11090
2008, 112, 11090–11092 Published on Web 07/04/2008
Thermal Stability of Tungsten Oxide Clusters Andriy Romanyuk,*,† Roland Steiner,† Peter Oelhafen,† Johannes Biskupek,‡ Ute Kaiser,‡ Daniel Mathys,§ and Vladislav Spassov| Department of Physics, Microscopy Center, UniVersity of Basel, 4056 Basel, Switzerland, Electron Microscopy Group of Materials Science, UniVersity of Ulm, 89069 Ulm, Germany, and Swiss Center for Electronics and Microtechnology, 2007 Neuchatel, Switzerland ReceiVed: May 1, 2008; ReVised Manuscript ReceiVed: June 19, 2008
Thermal reduction of tungsten oxide nanoclusters supported on thin silicon and aluminum oxide films and ranging in size from 2 to 8 nm was studied by in situ X-ray photoelectron spectroscopy. We observed that with a decrease in size oxide clusters demonstrate enhanced resistance to the oxide reduction when heated in vacuum. The effect is presumed to be related to the point defects that serve as nucleation sites in the oxide reduction process. It is suggested that due to the increased Laplace pressure small clusters contain lower defect density thus making them less susceptible to the thermal reduction. Preamorphization of the oxide particles by low-energy ion bombardment demonstrated a lowering of oxide decomposition temperature thus supporting the inference on the defect stimulated oxide reduction. These results provide a novel model system to understand the reduction behavior of bulk tungsten oxides and might be of crucial relevance for applications where the use of clusters is of special importance. Thin films of tungsten oxide exhibit different types of chromogenism (i.e., induced change of color), thus attracting considerable attention for potential applications ranging from architecture to optoelectronic devices.1,2 The coloration can be achieved in various ways, e.g., charge insertion, exposure to different types of irradiation, or temperature treatment resulting in the oxide reduction and appearance of lower oxidation states (W5+ and W4+). The coloration is then explained by intervalence charge transfer3 or small polaron transition between two nonequivalent sites of tungsten.4 While tungsten oxide bulk crystals are built from a cornersharing WO6 octahedra with a tungsten atom surrounded by six oxygen atoms, the local microstructure of thin tungsten oxide films does, however, rarely exhibit long-range periodicity. Previous studies of tungsten oxide films by high-resolution transmission electron microscopy revealed small clusters of different size, depending on the type of substrates and substrate temperature during film deposition.5–8 The clusters are arranged in space so that they constitute a continuous “film” whose properties are governed by the combination of the properties of individual clusters and their interface with the surrounding medium. Whereas the properties of thin film are close to that of the bulk oxide, electronic properties, chemical reactivity, or decomposition temperatures of individual oxide clusters may be different to those of bulk and significantly vary as a function of cluster size, geometry, and surrounding media. Although recently published first principle calculations and investigations of band structure of tungsten oxide clusters with photoelectron * Corresponding author. Tel: +41 61 267 37 20. Fax: +41 61 237 37 84. E-mail:
[email protected]. † Department of Physics, University of Basel. ‡ University of Ulm. § Microscopy Center, University of Basel. | Swiss Center for Electronics and Microtechnology.
10.1021/jp803844d CCC: $40.75
spectroscopy reveal that clusters containing only two tungsten and six oxygen atoms possess bulklike properties,9 to the best of our knowledge, no results on the chromogenic properties of single oxide clusters have been reported to date. In the present work, we investigate the size dependence of thermal reduction behavior of tungsten oxide nanoclusters. Stepwise heating of oxide clusters in vacuum allowed us to systematically change the tungsten oxidation state that was simultaneously monitored by in situ photoelectron spectroscopy. We found that isolated oxide clusters exhibited enhanced stability against thermal reduction demonstrating a monotonous increase in decomposition temperature with a decrease in cluster size. The present experiments improve the fundamental knowledge of tungsten oxides and provide more insight into the properties of nanoscaled oxide structures. Tungsten oxide clusters of different size were prepared by micellar approach using self-organization of salt-loaded micelles. The method allows the preparation of 2D nanoparticle arrays with definite particle size and large interparticle distance over macroscopic areas.10,11 The cluster size is controlled by the amount of precursor salt added to the solution while the distance between clusters is determined by the length of the block copolymer. In our experiments, diblock copolymer poly(styrene)block-poly2(2-vinyl-pyridine) of various block lengths such as PS(266)-b-P2VP(41), PS(227)-b-P2VP(99), and PS(50)-bP2P(16.5) obtained from Polymer Source Inc. was solved in toluene (99.9% p.a., Merck) in proportion of 5 mg/ml solvent thereby forming reverse micelles. The micelles’ core was then loaded with 0.05-0.5 equiv of WCl6 (purity 99.99%, Aldrich) per pyridine unit. In the subsequent step, a monomicellar layer was deposited onto a silicon wafer covered with 2 nm silicon native oxide film by a dip coating at a constant velocity of 15 mm/min. In order to remove the polymer matrix and form 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11091
Figure 1. SEM image of tungsten oxide particles with size of 8 nm prepared by micellar method.
isolated oxide clusters, the samples were exposed to the low temperature oxygen plasma (exposure for 30 min at 30 W and pressure of 2 Pa) at 250 °C. As an example, the scanning electron microscopy (SEM) image obtained with a Hitachi S-4800 of the investigated oxide particle with size of 8 ( 1.3 nm is presented in Figure 1. As seen from the image, the particles are mostly round in shape and well separated from each other. From noncontact atomic force microscopy (AFM) and transmission electron microscopy (TEM) images, Gaussian size distributions could be extracted with averages of 1.9 ( 0.5, 3.7 ( 0.9, 4.8 ( 1.0, 5.9 ( 1.1, and 8 ( 1.3 nm for the different micellar solutions used in the present study. The synthesized nanoparticles were structurally characterized on the atomic scale by a FEI Titan 80-300 CScorrected transmission electron microscope operating at 300 kV. Figure 2 demonstrates the cross-sectional high-resolution (HR) bright field TEM images of the smallest investigated particles together with corresponding local fast Fourier transform (FFT) shown as inset to the Figure 2b. Analysis of the electron diffraction rings confirmed that all investigated particles are single crystal tungsten trioxide with monocline structure. By analyzing HRTEM images and local FFTs of the nanoparticles, we can state that most of the nanoparticles (>80%) appear as single crystals whereas twinnings occur as a particle size increases. In order to avoid possible contamination from the atmosphere, after plasma exposure the specimens were introduced into the analysis chamber of the electron spectrometer (VG ESCALAB 210 equipped with monochromatized Al-KR radiation source emitting at 1486.6 eV) without breaking the vacuum and analyzed at different temperatures under UHV conditions. The oxygen plasma treatment results in completely vanished C-1s signal (not presented here) confirming an effective etching of the polymer matrix. Further, the exposure of metallic particles to oxygen plasma at given temperature leads to full oxidation of tungsten12 thus forming tungsten oxide particles. The evolution of W-4f line peak intensity with the substrate temperature measured on 8 and 2 nm oxide clusters is illustrated by means
Figure 2. High-resolution TEM images of the tungsten oxide particles. The inset in panel b shows the corresponding local FFT of the marked nanoparticle.
Figure 3. The intensity of W-4f core-level line recorded on the 8 nm (a) and on 2 nm tungsten oxide clusters (b) as a function of the substrate temperature. The dots represent the measured spectra and the red lines are the fit functions. The dashed horizontal lines indicate the position of the different oxidation states.
of the 2D diagram in Figure 3a,b, respectively. The spectrum on the left part of Figure 3a corresponds to the first temperature “slice” of the 2D representation. As seen, at 280 °C the tungsten still remains in W6+ oxidation state and the W-4f XPS line consists only of one doublet assigned to tungsten in W6+ oxidation state (binding energies 38.3 eV for the W-4 f5/2 and 36.1 eV for the W-4 f7/2 doublet component). Gradual increase in substrate temperature results in a continuous line broadening predominantly toward the lower binding energy side, indicating the appearance of lower oxidation states. A fit procedure applied to the spectrum acquired at 520 °C (right-hand side of Figure 3a) allowed to resolve the line into two doublets corresponding to W6+ and W5+ oxidation states with the W5+ state shifted toward lower binding energy on 1.2 eV with respect to W6+ (the position of the states are indicated with dashed horizontal lines). An increase in the full width at half-maximum (fwhm) of the separate components is associated with the decrease in lifetime of the photohole due to the change in the sample conductivity. An estimation of the peak integrated area of the high and low binding energy components yields a W5+/W6+ ratio of ∼0.4. The XPS intensity measurement on 2 nm oxide clusters recorded as a function of the substrate temperature is depicted in Figure 3b. In contrast to measurements on 8 nm clusters, the W-4f line of 2 nm clusters demonstrates no essential change upon increase in the temperature and the tungsten remains in W6+ oxidation state even at temperatures as high as 480 °C. Annealing the particles at 520 °C (spectrum on the right-hand side of Figure 3b) and applying the fit procedure results in the amount of W5+ oxidation states of only about 8%. In order to perform a quantitative analysis of the observed effect we defined a transition temperature Tc as the temperature at which the reduction of the oxide takes place with a minimal concentration of W5+ states of 25%. The development of Tc as a function of the particle size is depicted in Figure 4. As can be seen, the tendency of Tc toward higher values with a decrease in particle size is quite evident, demonstrating remarkable size-induced cluster stability. It is interesting to note that an opposite trend was expected since the majority of atoms in the cluster occupy less stable surface position and therefore with an increase in
11092 J. Phys. Chem. C, Vol. 112, No. 30, 2008
Figure 4. Transition temperature measured on tungsten oxide clusters with different size supported on SiO2 (blue open symbols) and on Al2O3 (green open symbols). Transition temperature measured on clusters irradiated with 1 keV Ar+ ion to a dose 1.5 × 1012 cm-2 is shown with red filled symbols. The points are connected with an eye-guide line. The horizontal error bars indicate the standard deviation of the cluster sizes. The vertical error bars reflect the uncertainty of temperature measurement.
surface to volume ratio the cluster tends to reduce the surface energy and becomes less stable. On the other hand, the dependence of thermodynamic properties of single clusters might stem from the structural effects. It is a matter of general experience that tungsten oxide is characterized by various defects such as dangling bonds, oxygen vacancies, or interstitials. Similarly to the melting of metals where point defects serve as nucleation sites for the melting process,13 we propose that the existence of defects inside the cluster can substantially influence the cluster stability promoting enhanced formation of W5+ state. Taking into account that Laplace pressure can be quite large in nanoclusters, it is reasonably safe to suggest that with a decrease in cluster size (i.e., increased compressive stress) the probability of point defect formation decreases. Therefore bigger clusters contain defects at higher densities making them less stable against thermal reduction. In order to prove this assumption, particles of different sizes were subjected to low-energy ion irradiation (Ar+, 1 keV) to a dose of 1.5 × 1012 cm-2 resulting in partial oxide amorphization as discussed previously by Lam.14 The transition temperature measured on the ion-irradiated particles is shown in Figure 4 with filled symbols and is constantly lower, even though the size of the particles is somewhat reduced, possibly due to physical sputtering. To a certain extent this observation supports the inference on the role of defects in cluster stability; however
Letters the error in temperature determination and variation in cluster size do not allow us to come to an unequivocal conclusion. In addition, strong cluster-surface interaction may also have an essential impact on the observed stability. In order to clarify the effect of cluster-surface interaction, the reduction behavior of WO3 clusters supported on Al2O3 films was studied. The tungsten oxide particles of different size were prepared on the top of the 5 nm thick Al2O3 films deposited on silicon substrate by conventional magnetron sputtering of aluminum target in oxygen atmosphere. The reduction of the WO3 particles supported on alumina (Figure 4, open green symbols) shows a similar tendency suggesting that the observed stability effect is rather general. In conclusion, it has been experimentally demonstrated that isolated tungsten oxide particles show a stability against thermal reduction in vacuum with a pronounced monotonous size dependence. We suggest that the stability effect is related to the presence of different types of defects inside the cluster, stimulating enhanced oxide decomposition. The lowering of the decomposition temperature observed on preamorphized particles gives evidence in favor of the proposed model. Acknowledgment. The authors would like to thank Professor Hans-Gerd Boyen (University of Hasselt, Belgium) and Professor Stefan Goedecker (University of Basel) for fruitful and helpful discussions. We also acknowledge S. Gro¨zinger (University of Ulm) for TEM sample preparation. Financial support by the Swiss National Foundation and the Swiss Federal Office of Energy is gratefully acknowledged. References and Notes (1) Bange, K. Sol. Energy Mater. Sol. Cells 1999, 58, 1. (2) Granqvist, C. G. Handbook on Inorganic Electrochromic Materials; Elsevier Science B.V: Amsterdam, 1995. (3) Faughnan, B. W.; Crandall, R. S.; Heyman, P. M. RCA ReV. 1975, 36, 177. (4) Schirmer, O. F. J. Phys. (Paris) 1980, 6, 479. (5) Shiojiri, M.; Miyano, T.; Kaito, C. Jpn. J. Appl. Phys. 1979, 18, 1937. (6) Kaito, C.; Shimizu, T.; Nakata, Y.; Saito, Y. Jpn. J. Appl. Phys. 1985, 24, 117. (7) Benson, D. K.; Tracy, C. E. Proc. Soc. Photo-Opt. Instrum. Engr. 1985, 562, 46. (8) Green, M.; Travlos, A. Philos. Mag. B 1985, 51, 501. (9) Sun, Q.; Rao, B. K.; Jena, P.; Stolcic, D.; Kim, Y. D.; Gantefor, G.; Castleman, A. W. J. Chem. Phys. 2004, 121, 9417. (10) Ka¨stle, G.; Boyen, H.-G.; Weigl, F.; Lengl, G.; Herzog, Th.; Ziemann, P.; Riethmu¨ller, S.; Meyer, O.; Hartmann, C.; Spatz, J. P.; Mo¨ller, M.; Ozawa, M.; Banhart, F.; Garnier, G.; Oelhafen, P. AdV. Funct. Mat. 2003, 13, 853. (11) Ka¨stle, G.; Boyen, H.-G.; Weigl, F.; Lengl, G.; Ziemann, P.; Riethmu¨ller, S.; Hartmann, C.; Spatz, J. P.; Mo¨ller, M.; Garnier, G.; Oelhafen, P. Phase Trans. 2003, 76, 307. (12) Romanyuk, A.; Melnik, V.; Oelhafen, P. Nucl. Instrum. Methods Phys. Res., Sect. B 2005, 232, 358. (13) Goedecker, S. Institute of Physics, University of Basel. Personal communication 2006. (14) Lam, N. Q.; Kelly, R. Can. J. Phys. 1972, 50, 1887.
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