NANO LETTERS
Preparation of TiO2 Nanocrystallites by Oxidation of Ti-Au(111) Surface Alloy
2009 Vol. 9, No. 6 2378-2383
Denis V. Potapenko and Richard M. Osgood* Department of Applied Physics and Applied Mathematics, Columbia UniVersity, New York, New York 10027 Received March 20, 2009; Revised Manuscript Received April 23, 2009
ABSTRACT Ti-Au surface alloy oxidation is used to form nanocrystals of TiO2 on Au(111). In situ scanning tunneling microscopy (STM) studies show that the approach yields arrays of 8-11 nm wide crystals with relatively narrow size dispersion and uniform crystallography. STM imaging shows that their crystallographic form is rutile with a triangular or hexagonal geometry. Scanning tunneling spectroscopy indicates that the crystals have a well-developed band gap, comparable to that in bulk TiO2.
Titanium dioxide is a semiconducting oxide of continuing interest for applications in solar-energy conversion and for environmental chemistry. As a result, its thermal chemistry and photochemistry are still the subject of much research.1,2 Ultrahigh vacuum (UHV) studies are an important phase of this research due to the fact that this approach can in principle yield well-defined surfaces and precise adsorbate conditions. Prior studies have included those directed toward the chemistry of TiO2 nanoparticles since they offer high surface area and, if sufficiently small, the potential of new and, possibly, enhanced chemical properties. For these studies, the Au(111) surface is an attractive choice of substrate due to its ease of handling, its chemical inertness, and the fact that its “herringbone” surface reconstruction may serve as a well-defined template for growth of nanoparticles. While wet-chemical methods are available for preparation of nanoparticles, these methods are not particularly useful in UHV studies because of difficulties related to full removal of the organic terminating groups from the surface of the particles and limited control of the nanoparticles’ arrangement on a substrate.3 As a result, a variety of new in situ techniques have emerged for preparation of nanometer-scale TiO2 particles. In one method, Ti was evaporated onto a Au(111) substrate in an oxygen atmosphere followed by annealing to 900 K. The resulting TiO2 crystallites showed a variety of shapes and structures with a characteristic size of 5 nm.4 At least three distinct structures were identified: needle-like, two-dimensional hexagon-shaped, and threedimensional triangular crystallites. In another approach, termed reactive-layer-assisted deposition (RLAD), titanium vapor was deposited on adsorbed multilayers of an oxygen-containing compound, such as H2O * Corresponding author. Tel.: 212-854-4462. Fax: 212-854-1909. E-mail:
[email protected]. 10.1021/nl900904s CCC: $40.75 Published on Web 05/07/2009
2009 American Chemical Society
and NO2, and the sample subsequently annealed at a higher temperature to remove the unreacted compound and form TiO2 crystallites.5,6 Specifically, in the case of a NO2 reactive layer, the resulting crystallites generally had a hexagonal symmetry with an atomic structure, which occasionally varied even within a single nanoparticle; in this case, regions of rutile and anatase structure were seen.6 On the other hand, when H2O was used as a reactive layer, 5-20 nm wide nanocrystallites of at least three distinct morphologies were formed.5 The majority of the crystallitessabout 70%shad a hexagonal geometry; the atomic structure of these nanocrystals was identified as rutile with a (100) face parallel to the substrate. The remainder of the crystallites had either a three-dimensional ridgelike morphology, also identified with rutile, or a structure consisting of nanosheets of octagonshaped TiO2.5 In general, each of the above methods of preparation of titania nanocrystals on Au(111) surface offer only limited control over the morphology and atomic structure of an ensemble of the crystallites. This limitation becomes an important issue for studies of crystal chemistry, which averages over the ensemble, when using typical large-area techniques such as temperature-programmed desorption, synchrotron radiation, or mass spectrometry studies of photoirradiation. In these cases, the clear interpretation of the measurements would require a high degree of structural homogeneity of crystallites. In the present work, we explore the formation of TiO2 nanocrystals using high-temperature treatment of a Ti-Au surface alloy on a Au(111) substrate in an oxygen atmosphere. We demonstrate that, compared to the other methods, such as those described above, surfacealloy oxidation brings superior results in terms of both greater nanoparticle structural homogeneity and a narrower size distribution.
Figure 1. Evolution of the ratio of Ti 387 eV and Au 69 eV AES signals resulted from consequential 5 min anneals of the sample to the indicated temperatures.
The experiments were performed in a customized UHV chamber,5 which was equipped with an Omicron VT-STM and a LEED/Auger system, sputtering gun, custom-built directed gas doser, and titanium-evaporation source. The base pressure in the STM chamber was 4 × 10-11 Torr. All STM images were acquired at room temperature using W tips; these tips were prepared by a drop-off technique described elsewhere.7 Each newly introduced tip was cleaned through heating by electron field emission at ∼40 µA. A pristine Au(111) surface was prepared by cycles of Ar+ sputtering (1 keV) and annealing at 900 K. Titanium was physicalvapor deposited onto the Au surface from a titanium-wrapped tungsten filament heated by direct current. Our Ti coverages were calculated from STM images of the surfaces before any annealing to temperatures above 350 K. To obtain each scanning tunneling spectroscopy (STS) spectrum, the tip was positioned over the area of interest, and the electronic feedback loop was then turned off while the sample bias voltage was swept from -3 to +3 V. The Itun (V) signal was recorded during the sweep. dItun/dV data presented here were obtained by numerical differentiation of Itun (V). At least five STS spectra were recorded at each point of interest to ensure consistency. Note that the STS I(V) data reflect equally the electronic properties of the sample and the tip.8 The atomic arrangement on the STM tip frequently changed due to occasional tip crashes. For this reason each spectrum presented in this work is shown together with a reference spectrum from a clean Au(111) surface obtained during the same experimental session. Figure 1 shows the change in the Ti surface concentration on Au(111) as a function of annealing temperature. Initially, 0.7 ML of Ti were deposited on the surface at 200 K, and the sample was then annealed stepwise for 5 min at the temperatures shown on the x-axis. After each annealing step, the sample was cooled to room temperature (or to 200 K for the first three points) and an Auger electron spectroscopy (AES) spectrum was taken. The ratio of Ti 387 eV and Au 69 eV signals for each spectrum is plotted in Figure 1. As seen in the figure, the surface concentration decreased with increasing annealing temperature, as expected for the entropydriven migration of Ti into the bulk of the gold crystal. Nano Lett., Vol. 9, No. 6, 2009
Figure 2. 80 × 80 nm STM image of Au(111) surface with 0.2 ML of Ti vapor deposited while the sample was kept at 250 K. The image contrast in the vicinity the substrate is exaggerated to demonstrate the surface reconstruction.
However, even after annealing at 900 K a Ti Auger signal was still detectable, indicating that titanium had not completely dissolved in the bulk at this temperature. According to the Au-Ti phase diagram9 titanium readily alloys with gold forming a number of binary phases including TiAu4, TiAu2, TiAu, etc. It would be expected, therefore, that a small surface concentration of Ti would be fully dissolved in the bulk of the Au sample once the temperature was high enough to activate the mobility of Ti atoms in the gold crystal lattice. The above AES observation that Ti still remains near the surface after a 900 K anneal suggests the existence of a thermodynamic driving force that favors subsurface localization of Ti. This conclusion is also supported by experimental observation of Ti surface segregation in a bulk Ti (4%)-Au alloy.10 This phenomenon may be similar in nature to the formation of near-surface Cu-Pt alloy on Pt(111)11 and in cases of some other bimetallic systems, as has also been predicted theoretically.12 Figure 2 shows room-temperature STM image of a Au(111) surface obtained at a relatively low coverage of 0.2 ML of Ti, deposited while the surfaces were kept ∼250 K, and then subsequently annealed at 350 K. As shown in Figure 2, after annealing titanium forms compact clusters arranged in rows due to preferential nucleation at the elbows of the underlying “herringbone” reconstruction on the Au(111) surface, a phenomena observed in a previous study of Ti/ Au(111).4 Many other metals including Co,13 Rh,14 Mn,15 Ru,16 Mo,17,18 Ni,19 and Fe20 have also been reported to show preferential or exclusive nucleation at the elbow sites of the reconstructed Au(111) surface during physical vapor deposition (PVD). The Ti islands in Figure 2 have an apparent height of 0.24 nm. Note, that the island’s height as measured via STM cannot alone be used to distinguish gold versus titanium because even in pure elemental forms the heights of a single close-packed layers are 0.236 and 0.242 nm, respectively,21,22 values that are too close for differentiation in our experiments. Figure 3a presents the results of the STS experiments conducted on a surface prepared by deposition of Ti on a room temperature Au(111) substratessimilar to that shown 2379
Figure 3. Representative differentiated STS spectra (dI/dV) recorded at various positions on different stages of surface preparation: (a) Ti deposited on Au(111) at room temperature; (b) the same sample annealed at 600 K; (c) the same sample annealed in O2 at 900 K.
in Figure 2. These spectra were measured over the voltage range from -3 to +3 V at two types of locationssat the substrate and at the top faces of the clusters. At least five spectra were recorded at each location type. Deviations within each group of spectra were significantly lower compared to the difference between the two groups, illustrated by the most representative spectra in Figure 3a. Specifically, the dI/dV values for bare regions of Au(111) were more than 2 times higher, for all values of negativebias voltage, and slightly lower for voltages greater than +2 V compared to the dI/dV signals obtained on Ti (or Ti-rich) clusters at the same values of bias voltage. These STS results are consistent with the known electronic band structure of Au and Ti.23 In particular, gold has a high density of states (DOS) bands starting below -1.7 V from the Fermi level, while titanium has a relatively symmetric DOS distribution with respect to the Fermi level. In interpreting these results it is important to point out that metal-gold surface alloying has been reported to occur for submonolayer Ti coverages on Au(111) substrates, at temperatures even below room temperature.14 The first three points of the AES experiment in Figure 1 also suggest the formation of titanium-gold alloy as the sample temperature is raised from 200 to 300 K. However, the STS measurements, discussed above, show that this alloying does not result in equal Ti concentration in the islands and the substrate. Even if Ti-Au alloying proceeds to some extent, the clusters, which are formed, are sufficiently Ti-rich to show different electronic structure than that of the substrate. To find conditions, at which complete dissolution of titanium in the gold substrate is achieved, experiments were carried out to investigate annealing to higher temperatures. Thus, Figure 4 shows STM images of Au(111) surfaces containing 0.4 ML of Ti, which were subsequently annealed at 600 K. The images show a small extent of redistribution of the surface islands that suggests surface Oswald ripening, i.e., lateral growth of larger islands at expense of the smaller ones. As a result some islands appear to have coalesced, while elsewhere islands on terraces in the close proximity to step edges have been depleted. However, no large-scale transport to or from the step edges is seen in the images, as is apparent from the absence of etch bays or terrace extensions after annealing.24 In addition, a comparison of fractional areas of the islands on large terraces before and after annealing shows no significant change as a result of 2380
Figure 4. 100 × 100 nm STM image of Au(111) surface with 0.4 ML of Ti deposited at RT followed by anneal to 600 K for 5 min.
the annealing. For example, in the case of the experiment with 0.4 ML Ti, the island fractional areas were calculated as 40 and 43% before and after annealing while in the 0.2 ML experiment the corresponding numbers were 21 and 15%. However, STS measurements on this surface did show a significant change in composition of the islands after this same 600 K annealing. The dI/dV scans over the islands and the substrate areas plotted in Figure 3b are practically indistinguishable. This observation indicates that the composition of the islands and substrate are, within the uncertainty of the method, the same, a result, which is consistent with extensive Ti-Au alloying or intermixing. Cumulatively our data presented in Figures 3a and 4 show that neither the total area of the islands nor the outline of the step edges changes during intermixing of Ti with Au. STS results also show that the composition of the intermixed system is uniform across the full surface and its features. These results suggest that the predominant mechanism of Au-Ti alloying on the Au(111) surface is direct atomic exchange between clusters and substrate; no separate stoichiometric Ti-Au phase was formed during the alloying process. Such a mechanism is different from many metalgold interaction structures formed on Au(111). For example, Mo on Au(111) has been shown to draw gold from the steps to form Mo-Au conglomerates.24 In the case of Co on Au(111), Co clusters are buried into the gold substrate by Nano Lett., Vol. 9, No. 6, 2009
Figure 5. 200 × 200 nm STM image of the surface shown in Figure 3 (with 0.4 ML of Ti) after annealing to 870 K for 5 min. The image contrast in the vicinity of the substrate is exaggerated to demonstrate the surface reconstruction.
expelling gold atoms that form either large islands or extend terraces.13 Finally for Ce on Au(111), cerium alloys with the gold but, unlike titanium, forms characteristic islands of stable stoichiometric surface phases.25 In the case of Ti on Au, the distinctive characteristic is that titanium and gold have interatomic distances that cause Ti-Au alloys to have a structure and atomic density very close to that of pure gold. For example, in going from pure gold through a Ti solid solution in Au, through TiAu4 to TiAu2 stoichiometric compounds the atomic density of the material changes from 59.0 to 60.1 atoms per nm3 based on the Au-Ti alloy crystallographic data.9 Note that this fact justifies our earlier mentioned method of calculating Ti coverages from STM images; namely, it is reasonable to assume that the fractional area of Ti islands can serve as a measure of Ti coverage even if considerable Ti-Au atomic interchange has occurred by 300 K. Annealing of the sample to higher temperatures was found to lead to more substantial merging of the Ti-Au islands. Figure 5 shows a surface with 0.4 ML of Ti after annealing to 870 K for 5 min, which contains large islands with characteristic 50-100 nm dimensions distributed on the surface. In a separate experiment with 0.7 ML of Ti, which was also annealed to 900 K for 5 min, some islands merged with the step edges to form terrace extensions (not shown). In cases when the islands were distinguishable, the total island fractional area was still approximately equal to the original Ti coverage. For example, the fractional area of the islands in Figure 6a is 35% for 0.4 ML of Ti. Finally, the surfaces of both the islands and the substrate show a very distorted herringbone reconstruction. The distortion is most likely due to the presence of impurity clusters, which are discussed in detail in a separate paper. The process of oxidation of this surface was then examined in order to investigate possible formation of TiO2 nanoparticles. Oxidation was carried out by exposing Au(111) samples after surface alloying to a flux of molecular oxygen at elevated temperatures. The alloyed samples were prepared by vapor deposition of Ti on the sample at ∼300 K and subsequently annealed to 900 K for 5 min. STM imaging Nano Lett., Vol. 9, No. 6, 2009
Figure 6. (a) 80 × 80 nm STM image of Ti-Au alloy surface with 0.7 ML of Ti, oxidized in O2 at 900 K and cooled to RT. (b) Histogram of linear dimensions of individual nanocrystallites in the STM image and (c) histogram of heights of the data points.
was carried out at room temperature after oxidation and the oxygen exposure was found to cause uniform growth of flat crystallites across the surface. In particular, Figure 6a show surfaces prepared with 0.7 Ti coverage, annealed, and then exposed to ∼500 langmuirs of O2 over 10 min, while the sample remained at 900 K. The crystal nature of such particles has been examined in several previous studies of the growth of nanoscale TiO2.4-6 The STM image in Figure 6a reveals that virtually all crystallites have either a triagonal or a hexagonal shape, with flat-top faces parallel to the surface. A statistical analysis of the sizes of the crystallites in a 100 × 100 nm STM image of the same surface as in Figure 6a yields a relatively narrow size distribution centered at 9.5 nm, as illustrated by the histogram in Figure 6b. The plot given in Figure 6c displays the distribution of heights for the data points of Figure 6a. The histogram shows a large peak at 0 nm corresponding to measurement on the substrate, followed by a series of peaks along the x-axis at intervals of 0.23 nm. These peaks originate from the flat-top faces of the hexagonal/triangular crystallites in the STM image. The histogram shows that the lowest 2381
apparent height of these TiO2 crystallites is 0.60 nm, while the height of thicker crystallites increases in units of 0.23 nm; such a progression implies that a single structural layer for these crystallites has 0.23 nm thickness and up to 5 additional TiO2 layers could grow on top of a 0.6 nm base at our experimental conditions. This quantum of height was also observed in our previous work for the “hexagonal” type of titania crystallites grown by the RLAD method.5 In that prior work the atomic structure of the “hexagonal” crystallites was identified as rutile with a (100) plane parallel to the surface of the substrate. The measured thickness of a single structural layer was the primary basis of such identification, since among stable bulk TiO2 phases (rutile and anatase) only a rutile interlayer distance of 0.229 nm along the [100] direction is close to the 0.23 nm observed in our experiments. Atomic rows on the top faces of the crystallites, with a spacing consistent with the expected row spacing for rutile(100) surface, provided additional evidence supporting this conclusion.5 Although in the present work the atomic structure of the top faces of the crystallites has not been resolved, the identical single-layer height and similar overall hexagonal symmetry of the crystallites allow us to identify the majority (about 95%) of the crystallites in Figure 6a with the “hexagonal” crystallites from the earlier work and, thus, with rutile atomic structure. The remaining 5% of the nanocrystals seen in area scans such as shown in Figure 6a resemble “three-dimensional” crystallites also seen in the same prior work: these were ridgelike structures, identified from their geometry also as rutile TiO2 but with two (110)-like faces making the two slopes of the ridge. The calculations based on the total volume of the crystallites in Figure 6 and on the known density of bulk TiO2 show that the total amount of Ti in the crystallites corresponds to 0.46 ML, i.e., 65% of the original amount of deposited titanium returned to the surface after alloying with Au followed by oxidation. This observation again supports our hypothesis that most of the titanium remains in the vicinity of the surface after alloying with gold. The STS spectra collected over a set of crystallites grown with 0.7 ML of Ti demonstrate that the TiO2 nanocrystals have nearly identical electronic structure. The representative STS spectrum of a TiO2 nanocrystal in Figure 3c shows a band gap in the electronic states generally consistent with the bulk rutile band gap value of ∼3.0 eV.26 The center of the band gap is shifted toward a negative sample bias, which signifies an n-type semiconductor. Reduced bulk TiO2 is known to be an n-type semiconductor due to presence of +3 titanium ions that act as electron donors: Ti3+ f Ti4+ + e-. In conclusion, the method of TiO2 nanocrystals growth from Au-Ti surface alloy, described in the present paper, gives more satisfactory nanocrystallites compared to other methods reported in the literature in terms of increased structural homogeneity and narrower size distribution. O2 oxidation of Ti deposited on Au(111) surface without preliminary thermal dissolution of titanium in gold leads to formation of titania crystallites of generally hexagonal 2382
symmetry, similar to our work.4,27 However, these crystallites appear to have had more irregular shapes and wider size distribution than the crystallites reported here. Reactive-layerassisted deposition (RLAD) methods also provided inferior homogeneity of the crystallites in comparison to those obtained here. In particular, when NO2 was used as a reactive layer, the resulting crystallites looked irregular in shape, although clearly with remnant hexagonal symmetry. In addition, in this case, the apparent atomic structure varied from one crystallite to another and even, in some instances, within single crystallites.6 With H2O as a reactive layer, a relatively narrow nanocrystallite size distribution could be achieved but at least three different crystallite structures were observed including the nanosheet TiO2 structure with octagonal symmetry, not observed in the present work.5 The fraction of the hexagonal crystallitessthe major crystallite structuresin H2O RLAD work was 60-75% versus ∼95% in the present study. Finally, the observations presented above allow us to offer a more general tentative conclusion regarding the relationship between the nanocrystallite geometries produced by different procedures and their growth mechanisms. In particular, in all the methods listed above, the final step of crystallite formation involved redistribution of TiO2 on the Au(111) surface in forms of clusters. Therefore, the shape and structure of the crystallites were influenced by the structures that the TiO2 clusters had assumed in the previous steps of the procedure. In contrast, growth of the crystallites from Ti-Au surface alloy proceeds through continuous supply of TiO2 produced via chemical interaction of Ti atoms emerging from the bulk with the O2 flux bombarding the sample. Thus in this procedure pre-existing surface structures cannot interfere with the growth process. Acknowledgment. R. Osgood and D. Potapenko gratefully acknowledge support from the U.S. Department of Energy, Contract No. DE-FG02-90ER14104. We wish to thank Nader Zaki for help with the experimental setup and for helpful discussions. In addition, we are indebted to Jan Hrbek for discussions of this work and for encouragement in the early stages of the experiments. References (1) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (2) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (3) Datar, S.; Kumar, P. M.; Sastry, M.; Dharmadhikari, C. V. Colloids Surf., A 2004, 232, 11. (4) Biener, J.; Farfan-Arribas, E.; Biener, M.; Friend, C. M.; Madix, R. J. J. Chem. Phys. 2005, 123, 094705. (5) Potapenko, D. V.; Hrbek, J.; Osgood, R. M. ACS Nano 2008, 2, 1353. (6) Song, Z.; Hrbek, J.; Osgood, R. Nano Lett. 2005, 5, 1327. (7) Muller, A. D.; Muller, F.; Hietschold, M.; Demming, F.; Jersch, J.; Dickmann, K. ReV. Sci. Instrum. 1999, 70, 3970. (8) Chen, C. J. Introduction to Scanning Tunneling Microscopy; Oxford University Press: Oxford, 2007. (9) Murray, J. L. J. Phase Equilib. 1983, 4, 278. (10) Viljoen, P. E.; Roux, J. P. Vacuum 1990, 41, 1746. (11) Knudsen, J.; Nilekar, A. U.; Vang, R. T.; Schnadt, J.; Kunkes, E. L.; Dumesic, J. A.; Mavrikakis, M.; Besenbacher, F. J. Am. Chem. Soc. 2007, 129, 6485. (12) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810. (13) Padovani, S.; Scheurer, F.; Bucher, J. P. Europhys. Lett. 1999, 45, 327. Nano Lett., Vol. 9, No. 6, 2009
(14) Chado, I.; Scheurer, F.; Bucher, J. P. Phys. ReV. B 2001, 6409. (15) Fonin, M.; Dedkov, Y. S.; Rudiger, U.; Guntherodt, G. Surf. Sci. 2003, 529, L275. (16) Cai, T. H.; Song, Z.; Rodriguez, J. A.; Hrbek, J. J. Am. Chem. Soc. 2004, 126, 8886. (17) Biener, M. M.; Biener, J.; Schalek, R.; Friend, C. M. J. Chem. Phys. 2004, 121, 12010. (18) Helveg, S.; Lauritsen, J. V.; Laegsgaard, E.; Stensgaard, I.; Norskov, J. K.; Clausen, B. S.; Topsoe, H.; Besenbacher, F. Phys. ReV. Lett. 2000, 84, 951. (19) Chambliss, D. D.; Wilson, R. J.; Chiang, S. Phys. ReV. Lett. 1991, 66, 1721. (20) Stroscio, J. A.; Pierce, D. T.; Dragoset, R. A.; First, P. N. J. Vac. Sci. Technol., A 1992, 10, 1981.
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(21) Maeland, A.; Flanagan, T. B. Can. J. Phys. 1964, 42, 2364. (22) Pawar, R. R.; Deshpand, V. T. Acta Crystallogr., Sect. A 1968, A24, 316. (23) Papaconstantopoulos, D. A. Handbook of the Band Structure of Elemental Solids; Plenum Press: New York, 1986. (24) Potapenko, D. V.; Horn, J. M.; Beuhler, R. J.; Song, Z.; White, M. G. Surf. Sci. 2005, 574, 244. (25) Ma, S.; Zhao, X.; Rodriguez, J. A.; Hrbek, J. J. Phys. Chem. C 2007, 111, 3685. (26) Pascual, J.; Camassel, J.; Mathieu, H. Phys. ReV. B 1978, 18, 5606. (27) Farfan-Arribas, E.; Biener, J.; Friend, C. M.; Madix, R. J. Surf. Sci. 2005, 591, 1.
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