Semiconductor Photocatalytic Composites

Sep 4, 2008 - Nano silver particles loaded on micrometer-size TiO2, nanosize TiO2, and BiVO4 were prepared by a silver mirror reaction (SMR), which is...
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J. Phys. Chem. C 2008, 112, 15423–15428

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Highly Effective Silver/Semiconductor Photocatalytic Composites Prepared by a Silver Mirror Reaction Zhichao Shan,†,‡ Jianjun Wu,† Fangfang Xu,† Fu-Qiang Huang,*,† and Hanming Ding*,‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China, and Department of Chemistry, East China Normal UniVersity, Shanghai 200062, P. R. China ReceiVed: May 21, 2008; ReVised Manuscript ReceiVed: July 21, 2008

Nano silver particles loaded on micrometer-size TiO2, nanosize TiO2, and BiVO4 were prepared by a silver mirror reaction (SMR), which is an old and well-known method. Morphology of silver on the surface of semiconductor particles was characterized by energy dispersion spectrum, transmission electron microscopy, and electron backscatter diffraction. The photocatalytic activities of these photocatalysts were evaluated by the degradation of methyl orange and hydrogen production under UV and visible light irradiation, respectively. The experimental results indicated that the photocatalytic activities of Ag/TiO2 and Ag/BiVO4 composites prepared by the SMR were remarkably higher than those prepared by the photoinduced deposition method. 1. Introduction The design, synthesis, and study of efficient photocatalysts for H2 production and pollutant degradation are the driving force and foundation of development of photocatalysis chemistry. A tremendous amount of research has been devoted to the design and synthesis of robust ligands, multimetallic active sites, secondary environments, and the functioning of the new catalysts. The performance of photocatalysis can be improved in several ways. One of the most effective approaches is to deposit metal particles on the surface of the semiconductors, which have been widely used as photocatalysts. These materials formed were represented as metal/semiconductor composites, and these metal particles were entitled “metal island”.1-10 The metal and the semiconductor generally have different Fermi level positions. A metal island contact with a semiconductor leads to the formation of a Schottky barrier, as illustrated in Figure 1. The electron migration from the semiconductor to the metal occurs until the two Fermi levels are aligned since the metal has a work function (φm) higher than that of the semiconductor (φs). The surface of the metal acquires an excess negative charge, while the semiconductor exhibits an excess positive charge as a result of electron migration away from the barrier region. A Schottky barrier forms at the metal-semiconductor interface. The height of the barrier (φb) is defined as the difference between the semiconductor conduction band and the metal Fermi level.11 The Schottky barrier formed at the metalsemiconductor interface can serve as an efficient electron trap to avoid the electron-hole recombination in a photocatalytic process. Figure 2 illustrates the mediating role of noble metals in storing and shuttling photogenerated electrons from the semiconductor to an acceptor in a photocatalytic process.11 Thus, the photoinduced electrons in the conduction band of the semiconductor are believed to readily transfer to the metal, which facilitates the separation of the photoinduced electron-hole * Corresponding author. Tel: +86-21-52411620. Fax: +86-21-52413903. E-mail: [email protected] (F.-Q.H.); [email protected] (H.D.). † Chinese Academy of Sciences. ‡ East China Normal University.

Figure 1. Schematic diagram of Schottky barrier.

pairs and effectively inhibits their recombination. The metal particles distributed on the surface of the semiconductors could greatly enhance the overall photocatalytic efficiency. Furthermore, metal loading on the semiconductor surface can shift the Fermi level to the more negative potential direction to improve the energetics of the composite system and the efficiency of the interfacial charge-transfer process.4,11 Silver ions and silver clusters were demonstrated to play a role in photochemical/photoelectrochemical waste oxidation and also water splitting experiments.12-14 Silver/semiconductor composites especially have excellent exhibitions to promote the photocatalytic activity under UV and visible light irradiation. The silver/semiconductor system, such as Ag/BiVO4,1 Ag/TiO2,6 Ag/AgCl12-14 and so forth, has a much higher photoactivity compared to the single semiconductor system. In these systems, silver can promote the charge-transfer process at the semiconductor/electrolyte interface to improve the photocatalytic oxidation capability of the semiconductor. However, to the best of our knowledge, the method employed to deposit silver particles onto semiconductor photocatalysts was either photoinduced deposition1,15-18 or calcination.19-23 The photocatalytic activity of silver photoinduced deposited semiconductor is usually much higher than that of a silver-loaded semiconductor prepared by calcination.24 Whereas the method

10.1021/jp804482k CCC: $40.75  2008 American Chemical Society Published on Web 09/04/2008

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Shan et al. UV irradiation for 4 h. The final product was filtered and dried in an oven at 60 °C for 1 h. The reaction equation is as follow:

2AgNO3 ) 2Ag + 2NO2 v + O2 v

Figure 2. Schematic diagram for charge separation in metal-loaded semiconductor.

using an irradiation source would cause aggregation of silver particles, calcination is tedious and energy-consuming because of the necessary thermal post-treatment, which may also result in sintering and/or aggregation of silver particles. To avoid these disadvantages and further improve the photocatalytic activity, in the present study, we developed a simple, convenient, readily available, and very effective method to deposit the small and uniform-distributed spherical silver nanoparticles over semiconductors by silver mirror reaction (SMR). In the SMR process, calcination or long-term UV irradiation are not needed, and the rate of silver nucleation is so high that the agglomeration of large-sized silver particles could be successfully avoided. The investigation results indicate that the silver-loaded photocatalysts prepared by the SMR show much higher photocatalytic activity than those by photoinduced deposition. Herein formation mechanisms for the silver/photocatalyst are also discussed. 2. Experimental Section 2.1. Chemical Reagents. All chemical reagents used in the present experiments are analytical reagents. AgNO3 (99.9%), HCHO (30%), Bi2O3 (99.5%), NH4VO3 (99.5%), and anatase TiO2 (0.5 µm, 99.5%) were purchased from Sinoreg. Nano TiO2 (Degussa, P25) consists of 30% rutile and 70% anatase with average size about 30 nm, and the BET specific surface area is 50 m2/g. 2.2. Photocatalyst Preparation. The Ag/TiO2 composites were synthesized by the SMR. Take the preparation of the 1 wt % Ag/TiO2 sample as an example: 50 mL of 0.0157 g of silver nitrate solution was placed in a 100-mL beaker, and then 5% ammonia was added dropwise into the silver nitrate solution under stirring until the brown precipitate was dissolved. One gram of TiO2 powders was introduced to the above solution under stirring for 30 min. Ten milliliters of 5% formaldehyde solution was dropped into the above system, and the stirring was kept at room temperature for 30 min. The following reaction took place:

2[Ag(NH3)2]+ + 2OH- + HCHO ) 2Ag V + 3NH3 v + HCOO- + H2O + NH4+ (1) Finally, the Ag/TiO2 composite was collected and filtered, washed with water and ethanol for several times, and then dried at 60 °C for 1 h in a vacuum oven. Ag/P25 was also prepared by the SMR. Ag/TiO2 photocatalyst was also prepared by the photoinduced deposition and used as a reference. In the photoinduced deposition process, AgNO3 was utilized as silver source and methanol was used to capture photoinduced holes induced by UV irradiation. The deposition process was carried out under

(2)

Powders of BiVO4 were prepared by a solid-state reaction process.25-27 Bi2O3 (99.5%) and NH4VO3 (99.5%) were selected as raw materials. Stoichiometric mixtures of the starting materials were ground and mixed thoroughly in an agate mortar. The well-mixed powders were preheated at 650 °C for 10 h and then calcined again in the same conditions for 10 h after being vigorously reground in a mortar. The composites Ag/BiVO4 were also prepared by the SMR and the photoinduced deposition mentioned above. The silver distribution was characterized by energy-dispersion spectrum (EDS, OXFORD-INCA), transmission electron microscopy (TEM, JEM-2100F), and electron back scatter diffraction (EBSD) mapping analysis system (Opal EBSD, Oxford) attached to a JEOL JSM-6700F scanning electron microscope. 2.3. Photocatalytic Experiments. The photocatalytic reactor of UV light-driven (λ < 420 nm) degradation reaction consists of two parts: a quartz cell with a circulating water jacket and a 500 W high-pressure mercury lamp with a maximum emission at 365 nm placed inside the quartz cell. In all experiments, the reaction temperature was kept at room temperature to prevent any thermal catalytic effect by using the circulating water. The volume of initial methyl orange (MO) solution was 300 mL, with a concentration of 30 mg/L. The Ag/TiO2 powders in the MO aqueous solution were 0.2 g. UV illumination was conducted after the suspension was strongly magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium of MO on the catalyst surface. Under irradiation, about 3 mL of suspension was continually taken from the reaction cell at given time intervals for subsequent MO concentration analysis after centrifuging by measuring its maximum absorption (464 nm) with a UV-vis spectrophotometer (Hitachi U-3010 spectrophotometer). Photocatalytic reactor over the photocatalyst Ag/BiVO4 under visible light irradiation was performed using another reactor system. The optical system for the catalytic reaction under visible light included a 300 W Xe arc lamp, a solution container, and a cutoff filter to exclude wavelengths shorter than 420 nm. The Xe arc lamp is appended above the container with a distance of 15 cm. In all the experiments, the reaction temperature was kept at room temperature to prevent any thermal catalytic effect by using the circulating water. The volume of initial MO solution was 200 mL, with a concentration of 10 mg/L. The powder concentration in the MO aqueous solution was 0.20 g/100 mL. Visible light illumination was conducted after the suspension was strongly magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium of MO on catalyst surfaces. Furthermore, we also tested the H2 production performance of the photocatalysts. The reactions were carried out in a gasclosed system, which was abounded with nitrogen. The 0.2 g of photocatalyst powders was dispersed in 250 mL of pure water and 50 mL of methanol for H2 production in the present suspension system by a magnetic stirrer in an inner irradiation cell made of quartz. The light source was a 500 W high-pressure mercury lamp. Gas production from an aqueous methanol was detected on a GC-7900 gas chromatograph with argon gas as carrier.

Photocatalytic Composites Prepared by Ag Mirror Reaction

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ln(C0 /C) ) kt

Figure 3. EDS of (a) Ag/TiO2 and (b) TiO2.

3. Experiment Results 3.1. Morphology of Nano Silver on the Surface of TiO2. To explain the difference in the photocatalytic activities between the samples prepared by using the SMR and the samples prepared by the photoinduced deposition, the microstructures of these samples were characterized. As shown in Figure 3, EDS reveal that the composite is composed of Ag and TiO2. The Cu element is from the copper net, on which the samples were loaded. EBSD and TEM images of the 3 wt % Ag/TiO2 composite are shown in Figure 4. In Figure 4a,b, the large particles with the average size about 0.5 µm are TiO2, and the bright dots are silver nanoparticles dispersed on the TiO2 particle surface. Silver particles with a uniform size of 20 nm were produced by the SMR, as shown in Figure 4c,d. The high dispersion of nanosized silver particles on TiO2 particles induces the SMR samples to possess superior photocatalytic activity compared to those of the photoinduced deposition ones, which contain bigger and agglomerate silver particles (Figure 4b). 3.2. Photocatalytic Activity. SMR provides an easy synthetic procedure for the Ag/TiO2 (microsized), Ag/P25 (nanosized), and Ag/BiVO4 composites with better photocatalytic performance at room temperature. The UV-assisted (λ e 420 nm) photocatalytic activities of the 2 wt % Ag loaded samples are shown in Figure 5. The decomposing MO over the photocatalysts is shown in Figure 5a,b. As shown in these figures, the photocatalytic activity of 2 wt % Ag/TiO2 (SMR) is higher than that of P25 and 2 wt % Ag/TiO2 (photoinduced deposition), and the activity of P25 can be further enhanced by SMR. Compared with the more expensive nanosized TiO2 (P25), commercially low-cost microsized TiO2 with the SMR Agloaded could be a more effective substitute. The H2 production over the 2 wt % Ag/TiO2 composite photocatalysts is shown in Figure 5c, and the H2 evolution rate of 90 µmol/h of the SMR sample is 30 µmol/h higher than that of the photoinduced deposition sample. Moreover, as shown in Figure 6, the photocatalytic activities of the samples loaded with a set of silver contents, such as 1, 2, and 3 wt %, prepared by the SMR and photoinduced deposition were investigated. The SMR photocatalysts exhibit much higher photocatalytic activity than the photoinduced deposition ones, and the photocatalytic activities follow the order of 2 wt % Ag/TiO2 > 3 wt % Ag/TiO2 > 1 wt % Ag/TiO2 > TiO2 in both SMR series and photoinduced deposition series. To quantitatively understand the reaction kinetics of the MO degradation in our experiments, we applied the pseudo-firstorder model as expressed by eq 3, which is generally used for the photocatalytic degradation process if the initial concentration of pollutant is low.

(3)

where C0 and C are the concentrations of dye in solution at time 0 and t, respectively, and k is the pseudo-first-order rate constant. Figure 7 is the photocatalytic reaction kinetics of MO degradation in solution based on the data plotted in Figure 4b. The rate constants obtained from Figure 7 are listed in Table 1. It can be seen that a rather good correlation to the pseudo-firstorder reaction kinetics (R > 0.99) was found. It is shown that the MO removal rate over 2 wt % Ag/P25 (SMR) is 0.207 min-1 and over 2 wt % Ag/TiO2 (SMR) it is 0.165 min-1; both values are higher than those of the reactions rate constants of P25 (0.129 min-1), 2 wt % Ag/TiO2 (photoinduced deposition) (0.084 min-1), and TiO2 (0.012 min-1). In particular, the constant remarkably rises from 0.012 min-1 for the single TiO2 sample to 0.165 min-1 for SMR, increased by almost 14 times. Moreover, to investigate the effectivity of the SMR on other photocatalysts, we extended the SMR to the other systems, such as BiVO4. Here, the visible light-assisted (λ g 420 nm) photocatalytic activities for decolorizing MO on 3 wt % Ag/BiVO4 photocatalysts are displayed in Figure 8. Similar to the case of UV light irradiation, Ag/BiVO4 from SMR is more photocatalytically active than that from photoinduced deposition for MO degradation. Additionally, the photodegradation data over the 1-3 wt % Ag/BiVO4 samples are shown in Figure 9. Among all of the samples, Ag/BiVO4 prepared by the SMR showed higher photocatalytic activity than that by the photoinduced deposition. The fact has also testified that the SMR can be broadly applied to the photocatalysts to obtain composites with higher performance. 4. Mechanism Discussions 4.1. Effect of ζ-Potential of Semiconductor. From the viewpoint of dynamic evolution of silver particles on the TiO2 particles and surface chemistry, several features of the SMR can explain the photocatalytic superiority. The ζ-potentials of semiconductors greatly affect the nucleation of Ag particles onto the semiconductors surface. Particles in solution usually develop charges at the solid/liquid interface. The interaction between two charged particles in a liquid is related to the osmotic pressure created by the increase of ion concentration between the particles whose electrical double layer overlaps and as a result detaches the particles.28-31 The charge of these surfaces becomes dependent on the degree of ionization of some functional groups on the surface, such as -OH, -COOH, and -H. The different pH values can have a diverse effect on the surface charge of TiO2 and play a key role in the adsorption of silver species.32-34 The pH of the isoelectric point (IEP) for anatase is estimated to be about 6.8.35-38 Therefore, the surface of TiO2 is positively charged in acidic media and negatively charged in alkaline media as shown in the following reactions:

Ti4+-OH + H+ f Ti4+-OH2+,

pH < pHIEP

Ti4+-OH + OH- f Ti4+-O- + H2O,

(4)

pH > pHIEP (5)

Alkaline Ag(NH3)2+ solution leads to the negative charge on the surface of TiO2 solid-phase (Ti4+-O-). Since Ag(NH3)2+ possesses positive charge, the Coulombic force between negative and positive charge is propitious to the adsorption of Ag(NH3)2+ and Ag nucleation onto the TiO2 surface. In acidic silver nitrate solution, on the contrary, the surface of TiO2 is positively charged (Ti4+-OH2+), which is repulsive to Ag+ and disadvantageous to the silver nucleation onto the TiO2 surface.

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Figure 4. (a) EBSD image of the 3 wt % SMR sample, (b) EBSD image of the 3 wt % photoinduced deposition sample, and (c, d) TEM images of the 3 wt % SMR sample at different scales.

Figure 5. (a) MO photodegradation within 15 min, (b) MO photodegradation, and (c) H2 photogeneration over 2 wt % Ag/TiO2 under UV light irradiation (λ e 420 nm) (photoinduced deposition, PD).

4.2. Effect of Silver Particle Size and Silver Contents. Furthermore, there is a competition between the nucleation and crystal growth. With high reaction speed, the tendency of nucleation is higher than that of crystal growth, implying that more crystal particles with smaller diameter will be obtained in the system. Generally, the velocity of chemical reaction is

Figure 6. MO photodegradations on 1, 2, and 3 wt % Ag/TiO2 under UV light irradiation. (a) SMR and (b) PD.

much higher than that of photoinduced reaction. In this experiment, silver was created within seconds in the SMR reaction, which is much faster than that in the photoinduced deposition reaction completed in hours. Therefore, it leads to the formation of nanosized silver particles with uniform size and homogeneous dispersion on the TiO2 surface. Charge equilibrium between photoirradiated TiO2 and Ag nanoparticles was probed to elucidate the photocatalytic activity

Photocatalytic Composites Prepared by Ag Mirror Reaction

Figure 9. MO photodegradations under visble light irradiation on 1, 2, and 3 wt % Ag/BiVO4 photocatalysts prepared by (a) PD and (b) SMR.

Figure 7. Kinetics of MO decolorization in solutions.

TABLE 1: Pseudo-First-Order Rate Constants of MO Photodecomposition over Different Catalysts catalyst

k (min-1)

R

anatase TiO2 2 wt % Ag/TiO2 (photoinduced deposition) P25 2 wt % Ag/TiO2 (SMR) 2 wt % Ag/P25 (SMR)

0.01218 0.08409 0.12868 0.16543 0.20676

0.99453 0.99906 0.99578 0.99456 0.99313

of semiconductor-metal nanocomposites. In these composites, nanosize silver particles have the large total surface areas, and the high surface loading facilitates electron accumulation on the metal particles to favor the photocatalytic redox process.39 The metal deposits can strongly absorb molecules and ions in the solution to facilitate the chemical reaction.40 Furthermore, an efficient photocatalyst requires that the potential of the catalyst must be more negative than that of the acceptor dye. The larger difference, the larger the driving force for electron transfer.41 The charge distribution between the two phases (silver phase and solid phase) has a direct influence on the energetics of the composite by shifting the Fermi level to have more negative potentials. The size of the silver particles is one key factor that can potentially influence the electronic properties of the nanocomposite. The Fermi level (EF) of the semiconductor is directly related to the number of accumulated electrons, as illustrated in eq 6.4

EF ) ECB + kT ln nc/Nc

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(6)

ECB is the conduction band energy, nc is the density of accumulated electrons, and Nc is the charge carrier density of

Figure 8. MO photodegradation over BiVO4, 3% Ag/BiVO4 (PD), and 3% Ag/BiVO4 (SMR) under visible light irradiation (λ g 420 nm).

the semiconductor. If the TiO2 or Ag/TiO2 system accumulates more electrons, a negative shift in the Fermi level of the TiO2 would be expected. Obviously, smaller silver particles loading on TiO2 could accumulate many more electrons than larger silver particles. Higher degree of electron accumulation causes the composite system to shift Fermi level to a more negative potential, which is reflected in higher photocatalytic reduction efficiency and higher photocurrent generation. Therefore, very fast surface reaction rate for smaller particles could be attributed to the higher electron transfer caused by their larger Fermi level shift.42-44 This explains why the degradation rate is greatly enhanced as the number of smaller particles increase. In addition, the photocatalytic activity of Ag/TiO2 decreases when silver loading is more than 2 wt %. Athough Schottky barrier favors electron flow from TiO2 to silver islands, too many negatively charged silver sites can recombine positively charged holes to reduce the photocatalystic activity.45 In addition, the excessive silver source results in the growth and agglomeration of silver nanoparticles, which would absorb and scatter light and near-UV radiation. Thus, silver can provide a light-filtering effect to reduce the number of photons absorbed by the photocatalyst. This reduction lowers the apparent photoquantum efficiency of the photocatalytic reaction and also decreases the probability of holes reacting with absorbed species at the TiO2 surface. 4.3. Effect of Methanol Addition. TiO2 could not split water into H2 and O2 in a similar aqueous suspension, which might be attributed to the photogenerated electron and hole recombination reaction as well as the back reaction between the produced H2 and O2. Photoefficiency of the H2 production process can be improved by the addition of sacrificial reagents.46,47 Sacrificial reagents act as a potential electron donor to the valence band hole, separate the photoexcited electrons and holes, and prevent the O2 evolution from an aqueous suspension system, which would improve the efficiency of H2 production. For photocatalytic hydrogen generation, compounds such as methanol, ethanol, Na2S, Na2SO4, or ions such as I-, IO3-, CN-, and Fe3+ can be used as sacrificial reagents.48-50 Of all the sacrificial reagents used, methanol was found to be one of the best capture reagents for the photogenerated holes. Therefore, methanol was applied as electron donors for H2 production, which can capture the photogenerated holes and then be oxidized to CO2 as follows:

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hv, cat

CH2OH 98 HCHO + H2

(7)

hv, cat

HCHO + H2O 98 HCOOH + H2

(8)

hv, cat

HCOOH 98 CO2 + H2

(9)

5. Conclusions In summary, in the scope of silver/semiconductor composites, we used the most conventional and simple method (SMR) to achieve the highest level of photocatalytic activity. Compared with the photoinduced deposition, SMR can obviously produce much smaller homogenized silver particles with narrow size distribution and closer connection to semiconductors surface. These highly dispersed nanosized silver particles will lead to superior photocatalytic activity to the compositions prepared by the photoinduced deposition, in both degrading methyl orange and H2 production. Moreover, 2 wt % Ag/TiO2 (microsized) prepared by SMR showed higher performance than that of P25, indicating that the low-cost microsized TiO2 decorated with Ag by SMR is an economical way to enhance the photocatalytic activity. To summarize, highly dispersed nanosized metal particles, closely jointed solid surface, and suitable metal contents lead to superior photocatalytic activity for dye degradation and H2 production. Acknowledgment. The research was financially supported by National 973 Program of China Grant 2007CB936704, National Science Foundation of China Grant 50772123, and Science and Technology Commission of Shanghai Grant 0752nm016. References and Notes (1) Kohtani, S.; Hiro, J.; Yamamoto, N.; Kudo, A.; Tokumura, K.; Nakagaki, R. Catal. Commun. 2005, 6, 185–189. (2) Maeda, K.; Teramura, K. Nature 2006, 440, 295. (3) Yin, J.; Zou, Z. G.; Ye, J. H. J. Phys. Chem. B 2004, 108, 12790– 12794. (4) Subramanian, V.; Wolf, E. E.; Kamat, P. V. J. Am. Chem. Soc. 2004, 126, 4943–4950. (5) Zou, Z. G.; Ye, J. H.; Arakawa, H. Chem. Mater. 2001, 13, 1765– 1769. (6) Wold, A. Chem. Mater. 1993, 5, 280–283. (7) Yin, J.; Zou, Z. G.; Ye, J. H. Chem. Phys. Lett. 2003, 378, 24–28. (8) Linkous, C. A.; Carter, G. J.; Locuson, D. B.; Ouellette, A. J.; Slattery, D. K.; Smitha, L. A. EnViron. Sci. Technol. 2000, 34, 4754–4758. (9) Konta, R.; Ishii, T.; Kato, H.; Kudo, A. J. Phys. Chem. B 2004, 108, 8992–8995. (10) Tada, H.; Teranishi, K.; Ito, S.; Kobayashi, H.; Kitagawa, S. Langmuir 2000, 16, 6077–6080. (11) Linsebigler, A. L.; Lu, G.; Yates, J. T. Chem. ReV. 1995, 95, 735– 758. (12) Schurch, D.; Currao, A.; Sarkar, S.; Hodes, G.; Calzaferri, G. J. Phys. Chem. B 2002, 106, 12764–12775. (13) Glaus, S.; Calzaferri, G.; Hoffmann, R. Chem.-Eur. J. 2002, 8, 1785–1794. (14) Currao, A.; Reddy, V. R.; Veen, M. K.; Schropp, R. E. I.; Calzaferri, G. Photochem. Photobiol. Sci. 2004, 3, 1017–1025.

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