Oxidation of Aqueous Polyselenide Solutions. A Mechanistic Pulse Radiolysis Study

Article abstract of DOI:10.1021/jp994361g

The oxidation of aqueous polyselenide solutions was studied by pulse radiolysis in the presence of N₂O at pH 12.3; the hydroxyl radical OH was the predominant oxidant, while hydrogen selenide anions HSe^- and triselenide dianions Se₃^2- were the major selenide species in the starting solution. The progress of the oxidation was monitored by optical spectroscopy. Transient polyselenides appeared immediately after the electron pulse and rapidly proceeded to form adducts with HSe^-, i.e., HSe₂^2- and H₂Se₂^-, and a fairly long-lived intermediate that was identified as the diselenide radical anion Se₂^-. These radicals recombine to give eventually the tetraselenide dianion, Se₄^2-.

Full text of DOI:10.1021/jp994361g

J. Phys. Chem. A 2000, 104, 4011-4016
4011
Oxidation of Aqueous Polyselenide Solutions. A Mechanistic Pulse Radiolysis Study
Andreas Goldbach
UniVersity of Stuttgart, Nobelstrasse 12, D-70569 Stuttgart, Germany
Marie-Louise Saboungi* and J. A. Johnson
Argonne National Laboratory, Argonne, Illinois 60439
Andrew R. Cook
Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973
Dan Meisel
Radiation Laboratory and Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556
ReceiVed: December 10, 1999; In Final Form: February 17, 2000
The oxidation of aqueous polyselenide solutions was studied by pulse radiolysis in the presence of N2O at
pH 12.3; the hydroxyl radical OH was the predominant oxidant, while hydrogen selenide anions HSe- and
triselenide dianions Se32- were the major selenide species in the starting solution. The progress of the oxidation
was monitored by optical spectroscopy. Transient polyselenides appeared immediately after the electron pulse
-
and rapidly proceeded to form adducts with HSe-, i.e., HSe22- and H2Se2 , and a fairly long-lived intermediate
-
that was identified as the diselenide radical anion Se2 . These radicals recombine to give eventually the
2-
tetraselenide dianion, Se4
.
Introduction
determined. Only recently have we obtained evidence for the
-
formation of Se2 radical anions in aqueous polyselenide
Polyselenides are a frequently encountered but relatively little
studied species. They are particularly common in high efficiency
photoelectrochemical cells using II-VI compounds (often CdSe
or GaAs) as anodes, where a selenide salt is added to the
electrolyte to minimize anodic photocorrosion.1 The photo-
generated holes react with the selenide ions generated at the
solid/liquid interface to produce selenide radicals, which eventu-
ally produce polyselenides.2 Investigation of the chemistry and
dynamics of the polyselenides will provide important informa-
tion on their stability in different media and thus enable a better
control of the factors governing the longevity and performance
of the cells. At present, not much is known about the properties
and nature of the various species involved in the formation of
polyselenides in aqueous solution.3,4 The purpose of the
experimental work presented here is to fill this gap.
The absorption spectra of aqueous polyselenides have been
measured.5,6 It was recognized that complex equilibria exist
between the various dianions Sex2-, where x ) 1-4, and some
of the equilibrium constants were determined.6 The correspond-
ing redox potentials were obtained by potentiometric measure-
ments.5 A recent electrospray mass spectroscopy study7 revealed
that protonated species HSex- exist in these electrolyte solutions
electrolytes.9
The makeup of polyselenide electrolytes is very complicated
even in the absence of radicals: the diverse equilibria between
2-
-
the Sex dianions and the protonated HSex depend strongly
on the mean oxidation state of the electrolyte, the pH, and the
counterion.5-7 However, the dissociation constants Ka of
H2Se10-12 and H2Se2 are well established. The pKa values of
8
both molecules are the following:
H2Se h H+ + HSe-
HSe- h H+ + Se2-
pKa1 ) 3.9
(1)
(2)
pKa2 ) 15.0
-
H2Se2 h H+ + HSe2
HSe2- h H+ + Se2
pKa1 ) 1.7
pKa2 ) 9.3
(3)
(4)
2-
At pH 12.3, employed in this study, equilibrium 2 is completely
shifted to the left side.12 Hence, Se2- does not play a role at all
and fully reduced Se appears only as the monoprotonated HSe-.
On the other hand, the dissociation constants of H2Se2 are
significantly larger than those of the parent molecule, H2Se,
and equilibrium 4 is completely shifted to the right side. Thus,
the diselenide dianion is fully deprotonated at the pH relevant
to this study.8 Following this trend, and the well-established
correlation of pKa’s with the electrostatic proton-base interac-
tions, the pKa values of the higher polyselenides are assumed
to be similar to those of H2Se2 or even smaller. Therefore, we
2-
as well and even larger dianions such as Se5 were also
identified. However, Lyons and Young detected no paramagnetic
species in these solutions using ESR spectroscopy.5 An early
pulse radiolysis investigation of H2Se solutions is the only study
focusing on polyselenide radicals in aqueous environments.8 In
this pioneering work Se-, its protonated form HSe, and the
adduct to HSe-, i.e., H2Se2-, were identified and equilibria and
rate constants for some reactions of these radicals were
10.1021/jp994361g CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/27/2000

4012 J. Phys. Chem. A, Vol. 104, No. 17, 2000
Goldbach et al.
TABLE 1: Combination of Stock Solutions To Yield the
Polyselenide Solutions Used in the Present Study
conv ratio Se oxidation 0.084 M
0.01 M
0.1 M
H2O
(Se0/Se2-
)
level (%) H2Se (mL)a H2O2 (mL)b NaOH (mL)b (mL)b
0
0.14
0.91
0
12
48
5
10
15
0
10
60
20
40
60
75
140
165
a Deaereated with argon. b Deaereated with N2O.
conclude that at pH > 12 the larger polyselenides are also fully
deprotonated.
In the course of Raman spectroscopic studies we observed
-
the formation of Se2 in aqueous polyselenide solutions fol-
lowing excitation with blue or green laser light.9 The radical
was identified by its characteristic Raman band at 324 cm-1
.
2-
Se4 was the only other polyselenide detected by means of
Raman spectroscopy in that study. Hence, we speculated that
2-
the photolysis of Se4 leads to homolytic dissociation of the
Figure 1. Optical spectra of the polyselenide electrolytes used in the
present study. The degree of oxidation increases from the bottom to
the top. Experimental conditions: [OH-] ) 1.75 × 10-2 M, [H2Se]0
) 4.2 × 10-3 M.
molecule. In the present pulse radiolysis investigation we prepare
radicals by a well-defined route and study the oxidation
processes and the evolution of polyselenide species. One electron
oxidation of polyselenides was achieved using hydroxyl radicals,
which were generated by an electron pulse in the presence of
N2O. Optical spectroscopy was used to identify transient species
and end products, and a mechanism was devised for the
oxidation of selenide in aqueous solution.
-
water generates eaq and OH radicals as the primary products
at a yield of 2.8 radicals per 100 eV each. Other minor products,
H and H2O2, were neglected. When the solutions contain N2O,
-
all the eaq are converted to OH radicals via the reaction
eaq- + N2O + H2O f OH + OH- + N2
(5)
Experimental Section
Thus, the OH concentration is 5 × 10-5 M at the maximum
radiation dose used. Because of the high pH, some of these
radicals may dissociate to their basic form, the O- radicals (pKa
) 11.9). The latter are expected to react slower than the OH
radicals, and therefore most of the reaction would proceed via
oxidation by OH coupled to its acid-base equilibrium. N2O
concentrations were slightly below the saturation level at 1 atm
(2.0 × 10-2 M at 25 °C)14 because of the addition of argon-
saturated H2Se solution to the other solutions.
Preparation of Polyselenide Solutions. H2O2 was used for
partial oxidation of H2Se in alkaline solutions because it provides
controlled and reproducible conversion of selenide into poly-
selenide. Appropriate quantities of aqueous stock solutions of
H2Se, H2O2, and NaOH were mixed in water to yield the
polyselenide at the proper pH. All stock solutions were freshly
prepared in a glovebox under nitrogen atmosphere prior to each
preparation of the polyselenides. Water, 0.1 M NaOH, and 0.01
M H2O2 were deoxygenated by bubbling N2O for 15-20 min
in a septum-sealed flask. Saturated H2Se solutions (8.4 × 10-2
M at 28 °C)13 were prepared in the same way by bubbling the
gas into water that was previously deaerated with argon. All
chemicals were of the highest purity commercially available,
and Nanopure water from a Barnstead Ultrapure water system
was used throughout.
Results
The absorption spectra of the polyselenide solutions used in
the pulse radiolysis experiments are shown in Figure 1. As
discussed above, HSe- is the only species expected in the
solution prior to oxidation at pH 12.3. The hydrogen selenide
anion has a broad absorption band at around 250 nm. Upon
oxidation by hydrogen peroxide three bands appear at 330, 380,
and 530 nm, as can be seen in Figure 1. These bands indicate
the formation of Se32-, the triselenide dianion.5,6 For compari-
Following the procedure described above, sodium selenide
solutions were produced at a total Se concentration of 4.2 ×
10-3 M at pH ) 12.3 with 0%, 12%, and 48% of the Se formally
oxidized to Se0. Details are given in Table 1. Nominally, a 50%
2-
oxidation level corresponds to 100% conversion to Se2
;
,
however, higher oxidized polyselenides, such as Se32- or Se4
2-
2-
son, the Se4 anion has absorption bands at 380 and 470 nm
2-
and Se2 at 430 nm.5,6 Using the spectral parameters derived
may form as well, via the equilibria between these species in
water.5-7 Absorption spectra of the solutions were recorded on
a Varian Cary 5 spectrophotometer prior to the radiolysis
experiments. We limited the selenide conversion in order to
avoid complications caused by high oxidation levels, notably
the generation of colloidal selenium.
Pulse Radiolysis Experiments. All solutions, degassed with
ultrapure argon or N2O, were manipulated using the syringe
technique. Electron pulses of 4-40 ns width from the Argonne
20 MeV electron linear accelerator were used to produce 1.2 ×
10-6 to 2.5 × 10-5 M hydrated electrons. Measurements were
made using 1 cm Suprasil cells and a pulsed xenon lamp was
employed as analyzing light source. An N2O saturated solution
of 10-2 M KSCN was used for dosimetry. The radiolysis of
by Lyons and Young5 the best fit to these spectra was obtained
2-
with a mixture of 5% of Se2 and 95% of Se32-. Apart from
their intensity, the absorption spectra of the two oxidized H2Se
solutions were identical. The predominance of the triselenide
even at the lower Se oxidation level is consistent with the
electrospray mass spectroscopy study by Raymond et al., who
investigated the pH dependence of the polyselenide equilibria
in an aqueous Na2Se4 solution. They showed that at pH 7 and
below almost equal amounts of Se22-, Se32-, and Se42- coexist
in that highly oxidized polyselenide solution (75% conversion
to “Se0”). Upon raising the pH the concentrations of Se22- and
2-
Se3 decrease and the tetraselenide is clearly favored above
pH 11. The predominance of the triselenide at the low

Oxidation of Aqueous Polyselenide Solutions
J. Phys. Chem. A, Vol. 104, No. 17, 2000 4013
Figure 2. Difference absorption spectra obtained at various times
following a pulse that generates 5 × 10-5 M of OH radicals in the
solution with an initial Se conversion ratio of 0.14 at pH 12.3. Initial
conditions: [HSe-] ) 3.45 × 10-3 M, [Se32-]0 ) 2.5 × 10-4 M, N2O
saturated solutions.
Figure 3. Difference absorption spectra obtained at various times
following a pulse that generates 5 × 10-5 M of OH radicals in the
solution with an initial Se conversion ratio of 0.91 at pH 12.3. Initial
conditions: [HSe-] ) 1.2 × 10-3 M, [Se32-]0 ) 1 × 10-3 M, N2O
saturated solutions.
Despite these limitations the difference absorption spectra
provide several constructive details. First, the bands at 380 and
470 nm in Figure 3 indicate that the tetraselenide Se4 is the
conversion ratios in the two solutions prepared for the present
radiolysis experiments is in line with those observations.
2-
The spectral changes induced by the radiolysis experiments
are shown in Figures 2 and 3 (lower and higher initial Se
oxidation levels, respectively). These spectra are difference
spectra between the irradiated and the starting polyselenide
electrolytes. Within the first microsecond after the electron pulse,
new spectral features emerge and vanish very fast, mainly
around 400 nm. At 2 µs after the pulse, both solutions give
rather similar absorption spectra with a broad absorption band
near 500 nm, a shoulder at around 400 nm, and bleaching of
the parent molecule band around 340 nm. These bands further
disappear, and at 100 µs after the pulse, the difference absorption
spectrum of the highly oxidized electrolyte (Figure 3) shows
only a prominent band at 380 nm and a shoulder at 470 nm. In
addition to these bands, the less oxidized electrolyte exhibits
two more bands at 430 and 520 nm after 100 µs (Figure 2).
end product of the radiolytic oxidation on the time scale of our
experiments. In the solution with a lower initial Se oxidation
level the conversion into Se4 is incomplete, as indicated by
2-
the additional bands in the 100 µs spectrum of Figure 2. These
2-
additional features are attributed to Se2 (430 nm) and the
intermediate, which dominates the 2 µs spectra with an intense
band around 500 nm (Figures 2 and 3). The latter band is
centered at 510 nm in Figure 3 and therefore is attributed to
Se2-. This radical anion appears to be the direct precursor of
the tetraselenide, since no other intermediate band is observed
en route to Se42-. Another notable feature in the 2 µs spectra is
2-
the bleaching around 340 nm. In this region Se3 dominates
the absorption spectrum of the parent polyselenide solution.
Hence, the bleaching is attributed to the removal of the
triselenide upon reaction with the hydroxyl radicals. Figures 4
and 5 show the temporal evolution of the absorbance at 340
and 510 nm of both solutions, low and high conversions,
following the radiation pulse. Significantly, the level of bleach-
ing at 340 nm and the gain in absorbance at 510 nm are equal
Identification of the polyselenide intermediates from these
spectra is difficult because of the limited number of well-
established optical spectra of reactive polyselenides. The radical
-
anion Se2 is best characterized among these since it can be
easily implanted into ultramarines,15 zeolites,16 and halogenide
crystals.17 It has two characteristic absorption bands in the green
2-
in both solutions. This indicates that the consumption of Se3
and the formation of Se2- depend on the radiation dose but not
on the initial Se oxidation level of the investigated solutions.
The optical spectrum of Se4 displayed in Figure 3 is
and the UV spectral region. For instance, these bands are found
- 17
2-
at 345 and 515 nm in KI crystals doped with Se2
.
An
absorption band centered at 450 nm has been attributed to Se-
distorted when compared to that reported by Lyons and Young.5
This results from the overlap of the bleached absorption of the
parent molecule and that of the oxidation product: The bleached
Se32- bands and the spectrum of the newly formed Se42- blend
together to produce the 100 µs spectrum. Indeed, this spectrum
can be reproduced satisfactorily by employing the spectral
parameters of Se32- and Se42- determined by Lyons and Young5
and assuming the consumption of one Se32- per Se42- molecule.
Also, the level of bleaching at 340 nm after 2 µs translates into
in the pulse radiolysis study of H2Se solutions.8 Other bands
-
were assigned to the radicals HSe (350 nm) and H2Se2 (410
nm). We have no knowledge of any absorption spectra of other
selenide radicals in aqueous milieu. The assignment of the
difference absorption spectra in Figures 2 and 3 is further
complicated by the fact that most of the absorption bands of
the reactive intermediates and dianions Sex2- overlap below 500
nm.

4014 J. Phys. Chem. A, Vol. 104, No. 17, 2000
Goldbach et al.
difference spectra makes it very difficult to extract reliable rate
constants from the absorbance changes. The intermediate band
at 510 nm is the least affected by overlap effects because the
2-
2-
optical spectra of Se3 and Se4 are relatively weak in this
spectral range. Notably, the corresponding species Se2- disap-
pears in a radiation dose dependent process, which suggests a
recombination of these radicals. However, since we do not know
-
the absolute value of the absorption coefficient of Se2 at 510
nm, we are unable to determine a rate constant for the
recombination.
Discussion
A detailed mechanism can be outlined for the oxidation of
polyselenide solutions by OH radicals. The major selenide
species in the solutions, prior to the electron pulse, are HSe-
and Se32-. One electron oxidation of these selenides by the
hydroxyl radicals is the initial step in a sequence that eventually
leads to the formation of Se42-. The pH used is close to the
pKa ) 11.9 of the hydroxyl radical18 and its corresponding base,
O-, may also participate in the oxidation. The rate constants
2-
for reactions of O- with HSe- and Se3 are expected to be
significantly smaller than those of OH, but the OH/O- acid-
base equilibrium is rapidly achieved on the time scale of the
oxidation reactions
Figure 4. Absorbance changes at 340 and 510 nm of the solution
described in Figure 2 at the same radiation dose.
OH + HSe- f HSe + OH-
OH + Se32- f Se3- + OH-
(6)
(7)
At the lower Se oxidation level the concentrations of Se32- and
HSe- are 2.5 × 10-4 and 3.45 × 10-3 M, respectively, and
reaction 7 cannot compete with reaction 6 provided that the
rate constants are similar. At the higher Se conversion level
the concentrations of both Se3 and HSe- are approximately
2-
10-3 M and both oxidation routes may be important. The
reaction of OH with HSe- was investigated by Henglein and
co-workers and the rate constant for reaction 6 has been
determined as k6 ) 5.5 × 109 M-1 s-1.8 Note that the 350 nm
band of HSe can be recognized in the 0.2 µs spectrum of Figure
2-
3. It is much weaker than those of Se3 in this spectral range
and is masked in the difference absorption spectra when
significant amounts of Se32- are depleted (2 µs spectra, Figures
2-
2 and 3). The oxidation of Se3 by OH has not been studied,
and the absorption spectrum of the one electron oxidation
-
product Se3 is not known either. Note, however, that the pKa
of HSe
HSe + OH- f Se- + H2O
(8)
Figure 5. Absorbance changes at 340 and 510 nm of the solution
described in Figure 3 at the same radiation dose.
is expected to be considerably smaller than that of the hydroxyl
radical. Therefore, HSe should be mostly ionized at pH 12.3 at
equilibrium. The 450 nm absorption due to Se- (ꢀmax ) 380
M-1 cm-1)8 is responsible for some of the bands in the 0.2 and
2 µs spectra of Figures 2 and 3.
a Se3 concentration reduction of 5 × 10-6 M using the
2-
2-
extinction coefficient of Se3 at this wavelength taken from
the investigation of Lyons and Young.5 This corresponds to
roughly 10% of the expected absorption depletion at the dose
used if we assume the consumption of one molecule Se32- per
OH radical. Clearly, the oxidation products must contribute to
the absorption at this wavelength also. Although the absorption
at 350 nm in the 0.2 µs spectrum of Figure 3 points to HSe,
another species has to be involved because HSe (ꢀmax ) 300
M-1 cm-1)8 is a much weaker absorber in this spectral range
than Se32- (ꢀmax ) 3000 M-1 cm-1).5 It is likely that the hidden
band is the second optical transition of the radical Se2-, which
is found at 345 nm in the KI matrix.17 The intricacy of the
The primary oxidation products HSe, Se-, and Se3 are
-
rapidly removed by subsequent reactions. HSe has to be
considered as well because it may be intercepted by another
reactant before it equilibrates according to eq 8. The initial
concentration of OH sets an upper limit for the total yield of
these intermediates, which is 5 × 10-5 M following the most
intense radiation pulse. Because of the much larger concentration
of HSe- and Se32-, the reactions that follow the initial oxidation
step involve these species:

Oxidation of Aqueous Polyselenide Solutions
J. Phys. Chem. A, Vol. 104, No. 17, 2000 4015
OH + Se32- + HSe- f Se22- + Se2- + H2O (18)
Reaction 14 leads to an increase in the polyselenide chain length,
i.e., the formation of Se4 from two Se3 parent molecules.
This pathway seems to be of minor importance in the scheme
outlined above. However, the reactive paramagnetic species,
Se2-, which results from any of the pathways shown above,
eventually recombines to yield the Se42-. This is evident from
the dose dependent decay of the absorbance at 510 nm:
HSe + HSe- h H2Se2
(9)
(10)
(11)
(12)
(13)
(14)
-
HSe + Se32- f Se22- + Se2- + H+
2-
2-
Se- + HSe- h HSe2
2-
Se- + Se32- f Se22- + Se2
-
Se3- + HSe- f Se22- + Se2- + H+
2-
Se2- + Se2- f Se4
(19)
-
Se3- + Se32- f Se42- + Se2
Since the absorption coefficient of Se2- at 510 nm is not known,
we are unable to determine the rate constant 2k19 of the
recombination. However, it is expected to be of the same order
as the rate constant found for the recombination of Se-:
With the exception of the equilibria 9 and 11 these reactions
yield a stable polyselenide dianion and the dimeric radical Se2
-
,
which was postulated above as an intermediate based on the 2
µs difference absorption spectra. The adducts defined by the
equilibria 9 and 11 lead eventually to Se22- and Se2- also (vide
infra). Equilibrium 9 was studied by Henglein and co-workers.8
They determined the equilibrium constant K9 ) 1.7 × 104 M-1
2-
Se- + Se- f Se2
(20)
where 2k20 ) 4.6 × 109 M-1 s-1.8 According to eqs 18 and 19
2-
two molecules of Se32- are consumed in the formation of Se4
.
-
and the absorption spectrum of the complex H2Se2 shows an
absorption band centered at 410 nm (ꢀmax ) 8600 M-1 cm-1).8
Equilibrium 9 is strongly pH dependent and at pH > 12 the
410 nm band is very weak,8 suggesting that this complex is of
minor importance at very high pH values. Nonetheless, this band
may contribute to the absorption around 400 nm in the 0.2 and
2 µs spectra of Figures 2 and 3. No information is available on
equilibrium 11 or the spectral characteristics of the complex
However, the 100 µs spectrum of Figure 3 implies the loss of
just one Se32- molecule in this process and it does not indicate
2-
the formation of two Se2 as byproduct either. Nonetheless,
the interim formation of Se22- is supported by the 430 nm band
in the 100 µs spectrum of Figure 2. Notably, the Se22- band is
accompanied by the 510 nm band of Se2- and the 380 and 470
nm bands of Se42- in Figure 2, which suggests that the formation
2-
-
HSe22-. However, it is tied to H2Se2 via the acid-base
of Se4 is considerably slower in the solution with the lower
initial Se oxidation level. Here the HSe/Se- reservoir character
of the equilibria 9, 11, and 15 becomes evident. Since the
equilibrium:
concentration of HSe- exceeds that of Se3 by more than an
2-
HSe22- + H+ h H2Se2
(15)
-
order of magnitude in this solution, most of the primary radicals
are captured by HSe-. The release of HSe/Se- radicals by the
Equilibria 9, 11, and 15 then becomes the step controlling the
where the doubly ionized species, HSe22-, is expected to
dominate at the pH value of this study.
2-
overall rate. Note that these “reservoir” species, (HSe2 and
The adducts to selenide stabilize the primary oxidation
products, HSe and Se-, and act as a reservoir for these radicals.
As these species are consumed via reactions 10 and 12,
equilibria 9, 11, and 15 are shifted to the left side, feeding the
restored primary radicals into the consuming reaction channels
10 and 12. Of course, it is possible that the adducts themselves
H2Se2-) retard the depletion of Se32-, which contributes to the
relatively weak bleaching at 340 nm. Finally, the absence of
430 nm bands in the 100 µs spectrum of Figure 3 and the
2-
“missing” Se3 molecule can be rationalized by taking the
following equilibrium into account:
react with Se32-, but these routes yield the same products Se2
2-
OH- + Se32- + HSe- h 2Se22- + H2O
(21)
-
and Se2 as reactions 10 and 12:
The electrospray mass spectroscopy study of Raymond et al.7
showed that at pH > 7 this equilibrium is strongly shifted to
the left. So, one molecule Se32- is recovered while the Se22- is
completely consumed and the stoichiometric combination of eqs
18, 19, and 21 gives the overall equation for the selenide
oxidation triggered by the radiolysis pulse:
Se32- + H2Se2- f Se22- + Se2- + H2Se
Se32- + HSe22- f Se22- + Se2- + HSe-
(16)
(17)
The bleaching level at 340 nm depends on the radiation dose
but not on the initial Se oxidation level. The sequence of
reactions 6-17 is, in particular, consistent with that observation.
2OH + Se32- + HSe- f Se42- + OH- + H2O (22)
2-
In this mechanism the primary oxidation of Se3 by OH (eq
2-
7) is important only in the solution with high initial Se
conversion. Nonetheless, in the mechanism outlined above, the
Equation 22 is also consistent with other Se4 delivering
reactions, such as the direct association of Se- and Se3
-
2-
-
2-
formation of Se2 and Se2 consumes exactly one Se3 and
one HSe- molecule regardless of the primary oxidation step.
Primary oxidation of HSe- by OH radicals (reactions 6 and 8)
is followed by (i) 10 and 12, respectively, or (ii) the detour via
the adduct to the parent selenide (reactions 9, 11, and 15)
followed by reaction with the parent trimer (reactions 10, 12,
16, and 17, respectively). On the other hand, the sequence that
produced in eqs 6 and 7 or reaction 20. However, because the
concentrations of the short-lived radicals are much smaller than
those of the parent molecules, HSe- and Se32-, intervention by
the latter dominates. Whereas this study focuses on the oxidation
of Se32-, the results are of general importance for the under-
-
standing of aqueous polyselenides. First, Se2 is evidently a
key species in the redox chemistry of polyselenides. Its
formation favors the conversion to Se42-. Other Sex2- dianions
that may be thermodynamically favored form subsequently
according to the established polyselenide equilibria.5-7 Second,
2-
is initiated by the oxidation of the trimer Se3 (reaction 7) is
followed by a reaction with the parent selenide HSe- (reaction
13). Either of these sequences leads to the same net reaction:

4016 J. Phys. Chem. A, Vol. 104, No. 17, 2000
Goldbach et al.
2-
the adduct radicals H2Se2-/HSe2 become of crucial impor-
Office of Basic Energy Sciences, under Contract No. W-31-
109-ENG-38. A.G. gratefully acknowledges support by the
Deutsche Forschungsgemeinschaft. Work at BNL and NDRL
is supported by the US-DOE, Division of Chemical Sciences.
This is document NDRL No. 4139.
tance. They serve as a reservoir for the primary radical, HSe/
Se-, and the release of that radical from the adduct may control
the overall rate.
Conclusion
References and Notes
The oxidation of aqueous polyselenide electrolytes has been
studied by pulse radiolysis experiments on N2O saturated
solutions so that the hydroxyl radical OH was the main oxidizing
agent. The electrolytes were characterized by optical spectros-
copy. The hydrogen selenide anion HSe- and the triselenide
(1) Licht, S. Sol. Energy Mater. Sol. Cells 1995, 38, 305.
(2) Dimitrijevic, N. M.; Kamat, P. V. Langmuir 1987, 3, 1004.
(3) Kanatzidis, M. G. Comments Inorg. Chem. 1990, 10, 161.
(4) Ansari, M. A.; Ibers, J. A. Coord. Chem. ReV. 1990, 100, 223.
(5) Lyons, L. E.; Young, T. L. Austr. J. Chem. 1986, 39, 511.
(6) Licht, S.; Forouzan, F. J. Electrochem. Soc. 1995, 142, 1546.
2-
dianion Se3 were the major selenide species in the solutions
prior to the radiolysis. We have drawn up an oxidation
mechanism that accounts for all the crucial details of the
(7) Raymond, C. C.; Dick, D. L.; Dorhout, P. K. Inorg. Chem. 1997,
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2-
difference absorbance spectra, i.e., (i) the depletion of Se3
,
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(ii) the intermediate with an absorption band at 510 nm, (iii)
the end product Se42-, (iv) the interim product Se22-, (v) the
2-
depletion of 1 mol of Se3 per Se42-, and (vi) the formation
of Se42- via recombination of the intermediate. After the electron
-
pulse HSe, Se-, and probably Se3 form, but these are short-
lived and soon the optical spectra are dominated by an optical
band around 500 nm, which is attributed to the Se2-. This radical
anion is fairly stable on the time scale of our experiments and
eventually proceeds to form the tetraselenide Se42- via recom-
bination. In addition, we have evidence for the existence of
reservoir species H2Se2- and HSe22- that stabilize the primary
HSe and Se- radicals temporarily. The release of HSe or Se-
from these HSe- adducts is the rate-controlling step in
electrolytes with low initial oxidation level.
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Acknowledgment. The work at ANL is supported by the
U.S. Department of Energy, Division of Materials Sciences,
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