In static SIMS, the sputtering of material from a surface results from a “collision cascade” initiated by the impact of the primary ion. For organic adsorbates, it is generally assumed that in the central impact region, mostly atomic and non-characteristic small organic fragments are generated, but immediately outside this impact region, more extensive fragments that may have undergone some structural rearrangement are produced. Further away from the impact region, the energy available for rearrangement is lower, so that larger and minimally rearranged fragment ions (and even parent ions) might be expected. On this basis, it might be anticipated that the larger diagnostic secondary ions should better reflect species present at the surface prior to the primary ion impact. However, as discussed in Section 3.3
, the larger the secondary ion, the greater the need to extrapolate the mass range calibration; i.e.
, the more relevant the diagnostic ion, the more uncertain its measured m/z
2.4.1. Conditioned Cu Metal ToF-SIMS
For Cu metal surfaces that had been freshly abraded in air, then conditioned for 2 min in hydroxamate collector solution and subsequently rinsed with water, peaks of moderate intensity were observed in the positive secondary ion spectrum at m/z
38.964 and 39.961. The former could be assigned to K+
and the latter to Ca+
rather than KH+
). In making that assignment, it should be noted that 39
K with atomic mass 38.9637 accounts for more than 93.2% of the K stable isotopes, and 40
Ca with atomic mass 39.9626 accounts for 96.9% of the Ca stable isotopes. Mostly because of its low (~4.3 eV) ionisation potential, K has one of the highest secondary ion yields, so that a very low surface concentration would be expected to give rise to a discernible peak at m/z
38.964. Ca has a somewhat higher (~6.1 eV) ionisation potential and hence lower secondary ion yield. Indeed, measured secondary ion yields from Au+
primary ions reported by King et al.
] for selected elements in glass standards included 1400 for K, 154 for Ca and 45 for Fe. XPS analysis of Cu metal surfaces conditioned similarly to those characterised by ToF-SIMS showed that neither the Ca nor residual K surface concentration should have been significant (>0.1 atom %). Any Ca present below the XPS detection limit might have been an impurity introduced by the surface abrasion immediately before the specimen was conditioned in the collector solution or an impurity in the potassium hydrogen n
Also in each positive secondary ion spectrum was a low intensity peak at m/z
159.151 (Table 1
). This m/z
value is well outside the calibration range, so it might have arisen from parent hydroxamic acid ions of mass 159.126 amu. There was an even weaker peak at m/z
160.151 that might be assigned to (acid + H)+
ions of mass 160.137 amu. In each corresponding negative secondary ion spectrum, there was no peak near m/z
159.1 and only very weak peaks at m/z
158.125 and 157.118 that might have arisen from (acid − H)−
and (acid − 2H)−
ions with mass 158.118 and 157.110 amu, respectively. Assuming those assignments were correct, it cannot be concluded that a low concentration of hydroxamic acid was necessarily present at the surface, as it is conceivable that chemisorbed hydroximate (157.110 amu) might have captured one or two protons prior to abstraction from the impact zone. Thus, while the secondary ion mass spectra do not provide unequivocal support for the presence of hydroxamic acid at the conditioned surface and hence are unable to definitely corroborate the XPS evidence for the co-adsorption of the acid, they are certainly consistent with such co-adsorption.
Secondary ion abundances associated with the major Cu-containing peaks are also listed in Table 1
as a range across the eight 200 μm × 200 μm regions characterised. 63
Cu-containing positive secondary ion peaks near m/z
119.95 and 122.95 were of very low intensity, as they were for bulk Cu hydroximate. The major secondary ion peaks for conditioned (air-exposed) Cu metal were the same as those for Cu hydroximate, despite the fact that for the Cu metal, very little multilayer Cu hydroximate, as distinct from the monolayer (chemisorbed) collector, would have been present at the solid/vacuum interface. Thus, the diagnostic ions appear to be the same for both the monolayer and Cu hydroximate. However, for peak intensities relative to those for Cu+
, while the abundances of the positive diagnostic secondary ions were consistently lower for the adsorbed layer than for Cu hydroximate, the abundances of the negative ions were consistently higher for the adsorbed layer (Table 3
). Such a clear difference can be rationalised, if not predicted, by recognising that the negative diagnostic Cu-containing ions all contain one or two O atoms whereas the positive diagnostic ions contain no O atoms. The spectra indicated that for predominantly a chemisorbed monolayer, Cu-containing ions also including O were more abundant relative to Cu−
, while those not including O were less abundant relative to Cu+
than they were for bulk Cu hydroximate.
Intensities of Cu-containing diagnostic secondary ion peaks from conditioned surfaces of Cu metal, malachite and pseudomalachite compared with those from bulk Cu hydroximate (L: lower, H: higher); for each surface, peak intensities have been normalised relative to the intensities of both Cu ion and organic fragment ion peaks as in Table 1
and Table 4
Intensities of Cu-containing diagnostic secondary ion peaks from conditioned surfaces of Cu metal, malachite and pseudomalachite compared with those from bulk Cu hydroximate (L: lower, H: higher); for each surface, peak intensities have been normalised relative to the intensities of both Cu ion and organic fragment ion peaks as in Table 1 and Table 4.
|Peak m/z||Oxide surface/normalisation|
|Cu metal/||Cu metal/||Malachite/||Malachite/||Pseudomalachite/||Pseudomalachite/|
| Cu|| organic|| Cu|| organic|| Cu|| organic|
|+90.9||23% L||44% L||6% L||39% H||11% L||17% H|
|+104.9||39% L||46% L||13% L||43% H||16% L||17% H|
|+118.9||40% L||59% L||18% L||29% H||24% L||11% L|
|−105.9||13% H||73% L||20% L||57% L||5% L||53% L|
|−121.9||45% H||72% L||1% H||55% L||50% H||39% L|
|−130.9||7% H||80% L||21% L||63% L||3% L||58% L|
|−146.9||23% H||77% L||16% L||65% L||8% H||57% L|
For normalisation by the integrated intensities of the organic secondary ion peaks, both the positive and negative secondary ions were less abundant for the conditioned Cu metal than for Cu hydroximate (Table 3
). Different abundances for those two specimens would be consistent with the absence of a uniform multilayer of Cu hydroximate on the conditioned Cu metal surface. However, it is not immediately obvious whether the particular differences observed indicated a uniform chemisorbed monolayer, patches of chemisorbed monolayer, or patches of chemisorbed monolayer plus physically co-adsorbed hydroxamic acid on the conditioned Cu metal surface. As noted above, the positive secondary ion spectra were consistent with the presence of co-adsorbed hydroxamic acid, as the peak assigned to the acid parent positive ion (159.126 amu) was of barely detectable intensity for bulk Cu hydroximate but an order of magnitude greater for the conditioned Cu metal surface.
Peaks from CuO− and CuO2− were observed, but typically with intensities only ~27% and 8%, respectively, of the Cu− peaks. These intensities were lower than the normalised values observed for Cu hydroximate, and hence do not provide support for the possibility that oxidised Cu not covered by at least a monolayer of collector was a major surface species. Low intensity peaks near m/z 78.94 and 80.94 from Cu-containing positive secondary ions were observed, but these could be assigned to CuCH2+ and there was no evidence for CuO2+ ions of appreciable abundance.
As canvassed in Section 2.4
, it is the diagnostic secondary ion of highest mass that might be expected to best represent the species present at the surface prior to the primary ion impact. For conditioned oxide Cu surfaces, it is the negative ion of m/z
near 146.9 that meets this criterion, and the abundance of this ion also happens to be one of the largest of the diagnostic secondary ions. Relative to the Cu−
peak intensity, the m/z
146.9 peak was significantly more
abundant than for Cu hydroximate, whereas relative to organic fragment peak intensities it was much less
abundant than for Cu hydroximate. One possible interpretation of these observations is that the Cu metal native oxide was covered predominantly by chemisorbed hydroximate and possibly also co-adsorbed hydroxamic acid rather than multilayer Cu hydroximate. In other words, there would have been few Cu atoms at the solid/vacuum interface, so that most of the Cu atoms ejected would have been those to which the overlying hydroximate had chemisorbed (through its O atoms) whereas in bulk or multilayer Cu hydroximate, some Cu atoms would have been present at the solid/vacuum interface. By contrast, because of the surface excess of chemisorbed hydroximate and co-adsorbed acid, relative to organic fragment secondary ions, Cu-containing fragments would have been less abundant. The positive Cu-containing secondary ions might also be expected to have been relatively less abundant regardless of the normalisation because the positive ions do not contain O, so that only Cu atoms interacting with the N of any “horizontally” oriented hydroximate would have contributed to secondary ions that had undergone minimal fragmentation (whereas in bulk Cu hydroximate essentially all
Cu atoms interact with N atoms).
In summary, for conditioned Cu metal, while there is no static SIMS evidence to support a uniform coverage of multilayer Cu hydroximate or uncovered native oxide, the secondary ion spectra are consistent with monolayer coverage of chemisorbed hydroximate and co-adsorbed hydroxamic acid.
2.4.2. Conditioned Malachite and Pseudomalachite ToF-SIMS
The secondary ion mass spectra for the conditioned minerals were not markedly more complicated than those for the bulk Cu complex, probably because of the presence of multilayer Cu hydroximate at the mineral/vacuum interface. However, for the pseudomalachite, 31
(30.974 amu) and 31
(62.964 amu) peaks were observed, indicating that some P would have been near the solid/vacuum interface and consequently that the multilayer Cu hydroximate would have been in patches. The multilayer might also have been in patches on malachite. For each mineral, the abundance of 39
ions and 40
ions was only slightly greater than for the conditioned Cu metal surface (Table 4
). Peaks from hydroxamic acid ions were of slightly lower intensity than those for the conditioned Cu metal, consistent with slightly higher co-adsorbed acid on the conditioned Cu metal. It can be seen from Table 1
and Table 4
that the abundances of the Cu-containing diagnostic secondary ions were comparable for both minerals, but somewhat different from those for Cu hydroximate and the conditioned Cu metal.
For both minerals, the positive Cu-containing ions were moderately (≈15%) less abundant than for bulk Cu hydroximate relative to Cu+ ions, but relative to C5H11+ ions, the Cu-containing diagnostic ions were ≈35% more abundant for malachite and ≈15% more abundant for pseudomalachite. These observations would be broadly consistent with patches of adsorbed hydroximate on both minerals (allowing Cu species other than Cu hydroximate at the solid/vacuum interface), but fewer or smaller multilayer patches than monolayer patches on pseudomalachite, where a higher concentration of chemisorbed hydroximate relative to multilayer Cu hydroximate would result in a higher concentration of the organic ligand oriented towards the solid/vacuum interface.
Diagnostic ions and their observed m/z and relative abundance ranges for conditioned malachite and pseudomalachite surfaces.
Diagnostic ions and their observed m/z and relative abundance ranges for conditioned malachite and pseudomalachite surfaces.
|Ion||Mass (amu)||Malachite Observed m/z range||Malachite Abundance (rel. Cu)||Malachite Abundance (rel. organic)||Pseudo-malachite Observed m/z range||Pseudo-malachite Abundance (rel. Cu)||Pseudo-malachite Abundance (rel. organic)|
|31P||30.974||-||-||-||30.969–30.974||0.25 ± 0.1||0.1 ± 0.05|
|39K+||38.9637||38.963–38.965||15 ± 3||12 ± 6||38.961–38.966||22 ± 6||11.5 ± 3|
|40Ca+||39.9626||39.958–39.961||39 ± 4||22 ± 8||39.959–39.965||40 ± 15||18 ± 6|
|63Cu+||62.9296||62.919–62.926||1000||550 ± 160||62.923–62.933||1000||485 ± 90|
|C5H11+||71.086||71.084–71.090||2 ± 0.6||1||71.087–71.091||2 ± 0.5||1|
|63CuNCH2+||90.949||90.951–90.957||44 ± 2||25 ± 8||90.953–90.961||42 ± 3.5||21 ± 4|
|63CuNC2H4+||104.964||104.963–104.970||8.7 ± 0.6||5 ± 1.5||104.967–104.974||8.4 ± 0.6||4.1 ± 0.8|
|63CuNC3H6+||118.981||118.973–118.986||7 ± 1.5||4.5 ± 1.5||118.982–118.988||6.5 ± 1||3.1 ± 0.7|
|(hydroxamic acid)+||159.126||159.148–159.156||2.5 ± 1.5||2.4 ± 1.7||159.136–159.156||2 ± 1.6||1 ± 0.8|
|31P−||30.974||-||-||-||30.972–30.973||3 ± 2||1 ± 0.7|
|63Cu−||62.9296||62.928–62.933||10||3.6 ± 0.7||62.928–62.931||10||4 ± 1.4|
|PO2−||62.964||-||-||-||62.965–62.967||19 ± 11||5.6 ± 2.3|
|C3H3O2−||71.0133||71.014–71.021||2.8 ± 0.5||1||71.014–71.023||2.8 ± 1||1|
|63CuONCH−||105.935||105.940–105.949||4.8 ± 0.6||1.9 ± 0.3||105.943–105.948||5.7 ± 1.5||2.05 ± 0.25|
|63CuO2NCH−||121.930||121.936–121.945||10.1 ± 1.4||3.9 ± 0.9||121.941–121.947||15 ± 3.5||5.3 ± 1.2|
|63CuONC3H2−||130.943||130.936–130.946||5.5 ± 1||2.2 ± 0.8||130.942–130.949||6.8 ± 1||2.5 ± 0.6|
|63CuO2NC3H2−||146.938||146.931–146.946||21.8 ± 2.5||8.5 ± 2||146.940–146.948||28 ± 5||10.3 ± 2.2|
|(hydroxamic acid − H)−||158.118||158.119–158.126||0.55 ± 0.15||0.22 ± 0.08||158.128–158.139||0.9 ± 0.2||0.4 ± 0.1|
Relative to Cu−
ions, for malachite the negative Cu-containing ions were moderately less abundant than for Cu hydroximate, whereas for pseudomalachite they were comparable or even more abundant. Relative to C3
ions, for both minerals the negative Cu-containing ions were appreciably less abundant than for bulk Cu hydroximate. For the conditioned Cu metal surface (Section 2.4.1
), it was found that relative to Cu−
ions, the negative Cu-containing diagnostic ions (which also contain one or two O atoms) were more abundant than for Cu hydroximate. These observations can be rationalised by extensive monolayer coverage with multilayer patches on malachite, but not much more than monolayer patches on pseudomalachite.
2.4.3. Conditioned Magnetite ToF-SIMS
Abundant Fe-containing and selected organic fragment secondary ions are listed in Table 2
for comparison with the data for Fe hydroxamate. There was no obvious correlation between the intensity of the K+
peak and the intensity of the hydroxamic acid or hydroxamate ion peaks; in fact there was almost an inverse relationship, confirming that the K that was observed was unlikely to have been from residual collector solution that had not been rinsed from the surface at the end of the conditioning period. Also, there was no obvious correlation between the intensities of the peaks at m/z
38.96 and 39.96, so that the peak assigned to Ca+
ions (39.963 amu) was unlikely to have arisen instead solely from KH+
. However, some of the m/z
39.96 peak intensity might have been due to KH+
ions (39.972 amu), as otherwise Ca+
ions would have been more abundant than expected. For the conditioned magnetite, (hydroxamic acid)+
ions were relatively abundant (compared with not observed for Fe hydroxamate), but the (acid)−
and (acid − H)−
abundances were comparable with those for Fe hydroxamate. As for Fe hydroxamate, FeH−
ions were more abundant that Fe−
, unlike the relative abundance of CuH−
for the analogous conditioned oxide Cu surfaces. As for conditioned oxide Cu surfaces, there was no clear indication that the metal-containing diagnostic ions also contained N rather than CH2
or vice versa.
Differences in the Fe-containing diagnostic ion abundances for the conditioned magnetite surface relative to those for Fe hydroxamate are listed in Table 5
. It can be seen that the difference in abundance for each positive diagnostic ion is in fairly close agreement regardless of whether the peak intensities had been normalised by the Fe+
peak intensities. With one exception, the difference for each negative diagnostic ion is also in close agreement for normalisation by FeH−
Intensities of Fe-containing diagnostic secondary ion peaks from conditioned magnetite surfaces compared with those from bulk Fe hydroxamate (L: lower, H: higher); peak intensities have been normalised relative to the intensities of both Fe ion and organic fragment ion peaks as in Table 2
Intensities of Fe-containing diagnostic secondary ion peaks from conditioned magnetite surfaces compared with those from bulk Fe hydroxamate (L: lower, H: higher); peak intensities have been normalised relative to the intensities of both Fe ion and organic fragment ion peaks as in Table 2.
|+70.9||60% L||41% L|
|+84.9||15% L||10% L|
|+96.9||2% H||1% L|
|+112.9||41% L||37% L|
|+115.9||83% L||81% L|
|−98.9||77% L||90% L|
|−113.9||87% L||94% L|
|−114.9||90% L||54% L|
|−139.9||94% L||98% L|
The mid-range m/z Fe-containing positive diagnostic ions were of comparable abundance to those of bulk Fe hydroxamate, but the lower and higher m/z positive, and all the negative, diagnostic ions were up to an order of magnitude lower than for bulk Fe hydroxamate. Therefore, the secondary ion spectra indicated that the adsorbate on magnetite could not have been similar to a uniform Fe hydroxamate multilayer, but they were consistent with no greater than monolayer coverage or possibly sparse patches of multilayer Fe hydroxamate.
It might have been expected that FeOx(NCH) ions (where x > 4) would be more abundant for bulk Fe hydroxamate than for the adsorbed hydroxamate monolayer. For bulk Fe hydroxamate, relatively intense peaks attributable to ions such as 56FeO5NC3H−, 56FeO6NC− and 56FeO6NC3H− were observed with abundances ~45, 30 and 155, respectively, normalised by the FeH− intensity, whereas for conditioned magnetite the corresponding values were less than 1, 1.5 and 2.5.