Photoelectron Spectroscopic Study of the Surfaces of some High-performance Liquid Chromatography Substrates BY ALBERTF. CARLEY AND M. WYNROBERTS LEEMORONEY Department of Chemistry University College Cardiff CF1 1 XL Received 4th September 1980 An X-ray photoelectron spectroscopic study of seven different substrates used in high-perform- ance liquid chromatography is described. The substrates investigated are RP2 RP8 Permaphase ODs Permaphase ETH Spherisorb ODs Spherisorb-5-Amino and Spherisorb-5-Nitrile; in addi- tion and for comparative purposes only core-level spectra have been obtained for samples of quartz and standard silica. Wide-scan spectra in the electron binding energy range (0 to ca. 1000 eV) pro- vided a chemical characterisation of the respective “ surface regions ” i.e.within a depth of ca. 20 A. In addition to the anticipated photoelectron peaks owing to silicon oxygen and carbon clear evidence for the presence of sodium and titanium was obtained with some of the substrates. In the case of Spherisorb-5-Amino and Spherisorb-5-Nitrile high resolution N(1s) spectra are reported and two distinct nitrogen species are shown to be present. Estimates of the concentrations of the various species (nitrogen sodium and titanium) present within the “ probing depth ” are reported. Some general comments are made on the significance of these results to the surface modification of silica substrates used in h.p.1.c. and also on the liniitations of the present approach. The purpose of this work was to explore what information could be obtained from photoelectron spectroscopy on the nature of the surfaces of a series of solid substrates used in h.p.1.c.columns. The spectroscopic data for the chemically modified sub- strates are compared with data obtained with silica quartz and alumina. As far as we are aware there have been no previous studies aimed specifically at elucidating at the atomic level the chemical nature of such substrate surfaces. We regard the pre- sent paper to be exploratory in nature. Photoelectron spectroscopy and in particular X-ray photoelectron spectroscopy (X.P.S.) has emerged over the last decade as an important experimental approach for defining the nature of solid surfaces. By monitoring the kinetic energy EK,of photo- electrons produced by for example Al(Ko() radiation (hv = 1486 eV) the binding energy (EB)of the electrons within the atom involved in the photoemission process may be determined [eqn (l)] The binding energy EBis usually equated with the energy of the corresponding level in the neutral atom through Koopmans’ theorem (the “ frozen orbital ” approxinia-tion) which ignores relaxation of the other electrons in the atom (or molecule) during the photoemission process.A photoelectron spectrum therefore consists of a series of discrete peaks superimposed on a background of inelastically scattered photo- electrons. The value of EBis referred to the Fernii level of the spectrometer but spectra are calibrated with respect to a peak whose binding energy is accurately known and of general acceptance.Two such peaks are the C(1s) and Au(4f) peaks at 285 and 83.7 eV respectively. The binding energy value provides us immediately with information P.E.S. STUDY OF H.P.L.C. SUBSTRATES on the chemical nature of the " surface region ". We illustrate what is meant by " surface region '' by making recourse to eqn (2) Y = Y,(1 -which gives the photoelectron yield Yd from a solid of thickness din terms of Y, the photoelectron yield from an " infinitely thick " solid and the electron escape depth A. Using eqn (2) we can show that ca. 80% of the photoelectron signal is generated from a depth below the surface of 2A. A typical value of 3 is ca. 13 A. The contribution of the signal from the outermost surface layer is ca. 30% of the total intensity.It should be emphasised that these " model calculations " are for a flat atomically smooth solid surface and are included here to make clear what is meant by the " sur-face sensitivity of X.P.S.". Although there are limitations to the experimental approach when we attempt to quantify data obtained with " rough " surfaces X.P.S. nevertheless provides unique information on the chemistry of the outermost layer of a solid. The identity of the surface atoms can be ascertained unambiguously and under favourable conditions shifts in the binding energy values can be used to provide clues as to the chemical environment of the particular atom. Estimates can also be made of the atomic con- centration of each element present from a knowledge of the peak intensities photo- ionization cross-sections and electron escape-depth data.In some cases shifts in binding energy do not result in the emergence of a distinct second new peak and a mere broadening of the peak occurs. Therefore the " full width at half maximum height " (f.w.h.m.) value is a parameter which particularly if followed up by a curve-fitting procedure can allow detail to be extracted from a composite and particularly wide photoelectron peak. EXPERIMENTAL All the X.P.S. data reported were obtained using a Vacuum Generators (ESCA-3) instru- ment which has been described previous1y.l The samples were mounted in two ways (a) by pressing into a thin piece of indium foil and (b) by spreading over double-sided Scotch tape. The first method enabled more reliable calibration of the binding-energy scale and also the generation of better peaks from which intensities could be determined.It however suffered the disadvantage that the indium photoelectron peaks obscured the sodium and titanium peaks. The " Scotch tape " mounting can give rise to silicon and carbon peaks but enables emission from sodium and titanium to be observed. The photoelectron spectra obtained using both methods of sample mounting were internally consistent. Seven different substrates based on silica have been studied RP2 which is reported to have a C2 species bonded to the surface; RP8 which is reported to have a C8 hydrocarbon bonded to the surface; Permaphase ODs; Permaphase ETH; Spherisorb ODs; Spheri-sorb-5-Amino and Spherisorb-5-Nitrile.RESULTS AND DISCUSSION WIDE-SCAN SPECTRA The experimental procedure with all the samples was to obtain first a wide-scan spectrum up to ca. 700 eV binding energy (i.e. in the kinetic-energy range 800-1500 eV). This enabled the elements present within the " surface region " to be identified. The next stage was to obtain higher-resolution spectra of those regions of the spectrum that were considered worthy of more detailed study. In fig. 1 is shown a wide-scan spectrum plotted as a function of electron binding energy for a sample of Spherisorb-5-Amino. The spectrometer parameters are as A. F. CARLEY L. MORONEY AND M. W. ROBERTS I I I I I I I 0 100 200 300 400 500 600 binding energy/eV FIG.I .-Wide-scan spectrum from Spherisorb-5-Amino (analysing energy = 100 eV f.s.d.= 3 x lo4 c s-9. stated in the legend to the figure. The main peaks present have been assigned to photoemission from the Si(2p) Si(2s) C(ls) N(Is) Ti(2p) and O(1s) electron energy levels. On the other hand Permaphase ODS showed (fig. 2) the presence of only silicon carbon oxygen and sodium. Pure alumina as used as a column packing in chromatography also showed clear evidence for both titanium and sodium and a spectrum (fig. 3) is included here since the data may be of general relevance for under- standing the role of the surface or in particular partitioning at the surface in chroma- tography. Wide-scan core-level spectra were also obtained for RP-2 RP-8 Permaphase ODs Permaphase ETH Spherisorb ODs Spherisorb-5-Nitrile quartz and a standard V -T--7-~ I -0 100 200 300 400 500 600 700 binding energy /eV FIG.2.-Wide-scan spectrum from Permaphase ODS (analysing energy = 100 eV f.s.d.= 3 x lo4 c s-1). P.E.S. STUDY OF H.P.L.C. SUBSTRATES I I I I I I 0 100 200 300 460 5b0 600 700 800 binding energy/eV FIG.3.-Wide-scan spectrum from alumina (analysing energy = 100 eV f.s.d. = 3 x lo4c s-'). sample of silica. In table 1 are summarised the binding energies of the photoelectron peaks corresponding to the various elements found to be present i.e. silicon carbon oxygen nitrogen sodium and titanium. The unsuspected elements titanium and sodium were found in both RP-2 and RP-8 sodium in Permaphase ODS and Perma- phase ETH and titanium in both Spherisorb-5-Nitrile and Spherisorb-5-Amino.Also reported in table 1 are the respective f.w.h.m. values of the Si(2s) C(1s) and 7-7-7 , :-r-1 1 I I I 1 I 530.9 535.0 binding energy/eV FIG.4.-O(ls) spectrum from the RP-2 substrate (analysing energy = 20 eV f.s.d. = 1 x lo4c s-'). A. F. CARLEY L. MORONEY AND M. W. ROBERTS 43 TABLE OF BINDING ENERGIES, 1.-SUMMARY F.W.H.M. VALUES AND PEAK INTENSITY RATIOS species normali sed intensity rat i0s sample bonded to EB (f.w.h.m.)/eV relative to Si(2p) surface Si(2s) C(1s) O(1s) N(ls) C(1s) O(1s) N(ls) Na(ls)Ti(2p) 153.7 283.8 532.2 RP-2 c2 (3.7) (2.8) (2.8) -0.43 7.81 -0.2 0.1 153.9 284.1 532.5 RP-8 cs (3.7) (2.8) (2.8) -1.20 7.34 -0.2 0.1 154.3 284.6 532.6 Permaphase C18 (3.7) (2.6) (3.1) -6.42 9.58 -1.0 -ODS Permaphase -glycidoxy-154.0 285.1 532.4 ETH propyl (3.4) (3.5) (2.6) -3.58 11.53 -0.8 I 154.3 284.3 532.6 Spherisorb ODS C18 (3.6) (3.2) (2.9) -0.59 7.54 -0.1 Spherisorb-5-153.4 284.4 532.1 398 Amino -NH2 (3.4) (3.5) (2.8) 400.4 0.31 7.50 0.15 -0.1 Spherisorb-5-154.1 284.2 532.2 398.0 Nitrile -CN (3.6) (4.5) (2.9) 0.36 8.52 0.10 -0.1 154.0 284.5 532.3 quartz -(3.1) (2.3) (2.2) -1.60 8.14 --153.7 284.0 531.7 silica -(2.9) (2.5) (2.1) -0.79 7.34 --Assuming Si(2p) = 103.0 eV for Si02(Nordberg et al.Inovg. Chern. 1970 9 2469). O(ls) peaks. The binding energies have been calculated with reference to the Si(2p) value of 103.0 eV in quartz. HIGH-RESOLUTION SPECTRA The O(1s) spectra were generally uninformative with binding energies close to 532 eV for all the samples.The f.w.h.m. values vary from 2.1 eV (for silica and quartz) to between 2.8 and 3.1 eV for all the other substrates. A typical O(ls) spec- trum for the RP-2 substrate is shown in fig. 4. The C(1.s) spectra for RP-8 Perma- phase ETH Spherisorb-5-CN and Quartz are shown in fig. 5. We draw particular attention to the C(ls) spectrum for Permaphase ETH [fig. 5 spectrum (b)]which is characterised by a very large f.w.h.m. value (ca. 4.5 eV); the other Spherisorb sub- strates and Permaphase-ETH and f.w.h.m. values > 3 eV. Two Spherisorb substrates showed N(1s) peaks arising from nitrogen " atoms " bonded to the surface in one case as an amino group and in the other as a cyano group.Only these two samples in fact showed any evidence of intensity in the N(1s) spectral region and the high-resolution spectra are shown in fig. 6. Clearly there are significant differences in the N( 1s) profiles observed with Spherisorb-5-Amino and Spherisorb-5-Nitrile. P.E.S. STUDY OF H.P.L.C. SUBSTRATES 280.0 285.0 290.0 bndingi energy/eV FIG.5.-C(ls) spectra from (a)RP-8 substrate (b)Permaphase ETH (c) Spherisorb-5-CN (d)quartz (analysing energy = 20 eV f.s.d. = 3 x lo3c s-’). ASSIGNMENT OF N(l.5’) AND C(l.5’)SPECTRA In the N(1s) spectrum for Spherisorb-5-Nitrile [fig. 6 spectrum (a)] there are obviously two components one with a peak at a binding energy of 397.5 eV and the other the major component of the profile at ca.398.5 eV. We suggest that the cyano 395.0 400.0 405.0 binding energy/eV FIG.6.-N(1s) spectra from (a) Spherisorb-5-Nitrile (6) Spherisorb-5-Amino (analysing energy = 50 eV f.s.d. = 3 x lo3c s-’). A. F. CARLEY L. MORONEY AND M. W. ROBERTS species (CN) has the higher N(1s) value while the surface species responsible for the peak at 397.5 eV is a nitrogen adatom. There is considerable and unequivocal evi- dence2 for assigning an N(1s) value of ca. 397 eV to nitrogen adatoms chemisorbed on solid surfaces. Furthermore with the progressive addition of ligands to the adsorbed " nitrogen atom " (NH NH2 NH3) the N( 1s) value increased in approximately 1 eV steps.3 We therefore attribute the 398.5 eV peak to surface-CN species. In the case of the Spherisorb-5-Amino substrate the N( 1s) profile [fig.6 spectrum (b)]also shows two distinct components with peak maxima at binding energies of ca. 398.5 and 400 eV. We have therefore conclusive evidence for two kinds of " nitro-gen " present at the surface and the most likely are NH and NH species. Extensive studies of the adsorption of ammonia and hydrazine on solid surface^^.^ have estab- lished that NH2 and NH species are characterised by N(1s) values of ca. 399.5 and 398 eV respectively. The f.w.h.m. value of ca. 4 eV is also conclusive evidence for at least two nitrogen species being present. SURFACE ELEMENTAL COMPOSITION In addition to the observed binding energies and f.w.h.m. values reported in table 1 we include the normalised intensity ratios [relative to Si (2p)l of the C(ls) O(ls) N(ls) Na(1s) and Ti(2p) peaks.It is interesting to note that for substrates RP-2 Spherisorb ODs Spherisorb-5-Amino and Spherisorb-5-Nitrile quartz and silica the O(ls)/Si(2p) ratios fall within the range 7.5-8.5 (table 1) indicating a consistency for the 0:Si atomic ratios. Clearly for such substrates as Permaphase ETH and Permaphase ODS where substantial modification of the surface by long-chain hydrocarbons had occurred we would expect a significant variation in the observed 0:Si ratio. To convert peak intensities e.g. the oxygen and silicon peaks into atomic ratios nO/nsiwe have used eqn (3) where A is the area of the relevant photoelectron peak p the photoionization cross- section of the particular electron shell E the respective kinetic energies of the photo- electrons and p an asymmetry factor.To confirm the validity of our procedure we TABLE 2.-ATOMIC RATIOS o/si FOR SILICA AND QUARTZ CALCULATED FROM EQN (2) USING USING TWO SETS OF PHOTOIONISATION CROSS-SECTION DATA AND Si(2p) AND Si(2s) PEAKS A B O(lS)lSi(2P) O(1s)/Si(2s) O(1s)/Si(2p) O(ls)/Si(2s) silica 1.6 2.0 1.1 1.8 quartz 1.8 2.2 1.2 2.0 A using photoionisation cross-section data of Thomas and co~orkers.~ B using photoionisation cross-section data of Sc~field.~ analysed first (as a model system) the photoelectron spectra for silica and quartz. We have calculated the atomic ratios 0:S using the O( Is) Si(2p) and S(2s) peak areas and two sets of photoionization cross-section values those determined experimentally by Thomas and co~orkers,~ and also the calculated values of Scofield5 (table 2).The model calculations indicate clearly that it is preferable to make use of O(ls)/Si(2s) peak P.E.S. STUDY OF H.P.L.C. SUBSTRATES ratios rather than O(ls)/Si(2p) ratios since only then do we observe the expected value of ca. 2.0 for the 0:Si atomic ratio. There is little to choose between the two sets of cross-section data (A and B) but the values of 1.6 1.8 1.1 and 1.2 based on the Si(2p) peak areas clearly give an unacceptably low 0:Si ratio. In addition to silicon and oxygen carbon nitrogen sodium and titanium were shown to be present in the surface region Na in RP-2 RP-8 and the two Permaphase substrates; Ti in RP-2 and RP-8 and the three Spherisorb substrates and nitrogen ia the Spherisorb-5-Amino and Spherisorb-5-Nitrile substrates.Carbon was present in all cases. If we assume that the Na Ti and N are present at the surface then we can estimate their respective surface concentration 0using eqn (4) where the subscripts m and s refer to the adsorbed layer and substrate respectively Y is the observed signal strength from the atomic level monitored ,u is the appropriate sub-shell photoionisation cross-section vsis the number density of the relevant sub- strate atoms and 07 is the angle between the electron take-off direction and the surface normal. TABLE 3.-sURFACE CONCENTRATIONS OF Na Ti AND ‘N’ SPECIES (loi5Cm-2) CALCULATED BY MEANS OF EQN (4) sample Na Ti N RP-2 0.1 0.1 -RP-8 0.1 0.1 -Permaphase ODS 0.6 -Permaphase ETH 0.5 Spherisorb ODS -0.1 -Spherisorb-5-Amino -0.1 0.5 Spherisorb-5-Nitrile -0.1 0.4 In table 3 are summarised our results.The two Permaphase substrates contain appreciable surface concentrations of sodium (0.6 x 10’’ cm-2 for Permaphase EDS and 0.5 x lo1’ cm-2 for Permaphase ETH) RP-2 and RP-8 contain appreciably less sodium 0.1 x 10’’ cm-2 but some titanium is present (0.1 x 1015cm-2) as is also the case with the three Spherisorb samples (0.1 x 10’’ cm-2). Both titanium and sodium were also found to be present in the A1203sample (fig. 3). Nitrogen was found only with the Spherisorb-5-Amino and Spherisorb-5-Nitrile substrates at concentra- tions of ca. 0.5 x cm-2 and 0.4 x 10‘’ cme2 respectively.GENERAL COMMENTS The object of this study of various h.p.1.c. chemically bonded substrates by X.P.S. was to ascertain whether or not this approach could provide evidence for the atomic nature of the respective surfaces and therefore show how the surfaces had been modified by replacing surface hydroxyls with various polar or hydrophobic groups. The surface analytical data showed some interesting and unexpected features with significant concentrations of sodium and titanium present with some substrates. Both these elements were also present in very significant quantities in the surface of “pure ” alumina. The sodium was assigned by characteristic Na(1s) and Auger peaks. By making use of photoionization cross-section data estimates were made of their A.F. CARLEY L. MORONEY AND M. W. ROBERTS respective concentrations and in general when present the concentrations were in the range (0.1-0.5) x lo1’ atom cm-2. Definitive evidence was obtained from N(1s) spectra of Spherisorb-5-Amino and Spherisorb-5-Nitrile for surface nitrogen species ; the estimated “ nitrogen concentrations ” were 0.5 x and 0.4 x lo” ern-' res-pectively. The high-resolution spectra indicate the presence of two distinct types of nitrogen species; one of these is obviously related to the replacement of the hydroxyl group the other may be associated with the surface titanium present. It is also interesting to note that the surface concentration of hydroxyl groups pre- sent on the silica gels is estimated to be between 0.2 x lo1’ and 0.4 x 10‘’ cm-2 which suggests a high degree of OH replacement in Spherisorb-5-Nitrile and Spheri- sorb-5-Amino substrates.It should be emphasised that in estimating surface concentrations we have assumed a flat surface when there is substantial evidence for the usefulness and validity of eqn (4). However for surfaces of unknown topography there must of necessity be doubt regarding the accuracy of the calculated values6 Only in the case of N(1s) spectra was there obvious evidence for two different “ surface bonded ” species. The O(1s) and C(1s) spectra were broad and uninforma- tive with little or no evidence for “ chemical shifts ” from one sample to another. Information from these spectra was therefore limited to the trivial observation that C(ls) intensities generally increased with the Rumber of carbon atoms present in the bonded species and in only one case (Permaphase ETH) was there evidence for two different carbon species [fig.5 curve (b)]. The photoemission intensity from the Si(2s) is relatively low with Al(Kx) radiation and there would be advantages in using Zr(Lol) radiation where a distinction is possible’ between differently bonded silicon atoms. Studies of standard silica and quartz samples showed that it is preferable to make use of Si(2p) peaks for the quantitative estimate of silicon. That inner core- levels are preferable in quantitative studies seems to be of general significance.’ We are grateful to Prof. J. H. Purnell Dr. C. F. Simpson and Dr. R. A. C. Gray who supplied us with the substrates used in this study.C. R. Brundle M. W. Roberts D. Latham and K. Yates J. Electron Spectrosc. 1974 3 241. K. Kishi and M. W. Roberts Surf. Sci.,1977 62,252; A. F. Carley and M. W. Roberts Proc. R. Soc. London Ser. A 1978 363 403. M. H. Matloob and M. W. Roberts J. Chetn. Res. (S),1977 336. S. Evans R. G. Pritchard and J. M. Thomas J. Electron Spectrosc. 1978 14 341. J. H. Scofield J. Electron Spectrosc. 1976 8 129. Y. M. Cross and J. Dewing Surface and Interface Analysis 1979 1 26. ’J. E. Castle and R. H. West Surface Reactivity and Catalysis Group Meeting Cardiff 1980. ’A. F. Carley S. Rassias and M. W. Roberts J. Chern. Res. CS) 1979 208.