Gas adsorption on cadmium sulphide

SURFACE SCIENCE 9 (1968) 396--406 © North-Holland Publishin~ Co., .A,m:~terdam

GAS ADSORPTION O N CADMIUM S U L P H I D E G. A. BOOTSMA

Philips Research Laboratories, N.V. Philips' Glo~ilamF,enfabrieken, Eindhoven, The Netherlands Received 7 July 1967; revised manuscript received 26 September 1967 Gas volumetric measurements were performed in the study of the adsorption of gases on CdS powder surfaces. At room temperature and pressures of about 1 Tort, where oxygen is taken up to the extent of a few percents of a monolayer coverage, the hydrides of the type H~A are adsorbed to coverages of one third to one half of a monolayer. Oxygen reacts irreversibly with cadmium sulphide, while the reaction rate and ~he production of sulphur dioxide increase with the temperature. The adsorption of gases HI~A decreases with increasing temperature. The adsorption of krypton and argon is reduced by the presence of H~A on the surface of the powders. The effect of the adsorption of HxA on the conductivity of s~ngle crystals is qualitatively the same as the effect of oxygen; crystals showing a decrease of the conductivity with oxygen also exhibit this effect with H.rA.

1. Introduct ion

in the literature on cadmium sulphide tl~e effects of gases, mostl) electronegative, on the (photo-)electrical prop:rties of this semiconductor have frequently been discussed (cf. refs. 1-7). In many cases tt~ese effects, especially of oxygen, are ascribed to an adsorption-desorDtion mechanism under the influence of light. Absolute evidence for adsc,rption and desorption processes, however, is scarcely presented. Di~rect gas volumetric measurements of the reaction ef oxygen with CdS po~ ders have been performed by Kotel'nikovS)., whereas Haas, Fox and Katz~') used ,~he (0001) and (000-f) surfaces of si~agle c y~ :als. Their results suggest the possibility of photo~,dsorpt,oa of oxygen. ll|ar~

~l|lldil[Ulll

a l l l U bita~.,UH~ W J ~ , I ~

ttl~ au~Ut~L:v,1 oF vAff~,ll

a|iu

utitr~t

gases, like hyc3rides of the type H~A, has been the subject ,~f detailed studies~O, a~), the chemical reactivity of the CdS surface has hitF.erto been w:ac'icaily unexplored. Investigations in the field of colloid chemistry give some indications; here the stabilizing action of substances such as hydrogen sulphide, hydroge~a chloride and ammonia on CdS soles was attributed to adsorption (cL ref'. 12). ~n this paper th.• results of the measurements on the reaction cf CdS wkh 396

GAS ADSOkPTION OIN CADMIUM SULPHIDE

397

oxygen and on the adsorption of gases H~A on CdS will be reported. According to our findings there i~ a pronounced difference in behaviour between gases H~A and oxygen. At room temperature and pressures of about 1 Torr the quantities of the hydrides adsorbed amount to coverages of 509/0 of a monolayer, whereas oxygen is taken up only to the extent of a few percents. The reaction of oxygen with CdS is stimulated by an increase in temperature, whereas the adsorption of H~A then decreases. 2. Experimental

A complicated problem which arises in the case of a compound semiconductor is the definition of the surface. Apart from foreign impurities like adsorbed gases, and lattice imperfections such as cracks and crevices, deviations from stoichiometry can also cause a bad definition. Though "cleanliness" in the sense of being free from impurities, atomically fiat and stoichiometrically pure seems to be an unattainable goal, it is at least necessary in order to get reproducible results to defihe the surface always in the same way. In our adsorption measurements reproducibility was achieved by annealing the powders in vacuum at about 400°C (compare ref. 9). The polycrystalline material used was "'Suprapur" powder from E. Merck A. G. containing max. CI 5 x 10 -4, C~ 1 × 10 -6, Co 5 × 10 -6, Ni 5 × 10 -~, Fe 5 x 10 -6, Zn 5 x 10-3~o . The experiments were performed in a glass appa: ~ttus, essentially the same as described by Boonstra and Van Ruler :~). The pressure could be lowered to about 10 -.7 Torr. The working pressures varied from 10 -4 to 2 Torr and were measured by McLeod manomelcrs. When necessary blank experiments were carried out in order to correct tk~t adsorption on the glass walls (e.g. NH3). The general procedure was as follows. After annealing the powder tk~r about 15 hours at 10 - 7 Torr and 400°C, the adsorption isotherm of krypton was determined at 77°K. The surface: area of the powd r, calculated by the B.E.T. method18), was of the order of 3 m2/g CdS (mean diameter of the cr~,stalhtes 0.5 pro). After removal of the krypton and heating to the measuring temperature, the adsorption of o:~ygen or t4~A was me~surcd. For the calculation of the surface coverages two paramt'~ters have 1,~ bc known. In the first place the value of the cross-section of a krypt~n at~n (assumed to be 19.5 A2), secondly the cross-section of a sur~acc cadmium ,~r su!phur atom. The last number could be calculated if the distribution ~,! the pr:ncipal crystallographic planes on the surface of the crystallitcs (wurtzitc m,~dification) were known. For the pianes (11~0), (10]0) and (0001) the crosssections are 12.1, 14.0 and 14.8 A 2 respectively, in the subsequent calculations 'we attributed a value of 13 A 2 to the cross-section of a surface Cd or S

atom. U n ! ~ s ot'aerw~se stated, the coverages 0 in the f o l l m d n g sections denote the n u m ~ . r of molecui~-s HxA or O 2 adsorbed per surfa~.~ C d o r S atom. The e|eetdcal effects o f the reaction with oxygen or adsorption of HxA •a,x~ t e s t ~ q u a l i m t i ~ ! y by conductivity measurements on ,.tingle crystals.

3, Res~Rs ~n,t d i ~ m s i o . 3. I. THE

REACTION

't~TrH OXYGEN

On germanium and silicon and m--v compounds, such as G a A s mad inSb, amounts of oxygen corresponding to one oxygen atom per surface atom are taken up in a few minutes, even at tow pressures and room t,emlLmraturc-t°). The more ionic 14) it-vl compounds are much less reactive to oxygen. Though thermed~mmic d a m ts) suggest the possibility of the reaction CdS+ 1½0,+CdO+SO{

F=-90kcal

(25°C),

(1)

its reaction rate at room tempera~:xc i~ evidently very small.

3F;O'~C

o/ >lm

i / 2o.c j Y+

29s'e

~

"

zoo

soo

Fig. ~. R a:~: cf the r~-nction of oxygen ~iV'mmdmium sulphide, 235 °C to 495 °C, plotted ~cording to ~a!.{2).

vgpOp,.

.

.

.

.

.

.

.

.

;'777

-

S v po i~ arbi~ra~ ,,mi~ the de~ee of coverage thal wo'..t|dcorrespond to the amount of o~ygen consumed ~.~,) is 0.05 bem~ee'.~ , and 2, and 0.t betw~-een 3 and 4.

G .J ADSORPTION ON ¢~AD~IUM SULPHIDE

399

In order to get an idea of the mechani ~ of the reaction we determined tk,,e oxygen consumption at t e m ~ r a t u r o s yawing from t50 to 500°C by gas volUme~c measurements, T h e experiments were performed in a closed s~t¢,m (volume V about 3 ~ cm 3) With powder samples of almost equal surface areas (S= 10 m 2) and with initial oxygen pressures Po of the order o f 0.4 Tort. The reaction rate increases with the temperature, as is demonstrated in fig. 1, where some of the results are plotted. In the range 250-500°C straight lines are obtained if the order of the reaction with respect to oxygen is assumed to be 1½. Then the decrease of the oxygen pressure Pt with time t is descrit~d by

VJpo/p,-,

kt

(2)

s

if S, the clean part of the surface, "s considered to be constant during the reaction (cf. fig. 1" 0o, ~<0.1). In this temperature range the Arrhenius activa-

•e~ .... 500 "

"

i

I

t e m p e r a t u r e ('C)

350 ......

i . . . . . . . .

250

1.50

i. . . . . .

i

"

÷

3to,] k

I

2-

1-

2.0

2...=,

Fig. 2. Log k (albitrary units) versus 1/£ for the reaction of oxygen with cadmium sulphide. I50°C to 495°C. The point at 235°C corresponds to the dashed line in fig.l ; the point at 150°C was determined in a similar way.

~ . A. IgX'~TSMA

fion energy E~ defined by

d In k ........................

E, =

(3)

. . . . .

is 23 kc~Umole (fig. 2). A~ temperatures below about 250°C the ~ c t i o n p r ~ t s in a different , ~ y . P~o~ng the results a c c o r d i ~ to ~Xl. (2) does ~ot yield s~'aight lines, ~either d ~ plotting a c c o ~ i a g to eqt~ations valid for reactions that are :k~umed ~o be of the flint or second order with resgect to oxygen, or of the EIo~ich ty.~ (dp=a In t + b). F u r t ~ r m o r e the formation of a gaseous reaction product, condensable at - ! % X], is only observed above 250 °C. Omegatron measurements identified ~his ~ s as sulphur dioxide. At a given temperatm~ the value of ASOa/ ~O~, representing the amount of SOz produo~d divided by the amount of O: consume~l, is nearly constant over the reaction period. This ratio increases ~ith ta¢ lemperatur¢ (fig. 3), reaching the va|ue of~ teq. 11)) at about 500°C. On ~ e basis of these results the reaction mechanism at 250-500°C may be wrR~e~ as CdS. + 1½ O2 --* " C d O ' S O z "'*--*CdO, + S02~.

(1)'

it is clear that o~;ygen can bring ~bout t ~ , kinds of effects or CdS (cf. tefs At im~er temperatures ar~ oxidation t~kes pla
0.?!

!

.ta.

zoo

/"

*

300

i

[

~00

500

~--~,

__J

ter, N~ra~ur e (%)

Rc-~;~ ,,c a~vc,un~ of SO, produced d u r i n g ~he reactior~ of c a d m i u m sulphide w i [ h oxygen venus lem~ratu~,

GAS ADSORPTION ON CADMIUM SULPHIDE

401

vacancies, thus increasing the conductivity. At temperatures up to 500%C the oxidation predominates and the net effect on the conductivity will depend on the temperature. For comparison we studied the reaction of oxygen with powders of zinc sulphide (surface area 20 m2/g) using the same pre-tre.atment, V, S, and Po as with cadmium sulphide. The results were analogous. In the temperature range 250--500°C the order of the reaction with respect to oxygen is also 1½, the activation energy being, wRhin the limits of accuracy (4-10%), equal to that of the reaction of oxygen with cadmium sulphide. At a give, temperature, the reaction rate is higher for ZnS; in fig. 2 the corresponding straight line for zinc sulphide is shifted to lower temperatures over a distance A(1000/ T ) = +0.18. The production of sulphur dioxide starts at about 230°C, the ratio ASO2/AO2 reaching the value t at about 360°C. The effect of the heat treatment in an oxygen atmosphere on the electrical properties was studied by measuring the temperature dependence of the dark conductivity of undoped CdS single crystals in vacuum, helium and oxygen (pressures about 1 Ton'). The crystals (0.8 x 1.5 x 2 ram) were provided with gold electrodes and heated to 400°C in vacuum before the measurements were performed. At room temperature and ia vacuum, the dark resistances Ra were 10~-10 s ohm arid the photo-re~.istances R~ 10*-105 ohm (illumination with i.R. filtered l':ght of a tungsten, lamp). The activation energies associated wi ~: , the da-k conductivity wet. 0.1-0.3 eV. TEe increase of the conductivit' ith the ten,perature was smz'ler in oxygen than in vacuum or he~: :rn. A sharp irrever "~ie d r e - in the con,Juctivity, denoting the real onset of the o.ddation reaction, was noticed at about 250°C. After annealing in oxygen at: 250-400°C both Ro and Rt, measured at room temlx:rature, had incre~sed in such a way that the photo-sensitivities Rd[Rt of the crystals were about 10 times higher than before annealing. The results of pneliminary measurements of the effect of illumination on the adsorption of oxygen on isolating and photo-conductive CdS powders at room temperature were in agreement with those of Kotel'nikovS). After equilibration in the dark the adsorption continues upon illuminavon; no indication of photo-desorption was obtained. 3.2. ADSORPTIONOF HxA In coutrast with O2 and also CO2, gases of the type HxA are adsorbed on CdS in considerable amounts at room temperature. For the compounds given in table 1 isotherms were obtained similar to those shown in fig. 4 for H:S. The ,quantities adsorbed at a given pressure decrease with ~increasing temperature and are for all the gases negligible at 400°C. The greater part of the amounts adsorbed at equilibrium was taken up in about 10 mir~. The

402

~. A. BOOTSMA

i:f 03I-

.,.~-~

,,"

*-------,

-~--20"C . . . . . . . . . . . . . . . . .

~..._......--..-.-- * - ' - ' - - -

70"C

02~ ~ ,~,-.--90'C 01~..,+/~/~''''- ._.,_....-..--165"C " n ~ - ~ ~ - J . ~ '13 I~l 0.2 03 0/, 0,5 06 0.7 0.8 09 t0 1.1 '

,-

1.2

13

~ PH2$ (Torr)

Fig. 4. Adsorption isoiherms for hydrogen sulphide on cadmium sulphide, 20 °C to 165 °C,

values of 02 given in table 1 were o b t a i n e d after evacuation for one hour at r o o m temperature a n d ,give an impression of the rate o f desorption. F r o m these experiments it is n o t clear whether there are different types of adsorption, weakly and strongly bonded, or whether there is one uniformly adsorbed phase. If the last is a s s u m e d and if the adsorptions were reversible (although this is not supported b~ the data in table 1), values of the differential enthaipy TAm.I~. 1 Adsorption data of H~A on CdS al 20 C t%: degree of c~,veragc a~ p 0.6 'l orB, (,~u:~hc same a|ter evacuatio,1

1lBr HCI H2Se H2S NHa PH.~

0.3.~ 0.35 0.30 0.30 0.2~ 0.09

0.35 0.31 0.21 0.20 0.07 0.0~

of adsorption AH : o u l d be c a c u l a t e d from the a d s o r p t i o n isotherms at different temperatures with the C l a u s i u s - C l a p e y r o n e q u a t i o n Aii = R

T; T~ T2 -

TI

in

P2

{4)

Pl

~:or ce:,.:rag¢~ of a b o u [ ~0,;, " " of a mon~.,~klycn ' ~ ,:1~!twould vary from (~ 10 kcaJl mole for N H 3 to HBr. The adsorptio!~ of water was de~ermined for low eqt~ilibrium pwoessures only and is p r o b a b l y similar to that of H2S. T h e electrical effect of fl~e adsorption at room tcxr,,~pera~ure" ' was ~esled qualitatively by determining the conductivity of single crys~a~ in vacuum

GAS ADSORPTION ON CADMIUM SULPHIDE

403

and after admission of the gases (pressures about 1 Torr). The crystals were tl-tin ptatetets ~hich had been epitaxiaUy grown on germanium in our laboratory b y V a n Dijk~0). After being annealed in H , S at 700°C, the crystals were provided with indium electrodes (conducting channel 0.05 x 3 x 2 mm). The crystals were isolating with a dark resistivity of about 10 ~° £~ cm and a photo, resistivity of about I04 f~ ¢m. Replacement of the vacuum by helium had no effect on the corductivity. In cases where an effect with oxygen was found, effects were also observed with gases H~A; both were of the same sign and of approximately the same magnitude: the admission caused a decrease of the order of 20-50Vo in the (photo-)conductivity, which was (partly) reversible by evacuation under illumination. For comparison the adsorption of H , A gases was also measured on zinc sulphide powders, pre-treated in the same way as CdS. The results were analogous, the values of 0i being 0.40, 0.26 and 0.3l for HCI, H2S and NH3 respectively. The assumptions made ir~ the calculations of the degrees of coverage are that of the same cross-section of krypton and of a different cross-section of the adsorption sites for H.~A, namely 13.0 and 11.2 A 2 for CdS and ZnS. In other words, the krypton adsorption is treated as nonlocalized and the H.~A adsorption as localized with respect to the adsorbent. One could also assume that the character of the adsorption is the same. either localized or nor-localized, for both krypton and H~A. The quotients A V/v°,, of the volumes of H~A adsorbed at a pressure of 0.6 Tort and of kryplon adsorbed into the monolayer are approximately the same for HzS (0.45}, but differ consid:rably for NH 3 and HCI. No firm conclusit~ns can be drawn on the basis of these results as to the nature of the adsorplion. It is clear, however, that crystals that have been grown and cooled down in an atmosphere with hydrogen sulphide or etched in aqueous solutions of hydrogen chloride cannot be considered as being fi'ee from surface impurities, unless they have been awmealed at 400"C in vacuum. ~.3.

A I ) ~ O R P I I O N ()F KRYPI{}N AND ARGON

f:or the sur|'a~e delermination~ we made use of lhe B.E.T. isotherm p

=

1

+

c-lp

.

~5)

|~iere ~,,, is the votume adsorbed ink~ a mom:~laycr, ~ the volume of gas adsorbed at a pressure p, p, tile vapour pressure of the ads~,rbale at the lemperature T ( ' K ) of the measuremenls, c a ~~)nstant whicl~ depends on |he nature of lhe adsorbale adsorbent s~ s l e m . =

40~L

G.A. BOOTSMA

F r o m measurements made before and after adsorption of H,,A on tlhe powder, it appeared that the value of the constant c varied considerably with the state of the surface. According to the B.E.T. theory xz), to a first approximation c is given by

(Et - E,~

c = exp \--R-T--1'

(6)

where E 1 is the heat of adsorption of the first layer and E~ is the heat of liquefaction. The values of the B.E.T. constants, derived from isotherms determined at 77°K and 90°K for the adsorption of krypton on clean and gas-cc, vered CdS, are given in table 2. At both temperatures E t is 0.2 kcai/mote higher for the clean surface than for the surface covered with 0.3 monolayer of hydrogen cbloride. For surfaces covered with less HCI intermediate vahtes of c are found. The difference in "Jdsorptive behaviour is clearly expressed in the adsorption isotherms represented in figs. 5 and b for krypton and argon. The surface coverages in fig. 5 were calculated '~vith O=v/Vm. F o r the same powder sample and identical surface condition,; the values of Vm at 90 °K are 7% smaller than at TABLE 2

B.E.T. constants for krypton on CdS, "clean" or covered with HCi (0 ~--0.3~ clean surfi~ce

covered with HC!

T (~K)

p.~ (Torr)

C

F1 - - E t (kcal/mole)

¢

(kcal/rr ole)

77.4 90.2

2. [ 21

400 280

0.92 1.01

100 85

0.7 ! 0.80

*''-÷-'* ÷......-÷-'Z~2.~-o'---

:f .j+f~

0.1

E1 -- St

7Z4"K

_.o......../'--~

0.2

0.3 0.4 0.5 -----'~ PK, (Torr) Fig. 5. Adsorptk,n isotherms for krypton on cadmium stdph[de rat 77.,~°K and 90.2°K. Cr~sse~" c~ean surface, circles" surface covered with 0.3 monolayer HCI.

405

(3AS ~DSORPTION ON CADMIUM SULPHIDE

i

77.4"K

00"31

&

90,2'K i

0.1

0

z

0.1

i

..........

02

',

I

03

0.4

"L-PA(l?,rr)

,

0.5

Fig. 6. Adsormion isotherms for argon on cad rniu ~ sulphide at 77.4°K and 90.2°K. Crosses" c,an surface, circles- surface covered with 0.3 monolayer HCI. 77°K, probably owing to an increase of th.~ cross-section of the krypton atom with the temperature. After adsorption of 0.3 monolayer HC! the Vm'S were 8-9% smaller than before. The surface coverages in fig. 6 have been calculated with v,, values as determined by krvpton adso"ption, making allowance for the difference in cross-sections aK~/C:'A= 1.27 (ref. 21). With eq. (4) and the relation

-AS=-

AH

Pl

Tt + R lnp°

or

-AS=-

AH

p2

7"2 + R lnp°

(7)

values of the differential entropy AS and enthalpy A H can be calculated from the adsorpticn isotherms. Here pO represents the pressure of the gas in the standard state (760 Torr) and the pressures Pl (at Tt) and P2 (at T2) correspond to surface phases that are characterized by the same number of adsorbed molecules/cm 2. Both AH and AS are assumed to be constant over the ~:emperature range 7",- T2. The results of this treatment of the data of figs. 5 and 6 are as follows. T!le adsorption cnthalpy A H increases slightly with the degree of coverage, for krypton frcm 2.6 kcal/mole at 0=:0.3 to 2.9 kcal/mole at 0=0.7, lbr argor~ from 2.6 kcal/mole at 0=0.05 to 2.8 kcal/n~ole at 0=0.15. There is no signflicant difference between A l l for the clean and the HCI-covercd surface. For ~ rypton the values of AS vary from 8 to ! 5 e.u. for the clean surface, and fr:.ma 12 to 18 e.u. for the HCi covered surface, for 0 = 0 . 3 Iv 0,7 respectively. After conversion of these AS valves to entropies valid for the standard

406

G.A. BOO'rSMA

states that correspond to the models of non-localized and localized adsorption, a comparison can be m~te with entropies calculated tbr these 'two models (cf. refs. 22, __,23~}.In all cases the best agreement is found for the model of non-localized adsorption. W:ithin the scope of th~s model the presence of HCI on the surface tends to decrease the mobility c.f krypton anti argon on the surface. The phenomenon as such, natively that the adsorption isotherms for noble gases are dependent upon the n~ture of the surface, has also been found in other cases, cf. krypton on germanium ga), argon on germanium ~4), krypton on silicon ~), krypton an alkali t hloddev ~). It can be considered as a fairly good criterion for tlhe cleanlines,~ of the surface.

Aek nowledgemev~t I wish to thank M~ss M. C. Dusseljee for her assistance with t~e mt'asurements. References 1) 2) 3) 4) 5) 6) 7) 8) 9) I0,) 11) 12) 13) 14) 15) t6) 17) J8)

R. Williams, J. Phys. Chem. Solids 23 (1962) 1057. C E. Reed ,:nd C. G. Scott, Brit. J. Appl. Phys. 16(1965) 471. P. Mark, RCA Rev. 26 (1965) 46I. K. Sawamoto. Japan. J. Appl. Ph.~s. 4 (1965) 173. C. S6benne and M. Balkanski, Surface Sci. 5 (1966) 410. R. H. Bube, J. Electroch. Soc. ]13 (1966)793, A. Many and A. Katzir, Surfa,ze S:i. 6 (1967) 279. V. A. Kotel'nikov, Proc. Acad. Sci USSR. Phys. Chem. Section 155 (19(;4) 330. Sk. J. Haas, D. C. VOx and M. J. Katz, J. Phys. Chem. Solids 26 (1965) 1,'79. A. Many, Y. Goldstein and N. B. Grover, Semiconductor Surfaces (North-Holland Publ. Co., Amsterdam, 1965) p. 113. A. H. iBoonstra and J. van Ruler, Surface Sci. 4 (1966) 141. Gmeliv,s Handbuch der Anorganischen Ci~emie, 8 Auflage, Cd Erg. (VCdag Chemie, GmbH, Weinheim, 1959) p. :;84. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc. 60 (1938) 30~ J. D. Lev~ne and P. Mark, Phys. Roy. 144 (1966) 751. Handbook of Chemistry andPhysics, 45th Ed. (Chem. Rubber Co.~ t964)F. D-38. W. Muscheid, Anr~. Physik 13 (1953) 305. J. Woods, J. Electron. Control 5 (1958) 417. S. Kitamura, J. Phys. Soc. Japan 15 (1960) 2343.

20) H. Van Dijk ancl 3. G,'~ofissen, in: Crystal Groweh, Proc. tmern. Conj~ on Crystal Growth, 8oston, 1966, Ed. H. S. Peiser (Pergamo.~ Press, I ondon, 1967) p. 53t. 21 ) tq. K. Livingston, ,~. Colloid Sci. 4(1949) 447. 22) 3. H. de Boer and S. Kruyer, Proc. Kon. Ned. Akad. Wetens¢~ap. B 55 (1952) 451. 23) A.J. Rosenberg, J. Phys. Chem. 62 (1958) ] 112. 24) M.J. Sparnaay, Solid State Phys. Electron. Telecommun. 1 (1957) 613. 25) D. E. Meyer and ,g. E. Wells, J. C,£1oid Interf. Sci. 22 ('1966) 503. 26) T. Takaishi and Ma~ashi S~fito, ~. ~'Lys. Chem. 7~ (1967) 453.