The sodium order-disorder transition on the NaxWO3 (100) single crystal surface

Surface Science 110 (1981) 217-226 North-Holland Publishing Company

217

THE SODIUM ORDER-DISORDER TRANSITION ON THE NaxWO3 (100) SINGLE CRYSTAL SURFACE C.J. SCHRAMM, Jr., M.A. LANGELL * and S.L. BERNASEK Department of Chemtstry, Princeton Umverslty, Prmceton, New Jersey 08544, USA Received 24 February 1981 ; accepted for pubhcatlon 4 May 1981

An order-disorder transition revolving mobile sodmm on the (100) surface of single crystal NaxWO3 has been observed using low energy electron diffraction The kinetics of this transmon have been investigated by following the intensity and size of diffraction features assocmted with ordered sodium on the surface The activation energy for diffusion of sodium on the (100) surface of NaxWO3 is determined to be 4 kcal/mole.

1. Introduction The physical properties o f a material, including chemical reactivity and catalytic selectivity, ultimately depend upon the type and the arrangement of the constituent elements at the solid surface. Although extensive studies of metal surfaces by ultra high vacuum (UHV) techniques have been made [1], UHV studies of transition metal compounds are more limited [2]. Multlcomponent transition metal compounds are perhaps more appropriate than pure metals, as model systems for industrial heterogeneous catalysts, and actual material applications. The ABO3 perovskl.te-type oxides are of particular importance both in terms of their own catalytic actwlty and as useful prototypes for other more comphcated multlcomponent oxide compounds. The effects of third components on catalytic activity have been studied previously m less well defined systems, such as hthmm ion doped nickel oxide catalysts [3]. The sodmm tungsten bronze (NaxWO3)-tungsten trioxide (WO3) system has been the subject of studies directed toward determining the nature of the metal oxide surface and the effect a third component will have on the surface properties. Initial studies of the NaxWO3-WO3 system involved the surface characterization o f the cubic sodium tungsten bronze (100) single crystal surface [4]. These studies were extended to the investigation of the tungsten trioxide single crystal surface * Present address Molecular and Materials Research Dlvasion, Lawrence Berkeley Laboratory, Umversity of Cahfornla, Berkeley, Cahforma 94720, USA. 0 0 3 9 - 6 0 2 8 / 8 1 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © 1981 North-Holland

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C J Schramm et al. /Sodturn order-dtsorder transttton on NaxWO 3 (100)

[5] and the high energy electron loss spectroscopy of WO3(100) and NaxWOs(100) single crystal surfaces [6]. In this paper we present a detailed investigation of the surface sodium order-disorder transition observed at elevated temperatures Clean oxygen annealed Nao.sWO3 (100) crystals produce sharp low energy electron diffraction (LEED) patterns with cubic symmetry in agreement with simple bulk termination at the crystal surface [7] The crystal as believed to form in a sodium oxygen outerlayer [4] with the surface sodium m ordered rows spaced at twice the unit cell distance. The half order features of the (2 X 1) LEED pattern Indicate an ordering of surface sodium into (2 × 1) rows at room temperature. Studies of the kinetics of mobile species in ABO3 perovsklte-type oxides may have significant impact on various material apphcations. Mobile oxygen ions are an important specie in some Industrial multicomponent metal oxide catalysts [8]. Mobile species may act as catalytic promoters, facilitating a reaction by mediating between the surface and bulk. Such catalysts will be very sensitive to poisons which bind at empty sites necessary for ionic moblhty. Even more pertinent to the present study IS the Importance of mobile hydrogen, lithium and sodium species in metal bronzes utilized in electrochromlc devices [9].

2. Experimental The UHV apparatus and crystal preparation and cleaning procedures for the Nao.8WO3 (100) crystal have been described elsewhere [10]. The system was equipped with a Physical Electronics model 11-050 cylindrical mirror analyzer and related electronics for Auger spectroscopy. It was also equipped with 4-grid LEED optics. LEED patterns were observed visually through a glass port opposite the optics or, when hard copy was desired for kinetic analysis, photographed with an Olympus OM-1 35 mm camera using Kodak Trl-x black and white film. Intensity profiles of the diffraction beams were determined from the photographs with a Hamamatsu vldlcon TV camera interfaced to a Data General Nova 3 minicomputer [11]. The sample was heated radiatively by passing an ac current through a tungsten filament two centimeters behind the sample. Imtial Information on the temperature dependence of the Nao.8WO3 (100) LEED pattern was obtained by bringing the crystal to an elevated temperature (800 K) and observing the LEED changes as the crystal cooled to room temperature. The filament must necessarily be off during observation of the LEED pattern, to prevent distortions of the LEED pattern. Radiative heating also produces visible light, making it difficult to detect phosphorescence of the LEED screen. When the filament was turned off at 800 K the crystal cooled at the rate shown in fig. 1. The intensity of the LEED features are in part dependent upon the concentration of ordered surface species. Thus for any surface species to which a particular LEED feature can be assigned, the ordered concentration of that species may be monitored by following changes in intensity of the LEED feature. Changes in

CJ. Schramm et al. / Sodium order-disorder transznon on NaxWO 3 (100)

219

600

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550

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500

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0:450

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00000 0

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300

0000 250 I 0

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I 6

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Fig. 1. Temperature versus time cooling curves, both unassisted and wnh a cold finger using liqmd mtrogen, for a Nao.8WO3 sample heated ftrst to 800 K. The time reqmred for the sample to drop to the startmg point 600 K, 10 s, Is also that reqmred for the filament to cease glowing. ordered concentration versus time may be used in determining a kinetic model for the o r d e r - d i s o r d e r process. Additionally the width of the LEED feature is Inversely related to the domain size of the ordered array responsible for the LEED feature.

3. Experimental results 3.1. LEED and Auger studies o f the reversible thermal disordering o f sodium on Nao.sW03

No changes were observed in the Nao.sWOa (100)(2 × 1) LEED pattern until a threshold temperature of 580 + 25 K was reached. Above 580 K half order features from (2 X 1) ordering disappeared, leaving only the basic (1 × 1) diffraction pattern from the WO3 matrix. As the sample temperature cooled below 580 K, the sodium row features reappeared with streaking along the [010] and [001] directions. The streaking gradually coalesced into the half order beams of the (2 × 1) pattern, over approximately seven minutes. The LEED pattern symmetry after sodium ordering is identical to the initial pattern. This suggests that the o r d e r - d i s o r d e r process is completely reversible, although detailed comparisons of the I - V behavior of the patterns was not made. The Auger spectrum of the Nao.sWO3 (100) surface was monitored throughout the o r d e r - d i s o r d e r transition. No change in the ratio o f the sodium peak intensity to the tungsten peak lnfensity or in the peak shapes was observed. This precludes

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CJ. Schramm et al / Sodmm order-dzsorder transttlon on NaxWO 3 (100)

any diffusion of sodium into the bulk or desorption of sodlunr from the surface, thus insuring that the sodium remains a surface species over the temperature range of the experiments. In addition, no change in the ratio of oxygen peak intensity to the tungsten peak intensity was observed indicating that oxygen is not lost from the material at the temperatures needed for sodium disordering. The transformation of the (2 × 1) LEED pattern to a (I × 1) pattern above the threshold temperature of 580 K results from a disordering of sodium over the Nao.sWO3 sodium surface sites. The LEED pattern observed is characteristic of the undistorted WO3 lattice in the Nao.sWO3 (100) surface. There is no evidence in the LEED pattern of the distortion from tetragonal toward cubic with increasing temperature, which has been observed in the bulk [12] A structure with a disordered distribution of sodium IS preferred over formation of NaWO3 domains with complete sodium filhng accounting for the transformation to the (1 × 1) structure at elevated temperatures. Formation of ordered domains of NaWO3 and WO3, as required by complete disappearance of the half order features, would produce a pattern with sharp first order spots and low background intensity. Instead, as the temperature is increased above 580 K, the half order features disappear and the background intensity increases. This suggests that the sodium is actually disordered on the surface. 3.2. Kinetic studies o f Na ordering using L E E D

The rate of return to order after cooling below the transition temperature is expected to be temperature dependent. Kinetic studies were carried out at successively lower temperatures below the transition temperature. Thus for each data set collected at a constant temperature the rate of return to order should remain constant. For each data run the Nao.sWO3(100) sample was first oriented to yield a good room temperature (2 × 1) LEED pattern at 37 eV. The sample temperature was raised to the desired data set collection temperature and a reference photograph taken of the (2 × 1) LEED pattern at that temperature. The sample was then heated above the transition temperature (750 K) and allowed to cool. A liquid nitrogen cooled cold finger was employed to yield increased cooling rates. Thermal contact to the sample was made by a copper braid attached to the rear of the sample holder and the tip of the cold finger. The Increased cooling rates allowed the observation of a broader range of ordering rates. This cooling rate is also indicated in fig. 1. The first photograph (0 rain) of each data set was taken at 600 K. Further photographs were taken at half minute Intervals over an 8 rain period. Cooling was allowed to continue until the sample temperature reached the desired points. To maintain a "constant" temperature at that point the filament was turned on between photographs and off during photographs. Since the light from the filament interferes with the LEED phosphorescence, the filament must be turned off a few

C J . Schramm et al. / S o d t u m order-disorder transitton on N a x W O 3 (100)

221

Table 1 Observed rates for different ordering temperatures a) Temperature (K)

Rate (s-I )

Temperature (K)

Rate (S -1 )

330 337 345 365 376 383 389 399

7 6 × 10-2 4.1 × 10 -2 5.9 × 10 -2 6.4 × 10-2 8.2 × 10 -2 9.5 × 10 -2 8.4 × 10 -2 6 7 × 10 -2

404 410 413 430 438 457 466 475

1.1 × 10-1 1.1 × 10-1 1.8 × 10-I 1 6 × 10-I 1.6 × 10-1 2 2 × 10-1 l 9 × 10-1 2 7 × 10-I

a) This is the rate calculated on the basis of a first order rate law seconds (3 s) before each photograph is taken. Turning the filament on and off causes the temperature to fluctuate in a sawtooth manner rather than staying truly constant. The fluctuations in temperature ranged from +2 K for the lowest temperature data sets to +7 K for the highest temperature data sets The average temperatures for each of the data sets collected are shown m table 1. All subsequent calculations used this average temperature. Each data set was comprised of 18 exposures on 35 mm black and white film. The fihn was developed on site by standard procedures. The intensity analysis was performed on the LEED features on the negatives. The intensity profiles of the (Or) and (01) diffraction beam at each time interval were then determined from the photographs with a vxdlcon densitometer system [11]. The vidlcon system yields information much like that obtained from a mlcrodensltometer m determining diffraction Intensities, with the added benefit of Iterated scans and background corrections If desired. In the present analysis, the vldicon was directed to scan along the [010] direction, bisecting the (0~) and (01) diffraction beams. A plot along [010] was obtained. If the diffraction beams are syinmetrlc, the areas under the Intensity profile can be set proportional to the total beam Intensity. Also assuming intensity peak shapes to be relatively constant, the full width at half maximum (FWHM) will be inversely related to the domain size of the coherently scattering array responsible for the diffraction beam [13]. A plot of the full width at half maximum typical of all the data sets IS shown in fig. 2. The domain size grows rapidly and exceeds the coherence width (100 A) of the electron beam prior to complete sodium ordering. The concentration of ordered sodium at any time t ([Na]to) is proportional to the square root of the area under the (0~-) intensity profile as predicted by kinematic scattering theory [ 14]. Changes in the intensity of the (01) beam (which is dependent upon the basic tungsten oxide framework) with temperature are a function of the Debye-Waller factor [15]. To compensate the effect of this on the intensity of the (0~) profile, the area under the (0~) profile divided by the area

222

C.J. Schramm et al. / Sodium order-disorder transition on N a x W O 3 (100) O

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Fig. 2. Na0.sWO 3 (100) (0~) b e a m d i a m e t e r versus time m the (1 X 1) ~ (2 X 1) structural t r a n s m o n . Since the b e a m d i a m e t e r IS reversely p r o p o r t i o n a l to the d o m a i n size, the plot shows an increase m (2 X 1) ordered domain size with time.

under t_he (0i) profile ([Na]to cc [It(O~)/It(Oi) ] 1/2) was used. Even though the (03) and (0R) beams m principle have different Debye-Waller factors, this correction appears to be valid to a first approx]mation. The decrease in intensity of both beams as temperature is increased below the disordering temperature is observed to be well described by the same Debye-Waller factor. In addition, any contribution to the (0½) beam intensity due to multiple scattering from the tungsten oxide framework has been neglected. This is also reasonable, due to the very low intensity

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Fig. 3. Kinetic data for the sodium ordering on Na0.sWO 3 (100) The intensity of the (01) b e a m as a f u n c t i o n of time for two representative data sets. T h e a r r o w indicates the t i m e b e y o n d wbach the t e m p e r a t u r e is held c o n s t a n t

C.J Schramm et al. / Sodium order-disorder transttton on N a x W O a (100)

223

observed in this region at temperatures greater than the ordering temperature. Each data set was normalized with respect to the value of [Na]o formed for the reference photograph o f that data set. Two typical intensity versus time plots for the sodium ordering data are shown m fig. 3. Curve A is a low temperature data set (365 K) and curve B is a high temperature data set (466 K). Various simple kinetic rate laws were applied in order to suggest an appropriate model for the sodium ordering. The kinetic models tested were zero, first, and second order functions in the concentration of mobile sodium. More complicated approaches did not seem warranted based upon certain llm]tations in the klnet]c data. Those conditions which contribute to a certain amount of uncertainty in the data are: the sawtooth fluctuation of temperature during data collection mentloned previously; the lnablhty to collect data points closer than half minute intervals; and the restrictively short time interval over which the ordering transition occurred. With these limitations m mind, a fairly precise fit was obtained by utihzxng a simple first order rate law of the type:

d [Na]to~dr = k [Na] tm,

(1)

where [Na] t is the concentration o f mobile sodium at any time t. All data points were normahzed to a reference value taken before the experiment was started, therefore [Na]o = 1. Since [Na]tm = [Na]o - [Na]to we may rewrite eq. (1) completely in terms o f the concentration o f ordered sodium d[Na]to/dt = k(1 - [Nalto).

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Fig. 4. Integrated form of the kinetic data versus time for two representative data sets. The line IS the calculated least squares fit to the data representing the (1 × 1)~ (2 X 1) structural transition.

224

C.J. Schramm et al. /Sodtum order dtsorder transttton on NaxWO 3 (100)

32 2.8

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Fig 5. Arrhemus plot of -In k v e r s u s T -1 for all of the ordering data collected. A least squares lit of the data yields an activation energy of 4 kcal/mole.

The integrated form of this rate law ln[(1 - [Na]to) -1] = k t ,

(3)

was then employed in conjunction with the experimentally determined q u a n m y [Na]ot to find the rate at various temperatures A plot of the kinetic data of fig. 3 as calculated according to eq. (3) Is shown m fig. 4. For curve A the region of constant temperature starts at 3 mln, whde it starts at 0.5 nun for curve B. Ordering ts essentially complete by 7 mln for curve A and by 3 mln for curve B. Rates of ordering were calculated by determining the slope of a least square fit of the integrated kinetic data over the time interval that the temperature was held constant. The rates of ordering calculated in this manner for all data sets collected are hsted m table 1. The variation m rate constant wxth temperature is described by the Arrhenlus equation [16] k = k0 exp( E a / k T ) . A plot of - I n k against l I T is shown m fig 5. From the slope of the hne as determined by a least squares fit we have calculated an activation energy for the diffusion of sodium on the NaxWO3 (100) surface of 4 kcal/mole

C.J. Schramrn et al. / S o d t u m order-dtsorder transttton on N a x W O 3 [100)

225

4. Conclusions Our LEED data clearly show that the clean Nao.aWO3 (100) surface undergoes reversible sodmm &sorderlng at a temperature of 580 + 25 K. Auger data indicate that the disordering process is not accompanied b_y any change in overall surface composmon. The behavior of the FWHM of the (0½) intensity profile illustrates the rapid growth in domain size. The rapid decrease m FWHM suggests that ordering proceeds by island nucleation, rather than by random filling of the structure. The kinetics of the sodium ordering process is first order with respect to the concentration of disordered species. In a model where sodium ordering proceeds by ordered island nucleation, the fact that the process is first order indicates little interaction between disordered species. Thus the disordered spemes is not hkely to be a fully charged sodium ion. The actwatlon energy fol surface sodmm diffusion, 4 kcal/ mole, is quite small, making this class of material desirable for use m electrochromic devices. As expected this activation energy IS somewhat smaller than that found for the ordering of adsorbed oxygen on the R h ( l l l ) surface (13.5 kcal/ mole) [17] and the I r ( l l l ) surface ( 1 6 - 1 9 kcal/mole) [18] We plan to extend these measurements to other alkali tungsten bronzes, such as LlxWO3, in order to investigate the effect of alkah metal on the observed diffusion and ordering process.

Acknowledgements Partial support of this research by the Donors of the Petroleum Research Fund, Administered by the American Chemical Society, and by the US Department of Energy is gratefully acknowledged.

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[12] R. Clarke, Phys. Rev. Letters 39 (1977) 1550. [13] A. Gulmer, X-Ray Diffraction m Crystals (Freeman, San Francisco, 1963) [14] G. Ertl and J. Kuppers, Low Energy Electrons and Surface Chemistry (Verlag Chemle Wemhelm, 1974). [15] P.K. Johansson and B.N.J Persson, Sohd State Commun. 36 (1980) 271. [16] R.E. Watson, Jr. and H.H Swartz, Chemical Kinetics (Prentice Hall, Princeton, N J, 1972) [17] P.A. Thlel, J.T. Yates and W.H. Wemberg, Surface Scl. 82 (1979) 22 [18] U P. Ivanov, G.K. Boreskov, U I. Saucheuko, W.F. Egelhoff, Jr. and W.H. Weinberg, Surface SCl. 61 (1976) 25