X-Ray Photoelectron Spectroscopic Investigation of Group VI-A Elements William E. Swartz, Jr., Kenneth J. Wynne, and David M. Hercules Department of Chemistry, Unirersity of Georgia, Athens, Ga. 30601 X-RAY PHOTOELECTRON SPECTROSCOPY, ESCA, allows the direct measurement of electron binding energies in atoms and molecules. By interaction of the atom or molecule with a monoenergetic X-ray, electrons are ejected and their kinetic energies, Ek, are measured. T h e core-electron binding energies, E,, can be determined by applying the relationship Eb =
hv
- E&
(1)
where liv is the energy of the exciting X-ray. Core-electron binding energies are affected by the chemical environmentthat is, a chemical shift occurs ( I ) . Correlations between the increase in binding energy and the increase in positive charge induced by chemical environment have been shown
(0. Also, the shift in core-electron binding energy, AEb, between oxidation states decreases as one goes from chlorine to iodine in G r o u p VI11 ( 2 ) . It has been observed in our laboratory that the AEb’s between oxidation states for nitrogen are larger than for similar oxidation state changes in phosphorus. In addition, the range of chemical shifts observed for quaternary nitrogen (6-7 eV) (3) is much larger than that observed for quaternary phosphorus (2- 3 eV) ( 4 ) . Therefore, the question arises as to whether the chemical shifts, AEb’s, between oxidation states continue to decrease as one descends a given group in the Periodic Table. Since a large volume of binding energy data has been reported for the sulfur 2p electrons as a function of oxidation state ( I ) , we undertook a n investigation of the G r o u p VI elements selenium and tellurium, in similar oxidation states, to make comparisons of the chemical shifts with those of sulfur. EXPERIMENTAL Apparatus. The electron spectra were obtained using a 30-cm, double focusing, iron-free electron spectrometer of the split-coil solenoidal type. A detailed description of the apparatus has been previously reported (3). Reagents. The selenium and tellurium compounds investigated were either commercially available from Rocky Mountain Research, Inc., or Alfa Inorganics or were prepared by standard procedures in these laboratories. The ESCA spectra were obtained o n the compounds without further purification. Procedure. All of the ESCA spectra were obtained at a n instrumental resolution of 0.04 %. Instrumental resolution considers only instrumental parameters and ignores the natural width of the exciting radiation and the electron energy levels themselves. The aluminum Kal,? line (1486.6 eV) was used as the X-ray excitation source in all cases. All samples were run as powders dusted onto a backing of double backed cellophane tape at room temperature. Special sample handling techniques were necessary in obtaining the __ _ _ _ _ _ ~( I ) K . Siegbahn ~t d..“ESCA ‘Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy.’ ” Almquist and Wiksells. Uppsala, 1967. (2) D. M. Hercules, ANAL.C m w . 42 ( l ) , 20A ( 1970). (3) J. J. Jack and D. M. Hercules, ibid., 43, 729 (1971). (4) W. E. Swartz, Jr.. and D. M . Hercules, ibid.. p 1066. 1884
Table I. Electron Binding Energies as a Function of Oxidation State for the Group VI Elements Sulfur (I) Oxidation Compound state s (2P) Eb, eV Na2S -2 160.8 0 162.2 SS Na2S03 +4 165.8 +6 167.7 Na2S04 Selenium ComOxidation Se(3pl/) Eb, eV state pound Se(3p3,:) Eb, eV ZnSe -2 1658i02 1597104 165 1 1 0 3 159 3 1 0 3 -2 KSeCN 0 165 9 1 0 2 159 9 + 0 2 Se 169 6 + 0 1 164 1 1 0 1 NarSeOl +4 169 8 + 0 1 163 8 i 0 1 SeOn +4 170 1 z t 0 1 164 6 f 0 1 Na2Se04 +6 Tellurium Oxidation ComTe (3d51,) Eh. eV state pound Te(3d~/?) Etl, eV 573.1 1 0 . 4 583.6 i 0 . 4 -2 ZnTe 582.6 i 0 . 3 572.4 i 0 . 2 -2 NalTe 583.2 i 0 . 2 512.9 i 0 . 2 0 Te 575.7 + 0 . 2 586.1 i 0 . 2 KzTeOo +4 576.4 i 0 . 2 586.8 i 0 . 2 DrTe04 +6 587.3 It 0 . 1 +6 576.8 f 0 . 2 Te03 (NHdJTeOl t 6 587 4 f 0 2 576 7 i. 0 . 2
data o n NazTe. Since Na2Te reacts with atmospheric moisture t o produce toxic HaTe, it was necessary t o load the NazTe sample in a glove bag under dry nitrogen. To compensate for any sample charging which may occur during the ESCA measurement, calibration of the spectra was accomplished using the C(1s) electron line resulting from hydrocarbon contamination. These C(1s) electrons have been reported to have a binding energy of 285 eV ( I ) . Also, the Se 3~3;. and Te 3d5;, lines of the elements were used as external standards for selenium and tellurium, respectively. Siegbahn er a/. (I) have reported binding energies of 168.0 eV and 162.0 eV for Se (3~1,:)and ( 3 p ~ . ?electrons, ) respectively, and binding energies of 582.0 eV and 572.0 eV for Te (3d3;,) and ( 3 d ,~) electrons, respectively. These results differed by -2.0 eV for Se and 1.0 eV for Te from the results using C(l s) standards. Since only relative binding energies are important t o the present study, it is unimportant which set of data is used. Because we feel using the C(1s) calibration compensates best for charging effects, binding energies derived from this vafue are reported here. At least three replicate samples were used in determining the core-electron binding energies. The reported energies are averages of the replications while the error limits are standard deviations. In all cases, the calibration line was recorded immediately after the line of interest. RESULTS AND DISCUSSION Representative spectra for the selenium electrons in NapSeOR are shown in Figure 1 and for the tellurium electrons in
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1700
3700
I500
3300
tn 0
z
v)
n
z
0
0 W
v)
Y
0
130C
v)
2 2900
\
rn I-
\ v)
z 3
I-
0
z
0
3
0 0
I IOC
2500
-
9oc
i 2100
i
i
i
2.1600
1
1
1
1
1
1
1
2.6200
1
1
1
1
SPECTROMETER
CURRENT
Figure 2.
= =
169.6 eV 164.1 eV
K?TeO:, in Figure 2 . T h e complete data are tabulated in Table I. Sulfur data, as reported by Siegbahn et d . (I), have been included in Table I for comparison. Several workers have reported the importance of “counterion effects” in core-electron binding energy measurements (5, 6). For example, the N(1s) binding energy is different in K N O , from that in NaNO.,. This difference can arise from the differing counter-ions, Na and K . Thus, when one is comparing core-electron binding energy shifts between oxidation states for a n element, it is important that the counter-ions remain equivalent. The counter-ion effect is best seen in the data o n the G r o u p VI-A elements by comparing the Te(3d3/,, 3d5/J binding energies for K?TeOl and (NHr)rTeO.l. Tellurium exists in the f 6 oxidation state in both of these compounds, yet the binding energies differ by about 0.6 eV. The order of t h e binding energies ((NHJrTeOi > K,TeO,) are as one would predict since the N H l + ion is more electronegative, X = 3.3 ( I ) , then potassium, X = 0.8. A similar counter-ion effect has been observed by Siegbahn et ul. (I) for the S(2p) binding energies in NasSOl and Fe2(S04),I. The S(2p) binding energy in Fe,(SO,),, ( X I . , = 1.8) is 0.6 eV larger t h a n it is in N G O , (X,,, = 0.9). The intent of this investigation, was to study the coreelectron binding energy shifts as a function of oxidation state for the G r o u p VI elements. Table I1 summarizes the binding energy shifts for sulfur, selenium, and tellurium, as a function o f oxidation state. Only the sodium salts of the selenium oxyanions and the potassium salts of the tellurium oxyanions ( 5 ) J . J . Jack, 1’h.D. Thesis. Massachusetts Institute of Technology. Cambridge. Mass., September 1970. ( 6 ) R . G . Albridge, K . Hamrin. G. Johanssan, and A. Fahlman, Z . P l ~ . y s .209, . 419 ( 1968).
i
i
i
i
l
l
l
2.1800
CURRENT
(AMPS)
Te(3d31s-3d612)electron spectrum for K2Te03 Te(3d3i2)= 586.1 eV Te(3djjp)= 575.7 eV
(AMPS)
Figure 1. Se(3p, ,.-3p3, J electron spectrum for Na2SeOs Se(3pl 2) Se(3p32)
t 2.1700
SPECTROMETER
1
2.6300
i
have been considered for the +4 and +6 oxidation states t o minimize counter-ion effects. Using the “free-ion” model, the binding energy shift, AE, can be related to atomic charge, q, by the relation
AE
=
Kq
+U+C
(2)
where K is a proportionality constant for the inner shell studied due to different degrees of screening of the shell by the valence electrons, and C is a constant determined by the choice of reference level. U is the molecular potential contribution which is similar to the Madelung lattice correction in ionic solids. For a given atom i, U can be estimated by (3) where R,, is the internuclear distance between atoms i and j . Cox (7) has performed H F S semi-empirical molecular orbital calculation for the G r o u p VI oxyanions. T h e charges o n the G r o u p VI atoms in the anions which result from the HFS calculations are listed in Table 11. F o r the 0 -+ +4 change in oxidation state, the difference in atomic charges, Aq, increases from sulfur (Aq = 0.586) t o selenium (Aq = 0.712) to tellurium (Aq = 0.773). This implies that t h e core-electron binding energy shifts (AE,) should follow the order A&(S) > A&(%) > AEh(Te). However, the A&’s are: AEb(s) = +3.6 e v , A&(%) = $4.0 e v , and A&(Te) = f 2 . 8 eV or AEb(S) ‘V AEb(Se) > AEb(Te). Thus, the order predicted from the H F S calculations is not followed exactly for the 0 1 4 change in oxidation state. Considering t h e relationship of Aci to AE, as discussed above for the +4 -+ +6 change in oxidation state, one predicts that the binding energy shifts should be ordered such that A,??b(s)> AEa(Se) c= (7) M. Cox, Ph.D. Thesis, lndiana University, Blooniington, Ind. June 1970.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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Table 11. Chemical Shifts for the Group VI Elements Sulfur ( I )
Compound Na2S s8
Na2SOa Na2S04
Compound KSeCN Se Na2SeOa Na2SeOa
Oxidation state -2 0 +4
+6 Oxidation state -2 0 $4
+6
s(2P) Eb, eV 160.8 162.2 165.8 167.7
A&, eV
Se(3pvJ Eb, eV 165.1 165.9 169.6 170.1
A&, eV
4 (7)
... -1.4 +3.6 +1.9
0
$0.586 +O. 746 Selenium Se(3pa/,) Ea, eV 159.3 159.9 164.1 164.6
-0.8 $3.7 +O. 5
0: 586 0.160
A&, eV
4 (7)
A4
... +O. 5
+O. 712 +O. 813
... 0.712 0.101
AE,, eV
4 (7)
Aq
-0.5 t2.8 $0.7
0 +O. 773 +O. 888
... 0.773 0.116
-0.6 +4.2
Tellurium
Compound Na5e Te K2TeOa K2Te04
Oxidation state -2 0
t4 $6
Te(3dn/,) Eb, eV 582.6 583.2 586.1 586.8
A&, eV
-0.6 $2.9 +0.7
+
AEb(Te). This is the order followed for AEb(s) = 1.9 eV while A&(Se) II +0.8 eV and A&,(Te) N +0.7 e v . The fact that the order of binding energy shifts can be predicted for the + 4 + $6 change in oxidation state but not for the 0 +. +4 change in the G r o u p VI elements from H F S semi-empirical molecular orbital calculations is consistent with results reported elsewhere. Siegbahn et al. ( I ) have used a modified H F S calculation to obtain binding energies for different ionic states of sulfur in the range - 2 to +6. By drawing a smooth curve through the A& points a t integral-charge values, the experimental values could be made to correspond to a n effective nonintegral charge C. C is related to the oxidation state (V) of the sulfur atom by
C
=
0.08 V
(4)
By using this relationship, shifts could be predicted only to 1 1 . 0 eV out of about 5.0 eV for the IsI/,, 2sl/,, and 2~11, levels of sulfur in three compounds from -2 to +6 in V. Fadley et a / . (8) have reported some HFS calculations for fluorine, chlorine, and bromine that contained errors of 5 in binding energy shift values upon going from one ionic state to another. Davis et a/. (9) have reported some theoretical estimates of binding energy shifts resulting from SCF-MO calculations for carbon, nitrogen, and oxygen 1s electrons. They conclude that the calculations can be used to give estimates of the binding energy shifts in the molecules to within about 1 eV. In all other theoretical approaches which attempt to predict core-electron binding energy shifts, the results are not yet able to match the experimental observations. In fact, chemical shifts in binding energy can be approximated by the sum of empirically evaluated parameters characteristic of the attached atoms o r group ( I O ) . Finn et a/. (11) have used such a n approach for estimating binding (8) C. S. Fadley, S. B. M. Hagworn. M . P. Klein, and D. A. Shirley, J . Chrm. Phys.. 48, 3779 (1968). (9) D. W. Davis, J. M. Hollander, D. A. Shirley, and T.D. Thomas, ibid., 52, 3295 (1970). (10) W. L. Jolly, J. Amer. C ~ I ~ ISo,., P I I 92, I . 3260 (1970). (11) P. Finn, R. K . Pearsoil, J. M. Hollander, and W. L. Jolly, Znorg. Chem., 10, 378 (1971). 1886
*
Te(3da/,)Eb, eV 572.4 572.9 575.7 576.4
energy shifts for gaseous nitrogen compounds and found that the empirical parameter method is capable of predicting chemical shifts to about 1 0 . 2 eV. Thus, to date, the empirical approach is much more accurate than theoretical approaches in predicting binding energy shifts. Comparisons of the AEb (0 .-, -2) shifts are not as straightforward as in the case for the oxyanions. The Se and Te atoms have very different chemical environments in the compounds investigated; the Se is bound to both potassium and a cyanide moiety while the Te is bound only to sodium. The shift between KSeCN and elemental selenium is -0.8 eV for the 3 ~ 1 1electrons , and -0.6 eV for the selenium 3~31, electrons while the shift for NanTe relative to elemental tellurium is -0.6 eV for the 3dr/, electrons and -0.5 eV for the 3d5i, electrons. The shift is negative as one would predict for a negative oxidation state, since the effective nuclear charge felt by the core-electrons has been decreased by the addition of electrons, thus decreasing the binding energy relative to the elemental species. The A&’S for the 0 - - 2 change in oxidation state are approximately equal for selenium and tellurium, while both are smaller than for sulfur, as would be predicted qualitatively. AS mentioned above, interpretation of these shifts is complicated by the different chemical environments of the G r o u p VI elements and, therefore, direct comparisons with sulfur may not be valid. The data for ZnSe and ZnTe appear to be extraordinary. As discussed above, the binding energy shift for a species in a negative oxidation state should be negative relative to the neutral species. F o r instance the shift between So and S2- is - 1.4 eV. However, the AEb for the Se(3p) electrons between elemental Se and ZnSe is only -0.1 eV. This shift is much less negative than that between Se and KSeCN. The AE, for the Te(3d) electrons between elemental Te and ZnTe is positiue rather than negative (+0.4 eV) as expected. Since the spectra were calibrated via a n internal C(1s) electron line, the extraordinary behavior cannot be due to sample charging even though the ZnSe and ZnTe are known semiconductors while all other compounds studied here are insulators. The spectra for ZnSe and ZnTe contained a n in-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
tense O(1s) electron line a t binding energies of 530.6 and 530.9 eV, respectively. This implies that the samples have undergone surface oxidation. Formation of a n oxide would be expected to shift the binding energies positive relative to the neutral species (note the data for SeOl a n d T e 0 3 ) . Since the Se and Te binding energies d o not coincide with t h e other oxides studied here, the surface oxidation must be considered incomplete with the formation o f nonstoichiometric oxides. One of the principal difficulties in investigating structure and bonding in G r o u p VI has been the absence of a uniformly applicable spectroscopic technique. N M R studies are complicated by the low isotopic abundance and sensitivity of iiS, '7Se, Iz3Te, and " T e . Neither nuclear quadrapole
resonance (33S) nor Mossbauer spectroscopy (Te) have a wide scope of use in G r o u p VIA. In contrast, the present study demonstrates that ESCA provides valuable information o n compounds containing all the G r o u p VIA elements.
RECEIVED for review June 11, 1971. Accepted July 20, 1971. This work was supported in part through funds provided by the U. S. Atomic Energy Commission under Contract AT(38-1)-645 and t h e National Science Foundation, under G r a n t N o . GP 9486. One of US (W. E. S.) would like to thank the National Institutes oi Health for a pre-doctoral fellowship during the term of this work.
Determination of Trace Concentrations of Mercury in Environmetal Water Samples P. E. Doherty and R . S. Dorsett Sacannah Ril;er Laboratory, E . 1. du Pont de Netnours and Co., Aiken. S. C . 29801 DETERMINATION OF MERCURY in environmental water samples has received considerable attention in recent months. Information obtained from reports in the literature and consultations with other investigators revealed that some of the analytical techniques initially used were inadequate to measure mercury concentrations in the low parts per billion range. These techniques did not provide for complete recovery of organomercury species. The flameless atomic absorption method of Hatch and O t t ( I ) , modified ( 2 ) to include a persulfate oxidation step, can be used t o analyze environmental water samples for both ionic and organic forms of mercury. However, our studies have shown that the flameless atomic absorption method of Brandenberger and Bader (3, 4 ) with electrodeposition for isolating mercury is more directly applicable t o determining organic as well as ionic forms of mercury in environmental water samples because no oxidation step is required. This note reports the procedure and results.
2.0
I
,."_
I R5
0
I
1
I
0.2
0.5 Mercury, p p b
I
1
0.1
I .o
Figure 1. Mercury calibration curve for 5 0 4 samples and 90-min electrodeposition period, uncorrected for blank
EXPERIMENTAL
A sample aliquot (up t o 50 ml in volume) in a 100-ml glass beaker is diluted to approximately 50-ml volume and adjusted to about 0.1N with nitric acid. A copper coil cathode of the type described and illustrated by Brandenberger and Bader (3) is prepared with 3-in. leads; a 1/16-in. diameter platinum wire is used as the anode. The electrodes are immersed in the sample solution that is stirred magnetically. Mercury is electrodeposited onto the cathode a t a potential of 3 volts generated by a conventional power supply. Electrolysis for 90 minutes quantitatively deposits up to 50 ng of mercury. The recovery of mercury decreases as the electrolysis time decreases below 60 minutes; electrolysis time for quantitative electrodeposition can be de( 1 ) W. R . Hatch and W. L. Ott, ANAL.CHEM.. 40, 2085 (1968). (2) Provisional Method for Determination of Mercury issued by
the Federal Water Quality Administration, Analytical Quality Control Laboratory, Cincinnati, Ohio, 1970. ( 3 ) H. Brandenberger and H. Bader, At. Ahsorptimr Nrn,s/ett., 6, 101 (1967).
(4) Ihid., 7, 53 (1968).
creased for smaller volumes of more concentrated mercury solutions. A number of electrodepositions may be conducted simultaneously with a n electrical power supply of sufficient capacity. When electrodeposition is complete, the copper coil is removed from the sample solution, rinsed with distilled water and acetone, and air dried. The mercury isolated o n the coil is determined by flameless atomic absorption spectrophotometry using the static method ( 4 ) with a mercury vapor discharge lamp. Dilute solutions of mercuric chloride are prepared daily for use as standards. RESULTS AND DISCUSSION
Ionic forms of mercury in environmental water samples a r e readily collected by spontaneous amalgamation onto copper wire coils. Attempts t o collect organic forms of mercury by this technique gave low and erratic yields. With the electrodeposition method, however, both ionic and organic forms of mercury were readily and completely collected. N o separate, chemical oxidation step is required. The specific
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1887
