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Surface photovoltage and XPS studies of electronic structure in defective nickel oxide powders
L245
Surface Sctence 169 (1986) L2455L252 North-Holland.
SURFACE
Amsterdam
SCIENCE
LETTERS
SURFACE PHOTOVOLTAGE STRUCTURE IN DEFECTIVE
AND XPS STUDIES OF ELECTRONIC NICKEL OXIDE POWDERS
Neil R. HUCK School of Muthemarical 6150, Austrulru
and Ph_psrcol Screncess. Murdoch
timrwurv.
Murdwh,
Wesrern Ausrrrrlu
Roger St.C. SMART
and Stephen
M. THURGATE
School of Muthematlcal and Ph.vsrcol Scienc~es. Murdwh Western Australia 6150, Australro Received 9 April
1985; accepted for publication
Surface photovoltage revealed dtfferences
measurements
lJnirer
18 December
of polycrystalline
powder
samples of nickel oxtde have
tn band bending (k’,) related to the defect concentrations
powders prepared at 700°C
and 1450°C
of extended defects. but considerably
The defective
(700°C)
under UV illumination.
of the oxide. The
tn air consist of small single crystals with low concentra-
tions ( < 10’ cm-*) NtO
1985
different
nickel vacancy concentrations.
gave V, = 760 mV and the equiltbrated
NiO
(1450°C)
V, = 230 mV
possibly due to Fermi level pinntng of the Ni 3dX band near V;,
and k’;,.
respectively. These results are consistent wtth values of V, (i.e. 630 and 240 mV. respectively) these two samples obtained charging on temperature. equilibrated
NiO
(1450°C)
discussed in relation
previously
from XPS
measurements
of the dependence
for
of surface
A visible transition at = 2.5 eV. gtwng &#I = I35 mV. IS also found in the sample but not m the defecttve NiO
to other work on the electronic
(700°C‘)
sample. The results are
band structure and surface states of ntckel
oxtde.
In this work we report surface photovoltage measurements on nickel oxide powders. prepared with different defect levels controlled by decomposition and annealing [l-4], carried out in order to study the dependence of Fermi level pinning (or band bending), IJ’,, on the defect properties of the oxide. Two preparations of NiO, annealed at 700°C (i.e. NiO,,,,,) and 1450°C (i.e. Ni0,45,,) for 4 h in air, have been extensively characterised for purity. surface structure, morphology and reactivity, and defect properties [l-8]. High resolution transmission electron microscopy shows that the particles of the powders consist of 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
N.R. ttuck et aL / l)e/ectit'e NiO powdei'~
L246
highly perfect single crystals with relatively low concentrations of dislocations and other extended defects (i.e. < 10 ~ cm 2). Thus, bulk conductivity in these single crystals is primarily due to defect-controlled hole conduction in the p-type oxide and not to enhanced conduction at extended defects or grain boundaries. Clearly surface conductivity, also defect-controlled, is likely to be of major importance in these polycrystalline powder. The NiOv00 and NiO145o samples differ in surface area (i.e. 2.6 and 0.36 m 2 g i respectively) [1], the extent of surface facetting [1,2], estimated bulk conductivity (i.e. 3.1 × 10 2 and 5.0 × 10 5 ohm 1 m 1 respectively) [3], surface defect (i.e. l/yi, O and Ni 3+) concentrations and reactivity [3 5,7]. The property of interest here, however, is the difference between the two samples in nickel vacancies (and associated defects) in the bulk and at the surface. An important consideration in the work was the opportunity to compare the V, values with those obtained for the same samples from X-ray photoelectron spectroscopy (XPS). XPS studies of polycrystalline, p-type semiconducting nickel and manganese oxides have revealed dependence of the surface potential ( V h ) (obtained as kinetic energy shifts) on X-ray power [6,8], bulk oxide defect concentrations [3,4,7], electron donor and aceptor chemisorption [4,5,7] and temperature [4,8]. An explanation, based on consideration of the mechanisms for development of surface potential during X-irradiation, for the dependence of ~,h on X-ray tube current, band bending at the surface (V), and bulk oxide conductivity (o,) has been advanced [3,8]. At high X-ray power (i.e. above 200 W), 1/{.h is given by:
<"-
( X I - K~)
t' e ~ ' ~
expt
l
where K~, K 3 are constants, B ( % ) represents a functional dependence on bulk conductivity of the sample and ~ is the band bending due to Fermi level pinning at the surface. Linear experimental plots of In le[:h versus T-1 were obtained for samples of NiO and MnO with different defect levels controlled by decomposition and annealing [1--4]. From the XPS studies, NiO700 gave much larger values of B(oo) than NiOi450, as expected, and also larger values of ~ i.e. NiO700` ~ = 630 mV; niOi451 }, l/~ = 240 mV [8]. The XPS evidence for the values of V, is indirect and subject to the assumptions made in the explanation, principally that electron conduction to the irradiated surface depends mainly on conduction in the surface layer. The explanation of conduction in nickel oxide is the subject of intense investigation, e.g. refs. [9 12], and remains contentious. Other mechanisms have been suggested to explain the effect of temperature on the kinetic energy shifts in XPS spectra. For instance, charge removal in the temperature range 20 80°C from a (100) surface of nickel oxide has been attributed [13] to increased hole mobility. The concensus of opinion, however, suggests that hole mobility in the bulk of single crystals is not activated [9] although thermal activation may
N.R. Huck et al. / Defectiue NiO powders
L247
occur in the surface region. Different mechanisms of conduction in the surface involving small-polarons in a narrow Ni 3d ~ band, or activated hopping in localised nickel vacancy levels, have been suggested [4,10]. In this work we set out to measure V, directly for NiO700 and NiO145o samples using surface photovoltage techniques. The two values of Iz obtained from XPS are sufficiently different to suggest that evidence from a second technique might assist in evaluating the degree of confidence to be placed on the XPS determination of ~ values. The NiOv00 and NiO1450 powders were prepared and characterised as described previously [1 ]. The NiOT00 powder was pressed at 10 tonnes pressure into a 13 mm diameter, coherent pellet in a standard KBr die press. The NiO1450 pellet was not coherent after pressing and was sintered at 1000°C in a muffle furnace for 1 h before mounting. A linear I / V plot, obtained using a gold-tipped, flexible probe, showed that the contact of the NiO with the tantalum back plate on the photovoltage mount was ohmic. The mount was fixed inside a UHV apparatus and baked out at 150°C for 12 h to 2 × 10 ~9 Torr. The pellets were heated to 500-600°C in vacuo for 1 h before voltage measurements were made. This procedure has been shown previously [7~8] to remove adsorbed H 2 0 and the majority of surface hydroxyl and carbonate groups. Surface voltages were measured using a vibrating capacitor Kelvin probe technique. The reference probe was constructed from a gold plated circular metal grid of ~ 10 mm diameter, positioned as close to the oxide surface as possible without contact and set vibrating at a natural frequency of 500 Hz as indicated schematically in fig. 1. The modulated capacitance between the Au probe and oxide sample generates an AC current which is detected using a circuit similar to Brillson [14]. The signal was anaplified by an electrically shielded current-to-voltage preamplifier (10 8 A / V ) then fed to a Princeton Applied Research PAR-5206 lock-in amplifier. Negative feedback from the lock-in amplifier is applied to the insulated sample and used to null the CPD. Thus the workfunction difference, q,(sample)-~(Au), can be continuously monitored by following the lock-in output. Illumination of sample was made through a glass viewport that gave flat transmission in the energy range used. Before discussing the surface voltage measurements, some general observations on the insulating behaviour of the nickel oxide samples are necessary. Equilibration of surface voltages on the pellets without illumination is relatively slow and varies considerably with pretreatment of the sample. For NiOToo after outgassing at 25°C, a steady value for the work function (~) of 4.97 V (assuming ~ ( A u ) = 4.82 V) is obtained in < 5 min and this value is unchanged by illumination with visible (I2/quartz 100 W) or UV (Hg/quartz, 90 W) light. After evacuation at 150°C, equilibration is much slower, i.e. -- 4 h and, following evacuation at ~ 500°C, a steady value of ~(NiO700) = 5.04 V
L248
N.R. Huck et al. / Defecti~,e NiO powders AUDIO AMPLIFIER
SOLENOID
OSCILLATOR
<1
•
" U.H.V.
~ VIEW
PORT
SAMPLE
Au
REFERENCE ,N
PREAMPLIFIER 10 - 8 A I V
COMPENSATION VOLTAGE TO S E T I N T I A L ZERO VOLTAGE
PAR - 5206 LOCK-IN AMPLIFIER OUT
E
I '
RECORDER
Fig. 1. A schematic representation of the e x p e r i m e n t a l s } s t e n and circuit used for surface p h o t o v o l t a g e determination.
without illumination is obtained only after 9 h. NiO1450 after evacuation at = 500°C also requires > 10 h to reach a steady value of qb(NiO145o)= 5.30 V without illumination. If shorter equilibration periods are used the voltage measured is found to arise from undischarged, externally applied bias, either applied directly in the capacitance circuit or as electron current from the ion gauge filament in the UHV system. Surface photovoltages were measured on both samples after 500°C degassing with both visible and UV lamps. For NiOT00 the visible lamp had no obvious effects but the UV lamp shifted the voltage to a steady 5.80 V reproducibly in < 2 min. This shift corresponds to a Jg,(NiOT00)=760 mV, giving photoconduction at the fiat band potential. For NiOl4s0, the UV lamp shifted the voltage to a steady 5.57 V, reproduci-
N.R. Huck et al. / Defective NiO powders
L249
bly in < 2 min, corresponding to A~(NiO~450) = 230 inV. The visible lamp also gave a measurable, reproducible shift to 5.42 V or A~ = 135 mV. Using a set of band pass filters it was possible to isolate the transition giving rise to the photoconduction in the visible region to absorption at 480-490 nm or --- 2.5 eV. The absorption appeared to have a broad tail to longer wavelengths. It was evident that electron conduction to the surface was responsible for the Ag, shifts after UV illumination because surface charging due to electron current from the ion gauge filament always shifted the voltage in the same direction, i.e. to more positive values. Since the measurements were made on polycrystalline powder samples, surface species must be considered in addition to the band structure of the oxide. It is known from XPS studies [4,5,7] that, after the pretreatment used in these experiments, the surface contains considerable concentrations of defects as nickel vacancies (VNi) with associated O - and Ni -~+ ions. For NiO700 and NIO1450, O / 0 2- ratios of = 3.0 and =0.2, respectively, and similar Ni3+/Ni2+ ratios are found [7]. Small quantities of hydroxide ( < 2% monolayer), carbonate ( < 2% monolayer) and carbon ( < 5% monolayer) also remain but no other adsorbates are detected. It is therefore likely that the main contribution to Fermi level pinning is associated with the nickel vacancies and associated defect species. In order to discuss this possibility further, we need to consider the electronic band structure of NiO at the surface. The most recent reviews of the surface electronic band structure of NiO, summarising previous theoretical and experimental work [9-12], have been presented by Henrich [15], Tsukuda et al. [16] and Dare-Edwards et al. [17]. Henrich [15] notes that theoretical estimates for the NiO (100) surface indicate only slight difference between bulk and surface band structure using LCAO theory [18], whilst D V - X a cluster calculations [16], comparing (NiO 5)~- with (NiO6)l°_, suggest a narrowing of the bandgap (i.e. O 2 p - N i 3d 9) at the surface of = 0.8 eV. These estimates do not, however, provide work function values for comparison with our surface photovoltage measurements. DareEdwards et al. [17] have considered the available experimental and theoretical evidence to derive a semi-empirical band structure for NiO, including surface states known from capacitance and DC cyclic voltammetry measurements, reproduced in fig. 2. The comments that follow are based on the recognition that there are limitations on the extent to which a bulk band structure for equilibrated NiO (with departures from stoichiometry represented as energy levels for VN, and V ~ ) can be used to interpret a surface phenomenon. Nevertheless, the bulk structure represents the only available systematic description, based on experimental verification of theory, for consideration of work function values and changes observed in surface photovoltage measurements. In our case, to a first approximation, UV illumination giving photoconduction should correspond to the flat band situation in single crystals.
L250
N.R. ~luck et al. / Defecttt,e .¥1(_)powder.s
@(V)
3"5
3-6eV 2-2eV
5"3
Li;NiO
5.7
Ni
3d 8
EF
6"0
7.1
00000000000000000000000000000
000000000000000000000000000000( 0000000000000000000000000000000 000000000000000000000000000000~ 0000000000000000000000000000000 000000000000000000000000000000( ooooooooooooooooooooooooooooooo 000000000000000000000000000000( 0000000000000000000000000000000 000000000000000000000000000000( 0000000000000000000000000000000
occupied level
O0
0 2p
~
unoccupied level
surface state Fig. 2. A representation of the e l e c m m i c band structure of NiO, in the flat-band configuration, based on refs. [lO 16].
We will assume, as do others [17], that the F e r m i level is close to the top of the occupied Ni 3d ~ band. The values of ~(NiOT00)= 5.80 V and ~(NiO1450 ) = 5.57 V for p h o t o c o n d u c t i o n c o r r e s p o n d i n g , in theory, to the flat b a n d situation are sufficiently close to the value of 5.70 _+ 0.3 V in fig. 2 (i.e. the last o c c u p i e d electronic level in the Ni 3d ~ b a n d ) to give some confidence in the correlation with other results. A n y e x p l a n a t i o n for the difference in 'k for
N.R. Huck et al. / D e f e c t w e NiO powders
L251
p h o t o c o n d u c t i o n between the samples is speculative but it may be due to real differences in the band structure caused by the smaller crystal size and very defective structure [1,2,7,8] of the NiO700 sample leading to qualifications in assumptions of lattice periodicity and semi-infinite crystals. The Aq~ results of 760 and 230 mV for NiOT00 and NiOi450 are consistent with the values obtained from the XPS measurements, i.e. 630 and 240 mV, respectively. The lower value of Adp(NiO700 ) obtained from XPS probably arises from the relatively large contribution of B(o,,) in this defective oxide as explained in the earlier paper [8]. The J¢~ for NiOToo corresponds roughly, given the errors in these values, to pinning of the Ni 3d ~ band in fig. 2 at V~, (i.e. 4.9 eV)~ i.e. a Ni 3+ ion localised at a nickel vacancy, with a value of Aq,(NiOT00) = 800 mV. The A~(NiO1450 )value may be due to two Ni ~+ ions localised at a nickel vacancy but fig. 2 would then suggest a A~(NiO~450) of 600 inV. Dare-Edwards et al. [17] have pointed out that stabilisation can be achieved by Ni 3+ ordering in arrays on the surface. Surface states in their system have been stabilised by 200-500 mV and this would explain our result. This is not inconsistent with the known defect level occupancy of the two oxide samples. The much larger concentration of defects, manifested as nickel vacancies and adjacent Ni 3+ ions, in NiO7oo [1,3,4,7,19] might be expected to give rise to singly ionised nickel vacancies, whereas there is less tendency for ionisation of vacancies in the equilibrated NiO~45o. In the absence of more detailed information on surface electronic band structure and surface states, the above assignments can only be speculative but they do correlate with the semi-empirical band structure of fig. 2 [17]. The visible transition at ~ 2.5 eV, causing limited photoconduction in NiOi450 (which is not evident in NiO700) cannot be explained with confidence. Photoconduction in the visible region is not normally observed in single crystals. In our case, conduction is found to be due to an electron current to the surface presumably from electrons in the Ni 3d 9 band. The conduction may arise from promotion from an unspecified surface slate or, possibly from a N i 3 d S ~ N i 3 d 9 transition (fig. 2). The value of A4~(NiO1450) for this transition is 135 mV, i.e. significantly less than the 230 mV found for UV illumination. This is likely to be due to a difference in photon-capture cross-section between this transition and the O 2p -, Ni 3d ~ transition. The reason for the lack of effect of the visible illumination on NiOToo is not obvious. It is possible that the rate of discharging may have been too slow for observation and, again, the more defective nature of the small crystals may be responsible for lower mobilities of electrons in the Ni 3d ~ band. There is evidence that, under UV illumination, the NiOH50 is a better conductor than NiOT0 o. This accords with the view that the structure of the Ni 3d ~ and Ni 3d ~ bands is not as well defined in the defective NiO7oo crystals. Further, more detailed study is continuing to test the kinetics of discharge in both samples and to investigate the spectral response across both the visible and UV spectra.
L252
N.R. l l u c k el aL / Defi, ctire NlO powders
These results provide several conclusions: t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s in b a n d b e n d i n g , V,, b e t w e e n d e f e c t i v e N i O ( 7 0 0 ° C ) (i.e. V - 7 6 0 m V ) a n d e q u i l i b r a t e d N i O ( 1 4 5 0 ° C ) (i.e. V = 230 mV); the work function
values suggest that
1/ m a y a r i s e p r i m a r i l y
level p i n n i n g o r e n e r g y l e v e l s c o r r e s p o n d i n g surface singly (NiO7~0) t r a p p e d N i -~+ i o n s ; the surface photovoltage
1~ v a l u e s a r e c o n s i s t e n t w i t h 1/~ v a l u e s f r o m X P S
p l o t s o f In F~,~, v e r s u s T
~ and provide evidence that the XPS
useful for obtaining
or
doubly
from Fermi
t o e x c e s s n i c k e l v a c a n c i e s in t h e
(NiO145,)
compensated
by
adjacent
method
is
V estimates;
a v i s i b l e a b s o r p t i o n at = 2.5 e V f o r NiO~aso ( o n l y ) , g i v i n g l i m i t e d p h o t o c o n d u c t i o n , m a y a r i s e f r o m a n u n s p e c i f i e d s u r f a c e s t a t e o r a N i 3 d s ---, N i 3 d ~ transition. The
work
is s u p p o r t e d
financially
by
the
Australian
Research
Grants
Scheme.
References I1] C,F. Jones, R i . Segall, R.St.('. Smart and P.S. Turner, J. Chem. Soc. Farada'~ trans. 1. 73 (19771 171(/. 12] C.F. Jones, R.L. Segall, R.St.C Smart and P.S. l'urner, J. ('hem. Soc. Faraday Trans. I, 74 (1978) 1615. 13] M.W. Roberts and R.St.('. Smart, ('hem. Phys. Letters 69 I19811) 234. [41 M.W. Roberts and R.St.('. Smart, Surface Sci. 100 (19801 590. 151 M.W. Roberts and R.St.('. Smart, Surface Sci. 108 (19811 271. [61 R.St.C. Smart, Surface Sci. 122 (19821 L643. 17] M.W. Roberts and R.St.('. Smart, J. ('hem. Soc. Faraday trans. 1, 80 (19841 2957. [81 M.W. Roberts and R.St.('. Smart, Surface Sci. 151 (1985) I. 19] J.B. Goodenough, Progr. Solid State Chem. 5 (1971) 145. I1(1] A.J. Bosman and H.J. van Daal, Advan. Phys. 19 (1979) I. I11 ] Z.M. Jarzebski, Oxide Semiconductors ( Pergamon, Oxford, 1973 ). [121 I). Adler, The Imperfect Solid Transport Properties, in lreati:,e on Solid State ('hcmislr 3. Vol. 2. Ed. N.B. ttannay (Plenum, New York, 19751 p. 237. [13] F.P. Netzer and M. Prutton. J. Phys. ('8 (19751 2401. 1141 L.J. Brillson. Surface Sci. 51 (1975) 45. [151 V.E. Henrich, Progr. Surface Sci. 14 (19831 175. [161 M. Tsukuda, H. Adachi and ('. Satoko, Progr. Surface Sci. 14 (1983) 113. 117] M.P. l)are-Edwards. J.B. (~oodenough, A. tlamneU and N.I). Nicholson. ,I. Chem. Soc. Faraday ~Irans. II, 77 (19811 643. [18] V.-('. l,ee and H.-S. Wong. J. Phys. Soc. Japan 5(}(1981) 2351. [19] ,1. l)eren and J. Stoch, J. ('atalw, is 18 (19701 249.
Surface Sctence 169 (1986) L2455L252 North-Holland.
SURFACE
Amsterdam
SCIENCE
LETTERS
SURFACE PHOTOVOLTAGE STRUCTURE IN DEFECTIVE
AND XPS STUDIES OF ELECTRONIC NICKEL OXIDE POWDERS
Neil R. HUCK School of Muthemarical 6150, Austrulru
and Ph_psrcol Screncess. Murdoch
timrwurv.
Murdwh,
Wesrern Ausrrrrlu
Roger St.C. SMART
and Stephen
M. THURGATE
School of Muthematlcal and Ph.vsrcol Scienc~es. Murdwh Western Australia 6150, Australro Received 9 April
1985; accepted for publication
Surface photovoltage revealed dtfferences
measurements
lJnirer
18 December
of polycrystalline
powder
samples of nickel oxtde have
tn band bending (k’,) related to the defect concentrations
powders prepared at 700°C
and 1450°C
of extended defects. but considerably
The defective
(700°C)
under UV illumination.
of the oxide. The
tn air consist of small single crystals with low concentra-
tions ( < 10’ cm-*) NtO
1985
different
nickel vacancy concentrations.
gave V, = 760 mV and the equiltbrated
NiO
(1450°C)
V, = 230 mV
possibly due to Fermi level pinntng of the Ni 3dX band near V;,
and k’;,.
respectively. These results are consistent wtth values of V, (i.e. 630 and 240 mV. respectively) these two samples obtained charging on temperature. equilibrated
NiO
(1450°C)
discussed in relation
previously
from XPS
measurements
of the dependence
for
of surface
A visible transition at = 2.5 eV. gtwng &#I = I35 mV. IS also found in the sample but not m the defecttve NiO
to other work on the electronic
(700°C‘)
sample. The results are
band structure and surface states of ntckel
oxtde.
In this work we report surface photovoltage measurements on nickel oxide powders. prepared with different defect levels controlled by decomposition and annealing [l-4], carried out in order to study the dependence of Fermi level pinning (or band bending), IJ’,, on the defect properties of the oxide. Two preparations of NiO, annealed at 700°C (i.e. NiO,,,,,) and 1450°C (i.e. Ni0,45,,) for 4 h in air, have been extensively characterised for purity. surface structure, morphology and reactivity, and defect properties [l-8]. High resolution transmission electron microscopy shows that the particles of the powders consist of 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
N.R. ttuck et aL / l)e/ectit'e NiO powdei'~
L246
highly perfect single crystals with relatively low concentrations of dislocations and other extended defects (i.e. < 10 ~ cm 2). Thus, bulk conductivity in these single crystals is primarily due to defect-controlled hole conduction in the p-type oxide and not to enhanced conduction at extended defects or grain boundaries. Clearly surface conductivity, also defect-controlled, is likely to be of major importance in these polycrystalline powder. The NiOv00 and NiO145o samples differ in surface area (i.e. 2.6 and 0.36 m 2 g i respectively) [1], the extent of surface facetting [1,2], estimated bulk conductivity (i.e. 3.1 × 10 2 and 5.0 × 10 5 ohm 1 m 1 respectively) [3], surface defect (i.e. l/yi, O and Ni 3+) concentrations and reactivity [3 5,7]. The property of interest here, however, is the difference between the two samples in nickel vacancies (and associated defects) in the bulk and at the surface. An important consideration in the work was the opportunity to compare the V, values with those obtained for the same samples from X-ray photoelectron spectroscopy (XPS). XPS studies of polycrystalline, p-type semiconducting nickel and manganese oxides have revealed dependence of the surface potential ( V h ) (obtained as kinetic energy shifts) on X-ray power [6,8], bulk oxide defect concentrations [3,4,7], electron donor and aceptor chemisorption [4,5,7] and temperature [4,8]. An explanation, based on consideration of the mechanisms for development of surface potential during X-irradiation, for the dependence of ~,h on X-ray tube current, band bending at the surface (V), and bulk oxide conductivity (o,) has been advanced [3,8]. At high X-ray power (i.e. above 200 W), 1/{.h is given by:
<"-
( X I - K~)
t' e ~ ' ~
expt
l
where K~, K 3 are constants, B ( % ) represents a functional dependence on bulk conductivity of the sample and ~ is the band bending due to Fermi level pinning at the surface. Linear experimental plots of In le[:h versus T-1 were obtained for samples of NiO and MnO with different defect levels controlled by decomposition and annealing [1--4]. From the XPS studies, NiO700 gave much larger values of B(oo) than NiOi450, as expected, and also larger values of ~ i.e. NiO700` ~ = 630 mV; niOi451 }, l/~ = 240 mV [8]. The XPS evidence for the values of V, is indirect and subject to the assumptions made in the explanation, principally that electron conduction to the irradiated surface depends mainly on conduction in the surface layer. The explanation of conduction in nickel oxide is the subject of intense investigation, e.g. refs. [9 12], and remains contentious. Other mechanisms have been suggested to explain the effect of temperature on the kinetic energy shifts in XPS spectra. For instance, charge removal in the temperature range 20 80°C from a (100) surface of nickel oxide has been attributed [13] to increased hole mobility. The concensus of opinion, however, suggests that hole mobility in the bulk of single crystals is not activated [9] although thermal activation may
N.R. Huck et al. / Defectiue NiO powders
L247
occur in the surface region. Different mechanisms of conduction in the surface involving small-polarons in a narrow Ni 3d ~ band, or activated hopping in localised nickel vacancy levels, have been suggested [4,10]. In this work we set out to measure V, directly for NiO700 and NiO145o samples using surface photovoltage techniques. The two values of Iz obtained from XPS are sufficiently different to suggest that evidence from a second technique might assist in evaluating the degree of confidence to be placed on the XPS determination of ~ values. The NiOv00 and NiO1450 powders were prepared and characterised as described previously [1 ]. The NiOT00 powder was pressed at 10 tonnes pressure into a 13 mm diameter, coherent pellet in a standard KBr die press. The NiO1450 pellet was not coherent after pressing and was sintered at 1000°C in a muffle furnace for 1 h before mounting. A linear I / V plot, obtained using a gold-tipped, flexible probe, showed that the contact of the NiO with the tantalum back plate on the photovoltage mount was ohmic. The mount was fixed inside a UHV apparatus and baked out at 150°C for 12 h to 2 × 10 ~9 Torr. The pellets were heated to 500-600°C in vacuo for 1 h before voltage measurements were made. This procedure has been shown previously [7~8] to remove adsorbed H 2 0 and the majority of surface hydroxyl and carbonate groups. Surface voltages were measured using a vibrating capacitor Kelvin probe technique. The reference probe was constructed from a gold plated circular metal grid of ~ 10 mm diameter, positioned as close to the oxide surface as possible without contact and set vibrating at a natural frequency of 500 Hz as indicated schematically in fig. 1. The modulated capacitance between the Au probe and oxide sample generates an AC current which is detected using a circuit similar to Brillson [14]. The signal was anaplified by an electrically shielded current-to-voltage preamplifier (10 8 A / V ) then fed to a Princeton Applied Research PAR-5206 lock-in amplifier. Negative feedback from the lock-in amplifier is applied to the insulated sample and used to null the CPD. Thus the workfunction difference, q,(sample)-~(Au), can be continuously monitored by following the lock-in output. Illumination of sample was made through a glass viewport that gave flat transmission in the energy range used. Before discussing the surface voltage measurements, some general observations on the insulating behaviour of the nickel oxide samples are necessary. Equilibration of surface voltages on the pellets without illumination is relatively slow and varies considerably with pretreatment of the sample. For NiOToo after outgassing at 25°C, a steady value for the work function (~) of 4.97 V (assuming ~ ( A u ) = 4.82 V) is obtained in < 5 min and this value is unchanged by illumination with visible (I2/quartz 100 W) or UV (Hg/quartz, 90 W) light. After evacuation at 150°C, equilibration is much slower, i.e. -- 4 h and, following evacuation at ~ 500°C, a steady value of ~(NiO700) = 5.04 V
L248
N.R. Huck et al. / Defecti~,e NiO powders AUDIO AMPLIFIER
SOLENOID
OSCILLATOR
<1
•
" U.H.V.
~ VIEW
PORT
SAMPLE
Au
REFERENCE ,N
PREAMPLIFIER 10 - 8 A I V
COMPENSATION VOLTAGE TO S E T I N T I A L ZERO VOLTAGE
PAR - 5206 LOCK-IN AMPLIFIER OUT
E
I '
RECORDER
Fig. 1. A schematic representation of the e x p e r i m e n t a l s } s t e n and circuit used for surface p h o t o v o l t a g e determination.
without illumination is obtained only after 9 h. NiO1450 after evacuation at = 500°C also requires > 10 h to reach a steady value of qb(NiO145o)= 5.30 V without illumination. If shorter equilibration periods are used the voltage measured is found to arise from undischarged, externally applied bias, either applied directly in the capacitance circuit or as electron current from the ion gauge filament in the UHV system. Surface photovoltages were measured on both samples after 500°C degassing with both visible and UV lamps. For NiOT00 the visible lamp had no obvious effects but the UV lamp shifted the voltage to a steady 5.80 V reproducibly in < 2 min. This shift corresponds to a Jg,(NiOT00)=760 mV, giving photoconduction at the fiat band potential. For NiOl4s0, the UV lamp shifted the voltage to a steady 5.57 V, reproduci-
N.R. Huck et al. / Defective NiO powders
L249
bly in < 2 min, corresponding to A~(NiO~450) = 230 inV. The visible lamp also gave a measurable, reproducible shift to 5.42 V or A~ = 135 mV. Using a set of band pass filters it was possible to isolate the transition giving rise to the photoconduction in the visible region to absorption at 480-490 nm or --- 2.5 eV. The absorption appeared to have a broad tail to longer wavelengths. It was evident that electron conduction to the surface was responsible for the Ag, shifts after UV illumination because surface charging due to electron current from the ion gauge filament always shifted the voltage in the same direction, i.e. to more positive values. Since the measurements were made on polycrystalline powder samples, surface species must be considered in addition to the band structure of the oxide. It is known from XPS studies [4,5,7] that, after the pretreatment used in these experiments, the surface contains considerable concentrations of defects as nickel vacancies (VNi) with associated O - and Ni -~+ ions. For NiO700 and NIO1450, O / 0 2- ratios of = 3.0 and =0.2, respectively, and similar Ni3+/Ni2+ ratios are found [7]. Small quantities of hydroxide ( < 2% monolayer), carbonate ( < 2% monolayer) and carbon ( < 5% monolayer) also remain but no other adsorbates are detected. It is therefore likely that the main contribution to Fermi level pinning is associated with the nickel vacancies and associated defect species. In order to discuss this possibility further, we need to consider the electronic band structure of NiO at the surface. The most recent reviews of the surface electronic band structure of NiO, summarising previous theoretical and experimental work [9-12], have been presented by Henrich [15], Tsukuda et al. [16] and Dare-Edwards et al. [17]. Henrich [15] notes that theoretical estimates for the NiO (100) surface indicate only slight difference between bulk and surface band structure using LCAO theory [18], whilst D V - X a cluster calculations [16], comparing (NiO 5)~- with (NiO6)l°_, suggest a narrowing of the bandgap (i.e. O 2 p - N i 3d 9) at the surface of = 0.8 eV. These estimates do not, however, provide work function values for comparison with our surface photovoltage measurements. DareEdwards et al. [17] have considered the available experimental and theoretical evidence to derive a semi-empirical band structure for NiO, including surface states known from capacitance and DC cyclic voltammetry measurements, reproduced in fig. 2. The comments that follow are based on the recognition that there are limitations on the extent to which a bulk band structure for equilibrated NiO (with departures from stoichiometry represented as energy levels for VN, and V ~ ) can be used to interpret a surface phenomenon. Nevertheless, the bulk structure represents the only available systematic description, based on experimental verification of theory, for consideration of work function values and changes observed in surface photovoltage measurements. In our case, to a first approximation, UV illumination giving photoconduction should correspond to the flat band situation in single crystals.
L250
N.R. ~luck et al. / Defecttt,e .¥1(_)powder.s
@(V)
3"5
3-6eV 2-2eV
5"3
Li;NiO
5.7
Ni
3d 8
EF
6"0
7.1
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occupied level
O0
0 2p
~
unoccupied level
surface state Fig. 2. A representation of the e l e c m m i c band structure of NiO, in the flat-band configuration, based on refs. [lO 16].
We will assume, as do others [17], that the F e r m i level is close to the top of the occupied Ni 3d ~ band. The values of ~(NiOT00)= 5.80 V and ~(NiO1450 ) = 5.57 V for p h o t o c o n d u c t i o n c o r r e s p o n d i n g , in theory, to the flat b a n d situation are sufficiently close to the value of 5.70 _+ 0.3 V in fig. 2 (i.e. the last o c c u p i e d electronic level in the Ni 3d ~ b a n d ) to give some confidence in the correlation with other results. A n y e x p l a n a t i o n for the difference in 'k for
N.R. Huck et al. / D e f e c t w e NiO powders
L251
p h o t o c o n d u c t i o n between the samples is speculative but it may be due to real differences in the band structure caused by the smaller crystal size and very defective structure [1,2,7,8] of the NiO700 sample leading to qualifications in assumptions of lattice periodicity and semi-infinite crystals. The Aq~ results of 760 and 230 mV for NiOT00 and NiOi450 are consistent with the values obtained from the XPS measurements, i.e. 630 and 240 mV, respectively. The lower value of Adp(NiO700 ) obtained from XPS probably arises from the relatively large contribution of B(o,,) in this defective oxide as explained in the earlier paper [8]. The J¢~ for NiOToo corresponds roughly, given the errors in these values, to pinning of the Ni 3d ~ band in fig. 2 at V~, (i.e. 4.9 eV)~ i.e. a Ni 3+ ion localised at a nickel vacancy, with a value of Aq,(NiOT00) = 800 mV. The A~(NiO1450 )value may be due to two Ni ~+ ions localised at a nickel vacancy but fig. 2 would then suggest a A~(NiO~450) of 600 inV. Dare-Edwards et al. [17] have pointed out that stabilisation can be achieved by Ni 3+ ordering in arrays on the surface. Surface states in their system have been stabilised by 200-500 mV and this would explain our result. This is not inconsistent with the known defect level occupancy of the two oxide samples. The much larger concentration of defects, manifested as nickel vacancies and adjacent Ni 3+ ions, in NiO7oo [1,3,4,7,19] might be expected to give rise to singly ionised nickel vacancies, whereas there is less tendency for ionisation of vacancies in the equilibrated NiO~45o. In the absence of more detailed information on surface electronic band structure and surface states, the above assignments can only be speculative but they do correlate with the semi-empirical band structure of fig. 2 [17]. The visible transition at ~ 2.5 eV, causing limited photoconduction in NiOi450 (which is not evident in NiO700) cannot be explained with confidence. Photoconduction in the visible region is not normally observed in single crystals. In our case, conduction is found to be due to an electron current to the surface presumably from electrons in the Ni 3d 9 band. The conduction may arise from promotion from an unspecified surface slate or, possibly from a N i 3 d S ~ N i 3 d 9 transition (fig. 2). The value of A4~(NiO1450) for this transition is 135 mV, i.e. significantly less than the 230 mV found for UV illumination. This is likely to be due to a difference in photon-capture cross-section between this transition and the O 2p -, Ni 3d ~ transition. The reason for the lack of effect of the visible illumination on NiOToo is not obvious. It is possible that the rate of discharging may have been too slow for observation and, again, the more defective nature of the small crystals may be responsible for lower mobilities of electrons in the Ni 3d ~ band. There is evidence that, under UV illumination, the NiOH50 is a better conductor than NiOT0 o. This accords with the view that the structure of the Ni 3d ~ and Ni 3d ~ bands is not as well defined in the defective NiO7oo crystals. Further, more detailed study is continuing to test the kinetics of discharge in both samples and to investigate the spectral response across both the visible and UV spectra.
L252
N.R. l l u c k el aL / Defi, ctire NlO powders
These results provide several conclusions: t h e r e a r e s i g n i f i c a n t d i f f e r e n c e s in b a n d b e n d i n g , V,, b e t w e e n d e f e c t i v e N i O ( 7 0 0 ° C ) (i.e. V - 7 6 0 m V ) a n d e q u i l i b r a t e d N i O ( 1 4 5 0 ° C ) (i.e. V = 230 mV); the work function
values suggest that
1/ m a y a r i s e p r i m a r i l y
level p i n n i n g o r e n e r g y l e v e l s c o r r e s p o n d i n g surface singly (NiO7~0) t r a p p e d N i -~+ i o n s ; the surface photovoltage
1~ v a l u e s a r e c o n s i s t e n t w i t h 1/~ v a l u e s f r o m X P S
p l o t s o f In F~,~, v e r s u s T
~ and provide evidence that the XPS
useful for obtaining
or
doubly
from Fermi
t o e x c e s s n i c k e l v a c a n c i e s in t h e
(NiO145,)
compensated
by
adjacent
method
is
V estimates;
a v i s i b l e a b s o r p t i o n at = 2.5 e V f o r NiO~aso ( o n l y ) , g i v i n g l i m i t e d p h o t o c o n d u c t i o n , m a y a r i s e f r o m a n u n s p e c i f i e d s u r f a c e s t a t e o r a N i 3 d s ---, N i 3 d ~ transition. The
work
is s u p p o r t e d
financially
by
the
Australian
Research
Grants
Scheme.
References I1] C,F. Jones, R i . Segall, R.St.('. Smart and P.S. Turner, J. Chem. Soc. Farada'~ trans. 1. 73 (19771 171(/. 12] C.F. Jones, R.L. Segall, R.St.C Smart and P.S. l'urner, J. ('hem. Soc. Faraday Trans. I, 74 (1978) 1615. 13] M.W. Roberts and R.St.('. Smart, ('hem. Phys. Letters 69 I19811) 234. [41 M.W. Roberts and R.St.('. Smart, Surface Sci. 100 (19801 590. 151 M.W. Roberts and R.St.('. Smart, Surface Sci. 108 (19811 271. [61 R.St.C. Smart, Surface Sci. 122 (19821 L643. 17] M.W. Roberts and R.St.('. Smart, J. ('hem. Soc. Faraday trans. 1, 80 (19841 2957. [81 M.W. Roberts and R.St.('. Smart, Surface Sci. 151 (1985) I. 19] J.B. Goodenough, Progr. Solid State Chem. 5 (1971) 145. I1(1] A.J. Bosman and H.J. van Daal, Advan. Phys. 19 (1979) I. I11 ] Z.M. Jarzebski, Oxide Semiconductors ( Pergamon, Oxford, 1973 ). [121 I). Adler, The Imperfect Solid Transport Properties, in lreati:,e on Solid State ('hcmislr 3. Vol. 2. Ed. N.B. ttannay (Plenum, New York, 19751 p. 237. [13] F.P. Netzer and M. Prutton. J. Phys. ('8 (19751 2401. 1141 L.J. Brillson. Surface Sci. 51 (1975) 45. [151 V.E. Henrich, Progr. Surface Sci. 14 (19831 175. [161 M. Tsukuda, H. Adachi and ('. Satoko, Progr. Surface Sci. 14 (1983) 113. 117] M.P. l)are-Edwards. J.B. (~oodenough, A. tlamneU and N.I). Nicholson. ,I. Chem. Soc. Faraday ~Irans. II, 77 (19811 643. [18] V.-('. l,ee and H.-S. Wong. J. Phys. Soc. Japan 5(}(1981) 2351. [19] ,1. l)eren and J. Stoch, J. ('atalw, is 18 (19701 249.