Surface photovoltage and XPS studies of electronic structure in defective nickel oxide powders

Surface Science 169 (1986) L245-L252 North-Holland, Amsterdam

SURFACE

L245

SCIENCE LETTERS

SURFACE PHOTOVOLTAGE STRUCTURE IN DEFECTIVE

AND XPS STUDIES OF ELECTRONIC NICKEL OXIDE POWDERS

N e i l R. H U C K School of Mathematical and Physical Sciences, Murdoch University, Murdoch, Western A ustralia 6150, Australia

R o g e r St.C. S M A R T School of Science, Griffith Universi(v, Nathan, Queensland 4111. Australia

and S t e p h e n M. T H U R G A T E School of Mathematical and Physical Sciences, Murdoch University, Murdoch, Western Australia 6150, Australia

Received 9 April 1985; accepted for publication 18 December 1985

Surface photovoltage measurements of polycrystalline powder samples of nickel oxide have revealed differences in band bending (I{,) related to the defect concentrations of the oxide. The powders prepared at 700°C and 1450°C in air consist of small single crystals with low concentrations ( < 109 cm- 2) of extended defects, but considerably different nickel vacancy concentrations. The defective NiO (700°C) gave V~= 760 mV and the equilibrated NiO (1450°C) V ~ 230 mV under UV illumination, possibly due to Fermi level pinning of the Ni 3d 8 band near V~, and V~i, respectively. These results are consistent with values of V~ (i.e. 630 and 240 mV, respectively) for these two samples obtained previously from XPS measurements of the dependence of surface charging on temperature. A visible transition at ~ 2.5 eV, giving A,~ = 135 mV, is also found in the equilibrated NiO (1450°C) sample but not in the defective NiO (700°C) sample. The results are discussed in relation to other work on the electronic band structure and surface states of nickel oxide.

In this w o r k we r e p o r t s u r f a c e p h o t o v o l t a g e m e a s u r e m e n t s on nickel o x i d e p o w d e r s , p r e p a r e d w i t h d i f f e r e n t d e f e c t levels c o n t r o l l e d by d e c o m p o s i t i o n a n d a n n e a l i n g [1-4], c a r r i e d o u t in o r d e r to s t u d y the d e p e n d e n c e of F e r m i level p i n n i n g (or b a n d b e n d i n g ) , ~ , on the d e f e c t p r o p e r t i e s of the oxide. T w o p r e p a r a t i o n s of N i O , a n n e a l e d at 7 0 0 ° C (i.e. NiO700) a n d 1 4 5 0 ° C (i.e. NIO1451 ) ) for 4 h in air, h a v e b e e n e x t e n s i v e l y c h a r a c t e r i s e d for purity, s u r f a c e structure, m o r p h o l o g y a n d reactivity, a n d d e f e c t p r o p e r t i e s [1 8]. H i g h r e s o l u t i o n transm i s s i o n e l e c t r o n m i c r o s c o p y s h o w s that the p a r t i c l e s of the p o w d e r s consist of 0 0 3 9 - 6 0 2 8 / 8 6 / $ 0 3 . 5 0 © Elsevier Science P u b l i s h e r s B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g D i v i s i o n )

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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 NiO7o o and NiO14~. samples differ in surface area (i.e. 2.6 and 0.36 in2 g ~, respectively) [1], the extent of surface facetting [1,2], estimated bulk conductivity (i.e. 3.1 x 10 2 and 5.0 × 10 5 ohm t m i, respectively) [3], surface defect (i.e. VNi, 0 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 V, on X-ray tube current, band bending at the surface (V,), and bulk oxide conductivity (%) has been advanced [3,8]. At high X-ray power (i.e. above 200 W), V~,h is given by:

v,,

(1%-K~)

/ ,,V~ I

where K~, K 3 are constants, B(o,,) represents a functional dependence on bulk conductivity of the sample and V, is the band bending due to Fermi level pinning at the surface. Linear experimental plots of In ~,h versus T ~ were obtained for samples of NiO and MnO with different defect levels controlled by decomposition and annealing [1 4]. From the XPS studies, NiO7o o gave much larger values of B(oo) than NiO~45o, as expected, and also larger values of V~ i.e, NiOT0o, V = 630 mV: niO145t I, 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. / Defectioe NiO powders

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occur in the surface region. Different mechanisms of conduction in the surface involving small-polarons in a narrow Ni 3d 8 band, or activated hopping in localised nickel vacancy levels, have been suggested [4,10]. In this work we set out to measure ~ directly for NiOv00 and NiO~450 samples using surface photovoltage techniques. The two values of ~ 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 NiO70o 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 U H V 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 m m 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 amplified 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)-q~(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 NiOT00 after outgassing at 25°C, a steady value for the work function (,/,) of 4.97 V (assuming ,~(Au)= 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 ~(NiO7oo) = 5.04 V

L248

N.R. ttuck et al. / Defectit,e NiO powders

AUDIO AMPLIFIER

SOLENOID

--~"~1 "~~/,,

OSCILLATOR

<1 VACUUM BELLOWS

U.H.V.

VIEW ' 7 PORT

REFERENCE PAR - 5206 LOCK--IN AMPLIFIER

PREAMPLIFIER

IO-8A/V

OUT

COMPENSATION E VOLTAGE TO SETINTIAL ZERO VOLTAGE

I RECORDER

1

Fig. I. A s~.'hematic l-epresenlahon of the experimental sSstem and circuit used for surface photovoltagc 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 ~(NiO]454 ~) = 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 Aq,(NiO7o0)= 760 mV, giving photoconduction at the flat band potential. For NiO145o, the UV lamp shifted the voltage to a steady 5.57 V. reproduci-

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bly in < 2 min, corresponding to A~b(NiO1450) = 230 mV. 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 Aq~ 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 3+ 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 ai. [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 DV-Xa cluster calculations [16], comparing (NiOs) 8- with (NiO6) 1° , 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 V~, and V~i ) 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. Ituck et al. / Defectiue NiO powders

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Ni 4s

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Ni 3d 9

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Fig. 2. A representation of the electronic band structure of Ni(), in Itlc flat-band configuration, ba~,cd on refs.[10 16].

W e will assume, as do others [17], that the F e r m i level is close to the top of the o c c u p i e d Ni 3d ~ band. The values of q~(NiOv(:~))= 5.80 V and qs(NiO14511) = 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 c o r r e l a t i o n with o t h e r results. A n y e x p l a n a t i o n for the difference in ~ for

N.R. Huck et al. / Defective NiO powders

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photoconduction 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 A4, results of 760 and 230 mV for NiO700 and NiO1450 are consistent with the values obtained from the XPS measurements, i.e. 630 and 240 mV, respectively. The lower value of AqS(NiO700) obtained from XPS probably arises from the relatively large contribution of B(oo) in this defective oxide as explained in the earlier paper [8]. The zl~ for NiOvo0 corresponds roughly, given the errors in these values, to pinning of the Ni 3d s 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 A~(NiOT~ ) = 800 inV. The A~(NiO1450 ) value may be due to two Ni -~+ ions localised at a nickel vacancy but fig. 2 would then suggest a Aqb(NiO145o) of 600 mV. Dare-Edwards et ai. [17] have pointed out that stabilisation can be achieved by Ni -~~ 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 NiOT~ [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 NiO1450. 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 NiO1450 (which is not evident in NiOvo0) 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 state or, possibly from a Ni 3dS---,Ni 3d 9 transition (fig. 2). The value of Jqs(NiOi450 ) 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 NiOv0 o 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 9 band. There is evidence that, under UV illumination, the NiOi45o is a better conductor than NiO7oo. This accords with the view that the structure of the Ni 3d ~ and Ni 3d ~ bands is not as well defined in the defective NiOvoo 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.

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N.R. ttuck et al. / Dejectn,e NiO powders

These results provide several conclusions: there are significant differences in band bending, V,, between defective NiO (700°C) (i.e. V, = 760 mV) and equilibrated NiO (1450°C) (i.e. V , - 2 3 0 mV); the work function values suggest thai 1~ may arise primarily from Fermi level pruning or energy levels corresponding to excess nickel vacancies in the surface singly (NiOT00) or doubly (NiO145o) compensated by adjacent trapped Ni ~ + ions; the surface photovoltage 1~, values are consistent with 1/~, values from XPS plots of In ~,h versus T i and provide evidence that the XPS method is useful for obtaining V estimates; a visible absorption at --- 2.5 eV for Ni()14:, o (only), giving limited photoconduction, may arise from an unspecified surface state or a Ni 3d s ~ Ni 3d ~ transition. The work is supported financially by the Australian Scheme.

Research Grants

References [1] ('.F. Jones, R.L. Segall, R.St.('. Smart and P.S. l urncr. J. ('l/era. Soc. Farada'~ Iran~. I, 73 (1977~ 1710. [2] ('.F. Jones, R.L. Scgall, R.St.('. Smart and P.S. Turner, J. ('llcm. Soc. [ a r a d a \ I'ram,. 1, 74 (1978) 1615. 131 M.W. Roberls and R.St.('. Smart, ('hem. Phys. LcUcrs 69 (1980) 234. [41 M.W. Roberts and R.St.('. Smart, Surface Sci. 100 (1980) 590. [5] M.W. Roberts and R.SI.('. Smart, Surface Sci. 11)8 (1981) 271. 16] R.St.C. Smart, Surface Sci. 122 11'482) i..643. [7] M.W. Roberts and R.SI.('. Smart..I. ('laem. Soc. I:aradav trans. 1, 81) (lC)84) 2`457. [8] M.W. Roberts and R.SI.('. Smart, Surface Sci. 151 11985) 1. [`4] ,I.B. (}oodcnough. Progr. Solid State ('hem. 5 11971) 145. IlO] A.J. BOSIll~.llland H.J. '+an l)aal, Advan. Ph),s. 19 (197`4) I. J i l l Z.M. Jarzebski, Oxide S e m i c o n d u c t o r s (Pergamon, Oxfl~rd, 1973~. I121 D. Adlcr, "]'he hnpcrfcct Solid T r a n s p o r t Properiics, ill "1rcatisc on Solid Slate (_ hcmi~,tr\. Vol. 2, Ed. N.B. H a n n a v (Plenum, Nov, York, 19751 p. 237. [13] I'.P. Netzer and M. Prutton, J. Ph'~s. ('~ 11975) 2401. [14] I,.J. Brillson, SurfaceSci. 51 (1`475) 45. [151 V.E. Henrich, l'rogr. Surface Sci. 14 11983~ 175. 116] M. Tsukuda, H. Adachi and C. Satoko, Progr. Surface Sci. 1411983) 11~.. [17] M P . l)arc-l..dwards, J.P,. (Joodenough. A. I l a m n c t l and N.I). Nicholson, ,1. ( h c m . Soc. l:arada'~ Trans. I1, 77 (1981l 643. [lSJ V.-('. Lcc and tt.-S. Wong,.l. Ph',s. Soc. J a p a n 51) 11981) 2351. [1`4i J. D c r c n and .1. S l o t h , .1. ('alal','sis IN (1`470) 24`4.