Langmuir 1989, 5 , 22-26
22
observe these multiple layes directly by conventional vacuum surface techniques. Burke and co-workers have found electrochemical evidence for the formation of multiple-layer oxide films on the surface of iridium, rhodium, and gold as well as nickel.2g30 However, likewise, direct physical evidence for these layers is lacking. The angle-resolved IR spectroelectrochemistry provides an opportunity to obtain physical measurements of multiple-layer films in these difficult yet important electrochemical systems. No attempt has been made in this work to calculate the relative thickness of each layer. The quantitative measurements of the optical density of each layer require knowledge of optical constants of the materials in the film.
Conclusions
Metal Substrate
Compact
Open Surface Layer
Layer
NiOOH
HZO
Anion
Figure 12. Schematic diagram showing sandwich structure of hydrated nickel oxide film. two oxidized phases are very similar, there is some evidence for the presence of nickel oxyhydroxide a t the outer surface. This is expected from eq 1 since the oxidation of p nickel hydroxide in the outer layer of the film should yield p nickel oxyhydroxide. Accordingly, the bulk of the film in the compact inner layer would be y nickel oxyhydroxide formed by oxidation of a nickel hydroxide in part of the film. Of more general interest is the application of the angle-resolved IR spectroscopy to other hydrated metal oxide films. Nazri, Yeager, and Cahan have found electrochemical evidence for the formation of multiple layers on hydrous iron oxide films.27 However, it was not possible to (27) Nazri, G.;Yeager, E.; Cahan, B. D. Gou. Rep. AD-A116422,1982.
Angle-resolved infrared spectroelectrochemistry was developed and successfully used for in situ depth profiling at the electrode/electrolyte interface. The results of this work showed that the electrochromic nickel oxide in the oxidized (colored) state has an oxyhydroxide structure and in the reduced (bleached) state contains both and a phases of nickel hydroxides. A sandwich structure is evident, particularly in the reduced state with a nickel hydroxide in the inner part of the film near the nickel substrate and p nickel hydroxide in the outer part of the film near the electrolyte interface. It is expected that this technique can also be used to probe multiple layers in other metal oxide films. Acknowledgment. We thank Drs. D. M. MacArthur and M. K. Carpenter for helpful discussions, R. S. Conell for preparation of the bulk standard samples, and Dr. J. L. Johnson for XRD characterization of the bulk standards. Registry No. NiOOH, 12026-04-9; Ni(OH)2, 12054-48-7; ",OH, 1336-21-6;NiS04, 7786-81-4; K2S208,7727-21-1; Br2, 7726-95-6; KOH, 1310-58-3;nickel oxide, 11099-02-8;hydrated nickel oxide, 12627-60-0. (28) Burke, L. D.; Twomey, T. A. M. J.Electroanal. Chem. 1982,134, 353. (29) Burke, L. D.; OSullivan, E.J. M.J. Electroanal. Chem. 1981,117, 144. (30) Burke, L. D.; Hopkins, G. P. J. Appl. Electrochem. 1984,14,679.
Photoelectrochemistry in Particulate Systems. 11. Reduction of Phenosafranin Dye in Colloidal Ti02 and CdS Suspensions K. R. Gopidas and Prashant V. Kamat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received June 8, 1988 The photoelectrochemicalreduction of an azine dye has been carried out at Ti02 and CdS semiconductor colloids under band-gap excitation. A laser flash photolysis technique has been employed to characterize the transients formed after the laser pulse excitation and to elucidate the mechanism of the interfacial charge-transfer process in these colloidal semiconductor systems. The formation of the radical anion of phenosafranin confirmed the interfacial charge transfer to be a one-electron reduction process. The quantum yield for the reduction of phenosafranin was found to be 0.05 in colloidal TiOz and 0.02 in colloidal CdS suspensions. Steady-state photolysis of TiOz colloids containing phenosafranin, which led to the formation of the leuco dye, is also described. Introduction In recent years a considerable interest has been shown in employing semiconductor particles as photocatalysts to
carry out the chemical transformations of organic and inorganic compounds in aqueous and nonaqueous media (see, for example, ref 1). Under band-gap excitation, the
0743-746318912405-0022$01.50/0 0 1989 American Chemical Society
Photoelectrochemistry in Particulate Systems semiconductor particles act as short-circuited microelectrodes and initiate the oxidation and reduction processes of the adsorbed substrates. In order to study the dynamics and mechanistic details of the interfacial charge-transfer process in semiconductor particulate systems, various approaches are being considered by several research groups. Techniques such as laser flash p h o t o l y ~ i s ,pulse ~ , ~ radioly ~ i s microwave ,~ a b ~ o r p t i o n ,diffuse ~ reflectance! and Raman spectroscopy7 are found to be useful in the characterization of transients generated in such heterogeneous systems. In our earlier study3l8 we have demonstrated that thiazine and oxazine dyes can serve as excellent probes in studying the charge-transfer processes in colloidal semiconductor systems. These dyes were easily reduced in colloidal metal oxide and metal chalcogenide-type semiconductors with an efficiency of 110%. Phenosafranin is a member of the azine class of dye and is a good electron acceptor in the ground state. In a recent study we reported the charge injection from the excited phenosafranin into the conduction band of Ti02particles.6a Since this dye is potentially useful in developing a photoelectrochemical system,SJOit is important to study the behavior of this dye in irradiated semiconductor systems. A laser flash photolysis study which elucidates the reduction of phenosafranin in colloidal T i 0 2 and CdS suspensions under band-gap excitation of the semiconductor is presented here.
Langmuir, Vol. 5, No. 1, 1989 23
05t I\
WAVELENGTH (nm)
Figure 1. Absorption spectra of (a) M Ti02colloidal suspension, (b) 0.1 mM CdS colloidal suspension, and (c) 0.8 X 10"
M phenosafranin in acetonitrile.
.;;I, \ 1 ,
-00025
400
,
500
,
[
1
?."-',
600
700
BOO
WAVELENGTH (nrn)
Experimental Section Phenosafranin, 3,7-diamino-5-phenylphenazinium chloride (Sigma),was purified chromatographicallyover silica gel (Sigma, chromatography grade, 100-200 mesh) with benzene-methanol M) (41) mixed solvent as eluent? Stock solutions ((2-5) X of colloidal Ti02were prepared by the hydrolysis of titanium(1V) 2-propoxide in acetonitrile" and were diluted with acetonitrile to obtain the desired concentrations of Ti02colloid. Scanning electron micrographs showed that the Ti0 particles were spherical with diameters ranging from 200 to 600 with an average value of 350 50 A. A colloidal CdS suspension was prepared by exposing 2 mM Cd12solution in acetonitrile (containing 2% water) to Hfi. The average particle diameter was found to be 33 A. The details of the preparation and characterization of CdS colloids have been described elsewhere.12 All other chemicals were
*
1
(1) (a! Gratzel, M. Energy Resources through Photochemistry and Catalysts; Academic: New York, 1983. (b) Fox, M. A. Acc. Chem. Res. 1983, 16, 314. (c) Pichat, P. In ACS Symp. Ser. 1986, 278, 21. (d) Henglein, A. Top. Curr. Chem. 1988, 143, 115. (2) See, for example: (a) Kalyansundaram, K.; Gratzel, M.; Pelliietti, E. Coord. Chem. Reu. 1986, 69, 57. (b) Rothenberger, G.; Moaer, J.; Gratzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. SOC.1985,107, 8054. (c) Darwent, J. R. J. Chem. SOC.,Faraday Trans. 1 1984,80,183. (3) (a) Kamat, P. V. Langmuir 1985, I , 608. (b) Kamat, P. V.; Chauvet, J.-P.; Fessenden, R. W. J. Phys. Chem. 1986, 90,1389. (c) Dimitrijevic, N. M.; Kamat, P. V. J.Phys. Chem. 1987,91,2096. (d) Kamat, P. V.; Dimitrijevic, N. M.; Fessenden, R. W. J. Phys. Chem. 1988, 92, 2324. (4) See, for example: (a) Henglein, A. Pure Appl. Chem. 1984, 56, 1215. (b) Mills, G.; Zongguan, L.; Meisel, D. J.Phys. Chem. 1988,92,822. (c) Dimitrijevic, N. M.; Kamat, P. V. Radiat. Phys. Chem. 1988,32, 53. (5) Fessenden, R. W.; Kamat, P. V. Chem. Phys. Lett. 1986,123,233. ( 6 ) (a) Kamat, P. V.; Gopidas, K. R.; Weir, D. Chem. Phys. Lett. 1988, 149, 491. (b) Wilkinson, F. J. Chem. SOC.,Faraday Trans. 2 1986,82, 2073. (7) Rossetti, R.; Brus, L. J. Phys. Chem. 1982,86, 4470. (8) (a) Kamat, P. V. J. Photochem. 1985,28, 513. (b) Kamat, P. V. J. Chem. SOC.,Faraday Trans. 1 1986,81, 509. (9) Kamat, P. V.; Gopidas, K. R. J.Photochem. Photobiol., A , sub-
mitted for publication. (10) (a) Kaneko, M.; Yamada, A. J. Phys. Chem. 1977,81,1213. (b) Rohtagi-Mukherjee, K. K.; Roy, M.; Bhowmik, B. B. Solar Energy 1983,
31, 417. (11) Kamat, P. V.; Fox, M. A. Chem. Phys. Lett. 1983,102,379. (12) Kamat, P. V.; Dimitrijevic, N. M.; Feasenden, R. W. J. Phys. Chem. 1987,91, 396.
Figure 2. Transient absorption spectrum recorded 80 ps after
M TiOz colloidal sus337-nm laser pulse excitation of 2 X pension in acetonitrile containing 1X lob M phenosafranin. Insert shows the absorption-time profile recorded at 520 nm. analytical reagents and were used as supplied. Absorption spectra were recorded with a Perkin Elmer 3840 diode array spectrophotometer. Emission spectra were recorded with a SLM photon counting fluorescence spectrometer. Flash photolysis experiments were performed with a 337-nm laser pulse (2-3 mJ pulse width 8 ns) from a Molectron UV-400nitrogen laser system or a 355-nm laser pulse (10 mJ, pulse width 6 ns) from a Quanta-Ray DCR-1 Nd:YAG laser system. The details of the flash photolysis apparatus and procedures are described elsewhere.13 All solutions were deaerated by bubbling with argon. Quantum yields of phenosafranin radical anion (PHNSO-) were measured from the flash photolysis experiments with a 337-nm laser pulse (for Ti02colloids) or a 355-nm laser pulse (for CdS colloids) as the excitation source and anthracene triplet as the reference (h= 0.7, €422 = 64700 M-l cm-' in cyclohexane").
Results and Discussion Absorption and Emission Characteristics. The absorption spectra of Ti02, CdS colloidal suspension, and phenosafranin in acetonitrile are shown in Figure 1. The TiOz colloids absorb strongly a t wavelengths < 350 nm while the CdS colloids absorb a t wavelengths < 500 nm. As described earlier,12the observed shift in the apparent absorption edge can be attributed to the size quantization effects. Phenosafranin has a strong absorption in the visible with an absorption maximum a t 517 nm (E = 5.4 X lo4 M-' ~ m - 9 .A~polar environment a t the surface of these semiconductor colloids facilitates the adsorption of the cationic dye. Such an adsorption on the semiconductor surface did not affect the absorption characteristics of phenosafranin. We have chosen 337- and 355-nm laser pulses as the excitation source for Ti02 and CdS colloids, (13) Das, P. K.; Encinas, M. V.; Small,R. D., Jr.; Scaiano, J. C. J.Am. Chem. SOC.1979,101,6965. (14) Amand, B.; Bensasson, R. Chem. Phys. Lett. 1976, 34, 44.
24 Langmuir, Vol. 5, No. 1, 1989
Gopidas and Kamat
respectively, as these colloids have strong absorption a t these wavelengths. Also, the negligibly small absorption due to phenosafranin a t these excitation wavelengths makes the band-gap excitation of semiconductor colloids more selective. Reduction of Phenosafranin in Colloidal TiO, Suspension. It has been shown earlier that colloidal TiO, upon band-gap irradiation can reduce compounds with a standard reduction potential > -0.5 V vs The efficiency of such a reduction process was found to be dependent on the energy difference between the conduction band of the semiconductor and the reduction potential of the substrate. Phenosafranin, which has a standard reduction potential of 0.252 V vs NHE at neutral pH,15 is energetically capable of scavenging conduction band electrons from an irradiated TiOz semiconductor. The transient absorption obtained upon 337-nm laser pulse excitation of a colloidal TiOz suspension containing phenosafranin is shown in Figure 2. The transient exhibited an absorption maximum at 440 nm with a simultaneous bleaching in the region of 520 nm. These transient absorption characteristics matched well with those of the radical anion of phenosafranin (PHNSO-). (PHNS'- was generated in a photochemical experiment by quenching PHNS* (TI)with triethylamine, and its absorption characteristics were r e c ~ r d e d . ~The ) transient was long lived in deaerated acetonitrile solutions and exhibited little decay during the period of 100 NS. Band-gap excitation of the semiconductor TiO, colloid ( E g 3.2 eV) led to the charge separation (reaction 1):
O
/I
I/[PHNS],IO4 M - '
Figure 3. Dependence of the inverse of the observed quantum yield of the photoelectrochemical production of PHNS- on the inverse of the phenosafranin concentration in acetonitrile containing 2 mM TiOz (excitation with 337-nm laser pulse).
0 5 t
-
hv X
< 380 nm*
TiOz (h+.-e-)
It has been s h o ~ n that ~ - ~while a major fraction of the charges separated a t the conduction and valence bands undergoes recombination, a small fraction gets trapped within its band structure. These charge carriers can be scavenged with a suitable redox couple such as phenosafranin to perform the necessary reduction and oxidation processes (reactions 2 and 3):
+ PHNS TiOz (h+) + i-PrOH TiOz (e-)
-
PHNS'-
(2)
products
(3)
2-Propanol, which is present in the colloidal Ti02 suspension, acts as a sacrificial donor in the present experiments. As can be seen from the insert in Figure 2, the formation of PHNS'- was prompt and was completed within the duration of the pulse (-10 ns). The absence of the slow growth of PHNS'- indicated nonparticipation of isopropyl radicals in the secondary reduction of phenosafranin. These results further confirmed that the interfacial electron transfer from the conduction band of the semiconductor to phenosafranin adsorbed on TiOz colloid was a one-electron reduction process. Dependence of Quantum Yield of bduction on the Dye Concentration. The role of TiOz colloid in the reduction of phenosafranin was further elucidated by varying the concentration of the dye. An increase in the PHNS'yield was observed as the concentration of phenosafranin was increased. Since the absorbance of the colloidal TiOz suspension at 337 nm did not change upon increasing the concentration of phenosafranin, it can be assumed that the extinction coefficients of the TiOz colloid and the associated complex of the TiO, colloid and phenosafranin are the same a t the excitation wavelength. As described (15) Clark, W. M. Oridation-Reduction Potentials of Organic Systems; Williams and Wilkins: Baltimore, 1960; p 415.
WAVELENGTH (nm)
Figure 4. Absorption spectra of 0.8 X lo4 M phenosafranin and 2 X M colloidal TiOz in acetonitrile during steady-state photolysis (340 nm) at different time intervals: (a) 0, (b) 5, (c) 15, (d) 25, and (e) 35 min.
earlier,@one can determine the association constant, K, p, for the association between TiOz colloid and the dye gy determining the quantum yield (4obd) a t different concentrations of phenosafranin and by using eq 4: (4) where 4 is the true quantum yield and K,[D]/(l + K,[D]) is the fraction of the dye present in the associated form. The linearity of the dependence of l/40bsdversus 1/ [PHNS] (Figure 3) shows that the associated complex between TiOz and phenosafranin is responsible for the production of PHNS'-. The values of 4 and Kappdetermined from this plot were 0.05 and 21 OOO M-l, respectively. The low value of 4 indicated that a major fraction of the conduction band electrons was not utilized in this reduction process but was lost in its recombination with valence band holes. Selective scavenging of the holes with a suitable redox couple (e.g., f3CN-P made it possible for us to enhance the efficiency of the reduction process. Steady-State Photolysis. If indeed colloidal Ti02 is responsible for the production of PHNS-, it should be possible to observe the leuco form of phenosafranin, which is formed under steady-state irradiation conditions. The semireduced forms of these dyes are known to undergo a disproportionation reaction (reaction 5) to yield leuco dye 2PHNS'-
-
PHNS2- + PHNS
(5)
and the parent dye. The leuco form of the dye (PHNS2-) is stable in an inert atmosphere such as Nz or Ar.
Photoelectrochemistry in Particulate Systems
Langmuir, Vol. 5, No. 1, 1989 25
a
0001 -
0000-.
~
400
I
3' 56C 60C
65C
750
70C
- 0 002
5w
?OO
600
BOO
\ P WAVELENGTH, nm
WAVELENGTH (nml
Figure 6: Transient absorption spectrum o€ 2 mM colloidal CdS and 2 X M phenosafranin in acetonitrile, recorded 4 I.LS after
the 355-nm laser pulse excitation.
00
o
1
.
2
3
4
5
6
7
a
9
[ PHNS] , IOm6M Figure 5. (a) Quenching of 1 mM colloidal CdS emission with phenosafranin in acetonitrile. Concentrationsof phenosafranin were (a) 0, (b) 0.5 X lo", (c) 1.5 X lo", (d) 3.5 X lo*, (e) 5.5 x lo", and (0 8.5 X 10" M. Excitation wavelength was at 355 nm. (b) Dependence of In (40/d)on the concentration of colloidal CdS (experimental conditions same as in Figure 5a).
The changes in the absorption spectra recorded after irradiating Ti02 suspension containing phenosafranin with a monochromatic light (340 nm) are shown in Figure 4. The bleaching a t 520 nm and simultaneous formation of a product with an absorption below 400 nm confirmed the conversion of phenosafranin to its leuco form. Regeneration of phenosafranin could be seen when the photolyzed product was exposed to air. Quenching of CdS Emission by Phenosafranin. CdS colloids prepared in acetonitrile-water mixtures exhibit intense red emission at wavelengths > 550 nm. The red emission of CdS colloids has been explained on the basis of a sulfur vacancy lying within the band structure of CdS. A detailed discussion on the emission of CdS colloids can be found elswhere.lE Since the interfacial charge transfer to a suitable substrate affects the emission process, one could use the decrease in emission yield to monitm the charge-transfer process. The quenching of the red emission by PHNS is shown in Figure 5a. As described earlier," such a quenching behavior can be treated with the model based on Poisson statistics and can be expressed as In (40/4) = dPHNSl/[CdSl
(6)
where 4o and 4 are the emission yields of CdS colloid in the absence and in the presence of PHNS, respectively, and 7 is the number of CdS molecules per colloidal particle. (16) Ramsden, J. J.; Webber, S.E.; Gratzel, M. J. Phys. Chem. 1986, 89, 2740. (17)(a) Ramsden, J. J.; Gratzel, M. J. Chem. SOC.,Faraday Trans. 1 1984,80,919. (b) Turro, N. J.; Yekta, A. J. Am. Chem. SOC.1978,100, 5951.
The dependence of In (do/$) on the concentration of PHNS is shown in Figure 5b. The linearity of this plot confirmed the validity of the Poisson statistics in treating the quenching data. The value of 7 determined from this plot was 220 and was in good agreement with the aggregation number of 160-780 determined previously for CdS colloids prepared in acetonitrile.12 Reduction of Phenosafranin in Colloidal CdS Suspension. A transient absorption spectrum recorded after the 355-nm laser pulse excitation of a colloidal CdS suspension containing phenosafranin is shown in Figure 6. The transient exhibited absorption maxima at 400,440, 690, and >860 nm and a bleaching in the region of 520 nm. As observed in the case of colloidal Ti02, absorption at 440 and >860 nm and bleaching a t 520 nm can be attributed to the formation of a radical anion of phenosafranin (PHNS-) CdS -kCdS (h+.-e-)
-
(7)
CdS (e-) + PHNS PHNS'. (8) Again, the formation of PHNS*- was prompt and was completed within the duration of the laser pulse (
