Controlled modification of nanoporous gold: Chemical vapor deposition of TiO2 in ultrahigh vacuum

Applied Surface Science 282 (2013) 439–443

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Controlled modification of nanoporous gold: Chemical vapor deposition of TiO2 in ultrahigh vacuum A. Schaefer a,∗ , D. Ragazzon b , L.E. Walle c , M.H. Farstad c , A. Wichmann a , M. Bäumer a , A. Borg c , A. Sandell b a

Institute of Applied and Physical Chemistry, University of Bremen, Box 33 04 40, D-28359 Bremen, Germany Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 20 Uppsala, Sweden c Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway b

a r t i c l e

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Article history: Received 4 April 2013 Received in revised form 27 May 2013 Accepted 28 May 2013 Available online 3 June 2013 Keywords: Nanoporous gold Titania Chemical vapor deposition Heterogeneous catalysis Ultrahigh vacuum

a b s t r a c t TiO2 has been deposited in the first 400 nm of a nanoporous gold (NPG) structure using metal organic chemical vapor deposition with titanium-tetraisopropoxide as single source precursor in ultra high vacuum. The NPG has been pretreated by ozone to clean and stabilize the structure for deposition. The deposited oxide stabilizes the porous structure, otherwise prone to coarsening at elevated temperatures, up to 300 ◦ C. The study combines the controlled sample preparation with a functional test of the prepared catalyst under real conditions in a continuous gas flow reactor. The catalytic activity of the loaded NPG at 60 ◦ C for CO oxidation is found to be superior to unloaded as-prepared NPG. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Among the porous materials nanoporous metals constitute an especially versatile and fascinating class of materials. In the recent years a lot of work in various fields was devoted to nanoporous gold (NPG). Potential applications of NPG were proposed and shown in heterogeneous catalysis, optics or sensorics [1–6]. On the way to new applications present studies deal with the functionalization, controlled modification and tailoring of the nanoporous gold material. Beyond thermal treatments this employs structural control by depositing oxide materials using a variety of methods [7–10]. For catalytic purposes, the addition of TiO2 to NPG is an enticing prospect. The oxidation of CO on small, TiO2 supported gold particles has been found to take place at remarkably low temperatures, down to 200 K [11,12,13]. Consequently, this is the process that has attracted the most attention. Several studies have aimed at unraveling the origins of the high catalytic activity [14–16]. Among other important effects like the influence of the oxide support [17], the activity has mainly been explained both in terms of a high density of under-coordinated Au atoms at the particle surfaces [18] and Ti–Au dual sites at the particle perimeter [19]. In the case of TiO2 /Au(1 1 1), which can be viewed as the “inverse system”, a

∗ Corresponding author. Tel.: +49 421 218 63175. E-mail address: [email protected] (A. Schaefer). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.05.148

high performance in the water–gas shift (WGS) reaction has been found (H2 O + CO → H2 + CO2 ) [20]. The maximum efficiency, found to occur at TiO2 particle coverages of 20–30%, compares well to good WGS catalysts such as copper. The facts that neither bulk Au nor bulk TiO2 are efficient for WGS and that the maximum activity is found for a surface partially covered by TiO2 particles both underscores the importance of the TiO2 –Au boundary region [21]. From this follows that loading of NPG with nanometer-sized TiO2 particles can be expected to yield a system with high catalytic activity by virtue of a very high density of TiO2 –Au boundary regions. First studies on the electrocatalysis of loaded NPG indeed report an increased performance of the catalyst [10,22]. Efficient loading can be accomplished with the use of gaseous precursor molecules that penetrate the porous network. Consequently, chemical vapor deposition (CVD) is a method suitable for loading NPG with another material. That this concept has a high potential to generate a new family of high-area catalysts was recently demonstrated when NPG was loaded with alumina and titania by way of atomic layer deposition, a method similar to CVD [9]. First pilot studies in our lab (yet unpublished) show that NPG loaded with TiO2 by ALD is active for the water gas shift reaction while pure NPG does not show any activity. In this work we present an approach for the controlled modification of nanoporous gold by depositing TiO2 by means of metal-organic CVD from Ti-tetraisopropoxide (TTIP) as precursor in ultrahigh vacuum (UHV). After characterizing the prepared

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catalyst material with surface science methodology we performed a reactivity test for CO oxidation in a continuous gas flow reactor. That is, the controlled sample preparation and characterization in UHV is directly correlated to a functional test under real conditions. 2. Experimental Nanoporous gold was prepared by dealloying of a AgAu-alloy (70 at%:30 at%) as described elsewhere [6]. The resulting sample exhibited a diameter of 5 mm and an approximate thickness of 200 ␮m. The photoelectron and X-ray absorption studies where performed at the undulator beamline I311 at the Swedish synchrotron radiation facility MAX II in Lund. The endstation of beamline I311 is housing a SCIENTA SES200 electron energy analyzer. The base pressure during analysis was in the low 10−10 mbar range. The energy scale of the spectra was calibrated with respect to the position of the Fermi level of a gold crystal mounted on the opposite side of the same sample holder. For the X-ray absorption spectra the Auger yield of the main line of Ti at ∼386 eV kinetic energy was recorded at pass energy of 50 eV (resulting in an energy window of ∼5 eV). The sample was mounted and degassed together with the manipulator following the same procedure as described in Ref. [23]. The temperature was measured employing a type-K thermocouple placed in close proximity to the sample. Titanium(IV) isopropoxide, TTIP (Aldrich, purity 99.999%) was dosed using a stainless-steel tube positioned 15 mm from the sample. The dose was controlled by a leak valve and estimated quantitatively as the background pressure in the preparation chamber during dosing multiplied by the time of exposure. Both the high vacuum side and the UHV side were thoroughly passivated with TTIP prior to sample exposure. Prior to TTIP exposure the NPG sample was subjected to an oxygen/ozone gas mixture (7 vol% O3 ) for 120 min at 120 ◦ C sample temperature. As reported in our previous UHV study [23] this treatment has two positive effects, which we exploit here for the titania deposition by using TTIP. Firstly, the sample surface is cleaned from carbonaceous contaminations. Secondly, the NPG surface becomes oxidized and thus stabilized toward thermal treatments. The ligament structure of as-prepared NPG samples is usually stable up to 150 ◦ C. At higher temperatures coarsening of the structure sets in. However, ozone treatment in UHV is capable of stabilizing the NPG structure up to min. 230 ◦ C [7,23]. For the decomposition of TTIP temperatures above 200 ◦ C are required [24] so that an ozone treatment prior to TTIP deposition is an ideal way to clean and stabilize the NPG for TiOx deposition. Upon deposition, the pre-oxidized sample was heated to 150 ◦ C while setting the desired precursor pressure. Subsequently the sample temperature was increased to the final value of 250 ◦ C. The sample temperature during deposition was kept at 250 ◦ C while the TTIP pressure was kept at ∼2 × 10−8 mbar. Deposition was carried out in two consecutive steps of 40 min each. Scanning electron microscopy (SEM) was conducted post deposition using a Zeiss SUPRA 400 instrument. 3. Results and discussion Survey spectra after exposure of pre-oxidized NPG to TTIP are shown in Fig. 1. The progressive deposition of titania is clearly observed, e.g. by the appearance of pronounced Ti 2p and O 1s signals. Based on the kinetic energy of the Au 4f photoelectrons of 137 eV and the resulting inelastic mean free path of ∼6 A˚ a final average layer thickness of ∼25 A˚ can be estimated from the attenuation of the Au 4f signal. In addition, a weak C 1s signal can be discerned in the survey spectra. Two different carbon species can be observed, with C 1s binding energies of 284.3 eV

Fig. 1. Survey spectra after the first and second deposition of TiOx on NPG at 250 ◦ C. Deposition of Ti and oxygen is clearly visible. The amount of carbon detected is very small. The Ag 3d signal stems from residual Ag of the dealloying process. Inset: zoom into the C 1s region. The C 1s signal is smaller after the second deposition. (For interpretation of the references to spectra in figure legend, the reader is referred to the web version of the article.)

and 286.7 eV BE, respectively (inset of Fig. 1). The detailed spectra, shown in the inset, furthermore show that the C 1s signal appears to decrease after the second deposition. Unreacted precursor can be ruled out as origin for the signal since the C 1s peaks associated with adsorbed TTIP are expected to occur at higher BE [24]. Decomposition products could be another source for the observed carbon signal. However, those products would then only stick during the initial stages of growth since no increasing carbon content is observed after subsequent depositions. Another possible origin for the observed carbon content after the first deposition is the diffusion of carbon impurities from the interior of the porous structure to the near surface region, where they can be detected by PES. If this is the case, the decrease in the C 1s intensity observed suggests that the out-diffusing carbon is gradually covered by the deposited film. In any event the carbon content at the surface could potentially be reduced by an additional cleaning cycle. Turning next to the Ti 2p spectra, shown in Fig. 2 (top), the Ti 2p3/2 binding energy of 458.9 ± 0.1 eV is typical for Ti4+ , that is, consistent with TiO2 . However, a pronounced shoulder on the low BE side is evidence for a minor amount of reduced Ti. The relative amount of reduced Ti decreases with increasing coverage. This is shown by the inset of the top diagram of Fig. 2, displaying the Ti 2p3/2 spectra normalized to the peak maximum. From this follows that the reduced species is formed during the initial stages of growth and is located that the Au/TiOx interface. The O 1s spectra depicted in Fig. 2 show that all oxygen species generated upon the ozone treatment have already been removed during the first TTIP dosing. This supports the assumption of a direct TiOx /Au interface. This is also evidenced by the Au 4f7/2 spectra in Fig. 2: the oxide related signal has vanished after the first TTIP deposition. The Au bulk signal is almost completely attenuated after the second deposition. To obtain more information about the structure of the as deposited TiOx we conducted X-ray absorption (XAS) measurements at the Ti 2p edge, summarized in the top diagram of Fig. 3. The spectra after the two depositions appear to be very similar in shape and the difference spectrum of the two preparations shows that the polymorph of the deposited oxide resembles the anatase phase. This is evident when comparing to spectra of anatase and rutile from a reference experiment on silicon (bottom diagram) [25]

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Fig. 3. Top: X-ray absorption spectra of the Ti L2,3 edge. Taken as a fingerprint the difference spectrum points toward the evolution of the anatase phase. Bottom: reference spectra from MOCVD of TiO2 on Si(1 1 1). (For interpretation of the references to spectra in figure legend, the reader is referred to the web version of the article.)

the mesoscopic structure. The other half of the sample was imaged without further treatment after TiO2 deposition. Usually, unloaded NPG has to be activated in e.g. a stream of CO (4 vol%) and O2 (20 vol%) at 80 ◦ C [6]. In this case, no activity was observed after this activation procedure. Only after heating the sample to 300 ◦ C production of CO2 set in. The temperature could be lowered while the reaction kept going. The CO2 production observed at 60 ◦ C is by a factor of 1.5–2 higher than observed for unloaded NPG under comparable conditions, Fig. 4. Images showing the mesoscopic structure after deposition of titania and after the activation in catalysis at 300 ◦ C as obtained

Fig. 2. Top: Ti 2p spectra for the two depositions. A reduced component is visible as a shoulder on the low BE side. The inset shows a zoom into the Ti 2p3/2 region normalized to the peak maximum. The reduced component becomes attenuated with ongoing deposition. Middle: O 1s spectra after oxidation by ozone and the two TiOx depositions. No gold oxide is detected after the first deposition. Bottom: Au 4f7/2 spectra after ozone treatment and after the two depositions of TTIP. Also here no gold oxide is detected after the first deposition. The Au signal is almost completely attenuated after the second deposition. (For interpretation of the references to spectra in figure legend, the reader is referred to the web version of the article.)

and with previously reported data [26–28]. However, the individual peaks are broader than observed for pure, well-ordered anatase. This can be attributed to a degree of disorder in the structure and the presence of reduced Ti species. For the further analysis the sample was taken out of the vacuum chamber and broken into two parts. One part was tested for CO oxidation in a continuous gas flow reactor and subsequently imaged with scanning electron microscopy (SEM) to check for changes in

Fig. 4. Comparison of the CO2 production of an unloaded NPG sample and the NPG sample loaded with TiO2 at 60 ◦ C. The CO2 production increased in average by a factor of ∼1.5.

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the activation process does not involve major structural changes on the imaged length scale. It is feasible that the activation involves in comparison more subtle changes. For instance, the activation in the reactor at 300 ◦ C may lead to cracks in the generated TiO2 film or even formation of more isolated TiO2 particles like reported for thicker films produced by ALD and heated to 600 ◦ C [9]. This would generate more TiO2 /Au interface area, which has been found to be important for catalytic reactions [19,30]. 4. Conclusions

Fig. 5. Cross section SEM micrographs of (a) the loaded sample after transferring to atmosphere and (b) the sample after heating it to 300 ◦ C in the catalytic reactor. In the first 400 nm the structure has been stabilized by deposited TiO2 . Deeper inside the structure coarsening occurred during deposition of TiO2 at 250 ◦ C and proceeded during annealing to 300 ◦ C in the catalytic reactor. Numbers given in the figure denote the average ligament size in the respective area.

By loading NPG with TiO2 by CVD an inverse oxide/metal catalyst with a large surface area was generated upon activation. We regard the oxide loaded NPG as an ideal system to study the concept of inverse oxide/metal catalysts under real and UHV conditions in the future. Further studies on the wetting behavior and particle formation during TTIP growth on Au are currently under way. The possibility to prepare and characterize these loaded catalysts under controlled conditions in UHV prior to the catalytic test in a reactor provides an atomic level perspective of the topic. This perspective has often been overlooked in previous work but is needed to pave the way for guided design of new catalytic systems. It is furthermore to be expected that the approach adopted here for CVD of TiO2 can be extended to a wide range of different oxide materials for which the precursors require vacuum conditions. In summary, we have demonstrated the feasibility of applying metal organic CVD using the TTIP precursor in UHV to load NPG combined with a functional test of the prepared catalyst. With this approach coating of NPG with TiO2 to a depth of the first 400 nm inside the structure could be achieved. Although the first few ␮m of the NPG structure have been found to be the important regions for catalysis [29] already the coating of the first 400 nm led to a significant increase in the catalytic activity. This means that by coating NPG with TiO2 , thinner layers of the catalyst material could be employed while preserving the catalysts performance. Acknowledgments

by SEM of cross sections of the two sample parts are presented in Fig. 5. The image taken right after loading with TiO2 (5a) shows a cross-sectional view of the first 4 ␮m. It can be divided into three parts with different average ligament sizes. The borders between those regions are rather sharp, which is possibly related to transport phenomena in the nanoporous structure [29]. The average ligament diameter in the first 400 nm is around 54 ± 2 nm, which is slightly more than usual values for as prepared NPG [6]. We attribute the increased ligament size in that area to coarsening in the initial stages of the deposition process during which the exchange between AuOx to TiOx takes place. The next 800 nm exhibit an increased ligament diameter of 78 ± 3 nm in average before the last section with a ligament diameter of 132 ± 5 nm starts. Fig. 5b shows a cross section after heating the sample to 300 ◦ C in the catalytic reactor in the CO/O2 gas mixture. The structure in the first 400 nm has not changed; the average ligament diameter is still in the same range (50 ± 2 nm). However, looking deeper inside the sample, the ligament size in the next 800–1200 nm increased by a factor of 1.5 (115 ± 8 nm). Beyond that point, the ligament size increased roughly about the same factor (1.6) to 216 ± 13 nm. The results displayed by the SEM micrographs clearly show that TiOx was deposited inside the first 400 nm of the structure and is stabilizing the NPG to at least 300 ◦ C. Noteworthy is the section with an intermediate coarsening. This could be the result of having a region with less titania deposited but there may also be other effects, e.g. involving diffusion of contaminants, that affect the degree of coarsening. Only minor changes in the mesoscopic structure were found after the reactor experiment. Consequently

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