig,† Slavica Stankic,†,‡ Michael Sto¨ger-Pollach,§ Johannes Bernardi,§ Erich Kno¨zinger,† and Oliver Diwald*,† Institute of Materials Chemistry, Vienna UniVersity of Technology, Veterina¨rplatz 1/GA, A-1210 Vienna, Austria, Institut des Nanosciences de Paris, Campus de Boucicaut, 140 Rue de Lourmel, 75015 Paris, France, and UniVersity SerVice Centre for Transmission Electron Microscopy, Vienna UniVersity of Technology, Wiedner Hauptstrasse 8-10/ 052, 1040 Vienna, Austria ReceiVed: April 2, 2008; ReVised Manuscript ReceiVed: April 30, 2008

MgO nanocubes with an average particle size of 8 nm were used to support thermally stable CaO deposits. Energy-filtered transmission electron microscopy (EFTEM) reveals their unprecedented high dispersion with sizes significantly below 4 nm. CaO-specific photoluminescence emission results from the photoexcitation of oxygen anions in edges and oxygen-terminated corners that, for the first time, are available at a sufficiently high concentration to be detected by ensemble averaging techniques. The presented approach can be easily extended to a variety of other thermally labile oxides that find important applications in optics, sensing, and catalysis and, on this base, can be incisively characterized. Supported nanostructures with specific chemical and/or optical properties are an important research topic in surface chemistry, heterogeneous catalysis, and photonics.1–3 Moreover, in photovoltaics and photocatalysis, nanometer-sized deposits of metals and metal oxides have shown potential due to their beneficial influence on interfacial charge recombination dynamics.4 Highsurface-area solids of sufficient stability against annealinginduced coarsening and coalescence are usually employed as substrates. Ultimately, when thermally stable nanoparticles are used, their decoration with clusters, islands, or thin overlayers can lead to the maximum enrichment and dispersion of a supported material which otherwise undergoes substantial sintering during high-temperature exposure. Aside from the generation or enhancement of unexpected chemical or optical properties,5 such an approach is critical to surface studies on powders6 that need to contain a sufficient number of particularly active surface elements to be probed with ensemble averaging spectroscopic techniques.9,12,13 Chemical vapor deposition (CVD) is exceptionally useful for the production of small and isolated MgO crystals of uniform properties.7 Different from conventional flame combustion methods,8–10 significantly smaller particles in the range between 1 and 20 nm can be generated. In addition to its high dispersion, the structural complexity of the nanoparticle surface was found to decrease during vacuum annealing to temperatures above T ) 870 K, leading to well-facetted MgO nanocubes with predominant (100) surface planes. Distinct from MgO, more basic CaO, SrO, and BaO nanoparticles agglomerate under corresponding conditions and form larger and morphologically less-defined grains.11 The size of the cubically shaped MgO * To whom correspondence should be addressed. E-mail: odiwald@ mail.zserv.tuwien.ac.at. † Institute of Materials Chemistry, Vienna University of Technology. ‡ Institut des Nanosciences de Paris. § University Service Centre for Transmission Electron Microscopy, Vienna University of Technology.

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particles critically affects the ratio between specific surface elements such as corners and edges that can be probed via their characteristic absorption properties in the range of ultraviolet light.12 In the present paper, we show that MgO nanocubes with an average edge length of about 8 nm can effectively be employed to support another, thermally more labile oxide component such as CaO at an even higher dispersion. We will show below that such deposits represent an unprecedented type of material since the size of thermally stable CaO grains has only been reported to be around 30 nm.13 For the production of thermally stable CaO deposits, the following three-step process was applied (Figure 1): in step 1, CVD-grown and irregularly shaped MgO particles of perfect crystallinity were subjected to vacuum annealing to transform them into cubes. A maximum temperature of 1170 K was effective for surface cleaning via complete adsorbate removal.14 For Ca evaporation, that is, step 2, Ca grains previously kept in the finger of the quartz glass cell were transferred into the furnace zone without breaking the vacuum. As a result, the white powder turned blue, a change that is indicative of electron transfer from Ca atoms to the MgO surface upon surface color center formation.15,16 A very recent study on Ca adsorption on MgO(100) that includes microcalorimetric measurements and DFT calculations17 provides important information concerning the here-employed deposition process; apart from strong binding of Ca atoms to intrinsic defects such as steps and kink sites where Ca donates electron density to the MgO surface,18 it was found that Ca adatoms bind only transiently to (100) terraces, diffuse rapidly to stronger binding sites, and finally agglomerate to small 3D Ca clusters.17 We expect that such elements transform into oxides when step 3, that is, oxygen addition at T g 298K, is applied (Figure 1). Technically, the metal oxide coverage θCaO is adjustable in the range of 0 e θCaO e 1 monolayer equivalents19 via (a) the number of iterations applied (steps 2 and 3) and/or (b) the metal evaporation time prior to oxidation (step 2). After deposition and subsequent vacuum  2008 American Chemical Society

Letters

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Figure 1. Schematic of the three-step process employed for deposition of thin metal oxide crusts on MgO nanocube surfaces. Step 2 and 3 can be iterated to increase the CaO coverage in a controlled way.

Figure 2. Electron microscopy images (a and b) and a photoluminescence emission spectrum of CaO-covered MgO nanocubes (c). (a) TEM image and (b) the energy-filtered TEM where Ca is indicated in pink color. The elemental map represents an overlay over the corresponding TEM micrograph. The photoemission spectrum, acquired at T ) 298 K and p < 10-5 mbar, was obtained on the same powder sample and is compared to that of pure CaO grains using excitation light of λEXC ) 320 nm.12

annealing to T ) 1170 K, we checked for the abundance of adsorbates on the surface of CaO-coated MgO nanocubes using FT-IR spectroscopy and found that the spectral range between 1000 and 4000 cm-1 was exempt from any measurable absorption. A typical transmission electron microscopy image of MgO particles with a Ca concentration of 10 atom % is shown in Figure 2a and demonstrates that the deposition process has affected neither the shape nor the dispersion of the MgO nanocubes. Furthermore, the energy-filtered TEM image in Figure 2b, where the Ca L2,3 edge at 345 eV was selected for Ca detection, reveals thin CaO deposits (in pink) that are thinner than 4 nm and match the shape of the nanocubes having an average size of 8 ( 2 nm. Corresponding X-ray diffraction patterns show Bragg reflections consistent with those of crystalline MgO and CaO. Low-coordinated ions on alkaline earth oxides absorb and emit light at λ > 200 nm,19–22 and their enrichment and manipulation hold the key to produce nanocrystals with controlled optical and chemical characteristics.23,24 Already, the addition of 10 atom % CaO, which under complete wetting conditions corresponds to a MgO nanocube surface coverage of 0.6 ML, leads to significantly altered optical properties, as highlighted by the photoluminescence emission spectrum in Figure 2c. Photoexcitation at λ ) 320 nm produces an emission feature that is significantly red shifted with respect to that of pure and unsupported CaO particles (λEM ) 410 nm, hν ) 3.0 eV).12 It seemingly suggests the existence of surface excitation processes present neither on MgO (no PL emission) nor on CaO particles (λEM ) 410 nm; Figure 2c). We explored the PL properties of CaO-covered MgO cubes in comparison to those of pure CaO particles12 in more detail. As demonstrated by an earlier study on larger (d ) 30 nm) and

Figure 3. Photoluminescence properties of CaO-covered MgO nanocubes. The emission spectra plotted in (a) to (c) were measured on dehydroxylated nanoparticle powders at room temperature and pressures p < 10-5 mbar. The superimposition of two bands at λEM ) 410 and 460 nm perfectly reproduces each emission spectrum (a-c). The associated excitation curves in (d) are explained in terms of two separate PL processes involving oxygen anions in CaO edges (λEXC ) 290 f 410 nm) and oxygen-terminated CaO corners (λEXC ) 305f 460 nm) as excitation sites (e).

irregularly shaped CaO grains, monochromatic excitation in the range between λEXC ) 230 and 350 nm produces only one type of photoemission curve at λEM ) 410 nm (hν ) 3.0 eV; Figure 2c).12 Induced by photoexcitation of four-coordinated anions in step edges and mediated by subsequent energy transfer along these structures, this emission process has been attributed to the radiative exciton deactivation at oxygen-terminated kink sites.12 The situation is more complex for CaO-covered MgO nanocubes, where the entire set of spectra can be satisfactory reproduced by two Gaussian profiles with maxima at λEM ) 410 and 460 nm (Figure 3a-c). The excitation spectrum related to the band at λEM ) 410 nm (Figure 3d, top) is identical to that obtained on CaO nanoparticle powders12 and attributed to surface excitons generated at four-fold-coordinated oxygen

9122 J. Phys. Chem. C, Vol. 112, No. 25, 2008 anions in CaO edges. In addition, CaO deposits on MgO nanocubes produce a band that peaks at λEM ) 460 nm (hν ) 2.7 eV; gray shaded curve in Figure 3a-c) and, as derived from a band-fitting analysis, relates to a red-shifted excitation spectrum, the threshold of which reaches wavelengths as low as λEXC ) 350 nm (Figure 3d). As for the emission band at λEM ) 410 nm, its asymmetric shape suggests that more than one electronic transition must contribute to PL emission.23 We also looked for possible modifications of the MgO-specific emissions at λEM ) 370 nm (hν ) 3.4 eV) and λEM ) 382 nm (hν ) 3.2 eV), which are induced by photoexcitation of fourfold-coordinated anions in MgO edges and oxygen-terminated MgO corner sites using λEXC ) 240 nm (hν ) 5.1 eV) and λEXC ) 270 nm (hν ) 4.6 eV), respectively.7 As a result, we did not observe any significant depletion of both features but measured additional CaO-related contributions using excitation light of λEXC ) 240 nm (hν ) 5.1 eV) (Figure S1, Supporting Information). A red-shifted emission feature observed on CaOcovered MgO particles and induced at λEXC ) 270 nm is by a factor of six more intense than the MgO-specific emission band measured on undoped MgO nanocubes. This is in line with the recently observed substantial increase in PL emission intensity going from MgO to higher alkaline earth oxides.12,25 To explain the red shift in excitation, we can think of two possible scenarios: mixed “Mg-O-Ca” surface structures correspond to a situation where Ca2+ substitutes specific surface Mg2+ ions, which would certainly affect the surface-elementspecific HOMO and LUMO levels.23 This relates to a previous study where we have shown that the controlled cocombustion of Ca and Mg vapor in conjunction with annealing-induced Ca2+ ion segregation into the nanocrystal surface gives rise to intense PL emission features in the same energy range.5 With regard to the present approach (i.e., Ca addition to morphologically well-developed MgO nanocubes; Figure 1), the incorporation of larger Ca2+ ions into the first MgO layers that constitute the interface between the MgO nanoparticle and the CaO deposit cannot be excluded.26 An alternative explanation relies on the TEM evidence of CaO deposits at a so far unprecedented high dispersion. EFTEM has revealed their inhomogeneous distribution (Figure 2b) as opposed to smooth epitaxial films.27 This is consistent with the recent report on Ca growth on MgO(100) surfaces, demonstrating that Ca atoms form three-dimensional caps17 that, after exposure to O2, would transform into similarly shaped CaO deposits. It is important to note that the emission process related to the 410 nm band (Figure 3d, top) is also observed on pure and 30 nm sized CaO particles and is related to the photoexcitation of anions in edges (4C).12 Obviously, CaO-covered MgO nanocubes host such structures. Apart from them, at such a high dispersion, we also expect a significant abundance of CaOspecific and less coordinated sites such as oxygen-terminated corners with excitation energies below that of oxygen anions in extended edges (Figure 3e). It is known from the literature that high-surface-area CaO can be alternatively produced via the thermal decomposition of calcium carbonate.13 For related samples of unspecified morphology and adsorbate coverage, a shoulder at λ ) 330 nm was observed in the corresponding UV diffuse reflectance spectra.20,28 An independent PL study on the same type of material revealed an emission at λEM ) 410 nm that is related to an excitation band at λ ) 280 nm as well as a shoulder at 310 nm, an observation which is consistent with our results.12 We therefore suggest that the emission band at λEM ) 460 nm results from the photoexcitation of oxygenterminated CaO corners at wavelengths significantly above λ

Letters ) 290 nm (Figure 3d). Furthermore, recent ab initio calculations23,29 demonstrate clearly that an experimentally observed single excitation band should not be regarded as merely the signature of one single ion but ought to be assigned to a multiatomic topological feature.23 It would be therefore an oversimplification to exclusively attribute the excitation spectrum in the bottom of Figure 3d to three-coordinated oxygen anions in CaO corners. The excitation maximum at λ ) 305 nm in conjunction with the low-energy threshold that extends to 350 nm, however, strongly suggests that the underlying surface element is determined by Ca2+ ions in corner close edge positions and can be regarded as an oxygen-terminated CaO corner. For MgO, it has been demonstrated that only the smallest structures with particle sizes in the range of 1 e d e 15 nm exhibit a sufficient surface concentration of regular oxygenterminated corners in order to be detected with ensembleaveraging spectroscopic techniques.7 On the basis of these results, we suggest that a powder made of CaO nanocubes with particle diameters below 15 nm would also show an emission band at λEXC ) 460 nm with an excitation maximum at λEXC ) 305 nm (Figure 3a-d). Since the thermal stability of CaO-coated MgO nanocubes is sufficient to allow for dehydroxylation of the entire particle surface, the isolation of nanometer-sized CaO, SrO, and BaO deposits on MgO nanocubes enables the enhancement of certain topological features of interest to a level above the detection limit of various spectroscopies. In this context, the approach presented here opens the opportunity for in-depth electronic surface structure investigations of thermally labile oxide nanostructures. In summary, we report on the generation of nanometer-sized and adsorbate-free CaO deposits with thicknesses of less than 4 nm on top of MgO nanocubes which are used as a high-surface-area substrate. Aside from CaO-specific spectroscopic fingerprints, we also observed a so far unreported and significantly red-shifted photoluminescence process that is assigned to the photoexcitation of oxygen-terminated CaO corners. Its very first report in conjunction with CaO-covered MgO nanocubes is attributed to their sufficiently effective enrichment within the sampled particle powder, making them detectable by PL spectroscopy. We believe that the availability of thermally labile metal oxides in a highly dispersed form opens the way for substantially new insights into their spectroscopic properties and reactivity. The synthesis approach can easily be generalized to a variety of other materials that may find important applications for optics, sensing, and catalysis and, thus, represents a significant advance in the endeavor to investigate and control the surface properties of inorganic nanomaterials.30 Acknowledgment. We would like to thank P.V. Sushko and M. Sterrer for their comments on the manuscript. This work was financially supported by the Austrian Fonds zur Fo¨rderung der Wissenschaftlichen Forschung (FWF Project P19848-N20). Supporting Information Available: Descriptions of experimental details, spectroscopic and TEM measurements, bandfitting analysis, as well as photoluminescence emission spectra showing the effect of CaO deposition on the MgO-specific PL emission properties. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Lambert, R. M., Pacchioni G., Eds. Chemisorption and ReactiVity of Supported Clusters and Thin Films; NATO ASI Series E,

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