Langmuir 1988,4, 1156-1159
1156
Regardless of the term considered, we find that p varies as d when t = T - T,tends toward zero: p=pD+pA-d; t-0 (10) We also find experimentally that p varies as I’2l and, by reasoning, as d . We may conclude that d varies as rzland diverges as this term when t tends toward zero:
d
-
Itl-”+fl
(11)
It is of interest to note that the same result may be obtained in an even more simple manner: recalling that
d = v*/A“
(12)
where
A“ =
(r,+ r2)-1=
area for a mole of mixture in the surface and as vu cannot tend toward zero or infinity (as t is tending toward zero), one sees that d varies as (A“)-’
-
-
d (A“)-’; t 0 (13) Next, consider the behavior of A“. As the term
rZl= r2- rl(X2a/Xla) tends toward infinity, it is seen that rl and/or r2tend also toward infinity. A“, therefore, tends toward zero as Itl-*@ (or (A“)-’ r2J,as does d :
-
d
- (A“)-’ d
-
I’21
(tl-”+fl
(14) (15)
Finally, for a critical interface, the thickness d varies as the bulk correlation length 5
d
-5 -
Itl-”
(16)
However this is not the case here, just because our interface is not a critical one. In the surface phase one may visualize two parts. One part, in contact with the liquid phase in a critical state, has a probable very smooth concentration profile, which explains the total great thickness of the surface phase.’ The other part, in contact with the noncritical gas phase, has a sharper concentration profile, with a weak contribution to the thickness of the surface phase.5 The experimental result expressed by eq 15 may be a guide to choosing a concentration profile if one needs to: it will be necessary to include in such a profile a term in z-lI2 characterizing the contact with a critical phase14 and an additional term characterizing the contact with a noncritical gas phase as e ~ p ( - z / [ G ) . ~
Conclusion The liquid-vapor surface phase for the water-2,6lutidine system in the liquid monophase and near the critical point of this liquid phase has the critical behavior theoretically predicted by h o s Gomez and Widom and Fisher and de Gennes for the relative excess: a divergence as Itl-”+O. In addition, the analysis of the ellipticity coefficient yields an approximate law for the variation of the thickness of the surface phase with the reduced temperature: the thickness diverges as Itl-”+@, i.e., as the relative excess. This divergence is less than for a critical interface that varies as Itl-”. This law corresponds qualitatively with the comgiven for such an interface. position profiles in Registry No. 2,6-Lutidine, 108-48-5.
Effects of Surface Modification with Silicon Oxides on the Photochemical Properties of Powdered Ti02 Shinri Sat0 Research Institute for Catalysis, Hokkaido University, Sapporo, 060 Japan Received November 11, 1987. In Final Form: April 25, 1988 Surface hydroxyl groups of powdered Ti02 were replaced with silicon oxides to investigate their role in photocatalytic reactions. The replacement was monitored by IR spectroscopy. Photocatalytic activity of the surface-modifiedTi02as well as the nonmodified one was measured for CO and C2H, photooxidation, oxygen isotope exchange between Ti02and ‘802 (GSOIE), O2photoadsorption, O2evolution from an aqueous AgNO, solution, and H2 evolution from an aqueous CHBOHsolution in the presence of Pt. Although the surface modification gave rise to a decrease in the activity for these photoreactions, ita extent was smaller than the extent of the surface hydroxyls elimination. A marked decrease in the rate of GSOIE was observed on the modified Ti02,since the reaction involves the oxygen in the hydroxyls of Ti02. However, the rate constant of GSOIE was influenced little by the modification, suggesting that the hydroxyls participate little in the photoactivation of dioxygen. Introduction Metal oxide semiconductors are covered, more or less, with surface hydroxyls, and the hydroxyls have been thought to play an important role in some photocatalytic reactions. For example, Boonstra and Matsaers’ and (1) Boonstra, A. H.;Mutsaers, C. A. H. A. J. Phys. Chem. 1975, 79, 1694.
0743-746318812404-1156$01.50/0
Munuera et a1.2 have reported that the photoadsorption of oxygen on highly hydroxylated Ti02 decreases with reducing amount of the hydroxyls by calcination. Oosawa and Graetzel,, on the other hand, have reported that the (2) Munuera, G.; Rives-Amau,V.; Saucedo, A. J.Chem. SOC.,Faraday Trans. 1 1979, 75, 736. (3) Oosawa, Y . ; Graetzel, M. J. Chem. SOC.,Chem. Commun. 1984, 1629.
0 1988 American Chemical Society
, Langmuir, Vol. 4, No. 5, 1988 1157
Effects of Surface Modification of Powdered TiOz
'
rate of photocatalytic oxygen evolution from a T i 0 2 suspension in an aqueous AgN03 solution is enhanced by elevating the calcination temperature of the Ti02 sample from 500 to 900 "C,and they ascribed this result to a decrease in the concentration of surface hydroxyls on TiOP In photoelectrochemical cells, it has been proposed that the surface hydroxyls mediate electron transfer from the T i 0 2 surface to electron acceptors in a s o l ~ t i o n . It ~ ~is,~ however, noteworthy that the surface modification of a Ti02 photoelectrode with organosilane has little effect on semiconductor properties: though this modification also reduces surface hydroxyls density. While many arguments have been made, few papers have presented evidence enough to prove direct participation of the hydroxyls in photoreactions. Although the calcination of powdered sample was often used to reduce the concentration of surface hydroxyls on metal oxide surfaces, it may lead to a change in the surface and the bulk properties of semiconductors, which also affect the photocatalytic activity of semiconductors. It seems to us that a new and unexpected result in semiconductor photocatalysis experiments tends to be ascribed to the role of surface hydroxyls even though another explanation is possible. In the present experiments, the surface hydroxyls of Ti02 were replaced with silicon oxides to eliminate their effects on photocatalytic reactions. This modification of semiconductor surface may give rise to some loss of the photocatalytic activity, since the silicone oxides would be an insulator and would retard electron transfer to some extent. However, if the hydroxyls play an indispensable role in a reaction, then the modification could give a much larger effect on the reaction rate than in the simple insulation case. The reactions for which photocatalytic activity was measured were CO and C2Hs oxidation, oxygen isotope exchange between leg2and TiOz (gas-olid oxygen isotope exchange, GSOIE),O2 adsorption, O2evolution from an aqueous AgN03 solution, and H2 evolution from an aqueous CH30H solution. The role of the hydroxyls in each photoreaction was discussed on the basis of the activity difference before and after the modification.
Experimental Section The TiO, used in the present experiments was supplied by De(P-25),Nippon Aerosil (P-25): and Fuji Titanium (TP-2). The Ti02sample was also prepared by the hydrolysis of Ti(0(CH3)2)4,followed by calcination at 400 "C in an O2stream (this sample will be referred to as IP-400). The samples of P-25 and TP-2 were calcined at 400 OC in an 0, steam to remove organic impurities. BET surface area after 400 OC calcination was 40 m2/g for P-25,19 m2/g for TP-2,and 130 m2/g for IF'-400. These Ti02 samples were outgassed at 200 "C to remove adsorbed water, exposed to gaseous (CH3)$4iC1or SiCl, for ca. 10 min at room temperature, outgassed again at 200 OC to remove unreacted silicone compounds, and finally calcined at 400 OC for 6 h in an O2stream to oxidize surface silicon compounds to silicone oxides. For IR spectroscopic measurements, the TiO, samples were pressed into self-supporting pellets. The evacuable IR cell with CdF2windows was made of Pyrex glass. The pellets were outgassed at 200 "C in the cell, and then IR spectra were recorded on a Hitachi 270-30 IR spectrometer. The reactions were carried out in an evacuable, closed circulation system. The light source was a 500-W high-pressure Hg (4) Parkinson, B.; Decker, F.; Juliano, J. F.; Abramovich, M.; Chagas, H. C. Electrochim. Acta 1980,25, 521. (5) Salvador, P.; Gutierrez, C. Chem. Phys. Lett. 1982, 86, 131. (6) Dinklea, H.0.; Murray, R. W. J.Phys. Chem. 1979,83, 353.
(7) Nippon Aerosil P-25 is identical with Degussa P-25, and the former was mainly used in the present experiment.
n
4000 3800 3600 3400 32 x) Wave number (cm-')
Figure 1. IR spectra of the P-25 TiO, samples: a, nonmodified;
b, modified with gaseous (CH3)3SiCl,followed by oxidation at 400 "C; c, modified with gaseous SiCl,, followed by oxidation at 400 OC.
H
9
H
? ?
-T/-o-T/-o-T,i-
Sic1,
q
ClgiCl
p'
-Ti-0-Ti-0-TiI
p
CI
"SF'
H
l
l
?'
si
0
'gi'
OH
Q'
2 -?i-O-?i-o-\i-
Figure 2. Possible reaction schemes of the (CH3@iC1 treatment
(top) and the Sic&treatment (bottom).
lamp. In the CO and CzH6 photooxidation and the GSOIE reactions, 0.3 g of the sample was spread on the flat bottom of a quartz cell and illuminated from the top with a mirror. The total pressure of CO and O2was 3 Torr with the ratio 2 1 in the former, and in the latter 1 Torr of lsOz (99 atom %) was used. In the photoevolution reaction of 0, or H,, a test-tube-type reaction cell was employed, and 50 mg of the sample was suspended in 5 mL of an aqueous AgN03solution (10 mmol of Ag+/L) in the former or in 5 mL of an aqueous CH30Hsolution (20 vol %) in the latter. The suspensions were outgassed by repeating a freeze-pump cycle 2 times and then illuminated with constant magnetic stirring. The light intensity was reduced with a 20% neutral-density fdter, since these reactions were so fast that the rate was hardly followed under full illumination. For the Hz evolution reaction, platinization of the sample was carried out by an in situ photodeposition method, a calculated amount of K,PtC16 (0.5 wt % Pt when deposited) was added to the CH30Hsolution prior to the reaction. Since an induction period caused by the platinization was observed at the beginning of the 0, and the Hz evolution reactions, the photocatalytic activity was evaluated with the steady-state rate of O2 or H, evolution. The reaction products were sampled at appropriate intervals and analyzed with a mass spectrometer.
Results and Discussion IR Spectroscopic Measurements. Spectrum a in Figure 1 is the IR spectrum of the P-25 sample before the surface modification. The bands at around 3660 and 3440 cm-l are assigned to the surface hydroxyls of TiOP8p9 The (8) (a) Yates, D. J. C. J.Phys. Chem. 1961,65,746. (b) Lewis, K.E.; Parftt, G. D. Trans. Faraday SOC. 1965,62,2469. (c) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75, 1216.
1158 Langmuir, Vol. 4, No. 5, 1988
Sat0
Table I. Change in the Photocatalytic Activities of the TiOz Samples for CO and CeHs Photooxidation and Oxygen Isotope Exchange between leOz and TiOz (GSOIE) by Reolacement of the Surface Hydroxyls with Silicon Oxides photooxidation
TiOz type Aerosil (P-25)
treatmenta none SiC14
Fuji titanium (TP-2) IP-400
none SiCl,
(CH3)3SiC1
none
SiC14
GSOIE
CO
CPH6
0.28 0.10 0.09 0.12 0.10 0.37 0.05
2.6 1.3 0.9 1.0 0.9 6.0 4.8
6.4 4.5 4.2 3.5 3.3 21.3 16.1
"The sample was exposed to gaseous SiC1, or (CH3)$3iC1and then oxidized at 400 "C in 02.
IR spectrum of the P-25 sample treated with (CH3),SiC1 showed only a slight decrease of the OH peak at 3660 cm-', while the 3440-cm-' peak completely disappeared. By repeating the same treatment 3 times, a substantial decrease of the peaks in the 3660-cm-' region was obtained, whereas the 3730-cm-l peak markedly grew as shown in spectrum b in Figure 1. Since the 3730-cm-l band is assigned to the hydroxyl on Si02,9this result implies the increasing population of the SiOH group by the (CH,),SiCl treatment. We assume that (CH,),SiCl did not cover densely the surface of P-25 because of a hindrance effect of its bulky methyl groups, as illustrated in Figure 2, and as a result the hydroxyls were hardly eliminated by a single treatment. The formation of the SiOH group on the resultant silicone oxides by the (CH3),SiC1 treatment is reasonable, since the oxidation of adsorbed (CH,),Si inevitably produces water, which could turn into a hydroxyl group. The SiC14treatment, on the other hand, was successful in the efficient elimination of the hydroxyls on TiOz without appreciable formation of SiOH group, as shown in spectrum c in Figure 1. In this case Sic& would cover the surface more densely than (CH,),SiCl, and one SiC14 molecule could react with two adjacent hydroxyl groups, as illustrated in Figure 2. As a result, bridge-type silicon monoxide may be formed without SiOH formation. The IR spectra in Figure 1 show that the integral intensity of the OH band was reduced by the SiC14 treatment by a factor of ca. 4. The TiOz samples used mainly in the present study were, therefore, modified by the SiC14 treatment to eliminate the hydroxyls efficiently. For the TP-2 sample, IR spectroscopy revealed that it has much fewer hydroxyls than the P-25 sample, and the SiC14 treatment gave a slight effect on the hydroxyls population. IR transmission of the IP-400 sample was so poor in the OH region that its IR spectrum could not be recorded. Photooxidation Reactions. In Table I are listed the photocatalytic activities of the nonmodified and the modified TiOz samples for the oxidation of CO and CzH6 and the initial rate of the oxygen isotope exchange between and T i 0 2 (GSOIE). As for the photooxidation reactions, the change in activity due to the modification ranged from 6% to 50%, depending on the type of modification, reaction, and sample. The (CH,),SiCl treatment appears to lower the activity to a greater extent than the SiC14 treatment. It is reasonable that the activity drop was slight on the TP-2 sample, which has fewer surface hydroxyls than the others. On the P-25 and the IP-400 samples the activity drop remained less than 30% except for the CO oxidation over the P-25 sample. This extent of the activity (9) Jackson, P.; Parfitt, G. D. Trans. Faraday SOC.1971, 67, 2469.
Illumination time (min) Figure 3. Simulated first-order plots of oxygen isotope exchange between and TiOz (GSOIE)over the P-25 TiOzsamples: 0, nonmodified; A, modified by the SiC14treatment; 0,modified by the (CH3)3SiC1 treatment. xo, x , and x , are the atomic fraction of " 0 in dioxygen at time 0, at time t , and at equilibrium, respectively. drop is smaller than the amount of the eliminated hydroxyls. Oxygen Isotope Exchange (GSOIE). The initial rates of GSOIE on the P-25 and IP-400 samples dropped significantly by the modification as shown in Table I. A mechanism of GSOIE has been studied in a previous experiment and was found to involve oxygen of the surface hydroxyls on oxide semiconductors.1° Therefore, a marked decrease in the GSOIE rate due to the elimination of hydroxyls is quite reasonable. This result, however, does not signify the drop in photocatalytic activity, because photocatalytic activity should be evaluated with a rate constant of relevant reaction. An isotope-exchange reaction obeys the first-order kinetics irrespective of a reaction mechanism,l' and hence we have dx/dt = k(x, - X )
(1)
where x and x , are the atomic fraction of lSO in dioxygen at time t and at equilibrium, respectively, and k is the rate constant. Integrating eq 1, we get In ( x , - x ) / ( x , - xo) = -kt
(2)
where xo is the initial value of x ( x o = 0.99 in the present experiment). Although x , was not observed in this case, k can be obtained by a computer simulation using eq 2.1° Figure 3 shows the results of the simulation applied to the GSOIE reactions over the modified and the nonmodified P-25 samples. A good agreement of simulated line with the observed values was obtained. The best-fit parameter min-l and x , = 0.4 for the nonsets were k = 1.1 X min-' and x , = 0.79 for the modified sample, k = 1 x min-' and x , = SiCl,-treated sample, and k = 0.6 X 0.72 for the (CH,),SiCl-treated sample. Thus the drop in the rate constant, i.e., photoactivity loss, is 10% in the former and 45% in the latter, while the drop in the initial rate is 64% and 68%, respectively. The decrease in photoactivity is, therefore, less in the photooxidation of CO. (10) Sato, S. J . Phys. Chem. 1987, 91, 2895. (11) Ozaki, A. Isotope Studies of Heterogeneous Catalysis; Kodansha: Tokyo, 1977. (12) Sato, S.; Kadowaki, T. J. Catal. 1987, 106, 295.
Effects of Surface Modification of Powdered TiOa
-
I
I
t a
0' I
10
I
I
20
30
40
60 0 (min)
20
40
60
Time
40
Time (min)
Figure 4. Time dependence of O2 photouptake over the P-25 TiOz samples: 0,nonmodified; 0 , modified by the Sic&treatment.
A similar result was obtained for the IP-400 sample. The number of exchangeable oxygen atoms on the illuminated surface, N, is given as
N = Nab0 - x , ) / x ,
20
(3)
Figure 5. Time coupse of the O2photoevolution from the aqueous AgN03 solution over the P-25and the IP-400 Ti02 samples; 0, nonmodified; 0 , modified by the SiCl, treatment; A, modified by the (CH3),SiC1treatment.
limiting pro~ess.'~We have recently found that highly hydroxylated Ti02shows much lower photoactivity for the O2 evolution reaction than standard Ti02. The surface hydroxyls of Ti02,therefore, may prevent electron transfer from the bulk of Ti02to Ag ion. In addition, the electron transfer would be inhibited by covering the surface with an insulating material such as silicone oxides. Photoevolution of H2. H2 photoevolution from an aqueous CH30H solution was carried out on the platinized P-25 samples. The photoactivity drop due to the surface modification was ca. 30%. The electron transfer from the bulk of T i 0 2 to Pt is vital to the H2 photoevolution reaction, since the potential of photoinduced electron in Ti02 is so low as to need a catalyst to produce hydrogen from water,14and therefore, the activity drop is probably attributable to the insulating effect of the silicon oxides, which lie between Pt and Ti02. In summary, although the surface modification of powdered Ti02with silicone oxides gave rise to a decrease in the activity for various photocatalytic reactions, its effect was smaller than the extent of eliminated hydroxyls, which were monitored by IR or GSOIE. The activity loss due to the elimination of hydroxyls may be smaller than the present results, since the (CH3),SiC1and SiC14treatments may leave C1, which reduces the photocatalytic activity, on the surface. The result of the GSOIE experiment strongly suggests little participation of the hydroxyls in the photoactivation of dioxygen. The result of the O2 photoadsorption experiment brought out a question on an important role of the hydroxyls in O2photouptake so that it should be reinvestigated in more detail in a clean system. The surface hydroxyls may prevent electron transfer from the bulk of T i 0 2 to substrate, such as Ag+, adsorbed on Ti02 or present in a solution, but they do not seem to play a positive role in the photoelectrochemical processes studied in the present experiments.
where No is the number of '*Oatoms introduced into the reaction system.'" On nonmodified P-25, N is 1.5N0 and dropped to 0.27N0after the Sic&treatment. This decrease in the exchangeable oxygen atom due to the modification is virtually in agreement with that estimated from the IR spectra in Figure 1. Photoadsorption of 02.Figure 4 shows the time dependence curves of O2photoadsorption on the modified and the nonmodified P-25 samples. The rapid drop in O2 pressure was observed a t the beginning of illumination, followed by a slow and steady decrease in O2pressure. The O2photouptake was faster on the nonmodified sample than on the modified one. The 0, photoadsorption was, however, hardly observed when a liquid nitrogen trap was placed in front of the cell to cut off vacuum-grease vapor and the sample oxidized in situ a t 400 "C. The O2photouptake, therefore, may arise from the photoreaction of O2 with organic impurities adsorbed on Ti02 during the outgassing process or storage in air. The O2 photoadsorption on Ti02 should be reexamined in more detail in a clean system. Photoevolution of O p Figure 5 shows the time courses of the photoevolution of O2 from a AgN03 solution on the P-25 and the IP-400 samples. A marked drop in the photoactivity due to the modification was observed on the P-25 samples whereas little effect appeared on the IP-400 samples. The hydroxyls of IP-400 may be included in the bulk of Ti02-like crystal water, since the poor IR transmission of IP-400 in the OH region suggests that IP-400 contains water. The photoevolution of O2from aqueous Ag salt solutions Acknowledgment. This work was partly supported by over Ti02has been intensively studied in recent y e a r ~ , ~ J ~ the Ministry of Education, Science and Culture, Grantand electron transfer from photogenerated electrons in the in-Aid for Special Project Research No. 61223001. conduction band of T i 0 2 to Ag ions appears to be a rate(13) (a) Hada, H.; Yonezawa, Y.; Ishino, M.; Tanemura, H. J. Chem. SOC.,Faraday Trans. I 1982, 78, 2677. (b) Nishimoto, S.; Ohtani, B.; Kagiya, H. J. Chem. SOC.,Faraday Trans. I 1983, 79, 2685. (c) Ohtani, B.; Okugawa, Y.; Nishimoto, S.; Kagiya, T. J.Phys. Chem. 1987,91,3550.
Registry No. OH, 3352-57-6;Ti02, 13463-67-7; 3276718-3; AgN03, 7761-88-8;Hz, 1333-74-0;CH30H,67-56-1; silicon oxide, 11126-22-0. (14) Sato, S.; White, J. M. J.Phys. Chem. 1981, 85, 592.
