Photoelectrochemistry of HfSe2 involving two condition bands

J_ EIerrmanaL,Cheni;,138-(!982) 121-129 .. Fjsevier Sequoia S& fausann e - Printed in The Netherlands

. PH_@i@LEtiO-CHEl&lI B-s. .~

STRY .-

OF Hf.!&

12:

INVOLVING

TWO

CONDUCTION

. _ MILTON ;AERAMOVICH’* Lnb0raroir.e GEft&~~e (Racival

and .HtiMUT

TRlBU-ISCH

Inrerfaci~& du CN_RS.,

16th October

1981; ;

revis&

form

**

1, P&e

A. Briand, 9.2190 Meudon-Bellwue

18th February

(France)

1982)

During the investigation of n-type HISe, (AEa = 1.13 eV. with n = 12X lOI cm-3) in contact with aqueous clatrolytcr. both anodic and cathodic photocurreuts were detected. This inversion of the polarity of the photocurrents is only observed during iihunination with photon energies larger than hu =2.5 eV. For photons with hp (25 eV only. anodic photocurrents were found. These reSults are interpreted in

~emx of two narrow d-conduction bands placed at different distances from the Fermi level. the existence of which is compatible with data on optical transmission_ Additional results obtained concern the photocorrosion-of HCSe, to Hf(lV) and elemental selenium, as .well as a very characteristic transient photocurrent rtxponse arising at potentials considerably negative of the, flat-band potential on a newly cleaved surface- The latter effect is interpreted as a photo-cleintercalation phenomenon arising from hafnium atoms pushed on to intercalation or adsorption sites during cleavage. and released as ions during illumination.

INTRODUCTION

Hafmum

selenide

is a ‘layer-type

semiconductor

which

has not previously

been

considered for photoelectrochemical aptilications. It is not only a semiconducting material in the classical sense, able to undergo light-induced electronic processes, but can

also

accept

compounds.induced

through.

lumination

atoms

or

molecules

This intercalation

electrochemical

with a semiconducting

be coupled.

ties of HfSe,

the layers

mechanisms.

intercalation

to form

Since. these arc ionic process,

in: the dark

intercalation

the deintercalation,

material, photo-electronic

One. type .of such a light-induced

after eltitrochemical publication.[

between

and its inverse reaction,

dependent

electrolytes

on

photo-deintercalation

has been discussed

in a preceding proper-

will receive attention_

* On leave from Institute de Fsica UNICAMP CP 1170. 13100 Campinas, SP. Brazil. t* Peimatient-add+sr Hahn,Meitner-Iustitut fur Kemforschung Berlin GmbH. B&&h Strahlenchemie. D-1000 Berlin.39. Glienickcr Strase 100. F.R.G_ .. : ‘~

.0@2-02~~/82/00=~/~0~.7~

-‘a 1082 ECLyi& Se&oia

S-A.’

il-

and ionic processes can

I]:--In this work some of the-more genera! photoelectrochemical in contact with .aiueous

can be

122

EXPERIMENTAL

The

HE!+

ments

samples

showed

3.5 X lc)-’

were obtained

them to be n-type

from Prof.

with

I.2 X

Nitsche,

lOI

Freiburg.

electrons

O-cm and an electron mobility of 15 cm* V-’

cmF3,

Hall

measure-

a resistivity

SC’_ The layer-type

of

crystals

[space group D&(l), octahedral coordination] were prepared for the experiments by cicaving off a thin sheet of surface material by means of an adhesive tape. The crystal was then mounted clean

surface

contact contact

became

in a Teflon

exposed

cell (described

to the electrolyte

in ref. 1) in such a way that the

while

the opposite

surface

made

with a platinum sheet to which the electrolyte had no-access. The electric between the crystal and the platinum was accomplished through an exter-

nally applied pressure_ The crystal surface exposed to the electrolyte which was studied during the experiments was the surface I c, i.e. the van der Waals surface. Microscopic

studies showed

the existence of steps also exposing

surface areas II c.

The photoelectrochemical experiments were performed with a setup comprising a 900 W mercury-xenon lamp, a high-intensity monochromator, a chopper, potentiostatic control and a phase-sensirive

detection system. Only monochromatic

light was

used for the experiments_ Potential measurements were made against a me&rous sulphate electrode to avoid the presence of Clions (possible involvement in oxidation reactions). The potential of this MSE electrode is O-41 V more positive than that of the saturated SOLID

STATE

HfSG

PROPERTIES

calomel electrode_ OF HIS+

is a semiconductor

with an indirect band gap of A

E, =

1.13 eV [2]. Band

structures have been calculated [3.4]. The general features resemble those of Ti and Zr sulphides and selenides. Energetic positions of bands are listed in ref. 5. It is generally Optical

agreed

that the valence band

spectra have been provided

of HfSe,

is derived

by Greenaway

from selenium

and Nitsche

orbitals.

[2] and Wilson

and

Yoffe [4], as well as by Beal et al. [7]_ The latter authors observed a sharp absorption peak preceding an absorption band having a width of approximately 1.5 eV. Between both there is a transmission arising from replacing

for the existence of two narrow localized

d-conduction

nature of the d-orbitals

HE-sulphide

and

window.

From observations

selenide

which allow only a small metal-metal

the combined

before

the next conduction

evidence

bands. Their width is controlled

width

of

valence band states is estimated to be approximately then follows energy.

of spectral changes

Hf by Ti and Zr, as well as Se by S, they obtained

the lower

conduction

-1.5 eV_ A trar.smission

band is reached some 2.0-2.5

by the

overlap. and

In the

window

eV higher in

RESULTS

When a HfSe, crystal aqueous

electrolyte,

with a freshly cleaved surface

arm&c

photocurrents

is studied in contact

of the order of 10 pA

cm-*

with an

(monochro-

123

matic ilhunination at h = 579 nm) are observed at quite negative electrode potentials starting from -0.9 V (MSE) (or -0.5 V measured against a calomel electrode)_ A remarkable property of these photocurrents is that they are only seen during the first voltage sweep from negative to positive voltages. During subsequent runs the photocurrent decreases more than tenfold. This transient photocurrent response has been studied with a larger number of freshly prepared surfaces under well-controlled conditions_ The sweep started at - 1.1 V and proceeded at a rate of 10 mV s-’ in a positive direction. The results of four experiments are shown in Fig. 1. It can be clearly seen that a current peak is traversed which would seem typical for a one-time discharge of the electrode surface induced by illumination. Up to five current oscillations could be observed near the top in a reproducible way. This is a clear indication for an autocatalytic reaction occurring at the electrode-electrolyte interface. The charge released by this mechanism is somewhat larger than one monolayer if one electronic charge is considered per atom. After relaxation of this transient phenomenon to < 10% of its peak value during the next and subsequent sweeps, a stationary situation is finally reached in which photocurrents basically arise from two contributions-photo-deintercalation and anodic photocorrosion. The first phenomenon is observed after the electrode has cathodically been intercalated with suitable ions in the dark for some time and was discussed in the preceding publication [I]. It can be suppressed when electrode potentials more negative than -0.5 V (MSE) are avoided. The contribution to the photocurrent arising from the anodic photo-oxidation of the HfSe, electrode can be observed at electrode potentials more positive than -0.35 V (MSE). Figure 2 shows that a limiting anodic photocurrent

typical for n-type

semiconductors

light is from the visible or near-infrared rent is observed

is reached,

when

the illuminating

(IR) spectral region_ No cathodic

at more negative electrode potentials. However,

photocur-

cathodic photocur-

” (MSE)/V

Fig. I. Transient

photocurrent

during

potential

the

wavelength

first

response

sweep

with

of the incident light: X =579

(shown a newly

for four different cleaved

nm; potential

electrode

sweep:

crystal surface.

10 mV 5-I.

samples)which Electrolyte:

is observed 1M

NaOH:

124

“E

HfSaI(lM

1,

”

H,SOd FB 1

* <

$ ; :: t=i 5

/ ./;__-

0.5. p

z m c d

/

/

AZ519

-.__

.-----

_

__-----yar-__

/

I -a5, -1.

-1.0

- 0.5

0.5 U/MSE

;:

Fig. 2. Photocurrent-voltage response of HI&, of different wavelengths (lock-ln technique).

in contact

rents

occurs

are

found

Depending

on

consequently

when

illumination

the energy

behaves

of

incident

indicated

in Fig. 2. It should

flat-band

theoretically

I M H,SO,

with

light,

either as an n-type

intrinsic) electrode_ The apparent

with

under illumination

ultraviolet

(UV)

the semiconducting

electrode potential

or a p-type (FB)

of ZrSe,

be the potential

or HfSe,

with light

blue

light.

electrode

(compensated

or

in 1 A4 H,SQ

is

at which anodic currents

-1. ; . :

2

- O.? ”

0

s

z

-2.

\, 1.5

2.5

3.5

Fig. 3. Spectral dependence of photocurrents absorplion co&Gent Q is plotted, in logxritic

al electrode potentials scale. for comparison

be~wccn -0.3 and -03 (ref. 7. taken at-5 K).

V_- The

125

p&%i&ifcaChodic~

ones (protidedthat-smaller

deviations.(C

50 mVj

such as those.

~p+sibly‘ arising-_~~m~a-Dernber .potential are neglected). The flat-band potential &+ld ndt’t@cdnfirmed by.&pacily tieasurements owing io the relatively large dark .,.1 cui-rents of:ihe e&&de._j .. .:-, ,~.’ ~~:Figtire 31khows the spectral Vi where

-9.7

dependence

both %nodic,and

cathodic

of-the photoresponse photocurrents

at -0.3,

-0.5

and

are visible-~ It can be seen

that dathodic photocurrentsarise. only at photon energies hv > 2.5 eV with a maximum between 3 and ‘3.5 eV, while visible and near-IR light also contribute to the ariodic photocurrent. Two different energy bands are apparently involved with different phdt&&ctrochemical proper&s. -1 ’ The anodic phot-rrqsion-process of HfSe, has been studied in some detail. Elemental

selenium is liberated

according

to the.reaction:

HfSe,+~&~j,4e-(hv)-+Hf(IV)+2Se+4e-

The generation

(1)

of .a red layer of se!enium

surface. Since.it absdrbs

can readily be observed

on the electrode spectral region, ils formation is also

lighi in the blue-green

spectrum of HUSSY,taken at 0 V as shown in Fig. 4, which

evident in the photocur&

gives the. photocurrent response during -subsequent measurements. These spectra confirm the band gap of HfSe, to be 1.13 eV, as found by Greenaway and Nitsche

PI- : Cathodic photocurrents, which are observed dsring illumination with blue and UV light involve reduction of protons and oxygen, but can equally produce intercalation of positive ions, i.e. of protons in the presence of 1 M H,SO, in the electrolyte. The latter phenomenon is partly irreversible with an aqueous electrolyte and has not been studied in detail.

HfSe,

In .=:

(1M

H,SO,)

ov.

2-

5.

.

absorpllon,

AEG= 1,13 ev

t

2

lhm_qh

- - -

”

z :~ z Jz (1. : .l

-

-_)

selmium

\

FJz Fa3 k

scallermg

demenlal

1.

0 //I~:.

/-\ r,‘\ -’

\‘,

I

2\.‘\ \\

. . ,-

\‘, 1,s.

~.

L‘-,-

.‘-2

2.5

photon Fig: 4. Specti measur&n&ts

eneigy

/ eV

. .. .

~ddcpend&t~.oC anodic pho&&en&taken al.0 V. (MSE); 0. I, 2 indicate which SCOW the effect of~elemenlal selenium formed at the electrode surface_

~.

skbsequenr

~.

126

The electrode currents [ -0-37 is identified Dember

potential V (MSE)

at which

anodic photocurrents

in presence of 1 &f H,SO,,

as the flat-band

potential

of HfSe,.

effect which arises due to the possibly

holes. Since it is generally flat-band

potential

invert to cathodic

photo-

or +0.27 V against hydrogen]

An error could be induced different

mobilities

by. the

of electrons Andy

much smaller than 50 mV, i.e_ within the error of many

measurements,

it is neglected here.

DISCUSSION

The anodic photocorrosion

of HfSe,

into Hf(IV)

since the valence band of layer type HfSe,

and selenium

is not derived

is not surprising

from d-states as in MO!+

or WSe,, but from selenium p-states, as in CdSe. Passage of holes into the electrolyte is consequently associated with the Liberation of elemental seleuium and positive hafnium

ions.

However, mechanisms relaxation

a remarkable

phenomenon

which depend

on the energy of exciting photons.

or holes

to take part

states, where

of photoelectrochemical It is well known

that

to the lowest states occurs

within an energy band

of the order of 10-‘4-10-‘5

is the occurrence

within a very short time would leave no time for electrons

s. This fast relaxation

in electrochemical

reactions

the lifetime is considerably

longer.

before

In order

they occupy to explain

the lowest

the fact that

there are electronic carriers which differ in terms of the energy of excitation (Fig. 3) it has to be concluded

that two distinct energy bands

trochemical

Each has a lower band

which

processes.

is sufficiently

long

for

the electrochemical

Fig. 4 and Fig. 3 it can be deduced the valence simplified

band

evidence authors

and

the position

to approximately

region

or

of

turned

out to be strongly

the first

< 1.5 eV

temperature

[7]. It is possibly

spectra (Fig. 3). Excitons The second narrow

at approximately

at approximately

2.5 eV. A

of two energy bands found

absorption

a in Fig_ 3) and of

the second

additional

hafnium

disulphide

conduction

band

and some

peak at 2.73 eV ((r in Fig. 3) itself

dependent

might recombine

d-conduction

From

above the valence band [7]. These

d-band

and

for this reason

and was attributed

to excitonic

that it is absent in our photocurrent

without

generating

free charge carriers.

band with a peak near 3.3 eV is, on the other hand,

clearly visible. Its edge, which indicates

UV

d-bands

have a lifetime

to proceed.

by Beal et al. who (compare

eV higher in energy. The strong absorption

contributions

found

and the second

results obtained

near-WV

in the photoelec-

band is at 1.13 eV from

in Fig. 5. The recognition

for the presence of two narrow estimated

diselenide 2-2.5

transition),

with optical

in the visible

mechanisms

that the first conduction

energy scheme is depicted

is in agreement peaks

(indirect

are involved

edge where electrons

the width of the second energy gap, can be

2.5 eV.

Since the flat-band potential of HfSe, can be identified with the passage of the photocurrent through zero and since the material is n-type (n = 1.2 X 1019

cme3)

having

the Fermi level near the edge of the lower conduction

distance assumed), depicted

the relative position

(Fig. 5). It shows

band

(0.1 eV

of energy bands and redox potentials

that the hydrogen

potential

is situated higher

can be

than the

:

127

b)

a.1

Fig.5. Simplified scheme of density position and the position of the redar

of electronic states potential of different

and band structure of H&. The flat-band redox couples is shown on the right hand side.

edge of the lower conduction band. It is apparently for this reason why cathodic dark currents (in the presence of 1 M H,SO,) remain small and why cathodic photOeffects by way of the second conduction band become detectable. The energetic and kinetic situation at the HfSeJelectrolyte interface during

ENERGY/~

I

bYI -He/

H,

Se I

-

Hf(lVl

3eclrolyte

Electrolyte

~b)

a) Fig- 6. Enc@tic-and photckkucxhaui~

HlSe

kinetic’&atiod at anodically reactivity for the two different :

(a) and cathodically polarized claxronic t.&.nsitioos.

(b) Ht?+

explaining

128

anodic and cathodic polarization is further explained in Fig.6. During the anodic reaction, excitation into both conduction bands lead to electron-hole separation and photocorrosion. The spectral sensitivity-covers all the region from the UV to the IR. During cathodic polarization the photoelectrochemical reactions originating from excitation into the two conduction bands are different. Electron-excitation~into the lower conduction band does not lead to a photocurrent, since there are already many electrons in this energy band and since they cannot reduce protons in any case. Excitation into the second energy baud leads, on the other hand, to photoreduction processes, since the edge of the involved energy band is energetically sufficiently high. Thermodynamically speaking, we are deahng with an intrinsic or slightly p-type semiconductor when exciting into the second d-band (the Fermi level is lower than the middle of the gap) and with an n-type semiconductor when exciting inlo the lower band (the Fermi level is close to the conduction band). The remarkable transient photoelectrochemical phenomenon observed only once at a freshly cleaved HfSez surface remains to be explained (Fig. 1). Since photocurrents start to occur near - 1.0 V, several hundred mV more negative than the flat-band potential of the electrode, it _cannot be discussed in terms of the photoeorrosion reaction (1) leading to the liberation of elemental selenium. [A change from an acid to a basic electrolyte shifts the photocurrent onset only slightly towards negative potentials (100-200 mV).] Since similar negative photocurrents have been observed during the light-induced deintercalation of sodium from HfSe, it is tempting to assume that the transient phenomenon is also a photo-deintercalation reaction. However, the effect observed with sodium in the presence of an aqueous electrolyte was one order of magnitude smaller and did not show oscillatory behaviour. What we suggest as a working hypothesis is that during cleavage some of the Se-Hf-Se sandwiches are disrupted and hafnium is accumulated on intercalation sites or adsorption sites on the electrode surface in the atomic state. Since their transfer

into

the ionic state corresponds

to a fairly

low

positive

electrochemical

potentials than the flat-band potential. Thermodynamically speaking, it would only be necessary to lower the quasi-Fermi level of holes below the oxidation potential of these hafnium species. This mechanism is consistent with the arguments on photo-deintercalation expressed in ref_ 1; however, it has to be emphasized that many details, especially the oscillatory behaviour of photocurrents. will have to be studied in more detail. potential

it could

occur

at more negative

CONCLUSIONS

Hafnium is only 100 times less abundant than zirconium, which is the eleventh most abundant element on earth. It has, furthermore, to be separated from zirconium, when this metal is used as nuclear reactor material_ Therefore, in the future it will not be less available than, for example, cadmium. We have shown that hafnium selenide could be considered for photoelectrical purposes_ Of possible scientific value for photoelectrochemical research would be the participation of a higher conduction band in light-induced surface reactions. Besides being a novel physical-chemical

129

-system which is worth while being explored, reactions

involving

two energetically

new aspect into photocatalytical

it is evident

different

that photoelectrochemical

conduction

bands

will introduce

a

mechanisms.

ACKNOWLEDGEMENTS

The authors Conselho

would

Nacidnal

C.I.E.S., France (H-T.).

like to thank the following

de Desenvolvimento (M-A.);

The encouragement

The

Deutsche

organizations

Cientifico

for fellowshpis:

e Tecnologicc-Bras4

Forschungsgemeinschaft,

given by Dr. Roger

Parsons

is gratefully

Bonn,

and

the the

F.R.G.

acknowledged.

REFERENCES 1 2 3 4 5

M. Abramovich, U. Gorochov and H. Tribulwh. J. Electrochem. Sot.. submitted. D.L. Grecnaway and R Nitschc. J. Phys. Chem. Solid, 26 (1965) 1445. R.A. Bromley and R.B. Murray, J. Phys.. C5 (1972) 738. R.B. Murray, RA. Bromley and A.D. YofTe, J. Phys.. C5 (1972) 746. C.Y. Fong and M. Schliiter in TJ. Wieking and M. SchlGter (Eds.), Uecrrons nnd Phonons

Strucrures. Reidel. Dordrecht-Boston, 1979, p_ 166. 6 J.A. Wilson and A.D. Yoffe. Adv. Phys., It3 (1969) 193. 7 A.R.

Beal. J.C. Knights

and W.Y. Liang,

J. Phys. C: Solid State Phys.. 5 (1972).

in Layered