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Xps studies of donor and acceptor chemisorption of NO and CO on nickel oxide surfaces
Surface Science 100 (1980) 590-604 0 North-Holland Publishing Company
XP!3 STUDIES OF DONOR AND ACCEPTOR CHEMISORPTION OF NO AND CO ON NICKEL OXIDE SURFACES. M.W. ROBERTS * and R.St.C. SMART ** Department of Physical Chemistry, W. Yorkshire BD 7 I DP. UK
University of Bradford, Bradford,
Received 22 January 1980; accepted for publication 25 June 1980
Systematic variation on the defect (excess) oxygen concentration in the surface of nickel oxide preheated to 700, 1100 and 145O”C, has been revealed in O(ls) and Ni(2p3& X-ray photoelectron spectra. The magnitude of the surface charge, after evacuation at 5OO”C, is also directly related to the defect properties. The effect of chemisorption of NO and CO on the free hole concentration of the p-type semiconductor surface has been studied by monitoring the surface charge. NO adsorption, predominantly as negatively charged (electron acceptor) molecules removes much of the surface charge whereas CO adsorption, as CO”+ species (donor chemisorption), increases the surface charge on all samples. 1. Introduction
The electronic theory of semiconducting oxides, and its relationship to technical applications, is now well established [ 1,2]. Much less attention has been paid to the understanding of electronic factors controlling surface phenomena such as surface charge, modes of adsorption and rates of reaction although the work of Vol’kenstein [3] and collaborators represented an attempt to develop a coherent theory of electronic control in catalysis on semiconductors. More recently, the effects of bulk and surface defect concentrations on surface reactions over nickel oxide have been examined [4-71. Vol’kenstein’s electronic theory of catalysis [3] distinguishes two types of chemisorption differing in their effect on the free electrons and positive holes of the oxide semiconductor surface. The first, “weak” chemisorption, results in a neutral adsorbed molecule without participation of the charge carriers of the crystal lattice. The second, “strong” chemisorption, involves a charged particle formed by retention of either a free electron or a free hole in the chemisorption bond. This “strong” chemisorption can produce either a p-bond (donor bond) by hole capture or an n-bond (acceptor bond) by electron capture. In either case, the concentration of free charge carriers in the surface and bulk is changed. Kinetic studies to test this * Present address: Dept. Physical Chemistry, University College, P.O. Box 78, Cardiff CPl lXL, Wales, UK. ** Present address: School of Science, Griffith University, Nathan, Queensland 4111, Australia. 590
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explanation, although giving some support for the different modes of adsorption, provide rather indirect evidence for the reaction mechanisms. Recently, effects of chemisorption and reaction on the electronic properties of the catalysts have been determined by physical methods that measure surface species more directly. The observation of changed concentrations of charge carriers at the surface is most easily made by measurement of changes in surface charge. The origin of a surface charge, as seen in the shift in observed kinetic energies of the emitted core electrons, has been studied in general terms in our previous work [8]. Loss of primary photoelectrons and secondary electron emission causes positive charging of the surface until this current is counter-balanced by a neutralising electron current, due to photoelectrons (from the X-ray window) and surface conduction, to the surface. Ley et al. [9] have found a correlation between the magnitude of surface charge and the band gap of II-VI semiconductors. Where the charging is less than the band gap, it is expected that surface conductivity is the major contribution in the neutralising electron current. Correlation of surface charge values with differing defect concentrations, arising from different oxide pretreatments, and different temperatures of the sample during recording of spectra has also been demonstrated [8]. The electronic explanation of “strong” chemisorption is closely allied to explanations of the formation of surface states [lo]. For instance, in a p-type semiconducting oxide like nickel oxide the introduction of a narrow density of filled or partly-filled surface states above the Fermi level leads to downward bending of the valence and conduction bands at the surface, shifting the Fermi level (or surface state level) until a common level is established between the surface state and Fermi level. The surface states are now filled to a level below that originally prevailing for the neutral (zero surface charge) surface. Consequently a positive surface charge, related to the extent of band bending, is formed in a manner very similar to p-bond hole trapping in “strong” chemisorption. It seems worthwhile to explore the effect of such chemisorption on electronic changes in both the adsorbed molecules and the solid. We describe here direct observations, using XI’S, of changes in surface charge of nickel oxide due to chemisorption of donor and acceptor molecules (NO, CO). The effect of chemisorption on the electronic configuration of nickel and oxygen at the surface is related to the nature of the adsorbed species and the changes in electronic properties of the solid. We also show that the charge effects, due to “strong” chemisorption, depend on the properties of the solid oxide itself and can be changed by different defect concentrations introduced during pretreatment. 2. Experimental
details
2.1. Sample preparation Nickel oxide was obtained from the same high purity source (Univar AR Grade) used in previous work [4,8]. Samples with different properties were prepared by
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heating the oxide at 700°C (i.e. NiO,ee), 1lOO’C (i.e. NiOr roe) and 145O’C (i.e. NiOrJso) for 4 h in air. The purity, surface area and morphology, and defect properties have been discussed previously [4,5,8]. The only impurities at levels >2 ppm are cobalt (20 ppm) and iron (
3. Results 3.1. Nickel oxide surfaces before adsorption Figs. 1 and 2 Ni07e0, NiOrree, sample at 5OO’C ing degassing the
show the O(ls) and Ni(2ps,*) spectral regions respectively of and NiOrhse samples, recorded at 20°C after evacuation of each below 1 X 10m9 Torr. The spectra are shown as broken lines. Durmass spectrometer showed that loss of Hz0 predominates at tem-
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Fig. 1. O(h) spectra (pass energy 50 eV, full scale deflection, i.e. fsd 1 X lo4 c s-l) from (a) NiO,oo, (b) NiOl 100 and (c) Ni014so. Broken-line spectra were recorded after evacuation at 5OO’C and retooling to 2O’C; full-line spectra were recorded after admission of 5 Torr NO and evacuation to <4 X 10% Torr at room temperature; full-line arrow indicates O(ls) binding energy of 529.7 eV; broken-line arrow indicates binding energy of 531.4 eV.
peratures up to 750°C. although some CO and CO2 are also observed (approximately 10% of L_e Hz0 lost in each case). Between 300 and 5OO”C, loss of CO pre-
dominates with Hz0 and CO2 losses -50% of the CO loss. No O2 was observed even at the highest sensitivity. The method of estimating the magnitude of surface charge has been described [8]. C(ls) spectra of all three samples indicate two components arising from carbon contamination on the uncharged metal clip (KE 1203.5 eV) and on the sample (potentially variable KE). The shift in the KE of the very weak signal due to sample carbon contamination can then be estimated and the surface charge obtained directly. Estimates of corrected C/O intensity ratios, by the method used in our earlier work [8], i.e. using calculated emission cross sections, analyser energy response proportional to l/KE and neglecting escape depth variation with KE, gave values of -0.05. The surface charge on the samples shown in figs. 1 and 2 is t1.0, t3.0 and t3.6 eV for NiOToO, NiOr roe and NiO r4so respectively. These values agree with those for the KE shifts of the main 0( 1s) peak relative to the KE of uncharged O(ls) samples (i.e. KE 958.5 eV). The 0( 1s) spectra of NiO r r o. and NiO r4so oxides in fig. 1 show a strong, sharp peak at a binding energy of 529.7 eV with asymmetry on the high binding energy
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Fig. 2. Ni(2pan) spectra (pass energy and (c) NiOl4 s 0. Broken&e spectra 2&J, full-line spectra were recorded lQ* Torr at room temperature. The are shown on curve (c).
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50 eV, fsd 1 X lo4 c s-l) from (a) Ni07o0, (b) NiOl10o were recorded after evacuation at SOO’C and retooling to after admission of 5 Torr NO and evacuation to <4 X positions of the two binding energies 854.6 and 856.1 eV
side indicating a weak, broad peak close to the 529.7 eV peak. This is confirmed by the Ni0700 sample in which the 0( 1s) spectrum has a very broad peak with a maximum binding energy at 529.7 eV and a very broad shoulder at 531.4 eV. The 529.7 eV peaks are marked by full-line arrows while 531.4 eV peaks are marked by broken-line arrows in all spectra. Analysis of the O(ls) spectra by curve fitting with Gaussian peaks using a least squares fitting procedure [ 1 l] gives the following ratios for the intensity of the 531.4 eV peak relative to the 529.7 eV peak: Ni0700 0.54; NiOlloo 0.12; NiO14s0 0~10. The full widths at half maximum found by this method for the 529.7 and 531.4 eV peaks in Ni0700, NiOrIoo, and NiO14s0 are 2.0 and 2.5; 2.5 and 4.0; and 2.6 and 3.3 eV respectively. The Ni(2ps,2) spectra in fig. 2 similarly show two peaks in the main emission with binding energies of 854.6 and 856.1 eV. The Ni0r4s0 sample has a spectrum with maximum intensity in the 854.6 eV peak and a distinct shoulder at 856.1 eV which we have used to assign the second O(ls) peak whilst the NiOrloo and Ni0700 spectra both show very broad emission with a maximum at 854.6 eV but a broad shoulder on the high binding energy side. The satellite peak near 861.7 eV is also considerably sharper in the NiO 145o spectrum where the low binding energy O(ls) Spectra obtained after evacuation at and Ni(2ps,a) components predominate.
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500°C often have broad, rather ill-defined peaks, as is clear from NiOrrce and NiO,e,, spectra in fig. 2. The values of the binding energies quoted for each of the two components of O(ls) and Ni(2p,,,) spectra can, however, be obtained from spectra recorded with samples evacuated at increasing temperature from room temperature to 500°C. 3.2. Adsorption of nitric oxide In order to study the effect of NO adsorption on surface charge, the oxide samples were subjected in the preparation chamber to pressures of NO of about 5 Torr at room temperature for periods of between 15 and 45 min. The chamber was then evacuated and spectra recorded at pressures below 4 X 10m8Torr. For direct comparison, fig. 1 also shows the O(ls) spectral region of the NiO,ce, NiOlloo and NiO14s0 samples after exposure to NO. Spectra recorded after NO adsorption are shown in full-line presentation. Clearly all three samples show significant shifts to higher KE due to loss of surface charge. The magnitude of the shift is shown by the horizontal lines joining arrows marking the 529.7 eV binding energy peak positions. The surface charge on each oxide is now t0.6, tl .O and t1.4 eV respectively. These values are confirmed by the shifts in kinetic energy of the C( 1s) peak with a binding energy of 284.9 eV from carbon contamination of the sample
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Fig. 3. N(ls) spectra (pass energy 100 eV, fsd 3 X lo3 c s-l) from (a) Ni070o, (b) NiOl100 and (c) NiOl4 s 0. In each case the lower full line is the background spectrum and the upper full line the spectrum after admission of 5 Torr NO and evacuation to <4 X lo-* Torr at room temperature. The positions of binding energies 399 and 402 eV are shown on curve (a).
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and of the main Ni(2p,,J with binding energy of 854.6 eV (fig. 2). The peaks in all spectra after NO adsorption are noticeably narrower than those before adsorption (figs. 1 and 2). There has also been an apparent change in the relative intensities of 529.7 and 531.4 eV peaks in the NiO,,,e and NiOr r ee 0( 1s) spectra. In both cases, the peak of lower binding energy seems markedly increased in intensity. Although less obvious, the Ni01450 O(ls) spectrum shows the same effect. Ni(2p,,,) spectra after NO adsorption (fig. 2) indicate apparent loss of intensity in the 856.1 eV peak relative to the 854.6 eV peak although this is difficult to quantify in the broad spectral emissions in this region. There is marked sharpening of the satellite at 861.7 eV binding energy in all spectra after adsorption. Fig. 3 shows the N(ls) spectra from the samples as in figs. 1 and 2 above. This spectral region from the oxide after evacuation at 500°C and before NO adsorption showed a very broad intensity, barely detectable above background, in the 396399 eV binding energy region. The NiO ,ee spectrum of fig. 3a has the highest intensity and shows two very broad emissions with binding energies of 399 and 402 eV (most intense), after making allowance for the surface charge. Spectra from adsorbed nitric oxide on the NiOrree and NiO rd5e oxides clearly show the same basic features. There appears to be a loss of intensity in the 402 eV peak in going from NiO,ee to Ni0r4s0 samples. The smaller N(ls) intensities from adsorbed NO on the oxides heated at higher temperature are consistent with the surface areas of 2.64,0.54 and 0.36 m2 g-’ for Ni0700, NiOlloo and Ni01450 respectively. Adsorbed NO is very difficult to remove completely from all of these oxide surfaces. Even after evacuation at 500°C for prolonged periods, some intensity remains in the N(ls) spectra across the 397-400 eV binding energy region particularly near 397.5 eV. However, the surface charge does gradually increase to the values given in section 3.1 above with prolonged evacuation at 500°C. It is notable, with NiO,ee samples, that unless the N(ls) intensity is substantially reduced, subsequent CO adsorption does not raise the surface charge above +I .2 eV (cf. t2.6 eV discussed below). 3.3. Adsorption of carbon monoxide As with NO adsorption, CO adsorption was carried out in the preparation chamber at room temperature using pressures of approximately 5 Torr for 15-30 min before evacuation to below 1 X lo-* Torr. Fig. 4 shows the O(ls) spectral region. Broken-line spectra were recorded immediately before CO adsorption. No such spectrum is included for NiOr roe. Full-line spectra were recorded after CO adsorption. The surface charge associated with the three samples is about t2.7, t3.7, and t4.3 eV respectively after CO adsorption. Again, these values are confirmed by the shifts in the kinetic energies of the C(ls) and Ni(2p,,,) spectra. Ni(2p,,,) spectra of these samples also show sharp main emissions at 854.6 eV binding energy with no obvious second peak at about 856.1 eV binding energy. C( 1s) spectra at the highest sensitivity, but poor resolution (-2.5 ev), are shown
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Fig. 4. O(h) spectra (pass energy 50 eV, fsd 1 X lo4 c s-l) from (a) NiOToo, (b) NiOl1oo and (c) NiOt45o. Broken-line spectra were recorded after evacuation at 5OO’C and retooling to 2O’C; full-line spectra were recorded after admission of 5 Torr CO and evacuation at
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Fig. 5. C(ls) spectra (pass energy 100 eV, fsd 3 X lo3 c s-l) from (a) Ni07oo and (b) Ni014so after admission of CO at a pressure of 5 Torr and then evacuation to
in fig, 5 for NiO7ee and Ni0r45e samples after CO adsorption. The most intense peak, at i203.5 eV kinetic energy in both spectra (marked J) is due to uncharged carbon ~ont~~a~on on the metal &p_ There are broad ~nte~s~t~ Mama at f200.8 and f 195 eV (marked t) in th.e NiO y*e spectrum, but there is no discernable increase in ~~t$n~ty in these peaks from the spectrum observed before adsorption. Taking into remount the surface charge of i-2.7 GV,these peaks correspond to 285 and 290.8 eV binding energies respectively. Thus, the former is undoubtedly due to carbon ~o~ta~natio~ of the sample and the iatter to surface carbonate fo~at~on f12j +~~rn~~~~y~ with the NiO r4se s~~trum~ there is no reliable evidence for an intensity increase on CO adsorption and the very broad peaks with kinetic errergies of about 1199 and 1194 eV arise from the same soume, thfj difference in kinetic energy being the result of a surface charge of about +4.3 eV, i.e. about 0.6 eV greater than observed with the NiO ?ee sample. We have not been able to identify any specific binding energy corresponding to adsorbed CO on these samples, This is not ~r~~s~~g in view of the very srna8 surfaoe coverage of CO under the conditions used. Adsorbed CO seems to be re~a~~v~lyeasily removed from all NiO surfaces by evacuation at 500°C or by displacement in subsequent NO adsorption. This is sug gested by the return of surface charge vakes to thasc found for these treatments before CO adsorption
Values
of surface charge for all cases are given in table 1.
There exists a $~bst~~~ body of literature ~~~~~ng work on the ~ela~ons~ps between thermal pretreatment, stoicbiometric composition and chemisorptive prop
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erties, especially for oxygen chemisorption on nickel oxide [ 13,151. It is well established that heating nickel oxide at increasing temperature (i.e. 500-1500°C) gradually decreases the excess (over equilibrium) defect concentration first introduced by decomposition of the hydrated oxide starting material [ 131. Thus, differences in conductivity between NiOTOe, NiOrre,,, and Ni01d5e samples arise from changes in concentrations of nickel vacancies [8]. Ionisation of these nickel vacancies is directly related to hole concentrations in the valence band of the p-type oxide [ 161. Our previous work has demonstrated the relationship between surface charge, defect concentrations and conductivity of the oxide [8]. This relationship is exemplified again in figs. 1 and 2, where the variation in surface charge from t1.0 eV for NiO,ee to t3.6 eV for Ni01450 represents a conductivity change from 3.1 X 10m2to 5.0 X lo-’ ohm-’ m-l. In previous studies of bulk nickel oxide [ 17,181, and indeed, of oxidised nickel surfaces [ 19,201, the O(ls) spectra have commonly shown a main peak at 529.7 eV binding energy with a second peak, the intensity of which varies markedly with different treatments at 531.4 eV. Similarly the Ni(2ps,,) spectra [17,20] have shown two components at 854.6 and 856.1 eV binding energies. However, the results presented here represent the first observations of a systematic variation of the ratio of intensities of these two peaks with the pretreatment temperature of the oxide. Figs. 1 and 2 show that, for relatively clean nickel oxide surfaces (i.e. almost all adsorbed species removed after evacuation at 5OO”C), there is considerable intensity in the high binding energy O(ls) and Ni(2p,,,) peaks relative to the main emissions. The intensity in the high binding energy peak decreases in going from the more defective Ni07e0 surfaces to the more perfect Ni0r4s0 surface. However, our understanding of the nature of the two different electronic configurations of oxygen and nickel is by no means clear. The high binding energy O(ls) peak at 531.4 eV, for bulk NiO samples, has been variously assigned to Ni203 as a gross defect structure, hydroxyl groups and adsorbed O;, Of, O-, or Hz0 [17, 18,21-241. Significant surface hydroxylation and adsorbed water can be eliminated by the 500°C evacuation procedure used. in our work. The most comprehensive study, by Kim and Winograd [ 181, suggests that the high binding energy 0( 1s) peak arises from the presence of Ni203 as a surface defect structure observed in nickel oxide. This view is certainly consistent with the reduction in intensity of this peak as this oxide becomes closer to stoichiometry at high preheating temperatures. However, it has the interesting implication of a very large non-stoichiometry in the surface with oxygen excess varying from -30 at% in NiO,ee to -10 at% in NiO14se. Ni(2p,,,) spectra in fig. 2 show the same trends with loss of intensity in the high 856.1 eV binding energy peak paralleling loss of intensity in the O(ls) peak at 53 1.4 eV. The satellite at 861 .l eV associated with the Ni(2p) peak has been satisfactorily explained previously as arising from the charge transfer transition 02p e: + Ni3d tis (a, antibonding; b, bonding) when a 2p electron is emitted [ 171. The 1.7 eV satellite at 856.1 eV has been suggested to arise from multiplet splitting
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[ 17j. This is based on a calculation of such splitting in a NiFz- cluster. The observations of Oku and Hirokawa [25] that a symmetrical 854.6 eV peak with no shoulder at high binding energy is found for Ni2+ octahedrally co-ordinated in Ni,Mgr_,O solid solutions, argue strongly against an origin in multiplet splitting. It seems more likely that previous observations of intensity at 856.1 eV have arisen from the presence of defect structure in the oxide surface. Even with the Ni0r4s0 samples there is some intensity in both the O(ls) (531.4 eV) and Ni(2p,,,) (856.1 eV) peaks (figs. 1 and 2). 4.2. NO adsorption - acceptor chemisorption The most obvious effects of NO adsorption on all of the samples are (i) the loss of surface charge (see table 1) and (ii) the apparent increase in the relative intensity of both the O(ls) peak at 529.7 eV (fig. 1) and Ni(2ps,,) peak at 854.6 eV (fig. 2). The latter effect is particularly obvious with the NiO,ee 0( 1s) spectrum in fig. 1, in which a much sharper, more intense peak at 529.7 eV is observed after NO adsorption compared with the broad emission in fig. 1. There does not appear to have been a significant loss of intensity in the O(ls) peak at 531.4 eV in any sample. Curve fitting the NiO,ee spectrum in fig. 3 gives almost the same intensity ratio for the 531.4/529.7 eV peaks (i.e. 0.51) but the full width at half maximum (FWHM) of the 529.7 eV peak has decreased to 1.6 eV (cf. 2.0 eV before NO adsorption). The FWHM of the 53 1.4 eV peak has not changed significantly presumably because this reflects the defect oxygen found predominantly in the outermost layers. Thus this shift of intensity to the lower binding energy O(ls) peak is apparent rather than real. The same conclusion is likely to apply to the apparent loss of Ni3+ high binding energy species in Ni(2p3,,) spectra, namely that the intensity ratio is unchanged but the FWHM of the 854.6 eV peak has decreased. The N(1s) spectra show two forms of adsorbed NO with binding energies of -399 and 402 eV. Roberts and his colleagues [19] have discussed the nature of adsorbed NO species on oxidised nickel surfaces. Using arguments based on angleresolved photoemission data [26], on the structure of metal nitrosyl complexes [27] and on the behaviour of the peaks with temperature they assign the peak at 399 eV to negatively charged NO molecules adsorbed in a bent or tilted form relative to the surface normal. The precise tilt of an adsorbed NO molecule depends on the hybridisation and resulting formal charge on the NO molecule [27]. There are also well established relationships between formal charge and binding energy: the more negative the ligand the smaller is the value of the binding energy [28] so that the species with an N(ls) binding energy of 402 eV is likely to be an adsorbed NO molecule with much less formal charge (little back donation) and an orientation close to the surface normal. Finn and Jolly [29] have found binding energies for bent nitrosyl complexes with significant back donation of typically 399-400 eV whereas linear nitrosyl complexes with no back bonding to the n” NO orbitals were found to have binding energies near 401 eV. Theoretical calculations [30] of
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the shift to lower binding energy in going from a linear to bent configuration of NO adsorbed on nickel gave about 1.5 eV which is consistent with the direction and in reasonable agreement with the magnitudes of binding energy change between the Nls) values of our two adsorbed forms of nitric oxide. In a p-type semiconductor, a positive surface charge gives rise to bending of the valence and conduction bands downwards, reflecting a depletion of majority carriers (holes) in the spacecharge region to compensate for the surface charge. The loss of surface charge observed from even highly charged Ni0r4s0 surfaces suggests a considerable increase in surface conductivity due to NO adsorption. This will result from electron-trapping adsorbed species releasing more holes in the valence band. Hence, the overall effect of NO adsorption is to form adsorbed species on the surface capable of accepting an electron from the oxide. The “bent” NO speices with an N(ls) value of 399 eV fulfils this requirement. The species with an N(ls) value of 402 ‘eV has less negative charge (close to neutrality), similar to Vol’kenstein’s “weak” chemisorption 131. A previous XI’S study (Dianis and Lester [31]) of NO chemisorbed on NiO at -1OO’C with spectra recorded at increasing temperature up to 150°C, was similar. They observed predominantly the N(ls) peak at 399 eV which they also attributed to a molecular species with significant back donation. In addition, they noted shifts of the C( 1s) and Ni(2ps,J peaks with different NO coverage and suggested that this may be due to electron trapping changing the surface potential. Their O(ls) spectra at -1OO’C show initially an intensity increase at 532.1 eV but shifting to 529.6 eV as the sample is warmed to 150°C. 4.3. CO adsorption’ - donor chemisorption Adsorption of CO on ‘all samples produces the opppsite effect to that observed with NO, namely, a significant increase in positive surface charge (see table 1). This was the case even for the NiO,ae sample with relatively high conductivity. It is known that very little CO(<5% monolayer) adsorbs on NiO surfaces at room temperature [32]. We have, however, not been ableto isolate any spectral features corresponding to adsorbed molecular CO or carbon and oxygen adatoms. There is, however, a shift of intensity in the O(ls) spectra from the high binding energy (53 1.4 eV) to the low binding energy (529.7 eV) peaks, as there is in the Ni(2p,,,) spectra. This may be interpreted as reflecting CO adsorption over Ni* centres with “transfer of charge” leading to Ni*+. Associated with this will be the conversion of o-to o*-. However, the increase in surface charge is indicative of a chemisorbed species trapping a hole thereby lowering the hole concentration in the surface. We would expect a positively charged CO species and this is in accord with previous work function data [32]. Furthermore conductivity measurements during adsorption of CO on oxides [33] have also provided evidence for electron exchange processes involving the formation of CO& species with the creation of initiallypartially filled
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states more than 0.5 eV above the Fermi level at the surface. In order to maintain the Fermi level constant through bulk and surface, the bands will bend down further so that these levels may be maintained partially filled but to a level below the initial level. There is then effective net electron donation to the solid and the increased band bending further lowers the already depleted hole concentration at the surface. Our nickel oxide, prepared as relatively perfect single crystals with very little residual water or carbonate in the powder [4], is clearly rather different from the catalytically active nickel oxide, prepared by decomposition under vacuum at 150250°C (Gravelle and Teichner [34], and Guglielminotti et al. [35]). However, there are some comparisons worthy of note. We found no reduction of nickel (II) to nickel (0) due to CO adsorption at room temperature for periods up to 2 h. Heating NiOiiee in 4 Torr CO at 3OO’C for 15 h resulted in a distinct shoulder in the Ni(2p& spectrum at a binding energy of 853.1 eV due to the formation of nickel (0). This corresponded to approximately 40% of the intensity of the 854.6 eV peak. The surface charge remained at 3.7 eV after this treatment. There does not appear to be any loss of charge due to formation of an extensive surface metal layer. The binding energy of 853.1 eV is the same as that for nickel metal. This does not suggest individual, isolated metal atoms as we might expect some shifts due to fields from surrounding ions. The likely conclusion is that nickel (0) exists as small particles consisting of a few atoms. The catalytically active nickel oxide [34] in contrast, is almost entirely reduced in carbon monoxide at 200°C. There has been some controversy concerning whether room temperature adsorption of CO on active nickel oxide is reductive [34-361. Our results, admittedly with more perfect oxide surfaces, confirm the view of Gravelle, Teichner and co-workers [34,36], that NiO is not reduced by CO at room temperature. We also find that desorption at room temperature occurs as CO with very little COZ. On the evidence of our XPS data and work function measurements [32], we also cannot agree with the arguments put forward by Guglielminotti et al. [35], that CO adsorption as carbonyl type groups, is electron-withdrawing. The overall effect on our oxide samples is clearly electron donation. 4.4. Space charge effects of adsorption in relation to coverage Having examined the adsorption of NO and CO separately, there are finally some general points worthy of note. Although it is difficult to estimate coverage for NO(ads) reliably, it is clear that this coverage is much higher than that found for CO(ads) under similar conditions. By comparison of fig. 3 with N(ls) intensities from NO adsorbed (30 L) on oxidised nickel [ 191, we would estimate coverage at more than 30%. CO coverage is clearly <5% since this is roughly the limit of detection for C(ls) emission intensity and this accords with previous work [32]. This difference is consistent with cumulative adsorption of NO and depletive adsorption of CO. For a p-type semiconductor, hole injection (or electron extraction) gives an
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accumulation layer (reduced distance) leading to higher coverage i.e. the NO case. It is interesting that this is also possible where there is less electron transfer to the NO. Dianis and Lester [31] have argued that a covalent bond forms, trapping the electron, rather than a distinctly anionic species. Conversely, unless CO acts as a very strong reducing agent causing an inversion layer, coverage will be very low as further coverage will be opposed by an increasing double-layer potential. The results are consistent with this semiconductor band model.
Acknowledgement We are grateful to Albert Carley for assistance and advice.
References [l] Z.M. Jarzebski, Oxide Semiconductors
(Pergamon, Oxford, 1973). Goodenough, Metallic Oxides, in: Progress in Solid State Chemistry, Vol. 5, Ed. H. Reiss (1971) p. 145. (31 F.F. Vol’kenstein, The Electronic Theory of Catalysis on Semiconductors (Pergamon, Oxford, 1963). (41 C.F. Jones, R.L. Segall, R.St.C. Smart and P.S. Turner, J. Chem. Sot. Faraday I, 73 (1977) 1710. [5] C.F. Jones, R.L. Segall, R.St.C. Smart and P.S. Turner, J. Chem. Sot. Faraday I, 74 (1978) 1615. [6] C.F. Jones, R.L. Segall, R.St.C. Smart and P.S. Turner, J. Chem. Sot. Faraday I, 74 (1978) 1624. [7] R.L. SegaIl, R.St.C. Smart and P.S. Turner, Semiconductor Dissolution Kinetics, in: Physics of Materials, Eds. D.W. Borland, L.M. Clareborough and A.J.W. Moore (CSIRO, Melbourne, 1979). [8] M.W. Roberts and R.St.C. Smart, Chem. Phys. Letters 69 (1980) 234. [9] L. Ley, R.A. PoIlak, F.R. McFeely, S.P. Kowalczyk and D.A. Shirley, Phys. Rev. B9 (1974) 600. [lo] S.R. Morrison, The Chemical Physics of Surfaces (Plenum, 1977). [ll] A.F. Carley and M.W. Roberts, Proc. Roy. Sot. (London) A363 (1978) 403. [ 121 J.W. Robinson, Handbook of Spectroscopy, Vol. 1 (CRC Press, Cleveland, Ohio, 1974). [13] J. Deren and J. Stoch, J. Catalysis 18 (1970) 249. [14] E.R.S. Winter, Advan. Catalysis 10 (1958) 165. [ 151 G.K. Boreskov, Advan. Catalysis 15 (1964) 289. (161 J. Deren and S. Mrowec, J. Mater. Scl. 8 (1973) 545. [17] K.S. Kim and R.E. Davis, J. Electron Spectrosc. 1 (1973) 257. [18] K.S. Kim and N. Winograd, Surface Scl. 43 (1974) 625. [19] D.W. Johnson, M.H. Matloob and M.W. Roberts, J. Chem. Sot. Faraday I, 75 (1979) 2143; A.F. Carley, S. Rassias, M.W. Roberts and T.H. Wang, Surface Sci. 84 (1979) 1227. [20] S. Evans, J. Pielaszek and J.M. Thomas, Surface Scl. 56 (1976) 644. [21] T. Robert, M. Bartel and G. Offergold, Surface Sci. 33 (1972) 123. [22] R. Rosencwaig and G.K. Wertheim, J. Electron Spectrosc. 1 (1972) 493. [2]
J.B.
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[23] G. Schon, J. Electron Spectrosc.
and acceptor chemisorption
1 (1972) 705. K. Kishi and S. Ikeda, Chem. Letters (1972) 245. M. Oku and K. Hirokawa, J. Electron Spectrosc. 10 (1977) 103. E:W. PIummer and.T. Gustafsson, Science 198 (1977) 165. J.A. McGinnety, in: Organic Chemistry, Part 1, Ed. D.W. Sharp (Butterworths, 1972). K. Siegbahn, ESCA: Atomic, Molecular and Solid State Structure Studied by Electron Spectroscopy (Abnqvist and WikseII, UppsaIa, 1967). [29] P. Finn and W. Jolly, Inorg. Chem. 11 (1972) 893. [30] D.T. Clark, B.J. Crommarty and A. SgameIloti, Chem. Phys. Letters 68 (1979) 420. 1311 W. Dianis and J.E. Lester, Surface Sci. 43 (1974) 602. 1321 M.W. Roberts and B.R. We&, Faraday Disc. Chem. Sot. 41 (1966) 162. [33] K. KIier and M. Jiratova, in: Proc. 3rd Intern. Congr. on Catalysis, Amsterdam, 1964 (North-Holland, Amsterdam, 1974) p. 763. [34] P.C. GravelIe and S.J. Teichner, Advan. Catalysis 20 (1969) 167. [35] E. Gugliehninotti, L. Cerruti and E. BoreIlo, Gazz. Chim. Ital. 107 (1977) 447,503. [36] P.C. Gravelle, G. Marty, G. El Shobaky and S.J. Teichner, BuII. Sot. Chim. France (1969) 1517. [24] [25] [26] [27] [28]
XP!3 STUDIES OF DONOR AND ACCEPTOR CHEMISORPTION OF NO AND CO ON NICKEL OXIDE SURFACES. M.W. ROBERTS * and R.St.C. SMART ** Department of Physical Chemistry, W. Yorkshire BD 7 I DP. UK
University of Bradford, Bradford,
Received 22 January 1980; accepted for publication 25 June 1980
Systematic variation on the defect (excess) oxygen concentration in the surface of nickel oxide preheated to 700, 1100 and 145O”C, has been revealed in O(ls) and Ni(2p3& X-ray photoelectron spectra. The magnitude of the surface charge, after evacuation at 5OO”C, is also directly related to the defect properties. The effect of chemisorption of NO and CO on the free hole concentration of the p-type semiconductor surface has been studied by monitoring the surface charge. NO adsorption, predominantly as negatively charged (electron acceptor) molecules removes much of the surface charge whereas CO adsorption, as CO”+ species (donor chemisorption), increases the surface charge on all samples. 1. Introduction
The electronic theory of semiconducting oxides, and its relationship to technical applications, is now well established [ 1,2]. Much less attention has been paid to the understanding of electronic factors controlling surface phenomena such as surface charge, modes of adsorption and rates of reaction although the work of Vol’kenstein [3] and collaborators represented an attempt to develop a coherent theory of electronic control in catalysis on semiconductors. More recently, the effects of bulk and surface defect concentrations on surface reactions over nickel oxide have been examined [4-71. Vol’kenstein’s electronic theory of catalysis [3] distinguishes two types of chemisorption differing in their effect on the free electrons and positive holes of the oxide semiconductor surface. The first, “weak” chemisorption, results in a neutral adsorbed molecule without participation of the charge carriers of the crystal lattice. The second, “strong” chemisorption, involves a charged particle formed by retention of either a free electron or a free hole in the chemisorption bond. This “strong” chemisorption can produce either a p-bond (donor bond) by hole capture or an n-bond (acceptor bond) by electron capture. In either case, the concentration of free charge carriers in the surface and bulk is changed. Kinetic studies to test this * Present address: Dept. Physical Chemistry, University College, P.O. Box 78, Cardiff CPl lXL, Wales, UK. ** Present address: School of Science, Griffith University, Nathan, Queensland 4111, Australia. 590
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explanation, although giving some support for the different modes of adsorption, provide rather indirect evidence for the reaction mechanisms. Recently, effects of chemisorption and reaction on the electronic properties of the catalysts have been determined by physical methods that measure surface species more directly. The observation of changed concentrations of charge carriers at the surface is most easily made by measurement of changes in surface charge. The origin of a surface charge, as seen in the shift in observed kinetic energies of the emitted core electrons, has been studied in general terms in our previous work [8]. Loss of primary photoelectrons and secondary electron emission causes positive charging of the surface until this current is counter-balanced by a neutralising electron current, due to photoelectrons (from the X-ray window) and surface conduction, to the surface. Ley et al. [9] have found a correlation between the magnitude of surface charge and the band gap of II-VI semiconductors. Where the charging is less than the band gap, it is expected that surface conductivity is the major contribution in the neutralising electron current. Correlation of surface charge values with differing defect concentrations, arising from different oxide pretreatments, and different temperatures of the sample during recording of spectra has also been demonstrated [8]. The electronic explanation of “strong” chemisorption is closely allied to explanations of the formation of surface states [lo]. For instance, in a p-type semiconducting oxide like nickel oxide the introduction of a narrow density of filled or partly-filled surface states above the Fermi level leads to downward bending of the valence and conduction bands at the surface, shifting the Fermi level (or surface state level) until a common level is established between the surface state and Fermi level. The surface states are now filled to a level below that originally prevailing for the neutral (zero surface charge) surface. Consequently a positive surface charge, related to the extent of band bending, is formed in a manner very similar to p-bond hole trapping in “strong” chemisorption. It seems worthwhile to explore the effect of such chemisorption on electronic changes in both the adsorbed molecules and the solid. We describe here direct observations, using XI’S, of changes in surface charge of nickel oxide due to chemisorption of donor and acceptor molecules (NO, CO). The effect of chemisorption on the electronic configuration of nickel and oxygen at the surface is related to the nature of the adsorbed species and the changes in electronic properties of the solid. We also show that the charge effects, due to “strong” chemisorption, depend on the properties of the solid oxide itself and can be changed by different defect concentrations introduced during pretreatment. 2. Experimental
details
2.1. Sample preparation Nickel oxide was obtained from the same high purity source (Univar AR Grade) used in previous work [4,8]. Samples with different properties were prepared by
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heating the oxide at 700°C (i.e. NiO,ee), 1lOO’C (i.e. NiOr roe) and 145O’C (i.e. NiOrJso) for 4 h in air. The purity, surface area and morphology, and defect properties have been discussed previously [4,5,8]. The only impurities at levels >2 ppm are cobalt (20 ppm) and iron (
3. Results 3.1. Nickel oxide surfaces before adsorption Figs. 1 and 2 Ni07e0, NiOrree, sample at 5OO’C ing degassing the
show the O(ls) and Ni(2ps,*) spectral regions respectively of and NiOrhse samples, recorded at 20°C after evacuation of each below 1 X 10m9 Torr. The spectra are shown as broken lines. Durmass spectrometer showed that loss of Hz0 predominates at tem-
M. W. Roberts, R.St.C. Smart /Donor and acceptor chemisorption
g60
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eV 95o
Fig. 1. O(h) spectra (pass energy 50 eV, full scale deflection, i.e. fsd 1 X lo4 c s-l) from (a) NiO,oo, (b) NiOl 100 and (c) Ni014so. Broken-line spectra were recorded after evacuation at 5OO’C and retooling to 2O’C; full-line spectra were recorded after admission of 5 Torr NO and evacuation to <4 X 10% Torr at room temperature; full-line arrow indicates O(ls) binding energy of 529.7 eV; broken-line arrow indicates binding energy of 531.4 eV.
peratures up to 750°C. although some CO and CO2 are also observed (approximately 10% of L_e Hz0 lost in each case). Between 300 and 5OO”C, loss of CO pre-
dominates with Hz0 and CO2 losses -50% of the CO loss. No O2 was observed even at the highest sensitivity. The method of estimating the magnitude of surface charge has been described [8]. C(ls) spectra of all three samples indicate two components arising from carbon contamination on the uncharged metal clip (KE 1203.5 eV) and on the sample (potentially variable KE). The shift in the KE of the very weak signal due to sample carbon contamination can then be estimated and the surface charge obtained directly. Estimates of corrected C/O intensity ratios, by the method used in our earlier work [8], i.e. using calculated emission cross sections, analyser energy response proportional to l/KE and neglecting escape depth variation with KE, gave values of -0.05. The surface charge on the samples shown in figs. 1 and 2 is t1.0, t3.0 and t3.6 eV for NiOToO, NiOr roe and NiO r4so respectively. These values agree with those for the KE shifts of the main 0( 1s) peak relative to the KE of uncharged O(ls) samples (i.e. KE 958.5 eV). The 0( 1s) spectra of NiO r r o. and NiO r4so oxides in fig. 1 show a strong, sharp peak at a binding energy of 529.7 eV with asymmetry on the high binding energy
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Fig. 2. Ni(2pan) spectra (pass energy and (c) NiOl4 s 0. Broken&e spectra 2&J, full-line spectra were recorded lQ* Torr at room temperature. The are shown on curve (c).
and acceptor chemisorption
50 eV, fsd 1 X lo4 c s-l) from (a) Ni07o0, (b) NiOl10o were recorded after evacuation at SOO’C and retooling to after admission of 5 Torr NO and evacuation to <4 X positions of the two binding energies 854.6 and 856.1 eV
side indicating a weak, broad peak close to the 529.7 eV peak. This is confirmed by the Ni0700 sample in which the 0( 1s) spectrum has a very broad peak with a maximum binding energy at 529.7 eV and a very broad shoulder at 531.4 eV. The 529.7 eV peaks are marked by full-line arrows while 531.4 eV peaks are marked by broken-line arrows in all spectra. Analysis of the O(ls) spectra by curve fitting with Gaussian peaks using a least squares fitting procedure [ 1 l] gives the following ratios for the intensity of the 531.4 eV peak relative to the 529.7 eV peak: Ni0700 0.54; NiOlloo 0.12; NiO14s0 0~10. The full widths at half maximum found by this method for the 529.7 and 531.4 eV peaks in Ni0700, NiOrIoo, and NiO14s0 are 2.0 and 2.5; 2.5 and 4.0; and 2.6 and 3.3 eV respectively. The Ni(2ps,2) spectra in fig. 2 similarly show two peaks in the main emission with binding energies of 854.6 and 856.1 eV. The Ni0r4s0 sample has a spectrum with maximum intensity in the 854.6 eV peak and a distinct shoulder at 856.1 eV which we have used to assign the second O(ls) peak whilst the NiOrloo and Ni0700 spectra both show very broad emission with a maximum at 854.6 eV but a broad shoulder on the high binding energy side. The satellite peak near 861.7 eV is also considerably sharper in the NiO 145o spectrum where the low binding energy O(ls) Spectra obtained after evacuation at and Ni(2ps,a) components predominate.
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500°C often have broad, rather ill-defined peaks, as is clear from NiOrrce and NiO,e,, spectra in fig. 2. The values of the binding energies quoted for each of the two components of O(ls) and Ni(2p,,,) spectra can, however, be obtained from spectra recorded with samples evacuated at increasing temperature from room temperature to 500°C. 3.2. Adsorption of nitric oxide In order to study the effect of NO adsorption on surface charge, the oxide samples were subjected in the preparation chamber to pressures of NO of about 5 Torr at room temperature for periods of between 15 and 45 min. The chamber was then evacuated and spectra recorded at pressures below 4 X 10m8Torr. For direct comparison, fig. 1 also shows the O(ls) spectral region of the NiO,ce, NiOlloo and NiO14s0 samples after exposure to NO. Spectra recorded after NO adsorption are shown in full-line presentation. Clearly all three samples show significant shifts to higher KE due to loss of surface charge. The magnitude of the shift is shown by the horizontal lines joining arrows marking the 529.7 eV binding energy peak positions. The surface charge on each oxide is now t0.6, tl .O and t1.4 eV respectively. These values are confirmed by the shifts in kinetic energy of the C( 1s) peak with a binding energy of 284.9 eV from carbon contamination of the sample
N(k)
KINETIC
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Fig. 3. N(ls) spectra (pass energy 100 eV, fsd 3 X lo3 c s-l) from (a) Ni070o, (b) NiOl100 and (c) NiOl4 s 0. In each case the lower full line is the background spectrum and the upper full line the spectrum after admission of 5 Torr NO and evacuation to <4 X lo-* Torr at room temperature. The positions of binding energies 399 and 402 eV are shown on curve (a).
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and of the main Ni(2p,,J with binding energy of 854.6 eV (fig. 2). The peaks in all spectra after NO adsorption are noticeably narrower than those before adsorption (figs. 1 and 2). There has also been an apparent change in the relative intensities of 529.7 and 531.4 eV peaks in the NiO,,,e and NiOr r ee 0( 1s) spectra. In both cases, the peak of lower binding energy seems markedly increased in intensity. Although less obvious, the Ni01450 O(ls) spectrum shows the same effect. Ni(2p,,,) spectra after NO adsorption (fig. 2) indicate apparent loss of intensity in the 856.1 eV peak relative to the 854.6 eV peak although this is difficult to quantify in the broad spectral emissions in this region. There is marked sharpening of the satellite at 861.7 eV binding energy in all spectra after adsorption. Fig. 3 shows the N(ls) spectra from the samples as in figs. 1 and 2 above. This spectral region from the oxide after evacuation at 500°C and before NO adsorption showed a very broad intensity, barely detectable above background, in the 396399 eV binding energy region. The NiO ,ee spectrum of fig. 3a has the highest intensity and shows two very broad emissions with binding energies of 399 and 402 eV (most intense), after making allowance for the surface charge. Spectra from adsorbed nitric oxide on the NiOrree and NiO rd5e oxides clearly show the same basic features. There appears to be a loss of intensity in the 402 eV peak in going from NiO,ee to Ni0r4s0 samples. The smaller N(ls) intensities from adsorbed NO on the oxides heated at higher temperature are consistent with the surface areas of 2.64,0.54 and 0.36 m2 g-’ for Ni0700, NiOlloo and Ni01450 respectively. Adsorbed NO is very difficult to remove completely from all of these oxide surfaces. Even after evacuation at 500°C for prolonged periods, some intensity remains in the N(ls) spectra across the 397-400 eV binding energy region particularly near 397.5 eV. However, the surface charge does gradually increase to the values given in section 3.1 above with prolonged evacuation at 500°C. It is notable, with NiO,ee samples, that unless the N(ls) intensity is substantially reduced, subsequent CO adsorption does not raise the surface charge above +I .2 eV (cf. t2.6 eV discussed below). 3.3. Adsorption of carbon monoxide As with NO adsorption, CO adsorption was carried out in the preparation chamber at room temperature using pressures of approximately 5 Torr for 15-30 min before evacuation to below 1 X lo-* Torr. Fig. 4 shows the O(ls) spectral region. Broken-line spectra were recorded immediately before CO adsorption. No such spectrum is included for NiOr roe. Full-line spectra were recorded after CO adsorption. The surface charge associated with the three samples is about t2.7, t3.7, and t4.3 eV respectively after CO adsorption. Again, these values are confirmed by the shifts in the kinetic energies of the C(ls) and Ni(2p,,,) spectra. Ni(2p,,,) spectra of these samples also show sharp main emissions at 854.6 eV binding energy with no obvious second peak at about 856.1 eV binding energy. C( 1s) spectra at the highest sensitivity, but poor resolution (-2.5 ev), are shown
M. W. Roberts, R.St.C. Smart /Donor nnd acceptor chemisorption
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9-y g60
KINETIC
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eVgso
Fig. 4. O(h) spectra (pass energy 50 eV, fsd 1 X lo4 c s-l) from (a) NiOToo, (b) NiOl1oo and (c) NiOt45o. Broken-line spectra were recorded after evacuation at 5OO’C and retooling to 2O’C; full-line spectra were recorded after admission of 5 Torr CO and evacuation at
C(k)
1205
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Fig. 5. C(ls) spectra (pass energy 100 eV, fsd 3 X lo3 c s-l) from (a) Ni07oo and (b) Ni014so after admission of CO at a pressure of 5 Torr and then evacuation to
in fig, 5 for NiO7ee and Ni0r45e samples after CO adsorption. The most intense peak, at i203.5 eV kinetic energy in both spectra (marked J) is due to uncharged carbon ~ont~~a~on on the metal &p_ There are broad ~nte~s~t~ Mama at f200.8 and f 195 eV (marked t) in th.e NiO y*e spectrum, but there is no discernable increase in ~~t$n~ty in these peaks from the spectrum observed before adsorption. Taking into remount the surface charge of i-2.7 GV,these peaks correspond to 285 and 290.8 eV binding energies respectively. Thus, the former is undoubtedly due to carbon ~o~ta~natio~ of the sample and the iatter to surface carbonate fo~at~on f12j +~~rn~~~~y~ with the NiO r4se s~~trum~ there is no reliable evidence for an intensity increase on CO adsorption and the very broad peaks with kinetic errergies of about 1199 and 1194 eV arise from the same soume, thfj difference in kinetic energy being the result of a surface charge of about +4.3 eV, i.e. about 0.6 eV greater than observed with the NiO ?ee sample. We have not been able to identify any specific binding energy corresponding to adsorbed CO on these samples, This is not ~r~~s~~g in view of the very srna8 surfaoe coverage of CO under the conditions used. Adsorbed CO seems to be re~a~~v~lyeasily removed from all NiO surfaces by evacuation at 500°C or by displacement in subsequent NO adsorption. This is sug gested by the return of surface charge vakes to thasc found for these treatments before CO adsorption
Values
of surface charge for all cases are given in table 1.
There exists a $~bst~~~ body of literature ~~~~~ng work on the ~ela~ons~ps between thermal pretreatment, stoicbiometric composition and chemisorptive prop
M. W. Roberts, R.St.C. Smart /Donor
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erties, especially for oxygen chemisorption on nickel oxide [ 13,151. It is well established that heating nickel oxide at increasing temperature (i.e. 500-1500°C) gradually decreases the excess (over equilibrium) defect concentration first introduced by decomposition of the hydrated oxide starting material [ 131. Thus, differences in conductivity between NiOTOe, NiOrre,,, and Ni01d5e samples arise from changes in concentrations of nickel vacancies [8]. Ionisation of these nickel vacancies is directly related to hole concentrations in the valence band of the p-type oxide [ 161. Our previous work has demonstrated the relationship between surface charge, defect concentrations and conductivity of the oxide [8]. This relationship is exemplified again in figs. 1 and 2, where the variation in surface charge from t1.0 eV for NiO,ee to t3.6 eV for Ni01450 represents a conductivity change from 3.1 X 10m2to 5.0 X lo-’ ohm-’ m-l. In previous studies of bulk nickel oxide [ 17,181, and indeed, of oxidised nickel surfaces [ 19,201, the O(ls) spectra have commonly shown a main peak at 529.7 eV binding energy with a second peak, the intensity of which varies markedly with different treatments at 531.4 eV. Similarly the Ni(2ps,,) spectra [17,20] have shown two components at 854.6 and 856.1 eV binding energies. However, the results presented here represent the first observations of a systematic variation of the ratio of intensities of these two peaks with the pretreatment temperature of the oxide. Figs. 1 and 2 show that, for relatively clean nickel oxide surfaces (i.e. almost all adsorbed species removed after evacuation at 5OO”C), there is considerable intensity in the high binding energy O(ls) and Ni(2p,,,) peaks relative to the main emissions. The intensity in the high binding energy peak decreases in going from the more defective Ni07e0 surfaces to the more perfect Ni0r4s0 surface. However, our understanding of the nature of the two different electronic configurations of oxygen and nickel is by no means clear. The high binding energy O(ls) peak at 531.4 eV, for bulk NiO samples, has been variously assigned to Ni203 as a gross defect structure, hydroxyl groups and adsorbed O;, Of, O-, or Hz0 [17, 18,21-241. Significant surface hydroxylation and adsorbed water can be eliminated by the 500°C evacuation procedure used. in our work. The most comprehensive study, by Kim and Winograd [ 181, suggests that the high binding energy 0( 1s) peak arises from the presence of Ni203 as a surface defect structure observed in nickel oxide. This view is certainly consistent with the reduction in intensity of this peak as this oxide becomes closer to stoichiometry at high preheating temperatures. However, it has the interesting implication of a very large non-stoichiometry in the surface with oxygen excess varying from -30 at% in NiO,ee to -10 at% in NiO14se. Ni(2p,,,) spectra in fig. 2 show the same trends with loss of intensity in the high 856.1 eV binding energy peak paralleling loss of intensity in the O(ls) peak at 53 1.4 eV. The satellite at 861 .l eV associated with the Ni(2p) peak has been satisfactorily explained previously as arising from the charge transfer transition 02p e: + Ni3d tis (a, antibonding; b, bonding) when a 2p electron is emitted [ 171. The 1.7 eV satellite at 856.1 eV has been suggested to arise from multiplet splitting
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M. W.Roberts, R.St.C. Smart /Donor and acceptor chemisorption
[ 17j. This is based on a calculation of such splitting in a NiFz- cluster. The observations of Oku and Hirokawa [25] that a symmetrical 854.6 eV peak with no shoulder at high binding energy is found for Ni2+ octahedrally co-ordinated in Ni,Mgr_,O solid solutions, argue strongly against an origin in multiplet splitting. It seems more likely that previous observations of intensity at 856.1 eV have arisen from the presence of defect structure in the oxide surface. Even with the Ni0r4s0 samples there is some intensity in both the O(ls) (531.4 eV) and Ni(2p,,,) (856.1 eV) peaks (figs. 1 and 2). 4.2. NO adsorption - acceptor chemisorption The most obvious effects of NO adsorption on all of the samples are (i) the loss of surface charge (see table 1) and (ii) the apparent increase in the relative intensity of both the O(ls) peak at 529.7 eV (fig. 1) and Ni(2ps,,) peak at 854.6 eV (fig. 2). The latter effect is particularly obvious with the NiO,ee 0( 1s) spectrum in fig. 1, in which a much sharper, more intense peak at 529.7 eV is observed after NO adsorption compared with the broad emission in fig. 1. There does not appear to have been a significant loss of intensity in the O(ls) peak at 531.4 eV in any sample. Curve fitting the NiO,ee spectrum in fig. 3 gives almost the same intensity ratio for the 531.4/529.7 eV peaks (i.e. 0.51) but the full width at half maximum (FWHM) of the 529.7 eV peak has decreased to 1.6 eV (cf. 2.0 eV before NO adsorption). The FWHM of the 53 1.4 eV peak has not changed significantly presumably because this reflects the defect oxygen found predominantly in the outermost layers. Thus this shift of intensity to the lower binding energy O(ls) peak is apparent rather than real. The same conclusion is likely to apply to the apparent loss of Ni3+ high binding energy species in Ni(2p3,,) spectra, namely that the intensity ratio is unchanged but the FWHM of the 854.6 eV peak has decreased. The N(1s) spectra show two forms of adsorbed NO with binding energies of -399 and 402 eV. Roberts and his colleagues [19] have discussed the nature of adsorbed NO species on oxidised nickel surfaces. Using arguments based on angleresolved photoemission data [26], on the structure of metal nitrosyl complexes [27] and on the behaviour of the peaks with temperature they assign the peak at 399 eV to negatively charged NO molecules adsorbed in a bent or tilted form relative to the surface normal. The precise tilt of an adsorbed NO molecule depends on the hybridisation and resulting formal charge on the NO molecule [27]. There are also well established relationships between formal charge and binding energy: the more negative the ligand the smaller is the value of the binding energy [28] so that the species with an N(ls) binding energy of 402 eV is likely to be an adsorbed NO molecule with much less formal charge (little back donation) and an orientation close to the surface normal. Finn and Jolly [29] have found binding energies for bent nitrosyl complexes with significant back donation of typically 399-400 eV whereas linear nitrosyl complexes with no back bonding to the n” NO orbitals were found to have binding energies near 401 eV. Theoretical calculations [30] of
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the shift to lower binding energy in going from a linear to bent configuration of NO adsorbed on nickel gave about 1.5 eV which is consistent with the direction and in reasonable agreement with the magnitudes of binding energy change between the Nls) values of our two adsorbed forms of nitric oxide. In a p-type semiconductor, a positive surface charge gives rise to bending of the valence and conduction bands downwards, reflecting a depletion of majority carriers (holes) in the spacecharge region to compensate for the surface charge. The loss of surface charge observed from even highly charged Ni0r4s0 surfaces suggests a considerable increase in surface conductivity due to NO adsorption. This will result from electron-trapping adsorbed species releasing more holes in the valence band. Hence, the overall effect of NO adsorption is to form adsorbed species on the surface capable of accepting an electron from the oxide. The “bent” NO speices with an N(ls) value of 399 eV fulfils this requirement. The species with an N(ls) value of 402 ‘eV has less negative charge (close to neutrality), similar to Vol’kenstein’s “weak” chemisorption 131. A previous XI’S study (Dianis and Lester [31]) of NO chemisorbed on NiO at -1OO’C with spectra recorded at increasing temperature up to 150°C, was similar. They observed predominantly the N(ls) peak at 399 eV which they also attributed to a molecular species with significant back donation. In addition, they noted shifts of the C( 1s) and Ni(2ps,J peaks with different NO coverage and suggested that this may be due to electron trapping changing the surface potential. Their O(ls) spectra at -1OO’C show initially an intensity increase at 532.1 eV but shifting to 529.6 eV as the sample is warmed to 150°C. 4.3. CO adsorption’ - donor chemisorption Adsorption of CO on ‘all samples produces the opppsite effect to that observed with NO, namely, a significant increase in positive surface charge (see table 1). This was the case even for the NiO,ae sample with relatively high conductivity. It is known that very little CO(<5% monolayer) adsorbs on NiO surfaces at room temperature [32]. We have, however, not been ableto isolate any spectral features corresponding to adsorbed molecular CO or carbon and oxygen adatoms. There is, however, a shift of intensity in the O(ls) spectra from the high binding energy (53 1.4 eV) to the low binding energy (529.7 eV) peaks, as there is in the Ni(2p,,,) spectra. This may be interpreted as reflecting CO adsorption over Ni* centres with “transfer of charge” leading to Ni*+. Associated with this will be the conversion of o-to o*-. However, the increase in surface charge is indicative of a chemisorbed species trapping a hole thereby lowering the hole concentration in the surface. We would expect a positively charged CO species and this is in accord with previous work function data [32]. Furthermore conductivity measurements during adsorption of CO on oxides [33] have also provided evidence for electron exchange processes involving the formation of CO& species with the creation of initiallypartially filled
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M. W. Roberts, R.St.C. Smart /Donor
and acceptor chemisorption
states more than 0.5 eV above the Fermi level at the surface. In order to maintain the Fermi level constant through bulk and surface, the bands will bend down further so that these levels may be maintained partially filled but to a level below the initial level. There is then effective net electron donation to the solid and the increased band bending further lowers the already depleted hole concentration at the surface. Our nickel oxide, prepared as relatively perfect single crystals with very little residual water or carbonate in the powder [4], is clearly rather different from the catalytically active nickel oxide, prepared by decomposition under vacuum at 150250°C (Gravelle and Teichner [34], and Guglielminotti et al. [35]). However, there are some comparisons worthy of note. We found no reduction of nickel (II) to nickel (0) due to CO adsorption at room temperature for periods up to 2 h. Heating NiOiiee in 4 Torr CO at 3OO’C for 15 h resulted in a distinct shoulder in the Ni(2p& spectrum at a binding energy of 853.1 eV due to the formation of nickel (0). This corresponded to approximately 40% of the intensity of the 854.6 eV peak. The surface charge remained at 3.7 eV after this treatment. There does not appear to be any loss of charge due to formation of an extensive surface metal layer. The binding energy of 853.1 eV is the same as that for nickel metal. This does not suggest individual, isolated metal atoms as we might expect some shifts due to fields from surrounding ions. The likely conclusion is that nickel (0) exists as small particles consisting of a few atoms. The catalytically active nickel oxide [34] in contrast, is almost entirely reduced in carbon monoxide at 200°C. There has been some controversy concerning whether room temperature adsorption of CO on active nickel oxide is reductive [34-361. Our results, admittedly with more perfect oxide surfaces, confirm the view of Gravelle, Teichner and co-workers [34,36], that NiO is not reduced by CO at room temperature. We also find that desorption at room temperature occurs as CO with very little COZ. On the evidence of our XPS data and work function measurements [32], we also cannot agree with the arguments put forward by Guglielminotti et al. [35], that CO adsorption as carbonyl type groups, is electron-withdrawing. The overall effect on our oxide samples is clearly electron donation. 4.4. Space charge effects of adsorption in relation to coverage Having examined the adsorption of NO and CO separately, there are finally some general points worthy of note. Although it is difficult to estimate coverage for NO(ads) reliably, it is clear that this coverage is much higher than that found for CO(ads) under similar conditions. By comparison of fig. 3 with N(ls) intensities from NO adsorbed (30 L) on oxidised nickel [ 191, we would estimate coverage at more than 30%. CO coverage is clearly <5% since this is roughly the limit of detection for C(ls) emission intensity and this accords with previous work [32]. This difference is consistent with cumulative adsorption of NO and depletive adsorption of CO. For a p-type semiconductor, hole injection (or electron extraction) gives an
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603
accumulation layer (reduced distance) leading to higher coverage i.e. the NO case. It is interesting that this is also possible where there is less electron transfer to the NO. Dianis and Lester [31] have argued that a covalent bond forms, trapping the electron, rather than a distinctly anionic species. Conversely, unless CO acts as a very strong reducing agent causing an inversion layer, coverage will be very low as further coverage will be opposed by an increasing double-layer potential. The results are consistent with this semiconductor band model.
Acknowledgement We are grateful to Albert Carley for assistance and advice.
References [l] Z.M. Jarzebski, Oxide Semiconductors
(Pergamon, Oxford, 1973). Goodenough, Metallic Oxides, in: Progress in Solid State Chemistry, Vol. 5, Ed. H. Reiss (1971) p. 145. (31 F.F. Vol’kenstein, The Electronic Theory of Catalysis on Semiconductors (Pergamon, Oxford, 1963). (41 C.F. Jones, R.L. Segall, R.St.C. Smart and P.S. Turner, J. Chem. Sot. Faraday I, 73 (1977) 1710. [5] C.F. Jones, R.L. Segall, R.St.C. Smart and P.S. Turner, J. Chem. Sot. Faraday I, 74 (1978) 1615. [6] C.F. Jones, R.L. Segall, R.St.C. Smart and P.S. Turner, J. Chem. Sot. Faraday I, 74 (1978) 1624. [7] R.L. SegaIl, R.St.C. Smart and P.S. Turner, Semiconductor Dissolution Kinetics, in: Physics of Materials, Eds. D.W. Borland, L.M. Clareborough and A.J.W. Moore (CSIRO, Melbourne, 1979). [8] M.W. Roberts and R.St.C. Smart, Chem. Phys. Letters 69 (1980) 234. [9] L. Ley, R.A. PoIlak, F.R. McFeely, S.P. Kowalczyk and D.A. Shirley, Phys. Rev. B9 (1974) 600. [lo] S.R. Morrison, The Chemical Physics of Surfaces (Plenum, 1977). [ll] A.F. Carley and M.W. Roberts, Proc. Roy. Sot. (London) A363 (1978) 403. [ 121 J.W. Robinson, Handbook of Spectroscopy, Vol. 1 (CRC Press, Cleveland, Ohio, 1974). [13] J. Deren and J. Stoch, J. Catalysis 18 (1970) 249. [14] E.R.S. Winter, Advan. Catalysis 10 (1958) 165. [ 151 G.K. Boreskov, Advan. Catalysis 15 (1964) 289. (161 J. Deren and S. Mrowec, J. Mater. Scl. 8 (1973) 545. [17] K.S. Kim and R.E. Davis, J. Electron Spectrosc. 1 (1973) 257. [18] K.S. Kim and N. Winograd, Surface Scl. 43 (1974) 625. [19] D.W. Johnson, M.H. Matloob and M.W. Roberts, J. Chem. Sot. Faraday I, 75 (1979) 2143; A.F. Carley, S. Rassias, M.W. Roberts and T.H. Wang, Surface Sci. 84 (1979) 1227. [20] S. Evans, J. Pielaszek and J.M. Thomas, Surface Scl. 56 (1976) 644. [21] T. Robert, M. Bartel and G. Offergold, Surface Sci. 33 (1972) 123. [22] R. Rosencwaig and G.K. Wertheim, J. Electron Spectrosc. 1 (1972) 493. [2]
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