Interaction of Na with multilayer water on MgO(100)

Article abstract of DOI:10.1021/jp001375+

The reaction between Na and multilayer water on MgO(100)/Mo(100) has been studied by metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), and temperature-programmed desorption (TPD). A water multilayer was exposed to Na at 100 K. During exposure, an attenuation of the surface water concentration concomitant with the formation of surface hydroxyls and metallic Na clusters is observed. After an anneal to 155 K, the metallic Na clusters disappear and the surface hydroxyls are essentially removed from the outermost surface, leaving water as the dominant surface species. Further annealing to >170 K leads to massive water and NaOH desorption, as indicated by TPD. After desorption of all multilayer components the species stabilized at the surface is primarily hydrated NaOH, which is stable up to 530 K.

Full text of DOI:10.1021/jp001375+

J. Phys. Chem. B 2000, 104, 7977-7980
7977
Interaction of Na with Multilayer Water on MgO(100)
J. Gu1nster, S. Krischok, J. Stultz, and D. W. Goodman*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843-3255
ReceiVed: April 10, 2000; In Final Form: June 14, 2000
The reaction between Na and multilayer water on MgO(100)/Mo(100) has been studied by metastable impact
electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS), and temperature-programmed
desorption (TPD). A water multilayer was exposed to Na at 100 K. During exposure, an attenuation of the
surface water concentration concomitant with the formation of surface hydroxyls and metallic Na clusters is
observed. After an anneal to 155 K, the metallic Na clusters disappear and the surface hydroxyls are essentially
removed from the outermost surface, leaving water as the dominant surface species. Further annealing to
>170 K leads to massive water and NaOH desorption, as indicated by TPD. After desorption of all multilayer
components the species stabilized at the surface is primarily hydrated NaOH, which is stable up to 530 K.
1. Introduction
reason we have applied in the present study metastable impact
electron spectroscopy (MIES) as an electron spectroscopic
The chemistry between a solvent and a surface is a challeng-
ing field for surface science since it combines the complexities
of a three-dimensional system, i.e., the solvent, with those of a
two-dimensional system, i.e., the surface-adsorbate interface.
The investigation of an isolated molecular species on surfaces
is a common theme of many surface science studies, however
little is known about the interfacial chemistry of multilayer
systems, especially under well-defined conditions. In particular,
the interaction of single water molecules with a model basic
oxide, e.g., magnesium oxide, has attracted considerable recent
attention.1-5 In general, those studies have addressed submono-
layer-substrate interactions. However, in naturally occurring
systems, surface-adsorbate reactions occur in the multilayer
rather than the submonolayer coverage regime. Recently we
have shown a marked contrast between the interaction between
a MgO(100) surface with isolated water molecules compared
to a water multilayer system.6 For example, the adsorption of
submonolayer water on a MgO(100) crystal surface is found to
be nondissociative,1,2,7,8 however, due to polarization of the
solvent in a multilayer system, charged species formed at the
solid-liquid interface can be stabilized and the dissociation of
water can become energetically favorable.6 For chemistry at
surfaces that occurs in an aqueous environment, the balance
between solvation, chemical reaction, and stabilization of
solvents and reaction products at the adsorbate substrate interface
are important factors. Studying these processes can provide a
more in-depth understanding of the chemistry of simple
structured molecules with surfaces under conditions relevant
to environmental chemistry. For that reason, we have examined
in the present study the interaction between Na and multilayer
water on MgO covered Mo(100).
technique which provides superior surface sensitivity.
2. Experimental Section
The experiments were carried out in an ultrahigh vacuum
(UHV) system (base pressure <2 × 10-10 Torr). The UHV
system consists out of two interconnected chambers, one for
sample treatment, equipped with low energy electron diffraction
(LEED) and temperature-programmed desorption (TPD) and the
second, with facilities for X-ray and ultraviolet photoelectron
spectroscopy (XPS and UPS), Auger electron spectroscopy
(AES), and MIES. MIES and UPS spectra were measured
simultaneously using a cold-cathode discharge source which
provides both ultraviolet photons (HeI) and metastable He* 23S/
21S (E* ) 19.8/20.6 eV) atoms with thermal kinetic energy.
The triplet-to-singlet ratio has been measured by He*-Ar impact
as 7:1. Metastable and photon contributions within the beam
were separated by means of a time-of-flight method using a
mechanical chopper. MIES and UPS spectra were acquired with
incident photon/metastable beam 45° with respect to the surface
normal in a constant pass energy mode using a double pass
cylindrical mirror analyzer (CMA), PHI model 15-255G.
Collection of a MIES/UPS spectrum requires approximately 120
s. The energies denoted by EF in the spectra correspond to
electrons emitted from the Fermi level of the Mo(100) substrate.
In the following spectra all binding energies are referenced to
EF. Since the metallic Mo substrate and the analyzer are in
electrical contact, the Fermi energy appears, independent of the
substrate work function, at a constant position in the spectra.
An additional biasing of the sample (45 V, negative on the
sample) permits the work function change of the surface to be
measured directly from the high binding-energy cutoff of the
spectra.
The use of electron spectroscopic techniques to address the
chemistry of small molecules on surfaces has been proved to
be a very powerful approach. However, one general problem
in the investigation of multilayer systems is the limited surface
sensitivity of typical surface electron spectroscopies. The finite
escape depth of the exiting electrons associated with a particular
probe technique limits the sensitivity of the technique, often
making it difficult to distinguish between a species adsorbed
on and one solved in a multilayer, adsorbate system. For that
MgO films were grown by depositing Mg in 1 × 10-6 Torr
of O2 ambient on the Mo(100) surface at 550 K. The Mg source
was constructed from a high-purity Mg ribbon wrapped around
a tantalum filament. To ensure a stoichiometric MgO layer, the
as-prepared surface was further annealed to 900 K in a 1 ×
10-7 Torr of O2 background for 20 min. As shown in previous
investigations, MgO films prepared under these conditions grow
10.1021/jp001375+ CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/27/2000

7978 J. Phys. Chem. B, Vol. 104, No. 33, 2000
Gu¨nster et al.
directly with the Auger deexcitation process that is responsible
for most of the intensity between 6 and 18 eV. Thus, the
attenuation of the intensity between 6 and 18 eV with increasing
Na coverage is partially due to an increasing probability for
AU. However, AU becomes significant only at low work
function surfaces with sufficient electron density near EF.
From these observations the following picture for the adsorp-
tion of sodium on multilayer water at 100 K emerges. In the
initial stages of sodium adsorption, the lowering of the substrate
work function indicates the formation of a surface dipole layer
likely induced by complete or partial electron transfer from the
sodium adsorbate to the substrate. Since the water bands shift
in concert with the substrate work function, the ionization energy
of the water molecules remains essentially constant. Similar
results have been reported by Blass et al.15 with UPS (HeII)
while dosing water to a Ag(111) surface precovered by varying
amounts of potassium. As a function of the potassium precov-
erage (up to 0.77 ML) the water bands shifted to higher binding
energies. This shift was presumed to indicate the formation of
a dipole layer at the water/potassium/Ag interface with a large
fraction of the water located above the dipole layer.
Figure 1. MIES spectra of the multilayer water covered MgO(100)/
Mo(100) substrate as a function of the Na exposure. The uppermost
spectrum shows the water-covered surface without Na. The direction
of the data collection and the relative Na exposure are indicated by the
arrow.
In surface sensitive electron spectroscopies, spectral features
following the substrate work function change are typically
associated with species located within or on top of the dipole
layer responsible for the work function change. In the present
experiment, however, Na was added to a water multilayer. For
that reason, a dipole layer formation below the outermost water
layer is possible only if the Na penetrates into the ASW
multilayer, presumably through the microporous structure previ-
ously proposed for ASW.16
epitaxially on the Mo(100) substrate and their properties with
respect to the adsorption of water are comparable to those of
MgO single crystals.9,10 D2O (CIL, 99.9%) was dosed to the
surface by backfilling the vacuum system after purification via
several freeze-pump-thaw cycles. Na was deposited onto the
surface using a SAES Getters source.
The minimum work function of the surface is reached at the
forth MIES spectrum (dotted spectrum), in which the 1π and
AU features are apparent. Increasing the Na exposure results
in the enhancement and finally saturation of both structures.
Blass et al., observed a feature similar to the 1π with UPS-
(HeII), and interpreted it in terms of K+[(H2O)-n] complex
formation. This, however, requires mobility to some extent of
the water molecules. On the basis of the present data, an
unambiguous interpretation of the 1π structure is not possible,
since the formation of a hydroxide-like species (NaOH) at the
outermost water surface is plausible. With this in mind, the
anticipated position of the hydroxyl 3σ band at ∼4.1 eV binding
energy is indicated in the figure. The pronounced AU feature
indicates that metallic sodium clusters are formed from the
excess sodium at the vacuum-water interface. In this context
the small AU-satellite at 2 eV higher binding energies, most
likely originating from a surface plasmon loss from small Na
clusters, lends additional support to the formation of clusters
with metallic character. Since the surface plasmon energy
depends strongly on the cluster size, a uniform cluster size
distribution is apparent.
3. Results
The sequence of MIES spectra in Figure 1 was acquired
during Na exposure to a water precovered MgO(100)/Mo(100)
substrate held at a temperature of 100 K. Since a similar
experiment has been discussed in detail recently11 we will
introduce these data only briefly. Prior to the sodium exposure
(uppermost spectrum), water (appoximately 4 monolayers, ML)
was dosed to the MgO covered Mo(100) substrate at 100 K. At
this temperature, due to the limited mobility of the water
molecules, the resulting water multilayer typically exhibits an
amorphous structure designated as amorphous solid water
(ASW). Binding energies for the three water bands 1b2, 3a1,
and 1b112-14 are indicated in Figure 1. In the spectra of Figure
1 the sodium exposure increases linearly from top to bottom.
In the initial stages of Na exposure, the high energy cutoff of
the spectra shifts toward higher binding energies indicating a
reduction of the substrate work function; the binding energies
of the water bands show the same trend. With increasing sodium
exposure, three additional modifications in the sequence of
MIES spectra are noteworthy: (i) a lowering of the overall
spectral intensity of the water bands; (ii) at 8.3 eV the
appearance of a new feature, designated as 1π; and (iii) near
EF, the appearance of a prominent new feature (AU). The AU
feature does not correlate with a change in the surface density
of states (SDOS) but originates from resonant ionization of the
He* projectile due to the overall lowering of the surface work
function and the formation of sodium clusters with metallic
character. The formation of this pronounced AU feature indicates
that a significant portion of the impinging He* projectiles are
resonantly ionized (He*- formation) and ultimately decay via
an auto-detachment (AU) process into the He ground state. In
this context it is noteworthy that the AU process competes
Figure 2 presents a sequence of MIES and UPS spectra
obtained from the surface prepared in Figure 1, i.e., a sodium
covered water multilayer, as a function of the substrate
temperature. Spectral features assigned in the figure are the three
water bands (1b1, 3a1, and 1b2), the two hydroxyl bands (1π
and 3σ), and the AU peak. Before discussing Figure 2 in detail,
it is appropriate to define three temperature regions, each
corresponding to a particular structure of multilayer water: (I)
between 100 and 132 K, within which the adsorbed water
molecules are randomly oriented and have a low mobility
(ASW); (II) near 155 K, within which the mobility of the water
molecules increases significantly, enabling the phase transition
from ASW to thermodynamically favored crystalline ice;17,18

Na Interaction with Water on MgO(100)
J. Phys. Chem. B, Vol. 104, No. 33, 2000 7979
Figure 3. AMU 20, 23, and 40 TPD data taken from the surface
prepared in Figure 1, i.e., a Na covered water multilayer on MgO-
(100)/Mo(100). The temperature ramp was 2 K/s.
desorption becomes significant (over 155 K) at which point a
strong contribution from the MgO substrate is apparent.
We have performed TPD of Na dosed onto a multilayer water-
covered MgO(100)/Mo(100), similar to the experiment shown
in Figure 2. The TPD data for atomic mass units AMU 20, 23,
and 40 are presented in Figure 3. The plot for AMU 23,
corresponding to sodium, shows no desorption feature. On the
other hand, the AMU 40 desorption trace (as well as the AMU
39 trace, not shown here) with desorption maxima at 198 and
531 K strongly implies a chemical reaction between water and
sodium. In this context it is important to note that a mass
spectrum (QMS) of pure NaOH (using an electron impact
ionizer) showed significant intensity at AMU 23, Na, but no
intensity at AMU 39, NaO, or AMU 40, NaOH, i.e., essentially
all of the NaOH is fragmented during ionization. In the present
experiment it is unclear as to what primary species gives rise
to the AMU 39 and AMU 40 signals.
To clarify this point, an experiment was carried out in which
solid NaOH, saturated with water, was annealed in UHV while
detecting the decomposition products. Initially, a low-temper-
ature anneal (∼400 K) led to a QMS signal typical for water
(AMU 16, 17, 18) with significant intensity at AMU 39 and
40. While maintaining a constant sample temperature, the
intensities of AMU 16, 17, 18, 39, and 40 decrease gradually
suggesting desiccation of the NaOH. Finally, annealing to higher
temperatures (∼700 K) led to a strong AMU 23, Na, signal
indicative of NaOH sublimation. A plausible explanation for
this behavior is that during the anneal, the water contaminated
NaOH sample sublimes hydrated NaOH clusters, yielding
fragments of water, NaOH, and NaO in the QMS ionizer. From
the well-developed hydroxyl doublet in the MIES spectrum
(>170 K) in Figure 2 and Figure 4, we conclude that NaOH
does indeed form on the MgO surface and is solvated by water.
It is noteworthy that the temperature ramp rate of Figure 3 is
significantly more rapid than that of Figures 2 and 4. The higher
ramp rate of Figure 3 (2 K/s) compared with Figures 2 and 4
(∼0.05 K/s) may result in a significantly higher local water
partial pressure of water at the surface during the desorption
process. This, in turn, could account for the stabilization of
hydrated clusters of NaOH during the desorption process of
Figure 3.
Figure 2. MIES and UPS spectra of the surface prepared in Figure 1,
i.e., a Na covered water multilayer on MgO(100)/Mo(100), as a function
of the anneal temperature.
and (III) over 170 K, within which multilayer water desorption
becomes significant. Noting these three temperature regions,
major changes are apparent in the sequence of MIES spectra in
Figure 2. In region I, from 100 to 132 K, the attenuation of the
AU peak indicates the disappearance of metallic sodium clusters
formed at the outermost surface. The water and hydroxyl bands
show no significant change in intensity, but shift ∼1 eV toward
lower binding energies, in direct contrast to the shift observed
upon sodium exposure. The work function increases by about
1 eV. In region II, near 155 K, the 1π features is weakened
whereas the water bands increase in intensity. Apparently, the
increasing molecular mobility leads to reorganization such that
the water concentration on the surface increases relative to the
sodium. In this context formation and hydration of NaOH, as
suggested in ref 15 for KOH, appears most likely. In region
III, over 170 K, the hydroxyl bands become the overall dominant
features. Because of its limited surface sensitivity UPS allows
no clear discrimination between those three temperature regions
(see Figure 2). On the other hand, UPS allows a relatively
precise determination of the temperature at which multilayer
In addition, three temperatures are assigned in Figure 3: 144
and 170 K are the temperatures at which significant changes in

7980 J. Phys. Chem. B, Vol. 104, No. 33, 2000
Gu¨nster et al.
reappearance of the hydroxyl features in MIES at temperatures
>170 K. At 170 K, there is a desorption onset of multilayer
water and solvated NaOH (see Figure 3), however, a fraction
of the solvated hydroxyls appear to be stabilized at the MgO
surface; metallic Na is not found on the MgO surface. As shown
in Figure 4, the hydroxyl bands, 3σ and 1π, are the dominate
features in MIES up to 530 K. At this temperature a prominent
peak in the AMU 40 signal in TPD and a rapid attenuation of
the hydroxyl bands in MIES indicate NaOH desorption. Finally,
at 710 K water desorption from the MgO surface is complete
(see Figure 3) and the MgO surface is adsorbate-free as seen
by MIES (see Figure 4).
5. Conclusion
The electronic structure of multilayer water covered MgO/
Mo(100) has been monitored as a function of the Na exposure
by metastable impact electron spectroscopy (MIES) at 100 K.
During Na exposure, the appearance of a new feature at 8 eV
binding energy indicates a reaction between Na and water. In
addition the formation of Na clusters with metallic character is
observed. From MIES it is concluded that up to a temperature
of 132 K the water concentration at the surface does not change
significantly with respect to hydroxyls. However, the metallic
Na clusters disappear from the surface. At ∼155 K a significant
increase in the water concentration at the surface suggests
solvation of the surface hydroxyls in the multilayer water
system. At temperatures >170 K desorption of water and NaOH
begins. After desorption of all multilayer components the species
stabilized at the surface is NaOH, which is stable up to 530 K.
Figure 4. MIES spectra obtained from a surface similar to the one in
Figure 2, i.e., a Na covered water multilayer on MgO(100)/Mo(100)
as a function of the anneal temperature.
MIES are seen; 355 K is the expected desorption temperature
for Na from MgO/Mo(100).
4. Discussion
The sequence of experiments can be divided into three distinct
phases, each representing a single stage in the reaction of a
multilayer water system on MgO(100) with a coadsorbate such
as sodium: (i) the reaction between Na and water at the substrate
surface; (ii) the solvation and further reaction of Na and its
reaction products, and (iii) after multilayer desorption, the
stabilization of reaction products at the substrate adsorbate
interface. At low temperatures (100 K) the adsorption of Na on
amorphous solid water (ASW) is expected to be a vacuum-
water interface reaction. Because of the low mobility of the
water molecules at 100 K, intermixing of the dosed Na with
the ASW bulk can be excluded. This interpretation is supported
by previous work18 in which long-range diffusion in multilayer
water has been observed at temperatures close to the transition
temperature of ASW to crystalline ice at about 160 K, but not
below this temperature. In this picture, the effect of the Na
condensation enthalpy is not considered, since the water
condensation enthalpy apparently has no effect on water-water
intermixing.18 As discussed above, the adsorption of Na leads
to the formation of an hydroxyl-like species and metallic Na
clusters, together with undissociated water, at the ASW surface.
At elevated temperatures the mobility of the water molecules
increases significantly. This is, in the case of a pure water phase,
manifested in the irreversible phase transition from ASW to
thermodynamically favored crystalline ice at ∼160 K. Even at
∼155 K, an increase of the surface water concentration relative
to the hydroxyl indicates a change in the molecular arrangement
at the surface (Figure 2). Since there are no significant AMU
23 and AMU 40 signals seen in TPD before 170 K (Figure 3),
hydration, i.e., solvation of Na and hydroxyls in the multilayer
water system, is presumed. The liquidlike transitional diffusion
prior to the ASW crystalline ice transition at 155 K strongly
supports this model.18 This picture is further supported by the
Acknowledgment. This work was supported by the Depart-
ment of Energy, the Office of Energy Research, Division of
Chemical Sciences, and the Robert A. Welch Foundation.
References and Notes
(1) Ferry, D.; Glebov, A.; Senz, V.; Suzanne, J.; Toennies, J. P.; Weiss,
H. J. Chem. Phys. 1996, 105, 1697.
(2) Heidberg, J.; Redlich, B.; Wetter, D. Ber. Bunsen-Ges. Phys. Chem.
1995, 99, 1333.
(3) Karolewski, M. A.; Cavell, R. G. Surf. Sci. 1992, 271, 128.
(4) Peng, X. D.; Barteau, M. A. Surf. Sci. 1990, 233, 283.
(5) Xu, C.; Goodman, D. W. Chem. Phys. Lett. 1997, 265, 341.
(6) Johnson, M. A.; Stefanovich, E. V.; Truong, T. N.; Gu¨nster, J.;
Goodman, D. W. J. Phys. Chem. 1999, 103, 3391.
(7) Stirniman, M. J.; Huang, C.; Smith, S. R.; Joyce, S. A.; Kay, B. D.
J. Chem. Phys. 1996, 105, 1295.
(8) Xu, C.; Goodman, D. W. Chem. Phys. Lett. 1997, 265, 341.
(9) Wu, M.-C.; Corneille, J. S.; Estrada, C. A.; He, J. W.; Goodman,
D. W. Chem. Phys. Lett. 1991, 182, 472.
(10) Wu, M.-C.; Truong, C. M.; Goodman, D. W. Phys. ReV. B 1992,
46, 12688.
(11) Gu¨nster, J.; Liu, G.; Kempter, V.; Goodman, D. W. J. Vac. Sci.
Technol. A 1998, 16, 996.
(12) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular
Photoelectron Spectroscopy; Wiley: New York, 1970.
(13) Banna, M. S.; McQuaide, B. H.; Malutzki, R.; Schmidt, V. J. Chem.
Phys. 1986, 84, 4739.
(14) Cˇ erma´k, V.; Yencha, A. J. J. Electron Spectrosc. Relat. Phenom.
1977, 11, 67.
(15) Blass, P. M.; Zhou, X.-L.; White, M. J. J. Phys. Chem. 1990, 94,
3054.
(16) Smith, R. S.; Huang, C.; Wong, E. K. L.; Kay, B. D. Phys. ReV.
Lett. 1997, 79, 909.
(17) Stietz, F.; Tra¨ger, F. Philos. Mag. B 1999, 79, 1281.
(18) Sceats, M. G.; Rice, S. A. In Water A ComprehensiVe Treatise;
Franks, F., Ed.; Plenum Press: New York, 1982; Vol. 7, p 83.
(19) Smith, R. S.; Huang, C.; Kay, B. D. J. Phys. Chem. B 1997, 101,
6123.