Reactivity Screening of Anatase TiO2 Nanotubes Array and Anatase

Chapter 11

Reactivity Screening of Anatase TiO Nanotubes Array and Anatase Thin Films: A Surface Chemistry Point of View

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 16, 2015 | http://pubs.acs.org Publication Date: September 19, 2008 | doi: 10.1021/bk-2008-0996.ch011

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S. Funk , B. Hokkanen , T. Nurkic , J. Goering , E . Kadossov , U. Burghaus , A. Ghicov , P. Schmuki , Z. Q. Y u 3,4, S. Thevuthasan , and L . V. Saraf 1,*

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Department of Chemistry, North Dakota State University, Fargo, ND 58105 Department of Materials Science, University of Erlangen-Nuremberg, Erlangen 91058, Germany Pacific Northwest National Laboratory, Richland, WA 99352 Current address: Department of Chemistry, Nanjing Normal University, Nanjing, 210097 China 2

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As a reactivity screening we collected thermal desorption spectroscopy (TDS) data of iso-butane, O , CO , and CO adsorbed on an ordered TiO nanotubes (TiNTs) array. As a reference system iso-butane adsorption on an anatase TiO thin film has been considered as well. The as-grown TiNTs are vertically aligned and amorphous. Polycrystalline (poly.) anatase or poly. anatase/rutile mixed nanotubes are formed by annealing confirmed by x-ray diffraction (XRD) and scanning electron microscopy (SEM). The anatase thin film was grown on SrTiO (001) and characterized by XRD and atomic force microscopy (AFM). Surprisingly, oxygen distinctly interacts with the TiNTs whereas this process is not observed on fully oxidized single crystal rutile TiO (110). Desorption temperatures of 110-150 K and 100-120 K were observed for CO and CO, respectively, on the TiNTs. Variations in the binding energies of the alkanes on TiNTs and anatase thin films also were present, i.e., a kinetic structure-activity relationship (SAR) is evident. 2

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© 2008 American Chemical Society

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In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

140 Introduction Motivation for this Study on Anatase Ti0 Samples 2

Titanium dioxide has a wide range of applications that are as diverse as photo catalysis and cosmetics. T i 0 is used for the design of gas sensors (/), for solar cells (2), batteries (5), water decomposition (4), as a pigment in paints and cosmetics, as a protective coating (5), and as a catalyst for a number of surface reactions including CO oxidation (6) and the selective catalytic reduction (7) of NO . T i 0 exists in three different crystallographic phases. Due to technical difficulties to obtain anatase samples, most surface science studies have focused on rutile TiO (110) although technical titania powder catalysts consist of rutile and anatase crystallites. Furthermore, it has been proposed that the catalytically more active component for a number of reactions in these powders is the anatase polymorph (8). In addition, the surface energy of anatase crystallites is smaller than for rutile (9) consistent with the fact that most T i 0 nanostructures are of the anatase polymorph (10-12). Therefore, anatase thin films also can be considered as the planar counterpart of TiNTs and nanoparticles which has motivated our study to directly compare anatase thin films and TiNTs. Information about what particular crystallographic orientation dominates industrial T i 0 power catalysts is quite diverse. According to a transmission electron microscopy study (75), T i 0 P25 and T i 0 Merck powder catalysts are dominated by (001) and (010) anatase surface planes. Furthermore, it has been proposed that anatase (001) is the active phase for photocatalytic water decomposition (14). Thus, anatase thin films with a (001) surface orientation, as studied here, are relevant for technical applications.


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Ti0 Nanotubes for Catalysis Applications 2

Most projects on TiNTs focus on characterizing their morphology as well as materials science applications such as sensor design and photovoltaics (11,1518). To the best of our knowledge no ultra-high vacuum (UHV) surface chemistry studies have been published. However, the great potential of planar T i 0 for surface science and catalysis applications promises fascinating potential of TiNTs in heterogeneous catalysis. Depending on the preparation procedures, a large variety of TiNT morphologies can be obtained making them a perfect model system. Data have been collected for amorphous, poly, anatase, and poly, mixed 80%-anatase/20%-rutile TiNTs. Nanotubes generally have the following advantages as compared with powder or planar model catalysts: 2

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Large surface-to-volume ratio.



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The micro-porous structure of powder catalysts can lead to transport limitations for surface reactions. Nanotubes minimize this limitation (19).

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Deactivation of catalysts is caused by sintering of the active metal nanoclusters. Trapping metal clusters on/inside the tubes can prevent sintering (20-22).

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Large diffusion coefficients for gas/liquid transport through nanotubes can prevent catalyst poisoning (19).

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High purity avoids self-poisoning and leads to consistent and easily reproducible properties.

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High mechanical and thermal stability.

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Functionalization allows for catalyst tailoring.

Importance of Oxygen Adsorption The adsorption of oxygen is an important elementary reaction step for a large number of catalytic processes as well as for materials science applications. •

The gas-surface interaction of 0 with nanotubes has been studied as a potential technique for 0 -doping of electronic devices (23). 2

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Improvements to catalytic processes for the decomposition of chemical warfare agents and pesticides are urgently needed (24). One promising strategy is catalytic oxidation. Efficient catalysts useful in remote locations would need to operate at ambient conditions, to require only oxygen (air), to be a non-toxic catalyst (Ti0 ), and would eventually need a light source (sunlight). TiNTs are a promising candidate. 2

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One of the most prominent application for T i 0 is the Au-Ti0 , low temperature CO oxidation catalyst that can be used, for example, for exhaust emission reduction (6,25-27). An important reaction step is the adsorption of oxygen.

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T i 0 is important for PROX catalysts (preferential oxidation of CO in the presence of excess hydrogen) with the adsorption of oxygen as the primary reaction step (28). PROX is important for lean-burn gasoline engines (29).

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The removal of CO from syngas is required for fuel cell applications since trace amounts of CO can poison the surface of fuel cell electrodes (30). More efficient CO oxidation catalysts are required; for all of them, oxygen adsorption is one of the first reaction steps in the oxidation mechanism. A number of more exotic but important applications exist, such as closedcycle C 0 lasers (57).

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Why Studying Alkane Adsorption? The physicochemical properties of hydrocarbons have been studied extensively in surface science due to their importance for the petroleum industry as building blocks to convert raw materials into chemicals of greater value. Therefore, TDS studies characterizing the adsorption kinetics of alkanes have been conducted for a large variety of single crystal surfaces including graphite (52), sapphire (55), ZnO (34), Pt (35), Ru (36), Cu (37,38), and carbon nanotubes (12). Typically molecular and non-activated adsorption is obeyed. Analyzing TDS data reveals kinetics parameters such as coverage dependent binding energies and pre-exponential factors required for microkinetics modeling supporting catalyst development. At low adsorption temperatures, condensation in bi- and multilayers leads to additional TDS features following 0 order kinetics. th

Experimental Procedures Synthesis of the T i 0 Nanotubes Array (TiNTs) 2

At the laboratory at Erlangen-Nuernberg University TiNT layers were produced by anodization of titanium foils. The foils were sonicated in acetone, isopropanol, and methanol, followed by rinsing with deionized water (DI) and drying in a N stream. A three-electrode cell with a Haber-Luggin capillary, an Ag/AgCl (1M KC1) reference electrode, and platinum gauze as a counter electrode was used for the anodization. The electrolyte was 1M (NaH P0 ) + 0.5%w.t. HF (pH 4.5) and anodization was carried out at 300 K by ramping the potential from open circuit potential to 20 V (500 mV/s) and holding the potential at 20 V for 2 h. Afterwards, the samples were rinsed with DI and dried in a N stream. Thermal treatments (temperature ramping from RT to 750 K at a heating/cooling rate of 30 K/min) of the initially amorphous TiNTs layers were carried out. After the ramp-up, the temperature of 750 K was held for 1 h. This procedure converts the initially amorphous nanotubes into polycrystalline anatase TiNTs on an oxidized Ti foil. Further annealing treatments result in mixed anatase/rutile TiNTs. XRD, SEM, and x-ray photoelectron spectroscopy (XPS) have been used to characterize the sample morphology and chemical cleanliness. 2

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Growth of Anatase Thin Films The film was grown in a custom-designed ultrahigh vacuum (UHV) chamber at PNNL as described in detail in ref.(5P). Reflection high energy electron


143 diffraction (RHEED) was used to monitor the growth process. Single-crystal STO(OOl) substrates were ultrasonically cleaned in acetone and methanol prior to insertion into the UHV system through a load lock. In the UHV chamber, the substrates were cleaned using oxygen plasma (l.OxlO torr oxygen partial pressure, 200 W power) with a sample temperature of 900 K. During the growth, Ti was evaporated from an effusion cell in the presence of the oxygen plasma, with the substrate kept at a temperature at 900 K. The growth rate of the film was 0.01 nm/min and it was monitored by a quartz-crystal oscillator (Inficon). X-ray diffraction (XRD) and x-ray reflectivity (XRR) measurements were carried out using Philips X'pert 0-20 diffractometer at 40 kV and 50 mA followed by data analysis using JADE 6.0 program.


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Kinetics Experiments The kinetics experiments have been conducted at North Dakota State University (NDSU) in two different UHV systems; in a TDS chamber (34) (for the TiNTs project) and in a molecular beam scattering system (for the thin film project) (40). For TDS a number of precautions have been taken to assure that the results are not obscured by artifacts. (1) The mass spectrometer is equipped with an aperture and the sample to detector distance amounts to one mm. (2) The TiNTs samples were either mounted on a tantalum plate or directly spot welded on Ta wires. No significant differences in TDS curves have been observed for the two different setups. (3) No bulky parts have been used in designing the sample holder resulting in good cooling performance and negligible background contributions. The reading of the thermocouple has been calibrated in situ by TDS using the known heat of condensation of the alkanes. The uncertainty in the temperature reading amounts to ±5 K. The TiNTs have been cleaned by outgassing in UHV; the anatase thin films by mild sputter-anneal cycles. The anatase thin film sample has been further characterized by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). Molecular beam scattering data have also been collected for the anatase film but will be presented elsewhere (12,41).

Sample Characterization XRD. A l l samples have been characterized by XRD as summarized in Figure 1. For the amorphous TiNTs only the Ti substrate peaks are present whereas the polycrystalline TiNTs show the characteristic anatase XRD lines. Similarly, XRD of the anatase thin film sample consists of a superposition of


144 TiSrO (001) subtrate and anatase(OOl) structures. The XRD results clearly demonstrate that a high quality well oriented anatase T i 0 film has been grown. The thickness of the film was determined to be 123 nm using XRR. TiNTs. Additionally, the TiNT samples have been characterized by SEM and XPS. Figure 2 shows a SEM top view of a typical TiNT sample. Studied here have been TiNTs with a diameter of 0.1 fim and a length of 1 |um. The asgrown TiNTs show fluorine impurities (Figure 3) in XPS which can, however, be removed by annealing the samples. No desorption of fluorine has been detected in TDS after degassing the samples in UHV. Thinfilm.A F M results for the anatase thin film are shown in Figure 4. On a lateral resolution scale of 200 nm, the films appear rather defect free. However, AES data indicate a nonstoichiometic sample (Ti-to-O-AES ratio of one); no LEED pattern could be observed. Some anatase particles were observed with A F M on the surface. Since the RHEED results during growth clearly showed streaks throughout the film growth, we believe the particles are well isolated. In addition, since we did not observe any other peaks except the TiO (001) peak in the XRD spectrum from the film, the particles appear to be oriented along the same direction as the film. 3


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Kinetics of Oxygen Adsorption on T\Oi Nanotubes A detailed outline of oxygen adsorption experiments on TiNTs will be presented elsewhere (72). Briefly, the following data have been gathered. Oxygen distinctly interacts with the TiNTs whereas this process is not observed on fully oxidized rutile T i 0 single crystals (42). Thus, TiNTs show intrinsically a fundamentally different catalytic behavior than T i 0 single crystal model systems. Both molecularly and atomically bonded oxygen have been observed. In the case of the mixed polycrystalline anatase/rutile TiNTs, TDS peak shifts to lower temperatures with increasing exposure (at 90 K) indicate a deviation from l -order kinetics. Additionally, a competing dissociative adsorption pathway for oxygen was evident at low adsorption temperatures. For the amorphous TiNTs at low adsorption temperatures, molecular adsorption/desorption kinetics was present with a binding energy of 30.5 kJ/mol (pre-exponential lxl0 /sec). Dissociative adsorption was observed above adsorption temperatures of -150 K with a TDS peak just below 300 K. For applications in heterogeneous catalysis the amorphous system may be better suited since oxygen adsorbs close to room temperature which will be important for oxidations of toxic compounds. For low-temperature reactions, the larger adsorption probability of 0 observed on the polycrystalline TiNTs will be advantageous. 2

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-i—'—i—•—i—•—i—•—i—•—r-

poly, anatase TiNTs


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anatase thin film

amorphous TiNTs

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Figure 1. XRD scans of the samples studied.

Figure 2. SEM of TiNTs.



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binding energy (eV) Figure 3. XPS of TiNTs (as prepared).

Figure 4. AFM of an anatase thin film grown on SrTiO (001). 2


147 Adsorption of Iso-butane on TiNTs and Anatase Thin Films The adsorption of alkanes has been studied on amorphous, poly, anatase, and poly, mixed anatase/rutile TiNTs as well as on an anatase(OOl) thin film sample. Comprehensive data sets and a more detailed modeling will be presented elsewhere (10, 41). The most interesting result obtained is a clear kinetic structure activity relationship (SAR), i.e., the binding energy of iso-butane depends on the polymorph of Ti0 . Typical examples of TDS curves are shown in Figure 5. Data for small (0.1 L) and large (1 L) exposures of iso-butane are depicted. (1 L = 1 sec gas exposure at lxl0" torr.) The arrows in Figure 5 highlight the TDS peak position for small exposures which clearly depends on the particular sample studied. The reactivity decreases as anatase thin film > amorphous TiNTs > poly, anatase TiNTs > poly, mixed anatase/rutile TiNTs with zero coverage binding energies of E = 52, 40, 39, and 34 kJ/mol (v = lxl0 /s), respectively. Fitting (43, 44) the TDS data (10, 41) revealed the coverage dependence of the binding energy which follows the same trend. It appears that the anatase system is indeed the most reactive towards adsorption of iso-butane since in this case the largest binding energies are obtained. Although, we cannot provide an atomistic explanation, this result is intriguing concerning technical applications. The differences in the thin film data and the TiNTs may be strongly related with the density of surface defects which appears to be largest for the anatase thin film sample studied here. The TDS peaks shift to lower desorption temperatures with increasing coverage (consistent with repulsive lateral interactions and molecular adsorption kinetics) for all samples studied. Interestingly, also for the mixed TiNTs only one TDS feature is present in the monolayer coverage range suggesting a random mixture of anatase and rutile crystallites. At large exposures (not shown), common condensation peaks have been observed allowing for a precise calibration of the temperature reading.


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C O and C 0 Adsorption on TiNTs 2

TDS data of C 0 have been collected for the amorphous and poly, mixed TiNTs (Figure 6). The binding energies are systematically larger for the mixed TiNTs with E =35 kJ/mol. However, the deviations are smaller than 3 kJ/mol. The width of the TDS peaks is large indicating a number of kinetically distinct adsorption sites, for example, along the grain boundaries of the crystallites. Therefore it appears plausible that no pronounced defect TDS features are present as observed for rutile single crystals (45-47). CO binding energies of E = 30 and 38 kJ/mol were measured for amorphous and poly, anatase TiNTs, respectively. 2

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Figure 5. TDS of iso-butane for all samples studied.


In Nanoparticles: Synthesis, Stabilization, Passivation, and Functionalization; Nagarajan, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008. 2

Figure 6. C0 TDS as a function of exposure, as indicated.


150 Acknowledgements


Financial support by the DoE-EPSCoR (DE-FG02-06ER46292) and from ND NSF-EPSCoR IIP seed (EPS-047679) is acknowledged as well as by the German DFG (SPP). Work was also performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DoE.

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Reactivity Screening of Anatase TiO2 Nanotubes Array and Anatase

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