Formation of Ruthenium Cluster on Nanocrystalline Tungsten Trioxide

Article pubs.acs.org/JPCC

Formation of Ruthenium Cluster on Nanocrystalline Tungsten Trioxide Y. Fujioka,*,† J. Frantti,† R. M. Nieminen,† and A. M. Asiri‡ †

Department of Applied Physics, Aalto University School of Science, FI-00076 Aalto, Finland Chemistry Department and Center of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia

‡

ABSTRACT: Ruthenium (Ru)-modified (up to 1 wt %) tungsten trioxide (WO3) samples were synthesized and characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy, and Raman spectroscopy. Ru was found to form hexagonal clusters on {111} planes of the underlying WO3 crystals with similar arrangement as is found in metal Ru. XPS revealed that the valence state of ruthenium corresponds to the oxidized state. Results indicate that bonding between Ru and oxygen of the crystal surfaces is a key factor for the cluster formation. No evidence for a crystalline ruthenium oxide or metal phase formation was found instead Ru atoms were bonded on oxygen on the WO3 {111} planes.

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INTRODUCTION Tungsten trioxide (WO3) is widely applied in modern technology, such as in smart windows,1 gas sensors,2 and photocatalysis.3 The crystal symmetry and consequently physical properties of WO3 are very susceptible to temperature, stoichiometry, and particle size.4 The physical properties of WO3 are frequently modified by metal or metal oxide doping. The mechanisms responsible for the change are often rather complex, including changes in host crystal symmetry or even in structure (e.g., formation of different tungsten bronzes5−7 and formation of secondary phases). Processing conditions influence the outcome so that the same starting compounds can yield very different crystal structures. For instance, Ni doping of WO3 results in perovskite tungsten bronze at mild annealing temperatures, whereas the dense wolframite structure is formed at higher annealing temperatures.7 Noble metals including platinum and palladium are the most widely used dopants. Because of the scarcity and high price, substitutional metal(s) and the knowledge of mechanism of functioning performance are necessary. Ruthenium (Ru) is a promising dopant candidate though it oxidizes easily in contrast to noble metals. Thus it is more difficult to tune the properties by controlling the process parameters. Ru is known to form a rutile type compound (RuO2), which further forms hydrates. Both Ru metal and oxide have numerous applications: RuO2 being an excellent material for electrochemical capacitor8 and photocatalytic applications and both being used as catalyst materials. For example, Ru catalyst is important for ammonia synthesis9−11 or for the hydrolysis of cellulose.12 The Ru catalyst is often supported and is conventionally prepared by impregnation of support with the reduction of chemical agents, such as RuCl3·nH2O or Ru3(CO)12. The structure of supported © 2013 American Chemical Society

Ru nanoparticles and their interaction with the support are not well-known,9 although they are crucial information for maximizing the catalytic activity of Ru. Ru doping affects physical properties and morphology of WO3. The electrical conductivity of WO3 films is increased by Ru doping, which is consistent with the fact that RuO2 conducts well.13 Co-deposited Ru-doped WO3 thin films had larger number of grain boundaries,14 and Ru was believed to be segregated as an oxide phase at WO3 grain boundaries.15 Whether Ru enters into the ReO3-type structure of WO3 or forms a separate compound(s) strongly affects materials properties, stability, and applications. The present study addresses the atomic scale structure of Ru-doped WO 3 nanopowders to provide an experimental basis for understanding the low-temperature doping induced changes in functional properties.

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EXPERIMENTAL SECTION

Synthesis of Ru-Doped Tungsten Oxides. Commercial nanopowder WO3 (particle size below 100 nm, Sigma-Aldrich 550086) was dispersed into an aqueous solution of ruthenium chloride hydrate (RuCl3·xH2O Sigma-Aldrich 206229) with Ru concentration of 0.5 and 1 wt % of WO3 and mildly heated on a hot plate of 50 °C until the mixture was dried into powder. Dried powders were ground and subsequently annealed in air at 500 °C for 1 h. Powder had a gray-green color in contrast to the greenish-yellow color of the WO3 powder. Received: October 10, 2012 Revised: March 21, 2013 Published: March 21, 2013 7506

dx.doi.org/10.1021/jp310038a | J. Phys. Chem. C 2013, 117, 7506−7510

The Journal of Physical Chemistry C

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Characterization. Samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission and scanning electron microscopy. XRD data were collected using Cu Kα radiation at room temperature at the Laboratory of Inorganic Chemistry, Aalto University. LeBail extractions were carried out using the program General Structure Analysis System (GSAS).16 The purpose of the XRD measurements was to identify the phases present in the samples and also to have estimates for the lattice parameters. Since nanoparticle samples give broad peaks, accompanied by broad background, the XRD analysis is limited to LeBail analysis. The benefit of the LeBail fit is that it gives more reliable lattice parameter estimation and also serves as a neat way of checking if all Bragg reflections are properly indexed. Raman spectra were collected under a microscope with a backscattering configuration by a single monochromator Horiba−Jobin−Yvon LabRAM HR spectrometer equipped with 600 gr/mm and 1800 gr/mm gratings. The excitation laser wavelength was 514.5 nm. The laser beam power on the sample surface was limited to few microwatts to avoid local heating and the laser beam spot was roughly 2 μm in diameter. Transmission electron microscopy (TEM) was used for the studies of spatial composition and structure of Ru containing areas. Room-temperature TEM studies were conducted by a JEOL JEM-2200FS microscope equipped with 2× Cs correctors at the Nanomicroscopy Center of the Aalto University. The acceleration voltage was 200 kV. For composition analysis, an energy dispersive spectroscopy of Xrays (EDS) was employed. EDS facilitated with a field emission scanning electron microscope (FESEM) JEOL JSM-7500FA was also applied. Electrons were accelerated by 15 kV voltage. XPS measurements were conducted using the AXIS 165 facility (Kratos Analytical located at Aalto University, School of Chemical Technology, Department of Forest Products Technology), equipped with a 12 crystal monochromator and an effective neutralization system providing high-resolution information of the topmost 10 nm of surfaces. Analytical area was roughly 1 mm2. Cellulose was used as a reference sample.

Figure 1. LeBail fit of 0.5 wt % Ru doped WO3 sample sintered at 500 °C. XRD pattern shows no evidence for other than rhenium oxide structured WO3 phase. Insets show the regions in which the strongest reflections of the (b) RuO2 (the strongest reflection is at 28.1°) and hcp Ru (two strong reflections at 42.2 and 69.4°) would appear, (c) and (d). No signs of metallic Ru or RuO2 phase was evidenced.

and several ten measurement points were chosen. In each point, either the ruthenium content was below detection limit or was between 1 and 4 mass-percent in terms of a mRu/(mRu + mW) ratio. The ratio between ruthenium and tungsten masses mRu/ (mRu + mW) was estimated to be 2% at the point marked by a cross in panel b of Figure 3. Detailed profile of EDS spectrum is given in Figure 3c. The corresponding ratio just next to the bright particle (marked by the cross in a circle) was 0.8%. The X-rays originate from a volume with a diameter of the order of one micrometer mainly in depth direction. Thus, the large difference between the compositions indicates that ruthenium is only on a surface of the particle. We emphasize that no ruthenium rich area, such as metallic or RuO2 aggregate, were found. Results agree with that Ru forms a layer on WO3 surfaces. Figure 3b shows a WO3 particle covered by a ruthenium layer. Figure 4 shows a TEM picture of 0.5 wt % Ru-doped WO3 sample. TEM-EDS measurements revealed that the spatial distribution of Ru was inhomogeneous and that the particle on the upper-left of panel a in Figure 4 contains Ru. A closer inspection, Figure 4b, showed a small area of hexagonally arranged atoms, where the nearest neighbor distance was close to 2.7 Å. Further inspection by fast Fourier transformation of the area containing the hexagonally arranged atoms, indicated by yellow square in Figure 4, shows that the underlying WO3 lattice is one of the {111} planes. The geometry and nearest neighbor atomic distances do not match with Ru oxide structures. In the case of RuO2 (space group P42/mnm) the nearest neighbor Ru−O distances were 1.941(1) and 1.985(2) Å, the O−O distances were 2.476(1) Å and the Ru−Ru distance was 3.538(1) Å.18 In contrast, metallic Ru possesses hexagonal close packed (hcp) structure (space group P63/mmc) with lattice parameters aH = 2.70389 Å and cH = 4.28168 Å.19 Thus, both the geometry and the nearest neighbor distances

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RESULTS AND DISCUSSION XRD pattern, Figure 1, revealed only monoclinically distorted WO3 phase (space group P21/n). The number of Bragg reflections resulting from the monoclinic WO3 is rather large so that weak reflections may not be revealed by XRD due to overlapping peaks. The XRD study suggests that either Ru enters the crystalline lattice or forms a phase with very small crystal size. The lattice parameters, given in Table 1, were practically identical to the values found in Ni doped WO3 annealed at 500 °C7 suggesting that the latter explanation is more probable. XPS measurements (see spectra given in Figure 2) show that the average (corresponding to an area of 1 mm2) Ru content on the surface was 0.2 atom %. According to literature, Ru peak is shifted toward higher energies with increasing oxygen content: The peak positions of metallic, RuO2, RuO3, and RuO4 being at 280, 281, 282.5, and 283.5 eV, respectively.17 The inset given in Figure 2 shows the Ru 3d5/2 peak at 281 eV, which suggests an oxidation of Ru. To address the spatial and depth distribution of Ru FESEM and TEM studies were conducted. FESEM was utilized to study Ru distribution. To see spatial variation, picture was contrived from backscattered electrons 7507



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Table 1. The Lattice Parameters, Estimated from XRD Data, of the Ru-Modified WO3 Sample. The Estimated Standard Deviations for Lattice Parameters are Given in Braces. Statistical Figures-of-Merit Numbers are χ2 = 4.564, Rwp = 12.82 (%) Rp = 9.02 (%), Rbwp = 11.87 (%), and Rbp = 8.73 (%) structure type

space group

a (Å)

b (Å)

c (Å)

β (deg)

V(Å3)

ReO3

P21/c

7.29461(31)

7.51743(27)

7.67718(29)

90.665(6)

420.963(29)

Figure 4. TEM picture of 0.5 wt % Ru-doped WO3 particle sintered at 500 °C. Local areas with hexagonally arranged atoms were found in upper-left part of (a), an example being enclosed by a rectangular box. (b) Magnified picture. The hexagonally arranged atoms (emphasized by broken sphere) were assigned to Ru with a tentative interpretation on a Fourier transformation image in (c), see text for details. The arrows give the projection of the hexagonal a and b axes of the Rucluster on the picture plane.

Figure 2. (a−c) High-resolution spectra showing signal from O, Ru, and W, respectively. The strongest peak in (b) consists of C1s and Ru3d3/2 peaks. The weaker peak at 281 eV corresponds to the Ru3d5/2 peak and suggests that Ru is bonded to one or two oxygen ions. (d) A wide range spectrum revealing that the only elements revealed are W, O, Ru, and C (surface contamination present in the sample).

indicate that Ru atoms arranged hexagonally on {111} planes of WO3 surface. We cannot exclude the possibility that Ru is grown on other planes, though the present observations indicate that (111) is the preferred plane as the shortest O− O distances on the plane match well with the hexagonal axes of metal Ru. Raman spectra shown in Figure 5 support aforementioned cluster formation. Spectra collected from the Ru-doped WO3 samples are almost identical to undoped WO3, except for two additional peaks at around 490 and 960 cm−1. The two bands were stronger in nonheat treated samples. Only the lower

Figure 5. Raman spectra of (a) nondoped WO3, Ru-doped WO3 powders dried with (b) 0.5 and (c) 1 wt % Ru and heat-treated at 500 °C with (d) 0.5 and (e) 1 wt % Ru, excited by laser beam wavelength 514.5 nm. New features, detected from Ru-doped WO3 samples at around 490 and 960 cm−1, are indicated by black spheres.

Figure 3. (a) A picture of a powder sample in which Ru-covered WO3 particle is seen as a slightly brighter area. (b) The Ru-covered particle has a diameter over 60 nm, which is consistent with the estimation based on XRD measurements. EDS measurements showed that the point marked by a cross contained a significant amount of ruthenium (c), whereas ruthenium was not detected in a point marked by a circled cross. See text for discussion. 7508



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conditions, Ru ions prefers to adsorb on WO3 surface oxygen, since RuO2 does not decompose to metal Ru + O2 at such low temperature.26

frequency peak shifted with increasing Ru concentration from 496 (0.5%) to 488 cm−1 (1%). These bands are comparable to the reported value of 470 cm−1 for Ru−O bonding emerged by oxidization process of electrochemically deposited Ru thin film.20 In the case of reduction, vibration mode of terminal Ru and absorbed atomic oxygen was reported to be 600 cm−1 by surface-enhanced Raman spectroscopy.20 Formation of crystalline RuO2, possessing three Raman modes located at 528 cm−1 (Eg symmetry), 644 cm−1 (A1g symmetry), and 716 cm−1 (B1g symmetry)21 was not evidenced. High-frequency modes can be seen when Ru-cation is in higher oxidation state: For instance, peak at around 1000 cm−1 from zirconia-supported ruthenium oxide (RuOx/m-ZrO2) sample, formed by heat treatment at 673 K in air, was assigned to terminal Ru = O stretching 22,23 vibration in tetrahedral RuO2− In contrast, the new high4 . frequency band is rather broad in the present case, which does not correspond to a single phonon or vibrational mode. The broad feature at around 960 cm−1 can be attributed to crystal disorder activated second order phonon modes. Similar broad character was detected from Ni-doped WO3.7 Possible causes of disorder are Ru-nanoclusters or Ru-cations embedded in grain boundaries of WO3 matrix. We note that the high frequency peak enhanced in 1% doped sample. Aforementioned analysis was concentrated on 0.5% Ru-doped sample. Figure 6 shows one possible way to place Ru atoms in hcp arrangement on the WO3 (111) plane. Red lines outline the

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CONCLUSIONS Ruthenium-doped tungsten trioxide (WO3) samples were studied by XRD, XPS, Raman spectroscopy, and TEM and SEM techniques. XPS and Raman studies revealed that Ru was oxidized although no evidence of ruthenium oxides was found. Neither XRD nor electron microscopy detected secondary phase or aggregates of Ru. Composition analysis by SEM-EDS showed that Ru was dispersed on WO3 surface area. The results imply that hexagonally close packed Ru clusters, too small to be detected by XRD, were formed on {111} planes of WO3 by making a bond between Ru ion and surface oxygen under mild processing conditions with 0.5 wt % Ru concentration. The result support the idea that ruthenium grows epitaxially on top of the oxygen atoms on the {111} planes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: yukari.fujioka@aalto.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS XPS measurements were conducted with the help of Drs. Leena-Sisko Johansson and Joseph Campbell (Aalto University). The research work was supported by the collaboration project between the Center of Excellence for Advanced Materials Research at King Abdulaziz University in Saudi Arabia and the Aalto University (Project No. T-001/431) and the Academy of Finland (the Center of Excellence Program 2012−2017).

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REFERENCES

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Figure 6. An arrangement for epitaxial attachment of Ru-clusters on WO3 (111) plane. Oxygen atoms are at the vertices of the light blue triangles. The oxygen octahedron edge lengths of the monoclinic WO3 are 2.7082, 2.6989, and 2.6907 Å, close to the experimentally determined value aH = 2.70389 Å of the hcp Ru.19 Hexagonal aH and bH axes of Ru are shown. The red lines, outlining two connected hexagons, correspond to the cluster shown in Figure 4. For illustration purposes, an average octahedral edge lengths is shown. Figure was prepared by VESTA software.27

hexagonal clusters corresponding to the clusters shown in Figure 4. To estimate oxygen octahedron edge lengths, we used the lattice parameters given in Table 1. Because laboratory XRD measurements do not provide sufficiently accurate estimation for oxygen positions, fractional coordinates from the earlier neutron powder diffraction study were used.24 The O−O distances (triangle edges in Figure 6) are very close to the nearest neighbor Ru−Ru distances in the hexagonal ab plane. The results further suggest that Ru−O bond formation plays a crucial role for forming hcp Ru clusters on the (111) plane of WO3. It would be possible that surface of Ru clusters bond with terminal oxygen. No evidence for RuO2 formation was found, though hydrolysis of RuCl3·xH2O results in black precipitation of RuO2 ·xH2O that can be dehydrated at 500 °C.25 Our results suggest that under reported processing 7509



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Formation of Ruthenium Cluster on Nanocrystalline Tungsten Trioxide

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