J. Phys. Chem. 1992, 96, 3521-3526
3521
Semiconductor Photocatalysis.' Effective Photoreduction of Carbon Dioxide Catalyzed by ZnS Quantum Crystallites with Low Density of Surface Defects Masasbi Kanemoto, Tsutomu Shiragami, Chyongjin Pac, and Sbozo Yanagida* Chemical Process Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan (Received: June 6 , 1991; In Final Form: December 16, 1991) Freshly prepared colloidal ZnS suspensions (ZnS-0) effectively catalyze photoreductionof C 0 2in water at pH 7 with NaH2P02 in the coexistence of Na2S under UV irradiation. With competitive H2 evolution, formate and a very small quantity of CO were formed with the apparent quantum yield 2HCOO-, 0.24 at 313 nm, where H2P0; was quantitatively photoxidized to HPOt-. The efficiency strongly depends on the pf-l of the system, preparation methods of ZnS photocatalysts,and synergistic effects of electron donors. Quantized ZnS crystallites with low density of surface defects are indispensable for the effective C 0 2reduction. The synergistic effect in the use of both SH- and H2PO< ions is discussed in terms of spectral properties of the photocatalyst.
Photoreduction of carbon dioxide (CO,) to organic compounds togenerated electrons for successive electron-transfer processes. has attracted substantial interest as a means for fuel p r o d ~ c t i o n ~ * ~ Accordingly, several systems for the photoreduction of C 0 2 were and resolving the greenhouse e f f e ~ t . ~ Electrochemical and recently developed applying colloidal s e m i c o n d u ~ t o ror ~ ~met'~ photoelectrochemical as well as photochemical reductions of C 0 2 alized semiconductor as photocatalysts in the presence have been extensively studieda2 Development of artificial phoof different sacrificial electron donors. Henglein et al.lSafirst tosynthetic systems is one of the ultimate goals in the reduction reported the efficient photoreduction of C 0 2 to formic acid using of COz. Photoassisted reductions of C 0 2 in semiconductor SO2-stabilized quantized ZnS as photocatalyst and 2-propanol particulate systems have been s t ~ d i e d , ~ *since ~ - l ~Inoue et al.5 first as sacrificial electron donor. The quantum yield for the formation reported photoreductions of COz with suspensions of various of formic acid has been determined to be 0.80. A comparable semiconductor powders such as W03, TiO,, ZnO, CdS, Gap, and result was recently reported by Inoue et al.,ISbrevealing that the S i c . This system is also interesting in terms of abiotic photoSO,-stabilized quantized ZnS, prepared in the presence of excess synthesis." However, most semiconductor-catalyzed photoreZn2+ ([Znz+]/[S2-] = 3.0), exhibits high activity for the C 0 2 ductions of CO, using water as electron donor resulted in low photoreduction (@ = 0.3). conversion and selectivity to reduction products.s In a series of recent studies on semiconductor photocatalysis, Recent studies have revealed that quantized semiconductor we have demonstrated that ZnS quantum crystallites and their particles and their loose aggregates are effective photocatalysts loose aggregates catalyze the quantitative photoreduction of alsince their band gap is increased and the recombination of a iphatic ketones to alcohols using Na2S and NaZSO3as sacrificial photoformed electron and hole pair is relatively slowed d ~ w n . ' ~ - ' ~ electron donors under >3 13-nm light The use of In addition, the efficiency of the semiconductor photocatalysts quantized ZnS particles with low density of surface defects prois controlled by the migration of electrons and holes to the semvides novel photocatalysts due to stabilization of the electron-hole iconductor solution i n t e r f a ~ e . l ~Metalization -'~ of semiconductor pair against recombination and eliminates the deactivation of particles with noble metals is a general means to eliminate phoelectrons by surface traps. These observations encouraged us to extend the application of quantized ZnS particles for CO, photoreduction. The effective photoreduction of CO, to formic acid ( 1 ) Part 12: Kanemoto, M.; Shiragami, T.; Pac, C.; Yanagida, S. Chem. Let;. 1990, 931. was achieved at pH 7 using NaH2P02 as electron donor with (2) Taniguchi, 1. In Modern Aspects of Electrochemistry; Bockris, J. 0 . Na2S.l We find that photoreduction of CO, to formic acid M., White, R. E., Conway, B. E., Eds.; Plenum Publishing: New York, 1989; proceeds most effectively when Na2S is added to the photosystem. D 327. r -- The effects of added Na2S on the performance of the photocatalyst (3) Tanaka, K.; Wakita, R.; Tanaka, T . J . Am. Chem. SOC. 1989, 1 1 1 , 2428 and references therein. are discussed in terms of defects elimination from the photo(4) (a) Hanscn, J.; Johnson, D.; Lacis, A.; Lebedeff, S.; Lee, P.; Lind, D.; catalyst. Russel, G. Science 1981,213,957. (b) Hileman, B. Chem. Eng. News 1987, March 13, 25. (5) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637. (6) Ulman, M.; Aurian-Blajeni, B.; Halmann, M . Isr. J . Chem. 1982, 22, 177. (7) Chandrasekaran, K.; Thomas, J . K. Chem. Phys. Lett. 1983, 7 , 99. (8) Halmann, M.; Katzir, V.; Borgarello, E.; Kiwi, J . Sol. Energy Mater. 1984, IO, 85. (9) (a) Yamamura, S.; Kojima, H.; lyoda, J.; Kawai, W. J . Electroanal. Chem. 1987, 225,287. (b) Yamamura, S.; Kojima, H.; lyoda, J.; Kawai, W. J . Electroanal. Chem. 1989, 247, 333. (10) Eggins, B. R.; Imine, J . T. S.; Murphy, E. P.; Grimshaw, J. J . Chem. Soc., Chem. Commun. 1988, 1123. ( I I ) (a) Folsome, C.; Brittain, A . Nature 1981, 291, 482. (b) Brown, R. D. In Origin of Lye; Wolman, Y., Ed.; Reidel: Dordrecht, 1981; p I . (12) (a) Henglein, A.; Gutierrez, M. Eer. Bunsen-Ges. Phys. Chem. 1983, 87, 852. (b) Nedeljkovc, J. M.; Nenadovi, M . T.; Micic, D. I.; Nozik, A . J . J . Phys. Chem. 1986, 90, 12. (c) Bahnemann, D . W.; Kormann, C.; Hoffmann, M. R. J . Phys. Chem. 1987, 91, 3789. (d) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A . J . Am. Chem. Soc. 1987, 109, 5649. (e) Anpo, M.; Shima. T.; Kodama, S.; Kubokawa, Y . J . Phys. Chem. 1987, 91, 4305. (13) (a) Yanagida, S.; Ishimaru, Y.; Miyake, Y.; Shiragami, T.; Pac, C.; Hashimoto, K.; Sakata, T. J . Phys. Chem. 1988, 92, 3476. (b) Yanagida, S . ; Yoshiya, M.; Shiragami, T.; Pac, C . Ibid. 1990, 94, 3104. (14) (a) Shiragami, T.; Pac. C.; Yanagida, S. J . Chem. SOC.,Chem. Commun. 1989,831. (b) Shiragami, T.; Pac. C.; Yanagida, S.J . Phys. Chem. 1990, 94, 504.
0022-365419212096-3521$03.00/0
Experimental Section Preparation of ZnS Photocatalysts. Colloidal ZnS suspensions (ZnS-0) consisting of ZnS quantum crystallites and their aggregates were prepared in situ as previously reported.13 The ZnS suspensions (ZnS-0) were prepared under argon atmosphere by mixing equal amounts of aqueous 0.05 M solutions of ZnS04 and Na2S under cooling with an ice bath and stirring with magnetic stirrer in a 8-mm-diameter Pyrex tube. Electron microscopy of the ZnS-0 particles showed that they had a diameter ranging from 2 to 5 nm as previously reported.I3 Similarly, ZnS-100 suspensions were obtained by refluxing the ZnS-0 suspensions for 10 min in ( I 5 ) (a) Henglein, A.; Gutierrez, M.; Fischer, H . Eer. Bunsen-Ges. Phys. Chem. 1984, 88, 170. (b) Inoue, H.; Torimoto, T.; Sakata, T.; Mori, H.; Yoneyama, H . Chem. Letr. 1990, 1483. (16) Ulman, M.; Tinnemans, A . H . A.; Mackor, A,; Aurian-Blajeni, B.; Halmann. M . Int. J . Sol. Energy 1982, I , 213. (17) Rophael, M. W.; Malati, M . A . J . Chem. Soc., Chem. Commun. 1987, 1418. (18) Albers, P.; Kiwi, J . New J . Chem. 1990, 14, 135. (19) Goren, Z.; Willner, 1.; Nelson, A . J.; Flank, A . J . J . Phys. Chem. 1990, 94, 3784.
0 1992 American Chemical Society
3522 The Journal of Physical Chemistry, Vol. 96, No. 8, 1992
Kanemoto et al. 80
- 60 0
5 .
t; 40
a .u
k” 20 0 1 ’ 2 3 4 5 6
0 0
Reaction time / h Figure 1. Rates of products formation by ZnS-0 under CO,, pH 7, in the presence of SH- (0.24 M) and H 2 P O c (0.35 M): ( 0 )H C O c , (A) €42, (0) CO.
a water bath. ZnS-Op and ZnS-100P were obtained by drying ZnS-0 and ZnS-100 suspensions to powder, respectively. Highpurity ZnS (99.999%) (ZnS-Ald) and ZnS (99.9%) (ZnS-Nac) were commercially purchased from Aldrich and Nacalai Tesque, respectively. Analysis. Gas analyses (Le., H2 and CO) were carried out by gas chromatography using an active carbon column (3 mm X 3 m) on a Shimadzu GC-12A chromatograph at 100 OC. Formic acid analysis was carried out by ion chromatography using a TSK gel SCX(H+) column (7.8 mm X 30 cm) at 40 OC and a UV detector (TOSOH Model UV-8011). As eluent, an aqueous phosphate solution (2 mM) was employed with an eluent rate of 0.8 mL/min. Ionic products were analyzed by ion-exchange chromatography using a TSKgel IC-Anion-PW column at 30 OC and an ion conductivity detector (TOSOH Model CM-8010). An aqueous solution of tartaric acid (2 mM) was employed as an eluent with a flow rate of 2.0 mL/min. Emission spectra were recorded on a Hitachi Model 850 spectrofluorimeter. UV spectra of the ZnS samples were measured by reflectance spectrophotometry using a Photal (Otsuka Electronics) with a spectromultichannel photodetector (MCPD- 100). ZnS-0 and ZnS- 100 suspensions were adjusted to pH 7 by C 0 2 , and their wet coagulates obtained by centrifugation were used for the measurement of UV spectra. General Procedure of ZnS-Catalyzed Photoredox Reactions. To 1 mL of ZnS-0 or ZnS-100 suspensions (25 pmol) in a Pyrex tube (8 mm in diameter X 20 or 100 cm) was added 1 mL of a mixture of Na2S (0.48 M) and NaH2P02(0.70 M) solution. In the case of powdered ZnS (e.g., ZnS-Ald (Aldrich), ZnS-Nac (Nacalai), ZnS-OP, and ZnS-IOOP), 10 mg (25 pmol) of the respective ZnS powder was placed in a similar Pyrex tube with 1 mL of water and 1 mL of a mixture of the same Na2S and NaH2P02solutions. The resulting mixture was saturated with C 0 2to become neutral (pH 7) under cooling on an ice bath, closed with a rubber stopper, and then irradiated with 500-W highpressure mercury arc lamp under cooling with water (20 “C). Determination of Apparent Quantum Yields. The quantum yields for production of H C O y were determined at 3 13 nm by using 50 pmol of ZnS-O,4 mL of distilled water, NaH2POz(0.35 M), and Na2S (0.24 M) in 4-mL cuvette cell. CO, was introduced into the reaction mixture until the solution became pH 7. The monochromatic light (3 13 nm) was obtained by using a Wacom xenon short-arc lamp, Model KXL-1OOF. Monochromatic light was obtained by using a Shimadzu Bausch & Lomb monochromator. The intensity of incident light was monitored by tris(oxalate)iron(III) actinometry. The quantum yields were calculated by assuming that two photons produce one molecule of H 2 or HC02- but were not corrected for light scattering by the suspended catalyst. Results ZnS-0-Catalyzed Photoreduction of Carbon Dioxide with Na2S and NaH2P02under >290-nm Irradiation. Figure 1 shows the
rate of the formation of products in a photosystem that includes
100
50
150
Reaction time lmin Figure 2. Correlation between oxidation and reduction products in ZnS-0-catalyzed photoreduction of CO, in water (pH 7) in the presence of both SH- (0.24 M) and H 2 P 0 c (0.035 M): ( 0 )HC02-, (0) CO, (A) H2, (B) H2P02-, (0)HP012-.
1 120 - 1
- 80 5
l
-
n
\
I
u
0
1
2
3
4
5
6
Reaction time / h Figure 3. ZnS-0-catalyzed photolysis of H C 0 2 - in water (pH 7): ( 0 ) HCO2-, (0)CO, (A)H2.
freshly prepared colloidal ZnS suspensions (ZnS-0), under COz, pH 7, in the presence of both Na2S (0.24 M) and sodium hypophosphite (NaH2P02)(0.35 M) under >29@nm (mainly 313-nm) irradiation. Formate (HC02-) is formed efficiently accompanied by a small quantity of CO, and H2 formation proceeds effectively. The apparent quantum yield for formate formation is CP,j2HC00= 0.24 at 313 nm. The initial rate of H2 evolution is twice as high as that for HCOy, as shown in Figure 1, indicating that the quantum yield for the H 2 is 9 = 0.48. Hydrogen phosphite ion (HP032-) was detected as the sole oxidation product by ion chromatography, and neither S2032-nor S042-was detected. Although H2P02-has very negative redox potential (-1.2 V vs SCE at pH 7)20 and is known as a good reducing agent, the photoreduction never occurred either under dark or under the irradiation in the absence of ZnS-0 suspensions. The irradiation of this system in the absence of COz resulted in the exclusive H 2 evolution. (Quantum yield was determined to be CP = 0.73 at 313 nm.) In order to clarify the electron balance of the observed C 0 2 photoreduction, the photoreduction was carried out by diluting the concentration of H2POc to a tenth in the presence of a given concentration of SH- (0.24 M). Time-conversion plots for the oxidation and reduction products (Figure 2) show that HzPOF was constantly photooxidized to hydrogen phosphite ion (HPO3’-) with electron balancing with the reduction to HC02- and H2 until the complete consumption of H2P02-. The decrease of HC02- content in the photosystem was always observed after an appropriate time interval and usually started after the complete consumption of HZPO2-. This fact suggests that photogenerated HCOy once formed should act as an electron donor. In fact, by illumination of a HCOz- solution in the presence (20) (a) Muylder, J. V.; Pourbaix, M. Atlas of Elecrrochemictll Equilibria in Aqueous Solutions;Pourbaix, M., Ed.; Pergamon Press: Cebelcor, Brussels, 1966; p 507. (b) Valensi. G.;Muylder. J. V.; Pourbaix, M. fbid., p 547.
The Journal of Physical Chemistry, Vol. 96, No. 8. 1992 3523
Semiconductor Photocatalysis
-g
20
. a J
P 6 IO
" n
n 0
1
2
3
4
5
6
Reaction time / h Figure 4. Effect of H2P0Tconcentration on ZnS-0-catalyzedphotoreduction of C02 in water (pH 7) in the presence of SH- (0.24 M). H 2 P Oconcentrations ~ in various systems: (0)0.70 M, (0) 0.35 M, (A)
ZnS- 100 ZnS-Op ZnS- 1OOP ZnS-Ald ZnS-Nac
2
3
4
5
6
0
1
2
3
4
5
6
200
TABLE I: ZnS-Catalyzed Photoreductions of CO, in the Presence of SH- (0.24 M) and H2POr (0.35 M)
ZnS
1
Reaction time / h Reaction time / h Figure 5. ZnS-0-catalyzed photoreduction of C 0 2in water (pH 7). (a) In the presence of SH- (0.24 M). (b) In the presence of H2P02' (0.35 M): ( 0 )HCOC, (0)CO, (A)H,.
0.175 M, (A)0 M.
ZnS-0
0
initial rate/(rmol/h) HCOOH CO H2 75.1 27.6 5.3 6.8
1.7 1.6 0.3 2.7
0
0.9 0
4.7
86.0 80.4 63.8 40.8 8.0 22.8
@J~/~HCW-
0.24 0.088 0.017 0.022
$u
loo
L
0 0.015
of ZnS-0, H2 evolution is observedIZaas shown in Figure 3. The rate of formate production in the ZnS-0 photosystem depends on the H2P02-donor concentration. When the photoreduction was carried out in the presence of various concentrations of H2P02- with a given SH- (0.24 M), the rate of the H C O y formation increased with increasing concentration of H2P0y as shown in Figure 4. The quantum yield @ = 0.28 for HC02formation was obtained in the presence of 0.70 M H2P02-and 0.24 M SH-. Effect of Preparation Methods of ZnS Photocatalysts. When ZnS- 100 suspensions prepared by refluxing the ZnS-0 suspension for 10 min was used instead of ZnS-0, the reduction of C02 became inefficient, while the H2 evolution was almost unaffected. Table I summarizes the initial formation rate of HC02-, CO, and H2 with apparent quantum yields of HC02- (@,,2Hc00-) obtained by using different types of ZnS photocatalysts under comparable conditions (SH- (0.24 M) and H2P02- (0.35 M)). The ZnS-Op powder obtained by drying ZnS-0 suspensions to powder disclosed a poor activity, although it had a good activity for the H2 evolution as well as ZnS-100 suspensions. The case was true for the ZnS-100P (the powdered ZnS-100). It is interesting to note that one of the commercially available bulk ZnS-Nac (particle size 3-5 pm) showed activity comparable to ZnS-OP. Effects of Sacrificial Electron Donors. It is well-known that the dissolution of C 0 2 in water gives an aqueous solution of pH 3.7 under a prpsure of 1 atm. When NaHCO, was used instead of C02, none of the reduction products from N a H C 0 3 could be detected. On the other hand, ZnS is unstable under acidic conditions, decomposing into H2Sand Zn2+by the reaction with acid. Therefore, the ZnS-0-catalyzed photoreduction of C 0 2was camed out with some other electron donor by controlling the pH of the system to 5 or 7 (Table 11). Dissociation constants of conjugated acids from inorganic electron donors are reported as follows:*l HPH,02 (pK, 1.23), HS03- (pK, 7.18), and SH- (pK, 13.9). Thus, under neutral conditions, i.e., at pH 7, sulfide ion (S2-)and sulfite ion should be protonated, being converted to hydrogen sulfide ion (SH-) and hydrogen sulfite ion (HS03-), respectively. The SH- and H2P02- pair gave the most effective result as compared to other electron donors, but the separate use of either SH- or H2P02-led to the inefficient photoreduction of COz. The ~~
(21) Rossetti, R.; Brus,
-0 5 .
L. E. J . Chem. Phys. 1984, 80, 4464.
" n
0
2
4
6
8
0
1
2
.
1
4
5
6
Reaction time / h
Reaction time I h Figure 6. Effect of SH- concentration on photoproduction of (a) HC0,and (b) HI in ZnS-0-catalyzed photoreduction of COz in water (pH 7) in the presence of H2P0c (0.35 M): (0)0.30 M, (0) 0.24 M, (A)0.10 M, (A)0.05 M, (m) 0.025 M, ( 0 ) 0 M.
inefficient reduction of C 0 2 is also observed for photosystems including Na2S03,2-propanol, or triethylamine (TEA) as sacrificial electron donor. In a previous the synergistic effect on the coexistence of S2-and S032-was clarified in the ZnS-0-catalyzed photoreduction of aliphatic ketones to alcohols. Expecting a similar effect, the photoreduction was carried out with the combination of some sacrificial electron donors with Na2S. The initial rate of the HCOY formation increased only slightly in the coexistence of SH-. However, a remarkable enhancement in C 0 2 reduction was only observed for the combination with H2P0c/SH-. Synergistic Effect of Use of H2P02-as Electron Donor with SH-. Figure 5a,b shows time-conversion plots of photoproducts in the ZnS-0-catalyzed photoreduction of C 0 2 at pH 7 in the presence of either SH- or H2P0;. In the former case, a small amount of H2 and a negligible amount of HCOy were produced. In the system that includes H2P02-as a donor, H2 is the major photoproduct. These observations imply that SH- does not act as a good electron donor but contributes to the effective photoreduction of C 0 2 in the coexistence of H2P02-as electron donor. To make clear the synergistic effect of SH- in the effective HCOy formation in the ZnS-0 photocatalysis, the photoreduction of C 0 2 was attempted by changing the concentration of SH- in the presence of a given concentration of H 2 P 0 (0.35 ~ M). Figure 6a,b shows the sequence of the respective HC0,- and H2 production. Further, their initial rates were plotted against the SHconcentration as shown in Figure 7. Interestingly, the formation of HC0,- was increased with increasing the SH- concentration and has a tendency to level off around the concentration 0.14.30 M, while the H2 formation was initially increased and then decreased constantly.
Discussion It was clarified in the previous paper13bthat only defect-free quantized ZnS and their aggregates (ZnS-0) catalyze quantitative photoredox reactions of aliphatic ketones to alcohols in the presence
3524
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992
Kanemoto et al.
TABLE I!: ZnS-0-Catalyzed Photoreduction of C02 in Water with Sacrificial Electron Donors
initial rate/(rmol/h) electron donor HS- (0.24 M), H2P02- (0.35 M) H2P02- (0.35 M) HS-(0.24 M) HSOC (0.35 M) i-PrOH (1 M) triethylamine ( 1 M) HS- (0.24 M), HSOC (0.35 M) HS- (0.24 M), i-PrOH (1 M)
PH
HCOOH
co
H2
@I/ZHCOW
7
75.1 11.2 1.o
1.7 0.3 0 0.2 0.2 0.2 1 .o 0.3
86.0 141.3 3.3 21.7 22.5 8 .O 45.1 11.5
0.24 0.035 3.2 x 10-3
trace 1.6
trace 3.7 3.3
iIoob 1
5.1 x 10-3 0.012 0.0 10
'. \
!
- - \
'), ! t l /
1,
'
!
/I '\I
.-.-m d
C
I
0 0.0
0.1
0.2 0.3 [SH-] I M
0.4
Figure 7. Effect of SH- concentration on the rate of HCOOH photoformation in water (pH 7) in the presence of H2P0< (0.35 M): (0) HCOC, (0) H2.
of S2-and S032-under >290-nm irradiation. The present results suggest that ZnS-0 also catalyzes the effective photoreduction of CO,. Figure 8 shows the reflectance spectrum of wet ZnS-0 at pH 7 in comparison with the reflectance spectra of ZnS-100, ZnS-Op, and some commercially available bulk ZnS. The ZnS-0 at pH 7 is characterized by the most steep absorption threshold and the onset arising at the shortest wavelength, which supports that ZnS-0 should consist of quantized ZnS microcrystallites with low density of surface defects at pH 7. Compared with the onset of ZnS-0, the red-shifted onset of the aged or powdered ZnS-0 (ZnS-100 and ZnS-Op) suggests an increase in their particle sizes as reported earlier.i3a The difference in activity between ZnS-0 and other processed ZnS can be attributed to the difference in their particle size; Le., the larger the size becomes, the less the quantization effect becomes. As an additional interpretation, the growth in particle size should be accompanied by an increase of surface defects, decreasing the electron availability for CO, reduction and causing the difference in activity. The results clearly indicate that SH- does not act as electron donor in the photoreduction of C 0 2at pH 7. In order to clarify the role of SH-, the emission at A,, = 420 nm, called self-activated (SA) emission of ZnS-0, and the effect of added SH- were examined. After 6-fold dilution of the photolysates in Figure 6, each solution was excited at 290 nm, giving SA emissions with various strengths as shown in Figure 9. It is evident that addition of SH- quenches the SA emission of ZnS-0 and that the ZnS-0 emission is almost entirely quenched at a SH-concentration of 0.24 M. Interestingly, a weak emission band at X = 320 nm, which was assignable to the band gap emission,i3ais observed upon addition of SH-. The effects of added SH- on the emission characteristics of ZnS-0 are observed even in the absence of H,PO;, indicating that the latter compound does not participate in the overall quenching process. As pointed out by one of the reviewers, earlier quenching studies on emission in this field revealed that quenching by anionic species as electron donors is attributable to the scavenging of the charges from the trap sites or to the increase in the surface recombination velocity through formation of positive hole-anion pairs at the surface.** In addition, Henglein et aL2= reported that the emission (22) (a) Henglein, A . Ber. Bunsen-Ges. Phys. Chem. 1982.86, 301. (b) Weller, H.; Koch, M.; Cutierrez, M.; Henglein, A. Ibid. 1984, 88, 649.
I
I
300
400
I
500
Wavelength / nm Figure 8. Absorption spectra of ZnS photocatalysts by reflectance ZnS-100 spectrophotometryusing a Photal: (-) ZnS-0 at pH 7; at pH 7; ZnS-OF'; ZnS-Ald; ( - - - ) ZnS-Nac. (e-)
(-a*-)
(-e-)
400 500 Wavelength I nm Figure 9. Effect of SH-on emission spectra of ZnS-0: 1, H2POZ-(0.35 M); 2, none; 3, SH- (0.025 M) and H2PO2-(0.35 M); 4, SH- (0.05 M) and H,POT (0.35 M); 5, SH- (0.10 M) and H2P0< (0.35 M); 6, SH(0.24 M) and H2P02-(0.35 M); 7, SH- (0.24 M).
300
centers are anion vacancies because of the promoting effect of excess metal ions, explaining the strongest quenching by excess sulfide anion as due to the removal of the anion vacancies at the surface. In order to distinguish quenching modes at pH 7, quenching of the SA emission of ZnS-0 was examined by using various electron donors. The quenching ratios (I/[,,) are summarized in Table I11 with those in the presence of SH-. Interestingly, SH-quenched the SA emission much more than any other electron-donating species. We thus rationalize the strongest quenching of SH- in terms of the elimination of surface vacancies
The Journal of Physical Chemistry, Vol. 96, No. 8,1992 3525
Semiconductor Photocatalysis TABLE 111: Effect of Electron Donors on the SA Emission of ZnS-0 run no. electron donors Illno NaH2P02(0.35 M) 3.28 1 Na2S (0.24 M) 0.04 2 Na2S204(0.24 M) 0.5 1 3 4 5 6 7 8 9
IO 11
12
Na2S03(0.24 M) TEA (0.5 M) TEA (0.1 M) 2-propanol (0.5 M) 2-propanol (0.1 M)
1.24
NaH2P02(0.35 M) + Na2S (0.24 M) NaH2P02(0.35 M) + Na2S (0.10 M) NaH2P02(0.35 M) f Na2S (0.05 M) NaH2P02(0.35 M) + Na2S (0.025 M)
0.86 0.83 1.05 0.94 0.06 0.19
0.25 0.36
Quenching ratios (]/Io) were obtained by comparison of emission intensities with and without donors at pH 7. pH 7 was controlled by c02. Bulk ZnS C.B. f.b. -
competes with the H2 evolution process suggests that the rate of C 0 2 reduction by conduction band electrons is comparable to the H2 evolution rate. Such interpretation is quite consistent with the fact that the quantum yield (0.73) of H2evolved in the absence of C 0 2 was in agreement with the total quantum yield of HC02(0.24) and HI (0.48) observed in the presence of COz. It is now clear that photoformed electrons on the conduction band have potential negative enough to react with C 0 2 molecules. Under neutral conditions, Le., at pH 7, C02should exist without complete dissociation to HC03- as apparent from the dissociation constant of C02, pK, 6.51. Further, HCOc was reported not to be reduced electrochemi~ally.~~ These facts strongly suggest the direct injection of electron to CO2 from irradiated quantized ZnS-0 particles as far as holes are scavenged by effective electron donors like H2P02-. Hence, the overall reactions occurring at the semiconductor solution interface may be depicted as outlined in Scheme I. SCHEME I
ZnS -k. ZnS(ecB + hvB) ecB ess (trapped electron) hvB hDP(trapped hole)
--
C02 / HCOOH (4.66V)
co2
Figure 10. Relationship between energy structure of quantized ZnS-0 and redox potentials of related substrates at pH 7.
(defects) by SH-: the generated quantized ZnS-0 particles include surface defects in the form of vacancies. As a result, conduction band electrons are trapped by these vacancies, giving rise to the emission at A = 420 nm. Addition of SH- results in the elimination of surface lattice vacancies. Consequently, electrons from the conduction band are not trapped by the defect sites present in the absence of SH-, as evident by the blue-shifted emission of ZnS-0 in the presence of SH-. Taking into account the energy level of bulk ZnS and the 3.85-eV band gap of ZnS-0, the redox potential diagram of ZnS and respective potentials for reduction fo C 0 2 and H2 evolution are presented in Figure 10. The potential of the conduction band was estimated to be -2.3 V vs SCE, and the energy level of surface states (V,) ascribed to sulfur vacancies was estimated to be -1.3 V vs SCE based on the SA emis~i0n.I~ The potential actually required for single electron transfer to C 0 2 is reported to be more negative than -2.0 V vs SCE.2q23 Some related redox potentials and the anodic decomposition potential (DP) of ZnS (sphalerite a t pH 7) are also shown in Figure 10. The quantized ZnS-0 including vacancies as surface lattice defects traps conduction band electrons, resulting in electrons that have a more positive reduction potential than the original conduction band electrons. These trapped electrons can only reduce H+ to H2(Eo= -0.65 V vs SCE at pH 7), and consequently H2 is the major photoproducts with H 2 P 0 T as electron donor. Addition of SH-eliminates the surface vacancies, and photogenerated conduction band electrons are preserved. These electrons are characterized by a reduction potential of Eo = -2.3 V vs SCE at pH 7, capable of reducing COz. Thus, in the presence of SH-, photoreduction of C02.proceeds. The competitive H2 evolution is still a thermodynamically feasible process. The fact that reduction of C 0 2 proceeds effectively in the presence of SH- and (23) Lamy. E.; Nadjo, L.; Saveant. J. M. J . Electroanal. Chem. 1983, 148, 17.
H2P02-
+ 30H-
-H+
(2) (3)
HCOT
-co -
2hv$h,
co2
D.P. -
V.B. -
2 e a , H+
(1)
2ccs
2hWlhLP
HP032- + 2H20
(7) From a thermodynamic point of view, the photoreduction of C 0 2 to HC02- with H2P02-is a downhill reaction, Le., photocatalytic but not photosynthetic. On the other hand, taking into account the oxidation potentials of SH- (-0.41 V vs SCE at pH 7) and HS03- (-0.2 V vs SCE at pH 7),20bthe ZnS-0-catalyzed photoreductions with SH- and/or S032-are uphill reductions and may be regarded as artificial bacteria-like photosynthesis.' The low efficiency may be explained by the rational that both SHand HS03-ions do not function as good electron donors in the ZnS-catalyzed photo~atalysis.~~ The activity of quantized ZnS suspensions depends on preparation methods and particle size; Le., quantized ZnS with low density of surface defects like sulfur vacancies is indispensable for the photoreduction of C02. In the presence of poor electron donors, defects (Le., sulfur vacancies) may be formed by converting lattice S2-ion to S22-ion as anodic photocorrosion. The anodic decomposition potential (DP) of bulk ZnS is reported to be -0.15 V vs SCE.26 The D P may be regarded as hole trapping sites and function as oxidation sites of electron donors (Figure 10, eqs 3 and 7). Defects formed during the workup should be hardly eliminated by the presence of SH- as was observed for ZnS-OP and ZnS-100. The increase in particle size by workup may contribute to the formation of unrepairable deep defects, leading to the inefficiency even in the presence of SH-. The case may be similar for commercially available ZnS. It is worth noting that CO was formed in a small quantity in the ZnS-catalyzed photolysis of both C 0 2 and HC02-. The latter case may suggest the oxidative formation of C O from H C O c , Le., the disproportionation of 'C02H. However, oxalic acid which might form during the disproportionation could not be detected in the reaction products. There should be two possible routes for (24) Van Rysselberghe, P.; Alkire, G . J. J . Am. Chem. Soc. 1944,66,1801. (25) Reber and Meier also reported that the platinized CdS-catayzed H2 formation with Na2S as electron donor was reported to depend on the pH of the solution, i.e., the actual concentration of S2-.26aOn the other hand, the ZnS(bulk)-catalyzed H2 formation with Na2S is independent of pH only in the pH range from 8 to 14.26b (26) (a) Buhler, N.;Meier, K.; Reber, J.-F. J . Phys. Chem. 1984.88, 3261. (b) Reber. J.-F.; Meier, K . [bid. 1984, 88, 5903.
3526
J . Phys. Chem. 1992, 96, 3526-3531
the formation of CO: one alternative involves the direct photoreduction of C 0 2 to CO (eq 5 ) , and the second possibility includes CO elimination from the intermediate zinc formate as follows ZnCOOH
-
ZnOH
+ CO
(8)
Conclusion
The efficient photofmation of C 0 2 in semiconductor particulate systems can be achieved under control of pH 7 by using quantized ZnS (ZnS-0) in the presence of SH- as photocatalysts and NaH2P02as an electron donor. A large quantity of HC02accompanied by a small quantity of CO is formed, and simultaneous large quantity of H2 evolution as a result of water photoreduction observed. The remarkable efficiency can be obtained by controlling the concentration of the electron donor of NaH2P02 and the sulfur vacancies suppressor of Na2S. Total quantum yield for the formation of HCOy and H2as high as 0.84 was attained in the presence of H2P02- (0.70 M) and SH- (0.24 M). The fact that the singleelectron transfer to C02can be achieved effectively in the semiconductor particulate system as well as
electrochemical reductions suggests that development of artificial photosynthetic semiconductor systems should become promising in view of the simplicity of the system regarding semiconductor particles as micro-photocells. Improvement in selectivity of photoproducts and development of semiconducting materials sensitive to visible light could further lead to effective photofmtion of C 0 2 by solar light. Acknowledgment. We appreciate the reviewer’s comments suggesting the alternate mechanisms, Le., the quenching mode by SH- and the contribution of particle size to the difference in activity between ZnS-0 and ZnS-100.This research was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan (No. 022031 19). The research was also conducted as a theme of the Research Skiety for C02Fixation sponsored by Institute of Laser Technology under the commission of the Kansai Electric Power Co., Inc. Registry No. C02, 124-38-9; ZnS, 1314-98-3; H 2 0 , 7732-18-5; NaH2P02, 7681-53-0; Na2S, 1313-82-2; H2P02-, 15460-68-1; SH-, 15035-72-0; Hi, 1333-74-0; CO, 630-08-0; HCOO-, 71-47-6.
Measurement of Distance Distribution between Spin Labels in Spin-Labeled Hemoglobin Using an Electron Spin Echo Method A. Raitsimring,*?+J. Peisach, H. Caroline Lee, Department of Molecular Pharmacology, Albert Einstein College of Medicine, Bronx, New York 10461 and X. Chen Department of Chemistry, University of Houston, Houston, Texas 77204 (Received: November 15, 1991) One of the varieties of electron spin echo methods, that utilizing the “2 + 1“ pulse train, was used to determine the distribution of distance between a pair of nitroxide radicals in spin-labeled, tetrameric hemoglobin. The method allows one to obtain the spin echo kinetics determined solely by the dipoldipole interaction between a pair of labels located within a protein molecule and to suppress the dipoledipole interaction of the pair with other pairs in the bulk medium. The kinetic behavior was simulated using various distribution functions. The agreement between experimental and simulated kinetics was found for a spin-label distance distribution function centered at 35 A and with a half-width of 3 A.
Introduction
A problem of particular interest in the study of biomolecules is the determination of distances between structural components. Distance measurements have been performed using a variety of methods including X-ray crystallography, fluorescence quenching, Mossbauer spectroscopy, electron or X-ray scattering, and a number of magnetic resonance techniques (ESR, NMR, ENDOR).l-’ Application of electron spin resonance (ESR) methods for distance measurements can be used when the biomolecule of interest contains either natural (metals and free radicals) or artificial (metals and spin labels) paramagnetic centers. The distance dependence of the dipole-dipole interaction between paramagnetic centers has allowed for the use of the EPR technique for measurements of distances between paramagnetic metal ions5 and the determination of local concentrations of spin labels.s The dipole-dipole interaction contributes to the broadening of the continuous-wave (CW) ESR spectrum and the alteration of relaxation parameters of individual paramagnetic centers. C W ESR methods have not been used to measure distances exceeding 10-20 A. However, pulsed ESR methods have been developed recently9 that allow for the determination of distances between *To whom correspondence should be addressed. ‘Onleave from Institute of Chemical Kinetics and Combustion SB AS USSR, Novosibirsk, 630090.
0022-365419212096-3526$03.00/0
paramagnetic centers up to 100-150 A. These new methods which employ the “2 1 pulse train procedure have been successfully applied to the investigation of the distribution of paramagnetic centers in irradiated soliddoand catalysts containing paramagnetic metal ions.” However these methods have never been used to solve structural problems in systems such as biological materials where the state of aggregation depends strongly on the local environment of the material of interest. In this work we apply the ‘2 + 1” pulse train procedure to determine the distance distribution between nitroxide spin labels tethered to the cysteine sulfhydryls a t position 93 on each of the
+
( I ) Fermi, G.; Perutz. M. F. J. Mol. Biol. 1977, 114, 421.
(2)OHara, P.;Yeh, S.M.; Meares, C. F.; Berson, R. Biochemistry 1981, 20, 4704.
(3)Belonogova, 0.V.;Lichtenstein, G.R.; Goldanskii, V. I. Dokl. Akad. USSR 1918, 241, 219. (4) Nicolson, G. L.; Yanamigachi, R.; Yanamigachi, H. J . Cell Biol. 1975, 66, 263. ( 5 ) More, K. H.; Eaton, G.R.; Eaton, S.J . Magn. Reson. 1985,63, 151. (6)Zweir, J. L.; Wooten, J. B.; Cohen, J. S.Biochemistry 1981, 20, 3505. (7) Mustafi, D.;Sachleben, J. R.; Wells, G. B.; Makinen, M. W.J . Am. Chem. SOC.1990, 112, 2558. ( 8 ) (a) Zweir, J. L. J . Biol. Chem. 1983, 258, 13759. (b) Kulikov, A. V.; Cherepanova, E. S.;Lichtenshtein, G.R. Eiol. Membr. 1989, 6, 1085. (9)Raitsimring, A. M.;Salichov, K. M. Bull. Magn. Reson. 1985, 7, 184. (IO) Kurshev, V. V.; Raitsimring, A . M.; Ichikava, Ts.J. Phys. Chem. 1991, 95, 3563. ( I I ) Levi, Z.; Raitsimring, A.; Goldfarb, D. J . Phys. Chem. 1991, 95, 7830. Nauk.
0 1992 American Chemical Society
