J. Phys. Chem. 1986,90,4193-4198 and surface field effects which can influence the form of the rate law expression^.^^ These have not been quantified for the semiconductor/electrolyte system. One surprising result of this research is that the difference between the energies for the reactions of N 2 0 under illumination and in the dark at p G a A s is relatively light.'^,'^ Quite different kinetics behavior has been found, and the foot-of-the-wave analysis for dark currents provides an exponential best fit. The finding that N 2 0 does not reduce in the dark at p-InP electrodes at any of the pH's tested (namely, 3.0, 7.0, 10, 11, and 12) for doping levels 3.4 X 10'' and (2 and 5 ) X 118cm-3 implicates a possible role for surface hydride at p-GaAs e1e~trodes.l~However, the option cannot be discounted that the hydrogen may be reduced by a tunneled electron through the oxide film, as proposed by Schmi~kler.~~.~~ (34) Sass, J. K.; Gerischer, H.Photoemission and the Electronic Properties of Surfaces; Feuerbacher, B., Fitton, B., Willis, R. F., E&.; Wiley: New York, 1978; Chapter 16, p 491. (35) Schmickler, W. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 477.
4193
Photoemission is an important technique to characterize high-energy processes at semiconductor/electrolyte interfaces. For example, with wide-spectrum incident radiation, low band gap semiconductors used in photoelectrochemical cells may have side reactions involving radicals in addition to the main photofaradaic mechanism. Alternatively, with monochromatic light and suitable conditions these high-energy processes might provide an efficient means to synthesize radicals or products or to study their reaction kinetics.
Acknowledgment. We are grateful to Dr. A. J. Nozik, SERI, for providing many of the crystals used in the present work. (36) Roy Morrison, S. Electrochemistry at Semiconductor and Oxidized Metal Electrodes; Plenum: New York, 1980; Chapter 6. (37) In this paper the periodic group notation in parentheses is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the pblock elements comprise group 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., I11 3 and 13.)
-
Photoreactivity of WO:, Dispersions: Spin Trapping and Electron Spin Resonance Detection of Radical Intermediates A. Leaustic, F. Babonneau,* and J. Livage Laboratoire de Spectrochimie du Solide (UA 302), Universitt Paris VI, Tour 44, 28me ttage, 75230 Paris Cedex 05, France (Received: November 12, 1985)
Spin-trapping techniques were used to detect free radical intermediates formed upon irradiation of W 0 3 powders dispersed in a mixture of water and alcohol (methanol, ethanol, 1-propanol,or 2-propanol). Three spin traps were used: two nitrones, a-(l-oxo-4-pyridyl)-N-tert-butylnitrone (POBN) and 5,5-dimethyl-1-pyrrolineN-oxide (DMPO), and one nitroso, 2methyl-2-nitrosopropane (NtB). Hydroxyl and hydroxyalkyl adducts were unambiguously identified. They arise from the photooxidation of the solvent. During the irradiation, the tungsten oxide was reduced. Experiments performed with W 0 3 aqueous dispersions containing various amounts of ethanol suggest that ethanol should be very sensitive toward oxidation by the photogenerated holes. Nevertheless, water seems to play a key role in the oxidation process.
Introduction The photochemical reactions occurring on semiconducting powders or colloids have been the subject of numerous studies.'-14 Among the oxides, much work has been done on titanium dioxide. Dispersed in an aqueous or organic medium, it can lead to several photochemical reactions such as water photoc a t a l ~ s i s , ' ~or * ' photodegradation ~ of pollutant^.'^-^^ Few studies (1) Grltzel, M. Acc. Chem. Res. 1981, 14, 376. (2) Henglein, A. Pure Appl. Chem. 1984,56, 1215. (3) Borgarello, E.;Kiwi, J.; Pelizetti, E.;Visca, M.; GrBtzel, M. J. Am. Chem. Soc. 1981,103,6324. (4) Duonghong, D.; Borgarello, E.; GrBtzel, M. J . Am. Chem. SOC.1981, 103,4685. (5) Duonghong, D.; Ramsden, J.; Grltzel, M. J . Am. Chem. SOC.1982, 104, 2977. (6) Gritzel, M.; Frank, A. J. J . Phys. Chem. 1982, 86, 2964. (7) Moser, J.; Griitzel, M. J . Am. Chem. SOC.1983, 105, 6547. ( 8 ) Henglein, A. Eer. Bunsenges. Phys. Chem. 1982, 86, 241. (9) Bahnemann, D.; Henglein, A.; Lilie, J.; Spanhel, L.J. Phys. Chem. 1984,88, 709. (10) Dunn, W. W.; Aikawa, Y.; Bard, A. J. J. Am. Chem. Soc. 1981,103, 3456. (11) Ward, M. D.; Bard, A. J. J. Phys. Chem. 1982, 86, 3599. (12) Ward, M. D.; White, J. R.; Bard, A. J. J. Am. Chem. Soc. 1983,105, 27. (13) Brown, G. T.; Darwent, J. R. J. Phys. Chem. 1984,88,4955. (14) Albery, W. J.; Bartlett, P. N.;Wilde, C. P.; Danvent, J. R. J. Am. Chem. Soc. 1985, 107, 1854. (15) Kraeutler, B.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 2239. (16) Inoue, T.; Fujishima, A.; Konishi, S.;Honda, K. Nature (London) 1977, 22, 17.
have been undertaken on tungsten oxide.2&22 Its band gap (2.8 eV), smaller than that of T i 0 2 (3-3.2 eV), should however be slightly more suitable for solar energy conversion. Visiblelight-induced O2generation from aqueous dispersions of W 0 3 was already proved.20*22The first step of this oxidation seems to be in the generation of hydroxyl radicals whose formation was evidenced by spin-trapping techniques23 based on the reaction24 radical spin trap spin adduct
+
-
Spin traps have unsaturated functions designed to give persistent free radical adducts when reacting with short-lived radicals. These adducts can then be detected by ESR. They exhibit characteristic spectra from which information about the radicals can often be extracted. Nitrone and nitroso compounds are commonly used as spin traps. In the case of a nitrone, the radical is trapped on (17) Barbeni, M.; Pramauro, E.;Pelizetti, E.;Borgarello, E.;Gritzel, M.; Serpone, N.Nouv. J. Chim. 1984, 8, 547. (18) Frank, S.N.;Bard, A. J. J. Am. Chem. Soc. 1977, 99, 303. (19) Borgarello, E.;Kalyanasundaram, K.; Gritzel, M.; Pelizetti, E. Helu. Chim. Acta 1982, 65, 243. (20) Danvent, J. R.; Mills, A. J . Chem. Soc., Faradoy Tram. 2 1982, 78, 359. (21) Nenadovic, M. T.; Rajh, T.; Micic, 0. I.; Nozik,A. J. J. Phys . Chem. 1984, 88, 5827. (22) Erbs, W.; Desilvestro, J.; Borgarello, E.;Gritzel, M. J . Phys . Chem. 1984, 88, 4001. (23) Aurian-Blajeni, B.; Halmann, M.; Manassen, J. Phofochem. Photobiol. 1982, 35, 157. (24) Janzen, E. G. Acc. Chem. Res. 1971, 4, 31.
0022-3654/86/2090-4193$01.50/00 1986 American Chemical Society
4194
The Journal of Physical Chemistry, Vol. 90, No. 17. 1986
the a-carbon atom while with a nitroso it is directly fixed on the nitrogen atom. -0 nitrone
-CH=N-C-
I
+
/
-t R e
\
-
Leaustic et al. a
\
-C-N=O
/
+
R.
-
nm
6 I /
-CH-N-C-
I
/
b
R nitroso
A :5 0 0
A
\
-C-N-0
1 1 R
The ESR spectrum of the resulting nitroxide is dominated by a triplet signal due to the hyperfme coupling between the unpaired electron and the nitrogen nuclei. Radicals trapped on a nitrone such as a-(l-oxo-4-pyridyl)-N-tert-butylnitrone(POBN) generally lead to a signal made of three doublets. The doublet signal results from the hyperfine interaction between the unpaired electron and the &hydrogen that does not belong to the radical. The identification of the trapped radical is therefore not direct; it depends on the magnitude of this @-hydrogen coupling. With nitroso traps, the radical is directly bonded to the nitrogen so that hypefine couplings with the hydrogen nuclei of the trapped radical itself can be detected. Information is then much easier to extract from the ESR spectrum. This is one of the main advantages of using nitroso traps. The identification of the radical can be more easily extracted from the ESR spectrum. One major drawback, however, of nitroso compounds such as 2-methyl-2nitrosopropane (NtB) comes from their poor stability under irradiation, giving rise to nitroxides. Thus, many authors prefer to use nitrone traps which are much more stable even if the identification of trapped radicals is not so easy. This spin-trapping technique leads us to investigate the photoreactivity of W 0 3 powders dispersed in various alcohol-water mixtures. Using both nitrone and nitroso spin traps allowed an unambiguous identification of the radical intermediates. All spin traps are sensitive to W light and one advantage of W 0 3 vs. Ti02 in spin-trapping experiments comes from its narrower band gap that allows one to use longer wavelengths in the visible or near-UV region.
Experimental !+tion Samples. W 0 3 powders were purchased from Merck and had a particle diameter of about 10 pm. The powder was dispersed in distilled water by magnetic stirring (0.2 g of powder in 10 mL of water). e-(I-Oxo-4-pyridyl)-N-tert-butylnitrone(POBN), 2-methyl-2-nitrosopropane (NtB), and 5,Sdimethyl- 1-pyrroline N-oxide (DMPO) were purchased from Aldrich and used as received. All alcohols were analytical reagents purchased from Prolabo. In all the experiments, the concentration of POBN and N t B was 0.01 M, while that of DMPO was 0.44 M. Equipment. Electron spin resonance spectra were recorded on a Varian E 09 spectrometer. A Bruker N M R gauss meter was used to measure the magnetic field with diphenylpicrylhydrazyl (DPPH) as a standard. All samples were put in capillaries and illuminated with a 200-W H g lamp through the metal grid of the ESR cavity. UV and IR components of the light were removed by a UV glass filter (Oriel 5146, cutoff 345 nm) and a 40-mm liquid filter. An Oriel 1240 grating monochromator with a 20-nm band-pass was used in order to select wavelengths. Optical spectra were recorded by diffuse reflection on a Beckman 5270 spectrophotometer using an integrating sphere. Results A . Evidence for Photoreactivity of Aqueous Suspensions of WO,. No ESR signal is detected when an aqueous solution of POBN (0.01 M) is irradiated. A spectrum, however, is observed when the same experiment is performed in the presence of W 0 3 , but only when the wavelength is shorter than 450 nm (2.8 eV). The intensity of the signal increases as the wavelength decreases (Figure 1). The ESR spectrum exhibits a triplet of doublets with ON = 14.9 G and aHB= 1.6 G. It can be assigned to hydroxyl adducts.25 When recorded with a small modulation amplitude
C
Figure 1. ESR spectra of spin-trapped hydroxyl radicals obtained by irradiation (1 5 min) of aqueous dispersions of WO, containing POBN (0.01 M). The irradiation wavelengths are (a) 500, (b) 450, and (c) 350 nm. Spectrometer settings are as follows: receiver gain, 2 X lo4;modulation amplitude, 1 G ; scan time, 4 min; time constant, 0.25 s.
Y
Y
Figure 2. ESR spectrum of POBN hydroxyl adduct. Spectrometer settings are as follows: receiver gain, 2 X lo4;modulation amplitude, 0.1 G ; scan time, 8 min; time constant, 0.25 s.
(0.1 G), the spectrum shows another doublet, arising from the coupling aH = 0.3 G25with an hydroxyl hydrogen (Figure 2). Thus, W 0 3 aqueous suspensions appear to be photoreactive, but only if the photon energy is greater than the oxide band gap. The following reactions would occur:
W03 h+
-
+ hv
+ H,O
+ h+ OH' + H+ e-
A quadruplet with relative intensities 1-2-2-1 sometimes appears (aN = 14.4 G ) (Figure 1). It could be due to the decomposition of the nitroxide, leading to the formation of the tert-butyl hydronitroxide, ON(H)-C(CH3)3.24 In the following experiments, all the wavelengths up to 360 nm were generally used since the irradiation with a single wavelength greatly decreases the intensity of the observed signals. B. Reactivity of W 0 3Aqueous Suspensions in the Presence of Alcohols or Acetone. 1 . Spin Trapping with POBN. Irradiation of a suspension of tungsten oxide in a POBN solution containing methanol, ethanol, or 1-propanol gives rise immediately to an ESR spectrum that can be described as the superposition of two triplets of doublets (Figure 3). The hyperfine parameters are listed in Table I. The hyperfine parameters corresponding to one of these signals are close to those obtained upon irradiation of an aqueous solution, without alcohol, and it can thus be attributed to an hydroxyl adduct. The slight differences between the coupling values are due to the solvents which are different. It is actually well-known that the hyperfine parameters, for a given adduct, depend on the ( 2 5 ) Janzen, E. G.; Wang, Y. Y.; Shetty, R.U. J . Am. Chem. SOC.1978, 100, 2923.
Photoreactivity of W 0 3 Dispersions
The Journal of Physical Chemistry, Vol. 90, No. 17, I986 4195
---1
METHANOL
v
Y
i.....:
a___..
v ...... .
,
Figure 3. ESR spectrum obtained by irradiation of aqueous dispersions of WO,containing ethanol and POBN (receiver gain, 1.25 X lo4; modulation amplitude, 0.1 G; scan time, 4 min; time constant, 0.25 s). TABLE I: ESR Parameters of Radicals Formed upon Irradiation of Tungsten Oxide Aaueous Suspensions in the Presence of POBN solvent g a N ,G aHB,G assumed radical 1.6 OH' H20 2.0060 14.9 CH3OH 2.0059 14.4 2.0 OH' 15.1 2.8 'CH20H CH3CH2OH 2.0059 14.7 1.8 OH' 14.9 2.6 CHOH-CH, CH3-CH2-CH20H 2.0059 14.4 1.8 OH' 15.0 2.4 CHOH-CH2-CH3 CH,-CHOH-CH, 2.0060 14.8 1.6 OH'
solvent.26 The second signal is probably due to hydroxyalkyl adducts arising from reactions with the alcohols. In the literat~re?~ such an experiment performed with methanol was mentioned: the ESR spectrum was rather complex, exhibiting four signals: two triplets of doublets as in our experiments, one triplet of triplets (uN = 16.6 G, U H = 10.25 G ) assigned to the H' radical adduct, and one triplet (aN = 17 G ) whose origin was not obvious. We never detected the triplet of triplets. As for the triplet (aN = 17 G ) ,it appears in some spectra, increasing in intensity with irradiation, and is probably due to the decomposition of the spin trap. 2-Propanol only gives rise to hydroxyl adducts. The CH3COH-CH3 radical is certainly formed, but owing to an important steric effect, it could perhaps not be trapped. Radical intermediates formed upon irradiation of W 0 3 dispersed in a mixture of water and alcohol, in the presence of POBN, seem to give rise to two different ESR signals. As mentioned previously, their identification is rather difficult since (i) both signals are made of a triplet of doublets and (ii) their hyperfine parameters are so close that the spectra overlap. The variation of the proton hyperfine splitting depends on the structure of the trapped free radical. The magnitude of this interaction is governed by the Heller-McConnell equation*'
+ B2 cos2 0 Bo and B2 are constants (Bo= 0 and B2 = 26 G for nitroxides), AH@= Bo
and 0 is the dihedral angle between the C-N ?r orbital and the N-C-H@ plane. Each R group, added to the spin trap, gives rise to a different value for 0. In the case of POBN, the influence of R on AH@is rather small when R = OH', 'CH,OH, CH3'CHOH, or CH3-CH2-CHOH. The spin trap 5,s-dimethyl- 1-pyrroline N-oxide (DMPO) cCH3 H 3 Y q
bDMPO (26) Symons, M. Chemical and Biochemical Aspects of Electron-Spin Resonance Spectroscopy; Van Nostrand Reinhold: New York, 1978. (27) Heller, C.; McConnell, H. M. J. Chem. Phys. 1960, 32, 1535.
--ACETONE
f
d
I
1
1
Figure 4. ESR spectra of DMPO radical adducts obtained by irradiation (2 min) of aqueous dispersions of W 0 3 containing various solvents: (a) water, (b) methanol, (c) ethanol, (d) 1-propanol, (e) 2-propano1,and (f) acetone. Spectrometer settings are as follows: receiver gain, 1.25 X lo4 for (a) and 6.3 X lo4 for (b)-(f); modulation amplitude, 1 G; scan time, 4 min; time constant, 0.128 s. TABLE II: ESR Parameters of Radicals Formed upon Irradiation of Tungsten Oxide Aqueous Suspensions in the Presence of DMPO assumed radical aN, G G solvent g H20 2.0054 14.75 14.75 OH' 2.0052 14.75 14.75 OH' CHiOH 16.00 22.50 CHIOH CH3-CH20H 2.0052 14.75 14.75 OH' 15.80 22.80 CHOH-CH, CH,-CH,-CH,OH 2.0055 14.75 14.75 OH' 16.00 22.60 CH3-CH2-CHOH CHI-CHOH-CH, 2.0056 14.75 14.75 OH' 15.90 23.50 CH,-COH-CH, CHS-CO-CHS 2.0053 14.75 14.75 OH' 16.25 23.00 CH2-CO-CH3
is structured so that the conformation of its adducts placed the &hydrogen in a nearly eclipsing relationship with the nitrogen p orbital (Le., 0 is small and A H Bis large). As a result, large variations of AH@may occur when changing the trapped radical R.28 Using DMPO may enable an easier identification of the radicals, especially when several signals are revealed. 2. Spin Trapping with DMPO. Irradiation of an aqueous suspension of tungsten oxide in the presence of the DMPO spin trap gives rise to a quadruplet with an hyperfine value a = 14.75 G (Figure 4a). This signal is attributed to the hydroxyl adduct for which it has been shown that the nitrogen and hydrogen coupling values are accidentally The expected triplet of doublets then appears as a quadruplet with respective intensities 1-2-2- 1. (28) Anderson Evans, C. Aldrichimica Acta 1979, 12, 23. (29) Harbour, J. R.; Chow, V.; Bolton, J. R. Can. J . Chem. 1974,52, 3549.
Leaustic et al.
4196 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 ETHANOL
METmHANOL a
ACETONE Y
I
Y
"
PROPANOL 1
*
A
I
I
i
I
ACETONE
I
d
d
' I I
,
Figure 5. ESR spectra of NtB radical adducts obtained by irradiation
(15 min) of aqueous dispersions of W03containing various solvents: (a) methanol, (b) ethanol, (c) 1-propanol,and (d) acetone. Spectrometer settings are as follows: receiver gain, 2.5 X lo4 for (a) and (b) and 2 X lo4 for (c) and (d); modulation amplitude, 1 G; scan time, 4 min; time constant, 0.250 s. An asterisk indicates the signal arising from the spin-trap decomposition. When methanol, ethanol, 1-propanol, or 2-propanol is added to the aqueous suspension of tungsten oxide, a new triplet of doublets appears, superimposed to the previous quadruplet (Figure 4). N o overlap between both signals is observed, and the hyperfine parameters can easily be extracted. They are listed in Table 11. So two signals clearly appear; the first one is attributed to the hydroxyl adduct while the other one is attributed to one hydroxyalkyl a d d u ~ t . ~ In ~ ,the ~ ' presence of acetone, the irradiation of a suspension of tungsten oxide in a solution of POBN gives rise to the hydroxyl adduct only. When a DMPO spin trap is used, another signal is observed (aN = 16.25 G, uH@ = 23.00 G) that can be attributed to the CH,-CO-CH, radical adduct. Using DMPO lead to an easier analysis of the ESR spectrum, but as for nitrones, the identification of the radical trapped is not direct. 3. Spin Trapping with a Nitroso: NtB. As mentioned above, nitroso compounds are generally more convenient than nitrones for providing a "fingerprint" of the trapped radicals. This is because the added group lies closer to the unpaired electron. However, they are photochemically unstable. The 2-methyl-2nitrosopropane (NtB), for instance, is decomposed under irradiation and gives rise to the di-tert-butyl nitroxide (DTBN) that shows a triplet ESR spectrum aN = 16.5 G.32 (CH,),C-NO 'C(CH3),
-k N O + *C(CH3)3
+ (CH,),C-NO
+
[(CH3)3C],NO'
(30) Makino, K.; Mossoba, M. M.; Riesz, P. J . Phys. Chem. 1983, 87, 1369. (31) Sargent, F. P.; Gardy, E. M. Can. J . Chem. 1976, 54, 275.
Figure 6. ESR spectra of NtB radical adducts obtained after irradiation (15 min) at 360 nm, a selected wavelength, of aqueous dispersions of W 0 3 containing (a) ethanol and (b) acetone (receiver gain, 5 X 10); modulation amplitude, 1.25 G for (a) and 1 G for (b); scan time, 4 min; time constant, 0.25 s).
TABLE III: ESR Parameters of Radicals Formed upon Irradiation of Tungsten Oxide Aaueous Suswnsions in the Presence of NtB solvent g aN. G aH, G assumed radical ~
H2O CH3OH 2.0055 CHIOHCH, 2.0055 CH3CH2CH20H 2.0056 CH3COCH3 2.0056
15.0 15.6
5.9
15.5 15.4
1.6 8.5
1.8
a
CHI-OH CHOH-CH3 CHOH-CHz-CH, CH2-CO-CH3
No signal.
When an aqueous suspension of tungsten oxide is irradiated in the presence of the NtB spin trap, the ESR spectrum only exhibits a triplet due to the decomposition of NtB. Hydroxyl radicals do not seem to be stabilized by such a spin trap. This was confirmed when a solution of H202was irradiated. None of the hydroxyl radicals which are formed are trapped on the NtB spin trap. When adding solvents such as methanol, ethanol, 1-propanol, or acetone, new ESR signals appear after a few minutes of irradiation together with the triplet arising from the decomposition of the spin trap (Figure 5). Experiments have been performed at selected wavelengths in order to avoid the formation of this triplet. The NtB spin trap is photosensitive only for X ranging between 320-360 and 660-680 nm; it gives rise to di-tert-butyl nitroxide (DTBN). Upon irradiation at X = 390 nm, in the presence of ethanol and acetone, it should then be possible not to decompose the spin trap and to obtain the adduct signal only, as shown in Figure 6. These signals have rather low intensities, and for methanol and 1-propanol, no spectrum can be observed. Each spectrum exhibits a triplet of multiplets arising from the hyperfine interaction between the unpaired electron with both the nitrogen nucleus of the nitroso spin trap and the hydrogen nuclei of the trapped radical. Hyperfine parameters are listed in Table 111. The number of hydrogen nuclei can be determined, allowing a direct identification of the trapped radical. In the case of (32) Rosenthal, I.; Mossoba, M.; Riesz, P. Can.J . Chem. 1982, 60, 1486.
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4197
Photoreactivity of W 0 3 Dispersions 1
I
b
C
500
1000 1500 wavolength (nm)
2.4M
ETHANOL
3.4M
ETHANOL
5.7Y
ETHANOL
2000
Figure 7. Diffuse reflectance spectra of W 0 3 powders after irradiation in various aqueous mediums: (a) before irradiation; (b) pure water; (c) pure ethanol; (d) water and ethanol.
methanol, a triplet of triplets is observed that can be attributed to the 'CH20H adduct. In the presence of ethanol or 1-propanol, the triplet of doublets may arise from the 'CHOH-CH3 and CHOH-CH2-CH3 adducts, respectively. The formation of 'CH2-CO-CH3 radicals in the presence of acetone, which was suggested when the DMPO spin traps were used, is now fully confirmed by the presence of a triplet of triplets detected with the NtB spin trap. No signal is observed when the W 0 3 suspension is irradiated in the presence of 2-propanol, except the triplet due to the decqmposition of NtB. As in the experiments with POBN, the CH3-COH-CH3 radical cannot be trapped. This study, using various spin traps, fully confirms the formation of radical intermediates when irradiating a suspension of W 0 3 . These radicals arise from the oxidation of the solvent. C. Photoreduction of W 0 3Dispersed either in Water or in an Alcohol- Water Mixture. Upon irradiation, a tungsten oxide powder dispersed in water or an alcohol-water mixture turns blue. Optical spectra were recorded by reflection on the blue powder after drying. Experimental results with solvents such as pure water, pure ethanol, or an ethanol-water mixture are shown in Figure 7. In each case, a broad absorption band appears for wavelengths longer than 500 nm. This suggests that electrons are injected into the semiconducting oxide; the optical properties of such electrons are known to provide information on their localization. Free electrons give rise to free carriers absorption a t longer wavelengths; the absorption coefficient then increases with higher powers of the wavelength. Localized electrons usually give rise to absorption bands. Optical spectra recorded with irradiated tungsten oxide thus reveal free carriers absorption in agreement with the crystalline structure of our samples. ESR experiments do not reveal any W5+ signal even at very low temperatures down to 4 K. This is consistent with free electrons, delocalized over the whole lattice. Upon irradiation, some photoreduction of the tungsten oxide dispersed in water or alcohol occurs. It seems to be more efficient when the solvent is a mixture of water and ethanol, rather than when it is pure water or pure ethanol only.
Discussion Irradiation of a suspension of tungsten oxide in solvents such as water, alcohols, or water-alcohol mixtures, with wavelengths longer than 360 nm, gives rise to both oxidation of the solvent and reduction of the oxide. Irradiation of a semiconductor with photons having an energy greater than the band gap leads to the creation of electron-hole pairs. In crystalline W 0 3 , the top of the valence band is located at +3.1 V while the bottom of the conduction band lies at +0.3 V. Positive holes are then very strong oxidizing agents while electrons have only a moderate reducing power. In the presence of water and alcohols, two radical intermediates are detected: hydroxyl and hydroxyalkyl radicals. Hydroxyl radicals may result from the direct oxidation of water by a positive hole. The potentials for the oxidation of H 2 0or OH- into free OH' lie higher
-
Figure 8. ESR spectra of DMPO spin-trapped radicals obtained upon irradiation of aqueous dispersions of W 0 3 containing ethanol: (a) 0 M; (b) 2.4 M; (c) 3.4 M; (d) 5.7 M. Spectrometer settings are as follows: receiver gain, 1.25 X lo4 for (a) and 1 X IO4 for (b)-(d); modulation amplitude, 1 G; scan time, 4 min; time constant, 0.128 s. TABLE I V Concentrations (M) of the Spin-Trapped Radicals Formed upon Irradiation of Tungsten Oxide Aqueous Suspensions Containing DMPO and Various Amounts of Ethanol
[CIHSOH]
[H,O]
0 2.4 5.7
55.6 47.6 37.0
trapped radicals [CHICHOH] [OH'] 0 I x 10" 2.6 x 10-5 6.8 X 10" 10 x 10" i o x 10-5
than the top of the valence band of W 0 3 . As for hydroxyalkyl radicals, two mechanisms are possible: (i) a direct oxidation of the alcohol by a positive hole and/or (ii) a reaction between the hydroxyl radical and the alcohol. Evidence for such a reaction was provided by generating OH' radicals by sonolysis in the presence of ethanol. CH,-CHOH adducts were detected by spin trapping.30 W03
H 2 0 + h+ CH3-CH20H
OH'
-
+ hv
+ h+
+ CH3-CH20H
+ eOH' + H+
-
4
h+
CH3-CHOH CH3-CHOH
(1)
(2)
+ H+ + HzO
(3) (4)
It is not obvious to say which of the two possible reactions, (3) or (4),dominates. We performed spin-trapping experiments on aqueous suspensions of W 0 3 containing different concentrations of ethanol in order to determine the concentration of both hydroxyl and hydroxyalkyl radicals formed. The DMPO spin trap was used because the resulting signals do not superimpose, leading to clear ESR spectra. It is thus possible to integrate each signal separately in order to get the trapped radical concentrations. The carbamoyl 2,2,5,5-tetramethylpyrrolidinyl1-oxy radical (TEMPO) was used as a reference. ESR spectra are shown in Figure 8. They clearly show that the intensity of the CH3-CHOH-DMPO adduct increases with ethanol concentration. The concentrations of both OH' and CH3-CHOH adducts were determined by double integration of the last two peaks located at high magnetic field. Results are listed in Table IV. In this study, we may assume that the OH' adduct is as stable as the CH3CHOH adduct. This seems to be true since the two signals decrease in the same way when the light is off. The generation of hydroxyalkyl radicals
4198
The Journal of Physical Chemistry, Vol. 90,No. 17, 1986
Leaustic et al. of oxygen upon irradiation of a s e m i c o n d ~ c t o r . ~ ~ Figure 9 shows the evolution of the pH of an aqueous suspension of W 0 3 as a function of the irradiation time. The pH decreases w.ith the illumination until it reaches a stationary value. Such a generation of protons is predicted by the proposed mechanism (eq 2) according to which water is directly oxidized by the positive hole.
1
0
10
20 30 40 50 irradiation time cmni
!
Figure 9. Evolution of the pH of an aqueous solution (50 mL) containing WO, ( 3 g) under irradiation.
seems to be much more efficient than the generation of hydroxyl radicals. For a ratio [C2H50H]/[H20] of 0.15, the ratio of corresponding trapped radicals is 10. When W 0 3 dispersed in M hydroxyl radicals were pure water, is irradiated, 7 X trapped. When we used a water-ethanol mixture ([C2H50H]/ [H,O] = 0.19, 1.1 X lo4 M radicals were trapped. This difference in reactivity also appears for the reduction reaction. Irradiation of a W 0 3 suspension in a water-ethanol mixture leads to a more efficient photoreduction than irradiation in pure water (Figure 8). But in pure ethanol, the photoreduction rate decreases. Thus, direct oxidation of ethanol by the positive holes (eq 3) should occur, but the presence of water seems to greatly increase the photoactivity of the W 0 3 suspension. Equation 4 should be very efficient to generate hydroxyalkyl radicals. One possible reaction mechanism may be suggested. Irradiation can be described as the generation of an electron-hole pair, inducing a negative charge on a tungsten atom and a positive one on an oxygen atom. At the oxide-solvent interface, these excess charges could be reactive toward the redox couples present in solution. At pH 4,the conduction band edge lies at about +0.06 V: a photogenerated electron therefore could not reduce water. The potential for the formation of H' is very high (Eo= -2.1 vs. N H E at pH 0). The potential for the formation of H2 even lies above the conduction band edge. W 0 3 does not appear as a good candidate for H2 production. The excess electrons that cannot react with species in the solution are thus trapped inside the oxide which is photoreduced and turns blue. The photogenerated holes, however, are quite energetic: the valence band edge lies around +2.9 V at pH 4,below the potential for the formation of OH' (2.55 V vs. NHE). Oxidationof water can occur. In our experiments, with water at pH 4, the oxide surface is hydroxylated. (The pH decreases as the powder is dispersed in water (Figure 9).) The photogenerated positive holes can oxidize the OH- adsorbed ions, giving OH' radicals that can then generate H202or O2molecules. These radicals can also react with molecular species in solution such as alcohols. The hydroxyl radical seems to be the first intermediate in the production process
Conclusion When irradiating a suspension of W 0 3 in a water-alcohol mixture, spin-trapping experiments clearly give evidence for the photogeneration of hydroxyl and hydroxyalkyl radicals. Using various spin traps appears necessary in order to identify correctly the trapped radicals. Nitrones have a good stability; they can trap hydroxyl or alkyl radicals but do not allow a clear-cut analysis. Among these spin traps, DMPO is certainly the more convenient. Nitroso spin traps such as NtB enable a direct identification of the trapped alkyl radicals. They are, however, quite unstable under irradiation and do not trap hydroxyl radicals. A clear identification of both hydroxyl and hydroxyalkyl radicals can only be obtained when both DMPO and NtB spin traps are used. Tungsten oxide powders dispersed in aqueous solutions appear quite reactive toward oxidation since the photogenerated holes created under irradiation have a very oxidizing power. Hydroxyl radical formation seems to be the first step in the photooxidation process of water. Alcohols such as methanol, ethanol, or 1 propanol can be oxidized by these radicals, leading to the generation of hydroxyalkyl radicals. A direct oxidation of alcohol molecules by the photogenerated positive holes should not be excluded, however. As for reduction reactions, irradiated suspensions of tungsten oxide do not appear very reactive since the photogenerated electrons exhibit a quite low reducing power. These excess electrons thus remain trapped in the oxide that turns blue upon reduction. The oxidant reactivity of irradiated dispersions of W 0 3 was already used for light-induced O2 generation. A reducer must be present to drain off the photogenerated electrons. Ferric ion (E(FeZ+/Fe3+)= +0.46 V at pH 0) and silver ion (E(Ag/Ag+) = +0.8 V) were used, and the last one appeared more efficient to produce Because of its band gap, which enables using visible and near UV light, spin-trapping experiments could be easily undertaken with W 0 3 . This technique seems quite useful to understand the mechanisms of the reactions that take plzce with dispersed semiconducting powders. 0 2 . 2 0 1 2 2
Registry NO. POBN, 66893-81-0; DMPO, 3317-61-1; NtB, 917-95-3;
wo,, 1314-35-8;H20,7732-18-5; CHSOH,67-56-1; CH,CH20H, 64-
17-5; CH,-CH2-CH2OH, 7 1-23-8; CH&HOH-CHS,67-63-0; CHS-CO-CH3, 67-64-1; 'CH,OH, 2597-43-5; 'CHOH-CH,, 2348-46-1; 'CHOH-CH2CH3, 5723-77-3; CHyCOH-CH3, 5 1 3 1-95-3; 'CH2COCH,, 3122-07-4;.OH, 3352-57-6, ( 3 3 ) Jaeger, C. D.: Bard, A. J. J . Phys. Chem. 1979, 83, 3146.
