J. Phys. Chem. 1993,97, 11802-1 1810
11802
Sensitized Layered Metal Oxide Semiconductor Particles for Photochemical Hydrogen Evolution from Nonsacrificial Electron Donors Yeong fl Kim, Stephen J. Atherton,’ Elaine S. Brigham, and Thomas E. Mallouk’ Department of Chemistry and Biochemistry and Centerfor Fast Kinetics Research, The University of Texas at Austin, Austin, Texas 78712 Received: July 7. 1993”
Layered alkali-metal titanates (NazTisO7 and K~Ti409),niobates (KNbsOs and &Nb6017), and titanoniobates (KTiNbOs and CsTi2Nb07) were internally platinized, acid-exchanged, sensitized with ruthenium polypyridyl complexes, and studied as photocatalysts for the production of Hz and 13- from acidic alkali-metal iodide solutions. The titanates were inactive as photocatalysts, whereas the niobates and titanoniobates were active with quantum efficiencies up to 0.3% for H I photolysis with visible light. Calculations based on electronegativity showed that the conduction band edge potentials of the acid-exchanged titanates were too positive to prevent semiconductor-mediated recombination of photogenerated Hz and Is-. Laser flash photolysis/transient diffuse reflectance spectra established that iodide reduces the oxidized sensitizer, forming 12*-, which subsequently decays in a bimolecular reaction to form Is-. The inefficiency of HI photolysis can be attributed to charge recombination between Is- and conduction band electrons for the niobates and titanoniobates. Modulation of the layer spacing in the hexaniobate, L,H,Nb6017, by exchange with different alkali metals (A), showed that the hydrogen evolution rate decreased sharply as the average layer spacing increased. This result suggests that the competition between charge recombination and electron tunneling between layers determines the efficiency of the HI photolysis reaction.
SCHEME I: schematic Drawing rad Potentid Energy
Introduction
As photochemical catalysts, structured semiconductormaterials
Dirgru~of
RuLi,*+-Sensitized LMOS System for HI
Photolysis
such as layered niobates, titanates, and semiconductorssupported on solid materials have become increasingly interesting for water decomposition and selective photochemical reactions. Domen and co-worken have studied the photocatalyticactivitiesof several layered metal oxide semiconductors(LMOS’s) for water cleavage by bandgap excitation.’ LMOS’s such as &Nb607*3H@, CsTi2Nb07, CsTiNbOs, alkali-metal titanates, and several provskitetype layered niobates, under band gap illumination, showed a much higher activity for hydrogen evolutionin aqueous methanol solutions, even without metallic catalysts, than did nonporous H2 semiconductors such as Ti02. Moreover, &Nb6017-3H20, when internally loaded with elemental nickel or platinum as a hydrogen evolution catalyst,photodecomposes water to hydrogen and oxygen stoichiometrically with band gap (A I330 nm) excitation.Ik Recently, we reported the visible light photolysis of hydrogen iodide using tris(2,2’-bipyridyl-4,4’-dicarboxylate)ruthenium (RuL3~+)-sensitized&Nb& particles.2 Owing to very efficient interfacial electron-transfer quenching of photoexcited R u L ~ ~ + by &Nb&7 and spatial separation of the platinum catalytic sites from the anionic electron donor (iodide), hydrogen and triiodide were generated stoichiometrically from acidic iodide solutions without appreciabledark recombination of the products. In this paper we study further the activity of several sensitized LMOS’s.specificallyKNbsOs, KTiNbOs, CsTitNbO7,NazTi30.1, K2T&0grand &Nb601703H20 as photocatalysts for nonsacrificial hydrogen evolution. A schematic drawing and potential energy diagram for these systemsare shown in Scheme I. The conduction I band edge potentials of these semiconductors are presumed from VB their photoactivity to be negativeof the hydrogenlwater potential. Their layered structures allow one to confine metallic catalysts to their interlayer spaces, at sites that are inaccessible to anionic metrically to hydrogen and triiodideusingvisible light. The failure oxidants such as 13-. in thecasesofthetwotitanateswillbediswedfromtheviewpoint Except for NazTisOl and K2Ti409,all LMOS’s studied herein of conduction band energetics. The finding that the hydrogen were found to photodecompose acidic iodide solutions stoichioevolution rate depends strongly on pillaring cations implies that the low intrinsic conductivity between the layers is responsible t Center for Fast Kinetics Research. Abstract published in Advunce ACS Abstrocrs, October 15, 1993. for the low quantum yield of hydrogen evolution.
Ij
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0022-3654/93/2091- 1 1802$04.00/0
Q 1993 American Chemical Society
Sensitized Layered Metal Oxide Semiconductor Particles
The Journal of Physical Chemistry, Vol. 97, No. 45, 2993 11803
Experimental Section Materials. Reagent grade K2CO3, Na2CO3, Cs2CO3, Ti02 (99.9% anatase), and Nb2O5 (99.99%) were obtained from commercialsources. K4Nb6017, KTiNbO5, CsTizNbO7,and Na2Ti307 were prepared by heating intimate mixtures of stoichiometric amounts of the appropriate metal oxides and carbonates in alumina crucibles (Coors) at 1050 "C for 2 days. For KNb308, K2CO3 and Nb2O5 were mixed in a 1:3 mole ratio and heated to 600 "C for 2 h and then to 900 "C for 3 h. K2Ti409 was prepared by heating a mixture of K2CO3 and Ti02 in a 1:3.5 mole ratio at 800 "C for 20 h and then for another 20 h after grinding. The phase purity of all these materials was confirmed by X-ray powder diffraction.3 Microcrystallitesof these materials were invariably flat plates, and the long particle dimensions were roughly 2-10 pm as determined by SEM. Tris(2,2'-bipyridyl-4,4'-dicarboxy1ate)ruthenium ( R u L ~ ~ +dichloride ) was the same as used previously.2 [Ru( bpy)z(CN)2] 2Ru( bpy(COOH)2)2*PF6(bpy = 2,2'-bipyridine) was a generous gift of Dr. K. Nazeeruddin and Prof. M. Grltzel. HI was freshly distilled from 50%HI containing hypophosphorous acid. All other chemicals were reagent grade and used as received from commercial sources. Sample Preparation. Platinization of these layered semiconductors was carried out basically following the method of Domen et al.le Typically 5 g of the LMOS was suspended in 150 mL of H20, an appropriate amount (0.01-0.1 wt %) of Pt(NH3)4C12 was added to the aqueous solution, and the suspension was stirred for 3 days at room temperature. After being filtered and washed with copious quantities of deionized water, the samples were dried at 40 OC in air. Reduction to elemental Pt was carried out in a hydrogen stream at 200-600 "C, and the materials were subsequently treated with 3:l (volume) concentrated HCl/HNO3 at about 90 OC for 1h to remove externally sited platinum particles. Subsequent acid exchange was carried out in 0.5-1 .O M HCl solutions. The solutionswere filtered and replaced daily to ensure full ion exchange, which required 1-2 days for K4Nb6O17, KTiNbO5, CsTi2Nb0-1,and KNb308 and 1-2 weeks for Na2Ti307 and K2Ti409. The fully acid-exchanged materials gave pH 3-4 suspensions in deionized water, while the original alkalimetal forms gave pH 9-10. The alkali/transition-metal ratio in the acid-exchanged materials was determined by electron microprobe analysis, and the released potassium ion was measured gravimetrically from precipitation with tetraphenylborate. The photosensitizer R u L ~ ~was + adsorbed onto the acid-exchanged semiconductor powders from 1 X l t 5 to 2 X le5M aqueous solutions, and the pH was adjusted to about 3.0 using an aqueous HClO, solution. Apparatus. Steady-state photolyses were performed with a 500 W Xe (Hg) arc lamp, equipped with a Pyrex focusing lens, 450 f 50 nm interference and 400nm cutoff filters, and a Pyrex water filter. The light was focused onto the window of the photolysis cell with the same area as the light beam. The light intensity, measured with a power meter, was 20-25 mW/cm2 at the cell window. Procedures for photolysis and hydrogen analysis were previously described.2 Diffuse reflectance flash photolysis experiments were carried out at the Center for Fast Kinetics Research, University of Texas at Austin. The instrumentation and data analysis for diffuse reflectance flash photolysis are described in a previous paper? Concentrations of photogenerated 13-were measured spectrophotometrically using e(352 nm) = 26 400 M-l ~ m - l . ~
Results and Discussion Structures and Energetics of LMOS's. Six different types of layered oxide semiconductors were used in this study. These are alkali-metal niobates (bNb6017 and KNb&), titanates (Na2Ti307 and K2Ti409), and titanoniobates (KTiNbO5 and CsTi2Nb07). The structures of all these layered oxides (Figure 1) can be characterized by the nature of their octahedral slabs (Moa"-)
Na2Ti307
K2Ti409
K4Nb60 17
a
a
b
CsTi2Nb07
a KTiNbO,
KNb30,
Figure 1. Idealized structures of LMOS's used in this work. Hatched squares represent MO6 octahedra, and filled/empty circles represent alkali-metal ions at different elevations.
3-
h
2-
!5: 1-
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Wavelength (nm)
Figure 2. Diffuse reflectance UV spectra of several LMOS's, K4Nb60173H20 (A), KNb3Os (A), KTiNbOs (O), CsTi2NbO.r (0),NazTi307 (B), and K2Ti409 ( O ) , compared with anatase Ti02 (-), and Nb2O5 (- - -).
and the relative positions of these slabs with respect to one another. A double plane of edge-sharing octahedra is assembled by shearing in the direction perpendicular to the plane, every third octahedron for NaTi3O.l and CsTizNbO7, every fourth octahedron for K2Ti409, and every second octahedron for KTiNbO5 and &Nb6017.296 All these materials have acid-exchangeable alkali metals in their interlayers. When acid-exchanged, the interlayer spacingsof NazTi307 and KTiNbO5 decrea~e,~ while the spacings increase in CsTizNb0.1,KNb308, and KzTi409 because they are hydrated as HTi2Nb0702H20, HNb308=H20,and H2Ti409-H20, respectively.6av8 Among these materials, &Nb6OI7is unique in that it has two crystallographicallydistinct interlayers. One of these interlayers is easily hydrated while the other is not. The hydrated material has usually three water molecules per formula unit, i.e., &Nb6017*3H20.3a While all the other LMOS's can be acidexchanged (albeit very slowly in some cases) to the extent that almost 100% of their alkali-metal ions are replaced by protons, only about 50% of the potassium ions of &Nb6017*3H20can be exchanged at room temperature, even after periods of weeks.9 These samples are denoted "K2H2Nb6017" hereafter. Theaverage layer spacing of &Nb6017-3H20 also decreased upon acid exchange, as evidenced by a shift in the strongest (040)layer diffraction line. While their structures are well-known, these materials are not well-characterized as semiconductors. They are all referred to as n-type semiconductors, like TiO2, because of their intrinsic oxygen deficiency. Figure 2 shows diffuse reflectance UV absorption spectra of all these materials, compared with those of
11804 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
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Kim et al. typically employ solution-phase molecular probes, and photocurrent or luminescence is monitored as a function of solution pH. The Nernstian relationship between the flatband potential, Eb, and pH allows the former to be evaluated conveniently for oxide semiconductors. It is, however, impractical to apply these methods in the case of the LMOS’s, because their composition changeswith pH, and equilibrationwith the surroundingmedium is often extremely slow. In some of these materials, protonation of the interlayer space is irreversible over time periods of weeks. Additionally, since changing the pH of the solution affects not only the Helmholtz layer at the interface but also the LMOS’s interlayer composition, variation of the band edge potentials of the LMOS’s is not necessarily Nernstian. Although the band edge positions of the LMOS’s are not easily determined experimentally, a theoretical prediction is possible using concepts of electronegativity. The conduction band edge energy of a semiconductor at the point of zero charge (Eao) can be expressed by”
E,’ = E - X + II2Eg
& 2 1
0
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Figure 3. Diffuse reflectance UV spectra of (a) Na2Ti3O.l (m), KzTuOg (O),CsTi2Nbq(O)and(b) &bJb60173H20(m),KNb308(0),KTiNbO5 ( 0 ) .compared with their acid-exchanged forms (- - -).
TABLE I: Estimated Band Gaps and Conduction Band Edge Energies at the Point of Zero a r g e for LMOS’s Eao (V vs NHE) E, (ev) Ti02 (anatase) 3.22 -0.30 Nb205 3.0 -0.55 Na2TilO.l 3.47 -0.96 K2Ti409 3.48 -0.98 KTiNbO5 3.50 -0.79 CsTi~Nb07 3.67 -0.83 KNb3Os 3.53 -0.45 hNb6017 3.52 -0.76 HzTi307 3.27 0.015 H2T409 3.25 -0.044 HTiNbOs 3.47 0.021 HTi2NbO.r 3.37 -0.054 HNb3Os 3.58 -0.073 K2H2Nbb017 3.45 -0.265 anatase Ti02 and bulk Nb20-5. The absorption onset wavelengths of all these materials are blue-shifted by 30-40 and 50-60 nm with respect to those of anatase Ti02 and Nb205, respectively. Assuming that they are indirect gap semiconductorslike Ti02,I0 the LMOS band gaps can be estimated from plots of the square root of Kubelka-Munk functions F(R)vs photon energy. Table I shows the estimated band gaps of all these materials. The band g a p are all 0.3-0.5 eV larger than that of Ti02 anatase. Interestingly, the acid-exchanged forms of Na2Ti307, K~Ti409, and CsTizNbO7 show absorption onsets that are red-shifted by 20-55 nm (Figure 3a), while the spectra of KTiNbOs, hNb6017, and KNb3Os do not change appreciably upon acid exchange (Figure 3b). In the case of H2Ti307 and H2Ti409.H20r the band gaps become quite close to that of anatase TiOz. Needless to say, knowledge of the band edge potentials of these semiconductors is particularly important to their use in photochemical hydrogen evolution systems, whether they are photosensitized with visible-absorbing dyes or excited with band gap light. There are photoelectrochemical” and spectroscopic12 methods for experimental determination of conduction band edge potentials for particulate semiconductors. These measurements
where Ec is the energy of free electrons on the hydrogen scale (=4.5 eV), Xis the electronegativity of the semiconductor, and E, is band gap energy of the semiconductor. The electronegativity of a semiconductor can be calculated as the geometric mean of the electronegativities of its constituent atoms.14 The atomic electronegativityis also given by the Mulliken definition, that is, the arithmetic mean of the atomic electron affinity and the first ionization energy. Butler and Ginley15calculated band potentials for several oxide semiconductors using this method and showed that the predicted values were in reasonable agreement with the measured flat band potentials. Prediction of conduction band edge potentials of LMOS’s by this method may be interesting, especially in comparison with TiO2. Although this method cannot give reliable absolutevalues, because structural factors are neglected in the estimation of their electronegativity,it may give a rough estimate of their positions relative to TiO2. The conduction band edges of several LMOS’s at the point of zero charge (pzc) predicted by eq 1 are shown in Table I. Assuming that the pzc of Ti02 is the sameas its isoelectric pH (6)16 and that of the LMOS’s is very roughly 9 from the pH of their suspensions in H20, most of alkali-metal forms of LMOS’s show more negative conduction band edges at the same pH than TiOz. However, the acid-exchanged forms have more positive conduction band edge potentials than Ti02 owing to the high electronegativity of hydrogen relative to the alkali metals. In studies of the titanoniobates (CsTizNbO, and CsTiNbOs) using band gap excitation,“ the proton-exchanged forms showed lower photocatalytic activity for hydrogen production from aqueous methanol solution but higher activities for oxygen production from silver nitrate solution. In the case of the titanates (NaTi3O.l and KzT409) the proton-exchanged forms also had lower catalytic activity for hydrogen evolution,17despiteincreased light absorption due to their smaller band gaps. These results support the idea of positive band edge shiftsupon proton exchange, favoring oxidation reactions (e.g., oxygen evolution) while disfavoring reductions such as hydrogen evolution. Emission Quenching and Mff~useReflectance Flash Photolysis of Rutbeniom Polypyridyl Sensitizers on LMOS’s. RuLj2+was immediately adsorbed on the surface of all six acid-exchanged LMOS’s,and at solution pH’s of 3 or below the emission of the adsorbed complex was ca. 90% quenched by electron transfer to the semiconductor. Figure 4 compares the pH dependence of the steady-state emission intensity of RuLs2+ adsorbed on K2H2Nb6017 and Ti02. The emission behavior of this sensitizer on all the other LMOS’s was quite similar to that of K~H2Nb6017. While the emission quenching in the case of Ti02 begins at about
Sensitized Layered Metal Oxide Semiconductor Particles
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11805 0.018
g
-
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2
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4
--
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7.0
PH Figure 4. pH dependence of the emission intensity of RuL3+2 adsorbed onTiO2(Degussa)(1 X 10-6mol/g)andK~H2N&017(5X l@7mol/g). The emission was monitored at 630 nm, using 466-nm excitation. L
J
300
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Wavelength, nm
Figure 6. Diffuse reflectancetransient spectra of K Z H ~ N ~ O ~ ~ / R U L ~ ~ + ( 5 X le7mol/g) in 33 mM KI solution at pH 3.0 (a) uncorrected transient spectra recorded 11 (m) and 140 ps ( 0 )after excitation; (b) corrected spectra (sec Appendix).
Time (second) Fignre 5. Diffuse reflectance transient bleaching signals, monitored at 466 nm, of R u L ~ ~( 5+X lCr7 mol/g) adsorbed on (a) H2Ti307, (b) HNb&H20, (c) HTiNbOs, and (d) Kz&Nb&. pH 6, coincident with surface adsorption of the dye,18 with the LIMOS'S the emission quenching begins below pH 5 and is efficient only below pH 3. This shift is in qualitative agreement with the calculation of negative conduction band edge potentials, relative to Ti02, of the LMOS's. Time-resolved experiments were conducted to measure the emission quenching rate, which could not be resolved on the shortest instrumental time scale available (ca. 0.1 la). To monitor the kinetics of charge recombination following electron injection into the LMOS's, the transient bleaching of ground-state RuLs2+ was recorded in diffuse reflectance flash photolysis experiments. Figure 5 shows these bleaching recovery signals monitored at 466 nm. As previously found for K2H2Nb017,2 all LMOS's show charge recombination between R u L ~ ~ + and conduction band electrons on a time scale of hundreds of microseconds, and in all cases the decays are biphasic. In the earlier study of R~L3~+-sensitized colloidal Ti02 by Gratzel et a1.,19chargerecombinationwas found tooccur with a rateconstant of 4 X los s-l. Although it is difficult todefine the recombination rate in the LMOS cases because of their nonlinear decay, the slow component appears to be at least 100 times slower than colloidal TiO2. Considering the inverse MLCT lifetimeof RuLsZ+ (ca. 2 X lo6s-I) as a lower limit for the charge injection rate from the excited complex to the conduction bands of LMOS's, the charge recombination rate is at least 3 orders of magnitude slower than the forward electron transfer rate. When I- is added to the suspension as an electron donor, the bleaching at 460-470 nm disappears completely for all LMOS's on the shortest observable time scale (ca. 100 ns), and broad positive transients at 380 and 600-800 nm are instead seen. These spectra may be interpreted as follows: an electron is injected into the semiconductor by excited-state MLCT, and immediately thereafter the ground-state sensitizer is regenerated by electron transfer from I-. Figure 6a shows the transient spectrum of R U L ~ ~ + / K ~ Hin~0.05 N ~M~ KI O solution. I~ Before interpreting this spectrum in detail, we note that the relative absorption
0
50
100
150
200
Time @second) Figwe 7. Transient absorption signals monitored at 380 and 700 nm for the same sample shown in Figure 6b.
intensity in the diffuse reflectance mode is dependent on wavelength, because the total path length of the analyzing light through the sample is influenced by light scattering, which varies inversely with the fourth power of the wavelength. Thederivation for the spectral correction, based on previous theoretical treatments of transient diffuse reflectance spectroscopy,M is given in the Appendix. In the corrected spectra (Figure 6b) the nearinfrared band is depressed relative to a band at 380 nm, and an additional band at 350 nm becomes prominent. Figure 7 shows decays from the corrected spectra, monitored at 380 and 700 nm. Both decays follow second-order kinetics with a first half-life of 79 ps. We assign the band at 380 nm and broad near-infrared band to the same species,12',2l and the band at 350 nm to 13-. The following reaction sequence depicts the formation of the charge-separated state, 13--e-(CB):
-+ -
*RuLF RUL? + e-(CB) RuL,~+ IRUL,~' I' I' I- 12*-
+
21;-
+
-+r I,-
+
(2)
(3) (4) (5)
The decay of the 12'- signal can be complex, considering possible recombination with the injected conduction band electron and the disproportionation reaction (5).22 However, the observed
€1806 The Journal of Physical Chemistry, Volt 97, No. 45, 1993
Kim et al.
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Time (second) Figure 8. Transient bleaching signals at 470 nm for the trinuclear Ru complex adsorbedon K2H2Nh017 in (a) H2O and (b) 33 mM KI solution,
both at pH 3.
Figure9. Photochemical hydrogen evolutionfrom R~L3~+-sensitized (a) K2H2Nk017 (m), (b) HTiNbOs (A),(c) HTi2NbO~2H20(a),and (d) HNb3OsH20 ( 0 )in 0.1 M KI or CsI solution with 0.05 wt 4% Pt.
second-order decay of 12'- and concomitant growth of the 13band at 350 nm in the corrected transient spectrum imply that reaction 5 is the predominant decay pathway for bo-. Fitzmauriceand Frei have studied the photooxidationof iodide at R~L3~+-sensitized colloidal Ti02 by near-infrared absorption ~pectroscopy.~~ From the observation that the transient at 700800 nm has a much higher extinction coefficient than that of 12'and undergoes single-exponential decay, they proposed the formation of a surfacebound RuLp2+.12'- ion pair. The near-IR band absorption was assigned to the bipyridyl-12-charge-transfer absorption of the ion pair. In contrast with their results, in the LMOS case the transient at 700 nm decays via second-order kinetics, with same half-life as the transient at 380 nm. The transient spectra and kinetics therefore argue against ion-pair formation between IZ*-and RULJ~+ on the surface of the LMOS's. The trinuclear Ru complex [Ru(bpy)~(CN)2]zRu(bpy(COO)(COOH))2 has been shown togive unusually high incident light photocurrent quantum efficiency (almost 100%) with high surfacearea of polycrystallineTi02 electrodes.24This remarkable result has been attributed to the two antenna moieties of the sensitizer, which can harvest light efficiently, as well as to the high efficiency of charge injection into the semiconductor. These results suggested that this molecule was a promising candidate for photosensitization of the LMOS's. Figure 8a shows the transient bleaching of the ground state of the trinuclear Ru complex adsorbed on KzHzNb6017. The transient MLCT bleaching undergoes a decay very similar to that of R u L ~ ~ + , indicatinglong-lived charge separation between the oxidized form of the complex and the photoinjected electron. In the presence of iodide as an electron donor, however, the bleaching recovery was only slightly perturbed on a 100-ps time scale, indicating very slow electron transfer between I- and the oxidized complex (Figure 8b). This contrasts with the R U L J ~case, + in which the bleaching disappears completely within a few tenths of a microsecond at the same concentration of iodide. Slow reduction of the oxidized trinuclear complex could be a consequence of steric hindrance by the two bulky antenna moieties, which prevent access of the donor of the oxidized metal center, or of poor energetics for the electron-transfer reaction between iodide and the oxidized semiconductor. Apparently, this electron-transfer reaction occurs very efficiently in the electrochemical system, albeit on a longer time scale. Pbotocbemical Hydrogen Evolution. Among the LMOS's studied, the RuLP-sensitized K2HzNb6017, HTiNbOs, HTi2Nb07.2H20, and HNb308.H20 (all internally loaded with platinum) produced hydrogen and triiodide in stoichiometric amounts in acidic iodide solutions upon visible light (>400 nm) excitation. However, the sensitized titanates (H2Ti307 and HzTh09*H20) evolved only traces of hydrogen under the same conditi0ns.2~Figure9 shows the timecourseofhydrogenevolution from these LMOS's in 0.1 M KI or CsI (for CsTizNbO7) solutions. In all cases, the hydrogen evolution rate gradually decreased and
fell nearly to zero following photolysis. The highest initial quantum yield of product formation was ca. 0.3%, in the case of KzH~Nb6017,as previously reported.2 In our previous paper,2we showed that the leveling off of the hydrogen evolution rate was not due to degradation of catalysts, but to the accumulation of 13-. Additionally, it was found that the initial quantum yield for hydrogen evolution was more than an order of magnitude smaller than that of formation of Iz*-. If formation of 13- is the dominant decay pathway for I2*-, as shown in diffuse reflectance flash photolysis experiments, the loss of quantum yield for initial hydrogen evolution must arise from the recombination of 13- and conduction band electrons. The major difference between the titanates and the niobates or titanoniobates is that the exchange reaction between protons and alkali-metal cations in the titanates occurs much more slowly than in the other LMOS's. When the acid-exchanged niobates and titanoniobates were suspended in iodide solutions of alkalimetal cations, the pH of the suspension immediately dropped to 1-2 from 3-4 due to the release of interlayer protons; no change in pH was observed in the case of titanate suspensions. The XRD patterns of niobates and titanoniobates, separated from these suspensions by filtration, showed two different kinds of interlayer spacings, corresponding to acid-exchanged and alkalimetal forms. However, the titanates showed no change in their XRDpattems. Thisindicates that theniobatesand titanoniobates exist as partially proton-exchanged forms in alkali-metal iodide solutions during photolysis, although they are initially fully acidexchanged,while the titanatesexist in fully acid-exchangedforms. Our calculations (Table I) show that the conduction band edge potentials of these LMOS's shift dramatically to more positive potentials upon proton exchange. Therefore, the acid-exchanged titanates should have much more positive conduction band potentials than other LMOS's under the same photolysis conditions. The lack of activity of HI photolysis for the titanates, in contrast to the niobates and titanoniobates, owes to the fact that theconductionband potentialsof the acid-exchanged titanates are too close to the hydrogen/water potential. Effectively, the semiconductor conduction band can mediate electron-transfer reaction in the reverse sense, between hydrogen and triiodide, catalyzing the recombination of products formed.16 Consistent with this hypothesis, we find that hydrogen is evolved efficiently from RuLa2+-sensitized,internally platinized H~Ti409when a sacrificial electron donor (H2EDTA2-) is substituted for the reversible electron donor I-. Interestingly, the hydrogen evolution rate with platinized KzHzNb6017 depends strongly on identity of the alkali metal used as a counterion for iodide. Figure 10 shows hydrogen evolution/time curves from the sensitized hexaniobate in several different alkali-metal iodide solutions. The initial evolution rate increased in the order Na < Li < Cs < K < H. This increasing order of the hydrogen evolution rate corresponds to the order of interlayer spacings from the largest to the smallest one when the
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The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11807
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Fipe 10. Photochemical hydrogen evolution from R u L ~ ~ + / K ~ H ~ N ~ s O IFigwe . ~ / 12 Photochemical hydrogen evolution from R u L ~ * + / K ~ H ~ N ~ , ~ O I ~ / Pt(0.05 wt 5%) in 0.7 M alkali-metal iodide solutions at pH 3: KI (m), Pt(0.05 wt 96) in 0.1 M KI, showing the effect of different Pt(NH,),2+ HI (A), CsI
(o),LiI (O), and NaI (0).
0
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reduction temperatures (200,450, and 650 "C).
0.c
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Time (min)
Figwe 11. Photochemical hydrogen evolution from R U L ~ ~ + / K ~ H ~ N ~ , ~ O Figure I , / 13. Photochemical hydrogen evolution from RuLip2+/KzNbs0l7 Pt(O.05 wt %) in 0.1 M KI at RuLa2+loadings at 5.0 X lO-' (monolayer in 0.1 M KI, prepared with initial platinum loadings of 0.1 and 0.05 wt mol/g. coverage), 1.0 X lw, and 1.0 X %.
niobate is ion-exchanged with these alkali-metal ions.27 The interlayer spacing does not depend directly on the ionic radius of the alkali-metal cation because of hydration effects. In this system, the only source of conduction band electrons is the photosensitizer, which is confined to the particle external surface. Only near-surface platinum clusters are accessible to the photoinjected electrons, most of the platinum in bulk being inaccessible becauseof poor electronicconductivitybetween layers. Therefore, the dependence of the hydrogen evolution rate on interlayer spacing may be related to a greater interlayer electron tunneling rate with decreasing interlayer spacing. There are several factors which can affect hydrogen yields in this system. To optimize the system, hydrogen evolution rates were measured under several different conditions. Figure 11 shows the effect of varying the loading of the photosensitizer, RuL32+. At high loading of sensitizer, the reaction rate significantly decreased. This can be attributed to an inner filter effect, because of formation of sensitizer multilayers on the semiconductor surface. Electron injection is only efficient for the layer in contact with the semiconductor. This was confirmed in flash photolysis experiments, in which almost no long-lived bleaching of the ground-state absorption of RuLs2+ was observed in the samples with mol/g of loading. The optimum loading, ca. 5.0 X lo-' mol/g, is consistent with roughly monolayer coverage of the external surface of the semiconductor. The reduction temperature for the platinization step and the loading of platinum also affected the hydrogen production rate. Figure 12 shows results from samples prepared at three different temperatures. The optimum platinum reduction temperature is in the range 400-500 OC. High-temperature treatment results in stronger interaction between platinum and the semiconductor. possibly by producing an Ohmic, low-resistivity contact?* However, it also can cause platinum migration to the edges or outersurfaceofthecrystallites. Externallysitedplatinumclusten will serve to catalyze the dark recombination reaction between Hz and 13-, since they are physically accessible to I,-. These two
opposing effects of temperature should give an optimum temperature for photocatalyst preparation. The hydrogen evolution rate also depended on the initial platinum loading, prior to aqua regia treatment. Loading levels of platinum higher than 0.05 wt %usually resulted in lower rates of photochemical hydrogen evolution. Figure 13 shows typical hydrogen evolution curves for two samples prepared in the same conditions, but at different initial platinum loadings. Although both materials were treated with aqua regia to remove platinum on the external surface, this procedure is clearly ineffective at very high platinum loading. For water decomposition by the hexaniobate, using band gap excitation,Ic lower activity at the higher platinum loading was also observed. This suggests again that the aqua regia treatment does not completely remove externally sited platinum at high loading levels.
concl~iolM Layered metal oxide semiconductors, Ib,H,NbaO1,.nH20, K1-,HxNbsOs*nH20, K1-,HXTiNb05, and Csl-xHxTi2Nbo7.nH20 sensitized with RuLs2+ yield hydrogen photochemically from acidic aqueous iodide solutionswhen they are internally platinized. Two factors are responsible for hydrogen evolution from a nonsacrificial donor, iodide, in these systems. First, their structures and anionic framework separate the anionic electron donor from internal catalytic sites, and second, their conduction band edge potentials are significantly more negative than the hydrogen/water formal potential. Acid-exchange equilibria of these materials seems to play a crucial role in hydrogen production, because their conduction band edge potentials depend on the interlayer composition of alkali-metal ions and protons. The finding that HzTisO, and H2Ti409-H20r in which exchange reactions between protons and alkali-metal ions are extremely slow, do not produce hydrogen supports this hypothesis. The low interlayer conductivity in LMOS's favors charge recombination at the external surface over electronmigrationto platinum clusters
Kim et al.
11808 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
contained within the semiconductor particles. The strong dependence of the hydrogen production rate on interlayer spacing argues strongly for the importance of interlayer conductivity. Since low interlayerconductivity is inherent in twedimensional solids of this type, one might imagine that the use of oxide semiconductors that have zeolitic channel structures, rather than layered structures, might result in higher quantum yields for hydrogen evolution. Research in this area is currently in progress.
Acknowledgment. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Department of Energy, under Contract DE-FG05-87ER13789. Flash photolysis experimentswere carried out at the Center for Fast Kinetics Research (CFKR), University of Texas at Austin. CFKR is supported jointly by the Biomedical Research Technology Program of the Division of Research Resources of NIH (RR00886) and by the University of Texas at Austin. T.E.M. thanks the Camille and Henry Dreyfus Foundation for support in the form of a Teacher-Scholar Award.
that would be obtained in the hypothetical situation where S = 0, w is a geometrical constant, and P Ais the absorption coefficient of the species which generates the transient by absorbing at the excitation wavelength. Finally, the measured reflectance R.at the analyzing wavelength is proportional to the background reflectance P s ,according to eq (AS), in which y = a / R . ~ k6,
+
= 1 y -&Sa/&, and u = 2Pr(O)/k. The derivation of (AS) is given in ref 20b. Since the measured quantity N / J o is related to P / @ Bby eq A6, one may compute a spectral correction factor according to (A7), assuming the same Adf for both X p and Xc:
P A!!= 1 -JO
Appendix. Transient Diffuse Reflectance Spectral Correction Transient signals observed in the diffuse reflectance mode, measured as (Rae-P), where Raeis the background reflectance and Ra the transient reflectance at the analyzing wavelength, and relative absorption AJ/Jo, have been shown to be directly proportional to the amount of absorber present at low concentrations.20gWhile this relation is sufficientat a singleanalyzing wavelength, a correction must be implementedfor spectra,because scattering factors Sawill in general vary with wavelength of the analyzing light. For dilute suspensions of the type used in this work, Savaries inverselywith the fourth power of the wavelength. As long as the sample remains optically dense at all wavelengths (i.e., no light penetrates the entire sample), the scattering theory developed by Lin and KanZoband later used by Kessler et a1.20a for the special case where Sais constant should be applicable for spectral correction. In this Appendix we develop a theoretical spectral correction based on this model, which is appropriate for normal-incidence pump and probe light. Experimentally, this analysis was found not to apply to the translucent samples and experimental configuration used, and so an empirical spectral correction was also developed. Briefly, the Lin-Kan model assumes that an exciting pulse at wavelength Ae creates a distribution of transient that varies exponentially with distance into the sample, according to (Al), C(x) = C(0) exp(-kx)
('41)
where C(x)and C(0)are respectively the concentration at depth x and at the surface. The inverse scattering length k is related to the total reflectance Rc, and the scattering factor Scat the excitation wavelength by eq A2. Rc, is also related to the
absorption coefficientsKee and K'A for background and absorber species at the excitation wavelength by the Kubelka-Munk equation (A3). Theabsorption coefficient P ( x ) at the analyzing (A31 wavelength is the sum of contributions from background and transient absorbers Kag and PT(x), the former being constant and the latter being given by (A4), in which &is the absorbance
P B
correction factor
Computing the correction factor from (AS) and (A7) requires calculation of RB, 7, 6, and u at both Xa and Xc. R.Bcan be obtained from PB, the background absorption coefficient at the analyzing wavelength, through Kubelka-Munk equation (A8):
In principle, both Rcm and R.Bare measurable quantities. However, in practice it is convenient to choose a reasonablevalue for Rc, and to make some assumptions about the relative magnitudesofPB, Kce,and Kea. We will show that the theoretical correction factor is not particularly sensitive to these parameter values. We first assume a wavelength-independent background absorptioncoefficient,Le., PB= KC^. The absorptioncoefficient of the absorber at the excitation wavelength, KOA,is a factor n greater than the background absorption coefficient, Le., P A / P B = n. By combining (A3) and (AS) to obtain (A9), R.Bcan be calculated from Rem, using the relation Sa/P= (Xe/X*)4.
where
We define a quantity i%,, such that bcm = k / P , which can be calculated from I?, by means of (A2). Then y = (Sa/*)(bCmRaB)-*I 6 = + 1 - (*/*(R'B/bCm), and U = 211?~(0)/ ( p a )= [Adrw(l + P m ) b C m ] ( p A / * ) . The ratio (a/*) is ~ ) be calculated from taken as (Xc/Xa)4, and the ratio ( P A / S may (A3) using P A / K ' B= n. Thus, with the assumption that K.B= K'B, the correction factor may be calculated for any Xa and Xc, assuming reasonable parameter values for I?,, A m , and n. Figure 14 shows plots of theoretical correction factor vs analyzing wavelength, for both 532- and 355-nm laser excitation, using I?, = 0.7, Adf = 0.1,and w = 2. Variation of p, between 0.6 and 0.8 and the product &rw between 0.2 and 0.4 causes barely perceptible changes in these plots for n = I .O (Figure 14a). Varying n within reasonable limits (between 0.2 and 5.0) also
Sensitized Layered Metal Oxide Semiconductor Particles
The Journal of Physical Chemistry, Vol. 97, No, 45, 1993 11809 .-x e
~
2.01
...
.......>
-5
Y
E
U
14
........;y= 5 3 2
.-8
..._
I
E
s V
1.0-
......
M
-=KCA
..... -..-
v)
1 .o
KCB
__
... ....................
............
....
0 e
Y
I
0.0 300
400
500
600
700
800
300
Figure 14. Theoretical diffuse reflectance spectral correction factors, calculated from eq A7: (a) comparison of 532- and 355-nm excitation and (b) effect of variation of the absorbcr to background extinction coefficient ratio at the excitation wavelength.
causes rather small changes in the correction factors, as indicated in Figure 14b. We note that the theoretical correction factor varies significantly less sharply with analyzing wavelength than does the scattering factor 9, which varies with the inverse fourth power of An. This may be understood in terms of the variation of Rae with wavelength, which varies with Saaccording to (A8). If the background absorption coefficient KPB is small compared to Sa and roughly constant, then RBB = 1 - (2&/S8)IJ2. At short wavelengths where Sais large, there is relatively little variation in Rag,and the correction factor variesroughly as However, at wavelengths to the red of the excitation wavelength, Rae decreases with increasing wavelength (as does Ra),and there is therefore less variation in the correction factor. In terms of penetration depths kl,the analyzing light at sufficiently long wavelengths passes twice (once as incident light and once as reflected light) entirely through the front face of the sample, which contains all of the transient generated by the excitation pulse. This effect is quite evident in comparing the relatively flat correction curve for 355-nm excitation to that for 532-nm excitation. This model was tested experimentally by comparing transient spectra, measured in diffuse reflectance and transmission mode, for the same species produced in by laser flash photolysis. For this purpose, a R~(bpy)3~+-methylviologen (MV2+) aqueous solution was used. Photoexcitation of R~(bpy)3~+ at 532 nm produces an MLCT excited state, which is quenched by electron transfer to MV2+. The resulting charge-separated state has characteristic transient absorption bands at 400 and 610 nm ( M V ) and bleaching at 450-460 nm (Ru(bpy)s2+). These spectral features decay together on a time scale of several microseconds via thecharge recombination reaction. To eliminate spectral shifts and changes in extinction coefficients caused by adsorption, a scattering solid (neutral alumina) on which neither the reactants nor products adsorb in water was used in the diffuse reflectance experiments. Figure 15 compares normalized transient spectra for Ru(bpy)j2+-MV2+photoexcited at 532 nm, both in homogeneous aqueous solution and in an alumina suspension. The solution spectrum is consistent with the assignment of a R~(bpy)3~+MV'+chargeseparated state. In particular, therelativeintensity
500
400
Wavelength. nm
600
700
800
Wavelength (nm)
Figure 15. (a) Comparison of transient absorbance for aqueous Ru(bpy)32+-methylviolgen in aqueous solution and transient diffuse reflectancefor the samesolution to which neutral alumina has been added
as a scatterer. Both spectra are normalized to the excitationwavelength, 532 nm. (b) Ratio of the two normalized spectra (solution/suspension) used for empirical correction of transient diffuse reflectance spectra.
samp
m o noch r o m aio r
I
ser specular reflection \/
\
Xenon lamp analyzing light)
532 or 355 nm Nd:YAG (11 n s )
F w16. Diagramof the experimentalconfiguration for transient diffuse reflectanceexperiments. Light is collected from two faccs of the cell by the focusing lens. of reduced viologen transients are in accord with published extinction coefficient^^^ (e396 = 42 100 M-I cm-I, e610 = 13 700 M-L cm-I) for MV*+. In contrast, in the diffuse reflectance spectrum the 610-nm peak appears larger than the one at 400 nm. In Figure 15b, the ratio of the transmission to diffuse reflectance transient signals (again normalized to 532 nm) for R~(bpy)~j+-MV'+ is plotted; wavelengths for which the transient signals are close to zero (410-420 and 500 nm) and for which scattered laser light distorts the diffuse reflectance intensities (53&550nm) areeliminated from this plot. Notethat this figure is qualitatively similar to the theoretical curves shown in Figure 14b, except that in the experimental case a significantly larger spectral correction is needed in the blue part of the spectrum. The difference may arise because of the geometry of the experimental setup, shown in Figure 16. We note in particular that this configuration allows some analyzing light to be collected from the face of the cell at right angles to the one facing the laser and probe beams. Blue probe light emerging from this face might be expected to be highly attenuated because of the length of the path (several millimeters) it travels through the translucent suspension. The model used to produce the curves shown in Figure 14assumes
11810 The Journal of Physical Chemistry, Vol. 97, No.45, 1993
that the samples are optically dense and that the probe light is collected from the front face at normal incidence. The empirical spectral ratios (Xe = 532 nm) shown in Figure 15b may be adequately fitted to an exponential curve (A10) for X < 650 nm and to a straight line (All) in the 650-750-nm region where the correction factor is roughly constant. These empirical fits are shown as solid lines in Figure lSb, and were usedto obtain corrected diffuse reflectancespectrashown in Figure 6b. correction factor = 164.0 exp[(-9.59 X lO-’)X], X < 650 nm (A10) correction factor = 0.779 - (7.30 X lo4) A, X L 650 nm (A1 1)
References and Notes (1) (a) Domen, K.; Kudo, A.; Shibata, M.; Tanaka, A.; Maruya, K.; Onishi, T. Chem. Soc., Chem. Commun. 1986,1706. (b) Kudo, A.; Tanaka, A.; Domen, K.; Maruya, K.; Aika, K.; Onishi, T. J. Carol. 1988,111,67. (c) Kudo, A,; Tanaka, K. S.;Asakura, K.; Domen, K.; Maruya, K.; Onishi, T. J. Caral. 1989,120,337. (d) Sayacna, K.;Tanaka, A.; Domen, K.; Maruya, K.; Onishi,T. Cafal. Len. 1990,4,217. (e) Sayama, K.; Tanaka, A.; Domen, K.; Maruya, K.; Onishi, T. J. Phys. Chem. 1990, 95, 1345. (f)Sekine, T.; Yoshimura, J.; Tanaka, A.; Domen, K.; Maruya, K.; Onishi, T. Bull. Chem. Soc. Jpn. 1990,63,2107. (g) Domen, K.; Yoshimura, J.; Sekine, T.; Tanaka, A.; Onishi, T. Coral. Lett. 1990, 4, 339. (2) Kim, Y. I.; Salim, S.;Huq, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1991,113, 9561. (3) (a) Nassau, K.; Shiever, J. W.; Bemstein, J. L. J . Electrochem.Soc. 1969, 116, 348. (b) Wadsley, A. D. Acra Crysrallogr. 1964, 17, 623. (c) Andersson,S.; Wads1ey.A. D. Acra Crysrallogr. 1%1,14,1245. (d) Hervieu, M.;Raveau, B.J.SolidSrare. Chem. 1980,32,161. (e) Gasperin,P. M . Acra Crysrallogr. 1982,838,2024. ( f ) Berry, K. L.; Aftandilian, V. D.; Gilbert, W. W.; Meibohn,E. P. H.;Young, H. S.J. Inorg. Nucl. Chem. 1960,14,231. (4) Kim, Y. I.; Mallouk, T. E. J. Phys. Chem. 1992, 96, 2879. (5) Awtrey, A. D.; Connick, R. E. J. Am. Chem. SOC.1951, 73, 1842.
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~
NaZTi,@ wasalsoactive for HIphotolysisunder theseconditions. Nohydrogen evolution is observed from the fully acid-exchanged titanate. (26) Kim. Y. I.; Riley, R. L.; Hug, M. J.; Salim, S.;Le, A. N.;Mallouk, T. E. In Synthesisf Characterization and Nwel Applicarions of Molecular Sieve Materials; W a r d , R. L., Bein, T., Davis, M. E., Garces, J., Maroni, V. A., Stucky, G. D., Eds.;Mater. Res. Soc. Symp. Ser. 1991,233,145-156. (27) Kinomura, N.; Kumada, N.; Muto, F. J . Chem. Soc.,Dalron Trans. 1985,2349. (28) Hope, G. A.; Bard, A. J. J. Phys. Chem. 1983,87, 1979. (29) Watanabe, T.; Honda, K. J. Phys. Chem. 1982.86, 2617.
