Multielectron storage and hydrogen generation with

Proc. Natl Acad. Sci. USA Vol. 80, pp. 3129-3132, May 1983 Chemistry

Multielectron storage and hydrogen generation with colloidal semiconductors (energy conversion/charge transfer/semiconducting particles/amphiphilic viologens/photoreactions)

MICHAEL GRATZEL AND JACQUES MOSER Institut de Chimie Physique, Ecole Polytechnique FMderale, CH-1015 Lausanne, Switzerland Communicated by Melvin Calvin, January 17, 1983

ABSTRACT Multielectron storage and hydrogen generation by light is achieved in aqueous dispersions of ultrafine TiO2 particles (120-A diameter) when the amphiphilic viologen derivative N-tetradecyl-N'-methyl-4,4'-dipyridinium dichloride (CM4MV21) is used as an electron relay. Consecutive reduction of C14MV2+ to the radical ion (C14MV+) and neutral (C14MV0) was observed after band-gap excitation of the semiconductor particle. Through surface adsorption of the relay, these electron-transfer reactions can occur very rapidly and are completed within less than 100 pisec (pH 11). Two-electron reduction of C14MV2+ can be coupled with H2 generation in alkaline medium in the presence of Pt catalyst codeposited onto the TiO2 particle. Electron-relay-free systems are 1/15th as efficient in producing H2 at the same pH.

Colloidal semiconductors have several advantageous features (1-7) that make them attractive candidates to be used as lightharvesting units in solar energy devices. The idea of using colloidal semiconductors in photoconversion devices has been discussed by Nozik (8, 9). The concept of photoelectrochemical reactions on semiconductor particles dates back to the discoveries of Emil Baur (10). For a recent review, see ref. 11. Particularly intriguing is the possibility of surface modification of the semiconductor particle by chemisorption, derivatization, or catalyst deposition to achieve light-induced charge separation (12) and subsequent fuel-generating dark reactions. The present study illustrates successful molecular engineering with such systems. By using aqueous dispersions of ultrafine TiO2 particles in conjunction with the amphiphilic redox relay,

CH3-(CH2)l-

ruby laser. An Osram XBO 450-W Xe lamp equipped with a water jacket was used for continuous illuminations. C14MV2+ was synthesized as described (13). RESULTS AND DISCUSSION Illumination of an aqueous TiO2 sol containing C14MV2+ with light of wavelength A > 300 nm results in the immediate appearance of a blue color, which can be attributed to formation of viologen cation radicals (C14MV+) (Fig. 1). However, this color does not persist under continuing exposure to light. A striking and rather rapid change from blue to intense yellow is noted. The resultant spectrum, also shown in Fig. 1, exhibits a weak absorption at 500 nm and a strong peak at 380 nm. Whereas the former absorption can be attributed to viologen dimer radicals (C14MV+)2 (14) present at low concentration, the latter was identified as doubly reduced viologen-i.e., 1-methyl1'-tetradecyl-4,4'-bipyridylene (C14MV0). The two-electron re-

duction of the analogous compound methylviologen by chemical (15, 16) and electrochemical (17, 18) means yields dimethylbipyridylene with similar spectral features [solvent, acetonitrile: Ama, 404 nm, 340 sh, 315 nm (19); solvent, cyclohexane: Amax 348 nm (log E = 4.77), 377 nm (log e = 4.66) (20); Ama 400 nm, 376 sh (16)]. To substantiate the assignment of the 380-nm peak to C14MV0, two-electron reduction of C14MV2+ was performed chemically by using dithionite as a reductant (16) and aqueous micellar solutions of Triton X-100 as a reaction medium. The spectrum of the product obtained has features (Fig. 2) identical with those of the photoproduct shown in Fig. 1 except that the absorption maximum of the bipyridylene is at 400 instead of 380 nm. We attribute this shift of the absorption maximum to adsorption of the doubly reduced viologen to the surface of the TiO2 particles. Spectral shifts due to adsorption onto mineral supports have been shown for a large number of chromophores including Ru(bipy)2+ (21) and other transition metal ion complexes (22). Independent experiments involving oxygen confirmed the two-electron nature of the C14MV2' reduction in the colloidal TiO2 dispersion. When a solution containing the yellow product of C14MV2+ photoreduction by colloidal TiO2 is exposed to air, the blue color of C14MV2+ is formed first, which subsequently fades to yield colorless C14MV2+. (This reduction-reoxidation cycle can be repeated many times.) Apparently the reoxidation of C14MV0 occurs in two distinct steps, C14MV0 + 02 -* C14MV+ + 02 [1] C14MV+ + 02 - C14MV2 + [2] Interestingly, the first reaction was found to occur much less rapidly than the second one, showing that C14MV0, despite its more reducing nature, is less reactive towards 02 than is C14MV+.

Q--H3 (C14MV2+),

striking electron storage and hydrogen generation effects are demonstrated. MATERIALS AND METHODS Colloidal solutions of TiO2 were prepared by slowly adding 5 g of TiC14 (Fluka purissimum) distilled under vacuum to 200 ml of water at 0°C. The solution was dialyzed until the pH reached a value of ca. 3. Precise determination of the TiO2 content was carried out as described (5). Application of electron microscopy and quasielastic light-scattering technique showed that the sol consisted of 110-A-diameter particles. At pH > 3, poly(vinyl alcohol) (PVA) was used to stabilize the colloidal particles. Commercial PVA (Mowiol 98110, Hoechst, Federal Republic of Germany) was pretreated by UV light (5) to remove impurities that absorb 347.1-nm light. Laser photolysis experiments were carried out with 347.1-nm pulses (10-nsec duration) of a JK-2000 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Abbreviation: PVA, poly(vinyl alcohol). 3129

3130

Chemistry: Gratzel and Moser

Proc. Nad Acad. Sci. USA 80 (1983) pH = 11.0,

=

I

2.0

602nm

l

I I___ _. __ I___ ___ _A

A 0.8

LASER PULSE

0.6

1.0

0.4

Irradiation lamp 450W, 1min 30sec .____ Irradiation lamp 450W, 10sec 0.2

0.0 400

500

k [nm]

600

700

FIG. 1. Spectral changes observed upon illumination (A > 300 nm) of a solution of TiO2 (500 mg/liter) containing C14MV2+ (0.2 mM) and PVA (1 g/liter; Mowiol 10-98). Left ordinate scale; right ordinate scale. A solution of TiO2 (500 mg/liter; pH 11) was used in the reference light beam. (Inset) Laser flash photolysis (347.1 nm) of the preirradiated (1 min, 30 sec) solution: temporal behavior of the 602-nm absorption of the C14MV+ radical. All solutions were flushed with nitrogen prior to irradiation. ---,

-,

The mechanism of light-induced C14MV0 formation involves band-gap excitation of TiO2 particles generating electron-hole pairs (Fig. 3). Conduction band electrons, eCB, reduce C14MV2+ first to the radical cation

concentration does not appear to undergo two-electron reduction at pH ' 12.5. Only at high alkalinity does slow formation of doubly reduced product become apparent. Apart from kinetic reasons, there are thermodynamic reasons for this drasti-

ki ecB + C14MV2+ -C14MV+

cally different behavior of the two electron relays. The second reduction of methylviologen requires a potential of -0.83 V (vs. the normal hydrogen electrode). Taking into account that the conduction-band position of the TiO2 particle changes with pH according to ECB = -0.11-0.059 pH (2), one predicts that a pH of at least 12 is required to render this process thermodynamically feasible. In the case of the amphiphilic viologen, adsorption to the surface of TiO2 apparently shifts the potential of the C14MV+/Cl4MV0 redox couple by more than 100 mV, facilitating the second reduction step. Blank photolysis experiments with solutions of C14MV2' did not lead to appearance of C14MV0 or C14MV+, showing that light absorption by TiO2 particles produces the electron-transfer events. As to the valence-band process occurring in parallel with the reaction of conduction-band electrons, it is indicated in Fig. 2 that this comprises oxidation of water to oxygen, competing with hole-scavenging by the PVA polymer used to stabilize the sol. However, the latter process has been shown to be rather inefficient on TiO2 colloids (5). Therefore, hole reaction with water prevails even in the presence of PVA. [TiO2 acts as an oxygen carrier. From our own experiments (unpublished data), we conclude that the capacity of the carrier is ca. 20 ,ul of 02 per mg of TiO2.] However, it cannot be excluded that intermediates from the water oxidation attack subsequently the protective polymer. So far we have not analyzed these processes in more detail, our main interest being the storage of conductionband electrons by the adsorbed relay.

[3]

and subsequently to the bipyridylene

k2

C14MV+ + ecB C14MV0. [4] Adhesion of the amphiphilic relay to the surface of TiO2 particles allows both electron-transfer processes to occur at a very rapid rate. Application of the laser photolysis technique yields for pH 11 the rate constants k1 = 10' sec-1 and k2 = 5 x 104 sec 1.* Therefore, two-electron reduction is completed in less than 100 ,usec. This contrasts sharply with the behavior of simple methylviologent which at comparable light intensity and Because the reduction of ec( with C14MV2+ (C14MV+) occurs on individual TiO2particles, it fits in the general framework of fast electrontransfer processes in colloidal aggregates, which has been elaborated upon previously (23). Similar to intramicellar electron-transfer processes, such intraparticle reactions follow first-order kinetics. The observed rate constant is expressed in units of sec- and is expected to increase with C14MV2+ association of the particle until saturation of surface sites available to the electron relay is reached. Under our experimental conditions (temperature, pH, TiO2 concentration), the saturation is practically reached at 0.2 mM C14MV2+. t Simple methylviologen is not adsorbed to the TiO2 surface (2). The rate of electron transfer in this case is limited by the diffusional approach of the reactants. For both C14MV2' and MV2', the quantum yield of radical ion formation is approximately 1.

*

Proc. Natl. Acad. Sci. USA 80 (1983)


3131

A 0.8

F

0.6

F

CH3- (OH, )13-

N~tN

-

CH3

0.4 1-

0.2 h

0.0

500

400

AS [nm]

600

700

FIG. 2. UV/visible absorption spectrum of doubly reduced C14MV2" in Triton X-100 micellar solution.

A third valence-band process, which sets in once a major fraction of C14MV2" has undergone two-electron reduction, is reoxidation (hV = holes) of the doubly reduced product to the Colloidal particle Ti 02 \

00 C"4 > 2e

MV2+

C14 MV ,

,' .

....... I..........

I.......

..........

..... I.....

(PVAt)1O 2

2

+ 2H '

)

C14

H29-N

N -CI

C14

H29- Nc

C14 H29- N

13

E° (N H E) - 0.446 V

N-CH E3 &e

(C14MV0)

[5]

Laser photolysis technique was applied to preirradiated C14MV2+/TiO2 solutions in order to monitor this reaction. Fig. 1 Inset shows the temporal behavior of the absorption at 602 nm. The sharp rise in the signal after the laser pulse is due to C14MV+ formation by reaction 5, whereas the subsequent absorption decay indicates the time course of C14MV+ reduction by conduction-band electrons (Eq. 4). In accordance with this interpretation, one finds that the 380-nm absorption behaves as the exact mirror image of the optical events at 602 nm: rapid bleaching after the laser pulse is followed by recovery of the original absorption, which obeys the same time law as the 602-

decay. One concludes that, under illumination of TiO2/

C14MV2+ solutions, a state is approached where cyclic electron transfer (i.e., the sequence of reactions 4 and 5) occurring on individual TiO2 particles maintains the electron relay in the doubly reduced form. Attempts to exploit this two-electron storage system to achieve light-induced hydrogen generation in alkaline aqueous medium |e

(Cl4MV )

h+ + C14MV0 -> C14MV+.

nm

(PVA)

(C14 MV

viologen radical:

E' (NHHE) N

-

0.820 V

CH3

FIG. 3. Scheme for two-electron reduction of C14MV2"; the redox potentials given refer to simple methylviologen (MVh). CB, conduction band; VB, valence band; NHE, normal hydrogen electrode.

gave very encouraging results. TiO2 particles (500 mg/liter) loaded with 1.5% Pt by a published procedure (2) were used in these experiments. The solution volume was 5 ml, and the pH was adjusted to 12 with NaOH. Vigorous hydrogen generation (0.5 ml/hr) was observed in C14MV2+-containing samples, while blank experiments with TiO2/Pt dispersions in the absence of C14MV2+ yielded only 35 ,ul of H2 per hr. Apparently, the cooperation of functional electron relay and catalyst on the surface of the semiconductor particle gives much better yields of hydrogen than do catalyst deposits alone. In conclusion, we have illustrated two-electron storage and hydrogen generation by light in colloidal semiconductor dispersions, using an amphiphilic viologen as an electron relay.

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