Characterization of Nanoporous WO3 Films Grown via Ballistic

Article pubs.acs.org/JPCC

Characterization of Nanoporous WO3 Films Grown via Ballistic Deposition Břetislav Šmíd,† Zhenjun Li,§ Alice Dohnálková,‡ Bruce W. Arey,‡ R. Scott Smith,§ Vladimír Matolín,† Bruce D. Kay,*,§ and Zdenek Dohnálek*,§ †

Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Charles University, V Holešovičkách 2, 18000 Prague 8, Czech Republic ‡ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, PO Box 999, Mail Stop K8-93, Richland, Washington 99352, United States § Chemical and Materials Sciences Division, Fundamental and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific Northwest National Laboratory, PO Box 999, Mail Stop K8-88, Richland, Washington 99352, United States ABSTRACT: We report on the preparation and characterization of high surface area, supported nanoporous tungsten oxide films prepared under different conditions on polished polycrystalline Ta and Pt(111) substrates via direct sublimation of monodispersed gas phase clusters of cyclic (WO3)3. Scanning electron microscopy and transmission electron microscopy were used to investigate the film morphology on a nanometer scale. The films consist of arrays of separated filaments that are amorphous. The chemical composition and the thermal stability of the films were investigated by means of X-ray photoelectron spectroscopy. The surface area and the distribution of binding sites on the films were measured as functions of growth temperature, deposition angle, and annealing conditions using temperature-programmed desorption of Kr. Films deposited at 20 K and at an incident angle of 65° from the substrate normal display the greatest specific surface area of ∼560 m2/g. ing,22−25 sol−gel methods,26 oxidation of tungsten films,27 and pulsed laser deposition.28−30 More recently, well-ordered two layer thick tungsten oxide films with (3 × 3) periodicity have been prepared via sublimation of WO3 powder on Pt(111) at 600−800 K.31 While dense, ordered films are generally prepared at elevated substrate temperatures,3,4 low substrate temperatures limit the surface mobility of the deposited material, which is a key factor for the growth of porous films.6,7,32 Various porous films such as metals (Pd,33 Cr,34 Cu,34 Fe,35 Ti,36), oxides (ZnO,37 MgO,38 TiO2,39−41), and molecular solids (H2O7,42,43) with tailored optical, magnetic, electrical, and adsorption properties have been successfully prepared using the BD technique. Here we show that the deposition of gas-phase (WO3)3 clusters (produced by WO3 sublimation) at oblique angles of incidence at 20 K yields extremely porous films with surface areas as high as 560 m2/g. Deposition at 300 K leads to denser films, but these films still have a fairly high surface area of 250 m2/g. The films are amorphous and exhibit filament-like structure. Annealing studies show that the films are structurally

1. INTRODUCTION In the past several decades, a significant effort has been put into understanding the growth morphology and physical and chemical properties of thin oxide films. This is in part driven by the fact that such films often exhibit new physical and chemical properties that can be radically different from their bulk counterparts.1−5 Our group is interested in developing a molecular level understanding of the site-specific catalytic phenomena on oxide surfaces. In this context, nanoporous thin films prepared via ballistic deposition (BD)6,7 can be viewed as an ideal bridge between well-ordered single crystalline films and high surface area powders. Nanoporous thin films can be synthesized directly under ultrahigh vacuum (UHV) conditions, their specific surface area can be smoothly tuned by changing the growth conditions (e.g., deposition angle and temperature), and their tethered nature allows for easy access of the reactants to the pore structure. In this particular study we focus on the preparation of nanoporous films of tungsten oxide, which is a catalytically important oxide that is active for a number of reactions such as the isomerization of alkanes and alkenes,8−11 partial oxidation of alcohols,12 selective reduction of nitric oxide,13−16 metathesis of alkenes,17−19 and polymerization of aldehydes.20 Tungsten oxide thin films have been prepared by different deposition techniques such as chemical vapor deposition,21 sputter© 2012 American Chemical Society

Received: February 16, 2012 Revised: April 25, 2012 Published: April 27, 2012 10649

dx.doi.org/10.1021/jp3015723 | J. Phys. Chem. C 2012, 116, 10649−10655

The Journal of Physical Chemistry C

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Figure 1. SEM images of nanoporous WO3 films deposited by BD from a (WO3)3 source on polished polycrystalline Ta substrates at 20 K and incident angles of (A) 45°, (B) 65°, and (C) 85°. The insets in (A) and (B) show areas with defects in the film structure and expose the crosssectional morphology of the films. Deposited amounts correspond to 100 ML of a dense WO3 film. Length scales for all the images are identical and indicated in (A).

and compositionally stable up to ∼600 K, a temperature that is high enough for model catalytic studies.

All Kr TPD spectra were acquired using a line-of-sight quadrupole mass spectrometer (UTI) and a linear temperature ramp rate of 1.0 K/s. The prepared samples were transferred through air before SEM and TEM measurements. All SEM data were collected using the Helios NanoLab by FEI. TEM imaging and analyses were performed on a JEOL 2010 microscope operating at 200 keV. For the TEM analysis, portions of the WO3 films were removed from the Ta substrates and transferred on to lacey carbon grids (Electron Microscopy Sciences, Hatfield, PA).

2. EXPERIMENTAL DETAILS The experiments were performed in a UHV apparatus equipped with Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and quadrupole mass spectrometry. The polished polycrystalline Ta substrates were used for the growth of thick nanoporous WO3 films for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) ex situ studies, while single crystalline Pt(111) was used for repeated growth of thin nanoporous WO3 films used in our surface area measurements. Thin and thick films contain 10 monolayers (ML) and 100 ML of densified WO3, respectively. A single monolayer (ML) of WO3 is defined as the planar WO3 density as calculated from the density of WO3 solid44 and corresponds to 270.2 ng/cm2. The substrates were mounted on a manipulator cooled by a closed cycle helium cryostat. The temperature of the samples was measured using W-5% Re/W-26% Re thermocouple spotwelded to the back side and controlled by resistive heating from 20 to 1500 K. A new Ta substrate was used for the preparation of each WO3 film. The substrates were cooled down to a base temperature of 20 K, outgassed at 1100 K prior to film growth, and warmed to 300 K after film growth before removing from the vacuum for the ex situ SEM and TEM analysis. The base pressure in the chamber (unbaked) before the thick (100 ML) film growth was in the high 10−9 Torr range. The Pt(111) for the growth of thin (10 ML) WO3 films was cleaned using a standard procedure including a sequence of Ne+ bombardment at 300 K, O2 annealing at 1200 K (5 min, 2 × 10−7 Torr), and annealing in UHV at 1300 K. The base pressure in the baked UHV chamber during the adsorption studies on thin WO3 films on Pt(111) was ∼1 × 10−10 Torr. The (WO3)3 clusters were deposited by direct sublimation of WO3 powder (99.95%, Aldrich) using a high-temperature effusion cell (CreaTec).45,46 Complete removal of WO3 from the Pt(111) substrate required extended Ne+ sputtering (2.0 kV, 10 μA, 2 h). The effusion cell is mounted tilted upward limiting the minimum deposition angle to 45°. Sample rotation along the vertical axis allows for the deposition angle be varied between 45° and 90°. The deposition flux was calibrated using a quartz-crystal microbalance (QCM XTM/2-Inficon). The surface area of the nanoporous WO3 films was determined using Kr physisorption. Krypton (Air Liquide, 99.995%) was dosed as received and introduced using a neat, 300 K, quasieffusive molecular beam directed normal to the surface at 39 K.

3. RESULTS AND DISCUSSION 3.1. Ex Situ Film Structure Analysis. In our previous study we focused on the growth of thin epitaxial WO3 films on Pt(111) using (WO3)3 gas phase clusters prepared by WO3 sublimation in vacuum.31 While the growth of ordered thin films generally relies on fast surface diffusion of the deposited species at high substrate temperatures, the growth of porous films via ballistic deposition (BD) requires limited surface diffusion and therefore low substrate temperatures. Under such conditions atoms/molecules from the vapor phase are incorporated at and/or very near the site where they initially strike (hit and stick).6,7,43,47 Because of the random nature of the deposition process, higher and lower areas develop. The asperities preferentially capture the incoming flux, while the regions behind are shadowed. The length of the shadows is proportional to the tangent of the incident angle and increases dramatically at grazing incidence angles leading to the formation of columnar, porous films. In contrast, BD at or near normal incidence leads to the formation of denser films. This evolution can be seen in the SEM images of WO3 films in Figure 1. The images of WO3 films resulting from (WO3)3 deposition at 45°, 65°, and 85° angles of incidence with respect to the substrate normal on a Ta substrate at 20 K are shown in parts A, B, and C of Figure 1, respectively. The films were warmed to room temperature and exposed to air prior to ex situ SEM imaging. Systematic changes in film morphology with increasing deposition angle are apparent. While the 45° film appears continuous and rough on a sub-micrometer scale, the 65° film shows the development of small pores, and the 85° film is clearly composed of well-separated filaments. The insets for the 45° and 65° films show defect structures that provide further opportunity to expose the interior of the films and reveal their porous nature. To assess how the film morphology changes with substrate temperature, we show the SEM images of films grown at 85° as a function of growth temperatures in Figure 2. The samples grown at 300, 450, and 600 K (Figure 2A−C) exhibit very 10650



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pattern from individual filaments is shown in the inset of Figure 3A. The diffuse nature of the ring pattern indicates the amorphous character of the material. Only two very diffuse maxima corresponding to 1.7 ± 0.05 and 3.7 ± 0.05 Å can be identified. Interestingly, these values correspond well to the WO bond length and the W−W nonbonded distance in the cyclic (WO3)3 cluster.48,49 The TEM image and SAED pattern for the 300 K grown film (Figure 3B) are very similar to those for the 20 K deposited film (Figure 3A). In contrast, the TEM image of the film grown at 750 K (Figure 3C) and its SAED pattern are rather different. The film fragments also exhibit filament-like structure, but their surfaces are rather smooth in comparison to those prepared at 20 and 300 K. The well-defined SAED diffraction pattern in Figure 3C clearly shows that the film is crystalline and has the BCC structure of tungsten metal50 as further supported by the XPS results presented below. 3.2. Film Composition. To determine the chemical composition of the films as a function of deposition temperature, we performed in situ XPS studies prior to sample removal for the SEM and TEM ex situ analysis. Figure 4 shows

Figure 2. SEM images of nanoporous WO3 films deposited on optically polished polycrystalline Ta substrates at an incidence angle of 85° and substrate temperature of (A) 300, (B) 450, (C) 600, and (D) 750 K. Length scales for all the images are identical and indicated in (A). Deposited amounts correspond to 100 ML of dense WO3.

similar filament-like morphologies analogous to that observed on the sample grown at 20 K (Figure 1C). The persistence of the filament-like morphology indicates that on average the diffusion length of the (WO3)3 clusters is shorter than the average filament−filament spacing (∼100 nm). It is plausible that at higher temperatures, where larger diffusion rates are expected, the extent of diffusion of incident (WO3)3 clusters is limited by their reaction and covalent bond formation with the already deposited film. A dramatic difference in the morphology is observed for the film grown at 750 K (Figure 2D, the highest deposition temperature employed in our SEM studies) where round and much smaller filaments are observed. As demonstrated below, this change is caused by the nearly complete reduction of (WO3)3 to metallic tungsten. To further analyze the structure and crystallinity of the individual filaments, portions of three films grown at 85° and 20, 300, and 750 K were removed from the Ta substrates and deposited on lacey carbon grids for TEM imaging. Figure 3A shows several filaments from the film grown at 20 K and reveals that the individual filaments are not dense but contain smaller nanometer size features. The width and length of the filaments observed in the TEM images range from 50−500 nm and 1−2 μm, respectively. The selected area electron diffraction (SAED)

Figure 4. Black squares show XPS core level spectra of (A) W 4f and (B) O 1s from nanoporous WO3 films deposited at an 85° incident angle as a function of substrate growth temperature. The deconvolved contribution of W in (6+), (5+), and (0) oxidation states in (A) are plotted with red, green, and magenta lines, respectively. The orange lines in (B) show the contribution to the O 1s spectra due to surface hydroxyls. All films (100 ML) were deposited on optically polished polycrystalline Ta substrates.

Figure 3. TEM images of individual WO3 filaments removed from WO3 films deposited at an 85° angle of incidence and substrate temperatures of (A) 20, (B) 300, and (C) 750 K. Insets show the corresponding SAED patterns. Length scales for the TEM images are indicated in each panel; scales for the SAED patterns are indicated in (A). The shorter length of the filaments in (C) is caused by the higher density of W and the dense nature of the filaments. 10651



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W 4f and O 1s core level spectra taken after WO3 film growth at different substrate temperatures. The W 4f spectra for (WO3)3 deposition at 20, 150, 300, and 450 K (Figure 4A(a− d)) are rather broad and show a two-peak structure due to a 2.18 eV spin−orbit splitting with a 4:3 ratio between the W 4f7/2 and 4f5/2 levels for W in the (6+) oxidation state.31,51−54 The broad nature of the peaks is most likely due to the insulating nature of the films which is also supported by similar broadening observed in the corresponding O 1s spectra (Figure 4B(a−d)). Additionally, the highly disordered nature of the films can also contribute to the observed peak broadening. The shoulder to the high-energy side of the O 1s peak at 533.2 eV in Figure 4B(a) can be assigned to hydroxyls51,55,56 on the surface due to the relatively high background pressure in the unbaked UHV system. The origin of increased concentration of the hydroxyl groups in Figure 4B(e) is unclear but is possibly correlated with the onset of WO3 reduction in this temperature range. A significant change in the W 4f spectra is observed for the film deposited at 600 K (Figure 4A(e)). The spectrum is significantly sharper, and a small shoulder is observed on the low-energy side. To determine the contribution of various oxidation states, the spectra were fitted after subtracting a Shirley-type background using mixed Lorentzian and Gaussian doublet peak profiles. The low-energy shoulder can be fitted by a doublet (green) with the W 4f7/2 peak at 34.6 eV, indicating presence of tungsten in (5+) oxidation state.31,52,53 The doublet peak area assigned to W5+ state is ∼9% of the total W 4f peak area. A similar deconvolution (not shown) is performed for the O 1s peak also yielding ∼9% of reduced species (this excludes O 1s feature due to hydroxyl groups). Finally, the deposition of (WO3)3 on the substrate at 750 K (Figure 4A(f)) leads to the formation of a mostly metallic tungsten film, in agreement with SAED results from the same sample (see Figure 3C). For the surface area measurements, thinner nanoporous WO3 films (10 ML of dense WO3 equivalent) were deposited on a single crystalline Pt(111) substrate rather than on the polished polycrystalline Ta. As shown in our prior studies,31 ordered two-layer thick (WO3)3 films can be grown on Pt(111) between 600 and 800 K and their surface area can be compared with those of nanoporous WO3 films. Additionally, Pt(111) also minimizes the extent of reduction of the (WO3)3 films and allows for their removal by a simple sublimation above ∼1000 K. The difference in the reduction of the WO3 is clearly evident from the annealing temperature-dependent set of XPS W 4f core level spectra from 10 ML of WO3 grown at 20 K and a 65° angle of incidence shown in Figure 5A. We did not observe any significant change in the spectra until about 650 K, where a small but detectable doublet in W 4f is observed at 34.4 and 36.6 eV due to the presence of W5+, most likely at the Pt(111) interface.31,52,53 A further increase in the annealing temperature to 900 K does not lead to any significant increase in the reduction, but an increase in temperature above 1000 K leads to an overall signal decrease due to WO3 sublimation, leaving only reduced tungsten at the Pt(111) surface in the (5+), (4+), and (0) oxidation states. The remaining tungsten is removed by sputter−anneal cycles to restore the clean Pt(111) substrate. A significantly smaller broadening of the W 4f peaks in Figure 5A as compared to that in Figure 4A is observed. This is likely a result of smaller film thickness (10 ML vs 100 ML) that leads to a reduced level of film charging. The integrated peak areas for W6+, W5+, and total W as a function of annealing

Figure 5. (A) XPS spectra of W 4f core level from nanoporous WO3 (10 ML of dense WO3 equivalent) films deposited on Pt(111) at 20 K and at a 65° angle of incidence as a function of annealing temperature. The spectra are deconvolved using the procedure described in the text to determine the fractions of tungsten in various oxidation states. (B) Integrated area of the W 4f spectra (black squares) and the deconvolved W6+ and W5+ fractions plotted as a function of annealing temperature (red triangles and green diamonds, respectively).

temperature are shown in Figure 5B. The W peak area (black squares) does not change up to 600 K, but a significant increase is observed between 650 and 850 K. This is likely due to the film densification and increase in the external surface that defines the macroscopic surface plane of the film (as further demonstrated by our surface area measurements discussed below). As already discussed, a small level of reduction is also observed in the same temperature range (green diamonds, Figure 5B). Above 900 K, the peak area decreases dramatically due to WO3 desorption. 3.3. Film Surface Area. To characterize the surface area of the WO3 films, we used Kr physisorption. Extensive prior work at PNNL has shown that the physisorption of weakly bound species (e.g., Kr, N2) can be successfully used to study the evolution of surface morphology, the distribution of defect sites, and the kinetics of phase transitions.33,57,58 Physisorption also allows us to measure the adsorption capacity of the porous films, i.e., to measure the total number of adsorption sites.33,38,39 Figure 6 displays Kr TPD spectra obtained from nanoporous WO3 films grown by depositing the equivalent of 10652



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weak maximum is observed at 65°. This behavior is similar to the dependences observed previously on nanoporous TiO239 and ASW films42,43 but differs from the monotonic increase observed for Pd films.33 While the exact origin of the observed maxima is not well understood, it is likely connected to the differences in the distribution of pore sizes and some pore condensation of Kr within the films as supported by previous ballistic deposition simulations32,60 and experimental results.61,62 A study by Krause et al. revealed that TiO2 and SiO2 films exhibited significant mesoporosity in the 2−10 nm range, which was strongly deposition angle dependent.61 Similarly, the dependence of pore size distribution on the deposition angle has been observed previously by Mullins’ group.62 To investigate the morphology and thermal stability of the films, we conducted a series of Kr TPD experiments on films which were grown at substrate temperatures of 20, 150, 300, 450, and 600 K and annealed incrementally up to 900 K. The total amount of adsorbed Kr (referenced to a Kr monolayer on Pt(111)) is displayed in Figure 8. As seen from the descent of Figure 6. Kr TPD spectra obtained after a saturation dose of Kr at 39 K on nanoporous WO3 films deposited on Pt(111) at 20 K (10 ML dense WO3 equivalent) at incident angles ranging from 45 to 85°. A Kr TPD spectrum from clean Pt(111) is provided as a 1 monolayer reference (bottom).

10 ML of dense WO3 film at a substrate temperature of 20 K and various deposition angles. The Kr adsorption is carried out at 39 K to prevent condensation of Kr multilayers, and surface saturation is determined from the Kr partial pressure increase in the chamber background as measured by QMS.58,59 All the TPD spectra show very similar broad features (39−105 K) with a maximum peak at ∼47 K, indicating a broad range of adsorption sites available on the surface, as expected for amorphous films.38,39 A monolayer Kr TPD spectrum from flat Pt(111) is shown for comparison59 and is used as a reference to determine the areas of the nanoporous WO3 films. Figure 7 displays the integrated amount of adsorbed Kr from the TPD spectra presented in Figure 6 in terms of adsorbed monolayers relative to 1 ML Kr on Pt(111). While the amount of adsorbed Kr changes only slightly between 45° and 85°, a

Figure 8. Saturation coverage of Kr at 39 K on nanoporous WO3 films (10 ML dense WO3 equivalent) deposited at a 65° angle of incidence as a function of (WO3)3 deposition and annealing temperatures. The dashed line connects the saturation coverages of Kr on as-grown WO3 films. The dotted line indicates the amount of Kr adsorbed on Pt(111) which is defined as 1 ML.

the dashed line (connecting the first points of each curve in Figure 8), the surface area of “as-grown” films decreases faster with deposition temperature than with postannealing temperature (solid lines). For example, a film deposited at 20 K and annealed to 600 K has a surface area 8 times greater than a dense film, but a film grown at 600 K has a surface area only 2 times that of a dense film. This fact is consistent with a lower activation barrier for surface diffusion (which dominates during the growth) compared to bulk diffusion (which is important in annealing induced densification). The maximum amount of adsorbed Kr (15 ML) is found on the film grown at 65° and 20 K. The specific surface area of the films (generally quoted for high surface area materials) can be calculated from the area covered by adsorbed Kr (15 ML of Kr is equal to 15 cm2/cm2) and from the mass of WO3 within this area (10 ML of WO3 equals to 2702 ng/cm2). For WO3 deposited at 65° and 20 K this yields a specific surface area of ∼560 m2/g, which is comparable with the very high specific surface areas previously reported for other materials deposited via BD.38,39,42 For

Figure 7. Saturation coverage of Kr adsorbed at 39 K on nanoporous WO3 films (10 ML dense WO3 equivalent) as a function of (WO3)3 deposition angle obtained by integrating the TPD spectra shown in Figure 6. The (WO3)3 films were grown at 20 K. 1 ML of Kr is defined as the amount of Kr adsorbed on Pt(111) surface. 10653



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comparison, previously reported high surface area WO3 powders have surface areas between 2 and 40 m2/g,63 while commercially available WO3 powders have specific surface areas of only 5.2 m2/g measured by BET.64 To connect the surface area measurements on thin WO3 films to the morphology characterization studies (SEM and TEM) on thick WO3 films, we have carried out thickness dependence measurements of the film surface area. The thickness-dependent surface areas measured for WO3 film deposited at a growth angle of 65° and at 20 K are shown in Figure 9. The total amount of adsorbed Kr molecules increases

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AUTHOR INFORMATION

Corresponding Author

*Ph (509) 371-6143, Fax (509) 371-6145, e-mail Bruce.Kay@ pnnl.gov (B.D.K.); Ph (509) 371-6150, Fax (509) 371-6145, email [email protected] (Z.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A part of this work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences, and performed in Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle. B.Š. and V.M. were supported by the Ministry of Education of the Czech Republic under grant ME08056.

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Figure 9. Saturation coverage of Kr adsorbed at 39 K on WO3 films deposited at 20 K and 65° on Pt(111) as a function of film thickness (5−40 ML). The solid line through the data indicates the linear dependence of the surface area on the film thickness.

linearly with thickness of the WO3 film, indicating that the surface within the whole film is accessible from the outer surface of the film where the Kr flux impinges. Further, the line shape of the Kr TPD (data not shown) does not change with thickness, indicating the uniform structure of the pores within the filaments of different thickness.

4. SUMMARY We have studied the ballistic deposition of nanoporous WO3 films from a beam of (WO3)3 gas phase clusters prepared via sublimation of solid WO3. The effects of deposition angle, substrate temperature, and postannealing on the structure and morphology of the films have been investigated by a combination of SEM, TEM/SAED, XPS, and TPD. The SEM, TEM, and SAED reveal that the films grown at 20 K and oblique angles have filament-like structure and are amorphous. We have found that the films are extremely porous with specific surface area as high as 560 m2/g when deposited at 65° angle of incidence on the substrate at 20 K. The high surface area of such films results from the limited surface mobility of the deposited (WO3)3 molecules and the shadowing of low lying areas that develop due to the stochastic nature of the growth at low temperatures and high deposition angles. The films’ surface areas decrease with increasing deposition and annealing temperature due to enhanced surface and bulk diffusion. Upon annealing to 600 K, the films retain ∼50% of their surface area (250 m2/g), making them good candidates for catalytic studies. In contrast, growth at 600 K at the deposition rate of 5 × 1013 WO3/(s cm2) leads to practically dense WO3 films. 10654



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Characterization of Nanoporous WO3 Films Grown via Ballistic

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