ESR study on the interaction of water vapour with polycrystalline TiO2 under illumination

Colloids and Surfaces, 13 (1985) 287-293 Elsevier Science Publishers B.V., Amsterdam -Printed

287 in The Netherlands

ESR STUDY ON THE INTERACTION OF WATER VAPOUR WITH POLYCRYSTALLINE TiOz UNDER ILLUMINATION

EWA SERWICKA Institute of Catalysis and Surface Chemistry, Polish Academy Niezapominajek, 30-239 Krakow (Poland)

of Sciences, ul.

(Received 27 October 1983; accepted in final form 28 June 1984)

ABSTRACT Electron spin resonance has been used to study the interaction of water vapour with polycrystalline TiO, (anatase) under illumination. At 77 K, the illumination of TiO, containing adsorbed water leads to the appearance of a strong ESR spectrum characterized by g, = 2.036 and g, = 2.003, which is tentatively ascribed to the HO, radical. Room-temperature illumination of the TiO,-water vapour system results in the formation of a spectrum with g1 = 2.022, g, = 2.009 and g, = 2.003, which is ascribed to the 0; radical. The influence of the degree of reduction of TiO, on the observed photoeffects was examined. The use of light of wavelengths G 380 nm was necessary in order to observe radical formation. The results are interpreted in terms of the photocatalytic decomposition of water vapour at the TiO, surface.

INTRODUCTION

The photocatalytic decomposition of gaseous or liquid water on semiconducting solids has attracted much attention recently. Photocatalysts such as platinized TiOz and TiOz-RuOz mixtures are widely used [l-5]. There are, however, contradictory results concerning the photocatalytic properties of pure TiOz powders. Schrauzer and Guth reported that small amounts of H2 and O2 were produced by the decomposition of gaseous water over a TiOz catalyst [6]. Evolution of Hz and HzOz was observed by Rao et al. for Ti02 suspended in water [ 71. By means of spin trapping and ESR, Jaeger and Bard found the spin adducts of OH and HO2 radicals in illuminated aqueous suspensions of TiOz and concluded that photodecomposition of water did occur [8]. On the other hand, the studies of Kawai and Sakata [l], Van Damme and Hall [9], and Sato and White [2] indicate that pure TiOz is inactive in the photolysis of water and that the use of Pt and/or RuO, additives is necessary to enhance the photocatalysis. It seemed worthwile to investigate the nature of the interaction between This paper was presented at the IIIrd International Symposium on Magnetic Resonance in Colloid and Interface Science, Torufi, 29 June-2 July, 1983.

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pure TiOz and water vapour under illumination by means of ESR spectroscopy, which is a powerful technique for studying radiation-induced surface defects and adsorbed radicals. EXPERIMENTAL

Materials For each experiment, a new 0.0250-g sample of TiOz powder (anatase, optipur, Merck) was used. After pretreatment in air at 673 K for 2 h, the sample was activated in vacua (~10~~ Pa) at the same temperature. The degree of reduction depended on the duration of the vacuum treatment (0.5-6 h). Subsequently the sample was inserted into the ESR cavity whilst still connected to the vacuum line, which was equipped with greaseless stopcocks. Water used in the experiments was deionized, doubly distilled and purified by several freeze-pump thaw cycles. Adsorption Water vapour was introduced at room temperature under a pressure of 2.5 X lo3 Pa and remained in contact with the TiOz powder for 0.5 h. Subsequently the samples were evacuated down to -10-l Pa and the ESR spectra were taken. ESR analysis The ESR spectra were recorded at 77 K and at room temperature using a Varian E-line Century Series ESR spectrometer model E-112 equipped with a field dial, operating in the X-band mode (made available by the Institute of Physical Chemistry of Kiel University, F.R.G.). DPPH was used as a standard for g factor determination. Spin densities were estimated using a VOSO, standard. Illumination

experiments

The samples were illuminated in situ with a high pressure 450 W Xenon lamp, its light being focused onto an optical transmission cavity type E-234. A set of cut-off filters was used to check the wavelength dependence of the observed phenomena. RESULTS

AND DISCUSSION

A typical ESR spectrum of TiO, after reduction in vacua, registered at 77 K, is shown in Fig. 1 (solid line). It consists of two signals. The broad absorption centered around g = 1.967, AH = 4 mT, is characteristic for

Ti3+ ions (-1017 per gram) in a TiOz matrix. The other, with g = 2.003, is attributed to a bulk defect, probably an electron trapped on an oxygen vacancy [lo]. The intensity of the Ti3+ signal depended on the duration of the reducing treatment. With up to 4 h of reduction, the signal increased. With longer treatment, the height of the signal decreased slowly. Simultaneously the peak-to-peak width became larger. Both changes indicate the increasing dipole--dipole interaction of paramagnetic Ti3+ ions in more strongly reduced samples.

Fig. (- -

1. -)

ESR spectrum of reduced after H,O vapour adsorption.

(TiO, (anatase): Spectra were registered

) before

contact

with

H,O;

at 77 K.

Following contact with water vapour, no new signals appeared in the ESR spectrum taken at 77 K, but the intensity of Ti3+ decreased distinctly (Fig. 1, dashed line). Illumination at 77 K of the slightly reduced samples (up to 1 h of vacuum treatment) resulted in the immediate appearance of a new ESR absorption characterized by g, = 2.036 and g, = 2.003 (Fig. 2). The species responsible for this signal persisted in the dark (Fig. 3), but became unstable on warming up to room temperature. The new signal is tentatively ascribed to the hydroperoxyl radical HO*. The observed values of the g factor components are close to those characteristic for HO* radicals in solid matrices (g, = 2.039, g, = 2.004 in argon) [ll]. A feature of the solid-state spectrum is the small magnitude in the ‘H hyperfine splitting. In the present case the hyperfine structure was not resolved; this probably contributed to the line width. A similar signal, also attributed to the HO2 radical, was observed by Gonzalez-Elipe et al. after photoadsorption of oxygen on a hydrated TiOz surface [12]. No HO2 radical formation was observed either on the strongly reduced samples or on the unreduced Ti02. The experiments with cut-off filters showed that the onset of the photoeffect occurred at wavelengths < 380 nm (E > 3.26 eV). This may be compared with the band gap of anatase, E, = 3.23 eV [13]. It is clear that the transition across the band gap is necessary to induce the formation of HO* radicals.

290

Room-temperature illumination of the TiOz samples containing adsorbed Hz0 did not result in any immediate change in the ESR spectrum. However, prolonged illumination during the period of time, longer than 6 h, when TiO, interacted with water vapour under a pressure of -1 Pa, led to the appearance of a new paramagnetic species in the ESR spectrum (Fig. 4). This signal, with g, = 2.022, g, = 2.009 and g, = 2.003, is identical with the spectrum of the 0; radical obtained after oxygen adsorption on the same catalyst pretreated in vacua [lo]. No radicals were formed in the absence of water vapour. Also, in this case, the photoeffect depended on the energy of light quanta and occurred upon band gap excitation of the anatase sample. 2.003 I

I

Fig. 2. ESR spectrum of TiO, was registered at 77 K.

5 mT

containing

adsorbed

water,

illuminated

at 77 K. Spectrum

One may speculate on the possible reactions which could give rise to the formation of the observed HOz and 0, radicals. Irradiation results in the creation of photoelectrons and photoholes, which in turn may react with the products of the dissociative adsorption of water, i.e., protons and hydroxyl groups. The potential for oxidation of OH- to the free OH radical lies above the valence band edge of TiO 2, so that a photogenerated hole would be sufficiently energetic to produce this species [S]. Indeed, the observed wavelength dependence of the photoeffects indicates that, in

291

0

IO

5

15 time

Fig. 3. Photocreation

20 [mfn]

of the HO, radical at 77 K. 5 mT I

1

2.009

2.022

I

A

A/l 2.003

Fig. 4. ESR spectrum of TiO, illuminated for 10 h at room temperature in an atmosphere of water vapour (-1 Pa). Spectrum was registered at 77 K after evacuation down to 1O-‘-lO-2 Pa.

292

both cases, the primary process must be connected with the reaction Of photoholes with the hydroxyl groups at the surface to give the intermediate OH radicals. The latter, being highly reactive, undergo further transformation and may give rise to such reactions as OH’ +OH’ + H202 H202 + OH’ + HZ0 + HO; H,02 + h+ + H+ + HO;

(I) (2) (3)

or Hz02 --f HZ0 + l/2 0, + e + 0;

O2

(4) (5)

The appearance of the HO* species after illumination at 77 K suggests that the OH radiacal are mobile enough to diffuse and react even at that temperature. This would explain the lack of any signal which might be attributed for samples irradiated at to these species. No evidence for 0; was found 77 K. This is consistent with the data obtained by Gonzalez-Elipe et al. for oxygen photoadsorption on hydrated anatase [ 121. They demonstrated that the concentration of 0; depends on the degree of hydration of the Ti02 surface. In strongly hydrated samples, the formation of 0; is followed by protonation of these species according to the following equation 0;

+H++

HO;

(6)

where protons originate from the more acidic hydroxyl groups that are present at the surface in great excess. It is conceivable that such a situation exists in the experimental conditions at 77 K, where water vapour has been condensed at the catalyst surface, providing a highly hydrated sample. After illumination at 293 K no HO, radical could be seen in the ESR spectrum. This result is understandable, since the experiment shows that this species is unstable at room temperature. On the other hand, the appearance of the 0; radical after prolonged illumination indicates that the concentration of the acidic hydroxyl groups must have become low enough to let this species stabilize at the TiO, surface, This may be due to the lower initial sample hydration in comparison with the experiment performed at 77 K. Also, some other photoprocesses involving protons, e.g. hydrogen evolution, may compete with reaction (6), leading to the photodehydroxylation of the TiO* surface. The data presented in this study show that the photodecomposition of water vapour on pure anatase powder does occur and that the oxygen involved in the radical formation originates from the oxygen of water. The appearance of HO* and 0; in the ESR spectrum of samples illuminated at 77 and 293 K, respectively, leads to the conclusion that oxygen evolved in the course of water cleavage is easily reduced at the Ti02 surface. Both radicals are relatively stable. They persist in the dark and do not change upon evacuation, indicating that they are strongly bonded to the surface.

This could be the reason for the decay of photocatalytic activity of pure TiOz observed by others [6]. Recently, it has been demonstrated by Kawai and Sakata that TiOz powders mixed with RuOz, which is known to be one of the best electrode materials for oxygen evolution with the lowest excess potential, showed markedly improved catalytic efficiency [ 11. It is worthwhile noting the lack of radical formation on strongly reduced TiOz samples. This could be connected with the increased recombination of the cryst,alrate of the photogenerated carriers. Also, the occurrence lographic shear effect in the near-to-surface layers of heavily reduced Ti02, leading to the formation of lower titanium oxides, may contribute to the change in the photocatalytic properties of these samples. ACKNOWLEDGEMENT

The author would Humboldt Foundation this paper possible.

like to express her gratitude to the Alexander von for the research grant which made completion of

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