10883
J. Phys. Chem. 1995, 99, 10883-10889
Photochemistry of R~(bpy)2(dcbpy)~+ on A1203 and Ti02 Surfaces. An Insight into the Mechanism of Photosensitization K. Vinodgopal? Xiao Hua: Robin L. Dahlgren? A. G. Lappin: L. K. Patterson,l and Prashant V. Kamat*-' Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry, Indiana University Northwest, Gary, Indiana 46408 Received: December 13, 1994; In Final Form: April 16, 1995@
Diffuse reflectance studies have been undertaken to investigate the photochemical behavior of a ruthenium complex, bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine)~the~um(~) [R~(bpy)2(dcbpy)~+], on the surface of A1203 and Ti02 particles. Decreased emission yield and lifetime of excited R~(bpy)2(dcbpy)~+ on Ti02 surface indicated dominance of the charge injection process in deactivating excited sensitizer. The rate constant for the heterogeneous electron transfer rate constant as measured from the analysis of luminescence decay was in the range (1.0-5.5) x lo8 s-l. Upon irradiation with visible light, the degassed sample of Ru(bpy);?(dcbpy)2+-coatedTi02 particles turned blue as photogenerated electrons got trapped at the Ti02 surface. The blue color disappeared instantly upon exposure to air. Irreversible degradation of the sensitizer was noticeable when air-equilibrated samples were photolyzed with visible light for an extended period. Diffuse reflectance FT'IR studies confirmed the semiconductor surface assisted photodegradation of Ru(bpy)2(d~bpy)~+.
Introduction Semiconductor nanoparticles such as Ti02 are paving the way toward more efficient, cost-effective means of utilizing solar energy for the conversion of electricity, synthesis of specialty chemicals, and mineralization of organic contaminants.' Currently, efforts are being directed to synthesize thin nanocrystalline semiconductor films from colloidal suspension^.^-^ These transparent semiconductor thin films possess a highly porous morphology and can be readily modified with sensitizing pigments. The sensitization properties of a variety of organic and inorganic molecules adsorbed on TiO2, SnO2, and ZnO films have been explored by several researchers?-' Photosensitization is a convenient method to extend the photoresponse of large bandgap semiconductor materials into the visible region and has been widely employed in imaging science applications. The recent reports of attainment of power conversion efficiencies up to 10% in diffuse daylight have renewed interest in developing photoelectrochemical cells for the conversion of solar energy into ele~tricity.~~ The principle of photosensitization of a semiconductor nanocrystallite is illustrated in Scheme 1. The energy difference between the oxidation potential of the excited sensitizer and the conduction band of the semiconductor acts as a driving force for the charge injection process. In a photoelectrochemical cell employing a dye-modified nanocrystalline semiconductor electrode, the injected electrons are collected at the conducting surface to generate photocurrent, and the redox couple present in the electrolyte regenerates the sensitizer. However, in the absence of a regenerative system, the oxidized dye can either recombine with the injected electrons or undergo irreversible chemical changes. Such chemical events have severe repercussions on the long-term stability of the sensitizer employed in t Indiana University Northwest.
Department of Chemistry and Biochemistry. Radiation Laboratory. NSF Summer Research Fellow. * Address correspondence to this author. Abstract published in Advance ACS Absrracrs, June 1, 1995.
*
@
SCHEME 1: Photosensitization of a Semiconductor Particle with a Sensitizing Dyd
St VB
-
Products
a The conduction and valance bands of the semiconductor particle are indicated by CB and VB, respectively. The electron-donating energy levels (oxidation potential) of the ground and excited state of the sensitizer are indicated by S and S*. Electron injection from excited sensitizer results in the oxidation of the substrate (S+).
photoelectrochemicalcells. Basic understanding of the photochemical processes on semiconductor surface is essential for tackling the stability issues and designing efficient photoelectrochemical cells for practical purposes. Ruthenium complexes have so far been proved to be most efficient in sensitizing nanocrystalline semiconductor films. Much interest has recently been directed toward the synthesis and sensitizing properties of Ru(bpy)2(dcbpy)2f and related ruthenium complexes4-I2because of strong visible absorption and resistance to ligand substitution,8aas well as their ability to interact with semiconductor surfaces.'' Despite their impressive performance as photosensitizers, little effort has been made so far to understand the photochemical behavior of ruthenium complexes on Ti02 semiconductor surface. Our earlier surface photochemical studies with organic dyes have indicated that the intrinsic property of the oxide support plays an important role in controlling the photochemistry of adsorbed molecule^.'^-'^ In these studies it was shown that semiconducting oxides such as Ti02 directly participated in the surface photochemical reaction while nonreactive oxides such as Si02 or A1203 did not influence the excited behavior of adsorbed substrate. Ruthenium complexes adsorbed on semiconductor surfaces have also been shown to exhibit decreased emission lifetime^.^^^^^'^^'^ We have now carried out diffuse reflectance and luminescence
0022-36541951209910883$09.0010 0 1995 American Chemical Society
Vinodgopal et al.
10884 J. Phys. Chem., Vol. 99,No. 27, 1995
SCHEME 2: Design of the High-Vacuum Cell Employed in Diffuse Reflectance Absorption and Emission Measurements Vacuum Llne
if #7 Ace Thread
'
v
....... -Glass
Kontes H. Vac Stopcock
frit
L J
#? Ace Thread
-
40 x 6 x 3 mm optical cell
w measurements of Ru(bpy)2(d~bpy)~+ adsorbed on A1203 and Ti02 particles. Spectroscopic measurements of these powder samples not only elucidate the mechanism and kinetics of charge injection process but also provide valuable information concerning the chemical changes associated with long term photolysis.
Experimental Section Materials. Bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine)ruthenium(II), [Ru(bpy)2(dcbpy)](PF& was synthesized by literature methods8 The compound was purified by chromatography using silica gel column and ethanol:H20 (containing 10% NaC1) as eluent. All other chemicals were analytical reagents and were used as supplied. Sample Preparation. Both Ti02 and A1203 were gift samples from Degussa Corp. The particle diameter of Ti02 powder is 30 nm, and the BET surface area is 50 m2/g. A1203 has a smaller particle diameter, 20 nm, and twice the BET surface area, 100 m2/g. R~(bpy)2(dcbpy)~+-coated Ti02 and A1203 samples were prepared by adding a known amount of the oxide particles to the R~(bpy)2(dcbpy)~+ solution in ethanol. These aqueous suspensions were stirred for an hour. The suspensions were rotavaporated to dryness. This method produced R~(bpy)2(dcbpy)~+-coated particles (referred as TiOd Ru(I1) and A1203/Ru(II)), with comparable concentration of sensitizer in each sample. The concentration of the sensitizer was 0.5 mg of Ru(II)/g of Ti02 or A1203 powder, which ensured approximately monolayer coverage of the sensitizer on the oxide surface. Optical Measurements. The diffuse reflectance absorption spectra of R~(bpy)2(dcbpy)~+-coated oxide samples were recorded with a Milton Roy 3000 array spectrophotometer with a diffuse reflectance attachment. Corrected emission and excitation spectra of the solid samples were measured with an SLM 8000C photon counting spectrofluorometer in a front face configuration. The measurements on degassed samples were carried out in a vacuum-tight 6 x 3 x 40 mm3 rectangular quartz cell. The cell design illustrated in Scheme 2 was convenient to degass the powder samples and carry out the optical measurements. The whole assembly could be inserted into the sample compartment of the respective spectrophotometers
without disturbing the vacuum. Steady-state photolysis was executed using a high-intensity beam (24W halogen lamp) from a Fiber-Lite Model 190 fiber optic illuminator. The sample was either pressed between the two microscope slides or filled in a high vacuum spectrophotometer cell described above. Lifetime Measurements. Emission lifetimes of degassed samples of A1203/Ru(II)and Ti02/Ru(II) were performed by a time-correlated single-photon counting technique using an apparatus that has been described e1~ewhere.I~The excitation source was a mode-locked, Q-switched Quantronix 416 Nd: YAG laser which provided 80 ps pulses of 355 nm light with a frequency of 5 kHz and an integrated power of 10 mW. Diffuse Reflectance Laser Flash Photolysis Experiments. Time-resolved diffuse reflectance laser flash photolysis experiments were carried out with the setup described earlier.14a,18 The 532 nm laser pulse (10 d, pulse width 6 ns) from a Quanta Ray DCR-1 Nd:YAG laser system was used for the excitation of the sample. A 1000 W xenon lamp was used as the monitoring source. The diffusely reflected monitoring light from the sample was collected and focused onto a monochromator which was fitted to a photomultiplier tube, and the photomultiplier output was input to a Tektronix 7912A digitizer. Before triggering each laser pulse, the cell was shaken to expose a fresh surface for excitation. The principle of diffuse reflectance laser flash photolysis is described by Kessler et al.19 Diffuse Reflectance FTIR Experiments. IR absorbance spectra of air-equilibrated samples were measured in the region 4000-400 cm-l, at a resolution of 4 cm-' using a Bio-Rad FTS 165 FTIR spectrometer. In all cases, the spectra displayed were obtained using the Bio-Rad diffuse reflectance accessory with samples exposed to air using a diffuse reflectance accessory. Depending on the particular sample, KBr, Ti02, or alumina was used as the background. Because of strong absorbance by oxides, Ti02 and Al203, spectral data below 1200 cm-' for the adsorbed species were not measurable. The diffuse reflectance FTIR spectrum of the R~(bpy)2(dcbpy)~+ complex was monitored by mixing it with KBr crystals. The diffuse reflectance FTIR spectra of the R~(bpy)2(dcbpy)~+ complex coated on Ti02 and A1203were monitored while undergoing steady-state photolysis using a high-intensity beam from a FiberLite Model 190 fiber optic illuminator.
Results and Discussion Absorption and Emission Characteristics. The diffuse reflectance spectra of R~(bpy)2(dcbpy)~+ coated on Ti02 and A1203 surfaces are compared with the solution spectrum in Figure 1. The absorption maximum of R~(bpy)2(dcbpy)~+ on the oxide surfaces is around 465 nm, which is very similar to that in aqueous solution. However, the absorption band is relatively broad on both oxide surfaces, and the tail absorption extends up to 650 nm. Broadening of the absorption band is an indication that a charge transfer interaction is responsible for binding the ruthenium complex on the oxide surface. Similar broadening of the absorption spectra has been observed for several sensitizers adsorbed on Ti02 ~ u r f a c e . ~The ~ ?broaden~~ ing of the absorption band is also beneficial in extending the photoresponse of Ti02 up to 650 nm. The emission spectra of degassed samples of Ru(bpy)2(dcbpy)2+* on A1203 and Ti02 are shown in Figure 2. These samples exhibited emission maxima around 625 nm. Comparison of the relative quantum yields of R~(bpy)2(dcbpy)~+* on these oxide surfaces indicated that the fluorescence yields are significantly lower on a semiconductor (Ti02, Eg 3 eV) surface than on an insulator surface (alumina, Eg 9 eV). Although the absorption of the R~(bpy)2(dcbpy)~+ samples coated on Ti02 and A1203 are similar at the excitation
--
J. Phys. Chem., Vol. 99,No. 27, 1995 10885
Ru(bpy)2(dcbpy)2+ on A1203 and Ti02 Surfaces
I
a. In aaueous medium
Omo2 Q)
0.01
t
A
0
E Q
-e
0.00
P
2 -0.01
'I
-0.02
l
*
.
Wavelength, nm solution, (b) on Al203, and (c) on Ti02 surface. Spectra b and c were recorded with a diffuse reflectance assembly using corresponding metal oxide as reference. The ordinate scale is expressed as Kubelka-Munk units.
k Y I
600
650
I II
700
Wavelength, nm Figure 2. Emission spectra of R~(bpy)2(dcbpy)~+ (a) on A1203 and (b, b') on Ti02 surface. Emission spectra were recorded in front face configuration. Excitation wavelength was at 470 nm.
wavelength, the emission yield of R~(bpy)2(dcbpy)~+* on Ti02 is only -5% that of Al2O3. This shows that significant quenching of the excited state has occurred on the Ti02 surface. As shown earlier'4-'5 with sensitizing organic dyes and Ru(bpy)32f, such an excited state quenching represents direct injection of electrons into the semiconductor particles. The processes controlling the excited state deactivation on a metal oxide (MO) surface can be summarized as follows (reactions 1-3).
+
Ru(bpy),(d~bpy)~+ hv Ru(bpy)2(dcbpy)2+*
+
Ru(bpy)2(dcbpy)2f*
+ MO
+
I
.
'
.
.
l
.
I
.
500
.
I
600
Figure 3. Time-resolved transient absorption spectra recorded following 532 nm laser pulse excitation of degassed R~(bpy)2(dcbpy)~+ in deaerated aqueous solution: (a) 0, (b) 0.2, (c) 0.6, and (d) 1.0 ps.
400 0.0 550
.
Wavelength, nm
Figure 1. Absorption spectra of R~(bpy)2(dcbpy)~+ (a) in aqueous
w
.
400
300
R~(bpy),(dcbpy)~+* (1)
+
R~(bpy)~(dcbpy)~'hv'
(2)
+
Ru(b~y)~(dcbpy+ ) ~M + W ) (3) where MO(e) is the electron trapped within the metal oxide particle. While deactivation of Ru(bpy)2(dcbpy)2+* occurs mainly via reaction 2 on the A 1 2 0 3 surface, reaction 3 dominates the decay of the excited state over Ti02 surface. The difference between the oxidation potential of excited sensitizer Ru(bpy)2(dcbpy)2+*(E" = -0.92 V vs NHE) and the conduction band of Ti02 (ECB= -0.5 V vs NHE)provides the necessary driving force for the charge injection process (reaction 3). On the other hand, such a charge injection into an insulator support, A1203, is not energetically feasible since its conduction band is more negative than the oxidation potential of Ru(bpy)2(dcbpy)2+*.
I
I
500
600
700
Wavelength (nm) Figure 4. Transient absorption spectrum recorded following 532 nm laser pulse excitation of degassed sample of Ru(bpy)z(dcbpy)*+on TiOz. The transient spectrum was recorded using diffuse reflectance laser flash photolysis, 45 ,us after laser pulse excitation. Insert shows the absorption-time profile at 650 nm.
Laser Flash Photolysis. The time-resolved transient absorption spectra recorded following the 532 nm laser pulse excitation of R~(bpy)2(dcbpy)~+ in aqueous solution are shown in Figure 3. These spectra show a broad bleaching in the 470 nm region, suggesting the depletion of the ground state. A simultaneous appearance of the transient absorption band at 370 nm confirms the formation of excited Ru(bpy)2(d~bpy)~+.The excited lifetime determined from the exponential decay of this transient was 0.5 ,us. The complete recovery of the bleaching at 470 nm indicated that Ru(bpy)2(dcbpy)2+does not undergo any irreversible photochemical changes in aqueous solution when excited with visible light (532 nm laser pulse). Similar behavior was also observed when samples of R~(bpy)2(dcbpy)~+ adsorbed on A1203 surface were subjected to 532 nm laser pulse excitation in a diffuse reflectance laser flash photolysis setup. Efforts were also made to probe the excited state behavior of Ru(bpy)2(dcbpy)2+on Ti02 surface using diffuse reflectance laser flash photolysis. (The sample is referred to as TiOdRu(II).) The excited state on the Ti02 surface had a short lifetime, and the scattering effects prevented us from time-resolving the transient absorption measurements in a short time domain (< 100 ns). However, it was possible to monitor the photochemical changes occurring at longer times. The transient absorption spectrum recorded at 45 p s following 532 nm laser pulse excitation of TiOz/Ru(II) sample is shown in Figure 4. The transient absorption spectrum shows bleaching of the dye at 470 nm and a broad absorption band in the red region (wavelengths greater than 500 nm). Although thermal desorption of the sensitizer with laser excitation is possible, its
10886 J. Phys. Chem., Vol. 99, No. 27, 1995
Vinodgopal et al.
contribution to the transient spectrum (Figure 4) is minimal. If this was a major process, we would not have observed absorption in the red region due to trapping of electrons. While most of the injected electrons recombine back with the oxidized sensitizer, R~(bpy)2(dcbpy)~+, a small fraction of the injected electrons gets trapped within the Ti02 particles. The broad absorption band observed in the red region of the spectrum in Figure 4 supports this argument. (The trapped electrons in metal oxide semiconductor particles have already been shown to absorb in the red-infrared region.20q21)The oxidized sensitizer does not exhibit any characteristic absorption in the visible part of the spectrum except for the weak bleaching at 460 nm. The irreversibility of bleaching at 470 nm indicates some permanent changes caused due to the oxidation of Ru(bpy)2(d~bpy)~+ (reaction 3). Thus, it is possible to confirm the net electron transfer between R~(bpy)2(dcbpy)~+* and Ti02 by characterizing electron transfer products in a diffuse reflectance laser flash photolysis experiment. Kinetics of Electron Injection Process. The emission lifetime of the adsorbed sensitizer serves as a good probe to study the kinetics of heterogeneous electron transfer between the semiconductor and excited sensitizer (reaction 3).l On the basis of the luminescence spectra and lifetimes of Ru(bpy)32+* on various metal oxides, Hashimoto et a l . I 6 have concluded that the interaction between the sensitizer and semiconductor as well as energetics of the semiconductor controls the rate of heterogeneous electron transfer. It has been shown that the carboxylic acid group of R~(bpy)2(dcbpy)~+ in the present case can provide a strong ester linkage between the semiconductor and sen~ i t i z e r . ~ ~Figure , ” 5A shows the emission decay of Ru(bpy)z(dcbpy)2+* on Ti02 and A1203 surfaces. When adsorbed on a nonreactive surface such as alumina, the excited state of the sensitizer is long-lived. A slight deviation from exponential behavior which is seen at short times is attributed to the excited state annihilation process.Ik The major component of this decay had a lifetime of 259 ns and is similar to the one observed in aqueous solutions. The long-lived excited state on the A1203 surface rules out any direct participation of the oxide support in deactivating excited sensitizer. While the influence of surface interaction on the excited state deactivation is minimal for the A1203 sample, it is significant for the Ti02 sample. When adsorbed on the Ti02 surface, the emission decay of R~(bpy)2(dcbpy)~+* significantly deviates from the single-exponential behavior. The observed nonexponential behavior is attributed to the existence of multiple injection/adsorption sites on the Ti02 surface. Accordingly one would expect a wide range of lifetimes for the sensitizer adsorbed on Ti02. A simple biexponential kinetic analysis can provide an estimate of the upper and lower limits of emission lifetimes, z ’ ~and z”,. The fit of Ru(bpy)2(d~bpy)~+* decay to the biexponential decay is shown in Figure 5B. The lifetimes determined from this analysis are 1.69 f0.006 and 9.85 f0.044 ns for the fast and slow components, respectively. If the decrease in lifetime observed on Ti02 surface is entirely due to charge injection process (reaction 3), one could express lifetimes, z-‘~ or t”,as z’, = l/(kr
+ k,, + /Yet)
z?, = l/(k,
+ k,, + k”,,)
(4)
where k, and k,,, are the rate constants for radiative and nonradiative processes and Vet and ret are the upper and lower limits for the heterogeneous electron transfer process. The lifetime of Ru(bpy)2(d~bpy)~+* in the absence of electron transfer quenching can be estimated from the measurements on the alumina sample (TO = 259 ns). If we assume (k, k,,,)is the same on both oxide surfaces, one can obtain the rate constant for heterogeneous electron transfer from Ru(bpy)2(dcbpy)2+*
+
loooOk
-0
2
4 Time, ps
6
8
1000 .
v)
c
f 100:
8
10 0
20 30 40 1 D Time, ns Figure 5. (A, top) Luminescence decay of degassed samples of Ru(bpy)z(dcbpy)2f(a) on AI203 and (b) on Ti02 surfaces. The traces were recorded following 355 nm laser pulse excitation. (B, bottom) Kinetic analysis of the luminescence decay of Ru(bpy)z(dcbpy)2+on Ti02 surface. Dotted traces show the experimental decay profile of Ru(bpy)z(dcbpy)2f*emission and the laser scatter. The solid line shows the biexponential kinetic fit of the emission decay using the expression F(t) = U I exp(-t/sl) + u2 exp(-t/tz). The CHISQR for the fit was
10
1.25.
into Ti02 particles from the expression
K,, = 1 H , - l/z,
/Ye,= l/z?, - l/zo
(5) By substituting the values of ~s (1.69 ns) and r“,(9.85 ns), we obtain the values for k’,, and V e t as 5.5 x lo8 and 1.0 x lo8 s-I for the fast and slow component of the charge injection process. The charge injection from singlet excited sensitizer into the conduction band of a large bandgap semiconductor is usually considered to be an ultrafast process occurring in the picosecond time domain. The charge injection process in the case of organic dyes such as anthracene carboxylate,22 ~ q u a r a i n e sand , ~ ~cresyl violet24 has been shown to occur within 20 ps. Similar fast electron transfer has also been noted for Ru(H20)z2- on Ti02 surface at very low c o ~ e r a g e . ~On~ ,the ~ ~contrary, relatively smaller charge injection rate constants ( lo8- lo9 s-I) have been reported by several research groups investigating the photophysical behavior of ruthenium complexes adsorbed on various semiconductor Similarly, the charge injection from the triplet excited dyes into Ti02 and ZnO colloids has also been shown to occur on a slower time scale.27 The results presented here suggest that the electron transfer from the excited Ru(bpy)2(dcbpy)2+occurs with a relatively slower rate than the
J. Phys. Chem., Vol. 99, No. 27, 1995 10887
Ru(bpy)2(dcbpy)2f on A1203 and Ti02 Surfaces 0.09
,
*I
Durailon of Photolysis
a. Omln b. 10 mln c. 20 mln
i
Duration of Photolysis 0 min
0.06
k
b.30 min c.60 min
\
% Y
".""400
0.03
0.00 450
500
550
600
650
700
500
400
(dcbpy)2+-coatedTi02 particles with visible light. Diffuse reflectance spectra were recorded at different photolysis times using the untreated metal oxide sample as reference. The ordinate scale is expressed as Kubelka-Munk units.
singlet excited organic dyes but is comparable to the triplet excited dyes. It should be noted that the excited state of Ru(bpy)z(dcbpy)*+ involves metal-to-ligand charge transfer state, and implications are that such an electronic configuration of the excited state plays an important role in controlling the electron injection rates. An alternate explanation for the slower electron transfer stems from the work of Willig and co-workers.26 The release of electrons from trapping sites can also contribute to the regeneration of excited state and thus become a rate-limiting factor in the net electron transfer process. Although this is a possibility, the results described in the present work neither support nor rule out this argument. Whether electron trapping/detrapping at surface defects is a contributing factor or not, the emission lifetimes of several nanoseconds obtained for excited Ru(bpy)z(dcbpy)2+-modifiedTi02 particles indicate the experimental time frame of net electron transfer from Ru(bpy)2(dcbpy)2+* into Ti02 particles. Steady State Photolysis of Ru(I1) Adsorbed on A1203 and Ti02 Particle. ( a )Degassed Samples. When degassed samples were irradiated with visible light, the sample slowly turned blue as the injected electrons got trapped into Ti02 particles. Figure 6 shows the diffuse reflectance absorption spectrum recorded before and after 10 and 20 min of photolysis with visible light. The absorption features of these spectra are similar to the difference spectrum recorded in the laser flash photolysis experiment showing depletion of 470 nm band and a broad absorption in the red region. The growth in the absorption at wavelengths greater than 500 nm arises from the trapping of electrons at the Ti4+ sites on the Ti02 surface.20s21The broader absorption in the infrared region can be attributed to filling of the deep traps during the long-term illumination. The identity of these trapped electrons can be tested by exposing the photolyzed sample to air. The blue color disappears instantly as 0 2 scavenges the trapped electrons, restoring the original orange color. To the best of our knowledge this is the first visible demonstration of an electron trapping process in powder systems induced by a charge injection from excited sensitizer into the semiconductor particles. No such color changes were observed when degassed A1203/Ru(II)samples were photolyzed with visible light. ( b ) Air-Equilibrated Samples. In order to probe long-term irradiation effects on the adsorbed sensitizer, we probed the absorption characteristics of R~(bpy)2(dcbpy)~+ adsorbed on both Ti02 and A1203 before and after the photolysis with visible light (A > 400 nm). The changes in the absorption spectra of
6
Wavelength, nm
Wavelength, nm Figure 6. Steady state photolysis of degassed sample of Ru(bpy)z-
550
0.25
B
*2A Duratlonof Photolyair a. 0 min b.30 mln c.60 mln
Omo5
t
0.00' 400
'
'
'
450
500
'
I
550
'
'
600
.
6 io
Wavelength, nm Figure 7. Steady state photolysis of air-equilibrated samples of Ru(bpy)*(dcbpy)*+ with visible light: (A) on Ti02 and (B) on A1& Diffuse reflectance spectra were recorded at different photolysis times using the untreated metal oxide as reference. The ordinate scale is expressed as Kubelka-Munk units.
these two samples are illustrated in Figure 7A,B. When airequilibrateddry samples of R~(bpy)2(dcbpy)~+ adsorbed on Ti02 were irradiated with visible light for few hours, Ru(bpy);?(dcbpy)2+ underwent degradation. This is evident from the decreased absorption of R~(bpy)2(dcbpy)~+ in the 460 nm absorption band (Figure 7A). Under similar steady state photolysis conditions no changes in the absorption of Ru(bpy)z(dcbpy)2+ on A 1 2 0 3 surface could be seen (Figure 7B). These experiments highlight the direct participation of semiconductor Ti02 in the charge transfer process (reaction 3) and irreversibly degrading the sensitizer. While a majority of the injected charge recombines back with the oxidized sensitizer, (Ru(bpy)2( d ~ b p y ) ~ +a , small fraction undergoes irreversible changes (reactions 6 and 7).
+
R~(bpy),(dcbpy)~+ MO(e)
-
Ru(bpy)2(dcbpy)2f
+ MO (6)
-
R~(bpy),(dcbpy)~+ products (7) A similar sensitization mechanism has been shown to control the degradation of several dyes on semiconductor surfaces.I4 In the photoelectrochemical cell employing Ru(bpy)2(d~bpy)~+ as the sensitizing dye, the redox couple, D/Df (e.g., 13-h-), quickly regenerates the sensitizer (reaction 8).5e3f36-7
+
R~(bpy),(dcbpy)~+ D
+
Ru(bpy),(dcbpy)*+
+ D+
(8)
Achieving sensitizer stability with a redox couple is essential in extending its life for the operation of a photoelectrochemical cell. The experiments shown in Figure 7A further ascertain
Vinodgopal et al.
10888 J. Phys. Chem., Vol. 99, No. 27, 1995
.3
8
8
I
.2
t
4
4
*I
'2
0
0
I 2000
I
I
I
I
1
1
1
1900
1800
1700
1600
1600
1400
1300
1200
2000
Wavenumber (cm-1)
Figure 8. Diffuse reflectance FTIR spectra in the region 1200-2000 cm-I of Ru(bpy)2(dcbpy)2+ (a) on A1203, (b) on Ti02 surface, and (c) mixed with KBr.
the importance of sensitizer regeneration since in the absence of this step sensitizer undergoes irreversible degradation. Diffuse Reflectance FTIR Study. In order to probe the surface photochemical events, a diffuse reflectance FTIR study of the ruthenium complex R~(bpy)z(dcbpy)~+ adsorbed on Ti02 powders was canied out at various irradiation times. There have been several attempts to characterize the vibrational frequencies of the ground and excited states of Ru(bpy)32+complexes (see, for example, refs 28-30). In general, the bands in the highfrequency region mainly originate from the aromatic ring of the ligand, and they are not sensitive to the metal.3' In the present experiments, the strong absorption from the Ti02 and A1203 substrate below 1200 cm-' makes it difficult to probe the metal-ligand vibrations. In Figure 8, we compare the diffuse reflectance FTIR spectrum of Ru(bpy)2(dcbpy)2+on KBr powder with the FTIR spectrum of the complex adsorbed on Ti02 and A1203 oxide supports. On KBr, Ru(bpy)2(dcbpy)z+ shows a characteristic strong carbonyl peak at 1740 cm-' arising from the free carboxylic acid group on the bipyridyl ligand. A characteristic aromatic ring stretching vibration at 1605 cm-' is also evident. However, major changes result when the compound is adsorbed on the oxides titania and alumina. The presence of surface OH groups on the Ti02 and A1203 surface results in a broad absorbance around 3000 cm-I, making interpretation of spectral features above 2500 cm-' d i f f i c ~ l t . ~Otherwise, ~,~~ the observed spectrum on Ti02 and A1203 looks reasonably similar. In both cases, the spectrum in the region of interest (2000-1000 cm-I) is quite broad and almost featureless. The C=O stretching band at 1700 cm-I observed on KBr has disappeared, and in its place we now have two broad bands centered at 1650 and 1450 cm-' and another somewhat sharper peak at 1380 cm-I. The broad feature at 1650 cm-' probably arises from a combination of surface OH groups on the Ti02 surface and the asymmetric stretch of the ~arboxylate.~~ The sharper peak seen at 1380 cm-' is most likely the symmetric stretching counterpart of the carboxylate These observations suggest that the interaction of the ruthenium complex with the oxide surface is via a carboxylate link. The other aspect to the spectrum to the complex on the oxide surfaces is the lower intensity of aromatic ring vibrations at -1600 cm-I, suggesting thereby that, following adsorption on the oxide surface, the bipyridyl ligand moiety is quite distorted away from planarity. Significant changes are observed in the FTIR spectrum following irradiation of the Ru(bpy)z(dcbpy)2+complex on TiO, with visible light. The results from the steady state photolysis cited in the previous section suggest that irreversible changes are occurring to the complex on the Ti02 surface following
1800
1600
1400
1200
1000
Wavenumber (em-1)
Figure 9. In situ diffuse reflectance FlTR spectra of Ru(bpy)2(dcbpy)2+ coated on Ti02 surface during steady state photolysis. The spectra were recorded at time intervals (a) 0, (b) 2.5, and (c) 18 h of irradiation with visible light.
irradiation. The FTIR spectra provide complementary evidence of such changes. Some representative spectra recorded before and during photolysis with visible light are shown in Figure 9. Spectrum a, which corresponds to the ruthenium complex on Ti02 before photolysis, has been discussed above. Following irradiation the broad spectrum begins to sharpen, and what was a featureless spectrum now begins to exhibit distinct features. The fist of these features is the development of a reasonably sharp band at 1573 cm-I, which we associate with an aromatic ring vibration. The second is the evident growth in the broad absorption around 1630 cm-I. While the surface hydroxyl groups still contribute to this band, the observed growth in the absorbance suggests that the bipyridyl ligand is undergoing changes. The sharpening of the FTIR bands also suggests that the photoproduct is not strongly attached to the surface. Although it is difficult to specify the exact chemical changes that occur following irradiation and charge transfer, the evidence from the FTIR spectra seems to suggest that the complex is breaking up following photosensitization. Oxidation of the 2,2'bipyridyl moiety is expected to occur at the 2-position, resulting in 2-hydroxy pyridine or its keto t a ~ t o m e r . This ~ ~ , is ~ probably ~ responsible for at least some of the absorption at 1630 cm-l. The literature spectrum of 2-hydroxypyridine shows fairly good agreement with the photolyzed ruthenium complex on Ti02.36 In particular, the peaks at 1577 and 1650 cm-' correlate very well with both spectra recorded after photolysis. Since these experiments were carried out in air, the reduced oxygen species formed at the semiconductor surface could play a role in determining eventual course of photodegradation. Although these FTIR studies do not establish the exact identity of the product, the spectral measurements confirm the findings of the absorption studies in Figure 7a that the Ru(bpy)z(dcbpy)2+ is irreversibly photodegraded on the surface of TiO2. On the other hand, the FTIR spectrum of Ru(bpy)2(dcb~y)~+coated A1203 samples exhibited little changes upon visible light photolysis for an extended period. This observation again supports our argument that the semiconducting property of Ti02 is responsible for the photodegradation of R~(bpy)2(dcbpy)~+
Conclusions The photochemical behavior of a model sensitizer, Ru(bpy)2(dcbpy)2+, has been investigated on the solid metal oxide surfaces possessing semiconductor (TiOz) and insulator (AlzO3) properties. The quenching of excited sensitizer by the semiconductor Ti02 indicates the charge injection from excited sensitizer into the semiconductor particle with rate constants ranging from 5.5 x los to 1.0 x lo8 s-I. Both laser flash and
J. Phys. Chem., Vol. 99,No. 27, 1995 10889
Ru(bpy)2(d~bpy)~+ on A1203 and Ti02 Surfaces steady state photolysis have demonstrated trapping of electrons injected from excited sensitizer at the semiconductor surface. The color changes (orange to blue) can be readily observed upon exposing the degassed samples to visible light. In the absence of a regenerative redox couple, long-term visible light irradiation leads to irreversible changes of Ru(bpy)2(dcbpy)2+ on Ti02 surface. 2-Hydroxypyridine has been identified as one of the photoproducts from the diffuse reflectance FTIR. On the other hand, the inert surface of A1203 had no influence on the excited state behavior of the sensitizer. The degradation aspect of the sensitizer has to be kept in mind while designing dye-modified nanocrystalline semiconductor films for photoelectrochemical conversion of solar energy into electricity.
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