6448
J . Phys. Chem. 1989, 93, 6448-6453
ESR Spin-Trapping Investigation of Azide Oxidation on CdS and ZnO Suspensions Rossano Amadelli,* Andrea Maldotti, Carlo Bartocci, and Vittorio Carassiti Centro di Studio sulla Fotochimica e Reattivitri degli Stati Eccitati dei Composti di Coordinazione del CNR and Dipartimento di Chimica della Universitri degli Studi di Ferrara, Ferrara, Italy (Received: December 6, 1988)
The ESR spin-trappingtechnique has been used to investigate the oxidation of N< to N3' radical in CdS and ZnO suspensions using phenyl-tert-butylnitrone (PBN) as a spin trap. The initial growth rate of the ESR signal of the N3PBN adduct has been followed at different N3- concentrations. The effect of different added ionic species on the ESR signal intensity has also been tested. In CdS, cadmium ions enhanced the N,PBN signal, while several anions decreased it in the order F e ( C N ) t > I- > Br- > NO2- > HP042-> CI-. Somewhat analogous, but less pronounced, was the effect of some anions in ZnO. The effect of these anions cannot be generally explained as a competition for holes. The effects of HPOd2-(which cannot be oxidized), Br-, and NO2- (whose redox potentials are very close to that of the N3-/N3' couple) are better explained as an adsorption competition. Likewise, the decrease in the N3PBN ESR signal as the pH increases is explained by the preferential interaction of OH- with the surface.
Introduction The study of reactions at semiconductor surfaces continues to attract a great deal of interest.'V2 Studies that were initially carried out only on semiconductor electrodes now also exploit powder suspen~ions.~The surface reactions occurring in these systems are overwhelmingly of the redox type and can still be treated on the basis of an electrochemical A useful guideline to establish whether species can be oxidized or reduced at a particular semiconductor is the position of the redox energy level of the species in solution with respect to the semiconductor valence and conduction band. This thermodynamic factor is undoubtedly important, although, as for electrode reactions on metals, kinetic factors and adsorption should be taken into proper account.i0 Radicals are often intermediates in charge-transfer reactions at the surface of illuminated semiconductors and are known to be involved in the so-called current-doubling phenomenon.li The importance of radical formation has been duly stressed by numerous ESR studies, in particular those using the spin-trapping te~hnique.l~-~~ In a previous worki5we reported on a spin-trapping investigation of azide oxidation on TiOz that indicated the involvement of OH radicals in the formation of azidyl radicals which were the species trapped. The present work is an extension of that study to CdS and ZnO suspensions. It also discusses the effect of species added ( I ) Serpone, N.; Borgarello, E.; Pelizzetti, E. In Photocatalysis and Enuironment; Schiavello, M., Ed.; Kluwer: Dordrecht, The Netherlands, 1987; pp 499-58 1. (2) Tributsch, H. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O'M., Eds.; Plenum: New York, 1986; Vol. 17, Chapter 4. (3) Fox, M. A. In Topics in Organic Electrochemistry; Fry, A. J., Britton, W. E., Eds.; Plenum: New York, 1986; Chapter 4. (4) Spiro, M.; Freund, P. L. J . Chem. Soc., Faraday Trans. I 1983, 79, 1649. ( 5 ) Freund, P. I-.: Spiro, M. J . Chem. SOC.,Faraday Trans. 1 1983, 7 9 , 481. (6) Spiro, M.; Ravno, A. B. J . Chem. SOC.1965, 78. (7) Miller, S. D.; Bard, A. J.; McLendon. G.; Ferguson, J. J . A m . Chem. SOC.1981, 103, 5336. (8) Duonghong, D.; Ramsden, J.; Gratzel, M. J . A m . Chem. SOC.1982, 104, 2977. (9) Brown, G . T.; Darwent, J. R.; Fletcher, P. D. 1. J . A m . Chem. SOC. 1985, 107, 6446. (10) Kobayashi. T.; Yoneyama, H.; Tamura, H. J . Electroanal. Chem. 1981, 122, 133. ( I I ) Morrison, S . R. The Chemical Physics of Surfaces; Plenum: New York, 1977. (12) Harbour, J. R.; Hair, L. M . Adv. Colloid lnterface Sci. 1986, 24, 103. (13) Howe, R. F. Adu. Colloid Interface Sci. 1982, 18, 1 . (14) Walter, T. H.; Bancroft, E. E.; McIntire, G. L.; Davis, F. R.; Gierasch, L. M.; Blount, H. N . Can. J . Chem. 1982, 60, 1621. (15) Maldotti, A.; Amadelli, R.; Carassiti, V. Can. J . Chem. 1988, 66, 76.
to the suspension that lead to an increase or to a decrease in the rate of N3*formation. Interest in the electrooxidation of N3- on metal electrodes dates back to the work of Stout.I6 Lately, the reaction has been studied by Roscoe and Conway" on Pt and by Amadelli et al. on Ru02.18 Addition of electrogenerated N3* radicals to olefins has been recently investigated by Wendt and P l ~ a k ' while ~ * ~ a~work by Alfassi and Schuler2*emphasized selective oxidation by N3' of a number of organic and inorganic compounds.
Experimental Section The nitroxides phenyl-tert-butylnitrone(PBN) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidin-l-yloxy were purchased from Janssen. Cadmium sulfide (Fischer) and zinc oxide (Alfa Ventron) were used as received. All other chemicals, including NaN3, were obtained from Fluka. Electron spin resonance measurements were carried out with a Bruker ER 200 D spectrometer. Calibration was achieved as described in ref 22. Analysis of Zn2+ and Cd2+ in corrosion experiments was carried out by atomic absorption spectroscopy with a Perkin-Elmer 1100 spectrometer. Suspensions of CdS and ZnO were prepared by ultrasonically dispersing 0.07 g/mL semiconductor powder containing the required amount of NaN3 and 0.015 g/mL PBN. This quantity was established by performing experiments where the signal intensity of the adduct between N3*and PBN (henceforth N3PBN) was followed as a function of the amount of PBN in solution. The amount of PBN used corresponds to the plateau region. The suspension containing the required reagents was introduced into a Spectrosil flat cell and irradiated directly inside the ESR cavity using a medium-pressure mercury lamp (Hanau Q 600). Light of wavelength lower than 360 nm for ZnO and 400 nm for CdS was cut off by use of suitable glass filters. For the experiments where we examined the effect of cations and anions on the intensity of the N3PBN signal, the required amounts of the salts were added to the suspensions and stirred for 10-15 h in the dark before the ESR experiments. Before each experiment with illumination, runs were performed in the dark to check for possible radicals formed thermally. The ( I 6) Stout, H. P. Trans. Faraday SOC.1945, 41. 64. (17) Roscoe, S. G.; Conway, B. E. J . Electroanal. Chem. 1988,249,217. (18) Amadelli, R.; De Battisti, A,; Rocchini, G.; Svestka, M.Extended Abstracts, International Society of Electrochemistry, 39th Annual Meeting, Glasgow, 4-9 September, 1988. (19) Wendt, H.; Plazak, V . J . Electroanal. Chem. 1983, 154, 13. (20) Plzak, V.; Wendt, H. J . Electroanal. Chem. 1983, 154, 29. (21) Alfassi, Z. B.; Schuler, R. H. J . Phys. Chem. 1985, 89, 3359. (22) Cunningham, J.; Corkery, S. J . Phys. Chem. 1975, 79, 933.
0022-3654/89/2093-6448$0l.50/00 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6449
Azide Oxidation on CdS and ZnO Suspensions
F n
v) (-I rl
I
C
I I
3 >1
L 0 L (-I rl
n
4w
2w
0
600
L
800
P
t i m e (seconds]
v
Figure 1. Fixed-field ESR signal intensity of the N3PBN adduct as a
L
function of illumination time in a CdS suspension. Insert: the normal ESR spectrum with the arrow indicating the position at which the field was fixed.
0.8
0.4
0
WY' Figure 3. Initial rate of N3PBN adduct formation in an illuminated CdS suspension as a function of the relative light intensity; vo is the maximum light intensity. Azide concentration 0.025 mol/dm3.
8
6 0
4
[Ng x
8
12
10
12
PH
lo3 tmole/dm3)
Figure 2. Initial rate r of growth of the ESR signal due to the N,PBN adduct as a function of the N3- concentration in a CdS suspension. Insert: plot of l / r vs 1/[N3-].
pH of the suspensions was controlled before each ESR run using a Beckman Select Ion 5000 or a Amel 337 pH meter. Unless otherwise stated, the pH was adjusted to 7.5 with N a O H or HCIO,. Light intensities were measured by means of a radiometer (International Light). Additional experimental details can be found in our previous paperL5and in the works of Cunningham et al.22*23
Results Growth Rate of the N3PBN E S R Signal as a Function of N c Concentration and Light Intensity. The spin trapping of N3* radicals by PBN has been discussed in our previous work conIn that work we also discussed the cerning TiOz s~spensions.'~ adduct stability. We recall that the ESR spectrum of N3*with PBN (N3PBN) (Figure 1, insert) consists of a triplet of quartets. We compared this spectrum with that obtained when we generated N3*in a homogeneous phase by photochemical excitation of iron porphyrins in the presence of N3-.24 The spectrum obtained in the suspension compared with that obtained in the homogeneous hase does not show anysignificant line broadenin or distortion !hat would indicate an interaction of the spin ad uct with the sllrfaCe.l4
5
(23) Cunningham, J.; Zainal, H. J . Phys. Chem. 1972, 76, 2362. (24) This may bc no longer true under continuous illumination since once the adsorbed species are oxidized, reestablishment of an adsorption equilibrium may be a slow process.
Figure 4. Initial rate of N,PBN adduct formation in an illuminated CdS
suspension as a function of pH. Azide concentration 0.025 mol/dm3. Figure 1 also shows the growth of the N3PBN ESR signal at a fixed-field position in an 02-or air-saturated CdS suspension containing NaN,. It is apparent, in the latter case, that the decay of the signal intensity is due to depletion of 02,which is the electron scavenger. Figure 2 illustrates the initial rate r of the ESR signal growth for different concentrations of N < in a CdS suspension. The data of this figure, when plotted as 1/ r vs l/[N,-], yield a straight line (Figure 2, insert) as was the case with Ti02in the previous workL5 and also with ZnO in the present work. The behavior is indicative of a reaction controlled by Langmuir adsorption of the reactant, i.e.
h+
(N3-)ads
PBN
(N3')ads
N3PBN
(1)
with
r = k,,lJ
(2)
then KN,-[N3-l
lJ=
+ KN3-[N3-l
(3)
where kd is the charge-transfer constant for N< oxidation by holes and KN3- is the adsorption coefficient of N3-. Since the N,PBN FSR spectrum does not show interaction with the surface, it must be inferred that there is a desorption step preceding the trapping process: (N3')ads
+
(N3')aq
(4)
6450 The Journal of Physical Chemistry, Vol. 93, No. 17, 1989
CI
I
Amadelli et al.
!
nk
0
r(
1
3
5
CH COO-
3
Br-
7
1
[x-] x 103 t m o l e / c " ) Figure 5. Effect of added anions (X-) on initial rate of adduct formation in an illuminated CdS suspension. Azide concentration 0.025 mol/dm3.
2
5
7
9
l o 3 tmole/dm3) added anions (X-)on the initial rate of N3PBN
[X-]
x
Figure 7. Effect of adduct formation in an illuminated ZnO suspension. Azide concentration 0.03 mol/dm3
0 CI
%I
rl
C
a
3
;Il >
ill
.
5s Figure 6. Time dependence of the photocurrent for a CdS polycrystalline electrode in a solution containing 0.1 mol/dm3 NaCIO, and 0.02 mol/ dm3NaN3: effect of NaBr (7 X IO-' mol/dm') and Na2HP04(5 X lo4 mol/dm'). Electrode polarized at -0.05 V vs SCE.
Since the spin-trapping process does not have a 100% efficiency, the rate of the ESR signal growth r is lower than the true rate of N3' radical formation as discussed by us earlier.I5 Nevertheless, the linearity of the 1 / r vs 1 / [N,-] plot shows that there must be proportionality between the observed rate r a n d the true rate R. This is sufficient for the purposes of this work. Figure 3 shows that the initial rate of N3PBN formation is proportional to light intensity, indicating a negligible e-/h+ pair annihilati~n.~~ p H Dependence of N3PBN Formation. Figure 4 shows the signal intensity of the N3PBN adduct as a function of the pH of a CdS suspension. A significant decrease in the intensity occurs at pH's higher than 9.5. Indeed, at pH 10 the species trapped by PBN were OH' radicals even in CdS, as it was apparent from the triplet-of-doublet pattern with aN = 15.7 G and aH= 2.1 G.I5 The signal could be seen also at pH 7 but was much weaker. The behavior observed for ZnO was qualitatively similar, with the signal intensity being constant from pH 7 to pH 9 and decreasing sharply after pH 10. The data of Figure 4 are in remarkable agreement with the studies of I- photoelectrooxidation on CdS as a function of pH reported by Matsumura et aLZ6 Effect of Added Cations and Anions. The effect of different anions on the rate of N3PBN formation in a CdS suspension is shown in Figure 5 . The effect of both H2P04- and HP042- has been tested although only the result for HP042- is shown in the
L
I
,
,
,
,
0
w
L
2
4
L c ~ ~ + ] x 103
8
tmole/dm3)
Figure 8. Effect of Cd2*on the initial rate of N3PBN adduct formation in an illuminated CdS suspension. Azide concentration 0.025 mol/dm3.
figure. We noticed the inhibitive effect of phosphate when we tried to use a buffered solution at pH 7 and saw but a weak signal due to N,PBN. The effect of singly added N a 2 H P 0 4and NaH2PO4 was tested in the low concentration range since at higher concentrations they caused too large pH changes. The effect of S042- and NO< was not appreciable, while CH3COONa caused the same degree of inhibition as C1-. Qualitatively, the degree of inhibition due to competitive phenomena; Le., Fe(CN),@ > I- > Br- > NO2- > HP042-> H2P04> C1- is the one expected on the basis of the reactivity with holes of the above species reported in the l i t e r a t ~ r e . ~ ' - ~ ~ Figure 6 shows the photocurrent vs time for a CdS polycrystalline electrode in the presence of N3- only and N3- with HP042and Br-. The effect of these ions parallels that of Figure 3; the behavior reflects a decrease in the charge-transfer rate at the interface. Data for the effect of anions on ZnO are shown in Figure 7. A comparison between the two sets of data (Figures 7 and 3) reveals a striking difference in the effect of Br- which, we anticipate, cannot find a thermodynamic explanation. The only observed effect fotadded cations was that of Cd2+, which increased the rate of N,' formation in a CdS suspension (Figure 8).
(25) Ward, M. D.;White, J. R.: Bard, A. J. J . A m . Chem. SOC.1983, 105, 27.
(26) Matsumura, M.; Hiramoto, M.; Iehara, T.; Tsubomura, H. J . Phys. Chem. 1984, 88, 248.
(27) Domhech, J.; Prieto, A. J . Phys. Chem. 1986, 90, 1123. T.; Yoneyama; Tamura, H. Chem. Lett. 1979, 457. (29) Reichman, B.; Byvik, C. E. J . Phys. Chem. 1981.85, 2255. (28) Kobayashi,
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6451
Azide Oxidation on CdS and ZnO Suspensions
TABLE 1:
Corrosion" of ZnO and CdS in Different Media under Continuous Ihminationb
irradn time, min 5 10 15 20
ZnO/ H,O 3.8 6.2 9.0 12
ZnO/ N70.6 1.2 1.8 2.1
ZnO/
Br1.2 2.3 3.5 4.8
CdS/
Br7.0 15 25 34
CdS/ NOz2.5 4.2 6.1 8.3
CdS/ N71.5 3.0 4.5 6.0
"In ppm Zn2+ or Cd2+. bIrradiation wavelength 360 nm for ZnO and 400 nm for CdS. Amount of semiconductor 0.28 g/10 mL. Concentration of added anions 0.02 mol/dm'.
Estimation of the N3PBN Adduct Concentration versus the Degree of Semiconductor Corrosion. To estimate the concentration of the adduct of N3' with PBN, we compared the integrated signals with those obtained for different concentrations of a stable nitroxide in the same suspension. The nitroxide used for this purpose was 3-carbamoyl-2,2,5,5-tetramethylpyrrolidin1-yloxy
(Figure 2 , insert) gives kctKN3-/kc. In this treatment we have neglected the rate of electron-hole recombination. We have done this on the grounds that the linear dependence of the initial rate r on light intensityZ5(Figure 3) suggests that e-/h+ recombination is negligible. This is understandable since we measure an initial rate, corresponding to a situation where the adsorbed N3- is in large excess with respect to photogenerated holes. In addition, the data presented in this work show that the corrosion of both CdS and ZnO in the presence of azide can be neglected under initial conditions. Then, eq 7 becomes kctKN3-[N3-l rct = kct3 =
_ -_ - l
0
I
The comparison yields a value of 2 X M N3PBN for a CdS suspension, and the same order of magnitude was confirmed for ZnO. Semiconductor corrosion in different media are reported in Table I. The amount of N3- oxidized during a given irradiation time was typically 2 orders of magnitude higher than the amount of MZ+formed in the solution, with the same experimental conditions. We can thus neglect the role of corrosion as a significant side process, especially during the initial conditions to which our experiments refer. Moreover, since the complexes of N3- with both Cdz+and Zn2+are soluble, one can rule out surface precipitation as a cause of error in the determination of the photocorrosion data of Table I. N3PBN Adduct Formation as a Function of N3- Concentration and tight Intensity. The growth of the N3PBN ESR signal with N3- concentration (Figure 2) indicates, as do other data in this work, that azide is oxidized when adsorbed on the semiconductor particles. In a simplified model, we assume that corrosion occurs at sites that are not occupied by N3-. This implies that adsorbed N3- reacts faster with holes than does the lattice.24 Then, if 9 is the surface coverage by N3-, we write dh+/dt = ko
- kCth+9- k,h+(l
- 9) = 0
(5)
where ko is the rate of electron-hole pair generation, k,, is the charge-transfer rate, and k, is the corrosion rate. Under these conditions rct = kCth+9
(6)
or kCtk0
= k,,9
9
+ k,(l
- 9)
kc
1-9
(7)
and
_1 r,,
1
-
+
m(T)
or, with Langmuir conditions: 1 'ct
= -1+ ko
k, kokctK~,-[N3-]
(9)
where KN,- is the adsorption coefficient for N3-. Thus, the ratio of the intercept over the slope (i/s) of a l / r c t vs 1/[N3-] plot
1
+
ko
1
+ KN,-[N~-]
(11)
Thus, the ratio i/s gives the adsorption coefficient KN,- = 425 dm3/mol on CdS. The analogous data obtained for the N3-/Zn0 system gave KN3-= 220 dm3/mol. These values are sensibly higher which was 16.5 than the KN,-found with a TiOz ~uspension,'~ dm3/mol. Hydroxide Zon Effect. The observed pH dependence of N < oxidation on both CdS and ZnO is at variance with that we observed on TiO2.I5There, an increase in pH led to more azidyl radicals being produced, and as a plausible explanation, we proposed that the oxidation of N3- on TiOz is mediated by OH' according to TiOH'
-
+ (N3-)sds Ti-• 4- (N30H-)ads Ti-• + OH- TiOH-
(N30H-)ads
(N3*)aq + (OH-),,
(12) (13) (14)
where oxidation of N3- is a surface process since the ideal mechanism Ti-OH'
Discussion
(10)
and kct
' 0
+ KN3-[N3-l
+ (N3-)aq
TiOH-
+ (N3')aq
(15)
would imply a reaction order with respect to N3- of 1, which we did not find. Involvement of OH' radicals has been claimed by several authors in the oxidation of organic30and inorganic31compounds on TiOz. As it results from literature data, the case of ZnO is more controversial in this respect. Harbour and Hair32reported the formation of a relatively large concentration of OH-spin-trap adduct upon illumination of an aqueous ZnO suspension. On the basis of the fact that the OH-spin-trap ESR signal decreased in the presence of formate or oxalate, the authors concluded that the OH' radicals were involved in the oxidation of the carboxylate anions. Involvement of OH' radicals in the oxidation of methyl orange on ZnO was claimed by Darwent and L e ~ r e . These ~~ authors assumed that OH' radicals were formed by oxidation of OH- by holes. Their work was carried out a t pH 9.2, which corresponds approximately to the point of zero charge (pzc) for ZnO;34thus significant adsorption of OH- is not likely. Nevertheless, it is possible that OH- ions are oxidized at this pH, although it has been reportedz7that stabilization of ZnO by OHagainst corrosion occurs in more alkaline solutions.35 On the other hand, according the work of Cunningham and Corkery,22 an increase in pH, Le., an increase in the number of hydroxyl groups (30) Izumi, I.; Fan, F. F. R.; Bard, A. J. J . Phys. Chem. 1981, 85, 218. (31) Gutierrez, C.; Salvador, P. J . Electrochem. Soc. 1986, 133, 924. (32) Harbour, J. R.; Hair, M. L. J . Phys. Chem. 1979, 83, 652. (33) Darwent, J. R.; Lepre, A. J. Chem. Soc., Faraday Trans. 2 1986,82, 1457. (34) Parks, G. A. Chem. Reu. 1965, 65, 177. (35) Even in this case, it is not fully clear whether Zn2+ is not found in the solution phase because of a decrease in the corrosion rate or whether it is held on the surface an insoluble hydrate.
6452 The Journal of Physical Chemistry, Vol. 93. No. 17, 1989
Amadelli et al.
on the surface, caused a decrease in the conduction band electrons due to electron-hole recombination at O H sites
-
?
+ h+ Zn+-OH Zn+-OH + eCB ZnOH
Zn-OH
(16) (17)
which may be one reason for a decrease in the corrosion rate as the pH increases. In this framework, there are good reasons to believe that at pH's lower or equal to the pzc OH' radicals originate from corrosion.36 Indeed, an elegant work by Fujishima et al.,' showed that formate ions are oxidized on ZnO according to the following mechanism: ZnO
+ 2h+
-
Zn2+ + O*
O* + 2 H C 0 0 - 4 2COO'-
Lewis sites
-r' r
(18)
+ H20
(19)
They found that the amount of Zn2+in solution was the same even up to HCOONa concentrations as high as 0.4 M. ZnO dissolved at the same rate of the hole formation. Conversely, the above authors found no Zn2+ on illumination of ZnO in a NaI solution, which indicates a direct reaction of I- with holes. The data of Table I of this work show that Zn2+formed when ZnO was irradiated for 15 min with N3- amounts to 1.8 ppm. The amount of N3- that was oxidized during the same period was 2 orders of magnitude higher than the above value. The same was found in the case of CdS. We conclude, in the framework of the paper of Fujishima et al., that N3- is oxidized by direct capture of holes. The effect of an increase in pH is then that of substituting adsorbed N3- with OH- on ZnO as well as on CdS. Effect of Added Cations. With the exception of Cd2+,cations preadsorbed on CdS do not seem to have a significant effect on the rate of N,' formation. A possible explanation of the effect of Cd2+is that of inducing an increase in the N; surface coverage. Cadmium ions are potential determining for CdS. According to Park and Huang,38 at pH 7.5, the surface consists of both Lewis and Brwsted sites. Excess Cd2+can thus adsorb on S Lewis sites.
Bransted sites
r
0
2
4
6
[x-) x 1 0 3 t m o l e / u m 3 ) Figure 9. Ratio of the initial rate of Na,PBN adduct formation in the absence of added anions ( r o ) to that in the presence of anions ( r ) vs the concentration of added anions (X-).Data from Figure 5. changes from a value close to zero to a net positive value (according to ref 37 p(Cd,,,) is 5.6), thus favoring the attraction of N3- to the surface. Effect of Added Anions. On CdS, the effect of some added anions (Figure 5) such as I- and Fe(CN)64- may be ascribed to a competition with N< for the photogenerated holes, on the basis of their relative energy levels in solution (Figure 10). On the other hand, while the data of Table I seem to indicate that the degree of stabilization of ZnO and CdS by N
