10 Surface Aspects of Hydrogen Photogeneration on Titanium Oxides F. T. WAGNER, S. FERRER, and G. A. SOMORJAI
Downloaded by FUDAN UNIV on December 20, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch010
Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, CA 94720
Strontium titanate and titanium dioxide have received considerable attention as materials for photoanodes and photocatalysts in the dissociation of water (1,2,3), and in other photoassisted reactions. Knowledge of how the surface composition and electronic structure of these materials change under illumination when in contact with gases or liquid electrolytes is essential i f detailed understanding of the mechanisms of semiconductor photochemistry is to be achieved. Although these wide bandgap oxides do not exhibit gross photocorrosion under most reaction conditions (2) and would appear less susceptable to possible Fermi-level pinning than many semiconductors with smaller bandgaps (4), more subtle surface chemical effects have been documented. Evidence for photocorrosion (5,6,7), surface-state mediation of electron and hole transfer to electrolyte species (8,9), and the dependence of quantum efficiencies on surface preparation techniques (10), indicate important roles for surface species on wide bandgap materials. Most detailed studies of water photodissociation on SrTiO 3
and TiO have concentrated on photoelectrochemical cells (PEC cells) operating under conditions of optimum efficiency, that is with an external potential applied between the photoanode and 2
counterelectrode. We have become i n t e r e s t e d i n understanding and improving r e a c t i o n k i n e t i c s under c o n d i t i o n s of zero a p p l i e d p o t e n t i a l . Operation at zero a p p l i e d p o t e n t i a l permits simpler e l e c t r o d e c o n f i g u r a t i o n s (11) and i s e s s e n t i a l to the development of photochemistry at the gas-semiconductor i n t e r f a c e . Reactions at the gas-sold, rather than l i q u i d - s o l i d , i n t e r f a c e might permit the use o f m a t e r i a l s which photocorrode i n aqueous e l e c t r o l y t e . The g a s - s o l i d i n t e r f a c e i s a l s o more amenable to the a p p l i c a t i o n of u l t r a h i g h vacuum surface a n a l y t i c a l techniques. In t h i s paper the hydroxide concentration dependence of the r a t e of hydrogen production i n S r T i 0 systems (JL2) i s discussed i n l i g h t of s u r f a c e a n a l y t i c a l r e s u l t s . The s u r f a c e elemental composition before and a f t e r i l l u m i n a t i o n i n various aqueous e l e c t r o l y t e s has been monitored with Auger e l e c t r o n spectroscopy 3
0097-6156/81/0146-0159$05.00/0 © 1981 American Chemical Society Nozik; Photoeffects at Semiconductor-Electrolyte Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
160
PHOTOEFFECTS AT SEMICONDUCTOR-ELECTROLYTE
INTERFACES
and i s compared with the composition obtained by u l t r a h i g h vacuum s u r f a c e p r e p a r a t i o n techniques. Auger spectroscopy, while l e s s s e n s i t i v e than photoelectron spectroscopies to s u b t l e changes i n the o x i d a t i o n s t a t e s o f s u r f a c e s p e c i e s , i s more e a s i l y a p p l i e d to the i m p e r f e c t l y c l e a n surfaces obtained i n b a s i c aqueous e l e c t r o l y t e s using present technology. Carbon and s i l i c o n i m p u r i t i e s are found on surfaces exposed to e l e c t r o l y t e s ; the carbonaceous species have some f i l l e d s t a t e s which may make them e f f e c t i v e f o r the mediation o f charge t r a n s f e r across the i n t e r f a c e . The e f f e c t s o f s u r f a c e p l a t i n i z a t i o n on p h o t o a c t i v i t y a r e discussed and evidence f o r a thermal r e a c t i o n between Pd and T i 0 surfaces i s given. (13) A hydrogen-producing s t o i c h i o m e t r i c photoreaction occurs between pre-reduced S r T i 0 and ^.O" T o r r water vapor.(14) At these low pressures surface T I and hydroxyl species can be observed by photoelectron s p e c t r o s c o p i e s . Comparison of the r e a c t i o n c o n d i t i o n s r e q u i r e d f o r hydrogen photogeneration from low pressure water vapor and from aqueous e l e c t r o l y t e allows some s p e c u l a t i o n as to the r o l e s of hydroxyl s p e c i e s .
Downloaded by FUDAN UNIV on December 20, 2016 | http://pubs.acs.org Publication Date: March 2, 1981 | doi: 10.1021/bk-1981-0146.ch010
2
7
3
3
II.
Experimental
I I . 1 . L i q u i d - s o l i d I n t e r f a c e Experiments. S i n g l e c r y s t a l wafers f o r experiments i n l i q u i d e l e c t r o l y t e were cut to w i t h i n 2% o f the (111) face and etched 3-5 minutes i n molten NaOH h e l d i n a gold l i n e d c r u c i b l e . Wafers were then r i n s e d i n water, soaked 5 minutes i n aqua r e g i a , r i n s e d , soaked 5 minutes i n high p u r i t y 35% aqueous NaOH (Apache SP 7329), r i n s e d i n 7M-ft t r i p l y d i s t i l l e d water, and a i r d r i e d . L i q u i d phase hydrogen photogeneration experiments were c a r r i e d out with a gas chromatographic d e t e c t i o n system described i n more d e t a i l elsewhere.(12) C r y s t a l s rested i n a 2-10 ml pool of e l e c t r o l y t e w i t h i n a b o r o s i l i c a t e glass vacuum f l a s k . The det e c t i o n system was s e n s i t i v e t o r a t e s of a t l e a s t 5 x 1 0 molecules H /hr-cm S r T i 0 (=5 monolayers/hr), but slow r a t e s of oxygen production could not be followed. A 500W high pressure mercury lamp provided a f l u x of bandgap photons o f 1 0 c m s" . The e l e c t r o l y t e f o r most experiments was compounded from r e agent-grade m a t e r i a l s and low c o n d u c t i v i t y water. However, i n some experiments high p u r i t y (Apache SP7329 35% NaOH) or u l t r a p u r i t y ( A l f a 87864 30% NaOH) s o l u t i o n s were employed. Glassware f o r these experiments was prepared by soaking i n 1:1 H S0
