Successive change in work function of Al exposed to air

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 767-771

Successive change in work function of A1 exposed to air M. Uda a'b'*, Y. Nakagawa a, T. Yamamoto a,b, M. Kawasaki a, A. Nakamura a, T. Saito a, K. H i r o s e c aDepartment of Materials Science and Engineering, Waseda University 3-4-10hkubo, Shinjuku-ku, Tokyo 169, Japan bLaboratory for Materials, Science and Technology, Waseda University 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169, Japan Clnstitute of Space and Astronomical Science 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229, Japan

Abstract Aluminum surface freshly shaven with a steel knife showed significant changes in work functions from 3.25 to 3.60 eV within one day. Successive changes of the work functions were measured in air using the open counter. XPS spectra showed no change fundamentally when the fresh AI surface was exposed to air for 1 min and 24 h. Molecular orbital calculations by use of the DV-X a method predicted an appearance of an additional local density of state near the Fermi edge when an A1-AI distance becomes large. Based on the experimental results and calculations, we proposed formation of an interface between an A1 matrix and a surface oxide or oxyhydroxide, which has a stress induced density of state. This state appears at the beginning of air exposure and disappears after the lapse of time because such stress is released after rearrangement of surface and interface structures. © 1998 Elsevier Science B.V. Keywords: AI; Work function; Surface; Molecular orbital calculation; Interface; A10(OH)

1. Introduction Recent advances in investigation techniques of solid surfaces have stimulated interest in the observation of dynamical states of the surfaces. In heterogeneous systems of solids and gases, chemical reactions such as oxidation, reduction, catalytic and corrosive action, and so on, proceed on the solid surfaces. However, most of studies have been performed in an ultrahigh vacuum, giving information on static states of the surfaces. Our earlier work dealt with observations of the solid surfaces in air using the open counter [ 1 - 4 ] . The scanning tunneling microscope (STM) might also be used, only in limited cases, to investigate the solid surfaces in gas atmospheres. * Corresponding author.

A l u m i n u m is known to be a unique 'corrosionresistive' industrial material used in recent high technology fields, and extensive investigations have been carried out to examine its surface states. However, it is not straightforward to study chemical reactions o f AI with oxygen or water molecules in a dynamical means. In the present investigation an electronic state of the valence band of A1 exposed to air was successively or consecutively studied using the open counter. Based on experimental results and molecular orbital calculations, we propose formation of an interface between an A1 matrix and a surface oxide or oxyhydroxide, which has a stress induced density of state at the beginning of air exposure.

2. Experiments A1 used here was a cold rolled plate (JIS A1050;

0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0368-2048(97)00237-5

768

M. Uda et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 767-771

A1 99.55, Fe 0.34, Si 0.06, Ti 0.03 and Cu 0.02 wt%). Fresh A1 surfaces were prepared by (1) evaporating AI in a vacuum (--10 -8 Torr) and (2) shaving the plate in air by a steel knife. Photoemission measurements were made at room temperature both in vacuum and in air (temperature 23-24, humidity 55-60% and pressure 755-766 mmHg), where emitted photons from a deuterium lamp were monochromatized through a grating monochrometer (Kratos Schoffel Instruments GM 100). Emitted electrons were counted using a channeltron in a vacuum and an open counter (Riken Keiki, AC-1) in air, respectively, with a 0.05 eV energy step of incident photons. A glass filter (Sigma Koki, UTF 50-28U) was installed between the deuterium lamp and a sample in open air measurements, which served to reduce photons with energies of 4.7 eV or more. The number of photons after passing through the grating monochrometer were estimated using a photodiod (Hamamatsu Photonics, S1227-101BR) installed in a sample position in place of the sample. A vacuum evaporated A1 film (--2000 ~, in thickness), after photoemission measurements in vacuum, was first exposed to pure N 2 gas, and then to air. The photoelectron measurements of the evaporated film in air were made after 30 min, 2, 8 and 24 h of the air exposure. By using one and the same sample to get successive or consecutive photoelectron spectra, reliable data cannot be obtained from the sample exposed to air less than 30 min, because one measurement in an energy range from 3.0 to 4.4 eV with a 0.05 eV energy step needs 6 min under our experimental conditions. To get reliable data even from the sample exposed to air less than 30 min, each freshly shaven sample was prepared for each electron counting at a given energy, and then 29 samples were prepared for all the measurements covering the energy range from 3.0 to 4.4 eV with the 0.05 eV step for each of 0, 3, 10 and 30 min and 2 h exposures, i.e. 295 x 5 = 145 samples in all. For this purpose the A1 plate was used instead of the evaporated A1 film because this type of evaporation is time-consuming. To get good statistics, a measurement for 10 s at a given photon energy was repeated 20 times for the 0 min exposure sample, 15 times for the 3 min sample, and 12 times for the 10 and 30 min, and 2, 8 and 24 h samples. A total of 1756 freshly shaven samples were ultimately prepared.

The number of incident electrons per second into the counter, N i n [ 3 ] , is estimated by: N°bs Nin = Nero f - 1 - rNob----'~'-

(li

where Nero is the number of emitted electrons per second, f the fraction of emitted electrons into the counter, z the dead time and Nobs the number of observed electrons per second. The photoelectric quantum yield Y is obtained by dividing Nin by the number of incident photons [3]. The photoelectric work function was now determined through a procedure of plotting y l / 2 v e r s u s photon energy. Such plots yield a straight line with abscissa intercepts at a photon energy taken to be the work function. The local density of state (LDOS) was estimated by differentiating Y with the incident photon energy E. X-ray photoelectron spectra were measured with V.G. ESCALAB 220i-XL using monochromatized A1 Kc~ (1486.6 eV). Experimental conditions were as follows: (1) step size 0.05 eV, measuring time for each step 100 ms, pass energy 50 eV and scanning times 30 for a valence band measurement, and (2) 0.01 eV, 100 ms, 5 eV and 30 for O l S l / 2 and A1 2p 1/2,3/2 measurements, respectively.

3. Results and discussions

The evaporated film and shaven A1 plate show fundamentally the same features both in the photoelectron yield and XPS spectra within the frame of experimental error limits, suggesting that the freshly shaven A1 surface is typical of a clean surface. Here the photoelectric yield and the work function of the

~

3.8

o= 3.6

~ ~_~.~==~ - - - ~ . . . . .

3.4 m

~* - freshly shaven AI plate [ ~ vacuum-evapolated AI film

3.2'

!

3

.

0

,

I

5

.

.

.

.

I

10

.

.

.

.

I

15

.

.

.

.

I

20

.

.

.

.

25

duration of air exposures (hr.) Fig. 1. Work functions of a vacuum evaporated A1 film and a freshly shaven AI plate with a steel knife.

769

M. Uda et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 767-771

evaporated film were measured in a vacuum prior to air exposure, giving a typical work function of pure and clean AI, i.e. 4.30 eV [5]. In Fig. 1 observed work functions are plotted as a function of duration of exposing the vacuum evaporated AI film and the freshly shaven AI plate to air. Change in the work functions was clearly observed during very convenient duration, i.e. a few hours. To explain such a change in the work function from 3.25 eV for 0 h to 3.60 eV for 24 h exposure to air, XPS spectra were

(a)

-,..,,

o=

35

(b)

30

. . . .

I

25 20 15 10 5 Binding Energy (eV) . . . .

I

. . . .

I

. . . .

I

AI2pl/2, 3/2

. . . .

I

. . . .

I

L.

0

I

. . . .

I

. . . .

-5

}~

°~

,

77

•

I

I

. . . .

76

l

. . . .

I

. . . .

I

. . . .

/

,

75 74 73 72 Binding Energy (eV)

71

70

observed, where a valence band, A12pl/2,3/2 and O lsl/2 spectra were taken as a function of duration of air exposures and typical data are shown in Fig. 2(a), (b) and (c), respectively. Approximately 10% of decrease and increase in intensities was observed on XPS peaks of a valence band ranging from 0 to 5 eV in Fig. 2(a) and of A1 2pl/2, 3/2 ranging from 72.2 eV to 73.3 eV in Fig. 2(b) which are originated from metallic AI, and on XPS peaks of O 2s 1/2 at - 2 4 . 5 eV in Fig. 2(a), AI 2pl/2. 3/2 at --75.5 eV in Fig. 2(b) and O lSl/2 at ~532.6 eV in Fig. 2(c) which are originated from a surface layer (oxide or oxyhydroxide), respectively, after 24 h air exposure. Binding energies for broad peaks at ~75.5 eV (A1 2pl/2, 3/2) and ~ 5 3 2 . 6 e V (O lSl/2) are far from those for A1203 ( ~ 7 4 . 0 and 531.6 eV, respectively), and might be explained by intermediates between those for A1203 and AI(OH)3 [6], i.e. those for A10(OH). No fundamental difference among these spectra is, however, found, suggesting that the change in the wok functions of A1 is not due to the change in surface elemental concentrations and in basic electronic structures of the valence band, but due to a tiny change in the local density of state (LDOS) near the Fermi edge. Such a tiny change can only be detected through the photoelectron yield spectroscopy by use of the photon monochromatization method. LDOS was deduced from differentiating the observed photoelectron yields by the incident photon energies, and is shown in Fig. 3 as a function of duration of air exposures. At the beginning of the air exposure an additional LDOS located in the low

(c) 6

[ 10"7/eV]

duration of air exposure 4 [. OQ [ F ,..1 I

a.j

L 538

536

534 532 530 Binding Energy (eV)

" ° • •

0min. 3min. 10min. 30min.

o 2hr. | " 8hr. I o 24h.| I

• o . l .; ~*1 1 ~ ! ~

: • ' ol ~ j ~°°"

528

Fig. 2. XPS spectra of the shaven A1 plate just after air exposure (0 h) and after 24 h air exposure: (a) valence band; (b) AI 2p i/2.3/2; (c) O lsl~.

3

3.5 4 4.5 Binding energy referred to V.L. (eV)

Fig. 3. The local density of state (LDOS) for the shaven AI plate with a steel knife.

770

M. Uda et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 767-771 . . . . . . . . .

I

. . . . . . . . .

I

. . . . . . . . .

8tl u 7a l g

!

I

7tlu 6alg I

9tlu . . . .

I

.

,

.

0

I

. . . . . . . . . .

t

5

.

.

.

.

.

.

.

.

10

15

Binding energy (eV) Fig. 4. The local density of state (LDOS) of metallic AI (a0 = 4049 A) calculated by the DV-X ~ method.

2.4hr:. , ~,.... -

energy region centered at --3.5 eV is formed, and disappears after the lapse of time. To predict the appearance of the additional LDOS, molecular orbital calculations were performed using the DV-X c~ method [7]. The LDOS of A1 near the Fermi edge calculated here is shown in Fig. 4, which is composed of five molecular orbitals i.e. 6alg and 7alg due to A1 3s, and 7tlu, 8tlu and 9tlu due to A1 3p. Then the Fermi edge is strongly influenced by the shallowest energy level of 9tlu. If A1 atoms combine with O or OH at the surface, the energy of 9tlu becomes deeper in this calculation than that of A1 in a metal. This cannot explain an appearance of the low energy peak at 3.5 eV. If a lattice constant of an A1 metal or an A1-A1 distance becomes large the energy of 9tlu shifts to a shallower level, as shown in Fig. 5. Such a simple model is, of course, far from realistic atomic sequences of A1 and O or OH but is not inconsistent with the appearance of the low energy peaks at

-1.5 eV]

. . . .

-

3.5

I 4.5

4

Binding energy referred to V.L.(eV) Fig. 6. The observed density of state (LDOS) of AI exposed to air for 0-24 h.

3.5 eV. Improvement of an accuracy of the MO calculations is in due course. To relieve the decrease in the intensity of the peak located at 3.5 eV, the LDOSs at 4.15 eV were used for normalization of the LDOSs to compensate decrease in the LDOSs due to change in the photoelectron yield after prolonged air exposure. A series of the observed LDOS is shown in Fig. 6. XPS and photoelectron yield data tell us that within a very short period of time, i.e. much less than 1 s, the A1 surface is covered with oxygen or water molecules to form a thin layer of aluminum oxide or oxyhydroxide whose elemental compositions remain unchanged in a few hours though the work functions and the LDOS changes noticeably within a few hours. An intensity of the 1.2

I

. . . .

I

. . . .

I

. . . .

i

. . . .

I

. . . .

1

0.8

"~,

"~ 0.6

"'\

O

0.4

-0.5

0.2

\\

--~.

• w

0

. . . .

0

i

5

. . . .

l l l i l l l j . . I

10

15

. . . .

20

i

. . . .

25

L

30

increase in the lattice constant of A1 (%) Fig. 5. Shift of 9t~o as a function of increase in the lattice constant of A1.

0.01

i

i

llll

.I

0.1

. . . . . . . .

i

• . . . . . . . .

1

i

,

10

Duration of air exposure (hr.) Fig. 7. An intensity of the local density of state (LDOS) centered at 3.5 eV as a function of duration of air exposures (h).

"~L8 (8L61) ~17 "udf "aos "s£qd "f 'o>lm~S "2) '~p~:3nsj+"± '!qa~PV "H [L] "8L6I 'elosouu!IAI 'uo!mJodJoD JotuI~-U!~JOd '~doasoJolaods UOJl -aalaOlOqd 3[~J-X jo :~ooqpueH '('sp~) ~Joquol!nIN "~'D 'JapInoIN "d'f 's!Ae(I '~'q 'sgg!~I "IN'A~ 'JaU~eA~_ "(I"3 [9] "996I 'u!poa 'geIJaA Ja~u.udS 'sageoqas!(I seD uo!ss!tu~ umlaal~ 'IE "IOA 'sa!s~qd jo e!padoIa,{au~ 'a~.n.l~I "S [g] "817I (966I) £LgV 'qlaIN 'JlSUI"pnN 'ePfl "IN 'equoz "H 'qongoJ!qs "N '~mouns+L "& 'etn!qs~N "S [17] 'IE~ (1~66I) E'~V "qlolAI'JlsuI "IanN '~Pfl "IN '~tu!qsggeN "S '!qan~oN "J~ [g] "'e8E (gg6D ~E "s~qd "IddV "f "udf '~Pi'I 'IN [E] "89 ([86D gg -IlSUI "13S 'AaH 'llpf] "IA[ "ell~q!J!~ "H [I] soauoa~jo~I

'aJnsodxa Jm j o ~u!uu!~aq aql It altls j o ~l!suap paanpu! ssaJls t sgq qa!qA~ 'X!JlgtU IV aql p u t ap!xoJp£qfixo JO ap!xo a a t j J n s aql u a a ~ l a q aatjJalu! aql j o uo!lgtuJoj ol anp aq lq~!tu SO(3"1 aq& "suo!lNnalta Itl!qJo JplnaalOtU aql fiq pala!paJd s! q a ! q ~ 'a~pa !tuJa d aql Jpau altls j o ~l!suap IpaOI aql j o a a u m t a d d t aql fiq p a u ! t l d x a aq u t a sJnoq A~Oj g u ! q l ! ~ J~P ol pasodxa IV j o u o ! l a u n j >lJOA~ aql U[ a ~ u t q a paaunouoJ d •sluatua~ug.ut a!tUOlt aatjJalu! pup aapjJns aql gu!larulsuoaaJ fiq paAotuaJ aq O1 palaadxa "HO-IV-IV ao O-IV-IV jo oauanbas g "a'! '(11 I) IV uo (010) (HO)OIV jo (HO-IV JO O-IV) Ja£~l a!uJol~-o~l V "6 "~!~I (JO£e I ps£) (HO)O1V u! HO JO O : 0

'.laAOA~Oq 'S! aataLlalU ! aql U! poonpu! ssoJlS qans ' a m p j o osd N aql JalJV "Itlatn IV u! l t q l u t q l .[a~Jt I aq plnoqs u o ! l & o s q t H O Jo O .taljt ~Ialp!patutu! pamJoj aapjJalu! aql u! a a u t l s ! p IV-IV aql 'asea 3~ug u I "itlom IV pug Ja3~tI ootjJns oql u! oautls!p IV-IV OgtJoat Ut u t q l SSOI aq plnoqs ootjJOlU! oql u! oaupls!p IV-IV oql q 8 n o q l 'filOa!laodsoJ '6 "g!A ut u ~ o q s s t IPlOtu IV pug (HO)OIV u! %0g+ p u t I - p u t '8 "8!d u! UA~oqs St %08 + p u t %gg + St. ItlOtU IV U¢ OOUtlS!p lSalaoqs oql p u t ~O~IV u! sooutls!p IV-IV lsa -1Joqs puoaas pug lsJg oql u! oauoJojj!(:I "oatjJolu! oql uo ssoJls SOAPOI oJnlanJls oatjJns u! o 8 u t q a uoppns qan S "x!Jltm IV oql pup Jo3~pI oatjJns oql uooaxloq oapj.tolu! oql u! 1.IoI o:m lso.[ oql p u t '3~Ildtuqt HO JO O q l ! ~ ou!qtuoa Outld (I I I)IV oql m stuolg IV j o j l t q ouo £plttUTxoJddt 'IV uo H O JO O j o UoTldJospt JalJt lsnf "poloodxa oJp 6 p u t 8 Sg!d u! UA~oqs St HO -IV-IV Jo O-IV-IV j o saauanbas ' a u N d (I I I)IV ue uo pomJoj s! (HO p u t IV Jo O p u t IV) Jo3~PI a!tuolt-OA~l g ~IUO JI ' t u n u ! t u n l t >llnq oql u! l t q l u t q l JogJt I oq ol poloodxo s! oapjJolu! oql u! oaupls!p IV-IV u t osntaoq oaJoj oi!suol ol polao.fqns oJt x!Jlttu IV oql p u t op!xoJpfiqfixo Jo op!xo oatouns oql u o o ~ l o q oatjJalu! u t m s t u o l t IV mql p a t u n s s t o& suo!lglnaltO itl!qJo JtlnaOlOtU oql pug sllnsoJ ItlUOtU!Jodxo osoql tuoJ d •soJnsodxo J!t j o uo!ltJnp j o uo!launj t st L "8!d u! UA~OqSS! Ao g ' ~ lP poJoluoa >Itad SO(:Iq "O-IV-IV jo oauonbas ~ "o't '(I [ D IV uo (001) ~OtlV 7ojo (O-IV) Jolt[ a!mol~-o~l V 'g "~!~

(JMN pj~)~O~lVTO u! O : O

(aO~l PUZ)(HO)OIV u! IV : 0

(sageI puE)SOZlV~0 u! IV :

(Ja~(l~I~,SI)IVU.I IV: C)

0o,~I lsD IV u! IV : 0 y61,'£

VITS'~; '

+

V9g

I

v9sS

,

J

,

y86'9

,' ',', .' , , : , , ',

. ,,

"c;---'d- -'c;--

I

/1 \k

0 ILL

t ,

// \\

,

++

12Z-Z9L (g661) 16-ffg vuaut°ua~d Palpla~l pup £doas'oslaads uoa2oal3 fo lvuanof/'lV la vpD "14!